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+The Project Gutenberg EBook of An Elementary Study of Chemistry, by 
+William McPherson and William Edwards Henderson
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: An Elementary Study of Chemistry
+
+Author: William McPherson
+        William Edwards Henderson
+
+Release Date: March 18, 2007 [EBook #20848]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK AN ELEMENTARY STUDY OF CHEMISTRY ***
+
+
+
+
+Produced by Elaine Walker, Josephine Paolucci and the
+Online Distributed Proofreading Team at http://www.pgdp.net
+
+
+
+
+
+
+
+
+
+AN ELEMENTARY
+
+STUDY OF CHEMISTRY
+
+BY
+
+WILLIAM McPHERSON, PH.D.
+
+PROFESSOR OF CHEMISTRY, OHIO STATE UNIVERSITY
+
+AND
+
+WILLIAM EDWARDS HENDERSON, PH.D.
+
+ASSOCIATE PROFESSOR OF CHEMISTRY, OHIO STATE UNIVERSITY
+
+
+_REVISED EDITION_
+
+GINN & COMPANY
+BOSTON * NEW YORK * CHICAGO * LONDON
+
+
+COPYRIGHT, 1905, 1906, BY
+WILLIAM MCPHERSON AND WILLIAM E. HENDERSON
+
+ALL RIGHTS RESERVED
+
+
+The Athenæum Press
+GINN & COMPANY * PROPRIETORS * BOSTON * U.S.A.
+
+
+
+
+Transcriber's note:
+
+For Text: A word surrounded by a cedilla such as ~this~ signifies that
+the word is bolded in the text. A word surrounded by underscores like
+_this_ signifies the word is italics in the text. The italic and bold
+markup for single italized letters (such as variables in equations) and
+"foreign" abbreviations are deleted for easier reading.
+
+For numbers and equations: Parentheses have been added to clarify
+fractions. Underscores before bracketed numbers in equations denote a
+subscript. Superscripts are designated with a caret and brackets, e.g.
+11.1^{3} is 11.1 to the third power.
+
+Appendix A and B have been moved to the end of the book.
+Minor typos have been corrected.
+
+
+
+
+PREFACE
+
+
+In offering this book to teachers of elementary chemistry the authors
+lay no claim to any great originality. It has been their aim to prepare
+a text-book constructed along lines which have become recognized as best
+suited to an elementary treatment of the subject. At the same time they
+have made a consistent effort to make the text clear in outline, simple
+in style and language, conservatively modern in point of view, and
+thoroughly teachable.
+
+The question as to what shall be included in an elementary text on
+chemistry is perhaps the most perplexing one which an author must
+answer. While an enthusiastic chemist with a broad understanding of the
+science is very apt to go beyond the capacity of the elementary student,
+the authors of this text, after an experience of many years, cannot help
+believing that the tendency has been rather in the other direction. In
+many texts no mention at all is made of fundamental laws of chemical
+action because their complete presentation is quite beyond the
+comprehension of the student, whereas in many cases it is possible to
+present the essential features of these laws in a way that will be of
+real assistance in the understanding of the science. For example, it is
+a difficult matter to deduce the law of mass action in any very simple
+way; yet the elementary student can readily comprehend that reactions
+are reversible, and that the point of equilibrium depends upon, rather
+simple conditions. The authors believe that it is worth while to
+present such principles in even an elementary and partial manner because
+they are of great assistance to the general student, and because they
+make a foundation upon which the student who continues his studies to
+more advanced courses can securely build.
+
+The authors have no apologies to make for the extent to which they have
+made use of the theory of electrolytic dissociation. It is inevitable
+that in any rapidly developing science there will be differences of
+opinion in regard to the value of certain theories. There can be no
+question, however, that the outline of the theory of dissociation here
+presented is in accord with the views of the very great majority of the
+chemists of the present time. Moreover, its introduction to the extent
+to which the authors have presented it simplifies rather than increases
+the difficulties with which the development of the principles of the
+science is attended.
+
+The oxygen standard for atomic weights has been adopted throughout the
+text. The International Committee, to which is assigned the duty of
+yearly reporting a revised list of the atomic weights of the elements,
+has adopted this standard for their report, and there is no longer any
+authority for the older hydrogen standard. The authors do not believe
+that the adoption of the oxygen standard introduces any real
+difficulties in making perfectly clear the methods by which atomic
+weights are calculated.
+
+The problems appended to the various chapters have been chosen with a
+view not only of fixing the principles developed in the text in the mind
+of the student, but also of enabling him to answer such questions as
+arise in his laboratory work. They are, therefore, more or less
+practical in character. It is not necessary that all of them should be
+solved, though with few exceptions the lists are not long. The answers
+to the questions are not directly given in the text as a rule, but can
+be inferred from the statements made. They therefore require independent
+thought on the part of the student.
+
+With very few exceptions only such experiments are included in the text
+as cannot be easily carried out by the student. It is expected that
+these will be performed by the teacher at the lecture table. Directions
+for laboratory work by the student are published in a separate volume.
+
+While the authors believe that the most important function of the
+elementary text is to develop the principles of the science, they
+recognize the importance of some discussion of the practical application
+of these principles to our everyday life. Considerable space is
+therefore devoted to this phase of chemistry. The teacher should
+supplement this discussion whenever possible by having the class visit
+different factories where chemical processes are employed.
+
+Although this text is now for the first time offered to teachers of
+elementary chemistry, it has nevertheless been used by a number of
+teachers during the past three years. The present edition has been
+largely rewritten in the light of the criticisms offered, and we desire
+to express our thanks to the many teachers who have helped us in this
+respect, especially to Dr. William Lloyd Evans of this laboratory, a
+teacher of wide experience, for his continued interest and helpfulness.
+We also very cordially solicit correspondence with teachers who may find
+difficulties or inaccuracies in the text.
+
+The authors wish to make acknowledgments for the photographs and
+engravings of eminent chemists from which the cuts included in the text
+were taken; to Messrs. Elliott and Fry, London, England, for that of
+Ramsay; to The Macmillan Company for those of Davy and Dalton, taken
+from the Century Science Series; to the L. E. Knott Apparatus Company,
+Boston, for that of Bunsen.
+
+THE AUTHORS
+
+OHIO STATE UNIVERSITY
+
+COLUMBUS, OHIO
+
+
+
+
+CONTENTS
+
+
+CHAPTER                                                           PAGE
+     I. INTRODUCTION                                                 1
+
+    II. OXYGEN                                                      13
+
+   III. HYDROGEN                                                    28
+
+    IV. WATER AND HYDROGEN DIOXIDE                                  40
+
+     V. THE ATOMIC THEORY                                           59
+
+    VI. CHEMICAL EQUATIONS AND CALCULATIONS                         68
+
+   VII. NITROGEN AND THE RARE ELEMENTS IN THE ATMOSPHERE            78
+
+  VIII. THE ATMOSPHERE                                              83
+
+    IX. SOLUTIONS                                                   94
+
+     X. ACIDS, BASES, AND SALTS; NEUTRALIZATION                    106
+
+    XI. VALENCE                                                    116
+
+   XII. COMPOUNDS OF NITROGEN                                      122
+
+  XIII. REVERSIBLE REACTIONS AND CHEMICAL EQUILIBRIUM              137
+
+   XIV. SULPHUR AND ITS COMPOUNDS                                  143
+
+    XV. PERIODIC LAW                                               165
+
+   XVI. THE CHLORINE FAMILY                                        174
+
+  XVII. CARBON AND SOME OF ITS SIMPLER COMPOUNDS                   196
+
+ XVIII. FLAMES,--ILLUMINANTS                                       213
+
+   XIX. MOLECULAR WEIGHTS, ATOMIC WEIGHTS, FORMULAS                223
+
+    XX. THE PHOSPHORUS FAMILY                                      238
+
+   XXI. SILICON, TITANIUM, BORON                                   257
+
+  XXII. THE METALS                                                 267
+
+ XXIII. THE ALKALI METALS                                          274
+
+  XXIV. THE ALKALINE-EARTH FAMILY                                  300
+
+   XXV. THE MAGNESIUM FAMILY                                       316
+
+  XXVI. THE ALUMINIUM FAMILY                                       327
+
+ XXVII. THE IRON FAMILY                                            338
+
+XXVIII. COPPER, MERCURY, AND SILVER                                356
+
+  XXIX. TIN AND LEAD                                               370
+
+   XXX. MANGANESE AND CHROMIUM                                     379
+
+  XXXI. GOLD AND THE PLATINUM FAMILY                               390
+
+ XXXII. SOME SIMPLE ORGANIC COMPOUNDS                              397
+
+INDEX                                                              421
+
+APPENDIX A                                                Facing back cover
+
+APPENDIX B                                                Inside back cover
+
+
+
+
+LIST OF FULL-PAGE ILLUSTRATIONS
+
+
+                                                                  PAGE
+ANTOINE LAURENT LAVOISIER                                  _Frontispiece_
+
+JOSEPH PRIESTLEY                                                   14
+
+JOHN DALTON                                                        60
+
+WILLIAM RAMSAY                                                     82
+
+DMITRI IVANOVITCH MENDELÉEFF                                      166
+
+HENRI MOISSAN                                                     176
+
+SIR HUMPHRY DAVY                                                  276
+
+ROBERT WILHELM BUNSEN                                             298
+
+
+
+
+AN ELEMENTARY STUDY OF CHEMISTRY
+
+
+
+
+CHAPTER I
+
+INTRODUCTION
+
+
+~The natural sciences.~ Before we advance very far in the study of nature,
+it becomes evident that the one large study must be divided into a
+number of more limited ones for the convenience of the investigator as
+well as of the student. These more limited studies are called the
+_natural sciences_.
+
+Since the study of nature is divided in this way for mere convenience,
+and not because there is any division in nature itself, it often happens
+that the different sciences are very intimately related, and a thorough
+knowledge of any one of them involves a considerable acquaintance with
+several others. Thus the botanist must know something about animals as
+well as about plants; the student of human physiology must know
+something about physics as well as about the parts of the body.
+
+~Intimate relation of chemistry and physics.~ Physics and chemistry are
+two sciences related in this close way, and it is not easy to make a
+precise distinction between them. In a general way it may be said that
+they are both concerned with inanimate matter rather than with living,
+and more particularly with the changes which such matter may be made to
+undergo. These changes must be considered more closely before a
+definition of the two sciences can be given.
+
+~Physical changes.~ One class of changes is not accompanied by an
+alteration in the composition of matter. When a lump of coal is broken
+the pieces do not differ from the original lump save in size. A rod of
+iron may be broken into pieces; it may be magnetized; it may be heated
+until it glows; it may be melted. In none of these changes has the
+composition of the iron been affected. The pieces of iron, the
+magnetized iron, the glowing iron, the melted iron, are just as truly
+iron as was the original rod. Sugar may be dissolved in water, but
+neither the sugar nor the water is changed in composition. The resulting
+liquid has the sweet taste of sugar; moreover the water may be
+evaporated by heating and the sugar recovered unchanged. Such changes
+are called _physical changes_.
+
+DEFINITION: _Physical changes are those which do not involve a change in
+the composition of the matter._
+
+~Chemical changes.~ Matter may undergo other changes in which its
+composition is altered. When a lump of coal is burned ashes and
+invisible gases are formed which are entirely different in composition
+and properties from the original coal. A rod of iron when exposed to
+moist air is gradually changed into rust, which is entirely different
+from the original iron. When sugar is heated a black substance is formed
+which is neither sweet nor soluble in water. Such changes are evidently
+quite different from the physical changes just described, for in them
+new substances are formed in place of the ones undergoing change.
+Changes of this kind are called _chemical changes_.
+
+DEFINITION: _Chemical changes are those which involve a change in the
+composition of the matter._
+
+~How to distinguish between physical and chemical changes.~ It is not
+always easy to tell to which class a given change belongs, and many
+cases will require careful thought on the part of the student. The test
+question in all cases is, Has the composition of the substance been
+changed? Usually this can be answered by a study of the properties of
+the substance before and after the change, since a change in composition
+is attended by a change in properties. In some cases, however, only a
+trained observer can decide the question.
+
+~Changes in physical state.~ One class of physical changes should be noted
+with especial care, since it is likely to prove misleading. It is a
+familiar fact that ice is changed into water, and water into steam, by
+heating. Here we have three different substances,--the solid ice, the
+liquid water, and the gaseous steam,--the properties of which differ
+widely. The chemist can readily show, however, that these three bodies
+have exactly the same composition, being composed of the same substances
+in the same proportion. Hence the change from one of these substances
+into another is a physical change. Many other substances may, under
+suitable conditions, be changed from solids into liquids, or from
+liquids into gases, without change in composition. Thus butter and wax
+will melt when heated; alcohol and gasoline will evaporate when exposed
+to the air. _The three states--solid, liquid, and gas--are called the
+three physical states of matter._
+
+~Physical and chemical properties.~ Many properties of a substance can be
+noted without causing the substance to undergo chemical change, and are
+therefore called its _physical properties_. Among these are its physical
+state, color, odor, taste, size, shape, weight. Other properties are
+only discovered when the substance undergoes chemical change. These are
+called its _chemical properties_. Thus we find that coal burns in air,
+gunpowder explodes when ignited, milk sours when exposed to air.
+
+~Definition of physics and chemistry.~ It is now possible to make a
+general distinction between physics and chemistry.
+
+DEFINITION: _Physics is the science which deals with those changes in
+matter which do not involve a change in composition._
+
+DEFINITION: _Chemistry is the science which deals with those changes in
+matter which do involve a change in composition._
+
+~Two factors in all changes.~ In all the changes which matter can undergo,
+whether physical or chemical, two factors must be taken into account,
+namely, _energy_ and _matter_.
+
+~Energy.~ It is a familiar fact that certain bodies have the power to do
+work. Thus water falling from a height upon a water wheel turns the
+wheel and in this way does the work of the mills. Magnetized iron
+attracts iron to itself and the motion of the iron as it moves towards
+the magnet can be made to do work. When coal is burned it causes the
+engine to move and transports the loaded cars from place to place. When
+a body has this power to do work it is said to possess energy.
+
+~Law of conservation of energy.~ Careful experiments have shown that when
+one body parts with its energy the energy is not destroyed but is
+transferred to another body or system of bodies. Just as energy cannot
+be destroyed, neither can it be created. If one body gains a certain
+amount of energy, some other body has lost an equivalent amount. These
+facts are summed up in the law of conservation of energy which may be
+stated thus: _While energy can be changed from one form into another, it
+cannot be created or destroyed._
+
+~Transformations of energy.~ Although energy can neither be created nor
+destroyed, it is evident that it may assume many different forms. Thus
+the falling water may turn the electric generator and produce a current
+of electricity. The energy lost by the falling water is thus transformed
+into the energy of the electric current. This in turn may be changed
+into the energy of motion, as when the current is used for propelling
+the cars, or into the energy of heat and light, as when it is used for
+heating and lighting the cars. Again, the energy of coal may be
+converted into energy of heat and subsequently of motion, as when it is
+used as a fuel in steam engines.
+
+Since the energy possessed by coal only becomes available when the coal
+is made to undergo a chemical change, it is sometimes called _chemical
+energy_. It is this form of energy in which we are especially interested
+in the study of chemistry.
+
+~Matter.~ Matter may be defined as that which occupies space and possesses
+weight. Like energy, matter may be changed oftentimes from one form into
+another; and since in these transformations all the other physical
+properties of a substance save weight are likely to change, the inquiry
+arises, Does the weight also change? Much careful experimenting has
+shown that it does not. The weight of the products formed in any change
+in matter always equals the weight of the substances undergoing change.
+
+~Law of conservation of matter.~ The important truth just stated is
+frequently referred to as the law of conservation of matter, and this
+law may be briefly stated thus: _Matter can neither be created nor
+destroyed, though it can be changed from one form into another._
+
+~Classification of matter.~ At first sight there appears to be no limit to
+the varieties of matter of which the world is made. For convenience in
+study we may classify all these varieties under three heads, namely,
+_mechanical mixtures_, _chemical compounds_, and _elements_.
+
+[Illustration: Fig. 1]
+
+~Mechanical mixtures.~ If equal bulks of common salt and iron filings are
+thoroughly mixed together, a product is obtained which, judging by its
+appearance, is a new substance. If it is examined more closely, however,
+it will be seen to be merely a mixture of the salt and iron, each of
+which substances retains its own peculiar properties. The mixture tastes
+just like salt; the iron particles can be seen and their gritty
+character detected. A magnet rubbed in the mixture draws out the iron
+just as if the salt were not there. On the other hand, the salt can be
+separated from the iron quite easily. Thus, if several grams of the
+mixture are placed in a test tube, and the tube half filled with water
+and thoroughly shaken, the salt dissolves in the water. The iron
+particles can then be filtered from the liquid by pouring the entire
+mixture upon a piece of filter paper folded so as to fit into the
+interior of a funnel (Fig. 1). The paper retains the solid but allows
+the clear liquid, known as the _filtrate_, to drain through. The iron
+particles left upon the filter paper will be found to be identical with
+the original iron. The salt can be recovered from the filtrate by
+evaporation of the water. To accomplish this the filtrate is poured into
+a small evaporating dish and gently heated (Fig. 2) until the water has
+disappeared, or _evaporated_. The solid left in the dish is identical in
+every way with the original salt. Both the iron and the salt have thus
+been recovered in their original condition. It is evident that no new
+substance has been formed by rubbing the salt and iron together. The
+product is called a _mechanical mixture_. Such mixtures are very common
+in nature, almost all minerals, sands, and soils being examples of this
+class of substances. It is at once apparent that there is no law
+regulating the composition of a mechanical mixture, and no two mixtures
+are likely to have exactly the same composition. The ingredients of a
+mechanical mixture can usually be separated by mechanical means, such as
+sifting, sorting, magnetic attraction, or by dissolving one constituent
+and leaving the other unchanged.
+
+[Illustration: Fig. 2]
+
+DEFINITION: _A mechanical mixture is one in which the constituents
+retain their original properties, no chemical action having taken place
+when they were brought together._
+
+~Chemical compounds.~ If iron filings and powdered sulphur are thoroughly
+ground together in a mortar, a yellowish-green substance results. It
+might easily be taken to be a new body; but as in the case of the iron
+and salt, the ingredients can readily be separated. A magnet draws out
+the iron. Water does not dissolve the sulphur, but other liquids do, as,
+for example, the liquid called carbon disulphide. When the mixture is
+treated with carbon disulphide the iron is left unchanged, and the
+sulphur can be obtained again, after filtering off the iron, by
+evaporating the liquid. The substance is, therefore, a mechanical
+mixture.
+
+If now a new portion of the mixture is placed in a dry test tube and
+carefully heated in the flame of a Bunsen burner, as shown in Fig. 3, a
+striking change takes place. The mixture begins to glow at some point,
+the glow rapidly extending throughout the whole mass. If the test tube
+is now broken and the product examined, it will be found to be a hard,
+black, brittle substance, in no way recalling the iron or the sulphur.
+The magnet no longer attracts it; carbon disulphide will not dissolve
+sulphur from it. It is a new substance with new properties, resulting
+from the chemical union of iron and sulphur, and is called iron
+sulphide. Such substances are called _chemical compounds_, and differ
+from mechanical mixtures in that the substances producing them lose
+their own characteristic properties. We shall see later that the two
+also differ in that the composition of a chemical compound never varies.
+
+[Illustration: Fig. 3]
+
+DEFINITION: _A chemical compound is a substance the constituents of
+which have lost their own characteristic properties, and which cannot be
+separated save by a chemical change._
+
+~Elements.~ It has been seen that iron sulphide is composed of two
+entirely different substances,--iron and sulphur. The question arises,
+Do these substances in turn contain other substances, that is, are they
+also chemical compounds? Chemists have tried in a great many ways to
+decompose them, but all their efforts have failed. Substances which have
+resisted all efforts to decompose them into other substances are called
+_elements_. It is not always easy to prove that a given substance is
+really an element. Some way as yet untried may be successful in
+decomposing it into other simpler forms of matter, and the supposed
+element will then prove to be a compound. Water, lime, and many other
+familiar compounds were at one time thought to be elements.
+
+DEFINITION: _An element is a substance which cannot be separated into
+simpler substances by any known means._
+
+~Kinds of matter.~ While matter has been grouped in three classes for the
+purpose of study, it will be apparent that there are really but two
+distinct kinds of matter, namely, compounds and elements. A mechanical
+mixture is not a third distinct kind of matter, but is made up of
+varying quantities of either compounds or elements or both.
+
+~Alchemy.~ In olden times it was thought that some way could be found to
+change one element into another, and a great many efforts were made to
+accomplish this transformation. Most of these efforts were directed
+toward changing the commoner metals into gold, and many fanciful ways
+for doing this were described. The chemists of that time were called
+_alchemists_, and the art which they practiced was called _alchemy_. The
+alchemists gradually became convinced that the only way common metals
+could be changed into gold was by the wonderful power of a magic
+substance which they called the _philosopher's stone_, which would
+accomplish this transformation by its mere touch and would in addition
+give perpetual youth to its fortunate possessor. No one has ever found
+such a stone, and no one has succeeded in changing one metal into
+another.
+
+~Number of elements.~ The number of substances now considered to be
+elements is not large--about eighty in all. Many of these are rare, and
+very few of them make any large fraction of the materials in the
+earth's crust. Clarke gives the following estimate of the composition of
+the earth's crust:
+
+    Oxygen       47.0%       Calcium           3.5%
+    Silicon      27.9        Magnesium         2.5
+    Aluminium     8.1        Sodium            2.7
+    Iron          4.7        Potassium         2.4
+                Other elements       1.2%
+
+A complete list of the elements is given in the Appendix. In this list
+the more common of the elements are marked with an asterisk. It is not
+necessary to study more than a third of the total number of elements to
+gain a very good knowledge of chemistry.
+
+~Physical state of the elements.~ About ten of the elements are gases at
+ordinary temperatures. Two--mercury and bromine--are liquids. The others
+are all solids, though their melting points vary through wide limits,
+from cæsium which melts at 26° to elements which do not melt save in the
+intense heat of the electric furnace.
+
+~Occurrence of the elements.~ Comparatively few of the elements occur as
+uncombined substances in nature, most of them being found in the form of
+chemical compounds. When an element does occur by itself, as is the case
+with gold, we say that it occurs in the _free state_ or _native_; when
+it is combined with other substances in the form of compounds, we say
+that it occurs in the _combined state_, or _in combination_. In the
+latter case there is usually little about the compound to suggest that
+the element is present in it; for we have seen that elements lose their
+own peculiar properties when they enter into combination with other
+elements. It would never be suspected, for example, that the reddish,
+earthy-looking iron ore contains iron.
+
+~Names of elements.~ The names given to the elements have been selected in
+a great many different ways. (1) Some names are very old and their
+original meaning is obscure. Such names are iron, gold, and copper. (2)
+Many names indicate some striking physical property of the element. The
+name bromine, for example, is derived from a Greek word meaning a
+stench, referring to the extremely unpleasant odor of the substance. The
+name iodine comes from a word meaning violet, alluding to the beautiful
+color of iodine vapor. (3) Some names indicate prominent chemical
+properties of the elements. Thus, nitrogen means the producer of niter,
+nitrogen being a constituent of niter or saltpeter. Hydrogen means water
+former, signifying its presence in water. Argon means lazy or inert, the
+element being so named because of its inactivity. (4) Other elements are
+named from countries or localities, as germanium and scandium.
+
+~Symbols.~ In indicating the elements found in compounds it is
+inconvenient to use such long names, and hence chemists have adopted a
+system of abbreviations. These abbreviations are known as _symbols_,
+each element having a distinctive symbol. (1) Sometimes the initial
+letter of the name will suffice to indicate the element. Thus I stands
+for iodine, C for carbon. (2) Usually it is necessary to add some other
+characteristic letter to the symbol, since several names may begin with
+the same letter. Thus C stands for carbon, Cl for chlorine, Cd for
+cadmium, Ce for cerium, Cb for columbium. (3) Sometimes the symbol is an
+abbreviation of the old Latin name. In this way Fe (ferrum) indicates
+iron, Cu (cuprum), copper, Au (aurum), gold. The symbols are included in
+the list of elements given in the Appendix. They will become familiar
+through constant use.
+
+~Chemical affinity the cause of chemical combination.~ The agency which
+causes substances to combine and which holds them together when combined
+is called _chemical affinity_. The experiments described in this
+chapter, however, show that heat is often necessary to bring about
+chemical action. The distinction between the cause producing chemical
+action and the circumstances favoring it must be clearly made. Chemical
+affinity is always the cause of chemical union. Many agencies may make
+it possible for chemical affinity to act by overcoming circumstances
+which stand in its way. Among these agencies are heat, light, and
+electricity. As a rule, solution also promotes action between two
+substances. Sometimes these agencies may overcome chemical attraction
+and so occasion the decomposition of a compound.
+
+
+EXERCISES
+
+1. To what class of changes do the following belong? (a) The melting
+of ice; (b) the souring of milk; (c) the burning of a candle; (d)
+the explosion of gunpowder; (e) the corrosion of metals. What test
+question must be applied in each of the above cases?
+
+2. Give two additional examples (a) of chemical changes; (b) of
+physical changes.
+
+3. Is a chemical change always accompanied by a physical change? Is a
+physical change always accompanied by a chemical change?
+
+4. Give two or more characteristics of a chemical change.
+
+5. (a) When a given weight of water freezes, does it absorb or evolve
+heat? (b) When the resulting ice melts, is the total heat change the
+same or different from that of freezing?
+
+6. Give three examples of each of the following: (a) mechanical
+mixtures; (b) chemical compounds; (c) elements.
+
+7. Give the derivation of the names of the following elements: thorium,
+gallium, selenium, uranium. (Consult dictionary.)
+
+8. Give examples of chemical changes which are produced through the
+agency of heat; of light; of electricity.
+
+
+
+
+CHAPTER II
+
+OXYGEN
+
+
+~History.~ The discovery of oxygen is generally attributed to the English
+chemist Priestley, who in 1774 obtained the element by heating a
+compound of mercury and oxygen, known as red oxide of mercury. It is
+probable, however, that the Swedish chemist Scheele had previously
+obtained it, although an account of his experiments was not published
+until 1777. The name oxygen signifies acid former. It was given to the
+element by the French chemist Lavoisier, since he believed that all
+acids owe their characteristic properties to the presence of oxygen.
+This view we now know to be incorrect.
+
+~Occurrence.~ Oxygen is by far the most abundant of all the elements. It
+occurs both in the free and in the combined state. In the free state it
+occurs in the air, 100 volumes of dry air containing about 21 volumes of
+oxygen. In the combined state it forms eight ninths of water and nearly
+one half of the rocks composing the earth's crust. It is also an
+important constituent of the compounds which compose plant and animal
+tissues; for example, about 66% by weight of the human body is oxygen.
+
+~Preparation.~ Although oxygen occurs in the free state in the atmosphere,
+its separation from the nitrogen and other gases with which it is mixed
+is such a difficult matter that in the laboratory it has been found more
+convenient to prepare it from its compounds. The most important of the
+laboratory methods are the following:
+
+1. _Preparation from water._ Water is a compound, consisting of 11.18%
+hydrogen and 88.82% oxygen. It is easily separated into these
+constituents by passing an electric current through it under suitable
+conditions. The process will be described in the chapter on water. While
+this method of preparation is a simple one, it is not economical.
+
+2. _Preparation from mercuric oxide._ This method is of interest, since
+it is the one which led to the discovery of oxygen. The oxide, which
+consists of 7.4% oxygen and 92.6% mercury, is placed in a small, glass
+test tube and heated. The compound is in this way decomposed into
+mercury which collects on the sides of the glass tube, forming a silvery
+mirror, and oxygen which, being a gas, escapes from the tube. The
+presence of the oxygen is shown by lighting the end of a splint,
+extinguishing the flame and bringing the glowing coal into the mouth of
+the tube. The oxygen causes the glowing coal to burst into a flame.
+
+     In a similar way oxygen may be obtained from its compounds with
+     some of the other elements. Thus manganese dioxide, a black
+     compound of manganese and oxygen, when heated to about 700°,
+     loses one third of its oxygen, while barium dioxide, when
+     heated, loses one half of its oxygen.
+
+3. _Preparation from potassium chlorate (usual laboratory method)._
+Potassium chlorate is a white solid which consists of 31.9% potassium,
+28.9% chlorine, and 39.2% oxygen. When heated it undergoes a series of
+changes in which all the oxygen is finally set free, leaving a compound
+of potassium and chlorine called potassium chloride. The change may be
+represented as follows:
+
+   /potassium\
+  |           | (potassium     / potassium \ (potassium
+ {  chlorine   }            = {             }             + oxygen
+  |           |  chlorate)     \ chlorine  /  chloride)
+   \oxygen   /
+
+[Illustration: JOSEPH PRIESTLEY (English) (1733-1804)
+
+School-teacher, theologian, philosopher, scientist; friend of Benjamin
+Franklin; discoverer of oxygen; defender of the phlogiston theory; the
+first to use mercury in a pneumatic trough, by which means he first
+isolated in gaseous form hydrochloric acid, sulphur dioxide, and
+ammonia]
+
+The evolution of the oxygen begins at about 400°. It has been found,
+however, that if the potassium chlorate is mixed with about one fourth
+its weight of manganese dioxide, the oxygen is given off at a much lower
+temperature. Just how the manganese dioxide brings about this result is
+not definitely known. The amount of oxygen obtained from a given weight
+of potassium chlorate is exactly the same whether the manganese dioxide
+is present or not. So far as can be detected the manganese dioxide
+undergoes no change.
+
+[Illustration: Fig. 4]
+
+~Directions for preparing oxygen.~ The manner of preparing oxygen from
+potassium chlorate is illustrated in the accompanying diagram (Fig. 4).
+A mixture consisting of one part of manganese dioxide and four parts of
+potassium chlorate is placed in the flask A and gently heated. The
+oxygen is evolved and escapes through the tube B. It is collected by
+bringing over the end of the tube the mouth of a bottle completely
+filled with water and inverted in a vessel of water, as shown in the
+figure. The gas rises in the bottle and displaces the water. In the
+preparation of large quantities of oxygen, a copper retort (Fig. 5) is
+often substituted for the glass flask.
+
+[Illustration: Fig. 5]
+
+In the preparation of oxygen from potassium chlorate and manganese
+dioxide, the materials used must be pure, otherwise a violent explosion
+may occur. The purity of the materials is tested by heating a small
+amount of the mixture in a test tube.
+
+~The collection of gases.~ The method used for collecting oxygen
+illustrates the general method used for collecting such gases as are
+insoluble in water or nearly so. The vessel C (Fig. 4), containing the
+water in which the bottles are inverted, is called a _pneumatic trough._
+
+~Commercial methods of preparation.~ Oxygen can now be purchased stored
+under great pressure in strong steel cylinders (Fig. 6). It is prepared
+either by heating a mixture of potassium chlorate and manganese dioxide,
+or by separating it from the nitrogen and other gases with which it is
+mixed in the atmosphere. The methods employed for effecting this
+separation will be described in subsequent chapters.
+
+[Illustration: Fig. 6]
+
+~Physical properties.~ Oxygen is a colorless, odorless, tasteless gas,
+slightly heavier than air. One liter of it, measured at a temperature of
+0° and under a pressure of one atmosphere, weighs 1.4285 g., while under
+similar conditions one liter of air weighs 1.2923 g. It is but slightly
+soluble in water. Oxygen, like other gases, may be liquefied by applying
+very great pressure to the highly cooled gas. When the pressure is
+removed the liquid oxygen passes again into the gaseous state, since its
+boiling point under ordinary atmospheric pressure is -182.5°.
+
+~Chemical properties.~ At ordinary temperatures oxygen is not very active
+chemically. Most substances are either not at all affected by it, or the
+action is so slow as to escape notice. At higher temperatures, however,
+it is very active, and unites directly with most of the elements. This
+activity may be shown by heating various substances until just ignited
+and then bringing them into vessels of the gas, when they will burn with
+great brilliancy. Thus a glowing splint introduced into a jar of oxygen
+bursts into flame. Sulphur burns in the air with a very weak flame and
+feeble light; in oxygen, however, the flame is increased in size and
+brightness. Substances which readily burn in air, such as phosphorus,
+burn in oxygen with dazzling brilliancy. Even substances which burn in
+air with great difficulty, such as iron, readily burn in oxygen.
+
+The burning of a substance in oxygen is due to the rapid combination of
+the substance or of the elements composing it with the oxygen. Thus,
+when sulphur burns both the oxygen and sulphur disappear as such and
+there is formed a compound of the two, which is an invisible gas, having
+the characteristic odor of burning sulphur. Similarly, phosphorus on
+burning forms a white solid compound of phosphorus and oxygen, while
+iron forms a reddish-black compound of iron and oxygen.
+
+~Oxidation.~ The term _oxidation_ is applied to the chemical change which
+takes place when a substance, or one of its constituent parts, combines
+with oxygen. This process may take place rapidly, as in the burning of
+phosphorus, or slowly, as in the oxidation (or rusting) of iron when
+exposed to the air. It is always accompanied by the liberation of heat.
+The amount of heat liberated by the oxidation of a definite weight of
+any given substance is always the same, being entirely independent of
+the rapidity of the process. If the oxidation takes place slowly, the
+heat is generated so slowly that it is difficult to detect it. If the
+oxidation takes place rapidly, however, the heat is generated in such a
+short interval of time that the substance may become white hot or burst
+into a flame.
+
+~Combustion; kindling temperature.~ When oxidation takes place so rapidly
+that the heat generated is sufficient to cause the substance to glow or
+burst into a flame the process is called _combustion_. In order that any
+substance may undergo combustion, it is necessary that it should be
+heated to a certain temperature, known as the _kindling temperature._
+This temperature varies widely for different bodies, but is always
+definite for the same body. Thus the kindling temperature of phosphorus
+is far lower than that of iron, but is definite for each. When any
+portion of a substance is heated until it begins to burn the combustion
+will continue without the further application of heat, provided the heat
+generated by the process is sufficient to bring other parts of the
+substance to the kindling temperature. On the other hand, if the heat
+generated is not sufficient to maintain the kindling temperature,
+combustion ceases.
+
+~Oxides.~ The compounds formed by the oxidation of any element are called
+_oxides_. Thus in the combustion of sulphur, phosphorus, and iron, the
+compounds formed are called respectively oxide of sulphur, oxide of
+phosphorus, and oxide of iron. In general, then, _an oxide is a compound
+of oxygen with another element_. A great many substances of this class
+are known; in fact, the oxides of all the common elements have been
+prepared, with the exception of those of fluorine and bromine. Some of
+these are familiar compounds. Water, for example, is an oxide of
+hydrogen, and lime an oxide of the metal calcium.
+
+~Products of combustion.~ The particular oxides formed by the combustion
+of any substance are called _products of combustion_ of that substance.
+Thus oxide of sulphur is the product of the combustion of sulphur; oxide
+of iron is the product of the combustion of iron. It is evident that the
+products of the combustion of any substance must weigh more than the
+original substance, the increase in weight corresponding to the amount
+of oxygen taken up in the act of combustion. For example, when iron
+burns the oxide of iron formed weighs more than the original iron.
+
+In some cases the products of combustion are invisible gases, so that
+the substance undergoing combustion is apparently destroyed. Thus, when
+a candle burns it is consumed, and so far as the eye can judge nothing
+is formed during combustion. That invisible gases are formed, however,
+and that the weight of these is greater than the weight of the candle
+may be shown by the following experiment.
+
+[Illustration: Fig. 7]
+
+     A lamp chimney is filled with sticks of the compound known as
+     sodium hydroxide (caustic soda), and suspended from the beam of
+     the balance, as shown in Fig. 7. A piece of candle is placed on
+     the balance pan so that the wick comes just below the chimney,
+     and the balance is brought to a level by adding weights to the
+     other pan. The candle is then lighted. The products formed pass
+     up through the chimney and are absorbed by the sodium
+     hydroxide. Although the candle burns away, the pan upon which
+     it rests slowly sinks, showing that the combustion is attended
+     by an increase in weight.
+
+     ~Combustion in air and in oxygen.~ Combustion in air and in
+     oxygen differs only in rapidity, the products formed being
+     exactly the same. That the process should take place less
+     rapidly in the former is readily understood, for the air is
+     only about one fifth oxygen, the remaining four fifths being
+     inert gases. Not only is less oxygen available, but much of the
+     heat is absorbed in raising the temperature of the inert gases
+     surrounding the substance undergoing combustion, and the
+     temperature reached in the combustion is therefore less.
+
+     ~Phlogiston theory of combustion.~ The French chemist Lavoisier
+     (1743-1794), who gave to oxygen its name was the first to show
+     that combustion is due to union with oxygen. Previous to his
+     time combustion was supposed to be due to the presence of a
+     substance or principle called _phlogiston_. One substance was
+     thought to be more combustible than another because it
+     contained more phlogiston. Coal, for example, was thought to be
+     very rich in phlogiston. The ashes left after combustion would
+     not burn because all the phlogiston had escaped. If the
+     phlogiston could be restored in any way, the substance would
+     then become combustible again. Although this view seems absurd
+     to us in the light of our present knowledge, it formerly had
+     general acceptance. The discovery of oxygen led Lavoisier to
+     investigate the subject, and through his experiments he arrived
+     at the true explanation of combustion. The discovery of oxygen
+     together with the part it plays in combustion is generally
+     regarded as the most important discovery in the history of
+     chemistry. It marked the dawn of a new period in the growth of
+     the science.
+
+~Combustion in the broad sense.~ According to the definition given above,
+the presence of oxygen is necessary for combustion. The term is
+sometimes used, however, in a broader sense to designate any chemical
+change attended by the evolution of heat and light. Thus iron and
+sulphur, or hydrogen and chlorine under certain conditions, will combine
+so rapidly that light is evolved, and the action is called a combustion.
+Whenever combustion takes place in the air, however, the process is one
+of oxidation.
+
+     ~Spontaneous combustion.~ The temperature reached in a given
+     chemical action, such as oxidation, depends upon the rate at
+     which the reaction takes place. This rate is usually increased
+     by raising the temperature of the substances taking part in the
+     action.
+
+     When a slow oxidation takes place under such conditions that
+     the heat generated is not lost by being conducted away, the
+     temperature of the substance undergoing oxidation is raised,
+     and this in turn hastens the rate of oxidation. The rise in
+     temperature may continue in this way until the kindling
+     temperature of the substance is reached, when combustion
+     begins. Combustion occurring in this way is called _spontaneous
+     combustion_.
+
+     Certain oils, such as the linseed oil used in paints, slowly
+     undergo oxidation at ordinary temperatures, and not
+     infrequently the origin of fires has been traced to the
+     spontaneous combustion of oily rags. The spontaneous combustion
+     of hay has been known to set barns on fire. Heaps of coal have
+     been found to be on fire when spontaneous combustion offered
+     the only possible explanation.
+
+~Importance of oxygen.~ 1. Oxygen is essential to life. Among living
+organisms only certain minute forms of plant life can exist without it.
+In the process of respiration the air is taken into the lungs where a
+certain amount of oxygen is absorbed by the blood. It is then carried to
+all parts of the body, oxidizing the worn-out tissues and changing them
+into substances which may readily be eliminated from the body. The heat
+generated by this oxidation is the source of the heat of the body. The
+small amount of oxygen which water dissolves from the air supports all
+the varied forms of aquatic animals.
+
+2. Oxygen is also essential to decay. The process of decay is really a
+kind of oxidation, but it will only take place in the presence of
+certain minute forms of life known as bacteria. Just how these assist in
+the oxidation is not known. By this process the dead products of animal
+and vegetable life which collect on the surface of the earth are slowly
+oxidized and so converted into harmless substances. In this way oxygen
+acts as a great purifying agent.
+
+3. Oxygen is also used in the treatment of certain diseases in which the
+patient is unable to inhale sufficient air to supply the necessary
+amount of oxygen.
+
+
+OZONE
+
+~Preparation.~ When electric sparks are passed through oxygen or air a
+small percentage of the oxygen is converted into a substance called
+_ozone_, which differs greatly from oxygen in its properties. The same
+change can also be brought about by certain chemical processes. Thus, if
+some pieces of phosphorus are placed in a bottle and partially covered
+with water, the presence of ozone may soon be detected in the air
+contained in the bottle. The conversion of oxygen into ozone is attended
+by a change in volume, 3 volumes of oxygen forming 2 volumes of ozone.
+If the resulting ozone is heated to about 300°, the reverse change
+takes place, the 2 volumes of ozone being changed back into 3 volumes of
+oxygen. It is possible that traces of ozone exist in the atmosphere,
+although its presence there has not been definitely proved, the tests
+formerly used for its detection having been shown to be unreliable.
+
+~Properties.~ As commonly prepared, ozone is mixed with a large excess of
+oxygen. It is possible, however, to separate the ozone and thus obtain
+it in pure form. The gas so obtained has the characteristic odor noticed
+about electrical machines when in operation. By subjecting it to great
+pressure and a low temperature, the gas condenses to a bluish liquid,
+boiling at -119°. When unmixed with other gases ozone is very explosive,
+changing back into oxygen with the liberation of heat. Its chemical
+properties are similar to those of oxygen except that it is far more
+active. Air or oxygen containing a small amount of ozone is now used in
+place of oxygen in certain manufacturing processes.
+
+~The difference between oxygen and ozone.~ Experiments show that in
+changing oxygen into ozone no other kind of matter is either added to
+the oxygen or withdrawn from it. The question arises then, How can we
+account for the difference in their properties? It must be remembered
+that in all changes we have to take into account _energy_ as well as
+_matter_. By changing the amount of energy in a substance we change its
+properties. That oxygen and ozone contain different amounts of energy
+may be shown in a number of ways; for example, by the fact that the
+conversion of ozone into oxygen is attended by the liberation of heat.
+The passage of the electric sparks through oxygen has in some way
+changed the energy content of the element and thus it has acquired new
+properties. _Oxygen and ozone must, therefore, be regarded as identical
+so far as the kind of matter of which they are composed is concerned.
+Their different properties are due to their different energy contents._
+
+~Allotropic states or forms of matter.~ Other elements besides oxygen may
+exist in more than one form. These different forms of the same element
+are called _allotropic states_ or _forms_ of the element. These forms
+differ not only in physical properties but also in their energy
+contents. Elements often exist in a variety of forms which look quite
+different. These differences may be due to accidental causes, such as
+the size or shape of the particles or the way in which the element was
+prepared. Only such forms, however, as have different energy contents
+are properly called allotropic forms.
+
+
+MEASUREMENT OF GAS VOLUMES
+
+~Standard conditions.~ It is a well-known fact that the volume occupied by
+a definite weight of any gas can be altered by changing the temperature
+of the gas or the pressure to which it is subjected. In measuring the
+volume of gases it is therefore necessary, for the sake of accuracy, to
+adopt some standard conditions of temperature and pressure. The
+conditions agreed upon are (1) a temperature of 0°, and (2) a pressure
+equal to the average pressure exerted by the atmosphere at the sea
+level, that is, 1033.3 g. per square centimeter. These conditions of
+temperature and pressure are known as the _standard conditions_, and
+when the volume of a gas is given it is understood that the measurement
+was made under these conditions, unless it is expressly stated
+otherwise. For example, the weight of a liter of oxygen has been given
+as 1.4285 g. This means that one liter of oxygen, measured at a
+temperature of 0° and under a pressure of 1033.3 g. per square
+centimeter, weighs 1.4285 g.
+
+The conditions which prevail in the laboratory are never the standard
+conditions. It becomes necessary, therefore, to find a way to calculate
+the volume which a gas will occupy under standard conditions from the
+volume which it occupies under any other conditions. This may be done in
+accordance with the following laws.
+
+~Law of Charles.~ This law expresses the effect which a change in the
+temperature of a gas has upon its volume. It may be stated as follows:
+_For every degree the temperature of a gas rises above zero the volume
+of the gas is increased by 1/273 of the volume which it occupies at
+zero; likewise for every degree the temperature of the gas falls below
+zero the volume of the gas is decreased by 1/273 of the volume which it
+occupies at zero, provided in both cases that the pressure to which the
+gas is subjected remains constant._
+
+If V represents the volume of gas at 0°, then the volume at 1° will be
+V + 1/273 V; at 2° it will be V + 2/273 V; or, in general, the
+volume v, at the temperature t, will be expressed by the formula
+
+    (1) v = V + t/273 V,
+
+    or (2) v = V(1 + (t/273)).
+
+Since 1/273 = 0.00366, the formula may be written
+
+    (3) v = V(1 + 0.00366t).
+
+Since the value of V (volume under standard conditions) is the one
+usually sought, it is convenient to transpose the equation to the
+following form:
+
+    (4) V = v/(1 + 0.00366t).
+
+The following problem will serve as an illustration of the application
+of this equation.
+
+The volume of a gas at 20° is 750 cc.; find the volume it will occupy at
+0°, the pressure remaining constant.
+
+In this case, v = 750 cc. and t = 20. By substituting these values,
+equation (4) becomes
+
+    V = 750/(1 + 0.00366 × 20) = 698.9 cc.
+
+~Law of Boyle.~ This law expresses the relation between the volume
+occupied by a gas and the pressure to which it is subjected. It may be
+stated as follows: _The volume of a gas is inversely proportional to the
+pressure under which it is measured, provided the temperature of the gas
+remains constant._
+
+If V represents the volume when subjected to a pressure P and v
+represents its volume when the pressure is changed to p, then, in
+accordance with the above law, V : v :: p : P, or VP = vp.
+In other words, for a given weight of a gas the product of the numbers
+representing its volume and the pressure to which it is subjected is a
+constant.
+
+Since the pressure of the atmosphere at any point is indicated by the
+barometric reading, it is convenient in the solution of the problems to
+substitute the latter for the pressure measured in grams per square
+centimeter. The average reading of the barometer at the sea level is 760
+mm., which corresponds to a pressure of 1033.3 g. per square centimeter.
+The following problem will serve as an illustration of the application
+of Boyle's law.
+
+A gas occupies a volume of 500 cc. in a laboratory where the barometric
+reading is 740 mm. What volume would it occupy if the atmospheric
+pressure changed so that the reading became 750 mm.?
+
+Substituting the values in the equation VP = vp, we have 500 × 740 =
+v × 750, or v = 493.3 cc.
+
+~Variations in the volume of a gas due to changes both in temperature and
+pressure.~ Inasmuch as corrections must be made as a rule for both
+temperature and pressure, it is convenient to combine the equations
+given above for the corrections for each, so that the two corrections
+may be made in one operation. The following equation is thus obtained:
+
+    (5) V_{s} = vp/(760(1 + 0.00366t)),
+
+in which V_{s} represents the volume of a gas under standard
+conditions and v, p, and t the volume, pressure, and temperature
+respectively at which the gas was actually measured.
+
+The following problem will serve to illustrate the application of this
+equation.
+
+A gas having a temperature of 20° occupies a volume of 500 cc. when
+subjected to a pressure indicated by a barometric reading of 740 mm.
+What volume would this gas occupy under standard conditions?
+
+In this problem v = 500, p = 740, and t = 20. Substituting these
+values in the above equation, we get
+
+    V_{s} = (500 × 740)/(760 (1 + 0.00366 × 20)) = 453.6 cc.
+
+[Illustration: Fig. 8]
+
+~Variations in the volume of a gas due to the pressure of aqueous vapor.~
+In many cases gases are collected over water, as explained under the
+preparation of oxygen. In such cases there is present in the gas a
+certain amount of water vapor. This vapor exerts a definite pressure,
+which acts in opposition to the atmospheric pressure and which therefore
+must be subtracted from the latter in determining the effective pressure
+upon the gas. Thus, suppose we wish to determine the pressure to which
+the gas in tube A (Fig. 8) is subjected. The tube is raised or lowered
+until the level of the water inside and outside the tube is the same.
+The atmosphere presses down upon the surface of the water (as indicated
+by the arrows), thus forcing the water upward within the tube with a
+pressure equal to the atmospheric pressure. The full force of this
+upward pressure, however, is not spent in compressing the gas within the
+tube, for since it is collected over water it contains a certain amount
+of water vapor. This water vapor exerts a pressure (as indicated by the
+arrow within the tube) in opposition to the upward pressure. It is
+plain, therefore, that the effective pressure upon the gas is equal to
+the atmospheric pressure less the pressure exerted by the aqueous vapor.
+The pressure exerted by the aqueous vapor increases with the
+temperature. The figures representing the extent of this pressure (often
+called the _tension of aqueous vapor_) are given in the Appendix. They
+express the pressure or tension in millimeters of mercury, just as the
+atmospheric pressure is expressed in millimeters of mercury.
+Representing the pressure of the aqueous vapor by a, formula (5)
+becomes
+
+    (6) V_{s} = v(p - a)/(760(1 + 0.00366t)).
+
+The following problem will serve to illustrate the method of applying
+the correction for the pressure of the aqueous vapor.
+
+The volume of a gas measured over water in a laboratory where the
+temperature is 20° and the barometric reading is 740 mm. is 500 cc. What
+volume would this occupy under standard conditions?
+
+The pressure exerted by the aqueous vapor at 20° (see table in Appendix)
+is equal to the pressure exerted by a column of mercury 17.4 mm. in
+height. Substituting the values of v, t, p, and a in formula
+(6), we have
+
+    (6) V_{s} = 500(740 - 17.4)/(760(1 + 0.00366 × 20)) = 442.9 cc.
+
+~Adjustment of tubes before reading gas volumes.~ In measuring the volumes
+of gases collected in graduated tubes or other receivers, over a liquid
+as illustrated in Fig. 8, the reading should be taken after raising or
+lowering the tube containing the gas until the level of the liquid
+inside and outside the tube is the same; for it is only under these
+conditions that the upward pressure within the tube is the same as the
+atmospheric pressure.
+
+
+EXERCISES
+
+1. What is the meaning of the following words? phlogiston, ozone,
+phosphorus. (Consult dictionary.)
+
+2. Can combustion take place without the emission of light?
+
+3. Is the evolution of light always produced by combustion?
+
+4. (a) What weight of oxygen can be obtained from 100 g. of water?
+(b) What volume would this occupy under standard conditions?
+
+5. (a) What weight of oxygen can be obtained from 500g. of mercuric
+oxide? (b) What volume would this occupy under standard conditions?
+
+6. What weight of each of the following compounds is necessary to
+prepare 50 l. of oxygen? (a) water; (b) mercuric oxide; (c)
+potassium chlorate.
+
+7. Reduce the following volumes to 0°, the pressure remaining constant:
+(a) 150 cc. at 10°; (b) 840 cc. at 273°.
+
+8. A certain volume of gas is measured when the temperature is 20°. At
+what temperature will its volume be doubled?
+
+9. Reduce the following volumes to standard conditions of pressure, the
+temperature remaining constant: (a) 200 cc. at 740 mm.; (b) 500 l.
+at 380 mm.
+
+10. What is the weight of 1 l. of oxygen when the pressure is 750 mm.
+and the temperature 0°?
+
+11. Reduce the following volumes to standard conditions of temperature
+and pressure: (a) 340 cc. at 12° and 753 mm; (b) 500 cc. at 15° and
+740 mm.
+
+12. What weight of potassium chlorate is necessary to prepare 250 l. of
+oxygen at 20° and 750 mm.?
+
+13. Assuming the cost of potassium chlorate and mercuric oxide to be
+respectively $0.50 and $1.50 per kilogram, calculate the cost of
+materials necessary for the preparation of 50 l. of oxygen from each of
+the above compounds.
+
+14. 100 g. of potassium chlorate and 25 g. of manganese dioxide were
+heated in the preparation of oxygen. What products were left in the
+flask, and how much of each was present?
+
+
+
+
+CHAPTER III
+
+HYDROGEN
+
+
+~Historical.~ The element hydrogen was first clearly recognized as a
+distinct substance by the English investigator Cavendish, who in 1766
+obtained it in a pure state, and showed it to be different from the
+other inflammable airs or gases which had long been known. Lavoisier
+gave it the name hydrogen, signifying water former, since it had been
+found to be a constituent of water.
+
+~Occurrence.~ In the free state hydrogen is found in the atmosphere, but
+only in traces. In the combined state it is widely distributed, being a
+constituent of water as well as of all living organisms, and the
+products derived from them, such as starch and sugar. About 10% of the
+human body is hydrogen. Combined with carbon, it forms the substances
+which constitute petroleum and natural gas.
+
+     It is an interesting fact that while hydrogen in the free state
+     occurs only in traces on the earth, it occurs in enormous
+     quantities in the gaseous matter surrounding the sun and
+     certain other stars.
+
+~Preparation from water.~ Hydrogen can be prepared from water by several
+methods, the most important of which are the following.
+
+1. _By the electric current._ As has been indicated in the preparation
+of oxygen, water is easily separated into its constituents, hydrogen and
+oxygen, by passing an electric current through it under certain
+conditions.
+
+2. _By the action of certain metals._ When brought into contact with
+certain metals under appropriate conditions, water gives up a portion
+or the whole of its hydrogen, its place being taken by the metal. In the
+case of a few of the metals this change occurs at ordinary temperatures.
+Thus, if a bit of sodium is thrown on water, an action is seen to take
+place at once, sufficient heat being generated to melt the sodium, which
+runs about on the surface of the water. The change which takes place
+consists in the displacement of one half of the hydrogen of the water by
+the sodium, and may be represented as follows:
+
+          _        _            _        _
+         | hydrogen |          | sodium   |
+sodium + | hydrogen |(water) = | hydrogen |(sodium hydroxide) + hydrogen
+         |_oxygen  _|          |_oxygen  _|
+
+The sodium hydroxide formed is a white solid which remains dissolved in
+the undecomposed water, and may be obtained by evaporating the solution
+to dryness. The hydrogen is evolved as a gas and may be collected by
+suitable apparatus.
+
+Other metals, such as magnesium and iron, decompose water rapidly, but
+only at higher temperatures. When steam is passed over hot iron, for
+example, the iron combines with the oxygen of the steam, thus displacing
+the hydrogen. Experiments show that the change may be represented as
+follows:
+
+        _        _
+       | hydrogen |           _       _                 _        _
+iron + | hydrogen |(water) = | iron    |(iron oxide) + | hydrogen |
+       |_oxygen  _|          |_oxygen _|               |_hydrogen_|
+
+The iron oxide formed is a reddish-black compound, identical with that
+obtained by the combustion of iron in oxygen.
+
+     ~Directions for preparing hydrogen by the action of steam on
+     iron.~ The apparatus used in the preparation of hydrogen from
+     iron and steam is shown in Fig. 9. A porcelain or iron tube
+     B, about 50 cm. in length and 2 cm. or 3 cm. in diameter, is
+     partially filled with fine iron wire or tacks and connected as
+     shown in the figure. The tube B is heated, slowly at first,
+     until the iron is red-hot. Steam is then conducted through the
+     tube by boiling the water in the flask A. The hot iron
+     combines with the oxygen in the steam, setting free the
+     hydrogen, which is collected over water. The gas which first
+     passes over is mixed with the air previously contained in the
+     flask and tube, and is allowed to escape, _since a mixture of
+     hydrogen with oxygen or air explodes violently when brought in
+     contact with a flame_. It is evident that the flask A must be
+     disconnected from the tube before the heat is withdrawn.
+
+     That the gas obtained is different from air and oxygen may be
+     shown by holding a bottle of it mouth downward and bringing a
+     lighted splint into it. The hydrogen is ignited and burns with
+     an almost colorless flame.
+
+[Illustration Fig. 9]
+
+~Preparation from acids~ (_usual laboratory method_). While hydrogen can
+be prepared from water, either by the action of the electric current or
+by the action of certain metals, these methods are not economical and
+are therefore but little used. In the laboratory hydrogen is generally
+prepared from compounds known as acids, all of which contain hydrogen.
+When acids are brought in contact with certain metals, the metals
+dissolve and set free the hydrogen of the acid. Although this reaction
+is a quite general one, it has been found most convenient in preparing
+hydrogen by this method to use either zinc or iron as the metal and
+either hydrochloric or sulphuric acid as the acid. Hydrochloric acid is
+a compound consisting of 2.77% hydrogen and 97.23% chlorine, while
+sulphuric acid consists of 2.05% hydrogen, 32.70% sulphur, and 65.25%
+oxygen.
+
+The changes which take place in the preparation of hydrogen from zinc
+and sulphuric acid (diluted with water) may be represented as follows:
+
+        _        _               _       _
+       | hydrogen |(sulphuric   | zinc    |(zinc
+zinc + | sulphur  | acid)     = | sulphur | sulphate) + hydrogen
+       |_oxygen  _|             |_oxygen _|
+
+In other words, the zinc has taken the place of the hydrogen in
+sulphuric acid. The resulting compound contains zinc, sulphur, and
+oxygen, and is known as zinc sulphate. This remains dissolved in the
+water present in the acid. It may be obtained in the form of a white
+solid by evaporating the liquid left after the metal has passed into
+solution.
+
+When zinc and hydrochloric acid are used the following changes take
+place:
+
+        _        _                  _        _
+       | hydrogen |(hydrochloric   | zinc     |(zinc
+zinc + |_chlorine_| acid)        = |_chlorine_| chloride) + hydrogen
+
+When iron is used the changes which take place are exactly similar to
+those just given for zinc.
+
+[Illustration Fig. 10.]
+
+     ~Directions for preparing hydrogen from acids.~ The preparation
+     of hydrogen from acids is carried out in the laboratory as
+     follows: The metal is placed in a flask or wide-mouthed bottle
+     A (Fig. 10) and the acid is added slowly through the funnel
+     tube B. The metal dissolves in the acid, while the hydrogen
+     which is liberated escapes through the exit tube C and is
+     collected over water. It is evident that the hydrogen which
+     passes over first is mixed with the air from the bottle A.
+     Hence care must be taken not to bring a flame near the exit
+     tube, since, as has been stated previously, such a mixture
+     explodes with great violence when brought in contact with a
+     flame.
+
+     ~Precautions.~ Both sulphuric acid and zinc, if impure, are
+     likely to contain small amounts of arsenic. Such materials
+     should not be used in preparing hydrogen, since the arsenic
+     present combines with a portion of the hydrogen to form a very
+     poisonous gas known as arsine. On the other hand, chemically
+     pure sulphuric acid, i.e. sulphuric acid that is entirely free
+     from impurities, will not act upon chemically pure zinc. The
+     reaction may be started, however, by the addition of a few
+     drops of a solution of copper sulphate or platinum
+     tetrachloride.
+
+~Physical properties.~ Hydrogen is similar to oxygen in that it is a
+colorless, tasteless, odorless gas. It is characterized by its extreme
+lightness, being the lightest of all known substances. One liter of the
+gas weighs only 0.08984 g. On comparing this weight with that of an
+equal volume of oxygen, viz., 1.4285 g., the latter is found to be 15.88
+times as heavy as hydrogen. Similarly, air is found to be 14.38 times as
+heavy as hydrogen. Soap bubbles blown with hydrogen rapidly rise in the
+air. On account of its lightness it is possible to pour it upward from
+one bottle into another. Thus, if the bottle A (Fig. 11) is filled
+with hydrogen, placed mouth downward by the side of bottle _B_, filled
+with air, and is then gradually inverted under B as indicated in the
+figure, the hydrogen will flow upward into bottle _B_, displacing the
+air. Its presence in bottle B may then be shown by bringing a lighted
+splint to the mouth of the bottle, when the hydrogen will be ignited by
+the flame. It is evident, from this experiment, that in order to retain
+the gas in an open bottle the bottle must be placed mouth downward.
+
+[Illustration Fig. 11]
+
+Hydrogen is far more difficult to liquefy than any other gas, with the
+exception of helium, a rare element recently found to exist in the
+atmosphere. The English scientist Dewar, however, in 1898 succeeded not
+only in obtaining hydrogen in liquid state but also as a solid. Liquid
+hydrogen is colorless and has a density of only 0.07. Its boiling point
+under atmospheric pressure is -252°. Under diminished pressure the
+temperature has been reduced to -262°. The solubility of hydrogen in
+water is very slight, being still less than that of oxygen.
+
+Pure hydrogen produces no injurious results when inhaled. Of course one
+could not live in an atmosphere of the gas, since oxygen is essential to
+respiration.
+
+~Chemical properties.~ At ordinary temperatures hydrogen is not an active
+element. A mixture of hydrogen and chlorine, however, will combine with
+explosive violence at ordinary temperature if exposed to the sunlight.
+The union can be brought about also by heating. The product formed in
+either case is hydrochloric acid. Under suitable conditions hydrogen
+combines with nitrogen to form ammonia, and with sulphur to form the
+foul-smelling gas, hydrogen sulphide. The affinity of hydrogen for
+oxygen is so great that a mixture of hydrogen and oxygen or hydrogen
+and air explodes with great violence when heated to the kindling
+temperature (about 612°). Nevertheless under proper conditions hydrogen
+may be made to burn quietly in either oxygen or air. The resulting
+hydrogen flame is almost colorless and is very hot. The combustion of
+the hydrogen is, of course, due to its union with oxygen. The product of
+the combustion is therefore a compound of hydrogen and oxygen. That this
+compound is water may be shown easily by experiment.
+
+[Illustration Fig. 12]
+
+     ~Directions for burning hydrogen in air.~ The combustion of
+     hydrogen in air may be carried out safely as follows: The
+     hydrogen is generated in the bottle A (Fig. 12), is dried by
+     conducting it through the tube X, filled with some substance
+     (generally calcium chloride) which has a great attraction for
+     moisture, and escapes through the tube T, the end of which is
+     drawn out to a jet. The hydrogen first liberated mixes with the
+     air contained in the generator. If a flame is brought near the
+     jet before this mixture has all escaped, a violent and very
+     dangerous explosion results, since the entire apparatus is
+     filled with the explosive mixture. On the other hand, if the
+     flame is not applied until all the air has been expelled, the
+     hydrogen is ignited and burns quietly, since only the small
+     amount of it which escapes from the jet can come in contact
+     with the oxygen of the air at any one time. By holding a cold,
+     dry bell jar or bottle over the flame, in the manner shown in
+     the figure, the steam formed by the combustion of the hydrogen
+     is condensed, the water collecting in drops on the sides of the
+     jar.
+
+~Precautions.~ In order to avoid danger it is absolutely necessary to
+prove that the hydrogen is free from air before igniting it. This can be
+done by testing small amounts of the escaping gas. A convenient and safe
+method of doing this is to fill a test tube with the gas by inverting it
+over the jet. The hydrogen, on account of its lightness, collects in the
+tube, displacing the air. After holding it over the jet for a few
+moments in order that it may be filled with the gas, the tube is gently
+brought, mouth downward, to the flame of a burner placed not nearer than
+an arm's length from the jet. If the hydrogen is mixed with air a slight
+explosion occurs, but if pure it burns quietly in the tube. The
+operation is repeated until the gas burns quietly, when the tube is
+quickly brought back over the jet for an instant, whereby the escaping
+hydrogen is ignited by the flame in the tube.
+
+[Illustration. Fig. 13]
+
+~A mixture of hydrogen and oxygen is explosive.~ That a mixture of
+hydrogen and air is explosive may be shown safely as follows: A cork
+through which passes a short glass tube about 1 cm. in diameter is
+fitted air-tight into the tubule of a bell jar of 2 l. or 3 l. capacity.
+(A thick glass bottle with bottom removed may be used.) The tube is
+closed with a small rubber stopper and the bell jar filled with
+hydrogen, the gas being collected over water. When entirely filled with
+the gas the jar is removed from the water and supported by blocks of
+wood in order to leave the bottom of the jar open, as shown in Fig. 13.
+The stopper is now removed from the tube in the cork, and the hydrogen,
+which on account of its lightness escapes from the tube, is at once
+lighted. As the hydrogen escapes, the air flows in at the bottom of the
+jar and mixes with the remaining portion of the hydrogen, so that a
+mixture of the two soon forms, and a loud explosion results. The
+explosion is not dangerous, since the bottom of the jar is open, thus
+leaving room for the expansion of the hot gas.
+
+Since air is only one fifth oxygen, the remainder being inert gases, it
+may readily be inferred that a mixture of hydrogen with pure oxygen
+would be far more explosive than a mixture of hydrogen with air. Such
+mixtures should not be made except in small quantities and by
+experienced workers.
+
+~Hydrogen does not support combustion.~ While hydrogen is readily
+combustible, it is not a supporter of combustion. In other words,
+substances will not burn in it. This may be shown by bringing a lighted
+candle supported by a stiff wire into a bottle or cylinder of the pure
+gas, as shown in Fig. 14. The hydrogen is ignited by the flame of the
+candle and burns at the mouth of the bottle, where it comes in contact
+with the oxygen in the air. When the candle is thrust up into the gas,
+its flame is extinguished on account of the absence of oxygen. If slowly
+withdrawn, the candle is relighted as it passes through the layer of
+burning hydrogen.
+
+[Illustration: Fig. 14]
+
+[Illustration: Fig. 15]
+
+~Reduction.~ On account of its great affinity for oxygen, hydrogen has the
+power of abstracting it from many of its compounds. Thus, if a stream of
+hydrogen, dried by passing through the tube B (Fig. 15), filled with
+calcium chloride, is conducted through the tube C containing some
+copper oxide, heated to a moderate temperature, the hydrogen abstracts
+the oxygen from the copper oxide. The change may be represented as
+follows:
+
+     hydrogen + {copper}                 {hydrogen}
+                {oxygen}(copper oxide) = {oxygen  }(water) + copper
+
+The water formed collects in the cold portions of the tube C near its
+end. In this experiment the copper oxide is said to undergo reduction.
+_Reduction may therefore be defined as the process of withdrawing oxygen
+from a compound._
+
+~Relation of reduction to oxidation.~ At the same time that the copper
+oxide is reduced it is clear that the hydrogen is oxidized, for it
+combines with the oxygen given up by the copper oxide. The two processes
+are therefore very closely related, and it usually happens that when one
+substance is oxidized some other substance is reduced. That substance
+which gives up its oxygen is called an _oxidizing agent_, while the
+substance which unites with the oxygen is called a _reducing agent_.
+
+~The oxyhydrogen blowpipe.~ This is a form of apparatus used for burning
+hydrogen in pure oxygen. As has been previously stated, the flame
+produced by the combustion of hydrogen in the air is very hot. It is
+evident that if pure oxygen is substituted for air, the temperature
+reached will be much higher, since there are no inert gases to absorb
+the heat. The oxyhydrogen blowpipe, used to effect this combination,
+consists of a small tube placed within a larger one, as shown in Fig.
+16.
+
+[Illustration: Fig. 16]
+
+The hydrogen, stored under pressure, generally in steel cylinders, is
+first passed through the outer tube and ignited at the open end of the
+tube. The oxygen from a similar cylinder is then conducted through the
+inner tube, and mixes with the hydrogen at the end of the tube. In order
+to produce the maximum heat, the hydrogen and oxygen must be admitted to
+the blowpipe in the exact proportion in which they combine, viz., 2
+volumes of hydrogen to 1 of oxygen, or by weight, 1 part of hydrogen to
+7.94 parts of oxygen. The intensity of the heat may be shown by bringing
+into the flame pieces of metal such as iron wire or zinc. These burn
+with great brilliancy. Even platinum, having a melting point of 1779°,
+may be melted by the heat of the flame.
+
+While the oxyhydrogen flame is intensely hot, it is almost non-luminous.
+If directed against some infusible substance like ordinary lime (calcium
+oxide), the heat is so intense that the lime becomes incandescent and
+glows with a brilliant light. This is sometimes used as a source of
+light, under the name of _Drummond_ or _lime light_.
+
+[Illustration: Fig. 17]
+
+~The blast lamp.~ A similar form of apparatus is commonly used in the
+laboratory as a source of heat under the name _blast lamp_ (Fig. 17).
+This differs from the oxyhydrogen blowpipe only in the size of the
+tubes. In place of the hydrogen and oxygen the more accessible coal gas
+and air are respectively used. The former is composed largely of a
+mixture of free hydrogen and gaseous compounds of carbon and hydrogen.
+While the temperature of the flame is not so high as that of the
+oxyhydrogen blowpipe, it nevertheless suffices for most chemical
+operations carried out in the laboratory.
+
+~Uses of hydrogen.~ On account of its cost, hydrogen is but little used
+for commercial purposes. It is sometimes used as a material for the
+inflation of balloons, but usually the much cheaper coal gas is
+substituted for it. Even hot air is often used when the duration of
+ascension is very short. It has been used also as a source of heat and
+light in the oxyhydrogen blowpipe. Where the electric current is
+available, however, this form of apparatus has been displaced almost
+entirely by the electric light and electric furnace, which are much more
+economical and more powerful sources of light and heat.
+
+
+EXERCISES
+
+1. Will a definite weight of iron decompose an unlimited weight of
+steam?
+
+2. Why is oxygen passed through the inner tube of the oxyhydrogen
+blowpipe rather than the outer?
+
+3. In Fig. 14, will the flame remain at the mouth of the tube?
+
+4. From Fig. 15, suggest a way for determining experimentally the
+quantity of water formed in the reaction.
+
+5. Distinguish clearly between the following terms: oxidation,
+reduction, combustion, and kindling temperature.
+
+6. Is oxidation always accompanied by reduction?
+
+7. What is the source of heat in the lime light? What is the exact use
+of lime in this instrument?
+
+8. In Fig. 12, why is it necessary to dry the hydrogen by means of the
+calcium chloride in the tube X?
+
+9. At what pressure would the weight of 1 l. of hydrogen be equal to
+that of oxygen under standard conditions?
+
+10. (a) What weight of hydrogen can be obtained from 150 g. of
+sulphuric acid? (b) What volume would this occupy under standard
+conditions? (c) The density of sulphuric acid is 1.84. What volume
+would the 150 g. of the acid occupy?
+
+11. How many liters of hydrogen can be obtained from 50 cc. of sulphuric
+acid having a density of 1.84?
+
+12. Suppose you wish to fill five liter bottles with hydrogen, the gas
+to be collected over water in your laboratory, how many cubic
+centimeters of sulphuric acid would be required?
+
+
+
+
+CHAPTER IV
+
+COMPOUNDS OF HYDROGEN AND OXYGEN; WATER AND HYDROGEN DIOXIDE
+
+
+WATER
+
+~Historical.~ Water was long regarded as an element. In 1781 Cavendish
+showed that it is formed by the union of hydrogen and oxygen. Being a
+believer in the phlogiston theory, however, he failed to interpret his
+results correctly. A few years later Lavoisier repeated Cavendish's
+experiments and showed that water must be regarded as a compound of
+hydrogen and oxygen.
+
+~General methods employed for the determination of the composition of a
+compound.~ The composition of a compound may be determined by either of
+two general processes these are known as _analysis_ and _synthesis_.
+
+1. _Analysis_ is the process of decomposing a compound into its
+constituents and determining what these constituents are. The analysis
+is _qualitative_ when it results in merely determining what elements
+compose the compound; it is _quantitative_ when the exact percentage of
+each constituent is determined. Qualitative analysis must therefore
+precede quantitative analysis, for it must be known what elements, are
+in a compound before a method can be devised for determining exactly how
+much of each is present.
+
+2. _Synthesis_ is the process of forming a compound from its constituent
+parts. It is therefore the reverse of analysis. Like analysis, it may be
+either qualitative or quantitative.
+
+~Application of these methods to the determination of the composition of
+water.~ The determination of the composition of water is a matter of
+great interest not only because of the importance of the compound but
+also because the methods employed illustrate the general methods of
+analysis and synthesis.
+
+~Methods based on analysis.~ The methods based on analysis may be either
+qualitative or quantitative in character.
+
+[Illustration: Fig. 18]
+
+1. _Qualitative analysis._ As was stated in the study of oxygen, water
+may be separated into its component parts by means of the electric
+current. The form of apparatus ordinarily used for effecting this
+analysis is shown in Fig. 18. A platinum wire, to the end of which is
+attached a small piece of platinum foil (about 15 mm. by 25 mm.), is
+fused through each of the tubes B and D, as shown in the figure. The
+stopcocks at the ends of these tubes are opened and water, to which has
+been added about one tenth of its volume of sulphuric acid, is poured
+into the tube A until the side tubes B and D are completely
+filled. The stopcocks are then closed. The platinum wires extending into
+the tubes B and D are now connected with the wires leading from two
+or three dichromate cells joined in series. The pieces of platinum foil
+within the tubes thus become the electrodes, and the current flows from
+one to the other through the acidulated water. As soon as the current
+passes, bubbles of gas rise from each of the electrodes and collect in
+the upper part of the tubes. The gas rising from the negative electrode
+is found to be hydrogen, while that from the positive electrode is
+oxygen. It will be seen that the volume of the hydrogen is approximately
+double that of the oxygen. Oxygen is more soluble in water than
+hydrogen, and a very little of it is also lost by being converted into
+ozone and other substances. It has been found that when the necessary
+corrections are made for the error due to these facts, the volume of the
+hydrogen is exactly double that of the oxygen.
+
+Fig. 19 illustrates a simpler form of apparatus, which may be used in
+place of that shown in Fig. 18. A glass or porcelain dish is partially
+filled with water to which has been added the proper amount of acid. Two
+tubes filled with the same liquid are inverted over the electrodes. The
+gases resulting from the decomposition of the water collect in the
+tubes.
+
+[Illustration: Fig. 19]
+
+2. _Quantitative analysis._ The analysis just described is purely
+qualitative and simply shows that water contains hydrogen and oxygen. It
+does not prove the absence of other elements; indeed it does not prove
+that the hydrogen and oxygen are present in the proportion in which they
+are liberated by the electric current. The method may be made
+quantitative, however, by weighing the water decomposed and also the
+hydrogen and oxygen obtained in its decomposition. If the combined
+weights of the hydrogen and oxygen exactly equal the weight of the water
+decomposed, then it would be proved that the water consists of hydrogen
+and oxygen in the proportion in which they are liberated by the electric
+current. This experiment is difficult to carry out, however, so that the
+more accurate methods based on synthesis are used.
+
+~Methods based on synthesis.~ Two steps are necessary to ascertain the
+exact composition of water by synthesis: (1) to show by qualitative
+synthesis that water is formed by the union of oxygen with hydrogen; (2)
+to determine by quantitative synthesis in what proportion the two
+elements unite to form water. The fact that water is formed by the
+combination of oxygen with hydrogen was proved in the preceding chapter.
+The quantitative synthesis may be made as follows:
+
+[Illustration: Fig. 20]
+
+The combination of the two gases is brought about in a tube called a
+eudiometer. This is a graduated tube about 60 cm. long and 2 cm. wide,
+closed at one end (Fig. 20). Near the closed end two platinum wires are
+fused through the glass, the ends of the wires within the tube being
+separated by a space of 2 mm or 3 mm. The tube is entirely filled with
+mercury and inverted in a vessel of the same liquid. Pure hydrogen is
+passed into the tube until it is about one fourth filled. The volume of
+the gas is then read off on the scale and reduced to standard
+conditions. Approximately an equal volume of pure oxygen is then
+introduced and the volume again read off and reduced to standard
+conditions. This gives the total volume of the two gases. From this the
+volume of the oxygen introduced may be determined by subtracting from
+it the volume of the hydrogen. The combination of the two gases is now
+brought about by connecting the two platinum wires with an induction
+coil and passing a spark from one wire to the other. Immediately a
+slight explosion occurs. The mercury in the tube is at first depressed
+because of the expansion of the gases due to the heat generated, but at
+once rebounds, taking the place of the gases which have combined to form
+water. The volume of the water in the liquid state is so small that it
+may be disregarded in the calculations. In order that the temperature of
+the residual gas and the mercury may become uniform, the apparatus is
+allowed to stand for a few minutes. The volume of the gas is then read
+off and reduced to standard conditions, so that it may be compared with
+the volumes of the hydrogen and oxygen originally taken. The residual
+gas is then tested in order to ascertain whether it is hydrogen or
+oxygen, experiments having proved that it is never a mixture of the two.
+From the information thus obtained the composition of the water may be
+calculated. Thus, suppose the readings were as follows:
+
+Volume of hydrogen taken                                          20.3 cc.
+Volume of hydrogen and oxygen                                     38.7
+Volume of oxygen                                                  18.4
+Volume of gas left after combination has taken place (oxygen)      8.3
+
+The 20.3 cc. of hydrogen have combined with 18.4 cc. minus 8.3 cc. (or
+10.1 cc.) of oxygen; or approximately 2 volumes of hydrogen have
+combined with 1 of oxygen. Since oxygen is 15.88 times as heavy as
+hydrogen, the proportion by weight in which the two gases combine is 1
+part of hydrogen to 7.94 of oxygen.
+
+~Precaution.~ If the two gases are introduced into the eudiometer in the
+exact proportions in which they combine, after the combination has taken
+place the liquid will rise and completely fill the tube. Under these
+conditions, however, the tube is very likely to be broken by the sudden
+upward rush of the liquid. Hence in performing the experiment care is
+taken to introduce an excess of one of the gases.
+
+~A more convenient form of eudiometer.~ A form of eudiometer (Fig. 21)
+different from that shown on page 43 is sometimes used to avoid the
+calculations necessary in reducing the volumes of the gases to the same
+conditions of temperature and pressure in order to make comparisons.
+With this apparatus it is possible to take the readings of the volumes
+under the same conditions of temperature and pressure, and thus compare
+them directly. The apparatus (Fig. 21) is filled with mercury and the
+gases introduced into the tube A. The experiment is carried out as in
+the preceding one, except that before taking the reading of the gas
+volumes, mercury is either added to the tube B or withdrawn from it by
+means of the stopcock C, until it stands at exactly the same height in
+both tubes. The gas inclosed in tube A is then under atmospheric
+pressure; and since but a few minutes are required for performing the
+experiment, the conditions of temperature and pressure may be regarded
+as constant. Hence the volumes of the hydrogen and oxygen and of the
+residual gas may be read off from the tube and directly compared.
+
+[Illustration: Fig. 21]
+
+~Method used by Berzelius and Dumas.~ The method used by these
+investigators enables us to determine directly the proportion by weight
+in which the hydrogen and oxygen combine. Fig. 22 illustrates the
+apparatus used in making this determination. B is a glass tube
+containing copper oxide. C and D are glass tubes filled with calcium
+chloride, a substance which has great affinity for water. The tubes B
+and C, including their contents, are carefully weighed, and the
+apparatus connected as shown in the figure. A slow current of pure
+hydrogen is then passed through A, and that part of the tube B which
+contains copper oxide is carefully heated. The hydrogen combines with
+the oxygen present in the copper oxide to form water, which is absorbed
+by the calcium chloride in tube C. The calcium chloride in tube D
+prevents any moisture entering tube C from the air. The operation is
+continued until an appreciable amount of water has been formed. The
+tubes B and C are then weighed once more. The loss of weight in the
+tube B will exactly equal the weight of oxygen taken up from the
+copper oxide in the formation of the water. The gain in weight in the
+tube C will exactly equal the weight of the water formed. The
+difference in these weights will of course equal the weight of the
+hydrogen present in the water formed.
+
+[Illustration: Fig. 22]
+
+~Dumas' results.~ The above method for the determination of the
+composition of water was first used by Berzelius in 1820. The work was
+repeated in 1843 by Dumas, the average of whose results is as follows:
+
+Weight of water formed                  236.36 g.
+Oxygen given up by the copper oxide     210.04
+                                        ------
+Weight of hydrogen present in water      26.32
+
+According to this experiment the ratio of hydrogen to oxygen in water is
+therefore 26.32 to 210.04, or as l to 7.98
+
+~Morley's results.~ The American chemist Morley has recently determined
+the composition of water, extreme precautions being taken to use pure
+materials and to eliminate all sources of error. The hydrogen and oxygen
+which combined, as well as the water formed, were all accurately
+weighed. According to Morley's results, 1 part of hydrogen by weight
+combines with 7.94 parts of oxygen to form water.
+
+~Comparison of results obtained.~ From the above discussions it is easy to
+see that it is by experiment alone that the composition of a compound
+can be determined. Different methods may lead to slightly different
+results. The more accurate the method chosen and the greater the skill
+with which the experiment is carried out, the more accurate will be the
+results. It is generally conceded by chemists that the results obtained
+by Morley in reference to the composition of water are the most accurate
+ones. In accordance with these results, then, _water must be regarded as
+a compound containing hydrogen and oxygen in the proportion of 1 part by
+weight of hydrogen to 7.94 parts by weight of oxygen_.
+
+~Relation between the volume of aqueous vapor and the volumes of the
+hydrogen and oxygen which combine to form it.~ When the quantitative
+synthesis of water is carried out in the eudiometer as described above,
+the water vapor formed by the union of the hydrogen and oxygen at once
+condenses. The volume of the resulting liquid is so small that it may be
+disregarded in making the calculations. If, however, the experiment is
+carried out at a temperature of 100° or above, the water-vapor formed is
+not condensed and it thus becomes possible to compare the volume of the
+vapor with the volumes of hydrogen and oxygen which combined to form it.
+This can be accomplished by surrounding the arm A of the eudiometer
+(Fig. 23) with the tube B through which is passed the vapor obtained
+by boiling some liquid which has a boiling point above 100°. In this way
+it has been proved that 2 volumes of hydrogen and 1 volume of oxygen
+combine to form exactly 2 volumes of water vapor, the volumes all being
+measured under the same conditions of temperature and pressure. It will
+be noted that the relation between these volumes may be expressed by
+whole numbers. The significance of this very important fact will be
+discussed in a subsequent chapter.
+
+[Illustration: Fig. 23]
+
+~Occurrence of water.~ Water not only covers about three fourths of the
+surface of the earth, and is present in the atmosphere in the form of
+moisture, but it is also a common constituent of the soil and rocks and
+of almost every form of animal and vegetable organism. The human body is
+nearly 70% water. This is derived not only from the water which we drink
+but also from the food which we eat, most of which contains a large
+percentage of water. Thus potatoes contain about 78% of water, milk 85%,
+beef over 50%, apples 84%, tomatoes 94%.
+
+~Impurities in water.~ Chemically pure water contains only hydrogen and
+oxygen. Such a water never occurs in nature, however, for being a good
+solvent, it takes up certain substances from the rocks and soil with
+which it comes in contact. When such waters are evaporated these
+substances are deposited in the form of a residue. Even rain water,
+which is the purest form occurring in nature, contains dust particles
+and gases dissolved from the atmosphere. The foreign matter in water is
+of two kinds, namely, _mineral_, such as common salt and limestone, and
+_organic_, that is the products of animal and vegetable life.
+
+     ~Mineral matter in water.~ The amount and nature of the mineral
+     matter present in different waters vary greatly, depending on
+     the character of the rocks and soil with which the waters come
+     in contact. The more common of the substances present are
+     common salt and compounds of calcium, magnesium, and iron. One
+     liter of the average river water contains about 175 mg. of
+     mineral matter. Water from deep wells naturally contains more
+     mineral matter than river water, generally two or three times
+     as much, while sea water contains as much as 35,000 mg. to the
+     liter.
+
+~Effect of impurities on health.~ The mineral matter in water does not,
+save in very exceptional cases, render the water injurious to the human
+system. In fact the presence of a certain amount of such matter is
+advantageous, supplying the mineral constituents necessary for the
+formation of the solid tissues of the body. The presence of organic
+matter, on the other hand, must always be regarded with suspicion. This
+organic matter may consist not only of the products of animal and
+vegetable life but also of certain microscopic forms of living organisms
+which are likely to accompany such products. Contagious diseases are
+known to be due to the presence in the body of minute living organisms
+or germs. Each disease is caused by its own particular kind of germ.
+Through sewage these germs may find their way from persons afflicted
+with disease into the water supply, and it is principally through the
+drinking water that certain of these diseases, especially typhoid fever,
+are spread. It becomes of great importance, therefore, to be able to
+detect such matter when present in drinking water as well as to devise
+methods whereby it can be removed or at least rendered harmless.
+
+     ~Analysis of water.~ The mineral analysis of a water is, as the
+     name suggests, simply the determination of the mineral matter
+     present. Sanitary analysis, on the other hand, is the
+     determination of the organic matter present. The physical
+     properties of a water give no conclusive evidence as to its
+     purity, since a water may be unfit for drinking purposes and
+     yet be perfectly clear and odorless. Neither can any reliance
+     be placed on the simple methods often given for testing the
+     purity of water. Only the trained chemist can carry out such
+     methods of analysis as can be relied upon.
+
+[Illustration: Fig. 24]
+
+~Purification of water.~ Three general methods are used for the
+purification of water, namely, _distillation_, _filtration_, and
+_boiling_.
+
+1. _Distillation._ The most effective way of purifying natural waters is
+by the process of distillation. This consists in boiling the water and
+condensing the steam. Fig. 24 illustrates the process of distillation,
+as commonly conducted in the laboratory. Ordinary water is poured into
+the flask A and boiled. The steam is conducted through the condenser
+B, which consists essentially of a narrow glass tube sealed within a
+larger one, the space between the two being filled with cold water,
+which is admitted at C and escapes at D. The inner tube is thus kept
+cool and the steam in passing through it is condensed. The water formed
+by the condensation of the steam collects in the receiver E and is
+known as _distilled_ water. Such water is practically pure, since the
+impurities are nonvolatile and remain in the flask A.
+
+     ~Commercial distillation.~ In preparing distilled water on a
+     large scale, the steam is generated in a boiler or other metal
+     container and condensed by passing it through a pipe made of
+     metal, generally tin. This pipe is wound into a spiral and is
+     surrounded by a current of cold water. Distilled water is used
+     by the chemist in almost all of his work. It is also used in
+     the manufacture of artificial ice and for drinking water.
+
+     ~Fractional distillation.~ In preparing distilled water, it is
+     evident that if the natural water contains some substance which
+     is volatile its vapor will pass over and be condensed with the
+     steam, so that the distillate will not be pure water. Even such
+     mixtures, however, may generally be separated by repeated
+     distillation. Thus, if a mixture of water (boiling point 100°)
+     and alcohol (boiling point 78°) is distilled, the alcohol,
+     having the lower boiling point, tends to distill first,
+     followed by the water. The separation of the two is not
+     perfect, however, but may be made nearly so by repeated
+     distillations. The process of separating a mixture of volatile
+     substances by distillation is known as _fractional
+     distillation_.
+
+2. _Filtration._ The process of distillation practically removes all
+nonvolatile foreign matter, mineral as well as organic. In purifying
+water for drinking purposes, however, it is only necessary to eliminate
+the latter or to render it harmless. This is ordinarily done either by
+filtration or boiling. In filtration the water is passed through some
+medium which will retain the organic matter. Ordinary charcoal is a
+porous substance and will condense within its pores the organic matter
+in water if brought in contact with it. It is therefore well adapted to
+the construction of filters. Such filters to be effective must be kept
+clean, since it is evident that the charcoal is useless after its pores
+are filled. A more effective type of filter is the Chamberlain-Pasteur
+filter. In this the water is forced through a porous cylindrical cup,
+the pores being so minute as to strain out the organic matter.
+
+     ~City filtration beds.~ For purifying the water supply of cities,
+     large filtration beds are prepared from sand and gravel, and
+     the water is allowed to filter through these. Some of the
+     impurities are strained out by the filter, while others are
+     decomposed by the action of certain kinds of bacteria present
+     in the sand. Fig. 25 shows a cross section of a portion of the
+     filter used in purifying the water supply of Philadelphia. The
+     water filters through the sand and gravel and passes into the
+     porous pipe A, from which it is pumped into the city mains.
+     The filters are covered to prevent the water from freezing in
+     cold weather.
+
+[Illustration: Fig. 25]
+
+3. _Boiling._ A simpler and equally efficient method for purifying water
+for drinking purposes consists in boiling the water. It is the germs in
+water that render it dangerous to health. These germs are living forms
+of matter. If the water is boiled, the germs are killed and the water
+rendered safe. While these germs are destroyed by heat, cold has little
+effect upon them. Thus Dewar, in working with liquid hydrogen, exposed
+some of these minute forms of life to the temperature of boiling
+hydrogen (-252°) without killing them.
+
+~Self-purification of water.~ It has long been known that water
+contaminated with organic matter tends to purify itself when exposed to
+the air. This is due to the fact that the water takes up a small amount
+of oxygen from the air, which gradually oxidizes the organic matter
+present in the water. While water is undoubtedly purified in this way,
+the method cannot be relied upon to purify a contaminated water so as to
+render it safe for drinking purposes.
+
+~Physical properties.~ Pure water is an odorless and tasteless liquid,
+colorless in thin layers, but having a bluish tinge when observed
+through a considerable thickness. It solidifies at 0° and boils at 100°
+under the normal pressure of one atmosphere. If the pressure is
+increased, the boiling point is raised. When water is cooled it steadily
+contracts until the temperature of 4° is reached: it then expands. Water
+is remarkable for its ability to dissolve other substances, and is the
+best solvent known. Solutions of solids in water are more frequently
+employed in chemical work than are the solid substances, for chemical
+action between substances goes on more readily when they are in solution
+than it does when they are in the solid state.
+
+~Chemical properties.~ Water is a very stable substance, or, in other
+words, it does not undergo decomposition readily. To decompose it into
+its elements by heat alone requires a very high temperature; at 2500°,
+for example, only about 5% of the entire amount is decomposed. Though
+very stable towards heat, water can be decomposed in other ways, as by
+the action of the electrical current or by certain metals.
+
+     ~Heat of formation and heat of decomposition are equal.~ The fact
+     that a very high temperature is necessary to decompose water
+     into hydrogen and oxygen is in accord with the fact that a
+     great deal of heat is evolved by the union of hydrogen and
+     oxygen; for it has been proved that the heat necessary to
+     decompose a compound into its elements (heat of decomposition)
+     is equal to the heat evolved in the formation of a compound
+     from its elements (heat of formation).
+
+~Water of crystallization.~ When a solid is dissolved in water and the
+resulting solution is allowed to evaporate, the solid separates out,
+often in the form of crystals. It has been found that the crystals of
+many compounds, although perfectly dry, give up a definite amount of
+water when heated, the substance at the same time losing its crystalline
+form. Such water is called _water of crystallization_. This varies in
+amount with different compounds, but is perfectly definite for the same
+compound. Thus, if a perfectly dry crystal of copper sulphate is
+strongly heated in a tube, water is evolved and condenses on the sides
+of the tube, the crystal crumbling to a light powder. The weight of the
+water evolved is always equal to exactly 36.07% of the weight of copper
+sulphate crystals heated. The water must therefore be in chemical
+combination with the substance composing the crystal; for if simply
+mixed with it or adhering to it, not only would the substance appear
+moist but the amount present would undoubtedly vary. The combination,
+however, must be a very weak one, since the water is often expelled by
+even a gentle heat. Indeed, in some cases the water is given up on
+simple exposure to air. Such compounds are said to be _efflorescent_.
+Thus a crystal of sodium sulphate (Glauber's salt) on exposure to air
+crumbles to a fine powder, owing to the escape of its water of
+crystallization. Other substances have just the opposite property: they
+absorb moisture when exposed to the air. For example, if a bit of dry
+calcium chloride is placed in moist air, in the course of a few hours it
+will have absorbed sufficient moisture to dissolve it. Such substances
+are said to be _deliquescent_. A deliquescent body serves as a good
+drying or _desiccating_ agent. We have already employed calcium chloride
+as an agent for absorbing the moisture from hydrogen. Many substances,
+as for example quartz, form crystals which contain no water of
+crystallization.
+
+     ~Mechanically inclosed water.~ Water of crystallization must be
+     carefully distinguished from water which is mechanically
+     inclosed in a crystal and which can be removed by powdering the
+     crystal and drying. Thus, when crystals of common salt are
+     heated, the water inclosed in the crystal is changed into steam
+     and bursts the crystal with a crackling sound. Such crystals
+     are said to _decrepitate_. That this water is not combined is
+     proved by the fact that the amount present varies and that it
+     has all the properties of water.
+
+~Uses of water.~ The importance of water in its relation to life and
+commerce is too well known to require comment. Its importance to the
+chemist has also been pointed out. It remains to call attention to the
+fact that it is used as a standard in many physical measurements. Thus
+0° and 100° on the centigrade scale are respectively the freezing and
+the boiling points of water under normal pressure. The weight of 1 cc.
+of water at its point of greatest density is the unit of weight in the
+metric system, namely, the gram. It is also taken as the unit for the
+determination of the density of liquids and solids as well as for the
+measurement of amounts of heat.
+
+
+HYDROGEN DIOXIDE
+
+~Composition.~ As has been shown, 1 part by weight of hydrogen combines
+with 7.94 parts by weight of oxygen to form water. It is possible,
+however, to obtain a second compound of hydrogen and oxygen differing
+from water in composition in that 1 part by weight of hydrogen is
+combined with 2 × 7.94, or 15.88 parts, of oxygen. This compound is
+called _hydrogen dioxide_ or _hydrogen peroxide_, the prefixes _di-_ and
+_per-_ signifying that it contains more oxygen than hydrogen oxide,
+which is the chemical name for water.
+
+~Preparation.~ Hydrogen dioxide cannot be prepared cheaply by the direct
+union of hydrogen and oxygen, and indirect methods must therefore be
+used. It is commonly prepared by the action of a solution of sulphuric
+acid on barium dioxide. The change which takes place may be indicated as
+follows:
+
+sulphuric acid +  barium dioxide =  barium sulphate + hydrogen dioxide
+--------------    --------------    ---------------   ----------------
+  hydrogen          barium           barium             hydrogen
+  sulphur           oxygen           sulphur            oxygen
+  oxygen                             oxygen
+
+In other words, the barium and hydrogen in the two compounds exchange
+places. By this method a dilute solution of the dioxide in water is
+obtained. It is possible to separate the dioxide from the water by
+fractional distillation. This is attended with great difficulties,
+however, since the pure dioxide is explosive. The distillation is
+carried on under diminished pressure so as to lower the boiling points
+as much as possible; otherwise the high temperature would decompose the
+dioxide.
+
+~Properties.~ Pure hydrogen dioxide is a colorless sirupy liquid having a
+density of 1.49. Its most characteristic property is the ease with which
+it decomposes into water and oxygen. One part by weight of hydrogen is
+capable of holding firmly only 7.94 parts of oxygen. The additional 7.94
+parts of oxygen present in hydrogen dioxide are therefore easily
+evolved, the compound breaking down into water and oxygen. This
+decomposition is attended by the generation of considerable heat. In
+dilute solution hydrogen dioxide is fairly stable, although such a
+solution should be kept in a dark, cool place, since both heat and light
+aid in the decomposition of the dioxide.
+
+~Uses.~ Solutions of hydrogen dioxide are used largely as oxidizing
+agents. The solution sold by druggists contains 3% of the dioxide and is
+used in medicine as an antiseptic. Its use as an antiseptic depends upon
+its oxidizing properties.
+
+
+EXERCISES
+
+1. Why does the chemist use distilled water in making solutions, rather
+than filtered water?
+
+2. How could you determine the total amount of solid matter dissolved in
+a sample of water?
+
+3. How could you determine whether a given sample of water is distilled
+water?
+
+4. How could the presence of air dissolved in water be detected?
+
+5. How could the amount of water in a food such as bread or potato be
+determined?
+
+6. Would ice frozen from impure water necessarily be free from disease
+germs?
+
+7. Suppose that the maximum density of water were at 0° in place of 4°;
+what effect would this have on the formation of ice on bodies of water?
+
+8. Is it possible for a substance to contain both mechanically inclosed
+water and water of crystallization?
+
+9. If steam is heated to 2000° and again cooled, has any chemical change
+taken place in the steam?
+
+10. Why is cold water passed into C instead of D (Fig. 24)?
+
+11. Mention at least two advantages that a metal condenser has over a
+glass condenser.
+
+12. Draw a diagram of the apparatus used in your laboratory for
+supplying distilled water.
+
+13. 20 cc. of hydrogen and 7 cc. of oxygen are placed in a eudiometer
+and the mixture exploded. (a) How many cubic centimeters of aqueous
+vapor are formed? (b) What gas and how much of it remains in excess?
+
+14. (a) What weight of water can be formed by the combustion of 100 L
+of hydrogen, measured under standard conditions? (b)What volume of
+oxygen would be required in (a)? (c)What weight of potassium
+chlorate is necessary to prepare this amount of oxygen?
+
+15. What weight of oxygen is present in 1 kg. of the ordinary hydrogen
+dioxide solution? In the decomposition of this weight of the dioxide
+into water and oxygen, what volume of oxygen (measured under standard
+conditions) is evolved?
+
+
+
+
+CHAPTER V
+
+THE ATOMIC THEORY
+
+
+~Three fundamental laws of matter.~ Before we can gain any very definite
+idea in regard to the structure of matter, and the way in which
+different kinds of substances act chemically upon each other, it is
+necessary to have clearly in view three fundamental laws of matter.
+These laws have been established by experiment, and any conception which
+may be formed concerning matter must therefore be in harmony with them.
+The laws are as follows:
+
+~Law of conservation of matter.~ This law has already been touched upon in
+the introductory chapter, and needs no further discussion. It will be
+recalled that it may be stated thus: _Matter can neither be created nor
+destroyed, though it can be changed from one form into another._
+
+~Law of definite composition.~ In the earlier days of chemistry there was
+much discussion as to whether the composition of a given compound is
+always precisely the same or whether it is subject to some variation.
+Two Frenchmen, Berthollet and Proust, were the leaders in this
+discussion, and a great deal of most useful experimenting was done to
+decide the question. Their experiments, as well as all succeeding ones,
+have shown that the composition of a pure chemical compound is always
+exactly the same. Water obtained by melting pure ice, condensing steam,
+burning hydrogen in oxygen, has always 11.18% hydrogen and 88.82% oxygen
+in it. Red oxide of mercury, from whatever source it is obtained,
+contains 92.6% mercury and 7.4% oxygen. This truth is known as _the law
+of definite composition_, and may be stated thus: _The composition of a
+chemical compound never varies._
+
+~Law of multiple proportion.~ It has already been noted, however, that
+hydrogen and oxygen combine in two different ratios to form water and
+hydrogen dioxide respectively. It will be observed that this fact does
+not contradict the law of definite composition, for entirely different
+substances are formed. These compounds differ from each other in
+composition, but the composition of each one is always constant. This
+ability of two elements to unite in more than one ratio is very
+frequently observed. Carbon and oxygen combine in two different ratios;
+nitrogen and oxygen combine to form as many as five distinct compounds,
+each with its own precise composition.
+
+In the first decade of the last century John Dalton, an English
+school-teacher and philosopher, endeavored to find some rule which holds
+between the ratios in which two given substances combine. His studies
+brought to light a very simple relation, which the following examples
+will make clear. In water the hydrogen and oxygen are combined in the
+ratio of 1 part by weight of hydrogen to 7.94 parts by weight of oxygen.
+In hydrogen dioxide the 1 part by weight of hydrogen is combined with
+15.88 parts by weight of oxygen. The ratio between the amounts of oxygen
+which combine with the same amount of hydrogen to form water and
+hydrogen dioxide respectively is therefore 7.94: 15.88, or 1: 2.
+
+[Illustration: JOHN DALTON (English) (1766-1844)
+
+Developed the atomic theory; made many studies on the properties and the
+composition of gases. His book entitled "A New System of Chemical
+Philosophy" had a large influence on the development of chemistry]
+
+Similarly, the element iron combines with oxygen to form two oxides, one
+of which is black and the other red. By analysis it has been shown that
+the former contains 1 part by weight of iron combined with 0.286 parts
+by weight of oxygen, while the latter contains 1 part by weight of iron
+combined with 0.429 parts by weight of oxygen. Here again we find that
+the amounts of oxygen which combine with the same fixed amount of iron
+to form the two compounds are in the ratio of small whole numbers, viz.,
+2:3.
+
+Many other examples of this simple relation might be given, since it has
+been found to hold true in all cases where more than one compound is,
+formed from the same elements. Dalton's law of multiple proportion
+states these facts as follows: _When any two elements,_ A _and_ B,
+_combine to form more than one compound, the amounts of_ B _which unite
+with any fixed amount of_ A _bear the ratio of small whole numbers to
+each other_.
+
+~Hypothesis necessary to explain the laws of matter.~ These three
+generalizations are called _laws_, because they express in concise
+language truths which are found by careful experiment to hold good in
+all cases. They do not offer any explanation of the facts, but merely
+state them. The human mind, however, does not rest content with the mere
+bare facts, but seeks ever to learn the explanation of the facts. A
+suggestion which is offered to explain such a set of facts is called an
+_hypothesis_. The suggestion which Dalton offered to explain the three
+laws of matter, called the _atomic hypothesis_, was prompted by his view
+of the constitution of matter, and it involves three distinct
+assumptions in regard to the nature of matter and chemical action.
+Dalton could not prove these assumptions to be true, but he saw that if
+they were true the laws of matter become very easy to understand.
+
+~Dalton's atomic hypothesis.~ The three assumptions which Dalton made in
+regard to the nature of matter, and which together constitute the atomic
+hypothesis, are these:
+
+1. All elements are made up of minute, independent particles which
+Dalton designated as _atoms_.
+
+2. All atoms of the same element have equal masses; those of different
+elements have different masses; in any change to which an atom is
+subjected its mass does not change.
+
+3. When two or more elements unite to form a compound, the action
+consists in the union of a definite small number of atoms of each
+element to form a small particle of the compound. The smallest particles
+of a given compound are therefore exactly alike in the number and kinds
+of atoms which they contain, and larger masses of the substances are
+simply aggregations of these least particles.
+
+~Molecules and atoms.~ Dalton applied the name atom not only to the minute
+particles of the elements but also to the least particles of compounds.
+Later Avogadro, an Italian scientist, pointed out the fact that the two
+are different, since the smallest particle of an element is a unit,
+while that of a compound must have at least two units in it. He
+suggested the name _molecule_ for the least particle of a compound which
+can exist, retaining the name _atom_ for the smallest particle of an
+element. In accordance with this distinction, we may define the atom and
+the molecule as follows: _An atom is the smallest particle of an element
+which can exist. A molecule is the smallest particle of a compound which
+can exist._ It will be shown in a subsequent chapter that sometimes two
+or more atoms of the same element unite with each other to form
+molecules of the element. While the term atom, therefore, is applicable
+only to elements, the term molecule is applicable both to elements and
+compounds.
+
+~The atomic hypothesis and the laws of matter.~ Supposing the atomic
+hypothesis to be true, let us now see if it is in harmony with the laws
+of matter.
+
+1. _The atomic hypothesis and the law of conservation of matter._ It is
+evident that if the atoms never change their masses in any change which
+they undergo, the total quantity of matter can never change and the law
+of conservation of matter must follow.
+
+2. _The atomic hypothesis and the law of definite composition._
+According to the third supposition, when iron combines with sulphur the
+union is between definite numbers of the two kinds of atoms. In the
+simplest case one atom of the one element combines with one atom of the
+other. If the sulphur and the iron atoms never change their respective
+masses when they unite to form a molecule of iron sulphide, all iron
+sulphide molecules will have equal amounts of iron in them and also of
+sulphur. Consequently any mass made up of iron sulphide molecules will
+have the same fraction of iron by weight as do the individual iron
+sulphide molecules. Iron sulphide, from whatever source, will have the
+same composition, which is in accordance with the law of definite
+composition.
+
+3. _The atomic hypothesis and the law of multiple proportion._ But this
+simplest case may not always be the only one. Under other conditions one
+atom of iron might combine with two of sulphur to form a molecule of a
+second compound. In such a case the one atom of iron would be in
+combination with twice the mass of sulphur that is in the first
+compound, since the sulphur atoms all have equal masses. What is true
+for one molecule will be true for any number of them; consequently when
+such quantities of these two compounds are selected as are found to
+contain the same amount of iron, the one will contain twice as much
+sulphur as the other.
+
+The combination between the atoms may of course take place in other
+simple ratios. For example, two atoms of one element might combine with
+three or with five of the other. In all such cases it is clear that the
+law of multiple proportion must hold true. For on selecting such numbers
+of the two kinds of molecules as have the same number of the one kind of
+atoms, the numbers of the other kind of atoms will stand in some simple
+ratio to each other, and their weights will therefore stand in the same
+simple ratio.
+
+~Testing the hypothesis.~ Efforts have been made to find compounds which
+do not conform to these laws, but all such attempts have resulted in
+failure. If such compounds should be found, the laws would be no longer
+true, and the hypothesis of Dalton would cease to possess value. When an
+hypothesis has been tested in every way in which experiment can test it,
+and is still found to be in harmony with the facts in the case, it is
+termed a _theory_. We now speak of the atomic theory rather than of the
+atomic hypothesis.
+
+~Value of a theory.~ The value of a theory is twofold. It aids in the
+clear understanding of the laws of nature because it gives an
+intelligent idea as to why these laws should be in operation.
+
+A theory also leads to discoveries. It usually happens that in testing a
+theory much valuable work is done, and many new facts are discovered.
+Almost any theory in explaining given laws will involve a number of
+consequences apart from the laws it seeks to explain. Experiment will
+soon show whether these facts are as the theory predicts they will be.
+Thus Dalton's atomic theory predicted many properties of gases which
+experiment has since verified.
+
+~Atomic weights.~ It would be of great advantage in the study of chemistry
+if we could determine the weights of the different kinds of atoms. It is
+evident that this cannot be done directly. They are so small that they
+cannot be seen even with a most powerful microscope. It is calculated
+that it would take 200,000,000 hydrogen atoms placed side by side to
+make a row one centimeter long. No balance can weigh such minute
+objects. It is possible, however, to determine their relative
+weights,--that is, how much heavier one is than another. _These relative
+weights of the atoms are spoken of as the atomic weights of the
+elements._
+
+If elements were able to combine in only one way,--one atom of one with
+one atom of another,--the problem of determining the atomic weights
+would be very simple. We should merely have to take some one convenient
+element as a standard, and find by experiment how much of each other
+element would combine with a fixed weight of it. The ratios thus found
+would be the same ratios as those between the atoms of the elements, and
+thus we should have their relative atomic weights. The law of multiple
+proportion calls attention to the fact that the atoms combine in other
+ratios than 1: 1, and there is no direct way of telling which one, if
+any, of the several compounds in a given case is the one consisting of a
+single atom of each element.
+
+If some way were to be found of telling how much heavier the entire
+molecule of a compound is than the atom chosen as a standard,--that is,
+of determining the molecular weights of compounds,--the problem could be
+solved, though its solution would not be an entirely simple matter.
+There are ways of determining the molecular weights of compounds, and
+there are other experiments which throw light directly upon the relative
+weights of the atoms. These methods cannot be described until the facts
+upon which they rest have been studied. It will be sufficient for the
+present to assume that these methods are trustworthy.
+
+~Standard for atomic weights.~ Since the atomic weights are merely
+relative to some one element chosen as a standard, it is evident that
+any one of the elements may serve as this standard and that any
+convenient value may be assigned to its atom. At one time oxygen was
+taken as this standard, with the value 100, and the atomic weights of
+the other elements were expressed in terms of this standard. It would
+seem more rational to take the element of smallest atomic weight as the
+standard and give it unit value; accordingly hydrogen was taken as the
+standard with an atomic weight of 1. Very recently, however, this unit
+has been replaced by oxygen, with an atomic weight of 16.
+
+~Why oxygen is chosen as the standard for atomic weights.~ In the
+determination of the atomic weight of an element it is necessary to find
+the weight of the element which combines with a definite weight of
+another element, preferably the element chosen as the standard. Since
+oxygen combines with the elements far more readily than does hydrogen to
+form definite compounds, it is far better adapted for the standard
+element, and has accordingly replaced hydrogen as the standard. Any
+definite value might be given to the weight of the oxygen atom. In
+assigning a value to it, however, it is convenient to choose a whole
+number, and as small a number as possible without making the atomic
+weight of any other element less than unity. For these reasons the
+number 16 has been chosen as the atomic weight of oxygen. This makes
+the atomic weight of hydrogen equal to 1.008, so that there is but
+little difference between taking oxygen as 16 and hydrogen as 1 for the
+unit.
+
+The atomic weights of the elements are given in the Appendix.
+
+
+EXERCISES
+
+1. Two compounds were found to have the following compositions: (a)
+oxygen = 69.53%, nitrogen = 30.47%; (b) oxygen = 53.27%, nitrogen =
+46.73%. Show that the law of multiple proportion holds in this case.
+
+2. Two compounds were found to have the following compositions: (a)
+oxygen = 43.64%, phosphorus = 56.36%; (b) oxygen = 56.35%, phosphorus
+= 43.65%. Show that the law of multiple proportion holds in this case.
+
+3. Why did Dalton assume that all the atoms of a given element have the
+same weight?
+
+
+
+
+CHAPTER VI
+
+CHEMICAL EQUATIONS AND CALCULATIONS
+
+
+~Formulas.~ Since the molecule of any chemical compound consists of a
+definite number of atoms, and this number never changes without
+destroying the identity of the compound, it is very convenient to
+represent the composition of a compound by indicating the composition of
+its molecules. This can be done very easily by using the symbols of the
+atoms to indicate the number and the kind of the atoms which constitute
+the molecule. HgO will in this way represent mercuric oxide, a molecule
+of which has been found to contain 1 atom each of mercury and oxygen.
+H_{2}O will represent water, the molecules of which consist of 1 atom of
+oxygen and 2 of hydrogen, the subscript figure indicating the number of
+the atoms of the element whose symbol precedes it. H_{2}SO_{4} will
+stand for sulphuric acid, the molecules of which contain 2 atoms of
+hydrogen, 1 of sulphur, and 4 of oxygen. The combination of symbols
+which represents the molecule of a substance is called its _formula_.
+
+~Equations.~ When a given substance undergoes a chemical change it is
+possible to represent this change by the use of such symbols and
+formulas. In a former chapter it was shown that mercuric oxide
+decomposes when heated to form mercury and oxygen. This may be expressed
+very briefly in the form of the equation
+
+    (1) HgO = Hg + O.
+
+When water is electrolyzed two new substances, hydrogen and oxygen, are
+formed from it. This statement in the form of an equation is
+
+    (2) H_{2}O = 2H + O.
+
+The coefficient before the symbol for hydrogen indicates that a single
+molecule of water yields two atoms of hydrogen on decomposition.
+
+In like manner the combination of sulphur with iron is expressed by the
+equation
+
+    (3) Fe + S = FeS.
+
+The decomposition of potassium chlorate by heat takes place as
+represented by the equation
+
+    (4) KClO_{3} = KCl + 3O.
+
+~Reading of equations.~ Since equations are simply a kind of shorthand way
+of indicating chemical changes which occur under certain conditions, in
+reading an equation the full statement for which it stands should be
+given. Equation (1) should be read, "Mercuric oxide when heated gives
+mercury and oxygen"; equation (2) is equivalent to the statement, "When
+electrolyzed, water produces hydrogen and oxygen"; equation (3), "When
+heated together iron and sulphur unite to form iron sulphide"; equation
+(4), "Potassium chlorate when heated yields potassium chloride and
+oxygen."
+
+~Knowledge required for writing equations.~ In order to write such
+equations correctly, a considerable amount of exact knowledge is
+required. Thus, in equation (1) the fact that red oxide of mercury has
+the composition represented by the formula HgO, that it is decomposed by
+heat, that in this decomposition mercury and oxygen are formed and no
+other products,--all these facts must be ascertained by exact experiment
+before the equation can be written. An equation expressing these facts
+will then have much value.
+
+Having obtained an equation describing the conduct of mercuric oxide on
+being heated, it will not do to assume that other oxides will behave in
+like manner. Iron oxide (FeO) resembles mercuric oxide in many respects,
+but it undergoes no change at all when heated. Manganese dioxide, the
+black substance used in the preparation of oxygen, has the formula
+MnO_{2}. When this substance is heated oxygen is set free, but the metal
+manganese is not liberated; instead, a different oxide of manganese
+containing less oxygen is produced. The equation representing the
+reaction is
+
+    3MnO_{2} = Mn_{3}O_{4} + 2O.
+
+~Classes of reactions.~ When a chemical change takes place in a substance
+the substance is said to undergo a reaction. Although a great many
+different reactions will be met in the study of chemistry, they may all
+be grouped under the following heads.
+
+1. _Addition._ This is the simplest kind of chemical action. It consists
+in the union of two or more substances to produce a new substance. The
+combination of iron with sulphur is an example:
+
+    Fe + S = FeS.
+
+2. _Decomposition._ This is the reverse of addition, the substance
+undergoing reaction being parted into its constituents. The
+decomposition of mercuric oxide is an example: HgO = Hg + O.
+
+3. _Substitution._ It is sometimes possible for an element in the free
+state to act upon a compound in such a way that it takes the place of
+one of the elements of the compound, liberating it in turn. In the study
+of the element hydrogen it was pointed out that hydrogen is most
+conveniently prepared by the action of sulphuric or hydrochloric acid
+upon zinc. When sulphuric acid is used a substance called zinc sulphate,
+having the composition represented by the formula ZnSO_{4}, is formed
+together with hydrogen. The equation is
+
+    Zn + H_{2}SO_{4} = ZnSO_{4} + 2H.
+
+When hydrochloric acid is used zinc chloride and hydrogen are the
+products of reaction:
+
+    Zn + 2HCl = ZnCl_{2} + 2H.
+
+When iron is used in place of zinc the equation is
+
+    Fe + H_{2}SO_{4} = FeSO_{4} + 2H.
+
+These reactions are quite similar, as is apparent from an examination of
+the equations. In each case 1 atom of the metal replaces 2 atoms of
+hydrogen in the acid, and the hydrogen escapes as a gas. When an element
+in the free state, such as the zinc in the equations just given, takes
+the place of some one element in a compound, setting it free from
+chemical combination, the act is called _substitution_.
+
+Other reactions illustrating substitution are the action of sodium on
+water,
+
+    Na + H_{2}O = NaOH + H;
+
+and the action of heated iron upon water,
+
+    3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H.
+
+4. _Double decomposition._ When barium dioxide (BaO_{2}) is treated with
+sulphuric acid two compounds are formed, namely, hydrogen dioxide
+(H_{2}O_{2}) and barium sulphate (BaSO_{4}). The equation is
+
+    BaO_{2} + H_{2}SO_{4} = BaSO_{4} + H_{2}O_{2}.
+
+In this reaction it will be seen that the two elements barium and
+hydrogen simply exchange places. Such a reaction is called a _double
+decomposition_. We shall meet with many examples of this kind of
+chemical reactions.
+
+~Chemical equations are quantitative.~ The use of symbols and formulas in
+expressing chemical changes has another great advantage. Thus, according
+to the equation
+
+    H_{2}O = 2H + O,
+
+1 molecule of water is decomposed into 2 atoms of hydrogen and 1 atom of
+oxygen. But, as we have seen, the relative weights of the atoms are
+known, that of hydrogen being 1.008, while that of oxygen is 16. The
+molecule of water, being composed of 2 atoms of hydrogen and 1 atom of
+oxygen, must therefore weigh relatively 2.016 + 16, or 18.016. The
+amount of hydrogen in this molecule must be 2.016/18.016, or 11.18% of
+the whole, while the amount of oxygen must be 16/18.018, or 88.82% of
+the whole. Now, since any definite quantity of water is simply the sum
+of a great many molecules of water, it is plain that the fractions
+representing the relative amounts of hydrogen and oxygen present in a
+molecule must likewise express the relative amounts of hydrogen and
+oxygen present in any quantity of water. Thus, for example, in 20 g. of
+water there are 2.016/18.016 × 20, or 2.238 g. of hydrogen, and
+16/18.016 × 20, or 17.762 g. of oxygen. These results in reference to
+the composition of water of course agree exactly with the facts obtained
+by the experiments described in the chapter on water, for it is because
+of those experiments that the values 1.008 and 16 are given to hydrogen
+and oxygen respectively.
+
+It is often easier to make calculations of this kind in the form of a
+proportion rather than by fractions. Since the molecule of water and
+the two atoms of hydrogen which it contains have the ratio by weight of
+18.016: 2.016, any mass of water has the same ratio between its total
+weight and the weight of the hydrogen in it. Hence, to find the number
+of grams (x) of hydrogen in 20 g. of water, we have the proportion
+
+    18.016 : 2.016 :: 20 g. : x (grams of hydrogen).
+
+Solving for x, we get 2.238 for the number of grams of hydrogen.
+Similarly, to find the amount (x) of oxygen present in the 20 g. of
+water, we have the proportion
+
+    18.016 : 16 :: 20 : x
+
+from which we find that x = 17.762 g.
+
+Again, suppose we wish to find what weight of oxygen can be obtained
+from 15 g. of mercuric oxide. The equation representing the
+decomposition of mercuric oxide is
+
+    HgO = Hg + O.
+
+The relative weights of the mercury and oxygen atoms are respectively
+200 and 16. The relative weight of the mercuric oxide molecule must
+therefore be the sum of these, or 216. The molecule of mercuric oxide
+and the atom of oxygen which it contains have the ratio 216: 16. This
+same ratio must therefore hold between the weight of any given quantity
+of mercuric oxide and that of the oxygen which it contains. Hence, to
+find the weight of oxygen in 15 g. of mercuric oxide, we have the
+proportion
+
+    216 : 16 :: 15 : x (grams of oxygen).
+
+On the other hand, suppose we wish to prepare, say, 20 g. of oxygen. The
+problem is to find out what weight of mercuric oxide will yield 20 g. of
+oxygen. The following proportion evidently holds
+
+    216 : 16 :: x (grams of mercuric oxide) : 20;
+
+from which we get x = 270.
+
+In the preparation of hydrogen by the action of sulphuric acid upon
+zinc, according to the equation,
+
+    Zn + H_{2}SO_{4} = ZnSO_{4} + 2 H,
+
+suppose that 50 g. of zinc are available; let it be required to
+calculate the weight of hydrogen which can be obtained. It will be seen
+that 1 atom of zinc will liberate 2 atoms of hydrogen. The ratio by
+weight of a zinc to an hydrogen atom is 65.4: 1.008; of 1 zinc atom to 2
+hydrogen atoms, 65.4: 2.016. Zinc and hydrogen will be related in this
+reaction in this same ratio, however many atoms of zinc are concerned.
+Consequently in the proportion
+
+    65.4 : 2.016 :: 50 : x,
+
+x will be the weight of hydrogen set free by 50 g. of zinc. The weight
+of zinc sulphate produced at the same time can be found from the
+proportion
+
+    65.4 : 161.46 :: 50 : x;
+
+where 161.46 is the molecular weight of the zinc sulphate, and x the
+weight of zinc sulphate formed. In like manner, the weight of sulphuric
+acid used up can be calculated from the proportion
+
+    65.4 : 98.076 :: 50 : x.
+
+These simple calculations are possible because the symbols and formulas
+in the equations represent the relative weights of the substances
+concerned in a chemical reaction. When once the relative weights of the
+atoms have been determined, and it has been agreed to allow the symbols
+to stand for these relative weights, an equation or formula making use
+of the symbols becomes a statement of a definite numerical fact, and
+calculations can be based on it.
+
+~Chemical equations not algebraic.~ Although chemical equations are
+quantitative, it must be clearly understood that they are not algebraic.
+A glance at the equations
+
+    7 + 4 = 11, 8 + 5 = 9 + 4
+
+will show at once that they are true. The equations
+
+    HgO = Hg + O, FeO = Fe + O
+
+are equally true in an algebraic sense, but experiment shows that only
+the first is true chemically, for iron oxide (FeO) cannot be directly
+decomposed into iron and oxygen. Only such equations as have been found
+by careful experiment to express a real chemical transformation, true
+both for the kinds of substances as well as for the weights, have any
+value.
+
+_Chemical formulas and equations, therefore, are a concise way of
+representing qualitatively and quantitatively facts which have been
+found by experiment to be true in reference to the composition of
+substances and the changes which they undergo._
+
+~Formulas representing water of crystallization.~ An examination of
+substances containing water of crystallization has shown that in every
+case the water is present in such proportion by weight as can readily be
+represented by a formula. For example, copper sulphate (CuSO_{4}) and
+water combine in the ratio of 1 molecule of the sulphate to 5 of water;
+calcium sulphate (CaSO_{4}) and water combine in the ratio 1: 2 to form
+gypsum. These facts are expressed by writing the formulas for the two
+substances with a period between them. Thus the formula for crystallized
+copper sulphate is CuSO_{4}·5H_{2}O; that of gypsum is CaSO_{4}·2H_{2}O.
+
+~Heat of reaction.~ Attention has frequently been directed to the fact
+that chemical changes are usually accompanied by heat changes. In
+general it has been found that in every chemical action heat is either
+absorbed or given off. By adopting a suitable unit for the measurement
+of heat, the heat change during a chemical reaction can be expressed in
+the equation for the reaction.
+
+Heat cannot be measured by the use of a thermometer alone, since the
+thermometer measures the intensity of heat, not its quantity. The
+easiest way to measure a quantity of heat is to note how warm it will
+make a definite amount of a given substance chosen as a standard. Water
+has been chosen as the standard, and the unit of heat is called a
+_calorie. A calorie is defined as the amount of heat required to raise
+the temperature of one gram of water one degree._
+
+By means of this unit it is easy to indicate the heat changes in a given
+chemical reaction. The equation
+
+    2H + O = H_{2}O + 68,300 cal.
+
+means that when 2.016 g. of hydrogen combine with 16 g. of oxygen,
+18.016 g. of water are formed and 68,300 cal. are set free.
+
+    C + 2S = CS_{2} - 19,000 cal.
+
+means that an expenditure of 19,000 cal. is required to cause 12 g. of
+carbon to unite with 64.12 g. of sulphur to form 76.12 g. of carbon
+disulphide. In these equations it will be noted that the symbols stand
+for as many grams of the substance as there are units in the weights of
+the atoms represented by the symbols. This is always understood to be
+the case in equations where the heat of reaction is given.
+
+~Conditions of a chemical action are not indicated by equations.~
+Equations do not tell the conditions under which a reaction will take
+place. The equation
+
+    HgO = Hg + O
+
+does not tell us that it is necessary to keep the mercuric oxide at a
+high temperature in order that the decomposition may go on. The equation
+
+    Zn + 2HCl = ZnCl_{2} + 2H
+
+in no way indicates the fact that the hydrochloric acid must be
+dissolved in water before it will act upon the zinc. From the equation
+
+    H + Cl = HCl
+
+it would not be suspected that the two gases hydrogen and chlorine will
+unite instantly in the sunlight, but will stand mixed in the dark a long
+time without change. It will therefore be necessary to pay much
+attention to the details of the conditions under which a given reaction
+occurs, as well as to the expression of the reaction in the form of an
+equation.
+
+
+EXERCISES
+
+1. Calculate the percentage composition of the following substances:
+(a) mercuric oxide; (b) potassium chlorate; (c) hydrochloric acid;
+(d) sulphuric acid. Compare the results obtained with the compositions
+as given in Chapters II and III.
+
+2. Determine the percentage of copper, sulphur, oxygen, and water in
+copper sulphate crystals. What weight of water can be obtained from 150
+g. of this substance?
+
+3. What weight of zinc can be dissolved in 10 g. of sulphuric acid? How
+much zinc sulphate will be formed?
+
+4. How many liters of hydrogen measured under standard conditions can be
+obtained from the action of 8 g. of iron on 10 g. of sulphuric acid? How
+much iron sulphate (FeSO_{4}) will be formed?
+
+5. 10 g. of zinc were used in the preparation of hydrogen; what weight
+of iron will be required to prepare an equal volume?
+
+6. How many grams of barium dioxide will be required to prepare 1 kg. of
+common hydrogen dioxide solution? What weight of barium sulphate will be
+formed at the same time?
+
+7. What weight of the compound Mn_{3}O_{4} will be formed by strongly
+heating 25 g. of manganese dioxide? What volume of oxygen will be given
+off at the same time, measured under standard conditions?
+
+8. (a) What is the weight of 100 l. of hydrogen measured in a
+laboratory in which the temperature is 20° and pressure 750 mm.? (b)
+What weight of sulphuric acid is necessary to prepare this amount of
+hydrogen? (c) The density of sulphuric acid is 1.84. Express the acid
+required in (b) in cubic centimeters.
+
+9. What weight of potassium chlorate is necessary to furnish sufficient
+oxygen to fill four 200 cc. bottles in your laboratory (the gas to be
+collected over water)?
+
+
+
+
+CHAPTER VII
+
+NITROGEN AND THE RARE ELEMENTS: ARGON, HELIUM, NEON, KRYPTON, XENON
+
+
+~Historical.~ Nitrogen was discovered by the English chemist Rutherford in
+1772. A little later Scheele showed it to be a constituent of air, and
+Lavoisier gave it the name _azote_, signifying that it would not support
+life. The name _nitrogen_ was afterwards given it because of its
+presence in saltpeter or niter. The term azote and symbol Az are still
+retained by the French chemists.
+
+~Occurrence.~ Air is composed principally of oxygen and nitrogen in the
+free state, about 78 parts by volume out of every 100 parts being
+nitrogen. Nitrogen also occurs in nature in the form of potassium
+nitrate (KNO_{3})--commonly called saltpeter or niter--as well as in
+sodium nitrate (NaNO_{3}). Nitrogen is also an essential constituent of
+all living organisms; for example, the human body contains about 2.4% of
+nitrogen.
+
+~Preparation from air.~ Nitrogen can be prepared from air by the action of
+some substance which will combine with the oxygen, leaving the nitrogen
+free. Such a substance must be chosen, however, as will combine with the
+oxygen to form a product which is not a gas, and which can be readily
+separated from the nitrogen. The substances most commonly used for this
+purpose are phosphorus and copper.
+
+1. _By the action of phosphorus._ The method used for the preparation of
+nitrogen by the action of phosphorus is as follows:
+
+The phosphorus is placed in a little porcelain dish, supported on a cork
+and floated on water (Fig. 26). It is then ignited by contact with a hot
+wire, and immediately a bell jar or bottle is brought over it so as to
+confine a portion of the air. The phosphorus combines with the oxygen to
+form an oxide of phosphorus, known as phosphorus pentoxide. This is a
+white solid which floats about in the bell jar, but in a short time it
+is all absorbed by the water, leaving the nitrogen. The withdrawal of
+the oxygen is indicated by the rising of the water in the bell jar.
+
+[Illustration: Fig. 26]
+
+2. _By the action of copper._ The oxygen present in the air may also be
+removed by passing air slowly through a heated tube containing copper.
+The copper combines with the oxygen to form copper oxide, which is a
+solid. The nitrogen passes on and may be collected over water.
+
+     ~Nitrogen obtained from air is not pure.~ Inasmuch as air, in
+     addition to oxygen and nitrogen, contains small amounts of
+     other gases, and since the phosphorus as well as the copper
+     removes only the oxygen, it is evident that the nitrogen
+     obtained by these methods is never quite pure. About 1% of the
+     product is composed of other gases, from which it is very
+     difficult to separate the nitrogen. The impure nitrogen so
+     obtained may, however, be used for a study of most of the
+     properties of nitrogen, since these are not materially affected
+     by the presence of the other gases.
+
+~Preparation from compounds of nitrogen.~ Pure nitrogen may be obtained
+from certain compounds of the element. Thus, if heat is applied to the
+compound ammonium nitrite (NH_{4}NO_{2}), the change represented in the
+following equation takes place:
+
+    NH_{4}NO_{2} = 2H_{2}O + 2N.
+
+~Physical properties.~ Nitrogen is similar to oxygen and hydrogen in that
+it is a colorless, odorless, and tasteless gas. One liter of nitrogen
+weighs 1.2501 g. It is almost insoluble in water. It can be obtained in
+the form of a colorless liquid having a boiling point of -195° at
+ordinary pressure. At -214° it solidifies.
+
+~Chemical properties.~ Nitrogen is characterized by its inertness. It is
+neither combustible nor a supporter of combustion. At ordinary
+temperatures it will not combine directly with any of the elements
+except under rare conditions. At higher temperatures it combines with
+magnesium, lithium, titanium, and a number of other elements. The
+compounds formed are called _nitrides_, just as compounds of an element
+with oxygen are called _oxides_. When it is mixed with oxygen and
+subjected to the action of electric sparks, the two gases slowly combine
+forming oxides of nitrogen. A mixture of nitrogen and hydrogen when
+treated similarly forms ammonia, a gaseous compound of nitrogen and
+hydrogen. Since we are constantly inhaling nitrogen, it is evident that
+it is not poisonous. Nevertheless life would be impossible in an
+atmosphere of pure nitrogen on account of the exclusion of the necessary
+oxygen.
+
+     ~Argon, helium, neon, krypton, xenon.~ These are all rare
+     elements occurring in the air in very small quantities. Argon,
+     discovered in 1894, was the first one obtained. Lord Rayleigh,
+     an English scientist, while engaged in determining the exact
+     weights of various gases, observed that the nitrogen obtained
+     from the air is slightly heavier than pure nitrogen obtained
+     from its compounds. After repeating his experiments many times,
+     always with the same results, Rayleigh finally concluded that
+     the nitrogen which he had obtained from the air was not pure,
+     but was mixed with a small amount of some unknown gas, the
+     density of which is greater than that of nitrogen. Acting on
+     this assumption, Rayleigh, together with the English chemist
+     Ramsay, attempted to separate the nitrogen from the unknown
+     gas. Knowing that nitrogen would combine with magnesium, they
+     passed the nitrogen obtained from the air and freed from all
+     known substances through tubes containing magnesium heated to
+     the necessary temperature. After repeating this operation, they
+     finally succeeded in obtaining from the atmospheric nitrogen a
+     small volume of gas which would not combine with magnesium and
+     hence could not be nitrogen. This proved to be a new element,
+     to which they gave the name _argon_. As predicted, this new
+     element was found to be heavier than nitrogen, its density as
+     compared with hydrogen as a standard being approximately 20,
+     that of nitrogen being only 14. About 1% of the atmospheric
+     nitrogen proved to be argon. The new element is characterized
+     by having no affinity for other elements. Even under the most
+     favorable conditions it has not been made to combine with any
+     other element. On this account it was given the name argon,
+     signifying lazy or idle. Like nitrogen, it is colorless,
+     odorless, and tasteless. It has been liquefied and solidified.
+     Its boiling point is -187°.
+
+     Helium was first found in the gases expelled from certain
+     minerals by heating. Through the agency of the spectroscope it
+     had been known to exist in the sun long before its presence on
+     the earth had been demonstrated,--a fact suggested by the name
+     helium, signifying the sun. Its existence in traces in the
+     atmosphere has also been proven. It was first liquefied by
+     Onnes in July, 1908. Its boiling point, namely -269°, is the
+     lowest temperature yet reached.
+
+     The remaining elements of this group--neon, krypton, and
+     xenon--have been obtained from liquid air. When liquid air is
+     allowed to boil, the constituents which are the most difficult
+     to liquefy, and which therefore have the lowest boiling points,
+     vaporize first, followed by the others in the order of their
+     boiling points. It is possible in this way to make at least a
+     partial separation of the air into its constituents, and Ramsay
+     thus succeeded in obtaining from liquid air not only the known
+     constituents, including argon and helium, but also the new
+     elements, neon, krypton, and xenon. These elements, as well as
+     helium, all proved to be similar to argon in that they are
+     without chemical activity, apparently forming no compounds
+     whatever. The percentages present in the air are very small.
+     The names, neon, krypton, xenon, signify respectively, new,
+     hidden, stranger.
+
+
+EXERCISES
+
+1. How could you distinguish between oxygen, hydrogen, and nitrogen?
+
+2. Calculate the relative weights of nitrogen and oxygen; of nitrogen
+and hydrogen.
+
+3. In the preparation of nitrogen from the air, how would hydrogen do as
+a substance for the removal of the oxygen?
+
+4. What weight of nitrogen can be obtained from 10 l. of air measured
+under the conditions of temperature and pressure which prevail in your
+laboratory?
+
+5. How many grams of ammonium nitrite are necessary in the preparation
+of 20 l. of nitrogen measured over water under the conditions of
+temperature and pressure which prevail in your laboratory?
+
+6. If 10 l. of air, measured under standard conditions, is passed over
+100 g. of hot copper, how much will the copper gain in weight?
+
+[Illustration: WILLIAM RAMSAY (Scotch) (1855-)
+
+Has made many studies in the physical properties of substances;
+discovered helium; together with Lord Rayleigh and others he discovered
+argon, krypton, xenon, and neon; has contributed largely to the
+knowledge of radio-active substances, showing that radium gradually
+gives rise to helium; professor at University College, London]
+
+
+
+
+CHAPTER VIII
+
+THE ATMOSPHERE
+
+
+~Atmosphere and air.~ The term _atmosphere_ is applied to the gaseous
+envelope surrounding the earth. The term _air_ is generally applied to a
+limited portion of this envelope, although the two words are often used
+interchangeably. Many references have already been made to the
+composition and properties of the atmosphere. These statements must now
+be collected and discussed somewhat more in detail.
+
+~Air formerly regarded as an element.~ Like water, air was at first
+regarded as elementary in character. Near the close of the eighteenth
+century Scheele, Priestley, and Lavoisier showed by their experiments
+that it is a mixture of at least two gases,--those which we now call
+oxygen and nitrogen. By burning substances in an inclosed volume of air
+and noting the contraction in volume due to the removal of the oxygen,
+they were able to determine with some accuracy the relative volumes of
+oxygen and nitrogen present in the air.
+
+~The constituents of the atmosphere.~ The constituents of the atmosphere
+may be divided into two general groups: those which are essential to
+life and those which are not essential.
+
+1. _Constituents essential to life._ In addition to oxygen and nitrogen
+at least two other substances, namely, carbon dioxide and water vapor,
+must be present in the atmosphere in order that life may exist. The
+former of these is a gaseous compound of carbon and oxygen having the
+formula CO_{2}. Its properties will be discussed in detail in the
+chapter on the compounds of carbon. Its presence in the air may be shown
+by causing the air to bubble through a solution of calcium hydroxide
+(Ca(OH)_{2}), commonly called lime water. The carbon dioxide combines
+with the calcium hydroxide in accordance with the following equation:
+
+    Ca(OH)_{2} + CO_{2} = CaCO_{3} + H_{2}O.
+
+The resulting calcium carbonate (CaCO_{3}) is insoluble in water and
+separates in the form of a white powder, which causes the solution to
+appear milky.
+
+The presence of water vapor is readily shown by its condensation on cold
+objects as well as by the fact that a bit of calcium chloride when
+exposed to the air becomes moist, and may even dissolve in the water
+absorbed from the air.
+
+2. _Constituents not essential to life._ In addition to the essential
+constituents, the air contains small percentages of various other gases,
+the presence of which so far as is known is not essential to life. This
+list includes the rare elements, argon, helium, neon, krypton, and
+xenon; also hydrogen, ammonia, hydrogen dioxide, and probably ozone.
+Certain minute forms of life (germs) are also present, the decay of
+organic matter being due to their presence.
+
+     ~Function of each of the essential constituents.~ (1) The oxygen
+     directly supports life through respiration. (2) The nitrogen,
+     on account of its inactivity, serves to dilute the oxygen, and
+     while contrary to the older views, it is possible that life
+     might continue to exist in the absence of the atmospheric
+     nitrogen, yet the conditions of life would be entirely changed.
+     Moreover, nitrogen is an essential constituent of all animal
+     and plant life. It was formerly supposed that neither animals
+     nor plants could assimilate the free nitrogen, but it has been
+     shown recently that the plants of at least one natural order,
+     the Leguminosæ, to which belong the beans, peas, and clover,
+     have the power of directly assimilating the free nitrogen from
+     the atmosphere. This is accomplished through the agency of
+     groups of bacteria, which form colonies in little tubercles on
+     the roots of the plants. These bacteria probably assist in the
+     absorption of nitrogen by changing the free nitrogen into
+     compounds which can be assimilated by the plant. Fig. 27 shows
+     the tubercles on the roots of a variety of bean. (3) The
+     presence of water vapor in the air is necessary to prevent
+     excessive evaporation from both plants and animals. (4) Carbon
+     dioxide is an essential plant food.
+
+[Illustration: Fig. 27]
+
+~The quantitative analysis of air.~ A number of different methods have
+been devised for the determination of the percentages of the
+constituents present in the atmosphere. Among these are the following.
+
+1. _Determination of oxygen._ (1) The oxygen is withdrawn from a
+measured volume of air inclosed in a tube, by means of phosphorus.
+
+     To make the determination, a graduated tube is filled with
+     water and inverted in a vessel of water. Air is introduced into
+     the tube until it is partially filled with the gas. The volume
+     of the inclosed air is carefully noted and reduced to standard
+     conditions. A small piece of phosphorus is attached to a wire
+     and brought within the tube as shown in Fig. 28. After a few
+     hours the oxygen in the inclosed air will have combined with
+     the phosphorus, the water rising to take its place. The
+     phosphorus is removed and the volume is again noted and reduced
+     to standard conditions. The contraction in the volume of the
+     air is equal to the volume of oxygen absorbed.
+
+[Illustration: Fig. 28]
+
+(2) The oxygen may also be estimated by passing a measured volume of air
+through a tube containing copper heated to a high temperature. The
+oxygen in the air combines with the copper to form copper oxide (CuO).
+Hence the increase in the weight of the copper equals the weight of the
+oxygen in the volume of air taken.
+
+(3) A more accurate method is the following. A eudiometer tube is filled
+with mercury and inverted in a vessel of the same liquid. A convenient
+amount of air is then introduced into the tube and its volume accurately
+noted. There is then introduced more than sufficient hydrogen to combine
+with the oxygen present in the inclosed air, and the volume is again
+accurately noted. The mixture is then exploded by an electric spark, and
+the volume is once more taken. By subtracting this volume from the total
+volume of the air and hydrogen there is obtained the contraction in
+volume due to the union of the oxygen and hydrogen. The volume occupied
+by the water formed by the union of the two gases is so small that it
+may be disregarded in the calculation. Since oxygen and hydrogen combine
+in the ratio 1: 2 by volume, it is evident that the contraction in
+volume due to the combination is equal to the volume occupied by the
+oxygen in the air contained in the tube, plus twice this volume of
+hydrogen. In other words, one third of the total contraction is equal to
+the volume occupied by the oxygen in the inclosed air. The following
+example will make this clear:
+
+Volume of air in tube                                        50.0 cc.
+Volume after introducing hydrogen                            80.0
+Volume after combination of oxygen and hydrogen              48.5
+Contraction in volume due to combination (80 cc.-48.5 cc.)   31.5
+Volume of oxygen in 50 cc. of air (1/3 of 31.5)              10.5
+
+All these methods agree in showing that 100 volumes of dry air contain
+approximately 21 volumes of oxygen.
+
+2. _Determination of nitrogen._ If the gas left after the removal of
+oxygen from a portion of air is passed over heated magnesium, the
+nitrogen is withdrawn, argon and the other rare elements being left. It
+may thus be shown that of the 79 volumes of gas left after the removal
+of the oxygen from 100 volumes of air, approximately 78 are nitrogen and
+0.93 argon. The other elements are present in such small quantities that
+they may be neglected.
+
+3. _Determination of carbon dioxide._ The percentage of carbon dioxide
+in any given volume of air may be determined by passing the air over
+calcium hydroxide or some other compound which will combine with the
+carbon dioxide. The increase in the weight of the hydroxide equals the
+weight of the carbon dioxide absorbed. The amount present in the open
+normal air is from 3 to 4 parts by volume in 10,000 volumes of air, or
+about 0.04%.
+
+4. _Determination of water vapor._ The water vapor present in a given
+volume of air may be determined by passing the air over calcium chloride
+(or some other compound which has a strong affinity for water), and
+noting the increase in the weight of the chloride. The amount present
+varies not only with the locality, but there is a wide variation from
+day to day in the same locality because of the winds and changes in
+temperature.
+
+~Processes affecting the composition of the air.~ The most important of
+these processes are the following.
+
+1. _Respiration._ In the process of respiration some of the oxygen in
+the inhaled air is absorbed by the blood and carried to all parts of the
+body, where it combines with the carbon of the worn-out tissues. The
+products of oxidation are carried back to the lungs and exhaled in the
+form of carbon dioxide. The amount exhaled by an adult averages about 20
+l. per hour. Hence in a poorly ventilated room occupied by a number of
+people the amount of carbon dioxide rapidly increases. While this gas is
+not poisonous unless present in large amounts, nevertheless air
+containing more than 15 parts in 10,000 is not fit for respiration.
+
+2. _Combustion._ All of the ordinary forms of fuel contain large
+percentages of carbon. On burning, this carbon combines with oxygen in
+the air, forming carbon dioxide. Combustion and respiration, therefore,
+tend to diminish the amount of oxygen in the air and to increase the
+amount of carbon dioxide.
+
+3. _Action of plants._ Plants have the power, when in the sunlight, of
+absorbing carbon dioxide from the air, retaining the carbon and
+returning at least a portion of the oxygen to the air. It will be
+observed that these changes are just the opposite of those brought about
+by the processes of respiration and combustion.
+
+     ~Poisonous effect of exhaled air.~ The differences in the
+     percentages of oxygen, carbon dioxide, and moisture present in
+     inhaled air and exhaled air are shown in the following
+     analyses.
+
+                       INHALED AIR     EXHALED AIR
+       Oxygen            21.00%          16.00%
+       Carbon dioxide     0.04            4.38
+       Moisture         variable        saturated
+
+     The foul odor of respired air is due to the presence of a
+     certain amount of organic matter. It is possible that this
+     organic matter rather than the carbon dioxide is responsible
+     for the injurious effects which follow the respiration of
+     impure air. The extent of such organic impurities present may
+     be judged, however, by the amount of carbon dioxide present,
+     since the two are exhaled together.
+
+     ~The cycle of carbon in nature.~ Under the influence of sunlight,
+     the carbon dioxide absorbed from the air by plants reacts with
+     water and small amounts of other substances absorbed from the
+     soil to form complex compounds of carbon which constitute the
+     essential part of the plant tissue. This reaction is attended
+     by the evolution of oxygen, which is restored to the air. The
+     compounds resulting from these changes are much richer in their
+     energy content than are the substances from which they are
+     formed; hence a certain amount of energy must have been
+     absorbed in their formation. The source of this energy is the
+     sun's rays.
+
+     If the plant is burned, the changes which took place in the
+     formation of the compounds present are largely reversed. The
+     carbon and hydrogen present combine with oxygen taken from the
+     air to form carbon dioxide and water, while the energy absorbed
+     from the sun's rays is liberated in the form of energy of heat.
+     If, on the other hand, the plant is used as food, the compounds
+     present are used in building up the tissues of the body. When
+     this tissue breaks down, the changes which it undergoes are
+     very similar to those which take place when the plant is
+     burned. The carbon and hydrogen combine with the inhaled oxygen
+     to form carbon dioxide and water, which are exhaled. The energy
+     possessed by the complex substances is liberated partly in the
+     form of energy of heat, which maintains the heat of the body,
+     and partly in the various forms of muscular energy. The carbon
+     originally absorbed from the air by the plant in the form of
+     carbon dioxide is thus restored to the air and is ready to
+     repeat the cycle of changes.
+
+~The composition of the air is constant.~ Notwithstanding the changes
+constantly taking place which tend to alter the composition of the air,
+the results of a great many analyses of air collected in the open fields
+show that the percentages of oxygen and nitrogen as well as of carbon
+dioxide are very nearly constant. Indeed, so constant are the
+percentages of oxygen and nitrogen that the question has arisen, whether
+these two elements are not combined in the air, forming a definite
+chemical compound. That the two are not combined but are simply mixed
+together can be shown in a number of ways, among which are the
+following.
+
+1. When air dissolves in water it has been found that the ratio of
+oxygen to nitrogen in the dissolved air is no longer 21: 78, but more
+nearly 35: 65. If it were a chemical compound, the ratio of oxygen to
+nitrogen would not be changed by solution in water.
+
+2. A chemical compound in the form of a liquid has a definite boiling
+point. Water, for example, boils at 100°. Moreover the steam which is
+thus formed has the same composition as the water. The boiling point of
+liquid air, on the other hand, gradually rises as the liquid boils, the
+nitrogen escaping first followed by the oxygen. If the two were
+combined, they would pass off together in the ratio in which they are
+found in the air.
+
+~Why the air has a constant composition.~ If air is a mixture and changes
+are constantly taking place which tend to modify its composition, how,
+then, do we account for the constancy of composition which the analyses
+reveal? This is explained by several facts. (1) The changes which are
+caused by the processes of combustion and respiration, on the one hand,
+and the action of plants, on the other, tend to equalize each other. (2)
+The winds keep the air in constant motion and so prevent local changes.
+(3) The volume of the air is so vast and the changes which occur are so
+small compared with the total amount of air that they cannot be readily
+detected. (4) Finally it must be noted that only air collected in the
+open fields shows this constancy in composition. The air in a poorly
+ventilated room occupied by a number of people rapidly changes in
+composition.
+
+~The properties of the air.~ Inasmuch as air is composed principally of a
+mixture of oxygen and nitrogen, which elements have already been
+discussed, its properties may be inferred largely from those of the two
+gases. One liter weighs 1.2923 g. It is thus 14.38 times as heavy as
+hydrogen. At the sea level it exerts an average pressure sufficient to
+sustain a column of mercury 760 mm. in height. This is taken as the
+standard pressure in determining the volumes of gases as well as the
+boiling points of liquids. Water may be made to boil at any temperature
+between 0° and considerably above 100° by simply varying the pressure.
+It is only when the pressure upon it is equal to the normal pressure of
+the atmosphere at the sea level, as indicated by a barometric reading of
+760 mm., that it boils at 100°.
+
+~Preparation of liquid air.~ Attention has been called to the fact that
+both oxygen and nitrogen can be obtained in the liquid state by strongly
+cooling the gases and applying great pressure to them. Since air is
+largely a mixture of these two gases, it can be liquefied by the same
+methods.
+
+     The methods for liquefying air have been simplified greatly in
+     that the low temperature required is obtained by allowing a
+     portion of the compressed air to expand. The expansion of a gas
+     is always attended by the absorption of heat. In liquefying air
+     the apparatus is so constructed that the heat absorbed is
+     withdrawn from air already under great pressure. This process
+     is continued until the temperature is lowered to the point of
+     liquefaction.
+
+[Illustration: Fig. 29]
+
+~The Dewar bulb.~ It is not possible to preserve air in the liquid state
+in a closed vessel, on account of the enormous pressure exerted by it in
+its tendency to pass into the gaseous state. It may however be preserved
+for some hours or even days before it will completely evaporate, by
+simply placing it in an open vessel surrounded by a nonconducting
+material. The most efficient vessel for this purpose is the _Dewar bulb_
+shown in Fig. 29. The air is withdrawn from the space between the two
+walls, thus making it nonconducting.
+
+~Properties and uses of liquid air.~ When first prepared, liquid air is
+cloudy because of the presence of particles of solid carbon dioxide.
+These may be filtered off, leaving a liquid of slightly bluish color. It
+begins to boil at about -190°, the nitrogen passing off first, gradually
+followed by the oxygen, the last portions being nearly pure oxygen. To a
+certain extent oxygen is now prepared in this way for commercial
+purposes.
+
+The extremely low temperature of liquid air may be inferred from the
+fact that mercury when cooled by it is frozen to a mass so hard that it
+may be used for driving nails.
+
+Liquid air is used in the preparation of oxygen and as a cooling agent
+in the study of the properties of matter at low temperatures. It has
+thus been found that elements at extremely low temperatures largely lose
+their chemical activity.
+
+
+EXERCISES
+
+1. When oxygen and nitrogen are mixed in the proportion in which they
+exist in the atmosphere, heat is neither evolved nor absorbed by the
+process. What important point does this suggest?
+
+2. What essential constituent of the air is found in larger amount in
+manufacturing districts than in the open country?
+
+3. Can you suggest any reason why the growth of clover in a field
+improves the soil?
+
+4. Why are the inner walls of a Dewar bulb sometimes coated with a film
+of silver?
+
+5. To what is the blue color of liquid air due? Does this color increase
+in intensity on standing?
+
+6. When ice is placed in a vessel containing liquid air, the latter
+boils violently. Explain.
+
+7. Taking the volumes of the oxygen and nitrogen in 100 volumes of air
+as 21 and 78 respectively, calculate the percentages of these elements
+present by weight.
+
+8. Would combustion be more intense in liquid air than in the gaseous
+substance?
+
+9. A tube containing calcium chloride was found to weigh 30.1293 g. A
+volume of air which weighed 15.2134 g. was passed through, after which
+the weight of the tube was found to be 30.3405 g. What was the
+percentage amount of moisture present in the air?
+
+10. 10 l. of air measured at 20° and 740 mm. passed through lime water
+caused the precipitation of 0.0102 g. of CaCO_{3}. Find the number of
+volumes of carbon dioxide in 10,000 volumes of the air.
+
+
+
+
+CHAPTER IX
+
+SOLUTIONS
+
+
+~Definitions.~ When a substance disappears in a liquid in such a way as to
+thoroughly mix with it and to be lost to sight as an individual body,
+the resulting liquid is called a _solution_. The liquid in which the
+substance dissolves is called the _solvent_, while the dissolved
+substance is called the _solute_.
+
+~Classes of solutions.~ Matter in any one of its physical states may
+dissolve in a liquid, so that we may have solutions of gases, of
+liquids, and of solids. Solutions of liquids in liquids are not often
+mentioned in the following pages, but the other two classes will become
+very familiar in the course of our study, and deserve special attention.
+
+
+SOLUTION OF GASES IN LIQUIDS
+
+[Illustration: Fig. 30]
+
+It has already been stated that oxygen, hydrogen, and nitrogen are
+slightly soluble in water. Accurate study has led to the conclusion that
+all gases are soluble to some extent not only in water but in many other
+liquids. The amount of a gas which will dissolve in a liquid depends
+upon a number of conditions, and these can best be understood by
+supposing a vessel B (Fig. 30), to be filled with the gas and inverted
+over the liquid. Under these circumstances the gas cannot escape or
+become mixed with another gas.
+
+~Circumstances affecting the solubility of gases.~ A number of
+circumstances affect the solubility of a gas in a liquid.
+
+1. _Nature of the gas._ Other conditions being equal, each gas has its
+own peculiar solubility, just as it has its own special taste or odor.
+The solubility of gases varies between wide limits, as will be seen from
+the following table, but as a rule a given volume of a liquid will not
+dissolve more than two or three times its own volume of a gas.
+
+
+_Solubility of Gases in Water_
+
+1 l. of water at 760 mm. pressure and at 0° will dissolve:
+
+    Ammonia                 1148.00 l.
+    Hydrochloric acid        503.00
+    Sulphur dioxide           79.79
+    Carbon dioxide             1.80
+    Oxygen                    41.14 cc.
+    Hydrogen                  21.15
+    Nitrogen                  20.03
+
+In the case of very soluble gases, such as the first three in the table,
+it is probable that chemical combination between the liquid and the gas
+takes place.
+
+2. _Nature of the liquid._ The character of the liquid has much
+influence upon the solubility of a gas. Water, alcohol, and ether have
+each its own peculiar solvent power. From the solubility of a gas in
+water, no prediction can be made as to its solubility in other liquids.
+
+3. _Influence of pressure._ It has been found that the weight of gas
+which dissolves in a given case is proportional to the pressure exerted
+upon the gas. If the pressure is doubled, the weight of gas going into
+solution is doubled; if the pressure is diminished to one half of its
+original value, half of the dissolved gas will escape. Under high
+pressure, large quantities of gas can be dissolved in a liquid, and when
+the pressure is removed the gas escapes, causing the liquid to foam or
+_effervesce_.
+
+4. _Influence of temperature._ In general, the lower the temperature of
+the liquid, the larger the quantity of gas which it can dissolve. 1000
+volumes of water at 0° will dissolve 41.14 volumes of oxygen; at 50°,
+18.37 volumes; at 100° none at all. While most gases can be expelled
+from a liquid by boiling the solution, some cannot. For example, it is
+not possible to expel hydrochloric acid gas completely from its solution
+by boiling.
+
+
+SOLUTION OF SOLIDS IN LIQUIDS
+
+This is the most familiar class of solutions, since in the laboratory
+substances are much more frequently used in the form of solutions than
+in the solid state.
+
+~Circumstances affecting the solubility of a solid.~ The solubility of a
+solid in a liquid depends upon several factors.
+
+1. _Nature of the solid._ Other conditions being the same, solids vary
+greatly in their solubility in liquids. This is illustrated in the
+following table:
+
+
+_Table of Solubility of Solids at 18°_
+
+100 cc. of water will dissolve:
+
+    Calcium chloride        71.0 g.
+    Sodium chloride         35.9
+    Potassium nitrate       29.1
+    Copper sulphate         21.4
+    Calcium sulphate         0.207
+
+No solids are absolutely insoluble, but the amount dissolved may be so
+small as to be of no significance for most purposes. Thus barium
+sulphate, one of the most insoluble of common substances, dissolves in
+water to the extent of 1 part in 400,000.
+
+2. _Nature of the solvent._ Liquids vary much in their power to dissolve
+solids. Some are said to be good solvents, since they dissolve a great
+variety of substances and considerable quantities of them. Others have
+small solvent power, dissolving few substances, and those to a slight
+extent only. Broadly speaking, water is the most general solvent, and
+alcohol is perhaps second in solvent power.
+
+3. _Temperature._ The weight of a solid which a given liquid can
+dissolve varies with the temperature. Usually it increases rapidly as
+the temperature rises, so that the boiling liquid dissolves several
+times the weight which the cold liquid will dissolve. In some instances,
+as in the case of common salt dissolved in water, the temperature has
+little influence upon the solubility, and a few solids are more soluble
+in cold water than in hot. The following examples will serve as
+illustrations:
+
+
+_Table of Solubility at 0° and at 100°_
+
+100 cc. of water will dissolve:
+
+                                At 0°           At 100°
+
+    Calcium chloride            49.6 g.        155.0 g.
+    Sodium chloride             35.7            39.8
+    Potassium nitrate           13.3           247.0
+    Copper sulphate             15.5            73.5
+    Calcium sulphate             0.205           0.217
+    Calcium hydroxide            0.173           0.079
+
+~Saturated solutions.~ A liquid will not dissolve an unlimited quantity of
+a solid. On adding the solid to the liquid in small portions at a time,
+it will be found that a point is reached at which the liquid will not
+dissolve more of the solid at that temperature. The solid and the
+solution remain in contact with each other unchanged. This condition may
+be described by saying that they are in equilibrium with each other. A
+solution is said to be _saturated_ when it remains unchanged in
+concentration in contact with some of the solid. The weight of the solid
+which will completely saturate a definite volume of a liquid at a given
+temperature is called the _solubility_ of the substance at that
+temperature.
+
+~Supersaturated solutions.~ When a solution, saturated at a given
+temperature, is allowed to cool it sometimes happens that no solid
+crystallizes out. This is very likely to occur when the vessel used is
+perfectly smooth and the solution is not disturbed in any way. Such a
+solution is said to be _supersaturated_. That this condition is unstable
+can be shown by adding a crystal of the solid to the solution. All of
+the solid in excess of the quantity required to saturate the solution at
+this temperature will at once crystallize out, leaving the solution
+saturated. Supersaturation may also be overcome in many cases by
+vigorously shaking or stirring the solution.
+
+~General physical properties of solutions.~ A few general statements may
+be made in reference to the physical properties of solutions.
+
+1. _Distribution of the solid in the liquid._ A solid, when dissolved,
+tends to distribute itself uniformly through the liquid, so that every
+part of the solution has the same concentration. The process goes on
+very slowly unless hastened by stirring or shaking the solution. Thus,
+if a few crystals of a highly colored substance such as copper sulphate
+are placed in the bottom of a tall vessel full of water, it will take
+weeks for the solution to become uniformly colored.
+
+2. _Boiling points of solutions._ The boiling point of a liquid is
+raised by the presence of a substance dissolved in it. In general the
+extent to which the boiling point of a solvent is raised by a given
+substance is proportional to the concentration of the solution, that
+is, to the weight of the substance dissolved in a definite weight of the
+solvent.
+
+3. _Freezing points of solutions._ A solution freezes at a lower
+temperature than the pure solvent. The lowering of the freezing point
+obeys the same law which holds for the raising of the boiling point: the
+extent of lowering is proportional to the weight of dissolved substance,
+that is, to the concentration of the solution.
+
+~Electrolysis of solutions.~ Pure water does not appreciably conduct the
+electric current. If, however, certain substances such as common salt
+are dissolved in the water, the resulting solutions are found to be
+conductors of electricity. Such solutions are called _electrolytes_.
+When the current passes through an electrolyte some chemical change
+always takes place. This change is called _electrolysis_.
+
+[Illustration: Fig. 31]
+
+The general method used in the electrolysis of a solution is illustrated
+in Fig. 31. The vessel D contains the electrolyte. Two plates or rods,
+A and B, made of suitable material, are connected with the wires
+from a battery (or dynamo) and dipped into the electrolyte, as shown in
+the figure. These plates or rods are called _electrodes_. The electrode
+connected with the zinc plate of the battery is the negative electrode
+or _cathode_, while that connected with the carbon plate is the positive
+electrode or _anode_.
+
+~Theory of electrolytic dissociation.~ The facts which have just been
+described in connection with solutions, together with many others, have
+led chemists to adopt a theory of solutions called _the theory of
+electrolytic dissociation_. The main assumptions in this theory are the
+following.
+
+1. _Formation of ions._ Many compounds when dissolved in water undergo
+an important change. A portion of their molecules fall apart, or
+_dissociate_, into two or more parts, called _ions_. Thus sodium nitrate
+(NaNO_{3}) dissociates into the ions Na and NO_{3}; sodium chloride,
+into the ions Na and Cl. These ions are free to move about in the
+solution independently of each other like independent molecules, and for
+this reason were given the name ion, which signifies a wanderer.
+
+2. _The electrical charge of ions._ Each ion carries a heavy electrical
+charge, and in this respect differs from an atom or molecule. It is
+evident that the sodium in the form of an ion must differ in some
+important way from ordinary sodium, for sodium ions, formed from sodium
+nitrate, give no visible evidence of their presence in water, whereas
+metallic sodium at once decomposes the water. The electrical charge,
+therefore, greatly modifies the usual chemical properties of the
+element.
+
+3. _The positive charges equal the negative charges._ The ions formed by
+the dissociation of any molecule are of two kinds. One kind is charged
+with positive electricity and the other with negative electricity;
+moreover the sum of all the positive charges is always equal to the sum
+of all the negative charges. The solution as a whole is therefore
+electrically neutral. If we represent dissociation by the usual chemical
+equations, with the electrical charges indicated by + and - signs
+following the symbols, the dissociation of sodium chloride molecules is
+represented thus:
+
+    NaCl --> Na^{+}, Cl^{-}.
+
+The positive charge on each sodium ion exactly equals the negative
+charge on each chlorine ion. Sodium sulphate dissociates, as shown in
+the equation
+
+    Na_{2}SO_{4} --> 2Na^{+}, SO_{4}^{--}.
+
+Here the positive charge on the two sodium ions equals the double
+negative charge on the SO_{4} ion.
+
+4. _Not all compounds dissociate._ Only those compounds dissociate whose
+solutions form electrolytes. Thus salt dissociates when dissolved in
+water, the resulting solution being an electrolyte. Sugar, on the other
+hand, does not dissociate and its solution is not a conductor of the
+electric current.
+
+5. _Extent of dissociation differs in different liquids._ While
+compounds most readily undergo dissociation in water, yet dissociation
+often occurs to a limited extent when solution takes place in liquids
+other than water. In the discussion of solutions it will be understood
+that the solvent is water unless otherwise noted.
+
+~The theory of electrolytic dissociation and the properties of solutions.~
+In order to be of value, this theory must give a reasonable explanation
+of the properties of solutions. Let us now see if the theory is in
+harmony with certain of these properties.
+
+~The theory of electrolytic dissociation and the boiling and freezing
+points of solutions.~ We have seen that the boiling point of a solution
+of a substance is raised in proportion to the concentration of the
+dissolved substance. This is but another way of saying that the change
+in the boiling point of the solution is proportional to the number of
+molecules of the dissolved substance present in the solution.
+
+It has been found, however, that in the case of electrolytes the boiling
+point is raised more than it should be to conform to this law. If the
+solute dissociates into ions, the reason for this becomes clear. Each
+ion has the same effect on the boiling point as a molecule, and since
+their number is greater than the number of molecules from which they
+were formed, the effect on the boiling point is abnormally great.
+
+In a similar way, the theory furnishes an explanation of the abnormal
+lowering of the freezing point of electrolytes.
+
+~The theory of electrolytic dissociation and electrolysis.~ The changes
+taking place during electrolysis harmonize very completely with the
+theory of dissociation. This will become clear from a study of the
+following examples.
+
+[Illustration: Fig. 32]
+
+1. _Electrolysis of sodium chloride._ Fig. 32 represents a vessel in
+which the electrolyte is a solution of sodium chloride (NaCl). According
+to the dissociation theory the molecules of sodium chloride dissociate
+into the ions Na^{+} and Cl^{-}. The Na^{+} ions are attracted to the
+cathode owing to its large negative charge. On coming into contact with
+the cathode, the Na^{+} ions give up their positive charge and are then
+ordinary sodium atoms. They immediately decompose the water according to
+the equation
+
+    Na + H_{2}O = NaOH + H,
+
+and hydrogen is evolved about the cathode.
+
+The chlorine ions on being discharged at the anode in similar manner may
+either be given off as chlorine gas, or may attack the water, as
+represented in the equation
+
+    2Cl + H_{2}O = 2HCl + O.
+
+2. _Electrolysis of water._ The reason for the addition of sulphuric
+acid to water in the preparation of oxygen and hydrogen by electrolysis
+will now be clear. Water itself is not an electrolyte to an appreciable
+extent; that is, it does not form enough ions to carry a current.
+Sulphuric acid dissolved in water is an electrolyte, and dissociates
+into the ions 2 H^{+} and SO_{4}^{--}. In the process of electrolysis of
+the solution, the hydrogen ions travel to the cathode, and on being
+discharged escape as hydrogen gas. The SO_{4} ions, when discharged at
+the anode, act upon water, setting free oxygen and once more forming
+sulphuric acid:
+
+    SO_{4} + H_{2}O = H_{2}SO_{4} + O.
+
+The sulphuric acid can again dissociate and the process repeat itself as
+long as any water is left. Hence the hydrogen and oxygen set free in the
+electrolysis of water really come directly from the acid but indirectly
+from the water.
+
+3. _Electrolysis of sodium sulphate._ In a similar way, sodium sulphate
+(Na_{2}SO_{4}), when in solution, gives the ions 2 Na^{+} and
+SO_{4}^{--}. On being discharged, the sodium atoms decompose water about
+the cathode, as in the case of sodium chloride, while the SO_{4} ions
+when discharged at the anode decompose the water, as represented in the
+equation
+
+    SO_{4} + H_{2}O = H_{2}SO_{4} + O
+
+[Illustration: Fig. 33]
+
+That new substances are formed at the cathode and anode may be shown in
+the following way. A U-tube, such as is represented in Fig. 33, is
+partially filled with a solution of sodium sulphate, and the liquid in
+one arm is colored with red litmus, that in the other with blue litmus.
+An electrode placed in the red solution is made to serve as cathode,
+while one in the blue solution is made the anode. On allowing the
+current to pass, the blue solution turns red, while the red solution
+turns blue. These are exactly the changes which would take place if
+sodium hydroxide and sulphuric acid were to be set free at the
+electrodes, as required by the theory.
+
+~The properties of electrolytes depend upon the ions present.~ When a
+substance capable of dissociating into ions is dissolved in water, the
+properties of the solution will depend upon two factors: (1) the ions
+formed from the substance; (2) the undissociated molecules. Since the
+ions are usually more active chemically than the molecules, most of the
+chemical properties of an electrolyte are due to the ions rather than to
+the molecules.
+
+The solutions of any two substances which give the same ion will have
+certain properties in common. Thus all solutions containing the copper
+ion (Cu^{++}) are blue, unless the color is modified by the presence of
+ions or molecules having some other color.
+
+
+EXERCISES
+
+1. Distinguish clearly between the following terms: electrolysis,
+electrolyte, electrolytic dissociation, ions, solute, solvent, solution,
+saturated solution, and supersaturated solution.
+
+2. Why does the water from some natural springs effervesce?
+
+3. (a) Why does not the water of the ocean freeze? (b) Why will ice
+and salt produce a lower temperature than ice alone?
+
+4. Why does shaking or stirring make a solid dissolve more rapidly in a
+liquid?
+
+5. By experiment it was found that a certain volume of water was
+saturated at 100° with 114 g. of potassium nitrate. On cooling to 0° a
+portion of the substance crystallized. (a) How many grams of the
+substance remained in solution? (b) What was the strength of the
+solution at 18°? (c) How much water had been used in the experiment?
+
+6. (a) 10 g. of common salt were dissolved in water and the solution
+evaporated to dryness; what weight of solid was left? (b) 10 g. of
+zinc were dissolved in hydrochloric acid and the solution evaporated to
+dryness; what weight of solid was left?
+
+7. Account for the fact that sugar sometimes deposits from molasses,
+even when no evaporation has taken place.
+
+8. (a) From the standpoint of the theory of electrolytic dissociation,
+write the simple equation for a dilute solution of copper sulphate
+(CuSO_{4}); this solution is blue. (b) In the same manner, write one
+for sodium sulphate; this solution is colorless. (c) How would you
+account for the color of the copper sulphate solution?
+
+9. (a) As in the preceding exercise, write a simple equation for a
+dilute solution of copper chloride (CuCl_{2}); this solution is blue.
+(b) In the same manner, write one for sodium chloride; this solution
+is colorless. To what is the blue color due?
+
+10. What component is present in concentrated sulphuric acid that is
+almost wanting in very dilute sulphuric acid?
+
+11. Why will vegetables cook faster when boiled in strong salt water
+than when boiled in pure water?
+
+12. How do you explain the foaming of soda water?
+
+
+
+
+CHAPTER X
+
+ACIDS, BASES, AND SALTS; NEUTRALIZATION
+
+
+~Acids, bases, and salts.~ The three classes of compounds known
+respectively as acids, bases, and salts include the great majority of
+the compounds with which we shall have to deal. It is important,
+therefore, for us to consider each of these classes in a systematic way.
+The individual members belonging to each class will be discussed in
+detail in the appropriate places, but a few representatives of each
+class will be described in this chapter with special reference to the
+common properties in accordance with which they are classified.
+
+~The familiar acids.~ _Hydrochloric acid_ is a gas composed of hydrogen and
+chlorine, and has the formula HCl. The substance is very soluble in
+water, and it is this solution which is usually called hydrochloric
+acid. _Nitric acid_ is a liquid composed of hydrogen, nitrogen, and
+oxygen, having the formula HNO_{3}. As sold commercially it is mixed
+with about 32% of water. _Sulphuric acid_, whose composition is
+represented by the formula H_{2}SO_{4}, is an oily liquid nearly twice
+as heavy as water, and is commonly called _oil of vitriol_.
+
+~Characteristics of acids.~ (1) All acids contain hydrogen. (2) When
+dissolved in water the molecules of the acid dissociate into two kinds
+of ions. One of these is always hydrogen and is the cation (+), while
+the other consists of the remainder of the molecule and is the anion
+(-). (3) The solution tastes sour. (4) It has the power to change the
+color of certain substances called _indicators_. Thus blue litmus is
+changed to red, and yellow methyl orange is changed to red. Since all
+acids produce hydrogen cations, while the anions of each are different,
+the properties which all acids have in common when in solution, such as
+taste and action on indicators, must be attributed to the hydrogen ions.
+
+DEFINITION: _An acid is a substance which produces hydrogen ions when
+dissolved in water or other dissociating liquids._
+
+~Undissociated acids.~ When acids are perfectly free from water, or are
+dissolved in liquids like benzene which do not have the power of
+dissociating them into ions, they should have no real acid properties.
+This is found to be the case. Under these circumstances they do not
+affect the color of indicators or have any of the properties
+characteristic of acids.
+
+The familiar bases. The bases most used in the laboratory are sodium
+hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide
+(Ca(OH)_{2}). These are white solids, soluble in water, the latter
+sparingly so. Some bases are very difficultly soluble in water. The very
+soluble ones with most pronounced basic properties are sometimes called
+the _alkalis_.
+
+~Characteristics of bases.~ (1) All bases contain hydrogen and oxygen. (2)
+When dissolved in water the molecules of the base dissociate into two
+kinds of ions. One of these is always composed of oxygen and hydrogen
+and is the anion. It has the formula OH and is called the _hydroxyl
+ion_. The remainder of the molecule, which usually consists of a single
+atom, is the cation. (3) The solution of a base has a soapy feel and a
+brackish taste. (4) It reverses the color change produced in indicators
+by acids, turning red litmus blue, and red methyl orange yellow. Since
+all bases produce hydroxyl anions, while the cations of each are
+different, the properties which all bases have in common when in
+solution must be due to the hydroxyl ions.
+
+DEFINITION: _A base is a substance which produces hydroxyl ions when
+dissolved in water or other dissociating liquids._
+
+~Undissociated bases.~ Bases, in the absence of water or when dissolved in
+liquids which do not dissociate them, should have none of the properties
+characteristic of this class of substances. This has been found to be
+the case. For example, they have no effect upon indicators under these
+circumstances.
+
+~Neutralization.~ When an acid and a base are brought together in solution
+in proper proportion, the characteristic properties of each disappear.
+The solution tastes neither sour nor brackish; it has no effect upon
+indicators. There can therefore be neither hydrogen nor hydroxyl ions
+present in the solution. A study of reactions of this kind has shown
+that the hydrogen ions of the acid combine with the hydroxyl ions of the
+base to form molecules of water, water being a substance which is not
+appreciably dissociated into ions. This action of an acid on a base is
+called _neutralization_. The following equations express the
+neutralization of the three acids by three bases, water being formed in
+each case.
+
+    Na^{+}, OH^{-} + H^{+}, Cl^{-} = Na^{+}, Cl^{-} + H_{2}O.
+
+    K^{+}, OH^{-} + H^{+}, NO_{3}^{-} = K^{+}, NO_{3}^{-} + H_{2}O.
+
+    Ca^{++}, (OH)_{2}^{--} + H_{2}^{++}, SO_{4}^{--}
+         = Ca^{++}, SO_{4}^{--} + 2H_{2}O.
+
+DEFINITION: _Neutralization consists in the union of the hydrogen ion of
+an acid with the hydroxyl ion of a base to form water._
+
+~Salts.~ It will be noticed that in neutralization the anion of the acid
+and the cation of the base are not changed. If, however, the water is
+expelled by evaporation, these two ions slowly unite, and when the water
+becomes saturated with the substance so produced, it separates in the
+form of a solid called a _salt_.
+
+DEFINITION: _A salt is a substance formed by the union of the anion of
+an acid with the cation of a base._
+
+~Characteristics of salts.~ (1) From the definition of a salt it will be
+seen that there is no element or group of elements which characterize
+salts. (2) Salts as a class have no peculiar taste. (3) In the absence
+of all other substances they are without action on indicators. (4) When
+dissolved in water they form two kinds of ions.
+
+     ~Heat of neutralization.~ If neutralization is due to the union
+     of hydrogen ions with hydroxyl ions, and nothing more, it
+     follows that when a given weight of water is formed in
+     neutralization, the heat set free should always be the same, no
+     matter from what acid and base the two kinds of ions have been
+     supplied. Careful experiments have shown that this is the case,
+     provided no other reactions take place at the same time. When
+     18g. of water are formed in neutralization, 13,700 cal. of heat
+     are set free. This is represented in the equations
+
+    Na^{+}, OH^{-} + H^{+}, Cl^{-}
+         = Na^{+}, Cl^{-} + H_{2}O + 13,700 cal.
+
+    K^{+}, OH^{-} + H^{+}, NO_{3}^{-}
+         = K^{+}, NO_{3}^{-} + H_{2}O + 13,700 cal.
+
+    Ca^{++}, (OH)_{2}^{--} + H_{2}^{++}, SO_{4}^{--}
+         = Ca^{++}, SO_{4}^{--} + 2H_{2}O + 2 × 13,700 cal.
+
+~Neutralization a quantitative act.~ Since neutralization is a definite
+chemical act, each acid will require a perfectly definite weight of each
+base for its neutralization. For example, a given weight of sulphuric
+acid will always require a definite weight of sodium hydroxide, in
+accordance with the equation
+
+    H_{2}, SO_{4} + 2Na, OH = Na_{2}, SO_{4} + 2H_{2}O.
+
+     ~Determination of the ratio in neutralization.~ The quantities of
+     acid and base required in neutralization may be determined in
+     the following way. Dilute solutions of the two substances are
+     prepared, the sulphuric acid being placed in one of the
+     burettes (Fig. 34) and the sodium hydroxide in the other. The
+     levels of the two liquids are then brought to the zero marks of
+     the burettes by means of the stopcocks. A measured volume of
+     the acid is drawn off into a beaker, a few drops of litmus
+     solution added, and the sodium hydroxide is run in drop by drop
+     until the red litmus just turns blue. The volume of the sodium
+     hydroxide consumed is then noted. If the concentrations of the
+     two solutions are known, it is easy to calculate what weight of
+     sodium hydroxide is required to neutralize a given weight of
+     sulphuric acid. By evaporating the neutralized solution to
+     dryness, the weight of the sodium sulphate formed can be
+     determined directly. Experiment shows that the weights are
+     always in accordance with the equation in the preceding
+     paragraph.
+
+[Illustration: Fig. 34]
+
+~Extent of dissociation.~ The question will naturally arise, When an acid,
+base, or salt dissolves in water, do all the molecules dissociate into
+ions, or only a part of them? The experiments by which this question can
+be answered cannot be described here. It has been found, however, that
+only a fraction of the molecules dissociate. The percentage which will
+dissociate in a given case depends upon several conditions, the chief of
+which are: (1) The concentration of the solution. In concentrated
+solutions only a very small percentage of dissociation occurs. As the
+solution is diluted the percentage increases, and in very dilute
+solutions it may be very large, though it is never complete in any
+ordinary solution. (2) The nature of the dissolved compound. At equal
+concentrations substances differ much among themselves in the percentage
+of dissociation. The great majority of salts are about equally
+dissociated. Acids and bases, on the contrary, show great differences.
+Some are freely dissociated, while others are dissociated to but a
+slight extent.
+
+~Strength of acids and bases.~ Since acid and basic properties are due to
+hydrogen and hydroxyl ions respectively, the acid or base which will
+produce the greatest percentage of these ions at a given concentration
+must be regarded as the strongest representative of its class. The acids
+and bases described in the foregoing paragraphs are all quite strong. In
+10% solutions they are dissociated to about 50%, and this is also
+approximately the extent to which most salts are dissociated at this
+same concentration.
+
+     ~Partial neutralization.~ 1. _Basic salts._ The chemical action
+     between an acid and a base is not always as complete as has
+     been represented in the foregoing paragraphs. For example, if
+     the base magnesium hydroxide (Mg(OH)_{2}) and hydrochloric acid
+     (HCl) are brought together in the ratio of an equal number of
+     molecules of each, there will be only half enough hydrogen ions
+     for the hydroxyl ions present.
+
+         Mg, (OH)_{2} + H, Cl = Mg, OH, Cl + H_{2}O.
+
+     Magnesium, hydroxyl, and chlorine ions are left at the close of
+     the reaction, and under the proper conditions unite to form
+     molecules of the compound Mg(OH)Cl. This compound, when
+     dissolved, can form hydroxyl ions and therefore possesses basic
+     properties; it can also form the ions of a salt (Mg and Cl),
+     and has properties characteristic of salts. Substances of this
+     kind are called _basic salts._
+
+     DEFINITION: _A basic salt is a substance which can give the
+     ions both of a base and of a salt when dissolved in water._
+
+     2. _Acid salts._ In a similar way, when sulphuric acid and
+     sodium hydroxide are brought together in the ratio of equal
+     numbers of the molecules of each, it is possible to have a
+     reaction expressed by the equation
+
+        Na, OH + H_{2}, SO_{4} = Na, H, SO_{4} + H_{2}O.
+
+     The ions remaining after all the hydroxyl ions have been used
+     up are those of an acid (H) and those of a salt (Na and
+     SO_{4}). These unite to form the substance NaHSO_{4}, and as
+     the solution becomes saturated with this substance through
+     evaporation, it separates in the form of crystals. In solution
+     this substance can give hydrogen ions, and therefore possesses
+     acid properties; it can also give the ions characteristic of a
+     salt. It is therefore called an _acid salt_.
+
+     DEFINITION: _An acid salt is one which can give the ions of an
+     acid and of a salt when in solution._
+
+     3. _Normal salts._ Salts which are the products of complete
+     neutralization, such as Na_{2}SO_{4}, and which in solution can
+     give neither hydrogen nor hydroxyl ions, but only the ions of a
+     salt, are called _normal salts_ to distinguish them from acid
+     and basic salts.
+
+~Methods of expressing reactions between compounds in solution.~ Chemical
+equations representing reactions between substances in solution may
+represent the details of the reaction, or they may simply indicate the
+final products formed. In the latter case the formation of ions is not
+indicated. Thus, if we wish to call attention to the details of the
+reaction between sodium hydroxide and hydrochloric acid in solution, the
+equation is written as follows:
+
+    Na^{+}, OH^{-} + H^{+}, Cl^{-} = Na^{+}, Cl^{-} + H_{2}O.
+
+On the other hand, if we wish simply to represent the final products
+formed, the following is used.
+
+    NaOH + HCl = NaCl + H_{2}O.
+
+Both of these methods will therefore be used:
+
+~Radicals.~ It has been emphasized that the hydroxyl group (OH) always
+forms the anion of a base, while the group NO_{3} forms the anion of
+nitric acid and sodium nitrate; the group SO_{4}, the anion of sulphuric
+acid and calcium sulphate. A group of elements which in this way
+constitutes a part of a molecule, acting as a unit in a chemical change,
+or forming ions in solution, is called a _radical_. Some of these
+radicals have been given special names, the names signifying the
+elements present in the radical. Thus we have the hydroxyl radical (OH)
+and the nitrate radical (NO_{3}).
+
+DEFINITION: _A radical is a group of elements forming part of a
+molecule, and acting as a unit in chemical reactions._
+
+~Names of acids, bases, and salts.~ Since acids, bases, and salts are so
+intimately related to each other, it is very advantageous to give names
+to the three classes in accordance with some fixed system. The system
+universally adopted is as follows:
+
+~Naming of bases.~ All bases are called _hydroxides_. They are
+distinguished from each other by prefixing the name of the element which
+is in combination with the hydroxyl group. Examples: sodium hydroxide
+(NaOH); calcium hydroxide (Ca(OH)_{2}); copper hydroxide (Cu(OH)_{2}).
+
+~Naming of acids.~ The method of naming acids depends upon whether the
+acid consists of two elements or three.
+
+1. _Binary acids._ Acids containing only one element in addition to
+hydrogen are called _binary acids_. They are given names consisting of
+the prefix _hydro-_, the name of the second element present, and the
+termination _-ic_. Examples: hydrochloric acid (HCl); hydrosulphuric
+acid (H_{2}S).
+
+2. _Ternary acids._ In addition to the two elements present in binary
+acids, the great majority of acids also contain oxygen. They therefore
+consist of three elements and are called _ternary acids_. It usually
+happens that the same three elements can unite in different proportions
+to make several different acids. The most familiar one of these is given
+a name ending in the suffix _-ic_, while the one with less oxygen is
+given a similar name, but ending in the suffix _-ous_. Examples: nitric
+acid (HNO_{3}); nitrous acid (HNO_{2}). In cases where more than two
+acids are known, use is made of prefixes in addition to the two suffixes
+_-ic_ and _-ous_. Thus the prefix _per-_ signifies an acid still richer
+in oxygen; the prefix _hypo-_ signifies one with less oxygen.
+
+~Naming of salts.~ A salt derived from a binary acid is given a name
+consisting of the names of the two elements composing it, with the
+termination _-ide_. Example: sodium chloride (NaCl). All other binary
+compounds are named in the same way.
+
+A salt of a ternary acid is named in accordance with the acid from which
+it is derived. A ternary acid with the termination _-ic_ gives a salt
+with the name ending in _-ate_, while an acid with termination _-ous_
+gives a salt with the name ending in _-ite_. The following table will
+make the application of these principles clear:
+
+     ACIDS          SYMBOL       SALTS                SYMBOL
+
+    Hydrochloric   HCl          Sodium chloride      NaCl
+    Hypochlorous   HClO         Sodium hypochlorite  NaClO
+    Chlorous       HClO_{2}     Sodium chlorite      NaClO_{2}
+    Chloric        HClO_{3}     Sodium chlorate      NaClO_{3}
+    Perchloric     HClO_{4}     Sodium perchlorate   NaClO_{4}
+
+
+EXERCISES
+
+1. 25 cc. of a solution containing 40 g. of sodium hydroxide per liter
+was found to neutralize 25 cc. of a solution of hydrochloric acid. What
+was the strength of the acid solution?
+
+2. After neutralizing a solution of sodium hydroxide with nitric acid,
+there remained after evaporation 100 g. of sodium nitrate. How much of
+each substance had been used?
+
+3. A solution contains 18 g. of hydrochloric acid per 100 cc. It
+required 25 cc. of this solution to neutralize 30 cc. of a solution of
+sodium hydroxide. What was the strength of the sodium hydroxide solution
+in parts per hundred?
+
+4. When perfectly dry sulphuric acid is treated with perfectly dry
+sodium hydroxide, no chemical change takes place. Explain.
+
+5. When cold, concentrated sulphuric acid is added to zinc, no change
+takes place. Recall the action of dilute sulphuric acid on the same
+metal. How do you account for the difference?
+
+6. A solution of hydrochloric acid in benzene does not conduct the
+electric current. When this solution is treated with zinc, will hydrogen
+be evolved? Explain.
+
+7. (a) Write equation for preparation of hydrogen from zinc and dilute
+sulphuric acid. (b) Rewrite the same equation from the standpoint of
+the theory of electrolytic dissociation, (c) Subtract the common
+SO_{4} ion from both members of the equation, (d) From the resulting
+equation, explain in what the preparation of hydrogen consists when
+examined from the standpoint of this theory.
+
+8. In the same manner as in the preceding exercise, explain in what the
+action of sodium on water to give hydrogen consists.
+
+
+
+
+CHAPTER XI
+
+VALENCE
+
+
+~Definition of valence.~ A study of the formulas of various binary
+compounds shows that the elements differ between themselves in the
+number of atoms of other elements which they are able to hold in
+combination. This is illustrated in the formulas
+
+            HCl,        H_{2}O,   H_{3}N,   H_{4}C.
+    (hydrochloric acid) (water) (ammonia) (marsh gas)
+
+It will be noticed that while one atom of chlorine combines with one
+atom of hydrogen, an atom of oxygen combines with two, an atom of
+nitrogen with three, one of carbon with four. The number which expresses
+this combining ratio between atoms is a definite property of each
+element and is called its _valence_.
+
+DEFINITION: _The valence of an element is that property which determines
+the number of the atoms of another element which its atom can hold in
+combination._
+
+~Valence a numerical property.~ Valence is therefore merely a numerical
+relation and does not convey any information in regard to the intensity
+of the affinity between atoms. Judging by the heat liberated in their
+union, oxygen has a far stronger affinity for hydrogen than does
+nitrogen, but an atom of oxygen can combine with two atoms only of
+hydrogen, while an atom of nitrogen can combine with three.
+
+~Measure of valence.~ In expressing the valence of an element we must
+select some standard for comparison, just as in the measurement of any
+other numerical quantity. It has been found that an atom of hydrogen is
+never able to hold in combination more than one atom of any other
+element. Hydrogen is therefore taken as the standard, and other elements
+are compared with it in determining their valence. A number of other
+elements are like hydrogen in being able to combine with at most one
+atom of other elements, and such elements are called _univalent_. Among
+these are chlorine, iodine, and sodium. Elements such as oxygen,
+calcium, and zinc, which can combine with two atoms of hydrogen or other
+univalent elements, are said to be _divalent_. Similarly, we have
+_trivalent, tetravalent, pentavalent_ elements. None have a valence of
+more than 8.
+
+~Indirect measure of valence.~ Many elements, especially among the metals,
+do not readily form compounds with hydrogen, and their valence is not
+easy to determine by direct comparison with the standard element. These
+elements, however, combine with other univalent elements, such as
+chlorine, and their valence can be determined from the compounds so
+formed.
+
+~Variable valence.~ Many elements are able to exert different valences
+under differing circumstances. Thus we have the compounds Cu_{2}O and
+CuO, CO and CO_{2}, FeCl_{2} and FeCl_{3}. It is not always possible to
+assign a fixed valence to an element. Nevertheless each element tends to
+exert some normal valence, and the compounds in which it has a valence
+different from this are apt to be unstable and easily changed into
+compounds in which the valence of the element is normal. The valences of
+the various elements will become familiar as the elements are studied in
+detail.
+
+~Valence and combining ratios.~ When elements combine to form compounds,
+the ratio in which they combine will be determined by their valences. In
+those compounds which consist of two elements directly combined, the
+union is between such numbers of the two atoms as have equal valences.
+Elements of the same valence will therefore combine atom for atom.
+Designating the valence of the atoms by Roman numerals placed above
+their symbols, we have the formulas
+
+    II II   II III   III   IV IV
+    HCl,     ZnO,     BN,   CSi.
+
+A divalent element, on the other hand, will combine with two atoms of a
+univalent element. Thus we have
+
+    II II         II II
+    ZnCl_{2} and H_{2}O
+
+(the numerals above each symbol representing the sum of the valences of
+the atoms of the element present). A trivalent atom will combine with
+three atoms of a univalent element, as in the compound
+
+    III III
+     H_{3}N.
+
+If a trivalent element combines with a divalent element, the union will
+be between two atoms of the trivalent element and three of the divalent
+element, since these numbers are the smallest which have equal valences.
+Thus the oxide of the trivalent metal aluminium has the formula
+Al_{2}O_{3}. Finally one atom of a tetravalent element such as carbon
+will combine with four atoms of a univalent element, as in the compound
+CH_{4}, or with two atoms of a divalent element, as in the compound
+CO_{2}.
+
+We have no knowledge as to why elements differ in their combining power,
+and there is no way to determine their valences save by experiment.
+
+     ~Valence and the structure of compounds.~ Compounds will be met
+     from time to time which are apparent exceptions to the general
+     statements just made in regard to valence. Thus, from the
+     formula for hydrogen dioxide (H_{2}O_{2}), it might be
+     supposed that the oxygen is univalent; yet it is certainly
+     divalent in water (H_{2}O). That it may also be divalent in
+     H_{2}O_{2} may be made clear as follows: The unit valence of
+     each element may be represented graphically by a line attached
+     to its symbol. Univalent hydrogen and divalent oxygen will then
+     have the symbols H- and -O-. When atoms combine, each unit
+     valence of one atom combines with a unit valence of another
+     atom. Thus the composition of water may be expressed by the
+     formula H-O-H, which is meant to show that each of the unit
+     valences of oxygen is satisfied with the unit valence of a
+     single hydrogen atom.
+
+     The chemical conduct of hydrogen dioxide leads to the
+     conclusion that the two oxygen atoms of its molecule are in
+     direct combination with each other, and in addition each is in
+     combination with a hydrogen atom. This may be expressed by the
+     formula H-O-O-H. The oxygen in the compound is therefore
+     divalent, just as it is in water. It will thus be seen that the
+     structure of a compound must be known before the valences of
+     the atoms making up the compound can be definitely decided
+     upon.
+
+     Such formulas as H-O-H and H-O-O-H are known as _structural
+     formulas_, because they are intended to show what is known in
+     regard to the arrangement of the atoms in the molecules.
+
+~Valence and the replacing power of atoms.~ Just as elements having the
+same valence combine with each other atom for atom, so if they replace
+each other in a chemical reaction they will do so in the same ratio.
+This is seen in the following equations, in which a univalent hydrogen
+atom is replaced by a univalent sodium atom:
+
+    NaOH + HCl = NaCl + H_{2}O.
+
+    2NaOH + H_{2}SO_{4} = Na_{2}SO_{4} + 2H_{2}O.
+
+    Na + H_{2}O = NaOH + H.
+
+Similarly, one atom of divalent calcium will replace two atoms of
+univalent hydrogen or one of divalent zinc:
+
+    Ca(OH)_{2} + 2 HCl = CaCl_{2} + 2H_{2}O.
+
+    CaCl_{2} + ZnSO_{4} = CaSO_{4} + ZnCl_{2}.
+
+In like manner, one atom of a trivalent element will replace three of a
+univalent element, or two atoms will replace three atoms of a divalent
+element.
+
+~Valence and its applications to formulas of salts.~ While the true nature
+of valence is not understood and many questions connected with the
+subject remain unanswered, yet many of the main facts are of much help
+to the student. Thus the formula of a salt, differs from that of the
+acid from which it is derived in that the hydrogen of the acid has been
+replaced by a metal. If, then, it is known that a given metal forms a
+normal salt with a certain acid, the formula of the salt can at once be
+determined if the valence of the metal is known. Since sodium is
+univalent, the sodium salts of the acids HCl and H_{2}SO_{4} will be
+respectively NaCl and Na_{2}SO_{4}. One atom of divalent zinc will
+replace 2 hydrogen atoms, so that the corresponding zinc salts will be
+ZnCl_{2} and ZnSO_{4}.
+
+The formula for aluminium sulphate is somewhat more difficult to
+determine. Aluminium is trivalent, and the simplest ratio in which the
+aluminium atom can replace the hydrogen in sulphuric acid is 2 atoms of
+aluminium (6 valences) to 3 molecules of sulphuric acid (6 hydrogen
+atoms). The formula of the sulphate will then be Al_{2}(SO_{4})_{3}.
+
+~Valence and its application to equation writing.~ It will be readily seen
+that a knowledge of valence is also of very great assistance in writing
+the equations for reactions of double decomposition. Thus, in the
+general reaction between an acid and a base, the essential action is
+between the univalent hydrogen ion and the univalent hydroxyl ion. The
+base and the acid must always be taken in such proportions as to secure
+an equal number of each of these ions. Thus, in the reaction between
+ferric hydroxide (Fe(OH)_{3}) and sulphuric acid (H_{2}SO_{4}), it will
+be necessary to take 2 molecules of the former and 3 of the latter in
+order to have an equal number of the two ions, namely, 6. The equation
+will then be
+
+    2Fe(OH)_{3} + 3H_{2}SO_{4} = Fe_{2}(SO_{4})_{3} + 6H_{2}O.
+
+Under certain conditions the salts Al_{2}(SO_{4})_{3} and CaCl_{2}
+undergo double decomposition, the two metals, aluminium and calcium,
+exchanging places. The simplest ratio of exchange in this case is 2
+atoms of aluminium (6 valences) and 3 atoms of calcium (6 valences).
+The reaction will therefore take place between 1 molecule of
+Al_{2}(SO_{4})_{3} and 3 of CaCl_{2}, and the equation is as follows:
+
+    Al_{2}(SO_{4})_{3} + 3 CaCl_{2} = 3CaSO_{4} + 2AlCl_{3}.
+
+
+EXERCISES
+
+1. Sodium, calcium, and aluminium have valences of 1, 2, and 3
+respectively; write the formulas of their chlorides, sulphates, and
+phosphates (phosphoric acid = H_{3}PO_{4}), on the supposition that they
+form salts having the normal composition.
+
+2. Iron forms one series of salts in which it has a valence of 2, and
+another series in which it has a valence of 3; write the formulas for
+the two chlorides of iron, also for the two sulphates, on the
+supposition that these have the normal composition.
+
+3. Write the equation representing the neutralization of each of the
+following bases by each of the acids whose formulas are given:
+
+    NaOH           HCl
+    Ba(OH)_{2}     H_{2}SO_{4}
+    Al(OH)_{3}     H_{3}PO_{4}
+
+4. Silver acts as a univalent element and calcium as a divalent element
+in the formation of their respective nitrates and chlorides. (a) Write
+the formula for silver nitrate; for calcium chloride. (b) When
+solutions of these two salts are mixed, the two metals, silver and
+calcium, exchange places; write the equation for the reaction.
+
+_5._ Antimony acts as a trivalent element in the formation of a
+chloride. (a) What is the formula for antimony chloride? (b) When
+hydrosulphuric acid (H_{2}S) is passed into a solution of this chloride
+the hydrogen and antimony exchange places; write the equation for the
+reaction.
+
+6. Lead has a valence of 2 and iron of 3 in the compounds known
+respectively as lead nitrate and ferric sulphate. (a) Write the
+formulas for these two compounds. (b) When their solutions are mixed
+the two metals exchange places; write the equation for the reaction.
+
+
+
+
+CHAPTER XII
+
+COMPOUNDS OF NITROGEN
+
+
+~Occurrence.~ As has been stated in a former chapter, nitrogen constitutes
+a large fraction of the atmosphere. The compounds of nitrogen, however,
+cannot readily be obtained from this source, since at any ordinary
+temperature nitrogen is able to combine directly with very few of the
+elements.
+
+In certain forms of combination nitrogen occurs in the soil from which
+it is taken up by plants and built into complex substances composed
+chiefly of carbon, hydrogen, oxygen, and nitrogen. Animals feeding on
+these plants assimilate the nitrogenous matter, so that this element is
+an essential constituent of both plants and animals.
+
+~Decomposition of organic matter by bacteria.~ When living matter dies and
+undergoes decay complicated chemical reactions take place, one result of
+which is that the nitrogen of the organic matter is set free either as
+the element nitrogen, or in the form of simple compounds, such as
+ammonia (NH_{3}) or oxides of nitrogen. Experiment has shown that all
+such processes of decay are due to the action of different kinds of
+bacteria, each particular kind effecting a different change.
+
+~Decomposition of organic matter by heat.~ When organic matter is strongly
+heated decomposition into simpler substances takes place in much the
+same way as in the case of bacterial decomposition. Coal is a complex
+substance of vegetable origin, consisting largely of carbon, but also
+containing hydrogen, oxygen, and nitrogen. When this is heated in a
+closed vessel so that air is excluded, about one seventh of the nitrogen
+is converted into ammonia, and this is the chief source from which
+ammonia and its compounds are obtained.
+
+
+COMPOUNDS OF NITROGEN WITH HYDROGEN
+
+~Ammonia~ (NH_{3}). Several compounds consisting exclusively of nitrogen
+and hydrogen are known, but only one, ammonia, need be considered here.
+
+~Preparation of ammonia.~ Ammonia is prepared in the laboratory by a
+different method from the one which is used commercially.
+
+1. _Laboratory method._ In the laboratory ammonia is prepared from
+ammonium chloride, a compound having the formula NH_{4}Cl, and obtained
+in the manufacture of coal gas. As will be shown later in the chapter,
+the group NH_{4} in this compound acts as a univalent radical and is
+known as _ammonium_. When ammonium chloride is warmed with sodium
+hydroxide, the ammonium and sodium change places, the reaction being
+expressed in the following equation.
+
+    NH_{4}Cl + NaOH = NaCl + NH_{4}OH.
+
+The ammonium hydroxide (NH_{4}OH) so formed is unstable and breaks down
+into water and ammonia.
+
+    NH_{4}OH = NH_{3} + H_{2}O.
+
+Calcium hydroxide (Ca(OH)_{2}) is frequently used in place of the more
+expensive sodium hydroxide, the equations being
+
+    2NH_{4}Cl + Ca(OH)_{2} = CaCl_{2} + 2NH_{4}OH,
+
+    2NH_{4}OH = 2H_{2}O + 2NH_{3}.
+
+     In the preparation, the ammonium chloride and calcium hydroxide
+     are mixed together and placed in a flask arranged as shown in
+     Fig. 35. The mixture is gently warmed, when ammonia is evolved
+     as a gas and is collected by displacement of air.
+
+[Illustration: Fig. 35]
+
+2. _Commercial method._ Nearly all the ammonia of commerce comes from
+the gasworks. Ordinary illuminating gas is made by distilling coal, as
+will be explained later, and among the products of this distillation a
+solution of ammonia in water is obtained. This solution, known as _gas
+liquor_, contains not only ammonia but other soluble substances. Most of
+these combine chemically with lime, while ammonia does not; if then lime
+is added to the gas liquor and the liquor is heated, the ammonia is
+driven out from the mixture. It may be dissolved again in pure, cold
+water, forming _aqua ammonia_, or the ammonia water of commerce.
+
+     ~Preparation from hydrogen and nitrogen.~ When electric sparks
+     are passed for some time through a mixture of hydrogen and
+     nitrogen, a small percentage of the two elements in the mixture
+     is changed into ammonia. The action soon ceases, however, for
+     the reason that ammonia is decomposed by the electric
+     discharge. The reaction expressed in the equation
+
+         N + 3H = NH_{3}
+
+     can therefore go in either direction depending upon the
+     relative quantities of the substances present. This recalls the
+     similar change from oxygen into ozone, which soon ceases
+     because the ozone is in turn decomposed into oxygen.
+
+~Physical properties.~ Under ordinary conditions ammonia is a gas whose
+density is 0.59. It is therefore little more than half as heavy as air.
+It is easily condensed into a colorless liquid, and can now be purchased
+in liquid form in steel cylinders. The gas is colorless and has a
+strong, suffocating odor. It is extremely soluble in water, 1 l. of
+water at 0° and 760 mm. pressure dissolving 1148 l. of the gas. In
+dissolving this large volume of gas the water expands considerably, so
+that the density of the solution is less than that of water, the
+strongest solutions having a density of 0.88.
+
+~Chemical properties.~ Ammonia will not support combustion, nor will it
+burn under ordinary conditions. In an atmosphere of oxygen it burns with
+a feeble, yellowish flame. When quite dry it is not a very active
+substance, but when moist it combines with a great many substances,
+particularly with acids.
+
+~Uses.~ It has been stated that ammonia can be condensed to a liquid by
+the application of pressure. If the pressure is removed from the liquid
+so obtained, it rapidly passes again into the gaseous state and in so
+doing absorbs a large amount of heat. Advantage is taken of this fact in
+the preparation of artificial ice. Large quantities of ammonia are also
+used in the preparation of ammonium compounds.
+
+     ~The manufacture of artificial ice.~ Fig. 36 illustrates the
+     method of preparing artificial ice. The ammonia gas is
+     liquefied in the pipes X by means of the pump Y. The heat
+     generated is absorbed by water flowing over the pipes. The
+     pipes lead into a large brine tank, a cross section of which is
+     shown in the figure. Into the brine (concentrated solution of
+     common salt) contained in this tank are dipped the vessels A,
+     B, C, filled with pure water. The pressure is removed from
+     the liquid ammonia as it passes into the pipes immersed in the
+     brine, and the heat absorbed by the rapid evaporation of the
+     liquid lowers the temperature of the brine below zero. The
+     water in A, B, C is thereby frozen into cakes of ice. The
+     gaseous ammonia resulting from the evaporation of the liquid
+     ammonia is again condensed, so that the process is continuous.
+
+[Illustration Fig. 36]
+
+~Ammonium hydroxide~ (NH_{4}OH). The solution of ammonia in water is found
+to have strong basic properties and therefore contains hydroxyl ions. It
+turns red litmus blue; it has a soapy feel; it neutralizes acids,
+forming salts with them. It seems probable, therefore, that when ammonia
+dissolves in water it combines chemically with it according to the
+equation
+
+    NH_{3} + H_{2}O = NH_{4}OH,
+
+and that it is the substance NH_{4}OH, called ammonium hydroxide, which
+has the basic properties, dissociating into the ions NH_{4} and OH.
+Ammonium hydroxide has never been obtained in a pure state. At every
+attempt to isolate it the substance breaks up into water and ammonia,--
+
+    NH_{4}OH = NH_{3} + H_{2}O.
+
+~The ammonium radical.~ The radical NH_{4} plays the part of a metal in
+many chemical reactions and is called ammonium. The ending _-ium_ is
+given to the name to indicate the metallic properties of the substance,
+since the names of the metals in general have that ending. The salts
+formed by the action of the base ammonium hydroxide on acids are called
+ammonium salts. Thus, with hydrochloric acid, ammonium chloride is
+formed in accordance with the equation
+
+    NH_{4}OH + HCl = NH_{4}Cl + H_{2}O.
+
+Similarly, with nitric acid, ammonium nitrate (NH_{4}NO_{3}) is formed,
+and with sulphuric acid, ammonium sulphate ((NH_{4})_{2}S0_{4}).
+
+It will be noticed that in the neutralization of ammonium hydroxide by
+acids the group NH_{4} replaces one hydrogen atom of the acid, just as
+sodium does. The group therefore acts as a univalent metal.
+
+~Combination of nitrogen with hydrogen by volume.~ Under suitable
+conditions ammonia can be decomposed into nitrogen and hydrogen by
+passing electric sparks through the gas. Accurate measurement has shown
+that when ammonia is decomposed, two volumes of the gas yield one volume
+of nitrogen and three volumes of hydrogen. Consequently, if the two
+elements were to combine directly, one volume of nitrogen would combine
+with three volumes of hydrogen to form two volumes of ammonia. Here, as
+in the formation of steam from hydrogen and oxygen, small whole numbers
+serve to indicate the relation between the volumes of combining gases
+and that of the gaseous product.
+
+
+COMPOUNDS OF NITROGEN WITH OXYGEN AND HYDROGEN
+
+In addition to ammonium hydroxide, nitrogen forms several compounds with
+hydrogen and oxygen, of which nitric acid (HNO_{3}) and nitrous acid
+(HNO_{2}) are the most familiar.
+
+~Nitric acid~ (HNO_{3}). Nitric acid is not found to any extent in nature,
+but some of its salts, especially sodium nitrate (NaNO_{3}) and
+potassium nitrate (KNO_{3}) are found in large quantities. From these
+salts nitric acid can be obtained.
+
+[Illustration Fig. 37]
+
+~Preparation of nitric acid.~ When sodium nitrate is treated with
+concentrated cold sulphuric acid, no chemical action seems to take
+place. If, however, the mixture is heated in a retort, nitric acid is
+given off as a vapor and may be easily condensed to a liquid by passing
+the vapor into a tube surrounded by cold water, as shown in Fig. 37. An
+examination of the liquid left in the retort shows that it contains
+sodium acid sulphate (NaHSO_{4}), so that the reaction may be
+represented by the equation
+
+    NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}.
+
+     If a smaller quantity of sulphuric acid is taken and the
+     mixture is heated to a high temperature, normal sodium sulphate
+     is formed:
+
+         2NaNO_{3} + H_{2}SO_{4} = Na_{2}SO_{4} + 2HNO_{3}.
+
+     In this case, however, the higher temperature required
+     decomposes a part of the nitric acid.
+
+     ~The commercial preparation of nitric acid.~ Fig. 38 illustrates
+     a form of apparatus used in the preparation of nitric acid on a
+     large scale. Sodium nitrate and sulphuric acid are heated in
+     the iron retort A. The resulting acid vapors pass in the
+     direction indicated by the arrows, and are condensed in the
+     glass tubes B, which are covered with cloth kept cool by
+     streams of water. These tubes are inclined so that the liquid
+     resulting from the condensation of the vapors runs back into
+     C and is drawn off into large vessels (D).
+
+[Illustration Fig. 38]
+
+~Physical properties of nitric acid.~ Pure nitric acid is a colorless
+liquid, which boils at about 86° and has a density of 1.56. The
+concentrated acid of commerce contains about 68% of the acid, the
+remainder being water. Such a mixture has a density of 1.4. The
+concentrated acid fumes somewhat in moist air, and has a sharp choking
+odor.
+
+~Chemical properties.~ The most important chemical properties of nitric
+acid are the following.
+
+1. _Acid properties._ As the name indicates, this substance is an acid,
+and has all the properties of that class of substances. It changes blue
+litmus red and has a sour taste in dilute solutions. It forms hydrogen
+ions in solution and neutralizes bases forming salts. It also acts upon
+the oxides of most metals, forming a salt and water. It is one of the
+strongest acids.
+
+2. _Decomposition on heating._ When boiled, or exposed for some time to
+sunlight, it suffers a partial decomposition according to the equation
+
+    2HNO_{3} = H_{2}O + 2NO_{2} + O.
+
+The substance NO_{2}, called nitrogen peroxide, is a brownish gas, which
+is readily soluble in water and in nitric acid. It therefore dissolves
+in the undecomposed acid, and imparts a yellowish or reddish color to
+it. Concentrated nitric acid highly charged with this substance is
+called _fuming nitric acid_.
+
+3. _Oxidizing action._ According to its formula, nitric acid contains a
+large percentage of oxygen, and the reaction just mentioned shows that
+the compound is not a very stable one, easily undergoing decomposition.
+These properties should make it a good oxidizing agent, and we find that
+this is the case. Under ordinary circumstances, when acting as an
+oxidizing agent, it is decomposed according to the equation
+
+    2HNO_{3} = H_{2}O + 2NO + 3O.
+
+The oxygen is taken up by the substance oxidized, and not set free, as
+is indicated in the equation. Thus, if carbon is oxidized by nitric
+acid, the oxygen combines with carbon, forming carbon dioxide (CO_{2}):
+
+    C + 2O = CO_{2}.
+
+4. _Action on metals._ We have seen that when an acid acts upon a metal
+hydrogen is set free. Accordingly, when nitric acid acts upon a metal,
+such as copper, we should expect the reaction to take place which is
+expressed in the equation
+
+    Cu + 2HNO_{3} = Cu(NO_{3})_{2} + 2H.
+
+This reaction does take place, but the hydrogen set free is immediately
+oxidized to water by another portion of the nitric acid according to the
+equation
+
+    HNO_{3} + 3H = 2H_{2}O + NO.
+
+As these two equations are written, two atoms of hydrogen are given off
+in the first equation, while three are used up in the second. In order
+that the hydrogen may be equal in the two equations, we must multiply
+the first by 3 and the second by 2. We shall then have
+
+    3Cu + 6HNO_{3} = 3Cu(NO_{3})_{2} + 6H,
+
+    2HNO_{3} + 6H = 4H_{2}O + 2NO.
+
+The two equations may now be combined into one by adding the quantities
+on each side of the equality sign, canceling the hydrogen which is given
+off in the one reaction and used up in the other. We shall then have the
+equation
+
+    3Cu + 8HNO_{3} = 3Cu(NO_{3})_{2} + 2NO + 4H_{2}O.
+
+A number of other reactions may take place when nitric acid acts upon
+metals, resulting in the formation of other oxides of nitrogen, free
+nitrogen, or even ammonia. The reaction just given is, however, the
+usual one.
+
+     ~Importance of steps in a reaction.~ This complete equation has
+     the advantage of making it possible to calculate very easily
+     the proportions in which the various substances enter into the
+     reaction or are formed in it. It is unsatisfactory in that it
+     does not give full information about the way in which the
+     reaction takes place. For example, it does not suggest that
+     hydrogen is at first formed, and subsequently transformed into
+     water. It is always much more important to remember the steps
+     in a chemical reaction than to remember the equation expressing
+     the complete action; for if these steps in the reaction are
+     understood, the complete equation is easily obtained in the
+     manner just described.
+
+~Salts of nitric acid,--nitrates.~ The salts of nitric acid are called
+nitrates. Many of these salts will be described in the study of the
+metals. They are all soluble in water, and when heated to a high
+temperature undergo decomposition. In a few cases a nitrate on being
+heated evolves oxygen, forming a nitrite:
+
+    NaNO_{3} = NaNO_{2} + O.
+
+In other cases the decomposition goes further, and the metal is left as
+oxide:
+
+    Cu(NO_{3})_{2} = CuO + 2NO_{2} + O.
+
+~Nitrous acid~ (HNO_{2}). It is an easy matter to obtain sodium nitrite
+(NaNO_{2}), as the reaction given on the previous page indicates.
+Instead of merely heating the nitrate, it is better to heat it together
+with a mild reducing agent, such as lead, when the reaction takes place
+which is expressed by the equation
+
+    NaNO_{3} + Pb = PbO + NaNO_{2}.
+
+When sodium nitrite is treated with an acid, such as sulphuric acid, it
+is decomposed and nitrous acid is set free:
+
+    NaNO_{2} + H_{2}SO_{4} = NaHSO_{4} + HNO_{2}.
+
+The acid is very unstable, however, and decomposes readily into water
+and nitrogen trioxide (N_{2}O_{3}):
+
+    2HNO_{2} = H_{2}O + N_{2}O_{3}.
+
+Dilute solutions of the acid, however, can be obtained.
+
+
+COMPOUNDS OF NITROGEN WITH OXYGEN
+
+Nitrogen combines with oxygen to form five different oxides. The
+formulas and names of these are as follows:
+
+    N_{2}O       nitrous oxide.
+    NO           nitric oxide.
+    NO_{2}       nitrogen peroxide.
+    N_{2}O_{3}   nitrogen trioxide, or nitrous anhydride.
+    N_{2}O_{5}   nitrogen pentoxide, or nitric anhydride.
+
+These will now be briefly discussed.
+
+~Nitrous oxide~ (_laughing gas_) (N_{2}O). Ammonium nitrate, like all
+nitrates, undergoes decomposition when heated; and owing to the fact
+that it contains no metal, but does contain both oxygen and hydrogen,
+the reaction is a peculiar one. It is represented by the equation
+
+    NH_{4}NO_{3} = 2H_{2}O + N_{2}O.
+
+The oxide of nitrogen so formed is called nitrous oxide or laughing gas.
+It is a colorless gas having a slight odor. It is somewhat soluble in
+water, and in solution has a slightly sweetish taste. It is easily
+converted into a liquid and can be purchased in this form. When inhaled
+it produces a kind of hysteria (hence the name "laughing gas"), and even
+unconsciousness and insensibility to pain if taken in large amounts. It
+has long been used as an anæsthetic for minor surgical operations, such
+as those of dentistry, but owing to its unpleasant after effects it is
+not so much in use now as formerly.
+
+Chemically, nitrous oxide is remarkable for the fact that it is a very
+energetic oxidizing agent. Substances such as carbon, sulphur, iron, and
+phosphorus burn in it almost as brilliantly as in oxygen, forming oxides
+and setting free nitrogen. Evidently the oxygen in nitrous oxide cannot
+be held in very firm combination by the nitrogen.
+
+[Illustration Fig. 39]
+
+~Nitric oxide~ (NO). We have seen that when nitric acid acts upon metals,
+such as copper, the reaction represented by the following equation takes
+place:
+
+    3Cu + 8HNO_{3} = 3Cu(NO_{3})_{3} + 2NO + 4H_{2}O.
+
+Nitric oxide is most conveniently prepared in this way. The metal is
+placed in the flask A (Fig. 39) and the acid added slowly through the
+funnel tube B. The gas escapes through C and is collected over
+water.
+
+Pure nitric oxide is a colorless gas, slightly heavier than air, and is
+practically insoluble in water. It is a difficult gas to liquefy. Unlike
+nitrous oxide, nitric oxide does not part with its oxygen easily, and
+burning substances introduced into this gas are usually extinguished. A
+few substances like phosphorus, which have a very strong affinity for
+oxygen and which are burning energetically in the air, will continue to
+burn in an atmosphere of nitric oxide. In this case the nitric oxide
+loses all of its oxygen and the nitrogen is set free as gas.
+
+~Action of nitric oxide with oxygen.~ When nitric oxide comes into contact
+with oxygen or with the air, it at once combines with the oxygen even at
+ordinary temperatures, forming a reddish-yellow gas of the formula
+NO_{2}, which is called nitrogen peroxide. This action is not energetic
+enough to produce a flame, though considerable heat is set free.
+
+~Nitrogen peroxide~ (NO_{2}). This gas, as we have just seen, is formed by
+allowing nitric oxide to come into contact with oxygen. It can also be
+made by heating certain nitrates, such as lead nitrate:
+
+    Pb(NO_{3})_{2} = PbO + 2NO_{2} + O.
+
+It is a reddish-yellow gas of unpleasant odor, which is quite poisonous
+when inhaled. It is heavier than air and is easily condensed to a
+liquid. It dissolves in water, but this solution is not a mere physical
+solution; the nitrogen peroxide is decomposed, forming a mixture of
+nitric and nitrous acids:
+
+    2NO_{2} + H_{2}O = HNO_{2} + HNO_{3}.
+
+Nitrogen peroxide will not combine with more oxygen; it will, however,
+give up a part of its oxygen to burning substances, acting as an
+oxidizing agent:
+
+    NO_{2} = NO + O.
+
+~Acid anhydrides.~ The oxides N_{2}O_{3} (nitrogen trioxide) and
+N_{2}O_{5} (nitrogen pentoxide) are rarely prepared and need not be
+separately described. They bear a very interesting relation to the acids
+of nitrogen. When dissolved in water they combine with the water,
+forming acids:
+
+    N_{2}O_{3} + H_{2}O = 2HNO_{2},
+
+    N_{2}O_{5} + H_{2}O = 2HNO_{3}.
+
+On the other hand, nitrous acid very easily decomposes, yielding water
+and nitrogen trioxide, and by suitable means nitric acid likewise may be
+decomposed into water and nitrogen pentoxide:
+
+    2HNO_{2} = H_{2}O + N_{2}O_{3},
+
+    2HNO_{3} = H_{2}O + N_{2}O_{5}.
+
+In view of the close relation between these oxides and the corresponding
+acids, they are called _anhydrides_ of the acids, N_{2}O_{3} being
+nitrous anhydride and N_{2}O_{5} nitric anhydride.
+
+DEFINITION: _Any oxide which will combine with water to form an acid, or
+which together with water is formed by the decomposition of an acid, is
+called an anhydride of that acid._
+
+
+EXERCISES
+
+1. Perfectly dry ammonia does not affect litmus paper. Explain.
+
+2. Can ammonia be dried by passing the gas through concentrated
+sulphuric acid? Explain.
+
+3. Ammonium hydroxide is a weak base, i.e. it is not highly dissociated.
+When it is neutralized by strong acids the heat of reaction is less than
+when strong bases are so neutralized. Suggest some possible cause for
+this.
+
+4. Why is brine used in the manufacture of artificial ice?
+
+5. Discuss the energy changes which take place in the manufacture of
+artificial ice.
+
+6. What weight of ammonium chloride is necessary to furnish enough
+ammonia to saturate 1 l. of water at 0° and 760 mm.?
+
+7. What weight of sodium nitrate is necessary to prepare 100 cc. of
+commercial nitric acid? What weight of potassium nitrate is necessary to
+furnish the same weight of acid?
+
+8. 100 l. of nitrogen peroxide were dissolved in water and neutralized
+with sodium hydroxide. What substances were formed and how much of
+each?(1 l. nitrogen peroxide weighs 2.05 grams.)
+
+9. How many liters of nitrous oxide, measured under standard conditions,
+can be prepared from 10 g. of ammonium nitrate?
+
+10. What weight of copper is necessary to prepare 50 l. of nitric oxide
+under standard conditions?
+
+11. (a) Calculate the percentage composition of the oxides of
+nitrogen. (b) What important law does this series of substances
+illustrate?
+
+12. Write the equations representing the reactions between ammonium
+hydroxide, and sulphuric acid and nitric acid respectively, in
+accordance with the theory of electrolytic dissociation.
+
+13. In the same way, write the equations representing the reactions
+between nitric acid and each of the following bases: NaOH, KOH,
+NH_{4}OH, Ca(OH)_{2}.
+
+
+
+
+CHAPTER XIII
+
+REVERSIBLE REACTIONS AND CHEMICAL EQUILIBRIUM
+
+
+~Reversible reactions.~ The reactions so far considered have been
+represented as continuing, when once started, until one or the other
+substance taking part in the reaction has been used up. In some
+reactions this is not the case. For example, we have seen that when
+steam is passed over hot iron the reaction is represented by the
+equation
+
+    3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H.
+
+On the other hand, when hydrogen is passed over hot iron oxide the
+reverse reaction takes place:
+
+    Fe_{3}O_{4} +8H = 3Fe + 4H_{2}O.
+
+The reaction can therefore go in either direction, depending upon the
+conditions of the experiment. Such a reaction is called a _reversible
+reaction_. It is represented by an equation with double arrows in place
+of the equality sign, thus:
+
+    3Fe + 4H_{2}O <--> Fe_{3}O_{4} + 8H.
+
+In a similar way, the equation
+
+    N + 3H <--> NH_{3}
+
+expresses the fact that under some conditions nitrogen may unite with
+hydrogen to form ammonia, while under other conditions ammonia
+decomposes into nitrogen and hydrogen.
+
+The conversion of oxygen into ozone is also reversible and may be
+represented thus:
+
+    oxygen <--> ozone.
+
+~Chemical equilibrium.~ Reversible reactions do not usually go on to
+completion in one direction unless the conditions under which the
+reaction takes place are very carefully chosen. Thus, if iron and steam
+are confined in a heated tube, the steam acts upon the iron, producing
+iron oxide and hydrogen. But these substances in turn act upon each
+other to form iron and steam once more. When these two opposite
+reactions go on at such rates that the weight of the iron changed into
+iron oxide is just balanced by the weight of the iron oxide changed into
+iron, there will be no further change in the relative weights of the
+four substances present in the tube. The reaction is then said to have
+reached an equilibrium.
+
+~Factors which determine the point of equilibrium.~ There are two factors
+which have a great deal of influence in determining the point at which a
+given reaction will reach equilibrium.
+
+1. _Influence of the chemical nature of the substances._ If two
+reversible reactions of the same general kind are selected, it has been
+found that the point of equilibrium is different in the two cases. For
+example, in the reactions represented by the equations
+
+    3Fe + 4H_{2}O <--> Fe_{3}O_{4} + 8H,
+
+    Zn + H_{2}O <--> ZnO + 2H,
+
+the equilibrium will be reached when very different quantities of the
+iron and zinc have been changed into oxides. The individual chemical
+properties of the iron and zinc have therefore marked influence upon the
+point at which equilibrium will be reached.
+
+2. _Influence of relative mass._ If the tube in which the reaction
+
+    3Fe + 4H_{2}O <--> Fe_{3}O_{4} + 8H
+
+has come to an equilibrium is opened and more steam is admitted, an
+additional quantity of the iron will be changed into iron oxide. If more
+hydrogen is admitted, some of the oxide will be reduced to metal. The
+point of equilibrium is therefore dependent upon the relative masses of
+the substances taking part in the reaction. When one of the substances
+is a solid, however, its mass has little influence, since it is only the
+extent of its surface which can affect the reaction.
+
+~Conditions under which reversible reactions are complete.~ If, when the
+equilibrium between iron and steam has been reached, the tube is opened
+and a current of steam is passed in, the hydrogen is swept away as fast
+as it is formed. The opposing reaction of hydrogen upon iron oxide must
+therefore cease, and the action of steam on the iron will go on until
+all of the iron has been transformed into iron oxide.
+
+On the other hand, if a current of hydrogen is admitted into the tube,
+the steam will be swept away by the hydrogen, and all of the iron oxide
+will be reduced to iron. _A reversible reaction can therefore be
+completed in either direction when one of the products of the reaction
+is removed as fast as it is formed._
+
+~Equilibrium in solution.~ When reactions take place in solution in water
+the same general principles hold good. The matter is not so simple,
+however, as in the case just described, owing to the fact that many of
+the reactions in solution are due to the presence of ions. The
+substances most commonly employed in solution are acids, bases, or
+salts, and all of these undergo dissociation. Any equilibrium which may
+be reached in solutions of these substances must take place between the
+various ions formed, on the one hand, and the undissociated molecules,
+on the other. Thus, when nitric acid is dissolved in water, equilibrium
+is reached in accordance with the equation
+
+    H^{+} + NO_{3}^{-} <--> HNO_{3}.
+
+~Conditions under which reversible reactions in solution are complete.~
+The equilibrium between substances in solution may be disturbed and the
+reaction caused to go on in one direction to completion in either of
+three ways.
+
+1. _A gas may be formed which escapes from the solution._ When sodium
+nitrate and sulphuric acid are brought together in solution all four
+ions, Na^{+}, NO_{3}^{-}, H^{+}, SO_{4}^{--}, are formed. These ions are
+free to rearrange themselves in various combinations. For example, the
+H^{+} and the NO_{3}^{-} ions will reach the equilibrium
+
+    H^{+} + NO_{3}^{-} <--> HNO_{3}.
+
+If the experiment is performed with very little water present, as is the
+case in the preparation of nitric acid, the equilibrium will be reached
+when most of the H^{+} and the NO_{3}^{-} ions have combined to form
+undissociated HNO_{3}.
+
+Finally, if the mixture is now heated above the boiling point of nitric
+acid, the acid distills away as fast as it is formed. More and more
+H^{+} and NO_{3}^{-} ions will then combine, and the process will
+continue until one or the other of them has all been removed from the
+solution. The substance remaining is sodium acid sulphate (NaHSO_{4}),
+and the reaction can therefore be expressed by the equation
+
+    NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}.
+
+2. _An insoluble solid may be formed._ When hydrochloric acid (HCl) and
+silver nitrate (AgNO_{3}) are brought together in solution the
+following ions will be present: H^{+}, Cl^{-}, Ag^{+}, NO_{3}^{-}. The
+ions Ag^{+} and Cl^{-} will then set up the equilibrium
+
+    Ag^{+} + Cl^{-} <--> AgCl.
+
+But silver chloride (AgCl) is almost completely insoluble in water, and
+as soon as a very little of it has formed the solution becomes
+supersaturated, and the excess of the salt precipitates. More silver and
+chlorine ions then unite, and this continues until practically all of
+the silver or the chlorine ions have been removed from the solution. We
+then say that the following reaction is complete:
+
+    AgNO_{3} + HCl = AgCl + HNO_{3}.
+
+3. _Two different ions may form undissociated molecules._ In the
+neutralization of sodium hydroxide by hydrochloric acid the ions H^{+}
+and OH^{-} come to the equilibrium
+
+    H^{+} + OH^{-} <--> H_{2}O.
+
+But since water is almost entirely undissociated, equilibrium can only
+be reached when there are very few hydroxyl or hydrogen ions present.
+Consequently the two ions keep uniting until one or the other of them is
+practically removed from the solution. When this occurs the
+neutralization expressed in the following equation is complete:
+
+    NaOH + HCl = H_{2}O + NaCl.
+
+~Preparation of acids.~ The principle of reversible reactions finds
+practical application in the preparation of most of the common acids. An
+acid is usually prepared by treating the most common of its salts with
+some other acid of high boiling point. The mixture is then heated until
+the lower boiling acid desired distills out. Owing to its high boiling
+point (338°), sulphuric acid is usually employed for this purpose, most
+other acids boiling below that temperature.
+
+
+EXERCISES
+
+1. What would take place when solutions of silver nitrate and sodium
+chloride are brought together? What other chlorides would act in the
+same way?
+
+2. Is the reaction expressed by the equation NH_{3} + H_{2}O = NH_{4}OH
+reversible? If so, state the conditions under which it will go in each
+direction.
+
+3. Is the reaction expressed by the equation 2H + O = H_{2}O reversible?
+If so, state the conditions under which it will go in each direction.
+
+4. Suggest a method for the preparation of hydrochloric acid.
+
+
+
+
+CHAPTER XIV
+
+SULPHUR AND ITS COMPOUNDS
+
+
+~Occurrence.~ The element sulphur has been known from the earliest times,
+since it is widely distributed in nature and occurs in large quantities
+in the uncombined form, especially in the neighborhood of volcanoes.
+Sicily has long been famous for its sulphur mines, and smaller deposits
+are found in Italy, Iceland, Mexico, and especially in Louisiana, where
+it is mined extensively. In combination, sulphur occurs abundantly in
+the form of sulphides and sulphates. In smaller amounts it is found in a
+great variety of minerals, and it is a constituent of many animal and
+vegetable substances.
+
+~Extraction of sulphur.~ Sulphur is prepared from the native substance,
+the separation of crude sulphur from the rock and earthy materials with
+which it is mixed being a very simple process. The ore from the mines is
+merely heated until the sulphur melts and drains away from the earthy
+impurities. The crude sulphur obtained in this way is distilled in a
+retort-shaped vessel made of iron, the exit tube of which opens into a
+cooling chamber of brickwork. When the sulphur vapor first enters the
+cooling chamber it condenses as a fine crystalline powder called
+_flowers of sulphur_. As the condensing chamber becomes warm, the
+sulphur collects as a liquid in it, and is drawn off into cylindrical
+molds, the product being called _roll sulphur_ or _brimstone_.
+
+~Physical properties.~ Roll sulphur is a pale yellow, crystalline solid,
+without marked taste and with but a faint odor. It is insoluble in
+water, but is freely soluble in a few liquids, notably in carbon
+disulphide. Roll sulphur melts at 114.8°. Just above the melting point
+it forms a rather thin, straw-colored liquid. As the temperature is
+raised, this liquid turns darker in color and becomes thicker, until at
+about 235° it is almost black and is so thick that the vessel containing
+it can be inverted without danger of the liquid running out. At higher
+temperatures it becomes thin once more, and boils at 448°, forming a
+yellowish vapor. On cooling the same changes take place in reverse
+order.
+
+~Varieties of sulphur.~ Sulphur is known in two general forms, crystalline
+and amorphous. Each of these forms exists in definite modifications.
+
+~Crystalline sulphur.~ Sulphur occurs in two crystalline forms, namely,
+rhombic sulphur and monoclinic sulphur.
+
+1. _Rhombic sulphur._ When sulphur crystallizes from its solution in
+carbon disulphide it separates in crystals which have the same color and
+melting point as roll sulphur, and are rhombic in shape. Roll sulphur is
+made up of minute rhombic crystals.
+
+2. _Monoclinic sulphur._ When melted sulphur is allowed to cool until a
+part of the liquid has solidified, and the remaining liquid is then
+poured off, it is found that the solid sulphur remaining in the vessel
+has assumed the form of fine needle-shaped crystals. These differ much
+in appearance from the rhombic crystals obtained by crystallizing
+sulphur from its solution in carbon disulphide. The needle-shaped form
+is called _monoclinic sulphur_. The two varieties differ also in density
+and in melting point, the monoclinic sulphur melting at 120°.
+
+Monoclinic and rhombic sulphur remain unchanged in contact with each
+other at 96°. Above this temperature the rhombic changes into
+monoclinic; at lower temperatures the monoclinic changes into rhombic.
+The temperature 96° is therefore called the transition point of sulphur.
+Heat is set free when monoclinic sulphur changes into rhombic.
+
+~Amorphous sulphur.~ Two varieties of amorphous sulphur can be readily
+obtained. These are white sulphur and plastic sulphur.
+
+1. _White sulphur._ Flowers of sulphur, the preparation of which has
+been described, consists of a mixture of rhombic crystals and amorphous
+particles. When treated with carbon disulphide, the crystals dissolve,
+leaving the amorphous particles as a white residue.
+
+2. _Plastic sulphur._ When boiling sulphur is poured into cold water it
+assumes a gummy, doughlike form, which is quite elastic. This can be
+seen in a very striking manner by distilling sulphur from a small,
+short-necked retort, such as is represented in Fig. 40, and allowing the
+liquid to run directly into water. In a few days it becomes quite
+brittle and passes over into ordinary rhombic sulphur.
+
+[Illustration Fig. 40]
+
+~Chemical properties of sulphur.~ When sulphur is heated to its kindling
+temperature in oxygen or in the air it burns with a pale blue flame,
+forming sulphur dioxide (SO_{2}). Small quantities of sulphur trioxide
+(SO_{3}) may also be formed in the combustion of sulphur. Most metals
+when heated with sulphur combine directly with it, forming metallic
+sulphides. In some cases the action is so energetic that the mass
+becomes incandescent, as has been seen in the case of iron uniting with
+sulphur. This property recalls the action of oxygen upon metals, and in
+general the metals which combine readily with oxygen are apt to combine
+quite readily with sulphur.
+
+~Uses of sulphur.~ Large quantities of sulphur are used as a germicide in
+vineyards, also in the manufacture of gunpowder, matches, vulcanized
+rubber, and sulphuric acid.
+
+
+COMPOUNDS OF SULPHUR WITH HYDROGEN
+
+~Hydrosulphuric acid~ (H_{2}S). This substance is a gas having the
+composition expressed by the formula H_{2}S and is commonly called
+hydrogen sulphide. It is found in the vapors issuing from volcanoes, and
+in solution in the so-called sulphur waters of many springs. It is
+formed when organic matter containing sulphur undergoes decay, just as
+ammonia is formed under similar circumstances from nitrogenous matter.
+
+~Preparation.~ Hydrosulphuric acid is prepared in the laboratory by
+treating a sulphide with an acid. Iron sulphide (FeS) is usually
+employed:
+
+    FeS + 2HCl = FeCl_{2} + H_{2}S.
+
+A convenient apparatus is shown in Fig. 41. A few lumps of iron sulphide
+are placed in the bottle A, and dilute acid is added in small
+quantities at a time through the funnel tube B, the gas escaping
+through the tube C.
+
+[Illustration: Fig. 41]
+
+     ~Explanation of the reaction.~ Iron sulphide is a salt of
+     hydrosulphuric acid, and this reaction is therefore similar to
+     the one which takes place when sulphuric acid acts upon a
+     nitrate. In both cases a salt and an acid are brought together,
+     and there is a tendency for the reaction to go on until a state
+     of equilibrium is reached. This equilibrium is constantly
+     disturbed by the escape of the gaseous acid set free, so that
+     the reaction goes on until all of the original salt has been
+     decomposed. The two reactions differ in that the first one is
+     complete at ordinary temperatures, while in the case of
+     sulphuric acid acting upon sodium nitrate, the reacting
+     substances must be heated so as to secure a temperature at
+     which nitric acid is a gas.
+
+~Physical properties.~ Hydrosulphuric acid is a colorless gas, having a
+weak, disagreeable taste and an exceedingly offensive odor. It is rather
+sparingly soluble in water at ordinary temperatures, about three volumes
+dissolving in one of water. In boiling water it is not soluble at all.
+In pure form it acts as a violent poison, and even when diluted largely
+with air produces headache, dizziness, and nausea. It is a little
+heavier than air, having a density of 1.18.
+
+~Chemical properties.~ The most important chemical properties of
+hydrosulphuric acid are the following:
+
+1. _Acid properties._ Hydrosulphuric acid is a weak acid. In solution in
+water it turns blue litmus red and neutralizes bases, forming salts
+called _sulphides_.
+
+2. _Action on oxygen._ The elements composing hydrosulphuric acid have
+each a strong affinity for oxygen, and are not held together very
+firmly. Consequently the gas burns readily in oxygen or the air,
+according to the equation
+
+    H_{2}S + 3O = H_{2}O + SO_{2}.
+
+When there is not enough oxygen for both the sulphur and the hydrogen,
+the latter element combines with the oxygen and the sulphur is set free:
+
+    H_{2}S + O = H_{2}O + S.
+
+3. _Reducing action._ Owing to the ease with which hydrosulphuric acid
+decomposes and the strong affinity of both sulphur and hydrogen for
+oxygen, the substance is a strong reducing agent, taking oxygen away
+from many substances which contain it.
+
+4. _Action on metals._ Hydrosulphuric acid acts towards metals in a way
+very similar to water. Thus, when it is passed over heated iron in a
+tube, the reaction is represented by the equation
+
+    3Fe + 4H_{2}S = Fe_{3}S_{4} + 8H.
+
+Water in the form of steam, under similar circumstances, acts according
+to the equation
+
+    3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H.
+
+~Salts of hydrosulphuric acid,--sulphides.~ The salts of hydrosulphuric
+acid, called sulphides, form an important class of salts. Many of them
+are found abundantly in nature, and some of them are important ores.
+They will be frequently mentioned in connection with the metals.
+
+Most of the sulphides are insoluble in water, and some of them are
+insoluble in acids. Consequently, when hydrosulphuric acid is passed
+into a solution of a salt, it often happens that a sulphide is
+precipitated. With copper chloride the equation is
+
+    CuCl_{2} + H_{2}S = CuS + 2HCl.
+
+Because of the fact that some metals are precipitated in this way as
+sulphides while others are not, hydrosulphuric acid is extensively used
+in the separation of the metals in the laboratory.
+
+     ~Explanation of the reaction.~ When hydrosulphuric acid and
+     copper chloride are brought together in solution, both copper
+     and sulphur ions are present, and these will come to an
+     equilibrium, as represented in the equation
+
+         Cu^{+} + S^{-} <--> CuS.
+
+     Since copper sulphide is almost insoluble in water, as soon as
+     a very small quantity has formed the solution becomes
+     supersaturated, and the excess keeps precipitating until nearly
+     all the copper or sulphur ions have been removed from the
+     solution. With some other ions, such as iron, the sulphide
+     formed does not saturate the solution, and no precipitate
+     results.
+
+
+OXIDES OF SULPHUR
+
+Sulphur forms two well-known compounds with oxygen: sulphur dioxide
+(SO_{2}), sometimes called sulphurous anhydride; and sulphur trioxide
+(SO_{3}), frequently called sulphuric anhydride.
+
+~Sulphur dioxide~ (SO_{2}). Sulphur dioxide occurs in nature in the gases
+issuing from volcanoes, and in solution in the water of many springs. It
+is likely to be found wherever sulphur compounds are undergoing
+oxidation.
+
+~Preparation.~ Three general ways may be mentioned for the preparation of
+sulphur dioxide:
+
+1. _By the combustion of sulphur._ Sulphur dioxide is readily formed by
+the combustion of sulphur in oxygen or the air:
+
+    S + 2O = SO_{2}.
+
+It is also formed when substances containing sulphur are burned:
+
+    ZnS + 3O = ZnO + SO_{2}.
+
+2. _By the reduction of sulphuric acid._ When concentrated sulphuric
+acid is heated with certain metals, such as copper, part of the acid is
+changed into copper sulphate, and part is reduced to sulphurous acid.
+The latter then decomposes into sulphur dioxide and water, the complete
+equation being
+
+    Cu + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O.
+
+3. _By the action of an acid on a sulphite._ Sulphites are salts of
+sulphurous acid (H_{2}SO_{3}). When a sulphite is treated with an acid,
+sulphurous acid is set free, and being very unstable, decomposes into
+water and sulphur dioxide. These reactions are expressed in the
+equations
+
+    Na_{2}SO_{3} + 2HCl = 2NaCl + H_{2}SO_{3},
+
+    H_{2}SO_{3} = H_{2}O + SO_{2}.
+
+~Explanation of the reaction.~ In this case we have two reversible
+reactions depending on each other. In the first reaction,
+
+    (1) Na_{2}SO_{3} + 2HCl <--> 2NaCl + H_{2}SO_{3},
+
+we should expect an equilibrium to result, for none of the four
+substances in the equation are insoluble or volatile when water is
+present to hold them in solution. But the quantity of the H_{2}SO_{3} is
+constantly diminishing, owing to the fact that it decomposes, as
+represented in the equation
+
+    (2) H_{2}SO_{3} <--> H_{2}O + SO_{2},
+
+and the sulphur dioxide, being a gas, escapes. No equilibrium can
+therefore result, since the quantity of the sulphurous acid is
+constantly being diminished because of the escape of sulphur dioxide.
+
+~Physical properties.~ Sulphur dioxide is a colorless gas, which at
+ordinary temperatures is 2.2 times as heavy as air. It has a peculiar,
+irritating odor. The gas is very soluble in water, one volume of water
+dissolving eighty of the gas under standard conditions. It is easily
+condensed to a colorless liquid, and can be purchased in this condition
+stored in strong bottles, such as the one represented in Fig. 42.
+
+[Illustration: Fig. 42]
+
+~Chemical properties.~ Sulphur dioxide has a marked tendency to combine
+with other substances, and is therefore an active substance chemically.
+It combines with oxygen gas, but not very easily. It can, however, take
+oxygen away from some other substances, and is therefore a good reducing
+agent. Its most marked chemical property is its ability to combine with
+water to form sulphurous acid (H_{2}SO_{3}).
+
+~Sulphurous acid~ (H_{2}SO_{3}). When sulphur dioxide dissolves in water
+it combines chemically with it to form sulphurous acid, an unstable
+substance having the formula H_{3}SO_{3}. It is impossible to prepare
+this acid in pure form, as it breaks down very easily into water and
+sulphur dioxide. The reaction is therefore reversible, and is expressed
+by the equation
+
+    H_{2}O + SO_{2} <--> H_{2}SO_{3}.
+
+Solutions of the acid in water have a number of interesting properties.
+
+1. _Acid properties._ The solution has all the properties typical of an
+acid. When neutralized by bases, sulphurous acid yields a series of
+salts called _sulphites_.
+
+2. _Reducing properties._ Solutions of sulphurous acid act as good
+reducing agents. This is due to the fact that sulphurous acid has the
+power of taking up oxygen from the air, or from substances rich in
+oxygen, and is changed by this reaction into sulphuric acid:
+
+    H_{2}SO_{3} + O = H_{2}SO_{4},
+
+    H_{2}SO_{3} + H_{2}O_{2} = H_{2}S0_{4} + H_{2}O.
+
+3. _Bleaching properties._ Sulphurous acid has strong bleaching
+properties, acting upon many colored substances in such a way as to
+destroy their color. It is on this account used to bleach paper, straw
+goods, and even such foods as canned corn.
+
+4. _Antiseptic properties._ Sulphurous acid has marked antiseptic
+properties, and on this account has the power of arresting
+fermentation. It is therefore used as a preservative.
+
+~Salts of sulphurous acid,--sulphites.~ The sulphites, like sulphurous
+acid, have the power of taking up oxygen very readily, and are good
+reducing agents. On account of this tendency, commercial sulphites are
+often contaminated with sulphates. A great deal of sodium sulphite is
+used in the bleaching industry, and as a reagent for softening paper
+pulp.
+
+~Sulphur trioxide~ (SO_{3}). When sulphur dioxide and oxygen are heated
+together at a rather high temperature, a small amount of sulphur
+trioxide (SO_{3}) is formed, but the reaction is slow and incomplete.
+If, however, the heating takes place in the presence of very fine
+platinum dust, the reaction is rapid and nearly complete.
+
+[Illustration: Fig. 43]
+
+     ~ Experimental preparation of sulphur trioxide.~ The experiment
+     can be performed by the use of the apparatus shown in Fig. 43,
+     the fine platinum being secured by moistening asbestos fiber
+     with a solution of platinum chloride and igniting it in a
+     flame. The fiber, covered with fine platinum, is placed in a
+     tube of hard glass, which is then heated with a burner to about
+     350°, while sulphur dioxide and air are passed into the tube.
+     Union takes place at once, and the strongly fuming sulphur
+     trioxide escapes from the jet at the end of the tube, and may
+     be condensed by surrounding the receiving tube with a freezing
+     mixture.
+
+~Properties of sulphur trioxide.~ Sulphur trioxide is a colorless liquid,
+which solidifies at about 15° and boils at 46°. A trace of moisture
+causes it to solidify into a mass of silky white crystals, somewhat
+resembling asbestos fiber in appearance. In contact with the air it
+fumes strongly, and when thrown upon water it dissolves with a hissing
+sound and the liberation of a great deal of heat. The product of this
+reaction is sulphuric acid, so that sulphur trioxide is the anhydride of
+that acid:
+
+    SO_{3} + H_{2}O = H_{2}SO_{4}.
+
+~Catalysis.~ It has been found that many chemical reactions, such as the
+union of sulphur dioxide with oxygen, are much influenced by the
+presence of substances which do not themselves seem to take a part in
+the reaction, and are left apparently unchanged after it has ceased.
+These reactions go on very slowly under ordinary circumstances, but are
+greatly hastened by the presence of the foreign substance. Substances
+which hasten very slow reactions in this way are said to act as
+catalytic agents or _catalyzers_, and the action is called _catalysis_.
+Just how the action is brought about is not well understood.
+
+DEFINITION: _A catalyzer is a substance which changes the velocity of a
+reaction, but does not change its products._
+
+~Examples of Catalysis.~ We have already had several instances of such
+action. Oxygen and hydrogen combine with each other at ordinary
+temperatures in the presence of platinum powder, while if no catalytic
+agent is present they do not combine in appreciable quantities until a
+rather high temperature is reached. Potassium chlorate, when heated with
+manganese dioxide, gives up its oxygen at a much lower temperature than
+when heated alone. Hydrogen dioxide decomposes very rapidly when
+powdered manganese dioxide is sifted into its concentrated solution.
+
+On the other hand, the catalytic agent sometimes retards chemical
+action. For example, a solution of hydrogen dioxide decomposes more
+slowly when it contains a little phosphoric acid than when perfectly
+pure. For this reason commercial hydrogen dioxide always contains
+phosphoric acid.
+
+Many reactions are brought about by the catalytic action of traces of
+water. For example, phosphorus will not burn in oxygen in the absence of
+all moisture. Hydrochloric acid will not unite with ammonia if the
+reagents are perfectly dry. It is probable that many of the chemical
+transformations in physiological processes, such as digestion, are
+assisted by certain substances acting as catalytic agents. The principle
+of catalysis is therefore very important.
+
+~Sulphuric acid~ (_oil of vitriol_) (H_{2}SO_{4}). Sulphuric acid is one
+of the most important of all manufactured chemicals. Not only is it one
+of the most common reagents in the laboratory, but enormous quantities
+of it are used in many of the industries, especially in the refining of
+petroleum, the manufacture of nitroglycerin, sodium carbonate, and
+fertilizers.
+
+~Manufacture of sulphuric acid.~ 1. _Contact process_. The reactions
+taking place in this process are represented by the following equations:
+
+    SO_{2} + O = SO_{3},
+
+    SO_{3} + H_{2}O = H_{2}SO_{4}.
+
+To bring about the first of these reactions rapidly, a catalyzer is
+employed, and the process is carried out in the following way: Large
+iron tubes are packed with some porous material, such as calcium and
+magnesium sulphates, which contains a suitable catalytic substance
+scattered through it. The catalyzers most used are platinum powder,
+vanadium oxide, and iron oxide. Purified sulphur dioxide and air are
+passed through the tubes, which are kept at a temperature of about 350°.
+Sulphur trioxide is formed, and as it issues from the tube it is
+absorbed in water or dilute sulphuric acid. The process is continued
+until all the water in the absorbing vessel has been changed into
+sulphuric acid, so that a very concentrated acid is made in this way. An
+excess of the trioxide may dissolve in the strong sulphuric acid,
+forming what is known as _fuming sulphuric acid_.
+
+2. _Chamber process._ The method of manufacture exclusively employed
+until recent years, and still in very extensive use, is much more
+complicated. The reactions are quite involved, but the conversion of
+water, sulphur dioxide, and oxygen into sulphuric acid is accomplished
+by the catalytic action of oxides of nitrogen. The reactions are brought
+about in large lead-lined chambers, into which oxides of nitrogen,
+sulphur dioxide, steam, and air are introduced in suitable proportions.
+
+     ~Reactions of the chamber process.~ In a very general way, the
+     various reactions which take place in the lead chambers may be
+     expressed in two equations. In the first reaction sulphur
+     dioxide, nitrogen peroxide, steam, and oxygen unite, as shown
+     in the equation
+
+    (1) 2SO_{2} + 2NO_{2} + H_{2}O + O = 2SO_{2} (OH) (NO_{2}).
+
+     The product formed in this reaction is called nitrosulphuric
+     acid or "chamber crystals." It actually separates on the walls
+     of the chambers when the process is not working properly. Under
+     normal conditions, it is decomposed as fast as it is formed by
+     the action of excess of steam, as shown in the equation
+
+    (2) 2SO_{2} (OH) (NO_{2}) + H_{2}O + O = 2H_{2}SO_{4} + 2NO_{2}.
+
+     The nitrogen dioxide formed in this reaction can now enter into
+     combination with a new quantity of sulphur dioxide, steam, and
+     oxygen, and the series of reactions go on indefinitely. Many
+     other reactions occur, but these two illustrate the principle
+     of the process.
+
+The relation between sulphuric acid and nitrosulphuric acid can be seen
+by comparing their structural formulas:
+
+    O=  -OH      O=  -OH
+       S            S
+    O=  -OH      O=  -NO_{2}
+
+The latter may be regarded as derived from the former by the
+substitution of the nitro group (NO_{2}) for the hydroxyl group (OH).
+
+[Illustration: Fig. 44]
+
+~The sulphuric acid plant.~ Fig. 44 illustrates the simpler parts of a
+plant used in the manufacture of sulphuric acid by the chamber process.
+Sulphur or some sulphide, as FeS_{2}, is burned in furnace A. The
+resulting sulphur dioxide, together with air and some nitrogen peroxide,
+are conducted into the large chambers, the capacity of each chamber
+being about 75,000 cu. ft. Steam is also admitted into these chambers at
+different points. These compounds react to form sulphuric acid,
+according to the equations given above. The nitrogen left after the
+withdrawal of the oxygen from the admitted air escapes through the
+Gay-Lussac tower X. In order to prevent the escape of the oxides of
+nitrogen regenerated in the reaction, the tower is filled with lumps of
+coke, over which trickles concentrated sulphuric acid admitted from Y.
+The nitrogen peroxide dissolves in the acid and the resulting solution
+collects in H. This is pumped into E, where it is mixed with dilute
+acid and allowed to trickle down through the chamber D (Glover tower),
+which is filled with some acid-resisting rock. Here the nitrogen
+peroxide is expelled from the solution by the action of the hot gases
+entering from A, and together with them enters the first chamber
+again. The acid from which the nitrogen peroxide is expelled collects in
+F. Theoretically, a small amount of nitrogen peroxide would suffice to
+prepare an unlimited amount of sulphuric acid; practically, some of it
+escapes, and this is replaced by small amounts admitted at B.
+
+The sulphuric acid so formed, together with the excess of condensed
+steam, collect upon the floor of the chambers in the form of a liquid
+containing from 62% to 70% of sulphuric acid. The product is called
+_chamber acid_ and is quite impure; but for many purposes, such as the
+manufacture of fertilizers, it needs no further treatment. It can be
+concentrated by boiling it in vessels made of iron or platinum, which
+resist the action of the acid, nearly all the water boiling off. Pure
+concentrated acid can be made best by the contact process, while the
+chamber process is cheaper for the dilute impure acid.
+
+~Physical properties.~ Sulphuric acid is a colorless, oily liquid, nearly
+twice as heavy as water. The ordinary concentrated acid contains about
+2% of water, has a density of 1.84, and boils at 338°. It is sometimes
+called _oil of vitriol_, since it was formerly made by distilling a
+substance called _green vitriol_.
+
+~Chemical properties.~ Sulphuric acid possesses chemical properties which
+make it one of the most important of chemical substances.
+
+1. _Action as an acid._ In dilute solution sulphuric acid acts as any
+other acid, forming salts with oxides and hydroxides.
+
+2. _Action as an oxidizing agent._ Sulphuric acid contains a large
+percentage of oxygen and is, like nitric acid, a very good oxidizing
+agent. When the concentrated acid is heated with sulphur, carbon, and
+many other substances, oxidation takes place, the sulphuric acid
+decomposing according to the equation
+
+    H_{2}SO_{4} = H_{2}SO_{3} + O.
+
+3. _Action on metals._ In dilute solution sulphuric acid acts upon many
+metals, such as zinc, forming a sulphate and liberating hydrogen. When
+the concentrated acid is employed the hydrogen set free is oxidized by a
+new portion of the acid, with the liberation of sulphur dioxide. With
+copper the reactions are expressed by the equations
+
+    (1) Cu + H_{2}SO_{4} = CuSO_{4} + 2H,
+
+    (2) H_{2}SO_{4} + 2H = H_{2}SO_{3} + H_{2}O,
+
+    (3) H_{2}SO_{3} = H_{2}O + SO_{2}.
+
+By combining these equations the following one is obtained:
+
+    Cu + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O.
+
+4. _Action on salts._ We have repeatedly seen that an acid of high
+boiling point heated with the salt of some acid of lower boiling point
+will drive out the low boiling acid. The boiling point of sulphuric acid
+(338°) is higher than that of almost any common acid; hence it is used
+largely in the preparation of other acids.
+
+5. _Action on water._ Concentrated sulphuric acid has a very great
+affinity for water, and is therefore an effective dehydrating agent.
+Gases which have no chemical action upon sulphuric acid can be freed
+from water vapor by bubbling them through the strong acid. When the acid
+is diluted with water much heat is set free, and care must be taken to
+keep the liquid thoroughly stirred during the mixing, and to pour the
+acid into the water,--never the reverse.
+
+Not only can sulphuric acid absorb water, but it will often withdraw the
+elements hydrogen and oxygen from a compound containing them,
+decomposing the compound, and combining with the water so formed. For
+this reason most organic substances, such as sugar, wood, cotton, and
+woolen fiber, and even flesh, all of which contain much oxygen and
+hydrogen in addition to carbon, are charred or burned by the action of
+the concentrated acid.
+
+~Salts of sulphuric acid,--sulphates.~ The sulphates form a very important
+class of salts, and many of them have commercial uses. Copperas (iron
+sulphate), blue vitriol (copper sulphate), and Epsom salt (magnesium
+sulphate) serve as examples. Many sulphates are important minerals,
+prominent among these being gypsum (calcium sulphate) and barytes
+(barium sulphate).
+
+     ~Thiosulphuric acid~ (H_{2}S_{2}O_{3}); ~Thiosulphates.~ Many other
+     acids of sulphur containing oxygen are known, but none of them
+     are of great importance. Most of them cannot be prepared in a
+     pure state, and are known only through their salts. The most
+     important of these is thiosulphuric acid.
+
+     When sodium sulphite is boiled with sulphur the two substances
+     combine, forming a salt which has the composition represented
+     in the formula Na_{2}S_{2}O_{3}:
+
+     Na_{2}SO_{3} + S = Na_{2}S_{2}O_{3}.
+
+     The substance is called sodium thiosulphate, and is a salt of
+     the easily decomposed acid H_{2}S_{2}O_{3}, called
+     thiosulphuric acid. This reaction is quite similar to the
+     action of oxygen upon sulphites:
+
+     Na_{2}SO_{3} + O = Na_{2}SO_{4}.
+
+     More commonly the salt is called sodium hyposulphite, or merely
+     "hypo." It is a white solid and is extensively used in
+     photography, in the bleaching industry, and as a disinfectant.
+
+~Monobasic and dibasic acids.~ Such acids as hydrochloric and nitric
+acids, which have only one replaceable hydrogen atom in the molecule, or
+in other words yield one hydrogen ion in solution, are called monobasic
+acids. Acids yielding two hydrogen ions in solution are called dibasic
+acids. Similarly, we may have tribasic and tetrabasic acids. The three
+acids of sulphur are dibasic acids. It is therefore possible for each of
+them to form both normal and acid salts. The acid salts can be made in
+two ways: the acid may be treated with only half enough base to
+neutralize it,--
+
+    NaOH + H_{2}SO_{4} = NaHSO_{4} + H_{2}O;
+
+or a normal salt may be treated with the free acid,--
+
+    Na_{2}SO_{4} + H_{2}SO_{4} = 2NaHSO_{4}.
+
+Acid sulphites and sulphides may be made in the same ways.
+
+~Carbon disulphide~ (CS_{2}). When sulphur vapor is passed over highly
+heated carbon the two elements combine, forming carbon disulphide
+(CS_{2}), just as oxygen and carbon unite to form carbon dioxide
+(CO_{2}). The substance is a heavy, colorless liquid, possessing, when
+pure, a pleasant ethereal odor. On standing for some time, especially
+when exposed to sunlight, it undergoes a slight decomposition and
+acquires a most disagreeable, rancid odor. It has the property of
+dissolving many substances, such as gums, resins, and waxes, which are
+insoluble in most liquids, and it is extensively used as a solvent for
+such substances. It is also used as an insecticide. It boils at a low
+temperature (46°), and its vapor is very inflammable, burning in the air
+to form carbon dioxide and sulphur dioxide, according to the equation
+
+    CS_{2} + 6O = CO_{2} + 2SO_{2}.
+
+[Illustration: Fig. 45]
+
+     ~Commercial preparation of carbon disulphide.~ In the preparation
+     of carbon disulphide an electrical furnace is employed, such as
+     is represented in Fig. 45. The furnace is packed with carbon
+     C, and this is fed in through the hoppers B, as fast as
+     that which is present in the hearth of the furnace is used up.
+     Sulphur is introduced at A, and at the lower ends of the
+     tubes it is melted by the heat of the furnace and flows into
+     the hearth as a liquid. An electrical current is passed through
+     the carbon and melted sulphur from the electrodes E, heating
+     the charge. The vapors of carbon disulphide pass up through the
+     furnace and escape at D, from which they pass to a suitable
+     condensing apparatus.
+
+~Comparison of sulphur and oxygen.~ A comparison of the formulas and the
+chemical properties of corresponding compounds of oxygen and sulphur
+brings to light many striking similarities. The conduct of
+hydrosulphuric acid and water toward many substances has been seen to be
+very similar; the oxides and sulphides of the metals have analogous
+formulas and undergo many parallel reactions. Carbon dioxide and
+disulphide are prepared in similar ways and undergo many analogous
+reactions. It is clear, therefore, that these two elements are far more
+closely related to each other than to any of the other elements so far
+studied.
+
+~Selenium and tellurium.~ These two very uncommon elements are still more
+closely related to sulphur than is oxygen. They occur in comparatively
+small quantities and are usually found associated with sulphur and
+sulphides, either as the free elements or more commonly in combination
+with metals. They form compounds with hydrogen of the formulas H_{2}Se
+and H_{2}Te; these bodies are gases with properties very similar to
+those of H_{2}S. They also form oxides and oxygen acids which resemble
+the corresponding sulphur compounds. The elements even have allotropic
+forms corresponding very closely to those of sulphur. Tellurium is
+sometimes found in combination with gold and copper, and occasions some
+difficulties in the refining of these metals. The elements have very few
+practical applications.
+
+~Crystallography.~ In order to understand the difference between the two
+kinds of sulphur crystals, it is necessary to know something about
+crystals in general and the forms which they may assume. An examination
+of a large number of crystals has shown that although they may differ
+much in geometric form, they can all be considered as modifications of a
+few simple plans. The best way to understand the relation of one crystal
+to another is to look upon every crystal as having its faces and angles
+arranged in definite fashion about certain imaginary lines drawn
+through the crystal. These lines are called axes, and bear much the same
+relation to a crystal as do the axis and parallels of latitude and
+longitude to the earth and a geographical study of it. All crystals can
+be referred to one of six simple plans or systems, which have their axes
+as shown in the following drawings.
+
+The names and characteristics of these systems are as follows:
+
+1. Isometric or regular system (Fig. 46). Three equal axes, all at right
+angles.
+
+[Illustration: Fig. 46]
+
+2. Tetragonal system (Fig. 47). Two equal axes and one of different
+length, all at right angles to each other.
+
+[Illustration: Fig. 47]
+
+3. Orthorhombic system (Fig. 48). Three unequal axes, all at right
+angles to each other.
+
+[Illustration: Fig. 48]
+
+4. Monoclinic system (Fig. 49). Two axes at right angles, and a third at
+right angles to one of these, but inclined to the other.
+
+[Illustration: Fig. 49]
+
+5. Triclinic system (Fig. 50). Three axes, all inclined to each other.
+
+[Illustration: Fig. 50]
+
+6. Hexagonal system (Fig. 51). Three equal axes in the same plane
+intersecting at angles of 60°, and a fourth at right angles to all of
+these.
+
+[Illustration: Fig. 51]
+
+Every crystal can be imagined to have its faces and angles arranged in a
+definite way around one of these systems of axes. A cube, for instance,
+is referred to Plan 1, an axis ending in the center of each face; while
+in a regular octohedron an axis ends in each solid angle. These forms
+are shown in Fig. 46. It will be seen that both of these figures belong
+to the same system, though they are very different in appearance. In the
+same way, many geometric forms may be derived from each of the systems,
+and the light lines about the axes in the drawings show two of the
+simplest forms of each of the systems.
+
+In general a given substance always crystallizes in the same system, and
+two corresponding faces of each crystal of it always make the same angle
+with each other. A few substances, of which sulphur is an example,
+crystallize in two different systems, and the crystals differ in such
+physical properties as melting point and density. Such substances are
+said to be _dimorphous_.
+
+
+EXERCISES
+
+1. (a) Would the same amount of heat be generated by the combustion of
+1 g. of each of the allotropic modifications of sulphur? (b) Would the
+same amount of sulphur dioxide be formed in each case?
+
+2. Is the equation for the preparation of hydrosulphuric acid a
+reversible one? As ordinarily carried out, does the reaction complete
+itself?
+
+3. Suppose that hydrosulphuric acid were a liquid, would it be necessary
+to modify the method of preparation?
+
+4. Can sulphuric acid be used to dry hydrosulphuric acid? Give reason
+for answer.
+
+5. Does dry hydrosulphuric acid react with litmus paper? State reason
+for answer.
+
+6. How many grams of iron sulphide are necessary to prepare 100 l. of
+hydrosulphuric acid when the laboratory conditions are 17° and 740 mm.
+pressure?
+
+7. Suppose that the hydrogen in 1 l. of hydrosulphuric acid were
+liberated; what volume would it occupy, the gases being measured under
+the same conditions?
+
+8. Write the equations representing the reaction between hydrosulphuric
+acid and sodium hydroxide and ammonium hydroxide respectively.
+
+9. Show that the preparation of sulphur dioxide from a sulphite is
+similar in principle to the preparation of hydrogen sulphide.
+
+10. (a) Does dry sulphur dioxide react with litmus paper? (b) How
+can it be shown that a solution of sulphur dioxide in water acts like an
+acid?
+
+11. (a) Calculate the percentage composition of sulphurous anhydride
+and sulphuric anhydride. (b) Show how these two substances are in
+harmony with the law of multiple proportion.
+
+12. How many pounds of sulphur would be necessary in the preparation of
+100 lb. of 98% sulphuric acid?
+
+13. What weight of sulphur dioxide is necessary in the preparation of 1
+kg. of sodium sulphite?
+
+14. What weight of copper sulphate crystals can be obtained by
+dissolving 1 kg. of copper in sulphuric acid and crystallizing the
+product from water?
+
+15. Write the names and formulas of the oxides and oxygen acids of
+selenium and tellurium.
+
+16. In the commercial preparation of carbon disulphide, what is the
+function of the electric current?
+
+17. If the Gay-Lussac tower were omitted from the sulphuric acid
+factory, what effect would this have on the cost of production of
+sulphuric acid?
+
+
+
+
+CHAPTER XV
+
+PERIODIC LAW
+
+
+A number of the elements have now been studied somewhat closely. The
+first three of these, oxygen, hydrogen, and nitrogen, while having some
+physical properties in common with each other, have almost no point of
+similarity as regards their chemical conduct. On the other hand, oxygen
+and sulphur, while quite different physically, have much in common in
+their chemical properties.
+
+About eighty elements are now known. If all of these should have
+properties as diverse as do oxygen, hydrogen, and nitrogen, the study of
+chemistry would plainly be a very difficult and complicated one. If,
+however, the elements can be classified in groups, the members of which
+have very similar properties, the study will be very much simplified.
+
+~Earlier classification of the elements.~ Even at an early period efforts
+were made to discover some natural principle in accordance with which
+the elements could be classified. Two of these classifications may be
+mentioned here.
+
+1. _Classification into metals and non-metals._ The classification into
+metals and non-metals most naturally suggested itself. This grouping was
+based largely on physical properties, the metals being heavy, lustrous,
+malleable, ductile, and good conductors of heat and electricity.
+Elements possessing these properties are usually base-forming in
+character, and the ability to form bases came to be regarded as a
+characteristic property of the metals. The non-metals possessed
+physical properties which were the reverse of those of the metals, and
+were acid-forming in character.
+
+Not much was gained by this classification, and it was very imperfect.
+Some metals, such as potassium, are very light; some non-metals, such as
+iodine, have a high luster; some elements can form either an acid or a
+base.
+
+2. _Classification into triad families._ In 1825 Döbereiner observed
+that an interesting relation exists between the atomic weights of
+chemically similar elements. To illustrate, lithium, sodium, and
+potassium resemble each other very closely, and the atomic weight of
+sodium is almost exactly an arithmetical mean between those of the other
+two: (7.03 + 39.15)/2 = 23.09. In many chemical and physical properties
+sodium is midway between the other two.
+
+A number of triad families were found, but among eighty elements, whose
+atomic weights range all the way from 1 to 240, such agreements might be
+mere chance. Moreover many elements did not appear to belong to such
+families.
+
+~Periodic division.~ In 1869 the Russian chemist Mendeléeff devised an
+arrangement of the elements based on their atomic weights, which has
+proved to be of great service in the comparative study of the elements.
+A few months later the German, Lothar Meyer, independently suggested the
+same ideas. This arrangement brought to light a great generalization,
+now known as the _periodic law_. An exact statement of the law will be
+given after the method of arranging the elements has been described.
+
+[Illustration: DMITRI IVANOVITCH MENDELÉEFF (Russian) (1834-1907)
+
+Author of the periodic law; made many investigations on the physical
+constants of elements and compounds; wrote an important book entitled
+"Principles of Chemistry"; university professor and government
+official]
+
+~Arrangement of the periodic table.~ The arrangement suggested by
+Mendeléeff, modified somewhat by more recent investigations, is as
+follows: Beginning with lithium, which has an atomic weight of 7, the
+elements are arranged in a horizontal row in the order of their atomic
+weights, thus:
+
+~Li (7.03), Be (9.1), B (11), C (12), N (14.04), O (16), F (19).~
+
+These seven elements all differ markedly from each other. The eighth
+element, sodium, is very similar to lithium. It is placed just under
+lithium, and a new row follows:
+
+~Na(23.05), Mg (24.36), Al (27.1), Si (28.4), P (31), S (32.06),
+Cl(35.45).~
+
+When the fifteenth element, potassium, is reached, it is placed under
+sodium, to which it is very similar, and serves to begin a third row:
+
+~K (39.15), Ca (40.1), Sc (44.1,) Ti (48.1), V (51.2), Cr (52.1), Mn(55).~
+
+Not only is there a strong similarity between lithium, sodium, and
+potassium, which have been placed in a vertical row because of this
+resemblance, but the elements in the other vertical rows exhibit much of
+the same kind of similarity among themselves, and evidently form little
+natural groups.
+
+The three elements following manganese, namely, iron, nickel, and
+cobalt, have atomic weights near together, and are very similar
+chemically. They do not strongly resemble any of the elements so far
+considered, and are accordingly placed in a group by themselves,
+following manganese. A new row is begun with copper, which somewhat
+resembles the elements of the first vertical column. Following the fifth
+and seventh rows are groups of three closely related elements, so that
+the completed arrangement has the appearance represented in the table on
+page 168.
+
+
+THE PERIODIC ARRANGEMENT OF THE ELEMENTS
+
+--------+-----------+-----------+-----------+-----------+-----------+
+Periods |   GROUP   |   GROUP   |   GROUP   |   GROUP   |   GROUP   |
+        |     0     |     I     |     II    |    III    |     IV    |
+        |A         B|A         B|A         B|A         B|A         B|
+--------+-----------+-----------+-----------+-----------+-----------+
+1       |H==1.008   |           |           |           |           |
+2       |He=4       |Li=7.03    |Be=9.1     |B=11       |C=12       |
+--------+-----------+-----------+-----------+-----------+-----------+
+3       |      Ne=20|Na=23.05   |   Mg=24.36|    AL=27.1|    Si=28.4|
+--------+-----------+-----------+-----------+-----------+-----------+
+4       |A=39.9     |K=39.15    |Ca=40.1    |Sc=44.1    |Ti=48.1    |
+        |           |           |           |           |           |
+        |           |           |           |           |           |
+--------+-----------+-----------+-----------+-----------+-----------+
+5       |           |    Cu=63.6|    Zn=65.4|      Ga=70|    Ge=72.5|
+--------+-----------+-----------+-----------+-----------+-----------+
+6       |Kr=81.8    |Rb=85.5    |Sr=87.6    |Y=89       |Zr=90.6    |
+        |           |           |           |           |           |
+        |           |           |           |           |           |
+--------+-----------+-----------+-----------+-----------+-----------+
+7       |           |  Ag=107.93|   Cd=112.4|     In=115|     Sn=119|
+--------+-----------+-----------+-----------+-----------+-----------+
+8       |X=128      |Cs=132.9   |Ba=137.4   |La=138.9   |Ce=Yb*     |
+        |           |           |           |           |140.25-173 |
+        |           |           |           |           |           |
+--------+-----------+-----------+-----------+-----------+-----------+
+9       |   Au=197.2|     Hg=200|   Tl=204.1|   Pb=206.9|   Bi=208.5|
+--------+-----------+-----------+-----------+-----------+-----------+
+10      |           |           |Ra=225     |           |Th=232.5   |
+--------+-----------+-----------+-----------+-----------+-----------+
+        |           |   R_{2}O  |     RO    |R_{2}O_{3} |   RO_{2}  |
+        |           |     RH    |   RH_{2}  |  RH_{3}   |   RH_{4}  |
+--------+-----------+-----------+-----------+-----------+-----------+
+
+==================part 2==============
+
+--------+-----------+-----------+-----------+-----------+
+Periods |   GROUP   |   GROUP   |   GROUP   |   GROUP   |
+        |     V     |     VI    |    VII    |    VIII   |
+        |A         B|A         B|A         B|           |
+--------+-----------+-----------+-----------+-----------+
+1       |           |           |           |           |
+2       |N=14.04    |O=16       |F=19       |           |
+--------+-----------+-----------+-----------+-----------+
+3       |       P=31|    S=32.06|   Cl=35.45|           |
+--------+-----------+-----------+-----------+-----------+
+4       |V=51.2     |Cr=52.1    |Mn=55      |Fe=55.9    |
+        |           |           |           |Ni=58.7    |
+        |           |           |           |Co=59      |
+--------+-----------+-----------+-----------+-----------+
+5       |      As=75|    Se=79.2|   Br=79.96|           |
+--------+-----------+-----------+-----------+-----------+
+6       |Cb=94      |Mo=96      |           |Ru=101.7   |
+        |           |           |           |Rh=103     |
+        |           |           |           |Pd=106.5   |
+--------+-----------+-----------+-----------+-----------+
+7       |   Sb=120.2|   Te=127.6|   I=126.97|           |
+--------+-----------+-----------+-----------+-----------+
+8       |Ta=183     |W=184      |           |Os=191     |
+        |           |           |           |Ir=193     |
+        |           |           |           |Pt=194.8   |
+--------+-----------+-----------+-----------+-----------+
+9       |           |           |           |           |
+--------+-----------+-----------+-----------+-----------+
+10      | U=238.5   |           |           |           |
+--------+-----------+-----------+-----------+-----------+
+        | R_{2}O_{5}|   RO_{3}  | R_{2}O_{7}|   RO_{4}  |
+        |   RH_{3}  |   RH_{2}  |     RH    |           |
+--------+-----------+-----------+-----------+-----------+
+
+[* This includes a number of elements whose atomic weights lie
+between 140 and 173, but which have not been accurately studied, and
+so their proper arrangement is uncertain.]
+
+~Place of the atmospheric elements.~ When argon was discovered it was seen
+at once that there was no place in the table for an element of atomic
+weight approximately 40. When the other inactive elements were found,
+however, it became apparent that they form a group just preceding Group
+1. They are accordingly arranged in this way in Group 0 (see table on
+opposite page). A study of this table brings to light certain very
+striking facts.
+
+~Properties of elements vary with atomic weights.~ There is evidently a
+close relation between the properties of an element and its atomic
+weight. Lithium, at the beginning of the first group, is a very strong
+base-forming element, with pronounced metallic properties. Beryllium,
+following lithium, is less strongly base-forming, while boron has some
+base-forming and some acid-forming properties. In carbon all
+base-forming properties have disappeared, and the acid-forming
+properties are more marked than in boron. These become still more
+emphasized as we pass through nitrogen and oxygen, until on reaching
+fluorine we have one of the strongest acid-forming elements. The
+properties of these seven elements therefore vary regularly with their
+atomic weights, or, in mathematical language, are regular functions of
+them.
+
+~Periodic law.~ The properties of the first seven elements vary
+_continuously_--that is steadily--away from base-forming and toward
+acid-forming properties. If lithium had the smallest atomic weight of
+any of the elements, and fluorine the greatest, so that in passing from
+one to the other we had included all the elements, we could say that the
+properties of elements are continuous functions of their atomic weights.
+But fluorine is an element of small atomic weight, and the one following
+it, sodium, breaks the regular order, for in it reappear all the
+characteristic properties of lithium. Magnesium, following sodium, bears
+much the same relation to beryllium that sodium does to lithium, and
+the properties of the elements in the second row vary much as they do in
+the first row until potassium is reached, when another repetition
+begins. The properties of the elements do not vary continuously,
+therefore, with atomic weights, but at regular intervals there is a
+repetition, or _period_. This generalization is known as the _periodic
+law_, and may be stated thus: _The properties of elements are periodic
+functions of their atomic weights._
+
+~The two families in a group.~ While all the elements in a given vertical
+column bear a general resemblance to each other, it has been noticed
+that those belonging to periods having even numbers are very strikingly
+similar to each other. They are placed at the left side of the group
+columns. In like manner, the elements belonging to the odd periods are
+very similar and are arranged at the right side of the group columns.
+Thus calcium, strontium, and barium are very much alike; so, too, are
+magnesium, zinc, and cadmium. The resemblance between calcium and
+magnesium, or strontium and zinc, is much less marked. This method of
+arrangement therefore divides each group into two families, each
+containing four or five members, between which there is a great
+similarity.
+
+~Family resemblances.~ Let us now inquire more closely in what respects
+the elements of a family resemble each other.
+
+1. _Valence._ In general the valence of the elements in a family is the
+same, and the formulas of their compounds are therefore similar. If we
+know that the formula of sodium chloride is NaCl, it is pretty certain
+that the formula of potassium chloride will be KCl--not KCl_{2} or
+KCl_{3}. The general formulas R_{2}O, RO, etc., placed below the
+columns show the formulas of the oxides of the elements in the column
+provided they form oxides. In like manner the formulas RH, RH_{2}, etc.,
+show the composition of the compounds formed with hydrogen or chlorine.
+
+2. _Chemical properties._ The chemical properties of the members of a
+family are quite similar. If one member is a metal, the others usually
+are; if one is a non-metal, so, too, are the others. The families in the
+first two columns consist of metals, while the elements found in the
+last two columns form acids. There is in addition a certain regularity
+in properties of the elements in each family. If the element at the head
+of the family is a strong acid-forming element, this property is likely
+to diminish gradually, as we pass to the members of the family with
+higher atomic weights. Thus phosphorus is strongly acid-forming, arsenic
+less so, antimony still less so, while bismuth has almost no
+acid-forming properties. We shall meet with many illustrations of this
+fact.
+
+3. _Physical properties._ In the same way, the physical properties of
+the members of a family are in general somewhat similar, and show a
+regular gradation as we pass from element to element in the family. Thus
+the densities of the members of the magnesium family are
+
+    Mg = 1.75, Zn = 7.00, Cd = 8.67, Hg = 13.6.
+
+Their melting points are
+
+    Mg = 750°, Zn = 420°, Cd = 320°, Hg = -39.5°.
+
+~Value of the periodic law.~ The periodic law has proved of much value in
+the development of the science of chemistry.
+
+1. _It simplifies study._ It is at once evident that such regularities
+very much simplify the study of chemistry. A thorough study of one
+element of a family makes the study of the other members a much easier
+task, since so many of the properties and chemical reactions of the
+elements are similar. Thus, having studied the element sulphur in some
+detail, it is not necessary to study selenium and tellurium so closely,
+for most of their properties can be predicted from the relation which
+they sustain to sulphur.
+
+2. _It predicts new elements._ When the periodic law was first
+formulated there were a number of vacant places in the table which
+evidently belonged to elements at that time unknown. From their position
+in the table, Mendeléeff predicted with great precision the properties
+of the elements which he felt sure would one day be discovered to fill
+these places. Three of them, scandium, germanium, and gallium, were
+found within fifteen years, and their properties agreed in a remarkable
+way with the predictions of Mendeléeff. There are still some vacant
+places in the table, especially among the heavier elements.
+
+3. _It corrects errors._ The physical constants of many of the elements
+did not at first agree with those demanded by the periodic law, and a
+further study of many such cases showed that errors had been made. The
+law has therefore done much service in indicating probable error.
+
+~Imperfections of the law.~ There still remain a good many features which
+must be regarded as imperfections in the law. Most conspicuous is the
+fact that the element hydrogen has no place in the table. In some of the
+groups elements appear in one of the families, while all of their
+properties show that they belong in the other. Thus sodium belongs with
+lithium and not with copper; fluorine belongs with chlorine and not with
+manganese. There are two instances where the elements must be
+transposed in order to make them fit into their proper group. According
+to their atomic weights, tellurium should follow iodine, and argon
+should follow potassium. Their properties show in each case that this
+order must be reversed. The table separates some elements altogether
+which, in many respects have closely agreeing properties. Iron,
+chromium, and manganese are all in different groups, although they are
+similar in many respects.
+
+The system is therefore to be regarded as but a partial and imperfect
+expression of some very important and fundamental relation between the
+substances which we know as elements, the exact nature of this relation
+being as yet not completely clear to us.
+
+
+EXERCISES
+
+1. Suppose that an element were discovered that filled the blank in
+Group O, Period 5; what properties would it probably have?
+
+2. Suppose that an element were discovered that filled the blank in
+Group VI, Period 9, family B; what properties would it have?
+
+3. Sulphur and oxygen both belong in Group VI, although in different
+families; in what respects are the two similar?
+
+
+
+
+CHAPTER XVI
+
+THE CHLORINE FAMILY
+
+
+==================================================================
+              |        |         |         |
+              | ATOMIC | MELTING | BOILING | COLOR AND STATE
+              | WEIGHT |  POINT  |  POINT  |
+______________|________|_________|_________|______________________
+              |        |         |         |
+Fluorine (F)  |  19.00 |   -223° |   -187° | Pale yellowish gas.
+Chlorine (Cl) |  35.45 |   -102° |  -33.6° | Greenish-yellow gas.
+Bromine (Br)  |  79.96 |     -7° |     59° | Red liquid.
+Iodine (I)    | 126.97 |    107° |    175° | Purplish-black solid.
+==================================================================
+
+~The family.~ The four elements named in the above table form a strongly
+marked family of elements and illustrate very clearly the way in which
+the members of a family in a periodic group resemble each other, as well
+as the character of the differences which we may expect to find between
+the individual members.
+
+1. _Occurrence._ These elements do not occur in nature in the free
+state. The compounds of the last three elements of the family are found
+extensively in sea water, and on this account the name _halogens_,
+signifying "producers of sea salt," is sometimes applied to the family.
+
+2. _Properties._ As will be seen by reference to the table, the melting
+points and boiling points of the elements of the family increase with
+their atomic weights. A somewhat similar gradation is noted in their
+color and state. One atom of each of the elements combines with one atom
+of hydrogen to form acids, which are gases very soluble in water. The
+affinity of the elements for hydrogen is in the inverse order of their
+atomic weights, fluorine having the strongest affinity and iodine the
+weakest. Only chlorine and iodine form oxides, and those of the former
+element are very unstable. The elements of the group are univalent in
+their compounds with hydrogen and the metals.
+
+
+FLUORINE
+
+~Occurrence.~ The element fluorine occurs in nature most abundantly as the
+mineral fluorspar (CaF_{2}), as cryolite (Na_{3}AlF_{6}), and in the
+complex mineral apatite (3 Ca_{3}(PO_{4})_{2}·CaF_{2}).
+
+~Preparation.~ All attempts to isolate the element resulted in failure
+until recent years. Methods similar to those which succeed in the
+preparation of the other elements of the family cannot be used; for as
+soon as the fluorine is liberated it combines with the materials of
+which the apparatus is made or with the hydrogen of the water which is
+always present. The preparation of fluorine was finally accomplished by
+the French chemist Moissan by the electrolysis of hydrofluoric acid.
+Perfectly dry hydrofluoric acid (HF) was condensed to a liquid and
+placed in a U-shaped tube made of platinum (or copper), which was
+furnished with electrodes and delivery tubes, as shown in Fig. 52. This
+liquid is not an electrolyte, but becomes such when potassium fluoride
+is dissolved in it. When this solution was electrolyzed hydrogen was set
+free at the cathode and fluorine at the anode.
+
+[Illustration: Fig. 52]
+
+~Properties.~ Fluorine is a gas of slightly yellowish color, and can be
+condensed to a liquid boiling at -187° under atmospheric pressure. It
+solidifies at -223°. It is extremely active chemically, being the most
+active of all the elements at ordinary temperatures.
+
+It combines with all the common elements save oxygen, very often with
+incandescence and the liberation of much heat. It has a strong affinity
+for hydrogen and is able to withdraw it from its compounds with other
+elements. Because of its great activity it is extremely poisonous.
+Fluorine does not form any oxides, neither does it form any oxygen
+acids, in which respects it differs from the other members of the
+family.
+
+~Hydrofluoric acid~ (HF). Hydrofluoric acid is readily obtained from
+fluorspar by the action of concentrated sulphuric acid. The equation is
+
+    CaF_{2} + H_{2}SO_{4} = CaSO_{4} + 2HF.
+
+In its physical properties it resembles the binary acids of the other
+elements of this family, being, however, more easily condensed to a
+liquid. The anhydrous acid boils at 19° and can therefore be prepared at
+ordinary pressures. It is soluble in all proportions in water, and a
+concentrated solution--about 50%--is prepared for the market. Its fumes
+are exceedingly irritating to the respiratory organs, and several
+chemists have lost their lives by accidentally breathing them.
+
+[Illustration: HENRI MOISSAN (French) (1853-1907)
+
+Famous for his work with the electric furnace at high temperatures;
+prepared artificial diamonds, together with many new binary compounds
+such as carbides, silicides, borides, and nitrides; isolated fluorine
+and studied its properties and its compounds very thoroughly]
+
+~Chemical properties.~ Hydrofluoric acid, like other strong acids, readily
+acts on bases and metallic oxides and forms the corresponding fluorides.
+It also dissolves certain metals such as silver and copper. It acts very
+vigorously upon organic matter, a single drop of the concentrated acid
+making a sore on the skin which is very painful and slow in healing. Its
+most characteristic property is its action upon silicon dioxide
+(SiO_{2}), with which it forms water and the gas silicon tetrafluoride
+(SiF_{4}), as shown in the equation
+
+    SiO_{2} + 4HF = SiF_{4} + 2H_{2}O.
+
+Glass consists of certain compounds of silicon, which are likewise acted
+on by the acid so that it cannot be kept in glass bottles. It is
+preserved in flasks made of wax or gutta-percha.
+
+     ~Etching.~ Advantage is taken of this reaction in etching designs
+     upon glass. The glass vessel is painted over with a protective
+     paint upon which the acid will not act, the parts which it is
+     desired to make opaque being left unprotected. A mixture of
+     fluorspar and sulphuric acid is then painted over the vessel
+     and after a few minutes the vessel is washed clean. Wherever
+     the hydrofluoric acid comes in contact with the glass it acts
+     upon it, destroying its luster and making it opaque, so that
+     the exposed design will be etched upon the clear glass. Frosted
+     glass globes are often made in this way.
+
+     The etching may also be effected by covering the glass with a
+     thin layer of paraffin, cutting the design through the wax and
+     then exposing the glass to the fumes of the acid.
+
+~Salts of hydrofluoric acid,--fluorides.~ A number of the fluorides are
+known, but only one of them, calcium fluoride (CaF_{2}), is of
+importance. This is the well-known mineral fluorspar.
+
+
+CHLORINE
+
+~Historical.~ While studying the action of hydrochloric acid upon the
+mineral pyrolusite, in 1774, Scheele obtained a yellowish, gaseous
+substance to which he gave a name in keeping with the phlogiston theory
+then current. Later it was supposed to be a compound containing oxygen.
+In 1810, however, the English chemist Sir Humphry Davy proved it to be
+an element and named it chlorine.
+
+~Occurrence.~ Chlorine does not occur free in nature, but its compounds
+are widely distributed. For the most part it occurs in combination with
+the metals in the form of chlorides, those of sodium, potassium, and
+magnesium being most abundant. Nearly all salt water contains these
+substances, particularly sodium chloride, and very large salt beds
+consisting of chlorides are found in many parts of the world.
+
+~Preparation.~ Two general methods of preparing chlorine may be mentioned,
+namely, the laboratory method and the electrolytic method.
+
+1. _Laboratory method._ In the laboratory chlorine is made by warming
+the mineral pyrolusite (manganese dioxide, MnO_{2}) with concentrated
+hydrochloric acid. The first reaction, which seems to be similar to the
+action of acids upon oxides in general, is expressed in the equation
+
+    MnO_{2} + 4HCl = MnCl_{4} + 2H_{2}O.
+
+The manganese compound so formed is very unstable, however, and breaks
+clown according to the equation
+
+    MnCl_{4} = MnCl_{2} + 2Cl.
+
+Instead of using hydrochloric acid in the preparation of chlorine it
+will serve just as well to use a mixture of sodium chloride and
+sulphuric acid, since these two react to form hydrochloric acid. The
+following equations will then express the changes:
+
+    (1) 2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
+
+    (2) MnO_{2} + 4 HCl = MnCl_{2} + 2Cl + 2H_{2}O.
+
+    (3) MnCl_{2} + H_{2}SO_{4} = MnSO_{4} + 2HCl.
+
+Combining these equations, the following equation expressing the
+complete reaction is obtained:
+
+    2NaCl + MnO_{2} + 2H_{2}SO_{4} = MnSO_{4} + Na_{2}SO_{4} + 2H_{2}O + 2Cl.
+
+Since the hydrochloric acid liberated in the third equation is free to
+act upon manganese dioxide, it will be seen that all of the chlorine
+originally present in the sodium chloride is set free.
+
+     The manganese dioxide and the hydrochloric acid are brought
+     together in a flask, as represented in Fig. 53, and a gentle
+     heat is applied. The rate of evolution of the gas is regulated
+     by the amount of heat applied, and the gas is collected by
+     displacement of air. As the equations show, only half of the
+     chlorine present in the hydrochloric acid is liberated.
+
+[Illustration: Fig. 53]
+
+2. _Electrolytic method._ Under the discussion of electrolysis (p. 102)
+it was shown that when a solution of sodium chloride is electrolyzed
+chlorine is evolved at the anode, while the sodium set free at the
+cathode reacts with the water to form hydrogen, which is evolved, and
+sodium hydroxide, which remains in solution. A great deal of the
+chlorine required in the chemical industries is now made in this way in
+connection with the manufacture of sodium hydroxide.
+
+~Physical properties.~ Chlorine is a greenish-yellow gas, which has a
+peculiar suffocating odor and produces a very violent effect upon the
+throat and lungs. Even when inhaled in small quantities it often
+produces all the symptoms of a hard cold, and in larger quantities may
+have serious and even fatal action. It is quite heavy (density = 2.45)
+and can therefore be collected by displacement of air. One volume of
+water under ordinary conditions dissolves about three volumes of
+chlorine. The gas is readily liquefied, a pressure of six atmospheres
+serving to liquefy it at 0°. It forms a yellowish liquid which
+solidifies at -102°.
+
+~Chemical properties.~ At ordinary temperatures chlorine is far more
+active chemically than any of the elements we have so far considered,
+with the exception of fluorine; indeed, it is one of the most active of
+all elements.
+
+1. _Action on metals._ A great many metals combine directly with
+chlorine, especially when hot. A strip of copper foil heated in a burner
+flame and then dropped into chlorine burns with incandescence. Sodium
+burns brilliantly when heated strongly in slightly moist chlorine. Gold
+and silver are quickly tarnished by the gas.
+
+2. _Action on non-metals._ Chlorine has likewise a strong affinity for
+many of the non-metals. Thus phosphorus burns in a current of the gas,
+while antimony and arsenic in the form of a fine powder at once burst
+into flame when dropped into jars of the gas. The products formed in all
+cases where chlorine combines with another element are called
+_chlorides_.
+
+3. _Action on hydrogen._ Chlorine has a strong affinity for hydrogen,
+uniting with it to form hydrochloric acid. A jet of hydrogen burning in
+the air continues to burn when introduced into a jar of chlorine, giving
+a somewhat luminous flame. A mixture of the two gases explodes violently
+when a spark is passed through it or when it is exposed to bright
+sunlight. In the latter case it is the light and not the heat which
+starts the action.
+
+4. _Action on substances containing hydrogen._ Not only will chlorine
+combine directly with free hydrogen but it will often abstract the
+element from its compounds. Thus, when chlorine is passed into a
+solution containing hydrosulphuric acid, sulphur is precipitated and
+Hydrochloric acid formed. The reaction is shown by the following
+equation:
+
+    H_{2}S + 2Cl = 2HCl + S.
+
+With ammonia the action is similar:
+
+    NH_{3} + 3Cl = 3HCl + N.
+
+The same tendency is very strikingly seen in the action of chlorine upon
+turpentine. The latter substance is largely made up of compounds having
+the composition represented by the formula C_{10}H_{16}. When a strip of
+paper moistened with warm turpentine is placed in a jar of chlorine
+dense fumes of hydrochloric acid appear and a black deposit of carbon is
+formed. Even water, which is a very stable compound, can be decomposed
+by chlorine, the oxygen being liberated. This may be shown in the
+following way:
+
+[Illustration: Fig. 54]
+
+     If a long tube of rather large diameter is filled with a strong
+     solution of chlorine in water and inverted in a vessel of the
+     same solution, as shown in Fig. 54, and the apparatus is placed
+     in bright sunlight, very soon bubbles of a gas will be observed
+     to rise through the solution and collect in the tube. An
+     examination of this gas will show that it is oxygen. It is
+     liberated from water in accordance with the following equation:
+
+     H_{2}O + 2Cl = 2HCl + O.
+
+5. _Action on color substances,--bleaching action._ If strips of
+brightly colored cloth or some highly colored flowers are placed in
+quite dry chlorine, no marked change in color is noticed as a rule. If,
+however, the cloth and flowers are first moistened, the color rapidly
+disappears, that is, the objects are bleached. Evidently the moisture as
+well as the chlorine is concerned in the action, and a study of the case
+shows that the chlorine has combined with the hydrogen of the water. The
+oxygen set free oxidizes the color substance, converting it into a
+colorless compound. It is evident from this explanation that chlorine
+will only bleach those substances which are changed into colorless
+compounds by oxidation.
+
+6. _Action as a disinfectant._ Chlorine has also marked germicidal
+properties, and the free element, as well as compounds from which it is
+easily liberated, are used as disinfectants.
+
+~Nascent state.~ It will be noticed that oxygen when set free from water
+by chlorine is able to do what ordinary oxygen cannot do, for both the
+cloth and the flowers are unchanged in the air which contains oxygen. It
+is generally true that the activity of an element is greatest at the
+instant of liberation from its compounds. To express this fact elements
+at the instant of liberation are said to be in the _nascent state_. It
+is nascent oxygen which does the bleaching.
+
+~Hydrochloric acid~ (_muriatic acid_) (HCl). The preparation of
+hydrochloric acid may be discussed under two general heads:
+
+1. _Laboratory preparation._ The product formed by the burning of
+hydrogen in chlorine is the gas hydrochloric acid. This substance is
+much more easily obtained, however, by treating common salt (sodium
+chloride) with sulphuric acid. The following equation shows the
+reaction:
+
+    2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
+
+The dry salt is placed in a flask furnished with a funnel tube and an
+exit tube, the sulphuric acid is added, and the flask gently warmed. The
+hydrochloric acid gas is rapidly given off and can be collected by
+displacement of air. The same apparatus can be used as was employed in
+the preparation of chlorine (Fig. 53).
+
+     When a _solution_ of salt is treated with sulphuric acid there
+     is no very marked action. The hydrochloric acid formed is very
+     soluble in water, and so does not escape from the solution;
+     hence a state of equilibrium is soon reached between the four
+     substances represented in the equation. When _concentrated_
+     sulphuric acid, in which hydrochloric acid is not soluble, is
+     poured upon dry salt the reaction is complete.
+
+2. _Commercial preparation._ Commercially, hydrochloric acid is prepared
+in connection with the manufacture of sodium sulphate, the reaction
+being the same as that just given. The reaction is carried out in a
+furnace, and the hydrochloric acid as it escapes in the form of gas is
+passed into water in which it dissolves, the solution forming the
+hydrochloric acid of commerce. When the materials are pure a colorless
+solution is obtained. The most concentrated solution has a density of
+1.2 and contains 40% HCl. The commercial acid, often called _muriatic
+acid_, is usually colored yellow by impurities.
+
+~Composition of hydrochloric acid.~ When a solution of hydrochloric acid
+is electrolyzed in an apparatus similar to the one in which water was
+electrolyzed (Fig. 18), chlorine collects at the anode and hydrogen at
+the cathode. At first the chlorine dissolves in the water, but soon the
+water in the one tube becomes saturated with it, and if the stopcocks
+are left open until this is the case, and are then closed, it will be
+seen that the two gases are set free in equal volumes.
+
+When measured volumes of the two gases are caused to unite it is found
+that one volume of hydrogen combines with one of chlorine. Other
+experiments show that the volume of hydrochloric acid formed is just
+equal to the sum of the volumes of hydrogen and chlorine. Therefore one
+volume of hydrogen combines with one volume of chlorine to form two
+volumes of hydrochloric acid gas. Since chlorine is 35.18 times as heavy
+as hydrogen, it follows that one part of hydrogen by weight combines
+with 35.18 parts of chlorine to form 36.18 parts of hydrochloric acid.
+
+~Physical properties.~ Hydrochloric acid is a colorless gas which has an
+irritating effect when inhaled, and possesses a sour, biting taste, but
+no marked odor. It is heavier than air (density = 1.26) and is very
+soluble in water. Under standard conditions 1 volume of water dissolves
+about 500 volumes of the gas. On warming such a solution the gas
+escapes, until at the boiling point the solution contains about 20% by
+weight of HCl. Further boiling will not drive out any more acid, but the
+solution will distill with unchanged concentration. A more dilute
+solution than this will lose water on boiling until it has reached the
+same concentration, 20%, and will then distill unchanged. Under high
+pressure the gas can be liquefied, 28 atmospheres being required at 0°.
+Under these conditions it forms a colorless liquid which is not very
+active chemically. It boils at -80° and solidifies at -113°. The
+solution of the gas in water is used almost entirely in the place of the
+gas itself, since it is not only far more convenient but also more
+active.
+
+~Chemical properties.~ The most important chemical properties of
+hydrochloric acid are the following:
+
+1. _Action as an acid._ In aqueous solution hydrochloric acid has very
+strong acid properties; indeed, it is one of the strongest acids. It
+acts upon oxides and hydroxides, converting them into salts:
+
+    NaOH + HCl = NaCl + H_{2}O,
+    CuO + 2HCl = CuCl_{2} + H_{2}O.
+
+It acts upon many metals, forming chlorides and liberating hydrogen:
+
+    Zn + 2HCl = ZnCl_{2} + 2H,
+    Al + 3HCl = AlCl_{3} + 3H.
+
+Unlike nitric and sulphuric acids it has no oxidizing action, so that
+when it acts on metals hydrogen is always given off.
+
+2. _Relation to combustion._ Hydrochloric acid gas is not readily
+decomposed, and is therefore neither combustible nor a supporter of
+combustion.
+
+3. _Action on oxidizing agents._ Although hydrochloric acid is
+incombustible, it can be oxidized under some circumstances, in which
+case the hydrogen combines with oxygen, while the chlorine is set free.
+Thus, when a solution of hydrochloric acid acts upon manganese dioxide
+part of the chlorine is set free:
+
+    MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + 2Cl.
+
+~Aqua regia.~ It has been seen that when nitric acid acts as an oxidizing
+agent it usually decomposes, as represented in the equation
+
+    2HNO_{3} = H_{2}O + 2NO + 3O.
+
+The oxygen so set free may act on hydrochloric acid:
+
+    6HCl + 3O = 3H_{2}O + 6Cl.
+
+The complete equation therefore is
+
+    2HNO_{3} + 6HCl = 4H_{2}O + 2NO + 6Cl.
+
+When concentrated nitric and hydrochloric acids are mixed this reaction
+goes on slowly, chlorine and some other substances not represented in
+the equation being formed. The mixture is known as _aqua regia_ and is
+commonly prepared by adding one volume of nitric acid to three volumes
+of hydrochloric acid. It acts more powerfully upon metals and other
+substances than either of the acids separately, and owes its strength
+not to acid properties but to the action of the nascent chlorine which
+it liberates. Consequently, when it acts upon metals such as gold it
+converts them into chlorides, and the reaction can be represented by
+such equations as
+
+    Au + 3Cl = AuCl_{3}.
+
+~Salts of hydrochloric acid,--chlorides.~ The chlorides of all the metals
+are known and many of them are very important compounds. Some of them
+are found in nature, and all can be prepared by the general method of
+preparing salts. Silver chloride, lead chloride, and mercurous chloride
+are insoluble in water and acids, and can be prepared by adding
+hydrochloric acid to solutions of compounds of the respective elements.
+While the chlorides have formulas similar to the fluorides, their
+properties are often quite different. This is seen in the solubility of
+the salts. Those metals whose chlorides are insoluble form soluble
+fluorides, while many of the metals which form soluble chlorides form
+insoluble fluorides.
+
+~Compounds of chlorine with oxygen and hydrogen.~ Chlorine combines with
+oxygen and hydrogen to form four different acids. They are all quite
+unstable, and most of them cannot be prepared in pure form; their salts
+can easily be made, however, and some of them will be met with in the
+study of the metals. The formulas and names of these acids are as
+follows:
+
+    HClO       hypochlorous acid.
+
+    HClO_{2}   chlorous acid.
+
+    HClO_{3}   chloric acid.
+
+    HClO_{4}   perchloric acid.
+
+~Oxides of chlorine.~ Two oxides are known, having the formulas Cl_{2}O
+and ClO_{2}. They decompose very easily and are good oxidizing agents.
+
+
+BROMINE
+
+~Historical.~ Bromine was discovered in 1826 by the French chemist
+Ballard, who isolated it from sea salt. He named it bromine (stench)
+because of its unbearable fumes.
+
+~Occurrence.~ Bromine occurs almost entirely in the form of bromides,
+especially as sodium bromide and magnesium bromide, which are found in
+many salt springs and salt deposits. The Stassfurt deposits in Germany
+and the salt waters of Ohio and Michigan are especially rich in
+bromides.
+
+~Preparation of bromine.~ The laboratory method of preparing bromine is
+essentially different from the commercial method.
+
+[Illustration Fig. 55]
+
+1. _Laboratory method._ As in the case of chlorine, bromine can be
+prepared by the action of hydrobromic acid (HBr) on manganese dioxide.
+Since hydrobromic acid is not an article of commerce, a mixture of
+sulphuric acid and a bromide is commonly substituted for it. The
+materials are placed in a retort arranged as shown in Fig. 55. The end
+of the retort just touches the surface of the water in the test tube. On
+heating, the bromine distills over and is collected in the cold
+receiver. The equation is
+
+    2NaBr + 2H_{2}SO_{4} + MnO_{2} = Na_{2}SO_{4} + MnSO_{4}
+      + 2H_{2}O + 2Br.
+
+2. _Commercial method._ Bromine is prepared commercially from the waters
+of salt wells which are especially rich in bromides. On passing a
+current of electricity through such waters the bromine is first
+liberated. Any chlorine liberated, however, will assist in the reaction,
+since free chlorine decomposes bromides, as shown in the equation
+
+    NaBr + Cl = NaCl + Br.
+
+When the water containing the bromine is heated, the liberated bromine
+distills over into the receiver.
+
+~Physical properties.~ Bromine is a dark red liquid about three times as
+heavy as water. Its vapor has a very offensive odor and is most
+irritating to the eyes and throat. The liquid boils at 59° and
+solidifies at -7°; but even at ordinary temperatures it evaporates
+rapidly, forming a reddish-brown gas very similar to nitrogen peroxide
+in appearance. Bromine is somewhat soluble in water, 100 volumes of
+water under ordinary conditions dissolving 1 volume of the liquid. It is
+readily soluble in carbon disulphide, forming a yellow solution.
+
+~Chemical properties and uses.~ In chemical action bromine is very similar
+to chlorine. It combines directly with many of the same elements with
+which chlorine unites, but with less energy. It combines with hydrogen
+and takes away the latter element from some of its compounds, but not
+so readily as does chlorine. Its bleaching properties are also less
+marked.
+
+Bromine finds many uses in the manufacture of organic drugs and
+dyestuffs and in the preparation of bromides.
+
+~Hydrobromic acid (HBr).~ When sulphuric acid acts upon a bromide
+hydrobromic acid is set free:
+
+    2NaBr + H_{2}SO_{4} = Na_{2}SO_{4} + 2HBr.
+
+At the same time some bromine is set free, as may be seen from the red
+fumes which appear, and from the odor. The explanation of this is found
+in the fact that hydrobromic acid is much less stable than hydrochloric
+acid, and is therefore more easily oxidized. Concentrated sulphuric acid
+is a good oxidizing agent, and oxidizes a part of the hydrobromic acid,
+liberating bromine:
+
+    H_{2}SO_{4} + 2HBr = 2H_{2}O + SO_{2} + 2Br.
+
+     ~Preparation of pure hydrobromic acid.~ A convenient way to make
+     pure hydrobromic acid is by the action of bromine upon moist
+     red phosphorus. This can be done with the apparatus shown in
+     Fig. 56. Bromine is put into the dropping funnel A, and red
+     phosphorus, together with enough water to cover it, is placed
+     in the flask B. By means of the stopcock the bromine is
+     allowed to flow drop by drop into the flask, the reaction
+     taking place without the application of heat. The equations are
+
+    (1) P + 3Br = PBr_{3},
+
+    (2) PBr_{3} + 3H_{2}O = P(OH)_{3} + 3HBr.
+
+[Illustration Fig. 56]
+
+     The U-tube C contains glass beads which have been moistened
+     with water and rubbed in red phosphorus. Any bromine escaping
+     action in the flask acts upon the phosphorus in the U-tube. The
+     hydrobromic acid is collected in the same way as hydrochloric
+     acid.
+
+~Properties.~ Hydrobromic acid very strikingly resembles hydrochloric acid
+in physical and chemical properties. It is a colorless, strongly fuming
+gas, heavier than hydrochloric acid and, like it, is very soluble in
+water. Under standard conditions 1 volume of water dissolves 610 volumes
+of the gas. Chemically, the chief point in which it differs from
+hydrochloric acid is in the fact that it is much more easily oxidized,
+so that bromine is more readily set free from it than chlorine is from
+hydrochloric acid.
+
+~Salts of hydrobromic acid,--bromides.~ The bromides are very similar to
+the chlorides in their properties. Chlorine acts upon both bromides and
+free hydrobromic acid, liberating bromine from them:
+
+    KBr + Cl = KCl + Br,
+
+    HBr + Cl = HCl + Br.
+
+Silver bromide is extensively used in photography, and the bromides of
+sodium and potassium are used as drugs.
+
+     ~Oxygen compounds.~ No oxides of bromine are surely known, and
+     bromine does not form so many oxygen acids as chlorine does.
+     Salts of hypobromous acid (HBrO) and bromic acid (HBrO_{3}) are
+     known.
+
+
+IODINE
+
+~Historical.~ Iodine was discovered in 1812 by Courtois in the ashes of
+certain sea plants. Its presence was revealed by its beautiful violet
+vapor, and this suggested the name iodine (from the Greek for violet
+appearance).
+
+~Occurrence.~ In the combined state iodine occurs in very small quantities
+in sea water, from which it is absorbed by certain sea plants, so that
+it is found in their ashes. It occurs along with bromine in salt springs
+and beds, and is also found in Chili saltpeter.
+
+~Preparation.~ Iodine may be prepared in a number of ways, the principal
+methods being the following:
+
+1. _Laboratory method._ Iodine can readily be prepared in the laboratory
+from an iodide by the method used in preparing bromine, except that
+sodium iodide is substituted for sodium bromide. It can also be made by
+passing chlorine into a solution of an iodide.
+
+[Illustration: Fig. 57]
+
+2. _Commercial method._ Commercially iodine was formerly prepared from
+seaweed (kelp), but is now obtained almost entirely from the deposits of
+Chili saltpeter. The crude saltpeter is dissolved in water and the
+solution evaporated until the saltpeter crystallizes. The remaining
+liquors, known as the "mother liquors," contain sodium iodate
+(NaIO_{3}), in which form the iodine is present in the saltpeter. The
+chemical reaction by which the iodine is liberated from this compound is
+a complicated one, depending on the fact that sulphurous acid acts upon
+iodic acid, setting iodine free. This reaction is shown as follows:
+
+    2HIO_{3} + 5H_{2}SO_{3} = 5H_{2}SO_{4} + H_{2}O + 2I.
+
+     ~Purification of iodine.~ Iodine can be purified very
+     conveniently in the following way. The crude iodine is placed
+     in an evaporating dish E (Fig. 57), and the dish is set upon
+     the sand bath S. The iodine is covered with the inverted
+     funnel F, and the sand bath is gently heated with a Bunsen
+     burner. As the dish becomes warm the iodine rapidly evaporates
+     and condenses again on the cold surface of the funnel in
+     shining crystals.
+
+     This process, in which a solid is converted into a vapor and is
+     again condensed into a solid without passing through the liquid
+     state, is called _sublimation_.
+
+~Physical properties.~ Iodine is a purplish-black, shining, heavy solid
+which crystallizes in brilliant plates. Even at ordinary temperatures it
+gives off a beautiful violet vapor, which increases in amount as heat is
+applied. It melts at 107° and boils at 175°. It is slightly soluble in
+water, but readily dissolves in alcohol, forming a brown solution
+(tincture of iodine), and in carbon disulphide, forming a violet
+solution. The element has a strong, unpleasant odor, though by no means
+as irritating as that of chlorine and bromine.
+
+~Chemical properties.~ Chemically iodine is quite similar to chlorine and
+bromine, but is still less active than bromine. It combines directly
+with many elements at ordinary temperatures. At elevated temperatures it
+combines with hydrogen, but the reaction is reversible and the compound
+formed is quite easily decomposed. Both chlorine and bromine displace it
+from its salts:
+
+    KI + Br = KBr + I,
+
+    KI + Cl = KCl + I.
+
+When even minute traces of iodine are added to thin starch paste a very
+intense blue color develops, and this reaction forms a delicate test for
+iodine. Iodine is extensively used in medicine, especially in the form
+of a tincture. It is also largely used in the preparation of dyes and
+organic drugs, iodoform, a substance used as an antiseptic, has the
+formula CHI_{3}.
+
+~Hydriodic acid (HI).~ This acid cannot be prepared in pure condition by
+the action of sulphuric acid upon an iodide, since the hydriodic acid
+set free is oxidized by the sulphuric acid just as in the case of
+hydrobromic acid, but to a much greater extent. It can be prepared in
+exactly the same way as hydrobromic acid, iodine being substituted for
+bromine. It can also be prepared by passing hydrosulphuric acid into
+water in which iodine is suspended. The equation is
+
+    H_{2}S + 2I = 2HI + S.
+
+The hydriodic acid formed in this way dissolves in the water.
+
+~Properties and uses.~ Hydriodic acid resembles the corresponding acids of
+chlorine and bromine in physical properties, being a strongly fuming,
+colorless gas, readily soluble in water. Under standard conditions 1
+volume of water dissolves about 460 volumes of the gas. It is, however,
+more unstable than either hydrochloric or hydrobromic acids, and on
+exposure to the air it gradually decomposes in accordance with the
+equation
+
+    2HI + O = H_{2}O + 2I.
+
+Owing to the slight affinity between iodine and hydrogen the acid easily
+gives up its hydrogen and is therefore a strong reducing agent. This is
+seen in its action on sulphuric acid.
+
+The salts of hydriodic acid, the iodides, are, in general, similar to
+the chlorides and bromides. Potassium iodide (KI) is the most familiar
+of the iodides and is largely used in medicine.
+
+     ~Oxygen compounds.~ Iodine has a much greater affinity for oxygen
+     than has either chlorine or bromine. When heated with nitric
+     acid it forms a stable oxide (I_{2}O_{5}). Salts of iodic acid
+     (HIO_{3}) and periodic acid (HIO_{4}) are easily prepared, and
+     the free acids are much more stable than the corresponding
+     acids of the other members of this family.
+
+
+GAY-LUSSAC'S LAW OF VOLUMES
+
+In the discussion of the composition of hydrochloric acid it was stated
+that one volume of hydrogen combines with one volume of chlorine to form
+two volumes of hydrochloric acid. With bromine and iodine similar
+combining ratios hold good. These facts recall the simple volume
+relations already noted in the study of the composition of steam and
+ammonia. These relations may be represented graphically in the following
+way:
+
+              +---+   +----+    +------+    +------+
+              | H | + | Cl | =  | H Cl |  + | H Cl |
+              +---+   +----+    +------+    +------+
+
+       +---+  +---+   +---+    +--------+   +--------+
+       | H |  | H | + | O |  = | H_{2}O | + | H_{2}O |
+       +---+  +---+   +---+    +--------+   +--------+
+
++---+  +---+  +---+   +---+    +--------+   +--------+
+| H |  | H |  | H | + | N |  = | NH_{3} | + | NH_{3} |
++---+  +---+  +---+   +---+    +--------+   +--------+
+
+In the early part of the past century Gay-Lussac, a distinguished French
+chemist, studied the volume relations of many combining gases, and
+concluded that similar relations always hold. His observations are
+summed up in the following law: _When two gases combine chemically there
+is always a simple ratio between their volumes, and between the volume
+of either one of them and that of the product, provided it is a gas._ By
+a simple ratio is meant of course the ratio of small whole numbers, as
+1 : 2, 2 : 3.
+
+
+EXERCISES
+
+1. How do we account for the fact that liquid hydrofluoric acid is not
+an electrolyte?
+
+2. Why does sulphuric acid liberate hydrofluoric acid from its salts?
+
+3. In the preparation of chlorine, what advantages are there in treating
+manganese dioxide with a mixture of sodium chloride and sulphuric acid
+rather than with hydrochloric acid?
+
+4. Why must chlorine water be kept in the dark?
+
+5. What is the derivation of the word nascent?
+
+6. What substances studied are used as bleaching agents? To what is the
+bleaching action due in each case?
+
+7. What substances studied are used as disinfecting agents?
+
+8. What is meant by the statement that hydrochloric acid is one of the
+strongest acids?
+
+9. What is the meaning of the phrase _aqua regia_?
+
+10. Cl_{2}O is the anhydride of what acid?
+
+11. A solution of hydriodic acid on standing turns brown. How is this
+accounted for?
+
+12. How can bromine vapor and nitrogen peroxide be distinguished from
+each other?
+
+13. Write the equations for the reaction taking place when hydriodic
+acid is prepared from iodine, phosphorus, and water.
+
+14. From their behavior toward sulphuric acid, to what class of agents
+do hydrobromic and hydriodic acids belong?
+
+15. Give the derivation of the names of the elements of the chlorine
+family.
+
+16. Write the names and formulas for the binary acids of the group in
+the order of the stability of the acids.
+
+17. What is formed when a metal dissolves in each of the following?
+nitric acid; dilute sulphuric acid; concentrated sulphuric acid;
+hydrochloric acid; aqua regia.
+
+18. How could you distinguish between a chloride, a bromide, and an
+iodide?
+
+19. What weight of sodium chloride is necessary to prepare sufficient
+hydrochloric acid to saturate 1 l. of water under standard conditions?
+
+20. On decomposition 100 l. of hydrochloric acid would yield how many
+liters of hydrogen and chlorine respectively, the gases being measured
+under the same conditions? Are your results in accord with the
+experimental facts?
+
+
+
+
+CHAPTER XVII
+
+CARBON AND SOME OF ITS SIMPLER COMPOUNDS
+
+
+~The family.~ Carbon stands at the head of a family of elements in the
+fourth group in the periodic table. The resemblances between the
+elements of this family, while quite marked, are not so striking as in
+the case of the elements of the chlorine family. With the exception of
+carbon, these elements are comparatively rare, and need not be taken up
+in detail in this chapter. Titanium will be referred to again in
+connection with silicon which it very closely resembles.
+
+~Occurrence.~ Carbon is found in nature in the uncombined state in several
+forms. The diamond is practically pure carbon, while graphite and coal
+are largely carbon, but contain small amounts of other substances. Its
+natural compounds are exceedingly numerous and occur as gases, liquids,
+and solids. Carbon dioxide is its most familiar gaseous compound.
+Natural gas and petroleum are largely compounds of carbon with hydrogen.
+The carbonates, especially calcium carbonate, constitute great strata of
+rocks, and are found in almost every locality. All living organisms,
+both plant and animal, contain a large percentage of this element, and
+the number of its compounds which go to make up all the vast variety of
+animate nature is almost limitless. Over one hundred thousand definite
+compounds containing carbon have been prepared. In the free state carbon
+occurs in three allotropic forms, two of which are crystalline and one
+amorphous.
+
+~Crystalline carbon.~ Crystalline carbon occurs in two forms,--diamond and
+graphite.
+
+1. _Diamond._ Diamonds are found in considerable quantities in several
+localities, especially in South Africa, the East Indies, and Brazil. The
+crystals belong to the regular system, but the natural stones do not
+show this very clearly. When found they are usually covered with a rough
+coating which is removed in the process of cutting. Diamond cutting is
+carried on most extensively in Holland.
+
+The density of the diamond is 3.5, and, though brittle, it is one of the
+hardest of substances. Black diamonds, as well as broken and imperfect
+stones which are valueless as gems, are used for grinding hard
+substances. Few chemical reagents have any action on the diamond, but
+when heated in oxygen or the air it blackens and burns, forming carbon
+dioxide.
+
+Lavoisier first showed that carbon dioxide is formed by the combustion
+of the diamond; and Sir Humphry Davy in 1814 showed that this is the
+only product of combustion, and that the diamond is pure carbon.
+
+     ~The diamond as a gem.~ The pure diamond is perfectly transparent
+     and colorless, but many are tinted a variety of colors by
+     traces of foreign substances. Usually the colorless ones are
+     the most highly prized, although in some instances the color
+     adds to the value; thus the famous Hope diamond is a beautiful
+     blue. Light passing through a diamond is very much refracted,
+     and to this fact the stone owes its brilliancy and sparkle.
+
+     ~Artificial preparation of diamonds.~ Many attempts have been
+     made to produce diamonds artificially, but for a long time
+     these always ended in failure, graphite and not diamonds being
+     the product obtained. The French chemist Moissan, in his
+     extended study of chemistry at high temperatures, finally
+     succeeded (1893) in making some small ones. He accomplished
+     this by dissolving carbon in boiling iron and plunging the
+     crucible containing the mixture into water, as shown in Fig.
+     58. Under these conditions the carbon crystallized in the iron
+     in the form of the diamond. The diamonds were then obtained by
+     dissolving away the iron in hydrochloric acid.
+
+[Illustration: Fig. 58]
+
+2. _Graphite._ This form of carbon is found in large quantities,
+especially in Ceylon, Siberia, and in some localities of the United
+States and Canada. It is a shining black substance, very soft and greasy
+to the touch. Its density is about 2.15. It varies somewhat in
+properties according to the locality in which it is found, and is more
+easily attacked by reagents than is the diamond. It is also manufactured
+by heating carbon with a small amount of iron (3%) in an electric
+furnace. It is used in the manufacture of lead pencils and crucibles, as
+a lubricant, and as a protective covering for iron in the form of a
+polish or a paint.
+
+~Amorphous carbon.~ Although there are many varieties of amorphous carbon
+known, they are not true allotropic modifications. They differ merely in
+their degree of purity, their fineness of division, and in their mode of
+preparation. These substances are of the greatest importance, owing to
+their many uses in the arts and industries. As they occur in nature, or
+are made artificially, they are nearly all impure carbon, the impurity
+depending on the particular substance in question.
+
+1. _Pure carbon._ Pure amorphous carbon is best prepared by charring
+sugar. This is a substance consisting of carbon, hydrogen, and oxygen,
+the latter two elements being present in the ratio of one oxygen atom to
+two of hydrogen. When sugar is strongly heated the oxygen and hydrogen
+are driven off in the form of water and pure carbon is left behind.
+Prepared in this way it is a soft, lustrous, very bulky, black powder.
+
+2. _Coal and coke._ Coals of various kinds were probably formed from
+vast accumulations of vegetable matter in former ages, which became
+covered over with earthy material and were thus protected from rapid
+decay. Under various natural agencies the organic matter was slowly
+changed into coal. In anthracite these changes have gone the farthest,
+and this variety of coal is nearly pure carbon. Soft or bituminous coals
+contain considerable organic matter besides carbon and mineral
+substances. When heated strongly out of contact with air the organic
+matter is decomposed and the resulting volatile matter is driven off in
+the form of gases and vapors, and only the mineral matter and carbon
+remain behind. The gaseous product is chiefly illuminating gas and the
+solid residue is _coke_. Some of the coke is found as a dense cake on
+the sides and roof of the retort. This is called retort carbon and is
+quite pure.
+
+3. _Charcoal._ This is prepared from wood in the same way that coke is
+made from coal. When the process is carried on in retorts the products
+expelled by the heat are saved. Among these are many valuable substances
+such as wood alcohol and acetic acid. Where timber is abundant the
+process is carried out in a wasteful way, by merely covering piles of
+wood with sod and setting the wood on fire. Some wood burns and the heat
+from this decomposes the wood not burned, forming charcoal from it. The
+charcoal, of course, contains the mineral part of the wood from which it
+is formed.
+
+4. _Bone black._ This is sometimes called animal charcoal, and is made
+by charring bones and animal refuse. The organic part of the materials
+is thus decomposed and carbon is left in a very finely divided state,
+scattered through the mineral part which consists largely of calcium
+phosphate. For some uses this mineral part is removed by treatment with
+hydrochloric acid and prolonged washing.
+
+5. _Lampblack._ Lampblack and soot are products of imperfect combustion
+of oil and coal, and are deposited from a smoky flame on a cold surface.
+The carbon in this form is very finely divided and usually contains
+various oily materials.
+
+~Properties.~ While the various forms of carbon differ in many properties,
+especially in color and hardness, yet they are all odorless, tasteless
+solids, insoluble in water and characterized by their stability towards
+heat. Only in the intense heat of the electric arc does carbon
+volatilize, passing directly from the solid state into a vapor. Owing to
+this fact the inside surface of an incandescent light bulb after being
+used for some time becomes coated with a dark film of carbon. It is not
+acted on at ordinary temperatures by most reagents, but at a higher
+temperature it combines directly with many of the elements, forming
+compounds called _carbides_. When heated in the presence of sufficient
+oxygen it burns, forming carbon dioxide.
+
+~Uses of carbon.~ The chief use of amorphous carbon is for fuel to furnish
+heat and power for all the uses of civilization. An enormous quantity of
+carbon in the form of the purer coals, coke, and charcoal is used as a
+reducing agent in the manufacture of the various metals, especially in
+the metallurgy of iron. Most of the metals are found in nature as
+oxides, or in forms which can readily be converted into oxides. When
+these oxides are heated with carbon the oxygen is abstracted, leaving
+the metal. Retort carbon and coke are used to make electric light
+carbons and battery plates, while lampblack is used for indelible inks,
+printer's ink, and black varnishes. Bone black and charcoal have the
+property of absorbing large volumes of certain gases, as well as smaller
+amounts of organic matter; hence they are used in filters to remove
+noxious gases and objectionable colors and odors from water. Bone black
+is used extensively in the sugar refineries to remove coloring matter
+from the impure sugars.
+
+~Chemistry of carbon compounds.~ Carbon is remarkable for the very large
+number of compounds which it forms with the other elements, especially
+with oxygen and hydrogen. Compounds containing carbon are more numerous
+than all others put together, and the chemistry of these substances
+presents peculiarities not met with in the study of other substances.
+For these reasons the systematic study of carbon compounds, or of
+_organic chemistry_ as it is usually called, must be deferred until the
+student has gained some knowledge of the chemistry of other elements. An
+acquaintance with a few of the most familiar carbon compounds is,
+however, essential for the understanding of the general principles of
+chemistry.
+
+~Compounds of carbon with hydrogen,--the hydrocarbons.~ Carbon unites with
+hydrogen to form a very large number of compounds called _hydrocarbons_.
+Petroleum and natural gas are essentially mixtures of a great variety of
+these hydrocarbons. Many others are found in living plants, and still
+others are produced by the decay of organic matter in the absence of
+air. Only two of them, methane and acetylene, will be discussed here.
+
+~Methane~ (_marsh gas_) (CH_{4}). This is one of the most important of
+these hydrocarbons, and constitutes about nine tenths of natural gas. As
+its name suggests, it is formed in marshes by the decay of vegetable
+matter under water, and bubbles of the gas are often seen to rise when
+the dead leaves on the bottom of pools are stirred. It also collects in
+mines, and, when mixed with air, is called _fire damp_ by the miners
+because of its great inflammability, damp being an old name for a gas.
+It is formed when organic matter, such as coal or wood, is heated in
+closed vessels, and is therefore a principal constituent of coal gas.
+
+~Preparation.~ Methane is prepared in the laboratory by heating sodium or
+calcium acetate with soda-lime. Equal weights of fused sodium acetate
+and soda-lime are thoroughly dried, then mixed and placed in a
+good-sized, hard-glass test tube fitted with a one-holed stopper and
+delivery tube. The mixture is gradually heated, and when the air has
+been displaced from the tube the gas is collected in bottles by
+displacement of water. Soda-lime is a mixture of sodium and calcium
+hydroxides. Regarding it as sodium hydroxide alone, the equation is
+
+    NaC_{2}H_{3}O_{2} + NaOH = Na_{2}CO_{3} + CH_{4}.
+
+~Properties.~ Methane is a colorless, odorless gas whose density is 0.55.
+It is difficult to liquefy, boiling at -155° under standard pressure,
+and is almost insoluble in water. It burns with a pale blue flame,
+liberating much heat, and when mixed with oxygen is very explosive.
+
+~Davy's safety lamp.~ In 1815 Sir Humphry Davy invented a lamp for the use
+of miners, to prevent the dreadful mine explosions then common, due to
+methane mixed with air. The invention consisted in surrounding the upper
+part of the common miner's lamp with a mantle of wire gauze and the
+lower part with glass (Fig. 59). It has been seen that two gases will
+not combine until raised to their kindling temperature, and if while
+combining they are cooled below this point, the combination ceases. A
+flame will not pass through a wire gauze because the metal, being a good
+conductor of heat, takes away so much heat from the flame that the gases
+are cooled below the kindling temperature. When a lamp so protected is
+brought into an explosive mixture the gases inside the wire mantle burn
+in a series of little explosions, giving warning to the miner that the
+air is unsafe.
+
+[Illustration: Fig. 59]
+
+~Acetylene~ (C_{2}H_{2}). This is a colorless gas usually having a
+disagreeable odor due to impurities. It is now made in large quantities
+from calcium carbide (CaC_{2}). This substance is formed when coal and
+lime are heated together in an electric furnace. When treated with water
+the carbide is decomposed, yielding acetylene:
+
+    CaC_{2} + 2H_{2}O = C_{2}H_{2} + Ca(OH)_{2}.
+
+Under ordinary conditions the gas burns with a very smoky flame; in
+burners constructed so as to secure a large amount of oxygen it burns
+with a very brilliant white light, and hence is used as an illuminant.
+
+~Laboratory preparation.~ The gas can be prepared readily in a generator
+such as is shown in Fig. 60. The inner tube contains fragments of
+calcium carbide, while the outer one is filled with water. As long as
+the stopcock is closed the water cannot rise in the inner tube. When the
+stopcock is open the water rises, and, coming into contact with the
+carbide in the inner tube, generates acetylene. This escapes through the
+stopcock, and after the air has been expelled may be lighted as it
+issues from the burner.
+
+[Illustration: Fig. 60]
+
+Carbon forms two oxides, namely, carbon dioxide (CO_{2}) and carbon
+monoxide (CO).
+
+~Carbon dioxide~ (CO_{2}). Carbon dioxide is present in the air to the
+extent of about 3 parts in 10,000, and this apparently small amount is
+of fundamental importance in nature. In some localities it escapes from
+the earth in great quantities, and many spring waters carry large
+amounts of it in solution. When these highly charged spring waters reach
+the surface of the earth, and the pressure on them is removed, the
+carbon dioxide escapes with effervescence. It is a product of the
+oxidation of all organic matter, and is therefore formed in fires as
+well as in the process of decay. It is thrown off from the lungs of all
+animals in respiration, and is a product of many fermentation processes
+such as vinegar making and brewing. Combined with metallic oxides it
+forms vast deposits of carbonates in nature.
+
+
+~Preparation.~ In the laboratory carbon dioxide is always prepared by the
+action of an acid upon a carbonate, usually calcium carbonate, the
+apparatus shown in Fig. 39 serving the purpose very well. This reaction
+might be expected to produce carbonic acid, thus:
+
+    CaCO_{3} + 2HCl = CaCl_{2} + H_{2}CO_{3}.
+
+Carbonic acid is very unstable, however, and decomposes into its
+anhydride, CO_{2}, and water, thus:
+
+    H_{2}CO_{3} = H_{2}O + CO_{2}.
+
+The complete reaction is represented by the equation
+
+    CaCO_{3} + 2HCl = CaCl_{2} + CO_{2} + H_{2}O.
+
+~Physical properties.~ Carbon dioxide is a colorless, practically odorless
+gas whose density is 1.5. Its weight may be inferred from the fact that
+it can be siphoned, or poured like water, from one vessel downward into
+another. At 15° and under ordinary pressure it dissolves in its own
+volume of water and imparts a somewhat biting, pungent taste to it. It
+is easily condensed, and is now prepared commercially in this form by
+pumping the gas into steel cylinders (see Fig. 6) which are kept cold
+during the process. When the liquid is permitted to escape into the air
+part of it instantly evaporates, and in so doing absorbs so much heat
+that another portion is solidified, the solid form strikingly resembling
+snow in appearance. This snow is very cold and mercury can easily be
+frozen with it.
+
+~Solid carbon dioxide.~ Cylinders of liquid carbon dioxide are
+inexpensive, and should be available in every school. To demonstrate the
+properties of solid carbon dioxide, the cylinder should be placed across
+the table and supported in such a way that the stopcock end is several
+inches lower than the other end. A loose bag is made by holding the
+corners of a handkerchief around the neck of the stopcock, and the cock
+is then turned on so that the gas rushes out in large quantities. Very
+quickly a considerable quantity of the snow collects in the
+handkerchief. To freeze mercury, press a piece of filter paper into a
+small evaporating dish and pour the mercury upon it. Coil a flat spiral
+upon the end of a wire, and dip the spiral into the mercury. Place a
+quantity of solid carbon dioxide upon the mercury and pour 10 cc.-15 cc.
+of ether over it. In a minute or two the mercury will solidify and may
+be removed from the dish by the wire serving as a handle. The filter
+paper is to prevent the mercury from sticking to the dish; the ether
+dissolves the solid carbon dioxide and promotes its rapid conversion
+into gas.
+
+~Chemical properties.~ Carbon dioxide is incombustible, since it is, like
+water, a product of combustion. It does not support combustion, as does
+nitrogen peroxide, because the oxygen in it is held in very firm
+chemical union with the carbon. Very strong reducing agents, such as
+highly heated carbon, can take away half of its oxygen:
+
+    CO_{2} + C = 2CO.
+
+~Uses.~ The relation of carbon dioxide to plant life has been discussed in
+a previous chapter. Water highly charged with carbon dioxide is used for
+making soda water and similar beverages. Since it is a non-supporter of
+combustion and can be generated readily, carbon dioxide is also used as
+a fire extinguisher. Some of the portable fire extinguishers are simply
+devices for generating large amounts of the gas. It is not necessary
+that all the oxygen should be kept away from the fire in order to
+smother it. A burning candle is extinguished in air which contains only
+2.5% of carbon dioxide.
+
+~Carbonic acid~ (H_{2}CO_{3}). Like most of the oxides of the non-metallic
+elements, carbon dioxide is an acid anhydride. It combines with water to
+form an acid of the formula H_{2}CO_{3}, called carbonic acid:
+
+    H_{2}O + CO_{2} = H_{2}CO_{3}.
+
+The acid is, however, very unstable and cannot be isolated. Only a very
+small amount of it is actually formed when carbon dioxide is passed into
+water, as is evident from the small solubility of the gas. If, however,
+a base is present in the water, salts of carbonic acid are formed, and
+these are quite stable:
+
+    2NaOH + H_{2}O + CO_{2} = Na_{2}CO_{3} + 2H_{2}O.
+
+~Action of carbon dioxide on bases.~ This conduct is explained by the
+principles of reversible reactions. The equation
+
+    H_{2}O +CO_{2} <--> H_{2}CO_{3}
+
+is a reversible equation, and the extent to which the reaction
+progresses depends upon the relative concentrations of each of the three
+factors in it. Equilibrium is ordinarily reached when very little
+H_{2}CO_{3} is formed. If a base is present in the water to combine with
+the H_{2}CO_{3} as fast as it is formed, all of the CO_{2} is converted
+into H_{2}CO_{3}, and thence into a carbonate.
+
+~Salts of carbonic acid,--carbonates.~ The carbonates form a very
+important class of salts. They are found in large quantities in nature,
+and are often used in chemical processes. Only the carbonates of sodium,
+potassium, and ammonium are soluble, and these can be made by the action
+of carbon dioxide on solutions of the bases, as has just been explained.
+
+The insoluble carbonates are formed as precipitates when soluble salts
+are treated with a solution of a soluble carbonate. Thus the insoluble
+calcium carbonate can be made by bringing together solutions of calcium
+chloride and sodium carbonate:
+
+    CaCl_{2} + Na_{2}CO_{3} = CaCO_{3} + 2NaCl.
+
+Most of the carbonates are decomposed by heat, yielding an oxide of the
+metal and carbon dioxide. Thus lime (calcium oxide) is made by strongly
+heating calcium carbonate:
+
+    CaCO_{3} = CaO + CO_{2}.
+
+~Acid carbonates.~ Like all acids containing two acid hydrogen atoms,
+carbonic acid can form both normal and acid salts. The acid carbonates
+are made by treating a normal carbonate with an excess of carbonic acid.
+With few exceptions they are very unstable, heat decomposing them even
+when in solution.
+
+
+~Action of carbon dioxide on calcium hydroxide.~ If carbon dioxide is
+passed into clear lime water, calcium carbonate is at first
+precipitated:
+
+    H_{2}O + CO_{2} = H_{2}CO_{3},
+
+    Ca(OH)_{2} + H_{2}CO_{3} = CaCO_{3} + 2H_{2}O.
+
+Advantage is taken of this reaction in testing for the presence of
+carbon dioxide, as already explained in the chapter on the atmosphere.
+If the current of carbon dioxide is continued, the precipitate soon
+dissolves, because the excess of carbonic acid forms calcium acid
+carbonate which is soluble:
+
+    CaCO_{3} + H_{2}CO_{3} = Ca(HCO_{3})_{2}.
+
+If now the solution is heated, the acid carbonate is decomposed and
+calcium carbonate once more precipitated:
+
+    Ca(HCO_{3})_{2} = CaCO_{3} + H_{2}CO_{3}.
+
+~Carbon monoxide (CO).~ Carbon monoxide can be made in a number of ways,
+the most important of which are the three following:
+
+1. _By the partial oxidation of carbon._ If a slow current of air is
+conducted over highly heated carbon, the monoxide is formed, thus:
+
+    C + O = CO
+
+It is therefore often formed in stoves when the air draught is
+insufficient. Water gas, which contains large amounts of carbon
+monoxide, is made by partially oxidizing carbon with steam:
+
+    C + H_{2}O = CO + 2H.
+
+2. _By the partial reduction of carbon dioxide._ When carbon dioxide is
+conducted over highly heated carbon it is reduced to carbon monoxide by
+the excess of carbon:
+
+    CO_{2} + C = 2CO.
+
+When coal is burning in a stove or grate carbon dioxide is at first
+formed in the free supply of air, but as the hot gas rises through the
+glowing coal it is reduced to carbon monoxide. When the carbon monoxide
+reaches the free air above the coal it takes up oxygen to form carbon
+dioxide, burning with the blue flame so familiar above a bed of coals,
+especially in the case of hard coals.
+
+3. _By the decomposition of oxalic acid._ In the laboratory carbon
+monoxide is usually prepared by the action of concentrated sulphuric
+acid upon oxalic acid. The latter substance has the formula
+C_{2}H_{2}O_{4}. The sulphuric acid, owing to its affinity for water,
+decomposes the oxalic acid, as represented in the equation
+
+    C_{2}H_{2}O_{4} + (H_{2}SO_{4}) = (H_{2}SO_{4}) + H_{2}O + CO_{2} + CO.
+
+~Properties.~ Carbon monoxide is a light, colorless, almost odorless gas,
+very difficult to liquefy. Chemically it is very active, combining
+directly with a great many substances. It has a great affinity for
+oxygen and is therefore combustible and a good reducing agent. Thus, if
+carbon monoxide is passed over hot copper oxide, the copper is reduced
+to the metallic state:
+
+    CuO + CO = Cu + CO_{2}.
+
+When inhaled it combines with the red coloring matter of the blood and
+in this way prevents the absorption of oxygen, so that even a small
+quantity of the gas may prove fatal.
+
+[Illustration: Fig. 61]
+
+     ~The reducing power of carbon monoxide.~ Fig. 61 illustrates a
+     method of showing the reducing power of carbon monoxide. The
+     gas is generated by gently heating 7 or 8 g. of oxalic acid
+     with 25 cc. of concentrated sulphuric acid in a 200 cc. flask
+     A. The bottle B contains a solution of sodium hydroxide,
+     which removes the carbon dioxide formed along with the
+     monoxide. C contains a solution of calcium hydroxide to show
+     that the carbon dioxide is completely removed. E is a
+     hard-glass tube containing 1 or 2 g. of copper oxide, which is
+     heated by a burner. The black copper oxide is reduced to
+     reddish metallic copper by the carbon monoxide, which is
+     thereby changed to carbon dioxide. The presence of the carbon
+     dioxide is shown by the precipitate in the calcium hydroxide
+     solution in D. Any unchanged carbon monoxide is collected
+     over water in F.
+
+~Carbon disulphide~ (CS_{2}). Just as carbon combines with oxygen to form
+carbon dioxide, so it combines with sulphur to form carbon disulphide
+(CS_{2}). This compound has been described in the chapter on sulphur.
+
+~Hydrocyanic acid~ (_prussic acid_)(HCN). Under the proper conditions
+carbon unites with nitrogen and hydrogen to form the acid HCN, called
+hydrocyanic acid. It is a weak, volatile acid, and is therefore easily
+prepared by treating its salts with sulphuric acid:
+
+    KCN + H_{2}SO_{4} = KHSO_{4} + HCN.
+
+It is most familiar as a gas, though it condenses to a colorless liquid
+boiling at 26°. It has a peculiar odor, suggesting bitter almonds, and
+is extremely poisonous either when inhaled or when taken into the
+stomach. A single drop may cause death. It dissolves readily in water,
+its solution being commonly called prussic acid.
+
+The salts of hydrocyanic acid are called _cyanides_, the cyanides of
+sodium and potassium being the best known. These are white solids and
+are extremely poisonous.
+
+~Solutions of potassium cyanide are alkaline.~ A solution of potassium
+cyanide turns red litmus blue, and must therefore contain hydroxyl ions.
+The presence of these ions is accounted for in the following way.
+
+Although water is so little dissociated into its ions H^{+} and OH^{-}
+that for most purposes we may neglect the dissociation, it is
+nevertheless measurably dissociated. Hydrocyanic acid is one of the
+weakest of acids, and dissociates to an extremely slight extent. When a
+cyanide such as potassium cyanide dissolves it freely dissociates, and
+the CN^{-} ions must come to an equilibrium with the H^{+} ions derived
+from the water:
+
+    H^{+} + CN^{-} <--> HCN.
+
+The result of this equilibrium is that quite a number of H^{+} ions from
+the water are converted into undissociated HCN molecules. But for every
+H^{+} ion so removed an OH^{-} ion remains free, and this will give the
+solution alkaline properties.
+
+
+EXERCISES
+
+1. How can you prove that the composition of the different allotropic
+forms of carbon is the same?
+
+2. Are lampblack and bone black allotropic forms of carbon? Will equal
+amounts of heat be liberated in the combustion of 1 g. of each?
+
+3. How could you judge of the relative purity of different forms of
+carbon?
+
+4. Apart from its color, why should carbon be useful in the preparation
+of inks and paints?
+
+5. Could asbestos fibers be used to replace the wire in a safety lamp?
+
+6. Why do most acids decompose carbonates?
+
+7. What effect would doubling the pressure have upon the solubility of
+carbon dioxide in water?
+
+8. What compound would be formed by passing carbon dioxide into a
+solution of ammonium hydroxide? Write the equation.
+
+9. Write equations for the preparation of K_{2}CO_{3}; of BaCO_{3}; of
+MgCO_{3}.
+
+10. In what respects are carbonic and sulphurous acids similar?
+
+11. Give three reasons why the reaction which takes place when a
+solution of calcium acid carbonate is heated, completes itself.
+
+12. How could you distinguish between carbonates and sulphites?
+
+13. How could you distinguish between oxygen, hydrogen, nitrogen,
+nitrous oxide, and carbon dioxide?
+
+14. Could a solution of sodium hydroxide be substituted for the solution
+of calcium hydroxide in testing for carbon dioxide?
+
+15. What weight of sodium hydroxide is necessary to neutralize the
+carbonic acid formed by the action of hydrochloric acid on 100 g. of
+calcium carbonate?
+
+16. What weight of calcium carbonate would be necessary to prepare
+sufficient carbon dioxide to saturate 10 l. of water at 15° and under
+ordinary pressure?
+
+17. On the supposition that calcium carbide costs 12 cents a kilogram,
+what would be the cost of an amount sufficient to generate 100 l. of
+acetylene measured at 20° and 740 mm.?
+
+18. How would the volume of a definite amount of carbon monoxide compare
+with the volume of carbon dioxide formed by its combustion, the
+measurements being made under the same conditions?
+
+
+
+
+CHAPTER XVIII
+
+FLAMES,--ILLUMINANTS
+
+
+~Conditions necessary for flames.~ It has been seen that when two
+substances unite chemically, with the production of light and heat, the
+act of union is called combustion. When one of the substances undergoing
+combustion remains solid at the temperature occasioned by the
+combustion, light may be given off, but there is no flame. Thus iron
+wire burning in oxygen throws off a shower of sparks and is brilliantly
+incandescent, but no flame is seen. When, however, both of the
+substances are gases or vapors at the temperature reached in the
+combustion, the act of union is accompanied by a flame.
+
+~Flames from burning liquids or solids.~ Many substances which are liquids
+or solids at ordinary temperatures burn with a flame because the heat of
+combustion vaporizes them slowly, and the flame is due to the union of
+this vapor with the gas supporting the combustion.
+
+~Supporter of combustion.~ That gas which surrounds the flame and
+constitutes the atmosphere in which the combustion occurs is said to
+support the combustion. The other gas which issues into this atmosphere
+is said to be the combustible gas. Thus, in the ordinary combustion of
+coal gas in the air the coal gas is said to be combustible, while the
+air is regarded as the supporter of combustion. These terms are entirely
+relative, however, for a jet of air issuing into an atmosphere of coal
+gas will burn when ignited, the coal gas supporting the combustion.
+Ordinarily, when we say that a gas is combustible we mean that it is
+combustible in an atmosphere of air.
+
+[Illustration: Fig. 62]
+
+~Either gas may be the supporter of combustion.~ That the terms
+_combustible_ and _supporter of combustion_ are merely relative may be
+shown in the following way: A lamp chimney A is fitted with a cork and
+glass tubes, as shown in Fig. 62. The tube C should have a diameter of
+from 12 to 15 mm. A thin sheet of asbestos in which is cut a circular
+opening about 2 cm. in diameter is placed over the top of the chimney.
+The opening in the asbestos is closed with the palm of the hand, and gas
+is admitted to the chimney through the tube B. The air in the chimney
+is soon expelled through the tube C, and the gas itself is then
+lighted at the lower end of this tube. The hand is now removed from the
+opening in the asbestos, when the flame at the end of the tube at once
+rises and appears at the end within the chimney, as shown in the figure.
+The excess of coal gas now escapes from the opening in the asbestos and
+may be lighted. The flame at the top of the asbestos board is due to the
+combustion of coal gas in air, while the flame within the chimney is due
+to the combustion of air in coal gas, the air being drawn up through the
+tube by the escaping gas.
+
+~Appearance of flames.~ The flame caused by the union of hydrogen and
+oxygen is almost colorless and invisible. Chlorine and hydrogen combine
+with a pale violet flame, carbon monoxide burns in oxygen with a blue
+flame, while ammonia burns with a deep yellow flame. The color and
+appearance of flames are therefore often quite characteristic of the
+particular combustion which occasions them.
+
+~Structure of flames.~ When the gas undergoing combustion issues from a
+round opening into an atmosphere of the gas supporting combustion, as is
+the case with the burning Bunsen burner (Fig. 63), the flame is
+generally conical in outline. It consists of several distinct cones,
+one within the other, the boundary between them being marked by
+differences of color or luminosity. In the simplest flame, of which
+hydrogen burning in oxygen is a good example, these cones are two in
+number,--an inner one, formed by unburned gas, and an outer one, usually
+more or less luminous, consisting of the combining gases. This outer one
+is in turn surrounded by a third envelope of the products of combustion;
+this envelope is sometimes invisible, as in the present case, but is
+sometimes faintly luminous. The lower part of the inner cone of the
+flame is quite cool and consists of unburned gas. Toward the top of the
+inner cone the gas has become heated to a high temperature by the
+burning envelope surrounding it. On reaching the supporter of combustion
+on the outside it is far above its kindling temperature, and combustion
+follows with the evolution of much heat. The region of combustion just
+outside the inner cone is therefore the hottest part of the flame.
+
+[Illustration: Fig. 63]
+
+~Oxidizing and reducing flames.~ Since the tip of the outside cone
+consists of very hot products of combustion mixed with oxygen from the
+air, a substance capable of oxidation placed in this part of the flame
+becomes very hot and is easily oxidized. The oxygen with which it
+combines comes, of course, from the atmosphere, and not from the
+products of combustion. This outer tip of the flame is called the
+_oxidizing flame_.
+
+At the tip of the inner cone the conditions are quite different. This
+region consists of a highly heated combustible gas, which has not yet
+reached a supply of oxygen.
+
+If a substance rich in oxygen, such as a metallic oxide, is placed in
+this region of the flame, the heated gases combine with its oxygen and
+the substance is reduced. This part of the flame is called the _reducing
+flame_. These flames are used in testing certain substances, especially
+minerals. For this purpose they are produced by blowing into a small
+luminous Bunsen flame from one side through a blowpipe. This is a tube
+of the shape shown in Fig. 64. The flame is directed in any desired way
+and has the oxidizing and reducing regions very clearly marked (Fig.
+65). It is non-luminous from the same causes which render the open
+Bunsen burner flame non-luminous, the gases from the lungs serving to
+furnish oxygen and to dilute the combustible gas.
+
+[Illustration: Fig. 64]
+
+[Illustration: Fig. 65]
+
+~Luminosity of flames.~ The luminosity of flames is due to a number of
+distinct causes, and may therefore be increased or diminished in several
+ways.
+
+1. _Presence of solid matter._ The most obvious of these causes is the
+presence in the flame of incandescent solid matter. Thus chalk dust
+sifted into a non-luminous flame renders it luminous. When hydrocarbons
+form a part of the combustible gas, as they do in nearly all
+illuminating gases and oils, some carbon is usually set free in the
+process of combustion. This is made very hot by the flame and becomes
+incandescent, giving out light. In a well-regulated flame it is
+afterward burned up, but when the supply of oxygen is insufficient it
+escapes from the flame as lampblack or soot. That it is temporarily
+present in a well-burning luminous flame may be demonstrated by holding
+a cold object, such as a small evaporating dish, in the flame for a few
+seconds. This cold object cools the carbon below its kindling
+temperature, and it is deposited on the object as soot.
+
+2. _Pressure._ A second factor in the luminosity of flames is the
+pressure under which the gases are burning. Under increased pressure
+there is more matter in a given volume of a gas, and the chemical action
+is more energetic than when the gases are rarefied. Consequently there
+is more heat and light. A candle burning on a high mountain gives less
+light than when it burns at the sea level.
+
+If the gas is diluted with a non-combustible gas, the effect is the same
+as if it is rarefied, for under these conditions there is less
+combustible gas in a given volume.
+
+3. _Temperature._ The luminosity also depends upon the temperature
+attained in the combustion. In general the hotter the flame the greater
+the luminosity; hence cooling the gases before combustion diminishes the
+luminosity of the flame they will make, because it diminishes the
+temperature attained in the combustion. Thus the luminosity of the
+Bunsen flame is largely diminished by the air drawn up with the gas.
+This is due in part to the fact that the burning gas is diluted and
+cooled by the air drawn in. The oxygen thus introduced into the flame
+also causes the combustion of the hot particles of carbon which would
+otherwise tend to make the flame luminous.
+
+~Illuminating and fuel gases.~ A number of mixtures of combustible gases,
+consisting largely of carbon compounds and hydrogen, find extensive use
+for the production of light and heat. The three chief varieties are coal
+gas, water gas, and natural gas. The use of acetylene gas has already
+been referred to.
+
+~Coal gas.~ Coal gas is made by heating bituminous coal in large retorts
+out of contact with the air. Soft or bituminous coal contains, in
+addition to large amounts of carbon, considerable quantities of
+compounds of hydrogen, oxygen, nitrogen, and sulphur. When distilled the
+nitrogen is liberated partly in the form of ammonia and cyanides and
+partly as free nitrogen gas; the sulphur is converted into hydrogen
+sulphide, carbon disulphide, and oxides of sulphur; the oxygen into
+water and oxides of carbon. The remaining hydrogen is set free partly
+as hydrogen and partly in combination with carbon in the form of
+hydrocarbons. The most important of these is methane, with smaller
+quantities of many others, some of which are liquids or solids at
+ordinary temperatures. The great bulk of the carbon remains behind as
+coke and retort carbon.
+
+~The manufacture of coal gas.~ In the manufacture of coal gas it is
+necessary to separate from the volatile constituents formed by the
+heating of the coal all those substances which are either solid or
+liquid at ordinary temperature, since these would clog the gas pipes.
+Certain gaseous constituents, such as hydrogen sulphide and ammonia,
+must also be removed. The method used to accomplish this is shown in
+Fig. 66. The coal is heated in air-tight retorts illustrated by A. The
+volatile products escape through the pipe X and bubble into the tarry
+liquid in the large pipe B, known as the _hydraulic main_, which runs
+at right angles to the retorts. Here is deposited the greater portion of
+the solid and liquid products, forming a tarry mass known as _coal tar_.
+Much of the ammonia also remains dissolved in this liquid. The partially
+purified gas then passes into the pipes C, which serve to cool it and
+further remove the solid and liquid matter. The gas then passes into
+D, which is filled with coke over which a jet of water is sprayed. The
+water still further cools the gas and at the same time partially removes
+such gaseous products as hydrogen sulphide and ammonia, which are
+soluble in water. In E the gas passes over some material such as lime,
+which removes the last portions of the sulphur compounds as well as much
+of the carbon dioxide present. From E the gas passes into the large
+gas holder F, from which it is distributed through pipes to the places
+where it is burned.
+
+[Illustration: Fig. 66]
+
+     One ton of good gas coal yields approximately 10,000 cu. ft. of
+     gas, 1400 lb. of coke, 120 lb. of tar, and 20 gal. of
+     ammoniacal liquor.
+
+     Not only is the ammonia obtained in the manufacture of the gas
+     of great importance, but the coal tar also serves as the source
+     of many very useful substances, as will be explained in Chapter
+     XXXII.
+
+~Water gas.~ Water gas is essentially a mixture of carbon monoxide and
+hydrogen. It is made by passing steam over very hot anthracite coal,
+when the reaction shown in the following equation takes place:
+
+    C + H_{2}O = CO + 2H.
+
+When required merely to produce heat the gas is at once ready for use.
+When made for illuminating purposes it must be enriched, that is,
+illuminants must be added, since both carbon monoxide and hydrogen burn
+with non-luminous flames. This is accomplished by passing it into
+heaters containing highly heated petroleum oils. The gas takes up
+hydrocarbon gases formed in the decomposition of the petroleum oils,
+which make it burn with a luminous flame.
+
+Water gas is very effective as a fuel, since both carbon monoxide and
+hydrogen burn with very hot flames. It has little odor and is very
+poisonous. Its use is therefore attended with some risk, since leaks in
+pipes are very likely to escape notice.
+
+~Natural gas.~ This substance, so abundant in many localities, varies much
+in composition, but is composed principally of methane. When used for
+lighting purposes it is usually burned in a burner resembling an open
+Bunsen, the illumination being furnished by an incandescent mantle. This
+is the case in the familiar Welsbach burner. Contrary to statements
+frequently made, natural gas contains no free hydrogen.
+
+
+TABLE SHOWING COMPOSITION OF GASES
+
+=====================+================+========+========+==========
+                     |  PENNSYLVANIA  |  COAL  | WATER  | ENRICHED
+                     |    NATURAL     |   GAS  |  GAS   |  WATER
+                     |      GAS       |        |        |   GAS
+---------------------+----------------+--------+--------+----------
+Hydrogen             |                |  41.3  | 52.88  |  30.00
+Methane              |    90.64       |  43.6  |  2.16  |  24.00
+Illuminants          |                |   3.9  |        |  12.05
+Carbon monoxide      |                |   6.4  | 36.80  |  29.00
+Carbon dioxide       |     0.30       |   2.0  |  3.47  |   0.30
+Nitrogen             |     9.06       |   1.2  |  4.69  |   2.50
+Oxygen               |                |   0.3  |        |   1.50
+Hydrocarbon vapors   |                |   1.5  |        |   1.50
+=====================+================+========+========+==========
+
+     These are analyses of actual samples, and may be taken as about
+     the average for the various kinds of gases. Any one of these
+     may vary considerably. The nitrogen and oxygen in most cases is
+     due to a slight admixture of air which is difficult to exclude
+     entirely in the manufacture and handling of gases.
+
+~Fuels.~ A variety of substances are used as fuels, the most important of
+them being wood, coal, and the various gases mentioned above. Wood
+consists mainly of compounds of carbon, hydrogen, and oxygen. The
+composition of coal and the fuel gases has been given. Since these fuels
+are composed principally of carbon and hydrogen or their compounds, the
+chief products of combustion are carbon dioxide and water. The practice
+of heating rooms with portable gas or oil stoves with no provision for
+removing the products of combustion is to be condemned, since the carbon
+dioxide is generated in sufficient quantities to render the air unfit
+for breathing. Rooms so heated also become very damp from the large
+amount of water vapor formed in the combustion, and which in cold
+weather condenses on the window glass, causing the glass to "sweat."
+Both coal and wood contain a certain amount of mineral substances which
+constitute the ashes.
+
+~The electric furnace.~ In recent years electric furnaces have come into
+wide use in operations requiring a very high temperature. Temperatures
+as high as 3500° can be easily reached, whereas the hottest oxyhydrogen
+flame is not much above 2000°. These furnaces are constructed on one of
+two general principles.
+
+[Illustration: Fig. 67]
+
+1. _Arc furnaces._ In the one type the source of heat is an electric arc
+formed between carbon electrodes separated a little from each other, as
+shown in Fig. 67. The substance to be heated is placed in a vessel,
+usually a graphite crucible, just below the arc. The electrodes and
+crucible are surrounded by materials which fuse with great difficulty,
+such as magnesium oxide, the walls of the furnace being so shaped as to
+reflect the heat downwards upon the contents of the crucible.
+
+[Illustration: Fig. 68]
+
+2. _Resistance furnaces._ In the other type of furnace the heat is
+generated by the resistance offered to the current in its passage
+through the furnace. In its simplest form it may be represented by Fig.
+68. The furnace is merely a rectangular box built up of loose bricks.
+The electrodes E, each consisting of a bundle of carbon rods, are
+introduced through the sides of the furnace. The materials to be heated,
+C, are filled into the furnace up to the electrodes, and a layer of
+broken coke is arranged so as to extend from one electrode to the other.
+More of the charge is then placed on top of the coke. In passing through
+the broken coke the electrical current encounters great resistance. This
+generates great heat, and the charge surrounding the coke is brought to
+a very high temperature. The advantage of this type of furnace is that
+the temperature can be regulated to any desired intensity.
+
+
+EXERCISES
+
+1. Why does charcoal usually burn with no flame? How do you account for
+the flame sometimes observed when it burns?
+
+2. How do you account for the fact that a candle burns with a flame?
+
+3. What two properties must the mantle used in the Welsbach lamp
+possess?
+
+4. (a) In what respects does the use of the Welsbach mantle resemble
+that of lime in the calcium light? (b) If the mantle were made of
+carbon, would it serve the same purpose?
+
+5. Would anthracite coal be suitable for the manufacture of coal gas?
+
+6. How could you prove the formation of carbon dioxide and water in the
+combustion of illuminating gases?
+
+7. Suggest a probable way in which natural gas has been formed.
+
+8. Coal frequently contains a sulphide of iron. (a) What two sulphur
+compounds are likely to be formed when gas is made from such coal? (b)
+Suggest some suitable method for the removal of these compounds.
+
+9. Why does the use of the bellows on the blacksmith's forge cause a
+more intense heat?
+
+10. What volume of oxygen is necessary to burn 100 l. of marsh gas and
+what volume of carbon dioxide would be formed, all of the gases being
+measured under standard conditions?
+
+11. Suppose a cubic meter of Pennsylvania natural gas, measured under
+standard conditions, were to be burned. How much water by weight would
+result?
+
+
+
+
+CHAPTER XIX
+
+MOLECULAR WEIGHTS, ATOMIC WEIGHTS, FORMULAS
+
+
+~Introduction.~ In the chapter on The Atomic Theory, it was shown that if
+it were true that two elements uniting to form a compound always
+combined in the ratio of one atom of one element to one atom of the
+other element, it would be a very easy matter to decide upon figures
+which would represent the relative weights of the different atoms. It
+would only be necessary to select some one element as a standard and
+determine the weight of every element which combines with a definite
+weight (say 1 g.) of the standard element. The figures so obtained would
+evidently represent the relative weights of the atoms.
+
+But the law of multiple proportion at once reminds us that two elements
+may unite in several proportions; and there is no simple way to
+determine the number of atoms present in the molecule of any compound.
+Consequently the problem of deciding upon the relative atomic weights is
+not an easy one. To the solution of this problem we must now turn.
+
+~Dalton's method of determining atomic weights.~ When Dalton first
+advanced the atomic theory he attempted to solve this problem by very
+simple methods. He thought that when only one compound of two elements
+is known it is reasonable to suppose that it contains one atom of each
+element. He therefore gave the formula HO to water, and HN to ammonia.
+When more than two compounds were known he assumed that the most
+familiar or the most stable one had the simple formula. He then
+determined the atomic weight as explained above. The results he
+obtained were contradictory and very far from satisfactory, and it was
+soon seen that some other method, resting on much more scientific
+grounds, must be found to decide what compounds, if any, have a single
+atom of each element present.
+
+~Determination of atomic weights.~ Three distinct steps are involved in
+the determination of the atomic weight of an element: (1) determination
+of the equivalent, (2) determination of molecular weights of its
+compounds, and (3) deduction of the exact atomic weight from the
+equivalent and molecular weights.
+
+~1. Determination of the equivalent.~ By the equivalent of an element is
+meant the weight of the element which will combine with a fixed weight
+of some other element chosen as a standard. It has already been
+explained that oxygen has been selected as the standard element for
+atomic weights, with a weight of 16. This same standard will serve very
+well as a standard for equivalents. _The equivalent of an element is the
+weight of the element which will combine with 16 g. of oxygen._ Thus 16
+g. of oxygen combines with 16.03 g. of sulphur, 65.4 g. of zinc, 215.86
+g. of silver, 70.9 g. of chlorine. These figures, therefore, represent
+the equivalent weights of these elements.
+
+~Relation of atomic weights to equivalents.~ According to the atomic
+theory combination always takes place between whole numbers of atoms.
+Thus one atom unites with one other, or with two or three; or two atoms
+may unite with three, or three with five, and so on.
+
+When oxygen combines with zinc the combination must be between definite
+numbers of the two kinds of atoms. Experiment shows that these two
+elements combine in the ratio of 16 g. of oxygen to 65.4 g. of zinc. If
+one atom of oxygen combines with one atom of zinc, then this ratio must
+be the ratio between the weights of the two atoms. If one atom of oxygen
+combines with two atoms of zinc, then the ratio between the weights of
+the two atoms will be 16: 32.7. If two atoms of oxygen combine with one
+atom of zinc, the ratio by weight between the two atoms will be 8: 65.4.
+It is evident, therefore, that the real atomic weight of an element must
+be some multiple or submultiple of the equivalent; in other words, the
+equivalent multiplied by 1/2, 1, 2, or 3 will give the atomic weight.
+
+~Combining weights.~ A very interesting relation holds good between the
+equivalents of the various elements. We have just seen that the figures
+16.03, 65.4, 215.86, and 70.9 are the equivalents respectively of
+sulphur, zinc, silver, and chlorine. These same figures represent the
+ratios by weight in which these elements combine among themselves. Thus
+215.86 g. of silver combine with 70.9 g. of chlorine and with 2 × 16.03
+g. of sulphur. 65.4 g. of zinc combine with 70.9 g. of chlorine and 2 ×
+16.03 g. of sulphur.
+
+By taking the equivalent or some multiple of it a value can be obtained
+for each element which will represent its combining value, and for this
+reason is called its _combining weight_. It is important to notice that
+the fact that a combining weight can be obtained for each element is not
+a part of a theory, but is the direct result of experiment.
+
+~Elements with more than one equivalent.~ It will be remembered that
+oxygen combines with hydrogen in two ratios. In one case 16 g. of oxygen
+combine with 2.016 g. of hydrogen to form water; in the other 16 g. of
+oxygen combine with 1.008 g. of hydrogen to form hydrogen dioxide. The
+equivalents of hydrogen are therefore 2.016 and 1.008. Barium combines
+with oxygen in two proportions: in barium oxide the proportion is 16 g.
+of oxygen to 137.4 g. of barium; in barium dioxide the proportion is 16
+g. of oxygen to 68.7 g. of barium.
+
+In each case one equivalent is a simple multiple of the other, so the
+fact that there may be two equivalents does not add to the uncertainty.
+All we knew before was that the true atomic weight is some multiple of
+the equivalent.
+
+~2. The determination of molecular weights.~ To decide the question as to
+which multiple of the equivalent correctly represents the atomic weight
+of an element, it has been found necessary to devise a method of
+determining the molecular weights of compounds containing the element in
+question. Since the molecular weight of a compound is merely the sum of
+the weights of all the atoms present in it, it would seem to be
+impossible to determine the molecular weight of a compound without first
+knowing the atomic weights of the constituent atoms, and how many atoms
+of each element are present in the molecule. But certain facts have been
+discovered which suggest a way in which this can be done.
+
+~Avogadro's hypothesis.~ We have seen that the laws of Boyle, Charles, and
+Gay-Lussac apply to all gases irrespective of their chemical character.
+This would lead to the inference that the structure of gases must be
+quite simple, and that it is much the same in all gases.
+
+In 1811 Avogadro, an Italian physicist, suggested that if we assume all
+gases under the same conditions of temperature and pressure to have the
+same number of molecules in a given volume, we shall have a probable
+explanation of the simplicity of the gas laws. It is difficult to prove
+the truth of this hypothesis by a simple experiment, but there are so
+many facts known which are in complete harmony with this suggestion that
+there is little doubt that it expresses the truth. Avogadro's hypothesis
+may be stated thus: _Equal volumes of all gases under the same
+conditions of temperature and pressure contain the same number of
+molecules._
+
+~Avogadro's hypothesis and molecular weights.~ Assuming that Avogadro's
+hypothesis is correct, we have a very simple means for deciding upon the
+relative weights of molecules; for if equal volumes of two gases contain
+the same number of molecules, the weights of the two volumes must be in
+the same ratio as the weights of the individual molecules which they
+contain. If we adopt some one gas as a standard, we can express the
+weights of all other gases as compared with this one, and the same
+figures will express the relative weights of the molecules of which the
+gases are composed.
+
+~Oxygen as the standard.~ It is important that the same standard should be
+adopted for the determination of molecular weights as has been decided
+upon for atomic weights and equivalents, so that the three values may be
+in harmony with each other. Accordingly it is best to adopt oxygen as
+the standard element with which to compare the molecular weights of
+other gases, being careful to keep the oxygen atom equal to 16.
+
+~The oxygen molecule contains two atoms.~ One point must not be
+overlooked, however. We desire to have our unit, the oxygen _atom_,
+equal to 16. The method of comparing the weights of gases just suggested
+compares the molecules of the gases with the _molecule_ of oxygen. Is
+the molecule and the atom of oxygen the same thing? This question is
+answered by the following considerations.
+
+We have seen that when steam is formed by the union of oxygen and
+hydrogen, two volumes of hydrogen combine with one volume of oxygen to
+form two volumes of steam. Let us suppose that the one volume of oxygen
+contains 100 molecules; then the two volumes of steam must, according
+to Avogadro's hypothesis, contain 200 molecules. But each of these 200
+molecules must contain at least one atom of oxygen, or 200 in all, and
+these 200 atoms came from 100 molecules of oxygen. It follows that each
+molecule of oxygen must contain at least two atoms of oxygen.
+
+Evidently this reasoning merely shows that there are _at least_ two
+atoms in the oxygen molecule. There may be more than that, but as there
+is no evidence to this effect, we assume that the molecule contains two
+atoms only.
+
+It is evident that if we wish to retain the value 16 for the atom of
+oxygen we must take twice this value, or 32, for the value of the oxygen
+molecule, when using it as a standard for molecular weights.
+
+~Determination of the molecular weights of gases from their weights
+compared with oxygen.~ Assuming the molecular weight of oxygen to be 32,
+Avogadro's hypothesis gives us a ready means for determining the
+molecular weight of any other gas, for all that is required is to know
+its weight compared with that of an equal volume of oxygen. For example,
+1 l. of chlorine is found by experiment to weigh 2.216 times as much as
+1 l. of oxygen. The molecular weight of chlorine must therefore be 2.216
+× 32, or 70.91.
+
+If, instead of comparing the relative weights of 1 l. of the two gases,
+we select such a volume of oxygen as will weigh 32 g., or the weight in
+grams corresponding to the molecular weight of the gas, the calculation
+is much simplified. It has been found that 32 g. of oxygen, under
+standard conditions, measure 22.4 l. This same volume of hydrogen weighs
+2.019 g.; of chlorine 70.9 g.; of hydrochloric acid 36.458 g. The
+weights of these equal volumes must be proportional to their molecular
+weights, and since the weight of the oxygen is the same as the value of
+its molecular weight, so too will the weights of the 22.4 l. of the
+other gases be equal to the value of their molecular weights.
+
+As a summary we can then make the following statement: _The molecular
+weight of any gas may be determined by calculating the weight of 22.4 l.
+of the gas, measured under standard conditions._
+
+~Determination of molecular weights from density of gases.~ In an actual
+experiment it is easier to determine the density of a gas than the
+weight of a definite volume of it. The density of a gas is usually
+defined as its weight compared with that of an equal volume of air.
+Having determined the density of a gas, its weight compared with oxygen
+may be determined by multiplying its density by the ratio between the
+weights of air and oxygen. This ratio is 0.9046. To compare it with our
+standard for atomic weights we must further multiply it by 32, since the
+standard is 1/32 the weight of oxygen molecules. The steps then are
+these:
+
+1. Determine the density of the gas (its weight compared with air).
+
+2. Multiply by 0.9046 to make the comparison with oxygen molecules.
+
+3. Multiply by 32 to make the comparison with the unit for atomic
+weights.
+
+We have, then, the formula:
+
+    molecular weight = density × 0.9046 × 32;
+
+or, still more briefly,
+
+    M. = D. × 28.9.
+
+The value found by this method for the determination of molecular
+weights will of course agree with those found by calculating the weight
+of 22.4 l. of the gas, since both methods depend on the same principles.
+
+[Illustration: Fig. 69]
+
+     ~Determination of densities of gases.~ The relative weights of
+     equal volumes of two gases can be easily determined. The
+     following is one of the methods used. A small flask, such as is
+     shown in Fig. 69, is filled with one of the gases, and after
+     the temperature and pressure have been noted the flask is
+     sealed up and weighed. The tip of the sealed end is then broken
+     off, the flask filled with the second gas, and its weight
+     determined. If the weight of the empty flask is subtracted from
+     these two weighings, the relative weights of the gases is
+     readily found.
+
+~3. Deduction of atomic weights from molecular weights and equivalents.~
+We have now seen how the equivalent of an element and the molecular
+weight of compounds containing the element can be obtained. Let us see
+how it is possible to decide which multiple of the equivalent really is
+the true atomic weight. As an example, let us suppose that the
+equivalent of nitrogen has been found to be 7.02 and that it is desired
+to obtain its atomic weight. The next step is to obtain the molecular
+weights of a large number of compounds containing nitrogen. The
+following will serve:
+
+==================+============+=============+================+==============
+                  |            | APPROXIMATE |  PERCENTAGE OF |    PART OF
+                  | DENSITY BY |  MOLECULAR  |   NITROGEN BY  |   MOLECULAR
+                  | EXPERIMENT |   WEIGHT    |    EXPERIMENT  |   WEIGHT DUE
+                  |            | (D. × 28.9) |                |  TO NITROGEN
+------------------+------------+-------------+----------------+--------------
+Nitrogen gas      |   0.9671   |    27.95    |    100.00      |    27.95
+Nitrous oxide     |   1.527    |    44.13    |     63.70      |    27.11
+Nitric oxide      |   1.0384   |    30.00    |     46.74      |    14.02
+Nitrogen peroxide |   1.580    |    45.66    |     30.49      |    13.90
+Ammonia           |   0.591    |    17.05    |     82.28      |    14.03
+Nitric acid       |   2.180    |    63.06    |     22.27      |    14.03
+Hydrocyanic acid  |   0.930    |    26.87    |     51.90      |    13.94
+==================+============+=============+================+==============
+
+~Method of calculation.~ The densities of the various gases in the first
+column of this table are determined by experiment, and are fairly
+accurate but not entirely so. By multiplying these densities by 28.9 the
+molecular weights of the compounds as given in the second column are
+obtained. By chemical analysis it is possible to determine the
+percentage composition of these substances, and the percentages of
+nitrogen in them as determined by analysis are given in the third
+column. If each of these molecular weights is multiplied in turn by the
+percentage of nitrogen in the compound, the product will be the weight
+of the nitrogen in the molecular weight of the compound. This will be
+the sum of the weights of the nitrogen atoms in the molecule. These
+values are given in the fourth column in the table.
+
+If a large number of compounds containing nitrogen are studied in this
+way, it is probable that there will be included in the list at least one
+substance whose molecule contains a single nitrogen atom. In this case
+the number in the fourth column will be the approximate atomic weight of
+nitrogen. On comparing the values for nitrogen in the table it will be
+seen that a number which is approximately 14 is the smallest, and that
+the others are multiples of this. These compounds of higher value,
+therefore, contain more than one nitrogen atom in the molecule.
+
+~Accurate determination of atomic weights.~ Molecular weights cannot be
+determined very accurately, and consequently the part in them due to
+nitrogen is a little uncertain, as will be seen in the table. All we can
+tell by this method is that the true weight is very near 14. The
+equivalent can however be determined very accurately, and we have seen
+that it is some multiple or submultiple of the true atomic weight.
+Since molecular-weight determinations have shown that in the case of
+nitrogen the atomic weight is near 14, and we have found the equivalent
+to be 7.02, it is evident that the true atomic weight is twice the
+equivalent, or 7.02 × 2 = 14.04.
+
+~Summary.~ These, then, are the steps necessary to establish the atomic
+weight of an element.
+
+1. Determine the equivalent accurately by analysis.
+
+2. Determine the molecular weight of a large number of compounds of the
+element, and by analysis the part of the molecular weight due to the
+element. The smallest number so obtained will be approximately the
+atomic weight.
+
+3. Multiply the equivalent by the small whole number (usually 1, 2, or
+3), which will make a number very close to the approximate atomic
+weight. The figure so obtained will be the true atomic weight.
+
+~Molecular weights of the elements.~ It will be noticed that the molecular
+weight of nitrogen obtained by multiplying its density by 28.9 is 28.08.
+Yet the atomic weight of nitrogen as deduced from a study of its gaseous
+compounds is 14.04. The simplest explanation that can be given for this
+is that the gaseous nitrogen is made up of molecules, each of which
+contains two atoms. In this respect it resembles oxygen; for we have
+seen that an entirely different line of reasoning leads us to believe
+that the molecule of oxygen contains two atoms. When we wish to indicate
+molecules of these gases the symbols N_{2} and O_{2} should be used.
+When we desire to merely show the weights taking part in a reaction this
+is not necessary.
+
+The vapor densities of many of the elements show that, like oxygen and
+nitrogen, their molecules consist of two atoms. In other cases,
+particularly among the metals, the molecule and the atom are identical.
+Still other elements have four atoms in their molecules.
+
+While oxygen contains two atoms in its molecules, a study of ozone has
+led to the conclusion that it has three. The formation of ozone from
+oxygen can therefore be represented by the equation
+
+    3O_{2} = 2O_{3}.
+
+~Other methods of determining molecular weights.~ It will be noticed that
+Avogadro's law gives us a method by which we can determine the relative
+weights of the molecules of two gases because it enables us to tell when
+we are dealing with an equal number of the two kinds of molecules. If by
+any other means we can get this information, we can make use of the
+knowledge so gained to determine the molecular weights of the two
+substances.
+
+~Raoult's laws.~ Two laws have been discovered which give us just such
+information. They are known as Raoult's laws, and can be stated as
+follows:
+
+1. _When weights of substances which are proportional to their molecular
+weights are dissolved in the same weight of solvent, the rise of the
+boiling point is the same in each case._
+
+2. _When weights of substances which are proportional to their molecular
+weights are dissolved in the same weight of solvent, the lowering of the
+freezing point is the same in each case._
+
+By taking advantage of these laws it is possible to determine when two
+solutions contain the same number of molecules of two dissolved
+substances, and consequently the relative molecular weights of the two
+substances.
+
+~Law of Dulong and Petit.~ In 1819 Dulong and Petit discovered a very
+interesting relation between the atomic weight of an element and its
+specific heat, which holds true for elements in the solid state. If
+equal weights of two solids, say, lead and silver, are heated through
+the same range of temperature, as from 10° to 20°, it is found that very
+different amounts of heat are required. The amount of heat required to
+change the temperature of a solid or a liquid by a definite amount
+compared with the amount required to change the temperature of an equal
+weight of water by the same amount is called its specific heat. Dulong
+and Petit discovered the following law: _The specific heat of an element
+in the solid form multiplied by its atomic weight is approximately equal
+to the constant 6.25._ That is,
+
+    at. wt. × sp. ht. = 6.25.
+
+
+Consequently,
+
+                6.25
+    at. wt. = --------
+               sp. ht.
+
+This law is not very accurate, but it is often possible by means of it
+to decide upon what multiple of the equivalent is the real atomic
+weight. Thus the specific heat of iron is found by experiment to be
+0.112, and its equivalent is 27.95. 6.25 ÷ 0.112 = 55.8. We see,
+therefore, that the atomic weight is twice the equivalent, or 55.9.
+
+~How formulas are determined.~ It will be well in connection with
+molecular weights to consider how the formula of a compound is decided
+upon, for the two subjects are very closely associated. Some examples
+will make clear the method followed.
+
+The molecular weight of a substance containing hydrogen and chlorine was
+36.4. By analysis 36.4 parts of the substance was found to contain 1
+part of hydrogen and 35.4 parts of chlorine. As these are the simple
+atomic weights of the two elements, the formula of the compound must be
+HCl.
+
+A substance consisting of oxygen and hydrogen was found to have a
+molecular weight of 34. Analysis showed that in 34 parts of the
+substance there were 2 parts of hydrogen and 32 parts of oxygen.
+Dividing these figures by the atomic weights of the two elements, we get
+2 ÷ 1 = 2 for H; 32 ÷ 16 = 2 for O. The formula is therefore H_{2}O_{2}.
+
+A substance containing 2.04% H, 32.6% S, and 65.3% O was found to have a
+molecular weight of 98. In these 98 parts of the substance there are 98
+× 2.04% = 2 parts of H, 98 × 32.6% = 32 parts of S, and 98 × 65.3% = 64
+parts of O. If the molecule weighs 98, the hydrogen atoms present must
+together weigh 2, the sulphur atoms 32, and the oxygen atoms 64.
+Dividing these figures by the respective atomic weights of the three
+elements, we have, for H, 2 ÷ 1 = 2 atoms; for S, 32 ÷ 32 = 1 atom; for
+O, 64 ÷ 16 = 4 atoms. Hence the formula is H_{2}SO_{4}.
+
+We have, then, this general procedure: Find the percentage composition
+of the substance and also its molecular weight. Multiply the molecular
+weight successively by the percentage of each element present, to find
+the amount of the element in the molecular weight of the compound. The
+figures so obtained will be the respective parts of the molecular weight
+due to the several atoms. Divide by the atomic weights of the respective
+elements, and the quotient will be the number of atoms present.
+
+~Avogadro's hypothesis and chemical calculations.~ This law simplifies
+many chemical calculations.
+
+1. _Application to volume relations in gaseous reactions._ Since equal
+volumes of gases contain an equal number of molecules, it follows that
+when an equal number of gaseous molecules of two or more gases take part
+in a reaction, the reaction will involve equal volumes of the gases. In
+the equation
+
+    C_{2}H_{2}O_{4} = H_{2}O + CO_{2} + CO,
+
+since 1 molecule of each of the gases CO_{2} and CO is set free from
+each molecule of oxalic acid, the two substances must always be set free
+in equal volumes.
+
+Acetylene burns in accordance with the equation
+
+    2C_{2}H_{2} + 5O_{2} = 4CO_{2} + 2H_{2}O.
+
+Hence 2 volumes of acetylene will react with 5 volumes of oxygen to form
+4 volumes of carbon dioxide and 2 volumes of steam. That the volume
+relations may be correct a gaseous element must be given its molecular
+formula. Thus oxygen must be written O_{2} and not 2O.
+
+2. _Application to weights of gases._ It will be recalled that the
+molecular weight of a gas is determined by ascertaining the weight of
+22.4 l. of the gas. This weight in grams is called the _gram-molecular
+weight_ of a gas. If the molecular weight of any gas is known, the
+weight of a liter of the gas under standard conditions may be determined
+by dividing its gram-molecular weight by 22.4. Thus the gram-molecular
+weight of a hydrochloric acid gas is 36.458. A liter of the gas will
+therefore weigh 36.458 ÷ 22.4 = 1.627 g.
+
+
+EXERCISES
+
+1. From the following data calculate the atomic weight of sulphur. The
+equivalent, as obtained by an analysis of sulphur dioxide, is 16.03. The
+densities and compositions of a number of compounds containing sulphur
+are as follows:
+
+      NAME             DENSITY      COMPOSITION BY PERCENTAGE
+Hydrosulphuric acid    1.1791     S = 94.11   H  =  5.89
+Sulphur dioxide        2.222      S = 50.05   O  = 49.95
+Sulphur trioxide       2.74       S = 40.05   O  = 59.95
+Sulphur chloride       4.70       S = 47.48   Cl = 52.52
+Sulphuryl chloride     4.64       S = 23.75   Cl = 52.53    O = 23.70
+Carbon disulphide      2.68       S = 84.24   C  = 15.76
+
+2. Calculate the formulas for compounds of the following compositions:
+
+                                            MOLECULAR
+                                              WEIGHT
+(1) S  = 39.07%   O = 58.49%   H =  2.44%      81.0
+(2) Ca = 29.40    S = 23.56    O = 47.04      136.2
+(3) K  = 38.67    N = 13.88    O = 47.45      101.2
+
+3. The molecular weight of ammonia is 17.06; of sulphur dioxide is
+64.06; of chlorine is 70.9. From the molecular weight calculate the
+weight of 1 l. of each of these gases. Compare your results with the
+table on the back cover of the book.
+
+4. From the molecular weight of the same gases calculate the density of
+each, referred to air as a standard.
+
+5. A mixture of 50 cc. of carbon monoxide and 50 cc. of oxygen was
+exploded in a eudiometer, (a) What gases remained in the tube after
+the explosion? (b) What was the volume of each?
+
+6. In what proportion must acetylene and oxygen be mixed to produce the
+greatest explosion?
+
+7. Solve Problem 18, Chapter XVII, without using molecular weights.
+Compare your results.
+
+8. Solve Problem 10, Chapter XVIII, without using molecular weights.
+Compare your results.
+
+9. The specific heat of aluminium is 0.214; of lead is 0.031. From these
+specific heats calculate the atomic weights of each of the elements.
+
+
+
+
+CHAPTER XX
+
+THE PHOSPHORUS FAMILY
+
+
+==================================================
+           |        | ATOMIC  |         | MELTING
+           | SYMBOL | WEIGHT  | DENSITY |  POINT
+-----------+--------+---------+---------+---------
+Phosphorus |   P    |    31.0 |    1.8  |    43.3°
+Arsenic    |   As   |    75.0 |    5.73 |     ---
+Antimony   |   Sb   |   120.2 |    6.7  |     432°
+Bismuth    |   Bi   |   208.5 |    9.8  |     270°
+==================================================
+
+~The family.~ The elements constituting this family belong in the same
+group with nitrogen and therefore resemble it in a general way. They
+exhibit a regular gradation of physical properties, as is shown in the
+above table. The same general gradation is also found in their chemical
+properties, phosphorus being an acid-forming element, while bismuth is
+essentially a metal. The other two elements are intermediate in
+properties.
+
+~Compounds.~ In general the elements of the family form compounds having
+similar composition, as is shown in the following table:
+
+    PH_{3}  PCl_{3}   PCl_{5}   P_{2}O_{3}   P_{2}O_{5}
+    AsH_{3} AsCl_{3}  AsCl_{5}  As_{2}O_{3}  As_{2}O_{5}
+    SbH_{3} SbCl_{3}  SbCl_{5}  Sb_{2}O_{3}  Sb_{2}O_{5}
+     ....   BiCl_{3}  BiCl_{5}  Bi_{2}O_{3}  Bi_{2}O_{5}
+
+In the case of phosphorus, arsenic, and antimony the oxides are acid
+anhydrides. Salts of at least four acids of each of these three elements
+are known, the free acid in some instances being unstable. The relation
+of these acids to the corresponding anhydrides may be illustrated as
+follows, phosphorus being taken as an example:
+
+    P_{2}O_{3} + 3H_{2}O = 2H_{3}PO_{3} (phosphorous acid).
+
+    P_{2}O_{5} + 3H_{2}O = 2H_{3}PO_{4} (phosphoric acid).
+
+    P_{2}O_{5} + 2H_{2}O = H_{4}P_{2}O_{7} (pyrophosphoric acid).
+
+    P_{2}O_{5} + H_{2}O = 2HPO_{3} (metaphosphoric acid).
+
+
+PHOSPHORUS
+
+~History.~ The element phosphorus was discovered by the alchemist Brand,
+of Hamburg, in 1669, while searching for the philosopher's stone. Owing
+to its peculiar properties and the secrecy which was maintained about
+its preparation, it remained a very rare and costly substance until the
+demand for it in the manufacture of matches brought about its production
+on a large scale.
+
+~Occurrence.~ Owing to its great chemical activity phosphorus never occurs
+free in nature. In the form of phosphates it is very abundant and widely
+distributed. _Phosphorite_ and _sombrerite_ are mineral forms of calcium
+phosphate, while _apatite_ consists of calcium phosphate together with
+calcium fluoride or chloride. These minerals form very large deposits
+and are extensively mined for use as fertilizers. Calcium phosphate is a
+constituent of all fertile soil, having been supplied to the soil by the
+disintegration of rocks containing it. It is the chief mineral
+constituent of bones of animals, and bone ash is therefore nearly pure
+calcium phosphate.
+
+~Preparation.~ Phosphorus is now manufactured from bone ash or a pure
+mineral phosphate by heating the phosphate with sand and carbon in an
+electric furnace. The materials are fed in at M (Fig. 70) by the feed
+screw F. The phosphorus vapor escapes at P and is condensed under
+water, while the calcium silicate is tapped off as a liquid at S. The
+phosphorus obtained in this way is quite impure, and is purified by
+distillation.
+
+[Illustration: Fig. 70]
+
+     ~Explanation of the reaction.~ To understand the reaction which
+     occurs, it must be remembered that a volatile acid anhydride is
+     expelled from its salts when heated with an anhydride which is
+     not volatile. Thus, when sodium carbonate and silicon dioxide
+     are heated together the following reaction takes place:
+
+         Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2}.
+
+     Silicon dioxide is a less volatile anhydride than phosphoric
+     anhydride (P_{2}O_{5}), and when strongly heated with a
+     phosphate the phosphoric anhydride is driven out, thus:
+
+         Ca_{3}(PO_{4})_{2} + 3SiO_{2} = 3CaSiO_{3} + P_{2}O_{5}.
+
+     If carbon is added before the heat is applied, the P_{2}O_{5}
+     is reduced to phosphorus at the same time, according to the
+     equation
+
+         P_{2}O_{5} + 5C = 2P + 5CO.
+
+~Physical properties.~ The purified phosphorus is a pale yellowish,
+translucent, waxy solid which melts at 43.3° and boils at 269°. It can
+therefore be cast into any convenient form under warm water, and is
+usually sold in the market in the form of sticks. It is quite soft and
+can be easily cut with a knife, but this must always be done while the
+element is covered with water, since it is extremely inflammable, and
+the friction of the knife blade is almost sure to set it on fire if cut
+in the air. It is not soluble in water, but is freely soluble in some
+other liquids, notably in carbon disulphide. Its density is 1.8.
+
+~Chemical properties.~ Exposed to the air phosphorus slowly combines with
+oxygen, and in so doing emits a pale light, or phosphorescence, which
+can be seen only in a dark place. The heat of the room may easily raise
+the temperature to the kindling point of phosphorus, when it burns with
+a sputtering flame, giving off dense fumes of oxide of phosphorus. It
+burns with dazzling brilliancy in oxygen, and combines directly with
+many other elements, especially with sulphur and the halogens. On
+account of its great affinity for oxygen it is always preserved under
+water.
+
+Phosphorus is very poisonous, from 0.2 to 0.3 gram being a fatal dose.
+Ground up with flour and water or similar substances, it is often used
+as a poison for rats and other vermin.
+
+     ~Precaution.~ The heat of the body is sufficient to raise
+     phosphorus above its kindling temperature, and for this reason
+     it should always be handled with forceps and never with the
+     bare fingers. Burns occasioned by it are very painful and slow
+     in healing.
+
+~Red phosphorus.~ On standing, yellow phosphorus gradually undergoes a
+remarkable change, being converted into a dark red powder which has a
+density of 2.1. It no longer takes fire easily, neither does it dissolve
+in carbon disulphide. It is not poisonous and, in fact, seems to be an
+entirely different substance. The velocity of this change increases with
+rise in temperature, and the red phosphorus is therefore prepared by
+heating the yellow just below the boiling point (250°-300°). When
+distilled and quickly condensed the red form changes back to the yellow.
+This is in accordance with the general rule that when a substance
+capable of existing in several allotropic forms is condensed from a gas
+or crystallized from the liquid state, the more unstable variety forms
+first, and this then passes into the more stable forms.
+
+     ~Matches.~ The chief use of phosphorus is in the manufacture of
+     matches. Common matches are made by first dipping the match
+     sticks into some inflammable substance, such as melted
+     paraffin, and afterward into a paste consisting of (1)
+     phosphorus, (2) some oxidizing substance, such as manganese
+     dioxide or potassium chlorate, and (3) a binding material,
+     usually some kind of glue. On friction the phosphorus is
+     ignited, the combustion being sustained by the oxidizing agent
+     and communicated to the wood by the burning paraffin. In
+     sulphur matches the paraffin is replaced by sulphur.
+
+     In safety matches _red_ phosphorus, an oxidizing agent, and
+     some gritty material such as emery is placed on the side of the
+     box, while the match tip is provided as before with an
+     oxidizing agent and an easily oxidized substance, usually
+     antimony sulphide. The match cannot be ignited easily by
+     friction, save on the prepared surface.
+
+~Compounds of phosphorus with hydrogen.~ Phosphorus forms several
+compounds with hydrogen, the best known of which is phosphine (PH_{3})
+analogous to ammonia (NH_{3}).
+
+~Preparation of phosphine.~ Phosphine is usually made by heating
+phosphorus with a strong solution of potassium hydroxide, the reaction
+being a complicated one.
+
+[Illustration: Fig. 71]
+
+     The experiment can be conveniently made in the apparatus shown
+     in Fig. 71. A strong solution of potassium hydroxide together
+     with several small bits of phosphorus are placed in the flask
+     A, and a current of coal gas is passed into the flask through
+     the tube B until all the air has been displaced. The gas is
+     then turned off and the flask is heated. Phosphine is formed in
+     small quantities and escapes through the delivery tube, the
+     exit of which is just covered by the water in the vessel C.
+     Each bubble of the gas as it escapes into the air takes fire,
+     and the product of combustion (P_{2}O_{5}) forms beautiful
+     small rings, which float unbroken for a considerable time in
+     quiet air. The pure phosphine does not take fire spontaneously.
+     When prepared as directed above, impurities are present which
+     impart this property.
+
+~Properties.~ Phosphine is a gas of unpleasant odor and is exceedingly
+poisonous. Like ammonia it forms salts with the halogen acids. Thus we
+have phosphonium chloride (PH_{4}Cl) analogous to ammonium chloride
+(NH_{4}Cl). The phosphonium salts are of but little importance.
+
+~Oxides of phosphorus.~ Phosphorus forms two well-known oxides,--the
+trioxide (P_{2}O_{3}) and the pentoxide (P_{2}O_{5}), sometimes called
+phosphoric anhydride. When phosphorus burns in an insufficient supply of
+air the product is partially the trioxide; in oxygen or an excess of air
+the pentoxide is formed. The pentoxide is much the better known of the
+two. It is a snow-white, voluminous powder whose most marked property is
+its great attraction for water. It has no chemical action upon most
+gases, so that they can be very thoroughly dried by allowing them to
+pass through properly arranged vessels containing phosphorus pentoxide.
+
+~Acids of phosphorus.~ The important acids of phosphorus are the
+following:
+
+    H_{3}PO_{3}       phosphorous acid.
+    H_{3}PO_{4}       phosphoric acid.
+    H_{4}P_{2}O_{7}   pyrophosphoric acid.
+    HPO_{3}           metaphosphoric acid.
+
+These may be regarded as combinations of the oxides of phosphorus with
+water according to the equations given in the discussion of the
+characteristics of the family.
+
+1. _Phosphorous acid_ (H_{3}PO_{3}). Neither the acid nor its salts are
+at all frequently met with in chemical operations. It can be easily
+obtained, however, in the form of transparent crystals when phosphorus
+trichloride is treated with water and the resulting solution is
+evaporated:
+
+    PCl_{3} + 3H_{2}O = H_{3}PO_{3} + 3HCl.
+
+Its most interesting property is its tendency to take up oxygen and pass
+over into phosphoric acid.
+
+2. _Orthophosphoric acid (phosphoric acid)_ (H_{3}PO_{4}). This acid can
+be obtained by dissolving phosphorus pentoxide in boiling water, as
+represented in the equation
+
+    P_{2}O_{5} + 3H_{2}O = 2H_{3}PO_{4}.
+
+It is usually made by treating calcium phosphate with concentrated
+sulphuric acid. The calcium sulphate produced in the reaction is nearly
+insoluble, and can be filtered off, leaving the phosphoric acid in
+solution. Very pure acid is made by oxidizing phosphorus with nitric
+acid. It forms large colorless crystals which are exceedingly soluble in
+water. Being a tribasic acid, it forms acid as well as normal salts.
+Thus the following compounds of sodium are known:
+
+    NaH_{2}PO_{4}   monosodium hydrogen phosphate.
+    Na_{2}HPO_{4}   disodium hydrogen phosphate.
+    Na_{3}PO_{4}    normal sodium phosphate.
+
+These salts are sometimes called respectively primary, secondary, and
+tertiary phosphates. They may be prepared by bringing together
+phosphoric acid and appropriate quantities of sodium hydroxide.
+Phosphoric acid also forms mixed salts, that is, salts containing two
+different metals. The most familiar compound of this kind is microcosmic
+salt, which has the formula Na(NH_{4})HPO_{4}.
+
+_Orthophosphates._ The orthophosphates form an important class of salts.
+The normal salts are nearly all insoluble and many of them occur in
+nature. The secondary phosphates are as a rule insoluble, while most of
+the primary salts are soluble.
+
+3. _Pyrophosphoric acid_ (H_{4}P_{2}O_{7}). On heating orthophosphoric
+acid to about 225° pyrophosphoric acid is formed in accordance with the
+following equation:
+
+    2H_{3}PO_{4} = H_{4}P_{2}O_{7} + H_{2}O.
+
+It is a white crystalline solid. Its salts can be prepared by heating a
+secondary phosphate:
+
+    2Na_{2}HPO_{4} = Na_{4}P_{2}O_{7} + H_{2}O.
+
+4. _Metaphosphoric acid (glacial phosphoric acid)_ (HPO_{3}). This acid
+is formed when orthophosphoric acid is heated above 400°:
+
+    H_{3}PO_{4} = HPO_{3} + H_{2}O.
+
+It is also formed when phosphorus pentoxide is treated with cold water:
+
+    P_{2}O_{5} + H_{2}O = 2HPO_{3}.
+
+It is a white crystalline solid, and is so stable towards heat that it
+can be fused and even volatilized without decomposition. On cooling from
+the fused state it forms a glassy solid, and on this account is often
+called glacial phosphoric acid. It possesses the property of dissolving
+small quantities of metallic oxides, with the formation of compounds
+which, in the case of certain metals, have characteristic colors. It is
+therefore used in the detection of these metals.
+
+While the secondary phosphates, on heating, give salts of pyrophosphoric
+acid, the primary phosphates yield salts of metaphosphoric acid. The
+equations representing these reactions are as follows:
+
+    2Na_{2}HPO_{4} = Na_{4}P_{3}O_{7} + H_{2}O,
+
+      NaH_{2}PO_{4} = NaPO_{3} + H_{2}O.
+
+~Fertilizers.~ When crops are produced year after year on the same field
+certain constituents of the soil essential to plant growth are removed,
+and the soil becomes impoverished and unproductive. To make the land
+once more fertile these constituents must be replaced. The calcium
+phosphate of the mineral deposits or of bone ash serves well as a
+material for restoring phosphorus to soils exhausted of that essential
+element; but a more soluble substance, which the plants can more readily
+assimilate, is desirable. It is better, therefore, to convert the
+insoluble calcium phosphate into the soluble primary phosphate before it
+is applied as fertilizer. It will be seen by reference to the formulas
+for the orthophosphates (see page 244) that in a primary phosphate only
+one hydrogen atom of phosphoric acid is replaced by a metal. Since the
+calcium atom always replaces two hydrogen atoms, it might be thought
+that there could be no primary calcium phosphate; but if the calcium
+atom replaces one hydrogen atom from each of two molecules of phosphoric
+acid, the salt Ca(H_{2}PO_{4})_{2} will result, and this is a primary
+phosphate. It can be made by treatment of the normal phosphate with the
+necessary amount of sulphuric acid, calcium sulphate being formed at the
+same time, thus:
+
+    Ca_{3}(PO_{4})_{2} + 2H_{2}SO_{4} = Ca(H_{2}PO_{4})_{2} + 2CaSO_{4}.
+
+The resulting mixture is a powder, which is sold as a fertilizer under
+the name of "superphosphate of lime."
+
+
+ARSENIC
+
+~Occurrence.~ Arsenic occurs in considerable quantities in nature as the
+native element, as the sulphides realgar (As_{2}S_{2}) and orpiment
+(As_{2}S_{3}), as oxide (As_{2}O_{3}), and as a constituent of many
+metallic sulphides, such as arsenopyrite (FeAsS).
+
+~Preparation.~ The element is prepared by purifying the native arsenic, or
+by heating the arsenopyrite in iron tubes, out of contact with air,
+when the reaction expressed by the following equation occurs:
+
+    FeAsS = FeS + As.
+
+The arsenic, being volatile, condenses in chambers connected with the
+heated tubes. It is also made from the oxide by reduction with carbon:
+
+    2As_{2}O_{3} + 3C = 4As + 3CO_{2}.
+
+~Properties.~ Arsenic is a steel-gray, metallic-looking substance of
+density 5.73. Though resembling metals in appearance, it is quite
+brittle, being easily powdered in a mortar. When strongly heated it
+sublimes, that is, it passes into a vapor without melting, and condenses
+again to a crystalline solid when the vapor is cooled. Like phosphorus
+it can be obtained in several allotropic forms. It alloys readily with
+some of the metals, and finds its chief use as an alloy with lead, which
+is used for making shot, the alloy being harder than pure lead. When
+heated on charcoal with the blowpipe it is converted into an oxide which
+volatilizes, leaving the charcoal unstained by any oxide coating. It
+burns readily in chlorine gas, forming arsenic trichloride,--
+
+    As + 3Cl = AsCl_{3}.
+
+Unlike most of its compounds, the element itself is not poisonous.
+
+~Arsine~ (AsH_{3}). When any compound containing arsenic is brought into
+the presence of nascent hydrogen, arsine (AsH_{3}), corresponding to
+phosphine and ammonia, is formed. The reaction when oxide of arsenic is
+so treated is
+
+    As_{2}O_{3} + 12H = 2AsH_{3} + 3H_{2}O.
+
+Arsine is a gas with a peculiar garlic-like odor, and is intensely
+poisonous. A single bubble of pure gas has been known to prove fatal. It
+is an unstable compound, decomposing into its elements when heated to a
+moderate temperature. It is combustible, burning with a pale
+bluish-white flame to form arsenic trioxide and water when air is in
+excess:
+
+    2AsH_{3} + 6O = As_{2}O_{3} + 3H_{2}O.
+
+When the supply of air is deficient water and metallic arsenic are
+formed:
+
+    2AsH_{3} + 3O = 3H_{2}O + 2As.
+
+These reactions make the detection of even minute quantities of arsenic
+a very easy problem.
+
+[Illustration: Fig. 72]
+
+     ~Marsh's test for arsenic.~ The method devised by Marsh for
+     detecting arsenic is most frequently used, the apparatus being
+     shown in Fig. 72. Hydrogen is generated in the flask A by the
+     action of dilute sulphuric acid on zinc, is dried by passing
+     over calcium chloride in the tube B, and after passing
+     through the hard-glass tube C is ignited at the jet D. If a
+     substance containing arsenic is now introduced into the
+     generator A, the arsenic is converted into arsine by the
+     action of the nascent hydrogen, and passes to the jet along
+     with the hydrogen. If the tube C is strongly heated at some
+     point near the middle, the arsine is decomposed while passing
+     this point and the arsenic is deposited just beyond the heated
+     point in the form of a shining, brownish-black mirror. If the
+     tube is not heated, the arsine burns along with the hydrogen at
+     the jet. Under these conditions a small porcelain dish crowded
+     down into the flame is blackened by a spot of metallic arsenic,
+     for the arsine is decomposed by the heat of the flame, and the
+     arsenic, cooled below its kindling temperature by the cold
+     porcelain, deposits upon it as a black spot. Antimony conducts
+     itself in the same way as arsenic, but the antimony deposit is
+     more sooty in appearance. The two can also be distinguished by
+     the fact that sodium hypochlorite (NaClO) dissolves the arsenic
+     deposit, but not that formed by antimony.
+
+~Oxides of arsenic.~ Arsenic forms two oxides, As_{2}O_{3} and
+As_{2}O_{5}, corresponding to those of phosphorus. Of these arsenious
+oxide, or arsenic trioxide (As_{2}O_{3}), is much better known, and is
+the substance usually called white arsenic, or merely arsenic. It is
+found as a mineral, but is usually obtained as a by-product in burning
+pyrite in the sulphuric-acid industry. The pyrite has a small amount of
+arsenopyrite in it, and when this is burned arsenious oxide is formed as
+a vapor together with sulphur dioxide:
+
+    2FeAsS + 10O = Fe_{2}O_{3} + As_{2}O_{3} + 2SO_{2}.
+
+The arsenious oxide is condensed in appropriate chambers. It is a rather
+heavy substance, obtained either as a crystalline powder or as large,
+vitreous lumps, resembling lumps of porcelain in appearance. It is very
+poisonous, from 0.2 to 0.3 g. being a fatal dose. It is frequently given
+as a poison, since it is nearly tasteless and does not act very rapidly.
+This slow action is due to the fact that it is not very soluble, and
+hence is absorbed slowly by the system. Arsenious oxide is also used as
+a chemical reagent in glass making and in the dye industry.
+
+~Acids of arsenic.~ Like the corresponding oxides of phosphorus, the
+oxides of arsenic are acid anhydrides. In solution they combine with
+bases to form salts, corresponding to the salts of the acids of
+phosphorus. Thus we have salts of the following acids:
+
+    H_{3}AsO_{3} arsenious acid.
+
+    H_{3}AsO_{4} orthoarsenic acid.
+
+    H_{4}As_{2}O_{3} pyroarsenic acid.
+
+    HAsO_{3} metarsenic acid.
+
+Several other acids of arsenic are also known. Not all of these can be
+obtained as free acids, since they tend to lose water and form the
+oxides. Thus, instead of obtaining arsenious acid (H_{3}AsO_{3}), the
+oxide As_{2}O_{3} is obtained:
+
+    2H_{3}AsO_{3} = As_{2}O_{3} + 3H_{2}O.
+
+Salts of all the acids are known, however, and some of them have
+commercial value. Most of them are insoluble, and some of the copper
+salts, which are green, are used as pigments. Paris green, which has a
+complicated formula, is a well-known insecticide.
+
+~Antidote for arsenical poisoning.~ The most efficient antidote for
+arsenic poisoning is ferric hydroxide. It is prepared as needed,
+according to the equation
+
+    Fe_{2}(SO_{4})_{3} + 3Mg(OH)_{2} = 2Fe(OH)_{3} + 3MgSO_{4}.
+
+~Sulphides of arsenic.~ When hydrogen sulphide is passed into an acidified
+solution containing an arsenic compound the arsenic is precipitated as a
+bright yellow sulphide, thus:
+
+    2H_{3}AsO_{3} + 3H_{2}S = As_{2}S_{3} + 6H_{2}O,
+
+    2H_{3}AsO_{4} + 5H_{2}S = As_{2}S_{5} + 8H_{2}O.
+
+In this respect arsenic resembles the metallic elements, many of which
+produce sulphides under similar conditions. The sulphides of arsenic,
+both those produced artificially and those found in nature, are used as
+yellow pigments.
+
+
+ANTIMONY
+
+~Occurrence.~ Antimony occurs in nature chiefly as the sulphide
+(Sb_{2}S_{3}), called stibnite, though it is also found as oxide and as
+a constituent of many complex minerals.
+
+~Preparation.~ Antimony is prepared from the sulphide in a very simple
+manner. The sulphide is melted with scrap iron in a furnace, when the
+iron combines with the sulphur to form a slag, or liquid layer of melted
+iron sulphide, while the heavier liquid, antimony, settles to the bottom
+and is drawn off from time to time. The reaction involved is represented
+by the equation
+
+    Sb_{2}S_{3} + 3Fe = 2Sb + 3FeS.
+
+~Physical properties.~ Antimony is a bluish-white, metallic-looking
+substance whose density is 6.7. It is highly crystalline, hard, and very
+brittle. It has a rather low melting point (432°) and expands very
+noticeably on solidifying.
+
+~Chemical properties.~ In chemical properties antimony resembles arsenic
+in many particulars. It forms the oxides Sb_{2}O_{3} and Sb_{2}O_{5},
+and in addition Sb_{2}O_{4}. It combines with the halogen elements with
+great energy, burning brilliantly in chlorine to form antimony
+trichloride (SbCl_{3}). When heated on charcoal with the blowpipe it is
+oxidized and forms a coating of antimony oxide on the charcoal which has
+a characteristic bluish-white color.
+
+~Stibine~ (SbH_{3}). The gas stibine (SbH_{3}) is formed under conditions
+which are very similar to those which produce arsine, and it closely
+resembles the latter compound, though it is still less stable. It is
+very poisonous.
+
+     ~Acids of antimony.~ The oxides Sb_{2}O_{3} and Sb_{2}O_{5} are
+     weak acid anhydrides and are capable of forming two series of
+     acids corresponding in formulas to the acids of phosphorus and
+     arsenic. They are much weaker, however, and are of little
+     practical importance.
+
+     ~Sulphides of antimony.~ Antimony resembles arsenic in that
+     hydrogen sulphide precipitates it as a sulphide when conducted
+     into an acidified solution containing an antimony compound:
+
+        2SbCl_{3} + 3H_{2}S = Sb_{2}S_{3} + 6HCl,
+
+        2SbCl_{5} + 5H_{2}S = Sb_{2}S_{5} + 10HCl.
+
+     The two sulphides of antimony are called the trisulphide and
+     the pentasulphide respectively. When prepared in this way they
+     are orange-colored substances, though the mineral stibnite is
+     black.
+
+~Metallic properties of antimony.~ The physical properties of the element
+are those of a metal, and the fact that its sulphide is precipitated by
+hydrogen sulphide shows that it acts like a metal in a chemical way.
+Many other reactions show that antimony has more of the properties of a
+metal than of a non-metal. The compound Sb(OH)_{3}, corresponding to
+arsenious acid, while able to act as a weak acid is also able to act as
+a weak base with strong acids. For example, when treated with
+concentrated hydrochloric acid antimony chloride is formed:
+
+    Sb(OH)_{3} + 3HCl = SbCl_{3} + 3H_{2}O.
+
+A number of elements act in this same way, their hydroxides under some
+conditions being weak acids and under others weak bases.
+
+
+ALLOYS
+
+Some metals when melted together thoroughly intermix, and on cooling
+form a homogeneous, metallic-appearing substance called an _alloy_. Not
+all metals will mix in this way, and in some cases definite chemical
+compounds are formed and separate out as the mixture solidifies, thus
+destroying the uniform quality of the alloy. In general the melting
+point of the alloy is below the average of the melting points of its
+constituents, and it is often lower than any one of them.
+
+Antimony forms alloys with many of the metals, and its chief commercial
+use is for such purposes. It imparts to its alloys high density, rather
+low melting point, and the property of expanding on solidification.
+Such an alloy is especially useful in type founding, where fine lines
+are to be reproduced on a cast. Type metal consists of antimony, lead,
+and tin. Babbitt metal, used for journal bearings in machinery, contains
+the same metals in a different proportion together with a small
+percentage of copper.
+
+
+BISMUTH
+
+~Occurrence.~ Bismuth is usually found in the uncombined form in nature.
+It also occurs as oxide and sulphide. Most of the bismuth of commerce
+comes from Saxony, and from Mexico and Colorado, but it is not an
+abundant element.
+
+~Preparation.~ It is prepared by merely heating the ore containing the
+native bismuth and allowing the melted metal to run out into suitable
+vessels. Other ores are converted into oxides and reduced by heating
+with carbon.
+
+~Physical properties.~ Bismuth is a heavy, crystalline, brittle metal
+nearly the color of silver, but with a slightly rosy tint which
+distinguishes it from other metals. It melts at a low temperature (270°)
+and has a density of 9.8. It is not acted upon by the air at ordinary
+temperatures.
+
+~Chemical properties.~ When heated with the blowpipe on charcoal, bismuth
+gives a coating of the oxide Bi_{2}O_{3}. This has a yellowish-brown
+color which easily distinguishes it from the oxides formed by other
+metals. It combines very readily with the halogen elements, powdered
+bismuth burning readily in chlorine. It is not very easily acted upon by
+hydrochloric acid, but nitric and sulphuric acids act upon it in the
+same way that they do upon copper.
+
+~Uses.~ Bismuth finds its chief use as a constituent of alloys,
+particularly in those of low melting point. Some of these melt in hot
+water. For example, Wood's metal, consisting of bismuth, lead, tin, and
+cadmium, melts at 60.5°.
+
+~Compounds of bismuth.~ Unlike the other elements of this group, bismuth
+has almost no acid properties. Its chief oxide, Bi_{2}O_{3}, is basic in
+its properties. It dissolves in strong acids and forms salts of bismuth:
+
+    Bi_{2}O_{3} + 6HCl = 2BiCl_{3} + 3H_{2}O,
+
+    Bi_{2}O_{3} + 6HNO_{3} = 2Bi(NO_{3})_{3} + 3H_{2}O.
+
+The nitrate and chloride of bismuth can be obtained as well-formed
+colorless crystals. When treated with water the salts are decomposed in
+the manner explained in the following paragraph.
+
+
+HYDROLYSIS
+
+Many salts such as those of antimony and bismuth form solutions which
+are somewhat acid in reaction, and must therefore contain hydrogen ions.
+This is accounted for by the same principle suggested to explain the
+fact that solutions of potassium cyanide are alkaline in reaction (p.
+210). Water forms an appreciable number of hydrogen and hydroxyl ions,
+and very weak bases such as bismuth hydroxide are dissociated to but a
+very slight extent. When Bi^{+++} ions from bismuth chloride, which
+dissociates very readily, are brought in contact with the OH^{-} ions
+from water, the two come to the equilibrium expressed in the equation
+
+    Bi^{+++} + 3OH^{-} <--> Bi(OH)_{3}.
+
+For every hydroxyl ion removed from the solution in this way a hydrogen
+ion is left free, and the solution becomes acid in reaction.
+
+Reactions of this kind and that described under potassium cyanide are
+called _hydrolysis_.
+
+DEFINITION: _Hydrolysis is the action of water upon a salt to form an
+acid and a base, one of which is very slightly dissociated._
+
+~Conditions favoring hydrolysis.~ While hydrolysis is primarily due to the
+slight extent to which either the acid or the base formed is
+dissociated, several other factors have an influence upon the extent to
+which it will take place.
+
+1. _Influence of mass._ Since hydrolysis is a reversible reaction, the
+relative masses of the reacting substances influence the point at which
+equilibrium will be reached. In the equilibrium
+
+    BiCl_{3} + 3H_{2}O <--> Bi(OH)_{3} + 3HCl
+
+the addition of more water will result in the formation of more bismuth
+hydroxide and hydrochloric acid. The addition of more hydrochloric acid
+will convert some of the bismuth hydroxide into bismuth chloride.
+
+2. _Formation of insoluble substances._ When one of the products of
+hydrolysis is nearly insoluble in water the solution will become
+saturated with it as soon as a very little has been formed. All in
+excess of this will precipitate, and the reaction will go on until the
+acid set free increases sufficiently to bring about an equilibrium. Thus
+a considerable amount of bismuth and antimony hydroxides are
+precipitated when water is added to the chlorides of these elements. The
+greater the dilution the more hydroxide precipitates. The addition of
+hydrochloric acid in considerable quantity will, however, redissolve the
+precipitate.
+
+~Partial hydrolysis.~ In many cases the hydrolysis of a salt is only
+partial, resulting in the formation of basic salts instead of the free
+base. Most of these basic salts are insoluble in water, which accounts
+for their ready formation. Thus bismuth chloride may hydrolyze by
+successive steps, as shown in the equations
+
+    BiCl_{3} + H_{2}O = Bi(OH)Cl_{2} + HCl,
+
+    BiCl_{3} + 2H_{2}O = Bi(OH)_{2}Cl + 2HCl,
+
+    BiCl_{3} + 3H_{2}O = Bi(OH)_{3} + 3HCl.
+
+The basic salt so formed may also lose water, as shown in the equation
+
+    Bi(OH)_{2}Cl = BiOCl + H_{2}O.
+
+The salt represented in the last equation is sometimes called bismuth
+oxychloride, or bismuthyl chloride. The corresponding nitrate,
+BiONO_{3}, is largely used in medicine under the name of subnitrate of
+bismuth. In these two compounds the group of atoms, BiO, acts as a
+univalent metallic radical and is called _bismuthyl_. Similar basic
+salts are formed by the hydrolysis of antimony salts.
+
+
+EXERCISES
+
+1. Name all the elements so far studied which possess allotropic forms.
+
+2. What compounds would you expect phosphorus to form with bromine and
+iodine? Write the equations showing the action of water on these
+compounds.
+
+3. In the preparation of phosphine, why is coal gas passed into the
+flask? What other gases would serve the same purpose?
+
+4. Give the formula for the salt which phosphine forms with hydriodic
+acid. Give the name of the compound.
+
+5. Could phosphoric acid be substituted for sulphuric acid in the
+preparation of the common acids?
+
+6. Write the equations for the preparation of the three sodium salts of
+orthophosphoric acid.
+
+7. Why does a solution of disodium hydrogen phosphate react alkaline?
+
+8. On the supposition that bone ash is pure calcium phosphate, what
+weight of it would be required in the preparation of 1 kg. of
+phosphorus?
+
+9. If arsenopyrite is heated in a current of air, what products are
+formed?
+
+10. (a) Write equations for the complete combustion of hydrosulphuric
+acid, methane, and arsine. (b) In what respects are the reactions
+similar?
+
+11. Write the equations for all the reactions involved in Marsh's test
+for arsenic.
+
+12. Write the names and formulas for the acids of antimony.
+
+13. Write the equations showing the hydrolysis of antimony trichloride;
+of bismuth nitrate.
+
+14. In what respects does nitrogen resemble the members of the
+phosphorus family?
+
+
+
+
+CHAPTER XXI
+
+SILICON, TITANIUM, BORON
+
+
+=================================================================
+            |        |        |         |           |
+            | SYMBOL | ATOMIC | DENSITY | CHLORIDES | OXIDES
+            |        | WEIGHT |         |           |
+____________|________|________|_________|___________|____________
+            |        |        |         |           |
+Silicon     |   Si   |  28.4  |   2.35  | SiCl_{4}  | SiO_{2}
+Titanium    |   Ti   |  48.1  |   3.5   | TiCl_{4}  | TiO_{2}
+Boron       |   B    |  11.0  |   2.45  | BCl_{3}   | B_{2}O_{3}
+=================================================================
+
+~General.~ Each of the three elements, silicon, titanium, and boron,
+belongs to a separate periodic family, but they occur near together in
+the periodic grouping and are very similar in both physical and chemical
+properties. Since the other elements in their families are either so
+rare that they cannot be studied in detail, or are best understood in
+connection with other elements, it is convenient to consider these three
+together at this point.
+
+The three elements are very difficult to obtain in the free state, owing
+to their strong attraction for other elements. They can be prepared by
+the action of aluminium or magnesium on their oxides and in impure state
+by reduction with carbon in an electric furnace. They are very hard and
+melt only at the highest temperatures. At ordinary temperatures they are
+not attacked by oxygen, but when strongly heated they burn with great
+brilliancy. Silicon and boron are not attacked by acids under ordinary
+conditions; titanium is easily dissolved by them.
+
+
+SILICON
+
+~Occurrence.~ Next to oxygen silicon is the most abundant element. It does
+not occur free in nature, but its compounds are very abundant and of the
+greatest importance. It occurs almost entirely in combination with
+oxygen as silicon dioxide (SiO_{2}), often called silica, or with oxygen
+and various metals in the form of salts of silicic acids, or silicates.
+These compounds form a large fraction of the earth's crust. Most plants
+absorb small amounts of silica from the soil, and it is also found in
+minute quantities in animal organisms.
+
+~Preparation.~ The element is most easily prepared by reducing pure
+powdered quartz with magnesium powder:
+
+    SiO_{2} + 2Mg = 2MgO + Si.
+
+~Properties.~ As would be expected from its place in the periodic table,
+silicon resembles carbon in many respects. It can be obtained in several
+allotropic forms, corresponding to those of carbon. The crystallized
+form is very hard, and is inactive toward reagents. The amorphous
+variety has, in general, properties more similar to charcoal.
+
+~Compounds of silicon with hydrogen and the halogens.~ Silicon hydride
+(SiH_{4}) corresponds in formula to methane (CH_{4}), but its properties
+are more like those of phosphine (PH_{3}). It is a very inflammable gas
+of disagreeable odor, and, as ordinarily prepared, takes fire
+spontaneously on account of the presence of impurities.
+
+Silicon combines with the elements of the chlorine family to form such
+compounds as SiCl_{4} and SiF_{4}. Of these silicon fluoride is the most
+familiar and interesting. As stated in the discussion of fluorine, it is
+formed when hydrofluoric acid acts upon silicon dioxide or a silicate.
+With silica the reaction is thus expressed:
+
+    SiO_{2} + 4HF = SiF_{4} + 2H_{2}O.
+
+It is a very volatile, invisible, poisonous gas. In contact with water
+it is partially decomposed, as shown in the equation
+
+    SiF_{4} + 4H_{2}O = 4HF + Si(OH)_{4}.
+
+The hydrofluoric acid so formed combines with an additional amount of
+silicon fluoride, forming the complex fluosilicic acid (H_{2}SiF_{6}),
+thus:
+
+    2HF + SiF_{4} = H_{2}SiF_{6}.
+
+~Silicides.~ As the name indicates, silicides are binary compounds
+consisting of silicon and some other element. They are very stable at
+high temperatures, and are usually made by heating the appropriate
+substances in an electric furnace. The most important one is
+_carborundum_, which is a silicide of carbon of the formula CSi. It is
+made by heating coke and sand, which is a form of silicon dioxide, in an
+electric furnace, the process being extensively carried on at Niagara
+Falls. The following equation represents the reaction
+
+    SiO_{2} + 3C = CSi + 2CO.
+
+The substance so prepared consists of beautiful purplish-black crystals,
+which are very hard. Carborundum is used as an abrasive, that is, as a
+material for grinding and polishing very hard substances. Ferrosilicon
+is a silicide of iron alloyed with an excess of iron, which finds
+extensive use in the manufacture of certain kinds of steel.
+
+~Manufacture of carborundum.~ The mixture of materials is heated in a
+large resistance furnace for about thirty-six hours. After the reaction
+is completed there is left a core of graphite G. Surrounding this core
+is a layer of crystallized carborundum C, about 16 in. thick. Outside
+this is a shell of amorphous carborundum A. The remaining materials
+M are unchanged and are used for a new charge.
+
+[Illustration: Fig. 73]
+
+~Silicon dioxide~ (_silica_) (SiO_{2}). This substance is found in a great
+variety of forms in nature, both in the amorphous and in the crystalline
+condition. In the form of quartz it is found in beautifully formed
+six-sided prisms, sometimes of great size. When pure it is perfectly
+transparent and colorless. Some colored varieties are given special
+names, as amethyst (violet), rose quartz (pale pink), smoky or milky
+quartz (colored and opaque). Other varieties of silicon dioxide, some of
+which also contain water, are chalcedony, onyx, jasper, opal, agate, and
+flint. Sand and sandstone are largely silicon dioxide.
+
+~Properties.~ As obtained by chemical processes silicon dioxide is an
+amorphous white powder. In the crystallized state it is very hard and
+has a density of 2.6. It is insoluble in water and in most chemical
+reagents, and requires the hottest oxyhydrogen flame for fusion. Acids,
+excepting hydrofluoric acid, have little action on it, and it requires
+the most energetic reducing agents to deprive it of oxygen. It is the
+anhydride of an acid, and consequently it dissolves in fused alkalis to
+form silicates. Being nonvolatile, it will drive out most other
+anhydrides when heated to a high temperature with their salts,
+especially when the silicates so formed are fusible. The following
+equations illustrate this property:
+
+    Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},
+
+    Na_{2}SO_{4} + SiO_{2} = Na_{2}SiO_{3} + SO_{3}.
+
+~Silicic acids.~ Silicon forms two simple acids, orthosilicic acid
+(H_{4}SiO_{4}) and metasilicic acid (H_{2}SiO_{3}). Orthosilicic acid is
+formed as a jelly-like mass when orthosilicates are treated with strong
+acids such as hydrochloric. On attempting to dry this acid it loses
+water, passing into metasilicic or common silicic acid:
+
+    H_{4}SiO_{4} = H_{2}SiO_{3} + H_{2}O.
+
+Metasilicic acid when heated breaks up into silica and water, thus:
+
+    H_{2}SiO_{3} = H_{2}O + SiO_{2}.
+
+     ~Salts of silicic acids,--silicates.~ A number of salts of the
+     orthosilicic and metasilicic acids occur in nature. Thus mica
+     (KAlSiO_{4}) is a salt of orthosilicic acid.
+
+~Polysilicic acids.~ Silicon has the power to form a great many complex
+acids which may be regarded as derived from the union of several
+molecules of the orthosilicic acid, with the loss of water. Thus we have
+
+    3H_{4}SiO_{4} = H_{4}Si_{3}O_{8} + 4H_{2}O.
+
+These acids cannot be prepared in the pure state, but their salts form
+many of the crystalline rocks in nature. Feldspar, for example, has the
+formula KAlSi_{3}O_{8}, and is a mixed salt of the acid
+H_{4}Si_{3}O_{8}, whose formation is represented in the equation above.
+Kaolin has the formula Al_{2}Si_{2}O_{7}·2H_{2}O. Many other examples
+will be met in the study of the metals.
+
+~Glass.~ When sodium and calcium silicates, together with silicon dioxide,
+are heated to a very high temperature, the mixture slowly fuses to a
+transparent liquid, which on cooling passes into the solid called glass.
+Instead of starting with sodium and calcium silicates it is more
+convenient and economical to heat sodium carbonate (or sulphate) and
+lime with an excess of clean sand, the silicates being formed during the
+heating:
+
+    Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},
+
+    CaO + SiO_{2} = CaSiO_{3}.
+
+[Illustration: Fig. 74]
+
+The mixture is heated below the fusing point for some time, so that the
+escaping carbon dioxide may not spatter the hot liquid; the heat is then
+increased and the mixture kept in a state of fusion until all gases
+formed in the reaction have escaped.
+
+_Molding and blowing of glass._ The way in which the melted mixture is
+handled in the glass factory depends upon the character of the article
+to be made. Many articles, such as bottles, are made by blowing the
+plastic glass into hollow molds of the desired shape. The mold is first
+opened, as shown in Fig. 74. A lump of plastic glass A on the hollow
+rod B is lowered into the mold, which is then closed by the handles
+C. By blowing into the tube the glass is blown into the shape of the
+mold. The mold is then opened and the bottle lifted out. The neck of the
+bottle must be cut off at the proper place and the sharp edges rounded
+off in a flame.
+
+Other objects, such as lamp chimneys, are made by getting a lump of
+plastic glass on the end of a hollow iron rod and blowing it into the
+desired shape without the help of a mold, great skill being required in
+the manipulation of the glass. Window glass is made by blowing large
+hollow cylinders about 6 ft. long and 1-1/2 ft. in diameter. These are
+cut longitudinally, and are then placed in an oven and heated until they
+soften, when they are flattened out into plates (Fig. 75). Plate glass
+is cast into flat slabs, which are then ground and polished to perfectly
+plane surfaces.
+
+_Varieties of glass._ The ingredients mentioned above make a soft,
+easily fusible glass. If potassium carbonate is substituted for the
+sodium carbonate, the glass is much harder and less easily fused;
+increasing the amount of sand has somewhat the same effect. Potassium
+glass is largely used in making chemical glassware, since it resists the
+action of reagents better than the softer sodium glass. If lead oxide is
+substituted for the whole or a part of the lime, the glass is very soft,
+but has a high index of refraction and is valuable for making optical
+instruments and artificial jewels.
+
+[Illustration: Fig. 75]
+
+_Coloring of glass._ Various substances fused along with the glass
+mixture give characteristic colors. The amber color of common bottles is
+due to iron compounds in the glass; in other cases iron colors the glass
+green. Cobalt compounds color it deep blue; those of manganese give it
+an amethyst tint and uranium compounds impart a peculiar yellowish green
+color. Since iron is nearly always present in the ingredients, glass is
+usually slightly yellow. This color can be removed by adding the proper
+amount of manganese dioxide, for the amethyst color of manganese and the
+yellow of iron together produce white light.
+
+_Nature of glass._ Glass is not a definite chemical compound and its
+composition varies between wide limits. Fused glass is really a solution
+of various silicates, such as those of calcium and lead, in fused sodium
+or potassium silicate. A certain amount of silicon dioxide is also
+present. This solution is then allowed to solidify under such conditions
+of cooling that the dissolved substances do not separate from the
+solvent. The compounds which are used to color the glass are sometimes
+converted into silicates, which then dissolve in the glass, giving it a
+uniform color. In other cases, as in the milky glasses which resemble
+porcelain in appearance, the color or opaqueness is due to the finely
+divided color material evenly distributed throughout the glass, but not
+dissolved in it. Milky glass is made by mixing calcium fluoride, tin
+oxide, or some other insoluble substance in the melted glass. Copper or
+gold in metallic form scattered through glass gives it shades of red.
+
+
+TITANIUM
+
+     Titanium is a very widely distributed element in nature, being
+     found in almost all soils, in many rocks, and even in plant and
+     animal tissues. It is not very abundant in any one locality,
+     and it possesses little commercial value save in connection
+     with the iron industry. Its most common ore is rutile
+     (TiO_{2}), which resembles silica in many respects.
+
+     In both physical and chemical properties titanium resembles
+     silicon, though it is somewhat more metallic in character. This
+     resemblance is most marked in the acids of titanium. It not
+     only forms metatitanic and orthotitanic acids but a great
+     variety of polytitanic acids as well.
+
+
+BORON
+
+~Occurrence.~ Boron is never found free in nature. It occurs as boric acid
+(H_{3}BO_{3}), and in salts of polyboric acids, which usually have very
+complicated formulas.
+
+~Preparation and properties.~ Boron can be prepared from its oxide by
+reduction with magnesium, exactly as in the case of silicon. It
+resembles silicon very strikingly in its properties. It occurs in
+several allotropic forms, is very hard when crystallized, and is rather
+inactive toward reagents. It forms a hydride, BH_{3}, and combines
+directly with the elements of the chlorine family. Boron fluoride
+(BF_{3}) is very similar to silicon fluoride in its mode of formation
+and chemical properties.
+
+~Boric oxide~ (B_{2}O_{3}). Boron forms one well-known oxide, B_{2}O_{3},
+called boric anhydride. It is formed as a glassy mass by heating boric
+acid to a high temperature. It absorbs water very readily, uniting with
+it to form boric acid again:
+
+    B_{2}O_{3} + 3H_{2}O = 2H_{3}BO_{3}.
+
+In this respect it differs from silicon dioxide, which will not combine
+directly with water.
+
+~Boric acid~ (H_{3}BO_{3}). This is found in nature in considerable
+quantities and forms one of the chief sources of boron compounds. It is
+found dissolved in the water of hot springs in some localities,
+particularly in Italy. Being volatile with steam, the vapor which
+escapes from these springs has some boric acid in it. It is easily
+obtained from these sources by condensation and evaporation, the
+necessary heat being supplied by other hot springs.
+
+Boric acid crystallizes in pearly flakes, which are greasy to the touch.
+In the laboratory it is easily prepared by treating a strong, hot
+solution of borax with sulphuric acid. Boric acid being sparingly
+soluble in water crystallizes out on cooling:
+
+    Na_{2}B_{4}O_{7} + 5H_{2}O + H_{2}SO_{4} = Na_{2}SO_{4} + 4H_{3}BO_{3}.
+
+The substance is a mild antiseptic, and on this account is often used in
+medicine and as a preservative for canned foods and milk.
+
+~Metaboric and polyboric acids.~ When boric acid is gently heated it is
+converted into metaboric acid (HBO_{2}):
+
+    H_{3}BO_{3} = HBO_{2} + H_{2}O.
+
+On heating metaboric acid to a somewhat higher temperature tetraboric
+acid (H_{2}B_{4}O_{7}) is formed:
+
+    4HBO_{2} = H_{2}B_{4}O_{7} + H_{2}O.
+
+Many other complex acids of boron are known.
+
+~Borax.~ Borax is the sodium salt of tetraboric acid, having the formula
+Na_{2}B_{4}O_{7}·10 H_{2}O. It is found in some arid countries, as
+southern California and Tibet, but is now made commercially from the
+mineral colemanite, which is the calcium salt of a complex boric acid.
+When this is treated with a solution of sodium carbonate, calcium
+carbonate is precipitated and borax crystallizes from the solution.
+
+When heated borax at first swells up greatly, owing to the expulsion of
+the water of crystallization, and then melts to a clear glass. This
+glass has the property of easily dissolving many metallic oxides, and on
+this account borax is used as a flux in soldering, for the purpose of
+removing from the metallic surfaces to be soldered the film of oxide
+with which they are likely to be covered. These oxides often give a
+characteristic color to the clear borax glass, and borax beads are
+therefore often used in testing for the presence of metals, instead of
+the metaphosphoric acid bead already described.
+
+     The reason that metallic oxides dissolve in borax is that borax
+     contains an excess of acid anhydride, as can be more easily
+     seen if its formula is written 2NaBO_{2} + B_{2}O_{3}. The
+     metallic oxide combines with this excess of acid anhydride,
+     forming a mixed salt of metaboric acid.
+
+Borax is extensively used as a constituent of enamels and glazes for
+both metal ware and pottery. It is also used as a flux in soldering and
+brazing, and in domestic ways it serves as a mild alkali, as a
+preservative for meats, and in a great variety of less important
+applications.
+
+
+EXERCISES
+
+1. Account for the fact that a solution of borax in water is alkaline.
+
+2. What weight of water of crystallization does 1 kg. of borax contain?
+
+3. When a concentrated solution of borax acts on silver nitrate a borate
+of silver is formed. If the solution of borax is dilute, however, an
+hydroxide of silver forms. Account for this difference in behavior.
+
+
+
+
+CHAPTER XXII
+
+THE METALS
+
+
+~The metals.~ The elements which remain to be considered are known
+collectively as the metals. They are also called the base-forming
+elements, since their hydroxides are bases. A metal may therefore be
+defined as an element whose hydroxide is a base. When a base dissolves
+in water the hydroxyl groups form the anions, while the metallic element
+forms the cations. From this standpoint a metal can be defined as an
+element capable of forming simple cations in solution.
+
+The distinction between a metal and a non-metal is not a very sharp one,
+since the hydroxides of a number of elements act as bases under some
+conditions and as acids under others. We have seen that antimony is an
+element of this kind.
+
+~Occurrence of metals in nature.~ A few of the metals are found in nature
+in the free state. Among these are gold, platinum, and frequently
+copper. They are usually found combined with other elements in the form
+of oxides or salts of various acids. Silicates, carbonates, sulphides,
+and sulphates are the most abundant salts. All inorganic substances
+occurring in nature, whether they contain a metal or not, are called
+_minerals_. Those minerals from which a useful substance can be
+extracted are called _ores_ of the substance. These two terms are most
+frequently used in connection with the metals.
+
+~Extraction of metals,--metallurgy.~ The process of extracting a metal
+from its ores is called the metallurgy of the metal. The metallurgy of
+each metal presents peculiarities of its own, but there are several
+methods of general application which are very frequently employed.
+
+1. _Reduction of an oxide with carbon._ Many of the metals occur in
+nature in the form of oxides. When these oxides are heated to a high
+temperature with carbon the oxygen combines with it and the metal is set
+free. Iron, for example, occurs largely in the form of the oxide
+Fe_{2}O_{3}. When this is heated with carbon the reaction expressed in
+the following equation takes place:
+
+    Fe_{2}O_{3} + 3 C = 2 Fe + 3 CO.
+
+Many ores other than oxides may be changed into oxides which can then be
+reduced by carbon. The conversion of such ores into oxides is generally
+accomplished by heating, and this process is called _roasting_. Many
+carbonates and hydroxides decompose directly into the oxide on heating.
+Sulphides, on the other hand, must be heated in a current of air, the
+oxygen of the air entering into the reaction. The following equations
+will serve to illustrate these changes in the case of the ores of iron:
+
+    FeCO_{3} = FeO + CO_{2},
+
+    2Fe(OH)_{3} = Fe_{2}O_{3} + 3H_{2}O,
+
+    2FeS_{2} + 11O = Fe_{2}O_{3} + 4SO_{2}.
+
+2. _Reduction of an oxide with aluminium._ Not all oxides, however, can
+be reduced by carbon. In such cases aluminium may be used. Thus chromium
+may be obtained in accordance with the following equation:
+
+    Cr_{2}O_{3} + 2 Al = 2 Cr + Al_{2}O_{3}.
+
+This method is a comparatively new one, having been brought into use by
+the German chemist Goldschmidt; hence it is sometimes called the
+Goldschmidt method.
+
+3. _Electrolysis._ In recent years increasing use is being made of the
+electric current in the preparation of metals. In some cases the
+separation of the metal from its compounds is accomplished by passing
+the current through a solution of a suitable salt of the metal, the
+metal usually being deposited upon the cathode. In other cases the
+current is passed through a fused salt of the metal, the chloride being
+best adapted to this purpose.
+
+~Electro-chemical industries.~ Most of the electro-chemical industries of
+the country are carried on where water power is abundant, since this
+furnishes the cheapest means for the generation of electrical energy.
+Niagara Falls is the most important locality in this country for such
+industries, and many different electro-chemical products are
+manufactured there. Some industries depend upon electrolytic processes,
+while in others the electrical energy is used merely as a source of heat
+in electric furnaces.
+
+~Preparation of compounds of the metals.~ Since the compounds of the
+metals are so numerous and varied in character, there are many ways of
+preparing them. In many cases the properties of the substance to be
+prepared, or the material available for its preparation, suggest a
+rather unusual way. There are, however, a number of general principles
+which are constantly applied in the preparation of the compounds of the
+metals, and a clear understanding of them will save much time and effort
+in remembering the details in any given case. The most important of
+these general methods for the preparation of compounds are the
+following:
+
+1. _By direct union of two elements._ This is usually accomplished by
+heating the two elements together. Thus the sulphides, chlorides, and
+oxides of a metal can generally be obtained in this way. The following
+equations serve as examples of this method:
+
+    Fe + S = FeS,
+
+    Mg + O = MgO,
+
+    Cu + 2Cl = CuCl_{2}.
+
+2. _By the decomposition of a compound._ This decomposition may be
+brought about either by heat alone or by the combined action of heat and
+a reducing agent. Thus when the nitrate of a metal is heated the oxide
+of the metal is usually obtained. Copper nitrate, for example,
+decomposes as follows:
+
+    Cu(NO_{3})_{2} = CuO + 2NO_{2} + O.
+
+Similarly the carbonates of the metals yield oxides, thus:
+
+    CaCO_{3} = CaO + CO_{2}.
+
+Most of the hydroxides form an oxide and water when heated:
+
+    2Al(OH)_{3} = Al_{2}O_{3} + 3H_{2}O.
+
+When heated with carbon, sulphates are reduced to sulphides, thus:
+
+    BaSO_{4} + 2C = BaS + 2CO_{2}.
+
+3. _Methods based on equilibrium in solution._ In the preparation of
+compounds the first requisite is that the reactions chosen shall be of
+such a kind as will go on to completion. In the chapter on chemical
+equilibrium it was shown that reactions in solution may become complete
+in either of three ways: (1) a gas may be formed which escapes from
+solution; (2) an insoluble solid may be formed which precipitates; (3)
+two different ions may combine to form undissociated molecules. By the
+judicious selection of materials these principles may be applied to the
+preparation of a great variety of compounds, and illustrations of such
+methods will very frequently be found in the subsequent pages.
+
+4. _By fusion methods._ It sometimes happens that substances which are
+insoluble in water and in acids, and which cannot therefore be brought
+into double decomposition in the usual way, are soluble in other
+liquids, and when dissolved in them can be decomposed and converted into
+other desired compounds. Thus barium sulphate is not soluble in water,
+and sulphuric acid, being less volatile than most other acids, cannot
+easily be driven out from this salt When brought into contact with
+melted sodium carbonate, however, it dissolves in it, and since barium
+carbonate is insoluble in melted sodium carbonate, double decomposition
+takes place:
+
+    Na_{2}CO_{3} + BaSO_{4} = BaCO_{3} + Na_{2}SO_{4}.
+
+On dissolving the cooled mixture in water the sodium sulphate formed in
+the reaction, together with any excess of sodium carbonate which may be
+present, dissolves. The barium carbonate can then be filtered off and
+converted into any desired salt by the processes already described.
+
+5. _By the action of metals on salts of other metals._ When a strip of
+zinc is placed in a solution of a copper salt the copper is precipitated
+and an equivalent quantity of zinc passes into solution:
+
+    Zn + CuSO_{4} = Cu + ZnSO_{4}.
+
+In like manner copper will precipitate silver from its salts:
+
+    Cu + Ag_{2}SO_{4} = 2Ag + CuSO_{4}.
+
+It is possible to tabulate the metals in such a way that any one of them
+in the table will precipitate any one following it from its salts. The
+following is a list of some of the commoner metals arranged in this way:
+
+    Zinc
+    Iron
+    Tin
+    Lead
+    Copper
+    Bismuth
+    Mercury
+    Silver
+    Gold
+
+According to this table copper will precipitate bismuth, mercury,
+silver, or gold from their salts, and will in turn be precipitated by
+zinc, iron, tin, or lead. Advantage is taken of this principle in the
+purification of some of the metals, and occasionally in the preparation
+of metals and their compounds.
+
+~Important insoluble compounds.~ Since precipitates play so important a
+part in the reactions which substances undergo, as well as in the
+preparation of many chemical compounds, it is important to know what
+substances are insoluble. Knowing this, we can in many cases predict
+reactions under certain conditions, and are assisted in devising ways to
+prepare desired compounds. While there is no general rule which will
+enable one to foretell the solubility of any given compound,
+nevertheless a few general statements can be made which will be of much
+assistance.
+
+1. _Hydroxides._ All hydroxides are insoluble save those of ammonium,
+sodium, potassium, calcium, barium, and strontium.
+
+2. _Nitrates._ All nitrates are soluble in water.
+
+3. _Chlorides._ All chlorides are soluble save silver and mercurous
+chlorides. (Lead chloride is but slightly soluble.)
+
+4. _Sulphates._ All sulphates are soluble save those of barium,
+strontium, and lead. (Sulphates of silver and calcium are only
+moderately soluble.)
+
+5. _Sulphides._ All sulphides are insoluble save those of ammonium,
+sodium, and potassium. The sulphides of calcium, barium, strontium, and
+magnesium are insoluble in water, but are changed by hydrolysis into
+acid sulphides which are soluble. On this account they cannot be
+prepared by precipitation.
+
+6. _Carbonates, phosphates, and silicates._ All normal carbonates,
+phosphates, and silicates are insoluble save those of ammonium, sodium
+and potassium.
+
+
+EXERCISES
+
+1. Write equations representing four different ways for preparing
+Cu(NO_{3})_{2}.
+
+2. Write equations representing six different ways for preparing
+ZnSO_{4}.
+
+3. Write equations for two reactions to illustrate each of the three
+ways in which reactions in solutions may become complete.
+
+4. Give one or more methods for preparing each of the following
+compounds: CaCl_{2}, PbCl_{2}, BaSO_{4}, CaCO_{3}, (NH_{4})_{2}S,
+Ag_{2}S, PbO, Cu(OH)_{2} (for solubilities, see last paragraph of
+chapter). State in each case the general principle involved in the
+method of preparation chosen.
+
+
+
+
+CHAPTER XXIII
+
+THE ALKALI METALS
+
+
+=================================================================
+          |        |        |         |         |
+          | SYMBOL | ATOMIC | DENSITY | MELTING | FIRST PREPARED
+          |        | WEIGHT |         |  POINT  |
+__________|________|________|_________|_________|________________
+          |        |        |         |         |
+Lithium   |   Li   |   7.03 |   0.59  |  186.°  | Davy     1820
+Sodium    |   Na   |  23.05 |   0.97  |   97.6° |  "       1807
+Potassium |   K    |  39.15 |   0.87  |   62.5° |  "       1807
+Rubidium  |   Rb   |  85.5  |   1.52  |   38.5° | Bunsen   1861
+Cæsium    |   Cs   | 132.9  |   1.88  |   26.5° |  "       1860
+=================================================================
+
+~The family.~ The metals listed in the above table constitute the even
+family in Group I in the periodic arrangement of the elements, and
+therefore form a natural family. The name alkali metals is commonly
+applied to the family for the reason that the hydroxides of the most
+familiar members of the family, namely sodium and potassium, have long
+been called alkalis.
+
+1. _Occurrence._ While none of these metals occur free in nature, their
+compounds are very widely distributed, being especially abundant in sea
+and mineral waters, in salt beds, and in many rocks. Only sodium and
+potassium occur in abundance, the others being rarely found in any
+considerable quantity.
+
+2. _Preparation._ The metals are most conveniently prepared by the
+electrolysis of their fused hydroxides or chlorides, though it is
+possible to prepare them by reducing their oxides or carbonates with
+carbon.
+
+3. _Properties._ They are soft, light metals, having low melting points
+and small densities, as is indicated in the table. Their melting points
+vary inversely with their atomic weights, while their densities (sodium
+excepted) vary directly with these. The pure metals have a silvery
+luster but tarnish at once when exposed to the air, owing to the
+formation of a film of oxide upon the surface of the metal. They are
+therefore preserved in some liquid, such as coal oil, which contains no
+oxygen. Because of their strong affinity for oxygen they decompose water
+with great ease, forming hydroxides and liberating hydrogen in
+accordance with the equation
+
+    M + H_{2}O = MOH + H,
+
+where M stands for any one of these metals. These hydroxides are white
+solids; they are readily soluble in water and possess very strong basic
+properties. These bases are nearly equal in strength, that is, they all
+dissociate in water to about the same extent.
+
+4. _Compounds._ The alkali metals almost always act as univalent
+elements in the formation of compounds, the composition of which can be
+represented by such formulas as MH, MCl, MNO_{3}, M_{2}SO_{4},
+M_{3}PO_{4}. These compounds, when dissolved in water, dissociate in
+such a way as to form simple, univalent metallic ions which are
+colorless. With the exception of lithium these metals form very few
+insoluble compounds, so that it is not often that precipitates
+containing them are obtained. Only sodium and potassium will be studied
+in detail, since the other metals of the family are of relatively small
+importance.
+
+The compounds of sodium and potassium are so similar in properties that
+they can be used interchangeably for most purposes. Other things being
+equal, the sodium compounds are prepared in preference to those of
+potassium, since they are cheaper. When a given sodium compound is
+deliquescent, or is so soluble that it is difficult to purify, the
+corresponding potassium compound is prepared in its stead, provided its
+properties are more desirable in these respects.
+
+
+SODIUM
+
+~Occurrence in nature.~ Large deposits of sodium chloride have been found
+in various parts of the world, and the water of the ocean and of many
+lakes and springs contains notable quantities of it. The element also
+occurs as a constituent of many rocks and is therefore present in the
+soil formed by their disintegration. The mineral cryolite
+(Na_{3}AlF_{6}) is an important substance, and the nitrate, carbonate,
+and borate also occur in nature.
+
+~Preparation.~ In 1807 Sir Humphry Davy succeeded in preparing very small
+quantities of metallic sodium by the electrolysis of the fused
+hydroxide. On account of the cost of electrical energy it was for many
+years found more economical to prepare it by reducing the carbonate with
+carbon in accordance with the following equation:
+
+    Na_{2}CO_{3} + 2C = 2Na + 3CO.
+
+The cost of generating the electric current has been diminished to such
+an extent, however, that it is now more economical to prepare sodium by
+Davy's original method, namely, by the electrolysis of the fused
+hydroxide or chloride. When the chloride is used the process is
+difficult to manage, owing to the higher temperature required to keep
+the electrolyte fused, and because of the corroding action of the fused
+chloride upon the containing vessel.
+
+[Illustration: SIR HUMPHRY DAVY (English) (1778-1829)
+
+Isolated sodium, lithium, potassium, barium, strontium, and calcium by
+means of electrolysis; demonstrated the elementary nature of chlorine;
+invented the safety lamp; discovered the stupefying effects of nitrous
+oxide]
+
+     ~Technical preparation.~ The sodium hydroxide is melted in a
+     cylindrical iron vessel (Fig. 76) through the bottom of which
+     rises the cathode K. The anodes A, several in number, are
+     suspended around the cathode from above. A cylindrical vessel
+     C floats in the fused alkali directly over the cathode, and
+     under this cap the sodium and hydrogen liberated at the cathode
+     collect. The hydrogen escapes by lifting the cover, and the
+     sodium, protected from the air by the hydrogen, is skimmed or
+     drained off from time to time. Oxygen is set free upon the
+     anode and escapes into the air through the openings O without
+     coming into contact with the sodium or hydrogen. This process
+     is carried on extensively at Niagara Falls.
+
+[Illustration: Fig. 76]
+
+~Properties.~ Sodium is a silver-white metal about as heavy as water, and
+so soft that it can be molded easily by the fingers or pressed into
+wire. It is very active chemically, combining with most of the
+non-metallic elements, such as oxygen and chlorine, with great energy.
+It will often withdraw these elements from combination with other
+elements, and is thus able to decompose water and the oxides and
+chlorides of many metals.
+
+~Sodium peroxide~ (NaO). Since sodium is a univalent element we should
+expect it to form an oxide of the formula Na_{2}O. While such an oxide
+can be prepared, the peroxide (NaO) is much better known. It is a
+yellowish-white powder made by burning sodium in air. Its chief use is
+as an oxidizing agent. When heated with oxidizable substances it gives
+up a part of its oxygen, as shown in the equation
+
+    2NaO = Na_{2}O + O.
+
+Water decomposes it in accordance with the equation
+
+    2NaO + 2H_{2}O = 2NaOH + H_{2}O_{2}.
+
+Acids act readily upon it, forming a sodium salt and hydrogen peroxide:
+
+    2NaO + 2HCl = 2NaCl + H_{2}O_{2}.
+
+In these last two reactions the hydrogen dioxide formed may decompose
+into water and oxygen if the temperature is allowed to rise:
+
+    H_{2}O_{2} = H_{2}O + O.
+
+     ~Peroxides.~ It will be remembered that barium dioxide (BaO_{2})
+     yields hydrogen dioxide when treated with acids, and that
+     manganese dioxide gives up oxygen when heated with sulphuric
+     acid. Oxides which yield either hydrogen dioxide or oxygen when
+     treated with water or an acid are called peroxides.
+
+~Sodium hydroxide~ (_caustic soda_) (NaOH). 1. _Preparation._ Sodium
+hydroxide is prepared commercially by several processes.
+
+(a) In the older process, still in extensive use, sodium carbonate is
+treated with calcium hydroxide suspended in water. Calcium carbonate is
+precipitated according to the equation
+
+    Na_{2}CO_{3} + Ca(OH)_{2} = CaCO_{3} + 2NaOH.
+
+The dilute solution of sodium hydroxide, filtered from the calcium
+carbonate, is evaporated to a paste and is then poured into molds to
+solidify. It is sold in the form of slender sticks.
+
+(b) The newer methods depend upon the electrolysis of sodium chloride.
+In the Castner process a solution of salt is electrolyzed, the reaction
+being expressed as follows:
+
+    NaCl + H_{2}O = NaOH + H + Cl.
+
+The chlorine escapes as a gas, and by an ingenious mechanical device the
+sodium hydroxide is prevented from mixing with the salt in the solution.
+
+In the Acker process the electrolyte is _fused_ sodium chloride. The
+chlorine is evolved as a gas at the anode, while the sodium alloys with
+the melted lead which forms the cathode. When this alloy is treated with
+water the following reaction takes place:
+
+    Na + H_{2}O = NaOH + H.
+
+[Illustration: Fig. 77]
+
+~Technical process.~ A sketch of an Acker furnace is represented in Fig.
+77. The furnace is an irregularly shaped cast-iron box, divided into
+three compartments, A, B, and C. Compartment A is lined with
+magnesia brick. Compartments B and C are filled with melted lead,
+which also covers the bottom of A to a depth of about an inch. Above
+this layer in A is fused salt, into which dip carbon anodes D. The
+metallic box and melted lead is the cathode.
+
+     When the furnace is in operation chlorine is evolved at the
+     anodes, and is drawn away through a pipe (not represented) to
+     the bleaching-powder chambers. Sodium is set free at the
+     surface of the melted lead in A, and at once alloys with it.
+     Through the pipe E a powerful jet of steam is driven through
+     the lead in B upwards into the narrow tube F. This forces
+     the lead alloy up through the tube and over into the chamber
+     G.
+
+     In this process the steam is decomposed by the sodium in the
+     alloy, forming melted sodium hydroxide and hydrogen. The melted
+     lead and sodium hydroxide separate into two layers in G, and
+     the sodium hydroxide, being on top, overflows into tanks from
+     which it is drawn off and packed in metallic drums. The lead is
+     returned to the other compartments of the furnace by a pipe
+     leading from H to I. Compartment C serves merely as a
+     reservoir for excess of melted lead.
+
+2. _Properties._ Sodium hydroxide is a white, crystalline, brittle
+substance which rapidly absorbs water and carbon dioxide from the air.
+As the name (caustic soda) indicates, it is a very corrosive substance,
+having a disintegrating action on most animal and vegetable tissues. It
+is a strong base. It is used in a great many chemical industries, and
+under the name of lye is employed to a small extent as a cleansing agent
+for household purposes.
+
+~Sodium chloride~ (_common salt_) (NaCl). 1. _Preparation._ Sodium
+chloride, or common salt, is very widely distributed in nature. Thick
+strata, evidently deposited at one time by the evaporation of salt
+water, are found in many places. In the United States the most important
+localities for salt are New York, Michigan, Ohio, and Kansas. Sometimes
+the salt is mined, especially if it is in the pure form called rock
+salt. More frequently a strong brine is pumped from deep wells sunk into
+the salt deposit, and is then evaporated in large pans until the salt
+crystallizes out. The crystals are in the form of small cubes and
+contain no water of crystallization; some water is, however, held in
+cavities in the crystals and causes the salt to decrepitate when heated.
+
+2. _Uses._ Since salt is so abundant in nature it forms the starting
+point in the preparation of all compounds containing either sodium or
+chlorine. This includes many substances of the highest importance to
+civilization, such as soap, glass, hydrochloric acid, soda, and
+bleaching powder. Enormous quantities of salt are therefore produced
+each year. Small quantities are essential to the life of man and
+animals. Pure salt does not absorb moisture; the fact that ordinary salt
+becomes moist in air is not due to a property of the salt, but to
+impurities commonly occurring in it, especially calcium and magnesium
+chlorides.
+
+~Sodium sulphate~ (_Glauber's salt_) (Na_{2}SO_{4}·10H_{2}O). This salt is
+prepared by the action of sulphuric acid upon sodium chloride,
+hydrochloric acid being formed at the same time:
+
+    2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
+
+Some sodium sulphate is prepared by the reaction represented in the
+equation
+
+    MgSO_{4} + 2NaCl = Na_{2}SO_{4} + MgCl_{2}.
+
+The magnesium sulphate required for this reaction is obtained in large
+quantities in the manufacture of potassium chloride, and being of little
+value for any other purpose is used in this way. The reaction depends
+upon the fact that sodium sulphate is the least soluble of any of the
+four factors in the equation, and therefore crystallizes out when hot,
+saturated solutions of magnesium sulphate and sodium chloride are mixed
+together and the resulting mixture cooled.
+
+Sodium sulphate forms large efflorescent crystals. The salt is
+extensively used in the manufacture of sodium carbonate and glass. Small
+quantities are used in medicine.
+
+~Sodium sulphite~ (Na_{2}SO_{3}·7H_{2}O). Sodium sulphite is prepared by
+the action of sulphur dioxide upon solutions of sodium hydroxide, the
+reaction being analogous to the action of carbon dioxide upon sodium
+hydroxide. Like the carbonate, the sulphite is readily decomposed by
+acids:
+
+    Na_{2}SO_{3} + 2HCl = 2NaCl + H_{2}O + SO_{2}.
+
+Because of this reaction sodium sulphite is used as a convenient source
+of sulphur dioxide. It is also used as a disinfectant and a
+preservative.
+
+~Sodium thiosulphate~ (_hyposulphite of soda or "hypo"_)
+(Na_{2}S_{2}O_{3}·5H_{2}O). This salt, commonly called sodium
+hyposulphite, or merely hypo, is made by boiling a solution of sodium
+sulphite with sulphur:
+
+    Na_{2}SO_{3} + S = Na_{2}S_{2}O_{3}.
+
+It is used in photography and in the bleaching industry, to absorb the
+excess of chlorine which is left upon the bleached fabrics.
+
+     ~Thio compounds.~ The prefix "thio" means sulphur. It is used to
+     designate substances which may be regarded as derived from
+     oxygen compounds by replacing the whole or a part of their
+     oxygen with sulphur. The thiosulphates may be regarded as
+     sulphates in which one atom of oxygen has been replaced by an
+     atom of sulphur. This may be seen by comparing the formula
+     Na_{2}SO_{4} (sodium sulphate) with the formula
+     Na_{2}S_{2}O_{3} (sodium thiosulphate).
+
+~Sodium carbonate~ (_sal soda_)(Na_{2}CO_{3}·10H_{2}O). There are two
+different methods now employed in the manufacture of this important
+substance.
+
+1. _Le Blanc process._ This older process involves several distinct
+reactions, as shown in the following equations.
+
+(a) Sodium chloride is first converted into sodium sulphate:
+
+    2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
+
+(b) The sodium sulphate is next reduced to sulphide by heating it with
+carbon:
+
+    Na_{2}SO_{4} + 2C = Na_{2}S + 2CO_{2}.
+
+(c) The sodium sulphide is then heated with calcium carbonate, when
+double decomposition takes place:
+
+    Na_{2}S + CaCO_{3} = CaS + Na_{2}CO_{3}.
+
+     ~Technical preparation of sodium carbonate.~ In a manufacturing
+     plant the last two reactions take place in one process. Sodium
+     sulphate, coal, and powdered limestone are heated together to a
+     rather high temperature. The coal reduces the sulphate to
+     sulphide, which in turn reacts upon the calcium carbonate. Some
+     limestone is decomposed by the heat, forming calcium oxide.
+     When treated with water the calcium oxide is changed into
+     hydroxide, and this prevents the water from decomposing the
+     insoluble calcium sulphide.
+
+     The crude product of the process is a hard black cake called
+     black ash. On digesting this mass with water the sodium
+     carbonate passes into solution. The pure carbonate is obtained
+     by evaporation of this solution, crystallizing from it in
+     crystals of the formula Na_{2}CO_{3}·10H_{2}O. Since over 60%
+     of this salt is water, the crystals are sometimes heated until
+     it is driven off. The product is called calcined soda, and is,
+     of course, more valuable than the crystallized salt.
+
+2. _Solvay process._ This more modern process depends upon the reactions
+represented in the equations
+
+    NaCl + NH_{4}HCO_{3} = NaHCO_{3} + NH_{4}Cl,
+
+    2NaHCO_{3} = Na_{2}CO_{3} + H_{2}O + CO_{2}.
+
+The reason the first reaction takes place is that sodium hydrogen
+carbonate is sparingly soluble in water, while the other compounds are
+freely soluble. When strong solutions of sodium chloride and of ammonium
+hydrogen carbonate are brought together the sparingly soluble sodium
+hydrogen carbonate is precipitated. This is converted into the normal
+carbonate by heating, the reaction being represented in the second
+equation.
+
+     ~Technical preparation.~ In the Solvay process a very
+     concentrated solution of salt is first saturated with ammonia
+     gas, and a current of carbon dioxide is then conducted into the
+     solution. In this way ammonium hydrogen carbonate is formed:
+
+     NH_{3} + H_{2}O + CO_{2} = NH_{4}HCO_{3}.
+
+     This enters into double decomposition with the salt, as shown
+     in the first equation under the Solvay process. After the
+     sodium hydrogen carbonate has been precipitated the mother
+     liquors containing ammonium chloride are treated with lime:
+
+     2NH_{4}Cl + CaO = CaCl_{2} + 2 NH_{3} + H_{2}O.
+
+     The lime is obtained by burning limestone:
+
+     CaCO_{3} = CaO + CO_{2}.
+
+     The ammonia and carbon dioxide evolved in the latter two
+     reactions are used in the preparation of an additional quantity
+     of ammonium hydrogen carbonate. It will thus be seen that there
+     is no loss of ammonia. The only materials permanently used up
+     are calcium carbonate and salt, while the only waste product is
+     calcium chloride.
+
+     ~Historical.~ In former times sodium carbonate was made by
+     burning seaweeds and extracting the carbonate from their ash.
+     On this account the salt was called _soda ash_, and the name is
+     still in common use. During the French Revolution this supply
+     was cut off, and in behalf of the French government Le Blanc
+     made a study of methods of preparing the carbonate directly
+     from salt. As a result he devised the method which bears his
+     name, and which was used exclusively for many years. It has
+     been replaced to a large extent by the Solvay process, which
+     has the advantage that the materials used are inexpensive, and
+     that the ammonium hydrogen carbonate used can be regenerated
+     from the products formed in the process. Much expense is also
+     saved in fuel, and the sodium hydrogen carbonate, which is the
+     first product of the process, has itself many commercial uses.
+     The Le Blanc process is still used, however, since the
+     hydrochloric acid generated is of value.
+
+     ~By-products.~ The substances obtained in a given process, aside
+     from the main product, are called the by-products. The success
+     of many processes depends upon the value of the by-products
+     formed.
+
+     Thus hydrochloric acid, a by-product in the Le Blanc process,
+     is valuable enough to make the process pay, even though sodium
+     carbonate can be made cheaper in other ways.
+
+~Properties of sodium carbonate.~ Sodium carbonate forms large crystals of
+the formula Na_{2}CO_{3} · 10 H_{2}O. It has a mild alkaline reaction
+and is used for laundry purposes under the name of washing soda. Mere
+mention of the fact that it is used in the manufacture of glass, soap,
+and many chemical reagents will indicate its importance in the
+industries. It is one of the few soluble carbonates.
+
+~Sodium hydrogen carbonate~ (_bicarbonate of soda_) (NaHCO_{3}). This
+salt, commonly called bicarbonate of soda, or baking soda, is made by
+the Solvay process, as explained above, or by passing carbon dioxide
+into strong solutions of sodium carbonate:
+
+    Na_{2}CO_{3} + H_{2}O + CO_{2} = 2NaHCO_{3}.
+
+The bicarbonate, being sparingly soluble, crystallizes out. A mixture of
+the bicarbonate with some substance (the compound known as cream of
+tartar is generally used) which slowly reacts with it, liberating carbon
+dioxide, is used largely in baking. The carbon dioxide generated forces
+its way through the dough, thus making it porous and light.
+
+~Sodium nitrate~ (_Chili saltpeter_) (NaNO_{3}). This substance is found
+in nature in arid regions in a number of places, where it has been
+formed apparently by the decay of organic substances in the presence of
+air and sodium salts. The largest deposits are in Chili, and most of the
+nitrate of commerce comes from that country. Smaller deposits occur in
+California and Nevada. The commercial salt is prepared by dissolving the
+crude nitrate in water, allowing the insoluble earthy materials to
+settle, and evaporating the clear solution so obtained to
+crystallization. The soluble impurities remain for the most part in the
+mother liquors.
+
+Since this salt is the only nitrate found extensively in nature, it is
+the material from which other nitrates as well as nitric acid are
+prepared. It is used in enormous quantities in the manufacture of
+sulphuric acid and potassium nitrate, and as a fertilizer.
+
+~Sodium phosphate~ (Na_{2}HPO_{4}·12H_{2}O). Since phosphoric acid has
+three replaceable hydrogen atoms, three sodium phosphates are
+possible,--two acid salts and one normal. All three can be made without
+difficulty, but disodium phosphate is the only one which is largely
+used, and is the salt which is commonly called sodium phosphate. It is
+made by the action of phosphoric acid on sodium carbonate:
+
+    Na_{2}CO_{3} + H_{3}PO_{4} = Na_{2}HPO_{4} + CO_{2} + H_{2}O.
+
+It is interesting as being one of the few phosphates which are soluble
+in water, and is the salt commonly used when a soluble phosphate is
+needed.
+
+~Normal sodium phosphate~ (Na_{3}PO_{4}). Although this is a normal salt
+its solution has a strongly alkaline reaction. This is due to the fact
+that the salt hydrolyzes in solution into sodium hydroxide and disodium
+phosphate, as represented in the equation
+
+    Na_{3}PO_{4} + H_{2}O = Na_{2}HPO_{4} + NaOH.
+
+Sodium hydroxide is strongly alkaline, while disodium phosphate is
+nearly neutral in reaction. The solution as a whole is therefore
+alkaline. The salt is prepared by adding a large excess of sodium
+hydroxide to a solution of disodium phosphate and evaporating to
+crystallization. The excess of the sodium hydroxide reverses the
+reaction of hydrolysis and the normal salt crystallizes out.
+
+~Sodium tetraborate ~(_borax_) (Na_{2}B_{4}O_{7}·10H_{2}O). The properties
+of this important compound have been discussed under the head of boron.
+
+
+POTASSIUM
+
+~Occurrence in nature.~ Potassium is a constituent of many common rocks
+and minerals, and is therefore a rather abundant element, though not so
+abundant as sodium. Feldspar, which occurs both by itself and as a
+constituent of granite, contains considerable potassium. The element is
+a constituent of all clay and of mica and also occurs in very large
+deposits at Stassfurt, Germany, in the form of the chloride and
+sulphate, associated with compounds of sodium and magnesium. In small
+quantities it is found as nitrate and in many other forms.
+
+The natural decomposition of rocks containing potassium gives rise to
+various compounds of the element in all fertile soils. Its soluble
+compounds are absorbed by growing plants and built up into complex
+vegetable substances; when these are burned the potassium remains in the
+ash in the form of the carbonate. Crude carbonate obtained from wood
+ashes was formerly the chief source of potassium compounds; they are now
+mostly prepared from the salts of the Stassfurt deposits.
+
+     ~Stassfurt salts.~ These salts form very extensive deposits in
+     middle and north Germany, the most noted locality for working
+     them being at Stassfurt. The deposits are very thick and rest
+     upon an enormous layer of common salt. They are in the form of
+     a series of strata, each consisting largely of a single mineral
+     salt. A cross section of these deposits is shown in Fig. 78.
+     While these strata are salts from a chemical standpoint, they
+     are as solid and hard as many kinds of stone, and are mined as
+     stone or coal would be. Since the strata differ in general
+     appearance, each can be mined separately, and the various
+     minerals can be worked up by methods adapted to each particular
+     case. The chief minerals of commercial importance in these
+     deposits are the following:
+
+        Sylvine       KCl.
+        Anhydrite     CaSO_{4}.
+        Carnallite    KCl·MgCl_{2}·6H_{2}O.
+        Kainite       K_{2}SO_{4}·MgSO_{4}·MgCl_{2}·6H_{2}O.
+        Polyhalite    K_{2}SO_{4}·MgSO_{4}·2CaSO_{4}·2H_{2}O.
+        Kieserite     MgSO_{4}·H_{2}O.
+        Schönite      K_{2}SO_{4}·MgSO_{4}·6H_{2}O.
+
+~Preparation and properties.~ The metal is prepared by the same method
+used in the preparation of sodium. In most respects it is very similar
+to sodium, the chief difference being that it is even more energetic in
+its action upon other substances. The freshly cut, bright surface
+instantly becomes dim through oxidation by the air. It decomposes water
+very vigorously, the heat of reaction being sufficient to ignite the
+hydrogen evolved. It is somewhat lighter than sodium and is preserved
+under gasoline.
+
+[Illustration: Fig. 78]
+
+~Potassium hydroxide~ (_caustic potash_) (KOH). Potassium hydroxide is
+prepared by methods exactly similar to those used in the preparation of
+sodium hydroxide, which compound it closely resembles in both physical
+and chemical properties. It is not used to any very great extent, being
+replaced by the cheaper sodium hydroxide.
+
+~Action of the halogen elements on potassium hydroxide.~ When any one of
+the three halogen elements--chlorine, bromine, and iodine--is added to a
+solution of potassium hydroxide a reaction takes place, the nature of
+which depends upon the conditions of the experiment. Thus, when chlorine
+is passed into a cold dilute solution of potassium hydroxide the
+reaction expressed by the following equation takes place:
+
+    (1) 2KOH + 2Cl = KCl + KClO + H_{2}O.
+
+If the solution of hydroxide is concentrated and hot, on the other hand,
+the potassium hypochlorite formed according to equation (1) breaks down
+as fast as formed:
+
+    (2) 3KClO = KClO_{3} + 2KCl.
+
+Equation (1), after being multiplied by 3, may be combined with equation
+(2), giving the following:
+
+    (3) 6KOH + 6Cl = 5KCl + KClO_{3} + 3H_{2}O.
+
+This represents in a single equation the action of chlorine on hot,
+concentrated solutions of potassium hydroxide. By means of these
+reactions one can prepare potassium chloride, potassium hypochlorite,
+and potassium chlorate. By substituting bromine or iodine for chlorine
+the corresponding compounds of these elements are obtained. Some of
+these compounds can be obtained in cheaper ways.
+
+If the halogen element is added to a solution of sodium hydroxide or
+calcium hydroxide, the reaction which takes place is exactly similar to
+that which takes place with potassium hydroxide. It is possible,
+therefore, to prepare in this way the sodium and calcium compounds
+corresponding to the potassium compounds given above.
+
+~Potassium chloride~ (KCl). This salt occurs in nature in sea water, in
+the mineral sylvine, and, combined with magnesium chloride, as
+carnallite (KCl·MgCl_{2}·6H_{2}O). It is prepared from carnallite by
+saturating boiling water with the mineral and allowing the solution to
+cool. The mineral decomposes while in solution, and the potassium
+chloride crystallizes out on cooling, while the very soluble magnesium
+chloride remains in solution. The salt is very similar to sodium
+chloride both in physical and chemical properties. It is used in the
+preparation of nearly all other potassium salts, and, together with
+potassium sulphate, is used as a fertilizer.
+
+~Potassium bromide~ (KBr). When bromine is added to a hot concentrated
+solution of potassium hydroxide there is formed a mixture of potassium
+bromide and potassium bromate in accordance with the reactions already
+discussed. There is no special use for the bromate, so the solution is
+evaporated to dryness, and the residue, consisting of a mixture of the
+bromate and bromide, is strongly heated. This changes the bromate to
+bromide, as follows:
+
+    KBrO_{3} = KBr +3O.
+
+The bromide is then crystallized from water, forming large colorless
+crystals. It is used in medicine and in photography.
+
+~Potassium iodide~ (KI). Potassium iodide may be made by exactly the same
+method as has just been described for the bromide, substituting iodine
+for bromine. It is more frequently made as follows. Iron filings are
+treated with iodine, forming the compound Fe_{3}I_{8}; on boiling this
+substance with potassium carbonate the reaction represented in the
+following equation occurs:
+
+    Fe_{3}I_{8} + 4K_{2}CO_{3} = Fe_{3}O_{4} + 8KI + 4CO_{2}.
+
+Potassium iodide finds its chief use in medicine.
+
+~Potassium chlorate~ (KClO_{3}). This salt, as has just been explained,
+can be made by the action of chlorine on strong potassium hydroxide
+solutions. The chief use of potassium chlorate is as an oxidizing agent
+in the manufacture of matches, fireworks, and explosives; it is also
+used in the preparation of oxygen and in medicine.
+
+     ~Commercial preparation.~ By referring to the reaction between
+     chlorine and hot concentrated solutions of potassium hydroxide,
+     it will be seen that only one molecule of potassium chlorate is
+     formed from six molecules of potassium hydroxide. Partly
+     because of this poor yield and partly because the potassium
+     hydroxide is rather expensive, this process is not an
+     economical one for the preparation of potassium chlorate. The
+     commercial method is the following. Chlorine is passed into hot
+     solutions of calcium hydroxide, a compound which is very cheap.
+     The resulting calcium chloride and chlorate are both very
+     soluble. To the solution of these salts potassium chloride is
+     added, and as the solution cools the sparingly soluble
+     potassium chlorate crystallizes out:
+
+          Ca(ClO_{3})_{2} + 2KCl = 2KClO_{3} + CaCl_{2}.
+
+     Electro-chemical processes are also used.
+
+~Potassium nitrate~ (_saltpeter_) (KNO_{3}). This salt was formerly made
+by allowing animal refuse to decompose in the open air in the presence
+of wood ashes or earthy materials containing potassium. Under these
+conditions the nitrogen in the organic matter is in part converted into
+potassium nitrate, which was obtained by extracting the mass with water
+and evaporating to crystallization. This crude and slow process is now
+almost entirely replaced by a manufacturing process in which the
+potassium salt is made from Chili saltpeter:
+
+    NaNO_{3} + KCl = NaCl + KNO_{3}.
+
+This process has been made possible by the discovery of the Chili niter
+beds and the potassium chloride of the Stassfurt deposits.
+
+     The reaction depends for its success upon the apparently
+     insignificant fact that sodium chloride is almost equally
+     soluble in cold and hot water. All four factors in the equation
+     are rather soluble in cold water, but in hot water sodium
+     chloride is far less soluble than the other three. When hot
+     saturated solutions of sodium nitrate and potassium chloride
+     are brought together, sodium chloride precipitates and can be
+     filtered off, leaving potassium nitrate in solution, together
+     with some sodium chloride. On cooling, potassium nitrate
+     crystallizes out, leaving small amounts of the other salts in
+     solution.
+
+Potassium nitrate is a colorless salt which forms very large crystals.
+It is stable in the air, and when heated is a good oxidizing agent,
+giving up oxygen quite readily. Its chief use is in the manufacture of
+gunpowder.
+
+     ~Gunpowder.~ The object sought for in the preparation of
+     gunpowder is to secure a solid substance which will remain
+     unchanged under ordinary conditions, but which will explode
+     readily when ignited, evolving a large volume of gas. When a
+     mixture of carbon and potassium nitrate is ignited a great deal
+     of gas is formed, as will be seen from the equation
+
+         2KNO_{3} + 3C = CO_{2} + CO + N_{2} + K_{2}CO_{3}.
+
+     By adding sulphur to the mixture the volume of gas formed in
+     the explosion is considerably increased:
+
+         2KNO_{3} + 3C + S = 3CO_{2} + N_{2} + K_{2}S.
+
+     Gunpowder is simply a mechanical mixture of these three
+     substances in the proportion required for the above reaction.
+     While the equation represents the principal reaction, other
+     reactions also take place. The gases formed in the explosion,
+     when measured under standard conditions, occupy about two
+     hundred and eighty times the volume of the original powder.
+     Potassium sulphide (K_{2}S) is a solid substance, and it is
+     largely due to it that gunpowder gives off smoke and soot when
+     it explodes. Smokeless powder consists of organic substances
+     which, on explosion, give only colorless gases, and hence
+     produce no smoke. Sodium nitrate is cheaper than potassium
+     nitrate, but it is not adapted to the manufacture of the best
+     grades of powder, since it is somewhat deliquescent and does
+     not give up its oxygen so readily as does potassium nitrate. It
+     is used, however, in the cheaper grades of powder, such as are
+     employed for blasting.
+
+~Potassium cyanide~ (KCN). When animal matter containing nitrogen is
+heated with iron and potassium carbonate, complicated changes occur
+which result in the formation of a substance commonly called yellow
+prussiate of potash, which has the formula K_{4}FeC_{6}N_{6}. When this
+substance is heated with potassium, potassium cyanide is formed:
+
+    K_{4}FeC_{6}N_{6} + 2 K = 6KCN + Fe.
+
+Since sodium is much cheaper than potassium it is often used in place of
+it:
+
+    K_{4}FeC_{6}N_{6} + 2Na = 4KCN + 2NaCN + Fe.
+
+The mixture of cyanides so resulting serves most of the purposes of the
+pure salt. It is used very extensively in several metallurgical
+processes, particularly in the extraction of gold. Potassium cyanide is
+a white solid characterized by its poisonous properties, and must be
+used with extreme caution.
+
+~Potassium carbonate~ (_potash_) (K_{2}CO_{3}). This compound occurs in
+wood ashes in small quantities. It cannot be prepared by the Solvay
+process, since the acid carbonate is quite soluble in water, but is made
+by the Le Blanc process. Its chief use is in the manufacture of other
+potassium salts.
+
+~Other salts of potassium.~ Among the other salts of potassium frequently
+met with are the sulphate (K_{2}SO_{4}), the acid carbonate (KHCO_{3}),
+the acid sulphate (KHSO_{4}), and the acid sulphite (KHSO_{3}). These
+are all white solids.
+
+
+LITHIUM, RUBIDIUM, CÆSIUM
+
+Of the three remaining elements of the family--lithium, rubidium, and
+cæsium--lithium is by far the most common, the other two being very
+rare. Lithium chloride and carbonate are not infrequently found in
+natural mineral waters, and as these substances are supposed to increase
+the medicinal value of the water, they are very often added to
+artificial mineral waters in small quantities.
+
+
+COMPOUNDS OF AMMONIUM
+
+~General.~ As explained in a previous chapter, when ammonia is passed into
+water the two compounds combine to form the base NH_{4}OH, known as
+ammonium hydroxide. When this base is neutralized with acids there are
+formed the corresponding salts, known as the ammonium salts. Since the
+ammonium group is univalent, ammonium salts resemble those of the alkali
+metals in formulas; they also resemble the latter salts very much in
+their chemical properties, and may be conveniently described in
+connection with them. Among the ammonium salts the chloride, sulphate,
+carbonate, and sulphide are the most familiar.
+
+~Ammonium chloride~ (_sal ammoniac_) (NH_{4}Cl). This substance is
+obtained by neutralizing ammonium hydroxide with hydrochloric acid. It
+is a colorless substance crystallizing in fine needles, and, like most
+ammonium salts, is very soluble in water. When placed in a tube and
+heated strongly it decomposes into hydrochloric acid and ammonia. When
+these gases reach a cooler portion of the tube they at once recombine,
+and the resulting ammonium chloride is deposited on the sides of the
+tube. In this way the salt can be separated from nonvolatile impurities.
+Ammonium chloride is sometimes used in preparation of ammonia; it is
+also used in making dry batteries and in the laboratory as a chemical
+reagent.
+
+~Ammonium sulphate~ ((NH_{4})_{2}SO_{4}). This salt resembles the chloride
+very closely, and, being cheaper, is used in place of it when possible.
+It is used in large quantity as a fertilizer, the nitrogen which it
+contains being a very valuable food for plants.
+
+~Ammonium carbonate~ ((NH_{4})_{2}CO_{3}). This salt, as well as the acid
+carbonate (NH_{4}HCO_{3}), is used as a chemical reagent. They are
+colorless solids, freely soluble in water. The normal carbonate is made
+by heating ammonium chloride with powdered limestone (calcium
+carbonate), the ammonium carbonate being obtained as a sublimate in
+compact hard masses:
+
+    2NH_{4}Cl + CaCO_{3} = (NH_{4})_{2}CO_{3} + CaCl_{2}.
+
+The salt always smells of ammonia, since it slowly decomposes, as shown
+in the equation
+
+    (NH_{4})_{2}CO_{3} = NH_{4}HCO_{3} + NH_{3}.
+
+The acid carbonate, or bicarbonate, is prepared by saturating a solution
+of ammonium hydroxide with carbon dioxide:
+
+    NH_{4}OH + CO_{2} = NH_{4}HCO_{3}.
+
+It is a well-crystallized stable substance.
+
+~Ammonium sulphide~ ((NH_{4})_{2}S). Ammonium sulphide is prepared by the
+action of hydrosulphuric acid upon ammonium hydroxide:
+
+    2NH_{4}OH + H_{2}S = (NH_{4})_{2}S + 2H_{2}O.
+
+If the action is allowed to continue until no more hydrosulphuric acid
+is absorbed, the product is the acid sulphide, sometimes called the
+hydrosulphide:
+
+    NH_{4}OH + H_{2}S = NH_{4}HS + H_{2}O.
+
+If equal amounts of ammonium hydroxide and ammonium acid sulphide are
+brought together, the normal sulphide is formed:
+
+    NH_{4}OH + NH_{4}HS = (NH_{4})_{2}S + H_{2}O
+
+It has been obtained in the solid state, but only with great difficulty.
+As used in the laboratory it is always in the form of a solution. It is
+much used in the process of chemical analysis because it is a soluble
+sulphide and easily prepared. On exposure to the air ammonium sulphide
+slowly decomposes, being converted into ammonia, water, and sulphur:
+
+    (NH_{4})_{2}S + O = 2NH_{3} + H_{2}O + S.
+
+As fast as the sulphur is liberated it combines with the unchanged
+sulphide to form several different ammonium sulphides in which there are
+from two to five sulphur atoms in the molecule, thus: (NH_{4})_{2}S_{2},
+(NH_{4})_{2}S_{3}, (NH_{4})_{2}S_{5}. These sulphides in turn decompose
+by further action of oxygen, so that the final products of the reaction
+are those given in the equation. A solution of these compounds is yellow
+and is sometimes called _yellow ammonium sulphide_.
+
+     FLAME REACTION--SPECTROSCOPE
+
+     When compounds of either sodium or potassium are brought into
+     the non-luminous flame of a Bunsen burner the flame becomes
+     colored. Sodium compounds color it intensely yellow, while
+     those of potassium color it pale violet. When only one of these
+     elements is present it is easy to identify it by this simple
+     test, but when both are present the intense color of the sodium
+     flame entirely conceals the pale tint characteristic of
+     potassium compounds.
+
+     It is possible to detect the potassium flame in such cases,
+     however, in the following way. When light is allowed to shine
+     through a very small hole or slit in some kind of a screen,
+     such as a piece of metal, upon a triangular prism of glass, the
+     light is bent or refracted out of its course instead of passing
+     straight through the glass. It thus comes out of the prism at
+     some angle to the line at which it entered. Yellow light is
+     bent more than red, and violet more than yellow. When light
+     made up of the yellow of sodium and the violet of potassium
+     shines through a slit upon such a prism, the yellow and the
+     violet lights come out at somewhat different angles, and so two
+     colored lines of light--a yellow line and a violet line--are
+     seen on looking into the prism in the proper direction. The
+     instrument used for separating the rays of light in this way is
+     called a _spectroscope_ (Fig. 79). The material to be tested is
+     placed on a platinum wire and held in the colorless Bunsen
+     flame. The resulting light passes through the slit in the end
+     of tube B, and then through B to the prism. The resulting
+     lines of light are seen by looking into the tube A, which
+     contains a magnifying lens. Most elements give more than one
+     image of the slit, each having a different color, and the
+     series of colored lines due to an element is called its
+     spectrum.
+
+[Illustration: Fig. 79]
+
+The spectra of the known elements have been carefully studied, and any
+element which imparts a characteristic color to a flame, or has a
+spectrum of its own, can be identified even when other elements are
+present. Through the spectroscopic examination of certain minerals a
+number of elements have been discovered by the observation of lines
+which did not belong to any known element. A study of the substance then
+brought to light the new element. Rubidium and cæsium were discovered in
+this way, rubidium having bright red lines and cæsium a very intense
+blue line. Lithium colors the flame deep red, and has a bright red line
+in its spectrum.
+
+
+EXERCISES
+
+1. What is an alkali? Can a metal itself be an alkali?
+
+2. Write equations showing how the following changes may be brought
+about, giving the general principle involved in each change: NaCl -->
+Na_{2}SO_{3}, Na_{2}SO_{3} --> NaCl, NaCl --> NaBr, Na_{2}SO_{4} -->
+NaNO_{3}, NaNO_{3} --> NaHCO_{3}.
+
+3. What carbonates are soluble?
+
+4. State the conditions under which the reaction represented by the
+following equation can be made to go in either direction:
+
+    Na_{2}CO_{3} + H_{2}O + CO_{2} <--> 2 NaHCO_{3}.
+
+5. Account for the fact that solutions of sodium carbonate and potassium
+carbonate are alkaline.
+
+6. What non-metallic element is obtained from the deposits of Chili
+saltpeter?
+
+7. Supposing concentrated hydrochloric acid (den. = 1.2) to be worth six
+cents a pound, what is the value of the acid generated in the
+preparation of 1 ton of sodium carbonate by the Le Blanc process?
+
+8. What weight of sodium carbonate crystals will 1 kg. of the anhydrous
+salt yield?
+
+9. Write equations for the preparation of potassium hydroxide by three
+different methods.
+
+10. What would take place if a bit of potassium hydroxide were left
+exposed to the air?
+
+11. Write the equations for the reactions between sodium hydroxide and
+bromine; between potassium hydroxide and iodine.
+
+12. Write equations for the preparation of potassium sulphate; of
+potassium acid carbonate.
+
+13. What weight of carnallite would be necessary in the preparation of 1
+ton of potassium carbonate?
+
+14. Write the equations showing how ammonium chloride, ammonium
+sulphate, ammonium carbonate, and ammonium nitrate may be prepared from
+ammonium hydroxide.
+
+15. Write an equation to represent the reaction involved in the
+preparation of ammonia from ammonium chloride.
+
+16. What substances already studied are prepared from the following
+compounds? ammonium chloride; ammonium nitrate; ammonium nitrite; sodium
+nitrate; sodium chloride.
+
+17. How could you prove that the water in crystals of common salt is not
+water of crystallization?
+
+18. How could you distinguish between potassium chloride and potassium
+iodide? between sodium chloride and ammonium chloride? between sodium
+nitrate and potassium nitrate?
+
+[Illustration: ROBERT WILHELM BUNSEN (German) (1811-1899)
+
+Invented many lecture-room and laboratory appliances (Bunsen burner);
+invented the spectroscope and with it discovered rubidium and cæsium;
+greatly perfected methods of electrolysis, inventing a new battery; made
+many investigations among metallic and organic substances]
+
+
+
+
+CHAPTER XXIV
+
+THE ALKALINE-EARTH FAMILY
+
+
+===========================================================================
+          |        |        |         |                  |
+          |        |        |         | MILLIGRAMS SOL-  |
+          |        |        |         |  UBLE IN 1 L.    |
+          |        |        |         | OF WATER AT 18°  |
+          | SYMBOL | ATOMIC | DENSITY |__________________| CARBONATE
+          |        | WEIGHT |         |         |        | DECOMPOSES
+          |        |        |         | SULPHATE| HYDROX-|
+          |        |        |         |         |   IDE  |
+__________|________|________|_________|_________|________|_________________
+          |        |        |         |         |        |
+Calcium   |   Ca   |  40.1  |   1.54  | 2070.00 |  1670. | At dull red heat
+Strontium |   Sr   |  87.6  |   2.50  |  170.00 |  7460. | At white heat
+Barium    |   Ba   | 137.4  |   3.75  |    2.29 | 36300. | Scarcely at all
+===========================================================================
+
+~The family.~ The alkaline-earth family consists of the very abundant
+element calcium and the much rarer elements strontium and barium. They
+are called the alkaline-earth metals because their properties are
+between those of the alkali metals and the earth metals. The earth
+metals will be discussed in a later chapter. The family is also
+frequently called the calcium family.
+
+1. _Occurrence._ These elements do not occur free in nature. Their most
+abundant compounds are the carbonates and sulphates; calcium also occurs
+in large quantities as the phosphate and silicate.
+
+2. _Preparation._ The metals were first prepared by Davy in 1808 by
+electrolysis. This method has again come into use in recent years.
+Strontium and barium have as yet been obtained only in small quantities
+and in the impure state, and many of their physical properties, such as
+their densities and melting points, are therefore imperfectly known.
+
+3. _Properties._ The three metals resemble each other very closely. They
+are silvery-white in color and are about as hard as lead. Their
+densities increase with their atomic weights, as is shown in the table
+on opposite page. Like the alkali metals they have a strong affinity for
+oxygen, tarnishing in the air through oxidation. They decompose water at
+ordinary temperatures, forming hydroxides and liberating hydrogen. When
+ignited in the air they burn with brilliancy, forming oxides of the
+general formula MO. These oxides readily combine with water, according
+to the equation
+
+    MO + H_{2}O = M(OH)_{2}.
+
+Each of the elements has a characteristic spectrum, and the presence of
+the metals can easily be detected by the spectroscope.
+
+4. _Compounds._ The elements are divalent in almost all of their
+compounds, and these compounds in solution give simple, divalent,
+colorless ions. The corresponding salts of the three elements are very
+similar to each other and show a regular variation in properties in
+passing from calcium to strontium and from strontium to barium. This is
+seen in the solubility of the sulphate and hydroxide, and in the ease of
+decomposition of the carbonates, as given in the table. Unlike the
+alkali metals, their normal carbonates and phosphates are insoluble in
+water.
+
+
+CALCIUM
+
+~Occurrence.~ The compounds of calcium are very abundant in nature, so
+that the total amount of calcium in the earth's crust is very large. A
+great many different compounds containing the clement are known, the
+most important of which are the following:
+
+    Calcite (marble)     CaCO_{3}.
+    Phosphorite          Ca_{3}(PO_{4})_{2}.
+    Fluorspar            CaF_{2}.
+    Wollastonite         CaSiO_{3}.
+    Gypsum               CaSO_{4}·2H_{2}O.
+    Anhydrite            CaSO_{4}.
+
+~Preparation.~ Calcium is now prepared by the electrolysis of the melted
+chloride, the metal depositing in solid condition on the cathode. It is
+a gray metal, considerably heavier and harder than sodium. It acts upon
+water, forming calcium hydroxide and hydrogen, but the action does not
+evolve sufficient heat to melt the metal. It promises to become a useful
+substance, though no commercial applications for it have as yet been
+found.
+
+~Calcium oxide~ (_lime, quicklime_) (CaO). Lime is prepared by strongly
+heating calcium carbonate (limestone) in large furnaces called kilns:
+
+    CaCO_{3} = CaO + CO_{2}.
+
+When pure, lime is a white amorphous substance. Heated intensely, as in
+the oxyhydrogen flame, it gives a brilliant light called the lime light.
+Although it is a very difficultly fusible substance, yet in the electric
+furnace it can be made to melt and even boil. Water acts upon lime with
+the evolution of a great deal of heat,--hence the name quicklime, or
+live lime,--the process being called slaking. The equation is
+
+    CaO + H_{2}O = Ca(OH)_{2}.
+
+Lime readily absorbs moisture from the air, and is used to dry moist
+gases, especially ammonia, which cannot be dried by the usual
+desiccating agents. It also absorbs carbon dioxide, forming the
+carbonate
+
+    CaO + CO_{2} = CaCO_{3}.
+
+Lime exposed to air is therefore gradually converted into hydroxide and
+carbonate, and will no longer slake with water. It is then said to be
+air-slaked.
+
+     ~Limekilns.~ The older kiln, still in common use, consists of a
+     large cylindrical stack in which the limestone is loosely
+     packed. A fire is built at the base of the stack, and when the
+     burning is complete it is allowed to die out and the lime is
+     removed from the kiln. The newer kilns are constructed as shown
+     in Fig. 80. A number of fire boxes are built around the lower
+     part of the kiln, one of which is shown at B. The fire is
+     built on the grate F and the hot products of combustion are
+     drawn up through the stack, decomposing the limestone. The kiln
+     is charged at C, and sometimes fuel is added with the
+     limestone to cause combustion throughout the contents of the
+     kiln. The burned lime is raked out through openings in the
+     bottom of the stack, one of which is shown at _D._ The
+     advantage of this kind of a kiln over the older form is that
+     the process is continuous, limestone being charged in at the
+     top as fast as the lime is removed at the bottom.
+
+[Illustration: Fig. 80]
+
+~Calcium hydroxide ~ (_slaked lime_) (Ca(OH)_{2}). Pure calcium hydroxide
+is a light white powder. It is sparingly soluble in water, forming a
+solution called _limewater_, which is often used in medicine as a mild
+alkali. Chemically, calcium hydroxide is a moderately strong base,
+though not so strong as sodium hydroxide. Owing to its cheapness it is
+much used in the industries whenever an alkali is desired. A number of
+its uses have already been mentioned. It is used in the preparation of
+ammonia, bleaching powder, and potassium hydroxide. It is also used to
+remove carbon dioxide and sulphur compounds from coal gas, to remove the
+hair from hides in the tanneries (this recalls the caustic or corrosive
+properties of sodium hydroxide), and for making mortar.
+
+~Mortar~ is a mixture of calcium hydroxide and sand. When it is exposed to
+the air or spread upon porous materials moisture is removed from it
+partly by absorption in the porous materials and partly by evaporation,
+and the mortar becomes firm, or _sets_. At the same time carbon dioxide
+is slowly absorbed from the air, forming hard calcium carbonate:
+
+    Ca(OH)_{2} + CO_{2} = CaCO_{3} + H_{2}O.
+
+By this combined action the mortar becomes very hard and adheres firmly
+to the surface upon which it is spread. The sand serves to give body to
+the mortar and makes it porous, so that the change into carbonate can
+take place throughout the mass. It also prevents too much shrinkage.
+
+~Cement.~ When limestone to which clay and sand have been added in certain
+proportions is burned until it is partly fused (some natural marl is
+already of about the right composition), and the clinker so produced is
+ground to powder, the product is called cement. When this material is
+moistened it sets to a hard stone-like mass which retains its hardness
+even when exposed to the continued action of water. It can be used for
+under-water work, such as bridge piers, where mortar would quickly
+soften. Several varieties of cement are made, the best known of which is
+Portland cement.
+
+~Growing importance of cement.~ Cement is rapidly coming into use for a
+great variety of purposes. It is often used in place of mortar in the
+construction of brick buildings. Mixed with crushed stone and sand it
+forms concrete which is used in foundation work. It is also used in
+making artificial stone, terra-cotta trimmings for buildings, artificial
+stone walks and floors, and the like. It is being used more and more for
+making many articles which were formerly made of wood or stone, and the
+entire walls of buildings are sometimes made of cement blocks or of
+concrete.
+
+~Calcium carbonate~ (CaCO_{3}). This substance is found in a great many
+natural forms to which various names have been given. They may be
+classified under three heads:
+
+1. _Amorphous carbonate._ This includes those forms which are not
+markedly crystalline. Limestone is the most familiar of these and is a
+grayish rock usually found in hard stratified masses. Whole mountain
+ranges are sometimes made up of this material. It is always impure,
+usually containing magnesium carbonate, clay, silica, iron and aluminium
+compounds, and frequently fossil remains. Marl is a mixture of limestone
+and clay. Pearls, chalk, coral, and shells are largely calcium
+carbonate.
+
+2. _Hexagonal carbonate._ Calcium carbonate crystallizes in the form of
+rhomb-shaped crystals which belong to the hexagonal system. When very
+pure and transparent the substance is called Iceland spar. Calcite is a
+similar form, but somewhat opaque or clouded. Mexican onyx is a massive
+variety, streaked or banded with colors due to impurities. Marble when
+pure is made up of minute calcite crystals. Stalactites and stalagmites
+are icicle-like forms sometimes found in caves.
+
+3. _Rhombic carbonate._ Calcium carbonate sometimes crystallizes in
+needle-shaped crystals belonging to the rhombic system. This is the
+unstable form and tends to go over into the other variety. Aragonite is
+the most familiar example of this form.
+
+~Preparation and uses of calcium carbonate.~ In the laboratory pure
+calcium carbonate can be prepared by treating a soluble calcium salt
+with a soluble carbonate:
+
+    Na_{2}CO_{3} + CaCl_{2} = CaCO_{3} + 2NaCl.
+
+When prepared in this way it is a soft white powder often called
+precipitated chalk, and is much used as a polishing powder. It is
+insoluble in water, but dissolves in water saturated with carbon
+dioxide, owing to the formation of the acid calcium carbonate which is
+slightly soluble:
+
+    CaCO_{3} + H_{2}CO_{3} = Ca(HCO_{3})_{2}.
+
+The natural varieties of calcium carbonate find many uses, such as in
+the preparation of lime and carbon dioxide; in metallurgical operations,
+especially in the blast furnaces; in the manufacture of soda, glass, and
+crayon (which, in addition to chalk, usually contains clay and calcium
+sulphate); for building stone and ballast for roads.
+
+~Calcium chloride~ (CaCl_{2}). This salt occurs in considerable quantity
+in sea water. It is obtained as a by-product in many technical
+processes, as in the Solvay soda process. When crystallized from its
+saturated solutions it forms colorless needles of the composition
+CaCl_{2}·6H_{2}O. By evaporating a solution to dryness and heating to a
+moderate temperature calcium chloride is obtained anhydrous as a white
+porous mass. In this condition it absorbs water with great energy and is
+a valuable drying agent.
+
+~Bleaching powder~ (CaOCl_{2}). When chlorine acts upon a solution of
+calcium hydroxide the reaction is similar to that which occurs between
+chlorine and potassium hydroxide:
+
+    2 Ca(OH)_{2} + 4 Cl = CaCl_{2} + Ca(ClO)_{2} + 2 H_{2}O.
+
+If, however, chlorine is conducted over calcium hydroxide in the form of
+a dry powder, it is absorbed and a substance is formed which appears to
+have the composition represented in the formula CaOCl_{2}. This
+substance is called bleaching powder, or hypochlorite of lime. It is
+probably the calcium salt of both hydrochloric and hypochlorous acids,
+so that its structure is represented by the formula
+
+       /ClO
+    Ca
+       \Cl.
+
+In solution this substance acts exactly like a mixture of calcium
+chloride (CaCl_{2}) and calcium hypochlorite (Ca(ClO)_{2}), since it
+dissociates to form the ions Ca^{++}, Cl^{-}, and ClO^{-}.
+
+Bleaching powder undergoes a number of reactions which make it an
+important substance.
+
+1. When treated with an acid it evolves chlorine:
+
+      /ClO
+    Ca + H_{2}SO_{4} = CaSO_{4} + HCl + HClO,
+      \Cl
+
+
+    HCl + HClO = H_{2}O + 2Cl.
+
+This reaction can be employed in the preparation of chlorine, or the
+nascent chlorine may be used as a bleaching agent.
+
+2. It is slowly decomposed by the carbon dioxide of the air, yielding
+calcium carbonate and chlorine:
+
+    CaOCl_{2} + CO_{2} = CaCO_{3} + 2Cl.
+
+Owing to this slow action the substance is a good disinfectant.
+
+3. When its solution is boiled the substance breaks down into calcium
+chloride and chlorate:
+
+    6CaOCl_{2} = 5CaCl_{2} + Ca(ClO_{3})_{2}.
+
+This reaction is used in the preparation of potassium chlorate.
+
+~Calcium fluoride~ (_fluorspar_) (CaF_{2}). Fluorspar has already been
+mentioned as the chief natural compound of fluorine. It is found in
+large quantities in a number of localities, and is often crystallized in
+perfect cubes of a light green or amethyst color. It can be melted
+easily in a furnace, and is sometimes used in the fused condition in
+metallurgical operations to protect a metal from the action of the air
+during its reduction. It is used as the chief source of fluorine
+compounds, especially hydrofluoric acid.
+
+~Calcium sulphate~ (_gypsum_) (CaSO_{4}·2H_{2}O). This abundant substance
+occurs in very perfectly formed crystals or in massive deposits. It is
+often found in solution in natural waters and in the sea water. Salts
+deposited from sea water are therefore likely to contain this substance
+(see Stassfurt salts).
+
+It is very sparingly soluble in water, and is thrown down as a fine
+white precipitate when any considerable amounts of a calcium salt and a
+soluble sulphate (or sulphuric acid) are brought together in solution.
+Its chief use is in the manufacture of plaster of Paris and of hollow
+tiles for fireproof walls. Such material is called _gypsite_. It is also
+used as a fertilizer.
+
+Calcium sulphate, like the carbonate, occurs in many forms in nature.
+Gypsum is a name given to all common varieties. Granular or massive
+specimens are called alabaster, while all those which are well
+crystallized are called selenite. Satin spar is still another variety
+often seen in mineral collections.
+
+~Plaster of Paris.~ When gypsum is heated to about 115° it loses a portion
+of its water of crystallization in accordance with the equation
+
+    2(CaSO_{4}·2H_{2}O) = 2CaSO_{4}·H_{2}O + 2H_{2}O.
+
+The product is a fine white powder called _plaster of Paris_. On being
+moistened it again takes up this water, and in so doing first forms a
+plastic mass, which soon becomes very firm and hard and regains its
+crystalline structure. These properties make it very valuable as a
+material for forming casts and stucco work, for cementing glass to
+metals, and for other similar purposes. If overheated so that all water
+is driven off, the process of taking up water is so slow that the
+material is worthless. Such material is said to be dead burned. Plaster
+of Paris is very extensively used as the finishing coat for plastered
+walls.
+
+~Hard water.~ Waters containing compounds of calcium and magnesium in
+solution are called hard waters because they feel harsh to the touch.
+The hardness of water may be of two kinds,--(1) temporary hardness and
+(2) permanent hardness.
+
+1. _Temporary hardness._ We have seen that when water charged with
+carbon dioxide comes in contact with limestone a certain amount of the
+latter dissolves, owing to the formation of the soluble acid carbonate
+of calcium. The hardness of such waters is said to be temporary, since
+it may be removed by boiling. The heat changes the acid carbonate into
+the insoluble normal carbonate which then precipitates, rendering the
+water soft:
+
+    Ca(HCO_{3})_{2} = CaCO_{3} + H_{2}O + CO_{2}.
+
+Such waters may also be softened by the addition of sufficient lime or
+calcium hydroxide to convert the acid carbonate of calcium into the
+normal carbonate. The equation representing the reaction is
+
+    Ca(HCO_{3})_{2} + Ca(OH)_{2} = 2CaCO_{3} + 2H_{2}O.
+
+2. _Permanent hardness._ The hardness of water may also be due to the
+presence of calcium and magnesium sulphates or chlorides. Boiling the
+water does not affect these salts; hence such waters are said to have
+permanent hardness. They may be softened, however, by the addition of
+sodium carbonate, which precipitates the calcium and magnesium as
+insoluble carbonates:
+
+    CaSO_{4} + Na_{2}CO_{3} = CaCO_{3} + Na_{2}SO_{4}.
+
+This process is sometimes called "breaking" the water.
+
+     ~Commercial methods for softening water.~ The average water of a
+     city supply contains not only the acid carbonates of calcium
+     and magnesium but also the sulphates and chlorides of these
+     metals, together with other salts in smaller quantities. Such
+     waters are softened on a commercial scale by the addition of
+     the proper quantities of calcium hydroxide and sodium
+     carbonate. The calcium hydroxide is added first to precipitate
+     all the acid carbonates. After a short time the sodium
+     carbonate is added to precipitate the other soluble salts of
+     calcium and magnesium, together with any excess of calcium
+     hydroxide which may have been added. The quantity of calcium
+     hydroxide and sodium carbonate required is calculated from a
+     chemical analysis of the water. It will be noticed that the
+     water softened in this way will contain sodium sulphate and
+     chloride, but the presence of these salts is not objectionable.
+
+~Calcium carbide~ (CaC_{2}). This substance is made by heating well-dried
+coke and lime in an electrical furnace. The equation is
+
+    CaO + 3C = CaC_{2} + CO.
+
+The pure carbide is a colorless, transparent, crystalline substance. In
+contact with water it is decomposed with the evolution of pure acetylene
+gas, having a pleasant ethereal odor. The commercial article is a dull
+gray porous substance which contains many impurities. The acetylene
+prepared from this substance has a very characteristic odor due to
+impurities, the chief of these being phosphine. It is used in
+considerable quantities as a source of acetylene gas for illuminating
+purposes.
+
+     ~Technical preparation.~ Fig. 81 represents a recent type of a
+     carbide furnace. The base of the furnace is provided with a
+     large block of carbon A, which serves as one of the
+     electrodes. The other electrodes B, several in number, are
+     arranged horizontally at some distance above this. A mixture of
+     coal and lime is fed into the furnace through the trap top C,
+     and in the lower part of the furnace this mixture becomes
+     intensely heated, forming liquid carbide. This is drawn off
+     through the taphole D.
+
+     The carbon monoxide formed in the reaction escapes through the
+     pipes E and is led back into the furnace. The pipes F
+     supply air, so that the monoxide burns as it reënters the
+     furnace and assists in heating the charge. The carbon dioxide
+     so formed, together with the nitrogen entering as air, escape
+     at G. An alternating current is used.
+
+[Illustration: Fig. 81]
+
+~Calcium phosphate~ (Ca_{3}(PO_{4})_{2}). This important substance
+occurs abundantly in nature as a constituent of apatite
+(3Ca_{3}(PO_{4})_{2}·CaF_{2}), in phosphate rock, and as the chief
+mineral constituent of bones. Bone ash is therefore nearly pure calcium
+phosphate. It is a white powder, insoluble in water, although it readily
+dissolves in acids, being decomposed by them and converted into soluble
+acid phosphates, as explained in connection with the acids of
+phosphorus.
+
+
+STRONTIUM
+
+~Occurrence.~ Strontium occurs sparingly in nature, usually as
+strontianite (SrCO_{3}) and as celestite (SrSO_{4}). Both minerals form
+beautiful colorless crystals, though celestite is sometimes colored a
+faint blue. Only a few of the compounds of strontium have any commercial
+applications.
+
+~Strontium hydroxide~ (Sr(OH)_{2}·8H_{2}O). The method of preparation of
+strontium hydroxide is analogous to that of calcium hydroxide. The
+substance has the property of forming an insoluble compound with sugar,
+which can easily be separated again into its constituents. It is
+therefore sometimes used in the sugar refineries to extract sugar from
+impure mother liquors from which the sugar will not crystallize.
+
+~Strontium nitrate~ (Sr(NO_{3})_{2}·4H_{2}O). This salt is prepared by
+treating the native carbonate with nitric acid. When ignited with
+combustible materials it imparts a brilliant crimson color to the flame,
+and because of this property it is used in the manufacture of red
+lights.
+
+
+BARIUM
+
+Barium is somewhat more abundant than strontium, occurring in nature
+largely as barytes, or heavy spar (BaSO_{4}), and witherite (BaCO_{3}).
+Like strontium, it closely resembles calcium both in the properties of
+the metal and in the compounds which it forms.
+
+~Oxides of barium.~ Barium oxide (BaO) can be obtained by strongly heating
+the nitrate:
+
+    Ba(NO_{3})_{2} = BaO + 2NO_{2} + O.
+
+Heated to a low red heat in the air, the oxide combines with oxygen,
+forming the peroxide (BaO_{2}). If the temperature is raised still
+higher, or the pressure is reduced, oxygen is given off and the oxide is
+once more formed. The reaction
+
+    BaO_{2} <--> BaO + O
+
+is reversible and has been used as a means of separating oxygen from the
+air. Treated with acids, barium peroxide yields hydrogen peroxide:
+
+    BaO_{2} + 2HCl = BaCl_{2} + H_{2}O_{2}.
+
+~Barium chloride~ (BaCl_{2}·2H_{2}O). Barium chloride is a white
+well-crystallized substance which is easily prepared from the native
+carbonate. It is largely used in the laboratory as a reagent to detect
+the presence of sulphuric acid or soluble sulphates.
+
+~Barium sulphate~ _(barytes)_ (BaSO_{4}). Barium sulphate occurs in nature
+in the form of heavy white crystals. It is precipitated as a crystalline
+powder when a barium salt is added to a solution of a sulphate or
+sulphuric acid:
+
+    BaCl_{2} + H_{2}SO_{4} = BaSO_{4} + 2HCl.
+
+This precipitate is used, as are also the finely ground native sulphate
+and carbonate, as a pigment in paints. On account of its low cost it is
+sometimes used as an adulterant of white lead, which is also a heavy
+white substance.
+
+Barium compounds color the flame green, and the nitrate (Ba(NO_{3})_{2})
+is used in the manufacture of green lights. Soluble barium compounds are
+poisonous.
+
+
+RADIUM
+
+~Historical.~ In 1896 the French scientist Becquerel observed that the
+mineral pitchblende possesses certain remarkable properties. It affects
+photographic plates even in complete darkness, and discharges a
+gold-leaf electroscope when brought close to it. In 1898 Madam Curie
+made a careful study of pitchblende to see if these properties belong to
+it or to some unknown substance contained in it. She succeeded in
+extracting from it a very small quantity of a substance containing a new
+element which she named radium.
+
+In 1910 Madam Curie succeeded in obtaining radium itself by the
+electrolysis of radium chloride. It is a silver-white metal melting at
+about 700°. It blackens in the air, forming a nitride, and decomposes
+water. Its atomic weight is about 226.5.
+
+~Properties.~ Compounds of radium affect a photographic plate or
+electroscope even through layers of paper or sheets of metal. They also
+bring about chemical changes in substances placed near them.
+Investigation of these strange properties has suggested that the radium
+atoms are unstable and undergo a decomposition. As a result of this
+decomposition very minute bodies, to which the name corpuscles has been
+given, are projected from the radium atom with exceedingly great
+velocity. It is to these corpuscles that the strange properties of
+radium are due. It seems probable that the gas helium is in some way
+formed during the decomposition of radium.
+
+Two or three other elements, particularly uranium and thorium, have been
+found to possess many of the properties of radium in smaller degree.
+
+~Radium and the atomic theory.~ If these views in regard to radium should
+prove to be well founded, it will be necessary to modify in some
+respects the conception of the atom as developed in a former chapter.
+The atom would have to be regarded as a compound unit made up of several
+parts. In a few cases, as in radium and uranium, it would appear that
+this unit is unstable and undergoes transformation into more stable
+combinations. This modification would not, in any essential way, be at
+variance with the atomic theory as propounded by Dalton.
+
+
+EXERCISES
+
+1. What properties have the alkaline-earth metals in common with the
+alkali metals? In what respects do they differ?
+
+2. Write the equation for the reaction between calcium carbide and
+water.
+
+3. For what is calcium chlorate used?
+
+4. Could limestone be completely decomposed if heated in a closed
+vessel?
+
+5. Caves often occur in limestone. Account for their formation.
+
+6. What is the significance of the term fluorspar? (Consult dictionary.)
+
+7. Could calcium chloride be used in place of barium chloride in testing
+for sulphates?
+
+8. What weight of water is necessary to slake the lime obtained from 1
+ton of pure calcium carbonate?
+
+9. What weight of gypsum is necessary in the preparation of 1 ton of
+plaster of Paris?
+
+10. Write equations to represent the reactions involved in the
+preparation of strontium hydroxide and strontium nitrate from
+strontianite.
+
+11. Write equations to represent the reactions involved in the
+preparation of barium chloride from heavy spar.
+
+12. Could barium hydroxide be used in place of calcium hydroxide in
+testing for carbon dioxide?
+
+
+
+
+CHAPTER XXV
+
+THE MAGNESIUM FAMILY
+
+
+===========================================================================
+           |SYMBOL   |ATOMIC   |DENSITY   |MELTING   |BOILING | OXIDE
+           |         |WEIGHT   |          | POINT    | POINT  |
+---------------------------------------------------------------------------
+Magnesium  | Mg      | 24.36   |  1.75    | 750°     | 920°   |   MgO
+Zinc       | Zn      | 65.4    |  7.00    | 420°     | 950°   |   ZnO
+Cadmium    | Cd      |112.4    |  8.67    | 320°     | 778°   |   CdO
+===========================================================================
+
+~The family.~ In the magnesium family are included the four elements:
+magnesium, zinc, cadmium, and mercury. Between the first three of these
+metals there is a close family resemblance, such as has been traced
+between the members of the two preceding families. Mercury in some
+respects is more similar to copper and will be studied in connection
+with that metal.
+
+1. _Properties._ When heated to a high temperature in the air each of
+these metals combines with oxygen to form an oxide of the general
+formula MO, in which M represents the metal. Magnesium decomposes
+boiling water slowly, while zinc and cadmium have but little action on
+it.
+
+2. _Compounds._ The members of this group are divalent in nearly all
+their compounds, so that the formulas of their salts resemble those of
+the alkaline-earth metals. Like the alkaline-earth metals, their
+carbonates and phosphates are insoluble in water. Their sulphates,
+however, are readily soluble. Unlike both the alkali and alkaline-earth
+metals, their hydroxides are nearly insoluble in water. Most of their
+compounds dissociate in such a way as to give a simple, colorless,
+metallic ion.
+
+
+MAGNESIUM
+
+~Occurrence.~ Magnesium is a very abundant element in nature, ranking a
+little below calcium in this respect. Like calcium, it is a constituent
+of many rocks and also occurs in the form of soluble salts.
+
+~Preparation.~ The metal magnesium, like most metals whose oxides are
+difficult to reduce with carbon, was formerly prepared by heating the
+anhydrous chloride with sodium:
+
+    MgCl_{2} + 2Na = 2NaCl + Mg.
+
+It is now made by electrolysis, but instead of using as the electrolyte
+the melted anhydrous chloride, which is difficult to obtain, the natural
+mineral carnallite is used. This is melted in an iron pot which also
+serves as the cathode in the electrolysis. A rod of carbon dipping into
+the melted salt serves as the anode. The apparatus is very similar to
+the one employed in the preparation of sodium.
+
+~Properties.~ Magnesium is a rather tough silvery-white metal of small
+density. Air does not act rapidly upon it, but a thin film of oxide
+forms upon its surface, dimming its bright luster. The common acids
+dissolve it with the formation of the corresponding salts. It can be
+ignited readily and in burning liberates much heat and gives a brilliant
+white light. This light is very rich in the rays which affect
+photographic plates, and the metal in the form of fine powder is
+extensively used in the production of flash lights and for white lights
+in pyrotechnic displays.
+
+~Magnesium oxide~ (_magnesia_) (MgO). Magnesium oxide, sometimes called
+magnesia or magnesia usta, resembles lime in many respects. It is much
+more easily formed than lime and can be made in the same way,--by
+igniting the carbonate. It is a white powder, very soft and light, and
+is unchanged by heat even at very high temperatures. For this reason it
+is used in the manufacture of crucibles, for lining furnaces, and for
+other purposes where a refractory substance is needed. It combines with
+water to form magnesium hydroxide, but much more slowly and with the
+production of much less heat than in the case of calcium oxide.
+
+~Magnesium hydroxide~ (Mg(OH)_{2}). The hydroxide formed in this way is
+very slightly soluble in water, but enough dissolves to give the water
+an alkaline reaction. Magnesium hydroxide is therefore a fairly strong
+base. It is an amorphous white substance. Neither magnesia nor magnesium
+salts have a very marked effect upon the system; and for this reason
+magnesia is a very suitable antidote for poisoning by strong acids,
+since any excess introduced into the system will have no injurious
+effect.
+
+     ~Magnesium cement.~ A paste of magnesium hydroxide and water
+     slowly absorbs carbon dioxide from the air and becomes very
+     hard. The hardness of the product is increased by the presence
+     of a considerable amount of magnesium chloride in the paste.
+     The hydroxide, with or without the chloride, is used in the
+     preparation of cements for some purposes.
+
+~Magnesium carbonate~ (MgCO_{3}). Magnesium carbonate is a very abundant
+mineral. It occurs in a number of localities as magnesite, which is
+usually amorphous, but sometimes forms pure crystals resembling calcite.
+More commonly it is found associated with calcium carbonate. The
+mineral dolomite has the composition CaCO_{3}·MgCO_{3}. Limestone
+containing smaller amounts of magnesium carbonate is known as dolomitic
+limestone. Dolomite is one of the most common rocks, forming whole
+mountain masses. It is harder and less readily attacked by acids than
+limestone. It is valuable as a building stone and as ballast for
+roadbeds and foundations. Like calcium carbonate, magnesium carbonate is
+insoluble in water, though easily dissolved by acids.
+
+~Basic carbonate of magnesium.~ We should expect to find magnesium
+carbonate precipitated when a soluble magnesium salt and a soluble
+carbonate are brought together:
+
+    Na_{2}CO_{3} + MgCl_{2} = MgCO_{3} + 2NaCl.
+
+Instead of this, some carbon dioxide escapes and the product is found to
+be a basic carbonate. The most common basic carbonate of magnesium has
+the formula 4MgCO_{3}·Mg(OH)_{2}, and is sometimes called magnesia alba.
+This compound is formed by the partial hydrolysis of the normal
+carbonate at first precipitated:
+
+    5MgCO_{3} + 2H_{2}O = 4MgCO_{3}·Mg(OH)_{2} + H_{2}CO_{3}.
+
+~Magnesium chloride~ (MgCl_{2}·6H_{2}O). Magnesium chloride is found in
+many natural waters and in many salt deposits (see Stassfurt salts). It
+is obtained as a by-product in the manufacture of potassium chloride
+from carnallite. As there is no very important use for it, large
+quantities annually go to waste. When heated to drive off the water of
+crystallization the chloride is decomposed as shown in the equation
+
+    MgCl_{2}·6H_{2}O = MgO + 2HCl + 5H_{2}O.
+
+Owing to the abundance of magnesium chloride, this reaction is being
+used to some extent in the preparation of both magnesium oxide and
+hydrochloric acid.
+
+     ~Boiler scale.~ When water which contains certain salts in
+     solution is evaporated in steam boilers, a hard insoluble
+     material called _scale_ deposits in the boiler. The formation
+     of this scale may be due to several distinct causes.
+
+     1. _To the deposit of calcium sulphate._ This salt, while
+     sparingly soluble in cold water, is almost completely insoluble
+     in superheated water. Consequently it is precipitated when
+     water containing it is heated in a boiler.
+
+     2. _To decomposition of acid carbonates._ As we have seen,
+     calcium and magnesium acid carbonates are decomposed on
+     heating, forming insoluble normal carbonates:
+
+     Ca(HCO_{3})_{2} = CaCO_{3} + H_{2}O + CO_{2}.
+
+     3. _To hydrolysis of magnesium salts._ Magnesium chloride, and
+     to some extent magnesium sulphate, undergo hydrolysis when
+     superheated in solution, and the magnesium hydroxide, being
+     sparingly soluble, precipitates:
+
+     MgCl_{2} + 2H_{2}O <--> Mg(OH)_{2} + 2HCl.
+
+     This scale adheres tightly to the boiler in compact layers and,
+     being a non-conductor of heat, causes much waste of fuel. It is
+     very difficult to remove, owing to its hardness and resistance
+     to reagents. Thick scale sometimes cracks, and the water coming
+     in contact with the overheated iron occasions an explosion.
+     Moreover, the acids set free in the hydrolysis of the magnesium
+     salts attack the iron tubes and rapidly corrode them. These
+     causes combine to make the formation of scale a matter which
+     occasions much trouble in cases where hard water is used in
+     steam boilers. Water containing such salts should be softened,
+     therefore, before being used in boilers.
+
+~Magnesium sulphate~ (_Epsom salt_) (MgSO_{4}·7H_{2}O). Like the chloride,
+magnesium sulphate is found rather commonly in springs and in salt
+deposits. A very large deposit of the almost pure salt has been found in
+Wyoming. Its name was given to it because of its abundant occurrence in
+the waters of the Epsom springs in England.
+
+Magnesium sulphate has many uses in the industries. It is used to a
+small extent in the preparation of sodium and potassium sulphates, as a
+coating for cotton cloth, in the dye industry, in tanning, and in the
+manufacture of paints and laundry soaps. To some extent it is used in
+medicine.
+
+~Magnesium silicates.~ Many silicates containing magnesium are known and
+some of them are important substances. Serpentine, asbestos, talc, and
+meerschaum are examples of such substances.
+
+
+ZINC
+
+~Occurrence.~ Zinc never occurs free in nature. Its compounds have been
+found in many different countries, but it is not a constituent of common
+rocks and minerals, and its occurrence is rather local and confined to
+definite deposits or pockets. It occurs chiefly in the following ores:
+
+    Sphalerite (zinc blende) ZnS.
+    Zincite ZnO.
+    Smithsonite ZnCO_{3}.
+    Willemite Zn_{2}SiO_{4}.
+    Franklinite ZnO·Fe_{2}O_{3}.
+
+One fourth of the world's output of zinc comes from the United States,
+Missouri being the largest producer.
+
+~Metallurgy.~ The ores employed in the preparation of zinc are chiefly the
+sulphide, oxide, and carbonate. They are first roasted in the air, by
+which process they are changed into oxide:
+
+    ZnCO_{3} = ZnO + CO_{2},
+    ZnS + 3O = ZnO + SO_{2}.
+
+The oxide is then mixed with coal dust, and the mixture is heated in
+earthenware muffles or retorts, natural gas being used as fuel in many
+cases. The oxide is reduced by this means to the metallic state, and the
+zinc, being volatile at the high temperature reached, distills and is
+collected in suitable receivers. At first the zinc collects in the form
+of fine powder, called zinc dust or flowers of zinc, recalling the
+formation under similar conditions of flowers of sulphur. Later, when
+the whole apparatus has become warm, the zinc condenses to a liquid in
+the receiver, from which it is drawn off into molds. Commercial zinc
+often contains a number of impurities, especially carbon, arsenic, and
+iron.
+
+~Physical properties.~ Pure zinc is a rather heavy bluish-white metal with
+a high luster. It melts at about 420°, and if heated much above this
+temperature in the air takes fire and burns with a very bright bluish
+flame. It boils at about 950° and can therefore be purified by
+distillation.
+
+Many of the physical properties of zinc are much influenced by the
+temperature and previous treatment of the metal. When cast into ingots
+from the liquid state it becomes at ordinary temperatures quite hard,
+brittle, and highly crystalline. At 150° it is malleable and can be
+rolled into thin sheets; at higher temperatures it again becomes very
+brittle. When once rolled into sheets it retains its softness and
+malleability at ordinary temperatures. When melted and poured into water
+it forms thin brittle flakes, and in this condition is called granulated
+or mossy zinc.
+
+~Chemical properties.~ Zinc is tarnished superficially by moist air, but
+beyond this is not affected by it. It does not decompose even boiling
+water. When the metal is quite pure, sulphuric and hydrochloric acids
+have scarcely any action upon it; when, however, it contains small
+amounts of other metals such as magnesium or arsenic, or when it is
+merely in contact with metallic platinum, brisk action takes place and
+hydrogen is evolved. For this reason, when pure zinc is used in the
+preparation of hydrogen a few drops of platinum chloride are often added
+to the solution to assist the chemical action. Nitric acid dissolves the
+metal readily, with the formation of zinc nitrate and various reduction
+products of nitric acid. The strong alkalis act upon zinc and liberate
+hydrogen:
+
+    Zn + 2KOH = Zn(OK)_{2} + 2H.
+
+The product of this reaction, potassium zincate, is a salt of zinc
+hydroxide, which is thus seen to have acid properties, though it usually
+acts as a base.
+
+~Uses of zinc.~ The metal has many familiar uses. Rolled into sheets, it
+is used as a lining for vessels which are to contain water. As a thin
+film upon the surface of iron (galvanized iron) it protects the iron
+from rust. Iron is usually galvanized by dipping it into a bath of
+melted zinc, but electrical methods are also employed. Zinc plates are
+used in many forms of electrical batteries. In the laboratory zinc is
+used in the preparation of hydrogen, and in the form of zinc dust as a
+reducing agent.
+
+One of the largest uses of zinc is in the manufacture of alloys. Brass,
+an alloy of zinc and copper, is the most important of these; German
+silver, consisting of copper, zinc, and nickel, has many uses; various
+bronzes, coin metals, and bearing metals also contain zinc. Its ability
+to alloy with silver finds application in the separation of silver from
+lead (see silver).
+
+~Compounds of zinc.~ In general, the compounds of zinc are similar in
+formula and appearance to those of magnesium, but in other properties
+they often differ markedly. A number of them have value in commercial
+ways.
+
+~Zinc oxide~ (_zinc white_) (ZnO). Zinc oxide occurs in impure form in
+nature, being colored red by manganese and iron compounds. It can be
+prepared just like magnesium oxide, but is more often made by burning
+the metal.
+
+Zinc oxide is a pure white powder which becomes yellow on heating and
+regains its white color when cold. It is much used as a white pigment in
+paints, under the name of zinc white, and has the advantage over white
+lead in that it is not changed in color by sulphur compounds, while lead
+turns black. It is also used in the manufacture of rubber goods.
+
+     ~Commercial preparation of zinc oxide.~ Commercially it is often
+     made from franklinite in the following way. The franklinite is
+     mixed with coal and heated to a high temperature in a furnace,
+     by which process the zinc is set free and converted into vapor.
+     As the vapor leaves the furnace through a conduit it meets a
+     current of air and takes fire in it, forming zinc oxide. The
+     oxide passes on and is filtered from the air through canvas
+     bags, which allow the air to pass but retain the oxide. It is
+     thus made by burning the metal, though the metal is not
+     actually isolated in the process.
+
+~Soluble salts.~ The soluble salts of zinc can be made by dissolving the
+metal or the oxide in the appropriate acid. They are all somewhat
+poisonous. The sulphate and chloride are the most familiar.
+
+~Zinc sulphate~ (_white vitriol_) (ZnSO_{4}·7H_{2}O). This salt is readily
+crystallized from strong solutions in transparent colorless crystals. It
+is prepared commercially by careful roasting of the sulphide:
+
+    ZnS + 4O = ZnSO_{4}.
+
+~Zinc chloride~ (ZnCl_{2}·H_{2}O). When a solution of zinc chloride is
+slowly evaporated a salt of the composition ZnCl_{2}·H_{2}O crystallizes
+out. If the water is completely expelled by heat and the residue
+distilled, the anhydrous chloride is obtained and may be cast into
+sticks or broken into lumps. In this distillation, just as in heating
+magnesium chloride, some of the chloride is decomposed:
+
+    ZnCl_{2}·H_{2}O = ZnO + 2HCl.
+
+The anhydrous chloride has a great affinity for water, and is used as a
+dehydrating agent. It is also a germicide, and wood which is to be
+exposed to conditions which favor decay, as, for example, railroad ties,
+is often soaked in solutions of this salt.
+
+~Insoluble compounds.~ The insoluble compounds of zinc can be prepared by
+precipitation. The most important are the sulphide, carbonate, and
+hydroxide.
+
+~Zinc sulphide~ (ZnS). This substance occurs as the mineral sphalerite,
+and is one of the most valued ores of zinc. Very large deposits occur in
+southwestern Missouri. The natural mineral is found in large crystals or
+masses, resembling resin in color and luster. When prepared by
+precipitation the sulphide is white.
+
+
+CADMIUM
+
+~The element.~ This element occurs in small quantities in some zinc ores.
+In the course of the metallurgy of zinc the cadmium compounds undergo
+chemical changes quite similar to those of the zinc compounds, and the
+cadmium distills along with the zinc. Being more volatile, it comes over
+with the first of the zinc and is prepared from the first portions of
+the distillate by special methods of purification. The element very
+closely resembles zinc in most respects. Some of its alloys are
+characterized by having low melting points.
+
+~Compounds of cadmium.~ Among the compounds of cadmium may be mentioned
+the chloride (CdCl_{2}·2H_{2}O), the sulphate (3CdSO_{4}·8H_{2}O), and
+the nitrate (Cd(NO_{3})_{2}·4H_{2}O). These are white solids soluble in
+water. The sulphide (CdS) is a bright yellow substance which is
+insoluble in water and in dilute acids. It is valuable as a pigment in
+fine paints.
+
+
+EXERCISES
+
+1. What properties have the metals of the magnesium family in common
+with the alkali metals; with the alkaline-earth metals?
+
+2. Compare the action of the metals of the magnesium group on water with
+that of the other metals studied.
+
+3. What metals already studied are prepared by electrolysis?
+
+4. Write the equations representing the reactions between magnesium and
+hydrochloric acid; between magnesium and dilute sulphuric acid.
+
+5. What property of magnesium was taken advantage of in the isolation of
+argon?
+
+6. With phosphoric acid magnesium forms salts similar to those of
+calcium. Write the names and formulas of the corresponding magnesium
+salts.
+
+7. How could you distinguish between magnesium chloride and magnesium
+sulphate? between Glauber's salts and Epsom salts?
+
+8. What weight of carnallite is necessary in the preparation of 500 g.
+of magnesium?
+
+9. Account for the fact that paints made of zinc oxide are not colored
+by hydrosulphuric acid.
+
+10. What hydroxide studied, other than zinc hydroxide, has both acid and
+basic properties?
+
+11. Write equations showing how the following compounds of zinc may be
+obtained from metallic zinc: the oxide, chloride, nitrate, carbonate,
+sulphate, sulphide, hydroxide.
+
+
+
+
+CHAPTER XXVI
+
+THE ALUMINIUM FAMILY
+
+
+~The family.~ The element aluminium is the most abundant member of the
+group of elements known as the aluminium family; indeed, the other
+members of the family--gallium, indium, and thallium--are of such rare
+occurrence that they need not be separately described. The elements of
+the family are ordinarily trivalent, so that the formulas for their
+compounds differ from those of the elements so far studied. Their
+hydroxides are practically insoluble in water and are very weak bases;
+indeed, the bases are so weak that their salts are often hydrolyzed into
+free base and free acid in solution. The salts formed from these bases
+usually contain water of crystallization, which cannot be driven off
+without decomposing them more or less.
+
+The trivalent metals, which in addition to aluminium include also iron
+and chromium, are sometimes called the _earth metals_. The name refers
+to the earthy appearance of the oxides of these metals, and to the fact
+that many earths, soils, and rocks are composed in part of these
+substances.
+
+
+ALUMINIUM
+
+~Occurrence.~ Aluminium never occurs in the free state in nature, owing to
+its great affinity for oxygen. In combined form, as oxides, silicates,
+and a few other salts, it is both abundant and widely distributed, being
+an essential constituent of all soils and of most rocks excepting
+limestone and sandstone. Cryolite (Na_{3}AlF_{6}), found in Greenland,
+and bauxite, which is an aluminium hydroxide usually mixed with some
+iron hydroxide, are important minerals. It is estimated that aluminium
+composes about 8% of the earth's crust. In the industries the metal is
+called aluminum, but its chemical name is aluminium.
+
+[Illustration: Fig. 82]
+
+~Preparation.~ Aluminium was first prepared by Wöhler, in 1827, by heating
+anhydrous aluminium chloride with potassium:
+
+    AlCl_{3} + 3K = 3KCl + Al.
+
+This method was tried after it was found impossible to reduce the oxide
+of aluminium with carbon. The metal possessed such interesting
+properties and promised to be so useful that many efforts were made to
+devise a cheap way of preparing it. The method which has proved most
+successful consists in the electrolysis of the oxide dissolved in melted
+cryolite.
+
+     ~Metallurgy.~ An iron box A (Fig. 82) about eight feet long and
+     six feet wide is connected with a powerful generator in such a
+     way as to serve as the cathode upon which the aluminium is
+     deposited. Three or four rows of carbon rods B dip into the
+     box and serve as the anodes. The box is partially filled with
+     cryolite and the current is turned on, generating enough heat
+     to melt the cryolite. Aluminium oxide is then added, and under
+     the influence of the electric current it decomposes into
+     aluminium and oxygen. The temperature is maintained above the
+     melting point of aluminium, and the liquid metal, being heavier
+     than cryolite, sinks to the bottom of the vessel, from which it
+     is tapped off from time to time through the tap hole C. The
+     oxygen in part escapes as gas, and in part combines with the
+     carbon of the anode, the combustion being very brilliant. The
+     process is carried on at Niagara Falls.
+
+     The largest expense in the process, apart from the cost of
+     electrical energy, is the preparation of aluminium oxide free
+     from other oxides, for most of the oxide found in nature is too
+     impure to serve without refining. Bauxite is the principal ore
+     used as a source of the aluminium because it is converted into
+     pure oxide without great difficulty. Since common clay is a
+     silicate of aluminium and is everywhere abundant, it might be
+     expected that this would be utilized in the preparation of
+     aluminium. It is, however, very difficult to extract the
+     aluminium from a silicate, and no practical method has been
+     found which will accomplish this.
+
+~Physical properties.~ Aluminium is a tin-white metal which melts at 640°
+and is very light, having a density of 2.68. It is stiff and strong, and
+with frequent annealing can be rolled into thin foil. It is a good
+conductor of heat and electricity, though not so good as copper for a
+given cross section of wire.
+
+~Chemical properties.~ Aluminium is not perceptibly acted on by boiling
+water, and moist air merely dims its luster. Further action is prevented
+in each case by the formation of an extremely thin film of oxide upon
+the surface of the metal. It combines directly with chlorine, and when
+heated in oxygen burns with great energy and the liberation of much
+heat. It is therefore a good reducing agent. Hydrochloric acid acts upon
+it, forming aluminium chloride: nitric acid and dilute sulphuric acid
+have almost no action on it, but hot, concentrated sulphuric acid acts
+upon it in the same way as upon copper:
+
+    2Al + 6H_{2}SO_{4} = Al_{2}(SO_{4})_{3} + 6H_{2}O + 3SO_{2}.
+
+Alkalis readily attack the metal, liberating hydrogen, as in the case of
+zinc:
+
+    Al + 3KOH = Al(OK)_{3} + 3H.
+
+Salt solutions, such as sea water, corrode the metal rapidly. It alloys
+readily with other metals.
+
+~Uses of aluminium.~ These properties suggest many uses for the metal. Its
+lightness, strength, and permanence make it well adapted for many
+construction purposes. These same properties have led to its extensive
+use in the manufacture of cooking utensils. The fact that it is easily
+corroded by salt solutions is, however, a disadvantage. Owing to its
+small resistance to electrical currents, it is replacing copper to some
+extent in electrical construction, especially for trolley and power
+wires. Some of its alloys have very valuable properties, and a
+considerable part of the aluminium manufactured is used for this
+purpose. Aluminium bronze, consisting of about 90% copper and 10%
+aluminium, has a pure golden color, is strong and malleable, is easily
+cast, and is permanent in the air. Considerable amounts of aluminium
+steel are also made.
+
+~Goldschmidt reduction process.~ Aluminium is frequently employed as a
+powerful reducing agent, many metallic oxides which resist reduction by
+carbon being readily reduced by it. The aluminium in the form of a fine
+powder is mixed with the metallic oxide, together with some substance
+such as fluorspar to act as a flux. The mixture is ignited, and the
+aluminium unites with the oxygen of the metallic oxide, liberating the
+metal. This collects in a fused condition under the flux.
+
+An enormous quantity of heat is liberated in this reaction, and a
+temperature as high as 3500° can be reached. The heat of the reaction is
+turned to practical account in welding car rails, steel castings, and in
+similar operations where an intense local heat is required. A mixture of
+aluminium with various metallic oxides, ready prepared for such
+purposes, is sold under the name of _thermite_.
+
+[Illustration: Fig. 83]
+
+     ~Preparation of chromium by the Goldschmidt method.~ A mixture of
+     chromium oxide and aluminium powder is placed in a Hessian
+     crucible (A, Fig. 83), and on top of it is placed a small
+     heap B of a mixture of sodium peroxide and aluminium, into
+     which is stuck a piece of magnesium ribbon C. Powdered
+     fluorspar D is placed around the sodium peroxide, after which
+     the crucible is set on a pan of sand and the magnesium ribbon
+     ignited. When the flame reaches the sodium peroxide mixture
+     combustion of the aluminium begins with almost explosive
+     violence, so that great care must be taken in the experiment.
+     The heat of this combustion starts the reaction in the chromium
+     oxide mixture, and the oxide is reduced to metallic chromium.
+     When the crucible has cooled a button of chromium will be found
+     in the bottom.
+
+~Aluminium oxide~ (Al_{2}O_{3}). This substance occurs in several forms in
+nature. The relatively pure crystals are called corundum, while emery is
+a variety colored dark gray or black, usually with iron compounds. In
+transparent crystals, tinted different colors by traces of impurities,
+it forms such precious stones as the sapphire, oriental ruby, topaz, and
+amethyst. All these varieties are very hard, falling little short of
+the diamond in this respect. Chemically pure aluminium oxide can be made
+by igniting the hydroxide, when it forms an amorphous white powder:
+
+    2Al(OH)_{3} = Al_{2}O_{3} + 3H_{2}O.
+
+The natural varieties, corundum and emery, are used for cutting and
+grinding purposes; the purest forms, together with the artificially
+prepared oxide, are largely used in the preparation of aluminium.
+
+~Aluminium hydroxide~ (Al(OH)_{3}). The hydroxide occurs in nature as the
+mineral hydrargyllite, and in a partially dehydrated form called
+bauxite. It can be prepared by adding ammonium hydroxide to any soluble
+aluminium salt, forming a semi-transparent precipitate which is
+insoluble in water but very hard to filter. It dissolves in most acids
+to form soluble salts, and in the strong bases to form aluminates, as
+indicated in the equations
+
+    Al(OH)_{3} + 3HCl = AlCl_{3} + 3H_{2}O,
+    Al(OH)_{3} + 3NaOH = Al(ONa)_{3} + 3H_{2}O.
+
+It may act, therefore, either as a weak base or as a weak acid, its
+action depending upon the character of the substances with which it is
+in contact. When heated gently the hydroxide loses part of its hydrogen
+and oxygen according to the equation
+
+    Al(OH)_{3} = AlO·OH + H_{2}O.
+
+This substance, the formula of which is frequently written HAlO_{2}, is
+a more pronounced acid than is the hydroxide, and its salts are
+frequently formed when aluminium compounds are fused with alkalis. The
+magnesium salt Mg(AlO_{2})_{2} is called spinel, and many other of its
+salts, called aluminates, are found in nature.
+
+When heated strongly the hydroxide is changed into oxide, which will not
+again take up water on being moistened.
+
+     ~Mordants and dyeing.~ Aluminium hydroxide has the peculiar
+     property of combining with many soluble coloring materials and
+     forming insoluble products with them. On this account it is
+     often used as a filter to remove objectionable colors from
+     water. This property also leads to its wide use in the dye
+     industry. Many dyes will not adhere to natural fibers such as
+     cotton and wool, that is, will not "dye fast." If, however, the
+     cloth to be dyed is soaked in a solution of aluminium compounds
+     and then treated with ammonia, the aluminium salts which have
+     soaked into the fiber will be converted into the hydroxide,
+     which, being insoluble, remains in the body of it. If the fiber
+     is now dipped into a solution of the dye, the aluminium
+     hydroxide combines with the color material and fastens, or
+     "fixes," it upon the fiber. A substance which serves this
+     purpose is called a _mordant_, and aluminium salts,
+     particularly the acetate, are used in this way.
+
+~Aluminium chloride~ (AlCl_{3}·6 H_{2}O). This substance is prepared by
+dissolving the hydroxide in hydrochloric acid and evaporating to
+crystallization. When heated it is converted into the oxide, resembling
+magnesium in this respect:
+
+    2(AlCl_{3}·6 H_{2}O) = Al_{2}O_{3} + 6HCl + 9H_{2}O.
+
+The anhydrous chloride, which has some important uses, is made by
+heating aluminium turnings in a current of chlorine.
+
+~Alums.~ Aluminium sulphate can be prepared by the action of sulphuric
+acid upon aluminium hydroxide. It has the property of combining with the
+sulphates of the alkali metals to form compounds called _alums_. Thus,
+with potassium sulphate the reaction is expressed by the equation
+
+    K_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O
+       = 2(KAl(SO_{4})_{2}·12H_{2}O).
+
+Under similar conditions ammonium sulphate yields ammonium alum:
+
+    (NH_{4})_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O
+        = 2(NH_{4}Al(SO_{4})_{2}·12H_{2}O).
+
+Other trivalent sulphates besides aluminium sulphate can form similar
+compounds with the alkali sulphates, and these compounds are also called
+alums, though they contain no aluminium. They all crystallize in
+octahedra and contain twelve molecules of water of crystallization. The
+alums most frequently prepared are the following:
+
+    Potassium alum               KAl(SO_{4})_{2}·12H_{2}O.
+    Ammonium alum                NH_{4}Al(SO_{4})_{2}·12H_{2}O.
+    Ammonium iron alum           NH_{4}Fe(SO_{4})_{2}·12H_{2}O.
+    Potassium chrome alum        KCr(SO_{4})_{2}·12H_{2}O.
+
+An alum may therefore be regarded as a compound derived from two
+molecules of sulphuric acid, in which one hydrogen atom has been
+displaced by the univalent alkali atom, and the other three hydrogen
+atoms by an atom of one of the trivalent metals, such as aluminium,
+iron, or chromium.
+
+     Very large, well-formed crystals of an alum can be prepared by
+     suspending a small crystal by a thread in a saturated solution
+     of the alum, as shown in Fig. 84. The small crystal slowly
+     grows and assumes a very perfect form.
+
+[Illustration: Fig. 84]
+
+~Other salts of aluminium.~ While aluminium hydroxide forms fairly stable
+salts with strong acids, it is such a weak base that its salts with weak
+acids are readily hydrolyzed. Thus, when an aluminium salt and a soluble
+carbonate are brought together in solution we should expect to have
+aluminium carbonate precipitated according to the equation
+
+    3Na_{2}CO_{3} + 2AlCl_{3} = Al_{2}(CO_{3})_{3} + 6NaCl.
+
+But if it is formed at all, it instantly begins to hydrolyze, the
+products of the hydrolysis being aluminium hydroxide and carbonic acid,
+
+    Al_{2}(CO_{3})_{3} + 6H_{2}O = 2Al(OH)_{3} + 3H_{2}CO_{3}.
+
+Similarly a soluble sulphide, instead of precipitating aluminium
+sulphide (Al_{2}S_{3}), precipitates aluminium hydroxide; for hydrogen
+sulphide is such a weak acid that the aluminium sulphide at first formed
+hydrolyzes at once, forming aluminium hydroxide and hydrogen sulphide:
+
+    3Na_{2}S + 2AlCl_{3} + 6H_{2}O = 2Al(OH)_{3} + 6NaCl + 3H_{2}S.
+
+~Alum baking powders.~ It is because of the hydrolysis of aluminium
+carbonate that alum is used as a constituent of some baking powders. The
+alum baking powders consist of a mixture of alum and sodium hydrogen
+carbonate. When water is added the two compounds react together, forming
+aluminium carbonate, which hydrolyzes into aluminium hydroxide and
+carbonic acid. The carbon dioxide from the latter escapes through the
+dough and in so doing raises it into a porous condition, which is the
+end sought in the use of a baking powder.
+
+~Aluminium silicates.~ One of the most common constituents of rocks is
+feldspar (KAlSi_{3}O_{8}), a mixed salt of potassium and aluminium with
+the polysilicic acid (H_{4}Si_{3}O_{8}). Under the influence of
+moisture, carbon dioxide, and changes of temperature this substance is
+constantly being broken down into soluble potassium compounds and
+hydrated aluminium silicate. This compound has the formula
+Al_{2}Si_{2}O_{7}·2H_{2}O. In relatively pure condition it is called
+kaolin; in the impure state, mixed with sand and other substances, it
+forms common clay. Mica is another very abundant mineral, having varying
+composition, but being essentially of the formula KAlSiO_{4}.
+Serpentine, talc, asbestos, and meerschaum are important complex
+silicates of aluminium and magnesium, and granite is a mechanical
+mixture of quartz, feldspar, and mica.
+
+     ~Ceramic industries.~ Many articles of greatest practical
+     importance, ranging from the roughest brick and tile to the
+     finest porcelain and chinaware, are made from some form of
+     kaolin, or clay. No very precise classification of such ware
+     can be made, as the products vary greatly in properties,
+     depending upon the materials used and the treatment during
+     manufacture.
+
+     Porcelain is made from the purest kaolin, to which must be
+     added some less pure, plastic kaolin, since the pure substance
+     is not sufficiently plastic. There is also added some more
+     fusible substance, such as feldspar, gypsum, or lime, together
+     with some pure quartz. The constituents must be ground very
+     fine, and when thoroughly mixed and moistened must make a
+     plastic mass which can be molded into any desired form. The
+     article molded from such materials is then burned. In this
+     process the article is slowly heated to a point at which it
+     begins to soften and almost fuse, and then it is allowed to
+     cool slowly. At this stage, a very thin vessel will be
+     translucent and have an almost glassy fracture; if, however, it
+     is somewhat thicker, or has not been heated quite so high, it
+     will still be porous, and partly on this account and partly to
+     improve its appearance it is usually glazed.
+
+     Glazing is accomplished by spreading upon the object a thin
+     layer of a more fusible mixture of the same materials as
+     compose the body of the object itself, and again heating until
+     the glaze melts to a transparent glassy coating upon the
+     surface of the vessel. In some cases fusible mixtures of quite
+     different composition from that used in fashioning the vessel
+     may be used as a glaze. Oxides of lead, zinc, and barium are
+     often used in this way.
+
+     When less carefully selected materials are used, or quite thick
+     vessels are made, various grades of stoneware are produced. The
+     inferior grades are glazed by throwing a quantity of common
+     salt into the kiln towards the end of the first firing. In the
+     form of vapor the salt attacks the surface of the baked ware
+     and forms an easily fusible sodium silicate upon it, which
+     constitutes a glaze.
+
+     Vitrified bricks, made from clay or ground shale, are burned
+     until the materials begin to fuse superficially, forming their
+     own glaze. Other forms of brick and tile are not glazed at all,
+     but are left porous. The red color of ordinary brick and
+     earthenware is due to an oxide of iron formed in the burning
+     process.
+
+     The decorations upon china are sometimes painted upon the baked
+     ware and then glazed over, and sometimes painted upon the glaze
+     and burned in by a third firing. Care must be taken to use such
+     pigments as are not affected by a high heat and do not react
+     chemically with the constituents of the baked ware or the
+     glaze.
+
+
+EXERCISES
+
+1. What metals and compounds studied are prepared by electrolysis?
+
+2. Write the equation for the reaction between aluminium and
+hydrochloric acid; between aluminium and sulphuric acid (in two steps).
+
+3. What hydroxides other than aluminium hydroxide have both acid and
+basic properties?
+
+4. Write equations showing the methods used for preparing aluminium
+hydroxide and sulphate.
+
+5. Write the general formula of an alum, representing an atom of an
+alkali metal by X and an atom of a trivalent metal by Y.
+
+6. What is meant by the term polysilicic acid, as used in the discussion
+of aluminium silicates?
+
+7. Compare the properties of the hydroxides of the different groups of
+metals so far studied.
+
+8. In what respects does aluminium oxide differ from calcium oxide in
+properties?
+
+9. Supposing bauxite to be 90% aluminium hydroxide, what weight of it is
+necessary for the preparation of 100 kg. of aluminium?
+
+
+
+
+CHAPTER XXVII
+
+THE IRON FAMILY
+
+
+===================================================================
+        |        |        |         |             |
+        |        |        |         | APPROXIMATE |
+        | SYMBOL | ATOMIC | DENSITY |   MELTING   | OXIDES
+        |        | WEIGHT |         |    POINT    |
+________|________|________|_________|_____________|________________
+        |        |        |         |             |
+Iron    |   Fe   |  55.9  |   7.93  |    1800°    | FeO, Fe_{2}O_{3}
+Cobalt  |   Co   |  59.0  |   8.55  |    1800°    | CoO, Co_{2}O_{3}
+Nickel  |   Ni   |  58.7  |   8.9   |    1600°    | NiO, Ni_{2}O_{3}
+===================================================================
+
+~The family.~ The elements iron, cobalt, and nickel form a group in the
+eighth column of the periodic table. The atomic weights of the three are
+very close together, and there is not the same gradual gradation in the
+properties of the three elements that is noticed in the families in
+which the atomic weights differ considerably in magnitude. The elements
+are very similar in properties, the similarity being so great in the
+case of nickel and cobalt that it is difficult to separate them by
+chemical analysis.
+
+The elements occur in nature chiefly as oxides and sulphides, though
+they have been found in very small quantities in the native state,
+usually in meteorites. Their sulphides, carbonates, and phosphates are
+insoluble in water, the other common salts being soluble. Their salts
+are usually highly colored, those of iron being yellow or light green as
+a rule, those of nickel darker green, while cobalt salts are usually
+rose colored. The metals are obtained by reducing the oxides with
+carbon.
+
+
+IRON
+
+~Occurrence.~ The element iron has long been known, since its ores are
+very abundant and it is not difficult to prepare the metal from them in
+fairly pure condition. It occurs in nature in many forms of
+combination,--in large deposits as oxides, sulphides, and carbonates,
+and in smaller quantities in a great variety of minerals. Indeed, very
+few rocks or soils are free from small amounts of iron, and it is
+assimilated by plants and animals playing an important part in life
+processes.
+
+~Metallurgy.~ It will be convenient to treat of the metallurgy of iron
+under two heads,--Materials Used and Process.
+
+~Materials used.~ Four distinct materials are used in the metallurgy of
+iron:
+
+     1. _Iron ore._ The ores most frequently used in the metallurgy
+     of iron are the following:
+
+         Hematite      Fe_{2}O_{3}.
+         Magnetite     Fe_{3}O_{4}.
+         Siderite      FeCO_{3}.
+         Limonite      2Fe_{2}O_{2}·3H_{2}O.
+
+     These ores always contain impurities, such as silica,
+     sulphides, and earthy materials. All ores, with the exception
+     of the oxides, are first roasted to expel any water and carbon
+     dioxide present and to convert any sulphide into oxide.
+
+     2. _Carbon._ Carbon in some form is necessary both as a fuel
+     and as a reducing agent. In former times wood charcoal was used
+     to supply the carbon, but now anthracite coal or coke is almost
+     universally used.
+
+     3. _Hot air._ To maintain the high temperature required for the
+     reduction of iron a very active combustion of fuel is
+     necessary. This is secured by forcing a strong blast of hot air
+     into the lower part of the furnace during the reduction
+     process.
+
+     4. _Flux._ (a) _Purpose of the flux._ All the materials which
+     enter the furnace must leave it again either in the form of
+     gases or as liquids. The iron is drawn off as the liquid metal
+     after its reduction. To secure the removal of the earthy matter
+     charged into the furnace along with the ore, materials are
+     added to the charge which will, at the high temperature of the
+     furnace, combine with the impurities in the ore, forming a
+     liquid. The material added for this purpose is called the
+     _flux_; the liquid produced from the flux and the ore is called
+     _slag_.
+
+     (b) _Function of the slag._ While the main purpose of adding
+     flux to the charge is to remove from the furnace in the form of
+     liquid slag the impurities originally present in the ore, the
+     slag thus produced serves several other functions. It keeps the
+     contents of the furnace in a state of fusion, thus preventing
+     clogging, and makes it possible for the small globules of iron
+     to run together with greater ease into one large liquid mass.
+
+     (c) _Character of the slag._ The slag is really a kind of
+     readily fusible glass, being essentially a calcium-aluminium
+     silicate. The ore usually contains silica and some aluminium
+     compounds, so that limestone (which also contains some silica
+     and aluminium) is added to furnish the calcium required for the
+     slag. If the ore and the limestone do not contain a sufficient
+     amount of silica and aluminium for the formation of the slag,
+     these ingredients are added in the form of sand and feldspar.
+     In the formation of slag from these materials the ore is freed
+     from the silica and aluminium which it contained.
+
+[Illustration: Fig. 85]
+
+~Process.~ The reduction of iron is carried out in large towers called
+blast furnaces. The blast furnace (Fig. 85) is usually about 80 ft. high
+and 20 ft. in internal diameter at its widest part, narrowing somewhat
+both toward the top and toward the bottom. The walls are built of steel
+and lined with fire-brick. The base is provided with a number of pipes
+T, called tuyers, through which hot air can be forced into the
+furnace. The tuyers are supplied from a large pipe S, which circles
+the furnace as a girdle. The base has also an opening M, through which
+the liquid metal can be drawn off from time to time, and a second
+opening P, somewhat above the first, through which the excess of slag
+overflows. The top is closed by a movable trap C and C, called the
+cone, and through this the materials to be used are introduced. The
+gases produced by the combustion of the fuel and the reduction of the
+ore, together with the nitrogen of the air forced in through the tuyers,
+escape through pipes D, called downcomer pipes, which leave the
+furnace near the top. These gases are very hot and contain combustible
+substances, principally carbon monoxide; they are therefore utilized as
+fuel for the engines and also to heat the blast admitted through the
+tuyers. The lower part of the furnace is often furnished with a water
+jacket. This consists of a series of pipes W built into the walls,
+through which water can be circulated to reduce their temperature.
+
+Charges consisting of coke (or anthracite coal), ore, and flux in proper
+proportions are introduced into the furnace at intervals through the
+trap top. The coke burns fiercely in the hot-air blast, giving an
+intense heat and forming carbon monoxide. The ore, working down in the
+furnace as the coke burns, becomes very hot, and by the combined
+reducing action of the carbon and carbon monoxide is finally reduced to
+metal and collects as a liquid in the bottom of the furnace, the slag
+floating on the molten iron. After a considerable amount of the iron has
+collected the slag is drawn off through the opening P. The molten iron
+is then drawn off into large ladles and taken to the converters for the
+manufacture of steel, or it is run out into sand molds, forming the bars
+or ingots called "pigs." The process is a continuous one, and when once
+started it is kept in operation for months or even years without
+interruption.
+
+     It seems probable that the first product of combustion of the
+     carbon, at the point where the tuyers enter the furnace, is
+     carbon dioxide. This is at once reduced to carbon monoxide by
+     the intensely heated carbon present, so that no carbon dioxide
+     can be found at that point. For practical purposes, therefore,
+     we may consider that carbon monoxide is the first product of
+     combustion.
+
+~Varieties of iron.~ The iron of commerce is never pure, but contains
+varying amounts of other elements, such as carbon, silicon, phosphorus,
+sulphur, and manganese. These elements may either be alloyed with the
+iron or may be combined with it in the form of definite chemical
+compounds. In some instances, as in the case of graphite, the mixture
+may be merely mechanical.
+
+The properties of iron are very much modified by the presence of these
+elements and by the form of the combination between them and the iron;
+the way in which the metal is treated during its preparation has also a
+marked influence on its properties. Owing to these facts many kinds of
+iron are recognized in commerce, the chief varieties being cast iron,
+wrought iron, and steel.
+
+~Cast iron.~ The product of the blast furnace, prepared as just described,
+is called cast iron. It varies considerably in composition, usually
+containing from 90 to 95% iron, the remainder being largely carbon and
+silicon with smaller amounts of phosphorus and sulphur. When the melted
+metal from the blast furnace is allowed to cool rapidly most of the
+carbon remains in chemical combination with the iron, and the product is
+called white cast iron. If the cooling goes on slowly, the carbon
+partially separates as flakes of graphite which remain scattered through
+the metal. This product is softer and darker in color and is called gray
+cast iron.
+
+~Properties of cast iron.~ Cast iron is hard, brittle, and rather easily
+melted (melting point about 1100°). It cannot be welded or forged into
+shape, but is easily cast in sand molds. It is strong and rigid but not
+elastic. It is used for making castings and in the manufacture of other
+kinds of iron. Cast iron, which contains the metal manganese up to the
+extent of 20%, together with about 3% carbon, is called spiegel iron;
+when more than this amount of manganese is present the product is called
+ferromanganese. The ferromanganese may contain as much as 80% manganese.
+These varieties of cast iron are much used in the manufacture of steel.
+
+~Wrought iron.~ Wrought iron is made by burning out from cast iron most of
+the carbon, silicon, phosphorus, and sulphur which it contains. The
+process is called _puddling_, and is carried out in a furnace
+constructed as represented in Fig. 86. The floor of the furnace F is
+somewhat concave and is made of iron covered with a layer of iron oxide.
+A long flame produced by burning fuel upon the grate G is directed
+downward upon the materials placed upon the floor, and the draught is
+maintained by the stack S. A is the ash box and T a trap to catch
+the solid particles carried into the stack by the draught. Upon the
+floor of the furnace is placed the charge of cast iron, together with a
+small amount of material to make a slag. The iron is soon melted by the
+flame directed upon it, and the sulphur, phosphorus, and silicon are
+oxidized by the iron oxide, forming oxides which are anhydrides of
+acids. These combine with the flux, which is basic in character, or with
+the iron oxide, to form a slag. The carbon is also oxidized and escapes
+as carbon dioxide. As the iron is freed from other elements it becomes
+pasty, owing to the higher melting point of the purer iron, and in this
+condition forms small lumps which are raked together into a larger one.
+The large lump is then removed from the furnace and rolled or hammered
+into bars, the slag; being squeezed out in this process. The product has
+a stranded or fibrous structure. _The product of a puddling furnace is
+called wrought iron._
+
+[Illustration: Fig. 86]
+
+~Properties of wrought iron.~ Wrought iron is nearly pure iron, usually
+containing about 0.3% of other substances, chiefly carbon. It is tough,
+malleable, and fibrous in structure. It is easily bent and is not
+elastic, so it will not sustain pressure as well as cast iron. It can be
+drawn out into wire of great tensile strength, and can also be rolled
+into thin sheets (sheet iron). It melts at a high temperature (about
+1600°) and is therefore forged into shape rather than cast. If melted,
+it would lose its fibrous structure and be changed into a low carbon
+steel.
+
+~Steel.~ Steel, like wrought iron, is made by burning out from cast iron a
+part of the carbon, silicon, phosphorus, and sulphur which it contains;
+but the process is carried out in a very different way, and usually,
+though not always, more carbon is found in steel than in wrought iron. A
+number of processes are in use, but nearly all the steel of commerce is
+made by one of the two following methods.
+
+[Illustration: Fig. 87]
+
+1. _Bessemer process._ This process, invented about 1860, is by far the
+most important. It is carried out in great egg-shaped crucibles called
+converters (Fig. 87), each one of which will hold as much as 15 tons of
+steel. The converter is built of steel and lined with silica. It is
+mounted on trunnions T, so that it can be tipped over on its side for
+filling and emptying. One of the trunnions is hollow and a pipe P
+connects it with an air chamber A, which forms a false bottom to the
+converter. The true bottom is perforated, so that air can be forced in
+by an air blast admitted through the trunnion and the air chamber.
+
+White-hot, liquid cast iron from a blast furnace is run into the
+converter through its open necklike top O, the converter being tipped
+over to receive it; the air blast is then turned on and the converter
+rotated to a nearly vertical position. The elements in the iron are
+rapidly oxidized, the silicon first and then the carbon. The heat
+liberated in the oxidation, largely due to the combustion of silicon,
+keeps the iron in a molten condition. When the carbon is practically all
+burned out cast iron or spiegel iron, containing a known percentage of
+carbon, is added and allowed to mix thoroughly with the fluid. The steel
+is then run into molds, and the ingots so formed are hammered or rolled
+into rails or other forms. By this process any desired percentage of
+carbon can be added to the steel. Low carbon steel, which does not
+differ much from wrought iron in composition, is now made in this way
+and is replacing the more expensive wrought iron for many purposes.
+
+     ~The basic lining process.~ When the cast iron contains
+     phosphorus and sulphur in appreciable quantities, the lining of
+     the converter is made of dolomite. The silicon and carbon burn,
+     followed by the phosphorus and sulphur, and the anhydrides of
+     acids so formed combine with the basic oxides of the lining,
+     forming a slag. This is known as the basic lining process.
+
+2. _Open-hearth process._ In this process a furnace very similar to a
+puddling furnace is used, but it is lined with silica or dolomite
+instead of iron oxide. A charge consisting in part of old scrap iron of
+any kind and in part of cast iron is melted in the furnace by a gas
+flame. The silicon and carbon are slowly burned away, and when a test
+shows that the desired percentage of carbon is present the steel is run
+out of the furnace. _Steel may therefore be defined as the product of
+the Bessemer or open-hearth processes._
+
+~Properties of steel.~ Bessemer and open-hearth steel usually contain only
+a few tenths of a per cent of carbon, less than 0.1% silicon, and a very
+much smaller quantity of phosphorus and sulphur. Any considerable amount
+of the latter elements makes the steel brittle, the sulphur affecting it
+when hot, and the phosphorus when cold. This kind of steel is used for
+structural purposes, for rails, and for nearly all large steel articles.
+It is hard, malleable, ductile, and melts at a lower temperature than
+wrought iron. It can be forged into shape, rolled into sheets, or cast
+in molds.
+
+~Relation of the three varieties of iron.~ It will be seen that wrought
+iron is usually very nearly pure iron, while steel contains an
+appreciable amount of alloy material, chiefly carbon, and cast iron
+still more of the same substances. It is impossible, however, to assign
+a given sample of iron to one of these three classes on the basis of its
+chemical composition alone. A low carbon steel, for example, may contain
+less carbon than a given sample of wrought iron. The real distinction
+between the three is the process by which they are made. The product of
+the blast furnace is cast iron; that of the puddling furnace is wrought
+iron; that of the Bessemer and open-hearth methods is steel.
+
+~Tool steel.~ Steel designed for use in the manufacture of edged tools and
+similar articles should be relatively free from silicon and phosphorus,
+but should contain from 0.5 to 1.5% carbon. The percentage of carbon
+should be regulated by the exact use to which the steel is to be put.
+Steel of this character is usually made in small lots from either
+Bessemer or open-hearth steel in the following way.
+
+A charge of melted steel is placed in a large crucible and the
+calculated quantity of pure carbon is added. The carbon dissolves in the
+steel, and when the solution is complete the metal is poured out of the
+crucible. This is sometimes called crucible steel.
+
+~Tempering of steel.~ Steel containing from 0.5 to 1.5% carbon is
+characterized by the property of "taking temper." When the hot steel is
+suddenly cooled by plunging it into water or oil it becomes very hard
+and brittle. On carefully reheating this hard form it gradually becomes
+less brittle and softer, so that by regulating the temperature to which
+steel is reheated in tempering almost any condition of temper demanded
+for a given purpose, such as for making springs or cutting tools, can be
+obtained.
+
+~Steel alloys.~ It has been found that small quantities of a number of
+different elements when alloyed with steel very much improve its quality
+for certain purposes, each element having a somewhat different effect.
+Among the elements most used in this connection are manganese, silicon,
+chromium, nickel, tungsten, and molybdenum.
+
+The usual method for adding these elements to the steel is to first
+prepare a very rich alloy of iron with the element to be added, and then
+add enough of this alloy to a large quantity of the steel to bring it to
+the desired composition. A rich alloy of iron with manganese or silicon
+can be prepared directly in a blast furnace, and is called
+ferromanganese or ferrosilicon. Similar alloys of iron with the other
+elements mentioned are made in an electric furnace by reducing the mixed
+oxides with carbon.
+
+~Pure iron.~ Perfectly pure iron is rarely prepared and is not adapted to
+commercial uses. It can be made by reducing pure oxide of iron in a
+current of hydrogen at a high temperature. Prepared in this way it
+forms a black powder; when melted it forms a tin-white metal which is
+less fusible and more malleable than wrought iron. It is easily acted
+upon by moist air.
+
+~Compounds of iron.~ Iron differs from the metals so far studied in that
+it is able to form two series of compounds in which the iron has two
+different valences. In the one series the iron is divalent and forms
+compounds which in formulas and many chemical properties are similar to
+the corresponding zinc compounds. It can also act as a trivalent metal,
+and in this condition forms salts similar to those of aluminium. Those
+compounds in which the iron is divalent are known as _ferrous_
+compounds, while those in which it is trivalent are known as _ferric_.
+
+~Oxides of iron.~ Iron forms several oxides. Ferrous oxide (FeO) is not
+found in nature, but can be prepared artificially in the form of a black
+powder which easily takes up oxygen, forming ferric oxide:
+
+    2FeO + O = Fe_{2}O_{3}.
+
+Ferric oxide is the most abundant ore of iron and occurs in great
+deposits, especially in the Lake Superior region. It is found in many
+mineral varieties which vary in density and color, the most abundant
+being hematite, which ranges in color from red to nearly black. When
+prepared by chemical processes it forms a red powder which is used as a
+paint pigment (Venetian red) and as a polishing powder (rouge).
+
+Magnetite has the formula Fe_{3}O_{4} and is a combination of FeO and
+Fe_{2}O_{3}. It is a very valuable ore, but is less abundant than
+hematite. It is sometimes called magnetic oxide of iron, or lodestone,
+since it is a natural magnet.
+
+~Ferrous salts.~ These salts are obtained by dissolving iron in the
+appropriate acid, or, when insoluble, by precipitation. They are usually
+light green in color and crystallize well. In chemical reactions they
+are quite similar to the salts of magnesium and zinc, but differ from
+them in one important respect, namely, that they are easily changed into
+compounds in which the metal is trivalent. Thus ferrous chloride treated
+with chlorine or aqua regia is changed into ferric chloride:
+
+    FeCl_{2} + Cl = FeCl_{3}.
+
+Ferrous hydroxide exposed to moist air is rapidly changed into ferric
+hydroxide:
+
+    2Fe(OH)_{2} + H_{2}O + O = 2Fe(OH)_{3}.
+
+~Ferrous sulphate~ _(copperas, green vitriol)_ (FeSO_{4}·7H_{2}O). Ferrous
+sulphate is the most familiar ferrous compound. It is prepared
+commercially as a by-product in the steel-plate mills. Steel plates are
+cleaned by the action of dilute sulphuric acid upon them, and in the
+process some of the iron dissolves. The liquors are concentrated and the
+green vitriol separates from them.
+
+~Ferrous sulphide~ (FeS). Ferrous sulphide is sometimes found in nature as
+a golden-yellow crystalline mineral. It is formed as a black precipitate
+when a soluble sulphide and an iron salt are brought together in
+solution:
+
+    FeSO_{4} + Na_{2}S = FeS + Na_{2}SO_{4}.
+
+It can also be made as a heavy dark-brown solid by fusing together the
+requisite quantities of sulphur and iron. It is obtained as a by-product
+in the metallurgy of lead:
+
+    PbS + Fe = FeS + Pb.
+
+It is used in the laboratory in the preparation of hydrosulphuric acid:
+
+    FeS + 2HCl = FeCl_{2} + H_{2}S.
+
+~Iron disulphide~ _(pyrites)_ (FeS_{2}). This substance bears the same
+relation to ferrous sulphide that hydrogen dioxide does to water. It
+occurs abundantly in nature in the form of brass-yellow cubical crystals
+and in compact masses. Sometimes the name "fool's gold" is applied to it
+from its superficial resemblance to the precious metal. It is used in
+very large quantities as a source of sulphur dioxide in the manufacture
+of sulphuric acid, since it burns readily in the air, forming ferric
+oxide and sulphur dioxide:
+
+    2FeS_{2} + 11O = Fe_{2}O_{3} + 4SO_{2}.
+
+~Ferrous carbonate~ (FeCO_{3}). This compound occurs in nature as
+siderite, and is a valuable ore. It will dissolve to some extent in
+water containing carbon dioxide, just as will calcium carbonate, and
+waters containing it are called chalybeate waters. These chalybeate
+waters are supposed to possess certain medicinal virtues and form an
+important class of mineral waters.
+
+~Ferric salts.~ Ferric salts are usually obtained by treating an acidified
+solution of a ferrous salt with an oxidizing agent:
+
+    2FeCl_{2} + 2HCl + O = 2FeCl_{3} + H_{2}O,
+
+    2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O.
+
+They are usually yellow or violet in color, are quite soluble, and as a
+rule do not crystallize well. Heated with water in the absence of free
+acid, they hydrolyze even more readily than the salts of aluminium. The
+most familiar ferric salts are the chloride and the sulphate.
+
+~Ferric chloride~ (FeCl_{3}). This salt can be obtained most conveniently
+by dissolving iron in hydrochloric acid and then passing chlorine into
+the solution:
+
+    Fe + 2HCl = FeCl_{2} + 2H,
+
+    FeCl_{2} + Cl = FeCl_{3}.
+
+When the pure salt is heated with water it is partly hydrolyzed:
+
+    FeCl_{3} + 3 H_{2}O <--> Fe(OH)_{3} + 3HCl.
+
+This is a reversible reaction, however, and hydrolysis can therefore be
+prevented by first adding a considerable amount of the soluble product
+of the reaction, namely, hydrochloric acid.
+
+~Ferric sulphate~ (Fe_{2}(SO_{4})_{3}). This compound can be made by
+treating an acid solution of green vitriol with an oxidizing agent. It
+is difficult to crystallize and hard to obtain in pure condition. When
+an alkali sulphate in proper quantity is added to ferric sulphate in
+solution an iron alum is formed, and is easily obtained in
+large crystals. The best known iron alums have the formulas
+KFe(SO_{4})_{2}·12H_{2}O and NH_{4}Fe(SO_{4})_{2}·12H_{2}O. They are
+commonly used when a pure ferric salt is required.
+
+~Ferric hydroxide~ (Fe(OH)_{3}). When solutions of ferric salts are
+treated with ammonium hydroxide, ferric hydroxide is formed as a
+rusty-red precipitate, insoluble in water.
+
+~Iron cyanides.~ A large number of complex cyanides containing iron are
+known, the most important being potassium ferrocyanide, or yellow
+prussiate of potash (K_{4}FeC_{6}N_{6}), and potassium ferricyanide, or
+red prussiate of potash (K_{3}FeC_{6}N_{6}). These compounds are the
+potassium salts of the complex acids of the formulas H_{4}FeC_{6}N_{6}
+and H_{3}FeC_{6}N_{6}.
+
+~Oxidation of ferrous salts.~ It has just been seen that when a ferrous
+salt is treated with an oxidizing agent in the presence of a free acid a
+ferric salt is formed:
+
+    2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O.
+
+In this reaction oxygen is used up, and the valence of the iron is
+changed from 2 to 3. The same equation may be written
+
+    2Fe^{++}, 2SO_{4}^{--} + 2H^{+}, SO_{4}^{--} + O
+      = 2Fe^{+++}, 3SO_{4}^{--} + H_{2}O.
+
+Hydrogen ions have been oxidized to water, while the charge of each iron
+ion has been increased from 2 to 3.
+
+In a similar way the conversion of ferrous chloride into ferric chloride
+may be written
+
+    Fe^{++}, 2Cl^{-} + Cl = Fe^{+++}, + 3Cl^{-}.
+
+Here again the valence of the iron and the charge on the iron ion has
+been increased from 2 to 3, though no oxygen has entered into the
+reaction. As a rule, however, changes of this kind are brought about by
+the use of an oxidizing agent, and are called oxidations.
+
+The term "oxidation" is applied to all reactions in which the valence of
+the metal of a compound is increased, or, in other words, to all
+reactions in which the charge of a cation is increased.
+
+~Reduction of ferric salts.~ The changes which take place when a ferric
+salt is converted into a ferrous salt are the reverse of the ones just
+described. This is seen in the equation
+
+    FeCl_{3} + H = FeCl_{2} + HCl
+
+In this reaction the valence of the iron has been changed from 3 to 2.
+The same equation may be written
+
+    Fe^{+++}, 3Cl_{-} + H = Fe^{++}, + H^{+} + 3Cl_{-}
+
+It will be seen that the charge of the iron ions has been diminished
+from 3 to 2. Since these changes are the reverse of the oxidation
+changes just considered, they are called reduction reactions. The term
+"reduction" is applied to all processes in which the valence of the
+metal of a compound is diminished, or, in other words, to all processes
+in which the charge on the cations is diminished.
+
+
+NICKEL AND COBALT
+
+These elements occur sparingly in nature, usually combined with arsenic
+or with arsenic and sulphur. Both elements have been found in the free
+state in meteorites. Like iron they form two series of compounds, but
+the salts corresponding to the ferrous salts are the most common, the
+ones corresponding to the ferric salts being difficult to obtain. Thus
+we have the chlorides NiCl_{2}·6H_{2}O and CoCl_{2}·6H_{2}O; the
+sulphates NiSO_{4}·7H_{2}O and CoSO_{4}·7H_{2}O; the nitrates
+Ni(NO_{3})_{2}·6H_{2}O and Co(NO_{3})_{2}·6H_{2}O.
+
+Nickel is largely used as an alloy with other metals. Alloyed with
+copper it forms coin metal from which five-cent pieces are made, with
+copper and zinc it forms German silver, and when added to steel in small
+quantities nickel steel is formed which is much superior to common steel
+for certain purposes. When deposited by electrolysis upon the surface of
+other metals such as iron, it forms a covering which will take a high
+polish and protects the metal from rust, nickel not being acted upon by
+moist air. Salts of nickel are usually green.
+
+Compounds of cobalt fused with glass give it an intensely blue color. In
+powdered form such glass is sometimes used as a pigment called smalt.
+Cobalt salts, which contain water of crystallization, are usually cherry
+red in color; when dehydrated they become blue.
+
+
+EXERCISES
+
+1. In the manufacture of cast iron, why is the air heated before being
+forced into the furnace?
+
+2. Write the equations showing how each of the following compounds of
+iron could be obtained from the metal itself: ferrous chloride, ferrous
+hydroxide, ferrous sulphate, ferrous sulphide, ferrous carbonate, ferric
+chloride, ferric sulphate, ferric hydroxide.
+
+3. Account for the fact that a solution of sodium carbonate, when added
+to a solution of a ferric salt, precipitates an hydroxide and not a
+carbonate.
+
+4. Calculate the percentage of iron in each of the common iron ores.
+
+5. One ton of steel prepared by the Bessemer process is found by
+analysis to contain 0.2% carbon. What is the minimum weight of carbon
+which must be added in order that the steel may be made to take a
+temper?
+
+
+
+
+CHAPTER XXVIII
+
+COPPER, MERCURY, AND SILVER
+
+
+==================================================================
+        |        |        |         |         |
+        |        |        |         |         | FORMULAS OF OXIDES
+        | SYMBOL | ATOMIC | DENSITY | MELTING |___________________
+        |        | WEIGHT |         |  POINT  |          |
+        |        |        |         |         |  "ous"   |  "ic"
+________|________|________|_________|_________|__________|________
+        |        |        |         |         |          |
+Copper  |   Cu   |  63.6  |   8.89  | 1084°   |  Cu_{2}O |  CuO
+Mercury |   Hg   | 200.00 |  13.596 |  -39.5° |  Hg_{2}O |  HgO
+Silver  |   Ag   | 107.93 |  10.5   |  960°   |  Ag_{2}O |  AgO
+==================================================================
+
+~The family.~ By referring to the periodic arrangement of the elements
+(page 168), it will be seen that mercury is not included in the same
+family with copper and silver. Since the metallurgy of the three
+elements is so similar, however, and since they resemble each other so
+closely in chemical properties, it is convenient to class them together
+for study.
+
+1. _Occurrence._ The three elements occur in nature to some extent in
+the free state, but are usually found as sulphides. Their ores are easy
+to reduce.
+
+2. _Properties._ They are heavy metals of high luster and are especially
+good conductors of heat and electricity. They are not very active
+chemically. Neither hydrochloric nor dilute sulphuric acid has any
+appreciable action upon them. Concentrated sulphuric acid attacks all
+three, forming metallic sulphates and evolving sulphur dioxide, while
+nitric acid, both dilute and concentrated, converts them into nitrates
+with the evolution of oxides of nitrogen.
+
+3. _Two series of salts._ Copper and mercury form oxides of the types
+M_{2}O and MO, as well as two series of salts. In one series the metals
+are univalent and the salts have formulas like those of the sodium
+salts. They are called cuprous and mercurous salts. In the other series
+the metals are divalent and resemble magnesium salts in formulas. These
+are called cupric and mercuric salts. Silver forms only one series of
+salts, being always a univalent metal.
+
+
+COPPER
+
+~Occurrence.~ The element copper has been used for various purposes since
+the earliest days of history. It is often found in the metallic state in
+nature, large masses of it occurring pure in the Lake Superior region
+and in other places to a smaller extent. The most valuable ores are the
+following:
+
+    Cuprite         Cu_{2}O.
+    Chalcocite      Cu_{2}S.
+    Chalcopyrite    CuFeS_{2}.
+    Bornite         Cu_{3}FeS_{3}.
+    Malachite       CuCO_{3}·Cu(OH)_{2}.
+    Azurite         2CuCO_{3}·Cu(OH)_{2}.
+
+~Metallurgy of copper.~ Ores containing little or no sulphur are easy to
+reduce. They are first crushed and the earthy impurities washed away.
+The concentrated ore is then mixed with carbon and heated in a furnace,
+metallic copper resulting from the reduction of the copper oxide by the
+hot carbon.
+
+     ~Metallurgy of sulphide ores.~ Much of the copper of commerce is
+     made from chalcopyrite and bornite, and these ores are more
+     difficult to work. They are first roasted in the air, by which
+     treatment much of the sulphur is burned to sulphur dioxide. The
+     roasted ore is then melted in a small blast furnace or in an
+     open one like a puddling furnace. In melting, part of the iron
+     combines with silica to form a slag of iron silicate. The
+     product, called crude matte, contains about 50% copper together
+     with sulphur and iron. Further purification is commonly carried
+     on by a process very similar to the Bessemer process for steel.
+     The converter is lined with silica, and a charge of matte from
+     the melting furnace, together with sand, is introduced, and air
+     is blown into the mass. By this means the sulphur is
+     practically all burned out by the air, and the remaining iron
+     combines with silica and goes off as slag. The copper is poured
+     out of the converter and molded into anode plates for refining.
+
+~Refining of copper.~ Impure copper is purified by electrolysis. A large
+plate of it, serving as an anode, is suspended in a tank facing a thin
+plate of pure copper, which is the cathode. The tank is filled with a
+solution of copper sulphate and sulphuric acid to serve as the
+electrolyte. A current from a dynamo passes from the anode to the
+cathode, and the copper, dissolving from the anode, is deposited upon
+the cathode in pure form, while the impurities collect on the bottom of
+the tank. Electrolytic copper is one of the purest of commercial metals
+and is very nearly pure copper.
+
+     ~Recovery of gold and silver.~ Gold and silver are often present
+     in small quantities in copper ores, and in electrolytic
+     refining these metals collect in the muddy deposit on the
+     bottom of the tank. The mud is carefully worked over from time
+     to time and the precious metals extracted from it. A surprising
+     amount of gold and silver is obtained in this way.
+
+~Properties of copper.~ Copper is a rather heavy metal of density 8.9, and
+has a characteristic reddish color. It is rather soft and is very
+malleable, ductile, and flexible, yet tough and strong; it melts at
+1084°. As a conductor of heat and electrical energy it is second only to
+silver.
+
+Hydrochloric acid, dilute sulphuric acid, and fused alkalis are almost
+without action upon it; nitric acid and hot, concentrated sulphuric
+acid, however, readily dissolve it. In moist air it slowly becomes
+covered with a thin layer of green basic carbonate; heated in the air it
+is easily oxidized to black copper oxide (CuO).
+
+~Uses.~ Copper is extensively used for electrical purposes, for roofs and
+cornices, for sheathing the bottom of ships, and for making alloys. In
+the following table the composition of some of these alloys is
+indicated:
+
+COMPOSITION OF ALLOYS OF COPPER IN PERCENTAGES
+
+    Aluminium bronze copper (90 to 97%), aluminium (3 to 10%).
+    Brass copper (63 to 73%), zinc (27 to 37%).
+    Bronze copper (70 to 95%), zinc (1 to 25%), tin (1 to 18%).
+    German silver copper (56 to 60%), zinc (20%), nickel (20 to 25%).
+    Gold coin copper (10%), gold (90%).
+    Gun metal copper (90%), tin (10%).
+    Nickel coin copper (75%), nickel (25%)
+    Silver coin copper (10%), silver (90%).
+
+     ~Electrotyping.~ Matter is often printed from electrotype plates
+     which are prepared as follows. The matter is set up in type and
+     wax is firmly pressed down upon the face of it until a clear
+     impression is obtained. The impressed side of the wax is coated
+     with graphite and the impression is made the cathode in an
+     electrolytic cell containing a copper salt in solution. When
+     connected with a current the copper is deposited as a thin
+     sheet upon the letters in wax, and when detached is a perfect
+     copy of the type, the under part of the letters being hollow.
+     The sheet is strengthened by pouring on the under surface a
+     suitable amount of molten metal (commercial lead is used). The
+     sheet so strengthened is then used in printing.
+
+~Two series of copper compounds.~ Copper, like iron, forms two series of
+compounds: in the cuprous compounds it is univalent; in the cupric it is
+divalent. The cupric salts are much the more common of the two, since
+the cuprous salts pass readily into cupric by oxidation.
+
+~Cuprous compounds.~ The most important cuprous compound is the oxide
+(Cu_{2}O), which occurs in nature as ruby copper or cuprite. It is a
+bright red substance and can easily be prepared by heating copper to a
+high temperature in a limited supply of air. It is used for imparting a
+ruby color to glass.
+
+By treating cuprous oxide with different acids a number of cuprous salts
+can be made. Many of these are insoluble in water, the chloride (CuCl)
+being the best known. When suspended in dilute hydrochloric acid it is
+changed into cupric chloride, the oxygen taking part in the reaction
+being absorbed from the air:
+
+    2CuCl + 2HCl + O = 2CuCl_{2} + H_{2}O.
+
+~Cupric compounds.~ Cupric salts are easily made by dissolving cupric
+oxide in acids, or, when insoluble, by precipitation. Most of them are
+blue or green in color, and the soluble ones crystallize well. Since
+they are so much more familiar than the cuprous salts, they are
+frequently called merely copper salts.
+
+~Cupric oxide~ (CuO). This is a black insoluble substance obtained by
+heating copper in excess of air, or by igniting the hydroxide or
+nitrate. It is used as an oxidizing agent.
+
+~Cupric hydroxide~ (Cu(OH)_{2}). The hydroxide prepared by treating a
+solution of a copper salt with sodium hydroxide is a light blue
+insoluble substance which easily loses water and changes into the oxide.
+Heat applied to the liquid containing the hydroxide suspended in it
+serves to bring about the reaction represented by the equation
+
+    Cu(OH)_{2} = CuO + H_{2}O.
+
+~Cupric sulphate~ (_blue vitriol_) (CuSO_{4}·5H_{2}O). This substance,
+called blue vitriol or bluestone, is obtained as a by-product in a
+number of processes and is produced in very large quantities. It forms
+large blue crystals, which lose water when heated and crumble to a white
+powder. The salt finds many uses, especially in electrotyping and in
+making electrical batteries.
+
+~Cupric sulphide~ (CuS). The insoluble black sulphide (CuS) is easily
+prepared by the action of hydrosulphuric acid upon a solution of a
+copper salt:
+
+    CuSO_{4} + H_{2}S = CuS + H_{2}SO_{4}.
+
+It is insoluble in water and dilute acids.
+
+
+MERCURY
+
+~Occurrence.~ Mercury occurs in nature chiefly as the sulphide (HgS)
+called cinnabar, and in globules of metal inclosed in the cinnabar. The
+mercury mines of Spain have long been famous, California being the next
+largest producer.
+
+~Metallurgy.~ Mercury is a volatile metal which has but little affinity
+for oxygen. Sulphur, on the other hand, readily combines with oxygen.
+These facts make the metallurgy of mercury very simple. The crushed ore,
+mixed with a small amount of carbon to reduce any oxide or sulphate that
+might be formed, is roasted in a current of air. The sulphur burns to
+sulphur dioxide, while the mercury is converted into vapor and is
+condensed in a series of condensing vessels. The metal is purified by
+distillation.
+
+~Properties.~ Mercury is a heavy silvery liquid with a density of 13.596.
+It boils at 357° and solidifies at -39.5°. Small quantities of many
+metals dissolve in it, forming liquid alloys, while with larger
+quantities it forms solid alloys. The alloys of mercury are called
+amalgams.
+
+Toward acids mercury conducts itself very much like copper; it is easily
+attacked by nitric and hot, concentrated sulphuric acids, while cold
+sulphuric and hydrochloric acids have no effect on it.
+
+~Uses.~ Mercury is extensively used in the construction of scientific
+instruments, such as the thermometer and barometer, and as a liquid over
+which to collect gases which are soluble in water. The readiness with
+which it alloys with silver and gold makes it very useful in the
+extraction of these elements.
+
+~Compounds of mercury.~ Like copper, mercury forms two series of
+compounds: the mercurous, of which mercurous chloride (HgCl) is an
+example; and the mercuric, represented by mercuric chloride (HgCl_{2}).
+
+~Mercuric oxide~ (HgO). Mercuric oxide can be obtained either as a
+brick-red or as a yellow substance. When mercuric nitrate is heated
+carefully the red modification is formed in accordance with the equation
+
+    Hg(NO_{3})_{2} = HgO + 2NO_{2} + O.
+
+The yellow modification is prepared by adding a solution of a mercuric
+salt to a solution of sodium or potassium hydroxide:
+
+    Hg(NO_{3})_{2} + 2NaOH = 2NaNO_{3} + Hg(OH)_{2},
+
+    Hg(OH)_{2} = HgO + H_{2}O.
+
+When heated the oxide darkens until it becomes almost black; at a higher
+temperature it decomposes into mercury and oxygen. It was by this
+reaction that oxygen was discovered.
+
+~Mercurous chloride~ (_calomel_) (HgCl). Being insoluble, mercurous
+chloride is precipitated as a white solid when a soluble chloride is
+added to a solution of mercurous nitrate:
+
+    HgNO_{3} + NaCl = HgCl + NaNO_{3}.
+
+Commercially it is manufactured by heating a mixture of mercuric
+chloride and mercury. When exposed to the light it slowly changes into
+mercuric chloride and mercury:
+
+    2HgCl = HgCl_{2} + Hg.
+
+It is therefore protected from the light by the use of colored bottles.
+It is used in medicine.
+
+Most mercurous salts are insoluble in water, the principal soluble one
+being the nitrate, which is made by the action of cold, dilute nitric
+acid on mercury.
+
+~Mercuric chloride~ (_corrosive sublimate_) (HgCl_{2}). This substance can
+be made by dissolving mercuric oxide in hydrochloric acid. On a
+commercial scale it is made by subliming a mixture of common salt and
+mercuric sulphate:
+
+    2NaCl + HgSO_{4} = HgCl_{2} + Na_{2}SO_{4}.
+
+The mercuric chloride, being readily volatile, vaporizes and is
+condensed again in cool vessels. Like mercurous chloride it is a white
+solid, but differs from it in that it is soluble in water. It is
+extremely poisonous and in dilute solutions is used as an antiseptic in
+dressing wounds.
+
+~Mercuric sulphide~ (HgS). As cinnabar this substance forms the chief
+native compound of mercury, occurring in red crystalline masses. By
+passing hydrosulphuric acid into a solution of a mercuric salt it is
+precipitated as a black powder, insoluble in water and acids. By other
+means it can be prepared as a brilliant red powder known as vermilion,
+which is used as a pigment in fine paints.
+
+     ~The iodides of mercury.~ If a solution of potassium iodide is
+     added to solutions of a mercurous and a mercuric salt
+     respectively, the corresponding iodides are precipitated.
+     Mercuric iodide is the more important of the two, and as
+     prepared above is a red powder which changes to yellow on
+     heating to 150°. The yellow form on cooling changes back again
+     to the red form, or may be made to do so by rubbing it with a
+     knife blade or some other hard object.
+
+
+SILVER
+
+~Occurrence.~ Silver is found in small quantities in the uncombined state;
+usually, however, it occurs in combination with sulphur, either as the
+sulphide (Ag_{2}S) or as a small constituent of other sulphides,
+especially those of lead and copper. It is also found alloyed with gold.
+
+~Metallurgy.~ _Parkes's process._ Silver is usually smelted in connection
+with lead. The ores are worked over together, as described under lead,
+and the lead and silver obtained as an alloy, the silver being present
+in small quantity. The alloy is melted and metallic zinc is stirred in.
+Zinc will alloy with silver but not with lead, and it is found that the
+silver leaves the lead and, in the form of an alloy with zinc, forms as
+a crust upon the lead and is skimmed off. This crust, which, of course,
+contains lead adhering to it, is partially melted and the most of the
+lead drained off. The zinc is removed by distillation, and the residue
+is melted on an open hearth in a current of air; by this means the zinc
+and lead remaining with the silver are changed into oxides and the
+silver remains behind unaltered.
+
+     ~Amalgamation process.~ In some localities the old amalgamation
+     process is used. The silver ore is treated with common salt and
+     ferrous compounds, which process converts the silver first into
+     chloride and then into metallic silver. Mercury is then added
+     and thoroughly mixed with the mass, forming an amalgam with the
+     silver. After some days the earthy materials are washed away
+     and the heavier amalgam is recovered. The mercury is distilled
+     off and the silver left in impure form.
+
+~Refining silver.~ The silver obtained by either of the above processes
+may still contain copper, gold, and iron, and is refined by "parting"
+with sulphuric acid. The metal is heated with strong sulphuric acid
+which dissolves the silver, copper, and iron present, but not the gold.
+In the solution of silver sulphate so obtained copper plates are
+suspended, upon which the pure silver precipitates, the copper going
+into solution as sulphate, as shown in the equation
+
+    Ag_{2}SO_{4} + Cu = 2Ag + CuSO_{4}.
+
+The solution obtained as a by-product in this process furnishes most of
+the blue vitriol of commerce. Silver is also refined by electrolytic
+methods similar to those used in refining copper.
+
+~Properties of silver.~ Silver is a heavy, rather soft, white metal, very
+ductile and malleable and capable of taking a high polish. It surpasses
+all other metals as a conductor of heat and electricity, but is too
+costly to find extensive use for such purposes. It melts at a little
+lower temperature than copper (961°). It alloys readily with other heavy
+metals, and when it is to be used for coinage a small amount of
+copper--from 8 to 10%--is nearly always melted with it to give it
+hardness.
+
+It is not acted upon by water or air, but is quickly tarnished when in
+contact with sulphur compounds, turning quite black in time.
+Hydrochloric acid and fused alkalis do not act upon it, but nitric acid
+and hot, concentrated sulphuric acid dissolve it with ease.
+
+[Illustration: Fig. 88]
+
+     ~Electroplating.~ Since silver is not acted upon by water or air,
+     and has a pleasing appearance, it is used to coat various
+     articles made of cheaper metals. Such articles are said to be
+     silver plated. The process by which this is done is called
+     electroplating. It is carried on as follows: The object to be
+     plated (such as a spoon) is attached to a wire and dipped into
+     a solution of a silver salt. Electrical connection is made in
+     such a way that the article to be plated serves as the cathode,
+     while the anode is made up of one or more plates of silver
+     (Fig. 88, A). When a current is passed through the
+     electrolyte silver dissolves from the anode plate and deposits
+     on the cathode in the form of a closely adhering layer. By
+     making the proper change in the electrolyte and anode plate
+     objects may be plated with gold and other metals.
+
+~Compounds of silver.~ Silver forms two oxides but only one series of
+salts, namely, the one which corresponds to the mercurous and cuprous
+series.
+
+~Silver nitrate~ (_lunar caustic_) (AgNO_{3}). This salt is easily
+prepared by dissolving silver in nitric acid and evaporating the
+resulting solution. It crystallizes in flat plates, and when heated
+carefully can be melted without decomposition. When cast into sticks it
+is called lunar caustic, for it has a very corrosive action on flesh,
+and is sometimes used in surgery to burn away abnormal growths.
+
+     The alchemists designated the metals by the names of the
+     heavenly bodies. The moon (luna) was the symbol for silver;
+     hence the name "lunar caustic."
+
+~Silver sulphide~ (Ag_{2}S). This occurs in nature and constitutes one of
+the principal ores of silver. It can be obtained in the form of a black
+solid by passing hydrosulphuric acid through a solution of silver
+nitrate.
+
+~Compounds of silver with the halogens.~ The chloride, bromide, and iodide
+of silver are insoluble in water and acids, and are therefore
+precipitated by bringing together a soluble halogen salt with silver
+nitrate:
+
+    AgNO_{3} + KCl = AgCl + KNO_{3}.
+
+They are remarkable for the fact that they are very sensitive to the
+action of light, undergoing a change of color and chemical composition
+when exposed to sunlight, especially if in contact with organic matter
+such as gelatin.
+
+     ~Photography.~ The art of photography is based on the fact that
+     the halogen compounds of silver are affected by the light,
+     particularly in the presence of organic matter. From a chemical
+     standpoint the processes involved may be described under two
+     heads: (1) the preparation of the negative; (2) the preparation
+     of the print.
+
+     1. _Preparation of the negative._ The plate used in the
+     preparation of the negative is made by spreading a thin layer
+     of gelatin, in which silver bromide is suspended (silver iodide
+     is sometimes added also), over a glass plate or celluloid film
+     and allowing it to dry. When the plate so prepared is placed in
+     a camera and the image of some object is focused upon it, the
+     silver salt undergoes a change which is proportional at each
+     point to the intensity of the light falling upon it. In this
+     way an image of the object photographed is produced upon the
+     plate, which is, however, invisible and is therefore called
+     "latent." It can be made visible by the process of developing.
+
+     To develop the image the exposed plate is immersed in a
+     solution of some reducing agent called the developer. The
+     developer reduces that portion of the silver salt which has
+     been affected by the light, depositing it in the form of black
+     metallic silver which closely adheres to the plate.
+
+     The unaffected silver salt, upon which the developer has no
+     action, must now be removed from the plate. This is done by
+     immersing the plate in a solution of sodium thiosulphate
+     (hypo). After the silver salt has been dissolved off, the plate
+     is washed with water and dried. The plate so prepared is
+     called the negative because it is a picture of the object
+     photographed, with the lights exactly reversed. This is called
+     fixing the negative.
+
+     2. _Preparation of the print._ The print is made from paper
+     which is prepared in the same way as the negative plate. The
+     negative is placed upon this paper and exposed to the light in
+     such a way that the light must pass through the negative before
+     striking the paper. If the paper is coated with silver
+     chloride, a visible image is produced, in which case a
+     developer is not needed. The proofs are made in this way. In
+     order to make them permanent the unchanged silver chloride must
+     be dissolved off with sodium thiosulphate. The print is then
+     toned by dipping it into a solution of gold or platinum salts.
+     The silver on the print passes into solution, while the gold or
+     platinum takes its place. These metals give a characteristic
+     color or tone to the print, the gold making it reddish brown,
+     while the platinum gives it a steel-gray tone. If a silver
+     bromide paper is used in making the print, a latent image is
+     produced which must be developed as in the case of the negative
+     itself. The silver bromide is much more sensitive than the
+     chloride, so that the printing can be done in artificial light.
+     Since the darkest places on the negative cut off the most
+     light, it is evident that the lights of the print will be the
+     reverse of those of the negative, and will therefore correspond
+     to those of the object photographed. The print is therefore
+     called the positive.
+
+
+EXERCISES
+
+1. Account for the fact that copper has been used for so long a time.
+
+2. Write equations for the action of concentrated sulphuric and nitric
+acids upon the metals of this family.
+
+3. How would you account for the fact that normal copper sulphate is
+slightly acid to litmus?
+
+4. Contrast the action of heat on cupric nitrate and mercuric nitrate.
+
+5. State reasons why mercury is adapted for use in thermometers and
+barometers.
+
+6. How could you distinguish between mercurous chloride and mercuric
+chloride?
+
+7. Write equations for the preparation of mercuric and mercurous
+iodides.
+
+8. How would you account for the fact that solutions of the different
+salts of a metal usually have the same color?
+
+9. Crude silver usually contains iron and lead. What would become of
+these metals in refining by parting with sulphuric acid?
+
+10. In the amalgamation process for extracting silver, how does ferrous
+chloride convert silver chloride into silver? Write equation. Why is the
+silver sulphide first changed into silver chloride?
+
+11. What impurities would you expect to find in the copper sulphate
+prepared from the refining of silver?
+
+12. How could you prepare pure silver chloride from a silver coin?
+
+13. Mercuric nitrate and silver nitrate are both white solids soluble in
+water. How could you distinguish between them?
+
+14. Account for the fact that sulphur waters turn a silver coin black;
+also for the fact that a silver spoon is blackened by foods (eggs, for
+example) containing sulphur.
+
+15. When a solution of silver nitrate is added to a solution of
+potassium chlorate no precipitate forms. How do you account for the fact
+that a precipitate of silver chloride is not formed?
+
+
+
+
+CHAPTER XXIX
+
+TIN AND LEAD
+
+
+====================================================================
+     |        |        |         |         |
+     | SYMBOL | ATOMIC | DENSITY | MELTING | COMMON OXIDES
+     |        | WEIGHT |         |  POINT  |
+_____|________|________|_________|_________|________________________
+     |        |        |         |         |
+Tin  |   Sn   |  119.0 |   7.35  |   235°  | SnO             SnO_{2}
+Lead |   Pb   |  206.9 |  11.38  |   327°  | PbO Pb_{3}O_{4} PbO_{2}
+====================================================================
+
+~The family.~ Tin and lead, together with silicon and germanium, form a
+family in Group IV of the periodic table. Silicon has been discussed
+along with the non-metals, while germanium, on account of its rarity,
+needs only to be mentioned.
+
+The other family of Group IV includes carbon, already described, and a
+number of rare elements.
+
+
+TIN
+
+~Occurrence.~ Tin is found in nature chiefly as the oxide (SnO_{2}),
+called cassiterite or tinstone. The most famous mines are those of
+Cornwall in England, and of the Malay Peninsula and East India Islands;
+in small amounts tinstone is found in many other localities.
+
+~Metallurgy.~ The metallurgy of tin is very simple. The ore, separated as
+far as possible from earthy materials, is mixed with carbon and heated
+in a furnace, the reduction taking place readily. The equation is
+
+    SnO_{2} + C = Sn + CO_{2}.
+
+The metal is often purified by carefully heating it until it is partly
+melted; the pure tin melts first and can be drained away from the
+impurities.
+
+~Properties.~ Pure tin, called block tin, is a soft white metal with a
+silver-like appearance and luster; it melts readily (235°) and is
+somewhat lighter than copper, having a density of 7.3. It is quite
+malleable and can be rolled out into very thin sheets, forming tin foil;
+most tin foil, however, contains a good deal of lead.
+
+Under ordinary conditions it is quite unchanged by air or moisture, but
+at a high temperature it burns in air, forming the oxide SnO_{2}. Dilute
+acids have no effect upon it, but concentrated acids attack it readily.
+Concentrated hydrochloric acid changes it into the chloride
+
+    Sn + 2HCl = SnCl_{2} + 2H.
+
+With sulphuric acid tin sulphate and sulphur dioxide are formed:
+
+    Sn + 2H_{2}SO_{4} = SnSO_{4} + SO_{2} + 2H_{2}O
+
+Concentrated nitric acid oxidizes it, forming a white insoluble compound
+of the formula H_{2}SnO_{3}, called metastannic acid:
+
+    3Sn + 4HNO_{3} + H_{2}O = 3H_{2}SnO_{3} + 4NO.
+
+~Uses of tin.~ A great deal of tin is made into tin plate by dipping thin
+steel sheets into the melted metal. Owing to the way in which tin
+resists the action of air and dilute acids, tin plate is used in many
+ways, such as in roofing, and in the manufacture of tin cans, cooking
+vessels, and similar articles.
+
+Many useful alloys contain tin, some of which have been mentioned in
+connection with copper. When tin is alloyed with other metals of low
+melting point, soft, easily melted alloys are formed which are used for
+friction bearings in machinery; tin, antimony, lead, and bismuth are the
+chief constituents of these alloys. Pewter and soft solder are alloys of
+tin and lead.
+
+~Compounds of tin.~ Tin forms two series of compounds: the stannous, in
+which the tin is divalent, illustrated in the compounds SnO, SnS,
+SnCl_{2}; the stannic, in which it is tetravalent as shown in the
+compounds SnO_{2}, SnS_{2}. There is also an acid, H_{2}SnO_{3}, called
+stannic acid, which forms a series of salts called stannates. While this
+acid has the same composition as metastannic acid, the two are quite
+different in their chemical properties. This difference is probably due
+to the different arrangement of the atoms in the molecules of the two
+substances. Only a few compounds of tin need be mentioned.
+
+~Stannic oxide~ (SnO_{2}). Stannic oxide is of interest, since it is the
+chief compound of tin found in nature. It is sometimes found in
+good-sized crystals, but as prepared in the laboratory is a white
+powder. When fused with potassium hydroxide it forms potassium stannate,
+acting very much like silicon dioxide:
+
+    SnO_{2} + 2KOH = K_{2}SnO_{3} + H_{2}O.
+
+~Chlorides of tin.~ Stannous chloride is prepared by dissolving tin in
+concentrated hydrochloric acid and evaporating the solution to
+crystallization. The crystals which are obtained have the composition
+SnCl_{2}·2H_{2}O, and are known as tin crystals. By treating a solution
+of stannous chloride with aqua regia, stannic chloride is formed:
+
+    SnCl_{2} + 2Cl = SnCl_{4}.
+
+The salt which crystallizes from such a solution has the composition
+SnCl_{4}·5H_{2}O, and is known commercially as oxymuriate of tin. If
+metallic tin is heated in a current of dry chlorine, the anhydrous
+chloride (SnCl_{4}) is obtained as a heavy colorless liquid which fumes
+strongly on exposure to air.
+
+The ease with which stannous chloride takes up chlorine to form stannic
+chloride makes it a good reducing agent in many reactions, changing the
+higher chlorides of metals to lower ones. Thus mercuric chloride is
+changed into mercurous chloride:
+
+    SnCl_{2} + 2HgCl_{2} = SnCl_{4} + 2HgCl.
+
+If the stannous chloride is in excess, the reaction may go further,
+producing metallic mercury:
+
+    SnCl_{2} + 2HgCl = SnCl_{4} + 2Hg.
+
+Ferric chloride is in like manner reduced to ferrous chloride:
+
+    SnCl_{3} + 2FeCl_{3} = SnCl_{4} + 2FeCl_{2}.
+
+The chlorides of tin, as well as the alkali stannates, are much used as
+mordants in dyeing processes. The hydroxides of tin and free stannic
+acid, which are easily liberated from these compounds, possess in very
+marked degree the power of fixing dyes upon fibers, as explained under
+aluminium.
+
+
+LEAD
+
+~Occurrence.~ Lead is found in nature chiefly as the sulphide (PbS),
+called galena; to a much smaller extent it occurs as carbonate,
+sulphate, chromate, and in a few other forms. Practically all the lead
+of commerce is made from galena, two general methods of metallurgy being
+in use.
+
+~Metallurgy.~ 1. The sulphide is melted with scrap iron, when iron
+sulphide and metallic lead are formed; the liquid lead, being the
+heavier, sinks to the bottom of the vessel and can be drawn off:
+
+    PbS + Fe = Pb + FeS.
+
+2. The sulphide is roasted in the air until a part of it has been
+changed into oxide and sulphate. The air is then shut off and the
+heating continued, the reactions indicated in the following equations
+taking place:
+
+    2PbO + PbS = 3Pb + SO_{2},
+
+    PbSO_{4} + PbS = 2Pb + 2SO_{2}.
+
+The lead so prepared usually contains small amounts of silver, arsenic,
+antimony, copper, and other metals. The silver is removed by Parkes's
+method, as described under silver, and the other metals in various ways.
+The lead of commerce is one of the purest commercial metals, containing
+as a rule only a few tenths per cent of impurities.
+
+~Properties.~ Lead is a heavy metal (den. = 11.33) which has a brilliant
+silvery luster on a freshly cut surface, but which soon tarnishes to a
+dull blue-gray color. It is soft, easily fused (melting at 327°), and
+quite malleable, but has little toughness or strength.
+
+It is not acted upon to any great extent by the oxygen of the air under
+ordinary conditions, but is changed into oxide at a high temperature.
+With the exception of hydrochloric and sulphuric acids, most acids, even
+very weak ones, act upon it, forming soluble lead salts. Hot,
+concentrated hydrochloric and sulphuric acids also attack it to a slight
+extent.
+
+~Uses.~ Lead is employed in the manufacture of lead pipes and in large
+storage batteries. In the form of sheet lead it is used in lining the
+chambers of sulphuric acid works and in the preparation of paint
+pigments. Some alloys of lead, such as solder and pewter (lead and tin),
+shot (lead and arsenic), and soft bearing metals, are widely used. Type
+metal consists of lead, antimony, and sometimes tin. Compounds of lead
+form several important pigments.
+
+~Compounds of lead.~ In nearly all its compounds lead has a valence of 2,
+but a few corresponding to stannic compounds have a valence of 4.
+
+~Lead oxides.~ Lead forms a number of oxides, the most important of which
+are litharge, red lead or minium, and lead peroxide.
+
+1. _Litharge_ (PbO). This oxide forms when lead is oxidized at a rather
+low temperature, and is obtained as a by-product in silver refining. It
+is a pale yellow powder, and has a number of commercial uses. It is
+easily soluble in nitric acid:
+
+    PbO + 2HNO_{3} = Pb(NO_{3})_{2} + H_{2}O.
+
+2. _Red lead, or minium_ (Pb_{3}O_{4}). Minium is prepared by heating
+lead (or litharge) to a high temperature in the air. It is a heavy
+powder of a beautiful red color, and is much used as a pigment.
+
+3. _Lead peroxide_ (PbO_{2}). This is left as a residue when minium is
+heated with nitric acid:
+
+Pb_{3}O_{4} + 4HNO_{3} = 2Pb(NO_{3})_{2} + PbO_{2} + 2H_{2}O.
+
+It is a brown powder which easily gives up a part of its oxygen and,
+like manganese dioxide and barium dioxide, is a good oxidizing agent.
+
+~Soluble salts of lead.~ The soluble salts of lead can be made by dissolving
+(Pb(C_{2}H_{3}O_{2})_{2}·3H_{2}O), litharge in acids. Lead acetate
+called sugar of lead, and lead nitrate (Pb(NO_{3})_{2}) are the most
+familiar examples. They are while crystalline solids and are poisonous
+in character.
+
+~Insoluble salts of lead; lead carbonate.~ While the normal carbonate of
+lead (PbCO_{3}) is found to some extent, in nature and can be prepared
+in the laboratory, basic carbonates of varying composition are much more
+easy to obtain. One of the simplest of these has the composition
+2PbCO_{3}·Pb(OH)_{2}. A mixture of such carbonates is called white lead.
+This is prepared on a large scale as a paint pigment and as a body for
+paints which are to be colored with other substances.
+
+     ~White lead.~ White lead is an amorphous white substance which,
+     when mixed with oil, has great covering power, that is, it
+     spreads out in an even waxy film, free from streaks and lumps,
+     and covers the entire surface upon which it is spread. Its
+     disadvantage as a pigment lies in the fact that it gradually
+     blackens when exposed to sulphur compounds, which are often
+     present in the air, forming black lead sulphide (PbS).
+
+     ~Technical preparation of white lead.~ Different methods are used
+     in the preparation of white lead, but the old one known as the
+     Dutch process is still the principal one employed. In this
+     process, earthenware pots about ten inches high and of the
+     shape shown in Fig. 89 are used. In the bottom A is placed a
+     3% solution of acetic acid (vinegar answers the purpose very
+     well). The space above this is filled with thin, perforated,
+     circular pieces of lead, supported by the flange B of the
+     pot. These pots are placed close together on a bed of tan bark
+     on the floor of a room known as the corroding room. They are
+     covered over with boards, upon which tan bark is placed, and
+     another row of pots is placed on this. In this way the room is
+     filled. The white lead is formed by the fumes of the acetic
+     acid, together with the carbon dioxide set free in the
+     fermentation of the tan bark acting on the lead. About three
+     months are required to complete the process.
+
+[Illustration 1: Fig. 89]
+
+~Lead sulphide~ (PbS). In nature this compound occurs in highly
+crystalline condition, the crystals having much the same luster as pure
+lead. It is readily prepared in the laboratory as a black precipitate,
+by the action of hydrosulphuric acid upon soluble lead salts:
+
+    Pb(NO_{3})_{2} + H_{2}S = PbS + 2HNO_{3}.
+
+It is insoluble both in water and in dilute acids.
+
+~Other insoluble salts.~ Lead chromate (PbCrO_{4}) is a yellow substance
+produced by the action of a soluble lead salt upon a soluble chromate,
+thus:
+
+    K_{2}CrO_{4} + Pb(NO_{3})_{2} = PbCrO_{4} + 2 KNO_{3}.
+
+It is used as a yellow pigment. Lead sulphate (PbSO_{4}) is a white
+substance sometimes found in nature and easily prepared by
+precipitation. Lead chloride (PbCl_{2}) is likewise a white substance
+nearly insoluble in cold water, but readily soluble in boiling water.
+
+     ~Thorium and cerium.~ These elements are found in a few rare
+     minerals, especially in the monazite sand of the Carolinas and
+     Brazil. The oxides of these elements are used in the
+     preparation of the Welsbach mantles for gas lights, because of
+     the intense light given out when a mixture of the oxides is
+     heated. These mantles contain the oxides of cerium and thorium
+     in the ratio of about 1% of the former to 99% of the latter.
+     Compounds of thorium, like those of radium, are found to
+     possess radio-activity, but in a less degree.
+
+
+EXERCISES
+
+1. How could you detect lead if present in tin foil?
+
+2. Stannous chloride reduces gold chloride (AuCl_{3}) to gold. Give
+equation.
+
+3. What are the products of hydrolysis when stannic chloride is used as
+a mordant?
+
+4. How could you detect arsenic, antimony, or copper in lead?
+
+5. Why is lead so extensively used for making water pipes?
+
+6. What sulphates other than lead are insoluble?
+
+7. Could lead nitrate be used in place of barium chloride in testing for
+sulphates?
+
+8. How much lead peroxide could be obtained from 1 kg. of minium?
+
+9. The purity of white lead is usually determined by observing the
+volume of carbon dioxide given off when it is treated with an acid. What
+acid should be used? On the supposition that it has the formula
+2PbCO_{3}·Pb(OH)_{2}, how nearly pure was a sample if 1 g. gave 30 cc.
+of carbon dioxide at 20° and 750 mm.?
+
+10. Silicon belongs in the same family with tin and lead. In what
+respects are these elements similar?
+
+11. What weight of tin could be obtained by the reduction of 1 ton of
+cassiterite?
+
+12. What reaction would you expect to take place when lead peroxide is
+treated with hydrochloric acid?
+
+13. White lead is often adulterated with barytes. Suggest a method for
+detecting it, if present, in a given example of white lead.
+
+
+
+
+CHAPTER XXX
+
+MANGANESE AND CHROMIUM
+
+
+====================================================================
+          |        |        |         |         |
+          | SYMBOL | ATOMIC | DENSITY | MELTING | FORMULAS OF ACIDS
+          |        | WEIGHT |         | POINT   |
+__________|________|________|_________|_________|___________________
+          |        |        |         |         |
+Manganese |   Mn   |  55.0  |  8.01   |  1900°  | H_{2}MnO_{4} and
+          |        |        |         |         | HMnO_{4}
+Chromium  |   Cr   |  52.1  |  7.3    |  3000°  | H_{2}CrO_{4} and
+          |        |        |         |         | H_{2}Cr_{2}O_{7}
+====================================================================
+
+~General.~ Manganese and chromium, while belonging to different families,
+have so many features in common in their chemical conduct that they may
+be studied together with advantage. They differ from most of the
+elements so far studied in that they can act either as acid-forming or
+base-forming elements. As base-forming elements each of the metals forms
+two series of salts. In the one series, designated by the suffix "ous,"
+the metal is divalent; in the other series, designated by the suffix
+"ic," the metal is trivalent. Only the manganous and the chromic salts,
+however, are of importance. The acids in which these elements play the
+part of a non-metal are unstable, but their salts are usually stable,
+and some of them are important compounds.
+
+
+MANGANESE
+
+~Occurrence.~ Manganese is found in nature chiefly as the dioxide MnO_{2},
+called pyrolusite. In smaller amounts it occurs as the oxides
+Mn_{2}O_{3} and Mn_{3}O_{4}, and as the carbonate MnCO_{3}. Some iron
+ores also contain manganese.
+
+~Preparation and properties.~ The element is difficult to prepare in pure
+condition and has no commercial applications. It can be prepared,
+however, by reducing the oxide with aluminium powder or by the use of
+the electric furnace, with carbon as the reducing agent. The metal
+somewhat resembles iron in appearance, but is harder, less fusible, and
+more readily acted upon by air and moisture. Acids readily dissolve it,
+forming manganous salts.
+
+~Oxides of manganese.~ The following oxides of manganese are known: MnO,
+Mn_{2}O_{3}, Mn_{3}O_{4}, MnO_{2}, and Mn_{2}O_{7}. Only one of these,
+the dioxide, needs special mention.
+
+~Manganese dioxide~ (_pyrolusite_) (MnO_{2}). This substance is the most
+abundant manganese compound found in nature, and is the ore from which
+all other compounds of manganese are made. It is a hard, brittle, black
+substance which is valuable as an oxidizing agent. It will be recalled
+that it is used in the preparation of chlorine and oxygen, in
+decolorizing glass which contains iron, and in the manufacture of
+ferromanganese.
+
+~Compounds containing manganese as a base-forming element.~ As has been
+stated previously, manganese forms two series of salts. The most
+important of these salts, all of which belong to the manganous series,
+are the following:
+
+    Manganous chloride           MnCl_{2}·4H_{2}O.
+    Manganous sulphide           MnS.
+    Manganous sulphate           MnSO_{4}·4H_{2}O.
+    Manganous carbonate          MnCO_{3}.
+    Manganous hydroxide          Mn(OH)_{2}.
+
+The chloride and sulphate may be prepared by heating the dioxide with
+hydrochloric and sulphuric acids respectively:
+
+    MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + 2Cl,
+
+    MnO_{2} + H_{2}SO_{4} = MnSO_{4} + H_{2}O + O.
+
+The sulphide, carbonate, and hydroxide, being insoluble, may be prepared
+from a solution of the chloride or sulphate by precipitation with the
+appropriate reagents. Most of the manganous salts are rose colored. They
+not only have formulas similar to the ferrous salts, but resemble them
+in many of their chemical properties.
+
+~Compounds containing manganese as an acid-forming element.~ Manganese
+forms two unstable acids, namely, manganic acid and permanganic acid.
+While these acids are of little interest, some of their salts,
+especially the permanganates, are important compounds.
+
+~Manganic acid and manganates.~ When manganese dioxide is fused with an
+alkali and an oxidizing agent a green compound is formed. The equation,
+when caustic potash is used, is as follows:
+
+    MnO_{2} + 2KOH + O = K_{2}MnO_{4} + H_{2}O.
+
+The green compound (K_{2}MnO_{4}) is called potassium manganate, and is
+a salt of the unstable manganic acid (H_{2}MnO_{4}). The manganates are
+all very unstable.
+
+~Permanganic acid and the permanganates.~ When carbon dioxide is passed
+through a solution of a manganate a part of the manganese is changed
+into manganese dioxide, while the remainder forms a salt of the unstable
+acid HMnO_{4}, called permanganic acid. The equation is
+
+    3K_{2}MnO_{4} + 2CO_{2} = MnO_{2} + 2KMnO_{4} + 2K_{2}CO_{3}.
+
+Potassium permanganate (KMnO_{4}) crystallizes in purple-black needles
+and is very soluble in water, forming an intensely purple solution. All
+other permanganates, as well as permanganic acid itself, give solutions
+of the same color.
+
+~Oxidizing properties of the permanganates.~ The permanganates are
+remarkable for their strong oxidizing properties. When used as an
+oxidizing agent the permanganate is itself reduced, the exact character
+of the products formed from it depending upon whether the oxidation
+takes place (1) in an alkaline or neutral solution, or (2) in an acid
+solution.
+
+1. _Oxidation in alkaline or neutral solution._ When the solution is
+either alkaline or neutral the potassium and the manganese of the
+permanganate are both converted into hydroxides, as shown in the
+equation
+
+    2KMnO_{4} + 5H_{2}O = 2Mn(OH)_{4} + 2KOH + 3O.
+
+2. _Oxidation in acid solution._ When free acid such as sulphuric is
+present, the potassium and the manganese are both changed into salts of
+the acid:
+
+    2KMnO_{4} + 3H_{2}SO_{4} = K_{2}SO_{4} + 2MnSO_{4} + 3H_{2}O + 5O.
+
+Under ordinary conditions, however, neither one of these reactions takes
+place except in the presence of a third substance which is capable of
+oxidation. The oxygen is not given off in the free state, as the
+equations show, but is used up in effecting oxidation.
+
+Potassium permanganate is particularly valuable as an oxidizing agent
+not only because it acts readily either in acid or in alkaline solution,
+but also because the reaction takes place so easily that often it is not
+even necessary to heat the solution to secure action. The substance
+finds many uses in the laboratory, especially in analytical work. It is
+also used as an antiseptic as well as a disinfectant.
+
+
+CHROMIUM
+
+~Occurrence.~ The ore from which all chromium compounds are made is
+chromite, or chrome iron ore (FeCr_{2}O_{4}). This is found most
+abundantly in New Caledonia and Turkey. The element also occurs in small
+quantities in many other minerals, especially in crocoisite (PbCrO_{4}),
+in which mineral it was first discovered.
+
+~Preparation.~ Chromium, like manganese, is very hard to reduce from its
+ores, owing to its great affinity for oxygen. It can, however, be made
+by the same methods which have proved successful with manganese.
+Considerable quantities of an alloy of chromium with iron, called
+ferrochromium, are now produced for the steel industry.
+
+~Properties.~ Chromium is a very hard metal of about the same density as
+iron. It is one of the most infusible of the metals, requiring a
+temperature little short of 3000° for fusion. At ordinary temperatures
+air has little action on it; at higher temperatures, however, it burns
+brilliantly. Nitric acid has no action on it, but hydrochloric and
+dilute sulphuric acids dissolve it, liberating hydrogen.
+
+~Compounds containing chromium as a base-forming element.~ While chromium
+forms two series of salts, chromous salts are difficult to prepare and
+are of little importance. The most important of the chromic series are
+the following:
+
+    Chromic hydroxide       Cr(OH)_{3}.
+    Chromic chloride        CrCl_{3}·6H_{2}O.
+    Chromic sulphate        Cr_{2}(SO_{4})_{3}.
+    Chrome alums
+
+~Chromic hydroxide~ (Cr(OH)_{3}). This substance, being insoluble, can be
+obtained by precipitating a solution of the chloride or sulphate with a
+soluble hydroxide. It is a greenish substance which, like aluminium
+hydroxide, dissolves in alkalis, forming soluble salts.
+
+     ~Dehydration of chromium hydroxide.~ When heated gently chromic
+     hydroxide loses a part of its oxygen and hydrogen, forming the
+     substance CrO·OH, which, like the corresponding aluminium
+     compound, has more pronounced acid properties than the
+     hydroxide. It forms a series of salts very similar to the
+     spinels; chromite is the ferrous salt of this acid, having the
+     formula Fe(CrO_{2})_{2}. When heated to a higher temperature
+     chromic hydroxide is completely dehydrated, forming the
+     trioxide Cr_{2}O_{3}. This resembles the corresponding oxides
+     of aluminium and iron in many respects. It is a bright green
+     powder, and when ignited strongly becomes almost insoluble in
+     acids, as is also the case with aluminium oxide.
+
+~Chromic sulphate~ (Cr_{2}(SO_{4})_{3}). This compound is a violet-colored
+solid which dissolves in water, forming a solution of the same color.
+This solution, however, turns green on heating, owing to the formation
+of basic salts. Chromic sulphate, like ferric and aluminium sulphates,
+unites with the sulphates of the alkali metals to form alums, of which
+the best known are potassium chrome alum (KCr(SO_{4})_{2}·12H_{2}O) and
+ammonium chrome alum (NH_{4}Cr(SO_{4})_{2}·12H_{2}O).
+
+These form beautiful dark purple crystals and have some practical uses
+in the tanning industry and in photography. A number of the salts of
+chromium are also used in the dyeing industry, for they hydrolyze like
+aluminium salts and the hydroxide forms a good mordant.
+
+     ~Hydrolysis of chromium salts.~ When ammonium sulphide is added
+     to a solution of a chromium salt, such as the sulphate,
+     chromium hydroxide precipitates instead of the sulphide. This
+     is due to the fact that chromic sulphide, like aluminium
+     sulphide, hydrolyzes in the presence of water, forming chromic
+     hydroxide and hydrosulphuric acid. Similarly, a soluble
+     carbonate precipitates a basic carbonate of chromium.
+
+~Compounds containing chromium as an acid-forming element.~ Like
+manganese, chromium forms two unstable acids, namely, chromic acid and
+dichromic acid. Their salts, the chromates and dichromates, are
+important compounds.
+
+~Chromates.~ When a chromium compound is fused with an alkali and an
+oxidizing agent a chromate is produced. When potassium hydroxide is used
+as the alkali the equation is
+
+    2Cr(OH)_{3} + 4KOH + 3O = 2K_{2}CrO_{4} + 5H_{2}O.
+
+This reaction recalls the formation of a manganate under similar
+conditions.
+
+~Properties of chromates.~ The chromates are salts of the unstable chromic
+acid (H_{2}CrO_{4}), and as a rule are yellow in color. Lead chromate
+(PbCrO_{4}) is the well-known pigment chrome yellow. Most of the
+chromates are insoluble and can therefore be prepared by precipitation.
+Thus, when a solution of potassium chromate is added to solutions of
+lead nitrate and barium nitrate respectively, the reactions expressed by
+the following equations occur:
+
+    Pb(NO_{3})_{2} + K_{2}CrO_{4} = PbCrO_{4} + 2KNO_{3},
+
+    Ba(NO_{3})_{2} + K_{2}CrO_{4} = BaCrO_{4} + 2KNO_{3}.
+
+The chromates of lead and barium separate as yellow precipitates. The
+presence of either of these two metals can be detected by taking
+advantage of these reactions.
+
+~Dichromates.~ When potassium chromate is treated with an acid the
+potassium salt of the unstable dichromic acid (H_{2}Cr_{2}O_{7}) is
+formed:
+
+    2K_{2}CrO_{4} + H_{2}SO_{4} = K_{2}Cr_{2}O_{7} + K_{2}SO_{4} + H_{2}O.
+
+The relation between the chromates and dichromates is the same as that
+between the phosphates and the pyrophosphates. Potassium dichromate
+might therefore be called potassium pyrochromate.
+
+~Potassium dichromate~ (K_{2}Cr_{2}O_{7}). This is the best known
+dichromate, and is the most familiar chromium compound. It forms large
+crystals of a brilliant red color, and is rather sparingly soluble in
+water. When treated with potassium hydroxide it is converted into the
+chromate
+
+    K_{2}Cr_{2}O_{7} + 2KOH = 2K_{2}CrO_{4} + H_{2}O.
+
+When added to a solution of lead or barium salt the corresponding
+chromates (not dichromates) are precipitated. With barium nitrate the
+equation is
+
+    2Ba(NO_{3})_{2} + K_{2}Cr_{2}O_{7} + H_{2}O
+        = 2BaCrO_{4} + 2KNO_{3} + 2HNO_{3}.
+
+Potassium dichromate finds use in many industries as an oxidizing agent,
+especially in the preparation of organic substances, such as the dye
+alizarin, and in the construction of several varieties of electric
+batteries.
+
+     ~Sodium chromates.~ The reason why the potassium salt rather than
+     the sodium compound is used is that sodium chromate and
+     dichromate are so soluble that it is hard to prepare them pure.
+     This difficulty is being overcome now, and the sodium compounds
+     are replacing the corresponding potassium salts. This is of
+     advantage, since a sodium salt is cheaper than a potassium
+     salt, so far as raw materials go.
+
+~Oxidizing action of chromates and dichromates.~ When a dilute solution of
+a chromate or dichromate is acidified with an acid, such as sulphuric
+acid, no reaction apparently takes place. However, if there is present a
+third substance capable of oxidation, the chromium compound gives up a
+portion of its oxygen to this substance. Since the chromate changes into
+a dichromate in the presence of an acid, it will be sufficient to study
+the action of the dichromates alone. The reaction takes place in two
+steps. Thus, when a solution of ferrous sulphate is added to a solution
+of potassium dichromate acidified with sulphuric acid, the reaction is
+expressed by the following equations:
+
+    (1) K_{2}Cr_{2}O_{7} + 4H_{2}SO_{4}
+           = K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 4H_{2}O + 3O,
+
+    (2) 6FeSO_{4} + 3H_{2}SO_{4} + 3O = 3Fe_{2}(SO_{4})_{3} + 3H_{2}O.
+
+The dichromate decomposes in very much the same way as a permanganate
+does, the potassium and chromium being both changed into salts in which
+they play the part of metals, while part of the oxygen of the dichromate
+is liberated.
+
+By combining equations (1) and (2), the following is obtained:
+
+    K_{2}Cr_{2}O_{7} + 7H_{2}SO_{4} + 6FeSO_{4}
+       = K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 3Fe_{2}(SO_{4})_{3} + 7H_{2}0.
+
+This reaction is often employed in the estimation of iron in iron ores.
+
+     ~Potassium chrome alum.~ It will be noticed that the oxidizing
+     action of potassium dichromate leaves potassium sulphate and
+     chromium sulphate as the products of the reaction. On
+     evaporating the solution these substances crystallize out as
+     potassium chrome alum, which substance is produced as a
+     by-product in the industries using potassium dichromate for
+     oxidizing purposes.
+
+~Chromic anhydride~ (CrO_{3}). When concentrated sulphuric acid is added
+to a strong solution of potassium dichromate, and the liquid allowed to
+stand, deep red needle-shaped crystals appear which have the formula
+CrO_{3}.This oxide of chromium is called chromic anhydride, since it
+combines readily with water to form chromic acid:
+
+    CrO_{3} + H_{2}O = H_{2}CrO_{4}.
+
+It is therefore analogous to sulphur trioxide which forms sulphuric acid
+in a similar way:
+
+    SO_{3} + H_{2}O = H_{2}SO_{4}.
+
+Chromic anhydride is a very strong oxidizing agent, giving up oxygen and
+forming chromic oxide:
+
+    2CrO_{3} = Cr_{2}O_{3} + 3O.
+
+     ~Rare elements of the family.~ Molybdenum, tungsten, and uranium
+     are three rather rare elements belonging in the same family
+     with chromium, and form many compounds which are similar in
+     formulas to the corresponding compounds of chromium. They can
+     play the part of metals and also form acids resembling chromic
+     acid in formula. Thus we have molybdic acid (H_{2}MoO_{4}), the
+     ammonium salt of which is (NH_{4})_{2}MoO_{4}. This salt has
+     the property of combining with phosphoric acid to form a very
+     complex substance which is insoluble in nitric acid. On this
+     account molybdic acid is often used in the estimation of the
+     phosphoric acid present in a substance. Like chromium, the
+     metals are difficult to prepare in pure condition. Alloys with
+     iron can be prepared by reducing the mixed oxides with carbon
+     in an electric furnace; these alloys are used to some extent in
+     preparing special kinds of steel.
+
+
+EXERCISES
+
+1. How does pyrolusite effect the decolorizing of glass containing iron?
+
+2. Write the equations for the preparation of manganous chloride,
+carbonate, and hydroxide.
+
+3. Write the equations representing the reactions which take place when
+ferrous sulphate is oxidized to ferric sulphate by potassium
+permanganate in the presence of sulphuric acid.
+
+4. In the presence of sulphuric acid, oxalic acid is oxidized by
+potassium permanganate according to the equation
+
+    C_{2}H_{2}O_{4} + O = 2CO_{2} + H_{2}O.
+
+Write the complete equation.
+
+5. 10 g. of iron were dissolved in sulphuric acid and oxidized to ferric
+sulphate by potassium permanganate. What weight of the permanganate was
+required?
+
+6. What weight of ferrochromium containing 40% chromium must be added to
+a ton of steel to produce an alloy containing 1% of chromium?
+
+7. Write the equation representing the action of ammonium sulphide upon
+chromium sulphate.
+
+8. Potassium chromate oxidizes hydrochloric acid, forming chlorine.
+Write the complete equation.
+
+9. Give the action of sulphuric acid on potassium dichromate (a) in
+the presence of a large amount of water; (b) in the presence of a
+small amount of water.
+
+
+
+
+CHAPTER XXXI
+
+GOLD AND THE PLATINUM FAMILY
+
+
+==============================================================================
+          |        |        |         |              |          |
+          |        | ATOMIC |         | HIGHEST      | HIGHEST  | MELTING
+          | SYMBOL | WEIGHT | DENSITY |  OXIDE       | CHLORIDE |  POINT
+__________|________|________|_________|_________     |__________|_____________
+          |        |        |         |              |          |
+Ruthenium |   Ru   |  101.7 |  12.26  | RuO_{4}      | RuCl_{4} | Electric arc
+Rhodium   |   Rh   |  103.  |  12.1   | RhO_{2}      | RhCl_{2} | Electric arc
+Palladium |   Pd   |  106.5 |  11.8   | PdO_{2}      | PdCl_{4} |   1500°
+Iridium   |   Ir   |  193.  |  22.42  | IrO_{2}      | IrCl_{4} |   1950°
+Osmium    |   Os   |  191.  |  22.47  | OsO_{4}      | OsCl_{4} | Electric arc
+Platinum  |   Pt   |  194.8 |  21.50  | PtO_{2}      | PtCl_{4} |   1779°
+Gold      |   Au   |  197.2 |  19.30  | Au_{2}O_{3}  | AuCl_{3} |   1064°
+==============================================================================
+
+~The family.~ Following iron, nickel, and cobalt in the eighth column of
+the periodic table are two groups of three elements each. The metals of
+the first of these groups--ruthenium, rhodium, and palladium--have
+atomic weights near 100 and densities near 12. The metals of the other
+group--iridium, osmium, and platinum--have atomic weights near 200 and
+densities near 21. These six rare elements have very similar physical
+properties and resemble each other chemically not only in the type of
+compounds which they form but also in the great variety of them. They
+occur closely associated in nature, usually as alloys of platinum in the
+form of irregular metallic grains in sand and gravel. Platinum is by far
+the most abundant of the six.
+
+Although the periodic classification assigns gold to the silver-copper
+group, its physical as well as many of its chemical properties much
+more closely resemble those of the platinum metals, and it can he
+conveniently considered along with them. The four elements gold,
+platinum, osmium, and iridium are the heaviest substances known, being
+about twice as heavy as lead.
+
+
+PLATINUM
+
+~Occurrence.~ About 90% of the platinum of commerce comes from Russia,
+small amounts being produced in California, Brazil, and Australia.
+
+~Preparation.~ Native platinum is usually alloyed with gold and the
+platinum metals. To separate the platinum the alloy is dissolved in aqua
+regia, which converts the platinum into chloroplatinic acid
+(H_{2}PtCl_{6}). Ammonium chloride is then added, which precipitates the
+platinum as insoluble ammonium chloroplatinate:
+
+    H_{2}PtCl_{6} + 2NH_{4}Cl = (NH_{4})_{2}PtCl_{6} + 2HCl.
+
+Some iridium is also precipitated as a similar compound. On ignition the
+double chloride is decomposed, leaving the platinum as a spongy metallic
+mass, which is melted in an electric furnace and rolled or hammered into
+the desired shape.
+
+~Physical properties.~ Platinum is a grayish-white metal of high luster,
+and is very malleable and ductile. It melts in the oxyhydrogen blowpipe
+and in the electric furnace; it is harder than gold and is a good
+conductor of electricity. In finely divided form it has the ability to
+absorb or occlude gases, especially oxygen and hydrogen. These gases,
+when occluded, are in a very active condition resembling the nascent
+state, and can combine with each other at ordinary temperatures. A jet
+of hydrogen or coal gas directed upon spongy platinum is at once
+ignited.
+
+     ~Platinum as a catalytic agent.~ Platinum is remarkable for its
+     property of acting as a catalytic agent in a large number of
+     chemical reactions, and mention has been made of this use of
+     the metal in connection with the manufacture of sulphuric acid.
+     When desired for this purpose some porous or fibrous substance,
+     such as asbestos, is soaked in a solution of platinic chloride
+     and then ignited. The platinum compound is decomposed and the
+     platinum deposited in very finely divided form. Asbestos
+     prepared in this way is called platinized asbestos. The
+     catalytic action seems to be in part connected with the
+     property of absorbing gases and rendering them nascent. Some
+     other metals possess this same power, notably palladium, which
+     is remarkable for its ability to absorb hydrogen.
+
+~Chemical properties.~ Platinum is a very inactive element chemically, and
+is not attacked by any of the common acids. Aqua regia slowly dissolves
+it, forming platinic chloride (PtCl_{4}), which in turn unites with the
+hydrochloric acid present in the aqua regia, forming the compound
+chloroplatinic acid (H_{2}PtCl_{6}). Platinum is attacked by fused
+alkalis. It combines at higher temperatures with carbon and phosphorus
+and alloys with many metals. It is readily attacked by chlorine but not
+by oxidizing agents.
+
+~Applications.~ Platinum is very valuable as a material for the
+manufacture of chemical utensils which are required to stand a high
+temperature or the action of strong reagents. Platinum crucibles,
+dishes, forceps, electrodes, and similar articles are indispensable in
+the chemical laboratory. In the industries it is used for such purposes
+as the manufacture of pans for evaporating sulphuric acid, wires for
+sealing through incandescent light bulbs, and for making a great variety
+of instruments. Unfortunately the supply of the metal is very limited,
+and the cost is steadily advancing, so that it is now more valuable than
+gold.
+
+~Compounds.~ Platinum forms two series of salts of which platinous
+chloride (PtCl_{2}) and platinic chloride (PtCl_{4}) are examples.
+Platinates are also known. While a great variety of compounds of
+platinum have been made, the substance is chiefly employed in the
+metallic state.
+
+~Platinic chloride (PtCl_{4}).~ Platinic chloride is an orange-colored,
+soluble compound made by heating chloroplatinic acid in a current of
+chlorine. If hydrochloric acid is added to a solution of the substance,
+the two combine, forming chloroplatinic acid (H_{2}PtCl_{6}):
+
+    2HCl + PtCl_{4} = H_{2}PtCl_{6}.
+
+The potassium and ammonium salts of this acid are nearly insoluble in
+water and alcohol. The acid is therefore used as a reagent to
+precipitate potassium in analytical work. With potassium chloride the
+equation is
+
+    2KCl + H_{2}PtCl_{6} = K_{2}PtCl_{6} + 2HCl.
+
+     ~Other metals of the family.~ The other members of the family
+     have few applications. Iridium is used in the form of a
+     platinum alloy, since the alloy is much harder than pure
+     platinum and is even less fusible. This alloy is sometimes used
+     to point gold pens. Osmium tetroxide (OsO_{4}) is a very
+     volatile liquid and is used under the name of osmic acid as a
+     stain for sections in microscopy.
+
+
+GOLD
+
+~Occurrence.~ Gold has been found in many localities, the most famous
+being South Africa, Australia, Russia, and the United States. In this
+country it is found in Alaska and in nearly half of the states of the
+union, notably in California, Colorado, and Nevada. It is usually found
+in the native condition, frequently alloyed with silver; in combination
+it is sometimes found as telluride (AuTe_{2}), and in a few other
+compounds.
+
+~Mining.~ Native gold occurs in the form of small grains or larger nuggets
+in the sands of old rivers, or imbedded in quartz veins in rocks. In the
+first case it is obtained in crude form by placer mining. The sand
+containing the gold is shaken or stirred in troughs of running waters
+called sluices. This sweeps away the sand but allows the heavier gold to
+sink to the bottom of the sluice. Sometimes the sand containing the gold
+is washed away from its natural location into the sluices by powerful
+streams of water delivered under pressure from pipes. This is called
+hydraulic mining. In vein mining the gold-bearing quartz is mined from
+the veins, stamped into fine powder in stamping mills, and the gold
+extracted by one of the processes to be described.
+
+~Extraction.~ 1. _Amalgamation process._ In the amalgamation process the
+powder containing the gold is washed over a series of copper plates
+whose surfaces have been amalgamated with mercury. The gold sticks to
+the mercury or alloys with it, and after a time the gold and mercury are
+scraped off and the mixture is distilled. The mercury distills off and
+the gold is left in the retort ready for refining.
+
+2. _Chlorination process._ When gold occurs along with metallic
+sulphides it is often extracted by chlorination. The ore is first
+roasted, and is then moistened and treated with chlorine. This dissolves
+the gold but not the metallic oxides:
+
+    Au + 3Cl = AuCl_{3}.
+
+The gold chloride, being soluble, is extracted from the mixture with
+water, and the gold is precipitated from the solution, usually by adding
+ferrous sulphate:
+
+    AuCl_{3} + 3FeSO_{4} = Au + FeCl_{3} + Fe_{2}(SO_{4})_{3}.
+
+3. _Cyanide process._ This process depends upon the fact that gold is
+soluble in a solution of potassium cyanide in the presence of the oxygen
+of the air. The powder from the stamping mills is treated with a very
+dilute potassium cyanide solution which extracts the gold:
+
+    2Au + 4KCN + H_{2}O + O = 2KOH + 2KAu(CN)_{2}.
+
+From this solution the gold can be obtained by electrolysis or by
+precipitation with metallic zinc:
+
+    2KAu(CN)_{2} + Zn = K_{2}Zn(CN)_{4} + 2Au.
+
+~Refining of gold.~ Gold is refined by three general methods:
+
+1. _Electrolysis._ When gold is dissolved in a solution of potassium
+cyanide, and the solution electrolyzed, the gold is deposited in very
+pure condition on the cathode.
+
+2. _Cupellation._ When the gold is alloyed with easily oxidizable
+metals, such as copper or lead, it may be refined by cupellation. The
+alloy is fused with an oxidizing flame on a shallow hearth made of bone
+ash, which substance has the property of absorbing metallic oxides but
+not the gold. Any silver which may be present remains alloyed with the
+gold.
+
+3. _Parting with sulphuric acid._ Gold may be separated from silver, as
+well as from many other metals, by heating the alloy with concentrated
+sulphuric acid. This dissolves the silver, while the gold is not
+attacked.
+
+~Physical properties.~ Gold is a very heavy bright yellow metal,
+exceedingly malleable and ductile, and a good conductor of electricity.
+It is quite soft and is usually alloyed with copper or silver to give it
+the hardness required for most practical uses. The degree of fineness is
+expressed in terms of carats, pure gold being twenty-four carats; the
+gold used for jewelry is usually eighteen carats, eighteen parts being
+gold and six parts copper or silver. Gold coinage is 90% gold and 10%
+copper.
+
+~Chemical properties.~ Gold is not attacked by any one of the common
+acids; aqua regia easily dissolves it, forming gold chloride (AuCl_{3}),
+which in turn combines with hydrochloric acid to form chlorauric acid
+(HAuCl_{4}). Fused alkalis also attack it. Most oxidizing agents are
+without action upon it, and in general it is not an active element.
+
+     ~Compounds.~ The compounds of gold, though numerous and varied in
+     character, are of comparatively little importance and need not
+     be described in detail. The element forms two series of salts
+     in which it acts as a metal: in the aurous series the gold is
+     univalent, the chloride having the formula AuCl; in the auric
+     series it is trivalent, auric chloride having the formula
+     AuCl_{3}. Gold also acts as an acid-forming element, forming
+     such compounds as potassium aurate (KAuO_{2}). Its compounds
+     are very easily decomposed, however, metallic gold separating
+     from them.
+
+
+EXERCISES
+
+1. From the method of preparation of platinum, what metal is likely to
+be alloyed with it?
+
+2. The "platinum chloride" of the laboratory is made by dissolving
+platinum in aqua regia. What is the compound?
+
+3. How would you expect potassium aurate and platinate to be formed?
+What precautions would this suggest in the use of platinum vessels?
+
+4. Why must gold ores be roasted in the chlorination process?
+
+
+
+
+CHAPTER XXXII
+
+SOME SIMPLE ORGANIC COMPOUNDS
+
+
+~Division of chemistry into organic and inorganic.~ Chemistry is usually
+divided into two great divisions,--organic and inorganic. The original
+significance of these terms was entirely different from the meaning
+which they have at the present time.
+
+1. _Original significance._ The division into organic and inorganic was
+originally made because it was believed that those substances which
+constitute the essential parts of living organisms were built up under
+the influence of the life force of the organism. Such substances,
+therefore, should be regarded as different from those compounds prepared
+in the laboratory or formed from the inorganic or mineral constituents
+of the earth. In accordance with this view organic chemistry included
+those substances formed by living organisms. Inorganic chemistry, on the
+other hand, included all substances formed from the mineral portions of
+the earth.
+
+In 1828 the German chemist Wöhler prepared urea, a typical organic
+compound, from inorganic materials. The synthesis of other so-called
+organic compounds followed, and at present it is known that the same
+chemical laws apply to all substances whether formed in the living
+organism or prepared in the laboratory from inorganic constituents. The
+terms "organic" and "inorganic" have therefore lost their original
+significance.
+
+2. _Present significance._ The great majority of the compounds found in
+living organisms contain carbon, and the term "organic chemistry," as
+used at present, includes not only these compounds but all compounds of
+carbon. _Organic chemistry_ has become, therefore, _the chemistry of the
+compounds of carbon_, all other substances being treated under the head
+of inorganic chemistry. This separation of the compounds of carbon into
+a group by themselves is made almost necessary by their great number,
+over one hundred thousand having been recorded. For convenience some of
+the simpler carbon compounds, such as the oxides and the carbonates, are
+usually discussed in inorganic chemistry.
+
+~The grouping of compounds in classes.~ The study of organic chemistry is
+much simplified by the fact that the large number of bodies included in
+this field may be grouped in classes of similar compounds. It thus
+becomes possible to study the properties of each class as a whole, in
+much the same way as we study a group of elements. The most important of
+these classes are the _hydrocarbons_, the _alcohols_, the _aldehydes_,
+the _acids_, the _ethereal salts_, the _ethers_, the _ketones_, the
+_organic bases_, and the _carbohydrates_. A few members of each of these
+classes will now be discussed briefly.
+
+THE HYDROCARBONS
+
+Carbon and hydrogen combine to form a large number of compounds. These
+compounds are known collectively as the _hydrocarbons_. They may be
+divided into a number of groups or series, each being named from its
+first member. Some of the groups are as follows:
+
+    METHANE SERIES
+    CH_{4}      methane
+    C_{2}H_{6}  ethane
+    C_{3}H_{8}  propane
+    C_{4}H_{10} butane
+    C_{5}H_{12} pentane
+    C_{6}H_{14} hexane
+    C_{7}H_{16} heptane
+    C_{8}H_{18} octane
+
+    ETHYLENE SERIES
+    C_{2}H_{4} ethylene
+    C_{3}H_{6} propylene
+    C_{4}H_{8} butylene
+
+    BENZENE SERIES
+    C_{6}H_{6}  benzene
+    C_{7}H_{8}  toluene
+    C_{8}H_{10} xylene
+
+    ACETYLENE SERIES
+    C_{2}H_{2} acetylene
+    C_{3}H_{4} allylene
+
+Only the lower members (that is, those which contain a small number of
+carbon atoms) of the above groups are given. The methane series is the
+most extensive, all of the compounds up to C_{24}H_{50} being known.
+
+It will be noticed that the successive members of each of the above
+series differ by the group of atoms (CH_{2}). Such a series is called an
+_homologous series_. In general, it may be stated that the members of an
+homologous series show a regular gradation in most physical properties
+and are similar in chemical properties. Thus in the methane group the
+first four members are gases at ordinary temperatures; those containing
+from five to sixteen carbon atoms are liquids, the boiling points of
+which increase with the number of carbon atoms present. Those containing
+more than sixteen carbon atoms are solids.
+
+~Sources of the hydrocarbons.~ There are two chief sources of the
+hydrocarbons, namely, (1) crude petroleum and (2) coal tar.
+
+1. _Crude petroleum._ This is a liquid pumped from wells driven into the
+earth in certain localities. Pennsylvania, Ohio, Kansas, California, and
+Texas are the chief oil-producing regions in the United States. The
+crude petroleum consists largely of liquid hydrocarbons in which are
+dissolved both gaseous and solid hydrocarbons. Before being used it must
+be refined. In this process the petroleum is run into large iron stills
+and subjected to fractional distillation. The various hydrocarbons
+distill over in the general order of their boiling points. The
+distillates which collect between certain limits of temperature are kept
+separate and serve for different uses; they are further purified,
+generally by washing with sulphuric acid, then with an alkali, and
+finally with water. Among the products obtained from crude petroleum in
+this way are the naphthas, including benzine and gasoline, kerosene or
+coal oil, lubricating oils, vaseline, and paraffin. None of these
+products are definite chemical compounds, but each consists of a mixture
+of hydrocarbons, the boiling points of which lie within certain limits.
+
+2. _Coal tar._ This product is obtained in the manufacture of coal gas,
+as already explained. It is a complex mixture and is refined by the same
+general method used in refining crude petroleum. The principal
+hydrocarbons obtained from the coal tar are benzene, toluene,
+naphthalene, and anthracene. In addition to the hydrocarbons, coal tar
+contains many other compounds, such as carbolic acid and aniline.
+
+~Properties of the hydrocarbons.~ The lower members of the first two
+series of hydrocarbons mentioned are all gases; the succeeding members
+are liquids. In some series, as the methane series, the higher members
+are solids. The preparation and properties of methane and acetylene have
+been discussed in a previous chapter. Ethylene is present in small
+quantities in coal gas and may be obtained in the laboratory by
+treating alcohol (C_{2}H_{6}O) with sulphuric acid:
+
+    C_{2}H_{6}O = C_{2}H_{4} + H_{2}O.
+
+Benzene, the first member of the benzene series, is a liquid boiling at
+80°.
+
+The hydrocarbons serve as the materials from which a large number of
+compounds can be prepared; indeed, it has been proposed to call organic
+chemistry _the chemistry of the hydrocarbon derivatives_.
+
+~Substitution products of the hydrocarbons.~ As a rule, at least a part of
+the hydrogen in any hydrocarbon can be displaced by an equivalent amount
+of certain elements or groups of elements. Thus the compounds CH_{3}Cl,
+CH_{2}Cl_{2}, CHCl_{3}, CCl_{4} can be obtained from methane by
+treatment with chlorine. Such compounds are called _substitution
+products_.
+
+~Chloroform~ (CHCl_{3}). This can be made by treating methane with
+chlorine, as just indicated, although a much easier method consists in
+treating alcohol or acetone (which see) with bleaching powder.
+Chloroform is a heavy liquid having a pleasant odor and a sweetish
+taste. It is largely used as a solvent and as an anæsthetic in surgery.
+
+~Iodoform~ (CHI_{3}). This is a yellow crystalline solid obtained by
+treating alcohol with iodine and an alkali. It has a characteristic odor
+and is used as an antiseptic.
+
+
+ALCOHOLS
+
+When such a compound as CH_{3}Cl is treated with silver hydroxide the
+reaction expressed by the following equation takes place:
+
+    CH_{3}Cl + AgOH = CH_{3}OH + AgCl.
+
+Similarly C_{2}H_{5}Cl will give C_{2}H_{5}OH and AgCl. The compounds
+CH_{3}OH and C_{2}H_{5}OH so obtained belong to the class of substances
+known as _alcohols_. From their formulas it will be seen that they may
+be regarded as derived from hydrocarbons by substituting the hydroxyl
+group (OH) for hydrogen. Thus the alcohol CH_{3}OH may be regarded as
+derived from methane (CH_{4}) by substituting the group OH for one atom
+of hydrogen. A great many alcohols are known, and, like the
+hydrocarbons, they may be grouped into series. The relation between the
+first three members of the methane series and the corresponding alcohols
+is shown in the following table:
+
+    CH_{4}     (methane)       CH_{3}OH       (methyl alcohol).
+    C_{2}H_{6} (ethane)        C_{2}H_{5}OH   (ethyl alcohol).
+    C_{3}H_{8} (propane)       C_{3}H_{7}OH   (propyl alcohol).
+
+~Methyl alcohol~ (_wood alcohol_) (CH_{3}OH). When wood is placed in an
+air-tight retort and heated, a number of compounds are evolved, the most
+important of which are the three liquids, methyl alcohol, acetic acid,
+and acetone. Methyl alcohol is obtained entirely from this source, and
+on this account is commonly called _wood alcohol_. It is a colorless
+liquid which has a density of 0.79 and boils at 67°. It burns with an
+almost colorless flame and is sometimes used for heating purposes, in
+place of the more expensive ethyl alcohol. It is a good solvent for
+organic substances and is used especially as a solvent in the
+manufacture of varnishes. It is very poisonous.
+
+~Ethyl alcohol~ (_common alcohol_) (C_{2}H_{5}OH). 1. _Preparation._ This
+compound may be prepared from glucose (C_{6}H_{12}O_{6}), a sugar easily
+obtained from starch. If some baker's yeast is added to a solution of
+glucose and the temperature is maintained at about 30°, bubbles of gas
+are soon evolved, showing that a change is taking place. The yeast
+contains a large number of minute organized bodies, which are really
+forms of plant life. The plant grows in the glucose solution, and in so
+doing secretes a substance known as _zymase_, which breaks down the
+glucose in accordance with the following equation:
+
+    C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}.
+
+     ~Laboratory preparation of alcohol.~ The formation of alcohol and
+     carbon dioxide from glucose may be shown as follows: About 100
+     g. of glucose are dissolved in a liter of water in flask A
+     (Fig. 90). This flask is connected with the bottle B, which
+     is partially filled with limewater. The tube C contains solid
+     sodium hydroxide. A little baker's yeast is now added to the
+     solution in flask A, and the apparatus is connected, as shown
+     in the figure. If the temperature is maintained at about 30°,
+     the reaction soon begins. The bubbles of gas escape through the
+     limewater in B. A precipitate of calcium carbonate soon forms
+     in the limewater, showing the presence of carbon dioxide. The
+     sodium hydroxide in tube C prevents the carbon dioxide in the
+     air from acting on the limewater. The alcohol remains in the
+     flask A and may be separated by fractional distillation.
+
+[Illustration: Fig. 90]
+
+2. _Properties._ Ethyl alcohol is a colorless liquid with a pleasant
+odor. It has a density of 0.78 and boils at 78°. It resembles methyl
+alcohol in its general properties. It is sometimes used as a source of
+heat, since its flame is very hot and does not deposit carbon, as the
+flame from oil does. When taken into the system in small quantities it
+causes intoxication; in larger quantities it acts as a poison. The
+intoxicating properties of such liquors as beer, wine, and whisky are
+due to the alcohol present. Beer contains from 2 to 5% of alcohol, wine
+from 5 to 20%, and whisky about 50%. The ordinary alcohol of the
+druggist contains 94% of alcohol and 6% of water. When this is boiled
+with lime and then distilled nearly all the water is removed, the
+distillate being called _absolute alcohol_.
+
+     ~Commercial preparation of alcohol.~ Alcohol is prepared
+     commercially from starch obtained from corn or potatoes. The
+     starch is first converted into a sugar known as maltose, by the
+     action of _malt_, a substance prepared by moistening barley
+     with water, allowing it to germinate, and then drying it. There
+     is present in the malt a substance known as diastase, which has
+     the property of changing starch into maltose. This sugar, like
+     glucose, breaks down into alcohol and carbon dioxide in the
+     presence of yeast. The resulting alcohol is separated by
+     fractional distillation.
+
+     ~Denatured alcohol.~ The 94% alcohol is prepared at present at a
+     cost of about 35 cents per gallon, which is about half the cost
+     of the preparation of methyl alcohol. The government, however,
+     imposes a tax on all ethyl alcohol which amounts to $2.08 per
+     gallon on the 94% product. This increases its cost to such an
+     extent that it is not economical to use it for many purposes
+     for which it is adapted, such as a solvent in the preparation
+     of paints and varnishes and as a material for the preparation
+     of many important organic compounds. By an act of Congress in
+     1906, the tax was removed from _denatured_ alcohol, that is
+     alcohol mixed with some substance which renders it unfit for
+     the purposes of a beverage but will not impair its use for
+     manufacturing purposes. Some of the European countries have
+     similar laws. The substances ordinarily used to denature
+     alcohol are wood alcohol and pyridine, the latter compound
+     having a very offensive odor.
+
+     ~Fermentation.~ The reaction which takes place in the preparation
+     of ethyl alcohol belongs to the class of changes known under
+     the general name of fermentation. Thus we say that the yeast
+     causes the glucose to ferment, and the process is known as
+     alcoholic fermentation. There are many kinds of fermentations,
+     and each is thought to be due to the presence of a definite
+     substance known as an _enzyme_, which acts by catalysis. In
+     many cases, as in alcoholic fermentation, the change is brought
+     about by the action of minute forms of life. These probably
+     secrete the enzymes which cause the fermentation to take place.
+     Thus the yeast plant is supposed to bring about alcoholic
+     fermentation by secreting the enzyme known as zymase.
+
+~Glycerin~ (C_{3}H_{5}(OH)_{3}). This compound may be regarded as derived
+from propane (C_{3}H_{8}) by displacing three atoms of hydrogen by three
+hydroxyl groups, and must therefore be regarded as an alcohol. It is
+formed in the manufacture of soaps, as will be explained later. It is an
+oily, colorless liquid having a sweetish taste. It is used in medicine
+and in the manufacture of the explosives nitroglycerin and dynamite.
+
+
+ALDEHYDES
+
+When alcohols are treated with certain oxidizing agents two hydrogen
+atoms are removed from each molecule of the alcohol. The resulting
+compounds are known as aldehydes. The relation of the aldehydes derived
+from methyl and ethyl alcohol to the alcohols themselves may be shown as
+follows:
+
+    Alcohols  {CH_{3}OH          Corresponding aldehydes  {CH_{2}O
+              {C_{2}H_{5}OH                               {C_{2}H_{4}O
+
+The first of these (CH_{2}O) is a gas known as formaldehyde. Its aqueous
+solution is largely used as an antiseptic and disinfectant under the
+name of _formalin_. Acetaldehyde (C_{2}H_{4}O) is a liquid boiling at
+21°.
+
+
+ACIDS
+
+Like the other classes of organic compounds, the organic acids may be
+arranged in homologous series. One of the most important of these series
+is the _fatty-acid series_, the name having been given to it because
+the derivatives of certain of its members are constituents of the fats.
+Some of the most important members of the series are given in the
+following table. They are all monobasic, and this fact is expressed in
+the formulas by separating the replaceable hydrogen atom from the rest
+of the molecule:
+
+    H·CHO_{2}            formic acid, a liquid boiling at 100°.
+    H·C_{2}H_{3}O        acetic acid, a liquid boiling at 118°.
+    H·C_{3}H_{5}O_{2}    propionic acid, a liquid boiling at 140°.
+    H·C_{4}H_{7}O_{2}    butyric acid, a liquid boiling at 163°.
+    H·C_{16}H_{31}O_{2}  palmitic acid, a solid melting at 62°.
+    H·C_{18}H_{35}O_{2}  stearic acid, a solid melting at 69°.
+
+~Formic acid~ (H·CHO_{2}). The name "formic" is derived from the Latin
+_formica_, signifying ant. This name was given to the acid because it
+was formerly obtained from a certain kind of ants. It is a colorless
+liquid and occurs in many plants such as the stinging nettles. The
+inflammation caused by the sting of the bee is due to formic acid.
+
+~Acetic acid~ (H·C_{2}H_{3}O_{2}). Acetic acid is the acid present in
+vinegar, the sour taste being due to it. It can be prepared by either of
+the following methods.
+
+1. _Acetic fermentation._ This consists in the change of alcohol into
+acetic acid through the agency of a minute organism commonly called
+mother of vinegar. The change is represented by the following equation:
+
+    C_{2}H_{5}OH + 2O = HC_{2}H_{3}O_{2} + H_{2}O.
+
+The various kinds of vinegars are all made by this process. In the
+manufacture of cider vinegar the sugar present in the cider first
+undergoes alcoholic fermentation; the resulting alcohol then undergoes
+acetic fermentation. The amount of acetic acid present in vinegars
+varies from 3 to 6%.
+
+2. _From the distillation of wood._ The liquid obtained by heating wood
+in the absence of air contains a large amount of acetic acid, and this
+can be separated readily in a pure state. This is the most economical
+method for the preparation of the concentrated acid.
+
+Acetic acid is a colorless liquid and has a strong pungent odor.
+Many of its salts are well-known compounds. Lead acetate
+(Pb(C_{2}H_{3}O_{2})_{2}) is the ordinary _sugar of lead_. Sodium
+acetate (NaC_{2}H_{3}O_{2}) is a white solid largely used in making
+chemical analyses. Copper acetate (Cu(C_{2}H_{3}O_{2})_{2}) is a blue
+solid. When copper is acted upon by acetic acid in the presence of air a
+green basic acetate of copper is formed. This is commonly known as
+verdigris. All acetates are soluble in water.
+
+~Butyric acid~ (H·C_{4}H_{7}O_{2}). Derivatives of butyric acid are
+present in butter and impart to it its characteristic flavor.
+
+~Palmitic and stearic acids.~ Ordinary fats consist principally of
+derivatives of palmitic and stearic acids. When the fats are heated with
+sodium hydroxide the sodium salts of these acids are formed. If
+hydrochloric acid is added to a solution of the sodium salts, the free
+palmitic and stearic acids are precipitated. They are white solids,
+insoluble in water. Stearic acid is often used in making candles.
+
+~Acids belonging to other series.~ In addition to members of the
+fatty-acid series, mention may be made of the following well-known
+acids.
+
+~Oxalic acid~ (H_{2}C_{2}O_{4}). This is a white solid which occurs in
+nature in many plants, such as the sorrels. Its ammonium salt
+((NH_{4})_{2}C_{2}O_{4}) is used as a reagent for the detection of
+calcium. When added to a solution of a calcium compound the white,
+insoluble calcium oxalate (CaC_{2}O_{4}) precipitates.
+
+~Tartaric acid~ (H_{2}·C_{4}H_{4}O_{6}). This compound occurs either in a
+free state or in the form of its salts in many fruits. The potassium
+acid salt (KHC_{4}H_{4}O_{6}) occurs in the juice of grapes. When the
+juice ferments in the manufacture of wine, this salt, being insoluble in
+alcohol, separates out on the sides of the cask and in this form is
+known as argol. This is more or less colored by the coloring matter of
+the grape. When purified it forms a white solid and is sold under the
+name of cream of tartar. The following are also well-known salts of
+tartaric acid: potassium sodium tartrate (Rochelle salt)
+(KNaC_{4}H_{4}O_{6}), potassium antimonyl tartrate (tartar emetic)
+(KSbOC_{4}H_{4}O_{6}).
+
+     ~Cream of tartar baking powders.~ The so-called cream of tartar
+     baking powders consist of a mixture of cream of tartar,
+     bicarbonate of soda, and some starch or flour. When water is
+     added to this mixture the cream of tartar slowly acts upon the
+     soda present liberating carbon dioxide in accordance with the
+     following equation:
+
+     KHC_{4}H_{4}O_{6} + NaHCO_{3} = KNaC_{4}H_{4}O_{6} + H_{2}O +
+     CO_{2}.
+
+     The carbon dioxide evolved escapes through the dough, thus
+     making it light and porous.
+
+~Citric acid~ (H_{3}·C_{6}H_{5}O_{7}). This acid occurs in many fruits,
+especially in lemons. It is a white solid, soluble in water, and is
+often used as a substitute for lemons in making lemonade.
+
+~Lactic acid~ (H·C_{3}H_{5}O_{3}). This is a liquid which is formed in the
+souring of milk.
+
+~Oleic acid~ (H·C_{18}H_{33}O_{2}). The derivatives of this acid
+constitute the principal part of many oils and liquid fats. The acid
+itself is an oily liquid.
+
+
+ETHEREAL SALTS
+
+When acids are brought in contact with alcohols under certain conditions
+a reaction takes place similar to that which takes place between acids
+and bases. The following equations will serve as illustrations:
+
+    KOH + HNO_{3} = KNO_{3} + H_{2}O,
+
+    CH_{3}OH + HNO_{3} = CH_{3}NO_{3} + H_{2}O.
+
+The resulting compounds of which methyl nitrate (CH_{3}NO_{3}) may be
+taken as the type belong to the class known as _ethereal salts_, the
+name having been given them because some of them possess pleasant
+ethereal odors. It will be seen that the ethereal salts differ from
+ordinary salts in that they contain a hydrocarbon radical, such as
+CH_{3}, C_{2}H_{5}, C_{3}H_{5}, in place of a metal.
+
+~The nitrates of glycerin~ (_nitroglycerin_). Nitric acid reacts with
+glycerin in the same way that it reacts with a base containing three
+hydroxyl groups such as Fe(OH)_{3}:
+
+    Fe(OH)_{3} + 3HNO_{3} = Fe(NO_{3})_{3} + 3H_{2}O,
+
+    C_{3}H_{5}(OH)_{3} + 3HNO_{3} = C_{3}H_{5}(NO_{3})_{3} + 3H_{2}O.
+
+The resulting nitrate (C_{3}H_{5}(NO_{3})_{3}) is the main constituent
+of _nitroglycerin_, a slightly yellowish oil characterized by its
+explosive properties. Dynamite consists of porous earth which has
+absorbed nitroglycerin, and its strength depends on the amount present.
+It is used much more largely than nitroglycerin itself, since it does
+not explode so readily by concussion and hence can be transported with
+safety.
+
+~The fats.~ These are largely mixtures of the ethereal salts known
+respectively as olein, palmitin, and stearin. These salts may be
+regarded as derived from oleic, palmitic, and stearic acids
+respectively, by replacing the hydrogen of the acid with the glycerin
+radical C_{3}H_{5}. Since this radical is trivalent and oleic, palmitic,
+and stearic acids contain only one replaceable hydrogen atom to the
+molecule, it is evident that three molecules of each acid must enter
+into each molecule of the ethereal salt. The formulas for the acids and
+the ethereal salts derived from each are as follows:
+
+    HC_{18}H_{33}O_{2}                 (oleic acid)
+    C_{8}H_{6}(C_{18}H_{33}O_{2})_{3}, (olein)
+
+    HC_{16}H_{31}O_{2}                (palmitic acid)
+    C_{3}H_{5}(C_{16}H_{31}0_{2})_{3} (palmitin)
+
+    HC_{18}H_{35}O_{2}                (stearic acid)
+    C_{3}H_{5}(C_{18}H_{35}O_{2})_{3} (stearin)
+
+Olein is a liquid and is the main constituent of liquid fats. Palmitin
+and stearin are solids.
+
+~Butter fat and oleomargarine.~ Butter fat consists principally of olein,
+palmitin, and stearin. The flavor of the fat is due to the presence of a
+small amount of butyrin, which is an ethereal salt of butyric acid.
+Oleomargarine differs from butter mainly in the fact that a smaller
+amount of butyrin is present. It is made from the fats obtained from
+cattle and hogs. This fat is churned up with milk, or a small amount of
+butter is added, in order to furnish sufficient butyrin to impart the
+butter flavor.
+
+~Saponification.~ When an ethereal salt is heated with an alkali a
+reaction expressed by the following equation takes place:
+
+    C_{2}H_{5}NO_{3} + KOH = C_{2}H_{5}OH + KNO_{3}.
+
+This process is known as _saponification_, since it is the one which
+takes place in the manufacture of soaps. The ordinary soaps are made by
+heating fats with a solution of sodium hydroxide. The reactions
+involved may be illustrated by the following equation representing the
+reaction between palmitin and sodium hydroxide:
+
+    C_{3}H_{5}(C_{16}H_{31}O_{2})_{3} + 3 NaOH
+         = 3 NaC_{16}H_{31}O_{2} + C_{3}H_{5}(OH)_{3}.
+
+In accordance with this equation the ethereal salts in the fats are
+converted into glycerin and the sodium salts of the corresponding acids.
+The sodium salts are separated and constitute the soaps. These salts are
+soluble in water. When added to water containing calcium salts the
+insoluble calcium palmitate and stearate are precipitated. Magnesium
+salts act in a similar way. It is because of these facts that soap is
+used up by hard waters.
+
+
+ETHERS
+
+When ethyl alcohol is heated to 140° with sulphuric acid the reaction
+expressed by the following equation takes place:
+
+    2C_{2}H_{5}OH = (C_{2}H_{5})_{2}O + H_{2}O.
+
+The resulting compound, (C_{2}H_{5})_{2}O, is ordinary ether and is the
+most important member of the class of compounds called _ethers_.
+Ordinarily ether is a light, very inflammable liquid boiling at 35°. It
+is used as a solvent for organic substances and as an anæsthetic in
+surgical operations.
+
+
+KETONES
+
+The most common member of this group is acetone (C_{3}H_{6}O), a
+colorless liquid obtained when wood is heated in the absence of air. It
+is used in the preparation of other organic compounds, especially
+chloroform.
+
+
+ORGANIC BASES
+
+This group includes a number of compounds, all of which contain nitrogen
+as well as carbon. They are characterized by combining directly with
+acids to form salts, and in this respect they resemble ammonia. They
+may, indeed, be regarded as derived from ammonia by displacing a part or
+all of the hydrogen present in ammonia by hydrocarbon radicals. Among
+the simplest of these compounds may be mentioned methylamine
+(CH_{3}NH_{2}) and ethylamine (C_{2}H_{5}NH_{2}). These two compounds
+are gases and are formed in the distillation of wood and bones. Pyridine
+(C_{5}H_{6}N) and quinoline (C_{9}H_{7}N) are liquids present in small
+amounts in coal tar, and also in the liquid obtained by the distillation
+of bones. Most of the compounds now classified under the general name of
+_alkaloids_ (which see) also belong to this group.
+
+
+CARBOHYDRATES
+
+The term "carbohydrate" is applied to a class of compounds which
+includes the sugars, starch, and allied bodies These compounds contain
+carbon, hydrogen, and oxygen the last two elements generally being
+present in the proportion in which they combine to form water. The most
+important members of this class are the following:
+
+    Cane sugar      C_{12}H_{22}O_{11}.
+    Milk sugar      C_{12}H_{22}O_{11}.
+    Dextrose        C_{6}H_{12}O_{6}.
+    Levulose        C_{6}H_{12}O_{6}.
+    Cellulose       C_{6}H_{10}O_{5}.
+    Starch          C_{6}H_{10}0_{5}.
+
+~Cane sugar~ (C_{12}H_{22}O_{11}). This is the well-known substance
+commonly called sugar. It occurs in many plants especially in the sugar
+cane and sugar beet. It was formerly obtained almost entirely from the
+sugar cane, but at present the greatest amount of it comes from the
+sugar beet. The juice from the cane or beet contains the sugar in
+solution along with many impurities. These impurities are removed, and
+the resulting solution is then evaporated until the sugar crystallizes
+out. The evaporation is conducted in closed vessels from which the air
+is partially exhausted. In this way the boiling point of the solution is
+lowered and the charring of the sugar is prevented. It is impossible to
+remove all the sugar from the solution. In preparing sugar from sugar
+cane the liquors left after separating as much of it as possible from
+the juice of the cane constitute ordinary molasses. Maple sugar is made
+by the evaporation of the sap obtained from a species of the maple tree.
+Its sweetness is due to the presence of cane sugar, other products
+present in the maple sap imparting the distinctive flavor.
+
+When a solution of cane sugar is heated with hydrochloric or other
+dilute mineral acid, two compounds, dextrose and levulose, are formed in
+accordance with the following equation:
+
+    C_{12}H_{22}O_{11} + H_{2}O = C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}.
+
+This same change is brought about by the action of an enzyme present in
+the yeast plant. When yeast is added to a solution of cane sugar
+fermentation is set up. The cane sugar, however, does not ferment
+directly: the enzyme in the yeast first transforms the sugar into
+dextrose and levulose, and these sugars then undergo alcoholic
+fermentation.
+
+When heated to 160° cane sugar melts; if the temperature is increased to
+about 215°, a partial decomposition takes place and a brown substance
+known as caramel forms. This is used largely as a coloring matter.
+
+~Milk sugar~ (C_{12}H_{22}O_{11}). This sugar is present in the milk of
+all mammals. The average composition of cow's milk is as follows:
+
+    Water                        87.17%
+    Casein (nitrogenous matter)   3.56
+    Butter fat                    3.64
+    Milk sugar                    4.88
+    Mineral matter                0.75
+
+When _rennin_, an enzyme obtained from the stomach of calves, is added
+to milk, the casein separates and is used in the manufacture of cheese.
+The remaining liquid contains the milk sugar which separates on
+evaporation; it resembles cane sugar in appearance but is not so sweet
+or soluble. The souring of milk is due to the fact that the milk sugar
+present undergoes _lactic fermentation_ in accordance with the equation
+
+    C_{12}H_{22}O_{11} + H_{2}O = 4C_{3}H_{6}O_{3}.
+
+The lactic acid formed causes the separation of the casein, thus giving
+the well-known appearance of sour milk.
+
+~Isomeric compounds.~ It will be observed that cane sugar and milk sugar
+have the same formulas. Their difference in properties is due to the
+different arrangement of the atoms in the molecule. Such compounds are
+said to be isomeric. Dextrose and levulose are also isomeric.
+
+~Dextrose~ (_grape sugar, glucose_) (C_{6}H_{12}O_{6}). This sugar is
+present in many fruits and is commonly called grape sugar because of its
+presence in grape juice. It can be obtained by heating cane sugar with
+dilute acids, as explained above; also by heating starch with dilute
+acids, the change being as follows:
+
+    C_{6}H_{10}6_{5} + H_{2}O = C_{6}H_{12}O_{6}.
+
+Pure dextrose is a white crystalline solid, readily soluble in water,
+and is not so sweet as cane sugar. In the presence of yeast it undergoes
+alcoholic fermentation. It is prepared from starch in large quantities,
+and being less expensive than cane sugar, is used as a substitute for it
+in the manufacture of jellies, jams, molasses, candy, and other sweets.
+The product commonly sold under the name of _glucose_ contains about 45%
+of dextrose.
+
+~Levulose~ _(fruit sugar)_(C_{6}H_{12}O_{6}). This sugar is a white solid
+which occurs along with dextrose in fruits and honey. It undergoes
+alcoholic fermentation in the presence of yeast.
+
+~Cellulose~ (C_{6}H_{10}O_{5}). This forms the basis of all woody fibers.
+Cotton and linen are nearly pure cellulose. It is insoluble in water,
+alcohol, and dilute acids. Sulphuric acid slowly converts it into
+dextrose. Nitric acid forms nitrates similar to nitroglycerin in
+composition and explosive properties. These nitrates are variously known
+as nitrocellulose, pyroxylin, and gun cotton. When exploded they yield
+only colorless gases; hence they are used especially in the manufacture
+of smokeless gunpowder. _Collodion_ is a solution of nitrocellulose in a
+mixture of alcohol and ether. _Celluloid_ is a mixture of nitrocellulose
+and camphor. _Paper_ consists mainly of cellulose, the finer grades
+being made from linen and cotton rags, and the cheaper grades from straw
+and wood.
+
+~Starch~ (C_{6}H_{10}O_{5}). This is by far the most abundant carbohydrate
+found in nature, being present especially in seeds and tubers. In the
+United States it is obtained chiefly from corn, nearly 80% of which is
+starch. In Europe it is obtained principally from the potato. It
+consists of minute granules and is practically insoluble in cold water.
+These granules differ somewhat in appearance, according to the source of
+the starch, so that it is often possible to determine from what plant
+the starch was obtained. When heated with water the granules burst and
+the starch partially dissolves. Dilute acids, as well as certain
+enzymes, convert it into dextrose or similar sugars. When seeds
+germinate the starch present is converted into soluble sugars, which are
+used as food for the growing plant.
+
+~Chemical changes in bread making.~ The average composition of wheat flour
+is as follows:
+
+    Water.                           13.8%
+    Protein (nitrogenous matter)      7.9
+    Fats                              1.4
+    Starch                           76.4
+    Mineral matter                    0.5
+
+In making bread the flour is mixed with water and yeast, and the
+resulting dough set aside in a warm place for a few hours. The yeast
+first converts a portion of the starch into dextrose or a similar sugar,
+which then undergoes alcoholic fermentation. The carbon dioxide formed
+escapes through the dough, making it light and porous. The yeast plant
+thrives best at about 30°; hence the necessity for having the dough in a
+warm place. If the temperature rises above 50°, the vitality of the
+yeast is destroyed and fermentation ceases. In baking the bread, the
+heat expels the alcohol and also expands the bubbles of carbon dioxide
+caught in the dough, thus increasing its lightness.
+
+
+SOME DERIVATIVES OF BENZENE
+
+Attention has been called to the complex nature of coal tar. Among the
+compounds present are the hydrocarbons, benzene, toluene, naphthalene,
+and anthracene. These compounds are not only useful in themselves but
+serve for the preparation of many other important compounds known under
+the general name of coal-tar products.
+
+~Nitrobenzene~ (_oil of myrbane_) (C_{6}H_{5}NO_{2}). When benzene is
+treated with nitric acid a reaction takes place which is expressed by
+the following equation:
+
+    C_{6}H_{6} + HNO_{3} = C_{6}H_{5}NO_{2} + H_{2}O.
+
+The product C_{6}H_{5}NO_{2} is called nitrobenzene. It is a slightly
+yellowish poisonous liquid, with a characteristic odor. Its main use is
+in the manufacture of aniline.
+
+~Aniline~ (C_{6}H_{5}NH_{2}). When nitrobenzene is heated with iron and
+hydrochloric acid the hydrogen evolved by the action of the iron upon
+the acid reduces the nitrobenzene in accordance with the following
+equation:
+
+    C_{6}H_{5}NO_{2} + 6H = C_{6}H_{5}NH_{2} + 2H_{2}O.
+
+The resulting compound is known as aniline, a liquid boiling at 182°.
+When first prepared it is colorless, but darkens on standing. Large
+quantities of it are used in the manufacture of the _aniline or coal-tar
+dyes_, which include many important compounds.
+
+~Carbolic acid~ (C_{6}H_{5}OH). This compound, sometimes known as
+_phenol_, occurs in coal tar, and is also prepared from benzene. It
+forms colorless crystals which are very soluble in water. It is strongly
+corrosive and very poisonous.
+
+~Naphthalene and anthracene.~ These are hydrocarbons occurring along with
+benzene in coal tar. They are white solids, insoluble in water. The
+well-known _moth balls_ are made of naphthalene. Large quantities of
+naphthalene are used in the preparation of _indigo_, a dye formerly
+obtained from the indigo plant, but now largely prepared by laboratory
+methods. Similarly anthracene is used in the preparation of the dye
+_alizarin_, which was formerly obtained from the madder root.
+
+
+THE ALKALOIDS
+
+This term is applied to a group of compounds found in many plants and
+trees. They all contain nitrogen, and most of them are characterized by
+their power to combine with acids to form salts. This property is
+indicated by the name alkaloids, which signifies alkali-like. The salts
+are soluble in water, and on this account are more largely used than the
+free alkaloids, which are insoluble in water. Many of the alkaloids are
+used in medicine, some of the more important ones being given below.
+
+~Quinine.~ This alkaloid occurs along with a number of others in the bark
+of certain trees which grow in districts in South America and also in
+Java and other tropical islands. It is a white solid, and its sulphate
+is used in medicine in the treatment of fevers.
+
+~Morphine.~ When incisions are made in the unripe capsules of one of the
+varieties of the poppy plant, a milky juice exudes which soon thickens.
+This is removed and partially dried. The resulting substance is the
+ordinary _opium_ which contains a number of alkaloids, the principal one
+being morphine. This alkaloid is a white solid and is of great service
+in medicine.
+
+Among the other alkaloids may be mentioned the following: _Nicotine_, a
+very poisonous liquid, the salts of which occur in the leaves of the
+tobacco plant; _cocaine_, a crystalline solid present in coca leaves and
+used in medicine as a local anæsthetic; _atropine_, a solid present in
+the berry of the deadly nightshade, and used in the treatment of
+diseases of the eye; _strychnine_, a white, intensely poisonous solid
+present in the seeds of the members of the _Strychnos_ family.
+
+
+
+
+INDEX
+
+
+Acetaldehyde 405
+
+Acetic acid 406
+
+Acetone 411
+
+Acetylene 203
+  series 399
+
+Acids 106
+  binary 113
+  characteristics 106
+  definition 107
+  dibasic 159
+  familiar 106
+  monobasic 159
+  nomenclature 113
+  organic 405
+  preparation 141
+  strength 111
+  ternary 113
+  undissociated 107
+
+Acker furnace, 279
+
+Agate 260
+
+Air 83
+  a mechanical mixture 89
+  carbon dioxide in 87
+  changes in composition 87
+  liquid 91
+  nitrogen in 87
+  oxygen in 85
+  poisonous effects of exhaled 88
+  properties 90
+  quantitative analysis of 85
+  regarded as an element 83
+  standard for density 229
+  water vapor in 87
+
+Alabaster 308
+
+Alchemists 9
+
+Alchemy 9
+
+Alcohol, common 402
+  denatured 404
+  ethyl 402
+  methyl 402
+  wood 402
+
+Alcohols 401
+
+Aldehydes 405
+
+Alizarin 418
+
+Alkali 107, 274
+  family 274
+
+Alkaline-earth family 300
+
+Alkaloids 418
+
+Allotropic forms 22
+
+Alloys 252
+
+Alum 333
+  ammonium 334
+  ammonium chrome 384
+  ammonium iron 352
+  baking powders 335
+  potassium 333
+  potassium chrome 384
+  potassium iron 352
+
+Aluminates 332
+
+Aluminium 327
+  bronze 330, 359
+  chloride 333
+  family 327
+  hydroxide 332
+  metallurgy 328
+  occurrence 327
+  oxide 331
+  preparation 328
+  properties 329
+  silicates 335
+  uses 330
+
+Amalgam 362
+
+Amethyst 260, 331
+
+Ammonia 123
+  composition 127
+  preparation 123
+  properties 124
+  uses 125
+
+Ammonium 126
+  acid carbonate 295
+  carbonate 295
+  chloride 294
+  compounds 294
+
+Ammonium hydrosulphide 296
+  hydroxide 126
+  molybdate 388
+  oxalate 407
+  sulphate 295
+  sulphide 295
+  sulphide, yellow 296
+
+Analysis 40
+
+Anhydride 135
+  carbonic 206
+  chromic 387
+  nitric 135
+  nitrous 135
+  phosphoric 243
+  sulphuric 153
+
+Anhydrite 288
+
+Aniline 417
+
+Anion 106
+
+Anode 99
+
+Anthracene 418
+
+Antimony 250
+  acids 251
+  alloys 253
+  chloride 252
+  metallic properties 252
+  occurrence 251
+  oxides 251
+  preparation 251
+  properties 251
+  sulphides 251
+
+Apatite 175, 239, 311
+
+Aqua ammonia 124
+
+Aqua regia 185
+
+Aqueous tension 25
+
+Argon 80
+
+Arsenic 246
+  acids 250
+  antidote 250
+  Marsh's test 248
+  occurrence 246
+  oxides 249
+  preparation 246
+  properties 247
+  sulphides 250
+  white 249
+
+Arsenopyrites 246
+
+Arsine 247
+
+Asbestos 321, 336
+
+Atmosphere 83
+  constituents 83
+  function of constituents 84
+
+Atomic hypothesis 61
+  theory 59
+    and laws of matter 63
+    and radium 314
+  weights, 65
+    accurate determination 231
+    and general properties 167
+    and specific heats 233
+    calculation of 231
+    Dalton's method 223
+    direct determination 233
+    from molecular weights 230
+    relation to equivalent 224
+    standard for 66
+    steps in determining 224
+
+Atoms 62
+  size 65
+
+Atropine 419
+
+Aurates 396
+
+Avogadro's hypothesis 226
+  and chemical calculations 235
+  and molecular weights 227
+
+Azote 78
+
+Azurite 357
+
+
+Babbitt metal 253
+
+Bacteria 85
+  decomposition of organic matter by 122
+  nitrifying 85
+
+Baking powders 285, 408
+  alum 335
+  soda 285
+
+Barium 312
+  chloride 313
+  nitrate 313
+  oxides 312
+  sulphate 313
+
+Barytes 312
+
+Bases 107
+  characteristics 107
+  definition 108
+  familiar 107
+  nomenclature 113
+  organic 412
+  strength 113
+  undissociated 108
+
+Basic lining process 346
+
+Bauxite 332
+
+Beer 404
+
+Benzene 417
+  derivatives 417
+  series 399
+
+Benzine 400
+
+Bessemer process 345
+
+Bismuth 253
+  basic salts 255
+  chloride 253
+  nitrate 253
+  occurrence 253
+  oxides 254
+  preparation 253
+  salts, hydrolysis of 254
+  subnitrate 256
+  uses 253
+
+Bismuthyl chloride 256
+
+Blast furnace 341
+  lamp 38
+
+Bleaching powder 306
+
+Bleaching by chlorine 181
+  by sulphurous acid 152
+
+Boiler scale 320
+
+Bone ash 311
+
+Bone black 200
+
+Borax 265
+  bead 266
+
+Bornite 357
+
+Boron 257, 264
+  acids 265
+  fluoride 264
+  hydride 264
+  occurrence 264
+  oxides 264
+  preparation 264
+  properties 264
+
+Brass 323
+
+Bread making 416
+
+Bromides 190
+
+Bromine 187
+  occurrence 187
+ oxygen compounds 190
+  preparation 187
+  properties 188
+
+Bronze 359
+  aluminium 330, 359
+
+Butter fat 410
+
+Butyric acid 407
+
+By-product 284
+
+
+Cadmium 325
+  compounds 326
+
+Cæsium 294
+
+Calamine 321
+
+Calcite 305
+
+Calcium 301
+  carbide 203, 310
+  carbonate 305
+  chloride 306
+  fluoride 308
+  hydroxide 303
+  occurrence 301
+  oxide 302
+  phosphate 246, 311
+  preparation 302
+  sulphate 308
+
+Calomel 363
+
+Calorie 76
+
+Caramel 414
+
+Carbohydrates 413
+
+Carbolic acid 417
+
+Carbon 196
+  allotropic forms 196
+  amorphous 198
+  compounds 196
+  crystalline forms 197
+  cycle in nature 88
+  dioxide 204
+    and bases 206
+    and plant life 88
+    in air 87
+    occurrence 204
+    preparation 204
+    properties 204
+    solid 204
+  disulphide 160, 210
+  family 196
+  hydrogen compounds 201
+  monoxide 208
+  occurrence 196
+  oxides 203
+  properties 200
+  pure 198
+  retort 199
+  uses 200
+
+Carbonates 207
+  acid 207
+
+Carbonic acid 206
+
+Carborundum 259
+
+Carnallite 288
+
+Casein 414
+
+Cassiterite 370
+
+Catalysis 153
+
+Catalyzers 153
+
+Cathode 99
+
+Cation 106
+
+Caustic potash 288
+  soda 278
+
+Celestite 312
+
+Celluloid 415
+
+Cellulose 415
+
+Cement 304
+
+Ceramic industries 336
+
+Cerium 377
+
+Chalcedony 260
+
+Chalcocite 357
+
+Chalcopyrite 357
+
+Chalk 305
+
+Chamber acid 157
+
+Changes, physical and chemical 2
+
+Charcoal 199
+
+Chemical affinity 12
+  changes 2
+  compounds 7
+  equilibrium 128
+  properties 3
+
+Chemistry, definition 4
+
+Chili saltpeter 191, 285
+
+Chinaware 336
+
+Chloric acid 187
+
+Chlorides 186
+
+Chlorine 177
+  bleaching action 181
+  chemical properties 180
+  family 174
+  historical 177
+  occurrence 178
+  oxides 187
+  oxygen acids 187
+  preparation 178
+  properties 179
+
+Chloroform 401
+
+Chloroplatinic acid 393
+
+Chlorous acid 187
+
+Chromates 385
+
+Chrome alum 384
+
+Chromic acid 388
+  anhydride 387
+  chloride 383
+  hydroxide 383
+  sulphate 384
+  sulphide 384
+
+Chromite 383
+
+Chromium 383
+  a base-forming element 383
+  an acid-forming element 385
+  occurrence 383
+
+Cinnabar 363
+
+Citric acid 408
+
+Clay 336
+
+Coal 199
+  gas 217
+  products 400
+  tar 218
+
+Cobalt 354
+  compounds 354
+
+Cocaine 419
+
+Coke 199
+
+Collodion 415
+
+Colemanite 265
+
+Combining weights 225
+
+Combustion 17
+  broad sense 20
+  in air 19
+  phlogiston theory 19
+  products 18
+  spontaneous 20
+  supporters 213
+
+Compounds, chemical 7
+  isomeric 414
+  of metals, preparation 265
+  structure of 118
+
+Conservation of energy 4
+  of matter 5
+
+Contact process 154
+
+Converter, Bessemer 345
+
+Copper 357
+  acetate 407
+  alloys of 359
+  family 356
+  hydroxide 360
+  metallurgy 357
+  occurrence 357
+  ores 357
+  oxide 360
+  properties 358
+  refining 358
+  sulphate 361
+  sulphide 361
+  uses 359
+
+Copperas 350
+
+Coral 305
+
+Corrosive sublimate 363
+
+Corundum 331
+
+Cream of tartar 408
+
+Crocoisite 383
+
+Cryolite 175, 328
+
+Crystallization 98
+  water of 54, 75
+
+Crystallography 161
+
+Crystals 161
+  axes of 161
+  systems 162
+
+Cupric compounds 360
+
+Cuprite 360
+
+Cuprous compounds 360
+  chloride 360
+  oxide 360
+
+Cyanides 210
+  solutions are alkaline 210
+
+
+Dalton's atomic hypothesis 61
+
+Decay 21
+
+Decomposition of organic matter 122
+
+Decrepitation 55
+
+Deliquescence 55
+
+Density of gases 230
+
+Desiccating agents 55
+
+Developers 367
+
+Dewar bulb 91
+
+Dextrose 414
+
+Diamond 197
+
+Dichromates 385
+
+Dichromic acid 385
+
+Dimorphous substances 163
+
+Dissociation 99
+  and boiling point 101
+  and freezing point 101
+  equations of 112
+  extent of 113
+
+Distillation 50
+
+Dogtooth spar 306
+
+Dolomite 319
+
+Double decomposition 71
+
+Drummond light 38
+
+Dyeing  333
+
+Dynamite 409
+
+
+Earth metals 327
+
+Efflorescence 54
+
+Electric furnace 221
+
+Electro-chemical industries 269
+
+Electrode 99
+
+Electrolysis 99
+  of sodium chloride 102
+  of sodium sulphate 103
+  of water 41, 102
+
+Electrolytes 99
+
+Electrolytic dissociation 99
+
+Electroplating 366
+
+Electrotyping 359
+
+Elements, definition 8
+  atomic weights 232
+  earlier classification 165
+  names 11
+  natural groups 165
+  number of 9
+  occurrence 10
+  periodic division 166
+  physical state 10
+  symbols of 11
+
+Emery 331
+
+Energy 4
+  and plant life 89
+  chemical 5
+  conservation of 4
+  transformation of 5
+
+Enzyme 405
+
+Epsom salts 320
+
+Equations 68
+  are quantitative 72
+  knowledge requisite for 69
+  not algebraic 74
+  reading of 69
+
+Equilibrium 138
+  chemical 138
+  in solution 139
+  point of 138
+
+Equivalent 224
+  determination of 224
+  elements with more than one 225
+  relation to atomic weight 224
+
+Etching 177
+
+Ether 411
+
+Ethereal salts 409
+
+Ethers 411
+
+Ethylamine 412
+
+Ethylene series 399
+
+Eudiometer 43
+
+Evaporation  11
+
+
+Families in periodic groups 170
+  triads 165
+
+Family resemblances 170
+
+Fats 409
+
+Fatty acid series 405
+
+Feldspar 261, 335
+
+Fermentation 404
+  acetic 406
+  alcoholic 404, 405
+  lactic 414
+
+Ferric chloride 352
+  hydroxide 352
+  salts 351
+    reduction 353
+  sulphate 352
+
+Ferrochromium, 383
+
+Ferromanganese 343
+
+Ferrosilicon 259
+
+Ferrous carbonate 351
+  salts 350
+    oxidation of 353
+  sulphate 350
+  sulphide 350
+
+Fertilizers 245
+
+Filtration 6, 51
+  beds 52
+
+Fire damp 202
+
+Flames 213
+  appearance 214
+  blowpipe 216
+  Bunsen 214
+  conditions for 213
+  hydrogen 34
+  luminosity 216
+  oxidizing 214
+  oxyhydrogen 37
+  reactions 296
+  reducing 214
+  structure 214
+
+Flash lights 317
+
+Flint 260
+
+Fluorides 177
+
+Fluorine 175
+
+Fluorspar 175, 308
+
+Fluosilicic acid 259
+
+Flux 340
+
+Fool's gold 351
+
+Formaldehyde 405
+
+Formalin 405
+
+Formic acid 406
+
+Formulas 68
+  how determined 234
+  structural 119
+
+Fractional distillation 51
+
+Franklinite 321
+
+Fuels 220
+
+Furnace, arc 221
+  electric 221
+  resistance 221
+
+Fusion methods 271
+
+
+Galena 373
+
+Gallium 327
+
+Galvanized iron 323
+
+Gas, collection of 15
+  coal 217
+  fuel 217
+  illuminating 217
+  measurement of 23
+  natural 219
+  purification of 218
+  water 219
+
+Gases, table 220
+
+Gasoline 400
+
+German silver 323, 359
+
+Germanium 370
+
+Germs, effect of cold on 53
+  in air 84
+  in water 52
+
+Glass 262
+  coloring of 263
+  etching of 177
+  molding of 263
+  nature of 263
+  varieties 263
+
+Glauber's salt 281
+
+Glazing 336
+
+Glucose 414
+
+Glycerin 405
+  nitrates of 409
+
+Gold 393
+  alloys 396
+  chloride 396
+  coin 359
+  extraction of 394
+  in copper 358
+  mining 394
+  occurrence 393
+  properties 396
+  refining of 395
+  telluride 394
+
+Goldschmidt method 269, 330
+
+Gram-molecular weight 236
+
+Granite 336
+
+Graphite 198
+
+Gun cotton 415
+  metal 359
+  powder 292
+
+Gypsite 308
+
+Gypsum 308
+
+
+Halogens 174
+
+Hard water 309
+
+Heat of reaction 75
+
+Helium 80, 314
+
+Hematite 339, 349
+
+Homologous series 398
+
+Hydriodic acid 193
+
+Hydrobromic acid 189
+
+Hydrocarbons 201, 398
+  properties 400
+  series 398
+  substitution products 401
+
+Hydrochloric acid 182
+  composition 183
+  oxidation of 185
+  preparation 182
+  properties 184
+  salts 186
+
+Hydrocyanic acid 210
+
+Hydrofluoric acid 176
+  etching by 177
+  salts of 177
+
+Hydrogen 28
+  dioxide 56
+  explosive with oxygen 35
+  occurrence 28
+  preparation from acids 30
+  preparation from water 28
+  properties 32
+  standard for atomic weights 66
+  standard for molecular weights 227
+  sulphide 146
+  uses 38
+
+Hydrolysis 254
+  conditions affecting 255
+  partial 255
+
+Hydrosulphuric acid 146
+
+Hydroxyl radical 112
+
+Hypochlorous acid 187
+
+Hypothesis 61
+  Avogadro's 226
+  Dalton's 61
+
+
+Ice manufacture 125
+
+Iceland spar 305
+
+Indigo 418
+
+Indium 327
+
+Insoluble compounds 272
+
+Iodic acid 194
+
+Iodides 193
+
+Iodine 190
+  oxygen compounds 193
+  preparation 191
+  properties 192
+  tincture 192
+
+Iodoform 192, 401
+
+Ions 100
+  and electrolytes 104
+
+Iridium 393
+
+Iron 339
+  alum 352
+  cast 343
+  compounds 349
+  cyanides 352
+  disulphide 351
+  family 338
+  metallurgy 339
+  occurrence 339
+  ores 339
+  oxides 349
+  pure 348
+  varieties 342, 347
+  wrought 343
+
+
+Jasper 260
+
+
+Kainite 288
+
+Kaolin 261, 335
+
+Kerosene 400
+
+Ketones 411
+
+Kieserite 288
+
+Kindling temperature 17
+
+Krypton 80
+
+
+Lactic acid 408
+
+Lampblack 200
+
+Laughing gas 132
+
+Law, definition 61
+  of Boyle 24
+  of Charles 23
+  of combining volumes 194
+  of conservation of energy 4
+  of conservation of matter 5, 59
+  of definite composition 59
+  of Dulong and Petit 233
+  of Gay-Lussac 194
+  of multiple proportion 60
+  of Raoult 233
+  periodic 169
+
+Lead 373
+  acetate 375, 407
+  alloys 375
+  basic carbonate 376
+  carbonate 376
+  chloride 377
+  chromate 377
+  insoluble compounds 376
+  metallurgy 373
+  nitrate 375
+  occurrence 373
+  oxides 375
+  peroxide 375
+  properties 374
+  red 375
+  soluble salts 375
+  sugar of 375
+  sulphate 377
+  sulphide 377
+  white 376
+
+Le Blanc soda process 282
+
+Levulose 415
+
+Lime 302
+  air-slaked 303
+  hypochlorite 307
+  kilns 303
+  slaked 303
+
+Lime light 38
+
+Limestone 305
+
+Limewater 303
+
+Limonite 339
+
+Litharge 375
+
+Lithium 294
+
+Luminosity of flames 216
+
+Lunar caustic 366
+
+
+Magnesia 318
+  alba 319
+  usta 318
+
+Magnesite 318
+
+Magnesium 317
+  basic carbonate 319
+  carbonate 318
+  cement 318
+  chloride 319
+  family 316
+  hydroxide 318
+  oxide 318
+  silicates 321
+  sulphate 320
+
+Magnetite 339, 349
+
+Malachite 357
+
+Manganates 381
+
+Manganese 379
+  a base-forming element 380
+  an acid-forming element 381
+  in glass 263
+  occurrence 379
+  oxides 380
+
+Manganic acid 381
+
+Manganous salts 380
+
+Marble 305
+
+Marl 305
+
+Marsh gas 202
+
+Matches 242
+
+Matte 358
+
+Matter, classification 6
+  conservation 5
+  definition 5
+  kinds 9
+
+Measurement of gases 23
+
+Mechanical mixtures 6
+
+Meerschaum 321, 336
+
+Mercuric chloride 363
+  iodide 364
+  oxide 14, 362
+  sulphide 363
+
+Mercurous chloride 363
+
+Mercury 361
+  iodides 364
+  metallurgy 361
+  occurrence 361
+  oxides 362
+  uses 362
+
+Metaboric acid 265
+
+Metallurgy 268
+
+Metals 165, 267
+  action on salts 271
+  definition 267
+  extraction 268
+  occurrence 267
+  preparation of compounds 269
+  reduction from ores 268
+
+Metaphosphoric acid 245
+
+Metarsenic acid 250
+
+Metasilicic acid 261
+
+Metastannic acid 371
+
+Methane 202, 399
+
+Methylamine 412
+
+Mexican onyx 305
+
+Mica 261, 336
+
+Microcosmic salt 244
+
+Milk 414
+
+Minerals 267
+
+Minium 375
+
+Mixed salts 244
+
+Molasses 413
+
+Molecular weights 226
+  boiling-point method 233
+  compared with oxygen 228
+  determination 226
+  freezing-point method 233
+  oxygen standard 227
+  of elements 232
+  vapor-density method 229
+
+Molecule 62
+
+Molybdenum 388
+
+Molybdic acid 388
+
+Monazite sand 377
+
+Mordants 333
+
+Morphine 418
+
+Mortar 304
+
+Moth balls 418
+
+Muriatic acid 182
+
+
+Naphthalene 418
+
+Naphthas 400
+
+Nascent state 182
+
+Natural gas 219
+  sciences 1
+
+Neon 80
+
+Neutralization 108
+  a definite act 109
+  definition 109
+  heat of 109
+  partial 111
+
+Niagara Falls 269, 329
+
+Nickel 354
+  coin 359
+  compounds 354
+  plating 354
+
+Nicotine 419
+
+Nitrates 131
+
+Nitric acid, 128
+  action on metals 130
+  decomposition 129
+  oxidizing action 130
+  preparation 128, 140
+  properties 129
+  salts 131
+
+Nitric oxide 133
+
+Nitrites 132
+
+Nitrobenzene 417
+
+Nitrocellulose 415
+
+Nitrogen 78
+  compounds 122
+  in air 87
+  occurrence 78, 122
+  oxides 132
+  preparation 78
+  properties 80
+
+Nitroglycerin 409
+
+Nitrosulphuric acid 155
+
+Nitrous acid 132
+  oxide 132
+
+Non-metals 165
+
+
+Oil of myrbane 417
+  of vitriol 154
+
+Oleic acid 408
+
+Olein 409
+
+Oleomargarine 410
+
+Onyx 260
+
+Opal 260
+
+Open-hearth process 346
+
+Opium 418
+
+Ores 267
+
+Organic bases 412
+  chemistry 201, 397
+  matter, decomposition 122
+
+Orpiment 246
+
+Orthoarsenic acid 250
+
+Orthophosphates 244
+
+Orthophosphoric acid 244
+
+Orthosilicic acid 261
+
+Osmic acid 393
+
+Osmium 393
+  tetroxide 393
+
+Oxalic acid 407
+
+Oxidation 17, 353
+  definition 18
+
+Oxidizing agent 37
+
+Oxygen 13
+  and ozone 22
+  commercial preparation 16
+  history 13
+  importance 21
+  in air estimation, 85
+  in air function, 84
+  occurrence 13
+  preparation 13
+  properties 16
+  standard  for  atomic weights 66
+  two atoms in molecule 227
+
+Oxyhydrogen blowpipe 37
+
+Ozone 21, 137
+
+
+Palladium 390
+
+Palmitic acid 407
+
+Palmitin 409
+
+Paraffin 400
+
+Paris green 250
+
+Parkes's method for silver 364
+
+Pearls 305
+
+Perchloric acid 187
+
+Periodic acid 194
+
+Periodic division 166
+  groups 167
+  law 169
+  law, imperfections 172
+  law, value 171
+  table 168
+  table, arrangement 166
+
+Permanent hardness 310
+
+Permanganates 381
+
+Permanganic acid 381
+
+Peroxides 278
+
+Petroleum 399
+
+Pewter 372
+
+Phenol 417
+
+Philosopher's stone 9
+
+Phlogiston 19
+
+Phosphates 245
+
+Phosphine 242
+
+Phosphonium compounds 243
+
+Phosphoric acid 244
+
+Phosphorite 239
+
+Phosphorous acid 244
+
+Phosphorus 239
+  acids 243
+  family 238
+  hydrogen compounds 242
+  occurrence 239
+  oxides 243
+  preparation 239
+  properties 240
+  red 241
+  yellow 240
+
+Photography 367
+
+Physical changes 2
+  properties 3
+  properties and periodic groups 171
+  state 3
+
+Physics 1, 4
+
+Pitchblende 314
+
+Plaster of Paris 308
+
+Platinic chloride 393
+
+Platinized asbestos 391
+
+Platinous chloride 393
+
+Platinum 391
+  a catalytic agent 152, 392
+
+Pneumatic trough 16
+
+Polyboric acid 265
+
+Polyhalite 288
+
+Polysilicic acids 261
+
+Porcelain 336
+
+Portland cement 304
+
+Potash 293
+
+Potassium 287
+  acid carbonate 294
+  acid sulphate 294
+  acid sulphite 294
+  alum, aluminium 334
+  alum, chrome 384
+  alum, iron 352
+  and plant life 287
+  aurate 396
+  bromide 290
+  carbonate 293
+  chlorate 291
+  chloride 290
+  chromate 385
+  cyanide 293
+  dichromate 386
+  ferricyanide 352
+  ferrocyanide 352
+  hydroxide 288
+  hydroxide, action of halogens 289
+  hypochlorite 289
+  iodide 290
+  manganate 381
+  nitrate 291
+  occurrence 287
+  permanganate 381
+  preparation 288
+  sulphate 294
+
+Precipitated chalk 306
+
+Precipitation 140
+
+Properties, chemical 3
+  physical 3
+
+Prussic acid 210
+
+Puddling 343
+  furnace 344
+
+Pyridine 412
+
+Pyrites 351
+
+Pyrolusite 380
+
+Pyrophosphoric acid 245
+
+
+Quantitative equations 72
+
+Quartz 260
+
+Quicklime 302
+
+Quinine 418
+
+Quinoline 412
+
+
+Radical 112
+
+Radium 313
+
+Reaction, classes 70
+  addition 70
+  completed 139
+  heat of 75
+  of decomposition 70
+  of double decomposition 71
+  of substitution 70
+  reversible 137
+  steps in 131
+
+Realgar 246
+
+Red lead 375
+  phosphorus 241
+
+Reducing agent 37
+
+Reduction 36, 354
+
+Rennin 414
+
+Resemblances, family 170
+
+Respiration 87
+
+Rhodium 390
+
+Rochelle salts 408
+
+Rouge 349
+
+Rubidium 294
+
+Ruby 331
+
+Ruthenium 390
+
+Rutile 264
+
+
+Safety lamp 202
+
+Sal ammoniac 294
+  soda 282
+
+Salt 280
+
+Saltpeter 291
+  Chili 285
+
+Salts, 109
+  acid, 112
+
+Salts basic 111
+  binary 114
+  characteristics 109
+  definition 109
+  insoluble 272
+  mixed 244
+  nomenclature 113
+  normal 112
+  preparation  by  precipitation 270
+
+Sand 260
+
+Sandstone 260
+
+Saponification 410
+
+Sapphire 331
+
+Satinspar 308
+
+Scale 320
+
+Schönite 288
+
+Selenite 308
+
+Selenium 161
+
+Serpentine 320, 336
+
+Shot 247, 375
+
+Siderite 339
+
+Silica 260
+
+Silicates 261
+
+Silicic acids 261
+
+Silicides 259
+
+Silicon 258
+  acids 261
+  dioxide 260
+  fluoride 258
+  hydride 258
+
+Silver 364
+  amalgamation process 364
+  bromide 367
+  chloride 367
+  coin 359
+  German 359
+  in copper ores 358
+  iodide 367
+  metallurgy 364
+  nitrate 366
+  oxide 366
+  parting of 365
+  refining 365
+  sulphide 366
+
+Slag 340
+
+Smalt 355
+
+Smithsonite 321
+
+Smokeless powder 293
+
+Soaps 410
+
+Soda ash 284
+
+Soda lime 202
+
+Sodium 276
+  acetate 407
+  bicarbonate 285
+  carbonate 282
+  carbonate, historical 284
+  chloride 280
+  chromates 386
+  hydrogen carbonate 285
+  hydroxide 278
+  hyposulphite 282
+  iodate 191
+  nitrate 285
+  occurrence 276
+  peroxide 277
+  phosphates 286
+  preparation 276
+  properties 277
+  sulphate 281
+  sulphite 281
+  tetraborate 287
+  thiosulphate 282
+
+Solder 372, 375
+
+Solubility of gases 95
+  of solids 96
+
+Solution 94
+  and chemical action 53
+  boiling point 98
+  classes 94
+  distribution of solids in 98
+  electrolysis of 99
+  freezing point 99
+  of gases in liquids 94
+  of solids in liquids 96
+  properties 98
+  saturated 97
+  supersaturated 98
+
+Solvay soda process 283
+
+Sombrerite 239
+
+Spectroscope 296
+
+Sphalerite 325
+
+Spiegel iron 343
+
+Spinel 332
+
+Spontaneous combustion 20
+
+Stalactites 305
+
+Stalagmites 305
+
+Standard conditions 23
+
+Stannates 372
+
+Stannic acid 372
+  chloride 372
+  oxide 372
+
+Stannous chloride 372
+
+Starch 415
+
+Stassfurt salts 287
+
+Stearic acid 407
+
+Stearin 409
+
+Steel 345
+  alloys 348
+  properties 347
+  tempering of 348
+  tool 347
+
+Stibine 251
+
+Stibnite 250
+
+Stoneware 336
+
+Strontianite 312
+
+Strontium 312
+  hydroxide 312
+  nitrate 312
+
+Structural formulas 119
+
+Structure of compounds 119
+
+Strychnine 419
+
+Substitution 70
+
+Sugars 412
+  cane 412
+  fruit 415
+  grape 414
+  milk 414
+
+Sulphates 159
+
+Sulphides 148
+
+Sulphites 152
+  action of acids on 150
+
+Sulphur 143
+  allotropic forms 144
+  chemical properties 145
+  comparison with oxygen 161
+  dioxide 149
+    preparation 149
+    properties 150
+  extraction 143
+  flowers of 143
+  occurrence 143
+  oxides 149
+  physical properties 144
+  trioxide 152
+  uses 146
+  varieties 144
+
+Sulphuric acid 154
+  action as an acid 157
+  action on metals 157
+  action on organic matter 158
+  action on salts 158
+  action on water 158
+  fuming 155
+  manufacture 154
+  oxidizing action 157
+  plant 156
+  properties 157
+  salts 159
+
+Sulphuric anhydride 153
+
+Sulphurous acid 151
+
+Superphosphate of lime 246
+
+Sylvine 288
+
+Symbols 11
+
+Synthesis 40
+
+
+Table, alkali metals 274
+  alkaline-earth metals 300
+  alloys of copper 359
+  aqueous tension Appendix B
+  atomic weights Appendix A
+  chlorine family 174
+  composition of earth's crust 10
+  composition of fuel gases 220
+  constants of elements Appendix B
+  copper family 356
+  elements Appendix A
+  gold and platinum metals 390
+  hydrocarbons 399
+  magnesium family 316
+  manganese and chromium 379
+  periodic arrangement 168
+  phosphorus family 238
+  silicon family 257
+  solubility of gases in water 95
+  solubility of salts 96
+  solubility of salts at different temperatures 97
+  tin and lead 370
+  weights of gases Appendix B
+
+Talc 321, 336
+
+Tartar emetic 408
+
+Tartaric acid 408
+
+Tellurium 161
+
+Temporary hardness 309
+
+Ternary acids 113
+  salts 114
+
+Tetraboric acid 265
+
+Thallium 327
+
+Theory, atomic 61
+  definition 64
+  value of 64
+
+Thermite 331
+
+Thio compounds 282
+
+Thiosulphates 159
+
+Thiosulphuric acid 159
+
+Thorium 377
+
+Tin 370
+  block 371
+  compounds 372
+  crystals 372
+  family 370
+  foil 371
+  metallurgy 370
+  plate 371
+  properties 371
+  uses 371
+
+Titanium 257, 264
+
+Topaz 331
+
+Triad families 166
+
+Tungsten 388
+
+Type metal 253, 375
+
+
+Uranium 388
+
+
+Valence 116
+  a numerical property 116
+  and combining ratios 118
+  and equations 120
+  and formulas 120
+  and periodic groups 162
+  and structure 118
+  definition 116
+  indirectly determined 117
+  measure of 117
+  variable 117
+
+Vaseline 400
+
+Venetian red 349
+
+Verdigris 407
+
+Vermilion 363
+
+Vinegar 406
+
+Vitriol, blue 361
+  green 350
+  oil of 154
+  white 324
+
+Volume and aqueous tension 25
+  and pressure 24
+  and temperature 23
+  of combining gases 194
+
+
+Water 40
+  a compound 40
+  and disease 49
+  catalytic action of 154
+  chalybeate 351
+  chemical properties 53
+  composition 47
+  composition by volume 44
+  composition by weight 47
+  dissociation of 210
+  distillation of 50
+  electrolysis of 41, 103
+  filtration of 51
+  gas 219
+  hard 309
+  historical 40
+  impurities in 48
+  in air 87
+  mineral 49
+  occurrence 48
+  of crystallization 54, 75
+  physical properties 53
+  purification of 50
+  qualitative analysis 41
+  quantitative analysis 42
+  river 49
+  sanitary analysis 50
+  self-purification 53
+  softening of 310
+  standard substance 55
+  synthesis 43
+  uses of 55
+
+Weights, atomic 65
+
+Welsbach mantles 219, 377
+
+Whisky 404
+
+Wine 404
+
+Witherite 312
+
+Wood alcohol 402
+  distillation 402
+
+Wood's metal 254
+
+
+Xenon 80
+
+
+Yeast 403
+
+
+Zinc 321
+  alloys of 323
+  blende 321
+  chloride 325
+  flowers of 322
+  metallurgy 321
+  occurrence 321
+  oxide 324
+  sulphate 324
+  sulphide 325
+  white 324
+
+
+Zymase, 403
+
+
+
+
+ANNOUNCEMENTS
+
+
+AN ELEMENTARY STUDY OF CHEMISTRY
+
+By WILLIAM McPHERSON, Professor of Chemistry in Ohio State University,
+and WILLIAM E. HENDERSON, Associate Professor of Chemistry in Ohio State
+University.
+
+12mo. Cloth. 434 pages. Illustrated. List price, $1.25; mailing price,
+$1.40
+
+This book is the outgrowth of many years of experience in the teaching
+of elementary chemistry. In its preparation the authors have steadfastly
+kept in mind the limitations of the student to whom chemistry is a new
+science. They have endeavored to present the subject in a clear,
+well-graded way, passing in a natural and logical manner from principles
+which are readily understood to those which are more difficult to grasp.
+The language is simple and as free as possible from unusual and
+technical phrases. Those which are unavoidable are carefully defined.
+The outline is made very plain, and the paragraphing is designed to be
+of real assistance to the student in his reading.
+
+The book is in no way radical, either in the subject-matter selected or
+in the method of treatment. At the same time it is in thorough harmony
+with the most recent developments in chemistry, both in respect to
+theory and discovery. Great care has been taken in the theoretical
+portions to make the treatment simple and well within the reach of the
+ability of an elementary student. The most recent discoveries have been
+touched upon where they come within the scope of an elementary text.
+Especial attention has been given to the practical applications of
+chemistry, and to the description of the manufacturing processes in use
+at the present time.
+
+
+EXERCISES IN CHEMISTRY. By WILLIAM McPHERSON and WILLIAM E. HENDERSON.
+(_In press._)
+
+GINN & COMPANY PUBLISHERS
+
+A FIRST COURSE IN PHYSICS
+
+By ROBERT A. MILLIKAN, Associate Professor of Physics, and HENRY G.
+GALE, Assistant Professor of Physics in The University of Chicago
+
+12mo, cloth, 488 pages, illustrated, $1.25
+
+
+A LABORATORY COURSE IN PHYSICS
+
+_FOR SECONDARY SCHOOLS_
+
+By ROBERT A. MILLIKAN and HENRY G. GALE 12mo, flexible cloth, 134 pages,
+illustrated, 40 cents
+
+This one-year course in physics has grown out of the experience of the
+authors in developing the work in physics at the School of Education of
+The University of Chicago, and in dealing with the physics instruction
+in affiliated high schools and academies.
+
+The book is a simple, objective presentation of the subject as opposed
+to a formal and mathematical one. It is intended for the third-year
+high-school pupils and is therefore adapted in style and method of
+treatment to the needs of students between the ages of fifteen and
+eighteen. It especially emphasizes the historical and practical aspects
+of the subject and connects the study very intimately with facts of
+daily observation and experience.
+
+The authors have made a careful distinction between the class of
+experiments which are essentially laboratory problems and those which
+belong more properly to the classroom and the lecture table. The former
+are grouped into a Laboratory Manual which is designed for use in
+connection with the text. The two books are not, however, organically
+connected, each being complete in itself.
+
+All the experiments included in the work have been carefully chosen with
+reference to their usefulness as effective classroom demonstrations.
+
+GINN AND COMPANY PUBLISHERS
+
+
+
+
+APPENDIX A
+
+
+LIST OF THE ELEMENTS, THEIR SYMBOLS, AND ATOMIC WEIGHTS
+
+The more important elements are marked with an asterisk
+
+O = 16
+
+*Antimony       Sb     120.2
+*Argon          A       39.9
+*Arsenic        As      75.0
+*Barium         Ba     137.4
+Beryllium       Be       9.1
+*Bismuth        Bi     208.5
+*Boron          B       11.0
+*Bromine        Br      79.96
+*Cadmium        Cd     112.4
+Cæsium          Cs     132.9
+*Calcium        Ca      40.1
+*Carbon         C       12.00
+Cerium          Ce     140.25
+*Chlorine       Cl      35.45
+*Chromium       Cr      52.1
+*Cobalt         Co      59.0
+Columbium       Cb      94.0
+*Copper         Cu      63.6
+Erbium          Er     166.0
+*Fluorine       F       19.0
+Gadolinium      Gd     156.0
+Gallium         Ga      70.0
+Germanium       Ge      72.5
+*Gold           Au     197.2
+Helium          He       4.0
+*Hydrogen       H        1.008
+Indium          In     115.0
+*Iodine         I      126.97
+Iridium         Ir     193.0
+*Iron           Fe      55.9
+Krypton         Kr      81.8
+Lanthanum       La     138.9
+*Lead           Pb     206.9
+Lithium         Li       7.03
+*Magnesium      Mg      24.36
+*Manganese      Mn      55.0
+*Mercury        Hg     200.0
+Molybdenum      Mo      96.0
+Neodymium       Nd     143.6
+Neon            Ne      20.0
+*Nickel         Ni      58.7
+*Nitrogen       N       14.04
+Osmium          Os     191.0
+*Oxygen         O       16.00
+Palladium       Pd     106.5
+*Phosphorus     P       31.0
+*Platinum       Pt     194.8
+*Potassium      K       39.15
+Praseodymium    Pr     140.5
+Radium          Ra     225.0
+Rhodium         Rh     103.0
+Rubidium        Rb      85.5
+Ruthenium       Ru     101.7
+Samarium        Sm     150.3
+Scandium        Sc      44.1
+Selenium        Se      79.2
+*Silicon        Si      28.4
+*Silver         Ag     107.93
+*Sodium         Na      23.05
+*Strontium      Sr      87.6
+*Sulphur        S       32.06
+Tantalum        Ta     183.0
+Tellurium       Te     127.6
+Terbium         Tb     160.0
+Thallium        Tl     204.1
+Thorium         Th     232.5
+Thulium         Tm     171.0
+*Tin            Sn     119.0
+Titanium        Ti      48.1
+Tungsten        W      184.0
+Uranium         U      238.5
+Vanadium        V       51.2
+Xenon           Xe     128.0
+Ytterbium       Yb     173.0
+Yttrium         Yt      89.0
+*Zinc           Zn      65.4
+Zirconium       Zr      90.6
+
+
+
+
+APPENDIX B
+
+
+Tension of Aqueous Vapor expressed in Millimeters of Mercury
+
+TEMPERATURE      PRESSURE
+16               13.5
+17               14.4
+18               15.3
+19               16.3
+20               17.4
+21               18.5
+22               19.6
+23               20.9
+24               22.2
+25               23.5
+
+
+Weight of 1 Liter of Various Gases measured under Standard Conditions
+
+Acetylene               1.1614
+Air                     1.2923
+Ammonia                 0.7617
+Carbon dioxide          1.9641
+Carbon monoxide         1.2499
+Chlorine                3.1650
+Hydrocyanic acid        1.2036
+Hydrochloric acid       1.6275
+Hydrogen                0.08984
+Hydrosulphuric acid     1.5211
+Methane                 0.7157
+Nitric oxide            1.3410
+Nitrogen                1.2501
+Nitrous oxide           1.9677
+Oxygen                  1.4285
+Sulphur dioxide         2.8596
+
+
+Densities and Melting Points of Some Common Elements
+
+                       DENSITY      MELTING POINT
+Aluminium                2.68           640
+Antimony                 6.70           432
+Arsenic                  5.73            --
+Barium                   3.75            --
+Bismuth                  9.80           270
+Boron                    2.45            --
+Cadmium                  8.67           320
+Cæsium                   1.88            26.5
+Calcium                  1.54            --
+Carbon, Diamond          3.50            --
+  "     Graphite         2.15            --
+  "     Charcoal         1.80            --
+Chromium                 7.30          3000
+Cobalt                   8.55          1800
+Copper                   8.89          1084
+Gold                    19.30          1064
+Iridium                 22.42          1950
+Iron                     7.93          1800
+Lead                    11.38           327
+Lithium                  0.59           186
+Magnesium                1.75           750
+Manganese                8.01          1900
+Mercury                 13.596          -39.5
+Nickel                   8.9           1600
+Osmium                  22.47            --
+Palladium               11.80          1500
+Phosphorus               1.80            45
+Platinum                21.50          1779
+Potassium                0.87            62.5
+Rhodium                 12.10            --
+Rubidium                 1.52            38.5
+Ruthenium               12.26            --
+Silicon                  2.35            --
+Silver                  10.5            960
+Sodium                   0.97            97.6
+Strontium                2.50            --
+Sulphur                  2.00           114.8
+Tin                      7.35           235
+Titanium                 3.50            --
+Zinc                     7.00           420
+
+
+
+
+
+End of the Project Gutenberg EBook of An Elementary Study of Chemistry, by 
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+<pre>
+
+The Project Gutenberg EBook of An Elementary Study of Chemistry, by 
+William McPherson and William Edwards Henderson
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: An Elementary Study of Chemistry
+
+Author: William McPherson
+        William Edwards Henderson
+
+Release Date: March 18, 2007 [EBook #20848]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK AN ELEMENTARY STUDY OF CHEMISTRY ***
+
+
+
+
+Produced by Elaine Walker, Josephine Paolucci and the
+Online Distributed Proofreading Team at http://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+<p><a name="frontispiece" id="frontispiece"></a></p>
+<div class="figcenter" style="width: 400px;">
+<img src="images/frontispiece.jpg" width="400" height="455" alt="ANTOINE LAURENT LAVOISIER
+
+Famous for his care in quantitative experiments, for demonstrating the true nature of
+combustion, for introducing system into the naming and grouping of chemical substances. Executed (1794)
+during the French Revolution because of his connection with the government.
+
+This picture is taken from a French engraving of 1799. The panel represents Lavoisier as he is being
+arrested in his laboratory by the Revolutionary Committee." title="" />
+<span class="caption">ANTOINE LAURENT LAVOISIER<br /><br />
+
+Famous for his care in quantitative experiments, for demonstrating the true nature of
+combustion, for introducing system into the naming and grouping of chemical substances. Executed (1794)
+during the French Revolution because of his connection with the government<br /><br />
+
+This picture is taken from a French engraving of 1799. The panel represents Lavoisier as he is being
+arrested in his laboratory by the Revolutionary Committee</span>
+</div>
+
+<h2>AN ELEMENTARY</h2>
+
+<h1>STUDY OF CHEMISTRY</h1>
+
+<h3>BY</h3>
+
+<h2>WILLIAM McPHERSON, PH.D.</h2>
+
+<h4>PROFESSOR OF CHEMISTRY, OHIO STATE UNIVERSITY</h4>
+
+<h3>AND</h3>
+
+<h2>WILLIAM EDWARDS HENDERSON, PH.D.</h2>
+
+<h4>ASSOCIATE PROFESSOR OF CHEMISTRY, OHIO STATE UNIVERSITY</h4>
+
+
+<h3><i>REVISED EDITION</i></h3>
+
+<p class="center">
+GINN &amp; COMPANY<br />
+BOSTON * NEW YORK * CHICAGO * LONDON<br />
+</p>
+
+
+<p class="center">
+COPYRIGHT, 1905, 1906, BY<br />
+WILLIAM MCPHERSON AND WILLIAM E. HENDERSON<br />
+<br />
+ALL RIGHTS RESERVED<br />
+<br />
+<br />
+The Athen&aelig;um Press<br />
+GINN &amp; COMPANY * PROPRIETORS * BOSTON * U.S.A.<br /></p>
+
+<hr style="width: 65%;" />
+
+<p>Transcriber's note: Minor typos have been corrected.</p>
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_iii" id="Page_iii">[Pg iii]</a></span></p>
+<h2><a name="PREFACE" id="PREFACE"></a>PREFACE</h2>
+
+
+<p>In offering this book to teachers of elementary chemistry the authors
+lay no claim to any great originality. It has been their aim to prepare
+a text-book constructed along lines which have become recognized as best
+suited to an elementary treatment of the subject. At the same time they
+have made a consistent effort to make the text clear in outline, simple
+in style and language, conservatively modern in point of view, and
+thoroughly teachable.</p>
+
+<p>The question as to what shall be included in an elementary text on
+chemistry is perhaps the most perplexing one which an author must
+answer. While an enthusiastic chemist with a broad understanding of the
+science is very apt to go beyond the capacity of the elementary student,
+the authors of this text, after an experience of many years, cannot help
+believing that the tendency has been rather in the other direction. In
+many texts no mention at all is made of fundamental laws of chemical
+action because their complete presentation is quite beyond the
+comprehension of the student, whereas in many cases it is possible to
+present the essential features of these laws in a way that will be of
+real assistance in the understanding of the science. For example, it is
+a difficult matter to deduce the law of mass action in any very simple
+way; yet the elementary student can readily comprehend that reactions
+are reversible, and that the point of equilibrium depends upon, rather
+simple conditions. The authors believe that it is worth while to<span class='pagenum'><a name="Page_iv" id="Page_iv">[Pg iv]</a></span>
+present such principles in even an elementary and partial manner because
+they are of great assistance to the general student, and because they
+make a foundation upon which the student who continues his studies to
+more advanced courses can securely build.</p>
+
+<p>The authors have no apologies to make for the extent to which they have
+made use of the theory of electrolytic dissociation. It is inevitable
+that in any rapidly developing science there will be differences of
+opinion in regard to the value of certain theories. There can be no
+question, however, that the outline of the theory of dissociation here
+presented is in accord with the views of the very great majority of the
+chemists of the present time. Moreover, its introduction to the extent
+to which the authors have presented it simplifies rather than increases
+the difficulties with which the development of the principles of the
+science is attended.</p>
+
+<p>The oxygen standard for atomic weights has been adopted throughout the
+text. The International Committee, to which is assigned the duty of
+yearly reporting a revised list of the atomic weights of the elements,
+has adopted this standard for their report, and there is no longer any
+authority for the older hydrogen standard. The authors do not believe
+that the adoption of the oxygen standard introduces any real
+difficulties in making perfectly clear the methods by which atomic
+weights are calculated.</p>
+
+<p>The problems appended to the various chapters have been chosen with a
+view not only of fixing the principles developed in the text in the mind
+of the student, but also of enabling him to answer such questions as
+arise in his laboratory work. They are, therefore, more or less
+practical in character. It is not necessary that all of them should<span class='pagenum'><a name="Page_v" id="Page_v">[Pg v]</a></span> be
+solved, though with few exceptions the lists are not long. The answers
+to the questions are not directly given in the text as a rule, but can
+be inferred from the statements made. They therefore require independent
+thought on the part of the student.</p>
+
+<p>With very few exceptions only such experiments are included in the text
+as cannot be easily carried out by the student. It is expected that
+these will be performed by the teacher at the lecture table. Directions
+for laboratory work by the student are published in a separate volume.</p>
+
+<p>While the authors believe that the most important function of the
+elementary text is to develop the principles of the science, they
+recognize the importance of some discussion of the practical application
+of these principles to our everyday life. Considerable space is
+therefore devoted to this phase of chemistry. The teacher should
+supplement this discussion whenever possible by having the class visit
+different factories where chemical processes are employed.</p>
+
+<p>Although this text is now for the first time offered to teachers of
+elementary chemistry, it has nevertheless been used by a number of
+teachers during the past three years. The present edition has been
+largely rewritten in the light of the criticisms offered, and we desire
+to express our thanks to the many teachers who have helped us in this
+respect, especially to Dr. William Lloyd Evans of this laboratory, a
+teacher of wide experience, for his continued interest and helpfulness.
+We also very cordially solicit correspondence with teachers who may find
+difficulties or inaccuracies in the text.</p>
+
+<p>The authors wish to make acknowledgments for the photographs and
+engravings of eminent chemists from which<span class='pagenum'><a name="Page_vi" id="Page_vi">[Pg vi]</a></span> the cuts included in the text
+were taken; to Messrs. Elliott and Fry, London, England, for that of
+Ramsay; to The Macmillan Company for those of Davy and Dalton, taken
+from the Century Science Series; to the L. E. Knott Apparatus Company,
+Boston, for that of Bunsen.</p>
+
+<p><span style="margin-left: 33em;">THE AUTHORS</span></p>
+
+<p>OHIO STATE UNIVERSITY</p>
+
+<p>COLUMBUS, OHIO</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_vii" id="Page_vii">[Pg vii]</a></span></p>
+<h2><a name="CONTENTS" id="CONTENTS"></a>CONTENTS</h2>
+
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>CHAPTER</td><td align='right'>PAGE</td></tr>
+<tr><td align='left'>I. INTRODUCTION</td><td align='right'><a href='#Page_1'>1</a></td></tr>
+<tr><td align='left'>II. OXYGEN</td><td align='right'><a href='#Page_13'>13</a></td></tr>
+<tr><td align='left'>III. HYDROGEN</td><td align='right'><a href='#Page_28'>28</a></td></tr>
+<tr><td align='left'>IV. WATER AND HYDROGEN DIOXIDE</td><td align='right'><a href='#Page_40'>40</a></td></tr>
+<tr><td align='left'>V. THE ATOMIC THEORY</td><td align='right'><a href='#Page_59'>59</a></td></tr>
+<tr><td align='left'>VI. CHEMICAL EQUATIONS AND CALCULATIONS</td><td align='right'><a href='#Page_68'>68</a></td></tr>
+<tr><td align='left'>VII. NITROGEN AND THE RARE ELEMENTS IN THE ATMOSPHERE</td><td align='right'><a href='#Page_78'>78</a></td></tr>
+<tr><td align='left'>VIII. THE ATMOSPHERE</td><td align='right'><a href='#Page_83'>83</a></td></tr>
+<tr><td align='left'>IX. SOLUTIONS</td><td align='right'><a href='#Page_94'>94</a></td></tr>
+<tr><td align='left'>X. ACIDS, BASES, AND SALTS; NEUTRALIZATION</td><td align='right'><a href='#Page_106'>106</a></td></tr>
+<tr><td align='left'>XI. VALENCE</td><td align='right'><a href='#Page_116'>116</a></td></tr>
+<tr><td align='left'>XII. COMPOUNDS OF NITROGEN</td><td align='right'><a href='#Page_122'>122</a></td></tr>
+<tr><td align='left'>XIII. REVERSIBLE REACTIONS AND CHEMICAL EQUILIBRIUM</td><td align='right'><a href='#Page_137'>137</a></td></tr>
+<tr><td align='left'>XIV. SULPHUR AND ITS COMPOUNDS</td><td align='right'><a href='#Page_143'>143</a></td></tr>
+<tr><td align='left'>XV. PERIODIC LAW</td><td align='right'><a href='#Page_165'>165</a></td></tr>
+<tr><td align='left'>XVI. THE CHLORINE FAMILY</td><td align='right'><a href='#Page_174'>174</a></td></tr>
+<tr><td align='left'>XVII. CARBON AND SOME OF ITS SIMPLER COMPOUNDS</td><td align='right'><a href='#Page_196'>196</a></td></tr>
+<tr><td align='left'> XVIII. FLAMES,&mdash;ILLUMINANTS</td><td align='right'><a href='#Page_213'>213</a></td></tr>
+<tr><td align='left'>XIX. MOLECULAR WEIGHTS, ATOMIC WEIGHTS, FORMULAS</td><td align='right'><a href='#Page_223'>223</a></td></tr>
+<tr><td align='left'>XX. THE PHOSPHORUS FAMILY</td><td align='right'><a href='#Page_238'>238</a></td></tr>
+<tr><td align='left'>XXI. SILICON, TITANIUM, BORON</td><td align='right'><a href='#Page_257'>257</a></td></tr>
+<tr><td align='left'>XXII. THE METALS</td><td align='right'><a href='#Page_267'>267</a></td></tr>
+<tr><td align='left'> XXIII. THE ALKALI METALS</td><td align='right'><a href='#Page_274'>274</a></td></tr>
+<tr><td align='left'>XXIV. THE ALKALINE-EARTH FAMILY</td><td align='right'><a href='#Page_300'>300</a></td></tr>
+<tr><td align='left'>XXV. THE MAGNESIUM FAMILY</td><td align='right'><a href='#Page_316'>316</a></td></tr>
+<tr><td align='left'>XXVI. THE ALUMINIUM FAMILY</td><td align='right'><a href='#Page_327'>327</a></td></tr>
+<tr><td align='left'> XXVII. THE IRON FAMILY</td><td align='right'><a href='#Page_338'>338</a></td></tr>
+<tr><td align='left'>XXVIII. COPPER, MERCURY, AND SILVER</td><td align='right'><a href='#Page_356'>356</a></td></tr>
+<tr><td align='left'>XXIX. TIN AND LEAD</td><td align='right'><a href='#Page_370'>370</a></td></tr>
+<tr><td align='left'>XXX. MANGANESE AND CHROMIUM</td><td align='right'><a href='#Page_379'>379</a></td></tr>
+<tr><td align='left'>XXXI. GOLD AND THE PLATINUM FAMILY</td><td align='right'><a href='#Page_390'>390</a></td></tr>
+<tr><td align='left'> XXXII. SOME SIMPLE ORGANIC COMPOUNDS</td><td align='right'><a href='#Page_397'>397</a></td></tr>
+<tr><td align='left'>INDEX</td><td align='right'><a href='#Page_421'>421</a></td></tr>
+<tr><td align='left'>APPENDIX A</td><td align='right'><a href="#APPENDIX_A">Facing back cover</a></td></tr>
+<tr><td align='left'>APPENDIX B</td><td align='right'><a href="#APPENDIX_B">Inside back cover</a></td></tr>
+</table></div>
+
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_ix" id="Page_ix">[Pg ix]</a></span></p>
+<h2>LIST OF FULL-PAGE ILLUSTRATIONS</h2>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='right'>PAGE</td></tr>
+<tr><td align='left'>ANTOINE LAURENT LAVOISIER</td><td align='right'><i><a href="#frontispiece">Frontispiece</a></i></td></tr>
+<tr><td align='left'>JOSEPH PRIESTLEY</td><td align='right'><a href="#Page_14">14</a></td></tr>
+<tr><td align='left'>JOHN DALTON</td><td align='right'><a href="#Page_60">60</a></td></tr>
+<tr><td align='left'>WILLIAM RAMSAY</td><td align='right'><a href="#Page_82">82</a></td></tr>
+<tr><td align='left'>DMITRI IVANOVITCH MENDEL&Eacute;EFF</td><td align='right'><a href="#Page_166">166</a></td></tr>
+<tr><td align='left'>HENRI MOISSAN</td><td align='right'><a href="#Page_176">176</a></td></tr>
+<tr><td align='left'>SIR HUMPHRY DAVY</td><td align='right'><a href="#Page_276">276</a></td></tr>
+<tr><td align='left'>ROBERT WILHELM BUNSEN</td><td align='right'><a href="#Page_298">298</a></td></tr>
+</table></div>
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+<h2>AN ELEMENTARY STUDY OF CHEMISTRY</h2>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I</h2>
+
+<h3>INTRODUCTION</h3>
+
+
+<p><b>The natural sciences.</b> Before we advance very far in the study of nature,
+it becomes evident that the one large study must be divided into a
+number of more limited ones for the convenience of the investigator as
+well as of the student. These more limited studies are called the
+<i>natural sciences</i>.</p>
+
+<p>Since the study of nature is divided in this way for mere convenience,
+and not because there is any division in nature itself, it often happens
+that the different sciences are very intimately related, and a thorough
+knowledge of any one of them involves a considerable acquaintance with
+several others. Thus the botanist must know something about animals as
+well as about plants; the student of human physiology must know
+something about physics as well as about the parts of the body.</p>
+
+<p><b>Intimate relation of chemistry and physics.</b> Physics and chemistry are
+two sciences related in this close way, and it is not easy to make a
+precise distinction between them. In a general way it may be said that
+they are both concerned with inanimate matter rather than with living,
+and more particularly with the changes which such matter<span class='pagenum'><a name="Page_2" id="Page_2">[Pg 2]</a></span> may be made to
+undergo. These changes must be considered more closely before a
+definition of the two sciences can be given.</p>
+
+<p><b>Physical changes.</b> One class of changes is not accompanied by an
+alteration in the composition of matter. When a lump of coal is broken
+the pieces do not differ from the original lump save in size. A rod of
+iron may be broken into pieces; it may be magnetized; it may be heated
+until it glows; it may be melted. In none of these changes has the
+composition of the iron been affected. The pieces of iron, the
+magnetized iron, the glowing iron, the melted iron, are just as truly
+iron as was the original rod. Sugar may be dissolved in water, but
+neither the sugar nor the water is changed in composition. The resulting
+liquid has the sweet taste of sugar; moreover the water may be
+evaporated by heating and the sugar recovered unchanged. Such changes
+are called <i>physical changes</i>.</p>
+
+<p>DEFINITION: <i>Physical changes are those which do not involve a change in
+the composition of the matter.</i></p>
+
+<p><b>Chemical changes.</b> Matter may undergo other changes in which its
+composition is altered. When a lump of coal is burned ashes and
+invisible gases are formed which are entirely different in composition
+and properties from the original coal. A rod of iron when exposed to
+moist air is gradually changed into rust, which is entirely different
+from the original iron. When sugar is heated a black substance is formed
+which is neither sweet nor soluble in water. Such changes are evidently
+quite different from the physical changes just described, for in them
+new substances are formed in place of the ones undergoing change.
+Changes of this kind are called <i>chemical changes</i>.<span class='pagenum'><a name="Page_3" id="Page_3">[Pg 3]</a></span></p>
+
+<p>DEFINITION: <i>Chemical changes are those which involve a change in the
+composition of the matter.</i></p>
+
+<p><b>How to distinguish between physical and chemical changes.</b> It is not
+always easy to tell to which class a given change belongs, and many
+cases will require careful thought on the part of the student. The test
+question in all cases is, Has the composition of the substance been
+changed? Usually this can be answered by a study of the properties of
+the substance before and after the change, since a change in composition
+is attended by a change in properties. In some cases, however, only a
+trained observer can decide the question.</p>
+
+<p><b>Changes in physical state.</b> One class of physical changes should be noted
+with especial care, since it is likely to prove misleading. It is a
+familiar fact that ice is changed into water, and water into steam, by
+heating. Here we have three different substances,&mdash;the solid ice, the
+liquid water, and the gaseous steam,&mdash;the properties of which differ
+widely. The chemist can readily show, however, that these three bodies
+have exactly the same composition, being composed of the same substances
+in the same proportion. Hence the change from one of these substances
+into another is a physical change. Many other substances may, under
+suitable conditions, be changed from solids into liquids, or from
+liquids into gases, without change in composition. Thus butter and wax
+will melt when heated; alcohol and gasoline will evaporate when exposed
+to the air. <i>The three states&mdash;solid, liquid, and gas&mdash;are called the
+three physical states of matter.</i></p>
+
+<p><b>Physical and chemical properties.</b> Many properties of a substance can be
+noted without causing the substance to undergo chemical change, and are
+therefore called its <i>physical properties</i>. Among these are its physical
+state, color, odor, taste, size, shape, weight. Other properties are
+only<span class='pagenum'><a name="Page_4" id="Page_4">[Pg 4]</a></span> discovered when the substance undergoes chemical change. These are
+called its <i>chemical properties</i>. Thus we find that coal burns in air,
+gunpowder explodes when ignited, milk sours when exposed to air.</p>
+
+<p><b>Definition of physics and chemistry.</b> It is now possible to make a
+general distinction between physics and chemistry.</p>
+
+<p>DEFINITION: <i>Physics is the science which deals with those changes in
+matter which do not involve a change in composition.</i></p>
+
+<p>DEFINITION: <i>Chemistry is the science which deals with those changes in
+matter which do involve a change in composition.</i></p>
+
+<p><b>Two factors in all changes.</b> In all the changes which matter can undergo,
+whether physical or chemical, two factors must be taken into account,
+namely, <i>energy</i> and <i>matter</i>.</p>
+
+<p><b>Energy.</b> It is a familiar fact that certain bodies have the power to do
+work. Thus water falling from a height upon a water wheel turns the
+wheel and in this way does the work of the mills. Magnetized iron
+attracts iron to itself and the motion of the iron as it moves towards
+the magnet can be made to do work. When coal is burned it causes the
+engine to move and transports the loaded cars from place to place. When
+a body has this power to do work it is said to possess energy.</p>
+
+<p><b>Law of conservation of energy.</b> Careful experiments have shown that when
+one body parts with its energy the energy is not destroyed but is
+transferred to another body or system of bodies. Just as energy cannot
+be destroyed, neither can it be created. If one body gains a certain
+amount of energy, some other body has lost an equivalent amount.<span class='pagenum'><a name="Page_5" id="Page_5">[Pg 5]</a></span> These
+facts are summed up in the law of conservation of energy which may be
+stated thus: <i>While energy can be changed from one form into another, it
+cannot be created or destroyed.</i></p>
+
+<p><b>Transformations of energy.</b> Although energy can neither be created nor
+destroyed, it is evident that it may assume many different forms. Thus
+the falling water may turn the electric generator and produce a current
+of electricity. The energy lost by the falling water is thus transformed
+into the energy of the electric current. This in turn may be changed
+into the energy of motion, as when the current is used for propelling
+the cars, or into the energy of heat and light, as when it is used for
+heating and lighting the cars. Again, the energy of coal may be
+converted into energy of heat and subsequently of motion, as when it is
+used as a fuel in steam engines.</p>
+
+<p>Since the energy possessed by coal only becomes available when the coal
+is made to undergo a chemical change, it is sometimes called <i>chemical
+energy</i>. It is this form of energy in which we are especially interested
+in the study of chemistry.</p>
+
+<p><b>Matter.</b> Matter may be defined as that which occupies space and possesses
+weight. Like energy, matter may be changed oftentimes from one form into
+another; and since in these transformations all the other physical
+properties of a substance save weight are likely to change, the inquiry
+arises, Does the weight also change? Much careful experimenting has
+shown that it does not. The weight of the products formed in any change
+in matter always equals the weight of the substances undergoing change.</p>
+
+<p><b>Law of conservation of matter.</b> The important truth just stated is
+frequently referred to as the law of conservation<span class='pagenum'><a name="Page_6" id="Page_6">[Pg 6]</a></span> of matter, and this
+law may be briefly stated thus: <i>Matter can neither be created nor
+destroyed, though it can be changed from one form into another.</i></p>
+
+<p><b>Classification of matter.</b> At first sight there appears to be no limit to
+the varieties of matter of which the world is made. For convenience in
+study we may classify all these varieties under three heads, namely,
+<i>mechanical mixtures</i>, <i>chemical compounds</i>, and <i>elements</i>.</p>
+
+<div class="figleft" style="width: 293px;">
+<img src="images/imagefig1.jpg" width="293" height="400" alt="Fig. 1" title="" />
+<span class="caption">Fig. 1</span>
+</div>
+
+<p><b>Mechanical mixtures.</b> If equal bulks of common salt and iron filings are
+thoroughly mixed together, a product is obtained which, judging by its
+appearance, is a new substance. If it is examined more closely, however,
+it will be seen to be merely a mixture of the salt and iron, each of
+which substances retains its own peculiar properties. The mixture tastes
+just like salt; the iron particles can be seen and their gritty
+character detected. A magnet rubbed in the mixture draws out the iron
+just as if the salt were not there. On the other hand, the salt can be
+separated from the iron quite easily. Thus, if several grams of the
+mixture are placed in a test tube, and the tube half filled with water
+and thoroughly shaken, the salt dissolves in the water. The iron
+particles can then be filtered from the liquid by pouring the entire
+mixture upon a piece of filter paper folded so as to fit into the
+interior of a funnel (Fig. 1). The paper retains the solid but allows
+the clear liquid, known as the <i>filtrate</i>, to drain through. The iron
+particles left upon the filter paper will be found to be identical with<span class='pagenum'><a name="Page_7" id="Page_7">[Pg 7]</a></span>
+the original iron. The salt can be recovered from the filtrate by
+evaporation of the water. To accomplish this the filtrate is poured into
+a small evaporating dish and gently heated (Fig. 2) until the water has
+disappeared, or <i>evaporated</i>. The solid left in the dish is identical in
+every way with the original salt. Both the iron and the salt have thus
+been recovered in their original condition. It is evident that no new
+substance has been formed by rubbing the salt and iron together. The
+product is called a <i>mechanical mixture</i>. Such mixtures are very common
+in nature, almost all minerals, sands, and soils being examples of this
+class of substances. It is at once apparent that there is no law
+regulating the composition of a mechanical mixture, and no two mixtures
+are likely to have exactly the same composition. The ingredients of a
+mechanical mixture can usually be separated by mechanical means, such as
+sifting, sorting, magnetic attraction, or by dissolving one constituent
+and leaving the other unchanged.</p>
+
+<div class="figright" style="width: 251px;">
+<img src="images/imagefig2.jpg" width="251" height="400" alt="Fig. 2" title="" />
+<span class="caption">Fig. 2</span>
+</div>
+
+<p>DEFINITION: <i>A mechanical mixture is one in which the constituents
+retain their original properties, no chemical action having taken place
+when they were brought together.</i></p>
+
+<p><b>Chemical compounds.</b> If iron filings and powdered sulphur are thoroughly
+ground together in a mortar, a yellowish-green substance results. It
+might easily be taken to be a new body; but as in the case of the iron
+and salt, the ingredients can readily be separated. A magnet draws out
+the iron. Water does not dissolve the sulphur, but other liquids do, as,
+for example, the liquid called carbon disulphide.<span class='pagenum'><a name="Page_8" id="Page_8">[Pg 8]</a></span> When the mixture is
+treated with carbon disulphide the iron is left unchanged, and the
+sulphur can be obtained again, after filtering off the iron, by
+evaporating the liquid. The substance is, therefore, a mechanical
+mixture.</p>
+
+<p>If now a new portion of the mixture is placed in a dry test tube and
+carefully heated in the flame of a Bunsen burner, as shown in Fig. 3, a
+striking change takes place. The mixture begins to glow at some point,
+the glow rapidly extending throughout the whole mass. If the test tube
+is now broken and the product examined, it will be found to be a hard,
+black, brittle substance, in no way recalling the iron or the sulphur.
+The magnet no longer attracts it; carbon disulphide will not dissolve
+sulphur from it. It is a new substance with new properties, resulting
+from the chemical union of iron and sulphur, and is called iron
+sulphide. Such substances are called <i>chemical compounds</i>, and differ
+from mechanical mixtures in that the substances producing them lose
+their own characteristic properties. We shall see later that the two
+also differ in that the composition of a chemical compound never varies.</p>
+
+<div class="figcenter" style="width: 360px;">
+<img src="images/imagefig3.jpg" width="360" height="400" alt="Fig. 3" title="" />
+<span class="caption">Fig. 3</span>
+</div>
+
+<p>DEFINITION: <i>A chemical compound is a substance the constituents of
+which have lost their own characteristic properties, and which cannot be
+separated save by a chemical change.</i></p>
+
+<p><b>Elements.</b> It has been seen that iron sulphide is composed of two
+entirely different substances,&mdash;iron and sulphur. The question arises,
+Do these substances in turn contain other substances, that is, are they
+also chemical compounds?<span class='pagenum'><a name="Page_9" id="Page_9">[Pg 9]</a></span> Chemists have tried in a great many ways to
+decompose them, but all their efforts have failed. Substances which have
+resisted all efforts to decompose them into other substances are called
+<i>elements</i>. It is not always easy to prove that a given substance is
+really an element. Some way as yet untried may be successful in
+decomposing it into other simpler forms of matter, and the supposed
+element will then prove to be a compound. Water, lime, and many other
+familiar compounds were at one time thought to be elements.</p>
+
+<p>DEFINITION: <i>An element is a substance which cannot be separated into
+simpler substances by any known means.</i></p>
+
+<p><b>Kinds of matter.</b> While matter has been grouped in three classes for the
+purpose of study, it will be apparent that there are really but two
+distinct kinds of matter, namely, compounds and elements. A mechanical
+mixture is not a third distinct kind of matter, but is made up of
+varying quantities of either compounds or elements or both.</p>
+
+<p><b>Alchemy.</b> In olden times it was thought that some way could be found to
+change one element into another, and a great many efforts were made to
+accomplish this transformation. Most of these efforts were directed
+toward changing the commoner metals into gold, and many fanciful ways
+for doing this were described. The chemists of that time were called
+<i>alchemists</i>, and the art which they practiced was called <i>alchemy</i>. The
+alchemists gradually became convinced that the only way common metals
+could be changed into gold was by the wonderful power of a magic
+substance which they called the <i>philosopher's stone</i>, which would
+accomplish this transformation by its mere touch and would in addition
+give perpetual youth to its fortunate possessor. No one has ever found
+such a stone, and no one has succeeded in changing one metal into
+another.</p>
+
+<p><b>Number of elements.</b> The number of substances now considered to be
+elements is not large&mdash;about eighty in all. Many of these are rare, and
+very few of them make any<span class='pagenum'><a name="Page_10" id="Page_10">[Pg 10]</a></span> large fraction of the materials in the
+earth's crust. Clarke gives the following estimate of the composition of
+the earth's crust:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Oxygen</td><td align='left'>47.0%</td><td align='left'>Calcium</td><td align='left'>3.5%</td></tr>
+<tr><td align='left'>Silicon</td><td align='left'>27.9</td><td align='left'>Magnesium</td><td align='left'>2.5</td></tr>
+<tr><td align='left'>Aluminium</td><td align='left'>8.1</td><td align='left'>Sodium</td><td align='left'>2.7</td></tr>
+<tr><td align='left'>Iron</td><td align='left'>4.7</td><td align='left'>Potassium</td><td align='left'>2.4</td></tr>
+<tr><td align='left'></td><td align='left'>Other elements</td><td align='left'>1.2%</td></tr>
+</table></div>
+
+<p>A complete list of the elements is given in the Appendix. In this list
+the more common of the elements are marked with an asterisk. It is not
+necessary to study more than a third of the total number of elements to
+gain a very good knowledge of chemistry.</p>
+
+<p><b>Physical state of the elements.</b> About ten of the elements are gases at
+ordinary temperatures. Two&mdash;mercury and bromine&mdash;are liquids. The others
+are all solids, though their melting points vary through wide limits,
+from c&aelig;sium which melts at 26&deg; to elements which do not melt save in the
+intense heat of the electric furnace.</p>
+
+<p><b>Occurrence of the elements.</b> Comparatively few of the elements occur as
+uncombined substances in nature, most of them being found in the form of
+chemical compounds. When an element does occur by itself, as is the case
+with gold, we say that it occurs in the <i>free state</i> or <i>native</i>; when
+it is combined with other substances in the form of compounds, we say
+that it occurs in the <i>combined state</i>, or <i>in combination</i>. In the
+latter case there is usually little about the compound to suggest that
+the element is present in it; for we have seen that elements lose their
+own peculiar properties when they enter into combination with other
+elements. It would never be suspected, for example, that the reddish,
+earthy-looking iron ore contains iron.<span class='pagenum'><a name="Page_11" id="Page_11">[Pg 11]</a></span></p>
+
+<p><b>Names of elements.</b> The names given to the elements have been selected in
+a great many different ways. (1) Some names are very old and their
+original meaning is obscure. Such names are iron, gold, and copper. (2)
+Many names indicate some striking physical property of the element. The
+name bromine, for example, is derived from a Greek word meaning a
+stench, referring to the extremely unpleasant odor of the substance. The
+name iodine comes from a word meaning violet, alluding to the beautiful
+color of iodine vapor. (3) Some names indicate prominent chemical
+properties of the elements. Thus, nitrogen means the producer of niter,
+nitrogen being a constituent of niter or saltpeter. Hydrogen means water
+former, signifying its presence in water. Argon means lazy or inert, the
+element being so named because of its inactivity. (4) Other elements are
+named from countries or localities, as germanium and scandium.</p>
+
+<p><b>Symbols.</b> In indicating the elements found in compounds it is
+inconvenient to use such long names, and hence chemists have adopted a
+system of abbreviations. These abbreviations are known as <i>symbols</i>,
+each element having a distinctive symbol. (1) Sometimes the initial
+letter of the name will suffice to indicate the element. Thus I stands
+for iodine, C for carbon. (2) Usually it is necessary to add some other
+characteristic letter to the symbol, since several names may begin with
+the same letter. Thus C stands for carbon, Cl for chlorine, Cd for
+cadmium, Ce for cerium, Cb for columbium. (3) Sometimes the symbol is an
+abbreviation of the old Latin name. In this way Fe (ferrum) indicates
+iron, Cu (cuprum), copper, Au (aurum), gold. The symbols are included in
+the list of elements given in the Appendix. They will become familiar
+through constant use.<span class='pagenum'><a name="Page_12" id="Page_12">[Pg 12]</a></span></p>
+
+<p><b>Chemical affinity the cause of chemical combination.</b> The agency which
+causes substances to combine and which holds them together when combined
+is called <i>chemical affinity</i>. The experiments described in this
+chapter, however, show that heat is often necessary to bring about
+chemical action. The distinction between the cause producing chemical
+action and the circumstances favoring it must be clearly made. Chemical
+affinity is always the cause of chemical union. Many agencies may make
+it possible for chemical affinity to act by overcoming circumstances
+which stand in its way. Among these agencies are heat, light, and
+electricity. As a rule, solution also promotes action between two
+substances. Sometimes these agencies may overcome chemical attraction
+and so occasion the decomposition of a compound.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> To what class of changes do the following belong? (<i>a</i>) The melting
+of ice; (<i>b</i>) the souring of milk; (<i>c</i>) the burning of a candle; (<i>d</i>)
+the explosion of gunpowder; (<i>e</i>) the corrosion of metals. What test
+question must be applied in each of the above cases?</p>
+
+<p><b>2.</b> Give two additional examples (<i>a</i>) of chemical changes; (<i>b</i>) of
+physical changes.</p>
+
+<p><b>3.</b> Is a chemical change always accompanied by a physical change? Is a
+physical change always accompanied by a chemical change?</p>
+
+<p><b>4.</b> Give two or more characteristics of a chemical change.</p>
+
+<p><b>5.</b> (<i>a</i>) When a given weight of water freezes, does it absorb or evolve
+heat? (<i>b</i>) When the resulting ice melts, is the total heat change the
+same or different from that of freezing?</p>
+
+<p><b>6.</b> Give three examples of each of the following: (<i>a</i>) mechanical
+mixtures; (<i>b</i>) chemical compounds; (<i>c</i>) elements.</p>
+
+<p><b>7.</b> Give the derivation of the names of the following elements: thorium,
+gallium, selenium, uranium. (Consult dictionary.)</p>
+
+<p><b>8.</b> Give examples of chemical changes which are produced through the
+agency of heat; of light; of electricity.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_13" id="Page_13">[Pg 13]</a></span></p>
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II</h2>
+
+<h3>OXYGEN</h3>
+
+
+<p><b>History.</b> The discovery of oxygen is generally attributed to the English
+chemist Priestley, who in 1774 obtained the element by heating a
+compound of mercury and oxygen, known as red oxide of mercury. It is
+probable, however, that the Swedish chemist Scheele had previously
+obtained it, although an account of his experiments was not published
+until 1777. The name oxygen signifies acid former. It was given to the
+element by the French chemist Lavoisier, since he believed that all
+acids owe their characteristic properties to the presence of oxygen.
+This view we now know to be incorrect.</p>
+
+<p><b>Occurrence.</b> Oxygen is by far the most abundant of all the elements. It
+occurs both in the free and in the combined state. In the free state it
+occurs in the air, 100 volumes of dry air containing about 21 volumes of
+oxygen. In the combined state it forms eight ninths of water and nearly
+one half of the rocks composing the earth's crust. It is also an
+important constituent of the compounds which compose plant and animal
+tissues; for example, about 66% by weight of the human body is oxygen.</p>
+
+<p><b>Preparation.</b> Although oxygen occurs in the free state in the atmosphere,
+its separation from the nitrogen and other gases with which it is mixed
+is such a difficult matter that in the laboratory it has been found more
+convenient to prepare it from its compounds. The most important of the
+laboratory methods are the following:<span class='pagenum'><a name="Page_14" id="Page_14">[Pg 14]</a></span></p>
+
+<p>1. <i>Preparation from water.</i> Water is a compound, consisting of 11.18%
+hydrogen and 88.82% oxygen. It is easily separated into these
+constituents by passing an electric current through it under suitable
+conditions. The process will be described in the chapter on water. While
+this method of preparation is a simple one, it is not economical.</p>
+
+<p>2. <i>Preparation from mercuric oxide.</i> This method is of interest, since
+it is the one which led to the discovery of oxygen. The oxide, which
+consists of 7.4% oxygen and 92.6% mercury, is placed in a small, glass
+test tube and heated. The compound is in this way decomposed into
+mercury which collects on the sides of the glass tube, forming a silvery
+mirror, and oxygen which, being a gas, escapes from the tube. The
+presence of the oxygen is shown by lighting the end of a splint,
+extinguishing the flame and bringing the glowing coal into the mouth of
+the tube. The oxygen causes the glowing coal to burst into a flame.</p>
+
+<div class="blockquot"><p>In a similar way oxygen may be obtained from its compounds with
+some of the other elements. Thus manganese dioxide, a black
+compound of manganese and oxygen, when heated to about 700&deg;,
+loses one third of its oxygen, while barium dioxide, when
+heated, loses one half of its oxygen.</p></div>
+
+<p>3. <i>Preparation from potassium chlorate (usual laboratory method).</i>
+Potassium chlorate is a white solid which consists of 31.9% potassium,
+28.9% chlorine, and 39.2% oxygen. When heated it undergoes a series of
+changes in which all the oxygen is finally set free, leaving a compound
+of potassium and chlorine called potassium chloride. The change may be
+represented as follows:</p>
+
+<pre>
+
+  / potassium \  (potassium       /potassium \ (potassium
+ { chlorine   }   chlorate) =    {            } chloride) + oxygen
+  \ oxygen    /                   \ chlorine /
+
+</pre>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/priestley.jpg" width="400" height="504" alt="JOSEPH PRIESTLEY (English) (1733-1804)
+
+School-teacher, theologian, philosopher, scientist; friend of Benjamin
+Franklin; discoverer of oxygen; defender of the phlogiston theory; the
+first to use mercury in a pneumatic trough, by which means he first
+isolated in gaseous form hydrochloric acid, sulphur dioxide, and
+ammonia" title="" />
+<span class="caption">JOSEPH PRIESTLEY (English) (1733-1804)<br /><br />
+
+School-teacher, theologian, philosopher, scientist; friend of Benjamin
+Franklin; discoverer of oxygen; defender of the phlogiston theory; the
+first to use mercury in a pneumatic trough, by which means he first
+isolated in gaseous form hydrochloric acid, sulphur dioxide, and
+ammonia</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_15" id="Page_15">[Pg 15]</a></span></p>
+<p>The evolution of the oxygen begins at about 400&deg;. It has been found,
+however, that if the potassium chlorate is mixed with about one fourth
+its weight of manganese dioxide, the oxygen is given off at a much lower
+temperature. Just how the manganese dioxide brings about this result is
+not definitely known. The amount of oxygen obtained from a given weight
+of potassium chlorate is exactly the same whether the manganese dioxide
+is present or not. So far as can be detected the manganese dioxide
+undergoes no change.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig4.jpg" width="400" height="359" alt="Fig. 4" title="" />
+<span class="caption">Fig. 4</span>
+</div>
+
+<p><b>Directions for preparing oxygen.</b> The manner of preparing oxygen from
+potassium chlorate is illustrated in the accompanying diagram (Fig. 4).
+A mixture consisting of one part of manganese dioxide and four parts of
+potassium chlorate is placed in the flask <i>A</i> and gently heated. The
+oxygen is evolved and escapes through the tube <i>B</i>. It is collected by
+bringing over the end of the tube the mouth of a bottle completely
+filled with water and inverted in a vessel of water, as shown in the
+figure. The gas rises in the bottle and displaces the water. In the
+preparation of large quantities of oxygen, a copper retort (Fig. 5) is
+often substituted for the glass flask.</p>
+
+<div class="figleft" style="width: 200px;">
+<img src="images/imagefig5.jpg" width="200" height="154" alt="Fig. 5" title="" />
+<span class="caption">Fig. 5</span>
+</div>
+
+<p>In the preparation of oxygen from potassium chlorate and manganese
+dioxide, the materials used must be pure, otherwise a violent explosion
+may occur. The purity of the materials is tested by heating a small
+amount of the mixture in a test tube.</p>
+
+<p><b>The collection of gases.</b> The method used for collecting oxygen
+illustrates the general method used for collecting such gases as are<span class='pagenum'><a name="Page_16" id="Page_16">[Pg 16]</a></span>
+insoluble in water or nearly so. The vessel <i>C</i> (Fig. 4), containing the
+water in which the bottles are inverted, is called a <i>pneumatic trough.</i></p>
+
+<p><b>Commercial methods of preparation.</b> Oxygen can now be purchased stored
+under great pressure in strong steel cylinders (Fig. 6). It is prepared
+either by heating a mixture of potassium chlorate and manganese dioxide,
+or by separating it from the nitrogen and other gases with which it is
+mixed in the atmosphere. The methods employed for effecting this
+separation will be described in subsequent chapters.</p>
+
+<div class="figright" style="width: 107px;">
+<img src="images/imagefig6.jpg" width="107" height="400" alt="Fig. 6" title="" />
+<span class="caption">Fig. 6</span>
+</div>
+
+<p><b>Physical properties.</b> Oxygen is a colorless, odorless, tasteless gas,
+slightly heavier than air. One liter of it, measured at a temperature of
+0&deg; and under a pressure of one atmosphere, weighs 1.4285 g., while under
+similar conditions one liter of air weighs 1.2923 g. It is but slightly
+soluble in water. Oxygen, like other gases, may be liquefied by applying
+very great pressure to the highly cooled gas. When the pressure is
+removed the liquid oxygen passes again into the gaseous state, since its
+boiling point under ordinary atmospheric pressure is -182.5&deg;.</p>
+
+<p><b>Chemical properties.</b> At ordinary temperatures oxygen is not very active
+chemically. Most substances are either not at all affected by it, or the
+action is so slow as to escape notice. At higher temperatures, however,
+it is very active, and unites directly with most of the elements. This
+activity may be shown by heating various substances until just ignited
+and then bringing them into vessels of the gas, when they will burn with
+great brilliancy. Thus a glowing splint introduced into a jar of oxygen
+bursts into flame. Sulphur burns in the air with a very weak flame and
+feeble light; in oxygen, however, the flame is increased in size and<span class='pagenum'><a name="Page_17" id="Page_17">[Pg 17]</a></span>
+brightness. Substances which readily burn in air, such as phosphorus,
+burn in oxygen with dazzling brilliancy. Even substances which burn in
+air with great difficulty, such as iron, readily burn in oxygen.</p>
+
+<p>The burning of a substance in oxygen is due to the rapid combination of
+the substance or of the elements composing it with the oxygen. Thus,
+when sulphur burns both the oxygen and sulphur disappear as such and
+there is formed a compound of the two, which is an invisible gas, having
+the characteristic odor of burning sulphur. Similarly, phosphorus on
+burning forms a white solid compound of phosphorus and oxygen, while
+iron forms a reddish-black compound of iron and oxygen.</p>
+
+<p><b>Oxidation.</b> The term <i>oxidation</i> is applied to the chemical change which
+takes place when a substance, or one of its constituent parts, combines
+with oxygen. This process may take place rapidly, as in the burning of
+phosphorus, or slowly, as in the oxidation (or rusting) of iron when
+exposed to the air. It is always accompanied by the liberation of heat.
+The amount of heat liberated by the oxidation of a definite weight of
+any given substance is always the same, being entirely independent of
+the rapidity of the process. If the oxidation takes place slowly, the
+heat is generated so slowly that it is difficult to detect it. If the
+oxidation takes place rapidly, however, the heat is generated in such a
+short interval of time that the substance may become white hot or burst
+into a flame.</p>
+
+<p><b>Combustion; kindling temperature.</b> When oxidation takes place so rapidly
+that the heat generated is sufficient to cause the substance to glow or
+burst into a flame the process is called <i>combustion</i>. In order that any
+substance may undergo combustion, it is necessary that it should be<span class='pagenum'><a name="Page_18" id="Page_18">[Pg 18]</a></span>
+heated to a certain temperature, known as the <i>kindling temperature.</i>
+This temperature varies widely for different bodies, but is always
+definite for the same body. Thus the kindling temperature of phosphorus
+is far lower than that of iron, but is definite for each. When any
+portion of a substance is heated until it begins to burn the combustion
+will continue without the further application of heat, provided the heat
+generated by the process is sufficient to bring other parts of the
+substance to the kindling temperature. On the other hand, if the heat
+generated is not sufficient to maintain the kindling temperature,
+combustion ceases.</p>
+
+<p><b>Oxides.</b> The compounds formed by the oxidation of any element are called
+<i>oxides</i>. Thus in the combustion of sulphur, phosphorus, and iron, the
+compounds formed are called respectively oxide of sulphur, oxide of
+phosphorus, and oxide of iron. In general, then, <i>an oxide is a compound
+of oxygen with another element</i>. A great many substances of this class
+are known; in fact, the oxides of all the common elements have been
+prepared, with the exception of those of fluorine and bromine. Some of
+these are familiar compounds. Water, for example, is an oxide of
+hydrogen, and lime an oxide of the metal calcium.</p>
+
+<p><b>Products of combustion.</b> The particular oxides formed by the combustion
+of any substance are called <i>products of combustion</i> of that substance.
+Thus oxide of sulphur is the product of the combustion of sulphur; oxide
+of iron is the product of the combustion of iron. It is evident that the
+products of the combustion of any substance must weigh more than the
+original substance, the increase in weight corresponding to the amount
+of oxygen taken up in the act of combustion. For example, when iron
+burns the oxide of iron formed weighs more than the original iron.<span class='pagenum'><a name="Page_19" id="Page_19">[Pg 19]</a></span></p>
+
+<p>In some cases the products of combustion are invisible gases, so that
+the substance undergoing combustion is apparently destroyed. Thus, when
+a candle burns it is consumed, and so far as the eye can judge nothing
+is formed during combustion. That invisible gases are formed, however,
+and that the weight of these is greater than the weight of the candle
+may be shown by the following experiment.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig7.jpg" width="400" height="375" alt="Fig. 7" title="" />
+<span class="caption">Fig. 7</span>
+</div>
+
+<div class="blockquot"><p>A lamp chimney is filled with sticks of the compound known as
+sodium hydroxide (caustic soda), and suspended from the beam of
+the balance, as shown in Fig. 7. A piece of candle is placed on
+the balance pan so that the wick comes just below the chimney,
+and the balance is brought to a level by adding weights to the
+other pan. The candle is then lighted. The products formed pass
+up through the chimney and are absorbed by the sodium
+hydroxide. Although the candle burns away, the pan upon which
+it rests slowly sinks, showing that the combustion is attended
+by an increase in weight.</p>
+
+<p><b>Combustion in air and in oxygen.</b> Combustion in air and in
+oxygen differs only in rapidity, the products formed being
+exactly the same. That the process should take place less
+rapidly in the former is readily understood, for the air is
+only about one fifth oxygen, the remaining four fifths being
+inert gases. Not only is less oxygen available, but much of the
+heat is absorbed in raising the temperature of the inert gases
+surrounding the substance undergoing combustion, and the
+temperature reached in the combustion is therefore less.</p>
+
+<p><b>Phlogiston theory of combustion.</b> The French chemist Lavoisier
+(1743-1794), who gave to oxygen its name was the first to show
+that combustion is due to union with oxygen. Previous to his
+time combustion was supposed to be due to the presence of a
+substance or principle called <i>phlogiston</i>. One substance was
+thought to be more combustible than another because it
+contained more phlogiston. Coal, for example, was thought to be
+very rich in phlogiston. The ashes<span class='pagenum'><a name="Page_20" id="Page_20">[Pg 20]</a></span> left after combustion would
+not burn because all the phlogiston had escaped. If the
+phlogiston could be restored in any way, the substance would
+then become combustible again. Although this view seems absurd
+to us in the light of our present knowledge, it formerly had
+general acceptance. The discovery of oxygen led Lavoisier to
+investigate the subject, and through his experiments he arrived
+at the true explanation of combustion. The discovery of oxygen
+together with the part it plays in combustion is generally
+regarded as the most important discovery in the history of
+chemistry. It marked the dawn of a new period in the growth of
+the science.</p></div>
+
+<p><b>Combustion in the broad sense.</b> According to the definition given above,
+the presence of oxygen is necessary for combustion. The term is
+sometimes used, however, in a broader sense to designate any chemical
+change attended by the evolution of heat and light. Thus iron and
+sulphur, or hydrogen and chlorine under certain conditions, will combine
+so rapidly that light is evolved, and the action is called a combustion.
+Whenever combustion takes place in the air, however, the process is one
+of oxidation.</p>
+
+<div class="blockquot"><p><b>Spontaneous combustion.</b> The temperature reached in a given
+chemical action, such as oxidation, depends upon the rate at
+which the reaction takes place. This rate is usually increased
+by raising the temperature of the substances taking part in the
+action.</p>
+
+<p>When a slow oxidation takes place under such conditions that
+the heat generated is not lost by being conducted away, the
+temperature of the substance undergoing oxidation is raised,
+and this in turn hastens the rate of oxidation. The rise in
+temperature may continue in this way until the kindling
+temperature of the substance is reached, when combustion
+begins. Combustion occurring in this way is called <i>spontaneous
+combustion</i>.</p>
+
+<p>Certain oils, such as the linseed oil used in paints, slowly
+undergo oxidation at ordinary temperatures, and not
+infrequently the origin of fires has been traced to the
+spontaneous combustion of oily rags. The spontaneous combustion
+of hay has been known to set barns on fire. Heaps of coal have
+been found to be on fire when spontaneous combustion offered
+the only possible explanation.</p></div><p><span class='pagenum'><a name="Page_21" id="Page_21">[Pg 21]</a></span></p>
+
+<p><b>Importance of oxygen.</b> 1. Oxygen is essential to life. Among living
+organisms only certain minute forms of plant life can exist without it.
+In the process of respiration the air is taken into the lungs where a
+certain amount of oxygen is absorbed by the blood. It is then carried to
+all parts of the body, oxidizing the worn-out tissues and changing them
+into substances which may readily be eliminated from the body. The heat
+generated by this oxidation is the source of the heat of the body. The
+small amount of oxygen which water dissolves from the air supports all
+the varied forms of aquatic animals.</p>
+
+<p>2. Oxygen is also essential to decay. The process of decay is really a
+kind of oxidation, but it will only take place in the presence of
+certain minute forms of life known as bacteria. Just how these assist in
+the oxidation is not known. By this process the dead products of animal
+and vegetable life which collect on the surface of the earth are slowly
+oxidized and so converted into harmless substances. In this way oxygen
+acts as a great purifying agent.</p>
+
+<p>3. Oxygen is also used in the treatment of certain diseases in which the
+patient is unable to inhale sufficient air to supply the necessary
+amount of oxygen.</p>
+
+
+<h3>OZONE</h3>
+
+<p><b>Preparation.</b> When electric sparks are passed through oxygen or air a
+small percentage of the oxygen is converted into a substance called
+<i>ozone</i>, which differs greatly from oxygen in its properties. The same
+change can also be brought about by certain chemical processes. Thus, if
+some pieces of phosphorus are placed in a bottle and partially covered
+with water, the presence of ozone may soon be detected in the air
+contained in the bottle. The conversion of oxygen into ozone is attended
+by a change in volume, 3 volumes of oxygen forming 2 volumes of ozone.
+If the resulting ozone is heated to about 300&deg;, the<span class='pagenum'><a name="Page_22" id="Page_22">[Pg 22]</a></span> reverse change
+takes place, the 2 volumes of ozone being changed back into 3 volumes of
+oxygen. It is possible that traces of ozone exist in the atmosphere,
+although its presence there has not been definitely proved, the tests
+formerly used for its detection having been shown to be unreliable.</p>
+
+<p><b>Properties.</b> As commonly prepared, ozone is mixed with a large excess of
+oxygen. It is possible, however, to separate the ozone and thus obtain
+it in pure form. The gas so obtained has the characteristic odor noticed
+about electrical machines when in operation. By subjecting it to great
+pressure and a low temperature, the gas condenses to a bluish liquid,
+boiling at -119&deg;. When unmixed with other gases ozone is very explosive,
+changing back into oxygen with the liberation of heat. Its chemical
+properties are similar to those of oxygen except that it is far more
+active. Air or oxygen containing a small amount of ozone is now used in
+place of oxygen in certain manufacturing processes.</p>
+
+<p><b>The difference between oxygen and ozone.</b> Experiments show that in
+changing oxygen into ozone no other kind of matter is either added to
+the oxygen or withdrawn from it. The question arises then, How can we
+account for the difference in their properties? It must be remembered
+that in all changes we have to take into account <i>energy</i> as well as
+<i>matter</i>. By changing the amount of energy in a substance we change its
+properties. That oxygen and ozone contain different amounts of energy
+may be shown in a number of ways; for example, by the fact that the
+conversion of ozone into oxygen is attended by the liberation of heat.
+The passage of the electric sparks through oxygen has in some way
+changed the energy content of the element and thus it has acquired new
+properties. <i>Oxygen and ozone must, therefore, be regarded as identical
+so far as the kind of matter of which they are composed is concerned.
+Their different properties are due to their different energy contents.</i></p>
+
+<p><b>Allotropic states or forms of matter.</b> Other elements besides oxygen may
+exist in more than one form. These different forms of the same element
+are called <i>allotropic states</i> or <i>forms</i> of the element. These forms
+differ not only in physical properties but also in their energy
+contents. Elements often exist in a variety of forms which look quite
+different. These differences may be due to accidental causes, such as
+the size or shape of the particles or the way in which the element was
+prepared. Only such forms, however, as have different energy contents
+are properly called allotropic forms.<span class='pagenum'><a name="Page_23" id="Page_23">[Pg 23]</a></span></p>
+
+
+<h3>MEASUREMENT OF GAS VOLUMES</h3>
+
+<p><b>Standard conditions.</b> It is a well-known fact that the volume occupied by
+a definite weight of any gas can be altered by changing the temperature
+of the gas or the pressure to which it is subjected. In measuring the
+volume of gases it is therefore necessary, for the sake of accuracy, to
+adopt some standard conditions of temperature and pressure. The
+conditions agreed upon are (1) a temperature of 0&deg;, and (2) a pressure
+equal to the average pressure exerted by the atmosphere at the sea
+level, that is, 1033.3 g. per square centimeter. These conditions of
+temperature and pressure are known as the <i>standard conditions</i>, and
+when the volume of a gas is given it is understood that the measurement
+was made under these conditions, unless it is expressly stated
+otherwise. For example, the weight of a liter of oxygen has been given
+as 1.4285 g. This means that one liter of oxygen, measured at a
+temperature of 0&deg; and under a pressure of 1033.3 g. per square
+centimeter, weighs 1.4285 g.</p>
+
+<p>The conditions which prevail in the laboratory are never the standard
+conditions. It becomes necessary, therefore, to find a way to calculate
+the volume which a gas will occupy under standard conditions from the
+volume which it occupies under any other conditions. This may be done in
+accordance with the following laws.</p>
+
+<p><b>Law of Charles.</b> This law expresses the effect which a change in the
+temperature of a gas has upon its volume. It may be stated as follows:
+<i>For every degree the temperature of a gas rises above zero the volume
+of the gas is increased by 1/273 of the volume which it occupies at
+zero; likewise for every degree the temperature of the gas falls below
+zero the volume of the gas is decreased by 1/273 of the volume which it
+occupies at zero, provided in both cases that the pressure to which the
+gas is subjected remains constant.</i></p>
+
+<p>If <i>V</i> represents the volume of gas at 0&deg;, then the volume at 1&deg; will be
+<i>V</i> + 1/273 <i>V</i>; at 2&deg; it will be <i>V</i> + 2/273 <i>V</i>; or, in general, the
+volume v, at the temperature <i>t</i>, will be expressed by the formula</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(1) <i>v</i> = <i>V</i> + <i>t</i>/273 <i>V</i>,<br /></span>
+</div><div class="stanza">
+<span class="i0">or (2) <i>v</i> = <i>V</i>(1 + (<i>t</i>/273)).<br /></span>
+</div></div>
+
+<p>Since 1/273 = 0.00366, the formula may be written</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(3) <i>v</i> = <i>V</i>(1 + 0.00366<i>t</i>).<br /></span>
+<span class='pagenum'><a name="Page_24" id="Page_24">[Pg 24]</a></span></div></div>
+
+<p>Since the value of <i>V</i> (volume under standard conditions) is the one
+usually sought, it is convenient to transpose the equation to the
+following form:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(4) <i>V</i> = <i>v</i>/(1 + 0.00366<i>t</i>).<br /></span>
+</div></div>
+
+<p>The following problem will serve as an illustration of the application
+of this equation.</p>
+
+<p>The volume of a gas at 20&deg; is 750 cc.; find the volume it will occupy at
+0&deg;, the pressure remaining constant.</p>
+
+<p>In this case, <i>v</i> = 750 cc. and <i>t</i> = 20. By substituting these values,
+equation (4) becomes</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0"><i>V</i> = 750/(1 + 0.00366 &times; 20) = 698.9 cc.<br /></span>
+</div></div>
+
+<p><b>Law of Boyle.</b> This law expresses the relation between the volume
+occupied by a gas and the pressure to which it is subjected. It may be
+stated as follows: <i>The volume of a gas is inversely proportional to the
+pressure under which it is measured, provided the temperature of the gas
+remains constant.</i></p>
+
+<p>If <i>V</i> represents the volume when subjected to a pressure <i>P</i> and <i>v</i>
+represents its volume when the pressure is changed to <i>p</i>, then, in
+accordance with the above law, <i>V</i> : <i>v</i> :: <i>p</i> : <i>P</i>, or <i>VP</i> = <i>vp</i>.
+In other words, for a given weight of a gas the product of the numbers
+representing its volume and the pressure to which it is subjected is a
+constant.</p>
+
+<p>Since the pressure of the atmosphere at any point is indicated by the
+barometric reading, it is convenient in the solution of the problems to
+substitute the latter for the pressure measured in grams per square
+centimeter. The average reading of the barometer at the sea level is 760
+mm., which corresponds to a pressure of 1033.3 g. per square centimeter.
+The following problem will serve as an illustration of the application
+of Boyle's law.</p>
+
+<p>A gas occupies a volume of 500 cc. in a laboratory where the barometric
+reading is 740 mm. What volume would it occupy if the atmospheric
+pressure changed so that the reading became 750 mm.?</p>
+
+<p>Substituting the values in the equation <i>VP</i> = <i>vp</i>, we have 500 &times; 740 =
+<i>v</i> &times; 750, or <i>v</i> = 493.3 cc.</p>
+
+<p><b>Variations in the volume of a gas due to changes both in temperature and
+pressure.</b> Inasmuch as corrections must be made as a rule<span class='pagenum'><a name="Page_25" id="Page_25">[Pg 25]</a></span> for both
+temperature and pressure, it is convenient to combine the equations
+given above for the corrections for each, so that the two corrections
+may be made in one operation. The following equation is thus obtained:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(5) <i>V<sub>s</sub></i> = <i>vp</i>/(760(1 + 0.00366<i>t</i>)),<br /></span>
+</div></div>
+
+<p>in which <i>V<sub>s</sub></i> represents the volume of a gas under standard
+conditions and <i>v</i>, <i>p</i>, and <i>t</i> the volume, pressure, and temperature
+respectively at which the gas was actually measured.</p>
+
+<p>The following problem will serve to illustrate the application of this
+equation.</p>
+
+<p>A gas having a temperature of 20&deg; occupies a volume of 500 cc. when
+subjected to a pressure indicated by a barometric reading of 740 mm.
+What volume would this gas occupy under standard conditions?</p>
+
+<p>In this problem <i>v</i> = 500, <i>p</i> = 740, and <i>t</i> = 20. Substituting these
+values in the above equation, we get</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0"><i>V<sub>s</sub></i> = (500 &times; 740)/(760 (1 + 0.00366 &times; 20)) = 453.6 cc.<br /></span>
+</div></div>
+
+<div class="figright" style="width: 144px;">
+<img src="images/imagefig8.jpg" width="144" height="400" alt="Fig. 8" title="" />
+<span class="caption">Fig. 8</span>
+</div>
+
+<p><b>Variations in the volume of a gas due to the pressure of aqueous vapor.</b>
+In many cases gases are collected over water, as explained under the
+preparation of oxygen. In such cases there is present in the gas a
+certain amount of water vapor. This vapor exerts a definite pressure,
+which acts in opposition to the atmospheric pressure and which therefore
+must be subtracted from the latter in determining the effective pressure
+upon the gas. Thus, suppose we wish to determine the pressure to which
+the gas in tube <i>A</i> (Fig. 8) is subjected. The tube is raised or lowered
+until the level of the water inside and outside the tube is the same.
+The atmosphere presses down upon the surface of the water (as indicated
+by the arrows), thus forcing the water upward within the tube with a
+pressure equal to the atmospheric pressure. The full force of this
+upward pressure, however, is not spent in compressing the gas within the
+tube, for since it is collected over water it contains a certain amount
+of water vapor. This water vapor exerts a pressure (as indicated by the
+arrow within the tube) in opposition to<span class='pagenum'><a name="Page_26" id="Page_26">[Pg 26]</a></span> the upward pressure. It is
+plain, therefore, that the effective pressure upon the gas is equal to
+the atmospheric pressure less the pressure exerted by the aqueous vapor.
+The pressure exerted by the aqueous vapor increases with the
+temperature. The figures representing the extent of this pressure (often
+called the <i>tension of aqueous vapor</i>) are given in the Appendix. They
+express the pressure or tension in millimeters of mercury, just as the
+atmospheric pressure is expressed in millimeters of mercury.
+Representing the pressure of the aqueous vapor by <i>a</i>, formula (5)
+becomes</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(6) <i>V<sub>s</sub></i> = <i>v</i>(<i>p</i> - <i>a</i>)/(760(1 + 0.00366<i>t</i>)).<br /></span>
+</div></div>
+
+<p>The following problem will serve to illustrate the method of applying
+the correction for the pressure of the aqueous vapor.</p>
+
+<p>The volume of a gas measured over water in a laboratory where the
+temperature is 20&deg; and the barometric reading is 740 mm. is 500 cc. What
+volume would this occupy under standard conditions?</p>
+
+<p>The pressure exerted by the aqueous vapor at 20&deg; (see table in Appendix)
+is equal to the pressure exerted by a column of mercury 17.4 mm. in
+height. Substituting the values of <i>v</i>, <i>t</i>, <i>p</i>, and <i>a</i> in formula
+(6), we have</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(6) <i>V<sub>s</sub></i> = 500(740 - 17.4)/(760(1 + 0.00366 &times; 20)) = 442.9 cc.<br /></span>
+</div></div>
+
+<p><b>Adjustment of tubes before reading gas volumes.</b> In measuring the volumes
+of gases collected in graduated tubes or other receivers, over a liquid
+as illustrated in Fig. 8, the reading should be taken after raising or
+lowering the tube containing the gas until the level of the liquid
+inside and outside the tube is the same; for it is only under these
+conditions that the upward pressure within the tube is the same as the
+atmospheric pressure.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> What is the meaning of the following words? phlogiston, ozone,
+phosphorus. (Consult dictionary.)</p>
+
+<p><b>2.</b> Can combustion take place without the emission of light?</p>
+
+<p><b>3.</b> Is the evolution of light always produced by combustion?</p>
+
+<p><b>4.</b> (<i>a</i>) What weight of oxygen can be obtained from 100 g. of water?
+(<i>b</i>) What volume would this occupy under standard conditions?<span class='pagenum'><a name="Page_27" id="Page_27">[Pg 27]</a></span></p>
+
+<p><b>5.</b> (<i>a</i>) What weight of oxygen can be obtained from 500g. of mercuric
+oxide? (<i>b</i>) What volume would this occupy under standard conditions?</p>
+
+<p><b>6.</b> What weight of each of the following compounds is necessary to
+prepare 50 l. of oxygen? (<i>a</i>) water; (<i>b</i>) mercuric oxide; (<i>c</i>)
+potassium chlorate.</p>
+
+<p><b>7.</b> Reduce the following volumes to 0&deg;, the pressure remaining constant:
+(<i>a</i>) 150 cc. at 10&deg;; (<i>b</i>) 840 cc. at 273&deg;.</p>
+
+<p><b>8.</b> A certain volume of gas is measured when the temperature is 20&deg;. At
+what temperature will its volume be doubled?</p>
+
+<p><b>9.</b> Reduce the following volumes to standard conditions of pressure, the
+temperature remaining constant: (<i>a</i>) 200 cc. at 740 mm.; (<i>b</i>) 500 l.
+at 380 mm.</p>
+
+<p><b>10.</b> What is the weight of 1 l. of oxygen when the pressure is 750 mm.
+and the temperature 0&deg;?</p>
+
+<p><b>11.</b> Reduce the following volumes to standard conditions of temperature
+and pressure: (<i>a</i>) 340 cc. at 12&deg; and 753 mm; (<i>b</i>) 500 cc. at 15&deg; and
+740 mm.</p>
+
+<p><b>12.</b> What weight of potassium chlorate is necessary to prepare 250 l. of
+oxygen at 20&deg; and 750 mm.?</p>
+
+<p><b>13.</b> Assuming the cost of potassium chlorate and mercuric oxide to be
+respectively $0.50 and $1.50 per kilogram, calculate the cost of
+materials necessary for the preparation of 50 l. of oxygen from each of
+the above compounds.</p>
+
+<p><b>14.</b> 100 g. of potassium chlorate and 25 g. of manganese dioxide were
+heated in the preparation of oxygen. What products were left in the
+flask, and how much of each was present?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_28" id="Page_28">[Pg 28]</a></span></p>
+<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III</h2>
+
+<h3>HYDROGEN</h3>
+
+
+<p><b>Historical.</b> The element hydrogen was first clearly recognized as a
+distinct substance by the English investigator Cavendish, who in 1766
+obtained it in a pure state, and showed it to be different from the
+other inflammable airs or gases which had long been known. Lavoisier
+gave it the name hydrogen, signifying water former, since it had been
+found to be a constituent of water.</p>
+
+<p><b>Occurrence.</b> In the free state hydrogen is found in the atmosphere, but
+only in traces. In the combined state it is widely distributed, being a
+constituent of water as well as of all living organisms, and the
+products derived from them, such as starch and sugar. About 10% of the
+human body is hydrogen. Combined with carbon, it forms the substances
+which constitute petroleum and natural gas.</p>
+
+<div class="blockquot"><p>It is an interesting fact that while hydrogen in the free state
+occurs only in traces on the earth, it occurs in enormous
+quantities in the gaseous matter surrounding the sun and
+certain other stars.</p></div>
+
+<p><b>Preparation from water.</b> Hydrogen can be prepared from water by several
+methods, the most important of which are the following.</p>
+
+<p>1. <i>By the electric current.</i> As has been indicated in the preparation
+of oxygen, water is easily separated into its constituents, hydrogen and
+oxygen, by passing an electric current through it under certain
+conditions.</p>
+
+<p>2. <i>By the action of certain metals.</i> When brought into contact with
+certain metals under appropriate conditions,<span class='pagenum'><a name="Page_29" id="Page_29">[Pg 29]</a></span> water gives up a portion
+or the whole of its hydrogen, its place being taken by the metal. In the
+case of a few of the metals this change occurs at ordinary temperatures.
+Thus, if a bit of sodium is thrown on water, an action is seen to take
+place at once, sufficient heat being generated to melt the sodium, which
+runs about on the surface of the water. The change which takes place
+consists in the displacement of one half of the hydrogen of the water by
+the sodium, and may be represented as follows:</p>
+
+<pre>
+          _        _            _        _
+         | hydrogen |          | sodium   |
+sodium + | hydrogen |(water) = | hydrogen |(sodium hydroxide) + hydrogen
+         |_oxygen  _|          |_oxygen  _|
+</pre>
+
+<p>The sodium hydroxide formed is a white solid which remains dissolved in
+the undecomposed water, and may be obtained by evaporating the solution
+to dryness. The hydrogen is evolved as a gas and may be collected by
+suitable apparatus.</p>
+
+<p>Other metals, such as magnesium and iron, decompose water rapidly, but
+only at higher temperatures. When steam is passed over hot iron, for
+example, the iron combines with the oxygen of the steam, thus displacing
+the hydrogen. Experiments show that the change may be represented as
+follows:</p>
+
+<pre>
+        _        _
+       | hydrogen |           _       _                 _        _
+iron + | hydrogen |(water) = | iron    |(iron oxide) + | hydrogen |
+       |_oxygen  _|          |_oxygen _|               |_hydrogen_|
+</pre>
+
+<p>The iron oxide formed is a reddish-black compound, identical with that
+obtained by the combustion of iron in oxygen.</p>
+
+<div class="blockquot"><p><b>Directions for preparing hydrogen by the action of steam on
+iron.</b> The apparatus used in the preparation of hydrogen from
+iron and<span class='pagenum'><a name="Page_30" id="Page_30">[Pg 30]</a></span> steam is shown in Fig. 9. A porcelain or iron tube
+<i>B</i>, about 50 cm. in length and 2 cm. or 3 cm. in diameter, is
+partially filled with fine iron wire or tacks and connected as
+shown in the figure. The tube <i>B</i> is heated, slowly at first,
+until the iron is red-hot. Steam is then conducted through the
+tube by boiling the water in the flask <i>A</i>. The hot iron
+combines with the oxygen in the steam, setting free the
+hydrogen, which is collected over water. The gas which first
+passes over is mixed with the air previously contained in the
+flask and tube, and is allowed to escape, <i>since a mixture of
+hydrogen with oxygen or air explodes violently when brought in
+contact with a flame</i>. It is evident that the flask <i>A</i> must be
+disconnected from the tube before the heat is withdrawn.</p>
+
+<p>That the gas obtained is different from air and oxygen may be
+shown by holding a bottle of it mouth downward and bringing a
+lighted splint into it. The hydrogen is ignited and burns with
+an almost colorless flame.</p></div>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig9.jpg" width="400" height="238" alt="Fig. 9" title="" />
+<span class="caption">Fig. 9</span>
+</div>
+
+<p><b>Preparation from acids</b> (<i>usual laboratory method</i>). While hydrogen can
+be prepared from water, either by the action of the electric current or
+by the action of certain metals, these methods are not economical and
+are therefore but little used. In the laboratory hydrogen is generally
+prepared from compounds known as acids, all of which contain hydrogen.
+When acids are brought in contact with certain metals, the metals
+dissolve and set free the hydrogen<span class='pagenum'><a name="Page_31" id="Page_31">[Pg 31]</a></span> of the acid. Although this reaction
+is a quite general one, it has been found most convenient in preparing
+hydrogen by this method to use either zinc or iron as the metal and
+either hydrochloric or sulphuric acid as the acid. Hydrochloric acid is
+a compound consisting of 2.77% hydrogen and 97.23% chlorine, while
+sulphuric acid consists of 2.05% hydrogen, 32.70% sulphur, and 65.25%
+oxygen.</p>
+
+<p>The changes which take place in the preparation of hydrogen from zinc
+and sulphuric acid (diluted with water) may be represented as follows:</p>
+
+<pre>
+        _        _               _       _
+       | hydrogen |(sulphuric   | zinc    |(zinc
+zinc + | sulphur  | acid)     = | sulphur | sulphate) + hydrogen
+       |_oxygen  _|             |_oxygen _|
+</pre>
+
+<p>In other words, the zinc has taken the place of the hydrogen in
+sulphuric acid. The resulting compound contains zinc, sulphur, and
+oxygen, and is known as zinc sulphate. This remains dissolved in the
+water present in the acid. It may be obtained in the form of a white
+solid by evaporating the liquid left after the metal has passed into
+solution.</p>
+
+<p>When zinc and hydrochloric acid are used the following changes take
+place:</p>
+
+<pre>
+        _        _                  _        _
+       | hydrogen |(hydrochloric   | zinc     |(zinc
+zinc + |_chlorine_| acid)        = |_chlorine_| chloride) + hydrogen
+</pre>
+
+<p>When iron is used the changes which take place are exactly similar to
+those just given for zinc.</p>
+
+<div class="figcenter" style="width: 432px;">
+<img src="images/imagefig10.jpg" width="432" height="262" alt="Fig. 10." title="" />
+<span class="caption">Fig. 10.</span>
+</div>
+
+<div class="blockquot"><p><b>Directions for preparing hydrogen from acids.</b> The preparation
+of hydrogen from acids is carried out in the laboratory as
+follows: The metal is placed in a flask or wide-mouthed bottle
+<i>A</i> (Fig. 10) and the acid is added slowly through the funnel
+tube <i>B</i>. The metal dissolves in the acid, while the hydrogen
+which is liberated escapes through the exit tube <i>C</i> and is
+collected over water. It is evident that the hydrogen<span class='pagenum'><a name="Page_32" id="Page_32">[Pg 32]</a></span> which
+passes over first is mixed with the air from the bottle <i>A</i>.
+Hence care must be taken not to bring a flame near the exit
+tube, since, as has been stated previously, such a mixture
+explodes with great violence when brought in contact with a
+flame.</p>
+
+<p><b>Precautions.</b> Both sulphuric acid and zinc, if impure, are
+likely to contain small amounts of arsenic. Such materials
+should not be used in preparing hydrogen, since the arsenic
+present combines with a portion of the hydrogen to form a very
+poisonous gas known as arsine. On the other hand, chemically
+pure sulphuric acid, i.e. sulphuric acid that is entirely free
+from impurities, will not act upon chemically pure zinc. The
+reaction may be started, however, by the addition of a few
+drops of a solution of copper sulphate or platinum
+tetrachloride.</p></div>
+
+<p><b>Physical properties.</b> Hydrogen is similar to oxygen in that it is a
+colorless, tasteless, odorless gas. It is characterized by its extreme
+lightness, being the lightest of all known substances. One liter of the
+gas weighs only 0.08984 g. On comparing this weight with that of an
+equal volume of oxygen, viz., 1.4285 g., the latter is found to be 15.88
+times as heavy as hydrogen. Similarly, air is found to be 14.38 times as
+heavy as hydrogen. Soap bubbles blown with hydrogen rapidly rise in the
+air. On account of its lightness it is possible to pour it upward from
+one bottle into another. Thus, if the bottle <i>A</i> (Fig. 11) is filled
+with hydrogen, placed mouth downward by the side of bottle <i>B</i>,<span class='pagenum'><a name="Page_33" id="Page_33">[Pg 33]</a></span> filled
+with air, and is then gradually inverted under <i>B</i> as indicated in the
+figure, the hydrogen will flow upward into bottle <i>B</i>, displacing the
+air. Its presence in bottle <i>B</i> may then be shown by bringing a lighted
+splint to the mouth of the bottle, when the hydrogen will be ignited by
+the flame. It is evident, from this experiment, that in order to retain
+the gas in an open bottle the bottle must be placed mouth downward.</p>
+
+<div class="figleft" style="width: 200px;">
+<img src="images/imagefig11.jpg" width="200" height="164" alt="Fig. 11" title="" />
+<span class="caption">Fig. 11</span>
+</div>
+
+<p>Hydrogen is far more difficult to liquefy than any other gas, with the
+exception of helium, a rare element recently found to exist in the
+atmosphere. The English scientist Dewar, however, in 1898 succeeded not
+only in obtaining hydrogen in liquid state but also as a solid. Liquid
+hydrogen is colorless and has a density of only 0.07. Its boiling point
+under atmospheric pressure is -252&deg;. Under diminished pressure the
+temperature has been reduced to -262&deg;. The solubility of hydrogen in
+water is very slight, being still less than that of oxygen.</p>
+
+<p>Pure hydrogen produces no injurious results when inhaled. Of course one
+could not live in an atmosphere of the gas, since oxygen is essential to
+respiration.</p>
+
+<p><b>Chemical properties.</b> At ordinary temperatures hydrogen is not an active
+element. A mixture of hydrogen and chlorine, however, will combine with
+explosive violence at ordinary temperature if exposed to the sunlight.
+The union can be brought about also by heating. The product formed in
+either case is hydrochloric acid. Under suitable conditions hydrogen
+combines with nitrogen to form ammonia, and with sulphur to form the
+foul-smelling gas, hydrogen sulphide. The affinity of hydrogen for
+oxygen is so great that<span class='pagenum'><a name="Page_34" id="Page_34">[Pg 34]</a></span> a mixture of hydrogen and oxygen or hydrogen
+and air explodes with great violence when heated to the kindling
+temperature (about 612&deg;). Nevertheless under proper conditions hydrogen
+may be made to burn quietly in either oxygen or air. The resulting
+hydrogen flame is almost colorless and is very hot. The combustion of
+the hydrogen is, of course, due to its union with oxygen. The product of
+the combustion is therefore a compound of hydrogen and oxygen. That this
+compound is water may be shown easily by experiment.</p>
+
+<div class="figcenter" style="width: 250px;">
+<img src="images/imagefig12.jpg" width="250" height="208" alt="Fig. 12" title="" />
+<span class="caption">Fig. 12</span>
+</div>
+
+<div class="blockquot"><p><b>Directions for burning hydrogen in air.</b> The combustion of
+hydrogen in air may be carried out safely as follows: The
+hydrogen is generated in the bottle <i>A</i> (Fig. 12), is dried by
+conducting it through the tube <i>X</i>, filled with some substance
+(generally calcium chloride) which has a great attraction for
+moisture, and escapes through the tube <i>T</i>, the end of which is
+drawn out to a jet. The hydrogen first liberated mixes with the
+air contained in the generator. If a flame is brought near the
+jet before this mixture has all escaped, a violent and very
+dangerous explosion results, since the entire apparatus is
+filled with the explosive mixture. On the other hand, if the
+flame is not applied until all the air has been expelled, the
+hydrogen is ignited and burns quietly, since only the small
+amount of it which escapes from the jet can come in contact
+with the oxygen of the air at any one time. By holding a cold,
+dry bell jar or bottle over the flame, in the manner shown in
+the figure, the steam formed by the combustion of the hydrogen
+is condensed, the water collecting in drops on the sides of the
+jar.</p></div><p><span class='pagenum'><a name="Page_35" id="Page_35">[Pg 35]</a></span></p>
+
+<p><b>Precautions.</b> In order to avoid danger it is absolutely necessary to
+prove that the hydrogen is free from air before igniting it. This can be
+done by testing small amounts of the escaping gas. A convenient and safe
+method of doing this is to fill a test tube with the gas by inverting it
+over the jet. The hydrogen, on account of its lightness, collects in the
+tube, displacing the air. After holding it over the jet for a few
+moments in order that it may be filled with the gas, the tube is gently
+brought, mouth downward, to the flame of a burner placed not nearer than
+an arm's length from the jet. If the hydrogen is mixed with air a slight
+explosion occurs, but if pure it burns quietly in the tube. The
+operation is repeated until the gas burns quietly, when the tube is
+quickly brought back over the jet for an instant, whereby the escaping
+hydrogen is ignited by the flame in the tube.</p>
+
+<div class="figright" style="width: 191px;">
+<img src="images/imagefig13.jpg" width="191" height="219" alt=". Fig. 13" title="" />
+<span class="caption">. Fig. 13</span>
+</div>
+
+<p><b>A mixture of hydrogen and oxygen is explosive.</b> That a mixture of
+hydrogen and air is explosive may be shown safely as follows: A cork
+through which passes a short glass tube about 1 cm. in diameter is
+fitted air-tight into the tubule of a bell jar of 2 l. or 3 l. capacity.
+(A thick glass bottle with bottom removed may be used.) The tube is
+closed with a small rubber stopper and the bell jar filled with
+hydrogen, the gas being collected over water. When entirely filled with
+the gas the jar is removed from the water and supported by blocks of
+wood in order to leave the bottom of the jar open, as shown in Fig. 13.
+The stopper is now removed from the tube in the cork, and the hydrogen,
+which on account of its lightness escapes from the tube, is at once
+lighted. As the hydrogen escapes, the air flows in at the bottom of the
+jar and mixes with the remaining portion of the hydrogen, so that a
+mixture of the two soon forms, and a loud explosion results. The
+explosion is not dangerous, since the bottom of the jar is open, thus
+leaving room for the expansion of the hot gas.</p>
+
+<p>Since air is only one fifth oxygen, the remainder being inert gases, it
+may readily be inferred that a mixture of hydrogen with pure oxygen
+would be far more explosive than a mixture of hydrogen with air. Such
+mixtures should not be made except in small quantities and by
+experienced workers.<span class='pagenum'><a name="Page_36" id="Page_36">[Pg 36]</a></span></p>
+
+<p><b>Hydrogen does not support combustion.</b> While hydrogen is readily
+combustible, it is not a supporter of combustion. In other words,
+substances will not burn in it. This may be shown by bringing a lighted
+candle supported by a stiff wire into a bottle or cylinder of the pure
+gas, as shown in Fig. 14. The hydrogen is ignited by the flame of the
+candle and burns at the mouth of the bottle, where it comes in contact
+with the oxygen in the air. When the candle is thrust up into the gas,
+its flame is extinguished on account of the absence of oxygen. If slowly
+withdrawn, the candle is relighted as it passes through the layer of
+burning hydrogen.</p>
+
+<div class="figleft" style="width: 76px;">
+<img src="images/imagefig14.jpg" width="76" height="300" alt="Fig. 14" title="" />
+<span class="caption">Fig. 14</span>
+</div>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/imagefig15.jpg" width="500" height="341" alt="Fig. 15" title="" />
+<span class="caption">Fig. 15</span>
+</div>
+
+<p><b>Reduction.</b> On account of its great affinity for oxygen, hydrogen has the
+power of abstracting it from many of its compounds. Thus, if a stream of
+hydrogen, dried by passing through the tube <i>B</i> (Fig. 15), filled with<span class='pagenum'><a name="Page_37" id="Page_37">[Pg 37]</a></span>
+calcium chloride, is conducted through the tube <i>C</i> containing some
+copper oxide, heated to a moderate temperature, the hydrogen abstracts
+the oxygen from the copper oxide. The change may be represented as
+follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i1">hydrogen + {copper}                 {hydrogen}<br /></span>
+<span class="i12">{oxygen}(copper oxide) = {oxygen  }(water) + copper<br /></span>
+</div></div>
+
+<p>The water formed collects in the cold portions of the tube <i>C</i> near its
+end. In this experiment the copper oxide is said to undergo reduction.
+<i>Reduction may therefore be defined as the process of withdrawing oxygen
+from a compound.</i></p>
+
+<p><b>Relation of reduction to oxidation.</b> At the same time that the copper
+oxide is reduced it is clear that the hydrogen is oxidized, for it
+combines with the oxygen given up by the copper oxide. The two processes
+are therefore very closely related, and it usually happens that when one
+substance is oxidized some other substance is reduced. That substance
+which gives up its oxygen is called an <i>oxidizing agent</i>, while the
+substance which unites with the oxygen is called a <i>reducing agent</i>.</p>
+
+<p><b>The oxyhydrogen blowpipe.</b> This is a form of apparatus used for burning
+hydrogen in pure oxygen. As has been previously stated, the flame
+produced by the combustion of hydrogen in the air is very hot. It is
+evident that if pure oxygen is substituted for air, the temperature
+reached will be much higher, since there are no inert gases to absorb
+the heat. The oxyhydrogen blowpipe, used to effect this combination,
+consists of a small tube placed within a larger one, as shown in Fig.
+16.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig16.jpg" width="400" height="136" alt="Fig. 16" title="" />
+<span class="caption">Fig. 16</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_38" id="Page_38">[Pg 38]</a></span></p>
+
+<p>The hydrogen, stored under pressure, generally in steel cylinders, is
+first passed through the outer tube and ignited at the open end of the
+tube. The oxygen from a similar cylinder is then conducted through the
+inner tube, and mixes with the hydrogen at the end of the tube. In order
+to produce the maximum heat, the hydrogen and oxygen must be admitted to
+the blowpipe in the exact proportion in which they combine, viz., 2
+volumes of hydrogen to 1 of oxygen, or by weight, 1 part of hydrogen to
+7.94 parts of oxygen. The intensity of the heat may be shown by bringing
+into the flame pieces of metal such as iron wire or zinc. These burn
+with great brilliancy. Even platinum, having a melting point of 1779&deg;,
+may be melted by the heat of the flame.</p>
+
+<p>While the oxyhydrogen flame is intensely hot, it is almost non-luminous.
+If directed against some infusible substance like ordinary lime (calcium
+oxide), the heat is so intense that the lime becomes incandescent and
+glows with a brilliant light. This is sometimes used as a source of
+light, under the name of <i>Drummond</i> or <i>lime light</i>.</p>
+
+<div class="figcenter" style="width: 398px;">
+<img src="images/imagefig17.jpg" width="398" height="400" alt="Fig. 17" title="" />
+<span class="caption">Fig. 17</span>
+</div>
+
+<p><b>The blast lamp.</b> A similar form of apparatus is commonly used in the
+laboratory as a source of heat under the name <i>blast lamp</i> (Fig. 17).
+This differs from the oxyhydrogen blowpipe only in the size of the
+tubes. In place of the hydrogen and oxygen the more accessible coal gas
+and air are respectively used. The former is composed largely of a
+mixture of free hydrogen and gaseous compounds of carbon and hydrogen.
+While the temperature of the flame is not so high as that of the
+oxyhydrogen blowpipe, it nevertheless suffices for most chemical
+operations carried out in the laboratory.</p>
+
+<p><b>Uses of hydrogen.</b> On account of its cost, hydrogen is but little used
+for commercial purposes. It is sometimes used as a material for the
+inflation of balloons, but usually the much cheaper coal gas is
+substituted for it. Even hot air is often used when the duration of
+ascension is very short. It has been used also as a source of heat and
+light<span class='pagenum'><a name="Page_39" id="Page_39">[Pg 39]</a></span> in the oxyhydrogen blowpipe. Where the electric current is
+available, however, this form of apparatus has been displaced almost
+entirely by the electric light and electric furnace, which are much more
+economical and more powerful sources of light and heat.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Will a definite weight of iron decompose an unlimited weight of
+steam?</p>
+
+<p><b>2.</b> Why is oxygen passed through the inner tube of the oxyhydrogen
+blowpipe rather than the outer?</p>
+
+<p><b>3.</b> In Fig. 14, will the flame remain at the mouth of the tube?</p>
+
+<p><b>4.</b> From Fig. 15, suggest a way for determining experimentally the
+quantity of water formed in the reaction.</p>
+
+<p><b>5.</b> Distinguish clearly between the following terms: oxidation,
+reduction, combustion, and kindling temperature.</p>
+
+<p><b>6.</b> Is oxidation always accompanied by reduction?</p>
+
+<p><b>7.</b> What is the source of heat in the lime light? What is the exact use
+of lime in this instrument?</p>
+
+<p><b>8.</b> In Fig. 12, why is it necessary to dry the hydrogen by means of the
+calcium chloride in the tube <i>X</i>?</p>
+
+<p><b>9.</b> At what pressure would the weight of 1 l. of hydrogen be equal to
+that of oxygen under standard conditions?</p>
+
+<p><b>10.</b> (<i>a</i>) What weight of hydrogen can be obtained from 150 g. of
+sulphuric acid? (<i>b</i>) What volume would this occupy under standard
+conditions? (<i>c</i>) The density of sulphuric acid is 1.84. What volume
+would the 150 g. of the acid occupy?</p>
+
+<p><b>11.</b> How many liters of hydrogen can be obtained from 50 cc. of sulphuric
+acid having a density of 1.84?</p>
+
+<p><b>12.</b> Suppose you wish to fill five liter bottles with hydrogen, the gas
+to be collected over water in your laboratory, how many cubic
+centimeters of sulphuric acid would be required?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_40" id="Page_40">[Pg 40]</a></span></p>
+<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV</h2>
+
+<h3>COMPOUNDS OF HYDROGEN AND OXYGEN; WATER AND HYDROGEN DIOXIDE</h3>
+
+
+<h3>WATER</h3>
+
+<p><b>Historical.</b> Water was long regarded as an element. In 1781 Cavendish
+showed that it is formed by the union of hydrogen and oxygen. Being a
+believer in the phlogiston theory, however, he failed to interpret his
+results correctly. A few years later Lavoisier repeated Cavendish's
+experiments and showed that water must be regarded as a compound of
+hydrogen and oxygen.</p>
+
+<p><b>General methods employed for the determination of the composition of a
+compound.</b> The composition of a compound may be determined by either of
+two general processes these are known as <i>analysis</i> and <i>synthesis</i>.</p>
+
+<p>1. <i>Analysis</i> is the process of decomposing a compound into its
+constituents and determining what these constituents are. The analysis
+is <i>qualitative</i> when it results in merely determining what elements
+compose the compound; it is <i>quantitative</i> when the exact percentage of
+each constituent is determined. Qualitative analysis must therefore
+precede quantitative analysis, for it must be known what elements, are
+in a compound before a method can be devised for determining exactly how
+much of each is present.</p>
+
+<p>2. <i>Synthesis</i> is the process of forming a compound from its constituent
+parts. It is therefore the reverse of analysis. Like analysis, it may be
+either qualitative or quantitative.<span class='pagenum'><a name="Page_41" id="Page_41">[Pg 41]</a></span></p>
+
+<p><b>Application of these methods to the determination of the composition of
+water.</b> The determination of the composition of water is a matter of
+great interest not only because of the importance of the compound but
+also because the methods employed illustrate the general methods of
+analysis and synthesis.</p>
+
+<p><b>Methods based on analysis.</b> The methods based on analysis may be either
+qualitative or quantitative in character.</p>
+
+<div class="figright" style="width: 281px;">
+<img src="images/imagefig18.jpg" width="281" height="400" alt="Fig. 18" title="" />
+<span class="caption">Fig. 18</span>
+</div>
+
+<p>1. <i>Qualitative analysis.</i> As was stated in the study of oxygen, water
+may be separated into its component parts by means of the electric
+current. The form of apparatus ordinarily used for effecting this
+analysis is shown in Fig. 18. A platinum wire, to the end of which is
+attached a small piece of platinum foil (about 15 mm. by 25 mm.), is
+fused through each of the tubes <i>B</i> and <i>D</i>, as shown in the figure. The
+stopcocks at the ends of these tubes are opened and water, to which has
+been added about one tenth of its volume of sulphuric acid, is poured
+into the tube <i>A</i> until the side tubes <i>B</i> and <i>D</i> are completely
+filled. The stopcocks are then closed. The platinum wires extending into
+the tubes <i>B</i> and <i>D</i> are now connected with the wires leading from two
+or three dichromate cells joined in series. The pieces of platinum foil
+within the tubes thus become the electrodes, and the current flows from
+one to the other through the acidulated water. As soon as the current
+passes, bubbles of gas rise from each of the electrodes and collect in
+the upper part of the tubes. The gas<span class='pagenum'><a name="Page_42" id="Page_42">[Pg 42]</a></span> rising from the negative electrode
+is found to be hydrogen, while that from the positive electrode is
+oxygen. It will be seen that the volume of the hydrogen is approximately
+double that of the oxygen. Oxygen is more soluble in water than
+hydrogen, and a very little of it is also lost by being converted into
+ozone and other substances. It has been found that when the necessary
+corrections are made for the error due to these facts, the volume of the
+hydrogen is exactly double that of the oxygen.</p>
+
+<p>Fig. 19 illustrates a simpler form of apparatus, which may be used in
+place of that shown in Fig. 18. A glass or porcelain dish is partially
+filled with water to which has been added the proper amount of acid. Two
+tubes filled with the same liquid are inverted over the electrodes. The
+gases resulting from the decomposition of the water collect in the
+tubes.</p>
+
+<div class="figleft" style="width: 250px;">
+<img src="images/imagefig19.jpg" width="250" height="278" alt="Fig. 19" title="" />
+<span class="caption">Fig. 19</span>
+</div>
+
+<p>2. <i>Quantitative analysis.</i> The analysis just described is purely
+qualitative and simply shows that water contains hydrogen and oxygen. It
+does not prove the absence of other elements; indeed it does not prove
+that the hydrogen and oxygen are present in the proportion in which they
+are liberated by the electric current. The method may be made
+quantitative, however, by weighing the water decomposed and also the
+hydrogen and oxygen obtained in its decomposition. If the combined
+weights of the hydrogen and oxygen exactly equal the weight of the water
+decomposed, then it would<span class='pagenum'><a name="Page_43" id="Page_43">[Pg 43]</a></span> be proved that the water consists of hydrogen
+and oxygen in the proportion in which they are liberated by the electric
+current. This experiment is difficult to carry out, however, so that the
+more accurate methods based on synthesis are used.</p>
+
+<p><b>Methods based on synthesis.</b> Two steps are necessary to ascertain the
+exact composition of water by synthesis: (1) to show by qualitative
+synthesis that water is formed by the union of oxygen with hydrogen; (2)
+to determine by quantitative synthesis in what proportion the two
+elements unite to form water. The fact that water is formed by the
+combination of oxygen with hydrogen was proved in the preceding chapter.
+The quantitative synthesis may be made as follows:</p>
+
+<div class="figright" style="width: 177px;">
+<img src="images/imagefig20.jpg" width="177" height="378" alt="Fig. 20" title="" />
+<span class="caption">Fig. 20</span>
+</div>
+
+<p>The combination of the two gases is brought about in a tube called a
+eudiometer. This is a graduated tube about 60 cm. long and 2 cm. wide,
+closed at one end (Fig. 20). Near the closed end two platinum wires are
+fused through the glass, the ends of the wires within the tube being
+separated by a space of 2 mm or 3 mm. The tube is entirely filled with
+mercury and inverted in a vessel of the same liquid. Pure hydrogen is
+passed into the tube until it is about one fourth filled. The volume of
+the gas is then read off on the scale and reduced to standard
+conditions. Approximately an equal volume of pure oxygen is then
+introduced and the volume again read off and reduced to standard
+conditions. This gives the total volume of the two gases. From this the
+volume of the oxygen introduced may be determined by<span class='pagenum'><a name="Page_44" id="Page_44">[Pg 44]</a></span> subtracting from
+it the volume of the hydrogen. The combination of the two gases is now
+brought about by connecting the two platinum wires with an induction
+coil and passing a spark from one wire to the other. Immediately a
+slight explosion occurs. The mercury in the tube is at first depressed
+because of the expansion of the gases due to the heat generated, but at
+once rebounds, taking the place of the gases which have combined to form
+water. The volume of the water in the liquid state is so small that it
+may be disregarded in the calculations. In order that the temperature of
+the residual gas and the mercury may become uniform, the apparatus is
+allowed to stand for a few minutes. The volume of the gas is then read
+off and reduced to standard conditions, so that it may be compared with
+the volumes of the hydrogen and oxygen originally taken. The residual
+gas is then tested in order to ascertain whether it is hydrogen or
+oxygen, experiments having proved that it is never a mixture of the two.
+From the information thus obtained the composition of the water may be
+calculated. Thus, suppose the readings were as follows:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Volume of hydrogen taken</td><td align='left'>20.3 cc.</td></tr>
+<tr><td align='left'>Volume of hydrogen and oxygen</td><td align='left'>38.7</td></tr>
+<tr><td align='left'>Volume of oxygen</td><td align='left'>18.4</td></tr>
+<tr><td align='left'>Volume of gas left after combination has taken place (oxygen)</td><td align='left'>&nbsp;8.3</td></tr>
+</table></div>
+
+<p>The 20.3 cc. of hydrogen have combined with 18.4 cc. minus 8.3 cc. (or
+10.1 cc.) of oxygen; or approximately 2 volumes of hydrogen have
+combined with 1 of oxygen. Since oxygen is 15.88 times as heavy as
+hydrogen, the proportion by weight in which the two gases combine is 1
+part of hydrogen to 7.94 of oxygen.<span class='pagenum'><a name="Page_45" id="Page_45">[Pg 45]</a></span></p>
+
+<p><b>Precaution.</b> If the two gases are introduced into the eudiometer in the
+exact proportions in which they combine, after the combination has taken
+place the liquid will rise and completely fill the tube. Under these
+conditions, however, the tube is very likely to be broken by the sudden
+upward rush of the liquid. Hence in performing the experiment care is
+taken to introduce an excess of one of the gases.</p>
+
+<p><b>A more convenient form of eudiometer.</b> A form of eudiometer (Fig. 21)
+different from that shown on page 43 is sometimes used to avoid the
+calculations necessary in reducing the volumes of the gases to the same
+conditions of temperature and pressure in order to make comparisons.
+With this apparatus it is possible to take the readings of the volumes
+under the same conditions of temperature and pressure, and thus compare
+them directly. The apparatus (Fig. 21) is filled with mercury and the
+gases introduced into the tube <i>A</i>. The experiment is carried out as in
+the preceding one, except that before taking the reading of the gas
+volumes, mercury is either added to the tube <i>B</i> or withdrawn from it by
+means of the stopcock <i>C</i>, until it stands at exactly the same height in
+both tubes. The gas inclosed in tube <i>A</i> is then under atmospheric
+pressure; and since but a few minutes are required for performing the
+experiment, the conditions of temperature and pressure may be regarded
+as constant. Hence the volumes of the hydrogen and oxygen and of the
+residual gas may be read off from the tube and directly compared.</p>
+
+<div class="figcenter" style="width: 176px;">
+<img src="images/imagefig21.jpg" width="176" height="400" alt="Fig. 21" title="" />
+<span class="caption">Fig. 21</span>
+</div>
+
+<p><b>Method used by Berzelius and Dumas.</b> The method used by these
+investigators enables us to determine directly the proportion by weight
+in which the hydrogen and oxygen combine. Fig. 22 illustrates the
+apparatus used in making this determination. <i>B</i> is a glass tube
+containing copper oxide. <i>C</i> and <i>D</i> are glass tubes filled with calcium
+chloride, a substance which has great affinity for water.<span class='pagenum'><a name="Page_46" id="Page_46">[Pg 46]</a></span> The tubes <i>B</i>
+and <i>C</i>, including their contents, are carefully weighed, and the
+apparatus connected as shown in the figure. A slow current of pure
+hydrogen is then passed through <i>A</i>, and that part of the tube <i>B</i> which
+contains copper oxide is carefully heated. The hydrogen combines with
+the oxygen present in the copper oxide to form water, which is absorbed
+by the calcium chloride in tube <i>C</i>. The calcium chloride in tube <i>D</i>
+prevents any moisture entering tube <i>C</i> from the air. The operation is
+continued until an appreciable amount of water has been formed. The
+tubes <i>B</i> and <i>C</i> are then weighed once more. The loss of weight in the
+tube <i>B</i> will exactly equal the weight of oxygen taken up from the
+copper oxide in the formation of the water. The gain in weight in the
+tube <i>C</i> will exactly equal the weight of the water formed. The
+difference in these weights will of course equal the weight of the
+hydrogen present in the water formed.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig22.jpg" width="400" height="134" alt="Fig. 22" title="" />
+<span class="caption">Fig. 22</span>
+</div>
+
+<p><b>Dumas' results.</b> The above method for the determination of the
+composition of water was first used by Berzelius in 1820. The work was
+repeated in 1843 by Dumas, the average of whose results is as follows:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Weight of water formed</td><td align='left'>236.36 g.</td></tr>
+<tr><td align='left'>Oxygen given up by the copper oxide</td><td align='left'>210.04</td></tr>
+<tr><td align='left'></td><td align='left'>&mdash;&mdash;&mdash;</td></tr>
+<tr><td align='left'>Weight of hydrogen present in water</td><td align='left'>26.32</td></tr>
+</table></div>
+
+<p><span class='pagenum'><a name="Page_47" id="Page_47">[Pg 47]</a></span></p>
+
+<p>According to this experiment the ratio of hydrogen to oxygen in water is
+therefore 26.32 to 210.04, or as l to 7.98</p>
+
+<p><b>Morley's results.</b> The American chemist Morley has recently determined
+the composition of water, extreme precautions being taken to use pure
+materials and to eliminate all sources of error. The hydrogen and oxygen
+which combined, as well as the water formed, were all accurately
+weighed. According to Morley's results, 1 part of hydrogen by weight
+combines with 7.94 parts of oxygen to form water.</p>
+
+<p><b>Comparison of results obtained.</b> From the above discussions it is easy to
+see that it is by experiment alone that the composition of a compound
+can be determined. Different methods may lead to slightly different
+results. The more accurate the method chosen and the greater the skill
+with which the experiment is carried out, the more accurate will be the
+results. It is generally conceded by chemists that the results obtained
+by Morley in reference to the composition of water are the most accurate
+ones. In accordance with these results, then, <i>water must be regarded as
+a compound containing hydrogen and oxygen in the proportion of 1 part by
+weight of hydrogen to 7.94 parts by weight of oxygen</i>.</p>
+
+<p><b>Relation between the volume of aqueous vapor and the volumes of the
+hydrogen and oxygen which combine to form it.</b> When the quantitative
+synthesis of water is carried out in the eudiometer as described above,
+the water vapor formed by the union of the hydrogen and oxygen at once
+condenses. The volume of the resulting liquid is so small that it may be
+disregarded in making the calculations. If, however, the experiment is
+carried out at a temperature of 100&deg; or above, the water-vapor formed is
+not condensed and it thus becomes possible to compare the volume of the<span class='pagenum'><a name="Page_48" id="Page_48">[Pg 48]</a></span>
+vapor with the volumes of hydrogen and oxygen which combined to form it.
+This can be accomplished by surrounding the arm <i>A</i> of the eudiometer
+(Fig. 23) with the tube <i>B</i> through which is passed the vapor obtained
+by boiling some liquid which has a boiling point above 100&deg;. In this way
+it has been proved that 2 volumes of hydrogen and 1 volume of oxygen
+combine to form exactly 2 volumes of water vapor, the volumes all being
+measured under the same conditions of temperature and pressure. It will
+be noted that the relation between these volumes may be expressed by
+whole numbers. The significance of this very important fact will be
+discussed in a subsequent chapter.</p>
+
+<div class="figright" style="width: 238px;">
+<img src="images/imagefig23.jpg" width="238" height="400" alt="Fig. 23" title="" />
+<span class="caption">Fig. 23</span>
+</div>
+
+<p><b>Occurrence of water.</b> Water not only covers about three fourths of the
+surface of the earth, and is present in the atmosphere in the form of
+moisture, but it is also a common constituent of the soil and rocks and
+of almost every form of animal and vegetable organism. The human body is
+nearly 70% water. This is derived not only from the water which we drink
+but also from the food which we eat, most of which contains a large
+percentage of water. Thus potatoes contain about 78% of water, milk 85%,
+beef over 50%, apples 84%, tomatoes 94%.</p>
+
+<p><b>Impurities in water.</b> Chemically pure water contains only hydrogen and
+oxygen. Such a water never occurs in nature, however, for being a good
+solvent, it takes up certain substances from the rocks and soil with
+which it comes in contact. When such waters are evaporated these<span class='pagenum'><a name="Page_49" id="Page_49">[Pg 49]</a></span>
+substances are deposited in the form of a residue. Even rain water,
+which is the purest form occurring in nature, contains dust particles
+and gases dissolved from the atmosphere. The foreign matter in water is
+of two kinds, namely, <i>mineral</i>, such as common salt and limestone, and
+<i>organic</i>, that is the products of animal and vegetable life.</p>
+
+<div class="blockquot"><p><b>Mineral matter in water.</b> The amount and nature of the mineral
+matter present in different waters vary greatly, depending on
+the character of the rocks and soil with which the waters come
+in contact. The more common of the substances present are
+common salt and compounds of calcium, magnesium, and iron. One
+liter of the average river water contains about 175 mg. of
+mineral matter. Water from deep wells naturally contains more
+mineral matter than river water, generally two or three times
+as much, while sea water contains as much as 35,000 mg. to the
+liter.</p></div>
+
+<p><b>Effect of impurities on health.</b> The mineral matter in water does not,
+save in very exceptional cases, render the water injurious to the human
+system. In fact the presence of a certain amount of such matter is
+advantageous, supplying the mineral constituents necessary for the
+formation of the solid tissues of the body. The presence of organic
+matter, on the other hand, must always be regarded with suspicion. This
+organic matter may consist not only of the products of animal and
+vegetable life but also of certain microscopic forms of living organisms
+which are likely to accompany such products. Contagious diseases are
+known to be due to the presence in the body of minute living organisms
+or germs. Each disease is caused by its own particular kind of germ.
+Through sewage these germs may find their way from persons afflicted
+with disease into the water supply, and it is principally through the
+drinking water that certain of these diseases, especially typhoid fever,
+are spread. It becomes of great importance, therefore, to be<span class='pagenum'><a name="Page_50" id="Page_50">[Pg 50]</a></span> able to
+detect such matter when present in drinking water as well as to devise
+methods whereby it can be removed or at least rendered harmless.</p>
+
+<div class="blockquot"><p><b>Analysis of water.</b> The mineral analysis of a water is, as the
+name suggests, simply the determination of the mineral matter
+present. Sanitary analysis, on the other hand, is the
+determination of the organic matter present. The physical
+properties of a water give no conclusive evidence as to its
+purity, since a water may be unfit for drinking purposes and
+yet be perfectly clear and odorless. Neither can any reliance
+be placed on the simple methods often given for testing the
+purity of water. Only the trained chemist can carry out such
+methods of analysis as can be relied upon.</p></div>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig24.jpg" width="400" height="278" alt="Fig. 24" title="" />
+<span class="caption">Fig. 24</span>
+</div>
+
+<p><b>Purification of water.</b> Three general methods are used for the
+purification of water, namely, <i>distillation</i>, <i>filtration</i>, and
+<i>boiling</i>.</p>
+
+<p>1. <i>Distillation.</i> The most effective way of purifying natural waters is
+by the process of distillation. This consists in boiling the water and
+condensing the steam. Fig. 24 illustrates the process of distillation,
+as commonly conducted<span class='pagenum'><a name="Page_51" id="Page_51">[Pg 51]</a></span> in the laboratory. Ordinary water is poured into
+the flask <i>A</i> and boiled. The steam is conducted through the condenser
+<i>B</i>, which consists essentially of a narrow glass tube sealed within a
+larger one, the space between the two being filled with cold water,
+which is admitted at <i>C</i> and escapes at <i>D</i>. The inner tube is thus kept
+cool and the steam in passing through it is condensed. The water formed
+by the condensation of the steam collects in the receiver <i>E</i> and is
+known as <i>distilled</i> water. Such water is practically pure, since the
+impurities are nonvolatile and remain in the flask <i>A</i>.</p>
+
+<div class="blockquot"><p><b>Commercial distillation.</b> In preparing distilled water on a
+large scale, the steam is generated in a boiler or other metal
+container and condensed by passing it through a pipe made of
+metal, generally tin. This pipe is wound into a spiral and is
+surrounded by a current of cold water. Distilled water is used
+by the chemist in almost all of his work. It is also used in
+the manufacture of artificial ice and for drinking water.</p>
+
+<p><b>Fractional distillation.</b> In preparing distilled water, it is
+evident that if the natural water contains some substance which
+is volatile its vapor will pass over and be condensed with the
+steam, so that the distillate will not be pure water. Even such
+mixtures, however, may generally be separated by repeated
+distillation. Thus, if a mixture of water (boiling point 100&deg;)
+and alcohol (boiling point 78&deg;) is distilled, the alcohol,
+having the lower boiling point, tends to distill first,
+followed by the water. The separation of the two is not
+perfect, however, but may be made nearly so by repeated
+distillations. The process of separating a mixture of volatile
+substances by distillation is known as <i>fractional
+distillation</i>.</p></div>
+
+<p>2. <i>Filtration.</i> The process of distillation practically removes all
+nonvolatile foreign matter, mineral as well as organic. In purifying
+water for drinking purposes, however, it is only necessary to eliminate
+the latter or to render it harmless. This is ordinarily done either by
+filtration or<span class='pagenum'><a name="Page_52" id="Page_52">[Pg 52]</a></span> boiling. In filtration the water is passed through some
+medium which will retain the organic matter. Ordinary charcoal is a
+porous substance and will condense within its pores the organic matter
+in water if brought in contact with it. It is therefore well adapted to
+the construction of filters. Such filters to be effective must be kept
+clean, since it is evident that the charcoal is useless after its pores
+are filled. A more effective type of filter is the Chamberlain-Pasteur
+filter. In this the water is forced through a porous cylindrical cup,
+the pores being so minute as to strain out the organic matter.</p>
+
+<div class="blockquot"><p><b>City filtration beds.</b> For purifying the water supply of cities,
+large filtration beds are prepared from sand and gravel, and
+the water is allowed to filter through these. Some of the
+impurities are strained out by the filter, while others are
+decomposed by the action of certain kinds of bacteria present
+in the sand. Fig. 25 shows a cross section of a portion of the
+filter used in purifying the water supply of Philadelphia. The
+water filters through the sand and gravel and passes into the
+porous pipe <i>A</i>, from which it is pumped into the city mains.
+The filters are covered to prevent the water from freezing in
+cold weather.</p></div>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig25.jpg" width="400" height="321" alt="Fig. 25" title="" />
+<span class="caption">Fig. 25</span>
+</div>
+
+<p>3. <i>Boiling.</i> A simpler and equally efficient method for purifying water
+for drinking purposes consists in boiling the water. It is the germs in
+water that render it dangerous to health. These germs are living forms
+of matter. If the<span class='pagenum'><a name="Page_53" id="Page_53">[Pg 53]</a></span> water is boiled, the germs are killed and the water
+rendered safe. While these germs are destroyed by heat, cold has little
+effect upon them. Thus Dewar, in working with liquid hydrogen, exposed
+some of these minute forms of life to the temperature of boiling
+hydrogen (-252&deg;) without killing them.</p>
+
+<p><b>Self-purification of water.</b> It has long been known that water
+contaminated with organic matter tends to purify itself when exposed to
+the air. This is due to the fact that the water takes up a small amount
+of oxygen from the air, which gradually oxidizes the organic matter
+present in the water. While water is undoubtedly purified in this way,
+the method cannot be relied upon to purify a contaminated water so as to
+render it safe for drinking purposes.</p>
+
+<p><b>Physical properties.</b> Pure water is an odorless and tasteless liquid,
+colorless in thin layers, but having a bluish tinge when observed
+through a considerable thickness. It solidifies at 0&deg; and boils at 100&deg;
+under the normal pressure of one atmosphere. If the pressure is
+increased, the boiling point is raised. When water is cooled it steadily
+contracts until the temperature of 4&deg; is reached: it then expands. Water
+is remarkable for its ability to dissolve other substances, and is the
+best solvent known. Solutions of solids in water are more frequently
+employed in chemical work than are the solid substances, for chemical
+action between substances goes on more readily when they are in solution
+than it does when they are in the solid state.</p>
+
+<p><b>Chemical properties.</b> Water is a very stable substance, or, in other
+words, it does not undergo decomposition readily. To decompose it into
+its elements by heat alone requires a very high temperature; at 2500&deg;,
+for example, only about 5% of the entire amount is decomposed. Though
+very<span class='pagenum'><a name="Page_54" id="Page_54">[Pg 54]</a></span> stable towards heat, water can be decomposed in other ways, as by
+the action of the electrical current or by certain metals.</p>
+
+<div class="blockquot"><p><b>Heat of formation and heat of decomposition are equal.</b> The fact
+that a very high temperature is necessary to decompose water
+into hydrogen and oxygen is in accord with the fact that a
+great deal of heat is evolved by the union of hydrogen and
+oxygen; for it has been proved that the heat necessary to
+decompose a compound into its elements (heat of decomposition)
+is equal to the heat evolved in the formation of a compound
+from its elements (heat of formation).</p></div>
+
+<p><b>Water of crystallization.</b> When a solid is dissolved in water and the
+resulting solution is allowed to evaporate, the solid separates out,
+often in the form of crystals. It has been found that the crystals of
+many compounds, although perfectly dry, give up a definite amount of
+water when heated, the substance at the same time losing its crystalline
+form. Such water is called <i>water of crystallization</i>. This varies in
+amount with different compounds, but is perfectly definite for the same
+compound. Thus, if a perfectly dry crystal of copper sulphate is
+strongly heated in a tube, water is evolved and condenses on the sides
+of the tube, the crystal crumbling to a light powder. The weight of the
+water evolved is always equal to exactly 36.07% of the weight of copper
+sulphate crystals heated. The water must therefore be in chemical
+combination with the substance composing the crystal; for if simply
+mixed with it or adhering to it, not only would the substance appear
+moist but the amount present would undoubtedly vary. The combination,
+however, must be a very weak one, since the water is often expelled by
+even a gentle heat. Indeed, in some cases the water is given up on
+simple exposure to air. Such compounds are said to be <i>efflorescent</i>.
+Thus a crystal of sodium sulphate<span class='pagenum'><a name="Page_55" id="Page_55">[Pg 55]</a></span> (Glauber's salt) on exposure to air
+crumbles to a fine powder, owing to the escape of its water of
+crystallization. Other substances have just the opposite property: they
+absorb moisture when exposed to the air. For example, if a bit of dry
+calcium chloride is placed in moist air, in the course of a few hours it
+will have absorbed sufficient moisture to dissolve it. Such substances
+are said to be <i>deliquescent</i>. A deliquescent body serves as a good
+drying or <i>desiccating</i> agent. We have already employed calcium chloride
+as an agent for absorbing the moisture from hydrogen. Many substances,
+as for example quartz, form crystals which contain no water of
+crystallization.</p>
+
+<div class="blockquot"><p><b>Mechanically inclosed water.</b> Water of crystallization must be
+carefully distinguished from water which is mechanically
+inclosed in a crystal and which can be removed by powdering the
+crystal and drying. Thus, when crystals of common salt are
+heated, the water inclosed in the crystal is changed into steam
+and bursts the crystal with a crackling sound. Such crystals
+are said to <i>decrepitate</i>. That this water is not combined is
+proved by the fact that the amount present varies and that it
+has all the properties of water.</p></div>
+
+<p><b>Uses of water.</b> The importance of water in its relation to life and
+commerce is too well known to require comment. Its importance to the
+chemist has also been pointed out. It remains to call attention to the
+fact that it is used as a standard in many physical measurements. Thus
+0&deg; and 100&deg; on the centigrade scale are respectively the freezing and
+the boiling points of water under normal pressure. The weight of 1 cc.
+of water at its point of greatest density is the unit of weight in the
+metric system, namely, the gram. It is also taken as the unit for the
+determination of the density of liquids and solids as well as for the
+measurement of amounts of heat.<span class='pagenum'><a name="Page_56" id="Page_56">[Pg 56]</a></span></p>
+
+
+<h3>HYDROGEN DIOXIDE</h3>
+
+<p><b>Composition.</b> As has been shown, 1 part by weight of hydrogen combines
+with 7.94 parts by weight of oxygen to form water. It is possible,
+however, to obtain a second compound of hydrogen and oxygen differing
+from water in composition in that 1 part by weight of hydrogen is
+combined with 2 &times; 7.94, or 15.88 parts, of oxygen. This compound is
+called <i>hydrogen dioxide</i> or <i>hydrogen peroxide</i>, the prefixes <i>di-</i> and
+<i>per-</i> signifying that it contains more oxygen than hydrogen oxide,
+which is the chemical name for water.</p>
+
+<p><b>Preparation.</b> Hydrogen dioxide cannot be prepared cheaply by the direct
+union of hydrogen and oxygen, and indirect methods must therefore be
+used. It is commonly prepared by the action of a solution of sulphuric
+acid on barium dioxide. The change which takes place may be indicated as
+follows:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>sulphuric acid</td><td align='left'>+</td><td align='left'>barium dioxide</td><td align='left'>=</td><td align='left'>barium sulphate</td><td align='left'>+</td><td align='left'>hydrogen dioxide</td></tr>
+<tr><td align='left'>&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;</td><td align='left'></td><td align='left'>&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;</td><td align='left'></td><td align='left'>&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;</td><td align='left'></td><td align='left'>&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;&mdash;</td></tr>
+<tr><td align='left'>hydrogen</td><td align='left'></td><td align='left'>barium</td><td align='left'></td><td align='left'>barium</td><td align='left'></td><td align='left'>hydrogen</td></tr>
+<tr><td align='left'>sulphur</td><td align='left'></td><td align='left'>oxygen</td><td align='left'></td><td align='left'>sulphur</td><td align='left'></td><td align='left'>oxygen</td></tr>
+<tr><td align='left'>oxygen</td><td align='left'></td><td align='left'>oxygen</td></tr>
+</table></div>
+
+
+<p>In other words, the barium and hydrogen in the two compounds exchange
+places. By this method a dilute solution of the dioxide in water is
+obtained. It is possible to separate the dioxide from the water by
+fractional distillation. This is attended with great difficulties,
+however, since the pure dioxide is explosive. The distillation is
+carried on under diminished pressure so as to lower the boiling points
+as much as possible; otherwise the high temperature would decompose the
+dioxide.<span class='pagenum'><a name="Page_57" id="Page_57">[Pg 57]</a></span></p>
+
+<p><b>Properties.</b> Pure hydrogen dioxide is a colorless sirupy liquid having a
+density of 1.49. Its most characteristic property is the ease with which
+it decomposes into water and oxygen. One part by weight of hydrogen is
+capable of holding firmly only 7.94 parts of oxygen. The additional 7.94
+parts of oxygen present in hydrogen dioxide are therefore easily
+evolved, the compound breaking down into water and oxygen. This
+decomposition is attended by the generation of considerable heat. In
+dilute solution hydrogen dioxide is fairly stable, although such a
+solution should be kept in a dark, cool place, since both heat and light
+aid in the decomposition of the dioxide.</p>
+
+<p><b>Uses.</b> Solutions of hydrogen dioxide are used largely as oxidizing
+agents. The solution sold by druggists contains 3% of the dioxide and is
+used in medicine as an antiseptic. Its use as an antiseptic depends upon
+its oxidizing properties.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Why does the chemist use distilled water in making solutions, rather
+than filtered water?</p>
+
+<p><b>2.</b> How could you determine the total amount of solid matter dissolved in
+a sample of water?</p>
+
+<p><b>3.</b> How could you determine whether a given sample of water is distilled
+water?</p>
+
+<p><b>4.</b> How could the presence of air dissolved in water be detected?</p>
+
+<p><b>5.</b> How could the amount of water in a food such as bread or potato be
+determined?</p>
+
+<p><b>6.</b> Would ice frozen from impure water necessarily be free from disease
+germs?</p>
+
+<p><b>7.</b> Suppose that the maximum density of water were at 0&deg; in place of 4&deg;;
+what effect would this have on the formation of ice on bodies of water?</p>
+
+<p><b>8.</b> Is it possible for a substance to contain both mechanically inclosed
+water and water of crystallization?<span class='pagenum'><a name="Page_58" id="Page_58">[Pg 58]</a></span></p>
+
+<p><b>9.</b> If steam is heated to 2000&deg; and again cooled, has any chemical change
+taken place in the steam?</p>
+
+<p><b>10.</b> Why is cold water passed into <i>C</i> instead of <i>D</i> (Fig. 24)?</p>
+
+<p><b>11.</b> Mention at least two advantages that a metal condenser has over a
+glass condenser.</p>
+
+<p><b>12.</b> Draw a diagram of the apparatus used in your laboratory for
+supplying distilled water.</p>
+
+<p><b>13.</b> 20 cc. of hydrogen and 7 cc. of oxygen are placed in a eudiometer
+and the mixture exploded. (<i>a</i>) How many cubic centimeters of aqueous
+vapor are formed? (<i>b</i>) What gas and how much of it remains in excess?</p>
+
+<p><b>14.</b> (<i>a</i>) What weight of water can be formed by the combustion of 100 L
+of hydrogen, measured under standard conditions? (<i>b</i>)What volume of
+oxygen would be required in (<i>a</i>)? (<i>c</i>)What weight of potassium
+chlorate is necessary to prepare this amount of oxygen?</p>
+
+<p><b>15.</b> What weight of oxygen is present in 1 kg. of the ordinary hydrogen
+dioxide solution? In the decomposition of this weight of the dioxide
+into water and oxygen, what volume of oxygen (measured under standard
+conditions) is evolved?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_59" id="Page_59">[Pg 59]</a></span></p>
+<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V</h2>
+
+<h3>THE ATOMIC THEORY</h3>
+
+
+<p><b>Three fundamental laws of matter.</b> Before we can gain any very definite
+idea in regard to the structure of matter, and the way in which
+different kinds of substances act chemically upon each other, it is
+necessary to have clearly in view three fundamental laws of matter.
+These laws have been established by experiment, and any conception which
+may be formed concerning matter must therefore be in harmony with them.
+The laws are as follows:</p>
+
+<p><b>Law of conservation of matter.</b> This law has already been touched upon in
+the introductory chapter, and needs no further discussion. It will be
+recalled that it may be stated thus: <i>Matter can neither be created nor
+destroyed, though it can be changed from one form into another.</i></p>
+
+<p><b>Law of definite composition.</b> In the earlier days of chemistry there was
+much discussion as to whether the composition of a given compound is
+always precisely the same or whether it is subject to some variation.
+Two Frenchmen, Berthollet and Proust, were the leaders in this
+discussion, and a great deal of most useful experimenting was done to
+decide the question. Their experiments, as well as all succeeding ones,
+have shown that the composition of a pure chemical compound is always
+exactly the same. Water obtained by melting pure ice, condensing steam,
+burning hydrogen in oxygen, has always 11.18% hydrogen and 88.82% oxygen
+in it. Red oxide of mercury, from whatever source it is obtained,
+contains 92.6%<span class='pagenum'><a name="Page_60" id="Page_60">[Pg 60]</a></span> mercury and 7.4% oxygen. This truth is known as <i>the law
+of definite composition</i>, and may be stated thus: <i>The composition of a
+chemical compound never varies.</i></p>
+
+<p><b>Law of multiple proportion.</b> It has already been noted, however, that
+hydrogen and oxygen combine in two different ratios to form water and
+hydrogen dioxide respectively. It will be observed that this fact does
+not contradict the law of definite composition, for entirely different
+substances are formed. These compounds differ from each other in
+composition, but the composition of each one is always constant. This
+ability of two elements to unite in more than one ratio is very
+frequently observed. Carbon and oxygen combine in two different ratios;
+nitrogen and oxygen combine to form as many as five distinct compounds,
+each with its own precise composition.</p>
+
+<p>In the first decade of the last century John Dalton, an English
+school-teacher and philosopher, endeavored to find some rule which holds
+between the ratios in which two given substances combine. His studies
+brought to light a very simple relation, which the following examples
+will make clear. In water the hydrogen and oxygen are combined in the
+ratio of 1 part by weight of hydrogen to 7.94 parts by weight of oxygen.
+In hydrogen dioxide the 1 part by weight of hydrogen is combined with
+15.88 parts by weight of oxygen. The ratio between the amounts of oxygen
+which combine with the same amount of hydrogen to form water and
+hydrogen dioxide respectively is therefore 7.94: 15.88, or 1: 2.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/dalton.jpg" width="400" height="520" alt="JOHN DALTON (English) (1766-1844)
+
+Developed the atomic theory; made many studies on the properties and the
+composition of gases. His book entitled &quot;A New System of Chemical
+Philosophy&quot; had a large influence on the development of chemistry" title="" />
+<span class="caption">JOHN DALTON (English) (1766-1844)<br /><br />
+
+Developed the atomic theory; made many studies on the properties and the
+composition of gases. His book entitled &quot;A New System of Chemical
+Philosophy&quot; had a large influence on the development of chemistry</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_61" id="Page_61">[Pg 61]</a></span></p>
+
+<p>Similarly, the element iron combines with oxygen to form two oxides, one
+of which is black and the other red. By analysis it has been shown that
+the former contains 1 part by weight of iron combined with 0.286 parts
+by weight of oxygen, while the latter contains 1 part by weight of iron
+combined with 0.429 parts by weight of oxygen. Here again we find that
+the amounts of oxygen which combine with the same fixed amount of iron
+to form the two compounds are in the ratio of small whole numbers, viz.,
+2:3.</p>
+
+<p>Many other examples of this simple relation might be given, since it has
+been found to hold true in all cases where more than one compound is,
+formed from the same elements. Dalton's law of multiple proportion
+states these facts as follows: <i>When any two elements,</i> A <i>and</i> B,
+<i>combine to form more than one compound, the amounts of</i> B <i>which unite
+with any fixed amount of</i> A <i>bear the ratio of small whole numbers to
+each other</i>.</p>
+
+<p><b>Hypothesis necessary to explain the laws of matter.</b> These three
+generalizations are called <i>laws</i>, because they express in concise
+language truths which are found by careful experiment to hold good in
+all cases. They do not offer any explanation of the facts, but merely
+state them. The human mind, however, does not rest content with the mere
+bare facts, but seeks ever to learn the explanation of the facts. A
+suggestion which is offered to explain such a set of facts is called an
+<i>hypothesis</i>. The suggestion which Dalton offered to explain the three
+laws of matter, called the <i>atomic hypothesis</i>, was prompted by his view
+of the constitution of matter, and it involves three distinct
+assumptions in regard to the nature of matter and chemical action.
+Dalton could not prove these assumptions to be true, but he saw that if
+they were true the laws of matter become very easy to understand.</p>
+
+<p><b>Dalton's atomic hypothesis.</b> The three assumptions which Dalton made in
+regard to the nature of matter, and which together constitute the atomic
+hypothesis, are these:<span class='pagenum'><a name="Page_62" id="Page_62">[Pg 62]</a></span></p>
+
+<p>1. All elements are made up of minute, independent particles which
+Dalton designated as <i>atoms</i>.</p>
+
+<p>2. All atoms of the same element have equal masses; those of different
+elements have different masses; in any change to which an atom is
+subjected its mass does not change.</p>
+
+<p>3. When two or more elements unite to form a compound, the action
+consists in the union of a definite small number of atoms of each
+element to form a small particle of the compound. The smallest particles
+of a given compound are therefore exactly alike in the number and kinds
+of atoms which they contain, and larger masses of the substances are
+simply aggregations of these least particles.</p>
+
+<p><b>Molecules and atoms.</b> Dalton applied the name atom not only to the minute
+particles of the elements but also to the least particles of compounds.
+Later Avogadro, an Italian scientist, pointed out the fact that the two
+are different, since the smallest particle of an element is a unit,
+while that of a compound must have at least two units in it. He
+suggested the name <i>molecule</i> for the least particle of a compound which
+can exist, retaining the name <i>atom</i> for the smallest particle of an
+element. In accordance with this distinction, we may define the atom and
+the molecule as follows: <i>An atom is the smallest particle of an element
+which can exist. A molecule is the smallest particle of a compound which
+can exist.</i> It will be shown in a subsequent chapter that sometimes two
+or more atoms of the same element unite with each other to form
+molecules of the element. While the term atom, therefore, is applicable
+only to elements, the term molecule is applicable both to elements and
+compounds.<span class='pagenum'><a name="Page_63" id="Page_63">[Pg 63]</a></span></p>
+
+<p><b>The atomic hypothesis and the laws of matter.</b> Supposing the atomic
+hypothesis to be true, let us now see if it is in harmony with the laws
+of matter.</p>
+
+<p>1. <i>The atomic hypothesis and the law of conservation of matter.</i> It is
+evident that if the atoms never change their masses in any change which
+they undergo, the total quantity of matter can never change and the law
+of conservation of matter must follow.</p>
+
+<p>2. <i>The atomic hypothesis and the law of definite composition.</i>
+According to the third supposition, when iron combines with sulphur the
+union is between definite numbers of the two kinds of atoms. In the
+simplest case one atom of the one element combines with one atom of the
+other. If the sulphur and the iron atoms never change their respective
+masses when they unite to form a molecule of iron sulphide, all iron
+sulphide molecules will have equal amounts of iron in them and also of
+sulphur. Consequently any mass made up of iron sulphide molecules will
+have the same fraction of iron by weight as do the individual iron
+sulphide molecules. Iron sulphide, from whatever source, will have the
+same composition, which is in accordance with the law of definite
+composition.</p>
+
+<p>3. <i>The atomic hypothesis and the law of multiple proportion.</i> But this
+simplest case may not always be the only one. Under other conditions one
+atom of iron might combine with two of sulphur to form a molecule of a
+second compound. In such a case the one atom of iron would be in
+combination with twice the mass of sulphur that is in the first
+compound, since the sulphur atoms all have equal masses. What is true
+for one molecule will be true for any number of them; consequently when
+such quantities of these two compounds are selected as are found to
+contain<span class='pagenum'><a name="Page_64" id="Page_64">[Pg 64]</a></span> the same amount of iron, the one will contain twice as much
+sulphur as the other.</p>
+
+<p>The combination between the atoms may of course take place in other
+simple ratios. For example, two atoms of one element might combine with
+three or with five of the other. In all such cases it is clear that the
+law of multiple proportion must hold true. For on selecting such numbers
+of the two kinds of molecules as have the same number of the one kind of
+atoms, the numbers of the other kind of atoms will stand in some simple
+ratio to each other, and their weights will therefore stand in the same
+simple ratio.</p>
+
+<p><b>Testing the hypothesis.</b> Efforts have been made to find compounds which
+do not conform to these laws, but all such attempts have resulted in
+failure. If such compounds should be found, the laws would be no longer
+true, and the hypothesis of Dalton would cease to possess value. When an
+hypothesis has been tested in every way in which experiment can test it,
+and is still found to be in harmony with the facts in the case, it is
+termed a <i>theory</i>. We now speak of the atomic theory rather than of the
+atomic hypothesis.</p>
+
+<p><b>Value of a theory.</b> The value of a theory is twofold. It aids in the
+clear understanding of the laws of nature because it gives an
+intelligent idea as to why these laws should be in operation.</p>
+
+<p>A theory also leads to discoveries. It usually happens that in testing a
+theory much valuable work is done, and many new facts are discovered.
+Almost any theory in explaining given laws will involve a number of
+consequences apart from the laws it seeks to explain. Experiment will
+soon show whether these facts are as the theory predicts they will be.
+Thus Dalton's atomic theory predicted many properties of gases which
+experiment has since verified.<span class='pagenum'><a name="Page_65" id="Page_65">[Pg 65]</a></span></p>
+
+<p><b>Atomic weights.</b> It would be of great advantage in the study of chemistry
+if we could determine the weights of the different kinds of atoms. It is
+evident that this cannot be done directly. They are so small that they
+cannot be seen even with a most powerful microscope. It is calculated
+that it would take 200,000,000 hydrogen atoms placed side by side to
+make a row one centimeter long. No balance can weigh such minute
+objects. It is possible, however, to determine their relative
+weights,&mdash;that is, how much heavier one is than another. <i>These relative
+weights of the atoms are spoken of as the atomic weights of the
+elements.</i></p>
+
+<p>If elements were able to combine in only one way,&mdash;one atom of one with
+one atom of another,&mdash;the problem of determining the atomic weights
+would be very simple. We should merely have to take some one convenient
+element as a standard, and find by experiment how much of each other
+element would combine with a fixed weight of it. The ratios thus found
+would be the same ratios as those between the atoms of the elements, and
+thus we should have their relative atomic weights. The law of multiple
+proportion calls attention to the fact that the atoms combine in other
+ratios than 1: 1, and there is no direct way of telling which one, if
+any, of the several compounds in a given case is the one consisting of a
+single atom of each element.</p>
+
+<p>If some way were to be found of telling how much heavier the entire
+molecule of a compound is than the atom chosen as a standard,&mdash;that is,
+of determining the molecular weights of compounds,&mdash;the problem could be
+solved, though its solution would not be an entirely simple matter.
+There are ways of determining the molecular weights of<span class='pagenum'><a name="Page_66" id="Page_66">[Pg 66]</a></span> compounds, and
+there are other experiments which throw light directly upon the relative
+weights of the atoms. These methods cannot be described until the facts
+upon which they rest have been studied. It will be sufficient for the
+present to assume that these methods are trustworthy.</p>
+
+<p><b>Standard for atomic weights.</b> Since the atomic weights are merely
+relative to some one element chosen as a standard, it is evident that
+any one of the elements may serve as this standard and that any
+convenient value may be assigned to its atom. At one time oxygen was
+taken as this standard, with the value 100, and the atomic weights of
+the other elements were expressed in terms of this standard. It would
+seem more rational to take the element of smallest atomic weight as the
+standard and give it unit value; accordingly hydrogen was taken as the
+standard with an atomic weight of 1. Very recently, however, this unit
+has been replaced by oxygen, with an atomic weight of 16.</p>
+
+<p><b>Why oxygen is chosen as the standard for atomic weights.</b> In the
+determination of the atomic weight of an element it is necessary to find
+the weight of the element which combines with a definite weight of
+another element, preferably the element chosen as the standard. Since
+oxygen combines with the elements far more readily than does hydrogen to
+form definite compounds, it is far better adapted for the standard
+element, and has accordingly replaced hydrogen as the standard. Any
+definite value might be given to the weight of the oxygen atom. In
+assigning a value to it, however, it is convenient to choose a whole
+number, and as small a number as possible without making the atomic
+weight of any other element less than unity. For these reasons the
+number 16 has been chosen as the atomic<span class='pagenum'><a name="Page_67" id="Page_67">[Pg 67]</a></span> weight of oxygen. This makes
+the atomic weight of hydrogen equal to 1.008, so that there is but
+little difference between taking oxygen as 16 and hydrogen as 1 for the
+unit.</p>
+
+<p>The atomic weights of the elements are given in the Appendix.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Two compounds were found to have the following compositions: (<i>a</i>)
+oxygen = 69.53%, nitrogen = 30.47%; (<i>b</i>) oxygen = 53.27%, nitrogen =
+46.73%. Show that the law of multiple proportion holds in this case.</p>
+
+<p><b>2.</b> Two compounds were found to have the following compositions: (<i>a</i>)
+oxygen = 43.64%, phosphorus = 56.36%; (<i>b</i>) oxygen = 56.35%, phosphorus
+= 43.65%. Show that the law of multiple proportion holds in this case.</p>
+
+<p><b>3.</b> Why did Dalton assume that all the atoms of a given element have the
+same weight?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_68" id="Page_68">[Pg 68]</a></span></p>
+<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI</h2>
+
+<h3>CHEMICAL EQUATIONS AND CALCULATIONS</h3>
+
+
+<p><b>Formulas.</b> Since the molecule of any chemical compound consists of a
+definite number of atoms, and this number never changes without
+destroying the identity of the compound, it is very convenient to
+represent the composition of a compound by indicating the composition of
+its molecules. This can be done very easily by using the symbols of the
+atoms to indicate the number and the kind of the atoms which constitute
+the molecule. HgO will in this way represent mercuric oxide, a molecule
+of which has been found to contain 1 atom each of mercury and oxygen.
+H<sub>2</sub>O will represent water, the molecules of which consist of 1 atom of
+oxygen and 2 of hydrogen, the subscript figure indicating the number of
+the atoms of the element whose symbol precedes it. H<sub>2</sub>SO<sub>4</sub> will
+stand for sulphuric acid, the molecules of which contain 2 atoms of
+hydrogen, 1 of sulphur, and 4 of oxygen. The combination of symbols
+which represents the molecule of a substance is called its <i>formula</i>.</p>
+
+<p><b>Equations.</b> When a given substance undergoes a chemical change it is
+possible to represent this change by the use of such symbols and
+formulas. In a former chapter it was shown that mercuric oxide
+decomposes when heated to form mercury and oxygen. This may be expressed
+very briefly in the form of the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(1) HgO = Hg + O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_69" id="Page_69">[Pg 69]</a></span></p>
+<p>When water is electrolyzed two new substances, hydrogen and oxygen, are
+formed from it. This statement in the form of an equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(2) H<sub>2</sub>O = 2H + O.<br /></span>
+</div></div>
+
+<p>The coefficient before the symbol for hydrogen indicates that a single
+molecule of water yields two atoms of hydrogen on decomposition.</p>
+
+<p>In like manner the combination of sulphur with iron is expressed by the
+equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(3) Fe + S = FeS.<br /></span>
+</div></div>
+
+<p>The decomposition of potassium chlorate by heat takes place as
+represented by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(4) KClO<sub>3</sub> = KCl + 3O.<br /></span>
+</div></div>
+
+<p><b>Reading of equations.</b> Since equations are simply a kind of shorthand way
+of indicating chemical changes which occur under certain conditions, in
+reading an equation the full statement for which it stands should be
+given. Equation (1) should be read, "Mercuric oxide when heated gives
+mercury and oxygen"; equation (2) is equivalent to the statement, "When
+electrolyzed, water produces hydrogen and oxygen"; equation (3), "When
+heated together iron and sulphur unite to form iron sulphide"; equation
+(4), "Potassium chlorate when heated yields potassium chloride and
+oxygen."</p>
+
+<p><b>Knowledge required for writing equations.</b> In order to write such
+equations correctly, a considerable amount of exact knowledge is
+required. Thus, in equation (1) the fact that red oxide of mercury has
+the composition represented by the formula HgO, that it is decomposed by
+heat, that in this decomposition mercury and oxygen are formed and<span class='pagenum'><a name="Page_70" id="Page_70">[Pg 70]</a></span> no
+other products,&mdash;all these facts must be ascertained by exact experiment
+before the equation can be written. An equation expressing these facts
+will then have much value.</p>
+
+<p>Having obtained an equation describing the conduct of mercuric oxide on
+being heated, it will not do to assume that other oxides will behave in
+like manner. Iron oxide (FeO) resembles mercuric oxide in many respects,
+but it undergoes no change at all when heated. Manganese dioxide, the
+black substance used in the preparation of oxygen, has the formula
+MnO<sub>2</sub>. When this substance is heated oxygen is set free, but the metal
+manganese is not liberated; instead, a different oxide of manganese
+containing less oxygen is produced. The equation representing the
+reaction is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3MnO<sub>2</sub> = Mn<sub>3</sub>O<sub>4</sub> + 2O.<br /></span>
+</div></div>
+
+<p><b>Classes of reactions.</b> When a chemical change takes place in a substance
+the substance is said to undergo a reaction. Although a great many
+different reactions will be met in the study of chemistry, they may all
+be grouped under the following heads.</p>
+
+<p>1. <i>Addition.</i> This is the simplest kind of chemical action. It consists
+in the union of two or more substances to produce a new substance. The
+combination of iron with sulphur is an example:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe + S = FeS.<br /></span>
+</div></div>
+
+<p>2. <i>Decomposition.</i> This is the reverse of addition, the substance
+undergoing reaction being parted into its constituents. The
+decomposition of mercuric oxide is an example: HgO = Hg + O.</p>
+
+<p>3. <i>Substitution.</i> It is sometimes possible for an element in the free
+state to act upon a compound in such a way that<span class='pagenum'><a name="Page_71" id="Page_71">[Pg 71]</a></span> it takes the place of
+one of the elements of the compound, liberating it in turn. In the study
+of the element hydrogen it was pointed out that hydrogen is most
+conveniently prepared by the action of sulphuric or hydrochloric acid
+upon zinc. When sulphuric acid is used a substance called zinc sulphate,
+having the composition represented by the formula ZnSO<sub>4</sub>, is formed
+together with hydrogen. The equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Zn + H<sub>2</sub>SO<sub>4</sub> = ZnSO<sub>4</sub> + 2H.<br /></span>
+</div></div>
+
+<p>When hydrochloric acid is used zinc chloride and hydrogen are the
+products of reaction:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Zn + 2HCl = ZnCl<sub>2</sub> + 2H.<br /></span>
+</div></div>
+
+<p>When iron is used in place of zinc the equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe + H<sub>2</sub>SO<sub>4</sub> = FeSO<sub>4</sub> + 2H.<br /></span>
+</div></div>
+
+<p>These reactions are quite similar, as is apparent from an examination of
+the equations. In each case 1 atom of the metal replaces 2 atoms of
+hydrogen in the acid, and the hydrogen escapes as a gas. When an element
+in the free state, such as the zinc in the equations just given, takes
+the place of some one element in a compound, setting it free from
+chemical combination, the act is called <i>substitution</i>.</p>
+
+<p>Other reactions illustrating substitution are the action of sodium on
+water,</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na + H<sub>2</sub>O = NaOH + H;<br /></span>
+</div></div>
+
+<p>and the action of heated iron upon water,</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Fe + 4H<sub>2</sub>O = Fe<sub>3</sub>O<sub>4</sub> + 8H.<br /></span>
+</div></div>
+
+<p>4. <i>Double decomposition.</i> When barium dioxide (BaO<sub>2</sub>) is treated with
+sulphuric acid two compounds are formed, namely, hydrogen dioxide
+(H<sub>2</sub>O<sub>2</sub>) and barium sulphate (BaSO<sub>4</sub>). The equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">BaO<sub>2</sub> + H<sub>2</sub>SO<sub>4</sub> = BaSO<sub>4</sub> + H<sub>2</sub>O<sub>2</sub>.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_72" id="Page_72">[Pg 72]</a></span></p>
+<p>In this reaction it will be seen that the two elements barium and
+hydrogen simply exchange places. Such a reaction is called a <i>double
+decomposition</i>. We shall meet with many examples of this kind of
+chemical reactions.</p>
+
+<p><b>Chemical equations are quantitative.</b> The use of symbols and formulas in
+expressing chemical changes has another great advantage. Thus, according
+to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>O = 2H + O,<br /></span>
+</div></div>
+
+<p>1 molecule of water is decomposed into 2 atoms of hydrogen and 1 atom of
+oxygen. But, as we have seen, the relative weights of the atoms are
+known, that of hydrogen being 1.008, while that of oxygen is 16. The
+molecule of water, being composed of 2 atoms of hydrogen and 1 atom of
+oxygen, must therefore weigh relatively 2.016 + 16, or 18.016. The
+amount of hydrogen in this molecule must be 2.016/18.016, or 11.18% of
+the whole, while the amount of oxygen must be 16/18.018, or 88.82% of
+the whole. Now, since any definite quantity of water is simply the sum
+of a great many molecules of water, it is plain that the fractions
+representing the relative amounts of hydrogen and oxygen present in a
+molecule must likewise express the relative amounts of hydrogen and
+oxygen present in any quantity of water. Thus, for example, in 20 g. of
+water there are 2.016/18.016 &times; 20, or 2.238 g. of hydrogen, and
+16/18.016 &times; 20, or 17.762 g. of oxygen. These results in reference to
+the composition of water of course agree exactly with the facts obtained
+by the experiments described in the chapter on water, for it is because
+of those experiments that the values 1.008 and 16 are given to hydrogen
+and oxygen respectively.</p>
+
+<p>It is often easier to make calculations of this kind in the form of a
+proportion rather than by fractions. Since the<span class='pagenum'><a name="Page_73" id="Page_73">[Pg 73]</a></span> molecule of water and
+the two atoms of hydrogen which it contains have the ratio by weight of
+18.016: 2.016, any mass of water has the same ratio between its total
+weight and the weight of the hydrogen in it. Hence, to find the number
+of grams (<i>x</i>) of hydrogen in 20 g. of water, we have the proportion</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">18.016 : 2.016 :: 20 g. : <i>x</i> (grams of hydrogen).<br /></span>
+</div></div>
+
+<p>Solving for <i>x</i>, we get 2.238 for the number of grams of hydrogen.
+Similarly, to find the amount (<i>x</i>) of oxygen present in the 20 g. of
+water, we have the proportion</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">18.016 : 16 :: 20 : <i>x</i><br /></span>
+</div></div>
+
+<p>from which we find that <i>x</i> = 17.762 g.</p>
+
+<p>Again, suppose we wish to find what weight of oxygen can be obtained
+from 15 g. of mercuric oxide. The equation representing the
+decomposition of mercuric oxide is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">HgO = Hg + O.<br /></span>
+</div></div>
+
+<p>The relative weights of the mercury and oxygen atoms are respectively
+200 and 16. The relative weight of the mercuric oxide molecule must
+therefore be the sum of these, or 216. The molecule of mercuric oxide
+and the atom of oxygen which it contains have the ratio 216: 16. This
+same ratio must therefore hold between the weight of any given quantity
+of mercuric oxide and that of the oxygen which it contains. Hence, to
+find the weight of oxygen in 15 g. of mercuric oxide, we have the
+proportion</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">216 : 16 :: 15 : <i>x</i> (grams of oxygen).<br /></span>
+</div></div>
+
+<p>On the other hand, suppose we wish to prepare, say, 20 g. of oxygen. The
+problem is to find out what weight of mercuric oxide will yield 20 g. of
+oxygen. The following proportion evidently holds</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">216 : 16 :: <i>x</i> (grams of mercuric oxide) : 20;<br /></span>
+</div></div>
+
+<p>from which we get <i>x</i> = 270.</p>
+
+<p>In the preparation of hydrogen by the action of sulphuric acid upon
+zinc, according to the equation,</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Zn + H<sub>2</sub>SO<sub>4</sub> = ZnSO<sub>4</sub> + 2 H,<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_74" id="Page_74">[Pg 74]</a></span></p>
+<p>suppose that 50 g. of zinc are available; let it be required to
+calculate the weight of hydrogen which can be obtained. It will be seen
+that 1 atom of zinc will liberate 2 atoms of hydrogen. The ratio by
+weight of a zinc to an hydrogen atom is 65.4: 1.008; of 1 zinc atom to 2
+hydrogen atoms, 65.4: 2.016. Zinc and hydrogen will be related in this
+reaction in this same ratio, however many atoms of zinc are concerned.
+Consequently in the proportion</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">65.4 : 2.016 :: 50 : <i>x</i>,<br /></span>
+</div></div>
+
+<p><i>x</i> will be the weight of hydrogen set free by 50 g. of zinc. The weight
+of zinc sulphate produced at the same time can be found from the
+proportion</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">65.4 : 161.46 :: 50 : <i>x</i>;<br /></span>
+</div></div>
+
+<p>where 161.46 is the molecular weight of the zinc sulphate, and <i>x</i> the
+weight of zinc sulphate formed. In like manner, the weight of sulphuric
+acid used up can be calculated from the proportion</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">65.4 : 98.076 :: 50 : <i>x</i>.<br /></span>
+</div></div>
+
+<p>These simple calculations are possible because the symbols and formulas
+in the equations represent the relative weights of the substances
+concerned in a chemical reaction. When once the relative weights of the
+atoms have been determined, and it has been agreed to allow the symbols
+to stand for these relative weights, an equation or formula making use
+of the symbols becomes a statement of a definite numerical fact, and
+calculations can be based on it.</p>
+
+<p><b>Chemical equations not algebraic.</b> Although chemical equations are
+quantitative, it must be clearly understood that they are not algebraic.
+A glance at the equations</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">7 + 4 = 11, 8 + 5 = 9 + 4<br /></span>
+</div></div>
+
+<p>will show at once that they are true. The equations</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">HgO = Hg + O, FeO = Fe + O<br /></span>
+</div></div>
+
+<p>are equally true in an algebraic sense, but experiment shows that only
+the first is true chemically, for iron oxide (FeO)<span class='pagenum'><a name="Page_75" id="Page_75">[Pg 75]</a></span> cannot be directly
+decomposed into iron and oxygen. Only such equations as have been found
+by careful experiment to express a real chemical transformation, true
+both for the kinds of substances as well as for the weights, have any
+value.</p>
+
+<p><i>Chemical formulas and equations, therefore, are a concise way of
+representing qualitatively and quantitatively facts which have been
+found by experiment to be true in reference to the composition of
+substances and the changes which they undergo.</i></p>
+
+<p><b>Formulas representing water of crystallization.</b> An examination of
+substances containing water of crystallization has shown that in every
+case the water is present in such proportion by weight as can readily be
+represented by a formula. For example, copper sulphate (CuSO<sub>4</sub>) and
+water combine in the ratio of 1 molecule of the sulphate to 5 of water;
+calcium sulphate (CaSO<sub>4</sub>) and water combine in the ratio 1: 2 to form
+gypsum. These facts are expressed by writing the formulas for the two
+substances with a period between them. Thus the formula for crystallized
+copper sulphate is CuSO<sub>4</sub>&middot;5H<sub>2</sub>O; that of gypsum is CaSO<sub>4</sub>&middot;2H<sub>2</sub>O.</p>
+
+<p><b>Heat of reaction.</b> Attention has frequently been directed to the fact
+that chemical changes are usually accompanied by heat changes. In
+general it has been found that in every chemical action heat is either
+absorbed or given off. By adopting a suitable unit for the measurement
+of heat, the heat change during a chemical reaction can be expressed in
+the equation for the reaction.</p>
+
+<p>Heat cannot be measured by the use of a thermometer alone, since the
+thermometer measures the intensity of heat, not its quantity. The
+easiest way to measure a quantity of heat is to note how warm it will
+make a definite amount of<span class='pagenum'><a name="Page_76" id="Page_76">[Pg 76]</a></span> a given substance chosen as a standard. Water
+has been chosen as the standard, and the unit of heat is called a
+<i>calorie. A calorie is defined as the amount of heat required to raise
+the temperature of one gram of water one degree.</i></p>
+
+<p>By means of this unit it is easy to indicate the heat changes in a given
+chemical reaction. The equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2H + O = H<sub>2</sub>O + 68,300 cal.<br /></span>
+</div></div>
+
+<p>means that when 2.016 g. of hydrogen combine with 16 g. of oxygen,
+18.016 g. of water are formed and 68,300 cal. are set free.</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C + 2S = CS<sub>2</sub> - 19,000 cal.<br /></span>
+</div></div>
+
+<p>means that an expenditure of 19,000 cal. is required to cause 12 g. of
+carbon to unite with 64.12 g. of sulphur to form 76.12 g. of carbon
+disulphide. In these equations it will be noted that the symbols stand
+for as many grams of the substance as there are units in the weights of
+the atoms represented by the symbols. This is always understood to be
+the case in equations where the heat of reaction is given.</p>
+
+<p><b>Conditions of a chemical action are not indicated by equations.</b>
+Equations do not tell the conditions under which a reaction will take
+place. The equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">HgO = Hg + O<br /></span>
+</div></div>
+
+<p>does not tell us that it is necessary to keep the mercuric oxide at a
+high temperature in order that the decomposition may go on. The equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Zn + 2HCl = ZnCl<sub>2</sub> + 2H<br /></span>
+</div></div>
+
+<p>in no way indicates the fact that the hydrochloric acid must be
+dissolved in water before it will act upon the zinc. From the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H + Cl = HCl<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_77" id="Page_77">[Pg 77]</a></span></p>
+<p>it would not be suspected that the two gases hydrogen and chlorine will
+unite instantly in the sunlight, but will stand mixed in the dark a long
+time without change. It will therefore be necessary to pay much
+attention to the details of the conditions under which a given reaction
+occurs, as well as to the expression of the reaction in the form of an
+equation.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Calculate the percentage composition of the following substances:
+(<i>a</i>) mercuric oxide; (<i>b</i>) potassium chlorate; (<i>c</i>) hydrochloric acid;
+(<i>d</i>) sulphuric acid. Compare the results obtained with the compositions
+as given in Chapters II and III.</p>
+
+<p><b>2.</b> Determine the percentage of copper, sulphur, oxygen, and water in
+copper sulphate crystals. What weight of water can be obtained from 150
+g. of this substance?</p>
+
+<p><b>3.</b> What weight of zinc can be dissolved in 10 g. of sulphuric acid? How
+much zinc sulphate will be formed?</p>
+
+<p><b>4.</b> How many liters of hydrogen measured under standard conditions can be
+obtained from the action of 8 g. of iron on 10 g. of sulphuric acid? How
+much iron sulphate (FeSO<sub>4</sub>) will be formed?</p>
+
+<p><b>5.</b> 10 g. of zinc were used in the preparation of hydrogen; what weight
+of iron will be required to prepare an equal volume?</p>
+
+<p><b>6.</b> How many grams of barium dioxide will be required to prepare 1 kg. of
+common hydrogen dioxide solution? What weight of barium sulphate will be
+formed at the same time?</p>
+
+<p><b>7.</b> What weight of the compound Mn<sub>3</sub>O<sub>4</sub> will be formed by strongly
+heating 25 g. of manganese dioxide? What volume of oxygen will be given
+off at the same time, measured under standard conditions?</p>
+
+<p><b>8.</b> (<i>a</i>) What is the weight of 100 l. of hydrogen measured in a
+laboratory in which the temperature is 20&deg; and pressure 750 mm.? (<i>b</i>)
+What weight of sulphuric acid is necessary to prepare this amount of
+hydrogen? (<i>c</i>) The density of sulphuric acid is 1.84. Express the acid
+required in (<i>b</i>) in cubic centimeters.</p>
+
+<p><b>9.</b> What weight of potassium chlorate is necessary to furnish sufficient
+oxygen to fill four 200 cc. bottles in your laboratory (the gas to be
+collected over water)?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_78" id="Page_78">[Pg 78]</a></span></p>
+<h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII</h2>
+
+<h3>NITROGEN AND THE RARE ELEMENTS: ARGON, HELIUM, NEON, KRYPTON, XENON</h3>
+
+
+<p><b>Historical.</b> Nitrogen was discovered by the English chemist Rutherford in
+1772. A little later Scheele showed it to be a constituent of air, and
+Lavoisier gave it the name <i>azote</i>, signifying that it would not support
+life. The name <i>nitrogen</i> was afterwards given it because of its
+presence in saltpeter or niter. The term azote and symbol Az are still
+retained by the French chemists.</p>
+
+<p><b>Occurrence.</b> Air is composed principally of oxygen and nitrogen in the
+free state, about 78 parts by volume out of every 100 parts being
+nitrogen. Nitrogen also occurs in nature in the form of potassium
+nitrate (KNO<sub>3</sub>)&mdash;commonly called saltpeter or niter&mdash;as well as in
+sodium nitrate (NaNO<sub>3</sub>). Nitrogen is also an essential constituent of
+all living organisms; for example, the human body contains about 2.4% of
+nitrogen.</p>
+
+<p><b>Preparation from air.</b> Nitrogen can be prepared from air by the action of
+some substance which will combine with the oxygen, leaving the nitrogen
+free. Such a substance must be chosen, however, as will combine with the
+oxygen to form a product which is not a gas, and which can be readily
+separated from the nitrogen. The substances most commonly used for this
+purpose are phosphorus and copper.</p>
+
+<p>1. <i>By the action of phosphorus.</i> The method used for the preparation of
+nitrogen by the action of phosphorus is as follows:<span class='pagenum'><a name="Page_79" id="Page_79">[Pg 79]</a></span></p>
+
+<p>The phosphorus is placed in a little porcelain dish, supported on a cork
+and floated on water (Fig. 26). It is then ignited by contact with a hot
+wire, and immediately a bell jar or bottle is brought over it so as to
+confine a portion of the air. The phosphorus combines with the oxygen to
+form an oxide of phosphorus, known as phosphorus pentoxide. This is a
+white solid which floats about in the bell jar, but in a short time it
+is all absorbed by the water, leaving the nitrogen. The withdrawal of
+the oxygen is indicated by the rising of the water in the bell jar.</p>
+
+<div class="figleft" style="width: 400px;">
+<img src="images/imagefig26.jpg" width="400" height="285" alt="Fig. 26" title="" />
+<span class="caption">Fig. 26</span>
+</div>
+
+<p>2. <i>By the action of copper.</i> The oxygen present in the air may also be
+removed by passing air slowly through a heated tube containing copper.
+The copper combines with the oxygen to form copper oxide, which is a
+solid. The nitrogen passes on and may be collected over water.</p>
+
+<div class="blockquot"><p><b>Nitrogen obtained from air is not pure.</b> Inasmuch as air, in
+addition to oxygen and nitrogen, contains small amounts of
+other gases, and since the phosphorus as well as the copper
+removes only the oxygen, it is evident that the nitrogen
+obtained by these methods is never quite pure. About 1% of the
+product is composed of other gases, from which it is very
+difficult to separate the nitrogen. The impure nitrogen so
+obtained may, however, be used for a study of most of the
+properties of nitrogen, since these are not materially affected
+by the presence of the other gases.</p></div>
+
+<p><b>Preparation from compounds of nitrogen.</b> Pure nitrogen may be obtained
+from certain compounds of the element. Thus, if heat is applied to the
+compound ammonium nitrite (NH<sub>4</sub>NO<sub>2</sub>), the change represented in the
+following equation takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>4</sub>NO<sub>2</sub> = 2H<sub>2</sub>O + 2N.<br /></span>
+<span class='pagenum'><a name="Page_80" id="Page_80">[Pg 80]</a></span></div></div>
+
+<p><b>Physical properties.</b> Nitrogen is similar to oxygen and hydrogen in that
+it is a colorless, odorless, and tasteless gas. One liter of nitrogen
+weighs 1.2501 g. It is almost insoluble in water. It can be obtained in
+the form of a colorless liquid having a boiling point of -195&deg; at
+ordinary pressure. At -214&deg; it solidifies.</p>
+
+<p><b>Chemical properties.</b> Nitrogen is characterized by its inertness. It is
+neither combustible nor a supporter of combustion. At ordinary
+temperatures it will not combine directly with any of the elements
+except under rare conditions. At higher temperatures it combines with
+magnesium, lithium, titanium, and a number of other elements. The
+compounds formed are called <i>nitrides</i>, just as compounds of an element
+with oxygen are called <i>oxides</i>. When it is mixed with oxygen and
+subjected to the action of electric sparks, the two gases slowly combine
+forming oxides of nitrogen. A mixture of nitrogen and hydrogen when
+treated similarly forms ammonia, a gaseous compound of nitrogen and
+hydrogen. Since we are constantly inhaling nitrogen, it is evident that
+it is not poisonous. Nevertheless life would be impossible in an
+atmosphere of pure nitrogen on account of the exclusion of the necessary
+oxygen.</p>
+
+<div class="blockquot"><p><b>Argon, helium, neon, krypton, xenon.</b> These are all rare
+elements occurring in the air in very small quantities. Argon,
+discovered in 1894, was the first one obtained. Lord Rayleigh,
+an English scientist, while engaged in determining the exact
+weights of various gases, observed that the nitrogen obtained
+from the air is slightly heavier than pure nitrogen obtained
+from its compounds. After repeating his experiments many times,
+always with the same results, Rayleigh finally concluded that
+the nitrogen which he had obtained from the air was not pure,
+but was mixed with a small amount of some unknown gas, the
+density of which is greater than that of nitrogen. Acting on
+this assumption, Rayleigh, together with the<span class='pagenum'><a name="Page_81" id="Page_81">[Pg 81]</a></span> English chemist
+Ramsay, attempted to separate the nitrogen from the unknown
+gas. Knowing that nitrogen would combine with magnesium, they
+passed the nitrogen obtained from the air and freed from all
+known substances through tubes containing magnesium heated to
+the necessary temperature. After repeating this operation, they
+finally succeeded in obtaining from the atmospheric nitrogen a
+small volume of gas which would not combine with magnesium and
+hence could not be nitrogen. This proved to be a new element,
+to which they gave the name <i>argon</i>. As predicted, this new
+element was found to be heavier than nitrogen, its density as
+compared with hydrogen as a standard being approximately 20,
+that of nitrogen being only 14. About 1% of the atmospheric
+nitrogen proved to be argon. The new element is characterized
+by having no affinity for other elements. Even under the most
+favorable conditions it has not been made to combine with any
+other element. On this account it was given the name argon,
+signifying lazy or idle. Like nitrogen, it is colorless,
+odorless, and tasteless. It has been liquefied and solidified.
+Its boiling point is -187&deg;.</p>
+
+<p>Helium was first found in the gases expelled from certain
+minerals by heating. Through the agency of the spectroscope it
+had been known to exist in the sun long before its presence on
+the earth had been demonstrated,&mdash;a fact suggested by the name
+helium, signifying the sun. Its existence in traces in the
+atmosphere has also been proven. It was first liquefied by
+Onnes in July, 1908. Its boiling point, namely -269&deg;, is the
+lowest temperature yet reached.</p>
+
+<p>The remaining elements of this group&mdash;neon, krypton, and
+xenon&mdash;have been obtained from liquid air. When liquid air is
+allowed to boil, the constituents which are the most difficult
+to liquefy, and which therefore have the lowest boiling points,
+vaporize first, followed by the others in the order of their
+boiling points. It is possible in this way to make at least a
+partial separation of the air into its constituents, and Ramsay
+thus succeeded in obtaining from liquid air not only the known
+constituents, including argon and helium, but also the new
+elements, neon, krypton, and xenon. These elements, as well as
+helium, all proved to be similar to argon in that they are
+without chemical activity, apparently forming no compounds
+whatever. The percentages present in the air are very small.
+The names, neon, krypton, xenon, signify respectively, new,
+hidden, stranger.</p></div><p><span class='pagenum'><a name="Page_82" id="Page_82">[Pg 82]</a></span></p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> How could you distinguish between oxygen, hydrogen, and nitrogen?</p>
+
+<p><b>2.</b> Calculate the relative weights of nitrogen and oxygen; of nitrogen
+and hydrogen.</p>
+
+<p><b>3.</b> In the preparation of nitrogen from the air, how would hydrogen do as
+a substance for the removal of the oxygen?</p>
+
+<p><b>4.</b> What weight of nitrogen can be obtained from 10 l. of air measured
+under the conditions of temperature and pressure which prevail in your
+laboratory?</p>
+
+<p><b>5.</b> How many grams of ammonium nitrite are necessary in the preparation
+of 20 l. of nitrogen measured over water under the conditions of
+temperature and pressure which prevail in your laboratory?</p>
+
+<p><b>6.</b> If 10 l. of air, measured under standard conditions, is passed over
+100 g. of hot copper, how much will the copper gain in weight?</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/ramsay.jpg" width="400" height="499" alt="WILLIAM RAMSAY (Scotch) (1855-)
+
+Has made many studies in the physical properties of substances;
+discovered helium; together with Lord Rayleigh and others he discovered
+argon, krypton, xenon, and neon; has contributed largely to the
+knowledge of radio-active substances, showing that radium gradually
+gives rise to helium; professor at University College, London" title="" />
+<span class="caption">WILLIAM RAMSAY (Scotch) (1855-)<br /><br />
+
+Has made many studies in the physical properties of substances;
+discovered helium; together with Lord Rayleigh and others he discovered
+argon, krypton, xenon, and neon; has contributed largely to the
+knowledge of radio-active substances, showing that radium gradually
+gives rise to helium; professor at University College, London</span>
+</div>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_83" id="Page_83">[Pg 83]</a></span></p>
+<h2><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a>CHAPTER VIII</h2>
+
+<h3>THE ATMOSPHERE</h3>
+
+
+<p><b>Atmosphere and air.</b> The term <i>atmosphere</i> is applied to the gaseous
+envelope surrounding the earth. The term <i>air</i> is generally applied to a
+limited portion of this envelope, although the two words are often used
+interchangeably. Many references have already been made to the
+composition and properties of the atmosphere. These statements must now
+be collected and discussed somewhat more in detail.</p>
+
+<p><b>Air formerly regarded as an element.</b> Like water, air was at first
+regarded as elementary in character. Near the close of the eighteenth
+century Scheele, Priestley, and Lavoisier showed by their experiments
+that it is a mixture of at least two gases,&mdash;those which we now call
+oxygen and nitrogen. By burning substances in an inclosed volume of air
+and noting the contraction in volume due to the removal of the oxygen,
+they were able to determine with some accuracy the relative volumes of
+oxygen and nitrogen present in the air.</p>
+
+<p><b>The constituents of the atmosphere.</b> The constituents of the atmosphere
+may be divided into two general groups: those which are essential to
+life and those which are not essential.</p>
+
+<p>1. <i>Constituents essential to life.</i> In addition to oxygen and nitrogen
+at least two other substances, namely, carbon dioxide and water vapor,
+must be present in the atmosphere in order that life may exist. The
+former of these is a<span class='pagenum'><a name="Page_84" id="Page_84">[Pg 84]</a></span> gaseous compound of carbon and oxygen having the
+formula CO<sub>2</sub>. Its properties will be discussed in detail in the
+chapter on the compounds of carbon. Its presence in the air may be shown
+by causing the air to bubble through a solution of calcium hydroxide
+(Ca(OH)<sub>2</sub>), commonly called lime water. The carbon dioxide combines
+with the calcium hydroxide in accordance with the following equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ca(OH)<sub>2</sub> + CO<sub>2</sub> = CaCO<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The resulting calcium carbonate (CaCO<sub>3</sub>) is insoluble in water and
+separates in the form of a white powder, which causes the solution to
+appear milky.</p>
+
+<p>The presence of water vapor is readily shown by its condensation on cold
+objects as well as by the fact that a bit of calcium chloride when
+exposed to the air becomes moist, and may even dissolve in the water
+absorbed from the air.</p>
+
+<p>2. <i>Constituents not essential to life.</i> In addition to the essential
+constituents, the air contains small percentages of various other gases,
+the presence of which so far as is known is not essential to life. This
+list includes the rare elements, argon, helium, neon, krypton, and
+xenon; also hydrogen, ammonia, hydrogen dioxide, and probably ozone.
+Certain minute forms of life (germs) are also present, the decay of
+organic matter being due to their presence.</p>
+
+<div class="blockquot"><p><b>Function of each of the essential constituents.</b> (1) The oxygen
+directly supports life through respiration. (2) The nitrogen,
+on account of its inactivity, serves to dilute the oxygen, and
+while contrary to the older views, it is possible that life
+might continue to exist in the absence of the atmospheric
+nitrogen, yet the conditions of life would be entirely changed.
+Moreover, nitrogen is an essential constituent of all animal
+and plant life. It was formerly supposed that neither animals
+nor plants could assimilate the free nitrogen, but it has been
+shown recently that the plants of at least one natural<span class='pagenum'><a name="Page_85" id="Page_85">[Pg 85]</a></span> order,
+the Leguminos&aelig;, to which belong the beans, peas, and clover,
+have the power of directly assimilating the free nitrogen from
+the atmosphere. This is accomplished through the agency of
+groups of bacteria, which form colonies in little tubercles on
+the roots of the plants. These bacteria probably assist in the
+absorption of nitrogen by changing the free nitrogen into
+compounds which can be assimilated by the plant. Fig. 27 shows
+the tubercles on the roots of a variety of bean. (3) The
+presence of water vapor in the air is necessary to prevent
+excessive evaporation from both plants and animals. (4) Carbon
+dioxide is an essential plant food.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig27.jpg" width="400" height="300" alt="Fig. 27" title="" />
+<span class="caption">Fig. 27</span>
+</div></div>
+
+<p><b>The quantitative analysis of air.</b> A number of different methods have
+been devised for the determination of the percentages of the
+constituents present in the atmosphere. Among these are the following.</p>
+
+<p>1. <i>Determination of oxygen.</i> (1) The oxygen is withdrawn from a
+measured volume of air inclosed in a tube, by means of phosphorus.</p>
+
+<div class="blockquot"><p>To make the determination, a graduated tube is filled with
+water and inverted in a vessel of water. Air is introduced into
+the tube until it is partially filled with the gas. The volume
+of the inclosed air is carefully noted and reduced to standard
+conditions. A small piece of phosphorus is attached to a wire
+and brought within the tube as shown in Fig. 28. After a few
+hours the oxygen in the inclosed air will have combined with
+the phosphorus, the water rising to take its place. The
+phosphorus is removed and the volume is again noted and reduced
+to standard conditions. The contraction in the volume of the
+air is equal to the volume of oxygen absorbed.</p></div>
+
+<div class="figright" style="width: 205px;">
+<img src="images/imagefig28.jpg" width="205" height="400" alt="Fig. 28" title="" />
+<span class="caption">Fig. 28</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_86" id="Page_86">[Pg 86]</a></span></p>
+
+<p>(2) The oxygen may also be estimated by passing a measured volume of air
+through a tube containing copper heated to a high temperature. The
+oxygen in the air combines with the copper to form copper oxide (CuO).
+Hence the increase in the weight of the copper equals the weight of the
+oxygen in the volume of air taken.</p>
+
+<p>(3) A more accurate method is the following. A eudiometer tube is filled
+with mercury and inverted in a vessel of the same liquid. A convenient
+amount of air is then introduced into the tube and its volume accurately
+noted. There is then introduced more than sufficient hydrogen to combine
+with the oxygen present in the inclosed air, and the volume is again
+accurately noted. The mixture is then exploded by an electric spark, and
+the volume is once more taken. By subtracting this volume from the total
+volume of the air and hydrogen there is obtained the contraction in
+volume due to the union of the oxygen and hydrogen. The volume occupied
+by the water formed by the union of the two gases is so small that it
+may be disregarded in the calculation. Since oxygen and hydrogen combine
+in the ratio 1: 2 by volume, it is evident that the contraction in
+volume due to the combination is equal to the volume occupied by the
+oxygen in the air contained in the tube, plus twice this volume of
+hydrogen. In other words, one third of the total contraction is equal to
+the volume occupied by the oxygen in the inclosed air. The following
+example will make this clear:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Volume of air in tube</td><td align='left'>50.0 cc.</td></tr>
+<tr><td align='left'>Volume after introducing hydrogen</td><td align='left'>80.0</td></tr>
+<tr><td align='left'>Volume after combination of oxygen and hydrogen</td><td align='left'>48.5</td></tr>
+<tr><td align='left'>Contraction in volume due to combination (80 cc.-48.5 cc.)</td><td align='left'>31.5</td></tr>
+<tr><td align='left'>Volume of oxygen in 50 cc. of air (1/3 of 31.5)</td><td align='left'>10.5</td></tr>
+</table></div>
+
+<p><span class='pagenum'><a name="Page_87" id="Page_87">[Pg 87]</a></span></p>
+
+<p>All these methods agree in showing that 100 volumes of dry air contain
+approximately 21 volumes of oxygen.</p>
+
+<p>2. <i>Determination of nitrogen.</i> If the gas left after the removal of
+oxygen from a portion of air is passed over heated magnesium, the
+nitrogen is withdrawn, argon and the other rare elements being left. It
+may thus be shown that of the 79 volumes of gas left after the removal
+of the oxygen from 100 volumes of air, approximately 78 are nitrogen and
+0.93 argon. The other elements are present in such small quantities that
+they may be neglected.</p>
+
+<p>3. <i>Determination of carbon dioxide.</i> The percentage of carbon dioxide
+in any given volume of air may be determined by passing the air over
+calcium hydroxide or some other compound which will combine with the
+carbon dioxide. The increase in the weight of the hydroxide equals the
+weight of the carbon dioxide absorbed. The amount present in the open
+normal air is from 3 to 4 parts by volume in 10,000 volumes of air, or
+about 0.04%.</p>
+
+<p>4. <i>Determination of water vapor.</i> The water vapor present in a given
+volume of air may be determined by passing the air over calcium chloride
+(or some other compound which has a strong affinity for water), and
+noting the increase in the weight of the chloride. The amount present
+varies not only with the locality, but there is a wide variation from
+day to day in the same locality because of the winds and changes in
+temperature.</p>
+
+<p><b>Processes affecting the composition of the air.</b> The most important of
+these processes are the following.</p>
+
+<p>1. <i>Respiration.</i> In the process of respiration some of the oxygen in
+the inhaled air is absorbed by the blood and carried to all parts of the
+body, where it combines with the carbon of the worn-out tissues. The
+products of oxidation<span class='pagenum'><a name="Page_88" id="Page_88">[Pg 88]</a></span> are carried back to the lungs and exhaled in the
+form of carbon dioxide. The amount exhaled by an adult averages about 20
+l. per hour. Hence in a poorly ventilated room occupied by a number of
+people the amount of carbon dioxide rapidly increases. While this gas is
+not poisonous unless present in large amounts, nevertheless air
+containing more than 15 parts in 10,000 is not fit for respiration.</p>
+
+<p>2. <i>Combustion.</i> All of the ordinary forms of fuel contain large
+percentages of carbon. On burning, this carbon combines with oxygen in
+the air, forming carbon dioxide. Combustion and respiration, therefore,
+tend to diminish the amount of oxygen in the air and to increase the
+amount of carbon dioxide.</p>
+
+<p>3. <i>Action of plants.</i> Plants have the power, when in the sunlight, of
+absorbing carbon dioxide from the air, retaining the carbon and
+returning at least a portion of the oxygen to the air. It will be
+observed that these changes are just the opposite of those brought about
+by the processes of respiration and combustion.</p>
+
+<div class="blockquot"><p><b>Poisonous effect of exhaled air.</b> The differences in the
+percentages of oxygen, carbon dioxide, and moisture present in
+inhaled air and exhaled air are shown in the following
+analyses.</p></div>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>INHALED AIR</td><td align='left'>EXHALED AIR</td></tr>
+<tr><td align='left'>Oxygen</td><td align='left'>21.00%</td><td align='left'>16.00%</td></tr>
+<tr><td align='left'>Carbon dioxide</td><td align='left'>0.04</td><td align='left'>4.38</td></tr>
+<tr><td align='left'>Moisture</td><td align='left'>variable</td><td align='left'>saturated</td></tr>
+</table></div>
+
+<div class="blockquot"><p>The foul odor of respired air is due to the presence of a
+certain amount of organic matter. It is possible that this
+organic matter rather than the carbon dioxide is responsible
+for the injurious effects which follow the respiration of
+impure air. The extent of such organic impurities present may
+be judged, however, by the amount of carbon dioxide present,
+since the two are exhaled together.</p>
+
+<p><b>The cycle of carbon in nature.</b> Under the influence of sunlight,
+the carbon dioxide absorbed from the air by plants reacts with
+water<span class='pagenum'><a name="Page_89" id="Page_89">[Pg 89]</a></span> and small amounts of other substances absorbed from the
+soil to form complex compounds of carbon which constitute the
+essential part of the plant tissue. This reaction is attended
+by the evolution of oxygen, which is restored to the air. The
+compounds resulting from these changes are much richer in their
+energy content than are the substances from which they are
+formed; hence a certain amount of energy must have been
+absorbed in their formation. The source of this energy is the
+sun's rays.</p>
+
+<p>If the plant is burned, the changes which took place in the
+formation of the compounds present are largely reversed. The
+carbon and hydrogen present combine with oxygen taken from the
+air to form carbon dioxide and water, while the energy absorbed
+from the sun's rays is liberated in the form of energy of heat.
+If, on the other hand, the plant is used as food, the compounds
+present are used in building up the tissues of the body. When
+this tissue breaks down, the changes which it undergoes are
+very similar to those which take place when the plant is
+burned. The carbon and hydrogen combine with the inhaled oxygen
+to form carbon dioxide and water, which are exhaled. The energy
+possessed by the complex substances is liberated partly in the
+form of energy of heat, which maintains the heat of the body,
+and partly in the various forms of muscular energy. The carbon
+originally absorbed from the air by the plant in the form of
+carbon dioxide is thus restored to the air and is ready to
+repeat the cycle of changes.</p></div>
+
+<p><b>The composition of the air is constant.</b> Notwithstanding the changes
+constantly taking place which tend to alter the composition of the air,
+the results of a great many analyses of air collected in the open fields
+show that the percentages of oxygen and nitrogen as well as of carbon
+dioxide are very nearly constant. Indeed, so constant are the
+percentages of oxygen and nitrogen that the question has arisen, whether
+these two elements are not combined in the air, forming a definite
+chemical compound. That the two are not combined but are simply mixed
+together can be shown in a number of ways, among which are the
+following.<span class='pagenum'><a name="Page_90" id="Page_90">[Pg 90]</a></span></p>
+
+<p>1. When air dissolves in water it has been found that the ratio of
+oxygen to nitrogen in the dissolved air is no longer 21: 78, but more
+nearly 35: 65. If it were a chemical compound, the ratio of oxygen to
+nitrogen would not be changed by solution in water.</p>
+
+<p>2. A chemical compound in the form of a liquid has a definite boiling
+point. Water, for example, boils at 100&deg;. Moreover the steam which is
+thus formed has the same composition as the water. The boiling point of
+liquid air, on the other hand, gradually rises as the liquid boils, the
+nitrogen escaping first followed by the oxygen. If the two were
+combined, they would pass off together in the ratio in which they are
+found in the air.</p>
+
+<p><b>Why the air has a constant composition.</b> If air is a mixture and changes
+are constantly taking place which tend to modify its composition, how,
+then, do we account for the constancy of composition which the analyses
+reveal? This is explained by several facts. (1) The changes which are
+caused by the processes of combustion and respiration, on the one hand,
+and the action of plants, on the other, tend to equalize each other. (2)
+The winds keep the air in constant motion and so prevent local changes.
+(3) The volume of the air is so vast and the changes which occur are so
+small compared with the total amount of air that they cannot be readily
+detected. (4) Finally it must be noted that only air collected in the
+open fields shows this constancy in composition. The air in a poorly
+ventilated room occupied by a number of people rapidly changes in
+composition.</p>
+
+<p><b>The properties of the air.</b> Inasmuch as air is composed principally of a
+mixture of oxygen and nitrogen, which elements have already been
+discussed, its properties may be inferred largely from those of the two
+gases.<span class='pagenum'><a name="Page_91" id="Page_91">[Pg 91]</a></span> One liter weighs 1.2923 g. It is thus 14.38 times as heavy as
+hydrogen. At the sea level it exerts an average pressure sufficient to
+sustain a column of mercury 760 mm. in height. This is taken as the
+standard pressure in determining the volumes of gases as well as the
+boiling points of liquids. Water may be made to boil at any temperature
+between 0&deg; and considerably above 100&deg; by simply varying the pressure.
+It is only when the pressure upon it is equal to the normal pressure of
+the atmosphere at the sea level, as indicated by a barometric reading of
+760 mm., that it boils at 100&deg;.</p>
+
+<p><b>Preparation of liquid air.</b> Attention has been called to the fact that
+both oxygen and nitrogen can be obtained in the liquid state by strongly
+cooling the gases and applying great pressure to them. Since air is
+largely a mixture of these two gases, it can be liquefied by the same
+methods.</p>
+
+<div class="blockquot"><p>The methods for liquefying air have been simplified greatly in
+that the low temperature required is obtained by allowing a
+portion of the compressed air to expand. The expansion of a gas
+is always attended by the absorption of heat. In liquefying air
+the apparatus is so constructed that the heat absorbed is
+withdrawn from air already under great pressure. This process
+is continued until the temperature is lowered to the point of
+liquefaction.</p></div>
+
+<div class="figleft" style="width: 265px;">
+<img src="images/imagefig29.jpg" width="265" height="400" alt="Fig. 29" title="" />
+<span class="caption">Fig. 29</span>
+</div>
+
+<p><b>The Dewar bulb.</b> It is not possible to preserve air in the liquid state
+in a closed vessel, on account of the enormous pressure exerted by it in
+its tendency to pass into the gaseous state. It may however be preserved
+for some hours or even days before it will completely evaporate, by
+simply placing it in an open vessel surrounded by a nonconducting
+material. The most efficient vessel for this purpose is the <i>Dewar bulb</i>
+shown in Fig. 29.<span class='pagenum'><a name="Page_92" id="Page_92">[Pg 92]</a></span> The air is withdrawn from the space between the two
+walls, thus making it nonconducting.</p>
+
+<p><b>Properties and uses of liquid air.</b> When first prepared, liquid air is
+cloudy because of the presence of particles of solid carbon dioxide.
+These may be filtered off, leaving a liquid of slightly bluish color. It
+begins to boil at about -190&deg;, the nitrogen passing off first, gradually
+followed by the oxygen, the last portions being nearly pure oxygen. To a
+certain extent oxygen is now prepared in this way for commercial
+purposes.</p>
+
+<p>The extremely low temperature of liquid air may be inferred from the
+fact that mercury when cooled by it is frozen to a mass so hard that it
+may be used for driving nails.</p>
+
+<p>Liquid air is used in the preparation of oxygen and as a cooling agent
+in the study of the properties of matter at low temperatures. It has
+thus been found that elements at extremely low temperatures largely lose
+their chemical activity.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> When oxygen and nitrogen are mixed in the proportion in which they
+exist in the atmosphere, heat is neither evolved nor absorbed by the
+process. What important point does this suggest?</p>
+
+<p><b>2.</b> What essential constituent of the air is found in larger amount in
+manufacturing districts than in the open country?</p>
+
+<p><b>3.</b> Can you suggest any reason why the growth of clover in a field
+improves the soil?</p>
+
+<p><b>4.</b> Why are the inner walls of a Dewar bulb sometimes coated with a film
+of silver?</p>
+
+<p><b>5.</b> To what is the blue color of liquid air due? Does this color increase
+in intensity on standing?</p>
+
+<p><b>6.</b> When ice is placed in a vessel containing liquid air, the latter
+boils violently. Explain.<span class='pagenum'><a name="Page_93" id="Page_93">[Pg 93]</a></span></p>
+
+<p><b>7.</b> Taking the volumes of the oxygen and nitrogen in 100 volumes of air
+as 21 and 78 respectively, calculate the percentages of these elements
+present by weight.</p>
+
+<p><b>8.</b> Would combustion be more intense in liquid air than in the gaseous
+substance?</p>
+
+<p><b>9.</b> A tube containing calcium chloride was found to weigh 30.1293 g. A
+volume of air which weighed 15.2134 g. was passed through, after which
+the weight of the tube was found to be 30.3405 g. What was the
+percentage amount of moisture present in the air?</p>
+
+<p><b>10.</b> 10 l. of air measured at 20&deg; and 740 mm. passed through lime water
+caused the precipitation of 0.0102 g. of CaCO<sub>3</sub>. Find the number of
+volumes of carbon dioxide in 10,000 volumes of the air.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_94" id="Page_94">[Pg 94]</a></span></p>
+<h2><a name="CHAPTER_IX" id="CHAPTER_IX"></a>CHAPTER IX</h2>
+
+<h3>SOLUTIONS</h3>
+
+
+<p><b>Definitions.</b> When a substance disappears in a liquid in such a way as to
+thoroughly mix with it and to be lost to sight as an individual body,
+the resulting liquid is called a <i>solution</i>. The liquid in which the
+substance dissolves is called the <i>solvent</i>, while the dissolved
+substance is called the <i>solute</i>.</p>
+
+<p><b>Classes of solutions.</b> Matter in any one of its physical states may
+dissolve in a liquid, so that we may have solutions of gases, of
+liquids, and of solids. Solutions of liquids in liquids are not often
+mentioned in the following pages, but the other two classes will become
+very familiar in the course of our study, and deserve special attention.</p>
+
+
+<h3>SOLUTION OF GASES IN LIQUIDS</h3>
+
+<div class="figright" style="width: 400px;">
+<img src="images/imagefig30.jpg" width="400" height="294" alt="Fig. 30" title="" />
+<span class="caption">Fig. 30</span>
+</div>
+
+<p>It has already been stated that oxygen, hydrogen, and nitrogen are
+slightly soluble in water. Accurate study has led to the conclusion that
+all gases are soluble to some extent not only in water but in many other
+liquids. The amount of a gas which will dissolve in a liquid depends
+upon a number of conditions, and these can best be understood by<span class='pagenum'><a name="Page_95" id="Page_95">[Pg 95]</a></span>
+supposing a vessel <i>B</i> (Fig. 30), to be filled with the gas and inverted
+over the liquid. Under these circumstances the gas cannot escape or
+become mixed with another gas.</p>
+
+<p><b>Circumstances affecting the solubility of gases.</b> A number of
+circumstances affect the solubility of a gas in a liquid.</p>
+
+<p>1. <i>Nature of the gas.</i> Other conditions being equal, each gas has its
+own peculiar solubility, just as it has its own special taste or odor.
+The solubility of gases varies between wide limits, as will be seen from
+the following table, but as a rule a given volume of a liquid will not
+dissolve more than two or three times its own volume of a gas.</p>
+
+<h3><i>Solubility of Gases in Water</i></h3>
+
+<h4>1 l. of water at 760 mm. pressure and at 0&deg; will dissolve:</h4>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Ammonia</td><td align='left'>1148.00 l.</td></tr>
+<tr><td align='left'>Hydrochloric acid</td><td align='left'>503.00</td></tr>
+<tr><td align='left'>Sulphur dioxide</td><td align='left'>79.79</td></tr>
+<tr><td align='left'>Carbon dioxide</td><td align='left'>1.80</td></tr>
+<tr><td align='left'>Oxygen</td><td align='left'>41.14 cc.</td></tr>
+<tr><td align='left'>Hydrogen</td><td align='left'>21.15</td></tr>
+<tr><td align='left'>Nitrogen</td><td align='left'>20.03</td></tr>
+</table></div>
+
+<p>In the case of very soluble gases, such as the first three in the table,
+it is probable that chemical combination between the liquid and the gas
+takes place.</p>
+
+<p>2. <i>Nature of the liquid.</i> The character of the liquid has much
+influence upon the solubility of a gas. Water, alcohol, and ether have
+each its own peculiar solvent power. From the solubility of a gas in
+water, no prediction can be made as to its solubility in other liquids.</p>
+
+<p>3. <i>Influence of pressure.</i> It has been found that the weight of gas
+which dissolves in a given case is proportional to the pressure exerted
+upon the gas. If the<span class='pagenum'><a name="Page_96" id="Page_96">[Pg 96]</a></span> pressure is doubled, the weight of gas going into
+solution is doubled; if the pressure is diminished to one half of its
+original value, half of the dissolved gas will escape. Under high
+pressure, large quantities of gas can be dissolved in a liquid, and when
+the pressure is removed the gas escapes, causing the liquid to foam or
+<i>effervesce</i>.</p>
+
+<p>4. <i>Influence of temperature.</i> In general, the lower the temperature of
+the liquid, the larger the quantity of gas which it can dissolve. 1000
+volumes of water at 0&deg; will dissolve 41.14 volumes of oxygen; at 50&deg;,
+18.37 volumes; at 100&deg; none at all. While most gases can be expelled
+from a liquid by boiling the solution, some cannot. For example, it is
+not possible to expel hydrochloric acid gas completely from its solution
+by boiling.</p>
+
+
+<h3>SOLUTION OF SOLIDS IN LIQUIDS</h3>
+
+<p>This is the most familiar class of solutions, since in the laboratory
+substances are much more frequently used in the form of solutions than
+in the solid state.</p>
+
+<p><b>Circumstances affecting the solubility of a solid.</b> The solubility of a
+solid in a liquid depends upon several factors.</p>
+
+<p>1. <i>Nature of the solid.</i> Other conditions being the same, solids vary
+greatly in their solubility in liquids. This is illustrated in the
+following table:</p>
+
+<h4><i>Table of Solubility of Solids at 18&deg;</i></h4>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>100 cc. of water will dissolve:</td></tr>
+<tr><td align='left'>Calcium chloride</td><td align='left'>71.0 g.</td></tr>
+<tr><td align='left'>Sodium chloride</td><td align='left'>35.9</td></tr>
+<tr><td align='left'>Potassium nitrate</td><td align='left'>29.1</td></tr>
+<tr><td align='left'>Copper sulphate</td><td align='left'>21.4</td></tr>
+<tr><td align='left'>Calcium sulphate</td><td align='left'>0.207</td></tr>
+</table></div>
+
+<p><span class='pagenum'><a name="Page_97" id="Page_97">[Pg 97]</a></span></p>
+
+<p>No solids are absolutely insoluble, but the amount dissolved may be so
+small as to be of no significance for most purposes. Thus barium
+sulphate, one of the most insoluble of common substances, dissolves in
+water to the extent of 1 part in 400,000.</p>
+
+<p>2. <i>Nature of the solvent.</i> Liquids vary much in their power to dissolve
+solids. Some are said to be good solvents, since they dissolve a great
+variety of substances and considerable quantities of them. Others have
+small solvent power, dissolving few substances, and those to a slight
+extent only. Broadly speaking, water is the most general solvent, and
+alcohol is perhaps second in solvent power.</p>
+
+<p>3. <i>Temperature.</i> The weight of a solid which a given liquid can
+dissolve varies with the temperature. Usually it increases rapidly as
+the temperature rises, so that the boiling liquid dissolves several
+times the weight which the cold liquid will dissolve. In some instances,
+as in the case of common salt dissolved in water, the temperature has
+little influence upon the solubility, and a few solids are more soluble
+in cold water than in hot. The following examples will serve as
+illustrations:</p>
+
+
+<h3><i>Table of Solubility at 0&deg; and at 100&deg;</i></h3>
+
+
+<h4>100 cc. of water will dissolve:</h4>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>At 0&deg;</td><td align='left'>At 100&deg;</td></tr>
+<tr><td align='left'>Calcium chloride</td><td align='left'>49.6 g.</td><td align='left'>155.0 g.</td></tr>
+<tr><td align='left'>Sodium chloride</td><td align='left'>35.7</td><td align='left'>39.8</td></tr>
+<tr><td align='left'>Potassium nitrate</td><td align='left'>13.3</td><td align='left'>247.0</td></tr>
+<tr><td align='left'>Copper sulphate</td><td align='left'>15.5</td><td align='left'>73.5</td></tr>
+<tr><td align='left'>Calcium sulphate</td><td align='left'>0.205</td><td align='left'>0.217</td></tr>
+<tr><td align='left'>Calcium hydroxide</td><td align='left'>0.173</td><td align='left'>0.079</td></tr>
+</table></div>
+
+<p><b>Saturated solutions.</b> A liquid will not dissolve an unlimited quantity of
+a solid. On adding the solid to the liquid in small portions at<span class='pagenum'><a name="Page_98" id="Page_98">[Pg 98]</a></span> a time,
+it will be found that a point is reached at which the liquid will not
+dissolve more of the solid at that temperature. The solid and the
+solution remain in contact with each other unchanged. This condition may
+be described by saying that they are in equilibrium with each other. A
+solution is said to be <i>saturated</i> when it remains unchanged in
+concentration in contact with some of the solid. The weight of the solid
+which will completely saturate a definite volume of a liquid at a given
+temperature is called the <i>solubility</i> of the substance at that
+temperature.</p>
+
+<p><b>Supersaturated solutions.</b> When a solution, saturated at a given
+temperature, is allowed to cool it sometimes happens that no solid
+crystallizes out. This is very likely to occur when the vessel used is
+perfectly smooth and the solution is not disturbed in any way. Such a
+solution is said to be <i>supersaturated</i>. That this condition is unstable
+can be shown by adding a crystal of the solid to the solution. All of
+the solid in excess of the quantity required to saturate the solution at
+this temperature will at once crystallize out, leaving the solution
+saturated. Supersaturation may also be overcome in many cases by
+vigorously shaking or stirring the solution.</p>
+
+
+<p><b>General physical properties of solutions.</b> A few general statements may
+be made in reference to the physical properties of solutions.</p>
+
+<p>1. <i>Distribution of the solid in the liquid.</i> A solid, when dissolved,
+tends to distribute itself uniformly through the liquid, so that every
+part of the solution has the same concentration. The process goes on
+very slowly unless hastened by stirring or shaking the solution. Thus,
+if a few crystals of a highly colored substance such as copper sulphate
+are placed in the bottom of a tall vessel full of water, it will take
+weeks for the solution to become uniformly colored.</p>
+
+<p>2. <i>Boiling points of solutions.</i> The boiling point of a liquid is
+raised by the presence of a substance dissolved in it. In general the
+extent to which the boiling point of a solvent is raised by a given
+substance is proportional to the<span class='pagenum'><a name="Page_99" id="Page_99">[Pg 99]</a></span> concentration of the solution, that
+is, to the weight of the substance dissolved in a definite weight of the
+solvent.</p>
+
+<p>3. <i>Freezing points of solutions.</i> A solution freezes at a lower
+temperature than the pure solvent. The lowering of the freezing point
+obeys the same law which holds for the raising of the boiling point: the
+extent of lowering is proportional to the weight of dissolved substance,
+that is, to the concentration of the solution.</p>
+
+<p><b>Electrolysis of solutions.</b> Pure water does not appreciably conduct the
+electric current. If, however, certain substances such as common salt
+are dissolved in the water, the resulting solutions are found to be
+conductors of electricity. Such solutions are called <i>electrolytes</i>.
+When the current passes through an electrolyte some chemical change
+always takes place. This change is called <i>electrolysis</i>.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig31.jpg" width="400" height="255" alt="Fig. 31" title="" />
+<span class="caption">Fig. 31</span>
+</div>
+
+<p>The general method used in the electrolysis of a solution is illustrated
+in Fig. 31. The vessel <i>D</i> contains the electrolyte. Two plates or rods,
+<i>A</i> and <i>B</i>, made of suitable material, are connected with the wires
+from a battery (or dynamo) and dipped into the electrolyte, as shown in
+the figure. These plates or rods are called <i>electrodes</i>. The electrode
+connected with the zinc plate of the battery is the negative electrode
+or <i>cathode</i>, while that connected with the carbon plate is the positive
+electrode or <i>anode</i>.</p>
+
+<p><b>Theory of electrolytic dissociation.</b> The facts which have just been
+described in connection with solutions, together with many others, have
+led chemists to adopt a theory of solutions called <i>the theory of
+electrolytic dissociation</i>. The main assumptions in this theory are the
+following.<span class='pagenum'><a name="Page_100" id="Page_100">[Pg 100]</a></span></p>
+
+<p>1. <i>Formation of ions.</i> Many compounds when dissolved in water undergo
+an important change. A portion of their molecules fall apart, or
+<i>dissociate</i>, into two or more parts, called <i>ions</i>. Thus sodium nitrate
+(NaNO<sub>3</sub>) dissociates into the ions Na and NO<sub>3</sub>; sodium chloride,
+into the ions Na and Cl. These ions are free to move about in the
+solution independently of each other like independent molecules, and for
+this reason were given the name ion, which signifies a wanderer.</p>
+
+<p>2. <i>The electrical charge of ions.</i> Each ion carries a heavy electrical
+charge, and in this respect differs from an atom or molecule. It is
+evident that the sodium in the form of an ion must differ in some
+important way from ordinary sodium, for sodium ions, formed from sodium
+nitrate, give no visible evidence of their presence in water, whereas
+metallic sodium at once decomposes the water. The electrical charge,
+therefore, greatly modifies the usual chemical properties of the
+element.</p>
+
+<p>3. <i>The positive charges equal the negative charges.</i> The ions formed by
+the dissociation of any molecule are of two kinds. One kind is charged
+with positive electricity and the other with negative electricity;
+moreover the sum of all the positive charges is always equal to the sum
+of all the negative charges. The solution as a whole is therefore
+electrically neutral. If we represent dissociation by the usual chemical
+equations, with the electrical charges indicated by + and - signs
+following the symbols, the dissociation of sodium chloride molecules is
+represented thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaCl --> Na<sup>+</sup>, Cl<sup>-</sup>.<br /></span>
+</div></div>
+
+<p>The positive charge on each sodium ion exactly equals the negative
+charge on each chlorine ion.<span class='pagenum'><a name="Page_101" id="Page_101">[Pg 101]</a></span> Sodium sulphate dissociates, as shown in
+the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>SO<sub>4</sub> --> 2Na<sup>+</sup>, SO<sub>4</sub><sup>-</sup>.<br /></span>
+</div></div>
+
+<p>Here the positive charge on the two sodium ions equals the double
+negative charge on the SO<sub>4</sub> ion.</p>
+
+<p>4. <i>Not all compounds dissociate.</i> Only those compounds dissociate whose
+solutions form electrolytes. Thus salt dissociates when dissolved in
+water, the resulting solution being an electrolyte. Sugar, on the other
+hand, does not dissociate and its solution is not a conductor of the
+electric current.</p>
+
+<p>5. <i>Extent of dissociation differs in different liquids.</i> While
+compounds most readily undergo dissociation in water, yet dissociation
+often occurs to a limited extent when solution takes place in liquids
+other than water. In the discussion of solutions it will be understood
+that the solvent is water unless otherwise noted.</p>
+
+<p><b>The theory of electrolytic dissociation and the properties of solutions.</b>
+In order to be of value, this theory must give a reasonable explanation
+of the properties of solutions. Let us now see if the theory is in
+harmony with certain of these properties.</p>
+
+<p><b>The theory of electrolytic dissociation and the boiling and freezing
+points of solutions.</b> We have seen that the boiling point of a solution
+of a substance is raised in proportion to the concentration of the
+dissolved substance. This is but another way of saying that the change
+in the boiling point of the solution is proportional to the number of
+molecules of the dissolved substance present in the solution.</p>
+
+<p>It has been found, however, that in the case of electrolytes the boiling
+point is raised more than it should be to<span class='pagenum'><a name="Page_102" id="Page_102">[Pg 102]</a></span> conform to this law. If the
+solute dissociates into ions, the reason for this becomes clear. Each
+ion has the same effect on the boiling point as a molecule, and since
+their number is greater than the number of molecules from which they
+were formed, the effect on the boiling point is abnormally great.</p>
+
+<p>In a similar way, the theory furnishes an explanation of the abnormal
+lowering of the freezing point of electrolytes.</p>
+
+<p><b>The theory of electrolytic dissociation and electrolysis.</b> The changes
+taking place during electrolysis harmonize very completely with the
+theory of dissociation. This will become clear from a study of the
+following examples.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/imagefig32.jpg" width="500" height="179" alt="Fig. 32" title="" />
+<span class="caption">Fig. 32</span>
+</div>
+
+<p>1. <i>Electrolysis of sodium chloride.</i> Fig. 32 represents a vessel in
+which the electrolyte is a solution of sodium chloride (NaCl). According
+to the dissociation theory the molecules of sodium chloride dissociate
+into the ions Na<sup>+</sup> and Cl<sup>-</sup>. The Na<sup>+</sup> ions are attracted to the
+cathode owing to its large negative charge. On coming into contact with
+the cathode, the Na<sup>+</sup> ions give up their positive charge and are then
+ordinary sodium atoms. They immediately decompose the water according to
+the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na + H<sub>2</sub>O = NaOH + H,<br /></span>
+</div></div>
+
+<p>and hydrogen is evolved about the cathode.</p>
+
+<p>The chlorine ions on being discharged at the anode in similar manner may
+either be given off as chlorine gas, or may attack the water, as
+represented in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Cl + H<sub>2</sub>O = 2HCl + O.<br /></span>
+<span class='pagenum'><a name="Page_103" id="Page_103">[Pg 103]</a></span></div></div>
+
+<p>2. <i>Electrolysis of water.</i> The reason for the addition of sulphuric
+acid to water in the preparation of oxygen and hydrogen by electrolysis
+will now be clear. Water itself is not an electrolyte to an appreciable
+extent; that is, it does not form enough ions to carry a current.
+Sulphuric acid dissolved in water is an electrolyte, and dissociates
+into the ions 2 H<sup>+</sup> and SO<sub>4</sub><sup>&mdash;</sup>. In the process of electrolysis of
+the solution, the hydrogen ions travel to the cathode, and on being
+discharged escape as hydrogen gas. The SO<sub>4</sub> ions, when discharged at
+the anode, act upon water, setting free oxygen and once more forming
+sulphuric acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SO<sub>4</sub> + H<sub>2</sub>O = H<sub>2</sub>SO<sub>4</sub> + O.<br /></span>
+</div></div>
+
+<p>The sulphuric acid can again dissociate and the process repeat itself as
+long as any water is left. Hence the hydrogen and oxygen set free in the
+electrolysis of water really come directly from the acid but indirectly
+from the water.</p>
+
+<p>3. <i>Electrolysis of sodium sulphate.</i> In a similar way, sodium sulphate
+(Na<sub>2</sub>SO<sub>4</sub>), when in solution, gives the ions 2 Na<sup>+</sup> and
+SO<sub>4</sub><sup>&mdash;</sup>. On being discharged, the sodium atoms decompose water about
+the cathode, as in the case of sodium chloride, while the SO<sub>4</sub> ions
+when discharged at the anode decompose the water, as represented in the
+equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SO<sub>4</sub> + H<sub>2</sub>O = H<sub>2</sub>SO<sub>4</sub> + O<br /></span>
+</div></div>
+
+<div class="figright" style="width: 314px;">
+<img src="images/imagefig33.jpg" width="314" height="400" alt="Fig. 33" title="" />
+<span class="caption">Fig. 33</span>
+</div>
+
+<p>That new substances are formed at the cathode and anode may be shown in
+the following way. A U-tube, such as is represented in Fig. 33, is
+partially filled with a solution of sodium sulphate, and the liquid in
+one arm is colored with red litmus, that in the other<span class='pagenum'><a name="Page_104" id="Page_104">[Pg 104]</a></span> with blue litmus.
+An electrode placed in the red solution is made to serve as cathode,
+while one in the blue solution is made the anode. On allowing the
+current to pass, the blue solution turns red, while the red solution
+turns blue. These are exactly the changes which would take place if
+sodium hydroxide and sulphuric acid were to be set free at the
+electrodes, as required by the theory.</p>
+
+<p><b>The properties of electrolytes depend upon the ions present.</b> When a
+substance capable of dissociating into ions is dissolved in water, the
+properties of the solution will depend upon two factors: (1) the ions
+formed from the substance; (2) the undissociated molecules. Since the
+ions are usually more active chemically than the molecules, most of the
+chemical properties of an electrolyte are due to the ions rather than to
+the molecules.</p>
+
+<p>The solutions of any two substances which give the same ion will have
+certain properties in common. Thus all solutions containing the copper
+ion (Cu<sup>++</sup>) are blue, unless the color is modified by the presence of
+ions or molecules having some other color.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Distinguish clearly between the following terms: electrolysis,
+electrolyte, electrolytic dissociation, ions, solute, solvent, solution,
+saturated solution, and supersaturated solution.</p>
+
+<p><b>2.</b> Why does the water from some natural springs effervesce?</p>
+
+<p><b>3.</b> (<i>a</i>) Why does not the water of the ocean freeze? (<i>b</i>) Why will ice
+and salt produce a lower temperature than ice alone?</p>
+
+<p><b>4.</b> Why does shaking or stirring make a solid dissolve more rapidly in a
+liquid?</p>
+
+<p><b>5.</b> By experiment it was found that a certain volume of water was
+saturated at 100&deg; with 114 g. of potassium nitrate. On cooling to 0&deg; a
+portion of the substance crystallized. (<i>a</i>) How many grams of the
+substance remained in solution? (<i>b</i>) What was the strength<span class='pagenum'><a name="Page_105" id="Page_105">[Pg 105]</a></span> of the
+solution at 18&deg;? (<i>c</i>) How much water had been used in the experiment?</p>
+
+<p><b>6.</b> (<i>a</i>) 10 g. of common salt were dissolved in water and the solution
+evaporated to dryness; what weight of solid was left? (<i>b</i>) 10 g. of
+zinc were dissolved in hydrochloric acid and the solution evaporated to
+dryness; what weight of solid was left?</p>
+
+<p><b>7.</b> Account for the fact that sugar sometimes deposits from molasses,
+even when no evaporation has taken place.</p>
+
+<p><b>8.</b> (<i>a</i>) From the standpoint of the theory of electrolytic dissociation,
+write the simple equation for a dilute solution of copper sulphate
+(CuSO<sub>4</sub>); this solution is blue. (<i>b</i>) In the same manner, write one
+for sodium sulphate; this solution is colorless. (<i>c</i>) How would you
+account for the color of the copper sulphate solution?</p>
+
+<p><b>9.</b> (<i>a</i>) As in the preceding exercise, write a simple equation for a
+dilute solution of copper chloride (CuCl<sub>2</sub>); this solution is blue.
+(<i>b</i>) In the same manner, write one for sodium chloride; this solution
+is colorless. To what is the blue color due?</p>
+
+<p><b>10.</b> What component is present in concentrated sulphuric acid that is
+almost wanting in very dilute sulphuric acid?</p>
+
+<p><b>11.</b> Why will vegetables cook faster when boiled in strong salt water
+than when boiled in pure water?</p>
+
+<p><b>12.</b> How do you explain the foaming of soda water?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_106" id="Page_106">[Pg 106]</a></span></p>
+<h2><a name="CHAPTER_X" id="CHAPTER_X"></a>CHAPTER X</h2>
+
+<h3>ACIDS, BASES, AND SALTS; NEUTRALIZATION</h3>
+
+
+<p><b>Acids, bases, and salts.</b> The three classes of compounds known
+respectively as acids, bases, and salts include the great majority of
+the compounds with which we shall have to deal. It is important,
+therefore, for us to consider each of these classes in a systematic way.
+The individual members belonging to each class will be discussed in
+detail in the appropriate places, but a few representatives of each
+class will be described in this chapter with special reference to the
+common properties in accordance with which they are classified.</p>
+
+<p><b>The familiar acids.</b> <i>Hydrochloric acid</i>is a gas composed of hydrogen and
+chlorine, and has the formula HCl. The substance is very soluble in
+water, and it is this solution which is usually called hydrochloric
+acid. <i>Nitric acid</i> is a liquid composed of hydrogen, nitrogen, and
+oxygen, having the formula HNO<sub>3</sub>. As sold commercially it is mixed
+with about 32% of water. <i>Sulphuric acid</i>, whose composition is
+represented by the formula H<sub>2</sub>SO<sub>4</sub>, is an oily liquid nearly twice
+as heavy as water, and is commonly called <i>oil of vitriol</i>.</p>
+
+<p><b>Characteristics of acids.</b> (1) All acids contain hydrogen. (2) When
+dissolved in water the molecules of the acid dissociate into two kinds
+of ions. One of these is always hydrogen and is the cation (+), while
+the other consists of the remainder of the molecule and is the anion
+(-). (3) The solution tastes sour. (4) It has the power to change the<span class='pagenum'><a name="Page_107" id="Page_107">[Pg 107]</a></span>
+color of certain substances called <i>indicators</i>. Thus blue litmus is
+changed to red, and yellow methyl orange is changed to red. Since all
+acids produce hydrogen cations, while the anions of each are different,
+the properties which all acids have in common when in solution, such as
+taste and action on indicators, must be attributed to the hydrogen ions.</p>
+
+<p>DEFINITION: <i>An acid is a substance which produces hydrogen ions when
+dissolved in water or other dissociating liquids.</i></p>
+
+<p><b>Undissociated acids.</b> When acids are perfectly free from water, or are
+dissolved in liquids like benzene which do not have the power of
+dissociating them into ions, they should have no real acid properties.
+This is found to be the case. Under these circumstances they do not
+affect the color of indicators or have any of the properties
+characteristic of acids.</p>
+
+<p>The familiar bases. The bases most used in the laboratory are sodium
+hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide
+(Ca(OH)<sub>2</sub>). These are white solids, soluble in water, the latter
+sparingly so. Some bases are very difficultly soluble in water. The very
+soluble ones with most pronounced basic properties are sometimes called
+the <i>alkalis</i>.</p>
+
+<p><b>Characteristics of bases.</b> (1) All bases contain hydrogen and oxygen. (2)
+When dissolved in water the molecules of the base dissociate into two
+kinds of ions. One of these is always composed of oxygen and hydrogen
+and is the anion. It has the formula OH and is called the <i>hydroxyl
+ion</i>. The remainder of the molecule, which usually consists of a single
+atom, is the cation. (3) The solution of a base has<span class='pagenum'><a name="Page_108" id="Page_108">[Pg 108]</a></span> a soapy feel and a
+brackish taste. (4) It reverses the color change produced in indicators
+by acids, turning red litmus blue, and red methyl orange yellow. Since
+all bases produce hydroxyl anions, while the cations of each are
+different, the properties which all bases have in common when in
+solution must be due to the hydroxyl ions.</p>
+
+<p>DEFINITION: <i>A base is a substance which produces hydroxyl ions when
+dissolved in water or other dissociating liquids.</i></p>
+
+<p><b>Undissociated bases.</b> Bases, in the absence of water or when dissolved in
+liquids which do not dissociate them, should have none of the properties
+characteristic of this class of substances. This has been found to be
+the case. For example, they have no effect upon indicators under these
+circumstances.</p>
+
+<p><b>Neutralization.</b> When an acid and a base are brought together in solution
+in proper proportion, the characteristic properties of each disappear.
+The solution tastes neither sour nor brackish; it has no effect upon
+indicators. There can therefore be neither hydrogen nor hydroxyl ions
+present in the solution. A study of reactions of this kind has shown
+that the hydrogen ions of the acid combine with the hydroxyl ions of the
+base to form molecules of water, water being a substance which is not
+appreciably dissociated into ions. This action of an acid on a base is
+called <i>neutralization</i>. The following equations express the
+neutralization of the three acids by three bases, water being formed in
+each case.</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sup>+</sup>, OH<sup>-</sup> + H<sup>+</sup>, Cl<sup>-</sup> = Na<sup>+</sup>, Cl<sup>-</sup> + H<sub>2</sub>O.<br /></span>
+</div><div class="stanza">
+<span class="i0">K<sup>+</sup>, OH<sup>-</sup> + H<sup>+</sup>, NO<sub>3</sub><sup>-</sup> = K<sup>+</sup>, NO<sub>3</sub><sup>-</sup> + H<sub>2</sub>O.<br /></span>
+</div><div class="stanza">
+<span class="i0">Ca<sup>++</sup>, (OH)<sub>2</sub><sup>&mdash;</sup> + H<sub>2</sub><sup>++</sup>, SO<sub>4</sub><sup>-</sup> = Ca<sup>++</sup>, SO<sub>4</sub><sup>&mdash;</sup> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_109" id="Page_109">[Pg 109]</a></span></p>
+<p>DEFINITION: <i>Neutralization consists in the union of the hydrogen ion of
+an acid with the hydroxyl ion of a base to form water.</i></p>
+
+<p><b>Salts.</b> It will be noticed that in neutralization the anion of the acid
+and the cation of the base are not changed. If, however, the water is
+expelled by evaporation, these two ions slowly unite, and when the water
+becomes saturated with the substance so produced, it separates in the
+form of a solid called a <i>salt</i>.</p>
+
+<p>DEFINITION: <i>A salt is a substance formed by the union of the anion of
+an acid with the cation of a base.</i></p>
+
+<p><b>Characteristics of salts.</b> (1) From the definition of a salt it will be
+seen that there is no element or group of elements which characterize
+salts. (2) Salts as a class have no peculiar taste. (3) In the absence
+of all other substances they are without action on indicators. (4) When
+dissolved in water they form two kinds of ions.</p>
+
+<div class="blockquot"><p><b>Heat of neutralization.</b> If neutralization is due to the union
+of hydrogen ions with hydroxyl ions, and nothing more, it
+follows that when a given weight of water is formed in
+neutralization, the heat set free should always be the same, no
+matter from what acid and base the two kinds of ions have been
+supplied. Careful experiments have shown that this is the case,
+provided no other reactions take place at the same time. When
+18g. of water are formed in neutralization, 13,700 cal. of heat
+are set free. This is represented in the equations</p></div>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sup>+</sup>, OH<sup>-</sup> + H<sup>+</sup>, Cl<sup>-</sup> = Na<sup>+</sup>, Cl<sup>-</sup> + H<sub>2</sub>O + 13,700 cal.<br /></span>
+</div><div class="stanza">
+<span class="i0">K<sup>+</sup>, OH<sup>-</sup> + H<sup>+</sup>, NO<sub>3</sub><sup>-</sup> = K<sup>+</sup>, NO<sub>3</sub><sup>-</sup> + H<sub>2</sub>O + 13,700 cal.<br /></span>
+</div><div class="stanza">
+<span class="i0">Ca<sup>++</sup>, (OH)<sub>2</sub><sup>-</sup> + H<sub>2</sub><sup>++</sup>, SO<sub>4</sub><sup>-</sup> = Ca<sup>++</sup>, SO<sub>4</sub><sup>-</sup> + 2H<sub>2</sub>O + 2 &times; 13,700 cal.<br /></span>
+</div></div>
+
+<p><b>Neutralization a quantitative act.</b> Since neutralization is a definite
+chemical act, each acid will require a perfectly definite weight of each
+base for its neutralization. For<span class='pagenum'><a name="Page_110" id="Page_110">[Pg 110]</a></span> example, a given weight of sulphuric
+acid will always require a definite weight of sodium hydroxide, in
+accordance with the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>, SO<sub>4</sub> + 2Na, OH = Na<sub>2</sub>, SO<sub>4</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<div class="blockquot"><p><b>Determination of the ratio in neutralization.</b> The quantities of
+acid and base required in neutralization may be determined in
+the following way. Dilute solutions of the two substances are
+prepared, the sulphuric acid being placed in one of the
+burettes (Fig. 34) and the sodium hydroxide in the other. The
+levels of the two liquids are then brought to the zero marks of
+the burettes by means of the stopcocks. A measured volume of
+the acid is drawn off into a beaker, a few drops of litmus
+solution added, and the sodium hydroxide is run in drop by drop
+until the red litmus just turns blue. The volume of the sodium
+hydroxide consumed is then noted. If the concentrations of the
+two solutions are known, it is easy to calculate what weight of
+sodium hydroxide is required to neutralize a given weight of
+sulphuric acid. By evaporating the neutralized solution to
+dryness, the weight of the sodium sulphate formed can be
+determined directly. Experiment shows that the weights are
+always in accordance with the equation in the preceding
+paragraph.</p></div>
+
+<div class="figleft" style="width: 209px;">
+<img src="images/imagefig34.jpg" width="209" height="400" alt="Fig. 34" title="" />
+<span class="caption">Fig. 34</span>
+</div>
+
+<p><b>Extent of dissociation.</b> The question will naturally arise, When an acid,
+base, or salt dissolves in water, do all the molecules dissociate into
+ions, or only a part of them? The experiments by which this question can
+be answered cannot be described here. It has been found, however, that
+only a fraction of the molecules dissociate. The percentage which will
+dissociate in a given case depends upon several conditions, the chief of
+which are: (1) The concentration of the solution. In concentrated
+solutions only a very small<span class='pagenum'><a name="Page_111" id="Page_111">[Pg 111]</a></span> percentage of dissociation occurs. As the
+solution is diluted the percentage increases, and in very dilute
+solutions it may be very large, though it is never complete in any
+ordinary solution. (2) The nature of the dissolved compound. At equal
+concentrations substances differ much among themselves in the percentage
+of dissociation. The great majority of salts are about equally
+dissociated. Acids and bases, on the contrary, show great differences.
+Some are freely dissociated, while others are dissociated to but a
+slight extent.</p>
+
+<p><b>Strength of acids and bases.</b> Since acid and basic properties are due to
+hydrogen and hydroxyl ions respectively, the acid or base which will
+produce the greatest percentage of these ions at a given concentration
+must be regarded as the strongest representative of its class. The acids
+and bases described in the foregoing paragraphs are all quite strong. In
+10% solutions they are dissociated to about 50%, and this is also
+approximately the extent to which most salts are dissociated at this
+same concentration.</p>
+
+<div class="blockquot"><p><b>Partial neutralization.</b> 1. <i>Basic salts.</i> The chemical action
+between an acid and a base is not always as complete as has
+been represented in the foregoing paragraphs. For example, if
+the base magnesium hydroxide (Mg(OH)<sub>2</sub>) and hydrochloric acid
+(HCl) are brought together in the ratio of an equal number of
+molecules of each, there will be only half enough hydrogen ions
+for the hydroxyl ions present.</p>
+
+<p>Mg, (OH)<sub>2</sub> + H, Cl = Mg, OH, Cl + H<sub>2</sub>O.</p>
+
+<p>Magnesium, hydroxyl, and chlorine ions are left at the close of
+the reaction, and under the proper conditions unite to form
+molecules of the compound Mg(OH)Cl. This compound, when
+dissolved, can form hydroxyl ions and therefore possesses basic
+properties; it can also form the ions of a salt (Mg and Cl),
+and has properties characteristic of salts. Substances of this
+kind are called <i>basic salts.</i></p>
+
+<p>DEFINITION: <i>A basic salt is a substance which can give the
+ions both of a base and of a salt when dissolved in water.</i><span class='pagenum'><a name="Page_112" id="Page_112">[Pg 112]</a></span></p>
+
+<p>2. <i>Acid salts.</i> In a similar way, when sulphuric acid and
+sodium hydroxide are brought together in the ratio of equal
+numbers of the molecules of each, it is possible to have a
+reaction expressed by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na, OH + H<sub>2</sub>, SO<sub>4</sub> = Na, H, SO<sub>4</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The ions remaining after all the hydroxyl ions have been used
+up are those of an acid (H) and those of a salt (Na and
+SO<sub>4</sub>). These unite to form the substance NaHSO<sub>4</sub>, and as
+the solution becomes saturated with this substance through
+evaporation, it separates in the form of crystals. In solution
+this substance can give hydrogen ions, and therefore possesses
+acid properties; it can also give the ions characteristic of a
+salt. It is therefore called an <i>acid salt</i>.</p>
+
+<p>DEFINITION: <i>An acid salt is one which can give the ions of an
+acid and of a salt when in solution.</i></p>
+
+<p>3. <i>Normal salts.</i> Salts which are the products of complete
+neutralization, such as Na<sub>2</sub>SO<sub>4</sub>, and which in solution can
+give neither hydrogen nor hydroxyl ions, but only the ions of a
+salt, are called <i>normal salts</i> to distinguish them from acid
+and basic salts.</p></div>
+
+<p><b>Methods of expressing reactions between compounds in solution.</b> Chemical
+equations representing reactions between substances in solution may
+represent the details of the reaction, or they may simply indicate the
+final products formed. In the latter case the formation of ions is not
+indicated. Thus, if we wish to call attention to the details of the
+reaction between sodium hydroxide and hydrochloric acid in solution, the
+equation is written as follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sup>+</sup>, OH<sup>-</sup> + H<sup>+</sup>, Cl<sup>-</sup> = Na<sup>+</sup>, Cl<sup>-</sup> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>On the other hand, if we wish simply to represent the final products
+formed, the following is used.</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaOH + HCl = NaCl + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Both of these methods will therefore be used:</p>
+
+<p><b>Radicals.</b> It has been emphasized that the hydroxyl group (OH) always
+<span class='pagenum'><a name="Page_113" id="Page_113">[Pg 113]</a></span>forms the anion of a base, while the group NO<sub>3</sub> forms the anion of
+nitric acid and sodium nitrate; the group SO<sub>4</sub>, the anion of sulphuric
+acid and calcium sulphate. A group of elements which in this way
+constitutes a part of a molecule, acting as a unit in a chemical change,
+or forming ions in solution, is called a <i>radical</i>. Some of these
+radicals have been given special names, the names signifying the
+elements present in the radical. Thus we have the hydroxyl radical (OH)
+and the nitrate radical (NO<sub>3</sub>).</p>
+
+<p>DEFINITION: <i>A radical is a group of elements forming part of a
+molecule, and acting as a unit in chemical reactions.</i></p>
+
+<p><b>Names of acids, bases, and salts.</b> Since acids, bases, and salts are so
+intimately related to each other, it is very advantageous to give names
+to the three classes in accordance with some fixed system. The system
+universally adopted is as follows:</p>
+
+<p><b>Naming of bases.</b> All bases are called <i>hydroxides</i>. They are
+distinguished from each other by prefixing the name of the element which
+is in combination with the hydroxyl group. Examples: sodium hydroxide
+(NaOH); calcium hydroxide (Ca(OH)<sub>2</sub>); copper hydroxide (Cu(OH)<sub>2</sub>).</p>
+
+<p><b>Naming of acids.</b> The method of naming acids depends upon whether the
+acid consists of two elements or three.</p>
+
+<p>1. <i>Binary acids.</i> Acids containing only one element in addition to
+hydrogen are called <i>binary acids</i>. They are given names consisting of
+the prefix <i>hydro-</i>, the name of the second element present, and the
+termination <i>-ic</i>. Examples: hydrochloric acid (HCl); hydrosulphuric
+acid (H<sub>2</sub>S).</p>
+
+<p>2. <i>Ternary acids.</i> In addition to the two elements present in binary
+acids, the great majority of acids also contain oxygen. They therefore
+consist of three elements and<span class='pagenum'><a name="Page_114" id="Page_114">[Pg 114]</a></span> are called <i>ternary acids</i>. It usually
+happens that the same three elements can unite in different proportions
+to make several different acids. The most familiar one of these is given
+a name ending in the suffix <i>-ic</i>, while the one with less oxygen is
+given a similar name, but ending in the suffix <i>-ous</i>. Examples: nitric
+acid (HNO<sub>3</sub>); nitrous acid (HNO<sub>2</sub>). In cases where more than two
+acids are known, use is made of prefixes in addition to the two suffixes
+<i>-ic</i>and <i>-ous</i>. Thus the prefix <i>per-</i> signifies an acid still richer
+in oxygen; the prefix <i>hypo-</i> signifies one with less oxygen.</p>
+
+<p><b>Naming of salts.</b> A salt derived from a binary acid is given a name
+consisting of the names of the two elements composing it, with the
+termination <i>-ide</i>. Example: sodium chloride (NaCl). All other binary
+compounds are named in the same way.</p>
+
+<p>A salt of a ternary acid is named in accordance with the acid from which
+it is derived. A ternary acid with the termination <i>-ic</i> gives a salt
+with the name ending in <i>-ate</i>, while an acid with termination <i>-ous</i>
+gives a salt with the name ending in <i>-ite</i>. The following table will
+make the application of these principles clear:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>ACIDS</td><td align='left'>SYMBOL</td><td align='left'>SALTS</td><td align='left'>SYMBOL</td></tr>
+<tr><td align='left'>Hydrochloric</td><td align='left'>HCl</td><td align='left'>Sodium chloride</td><td align='left'>NaCl</td></tr>
+<tr><td align='left'>Hypochlorous</td><td align='left'>HClO</td><td align='left'>Sodium hypochlorite</td><td align='left'>NaClO</td></tr>
+<tr><td align='left'>Chlorous</td><td align='left'>HClO2</td><td align='left'>Sodium chlorite</td><td align='left'>NaClO2</td></tr>
+<tr><td align='left'>Chloric</td><td align='left'>HClO3</td><td align='left'>Sodium chlorate</td><td align='left'>NaClO3</td></tr>
+<tr><td align='left'>Perchloric</td><td align='left'>HClO4</td><td align='left'>Sodium perchlorate</td><td align='left'>NaClO4</td></tr>
+</table></div>
+
+
+<p><span class='pagenum'><a name="Page_115" id="Page_115">[Pg 115]</a></span></p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> 25 cc. of a solution containing 40 g. of sodium hydroxide per liter
+was found to neutralize 25 cc. of a solution of hydrochloric acid. What
+was the strength of the acid solution?</p>
+
+<p><b>2.</b> After neutralizing a solution of sodium hydroxide with nitric acid,
+there remained after evaporation 100 g. of sodium nitrate. How much of
+each substance had been used?</p>
+
+<p><b>3.</b> A solution contains 18 g. of hydrochloric acid per 100 cc. It
+required 25 cc. of this solution to neutralize 30 cc. of a solution of
+sodium hydroxide. What was the strength of the sodium hydroxide solution
+in parts per hundred?</p>
+
+<p><b>4.</b> When perfectly dry sulphuric acid is treated with perfectly dry
+sodium hydroxide, no chemical change takes place. Explain.</p>
+
+<p><b>5.</b> When cold, concentrated sulphuric acid is added to zinc, no change
+takes place. Recall the action of dilute sulphuric acid on the same
+metal. How do you account for the difference?</p>
+
+<p><b>6.</b> A solution of hydrochloric acid in benzene does not conduct the
+electric current. When this solution is treated with zinc, will hydrogen
+be evolved? Explain.</p>
+
+<p><b>7.</b> (<i>a</i>) Write equation for preparation of hydrogen from zinc and dilute
+sulphuric acid. (<i>b</i>) Rewrite the same equation from the standpoint of
+the theory of electrolytic dissociation, (<i>c</i>) Subtract the common
+SO<sub>4</sub> ion from both members of the equation, (<i>d</i>) From the resulting
+equation, explain in what the preparation of hydrogen consists when
+examined from the standpoint of this theory.</p>
+
+<p><b>8.</b> In the same manner as in the preceding exercise, explain in what the
+action of sodium on water to give hydrogen consists.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_116" id="Page_116">[Pg 116]</a></span></p>
+<h2><a name="CHAPTER_XI" id="CHAPTER_XI"></a>CHAPTER XI</h2>
+
+<h3>VALENCE</h3>
+
+
+<p><b>Definition of valence.</b> A study of the formulas of various binary
+compounds shows that the elements differ between themselves in the
+number of atoms of other elements which they are able to hold in
+combination. This is illustrated in the formulas</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>HCl,</td><td align='left'>H<sub>2</sub>O,</td><td align='left'>H<sub>3</sub>N,</td><td align='left'>H<sub>4</sub>C.</td></tr>
+<tr><td align='left'>(hydrochloric acid)</td><td align='left'>(water)</td><td align='left'>(ammonia)</td><td align='left'>(marsh gas)</td></tr>
+</table></div>
+
+
+<p>It will be noticed that while one atom of chlorine combines with one
+atom of hydrogen, an atom of oxygen combines with two, an atom of
+nitrogen with three, one of carbon with four. The number which expresses
+this combining ratio between atoms is a definite property of each
+element and is called its <i>valence</i>.</p>
+
+<p>DEFINITION: <i>The valence of an element is that property which determines
+the number of the atoms of another element which its atom can hold in
+combination.</i></p>
+
+<p><b>Valence a numerical property.</b> Valence is therefore merely a numerical
+relation and does not convey any information in regard to the intensity
+of the affinity between atoms. Judging by the heat liberated in their
+union, oxygen has a far stronger affinity for hydrogen than does
+nitrogen, but an atom of oxygen can combine with two atoms only of
+hydrogen, while an atom of nitrogen can combine with three.<span class='pagenum'><a name="Page_117" id="Page_117">[Pg 117]</a></span></p>
+
+<p><b>Measure of valence.</b> In expressing the valence of an element we must
+select some standard for comparison, just as in the measurement of any
+other numerical quantity. It has been found that an atom of hydrogen is
+never able to hold in combination more than one atom of any other
+element. Hydrogen is therefore taken as the standard, and other elements
+are compared with it in determining their valence. A number of other
+elements are like hydrogen in being able to combine with at most one
+atom of other elements, and such elements are called <i>univalent</i>. Among
+these are chlorine, iodine, and sodium. Elements such as oxygen,
+calcium, and zinc, which can combine with two atoms of hydrogen or other
+univalent elements, are said to be <i>divalent</i>. Similarly, we have
+<i>trivalent, tetravalent, pentavalent</i> elements. None have a valence of
+more than 8.</p>
+
+<p><b>Indirect measure of valence.</b> Many elements, especially among the metals,
+do not readily form compounds with hydrogen, and their valence is not
+easy to determine by direct comparison with the standard element. These
+elements, however, combine with other univalent elements, such as
+chlorine, and their valence can be determined from the compounds so
+formed.</p>
+
+<p><b>Variable valence.</b> Many elements are able to exert different valences
+under differing circumstances. Thus we have the compounds Cu<sub>2</sub>O and
+CuO, CO and CO<sub>2</sub>, FeCl<sub>2</sub> and FeCl<sub>3</sub>. It is not always possible to
+assign a fixed valence to an element. Nevertheless each element tends to
+exert some normal valence, and the compounds in which it has a valence
+different from this are apt to be unstable and easily changed into
+compounds in which the valence of the element is normal. The valences of
+the various elements will become familiar as the elements are studied in
+detail.<span class='pagenum'><a name="Page_118" id="Page_118">[Pg 118]</a></span></p>
+
+<p><b>Valence and combining ratios.</b> When elements combine to form compounds,
+the ratio in which they combine will be determined by their valences. In
+those compounds which consist of two elements directly combined, the
+union is between such numbers of the two atoms as have equal valences.
+Elements of the same valence will therefore combine atom for atom.
+Designating the valence of the atoms by Roman numerals placed above
+their symbols, we have the formulas</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>II II</td><td align='left'>II III</td><td align='left'>I II</td><td align='left'>IV IV</td></tr>
+<tr><td align='left'>HCl,</td><td align='left'>ZnO,</td><td align='left'>BN,</td><td align='left'>CSi.</td></tr>
+</table></div>
+
+<p>A divalent element, on the other hand, will combine with two atoms of a
+univalent element. Thus we have</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>II II</td><td align='right'>II II</td></tr>
+<tr><td align='left'>ZnCl<sub>2</sub></td><td align='left'> and H<sub>2</sub>O</td></tr>
+</table></div>
+
+<p>(the numerals above each symbol representing the sum of the valences of
+the atoms of the element present). A trivalent atom will combine with
+three atoms of a univalent element, as in the compound</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">III III<br /></span>
+<span class="i1">H<sub>3</sub>N.<br /></span>
+</div></div>
+
+<p>If a trivalent element combines with a divalent element, the union will
+be between two atoms of the trivalent element and three of the divalent
+element, since these numbers are the smallest which have equal valences.
+Thus the oxide of the trivalent metal aluminium has the formula
+Al<sub>2</sub>O<sub>3</sub>. Finally one atom of a tetravalent element such as carbon
+will combine with four atoms of a univalent element, as in the compound
+CH<sub>4</sub>, or with two atoms of a divalent element, as in the compound
+CO<sub>2</sub>.</p>
+
+<p>We have no knowledge as to why elements differ in their combining power,
+and there is no way to determine their valences save by experiment.</p>
+
+<div class="blockquot"><p><b>Valence and the structure of compounds.</b> Compounds will be met
+from time to time which are apparent exceptions to the general
+statements just made in regard to valence. Thus, from the
+formula for hydrogen dioxide (H<sub>2</sub>O<sub>2</sub>), it might be
+supposed that the oxygen is univalent; yet it is certainly<span class='pagenum'><a name="Page_119" id="Page_119">[Pg 119]</a></span>
+divalent in water (H<sub>2</sub>O). That it may also be divalent in
+H<sub>2</sub>O<sub>2</sub> may be made clear as follows: The unit valence of
+each element may be represented graphically by a line attached
+to its symbol. Univalent hydrogen and divalent oxygen will then
+have the symbols H- and -O-. When atoms combine, each unit
+valence of one atom combines with a unit valence of another
+atom. Thus the composition of water may be expressed by the
+formula H-O-H, which is meant to show that each of the unit
+valences of oxygen is satisfied with the unit valence of a
+single hydrogen atom.</p>
+
+<p>The chemical conduct of hydrogen dioxide leads to the
+conclusion that the two oxygen atoms of its molecule are in
+direct combination with each other, and in addition each is in
+combination with a hydrogen atom. This may be expressed by the
+formula H-O-O-H. The oxygen in the compound is therefore
+divalent, just as it is in water. It will thus be seen that the
+structure of a compound must be known before the valences of
+the atoms making up the compound can be definitely decided
+upon.</p>
+
+<p>Such formulas as H-O-H and H-O-O-H are known as <i>structural
+formulas</i>, because they are intended to show what is known in
+regard to the arrangement of the atoms in the molecules.</p></div>
+
+<p><b>Valence and the replacing power of atoms.</b> Just as elements having the
+same valence combine with each other atom for atom, so if they replace
+each other in a chemical reaction they will do so in the same ratio.
+This is seen in the following equations, in which a univalent hydrogen
+atom is replaced by a univalent sodium atom:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaOH + HCl = NaCl + H<sub>2</sub>O.<br /></span>
+</div><div class="stanza">
+<span class="i0">2NaOH + H<sub>2</sub>SO<sub>4</sub> = Na<sub>2</sub>SO<sub>4</sub> + 2H<sub>2</sub>O.<br /></span>
+</div><div class="stanza">
+<span class="i0">Na + H<sub>2</sub>O = NaOH + H.<br /></span>
+</div></div>
+
+<p>Similarly, one atom of divalent calcium will replace two atoms of
+univalent hydrogen or one of divalent zinc:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ca(OH)<sub>2</sub> + 2 HCl = CaCl<sub>2</sub> + 2H<sub>2</sub>O.<br /></span>
+</div><div class="stanza">
+<span class="i0">CaCl<sub>2</sub> + ZnSO<sub>4</sub> = CaSO<sub>4</sub> + ZnCl<sub>2</sub>.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_120" id="Page_120">[Pg 120]</a></span></p>
+<p>In like manner, one atom of a trivalent element will replace three of a
+univalent element, or two atoms will replace three atoms of a divalent
+element.</p>
+
+<p><b>Valence and its applications to formulas of salts.</b> While the true nature
+of valence is not understood and many questions connected with the
+subject remain unanswered, yet many of the main facts are of much help
+to the student. Thus the formula of a salt, differs from that of the
+acid from which it is derived in that the hydrogen of the acid has been
+replaced by a metal. If, then, it is known that a given metal forms a
+normal salt with a certain acid, the formula of the salt can at once be
+determined if the valence of the metal is known. Since sodium is
+univalent, the sodium salts of the acids HCl and H<sub>2</sub>SO<sub>4</sub> will be
+respectively NaCl and Na<sub>2</sub>SO<sub>4</sub>. One atom of divalent zinc will
+replace 2 hydrogen atoms, so that the corresponding zinc salts will be
+ZnCl<sub>2</sub> and ZnSO<sub>4</sub>.</p>
+
+<p>The formula for aluminium sulphate is somewhat more difficult to
+determine. Aluminium is trivalent, and the simplest ratio in which the
+aluminium atom can replace the hydrogen in sulphuric acid is 2 atoms of
+aluminium (6 valences) to 3 molecules of sulphuric acid (6 hydrogen
+atoms). The formula of the sulphate will then be Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>.</p>
+
+<p><b>Valence and its application to equation writing.</b> It will be readily seen
+that a knowledge of valence is also of very great assistance in writing
+the equations for reactions of double decomposition. Thus, in the
+general reaction between an acid and a base, the essential action is
+between the univalent hydrogen ion and the univalent hydroxyl ion. The
+base and the acid must always be taken in such proportions as to secure
+an equal number of each of these ions. Thus, in the reaction between
+ferric hydroxide (Fe(OH)<sub>3</sub>) and sulphuric acid (H<sub>2</sub>SO<sub>4</sub>), it will
+be necessary to take 2 molecules of the former and 3 of the latter in
+order to have an equal number of the two ions, namely, 6. The equation
+will then be</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Fe(OH)<sub>3</sub> + 3H<sub>2</sub>SO<sub>4</sub> = Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 6H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Under certain conditions the salts Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> and CaCl<sub>2</sub>
+undergo double decomposition, the two metals, aluminium and calcium,
+exchanging places. The simplest ratio of exchange in this case is 2
+atoms of aluminium (6 valences) and 3 atoms of calcium (6 valences).<span class='pagenum'><a name="Page_121" id="Page_121">[Pg 121]</a></span>
+The reaction will therefore take place between 1 molecule of
+Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> and 3 of CaCl<sub>2</sub>, and the equation is as follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 3 CaCl<sub>2</sub> = 3CaSO<sub>4</sub> + 2AlCl<sub>3</sub>.<br /></span>
+</div></div>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Sodium, calcium, and aluminium have valences of 1, 2, and 3
+respectively; write the formulas of their chlorides, sulphates, and
+phosphates (phosphoric acid = H<sub>3</sub>PO<sub>4</sub>), on the supposition that they
+form salts having the normal composition.</p>
+
+<p><b>2.</b> Iron forms one series of salts in which it has a valence of 2, and
+another series in which it has a valence of 3; write the formulas for
+the two chlorides of iron, also for the two sulphates, on the
+supposition that these have the normal composition.</p>
+
+<p><b>3.</b> Write the equation representing the neutralization of each of the
+following bases by each of the acids whose formulas are given:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>NaOH</td><td align='left'>HCl</td></tr>
+<tr><td align='left'>Ba(OH)<sub>2</sub></td><td align='left'>H<sub>2</sub>SO<sub>4</sub></td></tr>
+<tr><td align='left'>Al(OH)<sub>3</sub></td><td align='left'>H<sub>3</sub>PO<sub>4</sub></td></tr>
+</table></div>
+
+
+<p><b>4.</b> Silver acts as a univalent element and calcium as a divalent element
+in the formation of their respective nitrates and chlorides. (<i>a</i>) Write
+the formula for silver nitrate; for calcium chloride. (<i>b</i>) When
+solutions of these two salts are mixed, the two metals, silver and
+calcium, exchange places; write the equation for the reaction.</p>
+
+<p><i>5.</i> Antimony acts as a trivalent element in the formation of a
+chloride. (<i>a</i>) What is the formula for antimony chloride? (<i>b</i>) When
+hydrosulphuric acid (H<sub>2</sub>S) is passed into a solution of this chloride
+the hydrogen and antimony exchange places; write the equation for the
+reaction.</p>
+
+<p><b>6.</b> Lead has a valence of 2 and iron of 3 in the compounds known
+respectively as lead nitrate and ferric sulphate. (<i>a</i>) Write the
+formulas for these two compounds. (<i>b</i>) When their solutions are mixed
+the two metals exchange places; write the equation for the reaction.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_122" id="Page_122">[Pg 122]</a></span></p>
+<h2><a name="CHAPTER_XII" id="CHAPTER_XII"></a>CHAPTER XII</h2>
+
+<h3>COMPOUNDS OF NITROGEN</h3>
+
+
+<p><b>Occurrence.</b> As has been stated in a former chapter, nitrogen constitutes
+a large fraction of the atmosphere. The compounds of nitrogen, however,
+cannot readily be obtained from this source, since at any ordinary
+temperature nitrogen is able to combine directly with very few of the
+elements.</p>
+
+<p>In certain forms of combination nitrogen occurs in the soil from which
+it is taken up by plants and built into complex substances composed
+chiefly of carbon, hydrogen, oxygen, and nitrogen. Animals feeding on
+these plants assimilate the nitrogenous matter, so that this element is
+an essential constituent of both plants and animals.</p>
+
+<p><b>Decomposition of organic matter by bacteria.</b> When living matter dies and
+undergoes decay complicated chemical reactions take place, one result of
+which is that the nitrogen of the organic matter is set free either as
+the element nitrogen, or in the form of simple compounds, such as
+ammonia (NH<sub>3</sub>) or oxides of nitrogen. Experiment has shown that all
+such processes of decay are due to the action of different kinds of
+bacteria, each particular kind effecting a different change.</p>
+
+<p><b>Decomposition of organic matter by heat.</b> When organic matter is strongly
+heated decomposition into simpler substances takes place in much the
+same way as in the case of bacterial decomposition. Coal is a complex
+substance of<span class='pagenum'><a name="Page_123" id="Page_123">[Pg 123]</a></span> vegetable origin, consisting largely of carbon, but also
+containing hydrogen, oxygen, and nitrogen. When this is heated in a
+closed vessel so that air is excluded, about one seventh of the nitrogen
+is converted into ammonia, and this is the chief source from which
+ammonia and its compounds are obtained.</p>
+
+
+<h3>COMPOUNDS OF NITROGEN WITH HYDROGEN</h3>
+
+<p><b>Ammonia</b> (NH<sub>3</sub>). Several compounds consisting exclusively of nitrogen
+and hydrogen are known, but only one, ammonia, need be considered here.</p>
+
+<p><b>Preparation of ammonia.</b> Ammonia is prepared in the laboratory by a
+different method from the one which is used commercially.</p>
+
+<p>1. <i>Laboratory method.</i> In the laboratory ammonia is prepared from
+ammonium chloride, a compound having the formula NH<sub>4</sub>Cl, and obtained
+in the manufacture of coal gas. As will be shown later in the chapter,
+the group NH<sub>4</sub> in this compound acts as a univalent radical and is
+known as <i>ammonium</i>. When ammonium chloride is warmed with sodium
+hydroxide, the ammonium and sodium change places, the reaction being
+expressed in the following equation.</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>4</sub>Cl + NaOH = NaCl + NH<sub>4</sub>OH.<br /></span>
+</div></div>
+
+<p>The ammonium hydroxide (NH<sub>4</sub>OH) so formed is unstable and breaks down
+into water and ammonia.</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>4</sub>OH = NH<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Calcium hydroxide (Ca(OH)<sub>2</sub>) is frequently used in place of the more
+expensive sodium hydroxide, the equations being</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NH<sub>4</sub>Cl + Ca(OH)<sub>2</sub> = CaCl<sub>2</sub> + 2NH<sub>4</sub>OH,<br /></span>
+</div><div class="stanza">
+<span class="i0">2NH<sub>4</sub>OH = 2H<sub>2</sub>O + 2NH<sub>3</sub>.<br /></span>
+<span class='pagenum'><a name="Page_124" id="Page_124">[Pg 124]</a></span></div></div>
+
+<div class="blockquot"><p>In the preparation, the ammonium chloride and calcium hydroxide
+are mixed together and placed in a flask arranged as shown in
+Fig. 35. The mixture is gently warmed, when ammonia is evolved
+as a gas and is collected by displacement of air.</p></div>
+
+<div class="figcenter" style="width: 365px;">
+<img src="images/imagefig35.jpg" width="365" height="400" alt="Fig. 35" title="" />
+<span class="caption">Fig. 35</span>
+</div>
+
+<p>2. <i>Commercial method.</i> Nearly all the ammonia of commerce comes from
+the gasworks. Ordinary illuminating gas is made by distilling coal, as
+will be explained later, and among the products of this distillation a
+solution of ammonia in water is obtained. This solution, known as <i>gas
+liquor</i>, contains not only ammonia but other soluble substances. Most of
+these combine chemically with lime, while ammonia does not; if then lime
+is added to the gas liquor and the liquor is heated, the ammonia is
+driven out from the mixture. It may be dissolved again in pure, cold
+water, forming <i>aqua ammonia</i>, or the ammonia water of commerce.</p>
+
+<div class="blockquot"><p><b>Preparation from hydrogen and nitrogen.</b> When electric sparks
+are passed for some time through a mixture of hydrogen and
+nitrogen, a small percentage of the two elements in the mixture
+is changed into ammonia. The action soon ceases, however, for
+the reason that ammonia is decomposed by the electric
+discharge. The reaction expressed in the equation</p>
+
+<p>N + 3H = NH<sub>3</sub></p>
+
+<p>can therefore go in either direction depending upon the
+relative quantities of the substances present. This recalls the
+similar change from oxygen into ozone, which soon ceases
+because the ozone is in turn decomposed into oxygen.</p></div>
+
+<p><span class='pagenum'><a name="Page_125" id="Page_125">[Pg 125]</a></span></p>
+
+<p><b>Physical properties.</b> Under ordinary conditions ammonia is a gas whose
+density is 0.59. It is therefore little more than half as heavy as air.
+It is easily condensed into a colorless liquid, and can now be purchased
+in liquid form in steel cylinders. The gas is colorless and has a
+strong, suffocating odor. It is extremely soluble in water, 1 l. of
+water at 0&deg; and 760 mm. pressure dissolving 1148 l. of the gas. In
+dissolving this large volume of gas the water expands considerably, so
+that the density of the solution is less than that of water, the
+strongest solutions having a density of 0.88.</p>
+
+<p><b>Chemical properties.</b> Ammonia will not support combustion, nor will it
+burn under ordinary conditions. In an atmosphere of oxygen it burns with
+a feeble, yellowish flame. When quite dry it is not a very active
+substance, but when moist it combines with a great many substances,
+particularly with acids.</p>
+
+<p><b>Uses.</b> It has been stated that ammonia can be condensed to a liquid by
+the application of pressure. If the pressure is removed from the liquid
+so obtained, it rapidly passes again into the gaseous state and in so
+doing absorbs a large amount of heat. Advantage is taken of this fact in
+the preparation of artificial ice. Large quantities of ammonia are also
+used in the preparation of ammonium compounds.</p>
+
+<div class="blockquot"><p><b>The manufacture of artificial ice.</b> Fig. 36 illustrates the
+method of preparing artificial ice. The ammonia gas is
+liquefied in the pipes <i>X</i> by means of the pump <i>Y</i>. The heat
+generated is absorbed by water flowing over the pipes. The
+pipes lead into a large brine tank, a cross section of which is
+shown in the figure. Into the brine (concentrated solution of
+common salt) contained in this tank are dipped the vessels <i>A</i>,
+<i>B</i>, <i>C</i>, filled with pure water. The pressure is removed from
+the liquid ammonia as it passes into the pipes immersed in the<span class='pagenum'><a name="Page_126" id="Page_126">[Pg 126]</a></span>
+brine, and the heat absorbed by the rapid evaporation of the
+liquid lowers the temperature of the brine below zero. The
+water in <i>A</i>, <i>B</i>, <i>C</i> is thereby frozen into cakes of ice. The
+gaseous ammonia resulting from the evaporation of the liquid
+ammonia is again condensed, so that the process is continuous.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig36.jpg" width="400" height="262" alt="Fig. 36" title="" />
+<span class="caption">Fig. 36</span>
+</div></div>
+
+<p><b>Ammonium hydroxide</b> (NH<sub>4</sub>OH). The solution of ammonia in water is found
+to have strong basic properties and therefore contains hydroxyl ions. It
+turns red litmus blue; it has a soapy feel; it neutralizes acids,
+forming salts with them. It seems probable, therefore, that when ammonia
+dissolves in water it combines chemically with it according to the
+equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>3</sub> + H<sub>2</sub>O = NH<sub>4</sub>OH,<br /></span>
+</div></div>
+
+<p>and that it is the substance NH<sub>4</sub>OH, called ammonium hydroxide, which
+has the basic properties, dissociating into the ions NH<sub>4</sub> and OH.
+Ammonium hydroxide has never been obtained in a pure state. At every
+attempt to isolate it the substance breaks up into water and ammonia,&mdash;</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>4</sub>OH = NH<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><b>The ammonium radical.</b> The radical NH<sub>4</sub> plays the part of a metal in
+many chemical reactions and is called ammonium. The ending <i>-ium</i> is
+given to the name to indicate the metallic properties of the substance,
+since the names<span class='pagenum'><a name="Page_127" id="Page_127">[Pg 127]</a></span> of the metals in general have that ending. The salts
+formed by the action of the base ammonium hydroxide on acids are called
+ammonium salts. Thus, with hydrochloric acid, ammonium chloride is
+formed in accordance with the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>4</sub>OH + HCl = NH<sub>4</sub>Cl + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Similarly, with nitric acid, ammonium nitrate (NH<sub>4</sub>NO<sub>3</sub>) is formed,
+and with sulphuric acid, ammonium sulphate ((NH<sub>4</sub>)<sub>2</sub>S0<sub>4</sub>).</p>
+
+<p>It will be noticed that in the neutralization of ammonium hydroxide by
+acids the group NH<sub>4</sub> replaces one hydrogen atom of the acid, just as
+sodium does. The group therefore acts as a univalent metal.</p>
+
+<p><b>Combination of nitrogen with hydrogen by volume.</b> Under suitable
+conditions ammonia can be decomposed into nitrogen and hydrogen by
+passing electric sparks through the gas. Accurate measurement has shown
+that when ammonia is decomposed, two volumes of the gas yield one volume
+of nitrogen and three volumes of hydrogen. Consequently, if the two
+elements were to combine directly, one volume of nitrogen would combine
+with three volumes of hydrogen to form two volumes of ammonia. Here, as
+in the formation of steam from hydrogen and oxygen, small whole numbers
+serve to indicate the relation between the volumes of combining gases
+and that of the gaseous product.</p>
+
+
+<h3>COMPOUNDS OF NITROGEN WITH OXYGEN AND HYDROGEN</h3>
+
+<p>In addition to ammonium hydroxide, nitrogen forms several compounds with
+hydrogen and oxygen, of which nitric acid (HNO<sub>3</sub>) and nitrous acid
+<span class='pagenum'><a name="Page_128" id="Page_128">[Pg 128]</a></span>(HNO<sub>2</sub>) are the most familiar.</p>
+
+<p><b>Nitric acid</b> (HNO<sub>3</sub>). Nitric acid is not found to any extent in nature,
+but some of its salts, especially sodium nitrate (NaNO<sub>3</sub>) and
+potassium nitrate (KNO<sub>3</sub>) are found in large quantities. From these
+salts nitric acid can be obtained.</p>
+
+<div class="figcenter" style="width: 368px;">
+<img src="images/imagefig37.jpg" width="368" height="400" alt="Fig. 37" title="" />
+<span class="caption">Fig. 37</span>
+</div>
+
+<p><b>Preparation of nitric acid.</b> When sodium nitrate is treated with
+concentrated cold sulphuric acid, no chemical action seems to take
+place. If, however, the mixture is heated in a retort, nitric acid is
+given off as a vapor and may be easily condensed to a liquid by passing
+the vapor into a tube surrounded by cold water, as shown in Fig. 37. An
+examination of the liquid left in the retort shows that it contains
+sodium acid sulphate (NaHSO<sub>4</sub>), so that the reaction may be
+represented by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaNO<sub>3</sub> + H<sub>2</sub>SO<sub>4</sub> = NaHSO<sub>4</sub> + HNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<div class="blockquot"><p>If a smaller quantity of sulphuric acid is taken and the
+mixture is heated to a high temperature, normal sodium sulphate
+is formed:</p>
+
+<p>2NaNO<sub>3</sub> + H<sub>2</sub>SO<sub>4</sub> = Na<sub>2</sub>SO<sub>4</sub> + 2HNO<sub>3</sub>.</p>
+
+<p>In this case, however, the higher temperature required
+decomposes a part of the nitric acid.</p>
+
+<p><b>The commercial preparation of nitric acid.</b> Fig. 38 illustrates
+a form of apparatus used in the preparation of nitric acid on a
+large scale. Sodium nitrate and sulphuric acid are heated in
+the iron retort <i>A</i>. The resulting acid vapors pass in the
+direction indicated by the arrows, and are condensed in the
+glass tubes <i>B</i>, which are covered with cloth kept cool by
+streams of water. These tubes are inclined so that the liquid
+resulting from the condensation of the vapors runs back into
+<i>C</i> and is drawn off into large vessels (<i>D</i>).</p></div><p><span class='pagenum'><a name="Page_129" id="Page_129">[Pg 129]</a></span></p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig38.jpg" width="400" height="323" alt="Fig. 38" title="" />
+<span class="caption">Fig. 38</span>
+</div>
+
+<p><b>Physical properties of nitric acid.</b> Pure nitric acid is a colorless
+liquid, which boils at about 86&deg; and has a density of 1.56. The
+concentrated acid of commerce contains about 68% of the acid, the
+remainder being water. Such a mixture has a density of 1.4. The
+concentrated acid fumes somewhat in moist air, and has a sharp choking
+odor.</p>
+
+<p><b>Chemical properties.</b> The most important chemical properties of nitric
+acid are the following.</p>
+
+<p>1. <i>Acid properties.</i> As the name indicates, this substance is an acid,
+and has all the properties of that class of substances. It changes blue
+litmus red and has a sour taste in dilute solutions. It forms hydrogen
+ions in solution and neutralizes bases forming salts. It also acts upon
+the oxides of most metals, forming a salt and water. It is one of the
+strongest acids.</p>
+
+<p>2. <i>Decomposition on heating.</i> When boiled, or exposed for some time to
+sunlight, it suffers a partial decomposition according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HNO<sub>3</sub> = H<sub>2</sub>O + 2NO<sub>2</sub> + O.<br /></span>
+</div></div>
+
+<p>The substance NO<sub>2</sub>, called nitrogen peroxide, is a brownish gas, which
+is readily soluble in water and in nitric acid. It therefore dissolves
+in the undecomposed acid, and imparts a yellowish or reddish color to
+it. Concentrated<span class='pagenum'><a name="Page_130" id="Page_130">[Pg 130]</a></span> nitric acid highly charged with this substance is
+called <i>fuming nitric acid</i>.</p>
+
+<p>3. <i>Oxidizing action.</i> According to its formula, nitric acid contains a
+large percentage of oxygen, and the reaction just mentioned shows that
+the compound is not a very stable one, easily undergoing decomposition.
+These properties should make it a good oxidizing agent, and we find that
+this is the case. Under ordinary circumstances, when acting as an
+oxidizing agent, it is decomposed according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HNO<sub>3</sub> = H<sub>2</sub>O + 2NO + 3O.<br /></span>
+</div></div>
+
+<p>The oxygen is taken up by the substance oxidized, and not set free, as
+is indicated in the equation. Thus, if carbon is oxidized by nitric
+acid, the oxygen combines with carbon, forming carbon dioxide (CO<sub>2</sub>):</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C + 2O = CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>4. <i>Action on metals.</i> We have seen that when an acid acts upon a metal
+hydrogen is set free. Accordingly, when nitric acid acts upon a metal,
+such as copper, we should expect the reaction to take place which is
+expressed in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Cu + 2HNO<sub>3</sub> = Cu(NO<sub>3</sub>)<sub>2</sub> + 2H.<br /></span>
+</div></div>
+
+<p>This reaction does take place, but the hydrogen set free is immediately
+oxidized to water by another portion of the nitric acid according to the
+equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">HNO<sub>3</sub> + 3H = 2H<sub>2</sub>O + NO.<br /></span>
+</div></div>
+
+<p>As these two equations are written, two atoms of hydrogen are given off
+in the first equation, while three are used up in the second. In order
+that the hydrogen may be equal in<span class='pagenum'><a name="Page_131" id="Page_131">[Pg 131]</a></span> the two equations, we must multiply
+the first by 3 and the second by 2. We shall then have</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Cu + 6HNO<sub>3</sub> = 3Cu(NO<sub>3</sub>)<sub>2</sub> + 6H,<br /></span>
+</div><div class="stanza">
+<span class="i0">2HNO<sub>3</sub> + 6H = 4H<sub>2</sub>O + 2NO.<br /></span>
+</div></div>
+
+<p>The two equations may now be combined into one by adding the quantities
+on each side of the equality sign, canceling the hydrogen which is given
+off in the one reaction and used up in the other. We shall then have the
+equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Cu + 8HNO<sub>3</sub> = 3Cu(NO<sub>3</sub>)<sub>2</sub> + 2NO + 4H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>A number of other reactions may take place when nitric acid acts upon
+metals, resulting in the formation of other oxides of nitrogen, free
+nitrogen, or even ammonia. The reaction just given is, however, the
+usual one.</p>
+
+<div class="blockquot"><p><b>Importance of steps in a reaction.</b> This complete equation has
+the advantage of making it possible to calculate very easily
+the proportions in which the various substances enter into the
+reaction or are formed in it. It is unsatisfactory in that it
+does not give full information about the way in which the
+reaction takes place. For example, it does not suggest that
+hydrogen is at first formed, and subsequently transformed into
+water. It is always much more important to remember the steps
+in a chemical reaction than to remember the equation expressing
+the complete action; for if these steps in the reaction are
+understood, the complete equation is easily obtained in the
+manner just described.</p></div>
+
+<p><b>Salts of nitric acid,&mdash;nitrates.</b> The salts of nitric acid are called
+nitrates. Many of these salts will be described in the study of the
+metals. They are all soluble in water, and when heated to a high
+temperature undergo decomposition. In a few cases a nitrate on being
+heated evolves oxygen, forming a nitrite:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaNO<sub>3</sub> = NaNO<sub>2</sub> + O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_132" id="Page_132">[Pg 132]</a></span></p>
+<p>In other cases the decomposition goes further, and the metal is left as
+oxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Cu(NO<sub>3</sub>)<sub>2</sub> = CuO + 2NO<sub>2</sub> + O.<br /></span>
+</div></div>
+
+<p><b>Nitrous acid</b> (HNO<sub>2</sub>). It is an easy matter to obtain sodium nitrite
+(NaNO<sub>2</sub>), as the reaction given on the previous page indicates.
+Instead of merely heating the nitrate, it is better to heat it together
+with a mild reducing agent, such as lead, when the reaction takes place
+which is expressed by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaNO<sub>3</sub> + Pb = PbO + NaNO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>When sodium nitrite is treated with an acid, such as sulphuric acid, it
+is decomposed and nitrous acid is set free:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaNO<sub>2</sub> + H<sub>2</sub>SO<sub>4</sub> = NaHSO<sub>4</sub> + HNO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>The acid is very unstable, however, and decomposes readily into water
+and nitrogen trioxide (N<sub>2</sub>O<sub>3</sub>):</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HNO<sub>2</sub> = H<sub>2</sub>O + N<sub>2</sub>O<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Dilute solutions of the acid, however, can be obtained.</p>
+
+
+<h3>COMPOUNDS OF NITROGEN WITH OXYGEN</h3>
+
+<p>Nitrogen combines with oxygen to form five different oxides. The
+formulas and names of these are as follows:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>N<sub>2</sub>O</td><td align='left'>nitrous oxide.</td></tr>
+<tr><td align='left'>NO</td><td align='left'>nitric oxide.</td></tr>
+<tr><td align='left'>NO<sub>2</sub></td><td align='left'>nitrogen peroxide.</td></tr>
+<tr><td align='left'>N<sub>2</sub>O<sub>3</sub></td><td align='left'>nitrogen trioxide, or nitrous anhydride.</td></tr>
+<tr><td align='left'>N<sub>2</sub>O<sub>5</sub></td><td align='left'>nitrogen pentoxide, or nitric anhydride.</td></tr>
+</table></div>
+
+<p>These will now be briefly discussed.</p>
+
+<p><b>Nitrous oxide</b> (<i>laughing gas</i>) (N<sub>2</sub>O). Ammonium nitrate, like all
+nitrates, undergoes decomposition when heated; and owing to the fact
+that it contains no metal, but does<span class='pagenum'><a name="Page_133" id="Page_133">[Pg 133]</a></span> contain both oxygen and hydrogen,
+the reaction is a peculiar one. It is represented by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>4</sub>NO<sub>3</sub> = 2H<sub>2</sub>O + N<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The oxide of nitrogen so formed is called nitrous oxide or laughing gas.
+It is a colorless gas having a slight odor. It is somewhat soluble in
+water, and in solution has a slightly sweetish taste. It is easily
+converted into a liquid and can be purchased in this form. When inhaled
+it produces a kind of hysteria (hence the name "laughing gas"), and even
+unconsciousness and insensibility to pain if taken in large amounts. It
+has long been used as an an&aelig;sthetic for minor surgical operations, such
+as those of dentistry, but owing to its unpleasant after effects it is
+not so much in use now as formerly.</p>
+
+<p>Chemically, nitrous oxide is remarkable for the fact that it is a very
+energetic oxidizing agent. Substances such as carbon, sulphur, iron, and
+phosphorus burn in it almost as brilliantly as in oxygen, forming oxides
+and setting free nitrogen. Evidently the oxygen in nitrous oxide cannot
+be held in very firm combination by the nitrogen.</p>
+
+<div class="figright" style="width: 208px;">
+<img src="images/imagefig39.jpg" width="208" height="400" alt="Fig. 39" title="" />
+<span class="caption">Fig. 39</span>
+</div>
+
+<p><b>Nitric oxide</b> (NO). We have seen that when nitric acid acts upon metals,
+such as copper, the reaction represented by the following equation takes
+place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Cu + 8HNO<sub>3</sub> = 3Cu(NO<sub>3</sub>)<sub>3</sub> + 2NO + 4H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Nitric oxide is most conveniently prepared in this way. The metal is
+placed in the flask <i>A</i> (Fig. 39) and the acid added slowly through the
+funnel tube <i>B</i>. The gas escapes through <i>C</i> and is collected over
+water.<span class='pagenum'><a name="Page_134" id="Page_134">[Pg 134]</a></span></p>
+
+<p>Pure nitric oxide is a colorless gas, slightly heavier than air, and is
+practically insoluble in water. It is a difficult gas to liquefy. Unlike
+nitrous oxide, nitric oxide does not part with its oxygen easily, and
+burning substances introduced into this gas are usually extinguished. A
+few substances like phosphorus, which have a very strong affinity for
+oxygen and which are burning energetically in the air, will continue to
+burn in an atmosphere of nitric oxide. In this case the nitric oxide
+loses all of its oxygen and the nitrogen is set free as gas.</p>
+
+<p><b>Action of nitric oxide with oxygen.</b> When nitric oxide comes into contact
+with oxygen or with the air, it at once combines with the oxygen even at
+ordinary temperatures, forming a reddish-yellow gas of the formula
+NO<sub>2</sub>, which is called nitrogen peroxide. This action is not energetic
+enough to produce a flame, though considerable heat is set free.</p>
+
+<p><b>Nitrogen peroxide</b> (NO<sub>2</sub>). This gas, as we have just seen, is formed by
+allowing nitric oxide to come into contact with oxygen. It can also be
+made by heating certain nitrates, such as lead nitrate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Pb(NO<sub>3</sub>)<sub>2</sub> = PbO + 2NO<sub>2</sub> + O.<br /></span>
+</div></div>
+
+<p>It is a reddish-yellow gas of unpleasant odor, which is quite poisonous
+when inhaled. It is heavier than air and is easily condensed to a
+liquid. It dissolves in water, but this solution is not a mere physical
+solution; the nitrogen peroxide is decomposed, forming a mixture of
+nitric and nitrous acids:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NO<sub>2</sub> + H<sub>2</sub>O = HNO<sub>2</sub> + HNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Nitrogen peroxide will not combine with more oxygen; it will, however,
+give up a part of its oxygen to burning substances, acting as an
+oxidizing agent:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NO<sub>2</sub> = NO + O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_135" id="Page_135">[Pg 135]</a></span></p>
+<p><b>Acid anhydrides.</b> The oxides N<sub>2</sub>O<sub>3</sub> (nitrogen trioxide) and
+N<sub>2</sub>O<sub>5</sub> (nitrogen pentoxide) are rarely prepared and need not be
+separately described. They bear a very interesting relation to the acids
+of nitrogen. When dissolved in water they combine with the water,
+forming acids:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">N<sub>2</sub>O<sub>3</sub> + H<sub>2</sub>O = 2HNO<sub>2</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">N<sub>2</sub>O<sub>5</sub> + H<sub>2</sub>O = 2HNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>On the other hand, nitrous acid very easily decomposes, yielding water
+and nitrogen trioxide, and by suitable means nitric acid likewise may be
+decomposed into water and nitrogen pentoxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HNO<sub>2</sub> = H<sub>2</sub>O + N<sub>2</sub>O<sub>3</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">2HNO<sub>3</sub> = H<sub>2</sub>O + N<sub>2</sub>O<sub>5</sub>.<br /></span>
+</div></div>
+
+<p>In view of the close relation between these oxides and the corresponding
+acids, they are called <i>anhydrides</i> of the acids, N<sub>2</sub>O<sub>3</sub> being
+nitrous anhydride and N<sub>2</sub>O<sub>5</sub> nitric anhydride.</p>
+
+<p>DEFINITION: <i>Any oxide which will combine with water to form an acid, or
+which together with water is formed by the decomposition of an acid, is
+called an anhydride of that acid.</i></p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Perfectly dry ammonia does not affect litmus paper. Explain.</p>
+
+<p><b>2.</b> Can ammonia be dried by passing the gas through concentrated
+sulphuric acid? Explain.</p>
+
+<p><b>3.</b> Ammonium hydroxide is a weak base, i.e. it is not highly dissociated.
+When it is neutralized by strong acids the heat of reaction is less than
+when strong bases are so neutralized. Suggest some possible cause for
+this.</p>
+
+<p><b>4.</b> Why is brine used in the manufacture of artificial ice?</p>
+
+<p><b>5.</b> Discuss the energy changes which take place in the manufacture of
+artificial ice.<span class='pagenum'><a name="Page_136" id="Page_136">[Pg 136]</a></span></p>
+
+<p><b>6.</b> What weight of ammonium chloride is necessary to furnish enough
+ammonia to saturate 1 l. of water at 0&deg; and 760 mm.?</p>
+
+<p><b>7.</b> What weight of sodium nitrate is necessary to prepare 100 cc. of
+commercial nitric acid? What weight of potassium nitrate is necessary to
+furnish the same weight of acid?</p>
+
+<p><b>8.</b> 100 l. of nitrogen peroxide were dissolved in water and neutralized
+with sodium hydroxide. What substances were formed and how much of
+each?(1 l. nitrogen peroxide weighs 2.05 grams.)</p>
+
+<p><b>9.</b> How many liters of nitrous oxide, measured under standard conditions,
+can be prepared from 10 g. of ammonium nitrate?</p>
+
+<p><b>10.</b> What weight of copper is necessary to prepare 50 l. of nitric oxide
+under standard conditions?</p>
+
+<p><b>11.</b> (<i>a</i>) Calculate the percentage composition of the oxides of
+nitrogen. (<i>b</i>) What important law does this series of substances
+illustrate?</p>
+
+<p><b>12.</b> Write the equations representing the reactions between ammonium
+hydroxide, and sulphuric acid and nitric acid respectively, in
+accordance with the theory of electrolytic dissociation.</p>
+
+<p><b>13.</b> In the same way, write the equations representing the reactions
+between nitric acid and each of the following bases: NaOH, KOH,
+NH<sub>4</sub>OH, Ca(OH)<sub>2</sub>.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_137" id="Page_137">[Pg 137]</a></span></p>
+<h2><a name="CHAPTER_XIII" id="CHAPTER_XIII"></a>CHAPTER XIII</h2>
+
+<h3>REVERSIBLE REACTIONS AND CHEMICAL EQUILIBRIUM</h3>
+
+
+<p><b>Reversible reactions.</b> The reactions so far considered have been
+represented as continuing, when once started, until one or the other
+substance taking part in the reaction has been used up. In some
+reactions this is not the case. For example, we have seen that when
+steam is passed over hot iron the reaction is represented by the
+equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Fe + 4H<sub>2</sub>O = Fe<sub>3</sub>O<sub>4</sub> + 8H.<br /></span>
+</div></div>
+
+<p>On the other hand, when hydrogen is passed over hot iron oxide the
+reverse reaction takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe<sub>3</sub>O<sub>4</sub> +8H = 3Fe + 4H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The reaction can therefore go in either direction, depending upon the
+conditions of the experiment. Such a reaction is called a <i>reversible
+reaction</i>. It is represented by an equation with double arrows in place
+of the equality sign, thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Fe + 4H<sub>2</sub>O &lt;--> Fe<sub>3</sub>O<sub>4</sub> + 8H.<br /></span>
+</div></div>
+
+<p>In a similar way, the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">N + 3H &lt;--> NH<sub>3</sub><br /></span>
+</div></div>
+
+<p>expresses the fact that under some conditions nitrogen may unite with
+hydrogen to form ammonia, while under other conditions ammonia
+decomposes into nitrogen and hydrogen.</p>
+
+<p>The conversion of oxygen into ozone is also reversible and may be
+represented thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">oxygen &lt;--> ozone.<br /></span></div></div>
+<p><span class='pagenum'><a name="Page_138" id="Page_138">[Pg 138]</a></span></p>
+
+<p><b>Chemical equilibrium.</b> Reversible reactions do not usually go on to
+completion in one direction unless the conditions under which the
+reaction takes place are very carefully chosen. Thus, if iron and steam
+are confined in a heated tube, the steam acts upon the iron, producing
+iron oxide and hydrogen. But these substances in turn act upon each
+other to form iron and steam once more. When these two opposite
+reactions go on at such rates that the weight of the iron changed into
+iron oxide is just balanced by the weight of the iron oxide changed into
+iron, there will be no further change in the relative weights of the
+four substances present in the tube. The reaction is then said to have
+reached an equilibrium.</p>
+
+<p><b>Factors which determine the point of equilibrium.</b> There are two factors
+which have a great deal of influence in determining the point at which a
+given reaction will reach equilibrium.</p>
+
+<p>1. <i>Influence of the chemical nature of the substances.</i> If two
+reversible reactions of the same general kind are selected, it has been
+found that the point of equilibrium is different in the two cases. For
+example, in the reactions represented by the equations</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Fe + 4H<sub>2</sub>O &lt;--> Fe<sub>3</sub>O<sub>4</sub> + 8H,<br /></span>
+</div><div class="stanza">
+<span class="i0">Zn + H<sub>2</sub>O &lt;--> ZnO + 2H,<br /></span>
+</div></div>
+
+<p>the equilibrium will be reached when very different quantities of the
+iron and zinc have been changed into oxides. The individual chemical
+properties of the iron and zinc have therefore marked influence upon the
+point at which equilibrium will be reached.</p>
+
+<p>2. <i>Influence of relative mass.</i> If the tube in which the reaction</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Fe + 4H<sub>2</sub>O &lt;--> Fe<sub>3</sub>O<sub>4</sub> + 8H<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_139" id="Page_139">[Pg 139]</a></span></p>
+<p>has come to an equilibrium is opened and more steam is admitted, an
+additional quantity of the iron will be changed into iron oxide. If more
+hydrogen is admitted, some of the oxide will be reduced to metal. The
+point of equilibrium is therefore dependent upon the relative masses of
+the substances taking part in the reaction. When one of the substances
+is a solid, however, its mass has little influence, since it is only the
+extent of its surface which can affect the reaction.</p>
+
+<p><b>Conditions under which reversible reactions are complete.</b> If, when the
+equilibrium between iron and steam has been reached, the tube is opened
+and a current of steam is passed in, the hydrogen is swept away as fast
+as it is formed. The opposing reaction of hydrogen upon iron oxide must
+therefore cease, and the action of steam on the iron will go on until
+all of the iron has been transformed into iron oxide.</p>
+
+<p>On the other hand, if a current of hydrogen is admitted into the tube,
+the steam will be swept away by the hydrogen, and all of the iron oxide
+will be reduced to iron. <i>A reversible reaction can therefore be
+completed in either direction when one of the products of the reaction
+is removed as fast as it is formed.</i></p>
+
+<p><b>Equilibrium in solution.</b> When reactions take place in solution in water
+the same general principles hold good. The matter is not so simple,
+however, as in the case just described, owing to the fact that many of
+the reactions in solution are due to the presence of ions. The
+substances most commonly employed in solution are acids, bases, or
+salts, and all of these undergo dissociation. Any equilibrium which may
+be reached in solutions of these substances must take place between the
+various ions formed, on the<span class='pagenum'><a name="Page_140" id="Page_140">[Pg 140]</a></span> one hand, and the undissociated molecules,
+on the other. Thus, when nitric acid is dissolved in water, equilibrium
+is reached in accordance with the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sup>+</sup> + NO<sub>3</sub><sup>-</sup> &lt;--> HNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><b>Conditions under which reversible reactions in solution are complete.</b>
+The equilibrium between substances in solution may be disturbed and the
+reaction caused to go on in one direction to completion in either of
+three ways.</p>
+
+<p>1. <i>A gas may be formed which escapes from the solution.</i> When sodium
+nitrate and sulphuric acid are brought together in solution all four
+ions, Na<sup>+</sup>, NO<sub>3</sub><sup>-</sup>, H<sup>+</sup>, SO<sub>4</sub><sup>-</sup>, are formed. These ions are
+free to rearrange themselves in various combinations. For example, the
+H<sup>+</sup> and the NO<sub>3</sub><sup>-</sup> ions will reach the equilibrium</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sup>+</sup> + NO<sub>3</sub><sup>-</sup> &lt;--> HNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>If the experiment is performed with very little water present, as is the
+case in the preparation of nitric acid, the equilibrium will be reached
+when most of the H<sup>+</sup> and the NO<sub>3</sub><sup>-</sup> ions have combined to form
+undissociated HNO<sub>3</sub>.</p>
+
+<p>Finally, if the mixture is now heated above the boiling point of nitric
+acid, the acid distills away as fast as it is formed. More and more
+H<sup>+</sup> and NO<sub>3</sub><sup>-</sup> ions will then combine, and the process will
+continue until one or the other of them has all been removed from the
+solution. The substance remaining is sodium acid sulphate (NaHSO<sub>4</sub>),
+and the reaction can therefore be expressed by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaNO<sub>3</sub> + H<sub>2</sub>SO<sub>4</sub> = NaHSO<sub>4</sub> + HNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>2. <i>An insoluble solid may be formed.</i> When hydrochloric acid (HCl) and
+<span class='pagenum'><a name="Page_141" id="Page_141">[Pg 141]</a></span>silver nitrate (AgNO<sub>3</sub>) are brought together in solution the
+following ions will be present: H<sup>+</sup>, Cl<sup>-</sup>, Ag<sup>+</sup>, NO<sub>3</sub><sup>-</sup>. The
+ions Ag<sup>+</sup> and Cl<sup>-</sup> will then set up the equilibrium</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ag<sup>+</sup> + Cl<sup>-</sup> &lt;--> AgCl.<br /></span>
+</div></div>
+
+<p>But silver chloride (AgCl) is almost completely insoluble in water, and
+as soon as a very little of it has formed the solution becomes
+supersaturated, and the excess of the salt precipitates. More silver and
+chlorine ions then unite, and this continues until practically all of
+the silver or the chlorine ions have been removed from the solution. We
+then say that the following reaction is complete:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">AgNO<sub>3</sub> + HCl = AgCl + HNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>3. <i>Two different ions may form undissociated molecules.</i> In the
+neutralization of sodium hydroxide by hydrochloric acid the ions H<sup>+</sup>
+and OH<sup>-</sup> come to the equilibrium</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sup>+</sup> + OH<sup>-</sup> &lt;--> H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>But since water is almost entirely undissociated, equilibrium can only
+be reached when there are very few hydroxyl or hydrogen ions present.
+Consequently the two ions keep uniting until one or the other of them is
+practically removed from the solution. When this occurs the
+neutralization expressed in the following equation is complete:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaOH + HCl = H<sub>2</sub>O + NaCl.<br /></span>
+</div></div>
+
+<p><b>Preparation of acids.</b> The principle of reversible reactions finds
+practical application in the preparation of most of the common acids. An
+acid is usually prepared by treating the most common of its salts with
+some other acid of high boiling point. The mixture is then heated until
+the lower boiling acid desired distills out. Owing to<span class='pagenum'><a name="Page_142" id="Page_142">[Pg 142]</a></span> its high boiling
+point (338&deg;), sulphuric acid is usually employed for this purpose, most
+other acids boiling below that temperature.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> What would take place when solutions of silver nitrate and sodium
+chloride are brought together? What other chlorides would act in the
+same way?</p>
+
+<p><b>2.</b> Is the reaction expressed by the equation NH<sub>3</sub> + H<sub>2</sub>O = NH<sub>4</sub>OH
+reversible? If so, state the conditions under which it will go in each
+direction.</p>
+
+<p><b>3.</b> Is the reaction expressed by the equation 2H + O = H<sub>2</sub>O reversible?
+If so, state the conditions under which it will go in each direction.</p>
+
+<p><b>4.</b> Suggest a method for the preparation of hydrochloric acid.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_143" id="Page_143">[Pg 143]</a></span></p>
+<h2><a name="CHAPTER_XIV" id="CHAPTER_XIV"></a>CHAPTER XIV</h2>
+
+<h3>SULPHUR AND ITS COMPOUNDS</h3>
+
+
+<p><b>Occurrence.</b> The element sulphur has been known from the earliest times,
+since it is widely distributed in nature and occurs in large quantities
+in the uncombined form, especially in the neighborhood of volcanoes.
+Sicily has long been famous for its sulphur mines, and smaller deposits
+are found in Italy, Iceland, Mexico, and especially in Louisiana, where
+it is mined extensively. In combination, sulphur occurs abundantly in
+the form of sulphides and sulphates. In smaller amounts it is found in a
+great variety of minerals, and it is a constituent of many animal and
+vegetable substances.</p>
+
+<p><b>Extraction of sulphur.</b> Sulphur is prepared from the native substance,
+the separation of crude sulphur from the rock and earthy materials with
+which it is mixed being a very simple process. The ore from the mines is
+merely heated until the sulphur melts and drains away from the earthy
+impurities. The crude sulphur obtained in this way is distilled in a
+retort-shaped vessel made of iron, the exit tube of which opens into a
+cooling chamber of brickwork. When the sulphur vapor first enters the
+cooling chamber it condenses as a fine crystalline powder called
+<i>flowers of sulphur</i>. As the condensing chamber becomes warm, the
+sulphur collects as a liquid in it, and is drawn off into cylindrical
+molds, the product being called <i>roll sulphur</i> or <i>brimstone</i>.<span class='pagenum'><a name="Page_144" id="Page_144">[Pg 144]</a></span></p>
+
+<p><b>Physical properties.</b> Roll sulphur is a pale yellow, crystalline solid,
+without marked taste and with but a faint odor. It is insoluble in
+water, but is freely soluble in a few liquids, notably in carbon
+disulphide. Roll sulphur melts at 114.8&deg;. Just above the melting point
+it forms a rather thin, straw-colored liquid. As the temperature is
+raised, this liquid turns darker in color and becomes thicker, until at
+about 235&deg; it is almost black and is so thick that the vessel containing
+it can be inverted without danger of the liquid running out. At higher
+temperatures it becomes thin once more, and boils at 448&deg;, forming a
+yellowish vapor. On cooling the same changes take place in reverse
+order.</p>
+
+<p><b>Varieties of sulphur.</b> Sulphur is known in two general forms, crystalline
+and amorphous. Each of these forms exists in definite modifications.</p>
+
+<p><b>Crystalline sulphur.</b> Sulphur occurs in two crystalline forms, namely,
+rhombic sulphur and monoclinic sulphur.</p>
+
+<p>1. <i>Rhombic sulphur.</i> When sulphur crystallizes from its solution in
+carbon disulphide it separates in crystals which have the same color and
+melting point as roll sulphur, and are rhombic in shape. Roll sulphur is
+made up of minute rhombic crystals.</p>
+
+<p>2. <i>Monoclinic sulphur.</i> When melted sulphur is allowed to cool until a
+part of the liquid has solidified, and the remaining liquid is then
+poured off, it is found that the solid sulphur remaining in the vessel
+has assumed the form of fine needle-shaped crystals. These differ much
+in appearance from the rhombic crystals obtained by crystallizing
+sulphur from its solution in carbon disulphide. The needle-shaped form
+is called <i>monoclinic sulphur</i>. The two varieties differ also in density
+and in melting point, the monoclinic sulphur melting at 120&deg;.<span class='pagenum'><a name="Page_145" id="Page_145">[Pg 145]</a></span></p>
+
+<p>Monoclinic and rhombic sulphur remain unchanged in contact with each
+other at 96&deg;. Above this temperature the rhombic changes into
+monoclinic; at lower temperatures the monoclinic changes into rhombic.
+The temperature 96&deg; is therefore called the transition point of sulphur.
+Heat is set free when monoclinic sulphur changes into rhombic.</p>
+
+<p><b>Amorphous sulphur.</b> Two varieties of amorphous sulphur can be readily
+obtained. These are white sulphur and plastic sulphur.</p>
+
+<p>1. <i>White sulphur.</i> Flowers of sulphur, the preparation of which has
+been described, consists of a mixture of rhombic crystals and amorphous
+particles. When treated with carbon disulphide, the crystals dissolve,
+leaving the amorphous particles as a white residue.</p>
+
+<p>2. <i>Plastic sulphur.</i> When boiling sulphur is poured into cold water it
+assumes a gummy, doughlike form, which is quite elastic. This can be
+seen in a very striking manner by distilling sulphur from a small,
+short-necked retort, such as is represented in Fig. 40, and allowing the
+liquid to run directly into water. In a few days it becomes quite
+brittle and passes over into ordinary rhombic sulphur.</p>
+
+<div class="figleft" style="width: 335px;">
+<img src="images/imagefig40.jpg" width="335" height="400" alt="Fig. 40" title="" />
+<span class="caption">Fig. 40</span>
+</div>
+
+<p><b>Chemical properties of sulphur.</b> When sulphur is heated to its kindling
+temperature in oxygen or in the air it burns with a pale blue flame,
+forming sulphur dioxide (SO<sub>2</sub>). Small quantities of sulphur trioxide
+<span class='pagenum'><a name="Page_146" id="Page_146">[Pg 146]</a></span>(SO<sub>3</sub>) may also be formed in the combustion of sulphur. Most metals
+when heated with sulphur combine directly with it, forming metallic
+sulphides. In some cases the action is so energetic that the mass
+becomes incandescent, as has been seen in the case of iron uniting with
+sulphur. This property recalls the action of oxygen upon metals, and in
+general the metals which combine readily with oxygen are apt to combine
+quite readily with sulphur.</p>
+
+<p><b>Uses of sulphur.</b> Large quantities of sulphur are used as a germicide in
+vineyards, also in the manufacture of gunpowder, matches, vulcanized
+rubber, and sulphuric acid.</p>
+
+
+<h3>COMPOUNDS OF SULPHUR WITH HYDROGEN</h3>
+
+<p><b>Hydrosulphuric acid</b> (H<sub>2</sub>S). This substance is a gas having the
+composition expressed by the formula H<sub>2</sub>S and is commonly called
+hydrogen sulphide. It is found in the vapors issuing from volcanoes, and
+in solution in the so-called sulphur waters of many springs. It is
+formed when organic matter containing sulphur undergoes decay, just as
+ammonia is formed under similar circumstances from nitrogenous matter.</p>
+
+<p><b>Preparation.</b> Hydrosulphuric acid is prepared in the laboratory by
+treating a sulphide with an acid. Iron sulphide (FeS) is usually
+employed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">FeS + 2HCl = FeCl<sub>2</sub> + H<sub>2</sub>S.<br /></span>
+</div></div>
+
+<p>A convenient apparatus is shown in Fig. 41. A few lumps of iron sulphide
+are placed in the bottle <i>A</i>, and dilute acid is added in small
+quantities at a time through the funnel tube <i>B</i>, the gas escaping
+through the tube <i>C</i>.</p>
+
+<div class="figleft" style="width: 205px;">
+<img src="images/imagefig41.jpg" width="205" height="400" alt="Fig. 41" title="" />
+<span class="caption">Fig. 41</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_147" id="Page_147">[Pg 147]</a></span></p>
+<div class="blockquot"><p><b>Explanation of the reaction.</b> Iron sulphide is a salt of
+hydrosulphuric acid, and this reaction is therefore similar to
+the one which takes place when sulphuric acid acts upon a
+nitrate. In both cases a salt and an acid are brought together,
+and there is a tendency for the reaction to go on until a state
+of equilibrium is reached. This equilibrium is constantly
+disturbed by the escape of the gaseous acid set free, so that
+the reaction goes on until all of the original salt has been
+decomposed. The two reactions differ in that the first one is
+complete at ordinary temperatures, while in the case of
+sulphuric acid acting upon sodium nitrate, the reacting
+substances must be heated so as to secure a temperature at
+which nitric acid is a gas.</p></div>
+
+<p><b>Physical properties.</b> Hydrosulphuric acid is a colorless gas, having a
+weak, disagreeable taste and an exceedingly offensive odor. It is rather
+sparingly soluble in water at ordinary temperatures, about three volumes
+dissolving in one of water. In boiling water it is not soluble at all.
+In pure form it acts as a violent poison, and even when diluted largely
+with air produces headache, dizziness, and nausea. It is a little
+heavier than air, having a density of 1.18.</p>
+
+<p><b>Chemical properties.</b> The most important chemical properties of
+hydrosulphuric acid are the following:</p>
+
+<p>1. <i>Acid properties.</i> Hydrosulphuric acid is a weak acid. In solution in
+water it turns blue litmus red and neutralizes bases, forming salts
+called <i>sulphides</i>.</p>
+
+<p>2. <i>Action on oxygen.</i> The elements composing hydrosulphuric acid have
+each a strong affinity for oxygen, and are not held together very
+firmly. Consequently the gas burns readily in oxygen or the air,
+according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>S + 3O = H<sub>2</sub>O + SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>When there is not enough oxygen for both the sulphur and the hydrogen,
+the latter element combines with the oxygen and the sulphur is set free:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>S + O = H<sub>2</sub>O + S.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_148" id="Page_148">[Pg 148]</a></span></p>
+<p>3. <i>Reducing action.</i> Owing to the ease with which hydrosulphuric acid
+decomposes and the strong affinity of both sulphur and hydrogen for
+oxygen, the substance is a strong reducing agent, taking oxygen away
+from many substances which contain it.</p>
+
+<p>4. <i>Action on metals.</i> Hydrosulphuric acid acts towards metals in a way
+very similar to water. Thus, when it is passed over heated iron in a
+tube, the reaction is represented by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Fe + 4H<sub>2</sub>S = Fe<sub>3</sub>S<sub>4</sub> + 8H.<br /></span>
+</div></div>
+
+<p>Water in the form of steam, under similar circumstances, acts according
+to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Fe + 4H<sub>2</sub>O = Fe<sub>3</sub>O<sub>4</sub> + 8H.<br /></span>
+</div></div>
+
+<p><b>Salts of hydrosulphuric acid,&mdash;sulphides.</b> The salts of hydrosulphuric
+acid, called sulphides, form an important class of salts. Many of them
+are found abundantly in nature, and some of them are important ores.
+They will be frequently mentioned in connection with the metals.</p>
+
+<p>Most of the sulphides are insoluble in water, and some of them are
+insoluble in acids. Consequently, when hydrosulphuric acid is passed
+into a solution of a salt, it often happens that a sulphide is
+precipitated. With copper chloride the equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CuCl<sub>2</sub> + H<sub>2</sub>S = CuS + 2HCl.<br /></span>
+</div></div>
+
+<p>Because of the fact that some metals are precipitated in this way as
+sulphides while others are not, hydrosulphuric acid is extensively used
+in the separation of the metals in the laboratory.</p>
+<p><span class='pagenum'><a name="Page_149" id="Page_149">[Pg 149]</a></span></p>
+<div class="blockquot"><p><b>Explanation of the reaction.</b> When hydrosulphuric acid and
+copper chloride are brought together in solution, both copper
+and sulphur ions are present, and these will come to an
+equilibrium, as represented in the equation</p>
+
+<p>Cu<sup>+</sup> + S<sup>-</sup> &lt;--> CuS.</p>
+
+<p>Since copper sulphide is almost insoluble in water, as soon as
+a very small quantity has formed the solution becomes
+supersaturated, and the excess keeps precipitating until nearly
+all the copper or sulphur ions have been removed from the
+solution. With some other ions, such as iron, the sulphide
+formed does not saturate the solution, and no precipitate
+results.</p></div>
+
+
+<h3>OXIDES OF SULPHUR</h3>
+
+<p>Sulphur forms two well-known compounds with oxygen: sulphur dioxide
+(SO<sub>2</sub>), sometimes called sulphurous anhydride; and sulphur trioxide
+(SO<sub>3</sub>), frequently called sulphuric anhydride.</p>
+
+<p><b>Sulphur dioxide</b> (SO<sub>2</sub>). Sulphur dioxide occurs in nature in the gases
+issuing from volcanoes, and in solution in the water of many springs. It
+is likely to be found wherever sulphur compounds are undergoing
+oxidation.</p>
+
+<p><b>Preparation.</b> Three general ways may be mentioned for the preparation of
+sulphur dioxide:</p>
+
+<p>1. <i>By the combustion of sulphur.</i> Sulphur dioxide is readily formed by
+the combustion of sulphur in oxygen or the air:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">S + 2O = SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>It is also formed when substances containing sulphur are burned:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">ZnS + 3O = ZnO + SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>2. <i>By the reduction of sulphuric acid.</i> When concentrated sulphuric
+acid is heated with certain metals, such as copper, part of the acid is
+changed into copper sulphate, and part is reduced to sulphurous acid.
+The latter then decomposes into sulphur dioxide and water, the complete
+equation being</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Cu + 2H<sub>2</sub>SO<sub>4</sub> = CuSO<sub>4</sub> + SO<sub>2</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_150" id="Page_150">[Pg 150]</a></span></p>
+<p>3. <i>By the action of an acid on a sulphite.</i> Sulphites are salts of
+sulphurous acid (H<sub>2</sub>SO<sub>3</sub>). When a sulphite is treated with an acid,
+sulphurous acid is set free, and being very unstable, decomposes into
+water and sulphur dioxide. These reactions are expressed in the
+equations</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>SO<sub>3</sub> + 2HCl = 2NaCl + H<sub>2</sub>SO<sub>3</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">H<sub>2</sub>SO<sub>3</sub> = H<sub>2</sub>O + SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p><b>Explanation of the reaction.</b> In this case we have two reversible
+reactions depending on each other. In the first reaction,</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(1) Na<sub>2</sub>SO<sub>3</sub> + 2HCl &lt;--> 2NaCl + H<sub>2</sub>SO<sub>3</sub>,<br /></span>
+</div></div>
+
+<p>we should expect an equilibrium to result, for none of the four
+substances in the equation are insoluble or volatile when water is
+present to hold them in solution. But the quantity of the H<sub>2</sub>SO<sub>3</sub> is
+constantly diminishing, owing to the fact that it decomposes, as
+represented in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(2) H<sub>2</sub>SO<sub>3</sub> &lt;--> H<sub>2</sub>O + SO<sub>2</sub>,<br /></span>
+</div></div>
+
+<p>and the sulphur dioxide, being a gas, escapes. No equilibrium can
+therefore result, since the quantity of the sulphurous acid is
+constantly being diminished because of the escape of sulphur dioxide.</p>
+
+<p><b>Physical properties.</b> Sulphur dioxide is a colorless gas, which at
+ordinary temperatures is 2.2 times as heavy as air. It has a peculiar,
+irritating odor. The gas is very soluble in water, one volume of water
+dissolving eighty of the gas under standard conditions. It is easily
+condensed to a colorless liquid, and can be purchased in this condition
+stored in strong bottles, such as the one represented in Fig. 42.</p>
+
+<div class="figright" style="width: 207px;">
+<img src="images/imagefig42.jpg" width="207" height="400" alt="Fig. 42" title="" />
+<span class="caption">Fig. 42</span>
+</div>
+
+<p><b>Chemical properties.</b> Sulphur dioxide has a marked tendency to combine
+with other substances, and is therefore an<span class='pagenum'><a name="Page_151" id="Page_151">[Pg 151]</a></span> active substance chemically.
+It combines with oxygen gas, but not very easily. It can, however, take
+oxygen away from some other substances, and is therefore a good reducing
+agent. Its most marked chemical property is its ability to combine with
+water to form sulphurous acid (H<sub>2</sub>SO<sub>3</sub>).</p>
+
+<p><b>Sulphurous acid</b> (H<sub>2</sub>SO<sub>3</sub>). When sulphur dioxide dissolves in water
+it combines chemically with it to form sulphurous acid, an unstable
+substance having the formula H<sub>3</sub>SO<sub>3</sub>. It is impossible to prepare
+this acid in pure form, as it breaks down very easily into water and
+sulphur dioxide. The reaction is therefore reversible, and is expressed
+by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>O + SO<sub>2</sub> &lt;--> H<sub>2</sub>SO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Solutions of the acid in water have a number of interesting properties.</p>
+
+<p>1. <i>Acid properties.</i> The solution has all the properties typical of an
+acid. When neutralized by bases, sulphurous acid yields a series of
+salts called <i>sulphites</i>.</p>
+
+<p>2. <i>Reducing properties.</i> Solutions of sulphurous acid act as good
+reducing agents. This is due to the fact that sulphurous acid has the
+power of taking up oxygen from the air, or from substances rich in
+oxygen, and is changed by this reaction into sulphuric acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>SO<sub>3</sub> + O = H<sub>2</sub>SO<sub>4</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">H<sub>2</sub>SO<sub>3</sub> + H<sub>2</sub>O<sub>2</sub> = H<sub>2</sub>S0<sub>4</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>3. <i>Bleaching properties.</i> Sulphurous acid has strong bleaching
+properties, acting upon many colored substances in such a way as to
+destroy their color. It is on this account used to bleach paper, straw
+goods, and even such foods as canned corn.</p>
+
+<p>4. <i>Antiseptic properties.</i> Sulphurous acid has marked antiseptic
+properties, and on this account has the power<span class='pagenum'><a name="Page_152" id="Page_152">[Pg 152]</a></span> of arresting
+fermentation. It is therefore used as a preservative.</p>
+
+<p><b>Salts of sulphurous acid,&mdash;sulphites.</b> The sulphites, like sulphurous
+acid, have the power of taking up oxygen very readily, and are good
+reducing agents. On account of this tendency, commercial sulphites are
+often contaminated with sulphates. A great deal of sodium sulphite is
+used in the bleaching industry, and as a reagent for softening paper
+pulp.</p>
+
+<p><b>Sulphur trioxide</b> (SO<sub>3</sub>). When sulphur dioxide and oxygen are heated
+together at a rather high temperature, a small amount of sulphur
+trioxide (SO<sub>3</sub>) is formed, but the reaction is slow and incomplete.
+If, however, the heating takes place in the presence of very fine
+platinum dust, the reaction is rapid and nearly complete.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/imagefig43.jpg" width="450" height="146" alt="Fig. 43" title="" />
+<span class="caption">Fig. 43</span>
+</div>
+
+<div class="blockquot"><p><b> Experimental preparation of sulphur trioxide.</b> The experiment
+can be performed by the use of the apparatus shown in Fig. 43,
+the fine platinum being secured by moistening asbestos fiber
+with a solution of platinum chloride and igniting it in a
+flame. The fiber, covered with fine platinum, is placed in a
+tube of hard glass, which is then heated with a burner to about
+350&deg;, while sulphur dioxide and air are passed into the tube.
+Union takes place at once, and the strongly fuming sulphur
+trioxide escapes from the jet at the end of the tube, and may
+be condensed by surrounding the receiving tube with a freezing
+mixture.</p></div>
+
+<p><b>Properties of sulphur trioxide.</b> Sulphur trioxide is a colorless liquid,
+which solidifies at about 15&deg; and boils at 46&deg;.<span class='pagenum'><a name="Page_153" id="Page_153">[Pg 153]</a></span> A trace of moisture
+causes it to solidify into a mass of silky white crystals, somewhat
+resembling asbestos fiber in appearance. In contact with the air it
+fumes strongly, and when thrown upon water it dissolves with a hissing
+sound and the liberation of a great deal of heat. The product of this
+reaction is sulphuric acid, so that sulphur trioxide is the anhydride of
+that acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SO<sub>3</sub> + H<sub>2</sub>O = H<sub>2</sub>SO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p><b>Catalysis.</b> It has been found that many chemical reactions, such as the
+union of sulphur dioxide with oxygen, are much influenced by the
+presence of substances which do not themselves seem to take a part in
+the reaction, and are left apparently unchanged after it has ceased.
+These reactions go on very slowly under ordinary circumstances, but are
+greatly hastened by the presence of the foreign substance. Substances
+which hasten very slow reactions in this way are said to act as
+catalytic agents or <i>catalyzers</i>, and the action is called <i>catalysis</i>.
+Just how the action is brought about is not well understood.</p>
+
+<p>DEFINITION: <i>A catalyzer is a substance which changes the velocity of a
+reaction, but does not change its products.</i></p>
+
+<p><b>Examples of Catalysis.</b> We have already had several instances of such
+action. Oxygen and hydrogen combine with each other at ordinary
+temperatures in the presence of platinum powder, while if no catalytic
+agent is present they do not combine in appreciable quantities until a
+rather high temperature is reached. Potassium chlorate, when heated with
+manganese dioxide, gives up its oxygen at a much lower temperature than
+when heated alone. Hydrogen dioxide decomposes very rapidly when
+powdered manganese dioxide is sifted into its concentrated solution.<span class='pagenum'><a name="Page_154" id="Page_154">[Pg 154]</a></span></p>
+
+<p>On the other hand, the catalytic agent sometimes retards chemical
+action. For example, a solution of hydrogen dioxide decomposes more
+slowly when it contains a little phosphoric acid than when perfectly
+pure. For this reason commercial hydrogen dioxide always contains
+phosphoric acid.</p>
+
+<p>Many reactions are brought about by the catalytic action of traces of
+water. For example, phosphorus will not burn in oxygen in the absence of
+all moisture. Hydrochloric acid will not unite with ammonia if the
+reagents are perfectly dry. It is probable that many of the chemical
+transformations in physiological processes, such as digestion, are
+assisted by certain substances acting as catalytic agents. The principle
+of catalysis is therefore very important.</p>
+
+<p><b>Sulphuric acid</b> (<i>oil of vitriol</i>) (H<sub>2</sub>SO<sub>4</sub>). Sulphuric acid is one
+of the most important of all manufactured chemicals. Not only is it one
+of the most common reagents in the laboratory, but enormous quantities
+of it are used in many of the industries, especially in the refining of
+petroleum, the manufacture of nitroglycerin, sodium carbonate, and
+fertilizers.</p>
+
+<p><b>Manufacture of sulphuric acid.</b> 1. <i>Contact process</i>. The reactions
+taking place in this process are represented by the following equations:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SO<sub>2</sub> + O = SO<sub>3</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">SO<sub>3</sub> + H<sub>2</sub>O = H<sub>2</sub>SO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>To bring about the first of these reactions rapidly, a catalyzer is
+employed, and the process is carried out in the following way: Large
+iron tubes are packed with some porous material, such as calcium and
+magnesium sulphates, which contains a suitable catalytic substance
+scattered through it. The catalyzers most used are platinum powder,<span class='pagenum'><a name="Page_155" id="Page_155">[Pg 155]</a></span>
+vanadium oxide, and iron oxide. Purified sulphur dioxide and air are
+passed through the tubes, which are kept at a temperature of about 350&deg;.
+Sulphur trioxide is formed, and as it issues from the tube it is
+absorbed in water or dilute sulphuric acid. The process is continued
+until all the water in the absorbing vessel has been changed into
+sulphuric acid, so that a very concentrated acid is made in this way. An
+excess of the trioxide may dissolve in the strong sulphuric acid,
+forming what is known as <i>fuming sulphuric acid</i>.</p>
+
+<p>2. <i>Chamber process.</i> The method of manufacture exclusively employed
+until recent years, and still in very extensive use, is much more
+complicated. The reactions are quite involved, but the conversion of
+water, sulphur dioxide, and oxygen into sulphuric acid is accomplished
+by the catalytic action of oxides of nitrogen. The reactions are brought
+about in large lead-lined chambers, into which oxides of nitrogen,
+sulphur dioxide, steam, and air are introduced in suitable proportions.</p>
+
+<div class="blockquot"><p><b>Reactions of the chamber process.</b> In a very general way, the
+various reactions which take place in the lead chambers may be
+expressed in two equations. In the first reaction sulphur
+dioxide, nitrogen peroxide, steam, and oxygen unite, as shown
+in the equation</p>
+
+<p>(1) 2SO<sub>2</sub> + 2NO<sub>2</sub> + H<sub>2</sub>O + O = 2SO<sub>2</sub> (OH) (NO<sub>2</sub>).</p>
+
+<p>The product formed in this reaction is called nitrosulphuric
+acid or "chamber crystals." It actually separates on the walls
+of the chambers when the process is not working properly. Under
+normal conditions, it is decomposed as fast as it is formed by
+the action of excess of steam, as shown in the equation</p>
+
+<p>(2) 2SO<sub>2</sub> (OH) (NO<sub>2</sub>) + H<sub>2</sub>O + O = 2H<sub>2</sub>SO<sub>4</sub> + 2NO<sub>2</sub>.</p>
+
+<p>The nitrogen dioxide formed in this reaction can now enter into
+combination with a new quantity of sulphur dioxide, steam, and
+oxygen, and the series of reactions go on indefinitely. Many
+other reactions occur, but these two illustrate the principle
+of the process.</p></div><p><span class='pagenum'><a name="Page_156" id="Page_156">[Pg 156]</a></span></p>
+
+<p>The relation between sulphuric acid and nitrosulphuric acid can be seen
+by comparing their structural formulas:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>O= -OH</td><td align='left'>O= -OH</td></tr>
+<tr><td align='center'> S</td><td align='center'>S</td></tr>
+<tr><td align='left'>O= -OH</td><td align='left'>O= -NO2</td></tr>
+</table></div>
+
+<p>The latter may be regarded as derived from the former by the
+substitution of the nitro group (NO<sub>2</sub>) for the hydroxyl group (OH).</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/imagefig44.jpg" width="450" height="242" alt="Fig. 44." title="" />
+<span class="caption">Fig. 44.</span>
+</div>
+
+<p><b>The sulphuric acid plant.</b> Fig. 44 illustrates the simpler parts of a
+plant used in the manufacture of sulphuric acid by the chamber process.
+Sulphur or some sulphide, as FeS<sub>2</sub>, is burned in furnace <i>A</i>. The
+resulting sulphur dioxide, together with air and some nitrogen peroxide,
+are conducted into the large chambers, the capacity of each chamber
+being about 75,000 cu. ft. Steam is also admitted into these chambers at
+different points. These compounds react to form sulphuric acid,
+according to the equations given above. The nitrogen left after the
+withdrawal of the oxygen from the admitted air escapes through the
+Gay-Lussac tower <i>X</i>. In order to prevent the escape of the oxides of
+nitrogen regenerated in the reaction, the tower is filled with lumps of
+coke, over which trickles concentrated sulphuric acid admitted from <i>Y</i>.
+The nitrogen peroxide dissolves in the acid and the resulting solution
+collects in <i>H</i>. This is pumped into <i>E</i>, where it is mixed with dilute
+acid and allowed to trickle down through the chamber <i>D</i> (Glover tower),
+which is filled with some acid-resisting rock. Here the nitrogen
+peroxide is expelled from the solution by the action of the hot gases
+entering from <i>A</i>, and together with them enters the first chamber
+again. The acid from which the nitrogen peroxide is expelled collects in
+<i>F</i>. Theoretically, a small amount of nitrogen peroxide would suffice to
+prepare an unlimited amount of sulphuric acid; practically, some of it
+escapes, and this is replaced by small amounts admitted at <i>B</i>.<span class='pagenum'><a name="Page_157" id="Page_157">[Pg 157]</a></span></p>
+
+<p>The sulphuric acid so formed, together with the excess of condensed
+steam, collect upon the floor of the chambers in the form of a liquid
+containing from 62% to 70% of sulphuric acid. The product is called
+<i>chamber acid</i> and is quite impure; but for many purposes, such as the
+manufacture of fertilizers, it needs no further treatment. It can be
+concentrated by boiling it in vessels made of iron or platinum, which
+resist the action of the acid, nearly all the water boiling off. Pure
+concentrated acid can be made best by the contact process, while the
+chamber process is cheaper for the dilute impure acid.</p>
+
+<p><b>Physical properties.</b> Sulphuric acid is a colorless, oily liquid, nearly
+twice as heavy as water. The ordinary concentrated acid contains about
+2% of water, has a density of 1.84, and boils at 338&deg;. It is sometimes
+called <i>oil of vitriol</i>, since it was formerly made by distilling a
+substance called <i>green vitriol</i>.</p>
+
+<p><b>Chemical properties.</b> Sulphuric acid possesses chemical properties which
+make it one of the most important of chemical substances.</p>
+
+<p>1. <i>Action as an acid.</i> In dilute solution sulphuric acid acts as any
+other acid, forming salts with oxides and hydroxides.</p>
+
+<p>2. <i>Action as an oxidizing agent.</i> Sulphuric acid contains a large
+percentage of oxygen and is, like nitric acid, a very good oxidizing
+agent. When the concentrated acid is heated with sulphur, carbon, and
+many other substances, oxidation takes place, the sulphuric acid
+decomposing according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>SO<sub>4</sub> = H<sub>2</sub>SO<sub>3</sub> + O.<br /></span>
+</div></div>
+
+<p>3. <i>Action on metals.</i> In dilute solution sulphuric acid acts upon many
+metals, such as zinc, forming a sulphate and liberating hydrogen. When
+the concentrated acid is employed the hydrogen set free is oxidized by a
+new portion<span class='pagenum'><a name="Page_158" id="Page_158">[Pg 158]</a></span> of the acid, with the liberation of sulphur dioxide. With
+copper the reactions are expressed by the equations</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(1) Cu + H<sub>2</sub>SO<sub>4</sub> = CuSO<sub>4</sub> + 2H,<br /></span>
+</div><div class="stanza">
+<span class="i0">(2) H<sub>2</sub>SO<sub>4</sub> + 2H = H<sub>2</sub>SO<sub>3</sub> + H<sub>2</sub>O,<br /></span>
+</div><div class="stanza">
+<span class="i0">(3) H<sub>2</sub>SO<sub>3</sub> = H<sub>2</sub>O + SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>By combining these equations the following one is obtained:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Cu + 2H<sub>2</sub>SO<sub>4</sub> = CuSO<sub>4</sub> + SO<sub>2</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>4. <i>Action on salts.</i> We have repeatedly seen that an acid of high
+boiling point heated with the salt of some acid of lower boiling point
+will drive out the low boiling acid. The boiling point of sulphuric acid
+(338&deg;) is higher than that of almost any common acid; hence it is used
+largely in the preparation of other acids.</p>
+
+<p>5. <i>Action on water.</i> Concentrated sulphuric acid has a very great
+affinity for water, and is therefore an effective dehydrating agent.
+Gases which have no chemical action upon sulphuric acid can be freed
+from water vapor by bubbling them through the strong acid. When the acid
+is diluted with water much heat is set free, and care must be taken to
+keep the liquid thoroughly stirred during the mixing, and to pour the
+acid into the water,&mdash;never the reverse.</p>
+
+<p>Not only can sulphuric acid absorb water, but it will often withdraw the
+elements hydrogen and oxygen from a compound containing them,
+decomposing the compound, and combining with the water so formed. For
+this reason most organic substances, such as sugar, wood, cotton, and
+woolen fiber, and even flesh, all of which contain much oxygen and
+hydrogen in addition to carbon, are charred or burned by the action of
+the concentrated acid.<span class='pagenum'><a name="Page_159" id="Page_159">[Pg 159]</a></span></p>
+
+<p><b>Salts of sulphuric acid,&mdash;sulphates.</b> The sulphates form a very important
+class of salts, and many of them have commercial uses. Copperas (iron
+sulphate), blue vitriol (copper sulphate), and Epsom salt (magnesium
+sulphate) serve as examples. Many sulphates are important minerals,
+prominent among these being gypsum (calcium sulphate) and barytes
+(barium sulphate).</p>
+
+<div class="blockquot"><p><b>Thiosulphuric acid</b> (H<sub>2</sub>S<sub>2</sub>O<sub>3</sub>); <b>Thiosulphates.</b> Many other
+acids of sulphur containing oxygen are known, but none of them
+are of great importance. Most of them cannot be prepared in a
+pure state, and are known only through their salts. The most
+important of these is thiosulphuric acid.</p>
+
+<p>When sodium sulphite is boiled with sulphur the two substances
+combine, forming a salt which has the composition represented
+in the formula Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>:</p>
+
+<p>Na<sub>2</sub>SO<sub>3</sub> + S = Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>.</p>
+
+<p>The substance is called sodium thiosulphate, and is a salt of
+the easily decomposed acid H<sub>2</sub>S<sub>2</sub>O<sub>3</sub>, called
+thiosulphuric acid. This reaction is quite similar to the
+action of oxygen upon sulphites:</p>
+
+<p>Na<sub>2</sub>SO<sub>3</sub> + O = Na<sub>2</sub>SO<sub>4</sub>.</p>
+
+<p>More commonly the salt is called sodium hyposulphite, or merely
+"hypo." It is a white solid and is extensively used in
+photography, in the bleaching industry, and as a disinfectant.</p></div>
+
+<p><b>Monobasic and dibasic acids.</b> Such acids as hydrochloric and nitric
+acids, which have only one replaceable hydrogen atom in the molecule, or
+in other words yield one hydrogen ion in solution, are called monobasic
+acids. Acids yielding two hydrogen ions in solution are called dibasic
+acids. Similarly, we may have tribasic and tetrabasic acids. The three
+acids of sulphur are dibasic acids. It is therefore possible for each of
+them to form both normal and acid salts. The acid salts can be made in
+two ways: the acid may be treated with only half enough base to
+neutralize it,&mdash;</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaOH + H<sub>2</sub>SO<sub>4</sub> = NaHSO<sub>4</sub> + H<sub>2</sub>O;<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_160" id="Page_160">[Pg 160]</a></span></p>
+<p>or a normal salt may be treated with the free acid,&mdash;</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>SO<sub>4</sub> + H<sub>2</sub>SO<sub>4</sub> = 2NaHSO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>Acid sulphites and sulphides may be made in the same ways.</p>
+
+<p><b>Carbon disulphide</b> (CS<sub>2</sub>). When sulphur vapor is passed over highly
+heated carbon the two elements combine, forming carbon disulphide
+(CS<sub>2</sub>), just as oxygen and carbon unite to form carbon dioxide
+(CO<sub>2</sub>). The substance is a heavy, colorless liquid, possessing, when
+pure, a pleasant ethereal odor. On standing for some time, especially
+when exposed to sunlight, it undergoes a slight decomposition and
+acquires a most disagreeable, rancid odor. It has the property of
+dissolving many substances, such as gums, resins, and waxes, which are
+insoluble in most liquids, and it is extensively used as a solvent for
+such substances. It is also used as an insecticide. It boils at a low
+temperature (46&deg;), and its vapor is very inflammable, burning in the air
+to form carbon dioxide and sulphur dioxide, according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CS<sub>2</sub> + 6O = CO<sub>2</sub> + 2SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<div class="figright" style="width: 398px;">
+<img src="images/imagefig45.jpg" width="398" height="400" alt="Fig. 45" title="" />
+<span class="caption">Fig. 45</span>
+</div>
+
+<div class="blockquot"><p><b>Commercial preparation of carbon disulphide.</b> In the preparation
+of carbon disulphide an electrical furnace is employed, such as
+is represented in Fig. 45. The furnace is packed with carbon
+<i>C</i>, and this is fed in through the hoppers <i>B</i>, as fast as
+that which is present in the hearth of the furnace is used up.
+Sulphur is introduced at <i>A</i>, and at the lower ends of the
+tubes it is melted by the heat of the furnace and flows into
+the hearth as a liquid. An electrical current is passed through
+the carbon and melted sulphur from the electrodes <i>E</i>, heating
+the charge. The vapors of carbon disulphide pass up through the
+furnace and escape at <i>D</i>, from which they pass to a suitable
+condensing apparatus.</p></div>
+
+<p><span class='pagenum'><a name="Page_161" id="Page_161">[Pg 161]</a></span></p>
+
+<p><b>Comparison of sulphur and oxygen.</b> A comparison of the formulas and the
+chemical properties of corresponding compounds of oxygen and sulphur
+brings to light many striking similarities. The conduct of
+hydrosulphuric acid and water toward many substances has been seen to be
+very similar; the oxides and sulphides of the metals have analogous
+formulas and undergo many parallel reactions. Carbon dioxide and
+disulphide are prepared in similar ways and undergo many analogous
+reactions. It is clear, therefore, that these two elements are far more
+closely related to each other than to any of the other elements so far
+studied.</p>
+
+<p><b>Selenium and tellurium.</b> These two very uncommon elements are still more
+closely related to sulphur than is oxygen. They occur in comparatively
+small quantities and are usually found associated with sulphur and
+sulphides, either as the free elements or more commonly in combination
+with metals. They form compounds with hydrogen of the formulas H<sub>2</sub>Se
+and H<sub>2</sub>Te; these bodies are gases with properties very similar to
+those of H<sub>2</sub>S. They also form oxides and oxygen acids which resemble
+the corresponding sulphur compounds. The elements even have allotropic
+forms corresponding very closely to those of sulphur. Tellurium is
+sometimes found in combination with gold and copper, and occasions some
+difficulties in the refining of these metals. The elements have very few
+practical applications.</p>
+
+<p><b>Crystallography.</b> In order to understand the difference between the two
+kinds of sulphur crystals, it is necessary to know something about
+crystals in general and the forms which they may assume. An examination
+of a large number of crystals has shown that although they may differ
+much in geometric form, they can all be considered as modifications of a
+few simple plans. The best way to understand the relation of one crystal
+to another is to look upon every crystal as having its faces and angles
+arranged in definite fashion about<span class='pagenum'><a name="Page_162" id="Page_162">[Pg 162]</a></span> certain imaginary lines drawn
+through the crystal. These lines are called axes, and bear much the same
+relation to a crystal as do the axis and parallels of latitude and
+longitude to the earth and a geographical study of it. All crystals can
+be referred to one of six simple plans or systems, which have their axes
+as shown in the following drawings.</p>
+
+<p>The names and characteristics of these systems are as follows:</p>
+
+<p>1. Isometric or regular system (Fig. 46). Three equal axes, all at right
+angles.</p>
+
+<div class="figcenter" style="width: 484px;">
+<img src="images/imagefig46.jpg" width="484" height="188" alt="Fig. 46" title="" />
+<span class="caption">Fig. 46</span>
+</div>
+
+<p>2. Tetragonal system (Fig. 47). Two equal axes and one of different
+length, all at right angles to each other.</p>
+
+<div class="figcenter" style="width: 387px;">
+<img src="images/imagefig47.jpg" width="387" height="260" alt="Fig. 47" title="" />
+<span class="caption">Fig. 47</span>
+</div>
+
+<p>3. Orthorhombic system (Fig. 48). Three unequal axes, all at right
+angles to each other.</p>
+
+<div class="figcenter" style="width: 456px;">
+<img src="images/imagefig48.jpg" width="456" height="256" alt="Fig. 48" title="" />
+<span class="caption">Fig. 48</span>
+</div>
+
+<p>4. Monoclinic system (Fig. 49). Two axes at right angles, and a third at
+right angles to one of these, but inclined to the other.</p>
+
+<div class="figcenter" style="width: 433px;">
+<img src="images/imagefig49.jpg" width="433" height="246" alt="Fig. 49" title="" />
+<span class="caption">Fig. 49</span>
+</div>
+
+<p>5. Triclinic system (Fig. 50). Three axes, all inclined to each other.</p>
+
+<div class="figcenter" style="width: 402px;">
+<img src="images/imagefig50.jpg" width="402" height="260" alt="Fig. 50" title="" />
+<span class="caption">Fig. 50</span>
+</div>
+
+<p>6. Hexagonal system (Fig. 51). Three equal axes in the same plane
+intersecting at angles of 60&deg;, and a fourth at right angles to all of
+these.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig51.jpg" width="400" height="203" alt="Fig. 51" title="" />
+<span class="caption">Fig. 51</span>
+</div>
+
+<p>Every crystal can be imagined to have its faces and angles arranged in a
+definite way around one of these systems of axes. A cube, for instance,
+is referred to Plan 1, an axis ending in the center of each face; while
+in a regular octohedron an axis ends in each solid angle. These forms
+are shown in Fig. 46. It will be seen that both of these figures belong
+to the same system, though they are very different in appearance. In the
+same way, many geometric<span class='pagenum'><a name="Page_163" id="Page_163">[Pg 163]</a></span> forms may be derived from each of the systems,
+and the light lines about the axes in the drawings show two of the
+simplest forms of each of the systems.</p>
+
+<p>In general a given substance always crystallizes in the same system, and
+two corresponding faces of each crystal of it always make the same angle
+with each other. A few substances, of which sulphur is an example,
+crystallize in two different systems, and the crystals differ in such
+physical properties as melting point and density. Such substances are
+said to be <i>dimorphous</i>.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> (<i>a</i>) Would the same amount of heat be generated by the combustion of
+1 g. of each of the allotropic modifications of sulphur? (<i>b</i>) Would the
+same amount of sulphur dioxide be formed in each case?</p>
+
+<p><b>2.</b> Is the equation for the preparation of hydrosulphuric acid a
+reversible one? As ordinarily carried out, does the reaction complete
+itself?</p>
+
+<p><b>3.</b> Suppose that hydrosulphuric acid were a liquid, would it be necessary
+to modify the method of preparation?</p>
+
+<p><b>4.</b> Can sulphuric acid be used to dry hydrosulphuric acid? Give reason
+for answer.</p>
+
+<p><b>5.</b> Does dry hydrosulphuric acid react with litmus paper? State reason
+for answer.</p>
+
+<p><b>6.</b> How many grams of iron sulphide are necessary to prepare 100 l. of
+hydrosulphuric acid when the laboratory conditions are 17&deg; and 740 mm.
+pressure?</p>
+
+<p><b>7.</b> Suppose that the hydrogen in 1 l. of hydrosulphuric acid were
+liberated; what volume would it occupy, the gases being measured under
+the same conditions?</p>
+
+<p><b>8.</b> Write the equations representing the reaction between hydrosulphuric
+acid and sodium hydroxide and ammonium hydroxide respectively.</p>
+
+<p><b>9.</b> Show that the preparation of sulphur dioxide from a sulphite is
+similar in principle to the preparation of hydrogen sulphide.</p>
+
+<p><b>10.</b> (<i>a</i>) Does dry sulphur dioxide react with litmus paper? (<i>b</i>) How
+can it be shown that a solution of sulphur dioxide in water acts like an
+acid?<span class='pagenum'><a name="Page_164" id="Page_164">[Pg 164]</a></span></p>
+
+<p><b>11.</b> (<i>a</i>) Calculate the percentage composition of sulphurous anhydride
+and sulphuric anhydride. (<i>b</i>) Show how these two substances are in
+harmony with the law of multiple proportion.</p>
+
+<p><b>12.</b> How many pounds of sulphur would be necessary in the preparation of
+100 lb. of 98% sulphuric acid?</p>
+
+<p><b>13.</b> What weight of sulphur dioxide is necessary in the preparation of 1
+kg. of sodium sulphite?</p>
+
+<p><b>14.</b> What weight of copper sulphate crystals can be obtained by
+dissolving 1 kg. of copper in sulphuric acid and crystallizing the
+product from water?</p>
+
+<p><b>15.</b> Write the names and formulas of the oxides and oxygen acids of
+selenium and tellurium.</p>
+
+<p><b>16.</b> In the commercial preparation of carbon disulphide, what is the
+function of the electric current?</p>
+
+<p><b>17.</b> If the Gay-Lussac tower were omitted from the sulphuric acid
+factory, what effect would this have on the cost of production of
+sulphuric acid?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_165" id="Page_165">[Pg 165]</a></span></p>
+<h2><a name="CHAPTER_XV" id="CHAPTER_XV"></a>CHAPTER XV</h2>
+
+<h3>PERIODIC LAW</h3>
+
+
+<p>A number of the elements have now been studied somewhat closely. The
+first three of these, oxygen, hydrogen, and nitrogen, while having some
+physical properties in common with each other, have almost no point of
+similarity as regards their chemical conduct. On the other hand, oxygen
+and sulphur, while quite different physically, have much in common in
+their chemical properties.</p>
+
+<p>About eighty elements are now known. If all of these should have
+properties as diverse as do oxygen, hydrogen, and nitrogen, the study of
+chemistry would plainly be a very difficult and complicated one. If,
+however, the elements can be classified in groups, the members of which
+have very similar properties, the study will be very much simplified.</p>
+
+<p><b>Earlier classification of the elements.</b> Even at an early period efforts
+were made to discover some natural principle in accordance with which
+the elements could be classified. Two of these classifications may be
+mentioned here.</p>
+
+<p>1. <i>Classification into metals and non-metals.</i> The classification into
+metals and non-metals most naturally suggested itself. This grouping was
+based largely on physical properties, the metals being heavy, lustrous,
+malleable, ductile, and good conductors of heat and electricity.
+Elements possessing these properties are usually base-forming in
+character, and the ability to form bases came to be regarded as a
+characteristic property of the metals. The<span class='pagenum'><a name="Page_166" id="Page_166">[Pg 166]</a></span> non-metals possessed
+physical properties which were the reverse of those of the metals, and
+were acid-forming in character.</p>
+
+<p>Not much was gained by this classification, and it was very imperfect.
+Some metals, such as potassium, are very light; some non-metals, such as
+iodine, have a high luster; some elements can form either an acid or a
+base.</p>
+
+<p>2. <i>Classification into triad families.</i> In 1825 D&ouml;bereiner observed
+that an interesting relation exists between the atomic weights of
+chemically similar elements. To illustrate, lithium, sodium, and
+potassium resemble each other very closely, and the atomic weight of
+sodium is almost exactly an arithmetical mean between those of the other
+two: (7.03 + 39.15)/2 = 23.09. In many chemical and physical properties
+sodium is midway between the other two.</p>
+
+<p>A number of triad families were found, but among eighty elements, whose
+atomic weights range all the way from 1 to 240, such agreements might be
+mere chance. Moreover many elements did not appear to belong to such
+families.</p>
+
+<p><b>Periodic division.</b> In 1869 the Russian chemist Mendel&eacute;eff devised an
+arrangement of the elements based on their atomic weights, which has
+proved to be of great service in the comparative study of the elements.
+A few months later the German, Lothar Meyer, independently suggested the
+same ideas. This arrangement brought to light a great generalization,
+now known as the <i>periodic law</i>. An exact statement of the law will be
+given after the method of arranging the elements has been described.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/mendeleeff.jpg" width="400" height="543" alt="DMITRI IVANOVITCH MENDEL&Eacute;EFF (Russian) (1834-1907)
+
+Author of the periodic law; made many investigations on the physical
+constants of elements and compounds; wrote an important book entitled
+&quot;Principles of Chemistry&quot;; university professor and government
+official" title="" />
+<span class="caption">DMITRI IVANOVITCH MENDEL&Eacute;EFF (Russian) (1834-1907)<br /><br />
+
+Author of the periodic law; made many investigations on the physical
+constants of elements and compounds; wrote an important book entitled
+&quot;Principles of Chemistry&quot;; university professor and government
+official</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_167" id="Page_167">[Pg 167]</a></span></p>
+
+<p><b>Arrangement of the periodic table.</b> The arrangement suggested by
+Mendel&eacute;eff, modified somewhat by more recent investigations, is as
+follows: Beginning with lithium, which has an atomic weight of 7, the
+elements are arranged in a horizontal row in the order of their atomic
+weights, thus:</p>
+
+<p><b>Li (7.03), Be (9.1), B (11), C (12), N (14.04), O (16), F (19).</b></p>
+
+<p>These seven elements all differ markedly from each other. The eighth
+element, sodium, is very similar to lithium. It is placed just under
+lithium, and a new row follows:</p>
+
+<p><b>Na(23.05), Mg (24.36), Al (27.1), Si (28.4), P (31), S (32.06),
+Cl(35.45).</b></p>
+
+<p>When the fifteenth element, potassium, is reached, it is placed under
+sodium, to which it is very similar, and serves to begin a third row:</p>
+
+<p><b>K (39.15), Ca (40.1), Sc (44.1,) Ti (48.1), V (51.2), Cr (52.1), Mn(55).</b></p>
+
+<p>Not only is there a strong similarity between lithium, sodium, and
+potassium, which have been placed in a vertical row because of this
+resemblance, but the elements in the other vertical rows exhibit much of
+the same kind of similarity among themselves, and evidently form little
+natural groups.</p>
+
+<p>The three elements following manganese, namely, iron, nickel, and
+cobalt, have atomic weights near together, and are very similar
+chemically. They do not strongly resemble any of the elements so far
+considered, and are accordingly placed in a group by themselves,
+following manganese. A new row is begun with copper, which somewhat
+resembles the elements of the first vertical column. Following the fifth
+and seventh rows are groups of three closely related elements, so that
+the completed arrangement has the appearance represented in the table on
+page 168.</p>
+
+<p><span class='pagenum'><a name="Page_168" id="Page_168">[Pg 168]</a></span></p>
+<div class="figcenter" style="width: 800px;">
+<img src="images/periodic.jpg" width="800" height="563" alt="THE PERIODIC ARRANGEMENT OF THE ELEMENTS" title="" />
+<span class="caption">THE PERIODIC ARRANGEMENT OF THE ELEMENTS</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_169" id="Page_169">[Pg 169]</a></span></p>
+<p><b>Place of the atmospheric elements.</b> When argon was discovered it was seen
+at once that there was no place in the table for an element of atomic
+weight approximately 40. When the other inactive elements were found,
+however, it became apparent that they form a group just preceding Group
+1. They are accordingly arranged in this way in Group 0 (see table on
+opposite page). A study of this table brings to light certain very
+striking facts.</p>
+
+<p><b>Properties of elements vary with atomic weights.</b> There is evidently a
+close relation between the properties of an element and its atomic
+weight. Lithium, at the beginning of the first group, is a very strong
+base-forming element, with pronounced metallic properties. Beryllium,
+following lithium, is less strongly base-forming, while boron has some
+base-forming and some acid-forming properties. In carbon all
+base-forming properties have disappeared, and the acid-forming
+properties are more marked than in boron. These become still more
+emphasized as we pass through nitrogen and oxygen, until on reaching
+fluorine we have one of the strongest acid-forming elements. The
+properties of these seven elements therefore vary regularly with their
+atomic weights, or, in mathematical language, are regular functions of
+them.</p>
+
+<p><b>Periodic law.</b> The properties of the first seven elements vary
+<i>continuously</i>&mdash;that is steadily&mdash;away from base-forming and toward
+acid-forming properties. If lithium had the smallest atomic weight of
+any of the elements, and fluorine the greatest, so that in passing from
+one to the other we had included all the elements, we could say that the
+properties of elements are continuous functions of their atomic weights.
+But fluorine is an element of small atomic weight, and the one following
+it, sodium, breaks the regular order, for in it reappear all the
+characteristic properties of lithium. Magnesium, following sodium, bears
+much the same relation to<span class='pagenum'><a name="Page_170" id="Page_170">[Pg 170]</a></span> beryllium that sodium does to lithium, and
+the properties of the elements in the second row vary much as they do in
+the first row until potassium is reached, when another repetition
+begins. The properties of the elements do not vary continuously,
+therefore, with atomic weights, but at regular intervals there is a
+repetition, or <i>period</i>. This generalization is known as the <i>periodic
+law</i>, and may be stated thus: <i>The properties of elements are periodic
+functions of their atomic weights.</i></p>
+
+<p><b>The two families in a group.</b> While all the elements in a given vertical
+column bear a general resemblance to each other, it has been noticed
+that those belonging to periods having even numbers are very strikingly
+similar to each other. They are placed at the left side of the group
+columns. In like manner, the elements belonging to the odd periods are
+very similar and are arranged at the right side of the group columns.
+Thus calcium, strontium, and barium are very much alike; so, too, are
+magnesium, zinc, and cadmium. The resemblance between calcium and
+magnesium, or strontium and zinc, is much less marked. This method of
+arrangement therefore divides each group into two families, each
+containing four or five members, between which there is a great
+similarity.</p>
+
+<p><b>Family resemblances.</b> Let us now inquire more closely in what respects
+the elements of a family resemble each other.</p>
+
+<p>1. <i>Valence.</i> In general the valence of the elements in a family is the
+same, and the formulas of their compounds are therefore similar. If we
+know that the formula of sodium chloride is NaCl, it is pretty certain
+that the formula of potassium chloride will be KCl&mdash;not KCl<sub>2</sub> or
+<span class='pagenum'><a name="Page_171" id="Page_171">[Pg 171]</a></span>KCl<sub>3</sub>. The general formulas R<sub>2</sub>O, RO, etc., placed below the
+columns show the formulas of the oxides of the elements in the column
+provided they form oxides. In like manner the formulas RH, RH<sub>2</sub>, etc.,
+show the composition of the compounds formed with hydrogen or chlorine.</p>
+
+<p>2. <i>Chemical properties.</i> The chemical properties of the members of a
+family are quite similar. If one member is a metal, the others usually
+are; if one is a non-metal, so, too, are the others. The families in the
+first two columns consist of metals, while the elements found in the
+last two columns form acids. There is in addition a certain regularity
+in properties of the elements in each family. If the element at the head
+of the family is a strong acid-forming element, this property is likely
+to diminish gradually, as we pass to the members of the family with
+higher atomic weights. Thus phosphorus is strongly acid-forming, arsenic
+less so, antimony still less so, while bismuth has almost no
+acid-forming properties. We shall meet with many illustrations of this
+fact.</p>
+
+<p>3. <i>Physical properties.</i> In the same way, the physical properties of
+the members of a family are in general somewhat similar, and show a
+regular gradation as we pass from element to element in the family. Thus
+the densities of the members of the magnesium family are</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Mg = 1.75, Zn = 7.00, Cd = 8.67, Hg = 13.6.<br /></span>
+</div></div>
+
+<p>Their melting points are</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Mg = 750&deg;, Zn = 420&deg;, Cd = 320&deg;, Hg = -39.5&deg;.<br /></span>
+</div></div>
+
+<p><b>Value of the periodic law.</b> The periodic law has proved of much value in
+the development of the science of chemistry.</p>
+
+<p>1. <i>It simplifies study.</i> It is at once evident that such regularities
+very much simplify the study of chemistry.<span class='pagenum'><a name="Page_172" id="Page_172">[Pg 172]</a></span> A thorough study of one
+element of a family makes the study of the other members a much easier
+task, since so many of the properties and chemical reactions of the
+elements are similar. Thus, having studied the element sulphur in some
+detail, it is not necessary to study selenium and tellurium so closely,
+for most of their properties can be predicted from the relation which
+they sustain to sulphur.</p>
+
+<p>2. <i>It predicts new elements.</i> When the periodic law was first
+formulated there were a number of vacant places in the table which
+evidently belonged to elements at that time unknown. From their position
+in the table, Mendel&eacute;eff predicted with great precision the properties
+of the elements which he felt sure would one day be discovered to fill
+these places. Three of them, scandium, germanium, and gallium, were
+found within fifteen years, and their properties agreed in a remarkable
+way with the predictions of Mendel&eacute;eff. There are still some vacant
+places in the table, especially among the heavier elements.</p>
+
+<p>3. <i>It corrects errors.</i> The physical constants of many of the elements
+did not at first agree with those demanded by the periodic law, and a
+further study of many such cases showed that errors had been made. The
+law has therefore done much service in indicating probable error.</p>
+
+<p><b>Imperfections of the law.</b> There still remain a good many features which
+must be regarded as imperfections in the law. Most conspicuous is the
+fact that the element hydrogen has no place in the table. In some of the
+groups elements appear in one of the families, while all of their
+properties show that they belong in the other. Thus sodium belongs with
+lithium and not with copper; fluorine belongs with chlorine and not with
+manganese. There are<span class='pagenum'><a name="Page_173" id="Page_173">[Pg 173]</a></span> two instances where the elements must be
+transposed in order to make them fit into their proper group. According
+to their atomic weights, tellurium should follow iodine, and argon
+should follow potassium. Their properties show in each case that this
+order must be reversed. The table separates some elements altogether
+which, in many respects have closely agreeing properties. Iron,
+chromium, and manganese are all in different groups, although they are
+similar in many respects.</p>
+
+<p>The system is therefore to be regarded as but a partial and imperfect
+expression of some very important and fundamental relation between the
+substances which we know as elements, the exact nature of this relation
+being as yet not completely clear to us.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Suppose that an element were discovered that filled the blank in
+Group O, Period 5; what properties would it probably have?</p>
+
+<p><b>2.</b> Suppose that an element were discovered that filled the blank in
+Group VI, Period 9, family <i>B</i>; what properties would it have?</p>
+
+<p><b>3.</b> Sulphur and oxygen both belong in Group VI, although in different
+families; in what respects are the two similar?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_174" id="Page_174">[Pg 174]</a></span></p>
+<h2><a name="CHAPTER_XVI" id="CHAPTER_XVI"></a>CHAPTER XVI</h2>
+
+<h3>THE CHLORINE FAMILY</h3>
+
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>ATOMIC WEIGHT</td><td align='left'>MELTING POINT</td><td align='left'>BOILING POINT</td><td align='left'>COLOR AND STATE</td></tr>
+<tr><td align='left'>Fluorine (F)</td><td align='center'>19.00</td><td align='center'>-223&deg;</td><td align='center'>-187&deg;</td><td align='center'>Pale yellowish gas.</td></tr>
+<tr><td align='left'>Chlorine (Cl)</td><td align='center'>35.45</td><td align='center'>-102&deg;</td><td align='center'>-33.6&deg;</td><td align='center'>Greenish-yellow gas.</td></tr>
+<tr><td align='left'>Bromine (Br)</td><td align='center'>79.96</td><td align='center'>-7&deg;</td><td align='center'>59&deg;</td><td align='center'>Red liquid.</td></tr>
+<tr><td align='left'>Iodine (I)</td><td align='center'>126.97</td><td align='center'>107&deg;</td><td align='center'>175&deg;</td><td align='center'>Purplish-black solid.</td></tr>
+</table></div>
+
+<p><b>The family.</b> The four elements named in the above table form a strongly
+marked family of elements and illustrate very clearly the way in which
+the members of a family in a periodic group resemble each other, as well
+as the character of the differences which we may expect to find between
+the individual members.</p>
+
+<p>1. <i>Occurrence.</i> These elements do not occur in nature in the free
+state. The compounds of the last three elements of the family are found
+extensively in sea water, and on this account the name <i>halogens</i>,
+signifying "producers of sea salt," is sometimes applied to the family.</p>
+
+<p>2. <i>Properties.</i> As will be seen by reference to the table, the melting
+points and boiling points of the elements of the family increase with
+their atomic weights. A somewhat similar gradation is noted in their
+color and state. One atom of each of the elements combines with one atom
+of hydrogen to form acids, which are gases very soluble in water. The
+affinity of the elements for hydrogen is in<span class='pagenum'><a name="Page_175" id="Page_175">[Pg 175]</a></span> the inverse order of their
+atomic weights, fluorine having the strongest affinity and iodine the
+weakest. Only chlorine and iodine form oxides, and those of the former
+element are very unstable. The elements of the group are univalent in
+their compounds with hydrogen and the metals.</p>
+
+<h3>FLUORINE</h3>
+
+<p><b>Occurrence.</b> The element fluorine occurs in nature most abundantly as the
+mineral fluorspar (CaF<sub>2</sub>), as cryolite (Na<sub>3</sub>AlF<sub>6</sub>), and in the
+complex mineral apatite (3 Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>&middot;CaF<sub>2</sub>).</p>
+
+<p><b>Preparation.</b> All attempts to isolate the element resulted in failure
+until recent years. Methods similar to those which succeed in the
+preparation of the other elements of the family cannot be used; for as
+soon as the fluorine is liberated it combines with the materials of
+which the apparatus is made or with the hydrogen of the water which is
+always present. The preparation of fluorine was finally accomplished by
+the French chemist Moissan by the electrolysis of hydrofluoric acid.
+Perfectly dry hydrofluoric acid (HF) was condensed to a liquid and
+placed in a U-shaped tube made of platinum (or copper), which was
+furnished with electrodes and delivery tubes, as shown in Fig. 52. This
+liquid is not an electrolyte, but becomes such when potassium fluoride
+is dissolved in it. When this solution was electrolyzed hydrogen was set
+free at the cathode and fluorine at the anode.</p>
+
+<div class="figleft" style="width: 248px;">
+<img src="images/imagefig52.jpg" width="248" height="400" alt="Fig. 52" title="" />
+<span class="caption">Fig. 52</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_176" id="Page_176">[Pg 176]</a></span></p>
+<p><b>Properties.</b> Fluorine is a gas of slightly yellowish color, and can be
+condensed to a liquid boiling at -187&deg; under atmospheric pressure. It
+solidifies at -223&deg;. It is extremely active chemically, being the most
+active of all the elements at ordinary temperatures.</p>
+
+<p>It combines with all the common elements save oxygen, very often with
+incandescence and the liberation of much heat. It has a strong affinity
+for hydrogen and is able to withdraw it from its compounds with other
+elements. Because of its great activity it is extremely poisonous.
+Fluorine does not form any oxides, neither does it form any oxygen
+acids, in which respects it differs from the other members of the
+family.</p>
+
+<p><b>Hydrofluoric acid</b> (HF). Hydrofluoric acid is readily obtained from
+fluorspar by the action of concentrated sulphuric acid. The equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaF<sub>2</sub> + H<sub>2</sub>SO<sub>4</sub> = CaSO<sub>4</sub> + 2HF.<br /></span>
+</div></div>
+
+<p>In its physical properties it resembles the binary acids of the other
+elements of this family, being, however, more easily condensed to a
+liquid. The anhydrous acid boils at 19&deg; and can therefore be prepared at
+ordinary pressures. It is soluble in all proportions in water, and a
+concentrated solution&mdash;about 50%&mdash;is prepared for the market. Its fumes
+are exceedingly irritating to the respiratory organs, and several
+chemists have lost their lives by accidentally breathing them.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/moissan.jpg" width="400" height="482" alt="HENRI MOISSAN (French) (1853-1907)
+
+Famous for his work with the electric furnace at high temperatures;
+prepared artificial diamonds, together with many new binary compounds
+such as carbides, silicides, borides, and nitrides; isolated fluorine
+and studied its properties and its compounds very thoroughly" title="" />
+<span class="caption">HENRI MOISSAN (French) (1853-1907)<br /><br />
+
+Famous for his work with the electric furnace at high temperatures;
+prepared artificial diamonds, together with many new binary compounds
+such as carbides, silicides, borides, and nitrides; isolated fluorine
+and studied its properties and its compounds very thoroughly</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_177" id="Page_177">[Pg 177]</a></span></p>
+<p><b>Chemical properties.</b> Hydrofluoric acid, like other strong acids, readily
+acts on bases and metallic oxides and forms the corresponding fluorides.
+It also dissolves certain metals such as silver and copper. It acts very
+vigorously upon organic matter, a single drop of the concentrated acid
+making a sore on the skin which is very painful and slow in healing. Its
+most characteristic property is its action upon silicon dioxide
+(SiO<sub>2</sub>), with which it forms water and the gas silicon tetrafluoride
+(SiF<sub>4</sub>), as shown in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SiO<sub>2</sub> + 4HF = SiF<sub>4</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Glass consists of certain compounds of silicon, which are likewise acted
+on by the acid so that it cannot be kept in glass bottles. It is
+preserved in flasks made of wax or gutta-percha.</p>
+
+<div class="blockquot"><p><b>Etching.</b> Advantage is taken of this reaction in etching designs
+upon glass. The glass vessel is painted over with a protective
+paint upon which the acid will not act, the parts which it is
+desired to make opaque being left unprotected. A mixture of
+fluorspar and sulphuric acid is then painted over the vessel
+and after a few minutes the vessel is washed clean. Wherever
+the hydrofluoric acid comes in contact with the glass it acts
+upon it, destroying its luster and making it opaque, so that
+the exposed design will be etched upon the clear glass. Frosted
+glass globes are often made in this way.</p>
+
+<p>The etching may also be effected by covering the glass with a
+thin layer of paraffin, cutting the design through the wax and
+then exposing the glass to the fumes of the acid.</p></div>
+
+<p><b>Salts of hydrofluoric acid,&mdash;fluorides.</b> A number of the fluorides are
+known, but only one of them, calcium fluoride (CaF<sub>2</sub>), is of
+importance. This is the well-known mineral fluorspar.</p>
+
+
+<h3>CHLORINE</h3>
+
+<p><b>Historical.</b> While studying the action of hydrochloric acid upon the
+mineral pyrolusite, in 1774, Scheele obtained a yellowish, gaseous
+substance to which he gave a name in keeping with the phlogiston theory
+then current. Later it was supposed to be a compound containing oxygen.
+In<span class='pagenum'><a name="Page_178" id="Page_178">[Pg 178]</a></span> 1810, however, the English chemist Sir Humphry Davy proved it to be
+an element and named it chlorine.</p>
+
+<p><b>Occurrence.</b> Chlorine does not occur free in nature, but its compounds
+are widely distributed. For the most part it occurs in combination with
+the metals in the form of chlorides, those of sodium, potassium, and
+magnesium being most abundant. Nearly all salt water contains these
+substances, particularly sodium chloride, and very large salt beds
+consisting of chlorides are found in many parts of the world.</p>
+
+<p><b>Preparation.</b> Two general methods of preparing chlorine may be mentioned,
+namely, the laboratory method and the electrolytic method.</p>
+
+<p>1. <i>Laboratory method.</i> In the laboratory chlorine is made by warming
+the mineral pyrolusite (manganese dioxide, MnO<sub>2</sub>) with concentrated
+hydrochloric acid. The first reaction, which seems to be similar to the
+action of acids upon oxides in general, is expressed in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">MnO<sub>2</sub> + 4HCl = MnCl<sub>4</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The manganese compound so formed is very unstable, however, and breaks
+clown according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">MnCl<sub>4</sub> = MnCl<sub>2</sub> + 2Cl.<br /></span>
+</div></div>
+
+<p>Instead of using hydrochloric acid in the preparation of chlorine it
+will serve just as well to use a mixture of sodium chloride and
+sulphuric acid, since these two react to form hydrochloric acid. The
+following equations will then express the changes:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(1) 2NaCl + H<sub>2</sub>SO<sub>4</sub> = Na<sub>2</sub>SO<sub>4</sub> + 2HCl.<br /></span>
+</div><div class="stanza">
+<span class="i0">(2) MnO<sub>2</sub> + 4 HCl = MnCl<sub>2</sub> + 2Cl + 2H<sub>2</sub>O.<br /></span>
+</div><div class="stanza">
+<span class="i0">(3) MnCl<sub>2</sub> + H<sub>2</sub>SO<sub>4</sub> = MnSO<sub>4</sub> + 2HCl.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_179" id="Page_179">[Pg 179]</a></span></p>
+<p>Combining these equations, the following equation expressing the
+complete reaction is obtained:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaCl + MnO<sub>2</sub> + 2H<sub>2</sub>SO<sub>4</sub> = MnSO<sub>4</sub> + Na<sub>2</sub>SO<sub>4</sub> + 2H<sub>2</sub>O + 2Cl.<br /></span>
+</div></div>
+
+<p>Since the hydrochloric acid liberated in the third equation is free to
+act upon manganese dioxide, it will be seen that all of the chlorine
+originally present in the sodium chloride is set free.</p>
+
+<div class="blockquot"><p>The manganese dioxide and the hydrochloric acid are brought
+together in a flask, as represented in Fig. 53, and a gentle
+heat is applied. The rate of evolution of the gas is regulated
+by the amount of heat applied, and the gas is collected by
+displacement of air. As the equations show, only half of the
+chlorine present in the hydrochloric acid is liberated.</p></div>
+
+<div class="figcenter" style="width: 260px;">
+<img src="images/imagefig53.jpg" width="260" height="400" alt="Fig. 53" title="" />
+<span class="caption">Fig. 53</span>
+</div>
+
+<p>2. <i>Electrolytic method.</i> Under the discussion of electrolysis (p. 102)
+it was shown that when a solution of sodium chloride is electrolyzed
+chlorine is evolved at the anode, while the sodium set free at the
+cathode reacts with the water to form hydrogen, which is evolved, and
+sodium hydroxide, which remains in solution. A great deal of the
+chlorine required in the chemical industries is now made in this way in
+connection with the manufacture of sodium hydroxide.</p>
+
+<p><b>Physical properties.</b> Chlorine is a greenish-yellow gas, which has a
+peculiar suffocating odor and produces a very violent effect upon the
+throat and lungs. Even when inhaled in small quantities it often
+produces all the symptoms of a<span class='pagenum'><a name="Page_180" id="Page_180">[Pg 180]</a></span> hard cold, and in larger quantities may
+have serious and even fatal action. It is quite heavy (density = 2.45)
+and can therefore be collected by displacement of air. One volume of
+water under ordinary conditions dissolves about three volumes of
+chlorine. The gas is readily liquefied, a pressure of six atmospheres
+serving to liquefy it at 0&deg;. It forms a yellowish liquid which
+solidifies at -102&deg;.</p>
+
+<p><b>Chemical properties.</b> At ordinary temperatures chlorine is far more
+active chemically than any of the elements we have so far considered,
+with the exception of fluorine; indeed, it is one of the most active of
+all elements.</p>
+
+<p>1. <i>Action on metals.</i> A great many metals combine directly with
+chlorine, especially when hot. A strip of copper foil heated in a burner
+flame and then dropped into chlorine burns with incandescence. Sodium
+burns brilliantly when heated strongly in slightly moist chlorine. Gold
+and silver are quickly tarnished by the gas.</p>
+
+<p>2. <i>Action on non-metals.</i> Chlorine has likewise a strong affinity for
+many of the non-metals. Thus phosphorus burns in a current of the gas,
+while antimony and arsenic in the form of a fine powder at once burst
+into flame when dropped into jars of the gas. The products formed in all
+cases where chlorine combines with another element are called
+<i>chlorides</i>.</p>
+
+<p>3. <i>Action on hydrogen.</i> Chlorine has a strong affinity for hydrogen,
+uniting with it to form hydrochloric acid. A jet of hydrogen burning in
+the air continues to burn when introduced into a jar of chlorine, giving
+a somewhat luminous flame. A mixture of the two gases explodes violently
+when a spark is passed through it or when it is exposed to bright
+sunlight. In the latter case it is the light and not the heat which
+starts the action.<span class='pagenum'><a name="Page_181" id="Page_181">[Pg 181]</a></span></p>
+
+<p>4. <i>Action on substances containing hydrogen.</i> Not only will chlorine
+combine directly with free hydrogen but it will often abstract the
+element from its compounds. Thus, when chlorine is passed into a
+solution containing hydrosulphuric acid, sulphur is precipitated and
+Hydrochloric acid formed. The reaction is shown by the following
+equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>S + 2Cl = 2HCl + S.<br /></span>
+</div></div>
+
+<p>With ammonia the action is similar:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>3</sub> + 3Cl = 3HCl + N.<br /></span>
+</div></div>
+
+<p>The same tendency is very strikingly seen in the action of chlorine upon
+turpentine. The latter substance is largely made up of compounds having
+the composition represented by the formula C<sub>10</sub>H<sub>16</sub>. When a strip of
+paper moistened with warm turpentine is placed in a jar of chlorine
+dense fumes of hydrochloric acid appear and a black deposit of carbon is
+formed. Even water, which is a very stable compound, can be decomposed
+by chlorine, the oxygen being liberated. This may be shown in the
+following way:</p>
+
+<div class="figright" style="width: 139px;">
+<img src="images/imagefig54.jpg" width="139" height="450" alt="Fig. 54" title="" />
+<span class="caption">Fig. 54</span>
+</div>
+
+<div class="blockquot"><p>If a long tube of rather large diameter is filled with a strong
+solution of chlorine in water and inverted in a vessel of the
+same solution, as shown in Fig. 54, and the apparatus is placed
+in bright sunlight, very soon bubbles of a gas will be observed
+to rise through the solution and collect in the tube. An
+examination of this gas will show that it is oxygen. It is
+liberated from water in accordance with the following equation:</p>
+
+<p>H<sub>2</sub>O + 2Cl = 2HCl + O.</p></div>
+
+<p>5. <i>Action on color substances,&mdash;bleaching action.</i> If strips of
+brightly colored cloth or some highly colored flowers are placed in
+quite dry chlorine, no marked change<span class='pagenum'><a name="Page_182" id="Page_182">[Pg 182]</a></span> in color is noticed as a rule. If,
+however, the cloth and flowers are first moistened, the color rapidly
+disappears, that is, the objects are bleached. Evidently the moisture as
+well as the chlorine is concerned in the action, and a study of the case
+shows that the chlorine has combined with the hydrogen of the water. The
+oxygen set free oxidizes the color substance, converting it into a
+colorless compound. It is evident from this explanation that chlorine
+will only bleach those substances which are changed into colorless
+compounds by oxidation.</p>
+
+<p>6. <i>Action as a disinfectant.</i> Chlorine has also marked germicidal
+properties, and the free element, as well as compounds from which it is
+easily liberated, are used as disinfectants.</p>
+
+<p><b>Nascent state.</b> It will be noticed that oxygen when set free from water
+by chlorine is able to do what ordinary oxygen cannot do, for both the
+cloth and the flowers are unchanged in the air which contains oxygen. It
+is generally true that the activity of an element is greatest at the
+instant of liberation from its compounds. To express this fact elements
+at the instant of liberation are said to be in the <i>nascent state</i>. It
+is nascent oxygen which does the bleaching.</p>
+
+<p><b>Hydrochloric acid</b> (<i>muriatic acid</i>) (HCl). The preparation of
+hydrochloric acid may be discussed under two general heads:</p>
+
+<p>1. <i>Laboratory preparation.</i> The product formed by the burning of
+hydrogen in chlorine is the gas hydrochloric acid. This substance is
+much more easily obtained, however, by treating common salt (sodium
+chloride) with sulphuric acid. The following equation shows the
+reaction:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaCl + H<sub>2</sub>SO<sub>4</sub> = Na<sub>2</sub>SO<sub>4</sub> + 2HCl.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_183" id="Page_183">[Pg 183]</a></span></p>
+<p>The dry salt is placed in a flask furnished with a funnel tube and an
+exit tube, the sulphuric acid is added, and the flask gently warmed. The
+hydrochloric acid gas is rapidly given off and can be collected by
+displacement of air. The same apparatus can be used as was employed in
+the preparation of chlorine (Fig. 53).</p>
+
+<div class="blockquot"><p>When a <i>solution</i> of salt is treated with sulphuric acid there
+is no very marked action. The hydrochloric acid formed is very
+soluble in water, and so does not escape from the solution;
+hence a state of equilibrium is soon reached between the four
+substances represented in the equation. When <i>concentrated</i>
+sulphuric acid, in which hydrochloric acid is not soluble, is
+poured upon dry salt the reaction is complete.</p></div>
+
+<p>2. <i>Commercial preparation.</i> Commercially, hydrochloric acid is prepared
+in connection with the manufacture of sodium sulphate, the reaction
+being the same as that just given. The reaction is carried out in a
+furnace, and the hydrochloric acid as it escapes in the form of gas is
+passed into water in which it dissolves, the solution forming the
+hydrochloric acid of commerce. When the materials are pure a colorless
+solution is obtained. The most concentrated solution has a density of
+1.2 and contains 40% HCl. The commercial acid, often called <i>muriatic
+acid</i>, is usually colored yellow by impurities.</p>
+
+<p><b>Composition of hydrochloric acid.</b> When a solution of hydrochloric acid
+is electrolyzed in an apparatus similar to the one in which water was
+electrolyzed (Fig. 18), chlorine collects at the anode and hydrogen at
+the cathode. At first the chlorine dissolves in the water, but soon the
+water in the one tube becomes saturated with it, and if the stopcocks
+are left open until this is the case, and are then closed, it will be
+seen that the two gases are set free in equal volumes.<span class='pagenum'><a name="Page_184" id="Page_184">[Pg 184]</a></span></p>
+
+<p>When measured volumes of the two gases are caused to unite it is found
+that one volume of hydrogen combines with one of chlorine. Other
+experiments show that the volume of hydrochloric acid formed is just
+equal to the sum of the volumes of hydrogen and chlorine. Therefore one
+volume of hydrogen combines with one volume of chlorine to form two
+volumes of hydrochloric acid gas. Since chlorine is 35.18 times as heavy
+as hydrogen, it follows that one part of hydrogen by weight combines
+with 35.18 parts of chlorine to form 36.18 parts of hydrochloric acid.</p>
+
+<p><b>Physical properties.</b> Hydrochloric acid is a colorless gas which has an
+irritating effect when inhaled, and possesses a sour, biting taste, but
+no marked odor. It is heavier than air (density = 1.26) and is very
+soluble in water. Under standard conditions 1 volume of water dissolves
+about 500 volumes of the gas. On warming such a solution the gas
+escapes, until at the boiling point the solution contains about 20% by
+weight of HCl. Further boiling will not drive out any more acid, but the
+solution will distill with unchanged concentration. A more dilute
+solution than this will lose water on boiling until it has reached the
+same concentration, 20%, and will then distill unchanged. Under high
+pressure the gas can be liquefied, 28 atmospheres being required at 0&deg;.
+Under these conditions it forms a colorless liquid which is not very
+active chemically. It boils at -80&deg; and solidifies at -113&deg;. The
+solution of the gas in water is used almost entirely in the place of the
+gas itself, since it is not only far more convenient but also more
+active.</p>
+
+<p><b>Chemical properties.</b> The most important chemical properties of
+hydrochloric acid are the following:</p>
+
+<p>1. <i>Action as an acid.</i> In aqueous solution hydrochloric acid has very
+strong acid properties; indeed, it is one of<span class='pagenum'><a name="Page_185" id="Page_185">[Pg 185]</a></span> the strongest acids. It
+acts upon oxides and hydroxides, converting them into salts:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaOH + HCl = NaCl + H<sub>2</sub>O,<br /></span>
+<span class="i0">CuO + 2HCl = CuCl<sub>2</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>It acts upon many metals, forming chlorides and liberating hydrogen:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Zn + 2HCl = ZnCl<sub>2</sub> + 2H,<br /></span>
+<span class="i0">Al + 3HCl = AlCl<sub>3</sub> + 3H.<br /></span>
+</div></div>
+
+<p>Unlike nitric and sulphuric acids it has no oxidizing action, so that
+when it acts on metals hydrogen is always given off.</p>
+
+<p>2. <i>Relation to combustion.</i> Hydrochloric acid gas is not readily
+decomposed, and is therefore neither combustible nor a supporter of
+combustion.</p>
+
+<p>3. <i>Action on oxidizing agents.</i> Although hydrochloric acid is
+incombustible, it can be oxidized under some circumstances, in which
+case the hydrogen combines with oxygen, while the chlorine is set free.
+Thus, when a solution of hydrochloric acid acts upon manganese dioxide
+part of the chlorine is set free:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">MnO<sub>2</sub> + 4HCl = MnCl<sub>2</sub> + 2H<sub>2</sub>O + 2Cl.<br /></span>
+</div></div>
+
+<p><b>Aqua regia.</b> It has been seen that when nitric acid acts as an oxidizing
+agent it usually decomposes, as represented in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HNO<sub>3</sub> = H<sub>2</sub>O + 2NO + 3O.<br /></span>
+</div></div>
+
+<p>The oxygen so set free may act on hydrochloric acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">6HCl + 3O = 3H<sub>2</sub>O + 6Cl.<br /></span>
+</div></div>
+
+<p>The complete equation therefore is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HNO<sub>3</sub> + 6HCl = 4H<sub>2</sub>O + 2NO + 6Cl.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_186" id="Page_186">[Pg 186]</a></span></p>
+<p>When concentrated nitric and hydrochloric acids are mixed this reaction
+goes on slowly, chlorine and some other substances not represented in
+the equation being formed. The mixture is known as <i>aqua regia</i> and is
+commonly prepared by adding one volume of nitric acid to three volumes
+of hydrochloric acid. It acts more powerfully upon metals and other
+substances than either of the acids separately, and owes its strength
+not to acid properties but to the action of the nascent chlorine which
+it liberates. Consequently, when it acts upon metals such as gold it
+converts them into chlorides, and the reaction can be represented by
+such equations as</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Au + 3Cl = AuCl<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><b>Salts of hydrochloric acid,&mdash;chlorides.</b> The chlorides of all the metals
+are known and many of them are very important compounds. Some of them
+are found in nature, and all can be prepared by the general method of
+preparing salts. Silver chloride, lead chloride, and mercurous chloride
+are insoluble in water and acids, and can be prepared by adding
+hydrochloric acid to solutions of compounds of the respective elements.
+While the chlorides have formulas similar to the fluorides, their
+properties are often quite different. This is seen in the solubility of
+the salts. Those metals whose chlorides are insoluble form soluble
+fluorides, while many of the metals which form soluble chlorides form
+insoluble fluorides.</p>
+
+<p><b>Compounds of chlorine with oxygen and hydrogen.</b> Chlorine combines with
+oxygen and hydrogen to form four different acids. They are all quite
+unstable, and most of them cannot be prepared in pure form; their salts
+can easily be made, however, and some of them will be met with in the<span class='pagenum'><a name="Page_187" id="Page_187">[Pg 187]</a></span>
+study of the metals. The formulas and names of these acids are as
+follows:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>HClO</td><td align='left'>hypochlorous acid.</td></tr>
+<tr><td align='left'>HClO<sub>2</sub></td><td align='left'>chlorous acid.</td></tr>
+<tr><td align='left'>HClO<sub>3</sub></td><td align='left'>chloric acid.</td></tr>
+<tr><td align='left'>HClO<sub>4</sub></td><td align='left'>perchloric acid.</td></tr>
+</table></div>
+
+<p><b>Oxides of chlorine.</b> Two oxides are known, having the formulas Cl<sub>2</sub>O
+and ClO<sub>2</sub>. They decompose very easily and are good oxidizing agents.</p>
+
+
+<h3>BROMINE</h3>
+
+<p><b>Historical.</b> Bromine was discovered in 1826 by the French chemist
+Ballard, who isolated it from sea salt. He named it bromine (stench)
+because of its unbearable fumes.</p>
+
+<p><b>Occurrence.</b> Bromine occurs almost entirely in the form of bromides,
+especially as sodium bromide and magnesium bromide, which are found in
+many salt springs and salt deposits. The Stassfurt deposits in Germany
+and the salt waters of Ohio and Michigan are especially rich in
+bromides.</p>
+
+<p><b>Preparation of bromine.</b> The laboratory method of preparing bromine is
+essentially different from the commercial method.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig55.jpg" width="400" height="400" alt="Fig. 55" title="" />
+<span class="caption">Fig. 55</span>
+</div>
+
+<p>1. <i>Laboratory method.</i> As in the case of chlorine, bromine can be
+prepared by the action of hydrobromic acid (HBr) on manganese dioxide.
+Since hydrobromic acid is not an article of commerce, a mixture of
+sulphuric acid<span class='pagenum'><a name="Page_188" id="Page_188">[Pg 188]</a></span> and a bromide is commonly substituted for it. The
+materials are placed in a retort arranged as shown in Fig. 55. The end
+of the retort just touches the surface of the water in the test tube. On
+heating, the bromine distills over and is collected in the cold
+receiver. The equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaBr + 2H<sub>2</sub>SO<sub>4</sub> + MnO<sub>2</sub> = Na<sub>2</sub>SO<sub>4</sub> + MnSO<sub>4</sub>
++ 2H<sub>2</sub>O + 2Br.<br /></span>
+</div></div>
+
+<p>2. <i>Commercial method.</i> Bromine is prepared commercially from the waters
+of salt wells which are especially rich in bromides. On passing a
+current of electricity through such waters the bromine is first
+liberated. Any chlorine liberated, however, will assist in the reaction,
+since free chlorine decomposes bromides, as shown in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaBr + Cl = NaCl + Br.<br /></span>
+</div></div>
+
+<p>When the water containing the bromine is heated, the liberated bromine
+distills over into the receiver.</p>
+
+<p><b>Physical properties.</b> Bromine is a dark red liquid about three times as
+heavy as water. Its vapor has a very offensive odor and is most
+irritating to the eyes and throat. The liquid boils at 59&deg; and
+solidifies at -7&deg;; but even at ordinary temperatures it evaporates
+rapidly, forming a reddish-brown gas very similar to nitrogen peroxide
+in appearance. Bromine is somewhat soluble in water, 100 volumes of
+water under ordinary conditions dissolving 1 volume of the liquid. It is
+readily soluble in carbon disulphide, forming a yellow solution.</p>
+
+<p><b>Chemical properties and uses.</b> In chemical action bromine is very similar
+to chlorine. It combines directly with many of the same elements with
+which chlorine unites, but with less energy. It combines with hydrogen
+and takes away<span class='pagenum'><a name="Page_189" id="Page_189">[Pg 189]</a></span> the latter element from some of its compounds, but not
+so readily as does chlorine. Its bleaching properties are also less
+marked.</p>
+
+<p>Bromine finds many uses in the manufacture of organic drugs and
+dyestuffs and in the preparation of bromides.</p>
+
+<p><b>Hydrobromic acid (HBr).</b> When sulphuric acid acts upon a bromide
+hydrobromic acid is set free:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaBr + H<sub>2</sub>SO<sub>4</sub> = Na<sub>2</sub>SO<sub>4</sub> + 2HBr.<br /></span>
+</div></div>
+
+<p>At the same time some bromine is set free, as may be seen from the red
+fumes which appear, and from the odor. The explanation of this is found
+in the fact that hydrobromic acid is much less stable than hydrochloric
+acid, and is therefore more easily oxidized. Concentrated sulphuric acid
+is a good oxidizing agent, and oxidizes a part of the hydrobromic acid,
+liberating bromine:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>SO<sub>4</sub> + 2HBr = 2H<sub>2</sub>O + SO<sub>2</sub> + 2Br.<br /></span>
+</div></div>
+
+<div class="blockquot"><p><b>Preparation of pure hydrobromic acid.</b> A convenient way to make
+pure hydrobromic acid is by the action of bromine upon moist
+red phosphorus. This can be done with the apparatus shown in
+Fig. 56. Bromine is put into the dropping funnel <i>A</i>, and red
+phosphorus, together with enough water to cover it, is placed
+in the flask <i>B</i>. By means of the stopcock the bromine is
+allowed to flow drop by drop into the flask, the reaction
+taking place without the application of heat. The equations are</p></div>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(1) P + 3Br = PBr<sub>3</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">(2) PBr<sub>3</sub> + 3H<sub>2</sub>O = P(OH)<sub>3</sub> + 3HBr.<br /></span>
+</div></div>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig56.jpg" width="400" height="304" alt="Fig. 56" title="" />
+<span class="caption">Fig. 56</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_190" id="Page_190">[Pg 190]</a></span></p>
+
+<div class="blockquot"><p>The U-tube <i>C</i> contains glass beads which have been moistened
+with water and rubbed in red phosphorus. Any bromine escaping
+action in the flask acts upon the phosphorus in the U-tube. The
+hydrobromic acid is collected in the same way as hydrochloric
+acid.</p></div>
+
+<p><b>Properties.</b> Hydrobromic acid very strikingly resembles hydrochloric acid
+in physical and chemical properties. It is a colorless, strongly fuming
+gas, heavier than hydrochloric acid and, like it, is very soluble in
+water. Under standard conditions 1 volume of water dissolves 610 volumes
+of the gas. Chemically, the chief point in which it differs from
+hydrochloric acid is in the fact that it is much more easily oxidized,
+so that bromine is more readily set free from it than chlorine is from
+hydrochloric acid.</p>
+
+<p><b>Salts of hydrobromic acid,&mdash;bromides.</b> The bromides are very similar to
+the chlorides in their properties. Chlorine acts upon both bromides and
+free hydrobromic acid, liberating bromine from them:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">KBr + Cl = KCl + Br,<br /></span>
+</div><div class="stanza">
+<span class="i0">HBr + Cl = HCl + Br.<br /></span>
+</div></div>
+
+<p>Silver bromide is extensively used in photography, and the bromides of
+sodium and potassium are used as drugs.</p>
+
+<div class="blockquot"><p><b>Oxygen compounds.</b> No oxides of bromine are surely known, and
+bromine does not form so many oxygen acids as chlorine does.
+Salts of hypobromous acid (HBrO) and bromic acid (HBrO<sub>3</sub>) are
+known.</p></div>
+
+
+<h3>IODINE</h3>
+
+<p><b>Historical.</b> Iodine was discovered in 1812 by Courtois in the ashes of
+certain sea plants. Its presence was revealed by its beautiful violet
+vapor, and this suggested the name iodine (from the Greek for violet
+appearance).</p>
+
+<p><b>Occurrence.</b> In the combined state iodine occurs in very small quantities
+in sea water, from which it is absorbed by<span class='pagenum'><a name="Page_191" id="Page_191">[Pg 191]</a></span> certain sea plants, so that
+it is found in their ashes. It occurs along with bromine in salt springs
+and beds, and is also found in Chili saltpeter.</p>
+
+<p><b>Preparation.</b> Iodine may be prepared in a number of ways, the principal
+methods being the following:</p>
+
+<p>1. <i>Laboratory method.</i> Iodine can readily be prepared in the laboratory
+from an iodide by the method used in preparing bromine, except that
+sodium iodide is substituted for sodium bromide. It can also be made by
+passing chlorine into a solution of an iodide.</p>
+
+<div class="figright" style="width: 368px;">
+<img src="images/imagefig57.jpg" width="368" height="400" alt="Fig. 57" title="" />
+<span class="caption">Fig. 57</span>
+</div>
+
+<p>2. <i>Commercial method.</i> Commercially iodine was formerly prepared from
+seaweed (kelp), but is now obtained almost entirely from the deposits of
+Chili saltpeter. The crude saltpeter is dissolved in water and the
+solution evaporated until the saltpeter crystallizes. The remaining
+liquors, known as the "mother liquors," contain sodium iodate
+(NaIO<sub>3</sub>), in which form the iodine is present in the saltpeter. The
+chemical reaction by which the iodine is liberated from this compound is
+a complicated one, depending on the fact that sulphurous acid acts upon
+iodic acid, setting iodine free. This reaction is shown as follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HIO<sub>3</sub> + 5H<sub>2</sub>SO<sub>3</sub> = 5H<sub>2</sub>SO<sub>4</sub> + H<sub>2</sub>O + 2I.<br /></span>
+</div></div>
+
+<div class="blockquot"><p><b>Purification of iodine.</b> Iodine can be purified very
+conveniently in the following way. The crude iodine is placed
+in an evaporating dish <i>E</i> (Fig. 57), and the dish is set upon
+the sand bath <i>S</i>. The iodine is covered with the inverted
+funnel <i>F</i>, and the sand bath is<span class='pagenum'><a name="Page_192" id="Page_192">[Pg 192]</a></span> gently heated with a Bunsen
+burner. As the dish becomes warm the iodine rapidly evaporates
+and condenses again on the cold surface of the funnel in
+shining crystals.</p>
+
+<p>This process, in which a solid is converted into a vapor and is
+again condensed into a solid without passing through the liquid
+state, is called <i>sublimation</i>.</p></div>
+
+<p><b>Physical properties.</b> Iodine is a purplish-black, shining, heavy solid
+which crystallizes in brilliant plates. Even at ordinary temperatures it
+gives off a beautiful violet vapor, which increases in amount as heat is
+applied. It melts at 107&deg; and boils at 175&deg;. It is slightly soluble in
+water, but readily dissolves in alcohol, forming a brown solution
+(tincture of iodine), and in carbon disulphide, forming a violet
+solution. The element has a strong, unpleasant odor, though by no means
+as irritating as that of chlorine and bromine.</p>
+
+<p><b>Chemical properties.</b> Chemically iodine is quite similar to chlorine and
+bromine, but is still less active than bromine. It combines directly
+with many elements at ordinary temperatures. At elevated temperatures it
+combines with hydrogen, but the reaction is reversible and the compound
+formed is quite easily decomposed. Both chlorine and bromine displace it
+from its salts:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">KI + Br = KBr + I,<br /></span>
+</div><div class="stanza">
+<span class="i0">KI + Cl = KCl + I.<br /></span>
+</div></div>
+
+<p>When even minute traces of iodine are added to thin starch paste a very
+intense blue color develops, and this reaction forms a delicate test for
+iodine. Iodine is extensively used in medicine, especially in the form
+of a tincture. It is also largely used in the preparation of dyes and
+organic drugs, iodoform, a substance used as an antiseptic, has the
+<span class='pagenum'><a name="Page_193" id="Page_193">[Pg 193]</a></span>formula CHI<sub>3</sub>.</p>
+
+<p><b>Hydriodic acid (HI).</b> This acid cannot be prepared in pure condition by
+the action of sulphuric acid upon an iodide, since the hydriodic acid
+set free is oxidized by the sulphuric acid just as in the case of
+hydrobromic acid, but to a much greater extent. It can be prepared in
+exactly the same way as hydrobromic acid, iodine being substituted for
+bromine. It can also be prepared by passing hydrosulphuric acid into
+water in which iodine is suspended. The equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>S + 2I = 2HI + S.<br /></span>
+</div></div>
+
+<p>The hydriodic acid formed in this way dissolves in the water.</p>
+
+<p><b>Properties and uses.</b> Hydriodic acid resembles the corresponding acids of
+chlorine and bromine in physical properties, being a strongly fuming,
+colorless gas, readily soluble in water. Under standard conditions 1
+volume of water dissolves about 460 volumes of the gas. It is, however,
+more unstable than either hydrochloric or hydrobromic acids, and on
+exposure to the air it gradually decomposes in accordance with the
+equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HI + O = H<sub>2</sub>O + 2I.<br /></span>
+</div></div>
+
+<p>Owing to the slight affinity between iodine and hydrogen the acid easily
+gives up its hydrogen and is therefore a strong reducing agent. This is
+seen in its action on sulphuric acid.</p>
+
+<p>The salts of hydriodic acid, the iodides, are, in general, similar to
+the chlorides and bromides. Potassium iodide (KI) is the most familiar
+of the iodides and is largely used in medicine.</p>
+
+<div class="blockquot"><p><b>Oxygen compounds.</b> Iodine has a much greater affinity for oxygen
+than has either chlorine or bromine. When heated with nitric
+<span class='pagenum'><a name="Page_194" id="Page_194">[Pg 194]</a></span>acid it forms a stable oxide (I<sub>2</sub>O<sub>5</sub>). Salts of iodic acid
+(HIO<sub>3</sub>) and periodic acid (HIO<sub>4</sub>) are easily prepared, and
+the free acids are much more stable than the corresponding
+acids of the other members of this family.</p></div>
+
+
+<h3>GAY-LUSSAC'S LAW OF VOLUMES</h3>
+
+<p>In the discussion of the composition of hydrochloric acid it was stated
+that one volume of hydrogen combines with one volume of chlorine to form
+two volumes of hydrochloric acid. With bromine and iodine similar
+combining ratios hold good. These facts recall the simple volume
+relations already noted in the study of the composition of steam and
+ammonia. These relations may be represented graphically in the following
+way:</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/graph.jpg" width="500" height="121" alt="Graph" title="" />
+<span class="caption">Graph</span>
+</div>
+
+<p>In the early part of the past century Gay-Lussac, a distinguished French
+chemist, studied the volume relations of many combining gases, and
+concluded that similar relations always hold. His observations are
+summed up in the following law: <i>When two gases combine chemically there
+is always a simple ratio between their volumes, and between the volume
+of either one of them and that of the product, provided it is a gas.</i> By
+a simple ratio is meant of course the ratio of small whole numbers, as
+1 : 2, 2 : 3.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> How do we account for the fact that liquid hydrofluoric acid is not
+an electrolyte?</p>
+
+<p><b>2.</b> Why does sulphuric acid liberate hydrofluoric acid from its salts?<span class='pagenum'><a name="Page_195" id="Page_195">[Pg 195]</a></span></p>
+
+<p><b>3.</b> In the preparation of chlorine, what advantages are there in treating
+manganese dioxide with a mixture of sodium chloride and sulphuric acid
+rather than with hydrochloric acid?</p>
+
+<p><b>4.</b> Why must chlorine water be kept in the dark?</p>
+
+<p><b>5.</b> What is the derivation of the word nascent?</p>
+
+<p><b>6.</b> What substances studied are used as bleaching agents? To what is the
+bleaching action due in each case?</p>
+
+<p><b>7.</b> What substances studied are used as disinfecting agents?</p>
+
+<p><b>8.</b> What is meant by the statement that hydrochloric acid is one of the
+strongest acids?</p>
+
+<p><b>9.</b> What is the meaning of the phrase <i>aqua regia</i>?</p>
+
+<p><b>10.</b> Cl<sub>2</sub>O is the anhydride of what acid?</p>
+
+<p><b>11.</b> A solution of hydriodic acid on standing turns brown. How is this
+accounted for?</p>
+
+<p><b>12.</b> How can bromine vapor and nitrogen peroxide be distinguished from
+each other?</p>
+
+<p><b>13.</b> Write the equations for the reaction taking place when hydriodic
+acid is prepared from iodine, phosphorus, and water.</p>
+
+<p><b>14.</b> From their behavior toward sulphuric acid, to what class of agents
+do hydrobromic and hydriodic acids belong?</p>
+
+<p><b>15.</b> Give the derivation of the names of the elements of the chlorine
+family.</p>
+
+<p><b>16.</b> Write the names and formulas for the binary acids of the group in
+the order of the stability of the acids.</p>
+
+<p><b>17.</b> What is formed when a metal dissolves in each of the following?
+nitric acid; dilute sulphuric acid; concentrated sulphuric acid;
+hydrochloric acid; aqua regia.</p>
+
+<p><b>18.</b> How could you distinguish between a chloride, a bromide, and an
+iodide?</p>
+
+<p><b>19.</b> What weight of sodium chloride is necessary to prepare sufficient
+hydrochloric acid to saturate 1 l. of water under standard conditions?</p>
+
+<p><b>20.</b> On decomposition 100 l. of hydrochloric acid would yield how many
+liters of hydrogen and chlorine respectively, the gases being measured
+under the same conditions? Are your results in accord with the
+experimental facts?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_196" id="Page_196">[Pg 196]</a></span></p>
+<h2><a name="CHAPTER_XVII" id="CHAPTER_XVII"></a>CHAPTER XVII</h2>
+
+<h3>CARBON AND SOME OF ITS SIMPLER COMPOUNDS</h3>
+
+
+<p><b>The family.</b> Carbon stands at the head of a family of elements in the
+fourth group in the periodic table. The resemblances between the
+elements of this family, while quite marked, are not so striking as in
+the case of the elements of the chlorine family. With the exception of
+carbon, these elements are comparatively rare, and need not be taken up
+in detail in this chapter. Titanium will be referred to again in
+connection with silicon which it very closely resembles.</p>
+
+<p><b>Occurrence.</b> Carbon is found in nature in the uncombined state in several
+forms. The diamond is practically pure carbon, while graphite and coal
+are largely carbon, but contain small amounts of other substances. Its
+natural compounds are exceedingly numerous and occur as gases, liquids,
+and solids. Carbon dioxide is its most familiar gaseous compound.
+Natural gas and petroleum are largely compounds of carbon with hydrogen.
+The carbonates, especially calcium carbonate, constitute great strata of
+rocks, and are found in almost every locality. All living organisms,
+both plant and animal, contain a large percentage of this element, and
+the number of its compounds which go to make up all the vast variety of
+animate nature is almost limitless. Over one hundred thousand definite
+compounds containing carbon have been prepared. In the free state carbon
+occurs in three allotropic forms, two of which are crystalline and one
+amorphous.<span class='pagenum'><a name="Page_197" id="Page_197">[Pg 197]</a></span></p>
+
+<p><b>Crystalline carbon.</b> Crystalline carbon occurs in two forms,&mdash;diamond and
+graphite.</p>
+
+<p>1. <i>Diamond.</i> Diamonds are found in considerable quantities in several
+localities, especially in South Africa, the East Indies, and Brazil. The
+crystals belong to the regular system, but the natural stones do not
+show this very clearly. When found they are usually covered with a rough
+coating which is removed in the process of cutting. Diamond cutting is
+carried on most extensively in Holland.</p>
+
+<p>The density of the diamond is 3.5, and, though brittle, it is one of the
+hardest of substances. Black diamonds, as well as broken and imperfect
+stones which are valueless as gems, are used for grinding hard
+substances. Few chemical reagents have any action on the diamond, but
+when heated in oxygen or the air it blackens and burns, forming carbon
+dioxide.</p>
+
+<p>Lavoisier first showed that carbon dioxide is formed by the combustion
+of the diamond; and Sir Humphry Davy in 1814 showed that this is the
+only product of combustion, and that the diamond is pure carbon.</p>
+
+<div class="blockquot"><p><b>The diamond as a gem.</b> The pure diamond is perfectly transparent
+and colorless, but many are tinted a variety of colors by
+traces of foreign substances. Usually the colorless ones are
+the most highly prized, although in some instances the color
+adds to the value; thus the famous Hope diamond is a beautiful
+blue. Light passing through a diamond is very much refracted,
+and to this fact the stone owes its brilliancy and sparkle.</p>
+
+<p><b>Artificial preparation of diamonds.</b> Many attempts have been
+made to produce diamonds artificially, but for a long time
+these always ended in failure, graphite and not diamonds being
+the product obtained. The French chemist Moissan, in his
+extended study of chemistry at high temperatures, finally
+succeeded (1893) in making some small ones. He accomplished
+this by dissolving carbon in boiling iron and plunging the
+crucible containing the mixture into water,<span class='pagenum'><a name="Page_198" id="Page_198">[Pg 198]</a></span> as shown in Fig.
+58. Under these conditions the carbon crystallized in the iron
+in the form of the diamond. The diamonds were then obtained by
+dissolving away the iron in hydrochloric acid.</p></div>
+
+<div class="figcenter" style="width: 377px;">
+<img src="images/imagefig58.jpg" width="377" height="400" alt="Fig. 58" title="" />
+<span class="caption">Fig. 58</span>
+</div>
+
+<p>2. <i>Graphite.</i> This form of carbon is found in large quantities,
+especially in Ceylon, Siberia, and in some localities of the United
+States and Canada. It is a shining black substance, very soft and greasy
+to the touch. Its density is about 2.15. It varies somewhat in
+properties according to the locality in which it is found, and is more
+easily attacked by reagents than is the diamond. It is also manufactured
+by heating carbon with a small amount of iron (3%) in an electric
+furnace. It is used in the manufacture of lead pencils and crucibles, as
+a lubricant, and as a protective covering for iron in the form of a
+polish or a paint.</p>
+
+<p><b>Amorphous carbon.</b> Although there are many varieties of amorphous carbon
+known, they are not true allotropic modifications. They differ merely in
+their degree of purity, their fineness of division, and in their mode of
+preparation. These substances are of the greatest importance, owing to
+their many uses in the arts and industries. As they occur in nature, or
+are made artificially, they are nearly all impure carbon, the impurity
+depending on the particular substance in question.</p>
+
+<p>1. <i>Pure carbon.</i> Pure amorphous carbon is best prepared by charring
+sugar. This is a substance consisting of carbon, hydrogen, and oxygen,
+the latter two elements being present in the ratio of one oxygen atom to
+two of hydrogen.<span class='pagenum'><a name="Page_199" id="Page_199">[Pg 199]</a></span> When sugar is strongly heated the oxygen and hydrogen
+are driven off in the form of water and pure carbon is left behind.
+Prepared in this way it is a soft, lustrous, very bulky, black powder.</p>
+
+<p>2. <i>Coal and coke.</i> Coals of various kinds were probably formed from
+vast accumulations of vegetable matter in former ages, which became
+covered over with earthy material and were thus protected from rapid
+decay. Under various natural agencies the organic matter was slowly
+changed into coal. In anthracite these changes have gone the farthest,
+and this variety of coal is nearly pure carbon. Soft or bituminous coals
+contain considerable organic matter besides carbon and mineral
+substances. When heated strongly out of contact with air the organic
+matter is decomposed and the resulting volatile matter is driven off in
+the form of gases and vapors, and only the mineral matter and carbon
+remain behind. The gaseous product is chiefly illuminating gas and the
+solid residue is <i>coke</i>. Some of the coke is found as a dense cake on
+the sides and roof of the retort. This is called retort carbon and is
+quite pure.</p>
+
+<p>3. <i>Charcoal.</i> This is prepared from wood in the same way that coke is
+made from coal. When the process is carried on in retorts the products
+expelled by the heat are saved. Among these are many valuable substances
+such as wood alcohol and acetic acid. Where timber is abundant the
+process is carried out in a wasteful way, by merely covering piles of
+wood with sod and setting the wood on fire. Some wood burns and the heat
+from this decomposes the wood not burned, forming charcoal from it. The
+charcoal, of course, contains the mineral part of the wood from which it
+is formed.<span class='pagenum'><a name="Page_200" id="Page_200">[Pg 200]</a></span></p>
+
+<p>4. <i>Bone black.</i> This is sometimes called animal charcoal, and is made
+by charring bones and animal refuse. The organic part of the materials
+is thus decomposed and carbon is left in a very finely divided state,
+scattered through the mineral part which consists largely of calcium
+phosphate. For some uses this mineral part is removed by treatment with
+hydrochloric acid and prolonged washing.</p>
+
+<p>5. <i>Lampblack.</i> Lampblack and soot are products of imperfect combustion
+of oil and coal, and are deposited from a smoky flame on a cold surface.
+The carbon in this form is very finely divided and usually contains
+various oily materials.</p>
+
+<p><b>Properties.</b> While the various forms of carbon differ in many properties,
+especially in color and hardness, yet they are all odorless, tasteless
+solids, insoluble in water and characterized by their stability towards
+heat. Only in the intense heat of the electric arc does carbon
+volatilize, passing directly from the solid state into a vapor. Owing to
+this fact the inside surface of an incandescent light bulb after being
+used for some time becomes coated with a dark film of carbon. It is not
+acted on at ordinary temperatures by most reagents, but at a higher
+temperature it combines directly with many of the elements, forming
+compounds called <i>carbides</i>. When heated in the presence of sufficient
+oxygen it burns, forming carbon dioxide.</p>
+
+<p><b>Uses of carbon.</b> The chief use of amorphous carbon is for fuel to furnish
+heat and power for all the uses of civilization. An enormous quantity of
+carbon in the form of the purer coals, coke, and charcoal is used as a
+reducing agent in the manufacture of the various metals, especially in
+the metallurgy of iron. Most of the metals are found in nature as
+oxides, or in forms which can readily be<span class='pagenum'><a name="Page_201" id="Page_201">[Pg 201]</a></span> converted into oxides. When
+these oxides are heated with carbon the oxygen is abstracted, leaving
+the metal. Retort carbon and coke are used to make electric light
+carbons and battery plates, while lampblack is used for indelible inks,
+printer's ink, and black varnishes. Bone black and charcoal have the
+property of absorbing large volumes of certain gases, as well as smaller
+amounts of organic matter; hence they are used in filters to remove
+noxious gases and objectionable colors and odors from water. Bone black
+is used extensively in the sugar refineries to remove coloring matter
+from the impure sugars.</p>
+
+<p><b>Chemistry of carbon compounds.</b> Carbon is remarkable for the very large
+number of compounds which it forms with the other elements, especially
+with oxygen and hydrogen. Compounds containing carbon are more numerous
+than all others put together, and the chemistry of these substances
+presents peculiarities not met with in the study of other substances.
+For these reasons the systematic study of carbon compounds, or of
+<i>organic chemistry</i>as it is usually called, must be deferred until the
+student has gained some knowledge of the chemistry of other elements. An
+acquaintance with a few of the most familiar carbon compounds is,
+however, essential for the understanding of the general principles of
+chemistry.</p>
+
+<p><b>Compounds of carbon with hydrogen,&mdash;the hydrocarbons.</b> Carbon unites with
+hydrogen to form a very large number of compounds called <i>hydrocarbons</i>.
+Petroleum and natural gas are essentially mixtures of a great variety of
+these hydrocarbons. Many others are found in living plants, and still
+others are produced by the decay of organic matter in the absence of
+air. Only two of them, methane and acetylene, will be discussed here.<span class='pagenum'><a name="Page_202" id="Page_202">[Pg 202]</a></span></p>
+
+<p><b>Methane</b> (<i>marsh gas</i>) (CH<sub>4</sub>). This is one of the most important of
+these hydrocarbons, and constitutes about nine tenths of natural gas. As
+its name suggests, it is formed in marshes by the decay of vegetable
+matter under water, and bubbles of the gas are often seen to rise when
+the dead leaves on the bottom of pools are stirred. It also collects in
+mines, and, when mixed with air, is called <i>fire damp</i> by the miners
+because of its great inflammability, damp being an old name for a gas.
+It is formed when organic matter, such as coal or wood, is heated in
+closed vessels, and is therefore a principal constituent of coal gas.</p>
+
+<p><b>Preparation.</b> Methane is prepared in the laboratory by heating sodium or
+calcium acetate with soda-lime. Equal weights of fused sodium acetate
+and soda-lime are thoroughly dried, then mixed and placed in a
+good-sized, hard-glass test tube fitted with a one-holed stopper and
+delivery tube. The mixture is gradually heated, and when the air has
+been displaced from the tube the gas is collected in bottles by
+displacement of water. Soda-lime is a mixture of sodium and calcium
+hydroxides. Regarding it as sodium hydroxide alone, the equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaC<sub>2</sub>H<sub>3</sub>O<sub>2</sub> + NaOH = Na<sub>2</sub>CO<sub>3</sub> + CH<sub>4</sub>.<br /></span>
+</div></div>
+
+<p><b>Properties.</b> Methane is a colorless, odorless gas whose density is 0.55.
+It is difficult to liquefy, boiling at -155&deg; under standard pressure,
+and is almost insoluble in water. It burns with a pale blue flame,
+liberating much heat, and when mixed with oxygen is very explosive.</p>
+
+<p><b>Davy's safety lamp.</b> In 1815 Sir Humphry Davy invented a lamp for the use
+of miners, to prevent the dreadful mine explosions then common, due to
+methane mixed with air. The invention consisted in surrounding the upper
+part of the common miner's lamp with a mantle of wire gauze and the
+lower part with glass (Fig. 59). It has been seen that two gases will
+not combine until raised to their<span class='pagenum'><a name="Page_203" id="Page_203">[Pg 203]</a></span> kindling temperature, and if while
+combining they are cooled below this point, the combination ceases. A
+flame will not pass through a wire gauze because the metal, being a good
+conductor of heat, takes away so much heat from the flame that the gases
+are cooled below the kindling temperature. When a lamp so protected is
+brought into an explosive mixture the gases inside the wire mantle burn
+in a series of little explosions, giving warning to the miner that the
+air is unsafe.</p>
+
+<div class="figright" style="width: 181px;">
+<img src="images/imagefig59.jpg" width="181" height="400" alt="Fig. 59" title="" />
+<span class="caption">Fig. 59</span>
+</div>
+
+<p><b>Acetylene</b> (C<sub>2</sub>H<sub>2</sub>). This is a colorless gas usually having a
+disagreeable odor due to impurities. It is now made in large quantities
+from calcium carbide (CaC<sub>2</sub>). This substance is formed when coal and
+lime are heated together in an electric furnace. When treated with water
+the carbide is decomposed, yielding acetylene:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaC<sub>2</sub> + 2H<sub>2</sub>O = C<sub>2</sub>H<sub>2</sub> + Ca(OH)<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>Under ordinary conditions the gas burns with a very smoky flame; in
+burners constructed so as to secure a large amount of oxygen it burns
+with a very brilliant white light, and hence is used as an illuminant.</p>
+
+<p><b>Laboratory preparation.</b> The gas can be prepared readily in a generator
+such as is shown in Fig. 60. The inner tube contains fragments of
+calcium carbide, while the outer one is filled with water. As long as
+the stopcock is closed the water cannot rise in the inner tube. When the
+stopcock is open the water rises, and, coming into contact with the
+carbide in the inner tube, generates acetylene. This escapes through the
+stopcock, and after the air has been expelled may be lighted as it
+issues from the burner.</p>
+
+<div class="figleft" style="width: 150px;">
+<img src="images/imagefig60.jpg" width="150" height="400" alt="Fig. 60" title="" />
+<span class="caption">Fig. 60</span>
+</div>
+
+<p>Carbon forms two oxides, namely, carbon dioxide (CO<sub>2</sub>) and carbon
+monoxide (CO).<span class='pagenum'><a name="Page_204" id="Page_204">[Pg 204]</a></span></p>
+
+<p><b>Carbon dioxide</b> (CO<sub>2</sub>). Carbon dioxide is present in the air to the
+extent of about 3 parts in 10,000, and this apparently small amount is
+of fundamental importance in nature. In some localities it escapes from
+the earth in great quantities, and many spring waters carry large
+amounts of it in solution. When these highly charged spring waters reach
+the surface of the earth, and the pressure on them is removed, the
+carbon dioxide escapes with effervescence. It is a product of the
+oxidation of all organic matter, and is therefore formed in fires as
+well as in the process of decay. It is thrown off from the lungs of all
+animals in respiration, and is a product of many fermentation processes
+such as vinegar making and brewing. Combined with metallic oxides it
+forms vast deposits of carbonates in nature.</p>
+
+
+<p><b>Preparation.</b> In the laboratory carbon dioxide is always prepared by the
+action of an acid upon a carbonate, usually calcium carbonate, the
+apparatus shown in Fig. 39 serving the purpose very well. This reaction
+might be expected to produce carbonic acid, thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaCO<sub>3</sub> + 2HCl = CaCl<sub>2</sub> + H<sub>2</sub>CO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Carbonic acid is very unstable, however, and decomposes into its
+anhydride, CO<sub>2</sub>, and water, thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>CO<sub>3</sub> = H<sub>2</sub>O + CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>The complete reaction is represented by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaCO<sub>3</sub> + 2HCl = CaCl<sub>2</sub> + CO<sub>2</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><b>Physical properties.</b> Carbon dioxide is a colorless, practically odorless
+gas whose density is 1.5. Its weight may be inferred from the fact that
+it can be siphoned, or poured like water, from one vessel downward into
+another. At 15&deg;<span class='pagenum'><a name="Page_205" id="Page_205">[Pg 205]</a></span> and under ordinary pressure it dissolves in its own
+volume of water and imparts a somewhat biting, pungent taste to it. It
+is easily condensed, and is now prepared commercially in this form by
+pumping the gas into steel cylinders (see Fig. 6) which are kept cold
+during the process. When the liquid is permitted to escape into the air
+part of it instantly evaporates, and in so doing absorbs so much heat
+that another portion is solidified, the solid form strikingly resembling
+snow in appearance. This snow is very cold and mercury can easily be
+frozen with it.</p>
+
+<p><b>Solid carbon dioxide.</b> Cylinders of liquid carbon dioxide are
+inexpensive, and should be available in every school. To demonstrate the
+properties of solid carbon dioxide, the cylinder should be placed across
+the table and supported in such a way that the stopcock end is several
+inches lower than the other end. A loose bag is made by holding the
+corners of a handkerchief around the neck of the stopcock, and the cock
+is then turned on so that the gas rushes out in large quantities. Very
+quickly a considerable quantity of the snow collects in the
+handkerchief. To freeze mercury, press a piece of filter paper into a
+small evaporating dish and pour the mercury upon it. Coil a flat spiral
+upon the end of a wire, and dip the spiral into the mercury. Place a
+quantity of solid carbon dioxide upon the mercury and pour 10 cc.-15 cc.
+of ether over it. In a minute or two the mercury will solidify and may
+be removed from the dish by the wire serving as a handle. The filter
+paper is to prevent the mercury from sticking to the dish; the ether
+dissolves the solid carbon dioxide and promotes its rapid conversion
+into gas.</p>
+
+<p><b>Chemical properties.</b> Carbon dioxide is incombustible, since it is, like
+water, a product of combustion. It does not support combustion, as does
+nitrogen peroxide, because the oxygen in it is held in very firm
+chemical union with the carbon. Very strong reducing agents, such as
+highly heated carbon, can take away half of its oxygen:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CO<sub>2</sub> + C = 2CO.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_206" id="Page_206">[Pg 206]</a></span></p>
+<p><b>Uses.</b> The relation of carbon dioxide to plant life has been discussed in
+a previous chapter. Water highly charged with carbon dioxide is used for
+making soda water and similar beverages. Since it is a non-supporter of
+combustion and can be generated readily, carbon dioxide is also used as
+a fire extinguisher. Some of the portable fire extinguishers are simply
+devices for generating large amounts of the gas. It is not necessary
+that all the oxygen should be kept away from the fire in order to
+smother it. A burning candle is extinguished in air which contains only
+2.5% of carbon dioxide.</p>
+
+<p><b>Carbonic acid</b> (H<sub>2</sub>CO<sub>3</sub>). Like most of the oxides of the non-metallic
+elements, carbon dioxide is an acid anhydride. It combines with water to
+form an acid of the formula H<sub>2</sub>CO<sub>3</sub>, called carbonic acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>O + CO<sub>2</sub> = H<sub>2</sub>CO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>The acid is, however, very unstable and cannot be isolated. Only a very
+small amount of it is actually formed when carbon dioxide is passed into
+water, as is evident from the small solubility of the gas. If, however,
+a base is present in the water, salts of carbonic acid are formed, and
+these are quite stable:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaOH + H<sub>2</sub>O + CO<sub>2</sub> = Na<sub>2</sub>CO<sub>3</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><b>Action of carbon dioxide on bases.</b> This conduct is explained by the
+principles of reversible reactions. The equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>O +CO<sub>2</sub> &lt;--> H<sub>2</sub>CO<sub>3</sub><br /></span>
+</div></div>
+
+<p>is a reversible equation, and the extent to which the reaction
+progresses depends upon the relative concentrations of each of the three
+factors in it. Equilibrium is ordinarily reached when very little
+H<sub>2</sub>CO<sub>3</sub> is formed. If a base is present in the water to combine with
+the H<sub>2</sub>CO<sub>3</sub> as fast as it is formed, all of the CO<sub>2</sub> is converted
+<span class='pagenum'><a name="Page_207" id="Page_207">[Pg 207]</a></span>into H<sub>2</sub>CO<sub>3</sub>, and thence into a carbonate.</p>
+
+<p><b>Salts of carbonic acid,&mdash;carbonates.</b> The carbonates form a very
+important class of salts. They are found in large quantities in nature,
+and are often used in chemical processes. Only the carbonates of sodium,
+potassium, and ammonium are soluble, and these can be made by the action
+of carbon dioxide on solutions of the bases, as has just been explained.</p>
+
+<p>The insoluble carbonates are formed as precipitates when soluble salts
+are treated with a solution of a soluble carbonate. Thus the insoluble
+calcium carbonate can be made by bringing together solutions of calcium
+chloride and sodium carbonate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaCl<sub>2</sub> + Na<sub>2</sub>CO<sub>3</sub> = CaCO<sub>3</sub> + 2NaCl.<br /></span>
+</div></div>
+
+<p>Most of the carbonates are decomposed by heat, yielding an oxide of the
+metal and carbon dioxide. Thus lime (calcium oxide) is made by strongly
+heating calcium carbonate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaCO<sub>3</sub> = CaO + CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p><b>Acid carbonates.</b> Like all acids containing two acid hydrogen atoms,
+carbonic acid can form both normal and acid salts. The acid carbonates
+are made by treating a normal carbonate with an excess of carbonic acid.
+With few exceptions they are very unstable, heat decomposing them even
+when in solution.</p>
+
+
+<p><b>Action of carbon dioxide on calcium hydroxide.</b> If carbon dioxide is
+passed into clear lime water, calcium carbonate is at first
+precipitated:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>O + CO<sub>2</sub> = H<sub>2</sub>CO<sub>3</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">Ca(OH)<sub>2</sub> + H<sub>2</sub>CO<sub>3</sub> = CaCO<sub>3</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Advantage is taken of this reaction in testing for the presence of
+carbon dioxide, as already explained in the chapter on the atmosphere.
+If the current of carbon dioxide is continued, the precipitate<span class='pagenum'><a name="Page_208" id="Page_208">[Pg 208]</a></span> soon
+dissolves, because the excess of carbonic acid forms calcium acid
+carbonate which is soluble:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaCO<sub>3</sub> + H<sub>2</sub>CO<sub>3</sub> = Ca(HCO<sub>3</sub>)<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>If now the solution is heated, the acid carbonate is decomposed and
+calcium carbonate once more precipitated:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ca(HCO<sub>3</sub>)<sub>2</sub> = CaCO<sub>3</sub> + H<sub>2</sub>CO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><b>Carbon monoxide (CO).</b> Carbon monoxide can be made in a number of ways,
+the most important of which are the three following:</p>
+
+<p>1. <i>By the partial oxidation of carbon.</i> If a slow current of air is
+conducted over highly heated carbon, the monoxide is formed, thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C + O = CO<br /></span>
+</div></div>
+
+<p>It is therefore often formed in stoves when the air draught is
+insufficient. Water gas, which contains large amounts of carbon
+monoxide, is made by partially oxidizing carbon with steam:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C + H<sub>2</sub>O = CO + 2H.<br /></span>
+</div></div>
+
+<p>2. <i>By the partial reduction of carbon dioxide.</i> When carbon dioxide is
+conducted over highly heated carbon it is reduced to carbon monoxide by
+the excess of carbon:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CO<sub>2</sub> + C = 2CO.<br /></span>
+</div></div>
+
+<p>When coal is burning in a stove or grate carbon dioxide is at first
+formed in the free supply of air, but as the hot gas rises through the
+glowing coal it is reduced to carbon monoxide. When the carbon monoxide
+reaches the free air above the coal it takes up oxygen to form carbon
+dioxide, burning with the blue flame so familiar above a bed of coals,
+especially in the case of hard coals.</p>
+
+<p>3. <i>By the decomposition of oxalic acid.</i> In the laboratory carbon
+monoxide is usually prepared by the action of<span class='pagenum'><a name="Page_209" id="Page_209">[Pg 209]</a></span> concentrated sulphuric
+acid upon oxalic acid. The latter substance has the formula
+C<sub>2</sub>H<sub>2</sub>O<sub>4</sub>. The sulphuric acid, owing to its affinity for water,
+decomposes the oxalic acid, as represented in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>2</sub>H<sub>2</sub>O<sub>4</sub> + (H<sub>2</sub>SO<sub>4</sub>) = (H<sub>2</sub>SO<sub>4</sub>) + H<sub>2</sub>O + CO<sub>2</sub> + CO.<br /></span>
+</div></div>
+
+<p><b>Properties.</b> Carbon monoxide is a light, colorless, almost odorless gas,
+very difficult to liquefy. Chemically it is very active, combining
+directly with a great many substances. It has a great affinity for
+oxygen and is therefore combustible and a good reducing agent. Thus, if
+carbon monoxide is passed over hot copper oxide, the copper is reduced
+to the metallic state:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CuO + CO = Cu + CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>When inhaled it combines with the red coloring matter of the blood and
+in this way prevents the absorption of oxygen, so that even a small
+quantity of the gas may prove fatal.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/imagefig61.jpg" width="500" height="223" alt="Fig. 61" title="" />
+<span class="caption">Fig. 61</span>
+</div>
+
+<div class="blockquot"><p><b>The reducing power of carbon monoxide.</b> Fig. 61 illustrates a
+method of showing the reducing power of carbon monoxide. The
+gas is generated by gently heating 7 or 8 g. of oxalic acid
+with 25 cc. of concentrated sulphuric acid in a 200 cc. flask
+<i>A</i>. The bottle <i>B</i> contains a solution of sodium hydroxide,
+which removes the carbon dioxide formed along with the
+monoxide. <i>C</i> contains a solution of calcium hydroxide to show
+that the carbon dioxide is completely removed. <i>E</i> is a
+hard-glass tube containing 1 or 2 g. of copper oxide, which is
+heated by a burner. The black copper oxide is reduced to
+reddish metallic copper by the carbon monoxide, which is
+thereby changed to carbon dioxide. The presence of the carbon
+dioxide is shown by the precipitate in the calcium hydroxide
+solution in <i>D</i>. Any unchanged carbon monoxide is collected
+over water in <i>F</i>.</p></div>
+
+<p><span class='pagenum'><a name="Page_210" id="Page_210">[Pg 210]</a></span></p>
+
+<p><b>Carbon disulphide</b> (CS<sub>2</sub>). Just as carbon combines with oxygen to form
+carbon dioxide, so it combines with sulphur to form carbon disulphide
+(CS<sub>2</sub>). This compound has been described in the chapter on sulphur.</p>
+
+<p><b>Hydrocyanic acid</b> (<i>prussic acid</i>)(HCN). Under the proper conditions
+carbon unites with nitrogen and hydrogen to form the acid HCN, called
+hydrocyanic acid. It is a weak, volatile acid, and is therefore easily
+prepared by treating its salts with sulphuric acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">KCN + H<sub>2</sub>SO<sub>4</sub> = KHSO<sub>4</sub> + HCN.<br /></span>
+</div></div>
+
+<p>It is most familiar as a gas, though it condenses to a colorless liquid
+boiling at 26&deg;. It has a peculiar odor, suggesting bitter almonds, and
+is extremely poisonous either when inhaled or when taken into the
+stomach. A single drop may cause death. It dissolves readily in water,
+its solution being commonly called prussic acid.</p>
+
+<p>The salts of hydrocyanic acid are called <i>cyanides</i>, the cyanides of
+sodium and potassium being the best known. These are white solids and
+are extremely poisonous.</p>
+
+<p><b>Solutions of potassium cyanide are alkaline.</b> A solution of potassium
+cyanide turns red litmus blue, and must therefore contain hydroxyl ions.
+The presence of these ions is accounted for in the following way.</p>
+
+<p>Although water is so little dissociated into its ions H<sup>+</sup> and OH<sup>-</sup>
+that for most purposes we may neglect the dissociation, it is
+nevertheless measurably dissociated. Hydrocyanic acid is one of the
+weakest of acids, and dissociates<span class='pagenum'><a name="Page_211" id="Page_211">[Pg 211]</a></span> to an extremely slight extent. When a
+cyanide such as potassium cyanide dissolves it freely dissociates, and
+the CN<sup>-</sup> ions must come to an equilibrium with the H<sup>+</sup> ions derived
+from the water:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sup>+</sup> + CN<sup>-</sup> &lt;--> HCN.<br /></span>
+</div></div>
+
+<p>The result of this equilibrium is that quite a number of H<sup>+</sup> ions from
+the water are converted into undissociated HCN molecules. But for every
+H<sup>+</sup> ion so removed an OH<sup>-</sup> ion remains free, and this will give the
+solution alkaline properties.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> How can you prove that the composition of the different allotropic
+forms of carbon is the same?</p>
+
+<p><b>2.</b> Are lampblack and bone black allotropic forms of carbon? Will equal
+amounts of heat be liberated in the combustion of 1 g. of each?</p>
+
+<p><b>3.</b> How could you judge of the relative purity of different forms of
+carbon?</p>
+
+<p><b>4.</b> Apart from its color, why should carbon be useful in the preparation
+of inks and paints?</p>
+
+<p><b>5.</b> Could asbestos fibers be used to replace the wire in a safety lamp?</p>
+
+<p><b>6.</b> Why do most acids decompose carbonates?</p>
+
+<p><b>7.</b> What effect would doubling the pressure have upon the solubility of
+carbon dioxide in water?</p>
+
+<p><b>8.</b> What compound would be formed by passing carbon dioxide into a
+solution of ammonium hydroxide? Write the equation.</p>
+
+<p><b>9.</b> Write equations for the preparation of K<sub>2</sub>CO<sub>3</sub>; of BaCO<sub>3</sub>; of
+MgCO<sub>3</sub>.</p>
+
+<p><b>10.</b> In what respects are carbonic and sulphurous acids similar?</p>
+
+<p><b>11.</b> Give three reasons why the reaction which takes place when a
+solution of calcium acid carbonate is heated, completes itself.</p>
+
+<p><b>12.</b> How could you distinguish between carbonates and sulphites?</p>
+
+<p><b>13.</b> How could you distinguish between oxygen, hydrogen, nitrogen,
+nitrous oxide, and carbon dioxide?<span class='pagenum'><a name="Page_212" id="Page_212">[Pg 212]</a></span></p>
+
+<p><b>14.</b> Could a solution of sodium hydroxide be substituted for the solution
+of calcium hydroxide in testing for carbon dioxide?</p>
+
+<p><b>15.</b> What weight of sodium hydroxide is necessary to neutralize the
+carbonic acid formed by the action of hydrochloric acid on 100 g. of
+calcium carbonate?</p>
+
+<p><b>16.</b> What weight of calcium carbonate would be necessary to prepare
+sufficient carbon dioxide to saturate 10 l. of water at 15&deg; and under
+ordinary pressure?</p>
+
+<p><b>17.</b> On the supposition that calcium carbide costs 12 cents a kilogram,
+what would be the cost of an amount sufficient to generate 100 l. of
+acetylene measured at 20&deg; and 740 mm.?</p>
+
+<p><b>18.</b> How would the volume of a definite amount of carbon monoxide compare
+with the volume of carbon dioxide formed by its combustion, the
+measurements being made under the same conditions?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_213" id="Page_213">[Pg 213]</a></span></p>
+<h2><a name="CHAPTER_XVIII" id="CHAPTER_XVIII"></a>CHAPTER XVIII</h2>
+
+<h3>FLAMES,&mdash;ILLUMINANTS</h3>
+
+
+<p><b>Conditions necessary for flames.</b> It has been seen that when two
+substances unite chemically, with the production of light and heat, the
+act of union is called combustion. When one of the substances undergoing
+combustion remains solid at the temperature occasioned by the
+combustion, light may be given off, but there is no flame. Thus iron
+wire burning in oxygen throws off a shower of sparks and is brilliantly
+incandescent, but no flame is seen. When, however, both of the
+substances are gases or vapors at the temperature reached in the
+combustion, the act of union is accompanied by a flame.</p>
+
+<p><b>Flames from burning liquids or solids.</b> Many substances which are liquids
+or solids at ordinary temperatures burn with a flame because the heat of
+combustion vaporizes them slowly, and the flame is due to the union of
+this vapor with the gas supporting the combustion.</p>
+
+<p><b>Supporter of combustion.</b> That gas which surrounds the flame and
+constitutes the atmosphere in which the combustion occurs is said to
+support the combustion. The other gas which issues into this atmosphere
+is said to be the combustible gas. Thus, in the ordinary combustion of
+coal gas in the air the coal gas is said to be combustible, while the
+air is regarded as the supporter of combustion. These terms are entirely
+relative, however, for a jet of air issuing into an atmosphere of coal
+gas will burn when ignited, the coal gas supporting the combustion.<span class='pagenum'><a name="Page_214" id="Page_214">[Pg 214]</a></span>
+Ordinarily, when we say that a gas is combustible we mean that it is
+combustible in an atmosphere of air.</p>
+
+<div class="figright" style="width: 219px;">
+<img src="images/imagefig62.jpg" width="219" height="400" alt="Fig. 62" title="" />
+<span class="caption">Fig. 62</span>
+</div>
+
+<p><b>Either gas may be the supporter of combustion.</b> That the terms
+<i>combustible</i> and <i>supporter of combustion</i> are merely relative may be
+shown in the following way: A lamp chimney <i>A</i> is fitted with a cork and
+glass tubes, as shown in Fig. 62. The tube <i>C</i> should have a diameter of
+from 12 to 15 mm. A thin sheet of asbestos in which is cut a circular
+opening about 2 cm. in diameter is placed over the top of the chimney.
+The opening in the asbestos is closed with the palm of the hand, and gas
+is admitted to the chimney through the tube <i>B</i>. The air in the chimney
+is soon expelled through the tube <i>C</i>, and the gas itself is then
+lighted at the lower end of this tube. The hand is now removed from the
+opening in the asbestos, when the flame at the end of the tube at once
+rises and appears at the end within the chimney, as shown in the figure.
+The excess of coal gas now escapes from the opening in the asbestos and
+may be lighted. The flame at the top of the asbestos board is due to the
+combustion of coal gas in air, while the flame within the chimney is due
+to the combustion of air in coal gas, the air being drawn up through the
+tube by the escaping gas.</p>
+
+<p><b>Appearance of flames.</b> The flame caused by the union of hydrogen and
+oxygen is almost colorless and invisible. Chlorine and hydrogen combine
+with a pale violet flame, carbon monoxide burns in oxygen with a blue
+flame, while ammonia burns with a deep yellow flame. The color and
+appearance of flames are therefore often quite characteristic of the
+particular combustion which occasions them.</p>
+
+<p><b>Structure of flames.</b> When the gas undergoing combustion issues from a
+round opening into an atmosphere of the gas supporting combustion, as is
+the case with the burning Bunsen burner (Fig. 63), the flame is
+generally<span class='pagenum'><a name="Page_215" id="Page_215">[Pg 215]</a></span> conical in outline. It consists of several distinct cones,
+one within the other, the boundary between them being marked by
+differences of color or luminosity. In the simplest flame, of which
+hydrogen burning in oxygen is a good example, these cones are two in
+number,&mdash;an inner one, formed by unburned gas, and an outer one, usually
+more or less luminous, consisting of the combining gases. This outer one
+is in turn surrounded by a third envelope of the products of combustion;
+this envelope is sometimes invisible, as in the present case, but is
+sometimes faintly luminous. The lower part of the inner cone of the
+flame is quite cool and consists of unburned gas. Toward the top of the
+inner cone the gas has become heated to a high temperature by the
+burning envelope surrounding it. On reaching the supporter of combustion
+on the outside it is far above its kindling temperature, and combustion
+follows with the evolution of much heat. The region of combustion just
+outside the inner cone is therefore the hottest part of the flame.</p>
+
+<div class="figleft" style="width: 112px;">
+<img src="images/imagefig63.jpg" width="112" height="400" alt="Fig. 63" title="" />
+<span class="caption">Fig. 63</span>
+</div>
+
+<p><b>Oxidizing and reducing flames.</b> Since the tip of the outside cone
+consists of very hot products of combustion mixed with oxygen from the
+air, a substance capable of oxidation placed in this part of the flame
+becomes very hot and is easily oxidized. The oxygen with which it
+combines comes, of course, from the atmosphere, and not from the
+products of combustion. This outer tip of the flame is called the
+<i>oxidizing flame</i>.</p>
+
+<p>At the tip of the inner cone the conditions are quite different. This
+region consists of a highly heated combustible gas, which has not yet
+reached a supply of oxygen.<span class='pagenum'><a name="Page_216" id="Page_216">[Pg 216]</a></span></p>
+
+<p>If a substance rich in oxygen, such as a metallic oxide, is placed in
+this region of the flame, the heated gases combine with its oxygen and
+the substance is reduced. This part of the flame is called the <i>reducing
+flame</i>. These flames are used in testing certain substances, especially
+minerals. For this purpose they are produced by blowing into a small
+luminous Bunsen flame from one side through a blowpipe. This is a tube
+of the shape shown in Fig. 64. The flame is directed in any desired way
+and has the oxidizing and reducing regions very clearly marked (Fig.
+65). It is non-luminous from the same causes which render the open
+Bunsen burner flame non-luminous, the gases from the lungs serving to
+furnish oxygen and to dilute the combustible gas.</p>
+
+<div class="figright" style="width: 174px;">
+<img src="images/imagefig64.jpg" width="174" height="400" alt="Fig. 64" title="" />
+<span class="caption">Fig. 64</span>
+</div>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig65.jpg" width="400" height="252" alt="Fig. 65" title="" />
+<span class="caption">Fig. 65</span>
+</div>
+
+<p><b>Luminosity of flames.</b> The luminosity of flames is due to a number of
+distinct causes, and may therefore be increased or diminished in several
+ways.</p>
+
+<p>1. <i>Presence of solid matter.</i> The most obvious of these causes is the
+presence in the flame of incandescent solid matter. Thus chalk dust
+sifted into a non-luminous flame renders it luminous. When hydrocarbons
+form a part of the combustible gas, as they do in nearly all
+illuminating gases and oils, some carbon is usually set free in the
+process of combustion. This is made very hot by the flame and becomes
+incandescent, giving out light. In a well-regulated flame it is
+afterward burned up, but when the supply of oxygen is insufficient it
+escapes from the flame as lampblack or soot. That it is temporarily
+present in a well-burning luminous flame may be demonstrated by holding
+a cold object, such as a small evaporating dish, in the flame for a few
+seconds. This cold object cools the carbon below its kindling
+temperature, and it is deposited on the object as soot.<span class='pagenum'><a name="Page_217" id="Page_217">[Pg 217]</a></span></p>
+
+<p>2. <i>Pressure.</i> A second factor in the luminosity of flames is the
+pressure under which the gases are burning. Under increased pressure
+there is more matter in a given volume of a gas, and the chemical action
+is more energetic than when the gases are rarefied. Consequently there
+is more heat and light. A candle burning on a high mountain gives less
+light than when it burns at the sea level.</p>
+
+<p>If the gas is diluted with a non-combustible gas, the effect is the same
+as if it is rarefied, for under these conditions there is less
+combustible gas in a given volume.</p>
+
+<p>3. <i>Temperature.</i> The luminosity also depends upon the temperature
+attained in the combustion. In general the hotter the flame the greater
+the luminosity; hence cooling the gases before combustion diminishes the
+luminosity of the flame they will make, because it diminishes the
+temperature attained in the combustion. Thus the luminosity of the
+Bunsen flame is largely diminished by the air drawn up with the gas.
+This is due in part to the fact that the burning gas is diluted and
+cooled by the air drawn in. The oxygen thus introduced into the flame
+also causes the combustion of the hot particles of carbon which would
+otherwise tend to make the flame luminous.</p>
+
+<p><b>Illuminating and fuel gases.</b> A number of mixtures of combustible gases,
+consisting largely of carbon compounds and hydrogen, find extensive use
+for the production of light and heat. The three chief varieties are coal
+gas, water gas, and natural gas. The use of acetylene gas has already
+been referred to.</p>
+
+<p><b>Coal gas.</b> Coal gas is made by heating bituminous coal in large retorts
+out of contact with the air. Soft or bituminous coal contains, in
+addition to large amounts of carbon, considerable quantities of
+compounds of hydrogen, oxygen, nitrogen, and sulphur. When distilled the
+nitrogen is liberated partly in the form of ammonia and cyanides and
+partly as free nitrogen gas; the sulphur is converted into hydrogen
+sulphide, carbon disulphide, and oxides of sulphur; the oxygen into
+water and oxides of carbon. The<span class='pagenum'><a name="Page_218" id="Page_218">[Pg 218]</a></span> remaining hydrogen is set free partly
+as hydrogen and partly in combination with carbon in the form of
+hydrocarbons. The most important of these is methane, with smaller
+quantities of many others, some of which are liquids or solids at
+ordinary temperatures. The great bulk of the carbon remains behind as
+coke and retort carbon.</p>
+
+<p><b>The manufacture of coal gas.</b> In the manufacture of coal gas it is
+necessary to separate from the volatile constituents formed by the
+heating of the coal all those substances which are either solid or
+liquid at ordinary temperature, since these would clog the gas pipes.
+Certain gaseous constituents, such as hydrogen sulphide and ammonia,
+must also be removed. The method used to accomplish this is shown in
+Fig. 66. The coal is heated in air-tight retorts illustrated by <i>A</i>. The
+volatile products escape through the pipe <i>X</i> and bubble into the tarry
+liquid in the large pipe <i>B</i>, known as the <i>hydraulic main</i>, which runs
+at right angles to the retorts. Here is deposited the greater portion of
+the solid and liquid products, forming a tarry mass known as <i>coal tar</i>.
+Much of the ammonia also remains dissolved in this liquid. The partially
+purified gas then passes into the pipes <i>C</i>, which serve to cool it and
+further remove the solid and liquid matter. The gas then passes into
+<i>D</i>, which is filled with coke over which a jet of water is sprayed. The
+water still further cools the gas and at the same time partially removes
+such gaseous products as hydrogen sulphide and ammonia, which are
+soluble in water. In <i>E</i> the gas passes over some material such as lime,
+which removes the last portions of the sulphur compounds as well as much
+of the carbon dioxide present. From <i>E</i> the gas passes into the large
+gas holder <i>F</i>, from which it is distributed through pipes to the places
+where it is burned.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/imagefig66.jpg" width="450" height="238" alt="Fig. 66" title="" />
+<span class="caption">Fig. 66</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_219" id="Page_219">[Pg 219]</a></span></p>
+
+<div class="blockquot"><p>One ton of good gas coal yields approximately 10,000 cu. ft. of
+gas, 1400 lb. of coke, 120 lb. of tar, and 20 gal. of
+ammoniacal liquor.</p>
+
+<p>Not only is the ammonia obtained in the manufacture of the gas
+of great importance, but the coal tar also serves as the source
+of many very useful substances, as will be explained in Chapter
+XXXII.</p></div>
+
+<p><b>Water gas.</b> Water gas is essentially a mixture of carbon monoxide and
+hydrogen. It is made by passing steam over very hot anthracite coal,
+when the reaction shown in the following equation takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C + H<sub>2</sub>O = CO + 2H.<br /></span>
+</div></div>
+
+<p>When required merely to produce heat the gas is at once ready for use.
+When made for illuminating purposes it must be enriched, that is,
+illuminants must be added, since both carbon monoxide and hydrogen burn
+with non-luminous flames. This is accomplished by passing it into
+heaters containing highly heated petroleum oils. The gas takes up
+hydrocarbon gases formed in the decomposition of the petroleum oils,
+which make it burn with a luminous flame.</p>
+
+<p>Water gas is very effective as a fuel, since both carbon monoxide and
+hydrogen burn with very hot flames. It has little odor and is very
+poisonous. Its use is therefore attended with some risk, since leaks in
+pipes are very likely to escape notice.</p>
+
+<p><b>Natural gas.</b> This substance, so abundant in many localities, varies much
+in composition, but is composed principally of methane. When used for
+lighting purposes it is usually burned in a burner resembling an open
+Bunsen, the illumination being furnished by an incandescent mantle. This
+is the case in the familiar Welsbach burner. Contrary to statements
+frequently made, natural gas contains no free hydrogen.<span class='pagenum'><a name="Page_220" id="Page_220">[Pg 220]</a></span></p>
+
+
+<h3>TABLE SHOWING COMPOSITION OF GASES</h3>
+
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>PENNSYLVANIA NATURAL GAS</td><td align='left'>COAL GAS</td><td align='left'>WATER GAS</td><td align='left'>ENRICHED WATER GAS</td></tr>
+<tr><td align='left'>Hydrogen</td><td align='left'>&nbsp;</td><td align='center'>41.3</td><td align='center'>52.88</td><td align='center'>30.00</td></tr>
+<tr><td align='left'>Methane</td><td align='center'>90.64</td><td align='left'>43.6</td><td align='center'>2.16</td><td align='center'>24.00</td></tr>
+<tr><td align='left'>Illuminants</td><td align='left'>&nbsp;</td><td align='center'>3.9</td><td align='left'>&nbsp;</td><td align='center'>12.05</td></tr>
+<tr><td align='left'>Carbon monoxide</td><td align='left'>&nbsp;</td><td align='center'>6.4</td><td align='center'>36.80</td><td align='center'>29.00</td></tr>
+<tr><td align='left'>Carbon dioxide</td><td align='center'>0.30</td><td align='center'>2.0</td><td align='center'>3.47</td><td align='center'>0.30</td></tr>
+<tr><td align='left'>Nitrogen</td><td align='center'>9.06</td><td align='center'>1.2</td><td align='center'>4.69</td><td align='center'>2.50</td></tr>
+<tr><td align='left'>Oxygen</td><td align='left'>&nbsp;</td><td align='center'>0.3</td><td align='left'>&nbsp;</td><td align='center'>1.50</td></tr>
+<tr><td align='left'>Hydrocarbon vapors</td><td align='left'>&nbsp;</td><td align='center'>1.5</td><td align='left'>&nbsp;</td><td align='center'>1.50</td></tr>
+</table></div>
+
+
+
+<div class="blockquot"><p>These are analyses of actual samples, and may be taken as about
+the average for the various kinds of gases. Any one of these
+may vary considerably. The nitrogen and oxygen in most cases is
+due to a slight admixture of air which is difficult to exclude
+entirely in the manufacture and handling of gases.</p></div>
+
+<p><b>Fuels.</b> A variety of substances are used as fuels, the most important of
+them being wood, coal, and the various gases mentioned above. Wood
+consists mainly of compounds of carbon, hydrogen, and oxygen. The
+composition of coal and the fuel gases has been given. Since these fuels
+are composed principally of carbon and hydrogen or their compounds, the
+chief products of combustion are carbon dioxide and water. The practice
+of heating rooms with portable gas or oil stoves with no provision for
+removing the products of combustion is to be condemned, since the carbon
+dioxide is generated in sufficient quantities to render the air unfit
+for breathing. Rooms so heated also become very damp from the large
+amount of water vapor formed in the combustion, and which in<span class='pagenum'><a name="Page_221" id="Page_221">[Pg 221]</a></span> cold
+weather condenses on the window glass, causing the glass to "sweat."
+Both coal and wood contain a certain amount of mineral substances which
+constitute the ashes.</p>
+
+<p><b>The electric furnace.</b> In recent years electric furnaces have come into
+wide use in operations requiring a very high temperature. Temperatures
+as high as 3500&deg; can be easily reached, whereas the hottest oxyhydrogen
+flame is not much above 2000&deg;. These furnaces are constructed on one of
+two general principles.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig67.jpg" width="400" height="279" alt="Fig. 67" title="" />
+<span class="caption">Fig. 67</span>
+</div>
+
+<p>1. <i>Arc furnaces.</i> In the one type the source of heat is an electric arc
+formed between carbon electrodes separated a little from each other, as
+shown in Fig. 67. The substance to be heated is placed in a vessel,
+usually a graphite crucible, just below the arc. The electrodes and
+crucible are surrounded by materials which fuse with great difficulty,
+such as magnesium oxide, the walls of the furnace being so shaped as to
+reflect the heat downwards upon the contents of the crucible.</p>
+
+<div class="figcenter" style="width: 450px;">
+<img src="images/imagefig68.jpg" width="450" height="153" alt="Fig. 68" title="" />
+<span class="caption">Fig. 68</span>
+</div>
+
+<p>2. <i>Resistance furnaces.</i> In the other type of furnace the heat is
+generated by the resistance offered to the current in its passage
+through the furnace. In its simplest form it may be represented by Fig.
+68. The furnace is merely a rectangular box built up of loose bricks.
+The electrodes <i>E</i>, each consisting of a bundle of carbon rods, are
+introduced through the sides of the furnace. The materials to be heated,
+<i>C</i>, are filled into the furnace up to the electrodes, and a layer of
+broken coke is arranged so as to extend from one electrode to the other.
+More of the charge is then placed on top of the coke. In passing through
+the broken coke the electrical current encounters great resistance. This
+generates great heat, and the charge surrounding the coke is brought to
+a very high temperature. The advantage of this type of furnace is that
+the temperature can be regulated to any desired intensity.<span class='pagenum'><a name="Page_222" id="Page_222">[Pg 222]</a></span></p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Why does charcoal usually burn with no flame? How do you account for
+the flame sometimes observed when it burns?</p>
+
+<p><b>2.</b> How do you account for the fact that a candle burns with a flame?</p>
+
+<p><b>3.</b> What two properties must the mantle used in the Welsbach lamp
+possess?</p>
+
+<p><b>4.</b> (<i>a</i>) In what respects does the use of the Welsbach mantle resemble
+that of lime in the calcium light? (<i>b</i>) If the mantle were made of
+carbon, would it serve the same purpose?</p>
+
+<p><b>5.</b> Would anthracite coal be suitable for the manufacture of coal gas?</p>
+
+<p><b>6.</b> How could you prove the formation of carbon dioxide and water in the
+combustion of illuminating gases?</p>
+
+<p><b>7.</b> Suggest a probable way in which natural gas has been formed.</p>
+
+<p><b>8.</b> Coal frequently contains a sulphide of iron. (<i>a</i>) What two sulphur
+compounds are likely to be formed when gas is made from such coal? (<i>b</i>)
+Suggest some suitable method for the removal of these compounds.</p>
+
+<p><b>9.</b> Why does the use of the bellows on the blacksmith's forge cause a
+more intense heat?</p>
+
+<p><b>10.</b> What volume of oxygen is necessary to burn 100 l. of marsh gas and
+what volume of carbon dioxide would be formed, all of the gases being
+measured under standard conditions?</p>
+
+<p><b>11.</b> Suppose a cubic meter of Pennsylvania natural gas, measured under
+standard conditions, were to be burned. How much water by weight would
+result?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_223" id="Page_223">[Pg 223]</a></span></p>
+<h2><a name="CHAPTER_XIX" id="CHAPTER_XIX"></a>CHAPTER XIX</h2>
+
+<h3>MOLECULAR WEIGHTS, ATOMIC WEIGHTS, FORMULAS</h3>
+
+
+<p><b>Introduction.</b> In the chapter on The Atomic Theory, it was shown that if
+it were true that two elements uniting to form a compound always
+combined in the ratio of one atom of one element to one atom of the
+other element, it would be a very easy matter to decide upon figures
+which would represent the relative weights of the different atoms. It
+would only be necessary to select some one element as a standard and
+determine the weight of every element which combines with a definite
+weight (say 1 g.) of the standard element. The figures so obtained would
+evidently represent the relative weights of the atoms.</p>
+
+<p>But the law of multiple proportion at once reminds us that two elements
+may unite in several proportions; and there is no simple way to
+determine the number of atoms present in the molecule of any compound.
+Consequently the problem of deciding upon the relative atomic weights is
+not an easy one. To the solution of this problem we must now turn.</p>
+
+<p><b>Dalton's method of determining atomic weights.</b> When Dalton first
+advanced the atomic theory he attempted to solve this problem by very
+simple methods. He thought that when only one compound of two elements
+is known it is reasonable to suppose that it contains one atom of each
+element. He therefore gave the formula HO to water, and HN to ammonia.
+When more than two compounds were known he assumed that the most
+familiar or the most stable one had the simple formula. He then
+determined the atomic weight as<span class='pagenum'><a name="Page_224" id="Page_224">[Pg 224]</a></span> explained above. The results he
+obtained were contradictory and very far from satisfactory, and it was
+soon seen that some other method, resting on much more scientific
+grounds, must be found to decide what compounds, if any, have a single
+atom of each element present.</p>
+
+<p><b>Determination of atomic weights.</b> Three distinct steps are involved in
+the determination of the atomic weight of an element: (1) determination
+of the equivalent, (2) determination of molecular weights of its
+compounds, and (3) deduction of the exact atomic weight from the
+equivalent and molecular weights.</p>
+
+<p><b>1. Determination of the equivalent.</b> By the equivalent of an element is
+meant the weight of the element which will combine with a fixed weight
+of some other element chosen as a standard. It has already been
+explained that oxygen has been selected as the standard element for
+atomic weights, with a weight of 16. This same standard will serve very
+well as a standard for equivalents. <i>The equivalent of an element is the
+weight of the element which will combine with 16 g. of oxygen.</i> Thus 16
+g. of oxygen combines with 16.03 g. of sulphur, 65.4 g. of zinc, 215.86
+g. of silver, 70.9 g. of chlorine. These figures, therefore, represent
+the equivalent weights of these elements.</p>
+
+<p><b>Relation of atomic weights to equivalents.</b> According to the atomic
+theory combination always takes place between whole numbers of atoms.
+Thus one atom unites with one other, or with two or three; or two atoms
+may unite with three, or three with five, and so on.</p>
+
+<p>When oxygen combines with zinc the combination must be between definite
+numbers of the two kinds of atoms. Experiment shows that these two
+elements combine in the ratio of 16 g. of oxygen to 65.4 g. of zinc. If
+one atom of<span class='pagenum'><a name="Page_225" id="Page_225">[Pg 225]</a></span> oxygen combines with one atom of zinc, then this ratio must
+be the ratio between the weights of the two atoms. If one atom of oxygen
+combines with two atoms of zinc, then the ratio between the weights of
+the two atoms will be 16: 32.7. If two atoms of oxygen combine with one
+atom of zinc, the ratio by weight between the two atoms will be 8: 65.4.
+It is evident, therefore, that the real atomic weight of an element must
+be some multiple or submultiple of the equivalent; in other words, the
+equivalent multiplied by 1/2, 1, 2, or 3 will give the atomic weight.</p>
+
+<p><b>Combining weights.</b> A very interesting relation holds good between the
+equivalents of the various elements. We have just seen that the figures
+16.03, 65.4, 215.86, and 70.9 are the equivalents respectively of
+sulphur, zinc, silver, and chlorine. These same figures represent the
+ratios by weight in which these elements combine among themselves. Thus
+215.86 g. of silver combine with 70.9 g. of chlorine and with 2 &times; 16.03
+g. of sulphur. 65.4 g. of zinc combine with 70.9 g. of chlorine and 2 &times;
+16.03 g. of sulphur.</p>
+
+<p>By taking the equivalent or some multiple of it a value can be obtained
+for each element which will represent its combining value, and for this
+reason is called its <i>combining weight</i>. It is important to notice that
+the fact that a combining weight can be obtained for each element is not
+a part of a theory, but is the direct result of experiment.</p>
+
+<p><b>Elements with more than one equivalent.</b> It will be remembered that
+oxygen combines with hydrogen in two ratios. In one case 16 g. of oxygen
+combine with 2.016 g. of hydrogen to form water; in the other 16 g. of
+oxygen combine with 1.008 g. of hydrogen to form hydrogen dioxide. The
+equivalents of hydrogen are therefore 2.016 and 1.008. Barium combines
+with oxygen in two proportions: in barium oxide the proportion is 16 g.
+of oxygen to 137.4 g. of barium; in barium dioxide the proportion is 16
+g. of oxygen to 68.7 g. of barium.<span class='pagenum'><a name="Page_226" id="Page_226">[Pg 226]</a></span></p>
+
+<p>In each case one equivalent is a simple multiple of the other, so the
+fact that there may be two equivalents does not add to the uncertainty.
+All we knew before was that the true atomic weight is some multiple of
+the equivalent.</p>
+
+<p><b>2. The determination of molecular weights.</b> To decide the question as to
+which multiple of the equivalent correctly represents the atomic weight
+of an element, it has been found necessary to devise a method of
+determining the molecular weights of compounds containing the element in
+question. Since the molecular weight of a compound is merely the sum of
+the weights of all the atoms present in it, it would seem to be
+impossible to determine the molecular weight of a compound without first
+knowing the atomic weights of the constituent atoms, and how many atoms
+of each element are present in the molecule. But certain facts have been
+discovered which suggest a way in which this can be done.</p>
+
+<p><b>Avogadro's hypothesis.</b> We have seen that the laws of Boyle, Charles, and
+Gay-Lussac apply to all gases irrespective of their chemical character.
+This would lead to the inference that the structure of gases must be
+quite simple, and that it is much the same in all gases.</p>
+
+<p>In 1811 Avogadro, an Italian physicist, suggested that if we assume all
+gases under the same conditions of temperature and pressure to have the
+same number of molecules in a given volume, we shall have a probable
+explanation of the simplicity of the gas laws. It is difficult to prove
+the truth of this hypothesis by a simple experiment, but there are so
+many facts known which are in complete harmony with this suggestion that
+there is little doubt that it expresses the truth. Avogadro's hypothesis
+may be stated thus: <i>Equal volumes of all gases under the same
+conditions of temperature and pressure contain the same number of
+molecules.</i></p>
+<p><span class='pagenum'><a name="Page_227" id="Page_227">[Pg 227]</a></span></p>
+
+<p><b>Avogadro's hypothesis and molecular weights.</b> Assuming that Avogadro's
+hypothesis is correct, we have a very simple means for deciding upon the
+relative weights of molecules; for if equal volumes of two gases contain
+the same number of molecules, the weights of the two volumes must be in
+the same ratio as the weights of the individual molecules which they
+contain. If we adopt some one gas as a standard, we can express the
+weights of all other gases as compared with this one, and the same
+figures will express the relative weights of the molecules of which the
+gases are composed.</p>
+
+<p><b>Oxygen as the standard.</b> It is important that the same standard should be
+adopted for the determination of molecular weights as has been decided
+upon for atomic weights and equivalents, so that the three values may be
+in harmony with each other. Accordingly it is best to adopt oxygen as
+the standard element with which to compare the molecular weights of
+other gases, being careful to keep the oxygen atom equal to 16.</p>
+
+<p><b>The oxygen molecule contains two atoms.</b> One point must not be
+overlooked, however. We desire to have our unit, the oxygen <i>atom</i>,
+equal to 16. The method of comparing the weights of gases just suggested
+compares the molecules of the gases with the <i>molecule</i> of oxygen. Is
+the molecule and the atom of oxygen the same thing? This question is
+answered by the following considerations.</p>
+
+<p>We have seen that when steam is formed by the union of oxygen and
+hydrogen, two volumes of hydrogen combine with one volume of oxygen to
+form two volumes of steam. Let us suppose that the one volume of oxygen
+contains 100<span class='pagenum'><a name="Page_228" id="Page_228">[Pg 228]</a></span> molecules; then the two volumes of steam must, according
+to Avogadro's hypothesis, contain 200 molecules. But each of these 200
+molecules must contain at least one atom of oxygen, or 200 in all, and
+these 200 atoms came from 100 molecules of oxygen. It follows that each
+molecule of oxygen must contain at least two atoms of oxygen.</p>
+
+<p>Evidently this reasoning merely shows that there are <i>at least</i> two
+atoms in the oxygen molecule. There may be more than that, but as there
+is no evidence to this effect, we assume that the molecule contains two
+atoms only.</p>
+
+<p>It is evident that if we wish to retain the value 16 for the atom of
+oxygen we must take twice this value, or 32, for the value of the oxygen
+molecule, when using it as a standard for molecular weights.</p>
+
+<p><b>Determination of the molecular weights of gases from their weights
+compared with oxygen.</b> Assuming the molecular weight of oxygen to be 32,
+Avogadro's hypothesis gives us a ready means for determining the
+molecular weight of any other gas, for all that is required is to know
+its weight compared with that of an equal volume of oxygen. For example,
+1 l. of chlorine is found by experiment to weigh 2.216 times as much as
+1 l. of oxygen. The molecular weight of chlorine must therefore be 2.216
+&times;32, or 70.91.</p>
+
+<p>If, instead of comparing the relative weights of 1 l. of the two gases,
+we select such a volume of oxygen as will weigh 32 g., or the weight in
+grams corresponding to the molecular weight of the gas, the calculation
+is much simplified. It has been found that 32 g. of oxygen, under
+standard conditions, measure 22.4 l. This same volume of hydrogen weighs
+2.019 g.; of chlorine 70.9 g.; of hydrochloric acid 36.458 g. The
+weights of these equal volumes must be proportional to their molecular
+weights, and since<span class='pagenum'><a name="Page_229" id="Page_229">[Pg 229]</a></span> the weight of the oxygen is the same as the value of
+its molecular weight, so too will the weights of the 22.4 l. of the
+other gases be equal to the value of their molecular weights.</p>
+
+<p>As a summary we can then make the following statement: <i>The molecular
+weight of any gas may be determined by calculating the weight of 22.4 l.
+of the gas, measured under standard conditions.</i></p>
+
+<p><b>Determination of molecular weights from density of gases.</b> In an actual
+experiment it is easier to determine the density of a gas than the
+weight of a definite volume of it. The density of a gas is usually
+defined as its weight compared with that of an equal volume of air.
+Having determined the density of a gas, its weight compared with oxygen
+may be determined by multiplying its density by the ratio between the
+weights of air and oxygen. This ratio is 0.9046. To compare it with our
+standard for atomic weights we must further multiply it by 32, since the
+standard is 1/32 the weight of oxygen molecules. The steps then are
+these:</p>
+
+<p>1. Determine the density of the gas (its weight compared with air).</p>
+
+<p>2. Multiply by 0.9046 to make the comparison with oxygen molecules.</p>
+
+<p>3. Multiply by 32 to make the comparison with the unit for atomic
+weights.</p>
+
+<p>We have, then, the formula:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">molecular weight = density &times; 0.9046 &times; 32;<br /></span>
+</div></div>
+
+<p>or, still more briefly,</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">M. = D. &times; 28.9.<br /></span>
+</div></div>
+
+<p>The value found by this method for the determination of molecular
+weights will of course agree with those found<span class='pagenum'><a name="Page_230" id="Page_230">[Pg 230]</a></span> by calculating the weight
+of 22.4 l. of the gas, since both methods depend on the same principles.</p>
+
+<div class="figright" style="width: 350px;">
+<img src="images/imagefig69.jpg" width="350" height="346" alt="Fig. 69" title="" />
+<span class="caption">Fig. 69</span>
+</div>
+
+<div class="blockquot"><p><b>Determination of densities of gases.</b> The relative weights of
+equal volumes of two gases can be easily determined. The
+following is one of the methods used. A small flask, such as is
+shown in Fig. 69, is filled with one of the gases, and after
+the temperature and pressure have been noted the flask is
+sealed up and weighed. The tip of the sealed end is then broken
+off, the flask filled with the second gas, and its weight
+determined. If the weight of the empty flask is subtracted from
+these two weighings, the relative weights of the gases is
+readily found.</p></div>
+
+<p><b>3. Deduction of atomic weights from molecular weights and equivalents.</b>
+We have now seen how the equivalent of an element and the molecular
+weight of compounds containing the element can be obtained. Let us see
+how it is possible to decide which multiple of the equivalent really is
+the true atomic weight. As an example, let us suppose that the
+equivalent of nitrogen has been found to be 7.02 and that it is desired
+to obtain its atomic weight. The next step is to obtain the molecular
+weights of a large number of compounds containing nitrogen. The
+following will serve:</p>
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='center'>DENSITY BY EXPERIMENT</td><td align='center'>APPROXIMATE MOLECULAR WEIGHT (D. &times; 28.9)</td><td align='center'>PERCENTAGE OF NITROGEN BY EXPERIMENT</td><td align='center'>PART OF MOLECULAR WEIGHT DUE TO NITROGEN</td></tr>
+<tr><td align='left'>Nitrogen gas</td><td align='center'>0.9671</td><td align='center'>27.95</td><td align='center'>100.00</td><td align='center'>27.95</td></tr>
+<tr><td align='left'>Nitrous oxide</td><td align='center'>1.527</td><td align='center'>44.13</td><td align='center'>63.70</td><td align='center'>27.11</td></tr>
+<tr><td align='left'>Nitric oxide</td><td align='center'>1.0384</td><td align='center'>30.00</td><td align='center'>46.74</td><td align='center'>14.02</td></tr>
+<tr><td align='left'>Nitrogen peroxide</td><td align='center'>1.580</td><td align='center'>45.66</td><td align='center'>30.49</td><td align='center'>13.90</td></tr>
+<tr><td align='left'>Ammonia</td><td align='center'>0.591</td><td align='center'>17.05</td><td align='center'>82.28</td><td align='center'>14.03</td></tr>
+<tr><td align='left'>Nitric acid</td><td align='center'>2.180</td><td align='center'>63.06</td><td align='center'>22.27</td><td align='center'>14.03</td></tr>
+<tr><td align='left'>Hydrocyanic acid</td><td align='center'>0.930</td><td align='center'>26.87</td><td align='center'>51.90</td><td align='center'>13.94</td></tr>
+</table></div>
+
+
+<p><span class='pagenum'><a name="Page_231" id="Page_231">[Pg 231]</a></span></p>
+
+<p><b>Method of calculation.</b> The densities of the various gases in the first
+column of this table are determined by experiment, and are fairly
+accurate but not entirely so. By multiplying these densities by 28.9 the
+molecular weights of the compounds as given in the second column are
+obtained. By chemical analysis it is possible to determine the
+percentage composition of these substances, and the percentages of
+nitrogen in them as determined by analysis are given in the third
+column. If each of these molecular weights is multiplied in turn by the
+percentage of nitrogen in the compound, the product will be the weight
+of the nitrogen in the molecular weight of the compound. This will be
+the sum of the weights of the nitrogen atoms in the molecule. These
+values are given in the fourth column in the table.</p>
+
+<p>If a large number of compounds containing nitrogen are studied in this
+way, it is probable that there will be included in the list at least one
+substance whose molecule contains a single nitrogen atom. In this case
+the number in the fourth column will be the approximate atomic weight of
+nitrogen. On comparing the values for nitrogen in the table it will be
+seen that a number which is approximately 14 is the smallest, and that
+the others are multiples of this. These compounds of higher value,
+therefore, contain more than one nitrogen atom in the molecule.</p>
+
+<p><b>Accurate determination of atomic weights.</b> Molecular weights cannot be
+determined very accurately, and consequently the part in them due to
+nitrogen is a little uncertain, as will be seen in the table. All we can
+tell by this method is that the true weight is very near 14. The
+equivalent can however be determined very accurately, and we have seen
+that it is some multiple or submultiple<span class='pagenum'><a name="Page_232" id="Page_232">[Pg 232]</a></span> of the true atomic weight.
+Since molecular-weight determinations have shown that in the case of
+nitrogen the atomic weight is near 14, and we have found the equivalent
+to be 7.02, it is evident that the true atomic weight is twice the
+equivalent, or 7.02 &times; 2 = 14.04.</p>
+
+<p><b>Summary.</b> These, then, are the steps necessary to establish the atomic
+weight of an element.</p>
+
+<p>1. Determine the equivalent accurately by analysis.</p>
+
+<p>2. Determine the molecular weight of a large number of compounds of the
+element, and by analysis the part of the molecular weight due to the
+element. The smallest number so obtained will be approximately the
+atomic weight.</p>
+
+<p>3. Multiply the equivalent by the small whole number (usually 1, 2, or
+3), which will make a number very close to the approximate atomic
+weight. The figure so obtained will be the true atomic weight.</p>
+
+<p><b>Molecular weights of the elements.</b> It will be noticed that the molecular
+weight of nitrogen obtained by multiplying its density by 28.9 is 28.08.
+Yet the atomic weight of nitrogen as deduced from a study of its gaseous
+compounds is 14.04. The simplest explanation that can be given for this
+is that the gaseous nitrogen is made up of molecules, each of which
+contains two atoms. In this respect it resembles oxygen; for we have
+seen that an entirely different line of reasoning leads us to believe
+that the molecule of oxygen contains two atoms. When we wish to indicate
+molecules of these gases the symbols N<sub>2</sub> and O<sub>2</sub> should be used.
+When we desire to merely show the weights taking part in a reaction this
+is not necessary.</p>
+
+<p>The vapor densities of many of the elements show that, like oxygen and
+nitrogen, their molecules consist of two atoms. In other cases,
+particularly among the metals,<span class='pagenum'><a name="Page_233" id="Page_233">[Pg 233]</a></span> the molecule and the atom are identical.
+Still other elements have four atoms in their molecules.</p>
+
+<p>While oxygen contains two atoms in its molecules, a study of ozone has
+led to the conclusion that it has three. The formation of ozone from
+oxygen can therefore be represented by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3O<sub>2</sub> = 2O<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><b>Other methods of determining molecular weights.</b> It will be noticed that
+Avogadro's law gives us a method by which we can determine the relative
+weights of the molecules of two gases because it enables us to tell when
+we are dealing with an equal number of the two kinds of molecules. If by
+any other means we can get this information, we can make use of the
+knowledge so gained to determine the molecular weights of the two
+substances.</p>
+
+<p><b>Raoult's laws.</b> Two laws have been discovered which give us just such
+information. They are known as Raoult's laws, and can be stated as
+follows:</p>
+
+<p>1. <i>When weights of substances which are proportional to their molecular
+weights are dissolved in the same weight of solvent, the rise of the
+boiling point is the same in each case.</i></p>
+
+<p>2. <i>When weights of substances which are proportional to their molecular
+weights are dissolved in the same weight of solvent, the lowering of the
+freezing point is the same in each case.</i></p>
+
+<p>By taking advantage of these laws it is possible to determine when two
+solutions contain the same number of molecules of two dissolved
+substances, and consequently the relative molecular weights of the two
+substances.</p>
+
+<p><b>Law of Dulong and Petit.</b> In 1819 Dulong and Petit discovered a very
+interesting relation between the atomic<span class='pagenum'><a name="Page_234" id="Page_234">[Pg 234]</a></span> weight of an element and its
+specific heat, which holds true for elements in the solid state. If
+equal weights of two solids, say, lead and silver, are heated through
+the same range of temperature, as from 10&deg; to 20&deg;, it is found that very
+different amounts of heat are required. The amount of heat required to
+change the temperature of a solid or a liquid by a definite amount
+compared with the amount required to change the temperature of an equal
+weight of water by the same amount is called its specific heat. Dulong
+and Petit discovered the following law: <i>The specific heat of an element
+in the solid form multiplied by its atomic weight is approximately equal
+to the constant 6.25.</i> That is,</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">at. wt. &times; sp. ht. = 6.25.<br /></span>
+</div></div>
+
+
+<p>Consequently,</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'></td><td align='left'>6.25</td></tr>
+<tr><td align='left'>at. wt.</td><td align='left'> = </td><td align='left'>&mdash;&mdash;&mdash;&mdash;</td></tr>
+<tr><td align='left'></td><td align='left'></td><td align='left'>sp. ht.</td></tr>
+</table></div>
+
+
+<p>This law is not very accurate, but it is often possible by means of it
+to decide upon what multiple of the equivalent is the real atomic
+weight. Thus the specific heat of iron is found by experiment to be
+0.112, and its equivalent is 27.95. 6.25 &divide; 0.112 = 55.8. We see,
+therefore, that the atomic weight is twice the equivalent, or 55.9.</p>
+
+<p><b>How formulas are determined.</b> It will be well in connection with
+molecular weights to consider how the formula of a compound is decided
+upon, for the two subjects are very closely associated. Some examples
+will make clear the method followed.</p>
+
+<p>The molecular weight of a substance containing hydrogen and chlorine was
+36.4. By analysis 36.4 parts of the substance was found to contain 1
+part of hydrogen and 35.4 parts of chlorine. As these are the simple
+atomic<span class='pagenum'><a name="Page_235" id="Page_235">[Pg 235]</a></span> weights of the two elements, the formula of the compound must be
+HCl.</p>
+
+<p>A substance consisting of oxygen and hydrogen was found to have a
+molecular weight of 34. Analysis showed that in 34 parts of the
+substance there were 2 parts of hydrogen and 32 parts of oxygen.
+Dividing these figures by the atomic weights of the two elements, we get
+2 &divide; 1 = 2 for H; 32 &divide; 16 = 2 for O. The formula is therefore H<sub>2</sub>O<sub>2</sub>.</p>
+
+<p>A substance containing 2.04% H, 32.6% S, and 65.3% O was found to have a
+molecular weight of 98. In these 98 parts of the substance there are 98
+&times; 2.04% = 2 parts of H, 98 &times; 32.6% = 32 parts of S, and 98 &times; 65.3% = 64
+parts of O. If the molecule weighs 98, the hydrogen atoms present must
+together weigh 2, the sulphur atoms 32, and the oxygen atoms 64.
+Dividing these figures by the respective atomic weights of the three
+elements, we have, for H, 2 &divide; 1 = 2 atoms; for S, 32 &divide; 32 = 1 atom; for
+O, 64 &divide; 16 = 4 atoms. Hence the formula is H<sub>2</sub>SO<sub>4</sub>.</p>
+
+<p>We have, then, this general procedure: Find the percentage composition
+of the substance and also its molecular weight. Multiply the molecular
+weight successively by the percentage of each element present, to find
+the amount of the element in the molecular weight of the compound. The
+figures so obtained will be the respective parts of the molecular weight
+due to the several atoms. Divide by the atomic weights of the respective
+elements, and the quotient will be the number of atoms present.</p>
+
+<p><b>Avogadro's hypothesis and chemical calculations.</b> This law simplifies
+many chemical calculations.</p>
+
+<p>1. <i>Application to volume relations in gaseous reactions.</i> Since equal
+volumes of gases contain an equal number of<span class='pagenum'><a name="Page_236" id="Page_236">[Pg 236]</a></span> molecules, it follows that
+when an equal number of gaseous molecules of two or more gases take part
+in a reaction, the reaction will involve equal volumes of the gases. In
+the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>2</sub>H<sub>2</sub>O<sub>4</sub> = H<sub>2</sub>O + CO<sub>2</sub> + CO,<br /></span>
+</div></div>
+
+<p>since 1 molecule of each of the gases CO<sub>2</sub> and CO is set free from
+each molecule of oxalic acid, the two substances must always be set free
+in equal volumes.</p>
+
+<p>Acetylene burns in accordance with the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2C<sub>2</sub>H<sub>2</sub> + 5O<sub>2</sub> = 4CO<sub>2</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Hence 2 volumes of acetylene will react with 5 volumes of oxygen to form
+4 volumes of carbon dioxide and 2 volumes of steam. That the volume
+relations may be correct a gaseous element must be given its molecular
+formula. Thus oxygen must be written O<sub>2</sub> and not 2O.</p>
+
+<p>2. <i>Application to weights of gases.</i> It will be recalled that the
+molecular weight of a gas is determined by ascertaining the weight of
+22.4 l. of the gas. This weight in grams is called the <i>gram-molecular
+weight</i> of a gas. If the molecular weight of any gas is known, the
+weight of a liter of the gas under standard conditions may be determined
+by dividing its gram-molecular weight by 22.4. Thus the gram-molecular
+weight of a hydrochloric acid gas is 36.458. A liter of the gas will
+therefore weigh 36.458 &divide; 22.4 = 1.627 g.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> From the following data calculate the atomic weight of sulphur. The
+equivalent, as obtained by an analysis of sulphur dioxide, is 16.03. The
+densities and compositions of a number of compounds containing sulphur
+are as follows:<span class='pagenum'><a name="Page_237" id="Page_237">[Pg 237]</a></span></p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='center'>NAME</td><td align='center'>DENSITY</td><td colspan="3" align='center'>COMPOSITION BY PERCENTAGE</td></tr>
+<tr><td align='center'>Hydrosulphuric acid</td><td align='center'>1.1791</td><td align='center'>S = 94.11</td><td align='center'>H = 5.89</td></tr>
+<tr><td align='center'>Sulphur dioxide</td><td align='center'>2.222</td><td align='center'>S = 50.05</td><td align='center'>O = 49.95</td></tr>
+<tr><td align='center'>Sulphur trioxide</td><td align='center'>2.74</td><td align='center'>S = 40.05</td><td align='center'>O = 59.95</td></tr>
+<tr><td align='center'>Sulphur chloride</td><td align='center'>4.70</td><td align='center'>S = 47.48</td><td align='center'>Cl = 52.52</td></tr>
+<tr><td align='center'>Sulphuryl chloride</td><td align='center'>4.64</td><td align='center'>S = 23.75</td><td align='center'>Cl = 52.53</td><td align='center'>O = 23.70</td></tr>
+<tr><td align='center'>Carbon disulphide</td><td align='center'>2.68</td><td align='center'>S = 84.24</td><td align='center'>C = 15.76</td></tr>
+</table></div>
+
+<p>2. Calculate the formulas for compounds of the following compositions:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'></td><td align='left'></td><td align='left'>MOLECULAR WEIGHT</td></tr>
+<tr><td align='left'>(1) S = 39.07%</td><td align='left'>O = 58.49%</td><td align='left'>H = 2.44%</td><td align='center'>81.0</td></tr>
+<tr><td align='left'>(2) Ca = 29.40</td><td align='left'>S = 23.56</td><td align='left'>O = 47.04</td><td align='center'>136.2</td></tr>
+<tr><td align='left'>(3) K = 38.67</td><td align='left'>N = 13.88</td><td align='left'>O = 47.45</td><td align='center'>101.2</td></tr>
+</table></div>
+
+<p><b>3.</b> The molecular weight of ammonia is 17.06; of sulphur dioxide is
+64.06; of chlorine is 70.9. From the molecular weight calculate the
+weight of 1 l. of each of these gases. Compare your results with the
+table on the back cover of the book.</p>
+
+<p><b>4.</b> From the molecular weight of the same gases calculate the density of
+each, referred to air as a standard.</p>
+
+<p><b>5.</b> A mixture of 50 cc. of carbon monoxide and 50 cc. of oxygen was
+exploded in a eudiometer, (<i>a</i>) What gases remained in the tube after
+the explosion? (<i>b</i>) What was the volume of each?</p>
+
+<p><b>6.</b> In what proportion must acetylene and oxygen be mixed to produce the
+greatest explosion?</p>
+
+<p><b>7.</b> Solve Problem 18, Chapter XVII, without using molecular weights.
+Compare your results.</p>
+
+<p><b>8.</b> Solve Problem 10, Chapter XVIII, without using molecular weights.
+Compare your results.</p>
+
+<p><b>9.</b> The specific heat of aluminium is 0.214; of lead is 0.031. From these
+specific heats calculate the atomic weights of each of the elements.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_238" id="Page_238">[Pg 238]</a></span></p>
+<h2><a name="CHAPTER_XX" id="CHAPTER_XX"></a>CHAPTER XX</h2>
+
+<h3>THE PHOSPHORUS FAMILY</h3>
+
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>SYMBOL</td><td align='left'>ATOMIC WEIGHT</td><td align='left'>DENSITY</td><td align='left'>MELTING POINT</td></tr>
+<tr><td align='left'>Phosphorus</td><td align='center'>P</td><td align='center'>31.0</td><td align='center'>1.8</td><td align='center'>43.3&deg;</td></tr>
+<tr><td align='left'>Arsenic</td><td align='center'>As</td><td align='center'>75.0</td><td align='center'>5.73</td><td align='center'>&mdash;</td></tr>
+<tr><td align='left'>Antimony</td><td align='center'>Sb</td><td align='center'>120.2</td><td align='center'>6.7</td><td align='center'>432&deg;</td></tr>
+<tr><td align='left'>Bismuth</td><td align='center'>Bi</td><td align='center'>208.5</td><td align='center'>9.8</td><td align='center'>270&deg;</td></tr>
+</table></div>
+
+<p><b>The family.</b> The elements constituting this family belong in the same
+group with nitrogen and therefore resemble it in a general way. They
+exhibit a regular gradation of physical properties, as is shown in the
+above table. The same general gradation is also found in their chemical
+properties, phosphorus being an acid-forming element, while bismuth is
+essentially a metal. The other two elements are intermediate in
+properties.</p>
+
+<p><b>Compounds.</b> In general the elements of the family form compounds having
+similar composition, as is shown in the following table:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>PH<sub>3</sub></td><td align='left'>PCl<sub>3</sub></td><td align='left'>PCl<sub>5</sub></td><td align='left'>P<sub>2</sub>O<sub>3</sub></td><td align='left'>P<sub>2</sub>O<sub>5</sub></td></tr>
+<tr><td align='left'>AsH<sub>3</sub></td><td align='left'>AsCl<sub>3</sub></td><td align='left'>AsCl<sub>5</sub></td><td align='left'>As<sub>2</sub>O<sub>3</sub></td><td align='left'>As<sub>2</sub>O<sub>5</sub></td></tr>
+<tr><td align='left'>SbH<sub>3</sub></td><td align='left'>SbCl<sub>3</sub></td><td align='left'>SbCl<sub>5</sub></td><td align='left'>Sb<sub>2</sub>O<sub>3</sub></td><td align='left'>Sb<sub>2</sub>O<sub>5</sub></td></tr>
+<tr><td align='left'></td><td align='left'>BiCl<sub>3</sub></td><td align='left'>BiCl<sub>5</sub></td><td align='left'>Bi<sub>2</sub>O<sub>3</sub></td><td align='left'>Bi<sub>2</sub>O<sub>5</sub></td></tr>
+</table></div>
+
+
+<p>In the case of phosphorus, arsenic, and antimony the oxides are acid
+anhydrides. Salts of at least four acids of each of these three elements
+are known, the free acid in<span class='pagenum'><a name="Page_239" id="Page_239">[Pg 239]</a></span> some instances being unstable. The relation
+of these acids to the corresponding anhydrides may be illustrated as
+follows, phosphorus being taken as an example:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">P<sub>2</sub>O<sub>3</sub> + 3H<sub>2</sub>O = 2H<sub>3</sub>PO<sub>3</sub> (phosphorous acid).<br /></span>
+</div><div class="stanza">
+<span class="i0">P<sub>2</sub>O<sub>5</sub> + 3H<sub>2</sub>O = 2H<sub>3</sub>PO<sub>4</sub> (phosphoric acid).<br /></span>
+</div><div class="stanza">
+<span class="i0">P<sub>2</sub>O<sub>5</sub> + 2H<sub>2</sub>O = H<sub>4</sub>P<sub>2</sub>O<sub>7</sub> (pyrophosphoric acid).<br /></span>
+</div><div class="stanza">
+<span class="i0">P<sub>2</sub>O<sub>5</sub> + H<sub>2</sub>O = 2HPO<sub>3</sub> (metaphosphoric acid).<br /></span>
+</div></div>
+
+
+<h3>PHOSPHORUS</h3>
+
+<p><b>History.</b> The element phosphorus was discovered by the alchemist Brand,
+of Hamburg, in 1669, while searching for the philosopher's stone. Owing
+to its peculiar properties and the secrecy which was maintained about
+its preparation, it remained a very rare and costly substance until the
+demand for it in the manufacture of matches brought about its production
+on a large scale.</p>
+
+<p><b>Occurrence.</b> Owing to its great chemical activity phosphorus never occurs
+free in nature. In the form of phosphates it is very abundant and widely
+distributed. <i>Phosphorite</i> and <i>sombrerite</i> are mineral forms of calcium
+phosphate, while <i>apatite</i> consists of calcium phosphate together with
+calcium fluoride or chloride. These minerals form very large deposits
+and are extensively mined for use as fertilizers. Calcium phosphate is a
+constituent of all fertile soil, having been supplied to the soil by the
+disintegration of rocks containing it. It is the chief mineral
+constituent of bones of animals, and bone ash is therefore nearly pure
+calcium phosphate.</p>
+
+<p><b>Preparation.</b> Phosphorus is now manufactured from bone ash or a pure
+mineral phosphate by heating the phosphate with sand and carbon in an
+electric furnace. The materials<span class='pagenum'><a name="Page_240" id="Page_240">[Pg 240]</a></span> are fed in at <i>M</i> (Fig. 70) by the feed
+screw <i>F</i>. The phosphorus vapor escapes at <i>P</i> and is condensed under
+water, while the calcium silicate is tapped off as a liquid at <i>S</i>. The
+phosphorus obtained in this way is quite impure, and is purified by
+distillation.</p>
+
+<div class="figcenter" style="width: 364px;">
+<img src="images/imagefig70.jpg" width="364" height="400" alt="Fig. 70" title="" />
+<span class="caption">Fig. 70</span>
+</div>
+
+<div class="blockquot"><p><b>Explanation of the reaction.</b> To understand the reaction which
+occurs, it must be remembered that a volatile acid anhydride is
+expelled from its salts when heated with an anhydride which is
+not volatile. Thus, when sodium carbonate and silicon dioxide
+are heated together the following reaction takes place:</p>
+
+<p>Na<sub>2</sub>CO<sub>3</sub> + SiO<sub>2</sub> = Na<sub>2</sub>SiO<sub>3</sub> + CO<sub>2</sub>.</p>
+
+<p>Silicon dioxide is a less volatile anhydride than phosphoric
+anhydride (P<sub>2</sub>O<sub>5</sub>), and when strongly heated with a
+phosphate the phosphoric anhydride is driven out, thus:</p>
+
+<p>Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> + 3SiO<sub>2</sub> = 3CaSiO<sub>3</sub> + P<sub>2</sub>O<sub>5</sub>.</p>
+
+<p>If carbon is added before the heat is applied, the P<sub>2</sub>O<sub>5</sub>
+is reduced to phosphorus at the same time, according to the
+equation</p>
+
+<p>P<sub>2</sub>O<sub>5</sub> + 5C = 2P + 5CO.</p></div>
+
+<p><b>Physical properties.</b> The purified phosphorus is a pale yellowish,
+translucent, waxy solid which melts at 43.3&deg; and boils at 269&deg;. It can
+therefore be cast into any convenient form under warm water, and is
+usually sold in the market in the form of sticks. It is quite soft and
+can be easily cut with a knife, but this must always be done while the
+element is covered with water, since it is extremely inflammable, and
+the friction of the knife blade is almost<span class='pagenum'><a name="Page_241" id="Page_241">[Pg 241]</a></span> sure to set it on fire if cut
+in the air. It is not soluble in water, but is freely soluble in some
+other liquids, notably in carbon disulphide. Its density is 1.8.</p>
+
+<p><b>Chemical properties.</b> Exposed to the air phosphorus slowly combines with
+oxygen, and in so doing emits a pale light, or phosphorescence, which
+can be seen only in a dark place. The heat of the room may easily raise
+the temperature to the kindling point of phosphorus, when it burns with
+a sputtering flame, giving off dense fumes of oxide of phosphorus. It
+burns with dazzling brilliancy in oxygen, and combines directly with
+many other elements, especially with sulphur and the halogens. On
+account of its great affinity for oxygen it is always preserved under
+water.</p>
+
+<p>Phosphorus is very poisonous, from 0.2 to 0.3 gram being a fatal dose.
+Ground up with flour and water or similar substances, it is often used
+as a poison for rats and other vermin.</p>
+
+<div class="blockquot"><p><b>Precaution.</b> The heat of the body is sufficient to raise
+phosphorus above its kindling temperature, and for this reason
+it should always be handled with forceps and never with the
+bare fingers. Burns occasioned by it are very painful and slow
+in healing.</p></div>
+
+<p><b>Red phosphorus.</b> On standing, yellow phosphorus gradually undergoes a
+remarkable change, being converted into a dark red powder which has a
+density of 2.1. It no longer takes fire easily, neither does it dissolve
+in carbon disulphide. It is not poisonous and, in fact, seems to be an
+entirely different substance. The velocity of this change increases with
+rise in temperature, and the red phosphorus is therefore prepared by
+heating the yellow just below the boiling point (250&deg;-300&deg;). When
+distilled and quickly condensed the red form changes back to the yellow.
+This is in accordance with the general rule that when a substance
+capable<span class='pagenum'><a name="Page_242" id="Page_242">[Pg 242]</a></span> of existing in several allotropic forms is condensed from a gas
+or crystallized from the liquid state, the more unstable variety forms
+first, and this then passes into the more stable forms.</p>
+
+<div class="blockquot"><p><b>Matches.</b> The chief use of phosphorus is in the manufacture of
+matches. Common matches are made by first dipping the match
+sticks into some inflammable substance, such as melted
+paraffin, and afterward into a paste consisting of (1)
+phosphorus, (2) some oxidizing substance, such as manganese
+dioxide or potassium chlorate, and (3) a binding material,
+usually some kind of glue. On friction the phosphorus is
+ignited, the combustion being sustained by the oxidizing agent
+and communicated to the wood by the burning paraffin. In
+sulphur matches the paraffin is replaced by sulphur.</p>
+
+<p>In safety matches <i>red</i> phosphorus, an oxidizing agent, and
+some gritty material such as emery is placed on the side of the
+box, while the match tip is provided as before with an
+oxidizing agent and an easily oxidized substance, usually
+antimony sulphide. The match cannot be ignited easily by
+friction, save on the prepared surface.</p></div>
+
+<p><b>Compounds of phosphorus with hydrogen.</b> Phosphorus forms several
+compounds with hydrogen, the best known of which is phosphine (PH<sub>3</sub>)
+analogous to ammonia (NH<sub>3</sub>).</p>
+
+<p><b>Preparation of phosphine.</b> Phosphine is usually made by heating
+phosphorus with a strong solution of potassium hydroxide, the reaction
+being a complicated one.</p>
+
+<div class="figcenter" style="width: 386px;">
+<img src="images/imagefig71.jpg" width="386" height="400" alt="Fig. 71" title="" />
+<span class="caption">Fig. 71</span>
+</div>
+
+<div class="blockquot"><p>The experiment can be conveniently made in the apparatus shown
+in Fig. 71. A strong solution of potassium hydroxide together
+with several small bits of phosphorus are placed in the flask
+<i>A</i>, and a current of coal gas is passed into the flask through
+the tube <i>B</i> until<span class='pagenum'><a name="Page_243" id="Page_243">[Pg 243]</a></span> all the air has been displaced. The gas is
+then turned off and the flask is heated. Phosphine is formed in
+small quantities and escapes through the delivery tube, the
+exit of which is just covered by the water in the vessel <i>C</i>.
+Each bubble of the gas as it escapes into the air takes fire,
+and the product of combustion (P<sub>2</sub>O<sub>5</sub>) forms beautiful
+small rings, which float unbroken for a considerable time in
+quiet air. The pure phosphine does not take fire spontaneously.
+When prepared as directed above, impurities are present which
+impart this property.</p></div>
+
+<p><b>Properties.</b> Phosphine is a gas of unpleasant odor and is exceedingly
+poisonous. Like ammonia it forms salts with the halogen acids. Thus we
+have phosphonium chloride (PH<sub>4</sub>Cl) analogous to ammonium chloride
+(NH<sub>4</sub>Cl). The phosphonium salts are of but little importance.</p>
+
+<p><b>Oxides of phosphorus.</b> Phosphorus forms two well-known oxides,&mdash;the
+trioxide (P<sub>2</sub>O<sub>3</sub>) and the pentoxide (P<sub>2</sub>O<sub>5</sub>), sometimes called
+phosphoric anhydride. When phosphorus burns in an insufficient supply of
+air the product is partially the trioxide; in oxygen or an excess of air
+the pentoxide is formed. The pentoxide is much the better known of the
+two. It is a snow-white, voluminous powder whose most marked property is
+its great attraction for water. It has no chemical action upon most
+gases, so that they can be very thoroughly dried by allowing them to
+pass through properly arranged vessels containing phosphorus pentoxide.</p>
+
+<p><b>Acids of phosphorus.</b> The important acids of phosphorus are the
+following:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>H<sub>3</sub>PO<sub>3</sub></td><td align='left'>phosphorous acid.</td></tr>
+<tr><td align='left'>H<sub>3</sub>PO<sub>4</sub></td><td align='left'>phosphoric acid.</td></tr>
+<tr><td align='left'>H<sub>4</sub>P<sub>2</sub>O<sub>7</sub></td><td align='left'>pyrophosphoric acid.</td></tr>
+<tr><td align='left'>HPO<sub>3</sub></td><td align='left'>metaphosphoric acid.</td></tr>
+</table></div>
+
+<p>These may be regarded as combinations of the oxides of phosphorus with
+water according to the equations given in the discussion of the
+characteristics of the family.<span class='pagenum'><a name="Page_244" id="Page_244">[Pg 244]</a></span></p>
+
+<p>1. <i>Phosphorous acid</i> (H<sub>3</sub>PO<sub>3</sub>). Neither the acid nor its salts are
+at all frequently met with in chemical operations. It can be easily
+obtained, however, in the form of transparent crystals when phosphorus
+trichloride is treated with water and the resulting solution is
+evaporated:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">PCl<sub>3</sub> + 3H<sub>2</sub>O = H<sub>3</sub>PO<sub>3</sub> + 3HCl.<br /></span>
+</div></div>
+
+<p>Its most interesting property is its tendency to take up oxygen and pass
+over into phosphoric acid.</p>
+
+<p>2. <i>Orthophosphoric acid (phosphoric acid)</i> (H<sub>3</sub>PO<sub>4</sub>). This acid can
+be obtained by dissolving phosphorus pentoxide in boiling water, as
+represented in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">P<sub>2</sub>O<sub>5</sub> + 3H<sub>2</sub>O = 2H<sub>3</sub>PO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>It is usually made by treating calcium phosphate with concentrated
+sulphuric acid. The calcium sulphate produced in the reaction is nearly
+insoluble, and can be filtered off, leaving the phosphoric acid in
+solution. Very pure acid is made by oxidizing phosphorus with nitric
+acid. It forms large colorless crystals which are exceedingly soluble in
+water. Being a tribasic acid, it forms acid as well as normal salts.
+Thus the following compounds of sodium are known:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>NaH<sub>2</sub>PO<sub>4</sub></td><td align='left'>monosodium hydrogen phosphate.</td></tr>
+<tr><td align='left'>Na<sub>2</sub>HPO<sub>4</sub></td><td align='left'>disodium hydrogen phosphate.</td></tr>
+<tr><td align='left'>Na<sub>3</sub>PO<sub>4</sub></td><td align='left'>normal sodium phosphate.</td></tr>
+</table></div>
+
+<p>These salts are sometimes called respectively primary, secondary, and
+tertiary phosphates. They may be prepared by bringing together
+phosphoric acid and appropriate quantities of sodium hydroxide.
+Phosphoric acid also forms mixed salts, that is, salts containing two
+different metals. The most familiar compound of this kind is microcosmic
+<span class='pagenum'><a name="Page_245" id="Page_245">[Pg 245]</a></span>salt, which has the formula Na(NH<sub>4</sub>)HPO<sub>4</sub>.</p>
+
+<p><i>Orthophosphates.</i> The orthophosphates form an important class of salts.
+The normal salts are nearly all insoluble and many of them occur in
+nature. The secondary phosphates are as a rule insoluble, while most of
+the primary salts are soluble.</p>
+
+<p>3. <i>Pyrophosphoric acid</i> (H<sub>4</sub>P<sub>2</sub>O<sub>7</sub>). On heating orthophosphoric
+acid to about 225&deg; pyrophosphoric acid is formed in accordance with the
+following equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2H<sub>3</sub>PO<sub>4</sub> = H<sub>4</sub>P<sub>2</sub>O<sub>7</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>It is a white crystalline solid. Its salts can be prepared by heating a
+secondary phosphate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Na<sub>2</sub>HPO<sub>4</sub> = Na<sub>4</sub>P<sub>2</sub>O<sub>7</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>4. <i>Metaphosphoric acid (glacial phosphoric acid)</i> (HPO<sub>3</sub>). This acid
+is formed when orthophosphoric acid is heated above 400&deg;:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>3</sub>PO<sub>4</sub> = HPO<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>It is also formed when phosphorus pentoxide is treated with cold water:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">P<sub>2</sub>O<sub>5</sub> + H<sub>2</sub>O = 2HPO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>It is a white crystalline solid, and is so stable towards heat that it
+can be fused and even volatilized without decomposition. On cooling from
+the fused state it forms a glassy solid, and on this account is often
+called glacial phosphoric acid. It possesses the property of dissolving
+small quantities of metallic oxides, with the formation of compounds
+which, in the case of certain metals, have characteristic colors. It is
+therefore used in the detection of these metals.</p>
+
+<p>While the secondary phosphates, on heating, give salts of pyrophosphoric
+acid, the primary phosphates yield salts of metaphosphoric acid. The
+equations representing these reactions are as follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Na<sub>2</sub>HPO<sub>4</sub> = Na<sub>4</sub>P<sub>3</sub>O<sub>7</sub> + H<sub>2</sub>O,<br /></span>
+</div><div class="stanza">
+<span class="i2">NaH<sub>2</sub>PO<sub>4</sub> = NaPO<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><b>Fertilizers.</b> When crops are produced year after year on the same field
+certain constituents of the soil essential to plant growth are removed,
+and the soil becomes impoverished and unproductive. To make the land
+once more<span class='pagenum'><a name="Page_246" id="Page_246">[Pg 246]</a></span> fertile these constituents must be replaced. The calcium
+phosphate of the mineral deposits or of bone ash serves well as a
+material for restoring phosphorus to soils exhausted of that essential
+element; but a more soluble substance, which the plants can more readily
+assimilate, is desirable. It is better, therefore, to convert the
+insoluble calcium phosphate into the soluble primary phosphate before it
+is applied as fertilizer. It will be seen by reference to the formulas
+for the orthophosphates (see page 244) that in a primary phosphate only
+one hydrogen atom of phosphoric acid is replaced by a metal. Since the
+calcium atom always replaces two hydrogen atoms, it might be thought
+that there could be no primary calcium phosphate; but if the calcium
+atom replaces one hydrogen atom from each of two molecules of phosphoric
+acid, the salt Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub> will result, and this is a primary
+phosphate. It can be made by treatment of the normal phosphate with the
+necessary amount of sulphuric acid, calcium sulphate being formed at the
+same time, thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> + 2H<sub>2</sub>SO<sub>4</sub> = Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub> + 2CaSO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>The resulting mixture is a powder, which is sold as a fertilizer under
+the name of "superphosphate of lime."</p>
+
+
+<h3>ARSENIC</h3>
+
+<p><b>Occurrence.</b> Arsenic occurs in considerable quantities in nature as the
+native element, as the sulphides realgar (As<sub>2</sub>S<sub>2</sub>) and orpiment
+(As<sub>2</sub>S<sub>3</sub>), as oxide (As<sub>2</sub>O<sub>3</sub>), and as a constituent of many
+metallic sulphides, such as arsenopyrite (FeAsS).</p>
+
+<p><b>Preparation.</b> The element is prepared by purifying the native arsenic, or
+by heating the arsenopyrite in iron tubes,<span class='pagenum'><a name="Page_247" id="Page_247">[Pg 247]</a></span> out of contact with air,
+when the reaction expressed by the following equation occurs:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">FeAsS = FeS + As.<br /></span>
+</div></div>
+
+<p>The arsenic, being volatile, condenses in chambers connected with the
+heated tubes. It is also made from the oxide by reduction with carbon:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2As<sub>2</sub>O<sub>3</sub> + 3C = 4As + 3CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p><b>Properties.</b> Arsenic is a steel-gray, metallic-looking substance of
+density 5.73. Though resembling metals in appearance, it is quite
+brittle, being easily powdered in a mortar. When strongly heated it
+sublimes, that is, it passes into a vapor without melting, and condenses
+again to a crystalline solid when the vapor is cooled. Like phosphorus
+it can be obtained in several allotropic forms. It alloys readily with
+some of the metals, and finds its chief use as an alloy with lead, which
+is used for making shot, the alloy being harder than pure lead. When
+heated on charcoal with the blowpipe it is converted into an oxide which
+volatilizes, leaving the charcoal unstained by any oxide coating. It
+burns readily in chlorine gas, forming arsenic trichloride,&mdash;</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">As + 3Cl = AsCl<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Unlike most of its compounds, the element itself is not poisonous.</p>
+
+<p><b>Arsine</b> (AsH<sub>3</sub>). When any compound containing arsenic is brought into
+the presence of nascent hydrogen, arsine (AsH<sub>3</sub>), corresponding to
+phosphine and ammonia, is formed. The reaction when oxide of arsenic is
+so treated is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">As<sub>2</sub>O<sub>3</sub> + 12H = 2AsH<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_248" id="Page_248">[Pg 248]</a></span></p>
+<p>Arsine is a gas with a peculiar garlic-like odor, and is intensely
+poisonous. A single bubble of pure gas has been known to prove fatal. It
+is an unstable compound, decomposing into its elements when heated to a
+moderate temperature. It is combustible, burning with a pale
+bluish-white flame to form arsenic trioxide and water when air is in
+excess:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2AsH<sub>3</sub> + 6O = As<sub>2</sub>O<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>When the supply of air is deficient water and metallic arsenic are
+formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2AsH<sub>3</sub> + 3O = 3H<sub>2</sub>O + 2As.<br /></span>
+</div></div>
+
+<p>These reactions make the detection of even minute quantities of arsenic
+a very easy problem.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig72.jpg" width="400" height="295" alt="Fig. 72" title="" />
+<span class="caption">Fig. 72</span>
+</div>
+
+<div class="blockquot"><p><b>Marsh's test for arsenic.</b> The method devised by Marsh for
+detecting arsenic is most frequently used, the apparatus being
+shown in Fig. 72. Hydrogen is generated in the flask <i>A</i> by the
+action of dilute sulphuric acid on zinc, is dried by passing
+over calcium chloride in the tube <i>B</i>, and after passing
+through the hard-glass tube <i>C</i> is ignited at the jet <i>D</i>. If a
+substance containing arsenic is now introduced into the
+generator <i>A</i>, the arsenic is converted into arsine by the
+action of the nascent hydrogen, and<span class='pagenum'><a name="Page_249" id="Page_249">[Pg 249]</a></span> passes to the jet along
+with the hydrogen. If the tube <i>C</i> is strongly heated at some
+point near the middle, the arsine is decomposed while passing
+this point and the arsenic is deposited just beyond the heated
+point in the form of a shining, brownish-black mirror. If the
+tube is not heated, the arsine burns along with the hydrogen at
+the jet. Under these conditions a small porcelain dish crowded
+down into the flame is blackened by a spot of metallic arsenic,
+for the arsine is decomposed by the heat of the flame, and the
+arsenic, cooled below its kindling temperature by the cold
+porcelain, deposits upon it as a black spot. Antimony conducts
+itself in the same way as arsenic, but the antimony deposit is
+more sooty in appearance. The two can also be distinguished by
+the fact that sodium hypochlorite (NaClO) dissolves the arsenic
+deposit, but not that formed by antimony.</p></div>
+
+<p><b>Oxides of arsenic.</b> Arsenic forms two oxides, As<sub>2</sub>O<sub>3</sub> and
+As<sub>2</sub>O<sub>5</sub>, corresponding to those of phosphorus. Of these arsenious
+oxide, or arsenic trioxide (As<sub>2</sub>O<sub>3</sub>), is much better known, and is
+the substance usually called white arsenic, or merely arsenic. It is
+found as a mineral, but is usually obtained as a by-product in burning
+pyrite in the sulphuric-acid industry. The pyrite has a small amount of
+arsenopyrite in it, and when this is burned arsenious oxide is formed as
+a vapor together with sulphur dioxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2FeAsS + 10O = Fe<sub>2</sub>O<sub>3</sub> + As<sub>2</sub>O<sub>3</sub> + 2SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>The arsenious oxide is condensed in appropriate chambers. It is a rather
+heavy substance, obtained either as a crystalline powder or as large,
+vitreous lumps, resembling lumps of porcelain in appearance. It is very
+poisonous, from 0.2 to 0.3 g. being a fatal dose. It is frequently given
+as a poison, since it is nearly tasteless and does not act very rapidly.
+This slow action is due to the fact that it is not very soluble, and
+hence is absorbed slowly by the system. Arsenious oxide is also used as
+a chemical reagent in glass making and in the dye industry.<span class='pagenum'><a name="Page_250" id="Page_250">[Pg 250]</a></span></p>
+
+<p><b>Acids of arsenic.</b> Like the corresponding oxides of phosphorus, the
+oxides of arsenic are acid anhydrides. In solution they combine with
+bases to form salts, corresponding to the salts of the acids of
+phosphorus. Thus we have salts of the following acids:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>H<sub>3</sub>AsO<sub>3</sub></td><td align='left'>arsenious acid.</td></tr>
+<tr><td align='left'>H<sub>3</sub>AsO<sub>4</sub></td><td align='left'>orthoarsenic acid.</td></tr>
+<tr><td align='left'>H<sub>4</sub>As<sub>2</sub>O<sub>3</sub></td><td align='left'>pyroarsenic acid.</td></tr>
+<tr><td align='left'>HAsO<sub>3</sub></td><td align='left'>metarsenic acid.</td></tr>
+</table></div>
+
+
+<p>Several other acids of arsenic are also known. Not all of these can be
+obtained as free acids, since they tend to lose water and form the
+oxides. Thus, instead of obtaining arsenious acid (H<sub>3</sub>AsO<sub>3</sub>), the
+oxide As<sub>2</sub>O<sub>3</sub> is obtained:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2H<sub>3</sub>AsO<sub>3</sub> = As<sub>2</sub>O<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Salts of all the acids are known, however, and some of them have
+commercial value. Most of them are insoluble, and some of the copper
+salts, which are green, are used as pigments. Paris green, which has a
+complicated formula, is a well-known insecticide.</p>
+
+<p><b>Antidote for arsenical poisoning.</b> The most efficient antidote for
+arsenic poisoning is ferric hydroxide. It is prepared as needed,
+according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 3Mg(OH)<sub>2</sub> = 2Fe(OH)<sub>3</sub> + 3MgSO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p><b>Sulphides of arsenic.</b> When hydrogen sulphide is passed into an acidified
+solution containing an arsenic compound the arsenic is precipitated as a
+bright yellow sulphide, thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2H<sub>3</sub>AsO<sub>3</sub> + 3H<sub>2</sub>S = As<sub>2</sub>S<sub>3</sub> + 6H<sub>2</sub>O,<br /></span>
+</div><div class="stanza">
+<span class="i0">2H<sub>3</sub>AsO<sub>4</sub> + 5H<sub>2</sub>S = As<sub>2</sub>S<sub>5</sub> + 8H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>In this respect arsenic resembles the metallic elements, many of which
+produce sulphides under similar conditions. The sulphides of arsenic,
+both those produced artificially and those found in nature, are used as
+yellow pigments.</p>
+
+
+<h3>ANTIMONY</h3>
+
+<p><b>Occurrence.</b> Antimony occurs in nature chiefly as the sulphide
+(Sb<sub>2</sub>S<sub>3</sub>), called stibnite, though it is also found as oxide and as
+a constituent of many complex minerals.<span class='pagenum'><a name="Page_251" id="Page_251">[Pg 251]</a></span></p>
+
+<p><b>Preparation.</b> Antimony is prepared from the sulphide in a very simple
+manner. The sulphide is melted with scrap iron in a furnace, when the
+iron combines with the sulphur to form a slag, or liquid layer of melted
+iron sulphide, while the heavier liquid, antimony, settles to the bottom
+and is drawn off from time to time. The reaction involved is represented
+by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Sb<sub>2</sub>S<sub>3</sub> + 3Fe = 2Sb + 3FeS.<br /></span>
+</div></div>
+
+<p><b>Physical properties.</b> Antimony is a bluish-white, metallic-looking
+substance whose density is 6.7. It is highly crystalline, hard, and very
+brittle. It has a rather low melting point (432&deg;) and expands very
+noticeably on solidifying.</p>
+
+<p><b>Chemical properties.</b> In chemical properties antimony resembles arsenic
+in many particulars. It forms the oxides Sb<sub>2</sub>O<sub>3</sub> and Sb<sub>2</sub>O<sub>5</sub>,
+and in addition Sb<sub>2</sub>O<sub>4</sub>. It combines with the halogen elements with
+great energy, burning brilliantly in chlorine to form antimony
+trichloride (SbCl<sub>3</sub>). When heated on charcoal with the blowpipe it is
+oxidized and forms a coating of antimony oxide on the charcoal which has
+a characteristic bluish-white color.</p>
+
+<p><b>Stibine</b> (SbH<sub>3</sub>). The gas stibine (SbH<sub>3</sub>) is formed under conditions
+which are very similar to those which produce arsine, and it closely
+resembles the latter compound, though it is still less stable. It is
+very poisonous.</p>
+
+<div class="blockquot"><p><b>Acids of antimony.</b> The oxides Sb_{2}O_{3} and Sb_{2}O_{5} are
+weak acid anhydrides and are capable of forming two series of
+acids corresponding in formulas to the acids of phosphorus and
+arsenic. They are much weaker, however, and are of little
+practical importance.</p>
+
+<p><b>Sulphides of antimony.</b> Antimony resembles arsenic in that
+hydrogen sulphide precipitates it as a sulphide when conducted
+into an acidified solution containing an antimony compound:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2SbCl<sub>3</sub> + 3H<sub>2</sub>S = Sb<sub>2</sub>S<sub>3</sub> + 6HCl,<br /></span>
+</div><div class="stanza">
+<span class="i0">2SbCl<sub>5</sub> + 5H<sub>2</sub>S = Sb<sub>2</sub>S<sub>5</sub> + 10HCl.<br /></span>
+<span class='pagenum'><a name="Page_252" id="Page_252">[Pg 252]</a></span></div></div>
+
+<p>The two sulphides of antimony are called the trisulphide and
+the pentasulphide respectively. When prepared in this way they
+are orange-colored substances, though the mineral stibnite is
+black.</p></div>
+
+<p><b>Metallic properties of antimony.</b> The physical properties of the element
+are those of a metal, and the fact that its sulphide is precipitated by
+hydrogen sulphide shows that it acts like a metal in a chemical way.
+Many other reactions show that antimony has more of the properties of a
+metal than of a non-metal. The compound Sb(OH)<sub>3</sub>, corresponding to
+arsenious acid, while able to act as a weak acid is also able to act as
+a weak base with strong acids. For example, when treated with
+concentrated hydrochloric acid antimony chloride is formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Sb(OH)<sub>3</sub> + 3HCl = SbCl<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>A number of elements act in this same way, their hydroxides under some
+conditions being weak acids and under others weak bases.</p>
+
+
+<h3>ALLOYS</h3>
+
+<p>Some metals when melted together thoroughly intermix, and on cooling
+form a homogeneous, metallic-appearing substance called an <i>alloy</i>. Not
+all metals will mix in this way, and in some cases definite chemical
+compounds are formed and separate out as the mixture solidifies, thus
+destroying the uniform quality of the alloy. In general the melting
+point of the alloy is below the average of the melting points of its
+constituents, and it is often lower than any one of them.</p>
+
+<p>Antimony forms alloys with many of the metals, and its chief commercial
+use is for such purposes. It imparts to its alloys high density, rather
+low melting point, and the<span class='pagenum'><a name="Page_253" id="Page_253">[Pg 253]</a></span> property of expanding on solidification.
+Such an alloy is especially useful in type founding, where fine lines
+are to be reproduced on a cast. Type metal consists of antimony, lead,
+and tin. Babbitt metal, used for journal bearings in machinery, contains
+the same metals in a different proportion together with a small
+percentage of copper.</p>
+
+
+<h3>BISMUTH</h3>
+
+<p><b>Occurrence.</b> Bismuth is usually found in the uncombined form in nature.
+It also occurs as oxide and sulphide. Most of the bismuth of commerce
+comes from Saxony, and from Mexico and Colorado, but it is not an
+abundant element.</p>
+
+<p><b>Preparation.</b> It is prepared by merely heating the ore containing the
+native bismuth and allowing the melted metal to run out into suitable
+vessels. Other ores are converted into oxides and reduced by heating
+with carbon.</p>
+
+<p><b>Physical properties.</b> Bismuth is a heavy, crystalline, brittle metal
+nearly the color of silver, but with a slightly rosy tint which
+distinguishes it from other metals. It melts at a low temperature (270&deg;)
+and has a density of 9.8. It is not acted upon by the air at ordinary
+temperatures.</p>
+
+<p><b>Chemical properties.</b> When heated with the blowpipe on charcoal, bismuth
+gives a coating of the oxide Bi<sub>2</sub>O<sub>3</sub>. This has a yellowish-brown
+color which easily distinguishes it from the oxides formed by other
+metals. It combines very readily with the halogen elements, powdered
+bismuth burning readily in chlorine. It is not very easily acted upon by
+hydrochloric acid, but nitric and sulphuric acids act upon it in the
+same way that they do upon copper.</p>
+
+<p><b>Uses.</b> Bismuth finds its chief use as a constituent of alloys,
+particularly in those of low melting point. Some<span class='pagenum'><a name="Page_254" id="Page_254">[Pg 254]</a></span> of these melt in hot
+water. For example, Wood's metal, consisting of bismuth, lead, tin, and
+cadmium, melts at 60.5&deg;.</p>
+
+<p><b>Compounds of bismuth.</b> Unlike the other elements of this group, bismuth
+has almost no acid properties. Its chief oxide, Bi<sub>2</sub>O<sub>3</sub>, is basic in
+its properties. It dissolves in strong acids and forms salts of bismuth:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Bi<sub>2</sub>O<sub>3</sub> + 6HCl = 2BiCl<sub>3</sub> + 3H<sub>2</sub>O,<br /></span>
+</div><div class="stanza">
+<span class="i0">Bi<sub>2</sub>O<sub>3</sub> + 6HNO<sub>3</sub> = 2Bi(NO<sub>3</sub>)<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The nitrate and chloride of bismuth can be obtained as well-formed
+colorless crystals. When treated with water the salts are decomposed in
+the manner explained in the following paragraph.</p>
+
+
+<h3>HYDROLYSIS</h3>
+
+<p>Many salts such as those of antimony and bismuth form solutions which
+are somewhat acid in reaction, and must therefore contain hydrogen ions.
+This is accounted for by the same principle suggested to explain the
+fact that solutions of potassium cyanide are alkaline in reaction (p.
+210). Water forms an appreciable number of hydrogen and hydroxyl ions,
+and very weak bases such as bismuth hydroxide are dissociated to but a
+very slight extent. When Bi<sup>+++</sup> ions from bismuth chloride, which
+dissociates very readily, are brought in contact with the OH<sup>-</sup> ions
+from water, the two come to the equilibrium expressed in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Bi<sup>+++</sup> + 3OH<sup>-</sup> &lt;--> Bi(OH)<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>For every hydroxyl ion removed from the solution in this way a hydrogen
+ion is left free, and the solution becomes acid in reaction.<span class='pagenum'><a name="Page_255" id="Page_255">[Pg 255]</a></span></p>
+
+<p>Reactions of this kind and that described under potassium cyanide are
+called <i>hydrolysis</i>.</p>
+
+<p>DEFINITION: <i>Hydrolysis is the action of water upon a salt to form an
+acid and a base, one of which is very slightly dissociated.</i></p>
+
+<p><b>Conditions favoring hydrolysis.</b> While hydrolysis is primarily due to the
+slight extent to which either the acid or the base formed is
+dissociated, several other factors have an influence upon the extent to
+which it will take place.</p>
+
+<p>1. <i>Influence of mass.</i> Since hydrolysis is a reversible reaction, the
+relative masses of the reacting substances influence the point at which
+equilibrium will be reached. In the equilibrium</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">BiCl<sub>3</sub> + 3H<sub>2</sub>O &lt;--> Bi(OH)<sub>3</sub> + 3HCl<br /></span>
+</div></div>
+
+<p>the addition of more water will result in the formation of more bismuth
+hydroxide and hydrochloric acid. The addition of more hydrochloric acid
+will convert some of the bismuth hydroxide into bismuth chloride.</p>
+
+<p>2. <i>Formation of insoluble substances.</i> When one of the products of
+hydrolysis is nearly insoluble in water the solution will become
+saturated with it as soon as a very little has been formed. All in
+excess of this will precipitate, and the reaction will go on until the
+acid set free increases sufficiently to bring about an equilibrium. Thus
+a considerable amount of bismuth and antimony hydroxides are
+precipitated when water is added to the chlorides of these elements. The
+greater the dilution the more hydroxide precipitates. The addition of
+hydrochloric acid in considerable quantity will, however, redissolve the
+precipitate.</p>
+
+<p><b>Partial hydrolysis.</b> In many cases the hydrolysis of a salt is only
+partial, resulting in the formation of basic salts instead of the free
+base. Most of these basic salts are insoluble in water, which accounts
+for their ready formation. Thus bismuth chloride may hydrolyze by
+successive steps, as shown in the equations</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">BiCl<sub>3</sub> + H<sub>2</sub>O = Bi(OH)Cl<sub>2</sub> + HCl,<br /></span>
+</div><div class="stanza">
+<span class="i0">BiCl<sub>3</sub> + 2H<sub>2</sub>O = Bi(OH)<sub>2</sub>Cl + 2HCl,<br /></span>
+</div><div class="stanza">
+<span class="i0">BiCl<sub>3</sub> + 3H<sub>2</sub>O = Bi(OH)<sub>3</sub> + 3HCl.<br /></span>
+</div></div>
+
+<p>The basic salt so formed may also lose water, as shown in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Bi(OH)<sub>2</sub>Cl = BiOCl + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_256" id="Page_256">[Pg 256]</a></span></p>
+<p>The salt represented in the last equation is sometimes called bismuth
+oxychloride, or bismuthyl chloride. The corresponding nitrate,
+BiONO<sub>3</sub>, is largely used in medicine under the name of subnitrate of
+bismuth. In these two compounds the group of atoms, BiO, acts as a
+univalent metallic radical and is called <i>bismuthyl</i>. Similar basic
+salts are formed by the hydrolysis of antimony salts.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Name all the elements so far studied which possess allotropic forms.</p>
+
+<p><b>2.</b> What compounds would you expect phosphorus to form with bromine and
+iodine? Write the equations showing the action of water on these
+compounds.</p>
+
+<p><b>3.</b> In the preparation of phosphine, why is coal gas passed into the
+flask? What other gases would serve the same purpose?</p>
+
+<p><b>4.</b> Give the formula for the salt which phosphine forms with hydriodic
+acid. Give the name of the compound.</p>
+
+<p><b>5.</b> Could phosphoric acid be substituted for sulphuric acid in the
+preparation of the common acids?</p>
+
+<p><b>6.</b> Write the equations for the preparation of the three sodium salts of
+orthophosphoric acid.</p>
+
+<p><b>7.</b> Why does a solution of disodium hydrogen phosphate react alkaline?</p>
+
+<p><b>8.</b> On the supposition that bone ash is pure calcium phosphate, what
+weight of it would be required in the preparation of 1 kg. of
+phosphorus?</p>
+
+<p><b>9.</b> If arsenopyrite is heated in a current of air, what products are
+formed?</p>
+
+<p><b>10.</b> (<i>a</i>) Write equations for the complete combustion of hydrosulphuric
+acid, methane, and arsine. (<i>b</i>) In what respects are the reactions
+similar?</p>
+
+<p><b>11.</b> Write the equations for all the reactions involved in Marsh's test
+for arsenic.</p>
+
+<p><b>12.</b> Write the names and formulas for the acids of antimony.</p>
+
+<p><b>13.</b> Write the equations showing the hydrolysis of antimony trichloride;
+of bismuth nitrate.</p>
+
+<p><b>14.</b> In what respects does nitrogen resemble the members of the
+phosphorus family?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_257" id="Page_257">[Pg 257]</a></span></p>
+<h2><a name="CHAPTER_XXI" id="CHAPTER_XXI"></a>CHAPTER XXI</h2>
+
+<h3>SILICON, TITANIUM, BORON</h3>
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>SYMBOL</td><td align='left'>ATOMIC WEIGHT</td><td align='left'>DENSITY</td><td align='left'>CHLORIDES</td><td align='left'>OXIDES</td></tr>
+<tr><td align='left'>Silicon</td><td align='center'>Si</td><td align='center'>28.4</td><td align='center'>2.35</td><td align='center'>SiCl<sub>4</sub></td><td align='center'>SiO</td></tr>
+<tr><td align='left'>Titanium</td><td align='center'>Ti</td><td align='center'>48.1</td><td align='center'>3.5</td><td align='center'>TiCl<sub>4</sub></td><td align='center'>TiO</td></tr>
+<tr><td align='left'>Boron</td><td align='center'>B</td><td align='center'>11.0</td><td align='center'>2.45</td><td align='center'>BCl<sub>3</sub></td><td align='center'>B<sub>2</sub>O<sub>3</sub></td></tr>
+</table></div>
+
+
+<p><b>General.</b> Each of the three elements, silicon, titanium, and boron,
+belongs to a separate periodic family, but they occur near together in
+the periodic grouping and are very similar in both physical and chemical
+properties. Since the other elements in their families are either so
+rare that they cannot be studied in detail, or are best understood in
+connection with other elements, it is convenient to consider these three
+together at this point.</p>
+
+<p>The three elements are very difficult to obtain in the free state, owing
+to their strong attraction for other elements. They can be prepared by
+the action of aluminium or magnesium on their oxides and in impure state
+by reduction with carbon in an electric furnace. They are very hard and
+melt only at the highest temperatures. At ordinary temperatures they are
+not attacked by oxygen, but when strongly heated they burn with great
+brilliancy. Silicon and boron are not attacked by acids under ordinary
+conditions; titanium is easily dissolved by them.<span class='pagenum'><a name="Page_258" id="Page_258">[Pg 258]</a></span></p>
+
+
+<h3>SILICON</h3>
+
+<p><b>Occurrence.</b> Next to oxygen silicon is the most abundant element. It does
+not occur free in nature, but its compounds are very abundant and of the
+greatest importance. It occurs almost entirely in combination with
+oxygen as silicon dioxide (SiO<sub>2</sub>), often called silica, or with oxygen
+and various metals in the form of salts of silicic acids, or silicates.
+These compounds form a large fraction of the earth's crust. Most plants
+absorb small amounts of silica from the soil, and it is also found in
+minute quantities in animal organisms.</p>
+
+<p><b>Preparation.</b> The element is most easily prepared by reducing pure
+powdered quartz with magnesium powder:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SiO<sub>2</sub> + 2Mg = 2MgO + Si.<br /></span>
+</div></div>
+
+<p><b>Properties.</b> As would be expected from its place in the periodic table,
+silicon resembles carbon in many respects. It can be obtained in several
+allotropic forms, corresponding to those of carbon. The crystallized
+form is very hard, and is inactive toward reagents. The amorphous
+variety has, in general, properties more similar to charcoal.</p>
+
+<p><b>Compounds of silicon with hydrogen and the halogens.</b> Silicon hydride
+(SiH<sub>4</sub>) corresponds in formula to methane (CH<sub>4</sub>), but its properties
+are more like those of phosphine (PH<sub>3</sub>). It is a very inflammable gas
+of disagreeable odor, and, as ordinarily prepared, takes fire
+spontaneously on account of the presence of impurities.</p>
+
+<p>Silicon combines with the elements of the chlorine family to form such
+compounds as SiCl<sub>4</sub> and SiF<sub>4</sub>. Of these silicon fluoride is the most
+familiar and interesting. As stated in the discussion of fluorine, it is
+formed when<span class='pagenum'><a name="Page_259" id="Page_259">[Pg 259]</a></span> hydrofluoric acid acts upon silicon dioxide or a silicate.
+With silica the reaction is thus expressed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SiO<sub>2</sub> + 4HF = SiF<sub>4</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>It is a very volatile, invisible, poisonous gas. In contact with water
+it is partially decomposed, as shown in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SiF<sub>4</sub> + 4H<sub>2</sub>O = 4HF + Si(OH)<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>The hydrofluoric acid so formed combines with an additional amount of
+silicon fluoride, forming the complex fluosilicic acid (H<sub>2</sub>SiF<sub>6</sub>),
+thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HF + SiF<sub>4</sub> = H<sub>2</sub>SiF<sub>6</sub>.<br /></span>
+</div></div>
+
+<p><b>Silicides.</b> As the name indicates, silicides are binary compounds
+consisting of silicon and some other element. They are very stable at
+high temperatures, and are usually made by heating the appropriate
+substances in an electric furnace. The most important one is
+<i>carborundum</i>, which is a silicide of carbon of the formula CSi. It is
+made by heating coke and sand, which is a form of silicon dioxide, in an
+electric furnace, the process being extensively carried on at Niagara
+Falls. The following equation represents the reaction</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SiO<sub>2</sub> + 3C = CSi + 2CO.<br /></span>
+</div></div>
+
+<p>The substance so prepared consists of beautiful purplish-black crystals,
+which are very hard. Carborundum is used as an abrasive, that is, as a
+material for grinding and polishing very hard substances. Ferrosilicon
+is a silicide of iron alloyed with an excess of iron, which finds
+extensive use in the manufacture of certain kinds of steel.<span class='pagenum'><a name="Page_260" id="Page_260">[Pg 260]</a></span></p>
+
+<p><b>Manufacture of carborundum.</b> The mixture of materials is heated in a
+large resistance furnace for about thirty-six hours. After the reaction
+is completed there is left a core of graphite <i>G</i>. Surrounding this core
+is a layer of crystallized carborundum <i>C</i>, about 16 in. thick. Outside
+this is a shell of amorphous carborundum <i>A</i>. The remaining materials
+<i>M</i> are unchanged and are used for a new charge.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig73.jpg" width="400" height="172" alt="Fig. 73" title="" />
+<span class="caption">Fig. 73</span>
+</div>
+
+<p><b>Silicon dioxide</b> (<i>silica</i>) (SiO<sub>2</sub>). This substance is found in a great
+variety of forms in nature, both in the amorphous and in the crystalline
+condition. In the form of quartz it is found in beautifully formed
+six-sided prisms, sometimes of great size. When pure it is perfectly
+transparent and colorless. Some colored varieties are given special
+names, as amethyst (violet), rose quartz (pale pink), smoky or milky
+quartz (colored and opaque). Other varieties of silicon dioxide, some of
+which also contain water, are chalcedony, onyx, jasper, opal, agate, and
+flint. Sand and sandstone are largely silicon dioxide.</p>
+
+<p><b>Properties.</b> As obtained by chemical processes silicon dioxide is an
+amorphous white powder. In the crystallized state it is very hard and
+has a density of 2.6. It is insoluble in water and in most chemical
+reagents, and requires the hottest oxyhydrogen flame for fusion. Acids,
+excepting hydrofluoric acid, have little action on it, and it requires
+the most energetic reducing agents to deprive it of oxygen. It is the
+anhydride of an acid, and consequently it dissolves in fused alkalis to
+form silicates. Being nonvolatile, it will drive out most other
+anhydrides when heated<span class='pagenum'><a name="Page_261" id="Page_261">[Pg 261]</a></span> to a high temperature with their salts,
+especially when the silicates so formed are fusible. The following
+equations illustrate this property:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + SiO<sub>2</sub> = Na<sub>2</sub>SiO<sub>3</sub> + CO<sub>2</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">Na<sub>2</sub>SO<sub>4</sub> + SiO<sub>2</sub> = Na<sub>2</sub>SiO<sub>3</sub> + SO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><b>Silicic acids.</b> Silicon forms two simple acids, orthosilicic acid
+(H<sub>4</sub>SiO<sub>4</sub>) and metasilicic acid (H<sub>2</sub>SiO<sub>3</sub>). Orthosilicic acid is
+formed as a jelly-like mass when orthosilicates are treated with strong
+acids such as hydrochloric. On attempting to dry this acid it loses
+water, passing into metasilicic or common silicic acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>4</sub>SiO<sub>4</sub> = H<sub>2</sub>SiO<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Metasilicic acid when heated breaks up into silica and water, thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>SiO<sub>3</sub> = H<sub>2</sub>O + SiO<sub>2</sub>.<br /></span>
+</div></div>
+
+<div class="blockquot"><p><b>Salts of silicic acids,&mdash;silicates.</b> A number of salts of the
+orthosilicic and metasilicic acids occur in nature. Thus mica
+(KAlSiO<sub>4</sub>) is a salt of orthosilicic acid.</p></div>
+
+<p><b>Polysilicic acids.</b> Silicon has the power to form a great many complex
+acids which may be regarded as derived from the union of several
+molecules of the orthosilicic acid, with the loss of water. Thus we have</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3H<sub>4</sub>SiO<sub>4</sub> = H<sub>4</sub>Si<sub>3</sub>O<sub>8</sub> + 4H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>These acids cannot be prepared in the pure state, but their salts form
+many of the crystalline rocks in nature. Feldspar, for example, has the
+formula KAlSi<sub>3</sub>O<sub>8</sub>, and is a mixed salt of the acid
+H<sub>4</sub>Si<sub>3</sub>O<sub>8</sub>, whose formation is represented in the equation above.
+Kaolin has the formula Al<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>&middot;2H<sub>2</sub>O. Many other examples
+will be met in the study of the metals.<span class='pagenum'><a name="Page_262" id="Page_262">[Pg 262]</a></span></p>
+
+<p><b>Glass.</b> When sodium and calcium silicates, together with silicon dioxide,
+are heated to a very high temperature, the mixture slowly fuses to a
+transparent liquid, which on cooling passes into the solid called glass.
+Instead of starting with sodium and calcium silicates it is more
+convenient and economical to heat sodium carbonate (or sulphate) and
+lime with an excess of clean sand, the silicates being formed during the
+heating:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + SiO<sub>2</sub> = Na<sub>2</sub>SiO<sub>3</sub> + CO<sub>2</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">CaO + SiO<sub>2</sub> = CaSiO<sub>3</sub>.<br /></span>
+</div></div>
+
+<div class="figcenter" style="width: 287px;">
+<img src="images/imagefig74.jpg" width="287" height="400" alt="Fig. 74" title="" />
+<span class="caption">Fig. 74</span>
+</div>
+
+<p>The mixture is heated below the fusing point for some time, so that the
+escaping carbon dioxide may not spatter the hot liquid; the heat is then
+increased and the mixture kept in a state of fusion until all gases
+formed in the reaction have escaped.</p>
+
+<p><i>Molding and blowing of glass.</i> The way in which the melted mixture is
+handled in the glass factory depends upon the character of the article
+to be made. Many articles, such as bottles, are made by blowing the
+plastic glass into hollow molds of the desired shape. The mold is first
+opened, as shown in Fig. 74. A lump of plastic glass <i>A</i> on the hollow
+rod <i>B</i> is lowered into the mold, which is then closed by the handles
+<i>C</i>. By blowing into the tube the glass is blown into the shape of the
+mold. The mold is then opened and the bottle lifted out. The neck of the
+bottle must be cut off at the proper place and the sharp edges rounded
+off in a flame.</p>
+
+<p>Other objects, such as lamp chimneys, are made by getting a lump of
+plastic glass on the end of a hollow iron rod and blowing it into the
+desired shape without the help of a mold, great skill being required in
+the manipulation of the glass. Window glass is made by blowing large
+hollow cylinders about 6 ft. long and 1-1/2 ft. in diameter. These are
+cut longitudinally, and are then placed in an oven and heated until they
+soften, when they are flattened out into plates (Fig. 75). Plate glass
+is cast into flat slabs, which are then ground and polished to perfectly
+plane surfaces.<span class='pagenum'><a name="Page_263" id="Page_263">[Pg 263]</a></span></p>
+
+<p><i>Varieties of glass.</i> The ingredients mentioned above make a soft,
+easily fusible glass. If potassium carbonate is substituted for the
+sodium carbonate, the glass is much harder and less easily fused;
+increasing the amount of sand has somewhat the same effect. Potassium
+glass is largely used in making chemical glassware, since it resists the
+action of reagents better than the softer sodium glass. If lead oxide is
+substituted for the whole or a part of the lime, the glass is very soft,
+but has a high index of refraction and is valuable for making optical
+instruments and artificial jewels.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig75.jpg" width="400" height="134" alt="Fig. 75" title="" />
+<span class="caption">Fig. 75</span>
+</div>
+
+<p><i>Coloring of glass.</i> Various substances fused along with the glass
+mixture give characteristic colors. The amber color of common bottles is
+due to iron compounds in the glass; in other cases iron colors the glass
+green. Cobalt compounds color it deep blue; those of manganese give it
+an amethyst tint and uranium compounds impart a peculiar yellowish green
+color. Since iron is nearly always present in the ingredients, glass is
+usually slightly yellow. This color can be removed by adding the proper
+amount of manganese dioxide, for the amethyst color of manganese and the
+yellow of iron together produce white light.</p>
+
+<p><i>Nature of glass.</i> Glass is not a definite chemical compound and its
+composition varies between wide limits. Fused glass is really a solution
+of various silicates, such as those of calcium and lead, in fused sodium
+or potassium silicate. A certain amount of silicon dioxide is also
+present. This solution is then allowed to solidify under such conditions
+of cooling that the dissolved substances do not separate from the
+solvent. The compounds which are used to color the glass are sometimes
+converted into silicates, which then dissolve in the glass, giving it a
+uniform color. In other cases, as in the milky glasses which resemble
+porcelain in appearance, the color or opaqueness is due to the finely
+divided color material evenly distributed throughout the glass, but not
+dissolved in it. Milky glass is made by mixing calcium fluoride, tin
+oxide, or some other insoluble substance in the melted glass. Copper or
+gold in metallic form scattered through glass gives it shades of red.<span class='pagenum'><a name="Page_264" id="Page_264">[Pg 264]</a></span></p>
+
+
+<h3>TITANIUM</h3>
+
+<div class="blockquot"><p>Titanium is a very widely distributed element in nature, being
+found in almost all soils, in many rocks, and even in plant and
+animal tissues. It is not very abundant in any one locality,
+and it possesses little commercial value save in connection
+with the iron industry. Its most common ore is rutile
+(TiO<sub>2</sub>), which resembles silica in many respects.</p>
+
+<p>In both physical and chemical properties titanium resembles
+silicon, though it is somewhat more metallic in character. This
+resemblance is most marked in the acids of titanium. It not
+only forms metatitanic and orthotitanic acids but a great
+variety of polytitanic acids as well.</p></div>
+
+
+<h3>BORON</h3>
+
+<p><b>Occurrence.</b> Boron is never found free in nature. It occurs as boric acid
+(H<sub>3</sub>BO<sub>3</sub>), and in salts of polyboric acids, which usually have very
+complicated formulas.</p>
+
+<p><b>Preparation and properties.</b> Boron can be prepared from its oxide by
+reduction with magnesium, exactly as in the case of silicon. It
+resembles silicon very strikingly in its properties. It occurs in
+several allotropic forms, is very hard when crystallized, and is rather
+inactive toward reagents. It forms a hydride, BH<sub>3</sub>, and combines
+directly with the elements of the chlorine family. Boron fluoride
+(BF<sub>3</sub>) is very similar to silicon fluoride in its mode of formation
+and chemical properties.</p>
+
+<p><b>Boric oxide</b> (B<sub>2</sub>O<sub>3</sub>). Boron forms one well-known oxide, B<sub>2</sub>O<sub>3</sub>,
+called boric anhydride. It is formed as a glassy mass by heating boric
+acid to a high temperature. It absorbs water very readily, uniting with
+it to form boric acid again:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">B<sub>2</sub>O<sub>3</sub> + 3H<sub>2</sub>O = 2H<sub>3</sub>BO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>In this respect it differs from silicon dioxide, which will not combine
+directly with water.<span class='pagenum'><a name="Page_265" id="Page_265">[Pg 265]</a></span></p>
+
+<p><b>Boric acid</b> (H<sub>3</sub>BO<sub>3</sub>). This is found in nature in considerable
+quantities and forms one of the chief sources of boron compounds. It is
+found dissolved in the water of hot springs in some localities,
+particularly in Italy. Being volatile with steam, the vapor which
+escapes from these springs has some boric acid in it. It is easily
+obtained from these sources by condensation and evaporation, the
+necessary heat being supplied by other hot springs.</p>
+
+<p>Boric acid crystallizes in pearly flakes, which are greasy to the touch.
+In the laboratory it is easily prepared by treating a strong, hot
+solution of borax with sulphuric acid. Boric acid being sparingly
+soluble in water crystallizes out on cooling:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub> + 5H<sub>2</sub>O + H<sub>2</sub>SO<sub>4</sub> = Na<sub>2</sub>SO<sub>4</sub> + 4H<sub>3</sub>BO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>The substance is a mild antiseptic, and on this account is often used in
+medicine and as a preservative for canned foods and milk.</p>
+
+<p><b>Metaboric and polyboric acids.</b> When boric acid is gently heated it is
+converted into metaboric acid (HBO<sub>2</sub>):</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>3</sub>BO<sub>3</sub> = HBO<sub>2</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>On heating metaboric acid to a somewhat higher temperature tetraboric
+acid (H<sub>2</sub>B<sub>4</sub>O<sub>7</sub>) is formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">4HBO<sub>2</sub> = H<sub>2</sub>B<sub>4</sub>O<sub>7</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Many other complex acids of boron are known.</p>
+
+<p><b>Borax.</b> Borax is the sodium salt of tetraboric acid, having the formula
+Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub>&middot;10 H<sub>2</sub>O. It is found in some arid countries, as
+southern California and Tibet, but is now made commercially from the
+mineral colemanite, which is the calcium salt of a complex boric acid.
+When this is treated with a solution of sodium carbonate, calcium<span class='pagenum'><a name="Page_266" id="Page_266">[Pg 266]</a></span>
+carbonate is precipitated and borax crystallizes from the solution.</p>
+
+<p>When heated borax at first swells up greatly, owing to the expulsion of
+the water of crystallization, and then melts to a clear glass. This
+glass has the property of easily dissolving many metallic oxides, and on
+this account borax is used as a flux in soldering, for the purpose of
+removing from the metallic surfaces to be soldered the film of oxide
+with which they are likely to be covered. These oxides often give a
+characteristic color to the clear borax glass, and borax beads are
+therefore often used in testing for the presence of metals, instead of
+the metaphosphoric acid bead already described.</p>
+
+<div class="blockquot"><p>The reason that metallic oxides dissolve in borax is that borax
+contains an excess of acid anhydride, as can be more easily
+seen if its formula is written 2NaBO<sub>2</sub> + B<sub>2</sub>O<sub>3</sub>. The
+metallic oxide combines with this excess of acid anhydride,
+forming a mixed salt of metaboric acid.</p></div>
+
+<p>Borax is extensively used as a constituent of enamels and glazes for
+both metal ware and pottery. It is also used as a flux in soldering and
+brazing, and in domestic ways it serves as a mild alkali, as a
+preservative for meats, and in a great variety of less important
+applications.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Account for the fact that a solution of borax in water is alkaline.</p>
+
+<p><b>2.</b> What weight of water of crystallization does 1 kg. of borax contain?</p>
+
+<p><b>3.</b> When a concentrated solution of borax acts on silver nitrate a borate
+of silver is formed. If the solution of borax is dilute, however, an
+hydroxide of silver forms. Account for this difference in behavior.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_267" id="Page_267">[Pg 267]</a></span></p>
+<h2><a name="CHAPTER_XXII" id="CHAPTER_XXII"></a>CHAPTER XXII</h2>
+
+<h3>THE METALS</h3>
+
+
+<p><b>The metals.</b> The elements which remain to be considered are known
+collectively as the metals. They are also called the base-forming
+elements, since their hydroxides are bases. A metal may therefore be
+defined as an element whose hydroxide is a base. When a base dissolves
+in water the hydroxyl groups form the anions, while the metallic element
+forms the cations. From this standpoint a metal can be defined as an
+element capable of forming simple cations in solution.</p>
+
+<p>The distinction between a metal and a non-metal is not a very sharp one,
+since the hydroxides of a number of elements act as bases under some
+conditions and as acids under others. We have seen that antimony is an
+element of this kind.</p>
+
+<p><b>Occurrence of metals in nature.</b> A few of the metals are found in nature
+in the free state. Among these are gold, platinum, and frequently
+copper. They are usually found combined with other elements in the form
+of oxides or salts of various acids. Silicates, carbonates, sulphides,
+and sulphates are the most abundant salts. All inorganic substances
+occurring in nature, whether they contain a metal or not, are called
+<i>minerals</i>. Those minerals from which a useful substance can be
+extracted are called <i>ores</i> of the substance. These two terms are most
+frequently used in connection with the metals.<span class='pagenum'><a name="Page_268" id="Page_268">[Pg 268]</a></span></p>
+
+<p><b>Extraction of metals,&mdash;metallurgy.</b> The process of extracting a metal
+from its ores is called the metallurgy of the metal. The metallurgy of
+each metal presents peculiarities of its own, but there are several
+methods of general application which are very frequently employed.</p>
+
+<p>1. <i>Reduction of an oxide with carbon.</i> Many of the metals occur in
+nature in the form of oxides. When these oxides are heated to a high
+temperature with carbon the oxygen combines with it and the metal is set
+free. Iron, for example, occurs largely in the form of the oxide
+Fe<sub>2</sub>O<sub>3</sub>. When this is heated with carbon the reaction expressed in
+the following equation takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe<sub>2</sub>O<sub>3</sub> + 3 C = 2 Fe + 3 CO.<br /></span>
+</div></div>
+
+<p>Many ores other than oxides may be changed into oxides which can then be
+reduced by carbon. The conversion of such ores into oxides is generally
+accomplished by heating, and this process is called <i>roasting</i>. Many
+carbonates and hydroxides decompose directly into the oxide on heating.
+Sulphides, on the other hand, must be heated in a current of air, the
+oxygen of the air entering into the reaction. The following equations
+will serve to illustrate these changes in the case of the ores of iron:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">FeCO<sub>3</sub> = FeO + CO<sub>2</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">2Fe(OH)<sub>3</sub> = Fe<sub>2</sub>O<sub>3</sub> + 3H<sub>2</sub>O,<br /></span>
+</div><div class="stanza">
+<span class="i0">2FeS<sub>2</sub> + 11O = Fe<sub>2</sub>O<sub>3</sub> + 4SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>2. <i>Reduction of an oxide with aluminium.</i> Not all oxides, however, can
+be reduced by carbon. In such cases aluminium may be used. Thus chromium
+may be obtained in accordance with the following equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Cr<sub>2</sub>O<sub>3</sub> + 2 Al = 2 Cr + Al<sub>2</sub>O<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_269" id="Page_269">[Pg 269]</a></span></p>
+<p>This method is a comparatively new one, having been brought into use by
+the German chemist Goldschmidt; hence it is sometimes called the
+Goldschmidt method.</p>
+
+<p>3. <i>Electrolysis.</i> In recent years increasing use is being made of the
+electric current in the preparation of metals. In some cases the
+separation of the metal from its compounds is accomplished by passing
+the current through a solution of a suitable salt of the metal, the
+metal usually being deposited upon the cathode. In other cases the
+current is passed through a fused salt of the metal, the chloride being
+best adapted to this purpose.</p>
+
+<p><b>Electro-chemical industries.</b> Most of the electro-chemical industries of
+the country are carried on where water power is abundant, since this
+furnishes the cheapest means for the generation of electrical energy.
+Niagara Falls is the most important locality in this country for such
+industries, and many different electro-chemical products are
+manufactured there. Some industries depend upon electrolytic processes,
+while in others the electrical energy is used merely as a source of heat
+in electric furnaces.</p>
+
+<p><b>Preparation of compounds of the metals.</b> Since the compounds of the
+metals are so numerous and varied in character, there are many ways of
+preparing them. In many cases the properties of the substance to be
+prepared, or the material available for its preparation, suggest a
+rather unusual way. There are, however, a number of general principles
+which are constantly applied in the preparation of the compounds of the
+metals, and a clear understanding of them will save much time and effort
+in remembering the details in any given case. The most important of
+these general methods for the preparation of compounds are the
+following:<span class='pagenum'><a name="Page_270" id="Page_270">[Pg 270]</a></span></p>
+
+<p>1. <i>By direct union of two elements.</i> This is usually accomplished by
+heating the two elements together. Thus the sulphides, chlorides, and
+oxides of a metal can generally be obtained in this way. The following
+equations serve as examples of this method:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe + S = FeS,<br /></span>
+</div><div class="stanza">
+<span class="i0">Mg + O = MgO,<br /></span>
+</div><div class="stanza">
+<span class="i0">Cu + 2Cl = CuCl<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>2. <i>By the decomposition of a compound.</i> This decomposition may be
+brought about either by heat alone or by the combined action of heat and
+a reducing agent. Thus when the nitrate of a metal is heated the oxide
+of the metal is usually obtained. Copper nitrate, for example,
+decomposes as follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Cu(NO<sub>3</sub>)<sub>2</sub> = CuO + 2NO<sub>2</sub> + O.<br /></span>
+</div></div>
+
+<p>Similarly the carbonates of the metals yield oxides, thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaCO<sub>3</sub> = CaO + CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>Most of the hydroxides form an oxide and water when heated:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Al(OH)<sub>3</sub> = Al<sub>2</sub>O<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>When heated with carbon, sulphates are reduced to sulphides, thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">BaSO<sub>4</sub> + 2C = BaS + 2CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>3. <i>Methods based on equilibrium in solution.</i> In the preparation of
+compounds the first requisite is that the reactions chosen shall be of
+such a kind as will go on to completion. In the chapter on chemical
+equilibrium it was shown that reactions in solution may become complete
+in either of three ways: (1) a gas may be formed which escapes from
+solution; (2) an insoluble solid may be formed which precipitates; (3)
+two different ions may combine to form<span class='pagenum'><a name="Page_271" id="Page_271">[Pg 271]</a></span> undissociated molecules. By the
+judicious selection of materials these principles may be applied to the
+preparation of a great variety of compounds, and illustrations of such
+methods will very frequently be found in the subsequent pages.</p>
+
+<p>4. <i>By fusion methods.</i> It sometimes happens that substances which are
+insoluble in water and in acids, and which cannot therefore be brought
+into double decomposition in the usual way, are soluble in other
+liquids, and when dissolved in them can be decomposed and converted into
+other desired compounds. Thus barium sulphate is not soluble in water,
+and sulphuric acid, being less volatile than most other acids, cannot
+easily be driven out from this salt When brought into contact with
+melted sodium carbonate, however, it dissolves in it, and since barium
+carbonate is insoluble in melted sodium carbonate, double decomposition
+takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + BaSO<sub>4</sub> = BaCO<sub>3</sub> + Na<sub>2</sub>SO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>On dissolving the cooled mixture in water the sodium sulphate formed in
+the reaction, together with any excess of sodium carbonate which may be
+present, dissolves. The barium carbonate can then be filtered off and
+converted into any desired salt by the processes already described.</p>
+
+<p>5. <i>By the action of metals on salts of other metals.</i> When a strip of
+zinc is placed in a solution of a copper salt the copper is precipitated
+and an equivalent quantity of zinc passes into solution:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Zn + CuSO<sub>4</sub> = Cu + ZnSO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>In like manner copper will precipitate silver from its salts:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Cu + Ag<sub>2</sub>SO<sub>4</sub> = 2Ag + CuSO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_272" id="Page_272">[Pg 272]</a></span></p>
+<p>It is possible to tabulate the metals in such a way that any one of them
+in the table will precipitate any one following it from its salts. The
+following is a list of some of the commoner metals arranged in this way:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Zinc<br /></span>
+<span class="i0">Iron<br /></span>
+<span class="i0">Tin<br /></span>
+<span class="i0">Lead<br /></span>
+<span class="i0">Copper<br /></span>
+<span class="i0">Bismuth<br /></span>
+<span class="i0">Mercury<br /></span>
+<span class="i0">Silver<br /></span>
+<span class="i0">Gold<br /></span>
+</div></div>
+
+<p>According to this table copper will precipitate bismuth, mercury,
+silver, or gold from their salts, and will in turn be precipitated by
+zinc, iron, tin, or lead. Advantage is taken of this principle in the
+purification of some of the metals, and occasionally in the preparation
+of metals and their compounds.</p>
+
+<p><b>Important insoluble compounds.</b> Since precipitates play so important a
+part in the reactions which substances undergo, as well as in the
+preparation of many chemical compounds, it is important to know what
+substances are insoluble. Knowing this, we can in many cases predict
+reactions under certain conditions, and are assisted in devising ways to
+prepare desired compounds. While there is no general rule which will
+enable one to foretell the solubility of any given compound,
+nevertheless a few general statements can be made which will be of much
+assistance.</p>
+
+<p>1. <i>Hydroxides.</i> All hydroxides are insoluble save those of ammonium,
+sodium, potassium, calcium, barium, and strontium.</p>
+
+<p>2. <i>Nitrates.</i> All nitrates are soluble in water.</p>
+
+<p>3. <i>Chlorides.</i> All chlorides are soluble save silver and mercurous
+chlorides. (Lead chloride is but slightly soluble.)</p>
+
+<p>4. <i>Sulphates.</i> All sulphates are soluble save those of barium,
+strontium, and lead. (Sulphates of silver and calcium are only
+moderately soluble.)<span class='pagenum'><a name="Page_273" id="Page_273">[Pg 273]</a></span></p>
+
+<p>5. <i>Sulphides.</i> All sulphides are insoluble save those of ammonium,
+sodium, and potassium. The sulphides of calcium, barium, strontium, and
+magnesium are insoluble in water, but are changed by hydrolysis into
+acid sulphides which are soluble. On this account they cannot be
+prepared by precipitation.</p>
+
+<p>6. <i>Carbonates, phosphates, and silicates.</i> All normal carbonates,
+phosphates, and silicates are insoluble save those of ammonium, sodium
+and potassium.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Write equations representing four different ways for preparing
+Cu(NO<sub>3</sub>)<sub>2</sub>.</p>
+
+<p><b>2.</b> Write equations representing six different ways for preparing
+ZnSO<sub>4</sub>.</p>
+
+<p><b>3.</b> Write equations for two reactions to illustrate each of the three
+ways in which reactions in solutions may become complete.</p>
+
+<p><b>4.</b> Give one or more methods for preparing each of the following
+compounds: CaCl<sub>2</sub>, PbCl<sub>2</sub>, BaSO<sub>4</sub>, CaCO<sub>3</sub>, (NH<sub>4</sub>)<sub>2</sub>S,
+Ag<sub>2</sub>S, PbO, Cu(OH)<sub>2</sub> (for solubilities, see last paragraph of
+chapter). State in each case the general principle involved in the
+method of preparation chosen.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_274" id="Page_274">[Pg 274]</a></span></p>
+<h2><a name="CHAPTER_XXIII" id="CHAPTER_XXIII"></a>CHAPTER XXIII</h2>
+
+<h3>THE ALKALI METALS</h3>
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>SYMBOL</td><td align='left'>ATOMIC WEIGHT</td><td align='left'>DENSITY</td><td align='left'>MELTING POINT</td><td align='left'>FIRST PREPARED</td></tr>
+<tr><td align='left'>Lithium</td><td align='left'>Li</td><td align='left'>7.03</td><td align='left'>0.59</td><td align='left'>186.&deg;</td><td align='left'>Davy 1820</td></tr>
+<tr><td align='left'>Sodium</td><td align='left'>Na</td><td align='left'>23.05</td><td align='left'>0.97</td><td align='left'>97.6&deg;</td><td align='left'>&nbsp;&nbsp;"&nbsp;&nbsp; 1807</td></tr>
+<tr><td align='left'>Potassium</td><td align='left'>K</td><td align='left'>39.15</td><td align='left'>0.87</td><td align='left'>62.5&deg;</td><td align='left'>&nbsp;&nbsp;"&nbsp;&nbsp; 1807</td></tr>
+<tr><td align='left'>Rubidium</td><td align='left'>Rb</td><td align='left'>85.5</td><td align='left'>1.52</td><td align='left'>38.5&deg;</td><td align='left'>Bunsen 1861</td></tr>
+<tr><td align='left'>C&aelig;sium</td><td align='left'>Cs</td><td align='left'>132.9</td><td align='left'>1.88</td><td align='left'>26.5&deg;</td><td align='left'>&nbsp;&nbsp;"&nbsp;&nbsp; 1860</td></tr>
+</table></div>
+
+<p><b>The family.</b> The metals listed in the above table constitute the even
+family in Group I in the periodic arrangement of the elements, and
+therefore form a natural family. The name alkali metals is commonly
+applied to the family for the reason that the hydroxides of the most
+familiar members of the family, namely sodium and potassium, have long
+been called alkalis.</p>
+
+<p>1. <i>Occurrence.</i> While none of these metals occur free in nature, their
+compounds are very widely distributed, being especially abundant in sea
+and mineral waters, in salt beds, and in many rocks. Only sodium and
+potassium occur in abundance, the others being rarely found in any
+considerable quantity.</p>
+
+<p>2. <i>Preparation.</i> The metals are most conveniently prepared by the
+electrolysis of their fused hydroxides or chlorides, though it is
+possible to prepare them by reducing their oxides or carbonates with
+carbon.<span class='pagenum'><a name="Page_275" id="Page_275">[Pg 275]</a></span></p>
+
+<p>3. <i>Properties.</i> They are soft, light metals, having low melting points
+and small densities, as is indicated in the table. Their melting points
+vary inversely with their atomic weights, while their densities (sodium
+excepted) vary directly with these. The pure metals have a silvery
+luster but tarnish at once when exposed to the air, owing to the
+formation of a film of oxide upon the surface of the metal. They are
+therefore preserved in some liquid, such as coal oil, which contains no
+oxygen. Because of their strong affinity for oxygen they decompose water
+with great ease, forming hydroxides and liberating hydrogen in
+accordance with the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">M + H<sub>2</sub>O = MOH + H,<br /></span>
+</div></div>
+
+<p>where M stands for any one of these metals. These hydroxides are white
+solids; they are readily soluble in water and possess very strong basic
+properties. These bases are nearly equal in strength, that is, they all
+dissociate in water to about the same extent.</p>
+
+<p>4. <i>Compounds.</i> The alkali metals almost always act as univalent
+elements in the formation of compounds, the composition of which can be
+represented by such formulas as MH, MCl, MNO<sub>3</sub>, M<sub>2</sub>SO<sub>4</sub>,
+M<sub>3</sub>PO<sub>4</sub>. These compounds, when dissolved in water, dissociate in
+such a way as to form simple, univalent metallic ions which are
+colorless. With the exception of lithium these metals form very few
+insoluble compounds, so that it is not often that precipitates
+containing them are obtained. Only sodium and potassium will be studied
+in detail, since the other metals of the family are of relatively small
+importance.</p>
+
+<p>The compounds of sodium and potassium are so similar in properties that
+they can be used interchangeably for<span class='pagenum'><a name="Page_276" id="Page_276">[Pg 276]</a></span> most purposes. Other things being
+equal, the sodium compounds are prepared in preference to those of
+potassium, since they are cheaper. When a given sodium compound is
+deliquescent, or is so soluble that it is difficult to purify, the
+corresponding potassium compound is prepared in its stead, provided its
+properties are more desirable in these respects.</p>
+
+
+<h3>SODIUM</h3>
+
+<p><b>Occurrence in nature.</b> Large deposits of sodium chloride have been found
+in various parts of the world, and the water of the ocean and of many
+lakes and springs contains notable quantities of it. The element also
+occurs as a constituent of many rocks and is therefore present in the
+soil formed by their disintegration. The mineral cryolite
+(Na<sub>3</sub>AlF<sub>6</sub>) is an important substance, and the nitrate, carbonate,
+and borate also occur in nature.</p>
+
+<p><b>Preparation.</b> In 1807 Sir Humphry Davy succeeded in preparing very small
+quantities of metallic sodium by the electrolysis of the fused
+hydroxide. On account of the cost of electrical energy it was for many
+years found more economical to prepare it by reducing the carbonate with
+carbon in accordance with the following equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + 2C = 2Na + 3CO.<br /></span>
+</div></div>
+
+<p>The cost of generating the electric current has been diminished to such
+an extent, however, that it is now more economical to prepare sodium by
+Davy's original method, namely, by the electrolysis of the fused
+hydroxide or chloride. When the chloride is used the process is
+difficult to manage, owing to the higher temperature required to keep
+the electrolyte fused, and because of the corroding action of the fused
+chloride upon the containing vessel.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/davy.jpg" width="400" height="481" alt="SIR HUMPHRY DAVY (English) (1778-1829)
+
+Isolated sodium, lithium, potassium, barium, strontium, and calcium by
+means of electrolysis; demonstrated the elementary nature of chlorine;
+invented the safety lamp; discovered the stupefying effects of nitrous
+oxide" title="" />
+<span class="caption">SIR HUMPHRY DAVY (English) (1778-1829)<br /><br />
+
+Isolated sodium, lithium, potassium, barium, strontium, and calcium by
+means of electrolysis; demonstrated the elementary nature of chlorine;
+invented the safety lamp; discovered the stupefying effects of nitrous
+oxide</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_277" id="Page_277">[Pg 277]</a></span></p>
+<div class="blockquot"><p><b>Technical preparation.</b> The sodium hydroxide is melted in a
+cylindrical iron vessel (Fig. 76) through the bottom of which
+rises the cathode <i>K</i>. The anodes <i>A</i>, several in number, are
+suspended around the cathode from above. A cylindrical vessel
+<i>C</i> floats in the fused alkali directly over the cathode, and
+under this cap the sodium and hydrogen liberated at the cathode
+collect. The hydrogen escapes by lifting the cover, and the
+sodium, protected from the air by the hydrogen, is skimmed or
+drained off from time to time. Oxygen is set free upon the
+anode and escapes into the air through the openings <i>O</i> without
+coming into contact with the sodium or hydrogen. This process
+is carried on extensively at Niagara Falls.</p></div>
+
+<div class="figcenter" style="width: 322px;">
+<img src="images/imagefig76.jpg" width="322" height="400" alt="Fig. 76" title="" />
+<span class="caption">Fig. 76</span>
+</div>
+
+<p><b>Properties.</b> Sodium is a silver-white metal about as heavy as water, and
+so soft that it can be molded easily by the fingers or pressed into
+wire. It is very active chemically, combining with most of the
+non-metallic elements, such as oxygen and chlorine, with great energy.
+It will often withdraw these elements from combination with other
+elements, and is thus able to decompose water and the oxides and
+chlorides of many metals.</p>
+
+<p><b>Sodium peroxide</b> (NaO). Since sodium is a univalent element we should
+expect it to form an oxide of the formula Na<sub>2</sub>O. While such an oxide
+can be prepared, the peroxide (NaO) is much better known. It is a
+yellowish-white powder made by burning sodium in air. Its chief use is
+as an oxidizing agent. When heated with oxidizable substances it gives
+up a part of its oxygen, as shown in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaO = Na<sub>2</sub>O + O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_278" id="Page_278">[Pg 278]</a></span></p>
+<p>Water decomposes it in accordance with the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaO + 2H<sub>2</sub>O = 2NaOH + H<sub>2</sub>O<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>Acids act readily upon it, forming a sodium salt and hydrogen peroxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaO + 2HCl = 2NaCl + H<sub>2</sub>O<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>In these last two reactions the hydrogen dioxide formed may decompose
+into water and oxygen if the temperature is allowed to rise:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>O<sub>2</sub> = H<sub>2</sub>O + O.<br /></span>
+</div></div>
+
+<div class="blockquot"><p><b>Peroxides.</b> It will be remembered that barium dioxide (BaO_{2})
+yields hydrogen dioxide when treated with acids, and that
+manganese dioxide gives up oxygen when heated with sulphuric
+acid. Oxides which yield either hydrogen dioxide or oxygen when
+treated with water or an acid are called peroxides.</p></div>
+
+<p><b>Sodium hydroxide</b> (<i>caustic soda</i>) (NaOH). 1. <i>Preparation.</i> Sodium
+hydroxide is prepared commercially by several processes.</p>
+
+<p>(<i>a</i>) In the older process, still in extensive use, sodium carbonate is
+treated with calcium hydroxide suspended in water. Calcium carbonate is
+precipitated according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + Ca(OH)<sub>2</sub> = CaCO<sub>3</sub> + 2NaOH.<br /></span>
+</div></div>
+
+<p>The dilute solution of sodium hydroxide, filtered from the calcium
+carbonate, is evaporated to a paste and is then poured into molds to
+solidify. It is sold in the form of slender sticks.</p>
+
+<p>(<i>b</i>) The newer methods depend upon the electrolysis of sodium chloride.
+In the Castner process a solution of salt is electrolyzed, the reaction
+being expressed as follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaCl + H<sub>2</sub>O = NaOH + H + Cl.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_279" id="Page_279">[Pg 279]</a></span></p>
+<p>The chlorine escapes as a gas, and by an ingenious mechanical device the
+sodium hydroxide is prevented from mixing with the salt in the solution.</p>
+
+<p>In the Acker process the electrolyte is <i>fused</i> sodium chloride. The
+chlorine is evolved as a gas at the anode, while the sodium alloys with
+the melted lead which forms the cathode. When this alloy is treated with
+water the following reaction takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na + H<sub>2</sub>O = NaOH + H.<br /></span>
+</div></div>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig77.jpg" width="400" height="223" alt="Fig. 77" title="" />
+<span class="caption">Fig. 77</span>
+</div>
+
+<p><b>Technical process.</b> A sketch of an Acker furnace is represented in Fig.
+77. The furnace is an irregularly shaped cast-iron box, divided into
+three compartments, <i>A</i>, <i>B</i>, and <i>C</i>. Compartment <i>A</i> is lined with
+magnesia brick. Compartments <i>B</i> and <i>C</i> are filled with melted lead,
+which also covers the bottom of <i>A</i> to a depth of about an inch. Above
+this layer in <i>A</i> is fused salt, into which dip carbon anodes <i>D</i>. The
+metallic box and melted lead is the cathode.</p>
+
+<div class="blockquot"><p>When the furnace is in operation chlorine is evolved at the
+anodes, and is drawn away through a pipe (not represented) to
+the bleaching-powder chambers. Sodium is set free at the
+surface of the melted lead in <i>A</i>, and at once alloys with it.
+Through the pipe <i>E</i> a powerful jet of steam is driven through
+the lead in <i>B</i> upwards<span class='pagenum'><a name="Page_280" id="Page_280">[Pg 280]</a></span> into the narrow tube <i>F</i>. This forces
+the lead alloy up through the tube and over into the chamber
+<i>G</i>.</p>
+
+<p>In this process the steam is decomposed by the sodium in the
+alloy, forming melted sodium hydroxide and hydrogen. The melted
+lead and sodium hydroxide separate into two layers in <i>G</i>, and
+the sodium hydroxide, being on top, overflows into tanks from
+which it is drawn off and packed in metallic drums. The lead is
+returned to the other compartments of the furnace by a pipe
+leading from <i>H</i> to <i>I</i>. Compartment <i>C</i> serves merely as a
+reservoir for excess of melted lead.</p></div>
+
+<p>2. <i>Properties.</i> Sodium hydroxide is a white, crystalline, brittle
+substance which rapidly absorbs water and carbon dioxide from the air.
+As the name (caustic soda) indicates, it is a very corrosive substance,
+having a disintegrating action on most animal and vegetable tissues. It
+is a strong base. It is used in a great many chemical industries, and
+under the name of lye is employed to a small extent as a cleansing agent
+for household purposes.</p>
+
+<p><b>Sodium chloride</b> (<i>common salt</i>) (NaCl). 1. <i>Preparation.</i> Sodium
+chloride, or common salt, is very widely distributed in nature. Thick
+strata, evidently deposited at one time by the evaporation of salt
+water, are found in many places. In the United States the most important
+localities for salt are New York, Michigan, Ohio, and Kansas. Sometimes
+the salt is mined, especially if it is in the pure form called rock
+salt. More frequently a strong brine is pumped from deep wells sunk into
+the salt deposit, and is then evaporated in large pans until the salt
+crystallizes out. The crystals are in the form of small cubes and
+contain no water of crystallization; some water is, however, held in
+cavities in the crystals and causes the salt to decrepitate when heated.</p>
+
+<p>2. <i>Uses.</i> Since salt is so abundant in nature it forms the starting
+point in the preparation of all compounds<span class='pagenum'><a name="Page_281" id="Page_281">[Pg 281]</a></span> containing either sodium or
+chlorine. This includes many substances of the highest importance to
+civilization, such as soap, glass, hydrochloric acid, soda, and
+bleaching powder. Enormous quantities of salt are therefore produced
+each year. Small quantities are essential to the life of man and
+animals. Pure salt does not absorb moisture; the fact that ordinary salt
+becomes moist in air is not due to a property of the salt, but to
+impurities commonly occurring in it, especially calcium and magnesium
+chlorides.</p>
+
+<p><b>Sodium sulphate</b> (<i>Glauber's salt</i>) (Na<sub>2</sub>SO<sub>4</sub>&middot;10H<sub>2</sub>O). This salt is
+prepared by the action of sulphuric acid upon sodium chloride,
+hydrochloric acid being formed at the same time:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaCl + H<sub>2</sub>SO<sub>4</sub> = Na<sub>2</sub>SO<sub>4</sub> + 2HCl.<br /></span>
+</div></div>
+
+<p>Some sodium sulphate is prepared by the reaction represented in the
+equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">MgSO<sub>4</sub> + 2NaCl = Na<sub>2</sub>SO<sub>4</sub> + MgCl<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>The magnesium sulphate required for this reaction is obtained in large
+quantities in the manufacture of potassium chloride, and being of little
+value for any other purpose is used in this way. The reaction depends
+upon the fact that sodium sulphate is the least soluble of any of the
+four factors in the equation, and therefore crystallizes out when hot,
+saturated solutions of magnesium sulphate and sodium chloride are mixed
+together and the resulting mixture cooled.</p>
+
+<p>Sodium sulphate forms large efflorescent crystals. The salt is
+extensively used in the manufacture of sodium carbonate and glass. Small
+quantities are used in medicine.</p>
+
+<p><b>Sodium sulphite</b> (Na<sub>2</sub>SO<sub>3</sub>&middot;7H<sub>2</sub>O). Sodium sulphite is prepared by
+the action of sulphur dioxide upon solutions<span class='pagenum'><a name="Page_282" id="Page_282">[Pg 282]</a></span> of sodium hydroxide, the
+reaction being analogous to the action of carbon dioxide upon sodium
+hydroxide. Like the carbonate, the sulphite is readily decomposed by
+acids:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>SO<sub>3</sub> + 2HCl = 2NaCl + H<sub>2</sub>O + SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>Because of this reaction sodium sulphite is used as a convenient source
+of sulphur dioxide. It is also used as a disinfectant and a
+preservative.</p>
+
+<p><b>Sodium thiosulphate</b> (<i>hyposulphite of soda or "hypo"</i>)
+(Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>&middot;5H<sub>2</sub>O). This salt, commonly called sodium
+hyposulphite, or merely hypo, is made by boiling a solution of sodium
+sulphite with sulphur:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>SO<sub>3</sub> + S = Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>It is used in photography and in the bleaching industry, to absorb the
+excess of chlorine which is left upon the bleached fabrics.</p>
+
+<div class="blockquot"><p><b>Thio compounds.</b> The prefix "thio" means sulphur. It is used to
+designate substances which may be regarded as derived from
+oxygen compounds by replacing the whole or a part of their
+oxygen with sulphur. The thiosulphates may be regarded as
+sulphates in which one atom of oxygen has been replaced by an
+atom of sulphur. This may be seen by comparing the formula
+Na<sub>2</sub>SO<sub>4</sub> (sodium sulphate) with the formula
+Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub> (sodium thiosulphate).</p></div>
+
+<p><b>Sodium carbonate</b> (<i>sal soda</i>)(Na<sub>2</sub>CO<sub>3</sub>&middot;10H<sub>2</sub>O). There are two
+different methods now employed in the manufacture of this important
+substance.</p>
+
+<p>1. <i>Le Blanc process.</i> This older process involves several distinct
+reactions, as shown in the following equations.</p>
+
+<p>(<i>a</i>) Sodium chloride is first converted into sodium sulphate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaCl + H<sub>2</sub>SO<sub>4</sub> = Na<sub>2</sub>SO<sub>4</sub> + 2HCl.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_283" id="Page_283">[Pg 283]</a></span></p>
+<p>(<i>b</i>) The sodium sulphate is next reduced to sulphide by heating it with
+carbon:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>SO<sub>4</sub> + 2C = Na<sub>2</sub>S + 2CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>(<i>c</i>) The sodium sulphide is then heated with calcium carbonate, when
+double decomposition takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>S + CaCO<sub>3</sub> = CaS + Na<sub>2</sub>CO<sub>3</sub>.<br /></span>
+</div></div>
+
+<div class="blockquot"><p><b>Technical preparation of sodium carbonate.</b> In a manufacturing
+plant the last two reactions take place in one process. Sodium
+sulphate, coal, and powdered limestone are heated together to a
+rather high temperature. The coal reduces the sulphate to
+sulphide, which in turn reacts upon the calcium carbonate. Some
+limestone is decomposed by the heat, forming calcium oxide.
+When treated with water the calcium oxide is changed into
+hydroxide, and this prevents the water from decomposing the
+insoluble calcium sulphide.</p>
+
+<p>The crude product of the process is a hard black cake called
+black ash. On digesting this mass with water the sodium
+carbonate passes into solution. The pure carbonate is obtained
+by evaporation of this solution, crystallizing from it in
+crystals of the formula Na<sub>2</sub>CO<sub>3</sub>&middot;10H<sub>2</sub>O. Since over 60%
+of this salt is water, the crystals are sometimes heated until
+it is driven off. The product is called calcined soda, and is,
+of course, more valuable than the crystallized salt.</p></div>
+
+<p>2. <i>Solvay process.</i> This more modern process depends upon the reactions
+represented in the equations</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaCl + NH<sub>4</sub>HCO<sub>3</sub> = NaHCO<sub>3</sub> + NH<sub>4</sub>Cl,<br /></span>
+</div><div class="stanza">
+<span class="i0">2NaHCO<sub>3</sub> = Na<sub>2</sub>CO<sub>3</sub> + H<sub>2</sub>O + CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>The reason the first reaction takes place is that sodium hydrogen
+carbonate is sparingly soluble in water, while the other compounds are
+freely soluble. When strong solutions of sodium chloride and of ammonium
+hydrogen carbonate are brought together the sparingly soluble sodium
+hydrogen carbonate is precipitated. This is converted into the normal
+carbonate by heating, the reaction being represented in the second
+equation.<span class='pagenum'><a name="Page_284" id="Page_284">[Pg 284]</a></span></p>
+
+<div class="blockquot"><p><b>Technical preparation.</b> In the Solvay process a very
+concentrated solution of salt is first saturated with ammonia
+gas, and a current of carbon dioxide is then conducted into the
+solution. In this way ammonium hydrogen carbonate is formed:</p>
+
+<p>NH<sub>3</sub> + H<sub>2</sub>O + CO<sub>2</sub> = NH<sub>4</sub>HCO<sub>3</sub>.</p>
+
+<p>This enters into double decomposition with the salt, as shown
+in the first equation under the Solvay process. After the
+sodium hydrogen carbonate has been precipitated the mother
+liquors containing ammonium chloride are treated with lime:</p>
+
+<p>2NH<sub>4</sub>Cl + CaO = CaCl<sub>2</sub> + 2 NH<sub>3</sub> + H<sub>2</sub>O.</p>
+
+<p>The lime is obtained by burning limestone:</p>
+
+<p>CaCO<sub>3</sub> = CaO + CO<sub>2</sub>.</p>
+
+<p>The ammonia and carbon dioxide evolved in the latter two
+reactions are used in the preparation of an additional quantity
+of ammonium hydrogen carbonate. It will thus be seen that there
+is no loss of ammonia. The only materials permanently used up
+are calcium carbonate and salt, while the only waste product is
+calcium chloride.</p>
+
+<p><b>Historical.</b> In former times sodium carbonate was made by
+burning seaweeds and extracting the carbonate from their ash.
+On this account the salt was called <i>soda ash</i>, and the name is
+still in common use. During the French Revolution this supply
+was cut off, and in behalf of the French government Le Blanc
+made a study of methods of preparing the carbonate directly
+from salt. As a result he devised the method which bears his
+name, and which was used exclusively for many years. It has
+been replaced to a large extent by the Solvay process, which
+has the advantage that the materials used are inexpensive, and
+that the ammonium hydrogen carbonate used can be regenerated
+from the products formed in the process. Much expense is also
+saved in fuel, and the sodium hydrogen carbonate, which is the
+first product of the process, has itself many commercial uses.
+The Le Blanc process is still used, however, since the
+hydrochloric acid generated is of value.</p>
+
+<p><b>By-products.</b> The substances obtained in a given process, aside
+from the main product, are called the by-products. The success
+of many processes depends upon the value of the by-products
+formed.</p>
+
+<p>Thus hydrochloric acid, a by-product in the Le Blanc process,
+is valuable enough to make the process pay, even though sodium
+carbonate can be made cheaper in other ways.</p></div>
+
+<p><span class='pagenum'><a name="Page_285" id="Page_285">[Pg 285]</a></span></p>
+<p><b>Properties of sodium carbonate.</b> Sodium carbonate forms large crystals of
+the formula Na<sub>2</sub>CO<sub>3</sub> &middot; 10 H<sub>2</sub>O. It has a mild alkaline reaction
+and is used for laundry purposes under the name of washing soda. Mere
+mention of the fact that it is used in the manufacture of glass, soap,
+and many chemical reagents will indicate its importance in the
+industries. It is one of the few soluble carbonates.</p>
+
+<p><b>Sodium hydrogen carbonate</b> (<i>bicarbonate of soda</i>) (NaHCO<sub>3</sub>). This
+salt, commonly called bicarbonate of soda, or baking soda, is made by
+the Solvay process, as explained above, or by passing carbon dioxide
+into strong solutions of sodium carbonate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + H<sub>2</sub>O + CO<sub>2</sub> = 2NaHCO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>The bicarbonate, being sparingly soluble, crystallizes out. A mixture of
+the bicarbonate with some substance (the compound known as cream of
+tartar is generally used) which slowly reacts with it, liberating carbon
+dioxide, is used largely in baking. The carbon dioxide generated forces
+its way through the dough, thus making it porous and light.</p>
+
+<p><b>Sodium nitrate</b> (<i>Chili saltpeter</i>) (NaNO<sub>3</sub>). This substance is found
+in nature in arid regions in a number of places, where it has been
+formed apparently by the decay of organic substances in the presence of
+air and sodium salts. The largest deposits are in Chili, and most of the
+nitrate of commerce comes from that country. Smaller deposits occur in
+California and Nevada. The commercial salt is prepared by dissolving the
+crude nitrate in water,<span class='pagenum'><a name="Page_286" id="Page_286">[Pg 286]</a></span> allowing the insoluble earthy materials to
+settle, and evaporating the clear solution so obtained to
+crystallization. The soluble impurities remain for the most part in the
+mother liquors.</p>
+
+<p>Since this salt is the only nitrate found extensively in nature, it is
+the material from which other nitrates as well as nitric acid are
+prepared. It is used in enormous quantities in the manufacture of
+sulphuric acid and potassium nitrate, and as a fertilizer.</p>
+
+<p><b>Sodium phosphate</b> (Na<sub>2</sub>HPO<sub>4</sub>&middot;12H<sub>2</sub>O). Since phosphoric acid has
+three replaceable hydrogen atoms, three sodium phosphates are
+possible,&mdash;two acid salts and one normal. All three can be made without
+difficulty, but disodium phosphate is the only one which is largely
+used, and is the salt which is commonly called sodium phosphate. It is
+made by the action of phosphoric acid on sodium carbonate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + H<sub>3</sub>PO<sub>4</sub> = Na<sub>2</sub>HPO<sub>4</sub> + CO<sub>2</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>It is interesting as being one of the few phosphates which are soluble
+in water, and is the salt commonly used when a soluble phosphate is
+needed.</p>
+
+<p><b>Normal sodium phosphate</b> (Na<sub>3</sub>PO<sub>4</sub>). Although this is a normal salt
+its solution has a strongly alkaline reaction. This is due to the fact
+that the salt hydrolyzes in solution into sodium hydroxide and disodium
+phosphate, as represented in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>3</sub>PO<sub>4</sub> + H<sub>2</sub>O = Na<sub>2</sub>HPO<sub>4</sub> + NaOH.<br /></span>
+</div></div>
+
+<p>Sodium hydroxide is strongly alkaline, while disodium phosphate is
+nearly neutral in reaction. The solution as a whole is therefore
+alkaline. The salt is prepared by adding a large excess of sodium
+hydroxide to a solution of disodium<span class='pagenum'><a name="Page_287" id="Page_287">[Pg 287]</a></span> phosphate and evaporating to
+crystallization. The excess of the sodium hydroxide reverses the
+reaction of hydrolysis and the normal salt crystallizes out.</p>
+
+<p><b>Sodium tetraborate </b>(<i>borax</i>) (Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub>&middot;10H<sub>2</sub>O). The properties
+of this important compound have been discussed under the head of boron.</p>
+
+
+<h3>POTASSIUM</h3>
+
+<p><b>Occurrence in nature.</b> Potassium is a constituent of many common rocks
+and minerals, and is therefore a rather abundant element, though not so
+abundant as sodium. Feldspar, which occurs both by itself and as a
+constituent of granite, contains considerable potassium. The element is
+a constituent of all clay and of mica and also occurs in very large
+deposits at Stassfurt, Germany, in the form of the chloride and
+sulphate, associated with compounds of sodium and magnesium. In small
+quantities it is found as nitrate and in many other forms.</p>
+
+<p>The natural decomposition of rocks containing potassium gives rise to
+various compounds of the element in all fertile soils. Its soluble
+compounds are absorbed by growing plants and built up into complex
+vegetable substances; when these are burned the potassium remains in the
+ash in the form of the carbonate. Crude carbonate obtained from wood
+ashes was formerly the chief source of potassium compounds; they are now
+mostly prepared from the salts of the Stassfurt deposits.</p>
+
+<div class="blockquot"><p><b>Stassfurt salts.</b> These salts form very extensive deposits in
+middle and north Germany, the most noted locality for working
+them being at Stassfurt. The deposits are very thick and rest
+upon an enormous layer of common salt. They are in the form of
+a series of strata, each consisting largely of a single mineral
+salt. A cross section of<span class='pagenum'><a name="Page_288" id="Page_288">[Pg 288]</a></span> these deposits is shown in Fig. 78.
+While these strata are salts from a chemical standpoint, they
+are as solid and hard as many kinds of stone, and are mined as
+stone or coal would be. Since the strata differ in general
+appearance, each can be mined separately, and the various
+minerals can be worked up by methods adapted to each particular
+case. The chief minerals of commercial importance in these
+deposits are the following:</p></div>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Sylvine</td><td align='left'>KCl.</td></tr>
+<tr><td align='left'>Anhydrite</td><td align='left'>CaSO<sub>4</sub>.</td></tr>
+<tr><td align='left'>Carnallite</td><td align='left'>KCl&middot;MgCl<sub>2</sub>&middot;6H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Kainite</td><td align='left'>K<sub>2</sub>SO<sub>4</sub>&middot;MgSO<sub>4</sub>&middot;MgCl<sub>2</sub>&middot;6H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Polyhalite</td><td align='left'>K<sub>2</sub>SO<sub>4</sub>&middot;MgSO<sub>4</sub>&middot;2CaSO<sub>4</sub>&middot;2H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Kieserite</td><td align='left'>MgSO<sub>4</sub>&middot;H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Sch&ouml;nite</td><td align='left'>K<sub>2</sub>SO<sub>4</sub>&middot;MgSO<sub>4</sub>&middot;6H<sub>2</sub>O.</td></tr>
+</table></div>
+
+<p><b>Preparation and properties.</b> The metal is prepared by the same method
+used in the preparation of sodium. In most respects it is very similar
+to sodium, the chief difference being that it is even more energetic in
+its action upon other substances. The freshly cut, bright surface
+instantly becomes dim through oxidation by the air. It decomposes water
+very vigorously, the heat of reaction being sufficient to ignite the
+hydrogen evolved. It is somewhat lighter than sodium and is preserved
+under gasoline.</p>
+
+<div class="figcenter" style="width: 346px;">
+<img src="images/imagefig78.jpg" width="346" height="400" alt="Fig. 78" title="" />
+<span class="caption">Fig. 78</span>
+</div>
+
+<p><b>Potassium hydroxide</b> (<i>caustic potash</i>) (KOH). Potassium hydroxide is
+prepared by methods exactly similar to those<span class='pagenum'><a name="Page_289" id="Page_289">[Pg 289]</a></span> used in the preparation of
+sodium hydroxide, which compound it closely resembles in both physical
+and chemical properties. It is not used to any very great extent, being
+replaced by the cheaper sodium hydroxide.</p>
+
+<p><b>Action of the halogen elements on potassium hydroxide.</b> When any one of
+the three halogen elements&mdash;chlorine, bromine, and iodine&mdash;is added to a
+solution of potassium hydroxide a reaction takes place, the nature of
+which depends upon the conditions of the experiment. Thus, when chlorine
+is passed into a cold dilute solution of potassium hydroxide the
+reaction expressed by the following equation takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(1) 2KOH + 2Cl = KCl + KClO + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>If the solution of hydroxide is concentrated and hot, on the other hand,
+the potassium hypochlorite formed according to equation (1) breaks down
+as fast as formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(2) 3KClO = KClO<sub>3</sub> + 2KCl.<br /></span>
+</div></div>
+
+<p>Equation (1), after being multiplied by 3, may be combined with equation
+(2), giving the following:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(3) 6KOH + 6Cl = 5KCl + KClO<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>This represents in a single equation the action of chlorine on hot,
+concentrated solutions of potassium hydroxide. By means of these
+reactions one can prepare potassium chloride, potassium hypochlorite,
+and potassium chlorate. By substituting bromine or iodine for chlorine
+the corresponding compounds of these elements are obtained. Some of
+these compounds can be obtained in cheaper ways.</p>
+
+<p>If the halogen element is added to a solution of sodium hydroxide or
+calcium hydroxide, the reaction which takes place is exactly similar to
+that which takes place with<span class='pagenum'><a name="Page_290" id="Page_290">[Pg 290]</a></span> potassium hydroxide. It is possible,
+therefore, to prepare in this way the sodium and calcium compounds
+corresponding to the potassium compounds given above.</p>
+
+<p><b>Potassium chloride</b> (KCl). This salt occurs in nature in sea water, in
+the mineral sylvine, and, combined with magnesium chloride, as
+carnallite (KCl&middot;MgCl<sub>2</sub>&middot;6H<sub>2</sub>O). It is prepared from carnallite by
+saturating boiling water with the mineral and allowing the solution to
+cool. The mineral decomposes while in solution, and the potassium
+chloride crystallizes out on cooling, while the very soluble magnesium
+chloride remains in solution. The salt is very similar to sodium
+chloride both in physical and chemical properties. It is used in the
+preparation of nearly all other potassium salts, and, together with
+potassium sulphate, is used as a fertilizer.</p>
+
+<p><b>Potassium bromide</b> (KBr). When bromine is added to a hot concentrated
+solution of potassium hydroxide there is formed a mixture of potassium
+bromide and potassium bromate in accordance with the reactions already
+discussed. There is no special use for the bromate, so the solution is
+evaporated to dryness, and the residue, consisting of a mixture of the
+bromate and bromide, is strongly heated. This changes the bromate to
+bromide, as follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">KBrO<sub>3</sub> = KBr +3O.<br /></span>
+</div></div>
+
+<p>The bromide is then crystallized from water, forming large colorless
+crystals. It is used in medicine and in photography.</p>
+
+<p><b>Potassium iodide</b> (KI). Potassium iodide may be made by exactly the same
+method as has just been described for the bromide, substituting iodine
+for bromine. It is more frequently made as follows. Iron filings are
+<span class='pagenum'><a name="Page_291" id="Page_291">[Pg 291]</a></span>treated with iodine, forming the compound Fe<sub>3</sub>I<sub>8</sub>; on boiling this
+substance with potassium carbonate the reaction represented in the
+following equation occurs:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe<sub>3</sub>I<sub>8</sub> + 4K<sub>2</sub>CO<sub>3</sub> = Fe<sub>3</sub>O<sub>4</sub> + 8KI + 4CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>Potassium iodide finds its chief use in medicine.</p>
+
+<p><b>Potassium chlorate</b> (KClO<sub>3</sub>). This salt, as has just been explained,
+can be made by the action of chlorine on strong potassium hydroxide
+solutions. The chief use of potassium chlorate is as an oxidizing agent
+in the manufacture of matches, fireworks, and explosives; it is also
+used in the preparation of oxygen and in medicine.</p>
+
+<div class="blockquot"><p><b>Commercial preparation.</b> By referring to the reaction between
+chlorine and hot concentrated solutions of potassium hydroxide,
+it will be seen that only one molecule of potassium chlorate is
+formed from six molecules of potassium hydroxide. Partly
+because of this poor yield and partly because the potassium
+hydroxide is rather expensive, this process is not an
+economical one for the preparation of potassium chlorate. The
+commercial method is the following. Chlorine is passed into hot
+solutions of calcium hydroxide, a compound which is very cheap.
+The resulting calcium chloride and chlorate are both very
+soluble. To the solution of these salts potassium chloride is
+added, and as the solution cools the sparingly soluble
+potassium chlorate crystallizes out:</p>
+
+<p>Ca(ClO<sub>3</sub>)<sub>2</sub> + 2KCl = 2KClO<sub>3</sub> + CaCl<sub>2</sub>.</p>
+
+<p>Electro-chemical processes are also used.</p></div>
+
+<p><b>Potassium nitrate</b> (<i>saltpeter</i>) (KNO<sub>3</sub>). This salt was formerly made
+by allowing animal refuse to decompose in the open air in the presence
+of wood ashes or earthy materials containing potassium. Under these
+conditions the nitrogen in the organic matter is in part converted into
+potassium nitrate, which was obtained by extracting the mass with water
+and evaporating to crystallization. This crude and<span class='pagenum'><a name="Page_292" id="Page_292">[Pg 292]</a></span> slow process is now
+almost entirely replaced by a manufacturing process in which the
+potassium salt is made from Chili saltpeter:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NaNO<sub>3</sub> + KCl = NaCl + KNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>This process has been made possible by the discovery of the Chili niter
+beds and the potassium chloride of the Stassfurt deposits.</p>
+
+<div class="blockquot"><p>The reaction depends for its success upon the apparently
+insignificant fact that sodium chloride is almost equally
+soluble in cold and hot water. All four factors in the equation
+are rather soluble in cold water, but in hot water sodium
+chloride is far less soluble than the other three. When hot
+saturated solutions of sodium nitrate and potassium chloride
+are brought together, sodium chloride precipitates and can be
+filtered off, leaving potassium nitrate in solution, together
+with some sodium chloride. On cooling, potassium nitrate
+crystallizes out, leaving small amounts of the other salts in
+solution.</p></div>
+
+<p>Potassium nitrate is a colorless salt which forms very large crystals.
+It is stable in the air, and when heated is a good oxidizing agent,
+giving up oxygen quite readily. Its chief use is in the manufacture of
+gunpowder.</p>
+
+<div class="blockquot"><p><b>Gunpowder.</b> The object sought for in the preparation of
+gunpowder is to secure a solid substance which will remain
+unchanged under ordinary conditions, but which will explode
+readily when ignited, evolving a large volume of gas. When a
+mixture of carbon and potassium nitrate is ignited a great deal
+of gas is formed, as will be seen from the equation</p>
+
+<p>2KNO<sub>3</sub> + 3C = CO<sub>2</sub> + CO + N<sub>2</sub> + K<sub>2</sub>CO<sub>3</sub>.</p>
+
+<p>By adding sulphur to the mixture the volume of gas formed in
+the explosion is considerably increased:</p>
+
+<p>2KNO<sub>3</sub> + 3C + S = 3CO<sub>2</sub> + N<sub>2</sub> + K<sub>2</sub>S.</p>
+
+<p>Gunpowder is simply a mechanical mixture of these three
+substances in the proportion required for the above reaction.
+While the equation represents the principal reaction, other
+reactions also take place.<span class='pagenum'><a name="Page_293" id="Page_293">[Pg 293]</a></span> The gases formed in the explosion,
+when measured under standard conditions, occupy about two
+hundred and eighty times the volume of the original powder.
+Potassium sulphide (K<sub>2</sub>S) is a solid substance, and it is
+largely due to it that gunpowder gives off smoke and soot when
+it explodes. Smokeless powder consists of organic substances
+which, on explosion, give only colorless gases, and hence
+produce no smoke. Sodium nitrate is cheaper than potassium
+nitrate, but it is not adapted to the manufacture of the best
+grades of powder, since it is somewhat deliquescent and does
+not give up its oxygen so readily as does potassium nitrate. It
+is used, however, in the cheaper grades of powder, such as are
+employed for blasting.</p></div>
+
+<p><b>Potassium cyanide</b> (KCN). When animal matter containing nitrogen is
+heated with iron and potassium carbonate, complicated changes occur
+which result in the formation of a substance commonly called yellow
+prussiate of potash, which has the formula K<sub>4</sub>FeC<sub>6</sub>N<sub>6</sub>. When this
+substance is heated with potassium, potassium cyanide is formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">K<sub>4</sub>FeC<sub>6</sub>N<sub>6</sub> + 2 K = 6KCN + Fe.<br /></span>
+</div></div>
+
+<p>Since sodium is much cheaper than potassium it is often used in place of
+it:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">K<sub>4</sub>FeC<sub>6</sub>N<sub>6</sub> + 2Na = 4KCN + 2NaCN + Fe.<br /></span>
+</div></div>
+
+<p>The mixture of cyanides so resulting serves most of the purposes of the
+pure salt. It is used very extensively in several metallurgical
+processes, particularly in the extraction of gold. Potassium cyanide is
+a white solid characterized by its poisonous properties, and must be
+used with extreme caution.</p>
+
+<p><b>Potassium carbonate</b> (<i>potash</i>) (K<sub>2</sub>CO<sub>3</sub>). This compound occurs in
+wood ashes in small quantities. It cannot be prepared by the Solvay
+process, since the acid carbonate is quite soluble in water, but is made
+by the Le Blanc process. Its chief use is in the manufacture of other
+potassium salts.<span class='pagenum'><a name="Page_294" id="Page_294">[Pg 294]</a></span></p>
+
+<p><b>Other salts of potassium.</b> Among the other salts of potassium frequently
+met with are the sulphate (K<sub>2</sub>SO<sub>4</sub>), the acid carbonate (KHCO<sub>3</sub>),
+the acid sulphate (KHSO<sub>4</sub>), and the acid sulphite (KHSO<sub>3</sub>). These
+are all white solids.</p>
+
+
+<h3>LITHIUM, RUBIDIUM, C&AElig;SIUM</h3>
+
+<p>Of the three remaining elements of the family&mdash;lithium, rubidium, and
+c&aelig;sium&mdash;lithium is by far the most common, the other two being very
+rare. Lithium chloride and carbonate are not infrequently found in
+natural mineral waters, and as these substances are supposed to increase
+the medicinal value of the water, they are very often added to
+artificial mineral waters in small quantities.</p>
+
+
+<h3>COMPOUNDS OF AMMONIUM</h3>
+
+<p><b>General.</b> As explained in a previous chapter, when ammonia is passed into
+water the two compounds combine to form the base NH<sub>4</sub>OH, known as
+ammonium hydroxide. When this base is neutralized with acids there are
+formed the corresponding salts, known as the ammonium salts. Since the
+ammonium group is univalent, ammonium salts resemble those of the alkali
+metals in formulas; they also resemble the latter salts very much in
+their chemical properties, and may be conveniently described in
+connection with them. Among the ammonium salts the chloride, sulphate,
+carbonate, and sulphide are the most familiar.</p>
+
+<p><b>Ammonium chloride</b> (<i>sal ammoniac</i>) (NH<sub>4</sub>Cl). This substance is
+obtained by neutralizing ammonium hydroxide with hydrochloric acid. It
+is a colorless substance crystallizing in fine needles, and, like most
+ammonium salts, is very soluble in water. When placed in a tube and
+heated strongly it decomposes into hydrochloric acid and ammonia. When
+these gases reach a cooler portion of the tube they<span class='pagenum'><a name="Page_295" id="Page_295">[Pg 295]</a></span> at once recombine,
+and the resulting ammonium chloride is deposited on the sides of the
+tube. In this way the salt can be separated from nonvolatile impurities.
+Ammonium chloride is sometimes used in preparation of ammonia; it is
+also used in making dry batteries and in the laboratory as a chemical
+reagent.</p>
+
+<p><b>Ammonium sulphate</b> ((NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>). This salt resembles the chloride
+very closely, and, being cheaper, is used in place of it when possible.
+It is used in large quantity as a fertilizer, the nitrogen which it
+contains being a very valuable food for plants.</p>
+
+<p><b>Ammonium carbonate</b> ((NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub>). This salt, as well as the acid
+carbonate (NH<sub>4</sub>HCO<sub>3</sub>), is used as a chemical reagent. They are
+colorless solids, freely soluble in water. The normal carbonate is made
+by heating ammonium chloride with powdered limestone (calcium
+carbonate), the ammonium carbonate being obtained as a sublimate in
+compact hard masses:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NH<sub>4</sub>Cl + CaCO<sub>3</sub> = (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> + CaCl<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>The salt always smells of ammonia, since it slowly decomposes, as shown
+in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> = NH<sub>4</sub>HCO<sub>3</sub> + NH<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>The acid carbonate, or bicarbonate, is prepared by saturating a solution
+of ammonium hydroxide with carbon dioxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>4</sub>OH + CO<sub>2</sub> = NH<sub>4</sub>HCO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>It is a well-crystallized stable substance.</p>
+
+<p><b>Ammonium sulphide</b> ((NH<sub>4</sub>)<sub>2</sub>S). Ammonium sulphide is prepared by the
+action of hydrosulphuric acid upon ammonium hydroxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NH<sub>4</sub>OH + H<sub>2</sub>S = (NH<sub>4</sub>)<sub>2</sub>S + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_296" id="Page_296">[Pg 296]</a></span></p>
+<p>If the action is allowed to continue until no more hydrosulphuric acid
+is absorbed, the product is the acid sulphide, sometimes called the
+hydrosulphide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>4</sub>OH + H<sub>2</sub>S = NH<sub>4</sub>HS + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>If equal amounts of ammonium hydroxide and ammonium acid sulphide are
+brought together, the normal sulphide is formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">NH<sub>4</sub>OH + NH<sub>4</sub>HS = (NH<sub>4</sub>)<sub>2</sub>S + H<sub>2</sub>O<br /></span>
+</div></div>
+
+<p>It has been obtained in the solid state, but only with great difficulty.
+As used in the laboratory it is always in the form of a solution. It is
+much used in the process of chemical analysis because it is a soluble
+sulphide and easily prepared. On exposure to the air ammonium sulphide
+slowly decomposes, being converted into ammonia, water, and sulphur:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(NH<sub>4</sub>)<sub>2</sub>S + O = 2NH<sub>3</sub> + H<sub>2</sub>O + S.<br /></span>
+</div></div>
+
+<p>As fast as the sulphur is liberated it combines with the unchanged
+sulphide to form several different ammonium sulphides in which there are
+from two to five sulphur atoms in the molecule, thus: (NH<sub>4</sub>)<sub>2</sub>S<sub>2</sub>,
+(NH<sub>4</sub>)<sub>2</sub>S<sub>3</sub>, (NH<sub>4</sub>)<sub>2</sub>S<sub>5</sub>. These sulphides in turn decompose
+by further action of oxygen, so that the final products of the reaction
+are those given in the equation. A solution of these compounds is yellow
+and is sometimes called <i>yellow ammonium sulphide</i>.</p>
+
+<div class="blockquot"><p>FLAME REACTION&mdash;SPECTROSCOPE</p>
+
+<p>When compounds of either sodium or potassium are brought into
+the non-luminous flame of a Bunsen burner the flame becomes
+colored. Sodium compounds color it intensely yellow, while
+those of potassium color it pale violet. When only one of these
+elements is present it is<span class='pagenum'><a name="Page_297" id="Page_297">[Pg 297]</a></span> easy to identify it by this simple
+test, but when both are present the intense color of the sodium
+flame entirely conceals the pale tint characteristic of
+potassium compounds.</p>
+
+<p>It is possible to detect the potassium flame in such cases,
+however, in the following way. When light is allowed to shine
+through a very small hole or slit in some kind of a screen,
+such as a piece of metal, upon a triangular prism of glass, the
+light is bent or refracted out of its course instead of passing
+straight through the glass. It thus comes out of the prism at
+some angle to the line at which it entered. Yellow light is
+bent more than red, and violet more than yellow. When light
+made up of the yellow of sodium and the violet of potassium
+shines through a slit upon such a prism, the yellow and the
+violet lights come out at somewhat different angles, and so two
+colored lines of light&mdash;a yellow line and a violet line&mdash;are
+seen on looking into the prism in the proper direction. The
+instrument used for separating the rays of light in this way is
+called a <i>spectroscope</i> (Fig. 79). The material to be tested is
+placed on a platinum wire and held in the colorless Bunsen
+flame. The resulting light passes through the slit in the end
+of tube <i>B</i>, and then through <i>B</i> to the prism. The resulting
+lines of light are seen by looking into the tube <i>A</i>, which
+contains a magnifying lens. Most elements give more than one
+image of the slit, each having a different color, and the
+series of colored lines due to an element is called its
+spectrum.</p></div>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig79.jpg" width="400" height="248" alt="Fig. 79" title="" />
+<span class="caption">Fig. 79</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_298" id="Page_298">[Pg 298]</a></span></p>
+<p>The spectra of the known elements have been carefully studied, and any
+element which imparts a characteristic color to a flame, or has a
+spectrum of its own, can be identified even when other elements are
+present. Through the spectroscopic examination of certain minerals a
+number of elements have been discovered by the observation of lines
+which did not belong to any known element. A study of the substance then
+brought to light the new element. Rubidium and c&aelig;sium were discovered in
+this way, rubidium having bright red lines and c&aelig;sium a very intense
+blue line. Lithium colors the flame deep red, and has a bright red line
+in its spectrum.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> What is an alkali? Can a metal itself be an alkali?</p>
+
+<p><b>2.</b> Write equations showing how the following changes may be brought
+about, giving the general principle involved in each change: NaCl -->
+Na<sub>2</sub>SO<sub>3</sub>, Na<sub>2</sub>SO<sub>3</sub> --> NaCl, NaCl --> NaBr, Na<sub>2</sub>SO<sub>4</sub> -->
+NaNO<sub>3</sub>, NaNO<sub>3</sub> --> NaHCO<sub>3</sub>.</p>
+
+<p><b>3.</b> What carbonates are soluble?</p>
+
+<p><b>4.</b> State the conditions under which the reaction represented by the
+following equation can be made to go in either direction:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + H<sub>2</sub>O + CO<sub>2</sub> &lt;--> 2 NaHCO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><b>5.</b> Account for the fact that solutions of sodium carbonate and potassium
+carbonate are alkaline.</p>
+
+<p><b>6.</b> What non-metallic element is obtained from the deposits of Chili
+saltpeter?</p>
+
+<p><b>7.</b> Supposing concentrated hydrochloric acid (den. = 1.2) to be worth six
+cents a pound, what is the value of the acid generated in the
+preparation of 1 ton of sodium carbonate by the Le Blanc process?</p>
+
+<p><b>8.</b> What weight of sodium carbonate crystals will 1 kg. of the anhydrous
+salt yield?</p>
+
+<p><b>9.</b> Write equations for the preparation of potassium hydroxide by three
+different methods.</p>
+
+<p><b>10.</b> What would take place if a bit of potassium hydroxide were left
+exposed to the air?</p>
+
+<p><b>11.</b> Write the equations for the reactions between sodium hydroxide and
+bromine; between potassium hydroxide and iodine.</p>
+
+<p><b>12.</b> Write equations for the preparation of potassium sulphate; of
+potassium acid carbonate.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/bunsen.jpg" width="400" height="513" alt="ROBERT WILHELM BUNSEN (German) (1811-1899)
+
+Invented many lecture-room and laboratory appliances (Bunsen burner);
+invented the spectroscope and with it discovered rubidium and c&aelig;sium;
+greatly perfected methods of electrolysis, inventing a new battery; made
+many investigations among metallic and organic substances" title="" />
+<span class="caption">ROBERT WILHELM BUNSEN (German) (1811-1899)<br /><br />
+
+Invented many lecture-room and laboratory appliances (Bunsen burner);
+invented the spectroscope and with it discovered rubidium and c&aelig;sium;
+greatly perfected methods of electrolysis, inventing a new battery; made
+many investigations among metallic and organic substances</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_299" id="Page_299">[Pg 299]</a></span></p>
+<p><b>13.</b> What weight of carnallite would be necessary in the preparation of 1
+ton of potassium carbonate?</p>
+
+<p><b>14.</b> Write the equations showing how ammonium chloride, ammonium
+sulphate, ammonium carbonate, and ammonium nitrate may be prepared from
+ammonium hydroxide.</p>
+
+<p><b>15.</b> Write an equation to represent the reaction involved in the
+preparation of ammonia from ammonium chloride.</p>
+
+<p><b>16.</b> What substances already studied are prepared from the following
+compounds? ammonium chloride; ammonium nitrate; ammonium nitrite; sodium
+nitrate; sodium chloride.</p>
+
+<p><b>17.</b> How could you prove that the water in crystals of common salt is not
+water of crystallization?</p>
+
+<p><b>18.</b> How could you distinguish between potassium chloride and potassium
+iodide? between sodium chloride and ammonium chloride? between sodium
+nitrate and potassium nitrate?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_300" id="Page_300">[Pg 300]</a></span></p>
+<h2><a name="CHAPTER_XXIV" id="CHAPTER_XXIV"></a>CHAPTER XXIV</h2>
+
+<h3>THE ALKALINE-EARTH FAMILY</h3>
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>SYMBOL</td><td align='left'>ATOMIC WEIGHT</td><td align='left'>DENSITY</td><td colspan="2" align='left'>MILLIGRAMS SOLUBLE IN 1 L OF WATER AT 18&deg;</td><td align='left'>CARBONATE DECOMPOSES</td></tr>
+<tr><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='left'>SULPHATE</td><td align='left'>HYDROXIDE</td><td align='left'>&nbsp;</td></tr>
+<tr><td align='left'>Calcium</td><td align='left'>Ca</td><td align='left'>40.1</td><td align='left'>1.54</td><td align='left'>2070.00</td><td align='left'>1670.</td><td align='left'>At dull red heat</td></tr>
+<tr><td align='left'>Strontium</td><td align='left'>Sr</td><td align='left'>87.6</td><td align='left'>2.50</td><td align='left'>170.00</td><td align='left'>7460.</td><td align='left'>At white heat</td></tr>
+<tr><td align='left'>Barium</td><td align='left'>Ba</td><td align='left'>137.4</td><td align='left'>3.75</td><td align='left'>2.29</td><td align='left'>36300.</td><td align='left'>Scarcely at all</td></tr>
+</table></div>
+
+
+<p><b>The family.</b> The alkaline-earth family consists of the very abundant
+element calcium and the much rarer elements strontium and barium. They
+are called the alkaline-earth metals because their properties are
+between those of the alkali metals and the earth metals. The earth
+metals will be discussed in a later chapter. The family is also
+frequently called the calcium family.</p>
+
+<p>1. <i>Occurrence.</i> These elements do not occur free in nature. Their most
+abundant compounds are the carbonates and sulphates; calcium also occurs
+in large quantities as the phosphate and silicate.</p>
+
+<p>2. <i>Preparation.</i> The metals were first prepared by Davy in 1808 by
+electrolysis. This method has again come into use in recent years.
+Strontium and barium have as yet been obtained only in small quantities
+and in the impure state, and many of their physical properties,<span class='pagenum'><a name="Page_301" id="Page_301">[Pg 301]</a></span> such as
+their densities and melting points, are therefore imperfectly known.</p>
+
+<p>3. <i>Properties.</i> The three metals resemble each other very closely. They
+are silvery-white in color and are about as hard as lead. Their
+densities increase with their atomic weights, as is shown in the table
+on opposite page. Like the alkali metals they have a strong affinity for
+oxygen, tarnishing in the air through oxidation. They decompose water at
+ordinary temperatures, forming hydroxides and liberating hydrogen. When
+ignited in the air they burn with brilliancy, forming oxides of the
+general formula MO. These oxides readily combine with water, according
+to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">MO + H<sub>2</sub>O = M(OH)<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>Each of the elements has a characteristic spectrum, and the presence of
+the metals can easily be detected by the spectroscope.</p>
+
+<p>4. <i>Compounds.</i> The elements are divalent in almost all of their
+compounds, and these compounds in solution give simple, divalent,
+colorless ions. The corresponding salts of the three elements are very
+similar to each other and show a regular variation in properties in
+passing from calcium to strontium and from strontium to barium. This is
+seen in the solubility of the sulphate and hydroxide, and in the ease of
+decomposition of the carbonates, as given in the table. Unlike the
+alkali metals, their normal carbonates and phosphates are insoluble in
+water.</p>
+
+
+<h3>CALCIUM</h3>
+
+<p><b>Occurrence.</b> The compounds of calcium are very abundant in nature, so
+that the total amount of calcium in the earth's crust is very large. A
+great many different compounds<span class='pagenum'><a name="Page_302" id="Page_302">[Pg 302]</a></span> containing the clement are known, the
+most important of which are the following:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Calcite (marble)</td><td align='left'>CaCO3.</td></tr>
+<tr><td align='left'>Phosphorite</td><td align='left'>Ca3(PO4)2.</td></tr>
+<tr><td align='left'>Fluorspar</td><td align='left'>CaF2.</td></tr>
+<tr><td align='left'>Wollastonite</td><td align='left'>CaSiO3.</td></tr>
+<tr><td align='left'>Gypsum</td><td align='left'>CaSO4&middot;2H2O.</td></tr>
+<tr><td align='left'>Anhydrite</td><td align='left'>CaSO4.</td></tr>
+</table></div>
+
+<p><b>Preparation.</b> Calcium is now prepared by the electrolysis of the melted
+chloride, the metal depositing in solid condition on the cathode. It is
+a gray metal, considerably heavier and harder than sodium. It acts upon
+water, forming calcium hydroxide and hydrogen, but the action does not
+evolve sufficient heat to melt the metal. It promises to become a useful
+substance, though no commercial applications for it have as yet been
+found.</p>
+
+<p><b>Calcium oxide</b> (<i>lime, quicklime</i>) (CaO). Lime is prepared by strongly
+heating calcium carbonate (limestone) in large furnaces called kilns:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaCO<sub>3</sub> = CaO + CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>When pure, lime is a white amorphous substance. Heated intensely, as in
+the oxyhydrogen flame, it gives a brilliant light called the lime light.
+Although it is a very difficultly fusible substance, yet in the electric
+furnace it can be made to melt and even boil. Water acts upon lime with
+the evolution of a great deal of heat,&mdash;hence the name quicklime, or
+live lime,&mdash;the process being called slaking. The equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaO + H<sub>2</sub>O = Ca(OH)<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>Lime readily absorbs moisture from the air, and is used to dry moist
+gases, especially ammonia, which cannot be<span class='pagenum'><a name="Page_303" id="Page_303">[Pg 303]</a></span> dried by the usual
+desiccating agents. It also absorbs carbon dioxide, forming the
+carbonate</p>
+
+<p>CaO + CO<sub>2</sub> = CaCO<sub>3</sub>.</p>
+
+<p>Lime exposed to air is therefore gradually converted into hydroxide and
+carbonate, and will no longer slake with water. It is then said to be
+air-slaked.</p>
+
+<div class="blockquot"><p><b>Limekilns.</b> The older kiln, still in common use, consists of a
+large cylindrical stack in which the limestone is loosely
+packed. A fire is built at the base of the stack, and when the
+burning is complete it is allowed to die out and the lime is
+removed from the kiln. The newer kilns are constructed as shown
+in Fig. 80. A number of fire boxes are built around the lower
+part of the kiln, one of which is shown at <i>B</i>. The fire is
+built on the grate <i>F</i> and the hot products of combustion are
+drawn up through the stack, decomposing the limestone. The kiln
+is charged at <i>C</i>, and sometimes fuel is added with the
+limestone to cause combustion throughout the contents of the
+kiln. The burned lime is raked out through openings in the
+bottom of the stack, one of which is shown at <i>D.</i> The
+advantage of this kind of a kiln over the older form is that
+the process is continuous, limestone being charged in at the
+top as fast as the lime is removed at the bottom.</p></div>
+
+<div class="figcenter" style="width: 183px;">
+<img src="images/imagefig80.jpg" width="183" height="361" alt="Fig. 80" title="" />
+<span class="caption">Fig. 80</span>
+</div>
+
+<p><b>Calcium hydroxide </b> (<i>slaked lime</i>) (Ca(OH)<sub>2</sub>). Pure calcium hydroxide
+is a light white powder. It is sparingly soluble in water, forming a
+solution called <i>limewater</i>, which is often used in medicine as a mild
+alkali. Chemically, calcium hydroxide is a moderately strong base,
+though not so strong as sodium hydroxide. Owing to its cheapness it is
+much used in the<span class='pagenum'><a name="Page_304" id="Page_304">[Pg 304]</a></span> industries whenever an alkali is desired. A number of
+its uses have already been mentioned. It is used in the preparation of
+ammonia, bleaching powder, and potassium hydroxide. It is also used to
+remove carbon dioxide and sulphur compounds from coal gas, to remove the
+hair from hides in the tanneries (this recalls the caustic or corrosive
+properties of sodium hydroxide), and for making mortar.</p>
+
+<p><b>Mortar</b> is a mixture of calcium hydroxide and sand. When it is exposed to
+the air or spread upon porous materials moisture is removed from it
+partly by absorption in the porous materials and partly by evaporation,
+and the mortar becomes firm, or <i>sets</i>. At the same time carbon dioxide
+is slowly absorbed from the air, forming hard calcium carbonate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ca(OH)<sub>2</sub> + CO<sub>2</sub> = CaCO<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>By this combined action the mortar becomes very hard and adheres firmly
+to the surface upon which it is spread. The sand serves to give body to
+the mortar and makes it porous, so that the change into carbonate can
+take place throughout the mass. It also prevents too much shrinkage.</p>
+
+<p><b>Cement.</b> When limestone to which clay and sand have been added in certain
+proportions is burned until it is partly fused (some natural marl is
+already of about the right composition), and the clinker so produced is
+ground to powder, the product is called cement. When this material is
+moistened it sets to a hard stone-like mass which retains its hardness
+even when exposed to the continued action of water. It can be used for
+under-water work, such as bridge piers, where mortar would quickly
+soften. Several varieties of cement are made, the best known of which is
+Portland cement.<span class='pagenum'><a name="Page_305" id="Page_305">[Pg 305]</a></span></p>
+
+<p><b>Growing importance of cement.</b> Cement is rapidly coming into use for a
+great variety of purposes. It is often used in place of mortar in the
+construction of brick buildings. Mixed with crushed stone and sand it
+forms concrete which is used in foundation work. It is also used in
+making artificial stone, terra-cotta trimmings for buildings, artificial
+stone walks and floors, and the like. It is being used more and more for
+making many articles which were formerly made of wood or stone, and the
+entire walls of buildings are sometimes made of cement blocks or of
+concrete.</p>
+
+<p><b>Calcium carbonate</b> (CaCO<sub>3</sub>). This substance is found in a great many
+natural forms to which various names have been given. They may be
+classified under three heads:</p>
+
+<p>1. <i>Amorphous carbonate.</i> This includes those forms which are not
+markedly crystalline. Limestone is the most familiar of these and is a
+grayish rock usually found in hard stratified masses. Whole mountain
+ranges are sometimes made up of this material. It is always impure,
+usually containing magnesium carbonate, clay, silica, iron and aluminium
+compounds, and frequently fossil remains. Marl is a mixture of limestone
+and clay. Pearls, chalk, coral, and shells are largely calcium
+carbonate.</p>
+
+<p>2. <i>Hexagonal carbonate.</i> Calcium carbonate crystallizes in the form of
+rhomb-shaped crystals which belong to the hexagonal system. When very
+pure and transparent the substance is called Iceland spar. Calcite is a
+similar form, but somewhat opaque or clouded. Mexican onyx is a massive
+variety, streaked or banded with colors due to impurities. Marble when
+pure is made up of minute calcite crystals. Stalactites and stalagmites
+are icicle-like forms sometimes found in caves.</p>
+
+<p>3. <i>Rhombic carbonate.</i> Calcium carbonate sometimes crystallizes in
+needle-shaped crystals belonging to the rhombic system. This is the
+unstable form and tends to<span class='pagenum'><a name="Page_306" id="Page_306">[Pg 306]</a></span> go over into the other variety. Aragonite is
+the most familiar example of this form.</p>
+
+<p><b>Preparation and uses of calcium carbonate.</b> In the laboratory pure
+calcium carbonate can be prepared by treating a soluble calcium salt
+with a soluble carbonate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + CaCl<sub>2</sub> = CaCO<sub>3</sub> + 2NaCl.<br /></span>
+</div></div>
+
+<p>When prepared in this way it is a soft white powder often called
+precipitated chalk, and is much used as a polishing powder. It is
+insoluble in water, but dissolves in water saturated with carbon
+dioxide, owing to the formation of the acid calcium carbonate which is
+slightly soluble:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaCO<sub>3</sub> + H<sub>2</sub>CO<sub>3</sub> = Ca(HCO<sub>3</sub>)<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>The natural varieties of calcium carbonate find many uses, such as in
+the preparation of lime and carbon dioxide; in metallurgical operations,
+especially in the blast furnaces; in the manufacture of soda, glass, and
+crayon (which, in addition to chalk, usually contains clay and calcium
+sulphate); for building stone and ballast for roads.</p>
+
+<p><b>Calcium chloride</b> (CaCl<sub>2</sub>). This salt occurs in considerable quantity
+in sea water. It is obtained as a by-product in many technical
+processes, as in the Solvay soda process. When crystallized from its
+saturated solutions it forms colorless needles of the composition
+CaCl<sub>2</sub>&middot;6H<sub>2</sub>O. By evaporating a solution to dryness and heating to a
+moderate temperature calcium chloride is obtained anhydrous as a white
+porous mass. In this condition it absorbs water with great energy and is
+a valuable drying agent.</p>
+
+<p><b>Bleaching powder</b> (CaOCl<sub>2</sub>). When chlorine acts upon a solution of
+calcium hydroxide the reaction is similar to that which occurs between
+chlorine and potassium hydroxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Ca(OH)<sub>2</sub> + 4Cl = CaCl<sub>2</sub> + Ca(ClO)<sub>2</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_307" id="Page_307">[Pg 307]</a></span></p>
+<p>If, however, chlorine is conducted over calcium hydroxide in the form of
+a dry powder, it is absorbed and a substance is formed which appears to
+have the composition represented in the formula CaOCl<sub>2</sub>. This
+substance is called bleaching powder, or hypochlorite of lime. It is
+probably the calcium salt of both hydrochloric and hypochlorous acids,
+so that its structure is represented by the formula</p>
+
+<div class="poem"><div class="stanza">
+<span class="i1">/ClO<br /></span>
+<span class="i0">Ca<br /></span>
+<span class="i1">\Cl.<br /></span>
+</div></div>
+
+<p>In solution this substance acts exactly like a mixture of calcium
+chloride (CaCl<sub>2</sub>) and calcium hypochlorite (Ca(ClO)<sub>2</sub>), since it
+dissociates to form the ions Ca<sup>++</sup>, Cl<sup>-</sup>, and ClO<sup>-</sup>.</p>
+
+<p>Bleaching powder undergoes a number of reactions which make it an
+important substance.</p>
+
+<p>1. When treated with an acid it evolves chlorine:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i1">/ClO<br /></span>
+<span class="i0">Ca &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ H<sub>2</sub>SO<sub>4</sub> = CaSO<sub>4</sub> + HCl + HClO,<br /></span>
+<span class="i1">\Cl<br /></span>
+</div><div class="stanza">
+
+<span class="i0">HCl + HClO = H<sub>2</sub>O + 2Cl.<br /></span>
+</div></div>
+
+<p>This reaction can be employed in the preparation of chlorine, or the
+nascent chlorine may be used as a bleaching agent.</p>
+
+<p>2. It is slowly decomposed by the carbon dioxide of the air, yielding
+calcium carbonate and chlorine:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaOCl<sub>2</sub> + CO<sub>2</sub> = CaCO<sub>3</sub> + 2Cl.<br /></span>
+</div></div>
+
+<p>Owing to this slow action the substance is a good disinfectant.</p>
+
+<p>3. When its solution is boiled the substance breaks down into calcium
+chloride and chlorate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">6CaOCl<sub>2</sub> = 5CaCl<sub>2</sub> + Ca(ClO<sub>3</sub>)<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>This reaction is used in the preparation of potassium chlorate.<span class='pagenum'><a name="Page_308" id="Page_308">[Pg 308]</a></span></p>
+
+<p><b>Calcium fluoride</b> (<i>fluorspar</i>) (CaF<sub>2</sub>). Fluorspar has already been
+mentioned as the chief natural compound of fluorine. It is found in
+large quantities in a number of localities, and is often crystallized in
+perfect cubes of a light green or amethyst color. It can be melted
+easily in a furnace, and is sometimes used in the fused condition in
+metallurgical operations to protect a metal from the action of the air
+during its reduction. It is used as the chief source of fluorine
+compounds, especially hydrofluoric acid.</p>
+
+<p><b>Calcium sulphate</b> (<i>gypsum</i>) (CaSO<sub>4</sub>&middot;2H<sub>2</sub>O). This abundant substance
+occurs in very perfectly formed crystals or in massive deposits. It is
+often found in solution in natural waters and in the sea water. Salts
+deposited from sea water are therefore likely to contain this substance
+(see Stassfurt salts).</p>
+
+<p>It is very sparingly soluble in water, and is thrown down as a fine
+white precipitate when any considerable amounts of a calcium salt and a
+soluble sulphate (or sulphuric acid) are brought together in solution.
+Its chief use is in the manufacture of plaster of Paris and of hollow
+tiles for fireproof walls. Such material is called <i>gypsite</i>. It is also
+used as a fertilizer.</p>
+
+<p>Calcium sulphate, like the carbonate, occurs in many forms in nature.
+Gypsum is a name given to all common varieties. Granular or massive
+specimens are called alabaster, while all those which are well
+crystallized are called selenite. Satin spar is still another variety
+often seen in mineral collections.</p>
+
+<p><b>Plaster of Paris.</b> When gypsum is heated to about 115&deg; it loses a portion
+of its water of crystallization in accordance with the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2(CaSO<sub>4</sub>&middot;2H<sub>2</sub>O) = 2CaSO<sub>4</sub>&middot;H<sub>2</sub>O + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_309" id="Page_309">[Pg 309]</a></span></p>
+
+<p>The product is a fine white powder called <i>plaster of Paris</i>. On being
+moistened it again takes up this water, and in so doing first forms a
+plastic mass, which soon becomes very firm and hard and regains its
+crystalline structure. These properties make it very valuable as a
+material for forming casts and stucco work, for cementing glass to
+metals, and for other similar purposes. If overheated so that all water
+is driven off, the process of taking up water is so slow that the
+material is worthless. Such material is said to be dead burned. Plaster
+of Paris is very extensively used as the finishing coat for plastered
+walls.</p>
+
+<p><b>Hard water.</b> Waters containing compounds of calcium and magnesium in
+solution are called hard waters because they feel harsh to the touch.
+The hardness of water may be of two kinds,&mdash;(1) temporary hardness and
+(2) permanent hardness.</p>
+
+<p>1. <i>Temporary hardness.</i> We have seen that when water charged with
+carbon dioxide comes in contact with limestone a certain amount of the
+latter dissolves, owing to the formation of the soluble acid carbonate
+of calcium. The hardness of such waters is said to be temporary, since
+it may be removed by boiling. The heat changes the acid carbonate into
+the insoluble normal carbonate which then precipitates, rendering the
+water soft:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ca(HCO<sub>3</sub>)<sub>2</sub> = CaCO<sub>3</sub> + H<sub>2</sub>O + CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>Such waters may also be softened by the addition of sufficient lime or
+calcium hydroxide to convert the acid carbonate of calcium into the
+normal carbonate. The equation representing the reaction is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ca(HCO<sub>3</sub>)<sub>2</sub> + Ca(OH)<sub>2</sub> = 2CaCO<sub>3</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_310" id="Page_310">[Pg 310]</a></span></p>
+
+<p>2. <i>Permanent hardness.</i> The hardness of water may also be due to the
+presence of calcium and magnesium sulphates or chlorides. Boiling the
+water does not affect these salts; hence such waters are said to have
+permanent hardness. They may be softened, however, by the addition of
+sodium carbonate, which precipitates the calcium and magnesium as
+insoluble carbonates:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaSO<sub>4</sub> + Na<sub>2</sub>CO<sub>3</sub> = CaCO<sub>3</sub> + Na<sub>2</sub>SO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>This process is sometimes called "breaking" the water.</p>
+
+<div class="blockquot"><p><b>Commercial methods for softening water.</b> The average water of a
+city supply contains not only the acid carbonates of calcium
+and magnesium but also the sulphates and chlorides of these
+metals, together with other salts in smaller quantities. Such
+waters are softened on a commercial scale by the addition of
+the proper quantities of calcium hydroxide and sodium
+carbonate. The calcium hydroxide is added first to precipitate
+all the acid carbonates. After a short time the sodium
+carbonate is added to precipitate the other soluble salts of
+calcium and magnesium, together with any excess of calcium
+hydroxide which may have been added. The quantity of calcium
+hydroxide and sodium carbonate required is calculated from a
+chemical analysis of the water. It will be noticed that the
+water softened in this way will contain sodium sulphate and
+chloride, but the presence of these salts is not objectionable.</p></div>
+
+<p><b>Calcium carbide</b> (CaC<sub>2</sub>). This substance is made by heating well-dried
+coke and lime in an electrical furnace. The equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CaO + 3C = CaC<sub>2</sub> + CO.<br /></span>
+</div></div>
+
+<p>The pure carbide is a colorless, transparent, crystalline substance. In
+contact with water it is decomposed with the evolution of pure acetylene
+gas, having a pleasant ethereal odor. The commercial article is a dull
+gray porous substance which contains many impurities. The acetylene
+prepared from this substance has a very characteristic odor<span class='pagenum'><a name="Page_311" id="Page_311">[Pg 311]</a></span> due to
+impurities, the chief of these being phosphine. It is used in
+considerable quantities as a source of acetylene gas for illuminating
+purposes.</p>
+
+<div class="blockquot"><p><b>Technical preparation.</b> Fig. 81 represents a recent type of a
+carbide furnace. The base of the furnace is provided with a
+large block of carbon <i>A</i>, which serves as one of the
+electrodes. The other electrodes <i>B</i>, several in number, are
+arranged horizontally at some distance above this. A mixture of
+coal and lime is fed into the furnace through the trap top <i>C</i>,
+and in the lower part of the furnace this mixture becomes
+intensely heated, forming liquid carbide. This is drawn off
+through the taphole <i>D</i>.</p>
+
+<p>The carbon monoxide formed in the reaction escapes through the
+pipes <i>E</i> and is led back into the furnace. The pipes <i>F</i>
+supply air, so that the monoxide burns as it re&euml;nters the
+furnace and assists in heating the charge. The carbon dioxide
+so formed, together with the nitrogen entering as air, escape
+at <i>G</i>. An alternating current is used.</p></div>
+
+<div class="figcenter" style="width: 318px;">
+<img src="images/imagefig81.jpg" width="318" height="400" alt="Fig. 81" title="" />
+<span class="caption">Fig. 81</span>
+</div>
+
+<p><b>Calcium phosphate</b> (Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>). This important substance occurs
+abundantly in nature as a constituent of apatite (3
+Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>&middot;CaF<sub>2</sub>), in phosphate rock, and as the chief mineral
+constituent of bones. Bone ash is therefore nearly pure calcium
+phosphate. It is a white powder, insoluble in water, although it readily
+dissolves in acids, being decomposed by them and converted into soluble
+acid phosphates, as explained in connection with the acids of
+phosphorus.<span class='pagenum'><a name="Page_312" id="Page_312">[Pg 312]</a></span></p>
+
+
+<h3>STRONTIUM</h3>
+
+<p><b>Occurrence.</b> Strontium occurs sparingly in nature, usually as
+strontianite (SrCO<sub>3</sub>) and as celestite (SrSO<sub>4</sub>). Both minerals form
+beautiful colorless crystals, though celestite is sometimes colored a
+faint blue. Only a few of the compounds of strontium have any commercial
+applications.</p>
+
+<p><b>Strontium hydroxide</b> (Sr(OH)<sub>2</sub>&middot;8H<sub>2</sub>O). The method of preparation of
+strontium hydroxide is analogous to that of calcium hydroxide. The
+substance has the property of forming an insoluble compound with sugar,
+which can easily be separated again into its constituents. It is
+therefore sometimes used in the sugar refineries to extract sugar from
+impure mother liquors from which the sugar will not crystallize.</p>
+
+<p><b>Strontium nitrate</b> (Sr(NO<sub>3</sub>)<sub>2</sub>&middot;4H<sub>2</sub>O). This salt is prepared by
+treating the native carbonate with nitric acid. When ignited with
+combustible materials it imparts a brilliant crimson color to the flame,
+and because of this property it is used in the manufacture of red
+lights.</p>
+
+
+<h3>BARIUM</h3>
+
+<p>Barium is somewhat more abundant than strontium, occurring in nature
+largely as barytes, or heavy spar (BaSO<sub>4</sub>), and witherite (BaCO<sub>3</sub>).
+Like strontium, it closely resembles calcium both in the properties of
+the metal and in the compounds which it forms.</p>
+
+<p><b>Oxides of barium.</b> Barium oxide (BaO) can be obtained by strongly heating
+the nitrate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ba(NO<sub>3</sub>)<sub>2</sub> = BaO + 2NO<sub>2</sub> + O.<br /></span>
+</div></div>
+
+<p>Heated to a low red heat in the air, the oxide combines with oxygen,
+<span class='pagenum'><a name="Page_313" id="Page_313">[Pg 313]</a></span>forming the peroxide (BaO<sub>2</sub>). If the temperature is raised still
+higher, or the pressure is reduced, oxygen is given off and the oxide is
+once more formed. The reaction</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">BaO<sub>2</sub> &lt;--> BaO + O<br /></span>
+</div></div>
+
+<p>is reversible and has been used as a means of separating oxygen from the
+air. Treated with acids, barium peroxide yields hydrogen peroxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">BaO<sub>2</sub> + 2HCl = BaCl<sub>2</sub> + H<sub>2</sub>O<sub>2</sub>.<br /></span>
+</div></div>
+
+<p><b>Barium chloride</b> (BaCl<sub>2</sub>&middot;2H<sub>2</sub>O). Barium chloride is a white
+well-crystallized substance which is easily prepared from the native
+carbonate. It is largely used in the laboratory as a reagent to detect
+the presence of sulphuric acid or soluble sulphates.</p>
+
+<p><b>Barium sulphate</b> <i>(barytes)</i> (BaSO<sub>4</sub>). Barium sulphate occurs in nature
+in the form of heavy white crystals. It is precipitated as a crystalline
+powder when a barium salt is added to a solution of a sulphate or
+sulphuric acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">BaCl<sub>2</sub> + H<sub>2</sub>SO<sub>4</sub> = BaSO<sub>4</sub> + 2HCl.<br /></span>
+</div></div>
+
+<p>This precipitate is used, as are also the finely ground native sulphate
+and carbonate, as a pigment in paints. On account of its low cost it is
+sometimes used as an adulterant of white lead, which is also a heavy
+white substance.</p>
+
+<p>Barium compounds color the flame green, and the nitrate (Ba(NO<sub>3</sub>)<sub>2</sub>)
+is used in the manufacture of green lights. Soluble barium compounds are
+poisonous.</p>
+
+
+<h3>RADIUM</h3>
+
+<p><b>Historical.</b> In 1896 the French scientist Becquerel observed that the
+mineral pitchblende possesses certain remarkable properties. It affects
+photographic plates even in complete darkness, and discharges<span class='pagenum'><a name="Page_314" id="Page_314">[Pg 314]</a></span> a
+gold-leaf electroscope when brought close to it. In 1898 Madam Curie
+made a careful study of pitchblende to see if these properties belong to
+it or to some unknown substance contained in it. She succeeded in
+extracting from it a very small quantity of a substance containing a new
+element which she named radium.</p>
+
+<p>In 1910 Madam Curie succeeded in obtaining radium itself by the
+electrolysis of radium chloride. It is a silver-white metal melting at
+about 700&deg;. It blackens in the air, forming a nitride, and decomposes
+water. Its atomic weight is about 226.5.</p>
+
+<p><b>Properties.</b> Compounds of radium affect a photographic plate or
+electroscope even through layers of paper or sheets of metal. They also
+bring about chemical changes in substances placed near them.
+Investigation of these strange properties has suggested that the radium
+atoms are unstable and undergo a decomposition. As a result of this
+decomposition very minute bodies, to which the name corpuscles has been
+given, are projected from the radium atom with exceedingly great
+velocity. It is to these corpuscles that the strange properties of
+radium are due. It seems probable that the gas helium is in some way
+formed during the decomposition of radium.</p>
+
+<p>Two or three other elements, particularly uranium and thorium, have been
+found to possess many of the properties of radium in smaller degree.</p>
+
+<p><b>Radium and the atomic theory.</b> If these views in regard to radium should
+prove to be well founded, it will be necessary to modify in some
+respects the conception of the atom as developed in a former chapter.
+The atom would have to be regarded as a compound unit made up of several
+parts. In a few cases, as in radium and uranium, it would appear that
+this unit is unstable and undergoes transformation into more stable
+combinations. This modification would not, in any essential way, be at
+variance with the atomic theory as propounded by Dalton.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> What properties have the alkaline-earth metals in common with the
+alkali metals? In what respects do they differ?</p>
+
+<p><b>2.</b> Write the equation for the reaction between calcium carbide and
+water.</p>
+
+<p><b>3.</b> For what is calcium chlorate used?<span class='pagenum'><a name="Page_315" id="Page_315">[Pg 315]</a></span></p>
+
+<p><b>4.</b> Could limestone be completely decomposed if heated in a closed
+vessel?</p>
+
+<p><b>5.</b> Caves often occur in limestone. Account for their formation.</p>
+
+<p><b>6.</b> What is the significance of the term fluorspar? (Consult dictionary.)</p>
+
+<p><b>7.</b> Could calcium chloride be used in place of barium chloride in testing
+for sulphates?</p>
+
+<p><b>8.</b> What weight of water is necessary to slake the lime obtained from 1
+ton of pure calcium carbonate?</p>
+
+<p><b>9.</b> What weight of gypsum is necessary in the preparation of 1 ton of
+plaster of Paris?</p>
+
+<p><b>10.</b> Write equations to represent the reactions involved in the
+preparation of strontium hydroxide and strontium nitrate from
+strontianite.</p>
+
+<p><b>11.</b> Write equations to represent the reactions involved in the
+preparation of barium chloride from heavy spar.</p>
+
+<p><b>12.</b> Could barium hydroxide be used in place of calcium hydroxide in
+testing for carbon dioxide?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_316" id="Page_316">[Pg 316]</a></span></p>
+<h2><a name="CHAPTER_XXV" id="CHAPTER_XXV"></a>CHAPTER XXV</h2>
+
+<h3>THE MAGNESIUM FAMILY</h3>
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>SYMBOL</td><td align='left'>ATOMIC WEIGHT</td><td align='left'>DENSITY</td><td align='left'>MELTING POINT</td><td align='left'>BOILING POINT</td><td align='left'>OXIDE</td></tr>
+<tr><td align='left'>Magnesium</td><td align='center'>Mg</td><td align='center'>24.36</td><td align='center'>1.75</td><td align='center'>750&deg;</td><td align='center'>920&deg;</td><td align='center'>MgO</td></tr>
+<tr><td align='left'>Zinc</td><td align='center'>Zn</td><td align='center'>65.4</td><td align='center'>7.00</td><td align='center'>420&deg;</td><td align='center'>950&deg;</td><td align='center'>ZnO</td></tr>
+<tr><td align='left'>Cadmium</td><td align='center'>Cd</td><td align='center'>112.4</td><td align='center'>8.67</td><td align='center'>320&deg;</td><td align='center'>778&deg;</td><td align='center'>CdO</td></tr>
+</table></div>
+
+
+<p><b>The family.</b> In the magnesium family are included the four elements:
+magnesium, zinc, cadmium, and mercury. Between the first three of these
+metals there is a close family resemblance, such as has been traced
+between the members of the two preceding families. Mercury in some
+respects is more similar to copper and will be studied in connection
+with that metal.</p>
+
+<p>1. <i>Properties.</i> When heated to a high temperature in the air each of
+these metals combines with oxygen to form an oxide of the general
+formula MO, in which M represents the metal. Magnesium decomposes
+boiling water slowly, while zinc and cadmium have but little action on
+it.</p>
+
+<p>2. <i>Compounds.</i> The members of this group are divalent in nearly all
+their compounds, so that the formulas of their salts resemble those of
+the alkaline-earth metals. Like the alkaline-earth metals, their
+carbonates and phosphates are insoluble in water. Their sulphates,
+however, are readily soluble. Unlike both the alkali and alkaline-earth<span class='pagenum'><a name="Page_317" id="Page_317">[Pg 317]</a></span>
+metals, their hydroxides are nearly insoluble in water. Most of their
+compounds dissociate in such a way as to give a simple, colorless,
+metallic ion.</p>
+
+
+<h3>MAGNESIUM</h3>
+
+<p><b>Occurrence.</b> Magnesium is a very abundant element in nature, ranking a
+little below calcium in this respect. Like calcium, it is a constituent
+of many rocks and also occurs in the form of soluble salts.</p>
+
+<p><b>Preparation.</b> The metal magnesium, like most metals whose oxides are
+difficult to reduce with carbon, was formerly prepared by heating the
+anhydrous chloride with sodium:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">MgCl<sub>2</sub> + 2Na = 2NaCl + Mg.<br /></span>
+</div></div>
+
+<p>It is now made by electrolysis, but instead of using as the electrolyte
+the melted anhydrous chloride, which is difficult to obtain, the natural
+mineral carnallite is used. This is melted in an iron pot which also
+serves as the cathode in the electrolysis. A rod of carbon dipping into
+the melted salt serves as the anode. The apparatus is very similar to
+the one employed in the preparation of sodium.</p>
+
+<p><b>Properties.</b> Magnesium is a rather tough silvery-white metal of small
+density. Air does not act rapidly upon it, but a thin film of oxide
+forms upon its surface, dimming its bright luster. The common acids
+dissolve it with the formation of the corresponding salts. It can be
+ignited readily and in burning liberates much heat and gives a brilliant
+white light. This light is very rich in the rays which affect
+photographic plates, and the metal in the form of fine powder is
+extensively used in the production of flash lights and for white lights
+in pyrotechnic displays.<span class='pagenum'><a name="Page_318" id="Page_318">[Pg 318]</a></span></p>
+
+<p><b>Magnesium oxide</b> (<i>magnesia</i>) (MgO). Magnesium oxide, sometimes called
+magnesia or magnesia usta, resembles lime in many respects. It is much
+more easily formed than lime and can be made in the same way,&mdash;by
+igniting the carbonate. It is a white powder, very soft and light, and
+is unchanged by heat even at very high temperatures. For this reason it
+is used in the manufacture of crucibles, for lining furnaces, and for
+other purposes where a refractory substance is needed. It combines with
+water to form magnesium hydroxide, but much more slowly and with the
+production of much less heat than in the case of calcium oxide.</p>
+
+<p><b>Magnesium hydroxide</b> (Mg(OH)<sub>2</sub>). The hydroxide formed in this way is
+very slightly soluble in water, but enough dissolves to give the water
+an alkaline reaction. Magnesium hydroxide is therefore a fairly strong
+base. It is an amorphous white substance. Neither magnesia nor magnesium
+salts have a very marked effect upon the system; and for this reason
+magnesia is a very suitable antidote for poisoning by strong acids,
+since any excess introduced into the system will have no injurious
+effect.</p>
+
+<div class="blockquot"><p><b>Magnesium cement.</b> A paste of magnesium hydroxide and water
+slowly absorbs carbon dioxide from the air and becomes very
+hard. The hardness of the product is increased by the presence
+of a considerable amount of magnesium chloride in the paste.
+The hydroxide, with or without the chloride, is used in the
+preparation of cements for some purposes.</p></div>
+
+<p><b>Magnesium carbonate</b> (MgCO<sub>3</sub>). Magnesium carbonate is a very abundant
+mineral. It occurs in a number of localities as magnesite, which is
+usually amorphous, but sometimes forms pure crystals resembling calcite.
+More commonly it is found associated with calcium carbonate.<span class='pagenum'><a name="Page_319" id="Page_319">[Pg 319]</a></span> The
+mineral dolomite has the composition CaCO<sub>3</sub>&middot;MgCO<sub>3</sub>. Limestone
+containing smaller amounts of magnesium carbonate is known as dolomitic
+limestone. Dolomite is one of the most common rocks, forming whole
+mountain masses. It is harder and less readily attacked by acids than
+limestone. It is valuable as a building stone and as ballast for
+roadbeds and foundations. Like calcium carbonate, magnesium carbonate is
+insoluble in water, though easily dissolved by acids.</p>
+
+<p><b>Basic carbonate of magnesium.</b> We should expect to find magnesium
+carbonate precipitated when a soluble magnesium salt and a soluble
+carbonate are brought together:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Na<sub>2</sub>CO<sub>3</sub> + MgCl<sub>2</sub> = MgCO<sub>3</sub> + 2NaCl.<br /></span>
+</div></div>
+
+<p>Instead of this, some carbon dioxide escapes and the product is found to
+be a basic carbonate. The most common basic carbonate of magnesium has
+the formula 4MgCO<sub>3</sub>&middot;Mg(OH)<sub>2</sub>, and is sometimes called magnesia alba.
+This compound is formed by the partial hydrolysis of the normal
+carbonate at first precipitated:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">5MgCO<sub>3</sub> + 2H<sub>2</sub>O = 4MgCO<sub>3</sub>&middot;Mg(OH)<sub>2</sub> + H<sub>2</sub>CO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><b>Magnesium chloride</b> (MgCl<sub>2</sub>&middot;6H<sub>2</sub>O). Magnesium chloride is found in
+many natural waters and in many salt deposits (see Stassfurt salts). It
+is obtained as a by-product in the manufacture of potassium chloride
+from carnallite. As there is no very important use for it, large
+quantities annually go to waste. When heated to drive off the water of
+crystallization the chloride is decomposed as shown in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">MgCl<sub>2</sub>&middot;6H<sub>2</sub>O = MgO + 2HCl + 5H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_320" id="Page_320">[Pg 320]</a></span></p>
+
+<p>Owing to the abundance of magnesium chloride, this reaction is being
+used to some extent in the preparation of both magnesium oxide and
+hydrochloric acid.</p>
+
+<div class="blockquot"><p><b>Boiler scale.</b> When water which contains certain salts in
+solution is evaporated in steam boilers, a hard insoluble
+material called <i>scale</i> deposits in the boiler. The formation
+of this scale may be due to several distinct causes.</p>
+
+<p>1. <i>To the deposit of calcium sulphate.</i> This salt, while
+sparingly soluble in cold water, is almost completely insoluble
+in superheated water. Consequently it is precipitated when
+water containing it is heated in a boiler.</p>
+
+<p>2. <i>To decomposition of acid carbonates.</i> As we have seen,
+calcium and magnesium acid carbonates are decomposed on
+heating, forming insoluble normal carbonates:</p>
+
+<p>Ca(HCO<sub>3</sub>)<sub>2</sub> = CaCO<sub>3</sub> + H<sub>2</sub>O + CO<sub>2</sub>.</p>
+
+<p>3. <i>To hydrolysis of magnesium salts.</i> Magnesium chloride, and
+to some extent magnesium sulphate, undergo hydrolysis when
+superheated in solution, and the magnesium hydroxide, being
+sparingly soluble, precipitates:</p>
+
+<p>MgCl<sub>2</sub> + 2H<sub>2</sub>O &lt;--> Mg(OH)<sub>2</sub> + 2HCl.</p>
+
+<p>This scale adheres tightly to the boiler in compact layers and,
+being a non-conductor of heat, causes much waste of fuel. It is
+very difficult to remove, owing to its hardness and resistance
+to reagents. Thick scale sometimes cracks, and the water coming
+in contact with the overheated iron occasions an explosion.
+Moreover, the acids set free in the hydrolysis of the magnesium
+salts attack the iron tubes and rapidly corrode them. These
+causes combine to make the formation of scale a matter which
+occasions much trouble in cases where hard water is used in
+steam boilers. Water containing such salts should be softened,
+therefore, before being used in boilers.</p></div>
+
+<p><b>Magnesium sulphate</b> (<i>Epsom salt</i>) (MgSO<sub>4</sub>&middot;7H<sub>2</sub>O). Like the chloride,
+magnesium sulphate is found rather commonly in springs and in salt
+deposits. A very large deposit of the almost pure salt has been found in
+Wyoming. Its name<span class='pagenum'><a name="Page_321" id="Page_321">[Pg 321]</a></span> was given to it because of its abundant occurrence in
+the waters of the Epsom springs in England.</p>
+
+<p>Magnesium sulphate has many uses in the industries. It is used to a
+small extent in the preparation of sodium and potassium sulphates, as a
+coating for cotton cloth, in the dye industry, in tanning, and in the
+manufacture of paints and laundry soaps. To some extent it is used in
+medicine.</p>
+
+<p><b>Magnesium silicates.</b> Many silicates containing magnesium are known and
+some of them are important substances. Serpentine, asbestos, talc, and
+meerschaum are examples of such substances.</p>
+
+
+<h3>ZINC</h3>
+
+<p><b>Occurrence.</b> Zinc never occurs free in nature. Its compounds have been
+found in many different countries, but it is not a constituent of common
+rocks and minerals, and its occurrence is rather local and confined to
+definite deposits or pockets. It occurs chiefly in the following ores:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Sphalerite (zinc blende)</td><td align='left'>ZnS.</td></tr>
+<tr><td align='left'>Zincite</td><td align='left'>ZnO.</td></tr>
+<tr><td align='left'>Smithsonite</td><td align='left'>ZnCO<sub>3</sub>.</td></tr>
+<tr><td align='left'>Willemite</td><td align='left'>Zn<sub>2</sub>SiO<sub>4</sub>.</td></tr>
+<tr><td align='left'>Franklinite</td><td align='left'>ZnO&middot;Fe<sub>2</sub>O<sub>3</sub>.</td></tr>
+</table></div>
+
+<p>One fourth of the world's output of zinc comes from the United States,
+Missouri being the largest producer.</p>
+
+<p><b>Metallurgy.</b> The ores employed in the preparation of zinc are chiefly the
+sulphide, oxide, and carbonate. They are first roasted in the air, by
+which process they are changed into oxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">ZnCO<sub>3</sub> = ZnO + CO<sub>2</sub>,<br /></span>
+<span class="i0">ZnS + 3O = ZnO + SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_322" id="Page_322">[Pg 322]</a></span></p>
+
+<p>The oxide is then mixed with coal dust, and the mixture is heated in
+earthenware muffles or retorts, natural gas being used as fuel in many
+cases. The oxide is reduced by this means to the metallic state, and the
+zinc, being volatile at the high temperature reached, distills and is
+collected in suitable receivers. At first the zinc collects in the form
+of fine powder, called zinc dust or flowers of zinc, recalling the
+formation under similar conditions of flowers of sulphur. Later, when
+the whole apparatus has become warm, the zinc condenses to a liquid in
+the receiver, from which it is drawn off into molds. Commercial zinc
+often contains a number of impurities, especially carbon, arsenic, and
+iron.</p>
+
+<p><b>Physical properties.</b> Pure zinc is a rather heavy bluish-white metal with
+a high luster. It melts at about 420&deg;, and if heated much above this
+temperature in the air takes fire and burns with a very bright bluish
+flame. It boils at about 950&deg; and can therefore be purified by
+distillation.</p>
+
+<p>Many of the physical properties of zinc are much influenced by the
+temperature and previous treatment of the metal. When cast into ingots
+from the liquid state it becomes at ordinary temperatures quite hard,
+brittle, and highly crystalline. At 150&deg; it is malleable and can be
+rolled into thin sheets; at higher temperatures it again becomes very
+brittle. When once rolled into sheets it retains its softness and
+malleability at ordinary temperatures. When melted and poured into water
+it forms thin brittle flakes, and in this condition is called granulated
+or mossy zinc.</p>
+
+<p><b>Chemical properties.</b> Zinc is tarnished superficially by moist air, but
+beyond this is not affected by it. It does not decompose even boiling
+water. When the metal is quite pure, sulphuric and hydrochloric acids
+have scarcely any action upon it; when, however, it contains small<span class='pagenum'><a name="Page_323" id="Page_323">[Pg 323]</a></span>
+amounts of other metals such as magnesium or arsenic, or when it is
+merely in contact with metallic platinum, brisk action takes place and
+hydrogen is evolved. For this reason, when pure zinc is used in the
+preparation of hydrogen a few drops of platinum chloride are often added
+to the solution to assist the chemical action. Nitric acid dissolves the
+metal readily, with the formation of zinc nitrate and various reduction
+products of nitric acid. The strong alkalis act upon zinc and liberate
+hydrogen:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Zn + 2KOH = Zn(OK)<sub>2</sub> + 2H.<br /></span>
+</div></div>
+
+<p>The product of this reaction, potassium zincate, is a salt of zinc
+hydroxide, which is thus seen to have acid properties, though it usually
+acts as a base.</p>
+
+<p><b>Uses of zinc.</b> The metal has many familiar uses. Rolled into sheets, it
+is used as a lining for vessels which are to contain water. As a thin
+film upon the surface of iron (galvanized iron) it protects the iron
+from rust. Iron is usually galvanized by dipping it into a bath of
+melted zinc, but electrical methods are also employed. Zinc plates are
+used in many forms of electrical batteries. In the laboratory zinc is
+used in the preparation of hydrogen, and in the form of zinc dust as a
+reducing agent.</p>
+
+<p>One of the largest uses of zinc is in the manufacture of alloys. Brass,
+an alloy of zinc and copper, is the most important of these; German
+silver, consisting of copper, zinc, and nickel, has many uses; various
+bronzes, coin metals, and bearing metals also contain zinc. Its ability
+to alloy with silver finds application in the separation of silver from
+lead (see silver).</p>
+
+<p><b>Compounds of zinc.</b> In general, the compounds of zinc are similar in
+formula and appearance to those of magnesium,<span class='pagenum'><a name="Page_324" id="Page_324">[Pg 324]</a></span> but in other properties
+they often differ markedly. A number of them have value in commercial
+ways.</p>
+
+<p><b>Zinc oxide</b> (<i>zinc white</i>) (ZnO). Zinc oxide occurs in impure form in
+nature, being colored red by manganese and iron compounds. It can be
+prepared just like magnesium oxide, but is more often made by burning
+the metal.</p>
+
+<p>Zinc oxide is a pure white powder which becomes yellow on heating and
+regains its white color when cold. It is much used as a white pigment in
+paints, under the name of zinc white, and has the advantage over white
+lead in that it is not changed in color by sulphur compounds, while lead
+turns black. It is also used in the manufacture of rubber goods.</p>
+
+<div class="blockquot"><p><b>Commercial preparation of zinc oxide.</b> Commercially it is often
+made from franklinite in the following way. The franklinite is
+mixed with coal and heated to a high temperature in a furnace,
+by which process the zinc is set free and converted into vapor.
+As the vapor leaves the furnace through a conduit it meets a
+current of air and takes fire in it, forming zinc oxide. The
+oxide passes on and is filtered from the air through canvas
+bags, which allow the air to pass but retain the oxide. It is
+thus made by burning the metal, though the metal is not
+actually isolated in the process.</p></div>
+
+<p><b>Soluble salts.</b> The soluble salts of zinc can be made by dissolving the
+metal or the oxide in the appropriate acid. They are all somewhat
+poisonous. The sulphate and chloride are the most familiar.</p>
+
+<p><b>Zinc sulphate</b> (<i>white vitriol</i>) (ZnSO<sub>4</sub>&middot;7H<sub>2</sub>O). This salt is readily
+crystallized from strong solutions in transparent colorless crystals. It
+is prepared commercially by careful roasting of the sulphide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">ZnS + 4O = ZnSO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_325" id="Page_325">[Pg 325]</a></span></p>
+<p><b>Zinc chloride</b> (ZnCl<sub>2</sub>&middot;H<sub>2</sub>O). When a solution of zinc chloride is
+slowly evaporated a salt of the composition ZnCl<sub>2</sub>&middot;H<sub>2</sub>O crystallizes
+out. If the water is completely expelled by heat and the residue
+distilled, the anhydrous chloride is obtained and may be cast into
+sticks or broken into lumps. In this distillation, just as in heating
+magnesium chloride, some of the chloride is decomposed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">ZnCl<sub>2</sub>&middot;H<sub>2</sub>O = ZnO + 2HCl.<br /></span>
+</div></div>
+
+<p>The anhydrous chloride has a great affinity for water, and is used as a
+dehydrating agent. It is also a germicide, and wood which is to be
+exposed to conditions which favor decay, as, for example, railroad ties,
+is often soaked in solutions of this salt.</p>
+
+<p><b>Insoluble compounds.</b> The insoluble compounds of zinc can be prepared by
+precipitation. The most important are the sulphide, carbonate, and
+hydroxide.</p>
+
+<p><b>Zinc sulphide</b> (ZnS). This substance occurs as the mineral sphalerite,
+and is one of the most valued ores of zinc. Very large deposits occur in
+southwestern Missouri. The natural mineral is found in large crystals or
+masses, resembling resin in color and luster. When prepared by
+precipitation the sulphide is white.</p>
+
+
+<h3>CADMIUM</h3>
+
+<p><b>The element.</b> This element occurs in small quantities in some zinc ores.
+In the course of the metallurgy of zinc the cadmium compounds undergo
+chemical changes quite similar to those of the zinc compounds, and the
+cadmium distills along with the zinc. Being more volatile, it comes over
+with the first of the zinc and is prepared from the first portions of
+the distillate by special methods of purification.<span class='pagenum'><a name="Page_326" id="Page_326">[Pg 326]</a></span> The element very
+closely resembles zinc in most respects. Some of its alloys are
+characterized by having low melting points.</p>
+
+<p><b>Compounds of cadmium.</b> Among the compounds of cadmium may be mentioned
+the chloride (CdCl<sub>2</sub>&middot;2H<sub>2</sub>O), the sulphate (3CdSO<sub>4</sub>&middot;8H<sub>2</sub>O), and
+the nitrate (Cd(NO<sub>3</sub>)<sub>2</sub>&middot;4H<sub>2</sub>O). These are white solids soluble in
+water. The sulphide (CdS) is a bright yellow substance which is
+insoluble in water and in dilute acids. It is valuable as a pigment in
+fine paints.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> What properties have the metals of the magnesium family in common
+with the alkali metals; with the alkaline-earth metals?</p>
+
+<p><b>2.</b> Compare the action of the metals of the magnesium group on water with
+that of the other metals studied.</p>
+
+<p><b>3.</b> What metals already studied are prepared by electrolysis?</p>
+
+<p><b>4.</b> Write the equations representing the reactions between magnesium and
+hydrochloric acid; between magnesium and dilute sulphuric acid.</p>
+
+<p><b>5.</b> What property of magnesium was taken advantage of in the isolation of
+argon?</p>
+
+<p><b>6.</b> With phosphoric acid magnesium forms salts similar to those of
+calcium. Write the names and formulas of the corresponding magnesium
+salts.</p>
+
+<p><b>7.</b> How could you distinguish between magnesium chloride and magnesium
+sulphate? between Glauber's salts and Epsom salts?</p>
+
+<p><b>8.</b> What weight of carnallite is necessary in the preparation of 500 g.
+of magnesium?</p>
+
+<p><b>9.</b> Account for the fact that paints made of zinc oxide are not colored
+by hydrosulphuric acid.</p>
+
+<p><b>10.</b> What hydroxide studied, other than zinc hydroxide, has both acid and
+basic properties?</p>
+
+<p><b>11.</b> Write equations showing how the following compounds of zinc may be
+obtained from metallic zinc: the oxide, chloride, nitrate, carbonate,
+sulphate, sulphide, hydroxide.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_327" id="Page_327">[Pg 327]</a></span></p>
+<h2><a name="CHAPTER_XXVI" id="CHAPTER_XXVI"></a>CHAPTER XXVI</h2>
+
+<h3>THE ALUMINIUM FAMILY</h3>
+
+
+<p><b>The family.</b> The element aluminium is the most abundant member of the
+group of elements known as the aluminium family; indeed, the other
+members of the family&mdash;gallium, indium, and thallium&mdash;are of such rare
+occurrence that they need not be separately described. The elements of
+the family are ordinarily trivalent, so that the formulas for their
+compounds differ from those of the elements so far studied. Their
+hydroxides are practically insoluble in water and are very weak bases;
+indeed, the bases are so weak that their salts are often hydrolyzed into
+free base and free acid in solution. The salts formed from these bases
+usually contain water of crystallization, which cannot be driven off
+without decomposing them more or less.</p>
+
+<p>The trivalent metals, which in addition to aluminium include also iron
+and chromium, are sometimes called the <i>earth metals</i>. The name refers
+to the earthy appearance of the oxides of these metals, and to the fact
+that many earths, soils, and rocks are composed in part of these
+substances.</p>
+
+
+<h3>ALUMINIUM</h3>
+
+<p><b>Occurrence.</b> Aluminium never occurs in the free state in nature, owing to
+its great affinity for oxygen. In combined form, as oxides, silicates,
+and a few other salts, it is both abundant and widely distributed, being
+an essential<span class='pagenum'><a name="Page_328" id="Page_328">[Pg 328]</a></span> constituent of all soils and of most rocks excepting
+limestone and sandstone. Cryolite (Na<sub>3</sub>AlF<sub>6</sub>), found in Greenland,
+and bauxite, which is an aluminium hydroxide usually mixed with some
+iron hydroxide, are important minerals. It is estimated that aluminium
+composes about 8% of the earth's crust. In the industries the metal is
+called aluminum, but its chemical name is aluminium.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig82.jpg" width="400" height="244" alt="Fig. 82" title="" />
+<span class="caption">Fig. 82</span>
+</div>
+
+<p><b>Preparation.</b> Aluminium was first prepared by W&ouml;hler, in 1827, by heating
+anhydrous aluminium chloride with potassium:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">AlCl<sub>3</sub> + 3K = 3KCl + Al.<br /></span>
+</div></div>
+
+<p>This method was tried after it was found impossible to reduce the oxide
+of aluminium with carbon. The metal possessed such interesting
+properties and promised to be so useful that many efforts were made to
+devise a cheap way of preparing it. The method which has proved most
+successful consists in the electrolysis of the oxide dissolved in melted
+cryolite.</p>
+<p><span class='pagenum'><a name="Page_329" id="Page_329">[Pg 329]</a></span></p>
+<div class="blockquot"><p><b>Metallurgy.</b> An iron box <i>A</i> (Fig. 82) about eight feet long and
+six feet wide is connected with a powerful generator in such a
+way as to serve as the cathode upon which the aluminium is
+deposited. Three or four rows of carbon rods <i>B</i> dip into the
+box and serve as the anodes. The box is partially filled with
+cryolite and the current is turned on, generating enough heat
+to melt the cryolite. Aluminium oxide is then added, and under
+the influence of the electric current it decomposes into
+aluminium and oxygen. The temperature is maintained above the
+melting point of aluminium, and the liquid metal, being heavier
+than cryolite, sinks to the bottom of the vessel, from which it
+is tapped off from time to time through the tap hole <i>C</i>. The
+oxygen in part escapes as gas, and in part combines with the
+carbon of the anode, the combustion being very brilliant. The
+process is carried on at Niagara Falls.</p>
+
+<p>The largest expense in the process, apart from the cost of
+electrical energy, is the preparation of aluminium oxide free
+from other oxides, for most of the oxide found in nature is too
+impure to serve without refining. Bauxite is the principal ore
+used as a source of the aluminium because it is converted into
+pure oxide without great difficulty. Since common clay is a
+silicate of aluminium and is everywhere abundant, it might be
+expected that this would be utilized in the preparation of
+aluminium. It is, however, very difficult to extract the
+aluminium from a silicate, and no practical method has been
+found which will accomplish this.</p></div>
+
+<p><b>Physical properties.</b> Aluminium is a tin-white metal which melts at 640&deg;
+and is very light, having a density of 2.68. It is stiff and strong, and
+with frequent annealing can be rolled into thin foil. It is a good
+conductor of heat and electricity, though not so good as copper for a
+given cross section of wire.</p>
+
+<p><b>Chemical properties.</b> Aluminium is not perceptibly acted on by boiling
+water, and moist air merely dims its luster. Further action is prevented
+in each case by the formation of an extremely thin film of oxide upon
+the surface of the metal. It combines directly with chlorine, and when
+heated in oxygen burns with great energy and the liberation of much
+heat. It is therefore a good reducing agent. Hydrochloric acid acts upon
+it, forming aluminium chloride:<span class='pagenum'><a name="Page_330" id="Page_330">[Pg 330]</a></span> nitric acid and dilute sulphuric acid
+have almost no action on it, but hot, concentrated sulphuric acid acts
+upon it in the same way as upon copper:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Al + 6H<sub>2</sub>SO<sub>4</sub> = Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 6H<sub>2</sub>O + 3SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>Alkalis readily attack the metal, liberating hydrogen, as in the case of
+zinc:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Al + 3KOH = Al(OK)<sub>3</sub> + 3H.<br /></span>
+</div></div>
+
+<p>Salt solutions, such as sea water, corrode the metal rapidly. It alloys
+readily with other metals.</p>
+
+<p><b>Uses of aluminium.</b> These properties suggest many uses for the metal. Its
+lightness, strength, and permanence make it well adapted for many
+construction purposes. These same properties have led to its extensive
+use in the manufacture of cooking utensils. The fact that it is easily
+corroded by salt solutions is, however, a disadvantage. Owing to its
+small resistance to electrical currents, it is replacing copper to some
+extent in electrical construction, especially for trolley and power
+wires. Some of its alloys have very valuable properties, and a
+considerable part of the aluminium manufactured is used for this
+purpose. Aluminium bronze, consisting of about 90% copper and 10%
+aluminium, has a pure golden color, is strong and malleable, is easily
+cast, and is permanent in the air. Considerable amounts of aluminium
+steel are also made.</p>
+
+<p><b>Goldschmidt reduction process.</b> Aluminium is frequently employed as a
+powerful reducing agent, many metallic oxides which resist reduction by
+carbon being readily reduced by it. The aluminium in the form of a fine
+powder is mixed with the metallic oxide, together with some substance
+such as fluorspar to act as a flux. The mixture is ignited, and the
+aluminium unites with the<span class='pagenum'><a name="Page_331" id="Page_331">[Pg 331]</a></span> oxygen of the metallic oxide, liberating the
+metal. This collects in a fused condition under the flux.</p>
+
+<p>An enormous quantity of heat is liberated in this reaction, and a
+temperature as high as 3500&deg; can be reached. The heat of the reaction is
+turned to practical account in welding car rails, steel castings, and in
+similar operations where an intense local heat is required. A mixture of
+aluminium with various metallic oxides, ready prepared for such
+purposes, is sold under the name of <i>thermite</i>.</p>
+
+<div class="figcenter" style="width: 598px;">
+<img src="images/imagefig83.jpg" width="598" height="623" alt="Fig. 83" title="" />
+<span class="caption">Fig. 83</span>
+</div>
+
+<div class="blockquot"><p><b>Preparation of chromium by the Goldschmidt method.</b> A mixture of
+chromium oxide and aluminium powder is placed in a Hessian
+crucible (<i>A</i>, Fig. 83), and on top of it is placed a small
+heap <i>B</i> of a mixture of sodium peroxide and aluminium, into
+which is stuck a piece of magnesium ribbon <i>C</i>. Powdered
+fluorspar <i>D</i> is placed around the sodium peroxide, after which
+the crucible is set on a pan of sand and the magnesium ribbon
+ignited. When the flame reaches the sodium peroxide mixture
+combustion of the aluminium begins with almost explosive
+violence, so that great care must be taken in the experiment.
+The heat of this combustion starts the reaction in the chromium
+oxide mixture, and the oxide is reduced to metallic chromium.
+When the crucible has cooled a button of chromium will be found
+in the bottom.</p></div>
+
+<p><b>Aluminium oxide</b> (Al<sub>2</sub>O<sub>3</sub>). This substance occurs in several forms in
+nature. The relatively pure crystals are called corundum, while emery is
+a variety colored dark gray or black, usually with iron compounds. In
+transparent crystals, tinted different colors by traces of impurities,
+it forms such precious stones as the sapphire, oriental ruby, topaz, and
+amethyst. All these varieties are very<span class='pagenum'><a name="Page_332" id="Page_332">[Pg 332]</a></span> hard, falling little short of
+the diamond in this respect. Chemically pure aluminium oxide can be made
+by igniting the hydroxide, when it forms an amorphous white powder:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Al(OH)<sub>3</sub> = Al<sub>2</sub>O<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The natural varieties, corundum and emery, are used for cutting and
+grinding purposes; the purest forms, together with the artificially
+prepared oxide, are largely used in the preparation of aluminium.</p>
+
+<p><b>Aluminium hydroxide</b> (Al(OH)<sub>3</sub>). The hydroxide occurs in nature as the
+mineral hydrargyllite, and in a partially dehydrated form called
+bauxite. It can be prepared by adding ammonium hydroxide to any soluble
+aluminium salt, forming a semi-transparent precipitate which is
+insoluble in water but very hard to filter. It dissolves in most acids
+to form soluble salts, and in the strong bases to form aluminates, as
+indicated in the equations</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Al(OH)<sub>3</sub> + 3HCl = AlCl<sub>3</sub> + 3H<sub>2</sub>O,<br /></span>
+<span class="i0">Al(OH)<sub>3</sub> + 3NaOH = Al(ONa)<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>It may act, therefore, either as a weak base or as a weak acid, its
+action depending upon the character of the substances with which it is
+in contact. When heated gently the hydroxide loses part of its hydrogen
+and oxygen according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Al(OH)<sub>3</sub> = AlO&middot;OH + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>This substance, the formula of which is frequently written HAlO<sub>2</sub>, is
+a more pronounced acid than is the hydroxide, and its salts are
+frequently formed when aluminium compounds are fused with alkalis. The
+magnesium salt Mg(AlO<sub>2</sub>)<sub>2</sub> is called spinel, and many other of its
+salts, called aluminates, are found in nature.<span class='pagenum'><a name="Page_333" id="Page_333">[Pg 333]</a></span></p>
+
+<p>When heated strongly the hydroxide is changed into oxide, which will not
+again take up water on being moistened.</p>
+
+<div class="blockquot"><p><b>Mordants and dyeing.</b> Aluminium hydroxide has the peculiar
+property of combining with many soluble coloring materials and
+forming insoluble products with them. On this account it is
+often used as a filter to remove objectionable colors from
+water. This property also leads to its wide use in the dye
+industry. Many dyes will not adhere to natural fibers such as
+cotton and wool, that is, will not "dye fast." If, however, the
+cloth to be dyed is soaked in a solution of aluminium compounds
+and then treated with ammonia, the aluminium salts which have
+soaked into the fiber will be converted into the hydroxide,
+which, being insoluble, remains in the body of it. If the fiber
+is now dipped into a solution of the dye, the aluminium
+hydroxide combines with the color material and fastens, or
+"fixes," it upon the fiber. A substance which serves this
+purpose is called a <i>mordant</i>, and aluminium salts,
+particularly the acetate, are used in this way.</p></div>
+
+<p><b>Aluminium chloride</b> (AlCl<sub>3</sub>&middot;6 H<sub>2</sub>O). This substance is prepared by
+dissolving the hydroxide in hydrochloric acid and evaporating to
+crystallization. When heated it is converted into the oxide, resembling
+magnesium in this respect:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2(AlCl<sub>3</sub>&middot;6 H<sub>2</sub>O) = Al<sub>2</sub>O<sub>3</sub> + 6HCl + 9H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The anhydrous chloride, which has some important uses, is made by
+heating aluminium turnings in a current of chlorine.</p>
+
+<p><b>Alums.</b> Aluminium sulphate can be prepared by the action of sulphuric
+acid upon aluminium hydroxide. It has the property of combining with the
+sulphates of the alkali metals to form compounds called <i>alums</i>. Thus,
+with potassium sulphate the reaction is expressed by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">K<sub>2</sub>SO<sub>4</sub> + Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 24H<sub>2</sub>O = 2(KAl(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O).<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_334" id="Page_334">[Pg 334]</a></span></p>
+<p>Under similar conditions ammonium sulphate yields ammonium alum:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> + Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 24H<sub>2</sub>O = 2(NH<sub>4</sub>Al(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O).<br /></span>
+</div></div>
+
+<p>Other trivalent sulphates besides aluminium sulphate can form similar
+compounds with the alkali sulphates, and these compounds are also called
+alums, though they contain no aluminium. They all crystallize in
+octahedra and contain twelve molecules of water of crystallization. The
+alums most frequently prepared are the following:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Potassium alum</td><td align='left'>KAl(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Ammonium alum</td><td align='left'>NH<sub>4</sub>Al(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Ammonium iron alum</td><td align='left'>NH<sub>4</sub>Fe(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Potassium chrome alum</td><td align='left'>KCr(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O.</td></tr>
+</table></div>
+
+<p>An alum may therefore be regarded as a compound derived from two
+molecules of sulphuric acid, in which one hydrogen atom has been
+displaced by the univalent alkali atom, and the other three hydrogen
+atoms by an atom of one of the trivalent metals, such as aluminium,
+iron, or chromium.</p>
+
+<div class="blockquot"><p>Very large, well-formed crystals of an alum can be prepared by
+suspending a small crystal by a thread in a saturated solution
+of the alum, as shown in Fig. 84. The small crystal slowly
+grows and assumes a very perfect form.</p></div>
+
+<div class="figcenter" style="width: 272px;">
+<img src="images/imagefig84.jpg" width="272" height="400" alt="Fig. 84" title="" />
+<span class="caption">Fig. 84</span>
+</div>
+
+<p><b>Other salts of aluminium.</b> While aluminium hydroxide forms fairly stable
+salts with strong acids, it is such a weak base that its salts with weak
+acids are readily hydrolyzed. Thus, when an aluminium salt and a soluble
+carbonate are<span class='pagenum'><a name="Page_335" id="Page_335">[Pg 335]</a></span> brought together in solution we should expect to have
+aluminium carbonate precipitated according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Na<sub>2</sub>CO<sub>3</sub> + 2AlCl<sub>3</sub> = Al<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub> + 6NaCl.<br /></span>
+</div></div>
+
+<p>But if it is formed at all, it instantly begins to hydrolyze, the
+products of the hydrolysis being aluminium hydroxide and carbonic acid,</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Al<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub> + 6H<sub>2</sub>O = 2Al(OH)<sub>3</sub> + 3H<sub>2</sub>CO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Similarly a soluble sulphide, instead of precipitating aluminium
+sulphide (Al<sub>2</sub>S<sub>3</sub>), precipitates aluminium hydroxide; for hydrogen
+sulphide is such a weak acid that the aluminium sulphide at first formed
+hydrolyzes at once, forming aluminium hydroxide and hydrogen sulphide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Na<sub>2</sub>S + 2AlCl<sub>3</sub> + 6H<sub>2</sub>O = 2Al(OH)<sub>3</sub> + 6NaCl + 3H<sub>2</sub>S.<br /></span>
+</div></div>
+
+<p><b>Alum baking powders.</b> It is because of the hydrolysis of aluminium
+carbonate that alum is used as a constituent of some baking powders. The
+alum baking powders consist of a mixture of alum and sodium hydrogen
+carbonate. When water is added the two compounds react together, forming
+aluminium carbonate, which hydrolyzes into aluminium hydroxide and
+carbonic acid. The carbon dioxide from the latter escapes through the
+dough and in so doing raises it into a porous condition, which is the
+end sought in the use of a baking powder.</p>
+
+<p><b>Aluminium silicates.</b> One of the most common constituents of rocks is
+feldspar (KAlSi<sub>3</sub>O<sub>8</sub>), a mixed salt of potassium and aluminium with
+the polysilicic acid (H<sub>4</sub>Si<sub>3</sub>O<sub>8</sub>). Under the influence of
+moisture, carbon dioxide, and changes of temperature this substance is
+constantly being broken down into soluble potassium compounds and
+hydrated aluminium silicate. This compound has the formula
+Al<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>&middot;2H<sub>2</sub>O. In relatively pure condition it is called
+kaolin; in the impure state, mixed with sand and other<span class='pagenum'><a name="Page_336" id="Page_336">[Pg 336]</a></span> substances, it
+forms common clay. Mica is another very abundant mineral, having varying
+composition, but being essentially of the formula KAlSiO<sub>4</sub>.
+Serpentine, talc, asbestos, and meerschaum are important complex
+silicates of aluminium and magnesium, and granite is a mechanical
+mixture of quartz, feldspar, and mica.</p>
+
+<div class="blockquot"><p><b>Ceramic industries.</b> Many articles of greatest practical
+importance, ranging from the roughest brick and tile to the
+finest porcelain and chinaware, are made from some form of
+kaolin, or clay. No very precise classification of such ware
+can be made, as the products vary greatly in properties,
+depending upon the materials used and the treatment during
+manufacture.</p>
+
+<p>Porcelain is made from the purest kaolin, to which must be
+added some less pure, plastic kaolin, since the pure substance
+is not sufficiently plastic. There is also added some more
+fusible substance, such as feldspar, gypsum, or lime, together
+with some pure quartz. The constituents must be ground very
+fine, and when thoroughly mixed and moistened must make a
+plastic mass which can be molded into any desired form. The
+article molded from such materials is then burned. In this
+process the article is slowly heated to a point at which it
+begins to soften and almost fuse, and then it is allowed to
+cool slowly. At this stage, a very thin vessel will be
+translucent and have an almost glassy fracture; if, however, it
+is somewhat thicker, or has not been heated quite so high, it
+will still be porous, and partly on this account and partly to
+improve its appearance it is usually glazed.</p>
+
+<p>Glazing is accomplished by spreading upon the object a thin
+layer of a more fusible mixture of the same materials as
+compose the body of the object itself, and again heating until
+the glaze melts to a transparent glassy coating upon the
+surface of the vessel. In some cases fusible mixtures of quite
+different composition from that used in fashioning the vessel
+may be used as a glaze. Oxides of lead, zinc, and barium are
+often used in this way.</p>
+
+<p>When less carefully selected materials are used, or quite thick
+vessels are made, various grades of stoneware are produced. The
+inferior grades are glazed by throwing a quantity of common
+salt into the kiln towards the end of the first firing. In the
+form of vapor<span class='pagenum'><a name="Page_337" id="Page_337">[Pg 337]</a></span> the salt attacks the surface of the baked ware
+and forms an easily fusible sodium silicate upon it, which
+constitutes a glaze.</p>
+
+<p>Vitrified bricks, made from clay or ground shale, are burned
+until the materials begin to fuse superficially, forming their
+own glaze. Other forms of brick and tile are not glazed at all,
+but are left porous. The red color of ordinary brick and
+earthenware is due to an oxide of iron formed in the burning
+process.</p>
+
+<p>The decorations upon china are sometimes painted upon the baked
+ware and then glazed over, and sometimes painted upon the glaze
+and burned in by a third firing. Care must be taken to use such
+pigments as are not affected by a high heat and do not react
+chemically with the constituents of the baked ware or the
+glaze.</p></div>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> What metals and compounds studied are prepared by electrolysis?</p>
+
+<p><b>2.</b> Write the equation for the reaction between aluminium and
+hydrochloric acid; between aluminium and sulphuric acid (in two steps).</p>
+
+<p><b>3.</b> What hydroxides other than aluminium hydroxide have both acid and
+basic properties?</p>
+
+<p><b>4.</b> Write equations showing the methods used for preparing aluminium
+hydroxide and sulphate.</p>
+
+<p><b>5.</b> Write the general formula of an alum, representing an atom of an
+alkali metal by X and an atom of a trivalent metal by Y.</p>
+
+<p><b>6.</b> What is meant by the term polysilicic acid, as used in the discussion
+of aluminium silicates?</p>
+
+<p><b>7.</b> Compare the properties of the hydroxides of the different groups of
+metals so far studied.</p>
+
+<p><b>8.</b> In what respects does aluminium oxide differ from calcium oxide in
+properties?</p>
+
+<p><b>9.</b> Supposing bauxite to be 90% aluminium hydroxide, what weight of it is
+necessary for the preparation of 100 kg. of aluminium?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_338" id="Page_338">[Pg 338]</a></span></p>
+<h2><a name="CHAPTER_XXVII" id="CHAPTER_XXVII"></a>CHAPTER XXVII</h2>
+
+<h3>THE IRON FAMILY</h3>
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>SYMBOL</td><td align='left'>ATOMIC WEIGHT</td><td align='left'>DENSITY</td><td align='left'>APPROXIMATE MELTING POINT</td><td align='left'>OXIDES</td></tr>
+<tr><td align='left'>Iron</td><td align='center'>Fe</td><td align='center'>55.9</td><td align='center'>7.93</td><td align='center'>1800&deg;</td><td align='center'>FeO, Fe<sub>2</sub>O<sub>3</sub></td></tr>
+<tr><td align='left'>Cobalt</td><td align='center'>Co</td><td align='center'>59.0</td><td align='center'>8.55</td><td align='center'>1800&deg;</td><td align='center'>CoO, Co<sub>2</sub>O<sub>3</sub></td></tr>
+<tr><td align='left'>Nickel</td><td align='center'>Ni</td><td align='center'>58.7</td><td align='center'>8.9</td><td align='center'>1600&deg;</td><td align='center'>NiO, Ni<sub>2</sub>O<sub>3</sub></td></tr>
+</table></div>
+
+
+<p><b>The family.</b> The elements iron, cobalt, and nickel form a group in the
+eighth column of the periodic table. The atomic weights of the three are
+very close together, and there is not the same gradual gradation in the
+properties of the three elements that is noticed in the families in
+which the atomic weights differ considerably in magnitude. The elements
+are very similar in properties, the similarity being so great in the
+case of nickel and cobalt that it is difficult to separate them by
+chemical analysis.</p>
+
+<p>The elements occur in nature chiefly as oxides and sulphides, though
+they have been found in very small quantities in the native state,
+usually in meteorites. Their sulphides, carbonates, and phosphates are
+insoluble in water, the other common salts being soluble. Their salts
+are usually highly colored, those of iron being yellow or light green as
+a rule, those of nickel darker green, while cobalt salts are usually
+rose colored. The metals are obtained by reducing the oxides with
+carbon.<span class='pagenum'><a name="Page_339" id="Page_339">[Pg 339]</a></span></p>
+
+
+<h3>IRON</h3>
+
+<p><b>Occurrence.</b> The element iron has long been known, since its ores are
+very abundant and it is not difficult to prepare the metal from them in
+fairly pure condition. It occurs in nature in many forms of
+combination,&mdash;in large deposits as oxides, sulphides, and carbonates,
+and in smaller quantities in a great variety of minerals. Indeed, very
+few rocks or soils are free from small amounts of iron, and it is
+assimilated by plants and animals playing an important part in life
+processes.</p>
+
+<p><b>Metallurgy.</b> It will be convenient to treat of the metallurgy of iron
+under two heads,&mdash;Materials Used and Process.</p>
+
+<p><b>Materials used.</b> Four distinct materials are used in the metallurgy of
+iron:</p>
+
+<div class="blockquot"><p>1. <i>Iron ore.</i> The ores most frequently used in the metallurgy
+of iron are the following:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Hematite</td><td align='left'>Fe<sub>2</sub>O<sub>3</sub>.</td></tr>
+<tr><td align='left'>Magnetite</td><td align='left'>Fe<sub>3</sub>O<sub>4</sub>.</td></tr>
+<tr><td align='left'>Siderite</td><td align='left'>FeCO<sub>3</sub>.</td></tr>
+<tr><td align='left'>Limonite</td><td align='left'>2Fe<sub>2</sub>O<sub>2</sub>&middot;3H<sub>2</sub>O.</td></tr>
+</table></div>
+<p>These ores always contain impurities, such as silica,
+sulphides, and earthy materials. All ores, with the exception
+of the oxides, are first roasted to expel any water and carbon
+dioxide present and to convert any sulphide into oxide.</p>
+
+<p>2. <i>Carbon.</i> Carbon in some form is necessary both as a fuel
+and as a reducing agent. In former times wood charcoal was used
+to supply the carbon, but now anthracite coal or coke is almost
+universally used.</p>
+
+<p>3. <i>Hot air.</i> To maintain the high temperature required for the
+reduction of iron a very active combustion of fuel<span class='pagenum'><a name="Page_340" id="Page_340">[Pg 340]</a></span> is
+necessary. This is secured by forcing a strong blast of hot air
+into the lower part of the furnace during the reduction
+process.</p>
+
+<p>4. <i>Flux.</i> (<i>a</i>) <i>Purpose of the flux.</i> All the materials which
+enter the furnace must leave it again either in the form of
+gases or as liquids. The iron is drawn off as the liquid metal
+after its reduction. To secure the removal of the earthy matter
+charged into the furnace along with the ore, materials are
+added to the charge which will, at the high temperature of the
+furnace, combine with the impurities in the ore, forming a
+liquid. The material added for this purpose is called the
+<i>flux</i>; the liquid produced from the flux and the ore is called
+<i>slag</i>.</p>
+
+<p>(<i>b</i>) <i>Function of the slag.</i> While the main purpose of adding
+flux to the charge is to remove from the furnace in the form of
+liquid slag the impurities originally present in the ore, the
+slag thus produced serves several other functions. It keeps the
+contents of the furnace in a state of fusion, thus preventing
+clogging, and makes it possible for the small globules of iron
+to run together with greater ease into one large liquid mass.</p>
+
+<p>(<i>c</i>) <i>Character of the slag.</i> The slag is really a kind of
+readily fusible glass, being essentially a calcium-aluminium
+silicate. The ore usually contains silica and some aluminium
+compounds, so that limestone (which also contains some silica
+and aluminium) is added to furnish the calcium required for the
+slag. If the ore and the limestone do not contain a sufficient
+amount of silica and aluminium for the formation of the slag,
+these ingredients are added in the form of sand and feldspar.
+In the formation of slag from these materials the ore is freed
+from the silica and aluminium which it contained.</p></div><p><span class='pagenum'><a name="Page_341" id="Page_341">[Pg 341]</a></span></p>
+
+<div class="figleft" style="width: 183px;">
+<img src="images/imagefig85.jpg" width="183" height="400" alt="Fig. 85" title="" />
+<span class="caption">Fig. 85</span>
+</div>
+
+<p><b>Process.</b> The reduction of iron is carried out in large towers called
+blast furnaces. The blast furnace (Fig. 85) is usually about 80 ft. high
+and 20 ft. in internal diameter at its widest part, narrowing somewhat
+both toward the top and toward the bottom. The walls are built of steel
+and lined with fire-brick. The base is provided with a number of pipes
+<i>T</i>, called tuyers, through which hot air can be forced into the
+furnace. The tuyers are supplied from a large pipe <i>S</i>, which circles
+the furnace as a girdle. The base has also an opening <i>M</i>, through which
+the liquid metal can be drawn off from time to time, and a second
+opening <i>P</i>, somewhat above the first, through which the excess of slag
+overflows. The top is closed by a movable trap <i>C</i> and <i>C'</i>, called the
+cone, and through this the materials to be used are introduced. The
+gases produced by the combustion of the fuel and the reduction of the
+ore, together with the nitrogen of the air forced in through the tuyers,
+escape through pipes <i>D</i>, called downcomer pipes, which leave the
+furnace near the top. These gases are very hot and contain combustible
+substances, principally carbon monoxide; they are therefore utilized as
+fuel for the engines and also to heat the blast admitted through the
+tuyers. The lower part of the furnace is often furnished with a water
+jacket. This consists of a series of pipes <i>W</i> built into the walls,
+through which water can be circulated to reduce their temperature.<span class='pagenum'><a name="Page_342" id="Page_342">[Pg 342]</a></span></p>
+
+<p>Charges consisting of coke (or anthracite coal), ore, and flux in proper
+proportions are introduced into the furnace at intervals through the
+trap top. The coke burns fiercely in the hot-air blast, giving an
+intense heat and forming carbon monoxide. The ore, working down in the
+furnace as the coke burns, becomes very hot, and by the combined
+reducing action of the carbon and carbon monoxide is finally reduced to
+metal and collects as a liquid in the bottom of the furnace, the slag
+floating on the molten iron. After a considerable amount of the iron has
+collected the slag is drawn off through the opening <i>P</i>. The molten iron
+is then drawn off into large ladles and taken to the converters for the
+manufacture of steel, or it is run out into sand molds, forming the bars
+or ingots called "pigs." The process is a continuous one, and when once
+started it is kept in operation for months or even years without
+interruption.</p>
+
+<div class="blockquot"><p>It seems probable that the first product of combustion of the
+carbon, at the point where the tuyers enter the furnace, is
+carbon dioxide. This is at once reduced to carbon monoxide by
+the intensely heated carbon present, so that no carbon dioxide
+can be found at that point. For practical purposes, therefore,
+we may consider that carbon monoxide is the first product of
+combustion.</p></div>
+
+<p><b>Varieties of iron.</b> The iron of commerce is never pure, but contains
+varying amounts of other elements, such as carbon, silicon, phosphorus,
+sulphur, and manganese. These elements may either be alloyed with the
+iron or may be combined with it in the form of definite chemical
+compounds. In some instances, as in the case of graphite, the mixture
+may be merely mechanical.</p>
+
+<p>The properties of iron are very much modified by the presence of these
+elements and by the form of the combination between them and the iron;
+the way in which the<span class='pagenum'><a name="Page_343" id="Page_343">[Pg 343]</a></span> metal is treated during its preparation has also a
+marked influence on its properties. Owing to these facts many kinds of
+iron are recognized in commerce, the chief varieties being cast iron,
+wrought iron, and steel.</p>
+
+<p><b>Cast iron.</b> The product of the blast furnace, prepared as just described,
+is called cast iron. It varies considerably in composition, usually
+containing from 90 to 95% iron, the remainder being largely carbon and
+silicon with smaller amounts of phosphorus and sulphur. When the melted
+metal from the blast furnace is allowed to cool rapidly most of the
+carbon remains in chemical combination with the iron, and the product is
+called white cast iron. If the cooling goes on slowly, the carbon
+partially separates as flakes of graphite which remain scattered through
+the metal. This product is softer and darker in color and is called gray
+cast iron.</p>
+
+<p><b>Properties of cast iron.</b> Cast iron is hard, brittle, and rather easily
+melted (melting point about 1100&deg;). It cannot be welded or forged into
+shape, but is easily cast in sand molds. It is strong and rigid but not
+elastic. It is used for making castings and in the manufacture of other
+kinds of iron. Cast iron, which contains the metal manganese up to the
+extent of 20%, together with about 3% carbon, is called spiegel iron;
+when more than this amount of manganese is present the product is called
+ferromanganese. The ferromanganese may contain as much as 80% manganese.
+These varieties of cast iron are much used in the manufacture of steel.</p>
+
+<p><b>Wrought iron.</b> Wrought iron is made by burning out from cast iron most of
+the carbon, silicon, phosphorus, and sulphur which it contains. The
+process is called <i>puddling</i>, and is carried out in a furnace
+constructed as represented<span class='pagenum'><a name="Page_344" id="Page_344">[Pg 344]</a></span> in Fig. 86. The floor of the furnace <i>F</i> is
+somewhat concave and is made of iron covered with a layer of iron oxide.
+A long flame produced by burning fuel upon the grate <i>G</i> is directed
+downward upon the materials placed upon the floor, and the draught is
+maintained by the stack <i>S</i>. <i>A</i> is the ash box and <i>T</i> a trap to catch
+the solid particles carried into the stack by the draught. Upon the
+floor of the furnace is placed the charge of cast iron, together with a
+small amount of material to make a slag. The iron is soon melted by the
+flame directed upon it, and the sulphur, phosphorus, and silicon are
+oxidized by the iron oxide, forming oxides which are anhydrides of
+acids. These combine with the flux, which is basic in character, or with
+the iron oxide, to form a slag. The carbon is also oxidized and escapes
+as carbon dioxide. As the iron is freed from other elements it becomes
+pasty, owing to the higher melting point of the purer iron, and in this
+condition forms small lumps which are raked together into a larger one.
+The large lump is then removed from the furnace and rolled or hammered
+into bars, the slag; being squeezed out in this process. The product has
+a stranded or fibrous structure. <i>The product of a puddling furnace is
+called wrought iron.</i></p>
+
+<div class="figcenter" style="width: 364px;">
+<img src="images/imagefig86.jpg" width="364" height="333" alt="Fig. 86" title="" />
+<span class="caption">Fig. 86</span>
+</div>
+
+<p><b>Properties of wrought iron.</b> Wrought iron is nearly pure iron, usually
+containing about 0.3% of other substances, chiefly carbon. It is tough,
+malleable, and fibrous<span class='pagenum'><a name="Page_345" id="Page_345">[Pg 345]</a></span> in structure. It is easily bent and is not
+elastic, so it will not sustain pressure as well as cast iron. It can be
+drawn out into wire of great tensile strength, and can also be rolled
+into thin sheets (sheet iron). It melts at a high temperature (about
+1600&deg;) and is therefore forged into shape rather than cast. If melted,
+it would lose its fibrous structure and be changed into a low carbon
+steel.</p>
+
+<p><b>Steel.</b> Steel, like wrought iron, is made by burning out from cast iron a
+part of the carbon, silicon, phosphorus, and sulphur which it contains;
+but the process is carried out in a very different way, and usually,
+though not always, more carbon is found in steel than in wrought iron. A
+number of processes are in use, but nearly all the steel of commerce is
+made by one of the two following methods.</p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig87.jpg" width="400" height="338" alt="Fig. 87" title="" />
+<span class="caption">Fig. 87</span>
+</div>
+
+<p>1. <i>Bessemer process.</i> This process, invented about 1860, is by far the
+most important. It is carried out in great egg-shaped crucibles called
+converters (Fig. 87), each one of which will hold as much as 15 tons of
+steel. The converter is built of steel and lined with silica. It is
+mounted on trunnions <i>T</i>, so that it can be tipped over on its side for
+filling and emptying. One of the trunnions is hollow and a pipe <i>P</i>
+connects it with an air chamber <i>A</i>, which forms a false bottom to the
+converter. The true bottom is perforated, so that air<span class='pagenum'><a name="Page_346" id="Page_346">[Pg 346]</a></span> can be forced in
+by an air blast admitted through the trunnion and the air chamber.</p>
+
+<p>White-hot, liquid cast iron from a blast furnace is run into the
+converter through its open necklike top <i>O</i>, the converter being tipped
+over to receive it; the air blast is then turned on and the converter
+rotated to a nearly vertical position. The elements in the iron are
+rapidly oxidized, the silicon first and then the carbon. The heat
+liberated in the oxidation, largely due to the combustion of silicon,
+keeps the iron in a molten condition. When the carbon is practically all
+burned out cast iron or spiegel iron, containing a known percentage of
+carbon, is added and allowed to mix thoroughly with the fluid. The steel
+is then run into molds, and the ingots so formed are hammered or rolled
+into rails or other forms. By this process any desired percentage of
+carbon can be added to the steel. Low carbon steel, which does not
+differ much from wrought iron in composition, is now made in this way
+and is replacing the more expensive wrought iron for many purposes.</p>
+
+<div class="blockquot"><p><b>The basic lining process.</b> When the cast iron contains
+phosphorus and sulphur in appreciable quantities, the lining of
+the converter is made of dolomite. The silicon and carbon burn,
+followed by the phosphorus and sulphur, and the anhydrides of
+acids so formed combine with the basic oxides of the lining,
+forming a slag. This is known as the basic lining process.</p></div>
+
+<p>2. <i>Open-hearth process.</i> In this process a furnace very similar to a
+puddling furnace is used, but it is lined with silica or dolomite
+instead of iron oxide. A charge consisting in part of old scrap iron of
+any kind and in part of cast iron is melted in the furnace by a gas
+flame. The silicon and carbon are slowly burned away, and when a test
+shows that the desired percentage of carbon is present the steel<span class='pagenum'><a name="Page_347" id="Page_347">[Pg 347]</a></span> is run
+out of the furnace. <i>Steel may therefore be defined as the product of
+the Bessemer or open-hearth processes.</i></p>
+
+<p><b>Properties of steel.</b> Bessemer and open-hearth steel usually contain only
+a few tenths of a per cent of carbon, less than 0.1% silicon, and a very
+much smaller quantity of phosphorus and sulphur. Any considerable amount
+of the latter elements makes the steel brittle, the sulphur affecting it
+when hot, and the phosphorus when cold. This kind of steel is used for
+structural purposes, for rails, and for nearly all large steel articles.
+It is hard, malleable, ductile, and melts at a lower temperature than
+wrought iron. It can be forged into shape, rolled into sheets, or cast
+in molds.</p>
+
+<p><b>Relation of the three varieties of iron.</b> It will be seen that wrought
+iron is usually very nearly pure iron, while steel contains an
+appreciable amount of alloy material, chiefly carbon, and cast iron
+still more of the same substances. It is impossible, however, to assign
+a given sample of iron to one of these three classes on the basis of its
+chemical composition alone. A low carbon steel, for example, may contain
+less carbon than a given sample of wrought iron. The real distinction
+between the three is the process by which they are made. The product of
+the blast furnace is cast iron; that of the puddling furnace is wrought
+iron; that of the Bessemer and open-hearth methods is steel.</p>
+
+<p><b>Tool steel.</b> Steel designed for use in the manufacture of edged tools and
+similar articles should be relatively free from silicon and phosphorus,
+but should contain from 0.5 to 1.5% carbon. The percentage of carbon
+should be regulated by the exact use to which the steel is to be put.
+Steel of this character is usually made in small lots from either
+Bessemer or open-hearth steel in the following way.<span class='pagenum'><a name="Page_348" id="Page_348">[Pg 348]</a></span></p>
+
+<p>A charge of melted steel is placed in a large crucible and the
+calculated quantity of pure carbon is added. The carbon dissolves in the
+steel, and when the solution is complete the metal is poured out of the
+crucible. This is sometimes called crucible steel.</p>
+
+<p><b>Tempering of steel.</b> Steel containing from 0.5 to 1.5% carbon is
+characterized by the property of "taking temper." When the hot steel is
+suddenly cooled by plunging it into water or oil it becomes very hard
+and brittle. On carefully reheating this hard form it gradually becomes
+less brittle and softer, so that by regulating the temperature to which
+steel is reheated in tempering almost any condition of temper demanded
+for a given purpose, such as for making springs or cutting tools, can be
+obtained.</p>
+
+<p><b>Steel alloys.</b> It has been found that small quantities of a number of
+different elements when alloyed with steel very much improve its quality
+for certain purposes, each element having a somewhat different effect.
+Among the elements most used in this connection are manganese, silicon,
+chromium, nickel, tungsten, and molybdenum.</p>
+
+<p>The usual method for adding these elements to the steel is to first
+prepare a very rich alloy of iron with the element to be added, and then
+add enough of this alloy to a large quantity of the steel to bring it to
+the desired composition. A rich alloy of iron with manganese or silicon
+can be prepared directly in a blast furnace, and is called
+ferromanganese or ferrosilicon. Similar alloys of iron with the other
+elements mentioned are made in an electric furnace by reducing the mixed
+oxides with carbon.</p>
+
+<p><b>Pure iron.</b> Perfectly pure iron is rarely prepared and is not adapted to
+commercial uses. It can be made by reducing pure oxide of iron in a
+current of hydrogen at a<span class='pagenum'><a name="Page_349" id="Page_349">[Pg 349]</a></span> high temperature. Prepared in this way it
+forms a black powder; when melted it forms a tin-white metal which is
+less fusible and more malleable than wrought iron. It is easily acted
+upon by moist air.</p>
+
+<p><b>Compounds of iron.</b> Iron differs from the metals so far studied in that
+it is able to form two series of compounds in which the iron has two
+different valences. In the one series the iron is divalent and forms
+compounds which in formulas and many chemical properties are similar to
+the corresponding zinc compounds. It can also act as a trivalent metal,
+and in this condition forms salts similar to those of aluminium. Those
+compounds in which the iron is divalent are known as <i>ferrous</i>
+compounds, while those in which it is trivalent are known as <i>ferric</i>.</p>
+
+<p><b>Oxides of iron.</b> Iron forms several oxides. Ferrous oxide (FeO) is not
+found in nature, but can be prepared artificially in the form of a black
+powder which easily takes up oxygen, forming ferric oxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2FeO + O = Fe<sub>2</sub>O<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Ferric oxide is the most abundant ore of iron and occurs in great
+deposits, especially in the Lake Superior region. It is found in many
+mineral varieties which vary in density and color, the most abundant
+being hematite, which ranges in color from red to nearly black. When
+prepared by chemical processes it forms a red powder which is used as a
+paint pigment (Venetian red) and as a polishing powder (rouge).</p>
+
+<p>Magnetite has the formula Fe<sub>3</sub>O<sub>4</sub> and is a combination of FeO and
+Fe<sub>2</sub>O<sub>3</sub>. It is a very valuable ore, but is less abundant than
+hematite. It is sometimes called magnetic oxide of iron, or lodestone,
+since it is a natural magnet.<span class='pagenum'><a name="Page_350" id="Page_350">[Pg 350]</a></span></p>
+
+<p><b>Ferrous salts.</b> These salts are obtained by dissolving iron in the
+appropriate acid, or, when insoluble, by precipitation. They are usually
+light green in color and crystallize well. In chemical reactions they
+are quite similar to the salts of magnesium and zinc, but differ from
+them in one important respect, namely, that they are easily changed into
+compounds in which the metal is trivalent. Thus ferrous chloride treated
+with chlorine or aqua regia is changed into ferric chloride:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">FeCl<sub>2</sub> + Cl = FeCl<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Ferrous hydroxide exposed to moist air is rapidly changed into ferric
+hydroxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Fe(OH)<sub>2</sub> + H<sub>2</sub>O + O = 2Fe(OH)<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><b>Ferrous sulphate</b> <i>(copperas, green vitriol)</i> (FeSO<sub>4</sub>&middot;7H<sub>2</sub>O). Ferrous
+sulphate is the most familiar ferrous compound. It is prepared
+commercially as a by-product in the steel-plate mills. Steel plates are
+cleaned by the action of dilute sulphuric acid upon them, and in the
+process some of the iron dissolves. The liquors are concentrated and the
+green vitriol separates from them.</p>
+
+<p><b>Ferrous sulphide</b> (FeS). Ferrous sulphide is sometimes found in nature as
+a golden-yellow crystalline mineral. It is formed as a black precipitate
+when a soluble sulphide and an iron salt are brought together in
+solution:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">FeSO<sub>4</sub> + Na<sub>2</sub>S = FeS + Na<sub>2</sub>SO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>It can also be made as a heavy dark-brown solid by fusing together the
+requisite quantities of sulphur and iron. It is obtained as a by-product
+in the metallurgy of lead:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">PbS + Fe = FeS + Pb.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_351" id="Page_351">[Pg 351]</a></span></p>
+<p>It is used in the laboratory in the preparation of hydrosulphuric acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">FeS + 2HCl = FeCl<sub>2</sub> + H<sub>2</sub>S.<br /></span>
+</div></div>
+
+<p><b>Iron disulphide</b> <i>(pyrites)</i> (FeS<sub>2</sub>). This substance bears the same
+relation to ferrous sulphide that hydrogen dioxide does to water. It
+occurs abundantly in nature in the form of brass-yellow cubical crystals
+and in compact masses. Sometimes the name "fool's gold" is applied to it
+from its superficial resemblance to the precious metal. It is used in
+very large quantities as a source of sulphur dioxide in the manufacture
+of sulphuric acid, since it burns readily in the air, forming ferric
+oxide and sulphur dioxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2FeS<sub>2</sub> + 11O = Fe<sub>2</sub>O<sub>3</sub> + 4SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p><b>Ferrous carbonate</b> (FeCO<sub>3</sub>). This compound occurs in nature as
+siderite, and is a valuable ore. It will dissolve to some extent in
+water containing carbon dioxide, just as will calcium carbonate, and
+waters containing it are called chalybeate waters. These chalybeate
+waters are supposed to possess certain medicinal virtues and form an
+important class of mineral waters.</p>
+
+<p><b>Ferric salts.</b> Ferric salts are usually obtained by treating an acidified
+solution of a ferrous salt with an oxidizing agent:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2FeCl<sub>2</sub> + 2HCl + O = 2FeCl<sub>3</sub> + H<sub>2</sub>O,<br /></span>
+</div><div class="stanza">
+<span class="i0">2FeSO<sub>4</sub> + H<sub>2</sub>SO<sub>4</sub> + O = Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>They are usually yellow or violet in color, are quite soluble, and as a
+rule do not crystallize well. Heated with water in the absence of free
+acid, they hydrolyze even more readily than the salts of aluminium. The
+most familiar ferric salts are the chloride and the sulphate.<span class='pagenum'><a name="Page_352" id="Page_352">[Pg 352]</a></span></p>
+
+<p><b>Ferric chloride</b> (FeCl<sub>3</sub>). This salt can be obtained most conveniently
+by dissolving iron in hydrochloric acid and then passing chlorine into
+the solution:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe + 2HCl = FeCl<sub>2</sub> + 2H,<br /></span>
+</div><div class="stanza">
+<span class="i0">FeCl<sub>2</sub> + Cl = FeCl<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>When the pure salt is heated with water it is partly hydrolyzed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">FeCl<sub>3</sub> + 3 H<sub>2</sub>O &lt;--> Fe(OH)<sub>3</sub> + 3HCl.<br /></span>
+</div></div>
+
+<p>This is a reversible reaction, however, and hydrolysis can therefore be
+prevented by first adding a considerable amount of the soluble product
+of the reaction, namely, hydrochloric acid.</p>
+
+<p><b>Ferric sulphate</b> (Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>). This compound can be made by
+treating an acid solution of green vitriol with an oxidizing agent. It
+is difficult to crystallize and hard to obtain in pure condition. When
+an alkali sulphate in proper quantity is added to ferric sulphate in
+solution an iron alum is formed, and is easily obtained in large
+crystals. The best known iron alums have the formulas
+KFe(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O and NH<sub>4</sub>Fe(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O. They are
+commonly used when a pure ferric salt is required.</p>
+
+<p><b>Ferric hydroxide</b> (Fe(OH)<sub>3</sub>). When solutions of ferric salts are
+treated with ammonium hydroxide, ferric hydroxide is formed as a
+rusty-red precipitate, insoluble in water.</p>
+
+<p><b>Iron cyanides.</b> A large number of complex cyanides containing iron are
+known, the most important being potassium ferrocyanide, or yellow
+prussiate of potash (K<sub>4</sub>FeC<sub>6</sub>N<sub>6</sub>), and potassium ferricyanide, or
+red prussiate of potash (K<sub>3</sub>FeC<sub>6</sub>N<sub>6</sub>). These compounds are the
+potassium salts of the complex acids of the formulas H<sub>4</sub>FeC<sub>6</sub>N<sub>6</sub>
+<span class='pagenum'><a name="Page_353" id="Page_353">[Pg 353]</a></span>and H<sub>3</sub>FeC<sub>6</sub>N<sub>6</sub>.</p>
+
+<p><b>Oxidation of ferrous salts.</b> It has just been seen that when a ferrous
+salt is treated with an oxidizing agent in the presence of a free acid a
+ferric salt is formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2FeSO<sub>4</sub> + H<sub>2</sub>SO<sub>4</sub> + O = Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>In this reaction oxygen is used up, and the valence of the iron is
+changed from 2 to 3. The same equation may be written</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Fe<sup>++</sup>, 2SO<sub>4</sub><sup>-</sup> + 2H<sup>+</sup>, SO<sub>4</sub><sup>-</sup> + O = 2Fe<sup>+++</sup>, 3SO<sub>4</sub><sup>-</sup> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Hydrogen ions have been oxidized to water, while the charge of each iron
+ion has been increased from 2 to 3.</p>
+
+<p>In a similar way the conversion of ferrous chloride into ferric chloride
+may be written</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe<sup>++</sup>, 2Cl<sup>-</sup> + Cl = Fe<sup>+++</sup>, + 3Cl<sup>-</sup>.<br /></span>
+</div></div>
+
+<p>Here again the valence of the iron and the charge on the iron ion has
+been increased from 2 to 3, though no oxygen has entered into the
+reaction. As a rule, however, changes of this kind are brought about by
+the use of an oxidizing agent, and are called oxidations.</p>
+
+<p>The term "oxidation" is applied to all reactions in which the valence of
+the metal of a compound is increased, or, in other words, to all
+reactions in which the charge of a cation is increased.</p>
+
+<p><b>Reduction of ferric salts.</b> The changes which take place when a ferric
+salt is converted into a ferrous salt are the reverse of the ones just
+described. This is seen in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">FeCl<sub>3</sub> + H = FeCl<sub>2</sub> + HCl<br /></span>
+</div></div>
+
+<p>In this reaction the valence of the iron has been changed from 3 to 2.
+The same equation may be written</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe<sup>+++</sup>, 3Cl<sup>-</sup> + H = Fe<sup>++</sup>, + H<sup>+</sup> + 3Cl<sup>-</sup><br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_354" id="Page_354">[Pg 354]</a></span></p>
+<p>It will be seen that the charge of the iron ions has been diminished
+from 3 to 2. Since these changes are the reverse of the oxidation
+changes just considered, they are called reduction reactions. The term
+"reduction" is applied to all processes in which the valence of the
+metal of a compound is diminished, or, in other words, to all processes
+in which the charge on the cations is diminished.</p>
+
+
+<h3>NICKEL AND COBALT</h3>
+
+<p>These elements occur sparingly in nature, usually combined with arsenic
+or with arsenic and sulphur. Both elements have been found in the free
+state in meteorites. Like iron they form two series of compounds, but
+the salts corresponding to the ferrous salts are the most common, the
+ones corresponding to the ferric salts being difficult to obtain. Thus
+we have the chlorides NiCl<sub>2</sub>&middot;6H<sub>2</sub>O and CoCl<sub>2</sub>&middot;6H<sub>2</sub>O; the
+sulphates NiSO<sub>4</sub>&middot;7H<sub>2</sub>O and CoSO<sub>4</sub>&middot;7H<sub>2</sub>O; the nitrates
+Ni(NO<sub>3</sub>)<sub>2</sub>&middot;6H<sub>2</sub>O and Co(NO<sub>3</sub>)<sub>2</sub>&middot;6H<sub>2</sub>O.</p>
+
+<p>Nickel is largely used as an alloy with other metals. Alloyed with
+copper it forms coin metal from which five-cent pieces are made, with
+copper and zinc it forms German silver, and when added to steel in small
+quantities nickel steel is formed which is much superior to common steel
+for certain purposes. When deposited by electrolysis upon the surface of
+other metals such as iron, it forms a covering which will take a high
+polish and protects the metal from rust, nickel not being acted upon by
+moist air. Salts of nickel are usually green.</p>
+
+<p>Compounds of cobalt fused with glass give it an intensely blue color. In
+powdered form such glass is sometimes used<span class='pagenum'><a name="Page_355" id="Page_355">[Pg 355]</a></span> as a pigment called smalt.
+Cobalt salts, which contain water of crystallization, are usually cherry
+red in color; when dehydrated they become blue.</p>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> In the manufacture of cast iron, why is the air heated before being
+forced into the furnace?</p>
+
+<p><b>2.</b> Write the equations showing how each of the following compounds of
+iron could be obtained from the metal itself: ferrous chloride, ferrous
+hydroxide, ferrous sulphate, ferrous sulphide, ferrous carbonate, ferric
+chloride, ferric sulphate, ferric hydroxide.</p>
+
+<p><b>3.</b> Account for the fact that a solution of sodium carbonate, when added
+to a solution of a ferric salt, precipitates an hydroxide and not a
+carbonate.</p>
+
+<p><b>4.</b> Calculate the percentage of iron in each of the common iron ores.</p>
+
+<p><b>5.</b> One ton of steel prepared by the Bessemer process is found by
+analysis to contain 0.2% carbon. What is the minimum weight of carbon
+which must be added in order that the steel may be made to take a
+temper?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_356" id="Page_356">[Pg 356]</a></span></p>
+<h2><a name="CHAPTER_XXVIII" id="CHAPTER_XXVIII"></a>CHAPTER XXVIII</h2>
+
+<h3>COPPER, MERCURY, AND SILVER</h3>
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td align='left'>&nbsp;</td><td colspan="2" align='left'>FORMULAS OF OXIDES</td></tr>
+<tr><td align='left'>&nbsp;</td><td align='left'>SYMBOL</td><td align='left'>ATOMIC WEIGHT</td><td align='left'>DENSITY</td><td align='left'>MELTING POINT</td><td align='center'>"ous"</td><td align='center'>"ic"</td></tr>
+<tr><td align='left'>Copper</td><td align='center'>Cu</td><td align='center'>63.6</td><td align='center'>8.89</td><td align='center'>1084&deg;</td><td align='center'>Cu<sub>2</sub>O</td><td align='center'>CuO</td></tr>
+<tr><td align='left'>Mercury</td><td align='center'>Hg</td><td align='center'>200.00</td><td align='center'>13.596</td><td align='center'>-39.5&deg;</td><td align='center'>Hg<sub>2</sub>O</td><td align='center'>HgO</td></tr>
+<tr><td align='left'>Silver</td><td align='center'>Ag</td><td align='center'>107.93</td><td align='center'>10.5</td><td align='center'>960&deg;</td><td align='center'>Ag<sub>2</sub>O</td><td align='center'>AgO</td></tr>
+</table></div>
+
+<p><b>The family.</b> By referring to the periodic arrangement of the elements
+(page 168), it will be seen that mercury is not included in the same
+family with copper and silver. Since the metallurgy of the three
+elements is so similar, however, and since they resemble each other so
+closely in chemical properties, it is convenient to class them together
+for study.</p>
+
+<p>1. <i>Occurrence.</i> The three elements occur in nature to some extent in
+the free state, but are usually found as sulphides. Their ores are easy
+to reduce.</p>
+
+<p>2. <i>Properties.</i> They are heavy metals of high luster and are especially
+good conductors of heat and electricity. They are not very active
+chemically. Neither hydrochloric nor dilute sulphuric acid has any
+appreciable action upon them. Concentrated sulphuric acid attacks all
+three, forming metallic sulphates and evolving sulphur dioxide, while
+nitric acid, both dilute and concentrated, converts them into nitrates
+with the evolution of oxides of nitrogen.<span class='pagenum'><a name="Page_357" id="Page_357">[Pg 357]</a></span></p>
+
+<p>3. <i>Two series of salts.</i> Copper and mercury form oxides of the types
+M<sub>2</sub>O and MO, as well as two series of salts. In one series the metals
+are univalent and the salts have formulas like those of the sodium
+salts. They are called cuprous and mercurous salts. In the other series
+the metals are divalent and resemble magnesium salts in formulas. These
+are called cupric and mercuric salts. Silver forms only one series of
+salts, being always a univalent metal.</p>
+
+
+<h3>COPPER</h3>
+
+<p><b>Occurrence.</b> The element copper has been used for various purposes since
+the earliest days of history. It is often found in the metallic state in
+nature, large masses of it occurring pure in the Lake Superior region
+and in other places to a smaller extent. The most valuable ores are the
+following:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Cuprite</td><td align='left'>Cu<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Chalcocite</td><td align='left'>Cu<sub>2</sub>S.</td></tr>
+<tr><td align='left'>Chalcopyrite</td><td align='left'>CuFeS<sub>2</sub>.</td></tr>
+<tr><td align='left'>Bornite</td><td align='left'>Cu<sub>3</sub>FeS<sub>3</sub>.</td></tr>
+<tr><td align='left'>Malachite</td><td align='left'>CuCO<sub>3</sub>&middot;Cu(OH)<sub>2</sub>.</td></tr>
+<tr><td align='left'>Azurite</td><td align='left'>2CuCO<sub>3</sub>&middot;Cu(OH)<sub>2</sub>.</td></tr>
+</table></div>
+
+
+<p><b>Metallurgy of copper.</b> Ores containing little or no sulphur are easy to
+reduce. They are first crushed and the earthy impurities washed away.
+The concentrated ore is then mixed with carbon and heated in a furnace,
+metallic copper resulting from the reduction of the copper oxide by the
+hot carbon.</p>
+
+<div class="blockquot"><p><b>Metallurgy of sulphide ores.</b> Much of the copper of commerce is
+made from chalcopyrite and bornite, and these ores are more
+difficult to work. They are first roasted in the air, by which
+treatment much of the sulphur is burned to sulphur dioxide. The
+roasted ore is then<span class='pagenum'><a name="Page_358" id="Page_358">[Pg 358]</a></span> melted in a small blast furnace or in an
+open one like a puddling furnace. In melting, part of the iron
+combines with silica to form a slag of iron silicate. The
+product, called crude matte, contains about 50% copper together
+with sulphur and iron. Further purification is commonly carried
+on by a process very similar to the Bessemer process for steel.
+The converter is lined with silica, and a charge of matte from
+the melting furnace, together with sand, is introduced, and air
+is blown into the mass. By this means the sulphur is
+practically all burned out by the air, and the remaining iron
+combines with silica and goes off as slag. The copper is poured
+out of the converter and molded into anode plates for refining.</p></div>
+
+<p><b>Refining of copper.</b> Impure copper is purified by electrolysis. A large
+plate of it, serving as an anode, is suspended in a tank facing a thin
+plate of pure copper, which is the cathode. The tank is filled with a
+solution of copper sulphate and sulphuric acid to serve as the
+electrolyte. A current from a dynamo passes from the anode to the
+cathode, and the copper, dissolving from the anode, is deposited upon
+the cathode in pure form, while the impurities collect on the bottom of
+the tank. Electrolytic copper is one of the purest of commercial metals
+and is very nearly pure copper.</p>
+
+<div class="blockquot"><p><b>Recovery of gold and silver.</b> Gold and silver are often present
+in small quantities in copper ores, and in electrolytic
+refining these metals collect in the muddy deposit on the
+bottom of the tank. The mud is carefully worked over from time
+to time and the precious metals extracted from it. A surprising
+amount of gold and silver is obtained in this way.</p></div>
+
+<p><b>Properties of copper.</b> Copper is a rather heavy metal of density 8.9, and
+has a characteristic reddish color. It is rather soft and is very
+malleable, ductile, and flexible, yet tough and strong; it melts at
+1084&deg;. As a conductor of heat and electrical energy it is second only to
+silver.<span class='pagenum'><a name="Page_359" id="Page_359">[Pg 359]</a></span></p>
+
+<p>Hydrochloric acid, dilute sulphuric acid, and fused alkalis are almost
+without action upon it; nitric acid and hot, concentrated sulphuric
+acid, however, readily dissolve it. In moist air it slowly becomes
+covered with a thin layer of green basic carbonate; heated in the air it
+is easily oxidized to black copper oxide (CuO).</p>
+
+<p><b>Uses.</b> Copper is extensively used for electrical purposes, for roofs and
+cornices, for sheathing the bottom of ships, and for making alloys. In
+the following table the composition of some of these alloys is
+indicated:</p>
+
+<h3>COMPOSITION OF ALLOYS OF COPPER IN PERCENTAGES</h3>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Aluminium bronze</td><td align='left'>copper (90 to 97%), aluminium (3 to 10%).</td></tr>
+<tr><td align='left'>Brass</td><td align='left'>copper (63 to 73%), zinc (27 to 37%).</td></tr>
+<tr><td align='left'>Bronze</td><td align='left'>copper (70 to 95%), zinc (1 to 25%), tin (1 to 18%).</td></tr>
+<tr><td align='left'>German silver</td><td align='left'>copper (56 to 60%), zinc (20%), nickel (20 to 25%).</td></tr>
+<tr><td align='left'>Gold coin</td><td align='left'>copper (10%), gold (90%).</td></tr>
+<tr><td align='left'>Gun metal</td><td align='left'>copper (90%), tin (10%).</td></tr>
+<tr><td align='left'>Nickel coin</td><td align='left'>copper (75%), nickel (25%)</td></tr>
+<tr><td align='left'>Silver coin</td><td align='left'>copper (10%), silver (90%).</td></tr>
+</table></div>
+
+
+<div class="blockquot"><p><b>Electrotyping.</b> Matter is often printed from electrotype plates
+which are prepared as follows. The matter is set up in type and
+wax is firmly pressed down upon the face of it until a clear
+impression is obtained. The impressed side of the wax is coated
+with graphite and the impression is made the cathode in an
+electrolytic cell containing a copper salt in solution. When
+connected with a current the copper is deposited as a thin
+sheet upon the letters in wax, and when detached is a perfect
+copy of the type, the under part of the letters being hollow.
+The sheet is strengthened by pouring on the under surface a
+suitable amount of molten metal (commercial lead is used). The
+sheet so strengthened is then used in printing.</p></div>
+
+<p><b>Two series of copper compounds.</b> Copper, like iron, forms two series of
+compounds: in the cuprous compounds it is univalent; in the cupric it is
+divalent. The cupric salts<span class='pagenum'><a name="Page_360" id="Page_360">[Pg 360]</a></span> are much the more common of the two, since
+the cuprous salts pass readily into cupric by oxidation.</p>
+
+<p><b>Cuprous compounds.</b> The most important cuprous compound is the oxide
+(Cu<sub>2</sub>O), which occurs in nature as ruby copper or cuprite. It is a
+bright red substance and can easily be prepared by heating copper to a
+high temperature in a limited supply of air. It is used for imparting a
+ruby color to glass.</p>
+
+<p>By treating cuprous oxide with different acids a number of cuprous salts
+can be made. Many of these are insoluble in water, the chloride (CuCl)
+being the best known. When suspended in dilute hydrochloric acid it is
+changed into cupric chloride, the oxygen taking part in the reaction
+being absorbed from the air:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2CuCl + 2HCl + O = 2CuCl<sub>2</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><b>Cupric compounds.</b> Cupric salts are easily made by dissolving cupric
+oxide in acids, or, when insoluble, by precipitation. Most of them are
+blue or green in color, and the soluble ones crystallize well. Since
+they are so much more familiar than the cuprous salts, they are
+frequently called merely copper salts.</p>
+
+<p><b>Cupric oxide</b> (CuO). This is a black insoluble substance obtained by
+heating copper in excess of air, or by igniting the hydroxide or
+nitrate. It is used as an oxidizing agent.</p>
+
+<p><b>Cupric hydroxide</b> (Cu(OH)<sub>2</sub>). The hydroxide prepared by treating a
+solution of a copper salt with sodium hydroxide is a light blue
+insoluble substance which easily loses water and changes into the oxide.
+Heat applied to the liquid containing the hydroxide suspended in it
+serves to bring about the reaction represented by the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Cu(OH)<sub>2</sub> = CuO + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_361" id="Page_361">[Pg 361]</a></span></p>
+
+<p><b>Cupric sulphate</b> (<i>blue vitriol</i>) (CuSO<sub>4</sub>&middot;5H<sub>2</sub>O). This substance,
+called blue vitriol or bluestone, is obtained as a by-product in a
+number of processes and is produced in very large quantities. It forms
+large blue crystals, which lose water when heated and crumble to a white
+powder. The salt finds many uses, especially in electrotyping and in
+making electrical batteries.</p>
+
+<p><b>Cupric sulphide</b> (CuS). The insoluble black sulphide (CuS) is easily
+prepared by the action of hydrosulphuric acid upon a solution of a
+copper salt:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CuSO<sub>4</sub> + H<sub>2</sub>S = CuS + H<sub>2</sub>SO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>It is insoluble in water and dilute acids.</p>
+
+
+<h3>MERCURY</h3>
+
+<p><b>Occurrence.</b> Mercury occurs in nature chiefly as the sulphide (HgS)
+called cinnabar, and in globules of metal inclosed in the cinnabar. The
+mercury mines of Spain have long been famous, California being the next
+largest producer.</p>
+
+<p><b>Metallurgy.</b> Mercury is a volatile metal which has but little affinity
+for oxygen. Sulphur, on the other hand, readily combines with oxygen.
+These facts make the metallurgy of mercury very simple. The crushed ore,
+mixed with a small amount of carbon to reduce any oxide or sulphate that
+might be formed, is roasted in a current of air. The sulphur burns to
+sulphur dioxide, while the mercury is converted into vapor and is
+condensed in a series of condensing vessels. The metal is purified by
+distillation.</p>
+
+<p><b>Properties.</b> Mercury is a heavy silvery liquid with a density of 13.596.
+It boils at 357&deg; and solidifies at -39.5&deg;.<span class='pagenum'><a name="Page_362" id="Page_362">[Pg 362]</a></span> Small quantities of many
+metals dissolve in it, forming liquid alloys, while with larger
+quantities it forms solid alloys. The alloys of mercury are called
+amalgams.</p>
+
+<p>Toward acids mercury conducts itself very much like copper; it is easily
+attacked by nitric and hot, concentrated sulphuric acids, while cold
+sulphuric and hydrochloric acids have no effect on it.</p>
+
+<p><b>Uses.</b> Mercury is extensively used in the construction of scientific
+instruments, such as the thermometer and barometer, and as a liquid over
+which to collect gases which are soluble in water. The readiness with
+which it alloys with silver and gold makes it very useful in the
+extraction of these elements.</p>
+
+<p><b>Compounds of mercury.</b> Like copper, mercury forms two series of
+compounds: the mercurous, of which mercurous chloride (HgCl) is an
+example; and the mercuric, represented by mercuric chloride (HgCl<sub>2</sub>).</p>
+
+<p><b>Mercuric oxide</b> (HgO). Mercuric oxide can be obtained either as a
+brick-red or as a yellow substance. When mercuric nitrate is heated
+carefully the red modification is formed in accordance with the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Hg(NO<sub>3</sub>)<sub>2</sub> = HgO + 2NO<sub>2</sub> + O.<br /></span>
+</div></div>
+
+<p>The yellow modification is prepared by adding a solution of a mercuric
+salt to a solution of sodium or potassium hydroxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Hg(NO<sub>3</sub>)<sub>2</sub> + 2NaOH = 2NaNO<sub>3</sub> + Hg(OH)<sub>2</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">Hg(OH)<sub>2</sub> = HgO + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>When heated the oxide darkens until it becomes almost black; at a higher
+temperature it decomposes into mercury and oxygen. It was by this
+reaction that oxygen was discovered.<span class='pagenum'><a name="Page_363" id="Page_363">[Pg 363]</a></span></p>
+
+<p><b>Mercurous chloride</b> (<i>calomel</i>) (HgCl). Being insoluble, mercurous
+chloride is precipitated as a white solid when a soluble chloride is
+added to a solution of mercurous nitrate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">HgNO<sub>3</sub> + NaCl = HgCl + NaNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Commercially it is manufactured by heating a mixture of mercuric
+chloride and mercury. When exposed to the light it slowly changes into
+mercuric chloride and mercury:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HgCl = HgCl<sub>2</sub> + Hg.<br /></span>
+</div></div>
+
+<p>It is therefore protected from the light by the use of colored bottles.
+It is used in medicine.</p>
+
+<p>Most mercurous salts are insoluble in water, the principal soluble one
+being the nitrate, which is made by the action of cold, dilute nitric
+acid on mercury.</p>
+
+<p><b>Mercuric chloride</b> (<i>corrosive sublimate</i>) (HgCl<sub>2</sub>). This substance can
+be made by dissolving mercuric oxide in hydrochloric acid. On a
+commercial scale it is made by subliming a mixture of common salt and
+mercuric sulphate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2NaCl + HgSO<sub>4</sub> = HgCl<sub>2</sub> + Na<sub>2</sub>SO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>The mercuric chloride, being readily volatile, vaporizes and is
+condensed again in cool vessels. Like mercurous chloride it is a white
+solid, but differs from it in that it is soluble in water. It is
+extremely poisonous and in dilute solutions is used as an antiseptic in
+dressing wounds.</p>
+
+<p><b>Mercuric sulphide</b> (HgS). As cinnabar this substance forms the chief
+native compound of mercury, occurring in red crystalline masses. By
+passing hydrosulphuric acid into a solution of a mercuric salt it is
+precipitated as a black powder, insoluble in water and acids. By other
+means it can be prepared as a brilliant red powder known as vermilion,
+which is used as a pigment in fine paints.<span class='pagenum'><a name="Page_364" id="Page_364">[Pg 364]</a></span></p>
+
+<div class="blockquot"><p><b>The iodides of mercury.</b> If a solution of potassium iodide is
+added to solutions of a mercurous and a mercuric salt
+respectively, the corresponding iodides are precipitated.
+Mercuric iodide is the more important of the two, and as
+prepared above is a red powder which changes to yellow on
+heating to 150&deg;. The yellow form on cooling changes back again
+to the red form, or may be made to do so by rubbing it with a
+knife blade or some other hard object.</p></div>
+
+
+<h3>SILVER</h3>
+
+<p><b>Occurrence.</b> Silver is found in small quantities in the uncombined state;
+usually, however, it occurs in combination with sulphur, either as the
+sulphide (Ag<sub>2</sub>S) or as a small constituent of other sulphides,
+especially those of lead and copper. It is also found alloyed with gold.</p>
+
+<p><b>Metallurgy.</b> <i>Parkes's process.</i> Silver is usually smelted in connection
+with lead. The ores are worked over together, as described under lead,
+and the lead and silver obtained as an alloy, the silver being present
+in small quantity. The alloy is melted and metallic zinc is stirred in.
+Zinc will alloy with silver but not with lead, and it is found that the
+silver leaves the lead and, in the form of an alloy with zinc, forms as
+a crust upon the lead and is skimmed off. This crust, which, of course,
+contains lead adhering to it, is partially melted and the most of the
+lead drained off. The zinc is removed by distillation, and the residue
+is melted on an open hearth in a current of air; by this means the zinc
+and lead remaining with the silver are changed into oxides and the
+silver remains behind unaltered.</p>
+
+<div class="blockquot"><p><b>Amalgamation process.</b> In some localities the old amalgamation
+process is used. The silver ore is treated with common salt and
+ferrous compounds, which process converts the silver first into
+chloride and then into metallic silver. Mercury is then added
+and thoroughly mixed with the mass, forming an amalgam with the
+silver. After<span class='pagenum'><a name="Page_365" id="Page_365">[Pg 365]</a></span> some days the earthy materials are washed away
+and the heavier amalgam is recovered. The mercury is distilled
+off and the silver left in impure form.</p></div>
+
+<p><b>Refining silver.</b> The silver obtained by either of the above processes
+may still contain copper, gold, and iron, and is refined by "parting"
+with sulphuric acid. The metal is heated with strong sulphuric acid
+which dissolves the silver, copper, and iron present, but not the gold.
+In the solution of silver sulphate so obtained copper plates are
+suspended, upon which the pure silver precipitates, the copper going
+into solution as sulphate, as shown in the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Ag<sub>2</sub>SO<sub>4</sub> + Cu = 2Ag + CuSO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>The solution obtained as a by-product in this process furnishes most of
+the blue vitriol of commerce. Silver is also refined by electrolytic
+methods similar to those used in refining copper.</p>
+
+<p><b>Properties of silver.</b> Silver is a heavy, rather soft, white metal, very
+ductile and malleable and capable of taking a high polish. It surpasses
+all other metals as a conductor of heat and electricity, but is too
+costly to find extensive use for such purposes. It melts at a little
+lower temperature than copper (961&deg;). It alloys readily with other heavy
+metals, and when it is to be used for coinage a small amount of
+copper&mdash;from 8 to 10%&mdash;is nearly always melted with it to give it
+hardness.</p>
+
+<p>It is not acted upon by water or air, but is quickly tarnished when in
+contact with sulphur compounds, turning quite black in time.
+Hydrochloric acid and fused alkalis do not act upon it, but nitric acid
+and hot, concentrated sulphuric acid dissolve it with ease.<span class='pagenum'><a name="Page_366" id="Page_366">[Pg 366]</a></span></p>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig88.jpg" width="400" height="148" alt="Fig. 88" title="" />
+<span class="caption">Fig. 88</span>
+</div>
+
+<div class="blockquot"><p><b>Electroplating.</b> Since silver is not acted upon by water or air,
+and has a pleasing appearance, it is used to coat various
+articles made of cheaper metals. Such articles are said to be
+silver plated. The process by which this is done is called
+electroplating. It is carried on as follows: The object to be
+plated (such as a spoon) is attached to a wire and dipped into
+a solution of a silver salt. Electrical connection is made in
+such a way that the article to be plated serves as the cathode,
+while the anode is made up of one or more plates of silver
+(Fig. 88, <i>A</i>). When a current is passed through the
+electrolyte silver dissolves from the anode plate and deposits
+on the cathode in the form of a closely adhering layer. By
+making the proper change in the electrolyte and anode plate
+objects may be plated with gold and other metals.</p></div>
+
+<p><b>Compounds of silver.</b> Silver forms two oxides but only one series of
+salts, namely, the one which corresponds to the mercurous and cuprous
+series.</p>
+
+<p><b>Silver nitrate</b> (<i>lunar caustic</i>) (AgNO<sub>3</sub>). This salt is easily
+prepared by dissolving silver in nitric acid and evaporating the
+resulting solution. It crystallizes in flat plates, and when heated
+carefully can be melted without decomposition. When cast into sticks it
+is called lunar caustic, for it has a very corrosive action on flesh,
+and is sometimes used in surgery to burn away abnormal growths.</p>
+
+<div class="blockquot"><p>The alchemists designated the metals by the names of the
+heavenly bodies. The moon (luna) was the symbol for silver;
+hence the name "lunar caustic."</p></div>
+
+<p><b>Silver sulphide</b> (Ag<sub>2</sub>S). This occurs in nature and constitutes one of
+the principal ores of silver. It can be<span class='pagenum'><a name="Page_367" id="Page_367">[Pg 367]</a></span> obtained in the form of a black
+solid by passing hydrosulphuric acid through a solution of silver
+nitrate.</p>
+
+<p><b>Compounds of silver with the halogens.</b> The chloride, bromide, and iodide
+of silver are insoluble in water and acids, and are therefore
+precipitated by bringing together a soluble halogen salt with silver
+nitrate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">AgNO<sub>3</sub> + KCl = AgCl + KNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>They are remarkable for the fact that they are very sensitive to the
+action of light, undergoing a change of color and chemical composition
+when exposed to sunlight, especially if in contact with organic matter
+such as gelatin.</p>
+
+<div class="blockquot"><p><b>Photography.</b> The art of photography is based on the fact that
+the halogen compounds of silver are affected by the light,
+particularly in the presence of organic matter. From a chemical
+standpoint the processes involved may be described under two
+heads: (1) the preparation of the negative; (2) the preparation
+of the print.</p>
+
+<p>1. <i>Preparation of the negative.</i> The plate used in the
+preparation of the negative is made by spreading a thin layer
+of gelatin, in which silver bromide is suspended (silver iodide
+is sometimes added also), over a glass plate or celluloid film
+and allowing it to dry. When the plate so prepared is placed in
+a camera and the image of some object is focused upon it, the
+silver salt undergoes a change which is proportional at each
+point to the intensity of the light falling upon it. In this
+way an image of the object photographed is produced upon the
+plate, which is, however, invisible and is therefore called
+"latent." It can be made visible by the process of developing.</p>
+
+<p>To develop the image the exposed plate is immersed in a
+solution of some reducing agent called the developer. The
+developer reduces that portion of the silver salt which has
+been affected by the light, depositing it in the form of black
+metallic silver which closely adheres to the plate.</p>
+
+<p>The unaffected silver salt, upon which the developer has no
+action, must now be removed from the plate. This is done by
+immersing the plate in a solution of sodium thiosulphate
+(hypo). After the silver salt has been dissolved off, the plate
+is washed with water and<span class='pagenum'><a name="Page_368" id="Page_368">[Pg 368]</a></span> dried. The plate so prepared is
+called the negative because it is a picture of the object
+photographed, with the lights exactly reversed. This is called
+fixing the negative.</p>
+
+<p>2. <i>Preparation of the print.</i> The print is made from paper
+which is prepared in the same way as the negative plate. The
+negative is placed upon this paper and exposed to the light in
+such a way that the light must pass through the negative before
+striking the paper. If the paper is coated with silver
+chloride, a visible image is produced, in which case a
+developer is not needed. The proofs are made in this way. In
+order to make them permanent the unchanged silver chloride must
+be dissolved off with sodium thiosulphate. The print is then
+toned by dipping it into a solution of gold or platinum salts.
+The silver on the print passes into solution, while the gold or
+platinum takes its place. These metals give a characteristic
+color or tone to the print, the gold making it reddish brown,
+while the platinum gives it a steel-gray tone. If a silver
+bromide paper is used in making the print, a latent image is
+produced which must be developed as in the case of the negative
+itself. The silver bromide is much more sensitive than the
+chloride, so that the printing can be done in artificial light.
+Since the darkest places on the negative cut off the most
+light, it is evident that the lights of the print will be the
+reverse of those of the negative, and will therefore correspond
+to those of the object photographed. The print is therefore
+called the positive.</p></div>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> Account for the fact that copper has been used for so long a time.</p>
+
+<p><b>2.</b> Write equations for the action of concentrated sulphuric and nitric
+acids upon the metals of this family.</p>
+
+<p><b>3.</b> How would you account for the fact that normal copper sulphate is
+slightly acid to litmus?</p>
+
+<p><b>4.</b> Contrast the action of heat on cupric nitrate and mercuric nitrate.</p>
+
+<p><b>5.</b> State reasons why mercury is adapted for use in thermometers and
+barometers.</p>
+
+<p><b>6.</b> How could you distinguish between mercurous chloride and mercuric
+chloride?</p>
+
+<p><b>7.</b> Write equations for the preparation of mercuric and mercurous
+iodides.<span class='pagenum'><a name="Page_369" id="Page_369">[Pg 369]</a></span></p>
+
+<p><b>8.</b> How would you account for the fact that solutions of the different
+salts of a metal usually have the same color?</p>
+
+<p><b>9.</b> Crude silver usually contains iron and lead. What would become of
+these metals in refining by parting with sulphuric acid?</p>
+
+<p><b>10.</b> In the amalgamation process for extracting silver, how does ferrous
+chloride convert silver chloride into silver? Write equation. Why is the
+silver sulphide first changed into silver chloride?</p>
+
+<p><b>11.</b> What impurities would you expect to find in the copper sulphate
+prepared from the refining of silver?</p>
+
+<p><b>12.</b> How could you prepare pure silver chloride from a silver coin?</p>
+
+<p><b>13.</b> Mercuric nitrate and silver nitrate are both white solids soluble in
+water. How could you distinguish between them?</p>
+
+<p><b>14.</b> Account for the fact that sulphur waters turn a silver coin black;
+also for the fact that a silver spoon is blackened by foods (eggs, for
+example) containing sulphur.</p>
+
+<p><b>15.</b> When a solution of silver nitrate is added to a solution of
+potassium chlorate no precipitate forms. How do you account for the fact
+that a precipitate of silver chloride is not formed?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_370" id="Page_370">[Pg 370]</a></span></p>
+<h2><a name="CHAPTER_XXIX" id="CHAPTER_XXIX"></a>CHAPTER XXIX</h2>
+
+<h3>TIN AND LEAD</h3>
+
+
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>SYMBOL</td><td align='left'>ATOMIC WEIGHT</td><td align='left'>DENSITY</td><td align='left'>MELTING POINT</td><td align='left'>COMMON OXIDES</td></tr>
+<tr><td align='left'>Tin</td><td align='center'>Sn</td><td align='center'>119.0</td><td align='center'>7.35</td><td align='center'>235&deg;</td><td align='center'>SnO SnO<sub>2</sub></td></tr>
+<tr><td align='left'>Lead</td><td align='center'>Pb</td><td align='center'>206.9</td><td align='center'>11.38</td><td align='center'>327&deg;</td><td align='center'>PbO Pb<sub>3</sub>O<sub>4</sub> PbO<sub>2</sub></td></tr>
+</table></div>
+
+
+
+<p><b>The family.</b> Tin and lead, together with silicon and germanium, form a
+family in Group IV of the periodic table. Silicon has been discussed
+along with the non-metals, while germanium, on account of its rarity,
+needs only to be mentioned.</p>
+
+<p>The other family of Group IV includes carbon, already described, and a
+number of rare elements.</p>
+
+
+<h3>TIN</h3>
+
+<p><b>Occurrence.</b> Tin is found in nature chiefly as the oxide (SnO<sub>2</sub>),
+called cassiterite or tinstone. The most famous mines are those of
+Cornwall in England, and of the Malay Peninsula and East India Islands;
+in small amounts tinstone is found in many other localities.</p>
+
+<p><b>Metallurgy.</b> The metallurgy of tin is very simple. The ore, separated as
+far as possible from earthy materials, is mixed with carbon and heated
+in a furnace, the reduction taking place readily. The equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SnO<sub>2</sub> + C = Sn + CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_371" id="Page_371">[Pg 371]</a></span></p>
+<p>The metal is often purified by carefully heating it until it is partly
+melted; the pure tin melts first and can be drained away from the
+impurities.</p>
+
+<p><b>Properties.</b> Pure tin, called block tin, is a soft white metal with a
+silver-like appearance and luster; it melts readily (235&deg;) and is
+somewhat lighter than copper, having a density of 7.3. It is quite
+malleable and can be rolled out into very thin sheets, forming tin foil;
+most tin foil, however, contains a good deal of lead.</p>
+
+<p>Under ordinary conditions it is quite unchanged by air or moisture, but
+at a high temperature it burns in air, forming the oxide SnO<sub>2</sub>. Dilute
+acids have no effect upon it, but concentrated acids attack it readily.
+Concentrated hydrochloric acid changes it into the chloride</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Sn + 2HCl = SnCl<sub>2</sub> + 2H.<br /></span>
+</div></div>
+
+<p>With sulphuric acid tin sulphate and sulphur dioxide are formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Sn + 2H<sub>2</sub>SO<sub>4</sub> = SnSO<sub>4</sub> + SO<sub>2</sub> + 2H<sub>2</sub>O<br /></span>
+</div></div>
+
+<p>Concentrated nitric acid oxidizes it, forming a white insoluble compound
+of the formula H<sub>2</sub>SnO<sub>3</sub>, called metastannic acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3Sn + 4HNO<sub>3</sub> + H<sub>2</sub>O = 3H<sub>2</sub>SnO<sub>3</sub> + 4NO.<br /></span>
+</div></div>
+
+<p><b>Uses of tin.</b> A great deal of tin is made into tin plate by dipping thin
+steel sheets into the melted metal. Owing to the way in which tin
+resists the action of air and dilute acids, tin plate is used in many
+ways, such as in roofing, and in the manufacture of tin cans, cooking
+vessels, and similar articles.</p>
+
+<p>Many useful alloys contain tin, some of which have been mentioned in
+connection with copper. When tin is alloyed with other metals of low
+melting point, soft, easily<span class='pagenum'><a name="Page_372" id="Page_372">[Pg 372]</a></span> melted alloys are formed which are used for
+friction bearings in machinery; tin, antimony, lead, and bismuth are the
+chief constituents of these alloys. Pewter and soft solder are alloys of
+tin and lead.</p>
+
+<p><b>Compounds of tin.</b> Tin forms two series of compounds: the stannous, in
+which the tin is divalent, illustrated in the compounds SnO, SnS,
+SnCl<sub>2</sub>; the stannic, in which it is tetravalent as shown in the
+compounds SnO<sub>2</sub>, SnS<sub>2</sub>. There is also an acid, H<sub>2</sub>SnO<sub>3</sub>, called
+stannic acid, which forms a series of salts called stannates. While this
+acid has the same composition as metastannic acid, the two are quite
+different in their chemical properties. This difference is probably due
+to the different arrangement of the atoms in the molecules of the two
+substances. Only a few compounds of tin need be mentioned.</p>
+
+<p><b>Stannic oxide</b> (SnO<sub>2</sub>). Stannic oxide is of interest, since it is the
+chief compound of tin found in nature. It is sometimes found in
+good-sized crystals, but as prepared in the laboratory is a white
+powder. When fused with potassium hydroxide it forms potassium stannate,
+acting very much like silicon dioxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SnO<sub>2</sub> + 2KOH = K<sub>2</sub>SnO<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><b>Chlorides of tin.</b> Stannous chloride is prepared by dissolving tin in
+concentrated hydrochloric acid and evaporating the solution to
+crystallization. The crystals which are obtained have the composition
+SnCl<sub>2</sub>&middot;2H<sub>2</sub>O, and are known as tin crystals. By treating a solution
+of stannous chloride with aqua regia, stannic chloride is formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SnCl<sub>2</sub> + 2Cl = SnCl<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>The salt which crystallizes from such a solution has the composition
+<span class='pagenum'><a name="Page_373" id="Page_373">[Pg 373]</a></span>SnCl<sub>4</sub>&middot;5H<sub>2</sub>O, and is known commercially as oxymuriate of tin. If
+metallic tin is heated in a current of dry chlorine, the anhydrous
+chloride (SnCl<sub>4</sub>) is obtained as a heavy colorless liquid which fumes
+strongly on exposure to air.</p>
+
+<p>The ease with which stannous chloride takes up chlorine to form stannic
+chloride makes it a good reducing agent in many reactions, changing the
+higher chlorides of metals to lower ones. Thus mercuric chloride is
+changed into mercurous chloride:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SnCl<sub>2</sub> + 2HgCl<sub>2</sub> = SnCl<sub>4</sub> + 2HgCl.<br /></span>
+</div></div>
+
+<p>If the stannous chloride is in excess, the reaction may go further,
+producing metallic mercury:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SnCl<sub>2</sub> + 2HgCl = SnCl<sub>4</sub> + 2Hg.<br /></span>
+</div></div>
+
+<p>Ferric chloride is in like manner reduced to ferrous chloride:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SnCl<sub>3</sub> + 2FeCl<sub>3</sub> = SnCl<sub>4</sub> + 2FeCl<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>The chlorides of tin, as well as the alkali stannates, are much used as
+mordants in dyeing processes. The hydroxides of tin and free stannic
+acid, which are easily liberated from these compounds, possess in very
+marked degree the power of fixing dyes upon fibers, as explained under
+aluminium.</p>
+
+
+<h3>LEAD</h3>
+
+<p><b>Occurrence.</b> Lead is found in nature chiefly as the sulphide (PbS),
+called galena; to a much smaller extent it occurs as carbonate,
+sulphate, chromate, and in a few other forms. Practically all the lead
+of commerce is made from galena, two general methods of metallurgy being
+in use.</p>
+
+<p><b>Metallurgy.</b> 1. The sulphide is melted with scrap iron, when iron
+sulphide and metallic lead are formed; the<span class='pagenum'><a name="Page_374" id="Page_374">[Pg 374]</a></span> liquid lead, being the
+heavier, sinks to the bottom of the vessel and can be drawn off:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">PbS + Fe = Pb + FeS.<br /></span>
+</div></div>
+
+<p>2. The sulphide is roasted in the air until a part of it has been
+changed into oxide and sulphate. The air is then shut off and the
+heating continued, the reactions indicated in the following equations
+taking place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2PbO + PbS = 3Pb + SO<sub>2</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">PbSO<sub>4</sub> + PbS = 2Pb + 2SO<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>The lead so prepared usually contains small amounts of silver, arsenic,
+antimony, copper, and other metals. The silver is removed by Parkes's
+method, as described under silver, and the other metals in various ways.
+The lead of commerce is one of the purest commercial metals, containing
+as a rule only a few tenths per cent of impurities.</p>
+
+<p><b>Properties.</b> Lead is a heavy metal (den. = 11.33) which has a brilliant
+silvery luster on a freshly cut surface, but which soon tarnishes to a
+dull blue-gray color. It is soft, easily fused (melting at 327&deg;), and
+quite malleable, but has little toughness or strength.</p>
+
+<p>It is not acted upon to any great extent by the oxygen of the air under
+ordinary conditions, but is changed into oxide at a high temperature.
+With the exception of hydrochloric and sulphuric acids, most acids, even
+very weak ones, act upon it, forming soluble lead salts. Hot,
+concentrated hydrochloric and sulphuric acids also attack it to a slight
+extent.</p>
+
+<p><b>Uses.</b> Lead is employed in the manufacture of lead pipes and in large
+storage batteries. In the form of sheet lead it is used in lining the
+chambers of sulphuric acid<span class='pagenum'><a name="Page_375" id="Page_375">[Pg 375]</a></span> works and in the preparation of paint
+pigments. Some alloys of lead, such as solder and pewter (lead and tin),
+shot (lead and arsenic), and soft bearing metals, are widely used. Type
+metal consists of lead, antimony, and sometimes tin. Compounds of lead
+form several important pigments.</p>
+
+<p><b>Compounds of lead.</b> In nearly all its compounds lead has a valence of 2,
+but a few corresponding to stannic compounds have a valence of 4.</p>
+
+<p><b>Lead oxides.</b> Lead forms a number of oxides, the most important of which
+are litharge, red lead or minium, and lead peroxide.</p>
+
+<p>1. <i>Litharge</i> (PbO). This oxide forms when lead is oxidized at a rather
+low temperature, and is obtained as a by-product in silver refining. It
+is a pale yellow powder, and has a number of commercial uses. It is
+easily soluble in nitric acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">PbO + 2HNO<sub>3</sub> = Pb(NO<sub>3</sub>)<sub>2</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>2. <i>Red lead, or minium</i> (Pb<sub>3</sub>O<sub>4</sub>). Minium is prepared by heating
+lead (or litharge) to a high temperature in the air. It is a heavy
+powder of a beautiful red color, and is much used as a pigment.</p>
+
+<p>3. <i>Lead peroxide</i> (PbO<sub>2</sub>). This is left as a residue when minium is
+heated with nitric acid:</p>
+
+<p>Pb<sub>3</sub>O<sub>4</sub> + 4HNO<sub>3</sub> = 2Pb(NO<sub>3</sub>)<sub>2</sub> + PbO<sub>2</sub> + 2H<sub>2</sub>O.</p>
+
+<p>It is a brown powder which easily gives up a part of its oxygen and,
+like manganese dioxide and barium dioxide, is a good oxidizing agent.</p>
+
+<p><b>Soluble salts of lead.</b> The soluble salts of lead can be made by
+dissolving litharge in acids. Lead acetate
+(Pb(C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>)<sub>2</sub>&middot;3H<sub>2</sub>O), called sugar of lead, and lead
+<span class='pagenum'><a name="Page_376" id="Page_376">[Pg 376]</a></span>nitrate (Pb(NO<sub>3</sub>)<sub>2</sub>) are the most familiar examples. They are while
+crystalline solids and are poisonous in character.</p>
+
+<p><b>Insoluble salts of lead; lead carbonate.</b> While the normal carbonate of
+lead (PbCO<sub>3</sub>) is found to some extent, in nature and can be prepared
+in the laboratory, basic carbonates of varying composition are much more
+easy to obtain. One of the simplest of these has the composition
+2PbCO<sub>3</sub>&middot;Pb(OH)<sub>2</sub>. A mixture of such carbonates is called white lead.
+This is prepared on a large scale as a paint pigment and as a body for
+paints which are to be colored with other substances.</p>
+
+<div class="blockquot"><p><b>White lead.</b> White lead is an amorphous white substance which,
+when mixed with oil, has great covering power, that is, it
+spreads out in an even waxy film, free from streaks and lumps,
+and covers the entire surface upon which it is spread. Its
+disadvantage as a pigment lies in the fact that it gradually
+blackens when exposed to sulphur compounds, which are often
+present in the air, forming black lead sulphide (PbS).</p>
+
+<p><b>Technical preparation of white lead.</b> Different methods are used
+in the preparation of white lead, but the old one known as the
+Dutch process is still the principal one employed. In this
+process, earthenware pots about ten inches high and of the
+shape shown in Fig. 89 are used. In the bottom <i>A</i> is placed a
+3% solution of acetic acid (vinegar answers the purpose very
+well). The space above this is filled with thin, perforated,
+circular pieces of lead, supported by the flange <i>B</i> of the
+pot. These pots are placed close together on a bed of tan bark
+on the floor of a room known as the corroding room. They are
+covered over with boards, upon which tan bark is placed, and
+another row of pots is placed on this. In this way the room is
+filled. The white lead is formed by the fumes of the acetic
+acid, together with the carbon dioxide set free in the
+fermentation of the tan bark acting on the lead. About three
+months are required to complete the process.</p></div>
+
+<div class="figcenter" style="width: 300px;">
+<img src="images/imagefig89.jpg" width="300" height="400" alt="Fig. 89" title="" />
+<span class="caption">Fig. 89</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_377" id="Page_377">[Pg 377]</a></span></p>
+
+<p><b>Lead sulphide</b> (PbS). In nature this compound occurs in highly
+crystalline condition, the crystals having much the same luster as pure
+lead. It is readily prepared in the laboratory as a black precipitate,
+by the action of hydrosulphuric acid upon soluble lead salts:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Pb(NO<sub>3</sub>)<sub>2</sub> + H<sub>2</sub>S = PbS + 2HNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>It is insoluble both in water and in dilute acids.</p>
+
+<p><b>Other insoluble salts.</b> Lead chromate (PbCrO<sub>4</sub>) is a yellow substance
+produced by the action of a soluble lead salt upon a soluble chromate,
+thus:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">K<sub>2</sub>CrO<sub>4</sub> + Pb(NO<sub>3</sub>)<sub>2</sub> = PbCrO<sub>4</sub> + 2 KNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>It is used as a yellow pigment. Lead sulphate (PbSO<sub>4</sub>) is a white
+substance sometimes found in nature and easily prepared by
+precipitation. Lead chloride (PbCl<sub>2</sub>) is likewise a white substance
+nearly insoluble in cold water, but readily soluble in boiling water.</p>
+
+<div class="blockquot"><p><b>Thorium and cerium.</b> These elements are found in a few rare
+minerals, especially in the monazite sand of the Carolinas and
+Brazil. The oxides of these elements are used in the
+preparation of the Welsbach mantles for gas lights, because of
+the intense light given out when a mixture of the oxides is
+heated. These mantles contain the oxides of cerium and thorium
+in the ratio of about 1% of the former to 99% of the latter.
+Compounds of thorium, like those of radium, are found to
+possess radio-activity, but in a less degree.</p></div>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> How could you detect lead if present in tin foil?</p>
+
+<p><b>2.</b> Stannous chloride reduces gold chloride (AuCl<sub>3</sub>) to gold. Give
+equation.</p>
+
+<p><b>3.</b> What are the products of hydrolysis when stannic chloride is used as
+a mordant?</p>
+
+<p><b>4.</b> How could you detect arsenic, antimony, or copper in lead?<span class='pagenum'><a name="Page_378" id="Page_378">[Pg 378]</a></span></p>
+
+<p><b>5.</b> Why is lead so extensively used for making water pipes?</p>
+
+<p><b>6.</b> What sulphates other than lead are insoluble?</p>
+
+<p><b>7.</b> Could lead nitrate be used in place of barium chloride in testing for
+sulphates?</p>
+
+<p><b>8.</b> How much lead peroxide could be obtained from 1 kg. of minium?</p>
+
+<p><b>9.</b> The purity of white lead is usually determined by observing the
+volume of carbon dioxide given off when it is treated with an acid. What
+acid should be used? On the supposition that it has the formula
+2PbCO<sub>3</sub>&middot;Pb(OH)<sub>2</sub>, how nearly pure was a sample if 1 g. gave 30 cc.
+of carbon dioxide at 20&deg; and 750 mm.?</p>
+
+<p><b>10.</b> Silicon belongs in the same family with tin and lead. In what
+respects are these elements similar?</p>
+
+<p><b>11.</b> What weight of tin could be obtained by the reduction of 1 ton of
+cassiterite?</p>
+
+<p><b>12.</b> What reaction would you expect to take place when lead peroxide is
+treated with hydrochloric acid?</p>
+
+<p><b>13.</b> White lead is often adulterated with barytes. Suggest a method for
+detecting it, if present, in a given example of white lead.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_379" id="Page_379">[Pg 379]</a></span></p>
+<h2><a name="CHAPTER_XXX" id="CHAPTER_XXX"></a>CHAPTER XXX</h2>
+
+<h3>MANGANESE AND CHROMIUM</h3>
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>SYMBOL</td><td align='left'>ATOMIC</td><td align='left'>WEIGHT DENSITY</td><td align='left'>MELTING POINT</td><td align='left'>FORMULAS OF ACIDS</td></tr>
+<tr><td align='left'>Manganese</td><td align='center'>Mn</td><td align='center'>55.0</td><td align='center'>8.01</td><td align='center'>1900&deg;</td><td align='center'>H<sub>2</sub>MnO<sub>4</sub> and HMnO<sub>4</sub></td></tr>
+<tr><td align='left'>Chromium</td><td align='center'>Cr</td><td align='center'>52.1</td><td align='center'>7.3</td><td align='center'>3000&deg;</td><td align='center'>H<sub>2</sub>CrO<sub>4</sub> and H2Cr<sub>2</sub>O<sub>7</sub></td></tr>
+</table></div>
+
+<p><b>General.</b> Manganese and chromium, while belonging to different families,
+have so many features in common in their chemical conduct that they may
+be studied together with advantage. They differ from most of the
+elements so far studied in that they can act either as acid-forming or
+base-forming elements. As base-forming elements each of the metals forms
+two series of salts. In the one series, designated by the suffix "ous,"
+the metal is divalent; in the other series, designated by the suffix
+"ic," the metal is trivalent. Only the manganous and the chromic salts,
+however, are of importance. The acids in which these elements play the
+part of a non-metal are unstable, but their salts are usually stable,
+and some of them are important compounds.</p>
+
+
+<h3>MANGANESE</h3>
+
+<p><b>Occurrence.</b> Manganese is found in nature chiefly as the dioxide MnO<sub>2</sub>,
+called pyrolusite. In smaller amounts it occurs as the oxides
+Mn<sub>2</sub>O<sub>3</sub> and Mn<sub>3</sub>O<sub>4</sub>, and as the carbonate MnCO<sub>3</sub>. Some iron
+ores also contain manganese.<span class='pagenum'><a name="Page_380" id="Page_380">[Pg 380]</a></span></p>
+
+<p><b>Preparation and properties.</b> The element is difficult to prepare in pure
+condition and has no commercial applications. It can be prepared,
+however, by reducing the oxide with aluminium powder or by the use of
+the electric furnace, with carbon as the reducing agent. The metal
+somewhat resembles iron in appearance, but is harder, less fusible, and
+more readily acted upon by air and moisture. Acids readily dissolve it,
+forming manganous salts.</p>
+
+<p><b>Oxides of manganese.</b> The following oxides of manganese are known: MnO,
+Mn<sub>2</sub>O<sub>3</sub>, Mn<sub>3</sub>O<sub>4</sub>, MnO<sub>2</sub>, and Mn<sub>2</sub>O<sub>7</sub>. Only one of these,
+the dioxide, needs special mention.</p>
+
+<p><b>Manganese dioxide</b> (<i>pyrolusite</i>) (MnO<sub>2</sub>). This substance is the most
+abundant manganese compound found in nature, and is the ore from which
+all other compounds of manganese are made. It is a hard, brittle, black
+substance which is valuable as an oxidizing agent. It will be recalled
+that it is used in the preparation of chlorine and oxygen, in
+decolorizing glass which contains iron, and in the manufacture of
+ferromanganese.</p>
+
+<p><b>Compounds containing manganese as a base-forming element.</b> As has been
+stated previously, manganese forms two series of salts. The most
+important of these salts, all of which belong to the manganous series,
+are the following:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Manganous chloride</td><td align='left'>MnCl<sub>2</sub>&middot;4H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Manganous sulphide</td><td align='left'>MnS.</td></tr>
+<tr><td align='left'>Manganous sulphate</td><td align='left'>MnSO<sub>4</sub>&middot;4H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Manganous carbonate</td><td align='left'>MnCO<sub>3</sub>.</td></tr>
+<tr><td align='left'>Manganous hydroxide</td><td align='left'>Mn(OH)<sub>2</sub>.</td></tr>
+</table></div>
+
+
+<p>The chloride and sulphate may be prepared by heating the dioxide with
+hydrochloric and sulphuric acids respectively:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">MnO<sub>2</sub> + 4HCl = MnCl<sub>2</sub> + 2H<sub>2</sub>O + 2Cl,<br /></span>
+</div><div class="stanza">
+<span class="i0">MnO<sub>2</sub> + H<sub>2</sub>SO<sub>4</sub> = MnSO<sub>4</sub> + H<sub>2</sub>O + O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_381" id="Page_381">[Pg 381]</a></span></p>
+<p>The sulphide, carbonate, and hydroxide, being insoluble, may be prepared
+from a solution of the chloride or sulphate by precipitation with the
+appropriate reagents. Most of the manganous salts are rose colored. They
+not only have formulas similar to the ferrous salts, but resemble them
+in many of their chemical properties.</p>
+
+<p><b>Compounds containing manganese as an acid-forming element.</b> Manganese
+forms two unstable acids, namely, manganic acid and permanganic acid.
+While these acids are of little interest, some of their salts,
+especially the permanganates, are important compounds.</p>
+
+<p><b>Manganic acid and manganates.</b> When manganese dioxide is fused with an
+alkali and an oxidizing agent a green compound is formed. The equation,
+when caustic potash is used, is as follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">MnO<sub>2</sub> + 2KOH + O = K<sub>2</sub>MnO<sub>4</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The green compound (K<sub>2</sub>MnO<sub>4</sub>) is called potassium manganate, and is
+a salt of the unstable manganic acid (H<sub>2</sub>MnO<sub>4</sub>). The manganates are
+all very unstable.</p>
+
+<p><b>Permanganic acid and the permanganates.</b> When carbon dioxide is passed
+through a solution of a manganate a part of the manganese is changed
+into manganese dioxide, while the remainder forms a salt of the unstable
+acid HMnO<sub>4</sub>, called permanganic acid. The equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">3K<sub>2</sub>MnO<sub>4</sub> + 2CO<sub>2</sub> = MnO<sub>2</sub> + 2KMnO<sub>4</sub> + 2K<sub>2</sub>CO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Potassium permanganate (KMnO<sub>4</sub>) crystallizes in purple-black needles
+and is very soluble in water, forming an intensely purple solution. All
+other permanganates, as well as permanganic acid itself, give solutions
+of the same color.<span class='pagenum'><a name="Page_382" id="Page_382">[Pg 382]</a></span></p>
+
+<p><b>Oxidizing properties of the permanganates.</b> The permanganates are
+remarkable for their strong oxidizing properties. When used as an
+oxidizing agent the permanganate is itself reduced, the exact character
+of the products formed from it depending upon whether the oxidation
+takes place (1) in an alkaline or neutral solution, or (2) in an acid
+solution.</p>
+
+<p>1. <i>Oxidation in alkaline or neutral solution.</i> When the solution is
+either alkaline or neutral the potassium and the manganese of the
+permanganate are both converted into hydroxides, as shown in the
+equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2KMnO<sub>4</sub> + 5H<sub>2</sub>O = 2Mn(OH)<sub>4</sub> + 2KOH + 3O.<br /></span>
+</div></div>
+
+<p>2. <i>Oxidation in acid solution.</i> When free acid such as sulphuric is
+present, the potassium and the manganese are both changed into salts of
+the acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2KMnO<sub>4</sub> + 3H<sub>2</sub>SO<sub>4</sub> = K<sub>2</sub>SO<sub>4</sub> + 2MnSO<sub>4</sub> + 3H<sub>2</sub>O + 5O.<br /></span>
+</div></div>
+
+<p>Under ordinary conditions, however, neither one of these reactions takes
+place except in the presence of a third substance which is capable of
+oxidation. The oxygen is not given off in the free state, as the
+equations show, but is used up in effecting oxidation.</p>
+
+<p>Potassium permanganate is particularly valuable as an oxidizing agent
+not only because it acts readily either in acid or in alkaline solution,
+but also because the reaction takes place so easily that often it is not
+even necessary to heat the solution to secure action. The substance
+finds many uses in the laboratory, especially in analytical work. It is
+also used as an antiseptic as well as a disinfectant.<span class='pagenum'><a name="Page_383" id="Page_383">[Pg 383]</a></span></p>
+
+
+<h3>CHROMIUM</h3>
+
+<p><b>Occurrence.</b> The ore from which all chromium compounds are made is
+chromite, or chrome iron ore (FeCr<sub>2</sub>O<sub>4</sub>). This is found most
+abundantly in New Caledonia and Turkey. The element also occurs in small
+quantities in many other minerals, especially in crocoisite (PbCrO<sub>4</sub>),
+in which mineral it was first discovered.</p>
+
+<p><b>Preparation.</b> Chromium, like manganese, is very hard to reduce from its
+ores, owing to its great affinity for oxygen. It can, however, be made
+by the same methods which have proved successful with manganese.
+Considerable quantities of an alloy of chromium with iron, called
+ferrochromium, are now produced for the steel industry.</p>
+
+<p><b>Properties.</b> Chromium is a very hard metal of about the same density as
+iron. It is one of the most infusible of the metals, requiring a
+temperature little short of 3000&deg; for fusion. At ordinary temperatures
+air has little action on it; at higher temperatures, however, it burns
+brilliantly. Nitric acid has no action on it, but hydrochloric and
+dilute sulphuric acids dissolve it, liberating hydrogen.</p>
+
+<p><b>Compounds containing chromium as a base-forming element.</b> While chromium
+forms two series of salts, chromous salts are difficult to prepare and
+are of little importance. The most important of the chromic series are
+the following:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Chromic hydroxide</td><td align='left'>Cr(OH)<sub>3</sub>.</td></tr>
+<tr><td align='left'>Chromic chloride</td><td align='left'>CrCl<sub>3</sub>&middot;6H<sub>2</sub>O.</td></tr>
+<tr><td align='left'>Chromic sulphate</td><td align='left'>Cr<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>.</td></tr>
+<tr><td align='left'>Chrome alums</td></tr>
+</table></div>
+
+<p><b>Chromic hydroxide</b> (Cr(OH)<sub>3</sub>). This substance, being insoluble, can be
+obtained by precipitating a solution of the chloride or sulphate with a
+soluble hydroxide. It is a<span class='pagenum'><a name="Page_384" id="Page_384">[Pg 384]</a></span> greenish substance which, like aluminium
+hydroxide, dissolves in alkalis, forming soluble salts.</p>
+
+<div class="blockquot"><p><b>Dehydration of chromium hydroxide.</b> When heated gently chromic
+hydroxide loses a part of its oxygen and hydrogen, forming the
+substance CrO&middot;OH, which, like the corresponding aluminium
+compound, has more pronounced acid properties than the
+hydroxide. It forms a series of salts very similar to the
+spinels; chromite is the ferrous salt of this acid, having the
+formula Fe(CrO<sub>2</sub>)<sub>2</sub>. When heated to a higher temperature
+chromic hydroxide is completely dehydrated, forming the
+trioxide Cr<sub>2</sub>O<sub>3</sub>. This resembles the corresponding oxides
+of aluminium and iron in many respects. It is a bright green
+powder, and when ignited strongly becomes almost insoluble in
+acids, as is also the case with aluminium oxide.</p></div>
+
+<p><b>Chromic sulphate</b> (Cr<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>). This compound is a violet-colored
+solid which dissolves in water, forming a solution of the same color.
+This solution, however, turns green on heating, owing to the formation
+of basic salts. Chromic sulphate, like ferric and aluminium sulphates,
+unites with the sulphates of the alkali metals to form alums, of which
+the best known are potassium chrome alum (KCr(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O) and
+ammonium chrome alum (NH<sub>4</sub>Cr(SO<sub>4</sub>)<sub>2</sub>&middot;12H<sub>2</sub>O).</p>
+
+<p>These form beautiful dark purple crystals and have some practical uses
+in the tanning industry and in photography. A number of the salts of
+chromium are also used in the dyeing industry, for they hydrolyze like
+aluminium salts and the hydroxide forms a good mordant.</p>
+
+<div class="blockquot"><p><b>Hydrolysis of chromium salts.</b> When ammonium sulphide is added
+to a solution of a chromium salt, such as the sulphate,
+chromium hydroxide precipitates instead of the sulphide. This
+is due to the fact that chromic sulphide, like aluminium
+sulphide, hydrolyzes in the presence of water, forming chromic
+hydroxide and hydrosulphuric acid. Similarly, a soluble
+carbonate precipitates a basic carbonate of chromium.</p></div><p><span class='pagenum'><a name="Page_385" id="Page_385">[Pg 385]</a></span></p>
+
+<p><b>Compounds containing chromium as an acid-forming element.</b> Like
+manganese, chromium forms two unstable acids, namely, chromic acid and
+dichromic acid. Their salts, the chromates and dichromates, are
+important compounds.</p>
+
+<p><b>Chromates.</b> When a chromium compound is fused with an alkali and an
+oxidizing agent a chromate is produced. When potassium hydroxide is used
+as the alkali the equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Cr(OH)<sub>3</sub> + 4KOH + 3O = 2K<sub>2</sub>CrO<sub>4</sub> + 5H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>This reaction recalls the formation of a manganate under similar
+conditions.</p>
+
+<p><b>Properties of chromates.</b> The chromates are salts of the unstable chromic
+acid (H<sub>2</sub>CrO<sub>4</sub>), and as a rule are yellow in color. Lead chromate
+(PbCrO<sub>4</sub>) is the well-known pigment chrome yellow. Most of the
+chromates are insoluble and can therefore be prepared by precipitation.
+Thus, when a solution of potassium chromate is added to solutions of
+lead nitrate and barium nitrate respectively, the reactions expressed by
+the following equations occur:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Pb(NO<sub>3</sub>)<sub>2</sub> + K<sub>2</sub>CrO<sub>4</sub> = PbCrO<sub>4</sub> + 2KNO<sub>3</sub>,<br /></span>
+</div><div class="stanza">
+<span class="i0">Ba(NO<sub>3</sub>)<sub>2</sub> + K<sub>2</sub>CrO<sub>4</sub> = BaCrO<sub>4</sub> + 2KNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>The chromates of lead and barium separate as yellow precipitates. The
+presence of either of these two metals can be detected by taking
+advantage of these reactions.</p>
+
+<p><b>Dichromates.</b> When potassium chromate is treated with an acid the
+potassium salt of the unstable dichromic acid (H<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>) is
+formed:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2K<sub>2</sub>CrO<sub>4</sub> + H<sub>2</sub>SO<sub>4</sub> = K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + K<sub>2</sub>SO<sub>4</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_386" id="Page_386">[Pg 386]</a></span></p>
+<p>The relation between the chromates and dichromates is the same as that
+between the phosphates and the pyrophosphates. Potassium dichromate
+might therefore be called potassium pyrochromate.</p>
+
+<p><b>Potassium dichromate</b> (K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>). This is the best known
+dichromate, and is the most familiar chromium compound. It forms large
+crystals of a brilliant red color, and is rather sparingly soluble in
+water. When treated with potassium hydroxide it is converted into the
+chromate</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 2KOH = 2K<sub>2</sub>CrO<sub>4</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>When added to a solution of lead or barium salt the corresponding
+chromates (not dichromates) are precipitated. With barium nitrate the
+equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Ba(NO<sub>3</sub>)<sub>2</sub> + K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + H<sub>2</sub>O = 2BaCrO<sub>4</sub> + 2KNO<sub>3</sub> + 2HNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>Potassium dichromate finds use in many industries as an oxidizing agent,
+especially in the preparation of organic substances, such as the dye
+alizarin, and in the construction of several varieties of electric
+batteries.</p>
+
+<div class="blockquot"><p><b>Sodium chromates.</b> The reason why the potassium salt rather than
+the sodium compound is used is that sodium chromate and
+dichromate are so soluble that it is hard to prepare them pure.
+This difficulty is being overcome now, and the sodium compounds
+are replacing the corresponding potassium salts. This is of
+advantage, since a sodium salt is cheaper than a potassium
+salt, so far as raw materials go.</p></div>
+
+<p><b>Oxidizing action of chromates and dichromates.</b> When a dilute solution of
+a chromate or dichromate is acidified with an acid, such as sulphuric
+acid, no reaction apparently takes place. However, if there is present a
+third substance capable of oxidation, the chromium compound gives up a<span class='pagenum'><a name="Page_387" id="Page_387">[Pg 387]</a></span>
+portion of its oxygen to this substance. Since the chromate changes into
+a dichromate in the presence of an acid, it will be sufficient to study
+the action of the dichromates alone. The reaction takes place in two
+steps. Thus, when a solution of ferrous sulphate is added to a solution
+of potassium dichromate acidified with sulphuric acid, the reaction is
+expressed by the following equations:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">(1) K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 4H<sub>2</sub>SO<sub>4</sub> = K<sub>2</sub>SO<sub>4</sub> + Cr<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 4H<sub>2</sub>O + 3O,<br /></span>
+</div><div class="stanza">
+<span class="i0">(2) 6FeSO<sub>4</sub> + 3H<sub>2</sub>SO<sub>4</sub> + 3O = 3Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The dichromate decomposes in very much the same way as a permanganate
+does, the potassium and chromium being both changed into salts in which
+they play the part of metals, while part of the oxygen of the dichromate
+is liberated.</p>
+
+<p>By combining equations (1) and (2), the following is obtained:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 7H<sub>2</sub>SO<sub>4</sub> + 6FeSO<sub>4</sub> = K<sub>2</sub>SO<sub>4</sub> + Cr<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 3Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 7H<sub>2</sub>0.<br /></span>
+</div></div>
+
+<p>This reaction is often employed in the estimation of iron in iron ores.</p>
+
+<div class="blockquot"><p><b>Potassium chrome alum.</b> It will be noticed that the oxidizing
+action of potassium dichromate leaves potassium sulphate and
+chromium sulphate as the products of the reaction. On
+evaporating the solution these substances crystallize out as
+potassium chrome alum, which substance is produced as a
+by-product in the industries using potassium dichromate for
+oxidizing purposes.</p></div>
+
+<p><b>Chromic anhydride</b> (CrO<sub>3</sub>). When concentrated sulphuric acid is added
+to a strong solution of potassium dichromate, and the liquid allowed to
+stand, deep red needle-shaped crystals appear which have the formula
+<span class='pagenum'><a name="Page_388" id="Page_388">[Pg 388]</a></span>CrO<sub>3</sub>.This oxide of chromium is called chromic anhydride, since it
+combines readily with water to form chromic acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CrO<sub>3</sub> + H<sub>2</sub>O = H<sub>2</sub>CrO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>It is therefore analogous to sulphur trioxide which forms sulphuric acid
+in a similar way:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">SO<sub>3</sub> + H<sub>2</sub>O = H<sub>2</sub>SO<sub>4</sub>.<br /></span>
+</div></div>
+
+<p>Chromic anhydride is a very strong oxidizing agent, giving up oxygen and
+forming chromic oxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2CrO<sub>3</sub> = Cr<sub>2</sub>O<sub>3</sub> + 3O.<br /></span>
+</div></div>
+
+<div class="blockquot"><p><b>Rare elements of the family.</b> Molybdenum, tungsten, and uranium
+are three rather rare elements belonging in the same family
+with chromium, and form many compounds which are similar in
+formulas to the corresponding compounds of chromium. They can
+play the part of metals and also form acids resembling chromic
+acid in formula. Thus we have molybdic acid (H<sub>2</sub>MoO<sub>4</sub>), the
+ammonium salt of which is (NH<sub>4</sub>)<sub>2</sub>MoO<sub>4</sub>. This salt has
+the property of combining with phosphoric acid to form a very
+complex substance which is insoluble in nitric acid. On this
+account molybdic acid is often used in the estimation of the
+phosphoric acid present in a substance. Like chromium, the
+metals are difficult to prepare in pure condition. Alloys with
+iron can be prepared by reducing the mixed oxides with carbon
+in an electric furnace; these alloys are used to some extent in
+preparing special kinds of steel.</p></div>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> How does pyrolusite effect the decolorizing of glass containing iron?</p>
+
+<p><b>2.</b> Write the equations for the preparation of manganous chloride,
+carbonate, and hydroxide.</p>
+
+<p><b>3.</b> Write the equations representing the reactions which take place when
+ferrous sulphate is oxidized to ferric sulphate by potassium
+permanganate in the presence of sulphuric acid.<span class='pagenum'><a name="Page_389" id="Page_389">[Pg 389]</a></span></p>
+
+<p><b>4.</b> In the presence of sulphuric acid, oxalic acid is oxidized by
+potassium permanganate according to the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>2</sub>H<sub>2</sub>O<sub>4</sub> + O = 2CO<sub>2</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Write the complete equation.</p>
+
+<p><b>5.</b> 10 g. of iron were dissolved in sulphuric acid and oxidized to ferric
+sulphate by potassium permanganate. What weight of the permanganate was
+required?</p>
+
+<p><b>6.</b> What weight of ferrochromium containing 40% chromium must be added to
+a ton of steel to produce an alloy containing 1% of chromium?</p>
+
+<p><b>7.</b> Write the equation representing the action of ammonium sulphide upon
+chromium sulphate.</p>
+
+<p><b>8.</b> Potassium chromate oxidizes hydrochloric acid, forming chlorine.
+Write the complete equation.</p>
+
+<p><b>9.</b> Give the action of sulphuric acid on potassium dichromate (<i>a</i>) in
+the presence of a large amount of water; (<i>b</i>) in the presence of a
+small amount of water.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_390" id="Page_390">[Pg 390]</a></span></p>
+<h2><a name="CHAPTER_XXXI" id="CHAPTER_XXXI"></a>CHAPTER XXXI</h2>
+
+<h3>GOLD AND THE PLATINUM FAMILY</h3>
+
+<div class='center'>
+<table border="1" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>SYMBOL</td><td align='left'>ATOMIC WEIGHT</td><td align='left'>DENSITY</td><td align='left'>HIGHEST OXIDE</td><td align='left'>HIGHEST CHLORIDE</td><td align='left'>MELTING POINT</td></tr>
+<tr><td align='left'>Ruthenium</td><td align='center'>Ru</td><td align='center'>101.7</td><td align='center'>12.26</td><td align='center'>RuO<sub>4</sub></td><td align='center'>RuCl<sub>4</sub></td><td align='center'>Electric arc</td></tr>
+<tr><td align='left'>Rhodium</td><td align='center'>Rh</td><td align='center'>103.</td><td align='center'>12.1</td><td align='center'>RhO<sub>2</sub></td><td align='center'>RhCl<sub>2</sub></td><td align='center'>Electric arc</td></tr>
+<tr><td align='left'>Palladium</td><td align='center'>Pd</td><td align='center'>106.5</td><td align='center'>11.8</td><td align='center'>PdO<sub>2</sub></td><td align='center'>PdCl<sub>4</sub></td><td align='center'>1500&deg;</td></tr>
+<tr><td align='left'>Iridium</td><td align='center'>Ir</td><td align='center'>193.</td><td align='center'>22.42</td><td align='center'>IrO<sub>2</sub></td><td align='center'>IrCl<sub>4</sub></td><td align='center'>1950&deg;</td></tr>
+<tr><td align='left'>Osmium</td><td align='center'>Os</td><td align='center'>191.</td><td align='center'>22.47</td><td align='center'>OsO<sub>4</sub></td><td align='center'>OsCl<sub>4</sub></td><td align='center'>Electric arc</td></tr>
+<tr><td align='left'>Platinum</td><td align='center'>Pt</td><td align='center'>194.8</td><td align='center'>21.50</td><td align='center'>PtO<sub>2</sub></td><td align='center'>PtCl<sub>4</sub></td><td align='center'>1779&deg;</td></tr>
+<tr><td align='left'>Gold</td><td align='center'>Au</td><td align='center'>197.2</td><td align='center'>19.30</td><td align='center'>Au<sub>2</sub>O<sub>3</sub></td><td align='center'>AuCl<sub>3</sub></td><td align='center'>1064&deg;</td></tr>
+</table></div>
+<p><b>The family.</b> Following iron, nickel, and cobalt in the eighth column of
+the periodic table are two groups of three elements each. The metals of
+the first of these groups&mdash;ruthenium, rhodium, and palladium&mdash;have
+atomic weights near 100 and densities near 12. The metals of the other
+group&mdash;iridium, osmium, and platinum&mdash;have atomic weights near 200 and
+densities near 21. These six rare elements have very similar physical
+properties and resemble each other chemically not only in the type of
+compounds which they form but also in the great variety of them. They
+occur closely associated in nature, usually as alloys of platinum in the
+form of irregular metallic grains in sand and gravel. Platinum is by far
+the most abundant of the six.</p>
+
+<p>Although the periodic classification assigns gold to the silver-copper
+group, its physical as well as many of its<span class='pagenum'><a name="Page_391" id="Page_391">[Pg 391]</a></span> chemical properties much
+more closely resemble those of the platinum metals, and it can he
+conveniently considered along with them. The four elements gold,
+platinum, osmium, and iridium are the heaviest substances known, being
+about twice as heavy as lead.</p>
+
+
+<h3>PLATINUM</h3>
+
+<p><b>Occurrence.</b> About 90% of the platinum of commerce comes from Russia,
+small amounts being produced in California, Brazil, and Australia.</p>
+
+<p><b>Preparation.</b> Native platinum is usually alloyed with gold and the
+platinum metals. To separate the platinum the alloy is dissolved in aqua
+regia, which converts the platinum into chloroplatinic acid
+(H<sub>2</sub>PtCl<sub>6</sub>). Ammonium chloride is then added, which precipitates the
+platinum as insoluble ammonium chloroplatinate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">H<sub>2</sub>PtCl<sub>6</sub> + 2NH<sub>4</sub>Cl = (NH<sub>4</sub>)<sub>2</sub>PtCl<sub>6</sub> + 2HCl.<br /></span>
+</div></div>
+
+<p>Some iridium is also precipitated as a similar compound. On ignition the
+double chloride is decomposed, leaving the platinum as a spongy metallic
+mass, which is melted in an electric furnace and rolled or hammered into
+the desired shape.</p>
+
+<p><b>Physical properties.</b> Platinum is a grayish-white metal of high luster,
+and is very malleable and ductile. It melts in the oxyhydrogen blowpipe
+and in the electric furnace; it is harder than gold and is a good
+conductor of electricity. In finely divided form it has the ability to
+absorb or occlude gases, especially oxygen and hydrogen. These gases,
+when occluded, are in a very active condition resembling the nascent
+state, and can combine with each other at ordinary<span class='pagenum'><a name="Page_392" id="Page_392">[Pg 392]</a></span> temperatures. A jet
+of hydrogen or coal gas directed upon spongy platinum is at once
+ignited.</p>
+
+<div class="blockquot"><p><b>Platinum as a catalytic agent.</b> Platinum is remarkable for its
+property of acting as a catalytic agent in a large number of
+chemical reactions, and mention has been made of this use of
+the metal in connection with the manufacture of sulphuric acid.
+When desired for this purpose some porous or fibrous substance,
+such as asbestos, is soaked in a solution of platinic chloride
+and then ignited. The platinum compound is decomposed and the
+platinum deposited in very finely divided form. Asbestos
+prepared in this way is called platinized asbestos. The
+catalytic action seems to be in part connected with the
+property of absorbing gases and rendering them nascent. Some
+other metals possess this same power, notably palladium, which
+is remarkable for its ability to absorb hydrogen.</p></div>
+
+<p><b>Chemical properties.</b> Platinum is a very inactive element chemically, and
+is not attacked by any of the common acids. Aqua regia slowly dissolves
+it, forming platinic chloride (PtCl<sub>4</sub>), which in turn unites with the
+hydrochloric acid present in the aqua regia, forming the compound
+chloroplatinic acid (H<sub>2</sub>PtCl<sub>6</sub>). Platinum is attacked by fused
+alkalis. It combines at higher temperatures with carbon and phosphorus
+and alloys with many metals. It is readily attacked by chlorine but not
+by oxidizing agents.</p>
+
+<p><b>Applications.</b> Platinum is very valuable as a material for the
+manufacture of chemical utensils which are required to stand a high
+temperature or the action of strong reagents. Platinum crucibles,
+dishes, forceps, electrodes, and similar articles are indispensable in
+the chemical laboratory. In the industries it is used for such purposes
+as the manufacture of pans for evaporating sulphuric acid, wires for
+sealing through incandescent light bulbs, and for making a great variety
+of instruments. Unfortunately the supply<span class='pagenum'><a name="Page_393" id="Page_393">[Pg 393]</a></span> of the metal is very limited,
+and the cost is steadily advancing, so that it is now more valuable than
+gold.</p>
+
+<p><b>Compounds.</b> Platinum forms two series of salts of which platinous
+chloride (PtCl<sub>2</sub>) and platinic chloride (PtCl<sub>4</sub>) are examples.
+Platinates are also known. While a great variety of compounds of
+platinum have been made, the substance is chiefly employed in the
+metallic state.</p>
+
+<p><b>Platinic chloride (PtCl<sub>4</sub>).</b> Platinic chloride is an orange-colored,
+soluble compound made by heating chloroplatinic acid in a current of
+chlorine. If hydrochloric acid is added to a solution of the substance,
+the two combine, forming chloroplatinic acid (H<sub>2</sub>PtCl<sub>6</sub>):</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2HCl + PtCl<sub>4</sub> = H<sub>2</sub>PtCl<sub>6</sub>.<br /></span>
+</div></div>
+
+<p>The potassium and ammonium salts of this acid are nearly insoluble in
+water and alcohol. The acid is therefore used as a reagent to
+precipitate potassium in analytical work. With potassium chloride the
+equation is</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2KCl + H<sub>2</sub>PtCl<sub>6</sub> = K<sub>2</sub>PtCl<sub>6</sub> + 2HCl.<br /></span>
+</div></div>
+
+<div class="blockquot"><p><b>Other metals of the family.</b> The other members of the family
+have few applications. Iridium is used in the form of a
+platinum alloy, since the alloy is much harder than pure
+platinum and is even less fusible. This alloy is sometimes used
+to point gold pens. Osmium tetroxide (OsO<sub>4</sub>) is a very
+volatile liquid and is used under the name of osmic acid as a
+stain for sections in microscopy.</p></div>
+
+
+<h3>GOLD</h3>
+
+<p><b>Occurrence.</b> Gold has been found in many localities, the most famous
+being South Africa, Australia, Russia, and the United States. In this
+country it is found in Alaska and in nearly half of the states of the
+union, notably<span class='pagenum'><a name="Page_394" id="Page_394">[Pg 394]</a></span> in California, Colorado, and Nevada. It is usually found
+in the native condition, frequently alloyed with silver; in combination
+it is sometimes found as telluride (AuTe<sub>2</sub>), and in a few other
+compounds.</p>
+
+<p><b>Mining.</b> Native gold occurs in the form of small grains or larger nuggets
+in the sands of old rivers, or imbedded in quartz veins in rocks. In the
+first case it is obtained in crude form by placer mining. The sand
+containing the gold is shaken or stirred in troughs of running waters
+called sluices. This sweeps away the sand but allows the heavier gold to
+sink to the bottom of the sluice. Sometimes the sand containing the gold
+is washed away from its natural location into the sluices by powerful
+streams of water delivered under pressure from pipes. This is called
+hydraulic mining. In vein mining the gold-bearing quartz is mined from
+the veins, stamped into fine powder in stamping mills, and the gold
+extracted by one of the processes to be described.</p>
+
+<p><b>Extraction.</b> 1. <i>Amalgamation process.</i> In the amalgamation process the
+powder containing the gold is washed over a series of copper plates
+whose surfaces have been amalgamated with mercury. The gold sticks to
+the mercury or alloys with it, and after a time the gold and mercury are
+scraped off and the mixture is distilled. The mercury distills off and
+the gold is left in the retort ready for refining.</p>
+
+<p>2. <i>Chlorination process.</i> When gold occurs along with metallic
+sulphides it is often extracted by chlorination. The ore is first
+roasted, and is then moistened and treated with chlorine. This dissolves
+the gold but not the metallic oxides:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Au + 3Cl = AuCl<sub>3</sub>.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_395" id="Page_395">[Pg 395]</a></span></p>
+<p>The gold chloride, being soluble, is extracted from the mixture with
+water, and the gold is precipitated from the solution, usually by adding
+ferrous sulphate:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">AuCl<sub>3</sub> + 3FeSO<sub>4</sub> = Au + FeCl<sub>3</sub> + Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>3. <i>Cyanide process.</i> This process depends upon the fact that gold is
+soluble in a solution of potassium cyanide in the presence of the oxygen
+of the air. The powder from the stamping mills is treated with a very
+dilute potassium cyanide solution which extracts the gold:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2Au + 4KCN + H<sub>2</sub>O + O = 2KOH + 2KAu(CN)<sub>2</sub>.<br /></span>
+</div></div>
+
+<p>From this solution the gold can be obtained by electrolysis or by
+precipitation with metallic zinc:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2KAu(CN)<sub>2</sub> + Zn = K<sub>2</sub>Zn(CN)<sub>4</sub> + 2Au.<br /></span>
+</div></div>
+
+<p><b>Refining of gold.</b> Gold is refined by three general methods:</p>
+
+<p>1. <i>Electrolysis.</i> When gold is dissolved in a solution of potassium
+cyanide, and the solution electrolyzed, the gold is deposited in very
+pure condition on the cathode.</p>
+
+<p>2. <i>Cupellation.</i> When the gold is alloyed with easily oxidizable
+metals, such as copper or lead, it may be refined by cupellation. The
+alloy is fused with an oxidizing flame on a shallow hearth made of bone
+ash, which substance has the property of absorbing metallic oxides but
+not the gold. Any silver which may be present remains alloyed with the
+gold.</p>
+
+<p>3. <i>Parting with sulphuric acid.</i> Gold may be separated from silver, as
+well as from many other metals, by heating the alloy with concentrated
+sulphuric acid. This dissolves the silver, while the gold is not
+attacked.<span class='pagenum'><a name="Page_396" id="Page_396">[Pg 396]</a></span></p>
+
+<p><b>Physical properties.</b> Gold is a very heavy bright yellow metal,
+exceedingly malleable and ductile, and a good conductor of electricity.
+It is quite soft and is usually alloyed with copper or silver to give it
+the hardness required for most practical uses. The degree of fineness is
+expressed in terms of carats, pure gold being twenty-four carats; the
+gold used for jewelry is usually eighteen carats, eighteen parts being
+gold and six parts copper or silver. Gold coinage is 90% gold and 10%
+copper.</p>
+
+<p><b>Chemical properties.</b> Gold is not attacked by any one of the common
+acids; aqua regia easily dissolves it, forming gold chloride (AuCl<sub>3</sub>),
+which in turn combines with hydrochloric acid to form chlorauric acid
+(HAuCl<sub>4</sub>). Fused alkalis also attack it. Most oxidizing agents are
+without action upon it, and in general it is not an active element.</p>
+
+<div class="blockquot"><p><b>Compounds.</b> The compounds of gold, though numerous and varied in
+character, are of comparatively little importance and need not
+be described in detail. The element forms two series of salts
+in which it acts as a metal: in the aurous series the gold is
+univalent, the chloride having the formula AuCl; in the auric
+series it is trivalent, auric chloride having the formula
+AuCl<sub>3</sub>. Gold also acts as an acid-forming element, forming
+such compounds as potassium aurate (KAuO<sub>2</sub>). Its compounds
+are very easily decomposed, however, metallic gold separating
+from them.</p></div>
+
+
+<h3>EXERCISES</h3>
+
+<p><b>1.</b> From the method of preparation of platinum, what metal is likely to
+be alloyed with it?</p>
+
+<p><b>2.</b> The "platinum chloride" of the laboratory is made by dissolving
+platinum in aqua regia. What is the compound?</p>
+
+<p><b>3.</b> How would you expect potassium aurate and platinate to be formed?
+What precautions would this suggest in the use of platinum vessels?</p>
+
+<p><b>4.</b> Why must gold ores be roasted in the chlorination process?</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_397" id="Page_397">[Pg 397]</a></span></p>
+<h2><a name="CHAPTER_XXXII" id="CHAPTER_XXXII"></a>CHAPTER XXXII</h2>
+
+<h3>SOME SIMPLE ORGANIC COMPOUNDS</h3>
+
+
+<p><b>Division of chemistry into organic and inorganic.</b> Chemistry is usually
+divided into two great divisions,&mdash;organic and inorganic. The original
+significance of these terms was entirely different from the meaning
+which they have at the present time.</p>
+
+<p>1. <i>Original significance.</i> The division into organic and inorganic was
+originally made because it was believed that those substances which
+constitute the essential parts of living organisms were built up under
+the influence of the life force of the organism. Such substances,
+therefore, should be regarded as different from those compounds prepared
+in the laboratory or formed from the inorganic or mineral constituents
+of the earth. In accordance with this view organic chemistry included
+those substances formed by living organisms. Inorganic chemistry, on the
+other hand, included all substances formed from the mineral portions of
+the earth.</p>
+
+<p>In 1828 the German chemist W&ouml;hler prepared urea, a typical organic
+compound, from inorganic materials. The synthesis of other so-called
+organic compounds followed, and at present it is known that the same
+chemical laws apply to all substances whether formed in the living
+organism or prepared in the laboratory from inorganic constituents. The
+terms "organic" and "inorganic" have therefore lost their original
+significance.<span class='pagenum'><a name="Page_398" id="Page_398">[Pg 398]</a></span></p>
+
+<p>2. <i>Present significance.</i> The great majority of the compounds found in
+living organisms contain carbon, and the term "organic chemistry," as
+used at present, includes not only these compounds but all compounds of
+carbon. <i>Organic chemistry</i> has become, therefore, <i>the chemistry of the
+compounds of carbon</i>, all other substances being treated under the head
+of inorganic chemistry. This separation of the compounds of carbon into
+a group by themselves is made almost necessary by their great number,
+over one hundred thousand having been recorded. For convenience some of
+the simpler carbon compounds, such as the oxides and the carbonates, are
+usually discussed in inorganic chemistry.</p>
+
+<p><b>The grouping of compounds in classes.</b> The study of organic chemistry is
+much simplified by the fact that the large number of bodies included in
+this field may be grouped in classes of similar compounds. It thus
+becomes possible to study the properties of each class as a whole, in
+much the same way as we study a group of elements. The most important of
+these classes are the <i>hydrocarbons</i>, the <i>alcohols</i>, the <i>aldehydes</i>,
+the <i>acids</i>, the <i>ethereal salts</i>, the <i>ethers</i>, the <i>ketones</i>, the
+<i>organic bases</i>, and the <i>carbohydrates</i>. A few members of each of these
+classes will now be discussed briefly.</p>
+
+<h3>THE HYDROCARBONS</h3>
+
+<p>Carbon and hydrogen combine to form a large number of compounds. These
+compounds are known collectively as the <i>hydrocarbons</i>. They may be
+divided into a number of groups or series, each being named from its
+first member. Some of the groups are as follows:<span class='pagenum'><a name="Page_399" id="Page_399">[Pg 399]</a></span></p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>METHANE SERIES</td></tr>
+<tr><td align='left'>CH<sub>4</sub></td><td align='left'>methane</td></tr>
+<tr><td align='left'>C<sub>2</sub>H<sub>6</sub></td><td align='left'>ethane</td></tr>
+<tr><td align='left'>C<sub>3</sub>H<sub>8</sub></td><td align='left'>propane</td></tr>
+<tr><td align='left'>C<sub>4</sub>H<sub>10</sub></td><td align='left'>butane</td></tr>
+<tr><td align='left'>C<sub>5</sub>H<sub>12</sub></td><td align='left'>pentane</td></tr>
+<tr><td align='left'>C<sub>6</sub>H<sub>14</sub></td><td align='left'>hexane</td></tr>
+<tr><td align='left'>C<sub>7</sub>H<sub>16</sub></td><td align='left'>heptane</td></tr>
+<tr><td align='left'>C<sub>8</sub>H<sub>18</sub></td><td align='left'>octane</td></tr>
+<tr><td align='left'>ETHYLENE SERIES</td></tr>
+<tr><td align='left'>C<sub>2</sub>H<sub>4</sub></td><td align='left'>ethylene</td></tr>
+<tr><td align='left'>C<sub>3</sub>H<sub>6</sub></td><td align='left'>propylene</td></tr>
+<tr><td align='left'>C<sub>4</sub>H<sub>8</sub></td><td align='left'>butylene</td></tr>
+<tr><td align='left'>BENZENE SERIES</td></tr>
+<tr><td align='left'>C<sub>6</sub>H<sub>6</sub></td><td align='left'>benzene</td></tr>
+<tr><td align='left'>C<sub>7</sub>H<sub>8</sub></td><td align='left'>toluene</td></tr>
+<tr><td align='left'>C<sub>8</sub>H<sub>10</sub></td><td align='left'>xylene</td></tr>
+<tr><td align='left'>ACETYLENE SERIES</td></tr>
+<tr><td align='left'>C<sub>2</sub>H<sub>2</sub></td><td align='left'>acetylene</td></tr>
+<tr><td align='left'>C<sub>3</sub>H<sub>4</sub></td><td align='left'>allylene</td></tr>
+</table></div>
+<p>Only the lower members (that is, those which contain a small number of
+carbon atoms) of the above groups are given. The methane series is the
+most extensive, all of the compounds up to C<sub>24</sub>H<sub>50</sub> being known.</p>
+
+<p>It will be noticed that the successive members of each of the above
+series differ by the group of atoms (CH<sub>2</sub>). Such a series is called an
+<i>homologous series</i>. In general, it may be stated that the members of an
+homologous series show a regular gradation in most physical properties
+and are similar in chemical properties. Thus in the methane group the
+first four members are gases at ordinary temperatures; those containing
+from five to sixteen carbon atoms are liquids, the boiling points of
+which increase with the number of carbon atoms present. Those containing
+more than sixteen carbon atoms are solids.</p>
+
+<p><b>Sources of the hydrocarbons.</b> There are two chief sources of the
+hydrocarbons, namely, (1) crude petroleum and (2) coal tar.</p>
+
+<p>1. <i>Crude petroleum.</i> This is a liquid pumped from wells driven into the
+earth in certain localities. Pennsylvania, Ohio, Kansas, California, and
+Texas are the chief<span class='pagenum'><a name="Page_400" id="Page_400">[Pg 400]</a></span> oil-producing regions in the United States. The
+crude petroleum consists largely of liquid hydrocarbons in which are
+dissolved both gaseous and solid hydrocarbons. Before being used it must
+be refined. In this process the petroleum is run into large iron stills
+and subjected to fractional distillation. The various hydrocarbons
+distill over in the general order of their boiling points. The
+distillates which collect between certain limits of temperature are kept
+separate and serve for different uses; they are further purified,
+generally by washing with sulphuric acid, then with an alkali, and
+finally with water. Among the products obtained from crude petroleum in
+this way are the naphthas, including benzine and gasoline, kerosene or
+coal oil, lubricating oils, vaseline, and paraffin. None of these
+products are definite chemical compounds, but each consists of a mixture
+of hydrocarbons, the boiling points of which lie within certain limits.</p>
+
+<p>2. <i>Coal tar.</i> This product is obtained in the manufacture of coal gas,
+as already explained. It is a complex mixture and is refined by the same
+general method used in refining crude petroleum. The principal
+hydrocarbons obtained from the coal tar are benzene, toluene,
+naphthalene, and anthracene. In addition to the hydrocarbons, coal tar
+contains many other compounds, such as carbolic acid and aniline.</p>
+
+<p><b>Properties of the hydrocarbons.</b> The lower members of the first two
+series of hydrocarbons mentioned are all gases; the succeeding members
+are liquids. In some series, as the methane series, the higher members
+are solids. The preparation and properties of methane and acetylene have
+been discussed in a previous chapter. Ethylene is present in small
+quantities in coal gas and may be<span class='pagenum'><a name="Page_401" id="Page_401">[Pg 401]</a></span> obtained in the laboratory by
+treating alcohol (C<sub>2</sub>H<sub>6</sub>O) with sulphuric acid:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>2</sub>H<sub>6</sub>O = C<sub>2</sub>H<sub>4</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>Benzene, the first member of the benzene series, is a liquid boiling at
+80&deg;.</p>
+
+<p>The hydrocarbons serve as the materials from which a large number of
+compounds can be prepared; indeed, it has been proposed to call organic
+chemistry <i>the chemistry of the hydrocarbon derivatives</i>.</p>
+
+<p><b>Substitution products of the hydrocarbons.</b> As a rule, at least a part of
+the hydrogen in any hydrocarbon can be displaced by an equivalent amount
+of certain elements or groups of elements. Thus the compounds CH<sub>3</sub>Cl,
+CH<sub>2</sub>Cl<sub>2</sub>, CHCl<sub>3</sub>, CCl<sub>4</sub> can be obtained from methane by
+treatment with chlorine. Such compounds are called <i>substitution
+products</i>.</p>
+
+<p><b>Chloroform</b> (CHCl<sub>3</sub>). This can be made by treating methane with
+chlorine, as just indicated, although a much easier method consists in
+treating alcohol or acetone (which see) with bleaching powder.
+Chloroform is a heavy liquid having a pleasant odor and a sweetish
+taste. It is largely used as a solvent and as an an&aelig;sthetic in surgery.</p>
+
+<p><b>Iodoform</b> (CHI<sub>3</sub>). This is a yellow crystalline solid obtained by
+treating alcohol with iodine and an alkali. It has a characteristic odor
+and is used as an antiseptic.</p>
+
+
+<h3>ALCOHOLS</h3>
+
+<p>When such a compound as CH<sub>3</sub>Cl is treated with silver hydroxide the
+reaction expressed by the following equation takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">CH<sub>3</sub>Cl + AgOH = CH<sub>3</sub>OH + AgCl.<br /></span>
+</div></div>
+
+<p><span class='pagenum'><a name="Page_402" id="Page_402">[Pg 402]</a></span></p>
+<p>Similarly C<sub>2</sub>H<sub>5</sub>Cl will give C<sub>2</sub>H<sub>5</sub>OH and AgCl. The compounds
+CH<sub>3</sub>OH and C<sub>2</sub>H<sub>5</sub>OH so obtained belong to the class of substances
+known as <i>alcohols</i>. From their formulas it will be seen that they may
+be regarded as derived from hydrocarbons by substituting the hydroxyl
+group (OH) for hydrogen. Thus the alcohol CH<sub>3</sub>OH may be regarded as
+derived from methane (CH<sub>4</sub>) by substituting the group OH for one atom
+of hydrogen. A great many alcohols are known, and, like the
+hydrocarbons, they may be grouped into series. The relation between the
+first three members of the methane series and the corresponding alcohols
+is shown in the following table:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>CH<sub>4</sub></td><td align='left'>(methane)</td><td align='left'>CH<sub>3</sub>OH</td><td align='left'>(methyl alcohol).</td></tr>
+<tr><td align='left'>C<sub>2</sub>H<sub>6</sub></td><td align='left'>(ethane)</td><td align='left'>C<sub>2</sub>H<sub>5</sub>OH</td><td align='left'>(ethyl alcohol).</td></tr>
+<tr><td align='left'>C<sub>3</sub>H<sub>8</sub></td><td align='left'>(propane)</td><td align='left'>C<sub>3</sub>H<sub>7</sub>OH</td><td align='left'>(propyl alcohol).</td></tr>
+</table></div>
+
+
+<p><b>Methyl alcohol</b> (<i>wood alcohol</i>) (CH<sub>3</sub>OH). When wood is placed in an
+air-tight retort and heated, a number of compounds are evolved, the most
+important of which are the three liquids, methyl alcohol, acetic acid,
+and acetone. Methyl alcohol is obtained entirely from this source, and
+on this account is commonly called <i>wood alcohol</i>. It is a colorless
+liquid which has a density of 0.79 and boils at 67&deg;. It burns with an
+almost colorless flame and is sometimes used for heating purposes, in
+place of the more expensive ethyl alcohol. It is a good solvent for
+organic substances and is used especially as a solvent in the
+manufacture of varnishes. It is very poisonous.</p>
+
+<p><b>Ethyl alcohol</b> (<i>common alcohol</i>) (C<sub>2</sub>H<sub>5</sub>OH). 1. <i>Preparation.</i> This
+compound may be prepared from glucose (C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>), a sugar easily
+obtained from starch. If some baker's yeast is added to a solution of
+glucose and the temperature is maintained at about 30&deg;, bubbles of gas
+are<span class='pagenum'><a name="Page_403" id="Page_403">[Pg 403]</a></span> soon evolved, showing that a change is taking place. The yeast
+contains a large number of minute organized bodies, which are really
+forms of plant life. The plant grows in the glucose solution, and in so
+doing secretes a substance known as <i>zymase</i>, which breaks down the
+glucose in accordance with the following equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> = 2C<sub>2</sub>H<sub>5</sub>OH + 2CO<sub>2</sub>.<br /></span>
+</div></div>
+
+<div class="blockquot"><p><b>Laboratory preparation of alcohol.</b> The formation of alcohol and
+carbon dioxide from glucose may be shown as follows: About 100
+g. of glucose are dissolved in a liter of water in flask <i>A</i>
+(Fig. 90). This flask is connected with the bottle <i>B</i>, which
+is partially filled with limewater. The tube <i>C</i> contains solid
+sodium hydroxide. A little baker's yeast is now added to the
+solution in flask <i>A</i>, and the apparatus is connected, as shown
+in the figure. If the temperature is maintained at about 30&deg;,
+the reaction soon begins. The bubbles of gas escape through the
+limewater in <i>B</i>. A precipitate of calcium carbonate soon forms
+in the limewater, showing the presence of carbon dioxide. The
+sodium hydroxide in tube <i>C</i> prevents the carbon dioxide in the
+air from acting on the limewater. The alcohol remains in the
+flask <i>A</i> and may be separated by fractional distillation.</p></div>
+
+<div class="figcenter" style="width: 400px;">
+<img src="images/imagefig90.jpg" width="400" height="276" alt="Fig. 90" title="" />
+<span class="caption">Fig. 90</span>
+</div>
+
+<p>2. <i>Properties.</i> Ethyl alcohol is a colorless liquid with a pleasant
+odor. It has a density of 0.78 and boils at 78&deg;. It resembles methyl
+alcohol in its general properties. It is sometimes used as a source of
+heat, since its flame is very hot and does not deposit carbon, as the
+flame from oil does. When taken into the system in small quantities<span class='pagenum'><a name="Page_404" id="Page_404">[Pg 404]</a></span> it
+causes intoxication; in larger quantities it acts as a poison. The
+intoxicating properties of such liquors as beer, wine, and whisky are
+due to the alcohol present. Beer contains from 2 to 5% of alcohol, wine
+from 5 to 20%, and whisky about 50%. The ordinary alcohol of the
+druggist contains 94% of alcohol and 6% of water. When this is boiled
+with lime and then distilled nearly all the water is removed, the
+distillate being called <i>absolute alcohol</i>.</p>
+
+<div class="blockquot"><p><b>Commercial preparation of alcohol.</b> Alcohol is prepared
+commercially from starch obtained from corn or potatoes. The
+starch is first converted into a sugar known as maltose, by the
+action of <i>malt</i>, a substance prepared by moistening barley
+with water, allowing it to germinate, and then drying it. There
+is present in the malt a substance known as diastase, which has
+the property of changing starch into maltose. This sugar, like
+glucose, breaks down into alcohol and carbon dioxide in the
+presence of yeast. The resulting alcohol is separated by
+fractional distillation.</p>
+
+<p><b>Denatured alcohol.</b> The 94% alcohol is prepared at present at a
+cost of about 35 cents per gallon, which is about half the cost
+of the preparation of methyl alcohol. The government, however,
+imposes a tax on all ethyl alcohol which amounts to $2.08 per
+gallon on the 94% product. This increases its cost to such an
+extent that it is not economical to use it for many purposes
+for which it is adapted, such as a solvent in the preparation
+of paints and varnishes and as a material for the preparation
+of many important organic compounds. By an act of Congress in
+1906, the tax was removed from <i>denatured</i> alcohol, that is
+alcohol mixed with some substance which renders it unfit for
+the purposes of a beverage but will not impair its use for
+manufacturing purposes. Some of the European countries have
+similar laws. The substances ordinarily used to denature
+alcohol are wood alcohol and pyridine, the latter compound
+having a very offensive odor.</p>
+
+<p><b>Fermentation.</b> The reaction which takes place in the preparation
+of ethyl alcohol belongs to the class of changes known under
+the general name of fermentation. Thus we say that the yeast
+causes the glucose to ferment, and the process is known as
+alcoholic fermentation. There are many kinds of fermentations,
+and each is thought to be due to the presence of a definite
+substance known<span class='pagenum'><a name="Page_405" id="Page_405">[Pg 405]</a></span> as an <i>enzyme</i>, which acts by catalysis. In
+many cases, as in alcoholic fermentation, the change is brought
+about by the action of minute forms of life. These probably
+secrete the enzymes which cause the fermentation to take place.
+Thus the yeast plant is supposed to bring about alcoholic
+fermentation by secreting the enzyme known as zymase.</p></div>
+
+<p><b>Glycerin</b> (C<sub>3</sub>H<sub>5</sub>(OH)<sub>3</sub>). This compound may be regarded as derived
+from propane (C<sub>3</sub>H<sub>8</sub>) by displacing three atoms of hydrogen by three
+hydroxyl groups, and must therefore be regarded as an alcohol. It is
+formed in the manufacture of soaps, as will be explained later. It is an
+oily, colorless liquid having a sweetish taste. It is used in medicine
+and in the manufacture of the explosives nitroglycerin and dynamite.</p>
+
+
+<h3>ALDEHYDES</h3>
+
+<p>When alcohols are treated with certain oxidizing agents two hydrogen
+atoms are removed from each molecule of the alcohol. The resulting
+compounds are known as aldehydes. The relation of the aldehydes derived
+from methyl and ethyl alcohol to the alcohols themselves may be shown as
+follows:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td rowspan="2" align='left'>Alcohols</td><td align='left'>{CH<sub>3</sub>OH</td><td rowspan="2" align='left'>Corresponding aldehydes</td><td align='left'>{CH<sub>2</sub>O</td></tr>
+<tr><td align='left'>{C<sub>2</sub>H<sub>5</sub>OH</td><td align='left'>{C<sub>2</sub>H<sub>4</sub>O</td></tr>
+</table></div>
+
+<p>The first of these (CH<sub>2</sub>O) is a gas known as formaldehyde. Its aqueous
+solution is largely used as an antiseptic and disinfectant under the
+name of <i>formalin</i>. Acetaldehyde (C<sub>2</sub>H<sub>4</sub>O) is a liquid boiling at
+21&deg;.</p>
+
+
+<h3>ACIDS</h3>
+
+<p>Like the other classes of organic compounds, the organic acids may be
+arranged in homologous series. One of the most important of these series
+is the <i>fatty-acid series</i>, the<span class='pagenum'><a name="Page_406" id="Page_406">[Pg 406]</a></span> name having been given to it because
+the derivatives of certain of its members are constituents of the fats.
+Some of the most important members of the series are given in the
+following table. They are all monobasic, and this fact is expressed in
+the formulas by separating the replaceable hydrogen atom from the rest
+of the molecule:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>H&middot;CHO<sub>2</sub></td><td align='left'>formic acid, a liquid boiling at 100&deg;.</td></tr>
+<tr><td align='left'>H&middot;C<sub>2</sub>H<sub>3</sub>O</td><td align='left'>acetic acid, a liquid boiling at 118&deg;.</td></tr>
+<tr><td align='left'>H&middot;C<sub>3</sub>H<sub>5</sub>O<sub>2</sub></td><td align='left'>propionic acid, a liquid boiling at 140&deg;.</td></tr>
+<tr><td align='left'>H&middot;C<sub>4</sub>H<sub>7</sub>O<sub>2</sub></td><td align='left'>butyric acid, a liquid boiling at 163&deg;.</td></tr>
+<tr><td align='left'>H&middot;C<sub>16</sub>H<sub>31</sub>O<sub>2</sub></td><td align='left'>palmitic acid, a solid melting at 62&deg;.</td></tr>
+<tr><td align='left'>H&middot;C<sub>18</sub>H<sub>35</sub>O<sub>2</sub></td><td align='left'>stearic acid, a solid melting at 69&deg;.</td></tr>
+</table></div>
+
+<p><b>Formic acid</b> (H&middot;CHO<sub>2</sub>). The name "formic" is derived from the Latin
+<i>formica</i>, signifying ant. This name was given to the acid because it
+was formerly obtained from a certain kind of ants. It is a colorless
+liquid and occurs in many plants such as the stinging nettles. The
+inflammation caused by the sting of the bee is due to formic acid.</p>
+
+<p><b>Acetic acid</b> (H&middot;C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>). Acetic acid is the acid present in
+vinegar, the sour taste being due to it. It can be prepared by either of
+the following methods.</p>
+
+<p>1. <i>Acetic fermentation.</i> This consists in the change of alcohol into
+acetic acid through the agency of a minute organism commonly called
+mother of vinegar. The change is represented by the following equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>2</sub>H<sub>5</sub>OH + 2O = HC<sub>2</sub>H<sub>3</sub>O<sub>2</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The various kinds of vinegars are all made by this process. In the
+manufacture of cider vinegar the sugar present in the cider first
+undergoes alcoholic fermentation; the resulting alcohol then undergoes
+acetic fermentation. The amount of acetic acid present in vinegars
+varies from 3 to 6%.<span class='pagenum'><a name="Page_407" id="Page_407">[Pg 407]</a></span></p>
+
+<p>2. <i>From the distillation of wood.</i> The liquid obtained by heating wood
+in the absence of air contains a large amount of acetic acid, and this
+can be separated readily in a pure state. This is the most economical
+method for the preparation of the concentrated acid.</p>
+
+<p>Acetic acid is a colorless liquid and has a strong pungent odor. Many of
+its salts are well-known compounds. Lead acetate
+(Pb(C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>)<sub>2</sub>) is the ordinary <i>sugar of lead</i>. Sodium
+acetate (NaC<sub>2</sub>H<sub>3</sub>O<sub>2</sub>) is a white solid largely used in making
+chemical analyses. Copper acetate (Cu(C<sub>2</sub>H<sub>3</sub>O<sub>2</sub>)<sub>2</sub>) is a blue
+solid. When copper is acted upon by acetic acid in the presence of air a
+green basic acetate of copper is formed. This is commonly known as
+verdigris. All acetates are soluble in water.</p>
+
+<p><b>Butyric acid</b> (H&middot;C<sub>4</sub>H<sub>7</sub>O<sub>2</sub>). Derivatives of butyric acid are
+present in butter and impart to it its characteristic flavor.</p>
+
+<p><b>Palmitic and stearic acids.</b> Ordinary fats consist principally of
+derivatives of palmitic and stearic acids. When the fats are heated with
+sodium hydroxide the sodium salts of these acids are formed. If
+hydrochloric acid is added to a solution of the sodium salts, the free
+palmitic and stearic acids are precipitated. They are white solids,
+insoluble in water. Stearic acid is often used in making candles.</p>
+
+<p><b>Acids belonging to other series.</b> In addition to members of the
+fatty-acid series, mention may be made of the following well-known
+acids.</p>
+
+<p><b>Oxalic acid</b> (H<sub>2</sub>C<sub>2</sub>O<sub>4</sub>). This is a white solid which occurs in
+nature in many plants, such as the sorrels. Its ammonium salt
+((NH<sub>4</sub>)<sub>2</sub>C<sub>2</sub>O<sub>4</sub>) is used as a reagent for the detection of
+calcium. When added to a solution of a calcium<span class='pagenum'><a name="Page_408" id="Page_408">[Pg 408]</a></span> compound the white,
+insoluble calcium oxalate (CaC<sub>2</sub>O<sub>4</sub>) precipitates.</p>
+
+<p><b>Tartaric acid</b> (H<sub>2</sub>&middot;C<sub>4</sub>H<sub>4</sub>O<sub>6</sub>). This compound occurs either in a
+free state or in the form of its salts in many fruits. The potassium
+acid salt (KHC<sub>4</sub>H<sub>4</sub>O<sub>6</sub>) occurs in the juice of grapes. When the
+juice ferments in the manufacture of wine, this salt, being insoluble in
+alcohol, separates out on the sides of the cask and in this form is
+known as argol. This is more or less colored by the coloring matter of
+the grape. When purified it forms a white solid and is sold under the
+name of cream of tartar. The following are also well-known salts of
+tartaric acid: potassium sodium tartrate (Rochelle salt)
+(KNaC<sub>4</sub>H<sub>4</sub>O<sub>6</sub>), potassium antimonyl tartrate (tartar emetic)
+(KSbOC<sub>4</sub>H<sub>4</sub>O<sub>6</sub>).</p>
+
+<div class="blockquot"><p><b>Cream of tartar baking powders.</b> The so-called cream of tartar
+baking powders consist of a mixture of cream of tartar,
+bicarbonate of soda, and some starch or flour. When water is
+added to this mixture the cream of tartar slowly acts upon the
+soda present liberating carbon dioxide in accordance with the
+following equation:</p>
+
+<p>KHC<sub>4</sub>H<sub>4</sub>O<sub>6</sub> + NaHCO<sub>3</sub> = KNaC<sub>4</sub>H<sub>4</sub>O<sub>6</sub> + H<sub>2</sub>O +
+CO<sub>2</sub>.</p>
+
+<p>The carbon dioxide evolved escapes through the dough, thus
+making it light and porous.</p></div>
+
+<p><b>Citric acid</b> (H<sub>3</sub>&middot;C<sub>6</sub>H<sub>5</sub>O<sub>7</sub>). This acid occurs in many fruits,
+especially in lemons. It is a white solid, soluble in water, and is
+often used as a substitute for lemons in making lemonade.</p>
+
+<p><b>Lactic acid</b> (H&middot;C<sub>3</sub>H<sub>5</sub>O<sub>3</sub>). This is a liquid which is formed in the
+souring of milk.</p>
+
+<p><b>Oleic acid</b> (H&middot;C<sub>18</sub>H<sub>33</sub>O<sub>2</sub>). The derivatives of this acid
+constitute the principal part of many oils and liquid fats. The acid
+itself is an oily liquid.<span class='pagenum'><a name="Page_409" id="Page_409">[Pg 409]</a></span></p>
+
+
+<h3>ETHEREAL SALTS</h3>
+
+<p>When acids are brought in contact with alcohols under certain conditions
+a reaction takes place similar to that which takes place between acids
+and bases. The following equations will serve as illustrations:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">KOH + HNO<sub>3</sub> = KNO<sub>3</sub> + H<sub>2</sub>O,<br /></span>
+</div><div class="stanza">
+<span class="i0">CH<sub>3</sub>OH + HNO<sub>3</sub> = CH<sub>3</sub>NO<sub>3</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The resulting compounds of which methyl nitrate (CH<sub>3</sub>NO<sub>3</sub>) may be
+taken as the type belong to the class known as <i>ethereal salts</i>, the
+name having been given them because some of them possess pleasant
+ethereal odors. It will be seen that the ethereal salts differ from
+ordinary salts in that they contain a hydrocarbon radical, such as
+CH<sub>3</sub>, C<sub>2</sub>H<sub>5</sub>, C<sub>3</sub>H<sub>5</sub>, in place of a metal.</p>
+
+<p><b>The nitrates of glycerin</b> (<i>nitroglycerin</i>). Nitric acid reacts with
+glycerin in the same way that it reacts with a base containing three
+hydroxyl groups such as Fe(OH)<sub>3</sub>:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">Fe(OH)<sub>3</sub> + 3HNO<sub>3</sub> = Fe(NO<sub>3</sub>)<sub>3</sub> + 3H<sub>2</sub>O,<br /></span>
+</div><div class="stanza">
+<span class="i0">C<sub>3</sub>H<sub>5</sub>(OH)<sub>3</sub> + 3HNO<sub>3</sub> = C<sub>3</sub>H<sub>5</sub>(NO<sub>3</sub>)<sub>3</sub> + 3H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The resulting nitrate (C<sub>3</sub>H<sub>5</sub>(NO<sub>3</sub>)<sub>3</sub>) is the main constituent
+of <i>nitroglycerin</i>, a slightly yellowish oil characterized by its
+explosive properties. Dynamite consists of porous earth which has
+absorbed nitroglycerin, and its strength depends on the amount present.
+It is used much more largely than nitroglycerin itself, since it does
+not explode so readily by concussion and hence can be transported with
+safety.</p>
+
+<p><b>The fats.</b> These are largely mixtures of the ethereal salts known
+respectively as olein, palmitin, and stearin.<span class='pagenum'><a name="Page_410" id="Page_410">[Pg 410]</a></span> These salts may be
+regarded as derived from oleic, palmitic, and stearic acids
+respectively, by replacing the hydrogen of the acid with the glycerin
+radical C<sub>3</sub>H<sub>5</sub>. Since this radical is trivalent and oleic, palmitic,
+and stearic acids contain only one replaceable hydrogen atom to the
+molecule, it is evident that three molecules of each acid must enter
+into each molecule of the ethereal salt. The formulas for the acids and
+the ethereal salts derived from each are as follows:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>HC<sub>18</sub>H<sub>33</sub>O<sub>2</sub></td><td align='left'>(oleic acid)</td></tr>
+<tr><td align='left'>C<sub>8</sub>H<sub>6</sub>(C<sub>18</sub>H<sub>33</sub>O<sub>2</sub>)<sub>3</sub>,</td><td align='left'>(olein)</td></tr>
+<tr><td align='left'>HC<sub>16</sub>H<sub>31</sub>O<sub>2</sub></td><td align='left'>(palmitic acid)</td></tr>
+<tr><td align='left'>C<sub>3</sub>H<sub>5</sub>(C<sub>16</sub>H<sub>31</sub>0<sub>2</sub>)<sub>3</sub></td><td align='left'>(palmitin)</td></tr>
+<tr><td align='left'>HC<sub>18</sub>H<sub>35</sub>O<sub>2</sub></td><td align='left'>(stearic acid)</td></tr>
+<tr><td align='left'>C<sub>3</sub>H<sub>5</sub>(C<sub>18</sub>H<sub>35</sub>O<sub>2</sub>)<sub>3</sub></td><td align='left'>(stearin)</td></tr>
+</table></div>
+
+
+<p>Olein is a liquid and is the main constituent of liquid fats. Palmitin
+and stearin are solids.</p>
+
+<p><b>Butter fat and oleomargarine.</b> Butter fat consists principally of olein,
+palmitin, and stearin. The flavor of the fat is due to the presence of a
+small amount of butyrin, which is an ethereal salt of butyric acid.
+Oleomargarine differs from butter mainly in the fact that a smaller
+amount of butyrin is present. It is made from the fats obtained from
+cattle and hogs. This fat is churned up with milk, or a small amount of
+butter is added, in order to furnish sufficient butyrin to impart the
+butter flavor.</p>
+
+<p><b>Saponification.</b> When an ethereal salt is heated with an alkali a
+reaction expressed by the following equation takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>2</sub>H<sub>5</sub>NO<sub>3</sub> + KOH = C<sub>2</sub>H<sub>5</sub>OH + KNO<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>This process is known as <i>saponification</i>, since it is the one which
+takes place in the manufacture of soaps. The ordinary soaps are made by
+heating fats with a solution of<span class='pagenum'><a name="Page_411" id="Page_411">[Pg 411]</a></span> sodium hydroxide. The reactions
+involved may be illustrated by the following equation representing the
+reaction between palmitin and sodium hydroxide:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>3</sub>H<sub>5</sub>(C<sub>16</sub>H<sub>31</sub>O<sub>2</sub>)<sub>3</sub> + 3 NaOH = 3 NaC<sub>16</sub>H<sub>31</sub>O<sub>2</sub> + C<sub>3</sub>H<sub>5</sub>(OH)<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>In accordance with this equation the ethereal salts in the fats are
+converted into glycerin and the sodium salts of the corresponding acids.
+The sodium salts are separated and constitute the soaps. These salts are
+soluble in water. When added to water containing calcium salts the
+insoluble calcium palmitate and stearate are precipitated. Magnesium
+salts act in a similar way. It is because of these facts that soap is
+used up by hard waters.</p>
+
+
+<h3>ETHERS</h3>
+
+<p>When ethyl alcohol is heated to 140&deg; with sulphuric acid the reaction
+expressed by the following equation takes place:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">2C<sub>2</sub>H<sub>5</sub>OH = (C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>O + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The resulting compound, (C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>O, is ordinary ether and is the
+most important member of the class of compounds called <i>ethers</i>.
+Ordinarily ether is a light, very inflammable liquid boiling at 35&deg;. It
+is used as a solvent for organic substances and as an an&aelig;sthetic in
+surgical operations.</p>
+
+
+<h3>KETONES</h3>
+
+<p>The most common member of this group is acetone (C<sub>3</sub>H<sub>6</sub>O), a
+colorless liquid obtained when wood is heated in the absence of air. It
+is used in the preparation of other organic compounds, especially
+chloroform.<span class='pagenum'><a name="Page_412" id="Page_412">[Pg 412]</a></span></p>
+
+
+<h3>ORGANIC BASES</h3>
+
+<p>This group includes a number of compounds, all of which contain nitrogen
+as well as carbon. They are characterized by combining directly with
+acids to form salts, and in this respect they resemble ammonia. They
+may, indeed, be regarded as derived from ammonia by displacing a part or
+all of the hydrogen present in ammonia by hydrocarbon radicals. Among
+the simplest of these compounds may be mentioned methylamine
+(CH<sub>3</sub>NH<sub>2</sub>) and ethylamine (C<sub>2</sub>H<sub>5</sub>NH<sub>2</sub>). These two compounds
+are gases and are formed in the distillation of wood and bones. Pyridine
+(C<sub>5</sub>H<sub>6</sub>N) and quinoline (C<sub>9</sub>H<sub>7</sub>N) are liquids present in small
+amounts in coal tar, and also in the liquid obtained by the distillation
+of bones. Most of the compounds now classified under the general name of
+<i>alkaloids</i> (which see) also belong to this group.</p>
+
+
+<h3>CARBOHYDRATES</h3>
+
+<p>The term "carbohydrate" is applied to a class of compounds which
+includes the sugars, starch, and allied bodies These compounds contain
+carbon, hydrogen, and oxygen the last two elements generally being
+present in the proportion in which they combine to form water. The most
+important members of this class are the following:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Cane sugar</td><td align='left'>C<sub>12</sub>H<sub>22</sub>O<sub>11</sub>.</td></tr>
+<tr><td align='left'>Milk sugar</td><td align='left'>C<sub>12</sub>H<sub>22</sub>O<sub>11</sub>.</td></tr>
+<tr><td align='left'>Dextrose</td><td align='left'>C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>.</td></tr>
+<tr><td align='left'>Levulose</td><td align='left'>C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>.</td></tr>
+<tr><td align='left'>Cellulose</td><td align='left'>C<sub>6</sub>H<sub>10</sub>O<sub>5</sub>.</td></tr>
+<tr><td align='left'>Starch</td><td align='left'>C<sub>6</sub>H<sub>10</sub>0<sub>5</sub>.</td></tr>
+</table></div>
+
+
+<p><b>Cane sugar</b> (C<sub>12</sub>H<sub>22</sub>O<sub>11</sub>). This is the well-known substance
+commonly called sugar. It occurs in many plants<span class='pagenum'><a name="Page_413" id="Page_413">[Pg 413]</a></span> especially in the sugar
+cane and sugar beet. It was formerly obtained almost entirely from the
+sugar cane, but at present the greatest amount of it comes from the
+sugar beet. The juice from the cane or beet contains the sugar in
+solution along with many impurities. These impurities are removed, and
+the resulting solution is then evaporated until the sugar crystallizes
+out. The evaporation is conducted in closed vessels from which the air
+is partially exhausted. In this way the boiling point of the solution is
+lowered and the charring of the sugar is prevented. It is impossible to
+remove all the sugar from the solution. In preparing sugar from sugar
+cane the liquors left after separating as much of it as possible from
+the juice of the cane constitute ordinary molasses. Maple sugar is made
+by the evaporation of the sap obtained from a species of the maple tree.
+Its sweetness is due to the presence of cane sugar, other products
+present in the maple sap imparting the distinctive flavor.</p>
+
+<p>When a solution of cane sugar is heated with hydrochloric or other
+dilute mineral acid, two compounds, dextrose and levulose, are formed in
+accordance with the following equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>12</sub>H<sub>22</sub>O<sub>11</sub> + H<sub>2</sub>O = C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>.<br /></span>
+</div></div>
+
+<p>This same change is brought about by the action of an enzyme present in
+the yeast plant. When yeast is added to a solution of cane sugar
+fermentation is set up. The cane sugar, however, does not ferment
+directly: the enzyme in the yeast first transforms the sugar into
+dextrose and levulose, and these sugars then undergo alcoholic
+fermentation.</p>
+
+<p>When heated to 160&deg; cane sugar melts; if the temperature is increased to
+about 215&deg;, a partial decomposition<span class='pagenum'><a name="Page_414" id="Page_414">[Pg 414]</a></span> takes place and a brown substance
+known as caramel forms. This is used largely as a coloring matter.</p>
+
+<p><b>Milk sugar</b> (C<sub>12</sub>H<sub>22</sub>O<sub>11</sub>). This sugar is present in the milk of
+all mammals. The average composition of cow's milk is as follows:</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Water</td><td align='left'>87.17%</td></tr>
+<tr><td align='left'>Casein (nitrogenous matter)</td><td align='left'>3.56</td></tr>
+<tr><td align='left'>Butter fat</td><td align='left'>3.64</td></tr>
+<tr><td align='left'>Milk sugar</td><td align='left'>4.88</td></tr>
+<tr><td align='left'>Mineral matter</td><td align='left'>0.75</td></tr>
+</table></div>
+
+<p>When <i>rennin</i>, an enzyme obtained from the stomach of calves, is added
+to milk, the casein separates and is used in the manufacture of cheese.
+The remaining liquid contains the milk sugar which separates on
+evaporation; it resembles cane sugar in appearance but is not so sweet
+or soluble. The souring of milk is due to the fact that the milk sugar
+present undergoes <i>lactic fermentation</i> in accordance with the equation</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>12</sub>H<sub>22</sub>O<sub>11</sub> + H<sub>2</sub>O = 4C<sub>3</sub>H<sub>6</sub>O<sub>3</sub>.<br /></span>
+</div></div>
+
+<p>The lactic acid formed causes the separation of the casein, thus giving
+the well-known appearance of sour milk.</p>
+
+<p><b>Isomeric compounds.</b> It will be observed that cane sugar and milk sugar
+have the same formulas. Their difference in properties is due to the
+different arrangement of the atoms in the molecule. Such compounds are
+said to be isomeric. Dextrose and levulose are also isomeric.</p>
+
+<p><b>Dextrose</b> (<i>grape sugar, glucose</i>) (C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>). This sugar is
+present in many fruits and is commonly called grape sugar because of its
+presence in grape juice. It can be obtained by heating cane sugar with
+dilute acids, as<span class='pagenum'><a name="Page_415" id="Page_415">[Pg 415]</a></span> explained above; also by heating starch with dilute
+acids, the change being as follows:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>6</sub>H<sub>10</sub>6<sub>5</sub> + H<sub>2</sub>O = C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>.<br /></span>
+</div></div>
+
+<p>Pure dextrose is a white crystalline solid, readily soluble in water,
+and is not so sweet as cane sugar. In the presence of yeast it undergoes
+alcoholic fermentation. It is prepared from starch in large quantities,
+and being less expensive than cane sugar, is used as a substitute for it
+in the manufacture of jellies, jams, molasses, candy, and other sweets.
+The product commonly sold under the name of <i>glucose</i> contains about 45%
+of dextrose.</p>
+
+<p><b>Levulose</b> <i>(fruit sugar)</i>(C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>). This sugar is a white solid
+which occurs along with dextrose in fruits and honey. It undergoes
+alcoholic fermentation in the presence of yeast.</p>
+
+<p><b>Cellulose</b> (C<sub>6</sub>H<sub>10</sub>O<sub>5</sub>). This forms the basis of all woody fibers.
+Cotton and linen are nearly pure cellulose. It is insoluble in water,
+alcohol, and dilute acids. Sulphuric acid slowly converts it into
+dextrose. Nitric acid forms nitrates similar to nitroglycerin in
+composition and explosive properties. These nitrates are variously known
+as nitrocellulose, pyroxylin, and gun cotton. When exploded they yield
+only colorless gases; hence they are used especially in the manufacture
+of smokeless gunpowder. <i>Collodion</i> is a solution of nitrocellulose in a
+mixture of alcohol and ether. <i>Celluloid</i> is a mixture of nitrocellulose
+and camphor. <i>Paper</i> consists mainly of cellulose, the finer grades
+being made from linen and cotton rags, and the cheaper grades from straw
+and wood.</p>
+
+<p><b>Starch</b> (C<sub>6</sub>H<sub>10</sub>O<sub>5</sub>). This is by far the most abundant carbohydrate
+found in nature, being present especially in<span class='pagenum'><a name="Page_416" id="Page_416">[Pg 416]</a></span> seeds and tubers. In the
+United States it is obtained chiefly from corn, nearly 80% of which is
+starch. In Europe it is obtained principally from the potato. It
+consists of minute granules and is practically insoluble in cold water.
+These granules differ somewhat in appearance, according to the source of
+the starch, so that it is often possible to determine from what plant
+the starch was obtained. When heated with water the granules burst and
+the starch partially dissolves. Dilute acids, as well as certain
+enzymes, convert it into dextrose or similar sugars. When seeds
+germinate the starch present is converted into soluble sugars, which are
+used as food for the growing plant.</p>
+
+<p><b>Chemical changes in bread making.</b> The average composition of wheat flour
+is as follows:</p>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Water.</td><td align='left'>13.8%</td></tr>
+<tr><td align='left'>Protein (nitrogenous matter)</td><td align='left'>7.9</td></tr>
+<tr><td align='left'>Fats</td><td align='left'>1.4</td></tr>
+<tr><td align='left'>Starch</td><td align='left'>76.4</td></tr>
+<tr><td align='left'>Mineral matter</td><td align='left'>0.5</td></tr>
+</table></div>
+
+<p>In making bread the flour is mixed with water and yeast, and the
+resulting dough set aside in a warm place for a few hours. The yeast
+first converts a portion of the starch into dextrose or a similar sugar,
+which then undergoes alcoholic fermentation. The carbon dioxide formed
+escapes through the dough, making it light and porous. The yeast plant
+thrives best at about 30&deg;; hence the necessity for having the dough in a
+warm place. If the temperature rises above 50&deg;, the vitality of the
+yeast is destroyed and fermentation ceases. In baking the bread, the
+heat expels the alcohol and also expands the bubbles of carbon dioxide
+caught in the dough, thus increasing its lightness.<span class='pagenum'><a name="Page_417" id="Page_417">[Pg 417]</a></span></p>
+
+
+<h3>SOME DERIVATIVES OF BENZENE</h3>
+
+<p>Attention has been called to the complex nature of coal tar. Among the
+compounds present are the hydrocarbons, benzene, toluene, naphthalene,
+and anthracene. These compounds are not only useful in themselves but
+serve for the preparation of many other important compounds known under
+the general name of coal-tar products.</p>
+
+<p><b>Nitrobenzene</b> (<i>oil of myrbane</i>) (C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub>). When benzene is
+treated with nitric acid a reaction takes place which is expressed by
+the following equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>6</sub>H<sub>6</sub> + HNO<sub>3</sub> = C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> + H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The product C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> is called nitrobenzene. It is a slightly
+yellowish poisonous liquid, with a characteristic odor. Its main use is
+in the manufacture of aniline.</p>
+
+<p><b>Aniline</b> (C<sub>6</sub>H<sub>5</sub>NH<sub>2</sub>). When nitrobenzene is heated with iron and
+hydrochloric acid the hydrogen evolved by the action of the iron upon
+the acid reduces the nitrobenzene in accordance with the following
+equation:</p>
+
+<div class="poem"><div class="stanza">
+<span class="i0">C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> + 6H = C<sub>6</sub>H<sub>5</sub>NH<sub>2</sub> + 2H<sub>2</sub>O.<br /></span>
+</div></div>
+
+<p>The resulting compound is known as aniline, a liquid boiling at 182&deg;.
+When first prepared it is colorless, but darkens on standing. Large
+quantities of it are used in the manufacture of the <i>aniline or coal-tar
+dyes</i>, which include many important compounds.</p>
+
+<p><b>Carbolic acid</b> (C<sub>6</sub>H<sub>5</sub>OH). This compound, sometimes known as
+<i>phenol</i>, occurs in coal tar, and is also prepared from benzene. It
+forms colorless crystals which are very soluble in water. It is strongly
+corrosive and very poisonous.<span class='pagenum'><a name="Page_418" id="Page_418">[Pg 418]</a></span></p>
+
+<p><b>Naphthalene and anthracene.</b> These are hydrocarbons occurring along with
+benzene in coal tar. They are white solids, insoluble in water. The
+well-known <i>moth balls</i> are made of naphthalene. Large quantities of
+naphthalene are used in the preparation of <i>indigo</i>, a dye formerly
+obtained from the indigo plant, but now largely prepared by laboratory
+methods. Similarly anthracene is used in the preparation of the dye
+<i>alizarin</i>, which was formerly obtained from the madder root.</p>
+
+
+<h3>THE ALKALOIDS</h3>
+
+<p>This term is applied to a group of compounds found in many plants and
+trees. They all contain nitrogen, and most of them are characterized by
+their power to combine with acids to form salts. This property is
+indicated by the name alkaloids, which signifies alkali-like. The salts
+are soluble in water, and on this account are more largely used than the
+free alkaloids, which are insoluble in water. Many of the alkaloids are
+used in medicine, some of the more important ones being given below.</p>
+
+<p><b>Quinine.</b> This alkaloid occurs along with a number of others in the bark
+of certain trees which grow in districts in South America and also in
+Java and other tropical islands. It is a white solid, and its sulphate
+is used in medicine in the treatment of fevers.</p>
+
+<p><b>Morphine.</b> When incisions are made in the unripe capsules of one of the
+varieties of the poppy plant, a milky juice exudes which soon thickens.
+This is removed and partially dried. The resulting substance is the
+ordinary <i>opium</i> which contains a number of alkaloids, the principal one
+being morphine. This alkaloid is a white solid and is of great service
+in medicine.<span class='pagenum'><a name="Page_419" id="Page_419">[Pg 419]</a></span></p>
+
+<p>Among the other alkaloids may be mentioned the following: <i>Nicotine</i>, a
+very poisonous liquid, the salts of which occur in the leaves of the
+tobacco plant; <i>cocaine</i>, a crystalline solid present in coca leaves and
+used in medicine as a local an&aelig;sthetic; <i>atropine</i>, a solid present in
+the berry of the deadly nightshade, and used in the treatment of
+diseases of the eye; <i>strychnine</i>, a white, intensely poisonous solid
+present in the seeds of the members of the <i>Strychnos</i> family.<span class='pagenum'><a name="Page_421" id="Page_421">[Pg 421]</a></span></p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="INDEX" id="INDEX"></a>INDEX</h2>
+
+
+<p>
+Acetaldehyde <a href='#Page_405'>405</a><br />
+<br />
+Acetic acid <a href='#Page_406'>406</a><br />
+<br />
+Acetone <a href='#Page_411'>411</a><br />
+<br />
+Acetylene <a href='#Page_203'>203</a><br />
+<span style="margin-left: 1em;">series <a href='#Page_399'>399</a></span><br />
+<br />
+Acids <a href='#Page_106'>106</a><br />
+<span style="margin-left: 1em;">binary <a href='#Page_113'>113</a></span><br />
+<span style="margin-left: 1em;">characteristics <a href='#Page_106'>106</a></span><br />
+<span style="margin-left: 1em;">definition <a href='#Page_107'>107</a></span><br />
+<span style="margin-left: 1em;">dibasic <a href='#Page_159'>159</a></span><br />
+<span style="margin-left: 1em;">familiar <a href='#Page_106'>106</a></span><br />
+<span style="margin-left: 1em;">monobasic <a href='#Page_159'>159</a></span><br />
+<span style="margin-left: 1em;">nomenclature <a href='#Page_113'>113</a></span><br />
+<span style="margin-left: 1em;">organic <a href='#Page_405'>405</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_141'>141</a></span><br />
+<span style="margin-left: 1em;">strength <a href='#Page_111'>111</a></span><br />
+<span style="margin-left: 1em;">ternary <a href='#Page_113'>113</a></span><br />
+<span style="margin-left: 1em;">undissociated <a href='#Page_107'>107</a></span><br />
+<br />
+Acker furnace, <a href='#Page_279'>279</a><br />
+<br />
+Agate <a href='#Page_260'>260</a><br />
+<br />
+Air <a href='#Page_83'>83</a><br />
+<span style="margin-left: 1em;">a mechanical mixture <a href='#Page_89'>89</a></span><br />
+<span style="margin-left: 1em;">carbon dioxide in <a href='#Page_87'>87</a></span><br />
+<span style="margin-left: 1em;">changes in composition <a href='#Page_87'>87</a></span><br />
+<span style="margin-left: 1em;">liquid <a href='#Page_91'>91</a></span><br />
+<span style="margin-left: 1em;">nitrogen in <a href='#Page_87'>87</a></span><br />
+<span style="margin-left: 1em;">oxygen in <a href='#Page_85'>85</a></span><br />
+<span style="margin-left: 1em;">poisonous effects of exhaled <a href='#Page_88'>88</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_90'>90</a></span><br />
+<span style="margin-left: 1em;">quantitative analysis of <a href='#Page_85'>85</a></span><br />
+<span style="margin-left: 1em;">regarded as an element <a href='#Page_83'>83</a></span><br />
+<span style="margin-left: 1em;">standard for density <a href='#Page_229'>229</a></span><br />
+<span style="margin-left: 1em;">water vapor in <a href='#Page_87'>87</a></span><br />
+<br />
+Alabaster <a href='#Page_308'>308</a><br />
+<br />
+Alchemists <a href='#Page_9'>9</a><br />
+<br />
+Alchemy <a href='#Page_9'>9</a><br />
+<br />
+Alcohol, common <a href='#Page_402'>402</a><br />
+<span style="margin-left: 1em;">denatured <a href='#Page_404'>404</a></span><br />
+<span style="margin-left: 1em;">ethyl <a href='#Page_402'>402</a></span><br />
+<span style="margin-left: 1em;">methyl <a href='#Page_402'>402</a></span><br />
+<span style="margin-left: 1em;">wood <a href='#Page_402'>402</a></span><br />
+<br />
+Alcohols <a href='#Page_401'>401</a><br />
+<br />
+Aldehydes <a href='#Page_405'>405</a><br />
+<br />
+Alizarin <a href='#Page_418'>418</a><br />
+<br />
+Alkali <a href='#Page_107'>107</a>, <a href='#Page_274'>274</a><br />
+<span style="margin-left: 1em;">family <a href='#Page_274'>274</a></span><br />
+<br />
+Alkaline-earth family <a href='#Page_300'>300</a><br />
+<br />
+Alkaloids <a href='#Page_418'>418</a><br />
+<br />
+Allotropic forms <a href='#Page_22'>22</a><br />
+<br />
+Alloys <a href='#Page_252'>252</a><br />
+<br />
+Alum <a href='#Page_333'>333</a><br />
+<span style="margin-left: 1em;">ammonium <a href='#Page_334'>334</a></span><br />
+<span style="margin-left: 1em;">ammonium chrome <a href='#Page_384'>384</a></span><br />
+<span style="margin-left: 1em;">ammonium iron <a href='#Page_352'>352</a></span><br />
+<span style="margin-left: 1em;">baking powders <a href='#Page_335'>335</a></span><br />
+<span style="margin-left: 1em;">potassium <a href='#Page_333'>333</a></span><br />
+<span style="margin-left: 1em;">potassium chrome <a href='#Page_384'>384</a></span><br />
+<span style="margin-left: 1em;">potassium iron <a href='#Page_352'>352</a></span><br />
+<br />
+Aluminates <a href='#Page_332'>332</a><br />
+<br />
+Aluminium <a href='#Page_327'>327</a><br />
+<span style="margin-left: 1em;">bronze <a href='#Page_330'>330</a>, <a href='#Page_359'>359</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_333'>333</a></span><br />
+<span style="margin-left: 1em;">family <a href='#Page_327'>327</a></span><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_332'>332</a></span><br />
+<span style="margin-left: 1em;">metallurgy <a href='#Page_328'>328</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_327'>327</a></span><br />
+<span style="margin-left: 1em;">oxide <a href='#Page_331'>331</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_328'>328</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_329'>329</a></span><br />
+<span style="margin-left: 1em;">silicates <a href='#Page_335'>335</a></span><br />
+<span style="margin-left: 1em;">uses <a href='#Page_330'>330</a></span><br />
+<br />
+Amalgam <a href='#Page_362'>362</a><br />
+<br />
+Amethyst <a href="#Page_260">260</a>, <a href='#Page_331'>331</a><br />
+<br />
+Ammonia <a href='#Page_123'>123</a><br />
+<span style="margin-left: 1em;">composition <a href='#Page_127'>127</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_123'>123</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_124'>124</a></span><br />
+<span style="margin-left: 1em;">uses <a href='#Page_125'>125</a></span><br />
+<br />
+Ammonium <a href='#Page_126'>126</a><br />
+<span style="margin-left: 1em;">acid carbonate <a href='#Page_295'>295</a></span><br />
+<span style="margin-left: 1em;">carbonate <a href='#Page_295'>295</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_294'>294</a></span><br />
+<span class='pagenum'><a name="Page_422" id="Page_422">[Pg 422]</a></span><span style="margin-left: 1em;">compounds <a href='#Page_294'>294</a></span><br />
+<br />
+Ammonium hydrosulphide <a href='#Page_296'>296</a><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_126'>126</a></span><br />
+<span style="margin-left: 1em;">molybdate <a href='#Page_388'>388</a></span><br />
+<span style="margin-left: 1em;">oxalate <a href='#Page_407'>407</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_295'>295</a></span><br />
+<span style="margin-left: 1em;">sulphide <a href='#Page_295'>295</a></span><br />
+<span style="margin-left: 1em;">sulphide, yellow <a href='#Page_296'>296</a></span><br />
+<br />
+Analysis <a href='#Page_40'>40</a><br />
+<br />
+Anhydride <a href='#Page_135'>135</a><br />
+<span style="margin-left: 1em;">carbonic <a href='#Page_206'>206</a></span><br />
+<span style="margin-left: 1em;">chromic <a href='#Page_387'>387</a></span><br />
+<span style="margin-left: 1em;">nitric <a href='#Page_135'>135</a></span><br />
+<span style="margin-left: 1em;">nitrous <a href='#Page_135'>135</a></span><br />
+<span style="margin-left: 1em;">phosphoric <a href='#Page_243'>243</a></span><br />
+<span style="margin-left: 1em;">sulphuric <a href='#Page_153'>153</a></span><br />
+<br />
+Anhydrite <a href='#Page_288'>288</a><br />
+<br />
+Aniline <a href='#Page_417'>417</a><br />
+<br />
+Anion <a href='#Page_106'>106</a><br />
+<br />
+Anode <a href='#Page_99'>99</a><br />
+<br />
+Anthracene <a href='#Page_418'>418</a><br />
+<br />
+Antimony <a href='#Page_250'>250</a><br />
+<span style="margin-left: 1em;">acids <a href='#Page_251'>251</a></span><br />
+<span style="margin-left: 1em;">alloys <a href='#Page_253'>253</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_252'>252</a></span><br />
+<span style="margin-left: 1em;">metallic properties <a href='#Page_252'>252</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_251'>251</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_251'>251</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_251'>251</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_251'>251</a></span><br />
+<span style="margin-left: 1em;">sulphides <a href='#Page_251'>251</a></span><br />
+<br />
+Apatite <a href="#Page_175">175</a>, <a href="#Page_239">239</a>, <a href='#Page_311'>311</a><br />
+<br />
+Aqua ammonia <a href='#Page_124'>124</a><br />
+<br />
+Aqua regia <a href='#Page_185'>185</a><br />
+<br />
+Aqueous tension <a href='#Page_25'>25</a><br />
+<br />
+Argon <a href='#Page_80'>80</a><br />
+<br />
+Arsenic <a href='#Page_246'>246</a><br />
+<span style="margin-left: 1em;">acids <a href='#Page_250'>250</a></span><br />
+<span style="margin-left: 1em;">antidote <a href='#Page_250'>250</a></span><br />
+<span style="margin-left: 1em;">Marsh's test <a href='#Page_248'>248</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_246'>246</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_249'>249</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_246'>246</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_247'>247</a></span><br />
+<span style="margin-left: 1em;">sulphides <a href='#Page_250'>250</a></span><br />
+<span style="margin-left: 1em;">white <a href='#Page_249'>249</a></span><br />
+<br />
+Arsenopyrites <a href='#Page_246'>246</a><br />
+<br />
+Arsine <a href='#Page_247'>247</a><br />
+<br />
+Asbestos <a href="#Page_321">321</a>, <a href='#Page_336'>336</a><br />
+<br />
+Atmosphere <a href='#Page_83'>83</a><br />
+<span style="margin-left: 1em;">constituents <a href='#Page_83'>83</a></span><br />
+<span style="margin-left: 1em;">function of constituents <a href='#Page_84'>84</a></span><br />
+<br />
+Atomic hypothesis <a href='#Page_61'>61</a><br />
+<span style="margin-left: 1em;">theory <a href='#Page_59'>59</a></span><br />
+<span style="margin-left: 2em;">and laws of matter <a href='#Page_63'>63</a></span><br />
+<span style="margin-left: 2em;">and radium <a href='#Page_314'>314</a></span><br />
+<span style="margin-left: 1em;">weights, <a href='#Page_65'>65</a></span><br />
+<span style="margin-left: 2em;">accurate determination <a href='#Page_231'>231</a></span><br />
+<span style="margin-left: 2em;">and general properties <a href='#Page_167'>167</a></span><br />
+<span style="margin-left: 2em;">and specific heats <a href='#Page_233'>233</a></span><br />
+<span style="margin-left: 2em;">calculation of <a href='#Page_231'>231</a></span><br />
+<span style="margin-left: 2em;">Dalton's method <a href='#Page_223'>223</a></span><br />
+<span style="margin-left: 2em;">direct determination <a href='#Page_233'>233</a></span><br />
+<span style="margin-left: 2em;">from molecular weights <a href='#Page_230'>230</a></span><br />
+<span style="margin-left: 2em;">relation to equivalent <a href='#Page_224'>224</a></span><br />
+<span style="margin-left: 2em;">standard for <a href='#Page_66'>66</a></span><br />
+<span style="margin-left: 2em;">steps in determining <a href='#Page_224'>224</a></span><br />
+<br />
+Atoms <a href='#Page_62'>62</a><br />
+<span style="margin-left: 1em;">size <a href='#Page_65'>65</a></span><br />
+<br />
+Atropine <a href='#Page_419'>419</a><br />
+<br />
+Aurates <a href='#Page_396'>396</a><br />
+<br />
+Avogadro's hypothesis <a href='#Page_226'>226</a><br />
+<span style="margin-left: 1em;">and chemical calculations <a href='#Page_235'>235</a></span><br />
+<span style="margin-left: 1em;">and molecular weights <a href='#Page_227'>227</a></span><br />
+<br />
+Azote <a href='#Page_78'>78</a><br />
+<br />
+Azurite <a href='#Page_357'>357</a><br />
+<br />
+<br />
+Babbitt metal <a href='#Page_253'>253</a><br />
+<br />
+Bacteria <a href='#Page_85'>85</a><br />
+<span style="margin-left: 1em;">decomposition of organic matter by <a href='#Page_122'>122</a></span><br />
+<span style="margin-left: 1em;">nitrifying <a href='#Page_85'>85</a></span><br />
+<br />
+Baking powders <a href="#Page_285">285</a>, <a href='#Page_408'>408</a><br />
+<span style="margin-left: 1em;">alum <a href='#Page_335'>335</a></span><br />
+<span style="margin-left: 1em;">soda <a href='#Page_285'>285</a></span><br />
+<br />
+Barium <a href='#Page_312'>312</a><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_313'>313</a></span><br />
+<span style="margin-left: 1em;">nitrate <a href='#Page_313'>313</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_312'>312</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_313'>313</a></span><br />
+<br />
+Barytes <a href='#Page_312'>312</a><br />
+<br />
+Bases <a href='#Page_107'>107</a><br />
+<span style="margin-left: 1em;">characteristics <a href='#Page_107'>107</a></span><br />
+<span style="margin-left: 1em;">definition <a href='#Page_108'>108</a></span><br />
+<span style="margin-left: 1em;">familiar <a href='#Page_107'>107</a></span><br />
+<span style="margin-left: 1em;">nomenclature <a href='#Page_113'>113</a></span><br />
+<span style="margin-left: 1em;">organic <a href='#Page_412'>412</a></span><br />
+<span style="margin-left: 1em;">strength <a href='#Page_113'>113</a></span><br />
+<span class='pagenum'><a name="Page_423" id="Page_423">[Pg 423]</a></span><span style="margin-left: 1em;">undissociated <a href='#Page_108'>108</a></span><br />
+<br />
+Basic lining process <a href='#Page_346'>346</a><br />
+<br />
+Bauxite <a href='#Page_332'>332</a><br />
+<br />
+Beer <a href='#Page_404'>404</a><br />
+<br />
+Benzene <a href='#Page_417'>417</a><br />
+<span style="margin-left: 1em;">derivatives <a href='#Page_417'>417</a></span><br />
+<span style="margin-left: 1em;">series <a href='#Page_399'>399</a></span><br />
+<br />
+Benzine <a href='#Page_400'>400</a><br />
+<br />
+Bessemer process <a href='#Page_345'>345</a><br />
+<br />
+Bismuth <a href='#Page_253'>253</a><br />
+<span style="margin-left: 1em;">basic salts <a href='#Page_255'>255</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_253'>253</a></span><br />
+<span style="margin-left: 1em;">nitrate <a href='#Page_253'>253</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_253'>253</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_254'>254</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_253'>253</a></span><br />
+<span style="margin-left: 1em;">salts, hydrolysis of <a href='#Page_254'>254</a></span><br />
+<span style="margin-left: 1em;">subnitrate <a href='#Page_256'>256</a></span><br />
+<span style="margin-left: 1em;">uses <a href='#Page_253'>253</a></span><br />
+<br />
+Bismuthyl chloride <a href='#Page_256'>256</a><br />
+<br />
+Blast furnace <a href='#Page_341'>341</a><br />
+<span style="margin-left: 1em;">lamp <a href='#Page_38'>38</a></span><br />
+<br />
+Bleaching powder <a href='#Page_306'>306</a><br />
+<br />
+Bleaching by chlorine <a href='#Page_181'>181</a><br />
+<span style="margin-left: 1em;">by sulphurous acid <a href='#Page_152'>152</a></span><br />
+<br />
+Boiler scale <a href='#Page_320'>320</a><br />
+<br />
+Bone ash <a href='#Page_311'>311</a><br />
+<br />
+Bone black <a href='#Page_200'>200</a><br />
+<br />
+Borax <a href='#Page_265'>265</a><br />
+<span style="margin-left: 1em;">bead <a href='#Page_266'>266</a></span><br />
+<br />
+Bornite <a href='#Page_357'>357</a><br />
+<br />
+Boron <a href="#Page_257">257</a>, <a href='#Page_264'>264</a><br />
+<span style="margin-left: 1em;">acids <a href='#Page_265'>265</a></span><br />
+<span style="margin-left: 1em;">fluoride <a href='#Page_264'>264</a></span><br />
+<span style="margin-left: 1em;">hydride <a href='#Page_264'>264</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_264'>264</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_264'>264</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_264'>264</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_264'>264</a></span><br />
+<br />
+Brass <a href='#Page_323'>323</a><br />
+<br />
+Bread making <a href='#Page_416'>416</a><br />
+<br />
+Bromides <a href='#Page_190'>190</a><br />
+<br />
+Bromine <a href='#Page_187'>187</a><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_187'>187</a></span><br />
+<span style="margin-left: 0.5em;">oxygen compounds <a href='#Page_190'>190</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_187'>187</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_188'>188</a></span><br />
+<br />
+Bronze <a href='#Page_359'>359</a><br />
+<span style="margin-left: 1em;">aluminium <a href="#Page_330">330</a>, <a href='#Page_359'>359</a></span><br />
+<br />
+Butter fat <a href='#Page_410'>410</a><br />
+<br />
+Butyric acid <a href='#Page_407'>407</a><br />
+<br />
+By-product <a href='#Page_284'>284</a><br />
+<br />
+<br />
+Cadmium <a href='#Page_325'>325</a><br />
+<span style="margin-left: 1em;">compounds <a href='#Page_326'>326</a></span><br />
+<br />
+C&aelig;sium <a href='#Page_294'>294</a><br />
+<br />
+Calamine <a href='#Page_321'>321</a><br />
+<br />
+Calcite <a href='#Page_305'>305</a><br />
+<br />
+Calcium <a href='#Page_301'>301</a><br />
+<span style="margin-left: 1em;">carbide <a href="#Page_203">203</a>, <a href='#Page_310'>310</a></span><br />
+<span style="margin-left: 1em;">carbonate <a href='#Page_305'>305</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_306'>306</a></span><br />
+<span style="margin-left: 1em;">fluoride <a href='#Page_308'>308</a></span><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_303'>303</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_301'>301</a></span><br />
+<span style="margin-left: 1em;">oxide <a href='#Page_302'>302</a></span><br />
+<span style="margin-left: 1em;">phosphate 246, <a href='#Page_311'>311</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_302'>302</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_308'>308</a></span><br />
+<br />
+Calomel <a href='#Page_363'>363</a><br />
+<br />
+Calorie <a href='#Page_76'>76</a><br />
+<br />
+Caramel <a href='#Page_414'>414</a><br />
+<br />
+Carbohydrates <a href='#Page_413'>413</a><br />
+<br />
+Carbolic acid <a href='#Page_417'>417</a><br />
+<br />
+Carbon <a href='#Page_196'>196</a><br />
+<span style="margin-left: 1em;">allotropic forms <a href='#Page_196'>196</a></span><br />
+<span style="margin-left: 1em;">amorphous <a href='#Page_198'>198</a></span><br />
+<span style="margin-left: 1em;">compounds <a href='#Page_196'>196</a></span><br />
+<span style="margin-left: 1em;">crystalline forms <a href='#Page_197'>197</a></span><br />
+<span style="margin-left: 1em;">cycle in nature <a href='#Page_88'>88</a></span><br />
+<span style="margin-left: 1em;">dioxide <a href='#Page_204'>204</a></span><br />
+<span style="margin-left: 2em;">and bases <a href='#Page_206'>206</a></span><br />
+<span style="margin-left: 2em;">and plant life <a href='#Page_88'>88</a></span><br />
+<span style="margin-left: 2em;">in air <a href='#Page_87'>87</a></span><br />
+<span style="margin-left: 2em;">occurrence <a href='#Page_204'>204</a></span><br />
+<span style="margin-left: 2em;">preparation <a href='#Page_204'>204</a></span><br />
+<span style="margin-left: 2em;">properties <a href='#Page_204'>204</a></span><br />
+<span style="margin-left: 2em;">solid <a href='#Page_204'>204</a></span><br />
+<span style="margin-left: 1em;">disulphide <a href="#Page_160">160</a>, <a href='#Page_210'>210</a></span><br />
+<span style="margin-left: 1em;">family <a href='#Page_196'>196</a></span><br />
+<span style="margin-left: 1em;">hydrogen compounds <a href='#Page_201'>201</a></span><br />
+<span style="margin-left: 1em;">monoxide <a href='#Page_208'>208</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_196'>196</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_203'>203</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_200'>200</a></span><br />
+<span style="margin-left: 1em;">pure <a href='#Page_198'>198</a></span><br />
+<span style="margin-left: 1em;">retort <a href='#Page_199'>199</a></span><br />
+<span style="margin-left: 1em;">uses <a href='#Page_200'>200</a></span><br />
+<br />
+Carbonates <a href='#Page_207'>207</a><br />
+<span style="margin-left: 1em;">acid <a href='#Page_207'>207</a></span><br />
+<br />
+Carbonic acid <a href='#Page_206'>206</a><br />
+<br />
+Carborundum <a href='#Page_259'>259</a><br />
+<br />
+Carnallite <a href='#Page_288'>288</a><br />
+<br />
+<span class='pagenum'><a name="Page_424" id="Page_424">[Pg 424]</a></span>Casein <a href='#Page_414'>414</a><br />
+<br />
+Cassiterite <a href='#Page_370'>370</a><br />
+<br />
+Catalysis <a href='#Page_153'>153</a><br />
+<br />
+Catalyzers <a href='#Page_153'>153</a><br />
+<br />
+Cathode <a href='#Page_99'>99</a><br />
+<br />
+Cation <a href='#Page_106'>106</a><br />
+<br />
+Caustic potash <a href='#Page_288'>288</a><br />
+<span style="margin-left: 1em;">soda <a href='#Page_278'>278</a></span><br />
+<br />
+Celestite <a href='#Page_312'>312</a><br />
+<br />
+Celluloid <a href='#Page_415'>415</a><br />
+<br />
+Cellulose <a href='#Page_415'>415</a><br />
+<br />
+Cement <a href='#Page_304'>304</a><br />
+<br />
+Ceramic industries <a href='#Page_336'>336</a><br />
+<br />
+Cerium <a href='#Page_377'>377</a><br />
+<br />
+Chalcedony <a href='#Page_260'>260</a><br />
+<br />
+Chalcocite <a href='#Page_357'>357</a><br />
+<br />
+Chalcopyrite <a href='#Page_357'>357</a><br />
+<br />
+Chalk <a href='#Page_305'>305</a><br />
+<br />
+Chamber acid <a href='#Page_157'>157</a><br />
+<br />
+Changes, physical and chemical <a href='#Page_2'>2</a><br />
+<br />
+Charcoal <a href='#Page_199'>199</a><br />
+<br />
+Chemical affinity <a href='#Page_12'>12</a><br />
+<span style="margin-left: 1em;">changes <a href='#Page_2'>2</a></span><br />
+<span style="margin-left: 1em;">compounds <a href='#Page_7'>7</a></span><br />
+<span style="margin-left: 1em;">equilibrium <a href='#Page_128'>128</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_3'>3</a></span><br />
+<br />
+Chemistry, definition <a href='#Page_4'>4</a><br />
+<br />
+Chili saltpeter 191, <a href='#Page_285'>285</a><br />
+<br />
+Chinaware <a href='#Page_336'>336</a><br />
+<br />
+Chloric acid <a href='#Page_187'>187</a><br />
+<br />
+Chlorides <a href='#Page_186'>186</a><br />
+<br />
+Chlorine <a href='#Page_177'>177</a><br />
+<span style="margin-left: 1em;">bleaching action <a href='#Page_181'>181</a></span><br />
+<span style="margin-left: 1em;">chemical properties <a href='#Page_180'>180</a></span><br />
+<span style="margin-left: 1em;">family <a href='#Page_174'>174</a></span><br />
+<span style="margin-left: 1em;">historical <a href='#Page_177'>177</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_178'>178</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_187'>187</a></span><br />
+<span style="margin-left: 1em;">oxygen acids <a href='#Page_187'>187</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_178'>178</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_179'>179</a></span><br />
+<br />
+Chloroform <a href='#Page_401'>401</a><br />
+<br />
+Chloroplatinic acid <a href='#Page_393'>393</a><br />
+<br />
+Chlorous acid <a href='#Page_187'>187</a><br />
+<br />
+Chromates <a href='#Page_385'>385</a><br />
+<br />
+Chrome alum <a href='#Page_384'>384</a><br />
+<br />
+Chromic acid <a href='#Page_388'>388</a><br />
+<span style="margin-left: 1em;">anhydride <a href='#Page_387'>387</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_383'>383</a></span><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_383'>383</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_384'>384</a></span><br />
+<span style="margin-left: 1em;">sulphide <a href='#Page_384'>384</a></span><br />
+<br />
+Chromite <a href='#Page_383'>383</a><br />
+<br />
+Chromium <a href='#Page_383'>383</a><br />
+<span style="margin-left: 1em;">a base-forming element <a href='#Page_383'>383</a></span><br />
+<span style="margin-left: 1em;">an acid-forming element <a href='#Page_385'>385</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_383'>383</a></span><br />
+<br />
+Cinnabar <a href='#Page_363'>363</a><br />
+<br />
+Citric acid <a href='#Page_408'>408</a><br />
+<br />
+Clay <a href='#Page_336'>336</a><br />
+<br />
+Coal <a href='#Page_199'>199</a><br />
+<span style="margin-left: 1em;">gas <a href='#Page_217'>217</a></span><br />
+<span style="margin-left: 1em;">products <a href='#Page_400'>400</a></span><br />
+<span style="margin-left: 1em;">tar <a href='#Page_218'>218</a></span><br />
+<br />
+Cobalt <a href='#Page_354'>354</a><br />
+<span style="margin-left: 1em;">compounds <a href='#Page_354'>354</a></span><br />
+<br />
+Cocaine <a href='#Page_419'>419</a><br />
+<br />
+Coke <a href='#Page_199'>199</a><br />
+<br />
+Collodion <a href='#Page_415'>415</a><br />
+<br />
+Colemanite <a href='#Page_265'>265</a><br />
+<br />
+Combining weights <a href='#Page_225'>225</a><br />
+<br />
+Combustion <a href='#Page_17'>17</a><br />
+<span style="margin-left: 1em;">broad sense <a href='#Page_20'>20</a></span><br />
+<span style="margin-left: 1em;">in air <a href='#Page_19'>19</a></span><br />
+<span style="margin-left: 1em;">phlogiston theory <a href='#Page_19'>19</a></span><br />
+<span style="margin-left: 1em;">products <a href='#Page_18'>18</a></span><br />
+<span style="margin-left: 1em;">spontaneous <a href='#Page_20'>20</a></span><br />
+<span style="margin-left: 1em;">supporters <a href='#Page_213'>213</a></span><br />
+<br />
+Compounds, chemical <a href='#Page_7'>7</a><br />
+<span style="margin-left: 1em;">isomeric <a href='#Page_414'>414</a></span><br />
+<span style="margin-left: 1em;">of metals, preparation <a href='#Page_265'>265</a></span><br />
+<span style="margin-left: 1em;">structure of <a href='#Page_118'>118</a></span><br />
+<br />
+Conservation of energy <a href='#Page_4'>4</a><br />
+<span style="margin-left: 1em;">of matter <a href='#Page_5'>5</a></span><br />
+<br />
+Contact process <a href='#Page_154'>154</a><br />
+<br />
+Converter, Bessemer <a href='#Page_345'>345</a><br />
+<br />
+Copper <a href='#Page_357'>357</a><br />
+<span style="margin-left: 1em;">acetate <a href='#Page_407'>407</a></span><br />
+<span style="margin-left: 1em;">alloys of <a href='#Page_359'>359</a></span><br />
+<span style="margin-left: 1em;">family <a href='#Page_356'>356</a></span><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_360'>360</a></span><br />
+<span style="margin-left: 1em;">metallurgy <a href='#Page_357'>357</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_357'>357</a></span><br />
+<span style="margin-left: 1em;">ores <a href='#Page_357'>357</a></span><br />
+<span style="margin-left: 1em;">oxide <a href='#Page_360'>360</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_358'>358</a></span><br />
+<span style="margin-left: 1em;">refining <a href='#Page_358'>358</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_361'>361</a></span><br />
+<span style="margin-left: 1em;">sulphide <a href='#Page_361'>361</a></span><br />
+<span style="margin-left: 1em;">uses <a href='#Page_359'>359</a></span><br />
+<br />
+Copperas <a href='#Page_350'>350</a><br />
+<br />
+Coral <a href='#Page_305'>305</a><br />
+<br />
+<span class='pagenum'><a name="Page_425" id="Page_425">[Pg 425]</a></span>Corrosive sublimate <a href='#Page_363'>363</a><br />
+<br />
+Corundum <a href='#Page_331'>331</a><br />
+<br />
+Cream of tartar <a href='#Page_408'>408</a><br />
+<br />
+Crocoisite <a href='#Page_383'>383</a><br />
+<br />
+Cryolite <a href="#Page_175">175</a>, <a href='#Page_328'>328</a><br />
+<br />
+Crystallization <a href='#Page_98'>98</a><br />
+<span style="margin-left: 1em;">water of <a href="#Page_54">54</a>, <a href='#Page_75'>75</a></span><br />
+<br />
+Crystallography <a href='#Page_161'>161</a><br />
+<br />
+Crystals <a href='#Page_161'>161</a><br />
+<span style="margin-left: 1em;">axes of <a href='#Page_161'>161</a></span><br />
+<span style="margin-left: 1em;">systems <a href='#Page_162'>162</a></span><br />
+<br />
+Cupric compounds <a href='#Page_360'>360</a><br />
+<br />
+Cuprite <a href='#Page_360'>360</a><br />
+<br />
+Cuprous compounds <a href='#Page_360'>360</a><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_360'>360</a></span><br />
+<span style="margin-left: 1em;">oxide <a href='#Page_360'>360</a></span><br />
+<br />
+Cyanides <a href='#Page_210'>210</a><br />
+<span style="margin-left: 1em;">solutions are alkaline <a href='#Page_210'>210</a></span><br />
+<br />
+<br />
+Dalton's atomic hypothesis <a href='#Page_61'>61</a><br />
+<br />
+Decay <a href='#Page_21'>21</a><br />
+<br />
+Decomposition of organic matter <a href='#Page_122'>122</a><br />
+<br />
+Decrepitation <a href='#Page_55'>55</a><br />
+<br />
+Deliquescence <a href='#Page_55'>55</a><br />
+<br />
+Density of gases <a href='#Page_230'>230</a><br />
+<br />
+Desiccating agents <a href='#Page_55'>55</a><br />
+<br />
+Developers <a href='#Page_367'>367</a><br />
+<br />
+Dewar bulb <a href='#Page_91'>91</a><br />
+<br />
+Dextrose <a href='#Page_414'>414</a><br />
+<br />
+Diamond <a href='#Page_197'>197</a><br />
+<br />
+Dichromates <a href='#Page_385'>385</a><br />
+<br />
+Dichromic acid <a href='#Page_385'>385</a><br />
+<br />
+Dimorphous substances <a href='#Page_163'>163</a><br />
+<br />
+Dissociation <a href='#Page_99'>99</a><br />
+<span style="margin-left: 1em;">and boiling point <a href='#Page_101'>101</a></span><br />
+<span style="margin-left: 1em;">and freezing point <a href='#Page_101'>101</a></span><br />
+<span style="margin-left: 1em;">equations of <a href='#Page_112'>112</a></span><br />
+<span style="margin-left: 1em;">extent of <a href='#Page_113'>113</a></span><br />
+<br />
+Distillation <a href='#Page_50'>50</a><br />
+<br />
+Dogtooth spar <a href='#Page_306'>306</a><br />
+<br />
+Dolomite <a href='#Page_319'>319</a><br />
+<br />
+Double decomposition <a href='#Page_71'>71</a><br />
+<br />
+Drummond light <a href='#Page_38'>38</a><br />
+<br />
+Dyeing  <a href='#Page_333'>333</a><br />
+<br />
+Dynamite <a href='#Page_409'>409</a><br />
+<br />
+<br />
+Earth metals <a href='#Page_327'>327</a><br />
+<br />
+Efflorescence <a href='#Page_54'>54</a><br />
+<br />
+Electric furnace <a href='#Page_221'>221</a><br />
+<br />
+Electro-chemical industries <a href='#Page_269'>269</a><br />
+<br />
+Electrode <a href='#Page_99'>99</a><br />
+<br />
+Electrolysis <a href='#Page_99'>99</a><br />
+<span style="margin-left: 1em;">of sodium chloride <a href='#Page_102'>102</a></span><br />
+<span style="margin-left: 1em;">of sodium sulphate <a href='#Page_103'>103</a></span><br />
+<span style="margin-left: 1em;">of water <a href="#Page_41">41</a>, <a href='#Page_102'>102</a></span><br />
+<br />
+Electrolytes <a href='#Page_99'>99</a><br />
+<br />
+Electrolytic dissociation <a href='#Page_99'>99</a><br />
+<br />
+Electroplating <a href='#Page_366'>366</a><br />
+<br />
+Electrotyping <a href='#Page_359'>359</a><br />
+<br />
+Elements, definition <a href='#Page_8'>8</a><br />
+<span style="margin-left: 1em;">atomic weights <a href='#Page_232'>232</a></span><br />
+<span style="margin-left: 1em;">earlier classification <a href='#Page_165'>165</a></span><br />
+<span style="margin-left: 1em;">names <a href='#Page_11'>11</a></span><br />
+<span style="margin-left: 1em;">natural groups <a href='#Page_165'>165</a></span><br />
+<span style="margin-left: 1em;">number of <a href='#Page_9'>9</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_10'>10</a></span><br />
+<span style="margin-left: 1em;">periodic division <a href='#Page_166'>166</a></span><br />
+<span style="margin-left: 1em;">physical state <a href='#Page_10'>10</a></span><br />
+<span style="margin-left: 1em;">symbols of <a href='#Page_11'>11</a></span><br />
+<br />
+Emery <a href='#Page_331'>331</a><br />
+<br />
+Energy <a href='#Page_4'>4</a><br />
+<span style="margin-left: 1em;">and plant life <a href='#Page_89'>89</a></span><br />
+<span style="margin-left: 1em;">chemical <a href='#Page_5'>5</a></span><br />
+<span style="margin-left: 1em;">conservation of <a href='#Page_4'>4</a></span><br />
+<span style="margin-left: 1em;">transformation of <a href='#Page_5'>5</a></span><br />
+<br />
+Enzyme <a href='#Page_405'>405</a><br />
+<br />
+Epsom salts <a href='#Page_320'>320</a><br />
+<br />
+Equations <a href='#Page_68'>68</a><br />
+<span style="margin-left: 1em;">are quantitative <a href='#Page_72'>72</a></span><br />
+<span style="margin-left: 1em;">knowledge requisite for <a href='#Page_69'>69</a></span><br />
+<span style="margin-left: 1em;">not algebraic <a href='#Page_74'>74</a></span><br />
+<span style="margin-left: 1em;">reading of <a href='#Page_69'>69</a></span><br />
+<br />
+Equilibrium <a href='#Page_138'>138</a><br />
+<span style="margin-left: 1em;">chemical <a href='#Page_138'>138</a></span><br />
+<span style="margin-left: 1em;">in solution <a href='#Page_139'>139</a></span><br />
+<span style="margin-left: 1em;">point of <a href='#Page_138'>138</a></span><br />
+<br />
+Equivalent <a href='#Page_224'>224</a><br />
+<span style="margin-left: 1em;">determination of <a href='#Page_224'>224</a></span><br />
+<span style="margin-left: 1em;">elements with more than one <a href='#Page_225'>225</a></span><br />
+<span style="margin-left: 1em;">relation to atomic weight <a href='#Page_224'>224</a></span><br />
+<br />
+Etching <a href='#Page_177'>177</a><br />
+<br />
+Ether <a href='#Page_411'>411</a><br />
+<br />
+Ethereal salts <a href='#Page_409'>409</a><br />
+<br />
+Ethers <a href='#Page_411'>411</a><br />
+<br />
+Ethylamine <a href='#Page_412'>412</a><br />
+<br />
+Ethylene series <a href='#Page_399'>399</a><br />
+<br />
+Eudiometer <a href='#Page_43'>43</a><br />
+<br />
+Evaporation  <a href='#Page_11'>11</a><br />
+<br />
+<br />
+Families in periodic groups <a href='#Page_170'>170</a><br />
+<span class='pagenum'><a name="Page_426" id="Page_426">[Pg 426]</a></span><span style="margin-left: 1em;">triads <a href='#Page_165'>165</a></span><br />
+<br />
+Family resemblances <a href='#Page_170'>170</a><br />
+<br />
+Fats <a href='#Page_409'>409</a><br />
+<br />
+Fatty acid series <a href='#Page_405'>405</a><br />
+<br />
+Feldspar <a href="#Page_261">261</a>, <a href='#Page_335'>335</a><br />
+<br />
+Fermentation <a href='#Page_404'>404</a><br />
+<span style="margin-left: 1em;">acetic <a href='#Page_406'>406</a></span><br />
+<span style="margin-left: 1em;">alcoholic <a href="#Page_404">404</a>, <a href='#Page_405'>405</a></span><br />
+<span style="margin-left: 1em;">lactic <a href='#Page_414'>414</a></span><br />
+<br />
+Ferric chloride <a href='#Page_352'>352</a><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_352'>352</a></span><br />
+<span style="margin-left: 1em;">salts <a href='#Page_351'>351</a></span><br />
+<span style="margin-left: 2em;">reduction <a href='#Page_353'>353</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_352'>352</a></span><br />
+<br />
+Ferrochromium, <a href='#Page_383'>383</a><br />
+<br />
+Ferromanganese <a href='#Page_343'>343</a><br />
+<br />
+Ferrosilicon <a href='#Page_259'>259</a><br />
+<br />
+Ferrous carbonate <a href='#Page_351'>351</a><br />
+<span style="margin-left: 1em;">salts <a href='#Page_350'>350</a></span><br />
+<span style="margin-left: 2em;">oxidation of <a href='#Page_353'>353</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_350'>350</a></span><br />
+<span style="margin-left: 1em;">sulphide <a href='#Page_350'>350</a></span><br />
+<br />
+Fertilizers <a href='#Page_245'>245</a><br />
+<br />
+Filtration <a href="#Page_6">6</a>, <a href='#Page_51'>51</a><br />
+<span style="margin-left: 1em;">beds <a href='#Page_52'>52</a></span><br />
+<br />
+Fire damp <a href='#Page_202'>202</a><br />
+<br />
+Flames <a href='#Page_213'>213</a><br />
+<span style="margin-left: 1em;">appearance <a href='#Page_214'>214</a></span><br />
+<span style="margin-left: 1em;">blowpipe <a href='#Page_216'>216</a></span><br />
+<span style="margin-left: 1em;">Bunsen <a href='#Page_214'>214</a></span><br />
+<span style="margin-left: 1em;">conditions for <a href='#Page_213'>213</a></span><br />
+<span style="margin-left: 1em;">hydrogen <a href='#Page_34'>34</a></span><br />
+<span style="margin-left: 1em;">luminosity <a href='#Page_216'>216</a></span><br />
+<span style="margin-left: 1em;">oxidizing <a href='#Page_214'>214</a></span><br />
+<span style="margin-left: 1em;">oxyhydrogen <a href='#Page_37'>37</a></span><br />
+<span style="margin-left: 1em;">reactions <a href='#Page_296'>296</a></span><br />
+<span style="margin-left: 1em;">reducing <a href='#Page_214'>214</a></span><br />
+<span style="margin-left: 1em;">structure <a href='#Page_214'>214</a></span><br />
+<br />
+Flash lights <a href='#Page_317'>317</a><br />
+<br />
+Flint <a href='#Page_260'>260</a><br />
+<br />
+Fluorides <a href='#Page_177'>177</a><br />
+<br />
+Fluorine <a href='#Page_175'>175</a><br />
+<br />
+Fluorspar 175, <a href='#Page_308'>308</a><br />
+<br />
+Fluosilicic acid <a href='#Page_259'>259</a><br />
+<br />
+Flux <a href='#Page_340'>340</a><br />
+<br />
+Fool's gold <a href='#Page_351'>351</a><br />
+<br />
+Formaldehyde <a href='#Page_405'>405</a><br />
+<br />
+Formalin <a href='#Page_405'>405</a><br />
+<br />
+Formic acid <a href='#Page_406'>406</a><br />
+<br />
+Formulas <a href='#Page_68'>68</a><br />
+<span style="margin-left: 1em;">how determined <a href='#Page_234'>234</a></span><br />
+<span style="margin-left: 1em;">structural <a href='#Page_119'>119</a></span><br />
+<br />
+Fractional distillation <a href='#Page_51'>51</a><br />
+<br />
+Franklinite <a href='#Page_321'>321</a><br />
+<br />
+Fuels <a href='#Page_220'>220</a><br />
+<br />
+Furnace, arc <a href='#Page_221'>221</a><br />
+<span style="margin-left: 1em;">electric <a href='#Page_221'>221</a></span><br />
+<span style="margin-left: 1em;">resistance <a href='#Page_221'>221</a></span><br />
+<br />
+Fusion methods <a href='#Page_271'>271</a><br />
+<br />
+<br />
+Galena <a href='#Page_373'>373</a><br />
+<br />
+Gallium <a href='#Page_327'>327</a><br />
+<br />
+Galvanized iron <a href='#Page_323'>323</a><br />
+<br />
+Gas, collection of <a href='#Page_15'>15</a><br />
+<span style="margin-left: 1em;">coal <a href='#Page_217'>217</a></span><br />
+<span style="margin-left: 1em;">fuel <a href='#Page_217'>217</a></span><br />
+<span style="margin-left: 1em;">illuminating <a href='#Page_217'>217</a></span><br />
+<span style="margin-left: 1em;">measurement of <a href='#Page_23'>23</a></span><br />
+<span style="margin-left: 1em;">natural <a href='#Page_219'>219</a></span><br />
+<span style="margin-left: 1em;">purification of <a href='#Page_218'>218</a></span><br />
+<span style="margin-left: 1em;">water <a href='#Page_219'>219</a></span><br />
+<br />
+Gases, table <a href='#Page_220'>220</a><br />
+<br />
+Gasoline <a href='#Page_400'>400</a><br />
+<br />
+German silver <a href="#Page_323">323</a>, <a href='#Page_359'>359</a><br />
+<br />
+Germanium <a href='#Page_370'>370</a><br />
+<br />
+Germs, effect of cold on <a href='#Page_53'>53</a><br />
+<span style="margin-left: 1em;">in air <a href='#Page_84'>84</a></span><br />
+<span style="margin-left: 1em;">in water <a href='#Page_52'>52</a></span><br />
+<br />
+Glass <a href='#Page_262'>262</a><br />
+<span style="margin-left: 1em;">coloring of <a href='#Page_263'>263</a></span><br />
+<span style="margin-left: 1em;">etching of <a href='#Page_177'>177</a></span><br />
+<span style="margin-left: 1em;">molding of <a href='#Page_263'>263</a></span><br />
+<span style="margin-left: 1em;">nature of <a href='#Page_263'>263</a></span><br />
+<span style="margin-left: 1em;">varieties <a href='#Page_263'>263</a></span><br />
+<br />
+Glauber's salt <a href='#Page_281'>281</a><br />
+<br />
+Glazing <a href='#Page_336'>336</a><br />
+<br />
+Glucose <a href='#Page_414'>414</a><br />
+<br />
+Glycerin <a href='#Page_405'>405</a><br />
+<span style="margin-left: 1em;">nitrates of <a href='#Page_409'>409</a></span><br />
+<br />
+Gold <a href='#Page_393'>393</a><br />
+<span style="margin-left: 1em;">alloys <a href='#Page_396'>396</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_396'>396</a></span><br />
+<span style="margin-left: 1em;">coin <a href='#Page_359'>359</a></span><br />
+<span style="margin-left: 1em;">extraction of <a href='#Page_394'>394</a></span><br />
+<span style="margin-left: 1em;">in copper <a href='#Page_358'>358</a></span><br />
+<span style="margin-left: 1em;">mining <a href='#Page_394'>394</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_393'>393</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_396'>396</a></span><br />
+<span style="margin-left: 1em;">refining of <a href='#Page_395'>395</a></span><br />
+<span style="margin-left: 1em;">telluride <a href='#Page_394'>394</a></span><br />
+<br />
+Goldschmidt method <a href="#Page_269">269</a>, <a href='#Page_330'>330</a><br />
+<br />
+Gram-molecular weight <a href='#Page_236'>236</a><br />
+<br />
+<span class='pagenum'><a name="Page_427" id="Page_427">[Pg 427]</a></span>Granite <a href='#Page_336'>336</a><br />
+<br />
+Graphite <a href='#Page_198'>198</a><br />
+<br />
+Gun cotton <a href='#Page_415'>415</a><br />
+<span style="margin-left: 1em;">metal <a href='#Page_359'>359</a></span><br />
+<span style="margin-left: 1em;">powder <a href='#Page_292'>292</a></span><br />
+<br />
+Gypsite <a href='#Page_308'>308</a><br />
+<br />
+Gypsum <a href='#Page_308'>308</a><br />
+<br />
+<br />
+Halogens <a href='#Page_174'>174</a><br />
+<br />
+Hard water <a href='#Page_309'>309</a><br />
+<br />
+Heat of reaction <a href='#Page_75'>75</a><br />
+<br />
+Helium <a href="#Page_80">80</a>, <a href='#Page_314'>314</a><br />
+<br />
+Hematite <a href="#Page_339">339</a>, <a href='#Page_349'>349</a><br />
+<br />
+Homologous series <a href='#Page_398'>398</a><br />
+<br />
+Hydriodic acid <a href='#Page_193'>193</a><br />
+<br />
+Hydrobromic acid <a href='#Page_189'>189</a><br />
+<br />
+Hydrocarbons <a href="#Page_201">201</a>, <a href='#Page_398'>398</a><br />
+<span style="margin-left: 1em;">properties <a href='#Page_400'>400</a></span><br />
+<span style="margin-left: 1em;">series <a href='#Page_398'>398</a></span><br />
+<span style="margin-left: 1em;">substitution products <a href='#Page_401'>401</a></span><br />
+<br />
+Hydrochloric acid <a href='#Page_182'>182</a><br />
+<span style="margin-left: 1em;">composition <a href='#Page_183'>183</a></span><br />
+<span style="margin-left: 1em;">oxidation of <a href='#Page_185'>185</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_182'>182</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_184'>184</a></span><br />
+<span style="margin-left: 1em;">salts <a href='#Page_186'>186</a></span><br />
+<br />
+Hydrocyanic acid <a href='#Page_210'>210</a><br />
+<br />
+Hydrofluoric acid <a href='#Page_176'>176</a><br />
+<span style="margin-left: 1em;">etching by <a href='#Page_177'>177</a></span><br />
+<span style="margin-left: 1em;">salts of <a href='#Page_177'>177</a></span><br />
+<br />
+Hydrogen <a href='#Page_28'>28</a><br />
+<span style="margin-left: 1em;">dioxide <a href='#Page_56'>56</a></span><br />
+<span style="margin-left: 1em;">explosive with oxygen <a href='#Page_35'>35</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_28'>28</a></span><br />
+<span style="margin-left: 1em;">preparation from acids <a href='#Page_30'>30</a></span><br />
+<span style="margin-left: 1em;">preparation from water <a href='#Page_28'>28</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_32'>32</a></span><br />
+<span style="margin-left: 1em;">standard for atomic weights <a href='#Page_66'>66</a></span><br />
+<span style="margin-left: 1em;">standard for molecular weights <a href='#Page_227'>227</a></span><br />
+<span style="margin-left: 1em;">sulphide <a href='#Page_146'>146</a></span><br />
+<span style="margin-left: 1em;">uses <a href='#Page_38'>38</a></span><br />
+<br />
+Hydrolysis <a href='#Page_254'>254</a><br />
+<span style="margin-left: 1em;">conditions affecting <a href='#Page_255'>255</a></span><br />
+<span style="margin-left: 1em;">partial <a href='#Page_255'>255</a></span><br />
+<br />
+Hydrosulphuric acid <a href='#Page_146'>146</a><br />
+<br />
+Hydroxyl radical <a href='#Page_112'>112</a><br />
+<br />
+Hypochlorous acid <a href='#Page_187'>187</a><br />
+<br />
+Hypothesis <a href='#Page_61'>61</a><br />
+<span style="margin-left: 1em;">Avogadro's <a href='#Page_226'>226</a></span><br />
+<span style="margin-left: 1em;">Dalton's <a href='#Page_61'>61</a></span><br />
+<br />
+<br />
+Ice manufacture <a href='#Page_125'>125</a><br />
+<br />
+Iceland spar <a href='#Page_305'>305</a><br />
+<br />
+Indigo <a href='#Page_418'>418</a><br />
+<br />
+Indium <a href='#Page_327'>327</a><br />
+<br />
+Insoluble compounds <a href='#Page_272'>272</a><br />
+<br />
+Iodic acid <a href='#Page_194'>194</a><br />
+<br />
+Iodides <a href='#Page_193'>193</a><br />
+<br />
+Iodine <a href='#Page_190'>190</a><br />
+<span style="margin-left: 1em;">oxygen compounds <a href='#Page_193'>193</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_191'>191</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_192'>192</a></span><br />
+<span style="margin-left: 1em;">tincture <a href='#Page_192'>192</a></span><br />
+<br />
+Iodoform 192, <a href='#Page_401'>401</a><br />
+<br />
+Ions <a href='#Page_100'>100</a><br />
+<span style="margin-left: 1em;">and electrolytes <a href='#Page_104'>104</a></span><br />
+<br />
+Iridium <a href='#Page_393'>393</a><br />
+<br />
+Iron <a href='#Page_339'>339</a><br />
+<span style="margin-left: 1em;">alum <a href='#Page_352'>352</a></span><br />
+<span style="margin-left: 1em;">cast <a href='#Page_343'>343</a></span><br />
+<span style="margin-left: 1em;">compounds <a href='#Page_349'>349</a></span><br />
+<span style="margin-left: 1em;">cyanides <a href='#Page_352'>352</a></span><br />
+<span style="margin-left: 1em;">disulphide <a href='#Page_351'>351</a></span><br />
+<span style="margin-left: 1em;">family <a href='#Page_338'>338</a></span><br />
+<span style="margin-left: 1em;">metallurgy <a href='#Page_339'>339</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_339'>339</a></span><br />
+<span style="margin-left: 1em;">ores <a href='#Page_339'>339</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_349'>349</a></span><br />
+<span style="margin-left: 1em;">pure <a href='#Page_348'>348</a></span><br />
+<span style="margin-left: 1em;">varieties <a href="#Page_342">342</a>, <a href='#Page_347'>347</a></span><br />
+<span style="margin-left: 1em;">wrought <a href='#Page_343'>343</a></span><br />
+<br />
+<br />
+Jasper <a href='#Page_260'>260</a><br />
+<br />
+<br />
+Kainite <a href='#Page_288'>288</a><br />
+<br />
+Kaolin 261, <a href='#Page_335'>335</a><br />
+<br />
+Kerosene <a href='#Page_400'>400</a><br />
+<br />
+Ketones <a href='#Page_411'>411</a><br />
+<br />
+Kieserite <a href='#Page_288'>288</a><br />
+<br />
+Kindling temperature <a href='#Page_17'>17</a><br />
+<br />
+Krypton <a href='#Page_80'>80</a><br />
+<br />
+<br />
+Lactic acid <a href='#Page_408'>408</a><br />
+<br />
+Lampblack <a href='#Page_200'>200</a><br />
+<br />
+Laughing gas <a href='#Page_132'>132</a><br />
+<br />
+Law, definition <a href='#Page_61'>61</a><br />
+<span style="margin-left: 1em;">of Boyle <a href='#Page_24'>24</a></span><br />
+<span style="margin-left: 1em;">of Charles <a href='#Page_23'>23</a></span><br />
+<span style="margin-left: 1em;">of combining volumes <a href='#Page_194'>194</a></span><br />
+<span style="margin-left: 1em;">of conservation of energy <a href='#Page_4'>4</a></span><br />
+<span style="margin-left: 1em;">of conservation of matter <a href="#Page_5">5</a>, <a href='#Page_59'>59</a></span><br />
+<span class='pagenum'><a name="Page_428" id="Page_428">[Pg 428]</a></span><span style="margin-left: 1em;">of definite composition <a href='#Page_59'>59</a></span><br />
+<span style="margin-left: 1em;">of Dulong and Petit <a href='#Page_233'>233</a></span><br />
+<span style="margin-left: 1em;">of Gay-Lussac <a href='#Page_194'>194</a></span><br />
+<span style="margin-left: 1em;">of multiple proportion <a href='#Page_60'>60</a></span><br />
+<span style="margin-left: 1em;">of Raoult <a href='#Page_233'>233</a></span><br />
+<span style="margin-left: 1em;">periodic <a href='#Page_169'>169</a></span><br />
+<br />
+Lead <a href='#Page_373'>373</a><br />
+<span style="margin-left: 1em;">acetate <a href="#Page_375">375</a>, <a href='#Page_407'>407</a></span><br />
+<span style="margin-left: 1em;">alloys <a href='#Page_375'>375</a></span><br />
+<span style="margin-left: 1em;">basic carbonate <a href='#Page_376'>376</a></span><br />
+<span style="margin-left: 1em;">carbonate <a href='#Page_376'>376</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_377'>377</a></span><br />
+<span style="margin-left: 1em;">chromate <a href='#Page_377'>377</a></span><br />
+<span style="margin-left: 1em;">insoluble compounds <a href='#Page_376'>376</a></span><br />
+<span style="margin-left: 1em;">metallurgy <a href='#Page_373'>373</a></span><br />
+<span style="margin-left: 1em;">nitrate <a href='#Page_375'>375</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_373'>373</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_375'>375</a></span><br />
+<span style="margin-left: 1em;">peroxide <a href='#Page_375'>375</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_374'>374</a></span><br />
+<span style="margin-left: 1em;">red <a href='#Page_375'>375</a></span><br />
+<span style="margin-left: 1em;">soluble salts <a href='#Page_375'>375</a></span><br />
+<span style="margin-left: 1em;">sugar of <a href='#Page_375'>375</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_377'>377</a></span><br />
+<span style="margin-left: 1em;">sulphide <a href='#Page_377'>377</a></span><br />
+<span style="margin-left: 1em;">white <a href='#Page_376'>376</a></span><br />
+<br />
+Le Blanc soda process <a href='#Page_282'>282</a><br />
+<br />
+Levulose <a href='#Page_415'>415</a><br />
+<br />
+Lime <a href='#Page_302'>302</a><br />
+<span style="margin-left: 1em;">air-slaked <a href='#Page_303'>303</a></span><br />
+<span style="margin-left: 1em;">hypochlorite <a href='#Page_307'>307</a></span><br />
+<span style="margin-left: 1em;">kilns <a href='#Page_303'>303</a></span><br />
+<span style="margin-left: 1em;">slaked <a href='#Page_303'>303</a></span><br />
+<br />
+Lime light <a href='#Page_38'>38</a><br />
+<br />
+Limestone <a href='#Page_305'>305</a><br />
+<br />
+Limewater <a href='#Page_303'>303</a><br />
+<br />
+Limonite <a href='#Page_339'>339</a><br />
+<br />
+Litharge <a href='#Page_375'>375</a><br />
+<br />
+Lithium <a href='#Page_294'>294</a><br />
+<br />
+Luminosity of flames <a href='#Page_216'>216</a><br />
+<br />
+Lunar caustic <a href='#Page_366'>366</a><br />
+<br />
+<br />
+Magnesia <a href='#Page_318'>318</a><br />
+<span style="margin-left: 1em;">alba <a href='#Page_319'>319</a></span><br />
+<span style="margin-left: 1em;">usta <a href='#Page_318'>318</a></span><br />
+<br />
+Magnesite <a href='#Page_318'>318</a><br />
+<br />
+Magnesium <a href='#Page_317'>317</a><br />
+<span style="margin-left: 1em;">basic carbonate <a href='#Page_319'>319</a></span><br />
+<span style="margin-left: 1em;">carbonate <a href='#Page_318'>318</a></span><br />
+<span style="margin-left: 1em;">cement <a href='#Page_318'>318</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_319'>319</a></span><br />
+<span style="margin-left: 1em;">family <a href='#Page_316'>316</a></span><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_318'>318</a></span><br />
+<span style="margin-left: 1em;">oxide <a href='#Page_318'>318</a></span><br />
+<span style="margin-left: 1em;">silicates <a href='#Page_321'>321</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_320'>320</a></span><br />
+<br />
+Magnetite <a href="#Page_339">339</a>, <a href='#Page_349'>349</a><br />
+<br />
+Malachite <a href='#Page_357'>357</a><br />
+<br />
+Manganates <a href='#Page_381'>381</a><br />
+<br />
+Manganese <a href='#Page_379'>379</a><br />
+<span style="margin-left: 1em;">a base-forming element <a href='#Page_380'>380</a></span><br />
+<span style="margin-left: 1em;">an acid-forming element <a href='#Page_381'>381</a></span><br />
+<span style="margin-left: 1em;">in glass <a href='#Page_263'>263</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_379'>379</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_380'>380</a></span><br />
+<br />
+Manganic acid <a href='#Page_381'>381</a><br />
+<br />
+Manganous salts <a href='#Page_380'>380</a><br />
+<br />
+Marble <a href='#Page_305'>305</a><br />
+<br />
+Marl <a href='#Page_305'>305</a><br />
+<br />
+Marsh gas <a href='#Page_202'>202</a><br />
+<br />
+Matches <a href='#Page_242'>242</a><br />
+<br />
+Matte <a href='#Page_358'>358</a><br />
+<br />
+Matter, classification <a href='#Page_6'>6</a><br />
+<span style="margin-left: 1em;">conservation <a href='#Page_5'>5</a></span><br />
+<span style="margin-left: 1em;">definition <a href='#Page_5'>5</a></span><br />
+<span style="margin-left: 1em;">kinds <a href='#Page_9'>9</a></span><br />
+<br />
+Measurement of gases <a href='#Page_23'>23</a><br />
+<br />
+Mechanical mixtures <a href='#Page_6'>6</a><br />
+<br />
+Meerschaum <a href="#Page_321">321</a>, <a href='#Page_336'>336</a><br />
+<br />
+Mercuric chloride <a href='#Page_363'>363</a><br />
+<span style="margin-left: 1em;">iodide <a href='#Page_364'>364</a></span><br />
+<span style="margin-left: 1em;">oxide 14, <a href='#Page_362'>362</a></span><br />
+<span style="margin-left: 1em;">sulphide <a href='#Page_363'>363</a></span><br />
+<br />
+Mercurous chloride <a href='#Page_363'>363</a><br />
+<br />
+Mercury <a href='#Page_361'>361</a><br />
+<span style="margin-left: 1em;">iodides <a href='#Page_364'>364</a></span><br />
+<span style="margin-left: 1em;">metallurgy <a href='#Page_361'>361</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_361'>361</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_362'>362</a></span><br />
+<span style="margin-left: 1em;">uses <a href='#Page_362'>362</a></span><br />
+<br />
+Metaboric acid <a href='#Page_265'>265</a><br />
+<br />
+Metallurgy <a href='#Page_268'>268</a><br />
+<br />
+Metals 165, <a href='#Page_267'>267</a><br />
+<span style="margin-left: 1em;">action on salts <a href='#Page_271'>271</a></span><br />
+<span style="margin-left: 1em;">definition <a href='#Page_267'>267</a></span><br />
+<span style="margin-left: 1em;">extraction <a href='#Page_268'>268</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_267'>267</a></span><br />
+<span style="margin-left: 1em;">preparation of compounds <a href='#Page_269'>269</a></span><br />
+<span style="margin-left: 1em;">reduction from ores <a href='#Page_268'>268</a></span><br />
+<br />
+Metaphosphoric acid <a href='#Page_245'>245</a><br />
+<br />
+Metarsenic acid <a href='#Page_250'>250</a><br />
+<br />
+Metasilicic acid <a href='#Page_261'>261</a><br />
+<br />
+<span class='pagenum'><a name="Page_429" id="Page_429">[Pg 429]</a></span>Metastannic acid <a href='#Page_371'>371</a><br />
+<br />
+Methane 202, <a href='#Page_399'>399</a><br />
+<br />
+Methylamine <a href='#Page_412'>412</a><br />
+<br />
+Mexican onyx <a href='#Page_305'>305</a><br />
+<br />
+Mica 261, <a href='#Page_336'>336</a><br />
+<br />
+Microcosmic salt <a href='#Page_244'>244</a><br />
+<br />
+Milk <a href='#Page_414'>414</a><br />
+<br />
+Minerals <a href='#Page_267'>267</a><br />
+<br />
+Minium <a href='#Page_375'>375</a><br />
+<br />
+Mixed salts <a href='#Page_244'>244</a><br />
+<br />
+Molasses <a href='#Page_413'>413</a><br />
+<br />
+Molecular weights <a href='#Page_226'>226</a><br />
+<span style="margin-left: 1em;">boiling-point method <a href='#Page_233'>233</a></span><br />
+<span style="margin-left: 1em;">compared with oxygen <a href='#Page_228'>228</a></span><br />
+<span style="margin-left: 1em;">determination <a href='#Page_226'>226</a></span><br />
+<span style="margin-left: 1em;">freezing-point method <a href='#Page_233'>233</a></span><br />
+<span style="margin-left: 1em;">oxygen standard <a href='#Page_227'>227</a></span><br />
+<span style="margin-left: 1em;">of elements <a href='#Page_232'>232</a></span><br />
+<span style="margin-left: 1em;">vapor-density method <a href='#Page_229'>229</a></span><br />
+<br />
+Molecule <a href='#Page_62'>62</a><br />
+<br />
+Molybdenum <a href='#Page_388'>388</a><br />
+<br />
+Molybdic acid <a href='#Page_388'>388</a><br />
+<br />
+Monazite sand <a href='#Page_377'>377</a><br />
+<br />
+Mordants <a href='#Page_333'>333</a><br />
+<br />
+Morphine <a href='#Page_418'>418</a><br />
+<br />
+Mortar <a href='#Page_304'>304</a><br />
+<br />
+Moth balls <a href='#Page_418'>418</a><br />
+<br />
+Muriatic acid <a href='#Page_182'>182</a><br />
+<br />
+<br />
+Naphthalene <a href='#Page_418'>418</a><br />
+<br />
+Naphthas <a href='#Page_400'>400</a><br />
+<br />
+Nascent state <a href='#Page_182'>182</a><br />
+<br />
+Natural gas <a href='#Page_219'>219</a><br />
+<span style="margin-left: 1em;">sciences <a href='#Page_1'>1</a></span><br />
+<br />
+Neon <a href='#Page_80'>80</a><br />
+<br />
+Neutralization <a href='#Page_108'>108</a><br />
+<span style="margin-left: 1em;">a definite act <a href='#Page_109'>109</a></span><br />
+<span style="margin-left: 1em;">definition <a href='#Page_109'>109</a></span><br />
+<span style="margin-left: 1em;">heat of <a href='#Page_109'>109</a></span><br />
+<span style="margin-left: 1em;">partial <a href='#Page_111'>111</a></span><br />
+<br />
+Niagara Falls <a href="#Page_269">269</a>, <a href='#Page_329'>329</a><br />
+<br />
+Nickel <a href='#Page_354'>354</a><br />
+<span style="margin-left: 1em;">coin <a href='#Page_359'>359</a></span><br />
+<span style="margin-left: 1em;">compounds <a href='#Page_354'>354</a></span><br />
+<span style="margin-left: 1em;">plating <a href='#Page_354'>354</a></span><br />
+<br />
+Nicotine <a href='#Page_419'>419</a><br />
+<br />
+Nitrates <a href='#Page_131'>131</a><br />
+<br />
+Nitric acid, <a href='#Page_128'>128</a><br />
+<span style="margin-left: 1em;">action on metals <a href='#Page_130'>130</a></span><br />
+<span style="margin-left: 1em;">decomposition <a href='#Page_129'>129</a></span><br />
+<span style="margin-left: 1em;">oxidizing action <a href='#Page_130'>130</a></span><br />
+<span style="margin-left: 1em;">preparation <a href="#Page_128">128</a>, <a href='#Page_140'>140</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_129'>129</a></span><br />
+<span style="margin-left: 1em;">salts <a href='#Page_131'>131</a></span><br />
+<br />
+Nitric oxide <a href='#Page_133'>133</a><br />
+<br />
+Nitrites <a href='#Page_132'>132</a><br />
+<br />
+Nitrobenzene <a href='#Page_417'>417</a><br />
+<br />
+Nitrocellulose <a href='#Page_415'>415</a><br />
+<br />
+Nitrogen <a href='#Page_78'>78</a><br />
+<span style="margin-left: 1em;">compounds <a href='#Page_122'>122</a></span><br />
+<span style="margin-left: 1em;">in air <a href='#Page_87'>87</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href="#Page_78">78</a>, <a href='#Page_122'>122</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_132'>132</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_78'>78</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_80'>80</a></span><br />
+<br />
+Nitroglycerin <a href='#Page_409'>409</a><br />
+<br />
+Nitrosulphuric acid <a href='#Page_155'>155</a><br />
+<br />
+Nitrous acid <a href='#Page_132'>132</a><br />
+<span style="margin-left: 1em;">oxide <a href='#Page_132'>132</a></span><br />
+<br />
+Non-metals <a href='#Page_165'>165</a><br />
+<br />
+<br />
+Oil of myrbane <a href='#Page_417'>417</a><br />
+<span style="margin-left: 1em;">of vitriol <a href='#Page_154'>154</a></span><br />
+<br />
+Oleic acid <a href='#Page_408'>408</a><br />
+<br />
+Olein <a href='#Page_409'>409</a><br />
+<br />
+Oleomargarine <a href='#Page_410'>410</a><br />
+<br />
+Onyx <a href='#Page_260'>260</a><br />
+<br />
+Opal <a href='#Page_260'>260</a><br />
+<br />
+Open-hearth process <a href='#Page_346'>346</a><br />
+<br />
+Opium <a href='#Page_418'>418</a><br />
+<br />
+Ores <a href='#Page_267'>267</a><br />
+<br />
+Organic bases <a href='#Page_412'>412</a><br />
+<span style="margin-left: 1em;">chemistry <a href="#Page_201">201</a>, <a href='#Page_397'>397</a></span><br />
+<span style="margin-left: 1em;">matter, decomposition <a href='#Page_122'>122</a></span><br />
+<br />
+Orpiment <a href='#Page_246'>246</a><br />
+<br />
+Orthoarsenic acid <a href='#Page_250'>250</a><br />
+<br />
+Orthophosphates <a href='#Page_244'>244</a><br />
+<br />
+Orthophosphoric acid <a href='#Page_244'>244</a><br />
+<br />
+Orthosilicic acid <a href='#Page_261'>261</a><br />
+<br />
+Osmic acid <a href='#Page_393'>393</a><br />
+<br />
+Osmium <a href='#Page_393'>393</a><br />
+<span style="margin-left: 1em;">tetroxide <a href='#Page_393'>393</a></span><br />
+<br />
+Oxalic acid <a href='#Page_407'>407</a><br />
+<br />
+Oxidation 17, <a href='#Page_353'>353</a><br />
+<span style="margin-left: 1em;">definition <a href='#Page_18'>18</a></span><br />
+<br />
+Oxidizing agent <a href='#Page_37'>37</a><br />
+<br />
+Oxygen <a href='#Page_13'>13</a><br />
+<span style="margin-left: 1em;">and ozone <a href='#Page_22'>22</a></span><br />
+<span style="margin-left: 1em;">commercial preparation <a href='#Page_16'>16</a></span><br />
+<span style="margin-left: 1em;">history <a href='#Page_13'>13</a></span><br />
+<span style="margin-left: 1em;">importance <a href='#Page_21'>21</a></span><br />
+<span style="margin-left: 1em;">in air estimation, <a href='#Page_85'>85</a></span><br />
+<span class='pagenum'><a name="Page_430" id="Page_430">[Pg 430]</a></span><span style="margin-left: 1em;">in air function, <a href='#Page_84'>84</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_13'>13</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_13'>13</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_16'>16</a></span><br />
+<span style="margin-left: 1em;">standard&nbsp; for&nbsp; atomic weights <a href='#Page_66'>66</a></span><br />
+<span style="margin-left: 1em;">two atoms in molecule <a href='#Page_227'>227</a></span><br />
+<br />
+Oxyhydrogen blowpipe <a href='#Page_37'>37</a><br />
+<br />
+Ozone 21, <a href='#Page_137'>137</a><br />
+<br />
+<br />
+Palladium <a href='#Page_390'>390</a><br />
+<br />
+Palmitic acid <a href='#Page_407'>407</a><br />
+<br />
+Palmitin <a href='#Page_409'>409</a><br />
+<br />
+Paraffin <a href='#Page_400'>400</a><br />
+<br />
+Paris green <a href='#Page_250'>250</a><br />
+<br />
+Parkes's method for silver <a href='#Page_364'>364</a><br />
+<br />
+Pearls <a href='#Page_305'>305</a><br />
+<br />
+Perchloric acid <a href='#Page_187'>187</a><br />
+<br />
+Periodic acid <a href='#Page_194'>194</a><br />
+<br />
+Periodic division <a href='#Page_166'>166</a><br />
+<span style="margin-left: 1em;">groups <a href='#Page_167'>167</a></span><br />
+<span style="margin-left: 1em;">law <a href='#Page_169'>169</a></span><br />
+<span style="margin-left: 1em;">law, imperfections <a href='#Page_172'>172</a></span><br />
+<span style="margin-left: 1em;">law, value <a href='#Page_171'>171</a></span><br />
+<span style="margin-left: 1em;">table <a href='#Page_168'>168</a></span><br />
+<span style="margin-left: 1em;">table, arrangement <a href='#Page_166'>166</a></span><br />
+<br />
+Permanent hardness <a href='#Page_310'>310</a><br />
+<br />
+Permanganates <a href='#Page_381'>381</a><br />
+<br />
+Permanganic acid <a href='#Page_381'>381</a><br />
+<br />
+Peroxides <a href='#Page_278'>278</a><br />
+<br />
+Petroleum <a href='#Page_399'>399</a><br />
+<br />
+Pewter <a href='#Page_372'>372</a><br />
+<br />
+Phenol <a href='#Page_417'>417</a><br />
+<br />
+Philosopher's stone <a href='#Page_9'>9</a><br />
+<br />
+Phlogiston <a href='#Page_19'>19</a><br />
+<br />
+Phosphates <a href='#Page_245'>245</a><br />
+<br />
+Phosphine <a href='#Page_242'>242</a><br />
+<br />
+Phosphonium compounds <a href='#Page_243'>243</a><br />
+<br />
+Phosphoric acid <a href='#Page_244'>244</a><br />
+<br />
+Phosphorite <a href='#Page_239'>239</a><br />
+<br />
+Phosphorous acid <a href='#Page_244'>244</a><br />
+<br />
+Phosphorus <a href='#Page_239'>239</a><br />
+<span style="margin-left: 1em;">acids <a href='#Page_243'>243</a></span><br />
+<span style="margin-left: 1em;">family <a href='#Page_238'>238</a></span><br />
+<span style="margin-left: 1em;">hydrogen compounds <a href='#Page_242'>242</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_239'>239</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_243'>243</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_239'>239</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_240'>240</a></span><br />
+<span style="margin-left: 1em;">red <a href='#Page_241'>241</a></span><br />
+<span style="margin-left: 1em;">yellow <a href='#Page_240'>240</a></span><br />
+<br />
+Photography <a href='#Page_367'>367</a><br />
+<br />
+Physical changes <a href='#Page_2'>2</a><br />
+<span style="margin-left: 1em;">properties <a href='#Page_3'>3</a></span><br />
+<span style="margin-left: 1em;">properties and periodic groups <a href='#Page_171'>171</a></span><br />
+<span style="margin-left: 1em;">state <a href='#Page_3'>3</a></span><br />
+<br />
+Physics 1, <a href='#Page_4'>4</a><br />
+<br />
+Pitchblende <a href='#Page_314'>314</a><br />
+<br />
+Plaster of Paris <a href='#Page_308'>308</a><br />
+<br />
+Platinic chloride <a href='#Page_393'>393</a><br />
+<br />
+Platinized asbestos <a href='#Page_391'>391</a><br />
+<br />
+Platinous chloride <a href='#Page_393'>393</a><br />
+<br />
+Platinum <a href='#Page_391'>391</a><br />
+<span style="margin-left: 1em;">a catalytic agent <a href="#Page_152">152</a>, <a href='#Page_392'>392</a></span><br />
+<br />
+Pneumatic trough <a href='#Page_16'>16</a><br />
+<br />
+Polyboric acid <a href='#Page_265'>265</a><br />
+<br />
+Polyhalite <a href='#Page_288'>288</a><br />
+<br />
+Polysilicic acids <a href='#Page_261'>261</a><br />
+<br />
+Porcelain <a href='#Page_336'>336</a><br />
+<br />
+Portland cement <a href='#Page_304'>304</a><br />
+<br />
+Potash <a href='#Page_293'>293</a><br />
+<br />
+Potassium <a href='#Page_287'>287</a><br />
+<span style="margin-left: 1em;">acid carbonate <a href='#Page_294'>294</a></span><br />
+<span style="margin-left: 1em;">acid sulphate <a href='#Page_294'>294</a></span><br />
+<span style="margin-left: 1em;">acid sulphite <a href='#Page_294'>294</a></span><br />
+<span style="margin-left: 1em;">alum, aluminium <a href='#Page_334'>334</a></span><br />
+<span style="margin-left: 1em;">alum, chrome <a href='#Page_384'>384</a></span><br />
+<span style="margin-left: 1em;">alum, iron <a href='#Page_352'>352</a></span><br />
+<span style="margin-left: 1em;">and plant life <a href='#Page_287'>287</a></span><br />
+<span style="margin-left: 1em;">aurate <a href='#Page_396'>396</a></span><br />
+<span style="margin-left: 1em;">bromide <a href='#Page_290'>290</a></span><br />
+<span style="margin-left: 1em;">carbonate <a href='#Page_293'>293</a></span><br />
+<span style="margin-left: 1em;">chlorate <a href='#Page_291'>291</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_290'>290</a></span><br />
+<span style="margin-left: 1em;">chromate <a href='#Page_385'>385</a></span><br />
+<span style="margin-left: 1em;">cyanide <a href='#Page_293'>293</a></span><br />
+<span style="margin-left: 1em;">dichromate <a href='#Page_386'>386</a></span><br />
+<span style="margin-left: 1em;">ferricyanide <a href='#Page_352'>352</a></span><br />
+<span style="margin-left: 1em;">ferrocyanide <a href='#Page_352'>352</a></span><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_288'>288</a></span><br />
+<span style="margin-left: 1em;">hydroxide, action of halogens <a href='#Page_289'>289</a></span><br />
+<span style="margin-left: 1em;">hypochlorite <a href='#Page_289'>289</a></span><br />
+<span style="margin-left: 1em;">iodide <a href='#Page_290'>290</a></span><br />
+<span style="margin-left: 1em;">manganate <a href='#Page_381'>381</a></span><br />
+<span style="margin-left: 1em;">nitrate <a href='#Page_291'>291</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_287'>287</a></span><br />
+<span style="margin-left: 1em;">permanganate <a href='#Page_381'>381</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_288'>288</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_294'>294</a></span><br />
+<br />
+Precipitated chalk <a href='#Page_306'>306</a><br />
+<br />
+<span class='pagenum'><a name="Page_431" id="Page_431">[Pg 431]</a></span>Precipitation <a href='#Page_140'>140</a><br />
+<br />
+Properties, chemical <a href='#Page_3'>3</a><br />
+<span style="margin-left: 1em;">physical <a href='#Page_3'>3</a></span><br />
+<br />
+Prussic acid <a href='#Page_210'>210</a><br />
+<br />
+Puddling <a href='#Page_343'>343</a><br />
+<span style="margin-left: 1em;">furnace <a href='#Page_344'>344</a></span><br />
+<br />
+Pyridine <a href='#Page_412'>412</a><br />
+<br />
+Pyrites <a href='#Page_351'>351</a><br />
+<br />
+Pyrolusite <a href='#Page_380'>380</a><br />
+<br />
+Pyrophosphoric acid <a href='#Page_245'>245</a><br />
+<br />
+<br />
+Quantitative equations <a href='#Page_72'>72</a><br />
+<br />
+Quartz <a href='#Page_260'>260</a><br />
+<br />
+Quicklime <a href='#Page_302'>302</a><br />
+<br />
+Quinine <a href='#Page_418'>418</a><br />
+<br />
+Quinoline <a href='#Page_412'>412</a><br />
+<br />
+<br />
+Radical <a href='#Page_112'>112</a><br />
+<br />
+Radium <a href='#Page_313'>313</a><br />
+<br />
+Reaction, classes <a href='#Page_70'>70</a><br />
+<span style="margin-left: 1em;">addition <a href='#Page_70'>70</a></span><br />
+<span style="margin-left: 1em;">completed <a href='#Page_139'>139</a></span><br />
+<span style="margin-left: 1em;">heat of <a href='#Page_75'>75</a></span><br />
+<span style="margin-left: 1em;">of decomposition <a href='#Page_70'>70</a></span><br />
+<span style="margin-left: 1em;">of double decomposition <a href='#Page_71'>71</a></span><br />
+<span style="margin-left: 1em;">of substitution <a href='#Page_70'>70</a></span><br />
+<span style="margin-left: 1em;">reversible <a href='#Page_137'>137</a></span><br />
+<span style="margin-left: 1em;">steps in <a href='#Page_131'>131</a></span><br />
+<br />
+Realgar <a href='#Page_246'>246</a><br />
+<br />
+Red lead <a href='#Page_375'>375</a><br />
+<span style="margin-left: 1em;">phosphorus <a href='#Page_241'>241</a></span><br />
+<br />
+Reducing agent <a href='#Page_37'>37</a><br />
+<br />
+Reduction <a href="#Page_36">36</a>, <a href='#Page_354'>354</a><br />
+<br />
+Rennin <a href='#Page_414'>414</a><br />
+<br />
+Resemblances, family <a href='#Page_170'>170</a><br />
+<br />
+Respiration <a href='#Page_87'>87</a><br />
+<br />
+Rhodium <a href='#Page_390'>390</a><br />
+<br />
+Rochelle salts <a href='#Page_408'>408</a><br />
+<br />
+Rouge <a href='#Page_349'>349</a><br />
+<br />
+Rubidium <a href='#Page_294'>294</a><br />
+<br />
+Ruby <a href='#Page_331'>331</a><br />
+<br />
+Ruthenium <a href='#Page_390'>390</a><br />
+<br />
+Rutile <a href='#Page_264'>264</a><br />
+<br />
+<br />
+Safety lamp <a href='#Page_202'>202</a><br />
+<br />
+Sal ammoniac <a href='#Page_294'>294</a><br />
+<span style="margin-left: 1em;">soda <a href='#Page_282'>282</a></span><br />
+<br />
+Salt <a href='#Page_280'>280</a><br />
+<br />
+Saltpeter <a href='#Page_291'>291</a><br />
+<span style="margin-left: 1em;">Chili <a href='#Page_285'>285</a></span><br />
+<br />
+Salts, <a href='#Page_109'>109</a><br />
+<span style="margin-left: 1em;">acid, <a href='#Page_112'>112</a></span><br />
+<br />
+Salts basic <a href='#Page_111'>111</a><br />
+<span style="margin-left: 1em;">binary <a href='#Page_114'>114</a></span><br />
+<span style="margin-left: 1em;">characteristics <a href='#Page_109'>109</a></span><br />
+<span style="margin-left: 1em;">definition <a href='#Page_109'>109</a></span><br />
+<span style="margin-left: 1em;">insoluble <a href='#Page_272'>272</a></span><br />
+<span style="margin-left: 1em;">mixed <a href='#Page_244'>244</a></span><br />
+<span style="margin-left: 1em;">nomenclature <a href='#Page_113'>113</a></span><br />
+<span style="margin-left: 1em;">normal <a href='#Page_112'>112</a></span><br />
+<span style="margin-left: 1em;">preparation&nbsp; by&nbsp; precipitation <a href='#Page_270'>270</a></span><br />
+<br />
+Sand <a href='#Page_260'>260</a><br />
+<br />
+Sandstone <a href='#Page_260'>260</a><br />
+<br />
+Saponification <a href='#Page_410'>410</a><br />
+<br />
+Sapphire <a href='#Page_331'>331</a><br />
+<br />
+Satinspar <a href='#Page_308'>308</a><br />
+<br />
+Scale <a href='#Page_320'>320</a><br />
+<br />
+Sch&ouml;nite <a href='#Page_288'>288</a><br />
+<br />
+Selenite <a href='#Page_308'>308</a><br />
+<br />
+Selenium <a href='#Page_161'>161</a><br />
+<br />
+Serpentine <a href="#Page_320">320</a>, <a href='#Page_336'>336</a><br />
+<br />
+Shot 247, <a href='#Page_375'>375</a><br />
+<br />
+Siderite <a href='#Page_339'>339</a><br />
+<br />
+Silica <a href='#Page_260'>260</a><br />
+<br />
+Silicates <a href='#Page_261'>261</a><br />
+<br />
+Silicic acids <a href='#Page_261'>261</a><br />
+<br />
+Silicides <a href='#Page_259'>259</a><br />
+<br />
+Silicon <a href='#Page_258'>258</a><br />
+<span style="margin-left: 1em;">acids <a href='#Page_261'>261</a></span><br />
+<span style="margin-left: 1em;">dioxide <a href='#Page_260'>260</a></span><br />
+<span style="margin-left: 1em;">fluoride <a href='#Page_258'>258</a></span><br />
+<span style="margin-left: 1em;">hydride <a href='#Page_258'>258</a></span><br />
+<br />
+Silver <a href='#Page_364'>364</a><br />
+<span style="margin-left: 1em;">amalgamation process <a href='#Page_364'>364</a></span><br />
+<span style="margin-left: 1em;">bromide <a href='#Page_367'>367</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_367'>367</a></span><br />
+<span style="margin-left: 1em;">coin <a href='#Page_359'>359</a></span><br />
+<span style="margin-left: 1em;">German <a href='#Page_359'>359</a></span><br />
+<span style="margin-left: 1em;">in copper ores <a href='#Page_358'>358</a></span><br />
+<span style="margin-left: 1em;">iodide <a href='#Page_367'>367</a></span><br />
+<span style="margin-left: 1em;">metallurgy <a href='#Page_364'>364</a></span><br />
+<span style="margin-left: 1em;">nitrate <a href='#Page_366'>366</a></span><br />
+<span style="margin-left: 1em;">oxide <a href='#Page_366'>366</a></span><br />
+<span style="margin-left: 1em;">parting of <a href='#Page_365'>365</a></span><br />
+<span style="margin-left: 1em;">refining <a href='#Page_365'>365</a></span><br />
+<span style="margin-left: 1em;">sulphide <a href='#Page_366'>366</a></span><br />
+<br />
+Slag <a href='#Page_340'>340</a><br />
+<br />
+Smalt <a href='#Page_355'>355</a><br />
+<br />
+Smithsonite <a href='#Page_321'>321</a><br />
+<br />
+Smokeless powder <a href='#Page_293'>293</a><br />
+<br />
+Soaps <a href='#Page_410'>410</a><br />
+<br />
+<span class='pagenum'><a name="Page_432" id="Page_432">[Pg 432]</a></span>Soda ash <a href='#Page_284'>284</a><br />
+<br />
+Soda lime <a href='#Page_202'>202</a><br />
+<br />
+Sodium <a href='#Page_276'>276</a><br />
+<span style="margin-left: 1em;">acetate <a href='#Page_407'>407</a></span><br />
+<span style="margin-left: 1em;">bicarbonate <a href='#Page_285'>285</a></span><br />
+<span style="margin-left: 1em;">carbonate <a href='#Page_282'>282</a></span><br />
+<span style="margin-left: 1em;">carbonate, historical <a href='#Page_284'>284</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_280'>280</a></span><br />
+<span style="margin-left: 1em;">chromates <a href='#Page_386'>386</a></span><br />
+<span style="margin-left: 1em;">hydrogen carbonate <a href='#Page_285'>285</a></span><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_278'>278</a></span><br />
+<span style="margin-left: 1em;">hyposulphite <a href='#Page_282'>282</a></span><br />
+<span style="margin-left: 1em;">iodate <a href='#Page_191'>191</a></span><br />
+<span style="margin-left: 1em;">nitrate <a href='#Page_285'>285</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_276'>276</a></span><br />
+<span style="margin-left: 1em;">peroxide <a href='#Page_277'>277</a></span><br />
+<span style="margin-left: 1em;">phosphates <a href='#Page_286'>286</a></span><br />
+<span style="margin-left: 1em;">preparation <a href='#Page_276'>276</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_277'>277</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_281'>281</a></span><br />
+<span style="margin-left: 1em;">sulphite <a href='#Page_281'>281</a></span><br />
+<span style="margin-left: 1em;">tetraborate <a href='#Page_287'>287</a></span><br />
+<span style="margin-left: 1em;">thiosulphate <a href='#Page_282'>282</a></span><br />
+<br />
+Solder <a href="#Page_372">372</a>, <a href='#Page_375'>375</a><br />
+<br />
+Solubility of gases <a href='#Page_95'>95</a><br />
+<span style="margin-left: 1em;">of solids <a href='#Page_96'>96</a></span><br />
+<br />
+Solution <a href='#Page_94'>94</a><br />
+<span style="margin-left: 1em;">and chemical action <a href='#Page_53'>53</a></span><br />
+<span style="margin-left: 1em;">boiling point <a href='#Page_98'>98</a></span><br />
+<span style="margin-left: 1em;">classes <a href='#Page_94'>94</a></span><br />
+<span style="margin-left: 1em;">distribution of solids in <a href='#Page_98'>98</a></span><br />
+<span style="margin-left: 1em;">electrolysis of <a href='#Page_99'>99</a></span><br />
+<span style="margin-left: 1em;">freezing point <a href='#Page_99'>99</a></span><br />
+<span style="margin-left: 1em;">of gases in liquids <a href='#Page_94'>94</a></span><br />
+<span style="margin-left: 1em;">of solids in liquids <a href='#Page_96'>96</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_98'>98</a></span><br />
+<span style="margin-left: 1em;">saturated <a href='#Page_97'>97</a></span><br />
+<span style="margin-left: 1em;">supersaturated <a href='#Page_98'>98</a></span><br />
+<br />
+Solvay soda process <a href='#Page_283'>283</a><br />
+<br />
+Sombrerite <a href='#Page_239'>239</a><br />
+<br />
+Spectroscope <a href='#Page_296'>296</a><br />
+<br />
+Sphalerite <a href='#Page_325'>325</a><br />
+<br />
+Spiegel iron <a href='#Page_343'>343</a><br />
+<br />
+Spinel <a href='#Page_332'>332</a><br />
+<br />
+Spontaneous combustion <a href='#Page_20'>20</a><br />
+<br />
+Stalactites <a href='#Page_305'>305</a><br />
+<br />
+Stalagmites <a href='#Page_305'>305</a><br />
+<br />
+Standard conditions <a href='#Page_23'>23</a><br />
+<br />
+Stannates <a href='#Page_372'>372</a><br />
+<br />
+Stannic acid <a href='#Page_372'>372</a><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_372'>372</a></span><br />
+<span style="margin-left: 1em;">oxide <a href='#Page_372'>372</a></span><br />
+<br />
+Stannous chloride <a href='#Page_372'>372</a><br />
+<br />
+Starch <a href='#Page_415'>415</a><br />
+<br />
+Stassfurt salts <a href='#Page_287'>287</a><br />
+<br />
+Stearic acid <a href='#Page_407'>407</a><br />
+<br />
+Stearin <a href='#Page_409'>409</a><br />
+<br />
+Steel <a href='#Page_345'>345</a><br />
+<span style="margin-left: 1em;">alloys <a href='#Page_348'>348</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_347'>347</a></span><br />
+<span style="margin-left: 1em;">tempering of <a href='#Page_348'>348</a></span><br />
+<span style="margin-left: 1em;">tool <a href='#Page_347'>347</a></span><br />
+<br />
+Stibine <a href='#Page_251'>251</a><br />
+<br />
+Stibnite <a href='#Page_250'>250</a><br />
+<br />
+Stoneware <a href='#Page_336'>336</a><br />
+<br />
+Strontianite <a href='#Page_312'>312</a><br />
+<br />
+Strontium <a href='#Page_312'>312</a><br />
+<span style="margin-left: 1em;">hydroxide <a href='#Page_312'>312</a></span><br />
+<span style="margin-left: 1em;">nitrate <a href='#Page_312'>312</a></span><br />
+<br />
+Structural formulas <a href='#Page_119'>119</a><br />
+<br />
+Structure of compounds <a href='#Page_119'>119</a><br />
+<br />
+Strychnine <a href='#Page_419'>419</a><br />
+<br />
+Substitution <a href='#Page_70'>70</a><br />
+<br />
+Sugars <a href='#Page_412'>412</a><br />
+<span style="margin-left: 1em;">cane <a href='#Page_412'>412</a></span><br />
+<span style="margin-left: 1em;">fruit <a href='#Page_415'>415</a></span><br />
+<span style="margin-left: 1em;">grape <a href='#Page_414'>414</a></span><br />
+<span style="margin-left: 1em;">milk <a href='#Page_414'>414</a></span><br />
+<br />
+Sulphates <a href='#Page_159'>159</a><br />
+<br />
+Sulphides <a href='#Page_148'>148</a><br />
+<br />
+Sulphites <a href='#Page_152'>152</a><br />
+<span style="margin-left: 1em;">action of acids on <a href='#Page_150'>150</a></span><br />
+<br />
+Sulphur <a href='#Page_143'>143</a><br />
+<span style="margin-left: 1em;">allotropic forms <a href='#Page_144'>144</a></span><br />
+<span style="margin-left: 1em;">chemical properties <a href='#Page_145'>145</a></span><br />
+<span style="margin-left: 1em;">comparison with oxygen <a href='#Page_161'>161</a></span><br />
+<span style="margin-left: 1em;">dioxide <a href='#Page_149'>149</a></span><br />
+<span style="margin-left: 2em;">preparation <a href='#Page_149'>149</a></span><br />
+<span style="margin-left: 2em;">properties <a href='#Page_150'>150</a></span><br />
+<span style="margin-left: 1em;">extraction <a href='#Page_143'>143</a></span><br />
+<span style="margin-left: 1em;">flowers of <a href='#Page_143'>143</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_143'>143</a></span><br />
+<span style="margin-left: 1em;">oxides <a href='#Page_149'>149</a></span><br />
+<span style="margin-left: 1em;">physical properties <a href='#Page_144'>144</a></span><br />
+<span style="margin-left: 1em;">trioxide <a href='#Page_152'>152</a></span><br />
+<span style="margin-left: 1em;">uses <a href='#Page_146'>146</a></span><br />
+<span style="margin-left: 1em;">varieties <a href='#Page_144'>144</a></span><br />
+<br />
+Sulphuric acid <a href='#Page_154'>154</a><br />
+<span style="margin-left: 1em;">action as an acid <a href='#Page_157'>157</a></span><br />
+<span style="margin-left: 1em;">action on metals <a href='#Page_157'>157</a></span><br />
+<span style="margin-left: 1em;">action on organic matter <a href='#Page_158'>158</a></span><br />
+<span style="margin-left: 1em;">action on salts <a href='#Page_158'>158</a></span><br />
+<span class='pagenum'><a name="Page_433" id="Page_433">[Pg 433]</a></span><span style="margin-left: 1em;">action on water <a href='#Page_158'>158</a></span><br />
+<span style="margin-left: 1em;">fuming <a href='#Page_155'>155</a></span><br />
+<span style="margin-left: 1em;">manufacture <a href='#Page_154'>154</a></span><br />
+<span style="margin-left: 1em;">oxidizing action <a href='#Page_157'>157</a></span><br />
+<span style="margin-left: 1em;">plant <a href='#Page_156'>156</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_157'>157</a></span><br />
+<span style="margin-left: 1em;">salts <a href='#Page_159'>159</a></span><br />
+<br />
+Sulphuric anhydride <a href='#Page_153'>153</a><br />
+<br />
+Sulphurous acid <a href='#Page_151'>151</a><br />
+<br />
+Superphosphate of lime <a href='#Page_246'>246</a><br />
+<br />
+Sylvine <a href='#Page_288'>288</a><br />
+<br />
+Symbols <a href='#Page_11'>11</a><br />
+<br />
+Synthesis <a href='#Page_40'>40</a><br />
+<br />
+<br />
+Table, alkali metals <a href='#Page_274'>274</a><br />
+<span style="margin-left: 1em;">alkaline-earth metals <a href='#Page_300'>300</a></span><br />
+<span style="margin-left: 1em;">alloys of copper <a href='#Page_359'>359</a></span><br />
+<span style="margin-left: 1em;">aqueous tension <a href="#APPENDIX_B">Appendix B</a></span><br />
+<span style="margin-left: 1em;">atomic weights <a href="#APPENDIX_A">Appendix A</a></span><br />
+<span style="margin-left: 1em;">chlorine family <a href='#Page_174'>174</a></span><br />
+<span style="margin-left: 1em;">composition of earth's crust <a href='#Page_10'>10</a></span><br />
+<span style="margin-left: 1em;">composition of fuel gases <a href='#Page_220'>220</a></span><br />
+<span style="margin-left: 1em;">constants of elements <a href="#APPENDIX_B">Appendix B</a></span><br />
+<span style="margin-left: 1em;">copper family <a href='#Page_356'>356</a></span><br />
+<span style="margin-left: 1em;">elements<a href="#APPENDIX_A"> Appendix A</a></span><br />
+<span style="margin-left: 1em;">gold and platinum metals <a href='#Page_390'>390</a></span><br />
+<span style="margin-left: 1em;">hydrocarbons <a href='#Page_399'>399</a></span><br />
+<span style="margin-left: 1em;">magnesium family <a href='#Page_316'>316</a></span><br />
+<span style="margin-left: 1em;">manganese and chromium <a href='#Page_379'>379</a></span><br />
+<span style="margin-left: 1em;">periodic arrangement <a href='#Page_168'>168</a></span><br />
+<span style="margin-left: 1em;">phosphorus family <a href='#Page_238'>238</a></span><br />
+<span style="margin-left: 1em;">silicon family <a href='#Page_257'>257</a></span><br />
+<span style="margin-left: 1em;">solubility of gases in water <a href='#Page_95'>95</a></span><br />
+<span style="margin-left: 1em;">solubility of salts <a href='#Page_96'>96</a></span><br />
+<span style="margin-left: 1em;">solubility of salts at different temperatures <a href='#Page_97'>97</a></span><br />
+<span style="margin-left: 1em;">tin and lead <a href='#Page_370'>370</a></span><br />
+<span style="margin-left: 1em;">weights of gases <a href="#APPENDIX_B">Appendix B</a></span><br />
+<br />
+Talc <a href="#Page_321">321</a>, <a href='#Page_336'>336</a><br />
+<br />
+Tartar emetic <a href='#Page_408'>408</a><br />
+<br />
+Tartaric acid <a href='#Page_408'>408</a><br />
+<br />
+Tellurium <a href='#Page_161'>161</a><br />
+<br />
+Temporary hardness <a href='#Page_309'>309</a><br />
+<br />
+Ternary acids <a href='#Page_113'>113</a><br />
+<span style="margin-left: 1em;">salts <a href='#Page_114'>114</a></span><br />
+<br />
+Tetraboric acid <a href='#Page_265'>265</a><br />
+<br />
+Thallium <a href='#Page_327'>327</a><br />
+<br />
+Theory, atomic <a href='#Page_61'>61</a><br />
+<span style="margin-left: 1em;">definition <a href='#Page_64'>64</a></span><br />
+<span style="margin-left: 1em;">value of <a href='#Page_64'>64</a></span><br />
+<br />
+Thermite <a href='#Page_331'>331</a><br />
+<br />
+Thio compounds <a href='#Page_282'>282</a><br />
+<br />
+Thiosulphates <a href='#Page_159'>159</a><br />
+<br />
+Thiosulphuric acid <a href='#Page_159'>159</a><br />
+<br />
+Thorium <a href='#Page_377'>377</a><br />
+<br />
+Tin <a href='#Page_370'>370</a><br />
+<span style="margin-left: 1em;">block <a href='#Page_371'>371</a></span><br />
+<span style="margin-left: 1em;">compounds <a href='#Page_372'>372</a></span><br />
+<span style="margin-left: 1em;">crystals <a href='#Page_372'>372</a></span><br />
+<span style="margin-left: 1em;">family <a href='#Page_370'>370</a></span><br />
+<span style="margin-left: 1em;">foil <a href='#Page_371'>371</a></span><br />
+<span style="margin-left: 1em;">metallurgy <a href='#Page_370'>370</a></span><br />
+<span style="margin-left: 1em;">plate <a href='#Page_371'>371</a></span><br />
+<span style="margin-left: 1em;">properties <a href='#Page_371'>371</a></span><br />
+<span style="margin-left: 1em;">uses <a href='#Page_371'>371</a></span><br />
+<br />
+Titanium <a href="#Page_257">257</a>, <a href='#Page_264'>264</a><br />
+<br />
+Topaz <a href='#Page_331'>331</a><br />
+<br />
+Triad families <a href='#Page_166'>166</a><br />
+<br />
+Tungsten <a href='#Page_388'>388</a><br />
+<br />
+Type metal <a href="#Page_253">253</a>, <a href='#Page_375'>375</a><br />
+<br />
+<br />
+Uranium <a href='#Page_388'>388</a><br />
+<br />
+<br />
+Valence <a href='#Page_116'>116</a><br />
+<span style="margin-left: 1em;">a numerical property <a href='#Page_116'>116</a></span><br />
+<span style="margin-left: 1em;">and combining ratios <a href='#Page_118'>118</a></span><br />
+<span style="margin-left: 1em;">and equations <a href='#Page_120'>120</a></span><br />
+<span style="margin-left: 1em;">and formulas <a href='#Page_120'>120</a></span><br />
+<span style="margin-left: 1em;">and periodic groups <a href='#Page_162'>162</a></span><br />
+<span style="margin-left: 1em;">and structure <a href='#Page_118'>118</a></span><br />
+<span style="margin-left: 1em;">definition <a href='#Page_116'>116</a></span><br />
+<span style="margin-left: 1em;">indirectly determined <a href='#Page_117'>117</a></span><br />
+<span style="margin-left: 1em;">measure of <a href='#Page_117'>117</a></span><br />
+<span style="margin-left: 1em;">variable <a href='#Page_117'>117</a></span><br />
+<br />
+Vaseline <a href='#Page_400'>400</a><br />
+<br />
+Venetian red <a href='#Page_349'>349</a><br />
+<br />
+Verdigris <a href='#Page_407'>407</a><br />
+<br />
+Vermilion <a href='#Page_363'>363</a><br />
+<br />
+Vinegar <a href='#Page_406'>406</a><br />
+<br />
+Vitriol, blue <a href='#Page_361'>361</a><br />
+<span style="margin-left: 1em;">green <a href='#Page_350'>350</a></span><br />
+<span style="margin-left: 1em;">oil of <a href='#Page_154'>154</a></span><br />
+<span style="margin-left: 1em;">white <a href='#Page_324'>324</a></span><br />
+<br />
+Volume and aqueous tension <a href='#Page_25'>25</a><br />
+<span style="margin-left: 1em;">and pressure <a href='#Page_24'>24</a></span><br />
+<span style="margin-left: 1em;">and temperature <a href='#Page_23'>23</a></span><br />
+<span style="margin-left: 1em;">of combining gases <a href='#Page_194'>194</a></span><br />
+<br />
+<br />
+Water <a href='#Page_40'>40</a><br />
+<span style="margin-left: 1em;">a compound <a href='#Page_40'>40</a></span><br />
+<span class='pagenum'><a name="Page_434" id="Page_434">[Pg 434]</a></span><span style="margin-left: 1em;">and disease <a href='#Page_49'>49</a></span><br />
+<span style="margin-left: 1em;">catalytic action of <a href='#Page_154'>154</a></span><br />
+<span style="margin-left: 1em;">chalybeate <a href='#Page_351'>351</a></span><br />
+<span style="margin-left: 1em;">chemical properties <a href='#Page_53'>53</a></span><br />
+<span style="margin-left: 1em;">composition <a href='#Page_47'>47</a></span><br />
+<span style="margin-left: 1em;">composition by volume <a href='#Page_44'>44</a></span><br />
+<span style="margin-left: 1em;">composition by weight <a href='#Page_47'>47</a></span><br />
+<span style="margin-left: 1em;">dissociation of <a href='#Page_210'>210</a></span><br />
+<span style="margin-left: 1em;">distillation of <a href='#Page_50'>50</a></span><br />
+<span style="margin-left: 1em;">electrolysis of <a href="#Page_41">41</a>, <a href='#Page_103'>103</a></span><br />
+<span style="margin-left: 1em;">filtration of <a href='#Page_51'>51</a></span><br />
+<span style="margin-left: 1em;">gas <a href='#Page_219'>219</a></span><br />
+<span style="margin-left: 1em;">hard <a href='#Page_309'>309</a></span><br />
+<span style="margin-left: 1em;">historical <a href='#Page_40'>40</a></span><br />
+<span style="margin-left: 1em;">impurities in <a href='#Page_48'>48</a></span><br />
+<span style="margin-left: 1em;">in air <a href='#Page_87'>87</a></span><br />
+<span style="margin-left: 1em;">mineral <a href='#Page_49'>49</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_48'>48</a></span><br />
+<span style="margin-left: 1em;">of crystallization 54, <a href='#Page_75'>75</a></span><br />
+<span style="margin-left: 1em;">physical properties <a href='#Page_53'>53</a></span><br />
+<span style="margin-left: 1em;">purification of <a href='#Page_50'>50</a></span><br />
+<span style="margin-left: 1em;">qualitative analysis <a href='#Page_41'>41</a></span><br />
+<span style="margin-left: 1em;">quantitative analysis <a href='#Page_42'>42</a></span><br />
+<span style="margin-left: 1em;">river <a href='#Page_49'>49</a></span><br />
+<span style="margin-left: 1em;">sanitary analysis <a href='#Page_50'>50</a></span><br />
+<span style="margin-left: 1em;">self-purification <a href='#Page_53'>53</a></span><br />
+<span style="margin-left: 1em;">softening of <a href='#Page_310'>310</a></span><br />
+<span style="margin-left: 1em;">standard substance <a href='#Page_55'>55</a></span><br />
+<span style="margin-left: 1em;">synthesis <a href='#Page_43'>43</a></span><br />
+<span style="margin-left: 1em;">uses of <a href='#Page_55'>55</a></span><br />
+<br />
+Weights, atomic <a href='#Page_65'>65</a><br />
+<br />
+Welsbach mantles <a href="#Page_219">219</a>, <a href='#Page_377'>377</a><br />
+<br />
+Whisky <a href='#Page_404'>404</a><br />
+<br />
+Wine <a href='#Page_404'>404</a><br />
+<br />
+Witherite <a href='#Page_312'>312</a><br />
+<br />
+Wood alcohol <a href='#Page_402'>402</a><br />
+<span style="margin-left: 1em;">distillation <a href='#Page_402'>402</a></span><br />
+<br />
+Wood's metal <a href='#Page_254'>254</a><br />
+<br />
+<br />
+Xenon <a href='#Page_80'>80</a><br />
+<br />
+<br />
+Yeast <a href='#Page_403'>403</a><br />
+<br />
+<br />
+Zinc <a href='#Page_321'>321</a><br />
+<span style="margin-left: 1em;">alloys of <a href='#Page_323'>323</a></span><br />
+<span style="margin-left: 1em;">blende <a href='#Page_321'>321</a></span><br />
+<span style="margin-left: 1em;">chloride <a href='#Page_325'>325</a></span><br />
+<span style="margin-left: 1em;">flowers of <a href='#Page_322'>322</a></span><br />
+<span style="margin-left: 1em;">metallurgy <a href='#Page_321'>321</a></span><br />
+<span style="margin-left: 1em;">occurrence <a href='#Page_321'>321</a></span><br />
+<span style="margin-left: 1em;">oxide <a href='#Page_324'>324</a></span><br />
+<span style="margin-left: 1em;">sulphate <a href='#Page_324'>324</a></span><br />
+<span style="margin-left: 1em;">sulphide <a href='#Page_325'>325</a></span><br />
+<span style="margin-left: 1em;">white <a href='#Page_324'>324</a></span><br />
+<br />
+<br />
+Zymase, <a href='#Page_403'>403</a><br />
+</p>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="ANNOUNCEMENTS" id="ANNOUNCEMENTS"></a>ANNOUNCEMENTS</h2>
+
+
+<h3>AN ELEMENTARY STUDY OF CHEMISTRY</h3>
+
+<p>By WILLIAM McPHERSON, Professor of Chemistry in Ohio State University,
+and WILLIAM E. HENDERSON, Associate Professor of Chemistry in Ohio State
+University.</p>
+
+<p>12mo. Cloth. 434 pages. Illustrated. List price, $1.25; mailing price,
+$1.40</p>
+
+<p>This book is the outgrowth of many years of experience in the teaching
+of elementary chemistry. In its preparation the authors have steadfastly
+kept in mind the limitations of the student to whom chemistry is a new
+science. They have endeavored to present the subject in a clear,
+well-graded way, passing in a natural and logical manner from principles
+which are readily understood to those which are more difficult to grasp.
+The language is simple and as free as possible from unusual and
+technical phrases. Those which are unavoidable are carefully defined.
+The outline is made very plain, and the paragraphing is designed to be
+of real assistance to the student in his reading.</p>
+
+<p>The book is in no way radical, either in the subject-matter selected or
+in the method of treatment. At the same time it is in thorough harmony
+with the most recent developments in chemistry, both in respect to
+theory and discovery. Great care has been taken in the theoretical
+portions to make the treatment simple and well within the reach of the
+ability of an elementary student. The most recent discoveries have been
+touched upon where they come within the scope of an elementary text.
+Especial attention has been given to the practical applications of
+chemistry, and to the description of the manufacturing processes in use
+at the present time.</p>
+
+
+<h3>EXERCISES IN CHEMISTRY. By WILLIAM McPHERSON and WILLIAM E. HENDERSON.</h3>
+<h4>(<i>In press.</i>)</h4>
+
+<h4>GINN &amp; COMPANY PUBLISHERS</h4>
+
+<h3>A FIRST COURSE IN PHYSICS</h3>
+
+<p>By ROBERT A. MILLIKAN, Associate Professor of Physics, and HENRY G.
+GALE, Assistant Professor of Physics in The University of Chicago</p>
+
+<p>12mo, cloth, 488 pages, illustrated, $1.25</p>
+
+
+<h3>A LABORATORY COURSE IN PHYSICS</h3>
+
+<h4><i>FOR SECONDARY SCHOOLS</i></h4>
+
+<p>By ROBERT A. MILLIKAN and HENRY G. GALE 12mo, flexible cloth, 134 pages,
+illustrated, 40 cents</p>
+
+<p>This one-year course in physics has grown out of the experience of the
+authors in developing the work in physics at the School of Education of
+The University of Chicago, and in dealing with the physics instruction
+in affiliated high schools and academies.</p>
+
+<p>The book is a simple, objective presentation of the subject as opposed
+to a formal and mathematical one. It is intended for the third-year
+high-school pupils and is therefore adapted in style and method of
+treatment to the needs of students between the ages of fifteen and
+eighteen. It especially emphasizes the historical and practical aspects
+of the subject and connects the study very intimately with facts of
+daily observation and experience.</p>
+
+<p>The authors have made a careful distinction between the class of
+experiments which are essentially laboratory problems and those which
+belong more properly to the classroom and the lecture table. The former
+are grouped into a Laboratory Manual which is designed for use in
+connection with the text. The two books are not, however, organically
+connected, each being complete in itself.</p>
+
+<p>All the experiments included in the work have been carefully chosen with
+reference to their usefulness as effective classroom demonstrations.</p>
+
+<h4>GINN AND COMPANY PUBLISHERS</h4>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="APPENDIX_A" id="APPENDIX_A"></a>APPENDIX A</h2>
+
+
+<h3>LIST OF THE ELEMENTS, THEIR SYMBOLS, AND ATOMIC WEIGHTS</h3>
+
+<p>The more important elements are marked with an asterisk</p>
+
+<p>O = 16</p>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>*Antimony</td><td align='left'>Sb</td><td align='left'>120.2</td></tr>
+<tr><td align='left'>*Argon</td><td align='left'>A</td><td align='left'>39.9</td></tr>
+<tr><td align='left'>*Arsenic</td><td align='left'>As</td><td align='left'>75.0</td></tr>
+<tr><td align='left'>*Barium</td><td align='left'>Ba</td><td align='left'>137.4</td></tr>
+<tr><td align='left'>Beryllium</td><td align='left'>Be</td><td align='left'>9.1</td></tr>
+<tr><td align='left'>*Bismuth</td><td align='left'>Bi</td><td align='left'>208.5</td></tr>
+<tr><td align='left'>*Boron</td><td align='left'>B</td><td align='left'>11.0</td></tr>
+<tr><td align='left'>*Bromine</td><td align='left'>Br</td><td align='left'>79.96</td></tr>
+<tr><td align='left'>*Cadmium</td><td align='left'>Cd</td><td align='left'>112.4</td></tr>
+<tr><td align='left'>C&aelig;sium</td><td align='left'>Cs</td><td align='left'>132.9</td></tr>
+<tr><td align='left'>*Calcium</td><td align='left'>Ca</td><td align='left'>40.1</td></tr>
+<tr><td align='left'>*Carbon</td><td align='left'>C</td><td align='left'>12.00</td></tr>
+<tr><td align='left'>Cerium</td><td align='left'>Ce</td><td align='left'>140.25</td></tr>
+<tr><td align='left'>*Chlorine</td><td align='left'>Cl</td><td align='left'>35.45</td></tr>
+<tr><td align='left'>*Chromium</td><td align='left'>Cr</td><td align='left'>52.1</td></tr>
+<tr><td align='left'>*Cobalt</td><td align='left'>Co</td><td align='left'>59.0</td></tr>
+<tr><td align='left'>Columbium</td><td align='left'>Cb</td><td align='left'>94.0</td></tr>
+<tr><td align='left'>*Copper</td><td align='left'>Cu</td><td align='left'>63.6</td></tr>
+<tr><td align='left'>Erbium</td><td align='left'>Er</td><td align='left'>166.0</td></tr>
+<tr><td align='left'>*Fluorine</td><td align='left'>F</td><td align='left'>19.0</td></tr>
+<tr><td align='left'>Gadolinium</td><td align='left'>Gd</td><td align='left'>156.0</td></tr>
+<tr><td align='left'>Gallium</td><td align='left'>Ga</td><td align='left'>70.0</td></tr>
+<tr><td align='left'>Germanium</td><td align='left'>Ge</td><td align='left'>72.5</td></tr>
+<tr><td align='left'>*Gold</td><td align='left'>Au</td><td align='left'>197.2</td></tr>
+<tr><td align='left'>Helium</td><td align='left'>He</td><td align='left'>4.0</td></tr>
+<tr><td align='left'>*Hydrogen</td><td align='left'>H</td><td align='left'>1.008</td></tr>
+<tr><td align='left'>Indium</td><td align='left'>In</td><td align='left'>115.0</td></tr>
+<tr><td align='left'>*Iodine</td><td align='left'>I</td><td align='left'>126.97</td></tr>
+<tr><td align='left'>Iridium</td><td align='left'>Ir</td><td align='left'>193.0</td></tr>
+<tr><td align='left'>*Iron</td><td align='left'>Fe</td><td align='left'>55.9</td></tr>
+<tr><td align='left'>Krypton</td><td align='left'>Kr</td><td align='left'>81.8</td></tr>
+<tr><td align='left'>Lanthanum</td><td align='left'>La</td><td align='left'>138.9</td></tr>
+<tr><td align='left'>*Lead</td><td align='left'>Pb</td><td align='left'>206.9</td></tr>
+<tr><td align='left'>Lithium</td><td align='left'>Li</td><td align='left'>7.03</td></tr>
+<tr><td align='left'>*Magnesium</td><td align='left'>Mg</td><td align='left'>24.36</td></tr>
+<tr><td align='left'>*Manganese</td><td align='left'>Mn</td><td align='left'>55.0</td></tr>
+<tr><td align='left'>*Mercury</td><td align='left'>Hg</td><td align='left'>200.0</td></tr>
+<tr><td align='left'>Molybdenum</td><td align='left'>Mo</td><td align='left'>96.0</td></tr>
+<tr><td align='left'>Neodymium</td><td align='left'>Nd</td><td align='left'>143.6</td></tr>
+<tr><td align='left'>Neon</td><td align='left'>Ne</td><td align='left'>20.0</td></tr>
+<tr><td align='left'>*Nickel</td><td align='left'>Ni</td><td align='left'>58.7</td></tr>
+<tr><td align='left'>*Nitrogen</td><td align='left'>N</td><td align='left'>14.04</td></tr>
+<tr><td align='left'>Osmium</td><td align='left'>Os</td><td align='left'>191.0</td></tr>
+<tr><td align='left'>*Oxygen</td><td align='left'>O</td><td align='left'>16.00</td></tr>
+<tr><td align='left'>Palladium</td><td align='left'>Pd</td><td align='left'>106.5</td></tr>
+<tr><td align='left'>*Phosphorus</td><td align='left'>P</td><td align='left'>31.0</td></tr>
+<tr><td align='left'>*Platinum</td><td align='left'>Pt</td><td align='left'>194.8</td></tr>
+<tr><td align='left'>*Potassium</td><td align='left'>K</td><td align='left'>39.15</td></tr>
+<tr><td align='left'>Praseodymium</td><td align='left'>Pr</td><td align='left'>140.5</td></tr>
+<tr><td align='left'>Radium</td><td align='left'>Ra</td><td align='left'>225.0</td></tr>
+<tr><td align='left'>Rhodium</td><td align='left'>Rh</td><td align='left'>103.0</td></tr>
+<tr><td align='left'>Rubidium</td><td align='left'>Rb</td><td align='left'>85.5</td></tr>
+<tr><td align='left'>Ruthenium</td><td align='left'>Ru</td><td align='left'>101.7</td></tr>
+<tr><td align='left'>Samarium</td><td align='left'>Sm</td><td align='left'>150.3</td></tr>
+<tr><td align='left'>Scandium</td><td align='left'>Sc</td><td align='left'>44.1</td></tr>
+<tr><td align='left'>Selenium</td><td align='left'>Se</td><td align='left'>79.2</td></tr>
+<tr><td align='left'>*Silicon</td><td align='left'>Si</td><td align='left'>28.4</td></tr>
+<tr><td align='left'>*Silver</td><td align='left'>Ag</td><td align='left'>107.93</td></tr>
+<tr><td align='left'>*Sodium</td><td align='left'>Na</td><td align='left'>23.05</td></tr>
+<tr><td align='left'>*Strontium</td><td align='left'>Sr</td><td align='left'>87.6</td></tr>
+<tr><td align='left'>*Sulphur</td><td align='left'>S</td><td align='left'>32.06</td></tr>
+<tr><td align='left'>Tantalum</td><td align='left'>Ta</td><td align='left'>183.0</td></tr>
+<tr><td align='left'>Tellurium</td><td align='left'>Te</td><td align='left'>127.6</td></tr>
+<tr><td align='left'>Terbium</td><td align='left'>Tb</td><td align='left'>160.0</td></tr>
+<tr><td align='left'>Thallium</td><td align='left'>Tl</td><td align='left'>204.1</td></tr>
+<tr><td align='left'>Thorium</td><td align='left'>Th</td><td align='left'>232.5</td></tr>
+<tr><td align='left'>Thulium</td><td align='left'>Tm</td><td align='left'>171.0</td></tr>
+<tr><td align='left'>*Tin</td><td align='left'>Sn</td><td align='left'>119.0</td></tr>
+<tr><td align='left'>Titanium</td><td align='left'>Ti</td><td align='left'>48.1</td></tr>
+<tr><td align='left'>Tungsten</td><td align='left'>W</td><td align='left'>184.0</td></tr>
+<tr><td align='left'>Uranium</td><td align='left'>U</td><td align='left'>238.5</td></tr>
+<tr><td align='left'>Vanadium</td><td align='left'>V</td><td align='left'>51.2</td></tr>
+<tr><td align='left'>Xenon</td><td align='left'>Xe</td><td align='left'>128.0</td></tr>
+<tr><td align='left'>Ytterbium</td><td align='left'>Yb</td><td align='left'>173.0</td></tr>
+<tr><td align='left'>Yttrium</td><td align='left'>Yt</td><td align='left'>89.0</td></tr>
+<tr><td align='left'>*Zinc</td><td align='left'>Zn</td><td align='left'>65.4</td></tr>
+<tr><td align='left'>Zirconium</td><td align='left'>Zr</td><td align='left'>90.6</td></tr>
+</table></div>
+
+
+
+<hr style="width: 65%;" />
+<h2><a name="APPENDIX_B" id="APPENDIX_B"></a>APPENDIX B</h2>
+
+
+<h3>Tension of Aqueous Vapor expressed in Millimeters of Mercury</h3>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>TEMPERATURE</td><td align='left'>PRESSURE</td></tr>
+<tr><td align='left'>16</td><td align='left'>13.5</td></tr>
+<tr><td align='left'>17</td><td align='left'>14.4</td></tr>
+<tr><td align='left'>18</td><td align='left'>15.3</td></tr>
+<tr><td align='left'>19</td><td align='left'>16.3</td></tr>
+<tr><td align='left'>20</td><td align='left'>17.4</td></tr>
+<tr><td align='left'>21</td><td align='left'>18.5</td></tr>
+<tr><td align='left'>22</td><td align='left'>19.6</td></tr>
+<tr><td align='left'>23</td><td align='left'>20.9</td></tr>
+<tr><td align='left'>24</td><td align='left'>22.2</td></tr>
+<tr><td align='left'>25</td><td align='left'>23.5</td></tr>
+</table></div>
+
+
+<h3>Weight of 1 Liter of Various Gases measured under Standard Conditions</h3>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Acetylene</td><td align='left'>1.1614</td></tr>
+<tr><td align='left'>Air</td><td align='left'>1.2923</td></tr>
+<tr><td align='left'>Ammonia</td><td align='left'>0.7617</td></tr>
+<tr><td align='left'>Carbon dioxide</td><td align='left'>1.9641</td></tr>
+<tr><td align='left'>Carbon monoxide</td><td align='left'>1.2499</td></tr>
+<tr><td align='left'>Chlorine</td><td align='left'>3.1650</td></tr>
+<tr><td align='left'>Hydrocyanic acid</td><td align='left'>1.2036</td></tr>
+<tr><td align='left'>Hydrochloric acid</td><td align='left'>1.6275</td></tr>
+<tr><td align='left'>Hydrogen</td><td align='left'>0.08984</td></tr>
+<tr><td align='left'>Hydrosulphuric acid</td><td align='left'>1.5211</td></tr>
+<tr><td align='left'>Methane</td><td align='left'>0.7157</td></tr>
+<tr><td align='left'>Nitric oxide</td><td align='left'>1.3410</td></tr>
+<tr><td align='left'>Nitrogen</td><td align='left'>1.2501</td></tr>
+<tr><td align='left'>Nitrous oxide</td><td align='left'>1.9677</td></tr>
+<tr><td align='left'>Oxygen</td><td align='left'>1.4285</td></tr>
+<tr><td align='left'>Sulphur dioxide</td><td align='left'>2.8596</td></tr>
+</table></div>
+
+
+<h3>Densities and Melting Points of Some Common Elements</h3>
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='left'>DENSITY</td><td align='left'>MELTING POINT</td></tr>
+<tr><td align='left'>Aluminium</td><td align='left'>2.68</td><td align='left'>640</td></tr>
+<tr><td align='left'>Antimony</td><td align='left'>6.70</td><td align='left'>432</td></tr>
+<tr><td align='left'>Arsenic</td><td align='left'>5.73</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Barium</td><td align='left'>3.75</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Bismuth</td><td align='left'>9.80</td><td align='left'>270</td></tr>
+<tr><td align='left'>Boron</td><td align='left'>2.45</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Cadmium</td><td align='left'>8.67</td><td align='left'>320</td></tr>
+<tr><td align='left'>C&aelig;sium</td><td align='left'>1.88</td><td align='left'>26.5</td></tr>
+<tr><td align='left'>Calcium</td><td align='left'>1.54</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Carbon, Diamond</td><td align='left'>3.50</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>&nbsp;&nbsp;&nbsp;"&nbsp;&nbsp;&nbsp;Graphite</td><td align='left'>2.15</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>&nbsp;&nbsp;&nbsp;"&nbsp;&nbsp;&nbsp;&nbsp;Charcoal</td><td align='left'>1.80</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Chromium</td><td align='left'>7.30</td><td align='left'>3000</td></tr>
+<tr><td align='left'>Cobalt</td><td align='left'>8.55</td><td align='left'>1800</td></tr>
+<tr><td align='left'>Copper</td><td align='left'>8.89</td><td align='left'>1084</td></tr>
+<tr><td align='left'>Gold</td><td align='left'>19.30</td><td align='left'>1064</td></tr>
+<tr><td align='left'>Iridium</td><td align='left'>22.42</td><td align='left'>1950</td></tr>
+<tr><td align='left'>Iron</td><td align='left'>7.93</td><td align='left'>1800</td></tr>
+<tr><td align='left'>Lead</td><td align='left'>11.38</td><td align='left'>327</td></tr>
+<tr><td align='left'>Lithium</td><td align='left'>0.59</td><td align='left'>186</td></tr>
+<tr><td align='left'>Magnesium</td><td align='left'>1.75</td><td align='left'>750</td></tr>
+<tr><td align='left'>Manganese</td><td align='left'>8.01</td><td align='left'>1900</td></tr>
+<tr><td align='left'>Mercury</td><td align='left'>13.596</td><td align='left'>-39.5</td></tr>
+<tr><td align='left'>Nickel</td><td align='left'>8.9</td><td align='left'>1600</td></tr>
+<tr><td align='left'>Osmium</td><td align='left'>22.47</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Palladium</td><td align='left'>11.80</td><td align='left'>1500</td></tr>
+<tr><td align='left'>Phosphorus</td><td align='left'>1.80</td><td align='left'>45</td></tr>
+<tr><td align='left'>Platinum</td><td align='left'>21.50</td><td align='left'>1779</td></tr>
+<tr><td align='left'>Potassium</td><td align='left'>0.87</td><td align='left'>62.5</td></tr>
+<tr><td align='left'>Rhodium</td><td align='left'>12.10</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Rubidium</td><td align='left'>1.52</td><td align='left'>38.5</td></tr>
+<tr><td align='left'>Ruthenium</td><td align='left'>12.26</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Silicon</td><td align='left'>2.35</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Silver</td><td align='left'>10.5</td><td align='left'>960</td></tr>
+<tr><td align='left'>Sodium</td><td align='left'>0.97</td><td align='left'>97.6</td></tr>
+<tr><td align='left'>Strontium</td><td align='left'>2.50</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Sulphur</td><td align='left'>2.00</td><td align='left'>114.8</td></tr>
+<tr><td align='left'>Tin</td><td align='left'>7.35</td><td align='left'>235</td></tr>
+<tr><td align='left'>Titanium</td><td align='left'>3.50</td><td align='left'>&mdash;</td></tr>
+<tr><td align='left'>Zinc</td><td align='left'>7.00</td><td align='left'>420</td></tr>
+</table></div>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of An Elementary Study of Chemistry, by 
+William McPherson and William Edwards Henderson
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+</body>
+</html>
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+The Project Gutenberg EBook of An Elementary Study of Chemistry, by
+William McPherson and William Edwards Henderson
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: An Elementary Study of Chemistry
+
+Author: William McPherson
+        William Edwards Henderson
+
+Release Date: March 18, 2007 [EBook #20848]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK AN ELEMENTARY STUDY OF CHEMISTRY ***
+
+
+
+
+Produced by Elaine Walker, Josephine Paolucci and the
+Online Distributed Proofreading Team at http://www.pgdp.net
+
+
+
+
+
+
+
+
+
+AN ELEMENTARY
+
+STUDY OF CHEMISTRY
+
+BY
+
+WILLIAM McPHERSON, PH.D.
+
+PROFESSOR OF CHEMISTRY, OHIO STATE UNIVERSITY
+
+AND
+
+WILLIAM EDWARDS HENDERSON, PH.D.
+
+ASSOCIATE PROFESSOR OF CHEMISTRY, OHIO STATE UNIVERSITY
+
+
+_REVISED EDITION_
+
+GINN & COMPANY
+BOSTON * NEW YORK * CHICAGO * LONDON
+
+
+COPYRIGHT, 1905, 1906, BY
+WILLIAM MCPHERSON AND WILLIAM E. HENDERSON
+
+ALL RIGHTS RESERVED
+
+
+The Athenaeum Press
+GINN & COMPANY * PROPRIETORS * BOSTON * U.S.A.
+
+
+
+
+Transcriber's note:
+
+For Text: A word surrounded by a cedilla such as ~this~ signifies that
+the word is bolded in the text. A word surrounded by underscores like
+_this_ signifies the word is italics in the text. The italic and bold
+markup for single italized letters (such as variables in equations) and
+"foreign" abbreviations are deleted for easier reading.
+
+For numbers and equations: Parentheses have been added to clarify
+fractions. Underscores before bracketed numbers in equations denote a
+subscript. Superscripts are designated with a caret and brackets, e.g.
+11.1^{3} is 11.1 to the third power.
+
+Appendix A and B have been moved to the end of the book.
+Minor typos have been corrected.
+
+
+
+
+PREFACE
+
+
+In offering this book to teachers of elementary chemistry the authors
+lay no claim to any great originality. It has been their aim to prepare
+a text-book constructed along lines which have become recognized as best
+suited to an elementary treatment of the subject. At the same time they
+have made a consistent effort to make the text clear in outline, simple
+in style and language, conservatively modern in point of view, and
+thoroughly teachable.
+
+The question as to what shall be included in an elementary text on
+chemistry is perhaps the most perplexing one which an author must
+answer. While an enthusiastic chemist with a broad understanding of the
+science is very apt to go beyond the capacity of the elementary student,
+the authors of this text, after an experience of many years, cannot help
+believing that the tendency has been rather in the other direction. In
+many texts no mention at all is made of fundamental laws of chemical
+action because their complete presentation is quite beyond the
+comprehension of the student, whereas in many cases it is possible to
+present the essential features of these laws in a way that will be of
+real assistance in the understanding of the science. For example, it is
+a difficult matter to deduce the law of mass action in any very simple
+way; yet the elementary student can readily comprehend that reactions
+are reversible, and that the point of equilibrium depends upon, rather
+simple conditions. The authors believe that it is worth while to
+present such principles in even an elementary and partial manner because
+they are of great assistance to the general student, and because they
+make a foundation upon which the student who continues his studies to
+more advanced courses can securely build.
+
+The authors have no apologies to make for the extent to which they have
+made use of the theory of electrolytic dissociation. It is inevitable
+that in any rapidly developing science there will be differences of
+opinion in regard to the value of certain theories. There can be no
+question, however, that the outline of the theory of dissociation here
+presented is in accord with the views of the very great majority of the
+chemists of the present time. Moreover, its introduction to the extent
+to which the authors have presented it simplifies rather than increases
+the difficulties with which the development of the principles of the
+science is attended.
+
+The oxygen standard for atomic weights has been adopted throughout the
+text. The International Committee, to which is assigned the duty of
+yearly reporting a revised list of the atomic weights of the elements,
+has adopted this standard for their report, and there is no longer any
+authority for the older hydrogen standard. The authors do not believe
+that the adoption of the oxygen standard introduces any real
+difficulties in making perfectly clear the methods by which atomic
+weights are calculated.
+
+The problems appended to the various chapters have been chosen with a
+view not only of fixing the principles developed in the text in the mind
+of the student, but also of enabling him to answer such questions as
+arise in his laboratory work. They are, therefore, more or less
+practical in character. It is not necessary that all of them should be
+solved, though with few exceptions the lists are not long. The answers
+to the questions are not directly given in the text as a rule, but can
+be inferred from the statements made. They therefore require independent
+thought on the part of the student.
+
+With very few exceptions only such experiments are included in the text
+as cannot be easily carried out by the student. It is expected that
+these will be performed by the teacher at the lecture table. Directions
+for laboratory work by the student are published in a separate volume.
+
+While the authors believe that the most important function of the
+elementary text is to develop the principles of the science, they
+recognize the importance of some discussion of the practical application
+of these principles to our everyday life. Considerable space is
+therefore devoted to this phase of chemistry. The teacher should
+supplement this discussion whenever possible by having the class visit
+different factories where chemical processes are employed.
+
+Although this text is now for the first time offered to teachers of
+elementary chemistry, it has nevertheless been used by a number of
+teachers during the past three years. The present edition has been
+largely rewritten in the light of the criticisms offered, and we desire
+to express our thanks to the many teachers who have helped us in this
+respect, especially to Dr. William Lloyd Evans of this laboratory, a
+teacher of wide experience, for his continued interest and helpfulness.
+We also very cordially solicit correspondence with teachers who may find
+difficulties or inaccuracies in the text.
+
+The authors wish to make acknowledgments for the photographs and
+engravings of eminent chemists from which the cuts included in the text
+were taken; to Messrs. Elliott and Fry, London, England, for that of
+Ramsay; to The Macmillan Company for those of Davy and Dalton, taken
+from the Century Science Series; to the L. E. Knott Apparatus Company,
+Boston, for that of Bunsen.
+
+THE AUTHORS
+
+OHIO STATE UNIVERSITY
+
+COLUMBUS, OHIO
+
+
+
+
+CONTENTS
+
+
+CHAPTER                                                           PAGE
+     I. INTRODUCTION                                                 1
+
+    II. OXYGEN                                                      13
+
+   III. HYDROGEN                                                    28
+
+    IV. WATER AND HYDROGEN DIOXIDE                                  40
+
+     V. THE ATOMIC THEORY                                           59
+
+    VI. CHEMICAL EQUATIONS AND CALCULATIONS                         68
+
+   VII. NITROGEN AND THE RARE ELEMENTS IN THE ATMOSPHERE            78
+
+  VIII. THE ATMOSPHERE                                              83
+
+    IX. SOLUTIONS                                                   94
+
+     X. ACIDS, BASES, AND SALTS; NEUTRALIZATION                    106
+
+    XI. VALENCE                                                    116
+
+   XII. COMPOUNDS OF NITROGEN                                      122
+
+  XIII. REVERSIBLE REACTIONS AND CHEMICAL EQUILIBRIUM              137
+
+   XIV. SULPHUR AND ITS COMPOUNDS                                  143
+
+    XV. PERIODIC LAW                                               165
+
+   XVI. THE CHLORINE FAMILY                                        174
+
+  XVII. CARBON AND SOME OF ITS SIMPLER COMPOUNDS                   196
+
+ XVIII. FLAMES,--ILLUMINANTS                                       213
+
+   XIX. MOLECULAR WEIGHTS, ATOMIC WEIGHTS, FORMULAS                223
+
+    XX. THE PHOSPHORUS FAMILY                                      238
+
+   XXI. SILICON, TITANIUM, BORON                                   257
+
+  XXII. THE METALS                                                 267
+
+ XXIII. THE ALKALI METALS                                          274
+
+  XXIV. THE ALKALINE-EARTH FAMILY                                  300
+
+   XXV. THE MAGNESIUM FAMILY                                       316
+
+  XXVI. THE ALUMINIUM FAMILY                                       327
+
+ XXVII. THE IRON FAMILY                                            338
+
+XXVIII. COPPER, MERCURY, AND SILVER                                356
+
+  XXIX. TIN AND LEAD                                               370
+
+   XXX. MANGANESE AND CHROMIUM                                     379
+
+  XXXI. GOLD AND THE PLATINUM FAMILY                               390
+
+ XXXII. SOME SIMPLE ORGANIC COMPOUNDS                              397
+
+INDEX                                                              421
+
+APPENDIX A                                                Facing back cover
+
+APPENDIX B                                                Inside back cover
+
+
+
+
+LIST OF FULL-PAGE ILLUSTRATIONS
+
+
+                                                                  PAGE
+ANTOINE LAURENT LAVOISIER                                  _Frontispiece_
+
+JOSEPH PRIESTLEY                                                   14
+
+JOHN DALTON                                                        60
+
+WILLIAM RAMSAY                                                     82
+
+DMITRI IVANOVITCH MENDELEEFF                                      166
+
+HENRI MOISSAN                                                     176
+
+SIR HUMPHRY DAVY                                                  276
+
+ROBERT WILHELM BUNSEN                                             298
+
+
+
+
+AN ELEMENTARY STUDY OF CHEMISTRY
+
+
+
+
+CHAPTER I
+
+INTRODUCTION
+
+
+~The natural sciences.~ Before we advance very far in the study of nature,
+it becomes evident that the one large study must be divided into a
+number of more limited ones for the convenience of the investigator as
+well as of the student. These more limited studies are called the
+_natural sciences_.
+
+Since the study of nature is divided in this way for mere convenience,
+and not because there is any division in nature itself, it often happens
+that the different sciences are very intimately related, and a thorough
+knowledge of any one of them involves a considerable acquaintance with
+several others. Thus the botanist must know something about animals as
+well as about plants; the student of human physiology must know
+something about physics as well as about the parts of the body.
+
+~Intimate relation of chemistry and physics.~ Physics and chemistry are
+two sciences related in this close way, and it is not easy to make a
+precise distinction between them. In a general way it may be said that
+they are both concerned with inanimate matter rather than with living,
+and more particularly with the changes which such matter may be made to
+undergo. These changes must be considered more closely before a
+definition of the two sciences can be given.
+
+~Physical changes.~ One class of changes is not accompanied by an
+alteration in the composition of matter. When a lump of coal is broken
+the pieces do not differ from the original lump save in size. A rod of
+iron may be broken into pieces; it may be magnetized; it may be heated
+until it glows; it may be melted. In none of these changes has the
+composition of the iron been affected. The pieces of iron, the
+magnetized iron, the glowing iron, the melted iron, are just as truly
+iron as was the original rod. Sugar may be dissolved in water, but
+neither the sugar nor the water is changed in composition. The resulting
+liquid has the sweet taste of sugar; moreover the water may be
+evaporated by heating and the sugar recovered unchanged. Such changes
+are called _physical changes_.
+
+DEFINITION: _Physical changes are those which do not involve a change in
+the composition of the matter._
+
+~Chemical changes.~ Matter may undergo other changes in which its
+composition is altered. When a lump of coal is burned ashes and
+invisible gases are formed which are entirely different in composition
+and properties from the original coal. A rod of iron when exposed to
+moist air is gradually changed into rust, which is entirely different
+from the original iron. When sugar is heated a black substance is formed
+which is neither sweet nor soluble in water. Such changes are evidently
+quite different from the physical changes just described, for in them
+new substances are formed in place of the ones undergoing change.
+Changes of this kind are called _chemical changes_.
+
+DEFINITION: _Chemical changes are those which involve a change in the
+composition of the matter._
+
+~How to distinguish between physical and chemical changes.~ It is not
+always easy to tell to which class a given change belongs, and many
+cases will require careful thought on the part of the student. The test
+question in all cases is, Has the composition of the substance been
+changed? Usually this can be answered by a study of the properties of
+the substance before and after the change, since a change in composition
+is attended by a change in properties. In some cases, however, only a
+trained observer can decide the question.
+
+~Changes in physical state.~ One class of physical changes should be noted
+with especial care, since it is likely to prove misleading. It is a
+familiar fact that ice is changed into water, and water into steam, by
+heating. Here we have three different substances,--the solid ice, the
+liquid water, and the gaseous steam,--the properties of which differ
+widely. The chemist can readily show, however, that these three bodies
+have exactly the same composition, being composed of the same substances
+in the same proportion. Hence the change from one of these substances
+into another is a physical change. Many other substances may, under
+suitable conditions, be changed from solids into liquids, or from
+liquids into gases, without change in composition. Thus butter and wax
+will melt when heated; alcohol and gasoline will evaporate when exposed
+to the air. _The three states--solid, liquid, and gas--are called the
+three physical states of matter._
+
+~Physical and chemical properties.~ Many properties of a substance can be
+noted without causing the substance to undergo chemical change, and are
+therefore called its _physical properties_. Among these are its physical
+state, color, odor, taste, size, shape, weight. Other properties are
+only discovered when the substance undergoes chemical change. These are
+called its _chemical properties_. Thus we find that coal burns in air,
+gunpowder explodes when ignited, milk sours when exposed to air.
+
+~Definition of physics and chemistry.~ It is now possible to make a
+general distinction between physics and chemistry.
+
+DEFINITION: _Physics is the science which deals with those changes in
+matter which do not involve a change in composition._
+
+DEFINITION: _Chemistry is the science which deals with those changes in
+matter which do involve a change in composition._
+
+~Two factors in all changes.~ In all the changes which matter can undergo,
+whether physical or chemical, two factors must be taken into account,
+namely, _energy_ and _matter_.
+
+~Energy.~ It is a familiar fact that certain bodies have the power to do
+work. Thus water falling from a height upon a water wheel turns the
+wheel and in this way does the work of the mills. Magnetized iron
+attracts iron to itself and the motion of the iron as it moves towards
+the magnet can be made to do work. When coal is burned it causes the
+engine to move and transports the loaded cars from place to place. When
+a body has this power to do work it is said to possess energy.
+
+~Law of conservation of energy.~ Careful experiments have shown that when
+one body parts with its energy the energy is not destroyed but is
+transferred to another body or system of bodies. Just as energy cannot
+be destroyed, neither can it be created. If one body gains a certain
+amount of energy, some other body has lost an equivalent amount. These
+facts are summed up in the law of conservation of energy which may be
+stated thus: _While energy can be changed from one form into another, it
+cannot be created or destroyed._
+
+~Transformations of energy.~ Although energy can neither be created nor
+destroyed, it is evident that it may assume many different forms. Thus
+the falling water may turn the electric generator and produce a current
+of electricity. The energy lost by the falling water is thus transformed
+into the energy of the electric current. This in turn may be changed
+into the energy of motion, as when the current is used for propelling
+the cars, or into the energy of heat and light, as when it is used for
+heating and lighting the cars. Again, the energy of coal may be
+converted into energy of heat and subsequently of motion, as when it is
+used as a fuel in steam engines.
+
+Since the energy possessed by coal only becomes available when the coal
+is made to undergo a chemical change, it is sometimes called _chemical
+energy_. It is this form of energy in which we are especially interested
+in the study of chemistry.
+
+~Matter.~ Matter may be defined as that which occupies space and possesses
+weight. Like energy, matter may be changed oftentimes from one form into
+another; and since in these transformations all the other physical
+properties of a substance save weight are likely to change, the inquiry
+arises, Does the weight also change? Much careful experimenting has
+shown that it does not. The weight of the products formed in any change
+in matter always equals the weight of the substances undergoing change.
+
+~Law of conservation of matter.~ The important truth just stated is
+frequently referred to as the law of conservation of matter, and this
+law may be briefly stated thus: _Matter can neither be created nor
+destroyed, though it can be changed from one form into another._
+
+~Classification of matter.~ At first sight there appears to be no limit to
+the varieties of matter of which the world is made. For convenience in
+study we may classify all these varieties under three heads, namely,
+_mechanical mixtures_, _chemical compounds_, and _elements_.
+
+[Illustration: Fig. 1]
+
+~Mechanical mixtures.~ If equal bulks of common salt and iron filings are
+thoroughly mixed together, a product is obtained which, judging by its
+appearance, is a new substance. If it is examined more closely, however,
+it will be seen to be merely a mixture of the salt and iron, each of
+which substances retains its own peculiar properties. The mixture tastes
+just like salt; the iron particles can be seen and their gritty
+character detected. A magnet rubbed in the mixture draws out the iron
+just as if the salt were not there. On the other hand, the salt can be
+separated from the iron quite easily. Thus, if several grams of the
+mixture are placed in a test tube, and the tube half filled with water
+and thoroughly shaken, the salt dissolves in the water. The iron
+particles can then be filtered from the liquid by pouring the entire
+mixture upon a piece of filter paper folded so as to fit into the
+interior of a funnel (Fig. 1). The paper retains the solid but allows
+the clear liquid, known as the _filtrate_, to drain through. The iron
+particles left upon the filter paper will be found to be identical with
+the original iron. The salt can be recovered from the filtrate by
+evaporation of the water. To accomplish this the filtrate is poured into
+a small evaporating dish and gently heated (Fig. 2) until the water has
+disappeared, or _evaporated_. The solid left in the dish is identical in
+every way with the original salt. Both the iron and the salt have thus
+been recovered in their original condition. It is evident that no new
+substance has been formed by rubbing the salt and iron together. The
+product is called a _mechanical mixture_. Such mixtures are very common
+in nature, almost all minerals, sands, and soils being examples of this
+class of substances. It is at once apparent that there is no law
+regulating the composition of a mechanical mixture, and no two mixtures
+are likely to have exactly the same composition. The ingredients of a
+mechanical mixture can usually be separated by mechanical means, such as
+sifting, sorting, magnetic attraction, or by dissolving one constituent
+and leaving the other unchanged.
+
+[Illustration: Fig. 2]
+
+DEFINITION: _A mechanical mixture is one in which the constituents
+retain their original properties, no chemical action having taken place
+when they were brought together._
+
+~Chemical compounds.~ If iron filings and powdered sulphur are thoroughly
+ground together in a mortar, a yellowish-green substance results. It
+might easily be taken to be a new body; but as in the case of the iron
+and salt, the ingredients can readily be separated. A magnet draws out
+the iron. Water does not dissolve the sulphur, but other liquids do, as,
+for example, the liquid called carbon disulphide. When the mixture is
+treated with carbon disulphide the iron is left unchanged, and the
+sulphur can be obtained again, after filtering off the iron, by
+evaporating the liquid. The substance is, therefore, a mechanical
+mixture.
+
+If now a new portion of the mixture is placed in a dry test tube and
+carefully heated in the flame of a Bunsen burner, as shown in Fig. 3, a
+striking change takes place. The mixture begins to glow at some point,
+the glow rapidly extending throughout the whole mass. If the test tube
+is now broken and the product examined, it will be found to be a hard,
+black, brittle substance, in no way recalling the iron or the sulphur.
+The magnet no longer attracts it; carbon disulphide will not dissolve
+sulphur from it. It is a new substance with new properties, resulting
+from the chemical union of iron and sulphur, and is called iron
+sulphide. Such substances are called _chemical compounds_, and differ
+from mechanical mixtures in that the substances producing them lose
+their own characteristic properties. We shall see later that the two
+also differ in that the composition of a chemical compound never varies.
+
+[Illustration: Fig. 3]
+
+DEFINITION: _A chemical compound is a substance the constituents of
+which have lost their own characteristic properties, and which cannot be
+separated save by a chemical change._
+
+~Elements.~ It has been seen that iron sulphide is composed of two
+entirely different substances,--iron and sulphur. The question arises,
+Do these substances in turn contain other substances, that is, are they
+also chemical compounds? Chemists have tried in a great many ways to
+decompose them, but all their efforts have failed. Substances which have
+resisted all efforts to decompose them into other substances are called
+_elements_. It is not always easy to prove that a given substance is
+really an element. Some way as yet untried may be successful in
+decomposing it into other simpler forms of matter, and the supposed
+element will then prove to be a compound. Water, lime, and many other
+familiar compounds were at one time thought to be elements.
+
+DEFINITION: _An element is a substance which cannot be separated into
+simpler substances by any known means._
+
+~Kinds of matter.~ While matter has been grouped in three classes for the
+purpose of study, it will be apparent that there are really but two
+distinct kinds of matter, namely, compounds and elements. A mechanical
+mixture is not a third distinct kind of matter, but is made up of
+varying quantities of either compounds or elements or both.
+
+~Alchemy.~ In olden times it was thought that some way could be found to
+change one element into another, and a great many efforts were made to
+accomplish this transformation. Most of these efforts were directed
+toward changing the commoner metals into gold, and many fanciful ways
+for doing this were described. The chemists of that time were called
+_alchemists_, and the art which they practiced was called _alchemy_. The
+alchemists gradually became convinced that the only way common metals
+could be changed into gold was by the wonderful power of a magic
+substance which they called the _philosopher's stone_, which would
+accomplish this transformation by its mere touch and would in addition
+give perpetual youth to its fortunate possessor. No one has ever found
+such a stone, and no one has succeeded in changing one metal into
+another.
+
+~Number of elements.~ The number of substances now considered to be
+elements is not large--about eighty in all. Many of these are rare, and
+very few of them make any large fraction of the materials in the
+earth's crust. Clarke gives the following estimate of the composition of
+the earth's crust:
+
+    Oxygen       47.0%       Calcium           3.5%
+    Silicon      27.9        Magnesium         2.5
+    Aluminium     8.1        Sodium            2.7
+    Iron          4.7        Potassium         2.4
+                Other elements       1.2%
+
+A complete list of the elements is given in the Appendix. In this list
+the more common of the elements are marked with an asterisk. It is not
+necessary to study more than a third of the total number of elements to
+gain a very good knowledge of chemistry.
+
+~Physical state of the elements.~ About ten of the elements are gases at
+ordinary temperatures. Two--mercury and bromine--are liquids. The others
+are all solids, though their melting points vary through wide limits,
+from caesium which melts at 26 deg. to elements which do not melt save in the
+intense heat of the electric furnace.
+
+~Occurrence of the elements.~ Comparatively few of the elements occur as
+uncombined substances in nature, most of them being found in the form of
+chemical compounds. When an element does occur by itself, as is the case
+with gold, we say that it occurs in the _free state_ or _native_; when
+it is combined with other substances in the form of compounds, we say
+that it occurs in the _combined state_, or _in combination_. In the
+latter case there is usually little about the compound to suggest that
+the element is present in it; for we have seen that elements lose their
+own peculiar properties when they enter into combination with other
+elements. It would never be suspected, for example, that the reddish,
+earthy-looking iron ore contains iron.
+
+~Names of elements.~ The names given to the elements have been selected in
+a great many different ways. (1) Some names are very old and their
+original meaning is obscure. Such names are iron, gold, and copper. (2)
+Many names indicate some striking physical property of the element. The
+name bromine, for example, is derived from a Greek word meaning a
+stench, referring to the extremely unpleasant odor of the substance. The
+name iodine comes from a word meaning violet, alluding to the beautiful
+color of iodine vapor. (3) Some names indicate prominent chemical
+properties of the elements. Thus, nitrogen means the producer of niter,
+nitrogen being a constituent of niter or saltpeter. Hydrogen means water
+former, signifying its presence in water. Argon means lazy or inert, the
+element being so named because of its inactivity. (4) Other elements are
+named from countries or localities, as germanium and scandium.
+
+~Symbols.~ In indicating the elements found in compounds it is
+inconvenient to use such long names, and hence chemists have adopted a
+system of abbreviations. These abbreviations are known as _symbols_,
+each element having a distinctive symbol. (1) Sometimes the initial
+letter of the name will suffice to indicate the element. Thus I stands
+for iodine, C for carbon. (2) Usually it is necessary to add some other
+characteristic letter to the symbol, since several names may begin with
+the same letter. Thus C stands for carbon, Cl for chlorine, Cd for
+cadmium, Ce for cerium, Cb for columbium. (3) Sometimes the symbol is an
+abbreviation of the old Latin name. In this way Fe (ferrum) indicates
+iron, Cu (cuprum), copper, Au (aurum), gold. The symbols are included in
+the list of elements given in the Appendix. They will become familiar
+through constant use.
+
+~Chemical affinity the cause of chemical combination.~ The agency which
+causes substances to combine and which holds them together when combined
+is called _chemical affinity_. The experiments described in this
+chapter, however, show that heat is often necessary to bring about
+chemical action. The distinction between the cause producing chemical
+action and the circumstances favoring it must be clearly made. Chemical
+affinity is always the cause of chemical union. Many agencies may make
+it possible for chemical affinity to act by overcoming circumstances
+which stand in its way. Among these agencies are heat, light, and
+electricity. As a rule, solution also promotes action between two
+substances. Sometimes these agencies may overcome chemical attraction
+and so occasion the decomposition of a compound.
+
+
+EXERCISES
+
+1. To what class of changes do the following belong? (a) The melting
+of ice; (b) the souring of milk; (c) the burning of a candle; (d)
+the explosion of gunpowder; (e) the corrosion of metals. What test
+question must be applied in each of the above cases?
+
+2. Give two additional examples (a) of chemical changes; (b) of
+physical changes.
+
+3. Is a chemical change always accompanied by a physical change? Is a
+physical change always accompanied by a chemical change?
+
+4. Give two or more characteristics of a chemical change.
+
+5. (a) When a given weight of water freezes, does it absorb or evolve
+heat? (b) When the resulting ice melts, is the total heat change the
+same or different from that of freezing?
+
+6. Give three examples of each of the following: (a) mechanical
+mixtures; (b) chemical compounds; (c) elements.
+
+7. Give the derivation of the names of the following elements: thorium,
+gallium, selenium, uranium. (Consult dictionary.)
+
+8. Give examples of chemical changes which are produced through the
+agency of heat; of light; of electricity.
+
+
+
+
+CHAPTER II
+
+OXYGEN
+
+
+~History.~ The discovery of oxygen is generally attributed to the English
+chemist Priestley, who in 1774 obtained the element by heating a
+compound of mercury and oxygen, known as red oxide of mercury. It is
+probable, however, that the Swedish chemist Scheele had previously
+obtained it, although an account of his experiments was not published
+until 1777. The name oxygen signifies acid former. It was given to the
+element by the French chemist Lavoisier, since he believed that all
+acids owe their characteristic properties to the presence of oxygen.
+This view we now know to be incorrect.
+
+~Occurrence.~ Oxygen is by far the most abundant of all the elements. It
+occurs both in the free and in the combined state. In the free state it
+occurs in the air, 100 volumes of dry air containing about 21 volumes of
+oxygen. In the combined state it forms eight ninths of water and nearly
+one half of the rocks composing the earth's crust. It is also an
+important constituent of the compounds which compose plant and animal
+tissues; for example, about 66% by weight of the human body is oxygen.
+
+~Preparation.~ Although oxygen occurs in the free state in the atmosphere,
+its separation from the nitrogen and other gases with which it is mixed
+is such a difficult matter that in the laboratory it has been found more
+convenient to prepare it from its compounds. The most important of the
+laboratory methods are the following:
+
+1. _Preparation from water._ Water is a compound, consisting of 11.18%
+hydrogen and 88.82% oxygen. It is easily separated into these
+constituents by passing an electric current through it under suitable
+conditions. The process will be described in the chapter on water. While
+this method of preparation is a simple one, it is not economical.
+
+2. _Preparation from mercuric oxide._ This method is of interest, since
+it is the one which led to the discovery of oxygen. The oxide, which
+consists of 7.4% oxygen and 92.6% mercury, is placed in a small, glass
+test tube and heated. The compound is in this way decomposed into
+mercury which collects on the sides of the glass tube, forming a silvery
+mirror, and oxygen which, being a gas, escapes from the tube. The
+presence of the oxygen is shown by lighting the end of a splint,
+extinguishing the flame and bringing the glowing coal into the mouth of
+the tube. The oxygen causes the glowing coal to burst into a flame.
+
+     In a similar way oxygen may be obtained from its compounds with
+     some of the other elements. Thus manganese dioxide, a black
+     compound of manganese and oxygen, when heated to about 700 deg.,
+     loses one third of its oxygen, while barium dioxide, when
+     heated, loses one half of its oxygen.
+
+3. _Preparation from potassium chlorate (usual laboratory method)._
+Potassium chlorate is a white solid which consists of 31.9% potassium,
+28.9% chlorine, and 39.2% oxygen. When heated it undergoes a series of
+changes in which all the oxygen is finally set free, leaving a compound
+of potassium and chlorine called potassium chloride. The change may be
+represented as follows:
+
+   /potassium\
+  |           | (potassium     / potassium \ (potassium
+ {  chlorine   }            = {             }             + oxygen
+  |           |  chlorate)     \ chlorine  /  chloride)
+   \oxygen   /
+
+[Illustration: JOSEPH PRIESTLEY (English) (1733-1804)
+
+School-teacher, theologian, philosopher, scientist; friend of Benjamin
+Franklin; discoverer of oxygen; defender of the phlogiston theory; the
+first to use mercury in a pneumatic trough, by which means he first
+isolated in gaseous form hydrochloric acid, sulphur dioxide, and
+ammonia]
+
+The evolution of the oxygen begins at about 400 deg.. It has been found,
+however, that if the potassium chlorate is mixed with about one fourth
+its weight of manganese dioxide, the oxygen is given off at a much lower
+temperature. Just how the manganese dioxide brings about this result is
+not definitely known. The amount of oxygen obtained from a given weight
+of potassium chlorate is exactly the same whether the manganese dioxide
+is present or not. So far as can be detected the manganese dioxide
+undergoes no change.
+
+[Illustration: Fig. 4]
+
+~Directions for preparing oxygen.~ The manner of preparing oxygen from
+potassium chlorate is illustrated in the accompanying diagram (Fig. 4).
+A mixture consisting of one part of manganese dioxide and four parts of
+potassium chlorate is placed in the flask A and gently heated. The
+oxygen is evolved and escapes through the tube B. It is collected by
+bringing over the end of the tube the mouth of a bottle completely
+filled with water and inverted in a vessel of water, as shown in the
+figure. The gas rises in the bottle and displaces the water. In the
+preparation of large quantities of oxygen, a copper retort (Fig. 5) is
+often substituted for the glass flask.
+
+[Illustration: Fig. 5]
+
+In the preparation of oxygen from potassium chlorate and manganese
+dioxide, the materials used must be pure, otherwise a violent explosion
+may occur. The purity of the materials is tested by heating a small
+amount of the mixture in a test tube.
+
+~The collection of gases.~ The method used for collecting oxygen
+illustrates the general method used for collecting such gases as are
+insoluble in water or nearly so. The vessel C (Fig. 4), containing the
+water in which the bottles are inverted, is called a _pneumatic trough._
+
+~Commercial methods of preparation.~ Oxygen can now be purchased stored
+under great pressure in strong steel cylinders (Fig. 6). It is prepared
+either by heating a mixture of potassium chlorate and manganese dioxide,
+or by separating it from the nitrogen and other gases with which it is
+mixed in the atmosphere. The methods employed for effecting this
+separation will be described in subsequent chapters.
+
+[Illustration: Fig. 6]
+
+~Physical properties.~ Oxygen is a colorless, odorless, tasteless gas,
+slightly heavier than air. One liter of it, measured at a temperature of
+0 deg. and under a pressure of one atmosphere, weighs 1.4285 g., while under
+similar conditions one liter of air weighs 1.2923 g. It is but slightly
+soluble in water. Oxygen, like other gases, may be liquefied by applying
+very great pressure to the highly cooled gas. When the pressure is
+removed the liquid oxygen passes again into the gaseous state, since its
+boiling point under ordinary atmospheric pressure is -182.5 deg..
+
+~Chemical properties.~ At ordinary temperatures oxygen is not very active
+chemically. Most substances are either not at all affected by it, or the
+action is so slow as to escape notice. At higher temperatures, however,
+it is very active, and unites directly with most of the elements. This
+activity may be shown by heating various substances until just ignited
+and then bringing them into vessels of the gas, when they will burn with
+great brilliancy. Thus a glowing splint introduced into a jar of oxygen
+bursts into flame. Sulphur burns in the air with a very weak flame and
+feeble light; in oxygen, however, the flame is increased in size and
+brightness. Substances which readily burn in air, such as phosphorus,
+burn in oxygen with dazzling brilliancy. Even substances which burn in
+air with great difficulty, such as iron, readily burn in oxygen.
+
+The burning of a substance in oxygen is due to the rapid combination of
+the substance or of the elements composing it with the oxygen. Thus,
+when sulphur burns both the oxygen and sulphur disappear as such and
+there is formed a compound of the two, which is an invisible gas, having
+the characteristic odor of burning sulphur. Similarly, phosphorus on
+burning forms a white solid compound of phosphorus and oxygen, while
+iron forms a reddish-black compound of iron and oxygen.
+
+~Oxidation.~ The term _oxidation_ is applied to the chemical change which
+takes place when a substance, or one of its constituent parts, combines
+with oxygen. This process may take place rapidly, as in the burning of
+phosphorus, or slowly, as in the oxidation (or rusting) of iron when
+exposed to the air. It is always accompanied by the liberation of heat.
+The amount of heat liberated by the oxidation of a definite weight of
+any given substance is always the same, being entirely independent of
+the rapidity of the process. If the oxidation takes place slowly, the
+heat is generated so slowly that it is difficult to detect it. If the
+oxidation takes place rapidly, however, the heat is generated in such a
+short interval of time that the substance may become white hot or burst
+into a flame.
+
+~Combustion; kindling temperature.~ When oxidation takes place so rapidly
+that the heat generated is sufficient to cause the substance to glow or
+burst into a flame the process is called _combustion_. In order that any
+substance may undergo combustion, it is necessary that it should be
+heated to a certain temperature, known as the _kindling temperature._
+This temperature varies widely for different bodies, but is always
+definite for the same body. Thus the kindling temperature of phosphorus
+is far lower than that of iron, but is definite for each. When any
+portion of a substance is heated until it begins to burn the combustion
+will continue without the further application of heat, provided the heat
+generated by the process is sufficient to bring other parts of the
+substance to the kindling temperature. On the other hand, if the heat
+generated is not sufficient to maintain the kindling temperature,
+combustion ceases.
+
+~Oxides.~ The compounds formed by the oxidation of any element are called
+_oxides_. Thus in the combustion of sulphur, phosphorus, and iron, the
+compounds formed are called respectively oxide of sulphur, oxide of
+phosphorus, and oxide of iron. In general, then, _an oxide is a compound
+of oxygen with another element_. A great many substances of this class
+are known; in fact, the oxides of all the common elements have been
+prepared, with the exception of those of fluorine and bromine. Some of
+these are familiar compounds. Water, for example, is an oxide of
+hydrogen, and lime an oxide of the metal calcium.
+
+~Products of combustion.~ The particular oxides formed by the combustion
+of any substance are called _products of combustion_ of that substance.
+Thus oxide of sulphur is the product of the combustion of sulphur; oxide
+of iron is the product of the combustion of iron. It is evident that the
+products of the combustion of any substance must weigh more than the
+original substance, the increase in weight corresponding to the amount
+of oxygen taken up in the act of combustion. For example, when iron
+burns the oxide of iron formed weighs more than the original iron.
+
+In some cases the products of combustion are invisible gases, so that
+the substance undergoing combustion is apparently destroyed. Thus, when
+a candle burns it is consumed, and so far as the eye can judge nothing
+is formed during combustion. That invisible gases are formed, however,
+and that the weight of these is greater than the weight of the candle
+may be shown by the following experiment.
+
+[Illustration: Fig. 7]
+
+     A lamp chimney is filled with sticks of the compound known as
+     sodium hydroxide (caustic soda), and suspended from the beam of
+     the balance, as shown in Fig. 7. A piece of candle is placed on
+     the balance pan so that the wick comes just below the chimney,
+     and the balance is brought to a level by adding weights to the
+     other pan. The candle is then lighted. The products formed pass
+     up through the chimney and are absorbed by the sodium
+     hydroxide. Although the candle burns away, the pan upon which
+     it rests slowly sinks, showing that the combustion is attended
+     by an increase in weight.
+
+     ~Combustion in air and in oxygen.~ Combustion in air and in
+     oxygen differs only in rapidity, the products formed being
+     exactly the same. That the process should take place less
+     rapidly in the former is readily understood, for the air is
+     only about one fifth oxygen, the remaining four fifths being
+     inert gases. Not only is less oxygen available, but much of the
+     heat is absorbed in raising the temperature of the inert gases
+     surrounding the substance undergoing combustion, and the
+     temperature reached in the combustion is therefore less.
+
+     ~Phlogiston theory of combustion.~ The French chemist Lavoisier
+     (1743-1794), who gave to oxygen its name was the first to show
+     that combustion is due to union with oxygen. Previous to his
+     time combustion was supposed to be due to the presence of a
+     substance or principle called _phlogiston_. One substance was
+     thought to be more combustible than another because it
+     contained more phlogiston. Coal, for example, was thought to be
+     very rich in phlogiston. The ashes left after combustion would
+     not burn because all the phlogiston had escaped. If the
+     phlogiston could be restored in any way, the substance would
+     then become combustible again. Although this view seems absurd
+     to us in the light of our present knowledge, it formerly had
+     general acceptance. The discovery of oxygen led Lavoisier to
+     investigate the subject, and through his experiments he arrived
+     at the true explanation of combustion. The discovery of oxygen
+     together with the part it plays in combustion is generally
+     regarded as the most important discovery in the history of
+     chemistry. It marked the dawn of a new period in the growth of
+     the science.
+
+~Combustion in the broad sense.~ According to the definition given above,
+the presence of oxygen is necessary for combustion. The term is
+sometimes used, however, in a broader sense to designate any chemical
+change attended by the evolution of heat and light. Thus iron and
+sulphur, or hydrogen and chlorine under certain conditions, will combine
+so rapidly that light is evolved, and the action is called a combustion.
+Whenever combustion takes place in the air, however, the process is one
+of oxidation.
+
+     ~Spontaneous combustion.~ The temperature reached in a given
+     chemical action, such as oxidation, depends upon the rate at
+     which the reaction takes place. This rate is usually increased
+     by raising the temperature of the substances taking part in the
+     action.
+
+     When a slow oxidation takes place under such conditions that
+     the heat generated is not lost by being conducted away, the
+     temperature of the substance undergoing oxidation is raised,
+     and this in turn hastens the rate of oxidation. The rise in
+     temperature may continue in this way until the kindling
+     temperature of the substance is reached, when combustion
+     begins. Combustion occurring in this way is called _spontaneous
+     combustion_.
+
+     Certain oils, such as the linseed oil used in paints, slowly
+     undergo oxidation at ordinary temperatures, and not
+     infrequently the origin of fires has been traced to the
+     spontaneous combustion of oily rags. The spontaneous combustion
+     of hay has been known to set barns on fire. Heaps of coal have
+     been found to be on fire when spontaneous combustion offered
+     the only possible explanation.
+
+~Importance of oxygen.~ 1. Oxygen is essential to life. Among living
+organisms only certain minute forms of plant life can exist without it.
+In the process of respiration the air is taken into the lungs where a
+certain amount of oxygen is absorbed by the blood. It is then carried to
+all parts of the body, oxidizing the worn-out tissues and changing them
+into substances which may readily be eliminated from the body. The heat
+generated by this oxidation is the source of the heat of the body. The
+small amount of oxygen which water dissolves from the air supports all
+the varied forms of aquatic animals.
+
+2. Oxygen is also essential to decay. The process of decay is really a
+kind of oxidation, but it will only take place in the presence of
+certain minute forms of life known as bacteria. Just how these assist in
+the oxidation is not known. By this process the dead products of animal
+and vegetable life which collect on the surface of the earth are slowly
+oxidized and so converted into harmless substances. In this way oxygen
+acts as a great purifying agent.
+
+3. Oxygen is also used in the treatment of certain diseases in which the
+patient is unable to inhale sufficient air to supply the necessary
+amount of oxygen.
+
+
+OZONE
+
+~Preparation.~ When electric sparks are passed through oxygen or air a
+small percentage of the oxygen is converted into a substance called
+_ozone_, which differs greatly from oxygen in its properties. The same
+change can also be brought about by certain chemical processes. Thus, if
+some pieces of phosphorus are placed in a bottle and partially covered
+with water, the presence of ozone may soon be detected in the air
+contained in the bottle. The conversion of oxygen into ozone is attended
+by a change in volume, 3 volumes of oxygen forming 2 volumes of ozone.
+If the resulting ozone is heated to about 300 deg., the reverse change
+takes place, the 2 volumes of ozone being changed back into 3 volumes of
+oxygen. It is possible that traces of ozone exist in the atmosphere,
+although its presence there has not been definitely proved, the tests
+formerly used for its detection having been shown to be unreliable.
+
+~Properties.~ As commonly prepared, ozone is mixed with a large excess of
+oxygen. It is possible, however, to separate the ozone and thus obtain
+it in pure form. The gas so obtained has the characteristic odor noticed
+about electrical machines when in operation. By subjecting it to great
+pressure and a low temperature, the gas condenses to a bluish liquid,
+boiling at -119 deg.. When unmixed with other gases ozone is very explosive,
+changing back into oxygen with the liberation of heat. Its chemical
+properties are similar to those of oxygen except that it is far more
+active. Air or oxygen containing a small amount of ozone is now used in
+place of oxygen in certain manufacturing processes.
+
+~The difference between oxygen and ozone.~ Experiments show that in
+changing oxygen into ozone no other kind of matter is either added to
+the oxygen or withdrawn from it. The question arises then, How can we
+account for the difference in their properties? It must be remembered
+that in all changes we have to take into account _energy_ as well as
+_matter_. By changing the amount of energy in a substance we change its
+properties. That oxygen and ozone contain different amounts of energy
+may be shown in a number of ways; for example, by the fact that the
+conversion of ozone into oxygen is attended by the liberation of heat.
+The passage of the electric sparks through oxygen has in some way
+changed the energy content of the element and thus it has acquired new
+properties. _Oxygen and ozone must, therefore, be regarded as identical
+so far as the kind of matter of which they are composed is concerned.
+Their different properties are due to their different energy contents._
+
+~Allotropic states or forms of matter.~ Other elements besides oxygen may
+exist in more than one form. These different forms of the same element
+are called _allotropic states_ or _forms_ of the element. These forms
+differ not only in physical properties but also in their energy
+contents. Elements often exist in a variety of forms which look quite
+different. These differences may be due to accidental causes, such as
+the size or shape of the particles or the way in which the element was
+prepared. Only such forms, however, as have different energy contents
+are properly called allotropic forms.
+
+
+MEASUREMENT OF GAS VOLUMES
+
+~Standard conditions.~ It is a well-known fact that the volume occupied by
+a definite weight of any gas can be altered by changing the temperature
+of the gas or the pressure to which it is subjected. In measuring the
+volume of gases it is therefore necessary, for the sake of accuracy, to
+adopt some standard conditions of temperature and pressure. The
+conditions agreed upon are (1) a temperature of 0 deg., and (2) a pressure
+equal to the average pressure exerted by the atmosphere at the sea
+level, that is, 1033.3 g. per square centimeter. These conditions of
+temperature and pressure are known as the _standard conditions_, and
+when the volume of a gas is given it is understood that the measurement
+was made under these conditions, unless it is expressly stated
+otherwise. For example, the weight of a liter of oxygen has been given
+as 1.4285 g. This means that one liter of oxygen, measured at a
+temperature of 0 deg. and under a pressure of 1033.3 g. per square
+centimeter, weighs 1.4285 g.
+
+The conditions which prevail in the laboratory are never the standard
+conditions. It becomes necessary, therefore, to find a way to calculate
+the volume which a gas will occupy under standard conditions from the
+volume which it occupies under any other conditions. This may be done in
+accordance with the following laws.
+
+~Law of Charles.~ This law expresses the effect which a change in the
+temperature of a gas has upon its volume. It may be stated as follows:
+_For every degree the temperature of a gas rises above zero the volume
+of the gas is increased by 1/273 of the volume which it occupies at
+zero; likewise for every degree the temperature of the gas falls below
+zero the volume of the gas is decreased by 1/273 of the volume which it
+occupies at zero, provided in both cases that the pressure to which the
+gas is subjected remains constant._
+
+If V represents the volume of gas at 0 deg., then the volume at 1 deg. will be
+V + 1/273 V; at 2 deg. it will be V + 2/273 V; or, in general, the
+volume v, at the temperature t, will be expressed by the formula
+
+    (1) v = V + t/273 V,
+
+    or (2) v = V(1 + (t/273)).
+
+Since 1/273 = 0.00366, the formula may be written
+
+    (3) v = V(1 + 0.00366t).
+
+Since the value of V (volume under standard conditions) is the one
+usually sought, it is convenient to transpose the equation to the
+following form:
+
+    (4) V = v/(1 + 0.00366t).
+
+The following problem will serve as an illustration of the application
+of this equation.
+
+The volume of a gas at 20 deg. is 750 cc.; find the volume it will occupy at
+0 deg., the pressure remaining constant.
+
+In this case, v = 750 cc. and t = 20. By substituting these values,
+equation (4) becomes
+
+    V = 750/(1 + 0.00366 x 20) = 698.9 cc.
+
+~Law of Boyle.~ This law expresses the relation between the volume
+occupied by a gas and the pressure to which it is subjected. It may be
+stated as follows: _The volume of a gas is inversely proportional to the
+pressure under which it is measured, provided the temperature of the gas
+remains constant._
+
+If V represents the volume when subjected to a pressure P and v
+represents its volume when the pressure is changed to p, then, in
+accordance with the above law, V : v :: p : P, or VP = vp.
+In other words, for a given weight of a gas the product of the numbers
+representing its volume and the pressure to which it is subjected is a
+constant.
+
+Since the pressure of the atmosphere at any point is indicated by the
+barometric reading, it is convenient in the solution of the problems to
+substitute the latter for the pressure measured in grams per square
+centimeter. The average reading of the barometer at the sea level is 760
+mm., which corresponds to a pressure of 1033.3 g. per square centimeter.
+The following problem will serve as an illustration of the application
+of Boyle's law.
+
+A gas occupies a volume of 500 cc. in a laboratory where the barometric
+reading is 740 mm. What volume would it occupy if the atmospheric
+pressure changed so that the reading became 750 mm.?
+
+Substituting the values in the equation VP = vp, we have 500 x 740 =
+v x 750, or v = 493.3 cc.
+
+~Variations in the volume of a gas due to changes both in temperature and
+pressure.~ Inasmuch as corrections must be made as a rule for both
+temperature and pressure, it is convenient to combine the equations
+given above for the corrections for each, so that the two corrections
+may be made in one operation. The following equation is thus obtained:
+
+    (5) V_{s} = vp/(760(1 + 0.00366t)),
+
+in which V_{s} represents the volume of a gas under standard
+conditions and v, p, and t the volume, pressure, and temperature
+respectively at which the gas was actually measured.
+
+The following problem will serve to illustrate the application of this
+equation.
+
+A gas having a temperature of 20 deg. occupies a volume of 500 cc. when
+subjected to a pressure indicated by a barometric reading of 740 mm.
+What volume would this gas occupy under standard conditions?
+
+In this problem v = 500, p = 740, and t = 20. Substituting these
+values in the above equation, we get
+
+    V_{s} = (500 x 740)/(760 (1 + 0.00366 x 20)) = 453.6 cc.
+
+[Illustration: Fig. 8]
+
+~Variations in the volume of a gas due to the pressure of aqueous vapor.~
+In many cases gases are collected over water, as explained under the
+preparation of oxygen. In such cases there is present in the gas a
+certain amount of water vapor. This vapor exerts a definite pressure,
+which acts in opposition to the atmospheric pressure and which therefore
+must be subtracted from the latter in determining the effective pressure
+upon the gas. Thus, suppose we wish to determine the pressure to which
+the gas in tube A (Fig. 8) is subjected. The tube is raised or lowered
+until the level of the water inside and outside the tube is the same.
+The atmosphere presses down upon the surface of the water (as indicated
+by the arrows), thus forcing the water upward within the tube with a
+pressure equal to the atmospheric pressure. The full force of this
+upward pressure, however, is not spent in compressing the gas within the
+tube, for since it is collected over water it contains a certain amount
+of water vapor. This water vapor exerts a pressure (as indicated by the
+arrow within the tube) in opposition to the upward pressure. It is
+plain, therefore, that the effective pressure upon the gas is equal to
+the atmospheric pressure less the pressure exerted by the aqueous vapor.
+The pressure exerted by the aqueous vapor increases with the
+temperature. The figures representing the extent of this pressure (often
+called the _tension of aqueous vapor_) are given in the Appendix. They
+express the pressure or tension in millimeters of mercury, just as the
+atmospheric pressure is expressed in millimeters of mercury.
+Representing the pressure of the aqueous vapor by a, formula (5)
+becomes
+
+    (6) V_{s} = v(p - a)/(760(1 + 0.00366t)).
+
+The following problem will serve to illustrate the method of applying
+the correction for the pressure of the aqueous vapor.
+
+The volume of a gas measured over water in a laboratory where the
+temperature is 20 deg. and the barometric reading is 740 mm. is 500 cc. What
+volume would this occupy under standard conditions?
+
+The pressure exerted by the aqueous vapor at 20 deg. (see table in Appendix)
+is equal to the pressure exerted by a column of mercury 17.4 mm. in
+height. Substituting the values of v, t, p, and a in formula
+(6), we have
+
+    (6) V_{s} = 500(740 - 17.4)/(760(1 + 0.00366 x 20)) = 442.9 cc.
+
+~Adjustment of tubes before reading gas volumes.~ In measuring the volumes
+of gases collected in graduated tubes or other receivers, over a liquid
+as illustrated in Fig. 8, the reading should be taken after raising or
+lowering the tube containing the gas until the level of the liquid
+inside and outside the tube is the same; for it is only under these
+conditions that the upward pressure within the tube is the same as the
+atmospheric pressure.
+
+
+EXERCISES
+
+1. What is the meaning of the following words? phlogiston, ozone,
+phosphorus. (Consult dictionary.)
+
+2. Can combustion take place without the emission of light?
+
+3. Is the evolution of light always produced by combustion?
+
+4. (a) What weight of oxygen can be obtained from 100 g. of water?
+(b) What volume would this occupy under standard conditions?
+
+5. (a) What weight of oxygen can be obtained from 500g. of mercuric
+oxide? (b) What volume would this occupy under standard conditions?
+
+6. What weight of each of the following compounds is necessary to
+prepare 50 l. of oxygen? (a) water; (b) mercuric oxide; (c)
+potassium chlorate.
+
+7. Reduce the following volumes to 0 deg., the pressure remaining constant:
+(a) 150 cc. at 10 deg.; (b) 840 cc. at 273 deg..
+
+8. A certain volume of gas is measured when the temperature is 20 deg.. At
+what temperature will its volume be doubled?
+
+9. Reduce the following volumes to standard conditions of pressure, the
+temperature remaining constant: (a) 200 cc. at 740 mm.; (b) 500 l.
+at 380 mm.
+
+10. What is the weight of 1 l. of oxygen when the pressure is 750 mm.
+and the temperature 0 deg.?
+
+11. Reduce the following volumes to standard conditions of temperature
+and pressure: (a) 340 cc. at 12 deg. and 753 mm; (b) 500 cc. at 15 deg. and
+740 mm.
+
+12. What weight of potassium chlorate is necessary to prepare 250 l. of
+oxygen at 20 deg. and 750 mm.?
+
+13. Assuming the cost of potassium chlorate and mercuric oxide to be
+respectively $0.50 and $1.50 per kilogram, calculate the cost of
+materials necessary for the preparation of 50 l. of oxygen from each of
+the above compounds.
+
+14. 100 g. of potassium chlorate and 25 g. of manganese dioxide were
+heated in the preparation of oxygen. What products were left in the
+flask, and how much of each was present?
+
+
+
+
+CHAPTER III
+
+HYDROGEN
+
+
+~Historical.~ The element hydrogen was first clearly recognized as a
+distinct substance by the English investigator Cavendish, who in 1766
+obtained it in a pure state, and showed it to be different from the
+other inflammable airs or gases which had long been known. Lavoisier
+gave it the name hydrogen, signifying water former, since it had been
+found to be a constituent of water.
+
+~Occurrence.~ In the free state hydrogen is found in the atmosphere, but
+only in traces. In the combined state it is widely distributed, being a
+constituent of water as well as of all living organisms, and the
+products derived from them, such as starch and sugar. About 10% of the
+human body is hydrogen. Combined with carbon, it forms the substances
+which constitute petroleum and natural gas.
+
+     It is an interesting fact that while hydrogen in the free state
+     occurs only in traces on the earth, it occurs in enormous
+     quantities in the gaseous matter surrounding the sun and
+     certain other stars.
+
+~Preparation from water.~ Hydrogen can be prepared from water by several
+methods, the most important of which are the following.
+
+1. _By the electric current._ As has been indicated in the preparation
+of oxygen, water is easily separated into its constituents, hydrogen and
+oxygen, by passing an electric current through it under certain
+conditions.
+
+2. _By the action of certain metals._ When brought into contact with
+certain metals under appropriate conditions, water gives up a portion
+or the whole of its hydrogen, its place being taken by the metal. In the
+case of a few of the metals this change occurs at ordinary temperatures.
+Thus, if a bit of sodium is thrown on water, an action is seen to take
+place at once, sufficient heat being generated to melt the sodium, which
+runs about on the surface of the water. The change which takes place
+consists in the displacement of one half of the hydrogen of the water by
+the sodium, and may be represented as follows:
+
+          _        _            _        _
+         | hydrogen |          | sodium   |
+sodium + | hydrogen |(water) = | hydrogen |(sodium hydroxide) + hydrogen
+         |_oxygen  _|          |_oxygen  _|
+
+The sodium hydroxide formed is a white solid which remains dissolved in
+the undecomposed water, and may be obtained by evaporating the solution
+to dryness. The hydrogen is evolved as a gas and may be collected by
+suitable apparatus.
+
+Other metals, such as magnesium and iron, decompose water rapidly, but
+only at higher temperatures. When steam is passed over hot iron, for
+example, the iron combines with the oxygen of the steam, thus displacing
+the hydrogen. Experiments show that the change may be represented as
+follows:
+
+        _        _
+       | hydrogen |           _       _                 _        _
+iron + | hydrogen |(water) = | iron    |(iron oxide) + | hydrogen |
+       |_oxygen  _|          |_oxygen _|               |_hydrogen_|
+
+The iron oxide formed is a reddish-black compound, identical with that
+obtained by the combustion of iron in oxygen.
+
+     ~Directions for preparing hydrogen by the action of steam on
+     iron.~ The apparatus used in the preparation of hydrogen from
+     iron and steam is shown in Fig. 9. A porcelain or iron tube
+     B, about 50 cm. in length and 2 cm. or 3 cm. in diameter, is
+     partially filled with fine iron wire or tacks and connected as
+     shown in the figure. The tube B is heated, slowly at first,
+     until the iron is red-hot. Steam is then conducted through the
+     tube by boiling the water in the flask A. The hot iron
+     combines with the oxygen in the steam, setting free the
+     hydrogen, which is collected over water. The gas which first
+     passes over is mixed with the air previously contained in the
+     flask and tube, and is allowed to escape, _since a mixture of
+     hydrogen with oxygen or air explodes violently when brought in
+     contact with a flame_. It is evident that the flask A must be
+     disconnected from the tube before the heat is withdrawn.
+
+     That the gas obtained is different from air and oxygen may be
+     shown by holding a bottle of it mouth downward and bringing a
+     lighted splint into it. The hydrogen is ignited and burns with
+     an almost colorless flame.
+
+[Illustration Fig. 9]
+
+~Preparation from acids~ (_usual laboratory method_). While hydrogen can
+be prepared from water, either by the action of the electric current or
+by the action of certain metals, these methods are not economical and
+are therefore but little used. In the laboratory hydrogen is generally
+prepared from compounds known as acids, all of which contain hydrogen.
+When acids are brought in contact with certain metals, the metals
+dissolve and set free the hydrogen of the acid. Although this reaction
+is a quite general one, it has been found most convenient in preparing
+hydrogen by this method to use either zinc or iron as the metal and
+either hydrochloric or sulphuric acid as the acid. Hydrochloric acid is
+a compound consisting of 2.77% hydrogen and 97.23% chlorine, while
+sulphuric acid consists of 2.05% hydrogen, 32.70% sulphur, and 65.25%
+oxygen.
+
+The changes which take place in the preparation of hydrogen from zinc
+and sulphuric acid (diluted with water) may be represented as follows:
+
+        _        _               _       _
+       | hydrogen |(sulphuric   | zinc    |(zinc
+zinc + | sulphur  | acid)     = | sulphur | sulphate) + hydrogen
+       |_oxygen  _|             |_oxygen _|
+
+In other words, the zinc has taken the place of the hydrogen in
+sulphuric acid. The resulting compound contains zinc, sulphur, and
+oxygen, and is known as zinc sulphate. This remains dissolved in the
+water present in the acid. It may be obtained in the form of a white
+solid by evaporating the liquid left after the metal has passed into
+solution.
+
+When zinc and hydrochloric acid are used the following changes take
+place:
+
+        _        _                  _        _
+       | hydrogen |(hydrochloric   | zinc     |(zinc
+zinc + |_chlorine_| acid)        = |_chlorine_| chloride) + hydrogen
+
+When iron is used the changes which take place are exactly similar to
+those just given for zinc.
+
+[Illustration Fig. 10.]
+
+     ~Directions for preparing hydrogen from acids.~ The preparation
+     of hydrogen from acids is carried out in the laboratory as
+     follows: The metal is placed in a flask or wide-mouthed bottle
+     A (Fig. 10) and the acid is added slowly through the funnel
+     tube B. The metal dissolves in the acid, while the hydrogen
+     which is liberated escapes through the exit tube C and is
+     collected over water. It is evident that the hydrogen which
+     passes over first is mixed with the air from the bottle A.
+     Hence care must be taken not to bring a flame near the exit
+     tube, since, as has been stated previously, such a mixture
+     explodes with great violence when brought in contact with a
+     flame.
+
+     ~Precautions.~ Both sulphuric acid and zinc, if impure, are
+     likely to contain small amounts of arsenic. Such materials
+     should not be used in preparing hydrogen, since the arsenic
+     present combines with a portion of the hydrogen to form a very
+     poisonous gas known as arsine. On the other hand, chemically
+     pure sulphuric acid, i.e. sulphuric acid that is entirely free
+     from impurities, will not act upon chemically pure zinc. The
+     reaction may be started, however, by the addition of a few
+     drops of a solution of copper sulphate or platinum
+     tetrachloride.
+
+~Physical properties.~ Hydrogen is similar to oxygen in that it is a
+colorless, tasteless, odorless gas. It is characterized by its extreme
+lightness, being the lightest of all known substances. One liter of the
+gas weighs only 0.08984 g. On comparing this weight with that of an
+equal volume of oxygen, viz., 1.4285 g., the latter is found to be 15.88
+times as heavy as hydrogen. Similarly, air is found to be 14.38 times as
+heavy as hydrogen. Soap bubbles blown with hydrogen rapidly rise in the
+air. On account of its lightness it is possible to pour it upward from
+one bottle into another. Thus, if the bottle A (Fig. 11) is filled
+with hydrogen, placed mouth downward by the side of bottle _B_, filled
+with air, and is then gradually inverted under B as indicated in the
+figure, the hydrogen will flow upward into bottle _B_, displacing the
+air. Its presence in bottle B may then be shown by bringing a lighted
+splint to the mouth of the bottle, when the hydrogen will be ignited by
+the flame. It is evident, from this experiment, that in order to retain
+the gas in an open bottle the bottle must be placed mouth downward.
+
+[Illustration Fig. 11]
+
+Hydrogen is far more difficult to liquefy than any other gas, with the
+exception of helium, a rare element recently found to exist in the
+atmosphere. The English scientist Dewar, however, in 1898 succeeded not
+only in obtaining hydrogen in liquid state but also as a solid. Liquid
+hydrogen is colorless and has a density of only 0.07. Its boiling point
+under atmospheric pressure is -252 deg.. Under diminished pressure the
+temperature has been reduced to -262 deg.. The solubility of hydrogen in
+water is very slight, being still less than that of oxygen.
+
+Pure hydrogen produces no injurious results when inhaled. Of course one
+could not live in an atmosphere of the gas, since oxygen is essential to
+respiration.
+
+~Chemical properties.~ At ordinary temperatures hydrogen is not an active
+element. A mixture of hydrogen and chlorine, however, will combine with
+explosive violence at ordinary temperature if exposed to the sunlight.
+The union can be brought about also by heating. The product formed in
+either case is hydrochloric acid. Under suitable conditions hydrogen
+combines with nitrogen to form ammonia, and with sulphur to form the
+foul-smelling gas, hydrogen sulphide. The affinity of hydrogen for
+oxygen is so great that a mixture of hydrogen and oxygen or hydrogen
+and air explodes with great violence when heated to the kindling
+temperature (about 612 deg.). Nevertheless under proper conditions hydrogen
+may be made to burn quietly in either oxygen or air. The resulting
+hydrogen flame is almost colorless and is very hot. The combustion of
+the hydrogen is, of course, due to its union with oxygen. The product of
+the combustion is therefore a compound of hydrogen and oxygen. That this
+compound is water may be shown easily by experiment.
+
+[Illustration Fig. 12]
+
+     ~Directions for burning hydrogen in air.~ The combustion of
+     hydrogen in air may be carried out safely as follows: The
+     hydrogen is generated in the bottle A (Fig. 12), is dried by
+     conducting it through the tube X, filled with some substance
+     (generally calcium chloride) which has a great attraction for
+     moisture, and escapes through the tube T, the end of which is
+     drawn out to a jet. The hydrogen first liberated mixes with the
+     air contained in the generator. If a flame is brought near the
+     jet before this mixture has all escaped, a violent and very
+     dangerous explosion results, since the entire apparatus is
+     filled with the explosive mixture. On the other hand, if the
+     flame is not applied until all the air has been expelled, the
+     hydrogen is ignited and burns quietly, since only the small
+     amount of it which escapes from the jet can come in contact
+     with the oxygen of the air at any one time. By holding a cold,
+     dry bell jar or bottle over the flame, in the manner shown in
+     the figure, the steam formed by the combustion of the hydrogen
+     is condensed, the water collecting in drops on the sides of the
+     jar.
+
+~Precautions.~ In order to avoid danger it is absolutely necessary to
+prove that the hydrogen is free from air before igniting it. This can be
+done by testing small amounts of the escaping gas. A convenient and safe
+method of doing this is to fill a test tube with the gas by inverting it
+over the jet. The hydrogen, on account of its lightness, collects in the
+tube, displacing the air. After holding it over the jet for a few
+moments in order that it may be filled with the gas, the tube is gently
+brought, mouth downward, to the flame of a burner placed not nearer than
+an arm's length from the jet. If the hydrogen is mixed with air a slight
+explosion occurs, but if pure it burns quietly in the tube. The
+operation is repeated until the gas burns quietly, when the tube is
+quickly brought back over the jet for an instant, whereby the escaping
+hydrogen is ignited by the flame in the tube.
+
+[Illustration. Fig. 13]
+
+~A mixture of hydrogen and oxygen is explosive.~ That a mixture of
+hydrogen and air is explosive may be shown safely as follows: A cork
+through which passes a short glass tube about 1 cm. in diameter is
+fitted air-tight into the tubule of a bell jar of 2 l. or 3 l. capacity.
+(A thick glass bottle with bottom removed may be used.) The tube is
+closed with a small rubber stopper and the bell jar filled with
+hydrogen, the gas being collected over water. When entirely filled with
+the gas the jar is removed from the water and supported by blocks of
+wood in order to leave the bottom of the jar open, as shown in Fig. 13.
+The stopper is now removed from the tube in the cork, and the hydrogen,
+which on account of its lightness escapes from the tube, is at once
+lighted. As the hydrogen escapes, the air flows in at the bottom of the
+jar and mixes with the remaining portion of the hydrogen, so that a
+mixture of the two soon forms, and a loud explosion results. The
+explosion is not dangerous, since the bottom of the jar is open, thus
+leaving room for the expansion of the hot gas.
+
+Since air is only one fifth oxygen, the remainder being inert gases, it
+may readily be inferred that a mixture of hydrogen with pure oxygen
+would be far more explosive than a mixture of hydrogen with air. Such
+mixtures should not be made except in small quantities and by
+experienced workers.
+
+~Hydrogen does not support combustion.~ While hydrogen is readily
+combustible, it is not a supporter of combustion. In other words,
+substances will not burn in it. This may be shown by bringing a lighted
+candle supported by a stiff wire into a bottle or cylinder of the pure
+gas, as shown in Fig. 14. The hydrogen is ignited by the flame of the
+candle and burns at the mouth of the bottle, where it comes in contact
+with the oxygen in the air. When the candle is thrust up into the gas,
+its flame is extinguished on account of the absence of oxygen. If slowly
+withdrawn, the candle is relighted as it passes through the layer of
+burning hydrogen.
+
+[Illustration: Fig. 14]
+
+[Illustration: Fig. 15]
+
+~Reduction.~ On account of its great affinity for oxygen, hydrogen has the
+power of abstracting it from many of its compounds. Thus, if a stream of
+hydrogen, dried by passing through the tube B (Fig. 15), filled with
+calcium chloride, is conducted through the tube C containing some
+copper oxide, heated to a moderate temperature, the hydrogen abstracts
+the oxygen from the copper oxide. The change may be represented as
+follows:
+
+     hydrogen + {copper}                 {hydrogen}
+                {oxygen}(copper oxide) = {oxygen  }(water) + copper
+
+The water formed collects in the cold portions of the tube C near its
+end. In this experiment the copper oxide is said to undergo reduction.
+_Reduction may therefore be defined as the process of withdrawing oxygen
+from a compound._
+
+~Relation of reduction to oxidation.~ At the same time that the copper
+oxide is reduced it is clear that the hydrogen is oxidized, for it
+combines with the oxygen given up by the copper oxide. The two processes
+are therefore very closely related, and it usually happens that when one
+substance is oxidized some other substance is reduced. That substance
+which gives up its oxygen is called an _oxidizing agent_, while the
+substance which unites with the oxygen is called a _reducing agent_.
+
+~The oxyhydrogen blowpipe.~ This is a form of apparatus used for burning
+hydrogen in pure oxygen. As has been previously stated, the flame
+produced by the combustion of hydrogen in the air is very hot. It is
+evident that if pure oxygen is substituted for air, the temperature
+reached will be much higher, since there are no inert gases to absorb
+the heat. The oxyhydrogen blowpipe, used to effect this combination,
+consists of a small tube placed within a larger one, as shown in Fig.
+16.
+
+[Illustration: Fig. 16]
+
+The hydrogen, stored under pressure, generally in steel cylinders, is
+first passed through the outer tube and ignited at the open end of the
+tube. The oxygen from a similar cylinder is then conducted through the
+inner tube, and mixes with the hydrogen at the end of the tube. In order
+to produce the maximum heat, the hydrogen and oxygen must be admitted to
+the blowpipe in the exact proportion in which they combine, viz., 2
+volumes of hydrogen to 1 of oxygen, or by weight, 1 part of hydrogen to
+7.94 parts of oxygen. The intensity of the heat may be shown by bringing
+into the flame pieces of metal such as iron wire or zinc. These burn
+with great brilliancy. Even platinum, having a melting point of 1779 deg.,
+may be melted by the heat of the flame.
+
+While the oxyhydrogen flame is intensely hot, it is almost non-luminous.
+If directed against some infusible substance like ordinary lime (calcium
+oxide), the heat is so intense that the lime becomes incandescent and
+glows with a brilliant light. This is sometimes used as a source of
+light, under the name of _Drummond_ or _lime light_.
+
+[Illustration: Fig. 17]
+
+~The blast lamp.~ A similar form of apparatus is commonly used in the
+laboratory as a source of heat under the name _blast lamp_ (Fig. 17).
+This differs from the oxyhydrogen blowpipe only in the size of the
+tubes. In place of the hydrogen and oxygen the more accessible coal gas
+and air are respectively used. The former is composed largely of a
+mixture of free hydrogen and gaseous compounds of carbon and hydrogen.
+While the temperature of the flame is not so high as that of the
+oxyhydrogen blowpipe, it nevertheless suffices for most chemical
+operations carried out in the laboratory.
+
+~Uses of hydrogen.~ On account of its cost, hydrogen is but little used
+for commercial purposes. It is sometimes used as a material for the
+inflation of balloons, but usually the much cheaper coal gas is
+substituted for it. Even hot air is often used when the duration of
+ascension is very short. It has been used also as a source of heat and
+light in the oxyhydrogen blowpipe. Where the electric current is
+available, however, this form of apparatus has been displaced almost
+entirely by the electric light and electric furnace, which are much more
+economical and more powerful sources of light and heat.
+
+
+EXERCISES
+
+1. Will a definite weight of iron decompose an unlimited weight of
+steam?
+
+2. Why is oxygen passed through the inner tube of the oxyhydrogen
+blowpipe rather than the outer?
+
+3. In Fig. 14, will the flame remain at the mouth of the tube?
+
+4. From Fig. 15, suggest a way for determining experimentally the
+quantity of water formed in the reaction.
+
+5. Distinguish clearly between the following terms: oxidation,
+reduction, combustion, and kindling temperature.
+
+6. Is oxidation always accompanied by reduction?
+
+7. What is the source of heat in the lime light? What is the exact use
+of lime in this instrument?
+
+8. In Fig. 12, why is it necessary to dry the hydrogen by means of the
+calcium chloride in the tube X?
+
+9. At what pressure would the weight of 1 l. of hydrogen be equal to
+that of oxygen under standard conditions?
+
+10. (a) What weight of hydrogen can be obtained from 150 g. of
+sulphuric acid? (b) What volume would this occupy under standard
+conditions? (c) The density of sulphuric acid is 1.84. What volume
+would the 150 g. of the acid occupy?
+
+11. How many liters of hydrogen can be obtained from 50 cc. of sulphuric
+acid having a density of 1.84?
+
+12. Suppose you wish to fill five liter bottles with hydrogen, the gas
+to be collected over water in your laboratory, how many cubic
+centimeters of sulphuric acid would be required?
+
+
+
+
+CHAPTER IV
+
+COMPOUNDS OF HYDROGEN AND OXYGEN; WATER AND HYDROGEN DIOXIDE
+
+
+WATER
+
+~Historical.~ Water was long regarded as an element. In 1781 Cavendish
+showed that it is formed by the union of hydrogen and oxygen. Being a
+believer in the phlogiston theory, however, he failed to interpret his
+results correctly. A few years later Lavoisier repeated Cavendish's
+experiments and showed that water must be regarded as a compound of
+hydrogen and oxygen.
+
+~General methods employed for the determination of the composition of a
+compound.~ The composition of a compound may be determined by either of
+two general processes these are known as _analysis_ and _synthesis_.
+
+1. _Analysis_ is the process of decomposing a compound into its
+constituents and determining what these constituents are. The analysis
+is _qualitative_ when it results in merely determining what elements
+compose the compound; it is _quantitative_ when the exact percentage of
+each constituent is determined. Qualitative analysis must therefore
+precede quantitative analysis, for it must be known what elements, are
+in a compound before a method can be devised for determining exactly how
+much of each is present.
+
+2. _Synthesis_ is the process of forming a compound from its constituent
+parts. It is therefore the reverse of analysis. Like analysis, it may be
+either qualitative or quantitative.
+
+~Application of these methods to the determination of the composition of
+water.~ The determination of the composition of water is a matter of
+great interest not only because of the importance of the compound but
+also because the methods employed illustrate the general methods of
+analysis and synthesis.
+
+~Methods based on analysis.~ The methods based on analysis may be either
+qualitative or quantitative in character.
+
+[Illustration: Fig. 18]
+
+1. _Qualitative analysis._ As was stated in the study of oxygen, water
+may be separated into its component parts by means of the electric
+current. The form of apparatus ordinarily used for effecting this
+analysis is shown in Fig. 18. A platinum wire, to the end of which is
+attached a small piece of platinum foil (about 15 mm. by 25 mm.), is
+fused through each of the tubes B and D, as shown in the figure. The
+stopcocks at the ends of these tubes are opened and water, to which has
+been added about one tenth of its volume of sulphuric acid, is poured
+into the tube A until the side tubes B and D are completely
+filled. The stopcocks are then closed. The platinum wires extending into
+the tubes B and D are now connected with the wires leading from two
+or three dichromate cells joined in series. The pieces of platinum foil
+within the tubes thus become the electrodes, and the current flows from
+one to the other through the acidulated water. As soon as the current
+passes, bubbles of gas rise from each of the electrodes and collect in
+the upper part of the tubes. The gas rising from the negative electrode
+is found to be hydrogen, while that from the positive electrode is
+oxygen. It will be seen that the volume of the hydrogen is approximately
+double that of the oxygen. Oxygen is more soluble in water than
+hydrogen, and a very little of it is also lost by being converted into
+ozone and other substances. It has been found that when the necessary
+corrections are made for the error due to these facts, the volume of the
+hydrogen is exactly double that of the oxygen.
+
+Fig. 19 illustrates a simpler form of apparatus, which may be used in
+place of that shown in Fig. 18. A glass or porcelain dish is partially
+filled with water to which has been added the proper amount of acid. Two
+tubes filled with the same liquid are inverted over the electrodes. The
+gases resulting from the decomposition of the water collect in the
+tubes.
+
+[Illustration: Fig. 19]
+
+2. _Quantitative analysis._ The analysis just described is purely
+qualitative and simply shows that water contains hydrogen and oxygen. It
+does not prove the absence of other elements; indeed it does not prove
+that the hydrogen and oxygen are present in the proportion in which they
+are liberated by the electric current. The method may be made
+quantitative, however, by weighing the water decomposed and also the
+hydrogen and oxygen obtained in its decomposition. If the combined
+weights of the hydrogen and oxygen exactly equal the weight of the water
+decomposed, then it would be proved that the water consists of hydrogen
+and oxygen in the proportion in which they are liberated by the electric
+current. This experiment is difficult to carry out, however, so that the
+more accurate methods based on synthesis are used.
+
+~Methods based on synthesis.~ Two steps are necessary to ascertain the
+exact composition of water by synthesis: (1) to show by qualitative
+synthesis that water is formed by the union of oxygen with hydrogen; (2)
+to determine by quantitative synthesis in what proportion the two
+elements unite to form water. The fact that water is formed by the
+combination of oxygen with hydrogen was proved in the preceding chapter.
+The quantitative synthesis may be made as follows:
+
+[Illustration: Fig. 20]
+
+The combination of the two gases is brought about in a tube called a
+eudiometer. This is a graduated tube about 60 cm. long and 2 cm. wide,
+closed at one end (Fig. 20). Near the closed end two platinum wires are
+fused through the glass, the ends of the wires within the tube being
+separated by a space of 2 mm or 3 mm. The tube is entirely filled with
+mercury and inverted in a vessel of the same liquid. Pure hydrogen is
+passed into the tube until it is about one fourth filled. The volume of
+the gas is then read off on the scale and reduced to standard
+conditions. Approximately an equal volume of pure oxygen is then
+introduced and the volume again read off and reduced to standard
+conditions. This gives the total volume of the two gases. From this the
+volume of the oxygen introduced may be determined by subtracting from
+it the volume of the hydrogen. The combination of the two gases is now
+brought about by connecting the two platinum wires with an induction
+coil and passing a spark from one wire to the other. Immediately a
+slight explosion occurs. The mercury in the tube is at first depressed
+because of the expansion of the gases due to the heat generated, but at
+once rebounds, taking the place of the gases which have combined to form
+water. The volume of the water in the liquid state is so small that it
+may be disregarded in the calculations. In order that the temperature of
+the residual gas and the mercury may become uniform, the apparatus is
+allowed to stand for a few minutes. The volume of the gas is then read
+off and reduced to standard conditions, so that it may be compared with
+the volumes of the hydrogen and oxygen originally taken. The residual
+gas is then tested in order to ascertain whether it is hydrogen or
+oxygen, experiments having proved that it is never a mixture of the two.
+From the information thus obtained the composition of the water may be
+calculated. Thus, suppose the readings were as follows:
+
+Volume of hydrogen taken                                          20.3 cc.
+Volume of hydrogen and oxygen                                     38.7
+Volume of oxygen                                                  18.4
+Volume of gas left after combination has taken place (oxygen)      8.3
+
+The 20.3 cc. of hydrogen have combined with 18.4 cc. minus 8.3 cc. (or
+10.1 cc.) of oxygen; or approximately 2 volumes of hydrogen have
+combined with 1 of oxygen. Since oxygen is 15.88 times as heavy as
+hydrogen, the proportion by weight in which the two gases combine is 1
+part of hydrogen to 7.94 of oxygen.
+
+~Precaution.~ If the two gases are introduced into the eudiometer in the
+exact proportions in which they combine, after the combination has taken
+place the liquid will rise and completely fill the tube. Under these
+conditions, however, the tube is very likely to be broken by the sudden
+upward rush of the liquid. Hence in performing the experiment care is
+taken to introduce an excess of one of the gases.
+
+~A more convenient form of eudiometer.~ A form of eudiometer (Fig. 21)
+different from that shown on page 43 is sometimes used to avoid the
+calculations necessary in reducing the volumes of the gases to the same
+conditions of temperature and pressure in order to make comparisons.
+With this apparatus it is possible to take the readings of the volumes
+under the same conditions of temperature and pressure, and thus compare
+them directly. The apparatus (Fig. 21) is filled with mercury and the
+gases introduced into the tube A. The experiment is carried out as in
+the preceding one, except that before taking the reading of the gas
+volumes, mercury is either added to the tube B or withdrawn from it by
+means of the stopcock C, until it stands at exactly the same height in
+both tubes. The gas inclosed in tube A is then under atmospheric
+pressure; and since but a few minutes are required for performing the
+experiment, the conditions of temperature and pressure may be regarded
+as constant. Hence the volumes of the hydrogen and oxygen and of the
+residual gas may be read off from the tube and directly compared.
+
+[Illustration: Fig. 21]
+
+~Method used by Berzelius and Dumas.~ The method used by these
+investigators enables us to determine directly the proportion by weight
+in which the hydrogen and oxygen combine. Fig. 22 illustrates the
+apparatus used in making this determination. B is a glass tube
+containing copper oxide. C and D are glass tubes filled with calcium
+chloride, a substance which has great affinity for water. The tubes B
+and C, including their contents, are carefully weighed, and the
+apparatus connected as shown in the figure. A slow current of pure
+hydrogen is then passed through A, and that part of the tube B which
+contains copper oxide is carefully heated. The hydrogen combines with
+the oxygen present in the copper oxide to form water, which is absorbed
+by the calcium chloride in tube C. The calcium chloride in tube D
+prevents any moisture entering tube C from the air. The operation is
+continued until an appreciable amount of water has been formed. The
+tubes B and C are then weighed once more. The loss of weight in the
+tube B will exactly equal the weight of oxygen taken up from the
+copper oxide in the formation of the water. The gain in weight in the
+tube C will exactly equal the weight of the water formed. The
+difference in these weights will of course equal the weight of the
+hydrogen present in the water formed.
+
+[Illustration: Fig. 22]
+
+~Dumas' results.~ The above method for the determination of the
+composition of water was first used by Berzelius in 1820. The work was
+repeated in 1843 by Dumas, the average of whose results is as follows:
+
+Weight of water formed                  236.36 g.
+Oxygen given up by the copper oxide     210.04
+                                        ------
+Weight of hydrogen present in water      26.32
+
+According to this experiment the ratio of hydrogen to oxygen in water is
+therefore 26.32 to 210.04, or as l to 7.98
+
+~Morley's results.~ The American chemist Morley has recently determined
+the composition of water, extreme precautions being taken to use pure
+materials and to eliminate all sources of error. The hydrogen and oxygen
+which combined, as well as the water formed, were all accurately
+weighed. According to Morley's results, 1 part of hydrogen by weight
+combines with 7.94 parts of oxygen to form water.
+
+~Comparison of results obtained.~ From the above discussions it is easy to
+see that it is by experiment alone that the composition of a compound
+can be determined. Different methods may lead to slightly different
+results. The more accurate the method chosen and the greater the skill
+with which the experiment is carried out, the more accurate will be the
+results. It is generally conceded by chemists that the results obtained
+by Morley in reference to the composition of water are the most accurate
+ones. In accordance with these results, then, _water must be regarded as
+a compound containing hydrogen and oxygen in the proportion of 1 part by
+weight of hydrogen to 7.94 parts by weight of oxygen_.
+
+~Relation between the volume of aqueous vapor and the volumes of the
+hydrogen and oxygen which combine to form it.~ When the quantitative
+synthesis of water is carried out in the eudiometer as described above,
+the water vapor formed by the union of the hydrogen and oxygen at once
+condenses. The volume of the resulting liquid is so small that it may be
+disregarded in making the calculations. If, however, the experiment is
+carried out at a temperature of 100 deg. or above, the water-vapor formed is
+not condensed and it thus becomes possible to compare the volume of the
+vapor with the volumes of hydrogen and oxygen which combined to form it.
+This can be accomplished by surrounding the arm A of the eudiometer
+(Fig. 23) with the tube B through which is passed the vapor obtained
+by boiling some liquid which has a boiling point above 100 deg.. In this way
+it has been proved that 2 volumes of hydrogen and 1 volume of oxygen
+combine to form exactly 2 volumes of water vapor, the volumes all being
+measured under the same conditions of temperature and pressure. It will
+be noted that the relation between these volumes may be expressed by
+whole numbers. The significance of this very important fact will be
+discussed in a subsequent chapter.
+
+[Illustration: Fig. 23]
+
+~Occurrence of water.~ Water not only covers about three fourths of the
+surface of the earth, and is present in the atmosphere in the form of
+moisture, but it is also a common constituent of the soil and rocks and
+of almost every form of animal and vegetable organism. The human body is
+nearly 70% water. This is derived not only from the water which we drink
+but also from the food which we eat, most of which contains a large
+percentage of water. Thus potatoes contain about 78% of water, milk 85%,
+beef over 50%, apples 84%, tomatoes 94%.
+
+~Impurities in water.~ Chemically pure water contains only hydrogen and
+oxygen. Such a water never occurs in nature, however, for being a good
+solvent, it takes up certain substances from the rocks and soil with
+which it comes in contact. When such waters are evaporated these
+substances are deposited in the form of a residue. Even rain water,
+which is the purest form occurring in nature, contains dust particles
+and gases dissolved from the atmosphere. The foreign matter in water is
+of two kinds, namely, _mineral_, such as common salt and limestone, and
+_organic_, that is the products of animal and vegetable life.
+
+     ~Mineral matter in water.~ The amount and nature of the mineral
+     matter present in different waters vary greatly, depending on
+     the character of the rocks and soil with which the waters come
+     in contact. The more common of the substances present are
+     common salt and compounds of calcium, magnesium, and iron. One
+     liter of the average river water contains about 175 mg. of
+     mineral matter. Water from deep wells naturally contains more
+     mineral matter than river water, generally two or three times
+     as much, while sea water contains as much as 35,000 mg. to the
+     liter.
+
+~Effect of impurities on health.~ The mineral matter in water does not,
+save in very exceptional cases, render the water injurious to the human
+system. In fact the presence of a certain amount of such matter is
+advantageous, supplying the mineral constituents necessary for the
+formation of the solid tissues of the body. The presence of organic
+matter, on the other hand, must always be regarded with suspicion. This
+organic matter may consist not only of the products of animal and
+vegetable life but also of certain microscopic forms of living organisms
+which are likely to accompany such products. Contagious diseases are
+known to be due to the presence in the body of minute living organisms
+or germs. Each disease is caused by its own particular kind of germ.
+Through sewage these germs may find their way from persons afflicted
+with disease into the water supply, and it is principally through the
+drinking water that certain of these diseases, especially typhoid fever,
+are spread. It becomes of great importance, therefore, to be able to
+detect such matter when present in drinking water as well as to devise
+methods whereby it can be removed or at least rendered harmless.
+
+     ~Analysis of water.~ The mineral analysis of a water is, as the
+     name suggests, simply the determination of the mineral matter
+     present. Sanitary analysis, on the other hand, is the
+     determination of the organic matter present. The physical
+     properties of a water give no conclusive evidence as to its
+     purity, since a water may be unfit for drinking purposes and
+     yet be perfectly clear and odorless. Neither can any reliance
+     be placed on the simple methods often given for testing the
+     purity of water. Only the trained chemist can carry out such
+     methods of analysis as can be relied upon.
+
+[Illustration: Fig. 24]
+
+~Purification of water.~ Three general methods are used for the
+purification of water, namely, _distillation_, _filtration_, and
+_boiling_.
+
+1. _Distillation._ The most effective way of purifying natural waters is
+by the process of distillation. This consists in boiling the water and
+condensing the steam. Fig. 24 illustrates the process of distillation,
+as commonly conducted in the laboratory. Ordinary water is poured into
+the flask A and boiled. The steam is conducted through the condenser
+B, which consists essentially of a narrow glass tube sealed within a
+larger one, the space between the two being filled with cold water,
+which is admitted at C and escapes at D. The inner tube is thus kept
+cool and the steam in passing through it is condensed. The water formed
+by the condensation of the steam collects in the receiver E and is
+known as _distilled_ water. Such water is practically pure, since the
+impurities are nonvolatile and remain in the flask A.
+
+     ~Commercial distillation.~ In preparing distilled water on a
+     large scale, the steam is generated in a boiler or other metal
+     container and condensed by passing it through a pipe made of
+     metal, generally tin. This pipe is wound into a spiral and is
+     surrounded by a current of cold water. Distilled water is used
+     by the chemist in almost all of his work. It is also used in
+     the manufacture of artificial ice and for drinking water.
+
+     ~Fractional distillation.~ In preparing distilled water, it is
+     evident that if the natural water contains some substance which
+     is volatile its vapor will pass over and be condensed with the
+     steam, so that the distillate will not be pure water. Even such
+     mixtures, however, may generally be separated by repeated
+     distillation. Thus, if a mixture of water (boiling point 100 deg.)
+     and alcohol (boiling point 78 deg.) is distilled, the alcohol,
+     having the lower boiling point, tends to distill first,
+     followed by the water. The separation of the two is not
+     perfect, however, but may be made nearly so by repeated
+     distillations. The process of separating a mixture of volatile
+     substances by distillation is known as _fractional
+     distillation_.
+
+2. _Filtration._ The process of distillation practically removes all
+nonvolatile foreign matter, mineral as well as organic. In purifying
+water for drinking purposes, however, it is only necessary to eliminate
+the latter or to render it harmless. This is ordinarily done either by
+filtration or boiling. In filtration the water is passed through some
+medium which will retain the organic matter. Ordinary charcoal is a
+porous substance and will condense within its pores the organic matter
+in water if brought in contact with it. It is therefore well adapted to
+the construction of filters. Such filters to be effective must be kept
+clean, since it is evident that the charcoal is useless after its pores
+are filled. A more effective type of filter is the Chamberlain-Pasteur
+filter. In this the water is forced through a porous cylindrical cup,
+the pores being so minute as to strain out the organic matter.
+
+     ~City filtration beds.~ For purifying the water supply of cities,
+     large filtration beds are prepared from sand and gravel, and
+     the water is allowed to filter through these. Some of the
+     impurities are strained out by the filter, while others are
+     decomposed by the action of certain kinds of bacteria present
+     in the sand. Fig. 25 shows a cross section of a portion of the
+     filter used in purifying the water supply of Philadelphia. The
+     water filters through the sand and gravel and passes into the
+     porous pipe A, from which it is pumped into the city mains.
+     The filters are covered to prevent the water from freezing in
+     cold weather.
+
+[Illustration: Fig. 25]
+
+3. _Boiling._ A simpler and equally efficient method for purifying water
+for drinking purposes consists in boiling the water. It is the germs in
+water that render it dangerous to health. These germs are living forms
+of matter. If the water is boiled, the germs are killed and the water
+rendered safe. While these germs are destroyed by heat, cold has little
+effect upon them. Thus Dewar, in working with liquid hydrogen, exposed
+some of these minute forms of life to the temperature of boiling
+hydrogen (-252 deg.) without killing them.
+
+~Self-purification of water.~ It has long been known that water
+contaminated with organic matter tends to purify itself when exposed to
+the air. This is due to the fact that the water takes up a small amount
+of oxygen from the air, which gradually oxidizes the organic matter
+present in the water. While water is undoubtedly purified in this way,
+the method cannot be relied upon to purify a contaminated water so as to
+render it safe for drinking purposes.
+
+~Physical properties.~ Pure water is an odorless and tasteless liquid,
+colorless in thin layers, but having a bluish tinge when observed
+through a considerable thickness. It solidifies at 0 deg. and boils at 100 deg.
+under the normal pressure of one atmosphere. If the pressure is
+increased, the boiling point is raised. When water is cooled it steadily
+contracts until the temperature of 4 deg. is reached: it then expands. Water
+is remarkable for its ability to dissolve other substances, and is the
+best solvent known. Solutions of solids in water are more frequently
+employed in chemical work than are the solid substances, for chemical
+action between substances goes on more readily when they are in solution
+than it does when they are in the solid state.
+
+~Chemical properties.~ Water is a very stable substance, or, in other
+words, it does not undergo decomposition readily. To decompose it into
+its elements by heat alone requires a very high temperature; at 2500 deg.,
+for example, only about 5% of the entire amount is decomposed. Though
+very stable towards heat, water can be decomposed in other ways, as by
+the action of the electrical current or by certain metals.
+
+     ~Heat of formation and heat of decomposition are equal.~ The fact
+     that a very high temperature is necessary to decompose water
+     into hydrogen and oxygen is in accord with the fact that a
+     great deal of heat is evolved by the union of hydrogen and
+     oxygen; for it has been proved that the heat necessary to
+     decompose a compound into its elements (heat of decomposition)
+     is equal to the heat evolved in the formation of a compound
+     from its elements (heat of formation).
+
+~Water of crystallization.~ When a solid is dissolved in water and the
+resulting solution is allowed to evaporate, the solid separates out,
+often in the form of crystals. It has been found that the crystals of
+many compounds, although perfectly dry, give up a definite amount of
+water when heated, the substance at the same time losing its crystalline
+form. Such water is called _water of crystallization_. This varies in
+amount with different compounds, but is perfectly definite for the same
+compound. Thus, if a perfectly dry crystal of copper sulphate is
+strongly heated in a tube, water is evolved and condenses on the sides
+of the tube, the crystal crumbling to a light powder. The weight of the
+water evolved is always equal to exactly 36.07% of the weight of copper
+sulphate crystals heated. The water must therefore be in chemical
+combination with the substance composing the crystal; for if simply
+mixed with it or adhering to it, not only would the substance appear
+moist but the amount present would undoubtedly vary. The combination,
+however, must be a very weak one, since the water is often expelled by
+even a gentle heat. Indeed, in some cases the water is given up on
+simple exposure to air. Such compounds are said to be _efflorescent_.
+Thus a crystal of sodium sulphate (Glauber's salt) on exposure to air
+crumbles to a fine powder, owing to the escape of its water of
+crystallization. Other substances have just the opposite property: they
+absorb moisture when exposed to the air. For example, if a bit of dry
+calcium chloride is placed in moist air, in the course of a few hours it
+will have absorbed sufficient moisture to dissolve it. Such substances
+are said to be _deliquescent_. A deliquescent body serves as a good
+drying or _desiccating_ agent. We have already employed calcium chloride
+as an agent for absorbing the moisture from hydrogen. Many substances,
+as for example quartz, form crystals which contain no water of
+crystallization.
+
+     ~Mechanically inclosed water.~ Water of crystallization must be
+     carefully distinguished from water which is mechanically
+     inclosed in a crystal and which can be removed by powdering the
+     crystal and drying. Thus, when crystals of common salt are
+     heated, the water inclosed in the crystal is changed into steam
+     and bursts the crystal with a crackling sound. Such crystals
+     are said to _decrepitate_. That this water is not combined is
+     proved by the fact that the amount present varies and that it
+     has all the properties of water.
+
+~Uses of water.~ The importance of water in its relation to life and
+commerce is too well known to require comment. Its importance to the
+chemist has also been pointed out. It remains to call attention to the
+fact that it is used as a standard in many physical measurements. Thus
+0 deg. and 100 deg. on the centigrade scale are respectively the freezing and
+the boiling points of water under normal pressure. The weight of 1 cc.
+of water at its point of greatest density is the unit of weight in the
+metric system, namely, the gram. It is also taken as the unit for the
+determination of the density of liquids and solids as well as for the
+measurement of amounts of heat.
+
+
+HYDROGEN DIOXIDE
+
+~Composition.~ As has been shown, 1 part by weight of hydrogen combines
+with 7.94 parts by weight of oxygen to form water. It is possible,
+however, to obtain a second compound of hydrogen and oxygen differing
+from water in composition in that 1 part by weight of hydrogen is
+combined with 2 x 7.94, or 15.88 parts, of oxygen. This compound is
+called _hydrogen dioxide_ or _hydrogen peroxide_, the prefixes _di-_ and
+_per-_ signifying that it contains more oxygen than hydrogen oxide,
+which is the chemical name for water.
+
+~Preparation.~ Hydrogen dioxide cannot be prepared cheaply by the direct
+union of hydrogen and oxygen, and indirect methods must therefore be
+used. It is commonly prepared by the action of a solution of sulphuric
+acid on barium dioxide. The change which takes place may be indicated as
+follows:
+
+sulphuric acid +  barium dioxide =  barium sulphate + hydrogen dioxide
+--------------    --------------    ---------------   ----------------
+  hydrogen          barium           barium             hydrogen
+  sulphur           oxygen           sulphur            oxygen
+  oxygen                             oxygen
+
+In other words, the barium and hydrogen in the two compounds exchange
+places. By this method a dilute solution of the dioxide in water is
+obtained. It is possible to separate the dioxide from the water by
+fractional distillation. This is attended with great difficulties,
+however, since the pure dioxide is explosive. The distillation is
+carried on under diminished pressure so as to lower the boiling points
+as much as possible; otherwise the high temperature would decompose the
+dioxide.
+
+~Properties.~ Pure hydrogen dioxide is a colorless sirupy liquid having a
+density of 1.49. Its most characteristic property is the ease with which
+it decomposes into water and oxygen. One part by weight of hydrogen is
+capable of holding firmly only 7.94 parts of oxygen. The additional 7.94
+parts of oxygen present in hydrogen dioxide are therefore easily
+evolved, the compound breaking down into water and oxygen. This
+decomposition is attended by the generation of considerable heat. In
+dilute solution hydrogen dioxide is fairly stable, although such a
+solution should be kept in a dark, cool place, since both heat and light
+aid in the decomposition of the dioxide.
+
+~Uses.~ Solutions of hydrogen dioxide are used largely as oxidizing
+agents. The solution sold by druggists contains 3% of the dioxide and is
+used in medicine as an antiseptic. Its use as an antiseptic depends upon
+its oxidizing properties.
+
+
+EXERCISES
+
+1. Why does the chemist use distilled water in making solutions, rather
+than filtered water?
+
+2. How could you determine the total amount of solid matter dissolved in
+a sample of water?
+
+3. How could you determine whether a given sample of water is distilled
+water?
+
+4. How could the presence of air dissolved in water be detected?
+
+5. How could the amount of water in a food such as bread or potato be
+determined?
+
+6. Would ice frozen from impure water necessarily be free from disease
+germs?
+
+7. Suppose that the maximum density of water were at 0 deg. in place of 4 deg.;
+what effect would this have on the formation of ice on bodies of water?
+
+8. Is it possible for a substance to contain both mechanically inclosed
+water and water of crystallization?
+
+9. If steam is heated to 2000 deg. and again cooled, has any chemical change
+taken place in the steam?
+
+10. Why is cold water passed into C instead of D (Fig. 24)?
+
+11. Mention at least two advantages that a metal condenser has over a
+glass condenser.
+
+12. Draw a diagram of the apparatus used in your laboratory for
+supplying distilled water.
+
+13. 20 cc. of hydrogen and 7 cc. of oxygen are placed in a eudiometer
+and the mixture exploded. (a) How many cubic centimeters of aqueous
+vapor are formed? (b) What gas and how much of it remains in excess?
+
+14. (a) What weight of water can be formed by the combustion of 100 L
+of hydrogen, measured under standard conditions? (b)What volume of
+oxygen would be required in (a)? (c)What weight of potassium
+chlorate is necessary to prepare this amount of oxygen?
+
+15. What weight of oxygen is present in 1 kg. of the ordinary hydrogen
+dioxide solution? In the decomposition of this weight of the dioxide
+into water and oxygen, what volume of oxygen (measured under standard
+conditions) is evolved?
+
+
+
+
+CHAPTER V
+
+THE ATOMIC THEORY
+
+
+~Three fundamental laws of matter.~ Before we can gain any very definite
+idea in regard to the structure of matter, and the way in which
+different kinds of substances act chemically upon each other, it is
+necessary to have clearly in view three fundamental laws of matter.
+These laws have been established by experiment, and any conception which
+may be formed concerning matter must therefore be in harmony with them.
+The laws are as follows:
+
+~Law of conservation of matter.~ This law has already been touched upon in
+the introductory chapter, and needs no further discussion. It will be
+recalled that it may be stated thus: _Matter can neither be created nor
+destroyed, though it can be changed from one form into another._
+
+~Law of definite composition.~ In the earlier days of chemistry there was
+much discussion as to whether the composition of a given compound is
+always precisely the same or whether it is subject to some variation.
+Two Frenchmen, Berthollet and Proust, were the leaders in this
+discussion, and a great deal of most useful experimenting was done to
+decide the question. Their experiments, as well as all succeeding ones,
+have shown that the composition of a pure chemical compound is always
+exactly the same. Water obtained by melting pure ice, condensing steam,
+burning hydrogen in oxygen, has always 11.18% hydrogen and 88.82% oxygen
+in it. Red oxide of mercury, from whatever source it is obtained,
+contains 92.6% mercury and 7.4% oxygen. This truth is known as _the law
+of definite composition_, and may be stated thus: _The composition of a
+chemical compound never varies._
+
+~Law of multiple proportion.~ It has already been noted, however, that
+hydrogen and oxygen combine in two different ratios to form water and
+hydrogen dioxide respectively. It will be observed that this fact does
+not contradict the law of definite composition, for entirely different
+substances are formed. These compounds differ from each other in
+composition, but the composition of each one is always constant. This
+ability of two elements to unite in more than one ratio is very
+frequently observed. Carbon and oxygen combine in two different ratios;
+nitrogen and oxygen combine to form as many as five distinct compounds,
+each with its own precise composition.
+
+In the first decade of the last century John Dalton, an English
+school-teacher and philosopher, endeavored to find some rule which holds
+between the ratios in which two given substances combine. His studies
+brought to light a very simple relation, which the following examples
+will make clear. In water the hydrogen and oxygen are combined in the
+ratio of 1 part by weight of hydrogen to 7.94 parts by weight of oxygen.
+In hydrogen dioxide the 1 part by weight of hydrogen is combined with
+15.88 parts by weight of oxygen. The ratio between the amounts of oxygen
+which combine with the same amount of hydrogen to form water and
+hydrogen dioxide respectively is therefore 7.94: 15.88, or 1: 2.
+
+[Illustration: JOHN DALTON (English) (1766-1844)
+
+Developed the atomic theory; made many studies on the properties and the
+composition of gases. His book entitled "A New System of Chemical
+Philosophy" had a large influence on the development of chemistry]
+
+Similarly, the element iron combines with oxygen to form two oxides, one
+of which is black and the other red. By analysis it has been shown that
+the former contains 1 part by weight of iron combined with 0.286 parts
+by weight of oxygen, while the latter contains 1 part by weight of iron
+combined with 0.429 parts by weight of oxygen. Here again we find that
+the amounts of oxygen which combine with the same fixed amount of iron
+to form the two compounds are in the ratio of small whole numbers, viz.,
+2:3.
+
+Many other examples of this simple relation might be given, since it has
+been found to hold true in all cases where more than one compound is,
+formed from the same elements. Dalton's law of multiple proportion
+states these facts as follows: _When any two elements,_ A _and_ B,
+_combine to form more than one compound, the amounts of_ B _which unite
+with any fixed amount of_ A _bear the ratio of small whole numbers to
+each other_.
+
+~Hypothesis necessary to explain the laws of matter.~ These three
+generalizations are called _laws_, because they express in concise
+language truths which are found by careful experiment to hold good in
+all cases. They do not offer any explanation of the facts, but merely
+state them. The human mind, however, does not rest content with the mere
+bare facts, but seeks ever to learn the explanation of the facts. A
+suggestion which is offered to explain such a set of facts is called an
+_hypothesis_. The suggestion which Dalton offered to explain the three
+laws of matter, called the _atomic hypothesis_, was prompted by his view
+of the constitution of matter, and it involves three distinct
+assumptions in regard to the nature of matter and chemical action.
+Dalton could not prove these assumptions to be true, but he saw that if
+they were true the laws of matter become very easy to understand.
+
+~Dalton's atomic hypothesis.~ The three assumptions which Dalton made in
+regard to the nature of matter, and which together constitute the atomic
+hypothesis, are these:
+
+1. All elements are made up of minute, independent particles which
+Dalton designated as _atoms_.
+
+2. All atoms of the same element have equal masses; those of different
+elements have different masses; in any change to which an atom is
+subjected its mass does not change.
+
+3. When two or more elements unite to form a compound, the action
+consists in the union of a definite small number of atoms of each
+element to form a small particle of the compound. The smallest particles
+of a given compound are therefore exactly alike in the number and kinds
+of atoms which they contain, and larger masses of the substances are
+simply aggregations of these least particles.
+
+~Molecules and atoms.~ Dalton applied the name atom not only to the minute
+particles of the elements but also to the least particles of compounds.
+Later Avogadro, an Italian scientist, pointed out the fact that the two
+are different, since the smallest particle of an element is a unit,
+while that of a compound must have at least two units in it. He
+suggested the name _molecule_ for the least particle of a compound which
+can exist, retaining the name _atom_ for the smallest particle of an
+element. In accordance with this distinction, we may define the atom and
+the molecule as follows: _An atom is the smallest particle of an element
+which can exist. A molecule is the smallest particle of a compound which
+can exist._ It will be shown in a subsequent chapter that sometimes two
+or more atoms of the same element unite with each other to form
+molecules of the element. While the term atom, therefore, is applicable
+only to elements, the term molecule is applicable both to elements and
+compounds.
+
+~The atomic hypothesis and the laws of matter.~ Supposing the atomic
+hypothesis to be true, let us now see if it is in harmony with the laws
+of matter.
+
+1. _The atomic hypothesis and the law of conservation of matter._ It is
+evident that if the atoms never change their masses in any change which
+they undergo, the total quantity of matter can never change and the law
+of conservation of matter must follow.
+
+2. _The atomic hypothesis and the law of definite composition._
+According to the third supposition, when iron combines with sulphur the
+union is between definite numbers of the two kinds of atoms. In the
+simplest case one atom of the one element combines with one atom of the
+other. If the sulphur and the iron atoms never change their respective
+masses when they unite to form a molecule of iron sulphide, all iron
+sulphide molecules will have equal amounts of iron in them and also of
+sulphur. Consequently any mass made up of iron sulphide molecules will
+have the same fraction of iron by weight as do the individual iron
+sulphide molecules. Iron sulphide, from whatever source, will have the
+same composition, which is in accordance with the law of definite
+composition.
+
+3. _The atomic hypothesis and the law of multiple proportion._ But this
+simplest case may not always be the only one. Under other conditions one
+atom of iron might combine with two of sulphur to form a molecule of a
+second compound. In such a case the one atom of iron would be in
+combination with twice the mass of sulphur that is in the first
+compound, since the sulphur atoms all have equal masses. What is true
+for one molecule will be true for any number of them; consequently when
+such quantities of these two compounds are selected as are found to
+contain the same amount of iron, the one will contain twice as much
+sulphur as the other.
+
+The combination between the atoms may of course take place in other
+simple ratios. For example, two atoms of one element might combine with
+three or with five of the other. In all such cases it is clear that the
+law of multiple proportion must hold true. For on selecting such numbers
+of the two kinds of molecules as have the same number of the one kind of
+atoms, the numbers of the other kind of atoms will stand in some simple
+ratio to each other, and their weights will therefore stand in the same
+simple ratio.
+
+~Testing the hypothesis.~ Efforts have been made to find compounds which
+do not conform to these laws, but all such attempts have resulted in
+failure. If such compounds should be found, the laws would be no longer
+true, and the hypothesis of Dalton would cease to possess value. When an
+hypothesis has been tested in every way in which experiment can test it,
+and is still found to be in harmony with the facts in the case, it is
+termed a _theory_. We now speak of the atomic theory rather than of the
+atomic hypothesis.
+
+~Value of a theory.~ The value of a theory is twofold. It aids in the
+clear understanding of the laws of nature because it gives an
+intelligent idea as to why these laws should be in operation.
+
+A theory also leads to discoveries. It usually happens that in testing a
+theory much valuable work is done, and many new facts are discovered.
+Almost any theory in explaining given laws will involve a number of
+consequences apart from the laws it seeks to explain. Experiment will
+soon show whether these facts are as the theory predicts they will be.
+Thus Dalton's atomic theory predicted many properties of gases which
+experiment has since verified.
+
+~Atomic weights.~ It would be of great advantage in the study of chemistry
+if we could determine the weights of the different kinds of atoms. It is
+evident that this cannot be done directly. They are so small that they
+cannot be seen even with a most powerful microscope. It is calculated
+that it would take 200,000,000 hydrogen atoms placed side by side to
+make a row one centimeter long. No balance can weigh such minute
+objects. It is possible, however, to determine their relative
+weights,--that is, how much heavier one is than another. _These relative
+weights of the atoms are spoken of as the atomic weights of the
+elements._
+
+If elements were able to combine in only one way,--one atom of one with
+one atom of another,--the problem of determining the atomic weights
+would be very simple. We should merely have to take some one convenient
+element as a standard, and find by experiment how much of each other
+element would combine with a fixed weight of it. The ratios thus found
+would be the same ratios as those between the atoms of the elements, and
+thus we should have their relative atomic weights. The law of multiple
+proportion calls attention to the fact that the atoms combine in other
+ratios than 1: 1, and there is no direct way of telling which one, if
+any, of the several compounds in a given case is the one consisting of a
+single atom of each element.
+
+If some way were to be found of telling how much heavier the entire
+molecule of a compound is than the atom chosen as a standard,--that is,
+of determining the molecular weights of compounds,--the problem could be
+solved, though its solution would not be an entirely simple matter.
+There are ways of determining the molecular weights of compounds, and
+there are other experiments which throw light directly upon the relative
+weights of the atoms. These methods cannot be described until the facts
+upon which they rest have been studied. It will be sufficient for the
+present to assume that these methods are trustworthy.
+
+~Standard for atomic weights.~ Since the atomic weights are merely
+relative to some one element chosen as a standard, it is evident that
+any one of the elements may serve as this standard and that any
+convenient value may be assigned to its atom. At one time oxygen was
+taken as this standard, with the value 100, and the atomic weights of
+the other elements were expressed in terms of this standard. It would
+seem more rational to take the element of smallest atomic weight as the
+standard and give it unit value; accordingly hydrogen was taken as the
+standard with an atomic weight of 1. Very recently, however, this unit
+has been replaced by oxygen, with an atomic weight of 16.
+
+~Why oxygen is chosen as the standard for atomic weights.~ In the
+determination of the atomic weight of an element it is necessary to find
+the weight of the element which combines with a definite weight of
+another element, preferably the element chosen as the standard. Since
+oxygen combines with the elements far more readily than does hydrogen to
+form definite compounds, it is far better adapted for the standard
+element, and has accordingly replaced hydrogen as the standard. Any
+definite value might be given to the weight of the oxygen atom. In
+assigning a value to it, however, it is convenient to choose a whole
+number, and as small a number as possible without making the atomic
+weight of any other element less than unity. For these reasons the
+number 16 has been chosen as the atomic weight of oxygen. This makes
+the atomic weight of hydrogen equal to 1.008, so that there is but
+little difference between taking oxygen as 16 and hydrogen as 1 for the
+unit.
+
+The atomic weights of the elements are given in the Appendix.
+
+
+EXERCISES
+
+1. Two compounds were found to have the following compositions: (a)
+oxygen = 69.53%, nitrogen = 30.47%; (b) oxygen = 53.27%, nitrogen =
+46.73%. Show that the law of multiple proportion holds in this case.
+
+2. Two compounds were found to have the following compositions: (a)
+oxygen = 43.64%, phosphorus = 56.36%; (b) oxygen = 56.35%, phosphorus
+= 43.65%. Show that the law of multiple proportion holds in this case.
+
+3. Why did Dalton assume that all the atoms of a given element have the
+same weight?
+
+
+
+
+CHAPTER VI
+
+CHEMICAL EQUATIONS AND CALCULATIONS
+
+
+~Formulas.~ Since the molecule of any chemical compound consists of a
+definite number of atoms, and this number never changes without
+destroying the identity of the compound, it is very convenient to
+represent the composition of a compound by indicating the composition of
+its molecules. This can be done very easily by using the symbols of the
+atoms to indicate the number and the kind of the atoms which constitute
+the molecule. HgO will in this way represent mercuric oxide, a molecule
+of which has been found to contain 1 atom each of mercury and oxygen.
+H_{2}O will represent water, the molecules of which consist of 1 atom of
+oxygen and 2 of hydrogen, the subscript figure indicating the number of
+the atoms of the element whose symbol precedes it. H_{2}SO_{4} will
+stand for sulphuric acid, the molecules of which contain 2 atoms of
+hydrogen, 1 of sulphur, and 4 of oxygen. The combination of symbols
+which represents the molecule of a substance is called its _formula_.
+
+~Equations.~ When a given substance undergoes a chemical change it is
+possible to represent this change by the use of such symbols and
+formulas. In a former chapter it was shown that mercuric oxide
+decomposes when heated to form mercury and oxygen. This may be expressed
+very briefly in the form of the equation
+
+    (1) HgO = Hg + O.
+
+When water is electrolyzed two new substances, hydrogen and oxygen, are
+formed from it. This statement in the form of an equation is
+
+    (2) H_{2}O = 2H + O.
+
+The coefficient before the symbol for hydrogen indicates that a single
+molecule of water yields two atoms of hydrogen on decomposition.
+
+In like manner the combination of sulphur with iron is expressed by the
+equation
+
+    (3) Fe + S = FeS.
+
+The decomposition of potassium chlorate by heat takes place as
+represented by the equation
+
+    (4) KClO_{3} = KCl + 3O.
+
+~Reading of equations.~ Since equations are simply a kind of shorthand way
+of indicating chemical changes which occur under certain conditions, in
+reading an equation the full statement for which it stands should be
+given. Equation (1) should be read, "Mercuric oxide when heated gives
+mercury and oxygen"; equation (2) is equivalent to the statement, "When
+electrolyzed, water produces hydrogen and oxygen"; equation (3), "When
+heated together iron and sulphur unite to form iron sulphide"; equation
+(4), "Potassium chlorate when heated yields potassium chloride and
+oxygen."
+
+~Knowledge required for writing equations.~ In order to write such
+equations correctly, a considerable amount of exact knowledge is
+required. Thus, in equation (1) the fact that red oxide of mercury has
+the composition represented by the formula HgO, that it is decomposed by
+heat, that in this decomposition mercury and oxygen are formed and no
+other products,--all these facts must be ascertained by exact experiment
+before the equation can be written. An equation expressing these facts
+will then have much value.
+
+Having obtained an equation describing the conduct of mercuric oxide on
+being heated, it will not do to assume that other oxides will behave in
+like manner. Iron oxide (FeO) resembles mercuric oxide in many respects,
+but it undergoes no change at all when heated. Manganese dioxide, the
+black substance used in the preparation of oxygen, has the formula
+MnO_{2}. When this substance is heated oxygen is set free, but the metal
+manganese is not liberated; instead, a different oxide of manganese
+containing less oxygen is produced. The equation representing the
+reaction is
+
+    3MnO_{2} = Mn_{3}O_{4} + 2O.
+
+~Classes of reactions.~ When a chemical change takes place in a substance
+the substance is said to undergo a reaction. Although a great many
+different reactions will be met in the study of chemistry, they may all
+be grouped under the following heads.
+
+1. _Addition._ This is the simplest kind of chemical action. It consists
+in the union of two or more substances to produce a new substance. The
+combination of iron with sulphur is an example:
+
+    Fe + S = FeS.
+
+2. _Decomposition._ This is the reverse of addition, the substance
+undergoing reaction being parted into its constituents. The
+decomposition of mercuric oxide is an example: HgO = Hg + O.
+
+3. _Substitution._ It is sometimes possible for an element in the free
+state to act upon a compound in such a way that it takes the place of
+one of the elements of the compound, liberating it in turn. In the study
+of the element hydrogen it was pointed out that hydrogen is most
+conveniently prepared by the action of sulphuric or hydrochloric acid
+upon zinc. When sulphuric acid is used a substance called zinc sulphate,
+having the composition represented by the formula ZnSO_{4}, is formed
+together with hydrogen. The equation is
+
+    Zn + H_{2}SO_{4} = ZnSO_{4} + 2H.
+
+When hydrochloric acid is used zinc chloride and hydrogen are the
+products of reaction:
+
+    Zn + 2HCl = ZnCl_{2} + 2H.
+
+When iron is used in place of zinc the equation is
+
+    Fe + H_{2}SO_{4} = FeSO_{4} + 2H.
+
+These reactions are quite similar, as is apparent from an examination of
+the equations. In each case 1 atom of the metal replaces 2 atoms of
+hydrogen in the acid, and the hydrogen escapes as a gas. When an element
+in the free state, such as the zinc in the equations just given, takes
+the place of some one element in a compound, setting it free from
+chemical combination, the act is called _substitution_.
+
+Other reactions illustrating substitution are the action of sodium on
+water,
+
+    Na + H_{2}O = NaOH + H;
+
+and the action of heated iron upon water,
+
+    3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H.
+
+4. _Double decomposition._ When barium dioxide (BaO_{2}) is treated with
+sulphuric acid two compounds are formed, namely, hydrogen dioxide
+(H_{2}O_{2}) and barium sulphate (BaSO_{4}). The equation is
+
+    BaO_{2} + H_{2}SO_{4} = BaSO_{4} + H_{2}O_{2}.
+
+In this reaction it will be seen that the two elements barium and
+hydrogen simply exchange places. Such a reaction is called a _double
+decomposition_. We shall meet with many examples of this kind of
+chemical reactions.
+
+~Chemical equations are quantitative.~ The use of symbols and formulas in
+expressing chemical changes has another great advantage. Thus, according
+to the equation
+
+    H_{2}O = 2H + O,
+
+1 molecule of water is decomposed into 2 atoms of hydrogen and 1 atom of
+oxygen. But, as we have seen, the relative weights of the atoms are
+known, that of hydrogen being 1.008, while that of oxygen is 16. The
+molecule of water, being composed of 2 atoms of hydrogen and 1 atom of
+oxygen, must therefore weigh relatively 2.016 + 16, or 18.016. The
+amount of hydrogen in this molecule must be 2.016/18.016, or 11.18% of
+the whole, while the amount of oxygen must be 16/18.018, or 88.82% of
+the whole. Now, since any definite quantity of water is simply the sum
+of a great many molecules of water, it is plain that the fractions
+representing the relative amounts of hydrogen and oxygen present in a
+molecule must likewise express the relative amounts of hydrogen and
+oxygen present in any quantity of water. Thus, for example, in 20 g. of
+water there are 2.016/18.016 x 20, or 2.238 g. of hydrogen, and
+16/18.016 x 20, or 17.762 g. of oxygen. These results in reference to
+the composition of water of course agree exactly with the facts obtained
+by the experiments described in the chapter on water, for it is because
+of those experiments that the values 1.008 and 16 are given to hydrogen
+and oxygen respectively.
+
+It is often easier to make calculations of this kind in the form of a
+proportion rather than by fractions. Since the molecule of water and
+the two atoms of hydrogen which it contains have the ratio by weight of
+18.016: 2.016, any mass of water has the same ratio between its total
+weight and the weight of the hydrogen in it. Hence, to find the number
+of grams (x) of hydrogen in 20 g. of water, we have the proportion
+
+    18.016 : 2.016 :: 20 g. : x (grams of hydrogen).
+
+Solving for x, we get 2.238 for the number of grams of hydrogen.
+Similarly, to find the amount (x) of oxygen present in the 20 g. of
+water, we have the proportion
+
+    18.016 : 16 :: 20 : x
+
+from which we find that x = 17.762 g.
+
+Again, suppose we wish to find what weight of oxygen can be obtained
+from 15 g. of mercuric oxide. The equation representing the
+decomposition of mercuric oxide is
+
+    HgO = Hg + O.
+
+The relative weights of the mercury and oxygen atoms are respectively
+200 and 16. The relative weight of the mercuric oxide molecule must
+therefore be the sum of these, or 216. The molecule of mercuric oxide
+and the atom of oxygen which it contains have the ratio 216: 16. This
+same ratio must therefore hold between the weight of any given quantity
+of mercuric oxide and that of the oxygen which it contains. Hence, to
+find the weight of oxygen in 15 g. of mercuric oxide, we have the
+proportion
+
+    216 : 16 :: 15 : x (grams of oxygen).
+
+On the other hand, suppose we wish to prepare, say, 20 g. of oxygen. The
+problem is to find out what weight of mercuric oxide will yield 20 g. of
+oxygen. The following proportion evidently holds
+
+    216 : 16 :: x (grams of mercuric oxide) : 20;
+
+from which we get x = 270.
+
+In the preparation of hydrogen by the action of sulphuric acid upon
+zinc, according to the equation,
+
+    Zn + H_{2}SO_{4} = ZnSO_{4} + 2 H,
+
+suppose that 50 g. of zinc are available; let it be required to
+calculate the weight of hydrogen which can be obtained. It will be seen
+that 1 atom of zinc will liberate 2 atoms of hydrogen. The ratio by
+weight of a zinc to an hydrogen atom is 65.4: 1.008; of 1 zinc atom to 2
+hydrogen atoms, 65.4: 2.016. Zinc and hydrogen will be related in this
+reaction in this same ratio, however many atoms of zinc are concerned.
+Consequently in the proportion
+
+    65.4 : 2.016 :: 50 : x,
+
+x will be the weight of hydrogen set free by 50 g. of zinc. The weight
+of zinc sulphate produced at the same time can be found from the
+proportion
+
+    65.4 : 161.46 :: 50 : x;
+
+where 161.46 is the molecular weight of the zinc sulphate, and x the
+weight of zinc sulphate formed. In like manner, the weight of sulphuric
+acid used up can be calculated from the proportion
+
+    65.4 : 98.076 :: 50 : x.
+
+These simple calculations are possible because the symbols and formulas
+in the equations represent the relative weights of the substances
+concerned in a chemical reaction. When once the relative weights of the
+atoms have been determined, and it has been agreed to allow the symbols
+to stand for these relative weights, an equation or formula making use
+of the symbols becomes a statement of a definite numerical fact, and
+calculations can be based on it.
+
+~Chemical equations not algebraic.~ Although chemical equations are
+quantitative, it must be clearly understood that they are not algebraic.
+A glance at the equations
+
+    7 + 4 = 11, 8 + 5 = 9 + 4
+
+will show at once that they are true. The equations
+
+    HgO = Hg + O, FeO = Fe + O
+
+are equally true in an algebraic sense, but experiment shows that only
+the first is true chemically, for iron oxide (FeO) cannot be directly
+decomposed into iron and oxygen. Only such equations as have been found
+by careful experiment to express a real chemical transformation, true
+both for the kinds of substances as well as for the weights, have any
+value.
+
+_Chemical formulas and equations, therefore, are a concise way of
+representing qualitatively and quantitatively facts which have been
+found by experiment to be true in reference to the composition of
+substances and the changes which they undergo._
+
+~Formulas representing water of crystallization.~ An examination of
+substances containing water of crystallization has shown that in every
+case the water is present in such proportion by weight as can readily be
+represented by a formula. For example, copper sulphate (CuSO_{4}) and
+water combine in the ratio of 1 molecule of the sulphate to 5 of water;
+calcium sulphate (CaSO_{4}) and water combine in the ratio 1: 2 to form
+gypsum. These facts are expressed by writing the formulas for the two
+substances with a period between them. Thus the formula for crystallized
+copper sulphate is CuSO_{4}.5H_{2}O; that of gypsum is CaSO_{4}.2H_{2}O.
+
+~Heat of reaction.~ Attention has frequently been directed to the fact
+that chemical changes are usually accompanied by heat changes. In
+general it has been found that in every chemical action heat is either
+absorbed or given off. By adopting a suitable unit for the measurement
+of heat, the heat change during a chemical reaction can be expressed in
+the equation for the reaction.
+
+Heat cannot be measured by the use of a thermometer alone, since the
+thermometer measures the intensity of heat, not its quantity. The
+easiest way to measure a quantity of heat is to note how warm it will
+make a definite amount of a given substance chosen as a standard. Water
+has been chosen as the standard, and the unit of heat is called a
+_calorie. A calorie is defined as the amount of heat required to raise
+the temperature of one gram of water one degree._
+
+By means of this unit it is easy to indicate the heat changes in a given
+chemical reaction. The equation
+
+    2H + O = H_{2}O + 68,300 cal.
+
+means that when 2.016 g. of hydrogen combine with 16 g. of oxygen,
+18.016 g. of water are formed and 68,300 cal. are set free.
+
+    C + 2S = CS_{2} - 19,000 cal.
+
+means that an expenditure of 19,000 cal. is required to cause 12 g. of
+carbon to unite with 64.12 g. of sulphur to form 76.12 g. of carbon
+disulphide. In these equations it will be noted that the symbols stand
+for as many grams of the substance as there are units in the weights of
+the atoms represented by the symbols. This is always understood to be
+the case in equations where the heat of reaction is given.
+
+~Conditions of a chemical action are not indicated by equations.~
+Equations do not tell the conditions under which a reaction will take
+place. The equation
+
+    HgO = Hg + O
+
+does not tell us that it is necessary to keep the mercuric oxide at a
+high temperature in order that the decomposition may go on. The equation
+
+    Zn + 2HCl = ZnCl_{2} + 2H
+
+in no way indicates the fact that the hydrochloric acid must be
+dissolved in water before it will act upon the zinc. From the equation
+
+    H + Cl = HCl
+
+it would not be suspected that the two gases hydrogen and chlorine will
+unite instantly in the sunlight, but will stand mixed in the dark a long
+time without change. It will therefore be necessary to pay much
+attention to the details of the conditions under which a given reaction
+occurs, as well as to the expression of the reaction in the form of an
+equation.
+
+
+EXERCISES
+
+1. Calculate the percentage composition of the following substances:
+(a) mercuric oxide; (b) potassium chlorate; (c) hydrochloric acid;
+(d) sulphuric acid. Compare the results obtained with the compositions
+as given in Chapters II and III.
+
+2. Determine the percentage of copper, sulphur, oxygen, and water in
+copper sulphate crystals. What weight of water can be obtained from 150
+g. of this substance?
+
+3. What weight of zinc can be dissolved in 10 g. of sulphuric acid? How
+much zinc sulphate will be formed?
+
+4. How many liters of hydrogen measured under standard conditions can be
+obtained from the action of 8 g. of iron on 10 g. of sulphuric acid? How
+much iron sulphate (FeSO_{4}) will be formed?
+
+5. 10 g. of zinc were used in the preparation of hydrogen; what weight
+of iron will be required to prepare an equal volume?
+
+6. How many grams of barium dioxide will be required to prepare 1 kg. of
+common hydrogen dioxide solution? What weight of barium sulphate will be
+formed at the same time?
+
+7. What weight of the compound Mn_{3}O_{4} will be formed by strongly
+heating 25 g. of manganese dioxide? What volume of oxygen will be given
+off at the same time, measured under standard conditions?
+
+8. (a) What is the weight of 100 l. of hydrogen measured in a
+laboratory in which the temperature is 20 deg. and pressure 750 mm.? (b)
+What weight of sulphuric acid is necessary to prepare this amount of
+hydrogen? (c) The density of sulphuric acid is 1.84. Express the acid
+required in (b) in cubic centimeters.
+
+9. What weight of potassium chlorate is necessary to furnish sufficient
+oxygen to fill four 200 cc. bottles in your laboratory (the gas to be
+collected over water)?
+
+
+
+
+CHAPTER VII
+
+NITROGEN AND THE RARE ELEMENTS: ARGON, HELIUM, NEON, KRYPTON, XENON
+
+
+~Historical.~ Nitrogen was discovered by the English chemist Rutherford in
+1772. A little later Scheele showed it to be a constituent of air, and
+Lavoisier gave it the name _azote_, signifying that it would not support
+life. The name _nitrogen_ was afterwards given it because of its
+presence in saltpeter or niter. The term azote and symbol Az are still
+retained by the French chemists.
+
+~Occurrence.~ Air is composed principally of oxygen and nitrogen in the
+free state, about 78 parts by volume out of every 100 parts being
+nitrogen. Nitrogen also occurs in nature in the form of potassium
+nitrate (KNO_{3})--commonly called saltpeter or niter--as well as in
+sodium nitrate (NaNO_{3}). Nitrogen is also an essential constituent of
+all living organisms; for example, the human body contains about 2.4% of
+nitrogen.
+
+~Preparation from air.~ Nitrogen can be prepared from air by the action of
+some substance which will combine with the oxygen, leaving the nitrogen
+free. Such a substance must be chosen, however, as will combine with the
+oxygen to form a product which is not a gas, and which can be readily
+separated from the nitrogen. The substances most commonly used for this
+purpose are phosphorus and copper.
+
+1. _By the action of phosphorus._ The method used for the preparation of
+nitrogen by the action of phosphorus is as follows:
+
+The phosphorus is placed in a little porcelain dish, supported on a cork
+and floated on water (Fig. 26). It is then ignited by contact with a hot
+wire, and immediately a bell jar or bottle is brought over it so as to
+confine a portion of the air. The phosphorus combines with the oxygen to
+form an oxide of phosphorus, known as phosphorus pentoxide. This is a
+white solid which floats about in the bell jar, but in a short time it
+is all absorbed by the water, leaving the nitrogen. The withdrawal of
+the oxygen is indicated by the rising of the water in the bell jar.
+
+[Illustration: Fig. 26]
+
+2. _By the action of copper._ The oxygen present in the air may also be
+removed by passing air slowly through a heated tube containing copper.
+The copper combines with the oxygen to form copper oxide, which is a
+solid. The nitrogen passes on and may be collected over water.
+
+     ~Nitrogen obtained from air is not pure.~ Inasmuch as air, in
+     addition to oxygen and nitrogen, contains small amounts of
+     other gases, and since the phosphorus as well as the copper
+     removes only the oxygen, it is evident that the nitrogen
+     obtained by these methods is never quite pure. About 1% of the
+     product is composed of other gases, from which it is very
+     difficult to separate the nitrogen. The impure nitrogen so
+     obtained may, however, be used for a study of most of the
+     properties of nitrogen, since these are not materially affected
+     by the presence of the other gases.
+
+~Preparation from compounds of nitrogen.~ Pure nitrogen may be obtained
+from certain compounds of the element. Thus, if heat is applied to the
+compound ammonium nitrite (NH_{4}NO_{2}), the change represented in the
+following equation takes place:
+
+    NH_{4}NO_{2} = 2H_{2}O + 2N.
+
+~Physical properties.~ Nitrogen is similar to oxygen and hydrogen in that
+it is a colorless, odorless, and tasteless gas. One liter of nitrogen
+weighs 1.2501 g. It is almost insoluble in water. It can be obtained in
+the form of a colorless liquid having a boiling point of -195 deg. at
+ordinary pressure. At -214 deg. it solidifies.
+
+~Chemical properties.~ Nitrogen is characterized by its inertness. It is
+neither combustible nor a supporter of combustion. At ordinary
+temperatures it will not combine directly with any of the elements
+except under rare conditions. At higher temperatures it combines with
+magnesium, lithium, titanium, and a number of other elements. The
+compounds formed are called _nitrides_, just as compounds of an element
+with oxygen are called _oxides_. When it is mixed with oxygen and
+subjected to the action of electric sparks, the two gases slowly combine
+forming oxides of nitrogen. A mixture of nitrogen and hydrogen when
+treated similarly forms ammonia, a gaseous compound of nitrogen and
+hydrogen. Since we are constantly inhaling nitrogen, it is evident that
+it is not poisonous. Nevertheless life would be impossible in an
+atmosphere of pure nitrogen on account of the exclusion of the necessary
+oxygen.
+
+     ~Argon, helium, neon, krypton, xenon.~ These are all rare
+     elements occurring in the air in very small quantities. Argon,
+     discovered in 1894, was the first one obtained. Lord Rayleigh,
+     an English scientist, while engaged in determining the exact
+     weights of various gases, observed that the nitrogen obtained
+     from the air is slightly heavier than pure nitrogen obtained
+     from its compounds. After repeating his experiments many times,
+     always with the same results, Rayleigh finally concluded that
+     the nitrogen which he had obtained from the air was not pure,
+     but was mixed with a small amount of some unknown gas, the
+     density of which is greater than that of nitrogen. Acting on
+     this assumption, Rayleigh, together with the English chemist
+     Ramsay, attempted to separate the nitrogen from the unknown
+     gas. Knowing that nitrogen would combine with magnesium, they
+     passed the nitrogen obtained from the air and freed from all
+     known substances through tubes containing magnesium heated to
+     the necessary temperature. After repeating this operation, they
+     finally succeeded in obtaining from the atmospheric nitrogen a
+     small volume of gas which would not combine with magnesium and
+     hence could not be nitrogen. This proved to be a new element,
+     to which they gave the name _argon_. As predicted, this new
+     element was found to be heavier than nitrogen, its density as
+     compared with hydrogen as a standard being approximately 20,
+     that of nitrogen being only 14. About 1% of the atmospheric
+     nitrogen proved to be argon. The new element is characterized
+     by having no affinity for other elements. Even under the most
+     favorable conditions it has not been made to combine with any
+     other element. On this account it was given the name argon,
+     signifying lazy or idle. Like nitrogen, it is colorless,
+     odorless, and tasteless. It has been liquefied and solidified.
+     Its boiling point is -187 deg..
+
+     Helium was first found in the gases expelled from certain
+     minerals by heating. Through the agency of the spectroscope it
+     had been known to exist in the sun long before its presence on
+     the earth had been demonstrated,--a fact suggested by the name
+     helium, signifying the sun. Its existence in traces in the
+     atmosphere has also been proven. It was first liquefied by
+     Onnes in July, 1908. Its boiling point, namely -269 deg., is the
+     lowest temperature yet reached.
+
+     The remaining elements of this group--neon, krypton, and
+     xenon--have been obtained from liquid air. When liquid air is
+     allowed to boil, the constituents which are the most difficult
+     to liquefy, and which therefore have the lowest boiling points,
+     vaporize first, followed by the others in the order of their
+     boiling points. It is possible in this way to make at least a
+     partial separation of the air into its constituents, and Ramsay
+     thus succeeded in obtaining from liquid air not only the known
+     constituents, including argon and helium, but also the new
+     elements, neon, krypton, and xenon. These elements, as well as
+     helium, all proved to be similar to argon in that they are
+     without chemical activity, apparently forming no compounds
+     whatever. The percentages present in the air are very small.
+     The names, neon, krypton, xenon, signify respectively, new,
+     hidden, stranger.
+
+
+EXERCISES
+
+1. How could you distinguish between oxygen, hydrogen, and nitrogen?
+
+2. Calculate the relative weights of nitrogen and oxygen; of nitrogen
+and hydrogen.
+
+3. In the preparation of nitrogen from the air, how would hydrogen do as
+a substance for the removal of the oxygen?
+
+4. What weight of nitrogen can be obtained from 10 l. of air measured
+under the conditions of temperature and pressure which prevail in your
+laboratory?
+
+5. How many grams of ammonium nitrite are necessary in the preparation
+of 20 l. of nitrogen measured over water under the conditions of
+temperature and pressure which prevail in your laboratory?
+
+6. If 10 l. of air, measured under standard conditions, is passed over
+100 g. of hot copper, how much will the copper gain in weight?
+
+[Illustration: WILLIAM RAMSAY (Scotch) (1855-)
+
+Has made many studies in the physical properties of substances;
+discovered helium; together with Lord Rayleigh and others he discovered
+argon, krypton, xenon, and neon; has contributed largely to the
+knowledge of radio-active substances, showing that radium gradually
+gives rise to helium; professor at University College, London]
+
+
+
+
+CHAPTER VIII
+
+THE ATMOSPHERE
+
+
+~Atmosphere and air.~ The term _atmosphere_ is applied to the gaseous
+envelope surrounding the earth. The term _air_ is generally applied to a
+limited portion of this envelope, although the two words are often used
+interchangeably. Many references have already been made to the
+composition and properties of the atmosphere. These statements must now
+be collected and discussed somewhat more in detail.
+
+~Air formerly regarded as an element.~ Like water, air was at first
+regarded as elementary in character. Near the close of the eighteenth
+century Scheele, Priestley, and Lavoisier showed by their experiments
+that it is a mixture of at least two gases,--those which we now call
+oxygen and nitrogen. By burning substances in an inclosed volume of air
+and noting the contraction in volume due to the removal of the oxygen,
+they were able to determine with some accuracy the relative volumes of
+oxygen and nitrogen present in the air.
+
+~The constituents of the atmosphere.~ The constituents of the atmosphere
+may be divided into two general groups: those which are essential to
+life and those which are not essential.
+
+1. _Constituents essential to life._ In addition to oxygen and nitrogen
+at least two other substances, namely, carbon dioxide and water vapor,
+must be present in the atmosphere in order that life may exist. The
+former of these is a gaseous compound of carbon and oxygen having the
+formula CO_{2}. Its properties will be discussed in detail in the
+chapter on the compounds of carbon. Its presence in the air may be shown
+by causing the air to bubble through a solution of calcium hydroxide
+(Ca(OH)_{2}), commonly called lime water. The carbon dioxide combines
+with the calcium hydroxide in accordance with the following equation:
+
+    Ca(OH)_{2} + CO_{2} = CaCO_{3} + H_{2}O.
+
+The resulting calcium carbonate (CaCO_{3}) is insoluble in water and
+separates in the form of a white powder, which causes the solution to
+appear milky.
+
+The presence of water vapor is readily shown by its condensation on cold
+objects as well as by the fact that a bit of calcium chloride when
+exposed to the air becomes moist, and may even dissolve in the water
+absorbed from the air.
+
+2. _Constituents not essential to life._ In addition to the essential
+constituents, the air contains small percentages of various other gases,
+the presence of which so far as is known is not essential to life. This
+list includes the rare elements, argon, helium, neon, krypton, and
+xenon; also hydrogen, ammonia, hydrogen dioxide, and probably ozone.
+Certain minute forms of life (germs) are also present, the decay of
+organic matter being due to their presence.
+
+     ~Function of each of the essential constituents.~ (1) The oxygen
+     directly supports life through respiration. (2) The nitrogen,
+     on account of its inactivity, serves to dilute the oxygen, and
+     while contrary to the older views, it is possible that life
+     might continue to exist in the absence of the atmospheric
+     nitrogen, yet the conditions of life would be entirely changed.
+     Moreover, nitrogen is an essential constituent of all animal
+     and plant life. It was formerly supposed that neither animals
+     nor plants could assimilate the free nitrogen, but it has been
+     shown recently that the plants of at least one natural order,
+     the Leguminosae, to which belong the beans, peas, and clover,
+     have the power of directly assimilating the free nitrogen from
+     the atmosphere. This is accomplished through the agency of
+     groups of bacteria, which form colonies in little tubercles on
+     the roots of the plants. These bacteria probably assist in the
+     absorption of nitrogen by changing the free nitrogen into
+     compounds which can be assimilated by the plant. Fig. 27 shows
+     the tubercles on the roots of a variety of bean. (3) The
+     presence of water vapor in the air is necessary to prevent
+     excessive evaporation from both plants and animals. (4) Carbon
+     dioxide is an essential plant food.
+
+[Illustration: Fig. 27]
+
+~The quantitative analysis of air.~ A number of different methods have
+been devised for the determination of the percentages of the
+constituents present in the atmosphere. Among these are the following.
+
+1. _Determination of oxygen._ (1) The oxygen is withdrawn from a
+measured volume of air inclosed in a tube, by means of phosphorus.
+
+     To make the determination, a graduated tube is filled with
+     water and inverted in a vessel of water. Air is introduced into
+     the tube until it is partially filled with the gas. The volume
+     of the inclosed air is carefully noted and reduced to standard
+     conditions. A small piece of phosphorus is attached to a wire
+     and brought within the tube as shown in Fig. 28. After a few
+     hours the oxygen in the inclosed air will have combined with
+     the phosphorus, the water rising to take its place. The
+     phosphorus is removed and the volume is again noted and reduced
+     to standard conditions. The contraction in the volume of the
+     air is equal to the volume of oxygen absorbed.
+
+[Illustration: Fig. 28]
+
+(2) The oxygen may also be estimated by passing a measured volume of air
+through a tube containing copper heated to a high temperature. The
+oxygen in the air combines with the copper to form copper oxide (CuO).
+Hence the increase in the weight of the copper equals the weight of the
+oxygen in the volume of air taken.
+
+(3) A more accurate method is the following. A eudiometer tube is filled
+with mercury and inverted in a vessel of the same liquid. A convenient
+amount of air is then introduced into the tube and its volume accurately
+noted. There is then introduced more than sufficient hydrogen to combine
+with the oxygen present in the inclosed air, and the volume is again
+accurately noted. The mixture is then exploded by an electric spark, and
+the volume is once more taken. By subtracting this volume from the total
+volume of the air and hydrogen there is obtained the contraction in
+volume due to the union of the oxygen and hydrogen. The volume occupied
+by the water formed by the union of the two gases is so small that it
+may be disregarded in the calculation. Since oxygen and hydrogen combine
+in the ratio 1: 2 by volume, it is evident that the contraction in
+volume due to the combination is equal to the volume occupied by the
+oxygen in the air contained in the tube, plus twice this volume of
+hydrogen. In other words, one third of the total contraction is equal to
+the volume occupied by the oxygen in the inclosed air. The following
+example will make this clear:
+
+Volume of air in tube                                        50.0 cc.
+Volume after introducing hydrogen                            80.0
+Volume after combination of oxygen and hydrogen              48.5
+Contraction in volume due to combination (80 cc.-48.5 cc.)   31.5
+Volume of oxygen in 50 cc. of air (1/3 of 31.5)              10.5
+
+All these methods agree in showing that 100 volumes of dry air contain
+approximately 21 volumes of oxygen.
+
+2. _Determination of nitrogen._ If the gas left after the removal of
+oxygen from a portion of air is passed over heated magnesium, the
+nitrogen is withdrawn, argon and the other rare elements being left. It
+may thus be shown that of the 79 volumes of gas left after the removal
+of the oxygen from 100 volumes of air, approximately 78 are nitrogen and
+0.93 argon. The other elements are present in such small quantities that
+they may be neglected.
+
+3. _Determination of carbon dioxide._ The percentage of carbon dioxide
+in any given volume of air may be determined by passing the air over
+calcium hydroxide or some other compound which will combine with the
+carbon dioxide. The increase in the weight of the hydroxide equals the
+weight of the carbon dioxide absorbed. The amount present in the open
+normal air is from 3 to 4 parts by volume in 10,000 volumes of air, or
+about 0.04%.
+
+4. _Determination of water vapor._ The water vapor present in a given
+volume of air may be determined by passing the air over calcium chloride
+(or some other compound which has a strong affinity for water), and
+noting the increase in the weight of the chloride. The amount present
+varies not only with the locality, but there is a wide variation from
+day to day in the same locality because of the winds and changes in
+temperature.
+
+~Processes affecting the composition of the air.~ The most important of
+these processes are the following.
+
+1. _Respiration._ In the process of respiration some of the oxygen in
+the inhaled air is absorbed by the blood and carried to all parts of the
+body, where it combines with the carbon of the worn-out tissues. The
+products of oxidation are carried back to the lungs and exhaled in the
+form of carbon dioxide. The amount exhaled by an adult averages about 20
+l. per hour. Hence in a poorly ventilated room occupied by a number of
+people the amount of carbon dioxide rapidly increases. While this gas is
+not poisonous unless present in large amounts, nevertheless air
+containing more than 15 parts in 10,000 is not fit for respiration.
+
+2. _Combustion._ All of the ordinary forms of fuel contain large
+percentages of carbon. On burning, this carbon combines with oxygen in
+the air, forming carbon dioxide. Combustion and respiration, therefore,
+tend to diminish the amount of oxygen in the air and to increase the
+amount of carbon dioxide.
+
+3. _Action of plants._ Plants have the power, when in the sunlight, of
+absorbing carbon dioxide from the air, retaining the carbon and
+returning at least a portion of the oxygen to the air. It will be
+observed that these changes are just the opposite of those brought about
+by the processes of respiration and combustion.
+
+     ~Poisonous effect of exhaled air.~ The differences in the
+     percentages of oxygen, carbon dioxide, and moisture present in
+     inhaled air and exhaled air are shown in the following
+     analyses.
+
+                       INHALED AIR     EXHALED AIR
+       Oxygen            21.00%          16.00%
+       Carbon dioxide     0.04            4.38
+       Moisture         variable        saturated
+
+     The foul odor of respired air is due to the presence of a
+     certain amount of organic matter. It is possible that this
+     organic matter rather than the carbon dioxide is responsible
+     for the injurious effects which follow the respiration of
+     impure air. The extent of such organic impurities present may
+     be judged, however, by the amount of carbon dioxide present,
+     since the two are exhaled together.
+
+     ~The cycle of carbon in nature.~ Under the influence of sunlight,
+     the carbon dioxide absorbed from the air by plants reacts with
+     water and small amounts of other substances absorbed from the
+     soil to form complex compounds of carbon which constitute the
+     essential part of the plant tissue. This reaction is attended
+     by the evolution of oxygen, which is restored to the air. The
+     compounds resulting from these changes are much richer in their
+     energy content than are the substances from which they are
+     formed; hence a certain amount of energy must have been
+     absorbed in their formation. The source of this energy is the
+     sun's rays.
+
+     If the plant is burned, the changes which took place in the
+     formation of the compounds present are largely reversed. The
+     carbon and hydrogen present combine with oxygen taken from the
+     air to form carbon dioxide and water, while the energy absorbed
+     from the sun's rays is liberated in the form of energy of heat.
+     If, on the other hand, the plant is used as food, the compounds
+     present are used in building up the tissues of the body. When
+     this tissue breaks down, the changes which it undergoes are
+     very similar to those which take place when the plant is
+     burned. The carbon and hydrogen combine with the inhaled oxygen
+     to form carbon dioxide and water, which are exhaled. The energy
+     possessed by the complex substances is liberated partly in the
+     form of energy of heat, which maintains the heat of the body,
+     and partly in the various forms of muscular energy. The carbon
+     originally absorbed from the air by the plant in the form of
+     carbon dioxide is thus restored to the air and is ready to
+     repeat the cycle of changes.
+
+~The composition of the air is constant.~ Notwithstanding the changes
+constantly taking place which tend to alter the composition of the air,
+the results of a great many analyses of air collected in the open fields
+show that the percentages of oxygen and nitrogen as well as of carbon
+dioxide are very nearly constant. Indeed, so constant are the
+percentages of oxygen and nitrogen that the question has arisen, whether
+these two elements are not combined in the air, forming a definite
+chemical compound. That the two are not combined but are simply mixed
+together can be shown in a number of ways, among which are the
+following.
+
+1. When air dissolves in water it has been found that the ratio of
+oxygen to nitrogen in the dissolved air is no longer 21: 78, but more
+nearly 35: 65. If it were a chemical compound, the ratio of oxygen to
+nitrogen would not be changed by solution in water.
+
+2. A chemical compound in the form of a liquid has a definite boiling
+point. Water, for example, boils at 100 deg.. Moreover the steam which is
+thus formed has the same composition as the water. The boiling point of
+liquid air, on the other hand, gradually rises as the liquid boils, the
+nitrogen escaping first followed by the oxygen. If the two were
+combined, they would pass off together in the ratio in which they are
+found in the air.
+
+~Why the air has a constant composition.~ If air is a mixture and changes
+are constantly taking place which tend to modify its composition, how,
+then, do we account for the constancy of composition which the analyses
+reveal? This is explained by several facts. (1) The changes which are
+caused by the processes of combustion and respiration, on the one hand,
+and the action of plants, on the other, tend to equalize each other. (2)
+The winds keep the air in constant motion and so prevent local changes.
+(3) The volume of the air is so vast and the changes which occur are so
+small compared with the total amount of air that they cannot be readily
+detected. (4) Finally it must be noted that only air collected in the
+open fields shows this constancy in composition. The air in a poorly
+ventilated room occupied by a number of people rapidly changes in
+composition.
+
+~The properties of the air.~ Inasmuch as air is composed principally of a
+mixture of oxygen and nitrogen, which elements have already been
+discussed, its properties may be inferred largely from those of the two
+gases. One liter weighs 1.2923 g. It is thus 14.38 times as heavy as
+hydrogen. At the sea level it exerts an average pressure sufficient to
+sustain a column of mercury 760 mm. in height. This is taken as the
+standard pressure in determining the volumes of gases as well as the
+boiling points of liquids. Water may be made to boil at any temperature
+between 0 deg. and considerably above 100 deg. by simply varying the pressure.
+It is only when the pressure upon it is equal to the normal pressure of
+the atmosphere at the sea level, as indicated by a barometric reading of
+760 mm., that it boils at 100 deg..
+
+~Preparation of liquid air.~ Attention has been called to the fact that
+both oxygen and nitrogen can be obtained in the liquid state by strongly
+cooling the gases and applying great pressure to them. Since air is
+largely a mixture of these two gases, it can be liquefied by the same
+methods.
+
+     The methods for liquefying air have been simplified greatly in
+     that the low temperature required is obtained by allowing a
+     portion of the compressed air to expand. The expansion of a gas
+     is always attended by the absorption of heat. In liquefying air
+     the apparatus is so constructed that the heat absorbed is
+     withdrawn from air already under great pressure. This process
+     is continued until the temperature is lowered to the point of
+     liquefaction.
+
+[Illustration: Fig. 29]
+
+~The Dewar bulb.~ It is not possible to preserve air in the liquid state
+in a closed vessel, on account of the enormous pressure exerted by it in
+its tendency to pass into the gaseous state. It may however be preserved
+for some hours or even days before it will completely evaporate, by
+simply placing it in an open vessel surrounded by a nonconducting
+material. The most efficient vessel for this purpose is the _Dewar bulb_
+shown in Fig. 29. The air is withdrawn from the space between the two
+walls, thus making it nonconducting.
+
+~Properties and uses of liquid air.~ When first prepared, liquid air is
+cloudy because of the presence of particles of solid carbon dioxide.
+These may be filtered off, leaving a liquid of slightly bluish color. It
+begins to boil at about -190 deg., the nitrogen passing off first, gradually
+followed by the oxygen, the last portions being nearly pure oxygen. To a
+certain extent oxygen is now prepared in this way for commercial
+purposes.
+
+The extremely low temperature of liquid air may be inferred from the
+fact that mercury when cooled by it is frozen to a mass so hard that it
+may be used for driving nails.
+
+Liquid air is used in the preparation of oxygen and as a cooling agent
+in the study of the properties of matter at low temperatures. It has
+thus been found that elements at extremely low temperatures largely lose
+their chemical activity.
+
+
+EXERCISES
+
+1. When oxygen and nitrogen are mixed in the proportion in which they
+exist in the atmosphere, heat is neither evolved nor absorbed by the
+process. What important point does this suggest?
+
+2. What essential constituent of the air is found in larger amount in
+manufacturing districts than in the open country?
+
+3. Can you suggest any reason why the growth of clover in a field
+improves the soil?
+
+4. Why are the inner walls of a Dewar bulb sometimes coated with a film
+of silver?
+
+5. To what is the blue color of liquid air due? Does this color increase
+in intensity on standing?
+
+6. When ice is placed in a vessel containing liquid air, the latter
+boils violently. Explain.
+
+7. Taking the volumes of the oxygen and nitrogen in 100 volumes of air
+as 21 and 78 respectively, calculate the percentages of these elements
+present by weight.
+
+8. Would combustion be more intense in liquid air than in the gaseous
+substance?
+
+9. A tube containing calcium chloride was found to weigh 30.1293 g. A
+volume of air which weighed 15.2134 g. was passed through, after which
+the weight of the tube was found to be 30.3405 g. What was the
+percentage amount of moisture present in the air?
+
+10. 10 l. of air measured at 20 deg. and 740 mm. passed through lime water
+caused the precipitation of 0.0102 g. of CaCO_{3}. Find the number of
+volumes of carbon dioxide in 10,000 volumes of the air.
+
+
+
+
+CHAPTER IX
+
+SOLUTIONS
+
+
+~Definitions.~ When a substance disappears in a liquid in such a way as to
+thoroughly mix with it and to be lost to sight as an individual body,
+the resulting liquid is called a _solution_. The liquid in which the
+substance dissolves is called the _solvent_, while the dissolved
+substance is called the _solute_.
+
+~Classes of solutions.~ Matter in any one of its physical states may
+dissolve in a liquid, so that we may have solutions of gases, of
+liquids, and of solids. Solutions of liquids in liquids are not often
+mentioned in the following pages, but the other two classes will become
+very familiar in the course of our study, and deserve special attention.
+
+
+SOLUTION OF GASES IN LIQUIDS
+
+[Illustration: Fig. 30]
+
+It has already been stated that oxygen, hydrogen, and nitrogen are
+slightly soluble in water. Accurate study has led to the conclusion that
+all gases are soluble to some extent not only in water but in many other
+liquids. The amount of a gas which will dissolve in a liquid depends
+upon a number of conditions, and these can best be understood by
+supposing a vessel B (Fig. 30), to be filled with the gas and inverted
+over the liquid. Under these circumstances the gas cannot escape or
+become mixed with another gas.
+
+~Circumstances affecting the solubility of gases.~ A number of
+circumstances affect the solubility of a gas in a liquid.
+
+1. _Nature of the gas._ Other conditions being equal, each gas has its
+own peculiar solubility, just as it has its own special taste or odor.
+The solubility of gases varies between wide limits, as will be seen from
+the following table, but as a rule a given volume of a liquid will not
+dissolve more than two or three times its own volume of a gas.
+
+
+_Solubility of Gases in Water_
+
+1 l. of water at 760 mm. pressure and at 0 deg. will dissolve:
+
+    Ammonia                 1148.00 l.
+    Hydrochloric acid        503.00
+    Sulphur dioxide           79.79
+    Carbon dioxide             1.80
+    Oxygen                    41.14 cc.
+    Hydrogen                  21.15
+    Nitrogen                  20.03
+
+In the case of very soluble gases, such as the first three in the table,
+it is probable that chemical combination between the liquid and the gas
+takes place.
+
+2. _Nature of the liquid._ The character of the liquid has much
+influence upon the solubility of a gas. Water, alcohol, and ether have
+each its own peculiar solvent power. From the solubility of a gas in
+water, no prediction can be made as to its solubility in other liquids.
+
+3. _Influence of pressure._ It has been found that the weight of gas
+which dissolves in a given case is proportional to the pressure exerted
+upon the gas. If the pressure is doubled, the weight of gas going into
+solution is doubled; if the pressure is diminished to one half of its
+original value, half of the dissolved gas will escape. Under high
+pressure, large quantities of gas can be dissolved in a liquid, and when
+the pressure is removed the gas escapes, causing the liquid to foam or
+_effervesce_.
+
+4. _Influence of temperature._ In general, the lower the temperature of
+the liquid, the larger the quantity of gas which it can dissolve. 1000
+volumes of water at 0 deg. will dissolve 41.14 volumes of oxygen; at 50 deg.,
+18.37 volumes; at 100 deg. none at all. While most gases can be expelled
+from a liquid by boiling the solution, some cannot. For example, it is
+not possible to expel hydrochloric acid gas completely from its solution
+by boiling.
+
+
+SOLUTION OF SOLIDS IN LIQUIDS
+
+This is the most familiar class of solutions, since in the laboratory
+substances are much more frequently used in the form of solutions than
+in the solid state.
+
+~Circumstances affecting the solubility of a solid.~ The solubility of a
+solid in a liquid depends upon several factors.
+
+1. _Nature of the solid._ Other conditions being the same, solids vary
+greatly in their solubility in liquids. This is illustrated in the
+following table:
+
+
+_Table of Solubility of Solids at 18 deg._
+
+100 cc. of water will dissolve:
+
+    Calcium chloride        71.0 g.
+    Sodium chloride         35.9
+    Potassium nitrate       29.1
+    Copper sulphate         21.4
+    Calcium sulphate         0.207
+
+No solids are absolutely insoluble, but the amount dissolved may be so
+small as to be of no significance for most purposes. Thus barium
+sulphate, one of the most insoluble of common substances, dissolves in
+water to the extent of 1 part in 400,000.
+
+2. _Nature of the solvent._ Liquids vary much in their power to dissolve
+solids. Some are said to be good solvents, since they dissolve a great
+variety of substances and considerable quantities of them. Others have
+small solvent power, dissolving few substances, and those to a slight
+extent only. Broadly speaking, water is the most general solvent, and
+alcohol is perhaps second in solvent power.
+
+3. _Temperature._ The weight of a solid which a given liquid can
+dissolve varies with the temperature. Usually it increases rapidly as
+the temperature rises, so that the boiling liquid dissolves several
+times the weight which the cold liquid will dissolve. In some instances,
+as in the case of common salt dissolved in water, the temperature has
+little influence upon the solubility, and a few solids are more soluble
+in cold water than in hot. The following examples will serve as
+illustrations:
+
+
+_Table of Solubility at 0 deg. and at 100 deg._
+
+100 cc. of water will dissolve:
+
+                                At 0 deg.           At 100 deg.
+
+    Calcium chloride            49.6 g.        155.0 g.
+    Sodium chloride             35.7            39.8
+    Potassium nitrate           13.3           247.0
+    Copper sulphate             15.5            73.5
+    Calcium sulphate             0.205           0.217
+    Calcium hydroxide            0.173           0.079
+
+~Saturated solutions.~ A liquid will not dissolve an unlimited quantity of
+a solid. On adding the solid to the liquid in small portions at a time,
+it will be found that a point is reached at which the liquid will not
+dissolve more of the solid at that temperature. The solid and the
+solution remain in contact with each other unchanged. This condition may
+be described by saying that they are in equilibrium with each other. A
+solution is said to be _saturated_ when it remains unchanged in
+concentration in contact with some of the solid. The weight of the solid
+which will completely saturate a definite volume of a liquid at a given
+temperature is called the _solubility_ of the substance at that
+temperature.
+
+~Supersaturated solutions.~ When a solution, saturated at a given
+temperature, is allowed to cool it sometimes happens that no solid
+crystallizes out. This is very likely to occur when the vessel used is
+perfectly smooth and the solution is not disturbed in any way. Such a
+solution is said to be _supersaturated_. That this condition is unstable
+can be shown by adding a crystal of the solid to the solution. All of
+the solid in excess of the quantity required to saturate the solution at
+this temperature will at once crystallize out, leaving the solution
+saturated. Supersaturation may also be overcome in many cases by
+vigorously shaking or stirring the solution.
+
+~General physical properties of solutions.~ A few general statements may
+be made in reference to the physical properties of solutions.
+
+1. _Distribution of the solid in the liquid._ A solid, when dissolved,
+tends to distribute itself uniformly through the liquid, so that every
+part of the solution has the same concentration. The process goes on
+very slowly unless hastened by stirring or shaking the solution. Thus,
+if a few crystals of a highly colored substance such as copper sulphate
+are placed in the bottom of a tall vessel full of water, it will take
+weeks for the solution to become uniformly colored.
+
+2. _Boiling points of solutions._ The boiling point of a liquid is
+raised by the presence of a substance dissolved in it. In general the
+extent to which the boiling point of a solvent is raised by a given
+substance is proportional to the concentration of the solution, that
+is, to the weight of the substance dissolved in a definite weight of the
+solvent.
+
+3. _Freezing points of solutions._ A solution freezes at a lower
+temperature than the pure solvent. The lowering of the freezing point
+obeys the same law which holds for the raising of the boiling point: the
+extent of lowering is proportional to the weight of dissolved substance,
+that is, to the concentration of the solution.
+
+~Electrolysis of solutions.~ Pure water does not appreciably conduct the
+electric current. If, however, certain substances such as common salt
+are dissolved in the water, the resulting solutions are found to be
+conductors of electricity. Such solutions are called _electrolytes_.
+When the current passes through an electrolyte some chemical change
+always takes place. This change is called _electrolysis_.
+
+[Illustration: Fig. 31]
+
+The general method used in the electrolysis of a solution is illustrated
+in Fig. 31. The vessel D contains the electrolyte. Two plates or rods,
+A and B, made of suitable material, are connected with the wires
+from a battery (or dynamo) and dipped into the electrolyte, as shown in
+the figure. These plates or rods are called _electrodes_. The electrode
+connected with the zinc plate of the battery is the negative electrode
+or _cathode_, while that connected with the carbon plate is the positive
+electrode or _anode_.
+
+~Theory of electrolytic dissociation.~ The facts which have just been
+described in connection with solutions, together with many others, have
+led chemists to adopt a theory of solutions called _the theory of
+electrolytic dissociation_. The main assumptions in this theory are the
+following.
+
+1. _Formation of ions._ Many compounds when dissolved in water undergo
+an important change. A portion of their molecules fall apart, or
+_dissociate_, into two or more parts, called _ions_. Thus sodium nitrate
+(NaNO_{3}) dissociates into the ions Na and NO_{3}; sodium chloride,
+into the ions Na and Cl. These ions are free to move about in the
+solution independently of each other like independent molecules, and for
+this reason were given the name ion, which signifies a wanderer.
+
+2. _The electrical charge of ions._ Each ion carries a heavy electrical
+charge, and in this respect differs from an atom or molecule. It is
+evident that the sodium in the form of an ion must differ in some
+important way from ordinary sodium, for sodium ions, formed from sodium
+nitrate, give no visible evidence of their presence in water, whereas
+metallic sodium at once decomposes the water. The electrical charge,
+therefore, greatly modifies the usual chemical properties of the
+element.
+
+3. _The positive charges equal the negative charges._ The ions formed by
+the dissociation of any molecule are of two kinds. One kind is charged
+with positive electricity and the other with negative electricity;
+moreover the sum of all the positive charges is always equal to the sum
+of all the negative charges. The solution as a whole is therefore
+electrically neutral. If we represent dissociation by the usual chemical
+equations, with the electrical charges indicated by + and - signs
+following the symbols, the dissociation of sodium chloride molecules is
+represented thus:
+
+    NaCl --> Na^{+}, Cl^{-}.
+
+The positive charge on each sodium ion exactly equals the negative
+charge on each chlorine ion. Sodium sulphate dissociates, as shown in
+the equation
+
+    Na_{2}SO_{4} --> 2Na^{+}, SO_{4}^{--}.
+
+Here the positive charge on the two sodium ions equals the double
+negative charge on the SO_{4} ion.
+
+4. _Not all compounds dissociate._ Only those compounds dissociate whose
+solutions form electrolytes. Thus salt dissociates when dissolved in
+water, the resulting solution being an electrolyte. Sugar, on the other
+hand, does not dissociate and its solution is not a conductor of the
+electric current.
+
+5. _Extent of dissociation differs in different liquids._ While
+compounds most readily undergo dissociation in water, yet dissociation
+often occurs to a limited extent when solution takes place in liquids
+other than water. In the discussion of solutions it will be understood
+that the solvent is water unless otherwise noted.
+
+~The theory of electrolytic dissociation and the properties of solutions.~
+In order to be of value, this theory must give a reasonable explanation
+of the properties of solutions. Let us now see if the theory is in
+harmony with certain of these properties.
+
+~The theory of electrolytic dissociation and the boiling and freezing
+points of solutions.~ We have seen that the boiling point of a solution
+of a substance is raised in proportion to the concentration of the
+dissolved substance. This is but another way of saying that the change
+in the boiling point of the solution is proportional to the number of
+molecules of the dissolved substance present in the solution.
+
+It has been found, however, that in the case of electrolytes the boiling
+point is raised more than it should be to conform to this law. If the
+solute dissociates into ions, the reason for this becomes clear. Each
+ion has the same effect on the boiling point as a molecule, and since
+their number is greater than the number of molecules from which they
+were formed, the effect on the boiling point is abnormally great.
+
+In a similar way, the theory furnishes an explanation of the abnormal
+lowering of the freezing point of electrolytes.
+
+~The theory of electrolytic dissociation and electrolysis.~ The changes
+taking place during electrolysis harmonize very completely with the
+theory of dissociation. This will become clear from a study of the
+following examples.
+
+[Illustration: Fig. 32]
+
+1. _Electrolysis of sodium chloride._ Fig. 32 represents a vessel in
+which the electrolyte is a solution of sodium chloride (NaCl). According
+to the dissociation theory the molecules of sodium chloride dissociate
+into the ions Na^{+} and Cl^{-}. The Na^{+} ions are attracted to the
+cathode owing to its large negative charge. On coming into contact with
+the cathode, the Na^{+} ions give up their positive charge and are then
+ordinary sodium atoms. They immediately decompose the water according to
+the equation
+
+    Na + H_{2}O = NaOH + H,
+
+and hydrogen is evolved about the cathode.
+
+The chlorine ions on being discharged at the anode in similar manner may
+either be given off as chlorine gas, or may attack the water, as
+represented in the equation
+
+    2Cl + H_{2}O = 2HCl + O.
+
+2. _Electrolysis of water._ The reason for the addition of sulphuric
+acid to water in the preparation of oxygen and hydrogen by electrolysis
+will now be clear. Water itself is not an electrolyte to an appreciable
+extent; that is, it does not form enough ions to carry a current.
+Sulphuric acid dissolved in water is an electrolyte, and dissociates
+into the ions 2 H^{+} and SO_{4}^{--}. In the process of electrolysis of
+the solution, the hydrogen ions travel to the cathode, and on being
+discharged escape as hydrogen gas. The SO_{4} ions, when discharged at
+the anode, act upon water, setting free oxygen and once more forming
+sulphuric acid:
+
+    SO_{4} + H_{2}O = H_{2}SO_{4} + O.
+
+The sulphuric acid can again dissociate and the process repeat itself as
+long as any water is left. Hence the hydrogen and oxygen set free in the
+electrolysis of water really come directly from the acid but indirectly
+from the water.
+
+3. _Electrolysis of sodium sulphate._ In a similar way, sodium sulphate
+(Na_{2}SO_{4}), when in solution, gives the ions 2 Na^{+} and
+SO_{4}^{--}. On being discharged, the sodium atoms decompose water about
+the cathode, as in the case of sodium chloride, while the SO_{4} ions
+when discharged at the anode decompose the water, as represented in the
+equation
+
+    SO_{4} + H_{2}O = H_{2}SO_{4} + O
+
+[Illustration: Fig. 33]
+
+That new substances are formed at the cathode and anode may be shown in
+the following way. A U-tube, such as is represented in Fig. 33, is
+partially filled with a solution of sodium sulphate, and the liquid in
+one arm is colored with red litmus, that in the other with blue litmus.
+An electrode placed in the red solution is made to serve as cathode,
+while one in the blue solution is made the anode. On allowing the
+current to pass, the blue solution turns red, while the red solution
+turns blue. These are exactly the changes which would take place if
+sodium hydroxide and sulphuric acid were to be set free at the
+electrodes, as required by the theory.
+
+~The properties of electrolytes depend upon the ions present.~ When a
+substance capable of dissociating into ions is dissolved in water, the
+properties of the solution will depend upon two factors: (1) the ions
+formed from the substance; (2) the undissociated molecules. Since the
+ions are usually more active chemically than the molecules, most of the
+chemical properties of an electrolyte are due to the ions rather than to
+the molecules.
+
+The solutions of any two substances which give the same ion will have
+certain properties in common. Thus all solutions containing the copper
+ion (Cu^{++}) are blue, unless the color is modified by the presence of
+ions or molecules having some other color.
+
+
+EXERCISES
+
+1. Distinguish clearly between the following terms: electrolysis,
+electrolyte, electrolytic dissociation, ions, solute, solvent, solution,
+saturated solution, and supersaturated solution.
+
+2. Why does the water from some natural springs effervesce?
+
+3. (a) Why does not the water of the ocean freeze? (b) Why will ice
+and salt produce a lower temperature than ice alone?
+
+4. Why does shaking or stirring make a solid dissolve more rapidly in a
+liquid?
+
+5. By experiment it was found that a certain volume of water was
+saturated at 100 deg. with 114 g. of potassium nitrate. On cooling to 0 deg. a
+portion of the substance crystallized. (a) How many grams of the
+substance remained in solution? (b) What was the strength of the
+solution at 18 deg.? (c) How much water had been used in the experiment?
+
+6. (a) 10 g. of common salt were dissolved in water and the solution
+evaporated to dryness; what weight of solid was left? (b) 10 g. of
+zinc were dissolved in hydrochloric acid and the solution evaporated to
+dryness; what weight of solid was left?
+
+7. Account for the fact that sugar sometimes deposits from molasses,
+even when no evaporation has taken place.
+
+8. (a) From the standpoint of the theory of electrolytic dissociation,
+write the simple equation for a dilute solution of copper sulphate
+(CuSO_{4}); this solution is blue. (b) In the same manner, write one
+for sodium sulphate; this solution is colorless. (c) How would you
+account for the color of the copper sulphate solution?
+
+9. (a) As in the preceding exercise, write a simple equation for a
+dilute solution of copper chloride (CuCl_{2}); this solution is blue.
+(b) In the same manner, write one for sodium chloride; this solution
+is colorless. To what is the blue color due?
+
+10. What component is present in concentrated sulphuric acid that is
+almost wanting in very dilute sulphuric acid?
+
+11. Why will vegetables cook faster when boiled in strong salt water
+than when boiled in pure water?
+
+12. How do you explain the foaming of soda water?
+
+
+
+
+CHAPTER X
+
+ACIDS, BASES, AND SALTS; NEUTRALIZATION
+
+
+~Acids, bases, and salts.~ The three classes of compounds known
+respectively as acids, bases, and salts include the great majority of
+the compounds with which we shall have to deal. It is important,
+therefore, for us to consider each of these classes in a systematic way.
+The individual members belonging to each class will be discussed in
+detail in the appropriate places, but a few representatives of each
+class will be described in this chapter with special reference to the
+common properties in accordance with which they are classified.
+
+~The familiar acids.~ _Hydrochloric acid_ is a gas composed of hydrogen and
+chlorine, and has the formula HCl. The substance is very soluble in
+water, and it is this solution which is usually called hydrochloric
+acid. _Nitric acid_ is a liquid composed of hydrogen, nitrogen, and
+oxygen, having the formula HNO_{3}. As sold commercially it is mixed
+with about 32% of water. _Sulphuric acid_, whose composition is
+represented by the formula H_{2}SO_{4}, is an oily liquid nearly twice
+as heavy as water, and is commonly called _oil of vitriol_.
+
+~Characteristics of acids.~ (1) All acids contain hydrogen. (2) When
+dissolved in water the molecules of the acid dissociate into two kinds
+of ions. One of these is always hydrogen and is the cation (+), while
+the other consists of the remainder of the molecule and is the anion
+(-). (3) The solution tastes sour. (4) It has the power to change the
+color of certain substances called _indicators_. Thus blue litmus is
+changed to red, and yellow methyl orange is changed to red. Since all
+acids produce hydrogen cations, while the anions of each are different,
+the properties which all acids have in common when in solution, such as
+taste and action on indicators, must be attributed to the hydrogen ions.
+
+DEFINITION: _An acid is a substance which produces hydrogen ions when
+dissolved in water or other dissociating liquids._
+
+~Undissociated acids.~ When acids are perfectly free from water, or are
+dissolved in liquids like benzene which do not have the power of
+dissociating them into ions, they should have no real acid properties.
+This is found to be the case. Under these circumstances they do not
+affect the color of indicators or have any of the properties
+characteristic of acids.
+
+The familiar bases. The bases most used in the laboratory are sodium
+hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide
+(Ca(OH)_{2}). These are white solids, soluble in water, the latter
+sparingly so. Some bases are very difficultly soluble in water. The very
+soluble ones with most pronounced basic properties are sometimes called
+the _alkalis_.
+
+~Characteristics of bases.~ (1) All bases contain hydrogen and oxygen. (2)
+When dissolved in water the molecules of the base dissociate into two
+kinds of ions. One of these is always composed of oxygen and hydrogen
+and is the anion. It has the formula OH and is called the _hydroxyl
+ion_. The remainder of the molecule, which usually consists of a single
+atom, is the cation. (3) The solution of a base has a soapy feel and a
+brackish taste. (4) It reverses the color change produced in indicators
+by acids, turning red litmus blue, and red methyl orange yellow. Since
+all bases produce hydroxyl anions, while the cations of each are
+different, the properties which all bases have in common when in
+solution must be due to the hydroxyl ions.
+
+DEFINITION: _A base is a substance which produces hydroxyl ions when
+dissolved in water or other dissociating liquids._
+
+~Undissociated bases.~ Bases, in the absence of water or when dissolved in
+liquids which do not dissociate them, should have none of the properties
+characteristic of this class of substances. This has been found to be
+the case. For example, they have no effect upon indicators under these
+circumstances.
+
+~Neutralization.~ When an acid and a base are brought together in solution
+in proper proportion, the characteristic properties of each disappear.
+The solution tastes neither sour nor brackish; it has no effect upon
+indicators. There can therefore be neither hydrogen nor hydroxyl ions
+present in the solution. A study of reactions of this kind has shown
+that the hydrogen ions of the acid combine with the hydroxyl ions of the
+base to form molecules of water, water being a substance which is not
+appreciably dissociated into ions. This action of an acid on a base is
+called _neutralization_. The following equations express the
+neutralization of the three acids by three bases, water being formed in
+each case.
+
+    Na^{+}, OH^{-} + H^{+}, Cl^{-} = Na^{+}, Cl^{-} + H_{2}O.
+
+    K^{+}, OH^{-} + H^{+}, NO_{3}^{-} = K^{+}, NO_{3}^{-} + H_{2}O.
+
+    Ca^{++}, (OH)_{2}^{--} + H_{2}^{++}, SO_{4}^{--}
+         = Ca^{++}, SO_{4}^{--} + 2H_{2}O.
+
+DEFINITION: _Neutralization consists in the union of the hydrogen ion of
+an acid with the hydroxyl ion of a base to form water._
+
+~Salts.~ It will be noticed that in neutralization the anion of the acid
+and the cation of the base are not changed. If, however, the water is
+expelled by evaporation, these two ions slowly unite, and when the water
+becomes saturated with the substance so produced, it separates in the
+form of a solid called a _salt_.
+
+DEFINITION: _A salt is a substance formed by the union of the anion of
+an acid with the cation of a base._
+
+~Characteristics of salts.~ (1) From the definition of a salt it will be
+seen that there is no element or group of elements which characterize
+salts. (2) Salts as a class have no peculiar taste. (3) In the absence
+of all other substances they are without action on indicators. (4) When
+dissolved in water they form two kinds of ions.
+
+     ~Heat of neutralization.~ If neutralization is due to the union
+     of hydrogen ions with hydroxyl ions, and nothing more, it
+     follows that when a given weight of water is formed in
+     neutralization, the heat set free should always be the same, no
+     matter from what acid and base the two kinds of ions have been
+     supplied. Careful experiments have shown that this is the case,
+     provided no other reactions take place at the same time. When
+     18g. of water are formed in neutralization, 13,700 cal. of heat
+     are set free. This is represented in the equations
+
+    Na^{+}, OH^{-} + H^{+}, Cl^{-}
+         = Na^{+}, Cl^{-} + H_{2}O + 13,700 cal.
+
+    K^{+}, OH^{-} + H^{+}, NO_{3}^{-}
+         = K^{+}, NO_{3}^{-} + H_{2}O + 13,700 cal.
+
+    Ca^{++}, (OH)_{2}^{--} + H_{2}^{++}, SO_{4}^{--}
+         = Ca^{++}, SO_{4}^{--} + 2H_{2}O + 2 x 13,700 cal.
+
+~Neutralization a quantitative act.~ Since neutralization is a definite
+chemical act, each acid will require a perfectly definite weight of each
+base for its neutralization. For example, a given weight of sulphuric
+acid will always require a definite weight of sodium hydroxide, in
+accordance with the equation
+
+    H_{2}, SO_{4} + 2Na, OH = Na_{2}, SO_{4} + 2H_{2}O.
+
+     ~Determination of the ratio in neutralization.~ The quantities of
+     acid and base required in neutralization may be determined in
+     the following way. Dilute solutions of the two substances are
+     prepared, the sulphuric acid being placed in one of the
+     burettes (Fig. 34) and the sodium hydroxide in the other. The
+     levels of the two liquids are then brought to the zero marks of
+     the burettes by means of the stopcocks. A measured volume of
+     the acid is drawn off into a beaker, a few drops of litmus
+     solution added, and the sodium hydroxide is run in drop by drop
+     until the red litmus just turns blue. The volume of the sodium
+     hydroxide consumed is then noted. If the concentrations of the
+     two solutions are known, it is easy to calculate what weight of
+     sodium hydroxide is required to neutralize a given weight of
+     sulphuric acid. By evaporating the neutralized solution to
+     dryness, the weight of the sodium sulphate formed can be
+     determined directly. Experiment shows that the weights are
+     always in accordance with the equation in the preceding
+     paragraph.
+
+[Illustration: Fig. 34]
+
+~Extent of dissociation.~ The question will naturally arise, When an acid,
+base, or salt dissolves in water, do all the molecules dissociate into
+ions, or only a part of them? The experiments by which this question can
+be answered cannot be described here. It has been found, however, that
+only a fraction of the molecules dissociate. The percentage which will
+dissociate in a given case depends upon several conditions, the chief of
+which are: (1) The concentration of the solution. In concentrated
+solutions only a very small percentage of dissociation occurs. As the
+solution is diluted the percentage increases, and in very dilute
+solutions it may be very large, though it is never complete in any
+ordinary solution. (2) The nature of the dissolved compound. At equal
+concentrations substances differ much among themselves in the percentage
+of dissociation. The great majority of salts are about equally
+dissociated. Acids and bases, on the contrary, show great differences.
+Some are freely dissociated, while others are dissociated to but a
+slight extent.
+
+~Strength of acids and bases.~ Since acid and basic properties are due to
+hydrogen and hydroxyl ions respectively, the acid or base which will
+produce the greatest percentage of these ions at a given concentration
+must be regarded as the strongest representative of its class. The acids
+and bases described in the foregoing paragraphs are all quite strong. In
+10% solutions they are dissociated to about 50%, and this is also
+approximately the extent to which most salts are dissociated at this
+same concentration.
+
+     ~Partial neutralization.~ 1. _Basic salts._ The chemical action
+     between an acid and a base is not always as complete as has
+     been represented in the foregoing paragraphs. For example, if
+     the base magnesium hydroxide (Mg(OH)_{2}) and hydrochloric acid
+     (HCl) are brought together in the ratio of an equal number of
+     molecules of each, there will be only half enough hydrogen ions
+     for the hydroxyl ions present.
+
+         Mg, (OH)_{2} + H, Cl = Mg, OH, Cl + H_{2}O.
+
+     Magnesium, hydroxyl, and chlorine ions are left at the close of
+     the reaction, and under the proper conditions unite to form
+     molecules of the compound Mg(OH)Cl. This compound, when
+     dissolved, can form hydroxyl ions and therefore possesses basic
+     properties; it can also form the ions of a salt (Mg and Cl),
+     and has properties characteristic of salts. Substances of this
+     kind are called _basic salts._
+
+     DEFINITION: _A basic salt is a substance which can give the
+     ions both of a base and of a salt when dissolved in water._
+
+     2. _Acid salts._ In a similar way, when sulphuric acid and
+     sodium hydroxide are brought together in the ratio of equal
+     numbers of the molecules of each, it is possible to have a
+     reaction expressed by the equation
+
+        Na, OH + H_{2}, SO_{4} = Na, H, SO_{4} + H_{2}O.
+
+     The ions remaining after all the hydroxyl ions have been used
+     up are those of an acid (H) and those of a salt (Na and
+     SO_{4}). These unite to form the substance NaHSO_{4}, and as
+     the solution becomes saturated with this substance through
+     evaporation, it separates in the form of crystals. In solution
+     this substance can give hydrogen ions, and therefore possesses
+     acid properties; it can also give the ions characteristic of a
+     salt. It is therefore called an _acid salt_.
+
+     DEFINITION: _An acid salt is one which can give the ions of an
+     acid and of a salt when in solution._
+
+     3. _Normal salts._ Salts which are the products of complete
+     neutralization, such as Na_{2}SO_{4}, and which in solution can
+     give neither hydrogen nor hydroxyl ions, but only the ions of a
+     salt, are called _normal salts_ to distinguish them from acid
+     and basic salts.
+
+~Methods of expressing reactions between compounds in solution.~ Chemical
+equations representing reactions between substances in solution may
+represent the details of the reaction, or they may simply indicate the
+final products formed. In the latter case the formation of ions is not
+indicated. Thus, if we wish to call attention to the details of the
+reaction between sodium hydroxide and hydrochloric acid in solution, the
+equation is written as follows:
+
+    Na^{+}, OH^{-} + H^{+}, Cl^{-} = Na^{+}, Cl^{-} + H_{2}O.
+
+On the other hand, if we wish simply to represent the final products
+formed, the following is used.
+
+    NaOH + HCl = NaCl + H_{2}O.
+
+Both of these methods will therefore be used:
+
+~Radicals.~ It has been emphasized that the hydroxyl group (OH) always
+forms the anion of a base, while the group NO_{3} forms the anion of
+nitric acid and sodium nitrate; the group SO_{4}, the anion of sulphuric
+acid and calcium sulphate. A group of elements which in this way
+constitutes a part of a molecule, acting as a unit in a chemical change,
+or forming ions in solution, is called a _radical_. Some of these
+radicals have been given special names, the names signifying the
+elements present in the radical. Thus we have the hydroxyl radical (OH)
+and the nitrate radical (NO_{3}).
+
+DEFINITION: _A radical is a group of elements forming part of a
+molecule, and acting as a unit in chemical reactions._
+
+~Names of acids, bases, and salts.~ Since acids, bases, and salts are so
+intimately related to each other, it is very advantageous to give names
+to the three classes in accordance with some fixed system. The system
+universally adopted is as follows:
+
+~Naming of bases.~ All bases are called _hydroxides_. They are
+distinguished from each other by prefixing the name of the element which
+is in combination with the hydroxyl group. Examples: sodium hydroxide
+(NaOH); calcium hydroxide (Ca(OH)_{2}); copper hydroxide (Cu(OH)_{2}).
+
+~Naming of acids.~ The method of naming acids depends upon whether the
+acid consists of two elements or three.
+
+1. _Binary acids._ Acids containing only one element in addition to
+hydrogen are called _binary acids_. They are given names consisting of
+the prefix _hydro-_, the name of the second element present, and the
+termination _-ic_. Examples: hydrochloric acid (HCl); hydrosulphuric
+acid (H_{2}S).
+
+2. _Ternary acids._ In addition to the two elements present in binary
+acids, the great majority of acids also contain oxygen. They therefore
+consist of three elements and are called _ternary acids_. It usually
+happens that the same three elements can unite in different proportions
+to make several different acids. The most familiar one of these is given
+a name ending in the suffix _-ic_, while the one with less oxygen is
+given a similar name, but ending in the suffix _-ous_. Examples: nitric
+acid (HNO_{3}); nitrous acid (HNO_{2}). In cases where more than two
+acids are known, use is made of prefixes in addition to the two suffixes
+_-ic_ and _-ous_. Thus the prefix _per-_ signifies an acid still richer
+in oxygen; the prefix _hypo-_ signifies one with less oxygen.
+
+~Naming of salts.~ A salt derived from a binary acid is given a name
+consisting of the names of the two elements composing it, with the
+termination _-ide_. Example: sodium chloride (NaCl). All other binary
+compounds are named in the same way.
+
+A salt of a ternary acid is named in accordance with the acid from which
+it is derived. A ternary acid with the termination _-ic_ gives a salt
+with the name ending in _-ate_, while an acid with termination _-ous_
+gives a salt with the name ending in _-ite_. The following table will
+make the application of these principles clear:
+
+     ACIDS          SYMBOL       SALTS                SYMBOL
+
+    Hydrochloric   HCl          Sodium chloride      NaCl
+    Hypochlorous   HClO         Sodium hypochlorite  NaClO
+    Chlorous       HClO_{2}     Sodium chlorite      NaClO_{2}
+    Chloric        HClO_{3}     Sodium chlorate      NaClO_{3}
+    Perchloric     HClO_{4}     Sodium perchlorate   NaClO_{4}
+
+
+EXERCISES
+
+1. 25 cc. of a solution containing 40 g. of sodium hydroxide per liter
+was found to neutralize 25 cc. of a solution of hydrochloric acid. What
+was the strength of the acid solution?
+
+2. After neutralizing a solution of sodium hydroxide with nitric acid,
+there remained after evaporation 100 g. of sodium nitrate. How much of
+each substance had been used?
+
+3. A solution contains 18 g. of hydrochloric acid per 100 cc. It
+required 25 cc. of this solution to neutralize 30 cc. of a solution of
+sodium hydroxide. What was the strength of the sodium hydroxide solution
+in parts per hundred?
+
+4. When perfectly dry sulphuric acid is treated with perfectly dry
+sodium hydroxide, no chemical change takes place. Explain.
+
+5. When cold, concentrated sulphuric acid is added to zinc, no change
+takes place. Recall the action of dilute sulphuric acid on the same
+metal. How do you account for the difference?
+
+6. A solution of hydrochloric acid in benzene does not conduct the
+electric current. When this solution is treated with zinc, will hydrogen
+be evolved? Explain.
+
+7. (a) Write equation for preparation of hydrogen from zinc and dilute
+sulphuric acid. (b) Rewrite the same equation from the standpoint of
+the theory of electrolytic dissociation, (c) Subtract the common
+SO_{4} ion from both members of the equation, (d) From the resulting
+equation, explain in what the preparation of hydrogen consists when
+examined from the standpoint of this theory.
+
+8. In the same manner as in the preceding exercise, explain in what the
+action of sodium on water to give hydrogen consists.
+
+
+
+
+CHAPTER XI
+
+VALENCE
+
+
+~Definition of valence.~ A study of the formulas of various binary
+compounds shows that the elements differ between themselves in the
+number of atoms of other elements which they are able to hold in
+combination. This is illustrated in the formulas
+
+            HCl,        H_{2}O,   H_{3}N,   H_{4}C.
+    (hydrochloric acid) (water) (ammonia) (marsh gas)
+
+It will be noticed that while one atom of chlorine combines with one
+atom of hydrogen, an atom of oxygen combines with two, an atom of
+nitrogen with three, one of carbon with four. The number which expresses
+this combining ratio between atoms is a definite property of each
+element and is called its _valence_.
+
+DEFINITION: _The valence of an element is that property which determines
+the number of the atoms of another element which its atom can hold in
+combination._
+
+~Valence a numerical property.~ Valence is therefore merely a numerical
+relation and does not convey any information in regard to the intensity
+of the affinity between atoms. Judging by the heat liberated in their
+union, oxygen has a far stronger affinity for hydrogen than does
+nitrogen, but an atom of oxygen can combine with two atoms only of
+hydrogen, while an atom of nitrogen can combine with three.
+
+~Measure of valence.~ In expressing the valence of an element we must
+select some standard for comparison, just as in the measurement of any
+other numerical quantity. It has been found that an atom of hydrogen is
+never able to hold in combination more than one atom of any other
+element. Hydrogen is therefore taken as the standard, and other elements
+are compared with it in determining their valence. A number of other
+elements are like hydrogen in being able to combine with at most one
+atom of other elements, and such elements are called _univalent_. Among
+these are chlorine, iodine, and sodium. Elements such as oxygen,
+calcium, and zinc, which can combine with two atoms of hydrogen or other
+univalent elements, are said to be _divalent_. Similarly, we have
+_trivalent, tetravalent, pentavalent_ elements. None have a valence of
+more than 8.
+
+~Indirect measure of valence.~ Many elements, especially among the metals,
+do not readily form compounds with hydrogen, and their valence is not
+easy to determine by direct comparison with the standard element. These
+elements, however, combine with other univalent elements, such as
+chlorine, and their valence can be determined from the compounds so
+formed.
+
+~Variable valence.~ Many elements are able to exert different valences
+under differing circumstances. Thus we have the compounds Cu_{2}O and
+CuO, CO and CO_{2}, FeCl_{2} and FeCl_{3}. It is not always possible to
+assign a fixed valence to an element. Nevertheless each element tends to
+exert some normal valence, and the compounds in which it has a valence
+different from this are apt to be unstable and easily changed into
+compounds in which the valence of the element is normal. The valences of
+the various elements will become familiar as the elements are studied in
+detail.
+
+~Valence and combining ratios.~ When elements combine to form compounds,
+the ratio in which they combine will be determined by their valences. In
+those compounds which consist of two elements directly combined, the
+union is between such numbers of the two atoms as have equal valences.
+Elements of the same valence will therefore combine atom for atom.
+Designating the valence of the atoms by Roman numerals placed above
+their symbols, we have the formulas
+
+    II II   II III   III   IV IV
+    HCl,     ZnO,     BN,   CSi.
+
+A divalent element, on the other hand, will combine with two atoms of a
+univalent element. Thus we have
+
+    II II         II II
+    ZnCl_{2} and H_{2}O
+
+(the numerals above each symbol representing the sum of the valences of
+the atoms of the element present). A trivalent atom will combine with
+three atoms of a univalent element, as in the compound
+
+    III III
+     H_{3}N.
+
+If a trivalent element combines with a divalent element, the union will
+be between two atoms of the trivalent element and three of the divalent
+element, since these numbers are the smallest which have equal valences.
+Thus the oxide of the trivalent metal aluminium has the formula
+Al_{2}O_{3}. Finally one atom of a tetravalent element such as carbon
+will combine with four atoms of a univalent element, as in the compound
+CH_{4}, or with two atoms of a divalent element, as in the compound
+CO_{2}.
+
+We have no knowledge as to why elements differ in their combining power,
+and there is no way to determine their valences save by experiment.
+
+     ~Valence and the structure of compounds.~ Compounds will be met
+     from time to time which are apparent exceptions to the general
+     statements just made in regard to valence. Thus, from the
+     formula for hydrogen dioxide (H_{2}O_{2}), it might be
+     supposed that the oxygen is univalent; yet it is certainly
+     divalent in water (H_{2}O). That it may also be divalent in
+     H_{2}O_{2} may be made clear as follows: The unit valence of
+     each element may be represented graphically by a line attached
+     to its symbol. Univalent hydrogen and divalent oxygen will then
+     have the symbols H- and -O-. When atoms combine, each unit
+     valence of one atom combines with a unit valence of another
+     atom. Thus the composition of water may be expressed by the
+     formula H-O-H, which is meant to show that each of the unit
+     valences of oxygen is satisfied with the unit valence of a
+     single hydrogen atom.
+
+     The chemical conduct of hydrogen dioxide leads to the
+     conclusion that the two oxygen atoms of its molecule are in
+     direct combination with each other, and in addition each is in
+     combination with a hydrogen atom. This may be expressed by the
+     formula H-O-O-H. The oxygen in the compound is therefore
+     divalent, just as it is in water. It will thus be seen that the
+     structure of a compound must be known before the valences of
+     the atoms making up the compound can be definitely decided
+     upon.
+
+     Such formulas as H-O-H and H-O-O-H are known as _structural
+     formulas_, because they are intended to show what is known in
+     regard to the arrangement of the atoms in the molecules.
+
+~Valence and the replacing power of atoms.~ Just as elements having the
+same valence combine with each other atom for atom, so if they replace
+each other in a chemical reaction they will do so in the same ratio.
+This is seen in the following equations, in which a univalent hydrogen
+atom is replaced by a univalent sodium atom:
+
+    NaOH + HCl = NaCl + H_{2}O.
+
+    2NaOH + H_{2}SO_{4} = Na_{2}SO_{4} + 2H_{2}O.
+
+    Na + H_{2}O = NaOH + H.
+
+Similarly, one atom of divalent calcium will replace two atoms of
+univalent hydrogen or one of divalent zinc:
+
+    Ca(OH)_{2} + 2 HCl = CaCl_{2} + 2H_{2}O.
+
+    CaCl_{2} + ZnSO_{4} = CaSO_{4} + ZnCl_{2}.
+
+In like manner, one atom of a trivalent element will replace three of a
+univalent element, or two atoms will replace three atoms of a divalent
+element.
+
+~Valence and its applications to formulas of salts.~ While the true nature
+of valence is not understood and many questions connected with the
+subject remain unanswered, yet many of the main facts are of much help
+to the student. Thus the formula of a salt, differs from that of the
+acid from which it is derived in that the hydrogen of the acid has been
+replaced by a metal. If, then, it is known that a given metal forms a
+normal salt with a certain acid, the formula of the salt can at once be
+determined if the valence of the metal is known. Since sodium is
+univalent, the sodium salts of the acids HCl and H_{2}SO_{4} will be
+respectively NaCl and Na_{2}SO_{4}. One atom of divalent zinc will
+replace 2 hydrogen atoms, so that the corresponding zinc salts will be
+ZnCl_{2} and ZnSO_{4}.
+
+The formula for aluminium sulphate is somewhat more difficult to
+determine. Aluminium is trivalent, and the simplest ratio in which the
+aluminium atom can replace the hydrogen in sulphuric acid is 2 atoms of
+aluminium (6 valences) to 3 molecules of sulphuric acid (6 hydrogen
+atoms). The formula of the sulphate will then be Al_{2}(SO_{4})_{3}.
+
+~Valence and its application to equation writing.~ It will be readily seen
+that a knowledge of valence is also of very great assistance in writing
+the equations for reactions of double decomposition. Thus, in the
+general reaction between an acid and a base, the essential action is
+between the univalent hydrogen ion and the univalent hydroxyl ion. The
+base and the acid must always be taken in such proportions as to secure
+an equal number of each of these ions. Thus, in the reaction between
+ferric hydroxide (Fe(OH)_{3}) and sulphuric acid (H_{2}SO_{4}), it will
+be necessary to take 2 molecules of the former and 3 of the latter in
+order to have an equal number of the two ions, namely, 6. The equation
+will then be
+
+    2Fe(OH)_{3} + 3H_{2}SO_{4} = Fe_{2}(SO_{4})_{3} + 6H_{2}O.
+
+Under certain conditions the salts Al_{2}(SO_{4})_{3} and CaCl_{2}
+undergo double decomposition, the two metals, aluminium and calcium,
+exchanging places. The simplest ratio of exchange in this case is 2
+atoms of aluminium (6 valences) and 3 atoms of calcium (6 valences).
+The reaction will therefore take place between 1 molecule of
+Al_{2}(SO_{4})_{3} and 3 of CaCl_{2}, and the equation is as follows:
+
+    Al_{2}(SO_{4})_{3} + 3 CaCl_{2} = 3CaSO_{4} + 2AlCl_{3}.
+
+
+EXERCISES
+
+1. Sodium, calcium, and aluminium have valences of 1, 2, and 3
+respectively; write the formulas of their chlorides, sulphates, and
+phosphates (phosphoric acid = H_{3}PO_{4}), on the supposition that they
+form salts having the normal composition.
+
+2. Iron forms one series of salts in which it has a valence of 2, and
+another series in which it has a valence of 3; write the formulas for
+the two chlorides of iron, also for the two sulphates, on the
+supposition that these have the normal composition.
+
+3. Write the equation representing the neutralization of each of the
+following bases by each of the acids whose formulas are given:
+
+    NaOH           HCl
+    Ba(OH)_{2}     H_{2}SO_{4}
+    Al(OH)_{3}     H_{3}PO_{4}
+
+4. Silver acts as a univalent element and calcium as a divalent element
+in the formation of their respective nitrates and chlorides. (a) Write
+the formula for silver nitrate; for calcium chloride. (b) When
+solutions of these two salts are mixed, the two metals, silver and
+calcium, exchange places; write the equation for the reaction.
+
+_5._ Antimony acts as a trivalent element in the formation of a
+chloride. (a) What is the formula for antimony chloride? (b) When
+hydrosulphuric acid (H_{2}S) is passed into a solution of this chloride
+the hydrogen and antimony exchange places; write the equation for the
+reaction.
+
+6. Lead has a valence of 2 and iron of 3 in the compounds known
+respectively as lead nitrate and ferric sulphate. (a) Write the
+formulas for these two compounds. (b) When their solutions are mixed
+the two metals exchange places; write the equation for the reaction.
+
+
+
+
+CHAPTER XII
+
+COMPOUNDS OF NITROGEN
+
+
+~Occurrence.~ As has been stated in a former chapter, nitrogen constitutes
+a large fraction of the atmosphere. The compounds of nitrogen, however,
+cannot readily be obtained from this source, since at any ordinary
+temperature nitrogen is able to combine directly with very few of the
+elements.
+
+In certain forms of combination nitrogen occurs in the soil from which
+it is taken up by plants and built into complex substances composed
+chiefly of carbon, hydrogen, oxygen, and nitrogen. Animals feeding on
+these plants assimilate the nitrogenous matter, so that this element is
+an essential constituent of both plants and animals.
+
+~Decomposition of organic matter by bacteria.~ When living matter dies and
+undergoes decay complicated chemical reactions take place, one result of
+which is that the nitrogen of the organic matter is set free either as
+the element nitrogen, or in the form of simple compounds, such as
+ammonia (NH_{3}) or oxides of nitrogen. Experiment has shown that all
+such processes of decay are due to the action of different kinds of
+bacteria, each particular kind effecting a different change.
+
+~Decomposition of organic matter by heat.~ When organic matter is strongly
+heated decomposition into simpler substances takes place in much the
+same way as in the case of bacterial decomposition. Coal is a complex
+substance of vegetable origin, consisting largely of carbon, but also
+containing hydrogen, oxygen, and nitrogen. When this is heated in a
+closed vessel so that air is excluded, about one seventh of the nitrogen
+is converted into ammonia, and this is the chief source from which
+ammonia and its compounds are obtained.
+
+
+COMPOUNDS OF NITROGEN WITH HYDROGEN
+
+~Ammonia~ (NH_{3}). Several compounds consisting exclusively of nitrogen
+and hydrogen are known, but only one, ammonia, need be considered here.
+
+~Preparation of ammonia.~ Ammonia is prepared in the laboratory by a
+different method from the one which is used commercially.
+
+1. _Laboratory method._ In the laboratory ammonia is prepared from
+ammonium chloride, a compound having the formula NH_{4}Cl, and obtained
+in the manufacture of coal gas. As will be shown later in the chapter,
+the group NH_{4} in this compound acts as a univalent radical and is
+known as _ammonium_. When ammonium chloride is warmed with sodium
+hydroxide, the ammonium and sodium change places, the reaction being
+expressed in the following equation.
+
+    NH_{4}Cl + NaOH = NaCl + NH_{4}OH.
+
+The ammonium hydroxide (NH_{4}OH) so formed is unstable and breaks down
+into water and ammonia.
+
+    NH_{4}OH = NH_{3} + H_{2}O.
+
+Calcium hydroxide (Ca(OH)_{2}) is frequently used in place of the more
+expensive sodium hydroxide, the equations being
+
+    2NH_{4}Cl + Ca(OH)_{2} = CaCl_{2} + 2NH_{4}OH,
+
+    2NH_{4}OH = 2H_{2}O + 2NH_{3}.
+
+     In the preparation, the ammonium chloride and calcium hydroxide
+     are mixed together and placed in a flask arranged as shown in
+     Fig. 35. The mixture is gently warmed, when ammonia is evolved
+     as a gas and is collected by displacement of air.
+
+[Illustration: Fig. 35]
+
+2. _Commercial method._ Nearly all the ammonia of commerce comes from
+the gasworks. Ordinary illuminating gas is made by distilling coal, as
+will be explained later, and among the products of this distillation a
+solution of ammonia in water is obtained. This solution, known as _gas
+liquor_, contains not only ammonia but other soluble substances. Most of
+these combine chemically with lime, while ammonia does not; if then lime
+is added to the gas liquor and the liquor is heated, the ammonia is
+driven out from the mixture. It may be dissolved again in pure, cold
+water, forming _aqua ammonia_, or the ammonia water of commerce.
+
+     ~Preparation from hydrogen and nitrogen.~ When electric sparks
+     are passed for some time through a mixture of hydrogen and
+     nitrogen, a small percentage of the two elements in the mixture
+     is changed into ammonia. The action soon ceases, however, for
+     the reason that ammonia is decomposed by the electric
+     discharge. The reaction expressed in the equation
+
+         N + 3H = NH_{3}
+
+     can therefore go in either direction depending upon the
+     relative quantities of the substances present. This recalls the
+     similar change from oxygen into ozone, which soon ceases
+     because the ozone is in turn decomposed into oxygen.
+
+~Physical properties.~ Under ordinary conditions ammonia is a gas whose
+density is 0.59. It is therefore little more than half as heavy as air.
+It is easily condensed into a colorless liquid, and can now be purchased
+in liquid form in steel cylinders. The gas is colorless and has a
+strong, suffocating odor. It is extremely soluble in water, 1 l. of
+water at 0 deg. and 760 mm. pressure dissolving 1148 l. of the gas. In
+dissolving this large volume of gas the water expands considerably, so
+that the density of the solution is less than that of water, the
+strongest solutions having a density of 0.88.
+
+~Chemical properties.~ Ammonia will not support combustion, nor will it
+burn under ordinary conditions. In an atmosphere of oxygen it burns with
+a feeble, yellowish flame. When quite dry it is not a very active
+substance, but when moist it combines with a great many substances,
+particularly with acids.
+
+~Uses.~ It has been stated that ammonia can be condensed to a liquid by
+the application of pressure. If the pressure is removed from the liquid
+so obtained, it rapidly passes again into the gaseous state and in so
+doing absorbs a large amount of heat. Advantage is taken of this fact in
+the preparation of artificial ice. Large quantities of ammonia are also
+used in the preparation of ammonium compounds.
+
+     ~The manufacture of artificial ice.~ Fig. 36 illustrates the
+     method of preparing artificial ice. The ammonia gas is
+     liquefied in the pipes X by means of the pump Y. The heat
+     generated is absorbed by water flowing over the pipes. The
+     pipes lead into a large brine tank, a cross section of which is
+     shown in the figure. Into the brine (concentrated solution of
+     common salt) contained in this tank are dipped the vessels A,
+     B, C, filled with pure water. The pressure is removed from
+     the liquid ammonia as it passes into the pipes immersed in the
+     brine, and the heat absorbed by the rapid evaporation of the
+     liquid lowers the temperature of the brine below zero. The
+     water in A, B, C is thereby frozen into cakes of ice. The
+     gaseous ammonia resulting from the evaporation of the liquid
+     ammonia is again condensed, so that the process is continuous.
+
+[Illustration Fig. 36]
+
+~Ammonium hydroxide~ (NH_{4}OH). The solution of ammonia in water is found
+to have strong basic properties and therefore contains hydroxyl ions. It
+turns red litmus blue; it has a soapy feel; it neutralizes acids,
+forming salts with them. It seems probable, therefore, that when ammonia
+dissolves in water it combines chemically with it according to the
+equation
+
+    NH_{3} + H_{2}O = NH_{4}OH,
+
+and that it is the substance NH_{4}OH, called ammonium hydroxide, which
+has the basic properties, dissociating into the ions NH_{4} and OH.
+Ammonium hydroxide has never been obtained in a pure state. At every
+attempt to isolate it the substance breaks up into water and ammonia,--
+
+    NH_{4}OH = NH_{3} + H_{2}O.
+
+~The ammonium radical.~ The radical NH_{4} plays the part of a metal in
+many chemical reactions and is called ammonium. The ending _-ium_ is
+given to the name to indicate the metallic properties of the substance,
+since the names of the metals in general have that ending. The salts
+formed by the action of the base ammonium hydroxide on acids are called
+ammonium salts. Thus, with hydrochloric acid, ammonium chloride is
+formed in accordance with the equation
+
+    NH_{4}OH + HCl = NH_{4}Cl + H_{2}O.
+
+Similarly, with nitric acid, ammonium nitrate (NH_{4}NO_{3}) is formed,
+and with sulphuric acid, ammonium sulphate ((NH_{4})_{2}S0_{4}).
+
+It will be noticed that in the neutralization of ammonium hydroxide by
+acids the group NH_{4} replaces one hydrogen atom of the acid, just as
+sodium does. The group therefore acts as a univalent metal.
+
+~Combination of nitrogen with hydrogen by volume.~ Under suitable
+conditions ammonia can be decomposed into nitrogen and hydrogen by
+passing electric sparks through the gas. Accurate measurement has shown
+that when ammonia is decomposed, two volumes of the gas yield one volume
+of nitrogen and three volumes of hydrogen. Consequently, if the two
+elements were to combine directly, one volume of nitrogen would combine
+with three volumes of hydrogen to form two volumes of ammonia. Here, as
+in the formation of steam from hydrogen and oxygen, small whole numbers
+serve to indicate the relation between the volumes of combining gases
+and that of the gaseous product.
+
+
+COMPOUNDS OF NITROGEN WITH OXYGEN AND HYDROGEN
+
+In addition to ammonium hydroxide, nitrogen forms several compounds with
+hydrogen and oxygen, of which nitric acid (HNO_{3}) and nitrous acid
+(HNO_{2}) are the most familiar.
+
+~Nitric acid~ (HNO_{3}). Nitric acid is not found to any extent in nature,
+but some of its salts, especially sodium nitrate (NaNO_{3}) and
+potassium nitrate (KNO_{3}) are found in large quantities. From these
+salts nitric acid can be obtained.
+
+[Illustration Fig. 37]
+
+~Preparation of nitric acid.~ When sodium nitrate is treated with
+concentrated cold sulphuric acid, no chemical action seems to take
+place. If, however, the mixture is heated in a retort, nitric acid is
+given off as a vapor and may be easily condensed to a liquid by passing
+the vapor into a tube surrounded by cold water, as shown in Fig. 37. An
+examination of the liquid left in the retort shows that it contains
+sodium acid sulphate (NaHSO_{4}), so that the reaction may be
+represented by the equation
+
+    NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}.
+
+     If a smaller quantity of sulphuric acid is taken and the
+     mixture is heated to a high temperature, normal sodium sulphate
+     is formed:
+
+         2NaNO_{3} + H_{2}SO_{4} = Na_{2}SO_{4} + 2HNO_{3}.
+
+     In this case, however, the higher temperature required
+     decomposes a part of the nitric acid.
+
+     ~The commercial preparation of nitric acid.~ Fig. 38 illustrates
+     a form of apparatus used in the preparation of nitric acid on a
+     large scale. Sodium nitrate and sulphuric acid are heated in
+     the iron retort A. The resulting acid vapors pass in the
+     direction indicated by the arrows, and are condensed in the
+     glass tubes B, which are covered with cloth kept cool by
+     streams of water. These tubes are inclined so that the liquid
+     resulting from the condensation of the vapors runs back into
+     C and is drawn off into large vessels (D).
+
+[Illustration Fig. 38]
+
+~Physical properties of nitric acid.~ Pure nitric acid is a colorless
+liquid, which boils at about 86 deg. and has a density of 1.56. The
+concentrated acid of commerce contains about 68% of the acid, the
+remainder being water. Such a mixture has a density of 1.4. The
+concentrated acid fumes somewhat in moist air, and has a sharp choking
+odor.
+
+~Chemical properties.~ The most important chemical properties of nitric
+acid are the following.
+
+1. _Acid properties._ As the name indicates, this substance is an acid,
+and has all the properties of that class of substances. It changes blue
+litmus red and has a sour taste in dilute solutions. It forms hydrogen
+ions in solution and neutralizes bases forming salts. It also acts upon
+the oxides of most metals, forming a salt and water. It is one of the
+strongest acids.
+
+2. _Decomposition on heating._ When boiled, or exposed for some time to
+sunlight, it suffers a partial decomposition according to the equation
+
+    2HNO_{3} = H_{2}O + 2NO_{2} + O.
+
+The substance NO_{2}, called nitrogen peroxide, is a brownish gas, which
+is readily soluble in water and in nitric acid. It therefore dissolves
+in the undecomposed acid, and imparts a yellowish or reddish color to
+it. Concentrated nitric acid highly charged with this substance is
+called _fuming nitric acid_.
+
+3. _Oxidizing action._ According to its formula, nitric acid contains a
+large percentage of oxygen, and the reaction just mentioned shows that
+the compound is not a very stable one, easily undergoing decomposition.
+These properties should make it a good oxidizing agent, and we find that
+this is the case. Under ordinary circumstances, when acting as an
+oxidizing agent, it is decomposed according to the equation
+
+    2HNO_{3} = H_{2}O + 2NO + 3O.
+
+The oxygen is taken up by the substance oxidized, and not set free, as
+is indicated in the equation. Thus, if carbon is oxidized by nitric
+acid, the oxygen combines with carbon, forming carbon dioxide (CO_{2}):
+
+    C + 2O = CO_{2}.
+
+4. _Action on metals._ We have seen that when an acid acts upon a metal
+hydrogen is set free. Accordingly, when nitric acid acts upon a metal,
+such as copper, we should expect the reaction to take place which is
+expressed in the equation
+
+    Cu + 2HNO_{3} = Cu(NO_{3})_{2} + 2H.
+
+This reaction does take place, but the hydrogen set free is immediately
+oxidized to water by another portion of the nitric acid according to the
+equation
+
+    HNO_{3} + 3H = 2H_{2}O + NO.
+
+As these two equations are written, two atoms of hydrogen are given off
+in the first equation, while three are used up in the second. In order
+that the hydrogen may be equal in the two equations, we must multiply
+the first by 3 and the second by 2. We shall then have
+
+    3Cu + 6HNO_{3} = 3Cu(NO_{3})_{2} + 6H,
+
+    2HNO_{3} + 6H = 4H_{2}O + 2NO.
+
+The two equations may now be combined into one by adding the quantities
+on each side of the equality sign, canceling the hydrogen which is given
+off in the one reaction and used up in the other. We shall then have the
+equation
+
+    3Cu + 8HNO_{3} = 3Cu(NO_{3})_{2} + 2NO + 4H_{2}O.
+
+A number of other reactions may take place when nitric acid acts upon
+metals, resulting in the formation of other oxides of nitrogen, free
+nitrogen, or even ammonia. The reaction just given is, however, the
+usual one.
+
+     ~Importance of steps in a reaction.~ This complete equation has
+     the advantage of making it possible to calculate very easily
+     the proportions in which the various substances enter into the
+     reaction or are formed in it. It is unsatisfactory in that it
+     does not give full information about the way in which the
+     reaction takes place. For example, it does not suggest that
+     hydrogen is at first formed, and subsequently transformed into
+     water. It is always much more important to remember the steps
+     in a chemical reaction than to remember the equation expressing
+     the complete action; for if these steps in the reaction are
+     understood, the complete equation is easily obtained in the
+     manner just described.
+
+~Salts of nitric acid,--nitrates.~ The salts of nitric acid are called
+nitrates. Many of these salts will be described in the study of the
+metals. They are all soluble in water, and when heated to a high
+temperature undergo decomposition. In a few cases a nitrate on being
+heated evolves oxygen, forming a nitrite:
+
+    NaNO_{3} = NaNO_{2} + O.
+
+In other cases the decomposition goes further, and the metal is left as
+oxide:
+
+    Cu(NO_{3})_{2} = CuO + 2NO_{2} + O.
+
+~Nitrous acid~ (HNO_{2}). It is an easy matter to obtain sodium nitrite
+(NaNO_{2}), as the reaction given on the previous page indicates.
+Instead of merely heating the nitrate, it is better to heat it together
+with a mild reducing agent, such as lead, when the reaction takes place
+which is expressed by the equation
+
+    NaNO_{3} + Pb = PbO + NaNO_{2}.
+
+When sodium nitrite is treated with an acid, such as sulphuric acid, it
+is decomposed and nitrous acid is set free:
+
+    NaNO_{2} + H_{2}SO_{4} = NaHSO_{4} + HNO_{2}.
+
+The acid is very unstable, however, and decomposes readily into water
+and nitrogen trioxide (N_{2}O_{3}):
+
+    2HNO_{2} = H_{2}O + N_{2}O_{3}.
+
+Dilute solutions of the acid, however, can be obtained.
+
+
+COMPOUNDS OF NITROGEN WITH OXYGEN
+
+Nitrogen combines with oxygen to form five different oxides. The
+formulas and names of these are as follows:
+
+    N_{2}O       nitrous oxide.
+    NO           nitric oxide.
+    NO_{2}       nitrogen peroxide.
+    N_{2}O_{3}   nitrogen trioxide, or nitrous anhydride.
+    N_{2}O_{5}   nitrogen pentoxide, or nitric anhydride.
+
+These will now be briefly discussed.
+
+~Nitrous oxide~ (_laughing gas_) (N_{2}O). Ammonium nitrate, like all
+nitrates, undergoes decomposition when heated; and owing to the fact
+that it contains no metal, but does contain both oxygen and hydrogen,
+the reaction is a peculiar one. It is represented by the equation
+
+    NH_{4}NO_{3} = 2H_{2}O + N_{2}O.
+
+The oxide of nitrogen so formed is called nitrous oxide or laughing gas.
+It is a colorless gas having a slight odor. It is somewhat soluble in
+water, and in solution has a slightly sweetish taste. It is easily
+converted into a liquid and can be purchased in this form. When inhaled
+it produces a kind of hysteria (hence the name "laughing gas"), and even
+unconsciousness and insensibility to pain if taken in large amounts. It
+has long been used as an anaesthetic for minor surgical operations, such
+as those of dentistry, but owing to its unpleasant after effects it is
+not so much in use now as formerly.
+
+Chemically, nitrous oxide is remarkable for the fact that it is a very
+energetic oxidizing agent. Substances such as carbon, sulphur, iron, and
+phosphorus burn in it almost as brilliantly as in oxygen, forming oxides
+and setting free nitrogen. Evidently the oxygen in nitrous oxide cannot
+be held in very firm combination by the nitrogen.
+
+[Illustration Fig. 39]
+
+~Nitric oxide~ (NO). We have seen that when nitric acid acts upon metals,
+such as copper, the reaction represented by the following equation takes
+place:
+
+    3Cu + 8HNO_{3} = 3Cu(NO_{3})_{3} + 2NO + 4H_{2}O.
+
+Nitric oxide is most conveniently prepared in this way. The metal is
+placed in the flask A (Fig. 39) and the acid added slowly through the
+funnel tube B. The gas escapes through C and is collected over
+water.
+
+Pure nitric oxide is a colorless gas, slightly heavier than air, and is
+practically insoluble in water. It is a difficult gas to liquefy. Unlike
+nitrous oxide, nitric oxide does not part with its oxygen easily, and
+burning substances introduced into this gas are usually extinguished. A
+few substances like phosphorus, which have a very strong affinity for
+oxygen and which are burning energetically in the air, will continue to
+burn in an atmosphere of nitric oxide. In this case the nitric oxide
+loses all of its oxygen and the nitrogen is set free as gas.
+
+~Action of nitric oxide with oxygen.~ When nitric oxide comes into contact
+with oxygen or with the air, it at once combines with the oxygen even at
+ordinary temperatures, forming a reddish-yellow gas of the formula
+NO_{2}, which is called nitrogen peroxide. This action is not energetic
+enough to produce a flame, though considerable heat is set free.
+
+~Nitrogen peroxide~ (NO_{2}). This gas, as we have just seen, is formed by
+allowing nitric oxide to come into contact with oxygen. It can also be
+made by heating certain nitrates, such as lead nitrate:
+
+    Pb(NO_{3})_{2} = PbO + 2NO_{2} + O.
+
+It is a reddish-yellow gas of unpleasant odor, which is quite poisonous
+when inhaled. It is heavier than air and is easily condensed to a
+liquid. It dissolves in water, but this solution is not a mere physical
+solution; the nitrogen peroxide is decomposed, forming a mixture of
+nitric and nitrous acids:
+
+    2NO_{2} + H_{2}O = HNO_{2} + HNO_{3}.
+
+Nitrogen peroxide will not combine with more oxygen; it will, however,
+give up a part of its oxygen to burning substances, acting as an
+oxidizing agent:
+
+    NO_{2} = NO + O.
+
+~Acid anhydrides.~ The oxides N_{2}O_{3} (nitrogen trioxide) and
+N_{2}O_{5} (nitrogen pentoxide) are rarely prepared and need not be
+separately described. They bear a very interesting relation to the acids
+of nitrogen. When dissolved in water they combine with the water,
+forming acids:
+
+    N_{2}O_{3} + H_{2}O = 2HNO_{2},
+
+    N_{2}O_{5} + H_{2}O = 2HNO_{3}.
+
+On the other hand, nitrous acid very easily decomposes, yielding water
+and nitrogen trioxide, and by suitable means nitric acid likewise may be
+decomposed into water and nitrogen pentoxide:
+
+    2HNO_{2} = H_{2}O + N_{2}O_{3},
+
+    2HNO_{3} = H_{2}O + N_{2}O_{5}.
+
+In view of the close relation between these oxides and the corresponding
+acids, they are called _anhydrides_ of the acids, N_{2}O_{3} being
+nitrous anhydride and N_{2}O_{5} nitric anhydride.
+
+DEFINITION: _Any oxide which will combine with water to form an acid, or
+which together with water is formed by the decomposition of an acid, is
+called an anhydride of that acid._
+
+
+EXERCISES
+
+1. Perfectly dry ammonia does not affect litmus paper. Explain.
+
+2. Can ammonia be dried by passing the gas through concentrated
+sulphuric acid? Explain.
+
+3. Ammonium hydroxide is a weak base, i.e. it is not highly dissociated.
+When it is neutralized by strong acids the heat of reaction is less than
+when strong bases are so neutralized. Suggest some possible cause for
+this.
+
+4. Why is brine used in the manufacture of artificial ice?
+
+5. Discuss the energy changes which take place in the manufacture of
+artificial ice.
+
+6. What weight of ammonium chloride is necessary to furnish enough
+ammonia to saturate 1 l. of water at 0 deg. and 760 mm.?
+
+7. What weight of sodium nitrate is necessary to prepare 100 cc. of
+commercial nitric acid? What weight of potassium nitrate is necessary to
+furnish the same weight of acid?
+
+8. 100 l. of nitrogen peroxide were dissolved in water and neutralized
+with sodium hydroxide. What substances were formed and how much of
+each?(1 l. nitrogen peroxide weighs 2.05 grams.)
+
+9. How many liters of nitrous oxide, measured under standard conditions,
+can be prepared from 10 g. of ammonium nitrate?
+
+10. What weight of copper is necessary to prepare 50 l. of nitric oxide
+under standard conditions?
+
+11. (a) Calculate the percentage composition of the oxides of
+nitrogen. (b) What important law does this series of substances
+illustrate?
+
+12. Write the equations representing the reactions between ammonium
+hydroxide, and sulphuric acid and nitric acid respectively, in
+accordance with the theory of electrolytic dissociation.
+
+13. In the same way, write the equations representing the reactions
+between nitric acid and each of the following bases: NaOH, KOH,
+NH_{4}OH, Ca(OH)_{2}.
+
+
+
+
+CHAPTER XIII
+
+REVERSIBLE REACTIONS AND CHEMICAL EQUILIBRIUM
+
+
+~Reversible reactions.~ The reactions so far considered have been
+represented as continuing, when once started, until one or the other
+substance taking part in the reaction has been used up. In some
+reactions this is not the case. For example, we have seen that when
+steam is passed over hot iron the reaction is represented by the
+equation
+
+    3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H.
+
+On the other hand, when hydrogen is passed over hot iron oxide the
+reverse reaction takes place:
+
+    Fe_{3}O_{4} +8H = 3Fe + 4H_{2}O.
+
+The reaction can therefore go in either direction, depending upon the
+conditions of the experiment. Such a reaction is called a _reversible
+reaction_. It is represented by an equation with double arrows in place
+of the equality sign, thus:
+
+    3Fe + 4H_{2}O <--> Fe_{3}O_{4} + 8H.
+
+In a similar way, the equation
+
+    N + 3H <--> NH_{3}
+
+expresses the fact that under some conditions nitrogen may unite with
+hydrogen to form ammonia, while under other conditions ammonia
+decomposes into nitrogen and hydrogen.
+
+The conversion of oxygen into ozone is also reversible and may be
+represented thus:
+
+    oxygen <--> ozone.
+
+~Chemical equilibrium.~ Reversible reactions do not usually go on to
+completion in one direction unless the conditions under which the
+reaction takes place are very carefully chosen. Thus, if iron and steam
+are confined in a heated tube, the steam acts upon the iron, producing
+iron oxide and hydrogen. But these substances in turn act upon each
+other to form iron and steam once more. When these two opposite
+reactions go on at such rates that the weight of the iron changed into
+iron oxide is just balanced by the weight of the iron oxide changed into
+iron, there will be no further change in the relative weights of the
+four substances present in the tube. The reaction is then said to have
+reached an equilibrium.
+
+~Factors which determine the point of equilibrium.~ There are two factors
+which have a great deal of influence in determining the point at which a
+given reaction will reach equilibrium.
+
+1. _Influence of the chemical nature of the substances._ If two
+reversible reactions of the same general kind are selected, it has been
+found that the point of equilibrium is different in the two cases. For
+example, in the reactions represented by the equations
+
+    3Fe + 4H_{2}O <--> Fe_{3}O_{4} + 8H,
+
+    Zn + H_{2}O <--> ZnO + 2H,
+
+the equilibrium will be reached when very different quantities of the
+iron and zinc have been changed into oxides. The individual chemical
+properties of the iron and zinc have therefore marked influence upon the
+point at which equilibrium will be reached.
+
+2. _Influence of relative mass._ If the tube in which the reaction
+
+    3Fe + 4H_{2}O <--> Fe_{3}O_{4} + 8H
+
+has come to an equilibrium is opened and more steam is admitted, an
+additional quantity of the iron will be changed into iron oxide. If more
+hydrogen is admitted, some of the oxide will be reduced to metal. The
+point of equilibrium is therefore dependent upon the relative masses of
+the substances taking part in the reaction. When one of the substances
+is a solid, however, its mass has little influence, since it is only the
+extent of its surface which can affect the reaction.
+
+~Conditions under which reversible reactions are complete.~ If, when the
+equilibrium between iron and steam has been reached, the tube is opened
+and a current of steam is passed in, the hydrogen is swept away as fast
+as it is formed. The opposing reaction of hydrogen upon iron oxide must
+therefore cease, and the action of steam on the iron will go on until
+all of the iron has been transformed into iron oxide.
+
+On the other hand, if a current of hydrogen is admitted into the tube,
+the steam will be swept away by the hydrogen, and all of the iron oxide
+will be reduced to iron. _A reversible reaction can therefore be
+completed in either direction when one of the products of the reaction
+is removed as fast as it is formed._
+
+~Equilibrium in solution.~ When reactions take place in solution in water
+the same general principles hold good. The matter is not so simple,
+however, as in the case just described, owing to the fact that many of
+the reactions in solution are due to the presence of ions. The
+substances most commonly employed in solution are acids, bases, or
+salts, and all of these undergo dissociation. Any equilibrium which may
+be reached in solutions of these substances must take place between the
+various ions formed, on the one hand, and the undissociated molecules,
+on the other. Thus, when nitric acid is dissolved in water, equilibrium
+is reached in accordance with the equation
+
+    H^{+} + NO_{3}^{-} <--> HNO_{3}.
+
+~Conditions under which reversible reactions in solution are complete.~
+The equilibrium between substances in solution may be disturbed and the
+reaction caused to go on in one direction to completion in either of
+three ways.
+
+1. _A gas may be formed which escapes from the solution._ When sodium
+nitrate and sulphuric acid are brought together in solution all four
+ions, Na^{+}, NO_{3}^{-}, H^{+}, SO_{4}^{--}, are formed. These ions are
+free to rearrange themselves in various combinations. For example, the
+H^{+} and the NO_{3}^{-} ions will reach the equilibrium
+
+    H^{+} + NO_{3}^{-} <--> HNO_{3}.
+
+If the experiment is performed with very little water present, as is the
+case in the preparation of nitric acid, the equilibrium will be reached
+when most of the H^{+} and the NO_{3}^{-} ions have combined to form
+undissociated HNO_{3}.
+
+Finally, if the mixture is now heated above the boiling point of nitric
+acid, the acid distills away as fast as it is formed. More and more
+H^{+} and NO_{3}^{-} ions will then combine, and the process will
+continue until one or the other of them has all been removed from the
+solution. The substance remaining is sodium acid sulphate (NaHSO_{4}),
+and the reaction can therefore be expressed by the equation
+
+    NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}.
+
+2. _An insoluble solid may be formed._ When hydrochloric acid (HCl) and
+silver nitrate (AgNO_{3}) are brought together in solution the
+following ions will be present: H^{+}, Cl^{-}, Ag^{+}, NO_{3}^{-}. The
+ions Ag^{+} and Cl^{-} will then set up the equilibrium
+
+    Ag^{+} + Cl^{-} <--> AgCl.
+
+But silver chloride (AgCl) is almost completely insoluble in water, and
+as soon as a very little of it has formed the solution becomes
+supersaturated, and the excess of the salt precipitates. More silver and
+chlorine ions then unite, and this continues until practically all of
+the silver or the chlorine ions have been removed from the solution. We
+then say that the following reaction is complete:
+
+    AgNO_{3} + HCl = AgCl + HNO_{3}.
+
+3. _Two different ions may form undissociated molecules._ In the
+neutralization of sodium hydroxide by hydrochloric acid the ions H^{+}
+and OH^{-} come to the equilibrium
+
+    H^{+} + OH^{-} <--> H_{2}O.
+
+But since water is almost entirely undissociated, equilibrium can only
+be reached when there are very few hydroxyl or hydrogen ions present.
+Consequently the two ions keep uniting until one or the other of them is
+practically removed from the solution. When this occurs the
+neutralization expressed in the following equation is complete:
+
+    NaOH + HCl = H_{2}O + NaCl.
+
+~Preparation of acids.~ The principle of reversible reactions finds
+practical application in the preparation of most of the common acids. An
+acid is usually prepared by treating the most common of its salts with
+some other acid of high boiling point. The mixture is then heated until
+the lower boiling acid desired distills out. Owing to its high boiling
+point (338 deg.), sulphuric acid is usually employed for this purpose, most
+other acids boiling below that temperature.
+
+
+EXERCISES
+
+1. What would take place when solutions of silver nitrate and sodium
+chloride are brought together? What other chlorides would act in the
+same way?
+
+2. Is the reaction expressed by the equation NH_{3} + H_{2}O = NH_{4}OH
+reversible? If so, state the conditions under which it will go in each
+direction.
+
+3. Is the reaction expressed by the equation 2H + O = H_{2}O reversible?
+If so, state the conditions under which it will go in each direction.
+
+4. Suggest a method for the preparation of hydrochloric acid.
+
+
+
+
+CHAPTER XIV
+
+SULPHUR AND ITS COMPOUNDS
+
+
+~Occurrence.~ The element sulphur has been known from the earliest times,
+since it is widely distributed in nature and occurs in large quantities
+in the uncombined form, especially in the neighborhood of volcanoes.
+Sicily has long been famous for its sulphur mines, and smaller deposits
+are found in Italy, Iceland, Mexico, and especially in Louisiana, where
+it is mined extensively. In combination, sulphur occurs abundantly in
+the form of sulphides and sulphates. In smaller amounts it is found in a
+great variety of minerals, and it is a constituent of many animal and
+vegetable substances.
+
+~Extraction of sulphur.~ Sulphur is prepared from the native substance,
+the separation of crude sulphur from the rock and earthy materials with
+which it is mixed being a very simple process. The ore from the mines is
+merely heated until the sulphur melts and drains away from the earthy
+impurities. The crude sulphur obtained in this way is distilled in a
+retort-shaped vessel made of iron, the exit tube of which opens into a
+cooling chamber of brickwork. When the sulphur vapor first enters the
+cooling chamber it condenses as a fine crystalline powder called
+_flowers of sulphur_. As the condensing chamber becomes warm, the
+sulphur collects as a liquid in it, and is drawn off into cylindrical
+molds, the product being called _roll sulphur_ or _brimstone_.
+
+~Physical properties.~ Roll sulphur is a pale yellow, crystalline solid,
+without marked taste and with but a faint odor. It is insoluble in
+water, but is freely soluble in a few liquids, notably in carbon
+disulphide. Roll sulphur melts at 114.8 deg.. Just above the melting point
+it forms a rather thin, straw-colored liquid. As the temperature is
+raised, this liquid turns darker in color and becomes thicker, until at
+about 235 deg. it is almost black and is so thick that the vessel containing
+it can be inverted without danger of the liquid running out. At higher
+temperatures it becomes thin once more, and boils at 448 deg., forming a
+yellowish vapor. On cooling the same changes take place in reverse
+order.
+
+~Varieties of sulphur.~ Sulphur is known in two general forms, crystalline
+and amorphous. Each of these forms exists in definite modifications.
+
+~Crystalline sulphur.~ Sulphur occurs in two crystalline forms, namely,
+rhombic sulphur and monoclinic sulphur.
+
+1. _Rhombic sulphur._ When sulphur crystallizes from its solution in
+carbon disulphide it separates in crystals which have the same color and
+melting point as roll sulphur, and are rhombic in shape. Roll sulphur is
+made up of minute rhombic crystals.
+
+2. _Monoclinic sulphur._ When melted sulphur is allowed to cool until a
+part of the liquid has solidified, and the remaining liquid is then
+poured off, it is found that the solid sulphur remaining in the vessel
+has assumed the form of fine needle-shaped crystals. These differ much
+in appearance from the rhombic crystals obtained by crystallizing
+sulphur from its solution in carbon disulphide. The needle-shaped form
+is called _monoclinic sulphur_. The two varieties differ also in density
+and in melting point, the monoclinic sulphur melting at 120 deg..
+
+Monoclinic and rhombic sulphur remain unchanged in contact with each
+other at 96 deg.. Above this temperature the rhombic changes into
+monoclinic; at lower temperatures the monoclinic changes into rhombic.
+The temperature 96 deg. is therefore called the transition point of sulphur.
+Heat is set free when monoclinic sulphur changes into rhombic.
+
+~Amorphous sulphur.~ Two varieties of amorphous sulphur can be readily
+obtained. These are white sulphur and plastic sulphur.
+
+1. _White sulphur._ Flowers of sulphur, the preparation of which has
+been described, consists of a mixture of rhombic crystals and amorphous
+particles. When treated with carbon disulphide, the crystals dissolve,
+leaving the amorphous particles as a white residue.
+
+2. _Plastic sulphur._ When boiling sulphur is poured into cold water it
+assumes a gummy, doughlike form, which is quite elastic. This can be
+seen in a very striking manner by distilling sulphur from a small,
+short-necked retort, such as is represented in Fig. 40, and allowing the
+liquid to run directly into water. In a few days it becomes quite
+brittle and passes over into ordinary rhombic sulphur.
+
+[Illustration Fig. 40]
+
+~Chemical properties of sulphur.~ When sulphur is heated to its kindling
+temperature in oxygen or in the air it burns with a pale blue flame,
+forming sulphur dioxide (SO_{2}). Small quantities of sulphur trioxide
+(SO_{3}) may also be formed in the combustion of sulphur. Most metals
+when heated with sulphur combine directly with it, forming metallic
+sulphides. In some cases the action is so energetic that the mass
+becomes incandescent, as has been seen in the case of iron uniting with
+sulphur. This property recalls the action of oxygen upon metals, and in
+general the metals which combine readily with oxygen are apt to combine
+quite readily with sulphur.
+
+~Uses of sulphur.~ Large quantities of sulphur are used as a germicide in
+vineyards, also in the manufacture of gunpowder, matches, vulcanized
+rubber, and sulphuric acid.
+
+
+COMPOUNDS OF SULPHUR WITH HYDROGEN
+
+~Hydrosulphuric acid~ (H_{2}S). This substance is a gas having the
+composition expressed by the formula H_{2}S and is commonly called
+hydrogen sulphide. It is found in the vapors issuing from volcanoes, and
+in solution in the so-called sulphur waters of many springs. It is
+formed when organic matter containing sulphur undergoes decay, just as
+ammonia is formed under similar circumstances from nitrogenous matter.
+
+~Preparation.~ Hydrosulphuric acid is prepared in the laboratory by
+treating a sulphide with an acid. Iron sulphide (FeS) is usually
+employed:
+
+    FeS + 2HCl = FeCl_{2} + H_{2}S.
+
+A convenient apparatus is shown in Fig. 41. A few lumps of iron sulphide
+are placed in the bottle A, and dilute acid is added in small
+quantities at a time through the funnel tube B, the gas escaping
+through the tube C.
+
+[Illustration: Fig. 41]
+
+     ~Explanation of the reaction.~ Iron sulphide is a salt of
+     hydrosulphuric acid, and this reaction is therefore similar to
+     the one which takes place when sulphuric acid acts upon a
+     nitrate. In both cases a salt and an acid are brought together,
+     and there is a tendency for the reaction to go on until a state
+     of equilibrium is reached. This equilibrium is constantly
+     disturbed by the escape of the gaseous acid set free, so that
+     the reaction goes on until all of the original salt has been
+     decomposed. The two reactions differ in that the first one is
+     complete at ordinary temperatures, while in the case of
+     sulphuric acid acting upon sodium nitrate, the reacting
+     substances must be heated so as to secure a temperature at
+     which nitric acid is a gas.
+
+~Physical properties.~ Hydrosulphuric acid is a colorless gas, having a
+weak, disagreeable taste and an exceedingly offensive odor. It is rather
+sparingly soluble in water at ordinary temperatures, about three volumes
+dissolving in one of water. In boiling water it is not soluble at all.
+In pure form it acts as a violent poison, and even when diluted largely
+with air produces headache, dizziness, and nausea. It is a little
+heavier than air, having a density of 1.18.
+
+~Chemical properties.~ The most important chemical properties of
+hydrosulphuric acid are the following:
+
+1. _Acid properties._ Hydrosulphuric acid is a weak acid. In solution in
+water it turns blue litmus red and neutralizes bases, forming salts
+called _sulphides_.
+
+2. _Action on oxygen._ The elements composing hydrosulphuric acid have
+each a strong affinity for oxygen, and are not held together very
+firmly. Consequently the gas burns readily in oxygen or the air,
+according to the equation
+
+    H_{2}S + 3O = H_{2}O + SO_{2}.
+
+When there is not enough oxygen for both the sulphur and the hydrogen,
+the latter element combines with the oxygen and the sulphur is set free:
+
+    H_{2}S + O = H_{2}O + S.
+
+3. _Reducing action._ Owing to the ease with which hydrosulphuric acid
+decomposes and the strong affinity of both sulphur and hydrogen for
+oxygen, the substance is a strong reducing agent, taking oxygen away
+from many substances which contain it.
+
+4. _Action on metals._ Hydrosulphuric acid acts towards metals in a way
+very similar to water. Thus, when it is passed over heated iron in a
+tube, the reaction is represented by the equation
+
+    3Fe + 4H_{2}S = Fe_{3}S_{4} + 8H.
+
+Water in the form of steam, under similar circumstances, acts according
+to the equation
+
+    3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H.
+
+~Salts of hydrosulphuric acid,--sulphides.~ The salts of hydrosulphuric
+acid, called sulphides, form an important class of salts. Many of them
+are found abundantly in nature, and some of them are important ores.
+They will be frequently mentioned in connection with the metals.
+
+Most of the sulphides are insoluble in water, and some of them are
+insoluble in acids. Consequently, when hydrosulphuric acid is passed
+into a solution of a salt, it often happens that a sulphide is
+precipitated. With copper chloride the equation is
+
+    CuCl_{2} + H_{2}S = CuS + 2HCl.
+
+Because of the fact that some metals are precipitated in this way as
+sulphides while others are not, hydrosulphuric acid is extensively used
+in the separation of the metals in the laboratory.
+
+     ~Explanation of the reaction.~ When hydrosulphuric acid and
+     copper chloride are brought together in solution, both copper
+     and sulphur ions are present, and these will come to an
+     equilibrium, as represented in the equation
+
+         Cu^{+} + S^{-} <--> CuS.
+
+     Since copper sulphide is almost insoluble in water, as soon as
+     a very small quantity has formed the solution becomes
+     supersaturated, and the excess keeps precipitating until nearly
+     all the copper or sulphur ions have been removed from the
+     solution. With some other ions, such as iron, the sulphide
+     formed does not saturate the solution, and no precipitate
+     results.
+
+
+OXIDES OF SULPHUR
+
+Sulphur forms two well-known compounds with oxygen: sulphur dioxide
+(SO_{2}), sometimes called sulphurous anhydride; and sulphur trioxide
+(SO_{3}), frequently called sulphuric anhydride.
+
+~Sulphur dioxide~ (SO_{2}). Sulphur dioxide occurs in nature in the gases
+issuing from volcanoes, and in solution in the water of many springs. It
+is likely to be found wherever sulphur compounds are undergoing
+oxidation.
+
+~Preparation.~ Three general ways may be mentioned for the preparation of
+sulphur dioxide:
+
+1. _By the combustion of sulphur._ Sulphur dioxide is readily formed by
+the combustion of sulphur in oxygen or the air:
+
+    S + 2O = SO_{2}.
+
+It is also formed when substances containing sulphur are burned:
+
+    ZnS + 3O = ZnO + SO_{2}.
+
+2. _By the reduction of sulphuric acid._ When concentrated sulphuric
+acid is heated with certain metals, such as copper, part of the acid is
+changed into copper sulphate, and part is reduced to sulphurous acid.
+The latter then decomposes into sulphur dioxide and water, the complete
+equation being
+
+    Cu + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O.
+
+3. _By the action of an acid on a sulphite._ Sulphites are salts of
+sulphurous acid (H_{2}SO_{3}). When a sulphite is treated with an acid,
+sulphurous acid is set free, and being very unstable, decomposes into
+water and sulphur dioxide. These reactions are expressed in the
+equations
+
+    Na_{2}SO_{3} + 2HCl = 2NaCl + H_{2}SO_{3},
+
+    H_{2}SO_{3} = H_{2}O + SO_{2}.
+
+~Explanation of the reaction.~ In this case we have two reversible
+reactions depending on each other. In the first reaction,
+
+    (1) Na_{2}SO_{3} + 2HCl <--> 2NaCl + H_{2}SO_{3},
+
+we should expect an equilibrium to result, for none of the four
+substances in the equation are insoluble or volatile when water is
+present to hold them in solution. But the quantity of the H_{2}SO_{3} is
+constantly diminishing, owing to the fact that it decomposes, as
+represented in the equation
+
+    (2) H_{2}SO_{3} <--> H_{2}O + SO_{2},
+
+and the sulphur dioxide, being a gas, escapes. No equilibrium can
+therefore result, since the quantity of the sulphurous acid is
+constantly being diminished because of the escape of sulphur dioxide.
+
+~Physical properties.~ Sulphur dioxide is a colorless gas, which at
+ordinary temperatures is 2.2 times as heavy as air. It has a peculiar,
+irritating odor. The gas is very soluble in water, one volume of water
+dissolving eighty of the gas under standard conditions. It is easily
+condensed to a colorless liquid, and can be purchased in this condition
+stored in strong bottles, such as the one represented in Fig. 42.
+
+[Illustration: Fig. 42]
+
+~Chemical properties.~ Sulphur dioxide has a marked tendency to combine
+with other substances, and is therefore an active substance chemically.
+It combines with oxygen gas, but not very easily. It can, however, take
+oxygen away from some other substances, and is therefore a good reducing
+agent. Its most marked chemical property is its ability to combine with
+water to form sulphurous acid (H_{2}SO_{3}).
+
+~Sulphurous acid~ (H_{2}SO_{3}). When sulphur dioxide dissolves in water
+it combines chemically with it to form sulphurous acid, an unstable
+substance having the formula H_{3}SO_{3}. It is impossible to prepare
+this acid in pure form, as it breaks down very easily into water and
+sulphur dioxide. The reaction is therefore reversible, and is expressed
+by the equation
+
+    H_{2}O + SO_{2} <--> H_{2}SO_{3}.
+
+Solutions of the acid in water have a number of interesting properties.
+
+1. _Acid properties._ The solution has all the properties typical of an
+acid. When neutralized by bases, sulphurous acid yields a series of
+salts called _sulphites_.
+
+2. _Reducing properties._ Solutions of sulphurous acid act as good
+reducing agents. This is due to the fact that sulphurous acid has the
+power of taking up oxygen from the air, or from substances rich in
+oxygen, and is changed by this reaction into sulphuric acid:
+
+    H_{2}SO_{3} + O = H_{2}SO_{4},
+
+    H_{2}SO_{3} + H_{2}O_{2} = H_{2}S0_{4} + H_{2}O.
+
+3. _Bleaching properties._ Sulphurous acid has strong bleaching
+properties, acting upon many colored substances in such a way as to
+destroy their color. It is on this account used to bleach paper, straw
+goods, and even such foods as canned corn.
+
+4. _Antiseptic properties._ Sulphurous acid has marked antiseptic
+properties, and on this account has the power of arresting
+fermentation. It is therefore used as a preservative.
+
+~Salts of sulphurous acid,--sulphites.~ The sulphites, like sulphurous
+acid, have the power of taking up oxygen very readily, and are good
+reducing agents. On account of this tendency, commercial sulphites are
+often contaminated with sulphates. A great deal of sodium sulphite is
+used in the bleaching industry, and as a reagent for softening paper
+pulp.
+
+~Sulphur trioxide~ (SO_{3}). When sulphur dioxide and oxygen are heated
+together at a rather high temperature, a small amount of sulphur
+trioxide (SO_{3}) is formed, but the reaction is slow and incomplete.
+If, however, the heating takes place in the presence of very fine
+platinum dust, the reaction is rapid and nearly complete.
+
+[Illustration: Fig. 43]
+
+     ~ Experimental preparation of sulphur trioxide.~ The experiment
+     can be performed by the use of the apparatus shown in Fig. 43,
+     the fine platinum being secured by moistening asbestos fiber
+     with a solution of platinum chloride and igniting it in a
+     flame. The fiber, covered with fine platinum, is placed in a
+     tube of hard glass, which is then heated with a burner to about
+     350 deg., while sulphur dioxide and air are passed into the tube.
+     Union takes place at once, and the strongly fuming sulphur
+     trioxide escapes from the jet at the end of the tube, and may
+     be condensed by surrounding the receiving tube with a freezing
+     mixture.
+
+~Properties of sulphur trioxide.~ Sulphur trioxide is a colorless liquid,
+which solidifies at about 15 deg. and boils at 46 deg.. A trace of moisture
+causes it to solidify into a mass of silky white crystals, somewhat
+resembling asbestos fiber in appearance. In contact with the air it
+fumes strongly, and when thrown upon water it dissolves with a hissing
+sound and the liberation of a great deal of heat. The product of this
+reaction is sulphuric acid, so that sulphur trioxide is the anhydride of
+that acid:
+
+    SO_{3} + H_{2}O = H_{2}SO_{4}.
+
+~Catalysis.~ It has been found that many chemical reactions, such as the
+union of sulphur dioxide with oxygen, are much influenced by the
+presence of substances which do not themselves seem to take a part in
+the reaction, and are left apparently unchanged after it has ceased.
+These reactions go on very slowly under ordinary circumstances, but are
+greatly hastened by the presence of the foreign substance. Substances
+which hasten very slow reactions in this way are said to act as
+catalytic agents or _catalyzers_, and the action is called _catalysis_.
+Just how the action is brought about is not well understood.
+
+DEFINITION: _A catalyzer is a substance which changes the velocity of a
+reaction, but does not change its products._
+
+~Examples of Catalysis.~ We have already had several instances of such
+action. Oxygen and hydrogen combine with each other at ordinary
+temperatures in the presence of platinum powder, while if no catalytic
+agent is present they do not combine in appreciable quantities until a
+rather high temperature is reached. Potassium chlorate, when heated with
+manganese dioxide, gives up its oxygen at a much lower temperature than
+when heated alone. Hydrogen dioxide decomposes very rapidly when
+powdered manganese dioxide is sifted into its concentrated solution.
+
+On the other hand, the catalytic agent sometimes retards chemical
+action. For example, a solution of hydrogen dioxide decomposes more
+slowly when it contains a little phosphoric acid than when perfectly
+pure. For this reason commercial hydrogen dioxide always contains
+phosphoric acid.
+
+Many reactions are brought about by the catalytic action of traces of
+water. For example, phosphorus will not burn in oxygen in the absence of
+all moisture. Hydrochloric acid will not unite with ammonia if the
+reagents are perfectly dry. It is probable that many of the chemical
+transformations in physiological processes, such as digestion, are
+assisted by certain substances acting as catalytic agents. The principle
+of catalysis is therefore very important.
+
+~Sulphuric acid~ (_oil of vitriol_) (H_{2}SO_{4}). Sulphuric acid is one
+of the most important of all manufactured chemicals. Not only is it one
+of the most common reagents in the laboratory, but enormous quantities
+of it are used in many of the industries, especially in the refining of
+petroleum, the manufacture of nitroglycerin, sodium carbonate, and
+fertilizers.
+
+~Manufacture of sulphuric acid.~ 1. _Contact process_. The reactions
+taking place in this process are represented by the following equations:
+
+    SO_{2} + O = SO_{3},
+
+    SO_{3} + H_{2}O = H_{2}SO_{4}.
+
+To bring about the first of these reactions rapidly, a catalyzer is
+employed, and the process is carried out in the following way: Large
+iron tubes are packed with some porous material, such as calcium and
+magnesium sulphates, which contains a suitable catalytic substance
+scattered through it. The catalyzers most used are platinum powder,
+vanadium oxide, and iron oxide. Purified sulphur dioxide and air are
+passed through the tubes, which are kept at a temperature of about 350 deg..
+Sulphur trioxide is formed, and as it issues from the tube it is
+absorbed in water or dilute sulphuric acid. The process is continued
+until all the water in the absorbing vessel has been changed into
+sulphuric acid, so that a very concentrated acid is made in this way. An
+excess of the trioxide may dissolve in the strong sulphuric acid,
+forming what is known as _fuming sulphuric acid_.
+
+2. _Chamber process._ The method of manufacture exclusively employed
+until recent years, and still in very extensive use, is much more
+complicated. The reactions are quite involved, but the conversion of
+water, sulphur dioxide, and oxygen into sulphuric acid is accomplished
+by the catalytic action of oxides of nitrogen. The reactions are brought
+about in large lead-lined chambers, into which oxides of nitrogen,
+sulphur dioxide, steam, and air are introduced in suitable proportions.
+
+     ~Reactions of the chamber process.~ In a very general way, the
+     various reactions which take place in the lead chambers may be
+     expressed in two equations. In the first reaction sulphur
+     dioxide, nitrogen peroxide, steam, and oxygen unite, as shown
+     in the equation
+
+    (1) 2SO_{2} + 2NO_{2} + H_{2}O + O = 2SO_{2} (OH) (NO_{2}).
+
+     The product formed in this reaction is called nitrosulphuric
+     acid or "chamber crystals." It actually separates on the walls
+     of the chambers when the process is not working properly. Under
+     normal conditions, it is decomposed as fast as it is formed by
+     the action of excess of steam, as shown in the equation
+
+    (2) 2SO_{2} (OH) (NO_{2}) + H_{2}O + O = 2H_{2}SO_{4} + 2NO_{2}.
+
+     The nitrogen dioxide formed in this reaction can now enter into
+     combination with a new quantity of sulphur dioxide, steam, and
+     oxygen, and the series of reactions go on indefinitely. Many
+     other reactions occur, but these two illustrate the principle
+     of the process.
+
+The relation between sulphuric acid and nitrosulphuric acid can be seen
+by comparing their structural formulas:
+
+    O=  -OH      O=  -OH
+       S            S
+    O=  -OH      O=  -NO_{2}
+
+The latter may be regarded as derived from the former by the
+substitution of the nitro group (NO_{2}) for the hydroxyl group (OH).
+
+[Illustration: Fig. 44]
+
+~The sulphuric acid plant.~ Fig. 44 illustrates the simpler parts of a
+plant used in the manufacture of sulphuric acid by the chamber process.
+Sulphur or some sulphide, as FeS_{2}, is burned in furnace A. The
+resulting sulphur dioxide, together with air and some nitrogen peroxide,
+are conducted into the large chambers, the capacity of each chamber
+being about 75,000 cu. ft. Steam is also admitted into these chambers at
+different points. These compounds react to form sulphuric acid,
+according to the equations given above. The nitrogen left after the
+withdrawal of the oxygen from the admitted air escapes through the
+Gay-Lussac tower X. In order to prevent the escape of the oxides of
+nitrogen regenerated in the reaction, the tower is filled with lumps of
+coke, over which trickles concentrated sulphuric acid admitted from Y.
+The nitrogen peroxide dissolves in the acid and the resulting solution
+collects in H. This is pumped into E, where it is mixed with dilute
+acid and allowed to trickle down through the chamber D (Glover tower),
+which is filled with some acid-resisting rock. Here the nitrogen
+peroxide is expelled from the solution by the action of the hot gases
+entering from A, and together with them enters the first chamber
+again. The acid from which the nitrogen peroxide is expelled collects in
+F. Theoretically, a small amount of nitrogen peroxide would suffice to
+prepare an unlimited amount of sulphuric acid; practically, some of it
+escapes, and this is replaced by small amounts admitted at B.
+
+The sulphuric acid so formed, together with the excess of condensed
+steam, collect upon the floor of the chambers in the form of a liquid
+containing from 62% to 70% of sulphuric acid. The product is called
+_chamber acid_ and is quite impure; but for many purposes, such as the
+manufacture of fertilizers, it needs no further treatment. It can be
+concentrated by boiling it in vessels made of iron or platinum, which
+resist the action of the acid, nearly all the water boiling off. Pure
+concentrated acid can be made best by the contact process, while the
+chamber process is cheaper for the dilute impure acid.
+
+~Physical properties.~ Sulphuric acid is a colorless, oily liquid, nearly
+twice as heavy as water. The ordinary concentrated acid contains about
+2% of water, has a density of 1.84, and boils at 338 deg.. It is sometimes
+called _oil of vitriol_, since it was formerly made by distilling a
+substance called _green vitriol_.
+
+~Chemical properties.~ Sulphuric acid possesses chemical properties which
+make it one of the most important of chemical substances.
+
+1. _Action as an acid._ In dilute solution sulphuric acid acts as any
+other acid, forming salts with oxides and hydroxides.
+
+2. _Action as an oxidizing agent._ Sulphuric acid contains a large
+percentage of oxygen and is, like nitric acid, a very good oxidizing
+agent. When the concentrated acid is heated with sulphur, carbon, and
+many other substances, oxidation takes place, the sulphuric acid
+decomposing according to the equation
+
+    H_{2}SO_{4} = H_{2}SO_{3} + O.
+
+3. _Action on metals._ In dilute solution sulphuric acid acts upon many
+metals, such as zinc, forming a sulphate and liberating hydrogen. When
+the concentrated acid is employed the hydrogen set free is oxidized by a
+new portion of the acid, with the liberation of sulphur dioxide. With
+copper the reactions are expressed by the equations
+
+    (1) Cu + H_{2}SO_{4} = CuSO_{4} + 2H,
+
+    (2) H_{2}SO_{4} + 2H = H_{2}SO_{3} + H_{2}O,
+
+    (3) H_{2}SO_{3} = H_{2}O + SO_{2}.
+
+By combining these equations the following one is obtained:
+
+    Cu + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O.
+
+4. _Action on salts._ We have repeatedly seen that an acid of high
+boiling point heated with the salt of some acid of lower boiling point
+will drive out the low boiling acid. The boiling point of sulphuric acid
+(338 deg.) is higher than that of almost any common acid; hence it is used
+largely in the preparation of other acids.
+
+5. _Action on water._ Concentrated sulphuric acid has a very great
+affinity for water, and is therefore an effective dehydrating agent.
+Gases which have no chemical action upon sulphuric acid can be freed
+from water vapor by bubbling them through the strong acid. When the acid
+is diluted with water much heat is set free, and care must be taken to
+keep the liquid thoroughly stirred during the mixing, and to pour the
+acid into the water,--never the reverse.
+
+Not only can sulphuric acid absorb water, but it will often withdraw the
+elements hydrogen and oxygen from a compound containing them,
+decomposing the compound, and combining with the water so formed. For
+this reason most organic substances, such as sugar, wood, cotton, and
+woolen fiber, and even flesh, all of which contain much oxygen and
+hydrogen in addition to carbon, are charred or burned by the action of
+the concentrated acid.
+
+~Salts of sulphuric acid,--sulphates.~ The sulphates form a very important
+class of salts, and many of them have commercial uses. Copperas (iron
+sulphate), blue vitriol (copper sulphate), and Epsom salt (magnesium
+sulphate) serve as examples. Many sulphates are important minerals,
+prominent among these being gypsum (calcium sulphate) and barytes
+(barium sulphate).
+
+     ~Thiosulphuric acid~ (H_{2}S_{2}O_{3}); ~Thiosulphates.~ Many other
+     acids of sulphur containing oxygen are known, but none of them
+     are of great importance. Most of them cannot be prepared in a
+     pure state, and are known only through their salts. The most
+     important of these is thiosulphuric acid.
+
+     When sodium sulphite is boiled with sulphur the two substances
+     combine, forming a salt which has the composition represented
+     in the formula Na_{2}S_{2}O_{3}:
+
+     Na_{2}SO_{3} + S = Na_{2}S_{2}O_{3}.
+
+     The substance is called sodium thiosulphate, and is a salt of
+     the easily decomposed acid H_{2}S_{2}O_{3}, called
+     thiosulphuric acid. This reaction is quite similar to the
+     action of oxygen upon sulphites:
+
+     Na_{2}SO_{3} + O = Na_{2}SO_{4}.
+
+     More commonly the salt is called sodium hyposulphite, or merely
+     "hypo." It is a white solid and is extensively used in
+     photography, in the bleaching industry, and as a disinfectant.
+
+~Monobasic and dibasic acids.~ Such acids as hydrochloric and nitric
+acids, which have only one replaceable hydrogen atom in the molecule, or
+in other words yield one hydrogen ion in solution, are called monobasic
+acids. Acids yielding two hydrogen ions in solution are called dibasic
+acids. Similarly, we may have tribasic and tetrabasic acids. The three
+acids of sulphur are dibasic acids. It is therefore possible for each of
+them to form both normal and acid salts. The acid salts can be made in
+two ways: the acid may be treated with only half enough base to
+neutralize it,--
+
+    NaOH + H_{2}SO_{4} = NaHSO_{4} + H_{2}O;
+
+or a normal salt may be treated with the free acid,--
+
+    Na_{2}SO_{4} + H_{2}SO_{4} = 2NaHSO_{4}.
+
+Acid sulphites and sulphides may be made in the same ways.
+
+~Carbon disulphide~ (CS_{2}). When sulphur vapor is passed over highly
+heated carbon the two elements combine, forming carbon disulphide
+(CS_{2}), just as oxygen and carbon unite to form carbon dioxide
+(CO_{2}). The substance is a heavy, colorless liquid, possessing, when
+pure, a pleasant ethereal odor. On standing for some time, especially
+when exposed to sunlight, it undergoes a slight decomposition and
+acquires a most disagreeable, rancid odor. It has the property of
+dissolving many substances, such as gums, resins, and waxes, which are
+insoluble in most liquids, and it is extensively used as a solvent for
+such substances. It is also used as an insecticide. It boils at a low
+temperature (46 deg.), and its vapor is very inflammable, burning in the air
+to form carbon dioxide and sulphur dioxide, according to the equation
+
+    CS_{2} + 6O = CO_{2} + 2SO_{2}.
+
+[Illustration: Fig. 45]
+
+     ~Commercial preparation of carbon disulphide.~ In the preparation
+     of carbon disulphide an electrical furnace is employed, such as
+     is represented in Fig. 45. The furnace is packed with carbon
+     C, and this is fed in through the hoppers B, as fast as
+     that which is present in the hearth of the furnace is used up.
+     Sulphur is introduced at A, and at the lower ends of the
+     tubes it is melted by the heat of the furnace and flows into
+     the hearth as a liquid. An electrical current is passed through
+     the carbon and melted sulphur from the electrodes E, heating
+     the charge. The vapors of carbon disulphide pass up through the
+     furnace and escape at D, from which they pass to a suitable
+     condensing apparatus.
+
+~Comparison of sulphur and oxygen.~ A comparison of the formulas and the
+chemical properties of corresponding compounds of oxygen and sulphur
+brings to light many striking similarities. The conduct of
+hydrosulphuric acid and water toward many substances has been seen to be
+very similar; the oxides and sulphides of the metals have analogous
+formulas and undergo many parallel reactions. Carbon dioxide and
+disulphide are prepared in similar ways and undergo many analogous
+reactions. It is clear, therefore, that these two elements are far more
+closely related to each other than to any of the other elements so far
+studied.
+
+~Selenium and tellurium.~ These two very uncommon elements are still more
+closely related to sulphur than is oxygen. They occur in comparatively
+small quantities and are usually found associated with sulphur and
+sulphides, either as the free elements or more commonly in combination
+with metals. They form compounds with hydrogen of the formulas H_{2}Se
+and H_{2}Te; these bodies are gases with properties very similar to
+those of H_{2}S. They also form oxides and oxygen acids which resemble
+the corresponding sulphur compounds. The elements even have allotropic
+forms corresponding very closely to those of sulphur. Tellurium is
+sometimes found in combination with gold and copper, and occasions some
+difficulties in the refining of these metals. The elements have very few
+practical applications.
+
+~Crystallography.~ In order to understand the difference between the two
+kinds of sulphur crystals, it is necessary to know something about
+crystals in general and the forms which they may assume. An examination
+of a large number of crystals has shown that although they may differ
+much in geometric form, they can all be considered as modifications of a
+few simple plans. The best way to understand the relation of one crystal
+to another is to look upon every crystal as having its faces and angles
+arranged in definite fashion about certain imaginary lines drawn
+through the crystal. These lines are called axes, and bear much the same
+relation to a crystal as do the axis and parallels of latitude and
+longitude to the earth and a geographical study of it. All crystals can
+be referred to one of six simple plans or systems, which have their axes
+as shown in the following drawings.
+
+The names and characteristics of these systems are as follows:
+
+1. Isometric or regular system (Fig. 46). Three equal axes, all at right
+angles.
+
+[Illustration: Fig. 46]
+
+2. Tetragonal system (Fig. 47). Two equal axes and one of different
+length, all at right angles to each other.
+
+[Illustration: Fig. 47]
+
+3. Orthorhombic system (Fig. 48). Three unequal axes, all at right
+angles to each other.
+
+[Illustration: Fig. 48]
+
+4. Monoclinic system (Fig. 49). Two axes at right angles, and a third at
+right angles to one of these, but inclined to the other.
+
+[Illustration: Fig. 49]
+
+5. Triclinic system (Fig. 50). Three axes, all inclined to each other.
+
+[Illustration: Fig. 50]
+
+6. Hexagonal system (Fig. 51). Three equal axes in the same plane
+intersecting at angles of 60 deg., and a fourth at right angles to all of
+these.
+
+[Illustration: Fig. 51]
+
+Every crystal can be imagined to have its faces and angles arranged in a
+definite way around one of these systems of axes. A cube, for instance,
+is referred to Plan 1, an axis ending in the center of each face; while
+in a regular octohedron an axis ends in each solid angle. These forms
+are shown in Fig. 46. It will be seen that both of these figures belong
+to the same system, though they are very different in appearance. In the
+same way, many geometric forms may be derived from each of the systems,
+and the light lines about the axes in the drawings show two of the
+simplest forms of each of the systems.
+
+In general a given substance always crystallizes in the same system, and
+two corresponding faces of each crystal of it always make the same angle
+with each other. A few substances, of which sulphur is an example,
+crystallize in two different systems, and the crystals differ in such
+physical properties as melting point and density. Such substances are
+said to be _dimorphous_.
+
+
+EXERCISES
+
+1. (a) Would the same amount of heat be generated by the combustion of
+1 g. of each of the allotropic modifications of sulphur? (b) Would the
+same amount of sulphur dioxide be formed in each case?
+
+2. Is the equation for the preparation of hydrosulphuric acid a
+reversible one? As ordinarily carried out, does the reaction complete
+itself?
+
+3. Suppose that hydrosulphuric acid were a liquid, would it be necessary
+to modify the method of preparation?
+
+4. Can sulphuric acid be used to dry hydrosulphuric acid? Give reason
+for answer.
+
+5. Does dry hydrosulphuric acid react with litmus paper? State reason
+for answer.
+
+6. How many grams of iron sulphide are necessary to prepare 100 l. of
+hydrosulphuric acid when the laboratory conditions are 17 deg. and 740 mm.
+pressure?
+
+7. Suppose that the hydrogen in 1 l. of hydrosulphuric acid were
+liberated; what volume would it occupy, the gases being measured under
+the same conditions?
+
+8. Write the equations representing the reaction between hydrosulphuric
+acid and sodium hydroxide and ammonium hydroxide respectively.
+
+9. Show that the preparation of sulphur dioxide from a sulphite is
+similar in principle to the preparation of hydrogen sulphide.
+
+10. (a) Does dry sulphur dioxide react with litmus paper? (b) How
+can it be shown that a solution of sulphur dioxide in water acts like an
+acid?
+
+11. (a) Calculate the percentage composition of sulphurous anhydride
+and sulphuric anhydride. (b) Show how these two substances are in
+harmony with the law of multiple proportion.
+
+12. How many pounds of sulphur would be necessary in the preparation of
+100 lb. of 98% sulphuric acid?
+
+13. What weight of sulphur dioxide is necessary in the preparation of 1
+kg. of sodium sulphite?
+
+14. What weight of copper sulphate crystals can be obtained by
+dissolving 1 kg. of copper in sulphuric acid and crystallizing the
+product from water?
+
+15. Write the names and formulas of the oxides and oxygen acids of
+selenium and tellurium.
+
+16. In the commercial preparation of carbon disulphide, what is the
+function of the electric current?
+
+17. If the Gay-Lussac tower were omitted from the sulphuric acid
+factory, what effect would this have on the cost of production of
+sulphuric acid?
+
+
+
+
+CHAPTER XV
+
+PERIODIC LAW
+
+
+A number of the elements have now been studied somewhat closely. The
+first three of these, oxygen, hydrogen, and nitrogen, while having some
+physical properties in common with each other, have almost no point of
+similarity as regards their chemical conduct. On the other hand, oxygen
+and sulphur, while quite different physically, have much in common in
+their chemical properties.
+
+About eighty elements are now known. If all of these should have
+properties as diverse as do oxygen, hydrogen, and nitrogen, the study of
+chemistry would plainly be a very difficult and complicated one. If,
+however, the elements can be classified in groups, the members of which
+have very similar properties, the study will be very much simplified.
+
+~Earlier classification of the elements.~ Even at an early period efforts
+were made to discover some natural principle in accordance with which
+the elements could be classified. Two of these classifications may be
+mentioned here.
+
+1. _Classification into metals and non-metals._ The classification into
+metals and non-metals most naturally suggested itself. This grouping was
+based largely on physical properties, the metals being heavy, lustrous,
+malleable, ductile, and good conductors of heat and electricity.
+Elements possessing these properties are usually base-forming in
+character, and the ability to form bases came to be regarded as a
+characteristic property of the metals. The non-metals possessed
+physical properties which were the reverse of those of the metals, and
+were acid-forming in character.
+
+Not much was gained by this classification, and it was very imperfect.
+Some metals, such as potassium, are very light; some non-metals, such as
+iodine, have a high luster; some elements can form either an acid or a
+base.
+
+2. _Classification into triad families._ In 1825 Doebereiner observed
+that an interesting relation exists between the atomic weights of
+chemically similar elements. To illustrate, lithium, sodium, and
+potassium resemble each other very closely, and the atomic weight of
+sodium is almost exactly an arithmetical mean between those of the other
+two: (7.03 + 39.15)/2 = 23.09. In many chemical and physical properties
+sodium is midway between the other two.
+
+A number of triad families were found, but among eighty elements, whose
+atomic weights range all the way from 1 to 240, such agreements might be
+mere chance. Moreover many elements did not appear to belong to such
+families.
+
+~Periodic division.~ In 1869 the Russian chemist Mendeleeff devised an
+arrangement of the elements based on their atomic weights, which has
+proved to be of great service in the comparative study of the elements.
+A few months later the German, Lothar Meyer, independently suggested the
+same ideas. This arrangement brought to light a great generalization,
+now known as the _periodic law_. An exact statement of the law will be
+given after the method of arranging the elements has been described.
+
+[Illustration: DMITRI IVANOVITCH MENDELEEFF (Russian) (1834-1907)
+
+Author of the periodic law; made many investigations on the physical
+constants of elements and compounds; wrote an important book entitled
+"Principles of Chemistry"; university professor and government
+official]
+
+~Arrangement of the periodic table.~ The arrangement suggested by
+Mendeleeff, modified somewhat by more recent investigations, is as
+follows: Beginning with lithium, which has an atomic weight of 7, the
+elements are arranged in a horizontal row in the order of their atomic
+weights, thus:
+
+~Li (7.03), Be (9.1), B (11), C (12), N (14.04), O (16), F (19).~
+
+These seven elements all differ markedly from each other. The eighth
+element, sodium, is very similar to lithium. It is placed just under
+lithium, and a new row follows:
+
+~Na(23.05), Mg (24.36), Al (27.1), Si (28.4), P (31), S (32.06),
+Cl(35.45).~
+
+When the fifteenth element, potassium, is reached, it is placed under
+sodium, to which it is very similar, and serves to begin a third row:
+
+~K (39.15), Ca (40.1), Sc (44.1,) Ti (48.1), V (51.2), Cr (52.1), Mn(55).~
+
+Not only is there a strong similarity between lithium, sodium, and
+potassium, which have been placed in a vertical row because of this
+resemblance, but the elements in the other vertical rows exhibit much of
+the same kind of similarity among themselves, and evidently form little
+natural groups.
+
+The three elements following manganese, namely, iron, nickel, and
+cobalt, have atomic weights near together, and are very similar
+chemically. They do not strongly resemble any of the elements so far
+considered, and are accordingly placed in a group by themselves,
+following manganese. A new row is begun with copper, which somewhat
+resembles the elements of the first vertical column. Following the fifth
+and seventh rows are groups of three closely related elements, so that
+the completed arrangement has the appearance represented in the table on
+page 168.
+
+
+THE PERIODIC ARRANGEMENT OF THE ELEMENTS
+
+--------+-----------+-----------+-----------+-----------+-----------+
+Periods |   GROUP   |   GROUP   |   GROUP   |   GROUP   |   GROUP   |
+        |     0     |     I     |     II    |    III    |     IV    |
+        |A         B|A         B|A         B|A         B|A         B|
+--------+-----------+-----------+-----------+-----------+-----------+
+1       |H==1.008   |           |           |           |           |
+2       |He=4       |Li=7.03    |Be=9.1     |B=11       |C=12       |
+--------+-----------+-----------+-----------+-----------+-----------+
+3       |      Ne=20|Na=23.05   |   Mg=24.36|    AL=27.1|    Si=28.4|
+--------+-----------+-----------+-----------+-----------+-----------+
+4       |A=39.9     |K=39.15    |Ca=40.1    |Sc=44.1    |Ti=48.1    |
+        |           |           |           |           |           |
+        |           |           |           |           |           |
+--------+-----------+-----------+-----------+-----------+-----------+
+5       |           |    Cu=63.6|    Zn=65.4|      Ga=70|    Ge=72.5|
+--------+-----------+-----------+-----------+-----------+-----------+
+6       |Kr=81.8    |Rb=85.5    |Sr=87.6    |Y=89       |Zr=90.6    |
+        |           |           |           |           |           |
+        |           |           |           |           |           |
+--------+-----------+-----------+-----------+-----------+-----------+
+7       |           |  Ag=107.93|   Cd=112.4|     In=115|     Sn=119|
+--------+-----------+-----------+-----------+-----------+-----------+
+8       |X=128      |Cs=132.9   |Ba=137.4   |La=138.9   |Ce=Yb*     |
+        |           |           |           |           |140.25-173 |
+        |           |           |           |           |           |
+--------+-----------+-----------+-----------+-----------+-----------+
+9       |   Au=197.2|     Hg=200|   Tl=204.1|   Pb=206.9|   Bi=208.5|
+--------+-----------+-----------+-----------+-----------+-----------+
+10      |           |           |Ra=225     |           |Th=232.5   |
+--------+-----------+-----------+-----------+-----------+-----------+
+        |           |   R_{2}O  |     RO    |R_{2}O_{3} |   RO_{2}  |
+        |           |     RH    |   RH_{2}  |  RH_{3}   |   RH_{4}  |
+--------+-----------+-----------+-----------+-----------+-----------+
+
+==================part 2==============
+
+--------+-----------+-----------+-----------+-----------+
+Periods |   GROUP   |   GROUP   |   GROUP   |   GROUP   |
+        |     V     |     VI    |    VII    |    VIII   |
+        |A         B|A         B|A         B|           |
+--------+-----------+-----------+-----------+-----------+
+1       |           |           |           |           |
+2       |N=14.04    |O=16       |F=19       |           |
+--------+-----------+-----------+-----------+-----------+
+3       |       P=31|    S=32.06|   Cl=35.45|           |
+--------+-----------+-----------+-----------+-----------+
+4       |V=51.2     |Cr=52.1    |Mn=55      |Fe=55.9    |
+        |           |           |           |Ni=58.7    |
+        |           |           |           |Co=59      |
+--------+-----------+-----------+-----------+-----------+
+5       |      As=75|    Se=79.2|   Br=79.96|           |
+--------+-----------+-----------+-----------+-----------+
+6       |Cb=94      |Mo=96      |           |Ru=101.7   |
+        |           |           |           |Rh=103     |
+        |           |           |           |Pd=106.5   |
+--------+-----------+-----------+-----------+-----------+
+7       |   Sb=120.2|   Te=127.6|   I=126.97|           |
+--------+-----------+-----------+-----------+-----------+
+8       |Ta=183     |W=184      |           |Os=191     |
+        |           |           |           |Ir=193     |
+        |           |           |           |Pt=194.8   |
+--------+-----------+-----------+-----------+-----------+
+9       |           |           |           |           |
+--------+-----------+-----------+-----------+-----------+
+10      | U=238.5   |           |           |           |
+--------+-----------+-----------+-----------+-----------+
+        | R_{2}O_{5}|   RO_{3}  | R_{2}O_{7}|   RO_{4}  |
+        |   RH_{3}  |   RH_{2}  |     RH    |           |
+--------+-----------+-----------+-----------+-----------+
+
+[* This includes a number of elements whose atomic weights lie
+between 140 and 173, but which have not been accurately studied, and
+so their proper arrangement is uncertain.]
+
+~Place of the atmospheric elements.~ When argon was discovered it was seen
+at once that there was no place in the table for an element of atomic
+weight approximately 40. When the other inactive elements were found,
+however, it became apparent that they form a group just preceding Group
+1. They are accordingly arranged in this way in Group 0 (see table on
+opposite page). A study of this table brings to light certain very
+striking facts.
+
+~Properties of elements vary with atomic weights.~ There is evidently a
+close relation between the properties of an element and its atomic
+weight. Lithium, at the beginning of the first group, is a very strong
+base-forming element, with pronounced metallic properties. Beryllium,
+following lithium, is less strongly base-forming, while boron has some
+base-forming and some acid-forming properties. In carbon all
+base-forming properties have disappeared, and the acid-forming
+properties are more marked than in boron. These become still more
+emphasized as we pass through nitrogen and oxygen, until on reaching
+fluorine we have one of the strongest acid-forming elements. The
+properties of these seven elements therefore vary regularly with their
+atomic weights, or, in mathematical language, are regular functions of
+them.
+
+~Periodic law.~ The properties of the first seven elements vary
+_continuously_--that is steadily--away from base-forming and toward
+acid-forming properties. If lithium had the smallest atomic weight of
+any of the elements, and fluorine the greatest, so that in passing from
+one to the other we had included all the elements, we could say that the
+properties of elements are continuous functions of their atomic weights.
+But fluorine is an element of small atomic weight, and the one following
+it, sodium, breaks the regular order, for in it reappear all the
+characteristic properties of lithium. Magnesium, following sodium, bears
+much the same relation to beryllium that sodium does to lithium, and
+the properties of the elements in the second row vary much as they do in
+the first row until potassium is reached, when another repetition
+begins. The properties of the elements do not vary continuously,
+therefore, with atomic weights, but at regular intervals there is a
+repetition, or _period_. This generalization is known as the _periodic
+law_, and may be stated thus: _The properties of elements are periodic
+functions of their atomic weights._
+
+~The two families in a group.~ While all the elements in a given vertical
+column bear a general resemblance to each other, it has been noticed
+that those belonging to periods having even numbers are very strikingly
+similar to each other. They are placed at the left side of the group
+columns. In like manner, the elements belonging to the odd periods are
+very similar and are arranged at the right side of the group columns.
+Thus calcium, strontium, and barium are very much alike; so, too, are
+magnesium, zinc, and cadmium. The resemblance between calcium and
+magnesium, or strontium and zinc, is much less marked. This method of
+arrangement therefore divides each group into two families, each
+containing four or five members, between which there is a great
+similarity.
+
+~Family resemblances.~ Let us now inquire more closely in what respects
+the elements of a family resemble each other.
+
+1. _Valence._ In general the valence of the elements in a family is the
+same, and the formulas of their compounds are therefore similar. If we
+know that the formula of sodium chloride is NaCl, it is pretty certain
+that the formula of potassium chloride will be KCl--not KCl_{2} or
+KCl_{3}. The general formulas R_{2}O, RO, etc., placed below the
+columns show the formulas of the oxides of the elements in the column
+provided they form oxides. In like manner the formulas RH, RH_{2}, etc.,
+show the composition of the compounds formed with hydrogen or chlorine.
+
+2. _Chemical properties._ The chemical properties of the members of a
+family are quite similar. If one member is a metal, the others usually
+are; if one is a non-metal, so, too, are the others. The families in the
+first two columns consist of metals, while the elements found in the
+last two columns form acids. There is in addition a certain regularity
+in properties of the elements in each family. If the element at the head
+of the family is a strong acid-forming element, this property is likely
+to diminish gradually, as we pass to the members of the family with
+higher atomic weights. Thus phosphorus is strongly acid-forming, arsenic
+less so, antimony still less so, while bismuth has almost no
+acid-forming properties. We shall meet with many illustrations of this
+fact.
+
+3. _Physical properties._ In the same way, the physical properties of
+the members of a family are in general somewhat similar, and show a
+regular gradation as we pass from element to element in the family. Thus
+the densities of the members of the magnesium family are
+
+    Mg = 1.75, Zn = 7.00, Cd = 8.67, Hg = 13.6.
+
+Their melting points are
+
+    Mg = 750 deg., Zn = 420 deg., Cd = 320 deg., Hg = -39.5 deg..
+
+~Value of the periodic law.~ The periodic law has proved of much value in
+the development of the science of chemistry.
+
+1. _It simplifies study._ It is at once evident that such regularities
+very much simplify the study of chemistry. A thorough study of one
+element of a family makes the study of the other members a much easier
+task, since so many of the properties and chemical reactions of the
+elements are similar. Thus, having studied the element sulphur in some
+detail, it is not necessary to study selenium and tellurium so closely,
+for most of their properties can be predicted from the relation which
+they sustain to sulphur.
+
+2. _It predicts new elements._ When the periodic law was first
+formulated there were a number of vacant places in the table which
+evidently belonged to elements at that time unknown. From their position
+in the table, Mendeleeff predicted with great precision the properties
+of the elements which he felt sure would one day be discovered to fill
+these places. Three of them, scandium, germanium, and gallium, were
+found within fifteen years, and their properties agreed in a remarkable
+way with the predictions of Mendeleeff. There are still some vacant
+places in the table, especially among the heavier elements.
+
+3. _It corrects errors._ The physical constants of many of the elements
+did not at first agree with those demanded by the periodic law, and a
+further study of many such cases showed that errors had been made. The
+law has therefore done much service in indicating probable error.
+
+~Imperfections of the law.~ There still remain a good many features which
+must be regarded as imperfections in the law. Most conspicuous is the
+fact that the element hydrogen has no place in the table. In some of the
+groups elements appear in one of the families, while all of their
+properties show that they belong in the other. Thus sodium belongs with
+lithium and not with copper; fluorine belongs with chlorine and not with
+manganese. There are two instances where the elements must be
+transposed in order to make them fit into their proper group. According
+to their atomic weights, tellurium should follow iodine, and argon
+should follow potassium. Their properties show in each case that this
+order must be reversed. The table separates some elements altogether
+which, in many respects have closely agreeing properties. Iron,
+chromium, and manganese are all in different groups, although they are
+similar in many respects.
+
+The system is therefore to be regarded as but a partial and imperfect
+expression of some very important and fundamental relation between the
+substances which we know as elements, the exact nature of this relation
+being as yet not completely clear to us.
+
+
+EXERCISES
+
+1. Suppose that an element were discovered that filled the blank in
+Group O, Period 5; what properties would it probably have?
+
+2. Suppose that an element were discovered that filled the blank in
+Group VI, Period 9, family B; what properties would it have?
+
+3. Sulphur and oxygen both belong in Group VI, although in different
+families; in what respects are the two similar?
+
+
+
+
+CHAPTER XVI
+
+THE CHLORINE FAMILY
+
+
+==================================================================
+              |        |         |         |
+              | ATOMIC | MELTING | BOILING | COLOR AND STATE
+              | WEIGHT |  POINT  |  POINT  |
+______________|________|_________|_________|______________________
+              |        |         |         |
+Fluorine (F)  |  19.00 |   -223 deg. |   -187 deg. | Pale yellowish gas.
+Chlorine (Cl) |  35.45 |   -102 deg. |  -33.6 deg. | Greenish-yellow gas.
+Bromine (Br)  |  79.96 |     -7 deg. |     59 deg. | Red liquid.
+Iodine (I)    | 126.97 |    107 deg. |    175 deg. | Purplish-black solid.
+==================================================================
+
+~The family.~ The four elements named in the above table form a strongly
+marked family of elements and illustrate very clearly the way in which
+the members of a family in a periodic group resemble each other, as well
+as the character of the differences which we may expect to find between
+the individual members.
+
+1. _Occurrence._ These elements do not occur in nature in the free
+state. The compounds of the last three elements of the family are found
+extensively in sea water, and on this account the name _halogens_,
+signifying "producers of sea salt," is sometimes applied to the family.
+
+2. _Properties._ As will be seen by reference to the table, the melting
+points and boiling points of the elements of the family increase with
+their atomic weights. A somewhat similar gradation is noted in their
+color and state. One atom of each of the elements combines with one atom
+of hydrogen to form acids, which are gases very soluble in water. The
+affinity of the elements for hydrogen is in the inverse order of their
+atomic weights, fluorine having the strongest affinity and iodine the
+weakest. Only chlorine and iodine form oxides, and those of the former
+element are very unstable. The elements of the group are univalent in
+their compounds with hydrogen and the metals.
+
+
+FLUORINE
+
+~Occurrence.~ The element fluorine occurs in nature most abundantly as the
+mineral fluorspar (CaF_{2}), as cryolite (Na_{3}AlF_{6}), and in the
+complex mineral apatite (3 Ca_{3}(PO_{4})_{2}.CaF_{2}).
+
+~Preparation.~ All attempts to isolate the element resulted in failure
+until recent years. Methods similar to those which succeed in the
+preparation of the other elements of the family cannot be used; for as
+soon as the fluorine is liberated it combines with the materials of
+which the apparatus is made or with the hydrogen of the water which is
+always present. The preparation of fluorine was finally accomplished by
+the French chemist Moissan by the electrolysis of hydrofluoric acid.
+Perfectly dry hydrofluoric acid (HF) was condensed to a liquid and
+placed in a U-shaped tube made of platinum (or copper), which was
+furnished with electrodes and delivery tubes, as shown in Fig. 52. This
+liquid is not an electrolyte, but becomes such when potassium fluoride
+is dissolved in it. When this solution was electrolyzed hydrogen was set
+free at the cathode and fluorine at the anode.
+
+[Illustration: Fig. 52]
+
+~Properties.~ Fluorine is a gas of slightly yellowish color, and can be
+condensed to a liquid boiling at -187 deg. under atmospheric pressure. It
+solidifies at -223 deg.. It is extremely active chemically, being the most
+active of all the elements at ordinary temperatures.
+
+It combines with all the common elements save oxygen, very often with
+incandescence and the liberation of much heat. It has a strong affinity
+for hydrogen and is able to withdraw it from its compounds with other
+elements. Because of its great activity it is extremely poisonous.
+Fluorine does not form any oxides, neither does it form any oxygen
+acids, in which respects it differs from the other members of the
+family.
+
+~Hydrofluoric acid~ (HF). Hydrofluoric acid is readily obtained from
+fluorspar by the action of concentrated sulphuric acid. The equation is
+
+    CaF_{2} + H_{2}SO_{4} = CaSO_{4} + 2HF.
+
+In its physical properties it resembles the binary acids of the other
+elements of this family, being, however, more easily condensed to a
+liquid. The anhydrous acid boils at 19 deg. and can therefore be prepared at
+ordinary pressures. It is soluble in all proportions in water, and a
+concentrated solution--about 50%--is prepared for the market. Its fumes
+are exceedingly irritating to the respiratory organs, and several
+chemists have lost their lives by accidentally breathing them.
+
+[Illustration: HENRI MOISSAN (French) (1853-1907)
+
+Famous for his work with the electric furnace at high temperatures;
+prepared artificial diamonds, together with many new binary compounds
+such as carbides, silicides, borides, and nitrides; isolated fluorine
+and studied its properties and its compounds very thoroughly]
+
+~Chemical properties.~ Hydrofluoric acid, like other strong acids, readily
+acts on bases and metallic oxides and forms the corresponding fluorides.
+It also dissolves certain metals such as silver and copper. It acts very
+vigorously upon organic matter, a single drop of the concentrated acid
+making a sore on the skin which is very painful and slow in healing. Its
+most characteristic property is its action upon silicon dioxide
+(SiO_{2}), with which it forms water and the gas silicon tetrafluoride
+(SiF_{4}), as shown in the equation
+
+    SiO_{2} + 4HF = SiF_{4} + 2H_{2}O.
+
+Glass consists of certain compounds of silicon, which are likewise acted
+on by the acid so that it cannot be kept in glass bottles. It is
+preserved in flasks made of wax or gutta-percha.
+
+     ~Etching.~ Advantage is taken of this reaction in etching designs
+     upon glass. The glass vessel is painted over with a protective
+     paint upon which the acid will not act, the parts which it is
+     desired to make opaque being left unprotected. A mixture of
+     fluorspar and sulphuric acid is then painted over the vessel
+     and after a few minutes the vessel is washed clean. Wherever
+     the hydrofluoric acid comes in contact with the glass it acts
+     upon it, destroying its luster and making it opaque, so that
+     the exposed design will be etched upon the clear glass. Frosted
+     glass globes are often made in this way.
+
+     The etching may also be effected by covering the glass with a
+     thin layer of paraffin, cutting the design through the wax and
+     then exposing the glass to the fumes of the acid.
+
+~Salts of hydrofluoric acid,--fluorides.~ A number of the fluorides are
+known, but only one of them, calcium fluoride (CaF_{2}), is of
+importance. This is the well-known mineral fluorspar.
+
+
+CHLORINE
+
+~Historical.~ While studying the action of hydrochloric acid upon the
+mineral pyrolusite, in 1774, Scheele obtained a yellowish, gaseous
+substance to which he gave a name in keeping with the phlogiston theory
+then current. Later it was supposed to be a compound containing oxygen.
+In 1810, however, the English chemist Sir Humphry Davy proved it to be
+an element and named it chlorine.
+
+~Occurrence.~ Chlorine does not occur free in nature, but its compounds
+are widely distributed. For the most part it occurs in combination with
+the metals in the form of chlorides, those of sodium, potassium, and
+magnesium being most abundant. Nearly all salt water contains these
+substances, particularly sodium chloride, and very large salt beds
+consisting of chlorides are found in many parts of the world.
+
+~Preparation.~ Two general methods of preparing chlorine may be mentioned,
+namely, the laboratory method and the electrolytic method.
+
+1. _Laboratory method._ In the laboratory chlorine is made by warming
+the mineral pyrolusite (manganese dioxide, MnO_{2}) with concentrated
+hydrochloric acid. The first reaction, which seems to be similar to the
+action of acids upon oxides in general, is expressed in the equation
+
+    MnO_{2} + 4HCl = MnCl_{4} + 2H_{2}O.
+
+The manganese compound so formed is very unstable, however, and breaks
+clown according to the equation
+
+    MnCl_{4} = MnCl_{2} + 2Cl.
+
+Instead of using hydrochloric acid in the preparation of chlorine it
+will serve just as well to use a mixture of sodium chloride and
+sulphuric acid, since these two react to form hydrochloric acid. The
+following equations will then express the changes:
+
+    (1) 2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
+
+    (2) MnO_{2} + 4 HCl = MnCl_{2} + 2Cl + 2H_{2}O.
+
+    (3) MnCl_{2} + H_{2}SO_{4} = MnSO_{4} + 2HCl.
+
+Combining these equations, the following equation expressing the
+complete reaction is obtained:
+
+    2NaCl + MnO_{2} + 2H_{2}SO_{4} = MnSO_{4} + Na_{2}SO_{4} + 2H_{2}O + 2Cl.
+
+Since the hydrochloric acid liberated in the third equation is free to
+act upon manganese dioxide, it will be seen that all of the chlorine
+originally present in the sodium chloride is set free.
+
+     The manganese dioxide and the hydrochloric acid are brought
+     together in a flask, as represented in Fig. 53, and a gentle
+     heat is applied. The rate of evolution of the gas is regulated
+     by the amount of heat applied, and the gas is collected by
+     displacement of air. As the equations show, only half of the
+     chlorine present in the hydrochloric acid is liberated.
+
+[Illustration: Fig. 53]
+
+2. _Electrolytic method._ Under the discussion of electrolysis (p. 102)
+it was shown that when a solution of sodium chloride is electrolyzed
+chlorine is evolved at the anode, while the sodium set free at the
+cathode reacts with the water to form hydrogen, which is evolved, and
+sodium hydroxide, which remains in solution. A great deal of the
+chlorine required in the chemical industries is now made in this way in
+connection with the manufacture of sodium hydroxide.
+
+~Physical properties.~ Chlorine is a greenish-yellow gas, which has a
+peculiar suffocating odor and produces a very violent effect upon the
+throat and lungs. Even when inhaled in small quantities it often
+produces all the symptoms of a hard cold, and in larger quantities may
+have serious and even fatal action. It is quite heavy (density = 2.45)
+and can therefore be collected by displacement of air. One volume of
+water under ordinary conditions dissolves about three volumes of
+chlorine. The gas is readily liquefied, a pressure of six atmospheres
+serving to liquefy it at 0 deg.. It forms a yellowish liquid which
+solidifies at -102 deg..
+
+~Chemical properties.~ At ordinary temperatures chlorine is far more
+active chemically than any of the elements we have so far considered,
+with the exception of fluorine; indeed, it is one of the most active of
+all elements.
+
+1. _Action on metals._ A great many metals combine directly with
+chlorine, especially when hot. A strip of copper foil heated in a burner
+flame and then dropped into chlorine burns with incandescence. Sodium
+burns brilliantly when heated strongly in slightly moist chlorine. Gold
+and silver are quickly tarnished by the gas.
+
+2. _Action on non-metals._ Chlorine has likewise a strong affinity for
+many of the non-metals. Thus phosphorus burns in a current of the gas,
+while antimony and arsenic in the form of a fine powder at once burst
+into flame when dropped into jars of the gas. The products formed in all
+cases where chlorine combines with another element are called
+_chlorides_.
+
+3. _Action on hydrogen._ Chlorine has a strong affinity for hydrogen,
+uniting with it to form hydrochloric acid. A jet of hydrogen burning in
+the air continues to burn when introduced into a jar of chlorine, giving
+a somewhat luminous flame. A mixture of the two gases explodes violently
+when a spark is passed through it or when it is exposed to bright
+sunlight. In the latter case it is the light and not the heat which
+starts the action.
+
+4. _Action on substances containing hydrogen._ Not only will chlorine
+combine directly with free hydrogen but it will often abstract the
+element from its compounds. Thus, when chlorine is passed into a
+solution containing hydrosulphuric acid, sulphur is precipitated and
+Hydrochloric acid formed. The reaction is shown by the following
+equation:
+
+    H_{2}S + 2Cl = 2HCl + S.
+
+With ammonia the action is similar:
+
+    NH_{3} + 3Cl = 3HCl + N.
+
+The same tendency is very strikingly seen in the action of chlorine upon
+turpentine. The latter substance is largely made up of compounds having
+the composition represented by the formula C_{10}H_{16}. When a strip of
+paper moistened with warm turpentine is placed in a jar of chlorine
+dense fumes of hydrochloric acid appear and a black deposit of carbon is
+formed. Even water, which is a very stable compound, can be decomposed
+by chlorine, the oxygen being liberated. This may be shown in the
+following way:
+
+[Illustration: Fig. 54]
+
+     If a long tube of rather large diameter is filled with a strong
+     solution of chlorine in water and inverted in a vessel of the
+     same solution, as shown in Fig. 54, and the apparatus is placed
+     in bright sunlight, very soon bubbles of a gas will be observed
+     to rise through the solution and collect in the tube. An
+     examination of this gas will show that it is oxygen. It is
+     liberated from water in accordance with the following equation:
+
+     H_{2}O + 2Cl = 2HCl + O.
+
+5. _Action on color substances,--bleaching action._ If strips of
+brightly colored cloth or some highly colored flowers are placed in
+quite dry chlorine, no marked change in color is noticed as a rule. If,
+however, the cloth and flowers are first moistened, the color rapidly
+disappears, that is, the objects are bleached. Evidently the moisture as
+well as the chlorine is concerned in the action, and a study of the case
+shows that the chlorine has combined with the hydrogen of the water. The
+oxygen set free oxidizes the color substance, converting it into a
+colorless compound. It is evident from this explanation that chlorine
+will only bleach those substances which are changed into colorless
+compounds by oxidation.
+
+6. _Action as a disinfectant._ Chlorine has also marked germicidal
+properties, and the free element, as well as compounds from which it is
+easily liberated, are used as disinfectants.
+
+~Nascent state.~ It will be noticed that oxygen when set free from water
+by chlorine is able to do what ordinary oxygen cannot do, for both the
+cloth and the flowers are unchanged in the air which contains oxygen. It
+is generally true that the activity of an element is greatest at the
+instant of liberation from its compounds. To express this fact elements
+at the instant of liberation are said to be in the _nascent state_. It
+is nascent oxygen which does the bleaching.
+
+~Hydrochloric acid~ (_muriatic acid_) (HCl). The preparation of
+hydrochloric acid may be discussed under two general heads:
+
+1. _Laboratory preparation._ The product formed by the burning of
+hydrogen in chlorine is the gas hydrochloric acid. This substance is
+much more easily obtained, however, by treating common salt (sodium
+chloride) with sulphuric acid. The following equation shows the
+reaction:
+
+    2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
+
+The dry salt is placed in a flask furnished with a funnel tube and an
+exit tube, the sulphuric acid is added, and the flask gently warmed. The
+hydrochloric acid gas is rapidly given off and can be collected by
+displacement of air. The same apparatus can be used as was employed in
+the preparation of chlorine (Fig. 53).
+
+     When a _solution_ of salt is treated with sulphuric acid there
+     is no very marked action. The hydrochloric acid formed is very
+     soluble in water, and so does not escape from the solution;
+     hence a state of equilibrium is soon reached between the four
+     substances represented in the equation. When _concentrated_
+     sulphuric acid, in which hydrochloric acid is not soluble, is
+     poured upon dry salt the reaction is complete.
+
+2. _Commercial preparation._ Commercially, hydrochloric acid is prepared
+in connection with the manufacture of sodium sulphate, the reaction
+being the same as that just given. The reaction is carried out in a
+furnace, and the hydrochloric acid as it escapes in the form of gas is
+passed into water in which it dissolves, the solution forming the
+hydrochloric acid of commerce. When the materials are pure a colorless
+solution is obtained. The most concentrated solution has a density of
+1.2 and contains 40% HCl. The commercial acid, often called _muriatic
+acid_, is usually colored yellow by impurities.
+
+~Composition of hydrochloric acid.~ When a solution of hydrochloric acid
+is electrolyzed in an apparatus similar to the one in which water was
+electrolyzed (Fig. 18), chlorine collects at the anode and hydrogen at
+the cathode. At first the chlorine dissolves in the water, but soon the
+water in the one tube becomes saturated with it, and if the stopcocks
+are left open until this is the case, and are then closed, it will be
+seen that the two gases are set free in equal volumes.
+
+When measured volumes of the two gases are caused to unite it is found
+that one volume of hydrogen combines with one of chlorine. Other
+experiments show that the volume of hydrochloric acid formed is just
+equal to the sum of the volumes of hydrogen and chlorine. Therefore one
+volume of hydrogen combines with one volume of chlorine to form two
+volumes of hydrochloric acid gas. Since chlorine is 35.18 times as heavy
+as hydrogen, it follows that one part of hydrogen by weight combines
+with 35.18 parts of chlorine to form 36.18 parts of hydrochloric acid.
+
+~Physical properties.~ Hydrochloric acid is a colorless gas which has an
+irritating effect when inhaled, and possesses a sour, biting taste, but
+no marked odor. It is heavier than air (density = 1.26) and is very
+soluble in water. Under standard conditions 1 volume of water dissolves
+about 500 volumes of the gas. On warming such a solution the gas
+escapes, until at the boiling point the solution contains about 20% by
+weight of HCl. Further boiling will not drive out any more acid, but the
+solution will distill with unchanged concentration. A more dilute
+solution than this will lose water on boiling until it has reached the
+same concentration, 20%, and will then distill unchanged. Under high
+pressure the gas can be liquefied, 28 atmospheres being required at 0 deg..
+Under these conditions it forms a colorless liquid which is not very
+active chemically. It boils at -80 deg. and solidifies at -113 deg.. The
+solution of the gas in water is used almost entirely in the place of the
+gas itself, since it is not only far more convenient but also more
+active.
+
+~Chemical properties.~ The most important chemical properties of
+hydrochloric acid are the following:
+
+1. _Action as an acid._ In aqueous solution hydrochloric acid has very
+strong acid properties; indeed, it is one of the strongest acids. It
+acts upon oxides and hydroxides, converting them into salts:
+
+    NaOH + HCl = NaCl + H_{2}O,
+    CuO + 2HCl = CuCl_{2} + H_{2}O.
+
+It acts upon many metals, forming chlorides and liberating hydrogen:
+
+    Zn + 2HCl = ZnCl_{2} + 2H,
+    Al + 3HCl = AlCl_{3} + 3H.
+
+Unlike nitric and sulphuric acids it has no oxidizing action, so that
+when it acts on metals hydrogen is always given off.
+
+2. _Relation to combustion._ Hydrochloric acid gas is not readily
+decomposed, and is therefore neither combustible nor a supporter of
+combustion.
+
+3. _Action on oxidizing agents._ Although hydrochloric acid is
+incombustible, it can be oxidized under some circumstances, in which
+case the hydrogen combines with oxygen, while the chlorine is set free.
+Thus, when a solution of hydrochloric acid acts upon manganese dioxide
+part of the chlorine is set free:
+
+    MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + 2Cl.
+
+~Aqua regia.~ It has been seen that when nitric acid acts as an oxidizing
+agent it usually decomposes, as represented in the equation
+
+    2HNO_{3} = H_{2}O + 2NO + 3O.
+
+The oxygen so set free may act on hydrochloric acid:
+
+    6HCl + 3O = 3H_{2}O + 6Cl.
+
+The complete equation therefore is
+
+    2HNO_{3} + 6HCl = 4H_{2}O + 2NO + 6Cl.
+
+When concentrated nitric and hydrochloric acids are mixed this reaction
+goes on slowly, chlorine and some other substances not represented in
+the equation being formed. The mixture is known as _aqua regia_ and is
+commonly prepared by adding one volume of nitric acid to three volumes
+of hydrochloric acid. It acts more powerfully upon metals and other
+substances than either of the acids separately, and owes its strength
+not to acid properties but to the action of the nascent chlorine which
+it liberates. Consequently, when it acts upon metals such as gold it
+converts them into chlorides, and the reaction can be represented by
+such equations as
+
+    Au + 3Cl = AuCl_{3}.
+
+~Salts of hydrochloric acid,--chlorides.~ The chlorides of all the metals
+are known and many of them are very important compounds. Some of them
+are found in nature, and all can be prepared by the general method of
+preparing salts. Silver chloride, lead chloride, and mercurous chloride
+are insoluble in water and acids, and can be prepared by adding
+hydrochloric acid to solutions of compounds of the respective elements.
+While the chlorides have formulas similar to the fluorides, their
+properties are often quite different. This is seen in the solubility of
+the salts. Those metals whose chlorides are insoluble form soluble
+fluorides, while many of the metals which form soluble chlorides form
+insoluble fluorides.
+
+~Compounds of chlorine with oxygen and hydrogen.~ Chlorine combines with
+oxygen and hydrogen to form four different acids. They are all quite
+unstable, and most of them cannot be prepared in pure form; their salts
+can easily be made, however, and some of them will be met with in the
+study of the metals. The formulas and names of these acids are as
+follows:
+
+    HClO       hypochlorous acid.
+
+    HClO_{2}   chlorous acid.
+
+    HClO_{3}   chloric acid.
+
+    HClO_{4}   perchloric acid.
+
+~Oxides of chlorine.~ Two oxides are known, having the formulas Cl_{2}O
+and ClO_{2}. They decompose very easily and are good oxidizing agents.
+
+
+BROMINE
+
+~Historical.~ Bromine was discovered in 1826 by the French chemist
+Ballard, who isolated it from sea salt. He named it bromine (stench)
+because of its unbearable fumes.
+
+~Occurrence.~ Bromine occurs almost entirely in the form of bromides,
+especially as sodium bromide and magnesium bromide, which are found in
+many salt springs and salt deposits. The Stassfurt deposits in Germany
+and the salt waters of Ohio and Michigan are especially rich in
+bromides.
+
+~Preparation of bromine.~ The laboratory method of preparing bromine is
+essentially different from the commercial method.
+
+[Illustration Fig. 55]
+
+1. _Laboratory method._ As in the case of chlorine, bromine can be
+prepared by the action of hydrobromic acid (HBr) on manganese dioxide.
+Since hydrobromic acid is not an article of commerce, a mixture of
+sulphuric acid and a bromide is commonly substituted for it. The
+materials are placed in a retort arranged as shown in Fig. 55. The end
+of the retort just touches the surface of the water in the test tube. On
+heating, the bromine distills over and is collected in the cold
+receiver. The equation is
+
+    2NaBr + 2H_{2}SO_{4} + MnO_{2} = Na_{2}SO_{4} + MnSO_{4}
+      + 2H_{2}O + 2Br.
+
+2. _Commercial method._ Bromine is prepared commercially from the waters
+of salt wells which are especially rich in bromides. On passing a
+current of electricity through such waters the bromine is first
+liberated. Any chlorine liberated, however, will assist in the reaction,
+since free chlorine decomposes bromides, as shown in the equation
+
+    NaBr + Cl = NaCl + Br.
+
+When the water containing the bromine is heated, the liberated bromine
+distills over into the receiver.
+
+~Physical properties.~ Bromine is a dark red liquid about three times as
+heavy as water. Its vapor has a very offensive odor and is most
+irritating to the eyes and throat. The liquid boils at 59 deg. and
+solidifies at -7 deg.; but even at ordinary temperatures it evaporates
+rapidly, forming a reddish-brown gas very similar to nitrogen peroxide
+in appearance. Bromine is somewhat soluble in water, 100 volumes of
+water under ordinary conditions dissolving 1 volume of the liquid. It is
+readily soluble in carbon disulphide, forming a yellow solution.
+
+~Chemical properties and uses.~ In chemical action bromine is very similar
+to chlorine. It combines directly with many of the same elements with
+which chlorine unites, but with less energy. It combines with hydrogen
+and takes away the latter element from some of its compounds, but not
+so readily as does chlorine. Its bleaching properties are also less
+marked.
+
+Bromine finds many uses in the manufacture of organic drugs and
+dyestuffs and in the preparation of bromides.
+
+~Hydrobromic acid (HBr).~ When sulphuric acid acts upon a bromide
+hydrobromic acid is set free:
+
+    2NaBr + H_{2}SO_{4} = Na_{2}SO_{4} + 2HBr.
+
+At the same time some bromine is set free, as may be seen from the red
+fumes which appear, and from the odor. The explanation of this is found
+in the fact that hydrobromic acid is much less stable than hydrochloric
+acid, and is therefore more easily oxidized. Concentrated sulphuric acid
+is a good oxidizing agent, and oxidizes a part of the hydrobromic acid,
+liberating bromine:
+
+    H_{2}SO_{4} + 2HBr = 2H_{2}O + SO_{2} + 2Br.
+
+     ~Preparation of pure hydrobromic acid.~ A convenient way to make
+     pure hydrobromic acid is by the action of bromine upon moist
+     red phosphorus. This can be done with the apparatus shown in
+     Fig. 56. Bromine is put into the dropping funnel A, and red
+     phosphorus, together with enough water to cover it, is placed
+     in the flask B. By means of the stopcock the bromine is
+     allowed to flow drop by drop into the flask, the reaction
+     taking place without the application of heat. The equations are
+
+    (1) P + 3Br = PBr_{3},
+
+    (2) PBr_{3} + 3H_{2}O = P(OH)_{3} + 3HBr.
+
+[Illustration Fig. 56]
+
+     The U-tube C contains glass beads which have been moistened
+     with water and rubbed in red phosphorus. Any bromine escaping
+     action in the flask acts upon the phosphorus in the U-tube. The
+     hydrobromic acid is collected in the same way as hydrochloric
+     acid.
+
+~Properties.~ Hydrobromic acid very strikingly resembles hydrochloric acid
+in physical and chemical properties. It is a colorless, strongly fuming
+gas, heavier than hydrochloric acid and, like it, is very soluble in
+water. Under standard conditions 1 volume of water dissolves 610 volumes
+of the gas. Chemically, the chief point in which it differs from
+hydrochloric acid is in the fact that it is much more easily oxidized,
+so that bromine is more readily set free from it than chlorine is from
+hydrochloric acid.
+
+~Salts of hydrobromic acid,--bromides.~ The bromides are very similar to
+the chlorides in their properties. Chlorine acts upon both bromides and
+free hydrobromic acid, liberating bromine from them:
+
+    KBr + Cl = KCl + Br,
+
+    HBr + Cl = HCl + Br.
+
+Silver bromide is extensively used in photography, and the bromides of
+sodium and potassium are used as drugs.
+
+     ~Oxygen compounds.~ No oxides of bromine are surely known, and
+     bromine does not form so many oxygen acids as chlorine does.
+     Salts of hypobromous acid (HBrO) and bromic acid (HBrO_{3}) are
+     known.
+
+
+IODINE
+
+~Historical.~ Iodine was discovered in 1812 by Courtois in the ashes of
+certain sea plants. Its presence was revealed by its beautiful violet
+vapor, and this suggested the name iodine (from the Greek for violet
+appearance).
+
+~Occurrence.~ In the combined state iodine occurs in very small quantities
+in sea water, from which it is absorbed by certain sea plants, so that
+it is found in their ashes. It occurs along with bromine in salt springs
+and beds, and is also found in Chili saltpeter.
+
+~Preparation.~ Iodine may be prepared in a number of ways, the principal
+methods being the following:
+
+1. _Laboratory method._ Iodine can readily be prepared in the laboratory
+from an iodide by the method used in preparing bromine, except that
+sodium iodide is substituted for sodium bromide. It can also be made by
+passing chlorine into a solution of an iodide.
+
+[Illustration: Fig. 57]
+
+2. _Commercial method._ Commercially iodine was formerly prepared from
+seaweed (kelp), but is now obtained almost entirely from the deposits of
+Chili saltpeter. The crude saltpeter is dissolved in water and the
+solution evaporated until the saltpeter crystallizes. The remaining
+liquors, known as the "mother liquors," contain sodium iodate
+(NaIO_{3}), in which form the iodine is present in the saltpeter. The
+chemical reaction by which the iodine is liberated from this compound is
+a complicated one, depending on the fact that sulphurous acid acts upon
+iodic acid, setting iodine free. This reaction is shown as follows:
+
+    2HIO_{3} + 5H_{2}SO_{3} = 5H_{2}SO_{4} + H_{2}O + 2I.
+
+     ~Purification of iodine.~ Iodine can be purified very
+     conveniently in the following way. The crude iodine is placed
+     in an evaporating dish E (Fig. 57), and the dish is set upon
+     the sand bath S. The iodine is covered with the inverted
+     funnel F, and the sand bath is gently heated with a Bunsen
+     burner. As the dish becomes warm the iodine rapidly evaporates
+     and condenses again on the cold surface of the funnel in
+     shining crystals.
+
+     This process, in which a solid is converted into a vapor and is
+     again condensed into a solid without passing through the liquid
+     state, is called _sublimation_.
+
+~Physical properties.~ Iodine is a purplish-black, shining, heavy solid
+which crystallizes in brilliant plates. Even at ordinary temperatures it
+gives off a beautiful violet vapor, which increases in amount as heat is
+applied. It melts at 107 deg. and boils at 175 deg.. It is slightly soluble in
+water, but readily dissolves in alcohol, forming a brown solution
+(tincture of iodine), and in carbon disulphide, forming a violet
+solution. The element has a strong, unpleasant odor, though by no means
+as irritating as that of chlorine and bromine.
+
+~Chemical properties.~ Chemically iodine is quite similar to chlorine and
+bromine, but is still less active than bromine. It combines directly
+with many elements at ordinary temperatures. At elevated temperatures it
+combines with hydrogen, but the reaction is reversible and the compound
+formed is quite easily decomposed. Both chlorine and bromine displace it
+from its salts:
+
+    KI + Br = KBr + I,
+
+    KI + Cl = KCl + I.
+
+When even minute traces of iodine are added to thin starch paste a very
+intense blue color develops, and this reaction forms a delicate test for
+iodine. Iodine is extensively used in medicine, especially in the form
+of a tincture. It is also largely used in the preparation of dyes and
+organic drugs, iodoform, a substance used as an antiseptic, has the
+formula CHI_{3}.
+
+~Hydriodic acid (HI).~ This acid cannot be prepared in pure condition by
+the action of sulphuric acid upon an iodide, since the hydriodic acid
+set free is oxidized by the sulphuric acid just as in the case of
+hydrobromic acid, but to a much greater extent. It can be prepared in
+exactly the same way as hydrobromic acid, iodine being substituted for
+bromine. It can also be prepared by passing hydrosulphuric acid into
+water in which iodine is suspended. The equation is
+
+    H_{2}S + 2I = 2HI + S.
+
+The hydriodic acid formed in this way dissolves in the water.
+
+~Properties and uses.~ Hydriodic acid resembles the corresponding acids of
+chlorine and bromine in physical properties, being a strongly fuming,
+colorless gas, readily soluble in water. Under standard conditions 1
+volume of water dissolves about 460 volumes of the gas. It is, however,
+more unstable than either hydrochloric or hydrobromic acids, and on
+exposure to the air it gradually decomposes in accordance with the
+equation
+
+    2HI + O = H_{2}O + 2I.
+
+Owing to the slight affinity between iodine and hydrogen the acid easily
+gives up its hydrogen and is therefore a strong reducing agent. This is
+seen in its action on sulphuric acid.
+
+The salts of hydriodic acid, the iodides, are, in general, similar to
+the chlorides and bromides. Potassium iodide (KI) is the most familiar
+of the iodides and is largely used in medicine.
+
+     ~Oxygen compounds.~ Iodine has a much greater affinity for oxygen
+     than has either chlorine or bromine. When heated with nitric
+     acid it forms a stable oxide (I_{2}O_{5}). Salts of iodic acid
+     (HIO_{3}) and periodic acid (HIO_{4}) are easily prepared, and
+     the free acids are much more stable than the corresponding
+     acids of the other members of this family.
+
+
+GAY-LUSSAC'S LAW OF VOLUMES
+
+In the discussion of the composition of hydrochloric acid it was stated
+that one volume of hydrogen combines with one volume of chlorine to form
+two volumes of hydrochloric acid. With bromine and iodine similar
+combining ratios hold good. These facts recall the simple volume
+relations already noted in the study of the composition of steam and
+ammonia. These relations may be represented graphically in the following
+way:
+
+              +---+   +----+    +------+    +------+
+              | H | + | Cl | =  | H Cl |  + | H Cl |
+              +---+   +----+    +------+    +------+
+
+       +---+  +---+   +---+    +--------+   +--------+
+       | H |  | H | + | O |  = | H_{2}O | + | H_{2}O |
+       +---+  +---+   +---+    +--------+   +--------+
+
++---+  +---+  +---+   +---+    +--------+   +--------+
+| H |  | H |  | H | + | N |  = | NH_{3} | + | NH_{3} |
++---+  +---+  +---+   +---+    +--------+   +--------+
+
+In the early part of the past century Gay-Lussac, a distinguished French
+chemist, studied the volume relations of many combining gases, and
+concluded that similar relations always hold. His observations are
+summed up in the following law: _When two gases combine chemically there
+is always a simple ratio between their volumes, and between the volume
+of either one of them and that of the product, provided it is a gas._ By
+a simple ratio is meant of course the ratio of small whole numbers, as
+1 : 2, 2 : 3.
+
+
+EXERCISES
+
+1. How do we account for the fact that liquid hydrofluoric acid is not
+an electrolyte?
+
+2. Why does sulphuric acid liberate hydrofluoric acid from its salts?
+
+3. In the preparation of chlorine, what advantages are there in treating
+manganese dioxide with a mixture of sodium chloride and sulphuric acid
+rather than with hydrochloric acid?
+
+4. Why must chlorine water be kept in the dark?
+
+5. What is the derivation of the word nascent?
+
+6. What substances studied are used as bleaching agents? To what is the
+bleaching action due in each case?
+
+7. What substances studied are used as disinfecting agents?
+
+8. What is meant by the statement that hydrochloric acid is one of the
+strongest acids?
+
+9. What is the meaning of the phrase _aqua regia_?
+
+10. Cl_{2}O is the anhydride of what acid?
+
+11. A solution of hydriodic acid on standing turns brown. How is this
+accounted for?
+
+12. How can bromine vapor and nitrogen peroxide be distinguished from
+each other?
+
+13. Write the equations for the reaction taking place when hydriodic
+acid is prepared from iodine, phosphorus, and water.
+
+14. From their behavior toward sulphuric acid, to what class of agents
+do hydrobromic and hydriodic acids belong?
+
+15. Give the derivation of the names of the elements of the chlorine
+family.
+
+16. Write the names and formulas for the binary acids of the group in
+the order of the stability of the acids.
+
+17. What is formed when a metal dissolves in each of the following?
+nitric acid; dilute sulphuric acid; concentrated sulphuric acid;
+hydrochloric acid; aqua regia.
+
+18. How could you distinguish between a chloride, a bromide, and an
+iodide?
+
+19. What weight of sodium chloride is necessary to prepare sufficient
+hydrochloric acid to saturate 1 l. of water under standard conditions?
+
+20. On decomposition 100 l. of hydrochloric acid would yield how many
+liters of hydrogen and chlorine respectively, the gases being measured
+under the same conditions? Are your results in accord with the
+experimental facts?
+
+
+
+
+CHAPTER XVII
+
+CARBON AND SOME OF ITS SIMPLER COMPOUNDS
+
+
+~The family.~ Carbon stands at the head of a family of elements in the
+fourth group in the periodic table. The resemblances between the
+elements of this family, while quite marked, are not so striking as in
+the case of the elements of the chlorine family. With the exception of
+carbon, these elements are comparatively rare, and need not be taken up
+in detail in this chapter. Titanium will be referred to again in
+connection with silicon which it very closely resembles.
+
+~Occurrence.~ Carbon is found in nature in the uncombined state in several
+forms. The diamond is practically pure carbon, while graphite and coal
+are largely carbon, but contain small amounts of other substances. Its
+natural compounds are exceedingly numerous and occur as gases, liquids,
+and solids. Carbon dioxide is its most familiar gaseous compound.
+Natural gas and petroleum are largely compounds of carbon with hydrogen.
+The carbonates, especially calcium carbonate, constitute great strata of
+rocks, and are found in almost every locality. All living organisms,
+both plant and animal, contain a large percentage of this element, and
+the number of its compounds which go to make up all the vast variety of
+animate nature is almost limitless. Over one hundred thousand definite
+compounds containing carbon have been prepared. In the free state carbon
+occurs in three allotropic forms, two of which are crystalline and one
+amorphous.
+
+~Crystalline carbon.~ Crystalline carbon occurs in two forms,--diamond and
+graphite.
+
+1. _Diamond._ Diamonds are found in considerable quantities in several
+localities, especially in South Africa, the East Indies, and Brazil. The
+crystals belong to the regular system, but the natural stones do not
+show this very clearly. When found they are usually covered with a rough
+coating which is removed in the process of cutting. Diamond cutting is
+carried on most extensively in Holland.
+
+The density of the diamond is 3.5, and, though brittle, it is one of the
+hardest of substances. Black diamonds, as well as broken and imperfect
+stones which are valueless as gems, are used for grinding hard
+substances. Few chemical reagents have any action on the diamond, but
+when heated in oxygen or the air it blackens and burns, forming carbon
+dioxide.
+
+Lavoisier first showed that carbon dioxide is formed by the combustion
+of the diamond; and Sir Humphry Davy in 1814 showed that this is the
+only product of combustion, and that the diamond is pure carbon.
+
+     ~The diamond as a gem.~ The pure diamond is perfectly transparent
+     and colorless, but many are tinted a variety of colors by
+     traces of foreign substances. Usually the colorless ones are
+     the most highly prized, although in some instances the color
+     adds to the value; thus the famous Hope diamond is a beautiful
+     blue. Light passing through a diamond is very much refracted,
+     and to this fact the stone owes its brilliancy and sparkle.
+
+     ~Artificial preparation of diamonds.~ Many attempts have been
+     made to produce diamonds artificially, but for a long time
+     these always ended in failure, graphite and not diamonds being
+     the product obtained. The French chemist Moissan, in his
+     extended study of chemistry at high temperatures, finally
+     succeeded (1893) in making some small ones. He accomplished
+     this by dissolving carbon in boiling iron and plunging the
+     crucible containing the mixture into water, as shown in Fig.
+     58. Under these conditions the carbon crystallized in the iron
+     in the form of the diamond. The diamonds were then obtained by
+     dissolving away the iron in hydrochloric acid.
+
+[Illustration: Fig. 58]
+
+2. _Graphite._ This form of carbon is found in large quantities,
+especially in Ceylon, Siberia, and in some localities of the United
+States and Canada. It is a shining black substance, very soft and greasy
+to the touch. Its density is about 2.15. It varies somewhat in
+properties according to the locality in which it is found, and is more
+easily attacked by reagents than is the diamond. It is also manufactured
+by heating carbon with a small amount of iron (3%) in an electric
+furnace. It is used in the manufacture of lead pencils and crucibles, as
+a lubricant, and as a protective covering for iron in the form of a
+polish or a paint.
+
+~Amorphous carbon.~ Although there are many varieties of amorphous carbon
+known, they are not true allotropic modifications. They differ merely in
+their degree of purity, their fineness of division, and in their mode of
+preparation. These substances are of the greatest importance, owing to
+their many uses in the arts and industries. As they occur in nature, or
+are made artificially, they are nearly all impure carbon, the impurity
+depending on the particular substance in question.
+
+1. _Pure carbon._ Pure amorphous carbon is best prepared by charring
+sugar. This is a substance consisting of carbon, hydrogen, and oxygen,
+the latter two elements being present in the ratio of one oxygen atom to
+two of hydrogen. When sugar is strongly heated the oxygen and hydrogen
+are driven off in the form of water and pure carbon is left behind.
+Prepared in this way it is a soft, lustrous, very bulky, black powder.
+
+2. _Coal and coke._ Coals of various kinds were probably formed from
+vast accumulations of vegetable matter in former ages, which became
+covered over with earthy material and were thus protected from rapid
+decay. Under various natural agencies the organic matter was slowly
+changed into coal. In anthracite these changes have gone the farthest,
+and this variety of coal is nearly pure carbon. Soft or bituminous coals
+contain considerable organic matter besides carbon and mineral
+substances. When heated strongly out of contact with air the organic
+matter is decomposed and the resulting volatile matter is driven off in
+the form of gases and vapors, and only the mineral matter and carbon
+remain behind. The gaseous product is chiefly illuminating gas and the
+solid residue is _coke_. Some of the coke is found as a dense cake on
+the sides and roof of the retort. This is called retort carbon and is
+quite pure.
+
+3. _Charcoal._ This is prepared from wood in the same way that coke is
+made from coal. When the process is carried on in retorts the products
+expelled by the heat are saved. Among these are many valuable substances
+such as wood alcohol and acetic acid. Where timber is abundant the
+process is carried out in a wasteful way, by merely covering piles of
+wood with sod and setting the wood on fire. Some wood burns and the heat
+from this decomposes the wood not burned, forming charcoal from it. The
+charcoal, of course, contains the mineral part of the wood from which it
+is formed.
+
+4. _Bone black._ This is sometimes called animal charcoal, and is made
+by charring bones and animal refuse. The organic part of the materials
+is thus decomposed and carbon is left in a very finely divided state,
+scattered through the mineral part which consists largely of calcium
+phosphate. For some uses this mineral part is removed by treatment with
+hydrochloric acid and prolonged washing.
+
+5. _Lampblack._ Lampblack and soot are products of imperfect combustion
+of oil and coal, and are deposited from a smoky flame on a cold surface.
+The carbon in this form is very finely divided and usually contains
+various oily materials.
+
+~Properties.~ While the various forms of carbon differ in many properties,
+especially in color and hardness, yet they are all odorless, tasteless
+solids, insoluble in water and characterized by their stability towards
+heat. Only in the intense heat of the electric arc does carbon
+volatilize, passing directly from the solid state into a vapor. Owing to
+this fact the inside surface of an incandescent light bulb after being
+used for some time becomes coated with a dark film of carbon. It is not
+acted on at ordinary temperatures by most reagents, but at a higher
+temperature it combines directly with many of the elements, forming
+compounds called _carbides_. When heated in the presence of sufficient
+oxygen it burns, forming carbon dioxide.
+
+~Uses of carbon.~ The chief use of amorphous carbon is for fuel to furnish
+heat and power for all the uses of civilization. An enormous quantity of
+carbon in the form of the purer coals, coke, and charcoal is used as a
+reducing agent in the manufacture of the various metals, especially in
+the metallurgy of iron. Most of the metals are found in nature as
+oxides, or in forms which can readily be converted into oxides. When
+these oxides are heated with carbon the oxygen is abstracted, leaving
+the metal. Retort carbon and coke are used to make electric light
+carbons and battery plates, while lampblack is used for indelible inks,
+printer's ink, and black varnishes. Bone black and charcoal have the
+property of absorbing large volumes of certain gases, as well as smaller
+amounts of organic matter; hence they are used in filters to remove
+noxious gases and objectionable colors and odors from water. Bone black
+is used extensively in the sugar refineries to remove coloring matter
+from the impure sugars.
+
+~Chemistry of carbon compounds.~ Carbon is remarkable for the very large
+number of compounds which it forms with the other elements, especially
+with oxygen and hydrogen. Compounds containing carbon are more numerous
+than all others put together, and the chemistry of these substances
+presents peculiarities not met with in the study of other substances.
+For these reasons the systematic study of carbon compounds, or of
+_organic chemistry_ as it is usually called, must be deferred until the
+student has gained some knowledge of the chemistry of other elements. An
+acquaintance with a few of the most familiar carbon compounds is,
+however, essential for the understanding of the general principles of
+chemistry.
+
+~Compounds of carbon with hydrogen,--the hydrocarbons.~ Carbon unites with
+hydrogen to form a very large number of compounds called _hydrocarbons_.
+Petroleum and natural gas are essentially mixtures of a great variety of
+these hydrocarbons. Many others are found in living plants, and still
+others are produced by the decay of organic matter in the absence of
+air. Only two of them, methane and acetylene, will be discussed here.
+
+~Methane~ (_marsh gas_) (CH_{4}). This is one of the most important of
+these hydrocarbons, and constitutes about nine tenths of natural gas. As
+its name suggests, it is formed in marshes by the decay of vegetable
+matter under water, and bubbles of the gas are often seen to rise when
+the dead leaves on the bottom of pools are stirred. It also collects in
+mines, and, when mixed with air, is called _fire damp_ by the miners
+because of its great inflammability, damp being an old name for a gas.
+It is formed when organic matter, such as coal or wood, is heated in
+closed vessels, and is therefore a principal constituent of coal gas.
+
+~Preparation.~ Methane is prepared in the laboratory by heating sodium or
+calcium acetate with soda-lime. Equal weights of fused sodium acetate
+and soda-lime are thoroughly dried, then mixed and placed in a
+good-sized, hard-glass test tube fitted with a one-holed stopper and
+delivery tube. The mixture is gradually heated, and when the air has
+been displaced from the tube the gas is collected in bottles by
+displacement of water. Soda-lime is a mixture of sodium and calcium
+hydroxides. Regarding it as sodium hydroxide alone, the equation is
+
+    NaC_{2}H_{3}O_{2} + NaOH = Na_{2}CO_{3} + CH_{4}.
+
+~Properties.~ Methane is a colorless, odorless gas whose density is 0.55.
+It is difficult to liquefy, boiling at -155 deg. under standard pressure,
+and is almost insoluble in water. It burns with a pale blue flame,
+liberating much heat, and when mixed with oxygen is very explosive.
+
+~Davy's safety lamp.~ In 1815 Sir Humphry Davy invented a lamp for the use
+of miners, to prevent the dreadful mine explosions then common, due to
+methane mixed with air. The invention consisted in surrounding the upper
+part of the common miner's lamp with a mantle of wire gauze and the
+lower part with glass (Fig. 59). It has been seen that two gases will
+not combine until raised to their kindling temperature, and if while
+combining they are cooled below this point, the combination ceases. A
+flame will not pass through a wire gauze because the metal, being a good
+conductor of heat, takes away so much heat from the flame that the gases
+are cooled below the kindling temperature. When a lamp so protected is
+brought into an explosive mixture the gases inside the wire mantle burn
+in a series of little explosions, giving warning to the miner that the
+air is unsafe.
+
+[Illustration: Fig. 59]
+
+~Acetylene~ (C_{2}H_{2}). This is a colorless gas usually having a
+disagreeable odor due to impurities. It is now made in large quantities
+from calcium carbide (CaC_{2}). This substance is formed when coal and
+lime are heated together in an electric furnace. When treated with water
+the carbide is decomposed, yielding acetylene:
+
+    CaC_{2} + 2H_{2}O = C_{2}H_{2} + Ca(OH)_{2}.
+
+Under ordinary conditions the gas burns with a very smoky flame; in
+burners constructed so as to secure a large amount of oxygen it burns
+with a very brilliant white light, and hence is used as an illuminant.
+
+~Laboratory preparation.~ The gas can be prepared readily in a generator
+such as is shown in Fig. 60. The inner tube contains fragments of
+calcium carbide, while the outer one is filled with water. As long as
+the stopcock is closed the water cannot rise in the inner tube. When the
+stopcock is open the water rises, and, coming into contact with the
+carbide in the inner tube, generates acetylene. This escapes through the
+stopcock, and after the air has been expelled may be lighted as it
+issues from the burner.
+
+[Illustration: Fig. 60]
+
+Carbon forms two oxides, namely, carbon dioxide (CO_{2}) and carbon
+monoxide (CO).
+
+~Carbon dioxide~ (CO_{2}). Carbon dioxide is present in the air to the
+extent of about 3 parts in 10,000, and this apparently small amount is
+of fundamental importance in nature. In some localities it escapes from
+the earth in great quantities, and many spring waters carry large
+amounts of it in solution. When these highly charged spring waters reach
+the surface of the earth, and the pressure on them is removed, the
+carbon dioxide escapes with effervescence. It is a product of the
+oxidation of all organic matter, and is therefore formed in fires as
+well as in the process of decay. It is thrown off from the lungs of all
+animals in respiration, and is a product of many fermentation processes
+such as vinegar making and brewing. Combined with metallic oxides it
+forms vast deposits of carbonates in nature.
+
+
+~Preparation.~ In the laboratory carbon dioxide is always prepared by the
+action of an acid upon a carbonate, usually calcium carbonate, the
+apparatus shown in Fig. 39 serving the purpose very well. This reaction
+might be expected to produce carbonic acid, thus:
+
+    CaCO_{3} + 2HCl = CaCl_{2} + H_{2}CO_{3}.
+
+Carbonic acid is very unstable, however, and decomposes into its
+anhydride, CO_{2}, and water, thus:
+
+    H_{2}CO_{3} = H_{2}O + CO_{2}.
+
+The complete reaction is represented by the equation
+
+    CaCO_{3} + 2HCl = CaCl_{2} + CO_{2} + H_{2}O.
+
+~Physical properties.~ Carbon dioxide is a colorless, practically odorless
+gas whose density is 1.5. Its weight may be inferred from the fact that
+it can be siphoned, or poured like water, from one vessel downward into
+another. At 15 deg. and under ordinary pressure it dissolves in its own
+volume of water and imparts a somewhat biting, pungent taste to it. It
+is easily condensed, and is now prepared commercially in this form by
+pumping the gas into steel cylinders (see Fig. 6) which are kept cold
+during the process. When the liquid is permitted to escape into the air
+part of it instantly evaporates, and in so doing absorbs so much heat
+that another portion is solidified, the solid form strikingly resembling
+snow in appearance. This snow is very cold and mercury can easily be
+frozen with it.
+
+~Solid carbon dioxide.~ Cylinders of liquid carbon dioxide are
+inexpensive, and should be available in every school. To demonstrate the
+properties of solid carbon dioxide, the cylinder should be placed across
+the table and supported in such a way that the stopcock end is several
+inches lower than the other end. A loose bag is made by holding the
+corners of a handkerchief around the neck of the stopcock, and the cock
+is then turned on so that the gas rushes out in large quantities. Very
+quickly a considerable quantity of the snow collects in the
+handkerchief. To freeze mercury, press a piece of filter paper into a
+small evaporating dish and pour the mercury upon it. Coil a flat spiral
+upon the end of a wire, and dip the spiral into the mercury. Place a
+quantity of solid carbon dioxide upon the mercury and pour 10 cc.-15 cc.
+of ether over it. In a minute or two the mercury will solidify and may
+be removed from the dish by the wire serving as a handle. The filter
+paper is to prevent the mercury from sticking to the dish; the ether
+dissolves the solid carbon dioxide and promotes its rapid conversion
+into gas.
+
+~Chemical properties.~ Carbon dioxide is incombustible, since it is, like
+water, a product of combustion. It does not support combustion, as does
+nitrogen peroxide, because the oxygen in it is held in very firm
+chemical union with the carbon. Very strong reducing agents, such as
+highly heated carbon, can take away half of its oxygen:
+
+    CO_{2} + C = 2CO.
+
+~Uses.~ The relation of carbon dioxide to plant life has been discussed in
+a previous chapter. Water highly charged with carbon dioxide is used for
+making soda water and similar beverages. Since it is a non-supporter of
+combustion and can be generated readily, carbon dioxide is also used as
+a fire extinguisher. Some of the portable fire extinguishers are simply
+devices for generating large amounts of the gas. It is not necessary
+that all the oxygen should be kept away from the fire in order to
+smother it. A burning candle is extinguished in air which contains only
+2.5% of carbon dioxide.
+
+~Carbonic acid~ (H_{2}CO_{3}). Like most of the oxides of the non-metallic
+elements, carbon dioxide is an acid anhydride. It combines with water to
+form an acid of the formula H_{2}CO_{3}, called carbonic acid:
+
+    H_{2}O + CO_{2} = H_{2}CO_{3}.
+
+The acid is, however, very unstable and cannot be isolated. Only a very
+small amount of it is actually formed when carbon dioxide is passed into
+water, as is evident from the small solubility of the gas. If, however,
+a base is present in the water, salts of carbonic acid are formed, and
+these are quite stable:
+
+    2NaOH + H_{2}O + CO_{2} = Na_{2}CO_{3} + 2H_{2}O.
+
+~Action of carbon dioxide on bases.~ This conduct is explained by the
+principles of reversible reactions. The equation
+
+    H_{2}O +CO_{2} <--> H_{2}CO_{3}
+
+is a reversible equation, and the extent to which the reaction
+progresses depends upon the relative concentrations of each of the three
+factors in it. Equilibrium is ordinarily reached when very little
+H_{2}CO_{3} is formed. If a base is present in the water to combine with
+the H_{2}CO_{3} as fast as it is formed, all of the CO_{2} is converted
+into H_{2}CO_{3}, and thence into a carbonate.
+
+~Salts of carbonic acid,--carbonates.~ The carbonates form a very
+important class of salts. They are found in large quantities in nature,
+and are often used in chemical processes. Only the carbonates of sodium,
+potassium, and ammonium are soluble, and these can be made by the action
+of carbon dioxide on solutions of the bases, as has just been explained.
+
+The insoluble carbonates are formed as precipitates when soluble salts
+are treated with a solution of a soluble carbonate. Thus the insoluble
+calcium carbonate can be made by bringing together solutions of calcium
+chloride and sodium carbonate:
+
+    CaCl_{2} + Na_{2}CO_{3} = CaCO_{3} + 2NaCl.
+
+Most of the carbonates are decomposed by heat, yielding an oxide of the
+metal and carbon dioxide. Thus lime (calcium oxide) is made by strongly
+heating calcium carbonate:
+
+    CaCO_{3} = CaO + CO_{2}.
+
+~Acid carbonates.~ Like all acids containing two acid hydrogen atoms,
+carbonic acid can form both normal and acid salts. The acid carbonates
+are made by treating a normal carbonate with an excess of carbonic acid.
+With few exceptions they are very unstable, heat decomposing them even
+when in solution.
+
+
+~Action of carbon dioxide on calcium hydroxide.~ If carbon dioxide is
+passed into clear lime water, calcium carbonate is at first
+precipitated:
+
+    H_{2}O + CO_{2} = H_{2}CO_{3},
+
+    Ca(OH)_{2} + H_{2}CO_{3} = CaCO_{3} + 2H_{2}O.
+
+Advantage is taken of this reaction in testing for the presence of
+carbon dioxide, as already explained in the chapter on the atmosphere.
+If the current of carbon dioxide is continued, the precipitate soon
+dissolves, because the excess of carbonic acid forms calcium acid
+carbonate which is soluble:
+
+    CaCO_{3} + H_{2}CO_{3} = Ca(HCO_{3})_{2}.
+
+If now the solution is heated, the acid carbonate is decomposed and
+calcium carbonate once more precipitated:
+
+    Ca(HCO_{3})_{2} = CaCO_{3} + H_{2}CO_{3}.
+
+~Carbon monoxide (CO).~ Carbon monoxide can be made in a number of ways,
+the most important of which are the three following:
+
+1. _By the partial oxidation of carbon._ If a slow current of air is
+conducted over highly heated carbon, the monoxide is formed, thus:
+
+    C + O = CO
+
+It is therefore often formed in stoves when the air draught is
+insufficient. Water gas, which contains large amounts of carbon
+monoxide, is made by partially oxidizing carbon with steam:
+
+    C + H_{2}O = CO + 2H.
+
+2. _By the partial reduction of carbon dioxide._ When carbon dioxide is
+conducted over highly heated carbon it is reduced to carbon monoxide by
+the excess of carbon:
+
+    CO_{2} + C = 2CO.
+
+When coal is burning in a stove or grate carbon dioxide is at first
+formed in the free supply of air, but as the hot gas rises through the
+glowing coal it is reduced to carbon monoxide. When the carbon monoxide
+reaches the free air above the coal it takes up oxygen to form carbon
+dioxide, burning with the blue flame so familiar above a bed of coals,
+especially in the case of hard coals.
+
+3. _By the decomposition of oxalic acid._ In the laboratory carbon
+monoxide is usually prepared by the action of concentrated sulphuric
+acid upon oxalic acid. The latter substance has the formula
+C_{2}H_{2}O_{4}. The sulphuric acid, owing to its affinity for water,
+decomposes the oxalic acid, as represented in the equation
+
+    C_{2}H_{2}O_{4} + (H_{2}SO_{4}) = (H_{2}SO_{4}) + H_{2}O + CO_{2} + CO.
+
+~Properties.~ Carbon monoxide is a light, colorless, almost odorless gas,
+very difficult to liquefy. Chemically it is very active, combining
+directly with a great many substances. It has a great affinity for
+oxygen and is therefore combustible and a good reducing agent. Thus, if
+carbon monoxide is passed over hot copper oxide, the copper is reduced
+to the metallic state:
+
+    CuO + CO = Cu + CO_{2}.
+
+When inhaled it combines with the red coloring matter of the blood and
+in this way prevents the absorption of oxygen, so that even a small
+quantity of the gas may prove fatal.
+
+[Illustration: Fig. 61]
+
+     ~The reducing power of carbon monoxide.~ Fig. 61 illustrates a
+     method of showing the reducing power of carbon monoxide. The
+     gas is generated by gently heating 7 or 8 g. of oxalic acid
+     with 25 cc. of concentrated sulphuric acid in a 200 cc. flask
+     A. The bottle B contains a solution of sodium hydroxide,
+     which removes the carbon dioxide formed along with the
+     monoxide. C contains a solution of calcium hydroxide to show
+     that the carbon dioxide is completely removed. E is a
+     hard-glass tube containing 1 or 2 g. of copper oxide, which is
+     heated by a burner. The black copper oxide is reduced to
+     reddish metallic copper by the carbon monoxide, which is
+     thereby changed to carbon dioxide. The presence of the carbon
+     dioxide is shown by the precipitate in the calcium hydroxide
+     solution in D. Any unchanged carbon monoxide is collected
+     over water in F.
+
+~Carbon disulphide~ (CS_{2}). Just as carbon combines with oxygen to form
+carbon dioxide, so it combines with sulphur to form carbon disulphide
+(CS_{2}). This compound has been described in the chapter on sulphur.
+
+~Hydrocyanic acid~ (_prussic acid_)(HCN). Under the proper conditions
+carbon unites with nitrogen and hydrogen to form the acid HCN, called
+hydrocyanic acid. It is a weak, volatile acid, and is therefore easily
+prepared by treating its salts with sulphuric acid:
+
+    KCN + H_{2}SO_{4} = KHSO_{4} + HCN.
+
+It is most familiar as a gas, though it condenses to a colorless liquid
+boiling at 26 deg.. It has a peculiar odor, suggesting bitter almonds, and
+is extremely poisonous either when inhaled or when taken into the
+stomach. A single drop may cause death. It dissolves readily in water,
+its solution being commonly called prussic acid.
+
+The salts of hydrocyanic acid are called _cyanides_, the cyanides of
+sodium and potassium being the best known. These are white solids and
+are extremely poisonous.
+
+~Solutions of potassium cyanide are alkaline.~ A solution of potassium
+cyanide turns red litmus blue, and must therefore contain hydroxyl ions.
+The presence of these ions is accounted for in the following way.
+
+Although water is so little dissociated into its ions H^{+} and OH^{-}
+that for most purposes we may neglect the dissociation, it is
+nevertheless measurably dissociated. Hydrocyanic acid is one of the
+weakest of acids, and dissociates to an extremely slight extent. When a
+cyanide such as potassium cyanide dissolves it freely dissociates, and
+the CN^{-} ions must come to an equilibrium with the H^{+} ions derived
+from the water:
+
+    H^{+} + CN^{-} <--> HCN.
+
+The result of this equilibrium is that quite a number of H^{+} ions from
+the water are converted into undissociated HCN molecules. But for every
+H^{+} ion so removed an OH^{-} ion remains free, and this will give the
+solution alkaline properties.
+
+
+EXERCISES
+
+1. How can you prove that the composition of the different allotropic
+forms of carbon is the same?
+
+2. Are lampblack and bone black allotropic forms of carbon? Will equal
+amounts of heat be liberated in the combustion of 1 g. of each?
+
+3. How could you judge of the relative purity of different forms of
+carbon?
+
+4. Apart from its color, why should carbon be useful in the preparation
+of inks and paints?
+
+5. Could asbestos fibers be used to replace the wire in a safety lamp?
+
+6. Why do most acids decompose carbonates?
+
+7. What effect would doubling the pressure have upon the solubility of
+carbon dioxide in water?
+
+8. What compound would be formed by passing carbon dioxide into a
+solution of ammonium hydroxide? Write the equation.
+
+9. Write equations for the preparation of K_{2}CO_{3}; of BaCO_{3}; of
+MgCO_{3}.
+
+10. In what respects are carbonic and sulphurous acids similar?
+
+11. Give three reasons why the reaction which takes place when a
+solution of calcium acid carbonate is heated, completes itself.
+
+12. How could you distinguish between carbonates and sulphites?
+
+13. How could you distinguish between oxygen, hydrogen, nitrogen,
+nitrous oxide, and carbon dioxide?
+
+14. Could a solution of sodium hydroxide be substituted for the solution
+of calcium hydroxide in testing for carbon dioxide?
+
+15. What weight of sodium hydroxide is necessary to neutralize the
+carbonic acid formed by the action of hydrochloric acid on 100 g. of
+calcium carbonate?
+
+16. What weight of calcium carbonate would be necessary to prepare
+sufficient carbon dioxide to saturate 10 l. of water at 15 deg. and under
+ordinary pressure?
+
+17. On the supposition that calcium carbide costs 12 cents a kilogram,
+what would be the cost of an amount sufficient to generate 100 l. of
+acetylene measured at 20 deg. and 740 mm.?
+
+18. How would the volume of a definite amount of carbon monoxide compare
+with the volume of carbon dioxide formed by its combustion, the
+measurements being made under the same conditions?
+
+
+
+
+CHAPTER XVIII
+
+FLAMES,--ILLUMINANTS
+
+
+~Conditions necessary for flames.~ It has been seen that when two
+substances unite chemically, with the production of light and heat, the
+act of union is called combustion. When one of the substances undergoing
+combustion remains solid at the temperature occasioned by the
+combustion, light may be given off, but there is no flame. Thus iron
+wire burning in oxygen throws off a shower of sparks and is brilliantly
+incandescent, but no flame is seen. When, however, both of the
+substances are gases or vapors at the temperature reached in the
+combustion, the act of union is accompanied by a flame.
+
+~Flames from burning liquids or solids.~ Many substances which are liquids
+or solids at ordinary temperatures burn with a flame because the heat of
+combustion vaporizes them slowly, and the flame is due to the union of
+this vapor with the gas supporting the combustion.
+
+~Supporter of combustion.~ That gas which surrounds the flame and
+constitutes the atmosphere in which the combustion occurs is said to
+support the combustion. The other gas which issues into this atmosphere
+is said to be the combustible gas. Thus, in the ordinary combustion of
+coal gas in the air the coal gas is said to be combustible, while the
+air is regarded as the supporter of combustion. These terms are entirely
+relative, however, for a jet of air issuing into an atmosphere of coal
+gas will burn when ignited, the coal gas supporting the combustion.
+Ordinarily, when we say that a gas is combustible we mean that it is
+combustible in an atmosphere of air.
+
+[Illustration: Fig. 62]
+
+~Either gas may be the supporter of combustion.~ That the terms
+_combustible_ and _supporter of combustion_ are merely relative may be
+shown in the following way: A lamp chimney A is fitted with a cork and
+glass tubes, as shown in Fig. 62. The tube C should have a diameter of
+from 12 to 15 mm. A thin sheet of asbestos in which is cut a circular
+opening about 2 cm. in diameter is placed over the top of the chimney.
+The opening in the asbestos is closed with the palm of the hand, and gas
+is admitted to the chimney through the tube B. The air in the chimney
+is soon expelled through the tube C, and the gas itself is then
+lighted at the lower end of this tube. The hand is now removed from the
+opening in the asbestos, when the flame at the end of the tube at once
+rises and appears at the end within the chimney, as shown in the figure.
+The excess of coal gas now escapes from the opening in the asbestos and
+may be lighted. The flame at the top of the asbestos board is due to the
+combustion of coal gas in air, while the flame within the chimney is due
+to the combustion of air in coal gas, the air being drawn up through the
+tube by the escaping gas.
+
+~Appearance of flames.~ The flame caused by the union of hydrogen and
+oxygen is almost colorless and invisible. Chlorine and hydrogen combine
+with a pale violet flame, carbon monoxide burns in oxygen with a blue
+flame, while ammonia burns with a deep yellow flame. The color and
+appearance of flames are therefore often quite characteristic of the
+particular combustion which occasions them.
+
+~Structure of flames.~ When the gas undergoing combustion issues from a
+round opening into an atmosphere of the gas supporting combustion, as is
+the case with the burning Bunsen burner (Fig. 63), the flame is
+generally conical in outline. It consists of several distinct cones,
+one within the other, the boundary between them being marked by
+differences of color or luminosity. In the simplest flame, of which
+hydrogen burning in oxygen is a good example, these cones are two in
+number,--an inner one, formed by unburned gas, and an outer one, usually
+more or less luminous, consisting of the combining gases. This outer one
+is in turn surrounded by a third envelope of the products of combustion;
+this envelope is sometimes invisible, as in the present case, but is
+sometimes faintly luminous. The lower part of the inner cone of the
+flame is quite cool and consists of unburned gas. Toward the top of the
+inner cone the gas has become heated to a high temperature by the
+burning envelope surrounding it. On reaching the supporter of combustion
+on the outside it is far above its kindling temperature, and combustion
+follows with the evolution of much heat. The region of combustion just
+outside the inner cone is therefore the hottest part of the flame.
+
+[Illustration: Fig. 63]
+
+~Oxidizing and reducing flames.~ Since the tip of the outside cone
+consists of very hot products of combustion mixed with oxygen from the
+air, a substance capable of oxidation placed in this part of the flame
+becomes very hot and is easily oxidized. The oxygen with which it
+combines comes, of course, from the atmosphere, and not from the
+products of combustion. This outer tip of the flame is called the
+_oxidizing flame_.
+
+At the tip of the inner cone the conditions are quite different. This
+region consists of a highly heated combustible gas, which has not yet
+reached a supply of oxygen.
+
+If a substance rich in oxygen, such as a metallic oxide, is placed in
+this region of the flame, the heated gases combine with its oxygen and
+the substance is reduced. This part of the flame is called the _reducing
+flame_. These flames are used in testing certain substances, especially
+minerals. For this purpose they are produced by blowing into a small
+luminous Bunsen flame from one side through a blowpipe. This is a tube
+of the shape shown in Fig. 64. The flame is directed in any desired way
+and has the oxidizing and reducing regions very clearly marked (Fig.
+65). It is non-luminous from the same causes which render the open
+Bunsen burner flame non-luminous, the gases from the lungs serving to
+furnish oxygen and to dilute the combustible gas.
+
+[Illustration: Fig. 64]
+
+[Illustration: Fig. 65]
+
+~Luminosity of flames.~ The luminosity of flames is due to a number of
+distinct causes, and may therefore be increased or diminished in several
+ways.
+
+1. _Presence of solid matter._ The most obvious of these causes is the
+presence in the flame of incandescent solid matter. Thus chalk dust
+sifted into a non-luminous flame renders it luminous. When hydrocarbons
+form a part of the combustible gas, as they do in nearly all
+illuminating gases and oils, some carbon is usually set free in the
+process of combustion. This is made very hot by the flame and becomes
+incandescent, giving out light. In a well-regulated flame it is
+afterward burned up, but when the supply of oxygen is insufficient it
+escapes from the flame as lampblack or soot. That it is temporarily
+present in a well-burning luminous flame may be demonstrated by holding
+a cold object, such as a small evaporating dish, in the flame for a few
+seconds. This cold object cools the carbon below its kindling
+temperature, and it is deposited on the object as soot.
+
+2. _Pressure._ A second factor in the luminosity of flames is the
+pressure under which the gases are burning. Under increased pressure
+there is more matter in a given volume of a gas, and the chemical action
+is more energetic than when the gases are rarefied. Consequently there
+is more heat and light. A candle burning on a high mountain gives less
+light than when it burns at the sea level.
+
+If the gas is diluted with a non-combustible gas, the effect is the same
+as if it is rarefied, for under these conditions there is less
+combustible gas in a given volume.
+
+3. _Temperature._ The luminosity also depends upon the temperature
+attained in the combustion. In general the hotter the flame the greater
+the luminosity; hence cooling the gases before combustion diminishes the
+luminosity of the flame they will make, because it diminishes the
+temperature attained in the combustion. Thus the luminosity of the
+Bunsen flame is largely diminished by the air drawn up with the gas.
+This is due in part to the fact that the burning gas is diluted and
+cooled by the air drawn in. The oxygen thus introduced into the flame
+also causes the combustion of the hot particles of carbon which would
+otherwise tend to make the flame luminous.
+
+~Illuminating and fuel gases.~ A number of mixtures of combustible gases,
+consisting largely of carbon compounds and hydrogen, find extensive use
+for the production of light and heat. The three chief varieties are coal
+gas, water gas, and natural gas. The use of acetylene gas has already
+been referred to.
+
+~Coal gas.~ Coal gas is made by heating bituminous coal in large retorts
+out of contact with the air. Soft or bituminous coal contains, in
+addition to large amounts of carbon, considerable quantities of
+compounds of hydrogen, oxygen, nitrogen, and sulphur. When distilled the
+nitrogen is liberated partly in the form of ammonia and cyanides and
+partly as free nitrogen gas; the sulphur is converted into hydrogen
+sulphide, carbon disulphide, and oxides of sulphur; the oxygen into
+water and oxides of carbon. The remaining hydrogen is set free partly
+as hydrogen and partly in combination with carbon in the form of
+hydrocarbons. The most important of these is methane, with smaller
+quantities of many others, some of which are liquids or solids at
+ordinary temperatures. The great bulk of the carbon remains behind as
+coke and retort carbon.
+
+~The manufacture of coal gas.~ In the manufacture of coal gas it is
+necessary to separate from the volatile constituents formed by the
+heating of the coal all those substances which are either solid or
+liquid at ordinary temperature, since these would clog the gas pipes.
+Certain gaseous constituents, such as hydrogen sulphide and ammonia,
+must also be removed. The method used to accomplish this is shown in
+Fig. 66. The coal is heated in air-tight retorts illustrated by A. The
+volatile products escape through the pipe X and bubble into the tarry
+liquid in the large pipe B, known as the _hydraulic main_, which runs
+at right angles to the retorts. Here is deposited the greater portion of
+the solid and liquid products, forming a tarry mass known as _coal tar_.
+Much of the ammonia also remains dissolved in this liquid. The partially
+purified gas then passes into the pipes C, which serve to cool it and
+further remove the solid and liquid matter. The gas then passes into
+D, which is filled with coke over which a jet of water is sprayed. The
+water still further cools the gas and at the same time partially removes
+such gaseous products as hydrogen sulphide and ammonia, which are
+soluble in water. In E the gas passes over some material such as lime,
+which removes the last portions of the sulphur compounds as well as much
+of the carbon dioxide present. From E the gas passes into the large
+gas holder F, from which it is distributed through pipes to the places
+where it is burned.
+
+[Illustration: Fig. 66]
+
+     One ton of good gas coal yields approximately 10,000 cu. ft. of
+     gas, 1400 lb. of coke, 120 lb. of tar, and 20 gal. of
+     ammoniacal liquor.
+
+     Not only is the ammonia obtained in the manufacture of the gas
+     of great importance, but the coal tar also serves as the source
+     of many very useful substances, as will be explained in Chapter
+     XXXII.
+
+~Water gas.~ Water gas is essentially a mixture of carbon monoxide and
+hydrogen. It is made by passing steam over very hot anthracite coal,
+when the reaction shown in the following equation takes place:
+
+    C + H_{2}O = CO + 2H.
+
+When required merely to produce heat the gas is at once ready for use.
+When made for illuminating purposes it must be enriched, that is,
+illuminants must be added, since both carbon monoxide and hydrogen burn
+with non-luminous flames. This is accomplished by passing it into
+heaters containing highly heated petroleum oils. The gas takes up
+hydrocarbon gases formed in the decomposition of the petroleum oils,
+which make it burn with a luminous flame.
+
+Water gas is very effective as a fuel, since both carbon monoxide and
+hydrogen burn with very hot flames. It has little odor and is very
+poisonous. Its use is therefore attended with some risk, since leaks in
+pipes are very likely to escape notice.
+
+~Natural gas.~ This substance, so abundant in many localities, varies much
+in composition, but is composed principally of methane. When used for
+lighting purposes it is usually burned in a burner resembling an open
+Bunsen, the illumination being furnished by an incandescent mantle. This
+is the case in the familiar Welsbach burner. Contrary to statements
+frequently made, natural gas contains no free hydrogen.
+
+
+TABLE SHOWING COMPOSITION OF GASES
+
+=====================+================+========+========+==========
+                     |  PENNSYLVANIA  |  COAL  | WATER  | ENRICHED
+                     |    NATURAL     |   GAS  |  GAS   |  WATER
+                     |      GAS       |        |        |   GAS
+---------------------+----------------+--------+--------+----------
+Hydrogen             |                |  41.3  | 52.88  |  30.00
+Methane              |    90.64       |  43.6  |  2.16  |  24.00
+Illuminants          |                |   3.9  |        |  12.05
+Carbon monoxide      |                |   6.4  | 36.80  |  29.00
+Carbon dioxide       |     0.30       |   2.0  |  3.47  |   0.30
+Nitrogen             |     9.06       |   1.2  |  4.69  |   2.50
+Oxygen               |                |   0.3  |        |   1.50
+Hydrocarbon vapors   |                |   1.5  |        |   1.50
+=====================+================+========+========+==========
+
+     These are analyses of actual samples, and may be taken as about
+     the average for the various kinds of gases. Any one of these
+     may vary considerably. The nitrogen and oxygen in most cases is
+     due to a slight admixture of air which is difficult to exclude
+     entirely in the manufacture and handling of gases.
+
+~Fuels.~ A variety of substances are used as fuels, the most important of
+them being wood, coal, and the various gases mentioned above. Wood
+consists mainly of compounds of carbon, hydrogen, and oxygen. The
+composition of coal and the fuel gases has been given. Since these fuels
+are composed principally of carbon and hydrogen or their compounds, the
+chief products of combustion are carbon dioxide and water. The practice
+of heating rooms with portable gas or oil stoves with no provision for
+removing the products of combustion is to be condemned, since the carbon
+dioxide is generated in sufficient quantities to render the air unfit
+for breathing. Rooms so heated also become very damp from the large
+amount of water vapor formed in the combustion, and which in cold
+weather condenses on the window glass, causing the glass to "sweat."
+Both coal and wood contain a certain amount of mineral substances which
+constitute the ashes.
+
+~The electric furnace.~ In recent years electric furnaces have come into
+wide use in operations requiring a very high temperature. Temperatures
+as high as 3500 deg. can be easily reached, whereas the hottest oxyhydrogen
+flame is not much above 2000 deg.. These furnaces are constructed on one of
+two general principles.
+
+[Illustration: Fig. 67]
+
+1. _Arc furnaces._ In the one type the source of heat is an electric arc
+formed between carbon electrodes separated a little from each other, as
+shown in Fig. 67. The substance to be heated is placed in a vessel,
+usually a graphite crucible, just below the arc. The electrodes and
+crucible are surrounded by materials which fuse with great difficulty,
+such as magnesium oxide, the walls of the furnace being so shaped as to
+reflect the heat downwards upon the contents of the crucible.
+
+[Illustration: Fig. 68]
+
+2. _Resistance furnaces._ In the other type of furnace the heat is
+generated by the resistance offered to the current in its passage
+through the furnace. In its simplest form it may be represented by Fig.
+68. The furnace is merely a rectangular box built up of loose bricks.
+The electrodes E, each consisting of a bundle of carbon rods, are
+introduced through the sides of the furnace. The materials to be heated,
+C, are filled into the furnace up to the electrodes, and a layer of
+broken coke is arranged so as to extend from one electrode to the other.
+More of the charge is then placed on top of the coke. In passing through
+the broken coke the electrical current encounters great resistance. This
+generates great heat, and the charge surrounding the coke is brought to
+a very high temperature. The advantage of this type of furnace is that
+the temperature can be regulated to any desired intensity.
+
+
+EXERCISES
+
+1. Why does charcoal usually burn with no flame? How do you account for
+the flame sometimes observed when it burns?
+
+2. How do you account for the fact that a candle burns with a flame?
+
+3. What two properties must the mantle used in the Welsbach lamp
+possess?
+
+4. (a) In what respects does the use of the Welsbach mantle resemble
+that of lime in the calcium light? (b) If the mantle were made of
+carbon, would it serve the same purpose?
+
+5. Would anthracite coal be suitable for the manufacture of coal gas?
+
+6. How could you prove the formation of carbon dioxide and water in the
+combustion of illuminating gases?
+
+7. Suggest a probable way in which natural gas has been formed.
+
+8. Coal frequently contains a sulphide of iron. (a) What two sulphur
+compounds are likely to be formed when gas is made from such coal? (b)
+Suggest some suitable method for the removal of these compounds.
+
+9. Why does the use of the bellows on the blacksmith's forge cause a
+more intense heat?
+
+10. What volume of oxygen is necessary to burn 100 l. of marsh gas and
+what volume of carbon dioxide would be formed, all of the gases being
+measured under standard conditions?
+
+11. Suppose a cubic meter of Pennsylvania natural gas, measured under
+standard conditions, were to be burned. How much water by weight would
+result?
+
+
+
+
+CHAPTER XIX
+
+MOLECULAR WEIGHTS, ATOMIC WEIGHTS, FORMULAS
+
+
+~Introduction.~ In the chapter on The Atomic Theory, it was shown that if
+it were true that two elements uniting to form a compound always
+combined in the ratio of one atom of one element to one atom of the
+other element, it would be a very easy matter to decide upon figures
+which would represent the relative weights of the different atoms. It
+would only be necessary to select some one element as a standard and
+determine the weight of every element which combines with a definite
+weight (say 1 g.) of the standard element. The figures so obtained would
+evidently represent the relative weights of the atoms.
+
+But the law of multiple proportion at once reminds us that two elements
+may unite in several proportions; and there is no simple way to
+determine the number of atoms present in the molecule of any compound.
+Consequently the problem of deciding upon the relative atomic weights is
+not an easy one. To the solution of this problem we must now turn.
+
+~Dalton's method of determining atomic weights.~ When Dalton first
+advanced the atomic theory he attempted to solve this problem by very
+simple methods. He thought that when only one compound of two elements
+is known it is reasonable to suppose that it contains one atom of each
+element. He therefore gave the formula HO to water, and HN to ammonia.
+When more than two compounds were known he assumed that the most
+familiar or the most stable one had the simple formula. He then
+determined the atomic weight as explained above. The results he
+obtained were contradictory and very far from satisfactory, and it was
+soon seen that some other method, resting on much more scientific
+grounds, must be found to decide what compounds, if any, have a single
+atom of each element present.
+
+~Determination of atomic weights.~ Three distinct steps are involved in
+the determination of the atomic weight of an element: (1) determination
+of the equivalent, (2) determination of molecular weights of its
+compounds, and (3) deduction of the exact atomic weight from the
+equivalent and molecular weights.
+
+~1. Determination of the equivalent.~ By the equivalent of an element is
+meant the weight of the element which will combine with a fixed weight
+of some other element chosen as a standard. It has already been
+explained that oxygen has been selected as the standard element for
+atomic weights, with a weight of 16. This same standard will serve very
+well as a standard for equivalents. _The equivalent of an element is the
+weight of the element which will combine with 16 g. of oxygen._ Thus 16
+g. of oxygen combines with 16.03 g. of sulphur, 65.4 g. of zinc, 215.86
+g. of silver, 70.9 g. of chlorine. These figures, therefore, represent
+the equivalent weights of these elements.
+
+~Relation of atomic weights to equivalents.~ According to the atomic
+theory combination always takes place between whole numbers of atoms.
+Thus one atom unites with one other, or with two or three; or two atoms
+may unite with three, or three with five, and so on.
+
+When oxygen combines with zinc the combination must be between definite
+numbers of the two kinds of atoms. Experiment shows that these two
+elements combine in the ratio of 16 g. of oxygen to 65.4 g. of zinc. If
+one atom of oxygen combines with one atom of zinc, then this ratio must
+be the ratio between the weights of the two atoms. If one atom of oxygen
+combines with two atoms of zinc, then the ratio between the weights of
+the two atoms will be 16: 32.7. If two atoms of oxygen combine with one
+atom of zinc, the ratio by weight between the two atoms will be 8: 65.4.
+It is evident, therefore, that the real atomic weight of an element must
+be some multiple or submultiple of the equivalent; in other words, the
+equivalent multiplied by 1/2, 1, 2, or 3 will give the atomic weight.
+
+~Combining weights.~ A very interesting relation holds good between the
+equivalents of the various elements. We have just seen that the figures
+16.03, 65.4, 215.86, and 70.9 are the equivalents respectively of
+sulphur, zinc, silver, and chlorine. These same figures represent the
+ratios by weight in which these elements combine among themselves. Thus
+215.86 g. of silver combine with 70.9 g. of chlorine and with 2 x 16.03
+g. of sulphur. 65.4 g. of zinc combine with 70.9 g. of chlorine and 2 x
+16.03 g. of sulphur.
+
+By taking the equivalent or some multiple of it a value can be obtained
+for each element which will represent its combining value, and for this
+reason is called its _combining weight_. It is important to notice that
+the fact that a combining weight can be obtained for each element is not
+a part of a theory, but is the direct result of experiment.
+
+~Elements with more than one equivalent.~ It will be remembered that
+oxygen combines with hydrogen in two ratios. In one case 16 g. of oxygen
+combine with 2.016 g. of hydrogen to form water; in the other 16 g. of
+oxygen combine with 1.008 g. of hydrogen to form hydrogen dioxide. The
+equivalents of hydrogen are therefore 2.016 and 1.008. Barium combines
+with oxygen in two proportions: in barium oxide the proportion is 16 g.
+of oxygen to 137.4 g. of barium; in barium dioxide the proportion is 16
+g. of oxygen to 68.7 g. of barium.
+
+In each case one equivalent is a simple multiple of the other, so the
+fact that there may be two equivalents does not add to the uncertainty.
+All we knew before was that the true atomic weight is some multiple of
+the equivalent.
+
+~2. The determination of molecular weights.~ To decide the question as to
+which multiple of the equivalent correctly represents the atomic weight
+of an element, it has been found necessary to devise a method of
+determining the molecular weights of compounds containing the element in
+question. Since the molecular weight of a compound is merely the sum of
+the weights of all the atoms present in it, it would seem to be
+impossible to determine the molecular weight of a compound without first
+knowing the atomic weights of the constituent atoms, and how many atoms
+of each element are present in the molecule. But certain facts have been
+discovered which suggest a way in which this can be done.
+
+~Avogadro's hypothesis.~ We have seen that the laws of Boyle, Charles, and
+Gay-Lussac apply to all gases irrespective of their chemical character.
+This would lead to the inference that the structure of gases must be
+quite simple, and that it is much the same in all gases.
+
+In 1811 Avogadro, an Italian physicist, suggested that if we assume all
+gases under the same conditions of temperature and pressure to have the
+same number of molecules in a given volume, we shall have a probable
+explanation of the simplicity of the gas laws. It is difficult to prove
+the truth of this hypothesis by a simple experiment, but there are so
+many facts known which are in complete harmony with this suggestion that
+there is little doubt that it expresses the truth. Avogadro's hypothesis
+may be stated thus: _Equal volumes of all gases under the same
+conditions of temperature and pressure contain the same number of
+molecules._
+
+~Avogadro's hypothesis and molecular weights.~ Assuming that Avogadro's
+hypothesis is correct, we have a very simple means for deciding upon the
+relative weights of molecules; for if equal volumes of two gases contain
+the same number of molecules, the weights of the two volumes must be in
+the same ratio as the weights of the individual molecules which they
+contain. If we adopt some one gas as a standard, we can express the
+weights of all other gases as compared with this one, and the same
+figures will express the relative weights of the molecules of which the
+gases are composed.
+
+~Oxygen as the standard.~ It is important that the same standard should be
+adopted for the determination of molecular weights as has been decided
+upon for atomic weights and equivalents, so that the three values may be
+in harmony with each other. Accordingly it is best to adopt oxygen as
+the standard element with which to compare the molecular weights of
+other gases, being careful to keep the oxygen atom equal to 16.
+
+~The oxygen molecule contains two atoms.~ One point must not be
+overlooked, however. We desire to have our unit, the oxygen _atom_,
+equal to 16. The method of comparing the weights of gases just suggested
+compares the molecules of the gases with the _molecule_ of oxygen. Is
+the molecule and the atom of oxygen the same thing? This question is
+answered by the following considerations.
+
+We have seen that when steam is formed by the union of oxygen and
+hydrogen, two volumes of hydrogen combine with one volume of oxygen to
+form two volumes of steam. Let us suppose that the one volume of oxygen
+contains 100 molecules; then the two volumes of steam must, according
+to Avogadro's hypothesis, contain 200 molecules. But each of these 200
+molecules must contain at least one atom of oxygen, or 200 in all, and
+these 200 atoms came from 100 molecules of oxygen. It follows that each
+molecule of oxygen must contain at least two atoms of oxygen.
+
+Evidently this reasoning merely shows that there are _at least_ two
+atoms in the oxygen molecule. There may be more than that, but as there
+is no evidence to this effect, we assume that the molecule contains two
+atoms only.
+
+It is evident that if we wish to retain the value 16 for the atom of
+oxygen we must take twice this value, or 32, for the value of the oxygen
+molecule, when using it as a standard for molecular weights.
+
+~Determination of the molecular weights of gases from their weights
+compared with oxygen.~ Assuming the molecular weight of oxygen to be 32,
+Avogadro's hypothesis gives us a ready means for determining the
+molecular weight of any other gas, for all that is required is to know
+its weight compared with that of an equal volume of oxygen. For example,
+1 l. of chlorine is found by experiment to weigh 2.216 times as much as
+1 l. of oxygen. The molecular weight of chlorine must therefore be 2.216
+x 32, or 70.91.
+
+If, instead of comparing the relative weights of 1 l. of the two gases,
+we select such a volume of oxygen as will weigh 32 g., or the weight in
+grams corresponding to the molecular weight of the gas, the calculation
+is much simplified. It has been found that 32 g. of oxygen, under
+standard conditions, measure 22.4 l. This same volume of hydrogen weighs
+2.019 g.; of chlorine 70.9 g.; of hydrochloric acid 36.458 g. The
+weights of these equal volumes must be proportional to their molecular
+weights, and since the weight of the oxygen is the same as the value of
+its molecular weight, so too will the weights of the 22.4 l. of the
+other gases be equal to the value of their molecular weights.
+
+As a summary we can then make the following statement: _The molecular
+weight of any gas may be determined by calculating the weight of 22.4 l.
+of the gas, measured under standard conditions._
+
+~Determination of molecular weights from density of gases.~ In an actual
+experiment it is easier to determine the density of a gas than the
+weight of a definite volume of it. The density of a gas is usually
+defined as its weight compared with that of an equal volume of air.
+Having determined the density of a gas, its weight compared with oxygen
+may be determined by multiplying its density by the ratio between the
+weights of air and oxygen. This ratio is 0.9046. To compare it with our
+standard for atomic weights we must further multiply it by 32, since the
+standard is 1/32 the weight of oxygen molecules. The steps then are
+these:
+
+1. Determine the density of the gas (its weight compared with air).
+
+2. Multiply by 0.9046 to make the comparison with oxygen molecules.
+
+3. Multiply by 32 to make the comparison with the unit for atomic
+weights.
+
+We have, then, the formula:
+
+    molecular weight = density x 0.9046 x 32;
+
+or, still more briefly,
+
+    M. = D. x 28.9.
+
+The value found by this method for the determination of molecular
+weights will of course agree with those found by calculating the weight
+of 22.4 l. of the gas, since both methods depend on the same principles.
+
+[Illustration: Fig. 69]
+
+     ~Determination of densities of gases.~ The relative weights of
+     equal volumes of two gases can be easily determined. The
+     following is one of the methods used. A small flask, such as is
+     shown in Fig. 69, is filled with one of the gases, and after
+     the temperature and pressure have been noted the flask is
+     sealed up and weighed. The tip of the sealed end is then broken
+     off, the flask filled with the second gas, and its weight
+     determined. If the weight of the empty flask is subtracted from
+     these two weighings, the relative weights of the gases is
+     readily found.
+
+~3. Deduction of atomic weights from molecular weights and equivalents.~
+We have now seen how the equivalent of an element and the molecular
+weight of compounds containing the element can be obtained. Let us see
+how it is possible to decide which multiple of the equivalent really is
+the true atomic weight. As an example, let us suppose that the
+equivalent of nitrogen has been found to be 7.02 and that it is desired
+to obtain its atomic weight. The next step is to obtain the molecular
+weights of a large number of compounds containing nitrogen. The
+following will serve:
+
+==================+============+=============+================+==============
+                  |            | APPROXIMATE |  PERCENTAGE OF |    PART OF
+                  | DENSITY BY |  MOLECULAR  |   NITROGEN BY  |   MOLECULAR
+                  | EXPERIMENT |   WEIGHT    |    EXPERIMENT  |   WEIGHT DUE
+                  |            | (D. x 28.9) |                |  TO NITROGEN
+------------------+------------+-------------+----------------+--------------
+Nitrogen gas      |   0.9671   |    27.95    |    100.00      |    27.95
+Nitrous oxide     |   1.527    |    44.13    |     63.70      |    27.11
+Nitric oxide      |   1.0384   |    30.00    |     46.74      |    14.02
+Nitrogen peroxide |   1.580    |    45.66    |     30.49      |    13.90
+Ammonia           |   0.591    |    17.05    |     82.28      |    14.03
+Nitric acid       |   2.180    |    63.06    |     22.27      |    14.03
+Hydrocyanic acid  |   0.930    |    26.87    |     51.90      |    13.94
+==================+============+=============+================+==============
+
+~Method of calculation.~ The densities of the various gases in the first
+column of this table are determined by experiment, and are fairly
+accurate but not entirely so. By multiplying these densities by 28.9 the
+molecular weights of the compounds as given in the second column are
+obtained. By chemical analysis it is possible to determine the
+percentage composition of these substances, and the percentages of
+nitrogen in them as determined by analysis are given in the third
+column. If each of these molecular weights is multiplied in turn by the
+percentage of nitrogen in the compound, the product will be the weight
+of the nitrogen in the molecular weight of the compound. This will be
+the sum of the weights of the nitrogen atoms in the molecule. These
+values are given in the fourth column in the table.
+
+If a large number of compounds containing nitrogen are studied in this
+way, it is probable that there will be included in the list at least one
+substance whose molecule contains a single nitrogen atom. In this case
+the number in the fourth column will be the approximate atomic weight of
+nitrogen. On comparing the values for nitrogen in the table it will be
+seen that a number which is approximately 14 is the smallest, and that
+the others are multiples of this. These compounds of higher value,
+therefore, contain more than one nitrogen atom in the molecule.
+
+~Accurate determination of atomic weights.~ Molecular weights cannot be
+determined very accurately, and consequently the part in them due to
+nitrogen is a little uncertain, as will be seen in the table. All we can
+tell by this method is that the true weight is very near 14. The
+equivalent can however be determined very accurately, and we have seen
+that it is some multiple or submultiple of the true atomic weight.
+Since molecular-weight determinations have shown that in the case of
+nitrogen the atomic weight is near 14, and we have found the equivalent
+to be 7.02, it is evident that the true atomic weight is twice the
+equivalent, or 7.02 x 2 = 14.04.
+
+~Summary.~ These, then, are the steps necessary to establish the atomic
+weight of an element.
+
+1. Determine the equivalent accurately by analysis.
+
+2. Determine the molecular weight of a large number of compounds of the
+element, and by analysis the part of the molecular weight due to the
+element. The smallest number so obtained will be approximately the
+atomic weight.
+
+3. Multiply the equivalent by the small whole number (usually 1, 2, or
+3), which will make a number very close to the approximate atomic
+weight. The figure so obtained will be the true atomic weight.
+
+~Molecular weights of the elements.~ It will be noticed that the molecular
+weight of nitrogen obtained by multiplying its density by 28.9 is 28.08.
+Yet the atomic weight of nitrogen as deduced from a study of its gaseous
+compounds is 14.04. The simplest explanation that can be given for this
+is that the gaseous nitrogen is made up of molecules, each of which
+contains two atoms. In this respect it resembles oxygen; for we have
+seen that an entirely different line of reasoning leads us to believe
+that the molecule of oxygen contains two atoms. When we wish to indicate
+molecules of these gases the symbols N_{2} and O_{2} should be used.
+When we desire to merely show the weights taking part in a reaction this
+is not necessary.
+
+The vapor densities of many of the elements show that, like oxygen and
+nitrogen, their molecules consist of two atoms. In other cases,
+particularly among the metals, the molecule and the atom are identical.
+Still other elements have four atoms in their molecules.
+
+While oxygen contains two atoms in its molecules, a study of ozone has
+led to the conclusion that it has three. The formation of ozone from
+oxygen can therefore be represented by the equation
+
+    3O_{2} = 2O_{3}.
+
+~Other methods of determining molecular weights.~ It will be noticed that
+Avogadro's law gives us a method by which we can determine the relative
+weights of the molecules of two gases because it enables us to tell when
+we are dealing with an equal number of the two kinds of molecules. If by
+any other means we can get this information, we can make use of the
+knowledge so gained to determine the molecular weights of the two
+substances.
+
+~Raoult's laws.~ Two laws have been discovered which give us just such
+information. They are known as Raoult's laws, and can be stated as
+follows:
+
+1. _When weights of substances which are proportional to their molecular
+weights are dissolved in the same weight of solvent, the rise of the
+boiling point is the same in each case._
+
+2. _When weights of substances which are proportional to their molecular
+weights are dissolved in the same weight of solvent, the lowering of the
+freezing point is the same in each case._
+
+By taking advantage of these laws it is possible to determine when two
+solutions contain the same number of molecules of two dissolved
+substances, and consequently the relative molecular weights of the two
+substances.
+
+~Law of Dulong and Petit.~ In 1819 Dulong and Petit discovered a very
+interesting relation between the atomic weight of an element and its
+specific heat, which holds true for elements in the solid state. If
+equal weights of two solids, say, lead and silver, are heated through
+the same range of temperature, as from 10 deg. to 20 deg., it is found that very
+different amounts of heat are required. The amount of heat required to
+change the temperature of a solid or a liquid by a definite amount
+compared with the amount required to change the temperature of an equal
+weight of water by the same amount is called its specific heat. Dulong
+and Petit discovered the following law: _The specific heat of an element
+in the solid form multiplied by its atomic weight is approximately equal
+to the constant 6.25._ That is,
+
+    at. wt. x sp. ht. = 6.25.
+
+
+Consequently,
+
+                6.25
+    at. wt. = --------
+               sp. ht.
+
+This law is not very accurate, but it is often possible by means of it
+to decide upon what multiple of the equivalent is the real atomic
+weight. Thus the specific heat of iron is found by experiment to be
+0.112, and its equivalent is 27.95. 6.25 / 0.112 = 55.8. We see,
+therefore, that the atomic weight is twice the equivalent, or 55.9.
+
+~How formulas are determined.~ It will be well in connection with
+molecular weights to consider how the formula of a compound is decided
+upon, for the two subjects are very closely associated. Some examples
+will make clear the method followed.
+
+The molecular weight of a substance containing hydrogen and chlorine was
+36.4. By analysis 36.4 parts of the substance was found to contain 1
+part of hydrogen and 35.4 parts of chlorine. As these are the simple
+atomic weights of the two elements, the formula of the compound must be
+HCl.
+
+A substance consisting of oxygen and hydrogen was found to have a
+molecular weight of 34. Analysis showed that in 34 parts of the
+substance there were 2 parts of hydrogen and 32 parts of oxygen.
+Dividing these figures by the atomic weights of the two elements, we get
+2 / 1 = 2 for H; 32 / 16 = 2 for O. The formula is therefore H_{2}O_{2}.
+
+A substance containing 2.04% H, 32.6% S, and 65.3% O was found to have a
+molecular weight of 98. In these 98 parts of the substance there are 98
+x 2.04% = 2 parts of H, 98 x 32.6% = 32 parts of S, and 98 x 65.3% = 64
+parts of O. If the molecule weighs 98, the hydrogen atoms present must
+together weigh 2, the sulphur atoms 32, and the oxygen atoms 64.
+Dividing these figures by the respective atomic weights of the three
+elements, we have, for H, 2 / 1 = 2 atoms; for S, 32 / 32 = 1 atom; for
+O, 64 / 16 = 4 atoms. Hence the formula is H_{2}SO_{4}.
+
+We have, then, this general procedure: Find the percentage composition
+of the substance and also its molecular weight. Multiply the molecular
+weight successively by the percentage of each element present, to find
+the amount of the element in the molecular weight of the compound. The
+figures so obtained will be the respective parts of the molecular weight
+due to the several atoms. Divide by the atomic weights of the respective
+elements, and the quotient will be the number of atoms present.
+
+~Avogadro's hypothesis and chemical calculations.~ This law simplifies
+many chemical calculations.
+
+1. _Application to volume relations in gaseous reactions._ Since equal
+volumes of gases contain an equal number of molecules, it follows that
+when an equal number of gaseous molecules of two or more gases take part
+in a reaction, the reaction will involve equal volumes of the gases. In
+the equation
+
+    C_{2}H_{2}O_{4} = H_{2}O + CO_{2} + CO,
+
+since 1 molecule of each of the gases CO_{2} and CO is set free from
+each molecule of oxalic acid, the two substances must always be set free
+in equal volumes.
+
+Acetylene burns in accordance with the equation
+
+    2C_{2}H_{2} + 5O_{2} = 4CO_{2} + 2H_{2}O.
+
+Hence 2 volumes of acetylene will react with 5 volumes of oxygen to form
+4 volumes of carbon dioxide and 2 volumes of steam. That the volume
+relations may be correct a gaseous element must be given its molecular
+formula. Thus oxygen must be written O_{2} and not 2O.
+
+2. _Application to weights of gases._ It will be recalled that the
+molecular weight of a gas is determined by ascertaining the weight of
+22.4 l. of the gas. This weight in grams is called the _gram-molecular
+weight_ of a gas. If the molecular weight of any gas is known, the
+weight of a liter of the gas under standard conditions may be determined
+by dividing its gram-molecular weight by 22.4. Thus the gram-molecular
+weight of a hydrochloric acid gas is 36.458. A liter of the gas will
+therefore weigh 36.458 / 22.4 = 1.627 g.
+
+
+EXERCISES
+
+1. From the following data calculate the atomic weight of sulphur. The
+equivalent, as obtained by an analysis of sulphur dioxide, is 16.03. The
+densities and compositions of a number of compounds containing sulphur
+are as follows:
+
+      NAME             DENSITY      COMPOSITION BY PERCENTAGE
+Hydrosulphuric acid    1.1791     S = 94.11   H  =  5.89
+Sulphur dioxide        2.222      S = 50.05   O  = 49.95
+Sulphur trioxide       2.74       S = 40.05   O  = 59.95
+Sulphur chloride       4.70       S = 47.48   Cl = 52.52
+Sulphuryl chloride     4.64       S = 23.75   Cl = 52.53    O = 23.70
+Carbon disulphide      2.68       S = 84.24   C  = 15.76
+
+2. Calculate the formulas for compounds of the following compositions:
+
+                                            MOLECULAR
+                                              WEIGHT
+(1) S  = 39.07%   O = 58.49%   H =  2.44%      81.0
+(2) Ca = 29.40    S = 23.56    O = 47.04      136.2
+(3) K  = 38.67    N = 13.88    O = 47.45      101.2
+
+3. The molecular weight of ammonia is 17.06; of sulphur dioxide is
+64.06; of chlorine is 70.9. From the molecular weight calculate the
+weight of 1 l. of each of these gases. Compare your results with the
+table on the back cover of the book.
+
+4. From the molecular weight of the same gases calculate the density of
+each, referred to air as a standard.
+
+5. A mixture of 50 cc. of carbon monoxide and 50 cc. of oxygen was
+exploded in a eudiometer, (a) What gases remained in the tube after
+the explosion? (b) What was the volume of each?
+
+6. In what proportion must acetylene and oxygen be mixed to produce the
+greatest explosion?
+
+7. Solve Problem 18, Chapter XVII, without using molecular weights.
+Compare your results.
+
+8. Solve Problem 10, Chapter XVIII, without using molecular weights.
+Compare your results.
+
+9. The specific heat of aluminium is 0.214; of lead is 0.031. From these
+specific heats calculate the atomic weights of each of the elements.
+
+
+
+
+CHAPTER XX
+
+THE PHOSPHORUS FAMILY
+
+
+==================================================
+           |        | ATOMIC  |         | MELTING
+           | SYMBOL | WEIGHT  | DENSITY |  POINT
+-----------+--------+---------+---------+---------
+Phosphorus |   P    |    31.0 |    1.8  |    43.3 deg.
+Arsenic    |   As   |    75.0 |    5.73 |     ---
+Antimony   |   Sb   |   120.2 |    6.7  |     432 deg.
+Bismuth    |   Bi   |   208.5 |    9.8  |     270 deg.
+==================================================
+
+~The family.~ The elements constituting this family belong in the same
+group with nitrogen and therefore resemble it in a general way. They
+exhibit a regular gradation of physical properties, as is shown in the
+above table. The same general gradation is also found in their chemical
+properties, phosphorus being an acid-forming element, while bismuth is
+essentially a metal. The other two elements are intermediate in
+properties.
+
+~Compounds.~ In general the elements of the family form compounds having
+similar composition, as is shown in the following table:
+
+    PH_{3}  PCl_{3}   PCl_{5}   P_{2}O_{3}   P_{2}O_{5}
+    AsH_{3} AsCl_{3}  AsCl_{5}  As_{2}O_{3}  As_{2}O_{5}
+    SbH_{3} SbCl_{3}  SbCl_{5}  Sb_{2}O_{3}  Sb_{2}O_{5}
+     ....   BiCl_{3}  BiCl_{5}  Bi_{2}O_{3}  Bi_{2}O_{5}
+
+In the case of phosphorus, arsenic, and antimony the oxides are acid
+anhydrides. Salts of at least four acids of each of these three elements
+are known, the free acid in some instances being unstable. The relation
+of these acids to the corresponding anhydrides may be illustrated as
+follows, phosphorus being taken as an example:
+
+    P_{2}O_{3} + 3H_{2}O = 2H_{3}PO_{3} (phosphorous acid).
+
+    P_{2}O_{5} + 3H_{2}O = 2H_{3}PO_{4} (phosphoric acid).
+
+    P_{2}O_{5} + 2H_{2}O = H_{4}P_{2}O_{7} (pyrophosphoric acid).
+
+    P_{2}O_{5} + H_{2}O = 2HPO_{3} (metaphosphoric acid).
+
+
+PHOSPHORUS
+
+~History.~ The element phosphorus was discovered by the alchemist Brand,
+of Hamburg, in 1669, while searching for the philosopher's stone. Owing
+to its peculiar properties and the secrecy which was maintained about
+its preparation, it remained a very rare and costly substance until the
+demand for it in the manufacture of matches brought about its production
+on a large scale.
+
+~Occurrence.~ Owing to its great chemical activity phosphorus never occurs
+free in nature. In the form of phosphates it is very abundant and widely
+distributed. _Phosphorite_ and _sombrerite_ are mineral forms of calcium
+phosphate, while _apatite_ consists of calcium phosphate together with
+calcium fluoride or chloride. These minerals form very large deposits
+and are extensively mined for use as fertilizers. Calcium phosphate is a
+constituent of all fertile soil, having been supplied to the soil by the
+disintegration of rocks containing it. It is the chief mineral
+constituent of bones of animals, and bone ash is therefore nearly pure
+calcium phosphate.
+
+~Preparation.~ Phosphorus is now manufactured from bone ash or a pure
+mineral phosphate by heating the phosphate with sand and carbon in an
+electric furnace. The materials are fed in at M (Fig. 70) by the feed
+screw F. The phosphorus vapor escapes at P and is condensed under
+water, while the calcium silicate is tapped off as a liquid at S. The
+phosphorus obtained in this way is quite impure, and is purified by
+distillation.
+
+[Illustration: Fig. 70]
+
+     ~Explanation of the reaction.~ To understand the reaction which
+     occurs, it must be remembered that a volatile acid anhydride is
+     expelled from its salts when heated with an anhydride which is
+     not volatile. Thus, when sodium carbonate and silicon dioxide
+     are heated together the following reaction takes place:
+
+         Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2}.
+
+     Silicon dioxide is a less volatile anhydride than phosphoric
+     anhydride (P_{2}O_{5}), and when strongly heated with a
+     phosphate the phosphoric anhydride is driven out, thus:
+
+         Ca_{3}(PO_{4})_{2} + 3SiO_{2} = 3CaSiO_{3} + P_{2}O_{5}.
+
+     If carbon is added before the heat is applied, the P_{2}O_{5}
+     is reduced to phosphorus at the same time, according to the
+     equation
+
+         P_{2}O_{5} + 5C = 2P + 5CO.
+
+~Physical properties.~ The purified phosphorus is a pale yellowish,
+translucent, waxy solid which melts at 43.3 deg. and boils at 269 deg.. It can
+therefore be cast into any convenient form under warm water, and is
+usually sold in the market in the form of sticks. It is quite soft and
+can be easily cut with a knife, but this must always be done while the
+element is covered with water, since it is extremely inflammable, and
+the friction of the knife blade is almost sure to set it on fire if cut
+in the air. It is not soluble in water, but is freely soluble in some
+other liquids, notably in carbon disulphide. Its density is 1.8.
+
+~Chemical properties.~ Exposed to the air phosphorus slowly combines with
+oxygen, and in so doing emits a pale light, or phosphorescence, which
+can be seen only in a dark place. The heat of the room may easily raise
+the temperature to the kindling point of phosphorus, when it burns with
+a sputtering flame, giving off dense fumes of oxide of phosphorus. It
+burns with dazzling brilliancy in oxygen, and combines directly with
+many other elements, especially with sulphur and the halogens. On
+account of its great affinity for oxygen it is always preserved under
+water.
+
+Phosphorus is very poisonous, from 0.2 to 0.3 gram being a fatal dose.
+Ground up with flour and water or similar substances, it is often used
+as a poison for rats and other vermin.
+
+     ~Precaution.~ The heat of the body is sufficient to raise
+     phosphorus above its kindling temperature, and for this reason
+     it should always be handled with forceps and never with the
+     bare fingers. Burns occasioned by it are very painful and slow
+     in healing.
+
+~Red phosphorus.~ On standing, yellow phosphorus gradually undergoes a
+remarkable change, being converted into a dark red powder which has a
+density of 2.1. It no longer takes fire easily, neither does it dissolve
+in carbon disulphide. It is not poisonous and, in fact, seems to be an
+entirely different substance. The velocity of this change increases with
+rise in temperature, and the red phosphorus is therefore prepared by
+heating the yellow just below the boiling point (250 deg.-300 deg.). When
+distilled and quickly condensed the red form changes back to the yellow.
+This is in accordance with the general rule that when a substance
+capable of existing in several allotropic forms is condensed from a gas
+or crystallized from the liquid state, the more unstable variety forms
+first, and this then passes into the more stable forms.
+
+     ~Matches.~ The chief use of phosphorus is in the manufacture of
+     matches. Common matches are made by first dipping the match
+     sticks into some inflammable substance, such as melted
+     paraffin, and afterward into a paste consisting of (1)
+     phosphorus, (2) some oxidizing substance, such as manganese
+     dioxide or potassium chlorate, and (3) a binding material,
+     usually some kind of glue. On friction the phosphorus is
+     ignited, the combustion being sustained by the oxidizing agent
+     and communicated to the wood by the burning paraffin. In
+     sulphur matches the paraffin is replaced by sulphur.
+
+     In safety matches _red_ phosphorus, an oxidizing agent, and
+     some gritty material such as emery is placed on the side of the
+     box, while the match tip is provided as before with an
+     oxidizing agent and an easily oxidized substance, usually
+     antimony sulphide. The match cannot be ignited easily by
+     friction, save on the prepared surface.
+
+~Compounds of phosphorus with hydrogen.~ Phosphorus forms several
+compounds with hydrogen, the best known of which is phosphine (PH_{3})
+analogous to ammonia (NH_{3}).
+
+~Preparation of phosphine.~ Phosphine is usually made by heating
+phosphorus with a strong solution of potassium hydroxide, the reaction
+being a complicated one.
+
+[Illustration: Fig. 71]
+
+     The experiment can be conveniently made in the apparatus shown
+     in Fig. 71. A strong solution of potassium hydroxide together
+     with several small bits of phosphorus are placed in the flask
+     A, and a current of coal gas is passed into the flask through
+     the tube B until all the air has been displaced. The gas is
+     then turned off and the flask is heated. Phosphine is formed in
+     small quantities and escapes through the delivery tube, the
+     exit of which is just covered by the water in the vessel C.
+     Each bubble of the gas as it escapes into the air takes fire,
+     and the product of combustion (P_{2}O_{5}) forms beautiful
+     small rings, which float unbroken for a considerable time in
+     quiet air. The pure phosphine does not take fire spontaneously.
+     When prepared as directed above, impurities are present which
+     impart this property.
+
+~Properties.~ Phosphine is a gas of unpleasant odor and is exceedingly
+poisonous. Like ammonia it forms salts with the halogen acids. Thus we
+have phosphonium chloride (PH_{4}Cl) analogous to ammonium chloride
+(NH_{4}Cl). The phosphonium salts are of but little importance.
+
+~Oxides of phosphorus.~ Phosphorus forms two well-known oxides,--the
+trioxide (P_{2}O_{3}) and the pentoxide (P_{2}O_{5}), sometimes called
+phosphoric anhydride. When phosphorus burns in an insufficient supply of
+air the product is partially the trioxide; in oxygen or an excess of air
+the pentoxide is formed. The pentoxide is much the better known of the
+two. It is a snow-white, voluminous powder whose most marked property is
+its great attraction for water. It has no chemical action upon most
+gases, so that they can be very thoroughly dried by allowing them to
+pass through properly arranged vessels containing phosphorus pentoxide.
+
+~Acids of phosphorus.~ The important acids of phosphorus are the
+following:
+
+    H_{3}PO_{3}       phosphorous acid.
+    H_{3}PO_{4}       phosphoric acid.
+    H_{4}P_{2}O_{7}   pyrophosphoric acid.
+    HPO_{3}           metaphosphoric acid.
+
+These may be regarded as combinations of the oxides of phosphorus with
+water according to the equations given in the discussion of the
+characteristics of the family.
+
+1. _Phosphorous acid_ (H_{3}PO_{3}). Neither the acid nor its salts are
+at all frequently met with in chemical operations. It can be easily
+obtained, however, in the form of transparent crystals when phosphorus
+trichloride is treated with water and the resulting solution is
+evaporated:
+
+    PCl_{3} + 3H_{2}O = H_{3}PO_{3} + 3HCl.
+
+Its most interesting property is its tendency to take up oxygen and pass
+over into phosphoric acid.
+
+2. _Orthophosphoric acid (phosphoric acid)_ (H_{3}PO_{4}). This acid can
+be obtained by dissolving phosphorus pentoxide in boiling water, as
+represented in the equation
+
+    P_{2}O_{5} + 3H_{2}O = 2H_{3}PO_{4}.
+
+It is usually made by treating calcium phosphate with concentrated
+sulphuric acid. The calcium sulphate produced in the reaction is nearly
+insoluble, and can be filtered off, leaving the phosphoric acid in
+solution. Very pure acid is made by oxidizing phosphorus with nitric
+acid. It forms large colorless crystals which are exceedingly soluble in
+water. Being a tribasic acid, it forms acid as well as normal salts.
+Thus the following compounds of sodium are known:
+
+    NaH_{2}PO_{4}   monosodium hydrogen phosphate.
+    Na_{2}HPO_{4}   disodium hydrogen phosphate.
+    Na_{3}PO_{4}    normal sodium phosphate.
+
+These salts are sometimes called respectively primary, secondary, and
+tertiary phosphates. They may be prepared by bringing together
+phosphoric acid and appropriate quantities of sodium hydroxide.
+Phosphoric acid also forms mixed salts, that is, salts containing two
+different metals. The most familiar compound of this kind is microcosmic
+salt, which has the formula Na(NH_{4})HPO_{4}.
+
+_Orthophosphates._ The orthophosphates form an important class of salts.
+The normal salts are nearly all insoluble and many of them occur in
+nature. The secondary phosphates are as a rule insoluble, while most of
+the primary salts are soluble.
+
+3. _Pyrophosphoric acid_ (H_{4}P_{2}O_{7}). On heating orthophosphoric
+acid to about 225 deg. pyrophosphoric acid is formed in accordance with the
+following equation:
+
+    2H_{3}PO_{4} = H_{4}P_{2}O_{7} + H_{2}O.
+
+It is a white crystalline solid. Its salts can be prepared by heating a
+secondary phosphate:
+
+    2Na_{2}HPO_{4} = Na_{4}P_{2}O_{7} + H_{2}O.
+
+4. _Metaphosphoric acid (glacial phosphoric acid)_ (HPO_{3}). This acid
+is formed when orthophosphoric acid is heated above 400 deg.:
+
+    H_{3}PO_{4} = HPO_{3} + H_{2}O.
+
+It is also formed when phosphorus pentoxide is treated with cold water:
+
+    P_{2}O_{5} + H_{2}O = 2HPO_{3}.
+
+It is a white crystalline solid, and is so stable towards heat that it
+can be fused and even volatilized without decomposition. On cooling from
+the fused state it forms a glassy solid, and on this account is often
+called glacial phosphoric acid. It possesses the property of dissolving
+small quantities of metallic oxides, with the formation of compounds
+which, in the case of certain metals, have characteristic colors. It is
+therefore used in the detection of these metals.
+
+While the secondary phosphates, on heating, give salts of pyrophosphoric
+acid, the primary phosphates yield salts of metaphosphoric acid. The
+equations representing these reactions are as follows:
+
+    2Na_{2}HPO_{4} = Na_{4}P_{3}O_{7} + H_{2}O,
+
+      NaH_{2}PO_{4} = NaPO_{3} + H_{2}O.
+
+~Fertilizers.~ When crops are produced year after year on the same field
+certain constituents of the soil essential to plant growth are removed,
+and the soil becomes impoverished and unproductive. To make the land
+once more fertile these constituents must be replaced. The calcium
+phosphate of the mineral deposits or of bone ash serves well as a
+material for restoring phosphorus to soils exhausted of that essential
+element; but a more soluble substance, which the plants can more readily
+assimilate, is desirable. It is better, therefore, to convert the
+insoluble calcium phosphate into the soluble primary phosphate before it
+is applied as fertilizer. It will be seen by reference to the formulas
+for the orthophosphates (see page 244) that in a primary phosphate only
+one hydrogen atom of phosphoric acid is replaced by a metal. Since the
+calcium atom always replaces two hydrogen atoms, it might be thought
+that there could be no primary calcium phosphate; but if the calcium
+atom replaces one hydrogen atom from each of two molecules of phosphoric
+acid, the salt Ca(H_{2}PO_{4})_{2} will result, and this is a primary
+phosphate. It can be made by treatment of the normal phosphate with the
+necessary amount of sulphuric acid, calcium sulphate being formed at the
+same time, thus:
+
+    Ca_{3}(PO_{4})_{2} + 2H_{2}SO_{4} = Ca(H_{2}PO_{4})_{2} + 2CaSO_{4}.
+
+The resulting mixture is a powder, which is sold as a fertilizer under
+the name of "superphosphate of lime."
+
+
+ARSENIC
+
+~Occurrence.~ Arsenic occurs in considerable quantities in nature as the
+native element, as the sulphides realgar (As_{2}S_{2}) and orpiment
+(As_{2}S_{3}), as oxide (As_{2}O_{3}), and as a constituent of many
+metallic sulphides, such as arsenopyrite (FeAsS).
+
+~Preparation.~ The element is prepared by purifying the native arsenic, or
+by heating the arsenopyrite in iron tubes, out of contact with air,
+when the reaction expressed by the following equation occurs:
+
+    FeAsS = FeS + As.
+
+The arsenic, being volatile, condenses in chambers connected with the
+heated tubes. It is also made from the oxide by reduction with carbon:
+
+    2As_{2}O_{3} + 3C = 4As + 3CO_{2}.
+
+~Properties.~ Arsenic is a steel-gray, metallic-looking substance of
+density 5.73. Though resembling metals in appearance, it is quite
+brittle, being easily powdered in a mortar. When strongly heated it
+sublimes, that is, it passes into a vapor without melting, and condenses
+again to a crystalline solid when the vapor is cooled. Like phosphorus
+it can be obtained in several allotropic forms. It alloys readily with
+some of the metals, and finds its chief use as an alloy with lead, which
+is used for making shot, the alloy being harder than pure lead. When
+heated on charcoal with the blowpipe it is converted into an oxide which
+volatilizes, leaving the charcoal unstained by any oxide coating. It
+burns readily in chlorine gas, forming arsenic trichloride,--
+
+    As + 3Cl = AsCl_{3}.
+
+Unlike most of its compounds, the element itself is not poisonous.
+
+~Arsine~ (AsH_{3}). When any compound containing arsenic is brought into
+the presence of nascent hydrogen, arsine (AsH_{3}), corresponding to
+phosphine and ammonia, is formed. The reaction when oxide of arsenic is
+so treated is
+
+    As_{2}O_{3} + 12H = 2AsH_{3} + 3H_{2}O.
+
+Arsine is a gas with a peculiar garlic-like odor, and is intensely
+poisonous. A single bubble of pure gas has been known to prove fatal. It
+is an unstable compound, decomposing into its elements when heated to a
+moderate temperature. It is combustible, burning with a pale
+bluish-white flame to form arsenic trioxide and water when air is in
+excess:
+
+    2AsH_{3} + 6O = As_{2}O_{3} + 3H_{2}O.
+
+When the supply of air is deficient water and metallic arsenic are
+formed:
+
+    2AsH_{3} + 3O = 3H_{2}O + 2As.
+
+These reactions make the detection of even minute quantities of arsenic
+a very easy problem.
+
+[Illustration: Fig. 72]
+
+     ~Marsh's test for arsenic.~ The method devised by Marsh for
+     detecting arsenic is most frequently used, the apparatus being
+     shown in Fig. 72. Hydrogen is generated in the flask A by the
+     action of dilute sulphuric acid on zinc, is dried by passing
+     over calcium chloride in the tube B, and after passing
+     through the hard-glass tube C is ignited at the jet D. If a
+     substance containing arsenic is now introduced into the
+     generator A, the arsenic is converted into arsine by the
+     action of the nascent hydrogen, and passes to the jet along
+     with the hydrogen. If the tube C is strongly heated at some
+     point near the middle, the arsine is decomposed while passing
+     this point and the arsenic is deposited just beyond the heated
+     point in the form of a shining, brownish-black mirror. If the
+     tube is not heated, the arsine burns along with the hydrogen at
+     the jet. Under these conditions a small porcelain dish crowded
+     down into the flame is blackened by a spot of metallic arsenic,
+     for the arsine is decomposed by the heat of the flame, and the
+     arsenic, cooled below its kindling temperature by the cold
+     porcelain, deposits upon it as a black spot. Antimony conducts
+     itself in the same way as arsenic, but the antimony deposit is
+     more sooty in appearance. The two can also be distinguished by
+     the fact that sodium hypochlorite (NaClO) dissolves the arsenic
+     deposit, but not that formed by antimony.
+
+~Oxides of arsenic.~ Arsenic forms two oxides, As_{2}O_{3} and
+As_{2}O_{5}, corresponding to those of phosphorus. Of these arsenious
+oxide, or arsenic trioxide (As_{2}O_{3}), is much better known, and is
+the substance usually called white arsenic, or merely arsenic. It is
+found as a mineral, but is usually obtained as a by-product in burning
+pyrite in the sulphuric-acid industry. The pyrite has a small amount of
+arsenopyrite in it, and when this is burned arsenious oxide is formed as
+a vapor together with sulphur dioxide:
+
+    2FeAsS + 10O = Fe_{2}O_{3} + As_{2}O_{3} + 2SO_{2}.
+
+The arsenious oxide is condensed in appropriate chambers. It is a rather
+heavy substance, obtained either as a crystalline powder or as large,
+vitreous lumps, resembling lumps of porcelain in appearance. It is very
+poisonous, from 0.2 to 0.3 g. being a fatal dose. It is frequently given
+as a poison, since it is nearly tasteless and does not act very rapidly.
+This slow action is due to the fact that it is not very soluble, and
+hence is absorbed slowly by the system. Arsenious oxide is also used as
+a chemical reagent in glass making and in the dye industry.
+
+~Acids of arsenic.~ Like the corresponding oxides of phosphorus, the
+oxides of arsenic are acid anhydrides. In solution they combine with
+bases to form salts, corresponding to the salts of the acids of
+phosphorus. Thus we have salts of the following acids:
+
+    H_{3}AsO_{3} arsenious acid.
+
+    H_{3}AsO_{4} orthoarsenic acid.
+
+    H_{4}As_{2}O_{3} pyroarsenic acid.
+
+    HAsO_{3} metarsenic acid.
+
+Several other acids of arsenic are also known. Not all of these can be
+obtained as free acids, since they tend to lose water and form the
+oxides. Thus, instead of obtaining arsenious acid (H_{3}AsO_{3}), the
+oxide As_{2}O_{3} is obtained:
+
+    2H_{3}AsO_{3} = As_{2}O_{3} + 3H_{2}O.
+
+Salts of all the acids are known, however, and some of them have
+commercial value. Most of them are insoluble, and some of the copper
+salts, which are green, are used as pigments. Paris green, which has a
+complicated formula, is a well-known insecticide.
+
+~Antidote for arsenical poisoning.~ The most efficient antidote for
+arsenic poisoning is ferric hydroxide. It is prepared as needed,
+according to the equation
+
+    Fe_{2}(SO_{4})_{3} + 3Mg(OH)_{2} = 2Fe(OH)_{3} + 3MgSO_{4}.
+
+~Sulphides of arsenic.~ When hydrogen sulphide is passed into an acidified
+solution containing an arsenic compound the arsenic is precipitated as a
+bright yellow sulphide, thus:
+
+    2H_{3}AsO_{3} + 3H_{2}S = As_{2}S_{3} + 6H_{2}O,
+
+    2H_{3}AsO_{4} + 5H_{2}S = As_{2}S_{5} + 8H_{2}O.
+
+In this respect arsenic resembles the metallic elements, many of which
+produce sulphides under similar conditions. The sulphides of arsenic,
+both those produced artificially and those found in nature, are used as
+yellow pigments.
+
+
+ANTIMONY
+
+~Occurrence.~ Antimony occurs in nature chiefly as the sulphide
+(Sb_{2}S_{3}), called stibnite, though it is also found as oxide and as
+a constituent of many complex minerals.
+
+~Preparation.~ Antimony is prepared from the sulphide in a very simple
+manner. The sulphide is melted with scrap iron in a furnace, when the
+iron combines with the sulphur to form a slag, or liquid layer of melted
+iron sulphide, while the heavier liquid, antimony, settles to the bottom
+and is drawn off from time to time. The reaction involved is represented
+by the equation
+
+    Sb_{2}S_{3} + 3Fe = 2Sb + 3FeS.
+
+~Physical properties.~ Antimony is a bluish-white, metallic-looking
+substance whose density is 6.7. It is highly crystalline, hard, and very
+brittle. It has a rather low melting point (432 deg.) and expands very
+noticeably on solidifying.
+
+~Chemical properties.~ In chemical properties antimony resembles arsenic
+in many particulars. It forms the oxides Sb_{2}O_{3} and Sb_{2}O_{5},
+and in addition Sb_{2}O_{4}. It combines with the halogen elements with
+great energy, burning brilliantly in chlorine to form antimony
+trichloride (SbCl_{3}). When heated on charcoal with the blowpipe it is
+oxidized and forms a coating of antimony oxide on the charcoal which has
+a characteristic bluish-white color.
+
+~Stibine~ (SbH_{3}). The gas stibine (SbH_{3}) is formed under conditions
+which are very similar to those which produce arsine, and it closely
+resembles the latter compound, though it is still less stable. It is
+very poisonous.
+
+     ~Acids of antimony.~ The oxides Sb_{2}O_{3} and Sb_{2}O_{5} are
+     weak acid anhydrides and are capable of forming two series of
+     acids corresponding in formulas to the acids of phosphorus and
+     arsenic. They are much weaker, however, and are of little
+     practical importance.
+
+     ~Sulphides of antimony.~ Antimony resembles arsenic in that
+     hydrogen sulphide precipitates it as a sulphide when conducted
+     into an acidified solution containing an antimony compound:
+
+        2SbCl_{3} + 3H_{2}S = Sb_{2}S_{3} + 6HCl,
+
+        2SbCl_{5} + 5H_{2}S = Sb_{2}S_{5} + 10HCl.
+
+     The two sulphides of antimony are called the trisulphide and
+     the pentasulphide respectively. When prepared in this way they
+     are orange-colored substances, though the mineral stibnite is
+     black.
+
+~Metallic properties of antimony.~ The physical properties of the element
+are those of a metal, and the fact that its sulphide is precipitated by
+hydrogen sulphide shows that it acts like a metal in a chemical way.
+Many other reactions show that antimony has more of the properties of a
+metal than of a non-metal. The compound Sb(OH)_{3}, corresponding to
+arsenious acid, while able to act as a weak acid is also able to act as
+a weak base with strong acids. For example, when treated with
+concentrated hydrochloric acid antimony chloride is formed:
+
+    Sb(OH)_{3} + 3HCl = SbCl_{3} + 3H_{2}O.
+
+A number of elements act in this same way, their hydroxides under some
+conditions being weak acids and under others weak bases.
+
+
+ALLOYS
+
+Some metals when melted together thoroughly intermix, and on cooling
+form a homogeneous, metallic-appearing substance called an _alloy_. Not
+all metals will mix in this way, and in some cases definite chemical
+compounds are formed and separate out as the mixture solidifies, thus
+destroying the uniform quality of the alloy. In general the melting
+point of the alloy is below the average of the melting points of its
+constituents, and it is often lower than any one of them.
+
+Antimony forms alloys with many of the metals, and its chief commercial
+use is for such purposes. It imparts to its alloys high density, rather
+low melting point, and the property of expanding on solidification.
+Such an alloy is especially useful in type founding, where fine lines
+are to be reproduced on a cast. Type metal consists of antimony, lead,
+and tin. Babbitt metal, used for journal bearings in machinery, contains
+the same metals in a different proportion together with a small
+percentage of copper.
+
+
+BISMUTH
+
+~Occurrence.~ Bismuth is usually found in the uncombined form in nature.
+It also occurs as oxide and sulphide. Most of the bismuth of commerce
+comes from Saxony, and from Mexico and Colorado, but it is not an
+abundant element.
+
+~Preparation.~ It is prepared by merely heating the ore containing the
+native bismuth and allowing the melted metal to run out into suitable
+vessels. Other ores are converted into oxides and reduced by heating
+with carbon.
+
+~Physical properties.~ Bismuth is a heavy, crystalline, brittle metal
+nearly the color of silver, but with a slightly rosy tint which
+distinguishes it from other metals. It melts at a low temperature (270 deg.)
+and has a density of 9.8. It is not acted upon by the air at ordinary
+temperatures.
+
+~Chemical properties.~ When heated with the blowpipe on charcoal, bismuth
+gives a coating of the oxide Bi_{2}O_{3}. This has a yellowish-brown
+color which easily distinguishes it from the oxides formed by other
+metals. It combines very readily with the halogen elements, powdered
+bismuth burning readily in chlorine. It is not very easily acted upon by
+hydrochloric acid, but nitric and sulphuric acids act upon it in the
+same way that they do upon copper.
+
+~Uses.~ Bismuth finds its chief use as a constituent of alloys,
+particularly in those of low melting point. Some of these melt in hot
+water. For example, Wood's metal, consisting of bismuth, lead, tin, and
+cadmium, melts at 60.5 deg..
+
+~Compounds of bismuth.~ Unlike the other elements of this group, bismuth
+has almost no acid properties. Its chief oxide, Bi_{2}O_{3}, is basic in
+its properties. It dissolves in strong acids and forms salts of bismuth:
+
+    Bi_{2}O_{3} + 6HCl = 2BiCl_{3} + 3H_{2}O,
+
+    Bi_{2}O_{3} + 6HNO_{3} = 2Bi(NO_{3})_{3} + 3H_{2}O.
+
+The nitrate and chloride of bismuth can be obtained as well-formed
+colorless crystals. When treated with water the salts are decomposed in
+the manner explained in the following paragraph.
+
+
+HYDROLYSIS
+
+Many salts such as those of antimony and bismuth form solutions which
+are somewhat acid in reaction, and must therefore contain hydrogen ions.
+This is accounted for by the same principle suggested to explain the
+fact that solutions of potassium cyanide are alkaline in reaction (p.
+210). Water forms an appreciable number of hydrogen and hydroxyl ions,
+and very weak bases such as bismuth hydroxide are dissociated to but a
+very slight extent. When Bi^{+++} ions from bismuth chloride, which
+dissociates very readily, are brought in contact with the OH^{-} ions
+from water, the two come to the equilibrium expressed in the equation
+
+    Bi^{+++} + 3OH^{-} <--> Bi(OH)_{3}.
+
+For every hydroxyl ion removed from the solution in this way a hydrogen
+ion is left free, and the solution becomes acid in reaction.
+
+Reactions of this kind and that described under potassium cyanide are
+called _hydrolysis_.
+
+DEFINITION: _Hydrolysis is the action of water upon a salt to form an
+acid and a base, one of which is very slightly dissociated._
+
+~Conditions favoring hydrolysis.~ While hydrolysis is primarily due to the
+slight extent to which either the acid or the base formed is
+dissociated, several other factors have an influence upon the extent to
+which it will take place.
+
+1. _Influence of mass._ Since hydrolysis is a reversible reaction, the
+relative masses of the reacting substances influence the point at which
+equilibrium will be reached. In the equilibrium
+
+    BiCl_{3} + 3H_{2}O <--> Bi(OH)_{3} + 3HCl
+
+the addition of more water will result in the formation of more bismuth
+hydroxide and hydrochloric acid. The addition of more hydrochloric acid
+will convert some of the bismuth hydroxide into bismuth chloride.
+
+2. _Formation of insoluble substances._ When one of the products of
+hydrolysis is nearly insoluble in water the solution will become
+saturated with it as soon as a very little has been formed. All in
+excess of this will precipitate, and the reaction will go on until the
+acid set free increases sufficiently to bring about an equilibrium. Thus
+a considerable amount of bismuth and antimony hydroxides are
+precipitated when water is added to the chlorides of these elements. The
+greater the dilution the more hydroxide precipitates. The addition of
+hydrochloric acid in considerable quantity will, however, redissolve the
+precipitate.
+
+~Partial hydrolysis.~ In many cases the hydrolysis of a salt is only
+partial, resulting in the formation of basic salts instead of the free
+base. Most of these basic salts are insoluble in water, which accounts
+for their ready formation. Thus bismuth chloride may hydrolyze by
+successive steps, as shown in the equations
+
+    BiCl_{3} + H_{2}O = Bi(OH)Cl_{2} + HCl,
+
+    BiCl_{3} + 2H_{2}O = Bi(OH)_{2}Cl + 2HCl,
+
+    BiCl_{3} + 3H_{2}O = Bi(OH)_{3} + 3HCl.
+
+The basic salt so formed may also lose water, as shown in the equation
+
+    Bi(OH)_{2}Cl = BiOCl + H_{2}O.
+
+The salt represented in the last equation is sometimes called bismuth
+oxychloride, or bismuthyl chloride. The corresponding nitrate,
+BiONO_{3}, is largely used in medicine under the name of subnitrate of
+bismuth. In these two compounds the group of atoms, BiO, acts as a
+univalent metallic radical and is called _bismuthyl_. Similar basic
+salts are formed by the hydrolysis of antimony salts.
+
+
+EXERCISES
+
+1. Name all the elements so far studied which possess allotropic forms.
+
+2. What compounds would you expect phosphorus to form with bromine and
+iodine? Write the equations showing the action of water on these
+compounds.
+
+3. In the preparation of phosphine, why is coal gas passed into the
+flask? What other gases would serve the same purpose?
+
+4. Give the formula for the salt which phosphine forms with hydriodic
+acid. Give the name of the compound.
+
+5. Could phosphoric acid be substituted for sulphuric acid in the
+preparation of the common acids?
+
+6. Write the equations for the preparation of the three sodium salts of
+orthophosphoric acid.
+
+7. Why does a solution of disodium hydrogen phosphate react alkaline?
+
+8. On the supposition that bone ash is pure calcium phosphate, what
+weight of it would be required in the preparation of 1 kg. of
+phosphorus?
+
+9. If arsenopyrite is heated in a current of air, what products are
+formed?
+
+10. (a) Write equations for the complete combustion of hydrosulphuric
+acid, methane, and arsine. (b) In what respects are the reactions
+similar?
+
+11. Write the equations for all the reactions involved in Marsh's test
+for arsenic.
+
+12. Write the names and formulas for the acids of antimony.
+
+13. Write the equations showing the hydrolysis of antimony trichloride;
+of bismuth nitrate.
+
+14. In what respects does nitrogen resemble the members of the
+phosphorus family?
+
+
+
+
+CHAPTER XXI
+
+SILICON, TITANIUM, BORON
+
+
+=================================================================
+            |        |        |         |           |
+            | SYMBOL | ATOMIC | DENSITY | CHLORIDES | OXIDES
+            |        | WEIGHT |         |           |
+____________|________|________|_________|___________|____________
+            |        |        |         |           |
+Silicon     |   Si   |  28.4  |   2.35  | SiCl_{4}  | SiO_{2}
+Titanium    |   Ti   |  48.1  |   3.5   | TiCl_{4}  | TiO_{2}
+Boron       |   B    |  11.0  |   2.45  | BCl_{3}   | B_{2}O_{3}
+=================================================================
+
+~General.~ Each of the three elements, silicon, titanium, and boron,
+belongs to a separate periodic family, but they occur near together in
+the periodic grouping and are very similar in both physical and chemical
+properties. Since the other elements in their families are either so
+rare that they cannot be studied in detail, or are best understood in
+connection with other elements, it is convenient to consider these three
+together at this point.
+
+The three elements are very difficult to obtain in the free state, owing
+to their strong attraction for other elements. They can be prepared by
+the action of aluminium or magnesium on their oxides and in impure state
+by reduction with carbon in an electric furnace. They are very hard and
+melt only at the highest temperatures. At ordinary temperatures they are
+not attacked by oxygen, but when strongly heated they burn with great
+brilliancy. Silicon and boron are not attacked by acids under ordinary
+conditions; titanium is easily dissolved by them.
+
+
+SILICON
+
+~Occurrence.~ Next to oxygen silicon is the most abundant element. It does
+not occur free in nature, but its compounds are very abundant and of the
+greatest importance. It occurs almost entirely in combination with
+oxygen as silicon dioxide (SiO_{2}), often called silica, or with oxygen
+and various metals in the form of salts of silicic acids, or silicates.
+These compounds form a large fraction of the earth's crust. Most plants
+absorb small amounts of silica from the soil, and it is also found in
+minute quantities in animal organisms.
+
+~Preparation.~ The element is most easily prepared by reducing pure
+powdered quartz with magnesium powder:
+
+    SiO_{2} + 2Mg = 2MgO + Si.
+
+~Properties.~ As would be expected from its place in the periodic table,
+silicon resembles carbon in many respects. It can be obtained in several
+allotropic forms, corresponding to those of carbon. The crystallized
+form is very hard, and is inactive toward reagents. The amorphous
+variety has, in general, properties more similar to charcoal.
+
+~Compounds of silicon with hydrogen and the halogens.~ Silicon hydride
+(SiH_{4}) corresponds in formula to methane (CH_{4}), but its properties
+are more like those of phosphine (PH_{3}). It is a very inflammable gas
+of disagreeable odor, and, as ordinarily prepared, takes fire
+spontaneously on account of the presence of impurities.
+
+Silicon combines with the elements of the chlorine family to form such
+compounds as SiCl_{4} and SiF_{4}. Of these silicon fluoride is the most
+familiar and interesting. As stated in the discussion of fluorine, it is
+formed when hydrofluoric acid acts upon silicon dioxide or a silicate.
+With silica the reaction is thus expressed:
+
+    SiO_{2} + 4HF = SiF_{4} + 2H_{2}O.
+
+It is a very volatile, invisible, poisonous gas. In contact with water
+it is partially decomposed, as shown in the equation
+
+    SiF_{4} + 4H_{2}O = 4HF + Si(OH)_{4}.
+
+The hydrofluoric acid so formed combines with an additional amount of
+silicon fluoride, forming the complex fluosilicic acid (H_{2}SiF_{6}),
+thus:
+
+    2HF + SiF_{4} = H_{2}SiF_{6}.
+
+~Silicides.~ As the name indicates, silicides are binary compounds
+consisting of silicon and some other element. They are very stable at
+high temperatures, and are usually made by heating the appropriate
+substances in an electric furnace. The most important one is
+_carborundum_, which is a silicide of carbon of the formula CSi. It is
+made by heating coke and sand, which is a form of silicon dioxide, in an
+electric furnace, the process being extensively carried on at Niagara
+Falls. The following equation represents the reaction
+
+    SiO_{2} + 3C = CSi + 2CO.
+
+The substance so prepared consists of beautiful purplish-black crystals,
+which are very hard. Carborundum is used as an abrasive, that is, as a
+material for grinding and polishing very hard substances. Ferrosilicon
+is a silicide of iron alloyed with an excess of iron, which finds
+extensive use in the manufacture of certain kinds of steel.
+
+~Manufacture of carborundum.~ The mixture of materials is heated in a
+large resistance furnace for about thirty-six hours. After the reaction
+is completed there is left a core of graphite G. Surrounding this core
+is a layer of crystallized carborundum C, about 16 in. thick. Outside
+this is a shell of amorphous carborundum A. The remaining materials
+M are unchanged and are used for a new charge.
+
+[Illustration: Fig. 73]
+
+~Silicon dioxide~ (_silica_) (SiO_{2}). This substance is found in a great
+variety of forms in nature, both in the amorphous and in the crystalline
+condition. In the form of quartz it is found in beautifully formed
+six-sided prisms, sometimes of great size. When pure it is perfectly
+transparent and colorless. Some colored varieties are given special
+names, as amethyst (violet), rose quartz (pale pink), smoky or milky
+quartz (colored and opaque). Other varieties of silicon dioxide, some of
+which also contain water, are chalcedony, onyx, jasper, opal, agate, and
+flint. Sand and sandstone are largely silicon dioxide.
+
+~Properties.~ As obtained by chemical processes silicon dioxide is an
+amorphous white powder. In the crystallized state it is very hard and
+has a density of 2.6. It is insoluble in water and in most chemical
+reagents, and requires the hottest oxyhydrogen flame for fusion. Acids,
+excepting hydrofluoric acid, have little action on it, and it requires
+the most energetic reducing agents to deprive it of oxygen. It is the
+anhydride of an acid, and consequently it dissolves in fused alkalis to
+form silicates. Being nonvolatile, it will drive out most other
+anhydrides when heated to a high temperature with their salts,
+especially when the silicates so formed are fusible. The following
+equations illustrate this property:
+
+    Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},
+
+    Na_{2}SO_{4} + SiO_{2} = Na_{2}SiO_{3} + SO_{3}.
+
+~Silicic acids.~ Silicon forms two simple acids, orthosilicic acid
+(H_{4}SiO_{4}) and metasilicic acid (H_{2}SiO_{3}). Orthosilicic acid is
+formed as a jelly-like mass when orthosilicates are treated with strong
+acids such as hydrochloric. On attempting to dry this acid it loses
+water, passing into metasilicic or common silicic acid:
+
+    H_{4}SiO_{4} = H_{2}SiO_{3} + H_{2}O.
+
+Metasilicic acid when heated breaks up into silica and water, thus:
+
+    H_{2}SiO_{3} = H_{2}O + SiO_{2}.
+
+     ~Salts of silicic acids,--silicates.~ A number of salts of the
+     orthosilicic and metasilicic acids occur in nature. Thus mica
+     (KAlSiO_{4}) is a salt of orthosilicic acid.
+
+~Polysilicic acids.~ Silicon has the power to form a great many complex
+acids which may be regarded as derived from the union of several
+molecules of the orthosilicic acid, with the loss of water. Thus we have
+
+    3H_{4}SiO_{4} = H_{4}Si_{3}O_{8} + 4H_{2}O.
+
+These acids cannot be prepared in the pure state, but their salts form
+many of the crystalline rocks in nature. Feldspar, for example, has the
+formula KAlSi_{3}O_{8}, and is a mixed salt of the acid
+H_{4}Si_{3}O_{8}, whose formation is represented in the equation above.
+Kaolin has the formula Al_{2}Si_{2}O_{7}.2H_{2}O. Many other examples
+will be met in the study of the metals.
+
+~Glass.~ When sodium and calcium silicates, together with silicon dioxide,
+are heated to a very high temperature, the mixture slowly fuses to a
+transparent liquid, which on cooling passes into the solid called glass.
+Instead of starting with sodium and calcium silicates it is more
+convenient and economical to heat sodium carbonate (or sulphate) and
+lime with an excess of clean sand, the silicates being formed during the
+heating:
+
+    Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},
+
+    CaO + SiO_{2} = CaSiO_{3}.
+
+[Illustration: Fig. 74]
+
+The mixture is heated below the fusing point for some time, so that the
+escaping carbon dioxide may not spatter the hot liquid; the heat is then
+increased and the mixture kept in a state of fusion until all gases
+formed in the reaction have escaped.
+
+_Molding and blowing of glass._ The way in which the melted mixture is
+handled in the glass factory depends upon the character of the article
+to be made. Many articles, such as bottles, are made by blowing the
+plastic glass into hollow molds of the desired shape. The mold is first
+opened, as shown in Fig. 74. A lump of plastic glass A on the hollow
+rod B is lowered into the mold, which is then closed by the handles
+C. By blowing into the tube the glass is blown into the shape of the
+mold. The mold is then opened and the bottle lifted out. The neck of the
+bottle must be cut off at the proper place and the sharp edges rounded
+off in a flame.
+
+Other objects, such as lamp chimneys, are made by getting a lump of
+plastic glass on the end of a hollow iron rod and blowing it into the
+desired shape without the help of a mold, great skill being required in
+the manipulation of the glass. Window glass is made by blowing large
+hollow cylinders about 6 ft. long and 1-1/2 ft. in diameter. These are
+cut longitudinally, and are then placed in an oven and heated until they
+soften, when they are flattened out into plates (Fig. 75). Plate glass
+is cast into flat slabs, which are then ground and polished to perfectly
+plane surfaces.
+
+_Varieties of glass._ The ingredients mentioned above make a soft,
+easily fusible glass. If potassium carbonate is substituted for the
+sodium carbonate, the glass is much harder and less easily fused;
+increasing the amount of sand has somewhat the same effect. Potassium
+glass is largely used in making chemical glassware, since it resists the
+action of reagents better than the softer sodium glass. If lead oxide is
+substituted for the whole or a part of the lime, the glass is very soft,
+but has a high index of refraction and is valuable for making optical
+instruments and artificial jewels.
+
+[Illustration: Fig. 75]
+
+_Coloring of glass._ Various substances fused along with the glass
+mixture give characteristic colors. The amber color of common bottles is
+due to iron compounds in the glass; in other cases iron colors the glass
+green. Cobalt compounds color it deep blue; those of manganese give it
+an amethyst tint and uranium compounds impart a peculiar yellowish green
+color. Since iron is nearly always present in the ingredients, glass is
+usually slightly yellow. This color can be removed by adding the proper
+amount of manganese dioxide, for the amethyst color of manganese and the
+yellow of iron together produce white light.
+
+_Nature of glass._ Glass is not a definite chemical compound and its
+composition varies between wide limits. Fused glass is really a solution
+of various silicates, such as those of calcium and lead, in fused sodium
+or potassium silicate. A certain amount of silicon dioxide is also
+present. This solution is then allowed to solidify under such conditions
+of cooling that the dissolved substances do not separate from the
+solvent. The compounds which are used to color the glass are sometimes
+converted into silicates, which then dissolve in the glass, giving it a
+uniform color. In other cases, as in the milky glasses which resemble
+porcelain in appearance, the color or opaqueness is due to the finely
+divided color material evenly distributed throughout the glass, but not
+dissolved in it. Milky glass is made by mixing calcium fluoride, tin
+oxide, or some other insoluble substance in the melted glass. Copper or
+gold in metallic form scattered through glass gives it shades of red.
+
+
+TITANIUM
+
+     Titanium is a very widely distributed element in nature, being
+     found in almost all soils, in many rocks, and even in plant and
+     animal tissues. It is not very abundant in any one locality,
+     and it possesses little commercial value save in connection
+     with the iron industry. Its most common ore is rutile
+     (TiO_{2}), which resembles silica in many respects.
+
+     In both physical and chemical properties titanium resembles
+     silicon, though it is somewhat more metallic in character. This
+     resemblance is most marked in the acids of titanium. It not
+     only forms metatitanic and orthotitanic acids but a great
+     variety of polytitanic acids as well.
+
+
+BORON
+
+~Occurrence.~ Boron is never found free in nature. It occurs as boric acid
+(H_{3}BO_{3}), and in salts of polyboric acids, which usually have very
+complicated formulas.
+
+~Preparation and properties.~ Boron can be prepared from its oxide by
+reduction with magnesium, exactly as in the case of silicon. It
+resembles silicon very strikingly in its properties. It occurs in
+several allotropic forms, is very hard when crystallized, and is rather
+inactive toward reagents. It forms a hydride, BH_{3}, and combines
+directly with the elements of the chlorine family. Boron fluoride
+(BF_{3}) is very similar to silicon fluoride in its mode of formation
+and chemical properties.
+
+~Boric oxide~ (B_{2}O_{3}). Boron forms one well-known oxide, B_{2}O_{3},
+called boric anhydride. It is formed as a glassy mass by heating boric
+acid to a high temperature. It absorbs water very readily, uniting with
+it to form boric acid again:
+
+    B_{2}O_{3} + 3H_{2}O = 2H_{3}BO_{3}.
+
+In this respect it differs from silicon dioxide, which will not combine
+directly with water.
+
+~Boric acid~ (H_{3}BO_{3}). This is found in nature in considerable
+quantities and forms one of the chief sources of boron compounds. It is
+found dissolved in the water of hot springs in some localities,
+particularly in Italy. Being volatile with steam, the vapor which
+escapes from these springs has some boric acid in it. It is easily
+obtained from these sources by condensation and evaporation, the
+necessary heat being supplied by other hot springs.
+
+Boric acid crystallizes in pearly flakes, which are greasy to the touch.
+In the laboratory it is easily prepared by treating a strong, hot
+solution of borax with sulphuric acid. Boric acid being sparingly
+soluble in water crystallizes out on cooling:
+
+    Na_{2}B_{4}O_{7} + 5H_{2}O + H_{2}SO_{4} = Na_{2}SO_{4} + 4H_{3}BO_{3}.
+
+The substance is a mild antiseptic, and on this account is often used in
+medicine and as a preservative for canned foods and milk.
+
+~Metaboric and polyboric acids.~ When boric acid is gently heated it is
+converted into metaboric acid (HBO_{2}):
+
+    H_{3}BO_{3} = HBO_{2} + H_{2}O.
+
+On heating metaboric acid to a somewhat higher temperature tetraboric
+acid (H_{2}B_{4}O_{7}) is formed:
+
+    4HBO_{2} = H_{2}B_{4}O_{7} + H_{2}O.
+
+Many other complex acids of boron are known.
+
+~Borax.~ Borax is the sodium salt of tetraboric acid, having the formula
+Na_{2}B_{4}O_{7}.10 H_{2}O. It is found in some arid countries, as
+southern California and Tibet, but is now made commercially from the
+mineral colemanite, which is the calcium salt of a complex boric acid.
+When this is treated with a solution of sodium carbonate, calcium
+carbonate is precipitated and borax crystallizes from the solution.
+
+When heated borax at first swells up greatly, owing to the expulsion of
+the water of crystallization, and then melts to a clear glass. This
+glass has the property of easily dissolving many metallic oxides, and on
+this account borax is used as a flux in soldering, for the purpose of
+removing from the metallic surfaces to be soldered the film of oxide
+with which they are likely to be covered. These oxides often give a
+characteristic color to the clear borax glass, and borax beads are
+therefore often used in testing for the presence of metals, instead of
+the metaphosphoric acid bead already described.
+
+     The reason that metallic oxides dissolve in borax is that borax
+     contains an excess of acid anhydride, as can be more easily
+     seen if its formula is written 2NaBO_{2} + B_{2}O_{3}. The
+     metallic oxide combines with this excess of acid anhydride,
+     forming a mixed salt of metaboric acid.
+
+Borax is extensively used as a constituent of enamels and glazes for
+both metal ware and pottery. It is also used as a flux in soldering and
+brazing, and in domestic ways it serves as a mild alkali, as a
+preservative for meats, and in a great variety of less important
+applications.
+
+
+EXERCISES
+
+1. Account for the fact that a solution of borax in water is alkaline.
+
+2. What weight of water of crystallization does 1 kg. of borax contain?
+
+3. When a concentrated solution of borax acts on silver nitrate a borate
+of silver is formed. If the solution of borax is dilute, however, an
+hydroxide of silver forms. Account for this difference in behavior.
+
+
+
+
+CHAPTER XXII
+
+THE METALS
+
+
+~The metals.~ The elements which remain to be considered are known
+collectively as the metals. They are also called the base-forming
+elements, since their hydroxides are bases. A metal may therefore be
+defined as an element whose hydroxide is a base. When a base dissolves
+in water the hydroxyl groups form the anions, while the metallic element
+forms the cations. From this standpoint a metal can be defined as an
+element capable of forming simple cations in solution.
+
+The distinction between a metal and a non-metal is not a very sharp one,
+since the hydroxides of a number of elements act as bases under some
+conditions and as acids under others. We have seen that antimony is an
+element of this kind.
+
+~Occurrence of metals in nature.~ A few of the metals are found in nature
+in the free state. Among these are gold, platinum, and frequently
+copper. They are usually found combined with other elements in the form
+of oxides or salts of various acids. Silicates, carbonates, sulphides,
+and sulphates are the most abundant salts. All inorganic substances
+occurring in nature, whether they contain a metal or not, are called
+_minerals_. Those minerals from which a useful substance can be
+extracted are called _ores_ of the substance. These two terms are most
+frequently used in connection with the metals.
+
+~Extraction of metals,--metallurgy.~ The process of extracting a metal
+from its ores is called the metallurgy of the metal. The metallurgy of
+each metal presents peculiarities of its own, but there are several
+methods of general application which are very frequently employed.
+
+1. _Reduction of an oxide with carbon._ Many of the metals occur in
+nature in the form of oxides. When these oxides are heated to a high
+temperature with carbon the oxygen combines with it and the metal is set
+free. Iron, for example, occurs largely in the form of the oxide
+Fe_{2}O_{3}. When this is heated with carbon the reaction expressed in
+the following equation takes place:
+
+    Fe_{2}O_{3} + 3 C = 2 Fe + 3 CO.
+
+Many ores other than oxides may be changed into oxides which can then be
+reduced by carbon. The conversion of such ores into oxides is generally
+accomplished by heating, and this process is called _roasting_. Many
+carbonates and hydroxides decompose directly into the oxide on heating.
+Sulphides, on the other hand, must be heated in a current of air, the
+oxygen of the air entering into the reaction. The following equations
+will serve to illustrate these changes in the case of the ores of iron:
+
+    FeCO_{3} = FeO + CO_{2},
+
+    2Fe(OH)_{3} = Fe_{2}O_{3} + 3H_{2}O,
+
+    2FeS_{2} + 11O = Fe_{2}O_{3} + 4SO_{2}.
+
+2. _Reduction of an oxide with aluminium._ Not all oxides, however, can
+be reduced by carbon. In such cases aluminium may be used. Thus chromium
+may be obtained in accordance with the following equation:
+
+    Cr_{2}O_{3} + 2 Al = 2 Cr + Al_{2}O_{3}.
+
+This method is a comparatively new one, having been brought into use by
+the German chemist Goldschmidt; hence it is sometimes called the
+Goldschmidt method.
+
+3. _Electrolysis._ In recent years increasing use is being made of the
+electric current in the preparation of metals. In some cases the
+separation of the metal from its compounds is accomplished by passing
+the current through a solution of a suitable salt of the metal, the
+metal usually being deposited upon the cathode. In other cases the
+current is passed through a fused salt of the metal, the chloride being
+best adapted to this purpose.
+
+~Electro-chemical industries.~ Most of the electro-chemical industries of
+the country are carried on where water power is abundant, since this
+furnishes the cheapest means for the generation of electrical energy.
+Niagara Falls is the most important locality in this country for such
+industries, and many different electro-chemical products are
+manufactured there. Some industries depend upon electrolytic processes,
+while in others the electrical energy is used merely as a source of heat
+in electric furnaces.
+
+~Preparation of compounds of the metals.~ Since the compounds of the
+metals are so numerous and varied in character, there are many ways of
+preparing them. In many cases the properties of the substance to be
+prepared, or the material available for its preparation, suggest a
+rather unusual way. There are, however, a number of general principles
+which are constantly applied in the preparation of the compounds of the
+metals, and a clear understanding of them will save much time and effort
+in remembering the details in any given case. The most important of
+these general methods for the preparation of compounds are the
+following:
+
+1. _By direct union of two elements._ This is usually accomplished by
+heating the two elements together. Thus the sulphides, chlorides, and
+oxides of a metal can generally be obtained in this way. The following
+equations serve as examples of this method:
+
+    Fe + S = FeS,
+
+    Mg + O = MgO,
+
+    Cu + 2Cl = CuCl_{2}.
+
+2. _By the decomposition of a compound._ This decomposition may be
+brought about either by heat alone or by the combined action of heat and
+a reducing agent. Thus when the nitrate of a metal is heated the oxide
+of the metal is usually obtained. Copper nitrate, for example,
+decomposes as follows:
+
+    Cu(NO_{3})_{2} = CuO + 2NO_{2} + O.
+
+Similarly the carbonates of the metals yield oxides, thus:
+
+    CaCO_{3} = CaO + CO_{2}.
+
+Most of the hydroxides form an oxide and water when heated:
+
+    2Al(OH)_{3} = Al_{2}O_{3} + 3H_{2}O.
+
+When heated with carbon, sulphates are reduced to sulphides, thus:
+
+    BaSO_{4} + 2C = BaS + 2CO_{2}.
+
+3. _Methods based on equilibrium in solution._ In the preparation of
+compounds the first requisite is that the reactions chosen shall be of
+such a kind as will go on to completion. In the chapter on chemical
+equilibrium it was shown that reactions in solution may become complete
+in either of three ways: (1) a gas may be formed which escapes from
+solution; (2) an insoluble solid may be formed which precipitates; (3)
+two different ions may combine to form undissociated molecules. By the
+judicious selection of materials these principles may be applied to the
+preparation of a great variety of compounds, and illustrations of such
+methods will very frequently be found in the subsequent pages.
+
+4. _By fusion methods._ It sometimes happens that substances which are
+insoluble in water and in acids, and which cannot therefore be brought
+into double decomposition in the usual way, are soluble in other
+liquids, and when dissolved in them can be decomposed and converted into
+other desired compounds. Thus barium sulphate is not soluble in water,
+and sulphuric acid, being less volatile than most other acids, cannot
+easily be driven out from this salt When brought into contact with
+melted sodium carbonate, however, it dissolves in it, and since barium
+carbonate is insoluble in melted sodium carbonate, double decomposition
+takes place:
+
+    Na_{2}CO_{3} + BaSO_{4} = BaCO_{3} + Na_{2}SO_{4}.
+
+On dissolving the cooled mixture in water the sodium sulphate formed in
+the reaction, together with any excess of sodium carbonate which may be
+present, dissolves. The barium carbonate can then be filtered off and
+converted into any desired salt by the processes already described.
+
+5. _By the action of metals on salts of other metals._ When a strip of
+zinc is placed in a solution of a copper salt the copper is precipitated
+and an equivalent quantity of zinc passes into solution:
+
+    Zn + CuSO_{4} = Cu + ZnSO_{4}.
+
+In like manner copper will precipitate silver from its salts:
+
+    Cu + Ag_{2}SO_{4} = 2Ag + CuSO_{4}.
+
+It is possible to tabulate the metals in such a way that any one of them
+in the table will precipitate any one following it from its salts. The
+following is a list of some of the commoner metals arranged in this way:
+
+    Zinc
+    Iron
+    Tin
+    Lead
+    Copper
+    Bismuth
+    Mercury
+    Silver
+    Gold
+
+According to this table copper will precipitate bismuth, mercury,
+silver, or gold from their salts, and will in turn be precipitated by
+zinc, iron, tin, or lead. Advantage is taken of this principle in the
+purification of some of the metals, and occasionally in the preparation
+of metals and their compounds.
+
+~Important insoluble compounds.~ Since precipitates play so important a
+part in the reactions which substances undergo, as well as in the
+preparation of many chemical compounds, it is important to know what
+substances are insoluble. Knowing this, we can in many cases predict
+reactions under certain conditions, and are assisted in devising ways to
+prepare desired compounds. While there is no general rule which will
+enable one to foretell the solubility of any given compound,
+nevertheless a few general statements can be made which will be of much
+assistance.
+
+1. _Hydroxides._ All hydroxides are insoluble save those of ammonium,
+sodium, potassium, calcium, barium, and strontium.
+
+2. _Nitrates._ All nitrates are soluble in water.
+
+3. _Chlorides._ All chlorides are soluble save silver and mercurous
+chlorides. (Lead chloride is but slightly soluble.)
+
+4. _Sulphates._ All sulphates are soluble save those of barium,
+strontium, and lead. (Sulphates of silver and calcium are only
+moderately soluble.)
+
+5. _Sulphides._ All sulphides are insoluble save those of ammonium,
+sodium, and potassium. The sulphides of calcium, barium, strontium, and
+magnesium are insoluble in water, but are changed by hydrolysis into
+acid sulphides which are soluble. On this account they cannot be
+prepared by precipitation.
+
+6. _Carbonates, phosphates, and silicates._ All normal carbonates,
+phosphates, and silicates are insoluble save those of ammonium, sodium
+and potassium.
+
+
+EXERCISES
+
+1. Write equations representing four different ways for preparing
+Cu(NO_{3})_{2}.
+
+2. Write equations representing six different ways for preparing
+ZnSO_{4}.
+
+3. Write equations for two reactions to illustrate each of the three
+ways in which reactions in solutions may become complete.
+
+4. Give one or more methods for preparing each of the following
+compounds: CaCl_{2}, PbCl_{2}, BaSO_{4}, CaCO_{3}, (NH_{4})_{2}S,
+Ag_{2}S, PbO, Cu(OH)_{2} (for solubilities, see last paragraph of
+chapter). State in each case the general principle involved in the
+method of preparation chosen.
+
+
+
+
+CHAPTER XXIII
+
+THE ALKALI METALS
+
+
+=================================================================
+          |        |        |         |         |
+          | SYMBOL | ATOMIC | DENSITY | MELTING | FIRST PREPARED
+          |        | WEIGHT |         |  POINT  |
+__________|________|________|_________|_________|________________
+          |        |        |         |         |
+Lithium   |   Li   |   7.03 |   0.59  |  186. deg.  | Davy     1820
+Sodium    |   Na   |  23.05 |   0.97  |   97.6 deg. |  "       1807
+Potassium |   K    |  39.15 |   0.87  |   62.5 deg. |  "       1807
+Rubidium  |   Rb   |  85.5  |   1.52  |   38.5 deg. | Bunsen   1861
+Caesium    |   Cs   | 132.9  |   1.88  |   26.5 deg. |  "       1860
+=================================================================
+
+~The family.~ The metals listed in the above table constitute the even
+family in Group I in the periodic arrangement of the elements, and
+therefore form a natural family. The name alkali metals is commonly
+applied to the family for the reason that the hydroxides of the most
+familiar members of the family, namely sodium and potassium, have long
+been called alkalis.
+
+1. _Occurrence._ While none of these metals occur free in nature, their
+compounds are very widely distributed, being especially abundant in sea
+and mineral waters, in salt beds, and in many rocks. Only sodium and
+potassium occur in abundance, the others being rarely found in any
+considerable quantity.
+
+2. _Preparation._ The metals are most conveniently prepared by the
+electrolysis of their fused hydroxides or chlorides, though it is
+possible to prepare them by reducing their oxides or carbonates with
+carbon.
+
+3. _Properties._ They are soft, light metals, having low melting points
+and small densities, as is indicated in the table. Their melting points
+vary inversely with their atomic weights, while their densities (sodium
+excepted) vary directly with these. The pure metals have a silvery
+luster but tarnish at once when exposed to the air, owing to the
+formation of a film of oxide upon the surface of the metal. They are
+therefore preserved in some liquid, such as coal oil, which contains no
+oxygen. Because of their strong affinity for oxygen they decompose water
+with great ease, forming hydroxides and liberating hydrogen in
+accordance with the equation
+
+    M + H_{2}O = MOH + H,
+
+where M stands for any one of these metals. These hydroxides are white
+solids; they are readily soluble in water and possess very strong basic
+properties. These bases are nearly equal in strength, that is, they all
+dissociate in water to about the same extent.
+
+4. _Compounds._ The alkali metals almost always act as univalent
+elements in the formation of compounds, the composition of which can be
+represented by such formulas as MH, MCl, MNO_{3}, M_{2}SO_{4},
+M_{3}PO_{4}. These compounds, when dissolved in water, dissociate in
+such a way as to form simple, univalent metallic ions which are
+colorless. With the exception of lithium these metals form very few
+insoluble compounds, so that it is not often that precipitates
+containing them are obtained. Only sodium and potassium will be studied
+in detail, since the other metals of the family are of relatively small
+importance.
+
+The compounds of sodium and potassium are so similar in properties that
+they can be used interchangeably for most purposes. Other things being
+equal, the sodium compounds are prepared in preference to those of
+potassium, since they are cheaper. When a given sodium compound is
+deliquescent, or is so soluble that it is difficult to purify, the
+corresponding potassium compound is prepared in its stead, provided its
+properties are more desirable in these respects.
+
+
+SODIUM
+
+~Occurrence in nature.~ Large deposits of sodium chloride have been found
+in various parts of the world, and the water of the ocean and of many
+lakes and springs contains notable quantities of it. The element also
+occurs as a constituent of many rocks and is therefore present in the
+soil formed by their disintegration. The mineral cryolite
+(Na_{3}AlF_{6}) is an important substance, and the nitrate, carbonate,
+and borate also occur in nature.
+
+~Preparation.~ In 1807 Sir Humphry Davy succeeded in preparing very small
+quantities of metallic sodium by the electrolysis of the fused
+hydroxide. On account of the cost of electrical energy it was for many
+years found more economical to prepare it by reducing the carbonate with
+carbon in accordance with the following equation:
+
+    Na_{2}CO_{3} + 2C = 2Na + 3CO.
+
+The cost of generating the electric current has been diminished to such
+an extent, however, that it is now more economical to prepare sodium by
+Davy's original method, namely, by the electrolysis of the fused
+hydroxide or chloride. When the chloride is used the process is
+difficult to manage, owing to the higher temperature required to keep
+the electrolyte fused, and because of the corroding action of the fused
+chloride upon the containing vessel.
+
+[Illustration: SIR HUMPHRY DAVY (English) (1778-1829)
+
+Isolated sodium, lithium, potassium, barium, strontium, and calcium by
+means of electrolysis; demonstrated the elementary nature of chlorine;
+invented the safety lamp; discovered the stupefying effects of nitrous
+oxide]
+
+     ~Technical preparation.~ The sodium hydroxide is melted in a
+     cylindrical iron vessel (Fig. 76) through the bottom of which
+     rises the cathode K. The anodes A, several in number, are
+     suspended around the cathode from above. A cylindrical vessel
+     C floats in the fused alkali directly over the cathode, and
+     under this cap the sodium and hydrogen liberated at the cathode
+     collect. The hydrogen escapes by lifting the cover, and the
+     sodium, protected from the air by the hydrogen, is skimmed or
+     drained off from time to time. Oxygen is set free upon the
+     anode and escapes into the air through the openings O without
+     coming into contact with the sodium or hydrogen. This process
+     is carried on extensively at Niagara Falls.
+
+[Illustration: Fig. 76]
+
+~Properties.~ Sodium is a silver-white metal about as heavy as water, and
+so soft that it can be molded easily by the fingers or pressed into
+wire. It is very active chemically, combining with most of the
+non-metallic elements, such as oxygen and chlorine, with great energy.
+It will often withdraw these elements from combination with other
+elements, and is thus able to decompose water and the oxides and
+chlorides of many metals.
+
+~Sodium peroxide~ (NaO). Since sodium is a univalent element we should
+expect it to form an oxide of the formula Na_{2}O. While such an oxide
+can be prepared, the peroxide (NaO) is much better known. It is a
+yellowish-white powder made by burning sodium in air. Its chief use is
+as an oxidizing agent. When heated with oxidizable substances it gives
+up a part of its oxygen, as shown in the equation
+
+    2NaO = Na_{2}O + O.
+
+Water decomposes it in accordance with the equation
+
+    2NaO + 2H_{2}O = 2NaOH + H_{2}O_{2}.
+
+Acids act readily upon it, forming a sodium salt and hydrogen peroxide:
+
+    2NaO + 2HCl = 2NaCl + H_{2}O_{2}.
+
+In these last two reactions the hydrogen dioxide formed may decompose
+into water and oxygen if the temperature is allowed to rise:
+
+    H_{2}O_{2} = H_{2}O + O.
+
+     ~Peroxides.~ It will be remembered that barium dioxide (BaO_{2})
+     yields hydrogen dioxide when treated with acids, and that
+     manganese dioxide gives up oxygen when heated with sulphuric
+     acid. Oxides which yield either hydrogen dioxide or oxygen when
+     treated with water or an acid are called peroxides.
+
+~Sodium hydroxide~ (_caustic soda_) (NaOH). 1. _Preparation._ Sodium
+hydroxide is prepared commercially by several processes.
+
+(a) In the older process, still in extensive use, sodium carbonate is
+treated with calcium hydroxide suspended in water. Calcium carbonate is
+precipitated according to the equation
+
+    Na_{2}CO_{3} + Ca(OH)_{2} = CaCO_{3} + 2NaOH.
+
+The dilute solution of sodium hydroxide, filtered from the calcium
+carbonate, is evaporated to a paste and is then poured into molds to
+solidify. It is sold in the form of slender sticks.
+
+(b) The newer methods depend upon the electrolysis of sodium chloride.
+In the Castner process a solution of salt is electrolyzed, the reaction
+being expressed as follows:
+
+    NaCl + H_{2}O = NaOH + H + Cl.
+
+The chlorine escapes as a gas, and by an ingenious mechanical device the
+sodium hydroxide is prevented from mixing with the salt in the solution.
+
+In the Acker process the electrolyte is _fused_ sodium chloride. The
+chlorine is evolved as a gas at the anode, while the sodium alloys with
+the melted lead which forms the cathode. When this alloy is treated with
+water the following reaction takes place:
+
+    Na + H_{2}O = NaOH + H.
+
+[Illustration: Fig. 77]
+
+~Technical process.~ A sketch of an Acker furnace is represented in Fig.
+77. The furnace is an irregularly shaped cast-iron box, divided into
+three compartments, A, B, and C. Compartment A is lined with
+magnesia brick. Compartments B and C are filled with melted lead,
+which also covers the bottom of A to a depth of about an inch. Above
+this layer in A is fused salt, into which dip carbon anodes D. The
+metallic box and melted lead is the cathode.
+
+     When the furnace is in operation chlorine is evolved at the
+     anodes, and is drawn away through a pipe (not represented) to
+     the bleaching-powder chambers. Sodium is set free at the
+     surface of the melted lead in A, and at once alloys with it.
+     Through the pipe E a powerful jet of steam is driven through
+     the lead in B upwards into the narrow tube F. This forces
+     the lead alloy up through the tube and over into the chamber
+     G.
+
+     In this process the steam is decomposed by the sodium in the
+     alloy, forming melted sodium hydroxide and hydrogen. The melted
+     lead and sodium hydroxide separate into two layers in G, and
+     the sodium hydroxide, being on top, overflows into tanks from
+     which it is drawn off and packed in metallic drums. The lead is
+     returned to the other compartments of the furnace by a pipe
+     leading from H to I. Compartment C serves merely as a
+     reservoir for excess of melted lead.
+
+2. _Properties._ Sodium hydroxide is a white, crystalline, brittle
+substance which rapidly absorbs water and carbon dioxide from the air.
+As the name (caustic soda) indicates, it is a very corrosive substance,
+having a disintegrating action on most animal and vegetable tissues. It
+is a strong base. It is used in a great many chemical industries, and
+under the name of lye is employed to a small extent as a cleansing agent
+for household purposes.
+
+~Sodium chloride~ (_common salt_) (NaCl). 1. _Preparation._ Sodium
+chloride, or common salt, is very widely distributed in nature. Thick
+strata, evidently deposited at one time by the evaporation of salt
+water, are found in many places. In the United States the most important
+localities for salt are New York, Michigan, Ohio, and Kansas. Sometimes
+the salt is mined, especially if it is in the pure form called rock
+salt. More frequently a strong brine is pumped from deep wells sunk into
+the salt deposit, and is then evaporated in large pans until the salt
+crystallizes out. The crystals are in the form of small cubes and
+contain no water of crystallization; some water is, however, held in
+cavities in the crystals and causes the salt to decrepitate when heated.
+
+2. _Uses._ Since salt is so abundant in nature it forms the starting
+point in the preparation of all compounds containing either sodium or
+chlorine. This includes many substances of the highest importance to
+civilization, such as soap, glass, hydrochloric acid, soda, and
+bleaching powder. Enormous quantities of salt are therefore produced
+each year. Small quantities are essential to the life of man and
+animals. Pure salt does not absorb moisture; the fact that ordinary salt
+becomes moist in air is not due to a property of the salt, but to
+impurities commonly occurring in it, especially calcium and magnesium
+chlorides.
+
+~Sodium sulphate~ (_Glauber's salt_) (Na_{2}SO_{4}.10H_{2}O). This salt is
+prepared by the action of sulphuric acid upon sodium chloride,
+hydrochloric acid being formed at the same time:
+
+    2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
+
+Some sodium sulphate is prepared by the reaction represented in the
+equation
+
+    MgSO_{4} + 2NaCl = Na_{2}SO_{4} + MgCl_{2}.
+
+The magnesium sulphate required for this reaction is obtained in large
+quantities in the manufacture of potassium chloride, and being of little
+value for any other purpose is used in this way. The reaction depends
+upon the fact that sodium sulphate is the least soluble of any of the
+four factors in the equation, and therefore crystallizes out when hot,
+saturated solutions of magnesium sulphate and sodium chloride are mixed
+together and the resulting mixture cooled.
+
+Sodium sulphate forms large efflorescent crystals. The salt is
+extensively used in the manufacture of sodium carbonate and glass. Small
+quantities are used in medicine.
+
+~Sodium sulphite~ (Na_{2}SO_{3}.7H_{2}O). Sodium sulphite is prepared by
+the action of sulphur dioxide upon solutions of sodium hydroxide, the
+reaction being analogous to the action of carbon dioxide upon sodium
+hydroxide. Like the carbonate, the sulphite is readily decomposed by
+acids:
+
+    Na_{2}SO_{3} + 2HCl = 2NaCl + H_{2}O + SO_{2}.
+
+Because of this reaction sodium sulphite is used as a convenient source
+of sulphur dioxide. It is also used as a disinfectant and a
+preservative.
+
+~Sodium thiosulphate~ (_hyposulphite of soda or "hypo"_)
+(Na_{2}S_{2}O_{3}.5H_{2}O). This salt, commonly called sodium
+hyposulphite, or merely hypo, is made by boiling a solution of sodium
+sulphite with sulphur:
+
+    Na_{2}SO_{3} + S = Na_{2}S_{2}O_{3}.
+
+It is used in photography and in the bleaching industry, to absorb the
+excess of chlorine which is left upon the bleached fabrics.
+
+     ~Thio compounds.~ The prefix "thio" means sulphur. It is used to
+     designate substances which may be regarded as derived from
+     oxygen compounds by replacing the whole or a part of their
+     oxygen with sulphur. The thiosulphates may be regarded as
+     sulphates in which one atom of oxygen has been replaced by an
+     atom of sulphur. This may be seen by comparing the formula
+     Na_{2}SO_{4} (sodium sulphate) with the formula
+     Na_{2}S_{2}O_{3} (sodium thiosulphate).
+
+~Sodium carbonate~ (_sal soda_)(Na_{2}CO_{3}.10H_{2}O). There are two
+different methods now employed in the manufacture of this important
+substance.
+
+1. _Le Blanc process._ This older process involves several distinct
+reactions, as shown in the following equations.
+
+(a) Sodium chloride is first converted into sodium sulphate:
+
+    2NaCl + H_{2}SO_{4} = Na_{2}SO_{4} + 2HCl.
+
+(b) The sodium sulphate is next reduced to sulphide by heating it with
+carbon:
+
+    Na_{2}SO_{4} + 2C = Na_{2}S + 2CO_{2}.
+
+(c) The sodium sulphide is then heated with calcium carbonate, when
+double decomposition takes place:
+
+    Na_{2}S + CaCO_{3} = CaS + Na_{2}CO_{3}.
+
+     ~Technical preparation of sodium carbonate.~ In a manufacturing
+     plant the last two reactions take place in one process. Sodium
+     sulphate, coal, and powdered limestone are heated together to a
+     rather high temperature. The coal reduces the sulphate to
+     sulphide, which in turn reacts upon the calcium carbonate. Some
+     limestone is decomposed by the heat, forming calcium oxide.
+     When treated with water the calcium oxide is changed into
+     hydroxide, and this prevents the water from decomposing the
+     insoluble calcium sulphide.
+
+     The crude product of the process is a hard black cake called
+     black ash. On digesting this mass with water the sodium
+     carbonate passes into solution. The pure carbonate is obtained
+     by evaporation of this solution, crystallizing from it in
+     crystals of the formula Na_{2}CO_{3}.10H_{2}O. Since over 60%
+     of this salt is water, the crystals are sometimes heated until
+     it is driven off. The product is called calcined soda, and is,
+     of course, more valuable than the crystallized salt.
+
+2. _Solvay process._ This more modern process depends upon the reactions
+represented in the equations
+
+    NaCl + NH_{4}HCO_{3} = NaHCO_{3} + NH_{4}Cl,
+
+    2NaHCO_{3} = Na_{2}CO_{3} + H_{2}O + CO_{2}.
+
+The reason the first reaction takes place is that sodium hydrogen
+carbonate is sparingly soluble in water, while the other compounds are
+freely soluble. When strong solutions of sodium chloride and of ammonium
+hydrogen carbonate are brought together the sparingly soluble sodium
+hydrogen carbonate is precipitated. This is converted into the normal
+carbonate by heating, the reaction being represented in the second
+equation.
+
+     ~Technical preparation.~ In the Solvay process a very
+     concentrated solution of salt is first saturated with ammonia
+     gas, and a current of carbon dioxide is then conducted into the
+     solution. In this way ammonium hydrogen carbonate is formed:
+
+     NH_{3} + H_{2}O + CO_{2} = NH_{4}HCO_{3}.
+
+     This enters into double decomposition with the salt, as shown
+     in the first equation under the Solvay process. After the
+     sodium hydrogen carbonate has been precipitated the mother
+     liquors containing ammonium chloride are treated with lime:
+
+     2NH_{4}Cl + CaO = CaCl_{2} + 2 NH_{3} + H_{2}O.
+
+     The lime is obtained by burning limestone:
+
+     CaCO_{3} = CaO + CO_{2}.
+
+     The ammonia and carbon dioxide evolved in the latter two
+     reactions are used in the preparation of an additional quantity
+     of ammonium hydrogen carbonate. It will thus be seen that there
+     is no loss of ammonia. The only materials permanently used up
+     are calcium carbonate and salt, while the only waste product is
+     calcium chloride.
+
+     ~Historical.~ In former times sodium carbonate was made by
+     burning seaweeds and extracting the carbonate from their ash.
+     On this account the salt was called _soda ash_, and the name is
+     still in common use. During the French Revolution this supply
+     was cut off, and in behalf of the French government Le Blanc
+     made a study of methods of preparing the carbonate directly
+     from salt. As a result he devised the method which bears his
+     name, and which was used exclusively for many years. It has
+     been replaced to a large extent by the Solvay process, which
+     has the advantage that the materials used are inexpensive, and
+     that the ammonium hydrogen carbonate used can be regenerated
+     from the products formed in the process. Much expense is also
+     saved in fuel, and the sodium hydrogen carbonate, which is the
+     first product of the process, has itself many commercial uses.
+     The Le Blanc process is still used, however, since the
+     hydrochloric acid generated is of value.
+
+     ~By-products.~ The substances obtained in a given process, aside
+     from the main product, are called the by-products. The success
+     of many processes depends upon the value of the by-products
+     formed.
+
+     Thus hydrochloric acid, a by-product in the Le Blanc process,
+     is valuable enough to make the process pay, even though sodium
+     carbonate can be made cheaper in other ways.
+
+~Properties of sodium carbonate.~ Sodium carbonate forms large crystals of
+the formula Na_{2}CO_{3} . 10 H_{2}O. It has a mild alkaline reaction
+and is used for laundry purposes under the name of washing soda. Mere
+mention of the fact that it is used in the manufacture of glass, soap,
+and many chemical reagents will indicate its importance in the
+industries. It is one of the few soluble carbonates.
+
+~Sodium hydrogen carbonate~ (_bicarbonate of soda_) (NaHCO_{3}). This
+salt, commonly called bicarbonate of soda, or baking soda, is made by
+the Solvay process, as explained above, or by passing carbon dioxide
+into strong solutions of sodium carbonate:
+
+    Na_{2}CO_{3} + H_{2}O + CO_{2} = 2NaHCO_{3}.
+
+The bicarbonate, being sparingly soluble, crystallizes out. A mixture of
+the bicarbonate with some substance (the compound known as cream of
+tartar is generally used) which slowly reacts with it, liberating carbon
+dioxide, is used largely in baking. The carbon dioxide generated forces
+its way through the dough, thus making it porous and light.
+
+~Sodium nitrate~ (_Chili saltpeter_) (NaNO_{3}). This substance is found
+in nature in arid regions in a number of places, where it has been
+formed apparently by the decay of organic substances in the presence of
+air and sodium salts. The largest deposits are in Chili, and most of the
+nitrate of commerce comes from that country. Smaller deposits occur in
+California and Nevada. The commercial salt is prepared by dissolving the
+crude nitrate in water, allowing the insoluble earthy materials to
+settle, and evaporating the clear solution so obtained to
+crystallization. The soluble impurities remain for the most part in the
+mother liquors.
+
+Since this salt is the only nitrate found extensively in nature, it is
+the material from which other nitrates as well as nitric acid are
+prepared. It is used in enormous quantities in the manufacture of
+sulphuric acid and potassium nitrate, and as a fertilizer.
+
+~Sodium phosphate~ (Na_{2}HPO_{4}.12H_{2}O). Since phosphoric acid has
+three replaceable hydrogen atoms, three sodium phosphates are
+possible,--two acid salts and one normal. All three can be made without
+difficulty, but disodium phosphate is the only one which is largely
+used, and is the salt which is commonly called sodium phosphate. It is
+made by the action of phosphoric acid on sodium carbonate:
+
+    Na_{2}CO_{3} + H_{3}PO_{4} = Na_{2}HPO_{4} + CO_{2} + H_{2}O.
+
+It is interesting as being one of the few phosphates which are soluble
+in water, and is the salt commonly used when a soluble phosphate is
+needed.
+
+~Normal sodium phosphate~ (Na_{3}PO_{4}). Although this is a normal salt
+its solution has a strongly alkaline reaction. This is due to the fact
+that the salt hydrolyzes in solution into sodium hydroxide and disodium
+phosphate, as represented in the equation
+
+    Na_{3}PO_{4} + H_{2}O = Na_{2}HPO_{4} + NaOH.
+
+Sodium hydroxide is strongly alkaline, while disodium phosphate is
+nearly neutral in reaction. The solution as a whole is therefore
+alkaline. The salt is prepared by adding a large excess of sodium
+hydroxide to a solution of disodium phosphate and evaporating to
+crystallization. The excess of the sodium hydroxide reverses the
+reaction of hydrolysis and the normal salt crystallizes out.
+
+~Sodium tetraborate ~(_borax_) (Na_{2}B_{4}O_{7}.10H_{2}O). The properties
+of this important compound have been discussed under the head of boron.
+
+
+POTASSIUM
+
+~Occurrence in nature.~ Potassium is a constituent of many common rocks
+and minerals, and is therefore a rather abundant element, though not so
+abundant as sodium. Feldspar, which occurs both by itself and as a
+constituent of granite, contains considerable potassium. The element is
+a constituent of all clay and of mica and also occurs in very large
+deposits at Stassfurt, Germany, in the form of the chloride and
+sulphate, associated with compounds of sodium and magnesium. In small
+quantities it is found as nitrate and in many other forms.
+
+The natural decomposition of rocks containing potassium gives rise to
+various compounds of the element in all fertile soils. Its soluble
+compounds are absorbed by growing plants and built up into complex
+vegetable substances; when these are burned the potassium remains in the
+ash in the form of the carbonate. Crude carbonate obtained from wood
+ashes was formerly the chief source of potassium compounds; they are now
+mostly prepared from the salts of the Stassfurt deposits.
+
+     ~Stassfurt salts.~ These salts form very extensive deposits in
+     middle and north Germany, the most noted locality for working
+     them being at Stassfurt. The deposits are very thick and rest
+     upon an enormous layer of common salt. They are in the form of
+     a series of strata, each consisting largely of a single mineral
+     salt. A cross section of these deposits is shown in Fig. 78.
+     While these strata are salts from a chemical standpoint, they
+     are as solid and hard as many kinds of stone, and are mined as
+     stone or coal would be. Since the strata differ in general
+     appearance, each can be mined separately, and the various
+     minerals can be worked up by methods adapted to each particular
+     case. The chief minerals of commercial importance in these
+     deposits are the following:
+
+        Sylvine       KCl.
+        Anhydrite     CaSO_{4}.
+        Carnallite    KCl.MgCl_{2}.6H_{2}O.
+        Kainite       K_{2}SO_{4}.MgSO_{4}.MgCl_{2}.6H_{2}O.
+        Polyhalite    K_{2}SO_{4}.MgSO_{4}.2CaSO_{4}.2H_{2}O.
+        Kieserite     MgSO_{4}.H_{2}O.
+        Schoenite      K_{2}SO_{4}.MgSO_{4}.6H_{2}O.
+
+~Preparation and properties.~ The metal is prepared by the same method
+used in the preparation of sodium. In most respects it is very similar
+to sodium, the chief difference being that it is even more energetic in
+its action upon other substances. The freshly cut, bright surface
+instantly becomes dim through oxidation by the air. It decomposes water
+very vigorously, the heat of reaction being sufficient to ignite the
+hydrogen evolved. It is somewhat lighter than sodium and is preserved
+under gasoline.
+
+[Illustration: Fig. 78]
+
+~Potassium hydroxide~ (_caustic potash_) (KOH). Potassium hydroxide is
+prepared by methods exactly similar to those used in the preparation of
+sodium hydroxide, which compound it closely resembles in both physical
+and chemical properties. It is not used to any very great extent, being
+replaced by the cheaper sodium hydroxide.
+
+~Action of the halogen elements on potassium hydroxide.~ When any one of
+the three halogen elements--chlorine, bromine, and iodine--is added to a
+solution of potassium hydroxide a reaction takes place, the nature of
+which depends upon the conditions of the experiment. Thus, when chlorine
+is passed into a cold dilute solution of potassium hydroxide the
+reaction expressed by the following equation takes place:
+
+    (1) 2KOH + 2Cl = KCl + KClO + H_{2}O.
+
+If the solution of hydroxide is concentrated and hot, on the other hand,
+the potassium hypochlorite formed according to equation (1) breaks down
+as fast as formed:
+
+    (2) 3KClO = KClO_{3} + 2KCl.
+
+Equation (1), after being multiplied by 3, may be combined with equation
+(2), giving the following:
+
+    (3) 6KOH + 6Cl = 5KCl + KClO_{3} + 3H_{2}O.
+
+This represents in a single equation the action of chlorine on hot,
+concentrated solutions of potassium hydroxide. By means of these
+reactions one can prepare potassium chloride, potassium hypochlorite,
+and potassium chlorate. By substituting bromine or iodine for chlorine
+the corresponding compounds of these elements are obtained. Some of
+these compounds can be obtained in cheaper ways.
+
+If the halogen element is added to a solution of sodium hydroxide or
+calcium hydroxide, the reaction which takes place is exactly similar to
+that which takes place with potassium hydroxide. It is possible,
+therefore, to prepare in this way the sodium and calcium compounds
+corresponding to the potassium compounds given above.
+
+~Potassium chloride~ (KCl). This salt occurs in nature in sea water, in
+the mineral sylvine, and, combined with magnesium chloride, as
+carnallite (KCl.MgCl_{2}.6H_{2}O). It is prepared from carnallite by
+saturating boiling water with the mineral and allowing the solution to
+cool. The mineral decomposes while in solution, and the potassium
+chloride crystallizes out on cooling, while the very soluble magnesium
+chloride remains in solution. The salt is very similar to sodium
+chloride both in physical and chemical properties. It is used in the
+preparation of nearly all other potassium salts, and, together with
+potassium sulphate, is used as a fertilizer.
+
+~Potassium bromide~ (KBr). When bromine is added to a hot concentrated
+solution of potassium hydroxide there is formed a mixture of potassium
+bromide and potassium bromate in accordance with the reactions already
+discussed. There is no special use for the bromate, so the solution is
+evaporated to dryness, and the residue, consisting of a mixture of the
+bromate and bromide, is strongly heated. This changes the bromate to
+bromide, as follows:
+
+    KBrO_{3} = KBr +3O.
+
+The bromide is then crystallized from water, forming large colorless
+crystals. It is used in medicine and in photography.
+
+~Potassium iodide~ (KI). Potassium iodide may be made by exactly the same
+method as has just been described for the bromide, substituting iodine
+for bromine. It is more frequently made as follows. Iron filings are
+treated with iodine, forming the compound Fe_{3}I_{8}; on boiling this
+substance with potassium carbonate the reaction represented in the
+following equation occurs:
+
+    Fe_{3}I_{8} + 4K_{2}CO_{3} = Fe_{3}O_{4} + 8KI + 4CO_{2}.
+
+Potassium iodide finds its chief use in medicine.
+
+~Potassium chlorate~ (KClO_{3}). This salt, as has just been explained,
+can be made by the action of chlorine on strong potassium hydroxide
+solutions. The chief use of potassium chlorate is as an oxidizing agent
+in the manufacture of matches, fireworks, and explosives; it is also
+used in the preparation of oxygen and in medicine.
+
+     ~Commercial preparation.~ By referring to the reaction between
+     chlorine and hot concentrated solutions of potassium hydroxide,
+     it will be seen that only one molecule of potassium chlorate is
+     formed from six molecules of potassium hydroxide. Partly
+     because of this poor yield and partly because the potassium
+     hydroxide is rather expensive, this process is not an
+     economical one for the preparation of potassium chlorate. The
+     commercial method is the following. Chlorine is passed into hot
+     solutions of calcium hydroxide, a compound which is very cheap.
+     The resulting calcium chloride and chlorate are both very
+     soluble. To the solution of these salts potassium chloride is
+     added, and as the solution cools the sparingly soluble
+     potassium chlorate crystallizes out:
+
+          Ca(ClO_{3})_{2} + 2KCl = 2KClO_{3} + CaCl_{2}.
+
+     Electro-chemical processes are also used.
+
+~Potassium nitrate~ (_saltpeter_) (KNO_{3}). This salt was formerly made
+by allowing animal refuse to decompose in the open air in the presence
+of wood ashes or earthy materials containing potassium. Under these
+conditions the nitrogen in the organic matter is in part converted into
+potassium nitrate, which was obtained by extracting the mass with water
+and evaporating to crystallization. This crude and slow process is now
+almost entirely replaced by a manufacturing process in which the
+potassium salt is made from Chili saltpeter:
+
+    NaNO_{3} + KCl = NaCl + KNO_{3}.
+
+This process has been made possible by the discovery of the Chili niter
+beds and the potassium chloride of the Stassfurt deposits.
+
+     The reaction depends for its success upon the apparently
+     insignificant fact that sodium chloride is almost equally
+     soluble in cold and hot water. All four factors in the equation
+     are rather soluble in cold water, but in hot water sodium
+     chloride is far less soluble than the other three. When hot
+     saturated solutions of sodium nitrate and potassium chloride
+     are brought together, sodium chloride precipitates and can be
+     filtered off, leaving potassium nitrate in solution, together
+     with some sodium chloride. On cooling, potassium nitrate
+     crystallizes out, leaving small amounts of the other salts in
+     solution.
+
+Potassium nitrate is a colorless salt which forms very large crystals.
+It is stable in the air, and when heated is a good oxidizing agent,
+giving up oxygen quite readily. Its chief use is in the manufacture of
+gunpowder.
+
+     ~Gunpowder.~ The object sought for in the preparation of
+     gunpowder is to secure a solid substance which will remain
+     unchanged under ordinary conditions, but which will explode
+     readily when ignited, evolving a large volume of gas. When a
+     mixture of carbon and potassium nitrate is ignited a great deal
+     of gas is formed, as will be seen from the equation
+
+         2KNO_{3} + 3C = CO_{2} + CO + N_{2} + K_{2}CO_{3}.
+
+     By adding sulphur to the mixture the volume of gas formed in
+     the explosion is considerably increased:
+
+         2KNO_{3} + 3C + S = 3CO_{2} + N_{2} + K_{2}S.
+
+     Gunpowder is simply a mechanical mixture of these three
+     substances in the proportion required for the above reaction.
+     While the equation represents the principal reaction, other
+     reactions also take place. The gases formed in the explosion,
+     when measured under standard conditions, occupy about two
+     hundred and eighty times the volume of the original powder.
+     Potassium sulphide (K_{2}S) is a solid substance, and it is
+     largely due to it that gunpowder gives off smoke and soot when
+     it explodes. Smokeless powder consists of organic substances
+     which, on explosion, give only colorless gases, and hence
+     produce no smoke. Sodium nitrate is cheaper than potassium
+     nitrate, but it is not adapted to the manufacture of the best
+     grades of powder, since it is somewhat deliquescent and does
+     not give up its oxygen so readily as does potassium nitrate. It
+     is used, however, in the cheaper grades of powder, such as are
+     employed for blasting.
+
+~Potassium cyanide~ (KCN). When animal matter containing nitrogen is
+heated with iron and potassium carbonate, complicated changes occur
+which result in the formation of a substance commonly called yellow
+prussiate of potash, which has the formula K_{4}FeC_{6}N_{6}. When this
+substance is heated with potassium, potassium cyanide is formed:
+
+    K_{4}FeC_{6}N_{6} + 2 K = 6KCN + Fe.
+
+Since sodium is much cheaper than potassium it is often used in place of
+it:
+
+    K_{4}FeC_{6}N_{6} + 2Na = 4KCN + 2NaCN + Fe.
+
+The mixture of cyanides so resulting serves most of the purposes of the
+pure salt. It is used very extensively in several metallurgical
+processes, particularly in the extraction of gold. Potassium cyanide is
+a white solid characterized by its poisonous properties, and must be
+used with extreme caution.
+
+~Potassium carbonate~ (_potash_) (K_{2}CO_{3}). This compound occurs in
+wood ashes in small quantities. It cannot be prepared by the Solvay
+process, since the acid carbonate is quite soluble in water, but is made
+by the Le Blanc process. Its chief use is in the manufacture of other
+potassium salts.
+
+~Other salts of potassium.~ Among the other salts of potassium frequently
+met with are the sulphate (K_{2}SO_{4}), the acid carbonate (KHCO_{3}),
+the acid sulphate (KHSO_{4}), and the acid sulphite (KHSO_{3}). These
+are all white solids.
+
+
+LITHIUM, RUBIDIUM, CAESIUM
+
+Of the three remaining elements of the family--lithium, rubidium, and
+caesium--lithium is by far the most common, the other two being very
+rare. Lithium chloride and carbonate are not infrequently found in
+natural mineral waters, and as these substances are supposed to increase
+the medicinal value of the water, they are very often added to
+artificial mineral waters in small quantities.
+
+
+COMPOUNDS OF AMMONIUM
+
+~General.~ As explained in a previous chapter, when ammonia is passed into
+water the two compounds combine to form the base NH_{4}OH, known as
+ammonium hydroxide. When this base is neutralized with acids there are
+formed the corresponding salts, known as the ammonium salts. Since the
+ammonium group is univalent, ammonium salts resemble those of the alkali
+metals in formulas; they also resemble the latter salts very much in
+their chemical properties, and may be conveniently described in
+connection with them. Among the ammonium salts the chloride, sulphate,
+carbonate, and sulphide are the most familiar.
+
+~Ammonium chloride~ (_sal ammoniac_) (NH_{4}Cl). This substance is
+obtained by neutralizing ammonium hydroxide with hydrochloric acid. It
+is a colorless substance crystallizing in fine needles, and, like most
+ammonium salts, is very soluble in water. When placed in a tube and
+heated strongly it decomposes into hydrochloric acid and ammonia. When
+these gases reach a cooler portion of the tube they at once recombine,
+and the resulting ammonium chloride is deposited on the sides of the
+tube. In this way the salt can be separated from nonvolatile impurities.
+Ammonium chloride is sometimes used in preparation of ammonia; it is
+also used in making dry batteries and in the laboratory as a chemical
+reagent.
+
+~Ammonium sulphate~ ((NH_{4})_{2}SO_{4}). This salt resembles the chloride
+very closely, and, being cheaper, is used in place of it when possible.
+It is used in large quantity as a fertilizer, the nitrogen which it
+contains being a very valuable food for plants.
+
+~Ammonium carbonate~ ((NH_{4})_{2}CO_{3}). This salt, as well as the acid
+carbonate (NH_{4}HCO_{3}), is used as a chemical reagent. They are
+colorless solids, freely soluble in water. The normal carbonate is made
+by heating ammonium chloride with powdered limestone (calcium
+carbonate), the ammonium carbonate being obtained as a sublimate in
+compact hard masses:
+
+    2NH_{4}Cl + CaCO_{3} = (NH_{4})_{2}CO_{3} + CaCl_{2}.
+
+The salt always smells of ammonia, since it slowly decomposes, as shown
+in the equation
+
+    (NH_{4})_{2}CO_{3} = NH_{4}HCO_{3} + NH_{3}.
+
+The acid carbonate, or bicarbonate, is prepared by saturating a solution
+of ammonium hydroxide with carbon dioxide:
+
+    NH_{4}OH + CO_{2} = NH_{4}HCO_{3}.
+
+It is a well-crystallized stable substance.
+
+~Ammonium sulphide~ ((NH_{4})_{2}S). Ammonium sulphide is prepared by the
+action of hydrosulphuric acid upon ammonium hydroxide:
+
+    2NH_{4}OH + H_{2}S = (NH_{4})_{2}S + 2H_{2}O.
+
+If the action is allowed to continue until no more hydrosulphuric acid
+is absorbed, the product is the acid sulphide, sometimes called the
+hydrosulphide:
+
+    NH_{4}OH + H_{2}S = NH_{4}HS + H_{2}O.
+
+If equal amounts of ammonium hydroxide and ammonium acid sulphide are
+brought together, the normal sulphide is formed:
+
+    NH_{4}OH + NH_{4}HS = (NH_{4})_{2}S + H_{2}O
+
+It has been obtained in the solid state, but only with great difficulty.
+As used in the laboratory it is always in the form of a solution. It is
+much used in the process of chemical analysis because it is a soluble
+sulphide and easily prepared. On exposure to the air ammonium sulphide
+slowly decomposes, being converted into ammonia, water, and sulphur:
+
+    (NH_{4})_{2}S + O = 2NH_{3} + H_{2}O + S.
+
+As fast as the sulphur is liberated it combines with the unchanged
+sulphide to form several different ammonium sulphides in which there are
+from two to five sulphur atoms in the molecule, thus: (NH_{4})_{2}S_{2},
+(NH_{4})_{2}S_{3}, (NH_{4})_{2}S_{5}. These sulphides in turn decompose
+by further action of oxygen, so that the final products of the reaction
+are those given in the equation. A solution of these compounds is yellow
+and is sometimes called _yellow ammonium sulphide_.
+
+     FLAME REACTION--SPECTROSCOPE
+
+     When compounds of either sodium or potassium are brought into
+     the non-luminous flame of a Bunsen burner the flame becomes
+     colored. Sodium compounds color it intensely yellow, while
+     those of potassium color it pale violet. When only one of these
+     elements is present it is easy to identify it by this simple
+     test, but when both are present the intense color of the sodium
+     flame entirely conceals the pale tint characteristic of
+     potassium compounds.
+
+     It is possible to detect the potassium flame in such cases,
+     however, in the following way. When light is allowed to shine
+     through a very small hole or slit in some kind of a screen,
+     such as a piece of metal, upon a triangular prism of glass, the
+     light is bent or refracted out of its course instead of passing
+     straight through the glass. It thus comes out of the prism at
+     some angle to the line at which it entered. Yellow light is
+     bent more than red, and violet more than yellow. When light
+     made up of the yellow of sodium and the violet of potassium
+     shines through a slit upon such a prism, the yellow and the
+     violet lights come out at somewhat different angles, and so two
+     colored lines of light--a yellow line and a violet line--are
+     seen on looking into the prism in the proper direction. The
+     instrument used for separating the rays of light in this way is
+     called a _spectroscope_ (Fig. 79). The material to be tested is
+     placed on a platinum wire and held in the colorless Bunsen
+     flame. The resulting light passes through the slit in the end
+     of tube B, and then through B to the prism. The resulting
+     lines of light are seen by looking into the tube A, which
+     contains a magnifying lens. Most elements give more than one
+     image of the slit, each having a different color, and the
+     series of colored lines due to an element is called its
+     spectrum.
+
+[Illustration: Fig. 79]
+
+The spectra of the known elements have been carefully studied, and any
+element which imparts a characteristic color to a flame, or has a
+spectrum of its own, can be identified even when other elements are
+present. Through the spectroscopic examination of certain minerals a
+number of elements have been discovered by the observation of lines
+which did not belong to any known element. A study of the substance then
+brought to light the new element. Rubidium and caesium were discovered in
+this way, rubidium having bright red lines and caesium a very intense
+blue line. Lithium colors the flame deep red, and has a bright red line
+in its spectrum.
+
+
+EXERCISES
+
+1. What is an alkali? Can a metal itself be an alkali?
+
+2. Write equations showing how the following changes may be brought
+about, giving the general principle involved in each change: NaCl -->
+Na_{2}SO_{3}, Na_{2}SO_{3} --> NaCl, NaCl --> NaBr, Na_{2}SO_{4} -->
+NaNO_{3}, NaNO_{3} --> NaHCO_{3}.
+
+3. What carbonates are soluble?
+
+4. State the conditions under which the reaction represented by the
+following equation can be made to go in either direction:
+
+    Na_{2}CO_{3} + H_{2}O + CO_{2} <--> 2 NaHCO_{3}.
+
+5. Account for the fact that solutions of sodium carbonate and potassium
+carbonate are alkaline.
+
+6. What non-metallic element is obtained from the deposits of Chili
+saltpeter?
+
+7. Supposing concentrated hydrochloric acid (den. = 1.2) to be worth six
+cents a pound, what is the value of the acid generated in the
+preparation of 1 ton of sodium carbonate by the Le Blanc process?
+
+8. What weight of sodium carbonate crystals will 1 kg. of the anhydrous
+salt yield?
+
+9. Write equations for the preparation of potassium hydroxide by three
+different methods.
+
+10. What would take place if a bit of potassium hydroxide were left
+exposed to the air?
+
+11. Write the equations for the reactions between sodium hydroxide and
+bromine; between potassium hydroxide and iodine.
+
+12. Write equations for the preparation of potassium sulphate; of
+potassium acid carbonate.
+
+13. What weight of carnallite would be necessary in the preparation of 1
+ton of potassium carbonate?
+
+14. Write the equations showing how ammonium chloride, ammonium
+sulphate, ammonium carbonate, and ammonium nitrate may be prepared from
+ammonium hydroxide.
+
+15. Write an equation to represent the reaction involved in the
+preparation of ammonia from ammonium chloride.
+
+16. What substances already studied are prepared from the following
+compounds? ammonium chloride; ammonium nitrate; ammonium nitrite; sodium
+nitrate; sodium chloride.
+
+17. How could you prove that the water in crystals of common salt is not
+water of crystallization?
+
+18. How could you distinguish between potassium chloride and potassium
+iodide? between sodium chloride and ammonium chloride? between sodium
+nitrate and potassium nitrate?
+
+[Illustration: ROBERT WILHELM BUNSEN (German) (1811-1899)
+
+Invented many lecture-room and laboratory appliances (Bunsen burner);
+invented the spectroscope and with it discovered rubidium and caesium;
+greatly perfected methods of electrolysis, inventing a new battery; made
+many investigations among metallic and organic substances]
+
+
+
+
+CHAPTER XXIV
+
+THE ALKALINE-EARTH FAMILY
+
+
+===========================================================================
+          |        |        |         |                  |
+          |        |        |         | MILLIGRAMS SOL-  |
+          |        |        |         |  UBLE IN 1 L.    |
+          |        |        |         | OF WATER AT 18 deg.  |
+          | SYMBOL | ATOMIC | DENSITY |__________________| CARBONATE
+          |        | WEIGHT |         |         |        | DECOMPOSES
+          |        |        |         | SULPHATE| HYDROX-|
+          |        |        |         |         |   IDE  |
+__________|________|________|_________|_________|________|_________________
+          |        |        |         |         |        |
+Calcium   |   Ca   |  40.1  |   1.54  | 2070.00 |  1670. | At dull red heat
+Strontium |   Sr   |  87.6  |   2.50  |  170.00 |  7460. | At white heat
+Barium    |   Ba   | 137.4  |   3.75  |    2.29 | 36300. | Scarcely at all
+===========================================================================
+
+~The family.~ The alkaline-earth family consists of the very abundant
+element calcium and the much rarer elements strontium and barium. They
+are called the alkaline-earth metals because their properties are
+between those of the alkali metals and the earth metals. The earth
+metals will be discussed in a later chapter. The family is also
+frequently called the calcium family.
+
+1. _Occurrence._ These elements do not occur free in nature. Their most
+abundant compounds are the carbonates and sulphates; calcium also occurs
+in large quantities as the phosphate and silicate.
+
+2. _Preparation._ The metals were first prepared by Davy in 1808 by
+electrolysis. This method has again come into use in recent years.
+Strontium and barium have as yet been obtained only in small quantities
+and in the impure state, and many of their physical properties, such as
+their densities and melting points, are therefore imperfectly known.
+
+3. _Properties._ The three metals resemble each other very closely. They
+are silvery-white in color and are about as hard as lead. Their
+densities increase with their atomic weights, as is shown in the table
+on opposite page. Like the alkali metals they have a strong affinity for
+oxygen, tarnishing in the air through oxidation. They decompose water at
+ordinary temperatures, forming hydroxides and liberating hydrogen. When
+ignited in the air they burn with brilliancy, forming oxides of the
+general formula MO. These oxides readily combine with water, according
+to the equation
+
+    MO + H_{2}O = M(OH)_{2}.
+
+Each of the elements has a characteristic spectrum, and the presence of
+the metals can easily be detected by the spectroscope.
+
+4. _Compounds._ The elements are divalent in almost all of their
+compounds, and these compounds in solution give simple, divalent,
+colorless ions. The corresponding salts of the three elements are very
+similar to each other and show a regular variation in properties in
+passing from calcium to strontium and from strontium to barium. This is
+seen in the solubility of the sulphate and hydroxide, and in the ease of
+decomposition of the carbonates, as given in the table. Unlike the
+alkali metals, their normal carbonates and phosphates are insoluble in
+water.
+
+
+CALCIUM
+
+~Occurrence.~ The compounds of calcium are very abundant in nature, so
+that the total amount of calcium in the earth's crust is very large. A
+great many different compounds containing the clement are known, the
+most important of which are the following:
+
+    Calcite (marble)     CaCO_{3}.
+    Phosphorite          Ca_{3}(PO_{4})_{2}.
+    Fluorspar            CaF_{2}.
+    Wollastonite         CaSiO_{3}.
+    Gypsum               CaSO_{4}.2H_{2}O.
+    Anhydrite            CaSO_{4}.
+
+~Preparation.~ Calcium is now prepared by the electrolysis of the melted
+chloride, the metal depositing in solid condition on the cathode. It is
+a gray metal, considerably heavier and harder than sodium. It acts upon
+water, forming calcium hydroxide and hydrogen, but the action does not
+evolve sufficient heat to melt the metal. It promises to become a useful
+substance, though no commercial applications for it have as yet been
+found.
+
+~Calcium oxide~ (_lime, quicklime_) (CaO). Lime is prepared by strongly
+heating calcium carbonate (limestone) in large furnaces called kilns:
+
+    CaCO_{3} = CaO + CO_{2}.
+
+When pure, lime is a white amorphous substance. Heated intensely, as in
+the oxyhydrogen flame, it gives a brilliant light called the lime light.
+Although it is a very difficultly fusible substance, yet in the electric
+furnace it can be made to melt and even boil. Water acts upon lime with
+the evolution of a great deal of heat,--hence the name quicklime, or
+live lime,--the process being called slaking. The equation is
+
+    CaO + H_{2}O = Ca(OH)_{2}.
+
+Lime readily absorbs moisture from the air, and is used to dry moist
+gases, especially ammonia, which cannot be dried by the usual
+desiccating agents. It also absorbs carbon dioxide, forming the
+carbonate
+
+    CaO + CO_{2} = CaCO_{3}.
+
+Lime exposed to air is therefore gradually converted into hydroxide and
+carbonate, and will no longer slake with water. It is then said to be
+air-slaked.
+
+     ~Limekilns.~ The older kiln, still in common use, consists of a
+     large cylindrical stack in which the limestone is loosely
+     packed. A fire is built at the base of the stack, and when the
+     burning is complete it is allowed to die out and the lime is
+     removed from the kiln. The newer kilns are constructed as shown
+     in Fig. 80. A number of fire boxes are built around the lower
+     part of the kiln, one of which is shown at B. The fire is
+     built on the grate F and the hot products of combustion are
+     drawn up through the stack, decomposing the limestone. The kiln
+     is charged at C, and sometimes fuel is added with the
+     limestone to cause combustion throughout the contents of the
+     kiln. The burned lime is raked out through openings in the
+     bottom of the stack, one of which is shown at _D._ The
+     advantage of this kind of a kiln over the older form is that
+     the process is continuous, limestone being charged in at the
+     top as fast as the lime is removed at the bottom.
+
+[Illustration: Fig. 80]
+
+~Calcium hydroxide ~ (_slaked lime_) (Ca(OH)_{2}). Pure calcium hydroxide
+is a light white powder. It is sparingly soluble in water, forming a
+solution called _limewater_, which is often used in medicine as a mild
+alkali. Chemically, calcium hydroxide is a moderately strong base,
+though not so strong as sodium hydroxide. Owing to its cheapness it is
+much used in the industries whenever an alkali is desired. A number of
+its uses have already been mentioned. It is used in the preparation of
+ammonia, bleaching powder, and potassium hydroxide. It is also used to
+remove carbon dioxide and sulphur compounds from coal gas, to remove the
+hair from hides in the tanneries (this recalls the caustic or corrosive
+properties of sodium hydroxide), and for making mortar.
+
+~Mortar~ is a mixture of calcium hydroxide and sand. When it is exposed to
+the air or spread upon porous materials moisture is removed from it
+partly by absorption in the porous materials and partly by evaporation,
+and the mortar becomes firm, or _sets_. At the same time carbon dioxide
+is slowly absorbed from the air, forming hard calcium carbonate:
+
+    Ca(OH)_{2} + CO_{2} = CaCO_{3} + H_{2}O.
+
+By this combined action the mortar becomes very hard and adheres firmly
+to the surface upon which it is spread. The sand serves to give body to
+the mortar and makes it porous, so that the change into carbonate can
+take place throughout the mass. It also prevents too much shrinkage.
+
+~Cement.~ When limestone to which clay and sand have been added in certain
+proportions is burned until it is partly fused (some natural marl is
+already of about the right composition), and the clinker so produced is
+ground to powder, the product is called cement. When this material is
+moistened it sets to a hard stone-like mass which retains its hardness
+even when exposed to the continued action of water. It can be used for
+under-water work, such as bridge piers, where mortar would quickly
+soften. Several varieties of cement are made, the best known of which is
+Portland cement.
+
+~Growing importance of cement.~ Cement is rapidly coming into use for a
+great variety of purposes. It is often used in place of mortar in the
+construction of brick buildings. Mixed with crushed stone and sand it
+forms concrete which is used in foundation work. It is also used in
+making artificial stone, terra-cotta trimmings for buildings, artificial
+stone walks and floors, and the like. It is being used more and more for
+making many articles which were formerly made of wood or stone, and the
+entire walls of buildings are sometimes made of cement blocks or of
+concrete.
+
+~Calcium carbonate~ (CaCO_{3}). This substance is found in a great many
+natural forms to which various names have been given. They may be
+classified under three heads:
+
+1. _Amorphous carbonate._ This includes those forms which are not
+markedly crystalline. Limestone is the most familiar of these and is a
+grayish rock usually found in hard stratified masses. Whole mountain
+ranges are sometimes made up of this material. It is always impure,
+usually containing magnesium carbonate, clay, silica, iron and aluminium
+compounds, and frequently fossil remains. Marl is a mixture of limestone
+and clay. Pearls, chalk, coral, and shells are largely calcium
+carbonate.
+
+2. _Hexagonal carbonate._ Calcium carbonate crystallizes in the form of
+rhomb-shaped crystals which belong to the hexagonal system. When very
+pure and transparent the substance is called Iceland spar. Calcite is a
+similar form, but somewhat opaque or clouded. Mexican onyx is a massive
+variety, streaked or banded with colors due to impurities. Marble when
+pure is made up of minute calcite crystals. Stalactites and stalagmites
+are icicle-like forms sometimes found in caves.
+
+3. _Rhombic carbonate._ Calcium carbonate sometimes crystallizes in
+needle-shaped crystals belonging to the rhombic system. This is the
+unstable form and tends to go over into the other variety. Aragonite is
+the most familiar example of this form.
+
+~Preparation and uses of calcium carbonate.~ In the laboratory pure
+calcium carbonate can be prepared by treating a soluble calcium salt
+with a soluble carbonate:
+
+    Na_{2}CO_{3} + CaCl_{2} = CaCO_{3} + 2NaCl.
+
+When prepared in this way it is a soft white powder often called
+precipitated chalk, and is much used as a polishing powder. It is
+insoluble in water, but dissolves in water saturated with carbon
+dioxide, owing to the formation of the acid calcium carbonate which is
+slightly soluble:
+
+    CaCO_{3} + H_{2}CO_{3} = Ca(HCO_{3})_{2}.
+
+The natural varieties of calcium carbonate find many uses, such as in
+the preparation of lime and carbon dioxide; in metallurgical operations,
+especially in the blast furnaces; in the manufacture of soda, glass, and
+crayon (which, in addition to chalk, usually contains clay and calcium
+sulphate); for building stone and ballast for roads.
+
+~Calcium chloride~ (CaCl_{2}). This salt occurs in considerable quantity
+in sea water. It is obtained as a by-product in many technical
+processes, as in the Solvay soda process. When crystallized from its
+saturated solutions it forms colorless needles of the composition
+CaCl_{2}.6H_{2}O. By evaporating a solution to dryness and heating to a
+moderate temperature calcium chloride is obtained anhydrous as a white
+porous mass. In this condition it absorbs water with great energy and is
+a valuable drying agent.
+
+~Bleaching powder~ (CaOCl_{2}). When chlorine acts upon a solution of
+calcium hydroxide the reaction is similar to that which occurs between
+chlorine and potassium hydroxide:
+
+    2 Ca(OH)_{2} + 4 Cl = CaCl_{2} + Ca(ClO)_{2} + 2 H_{2}O.
+
+If, however, chlorine is conducted over calcium hydroxide in the form of
+a dry powder, it is absorbed and a substance is formed which appears to
+have the composition represented in the formula CaOCl_{2}. This
+substance is called bleaching powder, or hypochlorite of lime. It is
+probably the calcium salt of both hydrochloric and hypochlorous acids,
+so that its structure is represented by the formula
+
+       /ClO
+    Ca
+       \Cl.
+
+In solution this substance acts exactly like a mixture of calcium
+chloride (CaCl_{2}) and calcium hypochlorite (Ca(ClO)_{2}), since it
+dissociates to form the ions Ca^{++}, Cl^{-}, and ClO^{-}.
+
+Bleaching powder undergoes a number of reactions which make it an
+important substance.
+
+1. When treated with an acid it evolves chlorine:
+
+      /ClO
+    Ca + H_{2}SO_{4} = CaSO_{4} + HCl + HClO,
+      \Cl
+
+
+    HCl + HClO = H_{2}O + 2Cl.
+
+This reaction can be employed in the preparation of chlorine, or the
+nascent chlorine may be used as a bleaching agent.
+
+2. It is slowly decomposed by the carbon dioxide of the air, yielding
+calcium carbonate and chlorine:
+
+    CaOCl_{2} + CO_{2} = CaCO_{3} + 2Cl.
+
+Owing to this slow action the substance is a good disinfectant.
+
+3. When its solution is boiled the substance breaks down into calcium
+chloride and chlorate:
+
+    6CaOCl_{2} = 5CaCl_{2} + Ca(ClO_{3})_{2}.
+
+This reaction is used in the preparation of potassium chlorate.
+
+~Calcium fluoride~ (_fluorspar_) (CaF_{2}). Fluorspar has already been
+mentioned as the chief natural compound of fluorine. It is found in
+large quantities in a number of localities, and is often crystallized in
+perfect cubes of a light green or amethyst color. It can be melted
+easily in a furnace, and is sometimes used in the fused condition in
+metallurgical operations to protect a metal from the action of the air
+during its reduction. It is used as the chief source of fluorine
+compounds, especially hydrofluoric acid.
+
+~Calcium sulphate~ (_gypsum_) (CaSO_{4}.2H_{2}O). This abundant substance
+occurs in very perfectly formed crystals or in massive deposits. It is
+often found in solution in natural waters and in the sea water. Salts
+deposited from sea water are therefore likely to contain this substance
+(see Stassfurt salts).
+
+It is very sparingly soluble in water, and is thrown down as a fine
+white precipitate when any considerable amounts of a calcium salt and a
+soluble sulphate (or sulphuric acid) are brought together in solution.
+Its chief use is in the manufacture of plaster of Paris and of hollow
+tiles for fireproof walls. Such material is called _gypsite_. It is also
+used as a fertilizer.
+
+Calcium sulphate, like the carbonate, occurs in many forms in nature.
+Gypsum is a name given to all common varieties. Granular or massive
+specimens are called alabaster, while all those which are well
+crystallized are called selenite. Satin spar is still another variety
+often seen in mineral collections.
+
+~Plaster of Paris.~ When gypsum is heated to about 115 deg. it loses a portion
+of its water of crystallization in accordance with the equation
+
+    2(CaSO_{4}.2H_{2}O) = 2CaSO_{4}.H_{2}O + 2H_{2}O.
+
+The product is a fine white powder called _plaster of Paris_. On being
+moistened it again takes up this water, and in so doing first forms a
+plastic mass, which soon becomes very firm and hard and regains its
+crystalline structure. These properties make it very valuable as a
+material for forming casts and stucco work, for cementing glass to
+metals, and for other similar purposes. If overheated so that all water
+is driven off, the process of taking up water is so slow that the
+material is worthless. Such material is said to be dead burned. Plaster
+of Paris is very extensively used as the finishing coat for plastered
+walls.
+
+~Hard water.~ Waters containing compounds of calcium and magnesium in
+solution are called hard waters because they feel harsh to the touch.
+The hardness of water may be of two kinds,--(1) temporary hardness and
+(2) permanent hardness.
+
+1. _Temporary hardness._ We have seen that when water charged with
+carbon dioxide comes in contact with limestone a certain amount of the
+latter dissolves, owing to the formation of the soluble acid carbonate
+of calcium. The hardness of such waters is said to be temporary, since
+it may be removed by boiling. The heat changes the acid carbonate into
+the insoluble normal carbonate which then precipitates, rendering the
+water soft:
+
+    Ca(HCO_{3})_{2} = CaCO_{3} + H_{2}O + CO_{2}.
+
+Such waters may also be softened by the addition of sufficient lime or
+calcium hydroxide to convert the acid carbonate of calcium into the
+normal carbonate. The equation representing the reaction is
+
+    Ca(HCO_{3})_{2} + Ca(OH)_{2} = 2CaCO_{3} + 2H_{2}O.
+
+2. _Permanent hardness._ The hardness of water may also be due to the
+presence of calcium and magnesium sulphates or chlorides. Boiling the
+water does not affect these salts; hence such waters are said to have
+permanent hardness. They may be softened, however, by the addition of
+sodium carbonate, which precipitates the calcium and magnesium as
+insoluble carbonates:
+
+    CaSO_{4} + Na_{2}CO_{3} = CaCO_{3} + Na_{2}SO_{4}.
+
+This process is sometimes called "breaking" the water.
+
+     ~Commercial methods for softening water.~ The average water of a
+     city supply contains not only the acid carbonates of calcium
+     and magnesium but also the sulphates and chlorides of these
+     metals, together with other salts in smaller quantities. Such
+     waters are softened on a commercial scale by the addition of
+     the proper quantities of calcium hydroxide and sodium
+     carbonate. The calcium hydroxide is added first to precipitate
+     all the acid carbonates. After a short time the sodium
+     carbonate is added to precipitate the other soluble salts of
+     calcium and magnesium, together with any excess of calcium
+     hydroxide which may have been added. The quantity of calcium
+     hydroxide and sodium carbonate required is calculated from a
+     chemical analysis of the water. It will be noticed that the
+     water softened in this way will contain sodium sulphate and
+     chloride, but the presence of these salts is not objectionable.
+
+~Calcium carbide~ (CaC_{2}). This substance is made by heating well-dried
+coke and lime in an electrical furnace. The equation is
+
+    CaO + 3C = CaC_{2} + CO.
+
+The pure carbide is a colorless, transparent, crystalline substance. In
+contact with water it is decomposed with the evolution of pure acetylene
+gas, having a pleasant ethereal odor. The commercial article is a dull
+gray porous substance which contains many impurities. The acetylene
+prepared from this substance has a very characteristic odor due to
+impurities, the chief of these being phosphine. It is used in
+considerable quantities as a source of acetylene gas for illuminating
+purposes.
+
+     ~Technical preparation.~ Fig. 81 represents a recent type of a
+     carbide furnace. The base of the furnace is provided with a
+     large block of carbon A, which serves as one of the
+     electrodes. The other electrodes B, several in number, are
+     arranged horizontally at some distance above this. A mixture of
+     coal and lime is fed into the furnace through the trap top C,
+     and in the lower part of the furnace this mixture becomes
+     intensely heated, forming liquid carbide. This is drawn off
+     through the taphole D.
+
+     The carbon monoxide formed in the reaction escapes through the
+     pipes E and is led back into the furnace. The pipes F
+     supply air, so that the monoxide burns as it reenters the
+     furnace and assists in heating the charge. The carbon dioxide
+     so formed, together with the nitrogen entering as air, escape
+     at G. An alternating current is used.
+
+[Illustration: Fig. 81]
+
+~Calcium phosphate~ (Ca_{3}(PO_{4})_{2}). This important substance
+occurs abundantly in nature as a constituent of apatite
+(3Ca_{3}(PO_{4})_{2}.CaF_{2}), in phosphate rock, and as the chief
+mineral constituent of bones. Bone ash is therefore nearly pure calcium
+phosphate. It is a white powder, insoluble in water, although it readily
+dissolves in acids, being decomposed by them and converted into soluble
+acid phosphates, as explained in connection with the acids of
+phosphorus.
+
+
+STRONTIUM
+
+~Occurrence.~ Strontium occurs sparingly in nature, usually as
+strontianite (SrCO_{3}) and as celestite (SrSO_{4}). Both minerals form
+beautiful colorless crystals, though celestite is sometimes colored a
+faint blue. Only a few of the compounds of strontium have any commercial
+applications.
+
+~Strontium hydroxide~ (Sr(OH)_{2}.8H_{2}O). The method of preparation of
+strontium hydroxide is analogous to that of calcium hydroxide. The
+substance has the property of forming an insoluble compound with sugar,
+which can easily be separated again into its constituents. It is
+therefore sometimes used in the sugar refineries to extract sugar from
+impure mother liquors from which the sugar will not crystallize.
+
+~Strontium nitrate~ (Sr(NO_{3})_{2}.4H_{2}O). This salt is prepared by
+treating the native carbonate with nitric acid. When ignited with
+combustible materials it imparts a brilliant crimson color to the flame,
+and because of this property it is used in the manufacture of red
+lights.
+
+
+BARIUM
+
+Barium is somewhat more abundant than strontium, occurring in nature
+largely as barytes, or heavy spar (BaSO_{4}), and witherite (BaCO_{3}).
+Like strontium, it closely resembles calcium both in the properties of
+the metal and in the compounds which it forms.
+
+~Oxides of barium.~ Barium oxide (BaO) can be obtained by strongly heating
+the nitrate:
+
+    Ba(NO_{3})_{2} = BaO + 2NO_{2} + O.
+
+Heated to a low red heat in the air, the oxide combines with oxygen,
+forming the peroxide (BaO_{2}). If the temperature is raised still
+higher, or the pressure is reduced, oxygen is given off and the oxide is
+once more formed. The reaction
+
+    BaO_{2} <--> BaO + O
+
+is reversible and has been used as a means of separating oxygen from the
+air. Treated with acids, barium peroxide yields hydrogen peroxide:
+
+    BaO_{2} + 2HCl = BaCl_{2} + H_{2}O_{2}.
+
+~Barium chloride~ (BaCl_{2}.2H_{2}O). Barium chloride is a white
+well-crystallized substance which is easily prepared from the native
+carbonate. It is largely used in the laboratory as a reagent to detect
+the presence of sulphuric acid or soluble sulphates.
+
+~Barium sulphate~ _(barytes)_ (BaSO_{4}). Barium sulphate occurs in nature
+in the form of heavy white crystals. It is precipitated as a crystalline
+powder when a barium salt is added to a solution of a sulphate or
+sulphuric acid:
+
+    BaCl_{2} + H_{2}SO_{4} = BaSO_{4} + 2HCl.
+
+This precipitate is used, as are also the finely ground native sulphate
+and carbonate, as a pigment in paints. On account of its low cost it is
+sometimes used as an adulterant of white lead, which is also a heavy
+white substance.
+
+Barium compounds color the flame green, and the nitrate (Ba(NO_{3})_{2})
+is used in the manufacture of green lights. Soluble barium compounds are
+poisonous.
+
+
+RADIUM
+
+~Historical.~ In 1896 the French scientist Becquerel observed that the
+mineral pitchblende possesses certain remarkable properties. It affects
+photographic plates even in complete darkness, and discharges a
+gold-leaf electroscope when brought close to it. In 1898 Madam Curie
+made a careful study of pitchblende to see if these properties belong to
+it or to some unknown substance contained in it. She succeeded in
+extracting from it a very small quantity of a substance containing a new
+element which she named radium.
+
+In 1910 Madam Curie succeeded in obtaining radium itself by the
+electrolysis of radium chloride. It is a silver-white metal melting at
+about 700 deg.. It blackens in the air, forming a nitride, and decomposes
+water. Its atomic weight is about 226.5.
+
+~Properties.~ Compounds of radium affect a photographic plate or
+electroscope even through layers of paper or sheets of metal. They also
+bring about chemical changes in substances placed near them.
+Investigation of these strange properties has suggested that the radium
+atoms are unstable and undergo a decomposition. As a result of this
+decomposition very minute bodies, to which the name corpuscles has been
+given, are projected from the radium atom with exceedingly great
+velocity. It is to these corpuscles that the strange properties of
+radium are due. It seems probable that the gas helium is in some way
+formed during the decomposition of radium.
+
+Two or three other elements, particularly uranium and thorium, have been
+found to possess many of the properties of radium in smaller degree.
+
+~Radium and the atomic theory.~ If these views in regard to radium should
+prove to be well founded, it will be necessary to modify in some
+respects the conception of the atom as developed in a former chapter.
+The atom would have to be regarded as a compound unit made up of several
+parts. In a few cases, as in radium and uranium, it would appear that
+this unit is unstable and undergoes transformation into more stable
+combinations. This modification would not, in any essential way, be at
+variance with the atomic theory as propounded by Dalton.
+
+
+EXERCISES
+
+1. What properties have the alkaline-earth metals in common with the
+alkali metals? In what respects do they differ?
+
+2. Write the equation for the reaction between calcium carbide and
+water.
+
+3. For what is calcium chlorate used?
+
+4. Could limestone be completely decomposed if heated in a closed
+vessel?
+
+5. Caves often occur in limestone. Account for their formation.
+
+6. What is the significance of the term fluorspar? (Consult dictionary.)
+
+7. Could calcium chloride be used in place of barium chloride in testing
+for sulphates?
+
+8. What weight of water is necessary to slake the lime obtained from 1
+ton of pure calcium carbonate?
+
+9. What weight of gypsum is necessary in the preparation of 1 ton of
+plaster of Paris?
+
+10. Write equations to represent the reactions involved in the
+preparation of strontium hydroxide and strontium nitrate from
+strontianite.
+
+11. Write equations to represent the reactions involved in the
+preparation of barium chloride from heavy spar.
+
+12. Could barium hydroxide be used in place of calcium hydroxide in
+testing for carbon dioxide?
+
+
+
+
+CHAPTER XXV
+
+THE MAGNESIUM FAMILY
+
+
+===========================================================================
+           |SYMBOL   |ATOMIC   |DENSITY   |MELTING   |BOILING | OXIDE
+           |         |WEIGHT   |          | POINT    | POINT  |
+---------------------------------------------------------------------------
+Magnesium  | Mg      | 24.36   |  1.75    | 750 deg.     | 920 deg.   |   MgO
+Zinc       | Zn      | 65.4    |  7.00    | 420 deg.     | 950 deg.   |   ZnO
+Cadmium    | Cd      |112.4    |  8.67    | 320 deg.     | 778 deg.   |   CdO
+===========================================================================
+
+~The family.~ In the magnesium family are included the four elements:
+magnesium, zinc, cadmium, and mercury. Between the first three of these
+metals there is a close family resemblance, such as has been traced
+between the members of the two preceding families. Mercury in some
+respects is more similar to copper and will be studied in connection
+with that metal.
+
+1. _Properties._ When heated to a high temperature in the air each of
+these metals combines with oxygen to form an oxide of the general
+formula MO, in which M represents the metal. Magnesium decomposes
+boiling water slowly, while zinc and cadmium have but little action on
+it.
+
+2. _Compounds._ The members of this group are divalent in nearly all
+their compounds, so that the formulas of their salts resemble those of
+the alkaline-earth metals. Like the alkaline-earth metals, their
+carbonates and phosphates are insoluble in water. Their sulphates,
+however, are readily soluble. Unlike both the alkali and alkaline-earth
+metals, their hydroxides are nearly insoluble in water. Most of their
+compounds dissociate in such a way as to give a simple, colorless,
+metallic ion.
+
+
+MAGNESIUM
+
+~Occurrence.~ Magnesium is a very abundant element in nature, ranking a
+little below calcium in this respect. Like calcium, it is a constituent
+of many rocks and also occurs in the form of soluble salts.
+
+~Preparation.~ The metal magnesium, like most metals whose oxides are
+difficult to reduce with carbon, was formerly prepared by heating the
+anhydrous chloride with sodium:
+
+    MgCl_{2} + 2Na = 2NaCl + Mg.
+
+It is now made by electrolysis, but instead of using as the electrolyte
+the melted anhydrous chloride, which is difficult to obtain, the natural
+mineral carnallite is used. This is melted in an iron pot which also
+serves as the cathode in the electrolysis. A rod of carbon dipping into
+the melted salt serves as the anode. The apparatus is very similar to
+the one employed in the preparation of sodium.
+
+~Properties.~ Magnesium is a rather tough silvery-white metal of small
+density. Air does not act rapidly upon it, but a thin film of oxide
+forms upon its surface, dimming its bright luster. The common acids
+dissolve it with the formation of the corresponding salts. It can be
+ignited readily and in burning liberates much heat and gives a brilliant
+white light. This light is very rich in the rays which affect
+photographic plates, and the metal in the form of fine powder is
+extensively used in the production of flash lights and for white lights
+in pyrotechnic displays.
+
+~Magnesium oxide~ (_magnesia_) (MgO). Magnesium oxide, sometimes called
+magnesia or magnesia usta, resembles lime in many respects. It is much
+more easily formed than lime and can be made in the same way,--by
+igniting the carbonate. It is a white powder, very soft and light, and
+is unchanged by heat even at very high temperatures. For this reason it
+is used in the manufacture of crucibles, for lining furnaces, and for
+other purposes where a refractory substance is needed. It combines with
+water to form magnesium hydroxide, but much more slowly and with the
+production of much less heat than in the case of calcium oxide.
+
+~Magnesium hydroxide~ (Mg(OH)_{2}). The hydroxide formed in this way is
+very slightly soluble in water, but enough dissolves to give the water
+an alkaline reaction. Magnesium hydroxide is therefore a fairly strong
+base. It is an amorphous white substance. Neither magnesia nor magnesium
+salts have a very marked effect upon the system; and for this reason
+magnesia is a very suitable antidote for poisoning by strong acids,
+since any excess introduced into the system will have no injurious
+effect.
+
+     ~Magnesium cement.~ A paste of magnesium hydroxide and water
+     slowly absorbs carbon dioxide from the air and becomes very
+     hard. The hardness of the product is increased by the presence
+     of a considerable amount of magnesium chloride in the paste.
+     The hydroxide, with or without the chloride, is used in the
+     preparation of cements for some purposes.
+
+~Magnesium carbonate~ (MgCO_{3}). Magnesium carbonate is a very abundant
+mineral. It occurs in a number of localities as magnesite, which is
+usually amorphous, but sometimes forms pure crystals resembling calcite.
+More commonly it is found associated with calcium carbonate. The
+mineral dolomite has the composition CaCO_{3}.MgCO_{3}. Limestone
+containing smaller amounts of magnesium carbonate is known as dolomitic
+limestone. Dolomite is one of the most common rocks, forming whole
+mountain masses. It is harder and less readily attacked by acids than
+limestone. It is valuable as a building stone and as ballast for
+roadbeds and foundations. Like calcium carbonate, magnesium carbonate is
+insoluble in water, though easily dissolved by acids.
+
+~Basic carbonate of magnesium.~ We should expect to find magnesium
+carbonate precipitated when a soluble magnesium salt and a soluble
+carbonate are brought together:
+
+    Na_{2}CO_{3} + MgCl_{2} = MgCO_{3} + 2NaCl.
+
+Instead of this, some carbon dioxide escapes and the product is found to
+be a basic carbonate. The most common basic carbonate of magnesium has
+the formula 4MgCO_{3}.Mg(OH)_{2}, and is sometimes called magnesia alba.
+This compound is formed by the partial hydrolysis of the normal
+carbonate at first precipitated:
+
+    5MgCO_{3} + 2H_{2}O = 4MgCO_{3}.Mg(OH)_{2} + H_{2}CO_{3}.
+
+~Magnesium chloride~ (MgCl_{2}.6H_{2}O). Magnesium chloride is found in
+many natural waters and in many salt deposits (see Stassfurt salts). It
+is obtained as a by-product in the manufacture of potassium chloride
+from carnallite. As there is no very important use for it, large
+quantities annually go to waste. When heated to drive off the water of
+crystallization the chloride is decomposed as shown in the equation
+
+    MgCl_{2}.6H_{2}O = MgO + 2HCl + 5H_{2}O.
+
+Owing to the abundance of magnesium chloride, this reaction is being
+used to some extent in the preparation of both magnesium oxide and
+hydrochloric acid.
+
+     ~Boiler scale.~ When water which contains certain salts in
+     solution is evaporated in steam boilers, a hard insoluble
+     material called _scale_ deposits in the boiler. The formation
+     of this scale may be due to several distinct causes.
+
+     1. _To the deposit of calcium sulphate._ This salt, while
+     sparingly soluble in cold water, is almost completely insoluble
+     in superheated water. Consequently it is precipitated when
+     water containing it is heated in a boiler.
+
+     2. _To decomposition of acid carbonates._ As we have seen,
+     calcium and magnesium acid carbonates are decomposed on
+     heating, forming insoluble normal carbonates:
+
+     Ca(HCO_{3})_{2} = CaCO_{3} + H_{2}O + CO_{2}.
+
+     3. _To hydrolysis of magnesium salts._ Magnesium chloride, and
+     to some extent magnesium sulphate, undergo hydrolysis when
+     superheated in solution, and the magnesium hydroxide, being
+     sparingly soluble, precipitates:
+
+     MgCl_{2} + 2H_{2}O <--> Mg(OH)_{2} + 2HCl.
+
+     This scale adheres tightly to the boiler in compact layers and,
+     being a non-conductor of heat, causes much waste of fuel. It is
+     very difficult to remove, owing to its hardness and resistance
+     to reagents. Thick scale sometimes cracks, and the water coming
+     in contact with the overheated iron occasions an explosion.
+     Moreover, the acids set free in the hydrolysis of the magnesium
+     salts attack the iron tubes and rapidly corrode them. These
+     causes combine to make the formation of scale a matter which
+     occasions much trouble in cases where hard water is used in
+     steam boilers. Water containing such salts should be softened,
+     therefore, before being used in boilers.
+
+~Magnesium sulphate~ (_Epsom salt_) (MgSO_{4}.7H_{2}O). Like the chloride,
+magnesium sulphate is found rather commonly in springs and in salt
+deposits. A very large deposit of the almost pure salt has been found in
+Wyoming. Its name was given to it because of its abundant occurrence in
+the waters of the Epsom springs in England.
+
+Magnesium sulphate has many uses in the industries. It is used to a
+small extent in the preparation of sodium and potassium sulphates, as a
+coating for cotton cloth, in the dye industry, in tanning, and in the
+manufacture of paints and laundry soaps. To some extent it is used in
+medicine.
+
+~Magnesium silicates.~ Many silicates containing magnesium are known and
+some of them are important substances. Serpentine, asbestos, talc, and
+meerschaum are examples of such substances.
+
+
+ZINC
+
+~Occurrence.~ Zinc never occurs free in nature. Its compounds have been
+found in many different countries, but it is not a constituent of common
+rocks and minerals, and its occurrence is rather local and confined to
+definite deposits or pockets. It occurs chiefly in the following ores:
+
+    Sphalerite (zinc blende) ZnS.
+    Zincite ZnO.
+    Smithsonite ZnCO_{3}.
+    Willemite Zn_{2}SiO_{4}.
+    Franklinite ZnO.Fe_{2}O_{3}.
+
+One fourth of the world's output of zinc comes from the United States,
+Missouri being the largest producer.
+
+~Metallurgy.~ The ores employed in the preparation of zinc are chiefly the
+sulphide, oxide, and carbonate. They are first roasted in the air, by
+which process they are changed into oxide:
+
+    ZnCO_{3} = ZnO + CO_{2},
+    ZnS + 3O = ZnO + SO_{2}.
+
+The oxide is then mixed with coal dust, and the mixture is heated in
+earthenware muffles or retorts, natural gas being used as fuel in many
+cases. The oxide is reduced by this means to the metallic state, and the
+zinc, being volatile at the high temperature reached, distills and is
+collected in suitable receivers. At first the zinc collects in the form
+of fine powder, called zinc dust or flowers of zinc, recalling the
+formation under similar conditions of flowers of sulphur. Later, when
+the whole apparatus has become warm, the zinc condenses to a liquid in
+the receiver, from which it is drawn off into molds. Commercial zinc
+often contains a number of impurities, especially carbon, arsenic, and
+iron.
+
+~Physical properties.~ Pure zinc is a rather heavy bluish-white metal with
+a high luster. It melts at about 420 deg., and if heated much above this
+temperature in the air takes fire and burns with a very bright bluish
+flame. It boils at about 950 deg. and can therefore be purified by
+distillation.
+
+Many of the physical properties of zinc are much influenced by the
+temperature and previous treatment of the metal. When cast into ingots
+from the liquid state it becomes at ordinary temperatures quite hard,
+brittle, and highly crystalline. At 150 deg. it is malleable and can be
+rolled into thin sheets; at higher temperatures it again becomes very
+brittle. When once rolled into sheets it retains its softness and
+malleability at ordinary temperatures. When melted and poured into water
+it forms thin brittle flakes, and in this condition is called granulated
+or mossy zinc.
+
+~Chemical properties.~ Zinc is tarnished superficially by moist air, but
+beyond this is not affected by it. It does not decompose even boiling
+water. When the metal is quite pure, sulphuric and hydrochloric acids
+have scarcely any action upon it; when, however, it contains small
+amounts of other metals such as magnesium or arsenic, or when it is
+merely in contact with metallic platinum, brisk action takes place and
+hydrogen is evolved. For this reason, when pure zinc is used in the
+preparation of hydrogen a few drops of platinum chloride are often added
+to the solution to assist the chemical action. Nitric acid dissolves the
+metal readily, with the formation of zinc nitrate and various reduction
+products of nitric acid. The strong alkalis act upon zinc and liberate
+hydrogen:
+
+    Zn + 2KOH = Zn(OK)_{2} + 2H.
+
+The product of this reaction, potassium zincate, is a salt of zinc
+hydroxide, which is thus seen to have acid properties, though it usually
+acts as a base.
+
+~Uses of zinc.~ The metal has many familiar uses. Rolled into sheets, it
+is used as a lining for vessels which are to contain water. As a thin
+film upon the surface of iron (galvanized iron) it protects the iron
+from rust. Iron is usually galvanized by dipping it into a bath of
+melted zinc, but electrical methods are also employed. Zinc plates are
+used in many forms of electrical batteries. In the laboratory zinc is
+used in the preparation of hydrogen, and in the form of zinc dust as a
+reducing agent.
+
+One of the largest uses of zinc is in the manufacture of alloys. Brass,
+an alloy of zinc and copper, is the most important of these; German
+silver, consisting of copper, zinc, and nickel, has many uses; various
+bronzes, coin metals, and bearing metals also contain zinc. Its ability
+to alloy with silver finds application in the separation of silver from
+lead (see silver).
+
+~Compounds of zinc.~ In general, the compounds of zinc are similar in
+formula and appearance to those of magnesium, but in other properties
+they often differ markedly. A number of them have value in commercial
+ways.
+
+~Zinc oxide~ (_zinc white_) (ZnO). Zinc oxide occurs in impure form in
+nature, being colored red by manganese and iron compounds. It can be
+prepared just like magnesium oxide, but is more often made by burning
+the metal.
+
+Zinc oxide is a pure white powder which becomes yellow on heating and
+regains its white color when cold. It is much used as a white pigment in
+paints, under the name of zinc white, and has the advantage over white
+lead in that it is not changed in color by sulphur compounds, while lead
+turns black. It is also used in the manufacture of rubber goods.
+
+     ~Commercial preparation of zinc oxide.~ Commercially it is often
+     made from franklinite in the following way. The franklinite is
+     mixed with coal and heated to a high temperature in a furnace,
+     by which process the zinc is set free and converted into vapor.
+     As the vapor leaves the furnace through a conduit it meets a
+     current of air and takes fire in it, forming zinc oxide. The
+     oxide passes on and is filtered from the air through canvas
+     bags, which allow the air to pass but retain the oxide. It is
+     thus made by burning the metal, though the metal is not
+     actually isolated in the process.
+
+~Soluble salts.~ The soluble salts of zinc can be made by dissolving the
+metal or the oxide in the appropriate acid. They are all somewhat
+poisonous. The sulphate and chloride are the most familiar.
+
+~Zinc sulphate~ (_white vitriol_) (ZnSO_{4}.7H_{2}O). This salt is readily
+crystallized from strong solutions in transparent colorless crystals. It
+is prepared commercially by careful roasting of the sulphide:
+
+    ZnS + 4O = ZnSO_{4}.
+
+~Zinc chloride~ (ZnCl_{2}.H_{2}O). When a solution of zinc chloride is
+slowly evaporated a salt of the composition ZnCl_{2}.H_{2}O crystallizes
+out. If the water is completely expelled by heat and the residue
+distilled, the anhydrous chloride is obtained and may be cast into
+sticks or broken into lumps. In this distillation, just as in heating
+magnesium chloride, some of the chloride is decomposed:
+
+    ZnCl_{2}.H_{2}O = ZnO + 2HCl.
+
+The anhydrous chloride has a great affinity for water, and is used as a
+dehydrating agent. It is also a germicide, and wood which is to be
+exposed to conditions which favor decay, as, for example, railroad ties,
+is often soaked in solutions of this salt.
+
+~Insoluble compounds.~ The insoluble compounds of zinc can be prepared by
+precipitation. The most important are the sulphide, carbonate, and
+hydroxide.
+
+~Zinc sulphide~ (ZnS). This substance occurs as the mineral sphalerite,
+and is one of the most valued ores of zinc. Very large deposits occur in
+southwestern Missouri. The natural mineral is found in large crystals or
+masses, resembling resin in color and luster. When prepared by
+precipitation the sulphide is white.
+
+
+CADMIUM
+
+~The element.~ This element occurs in small quantities in some zinc ores.
+In the course of the metallurgy of zinc the cadmium compounds undergo
+chemical changes quite similar to those of the zinc compounds, and the
+cadmium distills along with the zinc. Being more volatile, it comes over
+with the first of the zinc and is prepared from the first portions of
+the distillate by special methods of purification. The element very
+closely resembles zinc in most respects. Some of its alloys are
+characterized by having low melting points.
+
+~Compounds of cadmium.~ Among the compounds of cadmium may be mentioned
+the chloride (CdCl_{2}.2H_{2}O), the sulphate (3CdSO_{4}.8H_{2}O), and
+the nitrate (Cd(NO_{3})_{2}.4H_{2}O). These are white solids soluble in
+water. The sulphide (CdS) is a bright yellow substance which is
+insoluble in water and in dilute acids. It is valuable as a pigment in
+fine paints.
+
+
+EXERCISES
+
+1. What properties have the metals of the magnesium family in common
+with the alkali metals; with the alkaline-earth metals?
+
+2. Compare the action of the metals of the magnesium group on water with
+that of the other metals studied.
+
+3. What metals already studied are prepared by electrolysis?
+
+4. Write the equations representing the reactions between magnesium and
+hydrochloric acid; between magnesium and dilute sulphuric acid.
+
+5. What property of magnesium was taken advantage of in the isolation of
+argon?
+
+6. With phosphoric acid magnesium forms salts similar to those of
+calcium. Write the names and formulas of the corresponding magnesium
+salts.
+
+7. How could you distinguish between magnesium chloride and magnesium
+sulphate? between Glauber's salts and Epsom salts?
+
+8. What weight of carnallite is necessary in the preparation of 500 g.
+of magnesium?
+
+9. Account for the fact that paints made of zinc oxide are not colored
+by hydrosulphuric acid.
+
+10. What hydroxide studied, other than zinc hydroxide, has both acid and
+basic properties?
+
+11. Write equations showing how the following compounds of zinc may be
+obtained from metallic zinc: the oxide, chloride, nitrate, carbonate,
+sulphate, sulphide, hydroxide.
+
+
+
+
+CHAPTER XXVI
+
+THE ALUMINIUM FAMILY
+
+
+~The family.~ The element aluminium is the most abundant member of the
+group of elements known as the aluminium family; indeed, the other
+members of the family--gallium, indium, and thallium--are of such rare
+occurrence that they need not be separately described. The elements of
+the family are ordinarily trivalent, so that the formulas for their
+compounds differ from those of the elements so far studied. Their
+hydroxides are practically insoluble in water and are very weak bases;
+indeed, the bases are so weak that their salts are often hydrolyzed into
+free base and free acid in solution. The salts formed from these bases
+usually contain water of crystallization, which cannot be driven off
+without decomposing them more or less.
+
+The trivalent metals, which in addition to aluminium include also iron
+and chromium, are sometimes called the _earth metals_. The name refers
+to the earthy appearance of the oxides of these metals, and to the fact
+that many earths, soils, and rocks are composed in part of these
+substances.
+
+
+ALUMINIUM
+
+~Occurrence.~ Aluminium never occurs in the free state in nature, owing to
+its great affinity for oxygen. In combined form, as oxides, silicates,
+and a few other salts, it is both abundant and widely distributed, being
+an essential constituent of all soils and of most rocks excepting
+limestone and sandstone. Cryolite (Na_{3}AlF_{6}), found in Greenland,
+and bauxite, which is an aluminium hydroxide usually mixed with some
+iron hydroxide, are important minerals. It is estimated that aluminium
+composes about 8% of the earth's crust. In the industries the metal is
+called aluminum, but its chemical name is aluminium.
+
+[Illustration: Fig. 82]
+
+~Preparation.~ Aluminium was first prepared by Woehler, in 1827, by heating
+anhydrous aluminium chloride with potassium:
+
+    AlCl_{3} + 3K = 3KCl + Al.
+
+This method was tried after it was found impossible to reduce the oxide
+of aluminium with carbon. The metal possessed such interesting
+properties and promised to be so useful that many efforts were made to
+devise a cheap way of preparing it. The method which has proved most
+successful consists in the electrolysis of the oxide dissolved in melted
+cryolite.
+
+     ~Metallurgy.~ An iron box A (Fig. 82) about eight feet long and
+     six feet wide is connected with a powerful generator in such a
+     way as to serve as the cathode upon which the aluminium is
+     deposited. Three or four rows of carbon rods B dip into the
+     box and serve as the anodes. The box is partially filled with
+     cryolite and the current is turned on, generating enough heat
+     to melt the cryolite. Aluminium oxide is then added, and under
+     the influence of the electric current it decomposes into
+     aluminium and oxygen. The temperature is maintained above the
+     melting point of aluminium, and the liquid metal, being heavier
+     than cryolite, sinks to the bottom of the vessel, from which it
+     is tapped off from time to time through the tap hole C. The
+     oxygen in part escapes as gas, and in part combines with the
+     carbon of the anode, the combustion being very brilliant. The
+     process is carried on at Niagara Falls.
+
+     The largest expense in the process, apart from the cost of
+     electrical energy, is the preparation of aluminium oxide free
+     from other oxides, for most of the oxide found in nature is too
+     impure to serve without refining. Bauxite is the principal ore
+     used as a source of the aluminium because it is converted into
+     pure oxide without great difficulty. Since common clay is a
+     silicate of aluminium and is everywhere abundant, it might be
+     expected that this would be utilized in the preparation of
+     aluminium. It is, however, very difficult to extract the
+     aluminium from a silicate, and no practical method has been
+     found which will accomplish this.
+
+~Physical properties.~ Aluminium is a tin-white metal which melts at 640 deg.
+and is very light, having a density of 2.68. It is stiff and strong, and
+with frequent annealing can be rolled into thin foil. It is a good
+conductor of heat and electricity, though not so good as copper for a
+given cross section of wire.
+
+~Chemical properties.~ Aluminium is not perceptibly acted on by boiling
+water, and moist air merely dims its luster. Further action is prevented
+in each case by the formation of an extremely thin film of oxide upon
+the surface of the metal. It combines directly with chlorine, and when
+heated in oxygen burns with great energy and the liberation of much
+heat. It is therefore a good reducing agent. Hydrochloric acid acts upon
+it, forming aluminium chloride: nitric acid and dilute sulphuric acid
+have almost no action on it, but hot, concentrated sulphuric acid acts
+upon it in the same way as upon copper:
+
+    2Al + 6H_{2}SO_{4} = Al_{2}(SO_{4})_{3} + 6H_{2}O + 3SO_{2}.
+
+Alkalis readily attack the metal, liberating hydrogen, as in the case of
+zinc:
+
+    Al + 3KOH = Al(OK)_{3} + 3H.
+
+Salt solutions, such as sea water, corrode the metal rapidly. It alloys
+readily with other metals.
+
+~Uses of aluminium.~ These properties suggest many uses for the metal. Its
+lightness, strength, and permanence make it well adapted for many
+construction purposes. These same properties have led to its extensive
+use in the manufacture of cooking utensils. The fact that it is easily
+corroded by salt solutions is, however, a disadvantage. Owing to its
+small resistance to electrical currents, it is replacing copper to some
+extent in electrical construction, especially for trolley and power
+wires. Some of its alloys have very valuable properties, and a
+considerable part of the aluminium manufactured is used for this
+purpose. Aluminium bronze, consisting of about 90% copper and 10%
+aluminium, has a pure golden color, is strong and malleable, is easily
+cast, and is permanent in the air. Considerable amounts of aluminium
+steel are also made.
+
+~Goldschmidt reduction process.~ Aluminium is frequently employed as a
+powerful reducing agent, many metallic oxides which resist reduction by
+carbon being readily reduced by it. The aluminium in the form of a fine
+powder is mixed with the metallic oxide, together with some substance
+such as fluorspar to act as a flux. The mixture is ignited, and the
+aluminium unites with the oxygen of the metallic oxide, liberating the
+metal. This collects in a fused condition under the flux.
+
+An enormous quantity of heat is liberated in this reaction, and a
+temperature as high as 3500 deg. can be reached. The heat of the reaction is
+turned to practical account in welding car rails, steel castings, and in
+similar operations where an intense local heat is required. A mixture of
+aluminium with various metallic oxides, ready prepared for such
+purposes, is sold under the name of _thermite_.
+
+[Illustration: Fig. 83]
+
+     ~Preparation of chromium by the Goldschmidt method.~ A mixture of
+     chromium oxide and aluminium powder is placed in a Hessian
+     crucible (A, Fig. 83), and on top of it is placed a small
+     heap B of a mixture of sodium peroxide and aluminium, into
+     which is stuck a piece of magnesium ribbon C. Powdered
+     fluorspar D is placed around the sodium peroxide, after which
+     the crucible is set on a pan of sand and the magnesium ribbon
+     ignited. When the flame reaches the sodium peroxide mixture
+     combustion of the aluminium begins with almost explosive
+     violence, so that great care must be taken in the experiment.
+     The heat of this combustion starts the reaction in the chromium
+     oxide mixture, and the oxide is reduced to metallic chromium.
+     When the crucible has cooled a button of chromium will be found
+     in the bottom.
+
+~Aluminium oxide~ (Al_{2}O_{3}). This substance occurs in several forms in
+nature. The relatively pure crystals are called corundum, while emery is
+a variety colored dark gray or black, usually with iron compounds. In
+transparent crystals, tinted different colors by traces of impurities,
+it forms such precious stones as the sapphire, oriental ruby, topaz, and
+amethyst. All these varieties are very hard, falling little short of
+the diamond in this respect. Chemically pure aluminium oxide can be made
+by igniting the hydroxide, when it forms an amorphous white powder:
+
+    2Al(OH)_{3} = Al_{2}O_{3} + 3H_{2}O.
+
+The natural varieties, corundum and emery, are used for cutting and
+grinding purposes; the purest forms, together with the artificially
+prepared oxide, are largely used in the preparation of aluminium.
+
+~Aluminium hydroxide~ (Al(OH)_{3}). The hydroxide occurs in nature as the
+mineral hydrargyllite, and in a partially dehydrated form called
+bauxite. It can be prepared by adding ammonium hydroxide to any soluble
+aluminium salt, forming a semi-transparent precipitate which is
+insoluble in water but very hard to filter. It dissolves in most acids
+to form soluble salts, and in the strong bases to form aluminates, as
+indicated in the equations
+
+    Al(OH)_{3} + 3HCl = AlCl_{3} + 3H_{2}O,
+    Al(OH)_{3} + 3NaOH = Al(ONa)_{3} + 3H_{2}O.
+
+It may act, therefore, either as a weak base or as a weak acid, its
+action depending upon the character of the substances with which it is
+in contact. When heated gently the hydroxide loses part of its hydrogen
+and oxygen according to the equation
+
+    Al(OH)_{3} = AlO.OH + H_{2}O.
+
+This substance, the formula of which is frequently written HAlO_{2}, is
+a more pronounced acid than is the hydroxide, and its salts are
+frequently formed when aluminium compounds are fused with alkalis. The
+magnesium salt Mg(AlO_{2})_{2} is called spinel, and many other of its
+salts, called aluminates, are found in nature.
+
+When heated strongly the hydroxide is changed into oxide, which will not
+again take up water on being moistened.
+
+     ~Mordants and dyeing.~ Aluminium hydroxide has the peculiar
+     property of combining with many soluble coloring materials and
+     forming insoluble products with them. On this account it is
+     often used as a filter to remove objectionable colors from
+     water. This property also leads to its wide use in the dye
+     industry. Many dyes will not adhere to natural fibers such as
+     cotton and wool, that is, will not "dye fast." If, however, the
+     cloth to be dyed is soaked in a solution of aluminium compounds
+     and then treated with ammonia, the aluminium salts which have
+     soaked into the fiber will be converted into the hydroxide,
+     which, being insoluble, remains in the body of it. If the fiber
+     is now dipped into a solution of the dye, the aluminium
+     hydroxide combines with the color material and fastens, or
+     "fixes," it upon the fiber. A substance which serves this
+     purpose is called a _mordant_, and aluminium salts,
+     particularly the acetate, are used in this way.
+
+~Aluminium chloride~ (AlCl_{3}.6 H_{2}O). This substance is prepared by
+dissolving the hydroxide in hydrochloric acid and evaporating to
+crystallization. When heated it is converted into the oxide, resembling
+magnesium in this respect:
+
+    2(AlCl_{3}.6 H_{2}O) = Al_{2}O_{3} + 6HCl + 9H_{2}O.
+
+The anhydrous chloride, which has some important uses, is made by
+heating aluminium turnings in a current of chlorine.
+
+~Alums.~ Aluminium sulphate can be prepared by the action of sulphuric
+acid upon aluminium hydroxide. It has the property of combining with the
+sulphates of the alkali metals to form compounds called _alums_. Thus,
+with potassium sulphate the reaction is expressed by the equation
+
+    K_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O
+       = 2(KAl(SO_{4})_{2}.12H_{2}O).
+
+Under similar conditions ammonium sulphate yields ammonium alum:
+
+    (NH_{4})_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O
+        = 2(NH_{4}Al(SO_{4})_{2}.12H_{2}O).
+
+Other trivalent sulphates besides aluminium sulphate can form similar
+compounds with the alkali sulphates, and these compounds are also called
+alums, though they contain no aluminium. They all crystallize in
+octahedra and contain twelve molecules of water of crystallization. The
+alums most frequently prepared are the following:
+
+    Potassium alum               KAl(SO_{4})_{2}.12H_{2}O.
+    Ammonium alum                NH_{4}Al(SO_{4})_{2}.12H_{2}O.
+    Ammonium iron alum           NH_{4}Fe(SO_{4})_{2}.12H_{2}O.
+    Potassium chrome alum        KCr(SO_{4})_{2}.12H_{2}O.
+
+An alum may therefore be regarded as a compound derived from two
+molecules of sulphuric acid, in which one hydrogen atom has been
+displaced by the univalent alkali atom, and the other three hydrogen
+atoms by an atom of one of the trivalent metals, such as aluminium,
+iron, or chromium.
+
+     Very large, well-formed crystals of an alum can be prepared by
+     suspending a small crystal by a thread in a saturated solution
+     of the alum, as shown in Fig. 84. The small crystal slowly
+     grows and assumes a very perfect form.
+
+[Illustration: Fig. 84]
+
+~Other salts of aluminium.~ While aluminium hydroxide forms fairly stable
+salts with strong acids, it is such a weak base that its salts with weak
+acids are readily hydrolyzed. Thus, when an aluminium salt and a soluble
+carbonate are brought together in solution we should expect to have
+aluminium carbonate precipitated according to the equation
+
+    3Na_{2}CO_{3} + 2AlCl_{3} = Al_{2}(CO_{3})_{3} + 6NaCl.
+
+But if it is formed at all, it instantly begins to hydrolyze, the
+products of the hydrolysis being aluminium hydroxide and carbonic acid,
+
+    Al_{2}(CO_{3})_{3} + 6H_{2}O = 2Al(OH)_{3} + 3H_{2}CO_{3}.
+
+Similarly a soluble sulphide, instead of precipitating aluminium
+sulphide (Al_{2}S_{3}), precipitates aluminium hydroxide; for hydrogen
+sulphide is such a weak acid that the aluminium sulphide at first formed
+hydrolyzes at once, forming aluminium hydroxide and hydrogen sulphide:
+
+    3Na_{2}S + 2AlCl_{3} + 6H_{2}O = 2Al(OH)_{3} + 6NaCl + 3H_{2}S.
+
+~Alum baking powders.~ It is because of the hydrolysis of aluminium
+carbonate that alum is used as a constituent of some baking powders. The
+alum baking powders consist of a mixture of alum and sodium hydrogen
+carbonate. When water is added the two compounds react together, forming
+aluminium carbonate, which hydrolyzes into aluminium hydroxide and
+carbonic acid. The carbon dioxide from the latter escapes through the
+dough and in so doing raises it into a porous condition, which is the
+end sought in the use of a baking powder.
+
+~Aluminium silicates.~ One of the most common constituents of rocks is
+feldspar (KAlSi_{3}O_{8}), a mixed salt of potassium and aluminium with
+the polysilicic acid (H_{4}Si_{3}O_{8}). Under the influence of
+moisture, carbon dioxide, and changes of temperature this substance is
+constantly being broken down into soluble potassium compounds and
+hydrated aluminium silicate. This compound has the formula
+Al_{2}Si_{2}O_{7}.2H_{2}O. In relatively pure condition it is called
+kaolin; in the impure state, mixed with sand and other substances, it
+forms common clay. Mica is another very abundant mineral, having varying
+composition, but being essentially of the formula KAlSiO_{4}.
+Serpentine, talc, asbestos, and meerschaum are important complex
+silicates of aluminium and magnesium, and granite is a mechanical
+mixture of quartz, feldspar, and mica.
+
+     ~Ceramic industries.~ Many articles of greatest practical
+     importance, ranging from the roughest brick and tile to the
+     finest porcelain and chinaware, are made from some form of
+     kaolin, or clay. No very precise classification of such ware
+     can be made, as the products vary greatly in properties,
+     depending upon the materials used and the treatment during
+     manufacture.
+
+     Porcelain is made from the purest kaolin, to which must be
+     added some less pure, plastic kaolin, since the pure substance
+     is not sufficiently plastic. There is also added some more
+     fusible substance, such as feldspar, gypsum, or lime, together
+     with some pure quartz. The constituents must be ground very
+     fine, and when thoroughly mixed and moistened must make a
+     plastic mass which can be molded into any desired form. The
+     article molded from such materials is then burned. In this
+     process the article is slowly heated to a point at which it
+     begins to soften and almost fuse, and then it is allowed to
+     cool slowly. At this stage, a very thin vessel will be
+     translucent and have an almost glassy fracture; if, however, it
+     is somewhat thicker, or has not been heated quite so high, it
+     will still be porous, and partly on this account and partly to
+     improve its appearance it is usually glazed.
+
+     Glazing is accomplished by spreading upon the object a thin
+     layer of a more fusible mixture of the same materials as
+     compose the body of the object itself, and again heating until
+     the glaze melts to a transparent glassy coating upon the
+     surface of the vessel. In some cases fusible mixtures of quite
+     different composition from that used in fashioning the vessel
+     may be used as a glaze. Oxides of lead, zinc, and barium are
+     often used in this way.
+
+     When less carefully selected materials are used, or quite thick
+     vessels are made, various grades of stoneware are produced. The
+     inferior grades are glazed by throwing a quantity of common
+     salt into the kiln towards the end of the first firing. In the
+     form of vapor the salt attacks the surface of the baked ware
+     and forms an easily fusible sodium silicate upon it, which
+     constitutes a glaze.
+
+     Vitrified bricks, made from clay or ground shale, are burned
+     until the materials begin to fuse superficially, forming their
+     own glaze. Other forms of brick and tile are not glazed at all,
+     but are left porous. The red color of ordinary brick and
+     earthenware is due to an oxide of iron formed in the burning
+     process.
+
+     The decorations upon china are sometimes painted upon the baked
+     ware and then glazed over, and sometimes painted upon the glaze
+     and burned in by a third firing. Care must be taken to use such
+     pigments as are not affected by a high heat and do not react
+     chemically with the constituents of the baked ware or the
+     glaze.
+
+
+EXERCISES
+
+1. What metals and compounds studied are prepared by electrolysis?
+
+2. Write the equation for the reaction between aluminium and
+hydrochloric acid; between aluminium and sulphuric acid (in two steps).
+
+3. What hydroxides other than aluminium hydroxide have both acid and
+basic properties?
+
+4. Write equations showing the methods used for preparing aluminium
+hydroxide and sulphate.
+
+5. Write the general formula of an alum, representing an atom of an
+alkali metal by X and an atom of a trivalent metal by Y.
+
+6. What is meant by the term polysilicic acid, as used in the discussion
+of aluminium silicates?
+
+7. Compare the properties of the hydroxides of the different groups of
+metals so far studied.
+
+8. In what respects does aluminium oxide differ from calcium oxide in
+properties?
+
+9. Supposing bauxite to be 90% aluminium hydroxide, what weight of it is
+necessary for the preparation of 100 kg. of aluminium?
+
+
+
+
+CHAPTER XXVII
+
+THE IRON FAMILY
+
+
+===================================================================
+        |        |        |         |             |
+        |        |        |         | APPROXIMATE |
+        | SYMBOL | ATOMIC | DENSITY |   MELTING   | OXIDES
+        |        | WEIGHT |         |    POINT    |
+________|________|________|_________|_____________|________________
+        |        |        |         |             |
+Iron    |   Fe   |  55.9  |   7.93  |    1800 deg.    | FeO, Fe_{2}O_{3}
+Cobalt  |   Co   |  59.0  |   8.55  |    1800 deg.    | CoO, Co_{2}O_{3}
+Nickel  |   Ni   |  58.7  |   8.9   |    1600 deg.    | NiO, Ni_{2}O_{3}
+===================================================================
+
+~The family.~ The elements iron, cobalt, and nickel form a group in the
+eighth column of the periodic table. The atomic weights of the three are
+very close together, and there is not the same gradual gradation in the
+properties of the three elements that is noticed in the families in
+which the atomic weights differ considerably in magnitude. The elements
+are very similar in properties, the similarity being so great in the
+case of nickel and cobalt that it is difficult to separate them by
+chemical analysis.
+
+The elements occur in nature chiefly as oxides and sulphides, though
+they have been found in very small quantities in the native state,
+usually in meteorites. Their sulphides, carbonates, and phosphates are
+insoluble in water, the other common salts being soluble. Their salts
+are usually highly colored, those of iron being yellow or light green as
+a rule, those of nickel darker green, while cobalt salts are usually
+rose colored. The metals are obtained by reducing the oxides with
+carbon.
+
+
+IRON
+
+~Occurrence.~ The element iron has long been known, since its ores are
+very abundant and it is not difficult to prepare the metal from them in
+fairly pure condition. It occurs in nature in many forms of
+combination,--in large deposits as oxides, sulphides, and carbonates,
+and in smaller quantities in a great variety of minerals. Indeed, very
+few rocks or soils are free from small amounts of iron, and it is
+assimilated by plants and animals playing an important part in life
+processes.
+
+~Metallurgy.~ It will be convenient to treat of the metallurgy of iron
+under two heads,--Materials Used and Process.
+
+~Materials used.~ Four distinct materials are used in the metallurgy of
+iron:
+
+     1. _Iron ore._ The ores most frequently used in the metallurgy
+     of iron are the following:
+
+         Hematite      Fe_{2}O_{3}.
+         Magnetite     Fe_{3}O_{4}.
+         Siderite      FeCO_{3}.
+         Limonite      2Fe_{2}O_{2}.3H_{2}O.
+
+     These ores always contain impurities, such as silica,
+     sulphides, and earthy materials. All ores, with the exception
+     of the oxides, are first roasted to expel any water and carbon
+     dioxide present and to convert any sulphide into oxide.
+
+     2. _Carbon._ Carbon in some form is necessary both as a fuel
+     and as a reducing agent. In former times wood charcoal was used
+     to supply the carbon, but now anthracite coal or coke is almost
+     universally used.
+
+     3. _Hot air._ To maintain the high temperature required for the
+     reduction of iron a very active combustion of fuel is
+     necessary. This is secured by forcing a strong blast of hot air
+     into the lower part of the furnace during the reduction
+     process.
+
+     4. _Flux._ (a) _Purpose of the flux._ All the materials which
+     enter the furnace must leave it again either in the form of
+     gases or as liquids. The iron is drawn off as the liquid metal
+     after its reduction. To secure the removal of the earthy matter
+     charged into the furnace along with the ore, materials are
+     added to the charge which will, at the high temperature of the
+     furnace, combine with the impurities in the ore, forming a
+     liquid. The material added for this purpose is called the
+     _flux_; the liquid produced from the flux and the ore is called
+     _slag_.
+
+     (b) _Function of the slag._ While the main purpose of adding
+     flux to the charge is to remove from the furnace in the form of
+     liquid slag the impurities originally present in the ore, the
+     slag thus produced serves several other functions. It keeps the
+     contents of the furnace in a state of fusion, thus preventing
+     clogging, and makes it possible for the small globules of iron
+     to run together with greater ease into one large liquid mass.
+
+     (c) _Character of the slag._ The slag is really a kind of
+     readily fusible glass, being essentially a calcium-aluminium
+     silicate. The ore usually contains silica and some aluminium
+     compounds, so that limestone (which also contains some silica
+     and aluminium) is added to furnish the calcium required for the
+     slag. If the ore and the limestone do not contain a sufficient
+     amount of silica and aluminium for the formation of the slag,
+     these ingredients are added in the form of sand and feldspar.
+     In the formation of slag from these materials the ore is freed
+     from the silica and aluminium which it contained.
+
+[Illustration: Fig. 85]
+
+~Process.~ The reduction of iron is carried out in large towers called
+blast furnaces. The blast furnace (Fig. 85) is usually about 80 ft. high
+and 20 ft. in internal diameter at its widest part, narrowing somewhat
+both toward the top and toward the bottom. The walls are built of steel
+and lined with fire-brick. The base is provided with a number of pipes
+T, called tuyers, through which hot air can be forced into the
+furnace. The tuyers are supplied from a large pipe S, which circles
+the furnace as a girdle. The base has also an opening M, through which
+the liquid metal can be drawn off from time to time, and a second
+opening P, somewhat above the first, through which the excess of slag
+overflows. The top is closed by a movable trap C and C, called the
+cone, and through this the materials to be used are introduced. The
+gases produced by the combustion of the fuel and the reduction of the
+ore, together with the nitrogen of the air forced in through the tuyers,
+escape through pipes D, called downcomer pipes, which leave the
+furnace near the top. These gases are very hot and contain combustible
+substances, principally carbon monoxide; they are therefore utilized as
+fuel for the engines and also to heat the blast admitted through the
+tuyers. The lower part of the furnace is often furnished with a water
+jacket. This consists of a series of pipes W built into the walls,
+through which water can be circulated to reduce their temperature.
+
+Charges consisting of coke (or anthracite coal), ore, and flux in proper
+proportions are introduced into the furnace at intervals through the
+trap top. The coke burns fiercely in the hot-air blast, giving an
+intense heat and forming carbon monoxide. The ore, working down in the
+furnace as the coke burns, becomes very hot, and by the combined
+reducing action of the carbon and carbon monoxide is finally reduced to
+metal and collects as a liquid in the bottom of the furnace, the slag
+floating on the molten iron. After a considerable amount of the iron has
+collected the slag is drawn off through the opening P. The molten iron
+is then drawn off into large ladles and taken to the converters for the
+manufacture of steel, or it is run out into sand molds, forming the bars
+or ingots called "pigs." The process is a continuous one, and when once
+started it is kept in operation for months or even years without
+interruption.
+
+     It seems probable that the first product of combustion of the
+     carbon, at the point where the tuyers enter the furnace, is
+     carbon dioxide. This is at once reduced to carbon monoxide by
+     the intensely heated carbon present, so that no carbon dioxide
+     can be found at that point. For practical purposes, therefore,
+     we may consider that carbon monoxide is the first product of
+     combustion.
+
+~Varieties of iron.~ The iron of commerce is never pure, but contains
+varying amounts of other elements, such as carbon, silicon, phosphorus,
+sulphur, and manganese. These elements may either be alloyed with the
+iron or may be combined with it in the form of definite chemical
+compounds. In some instances, as in the case of graphite, the mixture
+may be merely mechanical.
+
+The properties of iron are very much modified by the presence of these
+elements and by the form of the combination between them and the iron;
+the way in which the metal is treated during its preparation has also a
+marked influence on its properties. Owing to these facts many kinds of
+iron are recognized in commerce, the chief varieties being cast iron,
+wrought iron, and steel.
+
+~Cast iron.~ The product of the blast furnace, prepared as just described,
+is called cast iron. It varies considerably in composition, usually
+containing from 90 to 95% iron, the remainder being largely carbon and
+silicon with smaller amounts of phosphorus and sulphur. When the melted
+metal from the blast furnace is allowed to cool rapidly most of the
+carbon remains in chemical combination with the iron, and the product is
+called white cast iron. If the cooling goes on slowly, the carbon
+partially separates as flakes of graphite which remain scattered through
+the metal. This product is softer and darker in color and is called gray
+cast iron.
+
+~Properties of cast iron.~ Cast iron is hard, brittle, and rather easily
+melted (melting point about 1100 deg.). It cannot be welded or forged into
+shape, but is easily cast in sand molds. It is strong and rigid but not
+elastic. It is used for making castings and in the manufacture of other
+kinds of iron. Cast iron, which contains the metal manganese up to the
+extent of 20%, together with about 3% carbon, is called spiegel iron;
+when more than this amount of manganese is present the product is called
+ferromanganese. The ferromanganese may contain as much as 80% manganese.
+These varieties of cast iron are much used in the manufacture of steel.
+
+~Wrought iron.~ Wrought iron is made by burning out from cast iron most of
+the carbon, silicon, phosphorus, and sulphur which it contains. The
+process is called _puddling_, and is carried out in a furnace
+constructed as represented in Fig. 86. The floor of the furnace F is
+somewhat concave and is made of iron covered with a layer of iron oxide.
+A long flame produced by burning fuel upon the grate G is directed
+downward upon the materials placed upon the floor, and the draught is
+maintained by the stack S. A is the ash box and T a trap to catch
+the solid particles carried into the stack by the draught. Upon the
+floor of the furnace is placed the charge of cast iron, together with a
+small amount of material to make a slag. The iron is soon melted by the
+flame directed upon it, and the sulphur, phosphorus, and silicon are
+oxidized by the iron oxide, forming oxides which are anhydrides of
+acids. These combine with the flux, which is basic in character, or with
+the iron oxide, to form a slag. The carbon is also oxidized and escapes
+as carbon dioxide. As the iron is freed from other elements it becomes
+pasty, owing to the higher melting point of the purer iron, and in this
+condition forms small lumps which are raked together into a larger one.
+The large lump is then removed from the furnace and rolled or hammered
+into bars, the slag; being squeezed out in this process. The product has
+a stranded or fibrous structure. _The product of a puddling furnace is
+called wrought iron._
+
+[Illustration: Fig. 86]
+
+~Properties of wrought iron.~ Wrought iron is nearly pure iron, usually
+containing about 0.3% of other substances, chiefly carbon. It is tough,
+malleable, and fibrous in structure. It is easily bent and is not
+elastic, so it will not sustain pressure as well as cast iron. It can be
+drawn out into wire of great tensile strength, and can also be rolled
+into thin sheets (sheet iron). It melts at a high temperature (about
+1600 deg.) and is therefore forged into shape rather than cast. If melted,
+it would lose its fibrous structure and be changed into a low carbon
+steel.
+
+~Steel.~ Steel, like wrought iron, is made by burning out from cast iron a
+part of the carbon, silicon, phosphorus, and sulphur which it contains;
+but the process is carried out in a very different way, and usually,
+though not always, more carbon is found in steel than in wrought iron. A
+number of processes are in use, but nearly all the steel of commerce is
+made by one of the two following methods.
+
+[Illustration: Fig. 87]
+
+1. _Bessemer process._ This process, invented about 1860, is by far the
+most important. It is carried out in great egg-shaped crucibles called
+converters (Fig. 87), each one of which will hold as much as 15 tons of
+steel. The converter is built of steel and lined with silica. It is
+mounted on trunnions T, so that it can be tipped over on its side for
+filling and emptying. One of the trunnions is hollow and a pipe P
+connects it with an air chamber A, which forms a false bottom to the
+converter. The true bottom is perforated, so that air can be forced in
+by an air blast admitted through the trunnion and the air chamber.
+
+White-hot, liquid cast iron from a blast furnace is run into the
+converter through its open necklike top O, the converter being tipped
+over to receive it; the air blast is then turned on and the converter
+rotated to a nearly vertical position. The elements in the iron are
+rapidly oxidized, the silicon first and then the carbon. The heat
+liberated in the oxidation, largely due to the combustion of silicon,
+keeps the iron in a molten condition. When the carbon is practically all
+burned out cast iron or spiegel iron, containing a known percentage of
+carbon, is added and allowed to mix thoroughly with the fluid. The steel
+is then run into molds, and the ingots so formed are hammered or rolled
+into rails or other forms. By this process any desired percentage of
+carbon can be added to the steel. Low carbon steel, which does not
+differ much from wrought iron in composition, is now made in this way
+and is replacing the more expensive wrought iron for many purposes.
+
+     ~The basic lining process.~ When the cast iron contains
+     phosphorus and sulphur in appreciable quantities, the lining of
+     the converter is made of dolomite. The silicon and carbon burn,
+     followed by the phosphorus and sulphur, and the anhydrides of
+     acids so formed combine with the basic oxides of the lining,
+     forming a slag. This is known as the basic lining process.
+
+2. _Open-hearth process._ In this process a furnace very similar to a
+puddling furnace is used, but it is lined with silica or dolomite
+instead of iron oxide. A charge consisting in part of old scrap iron of
+any kind and in part of cast iron is melted in the furnace by a gas
+flame. The silicon and carbon are slowly burned away, and when a test
+shows that the desired percentage of carbon is present the steel is run
+out of the furnace. _Steel may therefore be defined as the product of
+the Bessemer or open-hearth processes._
+
+~Properties of steel.~ Bessemer and open-hearth steel usually contain only
+a few tenths of a per cent of carbon, less than 0.1% silicon, and a very
+much smaller quantity of phosphorus and sulphur. Any considerable amount
+of the latter elements makes the steel brittle, the sulphur affecting it
+when hot, and the phosphorus when cold. This kind of steel is used for
+structural purposes, for rails, and for nearly all large steel articles.
+It is hard, malleable, ductile, and melts at a lower temperature than
+wrought iron. It can be forged into shape, rolled into sheets, or cast
+in molds.
+
+~Relation of the three varieties of iron.~ It will be seen that wrought
+iron is usually very nearly pure iron, while steel contains an
+appreciable amount of alloy material, chiefly carbon, and cast iron
+still more of the same substances. It is impossible, however, to assign
+a given sample of iron to one of these three classes on the basis of its
+chemical composition alone. A low carbon steel, for example, may contain
+less carbon than a given sample of wrought iron. The real distinction
+between the three is the process by which they are made. The product of
+the blast furnace is cast iron; that of the puddling furnace is wrought
+iron; that of the Bessemer and open-hearth methods is steel.
+
+~Tool steel.~ Steel designed for use in the manufacture of edged tools and
+similar articles should be relatively free from silicon and phosphorus,
+but should contain from 0.5 to 1.5% carbon. The percentage of carbon
+should be regulated by the exact use to which the steel is to be put.
+Steel of this character is usually made in small lots from either
+Bessemer or open-hearth steel in the following way.
+
+A charge of melted steel is placed in a large crucible and the
+calculated quantity of pure carbon is added. The carbon dissolves in the
+steel, and when the solution is complete the metal is poured out of the
+crucible. This is sometimes called crucible steel.
+
+~Tempering of steel.~ Steel containing from 0.5 to 1.5% carbon is
+characterized by the property of "taking temper." When the hot steel is
+suddenly cooled by plunging it into water or oil it becomes very hard
+and brittle. On carefully reheating this hard form it gradually becomes
+less brittle and softer, so that by regulating the temperature to which
+steel is reheated in tempering almost any condition of temper demanded
+for a given purpose, such as for making springs or cutting tools, can be
+obtained.
+
+~Steel alloys.~ It has been found that small quantities of a number of
+different elements when alloyed with steel very much improve its quality
+for certain purposes, each element having a somewhat different effect.
+Among the elements most used in this connection are manganese, silicon,
+chromium, nickel, tungsten, and molybdenum.
+
+The usual method for adding these elements to the steel is to first
+prepare a very rich alloy of iron with the element to be added, and then
+add enough of this alloy to a large quantity of the steel to bring it to
+the desired composition. A rich alloy of iron with manganese or silicon
+can be prepared directly in a blast furnace, and is called
+ferromanganese or ferrosilicon. Similar alloys of iron with the other
+elements mentioned are made in an electric furnace by reducing the mixed
+oxides with carbon.
+
+~Pure iron.~ Perfectly pure iron is rarely prepared and is not adapted to
+commercial uses. It can be made by reducing pure oxide of iron in a
+current of hydrogen at a high temperature. Prepared in this way it
+forms a black powder; when melted it forms a tin-white metal which is
+less fusible and more malleable than wrought iron. It is easily acted
+upon by moist air.
+
+~Compounds of iron.~ Iron differs from the metals so far studied in that
+it is able to form two series of compounds in which the iron has two
+different valences. In the one series the iron is divalent and forms
+compounds which in formulas and many chemical properties are similar to
+the corresponding zinc compounds. It can also act as a trivalent metal,
+and in this condition forms salts similar to those of aluminium. Those
+compounds in which the iron is divalent are known as _ferrous_
+compounds, while those in which it is trivalent are known as _ferric_.
+
+~Oxides of iron.~ Iron forms several oxides. Ferrous oxide (FeO) is not
+found in nature, but can be prepared artificially in the form of a black
+powder which easily takes up oxygen, forming ferric oxide:
+
+    2FeO + O = Fe_{2}O_{3}.
+
+Ferric oxide is the most abundant ore of iron and occurs in great
+deposits, especially in the Lake Superior region. It is found in many
+mineral varieties which vary in density and color, the most abundant
+being hematite, which ranges in color from red to nearly black. When
+prepared by chemical processes it forms a red powder which is used as a
+paint pigment (Venetian red) and as a polishing powder (rouge).
+
+Magnetite has the formula Fe_{3}O_{4} and is a combination of FeO and
+Fe_{2}O_{3}. It is a very valuable ore, but is less abundant than
+hematite. It is sometimes called magnetic oxide of iron, or lodestone,
+since it is a natural magnet.
+
+~Ferrous salts.~ These salts are obtained by dissolving iron in the
+appropriate acid, or, when insoluble, by precipitation. They are usually
+light green in color and crystallize well. In chemical reactions they
+are quite similar to the salts of magnesium and zinc, but differ from
+them in one important respect, namely, that they are easily changed into
+compounds in which the metal is trivalent. Thus ferrous chloride treated
+with chlorine or aqua regia is changed into ferric chloride:
+
+    FeCl_{2} + Cl = FeCl_{3}.
+
+Ferrous hydroxide exposed to moist air is rapidly changed into ferric
+hydroxide:
+
+    2Fe(OH)_{2} + H_{2}O + O = 2Fe(OH)_{3}.
+
+~Ferrous sulphate~ _(copperas, green vitriol)_ (FeSO_{4}.7H_{2}O). Ferrous
+sulphate is the most familiar ferrous compound. It is prepared
+commercially as a by-product in the steel-plate mills. Steel plates are
+cleaned by the action of dilute sulphuric acid upon them, and in the
+process some of the iron dissolves. The liquors are concentrated and the
+green vitriol separates from them.
+
+~Ferrous sulphide~ (FeS). Ferrous sulphide is sometimes found in nature as
+a golden-yellow crystalline mineral. It is formed as a black precipitate
+when a soluble sulphide and an iron salt are brought together in
+solution:
+
+    FeSO_{4} + Na_{2}S = FeS + Na_{2}SO_{4}.
+
+It can also be made as a heavy dark-brown solid by fusing together the
+requisite quantities of sulphur and iron. It is obtained as a by-product
+in the metallurgy of lead:
+
+    PbS + Fe = FeS + Pb.
+
+It is used in the laboratory in the preparation of hydrosulphuric acid:
+
+    FeS + 2HCl = FeCl_{2} + H_{2}S.
+
+~Iron disulphide~ _(pyrites)_ (FeS_{2}). This substance bears the same
+relation to ferrous sulphide that hydrogen dioxide does to water. It
+occurs abundantly in nature in the form of brass-yellow cubical crystals
+and in compact masses. Sometimes the name "fool's gold" is applied to it
+from its superficial resemblance to the precious metal. It is used in
+very large quantities as a source of sulphur dioxide in the manufacture
+of sulphuric acid, since it burns readily in the air, forming ferric
+oxide and sulphur dioxide:
+
+    2FeS_{2} + 11O = Fe_{2}O_{3} + 4SO_{2}.
+
+~Ferrous carbonate~ (FeCO_{3}). This compound occurs in nature as
+siderite, and is a valuable ore. It will dissolve to some extent in
+water containing carbon dioxide, just as will calcium carbonate, and
+waters containing it are called chalybeate waters. These chalybeate
+waters are supposed to possess certain medicinal virtues and form an
+important class of mineral waters.
+
+~Ferric salts.~ Ferric salts are usually obtained by treating an acidified
+solution of a ferrous salt with an oxidizing agent:
+
+    2FeCl_{2} + 2HCl + O = 2FeCl_{3} + H_{2}O,
+
+    2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O.
+
+They are usually yellow or violet in color, are quite soluble, and as a
+rule do not crystallize well. Heated with water in the absence of free
+acid, they hydrolyze even more readily than the salts of aluminium. The
+most familiar ferric salts are the chloride and the sulphate.
+
+~Ferric chloride~ (FeCl_{3}). This salt can be obtained most conveniently
+by dissolving iron in hydrochloric acid and then passing chlorine into
+the solution:
+
+    Fe + 2HCl = FeCl_{2} + 2H,
+
+    FeCl_{2} + Cl = FeCl_{3}.
+
+When the pure salt is heated with water it is partly hydrolyzed:
+
+    FeCl_{3} + 3 H_{2}O <--> Fe(OH)_{3} + 3HCl.
+
+This is a reversible reaction, however, and hydrolysis can therefore be
+prevented by first adding a considerable amount of the soluble product
+of the reaction, namely, hydrochloric acid.
+
+~Ferric sulphate~ (Fe_{2}(SO_{4})_{3}). This compound can be made by
+treating an acid solution of green vitriol with an oxidizing agent. It
+is difficult to crystallize and hard to obtain in pure condition. When
+an alkali sulphate in proper quantity is added to ferric sulphate in
+solution an iron alum is formed, and is easily obtained in
+large crystals. The best known iron alums have the formulas
+KFe(SO_{4})_{2}.12H_{2}O and NH_{4}Fe(SO_{4})_{2}.12H_{2}O. They are
+commonly used when a pure ferric salt is required.
+
+~Ferric hydroxide~ (Fe(OH)_{3}). When solutions of ferric salts are
+treated with ammonium hydroxide, ferric hydroxide is formed as a
+rusty-red precipitate, insoluble in water.
+
+~Iron cyanides.~ A large number of complex cyanides containing iron are
+known, the most important being potassium ferrocyanide, or yellow
+prussiate of potash (K_{4}FeC_{6}N_{6}), and potassium ferricyanide, or
+red prussiate of potash (K_{3}FeC_{6}N_{6}). These compounds are the
+potassium salts of the complex acids of the formulas H_{4}FeC_{6}N_{6}
+and H_{3}FeC_{6}N_{6}.
+
+~Oxidation of ferrous salts.~ It has just been seen that when a ferrous
+salt is treated with an oxidizing agent in the presence of a free acid a
+ferric salt is formed:
+
+    2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O.
+
+In this reaction oxygen is used up, and the valence of the iron is
+changed from 2 to 3. The same equation may be written
+
+    2Fe^{++}, 2SO_{4}^{--} + 2H^{+}, SO_{4}^{--} + O
+      = 2Fe^{+++}, 3SO_{4}^{--} + H_{2}O.
+
+Hydrogen ions have been oxidized to water, while the charge of each iron
+ion has been increased from 2 to 3.
+
+In a similar way the conversion of ferrous chloride into ferric chloride
+may be written
+
+    Fe^{++}, 2Cl^{-} + Cl = Fe^{+++}, + 3Cl^{-}.
+
+Here again the valence of the iron and the charge on the iron ion has
+been increased from 2 to 3, though no oxygen has entered into the
+reaction. As a rule, however, changes of this kind are brought about by
+the use of an oxidizing agent, and are called oxidations.
+
+The term "oxidation" is applied to all reactions in which the valence of
+the metal of a compound is increased, or, in other words, to all
+reactions in which the charge of a cation is increased.
+
+~Reduction of ferric salts.~ The changes which take place when a ferric
+salt is converted into a ferrous salt are the reverse of the ones just
+described. This is seen in the equation
+
+    FeCl_{3} + H = FeCl_{2} + HCl
+
+In this reaction the valence of the iron has been changed from 3 to 2.
+The same equation may be written
+
+    Fe^{+++}, 3Cl_{-} + H = Fe^{++}, + H^{+} + 3Cl_{-}
+
+It will be seen that the charge of the iron ions has been diminished
+from 3 to 2. Since these changes are the reverse of the oxidation
+changes just considered, they are called reduction reactions. The term
+"reduction" is applied to all processes in which the valence of the
+metal of a compound is diminished, or, in other words, to all processes
+in which the charge on the cations is diminished.
+
+
+NICKEL AND COBALT
+
+These elements occur sparingly in nature, usually combined with arsenic
+or with arsenic and sulphur. Both elements have been found in the free
+state in meteorites. Like iron they form two series of compounds, but
+the salts corresponding to the ferrous salts are the most common, the
+ones corresponding to the ferric salts being difficult to obtain. Thus
+we have the chlorides NiCl_{2}.6H_{2}O and CoCl_{2}.6H_{2}O; the
+sulphates NiSO_{4}.7H_{2}O and CoSO_{4}.7H_{2}O; the nitrates
+Ni(NO_{3})_{2}.6H_{2}O and Co(NO_{3})_{2}.6H_{2}O.
+
+Nickel is largely used as an alloy with other metals. Alloyed with
+copper it forms coin metal from which five-cent pieces are made, with
+copper and zinc it forms German silver, and when added to steel in small
+quantities nickel steel is formed which is much superior to common steel
+for certain purposes. When deposited by electrolysis upon the surface of
+other metals such as iron, it forms a covering which will take a high
+polish and protects the metal from rust, nickel not being acted upon by
+moist air. Salts of nickel are usually green.
+
+Compounds of cobalt fused with glass give it an intensely blue color. In
+powdered form such glass is sometimes used as a pigment called smalt.
+Cobalt salts, which contain water of crystallization, are usually cherry
+red in color; when dehydrated they become blue.
+
+
+EXERCISES
+
+1. In the manufacture of cast iron, why is the air heated before being
+forced into the furnace?
+
+2. Write the equations showing how each of the following compounds of
+iron could be obtained from the metal itself: ferrous chloride, ferrous
+hydroxide, ferrous sulphate, ferrous sulphide, ferrous carbonate, ferric
+chloride, ferric sulphate, ferric hydroxide.
+
+3. Account for the fact that a solution of sodium carbonate, when added
+to a solution of a ferric salt, precipitates an hydroxide and not a
+carbonate.
+
+4. Calculate the percentage of iron in each of the common iron ores.
+
+5. One ton of steel prepared by the Bessemer process is found by
+analysis to contain 0.2% carbon. What is the minimum weight of carbon
+which must be added in order that the steel may be made to take a
+temper?
+
+
+
+
+CHAPTER XXVIII
+
+COPPER, MERCURY, AND SILVER
+
+
+==================================================================
+        |        |        |         |         |
+        |        |        |         |         | FORMULAS OF OXIDES
+        | SYMBOL | ATOMIC | DENSITY | MELTING |___________________
+        |        | WEIGHT |         |  POINT  |          |
+        |        |        |         |         |  "ous"   |  "ic"
+________|________|________|_________|_________|__________|________
+        |        |        |         |         |          |
+Copper  |   Cu   |  63.6  |   8.89  | 1084 deg.   |  Cu_{2}O |  CuO
+Mercury |   Hg   | 200.00 |  13.596 |  -39.5 deg. |  Hg_{2}O |  HgO
+Silver  |   Ag   | 107.93 |  10.5   |  960 deg.   |  Ag_{2}O |  AgO
+==================================================================
+
+~The family.~ By referring to the periodic arrangement of the elements
+(page 168), it will be seen that mercury is not included in the same
+family with copper and silver. Since the metallurgy of the three
+elements is so similar, however, and since they resemble each other so
+closely in chemical properties, it is convenient to class them together
+for study.
+
+1. _Occurrence._ The three elements occur in nature to some extent in
+the free state, but are usually found as sulphides. Their ores are easy
+to reduce.
+
+2. _Properties._ They are heavy metals of high luster and are especially
+good conductors of heat and electricity. They are not very active
+chemically. Neither hydrochloric nor dilute sulphuric acid has any
+appreciable action upon them. Concentrated sulphuric acid attacks all
+three, forming metallic sulphates and evolving sulphur dioxide, while
+nitric acid, both dilute and concentrated, converts them into nitrates
+with the evolution of oxides of nitrogen.
+
+3. _Two series of salts._ Copper and mercury form oxides of the types
+M_{2}O and MO, as well as two series of salts. In one series the metals
+are univalent and the salts have formulas like those of the sodium
+salts. They are called cuprous and mercurous salts. In the other series
+the metals are divalent and resemble magnesium salts in formulas. These
+are called cupric and mercuric salts. Silver forms only one series of
+salts, being always a univalent metal.
+
+
+COPPER
+
+~Occurrence.~ The element copper has been used for various purposes since
+the earliest days of history. It is often found in the metallic state in
+nature, large masses of it occurring pure in the Lake Superior region
+and in other places to a smaller extent. The most valuable ores are the
+following:
+
+    Cuprite         Cu_{2}O.
+    Chalcocite      Cu_{2}S.
+    Chalcopyrite    CuFeS_{2}.
+    Bornite         Cu_{3}FeS_{3}.
+    Malachite       CuCO_{3}.Cu(OH)_{2}.
+    Azurite         2CuCO_{3}.Cu(OH)_{2}.
+
+~Metallurgy of copper.~ Ores containing little or no sulphur are easy to
+reduce. They are first crushed and the earthy impurities washed away.
+The concentrated ore is then mixed with carbon and heated in a furnace,
+metallic copper resulting from the reduction of the copper oxide by the
+hot carbon.
+
+     ~Metallurgy of sulphide ores.~ Much of the copper of commerce is
+     made from chalcopyrite and bornite, and these ores are more
+     difficult to work. They are first roasted in the air, by which
+     treatment much of the sulphur is burned to sulphur dioxide. The
+     roasted ore is then melted in a small blast furnace or in an
+     open one like a puddling furnace. In melting, part of the iron
+     combines with silica to form a slag of iron silicate. The
+     product, called crude matte, contains about 50% copper together
+     with sulphur and iron. Further purification is commonly carried
+     on by a process very similar to the Bessemer process for steel.
+     The converter is lined with silica, and a charge of matte from
+     the melting furnace, together with sand, is introduced, and air
+     is blown into the mass. By this means the sulphur is
+     practically all burned out by the air, and the remaining iron
+     combines with silica and goes off as slag. The copper is poured
+     out of the converter and molded into anode plates for refining.
+
+~Refining of copper.~ Impure copper is purified by electrolysis. A large
+plate of it, serving as an anode, is suspended in a tank facing a thin
+plate of pure copper, which is the cathode. The tank is filled with a
+solution of copper sulphate and sulphuric acid to serve as the
+electrolyte. A current from a dynamo passes from the anode to the
+cathode, and the copper, dissolving from the anode, is deposited upon
+the cathode in pure form, while the impurities collect on the bottom of
+the tank. Electrolytic copper is one of the purest of commercial metals
+and is very nearly pure copper.
+
+     ~Recovery of gold and silver.~ Gold and silver are often present
+     in small quantities in copper ores, and in electrolytic
+     refining these metals collect in the muddy deposit on the
+     bottom of the tank. The mud is carefully worked over from time
+     to time and the precious metals extracted from it. A surprising
+     amount of gold and silver is obtained in this way.
+
+~Properties of copper.~ Copper is a rather heavy metal of density 8.9, and
+has a characteristic reddish color. It is rather soft and is very
+malleable, ductile, and flexible, yet tough and strong; it melts at
+1084 deg.. As a conductor of heat and electrical energy it is second only to
+silver.
+
+Hydrochloric acid, dilute sulphuric acid, and fused alkalis are almost
+without action upon it; nitric acid and hot, concentrated sulphuric
+acid, however, readily dissolve it. In moist air it slowly becomes
+covered with a thin layer of green basic carbonate; heated in the air it
+is easily oxidized to black copper oxide (CuO).
+
+~Uses.~ Copper is extensively used for electrical purposes, for roofs and
+cornices, for sheathing the bottom of ships, and for making alloys. In
+the following table the composition of some of these alloys is
+indicated:
+
+COMPOSITION OF ALLOYS OF COPPER IN PERCENTAGES
+
+    Aluminium bronze copper (90 to 97%), aluminium (3 to 10%).
+    Brass copper (63 to 73%), zinc (27 to 37%).
+    Bronze copper (70 to 95%), zinc (1 to 25%), tin (1 to 18%).
+    German silver copper (56 to 60%), zinc (20%), nickel (20 to 25%).
+    Gold coin copper (10%), gold (90%).
+    Gun metal copper (90%), tin (10%).
+    Nickel coin copper (75%), nickel (25%)
+    Silver coin copper (10%), silver (90%).
+
+     ~Electrotyping.~ Matter is often printed from electrotype plates
+     which are prepared as follows. The matter is set up in type and
+     wax is firmly pressed down upon the face of it until a clear
+     impression is obtained. The impressed side of the wax is coated
+     with graphite and the impression is made the cathode in an
+     electrolytic cell containing a copper salt in solution. When
+     connected with a current the copper is deposited as a thin
+     sheet upon the letters in wax, and when detached is a perfect
+     copy of the type, the under part of the letters being hollow.
+     The sheet is strengthened by pouring on the under surface a
+     suitable amount of molten metal (commercial lead is used). The
+     sheet so strengthened is then used in printing.
+
+~Two series of copper compounds.~ Copper, like iron, forms two series of
+compounds: in the cuprous compounds it is univalent; in the cupric it is
+divalent. The cupric salts are much the more common of the two, since
+the cuprous salts pass readily into cupric by oxidation.
+
+~Cuprous compounds.~ The most important cuprous compound is the oxide
+(Cu_{2}O), which occurs in nature as ruby copper or cuprite. It is a
+bright red substance and can easily be prepared by heating copper to a
+high temperature in a limited supply of air. It is used for imparting a
+ruby color to glass.
+
+By treating cuprous oxide with different acids a number of cuprous salts
+can be made. Many of these are insoluble in water, the chloride (CuCl)
+being the best known. When suspended in dilute hydrochloric acid it is
+changed into cupric chloride, the oxygen taking part in the reaction
+being absorbed from the air:
+
+    2CuCl + 2HCl + O = 2CuCl_{2} + H_{2}O.
+
+~Cupric compounds.~ Cupric salts are easily made by dissolving cupric
+oxide in acids, or, when insoluble, by precipitation. Most of them are
+blue or green in color, and the soluble ones crystallize well. Since
+they are so much more familiar than the cuprous salts, they are
+frequently called merely copper salts.
+
+~Cupric oxide~ (CuO). This is a black insoluble substance obtained by
+heating copper in excess of air, or by igniting the hydroxide or
+nitrate. It is used as an oxidizing agent.
+
+~Cupric hydroxide~ (Cu(OH)_{2}). The hydroxide prepared by treating a
+solution of a copper salt with sodium hydroxide is a light blue
+insoluble substance which easily loses water and changes into the oxide.
+Heat applied to the liquid containing the hydroxide suspended in it
+serves to bring about the reaction represented by the equation
+
+    Cu(OH)_{2} = CuO + H_{2}O.
+
+~Cupric sulphate~ (_blue vitriol_) (CuSO_{4}.5H_{2}O). This substance,
+called blue vitriol or bluestone, is obtained as a by-product in a
+number of processes and is produced in very large quantities. It forms
+large blue crystals, which lose water when heated and crumble to a white
+powder. The salt finds many uses, especially in electrotyping and in
+making electrical batteries.
+
+~Cupric sulphide~ (CuS). The insoluble black sulphide (CuS) is easily
+prepared by the action of hydrosulphuric acid upon a solution of a
+copper salt:
+
+    CuSO_{4} + H_{2}S = CuS + H_{2}SO_{4}.
+
+It is insoluble in water and dilute acids.
+
+
+MERCURY
+
+~Occurrence.~ Mercury occurs in nature chiefly as the sulphide (HgS)
+called cinnabar, and in globules of metal inclosed in the cinnabar. The
+mercury mines of Spain have long been famous, California being the next
+largest producer.
+
+~Metallurgy.~ Mercury is a volatile metal which has but little affinity
+for oxygen. Sulphur, on the other hand, readily combines with oxygen.
+These facts make the metallurgy of mercury very simple. The crushed ore,
+mixed with a small amount of carbon to reduce any oxide or sulphate that
+might be formed, is roasted in a current of air. The sulphur burns to
+sulphur dioxide, while the mercury is converted into vapor and is
+condensed in a series of condensing vessels. The metal is purified by
+distillation.
+
+~Properties.~ Mercury is a heavy silvery liquid with a density of 13.596.
+It boils at 357 deg. and solidifies at -39.5 deg.. Small quantities of many
+metals dissolve in it, forming liquid alloys, while with larger
+quantities it forms solid alloys. The alloys of mercury are called
+amalgams.
+
+Toward acids mercury conducts itself very much like copper; it is easily
+attacked by nitric and hot, concentrated sulphuric acids, while cold
+sulphuric and hydrochloric acids have no effect on it.
+
+~Uses.~ Mercury is extensively used in the construction of scientific
+instruments, such as the thermometer and barometer, and as a liquid over
+which to collect gases which are soluble in water. The readiness with
+which it alloys with silver and gold makes it very useful in the
+extraction of these elements.
+
+~Compounds of mercury.~ Like copper, mercury forms two series of
+compounds: the mercurous, of which mercurous chloride (HgCl) is an
+example; and the mercuric, represented by mercuric chloride (HgCl_{2}).
+
+~Mercuric oxide~ (HgO). Mercuric oxide can be obtained either as a
+brick-red or as a yellow substance. When mercuric nitrate is heated
+carefully the red modification is formed in accordance with the equation
+
+    Hg(NO_{3})_{2} = HgO + 2NO_{2} + O.
+
+The yellow modification is prepared by adding a solution of a mercuric
+salt to a solution of sodium or potassium hydroxide:
+
+    Hg(NO_{3})_{2} + 2NaOH = 2NaNO_{3} + Hg(OH)_{2},
+
+    Hg(OH)_{2} = HgO + H_{2}O.
+
+When heated the oxide darkens until it becomes almost black; at a higher
+temperature it decomposes into mercury and oxygen. It was by this
+reaction that oxygen was discovered.
+
+~Mercurous chloride~ (_calomel_) (HgCl). Being insoluble, mercurous
+chloride is precipitated as a white solid when a soluble chloride is
+added to a solution of mercurous nitrate:
+
+    HgNO_{3} + NaCl = HgCl + NaNO_{3}.
+
+Commercially it is manufactured by heating a mixture of mercuric
+chloride and mercury. When exposed to the light it slowly changes into
+mercuric chloride and mercury:
+
+    2HgCl = HgCl_{2} + Hg.
+
+It is therefore protected from the light by the use of colored bottles.
+It is used in medicine.
+
+Most mercurous salts are insoluble in water, the principal soluble one
+being the nitrate, which is made by the action of cold, dilute nitric
+acid on mercury.
+
+~Mercuric chloride~ (_corrosive sublimate_) (HgCl_{2}). This substance can
+be made by dissolving mercuric oxide in hydrochloric acid. On a
+commercial scale it is made by subliming a mixture of common salt and
+mercuric sulphate:
+
+    2NaCl + HgSO_{4} = HgCl_{2} + Na_{2}SO_{4}.
+
+The mercuric chloride, being readily volatile, vaporizes and is
+condensed again in cool vessels. Like mercurous chloride it is a white
+solid, but differs from it in that it is soluble in water. It is
+extremely poisonous and in dilute solutions is used as an antiseptic in
+dressing wounds.
+
+~Mercuric sulphide~ (HgS). As cinnabar this substance forms the chief
+native compound of mercury, occurring in red crystalline masses. By
+passing hydrosulphuric acid into a solution of a mercuric salt it is
+precipitated as a black powder, insoluble in water and acids. By other
+means it can be prepared as a brilliant red powder known as vermilion,
+which is used as a pigment in fine paints.
+
+     ~The iodides of mercury.~ If a solution of potassium iodide is
+     added to solutions of a mercurous and a mercuric salt
+     respectively, the corresponding iodides are precipitated.
+     Mercuric iodide is the more important of the two, and as
+     prepared above is a red powder which changes to yellow on
+     heating to 150 deg.. The yellow form on cooling changes back again
+     to the red form, or may be made to do so by rubbing it with a
+     knife blade or some other hard object.
+
+
+SILVER
+
+~Occurrence.~ Silver is found in small quantities in the uncombined state;
+usually, however, it occurs in combination with sulphur, either as the
+sulphide (Ag_{2}S) or as a small constituent of other sulphides,
+especially those of lead and copper. It is also found alloyed with gold.
+
+~Metallurgy.~ _Parkes's process._ Silver is usually smelted in connection
+with lead. The ores are worked over together, as described under lead,
+and the lead and silver obtained as an alloy, the silver being present
+in small quantity. The alloy is melted and metallic zinc is stirred in.
+Zinc will alloy with silver but not with lead, and it is found that the
+silver leaves the lead and, in the form of an alloy with zinc, forms as
+a crust upon the lead and is skimmed off. This crust, which, of course,
+contains lead adhering to it, is partially melted and the most of the
+lead drained off. The zinc is removed by distillation, and the residue
+is melted on an open hearth in a current of air; by this means the zinc
+and lead remaining with the silver are changed into oxides and the
+silver remains behind unaltered.
+
+     ~Amalgamation process.~ In some localities the old amalgamation
+     process is used. The silver ore is treated with common salt and
+     ferrous compounds, which process converts the silver first into
+     chloride and then into metallic silver. Mercury is then added
+     and thoroughly mixed with the mass, forming an amalgam with the
+     silver. After some days the earthy materials are washed away
+     and the heavier amalgam is recovered. The mercury is distilled
+     off and the silver left in impure form.
+
+~Refining silver.~ The silver obtained by either of the above processes
+may still contain copper, gold, and iron, and is refined by "parting"
+with sulphuric acid. The metal is heated with strong sulphuric acid
+which dissolves the silver, copper, and iron present, but not the gold.
+In the solution of silver sulphate so obtained copper plates are
+suspended, upon which the pure silver precipitates, the copper going
+into solution as sulphate, as shown in the equation
+
+    Ag_{2}SO_{4} + Cu = 2Ag + CuSO_{4}.
+
+The solution obtained as a by-product in this process furnishes most of
+the blue vitriol of commerce. Silver is also refined by electrolytic
+methods similar to those used in refining copper.
+
+~Properties of silver.~ Silver is a heavy, rather soft, white metal, very
+ductile and malleable and capable of taking a high polish. It surpasses
+all other metals as a conductor of heat and electricity, but is too
+costly to find extensive use for such purposes. It melts at a little
+lower temperature than copper (961 deg.). It alloys readily with other heavy
+metals, and when it is to be used for coinage a small amount of
+copper--from 8 to 10%--is nearly always melted with it to give it
+hardness.
+
+It is not acted upon by water or air, but is quickly tarnished when in
+contact with sulphur compounds, turning quite black in time.
+Hydrochloric acid and fused alkalis do not act upon it, but nitric acid
+and hot, concentrated sulphuric acid dissolve it with ease.
+
+[Illustration: Fig. 88]
+
+     ~Electroplating.~ Since silver is not acted upon by water or air,
+     and has a pleasing appearance, it is used to coat various
+     articles made of cheaper metals. Such articles are said to be
+     silver plated. The process by which this is done is called
+     electroplating. It is carried on as follows: The object to be
+     plated (such as a spoon) is attached to a wire and dipped into
+     a solution of a silver salt. Electrical connection is made in
+     such a way that the article to be plated serves as the cathode,
+     while the anode is made up of one or more plates of silver
+     (Fig. 88, A). When a current is passed through the
+     electrolyte silver dissolves from the anode plate and deposits
+     on the cathode in the form of a closely adhering layer. By
+     making the proper change in the electrolyte and anode plate
+     objects may be plated with gold and other metals.
+
+~Compounds of silver.~ Silver forms two oxides but only one series of
+salts, namely, the one which corresponds to the mercurous and cuprous
+series.
+
+~Silver nitrate~ (_lunar caustic_) (AgNO_{3}). This salt is easily
+prepared by dissolving silver in nitric acid and evaporating the
+resulting solution. It crystallizes in flat plates, and when heated
+carefully can be melted without decomposition. When cast into sticks it
+is called lunar caustic, for it has a very corrosive action on flesh,
+and is sometimes used in surgery to burn away abnormal growths.
+
+     The alchemists designated the metals by the names of the
+     heavenly bodies. The moon (luna) was the symbol for silver;
+     hence the name "lunar caustic."
+
+~Silver sulphide~ (Ag_{2}S). This occurs in nature and constitutes one of
+the principal ores of silver. It can be obtained in the form of a black
+solid by passing hydrosulphuric acid through a solution of silver
+nitrate.
+
+~Compounds of silver with the halogens.~ The chloride, bromide, and iodide
+of silver are insoluble in water and acids, and are therefore
+precipitated by bringing together a soluble halogen salt with silver
+nitrate:
+
+    AgNO_{3} + KCl = AgCl + KNO_{3}.
+
+They are remarkable for the fact that they are very sensitive to the
+action of light, undergoing a change of color and chemical composition
+when exposed to sunlight, especially if in contact with organic matter
+such as gelatin.
+
+     ~Photography.~ The art of photography is based on the fact that
+     the halogen compounds of silver are affected by the light,
+     particularly in the presence of organic matter. From a chemical
+     standpoint the processes involved may be described under two
+     heads: (1) the preparation of the negative; (2) the preparation
+     of the print.
+
+     1. _Preparation of the negative._ The plate used in the
+     preparation of the negative is made by spreading a thin layer
+     of gelatin, in which silver bromide is suspended (silver iodide
+     is sometimes added also), over a glass plate or celluloid film
+     and allowing it to dry. When the plate so prepared is placed in
+     a camera and the image of some object is focused upon it, the
+     silver salt undergoes a change which is proportional at each
+     point to the intensity of the light falling upon it. In this
+     way an image of the object photographed is produced upon the
+     plate, which is, however, invisible and is therefore called
+     "latent." It can be made visible by the process of developing.
+
+     To develop the image the exposed plate is immersed in a
+     solution of some reducing agent called the developer. The
+     developer reduces that portion of the silver salt which has
+     been affected by the light, depositing it in the form of black
+     metallic silver which closely adheres to the plate.
+
+     The unaffected silver salt, upon which the developer has no
+     action, must now be removed from the plate. This is done by
+     immersing the plate in a solution of sodium thiosulphate
+     (hypo). After the silver salt has been dissolved off, the plate
+     is washed with water and dried. The plate so prepared is
+     called the negative because it is a picture of the object
+     photographed, with the lights exactly reversed. This is called
+     fixing the negative.
+
+     2. _Preparation of the print._ The print is made from paper
+     which is prepared in the same way as the negative plate. The
+     negative is placed upon this paper and exposed to the light in
+     such a way that the light must pass through the negative before
+     striking the paper. If the paper is coated with silver
+     chloride, a visible image is produced, in which case a
+     developer is not needed. The proofs are made in this way. In
+     order to make them permanent the unchanged silver chloride must
+     be dissolved off with sodium thiosulphate. The print is then
+     toned by dipping it into a solution of gold or platinum salts.
+     The silver on the print passes into solution, while the gold or
+     platinum takes its place. These metals give a characteristic
+     color or tone to the print, the gold making it reddish brown,
+     while the platinum gives it a steel-gray tone. If a silver
+     bromide paper is used in making the print, a latent image is
+     produced which must be developed as in the case of the negative
+     itself. The silver bromide is much more sensitive than the
+     chloride, so that the printing can be done in artificial light.
+     Since the darkest places on the negative cut off the most
+     light, it is evident that the lights of the print will be the
+     reverse of those of the negative, and will therefore correspond
+     to those of the object photographed. The print is therefore
+     called the positive.
+
+
+EXERCISES
+
+1. Account for the fact that copper has been used for so long a time.
+
+2. Write equations for the action of concentrated sulphuric and nitric
+acids upon the metals of this family.
+
+3. How would you account for the fact that normal copper sulphate is
+slightly acid to litmus?
+
+4. Contrast the action of heat on cupric nitrate and mercuric nitrate.
+
+5. State reasons why mercury is adapted for use in thermometers and
+barometers.
+
+6. How could you distinguish between mercurous chloride and mercuric
+chloride?
+
+7. Write equations for the preparation of mercuric and mercurous
+iodides.
+
+8. How would you account for the fact that solutions of the different
+salts of a metal usually have the same color?
+
+9. Crude silver usually contains iron and lead. What would become of
+these metals in refining by parting with sulphuric acid?
+
+10. In the amalgamation process for extracting silver, how does ferrous
+chloride convert silver chloride into silver? Write equation. Why is the
+silver sulphide first changed into silver chloride?
+
+11. What impurities would you expect to find in the copper sulphate
+prepared from the refining of silver?
+
+12. How could you prepare pure silver chloride from a silver coin?
+
+13. Mercuric nitrate and silver nitrate are both white solids soluble in
+water. How could you distinguish between them?
+
+14. Account for the fact that sulphur waters turn a silver coin black;
+also for the fact that a silver spoon is blackened by foods (eggs, for
+example) containing sulphur.
+
+15. When a solution of silver nitrate is added to a solution of
+potassium chlorate no precipitate forms. How do you account for the fact
+that a precipitate of silver chloride is not formed?
+
+
+
+
+CHAPTER XXIX
+
+TIN AND LEAD
+
+
+====================================================================
+     |        |        |         |         |
+     | SYMBOL | ATOMIC | DENSITY | MELTING | COMMON OXIDES
+     |        | WEIGHT |         |  POINT  |
+_____|________|________|_________|_________|________________________
+     |        |        |         |         |
+Tin  |   Sn   |  119.0 |   7.35  |   235 deg.  | SnO             SnO_{2}
+Lead |   Pb   |  206.9 |  11.38  |   327 deg.  | PbO Pb_{3}O_{4} PbO_{2}
+====================================================================
+
+~The family.~ Tin and lead, together with silicon and germanium, form a
+family in Group IV of the periodic table. Silicon has been discussed
+along with the non-metals, while germanium, on account of its rarity,
+needs only to be mentioned.
+
+The other family of Group IV includes carbon, already described, and a
+number of rare elements.
+
+
+TIN
+
+~Occurrence.~ Tin is found in nature chiefly as the oxide (SnO_{2}),
+called cassiterite or tinstone. The most famous mines are those of
+Cornwall in England, and of the Malay Peninsula and East India Islands;
+in small amounts tinstone is found in many other localities.
+
+~Metallurgy.~ The metallurgy of tin is very simple. The ore, separated as
+far as possible from earthy materials, is mixed with carbon and heated
+in a furnace, the reduction taking place readily. The equation is
+
+    SnO_{2} + C = Sn + CO_{2}.
+
+The metal is often purified by carefully heating it until it is partly
+melted; the pure tin melts first and can be drained away from the
+impurities.
+
+~Properties.~ Pure tin, called block tin, is a soft white metal with a
+silver-like appearance and luster; it melts readily (235 deg.) and is
+somewhat lighter than copper, having a density of 7.3. It is quite
+malleable and can be rolled out into very thin sheets, forming tin foil;
+most tin foil, however, contains a good deal of lead.
+
+Under ordinary conditions it is quite unchanged by air or moisture, but
+at a high temperature it burns in air, forming the oxide SnO_{2}. Dilute
+acids have no effect upon it, but concentrated acids attack it readily.
+Concentrated hydrochloric acid changes it into the chloride
+
+    Sn + 2HCl = SnCl_{2} + 2H.
+
+With sulphuric acid tin sulphate and sulphur dioxide are formed:
+
+    Sn + 2H_{2}SO_{4} = SnSO_{4} + SO_{2} + 2H_{2}O
+
+Concentrated nitric acid oxidizes it, forming a white insoluble compound
+of the formula H_{2}SnO_{3}, called metastannic acid:
+
+    3Sn + 4HNO_{3} + H_{2}O = 3H_{2}SnO_{3} + 4NO.
+
+~Uses of tin.~ A great deal of tin is made into tin plate by dipping thin
+steel sheets into the melted metal. Owing to the way in which tin
+resists the action of air and dilute acids, tin plate is used in many
+ways, such as in roofing, and in the manufacture of tin cans, cooking
+vessels, and similar articles.
+
+Many useful alloys contain tin, some of which have been mentioned in
+connection with copper. When tin is alloyed with other metals of low
+melting point, soft, easily melted alloys are formed which are used for
+friction bearings in machinery; tin, antimony, lead, and bismuth are the
+chief constituents of these alloys. Pewter and soft solder are alloys of
+tin and lead.
+
+~Compounds of tin.~ Tin forms two series of compounds: the stannous, in
+which the tin is divalent, illustrated in the compounds SnO, SnS,
+SnCl_{2}; the stannic, in which it is tetravalent as shown in the
+compounds SnO_{2}, SnS_{2}. There is also an acid, H_{2}SnO_{3}, called
+stannic acid, which forms a series of salts called stannates. While this
+acid has the same composition as metastannic acid, the two are quite
+different in their chemical properties. This difference is probably due
+to the different arrangement of the atoms in the molecules of the two
+substances. Only a few compounds of tin need be mentioned.
+
+~Stannic oxide~ (SnO_{2}). Stannic oxide is of interest, since it is the
+chief compound of tin found in nature. It is sometimes found in
+good-sized crystals, but as prepared in the laboratory is a white
+powder. When fused with potassium hydroxide it forms potassium stannate,
+acting very much like silicon dioxide:
+
+    SnO_{2} + 2KOH = K_{2}SnO_{3} + H_{2}O.
+
+~Chlorides of tin.~ Stannous chloride is prepared by dissolving tin in
+concentrated hydrochloric acid and evaporating the solution to
+crystallization. The crystals which are obtained have the composition
+SnCl_{2}.2H_{2}O, and are known as tin crystals. By treating a solution
+of stannous chloride with aqua regia, stannic chloride is formed:
+
+    SnCl_{2} + 2Cl = SnCl_{4}.
+
+The salt which crystallizes from such a solution has the composition
+SnCl_{4}.5H_{2}O, and is known commercially as oxymuriate of tin. If
+metallic tin is heated in a current of dry chlorine, the anhydrous
+chloride (SnCl_{4}) is obtained as a heavy colorless liquid which fumes
+strongly on exposure to air.
+
+The ease with which stannous chloride takes up chlorine to form stannic
+chloride makes it a good reducing agent in many reactions, changing the
+higher chlorides of metals to lower ones. Thus mercuric chloride is
+changed into mercurous chloride:
+
+    SnCl_{2} + 2HgCl_{2} = SnCl_{4} + 2HgCl.
+
+If the stannous chloride is in excess, the reaction may go further,
+producing metallic mercury:
+
+    SnCl_{2} + 2HgCl = SnCl_{4} + 2Hg.
+
+Ferric chloride is in like manner reduced to ferrous chloride:
+
+    SnCl_{3} + 2FeCl_{3} = SnCl_{4} + 2FeCl_{2}.
+
+The chlorides of tin, as well as the alkali stannates, are much used as
+mordants in dyeing processes. The hydroxides of tin and free stannic
+acid, which are easily liberated from these compounds, possess in very
+marked degree the power of fixing dyes upon fibers, as explained under
+aluminium.
+
+
+LEAD
+
+~Occurrence.~ Lead is found in nature chiefly as the sulphide (PbS),
+called galena; to a much smaller extent it occurs as carbonate,
+sulphate, chromate, and in a few other forms. Practically all the lead
+of commerce is made from galena, two general methods of metallurgy being
+in use.
+
+~Metallurgy.~ 1. The sulphide is melted with scrap iron, when iron
+sulphide and metallic lead are formed; the liquid lead, being the
+heavier, sinks to the bottom of the vessel and can be drawn off:
+
+    PbS + Fe = Pb + FeS.
+
+2. The sulphide is roasted in the air until a part of it has been
+changed into oxide and sulphate. The air is then shut off and the
+heating continued, the reactions indicated in the following equations
+taking place:
+
+    2PbO + PbS = 3Pb + SO_{2},
+
+    PbSO_{4} + PbS = 2Pb + 2SO_{2}.
+
+The lead so prepared usually contains small amounts of silver, arsenic,
+antimony, copper, and other metals. The silver is removed by Parkes's
+method, as described under silver, and the other metals in various ways.
+The lead of commerce is one of the purest commercial metals, containing
+as a rule only a few tenths per cent of impurities.
+
+~Properties.~ Lead is a heavy metal (den. = 11.33) which has a brilliant
+silvery luster on a freshly cut surface, but which soon tarnishes to a
+dull blue-gray color. It is soft, easily fused (melting at 327 deg.), and
+quite malleable, but has little toughness or strength.
+
+It is not acted upon to any great extent by the oxygen of the air under
+ordinary conditions, but is changed into oxide at a high temperature.
+With the exception of hydrochloric and sulphuric acids, most acids, even
+very weak ones, act upon it, forming soluble lead salts. Hot,
+concentrated hydrochloric and sulphuric acids also attack it to a slight
+extent.
+
+~Uses.~ Lead is employed in the manufacture of lead pipes and in large
+storage batteries. In the form of sheet lead it is used in lining the
+chambers of sulphuric acid works and in the preparation of paint
+pigments. Some alloys of lead, such as solder and pewter (lead and tin),
+shot (lead and arsenic), and soft bearing metals, are widely used. Type
+metal consists of lead, antimony, and sometimes tin. Compounds of lead
+form several important pigments.
+
+~Compounds of lead.~ In nearly all its compounds lead has a valence of 2,
+but a few corresponding to stannic compounds have a valence of 4.
+
+~Lead oxides.~ Lead forms a number of oxides, the most important of which
+are litharge, red lead or minium, and lead peroxide.
+
+1. _Litharge_ (PbO). This oxide forms when lead is oxidized at a rather
+low temperature, and is obtained as a by-product in silver refining. It
+is a pale yellow powder, and has a number of commercial uses. It is
+easily soluble in nitric acid:
+
+    PbO + 2HNO_{3} = Pb(NO_{3})_{2} + H_{2}O.
+
+2. _Red lead, or minium_ (Pb_{3}O_{4}). Minium is prepared by heating
+lead (or litharge) to a high temperature in the air. It is a heavy
+powder of a beautiful red color, and is much used as a pigment.
+
+3. _Lead peroxide_ (PbO_{2}). This is left as a residue when minium is
+heated with nitric acid:
+
+Pb_{3}O_{4} + 4HNO_{3} = 2Pb(NO_{3})_{2} + PbO_{2} + 2H_{2}O.
+
+It is a brown powder which easily gives up a part of its oxygen and,
+like manganese dioxide and barium dioxide, is a good oxidizing agent.
+
+~Soluble salts of lead.~ The soluble salts of lead can be made by dissolving
+(Pb(C_{2}H_{3}O_{2})_{2}.3H_{2}O), litharge in acids. Lead acetate
+called sugar of lead, and lead nitrate (Pb(NO_{3})_{2}) are the most
+familiar examples. They are while crystalline solids and are poisonous
+in character.
+
+~Insoluble salts of lead; lead carbonate.~ While the normal carbonate of
+lead (PbCO_{3}) is found to some extent, in nature and can be prepared
+in the laboratory, basic carbonates of varying composition are much more
+easy to obtain. One of the simplest of these has the composition
+2PbCO_{3}.Pb(OH)_{2}. A mixture of such carbonates is called white lead.
+This is prepared on a large scale as a paint pigment and as a body for
+paints which are to be colored with other substances.
+
+     ~White lead.~ White lead is an amorphous white substance which,
+     when mixed with oil, has great covering power, that is, it
+     spreads out in an even waxy film, free from streaks and lumps,
+     and covers the entire surface upon which it is spread. Its
+     disadvantage as a pigment lies in the fact that it gradually
+     blackens when exposed to sulphur compounds, which are often
+     present in the air, forming black lead sulphide (PbS).
+
+     ~Technical preparation of white lead.~ Different methods are used
+     in the preparation of white lead, but the old one known as the
+     Dutch process is still the principal one employed. In this
+     process, earthenware pots about ten inches high and of the
+     shape shown in Fig. 89 are used. In the bottom A is placed a
+     3% solution of acetic acid (vinegar answers the purpose very
+     well). The space above this is filled with thin, perforated,
+     circular pieces of lead, supported by the flange B of the
+     pot. These pots are placed close together on a bed of tan bark
+     on the floor of a room known as the corroding room. They are
+     covered over with boards, upon which tan bark is placed, and
+     another row of pots is placed on this. In this way the room is
+     filled. The white lead is formed by the fumes of the acetic
+     acid, together with the carbon dioxide set free in the
+     fermentation of the tan bark acting on the lead. About three
+     months are required to complete the process.
+
+[Illustration 1: Fig. 89]
+
+~Lead sulphide~ (PbS). In nature this compound occurs in highly
+crystalline condition, the crystals having much the same luster as pure
+lead. It is readily prepared in the laboratory as a black precipitate,
+by the action of hydrosulphuric acid upon soluble lead salts:
+
+    Pb(NO_{3})_{2} + H_{2}S = PbS + 2HNO_{3}.
+
+It is insoluble both in water and in dilute acids.
+
+~Other insoluble salts.~ Lead chromate (PbCrO_{4}) is a yellow substance
+produced by the action of a soluble lead salt upon a soluble chromate,
+thus:
+
+    K_{2}CrO_{4} + Pb(NO_{3})_{2} = PbCrO_{4} + 2 KNO_{3}.
+
+It is used as a yellow pigment. Lead sulphate (PbSO_{4}) is a white
+substance sometimes found in nature and easily prepared by
+precipitation. Lead chloride (PbCl_{2}) is likewise a white substance
+nearly insoluble in cold water, but readily soluble in boiling water.
+
+     ~Thorium and cerium.~ These elements are found in a few rare
+     minerals, especially in the monazite sand of the Carolinas and
+     Brazil. The oxides of these elements are used in the
+     preparation of the Welsbach mantles for gas lights, because of
+     the intense light given out when a mixture of the oxides is
+     heated. These mantles contain the oxides of cerium and thorium
+     in the ratio of about 1% of the former to 99% of the latter.
+     Compounds of thorium, like those of radium, are found to
+     possess radio-activity, but in a less degree.
+
+
+EXERCISES
+
+1. How could you detect lead if present in tin foil?
+
+2. Stannous chloride reduces gold chloride (AuCl_{3}) to gold. Give
+equation.
+
+3. What are the products of hydrolysis when stannic chloride is used as
+a mordant?
+
+4. How could you detect arsenic, antimony, or copper in lead?
+
+5. Why is lead so extensively used for making water pipes?
+
+6. What sulphates other than lead are insoluble?
+
+7. Could lead nitrate be used in place of barium chloride in testing for
+sulphates?
+
+8. How much lead peroxide could be obtained from 1 kg. of minium?
+
+9. The purity of white lead is usually determined by observing the
+volume of carbon dioxide given off when it is treated with an acid. What
+acid should be used? On the supposition that it has the formula
+2PbCO_{3}.Pb(OH)_{2}, how nearly pure was a sample if 1 g. gave 30 cc.
+of carbon dioxide at 20 deg. and 750 mm.?
+
+10. Silicon belongs in the same family with tin and lead. In what
+respects are these elements similar?
+
+11. What weight of tin could be obtained by the reduction of 1 ton of
+cassiterite?
+
+12. What reaction would you expect to take place when lead peroxide is
+treated with hydrochloric acid?
+
+13. White lead is often adulterated with barytes. Suggest a method for
+detecting it, if present, in a given example of white lead.
+
+
+
+
+CHAPTER XXX
+
+MANGANESE AND CHROMIUM
+
+
+====================================================================
+          |        |        |         |         |
+          | SYMBOL | ATOMIC | DENSITY | MELTING | FORMULAS OF ACIDS
+          |        | WEIGHT |         | POINT   |
+__________|________|________|_________|_________|___________________
+          |        |        |         |         |
+Manganese |   Mn   |  55.0  |  8.01   |  1900 deg.  | H_{2}MnO_{4} and
+          |        |        |         |         | HMnO_{4}
+Chromium  |   Cr   |  52.1  |  7.3    |  3000 deg.  | H_{2}CrO_{4} and
+          |        |        |         |         | H_{2}Cr_{2}O_{7}
+====================================================================
+
+~General.~ Manganese and chromium, while belonging to different families,
+have so many features in common in their chemical conduct that they may
+be studied together with advantage. They differ from most of the
+elements so far studied in that they can act either as acid-forming or
+base-forming elements. As base-forming elements each of the metals forms
+two series of salts. In the one series, designated by the suffix "ous,"
+the metal is divalent; in the other series, designated by the suffix
+"ic," the metal is trivalent. Only the manganous and the chromic salts,
+however, are of importance. The acids in which these elements play the
+part of a non-metal are unstable, but their salts are usually stable,
+and some of them are important compounds.
+
+
+MANGANESE
+
+~Occurrence.~ Manganese is found in nature chiefly as the dioxide MnO_{2},
+called pyrolusite. In smaller amounts it occurs as the oxides
+Mn_{2}O_{3} and Mn_{3}O_{4}, and as the carbonate MnCO_{3}. Some iron
+ores also contain manganese.
+
+~Preparation and properties.~ The element is difficult to prepare in pure
+condition and has no commercial applications. It can be prepared,
+however, by reducing the oxide with aluminium powder or by the use of
+the electric furnace, with carbon as the reducing agent. The metal
+somewhat resembles iron in appearance, but is harder, less fusible, and
+more readily acted upon by air and moisture. Acids readily dissolve it,
+forming manganous salts.
+
+~Oxides of manganese.~ The following oxides of manganese are known: MnO,
+Mn_{2}O_{3}, Mn_{3}O_{4}, MnO_{2}, and Mn_{2}O_{7}. Only one of these,
+the dioxide, needs special mention.
+
+~Manganese dioxide~ (_pyrolusite_) (MnO_{2}). This substance is the most
+abundant manganese compound found in nature, and is the ore from which
+all other compounds of manganese are made. It is a hard, brittle, black
+substance which is valuable as an oxidizing agent. It will be recalled
+that it is used in the preparation of chlorine and oxygen, in
+decolorizing glass which contains iron, and in the manufacture of
+ferromanganese.
+
+~Compounds containing manganese as a base-forming element.~ As has been
+stated previously, manganese forms two series of salts. The most
+important of these salts, all of which belong to the manganous series,
+are the following:
+
+    Manganous chloride           MnCl_{2}.4H_{2}O.
+    Manganous sulphide           MnS.
+    Manganous sulphate           MnSO_{4}.4H_{2}O.
+    Manganous carbonate          MnCO_{3}.
+    Manganous hydroxide          Mn(OH)_{2}.
+
+The chloride and sulphate may be prepared by heating the dioxide with
+hydrochloric and sulphuric acids respectively:
+
+    MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + 2Cl,
+
+    MnO_{2} + H_{2}SO_{4} = MnSO_{4} + H_{2}O + O.
+
+The sulphide, carbonate, and hydroxide, being insoluble, may be prepared
+from a solution of the chloride or sulphate by precipitation with the
+appropriate reagents. Most of the manganous salts are rose colored. They
+not only have formulas similar to the ferrous salts, but resemble them
+in many of their chemical properties.
+
+~Compounds containing manganese as an acid-forming element.~ Manganese
+forms two unstable acids, namely, manganic acid and permanganic acid.
+While these acids are of little interest, some of their salts,
+especially the permanganates, are important compounds.
+
+~Manganic acid and manganates.~ When manganese dioxide is fused with an
+alkali and an oxidizing agent a green compound is formed. The equation,
+when caustic potash is used, is as follows:
+
+    MnO_{2} + 2KOH + O = K_{2}MnO_{4} + H_{2}O.
+
+The green compound (K_{2}MnO_{4}) is called potassium manganate, and is
+a salt of the unstable manganic acid (H_{2}MnO_{4}). The manganates are
+all very unstable.
+
+~Permanganic acid and the permanganates.~ When carbon dioxide is passed
+through a solution of a manganate a part of the manganese is changed
+into manganese dioxide, while the remainder forms a salt of the unstable
+acid HMnO_{4}, called permanganic acid. The equation is
+
+    3K_{2}MnO_{4} + 2CO_{2} = MnO_{2} + 2KMnO_{4} + 2K_{2}CO_{3}.
+
+Potassium permanganate (KMnO_{4}) crystallizes in purple-black needles
+and is very soluble in water, forming an intensely purple solution. All
+other permanganates, as well as permanganic acid itself, give solutions
+of the same color.
+
+~Oxidizing properties of the permanganates.~ The permanganates are
+remarkable for their strong oxidizing properties. When used as an
+oxidizing agent the permanganate is itself reduced, the exact character
+of the products formed from it depending upon whether the oxidation
+takes place (1) in an alkaline or neutral solution, or (2) in an acid
+solution.
+
+1. _Oxidation in alkaline or neutral solution._ When the solution is
+either alkaline or neutral the potassium and the manganese of the
+permanganate are both converted into hydroxides, as shown in the
+equation
+
+    2KMnO_{4} + 5H_{2}O = 2Mn(OH)_{4} + 2KOH + 3O.
+
+2. _Oxidation in acid solution._ When free acid such as sulphuric is
+present, the potassium and the manganese are both changed into salts of
+the acid:
+
+    2KMnO_{4} + 3H_{2}SO_{4} = K_{2}SO_{4} + 2MnSO_{4} + 3H_{2}O + 5O.
+
+Under ordinary conditions, however, neither one of these reactions takes
+place except in the presence of a third substance which is capable of
+oxidation. The oxygen is not given off in the free state, as the
+equations show, but is used up in effecting oxidation.
+
+Potassium permanganate is particularly valuable as an oxidizing agent
+not only because it acts readily either in acid or in alkaline solution,
+but also because the reaction takes place so easily that often it is not
+even necessary to heat the solution to secure action. The substance
+finds many uses in the laboratory, especially in analytical work. It is
+also used as an antiseptic as well as a disinfectant.
+
+
+CHROMIUM
+
+~Occurrence.~ The ore from which all chromium compounds are made is
+chromite, or chrome iron ore (FeCr_{2}O_{4}). This is found most
+abundantly in New Caledonia and Turkey. The element also occurs in small
+quantities in many other minerals, especially in crocoisite (PbCrO_{4}),
+in which mineral it was first discovered.
+
+~Preparation.~ Chromium, like manganese, is very hard to reduce from its
+ores, owing to its great affinity for oxygen. It can, however, be made
+by the same methods which have proved successful with manganese.
+Considerable quantities of an alloy of chromium with iron, called
+ferrochromium, are now produced for the steel industry.
+
+~Properties.~ Chromium is a very hard metal of about the same density as
+iron. It is one of the most infusible of the metals, requiring a
+temperature little short of 3000 deg. for fusion. At ordinary temperatures
+air has little action on it; at higher temperatures, however, it burns
+brilliantly. Nitric acid has no action on it, but hydrochloric and
+dilute sulphuric acids dissolve it, liberating hydrogen.
+
+~Compounds containing chromium as a base-forming element.~ While chromium
+forms two series of salts, chromous salts are difficult to prepare and
+are of little importance. The most important of the chromic series are
+the following:
+
+    Chromic hydroxide       Cr(OH)_{3}.
+    Chromic chloride        CrCl_{3}.6H_{2}O.
+    Chromic sulphate        Cr_{2}(SO_{4})_{3}.
+    Chrome alums
+
+~Chromic hydroxide~ (Cr(OH)_{3}). This substance, being insoluble, can be
+obtained by precipitating a solution of the chloride or sulphate with a
+soluble hydroxide. It is a greenish substance which, like aluminium
+hydroxide, dissolves in alkalis, forming soluble salts.
+
+     ~Dehydration of chromium hydroxide.~ When heated gently chromic
+     hydroxide loses a part of its oxygen and hydrogen, forming the
+     substance CrO.OH, which, like the corresponding aluminium
+     compound, has more pronounced acid properties than the
+     hydroxide. It forms a series of salts very similar to the
+     spinels; chromite is the ferrous salt of this acid, having the
+     formula Fe(CrO_{2})_{2}. When heated to a higher temperature
+     chromic hydroxide is completely dehydrated, forming the
+     trioxide Cr_{2}O_{3}. This resembles the corresponding oxides
+     of aluminium and iron in many respects. It is a bright green
+     powder, and when ignited strongly becomes almost insoluble in
+     acids, as is also the case with aluminium oxide.
+
+~Chromic sulphate~ (Cr_{2}(SO_{4})_{3}). This compound is a violet-colored
+solid which dissolves in water, forming a solution of the same color.
+This solution, however, turns green on heating, owing to the formation
+of basic salts. Chromic sulphate, like ferric and aluminium sulphates,
+unites with the sulphates of the alkali metals to form alums, of which
+the best known are potassium chrome alum (KCr(SO_{4})_{2}.12H_{2}O) and
+ammonium chrome alum (NH_{4}Cr(SO_{4})_{2}.12H_{2}O).
+
+These form beautiful dark purple crystals and have some practical uses
+in the tanning industry and in photography. A number of the salts of
+chromium are also used in the dyeing industry, for they hydrolyze like
+aluminium salts and the hydroxide forms a good mordant.
+
+     ~Hydrolysis of chromium salts.~ When ammonium sulphide is added
+     to a solution of a chromium salt, such as the sulphate,
+     chromium hydroxide precipitates instead of the sulphide. This
+     is due to the fact that chromic sulphide, like aluminium
+     sulphide, hydrolyzes in the presence of water, forming chromic
+     hydroxide and hydrosulphuric acid. Similarly, a soluble
+     carbonate precipitates a basic carbonate of chromium.
+
+~Compounds containing chromium as an acid-forming element.~ Like
+manganese, chromium forms two unstable acids, namely, chromic acid and
+dichromic acid. Their salts, the chromates and dichromates, are
+important compounds.
+
+~Chromates.~ When a chromium compound is fused with an alkali and an
+oxidizing agent a chromate is produced. When potassium hydroxide is used
+as the alkali the equation is
+
+    2Cr(OH)_{3} + 4KOH + 3O = 2K_{2}CrO_{4} + 5H_{2}O.
+
+This reaction recalls the formation of a manganate under similar
+conditions.
+
+~Properties of chromates.~ The chromates are salts of the unstable chromic
+acid (H_{2}CrO_{4}), and as a rule are yellow in color. Lead chromate
+(PbCrO_{4}) is the well-known pigment chrome yellow. Most of the
+chromates are insoluble and can therefore be prepared by precipitation.
+Thus, when a solution of potassium chromate is added to solutions of
+lead nitrate and barium nitrate respectively, the reactions expressed by
+the following equations occur:
+
+    Pb(NO_{3})_{2} + K_{2}CrO_{4} = PbCrO_{4} + 2KNO_{3},
+
+    Ba(NO_{3})_{2} + K_{2}CrO_{4} = BaCrO_{4} + 2KNO_{3}.
+
+The chromates of lead and barium separate as yellow precipitates. The
+presence of either of these two metals can be detected by taking
+advantage of these reactions.
+
+~Dichromates.~ When potassium chromate is treated with an acid the
+potassium salt of the unstable dichromic acid (H_{2}Cr_{2}O_{7}) is
+formed:
+
+    2K_{2}CrO_{4} + H_{2}SO_{4} = K_{2}Cr_{2}O_{7} + K_{2}SO_{4} + H_{2}O.
+
+The relation between the chromates and dichromates is the same as that
+between the phosphates and the pyrophosphates. Potassium dichromate
+might therefore be called potassium pyrochromate.
+
+~Potassium dichromate~ (K_{2}Cr_{2}O_{7}). This is the best known
+dichromate, and is the most familiar chromium compound. It forms large
+crystals of a brilliant red color, and is rather sparingly soluble in
+water. When treated with potassium hydroxide it is converted into the
+chromate
+
+    K_{2}Cr_{2}O_{7} + 2KOH = 2K_{2}CrO_{4} + H_{2}O.
+
+When added to a solution of lead or barium salt the corresponding
+chromates (not dichromates) are precipitated. With barium nitrate the
+equation is
+
+    2Ba(NO_{3})_{2} + K_{2}Cr_{2}O_{7} + H_{2}O
+        = 2BaCrO_{4} + 2KNO_{3} + 2HNO_{3}.
+
+Potassium dichromate finds use in many industries as an oxidizing agent,
+especially in the preparation of organic substances, such as the dye
+alizarin, and in the construction of several varieties of electric
+batteries.
+
+     ~Sodium chromates.~ The reason why the potassium salt rather than
+     the sodium compound is used is that sodium chromate and
+     dichromate are so soluble that it is hard to prepare them pure.
+     This difficulty is being overcome now, and the sodium compounds
+     are replacing the corresponding potassium salts. This is of
+     advantage, since a sodium salt is cheaper than a potassium
+     salt, so far as raw materials go.
+
+~Oxidizing action of chromates and dichromates.~ When a dilute solution of
+a chromate or dichromate is acidified with an acid, such as sulphuric
+acid, no reaction apparently takes place. However, if there is present a
+third substance capable of oxidation, the chromium compound gives up a
+portion of its oxygen to this substance. Since the chromate changes into
+a dichromate in the presence of an acid, it will be sufficient to study
+the action of the dichromates alone. The reaction takes place in two
+steps. Thus, when a solution of ferrous sulphate is added to a solution
+of potassium dichromate acidified with sulphuric acid, the reaction is
+expressed by the following equations:
+
+    (1) K_{2}Cr_{2}O_{7} + 4H_{2}SO_{4}
+           = K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 4H_{2}O + 3O,
+
+    (2) 6FeSO_{4} + 3H_{2}SO_{4} + 3O = 3Fe_{2}(SO_{4})_{3} + 3H_{2}O.
+
+The dichromate decomposes in very much the same way as a permanganate
+does, the potassium and chromium being both changed into salts in which
+they play the part of metals, while part of the oxygen of the dichromate
+is liberated.
+
+By combining equations (1) and (2), the following is obtained:
+
+    K_{2}Cr_{2}O_{7} + 7H_{2}SO_{4} + 6FeSO_{4}
+       = K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 3Fe_{2}(SO_{4})_{3} + 7H_{2}0.
+
+This reaction is often employed in the estimation of iron in iron ores.
+
+     ~Potassium chrome alum.~ It will be noticed that the oxidizing
+     action of potassium dichromate leaves potassium sulphate and
+     chromium sulphate as the products of the reaction. On
+     evaporating the solution these substances crystallize out as
+     potassium chrome alum, which substance is produced as a
+     by-product in the industries using potassium dichromate for
+     oxidizing purposes.
+
+~Chromic anhydride~ (CrO_{3}). When concentrated sulphuric acid is added
+to a strong solution of potassium dichromate, and the liquid allowed to
+stand, deep red needle-shaped crystals appear which have the formula
+CrO_{3}.This oxide of chromium is called chromic anhydride, since it
+combines readily with water to form chromic acid:
+
+    CrO_{3} + H_{2}O = H_{2}CrO_{4}.
+
+It is therefore analogous to sulphur trioxide which forms sulphuric acid
+in a similar way:
+
+    SO_{3} + H_{2}O = H_{2}SO_{4}.
+
+Chromic anhydride is a very strong oxidizing agent, giving up oxygen and
+forming chromic oxide:
+
+    2CrO_{3} = Cr_{2}O_{3} + 3O.
+
+     ~Rare elements of the family.~ Molybdenum, tungsten, and uranium
+     are three rather rare elements belonging in the same family
+     with chromium, and form many compounds which are similar in
+     formulas to the corresponding compounds of chromium. They can
+     play the part of metals and also form acids resembling chromic
+     acid in formula. Thus we have molybdic acid (H_{2}MoO_{4}), the
+     ammonium salt of which is (NH_{4})_{2}MoO_{4}. This salt has
+     the property of combining with phosphoric acid to form a very
+     complex substance which is insoluble in nitric acid. On this
+     account molybdic acid is often used in the estimation of the
+     phosphoric acid present in a substance. Like chromium, the
+     metals are difficult to prepare in pure condition. Alloys with
+     iron can be prepared by reducing the mixed oxides with carbon
+     in an electric furnace; these alloys are used to some extent in
+     preparing special kinds of steel.
+
+
+EXERCISES
+
+1. How does pyrolusite effect the decolorizing of glass containing iron?
+
+2. Write the equations for the preparation of manganous chloride,
+carbonate, and hydroxide.
+
+3. Write the equations representing the reactions which take place when
+ferrous sulphate is oxidized to ferric sulphate by potassium
+permanganate in the presence of sulphuric acid.
+
+4. In the presence of sulphuric acid, oxalic acid is oxidized by
+potassium permanganate according to the equation
+
+    C_{2}H_{2}O_{4} + O = 2CO_{2} + H_{2}O.
+
+Write the complete equation.
+
+5. 10 g. of iron were dissolved in sulphuric acid and oxidized to ferric
+sulphate by potassium permanganate. What weight of the permanganate was
+required?
+
+6. What weight of ferrochromium containing 40% chromium must be added to
+a ton of steel to produce an alloy containing 1% of chromium?
+
+7. Write the equation representing the action of ammonium sulphide upon
+chromium sulphate.
+
+8. Potassium chromate oxidizes hydrochloric acid, forming chlorine.
+Write the complete equation.
+
+9. Give the action of sulphuric acid on potassium dichromate (a) in
+the presence of a large amount of water; (b) in the presence of a
+small amount of water.
+
+
+
+
+CHAPTER XXXI
+
+GOLD AND THE PLATINUM FAMILY
+
+
+==============================================================================
+          |        |        |         |              |          |
+          |        | ATOMIC |         | HIGHEST      | HIGHEST  | MELTING
+          | SYMBOL | WEIGHT | DENSITY |  OXIDE       | CHLORIDE |  POINT
+__________|________|________|_________|_________     |__________|_____________
+          |        |        |         |              |          |
+Ruthenium |   Ru   |  101.7 |  12.26  | RuO_{4}      | RuCl_{4} | Electric arc
+Rhodium   |   Rh   |  103.  |  12.1   | RhO_{2}      | RhCl_{2} | Electric arc
+Palladium |   Pd   |  106.5 |  11.8   | PdO_{2}      | PdCl_{4} |   1500 deg.
+Iridium   |   Ir   |  193.  |  22.42  | IrO_{2}      | IrCl_{4} |   1950 deg.
+Osmium    |   Os   |  191.  |  22.47  | OsO_{4}      | OsCl_{4} | Electric arc
+Platinum  |   Pt   |  194.8 |  21.50  | PtO_{2}      | PtCl_{4} |   1779 deg.
+Gold      |   Au   |  197.2 |  19.30  | Au_{2}O_{3}  | AuCl_{3} |   1064 deg.
+==============================================================================
+
+~The family.~ Following iron, nickel, and cobalt in the eighth column of
+the periodic table are two groups of three elements each. The metals of
+the first of these groups--ruthenium, rhodium, and palladium--have
+atomic weights near 100 and densities near 12. The metals of the other
+group--iridium, osmium, and platinum--have atomic weights near 200 and
+densities near 21. These six rare elements have very similar physical
+properties and resemble each other chemically not only in the type of
+compounds which they form but also in the great variety of them. They
+occur closely associated in nature, usually as alloys of platinum in the
+form of irregular metallic grains in sand and gravel. Platinum is by far
+the most abundant of the six.
+
+Although the periodic classification assigns gold to the silver-copper
+group, its physical as well as many of its chemical properties much
+more closely resemble those of the platinum metals, and it can he
+conveniently considered along with them. The four elements gold,
+platinum, osmium, and iridium are the heaviest substances known, being
+about twice as heavy as lead.
+
+
+PLATINUM
+
+~Occurrence.~ About 90% of the platinum of commerce comes from Russia,
+small amounts being produced in California, Brazil, and Australia.
+
+~Preparation.~ Native platinum is usually alloyed with gold and the
+platinum metals. To separate the platinum the alloy is dissolved in aqua
+regia, which converts the platinum into chloroplatinic acid
+(H_{2}PtCl_{6}). Ammonium chloride is then added, which precipitates the
+platinum as insoluble ammonium chloroplatinate:
+
+    H_{2}PtCl_{6} + 2NH_{4}Cl = (NH_{4})_{2}PtCl_{6} + 2HCl.
+
+Some iridium is also precipitated as a similar compound. On ignition the
+double chloride is decomposed, leaving the platinum as a spongy metallic
+mass, which is melted in an electric furnace and rolled or hammered into
+the desired shape.
+
+~Physical properties.~ Platinum is a grayish-white metal of high luster,
+and is very malleable and ductile. It melts in the oxyhydrogen blowpipe
+and in the electric furnace; it is harder than gold and is a good
+conductor of electricity. In finely divided form it has the ability to
+absorb or occlude gases, especially oxygen and hydrogen. These gases,
+when occluded, are in a very active condition resembling the nascent
+state, and can combine with each other at ordinary temperatures. A jet
+of hydrogen or coal gas directed upon spongy platinum is at once
+ignited.
+
+     ~Platinum as a catalytic agent.~ Platinum is remarkable for its
+     property of acting as a catalytic agent in a large number of
+     chemical reactions, and mention has been made of this use of
+     the metal in connection with the manufacture of sulphuric acid.
+     When desired for this purpose some porous or fibrous substance,
+     such as asbestos, is soaked in a solution of platinic chloride
+     and then ignited. The platinum compound is decomposed and the
+     platinum deposited in very finely divided form. Asbestos
+     prepared in this way is called platinized asbestos. The
+     catalytic action seems to be in part connected with the
+     property of absorbing gases and rendering them nascent. Some
+     other metals possess this same power, notably palladium, which
+     is remarkable for its ability to absorb hydrogen.
+
+~Chemical properties.~ Platinum is a very inactive element chemically, and
+is not attacked by any of the common acids. Aqua regia slowly dissolves
+it, forming platinic chloride (PtCl_{4}), which in turn unites with the
+hydrochloric acid present in the aqua regia, forming the compound
+chloroplatinic acid (H_{2}PtCl_{6}). Platinum is attacked by fused
+alkalis. It combines at higher temperatures with carbon and phosphorus
+and alloys with many metals. It is readily attacked by chlorine but not
+by oxidizing agents.
+
+~Applications.~ Platinum is very valuable as a material for the
+manufacture of chemical utensils which are required to stand a high
+temperature or the action of strong reagents. Platinum crucibles,
+dishes, forceps, electrodes, and similar articles are indispensable in
+the chemical laboratory. In the industries it is used for such purposes
+as the manufacture of pans for evaporating sulphuric acid, wires for
+sealing through incandescent light bulbs, and for making a great variety
+of instruments. Unfortunately the supply of the metal is very limited,
+and the cost is steadily advancing, so that it is now more valuable than
+gold.
+
+~Compounds.~ Platinum forms two series of salts of which platinous
+chloride (PtCl_{2}) and platinic chloride (PtCl_{4}) are examples.
+Platinates are also known. While a great variety of compounds of
+platinum have been made, the substance is chiefly employed in the
+metallic state.
+
+~Platinic chloride (PtCl_{4}).~ Platinic chloride is an orange-colored,
+soluble compound made by heating chloroplatinic acid in a current of
+chlorine. If hydrochloric acid is added to a solution of the substance,
+the two combine, forming chloroplatinic acid (H_{2}PtCl_{6}):
+
+    2HCl + PtCl_{4} = H_{2}PtCl_{6}.
+
+The potassium and ammonium salts of this acid are nearly insoluble in
+water and alcohol. The acid is therefore used as a reagent to
+precipitate potassium in analytical work. With potassium chloride the
+equation is
+
+    2KCl + H_{2}PtCl_{6} = K_{2}PtCl_{6} + 2HCl.
+
+     ~Other metals of the family.~ The other members of the family
+     have few applications. Iridium is used in the form of a
+     platinum alloy, since the alloy is much harder than pure
+     platinum and is even less fusible. This alloy is sometimes used
+     to point gold pens. Osmium tetroxide (OsO_{4}) is a very
+     volatile liquid and is used under the name of osmic acid as a
+     stain for sections in microscopy.
+
+
+GOLD
+
+~Occurrence.~ Gold has been found in many localities, the most famous
+being South Africa, Australia, Russia, and the United States. In this
+country it is found in Alaska and in nearly half of the states of the
+union, notably in California, Colorado, and Nevada. It is usually found
+in the native condition, frequently alloyed with silver; in combination
+it is sometimes found as telluride (AuTe_{2}), and in a few other
+compounds.
+
+~Mining.~ Native gold occurs in the form of small grains or larger nuggets
+in the sands of old rivers, or imbedded in quartz veins in rocks. In the
+first case it is obtained in crude form by placer mining. The sand
+containing the gold is shaken or stirred in troughs of running waters
+called sluices. This sweeps away the sand but allows the heavier gold to
+sink to the bottom of the sluice. Sometimes the sand containing the gold
+is washed away from its natural location into the sluices by powerful
+streams of water delivered under pressure from pipes. This is called
+hydraulic mining. In vein mining the gold-bearing quartz is mined from
+the veins, stamped into fine powder in stamping mills, and the gold
+extracted by one of the processes to be described.
+
+~Extraction.~ 1. _Amalgamation process._ In the amalgamation process the
+powder containing the gold is washed over a series of copper plates
+whose surfaces have been amalgamated with mercury. The gold sticks to
+the mercury or alloys with it, and after a time the gold and mercury are
+scraped off and the mixture is distilled. The mercury distills off and
+the gold is left in the retort ready for refining.
+
+2. _Chlorination process._ When gold occurs along with metallic
+sulphides it is often extracted by chlorination. The ore is first
+roasted, and is then moistened and treated with chlorine. This dissolves
+the gold but not the metallic oxides:
+
+    Au + 3Cl = AuCl_{3}.
+
+The gold chloride, being soluble, is extracted from the mixture with
+water, and the gold is precipitated from the solution, usually by adding
+ferrous sulphate:
+
+    AuCl_{3} + 3FeSO_{4} = Au + FeCl_{3} + Fe_{2}(SO_{4})_{3}.
+
+3. _Cyanide process._ This process depends upon the fact that gold is
+soluble in a solution of potassium cyanide in the presence of the oxygen
+of the air. The powder from the stamping mills is treated with a very
+dilute potassium cyanide solution which extracts the gold:
+
+    2Au + 4KCN + H_{2}O + O = 2KOH + 2KAu(CN)_{2}.
+
+From this solution the gold can be obtained by electrolysis or by
+precipitation with metallic zinc:
+
+    2KAu(CN)_{2} + Zn = K_{2}Zn(CN)_{4} + 2Au.
+
+~Refining of gold.~ Gold is refined by three general methods:
+
+1. _Electrolysis._ When gold is dissolved in a solution of potassium
+cyanide, and the solution electrolyzed, the gold is deposited in very
+pure condition on the cathode.
+
+2. _Cupellation._ When the gold is alloyed with easily oxidizable
+metals, such as copper or lead, it may be refined by cupellation. The
+alloy is fused with an oxidizing flame on a shallow hearth made of bone
+ash, which substance has the property of absorbing metallic oxides but
+not the gold. Any silver which may be present remains alloyed with the
+gold.
+
+3. _Parting with sulphuric acid._ Gold may be separated from silver, as
+well as from many other metals, by heating the alloy with concentrated
+sulphuric acid. This dissolves the silver, while the gold is not
+attacked.
+
+~Physical properties.~ Gold is a very heavy bright yellow metal,
+exceedingly malleable and ductile, and a good conductor of electricity.
+It is quite soft and is usually alloyed with copper or silver to give it
+the hardness required for most practical uses. The degree of fineness is
+expressed in terms of carats, pure gold being twenty-four carats; the
+gold used for jewelry is usually eighteen carats, eighteen parts being
+gold and six parts copper or silver. Gold coinage is 90% gold and 10%
+copper.
+
+~Chemical properties.~ Gold is not attacked by any one of the common
+acids; aqua regia easily dissolves it, forming gold chloride (AuCl_{3}),
+which in turn combines with hydrochloric acid to form chlorauric acid
+(HAuCl_{4}). Fused alkalis also attack it. Most oxidizing agents are
+without action upon it, and in general it is not an active element.
+
+     ~Compounds.~ The compounds of gold, though numerous and varied in
+     character, are of comparatively little importance and need not
+     be described in detail. The element forms two series of salts
+     in which it acts as a metal: in the aurous series the gold is
+     univalent, the chloride having the formula AuCl; in the auric
+     series it is trivalent, auric chloride having the formula
+     AuCl_{3}. Gold also acts as an acid-forming element, forming
+     such compounds as potassium aurate (KAuO_{2}). Its compounds
+     are very easily decomposed, however, metallic gold separating
+     from them.
+
+
+EXERCISES
+
+1. From the method of preparation of platinum, what metal is likely to
+be alloyed with it?
+
+2. The "platinum chloride" of the laboratory is made by dissolving
+platinum in aqua regia. What is the compound?
+
+3. How would you expect potassium aurate and platinate to be formed?
+What precautions would this suggest in the use of platinum vessels?
+
+4. Why must gold ores be roasted in the chlorination process?
+
+
+
+
+CHAPTER XXXII
+
+SOME SIMPLE ORGANIC COMPOUNDS
+
+
+~Division of chemistry into organic and inorganic.~ Chemistry is usually
+divided into two great divisions,--organic and inorganic. The original
+significance of these terms was entirely different from the meaning
+which they have at the present time.
+
+1. _Original significance._ The division into organic and inorganic was
+originally made because it was believed that those substances which
+constitute the essential parts of living organisms were built up under
+the influence of the life force of the organism. Such substances,
+therefore, should be regarded as different from those compounds prepared
+in the laboratory or formed from the inorganic or mineral constituents
+of the earth. In accordance with this view organic chemistry included
+those substances formed by living organisms. Inorganic chemistry, on the
+other hand, included all substances formed from the mineral portions of
+the earth.
+
+In 1828 the German chemist Woehler prepared urea, a typical organic
+compound, from inorganic materials. The synthesis of other so-called
+organic compounds followed, and at present it is known that the same
+chemical laws apply to all substances whether formed in the living
+organism or prepared in the laboratory from inorganic constituents. The
+terms "organic" and "inorganic" have therefore lost their original
+significance.
+
+2. _Present significance._ The great majority of the compounds found in
+living organisms contain carbon, and the term "organic chemistry," as
+used at present, includes not only these compounds but all compounds of
+carbon. _Organic chemistry_ has become, therefore, _the chemistry of the
+compounds of carbon_, all other substances being treated under the head
+of inorganic chemistry. This separation of the compounds of carbon into
+a group by themselves is made almost necessary by their great number,
+over one hundred thousand having been recorded. For convenience some of
+the simpler carbon compounds, such as the oxides and the carbonates, are
+usually discussed in inorganic chemistry.
+
+~The grouping of compounds in classes.~ The study of organic chemistry is
+much simplified by the fact that the large number of bodies included in
+this field may be grouped in classes of similar compounds. It thus
+becomes possible to study the properties of each class as a whole, in
+much the same way as we study a group of elements. The most important of
+these classes are the _hydrocarbons_, the _alcohols_, the _aldehydes_,
+the _acids_, the _ethereal salts_, the _ethers_, the _ketones_, the
+_organic bases_, and the _carbohydrates_. A few members of each of these
+classes will now be discussed briefly.
+
+THE HYDROCARBONS
+
+Carbon and hydrogen combine to form a large number of compounds. These
+compounds are known collectively as the _hydrocarbons_. They may be
+divided into a number of groups or series, each being named from its
+first member. Some of the groups are as follows:
+
+    METHANE SERIES
+    CH_{4}      methane
+    C_{2}H_{6}  ethane
+    C_{3}H_{8}  propane
+    C_{4}H_{10} butane
+    C_{5}H_{12} pentane
+    C_{6}H_{14} hexane
+    C_{7}H_{16} heptane
+    C_{8}H_{18} octane
+
+    ETHYLENE SERIES
+    C_{2}H_{4} ethylene
+    C_{3}H_{6} propylene
+    C_{4}H_{8} butylene
+
+    BENZENE SERIES
+    C_{6}H_{6}  benzene
+    C_{7}H_{8}  toluene
+    C_{8}H_{10} xylene
+
+    ACETYLENE SERIES
+    C_{2}H_{2} acetylene
+    C_{3}H_{4} allylene
+
+Only the lower members (that is, those which contain a small number of
+carbon atoms) of the above groups are given. The methane series is the
+most extensive, all of the compounds up to C_{24}H_{50} being known.
+
+It will be noticed that the successive members of each of the above
+series differ by the group of atoms (CH_{2}). Such a series is called an
+_homologous series_. In general, it may be stated that the members of an
+homologous series show a regular gradation in most physical properties
+and are similar in chemical properties. Thus in the methane group the
+first four members are gases at ordinary temperatures; those containing
+from five to sixteen carbon atoms are liquids, the boiling points of
+which increase with the number of carbon atoms present. Those containing
+more than sixteen carbon atoms are solids.
+
+~Sources of the hydrocarbons.~ There are two chief sources of the
+hydrocarbons, namely, (1) crude petroleum and (2) coal tar.
+
+1. _Crude petroleum._ This is a liquid pumped from wells driven into the
+earth in certain localities. Pennsylvania, Ohio, Kansas, California, and
+Texas are the chief oil-producing regions in the United States. The
+crude petroleum consists largely of liquid hydrocarbons in which are
+dissolved both gaseous and solid hydrocarbons. Before being used it must
+be refined. In this process the petroleum is run into large iron stills
+and subjected to fractional distillation. The various hydrocarbons
+distill over in the general order of their boiling points. The
+distillates which collect between certain limits of temperature are kept
+separate and serve for different uses; they are further purified,
+generally by washing with sulphuric acid, then with an alkali, and
+finally with water. Among the products obtained from crude petroleum in
+this way are the naphthas, including benzine and gasoline, kerosene or
+coal oil, lubricating oils, vaseline, and paraffin. None of these
+products are definite chemical compounds, but each consists of a mixture
+of hydrocarbons, the boiling points of which lie within certain limits.
+
+2. _Coal tar._ This product is obtained in the manufacture of coal gas,
+as already explained. It is a complex mixture and is refined by the same
+general method used in refining crude petroleum. The principal
+hydrocarbons obtained from the coal tar are benzene, toluene,
+naphthalene, and anthracene. In addition to the hydrocarbons, coal tar
+contains many other compounds, such as carbolic acid and aniline.
+
+~Properties of the hydrocarbons.~ The lower members of the first two
+series of hydrocarbons mentioned are all gases; the succeeding members
+are liquids. In some series, as the methane series, the higher members
+are solids. The preparation and properties of methane and acetylene have
+been discussed in a previous chapter. Ethylene is present in small
+quantities in coal gas and may be obtained in the laboratory by
+treating alcohol (C_{2}H_{6}O) with sulphuric acid:
+
+    C_{2}H_{6}O = C_{2}H_{4} + H_{2}O.
+
+Benzene, the first member of the benzene series, is a liquid boiling at
+80 deg..
+
+The hydrocarbons serve as the materials from which a large number of
+compounds can be prepared; indeed, it has been proposed to call organic
+chemistry _the chemistry of the hydrocarbon derivatives_.
+
+~Substitution products of the hydrocarbons.~ As a rule, at least a part of
+the hydrogen in any hydrocarbon can be displaced by an equivalent amount
+of certain elements or groups of elements. Thus the compounds CH_{3}Cl,
+CH_{2}Cl_{2}, CHCl_{3}, CCl_{4} can be obtained from methane by
+treatment with chlorine. Such compounds are called _substitution
+products_.
+
+~Chloroform~ (CHCl_{3}). This can be made by treating methane with
+chlorine, as just indicated, although a much easier method consists in
+treating alcohol or acetone (which see) with bleaching powder.
+Chloroform is a heavy liquid having a pleasant odor and a sweetish
+taste. It is largely used as a solvent and as an anaesthetic in surgery.
+
+~Iodoform~ (CHI_{3}). This is a yellow crystalline solid obtained by
+treating alcohol with iodine and an alkali. It has a characteristic odor
+and is used as an antiseptic.
+
+
+ALCOHOLS
+
+When such a compound as CH_{3}Cl is treated with silver hydroxide the
+reaction expressed by the following equation takes place:
+
+    CH_{3}Cl + AgOH = CH_{3}OH + AgCl.
+
+Similarly C_{2}H_{5}Cl will give C_{2}H_{5}OH and AgCl. The compounds
+CH_{3}OH and C_{2}H_{5}OH so obtained belong to the class of substances
+known as _alcohols_. From their formulas it will be seen that they may
+be regarded as derived from hydrocarbons by substituting the hydroxyl
+group (OH) for hydrogen. Thus the alcohol CH_{3}OH may be regarded as
+derived from methane (CH_{4}) by substituting the group OH for one atom
+of hydrogen. A great many alcohols are known, and, like the
+hydrocarbons, they may be grouped into series. The relation between the
+first three members of the methane series and the corresponding alcohols
+is shown in the following table:
+
+    CH_{4}     (methane)       CH_{3}OH       (methyl alcohol).
+    C_{2}H_{6} (ethane)        C_{2}H_{5}OH   (ethyl alcohol).
+    C_{3}H_{8} (propane)       C_{3}H_{7}OH   (propyl alcohol).
+
+~Methyl alcohol~ (_wood alcohol_) (CH_{3}OH). When wood is placed in an
+air-tight retort and heated, a number of compounds are evolved, the most
+important of which are the three liquids, methyl alcohol, acetic acid,
+and acetone. Methyl alcohol is obtained entirely from this source, and
+on this account is commonly called _wood alcohol_. It is a colorless
+liquid which has a density of 0.79 and boils at 67 deg.. It burns with an
+almost colorless flame and is sometimes used for heating purposes, in
+place of the more expensive ethyl alcohol. It is a good solvent for
+organic substances and is used especially as a solvent in the
+manufacture of varnishes. It is very poisonous.
+
+~Ethyl alcohol~ (_common alcohol_) (C_{2}H_{5}OH). 1. _Preparation._ This
+compound may be prepared from glucose (C_{6}H_{12}O_{6}), a sugar easily
+obtained from starch. If some baker's yeast is added to a solution of
+glucose and the temperature is maintained at about 30 deg., bubbles of gas
+are soon evolved, showing that a change is taking place. The yeast
+contains a large number of minute organized bodies, which are really
+forms of plant life. The plant grows in the glucose solution, and in so
+doing secretes a substance known as _zymase_, which breaks down the
+glucose in accordance with the following equation:
+
+    C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}.
+
+     ~Laboratory preparation of alcohol.~ The formation of alcohol and
+     carbon dioxide from glucose may be shown as follows: About 100
+     g. of glucose are dissolved in a liter of water in flask A
+     (Fig. 90). This flask is connected with the bottle B, which
+     is partially filled with limewater. The tube C contains solid
+     sodium hydroxide. A little baker's yeast is now added to the
+     solution in flask A, and the apparatus is connected, as shown
+     in the figure. If the temperature is maintained at about 30 deg.,
+     the reaction soon begins. The bubbles of gas escape through the
+     limewater in B. A precipitate of calcium carbonate soon forms
+     in the limewater, showing the presence of carbon dioxide. The
+     sodium hydroxide in tube C prevents the carbon dioxide in the
+     air from acting on the limewater. The alcohol remains in the
+     flask A and may be separated by fractional distillation.
+
+[Illustration: Fig. 90]
+
+2. _Properties._ Ethyl alcohol is a colorless liquid with a pleasant
+odor. It has a density of 0.78 and boils at 78 deg.. It resembles methyl
+alcohol in its general properties. It is sometimes used as a source of
+heat, since its flame is very hot and does not deposit carbon, as the
+flame from oil does. When taken into the system in small quantities it
+causes intoxication; in larger quantities it acts as a poison. The
+intoxicating properties of such liquors as beer, wine, and whisky are
+due to the alcohol present. Beer contains from 2 to 5% of alcohol, wine
+from 5 to 20%, and whisky about 50%. The ordinary alcohol of the
+druggist contains 94% of alcohol and 6% of water. When this is boiled
+with lime and then distilled nearly all the water is removed, the
+distillate being called _absolute alcohol_.
+
+     ~Commercial preparation of alcohol.~ Alcohol is prepared
+     commercially from starch obtained from corn or potatoes. The
+     starch is first converted into a sugar known as maltose, by the
+     action of _malt_, a substance prepared by moistening barley
+     with water, allowing it to germinate, and then drying it. There
+     is present in the malt a substance known as diastase, which has
+     the property of changing starch into maltose. This sugar, like
+     glucose, breaks down into alcohol and carbon dioxide in the
+     presence of yeast. The resulting alcohol is separated by
+     fractional distillation.
+
+     ~Denatured alcohol.~ The 94% alcohol is prepared at present at a
+     cost of about 35 cents per gallon, which is about half the cost
+     of the preparation of methyl alcohol. The government, however,
+     imposes a tax on all ethyl alcohol which amounts to $2.08 per
+     gallon on the 94% product. This increases its cost to such an
+     extent that it is not economical to use it for many purposes
+     for which it is adapted, such as a solvent in the preparation
+     of paints and varnishes and as a material for the preparation
+     of many important organic compounds. By an act of Congress in
+     1906, the tax was removed from _denatured_ alcohol, that is
+     alcohol mixed with some substance which renders it unfit for
+     the purposes of a beverage but will not impair its use for
+     manufacturing purposes. Some of the European countries have
+     similar laws. The substances ordinarily used to denature
+     alcohol are wood alcohol and pyridine, the latter compound
+     having a very offensive odor.
+
+     ~Fermentation.~ The reaction which takes place in the preparation
+     of ethyl alcohol belongs to the class of changes known under
+     the general name of fermentation. Thus we say that the yeast
+     causes the glucose to ferment, and the process is known as
+     alcoholic fermentation. There are many kinds of fermentations,
+     and each is thought to be due to the presence of a definite
+     substance known as an _enzyme_, which acts by catalysis. In
+     many cases, as in alcoholic fermentation, the change is brought
+     about by the action of minute forms of life. These probably
+     secrete the enzymes which cause the fermentation to take place.
+     Thus the yeast plant is supposed to bring about alcoholic
+     fermentation by secreting the enzyme known as zymase.
+
+~Glycerin~ (C_{3}H_{5}(OH)_{3}). This compound may be regarded as derived
+from propane (C_{3}H_{8}) by displacing three atoms of hydrogen by three
+hydroxyl groups, and must therefore be regarded as an alcohol. It is
+formed in the manufacture of soaps, as will be explained later. It is an
+oily, colorless liquid having a sweetish taste. It is used in medicine
+and in the manufacture of the explosives nitroglycerin and dynamite.
+
+
+ALDEHYDES
+
+When alcohols are treated with certain oxidizing agents two hydrogen
+atoms are removed from each molecule of the alcohol. The resulting
+compounds are known as aldehydes. The relation of the aldehydes derived
+from methyl and ethyl alcohol to the alcohols themselves may be shown as
+follows:
+
+    Alcohols  {CH_{3}OH          Corresponding aldehydes  {CH_{2}O
+              {C_{2}H_{5}OH                               {C_{2}H_{4}O
+
+The first of these (CH_{2}O) is a gas known as formaldehyde. Its aqueous
+solution is largely used as an antiseptic and disinfectant under the
+name of _formalin_. Acetaldehyde (C_{2}H_{4}O) is a liquid boiling at
+21 deg..
+
+
+ACIDS
+
+Like the other classes of organic compounds, the organic acids may be
+arranged in homologous series. One of the most important of these series
+is the _fatty-acid series_, the name having been given to it because
+the derivatives of certain of its members are constituents of the fats.
+Some of the most important members of the series are given in the
+following table. They are all monobasic, and this fact is expressed in
+the formulas by separating the replaceable hydrogen atom from the rest
+of the molecule:
+
+    H.CHO_{2}            formic acid, a liquid boiling at 100 deg..
+    H.C_{2}H_{3}O        acetic acid, a liquid boiling at 118 deg..
+    H.C_{3}H_{5}O_{2}    propionic acid, a liquid boiling at 140 deg..
+    H.C_{4}H_{7}O_{2}    butyric acid, a liquid boiling at 163 deg..
+    H.C_{16}H_{31}O_{2}  palmitic acid, a solid melting at 62 deg..
+    H.C_{18}H_{35}O_{2}  stearic acid, a solid melting at 69 deg..
+
+~Formic acid~ (H.CHO_{2}). The name "formic" is derived from the Latin
+_formica_, signifying ant. This name was given to the acid because it
+was formerly obtained from a certain kind of ants. It is a colorless
+liquid and occurs in many plants such as the stinging nettles. The
+inflammation caused by the sting of the bee is due to formic acid.
+
+~Acetic acid~ (H.C_{2}H_{3}O_{2}). Acetic acid is the acid present in
+vinegar, the sour taste being due to it. It can be prepared by either of
+the following methods.
+
+1. _Acetic fermentation._ This consists in the change of alcohol into
+acetic acid through the agency of a minute organism commonly called
+mother of vinegar. The change is represented by the following equation:
+
+    C_{2}H_{5}OH + 2O = HC_{2}H_{3}O_{2} + H_{2}O.
+
+The various kinds of vinegars are all made by this process. In the
+manufacture of cider vinegar the sugar present in the cider first
+undergoes alcoholic fermentation; the resulting alcohol then undergoes
+acetic fermentation. The amount of acetic acid present in vinegars
+varies from 3 to 6%.
+
+2. _From the distillation of wood._ The liquid obtained by heating wood
+in the absence of air contains a large amount of acetic acid, and this
+can be separated readily in a pure state. This is the most economical
+method for the preparation of the concentrated acid.
+
+Acetic acid is a colorless liquid and has a strong pungent odor.
+Many of its salts are well-known compounds. Lead acetate
+(Pb(C_{2}H_{3}O_{2})_{2}) is the ordinary _sugar of lead_. Sodium
+acetate (NaC_{2}H_{3}O_{2}) is a white solid largely used in making
+chemical analyses. Copper acetate (Cu(C_{2}H_{3}O_{2})_{2}) is a blue
+solid. When copper is acted upon by acetic acid in the presence of air a
+green basic acetate of copper is formed. This is commonly known as
+verdigris. All acetates are soluble in water.
+
+~Butyric acid~ (H.C_{4}H_{7}O_{2}). Derivatives of butyric acid are
+present in butter and impart to it its characteristic flavor.
+
+~Palmitic and stearic acids.~ Ordinary fats consist principally of
+derivatives of palmitic and stearic acids. When the fats are heated with
+sodium hydroxide the sodium salts of these acids are formed. If
+hydrochloric acid is added to a solution of the sodium salts, the free
+palmitic and stearic acids are precipitated. They are white solids,
+insoluble in water. Stearic acid is often used in making candles.
+
+~Acids belonging to other series.~ In addition to members of the
+fatty-acid series, mention may be made of the following well-known
+acids.
+
+~Oxalic acid~ (H_{2}C_{2}O_{4}). This is a white solid which occurs in
+nature in many plants, such as the sorrels. Its ammonium salt
+((NH_{4})_{2}C_{2}O_{4}) is used as a reagent for the detection of
+calcium. When added to a solution of a calcium compound the white,
+insoluble calcium oxalate (CaC_{2}O_{4}) precipitates.
+
+~Tartaric acid~ (H_{2}.C_{4}H_{4}O_{6}). This compound occurs either in a
+free state or in the form of its salts in many fruits. The potassium
+acid salt (KHC_{4}H_{4}O_{6}) occurs in the juice of grapes. When the
+juice ferments in the manufacture of wine, this salt, being insoluble in
+alcohol, separates out on the sides of the cask and in this form is
+known as argol. This is more or less colored by the coloring matter of
+the grape. When purified it forms a white solid and is sold under the
+name of cream of tartar. The following are also well-known salts of
+tartaric acid: potassium sodium tartrate (Rochelle salt)
+(KNaC_{4}H_{4}O_{6}), potassium antimonyl tartrate (tartar emetic)
+(KSbOC_{4}H_{4}O_{6}).
+
+     ~Cream of tartar baking powders.~ The so-called cream of tartar
+     baking powders consist of a mixture of cream of tartar,
+     bicarbonate of soda, and some starch or flour. When water is
+     added to this mixture the cream of tartar slowly acts upon the
+     soda present liberating carbon dioxide in accordance with the
+     following equation:
+
+     KHC_{4}H_{4}O_{6} + NaHCO_{3} = KNaC_{4}H_{4}O_{6} + H_{2}O +
+     CO_{2}.
+
+     The carbon dioxide evolved escapes through the dough, thus
+     making it light and porous.
+
+~Citric acid~ (H_{3}.C_{6}H_{5}O_{7}). This acid occurs in many fruits,
+especially in lemons. It is a white solid, soluble in water, and is
+often used as a substitute for lemons in making lemonade.
+
+~Lactic acid~ (H.C_{3}H_{5}O_{3}). This is a liquid which is formed in the
+souring of milk.
+
+~Oleic acid~ (H.C_{18}H_{33}O_{2}). The derivatives of this acid
+constitute the principal part of many oils and liquid fats. The acid
+itself is an oily liquid.
+
+
+ETHEREAL SALTS
+
+When acids are brought in contact with alcohols under certain conditions
+a reaction takes place similar to that which takes place between acids
+and bases. The following equations will serve as illustrations:
+
+    KOH + HNO_{3} = KNO_{3} + H_{2}O,
+
+    CH_{3}OH + HNO_{3} = CH_{3}NO_{3} + H_{2}O.
+
+The resulting compounds of which methyl nitrate (CH_{3}NO_{3}) may be
+taken as the type belong to the class known as _ethereal salts_, the
+name having been given them because some of them possess pleasant
+ethereal odors. It will be seen that the ethereal salts differ from
+ordinary salts in that they contain a hydrocarbon radical, such as
+CH_{3}, C_{2}H_{5}, C_{3}H_{5}, in place of a metal.
+
+~The nitrates of glycerin~ (_nitroglycerin_). Nitric acid reacts with
+glycerin in the same way that it reacts with a base containing three
+hydroxyl groups such as Fe(OH)_{3}:
+
+    Fe(OH)_{3} + 3HNO_{3} = Fe(NO_{3})_{3} + 3H_{2}O,
+
+    C_{3}H_{5}(OH)_{3} + 3HNO_{3} = C_{3}H_{5}(NO_{3})_{3} + 3H_{2}O.
+
+The resulting nitrate (C_{3}H_{5}(NO_{3})_{3}) is the main constituent
+of _nitroglycerin_, a slightly yellowish oil characterized by its
+explosive properties. Dynamite consists of porous earth which has
+absorbed nitroglycerin, and its strength depends on the amount present.
+It is used much more largely than nitroglycerin itself, since it does
+not explode so readily by concussion and hence can be transported with
+safety.
+
+~The fats.~ These are largely mixtures of the ethereal salts known
+respectively as olein, palmitin, and stearin. These salts may be
+regarded as derived from oleic, palmitic, and stearic acids
+respectively, by replacing the hydrogen of the acid with the glycerin
+radical C_{3}H_{5}. Since this radical is trivalent and oleic, palmitic,
+and stearic acids contain only one replaceable hydrogen atom to the
+molecule, it is evident that three molecules of each acid must enter
+into each molecule of the ethereal salt. The formulas for the acids and
+the ethereal salts derived from each are as follows:
+
+    HC_{18}H_{33}O_{2}                 (oleic acid)
+    C_{8}H_{6}(C_{18}H_{33}O_{2})_{3}, (olein)
+
+    HC_{16}H_{31}O_{2}                (palmitic acid)
+    C_{3}H_{5}(C_{16}H_{31}0_{2})_{3} (palmitin)
+
+    HC_{18}H_{35}O_{2}                (stearic acid)
+    C_{3}H_{5}(C_{18}H_{35}O_{2})_{3} (stearin)
+
+Olein is a liquid and is the main constituent of liquid fats. Palmitin
+and stearin are solids.
+
+~Butter fat and oleomargarine.~ Butter fat consists principally of olein,
+palmitin, and stearin. The flavor of the fat is due to the presence of a
+small amount of butyrin, which is an ethereal salt of butyric acid.
+Oleomargarine differs from butter mainly in the fact that a smaller
+amount of butyrin is present. It is made from the fats obtained from
+cattle and hogs. This fat is churned up with milk, or a small amount of
+butter is added, in order to furnish sufficient butyrin to impart the
+butter flavor.
+
+~Saponification.~ When an ethereal salt is heated with an alkali a
+reaction expressed by the following equation takes place:
+
+    C_{2}H_{5}NO_{3} + KOH = C_{2}H_{5}OH + KNO_{3}.
+
+This process is known as _saponification_, since it is the one which
+takes place in the manufacture of soaps. The ordinary soaps are made by
+heating fats with a solution of sodium hydroxide. The reactions
+involved may be illustrated by the following equation representing the
+reaction between palmitin and sodium hydroxide:
+
+    C_{3}H_{5}(C_{16}H_{31}O_{2})_{3} + 3 NaOH
+         = 3 NaC_{16}H_{31}O_{2} + C_{3}H_{5}(OH)_{3}.
+
+In accordance with this equation the ethereal salts in the fats are
+converted into glycerin and the sodium salts of the corresponding acids.
+The sodium salts are separated and constitute the soaps. These salts are
+soluble in water. When added to water containing calcium salts the
+insoluble calcium palmitate and stearate are precipitated. Magnesium
+salts act in a similar way. It is because of these facts that soap is
+used up by hard waters.
+
+
+ETHERS
+
+When ethyl alcohol is heated to 140 deg. with sulphuric acid the reaction
+expressed by the following equation takes place:
+
+    2C_{2}H_{5}OH = (C_{2}H_{5})_{2}O + H_{2}O.
+
+The resulting compound, (C_{2}H_{5})_{2}O, is ordinary ether and is the
+most important member of the class of compounds called _ethers_.
+Ordinarily ether is a light, very inflammable liquid boiling at 35 deg.. It
+is used as a solvent for organic substances and as an anaesthetic in
+surgical operations.
+
+
+KETONES
+
+The most common member of this group is acetone (C_{3}H_{6}O), a
+colorless liquid obtained when wood is heated in the absence of air. It
+is used in the preparation of other organic compounds, especially
+chloroform.
+
+
+ORGANIC BASES
+
+This group includes a number of compounds, all of which contain nitrogen
+as well as carbon. They are characterized by combining directly with
+acids to form salts, and in this respect they resemble ammonia. They
+may, indeed, be regarded as derived from ammonia by displacing a part or
+all of the hydrogen present in ammonia by hydrocarbon radicals. Among
+the simplest of these compounds may be mentioned methylamine
+(CH_{3}NH_{2}) and ethylamine (C_{2}H_{5}NH_{2}). These two compounds
+are gases and are formed in the distillation of wood and bones. Pyridine
+(C_{5}H_{6}N) and quinoline (C_{9}H_{7}N) are liquids present in small
+amounts in coal tar, and also in the liquid obtained by the distillation
+of bones. Most of the compounds now classified under the general name of
+_alkaloids_ (which see) also belong to this group.
+
+
+CARBOHYDRATES
+
+The term "carbohydrate" is applied to a class of compounds which
+includes the sugars, starch, and allied bodies These compounds contain
+carbon, hydrogen, and oxygen the last two elements generally being
+present in the proportion in which they combine to form water. The most
+important members of this class are the following:
+
+    Cane sugar      C_{12}H_{22}O_{11}.
+    Milk sugar      C_{12}H_{22}O_{11}.
+    Dextrose        C_{6}H_{12}O_{6}.
+    Levulose        C_{6}H_{12}O_{6}.
+    Cellulose       C_{6}H_{10}O_{5}.
+    Starch          C_{6}H_{10}0_{5}.
+
+~Cane sugar~ (C_{12}H_{22}O_{11}). This is the well-known substance
+commonly called sugar. It occurs in many plants especially in the sugar
+cane and sugar beet. It was formerly obtained almost entirely from the
+sugar cane, but at present the greatest amount of it comes from the
+sugar beet. The juice from the cane or beet contains the sugar in
+solution along with many impurities. These impurities are removed, and
+the resulting solution is then evaporated until the sugar crystallizes
+out. The evaporation is conducted in closed vessels from which the air
+is partially exhausted. In this way the boiling point of the solution is
+lowered and the charring of the sugar is prevented. It is impossible to
+remove all the sugar from the solution. In preparing sugar from sugar
+cane the liquors left after separating as much of it as possible from
+the juice of the cane constitute ordinary molasses. Maple sugar is made
+by the evaporation of the sap obtained from a species of the maple tree.
+Its sweetness is due to the presence of cane sugar, other products
+present in the maple sap imparting the distinctive flavor.
+
+When a solution of cane sugar is heated with hydrochloric or other
+dilute mineral acid, two compounds, dextrose and levulose, are formed in
+accordance with the following equation:
+
+    C_{12}H_{22}O_{11} + H_{2}O = C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}.
+
+This same change is brought about by the action of an enzyme present in
+the yeast plant. When yeast is added to a solution of cane sugar
+fermentation is set up. The cane sugar, however, does not ferment
+directly: the enzyme in the yeast first transforms the sugar into
+dextrose and levulose, and these sugars then undergo alcoholic
+fermentation.
+
+When heated to 160 deg. cane sugar melts; if the temperature is increased to
+about 215 deg., a partial decomposition takes place and a brown substance
+known as caramel forms. This is used largely as a coloring matter.
+
+~Milk sugar~ (C_{12}H_{22}O_{11}). This sugar is present in the milk of
+all mammals. The average composition of cow's milk is as follows:
+
+    Water                        87.17%
+    Casein (nitrogenous matter)   3.56
+    Butter fat                    3.64
+    Milk sugar                    4.88
+    Mineral matter                0.75
+
+When _rennin_, an enzyme obtained from the stomach of calves, is added
+to milk, the casein separates and is used in the manufacture of cheese.
+The remaining liquid contains the milk sugar which separates on
+evaporation; it resembles cane sugar in appearance but is not so sweet
+or soluble. The souring of milk is due to the fact that the milk sugar
+present undergoes _lactic fermentation_ in accordance with the equation
+
+    C_{12}H_{22}O_{11} + H_{2}O = 4C_{3}H_{6}O_{3}.
+
+The lactic acid formed causes the separation of the casein, thus giving
+the well-known appearance of sour milk.
+
+~Isomeric compounds.~ It will be observed that cane sugar and milk sugar
+have the same formulas. Their difference in properties is due to the
+different arrangement of the atoms in the molecule. Such compounds are
+said to be isomeric. Dextrose and levulose are also isomeric.
+
+~Dextrose~ (_grape sugar, glucose_) (C_{6}H_{12}O_{6}). This sugar is
+present in many fruits and is commonly called grape sugar because of its
+presence in grape juice. It can be obtained by heating cane sugar with
+dilute acids, as explained above; also by heating starch with dilute
+acids, the change being as follows:
+
+    C_{6}H_{10}6_{5} + H_{2}O = C_{6}H_{12}O_{6}.
+
+Pure dextrose is a white crystalline solid, readily soluble in water,
+and is not so sweet as cane sugar. In the presence of yeast it undergoes
+alcoholic fermentation. It is prepared from starch in large quantities,
+and being less expensive than cane sugar, is used as a substitute for it
+in the manufacture of jellies, jams, molasses, candy, and other sweets.
+The product commonly sold under the name of _glucose_ contains about 45%
+of dextrose.
+
+~Levulose~ _(fruit sugar)_(C_{6}H_{12}O_{6}). This sugar is a white solid
+which occurs along with dextrose in fruits and honey. It undergoes
+alcoholic fermentation in the presence of yeast.
+
+~Cellulose~ (C_{6}H_{10}O_{5}). This forms the basis of all woody fibers.
+Cotton and linen are nearly pure cellulose. It is insoluble in water,
+alcohol, and dilute acids. Sulphuric acid slowly converts it into
+dextrose. Nitric acid forms nitrates similar to nitroglycerin in
+composition and explosive properties. These nitrates are variously known
+as nitrocellulose, pyroxylin, and gun cotton. When exploded they yield
+only colorless gases; hence they are used especially in the manufacture
+of smokeless gunpowder. _Collodion_ is a solution of nitrocellulose in a
+mixture of alcohol and ether. _Celluloid_ is a mixture of nitrocellulose
+and camphor. _Paper_ consists mainly of cellulose, the finer grades
+being made from linen and cotton rags, and the cheaper grades from straw
+and wood.
+
+~Starch~ (C_{6}H_{10}O_{5}). This is by far the most abundant carbohydrate
+found in nature, being present especially in seeds and tubers. In the
+United States it is obtained chiefly from corn, nearly 80% of which is
+starch. In Europe it is obtained principally from the potato. It
+consists of minute granules and is practically insoluble in cold water.
+These granules differ somewhat in appearance, according to the source of
+the starch, so that it is often possible to determine from what plant
+the starch was obtained. When heated with water the granules burst and
+the starch partially dissolves. Dilute acids, as well as certain
+enzymes, convert it into dextrose or similar sugars. When seeds
+germinate the starch present is converted into soluble sugars, which are
+used as food for the growing plant.
+
+~Chemical changes in bread making.~ The average composition of wheat flour
+is as follows:
+
+    Water.                           13.8%
+    Protein (nitrogenous matter)      7.9
+    Fats                              1.4
+    Starch                           76.4
+    Mineral matter                    0.5
+
+In making bread the flour is mixed with water and yeast, and the
+resulting dough set aside in a warm place for a few hours. The yeast
+first converts a portion of the starch into dextrose or a similar sugar,
+which then undergoes alcoholic fermentation. The carbon dioxide formed
+escapes through the dough, making it light and porous. The yeast plant
+thrives best at about 30 deg.; hence the necessity for having the dough in a
+warm place. If the temperature rises above 50 deg., the vitality of the
+yeast is destroyed and fermentation ceases. In baking the bread, the
+heat expels the alcohol and also expands the bubbles of carbon dioxide
+caught in the dough, thus increasing its lightness.
+
+
+SOME DERIVATIVES OF BENZENE
+
+Attention has been called to the complex nature of coal tar. Among the
+compounds present are the hydrocarbons, benzene, toluene, naphthalene,
+and anthracene. These compounds are not only useful in themselves but
+serve for the preparation of many other important compounds known under
+the general name of coal-tar products.
+
+~Nitrobenzene~ (_oil of myrbane_) (C_{6}H_{5}NO_{2}). When benzene is
+treated with nitric acid a reaction takes place which is expressed by
+the following equation:
+
+    C_{6}H_{6} + HNO_{3} = C_{6}H_{5}NO_{2} + H_{2}O.
+
+The product C_{6}H_{5}NO_{2} is called nitrobenzene. It is a slightly
+yellowish poisonous liquid, with a characteristic odor. Its main use is
+in the manufacture of aniline.
+
+~Aniline~ (C_{6}H_{5}NH_{2}). When nitrobenzene is heated with iron and
+hydrochloric acid the hydrogen evolved by the action of the iron upon
+the acid reduces the nitrobenzene in accordance with the following
+equation:
+
+    C_{6}H_{5}NO_{2} + 6H = C_{6}H_{5}NH_{2} + 2H_{2}O.
+
+The resulting compound is known as aniline, a liquid boiling at 182 deg..
+When first prepared it is colorless, but darkens on standing. Large
+quantities of it are used in the manufacture of the _aniline or coal-tar
+dyes_, which include many important compounds.
+
+~Carbolic acid~ (C_{6}H_{5}OH). This compound, sometimes known as
+_phenol_, occurs in coal tar, and is also prepared from benzene. It
+forms colorless crystals which are very soluble in water. It is strongly
+corrosive and very poisonous.
+
+~Naphthalene and anthracene.~ These are hydrocarbons occurring along with
+benzene in coal tar. They are white solids, insoluble in water. The
+well-known _moth balls_ are made of naphthalene. Large quantities of
+naphthalene are used in the preparation of _indigo_, a dye formerly
+obtained from the indigo plant, but now largely prepared by laboratory
+methods. Similarly anthracene is used in the preparation of the dye
+_alizarin_, which was formerly obtained from the madder root.
+
+
+THE ALKALOIDS
+
+This term is applied to a group of compounds found in many plants and
+trees. They all contain nitrogen, and most of them are characterized by
+their power to combine with acids to form salts. This property is
+indicated by the name alkaloids, which signifies alkali-like. The salts
+are soluble in water, and on this account are more largely used than the
+free alkaloids, which are insoluble in water. Many of the alkaloids are
+used in medicine, some of the more important ones being given below.
+
+~Quinine.~ This alkaloid occurs along with a number of others in the bark
+of certain trees which grow in districts in South America and also in
+Java and other tropical islands. It is a white solid, and its sulphate
+is used in medicine in the treatment of fevers.
+
+~Morphine.~ When incisions are made in the unripe capsules of one of the
+varieties of the poppy plant, a milky juice exudes which soon thickens.
+This is removed and partially dried. The resulting substance is the
+ordinary _opium_ which contains a number of alkaloids, the principal one
+being morphine. This alkaloid is a white solid and is of great service
+in medicine.
+
+Among the other alkaloids may be mentioned the following: _Nicotine_, a
+very poisonous liquid, the salts of which occur in the leaves of the
+tobacco plant; _cocaine_, a crystalline solid present in coca leaves and
+used in medicine as a local anaesthetic; _atropine_, a solid present in
+the berry of the deadly nightshade, and used in the treatment of
+diseases of the eye; _strychnine_, a white, intensely poisonous solid
+present in the seeds of the members of the _Strychnos_ family.
+
+
+
+
+INDEX
+
+
+Acetaldehyde 405
+
+Acetic acid 406
+
+Acetone 411
+
+Acetylene 203
+  series 399
+
+Acids 106
+  binary 113
+  characteristics 106
+  definition 107
+  dibasic 159
+  familiar 106
+  monobasic 159
+  nomenclature 113
+  organic 405
+  preparation 141
+  strength 111
+  ternary 113
+  undissociated 107
+
+Acker furnace, 279
+
+Agate 260
+
+Air 83
+  a mechanical mixture 89
+  carbon dioxide in 87
+  changes in composition 87
+  liquid 91
+  nitrogen in 87
+  oxygen in 85
+  poisonous effects of exhaled 88
+  properties 90
+  quantitative analysis of 85
+  regarded as an element 83
+  standard for density 229
+  water vapor in 87
+
+Alabaster 308
+
+Alchemists 9
+
+Alchemy 9
+
+Alcohol, common 402
+  denatured 404
+  ethyl 402
+  methyl 402
+  wood 402
+
+Alcohols 401
+
+Aldehydes 405
+
+Alizarin 418
+
+Alkali 107, 274
+  family 274
+
+Alkaline-earth family 300
+
+Alkaloids 418
+
+Allotropic forms 22
+
+Alloys 252
+
+Alum 333
+  ammonium 334
+  ammonium chrome 384
+  ammonium iron 352
+  baking powders 335
+  potassium 333
+  potassium chrome 384
+  potassium iron 352
+
+Aluminates 332
+
+Aluminium 327
+  bronze 330, 359
+  chloride 333
+  family 327
+  hydroxide 332
+  metallurgy 328
+  occurrence 327
+  oxide 331
+  preparation 328
+  properties 329
+  silicates 335
+  uses 330
+
+Amalgam 362
+
+Amethyst 260, 331
+
+Ammonia 123
+  composition 127
+  preparation 123
+  properties 124
+  uses 125
+
+Ammonium 126
+  acid carbonate 295
+  carbonate 295
+  chloride 294
+  compounds 294
+
+Ammonium hydrosulphide 296
+  hydroxide 126
+  molybdate 388
+  oxalate 407
+  sulphate 295
+  sulphide 295
+  sulphide, yellow 296
+
+Analysis 40
+
+Anhydride 135
+  carbonic 206
+  chromic 387
+  nitric 135
+  nitrous 135
+  phosphoric 243
+  sulphuric 153
+
+Anhydrite 288
+
+Aniline 417
+
+Anion 106
+
+Anode 99
+
+Anthracene 418
+
+Antimony 250
+  acids 251
+  alloys 253
+  chloride 252
+  metallic properties 252
+  occurrence 251
+  oxides 251
+  preparation 251
+  properties 251
+  sulphides 251
+
+Apatite 175, 239, 311
+
+Aqua ammonia 124
+
+Aqua regia 185
+
+Aqueous tension 25
+
+Argon 80
+
+Arsenic 246
+  acids 250
+  antidote 250
+  Marsh's test 248
+  occurrence 246
+  oxides 249
+  preparation 246
+  properties 247
+  sulphides 250
+  white 249
+
+Arsenopyrites 246
+
+Arsine 247
+
+Asbestos 321, 336
+
+Atmosphere 83
+  constituents 83
+  function of constituents 84
+
+Atomic hypothesis 61
+  theory 59
+    and laws of matter 63
+    and radium 314
+  weights, 65
+    accurate determination 231
+    and general properties 167
+    and specific heats 233
+    calculation of 231
+    Dalton's method 223
+    direct determination 233
+    from molecular weights 230
+    relation to equivalent 224
+    standard for 66
+    steps in determining 224
+
+Atoms 62
+  size 65
+
+Atropine 419
+
+Aurates 396
+
+Avogadro's hypothesis 226
+  and chemical calculations 235
+  and molecular weights 227
+
+Azote 78
+
+Azurite 357
+
+
+Babbitt metal 253
+
+Bacteria 85
+  decomposition of organic matter by 122
+  nitrifying 85
+
+Baking powders 285, 408
+  alum 335
+  soda 285
+
+Barium 312
+  chloride 313
+  nitrate 313
+  oxides 312
+  sulphate 313
+
+Barytes 312
+
+Bases 107
+  characteristics 107
+  definition 108
+  familiar 107
+  nomenclature 113
+  organic 412
+  strength 113
+  undissociated 108
+
+Basic lining process 346
+
+Bauxite 332
+
+Beer 404
+
+Benzene 417
+  derivatives 417
+  series 399
+
+Benzine 400
+
+Bessemer process 345
+
+Bismuth 253
+  basic salts 255
+  chloride 253
+  nitrate 253
+  occurrence 253
+  oxides 254
+  preparation 253
+  salts, hydrolysis of 254
+  subnitrate 256
+  uses 253
+
+Bismuthyl chloride 256
+
+Blast furnace 341
+  lamp 38
+
+Bleaching powder 306
+
+Bleaching by chlorine 181
+  by sulphurous acid 152
+
+Boiler scale 320
+
+Bone ash 311
+
+Bone black 200
+
+Borax 265
+  bead 266
+
+Bornite 357
+
+Boron 257, 264
+  acids 265
+  fluoride 264
+  hydride 264
+  occurrence 264
+  oxides 264
+  preparation 264
+  properties 264
+
+Brass 323
+
+Bread making 416
+
+Bromides 190
+
+Bromine 187
+  occurrence 187
+ oxygen compounds 190
+  preparation 187
+  properties 188
+
+Bronze 359
+  aluminium 330, 359
+
+Butter fat 410
+
+Butyric acid 407
+
+By-product 284
+
+
+Cadmium 325
+  compounds 326
+
+Caesium 294
+
+Calamine 321
+
+Calcite 305
+
+Calcium 301
+  carbide 203, 310
+  carbonate 305
+  chloride 306
+  fluoride 308
+  hydroxide 303
+  occurrence 301
+  oxide 302
+  phosphate 246, 311
+  preparation 302
+  sulphate 308
+
+Calomel 363
+
+Calorie 76
+
+Caramel 414
+
+Carbohydrates 413
+
+Carbolic acid 417
+
+Carbon 196
+  allotropic forms 196
+  amorphous 198
+  compounds 196
+  crystalline forms 197
+  cycle in nature 88
+  dioxide 204
+    and bases 206
+    and plant life 88
+    in air 87
+    occurrence 204
+    preparation 204
+    properties 204
+    solid 204
+  disulphide 160, 210
+  family 196
+  hydrogen compounds 201
+  monoxide 208
+  occurrence 196
+  oxides 203
+  properties 200
+  pure 198
+  retort 199
+  uses 200
+
+Carbonates 207
+  acid 207
+
+Carbonic acid 206
+
+Carborundum 259
+
+Carnallite 288
+
+Casein 414
+
+Cassiterite 370
+
+Catalysis 153
+
+Catalyzers 153
+
+Cathode 99
+
+Cation 106
+
+Caustic potash 288
+  soda 278
+
+Celestite 312
+
+Celluloid 415
+
+Cellulose 415
+
+Cement 304
+
+Ceramic industries 336
+
+Cerium 377
+
+Chalcedony 260
+
+Chalcocite 357
+
+Chalcopyrite 357
+
+Chalk 305
+
+Chamber acid 157
+
+Changes, physical and chemical 2
+
+Charcoal 199
+
+Chemical affinity 12
+  changes 2
+  compounds 7
+  equilibrium 128
+  properties 3
+
+Chemistry, definition 4
+
+Chili saltpeter 191, 285
+
+Chinaware 336
+
+Chloric acid 187
+
+Chlorides 186
+
+Chlorine 177
+  bleaching action 181
+  chemical properties 180
+  family 174
+  historical 177
+  occurrence 178
+  oxides 187
+  oxygen acids 187
+  preparation 178
+  properties 179
+
+Chloroform 401
+
+Chloroplatinic acid 393
+
+Chlorous acid 187
+
+Chromates 385
+
+Chrome alum 384
+
+Chromic acid 388
+  anhydride 387
+  chloride 383
+  hydroxide 383
+  sulphate 384
+  sulphide 384
+
+Chromite 383
+
+Chromium 383
+  a base-forming element 383
+  an acid-forming element 385
+  occurrence 383
+
+Cinnabar 363
+
+Citric acid 408
+
+Clay 336
+
+Coal 199
+  gas 217
+  products 400
+  tar 218
+
+Cobalt 354
+  compounds 354
+
+Cocaine 419
+
+Coke 199
+
+Collodion 415
+
+Colemanite 265
+
+Combining weights 225
+
+Combustion 17
+  broad sense 20
+  in air 19
+  phlogiston theory 19
+  products 18
+  spontaneous 20
+  supporters 213
+
+Compounds, chemical 7
+  isomeric 414
+  of metals, preparation 265
+  structure of 118
+
+Conservation of energy 4
+  of matter 5
+
+Contact process 154
+
+Converter, Bessemer 345
+
+Copper 357
+  acetate 407
+  alloys of 359
+  family 356
+  hydroxide 360
+  metallurgy 357
+  occurrence 357
+  ores 357
+  oxide 360
+  properties 358
+  refining 358
+  sulphate 361
+  sulphide 361
+  uses 359
+
+Copperas 350
+
+Coral 305
+
+Corrosive sublimate 363
+
+Corundum 331
+
+Cream of tartar 408
+
+Crocoisite 383
+
+Cryolite 175, 328
+
+Crystallization 98
+  water of 54, 75
+
+Crystallography 161
+
+Crystals 161
+  axes of 161
+  systems 162
+
+Cupric compounds 360
+
+Cuprite 360
+
+Cuprous compounds 360
+  chloride 360
+  oxide 360
+
+Cyanides 210
+  solutions are alkaline 210
+
+
+Dalton's atomic hypothesis 61
+
+Decay 21
+
+Decomposition of organic matter 122
+
+Decrepitation 55
+
+Deliquescence 55
+
+Density of gases 230
+
+Desiccating agents 55
+
+Developers 367
+
+Dewar bulb 91
+
+Dextrose 414
+
+Diamond 197
+
+Dichromates 385
+
+Dichromic acid 385
+
+Dimorphous substances 163
+
+Dissociation 99
+  and boiling point 101
+  and freezing point 101
+  equations of 112
+  extent of 113
+
+Distillation 50
+
+Dogtooth spar 306
+
+Dolomite 319
+
+Double decomposition 71
+
+Drummond light 38
+
+Dyeing  333
+
+Dynamite 409
+
+
+Earth metals 327
+
+Efflorescence 54
+
+Electric furnace 221
+
+Electro-chemical industries 269
+
+Electrode 99
+
+Electrolysis 99
+  of sodium chloride 102
+  of sodium sulphate 103
+  of water 41, 102
+
+Electrolytes 99
+
+Electrolytic dissociation 99
+
+Electroplating 366
+
+Electrotyping 359
+
+Elements, definition 8
+  atomic weights 232
+  earlier classification 165
+  names 11
+  natural groups 165
+  number of 9
+  occurrence 10
+  periodic division 166
+  physical state 10
+  symbols of 11
+
+Emery 331
+
+Energy 4
+  and plant life 89
+  chemical 5
+  conservation of 4
+  transformation of 5
+
+Enzyme 405
+
+Epsom salts 320
+
+Equations 68
+  are quantitative 72
+  knowledge requisite for 69
+  not algebraic 74
+  reading of 69
+
+Equilibrium 138
+  chemical 138
+  in solution 139
+  point of 138
+
+Equivalent 224
+  determination of 224
+  elements with more than one 225
+  relation to atomic weight 224
+
+Etching 177
+
+Ether 411
+
+Ethereal salts 409
+
+Ethers 411
+
+Ethylamine 412
+
+Ethylene series 399
+
+Eudiometer 43
+
+Evaporation  11
+
+
+Families in periodic groups 170
+  triads 165
+
+Family resemblances 170
+
+Fats 409
+
+Fatty acid series 405
+
+Feldspar 261, 335
+
+Fermentation 404
+  acetic 406
+  alcoholic 404, 405
+  lactic 414
+
+Ferric chloride 352
+  hydroxide 352
+  salts 351
+    reduction 353
+  sulphate 352
+
+Ferrochromium, 383
+
+Ferromanganese 343
+
+Ferrosilicon 259
+
+Ferrous carbonate 351
+  salts 350
+    oxidation of 353
+  sulphate 350
+  sulphide 350
+
+Fertilizers 245
+
+Filtration 6, 51
+  beds 52
+
+Fire damp 202
+
+Flames 213
+  appearance 214
+  blowpipe 216
+  Bunsen 214
+  conditions for 213
+  hydrogen 34
+  luminosity 216
+  oxidizing 214
+  oxyhydrogen 37
+  reactions 296
+  reducing 214
+  structure 214
+
+Flash lights 317
+
+Flint 260
+
+Fluorides 177
+
+Fluorine 175
+
+Fluorspar 175, 308
+
+Fluosilicic acid 259
+
+Flux 340
+
+Fool's gold 351
+
+Formaldehyde 405
+
+Formalin 405
+
+Formic acid 406
+
+Formulas 68
+  how determined 234
+  structural 119
+
+Fractional distillation 51
+
+Franklinite 321
+
+Fuels 220
+
+Furnace, arc 221
+  electric 221
+  resistance 221
+
+Fusion methods 271
+
+
+Galena 373
+
+Gallium 327
+
+Galvanized iron 323
+
+Gas, collection of 15
+  coal 217
+  fuel 217
+  illuminating 217
+  measurement of 23
+  natural 219
+  purification of 218
+  water 219
+
+Gases, table 220
+
+Gasoline 400
+
+German silver 323, 359
+
+Germanium 370
+
+Germs, effect of cold on 53
+  in air 84
+  in water 52
+
+Glass 262
+  coloring of 263
+  etching of 177
+  molding of 263
+  nature of 263
+  varieties 263
+
+Glauber's salt 281
+
+Glazing 336
+
+Glucose 414
+
+Glycerin 405
+  nitrates of 409
+
+Gold 393
+  alloys 396
+  chloride 396
+  coin 359
+  extraction of 394
+  in copper 358
+  mining 394
+  occurrence 393
+  properties 396
+  refining of 395
+  telluride 394
+
+Goldschmidt method 269, 330
+
+Gram-molecular weight 236
+
+Granite 336
+
+Graphite 198
+
+Gun cotton 415
+  metal 359
+  powder 292
+
+Gypsite 308
+
+Gypsum 308
+
+
+Halogens 174
+
+Hard water 309
+
+Heat of reaction 75
+
+Helium 80, 314
+
+Hematite 339, 349
+
+Homologous series 398
+
+Hydriodic acid 193
+
+Hydrobromic acid 189
+
+Hydrocarbons 201, 398
+  properties 400
+  series 398
+  substitution products 401
+
+Hydrochloric acid 182
+  composition 183
+  oxidation of 185
+  preparation 182
+  properties 184
+  salts 186
+
+Hydrocyanic acid 210
+
+Hydrofluoric acid 176
+  etching by 177
+  salts of 177
+
+Hydrogen 28
+  dioxide 56
+  explosive with oxygen 35
+  occurrence 28
+  preparation from acids 30
+  preparation from water 28
+  properties 32
+  standard for atomic weights 66
+  standard for molecular weights 227
+  sulphide 146
+  uses 38
+
+Hydrolysis 254
+  conditions affecting 255
+  partial 255
+
+Hydrosulphuric acid 146
+
+Hydroxyl radical 112
+
+Hypochlorous acid 187
+
+Hypothesis 61
+  Avogadro's 226
+  Dalton's 61
+
+
+Ice manufacture 125
+
+Iceland spar 305
+
+Indigo 418
+
+Indium 327
+
+Insoluble compounds 272
+
+Iodic acid 194
+
+Iodides 193
+
+Iodine 190
+  oxygen compounds 193
+  preparation 191
+  properties 192
+  tincture 192
+
+Iodoform 192, 401
+
+Ions 100
+  and electrolytes 104
+
+Iridium 393
+
+Iron 339
+  alum 352
+  cast 343
+  compounds 349
+  cyanides 352
+  disulphide 351
+  family 338
+  metallurgy 339
+  occurrence 339
+  ores 339
+  oxides 349
+  pure 348
+  varieties 342, 347
+  wrought 343
+
+
+Jasper 260
+
+
+Kainite 288
+
+Kaolin 261, 335
+
+Kerosene 400
+
+Ketones 411
+
+Kieserite 288
+
+Kindling temperature 17
+
+Krypton 80
+
+
+Lactic acid 408
+
+Lampblack 200
+
+Laughing gas 132
+
+Law, definition 61
+  of Boyle 24
+  of Charles 23
+  of combining volumes 194
+  of conservation of energy 4
+  of conservation of matter 5, 59
+  of definite composition 59
+  of Dulong and Petit 233
+  of Gay-Lussac 194
+  of multiple proportion 60
+  of Raoult 233
+  periodic 169
+
+Lead 373
+  acetate 375, 407
+  alloys 375
+  basic carbonate 376
+  carbonate 376
+  chloride 377
+  chromate 377
+  insoluble compounds 376
+  metallurgy 373
+  nitrate 375
+  occurrence 373
+  oxides 375
+  peroxide 375
+  properties 374
+  red 375
+  soluble salts 375
+  sugar of 375
+  sulphate 377
+  sulphide 377
+  white 376
+
+Le Blanc soda process 282
+
+Levulose 415
+
+Lime 302
+  air-slaked 303
+  hypochlorite 307
+  kilns 303
+  slaked 303
+
+Lime light 38
+
+Limestone 305
+
+Limewater 303
+
+Limonite 339
+
+Litharge 375
+
+Lithium 294
+
+Luminosity of flames 216
+
+Lunar caustic 366
+
+
+Magnesia 318
+  alba 319
+  usta 318
+
+Magnesite 318
+
+Magnesium 317
+  basic carbonate 319
+  carbonate 318
+  cement 318
+  chloride 319
+  family 316
+  hydroxide 318
+  oxide 318
+  silicates 321
+  sulphate 320
+
+Magnetite 339, 349
+
+Malachite 357
+
+Manganates 381
+
+Manganese 379
+  a base-forming element 380
+  an acid-forming element 381
+  in glass 263
+  occurrence 379
+  oxides 380
+
+Manganic acid 381
+
+Manganous salts 380
+
+Marble 305
+
+Marl 305
+
+Marsh gas 202
+
+Matches 242
+
+Matte 358
+
+Matter, classification 6
+  conservation 5
+  definition 5
+  kinds 9
+
+Measurement of gases 23
+
+Mechanical mixtures 6
+
+Meerschaum 321, 336
+
+Mercuric chloride 363
+  iodide 364
+  oxide 14, 362
+  sulphide 363
+
+Mercurous chloride 363
+
+Mercury 361
+  iodides 364
+  metallurgy 361
+  occurrence 361
+  oxides 362
+  uses 362
+
+Metaboric acid 265
+
+Metallurgy 268
+
+Metals 165, 267
+  action on salts 271
+  definition 267
+  extraction 268
+  occurrence 267
+  preparation of compounds 269
+  reduction from ores 268
+
+Metaphosphoric acid 245
+
+Metarsenic acid 250
+
+Metasilicic acid 261
+
+Metastannic acid 371
+
+Methane 202, 399
+
+Methylamine 412
+
+Mexican onyx 305
+
+Mica 261, 336
+
+Microcosmic salt 244
+
+Milk 414
+
+Minerals 267
+
+Minium 375
+
+Mixed salts 244
+
+Molasses 413
+
+Molecular weights 226
+  boiling-point method 233
+  compared with oxygen 228
+  determination 226
+  freezing-point method 233
+  oxygen standard 227
+  of elements 232
+  vapor-density method 229
+
+Molecule 62
+
+Molybdenum 388
+
+Molybdic acid 388
+
+Monazite sand 377
+
+Mordants 333
+
+Morphine 418
+
+Mortar 304
+
+Moth balls 418
+
+Muriatic acid 182
+
+
+Naphthalene 418
+
+Naphthas 400
+
+Nascent state 182
+
+Natural gas 219
+  sciences 1
+
+Neon 80
+
+Neutralization 108
+  a definite act 109
+  definition 109
+  heat of 109
+  partial 111
+
+Niagara Falls 269, 329
+
+Nickel 354
+  coin 359
+  compounds 354
+  plating 354
+
+Nicotine 419
+
+Nitrates 131
+
+Nitric acid, 128
+  action on metals 130
+  decomposition 129
+  oxidizing action 130
+  preparation 128, 140
+  properties 129
+  salts 131
+
+Nitric oxide 133
+
+Nitrites 132
+
+Nitrobenzene 417
+
+Nitrocellulose 415
+
+Nitrogen 78
+  compounds 122
+  in air 87
+  occurrence 78, 122
+  oxides 132
+  preparation 78
+  properties 80
+
+Nitroglycerin 409
+
+Nitrosulphuric acid 155
+
+Nitrous acid 132
+  oxide 132
+
+Non-metals 165
+
+
+Oil of myrbane 417
+  of vitriol 154
+
+Oleic acid 408
+
+Olein 409
+
+Oleomargarine 410
+
+Onyx 260
+
+Opal 260
+
+Open-hearth process 346
+
+Opium 418
+
+Ores 267
+
+Organic bases 412
+  chemistry 201, 397
+  matter, decomposition 122
+
+Orpiment 246
+
+Orthoarsenic acid 250
+
+Orthophosphates 244
+
+Orthophosphoric acid 244
+
+Orthosilicic acid 261
+
+Osmic acid 393
+
+Osmium 393
+  tetroxide 393
+
+Oxalic acid 407
+
+Oxidation 17, 353
+  definition 18
+
+Oxidizing agent 37
+
+Oxygen 13
+  and ozone 22
+  commercial preparation 16
+  history 13
+  importance 21
+  in air estimation, 85
+  in air function, 84
+  occurrence 13
+  preparation 13
+  properties 16
+  standard  for  atomic weights 66
+  two atoms in molecule 227
+
+Oxyhydrogen blowpipe 37
+
+Ozone 21, 137
+
+
+Palladium 390
+
+Palmitic acid 407
+
+Palmitin 409
+
+Paraffin 400
+
+Paris green 250
+
+Parkes's method for silver 364
+
+Pearls 305
+
+Perchloric acid 187
+
+Periodic acid 194
+
+Periodic division 166
+  groups 167
+  law 169
+  law, imperfections 172
+  law, value 171
+  table 168
+  table, arrangement 166
+
+Permanent hardness 310
+
+Permanganates 381
+
+Permanganic acid 381
+
+Peroxides 278
+
+Petroleum 399
+
+Pewter 372
+
+Phenol 417
+
+Philosopher's stone 9
+
+Phlogiston 19
+
+Phosphates 245
+
+Phosphine 242
+
+Phosphonium compounds 243
+
+Phosphoric acid 244
+
+Phosphorite 239
+
+Phosphorous acid 244
+
+Phosphorus 239
+  acids 243
+  family 238
+  hydrogen compounds 242
+  occurrence 239
+  oxides 243
+  preparation 239
+  properties 240
+  red 241
+  yellow 240
+
+Photography 367
+
+Physical changes 2
+  properties 3
+  properties and periodic groups 171
+  state 3
+
+Physics 1, 4
+
+Pitchblende 314
+
+Plaster of Paris 308
+
+Platinic chloride 393
+
+Platinized asbestos 391
+
+Platinous chloride 393
+
+Platinum 391
+  a catalytic agent 152, 392
+
+Pneumatic trough 16
+
+Polyboric acid 265
+
+Polyhalite 288
+
+Polysilicic acids 261
+
+Porcelain 336
+
+Portland cement 304
+
+Potash 293
+
+Potassium 287
+  acid carbonate 294
+  acid sulphate 294
+  acid sulphite 294
+  alum, aluminium 334
+  alum, chrome 384
+  alum, iron 352
+  and plant life 287
+  aurate 396
+  bromide 290
+  carbonate 293
+  chlorate 291
+  chloride 290
+  chromate 385
+  cyanide 293
+  dichromate 386
+  ferricyanide 352
+  ferrocyanide 352
+  hydroxide 288
+  hydroxide, action of halogens 289
+  hypochlorite 289
+  iodide 290
+  manganate 381
+  nitrate 291
+  occurrence 287
+  permanganate 381
+  preparation 288
+  sulphate 294
+
+Precipitated chalk 306
+
+Precipitation 140
+
+Properties, chemical 3
+  physical 3
+
+Prussic acid 210
+
+Puddling 343
+  furnace 344
+
+Pyridine 412
+
+Pyrites 351
+
+Pyrolusite 380
+
+Pyrophosphoric acid 245
+
+
+Quantitative equations 72
+
+Quartz 260
+
+Quicklime 302
+
+Quinine 418
+
+Quinoline 412
+
+
+Radical 112
+
+Radium 313
+
+Reaction, classes 70
+  addition 70
+  completed 139
+  heat of 75
+  of decomposition 70
+  of double decomposition 71
+  of substitution 70
+  reversible 137
+  steps in 131
+
+Realgar 246
+
+Red lead 375
+  phosphorus 241
+
+Reducing agent 37
+
+Reduction 36, 354
+
+Rennin 414
+
+Resemblances, family 170
+
+Respiration 87
+
+Rhodium 390
+
+Rochelle salts 408
+
+Rouge 349
+
+Rubidium 294
+
+Ruby 331
+
+Ruthenium 390
+
+Rutile 264
+
+
+Safety lamp 202
+
+Sal ammoniac 294
+  soda 282
+
+Salt 280
+
+Saltpeter 291
+  Chili 285
+
+Salts, 109
+  acid, 112
+
+Salts basic 111
+  binary 114
+  characteristics 109
+  definition 109
+  insoluble 272
+  mixed 244
+  nomenclature 113
+  normal 112
+  preparation  by  precipitation 270
+
+Sand 260
+
+Sandstone 260
+
+Saponification 410
+
+Sapphire 331
+
+Satinspar 308
+
+Scale 320
+
+Schoenite 288
+
+Selenite 308
+
+Selenium 161
+
+Serpentine 320, 336
+
+Shot 247, 375
+
+Siderite 339
+
+Silica 260
+
+Silicates 261
+
+Silicic acids 261
+
+Silicides 259
+
+Silicon 258
+  acids 261
+  dioxide 260
+  fluoride 258
+  hydride 258
+
+Silver 364
+  amalgamation process 364
+  bromide 367
+  chloride 367
+  coin 359
+  German 359
+  in copper ores 358
+  iodide 367
+  metallurgy 364
+  nitrate 366
+  oxide 366
+  parting of 365
+  refining 365
+  sulphide 366
+
+Slag 340
+
+Smalt 355
+
+Smithsonite 321
+
+Smokeless powder 293
+
+Soaps 410
+
+Soda ash 284
+
+Soda lime 202
+
+Sodium 276
+  acetate 407
+  bicarbonate 285
+  carbonate 282
+  carbonate, historical 284
+  chloride 280
+  chromates 386
+  hydrogen carbonate 285
+  hydroxide 278
+  hyposulphite 282
+  iodate 191
+  nitrate 285
+  occurrence 276
+  peroxide 277
+  phosphates 286
+  preparation 276
+  properties 277
+  sulphate 281
+  sulphite 281
+  tetraborate 287
+  thiosulphate 282
+
+Solder 372, 375
+
+Solubility of gases 95
+  of solids 96
+
+Solution 94
+  and chemical action 53
+  boiling point 98
+  classes 94
+  distribution of solids in 98
+  electrolysis of 99
+  freezing point 99
+  of gases in liquids 94
+  of solids in liquids 96
+  properties 98
+  saturated 97
+  supersaturated 98
+
+Solvay soda process 283
+
+Sombrerite 239
+
+Spectroscope 296
+
+Sphalerite 325
+
+Spiegel iron 343
+
+Spinel 332
+
+Spontaneous combustion 20
+
+Stalactites 305
+
+Stalagmites 305
+
+Standard conditions 23
+
+Stannates 372
+
+Stannic acid 372
+  chloride 372
+  oxide 372
+
+Stannous chloride 372
+
+Starch 415
+
+Stassfurt salts 287
+
+Stearic acid 407
+
+Stearin 409
+
+Steel 345
+  alloys 348
+  properties 347
+  tempering of 348
+  tool 347
+
+Stibine 251
+
+Stibnite 250
+
+Stoneware 336
+
+Strontianite 312
+
+Strontium 312
+  hydroxide 312
+  nitrate 312
+
+Structural formulas 119
+
+Structure of compounds 119
+
+Strychnine 419
+
+Substitution 70
+
+Sugars 412
+  cane 412
+  fruit 415
+  grape 414
+  milk 414
+
+Sulphates 159
+
+Sulphides 148
+
+Sulphites 152
+  action of acids on 150
+
+Sulphur 143
+  allotropic forms 144
+  chemical properties 145
+  comparison with oxygen 161
+  dioxide 149
+    preparation 149
+    properties 150
+  extraction 143
+  flowers of 143
+  occurrence 143
+  oxides 149
+  physical properties 144
+  trioxide 152
+  uses 146
+  varieties 144
+
+Sulphuric acid 154
+  action as an acid 157
+  action on metals 157
+  action on organic matter 158
+  action on salts 158
+  action on water 158
+  fuming 155
+  manufacture 154
+  oxidizing action 157
+  plant 156
+  properties 157
+  salts 159
+
+Sulphuric anhydride 153
+
+Sulphurous acid 151
+
+Superphosphate of lime 246
+
+Sylvine 288
+
+Symbols 11
+
+Synthesis 40
+
+
+Table, alkali metals 274
+  alkaline-earth metals 300
+  alloys of copper 359
+  aqueous tension Appendix B
+  atomic weights Appendix A
+  chlorine family 174
+  composition of earth's crust 10
+  composition of fuel gases 220
+  constants of elements Appendix B
+  copper family 356
+  elements Appendix A
+  gold and platinum metals 390
+  hydrocarbons 399
+  magnesium family 316
+  manganese and chromium 379
+  periodic arrangement 168
+  phosphorus family 238
+  silicon family 257
+  solubility of gases in water 95
+  solubility of salts 96
+  solubility of salts at different temperatures 97
+  tin and lead 370
+  weights of gases Appendix B
+
+Talc 321, 336
+
+Tartar emetic 408
+
+Tartaric acid 408
+
+Tellurium 161
+
+Temporary hardness 309
+
+Ternary acids 113
+  salts 114
+
+Tetraboric acid 265
+
+Thallium 327
+
+Theory, atomic 61
+  definition 64
+  value of 64
+
+Thermite 331
+
+Thio compounds 282
+
+Thiosulphates 159
+
+Thiosulphuric acid 159
+
+Thorium 377
+
+Tin 370
+  block 371
+  compounds 372
+  crystals 372
+  family 370
+  foil 371
+  metallurgy 370
+  plate 371
+  properties 371
+  uses 371
+
+Titanium 257, 264
+
+Topaz 331
+
+Triad families 166
+
+Tungsten 388
+
+Type metal 253, 375
+
+
+Uranium 388
+
+
+Valence 116
+  a numerical property 116
+  and combining ratios 118
+  and equations 120
+  and formulas 120
+  and periodic groups 162
+  and structure 118
+  definition 116
+  indirectly determined 117
+  measure of 117
+  variable 117
+
+Vaseline 400
+
+Venetian red 349
+
+Verdigris 407
+
+Vermilion 363
+
+Vinegar 406
+
+Vitriol, blue 361
+  green 350
+  oil of 154
+  white 324
+
+Volume and aqueous tension 25
+  and pressure 24
+  and temperature 23
+  of combining gases 194
+
+
+Water 40
+  a compound 40
+  and disease 49
+  catalytic action of 154
+  chalybeate 351
+  chemical properties 53
+  composition 47
+  composition by volume 44
+  composition by weight 47
+  dissociation of 210
+  distillation of 50
+  electrolysis of 41, 103
+  filtration of 51
+  gas 219
+  hard 309
+  historical 40
+  impurities in 48
+  in air 87
+  mineral 49
+  occurrence 48
+  of crystallization 54, 75
+  physical properties 53
+  purification of 50
+  qualitative analysis 41
+  quantitative analysis 42
+  river 49
+  sanitary analysis 50
+  self-purification 53
+  softening of 310
+  standard substance 55
+  synthesis 43
+  uses of 55
+
+Weights, atomic 65
+
+Welsbach mantles 219, 377
+
+Whisky 404
+
+Wine 404
+
+Witherite 312
+
+Wood alcohol 402
+  distillation 402
+
+Wood's metal 254
+
+
+Xenon 80
+
+
+Yeast 403
+
+
+Zinc 321
+  alloys of 323
+  blende 321
+  chloride 325
+  flowers of 322
+  metallurgy 321
+  occurrence 321
+  oxide 324
+  sulphate 324
+  sulphide 325
+  white 324
+
+
+Zymase, 403
+
+
+
+
+ANNOUNCEMENTS
+
+
+AN ELEMENTARY STUDY OF CHEMISTRY
+
+By WILLIAM McPHERSON, Professor of Chemistry in Ohio State University,
+and WILLIAM E. HENDERSON, Associate Professor of Chemistry in Ohio State
+University.
+
+12mo. Cloth. 434 pages. Illustrated. List price, $1.25; mailing price,
+$1.40
+
+This book is the outgrowth of many years of experience in the teaching
+of elementary chemistry. In its preparation the authors have steadfastly
+kept in mind the limitations of the student to whom chemistry is a new
+science. They have endeavored to present the subject in a clear,
+well-graded way, passing in a natural and logical manner from principles
+which are readily understood to those which are more difficult to grasp.
+The language is simple and as free as possible from unusual and
+technical phrases. Those which are unavoidable are carefully defined.
+The outline is made very plain, and the paragraphing is designed to be
+of real assistance to the student in his reading.
+
+The book is in no way radical, either in the subject-matter selected or
+in the method of treatment. At the same time it is in thorough harmony
+with the most recent developments in chemistry, both in respect to
+theory and discovery. Great care has been taken in the theoretical
+portions to make the treatment simple and well within the reach of the
+ability of an elementary student. The most recent discoveries have been
+touched upon where they come within the scope of an elementary text.
+Especial attention has been given to the practical applications of
+chemistry, and to the description of the manufacturing processes in use
+at the present time.
+
+
+EXERCISES IN CHEMISTRY. By WILLIAM McPHERSON and WILLIAM E. HENDERSON.
+(_In press._)
+
+GINN & COMPANY PUBLISHERS
+
+A FIRST COURSE IN PHYSICS
+
+By ROBERT A. MILLIKAN, Associate Professor of Physics, and HENRY G.
+GALE, Assistant Professor of Physics in The University of Chicago
+
+12mo, cloth, 488 pages, illustrated, $1.25
+
+
+A LABORATORY COURSE IN PHYSICS
+
+_FOR SECONDARY SCHOOLS_
+
+By ROBERT A. MILLIKAN and HENRY G. GALE 12mo, flexible cloth, 134 pages,
+illustrated, 40 cents
+
+This one-year course in physics has grown out of the experience of the
+authors in developing the work in physics at the School of Education of
+The University of Chicago, and in dealing with the physics instruction
+in affiliated high schools and academies.
+
+The book is a simple, objective presentation of the subject as opposed
+to a formal and mathematical one. It is intended for the third-year
+high-school pupils and is therefore adapted in style and method of
+treatment to the needs of students between the ages of fifteen and
+eighteen. It especially emphasizes the historical and practical aspects
+of the subject and connects the study very intimately with facts of
+daily observation and experience.
+
+The authors have made a careful distinction between the class of
+experiments which are essentially laboratory problems and those which
+belong more properly to the classroom and the lecture table. The former
+are grouped into a Laboratory Manual which is designed for use in
+connection with the text. The two books are not, however, organically
+connected, each being complete in itself.
+
+All the experiments included in the work have been carefully chosen with
+reference to their usefulness as effective classroom demonstrations.
+
+GINN AND COMPANY PUBLISHERS
+
+
+
+
+APPENDIX A
+
+
+LIST OF THE ELEMENTS, THEIR SYMBOLS, AND ATOMIC WEIGHTS
+
+The more important elements are marked with an asterisk
+
+O = 16
+
+*Antimony       Sb     120.2
+*Argon          A       39.9
+*Arsenic        As      75.0
+*Barium         Ba     137.4
+Beryllium       Be       9.1
+*Bismuth        Bi     208.5
+*Boron          B       11.0
+*Bromine        Br      79.96
+*Cadmium        Cd     112.4
+Caesium          Cs     132.9
+*Calcium        Ca      40.1
+*Carbon         C       12.00
+Cerium          Ce     140.25
+*Chlorine       Cl      35.45
+*Chromium       Cr      52.1
+*Cobalt         Co      59.0
+Columbium       Cb      94.0
+*Copper         Cu      63.6
+Erbium          Er     166.0
+*Fluorine       F       19.0
+Gadolinium      Gd     156.0
+Gallium         Ga      70.0
+Germanium       Ge      72.5
+*Gold           Au     197.2
+Helium          He       4.0
+*Hydrogen       H        1.008
+Indium          In     115.0
+*Iodine         I      126.97
+Iridium         Ir     193.0
+*Iron           Fe      55.9
+Krypton         Kr      81.8
+Lanthanum       La     138.9
+*Lead           Pb     206.9
+Lithium         Li       7.03
+*Magnesium      Mg      24.36
+*Manganese      Mn      55.0
+*Mercury        Hg     200.0
+Molybdenum      Mo      96.0
+Neodymium       Nd     143.6
+Neon            Ne      20.0
+*Nickel         Ni      58.7
+*Nitrogen       N       14.04
+Osmium          Os     191.0
+*Oxygen         O       16.00
+Palladium       Pd     106.5
+*Phosphorus     P       31.0
+*Platinum       Pt     194.8
+*Potassium      K       39.15
+Praseodymium    Pr     140.5
+Radium          Ra     225.0
+Rhodium         Rh     103.0
+Rubidium        Rb      85.5
+Ruthenium       Ru     101.7
+Samarium        Sm     150.3
+Scandium        Sc      44.1
+Selenium        Se      79.2
+*Silicon        Si      28.4
+*Silver         Ag     107.93
+*Sodium         Na      23.05
+*Strontium      Sr      87.6
+*Sulphur        S       32.06
+Tantalum        Ta     183.0
+Tellurium       Te     127.6
+Terbium         Tb     160.0
+Thallium        Tl     204.1
+Thorium         Th     232.5
+Thulium         Tm     171.0
+*Tin            Sn     119.0
+Titanium        Ti      48.1
+Tungsten        W      184.0
+Uranium         U      238.5
+Vanadium        V       51.2
+Xenon           Xe     128.0
+Ytterbium       Yb     173.0
+Yttrium         Yt      89.0
+*Zinc           Zn      65.4
+Zirconium       Zr      90.6
+
+
+
+
+APPENDIX B
+
+
+Tension of Aqueous Vapor expressed in Millimeters of Mercury
+
+TEMPERATURE      PRESSURE
+16               13.5
+17               14.4
+18               15.3
+19               16.3
+20               17.4
+21               18.5
+22               19.6
+23               20.9
+24               22.2
+25               23.5
+
+
+Weight of 1 Liter of Various Gases measured under Standard Conditions
+
+Acetylene               1.1614
+Air                     1.2923
+Ammonia                 0.7617
+Carbon dioxide          1.9641
+Carbon monoxide         1.2499
+Chlorine                3.1650
+Hydrocyanic acid        1.2036
+Hydrochloric acid       1.6275
+Hydrogen                0.08984
+Hydrosulphuric acid     1.5211
+Methane                 0.7157
+Nitric oxide            1.3410
+Nitrogen                1.2501
+Nitrous oxide           1.9677
+Oxygen                  1.4285
+Sulphur dioxide         2.8596
+
+
+Densities and Melting Points of Some Common Elements
+
+                       DENSITY      MELTING POINT
+Aluminium                2.68           640
+Antimony                 6.70           432
+Arsenic                  5.73            --
+Barium                   3.75            --
+Bismuth                  9.80           270
+Boron                    2.45            --
+Cadmium                  8.67           320
+Caesium                   1.88            26.5
+Calcium                  1.54            --
+Carbon, Diamond          3.50            --
+  "     Graphite         2.15            --
+  "     Charcoal         1.80            --
+Chromium                 7.30          3000
+Cobalt                   8.55          1800
+Copper                   8.89          1084
+Gold                    19.30          1064
+Iridium                 22.42          1950
+Iron                     7.93          1800
+Lead                    11.38           327
+Lithium                  0.59           186
+Magnesium                1.75           750
+Manganese                8.01          1900
+Mercury                 13.596          -39.5
+Nickel                   8.9           1600
+Osmium                  22.47            --
+Palladium               11.80          1500
+Phosphorus               1.80            45
+Platinum                21.50          1779
+Potassium                0.87            62.5
+Rhodium                 12.10            --
+Rubidium                 1.52            38.5
+Ruthenium               12.26            --
+Silicon                  2.35            --
+Silver                  10.5            960
+Sodium                   0.97            97.6
+Strontium                2.50            --
+Sulphur                  2.00           114.8
+Tin                      7.35           235
+Titanium                 3.50            --
+Zinc                     7.00           420
+
+
+
+
+
+End of the Project Gutenberg EBook of An Elementary Study of Chemistry, by
+William McPherson and William Edwards Henderson
+
+*** END OF THIS PROJECT GUTENBERG EBOOK AN ELEMENTARY STUDY OF CHEMISTRY ***
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