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diff --git a/26908-0.txt b/26908-0.txt new file mode 100644 index 0000000..8a922f0 --- /dev/null +++ b/26908-0.txt @@ -0,0 +1,22084 @@ +Project Gutenberg's Conversations on Chemistry, V. 1-2, by Jane Marcet + +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: Conversations on Chemistry, V. 1-2 + In Which the Elements of that Science Are Familiarly + Explained and Illustrated by Experiments + +Author: Jane Marcet + +Release Date: October 13, 2008 [EBook #26908] + +Language: English + +Character set encoding: UTF-8 + +*** START OF THIS PROJECT GUTENBERG EBOOK CONVERSATIONS ON CHEMISTRY, V. 1-2 *** + + + + +Produced by Louise Hope + + + + +[Transcriber’s Note: + +DO NOT TRY THIS AT HOME. + +This e-text comes in three different forms: unicode (UTF-8), Latin-1 and +ascii-7. Use the one that works best on your text reader. + + --If “œ” displays as a single character, and apostrophes and quotation + marks are “curly” or angled, you have the utf-8 version (best). If any + part of this paragraph displays as garbage, try changing your text + reader’s “character set” or “file encoding”. If that doesn’t work, + proceed to: + --In the Latin-1 version, “œ” is two letters, but the word “aëriform” + is usually written with dieresis (dots) over the “e”, and “æ” is a + single letter. Apostrophes and quotation marks will be straight + (“typewriter” form). Again, if you see any garbage in this paragraph + and can’t get it to display properly, use: + --The ASCII-7 or rock-bottom version. All necessary text will still be + there; it just won’t be as pretty. + +The full caption of each Plate is given after its first mention in the +text--generally a few pages before the Plate’s physical appearance, as +specified in the caption. + +Many terms used in this book are different from today’s standard +terminology. Note in particular: + + oxy-muriatic acid = the element chlorine + phosphat of lime = calcium diphosphate _or_ the element calcium + glucium = the element beryllium + + muriatic acid = hydrochloric acid + muriat of lime = calcium chloride + oxymuriate of potash = potassium chlorate + carbonic acid = carbon dioxide + +Further details and more examples are at the end of the e-text. + +Each Volume had its own table of contents. They have been merged for +this e-text, but the Vol. II title page was retained. Some Conversations +were renumbered between the 4th and 5th edition, resulting in the +apparent disappearance of Conversations XI and XII. + +Typographical errors are listed at the end of the text.] + + + * * * * * + * * * * + * * * * * + + + CONVERSATIONS + ON + CHEMISTRY; + + In Which + The Elements Of That Science + Are + _Familiarly Explained_ + And + Illustrated By Experiments. + + + IN TWO VOLUMES. + + _The Fifth Edition, revised, corrected,_ + _and considerably enlarged._ + + VOL. I. + ON SIMPLE BODIES. + + + _London:_ + Printed For Longman, Hurst, Rees, Orme, and Brown, + Paternoster-Row. + 1817. + + + + +Printed by A. Strahan, Printers-Street, London. + + + + +ADVERTISEMENT. + + +_The Author, in this fifth edition, has endeavoured to give an account +of the principal discoveries which have been made within the last four +years in Chemical Science, and of the various important applications, +such as the gas-lights, and the miner’s-lamp, to which they have given +rise. But in regard to doctrines or principles, the work has undergone +no material alteration._ + +_London_, _July_, 1817. + + + + +PREFACE. + + +In venturing to offer to the public, and more particularly to the female +sex, an Introduction to Chemistry, the author, herself a woman, +conceives that some explanation may be required; and she feels it the +more necessary to apologise for the present undertaking, as her +knowledge of the subject is but recent, and as she can have no real +claims to the title of chemist. + +On attending for the first time experimental lectures, the author found +it almost impossible to derive any clear or satisfactory information +from the rapid demonstrations which are usually, and perhaps +necessarily, crowded into popular courses of this kind. But frequent +opportunities having afterwards occurred of conversing with a friend on +the subject of chemistry, and of repeating a variety of experiments, she +became better acquainted with the principles of that science, and began +to feel highly interested in its pursuit. It was then that she +perceived, in attending the excellent lectures delivered at the Royal +Institution, by the present Professor of Chemistry, the great advantage +which her previous knowledge of the subject, slight as it was, gave her +over others who had not enjoyed the same means of private instruction. +Every fact or experiment attracted her attention, and served to explain +some theory to which she was not a total stranger; and she had the +gratification to find that the numerous and elegant illustrations, for +which that school is so much distinguished, seldom failed to produce on +her mind the effect for which they were intended. + +Hence it was natural to infer, that familiar conversation was, in +studies of this kind, a most useful auxiliary source of information; and +more especially to the female sex, whose education is seldom calculated +to prepare their minds for abstract ideas, or scientific language. + +As, however, there are but few women who have access to this mode of +instruction; and as the author was not acquainted with any book that +could prove a substitute for it, she thought that it might be useful for +beginners, as well as satisfactory to herself, to trace the steps by +which she had acquired her little stock of chemical knowledge, and to +record, in the form of dialogue, those ideas which she had first derived +from conversation. + +But to do this with sufficient method, and to fix upon a mode of +arrangement, was an object of some difficulty. After much hesitation, +and a degree of embarrassment, which, probably, the most competent +chemical writers have often felt in common with the most superficial, +a mode of division was adopted, which, though the most natural, does not +always admit of being strictly pursued--it is that of treating first of +the simplest bodies, and then gradually rising to the most intricate +compounds. + +It is not the author’s intention to enter into a minute vindication of +this plan. But whatever may be its advantages or inconveniences, the +method adopted in this work is such, that a young pupil, who should +occasionally recur to it, with a view to procure information on +particular subjects, might often find it obscure or unintelligible; for +its various parts are so connected with each other as to form an +uninterrupted chain of facts and reasonings, which will appear +sufficiently clear and consistent to those only who may have patience to +go through the whole work, or have previously devoted some attention to +the subject. + +It will, no doubt, be observed, that in the course of these +Conversations, remarks are often introduced, which appear much too acute +for the young pupils, by whom they are supposed to be made. Of this +fault the author is fully aware. But, in order to avoid it, it would +have been necessary either to omit a variety of useful illustrations, or +to submit to such minute explanations and frequent repetitions, as would +have rendered the work tedious, and therefore less suited to its +intended purpose. + +In writing these pages, the author was more than once checked in her +progress by the apprehension that such an attempt might be considered by +some, either as unsuited to the ordinary pursuits of her sex, or +ill-justified by her own recent and imperfect knowledge of the subject. +But, on the one hand, she felt encouraged by the establishment of those +public institutions, open to both sexes, for the dissemination of +philosophical knowledge, which clearly prove that the general opinion no +longer excludes women from an acquaintance with the elements of science; +and, on the other, she flattered herself that whilst the impressions +made upon her mind, by the wonders of Nature, studied in this new point +of view, were still fresh and strong, she might perhaps succeed the +better in communicating to others the sentiments she herself +experienced. + +The reader will soon perceive, in perusing this work, that he is often +supposed to have previously acquired some slight knowledge of natural +philosophy, a circumstance, indeed, which appears very desirable. The +author’s original intention was to commence this work by a small tract, +explaining, on a plan analogous to this, the most essential rudiments of +that science. This idea she has since abandoned; but the manuscript was +ready, and might, perhaps, have been printed at some future period, had +not an elementary work of a similar description, under the tide of +“Scientific Dialogues,” been pointed out to her, which, on a rapid +perusal, she thought very ingenious, and well calculated to answer its +intended object. + + + + + Contents Of + _The First Volume_. + + ON SIMPLE BODIES. + + +CONVERSATION I. + Page + + ON THE GENERAL PRINCIPLES OF CHEMISTRY. 1 + +Connexion between Chemistry and Natural Philosophy. --Improved State +of modern Chemistry. --Its use in the Arts. --The general Objects of +Chemistry. --Definition of Elementary Bodies. --Definition of +Decomposition. --Integrant and Constituent Particles. --Distinction +between Simple and Compound Bodies. --Classification of Simple +Bodies. --Of Chemical Affinity, or Attraction of Composition. +--Examples of Composition and Decomposition. + + +CONVERSATION II. + + ON LIGHT AND HEAT. 26 + +Light and Heat capable of being separated. --Dr. Herschel’s +Experiments. --Phosphorescence. --Of Caloric. --Its two +Modifications. --Free Caloric. --Of the three different States of +Bodies, solid, fluid, and aeriform. --Dilatation of solid Bodies. +--Pyrometer. --Dilatation of Fluids. --Thermometer. --Dilatation of +Elastic Fluids. --Air Thermometer. --Equal Diffusion of Caloric. +--Cold a Negative Quality. --Professor Prevost’s Theory of the +Radiation of Heat. --Professor Pictet’s Experiments on the Reflexion +of Heat. --Mr. Leslie’s Experiments on the Radiation of Heat. + + +CONVERSATION III. + + CONTINUATION OF THE SUBJECT. 70 + +Of the different Power of Bodies to conduct Heat. --Attempt to +account for this Power. --Count Rumford’s Theory of the +non-conducting Power of Fluids. --Phenomena of Boiling. --Of +Solution in general. --Solvent Power of Water. --Difference between +Solution and Mixture. --Solvent Power of Caloric. --Of Clouds, Rain, +Dr. Wells’ theory of Dew, Evaporation, &c. --Influence of +Atmospherical Pressure on Evaporation. --Ignition. + + +CONVERSATION IV. + + ON COMBINED CALORIC, COMPREHENDING SPECIFIC HEAT + AND LATENT HEAT. 122 + +Of Specific Heat. --Of the different Capacities of Bodies for Heat. +--Specific Heat not perceptible by the Senses. --How to be +ascertained. --Of Latent Heat. --Distinction between Latent and +Specific Heat. --Phenomena attending the Melting of Ice and the +Formation of Vapour. --Phenomena attending the Formation of Ice, and +the Condensation of Elastic Fluids. --Instances of Condensation, and +consequent Disengagement of Heat, produced by Mixtures, by the +Slaking of Lime. --General Remarks on Latent Heat. --Explanation of +the Phenomena of Ether boiling, and Water freezing, at the same +Temperature. --Of the Production of Cold by Evaporation. +--Calorimeter. --Meteorological Remarks. + + +CONVERSATION V. + + ON THE CHEMICAL AGENCIES OF ELECTRICITY. 160 + +Of Positive and Negative Electricity. --Galvani’s Discoveries. +--Voltaic Battery. --Electrical Machine. --Theory of Voltaic +Excitement. + + +CONVERSATION VI. + + ON OXYGEN AND NITROGEN. 181 + +The Atmosphere composed of Oxygen and Nitrogen in the State of Gas. +--Definition of Gas. --Distinction between Gas and Vapour. --Oxygen +essential to Combustion and Respiration. --Decomposition of the +Atmosphere by Combustion. --Nitrogen Gas obtained by this Process. +--Of Oxygenation in general. --Of the Oxydation of Metals. --Oxygen +Gas obtained from Oxyd of Manganese. --Description of a Water-Bath +for collecting and preserving Gases. --Combustion of Iron Wire in +Oxygen Gas. --Fixed and volatile Products of Combustion. --Patent +Lamps. --Decomposition of the Atmosphere by Respiration. +--Recomposition of the Atmosphere. + + +CONVERSATION VII. + + ON HYDROGEN. 214 + +Of Hydrogen. --Of the Formation of Water by the Combustion of +Hydrogen. --Of the Decomposition of Water. --Detonation of Hydrogen +Gas. --Description of Lavoisier’s Apparatus for the formation of +Water. --Hydrogen Gas essential to the Production of Flame. +--Musical Tones produced by the Combustion of Hydrogen Gas within a +Glass Tube. --Combustion of Candles explained. --Gas lights. +--Detonation of Hydrogen Gas in Soap Bubbles. --Air Balloons. +--Meteorological Phenomena ascribed to Hydrogen Gas. --Miner’s Lamp. + + [Transcriber’s Note: + The final two pages of the Table of Contents for Volume I were + missing; everything after “Decomposition of Water” was supplied + from earlier and later editions, compared against the body text. + The section marked “Diamond” (Conv. IX) was called “Diamond is + Carbon(e) in a state of perfect purity” in the 4th edn., “Diamond” + alone in later editions.] + + +CONVERSATION VIII. + + ON SULPHUR AND PHOSPHORUS. 256 + +Natural History of Sulphur. --Sublimation. --Alembic. --Combustion +of Sulphur in Atmospheric Air. --Of Acidification in general. +--Nomenclature of the Acids. --Combustion of Sulphur in Oxygen Gas. +--Sulphuric Acid. --Sulphurous Acid. --Decomposition of Sulphur. +--Sulphurated Hydrogen Gas. --Harrogate, or Hydro-sulphurated +Waters. --Phosphorus. --History of its Discovery. --Its Combustion +in Oxygen Gas. --Phosphoric Acid. --Phosphorus Acid. --Eudiometer. +--Combination of Phosphorus with Sulphur. --Phosphorated Hydrogen +Gas. --Nomenclature of Binary Compounds. --Phosphoret of Lime +burning under Water. + + +CONVERSATION IX. + + ON CARBON. 282 + +Method of obtaining pure Charcoal. --Method of making common +Charcoal. --Pure Carbon not to be obtained by Art. --Diamond. +--Properties of Carbon. --Combustion of Carbon. --Production of +Carbonic Acid Gas. --Carbon susceptible of only one Degree of +Acidification. --Gaseous Oxyd of Carbon. --Of Seltzer Water and +other Mineral Waters. --Effervescence. --Decomposition of Water by +Carbon. --Of Fixed and Essential Oils. --Of the Combustion of Lamps +and Candles. --Vegetable Acids. --Of the Power of Carbon to revive +Metals. + + +CONVERSATION X. + + ON METALS. 314 + +Natural History of Metals. --Of Roasting, Smelting, &c. --Oxydation +of metals by the Atmosphere. --Change of Colours produced by +different degrees of Oxydation. --Combustion of Metals. --Perfect +Metals burnt by Electricity only. --Some Metals revived by Carbon +and other Combustibles. --Perfect Metals revived by Heat alone. --Of +the Oxydation of certain Metals by the Decomposition of Water. Power +of Acids to promote this Effect. --Oxydation of Metals by Acids. +--Metallic Neutral Salts. --Previous oxydation of the Metal +requisite. --Crystallisation. --Solution distinguished from +Dissolution. --Five metals susceptible of acidification. --Meteoric +Stones. --Alloys, Soldering, Plating, &c. --Of Arsenic, and of the +caustic Effects of Oxygen. --Of Verdigris, Sympathetic Ink, &c. --Of +the new Metals discovered by Sir H. Davy. + + + Contents Of + _The Second Volume_. + + ON COMPOUND BODIES. + + +CONVERSATION XIII. + Page + + ON THE ATTRACTION OF COMPOSITION. 1 + +Of the laws which regulate the Phenomena of the Attraction of +Composition. --1. It takes place only between Bodies of a different +Nature. --2. Between the most minute Particles only. --3. Between 2, +3, 4, or more Bodies. --Of Compound or Neutral Salts. --4. Produces +a Change of Temperature. --5. The Properties which characterise +Bodies in their separate State, destroyed by Combination. --6. The +Force of Attraction estimated by that which is required by the +Separation of the Constituents. --7. Bodies have amongst themselves +different Degrees of Attraction. --Of simple elective and double +elective Attractions. --Of quiescent and divellent Forces. --Law of +definite Proportions. --Decomposition of Salts by Voltaic +Electricity. + + +CONVERSATION XIV. + + ON ALKALIES. 19 + +Of the Composition and general Properties of the Alkalies. --Of +Potash. --Manner of preparing it. --Pearlash. --Soap. --Carbonat of +Potash. --Chemical Nomenclature. --Solution of Potash. --Of Glass. +--Of Nitrat of Potash or Saltpetre. --Effect of Alkalies on +Vegetable Colours. --Of Soda. --Of Ammonia or Volatile Alkali. +--Muriat of Ammonia. --Ammoniacal Gas. --Composition of Ammonia. +--Hartshorn and Sal Volatile. --Combustion of Ammoniacal Gas. + + +CONVERSATION XV. + + ON EARTHS. 44 + +Composition of the Earths. --Of their Incombustibility. --Form the +Basis of all Minerals. --Their Alkaline Properties. --Silex; its +Properties and Uses in the Arts. --Alumine; its Uses in Pottery, &c. +--Alkaline Earths. --Barytes. --Lime; its extensive chemical +Properties and Uses in the Arts. --Magnesia. --Strontian. + + +CONVERSATION XVI. + + ON ACIDS. 69 + +Nomenclature of the Acids. --Of the Classification of Acids. --1st +Class --Acids of simple and known Radicals, or Mineral Acids. +--2d Class --Acids of double Radicals, or Vegetable Acids. +--3d Class --Acids of triple Radicals or Animal Acids. --Of the +Decomposition of Acids of the 1st Class by Combustible bodies. + + +CONVERSATION XVII. + + OF THE SULPHURIC AND PHOSPHORIC ACIDS: OR, THE COMBINATIONS OF + OXYGEN WITH SULPHUR AND WITH PHOSPHORUS; AND OF THE SULPHATS + AND PHOSPHATS. 80 + +Of the Sulphuric Acid. --Combustion of Animal or Vegetable Bodies by +this Acid. --Method of preparing it. -- The Sulphurous Acid obtained +in the Form of Gas. --May be obtained from Sulphuric Acid. --May be +reduced to Sulphur. --Is absorbable by Water. --Destroys Vegetable +Colours. --Oxyd of Sulphur. --Of Salts in general. --Sulphats. +--Sulphat of Potash, or Sal Polychrest. --Cold produced by the +melting of Salts. --Sulphat of Soda, or Glauber’s Salt. --Heat +evolved during the Formation of Salts. --Crystallisation of Salts. +--Water of Crystallisation. --Efflorescence and Deliquescence of +Salts. --Sulphat of Lime, Gypsum or Plaister of Paris. --Sulphat of +Magnesia. --Sulphat of Alumine, or Alum. --Sulphat of Iron. --Of +Ink. --Of the Phosphoric and Phosphorous Acids. --Phosphorus +obtained from Bones. --Phosphat of Lime. + + +CONVERSATION XVIII. + + OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATION OF + OXYGEN WITH NITROGEN AND WITH CARBON; AND OF THE NITRATS AND + CARBONATS. 100 + +Nitrogen susceptible of various Degrees of Acidification. --Of the +Nitric Acid. --Its Nature and Composition discovered by +Mr. Cavendish. --Obtained from Nitrat of Potash. --Aqua Fortis. +--Nitric Acid may be converted into Nitrous Acid. --Nitric Oxyd Gas. +--Its Conversion into Nitrous Acid Gas. --Used as an Eudiometrical +Test. --Gaseous Oxyd of Nitrogen, or exhilarating Gas, obtained from +Nitrat of Ammonia. --Its singular Effects on being respired. +--Nitrats. --Of Nitrat of Potash, Nitre or Saltpetre. --Of +Gunpowder. --Causes of Detonation. --Decomposition of Nitre. +--Deflagration. --Nitrat of Ammonia. --Nitrat of Silver. --Of the +Carbonic Acid. --Formed by the Combustion of Carbon. --Constitutes a +component Part of the Atmosphere. --Exhaled in some Caverns. +--Grotto del Cane. --Great Weight of this Gas. --Produced from +calcareous Stones by Sulphuric Acid. --Deleterious Effects of this +Gas when respired. --Sources which keep up a Supply of this Gas in +the Atmosphere. --Its Effects on Vegetation. --Of the Carbonats of +Lime; Marble, Chalk, Shells, Spars, and calcareous Stones. + + +CONVERSATION XIX. + + ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC ACIDS; + AND ON MURIATS. 131 + +On the Boracic Acid. --Its Decomposition by Sir H. Davy. --Its Basis +Boracium. --Its Recomposition. --Its Uses in the Arts. --Borax or +Borat of Soda. --Of the Fluoric Acid. --Obtained from Fluor; +corrodes Siliceous Earth; its supposed Composition. --Fluorine; its +supposed Basis. --Of the Muriatic Acid. --Obtained from Muriats. +--Its gaseous Form. --Is absorbable by Water. --Its Decomposition. +--Is susceptible of a stronger Degree of Oxygenation. --Oxygenated +Muriatic Acid. --Its gaseous Form and other Properties. --Combustion +of Bodies in this Gas. --It dissolves Gold. --Composition of Aqua +Regia. --Oxygenated Muriatic Acid destroys all Colours. --Sir H. +Davy’s Theory of the Nature of Muriatic and Oxymuriatic Acid. +--Chlorine. --Used for Bleaching and for Fumigations. --Its +offensive Smell, &c. --Muriats. --Muriat of Soda, or common Salt. +--Muriat of Ammonia. --Oxygenated Muriat of Potash. --Detonates with +Sulphur, Phosphorus, &c. --Experiment of burning Phosphorus under +Water by means of this Salt and of Sulphuric Acid. + + +CONVERSATION XX. + + ON THE NATURE AND COMPOSITION OF VEGETABLES. 162 + +Of organised Bodies. --Of the Functions of Vegetables. --Of the +Elements of Vegetables. --Of the Materials of Vegetables. --Analysis +of Vegetables. --Of Sap. --Mucilage, or Gum. --Sugar. --Manna, and +Honey. --Gluten. --Vegetable Oils. --Fixed Oils, Linseed, Nut, and +Olive Oils. --Volatile Oils, forming Essences and Perfumes. +--Camphor. --Resins and Varnishes. --Pitch, Tar, Copal, Mastic, &c. +--Gum Resins. --Myrrh, Assafœtida, &c. --Caoutchouc, or Gum Elastic. +--Extractive colouring Matter; its Use in the Arts of Dyeing and +Painting. --Tannin; its Use in the Art of preparing Leather. --Woody +Fibre. --Vegetable Acids. --The Alkalies and Salts contained in +Vegetables. + + +CONVERSATION XXI. + + ON THE DECOMPOSITION OF VEGETABLES. 202 + +Of Fermentation in general. --Of the Saccharine Fermentation, the +Product of which is Sugar. --Of the Vinous Fermentation, the Product +of which is Wine. --Alcohol, or Spirit of Wine. --Analysis of Wine +by Distillation. --Of Brandy, Rum, Arrack, Gin, &c. --Tartrit of +Potash, or Cream of Tartar. --Liqueurs. --Chemical Properties of +Alcohol. --Its Combustion. --Of Ether. --Of the Acetous +Fermentation, the Product of which is Vinegar. --Fermentation of +Bread. --Of the Putrid Fermentation, which reduces Vegetables to +their Elements. --Spontaneous Succession of these Fermentations. +--Of Vegetables said to be petrified. --Of Bitumens: Naphtha, +Asphaltum, Jet, Coal, Succin, or Yellow Amber. --Of Fossil Wood, +Peat, and Turf. + + +CONVERSATION XXII. + + HISTORY OF VEGETATION. 243 + +Connexion between the Vegetable and Animal Kingdoms. --Of Manures. +--Of Agriculture. --Inexhaustible Sources of Materials for the +Purposes of Agriculture. --Of sowing Seed. --Germination of the +Seed. --Function of the Leaves of Plants. --Effects of Light and Air +on Vegetation. --Effects of Water on Vegetation. --Effects of +Vegetation on the Atmosphere. --Formation of Vegetable Materials by +the Organs of Plants. --Vegetable Heat. --Of the Organs of Plants. +--Of the Bark, consisting of Epidermis, Parenchyma, and Cortical +Layers. --Of Alburnum, or Wood. --Leaves, Flowers, and Seeds. +--Effects of the Season on Vegetation. --Vegetation of Evergreens in +Winter. + + +CONVERSATION XXIII. + + ON THE COMPOSITION OF ANIMALS. 276 + +Elements of Animals. --Of the principal Materials of Animals, viz. +--Gelatine, Albumen, Fibrine, Mucus. --Of Animal Acids. --Of Animal +Colours, Prussian Blue, Carmine, and Ivory Black. + + +CONVERSATION XXIV. + + ON THE ANIMAL ECONOMY. 297 + +Of the principal Animal Organs. --Of Bones, Teeth, Horns, Ligaments, +and Cartilage. --Of the Muscles, constituting the Organs of Motion. +--Of the Vascular System, for the Conveyance of Fluids. --Of the +Glands, for the Secretion of Fluids. --Of the Nerves, constituting +the Organs of Sensation. --Of the Cellular Substance which connects +the several Organs. --Of the Skin. + + +CONVERSATION XXV. + + ON ANIMALISATION, NUTRITION, AND RESPIRATION. 314 + +Digestion. --Solvent Power of the Gastric Juice. --Formation of a +Chyle. --Its Assimilation, or Conversion into Blood. --Of +Respiration. --Mechanical Process of Respiration. --Chemical Process +of Respiration. --Of the Circulation of the Blood. --Of the +Functions of the Arteries, the Veins, and the Heart. --Of the Lungs. +--Effects of Respiration on the Blood. + + +CONVERSATION XXVI. + + ON ANIMAL HEAT; AND OF VARIOUS ANIMAL PRODUCTS. 336 + +Of the Analogy of Combustion and Respiration. --Animal Heat evolved +in the Lungs. --Animal Heat evolved in the Circulation. --Heat +produced by Fever. --Perspiration. --Heat produced by Exercise. +--Equal Temperature of Animals at all Seasons. --Power of the Animal +Body to resist the Effects of Heat. --Cold produced by Perspiration. +--Respiration of Fish and of Birds. --Effects of Respiration on +Muscular Strength. --Of several Animal Products, viz. Milk, Butter, +and Cheese; Spermaceti; Ambergris; Wax; Lac; Silk; Musk; Civet; +Castor. --Of the putrid Fermentation. --Conclusion. + + + + +CONVERSATIONS + +ON + +CHEMISTRY. + + + + +CONVERSATION I. + +ON THE GENERAL PRINCIPLES OF CHEMISTRY. + + +MRS. B. + +As you have now acquired some elementary notions of NATURAL PHILOSOPHY, +I am going to propose to you another branch of science, to which I am +particularly anxious that you should devote a share of your attention. +This is CHEMISTRY, which is so closely connected with Natural +Philosophy, that the study of the one must be incomplete without some +knowledge of the other; for, it is obvious that we can derive but a very +imperfect idea of bodies from the study of the general laws by which +they are governed, if we remain totally ignorant of their intimate +nature. + +CAROLINE. + +To confess the truth, Mrs. B., I am not disposed to form a very +favourable idea of chemistry, nor do I expect to derive much +entertainment from it. I prefer the sciences which exhibit nature on a +grand scale, to those that are confined to the minutiæ of petty details. +Can the studies which we have lately pursued, the general properties of +matter, or the revolutions of the heavenly bodies, be compared to the +mixing up of a few insignificant drugs? I grant, however, there may be +entertaining experiments in chemistry, and should not dislike to try +some of them: the distilling, for instance, of lavender, or rose +water . . . . . . + +MRS. B. + +I rather imagine, my dear Caroline, that your want of taste for +chemistry proceeds from the very limited idea you entertain of its +object. You confine the chemist’s laboratory to the narrow precincts of +the apothecary’s and perfumer’s shops, whilst it is subservient to an +immense variety of other useful purposes. Besides, my dear, chemistry is +by no means confined to works of art. Nature also has her laboratory, +which is the universe, and there she is incessantly employed in chemical +operations. You are surprised, Caroline, but I assure you that the most +wonderful and the most interesting phenomena of nature are almost all of +them produced by chemical powers. What Bergman, in the introduction to +his history of chemistry, has said of this science, will give you a more +just and enlarged idea of it. The knowledge of nature may be divided, he +observes, into three periods. The first was that in which the attention +of men was occupied in learning the external forms and characters of +objects, and this is called _Natural History_. In the second, they +considered the effects of bodies acting on each other by their +mechanical power, as their weight and motion, and this constitutes the +science of _Natural Philosophy_. The third period is that in which the +properties and mutual action of the elementary parts of bodies was +investigated. This last is the science of CHEMISTRY, and I have no doubt +you will soon agree with me in thinking it the most interesting. + +You may easily conceive, therefore, that without entering into the +minute details of practical chemistry, a woman may obtain such a +knowledge of the science as will not only throw an interest on the +common occurrences of life, but will enlarge the sphere of her ideas, +and render the contemplation of nature a source of delightful +instruction. + +CAROLINE. + +If this is the case, I have certainly been much mistaken in the notion I +had formed of chemistry. I own that I thought it was chiefly confined to +the knowledge and preparation of medicines. + +MRS. B. + +That is only a branch of chemistry which is called Pharmacy; and, though +the study of it is certainly of great importance to the world at large, +it belongs exclusively to professional men, and is therefore the last +that I should advise you to pursue. + +EMILY. + +But, did not the chemists formerly employ themselves in search of the +philosopher’s stone, or the secret of making gold? + +MRS. B. + +These were a particular set of misguided philosophers, who dignified +themselves with the name of Alchemists, to distinguish their pursuits +from those of the common chemists, whose studies were confined to the +knowledge of medicines. + +But, since that period, chemistry has undergone so complete a +revolution, that, from an obscure and mysterious art, it is now become a +regular and beautiful science, to which art is entirely subservient. It +is true, however, that we are indebted to the alchemists for many very +useful discoveries, which sprung from their fruitless attempts to make +gold, and which, undoubtedly, have proved of infinitely greater +advantage to mankind than all their chimerical pursuits. + +The modern chemists, instead of directing their ambition to the vain +attempt of producing any of the original substances in nature, rather +aim at analysing and imitating her operations, and have sometimes +succeeded in forming combinations, or effecting decompositions, no +instances of which occur in the chemistry of Nature. They have little +reason to regret their inability to make gold, whilst, by their +innumerable inventions and discoveries, they have so greatly stimulated +industry and facilitated labour, as prodigiously to increase the +luxuries as well as the necessaries of life. + +EMILY. + +But, I do not understand by what means chemistry can facilitate labour; +is not that rather the province of the mechanic? + +MRS. B. + +There are many ways by which labour may be rendered more easy, +independently of mechanics; but even the machine, the most wonderful in +its effects, the Steam-engine, cannot be understood without the +assistance of chemistry. In agriculture, a chemical knowledge of the +nature of soils, and of vegetation, is highly useful; and, in those arts +which relate to the comforts and conveniences of life, it would be +endless to enumerate the advantages which result from the study of this +science. + +CAROLINE. + +But, pray, tell us more precisely in what manner the discoveries of +chemists have proved so beneficial to society? + +MRS. B. + +That would be an injudicious anticipation; for you would not comprehend +the nature of such discoveries and useful applications, as well as you +will do hereafter. Without a due regard to method, we cannot expect to +make any progress in chemistry. I wish to direct your observations +chiefly to the chemical operations of Nature; but those of Art are +certainly of too high importance to pass unnoticed. We shall therefore +allow them also some share of our attention. + +EMILY. + +Well, then, let us now set to work regularly. I am very anxious to +begin. + +MRS. B. + +The object of chemistry is to obtain a knowledge of the intimate nature +of bodies, and of their mutual action on each other. You find therefore, +Caroline, that this is no narrow or confined science, which comprehends +every thing material within our sphere. + +CAROLINE. + +On the contrary, it must be inexhaustible; and I am a loss to conceive +how any proficiency can be made in a science whose objects are so +numerous. + +MRS. B. + +If every individual substance were formed of different materials, the +study of chemistry would, indeed, be endless; but you must observe that +the various bodies in nature are composed of certain elementary +principles, which are not very numerous. + +CAROLINE. + +Yes; I know that all bodies are composed of fire, air, earth, and water; +I learnt that many years ago. + +MRS. B. + +But you must now endeavour to forget it. I have already informed you +what a great change chemistry has undergone since it has become a +regular science. Within these thirty years especially, it has +experienced an entire revolution, and it is now proved, that neither +fire, air, earth, nor water, can be called elementary bodies. For an +elementary body is one that has never been decomposed, that is to say, +separated into other substances; and fire, air, earth, and water, are +all of them susceptible of decomposition. + +EMILY. + +I thought that decomposing a body was dividing it into its minutest +parts. And if so, I do not understand why an elementary substance is not +capable of being decomposed, as well as any other. + +MRS. B. + +You have misconceived the idea of _decomposition_; it is very different +from mere _division_. The latter simply reduces a body into parts, but +the former separates it into the various ingredients, or materials, of +which it is composed. If we were to take a loaf of bread, and separate +the several ingredients of which it is made, the flour, the yeast, the +salt, and the water, it would be very different from cutting or +crumbling the loaf into pieces. + +EMILY. + +I understand you now very well. To decompose a body is to separate from +each other the various elementary substances of which it consists. + +CAROLINE. + +But flour, water, and other materials of bread, according to our +definition, are not elementary substances? + +MRS. B. + +No, my dear; I mentioned bread rather as a familiar comparison, to +illustrate the idea, than as an example. + +The elementary substances of which a body is composed are called the +_constituent_ parts of that body; in decomposing it, therefore, we +separate its constituent parts. If, on the contrary, we divide a body by +chopping it to pieces, or even by grinding or pounding it to the finest +powder, each of these small particles will still consist of a portion of +the several constituent parts of the whole body: these are called the +_integrant_ parts; do you understand the difference? + +EMILY. + +Yes, I think, perfectly. We _decompose_ a body into its _constituent_ +parts; and _divide_ it into its _integrant_ parts. + +MRS. B. + +Exactly so. If therefore a body consists of only one kind of substance, +though it may be divided into its integrant parts, it is not possible to +decompose it. Such bodies are therefore called _simple_ or _elementary_, +as they are the elements of which all other bodies are composed. +_Compound bodies_ are such as consist of more than one of these +elementary principles. + +CAROLINE. + +But do not fire, air, earth, and water, consist, each of them, but of +one kind of substance? + +MRS. B. + +No, my dear; they are every one of them susceptible of being separated +into various simple bodies. Instead of four, chemists now reckon upwards +of forty elementary substances. The existence of most of these is +established by the clearest experiments; but, in regard to a few of +them, particularly the most subtle agents of nature, _heat_, _light_, +and _electricity_, there is yet much uncertainty, and I can only give +you the opinion which seems most probably deduced from the latest +discoveries. After I have given you a list of the elementary bodies, +classed according to their properties, we shall proceed to examine each +of them separately, and then consider them in their combinations with +each other. + +Excepting the more general agents of nature, heat, light, and +electricity, it would seem that the simple form of bodies is that of a +metal. + +CAROLINE. + +You astonish me! I thought the metals were only one class of minerals, +and that there were besides, earths, stones, rocks, acids, alkalies, +vapours, fluids, and the whole of the animal and vegetable kingdoms. + +MRS. B. + +You have made a tolerably good enumeration, though I fear not arranged +in the most scientific order. All these bodies, however, it is now +strongly believed, may be ultimately resolved into metallic substances. +Your surprise at this circumstance is not singular, as the decomposition +of some of them, which has been but lately accomplished, has excited the +wonder of the whole philosophical world. + +But to return to the list of simple bodies--these being usually found in +combination with oxygen, I shall class them according to their +properties when so combined. This will, I think, facilitate their future +investigation. + +EMILY. + +Pray what is oxygen? + +MRS. B. + +A simple body; at least one that is supposed to be so, as it has never +been decomposed. It is always found united with the negative +electricity. It will be one of the first of the elementary bodies whose +properties I shall explain to you, and, as you will soon perceive, it is +one of the most important in nature; but it would be irrelevant to enter +upon this subject at present. We must now confine our attention to the +enumeration and classification of the simple bodies in general. They may +be arranged as follows: + +CLASS I. + +_Comprehending the imponderable agents, viz._ + + HEAT or CALORIC, + LIGHT, + ELECTRICITY. + +CLASS II. + +_Comprehending agents capable of uniting with inflammable bodies, and in +most instances of effecting their combustion._ + + OXYGEN, + CHLORINE, + IODINE.* + + [Footnote *: It has been questioned by some eminent chemists, + whether these two last agents should not be classed among the + inflammable bodies, as they are capable of combining with oxygen, + as well as with inflammable bodies. But they seem to be more + distinctly characterised by their property of supporting + combustion than by any other quality.] + +CLASS III. + +_Comprehending bodies capable of uniting with oxygen, and, forming with +it various compounds. This class may be divided as follows:_ + +DIVISION 1. + + HYDROGEN, _forming_ water. + +DIVISION 2. + +_Bodies forming acids._ + + NITROGEN, _forming_ nitric acid. + SULPHUR, _forming_ sulphuric acid. + PHOSPHORUS, _forming_ phosphoric acid. + CARBON, _forming_ carbonic acid. + BORACIUM, _forming_ boracic acid. + FLUORIUM, _forming_ fluoric acid. + MURIATIUM, _forming_ muriatic acid. + +DIVISION 3. + +_Metallic bodies forming alkalies._ + + POTASSIUM, _forming_ potash. + SODIUM, _forming_ soda. + AMMONIUM, _forming_ ammonia. + +DIVISION 4. + +_Metallic bodies forming earths._ + + CALCIUM, _or metal forming_ lime. + MAGNIUM, _forming_ magnesia. + BARIUM, _forming_ barytes. + STRONTIUM, _forming_ strontites. + SILICIUM, _forming_ silex. + ALUMIUM, _forming_ alumine. + YTTRIUM, _forming_ yttria. + GLUCIUM, _forming_ glucina. + ZIRCONIUM, _forming_ zirconi.* + + [Footnote *: Of all these earths, three or four only have as yet + been distinctly decomposed.] + +DIVISION 5. + +_Metals, either naturally metallic, or yielding their oxygen to carbon +or to heat alone._ + +_Subdivision 1._ + +_Malleable Metals._ + + GOLD, + PLATINA, + PALLADIUM, + SILVER* + MERCURY† + TIN, + COPPER, + IRON, + LEAD, + NICKEL, + ZINC. + + [Footnote *: These first four metals have commonly been + distinguished by the appellation of perfect or noble metals, on + account of their possessing the characteristic properties of + ductility, malleability, inalterability, and great specific + gravity, in an eminent degree.] + + [Footnote †: Mercury, in its liquid state, cannot, of course, + be called a malleable metal. But when frozen, it possesses a + considerable degree of malleability.] + +_Subdiv. 2._ + +_Brittle Metals._ + + ARSENIC, + BISMUTH, + ANTIMONY, + MANGANESE, + TELLURIUM, + COBALT, + TUNGSTEN, + MOLYBDENUM, + TITANIUM, + CHROME, + URANIUM, + COLUMBIUM _or_ TANTALIUM, + IRIDIUM, + OSMIUM, + RHODIUM.* + + [Footnote *: These last four or five metallic bodies are placed + under this class for the sake of arrangement, though some of their + properties have not been yet fully investigated.] + +CAROLINE. + +Oh, what a formidable list! You will have much to do to explain it, +Mrs. B.; for I assure you it is perfectly unintelligible to me, and I +think rather perplexes than assists me. + +MRS. B. + +Do not let that alarm you, my dear; I hope that hereafter this +classification will appear quite clear, and, so far from perplexing you, +will assist you in arranging your ideas. It would be in vain to attempt +forming a division that would appear perfectly clear to a beginner: for +you may easily conceive that a chemical division being necessarily +founded on properties with which you are almost wholly unacquainted, it +is impossible that you should at once be able to understand its meaning +or appreciate its utility. + +But, before we proceed further, it will be necessary to give you some +idea of chemical attraction, a power on which the whole science depends. + +_Chemical Attraction_, or the _Attraction of Composition_, consists in +the peculiar tendency which bodies of a different nature have to unite +with each other. It is by this force that all the compositions, and +decompositions, are effected. + +EMILY. + +What is the difference between chemical attraction, and the attraction +of cohesion, or of aggregation, which you often mentioned to us, in +former conversations? + +MRS. B. + +The attraction of cohesion exists only between particles of the _same_ +nature, whether simple or compound; thus it unites the particles of a +piece of metal which is a simple substance, and likewise the particles +of a loaf of bread which is a compound. The attraction of composition, +on the contrary, unites and maintains, in a state of combination, +particles of a _dissimilar_ nature; it is this power that forms each of +the compound particles of which bread consists; and it is by the +attraction of cohesion that all these particles are connected into a +single mass. + +EMILY. + +The attraction of cohesion, then, is the power which unites the +integrant particles of a body: the attraction of composition that which +combines the constituent particles. Is it not so? + +MRS. B. + +Precisely: and observe that the attraction of cohesion unites particles +of a similar nature, without changing their original properties; the +result of such an union, therefore, is a body of the same kind as the +particles of which it is formed; whilst the attraction of composition, +by combining particles of a dissimilar nature, produces compound bodies, +quite different from any of their constituents. If, for instance, I pour +on the piece of copper, contained in this glass, some of this liquid +(which is called nitric acid), for which it has a strong attraction, +every particle of the copper will combine with a particle of acid, and +together they will form a new body, totally different from either the +copper or the acid. + +Do you observe the internal commotion that already begins to take place? +It is produced by the combination of these two substances; and yet the +acid has in this case to overcome not only the resistance which the +strong cohesion of the particles of copper opposes to their combination +with it, but also to overcome the weight of the copper, which makes it +sink to the bottom of the glass, and prevents the acid from having such +free access to it as it would if the metal were suspended in the liquid. + +EMILY. + +The acid seems, however, to overcome both these obstacles without +difficulty, and appears to be very rapidly dissolving the copper. + +MRS. B. + +By this means it reduces the copper into more minute parts than could +possibly be done by any mechanical power. But as the acid can act only +on the surface of the metal, it will be some time before the union of +these two bodies will be completed. + +You may, however, already see how totally different this compound is +from either of its ingredients. It is neither colourless, like the acid, +nor hard, heavy, and yellow like the copper. If you tasted it, you would +no longer perceive the sourness of the acid. It has at present the +appearance of a blue liquid; but when the union is completed, and the +water with which the acid is diluted is evaporated, the compound will +assume the form of regular crystals, of a fine blue colour, and +perfectly transparent*. Of these I can shew you a specimen, as I have +prepared some for that purpose. + + [Footnote *: These crystals are more easily obtained from a + mixture of sulphuric with a little nitric acid.] + +CAROLINE. + +How very beautiful they are, in colour, form, and transparency! + +EMILY. + +Nothing can be more striking than this example of chemical attraction. + +MRS. B. + +The term _attraction_ has been lately introduced into chemistry as a +substitute for the word _affinity_, to which some chemists have +objected, because it originated in the vague notion that chemical +combinations depended upon a certain resemblance, or relationship, +between particles that are disposed to unite; and this idea is not only +imperfect, but erroneous, as it is generally particles of the most +dissimilar nature, that have the greatest tendency to combine. + +CAROLINE. + +Besides, there seems to be no advantage in using a variety of terms to +express the same meaning; on the contrary it creates confusion; and as +we are well acquainted with the term Attraction in natural philosophy, +we had better adopt it in chemistry likewise. + +MRS. B. + +If you have a clear idea of the meaning, I shall leave you at liberty to +express it in the terms you prefer. For myself, I confess that I think +the word Attraction best suited to the general law that unites the +integrant particles of bodies; and Affinity better adapted to that which +combines the constituent particles, as it may convey an idea of the +preference which some bodies have for others, which the term _attraction +of composition_ does not so well express. + +EMILY. + +So I think; for though that preference may not result from any +relationship, or similitude, between the particles (as you say was once +supposed), yet, as it really exists, it ought to be expressed. + +MRS. B. + +Well, let it be agreed that you may use the terms _affinity_, _chemical +attraction_ and _attraction of composition_, indifferently, provided you +recollect that they have all the same meaning. + +EMILY. + +I do not conceive how bodies can be decomposed by chemical attraction. +That this power should be the means of composing them, is very obvious; +but that it should, at the same time, produce exactly the contrary +effect, appears to me very singular. + +MRS. B. + +To decompose a body is, you know, to separate its constituent parts, +which, as we have just observed, cannot be done by mechanical means. + +EMILY. + +No: because mechanical means separate only the integrant particles; they +act merely against the attraction of cohesion, and only divide a +compound into smaller parts. + +MRS. B. + +The decomposition of a body is performed by chemical powers. If you +present to a body composed of two principles, a third, which has a +greater affinity for one of them than the two first have for each other, +it will be decomposed, that is, its two principles will be separated by +means of the third body. Let us call two ingredients, of which the body +is composed, A and B. If we present to it another ingredient C, which +has a greater affinity for B than that which unites A and B, it +necessarily follows that B will quit A to combine with C. The new +ingredient, therefore, has effected a decomposition of the original body +A B; A has been left alone, and a new compound, B C, has been formed. + +EMILY. + +We might, I think, use the comparison of two friends, who were very +happy in each other’s society, till a third disunited them by the +preference which one of them gave to the new-comer. + +MRS. B. + +Very well. I shall now show you how this takes place in chemistry. + +Let us suppose that we wish to decompose the compound we have just +formed by the combination of the two ingredients, copper and nitric +acid; we may do this by presenting to it a piece of iron, for which the +acid has a stronger attraction than for copper; the acid will, +consequently, quit the copper to combine with the iron, and the copper +will be what the chemists call _precipitated_, that is to say, it will +be thrown down in its separate state, and reappear in its simple form. + +In order to produce this effect, I shall dip the blade of this knife +into the fluid, and, when I take it out, you will observe, that, instead +of being wetted with a bluish liquid, like that contained in the glass, +it will be covered with a thin coat of copper. + +CAROLINE. + +So it is really! but then is it not the copper, instead of the acid, +that has combined with the iron blade? + +MRS. B. + +No; you are deceived by appearances: it is the acid which combines with +the iron, and, in so doing, deposits or precipitates the copper on the +surface of the blade. + +EMILY. + +But, cannot three or more substances combine together, without any of +them being precipitated? + +MRS. B. + +That is sometimes the case; but, in general, the stronger affinity +destroys the weaker; and it seldom happens that the attraction of +several substances for each other is so equally balanced as to produce +such complicated compounds. + +CAROLINE. + +But, pray, Mrs. B., what is the cause of the chemical attraction of +bodies for each other? It appears to me more extraordinary or unnatural, +if I may use the expression, than the attraction of cohesion, which +unites particles of a similar nature. + +MRS. B. + +Chemical attraction may, like that of cohesion or gravitation, be one of +the powers inherent in matter which, in our present state of knowledge, +admits of no other satisfactory explanation than an immediate reference +to a divine cause. Sir H. Davy, however, whose important discoveries +have opened such improved views in chemistry, has suggested an +hypothesis which may throw great light upon that science. He supposes +that there are two kinds of electricity, with one or other of which all +bodies are united. These we distinguish by the names of _positive_ and +_negative_ electricity; those bodies are disposed to combine, which +possess opposite electricities, as they are brought together by the +attraction which these electricities have for each other. But, whether +this hypothesis be altogether founded on truth or not, it is impossible +to question the great influence of electricity in chemical combinations. + +EMILY. + +So, that we must suppose that the two electricities always attract each +other, and thus compel the bodies in which they exist to combine? + +CAROLINE. + +And may not this be also the cause of the attraction of cohesion? + +MRS. B. + +No, for in particles of the same nature the same electricities must +prevail, and it is only the different or opposite electric fluids that +attract each other. + +CAROLINE. + +These electricities seem to me to be a kind of chemical spirit, which +animates the particles of bodies, and draws them together. + +EMILY. + +If it is known, then, with which of the electricities bodies are united, +it can be inferred which will, and which will not, combine together? + +MRS. B. + +Certainly. --I should not omit to mention, that some doubts have been +entertained whether electricity be really a material agent, or whether +it might not be a power inherent in bodies, similar to, or, perhaps +identical with, attraction. + +EMILY. + +But what then would be the electric spark which is visible, and must +therefore be really material? + +MRS. B. + +What we call the electric spark, may, Sir H. Davy says, be merely the +heat and light, or fire produced by the chemical combinations with which +these phenomena are always connected. We will not, however, enter more +fully on this important subject at present, but reserve the principal +facts which relate to it to a future conversation. + +Before we part, however, I must recommend you to fix in your memory the +names of the simple bodies, against our next interview. + + + + +CONVERSATION II. + +ON LIGHT AND HEAT OR CALORIC. + + +CAROLINE. + +We have learned by heart the names of all the simple bodies which you +have enumerated, and we are now ready to enter on the examination of +each of them successively. You will begin, I suppose, with LIGHT? + +MRS. B. + +Respecting the nature of light we have little more than conjectures. It +is considered by most philosophers as a real substance, immediately +emanating from the sun, and from all luminous bodies, from which it is +projected in right lines with prodigious velocity. Light, however, being +imponderable, it cannot be confined and examined by itself; and +therefore it is to the effects it produces on other bodies, rather than +to its immediate nature, that we must direct our attention. + +The connection between light and heat is very obvious; indeed, it is +such, that it is extremely difficult to examine the one independently of +the other. + +EMILY. + +But, is it possible to separate light from heat; I thought they were +only different degrees of the same thing, fire? + +MRS. B. + +I told you that fire was not now considered as a simple element. Whether +light and heat be altogether different agents, or not, I cannot pretend +to decide; but, in many cases, light may be separated from heat. The +first discovery of this was made by a celebrated Swedish chemist, +Scheele. Another very striking illustration of the separation of heat +and light was long after pointed out by Dr. Herschell. This philosopher +discovered that these two agents were emitted in the rays of the sun, +and that heat was less refrangible than light; for, in separating the +different coloured rays of light by a prism (as we did some time ago), +he found that the greatest heat was beyond the spectrum, at a little +distance from the red rays, which, you may recollect, are the least +refrangible. + +EMILY. + +I should like to try that experiment. + +MRS. B. + +It is by no means an easy one: the heat of a ray of light, refracted by +a prism, is so small, that it requires a very delicate thermometer to +distinguish the difference of the degree of heat within and without the +spectrum. For in this experiment the heat is not totally separated from +the light, each coloured ray retaining a certain portion of it, though +the greatest part is not sufficiently refracted to fall within the +spectrum. + +EMILY. + +I suppose, then, that those coloured rays which are the least +refrangible, retain the greatest quantity of heat? + +MRS. B. + +They do so. + +EMILY. + +Though I no longer doubt that light and heat can be separated, Dr. +Herschell’s experiment does not appear to me to afford sufficient proof +that they are essentially different; for light, which you call a simple +body, may likewise be divided into the various coloured rays. + +MRS. B. + +No doubt there must be some difference in the various coloured rays. +Even their chemical powers are different. The blue rays, for instance, +have the greatest effect in separating oxygen from bodies, as was found +by Scheele; and there exist also, as Dr. Wollaston has shown, rays more +refrangible than the blue, which produce the same chemical effect, and, +what is very remarkable, are invisible. + +EMILY. + +Do you think it possible that heat may be merely a modification of +light? + +MRS. B. + +That is a supposition which, in the present state of natural philosophy, +can neither be positively affirmed nor denied. Let us, therefore, +instead of discussing theoretical points, be contented with examining +what is known respecting the chemical effects of light. + +Light is capable of entering into a kind of transitory union with +certain substances, and this is what has been called phosphorescence. +Bodies that are possessed of this property, after being exposed to the +sun’s rays, appear luminous in the dark. The shells of fish, the bones +of land animals, marble, limestone, and a variety of combinations of +earths, are more or less powerfully phosphorescent. + +CAROLINE. + +I remember being much surprised last summer with the phosphorescent +appearance of some pieces of rotten wood, which had just been dug out of +the ground; they shone so bright that I at first supposed them to be +glow-worms. + +EMILY. + +And is not the light of a glow-worm of a phosphorescent nature? + +MRS. B. + +It is a very remarkable instance of phosphorescence in living animals; +this property, however, is not exclusively possessed by the glow-worm. +The insect called the lanthorn-fly, which is peculiar to warm climates, +emits light as it flies, producing in the dark a remarkably sparkling +appearance. But it is more common to see animal matter in a dead state +possessed of a phosphorescent quality; sea fish is often eminently so. + +EMILY. + +I have heard that the sea has sometimes had the appearance of being +illuminated, and that the light is supposed to proceed from the spawn of +fishes floating on its surface. + +MRS. B. + +This light is probably owing to that or some other animal matter. Sea +water has been observed to become luminous from the substance of a fresh +herring having been immersed in it; and certain insects, of the Medusa +kind, are known to produce similar effects. + +But the strongest phosphorescence is produced by chemical compositions +prepared for the purpose, the most common of which consists of oyster +shells and sulphur, and is known by the name of Canton’s Phosphorus. + +EMILY. + +I am rather surprised, Mrs. B., that you should have said so much of the +light emitted by phosphorescent bodies without taking any notice of that +which is produced by burning bodies. + +MRS. B. + +The light emitted by the latter is so intimately connected with the +chemical history of combustion, that I must defer all explanation of it +till we come to the examination of that process, which is one of the +most interesting in chemical science. + +Light is an agent capable of producing various chemical changes. It is +essential to the welfare both of the animal and vegetable kingdoms; for +men and plants grow pale and sickly if deprived of its salutary +influence. It is likewise remarkable for its property of destroying +colour, which renders it of great consequence in the process of +bleaching. + +EMILY. + +Is it not singular that light, which in studying optics we were taught +to consider as the source and origin of colours, should have also the +power of destroying them? + +CAROLINE. + +It is a fact, however, that we every day experience; you know how it +fades the colours of linens and silks. + +EMILY. + +Certainly. And I recollect that endive is made to grow white instead of +green, by being covered up so as to exclude the light. But by what means +does light produce these effects? + +MRS. B. + +This I cannot attempt to explain to you until you have obtained a +further knowledge of chemistry. As the chemical properties of light can +be accounted for only in their reference to compound bodies, it would be +useless to detain you any longer on this subject; we may therefore pass +on to the examination of heat, or caloric, with which we are somewhat +better acquainted. + +HEAT and LIGHT may be always distinguished by the different sensations +they produce, _Light_ affects the sense of sight; _Caloric_ that of +feeling; the one produces _Vision_, the other the sensation of _Heat_. + +Caloric is found to exist in a variety of forms or modifications, and I +think it will be best to consider it under the two following heads, viz. + +1. FREE OR RADIANT CALORIC. + +2. COMBINED CALORIC. + +The first, FREE or RADIANT CALORIC, is also called HEAT OF TEMPERATURE; +it comprehends all heat which is perceptible to the senses, and affects +the thermometer. + +EMILY. + +You mean such as the heat of the sun, of fire, of candles, of stoves; in +short, of every thing that burns? + +MRS. B. + +And likewise of things that do not burn, as, for instance, the warmth of +the body; in a word, all heat that is _sensible_, whatever may be its +degree, or the source from which it is derived. + +CAROLINE. + +What then are the other modifications of caloric? It must be a strange +kind of heat that cannot be perceived by our senses. + +MRS. B. + +None of the modifications of caloric should properly be called _heat_; +for heat, strictly speaking, is the sensation produced by caloric, on +animated bodies; this word, therefore, in the accurate language of +science, should be confined to express the sensation. But custom has +adapted it likewise to inanimate matter, and we say _the heat of an +oven_, _the heat of the sun_, without any reference to the sensation +which they are capable of exciting. + +It was in order to avoid the confusion which arose from thus confounding +the cause and effect, that modern chemists adopted the new word +_caloric_, to denote the principle which produces heat; yet they do not +always, in compliance with their own language, limit the word _heat_ to +the expression of the sensation, since they still frequently employ it +in reference to the other modifications of caloric which are quite +independent of sensation. + +CAROLINE. + +But you have not yet explained to us what these other modifications of +caloric are. + +MRS. B. + +Because you are not acquainted with the properties of free caloric, and +you know that we have agreed to proceed with regularity. + +One of the most remarkable properties of free caloric is its power of +_dilating_ bodies. This fluid is so extremely subtle, that it enters and +pervades all bodies whatever, forces itself between their particles, and +not only separates them, but frequently drives them asunder to a +considerable distance from each other. It is thus that caloric dilates +or expands a body so as to make it occupy a greater space than it did +before. + +EMILY. + +The effect it has on bodies, therefore, is directly contrary to that of +the attraction of cohesion; the one draws the particles together, the +other drives them asunder. + +MRS. B. + +Precisely. There is a continual struggle between the attraction of +aggregation, and the expansive power of caloric; and from the action of +these two opposite forces, result all the various forms of matter, or +degrees of consistence, from the solid, to the liquid and aëriform +state. And accordingly we find that most bodies are capable of passing +from one of these forms to the other, merely in consequence of their +receiving different quantities of caloric. + +CAROLINE. + +That is very curious; but I think I understand the reason of it. If a +great quantity of caloric is added to a solid body, it introduces itself +between the particles in such a manner as to overcome, in a considerable +degree, the attraction of cohesion; and the body, from a solid, is then +converted into a fluid. + +MRS. B. + +This is the case whenever a body is fused or melted; but if you add +caloric to a liquid, can you tell me what is the consequence? + +CAROLINE. + +The caloric forces itself in greater abundance between the particles of +the fluid, and drives them to such a distance from each other, that +their attraction of aggregation is wholly destroyed: the liquid is then +transformed into vapour. + +MRS. B. + +Very well; and this is precisely the case with boiling water, when it is +converted into steam or vapour, and with all bodies that assume an +aëriform state. + +EMILY. + +I do not well understand the word aëriform? + +MRS. B. + +Any elastic fluid whatever, whether it be merely vapour or permanent +air, is called aëriform. + +But each of these various states, solid, liquid, and aëriform, admit of +many different degrees of density, or consistence, still arising +(chiefly at least) from the different quantities of caloric the bodies +contain. Solids are of various degrees of density, from that of gold, to +that of a thin jelly. Liquids, from the consistence of melted glue, or +melted metals, to that of ether, which is the lightest of all liquids. +The different elastic fluids (with which you are not yet acquainted) are +susceptible of no less variety in their degrees of density. + +EMILY. + +But does not every individual body also admit of different degrees of +consistence, without changing its state? + +MRS. B. + +Undoubtedly; and this I can immediately show you by a very simple +experiment. This piece of iron now exactly fits the frame, or ring, made +to receive it; but if heated red hot, it will no longer do so, for its +dimensions will be so much increased by the caloric that has penetrated +into it, that it will be much too large for the frame. + +The iron is now red hot; by applying it to the frame, we shall see how +much it is dilated. + +EMILY. + +Considerably so indeed! I knew that heat had this effect on bodies, but +I did not imagine that it could be made so conspicuous. + +MRS. B. + +By means of this instrument (called a Pyrometer) we may estimate, in the +most exact manner, the various dilatations of any solid body by heat. +The body we are now going to submit to trial is this small iron bar; +I fix it to this apparatus, (PLATE I. Fig. 1.) and then heat it by +lighting the three lamps beneath it: when the bar expands, it increases +in length as well as thickness; and, as one end communicates with this +wheel-work, whilst the other end is fixed and immoveable, no sooner does +it begin to dilate than it presses against the wheel-work, and sets in +motion the index, which points out the degrees of dilatation on the +dial-plate. + + [Illustration: Plate I. Vol. I. p. 38. + + Fig. 1. Pyrometer. + A.A Bar of Metal. + 1.2.3 Lamps burning. + B.B Wheel work. + C Index. + + Fig. 2 + A.A Glass tubes with bulbs. + B.B Glasses of water in which they are immersed.] + +EMILY. + +This is, indeed, a very curious instrument; but I do not understand the +use of the wheels: would it not be more simple, and answer the purpose +equally well, if the bar, in dilating, pressed against the index, and +put it in motion without the intervention of the wheels? + +MRS. B. + +The use of the wheels is merely to multiply the motion, and therefore +render the effect of the caloric more obvious; for if the index moved no +more than the bar increased in length, its motion would scarcely be +perceptible; but by means of the wheels it moves in a much greater +proportion, which therefore renders the variations far more conspicuous. + +By submitting different bodies to the test of the pyrometer, it is found +that they are far from dilating in the same proportion. Different metals +expand in different degrees, and other kinds of solid bodies vary still +more in this respect. But this different susceptibility of dilatation is +still more remarkable in fluids than in solid bodies, as I shall show +you. I have here two glass tubes, terminated at one end by large bulbs. +We shall fill the bulbs, the one with spirit of wine, the other with +water. I have coloured both liquids, in order that the effect may be +more conspicuous. The spirit of wine, you see, dilates by the warmth of +my hand as I hold the bulb. + +EMILY. + +It certainly does, for I see it is rising into the tube. But water, it +seems, is not so easily affected by heat; for scarcely any change is +produced on it by the warmth of the hand. + +MRS. B. + +True; we shall now plunge the bulbs into hot water, (PLATE I. Fig. 2.) +and you will see both liquids rise in the tubes; but the spirit of wine +will ascend highest. + +CAROLINE. + +How rapidly it expands! Now it has nearly reached the top of the tube, +though the water has hardly begun to rise. + +EMILY. + +The water now begins to dilate. Are not these glass tubes, with liquids +rising within them, very like thermometers? + +MRS. B. + +A thermometer is constructed exactly on the same principle, and these +tubes require only a scale to answer the purpose of thermometers: but +they would be rather awkward in their dimensions. The tubes and bulbs of +thermometers, though of various sizes, are in general much smaller than +these; the tube too is hermetically closed, and the air excluded from +it. The fluid most generally used in thermometers is mercury, commonly +called quicksilver, the dilatations and contractions of which correspond +more exactly to the additions, and subtractions, of caloric, than those +of any other fluid. + +CAROLINE. + +Yet I have often seen coloured spirit of wine used in thermometers. + +MRS. B. + +The expansions and contractions of that liquid are not quite so uniform +as those of mercury; but in cases in which it is not requisite to +ascertain the temperature with great precision, spirit of wine will +answer the purpose equally well, and indeed in some respects better, as +the expansion of the latter is greater, and therefore more conspicuous. +This fluid is used likewise in situations and experiments in which +mercury would be frozen; for mercury becomes a solid body, like a piece +of lead or any other metal, at a certain degree of cold: but no degree +of cold has ever been known to freeze spirit of wine. + +A thermometer, therefore, consists of a tube with a bulb, such as you +see here, containing a fluid whose degrees of dilatation and contraction +are indicated by a scale to which the tube is fixed. The degree which +indicates the boiling point, simply means that, when the fluid is +sufficiently dilated to rise to this point, the heat is such that water +exposed to the same temperature will boil. When, on the other hand, the +fluid is so much condensed as to sink to the freezing point, we know +that water will freeze at that temperature. The extreme points of the +scales are not the same in all thermometers, nor are the degrees always +divided in the same manner. In different countries philosophers have +chosen to adopt different scales and divisions. The two thermometers +most used are those of Fahrenheit, and of Reaumur; the first is +generally preferred by the English, the latter by the French. + +EMILY. + +The variety of scale must be very inconvenient, and I should think +liable to occasion confusion, when French and English experiments are +compared. + +MRS. B. + +The inconvenience is but very trifling, because the different gradations +of the scales do not affect the principle upon which thermometers are +constructed. When we know, for instance, that Fahrenheit’s scale is +divided into 212 degrees, in which 32° corresponds with the freezing +point, and 212° with the point of boiling water: and that Reaumur’s is +divided only into 80 degrees, in which 0° denotes the freezing point, +and 80° that of boiling water, it is easy to compare the two scales +together, and reduce the one into the other. But, for greater +convenience, thermometers are sometimes constructed with both these +scales, one on either side of the tube; so that the correspondence of +the different degrees of the two scales is thus instantly seen. Here +is one of these scales, (PLATE II. Fig. 1.) by which you can at +once perceive that each degree of Reaumur’s corresponds to 2¼ of +Fahrenheit’s division. But I believe the French have, of late, given +the preference to what they call the centigrade scale, in which the +space between the freezing and the boiling point is divided into 100 +degrees. + + [Illustration: Plate II. Vol. I. p. 42. + + Fig. 1. Thermometer. + Fahrenheit’s Scale. + Reaumur’s Scale. + Boiling point of Water + Freezing point of Water + + Fig. 2. Differential Thermometer.] + +CAROLINE. + +That seems to me the most reasonable division, and I cannot guess +why the freezing point is called 32°, or what advantage is derived +from it. + +MRS. B. + +There really is no advantage in it; and it originated in a mistaken +opinion of the instrument-maker, Fahrenheit, who first constructed these +thermometers. He mixed snow and salt together, and produced by that +means a degree of cold which he concluded was the greatest possible, and +therefore made his scale begin from that point. Between that and boiling +water he made 212 degrees, and the freezing point was found to be at +32°. + +EMILY. + +Are spirit of wine, and mercury, the only liquids used in the +construction of thermometers? + +MRS. B. + +I believe they are the only liquids now in use, though some others, such +as linseed oil, would make tolerable thermometers: but for experiments +in which a very quick and delicate test of the changes of temperature is +required, air is the fluid sometimes employed. The bulb of air +thermometers is filled with common air only, and its expansion and +contraction are indicated by a small drop of any coloured liquor, which +is suspended within the tube, and moves up and down, according as the +air within the bulb and tube expands or contracts. But in general, air +thermometers, however sensible to changes of temperature, are by no +means accurate in their indications. + +I can, however, show you an air thermometer of a very peculiar +construction, which is remarkably well adapted for some chemical +experiments, as it is equally delicate and accurate in its indications. + +CAROLINE. + +It looks like a double thermometer reversed, the tube being bent, and +having a large bulb at each of its extremities. (PLATE II. Fig. 2.) + +EMILY. + +Why do you call it an air thermometer; the tube contains a coloured +liquid? + +MRS. B. + +But observe that the bulbs are filled with air, the liquid being +confined to a portion of the tube, and answering only the purpose of +showing, by its motion in the tube, the comparative dilatation or +contraction of the air within the bulbs, which afford an indication of +their relative temperature. Thus if you heat the bulb A, by the warmth +of your hand, the fluid will rise towards the bulb B, and the contrary +will happen if you reverse the experiment. + +But if, on the contrary, both tubes are of the same temperature, as is +the case now, the coloured liquid, suffering an equal pressure on each +side, no change of level takes place. + +CAROLINE. + +This instrument appears, indeed, uncommonly delicate. The fluid is set +in motion by the mere approach of my hand. + +MRS. B. + +You must observe, however, that this thermometer cannot indicate the +temperature of any particular body, or of the medium in which it is +immersed; it serves only to point out the _difference_ of temperature +between the two bulbs, when placed under different circumstances. For +this reason it has been called _differential_ thermometer. You will see +by-and-bye to what particular purposes this instrument applies. + +EMILY. + +But do common thermometers indicate the exact quantity of caloric +contained either in the atmosphere, or in any body with which they are +in contact? + +MRS. B. + +No: first, because there are other modifications of caloric which do not +affect the thermometer; and, secondly, because the temperature of a +body, as indicated by the thermometer, is only relative. When, for +instance, the thermometer remains stationary at the freezing point, we +know that the atmosphere (or medium in which it is placed, whatever it +may be) is as cold as freezing water; and when it stands at the boiling +point, we know that this medium is as hot as boiling water; but we do +not know the positive quantity of heat contained either in freezing or +boiling water, any more than we know the real extremes of heat and cold; +and consequently we cannot determine that of the body in which the +thermometer is placed. + +CAROLINE. + +I do not quite understand this explanation. + +MRS. B. + +Let us compare a thermometer to a well, in which the water rises to +different heights, according as it is more or less supplied by the +spring which feeds it: if the depth of the well is unfathomable, it must +be impossible to know the absolute quantity of water it contains; yet we +can with the greatest accuracy measure the number of feet the water has +risen or fallen in the well at any time, and consequently know the +precise quantity of its increase or diminution, without having the least +knowledge of the whole quantity of water it contains. + +CAROLINE. + +Now I comprehend it very well; nothing appears to me to explain a thing +so clearly as a comparison. + +EMILY. + +But will thermometers bear any degree of heat? + +MRS. B. + +No; for if the temperature were much above the highest degree marked on +the scale of the thermometer, the mercury would burst the tube in an +attempt to ascend. And at any rate, no thermometer can be applied to +temperatures higher than the boiling point of the liquid used in its +construction, for the steam, on the liquid beginning to boil, would +burst the tube. In furnaces, or whenever any very high temperature is to +be measured, a pyrometer, invented by Wedgwood, is used for that +purpose. It is made of a certain composition of baked clay, which has +the peculiar property of contracting by heat, so that the degree of +contraction of this substance indicates the temperature to which it has +been exposed. + +EMILY. + +But is it possible for a body to contract by heat? I thought that heat +dilated all bodies whatever. + +MRS. B. + +This is not an exception to the rule. You must recollect that the bulk +of the clay is not compared, whilst hot, with that which it has when +cold; but it is from the change which the clay has undergone by _having +been_ heated that the indications of this instrument are derived. This +change consists in a beginning fusion which tends to unite the particles +of clay more closely, thus rendering it less pervious or spongy. + +Clay is to be considered as a spongy body, having many interstices or +pores, from its having contained water when soft. These interstices are +by heat lessened, and would by extreme heat be entirely obliterated. + +CAROLINE. + +And how do you ascertain the degrees of contraction of Wedgwood’s +pyrometer? + +MRS. B. + +The dimensions of a piece of clay are measured by a scale graduated on +the side of a tapered groove, formed in a brass ruler; the more the clay +is contracted by the heat, the further it will descend into the narrow +part of the tube. + +Before we quit the subject of expansion, I must observe to you that, as +liquids expand more readily than solids, so elastic fluids, whether air +or vapour, are the most expansible of all bodies. + +It may appear extraordinary that all elastic fluids whatever, undergo +the same degree of expansion from equal augmentations of temperature. + +EMILY. + +I suppose, then, that all elastic fluids are of the same density? + +MRS. B. + +Very far from it; they vary in density, more than either liquids or +solids. The uniformity of their expansibility, which at first may appear +singular, is, however, readily accounted for. For if the different +susceptibilities of expansion of bodies arise from their various degrees +of attraction of cohesion, no such difference can be expected in elastic +fluids, since in these the attraction of cohesion does not exist, their +particles being on the contrary possessed of an elastic or repulsive +power; they will therefore all be equally expanded by equal degrees of +caloric. + +EMILY. + +True; as there is no power opposed to the expansive force of caloric in +elastic bodies, its effect must be the same in all of them. + +MRS. B. + +Let us now proceed to examine the other properties of free caloric. + +Free caloric always tends to diffuse itself equally, that is to say, +when two bodies are of different temperatures, the warmer gradually +parts with its heat to the colder, till they are both brought to the +same temperature. Thus, when a thermometer is applied to a hot body, it +receives caloric; when to a cold one, it communicates part of its own +caloric, and this communication continues until the thermometer and the +body arrive at the same temperature. + +EMILY. + +Cold, then, is nothing but a negative quality, simply implying the +absence of heat. + +MRS. B. + +Not the total absence, but a diminution of heat; for we know of no body +in which some caloric may not be discovered. + +CAROLINE. + +But when I lay my hand on this marble table I feel it _positively_ cold, +and cannot conceive that there is any caloric in it. + +MRS. B. + +The cold you experience consists in the loss of caloric that your hand +sustains in an attempt to bring its temperature to an equilibrium with +the marble. If you lay a piece of ice upon it, you will find that the +contrary effect will take place; the ice will be melted by the heat +which it abstracts from the marble. + +CAROLINE. + +Is it not in this case the air of the room, which being warmer than the +marble, melts the ice? + +MRS. B. + +The air certainly acts on the surface which is exposed to it, but the +table melts that part with which it is in contact. + +CAROLINE. + +But why does caloric tend to an equilibrium? It cannot be on the same +principle as other fluids, since it has no weight? + +MRS. B. + +Very true, Caroline, that is an excellent objection. You might also, +with some propriety, object to the term _equilibrium_ being applied to a +body that is without weight; but I know of no expression that would +explain my meaning so well. You must consider it, however, in a +figurative rather than a literal sense; its strict meaning is an _equal +diffusion_. We cannot, indeed, well say by what power it diffuses itself +equally, though it is not surprising that it should go from the parts +which have the most to those which have the least. This subject is best +explained by a theory suggested by Professor Prevost of Geneva, which is +now, I believe, generally adopted. + +According to this theory, caloric is composed of particles perfectly +separate from each other, every one of which moves with a rapid velocity +in a certain direction. These directions vary as much as imagination can +conceive, the result of which is, that there are rays or lines of these +particles moving with immense velocity in every possible direction. +Caloric is thus universally diffused, so that when any portion of space +happens to be in the neighbourhood of another, which contains more +caloric, the colder portion receives a quantity of calorific rays from +the latter, sufficient to restore an equilibrium of temperature. This +radiation does not only take place in free space, but extends also to +bodies of every kind. Thus you may suppose all bodies whatever +constantly radiating caloric: those that are of the same temperature +give out and absorb equal quantities, so that no variation of +temperature is produced in them; but when one body contains more free +caloric than another, the exchange is always in favour of the colder +body, until an equilibrium is effected; this you found to be the case +when the marble table cooled your hand, and again when it melted the +ice. + +CAROLINE. + +This reciprocal radiation surprises me extremely; I thought, from what +you first said, that the hotter bodies alone emitted rays of caloric +which were absorbed by the colder; for it seems unnatural that a hot +body should receive any caloric from a cold one, even though it should +return a greater quantity. + +MRS. B. + +It may at first appear so, but it is no more extraordinary than that a +candle should send forth rays of light to the sun, which, you know, must +necessarily happen. + +CAROLINE. + +Well, Mrs. B--, I believe that I must give up the point. But I wish I +could _see_ these rays of caloric; I should then have greater faith in +them. + +MRS. B. + +Will you give no credit to any sense but that of sight? You may feel the +rays of caloric which you receive from any body of a temperature higher +than your own; the loss of the caloric you part with in return, it is +true, is not perceptible; for as you gain more than you lose, instead of +suffering a diminution, you are really making an acquisition of caloric. +It is, therefore, only when you are parting with it to a body of a lower +temperature, that you are sensible of the sensation of cold, because you +then sustain an absolute loss of caloric. + +EMILY. + +And in this case we cannot be sensible of the small quantity of heat we +receive in exchange from the colder body, because it serves only to +diminish the loss. + +MRS. B. + +Very well, indeed, Emily. Professor Pictet, of Geneva, has made some +very interesting experiments, which prove not only that caloric radiates +from all bodies whatever, but that these rays may be reflected, +according to the laws of optics, in the same manner as light. I shall +repeat these experiments before you, having procured mirrors fit for the +purpose; and it will afford us an opportunity of using the differential +thermometer, which is particularly well adapted for these experiments. +--I place an iron bullet, (PLATE III. Fig. 1.) about two inches in +diameter, and heated to a degree not sufficient to render it luminous, +in the focus of this large metallic concave mirror. The rays of heat +which fall on this mirror are reflected, agreeably to the property of +concave mirrors, in a parallel direction, so as to fall on a similar +mirror, which, you see, is placed opposite to the first, at the distance +of about ten feet; thence the rays converge to the focus of the second +mirror, in which I place one of the bulbs of this thermometer. Now, +observe in what manner it is affected by the caloric which is reflected +on it from the heated bullet. --The air is dilated in the bulb which we +placed in the focus of the mirror, and the liquor rises considerably in +the opposite leg. + + [Illustration: Plate III. Vol. I. p. 54 + Mr. Pictet’s Apparatus for the Reflection of Heat. + + Fig. 1. + A.A. & B.B Concave mirrors fixed on stands. + C Heated Bullet placed in the focus of the mirror A. + D Thermometer, with its bulb placed in the focus of the mirror B. + 1.2.3.4 Rays of Caloric radiating from the bullet & falling on the + mirror A. + 5.6.7.8 The same rays reflected from the mirror A to the mirror B. + 9.10.11.12 The same rays reflected by the mirror B to the + Thermometer.] + +EMILY. + +But would not the same effect take place, if the rays of caloric from +the heated bullet fell directly on the thermometer, without the +assistance of the mirrors? + +MRS. B. + +The effect would in that case be so trifling, at the distance at which +the bullet and the thermometer are from each other, that it would be +almost imperceptible. The mirrors, you know, greatly increase the +effect, by collecting a large quantity of rays into a focus; place your +hand in the focus of the mirror, and you will find it much hotter there +than when you remove it nearer to the bullet. + +EMILY. + +That is very true; it appears extremely singular to feel the heat +diminish in approaching the body from which it proceeds. + +CAROLINE. + +And the mirror which produces so much heat, by converging the rays, is +itself quite cold. + +MRS. B. + +The same number of rays that are dispersed over the surface of the +mirror are collected by it into the focus; but, if you consider how +large a surface the mirror presents to the rays, and, consequently, how +much they are diffused in comparison to what they are at the focus, +which is little more than a point, I think you can no longer wonder that +the focus should be so much hotter than the mirror. + +The principal use of the mirrors in this experiment is, to prove that +the calorific emanation is reflected in the same manner as light. + +CAROLINE. + +And the result, I think, is very conclusive. + +MRS. B. + +The experiment may be repeated with a wax taper instead of the bullet, +with a view of separating the light from the caloric. For this purpose a +transparent plate of glass must be interposed between the mirrors; for +light, you know, passes with great facility through glass, whilst the +transmission of caloric is almost wholly impeded by it. We shall find, +however, in this experiment, that some few of the calorific rays pass +through the glass together with the light, as the thermometer rises a +little; but, as soon as the glass is removed, and a free passage left to +the caloric, it will rise considerably higher. + +EMILY. + +This experiment, as well as that of Dr. Herschell’s, proves that light +and heat may be separated; for in the latter experiment the separation +was not perfect, any more than in that of Mr. Pictet. + +CAROLINE. + +I should like to repeat this experiment, with the difference of +substituting a cold body instead of the hot one, to see whether cold +would not be reflected as well as heat. + +MRS. B. + +That experiment was proposed to Mr. Pictet by an incredulous philosopher +like yourself, and he immediately tried it by substituting a piece of +ice in the place of the heated bullet. + +CAROLINE. + +Well, Mrs. B., and what was the result? + +MRS. B. + +That we shall see; I have procured some ice for the purpose. + +EMILY. + +The thermometer falls considerably! + +CAROLINE. + +And does not that prove that cold is not merely a _negative_ quality, +implying simply an inferior degree of heat? The cold must be _positive_, +since it is capable of reflection. + +MRS. B. + +So it at first appeared to Mr. Pictet; but upon a little consideration +he found that it afforded only an additional proof of the reflection of +heat: this I shall endeavour to explain to you. + +According to Mr. Prevost’s theory, we suppose that all bodies whatever +radiate caloric; the thermometer used in these experiments therefore +emits calorific rays in the same manner as any other substance. When its +temperature is in equilibrium with that of the surrounding bodies, it +receives as much caloric as it parts with, and no change of temperature +is produced. But when we introduce a body of a lower temperature, such +as a piece of ice, which parts with less caloric than it receives, the +consequence is, that its temperature is raised, whilst that of the +surrounding bodies is proportionally lowered. + +EMILY. + +If, for instance, I was to bring a large piece of ice into this room, +the ice would in time be melted, by absorbing caloric from the general +radiation which is going on throughout the room; and as it would +contribute very little caloric in return for what is absorbed, the room +would necessarily be cooled by it. + +MRS. B. + +Just so; and as in consequence of the mirrors, a more considerable +exchange of rays takes place between the ice and the thermometer, than +between these and any of the surrounding bodies, the temperature of the +thermometer must be more lowered than that of any other adjacent object. + +CAROLINE. + +I confess I do not perfectly understand your explanation. + +MRS. B. + +This experiment is exactly similar to that made with the heated bullet: +for, if we consider the thermometer as the hot body (which it certainly +is in comparison to the ice), you may then easily understand that it is +by the loss of the calorific rays which the thermometer sends to the +ice, and not by any cold rays received from it, that the fall of the +mercury is occasioned: for the ice, far from emitting rays of cold, +sends forth rays of caloric, which diminish the loss sustained by the +thermometer. + +Let us say, for instance, that the radiation of the thermometer towards +the ice is equal to 20, and that of the ice towards the thermometer to +10: the exchange in favour of the ice is as 20 is to 10, or the +thermometer absolutely loses 10, whilst the ice gains 10. + +CAROLINE. + +But if the ice actually sends rays of caloric to the thermometer, must +not the latter fall still lower when the ice is removed? + +MRS. B. + +No; for the space that the ice occupied, admits rays from all the +surrounding bodies to pass through it; and those being of the same +temperature as the thermometer, will not affect it, because as much heat +now returns to the thermometer as radiates from it. + +CAROLINE. + +I must confess that you have explained this in so satisfactory a manner, +that I cannot help being convinced now that cold has no real claim to +the rank of a positive being. + +MRS. B. + +Before I conclude the subject of radiation I must observe to you that +different bodies, (or rather surfaces,) possess the power of radiating +caloric in very different degrees. + +Some very curious experiments have been made by Mr. Leslie on this +subject, and it was for this purpose that he invented the differential +thermometer; with its assistance he ascertained that black surfaces +radiate most, glass next, and polished surfaces the least of all. + +EMILY. + +Supposing these surfaces, of course, to be all of the same temperature. + +MRS. B. + +Undoubtedly. I will now show you the very simple and ingenious +apparatus, by means of which he made these experiments. This cubical tin +vessel or canister, has each of its sides externally covered with +different materials; the one is simply blackened; the next is covered +with white paper; the third with a pane of glass, and in the fourth the +polished tin surface remains uncovered. We shall fill this vessel with +hot water, so that there can be no doubt but that all its sides will be +of the same temperature. Now let us place it in the focus of one of the +mirrors, making each of its sides front it in succession. We shall begin +with the black surface. + +CAROLINE. + +It makes the thermometer which is in the focus of the other mirror rise +considerably. Let us turn the paper surface towards the mirror. The +thermometer falls a little, therefore of course this side cannot emit or +radiate so much caloric as the blackened side. + +EMILY. + +This is very surprising; for the sides are exactly of the same size, and +must be of the same temperature. But let us try the glass surface. + +MRS. B. + +The thermometer continues falling, and with the plain surface it falls +still lower; these two surfaces therefore radiate less and less. + +CAROLINE. + +I think I have found out the reason of this. + +MRS. B. + +I should be very happy to hear it, for it has not yet (to my knowledge) +been accounted for. + +CAROLINE. + +The water within the vessel gradually cools, and the thermometer in +consequence gradually falls. + +MRS. B. + +It is true that the water cools, but certainly in much less proportion +than the thermometer descends, as you will perceive if you now change +the tin surface for the black one. + +CAROLINE. + +I was mistaken certainly, for the thermometer rises again now that the +black surface fronts the mirror. + +MRS. B. + +And yet the water in the vessel is still cooling, Caroline. + +EMILY. + +I am surprised that the tin surface should radiate the least caloric, +for a metallic vessel filled with hot water, a silver teapot, for +instance, feels much hotter to the hand than one of black earthen ware. + +MRS. B. + +That is owing to the different power which various bodies possess for +_conducting_ caloric, a property which we shall presently examine. Thus, +although a metallic vessel feels warmer to the hand, a vessel of this +kind is known to preserve the heat of the liquid within, better than one +of any other materials; it is for this reason that silver teapots make +better tea than those of earthen ware. + +EMILY. + +According to these experiments, light-coloured dresses, in cold weather, +should keep us warmer than black clothes, since the latter radiate so +much more than the former. + +MRS. B. + +And that is actually the case. + +EMILY. + +This property, of different surfaces to radiate in different degrees, +appears to me to be at variance with the equilibrium of caloric; since +it would imply that those bodies which radiate most, must ultimately +become coldest. + +Suppose that we were to vary this experiment, by using two metallic +vessels full of boiling water, the one blackened, the other not; would +not the black one cool the first? + +CAROLINE. + +True; but when they were both brought down to the temperature of the +room, the interchange of caloric between the canisters and the other +bodies of the room being then equal, their temperatures would remain the +same. + +EMILY. + +I do not see why that should be the case; for if different surfaces of +the same temperature radiate in different degrees when heated, why +should they not continue to do so when cooled down to the temperature of +the room? + +MRS. B. + +You have started a difficulty, Emily, which certainly requires +explanation. It is found by experiment that the power of absorption +corresponds with and is proportional to that of radiation; so that under +equal temperatures, bodies compensate for the greater loss they sustain +in consequence of their greater radiation by their greater absorption; +so that if you were to make your experiment in an atmosphere heated like +the canisters, to the temperature of boiling water, though it is true +that the canisters would radiate in different degrees, no change of +temperature would be produced in them, because they would each absorb +caloric in proportion to their respective radiation. + +EMILY. + +But would not the canisters of boiling water also absorb caloric in +different degrees in a room of the common temperature? + +MRS. B. + +Undoubtedly they would. But the various bodies in the room would not, at +a lower temperature, furnish either of the canisters with a sufficiency +of caloric to compensate for the loss they undergo; for, suppose the +black canister to absorb 400 rays of caloric, whilst the metallic one +absorbed only 200; yet if the former radiate 800, whilst the latter +radiates only 400, the black canister will be the first cooled down to +the temperature of the room. But from the moment the equilibrium of +temperature has taken place, the black canister, both receiving and +giving out 400 rays, and the metallic one 200, no change of temperature +will take place. + +EMILY. + +I now understand it extremely well. But what becomes of the surplus of +calorific rays, which good radiators emit and bad radiators refuse to +receive; they must wander about in search of a resting-place? + +MRS. B. + +They really do so; for they are rejected and sent back, or, in other +words, _reflected_ by the bodies which are bad radiators of caloric; and +they are thus transmitted to other bodies which happen to lie in their +way, by which they are either absorbed or again reflected, according as +the property of reflection, or that of absorption, predominates in these +bodies. + +CAROLINE. + +I do not well understand the difference between radiating and reflecting +caloric, for the caloric that is reflected from a body proceeds from it +in straight lines, and may surely be said to radiate from it? + +MRS. B. + +It is true that there at first appears to be a great analogy between +_radiation_ and _reflection_, as they equally convey the idea of the +transmission of caloric. + +But if you consider a little, you will perceive that when a body +_radiates_ caloric, the heat which it emits not only proceeds from, but +has its origin in the body itself. Whilst when a body _reflects_ +caloric, it parts with none of its own caloric, but only reflects that +which it receives from other bodies. + +EMILY. + +Of this difference we have very striking examples before us, in the tin +vessel of water, and the concave mirrors; the first radiates its own +heat, the latter reflect the heat which they receive from other bodies. + +CAROLINE. + +Now, that I understand the difference, it no longer surprises me that +bodies which radiate, or part with their own caloric freely, should not +have the power of transmitting with equal facility that which they +receive from other bodies. + +EMILY. + +Yet no body can be said to possess caloric of its own, if all caloric is +originally derived from the sun. + +MRS. B. + +When I speak of a body radiating its own caloric, I mean that which it +has absorbed and incorporated either immediately from the sun’s rays, or +through the medium of any other substance. + +CAROLINE. + +It seems natural enough that the power of absorption should be in +opposition to that of reflection, for the more caloric a body receives, +the less it will reject. + +EMILY. + +And equally so that the power of radiation should correspond with that +of absorption. It is, in fact, cause and effect; for a body cannot +radiate heat without having previously absorbed it; just as a spring +that is well fed flows abundantly. + +MRS. B. + +Fluids are in general very bad radiators of caloric; and air neither +radiates nor absorbs caloric in any sensible degree. + +We have not yet concluded our observations on free caloric. But I shall +defer, till our next meeting, what I have further to say on this +subject. I believe it will afford us ample conversation for another +interview. + + + + +CONVERSATION III. + +CONTINUATION OF THE SUBJECT. + + +MRS. B. + +In our last conversation, we began to examine the tendency of caloric to +restore an equilibrium of temperature. This property, when once well +understood, affords the explanation of a great variety of facts which +appeared formerly unaccountable. You must observe, in the first place, +that the effect of this tendency is gradually to bring all bodies that +are in contact to the same temperature. Thus, the fire which burns in +the grate, communicates its heat from one object to another, till every +part of the room has an equal proportion of it. + +EMILY. + +And yet this book is not so cold as the table on which it lies, though +both are at an equal distance from the fire, and actually in contact +with each other, so that, according to your theory, they should be +exactly of the same temperature. + +CAROLINE. + +And the hearth, which is much nearer the fire than the carpet, is +certainly the colder of the two. + +MRS. B. + +If you ascertain the temperature of these several bodies by a +thermometer (which is a much more accurate test than your feeling), you +will find that it is exactly the same. + +CAROLINE. + +But if they are of the same temperature, why should the one feel colder +than the other? + +MRS. B. + +The hearth and the table feel colder than the carpet or the book, +because the latter are not such good _conductors of heat_ as the former. +Caloric finds a more easy passage through marble and wood, than through +leather and worsted; the two former will therefore absorb heat more +rapidly from your hand, and consequently give it a stronger sensation of +cold than the two latter, although they are all of them really of the +same temperature. + +CAROLINE. + +So, then, the sensation I feel on touching a cold body, is in proportion +to the rapidity with which my hand yields its heat to that body? + +MRS. B. + +Precisely; and, if you lay your hand successively on every object in the +room, you will discover which are good, and which are bad conductors of +heat, by the different degrees of cold you feel. But, in order to +ascertain this point, it is necessary that the several substances should +be of the same temperature, which will not be the case with those that +are very near the fire, or those that are exposed to a current of cold +air from a window or door. + +EMILY. + +But what is the reason that some bodies are better conductors of heat +than others? + +MRS. B. + +This is a point not well ascertained. It has been conjectured that a +certain union or adherence takes place between the caloric and the +particles of the body through which it passes. If this adherence be +strong, the body detains the heat, and parts with it slowly and +reluctantly; if slight, it propagates it freely and rapidly. The +conducting power of a body is therefore, inversely, as its tendency to +unite with caloric. + +EMILY. + +That is to say, that the best conductors are those that have the least +affinity for caloric. + +MRS. B. + +Yes; but the term affinity is objectionable in this case, because, as +that word is used to express a chemical attraction (which can be +destroyed only by decomposition), it cannot be applicable to the slight +and transient union that takes place between free caloric and the bodies +through which it passes; an union which is so weak, that it constantly +yields to the tendency which caloric has to an equilibrium. Now you +clearly understand, that the passage of caloric, through bodies that are +good conductors, is much more rapid than through those that are bad +conductors, and that the former both give and receive it more quickly, +and therefore, in a given time, more abundantly, than bad conductors, +which makes them feel either hotter or colder, though they may be, in +fact, both of the same temperature. + +CAROLINE. + +Yes, I understand it now; the table, and the book lying upon it, being +really of the same temperature, would each receive, in the same space of +time, the same quantity of heat from my hand, were their conducting +powers equal; but as the table is the best conductor of the two, it will +absorb the heat from my hand more rapidly, and consequently produce a +stronger sensation of cold than the book. + +MRS. B. + +Very well, my dear; and observe, likewise, that if you were to heat the +table and the book an equal number of degrees above the temperature of +your body, the table, which before felt the colder, would now feel the +hotter of the two; for, as in the first case it took the heat most +rapidly from your hand, so it will now impart heat most rapidly to it. +Thus the marble table, which seems to us colder than the mahogany one, +will prove the hotter of the two to the ice; for, if it takes heat more +rapidly from our hands, which are warmer, it will give out heat more +rapidly to the ice, which is colder. Do you understand the reason of +these apparently opposite effects? + +EMILY. + +Perfectly. A body which is a good conductor of caloric, affords it a +free passage; so that it penetrates through that body more rapidly than +through one which is a bad conductor; and consequently, if it is colder +than your hand, you lose more caloric, and if it is hotter, you gain +more than with a bad conductor of the same temperature. + +MRS. B. + +But you must observe that this is the case only when the conductors are +either hotter or colder than your hand; for, if you heat different +conductors to the temperature of your body, they will all feel equally +warm, since the exchange of caloric between bodies of the same +temperature is equal. Now, can you tell me why flannel clothing, which +is a very bad conductor of heat, prevents our feeling cold? + +CAROLINE. + +It prevents the cold from penetrating . . . . . . . . + +MRS. B. + +But you forget that cold is only a negative quality. + +CAROLINE. + +True; it only prevents the heat of our bodies from escaping so rapidly +as it would otherwise do. + +MRS. B. + +Now you have explained it right; the flannel rather keeps in the heat, +than keeps out the cold. Were the atmosphere of a higher temperature +than our bodies, it would be equally efficacious in keeping their +temperature at the same degree, as it would prevent the free access of +the external heat, by the difficulty with which it conducts it. + +EMILY. + +This, I think, is very clear. Heat, whether external or internal, cannot +easily penetrate flannel; therefore in cold weather it keeps us warm; +and if the weather was hotter than our bodies, it would keep us cool. + +MRS. B. + +The most dense bodies are, generally speaking, the best conductors of +heat; probably because the denser the body the greater are the number of +points or particles that come in contact with caloric. At the common +temperature of the atmosphere a piece of metal will feel much colder +than a piece of wood, and the latter than a piece of woollen cloth; this +again will feel colder than flannel; and down, which is one of the +lightest, is at the same time one of the warmest bodies. + +CAROLINE. + +This is, I suppose, the reason that the plumage of birds preserves them +so effectually from the influence of cold in winter? + +MRS. B. + +Yes; but though feathers in general are an excellent preservative +against cold, down is a kind of plumage peculiar to aquatic birds, and +covers their chest, which is the part most exposed to the water; for +though the surface of the water is not of a lower temperature than the +atmosphere, yet, as it is a better conductor of heat, it feels much +colder, consequently the chest of the bird requires a warmer covering +than any other part of its body. Besides, the breasts of aquatic birds +are exposed to cold not only from the temperature of the water, but also +from the velocity with which the breast of the bird strikes against it; +and likewise from the rapid evaporation occasioned in that part by the +air against which it strikes, after it has been moistened by dipping +from time to time into the water. + +If you hold a finger of one hand motionless in a glass of water, and at +the same time move a finger of the other hand swiftly through water of +the same temperature, a different sensation will be soon perceived in +the different fingers. + +Most animal substances, especially those which Providence has assigned +as a covering for animals, such as fur, wool, hair, skin, &c. are bad +conductors of heat, and are, on that account, such excellent +preservatives against the inclemency of winter, that our warmest apparel +is made of these materials. + +EMILY. + +Wood is, I dare say, not so good a conductor as metal, and it is for +that reason, no doubt, that silver teapots have always wooden handles. + +MRS. B. + +Yes; and it is the facility with which metals conduct caloric that made +you suppose that a silver pot radiated more caloric than an earthen one. +The silver pot is in fact hotter to the hand when in contact with it; +but it is because its conducting power more than counterbalances its +deficiency in regard to radiation. + +We have observed that the most dense bodies are in general the best +conductors; and metals, you know, are of that class. Porous bodies, such +as the earths and wood, are worse conductors, chiefly, I believe, on +account of their pores being filled with air; for air is a remarkably +bad conductor. + +CAROLINE. + +It is a very fortunate circumstance that air should be a bad conductor, +as it tends to preserve the heat of the body when exposed to cold +weather. + +MRS. B. + +It is one of the many benevolent dispensations of Providence, in order +to soften the inclemency of the seasons, and to render almost all +climates habitable to man. + +In fluids of different densities, the power of conducting heat varies no +less remarkably; if you dip your hand into this vessel full of mercury, +you will scarcely conceive that its temperature is not lower than that +of the atmosphere. + +CAROLINE. + +Indeed I know not how to believe it, it feels so extremely cold. --But +we may easily ascertain its true temperature by the thermometer. --It is +really not colder than the air;--the apparent difference then is +produced merely by the difference of the conducting power in mercury and +in air. + +MRS. B. + +Yes; hence you may judge how little the sense of feeling is to be relied +on as a test of the temperature of bodies, and how necessary a +thermometer is for that purpose. + +It has indeed been doubted whether fluids have the power of conducting +caloric in the same manner as solid bodies. Count Rumford, a very few +years since, attempted to prove, by a variety of experiments, that +fluids, when at rest, were not at all endowed with this property. + +CAROLINE. + +How is that possible, since they are capable of imparting cold or heat +to us; for if they did not conduct heat, they would neither take it +from, nor give it to us? + +MRS. B. + +Count Rumford did not mean to say that fluids would not communicate +their heat to solid bodies; but only that heat does not pervade fluids, +that is to say, is not transmitted from one particle of a fluid to +another, in the same manner as in solid bodies. + +EMILY. + +But when you heat a vessel of water over the fire, if the particles of +water do not communicate heat to each other, how does the water become +hot throughout? + +MRS. B. + +By constant agitation. Water, as you have seen, expands by heat in the +same manner as solid bodies; the heated particles of water, therefore, +at the bottom of the vessel, become specifically lighter than the rest +of the liquid, and consequently ascend to the surface, where, parting +with some of their heat to the colder atmosphere, they are condensed, +and give way to a fresh succession of heated particles ascending from +the bottom, which having thrown off their heat at the surface, are in +their turn displaced. Thus every particle is successively heated at the +bottom, and cooled at the surface of the liquid; but as the fire +communicates heat more rapidly than the atmosphere cools the succession +of surfaces, the whole of the liquid in time becomes heated. + +CAROLINE. + +This accounts most ingeniously for the propagation of heat upwards. But +suppose you were to heat the upper surface of a liquid, the particles +being specifically lighter than those below, could not descend: how +therefore would the heat be communicated downwards? + +MRS. B. + +If there were no agitation to force the heated surface downwards, Count +Rumford assures us that the heat would not descend. In proof of this he +succeeded in making the upper surface of a vessel of water boil and +evaporate, while a cake of ice remained frozen at the bottom. + +CAROLINE. + +That is very extraordinary indeed! + +MRS. B. + +It appears so, because we are not accustomed to heat liquids by their +upper surface; but you will understand this theory better if I show you +the internal motion that takes place in liquids when they experience a +change of temperature. The motion of the liquid itself is indeed +invisible from the extreme minuteness of its particles; but if you mix +with it any coloured dust, or powder, of nearly the same specific +gravity as the liquid, you may judge of the internal motion of the +latter by that of the coloured dust it contains. --Do you see the small +pieces of amber moving about in the liquid contained in this phial? + +CAROLINE. + +Yes, perfectly. + +MRS. B. + +We shall now immerse the phial in a glass of hot water, and the motion +of the liquid will be shown, by that which it communicates to the amber. + +EMILY. + +I see two currents, the one rising along the sides of the phial, the +other descending in the centre: but I do not understand the reason of +this. + +MRS. B. + +The hot water communicates its caloric, through the medium of the phial, +to the particles of the fluid nearest to the glass; these dilate and +ascend laterally to the surface, where, in parting with their heat, they +are condensed, and in descending, form the central current. + +CAROLINE. + +This is indeed a very clear and satisfactory experiment; but how much +slower the currents now move than they did at first? + +MRS. B. + +It is because the circulation of particles has nearly produced an +equilibrium of temperature between the liquid in the glass and that in +the phial. + +CAROLINE. + +But these communicate laterally, and I thought that heat in liquids +could be propagated only upwards. + +MRS. B. + +You do not take notice that the heat is imparted from one liquid to the +other, through the medium of the phial itself, the external surface of +which receives the heat from the water in the glass, whilst its internal +surface transmits it to the liquid it contains. Now take the phial out +of the hot water, and observe the effect of its cooling. + +EMILY. + +The currents are reversed; the external current now descends, and the +internal one rises. --I guess the reason of this change:-- the phial +being in contact with cold air instead of hot water, the external +particles are cooled instead of being heated; they therefore descend and +force up the central particles, which, being warmer, are consequently +lighter. + +MRS. B. + +It is just so. Count Rumford hence infers that no alteration of +temperature can take place in a fluid, without an internal motion of its +particles, and as this motion is produced only by the comparative levity +of the heated particles, heat cannot be propagated downwards. + +But though I believe that Count Rumford’s theory as to heat being +incapable of pervading fluids is not strictly correct, yet there is, no +doubt, much truth in his observation, that the communication is +materially promoted by a motion of the parts; and this accounts for the +cold that is found to prevail at the bottom of the lakes in Switzerland, +which are fed by rivers issuing from the snowy Alps. The water of these +rivers being colder, and therefore more dense than that of the lakes, +subsides to the bottom, where it cannot be affected by the warmer +temperature of the surface; the motion of the waves may communicate this +temperature to some little depth, but it can descend no further than the +agitation extends. + +EMILY. + +But when the atmosphere is colder than the lake, the colder surface of +the water will descend, for the very reason that the warmer will not. + +MRS. B. + +Certainly: and it is on this account that neither a lake, nor any body +of water whatever, can be frozen until every particle of the water has +risen to the surface to give off its caloric to the colder atmosphere; +therefore the deeper a body of water is, the longer will be the time it +requires to be frozen. + +EMILY. + +But if the temperature of the whole body of water be brought down to the +freezing point, why is only the surface frozen? + +MRS. B. + +The temperature of the whole body is lowered, but not to the freezing +point. The diminution of heat, as you know, produces a contraction in +the bulk of fluids, as well as of solids. This effect, however, does not +take place in water below the temperature of 40 degrees, which is 8 +degrees above the freezing point. At that temperature, therefore, the +internal motion, occasioned by the increased specific gravity of the +condensed particles, ceases; for when the water at the surface no longer +condenses, it will no longer descend, and leave a fresh surface exposed +to the atmosphere: this surface alone, therefore, will be further +exposed to its severity, and will soon be brought down to the freezing +point, when it becomes ice, which being a bad conductor of heat, +preserves the water beneath a long time from being affected by the +external cold. + +CAROLINE. + +And the sea does not freeze, I suppose, because its depth is so great, +that a frost never lasts long enough to bring down the temperature of +such a great body of water to 40 degrees? + +MRS. B. + +That is one reason why the sea, as a large mass of water, does not +freeze. But, independently of this, salt water does not freeze till it +is cooled much below 32 degrees, and with respect to the law of +condensation, salt water is an exception, as it condenses even many +degrees below the freezing point. When the caloric of fresh water, +therefore, is imprisoned by the ice on its surface, the ocean still +continues throwing off heat into the atmosphere, which is a most signal +dispensation of Providence to moderate the intensity of the cold in +winter. + +CAROLINE. + +This theory of the non-conducting power of liquids, does not, I suppose, +hold good with respect to air, otherwise the atmosphere would not be +heated by the rays of the sun passing through it? + +MRS. B. + +Nor is it heated in that way. The pure atmosphere is a perfectly +transparent medium, which neither radiates, absorbs, nor conducts +caloric, but transmits the rays of the sun to us without in any way +diminishing their intensity. The air is therefore not more heated, by +the sun’s rays passing through it, than diamond, glass, water, or any +other transparent medium. + +CAROLINE. + +That is very extraordinary! Are glass windows not heated then by the sun +shining on them? + +MRS. B. + +No; not if the glass be perfectly transparent. A most convincing proof +that glass transmits the rays of the sun without being heated by them is +afforded by the burning lens, which by converging the rays to a focus +will set combustible bodies on fire, without its own temperature being +raised. + +EMILY. + +Yet, Mrs. B., if I hold a piece of glass near the fire it is almost +immediately warmed by it; the glass therefore must retain some of the +caloric radiated by the fire? Is it that the solar rays alone pass +freely through glass without paying tribute? It seems unaccountable that +the radiation of a common fire should have power to do what the sun’s +rays cannot accomplish. + +MRS. B. + +It is not because the rays from the fire have more power, but rather +because they have less, that they heat glass and other transparent +bodies. It is true, however, that as you approach the source of heat the +rays being nearer each other, the heat is more condensed, and can +produce effects of which the solar rays, from the great distance of +their source, are incapable. Thus we should find it impossible to roast +a joint of meat by the sun’s rays, though it is so easily done by +culinary heat. Yet caloric emanated from burning bodies, which is +commonly called _culinary heat_, has neither the intensity nor the +velocity of solar rays. All caloric, we have said, is supposed to +proceed originally from the sun; but after having been incorporated with +terrestrial bodies, and again given out by them, though its nature is +not essentially altered, it retains neither the intensity nor the +velocity with which it first emanated from that luminary; it has +therefore not the power of passing through transparent mediums, such as +glass and water, without being partially retained by those bodies. + +EMILY. + +I recollect that in the experiment on the reflection of heat, the glass +skreen which you interposed between the burning taper and the mirror, +arrested the rays of caloric, and suffered only those of light to pass +through it. + +CAROLINE. + +Glass windows, then, though they cannot be heated by the sun shining on +them, may be heated internally by a fire in the room? But, Mrs. B., +since the atmosphere is not warmed by the solar rays passing through it, +how does it obtain heat; for all the fires that are burning on the +surface of the earth would contribute very little towards warming it? + +EMILY. + +The radiation of heat is not confined to burning bodies: for all bodies, +you know, have that property; therefore, not only every thing upon the +surface of the earth, but the earth itself, must radiate heat; and this +terrestrial caloric, not having, I suppose, sufficient power to traverse +the atmosphere, communicates heat to it. + +MRS. B. + +Your inference is extremely well drawn, Emily; but the foundation on +which it rests is not sound; for the fact is, that terrestrial or +culinary heat, though it cannot pass through the denser transparent +mediums, such as glass or water, without loss, traverses the atmosphere +completely: so that all the heat which the earth radiates, unless it +meet with clouds or any foreign body to intercept its passage, passes +into the distant regions of the universe. + +CAROLINE. + +What a pity that so much heat should be wasted! + +MRS. B. + +Before you are tempted to object to any law of nature, reflect whether +it may not prove to be one of the numberless dispensations of Providence +for our good. If all the heat which the earth has received from the sun, +since the creation had been accumulated in it, its temperature by this +time would, no doubt, have been more elevated than any human being could +have borne. + +CAROLINE. + +I spoke indeed very inconsiderately. But, Mrs. B., though the earth, at +such a high temperature, might have scorched our feet, we should always +have had a cool refreshing air to breathe, since the radiation of the +earth does not heat the atmosphere. + +EMILY. + +The cool air would have afforded but very insufficient refreshment, +whilst our bodies were exposed to the burning radiation of the earth. + +MRS. B. + +Nor should we have breathed a cool air; for though it is true that heat +is not communicated to the atmosphere by radiation, yet the air is +warmed by contact with heated bodies, in the same manner as solids or +liquids. The stratum of air which is immediately in contact with the +earth is heated by it; it becomes specifically lighter and rises, making +way for another stratum of air which is in its turn heated and carried +upwards; and thus each successive stratum of air is warmed by coming in +contact with the earth. You may perceive this effect in a sultry day, if +you attentively observe the strata of air near the surface of the earth; +they appear in constant agitation, for though it is true the air is +itself invisible, yet the sun shining on the vapours floating in it, +render them visible, like the amber dust in the water. The temperature +of the surface of the earth is therefore the source from whence the +atmosphere derives its heat, though it is communicated neither by +radiation, nor transmitted from one particle of it to another by the +conducting power; but every particle of air must come in contact with +the earth in order to receive heat from it. + +EMILY. + +Wind then by agitating the air should contribute to cool the earth and +warm the atmosphere, by bringing a more rapid succession of fresh strata +of air in contact with the earth, and yet in general wind feels cooler +than still air? + +MRS. B. + +Because the agitation of the air carries off heat from the surface of +our bodies more rapidly than still air, by occasioning a greater number +of points of contact in a given time. + +EMILY. + +Since it is from the earth and not the sun that the atmosphere receives +its heat, I no longer wonder that elevated regions should be colder than +plains and valleys; it was always a subject of astonishment to me, that +in ascending a mountain and approaching the sun, the air became colder +instead of being more heated. + +MRS. B. + +At the distance of about a hundred million of miles, which we are from +the sun, the approach of a few thousand feet makes no sensible +difference, whilst it produces a very considerable effect with regard to +the warming the atmosphere at the surface of the earth. + +CAROLINE. + +Yet as the warm air rises from the earth and the cold air descends to +it, I should have supposed that heat would have accumulated in the upper +regions of the atmosphere, and that we should have felt the air warmer +as we ascended? + +MRS. B. + +The atmosphere, you know, diminishes in density, and consequently in +weight, as it is more distant from the earth; the warm air, therefore, +rises only till it meets with a stratum of air of its own density; and +it will not ascend into the upper regions of the atmosphere until all +the parts beneath have been previously heated. The length of summer even +in warm climates does not heat the air sufficiently to melt the snow +which has accumulated during the winter on very high mountains, although +they are almost constantly exposed to the heat of the sun’s rays, being +too much elevated to be often enveloped in clouds. + +EMILY. + +These explanations are very satisfactory; but allow me to ask you one +more question respecting the increased levity of heated liquids. You +said that when water was heated over the fire, the particles at the +bottom of the vessel ascended as soon as heated, in consequence of their +specific levity: why does not the same effect continue when the water +boils, and is converted into steam? and why does the steam rise from the +surface, instead of the bottom of the liquid? + +MRS. B. + +The steam or vapour does ascend from the bottom, though it seems to +arise from the surface of the liquid. We shall boil some water in this +Florence flask, (PLATE IV. Fig. 1.) in order that you may be well +acquainted with the process of ebullition;--you will then see, through +the glass, that the vapour rises in bubbles from the bottom. We shall +make it boil by means of a lamp, which is more convenient for this +purpose than the chimney fire. + + [Illustration: Plate IV. Vol. I. p. 84. + + Fig. 1. Pneumatic Pump. + Ether evaporated & water frozen in the air pump. + A Phial of Ether. + B Glass vessel containing water. + C.C Thermometers one in the Ether, the other in the water. + + Fig. 2. Boiling water in a flask over a Patent lamp.] + +EMILY. + +I see some small bubbles ascend, and a great many appear all over the +inside of the flask; does the water begin to boil already? + +MRS. B. + +No; what you now see are bubbles of air, which were either dissolved in +the water, or attached to the inner surface of the flask, and which, +being rarefied by the heat, ascend in the water. + +EMILY. + +But the heat which rarefies the air inclosed in the water must rarefy +the water at the same time; therefore, if it could remain stationary in +the water when both were cold, I do not understand why it should not +when both are equally heated? + +MRS. B. + +Air being much less dense than water, is more easily rarefied; the +former, therefore, expands to a great extent, whilst the latter +continues to occupy nearly the same space; for water dilates +comparatively but very little without changing its state and becoming +vapour. Now that the water in the flask begins to boil, observe what +large bubbles rise from the bottom of it. + +EMILY. + +I see them perfectly; but I wonder that they have sufficient power to +force themselves through the water. + +CAROLINE. + +They _must_ rise, you know, from their specific levity. + +MRS. B. + +You are right, Caroline; but vapour has not in all liquids (when brought +to the degree of vaporization) the power of overcoming the pressure of +the less heated surface. Metals, for instance, mercury excepted, +evaporate only from the surface; therefore no vapour will ascend from +them till the degree of heat which is necessary to form it has reached +the surface; that is to say, till the whole of the liquid is brought to +a state of ebullition. + +EMILY. + +I have observed that steam, immediately issuing from the spout of a +teakettle, is less visible than at a further distance from it; yet it +must be more dense when it first evaporates, than when it begins to +diffuse itself in the air. + +MRS. B. + +When the steam is first formed, it is so perfectly dissolved by caloric, +as to be invisible. In order however to understand this, it will be +necessary for me to enter into some explanation respecting the nature of +SOLUTION. Solution takes place whenever a body is melted in a fluid. In +this operation the body is reduced to such a minute state of division by +the fluid, as to become invisible in it, and to partake of its fluidity; +but in common solutions this happens without any decomposition, the body +being only divided into its integrant particles by the fluid in which it +is melted. + +CAROLINE. + +It is then a mode of destroying the attraction of aggregation. + +MRS. B. + +Undoubtedly. --The two principal solvent fluids are _water_, and +_caloric_. You may have observed that if you melt salt in water, it +totally disappears, and the water remains clear, and transparent as +before; yet though the union of these two bodies appears so perfect, it +is not produced by any chemical combination; both the salt and the water +remain unchanged; and if you were to separate them by evaporating the +latter, you would find the salt in the same state as before. + +EMILY. + +I suppose that water is a solvent for solid bodies, and caloric for +liquids? + +MRS. B. + +Liquids of course can only be converted into vapour by caloric. But the +solvent power of this agent is not at all confined to that class of +bodies; a great variety of solid substances are dissolved by heat: thus +metals, which are insoluble in water, can be dissolved by intense heat, +being first fused or converted into a liquid, and then rarefied into an +invisible vapour. Many other bodies, such as salt, gums, &c. yield to +either of these solvents. + +CAROLINE. + +And that, no doubt, is the reason why hot water will melt them so much +better than cold water? + +MRS. B. + +It is so. Caloric may, indeed, be considered as having, in every +instance, some share in the solution of a body by water, since water, +however low its temperature may be, always contains more or less +caloric. + +EMILY. + +Then, perhaps, water owes its solvent power merely to the caloric +contained in it? + +MRS. B. + +That, probably, would be carrying the speculation too far; I should +rather think that water and caloric unite their efforts to dissolve a +body, and that the difficulty or facility of effecting this, depend both +on the degree of attraction of aggregation to be overcome, and on the +arrangement of the particles which are more or less disposed to be +divided and penetrated by the solvent. + +EMILY. + +But have not all liquids the same solvent power as water? + +MRS. B. + +The solvent power of other liquids varies according to their nature, and +that of the substances submitted to their action. Most of these +solvents, indeed, differ essentially from water, as they do not merely +separate the integrant particles of the bodies which they dissolve, but +attack their constituent principles by the power of chemical attraction, +thus producing a true decomposition. These more complicated operations +we must consider in another place, and confine our attention at present +to the solutions by water and caloric. + +CAROLINE. + +But there are a variety of substances which, when dissolved in water, +make it thick and muddy, and destroy its transparency. + +MRS. B. + +In this case it is not a solution, but simply a mixture. I shall show +you the difference between a solution and a mixture, by putting some +common salt into one glass of water, and some powder of chalk into +another; both these substances are white, but their effect on the water +will be very different. + +CAROLINE. + +Very different indeed! The salt entirely disappears and leaves the water +transparent, whilst the chalk changes it into an opaque liquid like +milk. + +EMILY. + +And would lumps of chalk and salt produce similar effects on water? + +MRS. B. + +Yes, but not so rapidly; salt is, indeed, soon melted though in a lump; +but chalk, which does not mix so readily with water, would require a +much greater length of time; I therefore preferred showing you the +experiment with both substances reduced to powder, which does not in any +respect alter their nature, but facilitates the operation merely by +presenting a greater quantity of surface to the water. + +I must not forget to mention a very curious circumstance respecting +solutions, which is, that a fluid is not nearly so much increased in +bulk by holding a body in solution, as it would by mere mixture with the +body. + +CAROLINE. + +That seems impossible; for two bodies cannot exist together in the same +space. + +MRS. B. + +Two bodies may, by condensation, occupy less space when in union than +when separate, and this I can show you by an easy experiment. + +This phial, which contains some salt, I shall fill with water, pouring +it in quickly, so as not to dissolve much of the salt; and when it is +quite full I cork it. --If I now shake the phial till the salt is +dissolved, you will observe that it is no longer full. + +CAROLINE. + +I shall try to add a little more salt. --But now, you see, Mrs. B., the +water runs over. + +MRS. B. + +Yes; but observe that the last quantity of salt you put in remains solid +at the bottom, and displaces the water; for it has already melted all +the salt it is capable of holding in solution. This is called the point +of _saturation_; and the water in this case is said to be _saturated_ +with salt. + +EMILY. + +I think I now understand the solution of a solid body by water +perfectly: but I have not so clear an idea of the solution of a liquid +by caloric. + +MRS. B. + +It is probably of a similar nature; but as caloric is an invisible +fluid, its action as a solvent is not so obvious as that of water. +Caloric, we may conceive, dissolves water, and converts it into vapour +by the same process as water dissolves salt; that is to say, the +particles of water are so minutely divided by the caloric as to become +invisible. Thus, you are now enabled to understand why the vapour of +boiling water, when it first issues from the spout of a kettle, is +invisible; it is so, because it is then completely dissolved by caloric. +But the air with which it comes in contact, being much colder than the +vapour, the latter yields to it a quantity of its caloric. The particles +of vapour being thus in a great measure deprived of their solvent, +gradually collect, and become visible in the form of steam, which is +water in a state of imperfect solution; and if you were further to +deprive it of its caloric, it would return to its original liquid state. + +CAROLINE. + +That I understand very well. If you hold a cold plate over a tea-urn, +the steam issuing from it will be immediately converted into drops of +water by parting with its caloric to the plate; but in what state is the +steam, when it becomes invisible by being diffused in the air? + +MRS. B. + +It is not merely diffused, but is again dissolved by the air. + +EMILY. + +The air, then, has a solvent power, like water and caloric? + +MRS. B. + +This was formerly believed to be the case. But it appears from more +recent enquiries that the solvent power of the atmosphere depends solely +upon the caloric contained in it. Sometimes the watery vapour diffused +in the atmosphere is but imperfectly dissolved, as is the case in the +formation of clouds and fogs; but if it gets into a region sufficiently +warm, it becomes perfectly invisible. + +EMILY. + +Can any water dissolve in the atmosphere without its being previously +converted into vapour by boiling? + +MRS. B. + +Unquestionably; and this constitutes the difference between +_vaporization_ and _evaporation_. Water, when heated to the boiling +point, can no longer exist in the form of water, and must necessarily be +converted into vapour or steam, whatever may be the state and +temperature of the surrounding medium; this is called vaporization. But +the atmosphere, by means of the caloric it contains, can take up a +certain portion of water at any temperature, and hold it in a state of +solution. This is simply evaporation. Thus the atmosphere is continually +carrying off moisture from the surface of the earth, until it is +saturated with it. + +CAROLINE. + +That is the case, no doubt, when we feel the atmosphere damp. + +MRS. B. + +On the contrary, when the moisture is well dissolved it occasions no +humidity: it is only when in a state of imperfect solution and floating +in the atmosphere, in the form of watery vapour, that it produces +dampness. This happens more frequently in winter than in summer; for the +lower the temperature of the atmosphere, the less water it can dissolve; +and in reality it never contains so much moisture as in a dry hot +summer’s day. + +CAROLINE. + +You astonish me! But why, then, is the air so dry in frosty weather, +when its temperature is at the lowest? + +EMILY. + +This, I conjecture, proceeds not so much from the moisture being +dissolved, as from its being frozen; is not that the case? + +MRS. B. + +It is; and the freezing of the watery vapour which the atmospheric heat +could not dissolve, produces what is called a hoar frost; for the +particles descend in freezing, and attach themselves to whatever they +meet with on the surface of the earth. + +The tendency of free caloric to an equilibrium, together with its +solvent power, are likewise connected with the phenomena of rain, of +dew, &c. When moist air of a certain temperature happens to pass through +a colder region of the atmosphere, it parts with a portion of its heat +to the surrounding air; the quantity of caloric, therefore, which served +to keep the water in a state of vapour, being diminished, the watery +particles approach each other, and form themselves into drops of water, +which being heavier than the atmosphere, descend to the earth. There are +also other circumstances, and particularly the variation in the weight +of the atmosphere, which may contribute to the formation of rain. This, +however, is an intricate subject, into which we cannot more fully enter +at present. + +EMILY. + +In what manner do you account for the formation of dew? + +MRS. B. + +Dew is a deposition of watery particles or minute drops from the +atmosphere, precipitated by the coolness of the evening. + +CAROLINE. + +This precipitation is owing, I suppose, to the cooling of the +atmosphere, which prevents its retaining so great a quantity of watery +vapour in solution as during the heat of the day. + +MRS. B. + +Such was, from time immemorial, the generally received opinion +respecting the cause of dew; but it has been very recently proved by a +course of ingenious experiments of Dr. Wells, that the deposition of dew +is produced by the cooling of the surface of the earth, which he has +shown to take place previously to the cooling of the atmosphere; for on +examining the temperature of a plot of grass just before the dew-fall, +he found that it was considerably colder than the air a few feet above +it, from which the dew was shortly after precipitated. + +EMILY. + +But why should the earth cool in the evening sooner than the atmosphere? + +MRS. B. + +Because it parts with its heat more readily than the air; the earth is +an excellent radiator of caloric, whilst the atmosphere does not possess +that property, at least in any sensible degree. Towards evening, +therefore, when the solar heat declines, and when after sunset it +entirely ceases, the earth rapidly cools by radiating heat towards the +skies; whilst the air has no means of parting with its heat but by +coming into contact with the cooled surface of the earth, to which it +communicates its caloric. Its solvent power being thus reduced, it is +unable to retain so large a portion of watery vapour, and deposits those +pearly drops which we call dew. + +EMILY. + +If this be the cause of dew, we need not be apprehensive of receiving +any injury from it; for it can be deposited only on surfaces that are +colder than the atmosphere, which is never the case with our bodies. + +MRS. B. + +Very true; yet I would not advise you for this reason to be too +confident of escaping all the ill effects which may arise from exposure +to the dew; for it may be deposited on your clothes, and chill you +afterwards by its evaporation from them. Besides, whenever the dew is +copious, there is a chill in the atmosphere which it is not always safe +to encounter. + +CAROLINE. + +Wind, then, must promote the deposition of dew, by bringing a more rapid +succession of particles of air in contact with the earth, just as it +promotes the cooling of the earth and warming of the atmosphere during +the heat of the day? + +MRS. B. + +Yes; provided the wind be unattended with clouds, for these +accumulations of moisture not only prevent the free radiation of the +earth towards the upper regions, but themselves radiate towards the +earth; under these circumstances much less dew is formed than on fine +clear nights, when the radiation of the earth passes without obstacle +through the atmosphere to the distant regions of space, whence it +receives no caloric in exchange. The dew continues to be deposited +during the night, and is generally most abundant towards morning, when +the contrast between the temperature of the earth and that of the air is +greatest. After sunrise the equilibrium of temperature between these two +bodies is gradually restored by the solar rays passing freely through +the atmosphere to the earth; and later in the morning the temperature of +the earth gains the ascendency, and gives out caloric to the air by +contact, in the same manner as it receives it from the air during the +night. --Can you tell me, now, why a bottle of wine taken fresh from the +cellar (in summer particularly), will soon be covered with dew; and even +the glasses into which the wine is poured will be moistened with a +similar vapour? + +EMILY. + +The bottle being colder than the surrounding air, must absorb caloric +from it; the moisture therefore which that air contained becomes +visible, and forms the dew which is deposited on the bottle. + +MRS. B. + +Very well, Emily. Now, Caroline, can you inform me why, in a warm room, +or close carriage, the contrary effect takes place; that is to say, that +the inside of the windows is covered with vapour? + +CAROLINE. + +I have heard that it proceeds from the breath of those within the room +or the carriage; and I suppose it is occasioned by the windows which, +being colder than the breath, deprive it of part of its caloric, and by +this means convert it into watery vapour. + +MRS. B. + +You have both explained it extremely well. Bodies attract dew in +proportion as they are good radiators of caloric, as it is this quality +which reduces their temperature below that of the atmosphere; hence we +find that little or no dew is deposited on rocks, sand, water; while +grass and living vegetables, to which it is so highly beneficial, +attract it in abundance--another remarkable instance of the wise and +bountiful dispensations of Providence. + +EMILY. + +And we may again observe it in the abundance of dew in summer, and in +hot climates, when its cooling effects are so much required; but I do +not understand what natural cause increases the dew in hot weather? + +MRS. B. + +The more caloric the earth receives during the day, the more it will +radiate afterwards, and consequently the more rapidly its temperature +will be reduced in the evening, in comparison to that of the atmosphere. +In the West-Indies especially, where the intense heat of the day is +strongly contrasted with the coolness of the evening, the dew is +prodigiously abundant. During a drought, the dew is less plentiful, as +the earth is not sufficiently supplied with moisture to be able to +saturate the atmosphere. + +CAROLINE. + +I have often observed, Mrs. B., that when I walk out in frosty weather, +with a veil over my face, my breath freezes upon it. Pray what is the +reason of that? + +MRS. B. + +It is because the cold air immediately seizes on the caloric of your +breath, and, by robbing it of its solvent, reduces it to a denser fluid, +which is the watery vapour that settles on your veil, and there it +continues parting with its caloric till it is brought down to the +temperature of the atmosphere, and assumes the form of ice. + +You may, perhaps, have observed that the breath of animals, or rather +the moisture contained in it, is visible in damp weather, or during a +frost. In the former case, the atmosphere being over-saturated with +moisture, can dissolve no more. In the latter, the cold condenses it +into visible vapour; and for the same reason, the steam arising from +water that is warmer than the atmosphere, becomes visible. Have you +never taken notice of the vapour rising from your hands after having +dipped them into warm water? + +CAROLINE. + +Frequently, especially in frosty weather. + +MRS. B. + +We have already observed that pressure is an obstacle to evaporation: +there are liquids that contain so great a quantity of caloric, and whose +particles consequently adhere so slightly together, that they may be +rapidly converted into vapour without any elevation of temperature, +merely by taking off the weight of the atmosphere. In such liquids, you +perceive, it is the pressure of the atmosphere alone that connects their +particles, and keeps them in a liquid state. + +CAROLINE. + +I do not well understand why the particles of such fluids should be +disunited and converted into vapour, without any elevation of +temperature, in spite of the attraction of cohesion. + +MRS. B. + +It is because the degree of heat at which we usually observe these +fluids is sufficient to overcome their attraction of cohesion. Ether is +of this description; it will boil and be converted into vapour, at the +common temperature of the air, if the pressure of the atmosphere be +taken off. + +EMILY. + +I thought that ether would evaporate without either the pressure of the +atmosphere being taken away, or heat applied; and that it was for that +reason so necessary to keep it carefully corked up? + +MRS. B. + +It is true it will evaporate, but without ebullition; what I am now +speaking of is the vaporization of ether, or its conversion into vapour +by boiling. I am going to show you how suddenly the ether in this phial +will be converted into vapour, by means of the air-pump. --Observe with +what rapidity the bubbles ascend, as I take off the pressure of the +atmosphere. + +CAROLINE. + +It positively boils: how singular to see a liquid boil without heat! + +MRS. B. + +Now I shall place the phial of ether in this glass, which it nearly +fits, so as to leave only a small space, which I fill with water; and in +this state I put it again under the receiver. (PLATE IV. Fig. 1.)* You +will observe, as I exhaust the air from it, that whilst the ether boils, +the water freezes. + + [Footnote *: Two pieces of thin glass tubes, sealed at one end, + might answer this purpose better. The experiment, however, as here + described, is difficult, and requires a very nice apparatus. But + if, instead of phials or tubes, two watch-glasses be used, water + may be frozen almost instantly in the same manner. The two glasses + are placed over one another, with a few drops of water interposed + between them, and the uppermost glass is filled with ether. After + working the pump for a minute or two, the glasses are found to + adhere strongly together, and a thin layer of ice is seen between + them.] + +CAROLINE. + +It is indeed wonderful to see water freeze in contact with a boiling +fluid! + +EMILY. + +I am at a loss to conceive how the ether can pass to the state of vapour +without an addition of caloric. Does it not contain more caloric in a +state of vapour, than in a state of liquidity? + +MRS. B. + +It certainly does; for though it is the pressure of the atmosphere which +condenses it into a liquid, it is by forcing out the caloric that +belongs to it when in an aëriform state. + +EMILY. + +You have, therefore, two difficulties to explain, Mrs. B. --First, from +whence the ether obtains the caloric necessary to convert it into vapour +when it is relieved from the pressure of the atmosphere; and, secondly, +what is the reason that the water, in which the bottle of ether stands, +is frozen? + +CAROLINE. + +Now, I think, I can answer both these questions. The ether obtains the +addition of caloric required, from the water in the glass; and the loss +of caloric, which the latter sustains, is the occasion of its freezing. + +MRS. B. + +You are perfectly right; and if you look at the thermometer which I have +placed in the water, whilst I am working the pump, you will see that +every time bubbles of vapour are produced, the mercury descends; which +proves that the heat of the water diminishes in proportion as the ether +boils. + +EMILY. + +This I understand now very well; but if the water freezes in consequence +of yielding its caloric to the ether, the equilibrium of heat must, in +this case, be totally destroyed. Yet you have told us, that the exchange +of caloric between two bodies of equal temperature, was always equal; +how, then, is it that the water, which was originally of the same +temperature as the ether, gives out caloric to it, till the water is +frozen, and the ether made to boil? + +MRS. B. + +I suspected that you would make these objections; and, in order to +remove them, I enclosed two thermometers in the air-pump; one which +stands in the glass of water, the other in the phial of ether; and you +may see that the equilibrium of temperature is not destroyed; for as the +thermometer descends in the water, that in the ether sinks in the same +manner; so that both thermometers indicate the same temperature, though +one of them is in a boiling, the other in a freezing liquid. + +EMILY. + +The ether, then, becomes colder as it boils? This is so contrary to +common experience, that I confess it astonishes me exceedingly. + +CAROLINE. + +It is, indeed, a most extraordinary circumstance. But pray, how do you +account for it? + +MRS. B. + +I cannot satisfy your curiosity at present; for before we can attempt to +explain this apparent paradox, it is necessary to become acquainted with +the subject of LATENT HEAT: and that, I think, we must defer till our +next interview. + +CAROLINE. + +I believe, Mrs. B., that you are glad to put off the explanation; for it +must be a very difficult point to account for. + +MRS. B. + +I hope, however, that I shall do it to your complete satisfaction. + +EMILY. + +But before we part, give me leave to ask you one question. Would not +water, as well as ether, boil with less heat, if deprived of the +pressure of the atmosphere? + +MRS. B. + +Undoubtedly. You must always recollect that there are two forces to +overcome, in order to make a liquid boil or evaporate; the attraction of +aggregation, and the weight of the atmosphere. On the summit of a high +mountain (as Mr. De Saussure ascertained on Mount Blanc) much less heat +is required to make water boil, than in the plain, where the weight of +the atmosphere is greater.* Indeed if the weight of the atmosphere be +entirely removed by means of a good air-pump, and if water be placed in +the exhausted receiver, it will evaporate so fast, however cold it +maybe, as to give it the appearance of boiling from the surface. But +without the assistance of the air-pump, I can show you a very pretty +experiment, which proves the effect of the pressure of the atmosphere in +this respect. + +Observe, that this Florence flask is about half full of water, and the +upper half of invisible vapour, the water being in the act of boiling. +--I take it from the lamp, and cork it carefully--the water, you see, +immediately ceases boiling. --I shall now dip the flask into a bason of +cold water.† + + [Footnote *: On the top of Mount Blanc, water boiled when heated + only to 187 degrees, instead of 212 degrees.] + + [Footnote †: The same effect may be produced by wrapping a cold + wet linen cloth round the upper part of the flask. In order to + show how much the water cools whilst it is boiling, a thermometer, + graduated on the tube itself, may be introduced into the bottle + through the cork.] + +CAROLINE. + +But look, Mrs. B., the hot water begins to boil again, although the cold +water must rob it more and more of its caloric! What can be the reason +of that? + +MRS. B. + +Let us examine its temperature. You see the thermometer immersed in it +remains stationary at 180 degrees, which is about 30 degrees below the +boiling point. When I took the flask from the lamp, I observed to you +that the upper part of it was filled with vapour; this being compelled +to yield its caloric to the cold water, was again condensed into water-- +What, then, filled the upper part of the flask? + +EMILY. + +Nothing; for it was too well corked for the air to gain admittance, and +therefore the upper part of the flask must be a vacuum. + +MRS. B. + +The water below, therefore, no longer sustains the pressure of the +atmosphere, and will consequently boil at a much lower temperature. +Thus, you see, though it had lost many degrees of heat, it began boiling +again the instant the vacuum was formed above it. The boiling has now +ceased, the temperature of the water being still farther reduced; if it +had been ether, instead of water, it would have continued boiling much +longer, for ether boils, under the usual atmospheric pressure, at a +temperature as low as 100 degrees; and in a vacuum it boils at almost +any temperature; but water being a more dense fluid, requires a more +considerable quantity of caloric to make it evaporate quickly, even when +the pressure of the atmosphere is removed. + +EMILY. + +What proportion of vapour can the atmosphere contain in a state of +solution? + +MRS. B. + +I do not know whether it has been exactly ascertained by experiment; but +at any rate this proportion must vary, both according to the temperature +and the weight of the atmosphere; for the lower the temperature, and the +greater the pressure, the smaller must be the proportion of vapour that +the atmosphere can contain. + +To conclude the subject of free caloric, I should mention _Ignition_, by +which is meant that emission of light which is produced in bodies at a +very high temperature, and which is the effect of accumulated caloric. + +EMILY. + +You mean, I suppose, that light which is produced by a burning body? + +MRS. B. + +No: ignition is quite independent of combustion. Clay, chalk, and indeed +all incombustible substances, may be made red hot. When a body burns, +the light emitted is the effect of a chemical change which takes place, +whilst ignition is the effect of caloric alone, and no other change than +that of temperature is produced in the ignited body. + +All solid bodies, and most liquids, are susceptible of ignition, or, in +other words, of being heated so as to become luminous; and it is +remarkable that this takes place pretty nearly at the same temperature +in all bodies, that is, at about 800 degrees of Fahrenheit’s scale. + +EMILY. + +But how can liquids attain so high a temperature, without being +converted into vapour? + +MRS. B. + +By means of confinement and pressure. Water confined in a strong iron +vessel (called Papin’s digester) can have its temperature raised to +upwards of 400 degrees. Sir James Hall has made some very curious +experiments on the effects of heat assisted by pressure; by means of +strong gun-barrels, he succeeded in melting a variety of substances +which were considered as infusible: and it is not unlikely that, by +similar methods, water itself might be heated to redness. + +EMILY. + +I am surprised at that: for I thought that the force of steam was such +as to destroy almost all mechanical resistance. + +MRS. B. + +The expansive force of steam is prodigious; but in order to subject +water to such high temperatures, it is prevented by confinement from +being converted into steam, and the expansion of heated water is +comparatively trifling. --But we have dwelt so long on the subject of +free caloric, that we must reserve the other modifications of that agent +to our next meeting, when we shall endeavour to proceed more rapidly. + + + + +CONVERSATION IV. + +ON COMBINED CALORIC, COMPREHENDING SPECIFIC AND LATENT HEAT. + + +MRS. B. + +We are now to examine the other modifications of caloric. + +CAROLINE. + +I am very curious to know of what nature they can be; for I have no +notion of any kind of heat that is not perceptible to the senses. + +MRS. B. + +In order to enable you to understand them, it will be necessary to enter +into some previous explanations. + +It has been discovered by modern chemists, that bodies of a different +nature, heated to the same temperature, do not contain the same quantity +of caloric. + +CAROLINE. + +How could that be ascertained? Have you not told us that it is +impossible to discover the absolute quantity of caloric which bodies +contain? + +MRS. B. + +True; but at the same time I said that we were enabled to form a +judgment of the proportions which bodies bore to each other in this +respect. Thus it is found that, in order to raise the temperature of +different bodies the same number of degrees, different quantities of +caloric are required for each of them. If, for instance, you place a +pound of lead, a pound of chalk, and a pound of milk, in a hot oven, +they will be gradually heated to the temperature of the oven; but the +lead will attain it first, the chalk next, and the milk last. + +CAROLINE. + +That is a natural consequence of their different bulks; the lead being +the smallest body, will be heated soonest, and the milk, which is the +largest, will require the longest time. + +MRS. B. + +That explanation will not do, for if the lead be the least in bulk, it +offers also the least surface to the caloric, the quantity of heat +therefore which can enter into it in the same space of time is +proportionally smaller. + +EMILY. + +Why, then, do not the three bodies attain the temperature of the oven at +the same time? + +MRS. B. + +It is supposed to be on account of the different capacity of these +bodies for caloric. + +CAROLINE. + +What do you mean by the capacity of a body for caloric? + +MRS. B. + +I mean a certain disposition of bodies to require more or less caloric +for raising their temperature to any degree of heat. Perhaps the fact +may be thus explained: + +Let us put as many marbles into this glass as it will contain, and pour +some sand over them--observe how the sand penetrates and lodges between +them. We shall now fill another glass with pebbles of various forms--you +see that they arrange themselves in a more compact manner than the +marbles, which, being globular, can touch each other by a single point +only. The pebbles, therefore, will not admit so much sand between them; +and consequently one of these glasses will necessarily contain more sand +than the other, though both of them be equally full. + +CAROLINE. + +This I understand perfectly. The marbles and the pebbles represent two +bodies of different kinds, and the sand the caloric contained in them; +it appears very plain, from this comparison, that one body may admit of +more caloric between its particles than another. + +MRS. B. + +You can no longer be surprised, therefore, that bodies of a different +capacity for caloric should require different proportions of that fluid +to raise their temperatures equally. + +EMILY. + +But I do not conceive why the body that contains the most caloric should +not be of the highest temperature; that is to say, feel hot in +proportion to the quantity of caloric it contains? + +MRS. B. + +The caloric that is employed in filling the capacity of a body, is not +free caloric; but is imprisoned as it were in the body, and is therefore +imperceptible: for we can feel only the caloric which the body parts +with, and not that which it retains. + +CAROLINE. + +It appears to me very extraordinary that heat should be confined in a +body in such a manner as to be imperceptible. + +MRS. B. + +If you lay your hand on a hot body, you feel only the caloric which +leaves it, and enters your hand; for it is impossible that you should be +sensible of that which remains in the body. The thermometer, in the same +manner, is affected only by the free caloric which a body transmits to +it, and not at all by that which it does not part with. + +CAROLINE. + +I begin to understand it: but I confess that the idea of insensible heat +is so new and strange to me, that it requires some time to render it +familiar. + +MRS. B. + +Call it insensible caloric, and the difficulty will appear much less +formidable. It is indeed a sort of contradiction to call it heat, when +it is so situated as to be incapable of producing that sensation. Yet +this modification of caloric is commonly called SPECIFIC HEAT. + +CAROLINE. + +But it certainly would have been more correct to have called it +_specific caloric_. + +EMILY. + +I do not understand how the term _specific_ applies to this modification +of caloric? + +MRS. B. + +It expresses the relative quantity of caloric which different _species_ +of bodies of the same weight and temperature are capable of containing. +This modification is also frequently called _heat of capacity_, a term +perhaps preferable, as it explains better its own meaning. + +You now understand, I suppose, why the milk and chalk required a longer +portion of time than the lead to raise their temperature to that of the +oven? + +EMILY. + +Yes: the milk and chalk having a greater capacity for caloric than the +lead, a greater proportion of that fluid became insensible in those +bodies: and the more slowly, therefore, their temperature was raised. + +CAROLINE. + +But might not this difference proceed from the different conducting +powers of heat in these three bodies, since that which is the best +conductor must necessarily attain the temperature of the oven first? + +MRS. B. + +Very well observed, Caroline. This objection would be insurmountable, if +we could not, by reversing the experiment, prove that the milk, the +chalk, and the lead, actually absorbed different quantities of caloric, +and we know that if the different time they took in heating, proceeded +merely from their different conducting powers, they would each have +acquired an equal quantity of caloric. + +CAROLINE. + +Certainly. But how can you reverse this experiment? + +MRS. B. + +It may be done by cooling the several bodies to the same degree in an +apparatus adapted to receive and measure the caloric which they give +out. Thus, if you plunge them into three equal quantities of water, each +at the same temperature, you will be able to judge of the relative +quantity of caloric which the three bodies contained, by that, which, in +cooling, they communicated to their respective portions of water: for +the same quantity of caloric which they each absorbed to raise their +temperature, will abandon them in lowering it; and on examining the +three vessels of water, you will find the one in which you immersed the +lead to be the least heated; that which held the chalk will be the next; +and that which contained the milk will be heated the most of all. The +celebrated Lavoisier has invented a machine to estimate, upon this +principle, the specific heat of bodies in a more perfect manner; but I +cannot explain it to you, till you are acquainted with the next +modification of caloric. + +EMILY. + +The more dense a body is, I suppose, the less is its capacity for +caloric? + +MRS. B. + +This is not always the case with bodies of different nature; iron, for +instance, contains more specific heat than tin, though it is more dense. +This seems to show that specific heat does hot merely depend upon the +interstices between the particles; but, probably, also upon some +peculiar constitution of the bodies which we do not comprehend. + +EMILY. + +But, Mrs. B., it would appear to me more proper to compare bodies by +_measure_, rather than by _weight_, in order to estimate their specific +heat. Why, for instance, should we not compare _pints_ of milk, of +chalk, and of lead, rather than _pounds_ of those substances; for equal +weights may be composed of very different quantities? + +MRS. B. + +You are mistaken, my dear; equal weight must contain equal quantities of +matter; and when we wish to know what is the relative quantity of +caloric, which substances of various kinds are capable of containing +under the same temperature, we must compare equal weights, and not equal +bulks of those substances. Bodies of the same weight may undoubtedly be +of very different dimensions; but that does not change their real +quantity of matter. A pound of feathers does not contain one atom more +than a pound of lead. + +CAROLINE. + +I have another difficulty to propose. It appears to me, that if the +temperature of the three bodies in the oven did not rise equally, they +would never reach the same degree; the lead would always keep its +advantage over the chalk and milk, and would perhaps be boiling before +the others had attained the temperature of the oven. I think you might +as well say that, in the course of time, you and I should be of the same +age? + +MRS. B. + +Your comparison is not correct, Caroline. As soon as the lead reached +the temperature of the oven, it would remain stationary; for it would +then give out as much heat as it would receive. You should recollect +that the exchange of radiating heat, between two bodies of equal +temperature, is equal: it would be impossible, therefore, for the lead +to accumulate heat after having attained the temperature of the oven; +and that of the chalk and milk therefore would ultimately arrive at the +same standard. Now I fear that this will not hold good with respect to +our ages, and that, as long as I live, I shall never cease to keep my +advantage over you. + +EMILY. + +I think that I have found a comparison for specific heat, which is very +applicable. Suppose that two men of equal weight and bulk, but who +required different quantities of food to satisfy their appetites, sit +down to dinner, both equally hungry; the one would consume a much +greater quantity of provisions than the other, in order to be equally +satisfied. + +MRS. B. + +Yes, that is very fair; for the quantity of food necessary to satisfy +their respective appetites, varies in the same manner as the quantity of +caloric requisite to raise equally the temperature of different bodies. + +EMILY. + +The thermometer, then, affords no indication of the specific heat of +bodies? + +MRS. B. + +None at all: no more than satiety is a test of the quantity of food +eaten. The thermometer, as I have repeatedly said, can be affected only +by free caloric, which alone raises the temperature of bodies. + +But there is another mode of proving the existence of specific heat, +which affords a very satisfactory illustration of that modification. +This, however, I did not enlarge upon before, as I thought it might +appear to you rather complicated. --If you mix two fluids of different +temperatures, let us say the one at 50 degrees, and the other at 100 +degrees, of what temperature do you suppose the mixture will be? + +CAROLINE. + +It will be no doubt the medium between the two, that is to say, 75 +degrees. + +MRS. B. + +That will be the case if the two bodies happen to have the same capacity +for caloric; but if not, a different result will be obtained. Thus, for +instance, if you mix together a pound of mercury, heated at 50 degrees, +and a pound of water heated at 100 degrees, the temperature of the +mixture, instead of being 75 degrees, will be 80 degrees; so that the +water will have lost only 12 degrees, whilst the mercury will have +gained 38 degrees; from which you will conclude that the capacity of +mercury for heat is less than that of water. + +CAROLINE. + +I wonder that mercury should have so little specific heat. Did we not +see it was a much better conductor of heat than water? + +MRS. B. + +And it is precisely on that account that its specific heat is less. For +since the conductive power of bodies depends, as we have observed +before, on their readiness to receive heat and part with it, it is +natural to expect that those bodies which are the worst conductors +should absorb the most caloric before they are disposed to part with it +to other bodies. But let us now proceed to LATENT HEAT. + +CAROLINE. + +And pray what kind of heat is that? + +MRS. B. + +It is another modification of combined caloric, which is so analogous to +specific heat, that most chemists make no distinction between them; but +Mr. Pictet, in his Essay on Fire, has so clearly discriminated them, +that I am induced to adopt his view of the subject. We therefore call +_latent heat_ that portion of insensible caloric which is employed in +changing the state of bodies; that is to say, in converting solids into +liquids, or liquids; into vapour. When a body changes its state from +solid to liquid, or from liquid to vapour, its expansion occasions a +sudden and considerable increase of capacity for heat, in consequence of +which it immediately absorbs a quantity of caloric, which becomes fixed +in the body which it has transformed; and, as it is perfectly concealed +from our senses, it has obtained the name of _latent_ heat. + +CAROLINE. + +I think it would be much more correct to call this modification latent +caloric instead of latent heat, since it does not excite the sensation +of heat. + +MRS. B. + +This modification of heat was discovered and named by Dr. Black long +before the French chemists introduced the term caloric, and we must not +presume to alter it, as it is still used by much better chemists than +ourselves. And, besides, you are not to suppose that the nature of heat +is altered by being variously modified: for if latent heat and specific +heat do not excite the same sensations as free caloric, it is owing to +their being in a state of confinement, which prevents them from acting +upon our organs; and consequently, as soon as they are extricated from +the body in which they are imprisoned, they return to their state of +free caloric. + +EMILY. + +But I do not yet clearly see in what respect latent heat differs from +specific heat; for they are both of them imprisoned and concealed in +bodies. + +MRS. B. + +Specific heat is that which is employed in filling the capacity of a +body for caloric, in the state in which this body actually exists; while +latent heat is that which is employed only in effecting a change of +state, that is, in converting bodies from a solid to a liquid, or from a +liquid to an aëriform state. But I think that, in a general point of +view, both these modifications might be comprehended under the name of +_heat of capacity_, as in both cases the caloric is equally engaged in +filling the capacities of bodies. + +I shall now show you an experiment, which I hope will give you a clear +idea of what is understood by latent heat. + +The snow which you see in this phial has been cooled by certain chemical +means (which I cannot well explain to you at present), to 5 or 6 degrees +below the freezing point, as you will find indicated by the thermometer +which is placed in it. We shall expose it to the heat of a lamp, and you +will see the thermometer gradually rise, till it reaches the freezing +point---- + +EMILY. + +But there it stops, Mrs. B., and yet the lamp burns just as well as +before. Why is not its heat communicated to the thermometer? + +CAROLINE. + +And the snow begins to melt, therefore it must be rising above the +freezing point? + +MRS. B. + +The heat no longer affects the thermometer, because it is wholly +employed in converting the ice into water. As the ice melts, the caloric +becomes _latent_ in the new-formed liquid, and therefore cannot raise +its temperature; and the thermometer will consequently remain +stationary, till the whole of the ice be melted. + +CAROLINE. + +Now it is all melted, and the thermometer begins to rise again. + +MRS. B. + +Because the conversion of the ice into water being completed, the +caloric no longer becomes latent; and therefore the heat which the water +now receives raises its temperature, as you find the thermometer +indicates. + +EMILY. + +But I do not think that the thermometer rises so quickly in the water as +it did in the ice, previous to its beginning to melt, though the lamp +burns equally well? + +MRS. B. + +That is owing to the different specific heat of ice and water. The +capacity of water for caloric being greater than that of ice, more heat +is required to raise its temperature, and therefore the thermometer +rises slower in the water than in the ice. + +EMILY. + +True; you said that a solid body always increased its capacity for heat +by becoming fluid; and this is an instance of it. + +MRS. B. + +Yes, and the latent heat is that which is absorbed in consequence of the +greater capacity which the water has for heat, in comparison to ice. + +I must now tell you a curious calculation founded on that consideration. +I have before observed to you that though the thermometer shows us the +comparative warmth of bodies, and enables us to determine the same point +at different times and places, it gives us no idea of the absolute +quantity of heat in any body. We cannot tell how low it ought to fall by +the privation of all heat, but an attempt has been made to infer it in +the following manner. It has been found by experiment, that the capacity +of water for heat, when compared with that of ice, is as 10 to 9, so +that, at the same temperature, ice contains one tenth of caloric less +than water. By experiment also it is observed, that in order to melt +ice, there must be added to it as much heat, as would, if it did not +melt it, raise its temperature 140 degrees. This quantity of heat is +therefore absorbed when the ice, by being converted into water, is made +to contain one-ninth more caloric than it did before. Therefore 140 +degrees is a ninth part of the heat contained in ice at 30 degrees; and +the point of zero, or the absolute privation of heat, must consequently +be 1260 degrees below 32 degrees. + +This mode of investigating so curious a question is ingenious, but its +correctness is not yet established by similar calculations for other +bodies. The points of absolute cold, indicated by this method in various +bodies, are very remote from each other; it is however possible, that +this may arise from some imperfection in the experiments. + +CAROLINE. + +It is indeed very ingenious--but we must now attend to our present +experiment. The water begins to boil, and the thermometer is again +stationary. + +MRS. B. + +Well, Caroline, it is your turn to explain the phenomenon. + +CAROLINE. + +It is wonderfully curious! The caloric is now busy in changing the water +into steam, in which it hides itself, and becomes insensible. This is +another example of latent heat, producing a change of form. At first it +converted a solid body into a liquid, and now it turns the liquid into +vapour! + +MRS. B. + +You see, my dear, how easily you have become acquainted with these +modifications of insensible heat, which at first appeared so +unintelligible. If, now, we were to reverse these changes, and condense +the vapour into water, and the water into ice, the latent heat would +re-appear entirely, in the form of free caloric. + +EMILY. + +Pray do let us see the effect of latent heat returning to its free +state. + +MRS. B. + +For the purpose of showing this, we need simply conduct the vapour +through this tube into this vessel of cold water, where it will part +with its latent heat and return to its liquid form. + +EMILY. + +How rapidly the steam heats the water! + +MRS. B. + +That is because it does not merely impart its free caloric to the water, +but likewise its latent heat. This method of heating liquids, has been +turned to advantage, in several economical establishments. The +steam-kitchens, which are getting into such general use, are upon the +same principle. The steam is conveyed through a pipe in a similar +manner, into the several vessels which contain the provisions to be +dressed, where it communicates to them its latent caloric, and returns +to the state of water. Count Rumford makes great use of this principle +in many of his fire-places: his grand maxim is to avoid all unnecessary +waste of caloric, for which purpose he confines the heat in such a +manner, that not a particle of it shall unnecessarily escape; and while +he economises the free caloric, he takes care also to turn the latent +heat to advantage. It is thus that he is enabled to produce a degree of +heat superior to that which is obtained in common fire-places, though he +employs less fuel. + +EMILY. + +When the advantages of such contrivances are so clear and plain, +I cannot understand why they are not universally used. + +MRS. B. + +A long time is always required before innovations, however useful, can +be reconciled with the prejudices of the vulgar. + +EMILY. + +What a pity it is that there should be a prejudice against new +inventions; how much more rapidly the world would improve, if such +useful discoveries were immediately and universally adopted! + +MRS. B. + +I believe, my dear, that there are as many novelties attempted to be +introduced, the adoption of which would be prejudicial to society, as +there are of those which would be beneficial to it. The well-informed, +though by no means exempt from error, have an unquestionable advantage +over the illiterate, in judging what is likely or not to prove +serviceable; and therefore we find the former more ready to adopt such +discoveries as promise to be really advantageous, than the latter, who +having no other test of the value of a novelty but time and experience, +at first oppose its introduction. The well-informed, however, are +frequently disappointed in their most sanguine expectations, and the +prejudices of the vulgar, though they often retard the progress of +knowledge, yet sometimes, it must be admitted, prevent the propagation +of error. --But we are deviating from our subject. + +We have converted steam into water, and are now to change water into +ice, in order to render the latent heat sensible, as it escapes from the +water on its becoming solid. For this purpose we must produce a degree +of cold that will make water freeze. + +CAROLINE. + +That must be very difficult to accomplish in this warm room. + +MRS. B. + +Not so much as you think. There are certain chemical mixtures which +produce a rapid change from the solid to the fluid state, or the +reverse, in the substances combined, in consequence of which change +latent heat is either extricated or absorbed. + +EMILY. + +I do not quite understand you. + +MRS. B. + +This snow and salt, which you see me mix together, are melting rapidly; +heat, therefore, must be absorbed by the mixture, and cold produced. + +CAROLINE. + +It feels even colder than ice, and yet the snow is melted. This is very +extraordinary. + +MRS. B. + +The cause of the intense cold of the mixture is to be attributed to the +change from a solid to a fluid state. The union of the snow and salt +produces a new arrangement of their particles, in consequence of which +they become liquid; and the quantity of caloric, required to effect this +change, is seized upon by the mixture wherever it can be obtained. This +eagerness of the mixture for caloric, during its liquefaction, is such, +that it converts part of its own free caloric into latent heat, and it +is thus that its temperature is lowered. + +EMILY. + +Whatever you put in this mixture, therefore, would freeze? + +MRS. B. + +Yes; at least any fluid that is susceptible of freezing at that +temperature. I have prepared this mixture of salt and snow for the +purpose of freezing the water from which you are desirous of seeing the +latent heat escape. I have put a thermometer in the glass of water that +is to be frozen, in order that you may see how it cools. + +CAROLINE. + +The thermometer descends, but the heat which the water is now losing, is +its _free_, not its _latent_ heat. + +MRS. B. + +Certainly; it does not part with its latent heat till it changes its +state and is converted into ice. + +EMILY. + +But here is a very extraordinary circumstance! The thermometer is fallen +below the freezing point, and yet the water is not frozen. + +MRS. B. + +That is always the case previous to the freezing of water when it is in +a state of rest. Now it begins to congeal, and you may observe that the +thermometer again rises to the freezing point. + +CAROLINE. + +It appears to me very strange that the thermometer should rise the very +moment that the water freezes; for it seems to imply that the water was +colder before it froze than when in the act of freezing. + +MRS. B. + +It is so; and after our long dissertation on this circumstance, I did +not think it would appear so surprising to you. Reflect a little, and I +think you will discover the reason of it. + +CAROLINE. + +It must be, no doubt, the extrications of latent heat, at the instant +the water freezes, that raises the temperature. + +MRS. B. + +Certainly; and if you now examine the thermometer, you will find that +its rise was but temporary, and lasted only during the disengagement of +the latent heat--now that all the water is frozen it falls again, and +will continue to fall till the ice and mixture are of an equal +temperature. + +EMILY. + +And can you show us any experiments in which liquids, by being mixed, +become solid, and disengage latent heat? + +MRS. B. + +I could show you several; but you are not yet sufficiently advanced to +understand them well. I shall, however, try one, which will afford you a +striking instance of the fact. The fluid which you see in this phial +consists of a quantity of a certain salt called _muriat of lime_, +dissolved in water. Now, if I pour into it a few drops of this other +fluid, called _sulphuric acid_, the whole, or very nearly the whole, +will be instantaneously converted into a solid mass. + +EMILY. + +How white it turns! I feel the latent heat escaping, for the bottle is +warm, and the fluid is changed to a solid white substance like chalk! + +CAROLINE. + +This is, indeed, the most curious experiment we have seen yet. But pray +what is that white vapour that ascends from the mixture? + +MRS. B. + +You are not yet enough of a chemist to understand that. --But take care, +Caroline, do not approach too near it, for it has a very pungent smell. + +I shall show you another instance similar to that of the water, which +you observed to become warmer as it froze. I have in this phial a +solution of a salt called sulphat of soda or Glauber’s salt, made very +strong, and corked up when it was hot, and kept without agitation till +it became cold, as you may feel the phial is. Now when I take out the +cork and let the air fall upon it, (for being closed when boiling, there +was a vacuum in the upper part) observe that the salt will suddenly +crystallize. . . . + +CAROLINE. + +Surprising! how beautifully the needles of salt have shot through the +whole phial! + +MRS. B. + +Yes, it is very striking--but pray do not forget the object of the +experiment. Feel how warm the phial has become by the conversion of part +of the liquid into a solid. + +EMILY. + +Quite warm I declare! this is a most curious experiment of the +disengagement of latent heat. + +MRS. B. + +The slakeing of lime is another remarkable instance of the extrication +of latent heat. Have you never observed how quick-lime smokes when water +is poured upon it, and how much heat it produces? + +CAROLINE. + +Yes; but I do not understand what change of state takes place in the +lime that occasions its giving out latent heat; for the quick-lime, +which is solid, is (if I recollect right) reduced to powder, by this +operation, and is, therefore, rather expanded than condensed. + +MRS. B. + +It is from the water, not the lime, that the latent heat is set free. +The water incorporates with, and becomes solid in the lime; in +consequence of which, the heat, which kept it in a liquid state, is +disengaged, and escapes in a sensible form. + +CAROLINE. + +I always thought that the heat originated in the lime. It seems very +strange that water, and cold water too, should contain so much heat. + +EMILY. + +After this extrication of caloric, the water must exist in a state of +ice in the lime, since it parts with the heat which kept it liquid. + +MRS. B. + +It cannot properly be called ice, since ice implies a degree of cold, at +least equal to the freezing point. Yet as water, in combining with lime, +gives out more heat than in freezing, it must be in a state of still +greater solidity in the lime, than it is in the form of ice; and you may +have observed that it does not moisten or liquefy the lime in the +smallest degree. + +EMILY. + +But, Mrs. B., the smoke that rises is white; if it was only pure caloric +which escaped, we might feel, but could not see it. + +MRS. B. + +This white vapour is formed by some of the particles of lime, in a state +of fine dust, which are carried off by the caloric. + +EMILY. + +In all changes of state, then, a body either absorbs or disengages +latent heat? + +MRS. B. + +You cannot exactly say _absorbs latent heat_, as the heat becomes latent +only on being confined in the body; but you may say, generally, that +bodies, in passing from a solid to a liquid form, or from the liquid +state to that of vapour, absorb heat; and that when the reverse takes +place, heat is disengaged.* + + [Footnote *: This rule, if not universal, admits of very few + exceptions.] + +EMILY. + +We can now, I think, account for the ether boiling, and the water +freezing in vacuo, at the same temperature.† + + [Footnote †: See page 102.] + +MRS. B. + +Let me hear how you explain it. + +EMILY. + +The latent heat, which the water gave out in freezing, was immediately +absorbed by the ether, during its conversion into vapour; and therefore, +from a latent state in one liquid, it passed into a latent state in the +other. + +MRS. B. + +But this only partly accounts for the result of the experiment; it +remains to be explained why the temperature of the ether, while in a +state of ebullition, is brought down to the freezing temperature of the +water. --It is because the ether, during its evaporation, reduces its +own temperature, in the same proportion as that of the water, by +converting its free caloric into latent heat: so that, though one liquid +boils, and the other freezes, their temperatures remain in a state of +equilibrium. + +EMILY. + +But why does not water, as well as ether, reduce its own temperature by +evaporating? + +MRS. B. + +The fact is that it does, though much less rapidly than ether. Thus, for +instance, you may often have observed, in the heat of summer, how much +any particular spot may be cooled by watering, though the water used for +that purpose be as warm as the air itself. Indeed so much cold may be +produced by the mere evaporation of water, that the inhabitants of +India, by availing themselves of the most favourable circumstances for +this process which their warm climate can afford, namely, the cool of +the night, and situations most exposed to the night breeze, succeed in +causing water to freeze, though the temperature of the air be as high as +60 degrees. The water is put into shallow earthen trays, so as to expose +an extensive surface to the process of evaporation, and in the morning, +the water is found covered with a thin cake of ice, which is collected +in sufficient quantity to be used for purposes of luxury. + +CAROLINE. + +How delicious it must be to drink liquids so cold in those tropical +climates! But, Mrs. B., could we not try that experiment? + +MRS. B. + +If we were in the country, I have no doubt but that we should be able to +freeze water, by the same means, and under similar circumstances. But we +can do it immediately, upon a small scale, in this very room, in which +the thermometer stands at 70 degrees. For this purpose we need only +place some water in a little cup under the receiver of the air-pump +(PLATE V. fig. 1.), and exhaust the air from it. What will be the +consequence, Caroline? + + [Illustration: Plate V. Vol. I. page 138. + + Fig. 1. + The air-pump & receiver for Mr. Leslie’s experiment. + C a saucer with sulphuric Acid. + B a glass or earthen cup containing Water. + D a stand for the cup with its legs made of Glass. + A a Thermometer. + + Fig. 2. Dr. Wollaston’s Cryophorus. + + Fig. 5. Dr. Marcet’s mode of using the Cryophorus. + + Fig. 3. & 4. the different parts of Fig. 5. seen separate.] + +CAROLINE. + +Of course the water will evaporate more quickly, since there will no +longer be any atmospheric pressure on its surface: but will this be +sufficient to make the water freeze? + +MRS. B. + +Probably not, because the vapour will not be carried off fast enough; +but this will be accomplished without difficulty if we introduce into +the receiver (fig. 1.), in a saucer, or other large shallow vessel, some +strong sulphuric acid, a substance which has a great attraction for +water, whether in the form of vapour, or in the liquid state. This +attraction is such that the acid will instantly absorb the moisture as +it rises from the water, so as to make room for the formation of fresh +vapour; this will of course hasten the process, and the cold produced +from the rapid evaporation of the water, will, in a few minutes, be +sufficient to freeze its surface.* We shall now exhaust the air from the +receiver. + + [Footnote *: This experiment was first devised by Mr. Leslie, and + has since been modified in a variety of forms.] + +EMILY. + +Thousands of small bubbles already rise through the water from the +internal surface of the cup; what is the reason of this? + +MRS. B. + +These are bubbles of air which were partly attached to the vessel, and +partly diffused in the water itself; and they expand and rise in +consequence of the atmospheric pressure being removed. + +CAROLINE. + +See, Mrs. B.; the thermometer in the cup is sinking fast; it has already +descended to 40 degrees! + +EMILY. + +The water seems now and then violently agitated on the surface, as if it +was boiling; and yet the thermometer is descending fast! + +MRS. B. + +You may call it _boiling_, if you please, for this appearance is, as +well as boiling, owing to the rapid formation of vapour; but here, as +you have just observed, it takes place from the surface, for it is only +when heat is applied to the bottom of the vessel that the vapour is +formed there. --Now crystals of ice are actually shooting all over the +surface of the water. + +CAROLINE. + +How beautiful it is! The surface is now entirely frozen--but the +thermometer remains at 32 degrees. + +MRS. B. + +And so it will, conformably with our doctrine of latent heat, until the +whole of the water is frozen; but it will then again begin to descend +lower and lower, in consequence of the evaporation which goes on from +the surface of the ice. + +EMILY. + +This is a most interesting experiment; but it would be still more +striking if no sulphuric acid were required. + +MRS. B. + +I will show you a freezing instrument, contrived by Dr. Wollaston, upon +the same principle as Mr. Leslie’s experiment, by which water may be +frozen by its own evaporation alone, without the assistance of sulphuric +acid. + +This tube, which, as you see (PLATE V. fig. 2.), is terminated at each +extremity by a bulb, one of which is half full of water, is internally +perfectly exhausted of air; the consequence of this is, that the water +in the bulb is always much disposed to evaporate. This evaporation, +however, does not proceed sufficiently fast to freeze the water; but if +the empty ball be cooled by some artificial means, so as to condense +quickly the vapour which rises from the water, the process may be thus +so much promoted as to cause the water to freeze in the other ball. Dr. +Wollaston has called this instrument _Cryophorus_. + +CAROLINE. + +So that cold seems to perform here the same part which the sulphuric +acid acted in Mr. Leslie’s experiment? + +MRS. B. + +Exactly so; but let us try the experiment. + +EMILY. + +How will you cool the instrument? You have neither ice nor snow. + +MRS. B. + +True: but we have other means of effecting this.* You recollect what an +intense cold can be produced by the evaporation of ether in an exhausted +receiver. We shall inclose the bulb in this little bag of fine flannel +(fig. 3.), then soke it in ether, and introduce it into the receiver of +the air-pump. (Fig. 5.) For this purpose we shall find it more +convenient to use a cryophorus of this shape (fig. 4.), as its elongated +bulb passes easily through a brass plate which closes the top of the +receiver. If we now exhaust the receiver quickly, you will see, in less +than a minute, the water freeze in the other bulb, out of the receiver. + + [Footnote *: This mode of making the experiment was proposed, and + the particulars detailed, by Dr. Marcet, in the 34th vol. of + Nicholson’s Journal, page 119.] + +EMILY. + +The bulb already looks quite dim, and small drops of water are +condensing on its surface. + +CAROLINE. + +And now crystals of ice shoot all over the water. This is, indeed, +a very curious experiment! + +MRS. B. + +You will see, some other day, that, by a similar method, even +quicksilver may be frozen. --But we cannot at present indulge in any +further digression. + +Having advanced so far on the subject of heat, I may now give you an +account of the calorimeter, an instrument invented by Lavoisier, upon +the principles just explained, for the purpose of estimating the +specific heat of bodies. It consists of a vessel, the inner surface of +which is lined with ice, so as to form a sort of hollow globe of ice, in +the midst of which the body, whose specific heat is to be ascertained, +is placed. The ice absorbs caloric from this body, till it has brought +it down to the freezing point; this caloric converts into water a +certain portion of the ice which runs out through an aperture at the +bottom of the machine; and the quantity of ice changed to water is a +test of the quantity of caloric which the body has given out in +descending from a certain temperature to the freezing point. + +CAROLINE. + +In this apparatus, I suppose, the milk, chalk, and lead, would melt +different quantities of ice, in proportion to their different capacities +for caloric? + +MRS. B. + +Certainly: and thence we are able to ascertain, with precision, their +respective capacities for heat. But the calorimeter affords us no more +idea of the absolute quantity of heat contained in a body, than the +thermometer; for though by means of it we extricate both the free and +combined caloric, yet we extricate them only to a certain degree, which +is the freezing point; and we know not how much they contain of either +below that point. + +EMILY. + +According to the theory of latent heat, it appears to me that the +weather should be warm when it freezes, and cold in a thaw: for latent +heat is liberated from every substance that it freezes, and such a large +supply of heat must warm the atmosphere; whilst, during a thaw, that +very quantity of free heat must be taken from the atmosphere, and return +to a latent state in the bodies which it thaws. + +MRS. B. + +Your observation is very natural; but consider that in a frost the +atmosphere is so much colder than the earth, that all the caloric which +it takes from the freezing bodies is insufficient to raise its +temperature above the freezing point; otherwise the frost must cease. +But if the quantity of latent heat extricated does not destroy the +frost, it serves to moderate the suddenness of the change of temperature +of the atmosphere, at the commencement both of frost, and of a thaw. In +the first instance, its extrication diminishes the severity of the cold; +and, in the latter, its absorption moderates the warmth occasioned by a +thaw: it even sometimes produces a discernible chill, at the breaking up +of a frost. + +CAROLINE. + +But what are the general causes that produce those sudden changes in the +weather, especially from hot to cold, which we often experience? + +MRS. B. + +This question would lead us into meteorological discussions, to which I +am by no means competent. One circumstance, however, we can easily +understand. When the air has passed over cold countries, it will +probably arrive here at a temperature much below our own, and then it +must absorb heat from every object it meets with, which will produce a +general fall of temperature. + +CAROLINE. + +But pray, now that we know so much of the effects of heat, will you +inform us whether it is really a distinct body, or, as I have heard, +a peculiar kind of motion produced in bodies? + +MRS. B. + +As I before told you, there is yet much uncertainty as to the nature of +these subtle agents. But I am inclined to consider heat not as mere +motion, but as a separate substance. Late experiments too appear to make +it a compound body, consisting of the two electricities, and in our next +conversation I shall inform you of the principal facts on which that +opinion is founded. + + + + +CONVERSATION V. + +ON THE CHEMICAL AGENCIES OF ELECTRICITY. + + +MRS. B. + +Before we proceed further it will be necessary to give you some account +of certain properties of electricity, which have of late years been +discovered to have an essential connection with the phenomena of +chemistry. + +CAROLINE. + +It is ELECTRICITY, if I recollect right, which comes next in our list of +simple substances? + +MRS. B. + +I have placed electricity in that list, rather from the necessity of +classing it somewhere, than from any conviction that it has a right to +that situation, for we are as yet so ignorant of its intimate nature, +that we are unable to determine, not only whether it is simple or +compound, but whether it is in fact a material agent; or, as Sir H. Davy +has hinted, whether it may not be merely a property inherent in matter. +As, however, it is necessary to adopt some hypothesis for the +explanation of the discoveries which this agent has enabled us to make, +I have chosen the opinion, at present most prevalent, which supposes the +existence of two kinds of electricity, distinguished by the names of +_positive_ and _negative_ electricity. + +CAROLINE. + +Well, I must confess, I do not feel nearly so interested in a science in +which so much uncertainty prevails, as in those which rest upon +established principles; I never was fond of electricity, because, +however beautiful and curious the phenomena it exhibits may be, the +theories, by which they were explained, appeared to me so various, so +obscure and inadequate, that I always remained dissatisfied. I was in +hopes that the new discoveries in electricity had thrown so great a +light on the subject, that every thing respecting it would now have been +clearly explained. + +MRS. B. + +That is a point which we are yet far from having attained. But, in spite +of the imperfection of our theories, you will be amply repaid by the +importance and novelty of the subject. The number of new facts which +have already been ascertained, and the immense prospect of discovery +which has lately been opened to us, will, I hope, ultimately lead to a +perfect elucidation of this branch of natural science; but at present +you must be contented with studying the effects, and in some degree +explaining the phenomena, without aspiring to a precise knowledge of the +remote cause of electricity. + +You have already obtained some notions of electricity: in our present +conversation, therefore, I shall confine myself to that part of the +science which is of late discovery, and is more particularly connected +with chemistry. + +It was a trifling and accidental circumstance which first gave rise to +this new branch of physical science. Galvani, a professor of natural +philosophy at Bologna, being engaged (about twenty years ago) in some +experiments on muscular irritability, observed, that when a piece of +metal was laid on the nerve of a frog, recently dead, whilst the limb +supplied by that nerve rested upon some other metal, the limb suddenly +moved, on a communication being made between the two pieces of metal. + +EMILY. + +How is this communication made? + +MRS. B. + +Either by bringing the two metals into contact, or by connecting them by +means of a metallic conductor. But without subjecting a frog to any +cruel experiments, I can easily make you sensible of this kind of +electric action. Here is a piece of zinc, (one of the metals I mentioned +in the list of elementary bodies)--put it _under_ your tongue, and this +piece of silver _upon_ your tongue, and let both the metals project a +little beyond the tip of the tongue--very well--now make the projecting +parts of the metals touch each other, and you will instantly perceive a +peculiar sensation. + +EMILY. + +Indeed I did, a singular taste, and I think a degree of heat: but I can +hardly describe it. + +MRS. B. + +The action of these two pieces of metal on the tongue is, I believe, +precisely similar to that made on the nerve of a frog. I shall not +detain you by a detailed account of the theory by which Galvani +attempted to account for this fact, as his explanation was soon +overturned by subsequent experiments, which proved that _Galvanism_ (the +name this new power had obtained) was nothing more than electricity. +Galvani supposed that the virtue of this new agent resided in the nerves +of the frog, but Volta, who prosecuted this subject with much greater +success, shewed that the phenomena did not depend on the organs of the +frog, but upon the electrical agency of the metals, which is excited by +the moisture of the animal, the organs of the frog being only a delicate +test of the presence of electric influence. + +CAROLINE. + +I suppose, then, the saliva of the mouth answers the same purpose as the +moisture of the frog, in exciting the electricity of the pieces of +silver and zinc with which Emily tried the experiment on her tongue. + +MRS. B. + +Precisely. It does not appear, however, necessary that the fluid used +for this purpose should be of an animal nature. Water, and acids very +much diluted by water, are found to be the most effectual in promoting +the developement of electricity in metals; and, accordingly, the +original apparatus which Volta first constructed for this purpose, +consisted of a pile or succession of plates of zinc and copper, each +pair of which was connected by pieces of cloth or paper impregnated with +water; and this instrument, from its original inconvenient structure and +limited strength, has gradually arrived at its present state of power +and improvement, such as is exhibited in the Voltaic battery. In this +apparatus, a specimen of which you see before you (PLATE VI. fig. 1.), +the plates of zinc and copper are soldered together in pairs, each pair +being placed at regular distances in wooden troughs and the interstices +being filled with fluid. + + [Illustration: Plate VI. p. 151. + + Fig. 1. Voltaic Battery. + Fig. 2. + Fig. 4. + Fig. 1. 2. & 4. Voltaic Batteries + + Fig. 3. Electrical Machine. + A the Cylinder. + B the Conductor. + R the Rubber. + C the Chain.] + +CAROLINE. + +Though you will not allow us to enquire into the precise cause of +electricity, may we not ask in what manner the fluid acts on the metals +so as to produce it? + +MRS. B. + +The action of the fluid on the metals, whether water or acid be used, is +entirely of a chemical nature. But whether electricity is excited by +this chemical action, or whether it is produced by the contact of the +two metals, is a point upon which philosophers do not yet perfectly +agree. + +EMILY. + +But can the mere contact of two metals, without any intervening fluid, +produce electricity? + +MRS. B. + +Yes, if they are afterwards separated. It is an established fact, that +when two metals are put in contact, and afterwards separated, that which +has the strongest attraction for oxygen exhibits signs of positive, the +other of negative electricity. + +CAROLINE. + +It seems then but reasonable to infer that the power of the Voltaic +battery should arise from the contact of the plates of zinc and copper. + +MRS. B. + +It is upon this principle that Volta and Sir H. Davy explain the +phenomena of the pile; but notwithstanding these two great authorities, +many philosophers entertain doubts on the truth of this theory. The +principal difficulty which occurs in explaining the phenomena of the +Voltaic battery on this principle, is, that two such plates show no +signs of different states of electricity whilst in contact, but only on +being separated after contact. Now in the Voltaic battery, those plates +that are in contact always continue so, being soldered together: and +they cannot therefore receive a succession of charges. Besides, if we +consider the mere disturbance of the balance of electricity by the +contact of the plates, as the sole cause of the production of Voltaic +electricity, it remains to be explained how this disturbed balance +becomes an inexhaustible source of electrical energy, capable of pouring +forth a constant and copious supply of electrical fluid, though without +any means of replenishing itself from other sources. This subject, it +must be owned, is involved in too much obscurity to enable us to speak +very decidedly in favour of any theory. But, in order to avoid +perplexing you with different explanations, I shall confine myself to +one which appears to me to be least encumbered with difficulties, and +most likely to accord with truth.* + +This theory supposes the electricity to be excited by the chemical +action of the acid on the zinc; but you are yet such novices in +chemistry, that I think it will be necessary to give you some previous +explanation of the nature of this action. + +All metals have a strong attraction for oxygen, and this element is +found in great abundance both in water and in acids. The action of the +diluted acid on the zinc consists therefore in its oxygen combining with +it, and dissolving its surface. + + [Footnote *: This mode of explaining the phenomena of the Voltaic + pile is called the _chemical theory_ of electricity, because it + ascribes the cause of these phenomena to certain chemical changes + which take place during their appearance. In the preceding edition + of this work, the same theory was presented in a more elaborate, + but less easy form than it is in this. The mode of viewing the + subject which is here sketched was long since suggested by Dr. + Bostock, of whose theory, however, this is by no means to be + considered as a complete statement.] + +CAROLINE. + +In the same manner I suppose as we saw an acid dissolve copper? + +MRS. B. + +Yes; but in the Voltaic battery the diluted acid is not strong enough to +produce so complete an effect; it acts only on the surface of the zinc, +to which it yields its oxygen, forming upon it a film or crust, which is +a compound of the oxygen and the metal. + +EMILY. + +Since there is so strong a chemical attraction between oxygen and +metals, I suppose they are naturally in different states of electricity? + +MRS. B. + +Yes; it appears that all metals are united with the positive, and that +oxygen is the grand source of the negative electricity. + +CAROLINE. + +Does not then the acid act on the plates of copper, as well as on those +of zinc? + +MRS. B. + +No; for though copper has an affinity for oxygen, it is less strong than +that of zinc; and therefore the energy of the acid is only exerted upon +the zinc. + +It will be best, I believe, in order to render the action of the Voltaic +battery more intelligible, to confine our attention at first to the +effect produced on two plates only. (PLATE VI. fig. 2.) + +If a plate of zinc be placed opposite to one of copper, or any other +metal less attractive of oxygen, and the space between them (suppose of +half an inch in thickness), be filled with an acid or any fluid capable +of oxydating the zinc, the oxydated surface will have its capacity for +electricity diminished, so that a quantity of electricity will be +evolved from that surface. This electricity will be received by the +contiguous fluid, by which it will be transmitted to the opposite +metallic surface, the copper, which is not oxydated, and is therefore +disposed to receive it; so that the copper plate will thus become +positive, whilst the zinc plate will be in the negative state. + +This evolution of electrical fluid however will be very limited; for as +these two plates admit of but very little accumulation of electricity, +and are supposed to have no communication with other bodies, the action +of the acid, and further developement of electricity, will be +immediately stopped. + +EMILY. + +This action, I suppose, can no more continue to go on, than that of a +common electrical machine, which is not allowed to communicate with +other bodies? + +MRS. B. + +Precisely; the common electrical machine, when excited by the friction +of the rubber, gives out both the positive and negative electricities.-- +(PLATE VI. Fig. 3.) The positive, by the rotation of the glass cylinder, +is conveyed into the conductor, whilst the negative goes into the +rubber. But unless there is a communication made between the rubber and +the ground, but a very inconsiderable quantity of electricity can be +excited; for the rubber, like the plates of the battery, has too small a +capacity to admit of an accumulation of electricity. Unless therefore +the electricity can pass out of the rubber, it will not continue to go +into it, and consequently no additional accumulation will take place. +Now as one kind of electricity cannot be given out without the other, +the developement of the positive electricity is stopped as well as that +of the negative, and the conductor therefore cannot receive a succession +of charges. + +CAROLINE. + +But does not the conductor, as well as the rubber, require a +communication with the earth, in order to get rid of its electricity? + +MRS. B. + +No; for it is susceptible of receiving and containing a considerable +quantity of electricity, as it is much larger than the rubber, and +therefore has a greater capacity; and this continued accumulation of +electricity in the conductor is what is called a charge. + +EMILY. + +But when an electrical machine is furnished with two conductors to +receive the two electricities, I suppose no communication with the earth +is required? + +MRS. B. + +Certainly not, until the two are fully charged; for the two conductors +will receive equal quantities of electricity. + +CAROLINE. + +I thought the use of the chain had been to convey the electricity _from_ +the ground into the machine? + +MRS. B. + +That was the idea of Dr. Franklin, who supposed that there was but one +kind of electricity, and who, by the terms positive and negative (which +he first introduced), meant only different quantities of the same kind +of electricity. The chain was in that case supposed to convey +electricity _from_ the ground through the rubber into the conductor. But +as we have adopted the hypothesis of two electricities, we must consider +the chain as a vehicle to conduct the negative electricity into the +earth. + +EMILY. + +And are both kinds of electricity produced whenever electricity is +excited? + +MRS. B. + +Yes, invariably. If you rub a tube of glass with a woollen cloth, the +glass becomes positive, and the cloth negative. If, on the contrary, you +excite a stick of sealing-wax by the same means, it is the rubber which +becomes positive, and the wax negative. + +But with regard to the Voltaic battery, in order that the acid may act +freely on the zinc, and the two electricities be given out without +interruption, some method must be devised, by which the plates may part +with their electricities as fast as they receive them. --Can you think +of any means by which this might be effected? + +EMILY. + +Would not two chains or wires, suspended from either plate to the +ground, conduct the electricities into the earth, and thus answer the +purpose? + +MRS. B. + +It would answer the purpose of carrying off the electricity, I admit; +but recollect, that though it is necessary to find a vent for the +electricity, yet we must not lose it, since it is the power which we are +endeavouring to obtain. Instead, therefore, of conducting it into the +ground, let us make the wires, from either plate, meet: the two +electricities will thus be brought together, and will combine and +neutralize each other; and as long as this communication continues, the +two plates having a vent for their respective electricities, the action +of the acid will go on freely and uninterruptedly. + +EMILY. + +That is very clear, so far as two plates only are concerned; but I +cannot say I understand how the energy of the succession of plates, or +rather pairs of plates, of which the Galvanic trough is composed, is +propagated and accumulated throughout a battery? + +MRS. B. + +In order to shew you how the intensity of the electricity is increased +by increasing the number of plates, we will examine the action of four +plates; if you understand these, you will readily comprehend that of any +number whatever. In this figure (PLATE VI. Fig. 4.), you will observe +that the two central plates are united; they are soldered together, (as +we observed in describing the Voltaic trough,) so as to form but one +plate which offers two different surfaces, the one of copper, the other +of zinc. + +Now you recollect that, in explaining the action of two plates, we +supposed that a quantity of electricity was evolved from the surface of +the first zinc plate, in consequence of the action of the acid, and was +conveyed by the interposed fluid to the copper plate, No. 2, which thus +became positive. This copper plate communicates its electricity to the +contiguous zinc plate, No. 3, in which, consequently, some accumulation +of electricity takes place. When, therefore, the fluid in the next cell +acts upon the zinc plate, electricity is extricated from it in larger +quantity, and in a more concentrated form, than before. This +concentrated electricity is again conveyed by the fluid to the next pair +of plates, No. 4 and 5, when it is farther increased by the action of +the fluid in the third cell, and so on, to any number of plates of which +the battery may consist; so that the electrical energy will continue to +accumulate in proportion to the number of double plates, the first zinc +plate of the series being the most negative, and the last copper plate +the most positive. + +CAROLINE. + +But does the battery become more and more strongly charged, merely by +being allowed to stand undisturbed? + +MRS. B. + +No, for the action will soon stop, as was explained before, unless a +vent be given to the accumulated electricities. This is easily done, +however, by establishing a communication by means of the wires +(Fig. 1.), between the two ends of the battery: these being brought into +contact, the two electricities meet and neutralize each other, producing +the shock and other effects of electricity; and the action goes on with +renewed energy, being no longer obstructed by the accumulation of the +two electricities which impeded its progress. + +EMILY. + +Is it the union of the two electricities which produces the electric +spark? + +MRS. B. + +Yes; and it is, I believe, this circumstance which gave rise to Sir H. +Davy’s opinion that caloric may be a compound of the two electricities. + +CAROLINE. + +Yet surely caloric is very different from the electrical spark? + +MRS. B. + +The difference may consist probably only in intensity: for the heat of +the electric spark is considerably more intense, though confined to a +very minute spot, than any heat we can produce by other means. + +EMILY. + +Is it quite certain that the electricity of the Voltaic battery is +precisely of the same nature as that of the common electrical machine? + +MRS. B. + +Undoubtedly; the shock given to the human body, the spark, the +circumstance of the same substances which are conductors of the one +being also conductors of the other, and of those bodies, such as glass +and sealing-wax, which are non-conductors of the one, being also +non-conductors of the other, are striking proofs of it. Besides, Sir H. +Davy has shewn in his Lectures, that a Leyden jar, and a common electric +battery, can be charged with electricity obtained from a Voltaic +battery, the effect produced being perfectly similar to that obtained by +a common machine. + +Dr. Wollaston has likewise proved that similar chemical decompositions +are effected by the electric machine and by the Voltaic battery; and has +made other experiments which render it highly probable, that the origin +of both electricities is essentially the same, as they show that the +rubber of the common electrical machine, like the zinc in the Voltaic +battery, produces the two electricities by combining with oxygen. + +CAROLINE. + +But I do not see whence the rubber obtains oxygen, for there is neither +acid nor water used in the common machine, and I always understood that +the electricity was excited by the friction. + +MRS. B. + +It appears that by friction the rubber obtains oxygen from the +atmosphere, which is partly composed of that element. The oxygen +combines with the amalgam of the rubber, which is of a metallic nature, +much in the same way as the oxygen of the acid combines with the zinc in +the Voltaic battery, and it is thus that the two electricities are +disengaged. + +CAROLINE. + +But, if the electricities of both machines are similar, why not use the +common machine for chemical decompositions? + +MRS. B. + +Though its effects are similar to those of the Voltaic battery, they are +incomparably weaker. Indeed Dr. Wollaston, in using it for chemical +decompositions, was obliged to act upon the most minute quantities of +matter, and though the result was satisfactory in proving the similarity +of its effects to those of the Voltaic battery, these effects were too +small in extent to be in any considerable degree applicable to chemical +decomposition. + +CAROLINE. + +How terrible, then, the shock must be from a Voltaic battery, since it +is so much more powerful than an electrical machine! + +MRS. B. + +It is not nearly so formidable as you think; at least it is by no means +proportional to the chemical effect. The great superiority of the +Voltaic battery consists in the large _quantity_ of electricity that +passes; but in regard to the _rapidity_ or _intensity_ of the charge, it +is greatly surpassed by the common electrical machine. It would seem +that the shock or sensation depends chiefly upon the intensity; whilst, +on the contrary, for chemical purposes, it is quantity which is +required. In the Voltaic battery, the electricity, though copious, is so +weak as not to be able to force its way through the fluid which +separates the plates, whilst that of a common machine will pass through +any space of water. + +CAROLINE. + +Would not it be possible to increase the intensity of the Voltaic +battery till it should equal that of the common machine? + +MRS. B. + +It can actually be increased till it imitates a weak electrical machine, +so as to produce a visible spark when accumulated in a Leyden jar. But +it can never be raised sufficiently to pass through any considerable +extent of air, because of the ready communication through the fluids +employed. + +By increasing the number of plates of a battery, you increase its +_intensity_, whilst, by enlarging the dimensions of the plates, you +augment its _quantity_; and, as the superiority of the battery over the +common machine consists entirely in the quantity of electricity +produced, it was at first supposed that it was the size, rather than the +number of plates that was essential to the augmentation of power. It +was, however, found upon trial, that the quantity of electricity +produced by the Voltaic battery, even when of a very moderate size, was +sufficiently copious, and that the chief advantage in this apparatus was +obtained by increasing the intensity, which, however, still falls very +short of that of the common machine. + +I should not omit to mention, that a very splendid, and, at the same +time, most powerful battery, was, a few years ago, constructed under the +direction of Sir H. Davy, which he repeatedly exhibited in his course of +electro-chemical lectures. It consists of two thousand double plates of +zinc and copper, of six square inches in dimensions, arranged in troughs +of Wedgwood-ware, each of which contains twenty of these plates. The +troughs are furnished with a contrivance for lifting the plates out of +them in a very convenient and expeditious manner.* + + [Footnote *: A model of this mode of construction is exhibited in + PLATE XIII. Fig. 1.] + +CAROLINE. + +Well, now that we understand the nature of the action of the Voltaic +battery, I long to hear an account of the discoveries to which it has +given rise. + +MRS. B. + +You must restrain your impatience, my dear, for I cannot with any +propriety introduce the subject of these discoveries till we come to +them in the regular course of our studies. But, as almost every +substance in nature has already been exposed to the influence of the +Voltaic battery, we shall very soon have occasion to notice its effects. + + + + +CONVERSATION VI. + +ON OXYGEN AND NITROGEN. + + +MRS. B. + +To-day we shall examine the chemical properties of the ATMOSPHERE. + +CAROLINE. + +I thought that we were first to learn the nature of OXYGEN, which come +next in our table of simple bodies? + +MRS. B. + +And so you shall; the atmosphere being composed of two principles, +OXYGEN and NITROGEN, we shall proceed to analyse it, and consider its +component parts separately. + +EMILY. + +I always thought that the atmosphere had been a very complicated fluid, +composed of all the variety of exhalations from the earth. + +MRS. B. + +Such substances may be considered rather as heterogeneous and +accidental, than as forming any of its component parts; and the +proportion they bear to the whole mass is quite inconsiderable. + +ATMOSPHERICAL AIR is composed of two gasses, known by the names of +OXYGEN GAS and NITROGEN or AZOTIC GAS. + +EMILY. + +Pray what is a gas? + +MRS. B. + +The name of gas is given to any fluid capable of existing constantly in +an aeriform state, under the pressure and at the temperature of the +atmosphere. + +CAROLINE. + +Is not water, or any other substance, when evaporated by heat, called +gas? + +MRS. B. + +No, my dear; vapour is, indeed, an elastic fluid, and bears a strong +resemblance to a gas; there are, however, several points in which they +essentially differ, and by which you may always distinguish them. Steam, +or vapour, owes its elasticity merely to a high temperature, which is +equal to that of boiling water. And it differs from boiling water only +by being united with more caloric, which, as we before explained, is in +a latent state. When steam is cooled, it instantly returns to the form +of water; but air, or gas, has never yet been rendered liquid or solid +by any degree of cold. + +EMILY. + +But does not gas, as well as vapour, owe its elasticity to caloric? + +MRS. B. + +It was the prevailing opinion; and the difference of gas or vapour was +thought to depend on the different manner in which caloric was united +with the basis of these two kinds of elastic fluids. In vapour, it was +considered as in a latent state; in gas, it was said to be chemically +combined. But the late researches of Sir H. Davy have given rise to a +new theory respecting gasses; and there is now reason to believe that +these bodies owe their permanently elastic state, not solely to caloric, +but likewise to the prevalence of either the one or the other of the two +electricities. + +EMILY. + +When you speak, then, of the simple bodies oxygen and nitrogen, you mean +to express those substances which are the basis of the two gasses? + +MRS. B. + +Yes, in strict propriety, for they can properly be called gasses only +when brought to an aeriform state. + +CAROLINE. + +In what proportions are they combined in the atmosphere? + +MRS. B. + +The oxygen gas constitutes a little more than one-fifth, and the +nitrogen gas a little less than four-fifths. When separated, they are +found to possess qualities totally different from each other. For oxygen +gas is essential both to respiration and combustion, while neither of +these processes can be performed in nitrogen gas. + +CAROLINE. + +But if nitrogen gas is unfit for respiration, how does it happen that +the large proportion of it which enters into the composition of the +atmosphere is not a great impediment to breathing? + +MRS. B. + +We should breathe more freely than our lungs could bear, if we respired +oxygen gas alone. The nitrogen is no impediment to respiration, and +probably, on the contrary, answers some useful purpose, though we do not +know in what manner it acts in that process. + +EMILY. + +And by what means can the two gasses, which compose the atmospheric air, +be separated? + +MRS. B. + +There are many ways of analysing the atmosphere: the two gasses may be +separated first by combustion. + +EMILY. + +You surprise me! how is it possible that combustion should separate +them? + +MRS. B. + +I should previously remind you that oxygen is supposed to be the only +simple body naturally combined with negative electricity. In all the +other elements the positive electricity prevails, and they have +consequently, all of them, an attraction for oxygen.* + + [Footnote *: If chlorine or oxymuriatic gas be a simple body, + according to Sir H. Davy’s view of the subject, it must be + considered as an exception to this statement; but this subject + cannot be discussed till the properties and nature of chlorine + come under examination.] + +CAROLINE. + +Oxygen the only negatively electrified body! that surprises me +extremely; how then are the combinations of the other bodies performed, +if, according to your explanation of chemical attraction, bodies are +supposed only to combine in virtue of their opposite states of +electricity? + +MRS. B. + +Observe that I said, that oxygen was the only _simple_ body, naturally +negative. Compound bodies, in which oxygen prevails over the other +component parts, are also negative, but their negative energy is greater +or less in proportion as the oxygen predominates. Those compounds into +which oxygen enters in less proportion than the other constituents, are +positive, but their positive energy is diminished in proportion to the +quantity of oxygen which enters into their composition. + +All bodies, therefore, that are not already combined with oxygen, will +attract it, and, under certain circumstances, will absorb it from the +atmosphere, in which case the nitrogen gas will remain alone, and may +thus be obtained in its separate state. + +CAROLINE. + +I do not understand how a gas can be absorbed? + +MRS. B. + +It is only the oxygen, or basis of the gas, which is absorbed; and the +two electricities escaping, that is to say, the negative from the +oxygen, the positive from the burning body, unite and produce caloric. + +EMILY. + +And what becomes of this caloric? + +MRS. B. + +We shall make this piece of dry wood attract oxygen from the atmosphere, +and you will see what becomes of the caloric. + +CAROLINE. + +You are joking, Mrs. B--; you do not mean to decompose the atmosphere +with a piece of dry stick? + +MRS. B. + +Not the whole body of the atmosphere, certainly; but if we can make this +piece of wood attract any quantity of oxygen from it, a proportional +quantity of atmospherical air will be decomposed. + +CAROLINE. + +If wood has so strong an attraction for oxygen, why does it not +decompose the atmosphere spontaneously? + +MRS. B. + +It is found by experience, that an elevation of temperature is required +for the commencement of the union of the oxygen and the wood. + +This elevation of temperature was formerly thought to be necessary, in +order to diminish the cohesive attraction of the wood, and enable the +oxygen to penetrate and combine with it more readily. But since the +introduction of the new theory of chemical combination, another cause +has been assigned, and it is now supposed that the high temperature, by +exalting the electrical energies of bodies, and consequently their force +of attraction, facilitates their combination. + +EMILY. + +If it is true, that caloric is composed of the two electricities, an +elevation of temperature must necessarily augment the electric energies +of bodies. + +MRS. B. + +I doubt whether that would be a necessary consequence; for, admitting +this composition of caloric, it is only by its being decomposed that +electricity can be produced. Sir H. Davy, however, in his numerous +experiments, has found it to be an almost invariable rule that the +electrical energies of bodies are increased by elevation of temperature. + +What means then shall we employ to raise the temperature of the wood, so +as to enable it to attract oxygen from the atmosphere? + +CAROLINE. + +Holding it near the fire, I should think, would answer the purpose. + +MRS. B. + +It may, provided you hold it sufficiently close to the fire; for a very +considerable elevation of temperature is required. + +CAROLINE. + +It has actually taken fire, and yet I did not let it touch the coals, +but I held it so very close that I suppose it caught fire merely from +the intensity of the heat. + +MRS. B. + +Or you might say, in other words, that the caloric which the wood +imbibed, so much elevated its temperature, and exalted its electric +energy, as to enable it to attract oxygen very rapidly from the +atmosphere. + +EMILY. + +Does the wood absorb oxygen while it is burning? + +MRS. B. + +Yes, and the heat and light are produced by the union of the two +electricities which are set at liberty, in consequence of the oxygen +combining with the wood. + +CAROLINE. + +You astonish me! the heat of a burning body proceeds then as much from +the atmosphere as from the body itself? + +MRS. B. + +It was supposed that the caloric, given out during combustion, proceeded +entirely, or nearly so, from the decomposition of the oxygen gas; but, +according to Sir H. Davy’s new view of the subject, both the oxygen gas, +and the combustible body, concur in supplying the heat and light, by the +union of their opposite electricities. + +EMILY. + +I have not yet met with any thing in chemistry that has surprised or +delighted me so much as this explanation of combustion. I was at first +wondering what connection there could be between the affinity of a body +for oxygen and its combustibility; but I think I understand it now +perfectly. + +MRS. B. + +Combustion then, you see, is nothing more than the rapid combination of +a body with oxygen, attended by the disengagement of light and heat. + +EMILY. + +But are there no combustible bodies whose attraction for oxygen is so +strong, that they will combine with it, without the application of heat? + +CAROLINE. + +That cannot be; otherwise we should see bodies burning spontaneously. + +MRS. B. + +But there are some instances of this kind, such as phosphorus, +potassium, and some compound bodies, which I shall hereafter make you +acquainted with. These bodies, however, are prepared by art, for in +general, all the combustions that could occur spontaneously, at the +temperature of the atmosphere, have already taken place; therefore new +combustions cannot happen without the temperature of the body being +raised. Some bodies, however, will burn at a much lower temperature than +others. + +CAROLINE. + +But the common way of burning a body is not merely to approach it to one +already on fire, but rather to put the one in actual contact with the +other, as when I burn this piece of paper by holding it in the flame of +the fire. + +MRS. B. + +The closer it is in contact with the source of caloric, the sooner will +its temperature be raised to the degree necessary for it to burn. If you +hold it near the fire, the same effect will be produced; but more time +will be required, as you found to be the case with the piece of stick. + +EMILY. + +But why is it not necessary to continue applying caloric throughout the +process of combustion, in order to keep up the electric energy of the +wood, which is required to enable it to combine with the oxygen? + +MRS. B. + +The caloric which is gradually produced by the two electricities during +combustion, keeps up the temperature of the burning body; so that when +once combustion has begun, no further application of caloric is +required. + +CAROLINE. + +Since I have learnt this wonderful theory of combustion, I cannot take +my eyes from the fire; and I can scarcely conceive that the heat and +light, which I always supposed to proceed entirely from the coals, are +really produced as much by the atmosphere. + +EMILY. + +When you blow the fire, you increase the combustion, I suppose, by +supplying the coals with a greater quantity of oxygen gas? + +MRS. B. + +Certainly; but of course no blowing will produce combustion, unless the +temperature of the coals be first raised. A single spark, however, is +sometimes sufficient to produce that effect; for, as I said before, when +once combustion has commenced, the caloric disengaged is sufficient to +elevate the temperature of the rest of the body, provided that there be +a free access of oxygen. It however sometimes happens that if a fire be +ill made, it will be extinguished before all the fuel is consumed, from +the very circumstance of the combustion being so slow that the caloric +disengaged is insufficient to keep up the temperature of the fuel. You +must recollect that there are three things required in order to produce +combustion; a combustible body, oxygen, and a temperature at which the +one will combine with the other. + +EMILY. + +You said that combustion was one method of decomposing the atmosphere, +and obtaining the nitrogen gas in its simple state; but how do you +secure this gas, and prevent it from mixing with the rest of the +atmosphere? + +MRS. B. + +It is necessary for this purpose to burn the body within a close vessel, +which is easily done. --We shall introduce a small lighted taper (PLATE +VII. Fig. 1.) under this glass receiver, which stands in a bason over +water, to prevent all communication with the external air. + + [Illustration: Plate VII. Vol. I. p. 181. + + Fig. 1. Combustion of a taper under a receiver. + + Fig. 2. A Retort on a stand. + + Fig. 3. Preparation of oxygen gas. + A Furnace. + B Earthen Retort in the furnace. + C Water bath. + D Receiver. + E.E Tube conveying the gas from the Retort through the water into + the Receiver. + F.F.F Shelf perforated on which the Receiver stands. + + Fig. 4. Combustion of iron wire in oxygen gas.] + +CAROLINE. + +How dim the light burns already! --It is now extinguished. + +MRS. B. + +Can you tell us why it is extinguished? + +CAROLINE. + +Let me consider. --The receiver was full of atmospherical air; the +taper, in burning within it, must have combined with the oxygen +contained in that air, and the caloric that was disengaged produced the +light of the taper. But when the whole of the oxygen was absorbed, the +whole of its electricity was disengaged; consequently no more caloric +could be produced, the taper ceased to burn, and the flame was +extinguished. + +MRS. B. + +Your explanation is perfectly correct. + +EMILY. + +The two constituents of the oxygen gas being thus disposed of, what +remains under the receiver must be pure nitrogen gas? + +MRS. B. + +There are some circumstances which prevent the nitrogen gas, thus +obtained, from being perfectly pure; but we may easily try whether the +oxygen has disappeared, by putting another lighted taper under it. --You +see how instantaneously the flame is extinguished, for want of oxygen to +supply the negative electricity required for the formation of caloric; +and were you to put an animal under the receiver, it would immediately +be suffocated. But that is an experiment which I do not think your +curiosity will tempt you to try. + +EMILY. + +Certainly not. --But look, Mrs. B., the receiver is full of a thick +white smoke. Is that nitrogen gas? + +MRS. B. + +No, my dear; nitrogen gas is perfectly transparent and invisible, like +common air. This cloudiness proceeds from a variety of exhalations, +which arise from the burning taper, and the nature of which you cannot +yet understand. + +CAROLINE. + +The water within the receiver has now risen a little above its level in +the bason. What is the reason of this? + +MRS. B. + +With a moment’s reflection, I dare say, you would have explained it +yourself. The water rises in consequence of the oxygen gas within it +having been destroyed, or rather decomposed, by the combustion of the +taper. + +CAROLINE. + +Then why did not the water rise immediately when the oxygen gas was +destroyed? + +MRS. B. + +Because the heat of the taper, whilst burning, produced a dilatation of +the air in the vessel, which at first counteracted this effect. + +Another means of decomposing the atmosphere is the _oxygenation_ of +certain metals. This process is very analogous to combustion; it is, +indeed, only a more general term to express the combination of a body +with oxygen. + +CAROLINE. + +In what respect, then, does it differ from combustion? + +MRS. B. + +The combination of oxygen in combustion is always accompanied by a +disengagement of light and heat; whilst this circumstance is not a +necessary consequence of simple oxygenation. + +CAROLINE. + +But how can a body absorb oxygen without the combination of the two +electricities which produce caloric? + +MRS. B. + +Oxygen does not always present itself in a gaseous state; it is a +constituent part of a vast number of bodies, both solid and liquid, in +which it exists in a much denser state than in the atmosphere; and from +these bodies it may be obtained without much disengagement of caloric. +It may likewise, in some cases, be absorbed from the atmosphere without +any sensible production of light and heat; for, if the process be slow, +the caloric is disengaged in such small quantities, and so gradually, +that it is not capable of producing either light or heat. In this case +the absorption of oxygen is called _oxygenation_ or _oxydation_, instead +of _combustion_, as the production of sensible light and heat is +essential to the latter. + +EMILY. + +I wonder that metals can unite with oxygen; for, as they are so dense, +their attraction of aggregation must be very great; and I should have +thought that oxygen could never have penetrated such bodies. + +MRS. B. + +Their strong attraction for oxygen counterbalances this obstacle. Most +metals, however, require to be made red-hot before they are capable of +attracting oxygen in any considerable quantity. By this combination they +lose most of their metallic properties, and fall into a kind of powder, +formerly called _calx_, but now much more properly termed an _oxyd_; +thus we have _oxyd of lead_, _oxyd of iron_, &c. + +EMILY. + +And in the Voltaic battery, it is, I suppose, an oxyd of zinc, that is +formed by the union of the oxygen with that metal? + +MRS. B. + +Yes, it is. + +CAROLINE. + +The word oxyd, then, simply means a metal combined with oxygen? + +MRS. B. + +Yes; but the term is not confined to metals, though chiefly applied to +them. Any body whatever, that has combined with a certain quantity of +oxygen, either by means of oxydation or combustion, is called an _oxyd_, +and is said to be _oxydated_ or _oxygenated_. + +EMILY. + +Metals, when converted into oxyds, become, I suppose, negative? + +MRS. B. + +Not in general; because in most oxyds the positive energy of the metal +more than counterbalances the native energy of the oxygen with which it +combines. + +This black powder is an oxyd of manganese, a metal which has so strong +an affinity for oxygen, that it attracts that substance from the +atmosphere at any known temperature: it is therefore never found in its +metallic form, but always in that of an oxyd, in which state, you see, +it has very little of the appearance of a metal. It is now heavier than +it was before oxydation, in consequence of the additional weight of the +oxygen with which it has combined. + +CAROLINE. + +I am very glad to hear that; for I confess I could not help having some +doubts whether oxygen was really a substance, as it is not to be +obtained in a simple and palpable state; but its weight is, I think, +a decisive proof of its being a real body. + +MRS. B. + +It is easy to estimate its weight, by separating it from the manganese, +and finding how much the latter has lost. + +EMILY. + +But if you can take the oxygen from the metal, shall we not then have it +in its palpable simple state? + +MRS. B. + +No; for I can only separate the oxygen from the manganese, by presenting +to it some other body, for which it has a greater affinity than for the +manganese. Caloric affording the two electricities is decomposed, and +one of them uniting with the oxygen, restores it to the aëriform state. + +EMILY. + +But you said just now, that manganese would attract oxygen from the +atmosphere in which it is combined with the negative electricity; how, +therefore, can the oxygen have a superior affinity for that electricity, +since it abandons it to combine with the manganese? + +MRS. B. + +I give you credit for this objection, Emily; and the only answer I can +make to it is, that the mutual affinities of metals for oxygen, and of +oxygen for electricity, vary at different temperatures; a certain degree +of heat will, therefore, dispose a metal to combine with oxygen, whilst, +on the contrary, the former will be compelled to part with the latter, +when the temperature is further increased. I have put some oxyd of +manganese into a retort, which is an earthen vessel with a bent neck, +such as you see here. (PLATE VII. Fig. 2.) --The retort containing the +manganese you cannot see, as I have enclosed it in this furnace, where +it is now red-hot. But, in order to make you sensible of the escape of +the gas, which is itself invisible, I have connected the neck of the +retort with this bent tube, the extremity of which is immersed in this +vessel of water. (PLATE VII. Fig. 3.) --Do you see the bubbles of air +rise through the water? + +CAROLINE. + +Perfectly. This, then, is pure oxygen gas; what a pity it should be +lost! Could you not preserve it? + +MRS. B. + +We shall collect it in this receiver. --For this purpose, you observe, +I first fill it with water, in order to exclude the atmospherical air; +and then place it over the bubbles that issue from the retort, so as to +make them rise through the water to the upper part of the receiver. + +EMILY. + +The bubbles of oxygen gas rise, I suppose, from their specific levity? + +MRS. B. + +Yes; for though oxygen forms rather a heavy gas, it is light compared to +water. You see how it gradually displaces the water from the receiver. +It is now full of gas, and I may leave it inverted in water on this +shelf, where I can keep the gas as long as I choose, for future +experiments. This apparatus (which is indispensable in all experiments +in which gases are concerned) is called a water-bath. + +CAROLINE. + +It is a very clever contrivance, indeed; equally simple and useful. How +convenient the shelf is for the receiver to rest upon under water, and +the holes in it for the gas to pass into the receiver! I long to make +some experiments with this apparatus. + +MRS. B. + +I shall try your skill that way, when you have a little more experience. +I am now going to show you an experiment, which proves, in a very +striking manner, how essential oxygen is to combustion. You will see +that iron itself will burn in this gas, in the most rapid and brilliant +manner. + +CAROLINE. + +Really! I did not know that it was possible to burn iron. + +EMILY. + +Iron is a simple body, and you know, Caroline, that all simple bodies +are naturally positive, and therefore must have an affinity for oxygen. + +MRS. B. + +Iron will, however, not burn in atmospherical air without a very great +elevation of temperature; but it is eminently combustible in pure oxygen +gas; and what will surprise you still more, it can be set on fire +without any considerable rise of temperature. You see this spiral iron +wire--I fasten it at one end to this cork, which is made to fit an +opening at the top of the glass-receiver. (PLATE VII. Fig. 4.) + +EMILY. + +I see the opening in the receiver; but it is carefully closed by a +ground glass-stopper. + +MRS. B. + +That is in order to prevent the gas from escaping; but I shall take out +the stopper, and put in the cork, to which the wire hangs. --Now I mean +to burn this wire in the oxygen gas, but I must fix a small piece of +lighted tinder to the extremity of it, in order to give the first +impulse to combustion; for, however powerful oxygen is in promoting +combustion, you must recollect that it cannot take place without some +elevation of temperature. I shall now introduce the wire into the +receiver, by quickly changing the stoppers. + +CAROLINE. + +Is there no danger of the gas escaping while you change the stoppers? + +MRS. B. + +Oxygen gas is a little heavier than atmospherical air, therefore it will +not mix with it very rapidly; and, if I do not leave the opening +uncovered, we shall not lose any---- + +CAROLINE. + +Oh, what a brilliant and beautiful flame! + +EMILY. + +It is as white and dazzling as the sun! --Now a piece of the melted wire +drops to the bottom: I fear it is extinguished; but no, it burns again +as bright as ever. + +MRS. B. + +It will burn till the wire is entirely consumed, provided the oxygen is +not first expended: for you know it can burn only while there is oxygen +to combine with it. + +CAROLINE. + +I never saw a more beautiful light. My eyes can hardly bear it! How +astonishing to think that all this caloric was contained in the small +quantity of gas and iron that was enclosed in the receiver; and that, +without producing any sensible heat! + +CAROLINE. + +How wonderfully quick combustion goes on in pure oxygen gas! But pray, +are these drops of burnt iron as heavy as the wire was before? + +MRS. B. + +They are even heavier; for the iron, in burning, has acquired exactly +the weight of the oxygen which has disappeared, and is now combined with +it. It has become an oxyd of iron. + +CAROLINE. + +I do not know what you mean by saying that the oxygen has _disappeared_, +Mrs. B., for it was always invisible. + +MRS. B. + +True, my dear; the expression was incorrect. But though you could not +see the oxygen gas, I believe you had no doubt of its presence, as the +effect it produced on the wire was sufficiently evident. + +CAROLINE. + +Yes, indeed; yet you know it was the caloric, and not the oxygen gas +itself, that dazzled us so much. + +MRS. B. + +You are not quite correct in your turn, in saying the caloric dazzled +you; for caloric is invisible; it affects only the sense of feeling; it +was the light which dazzled you. + +CAROLINE. + +True; but light and caloric are such constant companions, that it is +difficult to separate them, even in idea. + +MRS. B. + +The easier it is to confound them, the more careful you should be in +making the distinction. + +CAROLINE. + +But why has the water now risen, and filled part of the receiver? + +MRS. B. + +Indeed, Caroline, I did not suppose you would have asked such a +question! I dare say, Emily, you can answer it. + +EMILY. + +Let me reflect . . . . . . The oxygen has combined with the wire; the +caloric has escaped; consequently nothing can remain in the receiver, +and the water will rise to fill the vacuum. + +CAROLINE. + +I wonder that I did not think of that. I wish that we had weighed the +wire and the oxygen gas before combustion; we might then have found +whether the weight of the oxyd was equal to that of both. + +MRS. B. + +You might try the experiment if you particularly wished it; but I can +assure you, that, if accurately performed, it never fails to show that +the additional weight of the oxyd is precisely equal to that of the +oxygen absorbed, whether the process has been a real combustion, or a +simple oxygenation. + +CAROLINE. + +But this cannot be the case with combustions in general; for when any +substance is burnt in the common air, so far from increasing in weight, +it is evidently diminished, and sometimes entirely consumed. + +MRS. B. + +But what do you mean by the expression _consumed_? You cannot suppose +that the smallest particle of any substance in nature can be actually +destroyed. A compound body is decomposed by combustion; some of its +constituent parts fly off in a gaseous form, while others remain in a +concrete state; the former are called the _volatile_, the latter the +_fixed products_ of combustion. But if we collect the whole of them, we +shall always find that they exceed the weight of the combustible body, +by that of the oxygen which has combined with them during combustion. + +EMILY. + +In the combustion of a coal fire, then, I suppose that the ashes are +what would be called the fixed product, and the smoke the volatile +product? + +MRS. B. + +Yet when the fire burns best, and the quantity of volatile products +should be the greatest, there is no smoke; how can you account for that? + +EMILY. + +Indeed I cannot; therefore I suppose that I was not right in my +conjecture. + +MRS. B. + +Not quite: ashes, as you supposed, are a fixed product of combustion; +but smoke, properly speaking, is not one of the volatile products, as it +consists of some minute undecomposed particles of the coals that are +carried off by the heated air without being burnt, and are either +deposited in the form of soot, or dispersed by the wind. Smoke, +therefore, ultimately, becomes one of the _fixed_ products of +combustion. And you may easily conceive that the stronger the fire is, +the less smoke is produced, because the fewer particles escape +combustion. On this principle depends the invention of Argand’s Patent +Lamps; a current of air is made to pass through the cylindrical wick of +the lamp, by which means it is so plentifully supplied with oxygen, that +scarcely a particle of oil escapes combustion, nor is there any smoke +produced. + +EMILY. + +But what then are the volatile products of combustion? + +MRS. B. + +Various new compounds, with which you are not yet acquainted, and which +being converted by caloric either into vapour or gas, are invisible; but +they can be collected, and we shall examine them at some future period. + +CAROLINE. + +There are then other gases, besides the oxygen and nitrogen gases. + +MRS. B. + +Yes, several: any substance that can assume and maintain the form of an +elastic fluid at the temperature of the atmosphere, is called a gas. We +shall examine the several gases in their respective places; but we must +now confine our attention to those that compose the atmosphere. + +I shall show you another method of decomposing the atmosphere, which is +very simple. In breathing, we retain a portion of the oxygen, and expire +the nitrogen gas; so that if we breathe in a closed vessel, for a +certain length of time, the air within it will be deprived of its oxygen +gas. Which of you will make the experiment? + +CAROLINE. + +I should be very glad to try it. + +MRS. B. + +Very well; breathe several times through this glass tube into the +receiver with which it is connected, until you feel that your breath is +exhausted. + +CAROLINE. + +I am quite out of breath already! + +MRS. B. + +Now let us try the gas with a lighted taper. + +EMILY. + +It is very pure nitrogen gas, for the taper is immediately extinguished. + +MRS. B. + +That is not a proof of its being pure, but only of the absence of +oxygen, as it is that principle alone which can produce combustion, +every other gas being absolutely incapable of it. + +EMILY. + +In the methods which you have shown us, for decomposing the atmosphere, +the oxygen always abandons the nitrogen; but is there no way of taking +the nitrogen from the oxygen, so as to obtain the latter pure from the +atmosphere? + +MRS. B. + +You must observe, that whenever oxygen is taken from the atmosphere, it +is by decomposing the oxygen gas; we cannot do the same with the +nitrogen gas, because nitrogen has a stronger affinity for caloric than +for any other known principle: it appears impossible therefore to +separate it from the atmosphere by the power of affinities. But if we +cannot obtain the oxygen gas, by this means, in its separate state, we +have no difficulty (as you have seen) to procure it in its gaseous form, +by taking it from those substances that have absorbed it from the +atmosphere, as we did with the oxyd of manganese. + +EMILY. + +Can atmospherical air be recomposed, by mixing due proportions of oxygen +and nitrogen gases? + +MRS. B. + +Yes: if about one part of oxygen gas be mixed with about four parts of +nitrogen gas, atmospherical air is produced.* + + [Footnote *: The proportion of oxygen in the atmosphere varies + from 21 to 22 per cent.] + +EMILY. + +The air, then, must be an oxyd of nitrogen? + +MRS. B. + +No, my dear; for there must be a chemical combination between oxygen and +nitrogen in order to produce an oxyd; whilst in the atmosphere these two +substances are separately combined with caloric, forming two distinct +gases, which are simply mixed in the formation of the atmosphere. + +I shall say nothing more of oxygen and nitrogen at present, as we shall +continually have occasion to refer to them in our future conversations. +They are both very abundant in nature; nitrogen is the most plentiful in +the atmosphere, and exists also in all animal substances; oxygen forms a +constituent part, both of the animal and vegetable kingdoms, from which +it may be obtained by a variety of chemical means. But it is now time to +conclude our lesson. I am afraid you have learnt more to-day than you +will be able to remember. + +CAROLINE. + +I assure you that I have been too much interested in it, ever to forget +it. In regard to nitrogen there seems to be but little to remember; it +makes a very insignificant figure in comparison to oxygen, although it +composes a much larger portion of the atmosphere. + +MRS. B. + +Perhaps this insignificance you complain of may arise from the compound +nature of nitrogen, for though I have hitherto considered it as a simple +body, because it is not known in any natural process to be decomposed, +yet from some experiments of Sir H. Davy, there appears to be reason for +suspecting that nitrogen is a compound body, as we shall see afterwards. +But even in its simple state, it will not appear so insignificant when +you are better acquainted with it; for though it seems to perform but a +passive part in the atmosphere, and has no very striking properties, +when considered in its separate state, yet you will see by-and-bye what +a very important agent it becomes, when combined with other bodies. But +no more of this at present; we must reserve it for its proper place. + + + + +CONVERSATION VII. + +ON HYDROGEN. + + +CAROLINE. + +The next simple bodies we come to are CHLORINE and IODINE. Pray what +kinds of substances are these; are they also invisible? + +MRS. B. + +No; for chlorine, in the state of gas, has a distinct greenish colour, +and is therefore visible; and iodine, in the same state, has a beautiful +claret-red colour. The knowledge of these two bodies, however, and the +explanation of their properties, imply various considerations, which you +would not yet be able to understand; we shall therefore defer their +examination to some future conversation, and we shall pass on to the +next simple substance, HYDROGEN, which we cannot, any more than oxygen, +obtain in a visible or palpable form. We are acquainted with it only in +its gaseous state, as we are with oxygen and nitrogen. + +CAROLINE. + +But in its gaseous state it cannot be called a simple substance, since +it is combined with heat and electricity? + +MRS. B. + +True, my dear; but as we do not know in nature of any substance which is +not more or less combined with caloric and electricity, we are apt to +say that a substance is in its pure state when combined with those +agents only. + +Hydrogen was formerly called _inflammable air_, as it is extremely +combustible, and burns with a great flame. Since the invention of the +new nomenclature, it has obtained the name of hydrogen, which is derived +from two Greek words, the meaning of which is, _to produce water_. + +EMILY. + +And how does hydrogen produce water? + +MRS. B. + +By its combustion. Water is composed of eighty-five parts, by weight, of +oxygen, combined with fifteen parts of hydrogen; or of two parts, by +bulk of hydrogen gas, to one part of oxygen gas. + +CAROLINE. + +Really! is it possible that water should be a combination of two gases, +and that one of these should be inflammable air! Hydrogen must be a most +extraordinary gas that will produce both fire and water. + +EMILY. + +But I thought you said that combustion could take place in no gas but +oxygen? + +MRS. B. + +Do you recollect what the process of combustion consists in? + +EMILY. + +In the combination of a body with oxygen, with disengagement of light +and heat. + +MRS. B. + +Therefore when I say that hydrogen is combustible, I mean that it has an +affinity for oxygen; but, like all other combustible substances, it +cannot burn unless supplied with oxygen, and also heated to a proper +temperature. + +CAROLINE. + +The simply mixing fifteen parts of hydrogen, with eighty-five parts of +oxygen gas, will not, therefore, produce water? + +MRS. B. + +No; water being a much denser fluid than gases, in order to reduce these +gases to a liquid, it is necessary to diminish the quantity of caloric +or electricity which maintains them in an elastic form. + +EMILY. + +That I should think might be done by combining the oxygen and hydrogen +together; for in combining they would give out their respective +electricities in the form of caloric, and by this means would be +condensed. + +CAROLINE. + +But you forget, Emily, that in order to make the oxygen and hydrogen +combine, you must begin by elevating their temperature, which increases, +instead of diminishing, their electric energies. + +MRS. B. + +Emily is, however, right; for though it is necessary to raise their +temperature, in order to make them combine, as that combination affords +them the means of parting with their electricities, it is eventually the +cause of the diminution of electric energy. + +CAROLINE. + +You love to deal in paradoxes to-day, Mrs. B. --Fire, then, produces +water? + +MRS. B. + +The combustion of hydrogen gas certainly does; but you do not seem to +have remembered the theory of combustion so well as you thought you +would. Can you tell me what happens in the combustion of hydrogen gas? + +CAROLINE. + +The hydrogen combines with the oxygen, and their opposite electricities +are disengaged in the form of caloric. --Yes, I think I understand it +now--by the loss of this caloric, the gases are condensed into a liquid. + +EMILY. + +Water, then, I suppose, when it evaporates and incorporates with the +atmosphere, is decomposed and converted into hydrogen and oxygen gases? + +MRS. B. + +No, my dear--there you are quite mistaken: the decomposition of water is +totally different from its evaporation; for in the latter case (as you +should recollect) water is only in a state of very minute division; and +is merely suspended in the atmosphere, without any chemical combination, +and without any separation of its constituent parts. As long as these +remain combined, they form WATER, whether in a state of liquidity, or in +that of an elastic fluid, as vapour, or under the solid form of ice. + +In our experiments on latent heat, you may recollect that we caused +water successively to pass through these three forms, merely by an +increase or diminution of caloric, without employing any power of +attraction, or effecting any decomposition. + +CAROLINE. + +But are there no means of decomposing water? + +MRS. B. + +Yes, several: charcoal, and metals, when heated red hot, will attract +the oxygen from water, in the same manner as they will from the +atmosphere. + +CAROLINE. + +Hydrogen, I see, is like nitrogen, a poor dependant friend of oxygen, +which is continually forsaken for greater favourites. + +MRS. B. + +The connection, or friendship, as you choose to call it, is much more +intimate between oxygen and hydrogen, in the state of water, than +between oxygen and nitrogen, in the atmosphere; for, in the first case, +there is a chemical union and condensation of the two substances; in the +latter, they are simply mixed together in their gaseous state. You will +find, however, that, in some cases, nitrogen is quite as intimately +connected with oxygen, as hydrogen is. --But this is foreign to our +present subject. + +EMILY. + +Water, then, is an oxyd, though the atmospherical air is not? + +MRS. B. + +It is not commonly called an oxyd, though, according to our definition, +it may, no doubt, be referred to that class of bodies. + +CAROLINE. + +I should like extremely to see water decomposed. + +MRS. B. + +I can gratify your curiosity by a much more easy process than the +oxydation of charcoal or metals: the decomposition of water by these +latter means takes up a great deal of time, and is attended with much +trouble; for it is necessary that the charcoal or metal should be made +red hot in a furnace, that the water should pass over them in a state of +vapour, that the gas formed should be collected over the water-bath, &c. +In short, it is a very complicated affair. But the same effect may be +produced with the greatest facility, by the action of the Voltaic +battery, which this will give me an opportunity of exhibiting. + +CAROLINE. + +I am very glad of that, for I longed to see the power of this apparatus +in decomposing bodies. + +MRS. B. + +For this purpose I fill this piece of glass-tube (PLATE VIII. fig. 1.) +with water, and cork it up at both ends; through one of the corks I +introduce that wire of the battery which conveys the positive +electricity; and the wire which conveys the negative electricity is made +to pass through the other cork, so that the two wires approach each +other sufficiently near to give out their respective electricities. + + [Illustration: Plate VIII. Vol. I. p. 206 + + Fig. 1. Apparatus for the decomposition of water by the Voltaic + Battery. + + Fig. 2. Apparatus for decomposing water by Voltaic Electricity + & obtaining the gasses separate. + + Fig. 3. Apparatus for preparing & collecting hydrogen gas. + + Fig. 4. Receiver full of hydrogen gas inverted over water. + + Fig. 5 Slow combustion of hydrogen gas. + + Fig. 6. Apparatus for illustrating the formation of water by the + combustion of hydrogen gas. + + Fig. 7. Apparatus for producing harmonic sounds by the combustion + of hydrogen gas.] + +CAROLINE. + +It does not appear to me that you approach the wires so near as you did +when you made the battery act by itself. + +MRS. B. + +Water being a better conductor of electricity than air, the two wires +will act on each other at a greater distance in the former than in the +latter. + +EMILY. + +Now the electrical effect appears: I see small bubbles of air emitted +from each wire. + +MRS. B. + +Each wire decomposes the water, the positive by combining with its +oxygen which is negative, the negative by combining with its hydrogen +which is positive. + +CAROLINE. + +That is wonderfully curious! But what are the small bubbles of air? + +MRS. B. + +Those that appear to proceed from the positive wire, are the result of +the decomposition of the water by that wire. That is to say, the +positive electricity having combined with some of the oxygen of the +water, the particles of hydrogen which were combined with that portion +of oxygen are set at liberty, and appear in the form of small bubbles of +gas or air. + +EMILY. + +And I suppose the negative fluid having in the same manner combined with +some of the hydrogen of the water, the particles of oxygen that were +combined with it, are set free, and emitted in a gaseous form. + +MRS. B. + +Precisely so. But I should not forget to observe, that the wires used in +this experiment are made of platina, a metal which is not capable of +combining with oxygen; for otherwise the wire would combine with the +oxygen, and the hydrogen alone would be disengaged. + +CAROLINE. + +But could not water be decomposed without the electric circle being +completed? If, for instance, you immersed only the positive wire in the +water, would it not combine with the oxygen, and the hydrogen gas be +given out? + +MRS. B. + +No; for as you may recollect, the battery cannot act unless the circle +be completed; since the positive wire will not give out its electricity, +unless attracted by that of the negative wire. + +CAROLINE. + +I understand it now. --But look, Mrs. B., the decomposition of the water +which has now been going on for some time, does not sensibly diminish +its quantity--what is the reason of that? + +MRS. B. + +Because the quantity decomposed is so extremely small. If you compare +the density of water with that of the gases into which it is resolved, +you must be aware that a single drop of water is sufficient to produce +thousands of such small bubbles as those you now perceive. + +CAROLINE. + +But in this experiment, we obtain the oxygen and hydrogen gases mixed +together. Is there any means of procuring the two gases separately? + +MRS. B. + +They can be collected separately with great ease, by modifying a little +the experiment. Thus if instead of one tube, we employ two, as you see +here, (c, d, PLATE VIII. fig. 2.) both tubes being closed at one end, +and open at the other; and if after filling these tubes with water, we +place them standing in a glass of water (e), with their open end +downwards, you will see that the moment we connect the wires (a, b) +which proceed upwards from the interior of each tube, the one with one +end of the battery, and the other with the other end, the water in the +tubes will be decomposed; hydrogen will be given out round the wire in +the tube connected with the positive end of the battery, and oxygen in +the other; and these gases will be evolved, exactly in the proportions +which I have before mentioned, namely, two measures of hydrogen for one +of oxygen. We shall now begin the experiment, but it will be some time +before any sensible quantity of the gases can be collected. + +EMILY. + +The decomposition of water in this way, slow as it is, is certainly very +striking; but I confess that I should be still more gratified, if you +could shew it us on a larger scale, and by a quicker process. I am sorry +that the decomposition of water by charcoal or metals is attended with +so much inconvenience. + +MRS. B. + +Water may be decomposed by means of metals without any difficulty; but +for this purpose the intervention of an acid is required. Thus, if we +add some sulphuric acid (a substance with the nature of which you are +not yet acquainted) to the water which the metal is to decompose, the +acid disposes the metal to combine with the oxygen of the water so +readily and abundantly, that no heat is required to hasten the process. +Of this I am going to shew you an instance. I put into this bottle the +water that is to be decomposed, as also the metal that is to effect that +decomposition by combining with the oxygen, and the acid which is to +facilitate the combination of the metal and the oxygen. You will see +with what violence these will act on each other. + +CAROLINE. + +But what metal is it that you employ for this purpose? + +MRS. B. + +It is iron; and it is used in the state of filings, as these present a +greater surface to the acid than a solid piece of metal. For as it is +the surface of the metal which is acted upon by the acid, and is +disposed to receive the oxygen produced by the decomposition of the +water, it necessarily follows that the greater is the surface, the more +considerable is the effect. The bubbles which are now rising are +hydrogen gas---- + +CAROLINE. + +How disagreeably it smells! + +MRS. B. + +It is indeed unpleasant, though, I believe, not particularly hurtful. We +shall not, however, suffer any more to escape, as it will be wanted for +experiments. I shall, therefore, collect it in a glass-receiver, by +making it pass through this bent tube, which will conduct it into the +water-bath. (PLATE VIII. fig. 3.) + +EMILY. + +How very rapidly the gas escapes! it is perfectly transparent, and +without any colour whatever. --Now the receiver is full---- + +MRS. B. + +We shall, therefore, remove it, and substitute another in its place. But +you must observe, that when the receiver is full, it is necessary to +keep it inverted with the mouth under water, otherwise the gas would +escape. And in order that it may not be in the way, I introduce within +the bath, under the water, a saucer, into which I slide the receiver, so +that it can be taken out of the bath and conveyed any where, the water +in the saucer being equally effectual in preventing its escape as that +in the bath. (PLATE VIII. fig. 4.) + +EMILY. + +I am quite surprised to see what a large quantity of hydrogen gas can be +produced by such a small quantity of water, especially as oxygen is the +principal constituent of water. + +MRS. B. + +In weight it is; but not in volume. For though the proportion, by +weight, is nearly six parts of oxygen to one of hydrogen, yet the +proportion of the volume of the gases, is about one part of oxygen to +two of hydrogen; so much heavier is the former than the latter. + +CAROLINE. + +But why is the vessel in which the water is decomposed so hot? As the +water changes from a liquid to a gaseous form, cold should be produced +instead of heat. + +MRS. B. + +No; for if one of the constituents of water is converted into a gas, the +other becomes solid in combining with the metal. + +EMILY. + +In this case, then, neither heat nor cold should be produced? + +MRS. B. + +True: but observe that the sensible heat which is disengaged in this +operation, is not owing to the decomposition of the water, but to an +extrication of heat produced by the mixture of water and sulphuric acid. +I will mix some water and sulphuric acid together in this glass, that +you may feel the surprising quantity of heat that is disengaged by their +union--now take hold of the glass---- + +CAROLINE. + +Indeed I cannot; it feels as hot as boiling water. I should have +imagined there would have been heat enough disengaged to have rendered +the liquid solid. + +MRS. B. + +As, however, it does not produce that effect, we cannot refer this heat +to the modification called latent heat. We may, however, I think, +consider it as heat of capacity, as the liquid is condensed by its loss; +and if you were to repeat the experiment, in a graduated tube, you would +find that the two liquids, when mixed, occupy considerably less space +than they did separately. --But we will reserve this to another +opportunity, and attend at present to the hydrogen gas which we have +been producing. + +If I now set the hydrogen gas, which is contained in this receiver, at +liberty all at once, and kindle it as soon as it comes in contact with +the atmosphere, by presenting it to a candle, it will so suddenly and +rapidly decompose the oxygen gas, by combining with its basis, that an +explosion, or a _detonation_ (as chemists commonly call it), will be +produced. For this purpose, I need only take up the receiver, and +quickly present its open mouth to the candle---- so . . . . + +CAROLINE. + +It produced only a sort of hissing noise, with a vivid flash of light. +I had expected a much greater report. + +MRS. B. + +And so it would have been, had the gases been closely confined at the +moment they were made to explode. If, for instance, we were to put in +this bottle a mixture of hydrogen gas and atmospheric air; and if, after +corking the bottle, we should kindle the mixture by a very small +orifice, from the sudden dilatation of the gases at the moment of their +combination, the bottle must either fly to pieces, or the cork be blown +out with considerable violence. + +CAROLINE. + +But in the experiment which we have just seen, if you did not kindle the +hydrogen gas, would it not equally combine with the oxygen? + +MRS. B. + +Certainly not; for, as I have just explained to you, it is necessary +that the oxygen and hydrogen gases be burnt together, in order to +combine chemically and produce water. + +CAROLINE. + +That is true; but I thought this was a different combination, for I see +no water produced. + +MRS. B. + +The water resulting from this detonation was so small in quantity, and +in such a state of minute division, as to be invisible. But water +certainly was produced; for oxygen is incapable of combining with +hydrogen in any other proportions than those that form water; therefore +water must always be the result of their combination. + +If, instead of bringing the hydrogen gas into sudden contact with the +atmosphere (as we did just now) so as to make the whole of it explode +the moment it is kindled, we allow but a very small surface of gas to +burn in contact with the atmosphere, the combustion goes on quietly and +gradually at the point of contact, without any detonation, because the +surfaces brought together are too small for the immediate union of +gases. The experiment is a very easy one. This phial, with a narrow +neck, (PLATE VIII. fig. 5.) is full of hydrogen gas, and is carefully +corked. If I take out the cork without moving the phial, and quickly +approach the candle to the orifice, you will see how different the +result will be---- + +EMILY. + +How prettily it burns, with a blue flame! The flame is gradually sinking +within the phial--now it has entirely disappeared. But does not this +combustion likewise produce water? + +MRS. B. + +Undoubtedly. In order to make the formation of the water sensible to +you, I shall procure a fresh supply of hydrogen gas, by putting into +this bottle (PLATE VIII. fig. 6.) iron filings, water, and sulphuric +acid, materials similar to those which we have just used for the same +purpose. I shall then cork up the bottle, leaving only a small orifice +in the cork, with a piece of glass-tube fixed to it, through which the +gas will issue in a continued rapid stream. + +CAROLINE. + +I hear already the hissing of the gas through the tube, and I can feel a +strong current against my hand. + +MRS. B. + +This current I am going to kindle with the candle--see how vividly it +burns---- + +EMILY. + +It burns like a candle with a long flame. But why does this combustion +last so much longer than in the former experiment? + +MRS. B. + +The combustion goes on uninterruptedly as long as the new gas continues +to be produced. Now if I invert this receiver over the flame, you will +soon perceive its internal surface covered with a very fine dew, which +is pure water---- + +CAROLINE. + +Yes, indeed; the glass is now quite dim with moisture! How glad I am +that we can see the water produced by this combustion. + +EMILY. + +It is exactly what I was anxious to see; for I confess I was a little +incredulous. + +MRS. B. + +If I had not held the glass-bell over the flame, the water would have +escaped in the state of vapour, as it did in the former experiment. We +have here, of course, obtained but a very small quantity of water; but +the difficulty of procuring a proper apparatus, with sufficient +quantities of gases, prevents my showing it you on a larger scale. + +The composition of water was discovered about the same period, both by +Mr. Cavendish, in this country, and by the celebrated French chemist +Lavoisier. The latter invented a very perfect and ingenious apparatus to +perform, with great accuracy, and upon a large scale, the formation of +water by the combination of oxygen and hydrogen gases. Two tubes, +conveying due proportions, the one of oxygen, the other of hydrogen gas, +are inserted at opposite sides of a large globe of glass, previously +exhausted of air; the two streams of gas are kindled within the globe, +by the electrical spark, at the point where they come in contact; they +burn together, that is to say, the hydrogen combines with the oxygen, +the caloric is set at liberty, and a quantity of water is produced +exactly equal, in weight, to that of the two gases introduced into the +globe. + +CAROLINE. + +And what was the greatest quantity of water ever formed in this +apparatus? + +MRS. B. + +Several ounces; indeed, very nearly a pound, if I recollect right; but +the operation lasted many days. + +EMILY. + +This experiment must have convinced all the world of the truth of the +discovery. Pray, if improper proportions of the gases were mixed and set +fire to, what would be the result? + +MRS. B. + +Water would equally be formed, but there would be a residue of either +one or other of the gases, because, as I have already told you, hydrogen +and oxygen will combine only in the proportions requisite for the +formation of water. + +EMILY. + +Look, Mrs. B., our experiment with the Voltaic battery (PLATE VIII. fig. +2.) has made great progress; a quantity of gas has been formed in each +tube, but in one of them there is twice as much gas as in the other. + +MRS. B. + +Yes; because, as I said before, water is composed of two volumes of +hydrogen to one of oxygen--and if we should now mix these gases together +and set fire to them by an electrical spark, both gases would entirely +disappear, and a small quantity of water would be formed. + +There is another curious effect produced by the combustion of hydrogen +gas, which I shall show you, though I must acquaint you first, that I +cannot well explain the cause of it. For this purpose, I must put some +materials into our apparatus, in order to obtain a stream of hydrogen +gas, just as we have done before. The process is already going on, and +the gas is rushing through the tube--I shall now kindle it with the +taper---- + +EMILY. + +It burns exactly as it did before---- What is the curious effect which +you were mentioning? + +MRS. B. + +Instead of the receiver, by means of which we have just seen the drops +of water form, we shall invert over the flame this piece of tube, which +is about two feet in length, and one inch in diameter (PLATE VIII. +fig. 7.); but you must observe that it is open at both ends. + +EMILY. + +What a strange noise it makes! something like the Æolian harp, but not +so sweet. + +CAROLINE. + +It is very singular, indeed; but I think rather too powerful to be +pleasing. And is not this sound accounted for? + +MRS. B. + +That the percussion of glass, by a rapid stream of gas, should produce a +sound, is not extraordinary: but the sound here is so peculiar, that no +other gas has a similar effect. Perhaps it is owing to a brisk vibratory +motion of the glass, occasioned by the successive formation and +condensation of small drops of water on the sides of the glass tube, and +the air rushing in to replace the vacuum formed.* + + [Footnote *: This ingenious explanation was first suggested by + Dr. Delarive. --See Journals of the Royal Institution, vol. i. + p. 259.] + +CAROLINE. + +How very much this flame resembles the burning of a candle. + +MRS. B. + +The burning of a candle is produced by much the same means. A great deal +of hydrogen is contained in candles, whether of tallow or wax. This +hydrogen being converted into gas by the heat of the candle, combines +with the oxygen of the atmosphere, and flame and water result from this +combination. So that, in fact, the flame of a candle is owing to the +combustion of hydrogen gas. An elevation of temperature, such as is +produced by a lighted match or taper, is required to give the first +impulse to the combustion; but afterwards it goes on of itself, because +the candle finds a supply of caloric in the successive quantities of +heat which results from the union of the two electricities given out by +the gases during their combustion. But there are other circumstances +connected with the combustion of candles and lamps, which I cannot +explain to you till you are acquainted with _carbon_, which is one of +their constituent parts. In general, however, whenever you see flame, +you may infer that it is owing to the formation and burning of hydrogen +gas*; for flame is the peculiar mode of burning hydrogen gas, which, +with only one or two apparent exceptions, does not belong to any other +combustible. + + [Footnote *: Or rather, _hydro-carbonat_, a gas composed of + hydrogen and carbon, which will be noticed under the head + _Carbon_.] + +EMILY. + +You astonish me! I understood that flame was the caloric produced by the +union of the two electricities, in all combustions whatever? + +MRS. B. + +Your error proceeded from your vague and incorrect idea of flame; you +have confounded it with light and caloric in general. Flame always +implies caloric, since it is produced by the combustion of hydrogen gas; +but all caloric does not imply flame. Many bodies burn with intense heat +without producing flame. Coals, for instance, burn with flame until all +the hydrogen which they contain is evaporated; but when they afterwards +become red hot, much more caloric is disengaged than when they produce +flame. + +CAROLINE. + +But the iron wire, which you burnt in oxygen gas, appeared to me to emit +flame; yet, as it was a simple metal, it could contain no hydrogen? + +MRS. B. + +It produced a sparkling dazzling blaze of light, but no real flame. + +EMILY. + +And what is the cause of the regular shape of the flame of a candle? + +MRS. B. + +The regular stream of hydrogen gas which exhales from its combustible +matter. + +CAROLINE. + +But the hydrogen gas must, from its great levity, ascend into the upper +regions of the atmosphere; why therefore does not the flame continue to +accompany it? + +MRS. B. + +The combustion of the hydrogen gas is completed at the point where the +flame terminates; it then ceases to be hydrogen gas, as it is converted +by its combination with oxygen into watery vapour; but in a state of +such minute division as to be invisible. + +CAROLINE. + +I do not understand what is the use of the wick of a candle, since the +hydrogen gas burns so well without it? + +MRS. B. + +The combustible matter of the candle must be decomposed in order to +emit the hydrogen gas, and the wick is instrumental in effecting this +decomposition. Its combustion first melts the combustible matter, +and . . . . + +CAROLINE. + +But in lamps the combustible matter is already fluid, and yet they also +require wicks? + +MRS. B. + +I am going to add that, afterwards, the burning wick (by the power of +capillary attraction) gradually draws up the fluid to the point where +combustion takes place; for you must have observed that the wick does +not burn quite to the bottom. + +CAROLINE. + +Yes; but I do not understand why it does not. + +MRS. B. + +Because the air has not so free an access to that part of the wick which +is immediately in contact with the candle, as to the part just above, so +that the heat there is not sufficient to produce its decomposition; the +combustion therefore begins a little above this point. + +CAROLINE. + +But, Mrs. B., in those beautiful lights, called _gas-lights_, which are +now seen in many streets, and will, I hope, be soon adopted every where, +I can perceive no wick at all. How are these lights managed? + +MRS. B. + +I am glad you have put me in mind of saying a few words on this very +useful and interesting improvement. In this mode of lighting, the gas is +conveyed to the extremity of a tube, where it is kindled, and burns as +long as the supply continues. There is, therefore, no occasion for a +wick, or any other fuel whatever. + +EMILY. + +But how is all this gas procured in such large quantities? + +MRS. B. + +It is obtained from coal, by distillation. --Coal, when exposed to heat +in a close vessel, is decomposed; and hydrogen, which is one of its +constituents, rises in the state of gas, combined with another of its +component parts, carbon, forming a compound gas, called _Hydrocarbonat_, +the nature of which we shall again have an opportunity of noticing when +we treat of carbon. This gas, like hydrogen, is perfectly transparent, +invisible, and highly inflammable; and in burning it emits that vivid +light which you have so often observed. + +CAROLINE. + +And does the process for procuring it require nothing but heating the +coals, and conveying the gas through tubes? + +MRS. B. + +Nothing else; except that the gas must be made to pass, immediately at +its formation, through two or three large vessels of water, in which it +deposits some other ingredients, and especially water, tar, and oil, +which also arise from the distillation of coals. The gas-light +apparatus, therefore, consists simply in a large iron vessel, in which +the coals are exposed to the heat of a furnace,--some reservoirs of +water, in which the gas deposits its impurities,--and tubes that convey +it to the desired spot, being propelled with uniform velocity through +the tubes by means of a certain degree of pressure which is made upon +the reservoir. + +EMILY. + +What an admirable contrivance! Do you not think, Mrs. B., that it will +soon get into universal use? + +MRS. B. + +Most probably, as to the lighting of streets, offices, and public +places, as it far surpasses any former invention for that purpose; but +as to the interior of private houses, this mode of lighting has not yet +been sufficiently tried to know whether it will be found generally +desirable, either in regard to economy or convenience. It may, however, +be considered as one of the happiest applications of chemistry to the +comforts of life; and there is every reason to suppose that it will +answer the full extent of public, expectation. + +I have another experiment to show you with hydrogen gas, which I think +will entertain you. Have you ever blown bubbles with soap and water? + +EMILY. + +Yes, often, when I was a child; and I used to make them float in the air +by blowing them upwards. + +MRS. B. + +We shall fill some such bubbles with hydrogen gas, instead of +atmospheric air, and you will see with what ease and rapidity they will +ascend, without the assistance of blowing, from the lightness of the +gas. --Will you mix some soap and water whilst I fill this bladder with +the gas contained in the receiver which stands on the shelf in the +water-bath? + +CAROLINE. + +What is the use of the brass-stopper and turn-cock at the top of the +receiver? + +MRS. B. + +It is to afford a passage to the gas when required. There is, you see, +a similar stop-cock fastened to this bladder, which is made to fit that +on the receiver. I screw them one on the other, and now turn the two +cocks, to open a communication between the receiver and the bladder; +then, by sliding the receiver off the shelf, and gently sinking it into +the bath, the water rises in the receiver and forces the gas into the +bladder. (PLATE IX. fig. 1.) + + [Illustration: Plate IX. Vol. I. p. 228 + + Fig. 1. Apparatus for transferring gases from a Receiver into a + bladder. + Fig. 2. Apparatus for blowing Soap bubbles.] + +CAROLINE. + +Yes, I see the bladder swell as the water rises in the receiver. + +MRS. B. + +I think that we have already a sufficient quantity in the bladder for +our purpose; we must be careful to stop both the cocks before we +separate the bladder from the receiver, lest the gas should escape. +--Now I must fix a pipe to the stopper of the bladder, and by dipping +its mouth into the soap and water, take up a few drops--then I again +turn the cock, and squeeze the bladder in order to force the gas into +the soap and water at the mouth of the pipe. (PLATE IX. fig. 2.) + +EMILY. + +There is a bubble--but it bursts before it leaves the mouth of the pipe. + +MRS. B. + +We must have patience and try again; it is not so easy to blow bubbles +by means of a bladder, as simply with the breath. + +CAROLINE. + +Perhaps there is not soap enough in the water; I should have had warm +water, it would have dissolved the soap better. + +EMILY. + +Does not some of the gas escape between the bladder and the pipe? + +MRS. B. + +No, they are perfectly air tight; we shall succeed presently, I dare +say. + +CAROLINE. + +Now a bubble ascends; it moves with the rapidity of a balloon. How +beautifully it refracts the light! + +EMILY. + +It has burst against the ceiling--you succeed now wonderfully; but why +do they all ascend and burst against the ceiling? + +MRS. B. + +Hydrogen gas is so much lighter than atmospherical air, that it ascends +rapidly with its very light envelope, which is burst by the force with +which it strikes the ceiling. + +Air-balloons are filled with this gas, and if they carried no other +weight than their covering, would ascend as rapidly as these bubbles. + +CAROLINE. + +Yet their covering must be much heavier than that of these bubbles? + +MRS. B. + +Not in proportion to the quantity of gas they contain. I do not know +whether you have ever been present at the filling of a large balloon. +The apparatus for that purpose is very simple. It consists of a number +of vessels, either jars or barrels, in which the materials for the +formation of the gas are mixed, each of these being furnished with a +tube, and communicating with a long flexible pipe, which conveys the gas +into the balloon. + +EMILY. + +But the fire-balloons which were first invented, and have been since +abandoned, on account of their being so dangerous, were constructed, +I suppose, on a different principle. + +MRS. B. + +They were filled simply with atmospherical air, considerably rarefied by +heat; and the necessity of having a fire underneath the balloon, in +order to preserve the rarefaction of the air within it, was the +circumstance productive of so much danger. + +If you are not yet tired of experiments, I have another to show you. It +consists in filling soap-bubbles with a mixture of hydrogen and oxygen +gases, in the proportions that form water; and afterwards setting fire +to them. + +EMILY. + +They will detonate, I suppose? + +MRS. B. + +Yes, they will. As you have seen the method of transferring the gas from +the receiver into the bladder, it is not necessary to repeat it. I have +therefore provided a bladder which contains a due proportion of oxygen +and hydrogen gases, and we have only to blow bubbles with it. + +CAROLINE. + +Here is a fine large bubble rising--shall I set fire to it with the +candle? + +MRS. B. + +If you please . . . . + +CAROLINE. + +Heavens, what an explosion! --It was like the report of a gun: I confess +it frightened me much. I never should have imagined it could be so loud. + +EMILY. + +And the flash was as vivid as lightning. + +MRS. B. + +The combination of the two gases takes place during that instant of time +that you see the flash, and hear the detonation. + +EMILY. + +This has a strong resemblance to thunder and lightning. + +MRS. B. + +These phenomena, however, are generally of an electrical nature. Yet +various meteorological effects may be attributed to accidental +detonations of hydrogen gas in the atmosphere; for nature abounds with +hydrogen: it constitutes a very considerable portion of the whole mass +of water belonging to our globe, and from that source almost every other +body obtains it. It enters into the composition of all animal +substances, and of a great number of minerals; but it is most abundant +in vegetables. From this immense variety of bodies, it is often +spontaneously disengaged; its great levity makes it rise into the +superior regions of the atmosphere; and when, either by an electrical +spark, or any casual elevation of temperature, it takes fire, it may +produce such meteors or luminous appearances as are occasionally seen in +the atmosphere. Of this kind are probably those broad flashes which we +often see on a summer-evening, without hearing any detonation. + +EMILY. + +Every flash, I suppose, must produce a quantity of water? + +CAROLINE. + +And this water, naturally, descends in the form of rain? + +MRS. B. + +That probably is often the case, though it is not a necessary +consequence; for the water may be dissolved by the atmosphere, as it +descends towards the lower regions, and remain there in the form of +clouds. + +The application of electrical attraction to chemical phenomena is likely +to lead to many very interesting discoveries in meteorology; for +electricity evidently acts a most important part in the atmosphere. This +subject however, is, as yet, not sufficiently developed for me to +venture enlarging upon it. The phenomena of the atmosphere are far from +being well understood; and even with the little that is known, I am but +imperfectly acquainted. + + +But before we take leave of hydrogen, I must not omit to mention to you +a most interesting discovery of Sir H. Davy, which is connected with +this subject. + +CAROLINE. + +You allude, I suppose, to the new miner’s lamp, which has of late been +so much talked of? I have long been desirous of knowing what that +discovery was, and what purpose it was intended to answer. + +MRS. B. + +It often happens in coal-mines, that quantities of the gas, called by +chemists _hydro-carbonat_, or by the miners _fire-damp_, (the same from +which the gas-lights are obtained,) ooze out from fissures in the beds +of coal, and fill the cavities in which the men are at work; and this +gas being inflammable, the consequence is, that when the men approach +those places with a lighted candle, the gas takes fire, and explosions +happen which destroy the men and horses employed in that part of the +colliery, sometimes in great numbers. + +EMILY. + +What tremendous accidents these must be! But whence does that gas +originate? + +MRS. B. + +Being the chief product of the combustion of coal, no wonder that +inflammable gas should occasionally appear in situations in which this +mineral abounds, since there can be no doubt that processes of +combustion are frequently taking place at a great depth under the +surface of the earth; and therefore those accumulations of gas may arise +either from combustions actually going on, or from former combustions, +the gas having perhaps been confined there for ages. + +CAROLINE. + +And how does Sir H. Davy’s lamp prevent those dreadful explosions? + +MRS. B. + +By a contrivance equally simple and ingenious; and one which does no +less credit to the philosophical views from which it was deduced, than +to the philanthropic motives from which the enquiry sprung. The +principle of the lamp is shortly this: It was ascertained, two or three +years ago, both by Mr. Tennant and by Sir Humphry himself, that the +combustion of inflammable gas could not be propagated through small +tubes; so that if a jet of an inflammable gaseous mixture, issuing from +a bladder or any other vessel, through a small tube, be set fire to, it +burns at the orifice of the tube, but the flame never penetrates into +the vessel. It is upon this fact that Sir Humphry’s safety-lamp is +founded. + +EMILY. + +But why does not the flame ever penetrate through the tube into the +vessel from which the gas issues, so as to explode at once the whole of +the gas? + +MRS. B. + +Because, no doubt, the inflamed gas is so much cooled in its passage +through a small tube as to cease to burn before the combustion reaches +the reservoir. + +CAROLINE. + +And how can this principle be applied to the construction of a lamp? + +MRS. B. + +Nothing easier. You need only suppose a lamp enclosed all round in glass +or horn, but having a number of small open tubes at the bottom, and +others at the top, to let the air in and out. Now, if such a lamp or +lanthorn be carried into an atmosphere capable of exploding, an +explosion or combustion of the gas will take place within the lamp; and +although the vent afforded by the tubes will save the lamp from +bursting, yet, from the principle just explained, the combustion will +not be propagated to the external air through the tubes, so that no +farther consequence will ensue. + +EMILY. + +And is that all the mystery of that valuable lamp? + +MRS. B. + +No; in the early part of the enquiry a lamp of this kind was actually +proposed; but it was but a rude sketch compared to its present state of +improvement. Sir H. Davy, after a succession of trials, by which he +brought his lamp nearer and nearer to perfection, at last conceived the +happy idea that if the lamp were surrounded with a wire-work or +wire-gauze, of a close texture, instead of glass or horn, the tubular +contrivance I have just described would be entirely superseded, since +each of the interstices of the gauze would act as a tube in preventing +the propagation of explosions; so that this pervious metallic covering +would answer the various purposes of transparency, of permeability to +air, and of protection against explosion. This idea, Sir Humphry +immediately submitted to the test of experiment, and the result has +answered his most sanguine expectations, both in his laboratory and in +the collieries, where it has already been extensively tried. And he has +now the happiness of thinking that his invention will probably be the +means of saving every year a number of lives, which would have been lost +in digging out of the bowels of the earth one of the most valuable +necessaries of life. Here is one of these lamps, every part of which you +will at once comprehend. (See PLATE X. fig. 1.) + + [Illustration: Plate X. + + Fig. 1. + A. the cistern containing the Oil + B. the rim or screw by which the gauze cage is fixed to the cistern. + C. apperture for supplying Oil. + E. a wire for trimming the wick. + D. F. the wire gauze cylinder. + G. a double top. + + Fig. 2. + A. the reservoir of condensed air. + B. the condensing Syringe. + C. the bladder for Oxygen. + D. the moveable jet.] + +CAROLINE. + +How very simple and ingenious! But I do not yet well see why an +explosion taking place within the lamp should not communicate to the +external air around it, through the interstices of the wire? + +MRS. B. + +This has been and is still a subject of wonder, even to philosophers; +and the only mode they have of explaining it is, that flame or ignition +cannot pass through a fine wire-work, because the metallic wire cools +the flame sufficiently to extinguish it in passing through the gauze. +This property of the wire-gauze is quite similar to that of the tubes +which I mentioned on introducing the subject; for you may consider each +interstice of the gauze as an extremely short tube of a very small +diameter. + +EMILY. + +But I should expect the wire would often become red-hot, by the burning +of the gas within the lamp? + +MRS. B. + +And this is actually the case, for the top of the lamp is very apt to +become red-hot. But, fortunately, inflammable gaseous mixtures cannot be +exploded by red-hot wire, the intervention of actual flame being +required for that purpose; so that the wire does not set fire to the +explosive gas around it. + +EMILY. + +I can understand that; but if the wire be red-hot, how can it cool the +flame within, and prevent its passing through the gauze? + +MRS. B. + +The gauze, though red-hot, is not so hot as the flame by which it has +been heated; and as metallic wire is a good conductor, the heat does not +much accumulate in it, as it passes off quickly to the other parts of +the lamp, as well as to any contiguous bodies. + +CAROLINE. + +This is indeed a most interesting discovery, and one which shows at once +the immense utility with which science may be practically applied to +some of the most important purposes. + + + + +CONVERSATION VIII. + +ON SULPHUR AND PHOSPHORUS. + + +MRS. B. + +SULPHUR is the next substance that comes under our consideration. It +differs in one essential point from the preceding, as it exists in a +solid form at the temperature of the atmosphere. + +CAROLINE. + +I am glad that we have at last a solid body to examine; one that we can +see and touch. Pray, is it not with sulphur that the points of matches +are covered, to make them easily kindle? + +MRS. B. + +Yes, it is; and you therefore already know that sulphur is a very +combustible substance. It is seldom discovered in nature in a pure +unmixed state; so great is its affinity for other substances, that it is +almost constantly found combined with some of them. It is most commonly +united with metals, under various forms, and is separated from them by a +very simple process. It exists likewise in many mineral waters, and some +vegetables yield it in various proportions, especially those of the +cruciform tribe. It is also found in animal matter; in short, it may be +discovered in greater or less quantity, in the mineral, vegetable, and +animal kingdoms. + +EMILY. + +I have heard of _flowers of sulphur_, are they the produce of any plant? + +MRS. B. + +By no means: they consist of nothing more than common sulphur, reduced +to a very fine powder by a process called _sublimation_. --You see some +of it in this phial; it is exactly the same substance as this lump of +sulphur, only its colour is a paler yellow, owing to its state of very +minute division. + +EMILY. + +Pray what is sublimation? + +MRS. B. + +It is the evaporation, or, more properly speaking, the volatilisation of +solid substances, which, in cooling, condense again in a concrete form. +The process, in this instance, must be performed in a closed vessel, +both to prevent combustion, which would take place if the access of air +were not carefully precluded, and likewise in order to collect the +substance after the operation. As it is rather a slow process, we shall +not try the experiment now; but you will understand it perfectly if I +show you the apparatus used for the purpose. (PLATE XI. fig. 1.) Some +lumps of sulphur are put into a receiver of this kind, which is called a +_cucurbit_. Its shape, you see, somewhat resembles that of a pear, and +is open at the top, so as to adapt itself exactly to a kind of conical +receiver of this sort, called the head. The cucurbit, thus covered with +its head, is placed over a sand-bath; this is nothing more than a vessel +full of sand, which is kept heated by a furnace, such as you see here, +so as to preserve the apparatus in a moderate and uniform temperature. +The sulphur then soon begins to melt, and immediately after this, +a thick white smoke rises, which is gradually deposited within the head, +or upper part of the apparatus, where it condenses against the sides, +somewhat in the form of a vegetation, whence it has obtained the name of +flowers of sulphur. This apparatus, which is called an _alembic_, is +highly useful in all kinds of distillations, as you will see when we +come to treat of those operations. Alembics are not commonly made of +glass, like this, which is applicable only to distillations upon a very +small scale. Those used in manufactures are generally made of copper, +and are, of course, considerably larger. The principal construction, +however, is always the same, although their shape admits of some +variation. + + [Illustration: Plate XI. Vol. I. p. 237. + + Fig. 1. Sublimation of Sulphur. + A Alembic. + B Sand-bath. + C Furnace. + + Fig. 2. Eudiometer. + + Fig. 3. Decomposition of water by Carbon. + A Retort containing water. + B Lamp to heat the water. + C.C Porcelain tube containing Carbone. + D Furnace through which the tube passes. + E Receiver for the gas produced. + F Water bath.] + +CAROLINE. + +What is the use of that neck, or tube, which bends down from the upper +piece of the apparatus? + +MRS. B. + +It is of no use in sublimations; but in distillations (the general +object of which is to evaporate, by heat, in closed vessels, the +volatile parts of a compound body, and to condense them again into a +liquid,) it serves to carry off the condensed fluid, which otherwise +would fall back into the cucurbit. But this is rather foreign to our +present subject. Let us return to the sulphur. You now perfectly +understand, I suppose, what is meant by sublimation? + +EMILY. + +I believe I do. Sublimation appears to consist in destroying, by means +of heat, the attraction of aggregation of the particles of a solid body, +which are thus volatilised; and as soon as they lose the caloric which +produced that effect, they are deposited in the form of a fine powder. + +CAROLINE. + +It seems to me to be somewhat similar to the transformation of water +into vapour, which returns to its liquid state when deprived of caloric. + +EMILY. + +There is this difference, however, that the sulphur does not return to +its former state, since, instead of lumps, it changes to a fine powder. + +MRS. B. + +Chemically speaking, it is exactly the same substance, whether in the +form of lump or powder. For if this powder be melted again by heat, it +will, in cooling, be restored to the same solid state in which it was +before its sublimation. + +CAROLINE. + +But if there be no real change, produced by the sublimation of the +sulphur, what is the use of that operation? + +MRS. B. + +It divides the sulphur into very minute parts, and thus disposes it to +enter more readily into combination with other bodies. It is used also +as a means of purification. + +CAROLINE. + +Sublimation appears to me like the beginning of combustion, for the +completion of which one circumstance only is wanting, the absorption of +oxygen. + +MRS. B. + +But that circumstance is every thing. No essential alteration is +produced in sulphur by sublimation; whilst in combustion it combines +with the oxygen, and forms a new compound totally different in every +respect from sulphur in its pure state. --We shall now _burn_ some +sulphur, and you will see how very different the result will be. For +this purpose I put a small quantity of flowers of sulphur into this cup, +and place it in a dish, into which I have poured a little water: I now +set fire to the sulphur with the point of this hot wire; for its +combustion will not begin unless its temperature be considerably raised. +--You see that it burns with a faint blueish flame; and as I invert over +it this receiver, white fumes arise from the sulphur, and fill the +vessel. --You will soon perceive that the water is rising within the +receiver, a little above its level in the plate. --Well, Emily, can you +account for this? + +EMILY. + +I suppose that the sulphur has absorbed the oxygen from the +atmospherical air within the receiver, and that we shall find some +oxygenated sulphur in the cup. As for the white smoke, I am quite at a +loss to guess what it may be. + +MRS. B. + +Your first conjecture is very right: but you are mistaken in the last; +for nothing will be left in the cup. The white vapour is the oxygenated +sulphur, which assumes the form of an elastic fluid of a pungent and +offensive smell, and is a powerful acid. Here you see a chemical +combination of oxygen and sulphur, producing a true gas, which would +continue such under the pressure and at the temperature of the +atmosphere, if it did not unite with the water in the plate, to which it +imparts its acid taste, and all its acid properties. --You see, now, +with what curious effects the combustion of sulphur is attended. + +CAROLINE. + +This is something quite new; and I confess that I do not perfectly +understand why the sulphur turns acid. + +MRS. B. + +It is because it unites with oxygen, which is the acidifying principle. +And, indeed, the word _oxygen_ is derived from two Greek words +signifying _to produce an acid_. + +CAROLINE. + +Why, then, is not water, which contains such a quantity of oxygen, acid? + +MRS. B. + +Because hydrogen, which is the other constituent of water, is not +susceptible of acidification. --I believe it will be necessary, before +we proceed further, to say a few words of the general nature of acids, +though it is rather a deviation from our plan of examining the simple +bodies separately, before we consider them in a state of combination. + +Acids may be considered as a peculiar class of _burnt_ bodies, which +during their combustion, or combination with oxygen, have acquired very +characteristic properties. They are chiefly discernible by their sour +taste, and by turning red most of the blue vegetable colours. These two +properties are common to the whole class of acids; but each of them is +distinguished by other peculiar qualities. Every acid consists of some +particular substance, (which constitutes its basis, and is different in +each,) and of oxygen, which is common to them all. + +EMILY. + +But I do not clearly see the difference between acids and oxyds. + +MRS. B. + +Acids were, in fact, oxyds, which, by the addition of a sufficient +quantity of oxygen, have been converted into acids. For acidification, +you must observe, always implies previous oxydation, as a body must have +combined with the quantity of oxygen requisite to constitute it an oxyd, +before it can combine with the greater quantity that is necessary to +render it an acid. + +CAROLINE. + +Are all oxyds capable of being converted into acids? + +MRS. B. + +Very far from it; it is only certain substances which will enter into +that peculiar kind of union with oxygen that produces acids, and the +number of these is proportionally very small; but all burnt bodies may +be considered as belonging either to the class of oxyds, or to that of +acids. At a future period, we shall enter more at large into this +subject. At present, I have but one circumstance further to point out to +your observation respecting acids: it is, that most of them are +susceptible of two degrees of acidification, according to the different +quantities of oxygen with which their basis combines. + +EMILY. + +And how are these two degrees of acidification distinguished? + +MRS. B. + +By the peculiar properties which result from them. The acid we have just +made is the first or weakest degree of acidification, and is called +_sulphureous acid_; if it were fully saturated with oxygen, it would be +called _sulphuric acid_. You must therefore remember, that in this, as +in all acids, the first degree of acidification is expressed by the +termination in _ous_; the stronger, by the termination in _ic_. + +CAROLINE. + +And how is the sulphuric acid made? + +MRS. B. + +By burning sulphur in pure oxygen gas, and thus rendering its combustion +much more complete. I have provided some oxygen gas for this purpose; it +is in that bottle, but we must first decant the gas into the glass +receiver which stands on the shelf in the bath, and is full of water. + +CAROLINE. + +Pray, let me try to do it, Mrs. B. + +MRS. B. + +It requires some little dexterity--hold the bottle completely under +water, and do not turn the mouth upwards, till it is immediately under +the aperture in the shelf, through which the gas is to pass into the +receiver, and then turn it up gradually. --Very well, you have only let +a few bubbles escape, and that must be expected at a first trial. --Now +I shall put this piece of sulphur into the receiver, through the opening +at the top, and introduce along with it a small piece of lighted tinder +to set fire to it. --This requires being done very quickly, lest the +atmospherical air should get in, and mix with the pure oxygen gas. + +EMILY. + +How beautifully it burns! + +CAROLINE. + +But it is already buried in the thick vapour. This, I suppose, is +sulphuric acid? + +EMILY. + +Are these acids always in a gaseous state? + +MRS. B. + +Sulphureous acid, as we have already observed, is a permanent gas, and +can be obtained in a liquid form only by condensing it in water. In its +pure state, the sulphureous acid is invisible, and it now appears in the +form of a white smoke, from its combining with the moisture. But the +vapour of sulphuric acid, which you have just seen to rise during the +combustion, is not a gas, but only a vapour, which condenses into liquid +sulphuric acid, by losing its caloric. But it appears from Sir H. Davy’s +experiments, that this formation and condensation of sulphuric acid +requires the presence of water, for which purpose the vapour is received +into cold water, which may afterwards be separated from the acid by +evaporation. + +Sulphur has hitherto been considered as a simple substance; but Sir H. +Davy has suspected that it contains a small portion of hydrogen, and +perhaps also of oxygen. + +On submitting sulphur to the action of the Voltaic battery, he observed +that the negative wire gave out hydrogen; and the existence of hydrogen +in sulphur was rendered still more probable by his observing that a +small quantity of water was produced during the combustion of sulphur. + +EMILY. + +And pray of what nature is sulphur when perfectly pure? + +MRS. B. + +Sulphur has probably never been obtained perfectly free from +combination, so that its radical may possibly possess properties very +different from those of common sulphur. It has been suspected to be of a +metallic nature; but this is mere conjecture. + +Before we quit the subject of sulphur, I must tell you that it is +susceptible of combining with a great variety of substances, and +especially with hydrogen, with which you are already acquainted. +Hydrogen gas can dissolve a small portion of it. + +EMILY. + +What! can a gas dissolve a solid substance? + +MRS. B. + +Yes; a solid substance may be so minutely divided by heat, as to become +soluble in a gas: and there are several instances of it. But you must +observe, that, in this case, a chemical union or combination of the +sulphur with the hydrogen gas is produced. In order to effect this, the +sulphur must be strongly heated in contact with the gas; the heat +reduces the sulphur to such a state of extreme division, and diffuses it +so thoroughly through the gas, that they combine and incorporate +together. And as a proof that there must be a chemical union between the +sulphur and the gas, it is sufficient to remark that they are not +separated when the sulphur loses the caloric by which it was +volatilized. Besides, it is evident, from the peculiar fetid smell of +this gas, that it is a new compound totally different from either of its +constituents; it is called _sulphuretted hydrogen gas_, and is contained +in great abundance in sulphureous mineral waters. + +CAROLINE. + +Are not the Harrogate waters of this nature? + +MRS. B. + +Yes; they are naturally impregnated with sulphuretted hydrogen gas, and +there are many other springs of the same kind, which shows that this gas +must often be formed in the bowels of the earth by spontaneous processes +of nature. + +CAROLINE. + +And could not such waters be made artificially by impregnating common +water with this gas? + +MRS. B. + +Yes; they can be so well imitated, as perfectly to resemble the +Harrogate waters. + +Sulphur combines likewise with phosphorus, and with the alkalies, and +alkaline earths, substances with which you are yet unacquainted. We +cannot, therefore, enter into these combinations at present. In our next +lesson we shall treat of phosphorus. + +EMILY. + +May we not begin that subject to-day; this lesson has been so short? + +MRS. B. + +I have no objection, if you are not tired. What do you say, Caroline? + +CAROLINE. + +I am as desirous as Emily of prolonging the lesson to-day, especially as +we are to enter on a new subject; for I confess that sulphur has not +appeared to me so interesting as the other simple bodies. + +MRS. B. + +Perhaps you may find phosphorus more entertaining. You must not, +however, be discouraged when you meet with some parts of a study less +amusing than others; it would answer no good purpose to select the most +pleasing parts, since, if we did not proceed with some method, in order +to acquire a general idea of the whole, we could scarcely expect to take +interest in any particular subjects. + + +PHOSPHORUS. + +PHOSPHORUS is considered as a simple body; though, like sulphur, it has +been suspected of containing hydrogen. It was not known by the earlier +chemists. It was first discovered by Brandt, a chemist of Hamburgh, +whilst employed in researches after the philosopher’s stone; but the +method of obtaining it remained a secret till it was a second time +discovered both by Kunckel and Boyle, in the year 1680. You see a +specimen of phosphorus in this phial; it is generally moulded into small +sticks of a yellowish colour, as you find it here. + +CAROLINE. + +I do not understand in what the discovery consisted; there may be a +secret method of making an artificial composition, but how can you talk +of _making_ a substance which naturally exists? + +MRS. B. + +A body may exist in nature so closely combined with other substances, as +to elude the observation of chemists, or render it extremely difficult +to obtain it in its separate state. This is the case with phosphorus, +which is always so intimately combined with other substances, that its +existence remained unnoticed till Brandt discovered the means of +obtaining it free from other combinations. It is found in all animal +substances, and is now chiefly extracted from bones, by a chemical +process. It exists also in some plants, that bear a strong analogy to +animal matter in their chemical composition. + +EMILY. + +But is it never found in its pure separate state? + +MRS. B. + +Never, and this is the reason that it has remained so long undiscovered. + +Phosphorus is eminently combustible; it melts and takes fire at the +temperature of one hundred degrees, and absorbs in its combustion nearly +once and a half its own weight of oxygen. + +CAROLINE. + +What! will a pound of phosphorus consume a pound and half of oxygen? + +MRS. B. + +So it appears from accurate experiments. I can show you with what +violence it combines with oxygen, by burning some of it in that gas. We +must manage the experiment in the same manner as we did the combustion +of sulphur. You see I am obliged to cut this little bit of phosphorus +under water, otherwise there would be danger of its taking fire by the +heat of my fingers. I now put into the receiver, and kindle it by means +of a hot wire. + +EMILY. + +What a blaze! I can hardly look at it. I never saw any thing so +brilliant. Does it not hurt your eyes, Caroline? + +CAROLINE. + +Yes; but still I cannot help looking at it. A prodigious quantity of +oxygen must indeed be absorbed, when so much light and caloric are +disengaged! + +MRS. B. + +In the combustion of a pound of phosphorus, a sufficient quantity of +caloric is set free to melt upwards of a hundred pounds of ice; this has +been computed by direct experiments with the calorimeter. + +EMILY. + +And is the result of this combustion, like that of sulphur, an acid? + +MRS. B. + +Yes; phosphoric acid. And had we duly proportioned the phosphorus and +the oxygen, they would have been completely converted into phosphoric +acid, weighing together, in this new state, exactly the sum of their +weights separately. The water would have ascended into the receiver, on +account of the vacuum formed, and would have filled it entirely. In this +case, as in the combustion of sulphur, the acid vapour formed is +absorbed and condensed in the water of the receiver. But when this +combustion is performed without any water or moisture being present, the +acid then appears in the form of concrete whitish flakes, which are, +however, extremely ready to melt upon the least admission of moisture. + +EMILY. + +Does phosphorus, in burning in atmospherical air, produce, like sulphur, +a weaker sort of the same acid? + +MRS. B. + +No: for it burns in atmospherical air, nearly at the same temperature as +in pure oxygen gas; and it is in both cases so strongly disposed to +combine with the oxygen, that the combustion is perfect, and the product +similar; only in atmospherical air, being less rapidly supplied with +oxygen, the process is performed in a slower manner. + +CAROLINE. + +But is there no method of acidifying phosphorus in a slighter manner, so +as to form _phosphorus_ acid? + +MRS. B. + +Yes, there is. When simply exposed to the atmosphere, phosphorus +undergoes a kind of slow combustion at any temperature above zero. + +EMILY. + +But is not the process in this case rather an oxydation than a +combustion? For if the oxygen is too slowly absorbed for a sensible +quantity of light and heat to be disengaged, it is not a true +combustion. + +MRS. B. + +The case is not as you suppose: a faint light is emitted which is very +discernible in the dark; but the heat evolved is not sufficiently strong +to be sensible: a whitish vapour arises from this combustion, which, +uniting with water, condenses into liquid phosphorus acid. + +CAROLINE. + +Is it not very singular that phosphorus should burn at so low a +temperature in atmospherical air, whilst it does not burn in pure oxygen +without the application of heat? + +MRS. B. + +So it at first appears. But this circumstance seems to be owing to the +nitrogen gas of the atmosphere. This gas dissolves small particles of +phosphorus, which being thus minutely divided and diffused in the +atmospherical air, combines with the oxygen, and undergoes this slow +combustion. But the same effect does not take place in oxygen gas, +because it is not capable of dissolving phosphorus; it is therefore +necessary, in this case, that heat should be applied to effect that +division of particles, which, in the former instance, is produced by the +nitrogen. + +EMILY. + +I have seen letters written with phosphorus, which are invisible by +day-light, but may be read in the dark by their own light. They look as +if they were written with fire; yet they do not seem to burn. + +MRS. B. + +But they do really burn; for it is by their slow combustion that the +light is emitted; and phosphorus acid is the result of this combustion. + +Phosphorus is sometimes used as a test to estimate the purity of +atmospherical air. For this purpose, it is burnt in a graduated tube, +called an _Eudiometer_ (PLATE XI. fig. 2.), and from the quantity of air +which the phosphorus absorbs, the proportion of oxygen in the air +examined is deduced; for the phosphorus will absorb all the oxygen, and +the nitrogen alone will remain. + +EMILY. + +And the more oxygen is contained in the atmosphere, the purer, +I suppose, it is esteemed? + +MRS. B. + +Certainly. Phosphorus, when melted, combines with a great variety of +substances. With sulphur it forms a compound so extremely combustible, +that it immediately takes fire on coming in contact with the air. It is +with this composition that phosphoric matches are prepared, which kindle +as soon as they are taken out of their case and are exposed to the air. + +EMILY. + +I have a box of these curious matches; but I have observed, that in very +cold weather, they will not take fire without being previously rubbed. + +MRS. B. + +By rubbing them you raise their temperature; for, you know, friction is +one of the means of extricating heat. + +EMILY. + +Will phosphorus combine with hydrogen gas, as sulphur does? + +MRS. B. + +Yes; and the compound gas which results from this combination has a +smell still more fetid than the sulphuretted hydrogen; it resembles that +of garlic. + +The _phosphoretted hydrogen gas_ has this remarkable peculiarity, that +it takes fire spontaneously in the atmosphere, at any temperature. It is +thus, probably, that are produced those transient flames, or flashes of +light, called by the vulgar _Will-of-the Whisp_, or more properly +_Ignes-fatui_, which are often seen in church-yards, and places where +the putrefactions of animal matter exhale phosphorus and hydrogen gas. + +CAROLINE. + +Country people, who are so much frightened by those appearances, would +soon be reconciled to them, if they knew from what a simple cause they +proceed. + +MRS. B. + +There are other combinations of phosphorus that have also very singular +properties, particularly that which results from its union with lime. + +EMILY. + +Is there any name to distinguish the combination of two substances, like +phosphorus and lime, neither of which are oxygen, and which cannot +therefore produce either an oxyd or an acid? + +MRS. B. + +The names of such combinations are composed from those of their +ingredients, merely by a slight change in their termination. Thus the +combination of sulphur with lime is called a _sulphuret_, and that of +phosphorus, a _phosphuret of lime_. This latter compound, I was going to +say, has the singular property of decomposing water, merely by being +thrown into it. It effects this by absorbing the oxygen of water, in +consequence of which bubbles of hydrogen gas ascend, holding in solution +a small quantity of phosphorus. + +EMILY. + +These bubbles then are _phosphoretted hydrogen gas_? + +MRS. B. + +Yes; and they produce the singular appearance of a flash of fire issuing +from water, as the bubbles kindle and detonate on the surface of the +water, at the instant that they come in contact with the atmosphere. + +CAROLINE. + +Is not this effect nearly similar to that produced by the combination of +phosphorus and sulphur, or, more properly speaking, the _phosphuret of +sulphur_? + +MRS. B. + +Yes; but the phenomenon appears more extraordinary in this case, from +the presence of water, and from the gaseous form of the combustible +compound. Besides, the experiment surprises by its great simplicity. You +only throw a piece of phosphoret of lime into a glass of water, and +bubbles of fire will immediately issue from it. + +CAROLINE. + +Cannot we try the experiment? + +MRS. B. + +Very easily: but we must do it in the open air; for the smell of the +phosphorated hydrogen gas is so extremely fetid, that it would be +intolerable in the house. But before we leave the room, we may produce, +by another process, some bubbles of the same gas, which are much less +offensive. + +There is in this little glass retort a solution of potash in water; +I add to it a small piece of phosphorus. We must now heat the retort +over the lamp, after having engaged its neck under water--you see it +begins to boil; in a few minutes bubbles will appear, which take fire +and detonate as they issue from the water. + +CAROLINE. + +There is one--and another. How curious it is! --But I do not understand +how this is produced. + +MRS. B. + +It is the consequence of a display of affinities too complicated, +I fear, to be made perfectly intelligible to you at present. + +In a few words, the reciprocal action of the potash, phosphorus, +caloric, and water are such, that some of the water is decomposed, and +the hydrogen gas thereby formed carries off some minute particles of +phosphorus, with which it forms phosphoretted hydrogen gas, a compound +which spontaneously takes fire at almost any temperature. + +EMILY. + +What is that circular ring of smoke which slowly rises from each bubble +after its detonation? + +MRS. B. + +It consists of water and phosphoric acid in vapour, which are produced +by the combustion of hydrogen and phosphorus. + + + + +CONVERSATION IX. + +ON CARBON. + + +CAROLINE. + +To-day, Mrs. B., I believe we are to learn the nature and properties of +CARBON. This substance is quite new to me; I never heard it mentioned +before. + +MRS. B. + +Not so new as you imagine; for carbon is nothing more than charcoal in a +state of purity, that is to say, unmixed with any foreign ingredients. + +CAROLINE. + +But charcoal is made by art, Mrs. B., and a body consisting of one +simple substance cannot be fabricated? + +MRS. B. + +You again confound the idea, of making a simple body, with that of +separating it from a compound. The chemical processes by which a simple +body is obtained in a state of purity, consist in _unmaking_ the +compound in which it is contained, in order to separate from it the +simple substance in question. The method by which charcoal is usually +obtained, is, indeed, commonly called _making_ it; but, upon +examination, you will find this process to consist simply in separating +it from other substances with which it is found combined in nature. + +Carbon forms a considerable part of the solid matter of all organised +bodies; but it is most abundant in the vegetable creation, and it is +chiefly obtained from wood. When the oil and water (which are other +constituents of vegetable matter) are evaporated, the black, porous, +brittle substance that remains, is charcoal. + +CAROLINE. + +But if heat be applied to the wood in order to evaporate the oil and +water, will not the temperature of the charcoal be raised so as to make +it burn; and if it combines with oxygen, can we any longer call it pure? + +MRS. B. + +I was going to say, that, in this operation, the air must be excluded. + +CAROLINE. + +How then can the vapour of the oil and water fly off? + +MRS. B. + +In order to produce charcoal in its purest state (which is, even then, +but a less imperfect sort of carbon), the operation should be performed +in an earthen retort. Heat being applied to the body of the retort, the +evaporable part of the wood will escape through its neck, into which no +air can penetrate as long as the heated vapour continues to fill it. And +if it be wished to collect these volatile products of the wood, this can +easily be done by introducing the neck of the retort into the water-bath +apparatus, with which you are acquainted. But the preparation of common +charcoal, such as is used in kitchens and manufactures, is performed on +a much larger scale, and by an easier and less expensive process. + +EMILY. + +I have seen the process of making common charcoal. The wood is ranged on +the ground in a pile of a pyramidical form, with a fire underneath; the +whole is then covered with clay, a few holes only being left for the +circulation of air. + +MRS. B. + +These holes are closed as soon as the wood is fairly lighted, so that +the combustion is checked, or at least continues but in a very imperfect +manner; but the heat produced by it is sufficient to force out and +volatilize, through the earthy cover, most part of the oily and watery +principles of the wood, although it cannot reduce it to ashes. + +EMILY. + +Is pure carbon as black as charcoal? + +MRS. B. + +The purest charcoal we can prepare is so; but chemists have never yet +been able to separate it entirely from hydrogen. Sir H. Davy says, that +the most perfect carbon that is prepared by art contains about five per +cent. of hydrogen; he is of opinion, that if we could obtain it quite +free from foreign ingredients, it would be metallic, in common with +other simple substances. + +But there is a form in which charcoal appears, that I dare say will +surprise you. --This ring, which I wear on my finger, owes its +brilliancy to a small piece of carbon. + +CAROLINE. + +Surely, you are jesting, Mrs. B.? + +EMILY. + +I thought your ring was diamond? + +MRS. B. + +It is so. But diamond is nothing more than carbon in a crystallized +state. + +EMILY. + +That is astonishing! Is it possible to see two things apparently more +different than diamond and charcoal? + +CAROLINE. + +It is, indeed, curious to think that we adorn ourselves with jewels of +charcoal! + +MRS. B. + +There are many other substances, consisting chiefly of carbon, that are +remarkably white. Cotton, for instance, is almost wholly carbon. + +CAROLINE. + +That, I own, I could never have imagined! --But pray, Mrs. B., since it +is known of what substance diamond and cotton are composed, why should +they not be manufactured, or imitated, by some chemical process, which +would render them much cheaper, and more plentiful than the present mode +of obtaining them? + +MRS. B. + +You might as well, my dear, propose that we should make flowers and +fruit, nay, perhaps even animals, by a chemical process; for it is known +of what these bodies consist, since every thing which we are acquainted +with in nature is formed from the various simple substances that we have +enumerated. But you must not suppose that a knowledge of the component +parts of a body will in every case enable us to imitate it. It is much +less difficult to decompose bodies, and discover of what materials they +are made, than it is to recompose them. The first of these processes is +called _analysis_, the last _synthesis_. When we are able to ascertain +the nature of a substance by both these methods, so that the result of +one confirms that of the other, we obtain the most complete knowledge of +it that we are capable of acquiring. This is the case with water, with +the atmosphere, with most of the oxyds, acids, and neutral salts, and +with many other compounds. But the more complicated combinations of +nature, even in the mineral kingdom, are in general beyond our reach, +and any attempt to imitate organised bodies must ever prove fruitless; +their formation is a secret that rests in the bosom of the Creator. You +see, therefore, how vain it would be to attempt to make cotton by +chemical means. But, surely, we have no reason to regret our inability +in this instance, when nature has so clearly pointed out a method of +obtaining it in perfection and abundance. + +CAROLINE. + +I did not imagine that the principle of life could be imitated by the +aid of chemistry; but it did not appear to me ridiculous to suppose that +chemists might attain a perfect imitation of inanimate nature. + +MRS. B. + +They have succeeded in this point in a variety of instances; but, as you +justly observe, the principle of life, or even the minute and intimate +organisation of the vegetable kingdom, are secrets that have almost +entirely eluded the researches of philosophers; nor do I imagine that +human art will ever be capable of investigating them with complete +success. + +EMILY. + +But diamond, since it consists of one simple unorganised substance, +might be, one would think, perfectly imitable by art? + +MRS. B. + +It is sometimes as much beyond our power to obtain a simple body in a +state of perfect purity, as it is to imitate a complicated combination; +for the operations by which nature separates bodies are frequently as +inimitable as those which she uses for their combination. This is the +case with carbon; all the efforts of chemists to separate it entirely +from other substances have been fruitless, and in the purest state in +which it can be obtained by art, it still retains a portion of hydrogen, +and probably of some other foreign ingredients. We are ignorant of the +means which nature employs to crystallize it. It may probably be the +work of ages, to purify, arrange, and unite the particles of carbon in +the form of diamond. Here is some charcoal in the purest state we can +procure it: you see that it is a very black, brittle, light, porous +substance, entirely destitute of either taste or smell. Heat, without +air, produces no alteration in it, as it is not volatile; but, on the +contrary, it invariably remains at the bottom of the vessel after all +the other parts of the vegetable are evaporated. + +EMILY. + +Yet carbon is, no doubt, combustible, since you say that charcoal would +absorb oxygen if air were admitted during its preparation? + +CAROLINE. + +Unquestionably. Besides, you know, Emily, how much it is used in +cooking. But pray what is the reason that charcoal burns without smoke, +whilst a wood fire smokes so much? + +MRS. B. + +Because, in the conversion of wood into charcoal, the volatile particles +of the former have been evaporated. + +CAROLINE. + +Yet I have frequently seen charcoal burn with flame; therefore it must, +in that case, contain some hydrogen. + +MRS. B. + +Very true; but you must recollect that charcoal, especially that which +is used for common purposes, is not perfectly pure. It generally retains +some remains of the various other component parts of vegetables, and +hydrogen particularly, which accounts for the flame in question. + +CAROLINE. + +But what becomes of the carbon itself during its combustion? + +MRS. B. + +It gradually combines with the oxygen of the atmosphere, in the same way +as sulphur and phosphorus, and, like those substances, it is converted +into a peculiar acid, which flies off in a gaseous form. There is this +difference, however, that the acid is not, in this instance, as in the +two cases just mentioned, a mere condensable vapour, but a permanent +elastic fluid, which always remains in the state of gas, under any +pressure and at any temperature. The nature of this acid was first +ascertained by Dr. Black, of Edinburgh; and, before the introduction of +the new nomenclature, it was called _fixed air_. It is now distinguished +by the more appropriate name of _carbonic acid gas_. + +EMILY. + +Carbon, then, can be volatilized by burning, though, by heat alone, no +such effect is produced? + +MRS. B. + +Yes; but then it is no longer simple carbon, but an acid of which carbon +forms the basis. In this state, carbon retains no more appearance of +solidity or corporeal form, than the basis of any other gas. And you +may, I think, from this instance, derive a more clear idea of the basis +of the oxygen, hydrogen, and nitrogen gases, the existence of which, as +real bodies, you seemed to doubt, because they were not to be obtained +simply in a solid form. + +EMILY. + +That is true; we may conceive the basis of the oxygen, and of the other +gases, to be solid, heavy substances, like carbon; but so much expanded +by caloric as to become invisible. + +CAROLINE. + +But does not the carbonic acid gas partake of the blackness of charcoal? + +MRS. B. + +Not in the least. Blackness, you know, does not appear to be essential +to carbon, and it is pure carbon, and not charcoal, that we must +consider as the basis of carbonic acid. We shall make some carbonic +acid, and, in order to hasten the process, we shall burn the carbon in +oxygen gas. + +EMILY. + +But do you mean then to burn diamond? + +MRS. B. + +Charcoal will answer the purpose still better, being softer and more +easy to inflame; besides the experiments on diamond are rather +expensive. + +CAROLINE. + +But is it possible to burn diamond? + +MRS. B. + +Yes, it is; and in order to effect this combustion, nothing more is +required than to apply a sufficient degree of heat by means of the +blow-pipe, and of a stream of oxygen gas. Indeed it is by burning +diamond that its chemical nature has been ascertained. It has long been +known as a combustible substance, but it is within these few years only +that the product of its combustion has been proved to be pure carbonic +acid. This remarkable discovery is due to Mr. Tennant. + +Now let us try to make some carbonic acid. --Will you, Emily, decant +some oxygen gas from this large jar into the receiver in which we are to +burn the carbon; and I shall introduce this small piece of charcoal, +with a little lighted tinder, which will be necessary to give the first +impulse to the combustion. + +EMILY. + +I cannot conceive how so small a piece of tinder, and that but just +lighted, can raise the temperature of the carbon sufficiently to set +fire to it; for it can produce scarcely any sensible heat, and it hardly +touches the carbon. + +MRS. B. + +The tinder thus kindled has only heat enough to begin its own +combustion, which, however, soon becomes so rapid in the oxygen gas, as +to raise the temperature of the charcoal sufficiently for this to burn +likewise, as you see is now the case. + +EMILY. + +I am surprised that the combustion of carbon is not more brilliant; it +does not give out near so much light or caloric as phosphorus, or +sulphur. Yet since it combines with so much oxygen, why is not a +proportional quantity of light and heat disengaged from the +decomposition of the oxygen gas, and the union of its electricity with +that of the charcoal? + +MRS. B. + +It is not surprising that less light and heat should be liberated in +this than in almost any other combustion, since the oxygen, instead of +entering into a solid or liquid combination, as it does in the +phosphoric and sulphuric acids, is employed in forming another elastic +fluid; it therefore parts with less of its caloric. + +EMILY. + +True; and, on second consideration, it appears, on the contrary, +surprising that the oxygen should, in its combination with carbon, +retain a sufficient portion of caloric to maintain both substances in a +gaseous state. + +CAROLINE. + +We may then judge of the degree of solidity in which oxygen is combined +in a burnt body, by the quantity of caloric liberated during its +combustion? + +MRS. B. + +Yes; provided that you take into the account the quantity of oxygen +absorbed by the combustible body, and observe the proportion which the +caloric bears to it. + +CAROLINE. + +But why should the water, after the combustion of carbon, rise in the +receiver, since the gas within it retains an aëriform state? + +MRS. B. + +Because the carbonic acid gas is gradually absorbed by the water; and +this effect would be promoted by shaking the receiver. + +EMILY. + +The charcoal is now extinguished, though it is not nearly consumed; it +has such an extraordinary avidity for oxygen, I suppose, that the +receiver did not contain enough to satisfy the whole. + +MRS. B. + +That is certainly the case; for if the combustion were performed in the +exact proportions of 28 parts of carbon to 72 of oxygen, both these +ingredients would disappear, and 100 parts of carbonic acid would be +produced. + +CAROLINE. + +Carbonic acid must be a very strong acid, since it contains so great a +proportion of oxygen? + +MRS. B. + +That is a very natural inference; yet it is erroneous. For the carbonic +is the weakest of all the acids. The strength of an acid seems to depend +upon the nature of its basis, and its mode of combination, as well as +upon the proportion of the acidifying principle. The same quantity of +oxygen that will convert some bodies into strong acids, will only be +sufficient simply to oxydate others. + +CAROLINE. + +Since this acid is so weak, I think chemists should have called it the +_carbonous_, instead of the _carbonic_ acid. + +EMILY. + +But, I suppose, the carbonous acid is still weaker, and is formed by +burning carbon in atmospherical air. + +MRS. B. + +It has been lately discovered, that carbon may be converted into a gas, +by uniting with a smaller proportion of oxygen; but as this gas does not +possess any acid properties, it is no more than an oxyd; it is called +_gaseous oxyd of carbon_. + +CAROLINE. + +Pray is not carbonic acid a very wholesome gas to breathe, as it +contains so much oxygen? + +MRS. B. + +On the contrary, it is extremely pernicious. Oxygen, when in a state of +combination with other substances, loses, in almost every instance, its +respirable properties, and the salubrious effects which it has on the +animal economy when in its unconfined state. Carbonic acid is not only +unfit for respiration, but extremely deleterious if taken into the +lungs. + +EMILY. + +You know, Caroline, how very unwholesome the fumes of burning charcoal +are reckoned. + +CAROLINE. + +Yes; but, to confess the truth, I did not consider that a charcoal fire +produced carbonic acid gas. --Can this gas be condensed into a liquid? + +MRS. B. + +No: for, as I told you before, it is a permanent elastic fluid. But +water can absorb a certain quantity of this gas, and can even be +impregnated with it, in a very strong degree, by the assistance of +agitation and pressure, as I am going to show you. I shall decant some +carbonic acid gas into this bottle, which I fill first with water, in +order to exclude the atmospherical air; the gas is then introduced +through the water, which you see it displaces, for it will not mix with +it in any quantity, unless strongly agitated, or allowed to stand over +it for some time. The bottle is now about half full of carbonic acid +gas, and the other half is still occupied by the water. By corking the +bottle, and then violently shaking it, in this way, I can mix the gas +and water together. --Now will you taste it? + +EMILY. + +It has a distinct acid taste. + +CAROLINE. + +Yes, it is sensibly sour, and appears full of little bubbles. + +MRS. B. + +It possesses likewise all the other properties of acids, but, of course, +in a less degree than the pure carbonic acid gas, as it is so much +diluted by water. + +This is a kind of artificial Seltzer water. By analysing that which is +produced by nature, it was found to contain scarcely any thing more than +common water impregnated with a certain proportion of carbonic acid gas. +We are, therefore, able to imitate it, by mixing those proportions of +water and carbonic acid. Here, my dear, is an instance, in which, by a +chemical process, we can exactly copy the operations of nature; for the +artificial Seltzer waters can be made in every respect similar to those +of nature; in one point, indeed, the former have an advantage, since +they may be prepared stronger, or weaker, as occasion requires. + +CAROLINE. + +I thought I had tasted such water before. But what renders it so brisk +and sparkling? + +MRS. B. + +This sparkling, or effervescence, as it is called, is always occasioned +by the action of an elastic fluid escaping from a liquid; in the +artifical Seltzer water, it is produced by the carbonic acid, which +being lighter than the water in which it was strongly condensed, flies +off with great rapidity the instant the bottle is uncorked; this makes +it necessary to drink it immediately. The bubbling that took place in +this bottle was but trifling, as the water was but very slightly +impregnated with carbonic acid. It requires a particular apparatus to +prepare the gaseous artificial mineral waters. + +EMILY. + +If, then, a bottle of Seltzer water remains for any length of time +uncorked, I suppose it returns to the state of common water? + +MRS. B. + +The whole of the carbonic acid gas, or very nearly so, will soon +disappear; but there is likewise in Seltzer water a very small quantity +of soda, and of a few other saline or earthy ingredients, which will +remain in the water, though it should be kept uncorked for any length of +time. + +CAROLINE. + +I have often heard of people drinking soda-water. Pray what sort of +water is that? + +MRS. B. + +It is a kind of artificial Seltzer water, holding in solution, besides +the gaseous acid, a particular saline substance, called soda, which +imparts to the water certain medicinal qualities. + +CAROLINE. + +But how can these waters be so wholesome, since carbonic acid is so +pernicious? + +MRS. B. + +A gas, we may conceive, though very prejudicial to breathe, may be +beneficial to the stomach. --But it would be of no use to attempt +explaining this more fully at present. + +CAROLINE. + +Are waters never impregnated with other gases? + +MRS. B. + +Yes; there are several kinds of gaseous waters. I forgot to tell you +that waters have, for some years past, been prepared, impregnated both +with oxygen and hydrogen gases. These are not an imitation of nature, +but are altogether obtained by artificial means. They have been lately +used medicinally, particularly on the continent, where, I understand, +they have acquired some reputation. + +EMILY. + +If I recollect right, Mrs. B., you told us that carbon was capable of +decomposing water; the affinity between oxygen and carbon must, +therefore, be greater than between oxygen and hydrogen? + +MRS. B. + +Yes; but this is not the case unless their temperature be raised to a +certain degree. It is only when carbon is red-hot, that it is capable of +separating the oxygen from the hydrogen. Thus, if a small quantity of +water be thrown on a red-hot fire, it will increase rather than +extinguish the combustion; for the coals or wood (both of which contain +a quantity of carbon) decompose the water, and thus supply the fire both +with oxygen and hydrogen gases. If, on the contrary, a large mass of +water be thrown over the fire, the diminution of heat thus produced is +such, that the combustible matter loses the power of decomposing the +water, and the fire is extinguished. + +EMILY. + +I have heard that fire-engines sometimes do more harm than good, and +that they actually increase the fire when they cannot throw water enough +to extinguish it. It must be owing, no doubt, to the decomposition of +the water by the carbon during the conflagration. + +MRS. B. + +Certainly. --The apparatus which you see here (PLATE XI. fig. 3.), may +be used to exemplify what we have just said. It consists in a kind of +open furnace, through which a porcelain tube, containing charcoal, +passes. To one end of the tube is adapted a glass retort with water in +it; and the other end communicates with a receiver placed on the +water-bath. A lamp being applied to the retort, and the water made to +boil, the vapour is gradually conveyed through the red-hot charcoal, by +which it is decomposed; and the hydrogen gas which results from this +decomposition is collected in the receiver. But the hydrogen thus +obtained is far from being pure; it retains in solution a minute portion +of carbon, and contains also a quantity of carbonic acid. This renders +it heavier than pure hydrogen gas, and gives it some peculiar +properties; it is distinguished by the name of _carbonated hydrogen +gas_. + +CAROLINE. + +And whence does it obtain the carbonic acid that is mixed with it? + +EMILY. + +I believe I can answer that question, Caroline. --From the union of the +oxygen (proceeding from the decomposed water) with the carbon, which, +you know, makes carbonic acid. + +CAROLINE. + +True; I should have recollected that. --The product of the decomposition +of water by red-hot charcoal, therefore, is carbonated hydrogen gas, and +carbonic acid gas. + +MRS. B. + +You are perfectly right now. + +Carbon is frequently found combined with hydrogen in a state of +solidity, especially in coals, which owe their combustible nature to +these two principles. + +EMILY. + +Is it the hydrogen, then, that produces the flame of coals? + +MRS. B. + +It is so; and when all the hydrogen is consumed, the carbon continues to +burn without flame. But again, as I mentioned when speaking of the +gas-lights, the hydrogen gas produced by the burning of coals is not +pure; for, during the combustion, particles of carbon are successively +volatilized with the hydrogen, with which they form what is called a +_hydro-carbonat_, which is the principal product of this combustion. + +Carbon is a very bad conductor of heat; for this reason, it is employed +(in conjunction with other ingredients) for coating furnaces and other +chemical apparatus. + +EMILY. + +Pray what is the use of coating furnaces? + +MRS. B. + +In most cases, in which a furnace is used, it is necessary to produce +and preserve a great degree of heat, for which purpose every possible +means are used to prevent the heat from escaping by communicating with +other bodies, and this object is attained by coating over the inside of +the furnace with a kind of plaster, composed of materials that are bad +conductors of heat. + +Carbon, combined with a small quantity of iron, forms a compound called +plumbago, or black-lead, of which pencils are made. This substance, +agreeably to the nomenclature, is _a carburet of iron_. + +EMILY. + +Why, then, is it called black-lead? + +MRS. B. + +It is an ancient name given to it by ignorant people, from its shining +metallic appearance; but it is certainly a most improper name for it, as +there is not a particle of lead in the composition. There is only one +mine of this mineral, which is in Cumberland. It is supposed to approach +as nearly to pure carbon as the best prepared charcoal does, as it +contains only five parts of iron, unadulterated by any other foreign +ingredients. There is another carburet of iron, in which the iron, +though united only to an extremely small proportion of carbon, acquires +very remarkable properties; this is steel. + +CAROLINE. + +Really; and yet steel is much harder than iron? + +MRS. B. + +But carbon is not ductile like iron, and therefore may render the steel +more brittle, and prevent its bending so easily. Whether it is that the +carbon, by introducing itself into the pores of the iron, and, by +filling them, makes the metal both harder and heavier; or whether this +change depends upon some chemical cause, I cannot pretend to decide. But +there is a subsequent operation, by which the hardness of steel is very +much increased, which simply consists in heating the steel till it is +red-hot, and then plunging it into cold water. + +Carbon, besides the combination just mentioned, enters into the +composition of a vast number of natural productions, such, for instance, +as all the various kinds of oils, which result from the combination of +carbon, hydrogen, and caloric, in various proportions. + +EMILY. + +I thought that carbon, hydrogen, and caloric, formed carbonated hydrogen +gas? + +MRS. B. + +That is the case when a small portion of carbonic acid gas is held in +solution by hydrogen gas. Different proportions of the same principles, +together with the circumstances of their union, produce very different +combinations; of this you will see innumerable examples. Besides, we are +not now talking of gases, but of carbon and hydrogen, combined only with +a quantity of caloric sufficient to bring them to the consistency of oil +or fat. + +CAROLINE. + +But oil and fat are not of the same consistence? + +MRS. B. + +Fat is only congealed oil; or oil, melted fat. The one requires a little +more heat to maintain it in a fluid state than the other. Have you never +observed the fat of meat turned to oil by the caloric it has imbibed +from the fire? + +EMILY. + +Yet oils in general, as salad-oil, and lamp-oil, do not turn to fat when +cold? + +MRS. B. + +Not at the common temperature of the atmosphere, because they retain too +much caloric to congeal at that temperature; but if exposed to a +sufficient degree of cold, their latent heat is extricated, and they +become solid fat substances. Have you never seen salad oil frozen in +winter? + +EMILY. + +Yes; but it appears to me in that state very different from animal fat. + +MRS. B. + +The essential constituent parts of either vegetable or animal oils are +the same, carbon and hydrogen; their variety arises from the different +proportions of these substances, and from other accessory ingredients +that may be mixed with them. The oil of a whale, and the oil of roses, +are, in their essential constituent parts, the same; but the one is +impregnated with the offensive particles of animal matter, the other +with the delicate perfume of a flower. + +The difference of _fixed oils_, and _volatile_ or _essential oils_, +consists also in the various proportions of carbon and hydrogen. Fixed +oils are those which will not evaporate without being decomposed; this +is the case with all common oils, which contain a greater proportion of +carbon than the essential oils. The essential oils (which comprehend the +whole class of essences and perfumes) are lighter; they contain more +equal proportions of carbon and hydrogen, and are volatilized or +evaporated without being decomposed. + +EMILY. + +When you say that one kind of oil will evaporate, and the other be +decomposed, you mean, I suppose, by the application of heat? + +MRS. B. + +Not necessarily; for there are oils that will evaporate slowly at the +common temperature of the atmosphere; but for a more rapid +volatilization, or for their decomposition, the assistance of heat is +required. + +CAROLINE. + +I shall now remember, I think, that fat and oil are really the same +substances, both consisting of carbon and hydrogen; that in fixed oils +the carbon preponderates, and heat produces a decomposition; while, in +essential oils, the proportion of hydrogen is greater, and heat produces +a volatilization only. + +EMILY. + +I suppose the reason why oil burns so well in lamps is because its two +constituents are so combustible? + +MRS. B. + +Certainly; the combustion of oil is just the same as that of a candle; +if tallow, it is only oil in a concrete state; if wax, or spermaceti, +its chief chemical ingredients are still hydrogen and carbon. + +EMILY. + +I wonder, then, there should be so great a difference between tallow and +wax? + +MRS. B. + +I must again repeat, that the same substances, in different proportions, +produce results that have sometimes scarcely any resemblance to each +other. But this is rather a general remark that I wish to impress upon +your minds, than one which is applicable to the present case; for tallow +and wax are far from being very dissimilar; the chief difference +consists in the wax being a purer compound of carbon and hydrogen than +the tallow, which retains more of the gross particles of animal matter. +The combustion of a candle, and that of a lamp, both produce water and +carbonic acid gas. Can you tell me how these are formed? + +EMILY. + +Let me reflect . . . . Both the candle and lamp burn by means of fixed +oil--this is decomposed as the combustion goes on; and the constituent +parts of the oil being thus separated, the carbon unites to a portion of +oxygen from the atmosphere to form carbonic acid gas, whilst the +hydrogen combines with another portion of oxygen, and forms with it +water. --The products, therefore, of the combustion of oils are water +and carbonic acid gas. + +CAROLINE. + +But we see neither water nor carbonic acid produced by the combustion of +a candle. + +MRS. B. + +The carbonic acid gas, you know, is invisible, and the water being in a +state of vapour, is so likewise. Emily is perfectly correct in her +explanation, and I am very much pleased with it. + +All the vegetable acids consist of various proportions of carbon and +hydrogen, acidified by oxygen. Gums, sugar, and starch, are likewise +composed of these ingredients; but, as the oxygen which they contain is +not sufficient to convert them into acids, they are classed with the +oxyds, and called vegetable oxyds. + +CAROLINE. + +I am very much delighted with all these new ideas; but, at the same +time, I cannot help being apprehensive that I may forget many of them. + +MRS. B. + +I would advise you to take notes, or, what would answer better still, to +write down, after every lesson, as much of it as you can recollect. And, +in order to give you a little assistance, I shall lend you the heads or +index, which I occasionally consult for the sake of preserving some +method and arrangement in these conversations. Unless you follow some +such plan, you cannot expect to retain nearly all that you learn, how +great soever be the impression it may make on you at first. + +EMILY. + +I will certainly follow your advice. --Hitherto I have found that I +recollected pretty well what you have taught us; but the history of +carbon is a more extensive subject than any of the simple bodies we have +yet examined. + +MRS. B. + +I have little more to say on carbon at present; but hereafter you will +see that it performs a considerable part in most chemical operations. + +CAROLINE. + +That is, I suppose, owing to its entering into the composition of so +great a variety of substances? + +MRS. B. + +Certainly; it is the basis, you have seen, of all vegetable matter; and +you will find that it is very essential to the process of animalization. +But in the mineral kingdom also, particularly in its form of carbonic +acid, we shall often discover it combined with a great variety of +substances. + +In chemical operations, carbon is particularly useful, from its very +great attraction for oxygen, as it will absorb this substance from many +oxygenated or burnt bodies, and thus deoxygenate, or _unburn_ them, and +restore them to their original combustible state. + +CAROLINE. + +I do not understand how a body can be _unburnt_, and restored to its +original state. This piece of tinder, for instance, that has been burnt, +if by any means the oxygen were extracted from it, would not be restored +to its former state of linen; for its texture is destroyed by burning, +and that must be the case with all organized or manufactured substances, +as you observed in a former conversation. + +MRS. B. + +A compound body is decomposed by combustion in a way which generally +precludes the possibility of restoring it to its former state; the +oxygen, for instance, does not become fixed in the tinder, but it +combines with its volatile parts, and flies off in the shape of gas, or +watery vapour. You see, therefore, how vain it would be to attempt the +recomposition of such bodies. But, with regard to simple bodies, or at +least bodies whose component parts are not disturbed by the process of +oxygenation or deoxygenation, it is often possible to restore them, +after combustion, to their original state. --The metals, for instance, +undergo no other alteration by combustion than a combination with +oxygen; therefore, when the oxygen is taken from them, they return to +their pure metallic state. But I shall say nothing further of this at +present, as the metals will furnish ample subject for another morning; +and they are the class of simple bodies that come next under +consideration. + + + + +CONVERSATION X. + +ON METALS. + + +MRS. B. + +The METALS, which we are now to examine, are bodies of a very different +nature from those which we have hitherto considered. They do not, like +the bases of gases, elude the immediate observation of our senses; for +they are the most brilliant, the most ponderous, and the most palpable +substances in nature. + +CAROLINE. + +I doubt, however, whether the metals will appear to us so interesting, +and give us so much entertainment as those mysterious elements which +conceal themselves from our view. Besides, they cannot afford so much +novelty; they are bodies with which we are already so well acquainted. + +MRS. B. + +You are not aware, my dear, of the interesting discoveries which were a +few years ago made by Sir H. Davy respecting this class of bodies. By +the aid of the Voltaic battery, he has obtained from a variety of +substances, metals before unknown, the properties of which are equally +new and curious. We shall begin, however, by noticing those metals with +which you profess to be so well acquainted. But the acquaintance, you +will soon perceive, is but very superficial; and I trust that you will +find both novelty and entertainment in considering the metals in a +chemical point of view. To treat of this subject fully, would require a +whole course of lectures; for metals form of themselves a most important +branch of practical chemistry. We must, therefore, confine ourselves to +a general view of them. These bodies are seldom found naturally in their +metallic form: they are generally more or less oxygenated or combined +with sulphur, earths, or acids, and are often blended with each other. +They are found buried in the bowels of the earth in most parts of the +world, but chiefly in mountainous districts, where the surface of the +globe has suffered from the earthquakes, volcanos, and other convulsions +of nature. They are spread in strata or beds, called veins, and these +veins are composed of a certain quantity of metal, combined with various +earthy substances, with which they form minerals of different nature and +appearance, which are called _ores_. + +CAROLINE. + +I now feel quite at home, for my father has a lead-mine in Yorkshire, +and I have heard a great deal about veins of ore, and of the _roasting_ +and _smelting_ of the lead; but, I confess, that I do not understand in +what these operations consist. + +MRS. B. + +Roasting is the process by which the volatile parts of the ore are +evaporated; smelting, that by which the pure metal is afterwards +separated from the earthy remains of the ore. This is done by throwing +the whole into a furnace, and mixing with it certain substances that +will combine with the earthy parts and other foreign ingredients of the +ore; the metal being the heaviest, falls to the bottom, and runs out by +proper openings in its pure metallic state. + +EMILY. + +You told us in a preceding lesson that metals had a great affinity for +oxygen. Do they not, therefore, combine with oxygen, when strongly +heated in the furnace, and run out in the state of oxyds? + +MRS. B. + +No; for the scoriæ, or oxyd, which soon forms on the surface of the +fused metal, when it is oxydable, prevents the air from having any +further influence on the mass; so that neither combustion nor +oxygenation can take place. + +CAROLINE. + +Are all the metals equally combustible? + +MRS. B. + +No; their attraction for oxygen varies extremely. There are some that +will combine with it only at a very high temperature, or by the +assistance of acids; whilst there are others that oxydate spontaneously +and with great rapidity, even at the lowest temperature; such is in +particular manganese, which scarcely ever exists in the metallic state, +as it immediately absorbs oxygen on being exposed to the air, and +crumbles to an oxyd in the course of a few hours. + +EMILY. + +Is not that the oxyd from which you extracted the oxygen gas? + +MRS. B. + +It is: so that, you see, this metal attracts oxygen at a low +temperature, and parts with it when strongly heated. + +EMILY. + +Is there any other metal that oxydates at the temperature of the +atmosphere? + +MRS. B. + +They all do, more or less, excepting gold, silver, and platina. + +Copper, lead, and iron, oxydate slowly in the air, and cover themselves +with a sort of rust, a process which depends on the gradual conversion +of the surface into an oxyd. This rusty surface preserves the interior +metal from oxydation, as it prevents the air from coming in contact with +it. Strictly speaking, however, the word rust applies only to the oxyd, +which forms on the surface of iron, when exposed to air and moisture, +which oxyd appears to be united with a small portion of carbonic acid. + +EMILY. + +When metals oxydate from the atmosphere without an elevation of +temperature, some light and heat, I suppose, must be disengaged, though +not in sufficient quantities to be sensible. + +MRS. B. + +Undoubtedly; and, indeed, it is not surprising that in this case the +light and heat should not be sensible, when you consider how extremely +slow, and, indeed, how imperfectly, most metals oxydate by mere exposure +to the atmosphere. For the quantity of oxygen with which metals are +capable of combining, generally depends upon their temperature; and the +absorption stops at various points of oxydation, according to the degree +to which their temperature is raised. + +EMILY. + +That seems very natural; for the greater the quantity of caloric +introduced into a metal, the more will its positive electricity be +exalted, and consequently the stronger will be its affinity for oxygen. + +MRS. B. + +Certainly. When the metal oxygenates with sufficient rapidity for light +and heat to become sensible, combustion actually takes place. But this +happens only at very high temperatures, and the product is nevertheless +an oxyd; for though, as I have just said, metals will combine with +different proportions of oxygen, yet with the exception of only five of +them, they are not susceptible of acidification. + +Metals change colour during the different degrees of oxydation which +they undergo. Lead, when heated in contact with the atmosphere, first +becomes grey; if its temperature be then raised, it turns yellow, and a +still stronger heat changes it to red. Iron becomes successively a +green, brown, and white oxyd. Copper changes from brown to blue, and +lastly green. + +EMILY. + +Pray, is the white lead with which houses are painted prepared by +oxydating lead? + +MRS. B. + +Not merely by oxydating, but by being also united with carbonic acid. It +is a carbonat of lead. The mere oxyd of lead is called red lead. +Litharge is another oxyd of lead, containing less oxygen. Almost all the +metallic oxyds are used as paints. The various sorts of ochres consist +chiefly of iron more or less oxydated. And it is a remarkable +circumstance, that if you burn metals rapidly, the light or flame they +emit during combustion partakes of the colours which the oxyd +successively assumes. + +CAROLINE. + +How is that accounted for, Mrs. B.? For light, you know, does not +proceed from the burning body, but from the decomposition of the oxygen +gas? + +MRS. B. + +The correspondence of the colour of the light with that of the oxyd +which emits it, is, in all probability, owing to some particles of the +metal which are volatilised and carried off by the caloric. + +CAROLINE. + +It is then a sort of metallic gas. + +EMILY. + +Why is it reckoned so unwholesome to breathe the air of a place in which +metals are melting? + +MRS. B. + +Perhaps the notion is too generally entertained. But it is true with +respect to lead, and some other noxious metals, because, unless care be +taken, the particles of the oxyd which are volatilised by the heat are +inhaled in with the breath, and may produce dangerous effects. + +I must show you some instances of the combustion of metals; it would +require the heat of a furnace to make them burn in the common air, but +if we supply them with a stream of oxygen gas, we may easily +accomplish it. + +CAROLINE. + +But it will still, I suppose, be necessary in some degree to raise their +temperature? + +MRS. B. + +This, as you shall see, is very easily done, particularly if the +experiment be tried upon a small scale. --I begin by lighting this piece +of charcoal with the candle, and then increase the rapidity of its +combustion by blowing upon it with a blow-pipe. (PLATE XII. fig. 1.) + + [Illustration: Plate XII. + Apparatus for the combustion of metals by means of oxygen gas. + + Fig. 1. + Igniting charcoal with a taper & blow-pipe. + + Fig. 2. + Combustion of metals by means of a blow-pipe conveying a stream of + oxygen gas from a gas holder.] + +EMILY. + +That I do not understand; for it is not every kind of air, but merely +oxygen gas, that produces combustion. Now you said that in breathing we +inspired, but did not expire oxygen gas. Why, therefore, should the air +which you breathe through the blow-pipe promote the combustion of the +charcoal? + +MRS. B. + +Because the air, which has but once passed through the lungs, is yet but +little altered, a small portion only of its oxygen being destroyed; so +that a great deal more is gained by increasing the rapidity of the +current, by means of the blow-pipe, than is lost in consequence of the +air passing once through the lungs, as you shall see-- + +EMILY. + +Yes, indeed, it makes the charcoal burn much brighter. + +MRS. B. + +Whilst it is red-hot, I shall drop some iron filings on it, and supply +them with a current of oxygen gas, by means of this apparatus, (PLATE +XII. fig 2.) which consists simply of a closed tin cylindrical vessel, +full of oxygen gas, with two apertures and stop-cocks, by one of which a +stream of water is thrown into the vessel through a long funnel, whilst +by the other the gas is forced out through a blow-pipe adapted to it, as +the water gains admittance. --Now that I pour water into the funnel, you +may hear the gas issuing from the blow-pipe--I bring the charcoal close +to the current, and drop the filings upon it-- + +CAROLINE. + +They emit much the same vivid light as the combustion of the iron wire +in oxygen gas. + +MRS. B. + +The process is, in fact, the same; there is only some difference in the +mode of conducting it. Let us burn some tin in the same manner--you see +that it is equally combustible. --Let us now try some copper-- + +CAROLINE. + +This burns with a greenish flame; it is, I suppose, owing to the colour +of the oxyd? + +EMILY. + +Pray, shall we not also burn some gold? + +MRS. B. + +That is not in our power, at least in this way. Gold, silver, and +platina, are incapable of being oxydated by the greatest heat that we +can produce by the common method. It is from this circumstance, that +they have been called perfect metals. Even these, however, have an +affinity for oxygen; but their oxydation or combustion can be performed +only by means of acids or by electricity. The spark given out by the +Voltaic battery produces at the point of contact a greater degree of +heat than any other process; and it is at this very high temperature +only that the affinity of these metals for oxygen will enable them to +act on each other. + +I am sorry that I cannot show you the combustion of the perfect metals +by this process, but it requires a considerable Voltaic battery. You +will see these experiments performed in the most perfect manner, when +you attend the chemical lectures of the Royal Institution. But in the +mean time I can, without difficulty, show you an ingenious apparatus +lately contrived for the purpose of producing intense heats, the power +of which nearly equals that of the largest Voltaic batteries. It simply +consists, you see, in a strong box, made of iron or copper, (PLATE X. +fig. 2.) to which may be adapted this air-syringe or condensing-pump, +and a stop-cock terminating in a small orifice similar to that of a +blow-pipe. By working the condensing syringe, up and down in this +manner, a quantity of air is accumulated in the vessel, which may be +increased to almost any extent; so that if we now turn the stop-cock, +the condensed air will rush out, forming a jet of considerable force; +and if we place the flame of a lamp in the current, you will see how +violently the flame is driven in that direction. + +CAROLINE. + +It seems to be exactly the same effect as that of a blow-pipe worked by +the mouth, only much stronger. + +EMILY. + +Yes; and this new instrument has this additional advantage, that it does +not fatigue the mouth and lungs like the common blow-pipe, and requires +no art in blowing. + +MRS. B. + +Unquestionably; but yet this blow-pipe would be of very limited utility, +if its energy and power could not be greatly increased by some other +contrivance. Can you imagine any mode of producing such an effect? + +EMILY. + +Could not the reservoir be charged with pure oxygen, instead of common +air, as in the case of the gas-holder? + +MRS. B. + +Undoubtedly; and this is precisely the contrivance I allude to. The +vessel need only be supplied with air from a bladder full of oxygen, +instead of the air of the room, and this, you see, may be easily done by +screwing the bladder on the upper part of the syringe, so that in +working the syringe the oxygen gas is forced from the bladder into the +condensing vessel. + +CAROLINE. + +With the aid of this small apparatus, therefore, we could obtain the +same effects as those we have just produced with the gas-holder, by +means of a column of water forcing the gas out of it? + +MRS. B. + +Yes; and much more conveniently so. But there is a mode of using this +apparatus by which more powerful effects still may be obtained. It +consists in condensing in the reservoir, not oxygen alone, but a mixture +of oxygen and hydrogen in the exact proportion in which they unite to +produce water; and then kindling the jet formed by the mixed gases. The +heat disengaged by this combustion, without the help of any lamp, is +probably the most intense known; and various effects are said to have +been obtained from it which exceed all expectation. + +CAROLINE. + +But why should we not try this experiment? + +MRS. B. + +Because it is not exempt from danger; the combustion (notwithstanding +various contrivances which have been resorted to with a view to prevent +accident) being apt to penetrate into the inside of the vessel, and to +produce a dangerous and violent explosion. --We shall, therefore, now +proceed in our subject. + +CAROLINE. + +I think you said the oxyds of metals could be restored to their metallic +state? + +MRS. B. + +Yes; this is called _reviving_ a metal. Metals are in general capable of +being revived by charcoal, when heated red hot, charcoal having a +greater attraction for oxygen than the metals. You need only, therefore, +decompose, or unburn the oxyd, by depriving it of its oxygen, and the +metal will be restored to its pure state. + +EMILY. + +But will the carbon, by this operation, be burnt, and be converted into +carbonic acid? + +MRS. B. + +Certainly. There are other combustible substances to which metals at a +high temperature will part with their oxygen. They will also yield it to +each other, according to their several degrees of attraction for it; and +if the oxygen goes into a more dense state in the metal which it enters, +than it existed in that which it quits, a proportional disengagement of +caloric will take place. + +CAROLINE. + +And cannot the oxyds of gold, silver, and platina, which are formed by +means of acids or of the electric fluid, be restored to their metallic +state? + +MRS. B. + +Yes, they may; and the intervention of a combustible body is not +required; heat alone will take the oxygen from them, convert it into a +gas, and revive the metal. + +EMILY. + +You said that rust was an oxyd of iron; how is it, then, that water, or +merely dampness, produces it, which, you know, it very frequently does +on steel grates, or any iron instruments? + +MRS. B. + +In that case the metal decomposes the water, or dampness (which is +nothing but water in a state of vapour), and obtains the oxygen from it. + +CAROLINE. + +I thought that it was necessary to bring metals to a very high +temperature to enable them to decompose water. + +MRS. B. + +It is so, if it is required that the process should be performed +rapidly, and if any considerable quantity is to be decomposed. Rust, you +knew, is sometimes months in forming, and then it is only the surface of +the metal that is oxydated. + +EMILY. + +Metals, then, that do not rust, are incapable of spontaneous oxydation, +either by air or water? + +MRS. B. + +Yes; and this is the case with the perfect metals, which, on that +account, preserve their metallic lustre so well. + +EMILY. + +Are all metals capable of decomposing water, provided their temperature +be sufficiently raised? + +MRS. B. + +No; a certain degree of attraction is requisite, besides the assistance +of heat. Water, you recollect, is composed of oxygen and hydrogen; and, +unless the affinity of the metal for oxygen be stronger than that of +hydrogen, it is in vain that we raise its temperature, for it cannot +take the oxygen from the hydrogen. Iron, zinc, tin, and antimony, have a +stronger affinity for oxygen than hydrogen has, therefore these four +metals are capable of decomposing water. But hydrogen having an +advantage over all the other metals with respect to its affinity for +oxygen, it not only withholds its oxygen from them, but is even capable, +under certain circumstances, of taking the oxygen from the oxyds of +these metals. + +EMILY. + +I confess that I do not quite understand why hydrogen can take oxygen +from those metals that do not decompose water. + +CAROLINE. + +Now I think I do perfectly. Lead, for instance, will not decompose +water, because it has not so strong an attraction for oxygen as hydrogen +has. Well, then, suppose the lead to be in a state of oxyd; hydrogen +will take the oxygen from the lead, and unite with it to form water, +because hydrogen has a stronger attraction for oxygen, than oxygen has +for lead; and it is the same with all the other metals which do not +decompose water. + +EMILY. + +I understand your explanation, Caroline, very well; and I imagine that +it is because lead cannot decompose water that it is so much employed +for pipes for conveying that fluid. + +MRS. B. + +Certainly; lead is, on that account, particularly appropriate to such +purposes; whilst, on the contrary, this metal, if it was oxydable by +water, would impart to it very noxious qualities, as all oxyds of lead +are more or less pernicious. + +But, with regard to the oxydation of metals, the most powerful mode of +effecting it is by means of acids. These, you know, contain a much +greater proportion of oxygen than either air or water; and will, most of +them, easily yield it to metals. Thus, you recollect, the zinc plates of +the Voltaic battery are oxydated by the acid and water, much more +effectually than by water alone. + +CAROLINE. + +And I have often observed that if I drop vinegar, lemon, or any acid on +the blade of a knife, or on a pair of scissars, it will immediately +produce a spot of rust. + +EMILY. + +Metals have, then, three ways of obtaining oxygen; from the atmosphere, +from water, and from acids. + +MRS. B. + +The two first you have already witnessed, and I shall now show you how +metals take the oxygen from an acid. This bottle contains nitric acid; +I shall pour some of it over this piece of copper-leaf . . . . . . . + +CAROLINE. + +Oh, what a disagreeable smell! + +EMILY. + +And what is it that produces the effervescency and that thick yellow +vapour? + +MRS. B. + +It is the acid, which being abandoned by the greatest part of its +oxygen, is converted into a weaker acid, which escapes in the form of +gas. + +CAROLINE. + +And whence proceeds this heat? + +MRS. B. + +Indeed, Caroline, I think you might now be able to answer that question +yourself. + +CAROLINE. + +Perhaps it is that the oxygen enters into the metal in a more solid +state than it existed in the acid, in consequence of which caloric is +disengaged. + +MRS. B. + +If the combination of the oxygen and the metal results from the union of +their opposite electricities, of course caloric must be given out. + +EMILY. + +The effervescence is over; therefore I suppose that the metal is now +oxydated. + +MRS. B. + +Yes. But there is another important connection between metals and acids, +with which I must now make you acquainted. Metals, when in the state of +oxyds, are capable of being dissolved by acids. In this operation they +enter into a chemical combination with the acid, and form an entirely +new compound. + +CAROLINE. + +But what difference is there between the _oxydation_ and the +_dissolution_ of the metal by an acid? + +MRS. B. + +In the first case, the metal merely combines with a portion of oxygen +taken from the acid, which is thus partly deoxygenated, as in the +instance you have just seen; in the second case, the metal, after being +previously oxydated, is actually dissolved in the acid, and enters into +a chemical combination with it, without producing any further +decomposition or effervescence. --This complete combination of an oxyd +and an acid forms a peculiar and important class of compound salts. + +EMILY. + +The difference between an oxyd and a compound salt, therefore, is very +obvious: the one consists of a metal and oxygen; the other of an oxyd +and an acid. + +MRS. B. + +Very well: and you will be careful to remember that the metals are +incapable of entering into this combination with acids, unless they are +previously oxydated; therefore, whenever you bring a metal in contact +with an acid, it will be first oxydated and afterwards dissolved, +provided that there be a sufficient quantity of acid for both +operations. + +There are some metals, however, whose solution is more easily +accomplished, by diluting the acid in water; and the metal will, in this +case, be oxydated, not by the acid, but by the water, which it will +decompose. But in proportion as the oxygen of the water oxydates the +surface of the metal, the acid combines with it, washes it off, and +leaves a fresh surface for the oxygen to act upon: then other coats of +oxyd are successively formed, and rapidly dissolved by the acid, which +continues combining with the new-formed surfaces of oxyd till the whole +of the metal is dissolved. During this process the hydrogen gas of the +water is disengaged, and flies off with effervescence. + +EMILY. + +Was not this the manner in which the sulphuric acid assisted the iron +filings in decomposing water? + +MRS. B. + +Exactly; and it is thus that several metals, which are incapable alone +of decomposing water, are enabled to do it by the assistance of an acid, +which, by continually washing off the covering of oxyd, as it is formed, +prepares a fresh surface of metal to act upon the water. + +CAROLINE. + +The acid here seems to act a part not very different from that of a +scrubbing-brush. --But pray would not this be a good method of cleaning +metallic utensils? + +MRS. B. + +Yes; on some occasions a weak acid, as vinegar, is used for cleaning +copper. Iron plates, too, are freed from the rust on their surface by +diluted muriatic acid, previous to their being covered with tin. You +must remember, however, that in this mode of cleaning metals the acid +should be quickly afterwards wiped off, otherwise it would produce fresh +oxyd. + +CAROLINE. + +Let us watch the dissolution of the copper in the nitric acid; for I am +very impatient to see the salt that is to result from it. The mixture is +now of a beautiful blue colour; but there is no appearance of the +formation of a salt; it seems to be a tedious operation. + +MRS. B. + +The crystallisation of the salt requires some length of time to be +completed; if, however, you are so impatient, I can easily show you a +metallic salt already formed. + +CAROLINE. + +But that would not satisfy my curiosity half so well as one of our own +manufacturing. + +MRS. B. + +It is one of our own preparing that I mean to show you. When we +decomposed water a few days since, by the oxydation of iron filings +through the assistance of sulphuric acid, in what did the process +consist? + +CAROLINE. + +In proportion as the water yielded its oxygen to the iron, the acid +combined with the new-formed oxyd, and the hydrogen escaped alone. + +MRS. B. + +Very well; the result, therefore, was a compound salt, formed by the +combination of sulphuric acid with oxyd of iron. It still remains in the +vessel in which the experiment was performed. Fetch it, and we shall +examine it. + +EMILY. + +What a variety of processes the decomposition of water, by a metal and +an acid, implies; 1st, the decomposition of the water; 2dly, the +oxydation of the metal; and 3dly, the formation of a compound salt. + +CAROLINE. + +Here it is, Mrs. B. --What beautiful green crystals! But we do not +perceive any crystals in the solution of copper in nitrous acid? + +MRS. B. + +Because the salt is now suspended in the water which the nitrous acid +contains, and will remain so till it is deposited in consequence of rest +and cooling. + +EMILY. + +I am surprised that a body so opake as iron can be converted into such +transparent crystals. + +MRS. B. + +It is the union with the acid that produces the transparency; for if the +pure metal were melted, and afterwards permitted to cool and +crystallise, it would be found just as opake as before. + +EMILY. + +I do not understand the exact meaning of _crystallisation_? + +MRS. B. + +You recollect that when a solid body is dissolved either by water or +caloric it is not decomposed; but that its integrant parts are only +suspended in the solvent. When the solution is made in water, the +integrant particles of the body will, on the water being evaporated, +again unite into a solid mass by the force of their mutual attraction. +But when the body is dissolved by caloric alone, nothing more is +necessary, in order to make its particles reunite, than to reduce its +temperature. And, in general, if the solvent, whether water or caloric, +be slowly separated by evaporation or by cooling, and care taken that +the particles be not agitated during their reunion, they will arrange +themselves in regular masses, each individual substance assuming a +peculiar form or arrangement; and this is what is called +crystallisation. + +EMILY. + +Crystallisation, therefore, is simply the reunion of the particles of a +solid body that has been dissolved in a fluid. + +MRS. B. + +That is a very good definition of it. But I must not forget to observe, +that _heat_ and _water_ may unite their solvent powers; and, in this +case, crystallisation may be hastened by cooling, as well as by +evaporating the liquid. + +CAROLINE. + +But if the body dissolved is of a volatile nature, will it not evaporate +with the fluid? + +MRS. B. + +A crystallised body held in solution only by water is scarcely ever so +volatile as the fluid itself, and care must be taken to manage the heat +so that it may be sufficient to evaporate the water only. + +I should not omit also to mention that bodies, in crystallising from +their watery solution, always retain a small portion of water, which +remains confined in the crystal in a solid form, and does not reappear +unless the body loses its crystalline state. This is called the _water +of crystallisation_. But you must observe, that whilst a body may be +separated from its solution in water or caloric simply by cooling or by +evaporation, an acid can be taken from a metal with which it is combined +only by stronger affinities, which produce a decomposition. + +EMILY. + +Are the perfect metals susceptible of being dissolved and converted into +compound salts by acids? + +MRS. B. + +Gold is acted upon by only one acid, the _oxygenated muriatic_, a very +remarkable acid, which, when in its most concentrated state, dissolves +gold or any other metal, by burning them rapidly. + +Gold can, it is true, be dissolved likewise by a mixture of two acids, +commonly called _aqua regia_; but this mixed solvent derives that +property from containing the peculiar acid which I have just mentioned. +Platina is also acted upon by this acid only; silver is dissolved by +nitric acid. + +CAROLINE. + +I think you said that some of the metals might be so strongly oxydated +as to become acid? + +MRS. B. + +There are five metals, arsenic, molybdena, chrome, tungsten, and +columbium, which are susceptible of combining with a sufficient quantity +of oxygen to be converted into acids. + +CAROLINE. + +Acids are connected with metals in such a variety of ways, that I am +afraid of some confusion in remembering them. --In the first place, +acids will yield their oxygen to metals. Secondly, they will combine +with them in their state of oxyds, to form compound salts; and lastly, +several of the metals are themselves susceptible of acidification. + +MRS. B. + +Very well; but though metals have so great an affinity for acids, it is +not with that class of bodies alone that they will combine. They are +most of them, in their simple state, capable of uniting with sulphur, +with phosphorus, with carbon, and with each other; these combinations, +according to the nomenclature which was explained to you on a former +occasion, are called _sulphurets_, _phosphorets_, _carburets_, &c. + +The metallic phosphorets offer nothing very remarkable. The sulphurets +form the peculiar kind of mineral called _pyrites_, from which certain +kinds of mineral waters, as those of Harrogate, derive their chief +chemical properties. In this combination, the sulphur, together with the +iron, have so strong an attraction for oxygen, that they obtain it both +from the air and from water, and by condensing it in a solid form, +produce the heat which raises the temperature of the water in such a +remarkable degree. + +EMILY. + +But if pyrites obtain oxygen from water, that water must suffer a +decomposition, and hydrogen gas be evolved. + +MRS. B. + +That is actually the case in the hot springs alluded to, which give out +an extremely fetid gas, composed of hydrogen impregnated with sulphur. + +CAROLINE. + +If I recollect right, steel and plumbago, which you mentioned in the +last lesson, are both carburets of iron? + +MRS. B. + +Yes; and they are the only carburets of much consequence. + +A curious combination of metals has lately very much attracted the +attention of the scientific world: I mean the meteoric stones that fall +from the atmosphere. They consist principally of native or pure iron, +which is never found in that state in the bowels of the earth; and +contain also a small quantity of nickel and chrome, a combination +likewise new in the mineral kingdom. + +These circumstances have led many scientific persons to believe that +those substances have fallen from the moon, or some other planet, while +others are of opinion either that they are formed in the atmosphere, or +are projected into it by some unknown volcano on the surface of our +globe. + +CAROLINE. + +I have heard much of these stones, but I believe many people are of +opinion that they are formed on the surface of the earth, and laugh at +their pretended celestial origin. + +MRS. B. + +The fact of their falling is so well ascertained, that I think no person +who has at all investigated the subject, can now entertain any doubt of +it. Specimens of these stones have been discovered in all parts of the +world, and to each of them some tradition or story of its fall has been +found connected. And as the analysis of all those specimens affords +precisely the same results, there is strong reason to conjecture that +they all proceed from the same source. It is to Mr. Howard that +philosophers are indebted for having first analysed these stones, and +directed their attention to this interesting subject. + +CAROLINE. + +But pray, Mrs. B., how can solid masses of iron and nickel be formed +from the atmosphere, which consists of the two airs, nitrogen and +oxygen? + +MRS. B. + +I really do not see how they could, and think it much more probable that +they fall from the moon. --But we must not suffer this digression to +take up too much of our time. + +The combinations of metals with each other are called alloys; thus brass +is an alloy of copper and zinc; bronze, of copper and tin, &c. + +EMILY. + +And is not pewter also a combination of metal? + +MRS. B. + +It is. The pewter made in this country is mostly composed of tin, with a +very small proportion of zinc and lead. + +CAROLINE. + +Block-tin is a kind of pewter, I believe? + +MRS. B. + +Properly speaking, block-tin means tin in blocks, or square massive +ingots; but in the sense in which it is used by ignorant workmen, it is +iron plated with tin, which renders it more durable, as tin will not so +easily rust. Tin alone, however, would be too soft a metal to be worked +for common use, and all tin-vessels and utensils are in fact made of +plates of iron, thinly coated with tin, which prevents the iron from +rusting. + +CAROLINE. + +Say rather _oxydating_, Mrs. B. --Rust is a word that should be exploded +in chemistry. + +MRS. B. + +Take care, however, not to introduce the word oxydate, instead of rust, +in general conversation; for you would probably not be understood, and +you might be suspected of affectation. + +Metals differ very much in their affinity for each other; some will not +unite at all, others readily combine together, and on this property of +metals the art of _soldering_ depends. + +EMILY. + +What is soldering? + +MRS. B. + +It is joining two pieces of metal together, by a more fusible metal +interposed between them. Thus tin is a solder for lead; brass, gold, or +silver, are solder for iron, &c. + +CAROLINE. + +And is not _plating_ metals something of the same nature? + +MRS. B. + +In the operation of plating, two metals are united, one being covered +with the other, but without the intervention of a third; iron or copper +may thus be covered with gold or silver. + +EMILY. + +Mercury appears to me of a very different nature from the other metals. + +MRS. B. + +One of its greatest peculiarities is, that it retains a fluid state at +the temperature of the atmosphere. All metals are fusible at different +degrees of heat, and they have likewise each the property of freezing or +becoming solid at a certain fixed temperature. Mercury congeals only at +seventy-two degrees below the freezing point. + +EMILY. + +That is to say, that in order to freeze, it requires a temperature of +seventy-two degrees colder than that at which water freezes. + +MRS. B. + +Exactly so. + +CAROLINE. + +But is the temperature of the atmosphere ever so low as that? + +MRS. B. + +Yes, often in Siberia; but happily never in this part of the globe. +Here, however, mercury may be congealed by artificial cold; I mean such +intense cold as can be produced by some chemical mixtures, or by the +rapid evaporation of ether under the air-pump.* + + [Footnote *: By a process analogous to that described, page 155. + of this volume.] + +CAROLINE. + +And can mercury be made to boil and evaporate? + +MRS. B. + +Yes, like any other liquid; only it requires a much greater degree of +heat. At the temperature of six hundred degrees, it begins to boil and +evaporate like water. + +Mercury combines with gold, silver, tin, and with several other metals; +and, if mixed with any of them in a sufficient proportion, it penetrates +the solid metal, softens it, loses its own fluidity, and forms an +_amalgam_, which is the name given to the combination of any metal with +mercury, forming a substance more or less solid, according as the +mercury or the other metal predominates. + +EMILY. + +In the list of metals there are some whose names I have never before +heard mentioned. + +MRS. B. + +Besides those which Sir H. Davy has obtained, there are several that +have been recently discovered, whose properties are yet but little +known, as for instance, titanium, which was discovered by the Rev. Mr. +Gregor, in the tin-mines of Cornwall; columbium or tantalium, which has +lately been discovered by Mr. Hatchett; and osmium, iridium, palladium, +and rhodium, all of which Dr. Wollaston and Mr. Tennant found mixed in +minute quantities with crude platina, and the distinct existence of +which they proved by curious and delicate experiments. + +CAROLINE. + +Arsenic has been mentioned amongst the metals. I had no notion that it +belonged to that class of bodies, for I had never seen it but as a +powder, and never thought of it but as a most deadly poison. + +MRS. B. + +In its pure metallic state, I believe, it is not so poisonous; but it +has such a great affinity for oxygen, that it absorbs it from the +atmosphere at its natural temperature: you have seen it, therefore, only +in its state of oxyd, when, from its combination with oxygen, it has +acquired its very poisonous properties. + +CAROLINE. + +Is it possible that oxygen can impart poisonous qualities? That valuable +substance which produces light and fire, and which all bodies in nature +are so eager to obtain? + +MRS. B. + +Most of the metallic oxyds are poisonous, and derive this property from +their union with oxygen. The white lead, so much used in paint, owes its +pernicious effects to oxygen. In general, oxygen, in a concrete state, +appears to be particularly destructive in its effects on flesh or any +animal matter; and those oxyds are most caustic that have an acrid +burning taste, which proceeds from the metal having but a slight +affinity for oxygen, and therefore easily yielding it to the flesh, +which it corrodes and destroys. + +EMILY. + +What is the meaning of the word _caustic_, which you have just used? + +MRS. B. + +It expresses that property which some bodies possess, of disorganizing +and destroying animal matter, by operating a kind of combustion, or at +least a chemical decomposition. You must often have heard of caustic +used to burn warts, or other animal excrescences; most of these bodies +owe their destructive power to the oxygen with which they are combined. +The common caustic, called _lunar caustic_, is a compound formed by the +union of nitric acid and silver; and it is supposed to owe its caustic +qualities to the oxygen contained in the nitric acid. + +CAROLINE. + +But, pray, are not acids still more caustic than oxyds, as they contain +a greater proportion of oxygen? + +MRS. B. + +Some of the acids are; but the caustic property of a body depends not +only upon the quantity of oxygen which it contains, but also upon its +slight affinity for that principle, and the consequent facility with +which it yields it. + +EMILY. + +Is not this destructive property of oxygen accounted for? + +MRS. B. + +It proceeds probably from the strong attraction of oxygen for hydrogen; +for if the one rapidly absorb the other from the animal fibre, +a disorganisation of the substance must ensue. + +EMILY. + +Caustics are, then, very properly said to burn the flesh, since the +combination of oxygen and hydrogen is an actual combustion. + +CAROLINE. + +Now, I think, this effect would be more properly termed an oxydation, as +there is no disengagement of light and heat. + +MRS. B. + +But there really is a sensation of heat produced by the action of +caustics. + +EMILY. + +If oxygen is so caustic, why does not that which is contained in the +atmosphere burn us? + +MRS. B. + +Because it is in a gaseous state, and has a greater attraction for its +electricity than for the hydrogen of our bodies. Besides, should the air +be slightly caustic, we are in a great measure sheltered from its +effects by the skin; you know how much a wound, however trifling, smarts +on being exposed to it. + +CAROLINE. + +It is a curious idea, however, that we should live in a slow fire. But, +if the air was caustic, would it not have an acrid taste? + +MRS. B. + +It possibly may have such a taste; though in so slight a degree, that +custom has rendered it insensible. + +CAROLINE. + +And why is not water caustic? When I dip my hand into water, though +cold, it ought to burn me from the caustic nature of its oxygen. + +MRS. B. + +Your hand does not decompose the water; the oxygen in that state is much +better supplied with hydrogen than it would be by animal matter, and if +its causticity depend on its affinity for that principle, it will be +very far from quitting its state of water to act upon your hand. You +must not forget that oxyds are caustic in proportion as the oxygen +adheres slightly to them. + +EMILY. + +Since the oxyd of arsenic is poisonous, its acid, I suppose, is fully as +much so? + +MRS. B. + +Yes; it is one of the strongest poisons in nature. + +EMILY. + +There is a poison called _verdigris_, which forms on brass and copper +when not kept very clean; and this, I have heard, is an objection to +these metals being made into kitchen utensils. Is this poison likewise +occasioned by oxygen? + +MRS. B. + +It is produced by the intervention of oxygen; for verdigris is a +compound salt formed by the union of vinegar and copper; it is of a +beautiful green colour, and much used in painting. + +EMILY. + +But, I believe, verdigris is often formed on copper when no vinegar has +been in contact with it. + +MRS. B. + +Not real verdigris, but compound salts, somewhat resembling it, may be +produced by the action of any acid on copper. + +The solution of copper in nitric acid, if evaporated, affords a salt +which produces an effect on tin that will surprise you, and I have +prepared some from the solution we made before, that I might show it to +you. I shall first sprinkle some water on this piece of tin-foil, and +then some of the salt. --Now observe that I fold it up suddenly, and +press it into one lump. + +CAROLINE. + +What a prodigious vapour issues from it--and sparks of fire I declare! + +MRS. B. + +I thought it would surprise you. The effect, however, I dare say you +could account for, since it is merely the consequence of the oxygen of +the salt rapidly entering into a closer combination with the tin. + +There is also a beautiful green salt too curious to be omitted; it is +produced by the combination of cobalt with muriatic acid, which has the +singular property of forming what is called _sympathetic ink_. +Characters written with this solution are invisible when cold, but when +a gentle heat is applied, they assume a fine bluish green colour. + +CAROLINE. + +I think one might draw very curious landscapes with the assistance of +this ink; I would first make a water-colour drawing of a winter-scene, +in which the trees should be leafless, and the grass scarcely green: +I would then trace all the verdure with the invisible ink, and whenever +I chose to create spring, I should hold it before the fire, and its +warmth would cover the landscape with a rich verdure. + +MRS. B. + +That will be a very amusing experiment, and I advise you by all means to +try it. + + [Transcriber’s Note: + Several cobalt compounds, including the cobalt chloride described + here, are still in use as invisible (“sympathetic”) inks. They are + safe if used appropriately.] + +Before we part, I must introduce to your acquaintance the curious metals +which Sir H. Davy has recently discovered. The history of these +extraordinary bodies is yet so much in its infancy, that I shall confine +myself to a very short account of them; it is more important to point +out to you the vast, and apparently inexhaustible, field of research +which has been thrown open to our view by Sir H. Davy’s memorable +discoveries, than to enter into a minute account of particular bodies or +experiments. + +CAROLINE. + +But I have heard that these discoveries, however splendid and +extraordinary, are not very likely to prove of any great benefit to the +world, as they are rather objects of curiosity than of use. + +MRS. B. + +Such may be the illiberal conclusions of the ignorant and narrow-minded; +but those who can duly estimate the advantages of enlarging the sphere +of science, must be convinced that the acquisition of every new fact, +however unconnected it may at first appear with practical utility, must +ultimately prove beneficial to mankind. But these remarks are scarcely +applicable to the present subject; for some of the new metals have +already proved eminently useful as chemical agents, and are likely soon +to be employed in the arts. For the enumeration of these metals, I must +refer you to our list of simple bodies; they are derived from the +alkalies, the earths, and three of the acids, all of which had been +hitherto considered as undecompoundable or simple bodies. + +When Sir H. Davy first turned his attention to the effects of the +Voltaic battery, he tried its power on a variety of compound bodies, and +gradually brought to light a number of new and interesting facts, which +led the way to more important discoveries. It would be highly +interesting to trace his steps in this new department of science, but it +would lead us too far from our principal object. A general view of his +most remarkable discoveries is all that I can aim at, or that you could, +at present, understand. + +The facility with which compound bodies yielded to the Voltaic +electricity, induced him to make trial of its effects on substances +hitherto considered as simple, but which he suspected of being compound, +and his researches were soon crowned with the most complete success. + +The body which he first submitted to the Voltaic battery, and which had +never yet been decomposed, was one of the fixed alkalies, called potash. +This substance gave out an elastic fluid at the positive wire, which was +ascertained to be oxygen, and at the negative wire, small globules of a +very high metallic lustre, very similar in appearance to mercury; thus +proving that potash, which had hitherto been considered as a simple +incombustible body, was in fact a metallic oxyd; and that its +incombustibility proceeded from its being already combined with oxygen. + +EMILY. + +I suppose the wires used in this experiment were of platina, as they +were when you decomposed water; for if of iron, the oxygen would have +combined with the wire, instead of appearing in the form of gas. + +MRS. B. + +Certainly: the metal, however, would equally have been disengaged. Sir +H. Davy has distinguished this new substance by the name of POTASSIUM, +which is derived from that of the alkali, from which it is procured. +I have some small pieces of it in this phial, but you have already seen +it, as it is the metal which we burnt in contact with sulphur. + +EMILY. + +What is the liquid in which you keep it? + +MRS. B. + +It is naptha, a bituminous liquid, with which I shall hereafter make you +acquainted. It is almost the only fluid in which potassium can be +preserved, as it contains no oxygen, and this metal has so powerful an +attraction for oxygen, that it will not only absorb it from the air, but +likewise from water, or any body whatever that contains it. + +EMILY. + +This, then, is one of the bodies that oxydates spontaneously without the +application of heat? + +MRS. B. + +Yes; and it has this remarkable peculiarity that it attracts oxygen much +more rapidly from water than from air; so that when thrown into water, +however cold, it actually bursts into flame. I shall now throw a small +piece, about the size of a pin’s head, on this drop of water. + +CAROLINE. + +It instantaneously exploded, producing a little flash of light! this is, +indeed, a most curious substance! + +MRS. B. + +By its combustion it is reconverted into potash; and as potash is now +decidedly a compound body, I shall not enter into any of its properties +till we have completed our review of the simple bodies; but we may here +make a few observations on its basis, potassium. If this substance is +left in contact with air, it rapidly returns to the state of potash, +with a disengagement of heat, but without any flash of light. + +EMILY. + +But is it not very singular that it should burn better in water than in +air? + +CAROLINE. + +I do not think so: for if the attraction of potassium for oxygen is so +strong that it finds no more difficulty in separating it from the +hydrogen in water, than in absorbing it from the air, it will no doubt +be more amply and rapidly supplied by water than by air. + +MRS. B. + +That cannot, however, be precisely the reason, for when potassium is +introduced under water, without contact of air, the combustion is not so +rapid, and indeed, in that case, there is no luminous appearance; but a +violent action takes place, much heat is excited, the potash is +regenerated, and hydrogen gas is evolved. + +Potassium is so eminently combustible, that instead of requiring, like +other metals, an elevation of temperature, it will burn rapidly in +contact with water, even below the freezing point. This you may witness +by throwing a piece on this lump of ice. + +CAROLINE. + +It again exploded with flame, and has made a deep hole in the ice. + +MRS. B. + +This hole contains a solution of potash; for the alkali being extremely +soluble, disappears in the water at the instant it is produced. Its +presence, however, may be easily ascertained, alkalies having the +property of changing paper, stained with turmeric, to a red colour; if +you dip one end of this slip of paper into the hole in the ice you will +see it change colour, and the same, if you wet it with the drop of water +in which the first piece of potassium was burnt. + +CAROLINE. + +It has indeed changed the paper from yellow to red. + +MRS. B. + +This metal will burn likewise in carbonic acid gas, a gas that had +always been supposed incapable of supporting combustion, as we were +unacquainted with any substance that had a greater attraction for oxygen +than carbon. Potassium, however, readily decomposes this gas, by +absorbing its oxygen, as I shall show you. This retort is filled with +carbonic acid gas. --I will put a small piece of potassium in it; but +for this combustion a slight elevation of temperature is required, for +which purpose I shall hold the retort over the lamp. + +CAROLINE. + +Now it has taken fire, and burns with violence! It has burst the retort. + +MRS. B. + +Here is the piece of regenerated potash; can you tell me why it is +become so black? + +EMILY. + +No doubt it is blackened by the carbon, which, when its oxygen entered +into combination with the potassium, was deposited on its surface. + +MRS. B. + +You are right. This metal is perfectly fluid at the temperature of one +hundred degrees; at fifty degrees it is solid, but soft and malleable; +at thirty-two degrees it is hard and brittle, and its fracture exhibits +an appearance of confused crystallization. It is scarcely more than half +as heavy as water; its specific gravity being about six when water is +reckoned at ten; so that this metal is actually lighter than any known +fluid, even than ether. + +Potassium combines with sulphur and phosphorus, forming sulphurets and +phosphurets; it likewise forms alloys with several metals, and +amalgamates with mercury. + +EMILY. + +But can a sufficient quantity of potassium be obtained, by means of the +Voltaic battery, to admit of all its properties and relations to other +bodies being satisfactorily ascertained? + +MRS. B. + +Not easily; but I must not neglect to inform you that a method of +obtaining this metal in considerable quantities has since been +discovered. Two eminent French chemists, Thenard and Gay Lussac, +stimulated by the triumph which Sir H. Davy had obtained, attempted to +separate potassium from its combination with oxygen, by common chemical +means, and without the aid of electricity. They caused red hot potash in +a state of fusion to filter through iron turnings in an iron tube, +heated to whiteness. Their experiment was crowned with the most complete +success; more potassium was obtained by this single operation, that +could have been collected in many weeks by the most diligent use of the +Voltaic battery. + +EMILY. + +In this experiment, I suppose, the oxygen quitted its combination with +the potassium to unite with the iron turnings? + +MRS. B. + +Exactly so; and the potassium was thus obtained in its simple state. +From that time it has become a most convenient and powerful instrument +of deoxygenation in chemical experiments. This important improvement, +engrafted on Sir H. Davy’s previous discoveries, served but to add to +his glory, since the facts which he had established, when possessed of +only a few atoms of this curious substance, and the accuracy of his +analytical statements, were all confirmed when an opportunity occurred +of repeating his experiments upon this substance, which can now be +obtained in unlimited quantities. + +CAROLINE. + +What a satisfaction Sir H. Davy must have felt, when by an effort of +genius he succeeded in bringing to light and actually giving existence, +to these curious bodies, which without him might perhaps have ever +remained concealed from our view! + +MRS. B. + +The next substance which Sir H. Davy submitted to the influence of the +Voltaic battery was _Soda_, the other fixed alkali, which yielded to the +same powers of decomposition; from this alkali too, a metallic substance +was obtained, very analogous in its properties to that which had been +discovered in potash; Sir H. Davy has called it SODIUM. It is rather +heavier than potassium, though considerably lighter than water; it is +not so easily fusible as potassium. + +Encouraged by these extraordinary results, Sir H. Davy next performed a +series of beautiful experiments on _Ammonia_, or the volatile alkali, +which, from analogy, he was led to suspect might also contain oxygen. +This he soon ascertained to be the fact, but he has not yet succeeded in +obtaining the basis of ammonia in a separate state; it is from analogy, +and from the power which the volatile alkali has, in its gaseous form, +to oxydate iron, and also from the amalgams which can be obtained from +ammonia by various processes, that the proofs of that alkali being also +a metallic oxyd are deduced. + +Thus, then, the three alkalies, two of which had always been considered +as simple bodies, have now lost all claim to that title, and I have +accordingly classed the alkalies amongst the compounds, whose properties +we shall treat of in a future conversation. + +EMILY. + +What are the other newly discovered metals which you have alluded to in +your list of simple bodies? + +MRS. B. + +They are the metals of the earths which became next the object of Sir H. +Davy’s researches; these bodies had never yet been decomposed, though +they were strongly suspected not only of being compounds, but of being +metallic oxyds. From the circumstance of their incombustibility it was +conjectured, with some plausibility, that they might possibly be bodies +that had been already burnt. + +CAROLINE. + +And metals, when oxydated, become, to all appearance, a kind of earthy +substance. + +MRS. B. + +They have, besides, several features of resemblance with metallic oxyds; +Sir H. Davy had therefore great reason to be sanguine in his +expectations of decomposing them, and he was not disappointed. He could +not, however, succeed in obtaining the basis of the earths in a pure +separate state; but metallic alloys were formed with other metals, which +sufficiently proved the existence of the metallic basis of the earths. + +The last class of new metallic bodies which Sir H. Davy discovered was +obtained from the three undecompounded acids, the boracic, the fluoric, +and the muriatic acids; but as you are entirely unacquainted with these +bodies, I shall reserve the account of their decomposition till we come +to treat of their properties as acids. + +Thus in the course of two years, by the unparalleled exertions of a +single individual, chemical science has assumed a new aspect. Bodies +have been brought to light which the human eye never before beheld, and +which might have remained eternally concealed under their impenetrable +disguise. + +It is impossible at the present period to appreciate to their full +extent the consequences which science or the arts may derive from these +discoveries; we may, however, anticipate the most important results. + +In chemical analysis we are now in possession of more energetic agents +of decomposition than were ever before known. + +In geology new views are opened, which will probably operate a +revolution in that obscure and difficult science. It is already proved +that all the earths, and, in fact, the solid surface of this globe, are +metallic bodies mineralized by oxygen, and as our planet has been +calculated to be considerably more dense upon the whole than on the +surface, it is reasonable to suppose that the interior part is composed +of a metallic mass, the surface of which only has been mineralized by +the atmosphere. + +The eruptions of volcanos, those stupendous problems of nature, admit +now of an easy explanation. For if the bowels of the earth are the grand +recess of these newly discovered inflammable bodies, whenever water +penetrates into them, combustions and explosions must take place; and it +is remarkable that the lava which is thrown out, is the very kind of +substance which might be expected to result from these combustions. + +I must now take my leave of you; we have had a very long conversation +to-day, and I hope you will be able to recollect what you have learnt. +At our next interview we shall enter on a new subject. + + +END OF THE FIRST VOLUME. + + + Printed by A. Strahan, + Printers-Street, London. + + * * * * * + * * * * + + CONVERSATIONS + ON + CHEMISTRY; + + In Which + The Elements Of That Science + Are + _Familiarly Explained_ + And + Illustrated By Experiments. + + + IN TWO VOLUMES. + + _The Fifth Edition, revised, corrected,_ + _and considerably enlarged._ + + VOL. II. + ON COMPOUND BODIES. + + + _London:_ + Printed For Longman, Hurst, Rees, Orme, and Brown, + Paternoster-Row. + 1817. + + + + +CONVERSATION XIII. + +ON THE ATTRACTION OF COMPOSITION. + + +MRS. B. + +Having completed our examination of the simple or elementary bodies, we +are now to proceed to those of a compound nature; but before we enter on +this extensive subject, it will be necessary to make you acquainted with +the principal laws by which chemical combinations are governed. + +You recollect, I hope, what we formerly said of the nature of the +attraction of composition, or chemical attraction, or affinity, as it is +also called? + +EMILY. + +Yes, I think perfectly; it is the attraction that subsists between +bodies of a different nature, which occasions them to combine and form a +compound, when they come in contact, and, according to Sir H. Davy’s +opinion, this effect is produced by the attraction of the opposite +electricities, which prevail in bodies of different kinds. + +MRS. B. + +Very well; your definition comprehends the first law of chemical +attraction, which is, that _it takes place only between bodies of a +different nature_; as, for instance, between an acid and an alkali; +between oxygen and a metal, &c. + +CAROLINE. + +That we understand of course; for the attraction between particles of a +similar nature is that of aggregation, or cohesion, which is independent +of any chemical power. + +MRS. B. + +The 2d law of chemical attraction is, that _it takes place only between +the most minute particles of bodies_; therefore, the more you divide the +particles of the bodies to be combined, the more readily they act upon +each other. + +CAROLINE. + +That is again a circumstance which we might have supposed, for the finer +the particles of the two substances are, the more easily and perfectly +they will come in contact with each other, which must greatly facilitate +their union. It was for this purpose, you said, that you used iron +filings, in preference to wires or pieces of iron, for the decomposition +of water. + +MRS. B. + +It was once supposed that no mechanical power could divide bodies into +particles sufficiently minute for them to act on each other; and that, +in order to produce the extreme division requisite for a chemical +action, one, if not both of the bodies, should be in a fluid state. +There are, however, a few instances in which two solid bodies, very +finely pulverized, exert a chemical action on one another; but such +exceptions to the general rule are very rare indeed. + +EMILY. + +In all the combinations that we have hitherto seen, one of the +constituents has, I believe, been either liquid or aëriform. In +combustions, for instance, the oxygen is taken from the atmosphere, in +which it existed in the state of gas; and whenever we have seen acids +combine with metals or with alkalies, they were either in a liquid or an +aëriform state. + +MRS. B. + +The 3d law of chemical attraction is, that _it can take place between +two, three, four, or even a greater number of bodies_. + +CAROLINE. + +Oxyds and acids are bodies composed of two constituents; but I recollect +no instance of the combination of a greater number of principles. + +MRS. B. + +The compound salts, formed by the union of the metals with acids, are +composed of three principles. And there are salts formed by the +combination of the alkalies with the earths which are of a similar +description. + +CAROLINE. + +Are they of the same kind as the metallic salts? + +MRS. B. + +Yes; they are very analogous in their nature, although different in many +of their properties. + +A methodical nomenclature, similar to that of the acids, has been +adopted for the compound salts. Each individual salt derives its name +from its constituent parts, so that every name implies a knowledge of +the composition of the salt. + +The three alkalies, the alkaline earths, and the metals, are called +_salifiable bases_ or _radicals_; and the acids, _salifying principles_. +The name of each salt is composed both of that of the acid and the +salifiable base; and it terminates in _at_ or _it_, according to the +degree of the oxygenation of the acid. Thus, for instance, all those +salts which are formed by the combination of the sulphuric acid with any +of the salifiable bases are called _sulphats_, and the name of the +radical is added for the specific distinction of the salt; if it be +potash, it will compose a _sulphat of potash_; if ammonia, _sulphat of +ammonia_, &c. + +EMILY. + +The crystals which we obtained from the combination of iron and +sulphuric acid were therefore _sulphat of iron_? + +MRS. B. + +Precisely; and those which we prepared by dissolving copper in nitric +acid, _nitrat of copper_, and so on. --But this is not all; if the salt +be formed by that class of acids which ends in _ous_, (which you know +indicates a less degree of oxygenation,) the termination of the name of +the salt will be in _it_, as _sulphit of potash_, _sulphit of +ammonia_, &c. + +EMILY. + +There must be an immense number of compound salts, since there is so +great a variety of salifiable radicals, as well as of salifying +principles. + +MRS. B. + +Their real number cannot be ascertained, since it increases every day. +But we must not proceed further in the investigation of the compound +salts, until we have completed the examination of the nature of the +ingredients of which they are composed. + +The 4th law of chemical attraction is, that _a change of temperature +always takes place at the moment of combination_. This arises from the +extrication of the two electricities in the form of caloric, which takes +place when bodies unite; and also sometimes in part from a change of +capacity of the bodies for heat, which always takes place when the +combination is attended with an increase of density, but more especially +when the compound passes from the liquid to the solid form. I shall now +show you a striking instance of a change of temperature from chemical +union, merely by pouring some nitrous acid on this small quantity of oil +of turpentine--the oil will instantly combine with the oxygen of the +acid, and produce a considerable change of temperature. + +CAROLINE. + +What a blaze! The temperature of the oil and the acid must be greatly +raised, indeed, to produce such a violent combustion. + +MRS. B. + +There is, however, a peculiarity in this combustion, which is, that the +oxygen, instead of being derived from the atmosphere alone, is +principally supplied by the acid itself. + +EMILY. + +And are not all combustions instances of the change of temperature +produced by the chemical combination of two bodies? + +MRS. B. + +Undoubtedly; when oxygen loses its gaseous form, in order to combine +with a solid body, it becomes condensed, and the caloric evolved +produces the elevation of temperature. The specific gravity of bodies is +at the same time altered by chemical combination; for in consequence of +a change of capacity for heat, a change of density must be produced. + +CAROLINE. + +That was the case with the sulphuric acid and water, which, by being +mixed together, gave out a great deal of heat, and increased in density. + +MRS. B. + +The 5th law of chemical attraction is, that _the properties which +characterise bodies, when separate, are altered or destroyed by their +combination_. + +CAROLINE. + +Certainly; what, for instance, can be so different from water as the +hydrogen and oxygen gases? + +EMILY. + +Or what more unlike sulphat of iron than iron or sulphuric acid? + +MRS. B. + +Every chemical combination is an illustration of this rule. But let us +proceed-- + +The 6th law is, that _the force of chemical affinity between the +constituents of a body is estimated by that which is required for their +separation_. This force is not always proportional to the facility with +which bodies unite; for manganese, for instance, which, you know, is so +much disposed to unite with oxygen that it is never found in a metallic +state, yields it more easily than any other metal. + +EMILY. + +But, Mrs. B., you speak of estimating the force of attraction between +bodies, by the force required to separate them; how can you measure +these forces? + +MRS. B. + +They cannot be precisely measured, but they are comparatively +ascertained by experiment, and can be represented by numbers which +express the relative degrees of attraction. + +The 7th law is, that _bodies have amongst themselves different degrees +of attraction_. Upon this law, (which you may have discovered yourselves +long since,) the whole science of chemistry depends; for it is by means +of the various degrees of affinity which bodies have for each other, +that all the chemical compositions and decompositions are effected. +Every chemical fact or experiment is an instance of the same kind; and +whenever the decomposition of a body is performed by the addition of any +single new substance, it is said to be effected by _simple elective +attractions_. But it often happens that no simple substance will +decompose a body, and that, in order to effect this, you must offer to +the compound a body which is itself composed of two, or sometimes three +principles, which would not, each separately, perform the decomposition. +In this case there are two new compounds formed in consequence of a +reciprocal decomposition and recomposition. All instances of this kind +are called _double elective attractions_. + +CAROLINE. + +I confess I do not understand this clearly. + +MRS. B. + +You will easily comprehend it by the assistance of this diagram, in +which the reciprocal forces of attraction are represented by numbers: + + _Original Compound_ + Sulphat of Soda. + + Soda 8 Sulphuric Acid + + | + | _Quies-_ + | + | _cent_ + | + _Result_ _Result_ + Nitrat 7 _Divellent Attractions_ 6} 13 Sulphat + of Soda of Lime + | + | + | _Attrac-_ + | + | _tions_ + | + + Nitric Acid 4 Lime + -- + 12 + + _Original Compound_ + Nitrat of Lime. + +We here suppose that we are to decompose sulphat of soda; that is, to +separate the acid from the alkali; if, for this purpose, we add some +lime, in order to make it combine with the acid, we shall fail in our +attempt, because the soda and the sulphuric acid attract each other by a +force which is superior, and (by way of supposition) is represented by +the number 8; while the lime tends to unite with this acid by an +affinity equal only to the number 6. It is plain, therefore, that the +sulphat of soda will not be decomposed, since a force equal to 8 cannot +be overcome by a force equal only to 6. + +CAROLINE. + +So far, this appears very clear. + +MRS. B. + +If, on the other hand, we endeavour to decompose this salt by nitric +acid, which tends to combine with soda, we shall be equally +unsuccessful, as nitric acid tends to unite with the alkali by a force +equal only to 7. + +In neither of these cases of simple elective attraction, therefore, can +we accomplish our purpose. But let us previously combine together the +lime and nitric acid, so as to form a nitrat of lime, a compound salt, +the constituents of which are united by a power equal to 4. If then we +present this compound to the sulphat of soda, a decomposition will +ensue, because the sum of the forces which tend to preserve the two +salts in their actual state is not equal to that of the forces which +tend to decompose them, and to form new combinations. The nitric acid, +therefore, will combine with the soda, and the sulphuric acid with the +lime. + +CAROLINE. + +I understand you now very well. This double effect takes place because +the numbers 8 and 4, which represent the degrees of attraction of the +constituents of the two original salts, make a sum less than the numbers +7 and 6, which represent the degrees of attraction of the two new +compounds that will in consequence be formed. + +MRS. B. + +Precisely so. + +CAROLINE. + +But what is the meaning of _quiescent_ and _divellent_ forces, which are +written in the diagram? + +MRS. B. + +Quiescent forces are those which tend to preserve compounds in a state +of rest, or such as they actually are: divellent forces, those which +tend to destroy that state of combination, and to form new compounds. + +These are the principal circumstances relative to the doctrine of +chemical attractions, which have been laid down as rules by modern +chemists; a few others might be mentioned respecting the same theory, +but of less importance, and such as would take us too far from our plan. +I should, however, not omit to mention that Mr. Berthollet, a celebrated +French chemist, has questioned the uniform operation of elective +attraction, and has advanced the opinion, that, in chemical +combinations, the changes which take place depend not only upon the +affinities, but also, in some degree, on the respective quantities of +the substances concerned, on the heat applied during the process, and +some other circumstances. + +CAROLINE. + +In that case, I suppose, there would hardly be two compounds exactly +similar, though composed of the same materials? + +MRS. B. + +On the contrary, it is found that a remarkable uniformity prevails, as +to proportions, between the ingredients of bodies of similar +composition. Thus water, as you may recollect to have seen in a former +conversation, is composed of two volumes of hydrogen gas to one of +oxygen, and this is always found to be precisely the proportion of its +constituents, from whatever source the water be derived. The same +uniformity prevails with regard to the various salts; the acid and +alkali, in each kind of salt, being always found to combine in the same +proportions. Sometimes, it is true, the same acid, and the same alkali, +are capable of making two distinct kinds of salts; but in all these +cases it is found that one of the salts contains just twice, or in some +instances, thrice as much acid, or alkali, as the other. + +EMILY. + +If the proportions in which bodies combine are so constant and so well +defined, how can Mr. Berthollet’s remark be reconciled with this uniform +system of combination? + +MRS. B. + +Great as that philosopher’s authority is in chemistry, it is now +generally supposed that his doubts on this subject were in a great +degree groundless, and that the exceptions he has observed in the laws +of definite proportions, have been only apparent, and may be accounted +for consistently with those laws. + +CAROLINE. + +Pray, Mrs. B., can you decompose a salt by means of electricity, in the +same way as we decompose water? + +MRS. B. + +Undoubtedly; and I am glad this question occurred to you, because it +gives me an opportunity of showing you some very interesting experiments +on the subject. + +If we dissolve a quantity, however small, of any salt in a glass of +water, and if we plunge into it the extremities of the wires which +proceed from the two ends of the Voltaic battery, the salt will be +gradually decomposed, the acid being attracted by the positive, and the +alkali by the negative wire. + +EMILY. + +But how can you render that decomposition perceptible? + +MRS. B. + +By placing in contact with the extremities of each wire, in the +solution, pieces of paper stained with certain vegetable colours, which +are altered by the contact of an acid or an alkali. Thus this blue +vegetable preparation called litmus becomes red when touched by an acid; +and the juice of violets becomes green by the contact of an alkali. + +But the experiment can be made in a much more distinct manner, by +receiving the extremities of the wires into two different vessels, so +that the alkali shall appear in one vessel and the acid in the other. + +CAROLINE. + +But then the Voltaic circle will not be completed; how can any effect be +produced? + +MRS. B. + +You are right; I ought to have added that the two vessels must be +connected together by some interposed substance capable of conducting +electricity. A piece of moistened cotton-wick answers this purpose very +well. You see that the cotton (PLATE XIII. fig. 2. c.) has one end +immersed in one glass and the other end in the other, so as to establish +a communication between any fluids contained in them. We shall now put +into each of the glasses a little glauber salt, or sulphat of soda, +(which consists of an acid and an alkali,) and then we shall fill the +glasses with water, which will dissolve the salt. Let us now connect the +glasses by means of the wires (e, d,) with the two ends of the battery, +thus . . . . + + [Illustration: Plate XIII. Vol. II. page 16. + + Fig. 1. Voltaic Battery of improved construction with the Plates + out of the Cells. + + Fig. 2. 3 & 4. Instances of Chemical decomposition by the Voltaic + Battery.] + +CAROLINE. + +The wires are already giving out small bubbles; is this owing to the +decomposition of the salt? + +MRS. B. + +No; these are bubbles produced by the decomposition of the water, as you +saw in a former experiment. In order to render the separation of the +acid from the alkali visible, I pour into the glass (a), which is +connected with the positive wire, a few drops of a solution of litmus, +which the least quantity of acid turns red; and into the other +glass (b), which is connected with the negative wire, I pour a few drops +of the juice of violets . . . . + +EMILY. + +The blue solution is already turning red all round the wire. + +CAROLINE. + +And the violet solution is beginning to turn green. This is indeed very +singular! + +MRS. B. + +You will be still more astonished when we vary the experiment in this +manner:-- These three glasses (fig. 3. f, g, h,) are, as in the former +instance, connected together by wetted cotton, but the middle one alone +contains a saline solution, the two others containing only distilled +water, coloured as before by vegetable infusions. Yet, on making the +connection with the battery, the alkali will appear in the negative +glass (h), and the acid in the positive glass (f), though neither of +them contained any saline matter. + +EMILY. + +So that the acid and alkali must be conveyed right and left from the +central glass, into the other glasses, by means of the connecting +moistened cotton? + +MRS. B. + +Exactly so; and you may render the experiment still more striking, by +putting into the central glass (k, fig. 3.) an alkaline solution, the +glauber salt being placed into the negative glass (l), and the positive +glass (i) containing only water. The acid will be attracted by the +positive wire (m), and will actually appear in the vessel (i), after +passing through the alkaline solution (k), without combining with it, +although, you know, acids and alkalies are so much disposed to combine. +--But this conversation has already much exceeded our usual limits, and +we cannot enlarge more upon this interesting subject at present. + + + + +CONVERSATION XIV. + +ON ALKALIES. + + +MRS. B. + +Having now given you some idea of the laws by which chemical attractions +are governed, we may proceed to the examination of bodies which are +formed in consequence of these attractions. + +The first class of compounds that present themselves to our notice, in +our gradual ascent to the most complicated combinations, are bodies +composed of only two principles. The sulphurets, phosphurets, carburets, +&c. are of this description; but the most numerous and important of +these compounds are the combinations of oxygen with the various simple +substances with which it has a tendency to unite. Of these you have +already acquired some knowledge, but it will be necessary to enter into +further particulars respecting the nature and properties of those most +deserving our notice. Of this class are the ALKALIES and the EARTHS, +which we shall successively examine. + +We shall first take a view of the alkalies, of which there are three, +viz. POTASH, SODA, and AMMONIA. The two first are called _fixed +alkalies_, because they exist in a solid form at the temperature of the +atmosphere, and require a great heat to be volatilised. They consist, as +you already know, of metallic bases combined with oxygen. In potash, the +proportions are about eighty-six parts of potassium to fourteen of +oxygen; and in soda, seventy-seven parts of sodium to twenty-three of +oxygen. The third alkali, ammonia, has been distinguished by the name of +_volatile alkali_, because its natural form is that of gas. Its +composition is of a more complicated nature, of which we shall speak +hereafter. + +Some of the earths bear so strong a resemblance in their properties to +the alkalies, that it is difficult to know under which head to place +them. The celebrated French chemist, Fourcroy, has classed two of them +(barytes and strontites) with the alkalies; but as lime and magnesia +have almost an equal title to that rank, I think it better not to +separate them, and therefore have adopted the common method of classing +them with the earths, and of distinguishing them by the name of +_alkaline earths_. + +The general properties of alkalies are, an acrid burning taste, +a pungent smell, and a caustic action on the skin and flesh. + +CAROLINE. + +I wonder they should be caustic, Mrs. B., since they contain so little +oxygen. + +MRS. B. + +Whatever substance has an affinity for any one of the constituents of +animal matter, sufficiently powerful to decompose it, is entitled to the +appellation of caustic. The alkalies, in their pure state, have a very +strong attraction for water, for hydrogen, and for carbon, which, you +know, are the constituent principles of oil, and it is chiefly by +absorbing these substances from animal matter that they effect its +decomposition; for, when diluted with a sufficient quantity of water, or +combined with any oily substance, they lose their causticity. + +But, to return to the general properties of alkalies--they change, as we +have already seen, the colour of syrup of violets, and other blue +vegetable infusions, to green; and have, in general, a very great +tendency to unite with acids, although the respective qualities of these +two classes of bodies form a remarkable contrast. + +We shall examine the result of the combination of acids and alkalies +more particularly hereafter. It will be sufficient at present to inform +you, that whenever acids are brought in contact with alkalies, or +alkaline earths, they unite with a remarkable eagerness, and form +compounds perfectly different from either of their constituents; these +bodies are called _neutral_ or _compound salts_. + +The dry white powder which you see in this phial is pure caustic POTASH; +it is very difficult to preserve it in this state, as it attracts, with +extreme avidity, the moisture from the atmosphere, and if the air were +not perfectly excluded, it would, in a very short time, be actually +melted. + +EMILY. + +It is then, I suppose, always found in a liquid state? + +MRS. B. + +No; it exists in nature in a great variety of forms and combinations, +but is never found in its pure separate state; it is combined with +carbonic acid, with which it exists in every part of the vegetable +kingdom, and is most commonly obtained from the ashes of vegetables, +which are the residue that remains after all the other parts have been +volatilised by combustion. + +CAROLINE. + +But you once said, that after all the volatile parts of a vegetable were +evaporated, the substance that remained was charcoal? + +MRS. B. + +I am surprised that you should still confound the processes of +volatilisation and combustion. In order to procure charcoal, we +evaporate such parts as can be reduced to vapour by the operation of +heat alone; but when we _burn_ the vegetable, we burn the carbon also, +and convert it into carbonic acid gas. + +CAROLINE. + +That is true; I hope I shall make no more mistakes in my favourite +theory of combustion. + +MRS. B. + +Potash derives its name from the _pots_ in which the vegetables, from +which it was obtained, used formerly to be burnt; the alkali remained +mixed with the ashes at the bottom, and was thence called potash. + +EMILY. + +The ashes of a wood-fire, then, are potash, since they are vegetable +ashes? + +MRS. B. + +They always contain more or less potash, but are very far from +consisting of that substance alone, as they are a mixture of various +earths and salts which remain after the combustion of vegetables, and +from which it is not easy to separate the alkali in its pure form. The +process by which potash is obtained, even in the imperfect state in +which it is used in the arts, is much more complicated than simple +combustion. It was once deemed impossible to separate it entirely from +all foreign substances, and it is only in chemical laboratories that it +is to be met with in the state of purity in which you find it in this +phial. Wood-ashes are, however, valuable for the alkali which they +contain, and are used for some purposes without any further preparation. +Purified in a certain degree, they make what is commonly called +_pearlash_, which is of great efficacy in taking out grease, in washing +linen, &c.; for potash combines readily with oil or fat, with which it +forms a compound well known to you under the name of _soap_. + +CAROLINE. + +Really! Then I should think it would be better to wash all linen with +pearlash than with soap, as, in the latter case, the alkali being +already combined with oil, must be less efficacious in extracting +grease. + +MRS. B. + +Its effect would be too powerful on fine linen, and would injure its +texture; pearlash is therefore only used for that which is of a strong +coarse kind. For the same reason you cannot wash your hands with plain +potash; but, when mixed with oil in the form of soap, it is soft as well +as cleansing, and is therefore much better adapted to the purpose. + +Caustic potash, as we already observed, acts on the skin, and animal +fibre, in virtue of its attraction for water and oil, and converts all +animal matter into a kind of saponaceous jelly. + +EMILY. + +Are vegetables the only source from which potash can be derived? + +MRS. B. + +No: for though far most abundant in vegetables, it is by no means +confined to that class of bodies, being found also on the surface of the +earth, mixed with various minerals, especially with earths and stones, +whence it is supposed to be conveyed into vegetables by the roots of the +plant. It is also met with, though in very small quantities, in some +animal substances. The most common state of potash is that of +_carbonat_; I suppose you understand what that is? + +EMILY. + +I believe so; though I do not recollect that you ever mentioned the word +before. If I am not mistaken, it must be a compound salt, formed by the +union of carbonic acid with potash. + +MRS. B. + +Very true; you see how admirably the nomenclature of modern chemistry is +adapted to assist the memory; when you hear the name of a compound, you +necessarily learn what are its constituent parts; and when you are +acquainted with these constituents, you can immediately name the +compound which they form. + +CAROLINE. + +Pray, how were bodies arranged and distinguished before this +nomenclature was introduced? + +MRS. B. + +Chemistry was then a much more difficult study; for every substance had +an arbitrary name, which it derived either from the person who +discovered it, as _Glauber’s salts_ for instance; or from some other +circumstance relative to it, though quite unconnected with its real +nature, as potash. + +These names have been retained for some of the simple bodies; for as +this class is not numerous, and therefore can easily be remembered, it +has not been thought necessary to change them. + +EMILY. + +Yet I think it would have rendered the new nomenclature more complete to +have methodised the names of the elementary, as well as of the compound +bodies, though it could not have been done in the same manner. But the +names of the simple substances might have indicated their nature, or, at +least, some of their principal properties; and if, like the acids and +compound salts, all the simple bodies had a similar termination, they +would have been immediately known as such. So complete and regular a +nomenclature would, I think, have given a clearer and more comprehensive +view of chemistry than the present, which is a medley of the old and new +terms. + +MRS. B. + +But you are not aware of the difficulty of introducing into science an +entire set of new terms; it obliges all the teachers and professors to +go to school again, and if some of the old names, that are least +exceptionable, were not left as an introduction to the new ones, few +people would have had industry and perseverance enough to submit to the +study of a completely new language; and the inferior classes of artists, +who can only act from habit and routine, would, at least for a time, +have felt material inconvenience from a total change of their habitual +terms. From these considerations, Lavoisier and his colleagues, who +invented the new nomenclature, thought it most prudent to leave a few +links of the old chain, in order to connect it with the new one. +Besides, you may easily conceive the inconvenience which might arise +from giving a regular nomenclature to substances, the simple nature of +which is always uncertain; for the new names might, perhaps, have proved +to have been founded in error. And, indeed, cautious as the inventors of +the modern chemical language have been, it has already been found +necessary to modify it in many respects. In those few cases, however, in +which new terms have been adopted to designate simple bodies, these +names have been so contrived as to indicate one of the chief properties +of the body in question; this is the case with oxygen, which, as I +explained to you, signifies generator of acids; and hydrogen generator +of water. If all the elementary bodies had a similar termination, as you +propose, it would be necessary to change the name of any that might +hereafter be found of a compound nature, which would be very +inconvenient in this age of discovery. + +But to return to the alkalies. --We shall now try to melt some of this +caustic potash in a little water, as a circumstance occurs during its +solution very worthy of observation. --Do you feel the heat that is +produced? + +CAROLINE. + +Yes, I do; but is not this directly contrary to our theory of latent +heat, according to which heat is disengaged when fluids become solid, +and cold produced when solids are melted? + +MRS. B. + +The latter is really the case in all solutions; and if the solution of +caustic alkalies seems to make an exception to the rule, it does not, +I believe, form any solid objection to the theory. The matter may be +explained thus: When water first comes in contact with the potash, it +produces an effect similar to the slaking of lime, that is, the water is +solidified in combining with the potash, and thus loses its latent heat; +this is the heat that you now feel, and which is, therefore, produced +not by the melting of the solid, but by the solidification of the fluid. +But when there is more water than the potash can absorb and solidify, +the latter then yields to the solvent power of the water; and if we do +not perceive the cold produced by its melting, it is because it is +counterbalanced by the heat previously disengaged.* + +A very remarkable property of potash is the formation of glass by its +fusion with siliceous earth. You are not yet acquainted with this last +substance, further than its being in the list of simple bodies. It is +sufficient, for the present, that you should know that sand and flint +are chiefly composed of it; alone, it is infusible, but mixed with +potash, it melts when exposed to the heat of a furnace, combines with +the alkali, and runs into glass. + + [Footnote *: This defence of the general theory, however + plausible, is liable to some obvious objections. The phenomenon + might perhaps be better accounted for by supposing that a solution + of alkali in water has less capacity for heat than either water or + alkali in their separate state.] + +CAROLINE. + +Who would ever have supposed that the same substance which converts +transparent oil into such an opake body as soap, should transform that +opake substance, sand, into transparent glass! + +MRS. B. + +The transparency, or opacity of bodies, does not, I conceive, depend so +much upon their intimate nature, as upon the arrangement of their +particles: we cannot have a more striking instance of this, than is +afforded by the different states of carbon, which, though it commonly +appears in the form of a black opake body, sometimes assumes the most +dazzling transparent form in nature, that of diamond, which, you +recollect, is carbon, and which, in all probability, derives its +beautiful transparency from the peculiar arrangement of its particles +during their crystallisation. + +EMILY. + +I never should have supposed that the formation of glass was so simple a +process as you describe it. + +MRS. B. + +It is by no means an easy operation to make perfect glass; for if the +sand, or flint, from which the siliceous earth is obtained, be mixed +with any metallic particles, or other substance, which cannot be +vitrified, the glass will be discoloured, or defaced, by opake specks. + +CAROLINE. + +That, I suppose, is the reason why objects so often appear irregular and +shapeless through a common glass-window. + +MRS. B. + +This species of imperfection proceeds, I believe, from another cause. It +is extremely difficult to prevent the lower part of the vessels, in +which the materials of glass are fused, from containing a more dense +vitreous matter than the upper, on account of the heavier ingredients +falling to the bottom. When this happens, it occasions the appearance of +veins or waves in the glass, from the difference of density in its +several parts, which produces an irregular refraction of the rays of +light that pass through it. + +Another species of imperfection sometimes arises from the fusion not +being continued for a length of time sufficient to combine the two +ingredients completely, or from the due proportion of potash and silex +(which are as two to one) not being carefully observed; the glass, in +those cases, will be liable to alteration from the action of the air, of +salts, and especially of acids, which will effect its decomposition by +combining with the potash, and forming compound salts. + +EMILY. + +What an extremely useful substance potash is! + +MRS. B. + +Besides the great importance of potash in the manufactures of glass and +soap, it is of very considerable utility in many of the other arts, and +in its combinations with several acids, particularly the nitric, with +which it forms saltpetre. + +CAROLINE. + +Then saltpetre must be a _nitrat of potash_? But we are not yet +acquainted with the nitric acid? + +MRS. B. + +We shall therefore defer entering into the particulars of these +combinations till we come to a general review of the compound salts. In +order to avoid confusion, it will be better at present to confine +ourselves to the alkalies. + +EMILY. + +Cannot you show us the change of colour which you said the alkalies +produced on blue vegetable infusions? + +MRS. B. + +Yes; very easily. I shall dip a piece of white paper into this syrup of +violets, which, you see, is of a deep blue, and dyes the paper of the +same colour. --As soon as it is dry, we shall dip it into a solution of +potash, which, though itself colourless, will turn the paper green-- + +CAROLINE. + +So it has, indeed! And do the other alkalies produce a similar effect? + +MRS. B. + +Exactly the same. --We may now proceed to SODA, which, however +important, will detain us but a very short time; as in all its general +properties it very strongly resembles potash; indeed, so great is their +similitude, that they have been long confounded, and they can now +scarcely be distinguished, except by the difference of the salts which +they form with acids. + +The great source of this alkali is the sea, where, combined with a +peculiar acid, it forms the salt with which the waters of the ocean are +so strongly impregnated. + +EMILY. + +Is not that the common table salt? + +MRS. B. + +The very same; but again we must postpone entering into the particulars +of this interesting combination, till we treat of the neutral salts. +Soda may be obtained from common salt; but the easiest and most usual +method of procuring it is by the combustion of marine plants, an +operation perfectly analogous to that by which potash is obtained from +vegetables. + +EMILY. + +From what does soda derive its name? + +MRS. B. + +From a plant called by us _soda_, and by the Arabs _kali_, which affords +it in great abundance. Kali has, indeed, given its name to the alkalies +in general. + +CAROLINE. + +Does soda form glass and soap in the same manner as potash? + +MRS. B. + +Yes, it does; it is of equal importance in the arts, and is even +preferred to potash for some purposes; but you will not be able to +distinguish their properties till we examine the compound salts which +they form with acids; we must therefore leave soda for the present, and +proceed to AMMONIA, or the VOLATILE ALKALI. + +EMILY. + +I long to hear something of this alkali; is it not of the same nature as +hartshorn? + +MRS. B. + +Yes, it is, as you will see by-and-bye. This alkali is seldom found in +nature in its pure state; it is most commonly extracted from a compound +salt, called _sal ammoniac_, which was formerly imported from _Ammonia_, +a region of Libya, from which both these salts and the alkali derive +their names. The crystals contained in this bottle are specimens of this +salt, which consists of a combination of ammonia and muriatic acid. + +CAROLINE. + +Then it should be called _muriat of ammonia_; for though I am ignorant +what muriatic acid is, yet I know that its combination with ammonia +cannot but be so called; and I am surprised to see sal ammoniac +inscribed on the label. + +MRS. B. + +That is the name by which it has been so long known, that the modern +chemists have not yet succeeded in banishing it altogether; and it is +still sold under that name by druggists, though by scientific chemists +it is more properly called muriat of ammonia. + +CAROLINE. + +Both the popular and the common name should be inscribed on labels--this +would soon introduce the new nomenclature. + +EMILY. + +By what means can the ammonia be separated from the muriatic acid? + +MRS. B. + +By chemical attractions; but this operation is too complicated for you +to understand, till you are better acquainted with the agency of +affinities. + +EMILY. + +And when extracted from the salt, what kind of substance is ammonia? + +MRS. B. + +Its natural form, at the temperature of the atmosphere, when free from +combination, is that of gas; and in this state it is called _ammoniacal +gas_. But it mixes very readily with water, and can be thus obtained in +a liquid form. + +CAROLINE. + +You said that ammonia was more complicated in its composition than the +other alkalies; pray of what principles does it consist? + +MRS. B. + +It was discovered a few years since, by Berthollet, a celebrated French +chemist, that it consisted of about one part of hydrogen to four parts +of nitrogen. Having heated ammoniacal gas under a receiver, by causing +the electrical spark to pass repeatedly through it, he found that it +increased considerably in bulk, lost all its alkaline properties, and +was actually converted into hydrogen and nitrogen gases; and from the +latest and most accurate experiments, the proportions appear to be, one +volume of nitrogen gas to three of hydrogen gas. + +CAROLINE. + +Ammonia, therefore, has not, like the two other alkalies, a metallic +basis? + +MRS. B. + +It is believed it has, though it is extremely difficult to reconcile +that idea with what I have just stated of its chemical nature. But the +fact is, that although this supposed metallic basis of ammonia has never +been obtained distinct and separate, yet both Professor Berzelius, of +Stockholm, and Sir H. Davy, have succeeded in forming a combination of +mercury with the basis of ammonia, which has so much the appearance of +an amalgam, that it strongly corroborates the idea of ammonia having a +metallic basis.* But these theoretical points are full of difficulties +and doubts, and it would be useless to dwell any longer upon them. + +Let us therefore return to the properties of volatile alkali. Ammoniacal +gas is considerably lighter than oxygen gas, and only about half the +weight of atmospherical air. It possesses most of the properties of the +fixed alkalies; but cannot be of so much use in the arts on account of +its volatile nature. It is, therefore, never employed in the manufacture +of glass, but it forms soap with oils equally as well as potash and +soda; it resembles them likewise in its strong attraction for water; for +which reason it can be collected in a receiver over mercury only. + + [Footnote *: This amalgam is easily obtained, by placing a globule + of mercury upon a piece of muriat, or carbonat of ammonia, and + electrifying this globule by the Voltaic battery. The globule + instantly begins to expand to three or four times its former size, + and becomes much less fluid, though without losing its metallic + lustre, a change which is ascribed to the metallic basis of + ammonia uniting with the mercury. This is an extremely curious + experiment.] + +CAROLINE. + +I do not understand this? + +MRS. B. + +Do you recollect the method which we used to collect gases in a +glass-receiver over water? + +CAROLINE. + +Perfectly. + +MRS. B. + +Ammoniacal gas has so strong a tendency to unite with water, that, +instead of passing through that fluid, it would be instantaneously +absorbed by it. We can therefore neither use water for that purpose, nor +any other liquid of which water is a component part; so that, in order +to collect this gas, we are obliged to have recourse to mercury, +(a liquid which has no action upon it,) and a mercurial bath is used +instead of a water bath, such as we employed on former occasions. Water +impregnated with this gas is nothing more than the fluid which you +mentioned at the beginning of the conversation--hartshorn; it is the +ammoniacal gas escaping from the water which gives it so powerful a +smell. + +EMILY. + +But there is no appearance of effervescence in hartshorn. + +MRS. B. + +Because the particles of gas that rise from the water are too subtle and +minute for their effect to be visible. + +Water diminishes in density, by being impregnated with ammoniacal gas; +and this augmentation of bulk increases its capacity for caloric. + +EMILY. + +In making hartshorn, then, or impregnating water with ammonia, heat must +be absorbed, and cold produced? + +MRS. B. + +That effect would take place if it was not counteracted by another +circumstance; the gas is liquefied by incorporating with the water, and +gives out its latent heat. The condensation of the gas more than +counterbalances the expansion of the water; therefore, upon the whole, +heat is produced. --But if you dissolve ammoniacal gas with ice or snow, +cold is produced. --Can you account for that? + +EMILY. + +The gas, in being condensed into a liquid, must give out heat; and, on +the other hand, the snow or ice, in being rarefied into a liquid, must +absorb heat; so that, between the opposite effects, I should have +supposed the original temperature would have been preserved. + +MRS. B. + +But you have forgotten to take into the account the rarefaction of the +water (or melted ice) by the impregnation of the gas; and this is the +cause of the cold which is ultimately produced. + +CAROLINE. + +Is the _sal volatile_ (the smell of which so strongly resembles +hartshorn) likewise a preparation of ammonia? + +MRS. B. + +It is carbonat of ammonia dissolved in water; and which, in its concrete +state, is commonly called salts of hartshorn. Ammonia is caustic, like +the fixed alkalies, as you may judge by the pungent effects of +hartshorn, which cannot be taken internally, nor applied to delicate +external parts, without being plentifully diluted with water. --Oil and +acids are very excellent antidotes for alkaline poisons; can you guess +why? + +CAROLINE. + +Perhaps, because the oil combines with the alkali, and forms soap, and +thus destroys its caustic properties; and the acid converts it into a +compound salt, which, I suppose, is not so pernicious as caustic alkali. + +MRS. B. + +Precisely so. + +Ammoniacal gas, if it be mixed with atmospherical air, and a burning +taper repeatedly plunged into it, will burn with a large flame of a +peculiar yellow colour. + +EMILY. + +But pray tell me, can ammonia be procured from this Lybian salt only? + +MRS. B. + +So far from it, that it is contained in, and may be extracted from, all +animal substances whatever. Hydrogen and nitrogen are two of the chief +constituents of animal matter; it is therefore not surprising that they +should occasionally meet and combine in those proportions that compose +ammonia. But this alkali is more frequently generated by the spontaneous +decomposition of animal substances; the hydrogen and nitrogen gases that +arise from putrefied bodies combine, and form the volatile alkali. + +Muriat of ammonia, instead of being exclusively brought from Lybia, as +it originally was, is now chiefly prepared in Europe, by chemical +processes. Ammonia, although principally extracted from this salt, can +also be produced by a great variety of other substances. The horns of +cattle, especially those of deer, yield it in abundance, and it is from +this circumstance that a solution of ammonia in water has been called +hartshorn. It may likewise be procured from wool, flesh, and bones; in a +word, any animal substance whatever yields it by decomposition. + +We shall now lay aside the alkalies, however important the subject may +be, till we treat of their combination with acids. The next time we meet +we shall examine the earths. + + + + +CONVERSATION XV. + +ON EARTHS. + + +MRS. B. + +The EARTHS, which we are to-day to examine, are nine in number: + + SILEX, + ALUMINE, + BARYTES, + LIME, + MAGNESIA, + STRONTITES, + YTTRIA, + GLUCINA, + ZIRCONIA. + +The last three are of late discovery; their properties are but +imperfectly known; and, as they have not yet been applied to use, it +will be unnecessary to enter into any particulars respecting them; we +shall confine our remarks, therefore, to the first five. They are +composed, as you have already learnt, of a metallic basis combined with +oxygen; and, from this circumstance, are incombustible. + +CAROLINE. + +Yet I have seen turf burnt in the country, and it makes an excellent +fire; the earth becomes red hot, and produces a very great quantity of +heat. + +MRS. B. + +It is not the earth that burns, my dear, but the roots, grass, and other +remnants of vegetables that are intermixed with it. The caloric, which +is produced by the combustion of these substances, makes the earth red +hot, and this being a bad conductor of heat, retains its caloric a long +time; but were you to examine it when cooled, you would find that it had +not absorbed one particle of oxygen, nor suffered any alteration from +the fire. Earth is, however, from the circumstance just mentioned, an +excellent radiator of heat, and owes its utility, when mixed with fuel, +solely to that property. It is in this point of view that Count Rumford +has recommended balls of incombustible substances to be arranged in +fire-places, and mixed with the coals, by which means the caloric +disengaged by the combustion of the latter is more perfectly reflected +into the room, and an expense of fuel is saved. + +EMILY. + +I expected that the list of earths would be much more considerable. When +I think of the great variety of soils, I am astonished that there is not +a greater number of earths to form them. + +MRS. B. + +You might, indeed, almost confine that number to four; for barytes, +strontites, and the others of late discovery, act but so small a part in +this great theatre, that they cannot be reckoned as essential to the +general formation of the globe. And you must not confine your idea of +earths to the formation of soil; for rock, marble, chalk, slate, sand, +flint, and all kinds of stones, from the precious jewels to the +commonest pebbles; in a word, all the immense variety of mineral +products, may be referred to some of these earths, either in a simple +state, or combined the one with the other, or blended with other +ingredients. + +CAROLINE. + +Precious stones composed of earth! That seems very difficult to +conceive. + +EMILY. + +Is it more extraordinary than that the most precious of all jewels, +diamond, should be composed of carbon? But diamond forms an exception, +Mrs. B.; for, though a stone, it is not composed of earth. + +MRS. B. + +I did not specify the exception, as I knew you were so well acquainted +with it. Besides, I would call a diamond a mineral rather than a stone, +as the latter term always implies the presence of some earth. + +CAROLINE. + +I cannot conceive how such coarse materials can be converted into such +beautiful productions. + +MRS. B. + +We are very far from understanding all the secret resources of nature; +but I do not think the spontaneous formation of the crystals, which we +call precious stones, one of the most difficult phenomena to comprehend. + +By the slow and regular work of ages, perhaps of hundreds of ages, these +earths may be gradually dissolved by water, and as gradually deposited +by their solvent in the undisturbed process of crystallisation. The +regular arrangement of their particles, during their reunion in a solid +mass, gives them that brilliancy, transparency, and beauty, for which +they are so much admired; and renders them in appearance so totally +different from their rude and primitive ingredients. + +CAROLINE. + +But how does it happen that they are spontaneously dissolved, and +afterwards crystallised? + +MRS. B. + +The scarcity of many kinds of crystals, as rubies, emeralds, topazes, +&c. shows that their formation is not an operation very easily carried +on in nature. But cannot you imagine that when water, holding in +solution some particles of earth, filters through the crevices of hills +or mountains, and at length dribbles into some cavern, each successive +drop may be slowly evaporated, leaving behind it the particle of earth +which it held in solution? You know that crystallisation is more regular +and perfect, in proportion as the evaporation of the solvent is slow and +uniform; nature, therefore, who knows no limit of time, has, in all +works of this kind, an infinite advantage over any artist who attempts +to imitate such productions. + +EMILY. + +I can now conceive that the arrangement of the particles of earth, +during crystallisation, may be such as to occasion transparency, by +admitting a free passage to the rays of light; but I cannot understand +why crystallised earths should assume such beautiful colours as most of +them do. Sapphire, for instance, is of a celestial blue; ruby, a deep +red; topaz, a brilliant yellow? + +MRS. B. + +Nothing is more simple than to suppose that the arrangement of their +particles is such, as to transmit some of the coloured rays of light, +and to reflect others, in which case the stone must appear of the colour +of the rays which it reflects. But besides, it frequently happens that +the colour of a stone is owing to a mixture of some metallic matter. + +CAROLINE. + +Pray, are the different kinds of precious stones each composed of one +individual earth, or are they formed of a combination of several earths? + +MRS. B. + +A great variety of materials enters into the composition of most of +them; not only several earths, but sometimes salts and metals. The +earths, however, in their simple state, frequently form very beautiful +crystals; and, indeed, it is in that state only that they can be +obtained perfectly pure. + +EMILY. + +Is not the Derbyshire spar produced by the crystallisation of earths, in +the way you have just explained? I have been in some of the +subterraneous caverns where it is found, which are similar to those you +have described. + +MRS. B. + +Yes; but this spar is a very imperfect specimen of crystallisation; it +consists of a variety of ingredients confusedly blended together, as you +may judge by its opacity, and by the various colours and appearances +which it exhibits. + +But, in examining the earths in their most perfect and agreeable form, +we must not lose sight of that state in which they are commonly found, +and which, if less pleasing to the eye, is far more interesting by its +utility. + +All the earths are more or less endowed with alkaline properties; but +there are four, barytes, magnesia, lime, and strontites, which are +called _alkaline earths_, because they possess those qualities in so +great a degree, as to entitle them, in most respects, to the rank of +alkalies. They combine and form compound salts with acids, in the same +way as alkalies; they are, like them, susceptible of a considerable +degree of causticity, and are acted upon in a similar manner by chemical +tests. --The remaining earths, silex and alumine, with one or two others +of late discovery, are in some degree more earthy, that is to say, they +possess more completely the properties common to all the earths, which +are, insipidity, dryness, unalterableness in the fire, infusibility, &c. + +CAROLINE. + +Yet, did you not tell us that silex, or siliceous earth, when mixed with +an alkali, was fusible, and run into glass? + +MRS. B. + +Yes, my dear; but the characteristic properties of earths, which I have +mentioned, are to be considered as belonging to them in a state of +purity only; a state in which they are very seldom to be met with in +nature. --Besides these general properties, each earth has its own +specific characters, by which it is distinguished from any other +substance. --Let us therefore review them separately. + + +SILEX, or SILICA, abounds in flint, sand, sandstone, agate, jasper, &c.; +it forms the basis of many precious stones, and particularly of those +which strike fire with steel. It is rough to the touch, scratches and +wears away metals; it is acted upon by no acid but the fluoric, and is +not soluble in water by any known process; but nature certainly +dissolves it by means with which we are unacquainted, and thus produces +a variety of siliceous crystals, and amongst these _rock crystal_, which +is the purest specimen of this earth. Silex appears to have been +intended by Providence to form the solid basis of the globe, to serve as +a foundation for the original mountains, and give them that hardness and +durability which has enabled them to resist the various revolutions +which the surface of the earth has successively undergone. From these +mountains siliceous rocks have, during the course of ages, been +gradually detached by torrents of water, and brought down in fragments; +these, in the violence and rapidity of their descent, are sometimes +crumbled to sand, and in this state form the beds of rivers and of the +sea, chiefly composed of siliceous materials. Sometimes the fragments +are broken without being pulverised by their fall, and assume the form +of pebbles, which gradually become rounded and polished. + +EMILY. + +Pray what is the true colour of silex, which forms such a variety of +different coloured substances? Sand is brown, flint is nearly black, and +precious stones are of all colours. + +MRS. B. + +Pure silex, such as is found only in the chemist’s laboratory, is +perfectly white, and the various colours which it assumes, in the +different substances you have just mentioned, proceed from the different +ingredients with which it is mixed in them. + +CAROLINE. + +I wonder that silex is not more valuable, since it forms the basis of so +many precious stones. + +MRS. B. + +You must not forget that the value we set upon precious stones depends +in a great measure upon the scarcity with which nature affords them; +for, were those productions either common or perfectly imitable by art, +they would no longer, notwithstanding their beauty, be so highly +esteemed. But the real value of siliceous earth, in many of the most +useful arts, is very extensive. Mixed with clay, it forms the basis of +all the various kinds of earthen ware, from the most common utensils to +the most refined ornaments. + +EMILY. + +And we must recollect its importance in the formation of glass with +potash. + +MRS. B. + +Nor should we omit to mention, likewise, many other important uses of +silex, such as being the chief ingredient of some of the most durable +cements, of mortar, &c. + +I said before, that siliceous earth combined with no acid but the +fluoric; it is for this reason that glass is liable to be attacked by +that acid only, which, from its strong affinity for silex, forces that +substance from its combination with the potash, and thus destroys the +glass. + +We will now hasten to proceed to the other earths, for I am rather +apprehensive of your growing weary of this part of our subject. + +CAROLINE. + +The history of the earths is not quite so entertaining as that of the +simple substances. + +MRS. B. + +Perhaps not; but it is absolutely indispensable that you should know +something of them; for they form the basis of so many interesting and +important compounds, that their total omission would throw great +obscurity on our general outline of chemical science. We shall, however, +review them in as cursory a manner as the subject can admit of. + + +ALUMINE derives its name from a compound salt called _alum_, of which it +forms the basis. + +CAROLINE. + +But it ought to be just the contrary, Mrs. B.; the simple body should +give, instead of taking, its name from the compound. + +MRS. B. + +That is true; but as the compound salt was known long before its basis +was discovered, it was very natural that when the earth was at length +separated from the acid, it should derive its name from the compound +from which it was obtained. However, to remove your scruples, we will +call the salt according to the new nomenclature, _sulphat of alumine_. +From this combination, alumine may be obtained in its pure state; it is +then soft to the touch, makes a paste with water, and hardens in the +fire. In nature, it is found chiefly in clay, which contains a +considerable proportion of this earth; it is very abundant in fuller’s +earth, slate, and a variety of other mineral productions. There is +indeed scarcely any mineral substance more useful to mankind than +alumine. In the state of clay, it forms large strata of the earth, gives +consistency to the soil of valleys, and of all low and damp spots, such +as swamps and marshes. The beds of lakes, ponds, and springs, are almost +entirely of clay; instead of allowing of the filtration of water, as +sand does, it forms an impenetrable bottom, and by this means water is +accumulated in the caverns of the earth, producing those reservoirs +whence springs issue, and spout out at the surface. + +EMILY. + +I always thought that these subterraneous reservoirs of water were +bedded by some hard stone, or rock, which the water could not penetrate. + +MRS. B. + +That is not the case; for in the course of time water would penetrate, +or wear away silex, or any other kind of stone, while it is effectually +stopped by clay, or alumine. + +The solid compact soils, such as are fit for corn, owe their consistence +in a great measure to alumine; this earth is therefore used to improve +sandy or chalky soils, which do not retain a sufficient quantity of +water for the purpose of vegetation. + +Alumine is the most essential ingredient in all potteries. It enters +into the composition of brick, as well as that of the finest porcelain; +the addition of silex and water hardens it, renders it susceptible of a +degree of vitrification, and makes it perfectly fit for its various +purposes. + +CAROLINE. + +I can scarcely conceive that brick and china should be made of the same +materials. + +MRS. B. + +Brick consists almost entirely of baked clay; but a certain proportion +of silex is essential to the formation of earthen or stone ware. In +common potteries sand is used for that purpose; a more pure silex is, +I believe, necessary for the composition of porcelain, as well as a +finer kind of clay; and these materials are, no doubt, more carefully +prepared, and curiously wrought, in the one case than in the other. +Porcelain owes its beautiful semitransparency to a commencement of +vitrification. + +EMILY. + +But the commonest earthen-ware, though not transparent, is covered with +a kind of glazing. + +MRS. B. + +That precaution is equally necessary for use as for beauty, as the ware +would be liable to be spoiled and corroded by a variety of substances, +if not covered with a coating of this kind. In porcelain it consists of +enamel, which is a fine white opake glass, formed of metallic oxyds, +sand, salts, and such other materials as are susceptible of +vitrification. The glazing of common earthen-ware is made chiefly of +oxyd of lead, or sometimes merely of salt, which, when thinly spread +over earthen vessels, will, at a certain heat, run into opake glass. + +CAROLINE. + +And of what nature are the colours which are used for painting +porcelain? + +MRS. B. + +They are all composed of metallic oxyds, so that these colours, instead +of receiving injury from the application of fire, are strengthened and +developed by its action, which causes them to undergo different degrees +of oxydation. + +Alumine and silex are not only often combined by art, but they have in +nature a very strong tendency to unite, and are found combined, in +different proportions, in various gems and other minerals. Indeed, many +of the precious stones, such as ruby, oriental sapphire, amethyst, &c. +consist chiefly of alumine. + + +We may now proceed to the alkaline earths, I shall say but a few words +on BARYTES, as it is hardly ever used, except in chemical laboratories. +It is remarkable for its great weight, and its strong alkaline +properties, such as destroying animal substances, turning green some +blue vegetable colours, and showing a powerful attraction for acids; +this last property it possesses to such a degree, particularly with +regard to the sulphuric acid, that it will always detect its presence in +any substance or combination whatever, by immediately uniting with it, +and forming a sulphat of barytes. This renders it a very valuable +chemical test. It is found pretty abundantly in nature in the state of +carbonat, from which the pure earth can be easily separated. + + +The next earth we have to consider is LIME. This is a substance of too +great and general importance to be passed over so slightly as the last. + +Lime is strongly alkaline. In nature it is not met with in its simple +state, as its affinity for water and carbonic acid is so great, that it +is always found combined with these substances, with which it forms the +common lime-stone; but it is separated in the kiln from these +ingredients, which are volatilised whenever a sufficient degree of heat +is applied. + +EMILY. + +Pure lime, then, is nothing but lime-stone, which has been deprived, in +the kiln, of its water and carbonic acid? + +MRS. B. + +Precisely: in this state it is called _quick-lime_, and it is so +caustic, that it is capable of decomposing the dead bodies of animals +very rapidly, without their undergoing the process of putrefaction. +--I have here some quick lime, which is kept carefully corked up in a +bottle to prevent the access of air; for were it at all exposed to the +atmosphere, it would absorb both moisture and carbonic acid gas from it, +and be soon slaked. Here is also some lime-stone--we shall pour a little +water on each, and observe the effects that result from it. + +CAROLINE. + +How the quick-lime hisses! It is become excessively hot! --It swells, +and now it bursts and crumbles to powder, while the water appears to +produce no kind of alteration on the lime-stone. + +MRS. B. + +Because the lime-stone is already saturated with water, whilst the +quick-lime, which has been deprived of it in the kiln, combines with it +with very great avidity, and produces this prodigious disengagement of +heat, the cause of which I formerly explained to you; do you +recollect it? + +EMILY. + +Yes; you said that the heat did not proceed from the lime, but from the +water which was _solidified_, and thus parted with its heat of +liquidity. + +MRS. B. + +Very well. If we continue to add successive quantities of water to the +lime after being slaked and crumbled as you see, it will then gradually +be diffused in the water, till it will at length be dissolved in it, and +entirely disappear; but for this purpose it requires no less than 700 +times its weight of water. This solution is called _lime-water_. + +CAROLINE. + +How very small, then, is the proportion of lime dissolved! + +MRS. B. + +Barytes is still of more difficult solution; it dissolves only in 900 +times its weight of water: but it is much more soluble in the state of +crystals. The liquid contained in this bottle is lime-water; it is often +used as a medicine, chiefly, I believe, for the purpose of combining +with, and neutralising, the superabundant acid which it meets with in +the stomach. + +EMILY. + +I am surprised that it is so perfectly clear; it does not at all partake +of the whiteness of the lime. + +MRS. B. + +Have you forgotten that, in solutions, the solid body is so minutely +subdivided by the fluid as to become invisible, and therefore will not +in the least degree impair the transparency of the solvent? + +I said that the attraction of lime for carbonic acid was so strong, that +it would absorb it from the atmosphere. We may see this effect by +exposing a glass of lime-water to the air; the lime will then separate +from the water, combine with the carbonic acid, and re-appear on the +surface in the form of a white film, which is carbonat of lime, commonly +called _chalk_. + +CAROLINE. + +Chalk is, then, a compound salt! I never should have supposed that those +immense beds of chalk, that we see in many parts of the country, were a +salt. --Now, the white film begins to appear on the surface of the +water; but it is far from resembling hard solid chalk. + +MRS. B. + +That is owing to its state of extreme division; in a little time it will +collect into a more compact mass, and subside at the bottom of the +glass. + +If you breathe into lime-water, the carbonic acid, which is mixed with +the air that you expire, will produce the same effect. It is an +experiment very easily made; --I shall pour some lime-water into this +glass tube, and, by breathing repeatedly into it, you will soon perceive +a precipitation of chalk-- + +EMILY. + +I see already a small white cloud formed. + +MRS. B. + +It is composed of minute particles of chalk; at present it floats in the +water, but it will soon subside. + +Carbonat of lime, or chalk, you see, is insoluble in water, since the +lime which was dissolved re-appears when converted into chalk; but you +must take notice of a very singular circumstance, which is, that chalk +is soluble in water impregnated with carbonic acid. + +CAROLINE. + +It is very curious, indeed, that carbonic acid gas should render lime +soluble in one instance, and insoluble in the other! + +MRS. B. + +I have here a bottle of Seltzer water, which, you know, is strongly +impregnated with carbonic acid:-- let us pour a little of it into a +glass of lime-water. You see that it immediately forms a precipitation +of carbonat of lime? + +EMILY. + +Yes, a white cloud appears. + +MRS. B. + +I shall now pour an additional quantity of the Seltzer water into the +lime-water-- + +EMILY. + +How singular! The cloud is re-dissolved, and the liquid is again +transparent. + +MRS. B. + +All the mystery depends upon this circumstance, that carbonat of lime is +soluble in carbonic acid, whilst it is insoluble in water; the first +quantity of carbonic acid, therefore, which I introduce into the +lime-water, was employed in forming the carbonat of lime, which remained +visible, until an additional quantity of carbonic acid dissolved it. +Thus, you see, when the lime and carbonic acid are in proper proportions +to form chalk, the white cloud appears, but when the acid predominates, +the chalk is no sooner formed than it is dissolved. + +CAROLINE. + +That is now the case; but let us try whether a further addition of +lime-water will again precipitate the chalk. + +EMILY. + +It does, indeed! The cloud re-appears, because, I suppose, there is now +no more of the carbonic acid than is necessary to form chalk; and, in +order to dissolve the chalk, a superabundance of acid is required. + +MRS. B. + +We have, I think, carried this experiment far enough; every repetition +would but exhibit the same appearances. + +Lime combines with most of the acids, to which the carbonic (as being +the weakest) readily yields it; but these combinations we shall have an +opportunity of noticing more particularly hereafter. It unites with +phosphorus, and with sulphur, in their simple state; in short, of all +the earths, lime is that which nature employs most frequently, and most +abundantly, in its innumerable combinations. It is the basis of all +calcareous earths and stones; we find it likewise in the animal and the +vegetable creations. + +EMILY. + +And in the arts is not lime of very great utility? + +MRS. B. + +Scarcely any substance more so; you know that it is a most essential +requisite in building, as it constitutes the basis of all cements, such +as mortar, stucco, plaister, &c. + +Lime is also of infinite importance in agriculture; it lightens and +warms soils that are too cold, and compact, in consequence of too great +a proportion of clay. --But it would be endless to enumerate the various +purposes for which it is employed; and you know enough of it to form +some idea of its importance; we shall, therefore, now proceed to the +third alkaline earth, MAGNESIA. + +CAROLINE. + +I am already pretty well acquainted with that earth; it is a medicine. + +MRS. B. + +It is in the state of carbonat that magnesia is usually employed +medicinally; it then differs but little in appearance from its simple +form, which is that of a very fine light white powder. It dissolves in +2000 times its weight of water, but forms with acids extremely soluble +salts. It has not so great an attraction for acids as lime, and +consequently yields them to the latter. It is found in a great variety +of mineral combinations, such as slate, mica, amianthus, and more +particularly in a certain lime stone, which has lately been discovered +by Mr. Tennant to contain it in very great quantities. It does not +attract and solidify water, like lime: but when mixed with water and +exposed to the atmosphere, it slowly absorbs carbonic acid from the +latter, and thus loses its causticity. Its chief use in medicine is, +like that of lime, derived from its readiness to combine with, and +neutralise, the acid which it meets with in the stomach. + +EMILY. + +Yet, you said that it was taken in the state of carbonat, in which case +it has already combined with an acid? + +MRS. B. + +Yes; but the carbonic is the last of all the acids in the order of +affinities; it will therefore yield the magnesia to any of the others. +It is, however, frequently taken in its caustic state as a remedy for +flatulence. Combined with sulphuric acid, magnesia forms another and +more powerful medicine, commonly called _Epsom salt_. + +CAROLINE. + +And properly, _sulphat of magnesia_, I suppose? Pray why was it ever +called Epsom salt? + +MRS. B. + +Because there is a spring in the neighbourhood of Epsom which contains +this salt in great abundance. + +The last alkaline earth which we have to mention is STRONTIAN, or +STRONTITES, discovered by Dr. Hope a few years ago. It so strongly +resembles barytes in its properties, and is so sparingly found in +nature, and of so little use in the arts, that it will not be necessary +to enter into any particulars respecting it. One of the remarkable +characteristic properties of strontites is, that its salts, when +dissolved in spirit of wine, tinge the flame of a deep red, or blood +colour. + + + + +CONVERSATION XVI. + +ON ACIDS. + + +MRS. B. + +We may now proceed to the acids. Of the metallic oxyds, you have already +acquired some general notions. This subject, though highly interesting +in its details, is not of sufficient importance to our concise view of +chemistry, to be particularly treated of; but it is absolutely necessary +that you should be better acquainted with the acids, and likewise with +their combinations with the alkalies, which form the triple compounds +called NEUTRAL SALTS. + +The class of acids is characterised by very distinct properties. They +all change blue vegetable infusions to a red colour: they are all more +or less sour to the taste; and have a general tendency to combine with +the earths, alkalies, and metallic oxyds. + +You have, I believe, a clear idea of the nomenclature by which the base +(or radical) of the acid, and the various degrees of acidification, are +expressed? + +EMILY. + +Yes, I think so; the acid is distinguished by the name of its base, and +its degree of oxydation, that is, the quantity of oxygen it contains, by +the termination of that name in _ous_ or _ic_; thus sulphure_ous_ acid +is that formed by the smallest proportion of oxygen combined with +sulphur; sulphur_ic_ acid that which results from the combination of +sulphur with the greatest quantity of oxygen. + +MRS. B. + +A still greater latitude may, in many cases, be allowed to the +proportions of oxygen than can be combined with acidifiable radicals; +for several of these radicals are susceptible of uniting with a quantity +of oxygen so small as to be insufficient to give them the properties of +acids; in these cases, therefore, they are converted into oxyds. Such is +sulphur, which by exposure to the atmosphere with a degree of heat +inadequate to produce inflammation, absorbs a small proportion of +oxygen, which colours it red or brown. This, therefore, is the first +degree of oxygenation of sulphur; the 2d converts it into sulphur_ous_ +acid; the 3d into the sulphur_ic_ acid; and 4thly, if it was found +capable of combining with a still larger proportion of oxygen, it would +then be termed _super-oxygenated sulphuric acid_. + +EMILY. + +Are these various degrees of oxygenation common to all the acids? + +MRS. B. + +No; they vary much in this respect: some are susceptible of only one +degree of oxygenation; others, of two, or three; there are but very few +that will admit of more. + +CAROLINE. + +The modern nomenclature must be of immense advantage in pointing out so +easily the nature of the acids, and their various degrees of +oxygenation. + +MRS. B. + +Till lately many of the acids had not been decomposed; but analogy +afforded so strong a proof of their compound nature, that I never could +reconcile myself to classing them with the simple bodies, though this +division has been adopted by several chemical writers. At present there +are only the muriatic and the fluoric acids, which have not had their +bases distinctly separated. + +CAROLINE. + +We have heard of a great variety of acids; pray how many are there in +all? + +MRS. B. + +I believe there are reckoned at present thirty-four, and their number is +constantly increasing, as the science improves; but the most important, +and those to which we shall almost entirely confine our attention, are +but few. I shall, however, give you a general view of the whole; and +then we shall more particularly examine those that are the most +essential. + +This class of bodies was formerly divided into mineral, vegetable, and +animal acids, according to the substances from which they were commonly +obtained. + +CAROLINE. + +That, I should think, must have been an excellent arrangement; why was +it altered? + +MRS. B. + +Because in many cases it produced confusion. In which class, for +instance, would you place carbonic acid? + +CAROLINE. + +Now I see the difficulty. I should be at a loss where to place it, as +you have told us that it exists in the animal, vegetable, and mineral +kingdoms. + +EMILY. + +There would be the same objection with respect to phosphoric acid, +which, though obtained chiefly from bones, can also, you said, be found +in small quantities in stones, and likewise in some plants. + +MRS. B. + +You see, therefore, the propriety of changing this mode of +classification. These objections do not exist in the present +nomenclature; for the composition and nature of each individual acid is +in some degree pointed out, instead of the class of bodies from which it +is extracted; and, with regard to the more general division of acids, +they are classed under these three heads: + +First, Acids of known or supposed simple bases, which are formed by the +union of these bases with oxygen. They are the following: + +The _Sulphuric_ + _Carbonic_ + _Nitric_ + _Phosphoric_ + _Arsenical_ Acids, of known and simple bases. + _Tungstenic_ + _Molybdenic_ + _Boracic_ + _Fluoric_ + _Muriatic_ + +This class comprehends the most anciently known and most important +acids. The sulphuric, nitric, and muriatic were formerly, and are still +frequently, called _mineral acids_. + +2dly, Acids that have double or binary radicals, and which consequently +consist of triple combinations. These are the vegetable acids, whose +common radical is a compound of hydrogen and carbon. + +CAROLINE. + +But if the basis of all the vegetable acids be the same, it should form +but one acid; it may indeed combine with different proportions of +oxygen, but the nature of the acid must be the same. + +MRS. B. + +The only difference that exists in the basis of vegetable acids, is the +various proportions of hydrogen and carbon from which they are severally +composed. But this is enough to produce a number of acids apparently +very dissimilar. That they do not, however, differ essentially, is +proved by their susceptibility of being converted into each other, by +the addition or subtraction of a portion of hydrogen or of carbon. The +names of these acids are, + +The _Acetic_ + _Oxalic_ + _Tartarous_ + _Citric_ + _Malic_ Acids, of double bases, being of vegetable origin. + _Gallic_ + _Mucous_ + _Benzoic_ + _Succinic_ + _Camphoric_ + _Suberic_ + +The 3d class of acids consists of those which have triple radicals, and +are therefore of a still more compound nature. This class comprehends +the animal acids, which are, + +The _Lactic_ + _Prussic_ + _Formic_ Acids, of triple bases, or animal acids. + _Bombic_ + _Sebacic_ + _Zoonic_ + _Lithic_ + +I have given you this summary account or enumeration of the acids, as +you may find it more satisfactory to have at once an outline or a +general notion of the extent of the subject; but we shall now confine +ourselves to the first class, which requires our more immediate +attention; and defer the few remarks which we shall have to make on the +others, till we treat of the chemistry of the animal and vegetable +kingdoms. + +The acids of simple and known radicals are all capable of being +decomposed by combustible bodies, to which they yield their oxygen. If, +for instance, I pour a drop of sulphuric acid on this piece of iron, it +will produce a spot of rust, you know what that is? + +CAROLINE. + +Yes; it is an oxyd, formed by the oxygen of the acid combining with the +iron. + +MRS. B. + +In this case you see the sulphur deposits the oxygen by which it was +acidified on the metal. And again, if we pour some acid on a compound +combustible substance, (we shall try it on this piece of wood,) it will +combine with one or more of the constituents of that substance, and +occasion a decomposition. + +EMILY. + +It has changed the colour of the wood to black. How is that? + +MRS. B. + +The oxygen deposited by the acid has burnt it; you know that wood in +burning becomes black before it is reduced to ashes. Whether it derives +the oxygen which burns it from the atmosphere, or from any other source, +the chemical effect on the wood is the same. In the case of real +combustion, wood becomes black, because it is reduced to the state of +charcoal by the evaporation of its other constituents. But can you tell +me the reason why wood turns black when burnt by the application of an +acid? + +CAROLINE. + +First, tell me what are the ingredients of wood? + +MRS. B. + +Hydrogen and carbon are the chief constituents of wood, as of all other +vegetable substances. + +CAROLINE. + +Well, then, I suppose that the oxygen of the acid combines with the +hydrogen of the wood, to form water; and that the carbon of the wood, +remaining alone, appears of its usual black colour. + +MRS. B. + +Very well indeed, my dear; that is certainly the most plausible +explanation. + +EMILY. + +Would not this be a good method of making charcoal? + +MRS. B. + +It would be an extremely expensive, and, I believe, very imperfect +method; for the action of the acid on the wood, and the heat produced by +it, are far from sufficient to deprive the wood of all its evaporable +parts. + +CAROLINE. + +What is the reason that vinegar, lemon, and the acid of fruits, do not +produce this effect on wood? + +MRS. B. + +They are vegetable acids, whose bases are composed of hydrogen and +carbon; the oxygen, therefore, will not be disposed to quit this +radical, where it is already united with hydrogen. The strongest of +these may, perhaps, yield a little of their oxygen to the wood, and +produce a stain upon it; but the carbon will not be sufficiently +uncovered to assume its black colour. Indeed, the several mineral acids +themselves possess this power of charring wood in very different +degrees. + +EMILY. + +Cannot vegetable acids be decomposed, by any combustibles? + +MRS. B. + +No; because their radical is composed of two substances which have a +greater attraction for oxygen than any known body. + +CAROLINE. + +And are those strong acids, which burn and decompose wood, capable of +producing similar effects on the skin and flesh of animals? + +MRS. B. + +Yes; all the mineral acids, and one of them more especially, possess +powerful caustic qualities. They actually corrode and destroy the skin +and flesh; but they do not produce upon these exactly the same +alteration they do on wood, probably because there is a great proportion +of nitrogen and other substances in animal matter, which prevents the +separation of carbon from being so conspicuous. + + + + +CONVERSATION XVII. + +OF THE SULPHURIC AND PHOSPHORIC ACIDS; OR THE COMBINATIONS OF OXYGEN +WITH SULPHUR AND PHOSPHORUS; AND OF THE SULPHATS AND PHOSPHATS. + + +MRS. B. + +In addition to the general survey which we have taken of acids, I think +you will find it interesting to examine individually a few of the most +important of them, and likewise some of their principal combinations +with the alkalies, alkaline earths, and metals. The first of the acids, +in point of importance, is the SULPHURIC, formerly called _oil of +vitriol_. + +CAROLINE. + +I have known it a long time by that name, but had no idea that it was +the same fluid as sulphuric acid. What resemblance or connection can +there be between oil of vitriol and this acid? + +MRS. B. + +Vitriol is the common name for sulphat of iron, a salt which is formed +by the combination of sulphuric acid and iron; the sulphuric acid was +formerly obtained by distillation from this salt, and it very naturally +received its name from the substance which afforded it. + +CAROLINE. + +But it is still usually called oil of vitriol? + +MRS. B. + +Yes; a sufficient length of time has not yet elapsed, since the +invention of the new nomenclature, for it to be generally disseminated; +but, as it is adopted by all scientific chemists, there is every reason +to suppose that it will gradually become universal. When I received this +bottle from the chemists, _oil of vitriol_ was inscribed on the label; +but, as I knew you were very punctilious in regard to the nomenclature, +I changed it, and substituted the words _sulphuric acid_. + +EMILY. + +This acid has neither colour nor smell, but it appears much thicker than +water. + +MRS. B. + +It is nearly twice as heavy as water, and has, you see, an oily +consistence. + +CAROLINE. + +And it is probably from this circumstance that it has been called an +oil, for it can have no real claim to that name, as it does not contain +either hydrogen or carbon, which are the essential constituents of oil. + +MRS. B. + +Certainly; and therefore it would be the more absurd to retain a name +which owed its origin to such a mistaken analogy. + +Sulphuric acid, in its purest state, would probably be a concrete +substance, but its attraction for water is such, that it is impossible +to obtain that acid perfectly free from it; it is, therefore, always +seen in a liquid form, such as you here find it. One of the most +striking properties of sulphuric acid is that of evolving a considerable +quantity of heat when mixed with water; this I have already shown you. + +EMILY. + +Yes, I recollect it; but what was the degree of heat produced by that +mixture? + +MRS. B. + +The thermometer may be raised by it to 300 degrees, which is +considerably above the temperature of boiling water. + +CAROLINE. + +Then water might be made to boil in that mixture? + +MRS. B. + +Nothing more easy, provided that you employ sufficient quantities of +acid and of water, and in the due proportions. The greatest heat is +produced by a mixture of one part of water to four of the acid: we shall +make a mixture of these proportions, and immerse in it this thin glass +tube, which is full of water. + +CAROLINE. + +The vessel feels extremely hot, but the water does not boil yet. + +MRS. B. + +You must allow some time for the heat to penetrate the tube, and raise +the temperature of the water to the boiling point-- + +CAROLINE. + +Now it boils--and with increasing violence. + +MRS. B. + +But it will not continue boiling long; for the mixture gives out heat +only while the particles of the water and the acid are mutually +penetrating each other: as soon as the new arrangement of those +particles is effected, the mixture will gradually cool, and the water +return to its former temperature. + +You have seen the manner in which sulphuric acid decomposes all +combustible substances, whether animal, vegetable, or mineral, and burns +them by means of its oxygen? + +CAROLINE. + +I have very unintentionally repeated the experiment on my gown, by +letting a drop of the acid fall upon it, and it has made a stain, which, +I suppose, will never wash out. + +MRS. B. + +No, certainly; for before you can put it into water, the spot will +become a hole, as the acid has literally burnt the muslin. + +CAROLINE. + +So it has, indeed! Well, I will fasten the stopper, and put the bottle +away, for it is a dangerous substance. --Oh, now I have done worse +still, for I have spilt some on my hand! + +MRS. B. + +It is then burned, as well as your gown, for you know that oxygen +destroys animal as well as vegetable matters; and, as far as the +decomposition of the skin of your finger is effected, there is no +remedy; but by washing it immediately in water, you will dilute the +acid, and prevent any further injury. + +CAROLINE. + +It feels extremely hot, I assure you. + +MRS. B. + +You have now learned, by experience, how cautiously this acid must be +used. You will soon become acquainted with another acid, the nitric, +which, though it produces less heat on the skin, destroys it still +quicker, and makes upon it an indelible stain. You should never handle +any substances of this kind, without previously dipping your fingers in +water, which will weaken their caustic effects. But, since you will not +repeat the experiment, I must put in the stopper, for the acid attracts +the moisture from the atmosphere, which would destroy its strength and +purity. + +EMILY. + +Pray, how can sulphuric acid be extracted from sulphat of iron by +distillation? + +MRS. B. + +The process of distillation, you know, consists in separating substances +from one another by means of their different degrees of volatility, and +by the introduction of a new chemical agent, caloric. Thus, if sulphat +of iron be exposed in a retort to a proper degree of heat, it will be +decomposed, and the sulphuric acid will be volatilised. + +EMILY. + +But now that the process of forming acids by the combustion of their +radicals is known, why should not this method be used for making +sulphuric acid? + +MRS. B. + +This is actually done in most manufactures; but the usual method of +preparing sulphuric acid does not consist in burning the sulphur in +oxygen gas (as we formerly did by the way of experiment), but in heating +it together with another substance, nitre, which yields oxygen in +sufficient abundance to render the combustion in common air rapid and +complete. + +CAROLINE. + +This substance, then, answers the same purpose as oxygen gas? + +MRS. B. + +Exactly. In manufactures the combustion is performed in a leaden +chamber, with water at the bottom, to receive the vapour and assist its +condensation. The combustion is, however, never so perfect but that a +quantity of _sulphureous_ acid is formed at the same time; for you +recollect that the sulphureous acid, according to the chemical +nomenclature, differs from the sulphuric only by containing less oxygen. + +From its own powerful properties, and from the various combinations into +which it enters, sulphuric acid is of great importance in many of the +arts. + +It is used also in medicine in a state of great dilution; for were it +taken internally, in a concentrated state, it would prove a most +dangerous poison. + +CAROLINE. + +I am sure it would burn the throat and stomach. + +MRS. B. + +Can you think of any thing that would prove an antidote to this poison? + +CAROLINE. + +A large draught of water to dilute it. + +MRS. B. + +That would certainly weaken the caustic power of the acid, but it would +increase the heat to an intolerable degree. Do you recollect nothing +that would destroy its deleterious properties more effectually? + +EMILY. + +An alkali might, by combining with it; but, then, a pure alkali is +itself a poison, on account of its causticity. + +MRS. B. + +There is no necessity that the alkali should be caustic. Soap, in which +it is combined with oil; or magnesia, either in the state of carbonat, +or mixed with water, would prove the best antidotes. + +EMILY. + +In those cases then, I suppose, the potash and the magnesia would quit +their combinations to form salts with the sulphuric acid? + +MRS. B. + +Precisely. + +We may now make a few observations on the sulphure_ous_ acid, which we +have found to be the product of sulphur slowly and imperfectly burnt. +This acid is distinguished by its pungent smell, and its gaseous form. + +CAROLINE. + +Its aëriform state is, I suppose, owing to the smaller proportion of +oxygen, which renders it lighter than sulphur_ic_ acid? + +MRS. B. + +Probably; for by adding oxygen to the weaker acid, it may be converted +into the stronger kind. But this change of state may also be connected +with a change of affinity with regard to caloric. + +EMILY. + +And may sulphureous acid be obtained from sulphuric acid by a diminution +of oxygen? + +MRS. B. + +Yes; it can be done by bringing any combustible substance in contact +with the acid. This decomposition is most easily performed by some of +the metals; these absorb a portion of the oxygen from the sulphuric +acid, which is thus converted into the sulphureous, and flies off in its +gaseous form. + +CAROLINE. + +And cannot the sulphureous acid itself be decomposed and reduced to +sulphur? + +MRS. B. + +Yes; if this gas be heated in contact with charcoal, the oxygen of the +gas will combine with it, and the pure sulphur is regenerated. + +Sulphureous acid is readily absorbed by water; and in this liquid state +it is found particularly useful in bleaching linen and woollen cloths, +and is much used in manufactures for those purposes. I can show you its +effect in destroying colours, by taking out vegetable stains--I think I +see a spot on your gown, Emily, on which we may try the experiment. + +EMILY. + +It is the stain of mulberries; but I shall be almost afraid of exposing +my gown to the experiment, after seeing the effect which the sulphuric +acid produced on that of Caroline-- + +MRS. B. + +There is no such danger from the sulphureous; but the experiment must be +made with great caution, for, during the formation of sulphureous acid +by combustion, there is always some sulphuric produced. + +CAROLINE. + +But where is your sulphureous acid? + +MRS. B. + +We may easily prepare some ourselves, simply by burning a match; we must +first wet the stain with water, and now hold it in this way, at a little +distance, over the lighted match: the vapour that arises from it is +sulphureous acid, and the stain, you see, gradually disappears. + +EMILY. + +I have frequently taken out stains by this means, without understanding +the nature of the process. But why is it necessary to wet the stain +before it is exposed to the acid fumes? + +MRS. B. + +The moisture attracts and absorbs the sulphureous acid; and it serves +likewise to dilute any particles of sulphuric acid which might injure +the linen. + +Sulphur is susceptible of a third combination with oxygen, in which the +proportion of the latter is too small to render the sulphur acid. It +acquires this slight oxygenation by mere exposure to the atmosphere, +without any elevation of temperature: in this case, the sulphur does not +change its natural form, but is only discoloured, being changed to red +or brown; and in this state it is an oxyd of sulphur. + +Before we take leave of the sulphuric acid, we shall say a few words of +its principal combinations. It unites with all the alkalies, alkaline +earths and metals, to form compound salts. + +CAROLINE. + +Pray, give me leave to interrupt you for a moment: you have never +mentioned any other salts than the compound or neutral salts; is there +no other kind? + +MRS. B. + +The term _salt_ has been used, from time immemorial, as a kind of +general name for any substance that has savour, odour, is soluble in +water, and crystallisable, whether it be of an acid, an alkaline, or +compound nature; but the compound salts alone retain that appellation in +modern chemistry. + +The most important of the salts, formed by the combinations of the +sulphuric acid, are, first, _sulphat of potash_, formerly called _sal +polychrest_: this is a very bitter salt, much used in medicine; it is +found in the ashes of most vegetables, but it may be prepared +artificially by the immediate combination of sulphuric acid and potash. +This salt is easily soluble in boiling water. Solubility is, indeed, +a property common to all salts; and they always produce cold in melting. + +EMILY. + +That must be owing to the caloric which they absorb in passing from a +solid to a fluid form. + +MRS. B. + +That is, certainly, the most probable explanation. + +_Sulphat of soda_, commonly called Glauber’s salt, is another medicinal +salt, which is still more bitter than the preceding. We must prepare +some of these compounds, that you may observe the phenomena which take +place during their formation. We need only pour some sulphuric acid over +the soda which I have put into this glass. + +CAROLINE. + +What an amazing heat is disengaged! --I thought you said that cold was +produced by the melting of salts? + +MRS. B. + +But you must observe that we are now _making_, not _melting_ a salt. +Heat is disengaged during the formation of compound salts, and a faint +light is also emitted, which may sometimes be perceived in the dark. + +EMILY. + +And is this heat and light produced by the union of the opposite +electricities of the alkali and the acid? + +MRS. B. + +No doubt it is, if that theory be true. + +CAROLINE. + +The union of an acid and an alkali is then an actual combustion? + +MRS. B. + +Not precisely, though there is certainly much analogy in these +processes. + +CAROLINE. + +Will this sulphat of soda become solid? + +MRS. B. + +We have not, I suppose, mixed the acid and the alkali in the exact +proportions that are required for the formation of the salt, otherwise +the mixture would have been almost immediately changed to a solid mass; +but, in order to obtain it in crystals, as you see it in this bottle, it +would be necessary first to dilute it with water, and afterwards to +evaporate the water, during which operation the salt would gradually +crystallise. + +CAROLINE. + +But of what use is the addition of water, if it is afterwards to be +evaporated? + +MRS. B. + +When suspended in water, the acid and the alkali are more at liberty to +act on each other, their union is more complete, and the salt assumes +the regular form of crystals during the slow evaporation of its solvent. + +Sulphat of soda liquefies by heat, and effloresces in the air. + +EMILY. + +Pray what is the meaning of the word _effloresces_? I do not recollect +your having mentioned it before. + +MRS. B. + +A salt is said to effloresce when it loses its water of crystallisation +on being exposed to the atmosphere, and is thus gradually converted into +a dry powder: you may observe that these crystals of sulphat of soda are +far from possessing the transparency which belongs to their crystalline +state; they are covered with a white powder, occasioned by their having +been exposed to the atmosphere, which has deprived their surface of its +lustre, by absorbing its water of crystallisation. Salts are, in +general, either _efflorescent_ or _deliquescent_: this latter property +is precisely the reverse of the former; that is to say, deliquescent +salts absorb water from the atmosphere, and are moistened and gradually +melted by it. Muriat of lime is an instance of great deliquescence. + +EMILY. + +But are there no salts that have the same degree of attraction for water +as the atmosphere, and that will consequently not be affected by it? + +MRS. B. + +Yes; there are many such salts, as, for instance, common salt, sulphat +of magnesia, and a variety of others. + +_Sulphat of lime_ is very frequently met with in nature, and constitutes +the well-known substance called _gypsum_, or _plaster of Paris_. + +_Sulphat of magnesia_, commonly called _Epsom salt_, is another very +bitter medicine, which is obtained from sea-water and from several +springs, or may be prepared by the direct combination of its +ingredients. + +We have formerly mentioned _sulphat of alumine_ as constituting the +common _alum_; it is found in nature chiefly in the neighbourhood of +volcanos, and is particularly useful in the arts, from its strong +astringent qualities. It is chiefly employed by dyers and +calico-printers, to fix colours; and is used also in the manufacture of +some kinds of leather. + +Sulphuric acid combines also with the metals. + +CAROLINE. + +One of these combinations, _sulphat of iron_, we are already well +acquainted with. + +MRS. B. + +That is the most important metallic salt formed by sulphuric acid, and +the only one that we shall here notice. It is of great use in the arts; +and, in medicine, it affords a very valuable tonic: it is of this salt +that most of those preparations called _steel medicines_ are composed. + +CAROLINE. + +But does any carbon enter into these compositions to form steel? + +MRS. B. + +Not an atom: they are, therefore, very improperly called steel: but it +is the vulgar appellation, and medical men themselves often comply with +the general custom. + +Sulphat of iron may be prepared, as you have seen, by dissolving iron in +sulphuric acid; but it is generally obtained from the natural production +called _Pyrites_, which being a sulphuret of iron, requires only +exposure to the atmosphere to be oxydated, in order to form the salt; +this, therefore, is much the most easy way of procuring it on a large +scale. + +EMILY. + +I am surprised to find that both acids and compound salts are generally +obtained from their various combinations, rather than from the immediate +union of their ingredients. + +MRS. B. + +Were the simple bodies always at hand, their combinations would +naturally be the most convenient method of forming compounds; but you +must consider that, in most instances, there is great difficulty and +expense in obtaining the simple ingredients from their combinations; it +is, therefore, often more expedient to procure compounds from the +decomposition of other compounds. But, to return to the sulphat of iron. +--There is a certain vegetable acid called _Gallic acid_, which has the +remarkable property of precipitating this salt black--I shall pour a few +drops of the gallic acid into this solution of sulphat of iron-- + +CAROLINE. + +It is become as black as ink! + +MRS. B. + +And it is ink in reality. Common writing ink is a precipitate of sulphat +of iron by gallic acid; the black colour is owing to the formation of +gallat of iron, which being insoluble, remains suspended in the fluid. + +This acid has also the property of altering the colour of iron in its +metallic state. You may frequently see its effect on the blade of a +knife, that has been used to cut certain kinds of fruits. + +CAROLINE. + +True; and that is, perhaps, the reason that a silver knife is preferred +to cut fruits; the gallic acid, I suppose, does not act upon silver. +--Is this acid found in all fruits? + +MRS. B. + +It is contained, more or less, in the rind of most fruits and roots, +especially the radish, which, if scraped with a steel or iron knife, has +its bright red colour changed to a deep purple, the knife being at the +same time blackened. But the vegetable substance in which the gallic +acid most abounds is _nutgall_, a kind of excrescence that grows on +oaks, and from which the acid is commonly obtained for its various +purposes. + + +MRS. B. + +We now come to the PHOSPHORIC and PHOSPHOROUS ACIDS. In treating of +phosphorus, you have seen how these acids may be obtained from it by +combustion? + +EMILY. + +Yes; but I should be much surprised if it was the usual method of +obtaining them, since it is so very difficult to procure phosphorus in +its pure state. + +MRS. B. + +You are right, my dear; the phosphoric acid, for general purposes, is +extracted from bones, in which it is contained in the state of phosphat +of lime; from this salt the phosphoric acid is separated by means of the +sulphuric, which combines with the lime. In its pure state, phosphoric +acid is either liquid or solid, according to its degree of +concentration. + +Among the salts formed by this acid, _phosphat of lime_ is the only one +that affords much interest; and this, we have already observed, +constitutes the basis of all bones. It is also found in very small +quantities in some vegetables. + + + + +CONVERSATION XVIII. + +OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATIONS OF OXYGEN WITH +NITROGEN AND CARBON; AND OF THE NITRATS AND CARBONATS. + + +MRS. B. + +I am almost afraid of introducing the subject of the NITRIC ACID, as I +am sure that I shall be blamed by Caroline for not having made her +acquainted with it before. + +CAROLINE. + +Why so, Mrs. B.? + +MRS. B. + +Because you have long known its radical, which is nitrogen or azote; and +in treating of that element, I did not even hint that it was the basis +of an acid. + +CAROLINE. + +And what could be your reason for not mentioning this acid sooner? + +MRS. B. + +I do not know whether you will think the reason sufficiently good to +acquit me; but the omission, I assure you, did not proceed from +negligence. You may recollect that nitrogen was one of the first simple +bodies which we examined; you were then ignorant of the theory of +combustion, which I believe was, for the first time, mentioned in that +lesson; and therefore it would have been in vain, at that time, to have +attempted to explain the nature and formation of acids. + +CAROLINE. + +I wonder, however, that it never occurred to us to enquire whether +nitrogen could be acidified; for, as we knew it was classed among the +combustible bodies, it was natural to suppose that it might produce an +acid. + +MRS. B. + +That is not a necessary consequence; for it might combine with oxygen +only in the degree requisite to form an oxyd. But you will find that +nitrogen is susceptible of various degrees of oxygenation, some of which +convert it merely into an oxyd, and others give it all the acid +properties. + +The acids, resulting from the combination of oxygen and nitrogen, are +called the NITROUS and NITRIC acids. We will begin with the NITRIC, in +which nitrogen is in the highest state of oxygenation. This acid +naturally exists in the form of gas; but is so very soluble in water, +and has so great an affinity for it, that one grain of water will absorb +and condense ten grains of acid gas, and form the limpid fluid which you +see in this bottle. + +CAROLINE. + +What a strong offensive smell it has! + +MRS. B. + +This acid contains a greater abundance of oxygen than any other, but it +retains it with very little force. + +EMILY. + +Then it must be a powerful caustic, both from the facility with which it +parts with its oxygen, and the quantity which it affords? + +MRS. B. + +Very well, Emily; both cause and effect are exactly such as you +describe: nitric acid burns and destroys all kinds of organised matter. +It even sets fire to some of the most combustible substances. --We shall +pour a little of it over this piece of dry warm charcoal--you see it +inflames it immediately; it would do the same with oil of turpentine, +phosphorus, and several other very combustible bodies. This shows you +how easily this acid is decomposed by combustible bodies, since these +effects must depend upon the absorption of its oxygen. + +Nitric acid has been used in the arts from time immemorial, but it is +only within these twenty-five years that its chemical nature has been +ascertained. The celebrated Mr. Cavendish discovered that it consisted +of about 10 parts of nitrogen and 25 of oxygen.* These principles, in +their gaseous state, combine at a high temperature; and this may be +effected by repeatedly passing the electrical spark through a mixture of +the two gases. + + [Footnote *: The proportion stated by Sir H. Davy, in his Chemical + Researches, is as 1 to 2.389.] + +EMILY. + +The nitrogen and oxygen gases, of which the atmosphere is composed, do +not combine, I suppose, because their temperature is not sufficiently +elevated? + +CAROLINE. + +But in a thunder-storm, when the lightning repeatedly passes through +them, may it not produce nitric acid? We should be in a strange +situation, if a violent storm should at once convert the atmosphere into +nitric acid. + +MRS. B. + +There is no danger of it, my dear; the lightning can affect but a very +small portion of the atmosphere, and though it were occasionally to +produce a little nitric acid, yet this never could happen to such an +extent as to be perceivable. + +EMILY. + +But how could the nitric acid be known, and used, before the method of +combining its constituents was discovered? + +MRS. B. + +Before that period the nitric acid was obtained, and it is indeed still +extracted, for the common purposes of art, from the compound salt which +it forms with potash, commonly called _nitre_. + +CAROLINE. + +Why is it so called? Pray, Mrs. B., let these old unmeaning names be +entirely given up, by us at least; and let us call this salt _nitrat of +potash_. + +MRS. B. + +With all my heart; but it is necessary that I should, at least, mention +the old names, and more especially those which are yet in common use; +otherwise, when you meet with them, you would not be able to understand +their meaning. + +EMILY. + +And how is the acid obtained from this salt? + +MRS. B. + +By the intervention of sulphuric acid, which combines with the potash, +and sets the nitric acid at liberty. This I can easily show you, by +mixing some nitrat of potash and sulphuric acid in this retort, and +heating it over a lamp; the nitric acid will come over in the form of +vapour, which we shall collect in a glass bell. This acid, diluted in +water, is commonly called _aqua fortis_, if Caroline will allow me to +mention that name. + +CAROLINE. + +I have often heard that aqua fortis will dissolve almost all metals; it +is no doubt because it yields its oxygen so easily. + +MRS. B. + +Yes; and from this powerful solvent property, it derived the name of +aqua fortis, or strong water. Do you not recollect that we oxydated, and +afterwards dissolved, some copper in this acid? + +EMILY. + +If I remember right, the nitrat of copper was the first instance you +gave us of a compound salt. + +CAROLINE. + +Can the nitric acid be completely decomposed and converted into nitrogen +and oxygen? + +EMILY. + +That cannot be the case, Caroline; since the acid can be decomposed only +by the combination of its constituents with other bodies. + +MRS. B. + +True; but caloric is sufficient for this purpose. By making the acid +pass through a red hot porcelain tube, it is decomposed; the nitrogen +and oxygen regain the caloric which they had lost in combining, and are +thus both restored to their gaseous state. + +The nitric acid may also be partly decomposed, and is by this means +converted into NITROUS ACID. + +CAROLINE. + +This conversion must be easily effected, as the oxygen is so slightly +combined with the nitrogen. + +MRS. B. + +The partial decomposition of nitric acid is readily effected by most +metals; but it is sufficient to expose the nitric acid to a very strong +light to make it give out oxygen gas, and thus be converted into nitrous +acid. Of this acid there are various degrees, according to the +proportions of oxygen which it contains; the strongest, and that into +which the nitric is first converted, is of a yellow colour, as you see +in this bottle. + +CAROLINE. + +How it fumes when the stopper is taken out! + +MRS. B. + +The acid exists naturally in a gaseous state, and is here so strongly +concentrated in water, that it is constantly escaping. + +Here is another bottle of nitrous acid, which, you see, is of an orange +red; this acid is weaker, the nitrogen being combined with a smaller +quantity of oxygen; and with a still less proportion of oxygen it is an +olive-green colour, as it appears in this third bottle. In short, the +weaker the acid, the deeper is its colour. + +Nitrous acid acts still more powerfully on some inflammable substances +than the nitric. + +EMILY. + +I am surprised at that, as it contains less oxygen. + +MRS. B. + +But, on the other hand, it parts with its oxygen much more readily: you +may recollect that we once inflamed oil with this acid. + +The next combinations of nitrogen and oxygen form only oxyds of +nitrogen, the first of which is commonly called _nitrous air_; or more +properly _nitric oxyd gas_. This may be obtained from nitric acid, by +exposing the latter to the action of metals, as in dissolving them it +does not yield the whole of its oxygen, but retains a portion of this +principle sufficient to convert it into this peculiar gas, a specimen of +which I have prepared, and preserved within this inverted glass bell. + +EMILY. + +It is a perfectly invisible elastic fluid. + +MRS. B. + +Yes; and it may be kept any length of time in this manner over water, as +it is not, like the nitric and nitrous acids, absorbable by it. It is +rather heavier than atmospherical air, and is incapable of supporting +either combustion or respiration. I am going to incline the glass gently +on one side, so as to let some of the gas escape-- + +EMILY. + +How very curious! --It produces orange fumes like the nitrous acid! that +is the more extraordinary, as the gas within the glass is perfectly +invisible. + +MRS. B. + +It would give me much pleasure if you could make out the reason of this +curious change without requiring any further explanation. + +CAROLINE. + +It seems, by the colour and smell, as if it were converted into nitrous +acid gas: yet that cannot be, unless it combines with more oxygen; and +how can it obtain oxygen the very instant it escapes from the glass? + +EMILY. + +From the atmosphere, no doubt. Is it not so, Mrs. B.? + +MRS. B. + +You have guessed it; as soon as it comes in contact with the atmosphere, +it absorbs from it the additional quantity of oxygen necessary to +convert it into nitrous acid gas. And, if I now remove the bottle +entirely from the water, so as to bring at once the whole of the gas +into contact with the atmosphere, this conversion will appear still more +striking-- + +EMILY. + +Look, Caroline, the whole capacity of the bottle is instantly tinged of +an orange colour! + +MRS. B. + +Thus, you see, it is the most easy process imaginable to convert +_nitrous oxyd gas_ into _nitrous acid gas_. The property of attracting +oxygen from the atmosphere, without any elevation of temperature, has +occasioned this gaseous oxyd being used as a test for ascertaining the +degree of purity of the atmosphere. I am going to show you how it is +applied to this purpose. --You see this graduated glass tube, which is +closed at one end, (PLATE X. Fig. 2.) --I first fill it with water, and +then introduce a certain measure of nitrous gas, which, not being +absorbable by water, passes through it, and occupies the upper part of +the tube. I must now add rather above two-thirds of oxygen gas, which +will just be sufficient to convert the nitrous oxyd gas into nitrous +acid gas. + +CAROLINE. + +So it has! --I saw it turn of an orange colour; but it immediately +afterwards disappeared entirely, and the water, you see, has risen, and +almost filled the tube. + +MRS. B. + +That is because the acid gas is absorbable by water, and in proportion +as the gas impregnates the water, the latter rises in the tube. When the +oxygen gas is very pure, and the required proportion of nitrous oxyd gas +very exact, the whole is absorbed by the water; but if any other gas be +mixed with the oxygen, instead of combining with the nitrous oxygen, it +will remain and occupy the upper part of the tube; or, if the gases be +not in the due proportion, there will be a residue of that which +predominates. --Before we leave this subject, I must not forget to +remark that nitrous acid may be formed by dissolving nitrous oxyd gas in +nitric acid. This solution may be effected simply by making bubbles of +nitrous oxyd gas pass through nitric acid. + +EMILY. + +That is to say, that nitrogen at its highest degree of oxygenation, +being mixed with nitrogen at its lowest degree of oxygenation, will +produce a kind of intermediate substance, which is nitrous acid. + +MRS. B. + +You have stated the fact with great precision. --There are various other +methods of preparing nitrous oxyd, and of obtaining it from compound +bodies; but it is not necessary to enter into these particulars. It +remains for me only to mention another curious modification of +oxygenated nitrogen, which has been distinguished by the name of +_gaseous oxyd of nitrogen_. It is but lately that this gas has been +accurately examined, and its properties have been investigated chiefly +by Sir H. Davy. It has obtained also the name of _exhilarating_ gas, +from the very singular property which that gentleman has discovered in +it, of elevating the animal spirits, when inhaled into the lungs, to a +degree sometimes resembling delirium or intoxication. + +CAROLINE. + +Is it respirable, then? + +MRS. B. + +It can scarcely be called respirable, as it would not support life for +any length of time; but it may be breathed for a few moments without any +other effects, than the singular exhilaration of spirits I have just +mentioned. It affects different people, however, in a very different +manner. Some become violent, even outrageous: others experience a +languor, attended with faintness; but most agree in opinion, that the +sensations it excites are extremely pleasant. + +CAROLINE. + +I think I should like to try it--how do you breathe it? + +MRS. B. + +By collecting the gas in a bladder, to which a short tube with a +stop-cock is adapted; this is applied to the mouth with one hand, whilst +the nostrils are kept closed with the other, that the common air may +have no access. You then alternately inspire, and expire the gas, till +you perceive its effects. But I cannot consent to your making the +experiment; for the nerves are sometimes unpleasantly affected by it, +and I would not run any risk of that kind. + +EMILY. + +I should like, at least, to see somebody breathe it; but pray by what +means is this curious gas obtained? + +MRS. B. + +It is procured from _nitrat of ammonia_, an artificial salt which yields +this gas on the application of a gentle heat. I have put some of the +salt into a retort, and by the aid of a lamp the gas will be +extricated.-- + +CAROLINE. + +Bubbles of air begin to escape through the neck of the retort into the +water apparatus; will you not collect them? + +MRS. B. + +The gas that first comes over need not be preserved, as it consists of +little more than the common air that was in the retort; besides, there +is always in this experiment a quantity of watery vapour which must come +away before the nitrous oxyd appears. + +EMILY. + +Watery vapour! Whence does that proceed? There is no water in nitrat of +ammonia? + +MRS. B. + +You must recollect that there is in every salt a quantity of water of +crystallisation, which may be evaporated by heat alone. But, besides +this, water is actually generated in this experiment, as you will see +presently. First tell me, what are the constituent parts of nitrat of +ammonia? + +EMILY. + +Ammonia, and nitric acid: this salt, therefore, contains three different +elements, nitrogen and hydrogen, which produce the ammonia; and oxygen, +which, with nitrogen, forms the acid. + +MRS. B. + +Well then, in this process the ammonia is decomposed; the hydrogen quits +the nitrogen to combine with some of the oxygen of the nitric acid, and +forms with it the watery vapour which is now coming over. When that is +effected, what will you expect to find? + +EMILY. + +Nitrous acid instead of nitric acid, and nitrogen instead of ammonia. + +MRS. B. + +Exactly so; and the nitrous acid and nitrogen combine, and form the +gaseous oxyd of nitrogen, in which the proportion of oxygen is 37 parts +to 63 of nitrogen. + +You may have observed, that for a little while no bubbles of air have +come over, and we have perceived only a stream of vapour condensing as +it issued into the water. --Now bubbles of air again make their +appearance, and I imagine that by this time all the watery vapour is +come away, and that we may begin to collect the gas. We may try whether +it is pure, by filling a phial with it, and plunging a taper into +it--yes, it will do now, for the taper burns brighter than in the common +air, and with a greenish flame. + +CAROLINE. + +But how is that? I thought no gas would support combustion but oxygen or +chlorine. + +MRS. B. + +Or any gas that contains oxygen, and is ready to yield it, which is the +case with this in a considerable degree; it is not, therefore, +surprising that it should accelerate the combustion of the taper. + +You see that the gas is now produced in great abundance; we shall +collect a large quantity of it, and I dare say that we shall find some +of the family who will be curious to make the experiment of respiring +it. Whilst this process is going on, we may take a general survey of the +most important combinations of the nitric and nitrous acids with the +alkalies. + +The first of these is _nitrat of potash_, commonly called _nitre_ or +_saltpetre_. + +CAROLINE. + +Is not that the salt with which gunpowder is made? + +MRS. B. + +Yes. Gunpowder is a mixture of five parts of nitre to one of sulphur, +and one of charcoal. --Nitre from its great proportion of oxygen, and +from the facility with which it yields it, is the basis of most +detonating compositions. + +EMILY. + +But what is the cause of the violent detonation of gunpowder when set +fire to? + +MRS. B. + +Detonation may proceed from two causes; the sudden formation or +destruction of an elastic fluid. In the first case, when either a solid +or liquid is instantaneously converted into an elastic fluid, the +prodigious and sudden expansion of the body strikes the air with great +violence, and this concussion produces the sound called detonation. + +CAROLINE. + +That I comprehend very well; but how can a similar effect be produced by +the destruction of a gas? + +MRS. B. + +A gas can be destroyed only by condensing it to a liquid or solid state; +when this takes place suddenly, the gas, in assuming a new and more +compact form, produces a vacuum, into which the surrounding air rushes +with great impetuosity; and it is by that rapid and violent motion that +the sound is produced. In all detonations, therefore, gases are either +suddenly formed, or destroyed. In that of gunpowder, can you tell me +which of these two circumstances takes place? + +EMILY. + +As gunpowder is a solid, it must, of course, produce the gases in its +detonation; but how, I cannot tell. + +MRS. B. + +The constituents of gunpowder, when heated to a certain degree, enter +into a number of new combinations, and are instantaneously converted +into a variety of gases, the sudden expansion of which gives rise to the +detonation. + +CAROLINE. + +And in what instance does the destruction or condensation of gases +produce detonation? + +MRS. B. + +I can give you one with which you are well acquainted; the sudden +combination of the oxygen and hydrogen gases. + +CAROLINE. + +True; I recollect perfectly that hydrogen detonates with oxygen when the +two gases are converted into water. + +MRS. B. + +But let us return to the nitrat of potash. --This salt is decomposed +when exposed to heat, and mixed with any combustible body, such as +carbon, sulphur, or metals, these substances oxydating rapidly at the +expense of the nitrat. I must show you an instance of this. --I expose +to the fire some of the salt in a small iron ladle, and, when it is +sufficiently heated, add to it some powdered charcoal; this will attract +the oxygen from the salt, and be converted into carbonic acid.-- + +EMILY. + +But what occasions that crackling noise, and those vivid flashes that +accompany it? + +MRS. B. + +The rapidity with which the carbonic acid gas is formed occasions a +succession of small detonations, which, together with the emission of +flame, is called _deflagration_. + +_Nitrat of ammonia_ we have already noticed, on account of the gaseous +oxyd of nitrogen which is obtained from it. + +_Nitrat of silver_ is the lunar caustic, so remarkable for its property +of destroying animal fibre, for which purpose it is often used by +surgeons. --We have said so much on a former occasion, on the mode in +which caustics act on animal matter, that I shall not detain you any +longer on this subject. + + +We now come to the CARBONIC ACID, which we have already had many +opportunities of noticing. You recollect that this acid may be formed by +the combustion of carbon, whether in its imperfect state of charcoal, or +in its purest form of diamond. And it is not necessary, for this +purpose, to burn the carbon in oxygen gas, as we did in the preceding +lecture; for you need only light a piece of charcoal and suspend it +under a receiver on the water bath. The charcoal will soon be +extinguished, and the air in the receiver will be found mixed with +carbonic acid. The process, however, is much more expeditious if the +combustion be performed in pure oxygen gas. + +CAROLINE. + +But how can you separate the carbonic acid, obtained in this manner, +from the air with which it is mixed? + +MRS. B. + +The readiest mode is to introduce under the receiver a quantity of +caustic lime, or caustic alkali, which soon attracts the whole of the +carbonic acid to form a carbonat. --The alkali is found increased in +weight, and the volume of the air is diminished by a quantity equal to +that of the carbonic acid which was mixed with it. + +EMILY. + +Pray is there no method of obtaining pure carbon from carbonic acid? + +MRS. B. + +For a long time it was supposed that carbonic acid was not +decompoundable; but Mr. Tennant discovered, a few years ago, that this +acid may be decomposed by burning phosphorus in a closed vessel with +carbonat of soda or carbonat of lime: the phosphorus absorbs the oxygen +from the carbonat, whilst the carbon is separated in the form of a black +powder. This decomposition, however, is not effected simply by the +attraction of the phosphorus for oxygen, since it is weaker than that of +charcoal; but the attraction of the alkali of lime for the phosphoric +acid, unites its power at the same time. + +CAROLINE. + +Cannot we make that experiment? + +MRS. B. + +Not easily; it requires being performed with extreme nicety, in order to +obtain any sensible quantity of carbon, and the experiment is much too +delicate for me to attempt it. But there can be no doubt of the accuracy +of Mr. Tennant’s results; and all chemists now agree, that one hundred +parts of carbonic acid gas consists of about twenty-eight parts of +carbon to seventy-two of oxygen gas. But if you recollect, we decomposed +carbonic acid gas the other day by burning potassium in it. + +CAROLINE. + +True, so we did; and found the carbon precipitated on the regenerated +potash. + +MRS. B. + +Carbonic acid gas is found very abundantly in nature; it is supposed to +form about one thousandth part of the atmosphere, and is constantly +produced by the respiration of animals; it exists in a great variety of +combinations, and is exhaled from many natural decompositions. It is +contained in a state of great purity in certain caves, such as the +_Grotto del Cane_, near Naples. + +EMILY. + +I recollect having read an account of that grotto, and of the cruel +experiments made on the poor dogs, to gratify the curiosity of +strangers. But I understood that the vapour exhaled by this cave was +called _fixed air_. + +MRS. B. + +That is the name by which carbonic acid was known before its chemical +composition was discovered. --This gas is more destructive of life than +any other; and if the poor animals that are submitted to its effects are +not plunged into cold water as soon as they become senseless, they do +not recover. It extinguishes flame instantaneously. I have collected +some in this glass, which I will pour over the candle. + +CAROLINE. + +This is extremely singular--it seems to extinguish it as it were by +enchantment, as the gas is invisible. I never should have imagined that +gas could have been poured like a liquid. + +MRS. B. + +It can be done with carbonic acid only, as no other gas is sufficiently +heavy to be susceptible of being poured out in the atmospherical air +without mixing with it. + +EMILY. + +Pray by what means did you obtain this gas? + +MRS. B. + +I procured it from marble. Carbonic acid gas has so strong an attraction +for all the alkalies and alkaline earths, that these are always found in +nature in the state of carbonats. Combined with lime, this acid forms +chalk, which may be considered as the basis of all kinds of marbles, and +calcareous stones. From these substances carbonic acid is easily +separated, as it adheres so slightly to its combinations, that the +carbonats are all decomposable by any of the other acids. I can easily +show you how I obtained this gas; I poured some diluted sulphuric acid +over pulverised marble in this bottle (the same which we used the other +day to prepare hydrogen gas), and the gas escaped through the tube +connected with it; the operation still continues, as you may easily +perceive-- + +EMILY. + +Yes, it does; there is a great fermentation in the glass vessel. What +singular commotion is excited by the sulphuric acid taking possession of +the lime, and driving out the carbonic acid! + +CAROLINE. + +But did the carbonic acid exist in a gaseous state in the marble? + +MRS. B. + +Certainly not; the acid, when in a state of combination, is capable of +existing in a solid form. + +CAROLINE. + +Whence, then, does it obtain the caloric necessary to convert it into +gas? + +MRS. B. + +It may be supplied in this case from the mixture of sulphuric acid and +water, which produces an evolution of heat, even greater than is +required for the purpose; since, as you may perceive by touching the +glass vessel, a considerable quantity of the caloric disengaged becomes +sensible. But a supply of caloric may be obtained also from a diminution +of capacity for heat, occasioned by the new combination which takes +place; and, indeed, this must be the case when other acids are employed +for the disengagement of carbonic acid gas, which do not, like the +sulphuric, produce heat on being mixed with water. Carbonic acid may +likewise be disengaged from its combinations by heat alone, which +restores it to its gaseous state. + +CAROLINE. + +It appears to me very extraordinary that the same gas, which is produced +by the burning of wood and coals, should exist also in such bodies as +marble, and chalk, which are incombustible substances. + +MRS. B. + +I will not answer that objection, Caroline, because I think I can put +you in a way of doing it yourself. Is carbonic acid combustible? + +CAROLINE. + +Why, no--because it is a body that has been already burnt; it is carbon +only, and not the acid, that is combustible. + +MRS. B. + +Well, and what inference do you draw from this? + +CAROLINE. + +That carbonic acid cannot render the bodies with which it is united +combustible; but that simple carbon does, and that it is in this +elementary state that it exists in wood, coals, and a great variety of +other combustible bodies. --Indeed, Mrs. B., you are very ungenerous; +you are not satisfied with convincing me that my objections are +frivolous, but you oblige me to prove them so myself. + +MRS. B. + +You must confess, however, that I make ample amends for the detection of +error, when I enable you to discover the truth. You, understand, now, +I hope, that carbonic acid is equally produced by the decomposition of +chalk, or by the combustion of charcoal. These processes are certainly +of a very different nature; in the first case the acid is already +formed, and requires nothing more than heat to restore it to its gaseous +state; whilst, in the latter, the acid is actually made by the process +of combustion. + +CAROLINE. + +I understand it now perfectly. But I have just been thinking of another +difficulty, which, I hope, you will excuse my not being able to remove +myself. How does the immense quantity of calcareous earth, which is +spread all over the globe, obtain the carbonic acid with which it is +combined? + +MRS. B. + +The question is, indeed, not very easy to answer; but I conceive that +the general carbonisation of calcareous matter may have been the effect +of a general combustion, occasioned by some revolution of our globe, and +producing an immense supply of carbonic acid, with which the calcareous +matter became impregnated; or that this may have been effected by a +gradual absorption of carbonic acid from the atmosphere. --But this +would lead us to discussions which we cannot indulge in, without +deviating too much from our subject. + +EMILY. + +How does it happen that we do not perceive the pernicious effects of the +carbonic acid which is floating in the atmosphere? + +MRS. B. + +Because of the state of very great dilution in which it exists there. +But can you tell me, Emily, what are the sources which keep the +atmosphere constantly supplied with this acid? + +EMILY. + +I suppose the combustion of wood, coals, and other substances, that +contain carbon. + +MRS. B. + +And also the breath of animals. + +CAROLINE. + +The breath of animals! I thought you said that this gas was not at all +respirable, but on the contrary, extremely poisonous. + +MRS. B. + +So it is; but although animals cannot breathe in carbonic acid gas, yet, +in the process of respiration, they have the power of forming this gas +in their lungs; so that the air which we _expire_, or reject from the +lungs, always contains a certain proportion of carbonic acid, which is +much greater than that which is commonly found in the atmosphere. + +CAROLINE. + +But what is it that renders carbonic acid such a deadly poison? + +MRS. B. + +The manner in which this gas destroys life, seems to be merely by +preventing the access of respirable air; for carbonic acid gas, unless +very much diluted with common air, does not penetrate into the lungs, as +the windpipe actually contracts and refuses it admittance. --But we must +dismiss this subject at present, as we shall have an opportunity of +treating of respiration much more fully, when we come to the chemical +functions of animals. + +EMILY. + +Is carbonic acid as destructive to the life of vegetables as it is to +that of animals? + +MRS. B. + +If a vegetable be completely immersed in it, I believe it generally +proves fatal to it; but mixed in certain proportions with atmospherical +air, it is, on the contrary, very favourable to vegetation. + +You remember, I suppose, our mentioning the mineral waters, both natural +and artificial, which contain carbonic acid gas? + +CAROLINE. + +You mean the Seltzer water? + +MRS. B. + +That is one of those which are the most used; there are, however, +a variety of others into which carbonic acid enters as an ingredient: +all these waters are usually distinguished by the name of _acidulous_ or +_gaseous mineral waters_. + +The class of salts called _carbonats_ is the most numerous in nature; we +must pass over them in a very cursory manner, as the subject is far too +extensive for us to enter on it in detail. The state of carbonat is the +natural state of a vast number of minerals, and particularly of the +alkalies and alkaline earths, as they have so great an attraction for +the carbonic acid, that they are almost always found combined with it; +and you may recollect that it is only by separating them from this acid, +that they acquire that causticity and those striking qualities which I +have formerly described. All marbles, chalks, shells, calcareous spars, +and lime-stones of every description, are neutral salts, in which +_lime_, their common basis, has lost all its characteristic properties. + +EMILY. + +But if all these various substances are formed by the union of lime with +carbonic acid, whence arises their diversity of form and appearance? + +MRS. B. + +Both from the different proportions of their component parts, and from a +variety of foreign ingredients which may be occasionally blended with +them: the veins and colours of marbles, for instance, proceed from a +mixture of metallic substances; silex and alumine also frequently enter +into these combinations. The various carbonats, therefore, that I have +enumerated, cannot be considered as pure unadulterated neutral salts, +although they certainly belong to that class of bodies. + + + + +CONVERSATION XIX. + +ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC ACIDS; AND ON +MURIATS. --ON IODINE AND IODIC ACID. + + +MRS. B. + +We now come to the three remaining acids with simple bases, the compound +nature of which, though long suspected, has been but recently proved. +The chief of these is the muriatic; but I shall first describe the two +others, as their bases have been obtained more distinctly than that of +the muriatic acid. + +You may recollect I mentioned the BORACIC ACID. This is found very +sparingly in some parts of Europe, but for the use of manufactures we +have always received it from the remote country of Thibet, where it is +found in some lakes, combined with soda. It is easily separated from the +soda by sulphuric acid, and appears in the form of shining scales, as +you see here. + +CAROLINE. + +I am glad to meet with an acid which we need not be afraid to touch; for +I perceive, from your keeping it in a piece of paper, that it is more +innocent than our late acquaintance, the sulphuric and nitric acids. + +MRS. B. + +Certainly; but being more inert, you will not find its properties so +interesting. However, its decomposition, and the brilliant spectacle it +affords when its basis again unites with oxygen, atones for its want of +other striking qualities. + +Sir H. Davy succeeded in decomposing the boracic acid, (which had till +then been considered as undecompoundable,) by various methods. On +exposing this acid to the Voltaic battery, the positive wire gave out +oxygen, and on the negative wire was deposited a black substance, in +appearance resembling charcoal. This was the basis of the acid, which +Sir H. Davy has called _Boracium_, or _Boron_. + +The same substance was obtained in more considerable quantities, by +exposing the acid to a great heat in an iron gun-barrel. + +A third method of decomposing the boracic acid consisted in burning +potassium in contact with it in vacuo. The potassium attracts the oxygen +from the acid, and leaves its basis in a separate state. + +The recomposition of this acid I shall show you, by burning some of its +basis, which you see here, in a retort full of oxygen gas. The heat of a +candle is all that is required for this combustion.-- + +EMILY. + +The light is astonishingly brilliant, and what beautiful sparks it +throws out! + +MRS. B. + +The result of this combustion is the boracic acid, the nature of which, +you see, is proved both by analytic and synthetic means. Its basis has +not, it is true, a metallic appearance; but it makes very hard alloys +with other metals. + +EMILY. + +But pray, Mrs. B., for what purpose is the boracic acid used in +manufactures? + +MRS. B. + +Its principal use is in conjunction with soda, that is, in the state of +_borat of soda_, which in the arts is commonly called borax. This salt +has a peculiar power of dissolving metallic oxyds, and of promoting the +fusion of substances capable of being melted; it is accordingly employed +in various metallic arts; it is used, for example, to remove the oxyd +from the surface of metals, and is often employed in the assaying of +metallic ores. + + +Let us now proceed to the FLUORIC ACID. This acid is obtained from a +substance which is found frequently in mines, and particularly in those +of Derbyshire, called _fluor_, a name which it acquired from the +circumstance of its being used to render the ores of metals more fluid +when heated. + +CAROLINE. + +Pray is not this the Derbyshire spar, of which so many ornaments are +made? + +MRS. B. + +The same; but though it has long been employed for a variety of +purposes, its nature was unknown until Scheele, the great Swedish +chemist, discovered that it consisted of lime united with a peculiar +acid, which obtained the name of _fluoric acid_. It is easily separated +from the lime by the sulphuric acid, and unless condensed in water, +ascends in the form of gas. A very peculiar property of this acid is its +union with siliceous earths, which I have already mentioned. If the +distillation of this acid is performed in glass vessels, they are +corroded, and the siliceous part of the glass comes over, united with +the gas; if water is then admitted, part of the silex is deposited, as +you may observe in this jar. + +CAROLINE. + +I see white flakes forming on the surface of the water; is that silex? + +MRS. B. + +Yes it is. This power of corroding glass has been used for engraving, or +rather etching, upon it. The glass is first covered with a coat of wax, +through which the figures to be engraved are to be scratched with a pin; +then pouring the fluoric acid over the wax, it corrodes the glass where +the scratches have been made. + +CAROLINE. + +I should like to have a bottle of this acid, to make engravings. + +MRS. B. + +But you could not have it in a _glass_ bottle, for in that case the acid +would be saturated with silex, and incapable of executing an engraving; +the same thing would happen were the acid kept in vessels of porcelain +or earthen-ware; this acid must therefore be both prepared and preserved +in vessels of silver. + +If it be distilled from fluor spar and vitriolic acid, in silver or +leaden vessels, the receiver being kept very cold during the +distillation, it assumes the form of a dense fluid, and in that state is +the most intensely corrosive substance known. This seems to be the acid +combined with a little water. It may be called _hydro-fluoric acid_; and +Sir H. Davy has been led, from some late experiments on the subject, to +consider _pure_ fluoric acid as a compound of a certain unknown +principle, which he calls _fluorine_, with hydrogen. + +Sir H. Davy has also attempted to decompose the fluoric acid by burning +potassium in contact with it; but he has not yet been able by this or +any other method, to obtain its basis in a distinct separate state. + + +We shall conclude our account of the acids with that of the MURIATIC +ACID, which is perhaps the most curious and interesting of all of them. +It is found in nature combined with soda, lime, and magnesia. _Muriat of +soda_ is the common sea-salt, and from this substance the acid is +usually disengaged by means of the sulphuric acid. The natural state of +the muriatic acid is that of an invisible permanent gas, at the common +temperature of the atmosphere; but it has a remarkably strong attraction +for water, and assumes the form of a whitish cloud whenever it meets any +moisture to combine with. This acid is remarkable for its peculiar and +very pungent smell, and possesses, in a powerful degree, most of the +acid properties. Here is a bottle containing muriatic acid in a liquid +state. + +CAROLINE. + +And how is it liquefied? + +MRS. B. + +By impregnating water with it; its strong attraction for water makes it +very easy to obtain it in a liquid form. Now, if I open the phial, you +may observe a kind of vapour rising from it, which is muriatic acid gas, +of itself invisible, but made apparent by combining with the moisture of +the atmosphere. + +EMILY. + +Have you not any of the pure muriatic acid gas? + +MRS. B. + +This jar is full of that acid in its gaseous state--it is inverted over +mercury instead of water, because, being absorbable by water, this gas +cannot be confined by it. --I shall now raise the jar a little on one +side, and suffer some of the gas to escape. --You see that it +immediately becomes visible in the form of a cloud. + +EMILY. + +It must be, no doubt, from its uniting with the moisture of the +atmosphere, that it is converted into this dewy vapour. + +MRS. B. + +Certainly; and for the same reason, that is to say, its extreme +eagerness to unite with water, this gas will cause snow to melt as +rapidly as an intense fire. + +This acid proved much more refractory when Sir H. Davy attempted to +decompose it than the other two undecompounded acids. It is singular +that potassium will burn in muriatic acid, and be converted into potash, +without decomposing the acid, and the result of this combustion is a +_muriat of potash_; for the potash, as soon as it is regenerated, +combines with the muriatic acid. + +CAROLINE. + +But how can the potash be regenerated if the muriatic acid does not +oxydate the potassium? + +MRS. B. + +The potassium, in this process, obtains oxygen from the moisture with +which the muriatic acid is always combined, and accordingly hydrogen, +resulting from the decomposition of the moisture, is invariably evolved. + +EMILY. + +But why not make these experiments with dry muriatic acid? + +MRS. B. + +Dry acids cannot be acted on by the Voltaic battery, because acids are +non-conductors of electricity, unless moistened. In the course of a +number of experiments which Sir H. Davy made upon acids in a state of +dryness, he observed that the presence of water appeared always +necessary to develop the acid properties, so that acids are not even +capable of reddening vegetable blues if they have been carefully +deprived of moisture. This remarkable circumstance led him to suspect, +that water, instead of oxygen, may be the acidifying principle; but this +he threw out rather as a conjecture than as an established point. + +Sir H. Davy obtained very curious results from burning potassium in a +mixture of phosphorus and muriatic acid, and also of sulphur and +muriatic acid; the latter detonates with great violence. All his +experiments, however, failed in presenting to his view the basis of the +muriatic acid, of which he was in search; and he was at last induced to +form an opinion respecting the nature of this acid, which I shall +presently explain. + +EMILY. + +Is this acid susceptible of different degrees of oxygenation? + +MRS. B. + +Yes, for though we cannot deoxygenate this acid, yet we may add oxygen +to it. + +CAROLINE. + +Why, then, is not the least degree of oxygenation of the acid called the +_muriatous_, and the higher degree the _muriatic_ acid? + +MRS. B. + +Because, instead of becoming, like other acids, more dense, and more +acid by an addition of oxygen, it is rendered on the contrary more +volatile, more pungent, but less acid, and less absorbable by water. +These circumstances, therefore, seem to indicate the propriety of making +an exception to the nomenclature. The highest degree of oxygenation of +this acid has been distinguished by the additional epithet of +_oxygenated_, or, for the sake of brevity, _oxy_, so that it is called +the _oxygenated_, or _oxy-muriatic acid_. This likewise exists in a +gaseous form, at the temperature of the atmosphere; it is also +susceptible of being absorbed by water, and can be congealed, or +solidified, by a certain degree of cold. + +EMILY. + +And how do you obtain the oxy-muriatic acid? + +MRS. B. + +In various ways; but it may be most conveniently obtained by distilling +liquid muriatic acid over oxyd of manganese, which supplies the acid +with the additional oxygen. One part of the acid being put into a +retort, with two parts of the oxyd of manganese, and the heat of a lamp +applied, the gas is soon disengaged, and may be received over water, as +it is but sparingly absorbed by it. --I have collected some in this +jar-- + +CAROLINE. + +It is not invisible, like the generality of gases; for it is of a +yellowish colour. + +MRS. B. + +The muriatic acid extinguishes flame, whilst, on the contrary, the +oxy-muriatic makes the flame larger, and gives it a dark red colour. Can +you account for this difference in the two acids? + +EMILY. + +Yes, I think so; the muriatic acid will not supply the flame with the +oxygen necessary for its support; but when this acid is further +oxygenated, it will part with its additional quantity of oxygen, and in +this way support combustion. + +MRS. B. + +That is exactly the case; indeed the oxygen added to the muriatic acid, +adheres so slightly to it, that it is separated by mere exposure to the +sun’s rays. This acid is decomposed also by combustible bodies, many of +which it burns, and actually inflames, without any previous increase of +temperature. + +CAROLINE. + +That is extraordinary, indeed! I hope you mean to indulge us with some +of these experiments? + +MRS. B. + +I have prepared several glass jars of oxy-muriatic acid gas for that +purpose. In the first we shall introduce some Dutch gold leaf. --Do you +observe that it takes fire? + +EMILY. + +Yes, indeed it does--how wonderful it is! It became immediately red hot, +but was soon smothered in a thick vapour. + +CAROLINE. + +What a disagreeable smell! + +MRS. B. + +We shall try the same experiment with phosphorus in another jar of this +acid. --You had better keep your handkerchief to your nose when I open +it--now let us drop into it this little piece of phosphorus-- + +CAROLINE. + +It burns really; and almost as brilliantly as in oxygen gas! But, what +is most extraordinary, these combustions take place without the metal or +phosphorus being previously lighted, or even in the least heated. + +MRS. B. + +All these curious effects are owing to the very great facility with +which this acid yields oxygen to such bodies as are strongly disposed to +combine with it. It appears extraordinary indeed to see bodies, and +metals in particular, melted down and inflamed, by a gas without any +increase of temperature, either of the gas, or of the combustible. The +phenomenon, however, is, you see, well accounted for. + +EMILY. + +Why did you burn a piece of Dutch gold leaf rather than a piece of any +other metal? + +MRS. B. + +Because, in the first place, it is a composition of metals (consisting +chiefly of copper) which burns readily; and I use a thin metallic leaf +in preference to a lump of metal, because it offers to the action of the +gas but a small quantity of matter under a large surface. Filings, or +shavings, would answer the purpose nearly as well; but a lump of metal, +though the surface would oxydate with great rapidity, would not take +fire. Pure gold is not inflamed by oxy-muriatic acid gas, but it is +rapidly oxydated, and dissolved by it; indeed, this acid is the only one +that will dissolve gold. + +EMILY. + +This, I suppose, is what is commonly called _aqua regia_, which you know +is the only thing that will act upon gold. + +MRS. B. + +That is not exactly the case either; for aqua regia is composed of a +mixture of muriatic acid and nitric acid. --But, in fact, the result of +this mixture is the formation of oxy-muriatic acid, as the muriatic acid +oxygenates itself at the expence of the nitric; this mixture, therefore, +though it bears the name of _nitro-muriatic acid_, acts on gold merely +in virtue of the oxy-muriatic acid which it contains. + +Sulphur, volatile oils, and many other substances, will burn in the same +manner in oxy-muriatic acid gas; but I have not prepared a sufficient +quantity of it, to show you the combustion of all these bodies. + +CAROLINE. + +There are several jars of the gas yet remaining. + +MRS. B. + +We must reserve these for future experiments. The oxy-muriatic acid does +not, like other acids, redden the blue vegetable colours; but it totally +destroys any colour, and turns all vegetables perfectly white. Let us +collect some vegetable substances to put into this glass, which is full +of gas. + +EMILY. + +Here is a sprig of myrtle-- + +CAROLINE. + +And here some coloured paper-- + +MRS. B. + +We shall also put in this piece of scarlet riband, and a rose-- + +EMILY. + +Their colours begin to fade immediately! But how does the gas produce +this effect? + +MRS. B. + +The oxygen combines with the colouring matter of these substances, and +destroys it; that is to say, destroys the property which these colours +had of reflecting only one kind of rays, and renders them capable of +reflecting them all, which, you know, will make them appear white. Old +prints may be cleaned by this acid, for the paper will be whitened +without injury to the impression, as printer’s ink is made of materials +(oil and lamp black) which are not acted upon by acids. + +This property of the oxy-muriatic acid has lately been employed in +manufactures in a variety of bleaching processes; but for these purposes +the gas must be dissolved in water, as the acid is thus rendered much +milder and less powerful in its effects; for, in a gaseous state, it +would destroy the texture, as well as the colour of the substance +submitted to its action. + +CAROLINE. + +Look at the things which we put into the gas; they have now entirely +lost their colour! + +MRS. B. + +The effect of the acid is almost completed; and, if we were to examine +the quantity that remains, we should find it to consist chiefly of +muriatic acid. + +The oxy-muriatic acid has been used to purify the air in fever hospitals +and prisons, as it burns and destroys putrid effluvia of every kind. The +infection of the small-pox is likewise destroyed by this gas, and matter +that has been submitted to its influence will no longer generate that +disorder. + +CAROLINE. + +Indeed, I think the remedy must be nearly as bad as the disease; the +oxy-muriatic acid has such a dreadfully suffocating smell. + +MRS. B. + +It is certainly extremely offensive; but by keeping the mouth shut, and +wetting the nostrils with liquid ammonia, in order to neutralize the +vapour as it reaches the nose, its prejudicial effects may be in some +degree prevented. At any rate, however, this mode of disinfection can +hardly be used in places that are inhabited. And as the vapour of nitric +acid, which is scarcely less efficacious for this purpose, is not at all +prejudicial, it is usually preferred on such occasions. + +CAROLINE. + +You have not told us yet what is Sir H. Davy’s new opinion respecting +the nature of muriatic acid, to which you alluded a few minutes ago? + +MRS. B. + +True; I avoided noticing it then, because you could not have understood +it without some previous knowledge of the oxy-muriatic acid, which I +have but just introduced to your acquaintance. + +Sir H. Davy’s idea is that muriatic acid, instead of being a compound, +consisting of an unknown basis and oxygen, is formed by the union of +oxy-muriatic gas with hydrogen. + +EMILY. + +Have you not told us just now that oxy-muriatic gas was itself a +compound of muriatic acid and oxygen? + +MRS. B. + +Yes; but according to Sir H. Davy’s hypothesis, oxy-muriatic gas is +considered as a simple body, which contains no oxygen--as a substance of +its own kind, which has a great analogy to oxygen in most of its +properties, though in others it differs entirely from it. --According to +this view of the subject, the name of _oxy-muriatic acid_ can no longer +be proper, and therefore Sir H. Davy has adopted that of _chlorine_, or +_chlorine gas_, a name which is simply expressive of its greenish +colour; and in compliance with that philosopher’s theory, we have placed +chlorine in our table among the simple bodies. + +CAROLINE. + +But what was Sir H. Davy’s reason for adopting an opinion so contrary to +that which had hitherto prevailed? + +MRS. B. + +There are many circumstances which are favourable to the new doctrine; +but the clearest and simplest fact in its support is, that if hydrogen +gas and oxy-muriatic gas be mixed together, both these gases disappear, +and muriatic acid gas is formed. + +EMILY. + +That seems to be a complete proof; is it not considered as perfectly +conclusive? + +MRS. B. + +Not so decisive as it appears at first sight; because it is argued by +those who still incline to the old doctrine, that muriatic acid gas, +however dry it may be, always contains a certain quantity of water, +which is supposed essential to its formation. So that, in the experiment +just mentioned, this water is supplied by the union of the hydrogen gas +with the oxygen of the oxy-muriatic acid; and therefore the mixture +resolves itself into the base of muriatic acid and water, that is, +muriatic acid gas. + +CAROLINE. + +I think the old theory must be the true one; for otherwise how could you +explain the formation of oxy-muriatic gas, from a mixture of muriatic +acid and oxyd of manganese? + +MRS. B. + +Very easily; you need only suppose that in this process the muriatic +acid is decomposed; its hydrogen unites with the oxygen of the manganese +to form water, and the chlorine appears in its separate state. + +EMILY. + +But how can you explain the various combustions which take place in +oxy-muriatic gas, if you consider it as containing no oxygen? + +MRS. B. + +We need only suppose that combustion is the result of intense chemical +action; so that chlorine, like oxygen, in combining with bodies, forms +compounds which have less capacity for caloric than their constituent +principles, and, therefore, caloric is evolved at the moment of their +combination. + +EMILY. + +If, then, we may explain every thing by either theory, to which of the +two shall we give the preference? + +MRS. B. + +It will, perhaps, be better to wait for more positive proofs, if such +can be obtained, before we decide positively upon the subject. The new +doctrine has certainly gained ground very rapidly, and may be considered +as nearly established; but several competent judges still refuse their +assent to it, and until that theory is very generally adopted, it may be +as well for us still occasionally to use the language to which chemists +have long been accustomed. --But let us proceed to the examination of +salts formed by muriatic acid. + +Among the compound salts formed by muriatic acid, the _muriat of soda_, +or common salt, is the most interesting.* The uses and properties of +this salt are too well known to require much comment. Besides the +pleasant flavour it imparts to the food, it is very wholesome, when not +used to excess, as it assists the process of digestion. + +Sea-water is the great source from which muriat of soda is extracted by +evaporation. But it is also found in large solid masses in the bowels of +the earth, in England, and in many other parts of the world. + + [Footnote *: According to Sir H. Davy’s views of the nature of the + muriatic and oxy-muriatic acids, dry muriat of soda is a compound + of sodium and chlorine, for it may be formed by the direct + combination of oxy-muriatic gas and sodium. In his opinion, + therefore, what we commonly call muriat of soda contains neither + soda nor muriatic acid.] + +EMILY. + +I thought that salts, when solid, were always in the state of crystals; +but the common table-salt is in the form of a coarse white powder. + +MRS. B. + +Crystallisation depends, as you may recollect, on the slow and regular +reunion of particles dissolved in a fluid; common sea-salt is only in a +state of imperfect crystallisation, because the process by which it is +prepared is not favourable to the formation of regular crystals. But if +you dissolve it, and afterwards evaporate the water slowly, you will +obtain a regular crystallisation. + +_Muriat of ammonia_ is another combination of this acid, which we have +already mentioned as the principal source from which ammonia is derived. + +I can at once show you the formation of this salt by the immediate +combination of muriatic acid with ammonia. --These two glass jars +contain, the one muriatic acid gas, the other ammoniacal gas, both of +which are perfectly invisible--now, if I mix them together, you see they +immediately form an opake white cloud, like smoke. --If a thermometer +was placed in the jar in which these gases are mixed, you would perceive +that some heat is at the same time produced. + +EMILY. + +The effects of chemical combinations are, indeed, wonderful! --How +extraordinary it is that two invisible bodies should become visible by +their union! + +MRS. B. + +This strikes you with astonishment, because it is a phenomenon which +nature seldom exhibits to our view; but the most common of her +operations are as wonderful, and it is their frequency only that +prevents our regarding them with equal admiration. What would be more +surprising, for instance, than combustion, were it not rendered so +familiar by custom? + +EMILY. + +That is true. --But pray, Mrs. B., is this white cloud the salt that +produces ammonia? How different it is from the solid muriat of ammonia +which you once showed us! + +MRS. B. + +It is the same substance which first appears in the state of vapour, but +will soon be condensed by cooling against the sides of the jar, in the +form of very minute crystals. + +We may now proceed to the _oxy-muriats_. In this class of salts the +_oxy-muriat of potash_ is the most worthy of our attention, for its +striking properties. The acid, in this state of combination, contains a +still greater proportion of oxygen than when alone. + +CAROLINE. + +But how can the oxy-muriatic acid acquire an increase of oxygen by +combining with potash? + +MRS. B. + +It does not really acquire an additional quantity of oxygen, but it +loses some of the muriatic acid, which produces the same effect, as the +acid which remains is proportionably super-oxygenated.* + +If this salt be mixed, and merely rubbed together with sulphur, +phosphorus, charcoal, or indeed any other combustible, it explodes +strongly. + + [Footnote *: According to Sir H. Davy’s new views, just explained, + oxy-muriat of potash is a compound of chlorine with oxyd of + potassium.] + +CAROLINE. + +Like gun-powder, I suppose, it is suddenly converted into elastic +fluids? + +MRS. B. + +Yes; but with this remarkable difference, that no increase of +temperature, any further than is produced by gentle friction, is +required in this instance. Can you tell me what gases are generated by +the detonation of this salt with charcoal? + +EMILY. + +Let me consider . . . . . The oxy-muriatic acid parts with its excess of +oxygen to the charcoal, by which means it is converted into muriatic +acid gas; whilst the charcoal, being burnt by the oxygen, is changed to +carbonic acid gas. --What becomes of the potash I cannot tell. + +MRS. B. + +That is a fixed product which remains in the vessel. + +CAROLINE. + +But since the potash does not enter into the new combinations, I do not +understand of what use it is in this operation. Would not the +oxy-muriatic acid and the charcoal produce the same effect without it? + +MRS. B. + +No; because there would not be that very great concentration of oxygen +which the combination with the potash produces, as I have just +explained. + +I mean to show you this experiment, but I would advise you not to repeat +it alone; for if care be not taken to mix only very small quantities at +a time, the detonation will be extremely violent, and may be attended +with dangerous effects. You see I mix an exceedingly small quantity of +the salt with a little powdered charcoal, in this Wedgwood mortar, and +rub them together with the pestle-- + +CAROLINE. + +Heavens! How can such a loud explosion be produced by so small a +quantity of matter? + +MRS. B. + +You must consider that an extremely small quantity of solid substance +may produce a very great volume of gases; and it is the sudden evolution +of these which occasions the sound. + +EMILY. + +Would not oxy-muriat of potash make stronger gunpowder than nitrat of +potash? + +MRS. B. + +Yes; but the preparation, as well as the use of this salt, is attended +with so much danger, that it is never employed for that purpose. + +CAROLINE. + +There is no cause to regret it, I think; for the common gunpowder is +quite sufficiently destructive. + +MRS. B. + +I can show you a very curious experiment with this salt; but it must +again be on condition that you will never attempt to repeat it by +yourselves. I throw a small piece of phosphorus into this glass of +water; then a little oxy-muriat of potash; and, lastly, I pour in (by +means of this funnel, so as to bring it in contact with the two other +ingredients at the bottom of the glass) a small quantity of sulphuric +acid-- + +CAROLINE. + +This is, indeed, a beautiful experiment! The phosphorus takes fire and +burns from the bottom of the water. + +EMILY. + +How wonderful it is to see flame bursting out under water, and rising +through it! Pray, how is this accounted for? + +MRS. B. + +Cannot you find it out, Caroline? + +EMILY. + +Stop--I think I can explain it. Is it not because the sulphuric acid +decomposes the salt by combining with the potash, so as to liberate the +oxy-muriatic acid gas by which the phosphoric is set on fire? + +MRS. B. + +Very well, Emily; and with a little more reflection you would have +discovered another concurring circumstance, which is, that an increase +of temperature is produced by the mixture of the sulphuric acid and +water, which assists in promoting the combustion of the phosphorus. + + +I must, before we part, introduce to your acquaintance the +newly-discovered substance IODINE, which you may recollect we placed +next to oxygen and chlorine in our table of simple bodies. + +CAROLINE. + +Is this also a body capable of maintaining combustion like oxygen and +chlorine? + +MRS. B. + +It is; and although it does not so generally disengage light and heat +from inflammable bodies, as oxygen and chlorine do, yet it is capable of +combining with most of them; and sometimes, as in the instance of +potassium and phosphorus, the combination is attended with an actual +appearance of light and heat. + +CAROLINE. + +But what sort of a substance is iodine: what is its form, and colour? + +MRS. B. + +It is a very singular body, in many respects. At the ordinary +temperature of the atmosphere, it commonly appears in the form of +blueish black crystalline scales, such as you see in this tube. + +CAROLINE. + +They shine like black lead, and some of the scales have the shape of +lozenges. + +MRS. B. + +That is actually the form which the crystals of iodine often assume. But +if we heat them gently, by holding the tube over the flame of a candle, +see what a change takes place in them. + +CAROLINE. + +How curious! They seem to melt, and the tube immediately fills with a +beautiful violet vapour. But look, Mrs. B., the same scales are now +appearing at the other end of the tube. + +MRS. B. + +This is in fact a sublimation of iodine, from one part of the tube to +another; but with this remarkable peculiarity, that, while in the +gaseous state, iodine assumes that bright violet colour, which, as you +may already perceive, it loses as the tube cools, and the substance +resumes its usual solid form. --It is from the violet colour of the gas +that iodine has obtained its name. + +CAROLINE. + +But how is this curious substance obtained? + +MRS. B. + +It is found in the ley of ashes of sea-weeds, after the soda has been +separated by crystallisation; and it is disengaged by means of sulphuric +acid, which expels it from the alkaline ley in the form of a violet gas, +which may be collected and condensed in the way you have just seen. +--This interesting discovery was made in the year 1812, by M. Courtois, +a manufacturer of saltpetre at Paris. + +CAROLINE. + +And pray, Mrs. B., what is the proof of iodine being a simple body? + +MRS. B. + +It is considered as a simple body, both because it is not capable of +being resolved into other ingredients; and because it is itself capable +of combining with other bodies, in a manner analogous to oxygen and +chlorine. The most curious of these combinations is that which it forms +with hydrogen gas, the result of which is a peculiar gaseous acid. + +CAROLINE. + +Just as chlorine and hydrogen gas form muriatic acid? In this respect +chlorine and iodine seem to bear a strong analogy to each other. + +MRS. B. + +That is indeed the case; so that if the theory of the constitution of +either of these two bodies be true, it must be true also in regard to +the other; if erroneous in the one, the theory must fall in both. + +But it is now time to conclude; we have examined such of the acids and +salts as I conceived would appear to you most interesting. --I shall not +enter into any particulars respecting the metallic acids, as they offer +nothing sufficiently striking for our present purpose. + + + + +CONVERSATION XX. + +ON THE NATURE AND COMPOSITION OF VEGETABLES. + + +MRS. B. + +We have hitherto treated only of the simplest combinations of elements, +such as alkalies, earths, acids, compound salts, stones, &c.; all of +which belong to the mineral kingdom. It is time now to turn our +attention to a more complicated class of compounds, that of ORGANISED +BODIES, which will furnish us with a new source of instruction and +amusement. + +EMILY. + +By organised bodies, I suppose, you mean the vegetable and animal +creation? I have, however, but a very vague idea of the word +_organisation_, and I have often wished to know more precisely what it +means. + +MRS. B. + +Organised bodies are such as are endowed by nature with various parts, +peculiarly constructed and adapted to perform certain functions +connected with life. Thus you may observe, that mineral compounds are +formed by the simple effect of mechanical or chemical attraction, and +may appear to some to be in a great measure the productions of chance; +whilst organised bodies bear the most striking and impressive marks of +design, and are eminently distinguished by that unknown principle, +called _life_, from which the various organs derive the power of +exercising their respective functions. + +CAROLINE. + +But in what manner does life enable these organs to perform their +several functions? + +MRS. B. + +That is a mystery which, I fear, is enveloped in such profound darkness +that there is very little hope of our ever being able to unfold it. We +must content ourselves with examining the effects of this principle; as +for the cause, we have been able only to give it a name, without +attaching any other meaning to it than the vague and unsatisfactory idea +of au unknown agent. + +CAROLINE. + +And yet I think I can form a very clear idea of life. + +MRS. B. + +Pray let me hear how you would define it? + +CAROLINE. + +It is perhaps more easy to conceive than to express--let me consider-- +Is not life the power which enables both the animal and the vegetable +creation to perform the various functions which nature has assigned to +them? + +MRS. B. + +I have nothing to object to your definition; but you will allow me to +observe, that you have only mentioned the effects which the unknown +cause produces, without giving us any notion of the cause itself. + +EMILY. + +Yes, Caroline, you have told us what life _does_, but you have not told +us what it _is_. + +MRS. B. + +We may study its operations, but we should puzzle ourselves to no +purpose by attempting to form an idea of its real nature. + +We shall begin with examining its effects in the vegetable world, which +constitutes the simplest class of organised bodies; these we shall find +distinguished from the mineral creation, not only by their more +complicated nature, but by the power which they possess within +themselves, of forming new chemical arrangements of their constituent +parts, by means of appropriate organs. Thus, though all vegetables are +ultimately composed of hydrogen, carbon, and oxygen, (with a few other +occasional ingredients,) they separate and combine these principles by +their various organs, in a thousand ways, and form, with them, different +kinds of juices and solid parts, which exist ready made in vegetables, +and may, therefore, be considered as their immediate materials. + +These are: + + _Sap_, + _Mucilage_, + _Sugar_, + _Fecula_, + _Gluten_, + _Fixed Oil_, + _Volatile Oil_, + _Camphor_, + _Resins_, + _Gum Resins_, + _Balsams_, + _Caoutchouc_, + _Extractive colouring Matter_, + _Tannin_, + _Woody Fibre_, + _Vegetable Acids_, _&c._ + +CAROLINE. + +What a long list of names! I did not suppose that a vegetable was +composed of half so many ingredients. + +MRS. B. + +You must not imagine that every one of these materials is formed in each +individual plant. I only mean to say, that they are all derived +exclusively from the vegetable kingdom. + +EMILY. + +But does each particular part of the plant, such as the root, the bark, +the stem, the seeds, the leaves, consist of one of these ingredients +only, or of several of them combined together? + +MRS. B. + +I believe there is no part of a plant which can be said to consist +solely of any one particular ingredient; a certain number of vegetable +materials must always be combined for the formation of any particular +part, (of a seed for instance,) and these combinations are carried on by +sets of vessels, or minute organs, which select from other parts, and +bring together, the several principles required for the development and +growth of those particular parts which they are intended to form and to +maintain. + +EMILY. + +And are not these combinations always regulated by the laws of chemical +attraction? + +MRS. B. + +No doubt; the organs of plants cannot force principles to combine that +have no attraction for each other; nor can they compel superior +attractions to yield to those of inferior power; they probably act +rather mechanically, by bringing into contact such principles, and in +such proportions, as will, by their chemical combination, form the +various vegetable products. + +CAROLINE. + +We may then consider each of these organs as a curiously constructed +apparatus, adapted for the performance of a variety of chemical +processes. + +MRS. B. + +Exactly so. As long as the plant lives and thrives, the carbon, +hydrogen, and oxygen, (the chief constituents of its immediate +materials,) are so balanced and connected together, that they are not +susceptible of entering into other combinations; but no sooner does +death take place, than this state of equilibrium is destroyed, and new +combinations produced. + +EMILY. + +But why should death destroy it; for these principles must remain in the +same proportions, and consequently, I should suppose, in the same order +of attractions? + +MRS. B. + +You must remember, that in the vegetable, as well as in the animal +kingdom, it is by the principle of _life_ that the organs are enabled to +act; when deprived of that agent or stimulus, their power ceases, and an +order of attractions succeeds similar to that which would take place in +mineral or unorganised matter. + +EMILY. + +It is this new order of attractions, I suppose, that destroys the +organisation of the plant after death; for if the same combinations +still continued to prevail, the plant would always remain in the state +in which it died? + +MRS. B. + +And that, you know, is never the case; plants may be partially preserved +for some time after death, by drying; but in the natural course of +events they all return to the state of simple elements; a wise and +admirable dispensation of Providence, by which dead plants are rendered +fit to enrich the soil, and become subservient to the nourishment of +living vegetables. + +CAROLINE. + +But we are talking of the dissolution of plants, before we have examined +them in their living state. + +MRS. B. + +That is true, my dear. But I wished to give you a general idea of the +nature of vegetation, before we entered into particulars. Besides, it is +not so irrelevant as you suppose to talk of vegetables in their dead +state, since we cannot analyse them without destroying life; and it is +only by hastening to submit them to examination, immediately after they +have ceased to live, that we can anticipate their natural decomposition. +There are two kinds of analysis of which vegetables are susceptible; +first, that which separates them into their immediate materials, such as +sap, resin, mucilage, &c.; secondly, that which decomposes them into +their primitive elements, as carbon, hydrogen, and oxygen. + +EMILY. + +Is there not a third kind of analysis of plants, which consists in +separating their various parts, as the stem, the leaves, and the several +organs of the flower? + +MRS. B. + +That, my dear, is rather the department of the botanist; we shall +consider these different parts of plants only, as the organs by which +the various secretions or separations are performed; but we must first +examine the nature of these secretions. + +The _sap_ is the principal material of vegetables, since it contains the +ingredients that nourish every part of the plant. The basis of this +juice, which the roots suck up from the soil, is water; this holds in +solution the various other ingredients required by the several parts of +the plant, which are gradually secreted from the sap by the different +organs appropriated to that purpose, as it passes them in circulating +through the plant. + + +_Mucus_, or _mucilage_, is a vegetable substance, which, like all the +others, is secreted from the sap; when in excess, it exudes from trees +in the form of gum. + +CAROLINE. + +Is that the gum so frequently used instead of paste or glue? + +MRS. B. + +It is; almost all fruit-trees yield some sort of gum, but that most +commonly used in the arts is obtained from a species of acacia-tree in +Arabia, and is called _gum arabic_; it forms the chief nourishment of +the natives of those parts, who obtain it in great quantities from +incisions which they make in the trees. + +CAROLINE. + +I did not know that gum was eatable. + +MRS. B. + +There is an account of a whole ship’s company being saved from starving +by feeding on the cargo, which was gum senegal. I should not, however, +imagine, that it would be either a pleasant or a particularly eligible +diet to those who have not, from their birth, been accustomed to it. It +is, however, frequently taken medicinally, and considered as very +nourishing. Several kinds of vegetable acids may be obtained, by +particular processes, from gum or mucilage, the principal of which is +called the _mucous acid_. + + +_Sugar_ is not found in its simple state in plants, but is always mixed +with gum, sap, or other ingredients; this saccharine matter is to be met +with in every vegetable, but abounds most in roots, fruits, and +particularly in the sugar-cane. + +EMILY. + +If all vegetables contain sugar, why is it extracted exclusively from +the sugar-cane? + +MRS. B. + +Because it is both most abundant in that plant, and most easily obtained +from it. Besides, the sugars produced by other vegetables differ a +little in their nature. + +During the late troubles in the West-Indies, when Europe was but +imperfectly supplied with sugar, several attempts were made to extract +it from other vegetables, and very good sugar was obtained from parsnips +and from carrots; but the process was too expensive to carry this +enterprize to any extent. + +CAROLINE. + +I should think that sugar might be more easily obtained from sweet +fruits, such as figs, dates, &c. + +MRS. B. + +Probably; but it would be still more expensive, from the high price of +those fruits. + +EMILY. + +Pray, in what manner is sugar obtained from the sugar-cane? + +MRS. B. + +The juice of this plant is first expressed by passing it between two +cylinders of iron. It is then boiled with lime-water, which makes a +thick scum rise to the surface. The clarified liquor is let off below +and evaporated to a very small quantity, after which it is suffered to +crystallise by standing in a vessel, the bottom of which is perforated +with holes, that are imperfectly stopped, in order that the syrup may +drain off. The sugar obtained by this process is a coarse brown powder, +commonly called raw or moist sugar; it undergoes another operation to be +refined and converted into loaf sugar. For this purpose it is dissolved +in water, and afterwards purified by an animal fluid called albumen. +White of eggs chiefly consist of this fluid, which is also one of the +constituent parts of blood; and consequently eggs, or bullocks’ blood, +are commonly used for this purpose. + +The albuminous fluid being diffused through the syrup, combines with all +the solid impurities contained in it, and rises with them to the +surface, where it forms a thick scum; the clear liquor is then again +evaporated to a proper consistence, and poured into moulds, in which, by +a confused crystallisation, it forms loaf-sugar. But an additional +process is required to whiten it; to this effect the mould is inverted, +and its open base is covered with clay, through which water is made to +pass; the water slowly trickling through the sugar, combines with and +carries off the colouring matter. + +CAROLINE. + +I am very glad to hear that the blood that is used to purify sugar does +not remain in it; it would be a disgusting idea. I have heard of some +improvements by the late Mr. Howard, in the process of refining sugar. +Pray what are they? + +MRS. B. + +It would be much too long to give you an account of the process in +detail. But the principal improvement relates to the mode of evaporating +the syrup, in order to bring it to the consistency of sugar. Instead of +boiling the syrup in a large copper, over a strong fire, Mr. Howard +carries off the water by means of a large air-pump, in a way similar to +that used in Mr. Leslie’s experiment for freezing water by evaporation; +that is, the syrup being exposed to a vacuum, the water evaporates +quickly, with no greater heat than that of a little steam, which is +introduced round the boiler. The air-pump is of course of large +dimensions, and is worked by a steam engine. A great saving is thus +obtained, and a striking instance afforded of the power of science in +suggesting useful economical improvements. + +EMILY. + +And pray how is sugar-candy and barley-sugar prepared? + +MRS. B. + +Candied sugar is nothing more than the regular crystals, obtained by +slow evaporation from a solution of sugar. Barley-sugar is sugar melted +by heat, and afterwards cooled in moulds of a spiral form. + +Sugar may be decomposed by a red heat, and, like all other vegetable +substances, resolved into carbonic acid and hydrogen. The formation and +the decomposition of sugar afford many very interesting particulars, +which we shall fully examine, after having gone through the other +materials of vegetables. We shall find that there is reason to suppose +that sugar is not, like the other materials, secreted from the sap by +appropriate organs; but that it is formed by a peculiar process with +which you are not yet acquainted. + +CAROLINE. + +Pray, is not honey of the same nature as sugar? + +MRS. B. + +Honey is a mixture of saccharine matter and gum. + +EMILY. + +I thought that honey was in some measure an animal substance, as it is +prepared by the bees. + +MRS. B. + +It is rather collected by them from flowers, and conveyed to their +store-houses, the hives. It is the wax only that undergoes a real +alteration in the body of the bee, and is thence converted into an +animal substance. + +Manna is another kind of sugar, which is united with a nauseous +extractive matter, to which it owes its peculiar taste and colour. It +exudes like gum from various trees in hot climates, some of which have +their leaves glazed by it. + +The next of the vegetable materials is _fecula_; this is the general +name given to the farinaceous substance contained in all seeds, and in +some roots, as the potatoe, parsnip, &c. It is intended by nature for +the first aliment of the young vegetable; but that of one particular +grain is become a favourite and most common food of a large part of +mankind. + +EMILY. + +You allude, I suppose, to bread, which is made of wheat-flower? + +MRS. B. + +Yes. The fecula of wheat contains also another vegetable substance which +seems peculiar to that seed, or at least has not as yet been obtained +from any other. This is _gluten_, which is of a sticky, ropy, elastic +nature; and it is supposed to be owing to the viscous qualities of this +substance, that wheat-flour forms a much better paste than any other. + +EMILY. + +Gluten, by your description, must be very like gum? + +MRS. B. + +In their sticky nature they certainly have some resemblance; but gluten +is essentially different from gum in other points, and especially in its +being insoluble in water, whilst gum, you know, is extremely soluble. + +The _oils_ contained in vegetables all consist of hydrogen and carbon in +various proportions. They are of two kinds, _fixed_ and _volatile_, both +of which we formerly mentioned. Do you remember in what the difference +between fixed and volatile oil consists? + +EMILY. + +If I recollect rightly, the former are decomposed by heat, whilst the +latter are merely volatilised by it. + +MRS. B. + +Very well. Fixed oil is contained only in the seeds of plants, excepting +in the olive, in which it is produced in, and expressed from, the fruit. +We have already observed that seeds contain also fecula; these two +substances, united with a little mucilage, form the white substance +contained in the seeds or kernels of plants, and is destined for the +nourishment of the young plant, to which the seed gives birth. The milk +of almonds, which is expressed from the seed of that name, is composed +of these three substances. + +EMILY. + +Pray, of what nature is the linseed oil which is used in painting? + +MRS. B. + +It is a fixed oil, obtained from the seed of flax. Nut oil, which is +frequently used for the same purpose, is expressed from walnuts. + +Olive oil is that which is best adapted to culinary purposes. + +CAROLINE. + +And what are the oils used for burning? + +MRS. B. + +Animal oils most commonly; but the preference given to them is owing to +their being less expensive; for vegetable oils burn equally well, and +are more pleasant, as their smell is not offensive. + +EMILY. + +Since oil is so good a combustible, what is the reason that lamps so +frequently require trimming? + +MRS. B. + +This sometimes proceeds from the construction of the lamp, which may not +be sufficiently favourable to a perfect combustion; but there is +certainly a defect in the nature of oil itself, which renders it +necessary for the best-constructed lamps to be occasionally trimmed. +This defect arises from a portion of mucilage which it is extremely +difficult to separate from the oil, and which being a bad combustible, +gathers round the wick, and thus impedes its combustion, and +consequently dims the light. + +CAROLINE. + +But will not oils burn without a wick? + +MRS. B. + +Not unless their temperature be elevated to five or six hundred degrees; +the wick answers this purpose, as I think I once before explained to +you. The oil rises between the fibres of the cotton by capillary +attraction, and the heat of the burning wick volatilises it, and brings +it successively to the temperature at which it is combustible. + +EMILY. + +I suppose the explanation which you have given with regard to the +necessity of trimming lamps, applies also to candles, which so often +require snuffing? + +MRS. B. + +I believe it does; at least, in some degree. But besides the +circumstance just explained, the common sorts of oils are not very +highly combustible, so that the heat produced by a candle, which is a +coarse kind of animal oil, being insufficient to volatilise them +completely, a quantity of soot is gradually deposited on the wick, which +dims the light, and retards the combustion. + +CAROLINE. + +Wax candles then contain no incombustible matter, since they do not +require snuffing? + +MRS. B. + +Wax is a much better combustible than tallow, but still not perfectly +so, since it likewise contains some particles that are unfit for +burning; but when these gather round the wick, (which in a wax light is +comparatively small,) they weigh it down on one side, and fall off +together with the burnt part of the wick. + +CAROLINE. + +As oils are such good combustibles, I wonder that they should require so +great an elevation of temperature before they begin to burn? + +MRS. B. + +Though fixed oils will not enter into actual combustion below the +temperature of about four hundred degrees, yet they will slowly absorb +oxygen at the common temperature of the atmosphere. Hence arises a +variety of changes in oils which modify their properties and uses in the +arts. + +If oil simply absorbs, and combines with oxygen, it thickens and changes +to a kind of wax. This change is observed to take place on the external +parts of certain vegetables, even during their life. But it happens in +many instances that the oil does not retain all the oxygen which it +attracts, but that part of it combines with, or burns, the hydrogen of +the oil, thus forming a quantity of water, which gradually goes off by +evaporation. In this case the alteration of the oil consists not only in +the addition of a certain quantity of oxygen, but in the diminution of +the hydrogen. These oils are distinguished by the name of _drying oils_. +Linseed, poppy, and nut-oils, are of this description. + +EMILY. + +I am well acquainted with drying oils, as I continually use them in +painting. But I do not understand why the acquisition of oxygen on one +hand, and a loss of hydrogen on the other, should render them drying? + +MRS. B. + +This, I conceive, may arise from two reasons; either from the oxygen +which is added being less favourable to the state of fluidity than the +hydrogen, which is subtracted; or from this additional quantity of +oxygen giving rise to new combinations, in consequence of which the most +fluid parts of the oil are liberated and volatilised. + +For the purpose of painting, the drying quality of oil is further +increased by adding a quantity of oxyd of lead to it, by which means it +is more rapidly oxygenated. + +The rancidity of oil is likewise owing to their oxygenation. In this +case a new order of attraction takes place, from which a peculiar acid +is formed, called the _sebacic acid_. + +CAROLINE. + +Since the nature and composition of oil is so well known, pray could not +oil be actually _made_, by combining its principles? + +MRS. B. + +That is by no means a necessary consequence; for there are innumerable +varieties of compound bodies which we can decompose, although we are +unable to reunite their ingredients. This, however, is not the case with +oil, as it has very lately been discovered, that it is possible to form +oil, by a peculiar process, from the action of oxygenated muriatic acid +gas on hydro-carbonate. + +We now pass to the _volatile_ or _essential oils_. These form the basis +of all the vegetable perfumes, and are contained, more or less, in every +part of the plant excepting the seed; they are, at least, never found in +that part of the seed which contains the embrio plant. + +EMILY. + +The smell of flowers, then, proceeds from volatile oil? + +MRS. B. + +Certainly; but this oil is often most abundant in the rind of fruits, as +in oranges, lemons, &c. from which it may be extracted by the slightest +pressure; it is found also in the leaves of plants, and even in the +wood. + +CAROLINE. + +Is it not very plentiful in the leaves of mint, and of thyme, and all +the sweet-smelling herbs? + +MRS. B. + +Yes, remarkably so; and in geranium leaves also, which have a much more +powerful odour than the flowers. + +The perfume of sandal fans is an instance of its existence in wood. In +short, all vegetable odours or perfumes are produced by the evaporation +of particles of these volatile oils. + +EMILY. + +They are, I suppose, very light, and of very thin consistence, since +they are so volatile? + +MRS. B. + +They vary very much in this respect, some of them being as thick as +butter, whilst others are as fluid as water. In order to be prepared for +perfumes, or essences, these oils are first properly purified, and then +either distilled with spirit of wine, as in the case with lavender +water, or simply mixed with a large proportion of water, as is often +done with regard to peppermint. Frequently, also, these odoriferous +waters are prepared merely by soaking the plants in water, and +distilling. The water then comes over impregnated with the volatile oil. + +CAROLINE. + +Such waters are frequently used to take spots of grease out of cloth, or +silk; how do they produce that effect? + +MRS. B. + +By combining with the substance that forms these stains; for volatile +oils, and likewise the spirit in which they are distilled, will dissolve +wax, tallow, spermaceti, and resins; if, therefore, the spot proceeds +from any of these substances, it will remove it. Insects of every kind +have a great aversion to perfumes, so that volatile oils are employed +with success in museums for the preservation of stuffed birds and other +species of animals. + +CAROLINE. + +Pray does not the powerful smell of camphor proceed from a volatile oil? + +MRS. B. + +_Camphor_ seems to be a substance of its own kind, remarkable by many +peculiarities. But if not exactly of the same nature as volatile oil, it +is at least very analogous to it. It is obtained chiefly from the +camphor-tree, a species of laurel which grows in China, and in the +Indian isles, from the stem and roots of which it is extracted. Small +quantities have also been distilled from thyme, sage, and other aromatic +plants; and it is deposited in pretty large quantities by some volatile +oils after long standing. It is extremely volatile and inflammable. It +is insoluble in water, but is soluble in oils, in which state, as well +as in its solid form, it is frequently applied to medicinal purposes. +Amongst the particular properties of camphor, there is one too singular +to be passed over in silence. If you take a small piece of camphor, and +place it on the surface of a bason of pure water, it will immediately +begin to move round and round with great rapidity; but if you pour into +the bason a single drop of any odoriferous fluid, it will instantly put +a stop to this motion. You can at any time try this very simple +experiment; but you must not expect that I shall be able to account for +this phenomenon, as nothing satisfactory has yet been advanced for its +explanation. + +CAROLINE. + +It is very singular indeed; and I will certainly try the experiment. +Pray what are _resins_, which you just now mentioned? + +MRS. B. + +They are volatile oils, that have been acted on, and peculiarly +modified, by oxygen. + +CAROLINE. + +They are, therefore, oxygenated volatile oils? + +MRS. B. + +Not exactly; for the process does not appear to consist so much in the +oxygenation of the oil, as in the combustion of a portion of its +hydrogen, and a small portion of its carbon. For when resins are +artificially made by the combination of volatile oils with oxygen, the +vessel in which the process is performed is bedewed with water, and the +air included within is loaded with carbonic acid. + +EMILY. + +This process must be, in some respects, similar to that for preparing +drying oils? + +MRS. B. + +Yes; and it is by this operation that both of them acquire a greater +degree of consistence. Pitch, tar, and turpentine, are the most common +resins; they exude from the pine and fir trees. Copal, mastic, and +frankincense, are also of this class of vegetable substances. + +EMILY. + +Is it of these resins that the mastic and copal varnishes, so much used +in painting, are made? + +MRS. B. + +Yes. Dissolved either in oil, or in alcohol, resins form varnishes. From +these solutions they may be precipitated by water, in which they are +insoluble. This I can easily show you. --If you will pour some water +into this glass of mastic varnish, it will combine with the alcohol in +which the resin is dissolved, and the latter will be precipitated in the +form of a white cloud-- + +EMILY. + +It is so. And yet how is it that pictures or drawings, varnished with +this solution, may safely be washed with water? + +MRS. B. + +As the varnish dries, the alcohol evaporates, and the dry varnish or +resin which remains, not being soluble in water, will not be acted on +by it. + +There is a class of compound resins called _gum-resins_, which are +precisely what their name denotes, that is to say, resins combined with +mucilage. Myrrh and assafœtida are of this description. + +CAROLINE. + +Is it possible that a substance of so disagreeable a smell as assafœtida +can be formed from a volatile oil? + +MRS. B. + +The odour of volatile oils is by no means always grateful. Onions and +garlic derive their smell from volatile oils, as well as roses and +lavender. + +There is still another form under which volatile oils present +themselves, which is that of _balsams_. These consist of resinous juices +combined with a peculiar acid, called the benzoic acid. Balsams appear +to have been originally volatile oils, the oxygenation of which has +converted one part into a resin, and the other part into an acid, which, +combined together, form a balsam; such are the balsams of Peru, Tolu, +&c. + + +We shall now take leave of the oils and their various modifications, and +proceed to the next vegetable substance, which is _caoutchouc_. This is +a white milky glutinous fluid, which acquires consistence, and blackens +in drying, in which state it forms the substance with which you are so +well acquainted, under the name of gum-elastic. + +CAROLINE. + +I am surprised to hear that gum-elastic was ever white, or ever fluid! +And from what vegetable is it procured? + +MRS. B. + +It is obtained from two or three different species of trees, in the +East-Indies, and South-America, by making incisions in the stem. The +juice is collected as it trickles from these incisions, and moulds of +clay, in the form of little bottles of gum-elastic, are dipped into it. +A layer of this juice adheres to the clay and dries on it; and several +layers are successively added by repeating this till the bottle is of +sufficient thickness. It is then beaten to break down the clay, which is +easily shaken out. The natives of the countries where this substance is +produced sometimes make shoes and boots of it by a similar process, and +they are said to be extremely pleasant and serviceable, both from their +elasticity, and their being water-proof. + + +The substance which comes next in our enumeration of the immediate +ingredients of vegetables, is _extractive matter_. This is a term, +which, in a general sense, may be applied to any substance extracted +from vegetables; but it is more particularly understood to relate to the +extractive _colouring matter_ of plants. A great variety of colours are +prepared from the vegetable kingdom, both for the purposes of painting +and of dying; all the colours called _lakes_ are of this description; +but they are less durable than mineral colours, for, by long exposure to +the atmosphere, they either darken or turn yellow. + +EMILY. + +I know that in painting, the lakes are reckoned far less durable colours +than the ochres; but what is the reason of it? + +MRS. B. + +The change which takes place in vegetable colours is owing chiefly to +the oxygen of the atmosphere slowly burning their hydrogen, and leaving, +in some measure, the blackness of the carbon exposed. Such change cannot +take place in ochre, which is altogether a mineral substance. + +Vegetable colours have a stronger affinity for animal than for vegetable +substances, and this is supposed to be owing to a small quantity of +nitrogen which they contain. Thus, silk and worsted will take a much +finer vegetable dye than linen and cotton. + +CAROLINE. + +Dying, then, is quite a chemical process? + +MRS. B. + +Undoubtedly. The condition required to form a good dye is, that the +colouring matter should be precipitated, or fixed, on the substance to +be dyed, and should form a compound not soluble in the liquids to which +it will probably be exposed. Thus, for instance, printed or dyed linens +or cottons must be able to resist the action of soap and water, to which +they must necessarily be subject in washing; and woollens and silks +should withstand the action of grease and acids, to which they may +accidentally be exposed. + +CAROLINE. + +But if linen and cotton have not a sufficient affinity for colouring +matter, how are they made to resist the action of washing, which they +always do when they are well printed? + +MRS. B. + +When the substance to be dyed has either no affinity for the colouring +matter, or not sufficient power to retain it, the combination is +effected, or strengthened, by the intervention of a third substance, +called a _mordant_, or basis. The mordant must have a strong affinity +both for the colouring matter and the substance to be dyed, by which +means it causes them to combine and adhere together. + +CAROLINE. + +And what are the substances that perform the office of thus reconciling +the two adverse parties? + +MRS. B. + +The most common mordant is sulphat of alumine, or alum. Oxyds of tin and +iron, in the state of compound salts, are likewise used for that +purpose. + +_Tannin_ is another vegetable ingredient of great importance in the +arts. It is obtained chiefly from the bark of trees; but it is found +also in nut-galls, and in some other vegetables. + +EMILY. + +Is that the substance commonly called _tan_, which is used in +hot-houses? + +MRS. B. + +Tan is the prepared bark in which the peculiar substance, tannin, is +contained. But the use of tan in hot-houses is of much less importance +than in the operation of _tanning_, by which skin is converted into +leather. + +EMILY. + +Pray, how is this operation performed? + +MRS. B. + +Various methods are employed for this purpose, which all consist in +exposing skin to the action of tannin, or of substances containing this +principle, in sufficient quantities, and disposed to yield it to the +skin. The most usual way is to infuse coarsely powdered oak bark in +water, and to keep the skin immersed in this infusion for a certain +length of time. During this process, which is slow and gradual, the skin +is found to have increased in weight, and to have acquired a +considerable tenacity and impermeability to water. This effect may be +much accelerated by using strong saturations of the tanning principle +(which can be extracted from bark), instead of employing the bark +itself. But this quick mode of preparation does not appear to make +equally good leather. + +Tannin is contained in a great variety of astringent vegetable +substances, as galls, the rose-tree, and wine; but it is nowhere so +plentiful as in bark. All these substances yield it to water, from which +it may be precipitated by a solution of isinglass, or glue, with which +it strongly unites and forms an insoluble compound. Hence its valuable +property of combining with skin (which consists chiefly of glue), and of +enabling it to resist the action of water. + +EMILY. + +Might we not see that effect by pouring a little melted isinglass into a +glass of wine, which you say contains tannin? + +MRS. B. + +Yes. I have prepared a solution of isinglass for that very purpose. --Do +you observe the thick muddy precipitate? --That is the tannin combined +with the isinglass. + +CAROLINE. + +This precipitate must then be of the same nature as leather? + +MRS. B. + +It is composed of the same ingredients; but the organisation and texture +of the skin being wanting, it has neither the consistence nor the +tenacity of leather. + +CAROLINE. + +One might suppose that men who drink large quantities of red wine stand +a chance of having the coats of their stomachs converted into leather, +since tannin has so strong an affinity for skin. + +MRS. B. + +It is not impossible but that the coats of their stomachs may be, in +some measure, tanned, or hardened by the constant use of this liquor; +but you must remember that where a number of other chemical agents are +concerned, and, above all, where life exists, no certain chemical +inference can be drawn. + +I must not dismiss this subject, without mentioning a recent discovery +of Mr. Hatchett, which relates to it. This gentleman found that a +substance very similar to tannin, possessing all its leading properties, +and actually capable of tanning leather, may be produced by exposing +carbon, or any substance containing carbonaceous matter, whether +vegetable, animal, or mineral, to the action of nitric acid. + +CAROLINE. + +And is not this discovery very likely to be of use to manufactures? + +MRS. B. + +That is very doubtful, because tannin, thus artificially prepared, must +probably always be more expensive than that which is obtained from bark. +But the fact is extremely curious, as it affords one of those very rare +instances of chemistry being able to imitate the proximate principles of +organised bodies. + + +The last of the vegetable materials is _woody fibre_; it is the hardest +part of plants. The chief source from which this substance is derived is +wood, but it is also contained, more or less, in every solid part of +that plant. It forms a kind of skeleton of the part to which it belongs, +and retains its shape after all the other materials have disappeared. It +consists chiefly of carbon, united with a small proportion of salts, and +the other constituents common to all vegetables. + +EMILY. + +It is of woody fibre, then, that the common charcoal is made? + +MRS. B. + +Yes. Charcoal, as you may recollect, is obtained from wood, by the +separation of all its evaporable parts. + +Before we take leave of the vegetable materials, it will be proper, at +least, to enumerate the several vegetable acids which we either have +had, or may have occasion to mention. I believe I formerly told you that +their basis, or radical, was uniformly composed of hydrogen and carbon, +and that their difference consisted only in the various proportions of +oxygen which they contained. + + +The following are the names of the vegetable acids: + +The _Mucous Acid_, obtained from gum or mucilage; + _Suberic_ - - - from cork; + _Camphoric_ - - - from camphor; + _Benzoic_ - - - from balsams; + _Gallic_ - - - from galls, bark, &c. + _Malic_ - - - from ripe fruits; + _Citric_ - - - from lemon juice; + _Oxalic_ - - - from sorrel; + _Succinic_ - - - from amber; + _Tartarous_ - - - from tartrit of potash: + _Acetic_ - - - from vinegar. + +They are all decomposable by heat, soluble in water, and turn vegetable +blue colours red. The _succinic_, the _tartarous_, and the _acetous +acids_, are the products of the decomposition of vegetables; we shall, +therefore, reserve their examination for a future period. + +The _oxalic acid_, distilled from sorrel, is the highest term of +vegetable acidification; for, if more oxygen be added to it, it loses +its vegetable nature, and is resolved into carbonic acid and water; +therefore, though all the other acids may be converted into the oxalic +by an addition of oxygen, the oxalic itself is not susceptible of a +further degree of oxygenation; nor can it be made, by any chemical +processes, to return to a state of lower acidification. + + +To conclude this subject, I have only to add a few words on the _gallic +acid_. . . . . + +CAROLINE. + +Is not this the same acid before mentioned, which forms ink, by +precipitating sulphat of iron from its solution? + +MRS. B. + +Yes. Though it is usually extracted from galls, on account of its being +most abundant in that vegetable substance, it may also be obtained from +a great variety of plants. It constitutes what is called the _astringent +principle_ of vegetables; it is generally combined with tannin, and you +will find that an infusion of tea, coffee, bark, red-wine, or any +vegetable substance that contains the astringent principle, will make a +black precipitate with a solution of sulphat of iron. + +CAROLINE. + +But pray what are galls? + +MRS. B. + +They are excrescences which grow on the bark of young oaks, and are +occasioned by an insect which wounds the bark of trees, and lays its +eggs in the aperture. The lacerated vessels of the tree then discharge +their contents, and form an excrescence, which affords a defensive +covering for these eggs. The insect, when come to life, first feeds on +this excrescence, and some time afterward eats its way out, as it +appears from a hole which is formed in all gall-nuts that no longer +contain an insect. It is in hot climates only that strongly astringent +gall-nuts are found; those which are used for the purpose of making ink +are brought from Aleppo. + +EMILY. + +But are not the oak-apples, which grow on the leaves of the oak in this +country, of a similar nature? + +MRS. B. + +Yes; only they are an inferior species of galls, containing less of the +astringent principle, and therefore less applicable to useful purposes. + +CAROLINE. + +Are the vegetable acids never found but in their pure uncombined state? + +MRS. B. + +By no means; on the contrary, they are frequently met with in the state +of compound salts; these, however, are in general not fully saturated +with the salifiable bases, so that the acid predominates; and, in this +state, they are called _acidulous_ salts. Of this kind is the salt +called cream of tartar. + +CAROLINE. + +Is not the salt of lemon, commonly used to take out ink-spots and +stains, of this nature? + +MRS. B. + +No; that salt consists of the oxalic acid, combined with a little +potash. It is found in that state in sorrel. + +CAROLINE. + +And pray how does it take out ink-spots? + +MRS. B. + +By uniting with the iron, and rendering it soluble in water. + + +Besides the vegetable materials which we have enumerated, a variety of +other substances, common to the three kingdoms, are found in vegetables, +such as potash, which was formerly supposed to belong exclusively to +plants, and was, in consequence, called the vegetable alkali. + +Sulphur, phosphorus, earths, and a variety of metallic oxyds, are also +found in vegetables, but only in small quantities. And we meet sometimes +with neutral salts, formed by the combination of these ingredients. + + + + +CONVERSATION XXI. + +ON THE DECOMPOSITION OF VEGETABLES. + + +CAROLINE. + +The account which you have given us, Mrs. B., of the materials of +vegetables, is, doubtless, very instructive; but it does not completely +satisfy my curiosity. I wish to know how plants obtain the principles +from which their various materials are formed; by what means these are +converted into vegetable matter, and how they are connected with the +life of the plant? + +MRS. B. + +This implies nothing less than a complete history of the chemistry and +physiology of vegetation, subjects on which we have yet but very +imperfect notions. Still I hope that I shall be able, in some measure, +to satisfy your curiosity. But, in order to render the subject more +intelligible, I must first make you acquainted with the various changes +which vegetables undergo, when the vital power no longer enables them to +resist the common laws of chemical attraction. + +The composition of vegetables being more complicated than that of +minerals, the former more readily undergo chemical changes than the +latter: for the greater the variety of attractions, the more easily is +the equilibrium destroyed, and a new order of combinations introduced. + +EMILY. + +I am surprised that vegetables should be so easily susceptible of +decomposition; for the preservation of the vegetable kingdom is +certainly far more important than that of minerals. + +MRS. B. + +You must consider, on the other hand, how much more easily the former is +renewed than the latter. The decomposition of the vegetable takes place +only after the death of the plant, which, in the common course of +nature, happens when it has yielded fruit and seeds to propagate its +species. If, instead of thus finishing its career, each plant was to +retain its form and vegetable state, it would become an useless burden +to the earth and its inhabitants. When vegetables, therefore, cease to +be productive, they cease to live, and nature then begins her process of +decomposition, in order to resolve them into their chemical +constituents, hydrogen, carbon, and oxygen; those simple and primitive +ingredients, which she keeps in store for all her combinations. + +EMILY. + +But since no system of combination can be destroyed, except by the +establishment of another order of attractions, how can the decomposition +of vegetables reduce them to their simple elements? + +MRS. B. + +It is a very long process, during which a variety of new combinations +are successively established and successively destroyed: but, in each of +these changes, the ingredients of vegetable matter tend to unite in a +more simple order of compounds, till they are at length brought to their +elementary state, or, at least, to their most simple order of +combinations. Thus you will find that vegetables are in the end almost +entirely reduced to water and carbonic acid; the hydrogen and carbon +dividing the oxygen between them, so as to form with it these two +substances. But the variety of intermediate combinations that take place +during the several stages of the decomposition of vegetables, present us +with a new set of compounds, well worthy of our examination. + +CAROLINE. + +How is it possible that vegetables, while putrefying, should produce any +thing worthy of observation? + +MRS. B. + +They are susceptible of undergoing certain changes before they arrive at +the state of putrefaction, which is the final term of decomposition; and +of these changes we avail ourselves for particular and important +purposes. But, in order to make you understand this subject, which is of +considerable importance, I must explain it more in detail. + +The decomposition of vegetables is always attended by a violent internal +motion, produced by the disunion of one order of particles, and the +combination of another. This is called FERMENTATION. There are several +periods at which this process stops, so that a state of rest appears to +be restored, and the new order of compounds fairly established. But, +unless means be used to secure these new combinations in their actual +state, their duration will be but transient, and a new fermentation will +take place, by which the compound last formed will be destroyed; and +another, and less complex order, will succeed. + +EMILY. + +The fermentations, then, appear to be only the successive steps by which +a vegetable descends to its final dissolution. + +MRS. B. + +Precisely so. Your definition is perfectly correct. + +CAROLINE. + +And how many fermentations, or new arrangements, does a vegetable +undergo before it is reduced to its simple ingredients? + +MRS. B. + +Chemists do not exactly agree in this point; but there are, I think, +four distinct fermentations, or periods, at which the decomposition of +vegetable matter stops and changes its course. But every kind of +vegetable matter is not equally susceptible of undergoing all these +fermentations. + +There are likewise several circumstances required to produce +fermentation. Water and a certain degree of heat are both essential to +this process, in order to separate the particles, and thus weaken their +force of cohesion, that the new chemical affinities may be brought into +action. + +CAROLINE. + +In frozen climates, then, how can the spontaneous decomposition of +vegetables take place? + +MRS. B. + +It certainly cannot; and, accordingly, we find scarcely any vestiges of +vegetation where a constant frost prevails. + +CAROLINE. + +One would imagine that, on the contrary, such spots would be covered +with vegetables; for, since they cannot be decomposed, their number must +always increase. + +MRS. B. + +But, my dear, heat and water are quite as essential to the formation of +vegetables, as they are to their decomposition. Besides, it is from the +dead vegetables, reduced to their elementary principles, that the rising +generation is supplied with sustenance. No young plant, therefore, can +grow unless its predecessors contribute both to its formation and +support; and these not only furnish the seed from which the new plant +springs, but likewise the food by which it is nourished. + +CAROLINE. + +Under the torrid zone, therefore, where water is never frozen, and the +heat is very great, both the processes of vegetation and of fermentation +must, I suppose, be extremely rapid? + +MRS. B. + +Not so much as you imagine: for in such climates great part of the water +which it requires for these processes is in an aëriform state, which is +scarcely more conducive either to the growth or formation of vegetables +than that of ice. In those latitudes, therefore, it is only in low damp +situations, sheltered by woods from the sun’s rays, that the smaller +tribes of vegetables can grow and thrive during the dry season, as dead +vegetables seldom retain water enough to produce fermentation, but are, +on the contrary, soon dried up by the heat of the sun, which enables +them to resist that process; so that it is not till the fall of the +autumnal rains (which are very violent in such climates), that +spontaneous fermentation can take place. + +The several fermentations derive their names from their principal +products. The first is called the _saccharine fermentation_, because its +product is _sugar_. + +CAROLINE. + +But sugar, you have told us, is found in all vegetables; it cannot, +therefore, be the product of their decomposition. + +MRS. B. + +It is true that this fermentation is not confined to the decomposition +of vegetables, as it continually takes place during their life; and, +indeed, this circumstance has, till lately, prevented it from being +considered as one of the fermentations. But the process appears so +analogous to the other fermentations, and the formation of sugar, +whether in living or dead vegetable matter is so evidently a new +compound, proceeding from the destruction of the previous order of +combinations, and essential to the subsequent fermentations, that it is +now, I believe, generally esteemed the first step, or necessary +preliminary, to decomposition, if not an actual commencement of that +process. + +CAROLINE. + +I recollect your hinting to us that sugar was supposed not to be +secreted from the sap, in the same manner as mucilage, fecula, oil, and +the other ingredients of vegetables. + +MRS. B. + +It is rather from these materials, than from the sap itself, that sugar +is formed; and it is developed at particular periods, as you may observe +in fruits, which become sweet in ripening, sometimes even after they +have been gathered. Life, therefore, is not essential to the formation +of sugar, whilst on the contrary, mucilage, fecula, and the other +vegetable materials that are secreted from the sap by appropriate +organs, whose powers immediately depend on the vital principle, cannot +be produced but during the existence of that principle. + +EMILY. + +The ripening of fruits is, then, their first step to destruction, as +well as their last towards perfection? + +MRS. B. + +Exactly. --A process analogous to the saccharine fermentation takes +place also during the cooking of certain vegetables. This is the case +with parsnips, carrots, potatoes, &c. in which sweetness is developed by +heat and moisture; and we know that if we carried the process a little +farther, a more complete decomposition would ensue. The same process +takes place also in seeds previous to their sprouting. + +CAROLINE. + +How do you reconcile this to your theory, Mrs. B.? Can you suppose that +a decomposition is the necessary precursor of life? + +MRS. B. + +That is indeed the case. The materials of the seed must be decomposed, +and the seed disorganized, before a plant can sprout from it. Seeds, +besides the embrio plant, contain (as we have already observed) fecula, +oil, and a little mucilage. These substances are destined for the +nourishment of the future plant; but they undergo some change before +they can be fit for this function. The seeds, when buried in the earth, +with a certain degree of moisture and of temperature, absorb water, +which dilates them, separates their particles, and introduces a new +order of attractions, of which sugar is the product. The substance of +the seed is thus softened, sweetened, and converted into a sort of white +milky pulp, fit for the nourishment of the embrio plant. + +The saccharine fermentation of seeds is artificially produced, for the +purpose of making _malt_, by the following process:-- A quantity of +barley is first soaked in water for two or three days: the water being +afterwards drained off, the grain heats spontaneously, swells, bursts, +sweetens, shows a disposition to germinate, and actually sprouts to the +length of an inch, when the process is stopped by putting it into a +kiln, where it is well dried at a gentle heat. In this state it is crisp +and friable, and constitutes the substance called _malt_, which is the +principal ingredient of beer. + +EMILY. + +But I hope you will tell us how malt is made into beer? + +MRS. B. + +Certainly; but I must first explain to you the nature of the second +fermentation, which is essential to that operation. This is called the +_vinous fermentation_, because its product is _wine_. + +EMILY. + +How very different the decomposition of vegetables is from what I had +imagined! The products of their disorganisation appear almost superior +to those which they yield during their state of life and perfection. + +MRS. B. + +And do you not, at the same time, admire the beautiful economy of +Nature, which, whether she creates, or whether she destroys, directs all +her operations to some useful and benevolent purpose? --It appears that +the saccharine fermentation is extremely favourable, if not absolutely +essential, as a previous step, to the vinous fermentation; so that if +sugar be not developed during the life of the plant, the saccharine +fermentation must be artificially produced before the vinous +fermentation can take place. This is the case with barley, which does +not yield any sugar until it is made into malt; and it is in that state +only that it is susceptible of undergoing the vinous fermentation by +which it is converted into beer. + +CAROLINE. + +But if the product of the vinous fermentation is always wine, beer +cannot have undergone that process, for beer is certainly not wine. + +MRS. B. + +Chemically speaking, beer may be considered as the wine of grain. For it +is the product of the fermentation of malt, just as wine is that of the +fermentation of grapes, or other fruits. + +The consequence of the vinous fermentation is the decomposition of the +saccharine matter, and the formation of a spirituous liquor from the +constituents of the sugar. But, in order to promote this fermentation, +not only water and a certain degree of heat are necessary, but also some +other vegetable ingredients, besides the sugar, as fecula, mucilage, +acids, salts, extractive matter, &c. all of which seem to contribute to +this process; and give to the liquor its peculiar taste. + +EMILY. + +It is, perhaps, for this reason that wine is not obtained from the +fermentation of pure sugar; but that fruits are chosen for that purpose, +as they contain not only sugar, but likewise the other vegetable +ingredients which promote the vinous fermentation, and give the peculiar +flavour. + +MRS. B. + +Certainly. And you must observe also, that the relative quantity of +sugar is not the only circumstance to be considered in the choice of +vegetable juices for the formation of wine; otherwise the sugar-cane +would be best adapted for that purpose. It is rather the manner and +proportion in which the sugar is mixed with other vegetable ingredients +that influences the production and qualities of wine. And it is found +that the juice of the grape not only yields the most considerable +proportion of wine, but that it likewise affords it of the most grateful +flavour. + +EMILY. + +I have seen a vintage in Switzerland, and I do not recollect that heat +was applied, or water added, to produce the fermentation of the grapes. + +MRS. B. + +The common temperature of the atmosphere in the cellars in which the +juice of the grape is fermented is sufficiently warm for this purpose; +and as the juice contains an ample supply of water, there is no occasion +for any addition of it. But when fermentation is produced in dry malt, +a quantity of water must necessarily be added. + +EMILY. + +But what are precisely the changes that happen during the vinous +fermentation? + +MRS. B. + +The sugar is decomposed, and its constituents are recombined into two +new substances; the one a peculiar liquid substance, called _alcohol_ or +_spirit of wine_, which remains in the fluid; the other, carbonic acid +gas, which escapes during the fermentation. Wine, therefore, as I before +observed, in a general point of view, may be considered as a liquid of +which alcohol constitutes the essential part. And the varieties of +strength and flavour of the different kinds of wine are to be attributed +to the different qualities of the fruits from which they are obtained, +independently of the sugar. + +CAROLINE. + +I am astonished to hear that so powerful a liquid as spirit of wine +should be obtained from so mild a substance as sugar. + +MRS. B. + +Can you tell me in what the principal difference consists between +alcohol and sugar? + +CAROLINE. + +Let me reflect . . . . . Sugar consists of carbon, hydrogen, and oxygen. +If carbonic acid be subtracted from it, during the formation of alcohol, +the latter will contain less carbon and oxygen than sugar does; +therefore hydrogen must be the prevailing principle of alcohol. + +MRS. B. + +It is exactly so. And this very large proportion of hydrogen accounts +for the lightness and combustible property of alcohol, and of spirits in +general, all of which consist of alcohol variously modified. + +EMILY. + +And can sugar be recomposed from the combination of alcohol and carbonic +acid? + +MRS. B. + +Chemists have never been able to succeed in effecting this; but from +analogy, I should suppose such a recomposition possible. Let us now +observe more particularly the phenomena that take place during the +vinous fermentation. At the commencement of this process, heat is +evolved, and the liquor swells considerably from the formation of the +carbonic acid, which is disengaged in such prodigious quantities as +would be fatal to any person who should unawares inspire it; an accident +which has sometimes happened. If the fermentation be stopped by putting +the liquor into barrels, before the whole of the carbonic acid is +evolved, the wine is brisk, like Champagne, from the carbonic acid +imprisoned in it, and it tastes sweet, like cyder, from the sugar not +being completely decomposed. + +EMILY. + +But I do not understand why heat should be evolved during this +operation. For, as there is a considerable formation of gas, in which a +proportionable quantity of heat must become insensible, I should have +imagined that cold, rather than heat, would have been produced. + +MRS. B. + +It appears so on first consideration; but you must recollect that +fermentation is a complicated chemical process; and that, during the +decompositions and recompositions attending it, a quantity of chemical +heat may be disengaged, sufficient both to develope the gas, and to +effect an increase of temperature. When the fermentation is completed, +the liquid cools and subsides, the effervescence ceases, and the thick, +sweet, sticky juice of the fruit is converted into a clear, transparent, +spirituous liquor, called wine. + +EMILY. + +How much I regret not having been acquainted with the nature of the +vinous fermentation, when I had an opportunity of seeing the process! + +MRS. B. + +You have an easy method of satisfying yourself in that respect by +observing the process of brewing, which, in every essential +circumstance, is similar to that of making wine, and is really a very +curious chemical operation. + +Although we cannot actually make wine at this moment, it will be easy to +show you the mode of analyzing it. This is done by distillation. When +wine of any kind is submitted to this operation, it is found to contain +brandy, water, tartar, extractive colouring matter, and some vegetable +acids. I have put a little port wine into this alembic of glass (PLATE +XIV. Fig. 1.), and on placing the lamp under it, you will soon see the +spirit and water successively come over-- + + [Illustration: Plate XIV. Vol. II. p. 213. + + Fig. 1. + A Alembic. + B Lamp. + C Wine glass. + + Fig. 2. Alcohol blowpipe. + D the Lamp. + E the vessel in which the Alcohol is boiling. + F a safety valve. + G the inflamed jet or steam of alcohol directed towards a glass + tube H.] + +EMILY. + +But you do not mention alcohol amongst the _products_ of the +distillation of wine; and yet that is its most essential ingredient? + +MRS. B. + +The alcohol is contained in the brandy which is now coming over, and +dropping from the still. Brandy is nothing more than a mixture of +alcohol and water; and in order to obtain the alcohol pure, we must +again distil it from brandy. + +CAROLINE. + +I have just taken a drop on my finger; it tastes like strong brandy, but +it is without colour, whilst brandy is of a deep yellow. + +MRS. B. + +It is not so naturally; in its pure state brandy is colourless, and it +obtains the yellow tint you observe, by extracting the colouring matter +from the new oaken casks in which it is kept. But if it does not acquire +the usual tinge in this way, it is the custom to colour the brandy used +in this country artificially, with a little burnt sugar, in order to +give it the appearance of having been long kept. + +CAROLINE. + +And is rum also distilled from wine? + +MRS. B. + +By no means; it is distilled from the sugar-cane, a plant which contains +so great a quantity of sugar, that it yields more alcohol than almost +any other vegetable. After the juice of the cane has been pressed out +for making sugar, what still remains in the bruised cane is extracted by +water, and this watery solution of sugar is fermented, and produces rum. + +The spirituous liquor called _arack_ is in a similar manner distilled +from the product of the vinous fermentation of rice. + +EMILY. + +But rice has no sweetness; does it contain any sugar? + +MRS. B. + +Like barley and most other seeds, it is insipid until it has undergone +the saccharine fermentation; and this, you must recollect, is always a +previous step to the vinous fermentation in those vegetables in which +sugar is not already formed. Brandy may in the same manner be obtained +from malt. + +CAROLINE. + +You mean from beer, I suppose; for the malt must have previously +undergone the vinous fermentation. + +MRS. B. + +Beer is not precisely the product of the vinous fermentation of malt. +For hops are a necessary ingredient for the formation of that liquor; +whilst brandy is distilled from pure fermented malt. But brandy might, +no doubt, be distilled from beer as well as from any other liquor that +has undergone the vinous fermentation; for since the basis of brandy is +alcohol, it may be obtained from any liquid that contains that +spirituous substance. + +EMILY. + +And pray, from what vegetable is the favourite spirit of the lower +orders of people, gin, extracted? + +MRS. B. + +The spirit (which is the same in all fermented liquors) may be obtained +from any kind of grain; but the peculiar flavour which distinguishes gin +is that of juniper berries, which are distilled together with the +grain-- + +I think the brandy contained in the wine which we are distilling must, +by this time, be all come over. Yes--taste the liquid that is now +dropping from the alembic-- + +CAROLINE. + +It is perfectly insipid, like water. + +MRS. B. + +It is water, which, as I was telling you, is the second product of wine, +and comes over after all the spirit, which is the lightest part, is +distilled. --The tartar and extractive colouring matter we shall find in +a solid form at the bottom of the alembic. + +EMILY. + +They look very like the lees of wine. + +MRS. B. + +And in many respects they are of a similar nature; for lees of wine +consist chiefly of tartrit of potash; a salt which exists in the juice +of the grape, and in many other vegetables, and is developed only by the +vinous fermentation. During this operation it is precipitated, and +deposits itself on the internal surface of the cask in which the wine is +contained. It is much used in medicine, and in various arts, +particularly dying, under the name of _cream of tartar_, and it is from +this salt that the tartarous acid is obtained. + +CAROLINE. + +But the medicinal cream of tartar is in appearance quite different from +these dark-coloured dregs; it is perfectly colourless. + +MRS. B. + +Because it consists of the pure salts only, in its crystallised form; +whilst in the instance before us it is mixed with the deep-coloured +extractive matter, and other foreign ingredients. + +EMILY. + +Pray cannot we now obtain pure alcohol from the brandy which we have +distilled? + +MRS. B. + +We might; but the process would be tedious: for in order to obtain +alcohol perfectly free from water, it is necessary to distil, or, as the +distillers call it, _rectify_ it several times. You must therefore allow +me to produce a bottle of alcohol that has been thus purified. This is a +very important ingredient, which has many striking properties, besides +its forming the basis of all spirituous liquors. + +EMILY. + +It is alcohol, I suppose, that produces intoxication? + +MRS. B. + +Certainly; but the stimulus and momentary energy it gives to the system, +and the intoxication it occasions when taken in excess, are +circumstances not yet accounted for. + +CAROLINE. + +I thought that it produced these effects by increasing the rapidity of +the circulation of the blood; for drinking wine or spirits, I have +heard, always quickens the pulse. + +MRS. B. + +No doubt; the spirit, by stimulating the nerves, increases the action of +the muscles; and the heart, which is one of the strongest muscular +organs, beats with augmented vigour, and propels the blood with +accelerated quickness. After such a strong excitation the frame +naturally suffers a proportional degree of depression, so that a state +of debility and languor is the invariable consequence of intoxication. +But though these circumstances are well ascertained, they are far from +explaining why alcohol should produce such effects. + +EMILY. + +Liqueurs are the only kind of spirits which I think pleasant. Pray of +what do they consist? + +MRS. B. + +They are composed of alcohol, sweetened with syrup, and flavoured with +volatile oil. + +The different kinds of odoriferous spirituous waters are likewise +solutions of volatile oil in alcohol, as lavender water, eau de +Cologne, &c. + +The chemical properties of alcohol are important and numerous. It is one +of the most powerful chemical agents, and is particularly useful in +dissolving a variety of substances, which are soluble neither by water +nor heat. + +EMILY. + +We have seen it dissolve copal and mastic to form varnishes; and these +resins are certainly not soluble in water, since water precipitates them +from their solution in alcohol. + +MRS. B. + +I am happy to find that you recollect these circumstances so well. The +same experiment affords also an instance of another property of +alcohol,--its tendency to unite with water; for the resin is +precipitated in consequence of losing the alcohol, which abandons it +from its preference for water. It is attended also, as you may +recollect, with the same peculiar circumstance of a disengagement of +heat and consequent diminution of bulk, which we have supposed to be +produced by a mechanical penetration of particles by which latent heat +is forced out. + +Alcohol unites thus readily not only with resins and with water, but +with oils and balsams; these compounds form the extensive class of +elixirs, tinctures, quintessences, &c. + +EMILY. + +I suppose that alcohol must be highly combustible, since it contains so +large a proportion of hydrogen? + +MRS. B. + +Extremely so; and it will burn at a very moderate temperature. + +CAROLINE. + +I have often seen both brandy and spirit of wine burnt; they produce a +great deal of flame, but not a proportional quantity of heat, and no +smoke whatever. + +MRS. B. + +The last circumstance arises from their combustion being complete; and +the disproportion between the flame and heat shows you that these are by +no means synonymous. + +The great quantity of flame proceeds from the combustion of the hydrogen +to which, you know, that manner of burning is peculiar. --Have you not +remarked also that brandy and alcohol will burn without a wick? --They +take fire at so low a temperature, that this assistance is not required +to concentrate the heat and volatilise the fluid. + +CAROLINE. + +I have sometimes seen brandy burnt by merely heating it in a spoon. + +MRS. B. + +The rapidity of the combustion of alcohol may, however, be prodigiously +increased by first volatilising it. An ingenious instrument has been +constructed on this principle to answer the purpose of a blow-pipe, +which may be used for melting glass, or other chemical purposes. It +consists of a small metallic vessel (PLATE XIV. Fig. 2.), of a spherical +shape, which contains the alcohol, and is heated by the lamp beneath it; +as soon as the alcohol is volatilised, it passes through the spout of +the vessel, and issues just above the wick of the lamp, which +immediately sets fire to the stream of vapour, as I shall show you-- + +EMILY. + +With what amazing violence it burns! The flame of alcohol, in the state +of vapour, is, I fancy, much hotter than when the spirit is merely burnt +in a spoon? + +MRS. B. + +Yes; because in this way the combustion goes on much quicker, and, of +course, the heat is proportionally increased. --Observe its effect on +this small glass tube, the middle of which I present to the extremity of +the flame, where the heat is greatest. + +CAROLINE. + +The glass, in that spot, is become red hot, and bends from its own +weight. + +MRS. B. + +I have now drawn it asunder, and am going to blow a ball at one of the +heated ends; but I must previously close it up, and flatten it with this +little metallic instrument, otherwise the breath would pass through the +tube without dilating any part of it. --Now, Caroline, will you blow +strongly into the tube whilst the closed end is red hot. + +EMILY. + +You blowed too hard; for the ball suddenly dilated to a great size, and +then burst in pieces. + +MRS. B. + +You will be more expert another time; but I must caution you, should you +ever use this blow-pipe, to be very careful that the combustion of the +alcohol does not go on with too great violence, for I have seen the +flame sometimes dart out with such force as to reach the opposite wall +of the room, and set the paint on fire. There is, however, no danger of +the vessel bursting, as it is provided with a safety tube, which affords +an additional vent for the vapour of alcohol when required. + +The products of the combustion of alcohol consist in a great proportion +of water, and a small quantity of carbonic acid. There is no smoke or +fixed remains whatever. --How do you account for that, Emily? + +EMILY. + +I suppose that the oxygen which the alcohol absorbs in burning, converts +its hydrogen into water and its carbon into carbonic acid gas, and thus +it is completely consumed. + +MRS. B. + +Very well. --_Ether_, the lightest of all fluids, and with which you are +well acquainted, is obtained from alcohol, of which it forms the +lightest and most volatile part. + +EMILY. + +Ether, then, is to alcohol, what alcohol is to brandy? + +MRS. B. + +No: there is an essential difference. In order to obtain alcohol from +brandy, you need only deprive the latter of its water; but for the +formation of ether, the alcohol must be decomposed, and one of its +constituents partly subtracted. I leave you to guess which of them +it is-- + +EMILY. + +It cannot be hydrogen, as ether is more volatile than alcohol, and +hydrogen is the lightest of all its ingredients: nor do I suppose that +it can be oxygen, as alcohol contains so small a proportion of that +principle; it is, therefore, most probably, carbon, a diminution of +which would not fail to render the new compound more volatile. + +MRS. B. + +You are perfectly right. The formation of ether consists simply in +subtracting from the alcohol a certain proportion of carbon; this is +effected by the action of the sulphuric, nitric, or muriatic acids, on +alcohol. The acid and carbon remain at the bottom of the vessel, whilst +the decarbonised alcohol flies off in the form of a condensable vapour, +which is ether. + +Ether is the most inflammable of all fluids, and burns at so slow a +temperature that the heat evolved during its combustion is more than is +required for its support, so that a quantity of ether is volatilised, +which takes fire, and gradually increases the violence of the +combustion. + + +Sir Humphry Davy has lately discovered a very singular fact respecting +the vapour of ether. If a few drops of ether be poured into a +wine-glass, and a fine platina wire, heated almost to redness, be held +suspended in the glass, close to the surface of the ether, the wire soon +becomes intensely red-hot, and remains so for any length of time. We may +easily try the experiment. . . . . + +CAROLINE. + +How very curious! The wire is almost white hot, and a pungent smell +rises from the glass. Pray how is this accounted for? + +MRS. B. + +This is owing to a very peculiar property of the vapour of ether, and +indeed of many other combustible gaseous bodies. At a certain +temperature lower than that of ignition, these vapours undergo a slow +and imperfect combustion, which does not give rise, in any sensible +degree, to the phenomena of light and flame, and yet extricates a +quantity of caloric sufficient to react upon the wire and make it +red-hot, and the wire in its turn keeps up the effect as long as the +emission of vapour continues. + +CAROLINE. + +But why should not an iron or silver wire produce the same effect? + +MRS. B. + +Because either iron or silver, being much better conductors of heat than +platina, the heat is carried off too fast by those metals to allow the +accumulation of caloric necessary to produce the effect in question. + + +Ether is so light that it evaporates at the common temperature of the +atmosphere; it is therefore necessary to keep it confined by a well +ground glass stopper. No degree of cold known has ever frozen it. + +CAROLINE. + +Is it not often taken medicinally? + +MRS. B. + +Yes; it is one of the most effectual antispasmodic medicines, and the +quickness of its effects, as such, probably depends on its being +instantly converted into vapour by the heat of the stomach, through the +intervention of which it acts on the nervous system. But the frequent +use of ether, like that of spirituous liquors, becomes prejudicial, and, +if taken to excess, it produces effects similar to those of +intoxication. + +We may now take our leave of the vinous fermentation, of which, I hope, +you have acquired a clear idea; as well as of the several products that +are derived from it. + +CAROLINE. + +Though this process appears, at first sight, so much complicated, it +may, I think, be summed up in a few words, as it consists in the +conversion of sugar and fermentable bodies into alcohol and carbonic +acid, which give rise both to the formation of wine, and of all kinds of +spirituous liquors. + +MRS. B. + +We shall now proceed to the _acetous fermentation_, which is thus +called, because it converts wine into vinegar, by the formation of the +acetous acid, which is the basis or radical of vinegar. + +CAROLINE. + +But is not the acidifying principle of the acetous acid the same as that +of all other acids, oxygen? + +MRS. B. + +Certainly; and on that account the contact of air is essential to this +fermentation, as it affords the necessary supply of oxygen. Vinegar, in +order to obtain pure acetous acid from it, must be distilled and +rectified by certain processes. + +EMILY. + +But pray, Mrs. B., is not the acetous acid frequently formed without +this fermentation taking place? Is it not, for instance, contained in +acid fruits, and in every substance that becomes sour? + +MRS. B. + +No, not in fruits; you confound it with the citric, the malic, the +oxalic, and other vegetable acids, to which living vegetables owe their +acidity. But whenever a vegetable substance turns sour, after it has +ceased to live, the acetous acid is developed by means of the acetous +fermentation, in which the substance advances a step towards its final +decomposition. + +Amongst the various instances of acetous fermentation, that of bread is +usually classed. + +CAROLINE. + +But the fermentation of bread is produced by yeast; how does that +effect it? + +MRS. B. + +It is found by experience that any substance that has already undergone +a fermentation, will readily excite it in one that is susceptible of +that process. If, for instance, you mix a little vinegar with wine, that +is intended to be acidified, it will absorb oxygen more rapidly, and the +process be completed much sooner, than if left to ferment spontaneously. +Thus yeast, which is a product of the fermentation of beer, is used to +excite and accelerate the fermentation of malt, which is to be converted +into beer, as well as that of paste which is to be made into bread. + +CAROLINE. + +But if bread undergoes the acetous fermentation, why is it not sour? + +MRS. B. + +It acquires a certain savour which corrects the heavy insipidity of +flour, and may be reckoned a first degree of acidification; or if the +process were carried further, the bread would become decidedly acid. + +There are, however, some chemists who do not consider the fermentation +of bread as being of the acetous kind, but suppose that it is a process +of fermentation peculiar to that substance. + +The _putrid fermentation_ is the final operation of Nature, and her last +step towards reducing organised bodies to their simplest combinations. +All vegetables spontaneously undergo this fermentation after death, +provided there be a sufficient degree of heat and moisture, together +with access of air; for it is well known that dead plants may be +preserved by drying, or by the total exclusion of air. + +CAROLINE. + +But do dead plants undergo the other fermentation previous to this last; +or do they immediately suffer the putrid fermentation? + +MRS. B. + +That depends on a variety of circumstances, such as the degrees of +temperature and of moisture, the nature of the plant itself, &c. But if +you were carefully to follow and examine the decomposition of plants +from their death to their final dissolution, you would generally find a +sweetness developed in the seeds, and a spirituous flavour in the fruits +(which have undergone the saccharine fermentation), previous to the +total disorganisation and separation of the parts. + +EMILY. + +I have sometimes remarked a kind of spirituous taste in fruits that were +over ripe, especially oranges; and this was just before they became +rotten. + +MRS. B. + +It was then the vinous fermentation which had succeeded the saccharine, +and had you followed up these changes attentively, you would probably +have found the spirituous taste followed by acidity, previous to the +fruit passing to the state of putrefaction. + +When the leaves fall from the trees in autumn, they do not (if there is +no great moisture in the atmosphere) immediately undergo a +decomposition, but are first dried and withered; as soon, however, as +the rain sets in, fermentation commences, their gaseous products are +imperceptibly evolved into the atmosphere, and their fixed remains mixed +with their kindred earth. + +Wood, when exposed to moisture, also undergoes the putrid fermentation +and becomes rotten. + +EMILY. + +But I have heard that the _dry rot_, which is so liable to destroy the +beams of houses, is prevented by a current of air; and yet you said that +air was essential to the putrid fermentation? + +MRS. B. + +True; but it must not be in such a proportion to the moisture as to +dissolve the latter, and this is generally the case when the rotting of +wood is prevented or stopped by the free access of air. What is commonly +called dry rot, however, is not I believe a true process of +putrefaction. It is supposed to depend on a peculiar kind of vegetation, +which, by feeding on the wood, gradually destroys it. + +Straw and all other kinds of vegetable matter undergo the putrid +fermentation more rapidly when mixed with animal matter. Much heat is +evolved during this process, and a variety of volatile products are +disengaged, as carbonic acid and hydrogen gas, the latter of which is +frequently either sulphurated or phosphorated. --When all these gases +have been evolved, the fixed products, consisting of carbon, salts, +potash, &c. form a kind of vegetable earth, which makes very fine +manure, as it is composed of those elements which form the immediate +materials of plants. + +CAROLINE. + +Pray are not vegetables sometimes preserved from decomposition by +petrification? I have seen very curious specimens of petrified +vegetables, in which state they perfectly preserve their form and +organisation, though in appearance they are changed to stone. + +MRS. B. + +That is a kind of metamorphosis, which, now that you are tolerably well +versed in the history of mineral and vegetable substances, I leave to +your judgment to explain. Do you imagine that vegetables can be +converted into stone? + +EMILY. + +No, certainly; but they might perhaps be changed to a substance in +appearance resembling stone. + +MRS. B. + +It is not so, however, with the substances that are called petrified +vegetables; for these are really stone, and generally of the hardest +kind, consisting chiefly of silex. The case is this: when a vegetable is +buried under water, or in wet earth, it is slowly and gradually +decomposed. As each successive particle of the vegetable is destroyed, +its place is supplied by a particle of siliceous earth, conveyed thither +by the water. In the course of time the vegetable is entirely destroyed, +but the silex has completely replaced it, having assumed its form and +apparent texture, as if the vegetable itself were changed to stone. + +CAROLINE. + +That is very curious! and I suppose that petrified animal substances are +of the same nature? + +MRS. B. + +Precisely. It is equally impossible for either animal or vegetable +substances to be converted into stone. They may be reduced, as we find +they are, by decomposition, to their constituent elements, but cannot be +changed to elements, which do not enter into their composition. + +There are, however, circumstances which frequently prevent the regular +and final decomposition of vegetables; as, for instance, when they are +buried either in the sea, or in the earth, where they cannot undergo the +putrid fermentation for want of air. In these cases they are subject to +a peculiar change, by which they are converted into a new class of +compounds, called _bitumens_. + +CAROLINE. + +These are substances I never heard of before. + +MRS. B. + +You will find, however, that some of them are very familiar to you. +Bitumens are vegetables so far decomposed as to retain no organic +appearance; but their origin is easily detected by their oily nature, +their combustibility, the products of their analysis, and the +impressions of the forms of leaves, grains, fibres of wood, and even of +animals, which they frequently bear. + +They are sometimes of an oily liquid consistence, as the substance +called _naptha_, in which we preserved potassium; it is a fine +transparent colourless fluid, that issues out of clays in some parts of +Persia. But more frequently bitumens are solid, as _asphaltum_, +a smooth, hard, brittle substance, which easily melts, and forms, in its +liquid state, a beautiful dark brown colour for oil painting. _Jet_, +which is of a still harder texture, is a peculiar bitumen, susceptible +of so fine a polish, that it is used for many ornamental purposes. + + +_Coal_ is also a bituminous substance, to the composition of which both +the mineral and animal kingdoms seem to concur. This most useful mineral +appears to consist chiefly of vegetable matter, mixed with the remains +of marine animals and marine salts, and occasionally containing a +quantity of sulphuret of iron, commonly called pyrites. + +EMILY. + +It is, I suppose, the earthly, the metallic, and the saline parts of +coals, that compose the cinders or fixed products of their combustion; +whilst the hydrogen and carbon, which they derive from vegetables, +constitute their volatile products. + +CAROLINE. + +Pray is not _coke_, (which I have heard is much used in some +manufactures,) also a bituminous substance? + +MRS. B. + +No; it is a kind of fuel artificially prepared from coals. It consists +of coals reduced to a substance analogous to charcoal, by the +evaporation of their bituminous parts. Coke, therefore, is composed of +carbon, with some earthy and saline ingredients. + +_Succin_, or _yellow amber_, is a bitumen which the ancients called +_electrum_, from whence the word electricity is derived, as that +substance is peculiarly, and was once supposed to be exclusively, +electric. It is found either deeply buried in the bowels of the earth, +or floating on the sea, and is supposed to be a resinous body which has +been acted on by sulphuric acid, as its analysis shows it to consist of +ah oil and an acid. The oil is called _oil of amber_, the acid the +_succinic_. + +EMILY. + +That oil I have sometimes used in painting, as it is reckoned to change +less than the other kinds of oils. + +MRS. B. + +The last class of vegetable substances that have changed their nature +are _fossil-wood_, _peat_, and _turf_. These are composed of wood and +roots of shrubs, that are partly decomposed by being exposed to moisture +under ground, and yet, in some measure, preserve their form and organic +appearance. The peat, or black earth of the moors, retains but few +vestiges of the roots to which it owes its richness and combustibility, +these substances being in the course of time reduced to the state of +vegetable earth. But in turf the roots of plants are still discernible, +and it equally answers the purpose of fuel. It is the combustible used +by the poor in heathy countries, which supply it abundantly. + +It is too late this morning to enter upon the history of vegetation. We +shall reserve this subject, therefore, for our next interview, when I +expect that it will furnish us with ample matter for another +conversation. + + + + +CONVERSATION XXII. + +HISTORY OF VEGETATION. + + +MRS. B. + +The VEGETABLE KINGDOM may be considered as the link which unites the +mineral and animal creation into one common chain of beings; for it is +through the means of vegetation alone that mineral substances are +introduced into the animal system, since, generally speaking, it is from +vegetables that all animals ultimately derive their sustenance. + +CAROLINE. + +I do not understand that; the human species subsists as much on animal +as on vegetable food, and there are some carnivorous animals that will +eat only animal food. + +MRS. B. + +That is true; but you do not consider that those that live on animal +food, derive their sustenance equally, though not so immediately, from +vegetables. The meat that we eat is formed from the herbs of the field, +and the prey of carnivorous animals proceeds, either directly or +indirectly, from the same source. It is, therefore, through this channel +that the simple elements become a part of the animal frame. We should in +vain attempt to derive nourishment from carbon, hydrogen, and oxygen, +either in their separate state, or combined in the mineral kingdom; for +it is only by being united in the form of vegetable combination, that +they become capable of conveying nourishment. + +EMILY. + +Vegetation, then, seems to be the method which Nature employs to prepare +the food of animals? + +MRS. B. + +That is certainly its principal object. The vegetable creation does not +exhibit more wisdom in that admirable system of organisation, by which +it is enabled to answer its own immediate ends of preservation, +nutrition, and propagation, than in its grand and ultimate object of +forming those arrangements and combinations of principles, which are so +well adapted for the nourishment of animals. + +EMILY. + +But I am very curious to know whence vegetables obtain those principles +which form their immediate materials? + +MRS. B. + +This is a point on which we are yet so much in the dark, that I cannot +hope fully to satisfy your curiosity; but what little I know on this +subject, I will endeavour to explain to you. + +The soil, which, at first view, appears to be the aliment of vegetables, +is found, on a closer investigation, to be little more than the channel +through which they receive their nourishment; so that it is very +possible to rear plants without any earth or soil. + +CAROLINE. + +Of that we have an instance in the hyacinth and other bulbous roots, +which will grow and blossom beautifully in glasses of water. But I +confess I should think it would be difficult to rear trees in a similar +manner. + +MRS. B. + +No doubt it would, as it is the burying of the roots in the earth that +supports the stem of the tree. But this office, besides that of +affording a vehicle for food, is far the most important part which the +earthy portion of the soil performs in the process of vegetation; for we +can discover, by analysis, but an extremely small proportion of earth in +vegetable compounds. + +CAROLINE. + +But if earths do not afford nourishment, why is it necessary to be so +attentive to the preparation of the soil? + +MRS. B. + +In order to impart to it those qualities which render it a proper +vehicle for the food of the plant. Water is the chief nourishment of +vegetables; if, therefore, the soil be too sandy, it will not retain a +quantity of water sufficient to supply the roots of the plants. If, on +the contrary, it abound too much with clay, the water will lodge in such +quantities as to threaten a decomposition of the roots. Calcareous soils +are, upon the whole, the most favourable to the growth of plants: soils +are, therefore, usually improved by chalk, which, you may recollect, is +a carbonat of lime. Different vegetables, however, require different +kinds of soils. Thus rice demands a moist retentive soil; potatoes a +soft sandy soil; wheat a firm and rich soil. Forest trees grow better in +fine sand than in a stiff clay; and a light ferruginous soil is best +suited to fruit-trees. + +CAROLINE. + +But pray what is the use of manuring the soil? + +MRS. B. + +Manure consists of all kinds of substances, whether of vegetable or +animal origin, which have undergone the putrid fermentation, and are +consequently decomposed, or nearly so, into their elementary principles. +And it is requisite that these vegetable matters should be in a state of +decay, or approaching decomposition. The addition of calcareous earth, +in the state of chalk or lime, is beneficial to such soils, as it +accelerates the dissolution of vegetable bodies. Now, I ask you, what is +the utility of supplying the soil with these decomposed substances? + +CAROLINE. + +It is, I suppose, in order to furnish vegetables with the principles +which enter into their composition. For manures not only contain carbon, +hydrogen, and oxygen, but by their decomposition supply the soil with +these principles in their elementary form. + +MRS. B. + +Undoubtedly; and it is for this reason that the finest crops are +produced in fields that were formerly covered with woods, because their +soil is composed of a rich mould, a kind of vegetable earth, which +abounds in those principles. + +EMILY. + +This accounts for the plentifulness of the crops produced in America, +where the country was but a few years since covered with wood. + +CAROLINE. + +But how is it that animal substances are reckoned to produce the best +manure? Does it not appear much more natural that the decomposed +elements of vegetables should be the most appropriate to the formation +of new vegetables? + +MRS. B. + +The addition of a much greater proportion of nitrogen, which constitutes +the chief difference between animal and vegetable matter, renders the +composition of the former more complicated, and consequently more +favourable to decomposition. The use of animal substances is chiefly to +give the first impulse to the fermentation of the vegetable ingredients +that enter into the composition of manures. The manure of a farm-yard is +of that description; but there is scarcely any substance susceptible of +undergoing the putrid fermentation that will not make good manure. The +heat produced by the fermentation of manure is another circumstance +which is extremely favourable to vegetation; yet this heat would be too +great if the manure was laid on the ground during the height of +fermentation; it is used in this state only for hot-beds, to produce +melons, cucumbers, and such vegetables as require a very high +temperature. + +CAROLINE. + +A difficulty has just occurred to me which I do not know how to remove. +Since all organised bodies are, in the common course of nature, +ultimately reduced to their elementary state, they must necessarily in +that state enrich the soil, and afford food for vegetation. How is it, +then, that agriculture, which cannot increase the quantity of those +elements that are required to manure the earth, can increase its produce +so wonderfully as is found to be the case in all cultivated countries? + +MRS. B. + +It is by suffering none of these decaying bodies to be dissipated, but +in applying them duly to the soil. It is by a judicious preparation of +the soil, which consists in fitting it either for the general purposes +of vegetation, or for that of the particular seed which is to be sown. +Thus, if the soil be too wet, it may be drained; if too loose and sandy, +it may be rendered more consistent and retentive of water by the +addition of clay or loam; it may be enriched by chalk, or any kind of +calcareous earth. On soils thus improved, manures will act with double +efficacy, and if attention be paid to spread them on the ground at a +proper season of the year, to mix them with the soil so that they may be +generally diffused through it, to destroy the weeds which might +appropriate these nutritive principles to their own use, to remove the +stones which would impede the growth of the plant, &c. we may obtain a +produce an hundred fold more abundant than the earth would spontaneously +supply. + +EMILY. + +We have a very striking instance of this in the scanty produce of +uncultivated commons, compared to the rich crops of meadows which are +occasionally manured. + +CAROLINE. + +But, Mrs. B., though experience daily proves the advantage of +cultivation, there is still a difficulty which I cannot get over. +A certain quantity of elementary principles exist in nature, which it is +not in the power of man either to augment or diminish. Of these +principles you have taught us that both the animal and vegetable +creation are composed. Now the more of them is taken up by the vegetable +kingdom, the less, it would seem, will remain for animals; and, +therefore, the more populous the earth becomes, the less it will +produce. + +MRS. B. + +Your reasoning is very plausible; but experience every where contradicts +the inference you would draw from it; for we find that the animal and +vegetable kingdoms, instead of thriving, as you would suppose, at each +other’s expense, always increase and multiply together. For you should +recollect that animals can derive the elements of which they are formed +only through the medium of vegetables. And you must allow that your +conclusion would be valid only if every particle of the several +principles that could possibly be spared from other purposes were +employed in the animal and vegetable creations. Now we have reason to +believe that a much greater proportion of these principles than is +required for such purposes remains either in an elementary state, or +engaged in a less useful mode of combination in the mineral kingdom. +Possessed of such immense resources as the atmosphere and the waters +afford us, for oxygen, hydrogen, and carbon, so far from being in danger +of working up all our simple materials, we cannot suppose that we shall +ever bring agriculture to such a degree of perfection as to require the +whole of what these resources could supply. + +Nature, however, in thus furnishing us with an inexhaustible stock of +raw materials, leaves it in some measure to the ingenuity of man to +appropriate them to its own purposes. But, like a kind parent, she +stimulates him to exertion, by setting the example and pointing out the +way. For it is on the operations of nature that all the improvements of +art are founded. The art of agriculture consists, therefore, in +discovering the readiest method of obtaining the several principles, +either from their grand sources, air and water, or from the +decomposition of organised bodies; and in appropriating them in the best +manner to the purposes of vegetation. + +EMILY. + +But, among the sources of nutritive principles, I am surprised that you +do not mention the earth itself, as it contains abundance of coals, +which are chiefly composed of carbon. + +MRS. B. + +Though coals abound in carbon, they cannot, on account of their hardness +and impermeable texture, be immediately subservient to the purposes of +vegetation. + +EMILY. + +No; but by their combustion carbonic acid is produced; and this entering +into various combinations on the surface of the earth, may, perhaps, +assist in promoting vegetation. + +MRS. B. + +Probably it may in some degree; but at any rate the quantity of +nourishment which vegetables may derive from that source can be but very +trifling, and must entirely depend on local circumstances. + +CAROLINE. + +Perhaps the smoky atmosphere of London is the cause of vegetation being +so forward and so rich in its vicinity? + +MRS. B. + +I rather believe that this circumstance proceeds from the very ample +supply of manure, assisted, perhaps, by the warmth and shelter which the +town affords. Far from attributing any good to the smoky atmosphere of +London, I confess I like to anticipate the time when we shall have made +such progress in the art of managing combustion, that every particle of +carbon will be consumed, and the smoke destroyed at the moment of its +production. We may then expect to have the satisfaction of seeing the +atmosphere of London as clear as that of the country. --But to return to +our subject: I hope that you are now convinced that we shall not easily +experience a deficiency of nutritive elements to fertilise the earth, +and that, provided we are but industrious in applying them to the best +advantage by improving the art of agriculture, no limits can be assigned +to the fruits that we may expect to reap from our labours. + +CAROLINE. + +Yes; I am perfectly satisfied in that respect, and I can assure you that +I feel already much more interested in the progress and improvement of +agriculture. + +EMILY. + +I have frequently thought that the culture of the land was not +considered as a concern of sufficient importance. Manufactures always +take the lead; and health and innocence are frequently sacrificed to the +prospect of a more profitable employment. It has often grieved me to see +the poor manufacturers crowded together in close rooms, and confined for +the whole day to the most uniform and sedentary employment, instead of +being engaged in that innocent and salutary kind of labour, which Nature +seems to have assigned to man for the immediate acquirement of comfort, +and for the preservation of his existence. I am sure that you agree with +me in thinking so, Mrs. B.? + +MRS. B. + +I am entirely of your opinion, my dear, in regard to the importance of +agriculture; but as the conveniences of life, which we are all enjoying, +are not derived merely from the soil, I am far from wishing to +depreciate manufactures. Besides, as the labour of one man is sufficient +to produce food for several, those whose industry is not required in +tillage must do something in return for the food that is provided for +them. They exchange, consequently, the accommodations for the +necessaries of life. Thus the carpenter and the weaver lodge and clothe +the peasant, who supplies them with their daily bread. The greater stock +of provisions, therefore, which the husbandman produces, the greater is +the quantity of accommodation which the artificer prepares. Such are the +happy effects which naturally result from civilised society. It would be +wiser, therefore, to endeavour to improve the situation of those who are +engaged in manufactures, than to indulge in vain declamations on the +hardships to which they are too frequently exposed. + +But we must not yet take our leave of the subject of agriculture; we +have prepared the soil, it remains for us now to sow the seed. In this +operation we must be careful not to bury it too deep in the ground, as +the access of air is absolutely necessary to its germination; the earth +must, therefore, lie loose and light over it, in order that the air may +penetrate. Hence the use of ploughing and digging, harrowing and raking, +&c. A certain degree of heat and moisture, such as usually takes place +in the spring, is likewise necessary. + +CAROLINE. + +One would imagine you were going to describe the decomposition of an old +plant, rather than the formation of a new one; for you have enumerated +all the requisites of fermentation. + +MRS. B. + +Do you forget, my dear, that the young plant derives its existence from +the destruction of the seed, and that it is actually by the saccharine +fermentation that the latter is decomposed? + +CAROLINE. + +True; I wonder that I did not recollect that. The temperature and +moisture required for the germination of the seed is then employed in +producing the saccharine fermentation within it? + +MRS. B. + +Certainly. But, in order to understand the nature of germination, you +should be acquainted with the different parts of which the seed is +composed. The external covering or envelope contains, besides the germ +of the future plant, the substance which is to constitute its first +nourishment; this substance, which is called the _parenchyma_, consists +of fecula, mucilage, and oil, as we formerly observed. + +The seed is generally divided into two compartments, called _lobes_, or +_cotyledons_, as is exemplified by this bean (PLATE XV. Fig. 1.)--the +dark-coloured kind of string which divides the lobes is called the +_radicle_, as it forms the root of the plant, and it is from a +contiguous substance, called _plumula_, which is enclosed within the +lobes, that the stem arises. The figure and size of the seed depend very +much upon the cotyledons; these vary in number in different seeds; some +have only one, as wheat, oats, barley, and all the grasses; some have +three, others six. But most seeds, as, for instance, all the varieties +of beans, have two cotyledons. When the seed is buried in the earth, at +any temperature above 40 degrees, it imbibes water, which softens and +swells the lobes; it then absorbs oxygen, which combines with some of +its carbon, and is returned in the form of carbonic acid. This loss of +carbon increases the comparative proportion of hydrogen and oxygen in +the seed, and excites the saccharine fermentation, by which the +parenchymatous matter is converted into a kind of sweet emulsion. In +this form it is carried into the radicle by vessels appropriated to that +purpose; and in the mean time, the fermentation having caused the seed +to burst, the cotyledons are rent asunder, the radicle strikes into the +ground and becomes the root of the plant, and hence the fermented liquid +is conveyed to the plumula, whose vessels have been previously distended +by the heat of the fermentation. The plumula being thus swelled, as it +were, by the emulsive fluid, raises itself and springs up to the surface +of the earth, bearing with it the cotyledons, which, as soon as they +come in contact with the air, spread themselves, and are transformed +into leaves. --If we go into the garden, we shall probably find some +seeds in the state which I have described-- + + [Illustration: Plate XV. Vol. II. p. 250 + + Germination. + Fig. 1 & 2. + A.B Cotyledons. + C Envelope. + D Radicle. + + Fig. 3. + A.B Cotyledons. + C Plumula. + D Radicle. + + Fig. 4. + A.B. Cotyledons. + C Plumula. + D Radicle. + + Fig. 5. Apparatus to illustrate the mechanism of breathing. + A.A Glass Bell. + B Bladder representing the lungs. + C Bladder representing the Diaphragm.] + +EMILY. + +Here are some lupines that are just making their appearance above +ground. + +MRS. B. + +We shall take up several of them to observe their different degrees of +progress in vegetation. Here is one that has but recently burst its +envelope--do you see the little radicle striking downwards? (PLATE XV. +Fig. 2.) In this the plumula is not yet visible. But here is another in +a greater state of forwardness--the plumula, or stem, has risen out of +the ground, and the cotyledons are converted into seed leaves. (PLATE +XV. Fig. 3.) + +CAROLINE. + +These leaves are very thick and clumsy, and unlike the other leaves, +which I perceive are just beginning to appear. + +MRS. B. + +It is because they retain the remains of the parenchyma, with which they +still continue to nourish the young plant, as it has not yet sufficient +roots and strength to provide for its sustenance from the soil. --But, +in this third lupine (PLATE XV. Fig. 4.), the radicle had sunk deep +into the earth, and sent out several shoots, each of which is furnished +with a mouth to suck up nourishment from the soil; the function of the +original leaves, therefore, being no longer required, they are gradually +decaying, and the plumula is become a regular stem, shooting out small +branches, and spreading its foliage. + +EMILY. + +There seems to be a very striking analogy between a seed and an egg; +both require an elevation of temperature to be brought to life; both at +first supply with aliment the organised being which they produce; and as +soon as this has attained sufficient strength to procure its own +nourishment, the egg-shell breaks, whilst in the plant the seed-leaves +fall off. + +MRS. B. + +There is certainly some resemblance between these processes; and when +you become acquainted with animal chemistry, you will frequently be +struck with its analogy to that of the vegetable kingdom. + +As soon as the young plant feeds from the soil, it requires the +assistance of leaves, which are the organs by which it throws off its +super-abundant fluid; this secretion is much more plentiful in the +vegetable than in the animal creation, and the great extent of surface +of the foliage of plants is admirably calculated for carrying it on in +sufficient quantities. This transpired fluid consists of little more +than water. The sap, by this process, is converted into a liquid of +greater consistence, which is fit to be assimilated to its several +parts. + +EMILY. + +Vegetation, then, must be essentially injured by destroying the leaves +of the plant? + +MRS. B. + +Undoubtedly; it not only diminishes the transpiration, but also the +absorption by the roots; for the quantity of sap absorbed is always in +proportion to the quantity of fluid thrown off by transpiration. You +see, therefore, the necessity that a young plant should unfold its +leaves as soon as it begins to derive its nourishment from the soil; +and, accordingly, you will find that those lupines which have dropped +their seed-leaves, and are no longer fed by the parenchyma, have spread +their foliage, in order to perform the office just described. + +But I should inform you that this function of transpiration seems to be +confined to the upper surface of the leaves, whilst, on the contrary, +the lower surface, which is more rough and uneven, and furnished with a +kind of hair or down, is destined to absorb moisture, or such other +ingredients as the plant derives from the atmosphere. + +As soon as a young plant makes its appearance above ground, light, as +well as air, becomes necessary to its preservation. Light is essential +to the development of the colours, and to the thriving of the plant. You +may have often observed what a predilection vegetables have for the +light. If you make any plants grow in a room, they all spread their +leaves, and extend their branches towards the windows. + +CAROLINE. + +And many plants close up their flowers as soon as it is dark. + +EMILY. + +But may not this be owing to the cold and dampness of the evening air? + +MRS. B. + +That does not appear to be the case; for in a course of curious +experiments, made by Mr. Senebier, of Geneva, on plants which he reared +by lamp-light, he found that the flowers closed their petals whenever +the lamps were extinguished. + +EMILY. + +But pray, why is air essential to vegetation, plants do not breathe it +like animals? + +MRS. B. + +At least not in the same manner; but they certainly derive some +principles from the atmosphere, and yield others to it. Indeed, it is +chiefly owing to the action of the atmosphere and the vegetable kingdom +on each other, that the air continues always fit for respiration. But +you will understand this better when I have explained the effect of +water on plants. + +I have said that water forms the chief nourishment of plants; it is the +basis not only of the sap, but of all the vegetable juices. Water is the +vehicle which carries into the plant the various salts and other +ingredients required for the formation and support of the vegetable +system. Nor is this all; part of the water itself is decomposed by the +organs of the plant; the hydrogen becomes a constituent part of oil, of +extract, of colouring matter, &c. whilst a portion of the oxygen enters +into the formation of mucilage, of fecula, of sugar, and of vegetable +acids. But the greater part of the oxygen, proceeding from the +decomposition of the water, is converted into a gaseous state by the +caloric disengaged from the hydrogen during its condensation in the +formation of the vegetable materials. In this state the oxygen is +transpired by the leaves of plants when exposed to the sun’s rays. Thus +you find that the decomposition of water, by the organs of the plant, is +not only a means of supplying it with its chief ingredient, hydrogen, +but at the same time of replenishing the atmosphere with oxygen, +a principle which requires continual renovation, to make up for the +great consumption of it occasioned by the numerous oxygenations, +combustions, and respirations, that are constantly taking place on the +surface of the globe. + +EMILY. + +What a striking instance of the harmony of nature. + +MRS. B. + +And how admirable the design of Providence, who makes every different +part of the creation thus contribute to the support and renovation of +each other! + +But the intercourse of the vegetable and animal kingdoms through the +medium of the atmosphere extends still further. Animals, in breathing, +not only consume the oxygen of the air, but load it with carbonic acid, +which, if accumulated in the atmosphere, would, in a short time, render +it totally unfit for respiration. Here the vegetable kingdom again +interferes; it attracts and decomposes the carbonic acid, retains the +carbon for its own purposes, and returns the oxygen for ours. + +CAROLINE. + +How interesting this is! I do not know a more beautiful illustration of +the wisdom which is displayed in the laws of nature. + +MRS. B. + +Faint and imperfect as are the ideas which our limited perceptions +enable us to form of divine wisdom, still they cannot fail to inspire us +with awe and admiration. What, then, would be our feelings, were the +complete system of nature at once displayed before us! So magnificent a +scene would probably be too great for our limited and imperfect +comprehension, and it is no doubt among the wise dispensations of +Providence, to veil the splendour of a glory with which we should be +overpowered. But it is well suited to the nature of a rational being to +explore, step by step, the works of the creation, to endeavour to +connect them into harmonious systems; and, in a word, to trace in the +chain of beings, the kindred ties and benevolent design which unites its +various links, and secure its preservation. + +CAROLINE. + +But of what nature are the organs of plants which are endued with such +wonderful powers? + +MRS. B. + +They are so minute that their structure, as well as the mode in which +they perform their functions, generally elude our examination; but we +may consider them as so many vessels or apparatus appropriated to +perform, with the assistance of the principle of life, certain chemical +processes, by means of which these vegetable compounds are generated. We +may, however, trace the tannin, resins, gum, mucilage, and some other +vegetable materials, in the organised arrangement of plants, in which +they form the bark, the wood, the leaves, flowers, and seeds. + +The _bark_ is composed of the _epidermis_, the _parenchyma_, and the +_cortical layers_. + +The epidermis is the external covering of the plant. It is a thin +transparent membrane, consisting of a number of slender fibres, crossing +each other, and forming a kind of net-work. When of a white glossy +nature, as in several species of trees, in the stems of corn and of +seeds, it is composed of a thin coating of siliceous earth, which +accounts for the strength and hardness of those long and slender stems. +Sir H. Davy was led to the discovery of the siliceous nature of the +epidermis of such plants, by observing the singular phenomenon of sparks +of fire emitted by the collision of ratan canes with which two boys were +fighting in a dark room. On analysing the epidermis of the cane, he +found it to be almost entirely siliceous. + +CAROLINE. + +With iron then, a cane, I suppose, will strike fire very easily? + +MRS. B. + +I understand that it will. --In ever-greens the epidermis is mostly +resinous, and in some few plants is formed of wax. The resin, from its +want of affinity for water, tends to preserve the plant from the +destructive effects of violent rains, severe climates, or inclement +seasons, to which this species of vegetables is peculiarly exposed. + +EMILY. + +Resin must preserve wood just like a varnish, as it is the essential +ingredient of varnishes? + +MRS. B. + +Yes; and by this means it prevents likewise all unnecessary expenditure +of moisture. + +The parenchyma is immediately beneath the epidermis; it is that green +rind which appears when you strip a branch of any tree or shrub of its +external coat of bark. The parenchyma is not confined to the stem or +branches, but extends over every part of the plant. It forms the green +matter of the leaves, and is composed of tubes filled with a peculiar +juice. + +The cortical layers are immediately in contact with the wood; they +abound with tannin and gallic acid, and consist of small vessels through +which the sap descends after being elaborated in the leaves. The +cortical layers are annually renewed, the old bark being converted into +wood. + +EMILY. + +But through what vessels does the sap ascend? + +MRS. B. + +That function is performed by the tubes of the alburnum, or wood, which +is immediately beneath the cortical layers. The wood is composed of +woody fibre, mucilage, and resin. The fibres are disposed in two ways; +some of them longitudinally, and these form what is called the silver +grain of the wood. The others, which are concentric, are called the +spurious grain. These last are disposed in layers, from the number of +which the age of the tree may be computed, a new one being produced +annually by the conversion of the bark into wood. The oldest, and +consequently most internal part of the alburnum, is called heart-wood; +it appears to be dead, at least no vital functions are discernible in +it. It is through the tubes of the living alburnum that the sap rises. +These, therefore, spread into the leaves, and there communicate with the +extremities of the vessels of the cortical layers, into which they pour +their contents. + +CAROLINE. + +Of what use, then, are the tubes of the parenchyma, since neither the +ascending nor descending sap passes through them? + +MRS. B. + +They are supposed to perform the important function of secreting from +the sap the peculiar juices from which the plant more immediately +derives its nourishment. These juices are very conspicuous, as the +vessels which contain them are much larger than those through which the +sap circulates. The peculiar juices of plants differ much in their +nature, not only in different species of vegetables, but frequently in +different parts of the same individual plant: they are sometimes +saccharine, as in the sugar-cane, sometimes resinous, as in firs and +evergreens, sometimes of a milky appearance, as in the laurel. + +EMILY. + +I have often observed, that in breaking a young shoot, or in bruising a +leaf of laurel, a milky juice will ooze out in great abundance. + +MRS. B. + +And it is by making incisions in the bark that pitch, tar, and +turpentine are obtained from fir-trees. The durability of this species +of wood is chiefly owing to the resinous nature of its peculiar juices. +The volatile oils have, in a great measure, the same preservative +effects, as they defend the parts, with which they are connected, from +the attack of insects. This tribe seems to have as great an aversion to +perfumes, as the human species have delight in them. They scarcely ever +attack any odoriferous parts of plants, and it is not uncommon to see +every leaf of a tree destroyed by a blight, whilst the blossoms remain +untouched. Cedar, sandal, and all aromatic woods, are on this account of +great durability. + +EMILY. + +But the wood of the oak, which is so much esteemed for its durability, +has, I believe, no smell. Does it derive this quality from its hardness +alone? + +MRS. B. + +Not entirely; for the chesnut, though considerably harder and firmer +than the oak, is not so lasting. The durability of the oak is, +I believe, in a great measure owing to its having very little +heart-wood, the alburnum preserving its vital functions longer than in +other trees. + +CAROLINE. + +If incisions are made into the alburnum and cortical layers, may not the +ascending and descending sap be procured in the same manner as the +peculiar juice is from the vessels of the parenchyma? + +MRS. B. + +Yes; but in order to obtain specimens of these fluids, in any quantity, +the experiment must be made in the spring, when the sap circulates with +the greatest energy. For this purpose a small bent glass tube should be +introduced into the incision, through which the sap may flow without +mixing with any of the other juices of the tree. From the bark the sap +will flow much more plentifully than from the wood, as the ascending sap +is much more liquid, more abundant, and more rapid in its motion than +that which descends; for the latter having been deprived by the +operation of the leaves of a considerable part of its moisture, contains +a much greater proportion of solid matter, which retards its motion. It +does not appear that there is any excess of descending sap, as none ever +exudes from the roots of plants; this process, therefore, seems to be +carried on only in proportion to the wants of the plant, and the sap +descends no further, and in no greater quantity, than is required to +nourish the several organs. Therefore, though the sap rises and descends +in the plant, it does not appear to undergo a real circulation. + +The last of the organs of plants is the _flower_, or _blossom_, which +produces the _fruits_ and _seed_. These may be considered as the +ultimate purpose of nature in the vegetable creation. From fruits and +seeds animals derive both a plentiful source of immediate nourishment, +and an ample provision for the reproduction of the same means of +subsistence. + +The seed which forms the final product of mature plants, we have already +examined as constituting the first rudiments of future vegetation. + +These are the principal organs of vegetation, by means of which the +several chemical processes which are carried on during the life of the +plant are performed. + +EMILY. + +But how are the several principles which enter into the composition of +vegetables so combined by the organs of the plant as to be converted +into vegetable matter? + +MRS. B. + +By chemical processes, no doubt; but the apparatus in which they are +performed is so extremely minute as completely to elude our examination. +We can form an opinion, therefore, only by the result of these +operations. The sap is evidently composed of water, absorbed by the +roots, and holding in solution the various principles which it derives +from the soil. From the roots the sap ascends through the tubes of the +alburnum into the stem, and thence branches out to every extremity of +the plant. Together with the sap circulates a certain quantity of +carbonic acid, which is gradually disengaged from the former by the +internal heat of the plant. + +CAROLINE. + +What! have vegetables a peculiar heat, analogous to animal heat? + +MRS. B. + +It is a circumstance that has long been suspected; but late experiments +have decided beyond a doubt that vegetable heat is considerably above +that of unorganised matter in winter, and below it in summer. The wood +of a tree is about sixty degrees, when the thermometer is seventy or +eighty degrees. And the bark, though so much exposed, is seldom below +forty in winter. + +It is from the sap, after it has been elaborated by the leaves, that +vegetables derive their nourishment; in its progress through the plant +from the leaves to the roots, it deposits in the several sets of vessels +with which it communicates, the materials on which the growth and +nourishment of each plant depends. It is thus that the various peculiar +juices, saccharine, oily, mucous, acid, and colouring, are formed; as +also the more solid parts, fecula, woody fibre, tannin, resins, concrete +salts; in a word, all the immediate materials of vegetables, as well as +the organised parts of plants, which latter, besides the power of +secreting these from the sap for the general purpose of the plant, have +also that of applying them to their own particular nourishment. + +EMILY. + +But why should the process of vegetation take place only at one season +of the year, whilst a total inaction prevails during the other? + +MRS. B. + +Heat is such an important chemical agent, that its effect, as such, +might perhaps alone account for the impulse which the spring gives to +vegetation. But, in order to explain the mechanism of that operation, it +has been supposed that the warmth of the spring dilates the vessels of +plants, and produces a kind of vacuum, into which the sap (which had +remained in a state of inaction in the trunk during the winter) rises: +this is followed by the ascent of the sap contained in the roots, and +room is thus made for fresh sap, which the roots, in their turn, pump up +from the soil. This process goes on till the plant blossoms and bears +fruit, which terminates its summer career: but when the cold weather +sets in, the fibres and vessels contract, the leaves wither, and are no +longer able to perform their office of transpiration; and, as this +secretion stops, the roots cease to absorb sap from the soil. If the +plant be an annual, its life then terminates; if not, it remains in a +state of torpid inaction during the winter; or the only internal motion +that takes place is that of a small quantity of resinous juice, which +slowly rises from the stem into the branches, and enlarges their buds +during the winter. + +CAROLINE. + +Yet, in evergreens, vegetation must continue throughout the year. + +MRS. B. + +Yes; but in winter it goes on in a very imperfect manner, compared to +the vegetation of spring and summer. + +We have dwelt much longer on the history of vegetable chemistry than I +had intended; but we have at length, I think, brought the subject to a +conclusion. + +CAROLINE. + +I rather wonder that you did not reserve the account of the +fermentations for the conclusion; for the decomposition of vegetables +naturally follows their death, and can hardly, it seems, be introduced +with so much propriety at any other period. + +MRS. B. + +It is difficult to determine at what point precisely it may be most +eligible to enter on the history of vegetation; every part of the +subject is so closely connected, and forms such an uninterrupted chain, +that it is by no means easy to divide it. Had I begun with the +germination of the seed, which, at first view, seems to be the most +proper arrangement, I could not have explained the nature and +fermentation of the seed, or have described the changes which manure +must undergo, in order to yield the vegetable elements. To understand +the nature of germination, it is necessary, I think, previously to +decompose the parent plant, in order to become acquainted with the +materials required for that purpose. I hope, therefore, that, upon +second consideration, you will find that the order which I have adopted, +though apparently less correct, is in fact the best calculated for the +elucidation of the subject. + + + + +CONVERSATION XXIII. + +ON THE COMPOSITION OF ANIMALS. + + +MRS. B. + +We are now come to the last branch of chemistry, which comprehends the +most complicated order of compound beings. This is the animal creation, +the history of which cannot but excite the highest degree of curiosity +and interest, though we often fail in attempting to explain the laws by +which it is governed. + +EMILY. + +But since all animals ultimately derive their nourishment from +vegetables, the chemistry of this order of beings must consist merely in +the conversion of vegetable into animal matter. + +MRS. B. + +Very true; but the manner in which this is effected is, in a great +measure, concealed from our observation. This process is called +_animalisation_, and is performed by peculiar organs. The difference of +the animal and vegetable kingdoms does not however depend merely on a +different arrangement of combinations. A new principle abounds in the +animal kingdom, which is but rarely and in very small quantities found +in vegetables; this is nitrogen. There is likewise in animal substances +a greater and more constant proportion of phosphoric acid, and other +saline matters. But these are not essential to the formation of animal +matter. + +CAROLINE. + +Animal compounds contain, then, four fundamental principles; oxygen, +hydrogen, carbon, and nitrogen? + +MRS. B. + +Yes; and these form the immediate materials of animals, which are +_gelatine_, _albumen_, and _fibrine_. + +EMILY. + +Are those all? I am surprised that animals should be composed of fewer +kinds of materials than vegetables; for they appear much more +complicated in their organisation. + +MRS. B. + +Their organisation is certainly more perfect and intricate, and the +ingredients that occasionally enter into their composition are more +numerous. But notwithstanding the wonderful variety observable in the +texture of the animal organs, we find that the original compounds, from +which all the varieties of animal matter are derived, may be reduced to +the three heads just mentioned. Animal substances being the most +complicated of all natural compounds, are most easily susceptible of +decomposition, as the scale of attractions increases in proportion to +the number of constituent principles. Their analysis is, however, both +difficult and imperfect; for as they cannot be examined in their living +state, and are liable to alteration immediately after death, it is +probable that, when submitted to the investigation of a chemist, they +are always more or less altered in their combinations and properties, +from what they were, whilst they made part of the living animal. + +EMILY. + +The mere diminution of temperature, which they experience by the +privation of animal heat, must, I should suppose, be sufficient to +derange the order of attractions that existed during life. + +MRS. B. + +That is one of the causes, no doubt: but there are many other +circumstances which prevent us from studying the nature of living animal +substances. We must therefore, in a considerable degree, confine our +researches to the phenomena of these compounds in their inanimate state. + +These three kinds of animal matter, gelatine, albumen, and fibrine, form +the basis of all the various parts of the animal system; either solid, +as the _skin_, _flesh_, _nerves_, _membranes_, _cartilages_, and +_bones_; or fluid, as _blood_, _chyle_, _milk_, _mucus_, the _gastric_ +and _pancreatic juices_, _bile_, _perspiration_, _saliva_, _tears_, &c. + +CAROLINE. + +Is it not surprising that so great a variety of substances, and so +different in their nature, should yet all arise from so few materials, +and from the same original elements? + +MRS. B. + +The difference in the nature of various bodies depends, as I have often +observed to you, rather on their state of combination, than on the +materials of which they are composed. Thus, in considering the chemical +nature of the creation in a general point of view, we observe that it is +throughout composed of a very small number of elements. But when we +divide it into the three kingdoms, we find that, in the mineral, the +combinations seem to result from the union of elements casually brought +together; whilst in the vegetable and animal kingdoms, the attractions +are peculiarly and regularly produced by appropriate organs, whose +action depends on the vital principle. And we may further observe, that +by means of certain spontaneous changes and decompositions, the elements +of one kind of matter become subservient to the reproduction of another; +so that the three kingdoms are intimately connected, and constantly +contributing to the preservation of each other. + +EMILY. + +There is, however, one very considerable class of elements, which seems +to be confined to the mineral kingdom: I mean metals. + +MRS. B. + +Not entirely; they are found, though in very minute quantities, both in +the vegetable and animal kingdoms. A small portion of earths and sulphur +enters also into the composition of organised bodies. Phosphorus, +however, is almost entirely confined to the animal kingdom; and +nitrogen, but with few exceptions, is extremely scarce in vegetables. + +Let us now proceed to examine the nature of the three principal +materials of the animal system. + +_Gelatine_, or _jelly_, is the chief ingredient of skin, and of all the +membranous parts of animals. It may be obtained from these substances, +by means of boiling water, under the forms of glue, size, isinglass, and +transparent jelly. + +CAROLINE. + +But these are of a very different nature; they cannot therefore be all +pure gelatine. + +MRS. B. + +Not entirely, but very nearly so. Glue is extracted from the skin of +animals. Size is obtained either from skin in its natural state, or from +leather. Isinglass is gelatine procured from a particular species of +fish; it is, you know, of this substance that the finest jelly is made, +and this is done by merely dissolving the isinglass in boiling water, +and allowing the solution to congeal. + +EMILY. + +The wine, lemon, and spices, are, I suppose, added only to flavour the +jelly? + +MRS. B. + +Exactly so. + +CAROLINE. + +But jelly is often made of hartshorn shavings, and of calves’ feet; do +these substances contain gelatine? + +MRS. B. + +Yes. Gelatine may be obtained from almost any animal substance, as it +enters more or less into the composition of all of them. The process for +obtaining it is extremely simple, as it consists merely in boiling the +substance that contains it with water. The gelatine dissolves in water, +and may be attained of any degree of consistence or strength, by +evaporating this solution. Bones in particular produce it very +plentifully, as they consist of phosphat of lime combined or cemented by +gelatine. Horns, which are a species of bone, will yield abundance of +gelatine. The horns of the hart are reckoned to produce gelatine of the +finest quality; they are reduced to the state of shavings in order that +the jelly may be more easily extracted by the water. It is of hartshorn +shavings that the jellies for invalids are usually made, as they are of +very easy digestion. + +CAROLINE. + +It appears singular that hartshorn, which yields such a powerful +ingredient as ammonia, should at the same time produce so mild and +insipid a substance as jelly? + +MRS. B. + +And (what is more surprising) it is from the gelatine of bones that +ammonia is produced. You must observe, however, that the processes by +which these two substances are obtained from bones are very different. +By the simple action of water and heat, the gelatine is separated; but +in order to procure the ammonia, or what is commonly called hartshorn, +the bones must be distilled, by which means the gelatine is decomposed, +and hydrogen and nitrogen combined in the form of ammonia. So that the +first operation is a mere separation of ingredients, whilst the second +requires a chemical decomposition. + +CAROLINE. + +But when jelly is made from hartshorn shavings, what becomes of the +phosphat of lime which constitutes the other part of bones? + +MRS. B. + +It is easily separated by straining. But the jelly is afterwards more +perfectly purified, and rendered transparent, by adding white of egg, +which being coagulated by heat, rises to the surface along with any +impurities. + +EMILY. + +I wonder that bones are not used by the common people to make jelly; +a great deal of wholesome nourishment, might, I should suppose, be +procured from them, though the jelly would perhaps not be quite so good +as if made from hartshorn shavings? + +MRS. B. + +There is a prejudice among the poor against a species of food that is +usually thrown to the dogs; and as we cannot expect them to enter into +chemical considerations, it is in some degree excusable. Besides, it +requires a prodigious quantity of fuel to dissolve bones and obtain the +gelatine from them. + +The solution of bones in water is greatly promoted by an accumulation of +heat. This may be effected by means of an extremely strong metallic +vessel, called _Papin’s digester_, in which the bones and water are +enclosed, without any possibility of the steam making its escape. A heat +can thus be applied much superior to that of boiling water; and bones, +by this means, are completely reduced to a pulp. But the process still +consumes too much fuel to be generally adopted among the lower classes. + +CAROLINE. + +And why should not a manufacture be established for grinding or +macerating bones, or at least for reducing them to the state of +shavings, when I suppose they would dissolve as readily as hartshorn +shavings? + +MRS. B. + +They could not be collected clean for such a purpose, but they are not +lost, as they are used for making hartshorn and sal ammoniac; and such +is the superior science and industry of this country, that we now send +sal ammoniac to the Levant, though it originally came to us from Egypt. + +EMILY. + +When jelly is made of isinglass, does it leave no sediment? + +MRS. B. + +No; nor does it so much require clarifying, as it consists almost +entirely of pure gelantine, and any foreign matter that is mixed with +it, is thrown off during the boiling in the form of scum. --These are +processes which you may see performed in great perfection in the +culinary laboratory, by that very able and most useful chemist the cook. + +CAROLINE. + +To what an immense variety of purposes chemistry is subservient! + +EMILY. + +It appears, in that respect, to have an advantage over most other arts +and sciences; for these, very often, have a tendency to confine the +imagination to their own particular object, whilst the pursuit of +chemistry is so extensive and diversified, that it inspires a general +curiosity, and a desire of enquiring into the nature of every object. + +CAROLINE. + +I suppose that soup is likewise composed of gelatine; for, when cold, it +often assumes the consistence of jelly? + +MRS. B. + +Not entirely; for though soups generally contain a quantity of gelatine, +the most essential ingredient is a mucous or extractive matter, +a peculiar animal substance, very soluble in water, which has a strong +taste, and is more nourishing than gelatine. The various kinds of +portable soup consist of this extractive matter in a dry state, which, +in order to be made into soup, requires only to be dissolved in water. + +Gelatine, in its solid state, is a semiductile transparent substance, +without either taste or smell. --When exposed to heat, in contact with +air and water, it first swells, then fuses, and finally burns. You may +have seen the first part of this operation performed in the carpenter’s +glue-pot. + +CAROLINE. + +But you said that gelatine had no smell, and glue has a very +disagreeable one. + +MRS. B. + +Glue is not pure gelatine; as it is not designed for eating, it is +prepared without attending to the state of the ingredients, which are +more or less contaminated by particles that have become putrid. + +Gelatine may be precipitated from its solution in water by alcohol. --We +shall try this experiment with a glass of warm jelly. --You see that the +gelatine subsides by the union of the alcohol and the water. + +EMILY. + +How is it, then, that jelly is flavoured with wine, without producing +any precipitation? + +MRS. B. + +Because the alcohol contained in wine is already combined with water, +and other ingredients, and is therefore not at liberty to act upon the +jelly as when in its separate state. Gelatine is soluble both in acids +and in alkalies; the former, you know, are frequently used to season +jellies. + +CAROLINE. + +Among the combinations of gelatine we must not forget one which you +formerly mentioned; that with tannin, to form leather. + +MRS. B. + +True; but you must observe that leather can be produced only by gelatine +in a membranous state; for though pure gelatine and tannin will produce +a substance chemically similar to leather, yet the texture of the skin +is requisite to make it answer the useful purposes of that substance. + +The next animal substance we are to examine is _albumen_; this, although +constituting a part of most of the animal compounds, is frequently found +insulated in the animal system; the white of egg, for instance, consists +almost entirely of albumen; the substance that composes the nerves, the +serum, or white part of the blood, and the curds of milk, are little +else than albumen variously modified. + +In its most simple state, albumen appears in the form of a transparent +viscous fluid, possessed of no distinct taste or smell; it coagulates at +the low temperature of 165 degrees, and, when once solidified, it will +never return to its fluid state. + +Sulphuric acid and alcohol are each of them capable of coagulating +albumen in the same manner as heat, as I am going to show you. + +EMILY. + +Exactly so. --Pray, Mrs. B., what kind of action is there between +albumen and silver? I have sometimes observed, that if the spoon with +which I eat an egg happens to be wetted, it becomes tarnished. + +MRS. B. + +It is because the white of egg (and, indeed, albumen in general) +contains a little sulphur, which, at the temperature of an egg just +boiled, will decompose the drop of water that wets the spoon, and +produce sulphurated hydrogen gas, which has the property of tarnishing +silver. + +We may now proceed to _fibrine_. This is an insipid and inodorous +substance, having somewhat the appearance of fine white threads adhering +together; it is the essential constituent of muscles or flesh, in which +it is mixed with and softened by gelatine. It is insoluble both in water +and alcohol, but sulphuric acid converts it into a substance very +analogous to gelatine. + + +These are the essential and general ingredients of animal matter; but +there are other substances, which, though not peculiar to the animal +system, usually enter into its composition, such as oils, acids, +salts, &c. + +_Animal oil_ is the chief constituent of fat; it is contained in +abundance in the cream of milk, whence it is obtained in the form of +butter. + +EMILY. + +Is animal oil the same in its composition as vegetable oils? + +MRS. B. + +Not the same, but very analogous. The chief difference is that animal +oil contains nitrogen, a principle which seldom enters into the +composition of vegetable oils, and never in so large a proportion. + +There are a few animal acids, that is to say, acids peculiar to animal +matter, from which they are almost exclusively obtained. + +The animal acids have triple bases of hydrogen, carbon, and nitrogen. +Some of them are found native in animal matter; others are produced +during its decomposition. + +Those that we find ready formed are: + +The _bombic acid_, which is obtained from silk-worms. + +The _formic acid_, from ants. + +The _lactic acid_, from the whey of milk. + +The _sebacic_, from oil or fat. + +Those produced during the decomposition of animal substances by heat, +are the _prussic_ and _zoonic_ acids. This last is produced by the +roasting of meat, and gives it a brisk flavour. + +CAROLINE. + +The class of animal acids is not very extensive? + +MRS. B. + +No; nor are they, generally speaking, of great importance. The _prussic +acid_ is, I think, the only one sufficiently interesting to require any +further comment. It can be formed by any artificial process, without the +presence of any animal matter; and it may likewise be obtained from a +variety of vegetables, particularly those of the narcotic kind, such as +poppies, laurel, &c. But it is commonly obtained from blood, by strongly +heating that substance with caustic potash; the alkali attracts the acid +from the blood, and forms with it a _prussiat of potash_. From this +state of combination the prussic acid can be obtained pure by means of +other substances which have the power of separating it from the alkali. + +EMILY. + +But if this acid does not exist ready formed in blood, how can the +alkali attract it from it? + +MRS. B. + +It is the triple basis only of this acid that exists in the blood; and +this is developed and brought to the state of acid, during the +combustion. The acid therefore is first formed, and it afterwards +combines with the potash. + +EMILY. + +Now I comprehend it. But how can the prussic acid be artificially made? + +MRS. B. + +By passing ammoniacal gas over red-hot charcoal; and hence we learn that +the constituents of this acid are hydrogen, nitrogen, and carbon. The +two first are derived from the volatile alkali, the last from the +combustion of the charcoal. + +CAROLINE. + +But this does not accord with the system of oxygen being the principle +of acidity. + +MRS. B. + +The colouring matter of prussian blue is called an acid, because it +unites with alkalies and metals, and not from any other characteristic +properties of acids; perhaps the name is not strictly appropriate. But +this circumstance, together with some others of the same kind, has +induced several chemists to think that oxygen may not be the exclusive +generator of acids. Sir H. Davy, I have already informed you, was led by +his experiments on dry acids to suspect that water might be essential to +acidity. And it is the opinion of some chemists that acidity may +possibly depend rather on the arrangement than on the presence of any +particular principles. But we have not yet done with the prussic acid. +It has a strong affinity for metallic oxyds, and precipitates the +solutions of iron in acids of a blue colour. This is the prussian blue, +or prussiat of iron, so much used in the arts, and with which I think +you must be acquainted. + +EMILY. + +Yes, I am; it is much used in painting, both in oil and in water +colours; but it is not reckoned a permanent oil-colour. + +MRS. B. + +That defect arises, I believe, in general, from its being badly +prepared, which is the case when the iron is not so fully oxydated as to +form a red oxyd. For a solution of green oxyd of iron (in which the +metal is more slightly oxydated), makes only a pale green, or even a +white precipitate, with prussiat of potash; and this gradually changes +to blue by being exposed to the air, as I can immediately show you. + +CAROLINE. + +It already begins to assume a pale blue colour. But how does the air +produce this change? + +MRS. B. + +By oxydating the iron more perfectly. If we pour some nitrous acid on +it, the prussian blue colour will be immediately produced, as the acid +will yield its oxygen to the precipitate, and fully saturate it with +this principle, as you shall see. + +CAROLINE. + +It is very curious to see a colour change so instantaneously. + +MRS. B. + +Hence you perceive that prussian blue cannot be a permanent colour, +unless prepared with red oxyd of iron, since by exposure to the +atmosphere it gradually darkens, and in a short time is no longer in +harmony with the other colours of the painting. + +CAROLINE. + +But it can never become darker, by exposure to the atmosphere, than the +true prussian blue, in which the oxyd is perfectly saturated? + +MRS. B. + +Certainly not. But in painting, the artist not reckoning upon partial +alterations in his colours, gives his blue tints that particular shade +which harmonises with the rest of the picture. If, afterwards, those +tints become darker, the harmony of the colouring must necessarily be +destroyed. + +CAROLINE. + +Pray, of what nature is the paint called _carmine_? + +MRS. B. + +It is an animal colour prepared from _cochineal_, an insect, the +infusion of which produces a very beautiful red. + +CAROLINE. + +Whilst we are on the subject of colours, I should like to learn what +_ivory black_ is? + +MRS. B. + +It is a carbonaceous substance obtained by the combustion of ivory. +A more common species of black is obtained from the burning of bone. + +CAROLINE. + +But during the combustion of ivory or bone, the carbon, I should have +imagined, must be converted into carbonic acid gas, instead of this +black substance? + +MRS. B. + +In this, as in most combustions, a considerable part of the carbon is +simply volatilised by the heat, and again obtained concrete on cooling. +This colour, therefore, may be called the soot produced by the burning +of ivory or bone. + + + + +CONVERSATION XXIV. + +ON THE ANIMAL ECONOMY. + + +MRS. B. + +We have now acquired some idea of the various materials that compose the +animal system; but if you are curious to know in what manner these +substances are formed by the animal organs, from vegetable, as well as +from animal substances, it will be necessary to have some previous +knowledge of the nature and functions of these organs, without which it +is impossible to form any distinct idea of the process of +_animalisation_ and _nutrition_. + +CAROLINE. + +I do not exactly understand the meaning of the word animalisation? + +MRS. B. + +Animalisation is the process by which the food is _assimilated_, that is +to say, converted into animal matter; and nutrition is that by which the +food thus assimilated is rendered subservient to the purposes of +nourishing and maintaining the animal system. + +EMILY. + +This, I am sure, must be the most interesting of all the branches of +chemistry! + +CAROLINE. + +So I think; particularly as I expect that we shall hear something of the +nature of respiration, and of the circulation of the blood? + +MRS. B. + +These functions undoubtedly occupy a most important place in the history +of the animal economy. --But I must previously give you a very short +account of the principal organs by which the various operations of the +animal system are performed. These are: + +The _Bones_; + _Muscles_, + _Blood vessels_, + _Lymphatic vessels_, + _Glands_, and + _Nerves_. + +The _bones_ are the most solid part of the animal frame, and in a great +measure determine its form and dimensions. You recollect, I suppose, +what are the ingredients which enter into their composition? + +CAROLINE. + +Yes; phosphat of lime, cemented by gelatine. + +MRS. B. + +During the earliest period of animal life, they consist almost entirely +of gelatinous membrane having the form of the bones, but of a loose +spongy texture, the cells or cavities of which are destined to be filled +with phosphat of lime; it is the gradual acquisition of this salt which +gives to the bones their subsequent hardness and durability. Infants +first receive it from their mother’s milk, and afterwards derive it from +all animal and from most vegetable food, especially farinaceous +substances, such as wheat-flour, which contain it in sensible +quantities. A portion of the phosphat, after the bones of the infant +have been sufficiently expanded and solidified, is deposited in the +teeth, which consist at first only of a gelatinous membrane or case, +fitted for the reception of this salt; and which, after acquiring +hardness within the gum, gradually protrude from it. + +CAROLINE. + +How very curious this is; and how ingeniously nature has first provided +for the solidification of such bones as are immediately wanted, and +afterwards for the formation of the teeth, which would not only be +useless, but detrimental in infancy! + +MRS. B. + +In quadrupeds the phosphat of lime is deposited likewise in their horns, +and in the hair or wool with which they are generally clothed. + +In birds it serves also to harden the beaks and the quills of their +feathers. + +When animals are arrived at a state of maturity, and their bones have +acquired a sufficient degree of solidity, the phosphat of lime which is +taken with the food is seldom assimilated, excepting when the female +nourishes her young; it is then all secreted into the milk, as a +provision for the tender bones of the nursling. + +EMILY. + +So that whatever becomes superfluous to one being, is immediately wanted +by another; and the child acquires strength precisely by the species of +nourishment which is no longer necessary to the mother. Nature is, +indeed, an admirable economist! + +CAROLINE. + +Pray, Mrs. B., does not the disease in the bones of children, called the +rickets, proceed from a deficiency of phosphat of lime? + +MRS. B. + +I have heard that this disease may arise from two causes; it is +sometimes occasioned by the growth of the muscles being too rapid in +proportion to that of the bones. In this case the weight of the flesh is +greater than the bones can support, and presses upon them so as to +produce a swelling of the joints, which is the great indication of the +rickets. The other cause of this disorder is supposed to be an imperfect +digestion and assimilation of the food, attended with an excess of acid, +which counteracts the formation of phosphat of lime. In both instances, +therefore, care should be taken to alter the child’s diet, not merely by +increasing the quantity of aliment containing phosphat of lime, but also +by avoiding all food that is apt to turn acid on the stomach, and to +produce indigestion. But the best preservative against complaints of +this kind is, no doubt, good nursing: when a child has plenty of air and +exercise, the digestion and assimilation will be properly performed, no +acid will be produced to interrupt these functions, and the muscles and +bones will grow together in just proportions. + +CAROLINE. + +I have often heard the rickets attributed to bad nursing, but I never +could have guessed what connection there was between exercise and the +formation of the bones. + +MRS. B. + +Exercise is generally beneficial to all the animal functions. If man is +destined to labour for his subsistence, the bread which he earns is +scarcely more essential to his health and preservation than the +exertions by which he obtains it. Those whom the gifts of fortune have +placed above the necessity of bodily labour are compelled to take +exercise in some mode or other, and when they cannot convert it into an +amusement, they must submit to it as a task, or their health will soon +experience the effects of their indolence. + +EMILY. + +That will never be my case: for exercise, unless it becomes fatigue, +always gives me pleasure; and, so far from being a task, is to me a +source of daily enjoyment. I often think what a blessing it is, that +exercise, which is so conducive to health, should be so delightful; +whilst fatigue, which is rather hurtful, instead of pleasure, occasions +painful sensations. So that fatigue, no doubt, was intended to moderate +our bodily exertions, as satiety puts a limit to our appetites. + +MRS. B. + +Certainly. --But let us not deviate too far from our subject. --The +bones are connected together by ligaments, which consist of a white +thick flexible substance, adhering to their extremities, so far as to +secure the joints firmly, though without impeding their motion. And the +joints are moreover covered by a solid, smooth, elastic, white +substance, called _cartilage_, the use of which is to allow, by its +smoothness and elasticity, the bones to slide easily over one another, +so that the joints may perform their office without difficulty or +detriment. + +Over the bones the _muscles_ are placed; they consist of bundles of +fibres which terminate in a kind of string, or ligament, by which they +are fastened to the bones. The muscles are the organs of motion; by +their power of dilatation and contraction they put into action the +bones, which act as levers, in all the motions of the body, and form the +solid support of its various parts. The muscles are of various degrees +of strength or consistence in different species of animals. The +mammiferous tribe, or those that suckle their young, seem in this +respect to occupy an intermediate place between birds and cold-blooded +animals, such as reptiles and fishes. + +EMILY. + +The different degrees of firmness and solidity in the muscles of these +several species of animals proceed, I imagine, from the different nature +of the food on which they subsist? + +MRS. B. + +No; that is not supposed to be the case: for the human species, who are +of the mammiferous tribe, live on more substantial food than birds, and +yet the latter exceed them in muscular strength. We shall hereafter +attempt to account for this difference; but let us now proceed in the +examination of the animal functions. + +The next class of organs is that of the _vessels_ of the body, the +office of which is to convey the various fluids throughout the frame. +These vessels are innumerable. The most considerable of them are those +through which the blood circulates, which are of two kinds: the +_arteries_, which convey it from the heart to the extremities of the +body, and the _veins_, which bring it back into the heart. + +Besides these, there are a numerous set of small transparent vessels, +destined to absorb and convey different fluids into the blood; they are +generally called the _absorbent_ or _lymphatic_ vessels: but it is to a +portion of them only that the function of conveying into the blood the +fluid called _lymph_ is assigned. + +EMILY. + +Pray what is the nature of that fluid? + +MRS. B. + +The nature and use of the lymph have, I believe, never been perfectly +ascertained; but it is supposed to consist of matter that has been +previously animalised, and which, after answering the purpose for which +it was intended, must, in regular rotation, make way for the fresh +supplies produced by nourishment. The lymphatic vessels pump up this +fluid from every part of the system, and convey it into the veins to be +mixed with the blood which runs through them, and which is commonly +called venous blood. + +CAROLINE. + +But does it not again enter into the animal system through that channel? + +MRS. B. + +Not entirely; for the venous blood does not return into the circulation +until it has undergone a peculiar change, in which it throws off +whatever is become useless. + +Another set of absorbent vessels pump up the _chyle_ from the stomach +and intestines, and convey it, after many circumvolutions, into the +great vein near the heart. + +EMILY. + +Pray what is chyle? + +MRS. B. + +It is the substance into which food is converted by digestion. + +CAROLINE. + +One set of the absorbent vessels, then, is employed in bringing away the +old materials that are no longer fit for use; whilst the other set is +busy in conveying into the blood the new materials that are to replace +them. + +EMILY. + +What a great variety of ingredients must enter into the composition of +the blood? + +MRS. B. + +You must observe that there is also a great variety of substances to be +secreted from it. We may compare the blood to a general receptacle or +storehouse for all kinds of commodities, which are afterwards fashioned, +arranged, and disposed of as circumstances require. + +There is another set of absorbent vessels in females which is destined +to secrete milk for the nourishment of the young. + +EMILY. + +Pray is not milk very analogous in its composition to blood; for, since +the nursling derives its nourishment from that source only, it must +contain every principle which the animal system requires? + +MRS. B. + +Very true. Milk is found, by its analysis, to contain the principal +materials of animal matter, albumen, oil, and phosphat of lime; so that +the suckling has but little trouble to digest and assimilate this +nourishment. But we shall examine the composition of milk more fully +afterwards. + +In many parts of the body numbers of small vessels are collected +together in little bundles called _glands_, from a Latin word meaning +acorn, on account of the resemblance which some of them bear in shape to +that fruit. The function of the glands is to _secrete_, or separate +certain matters from the blood. + +The secretions are not only mechanical, but chemical separations from +the blood; for the substances thus formed, though contained in the +blood, are not ready combined in that fluid. The secretions are of two +kinds, those which form peculiar animal fluids, as bile, tears, saliva, +&c.; and those which produce the general materials of the animal system, +for the purpose of recruiting and nourishing the several organs of the +body; such as albumen, gelatine, and fibrine; the latter may be +distinguished by the name of _nutritive secretions_. + +CAROLINE. + +I am quite astonished to hear that all the secretions should be derived +from the blood. + +EMILY. + +I thought that the bile was produced by the liver? + +MRS. B. + +So it is; but the liver is nothing more than a very large gland, which +secretes the bile from the blood. + +The last of the animal organs which we have mentioned are the _nerves_; +these are the vehicles of sensation, every other part of the body being, +of itself, totally insensible. + +CAROLINE. + +They must then be spread through every part of the frame, for we are +every where susceptible of feeling. + +EMILY. + +Excepting the nails and the hair. + +MRS. B. + +And those are almost the only parts in which nerves cannot be +discovered. The common source of all the nerves is the brain; thence +they descend, some of them through different holes of the skull, but the +greatest part through the back bone, and extend themselves by +innumerable ramifications throughout the whole body. They spread +themselves over the muscles, penetrate the glands, wind round the +vascular system, and even pierce into the interior of the bones. It is +most probably through them that the communication is carried on between +the mind and the other parts of the body; but in what manner they are +acted on by the mind, and made to re-act on the body, is still a +profound secret. Many hypotheses have been formed on this very obscure +subject, but they are all equally improbable, and it would be useless +for us to waste our time in conjectures on an enquiry, which, in all +probability, is beyond the reach of human capacity. + +CAROLINE. + +But you have not mentioned those particular nerves that form the senses +of hearing, seeing, smelling, and tasting? + +MRS. B. + +They are considered as being of the same nature as those which are +dispersed over every part of the body, and constitute the general sense +of feeling. The different sensations which they produce arise from their +peculiar situation and connection with the several organs of taste, +smell, and hearing. + +EMILY. + +But these senses appear totally different from that of feeling? + +MRS. B. + +They are all of them sensations, but variously modified according to the +nature of the different organs in which the nerves are situated. For, as +we have formerly observed, it is by contact only that the nerves are +affected. Thus odoriferous particles must strike upon the nerves of the +nose, in order to excite the sense of smelling; in the same manner that +taste is produced by the particular substance coming in contact with the +nerves of the palate. It is thus also that the sensation of sound is +produced by the concussion of the air striking against the auditory +nerve; and sight is the effect of the light falling upon the optic +nerve. These various senses, therefore, are affected only by the actual +contact of particles of matter, in the same manner as that of feeling. + +The different organs of the animal body, though easily separated and +perfectly distinct, are loosely connected together by a kind of spongy +substance, in texture somewhat resembling net-work, called the cellular +membrane; and the whole is covered by the skin. + +The _skin_, as well as the bark of vegetables, is formed of three coats. +The external one is called the _cuticle_ or _epidermis_; the second, +which is called the _mucous membrane_, is of a thin soft texture, and +consists of a mucous substance, which in negroes is black, and is the +cause of their skin appearing of that colour. + +CAROLINE. + +Is then the external skin of negroes white like ours? + +MRS. B. + +Yes; but as the cuticle is transparent, as well as porous, the blackness +of the mucous membrane is visible through it. The extremities of the +nerves are spread over this skin, so that the sensation of feeling is +transmitted through the cuticle. The internal covering of the muscles, +which is properly the skin, is the thickest, the toughest, and most +resisting of the whole; it is this membrane which is so essential in the +arts, by forming leather when combined with tannin. + +The skin which covers the animal body, as well as those membranes that +form the coats of the vessels, consists almost exclusively of gelatine; +and is capable of being converted into glue, size, or jelly. + +The cavities between the muscles and the skin are usually filled with +fat, which lodges in the cells of the membranous net before mentioned, +and gives to the external form (especially in the human figure) that +roundness, smoothness, and softness, so essential to beauty. + +EMILY. + +And the skin itself is, I think, a very ornamental part of the human +frame, both from the fineness of its texture, and the variety and +delicacy of its tints. + +MRS. B. + +This variety and harmonious graduation of colours, proceed, not so much +from the skin itself, as from the internal organs which transmit their +several colours through it, these being only softened and blended by the +colour of the skin, which is uniformly of a yellowish white. + +Thus modified, the darkness of the veins appears of a pale blue colour, +and the floridness of the arteries is changed to a delicate pink. In the +most transparent parts, the skin exhibits the bloom of the rose, whilst +where it is more opake its own colour predominates; and at the joints, +where the bones are most prominent, their whiteness is often +discernible. In a word, every part of the human frame seems to +contribute to its external grace; and this not merely by producing a +pleasing variety of tints, but by a peculiar kind of beauty which +belongs to each individual part. Thus it is to the solidity and +arrangement of the bones that the human figure owes the grandeur of its +stature, and its firm and dignified deportment. The muscles delineate +the form, and stamp it with energy and grace; and the soft substance +which is spread over them smooths their ruggedness, and gives to the +contours the gentle undulations of the line of beauty. Every organ of +sense is a peculiar and separate ornament; and the skin, which polishes +the surface, and gives it that charm of colouring so inimitable by art, +finally conspires to render the whole the fairest work of the creation. + +But now that we have seen in what manner the animal frame is formed, let +us observe how it provides for its support, and how the several organs, +which form so complete a whole, are nourished and maintained. + +This will lead us to a more particular explanation of the internal +organs: here we shall not meet with so much apparent beauty, because +these parts were not intended by nature to be exhibited to view; but the +beauty of design, in the internal organisation of the animal frame, is, +if possible, still more remarkable than that of the external parts. + +We shall defer this subject till our next interview. + + + + +CONVERSATION XXV. + +ON ANIMALISATION, NUTRITION, AND RESPIRATION. + + +MRS. B. + +We have now learnt of what materials the animal system is composed, and +have formed some idea of the nature of its organisation. In order to +complete the subject, it remains for us to examine in what manner it is +nourished and supported. + +Vegetables, we have observed, obtain their nourishment from various +substances, either in their elementary state, or in a very simple state +of combination; as carbon, water, and salts, which they pump up from the +soil; and carbonic acid and oxygen, which they absorb from the +atmosphere. + +Animals, on the contrary, feed on substances of the most complicated +kind; for they derive their sustenance, some from the animal creation, +others from the vegetable kingdom, and some from both. + +CAROLINE. + +And there is one species of animals, which, not satisfied with enjoying +either kind of food in its simple state, has invented the art of +combining them together in a thousand ways, and of rendering even the +mineral kingdom subservient to its refinements. + +EMILY. + +Nor is this all; for our delicacies are collected from the various +climates of the earth, so that the four quarters of the globe are often +obliged to contribute to the preparation of our simplest dishes. + +CAROLINE. + +But the very complicated substances which constitute the nourishment of +animals, do not, I suppose, enter into their system in their actual +state of combination? + +MRS. B. + +So far from it, that they not only undergo a new arrangement of their +parts, but a selection is made of such as are most proper for the +nourishment of the body, and those only enter into the system, and are +animalised. + +EMILY. + +And by what organs is this process performed? + +MRS. B. + +Chiefly by the stomach, which is the organ of digestion, and the prime +regulator of the animal frame. + +_Digestion_ is the first step towards nutrition. It consists in reducing +into one homogeneous mass the various substances that are taken as +nourishment; it is performed by first chewing and mixing the solid +aliment with the saliva, which reduces it to a soft mass, in which state +it is conveyed into the stomach, where it is more completely dissolved +by the _gastric juice_. + +This fluid (which is secreted into the stomach by appropriate glands) is +so powerful a solvent that scarcely any substances will resist its +action. + +EMILY. + +The coats of the stomach, however, cannot be attacked by it, otherwise +we should be in danger of having them destroyed when the stomach was +empty. + +MRS. B. + +They are probably not subject to its action; as long, at least, as life +continues. But it appears, that when the gastric juice has no foreign +substance to act upon, it is capable of occasioning a degree of +irritation in the coats of the stomach, which produces the sensation of +hunger. The gastric juice, together with the heat and muscular action of +the stomach, converts the aliment into an uniform pulpy mass called +chyme. This passes into the intestines, where it meets with the bile and +some other fluids, by the agency of which, and by the operation of other +causes hitherto unknown, the chyme is changed into chyle, a much thinner +substance, somewhat resembling milk, which is pumped by immense numbers +of small absorbent vessels spread over the internal surface of the +intestines. These, after many circumvolutions, gradually meet and unite +into large branches, till they at length collect the chyle into one +vessel, which pours its contents into the great vein near the heart, by +which means the food, thus prepared, enters into the circulation. + +CAROLINE. + +But I do not yet clearly understand how the blood, thus formed, +nourishes the body and supplies all the secretions? + +MRS. B. + +Before this can be explained to you, you must first allow me to complete +the formation of the blood. The chyle may, indeed, be considered as +forming the chief ingredient of blood; but this fluid is not perfect +until it has passed through the lungs, and undergone (together with the +blood that has already circulated) certain necessary changes that are +effected by RESPIRATION. + +CAROLINE. + +I am very glad that you are going to explain the nature of respiration: +I have often longed to understand it, for though we talk incessantly of +_breathing_, I never knew precisely what purpose it answered. + +MRS. B. + +It is indeed one of the most interesting processes imaginable; but, in +order to understand this function well, it will be necessary to enter +into some previous explanations. Tell me, Emily, --what do you +understand by respiration? + +EMILY. + +Respiration, I conceive, consists simply in alternately _inspiring_ air +into the lungs, and _expiring_ it from them. + +MRS. B. + +Your answer will do very well as a general definition. But, in order to +form a tolerably clear notion of the various phenomena of respiration, +there are many circumstances to be taken into consideration. + +In the first place, there are two things to be distinguished in +respiration, the _mechanical_ and the _chemical_ part of the process. + +The mechanism of breathing depends on the alternate expansions and +contractions of the chest, in which the lungs are contained. When the +chest dilates, the cavity is enlarged, and the air rushes in at the +mouth, to fill up the vacuum formed by this dilatation; when it +contracts, the cavity is diminished, and the air forced out again. + +CAROLINE. + +I thought that it was the lungs that contracted and expanded in +breathing? + +MRS. B. + +They do likewise; but their action is only the consequence of that of +the chest. The lungs, together with the heart and largest blood vessels, +in a manner fill up the cavity of the chest; they could not, therefore, +dilate if the chest did not previously expand; and, on the other hand, +when the chest contracts, it compresses the lungs and forces the air out +of them. + +CAROLINE. + +The lungs, then, are like bellows, and the chest is the power that works +them. + +MRS. B. + +Precisely so. Here is a curious little figure (PLATE XV. Fig. 5.), that +will assist me in explaining the mechanism of breathing. + +CAROLINE. + +What a droll figure! a little head fixed upon a glass bell, with a +bladder tied over the bottom of it! + +MRS. B. + +You must observe that there is another bladder within the glass, the +neck of which communicates with the mouth of the figure--this represents +the lungs contained within the chest; the other bladder, which you see +is tied loose, represents a muscular membrane, called the _diaphragm_, +which separates the chest from the lower part of the body. By the chest, +therefore, I mean that large cavity in the upper part of the body +contained within the ribs, the neck, and the diaphragm; this membrane is +muscular, and capable of contraction and dilatation. The contraction may +be imitated by drawing the bladder tight over the bottom of the +receiver, when the air in the bladder, which represents the lungs, will +be forced out through the mouth of the figure-- + +EMILY. + +See, Caroline, how it blows the flame of the candle in breathing! + +MRS. B. + +By letting the bladder loose again, we imitate the dilatation of the +diaphragm, and the cavity of the chest being enlarged, the lungs expand, +and the air rushes in to fill them. + +EMILY. + +This figure, I think, gives a very clear idea of the process of +breathing. + +MRS. B. + +It illustrates tolerably well the action of the lungs and diaphragm; but +those are not the only powers that are concerned in enlarging or +diminishing the cavity of the chest; the ribs are also possessed of a +muscular motion for the same purpose; they are alternately drawn in, +edgeways, to assist the contraction, and stretched out, like the hoops +of a barrel, to contribute to the dilatation of the chest. + +EMILY. + +I always supposed that the elevation and depression of the ribs were the +consequence, not the cause of breathing. + +MRS. B. + +It is exactly the reverse. The muscular action of the diaphragm, +together with that of the ribs, are the _causes_ of the contraction and +expansion of the chest; and the air rushing into, and being expelled +from the lungs, are only _consequences_ of those actions. + +CAROLINE. + +I confess that I thought the act of breathing began by opening the mouth +for the air to rush in, and that it was the air alone, which, by +alternately rushing in and out, occasioned the dilatations and +contractions of the lungs and chest. + +MRS. B. + +Try the experiment of merely opening your mouth; the air will not rush +in, till by an interior muscular action you produce a vacuum--yes, just +so, your diaphragm is now dilated, and the ribs expanded. But you will +not be able to keep them long in that state. Your lungs and chest are +already resuming their former state, and expelling the air with which +they had just been filled. This mechanism goes on more or less rapidly, +but, in general, a person at rest and in health will breathe between +fifteen and twenty-five times in a minute. + +We may now proceed to the chemical effects of respiration; but, for this +purpose, it is necessary that you should previously have some notion of +the _circulation_ of the blood. Tell me, Caroline, what do you +understand by the circulation of the blood? + +CAROLINE. + +I am delighted that you come to that subject, for it is one that has +long excited my curiosity. But I cannot conceive how it is connected +with respiration. The idea I have of the circulation is, that the blood +runs from the heart through the veins all over the body, and back again +to the heart. + +MRS. B. + +I could hardly have expected a better definition from you; it is, +however, not quite correct, for you do not distinguish the _arteries_ +from the _veins_, which, as we have already observed, are two distinct +sets of vessels, each having its own peculiar functions. The arteries +convey the blood from the heart to the extremities of the body; and the +veins bring it back into the heart. + +This sketch will give you an idea of the manner in which some of the +principal veins and arteries of the human body branch out of the heart, +which may be considered as a common centre to both sets of vessels. The +heart is a kind of strong elastic bag, or muscular cavity, which +possesses a power of dilating and contracting itself, for the purposes +of alternately receiving and expelling the blood, in order to carry on +the process of circulation. + +EMILY. + +Why are the arteries in this drawing painted red, and the veins purple? + +MRS. B. + +It is to point out the difference of the colour of the blood in these +two sets of vessels. + +CAROLINE. + +But if it is the same blood that flows from the arteries into the veins, +how can its colour be changed? + +MRS. B. + +This change arises from various circumstances. In the first place, +during its passage through the arteries, the blood undergoes a +considerable alteration, some of its constituent parts being gradually +separated from it for the purpose of nourishing the body, and of +supplying the various secretions. The consequence of this is, that the +florid arterial colour of the blood changes by degrees to a deep purple, +which is its constant colour in the veins. On the other hand, the blood +is recruited during its return through the veins by the fresh chyle, or +imperfect blood, which has been produced by food; and it receives also +lymph from the absorbent vessels, as we have before mentioned. In +consequence of these several changes, the blood returns to the heart in +a state very different from that in which it left it. It is loaded with +a greater proportion of hydrogen and carbon, and is no longer fit for +the nourishment of the body, or other purposes of circulation. + +EMILY. + +And in this state does it mix in the heart with the pure florid blood +that runs into the arteries? + +MRS. B. + +No. The heart is divided into two cavities or compartitions, called the +_right_ and _left ventricles_. The left ventricle is the receptacle for +the pure arterial blood previous to its circulation; whilst the venous, +or impure blood, which returns to the heart after having circulated, is +received into the right ventricle, previous to its purification, which I +shall presently explain. + +CAROLINE. + +For my part, I always thought that the same blood circulated again and +again through the body, without undergoing any change. + +MRS. B. + +Yet you must have supposed that the blood circulated for some purpose? + +CAROLINE. + +I knew that it was indispensable to life; but had no idea of its real +functions. + +MRS. B. + +But now that you understand that the blood conveys nourishment to every +part of the body, and supplies the various secretions, you must be +sensible that it cannot constantly answer these objects without being +proportionally renovated and purified. + +CAROLINE. + +But does not the chyle answer this purpose? + +MRS. B. + +Only in part. It renovates the nutritive principles of the blood, but +does not relieve it from the superabundance of water and carbon with +which it is encumbered. + +EMILY. + +How, then, is this effected? + +MRS. B. + +By RESPIRATION. This is one of the grand mysteries which modern +chemistry has disclosed. When the venous blood enters the right +ventricle of the heart, it contracts by its muscular power, and throws +the blood through a large vessel into the lungs, which are contiguous, +and through which it circulates by millions of small ramifications. Here +it comes in contact with the air which we breathe. The action of the air +on the blood in the lungs is, indeed, concealed, from our immediate +observation; but we are able to form a tolerably accurate judgment of it +from the changes which it effects not only in the blood, but also on the +air expired. + +The air, after passing through the lungs, is found to contain all the +nitrogen inspired, but to have lost part of its oxygen, and to have +acquired a portion of watery vapour and of carbonic acid gas. Hence it +is inferred, that when the air comes in contact with the venous blood in +the lungs, the oxygen attracts from it the superabundant quantity of +carbon with which it has impregnated itself during the circulation, and +converts it into carbonic acid. This gaseous acid, together with the +redundant moisture from the lungs*, being then expired, the blood is +restored to its former purity, that is, to the state of arterial blood, +and is thus again enabled to perform its various functions. + + [Footnote *: The quantity of moisture discharged by the lungs in + 24 hours, may be computed at eight or nine ounces.] + +CAROLINE. + +This is truly wonderful! Of all that we have yet learned, I do not +recollect any thing that has appeared to me so curious and interesting. +I almost believe that I should like to study anatomy now, though I have +hitherto had so disgusting an idea of it. Pray, to whom are we indebted +for these beautiful discoveries? + +MRS. B. + +Priestley and Crawford, in this country, and Lavoisier, in France, are +the principal inventors of the theory of respiration. Of late years the +subject has been farther illustrated and simplified by the accurate +experiments of Messrs. Allen and Pepys. But the still more important and +more admirable discovery of the circulation of the blood was made long +before by our immortal countryman Harvey. + +EMILY. + +Indeed I never heard any thing that delighted me so much as this theory +of respiration. But I hope, Mrs. B., that you will enter a little more +into particulars before you dismiss so interesting a subject. We left +the blood in the lungs to undergo the salutary change: but how does it +thence spread to all the parts of the body? + +MRS. B. + +After circulating through the lungs, the blood is collected into four +large vessels, by which it is conveyed into the left ventricle of the +heart, whence it is propelled to all the different parts of the body by +a large artery, which gradually ramifies into millions of small arteries +through the whole frame. From the extremities of these little +ramifications the blood is transmitted to the veins, which bring it back +to the heart and lungs, to go round again and again in the manner we +have just described. You see, therefore, that the blood actually +undergoes two circulations; the one, through the lungs, by which it is +converted into pure arterial blood; the other, or general circulation, +by which nourishment is conveyed to every part of the body; and these +are both equally indispensable to the support of animal life. + +EMILY. + +But whence proceeds the carbon with which the blood is impregnated when +it comes into the lungs? + +MRS. B. + +Carbon exists in a greater proportion in blood than in organised animal +matter. The blood, therefore, after supplying its various secretions, +becomes loaded with an excess of carbon, which is carried off by +respiration; and the formation of new chyle from the food affords a +constant supply of carbonaceous matter. + +CAROLINE. + +I wonder what quantity of carbon may be expelled from the blood by +respiration in the course of 24 hours? + +MRS. B. + +It appears by the experiments of Messrs. Allen and Pepys that about +40,000 cubic inches of carbonic acid gas are emitted from the lungs of a +healthy person, daily; which is equivalent to _eleven ounces_ of solid +carbon every 24 hours. + +EMILY. + +What an immense quantity! And pray how much of carbonic acid gas do we +expel from our lungs at each expiration? + +MRS. B. + +The quantity of air which we take into our lungs at each inspiration, is +about 40 cubic inches, which contain a little less than 10 cubic inches +of oxygen; and of those 10 inches, one-eighth is converted into carbonic +acid gas on passing once through the lungs*, a change which is +sufficient to prevent air which has only been breathed once from +suffering a taper to burn in it. + + [Footnote *: The bulk of carbonic acid gas formed by respiration, + is exactly the same as that of the oxygen gas which disappears.] + +CAROLINE. + +Pray, how does the air come in contact with the blood in the lungs? + +MRS. B. + +I cannot answer this question without entering into an explanation of +the nature and structure of the lungs. You recollect that the venous +blood, on being expelled from the right ventricle, enters the lungs to +go through what we may call the lesser circulation; the large trunk or +vessel that conveys it branches out, at its entrance into the lungs, +into an infinite number of very fine ramifications. The windpipe, which +conveys the air from the mouth into the lungs, likewise spreads out into +a corresponding number of air vessels, which follow the same course as +the blood vessels, forming millions of very minute air-cells. These two +sets of vessels are so interwoven as to form a sort of net-work, +connected into a kind of spongy mass, in which every particle of blood +must necessarily come in contact with a particle of air. + +CAROLINE. + +But since the blood and the air are contained in different vessels, how +can they come into contact? + +MRS. B. + +They act on each other through the membrane which forms the coats of +these vessels; for although this membrane prevents the blood and the air +from mixing together in the lungs, yet it is no impediment to their +chemical action on each other. + +EMILY. + +Are the lungs composed entirely of blood vessels and air vessels? + +MRS. B. + +I believe they are, with the addition only of nerves and of a small +quantity of the cellular substance before mentioned, which connects the +whole into an uniform mass. + +EMILY. + +Pray, why are the lungs always spoken of in the plural number? Are there +more than one? + +MRS. B. + +Yes; for though they form but one organ, they really consist of two +compartments called lobes, which are enclosed in separate membranes or +bags, each occupying one side of the chest, and being in close contact +with each other, but without communicating together. This is a beautiful +provision of nature, in consequence of which, if one of the lobes be +wounded, the other performs the whole process of respiration till the +first is healed. + +The blood, thus completed, by the process of respiration, forms the most +complex of all animal compounds, since it contains not only the numerous +materials necessary to form the various secretions, as saliva, tears, +&c. but likewise all those that are required to nourish the several +parts of the body, as the muscles, bones, nerves, glands, &c. + +EMILY. + +There seems to be a singular analogy between the blood of animals and +the sap of vegetables; for each of these fluids contains the several +materials destined for the nutrition of the numerous class of bodies to +which they respectively belong. + +MRS. B. + +Nor is the production of these fluids in the animal and vegetable +systems entirely different; for the absorbent vessels, which pump up the +chyle from the stomach and intestines, may be compared to the absorbents +of the roots of plants, which suck up the nourishment from the soil. And +the analogy between the sap and the blood may be still further traced, +if we follow the latter in the course of its circulation; for, in the +living animal, we find every where organs which are possessed of a power +to secrete from the blood and appropriate to themselves the ingredients +requisite for their support. + +CAROLINE. + +But whence do these organs derive their respective powers? + +MRS. B. + +From a peculiar organisation, the secret of which no one has yet been +able to unfold. But it must be ultimately by means of the vital +principle that both their mechanical and chemical powers are brought +into action. + +I cannot dismiss the subject of circulation without mentioning +_perspiration_, a secretion which is immediately connected with it, and +acts a most important part in the animal economy. + +CAROLINE. + +Is not this secretion likewise made by appropriate glands? + +MRS. B. + +No; it is performed by the extremities of the arteries, which penetrate +through the skin and terminate under the cuticle, through the pores of +which the perspiration issues. When this fluid is not secreted in +excess, it is _insensible_, because it is dissolved by the air as it +exudes from the pores; but when it is secreted faster than it can be +dissolved, it becomes _sensible_, as it assumes its liquid state. + +EMILY. + +This secretion bears a striking resemblance to the transpiration of the +sap of plants. They both consist of the most fluid part, and both exude +from the surface by the extremities of the vessels through which they +circulate. + +MRS. B. + +And the analogy does not stop there; for, since it has been ascertained +that the sap returns into the roots of the plants, the resemblance +between the animal and vegetable circulation is become still more +obvious. The latter, however, is far from being complete, since, as we +observed before, it consists only in a rising and descending of the sap, +whilst in animals the blood actually _circulates_ through every part of +the system. + +We have now, I think, traced the process of nutrition, from the +introduction of the food into the stomach to its finally becoming a +constituent part of the animal frame. This will, therefore, be a fit +period to conclude our present conversation. What further remarks we +have to make on the animal economy shall be reserved for our next +interview. + + + + +CONVERSATION XXVI. + +ON ANIMAL HEAT; AND ON VARIOUS ANIMAL PRODUCTS. + + +EMILY. + +Since our last interview, I have been thinking much of the theory of +respiration; and I cannot help being struck with the resemblance which +it appears to bear to the process of combustion. For in respiration, as +in most cases of combustion, the air suffers a change, and a portion of +its oxygen combines with carbon, producing carbonic acid gas. + +MRS. B. + +I am much pleased that this idea has occurred to you: these two +processes appear so very analogous, that it has been supposed that a +kind of combustion actually takes place in the lungs; not of the blood, +but of the superfluous carbon which the oxygen attracts from it. + +CAROLINE. + +A combustion in our lungs! that is a curious idea indeed! But, Mrs. B., +how can you call the action of the air on the blood in the lungs +combustion, when neither light nor heat are produced by it? + +EMILY. + +I was going to make the same objection. --Yet I do not conceive how the +oxygen can combine with the carbon, and produce carbonic acid, without +disengaging heat? + +MRS. B. + +The fact is, that heat is disengaged.* Whether any light be evolved, +I cannot pretend to determine; but that heat is produced in considerable +and very sensible quantities is certain, and this is the principal, if +not the only source of ANIMAL HEAT. + + [Footnote *: It has been calculated that the heat produced by + respiration in 12 hours, in the lungs of a healthy person, is such + as would melt about 100 pounds of ice.] + +EMILY. + +How wonderful! that the very process which purifies and elaborates the +blood, should afford an inexhaustible supply of internal heat? + +MRS. B. + +This is the theory of animal heat in its original simplicity, such +nearly as it was first proposed by Black and Lavoisier. It was equally +clear and ingenious; and was at first generally adopted. But it was +objected, on second consideration, that if the whole of the animal heat +was evolved in the lungs, it would necessarily be much less in the +extremities of the body than immediately at its source; which is not +found to be the case. This objection, however, which was by no means +frivolous, is now satisfactorily removed by the following +consideration:-- Venous blood has been found by experiment to have _less +capacity for heat_ than arterial blood; whence it follows that the +blood, in gradually passing from the arterial to the venous state, +during the circulation, parts with a portion of caloric, by means of +which heat is diffused through every part of the body. + +EMILY. + +More and more admirable! + +CAROLINE. + +The cause of animal heat was always a perfect mystery to me, and I am +delighted with its explanation. --But pray, Mrs. B., can you tell me +what is the reason of the increase of heat that takes place in a fever? + +EMILY. + +Is it not because we then breathe quicker, and therefore more heat is +disengaged in the system? + +MRS. B. + +That may be one reason: but I should think that the principal cause of +the heat experienced in fevers, is, that there is no vent for the +caloric which is generated in the body. One of the most considerable +secretions is the insensible perspiration; this is constantly carrying +off caloric in a latent state; but during the hot stage of a fever, the +pores are so contracted, that all perspiration ceases, and the +accumulation of caloric in the body occasions those burning sensations +which are so painful. + +EMILY. + +This is, no doubt, the reason why the perspiration that often succeeds +the hot stage of a fever affords so much relief. If I had known this +theory of animal heat when I had a fever last summer, I think I should +have found some amusement in watching the chemical processes that were +going on within me. + +CAROLINE. + +But exercise likewise produces animal heat, and that must be quite in a +different manner. + +MRS. B. + +Not so much so as you think; for the more exercise you take, the more +the body is stimulated, and requires recruiting. For this purpose the +circulation of the blood is quickened, the breath proportionably +accelerated, and consequently a greater quantity of caloric evolved. + +CAROLINE. + +True; after running very fast, I gasp for breath, my respiration is +quick and hard, and it is just then that I begin to feel hot. + +EMILY. + +It would seem, then, that violent exercise should produce fever. + +MRS. B. + +Not if the person is in a good state of health; for the additional +caloric is then carried off by the perspiration which succeeds. + +EMILY. + +What admirable resources nature has provided for us! By the production +of animal heat she has enabled us to keep up the temperature of our +bodies above that of inanimate objects; and whenever this source becomes +too abundant, the excess is carried off by perspiration. + +MRS. B. + +It is by the same law of nature that we are enabled, in all climates, +and in all seasons, to preserve our bodies of an equal temperature, or +at least very nearly so. + +CAROLINE. + +You cannot mean to say that our bodies are of the same temperature in +summer, and in winter, in England, and in the West-Indies. + +MRS. B. + +Yes, I do; at least if you speak of the temperature of the blood, and +the internal parts of the body; for those parts that are immediately in +contact with the atmosphere, such as the hands and face, will +occasionally get warmer, or colder, than the internal or more sheltered +parts. But if you put the bulb of a thermometer in your mouth, which is +the best way of ascertaining the real temperature of your body, you will +scarcely perceive any difference in its indication, whatever may be the +difference of temperature of the atmosphere. + +CAROLINE. + +And when I feel overcome by heat, I am really not hotter than when I am +shivering with cold? + +MRS. B. + +When a person in health feels very hot, whether from internal heat, from +violent exercise, or from the temperature of the atmosphere, his body is +certainly a little warmer than when he feels very cold; but this +difference is much smaller than our sensations would make us believe; +and the natural standard is soon restored by rest and by perspiration. +It is chiefly the external parts that are warmer, and I am sure that you +will be surprised to hear that the internal temperature of the body +scarcely ever descends below ninety-five or ninety-six degrees, and +seldom attains one hundred and four or one hundred and five degrees, +even in the most violent fevers. + +EMILY. + +The greater quantity of caloric, therefore, that we receive from the +atmosphere in summer, cannot raise the temperature of our bodies beyond +certain limits, as it does that of inanimate bodies, because an excess +of caloric is carried off by perspiration. + +CAROLINE. + +But the temperature of the atmosphere, and consequently that of +inanimate bodies, is surely never so high as that of animal heat? + +MRS. B. + +I beg your pardon. Frequently in the East and West Indies, and sometimes +in the southern parts of Europe, the atmosphere is above ninety-eight +degrees, which is the common temperature of animal heat. Indeed, even in +this country, it occasionally happens that the sun’s rays, setting full +on an object, elevate its temperature above that point. + +In illustration of the power which our bodies have to resist the effects +of external heat, Sir Charles Blagden, with some other gentlemen, made +several very curious experiments. He remained for some time in an oven +heated to a temperature not much inferior to that of boiling water, +without suffering any other inconvenience than a profuse perspiration, +which he supported by drinking plentifully. + +EMILY. + +He could scarcely consider the perspiration as an inconvenience, since +it saved him from being baked by giving vent to the excess of caloric. + +CAROLINE. + +I always thought, I confess, that it was from the heat of the +perspiration that we suffered in summer. + +MRS. B. + +You now find that you are quite mistaken. Whenever evaporation takes +place, cold, you know, is produced in consequence of a quantity of +caloric being carried off in a latent state; this is the case with +perspiration, and it is in this way that it affords relief. It is on +that account also that we are so apt to _catch cold_, when in a state of +profuse perspiration. It is for the same reason that tea is often +refreshing in summer, though it appears to heat you at the moment you +drink it. + +EMILY. + +And in winter, on the contrary, tea is pleasant on account of its heat. + +MRS. B. + +Yes; for we have then rather to guard against a deficiency than an +excess of caloric, and you do not find that tea will excite perspiration +in winter, unless after dancing, or any other violent exercise. + +CAROLINE. + +What is the reason that it is dangerous to eat ice after dancing, or to +drink any thing cold when one is very hot? + +MRS. B. + +Because the loss of heat arising from the perspiration, conjointly with +the chill occasioned by the cold draught, produce more cold than can be +borne with safety, unless you continue to use the same exercise after +drinking that you did before; for the heat occasioned by the exercise +will counteract the effects of the cold drink, and the danger will be +removed. You may, however, contrary to the common notion, consider it as +a rule, that cold liquids may, at all times, be drunk with perfect +safety, however hot you may feel, provided you are not at the moment in +a state of great perspiration, and on condition that you keep yourself +in gentle exercise afterwards. + +EMILY. + +But since we are furnished with such resources against the extremes of +heat or cold, I should have thought that all climates would have been +equally wholesome. + +MRS. B. + +That is true, in a certain degree, with regard to those who have been +accustomed to them from birth; for we find that the natives of those +climates, which we consider as most deleterious, are as healthy as +ourselves; and if such climates are unwholesome to those who are +habituated to a more moderate temperature, it is because the animal +economy does not easily accustom itself to considerable changes. + +CAROLINE. + +But pray, Mrs. B., if the circulation preserves the body of an uniform +temperature, how does it happen that animals are sometimes frozen? + +MRS. B. + +Because, if more heat be carried off by the atmosphere than the +circulation can supply, the cold will finally prevail, the heart will +cease to beat, and the animal will be frozen. And, likewise, if the body +remained long exposed to a degree of heat, greater than the perspiration +could carry off, it would at last lose the power of resisting its +destructive influence. + +CAROLINE. + +Fish, I suppose, have no animal heat, but only partake of the +temperature of the water in which they live? + +EMILY. + +And their coldness, no doubt, proceeds from their not breathing? + +MRS. B. + +All kinds of fish breathe more or less, though in a much smaller degree +than land animals. Nor are they entirely destitute of animal heat, +though, for the same reason, they are much colder than other creatures. +They have comparatively but a very small quantity of blood, therefore +but very little oxygen is required, and a proportionally small quantity +of animal heat is generated. + +CAROLINE. + +But how can fish breathe under water? + +MRS. B. + +They breathe by means of the air which is dissolved in the water, and if +you put them into water deprived of air by boiling, they are soon +suffocated. + +If a fish is confined in a vessel of water closed from the air, it soon +dies; and any fish put in afterwards would be killed immediately, as all +the air had been previously consumed. + +CAROLINE. + +Are there any species of animals that breathe more than we do? + +MRS. B. + +Yes; birds, of all animals, breathe the greatest quantity of air in +proportion to their size; and it is to this that they are supposed to +owe the peculiar firmness and strength of their muscles, by which they +are enabled to support the violent exertion of flying. + +This difference between birds and fish, which may be considered as the +two extremes of the scale of muscular strength, is well worth observing. +Birds residing constantly in the atmosphere, surrounded by oxygen, and +respiring it in greater proportions than any other species of animals, +are endowed with a superior degree of muscular strength, whilst the +muscles of fish, on the contrary, are flaccid and oily; these animals +are comparatively feeble in their motions, and their temperature is +scarcely above that of the water in which they live. This is, in all +probability, owing to their imperfect respiration; the quantity of +hydrogen and carbon, that is in consequence accumulated in their bodies, +forms the oil which is so strongly characteristic of that species of +animals, and which relaxes and softens the small quantity of fibrine +which their muscles contain. + +CAROLINE. + +But, Mrs. B., there are some species of birds that frequent both +elements, as, for instance, ducks and other water fowl. Of what nature +is the flesh of these? + +MRS. B. + +Such birds, in general, make but little use of their wings; if they fly, +it is but feebly, and only to a short distance. Their flesh, too, +partakes of the oily nature, and even in taste sometimes resembles that +of fish. This is the case not only with the various kinds of water +fowls, but with all other amphibious animals, as the otter, the +crocodile, the lizard, &c. + +CAROLINE. + +And what is the reason that reptiles are so deficient in muscular +strength? + +MRS. B. + +It is because they usually live under ground, and seldom come into the +atmosphere. They have imperfect, and sometimes no discernible organs of +respiration; they partake therefore of the soft oily nature of fish; +indeed, many of them are amphibious, as frogs, toads, and snakes, and +very few of them find any difficulty in remaining a length of time under +water. Whilst, on the contrary, the insect tribe, that are so strong in +proportion to their size, and alert in their motions, partake of the +nature of birds, air being their peculiar element, and their organs of +respiration being comparatively larger than in other classes of animals. + +I have now given you a short account of the principal animal functions. +However interesting the subject may appear to you, a fuller +investigation of it would, I fear, lead us too far from our object. + +EMILY. + +Yet I shall not quit it without much regret; for of all the branches of +chemistry, it is certainly the most curious and most interesting. + +CAROLINE. + +But, Mrs. B., I must remind you that you promised to give us some +account of the nature of _milk_. + +MRS. B. + +True. There are several other animal productions that deserve likewise +to be mentioned. We shall begin with milk, which is certainly the most +important and the most interesting of all the animal secretions. + +Milk, like all other animal substances, ultimately yields by analysis +oxygen, hydrogen, carbon, and nitrogen. These are combined in it under +the forms of albumen, gelatine, oil, and water. But milk contains, +besides, a considerable portion of phosphat of lime, the purposes of +which I have already pointed out. + +CAROLINE. + +Yes; it is this salt which serves to nourish the tender bones of the +suckling. + +MRS. B. + +To reduce milk to its elements, would be a very complicated, as well as +useless operation; but this fluid, without any chemical assistance, may +be decomposed into three parts, _cream_, _curds_, and _whey_. These +constituents of milk have but a very slight affinity for each other, and +you find accordingly that cream separates from milk by mere standing. It +consists chiefly of oil, which being lighter than the other parts of the +milk, gradually rises to the surface. It is of this, you know, that +butter is made, which is nothing more than oxygenated cream. + +CAROLINE. + +Butter, then, is somewhat analogous to the waxy substance formed by the +oxygenation of vegetable oils. + +MRS. B. + +Very much so. + +EMILY. + +But is the cream oxygenated by churning? + +MRS. B. + +Its oxygenation commences previous to churning, merely by standing +exposed to the atmosphere, from which it absorbs oxygen. The process is +afterwards completed by churning; the violent motion which this +operation occasions brings every particle of cream in contact with the +atmosphere, and thus facilitates its oxygenation. + +CAROLINE. + +But the effect of churning, I have often observed in the dairy, is to +separate the cream into two substances, butter and butter-milk. + +MRS. B. + +That is to say, in proportion as the oily particles of the cream become +oxygenated, they separate from the other constituent parts of the cream +in the form of butter. So by churning you produce, on the one hand, +butter, or oxygenated oil; and, on the other, butter-milk, or cream +deprived of oil. But if you make butter by churning new milk instead of +cream, the butter-milk will then be exactly similar in its properties to +creamed or skimmed milk. + +CAROLINE. + +Yet butter-milk is very different from common skimmed milk. + +MRS. B. + +Because you know it is customary, in order to save time and labour, to +make butter from cream alone. In this case, therefore, the butter-milk +is deprived of the creamed milk, which contains both the curd and whey. +Besides, in consequence of the milk remaining exposed to the atmosphere +during the separation of the cream, the latter becomes more or less +acid, as well as the butter-milk which it yields in churning. + +EMILY. + +Why should not the butter be equally acidified by oxygenation? + +MRS. B. + +Animal oil is not so easily acidified as the other ingredients of milk. +Butter, therefore, though usually made of sour cream, is not sour +itself, because the oily part of the cream had not been acidified. +Butter, however, is susceptible of becoming acid by an excess of oxygen; +it is then said to be rancid, and produces the sebacic acid, the same as +that which is obtained from fat. + +EMILY. + +If that be the case, might not rancid butter be sweetened by mixing with +it some substance that would take the acid from it? + +MRS. B. + +This idea has been suggested by Sir H. Davy, who supposes, that if +rancid butter were well washed in an alkaline solution, the alkali would +separate the acid from the butter. + +CAROLINE. + +You said just now that creamed milk consisted of curd and whey. Pray how +are these separated? + +MRS. B. + +They may be separated by standing for a certain length of time exposed +to the atmosphere; but this decomposition may be almost instantaneously +effected by the chemical agency of a variety of substances. Alkalies, +rennet*, and indeed almost all animal substances, decompose milk by +combining with the curds. + +Acids and spirituous liquors, on the other hand, produce a decomposition +by combining with the whey. In order, therefore, to obtain the whey +pure, rennet, or alkaline substances, must be used to attract the curds +from it. + +But if it be wished to obtain the curds pure, the whey must be separated +by acids, wine, or other spirituous liquors. + + [Footnote *: Rennet is the name given to a watery infusion of the + coats of the stomach of a sucking calf. Its remarkable efficacy in + promoting coagulation is supposed to depend on the gastric juice + with which it is impregnated.] + +EMILY. + +This is a very useful piece of information; for I find white-wine whey, +which I sometimes take when I have a cold, extremely heating; now, if +the whey were separated by means of an alkali instead of wine, it would +not produce that effect. + +MRS. B. + +Perhaps not. But I would strenuously advise you not to place too much +reliance on your slight chemical knowledge in medical matters. I do not +know why whey is not separated from curd by rennet, or by an alkali, for +the purpose which you mention; but I strongly suspect that there must be +some good reason why the preparation by means of wine is generally +preferred. I can, however, safely point out to you a method of obtaining +whey without either alkali, rennet, or wine; it is by substituting lemon +juice, a very small quantity of which will separate it from the curds. + +Whey, as an article of diet, is very wholesome, being remarkable light +of digestion. But its effect, taken medicinally, is chiefly, I believe, +to excite perspiration, by being drunk warm on going to bed. + +From whey a substance may be obtained in crystals by evaporation, called +_sugar of milk_. This substance is sweet to the taste, and in its +composition is so analogous to common sugar, that it is susceptible of +undergoing the vinous fermentation. + +CAROLINE. + +Why then is not wine, or alcohol, made from whey? + +MRS. B. + +The quantity of sugar contained in milk is so trifling, that it can +hardly answer that purpose. I have heard of only one instance of its +being used for the production of a spirituous liquor, and this is by the +Tartan Arabs; their abundance of horses, as well as their scarcity of +fruits, has introduced the fermentation of mares’ milk, by which they +produce a liquor called _koumiss_. Whey is likewise susceptible of being +acidified by combining with oxygen from the atmosphere. It then produces +the _lactic acid_, which you may recollect is mentioned amongst the +animal acids, as the acid of milk. + +Let us now see what are the properties of curds. + +EMILY. + +I know that they are made into cheese; but I have heard that for that +purpose they are separated from the whey by rennet, and yet this you +have just told us is not the method of obtaining pure curds? + +MRS. B. + +Nor are pure curds so well adapted for the formation of cheese. For the +nature and flavour of the cheese depend, in a great measure, upon the +cream or oily matter which is left in the curds; so that if every +particle of cream be removed from the curds, the cheese is scarcely +eatable. Rich cheeses, such as cream and Stilton cheeses, derive their +excellence from the quantity, as well as the quality, of the cream that +enters into their composition. + +CAROLINE. + +I had no idea that milk was such an interesting compound. In many +respects there appears to me to be a very striking analogy between milk +and the contents of an egg, both in respect to their nature and their +use. They are, each of them, composed of the various substances +necessary for the nourishment of the young animal, and equally destined +for that purpose. + +MRS. B. + +There is, however, a very essential difference. The young animal is +formed, as well as nourished, by the contents of the egg-shell; whilst +milk serves as nutriment to the suckling, only after it is born. + + +There are several peculiar animal substances which do not enter into the +general enumeration of animal compounds, and which, however, deserve to +be mentioned. + +_Spermaceti_ is of this class; it is a kind of oily substance obtained +from the head of the whale, which, however, must undergo a certain +preparation before it is in a fit state to be made into candles. It is +not much more combustible than tallow, but it is pleasanter to burn, as +it is less fusible and less greasy. + +_Ambergris_ is another peculiar substance derived from a species of +whale. It is, however, seldom obtained from the animal itself, but is +generally found floating on the surface of the sea. + +_Wax_, you know, is a concrete oil, the peculiar product of the bee, +part of the constituents of which may probably be derived from flowers, +but so prepared by the organs of the bee, and so mixed with its own +substance, as to be decidedly an animal product. Bees’ wax is naturally +of a yellow colour, but it is bleached by long exposure to the +atmosphere, or may be instantaneously whitened by the oxy-muriatic acid. +The combustion of wax is far more perfect than that of tallow, and +consequently produces a greater quantity of light and heat. + +_Lac_ is a substance very similar to wax in the manner of its formation; +it is the product of an insect, which collects its ingredients from +flowers, apparently for the purpose of protecting its eggs from injury. +It is formed into cells, fabricated with as much skill as those of the +honey-comb, but differently arranged. The principal use of lac is in the +manufacture of sealing-wax, and in making varnishes and lacquers. + +_Musk_, _civet_, and _castor_, are other particular productions, from +different species of quadrupeds. The two first are very powerful +perfumes; the latter has a nauseous smell and taste, and is only used +medicinally. + +CAROLINE. + +Is it from this substance that castor oil is obtained? + +MRS. B. + +No. Far from it, for castor oil is a vegetable oil, expressed from the +seeds of a particular plant; and has not the least resemblance to the +medicinal substance obtained from the castor. + +_Silk_ is a peculiar secretion of the silk-worm, with which it builds +its nest or cocoon. This insect was originally brought to Europe from +China. Silk, in its chemical nature, is very similar to the hair and +wool of animals; whilst in the insect it is a fluid, which is +coagulated, apparently by uniting with oxygen, as soon as it comes in +contact with the air. The moth of the silk-worm ejects a liquor which +appears to contain a particular acid, called _bombic_, the properties of +which are but very little known. + +EMILY. + +Before we conclude the subject of the animal economy, shall we not learn +by what steps dead animals return to their elementary state? + +MRS. B. + +Animal matter, although the most complicated of all natural substances, +returns to its elementary state by one single spontaneous process, the +_putrid fermentation_. By this, the albumen, fibrine, &c. are slowly +reduced to the state of oxygen, hydrogen, nitrogen, and carbon; and thus +the circle of changes through which these principles have passed is +finally completed. They first quitted their elementary form, or their +combination with unorganised matter, to enter into the vegetable system. +Hence they were transmitted to the animal kingdom; and from this they +return, again to their primitive simplicity, soon to re-enter the sphere +of organised existence. + +When all the circumstances necessary to produce fermentation do not take +place, animal, like vegetable matter, is liable to a partial or +imperfect decomposition, which converts it into a combustible substance +very like spermaceti. I dare say that Caroline, who is so fond of +analogies, will consider this as a kind of animal bitumen. + +CAROLINE. + +And why should I not, since the processes which produce these substances +are so similar? + +MRS. B. + +There is, however, one considerable difference; the state of bitumen +seems permanent, whilst that of animal substances, thus imperfectly +decomposed, is only transient; and unless precautions be taken to +preserve them in that state, a total dissolution infallibly ensues. This +circumstance, of the occasional conversion of animal matter into a kind +of spermaceti, is of late discovery. A manufacture has in consequence +been established near Bristol, in which, by exposing the carcases of +horses and other animals for a length of time under water, the muscular +parts are converted into this spermaceti-like substance. The bones +afterwards undergo a different process to produce hartshorn, or, more +properly, ammonia, and phosphorus; and the skin is prepared for leather. + +Thus art contrives to enlarge the sphere of useful purposes, for which +the elements were intended by nature; and the productions of the several +kingdoms are frequently arrested in their course, and variously +modified, by human skill, which compels them to contribute, under new +forms, to the necessities or luxuries of man. + +But all that we enjoy, whether produced by the spontaneous operations of +nature, or the ingenious efforts of art, proceed alike from the goodness +of Providence. --To GOD alone man owes the admirable faculties which +enable him to improve and modify the productions of nature, no less than +those productions themselves. In contemplating the works of the +creation, or studying the inventions of art, let us, therefore, never +forget the Divine Source from which they proceed; and thus every +acquisition of knowledge will prove a lesson of piety and virtue. + + + + +INDEX. + + +A + + Absorbent vessels, ii. 304 + Absorption of caloric, i. 59. 66 + Acetic acid, ii. 75. 197 + Acetous fermentation, ii. 232 + ---- acid, ii. 193. 232 + Acidulous gaseous mineral waters, ii. 129 + ---- salts, ii. 200 + Acids, i. 262. ii. 69 + Aeriform, i. 36 + Affinity, i. 19. ii. 1 + Agate, ii. 51 + Agriculture, ii. 252 + Air, i. 182. ii. 262 + Albumen, ii. 277. 288 + Alburnum, ii. 267 + Alchemists, i. 4 + Alcohol, or spirit of wine, ii. 215. 222 + Alembic, i. 258 + Alkalies, ii. 19 + Alkaline earths, ii. 50. 58 + Alloys, i. 344 + Alum, or sulphat of alumine, ii. 55. 95 + Alumine, ii. 54 + Alumium, i. 13 + Amalgam, i. 347 + Ambergris, ii. 358 + Amethyst, ii. 58 + Amianthus, ii. 66 + Ammonia, or volatile alkali, i. 363. ii. 20. 35 + Ammoniacal gas, ii. 36 + Ammonium, i. 13 + Analysis, i. 287 + ---- of vegetables, ii. 165 + Animals, ii. 276 + Animal acids, ii. 75. 290 + ---- colours, ii. 292 + ---- heat, ii. 337 + ---- oil, ii. 178. 283 + Animalization, ii. 276. 297. 315 + Antidotes, ii. 41. 87 + Antimony, i. 14 + Aqua fortis, ii. 105 + ---- regia, i. 340. ii. 144 + Arrack, ii. 220 + Argand’s Lamp, i. 208 + Arsenic, i. 14. 340. 348 + Arteries, ii. 304. 323 + Arterial blood, ii. 305. 326. 338 + Asphaltum, ii. 240 + Assafœtida, ii. 188 + Assimilation, ii. 298 + Astringent principle, ii. 198 + Atmosphere, i. 90. 181. ii. 262 + Atmospherical air, i. 182 + Attraction of aggregation, or cohesion, i. 16. ii. 2 + ---- of composition, i. 16. ii. 1 + Azot, or nitrogen, i. 182, ii. 100 + Azotic gas, i. 182 + +B + + Balsams, ii. 165. 188 + Balloons, i. 245 + Bark, ii. 193. 265 + Barytes, ii. 44. 58. 61 + Bases of acids, i. 263. ii. 69 + ---- gases, i. 183 + ---- salts, ii. 5 + Beer, ii. 212. 220 + Benzoic acid, ii. 74. 197 + Bile, ii. 308 + Birds, ii. 347 + Bismuth, i. 14 + Bitumens, ii. 239 + Black lead, or plumbago, i. 304 + Bleaching, i. 32. ii. 89. 140. + Blow-pipe, i. 324. ii. 226 + Blood, ii. 306. 317 + Blood-vessels, ii. 298 + Boiling water, i. 93 + Bombic acid, ii. 75. 290 + Bones, ii. 298, 299 + Boracic acid, i. 365. ii. 131 + Boracium, i. 13. ii. 132 + Borat of soda, ii. 133 + Brandy, ii. 218 + Brass, i. 344 + Bread, ii. 233 + Bricks, ii. 56 + Brittle-metals, i. 14 + Bronze, i. 341 + Butter, ii. 351 + Butter-milk, ii. 352 + +C + + Calcareous earths, ii. 65 + ---- stones, ii. 123 + Calcium, i. 13 + Caloric, i. 12. 33 + ----, absorption of, i. 66 + ----, conductors of, i. 70 + ----, combined, i. 122 + ----, expansive power of i. 35 + ----, equilibrium of, i. 50 + ----, reflexion of, i. 54. 67 + ----, radiation of, i. 52. 61 + ----, solvent power of, i. 96. 102 + ----, capacity for, i. 124 + Calorimeter, i. 156 + Calx, i. 183 + Camphor, ii. 165. 185 + Camphoric acid, ii. 74. 197 + Caoutchouc, ii. 165. 189 + Carbonats, ii. 25. 129 + Carbonat of ammonia, ii. 41 + ---- lead, i. 320 + ---- lime, ii. 59. 130 + ---- magnesia, ii. 67 + ---- potash, ii. 25 + Carbonated hydrogen gas, i. 302 + Carbon, i. 282. ii. 329 + Carbonic acid, i. 290. 359. ii. 327 + Carburet of iron, i. 304. 342 + Carmine, ii. 295 + Cartilage, ii. 303 + Castor, ii. 359 + Cellular membrane, ii. 311 + Caustics, i. 349 + Chalk, ii. 62. 123 + Charcoal, i. 282 + Cheese, ii. 356 + Chemical attraction, i. 15. ii. 9 + Chemistry, i. 3 + Chest, ii. 318 + China, ii. 54 + Chlorine, i. 214 + Chrome, i. 14. 340 + Chyle, ii. 305. 317 + Chyme, ii. 316 + Citric acid, ii. 74. 197 + Circulation of the blood, ii. 322 + Civet, ii. 359 + Clay, i. 48. ii. 55 + Coke, ii. 241 + Coal, ii. 240. 252 + Cobalt, i. 14 + Cochineal, ii. 295 + Cold, i. 50. 58 + ---- from evaporation, i. 102. 113. 150 + Colours of metallic oxyds, i. 319 + Columbium, i. 14. 340. 348 + Combined caloric, i. 122 + Combustion, i. 190 + ----, volatile products of, i. 207 + ----, fixed products of, i. 207 + ----, of alcohol, ii. 225 + ----, of ammoniacal gas, ii. 42 + ----, of boracium, ii. 133 + ----, by oxymuriatic acid or chlorine, ii. 142 + ----, of carbon, i. 289 + ----, of coals, i. 207. 297 + ----, of charcoal by nitric acid, ii. 102 + ----, of candles, i. 236. 309. ii. 179 + ----, of diamonds, i. 292 + ----, of ether, ii. 230 + ----, of hydrogen, i. 229. + ----, of iron, i. 200. 322 + ----, of metals, i. 321 + ----, of oils, i. 208. ii. 178. 309 + ----, of oil of turpentine by nitrous acid, ii. 6 + ----, of phosphorus, i. 272 + ----, of sulphur, i. 261 + ---- of potassium, i. 358. ii. 132. 138, 139 + Compound bodies, i. 9. ii. 14 + ---- or neutral salts i. 333. ii. 4 + Conductors of heat, i. 71 + ----, solids, i. 73 + ----, fluids, i. 78 + ----, Count Rumford’s theory, i. 79 + Constituent parts, i. 9 + Copper, i. 14. 331 + Copal, ii. 187. 224 + Cortical layers, ii. 265. 267 + Cotyledons, or lobes, ii. 256 + Cream, ii. 351 + Cream of tartar, or tartrit of potash, ii. 200. 222 + Cryophorus, i. 154 + Crystallisation, i. 338. ii. 47 + Cucurbit, i. 258 + Culinary heat, i. 88 + Curd, ii. 351. 354 + Cuticle, or epidermis, ii. 310 + +D + + Decomposition, i. 8. 20 + ---- of atmospherical air, i. 181. 209 + ---- of water by the Voltaic battery, i. 220 + ---- of salts by the Voltaic battery, ii. 14 + ---- of water by metals, i. 225. 334 + ---- ---- by carbon, i. 301 + ---- of vegetables, ii. 202 + ---- of potash, i. 356 + ---- of soda, i. 56 + ---- of ammonia, i. 363. ii. 37 + ---- of the boracic acid, ii. 132 + ---- of the fluoric acid, ii. 136 + ---- of the muriatic acid, ii. 139 + Deflagration, ii. 118 + Definite proportions, ii. 13 + Deliquescence, ii. 95 + Detonation, i. 219. ii. 116 + Dew, i. 105 + Diamond, i. 285 + Diaphragm, ii. 320 + Digestion, ii. 316 + Dissolution of metals, i. 165. 316. 333 + Distillation, i. 259. ii. 218 + ---- of red wine, ii. 218 + Divellent forces, ii. 12 + Division, i. 7 + Drying oils, ii. 181 + Dying, ii. 191 + +E + + Earths, ii. 44 + Earthen-ware, ii. 53. 57 + Effervescence, i. 298 + Efflorescence, ii. 94 + Elastic fluids, i. 37 + Electricity, i. 12. 25. 160. 220. ii. 139 + Electric machine, i. 169 + Elective attractions, ii. 9 + Elementary bodies, i. 8. 12 + Elixirs, tinctures, or quintessences, ii. 225 + Enamel, ii. 57 + Epidermis of vegetables, ii. 269 + ---- of animals, ii. 310 + Epsom salts, ii. 63. 95 + Equilibrium of caloric, i. 50 + Essences, i. 307. ii. 183. 224 + Essential, or volatile oils, i. 307. ii. 183 + Ether, i. 111. ii. 229 + Evaporation, i. 103 + Evergreens, ii. 274 + Eudiometer, i. 276 + Expansion of caloric, i. 36 + Extractive colouring matter, ii. 165. 190 + +F + + Falling stones, i. 319 + Fat, i. 306. ii. 311 + Feathers, ii. 300 + Fecula, ii. 176 + Fermentation, ii. 205 + Fibrine, ii. 277. 289 + Fire, i. 7. 27 + Fish, ii. 346 + Fixed air, or carbonic acid, i. 290. ii. 125 + ---- alkalies, ii. 20 + ---- oils, i. 307. ii. 165. 177 + ---- products of combustion, i. 207 + Flame, i. 237 + Flint, ii. 30. 51 + Flower or blossom, ii. 271 + Fluoric acid, ii. 54. 134 + Fluorium, or Fluorine, i. 12. ii. 136 + Formic acid, ii. 290 + Fossil wood, ii. 242 + Frankincense, ii. 187 + Free or radiant caloric, or heat of temperature, i. 33 + Freezing mixtures, i. 142 + ---- by evaporation, i. 104. 150, &c. + Frost, i. 94 + Fruit, ii. 271 + Fuller’s earth, ii. 55 + Furnace, i. 304 + +G + + Galls, ii. 199 + Gallat of iron, ii. 98 + Gallic acid, ii. 74. 197, 198 + Galvanism, i. 163 + Gas, i. 182 + Gas-lights, i. 240 + Gaseous oxyd of carbon, i. 296 + ---- nitrogen, ii. 111 + Gastric juice, ii. 316 + Gelatine, or jelly, ii. 277. 280 + Germination, ii. 256 + Gin, ii. 221 + Glands, ii. 298. 307 + Glass, ii. 30 + Glauber’s salts, or sulphat of soda, ii. 92 + Glazing, ii. 57 + Glucium, i. 13 + Glue, ii. 281. 287 + Gluten, ii. 165. 177 + Gold, i. 14. 323 + Gum, ii. 170 + ---- arabic, ii. 170 + ---- elastic, or caoutchouc, ii. 189 + ---- resins, ii. 165. 188 + Gunpowder, ii. 116 + Gypsum, or plaister of Paris, or sulphat of lime, ii. 95 + +H + + Hair, ii. 300 + Harrogate water, i. 268. 341 + Hartshorn, ii. 35. 39. 281. 285 + Heart, ii. 323 + ---- wood, ii. 268 + Heat, i. 26. 33 + ---- of capacity, i. 127. 135 + ---- of temperature, i. 33 + Honey, ii. 175 + Horns, ii. 282. 300 + Hydro-carbonat, i. 241. 303 + Hydrogen, i. 214 + ---- gas, i. 215 + +I + + Jasper, ii. 51 + Ice, i. 138 + Jelly, ii. 281 + Jet, ii. 240 + Ignes fatui, i. 277 + Ignition, i. 119 + Imponderable agents, i. 12 + Inflammable air, i. 215 + Ink, ii. 98. 199 + Insects, ii. 349 + Integrant pans, i. 9 + Iridium, i. 14 + Iron, i. 14. 319. 328 + Isinglass, ii. 194. 285 + Ivory black, ii. 295 + Iodine, i. 214. ii. 157 + +K + + Kali, ii. 34 + Koumiss, ii. 356 + +L + + Lac, ii. 358 + Lactic acid, ii. 75. 290. 356 + Lakes, colours, ii. 190 + Latent heat, i. 133 + Lavender water, ii. 184. 224 + Lead, i. 14. 318. 330 + Leather, ii. 193. 287 + Leaves, ii. 260 + Life, ii. 159. 168 + Ligaments, ii. 303 + Light, i. 12. 26. ii. 261 + Lightning, i. 248 + Lime, ii. 59 + ---- water, ii. 61 + Limestone, ii. 60 + Linseed oil, ii. 178 + Liqueurs, ii. 224 + Liver, ii. 308 + Lobes, ii. 256. 332 + Lunar caustic, or nitrat of silver, i. 350. ii. 119 + Lungs, ii. 319. 330 + Lymph, ii. 304 + Lymphatic vessels, ii. 304 + +M + + Magnesia, ii. 44. 66 + Magnium, i. 13 + Malic acid, ii. 74. 197 + Malt, ii. 211 + Malleable metals, i. 14 + Manganese, i. 14. 317 + Manna, ii. 176 + Manure, ii. 247 + Marble, ii. 123 + Marine acid, or muriatic acid, ii. 136 + Mastic, ii. 187. 224 + Materials of animals, ii. 277 + ---- of vegetables, ii. 165 + Mercury, i. 14. 346 + ----, new mode of freezing, i. 155. 347 + Metallic acids, i. 340 + ---- oxyds, i. 316 + Metals, i. 12. 314 + Meteoric stones, i. 342 + Mica, ii. 66 + Milk, ii. 299. 306. 350 + Minerals, i. 315. ii. 44. 158 + Mineral waters, i. 296. ii. 129 + ---- acids, ii. 73 + Miner’s lamp, i. 249 + Mixture, i. 99 + Molybdena, i. 14. 340 + Mordant, ii. 165. 192 + Mortar, ii. 53. 65 + Mucilage, ii. 170 + Mucous acid, ii. 74. 171. 197 + ---- membrane, ii. 311 + Muriatic acid, or marine acid, ii. 136 + Muriats, ii. 151 + Muriat of ammonia, ii. 35. 152 + ---- lime, i. 100 + ---- soda, or common salt, ii. 136. 151 + ---- potash, ii. 138 + Muriatium, i. 13 + Muscles of animals, ii. 298. 303 + Musk, ii. 359 + Myrrh, ii. 188 + +N. + + Naphtha, i. 357. ii. 240 + Negative electricity, i. 25. 161. 185 + Nerves, ii. 279. 298. 308 + Neutral, or compound salts, i. 333. ii. 4. 22. 69 + Nickel, i. 13. 343 + Nitre, or nitrat of potash, or saltpetre, ii. 32. 104. 116 + Nitric acid, ii. 100 + Nitrogen, or azot, i. 181. ii. 100 + ---- gas, i. 182. 211 + Nitro-muriatic acid, or aqua regia, ii. 144 + Nitrous acid gas, ii. 101. 106 + ---- air, or nitrit oxyd gas, ii. 107 + Nitrats, ii. 116 + Nitrat of copper, ii. 5 + ---- ammonia, ii. 113. 118 + ---- potash, or nitre, or saltpetre, ii. 32. 104. 116 + ---- silver, or lunar caustic, ii. 19 + Nomenclature of acids, i. 264. ii. 69 + ---- compound salts, ii. 4. 22 + ---- other binary compounds, i. 278 + Nut-galls, ii. 98. 199 + Nut-oil, ii. 178 + Nutrition, ii. 297 + +O + + Ochres, i. 320 + Oils, i. 285. ii. 306 + Oil of amber, ii. 241 + ---- vitriol, or sulphuric acid, ii. 80 + Olive oil, ii. 178 + Ores, i. 315 + Organized bodies, ii. 159 + Organs of animals, ii. 290. 310 + ---- vegetables, ii. 159. 265. 271 + Osmium, i. 14. 348 + Oxalic acid, ii. 74. 197 + Oxyds, i. 198 + Oxyd of manganese, i. 117. 317 + ---- iron, i. 204. 319 + ---- lead, i. 319 + ---- sulphur, ii. 91 + Oxydation, or oxygenation, i. 196 + Oxygen, i. 11. 181. 201. 211 + ---- gas, or vital air, i. 182. 201 + Oxy-muriatic acid, ii. 140 + Oxy-muriats, ii. 153 + Oxy-muriat of potash, ii. 155 + +P + + Palladium, i. 13. 348 + Papin’s digester, i. 120. ii. 284 + Parenchyma, ii. 256. 266 + Particles, i. 16 + Pearlash, ii. 24 + Peat, ii. 242 + Peculiar juice of plants, ii. 268 + Perfect metals, i. 14. 324 + Perfumes, i. 308. ii. 183 + Perspiration, ii. 333. 329 + Petrification, ii. 237 + Pewter, i. 344 + Pharmacy, i. 14 + Phosphat of lime, ii. 99. 299 + Phosphorated hydrogen gas, i. 277 + Phosphorescence, i. 29 + Phosphoric acid, i. 273. ii. 99 + Phosphorous acid, i. 274. ii. 99 + Phosphorus, i. 270 + Phosphoret of lime, i. 278. 341 + ---- sulphur, i. 279. 341 + Pitch, ii. 187 + Plaster, ii. 65 + Platina, i. 14. 323 + Plating, i. 345 + Plumbago, or black lead, i. 304 + Plumula, ii. 257 + Porcelain, ii. 56 + Positive electricity, i. 25. 161. 185 + Potassium, i. 13. 357. ii. 15 + Pottery, ii. 56 + Potash, i. 356. ii. 22 + Precipitate, i. 22 + Pressure of the atmosphere, i. 112. 116 + Printer’s ink, ii. 144 + Prussiat of iron, or prussian blue, ii. 291 + ---- potash, ii. 291 + Prussic acid, ii. 75. 290 + Putrid fermentation, ii. 235. 360 + Pyrites, i. 341. ii. 97 + Pyrometer, i. 38. 42 + +Q + + Quick lime, ii. 59 + Quiescent forces, ii. 12 + +R + + Radiation of caloric, i. 52 + ----, Prevost’s theory, i. 52 + ----, Pictet’s explanations, i. 54 + ----, Leslie’s illustrations, i. 61 + Radicals, ii. 5. 69 + Radicle; or root, ii. 257 + Rain, i. 104 + Rancidity, ii. 182 + Rectification, ii. 223 + Reflexion of caloric, i. 54. 64 + Reptiles, ii. 349 + Resins, ii. 165, 186. 266 + Respiration, ii. 317. 326 + Reviving of metals, i. 327 + Rhodium, i. 14. 348 + Roasting metals, i. 316 + Rock crystal, ii. 61 + Ruby, ii. 53 + Rum, ii. 219 + Rust, i. 318. 328 + +S + + Saccharine fermentation, ii. 208 + Sal ammoniac, or muriat of ammonia, ii. 35 + ---- polychrest, or sulphat of potash, ii. 91 + ---- volatile, or carbonat of ammonia, ii. 41 + Salifiable bases, ii. 5 + Salifying principles, ii. 5 + Saltpetre, or nitre, or nitrat of potash, ii. 32. 104. 116 + Salt, ii. 91 + Sand, ii. 30. 51 + Sandstone, ii. 51 + Sap of plants, ii. 165. 260. 262. 270. 272 + Sapphire, ii. 58 + Saturation, i. 101. + Sapphire, ii. 58 + Saturation, i. 101 + Seas, temperature of, i. 33. + Sebacic acid, ii. 75. 182. 290. 353 + Secretions, ii. 307 + Seeds of plants, ii. 210. 271 + Seltzer water, i. 289. ii. 63. 129 + Senses, ii. 310 + Silex, or silica, ii. 30. 51 + Silicium, i. 13. + Silk, ii. 359 + Silver, i. 321 + Simple bodies, i. 10. 12 + Size, ii. 281 + Skin, ii. 279. 310. 193 + Slakeing of lime, i. 147. ii. 56 + Slate, ii. 51. 66 + Smelting metals, i. 316 + Smoke i. 208 + Soap, ii. 24 + Soda, i. 363. ii. 33 + ---- water, i. 299 + Sodium, i. 13. 363 + Soils, i. 42. ii. 245 + Soldering, i. 345 + Solubility, ii. 92 + Solution, i. 96 + ---- by the air, i. 102 + ---- of potash, ii. 28 + Specific heat, i. 126 + Spermaceti, ii. 358 + Spirits, ii. 313 + Steam, i. 140. 182 + Steel, i. 305 + Stomach, ii. 315 + Stones, ii. 46 + Stucco, ii. 65 + Strontites, ii. 44. 68 + Strontium, i. 13 + Suberic acid, ii. 74. 197 + Sublimation, i. 257 + Succin, or yellow amber, ii. 241 + Succinic acid, ii. 74. 197. 241 + Sugar, ii. 165. 174. 208 + ---- of milk, ii. 355 + Sulphats, ii. 5. 91 + Super oxygenated sulphuric acid, ii. 70. + Sulphat of alumine, or alum, ii. 54. 95 + ---- barytes, ii. 58 + ---- iron, ii. 96 + ---- lime, or gypsum, or plaster of Paris, ii. 95 + ---- magnesia, or Epsom salt, ii. 67. 95 + ---- potash, or sal polychrest, ii. 91 + ---- soda, or Glauber’s salts, ii. 92 + Sulphur, i. 256 + ---- flowers of, i. 257 + Sulphurated hydrogen gas, i. 165. 268 + Sulphurets, i. 341 + Sulphurous acid, i. 254. ii. 88 + Sulphuric acid, i. 74. ii. 265 + Sympathetic ink, i. 354 + Synthesis, i. 287 + +T + + Tan, ii. 192 + Tannin, ii. 165. 192 + Tar, ii. 187 + Tartarous acid, ii. 74. 197 + Tartrit of potash, ii. 222 + Teeth, ii. 300 + Tellurium, i. 14 + Temperature, i. 33 + Thaw, i. 158 + Thermometers, i. 40 + ----, Fahrenheit’s, i. 42 + ----, Reaumur’s, i. 42 + ----, Centigrade, i. 43 + ----, air, i. 44 + ----, differential, i. 46 + Thunder, i. 248 + Tin, i. 14. 344 + Titanium, i. 14. 348 + Turf, ii. 242 + Turpentine, ii. 187 + Transpiration of plants, ii. 260 + Tungsten, i. 14. 340 + +V + + Vapour, i. 36. 49. 93. 182 + Vaporisation, i. 103 + Varnishes, ii. 187 + Vegetables, ii. 158 + Vegetable acid, i. 310. ii. 74. 197 + ---- colours, ii. 190 + ---- heat, ii. 272 + ---- oils, ii. 177 + Veins, ii. 304. 323. + Venous blood, ii. 305. 326. 338 + Ventricles, ii. 324 + Verdigris, i. 352 + Vessels, ii. 304 + Vinegar, ii. 232 + Vinous fermentation, ii. 212 + Vital air, or oxygen gas, i. 182 + Vitriol, or sulphat of iron, ii. 81 + Volatile oils, i. 307. ii. 165. 183. 224. 269 + ---- products of combustion, i. 207 + ---- alkali, i. 363. ii. 20. 35 + Voltaic battery, i. 164. 220. 356. ii. 15 + +U + + Uranium, i. 14 + +W + + Water, i. 215. ii. 262 + ----, decomposition of, by electricity, i. 200. 225 + ----, condensation of, i. 32 + ---- of the sea, i. 86 + ----, boiling, i. 93 + ----, solution by, i. 96 + ---- of crystallisation, i. 339 + Wax, i. 309. ii. 180. 358 + Whey, ii. 351 + Wine, ii. 212 + Wood, ii. 267 + Woody fibre, ii. 156. 196. 267 + Wool, ii. 300 + +Y + + Yeast, ii. 234. + Yttria, ii. 44. + Yttrium, i. 13. + +Z + + Zinc, ii. 14. 344 + Zirconia, ii. 44 + Zirconium, i. 14 + Zoonic acid, ii. 75. 220 + + +END. + + + Printed by A. Strahan, + Printers-Street, London + + * * * * * + * * * * + * * * * * + +Terminology + + oxy-muriatic acid = chlorine + (proposed as an element in 1815: see Conversation XIX) + “columbium or tantalium” = niobium and tantalum + (the two elements always occur together, and were not recognized + as separate until much later in the 19th century) + phosphat of lime = calcium diphosphate _or_ calcium + (the element calcium was isolated in 1808, but is named only once + in this 1817 edition) + glucium = beryllium (Humphry Davy’s name for the element) + + muriatic acid = hydrochloric acid + (still called “muriatic acid” for some commercial uses) + muriat of lime = calcium chloride + oxymuriate of potash = potassium chlorate + muriat of soda = sodium chloride (table salt) + carbonic acid = carbon dioxide + +Note also: + + simple body, fundamental principle = element + fecula = starch (usually spelled “fæcula”) + spirit of wine = alcohol + philosopher = scientist + arts = industry, manufacture, crafts etc. (seldom “fine arts”) + +Some essential concepts relating to living things--photosynthesis, +microorganisms, the cell, proteins--are either unknown or not mentioned. +The atom theory had been proposed, but not by Humphry Davy; it is not +mentioned in this book. + +The word “explode” is used at least once in its orginal, figurative +sense (“a word that should be exploded in chemistry”) but far more +often in its later, concrete one. The word “explosion” is always used +concretely (“an explosion, or a _detonation_ as chemists commonly call +it”). + +Calculated Values: + +“the point of zero, or the absolute privation of heat, must consequently +be 1260 degrees below 32 degrees” + + [-1228° F. The calculation is based on wrong premises; the correct + figure is about -460° F or -273° C.] + +“Mercury congeals only at seventy-two degrees below the freezing point.” + + [-40° F, which is also -40° C. This figure is correct, though + approximate.] + +“The proportion stated by Sir H. Davy, in his Chemical Researches, is as +1 to 2.389.” + +“[ammonia] consisted of about one part of hydrogen to four parts of +nitrogen.... and from the latest and most accurate experiments, the +proportions appear to be, one volume of nitrogen gas to three of +hydrogen gas” + + [These and similar calculations involving weight and volume make more + sense when one knows the elements’ atomic weights. For nitric acid, + HNO_3, the figures are 1:14:48, giving a proportion closer to 1:3.5. + For ammonia, NH_3 (not 4), the figures are 14:3.] + +“The _oxalic acid_, distilled from sorrel, is the highest term of +vegetable acidification; for, if more oxygen be added to it, it loses +its vegetable nature, and is resolved into carbonic acid and water;” + + [Oxalic acid = H_2C_2O_4; carbonic acid (carbon dioxide) = CO_2. + H_2C_2O_4 + O becomes H_2O + CO_2 + CO_2.] + + * * * * * + * * * * + +Contents: Numbering and Changes + +The 3rd and 4th editions used the same Conversation numbering. +Changes between the 4th and 5th (present text) edition are shown. +Some illustrations were also changed. + +_Volume I: On Simple Bodies_ + +(I, II, III no change) + + IV. On Specific Heat, Latent Heat, and Chemical Heat. + IV. On Combined Caloric, Comprehending Specific Heat + and Latent Heat. + + -- V. [New Chapter] On The Chemical Agencies Of Electricity. + + V. VI. On Oxygen And Nitrogen. + + VI. VII. On Hydrogen. + [5th edn: adds sections on Gas lights and Miner’s Lamp] + + VII. VIII. On Sulphur And Phosphorus. + [5th edn: adds section on Decomposition of Sulphur] + + VIII. On Carbone. + IX. On Carbon. + [4th edn: Section “Diamond is Carbon in a State of perfect + purity”; later edn: “Diamond” alone] + + IX. X. On Metals. + + X. XIV. On Alkalies. + XI. XV. On Earths. + [5th edn: both moved to Vol. II.] + +_Volume II. On Compound Bodies_ + + XII. XIII. On The Attraction Of Composition. + + [5th edn. XIV, XV = 4th edn. X, XI] + + XIII. On Compound Bodies. + XVI. On Acids. + [Most of XIII, On Compound Bodies, became XVI, On Acids. + Some introductory material was moved to XIV, On Alkalies.] + + XIV. On The Combinations of Oxygen with Sulphur and with + Phosphorus; and of the Sulphats And Phosphats. + XVII. Of the Sulphuric and Phosphoric Acids: or, The + Combinations of .... + + XV. On The Combination of Oxygen With Nitrogen and with + Carbone; and of The Nitrats And Carbonats. + XVIII. Of The Nitric And Carbonic Acids: Or The Combination ... + + XVI. On Muriatic And Oxygenated Muriatic Acids; and on Muriats. + XIX. On The Boracic, Fluoric, Muriatic, and Oxygenated Muriatic + Acids; and on Muriats. + + XVII. XX. On The Nature And Composition Of Vegetables. + +(remaining Conversations: 4th edn. + 3 = 5th edn.) + + * * * * * + * * * * + +ERRATA + +Inconsistencies are generally unchanged; see end of Errata list. + +Two items are noted in the printed Errata, immediately after the +Contents for Vol. I: + + Vol. I. page 56. last line but one, for “caloric,” read “calorific.” + 179. Note, for “Plate XII.” r. “Plate XIII.” + + I.56 + The principal use of the mirrors in this experiment is, to prove + that the calorific emanation ... + I.179 fn. + A model of this mode of construction is exhibited in PLATE XIII. + Fig. 1. + + +Errata Noted by Transcriber: + +Contents + II ... Dr. Herschel’s Experiments + [_body text has “Herschell”_] + XIV ... Hartshorn and Sal Volatile [ad] + +Conversation I + [Emily] the electric spark which is visible, and [aud] + +Conversation II + [Mrs. B., parenthesis] (PLATE I. Fig. 1.) [Fig. I.] + [Mrs. B.] it has been called _differential_ thermometer + [_missing “the”?_] + [Mrs. B., parenthesis] (PLATE III. Fig. 1.) [Fig. I.] + [Emily] the tin surface should radiate the least caloric [carolic] + +Conversation III + [Emily] the glass skreen [_spelling unchanged_] + [Mrs. B., parenthesis] (PLATE IV. Fig. 1.) [_error for Fig. 2._] + [Plate IV caption] Thermometers one in the Ether, the other + [_invisible comma after “Thermometers”?_] + [Mrs. B.] he found that it was considerably colder [is was] + +Conversation IV + [Caroline] But how can you reverse this experiment? + [_printed “expe-/periment” at line break_] + [Mrs. B.] instead of being 75 degrees, will be 80 degrees + [_error for 88?_] + [Emily, footnote] See page 102. [_in Conversation III_] + [Mrs. B.] then soke it in ether [_spelling unchanged_] + +Conversation V + [Mrs. B.] at regular distances in wooden troughs [throughs] + [Caroline] the nature of the action of the Voltaic battery [Votaic] + +Conversation VI + [Caroline] the nature of OXYGEN, which come next in our table + [_error for “comes”?_] + +Conversation VII + [Mrs. B., parenthesis] (c, d, PLATE VIII. fig. 2.) [fig. 2,] + [Caroline] be soon adopted every where, [every where.] + [Plate X] C. apperture for supplying Oil. [_spelling unchanged_] + +Conversation VIII + [Mrs. B.] sulphur is a very combustible substance [sulpur] + [Mrs. B.] I now put into the receiver [_missing “it”?_] + [Emily] What is ... after its detonation? [. for ?] + +Conversation IX + [Mrs. B.] we are to burn the carbon [bnrn] + [Mrs. B.] since they may be prepared [thay] + [Mrs. B.] artificial Seltzer water [artifical] + +Conversation X + [Mrs. B.] increase the rapidity of its combustion [of of] + [Caroline] a pair of scissars [_spelling unchanged_] + [Mrs. B.] as well as by evaporating the liquid. [? for .] + [Mrs. B., footnote] page 155. of this volume + [_near the end of Conversation IV_] + [Caroline] amongst the metals. I had no notion [, for .] + [Emily] But is it not very singular [singulr] + [Mrs. B.] Thenard and Gay Lussac [_usually hyphenated: “Gay-Lussac”_] + +Conversation XIII + [Emily] ... render that decomposition perceptible? [. for ?] + +Conversation XIV + [Mrs. B.] an acrid burning taste [on acrid] + [Caroline] according to which heat is disengaged + [_t in “heat” invisible_] + [Mrs. B.] one volume of nitrogen gas to three of hydrogen gas + [_text has “oxygen” for “hydrogen”_] + +Conversation XV + [Mrs. B.] so many interesting and important compounds [interesing] + [Caroline] And of what nature ... painting porcelain? [. for ?] + [_speaker’s name missing; supplied from other editions_] + +Conversation XVI + [Mrs. B.] combined with acidifiable radicals [acidificiable] + [Mrs. B.] this power of charring wood [charing] + +Conversation XVIII + [Caroline] not mentioning this acid + [_printed “mention-/this” at line break_] + [Mrs. B. (footnote)] 1 to 2.389.] + [_printed “2,389”: no other decimal numbers occur in the text, + but a comma appears once as a thousands separator_] + [Mrs. B.] You, understand, now, I hope, [_all commas in original_] + +Conversation XX + [Mrs. B.] of vegetables; we shall, therefore [, for ;] + +Conversation XXI + [Mrs. B.] the compound last formed will be destroyed; [detroyed] + [Mrs. B.] in such climates great part of the water + [_missing “a” before “great”?_] + [Mrs. B.] a state of debility and languor [langour] + [Mrs. B., parenthesis] (PLATE XIV. Fig. 2.) [PLATE XIII.] + [Mrs. B.] burns at so slow a temperature + [_text unchanged: error for “low” or correct as printed?_] + [Mrs. B.] the poor in heathy countries + [_not an error: “heath-y”, not “healthy”_] + +Conversation XXII + [Mrs. B., parenthesis] (PLATE XV. Fig. 4.) [PLATE XIV.] + [Mrs. B.] so many vessels or apparatus + [_not an error: “apparatus” is the Latin plural form_] + [Mrs. B.] chesnut [_common variant spelling_] + +Conversation XXIII + [Mrs. B.] pure gelatine [gelantine] + +Conversation XXV + [Mrs. B.] The muscular action of the diaphragm [diaphram] + +Conversation XXVI + [Mrs. B.] the air had been previously consumed [previouly] + [Mrs. B.] this is by the Tartan Arabs + [_text unchanged: Tartar?_] + [Mrs. B.] or studying the inventions of art, let us, therefore + [; for ,] + +Index + Arrack [_body text has “arack”_] + [Cold] ---- from evaporation, i. 102. 113. 150 + [_volume number missing_] + Culinary heat, i. 88 [_volume number missing_] + [Decomposition] ---- of ammonia, i. 363. ii. 37 + [ammonnia; _“i” invisible_] + Frankincense [Francincense] + [Freezing mixtures] ---- by evaporation, i. 104. 150, &c. + [_volume number invisible_] + Glue, ii. 281. 287 [_volume number missing_] + N. [_anomalous . unchanged_] + Phosphorous acid, i. 274. ii. 99 + [_volume I body text always has “Phosphorus acid”_] + [Sulphat] ---- lime, or gypsum, or [gypsum of] + [Sulphur] ---- flowers of, i. 257 [_volume number missing_] + [Thermometers] ----, Centigrade, i. 43 [Centrigade] + [Thermometers] ----, differential, i. 46 [differentiial] + V, U [_alphabetized as shown_] + Zirconia, ii. 44 [Zicornia] + Zirconium, i. 14 [Zicornium ... 13] + + +Inconsistencies and variant spellings: + +Standard spellings in this book include: + bason, judgment, embrio, volcanos (plural), potatoe (singular) + +Inconsistencies include: + capitalization of “Fig.” or “fig.” + hyphenization of words such as “oxy-muriatic” + “glauber salt” and “Glauber’s salt” both occur + +Variant forms include: + opake, opaque + aëriform, aeriform (with and without dieresis) + gasses, gases + phosphoret, phosphuret (but always carburet) + Libya, Lybia + dy(e)ing [from “dye”] + nap(h)tha + pla(i)ster + slak(e)ing + earthen-ware, earthen ware + “sulphurous”, “naphtha” are used in the Contents and the Index; + “sulphureous”, “naptha” in the body text + forms in “-xion” (such as “connexion”) appear only in the Contents + and the Index + +Volume I has more archaic forms than Volume II: + “shew”, “inclose” are sometimes used instead of “show”, “enclose” + “carbone” with final “e” appears in one Plate caption. (In the + same plate’s header, the “e” appears to have been removed by + the engraver.) + “develope(ment)” is more common in Volume I, “develop(ment)” in + Volume II + “-ize” and “-yze” forms (for later “-ise” and “-yse”) are common in + Volume I, rare in Volume II except in the Index + + +The “Dr. Marcet” mentioned in a few footnotes and figure captions is the +author’s husband. Humphry Davy (“Sir H. Davy”) was knighted in 1812, +between the 3rd and 4th editions of the book. + + +Reminder: + +DO NOT TRY THIS AT HOME. + + + * * * * * + * * * * + * * * * * + + + + + + +End of Project Gutenberg's Conversations on Chemistry, V. 1-2, by Jane Marcet + +*** END OF THIS PROJECT GUTENBERG EBOOK CONVERSATIONS ON CHEMISTRY, V. 1-2 *** + +***** This file should be named 26908-0.txt or 26908-0.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/2/6/9/0/26908/ + +Produced by Louise Hope + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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