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diff --git a/16593.txt b/16593.txt new file mode 100644 index 0000000..0307601 --- /dev/null +++ b/16593.txt @@ -0,0 +1,12111 @@ +The Project Gutenberg EBook of General Science, by Bertha M. Clark + +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: General Science + +Author: Bertha M. Clark + +Release Date: August 25, 2005 [EBook #16593] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK GENERAL SCIENCE *** + + + + +Produced by John Hagerson, Kevin Handy, Sankar Viswanathan +and the Online Distributed Proofreading Team at +https://www.pgdp.net + + + + + + + + + + + GENERAL SCIENCE + + + + + BY + + BERTHA M. CLARK, PH.D. + + + HEAD OF THE SCIENCE DEPARTMENT + + WILLIAM PENN HIGH SCHOOL FOR GIRLS, PHILADELPHIA + + + + + + + NEW YORK - CINCINNATI - CHICAGO + + AMERICAN BOOK COMPANY + + 1912 + + + + + +PREFACE + + +This book is not intended to prepare for college entrance +examinations; it will not, in fact, prepare for any of the present-day +stock examinations in physics, chemistry, or hygiene, but it should +prepare the thoughtful reader to meet wisely and actively some of +life's important problems, and should enable him to pass muster on the +principles and theories underlying scientific, and therefore economic, +management, whether in the shop or in the home. + +We hear a great deal about the conservation of our natural resources, +such as forests and waterways; it is hoped that this book will show +the vital importance of the conservation of human strength and health, +and the irreparable loss to society of energy uselessly dissipated, +either in idle worry or in aimless activity. Most of us would reproach +ourselves for lack of shrewdness if we spent for any article more than +it was worth, yet few of us consider that we daily expend on domestic +and business tasks an amount of energy far in excess of that actually +required. The farmer who flails his grain instead of threshing it +wastes time and energy; the housewife who washes with her hands alone +and does not aid herself by the use of washing machine and proper +bleaching agents dissipates energy sadly needed for other duties. + +The Chapter on machines is intended not only as a stimulus to the +invention of further labor-saving devices, but also as an eye opener +to those who, in the future struggle for existence, must perforce go +to the wall unless they understand how to make use of contrivances +whereby man's limited physical strength is made effective for larger +tasks. + +The Chapter on musical instruments is more detailed than seems +warranted at first sight; but interest in orchestral instruments is +real and general, and there is a persistent desire for intelligent +information relative to musical instruments. The child of the laborer +as well as the child of the merchant finds it possible to attend some +of the weekly orchestral concerts, with their tiers of cheap seats, +and nothing adds more to the enjoyment and instruction of such hours +than an intimate acquaintance with the leading instruments. Unless +this is given in the public schools, a large percentage of mankind is +deprived of it, and it is for this reason that so large a share of the +treatment of sound has been devoted to musical instruments. + +The treatment of electricity is more theoretical than that used in +preceding Chapters, but the subject does not lend itself readily to +popular presentation; and, moreover, it is assumed that the +information and training acquired in the previous work will give the +pupil power to understand the more advanced thought and method. + +The real value of a book depends not so much upon the information +given as upon the permanent interest stimulated and the initiative +aroused. The youthful mind, and indeed the average adult mind as +well, is singularly non-logical and incapable of continued +concentration, and loses interest under too consecutive thought and +sustained style. For this reason the author has sacrificed at times +detail to general effect, logical development to present-day interest +and facts, and has made use of a popular, light style of writing as +well as of the more formal and logical style common to books of +science. + +No claim is made to originality in subject matter. The actual facts, +theories, and principles used are such as have been presented in +previous textbooks of science, but the manner and sequence of +presentation are new and, so far as I know, untried elsewhere. These +are such as in my experience have aroused the greatest interest and +initiative, and such as have at the same time given the maximum +benefit from the informational standpoint. In no case, however, is +mental training sacrificed to information; but mental development is +sought through the student's willing and interested participation in +the actual daily happenings of the home and the shop and the field, +rather than through formal recitations and laboratory experiments. + +Practical laboratory work in connection with the study of this book is +provided for in my _Laboratory Manual in General Science_, which +contains directions for a series of experiments designed to make the +pupil familiar with the facts and theories discussed in the textbook. + +I have sought and have gained help from many of the standard +textbooks, new and old. The following firms have kindly placed cuts +at my disposal, and have thus materially aided in the preparation of +the illustrations: American Radiator Company; Commercial Museum, +Philadelphia; General Electric Company; Hershey Chocolate Company; +_Scientific American_; The Goulds Manufacturing Company; Victor +Talking Machine Company. Acknowledgment is also due to Professor Alvin +Davison for figures 19, 23, 29, 142, and 161. + +Mr. W.D. Lewis, Principal of the William Penn High School, has read +the manuscript and has given me the benefit of his experience and +interest. Miss. Helen Hill, librarian of the same school, has been of +invaluable service as regards suggestions and proof reading. Miss. +Droege, of the Baldwin School, Bryn Mawr, has also been of very great +service. Practically all of my assistants have given of their time and +skill to the preparation of the work, but the list is too long for +individual mention. + +BERTHA M. CLARK. + +WILLIAM PENN HIGH SCHOOL. + + + + +CONTENTS + + + CHAPTER + + I. HEAT + + II. TEMPERATURE AND HEAT + + III. OTHER FACTS ABOUT HEAT + + IV. BURNING OR OXIDATION + + V. FOOD + + VI. WATER + + VII. AIR + + VIII. GENERAL PROPERTIES OF GASES + + IX. INVISIBLE OBJECTS + + X. LIGHT + + XI. REFRACTION + + XII. PHOTOGRAPHY + + XIII. COLOR + + XIV. HEAT AND LIGHT AS COMPANIONS + + XV. ARTIFICIAL LIGHTING + + XVI. MAN'S WAY OF HELPING HIMSELF + + XVII. THE POWER BEHIND THE ENGINE + + XVIII. PUMPS AND THEIR VALUE TO MAN + + XIX. THE WATER PROBLEM OF A LARGE CITY + + XX. MAN'S CONQUEST OF SUBSTANCES + + XXI. FERMENTATION + + XXII. BLEACHING + + XXIII. DYEING + + XXIV. CHEMICALS AS DISINFECTANTS AND PRESERVATIVES + + XXV. DRUGS AND PATENT MEDICINES + + XXVI. NITROGEN AND ITS RELATION TO PLANTS + + XXVII. SOUND + + XXVIII. MUSICAL INSTRUMENTS + + XXIX. SPEAKING AND HEARING + + XXX. ELECTRICITY + + XXXI. SOME USES OF ELECTRICITY + + XXXII. MODERN ELECTRICAL INVENTIONS + + XXXIII. MAGNETS AND CURRENTS + + XXXIV. HOW ELECTRICITY MAY BE MEASURED + + XXXV. HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE + + + INDEX + + +GENERAL SCIENCE + + + + +CHAPTER I + +HEAT + + +I. Value of Fire. Every day, uncontrolled fire wipes out human +lives and destroys vast amounts of property; every day, fire, +controlled and regulated in stove and furnace, cooks our food and +warms our houses. Fire melts ore and allows of the forging of iron, as +in the blacksmith's shop, and of the fashioning of innumerable objects +serviceable to man. Heated boilers change water into the steam which +drives our engines on land and sea. Heat causes rain and wind, fog and +cloud; heat enables vegetation to grow and thus indirectly provides +our food. Whether heat comes directly from the sun or from artificial +sources such as coal, wood, oil, or electricity, it is vitally +connected with our daily life, and for this reason the facts and +theories relative to it are among the most important that can be +studied. Heat, if properly regulated and controlled, would never be +injurious to man; hence in the following paragraphs heat will be +considered merely in its helpful capacity. + +2. General Effect of Heat. _Expansion and Contraction_. One of the +best-known effects of heat is the change which it causes in the size +of a substance. Every housewife knows that if a kettle is filled with +cold water to begin with, there will be an overflow as soon as the +water becomes heated. Heat causes not only water, but all other +liquids, to occupy more space, or to expand, and in some cases the +expansion, or increase in size, is surprisingly large. For example, if +100 pints of ice water is heated in a kettle, the 100 pints will +steadily expand until, at the boiling point, it will occupy as much +space as 104 pints of ice water. + +The expansion of water can be easily shown by heating a flask (Fig. I) +filled with water and closed by a cork through which a narrow tube +passes. As the water is heated, it expands and forces its way up the +narrow tube. If the heat is removed, the liquid cools, contracts, and +slowly falls in the tube, resuming in time its original size or +volume. A similar observation can be made with alcohol, mercury, or +any other convenient liquid. + +[Illustration: FIG. 1.--As the water becomes warmer it expands and +rise in the narrow tube.] + +Not only liquids are affected by heat and cold, but solids also are +subject to similar changes. A metal ball which when cool will just +slip through a ring (Fig. 2) will, when heated, be too large to slip +through the ring. Telegraph and telephone wires which in winter are +stretched taut from pole to pole, sag in hot weather and are much too +long. In summer they are exposed to the fierce rays of the sun, become +strongly heated, and expand sufficiently to sag. If the wires were +stretched taut in the summer, there would not be sufficient leeway for +the contraction which accompanies cold weather, and in winter they +would snap. + +[Illustration: FIG. 2--When the ball is heated, it become too large to +slip through the ring.] + +Air expands greatly when heated (Fig. 3), but since air is practically +invisible, we are not ordinarily conscious of any change in it. The +expansion of air can be readily shown by putting a drop of ink in a +thin glass tube, inserting the tube in the cork of a flask, and +applying heat to the flask (Fig. 4). The ink is forced up the tube by +the expanding air. Even the warmth of the hand is generally sufficient +to cause the drop to rise steadily in the tube. The rise of the drop +of ink shows that the air in the flask occupies more space than +formerly, and since the quantity of air has not changed, each cubic +inch of space must hold less warm air than| it held of cold air; that +is, one cubic inch of warm air weighs less than one cubic inch of cold +air, or warm air is less dense than cold air. All gases, if not +confined, expand when heated and contract as they cool. Heat, in +general, causes substances to expand or become less dense. + +[Illustration: FIG. 3--As the air in _A_ is heated, it expands and +escapes in the form of bubbles.] + +3. Amount of Expansion and Contraction. While most substances expand +when heated and contract when cooled, they are not all affected +equally by the same changes in temperature. Alcohol expands more than +water, and water more than mercury. Steel wire which measures 1/4 mile +on a snowy day will gain 25 inches in length on a warm summer day, and +an aluminum wire under the same conditions would gain 50 inches in +length. + +[Illustration: FIG. 4.--As the air in _A_ is heated, it expands and +forces the drop of ink up the tube.] + +4. Advantages and Disadvantages of Expansion and Contraction. We owe +the snug fit of metal tires and bands to the expansion and contraction +resulting from heating and cooling. The tire of a wagon wheel is made +slightly smaller than the wheel which it is to protect; it is then +put into a very hot fire and heated until it has expanded sufficiently +to slip on the wheel. As the tire cools it contracts and fits the +wheel closely. + +In a railroad, spaces are usually left between consecutive rails in +order to allow for expansion during the summer. + +The unsightly cracks and humps in cement floors are sometimes due to +the expansion resulting from heat (Fig. 5). Cracking from this cause +can frequently be avoided by cutting the soft cement into squares, the +spaces between them giving opportunity for expansion just as do the +spaces between the rails of railroads. + +[Illustration: FIG. 5: A cement walk broken by expansion due to sun +heat.] + +In the construction of long wire fences provision must be made for +tightening the wire in summer, otherwise great sagging would occur. + +Heat plays an important part in the splitting of rocks and in the +formation of debris. Rocks in exposed places are greatly affected by +changes in temperature, and in regions where the changes in +temperature are sudden, severe, and frequent, the rocks are not able +to withstand the strain of expansion and contraction, and as a result +crack and split. In the Sahara Desert much crumbling of the rock into +sand has been caused by the intense heat of the day followed by the +sharp frost of night. The heat of the day causes the rocks to expand, +and the cold of night causes them to contract, and these two forces +constantly at work loosen the grains of the rock and force them out of +place, thus producing crumbling. + +[Illustration: FIG. 6.--Splitting and crumbling of rock caused by +alternating heat and cold.] + +The surface of the rock is the most exposed part, and during the day +the surface, heated by the sun's rays, expands and becomes too large +for the interior, and crumbling and splitting result from the strain. +With the sudden fall of temperature in the late afternoon and night, +the surface of the rock becomes greatly chilled and colder than the +rock beneath; the surface rock therefore contracts and shrinks more +than the underlying rock, and again crumbling results (Fig. 6). + +[Illustration: FIG. 7.--Debris formed from crumbled rock.] + +On bare mountains, the heating and cooling effects of the sun are very +striking(Fig. 7); the surface of many a mountain peak is covered with +cracked rock so insecure that a touch or step will dislodge the +fragments and start them down the mountain slope. The lower levels of +mountains are frequently buried several feet under debris which has +been formed in this way from higher peaks, and which has slowly +accumulated at the lower levels. + +5. Temperature. When an object feels hot to the touch, we say that +it has a high temperature; when it feels cold to the touch, that it +has a low temperature; but we are not accurate judges of heat. Ice +water seems comparatively warm after eating ice cream, and yet we know +that ice water is by no means warm. A room may seem warm to a person +who has been walking in the cold air, while it may feel decidedly cold +to some one who has come from a warmer room. If the hand is cold, +lukewarm water feels hot, but if the hand has been in very hot water +and is then transferred to lukewarm water, the latter will seem cold. +We see that the sensation or feeling of warmth is not an accurate +guide to the temperature of a substance; and yet until 1592, one +hundred years after the discovery of America, people relied solely +upon their sensations for the measurement of temperature. Very hot +substances cannot be touched without injury, and hence inconvenience +as well as the necessity for accuracy led to the invention of the +thermometer, an instrument whose operation depends upon the fact that +most substances expand when heated and contract when cooled. + +[Illustration: FIG. 8.--Making a thermometer.] + +6. The Thermometer. The modern thermometer consists of a glass tube +at the lower end of which is a bulb filled with mercury or colored +alcohol (Fig. 8). After the bulb has been filled with the mercury, it +is placed in a beaker of water and the water is heated by a Bunsen +burner. As the water becomes warmer and warmer the level of the +mercury in the tube steadily rises until the water boils, when the +level remains stationary (Fig. 9). A scratch is made on the tube to +indicate the point to which the mercury rises when the bulb is placed +in boiling water, and this point is marked 212 deg.. The tube is then +removed from the boiling water, and after cooling for a few minutes, +it is placed in a vessel containing finely chopped ice (Fig. 10). The +mercury column falls rapidly, but finally remains stationary, and at +this level another scratch is made on the tube and the point is marked +32 deg.. The space between these two points, which represent the +temperatures of boiling water and of melting ice, is divided into 180 +equal parts called degrees. The thermometer in use in the United +States is marked in this way and is called the Fahrenheit thermometer +after its designer. Before the degrees are etched on the thermometer +the open end of the tube is sealed. + +[Illustration: FIG. 9.--Determining one of the fixed points of a +thermometer.] + +The Centigrade thermometer, in use in foreign countries and in all +scientific work, is similar to the Fahrenheit except that the fixed +points are marked 100 deg. and 0 deg., and the interval between the points is +divided into 100 equal parts instead of into 180. + +_The boiling point of water is 212 deg. F. or 100 deg. C_. + +_The melting point of ice is 32 deg. F. or 0 deg. C_. + +Glass thermometers of the above type are the ones most generally used, +but there are many different types for special purposes. + +[Illustration: FIG. 10.--Determining the lower fixed point of a +thermometer.] + +7. Some Uses of a Thermometer. One of the chief values of a +thermometer is the service it has rendered to medicine. If a +thermometer is held for a few minutes under the tongue of a normal, +healthy person, the mercury will rise to about 98.4 deg. F. If the +temperature of the body registers several degrees above or below this +point, a physician should be consulted immediately. The temperature of +the body is a trustworthy indicator of general physical condition; +hence in all hospitals the temperature of patients is carefully taken +at stated intervals. + +Commercially, temperature readings are extremely important. In sugar +refineries the temperature of the heated liquids is observed most +carefully, since a difference in temperature, however slight, affects +not only the general appearance of sugars and sirups, but the quality +as well. The many varieties of steel likewise show the influence which +heat may have on the nature of a substance. By observation and tedious +experimentation it has been found that if hardened steel is heated to +about 450 deg. F. and quickly cooled, it gives the fine cutting edge of +razors; if it is heated to about 500 deg. F. and then cooled, the metal is +much coarser and is suitable for shears and farm implements; while if +it is heated but 50 deg. F. higher, that is, to 550 deg. F., it gives the fine +elastic steel of watch springs. + +[Illustration: FIG. 11.--A well-made commercial thermometer.] + +A thermometer could be put to good use in every kitchen; the +inexperienced housekeeper who cannot judge of the "heat" of the oven +would be saved bad bread, etc., if the thermometer were a part of her +equipment. The thermometer can also be used in detecting adulterants. +Butter should melt at 94 deg. F.; if it does not, you may be sure that it +is adulterated with suet or other cheap fat. Olive oil should be a +clear liquid above 75 deg. F.; if, above this temperature, it looks +cloudy, you may be sure that it too is adulterated with fat. + +8. Methods of Heating Buildings. _Open Fireplaces and Stoves._ +Before the time of stoves and furnaces, man heated his modest dwelling +by open fires alone. The burning logs gave warmth to the cabin and +served as a primitive cooking agent; and the smoke which usually +accompanies burning bodies was carried away by means of the chimney. +But in an open fireplace much heat escapes with the smoke and is lost, +and only a small portion streams into the room and gives warmth. + +When fuel is placed in an open fireplace (Fig. 12) and lighted, the +air immediately surrounding the fire becomes warmer and, because of +expansion, becomes lighter than the cold air above. The cold air, +being heavier, falls and forces the warmer air upward, and along with +the warm air goes the disagreeable smoke. The fall of the colder and +heavier air, and the rise of the warmer and hence lighter air, is +similar to the exchange which takes place when water is poured on oil; +the water, being heavier than oil, sinks to the bottom and forces the +oil to the surface. The warmer air which escapes up the chimney +carries with it the disagreeable smoke, and when all the smoke is got +rid of in this way, the chimney is said to draw well. + +[Illustration: FIG. 12.--The open fireplace as an early method of +heating.] + +As the air is heated by the fire it expands, and is pushed up the +chimney by the cold air which is constantly entering through loose +windows and doors. Open fireplaces are very healthful because the air +which is driven out is impure, while the air which rushes in is fresh +and brings oxygen to the human being. + +But open fireplaces, while pleasant to look at, are not efficient for +either heating or cooking. The possibilities for the latter are +especially limited, and the invention of stoves was a great advance in +efficiency, economy, and comfort. A stove is a receptacle for fire, +provided with a definite inlet for air and a definite outlet for +smoke, and able to radiate into the room most of the heat produced +from the fire which burns within. The inlet, or draft, admits enough +air to cause the fire to burn brightly or slowly as the case may be. +If we wish a hot fire, the draft is opened wide and enough air enters +to produce a strong glow. If we wish a low fire, the inlet is only +partially opened, and just enough air enters to keep the fuel +smoldering. + +When the fire is started, the damper should be opened wide in order to +allow the escape of smoke; but after the fire is well started there is +less smoke, and the damper may be partly closed. If the damper is kept +open, coal is rapidly consumed, and the additional heat passes out +through the chimney, and is lost to use. + +9. Furnaces. _Hot Air_. The labor involved in the care of numerous +stoves is considerable, and hence the advent of a central heating +stove, or furnace, was a great saving in strength and fuel. A furnace +is a stove arranged as in Figure 13. The stove _S_, like all other +stoves, has an inlet for air and an outlet _C_ for smoke; but in +addition, it has built around it a chamber in which air circulates and +is warmed. The air warmed by the stove is forced upward by cold air +which enters from outside. For example, cold air constantly entering +at _E_ drives the air heated by _S_ through pipes and ducts to the +rooms to be heated. + +The metal pipes which convey the heated air from the furnace to the +ducts are sometimes covered with felt, asbestos, or other +non-conducting material in order that heat may not be lost during +transmission. The ducts which receive the heated air from the pipes +are built in the non-conducting walls of the house, and hence lose +practically no heat. The air which reaches halls and rooms is +therefore warm, in spite of its long journey from the cellar. + +[Illustration: FIG. 13.--A furnace. Pipes conduct hot air to the +rooms.] + +Not only houses are warmed by a central heating stove, but whole +communities sometimes depend upon a central heating plant. In the +latter case, pipes closely wrapped with a non-conducting material +carry steam long distances underground to heat remote buildings. +Overbrook and Radnor, Pa., are towns in which such a system is used. + +10. Hot-water Heating. The heated air which rises from furnaces is +seldom hot enough to warm large buildings well; hence furnace heating +is being largely supplanted by hot-water heating. + +The principle of hot-water heating is shown by the following simple +experiment. Two flasks and two tubes are arranged as in Figure 15, the +upper flask containing a colored liquid and the lower flask clear +water. If heat is applied to _B_, one can see at the end of a few +seconds the downward circulation of the colored liquid and the upward +circulation of the clear water. If we represent a boiler by _B_, a +radiator by the coiled tube, and a safety tank by _C_, we shall have a +very fair illustration of the principle of a hot-water heating system. +The hot water in the radiators cools and, in cooling, gives up its +heat to the rooms and thus warms them. + +[Illustration: FIG. 14.--Hot-water heating.] + +In hot-water heating systems, fresh air is not brought to the rooms, +for the radiators are closed pipes containing hot water. It is largely +for this reason that thoughtful people are careful to raise windows at +intervals. Some systems of hot-water heating secure ventilation by +confining the radiators to the basement, to which cold air from +outside is constantly admitted in such a way that it circulates over +the radiators and becomes strongly heated. This warm fresh air then +passes through ordinary flues to the rooms above. + +[Illustration: FIG. 15.--The principle of hot-water heating.] + +In Figure 16, a radiator is shown in a boxlike structure in the +cellar. Fresh air from outside enters a flue at the right, passes the +radiator, where it is warmed, and then makes its way to the room +through a flue at the left. The warm air which thus enters the room is +thoroughly fresh. The actual labor involved in furnace heating and in +hot-water heating is practically the same, since coal must be fed to +the fire, and ashes must be removed; but the hot-water system has the +advantage of economy and cleanliness. + +[Illustration: FIG. 16.--Fresh air from outside circulates over the +radiators and then rises into the rooms to be heated.] + +11. Fresh Air. Fresh air is essential to normal healthy living, and +2000 cubic feet of air per hour is desirable for each individual. If a +gentle breeze is blowing, a barely perceptible opening of a window +will give the needed amount, even if there are no additional drafts of +fresh air into the room through cracks. Most houses are so loosely +constructed that fresh air enters imperceptibly in many ways, and +whether we will or no, we receive some fresh air. The supply is, +however, never sufficient in itself and should not be depended upon +alone. At night, or at any other time when gas lights are required, +the need for ventilation increases, because every gas light in a room +uses up the same amount of air as four people. + +[Illustration: FIG. 17.--The air which goes to the schoolrooms is +warmed by passage over the radiators.] + +In the preceding Section, we learned that many houses heated by hot +water are supplied with fresh-air pipes which admit fresh air into +separate rooms or into suites of rooms. In some cases the amount which +enters is so great that the air in a room is changed three or four +times an hour. The constant inflow of cold air and exit of warm air +necessitates larger radiators and more hot water and hence more coal +to heat the larger quantity of water, but the additional expense is +more than compensated by the gain in health. + +12. Winds and Currents. The gentlest summer breezes and the fiercest +blasts of winter are produced by the unequal heating of air. We have +seen that the air nearest to a stove or hot object becomes hotter than +the adjacent air, that it tends to expand and is replaced and pushed +upward and outward by colder, heavier air falling downward. We have +learned also that the moving liquid or gas carries with it heat which +it gradually gives out to surrounding bodies. + +When a liquid or a gas moves away from a hot object, carrying heat +with it, the process is called _convection_. + +Convection is responsible for winds and ocean currents, for land and +sea breezes, and other daily phenomena. + +The Gulf Stream illustrates the transference of heat by convection. A +large body of water is strongly heated at the equator, and then moves +away, carrying heat with it to distant regions, such as England and +Norway. + +Owing to the shape of the earth and its position with respect to the +sun, different portions of the earth are unequally heated. In those +portions where the earth is greatly heated, the air likewise will be +heated; there will be a tendency for the air to rise, and for the cold +air from surrounding regions to rush in to fill its place. In this way +winds are produced. There are many circumstances which modify winds +and currents, and it is not always easy to explain their direction +and velocity, but one very definite cause is the unequal heating of +the surface of the earth. + +13. Conduction. A poker used in stirring a fire becomes hot and +heats the hand grasping the poker, although only the opposite end of +the poker has actually been in the fire. Heat from the fire passed +into the poker, traveled along it, and warmed it. When heat flows in +this way from a warm part of a body to a colder part, the process is +called _conduction_. A flatiron is heated by conduction, the heat from +the warm stove passing into the cold flatiron and gradually heating +it. + +In convection, air and water circulate freely, carrying heat with +them; in conduction, heat flows from a warm region toward a cold +region, but there is no apparent motion of any kind. + +Heat travels more readily through some substances than through others. +All metals conduct heat well; irons placed on the fire become heated +throughout and cannot be grasped with the bare hand; iron utensils are +frequently made with wooden handles, because wood is a poor conductor +and does not allow heat from the iron to pass through it to the hand. +For the same reason a burning match may be held without discomfort +until the flame almost reaches the hand. + +Stoves and radiators are made of metal, because metals conduct heat +readily, and as fast as heat is generated within the stove by the +burning of fuel, or introduced into the radiator by the hot water, the +heat is conducted through the metal and escapes into the room. + +Hot-water pipes and steam pipes are usually wrapped with a +non-conducting substance, or insulator, such as asbestos, in order +that the heat may not escape, but shall be retained within the pipes +until it reaches the radiators within the rooms. + +The invention of the "Fireless Cooker" depended in part upon the +principle of non-conduction. Two vessels, one inside the other, are +separated by sawdust, asbestos, or other poor conducting material +(Fig. 18). Foods are heated in the usual way to the boiling point or +to a high temperature, and are then placed in the inner vessel. The +heat of the food cannot escape through the non-conducting material +which surrounds it, and hence remains in the food and slowly cooks it. + +[Illustration: FIG. 18.--A fireless cooker.] + +A very interesting experiment for the testing of the efficacy of +non-conductors may be easily performed. Place hot water in a metal +vessel, and note by means of a thermometer the _rapidity_ with which +the water cools; then place water of the same temperature in a second +metal vessel similar to the first, but surrounded by asbestos or other +non-conducting material, and note the _slowness_ with which the +temperature falls. + +Chemical Change, an Effect of Heat. This effect of heat has a vital +influence on our lives, because the changes which take place when food +is cooked are due to it. The doughy mass which goes into the oven, +comes out a light spongy loaf; the small indigestible rice grain comes +out the swollen, fluffy, digestible grain. Were it not for the +chemical changes brought about by heat, many of our present foods +would be useless to man. Hundreds of common materials like glass, +rubber, iron, aluminum, etc., are manufactured by processes which +involve chemical action caused by heat. + + + + +CHAPTER II + +TEMPERATURE AND HEAT + + +14. Temperature not a Measure of the Amount of Heat Present. If two +similar basins containing unequal quantities of water are placed in +the sunshine on a summer day, the smaller quantity of water will +become quite warm in a short period of time, while the larger quantity +will become only lukewarm. Both vessels receive the same amount of +heat from the sun, but in one case the heat is utilized in heating to +a high temperature a small quantity of water, while in the second case +the heat is utilized in warming to a lower degree a larger quantity of +water. Equal amounts of heat do not necessarily produce equivalent +temperatures, and equal temperatures do not necessarily indicate equal +amounts of heat. It takes more heat to raise a gallon of water to the +boiling point than it does to raise a pint of water to the boiling +point, but a thermometer would register the same temperature in the +two cases. The temperature of boiling water is 100 deg. C. whether there +is a pint of it or a gallon. Temperature is independent of the +quantity of matter present; but the amount of heat contained in a +substance at any temperature is not independent of quantity, being +greater in the larger quantity. + +15. The Unit of Heat. It is necessary to have a unit of heat just as +we have a unit of length, or a unit of mass, or a unit of time. One +unit of heat is called a _calorie_, and is the amount of heat which +will change the temperature of 1 gram of water 1 deg. C. It is the amount +of heat given out by 1 gram of water when its temperature falls 1 deg. C., +or the amount of heat absorbed by 1 gram of water when its temperature +rises 1 deg. C. If 400 grams of water are heated from 0 deg. to 5 deg. C., the +amount of heat which has entered the water is equivalent to 5 x 400 or +2000 calories; if 200 grams of water cool from 25 deg. to 20 deg. C., the heat +given out by the water is equivalent to 5 x 200 or 1000 calories. + +16. Some Substances Heat more readily than Others. If two equal +quantities of water at the same temperature are exposed to the sun for +the same length of time, their final temperatures will be the same. +If, however, equal quantities of different substances are exposed, the +temperatures resulting from the heating will not necessarily be the +same. If a basin containing 1 lb. of mercury is put on the fire, side +by side with a basin containing an equal quantity of water, the +temperatures of the two substances will vary greatly at the end of a +short time. The mercury will have a far higher temperature than the +water, in spite of the fact that the amount of mercury is as great as +the amount of water and that the heat received from the fire has been +the same in each case. Mercury is not so difficult to heat as water; +less heat being required to raise its temperature 1 deg. than is required +to raise the temperature of an equal quantity of water 1 deg.. In fact, +mercury is 30 times as easy to heat as water, and it requires only one +thirtieth as much fire to heat a given quantity of mercury 1 deg. as to +heat the same quantity of water 1 deg.. + +17. Specific Heat. We know that different substances are differently +affected by heat. Some substances, like water, change their +temperature slowly when heated; others, like mercury, change their +temperature very rapidly when heated. The number of calories needed by +1 gram of a substance in order that its temperature may be increased +1 deg. C. is called the _specific heat_ of a substance; or, specific heat +is the number of calories given out by 1 gram of a substance when its +temperature falls 1 deg. C. For experiments on the determination of +specific heat, see Laboratory Manual. + +Water has the highest specific heat of any known substance except +hydrogen; that is, it requires more heat to raise the temperature of +water a definite number of degrees than it does to raise the +temperature of an equal amount of any other substance the same number +of degrees. Practically this same thing can be stated in another way: +Water in cooling gives out more heat than any other substance in +cooling through the same number of degrees. For this reason water is +used in foot warmers and in hot-water bags. If a copper lid were used +as a foot warmer, it would give the feet only .095 as much heat as an +equal weight of water; a lead weight only .031 as much heat as water. +Flatirons are made of iron because of the relatively high specific +heat of iron. The flatiron heats slowly and cools slowly, and, because +of its high specific heat, not only supplies the laundress with +considerable heat, but eliminates for her the frequent changing of the +flatiron. + +18. Water and Weather. About four times as much heat is required to +heat a given quantity of water one degree as to heat an equal quantity +of earth. In summer, when the rocks and the sand along the shore are +burning hot, the ocean and lakes are pleasantly cool, although the +amount of heat present in the water is as great as that present in the +earth. In winter, long after the rocks and sand have given out their +heat and have become cold, the water continues to give out the vast +store of heat accumulated during the summer. This explains why lands +situated on or near large bodies of water usually have less variation +in temperature than inland regions. In the summer the water cools the +region; in the winter, on the contrary, the water heats the region, +and hence extremes of temperature are practically unknown. + +19. Sources of Heat. Most of the heat which we enjoy and use we owe +to the sun. The wood which blazes on the hearth, the coal which glows +in the furnace, and the oil which burns in the stove owe their +existence to the sun. + +Without the warmth of the sun seeds could not sprout and develop into +the mighty trees which yield firewood. Even coal, which lies buried +thousands of feet below the earth's surface, owes its existence in +part to the sun. Coal is simply buried vegetation,--vegetation which +sprouted and grew under the influence of the sun's warm rays. Ages ago +trees and bushes grew "thick and fast," and the ground was always +covered with a deep layer of decaying vegetable matter. In time some +of this vast supply sank into the moist soil and became covered with +mud. Then rock formed, and the rock pressed down upon the sunken +vegetation. The constant pressure, the moisture in the ground, and +heat affected the underground vegetable mass, and slowly changed it +into coal. + +The buried forest and thickets were not all changed into coal. Some +were changed into oil and gas. Decaying animal matter was often mixed +with the vegetable mass. When the mingled animal and vegetable matter +sank into moist earth and came under the influence of pressure, it was +slowly changed into oil and gas. + +The heat of our bodies comes from the foods which we eat. Fruits, +grain, etc., could not grow without the warmth and the light of the +sun. The animals which supply our meats likewise depend upon the sun +for light and warmth. + +The sun, therefore, is the great source of heat; whether it is the +heat which comes directly from the sun and warms the atmosphere, or +the heat which comes from burning coal, wood, and oil. + + + + +CHAPTER III + +OTHER FACTS ABOUT HEAT + + +20. Boiling. _Heat absorbed in Boiling_. If a kettle of water is +placed above a flame, the temperature of the water gradually +increases, and soon small bubbles form at the bottom of the kettle and +begin to rise through the water. At first the bubbles do not get far +in their ascent, but disappear before they reach the surface; later, +as the water gets hotter and hotter, the bubbles become larger and +more numerous, rise higher and higher, and finally reach the surface +and pass from the water into the air; steam comes from the vessel, and +the water is said to _boil_. The temperature at which a liquid boils +is called the boiling point. + +While the water is heating, the temperature steadily rises, but as +soon as the water begins to boil the thermometer reading becomes +stationary and does not change, no matter how hard the water boils and +in spite of the fact that heat from the flame is constantly passing +into the water. + +If the flame is removed from the boiling water for but a second, the +boiling ceases; if the flame is replaced, the boiling begins again +immediately. Unless heat is constantly supplied, water at the boiling +point cannot be transformed into steam. + +_The number of calories which must be supplied to 1 gram of water at +the boiling point in order to change it into steam at the same +temperature is called the heat of vaporization_; it is the heat +necessary to change 1 gram of water at the boiling point into steam of +the same temperature. + +21. The Amount of Heat Absorbed. The amount of heat which must be +constantly supplied to water at the boiling point in order to change +it into steam is far greater than we realize. If we put a beaker of +ice water (water at 0 deg. C.) over a steady flame, and note (1) the time +which elapses before the water begins to boil, and (2) the time which +elapses before the boiling water completely boils away, we shall see +that it takes about 5-1/4 times as long to change water into steam as +it does to change its temperature from 0 deg. C. to 100 deg. C. Since, with a +steady flame, it takes 5-1/4 times as long to change water into steam +as it does to change its temperature from 0 deg. C. to the boiling point, +we conclude that it takes 5-1/4 times as much heat to convert water at +the boiling point into steam as it does to raise it from the +temperature of ice water to that of boiling water. + +The amount of heat necessary to raise the temperature of 1 gram of +water 1 deg. C. is equal to 1 calorie, and the amount necessary to raise +the temperature 100 deg. C. is equal to 100 calories; hence the amount of +heat necessary to convert 1 gram of water at the boiling point into +steam at that same temperature is equal to approximately 525 calories. +Very careful experiments show the exact heat of vaporization to be +536.1 calories. (See Laboratory Manual.) + +22. General Truths. Statements similar to the above hold for other +liquids and for solutions. If milk is placed upon a stove, the +temperature rises steadily until the boiling point is reached; further +heating produces, not a change in temperature, but a change of the +water of the milk into steam. As soon as the milk, or any other liquid +food, comes to a boil, the gas flame should be lowered until only an +occasional bubble forms, because so long as any bubbles form the +temperature is that of the boiling point, and further heat merely +results in waste of fuel. + +We find by experiment that every liquid has its own specific boiling +point; for example, alcohol boils at 78 deg. C. and brine at 103 deg. C. Both +specific heat and the heat of vaporization vary with the liquid used. + +23. Condensation. If one holds a cold lid in the steam of boiling +water, drops of water gather on the lid; the steam is cooled by +contact with the cold lid and _condenses_ into water. Bottles of water +brought from a cold cellar into a warm room become covered with a mist +of fine drops of water, because the moisture in the air, chilled by +contact with the cold bottles, immediately condenses into drops of +water. Glasses filled with ice water show a similar mist. + +In Section 21, we saw that 536 calories are required to change 1 gram +of water into steam; if, now, the steam in turn condenses into water, +it is natural to expect a release of the heat used in transforming +water into steam. Experiment shows not only that vapor gives out heat +during condensation, but that the amount of heat thus set free is +exactly equal to the amount absorbed during vaporization. (See +Laboratory Manual.) + +We learn that the heat of vaporization is the same whether it is +considered as the heat absorbed by 1 gram of water in its change to +steam, or as the heat given out by 1 gram of steam during its +condensation into water. + +24. Practical Application. We understand now the value of steam as a +heating agent. Water is heated in a boiler in the cellar, and the +steam passes through pipes which run to the various rooms; there the +steam condenses into water in the radiators, each gram of steam +setting free 536 calories of heat. When we consider the size of the +radiators and the large number of grams of steam which they contain, +and consider further that each gram in condensing sets free 536 +calories, we understand the ease with which buildings are heated by +steam. + +Most of us have at times profited by the heat of condensation. In cold +weather, when there is a roaring fire in the range, the water +frequently becomes so hot that it "steams" out of open faucets. If, at +such times, the hot water is turned on in a small cold bathroom, and +is allowed to run until the tub is well filled, vapor condenses on +windows, mirrors, and walls, and the cold room becomes perceptibly +warmer. The heat given out by the condensing steam passes into the +surrounding air and warms the room. + +There is, however, another reason for the rise in temperature. If a +large pail of hot soup is placed in a larger pail of cold water, the +soup will gradually cool and the cold water will gradually become +warmer. A red-hot iron placed on a stand gradually cools, but warms +the stand. A hot body loses heat so long as a cooler body is near it; +the cold object is heated at the expense of the warmer object, and one +loses heat and the other gains heat until the temperature of both is +the same. Now the hot water in the tub gradually loses heat and the +cold air of the room gradually gains heat by convection, but the +amount given the room by convection is relatively small compared with +the large amount set free by the condensing steam. + +25. Distillation. If impure, muddy water is boiled, drops of water +will collect on a cold plate held in the path of the steam, but the +drops will be clear and pure. When impure water is boiled, the steam +from it does not contain any of the impurities because these are left +behind in the vessel. If all the water were allowed to boil away, a +layer of mud or of other impurities would be found at the bottom of +the vessel. Because of this fact, it is possible to purify water in a +very simple way. Place over a fire a large kettle closed except for a +spout which is long enough to reach across the stove and dip into a +bottle. As the liquid boils, steam escapes through the spout, and on +reaching the cold bottle condenses and drops into the bottle as pure +water. The impurities remain behind in the kettle. Water freed from +impurities in this way is called _distilled water_, and the process is +called _distillation_ (Fig. 19). By this method, the salt water of the +ocean may be separated into pure drinking water and salt, and many of +the large ocean liners distill from the briny deep all the drinking +water used on their ocean voyages. + +[Illustration: FIG. 19.--In order that the steam which passes through +the coiled tube may be quickly cooled and condensed, cold water is +made to circulate around the coil. The condensed steam escapes at +_w_.] + +Commercially, distillation is a very important process. Turpentine, +for example, is made by distilling the sap of pine trees. Incisions +are cut in the bark of the long-leaf pine trees, and these serve as +channels for the escape of crude resin. This crude liquid is collected +in barrels and taken to a distillery, where it is distilled into +turpentine and rosin. The turpentine is the product which passes off +as vapor, and the rosin is the mass left in the boiler after the +distillation of the turpentine. + +26. Evaporation. If a stopper is left off a cologne bottle, the +contents of the bottle will slowly evaporate; if a dish of water is +placed out of doors on a hot day, evaporation occurs very rapidly. The +liquids which have disappeared from the bottle and the dish have +passed into the surrounding air in the form of vapor. In Section 20, +we saw that water could not pass into vapor without the addition of +heat; now the heat necessary for the evaporation of the cologne and +water was taken from the air, leaving it slightly cooler. If wet hands +are not dried with a towel, but are left to dry by evaporation, heat +is taken from the hand in the process, leaving a sensation of +coolness. Damp clothing should never be worn, because the moisture in +it tends to evaporate at the expense of the bodily heat, and this +undue loss of heat from the body produces chills. After a bath the +body should be well rubbed, otherwise evaporation occurs at the +expense of heat which the body cannot ordinarily afford to lose. + +Evaporation is a slow process occurring at all times; it is hastened +during the summer, because of the large amount of heat present in the +atmosphere. Many large cities make use of the cooling effect of +evaporation to lower the temperature of the air in summer; streets are +sprinkled not only to lay the dust, but in order that the surrounding +air may be cooled by the evaporation of the water. + +Some thrifty housewives economize by utilizing the cooling effects of +evaporation. Butter, cheese, and other foods sensitive to heat are +placed in porous vessels wrapped in wet cloths. Rapid evaporation of +the water from the wet cloths keeps the contents of the jars cool, and +that without expense other than the muscular energy needed for wetting +the cloths frequently. + +27. Rain, Snow, Frost, Dew. The heat of the sun causes constant +evaporation of the waters of oceans, rivers, streams, and marshes, and +the water vapor set free by evaporation passes into the air, which +becomes charged with vapor or is said to be humid. Constant, unceasing +evaporation of our lakes, streams, and pools would mean a steady +decrease in the supply of water available for daily use, if the +escaped water were all retained by the atmosphere and lost to the +earth. But although the escaped vapor mingles with the atmosphere, +hovering near the earth's surface, or rising far above the level of +the mountains, it does not remain there permanently. When this vapor +meets a cold wind or is chilled in any way, condensation takes place, +and a mass of tiny drops of water or of small particles of snow is +formed. When these drops or particles become large enough, they fall +to the earth as rain or snow, and in this way the earth is compensated +for the great loss of moisture due to evaporation. Fog is formed when +vapor condenses near the surface of the earth, and when the drops are +so small that they do not fall but hover in the air, the fog is said +"not to lift" or "not to clear." + +If ice water is poured into a glass, a mist will form on the outside +of the glass. This is because the water vapor in the air becomes +chilled by contact with the glass and condenses. Often leaves and +grass and sidewalks are so cold that the water vapor in the atmosphere +condenses on them, and we say a heavy dew has formed. If the +temperature of the air falls to the freezing point while the dew is +forming, the vapor is frozen and frost is seen instead of dew. + +The daily evaporation of moisture into the atmosphere keeps the +atmosphere more or less full of water vapor; but the atmosphere can +hold only a definite amount of vapor at a given temperature, and as +soon as it contains the maximum amount for that temperature, further +evaporation ceases. If clothes are hung out on a damp, murky day they +do not dry, because the air contains all the moisture it can hold, and +the moisture in the clothes has no chance to evaporate. When the air +contains all the moisture it can hold, it is said to be saturated, and +if a slight fall in temperature occurs when the air is saturated, +condensation immediately begins in the form of rain, snow, or fog. If, +however, the air is not saturated, a fall in temperature may occur +without producing precipitation. The temperature at which air is +saturated and condensation begins is called the _dew point_. + +28. How Chills are Caused. The discomfort we feel in an overcrowded +room is partly due to an excess of moisture in the air, resulting from +the breathing and perspiration of many persons. The air soon becomes +saturated with vapor and cannot take away the perspiration from our +bodies, and our clothing becomes moist and our skin tender. When we +leave the crowded "tea" or lecture and pass into the colder, drier, +outside air, clothes and skin give up their load of moisture through +sudden evaporation. But evaporation requires heat, and this heat is +taken from our bodies, and a chill results. + +Proper ventilation would eliminate much of the physical danger of +social events; fresh, dry air should be constantly admitted to crowded +rooms in order to replace the air saturated by the breath and +perspiration of the occupants. + +29. Weather Forecasts. When the air is near the saturation point, +the weather is oppressive and is said to be very humid. For comfort +and health, the air should be about two thirds saturated. The presence +of some water vapor in the air is absolutely necessary to animal and +plant life. In desert regions where vapor is scarce the air is so dry +that throat trouble accompanied by disagreeable tickling is prevalent; +fallen leaves become so dry that they crumble to dust; plants lose +their freshness and beauty. + +The likelihood of rain or frost is often determined by temperature and +humidity. If the air is near saturation and the temperature is +falling, it is safe to predict bad weather, because the fall of +temperature will probably cause rapid condensation, and hence rain. +If, however, the air is not near the saturation point, a fall in +temperature will not necessarily produce bad weather. + +The measurement of humidity is of far wider importance than the mere +forecasting of local weather conditions. The close relation between +humidity and health has led many institutions, such as hospitals, +schools, and factories, to regulate the humidity of the atmosphere as +carefully as they do the temperature. Too great humidity is +enervating, and not conducive to either mental or physical exertion; +on the other hand, too dry air is equally harmful. In summer the +humidity conditions cannot be well regulated, but in winter, when +houses are artificially heated, the humidity of a room can be +increased by placing pans of water near the registers or on radiators. + +30. Heat Needed to Melt Substances. If a spoon is placed in a vessel +of hot water for a few seconds and then removed, it will be warmer +than before it was placed in the hot water. If a lump of melting ice +is placed in the vessel of hot water and then removed, the ice will +not be warmer than before, but there will be less of it. The heat of +the water has been used in melting the ice, not in changing its +temperature. + +If, on a bitter cold day, a pail of snow is brought into a warm room +and a thermometer is placed in the snow, the temperature rises +gradually until 32 deg. F. is reached, when it becomes stationary, and the +snow begins to melt. If the pail is put on the fire, the temperature +still remains 32 deg.F., but the snow melts more rapidly. As soon as all +the snow is completely melted, however, the temperature begins to rise +and rises steadily until the water boils, when it again becomes +stationary and remains so during the passage of water into vapor. + +We see that heat must be supplied to ice at 0 deg. C. or 32 deg. F. in order +to change it into water, and further, that the temperature of the +mixture does not rise so long as any ice is present, no matter how +much heat is supplied. The amount of heat necessary to melt 1 gram of +ice is easily calculated. (See Laboratory Manual.) + +Heat must be supplied to ice to melt it. On the other hand, water, in +freezing, loses heat, and the amount of heat lost by freezing water is +exactly equal to the amount of heat absorbed by melting ice. + +The number of units of heat required to melt a unit mass of ice is +called the _heat of fusion_ of water. + +31. Climate. Water, in freezing, loses heat, even though its +temperature remains at 0 deg. C. Because water loses heat when it freezes, +the presence of large streams of water greatly influences the climate +of a region. In winter the heat from the freezing water keeps the +temperature of the surrounding higher than it would naturally be, and +consequently the cold weather is less severe. In summer water +evaporates, heat is taken from the air, and consequently the warm +weather is less intense. + +32. Molding of Glass and Forging of Iron. The fire which is hot +enough to melt a lump of ice may not be hot enough to melt an iron +poker; on the other hand, it may be sufficiently hot to melt a tin +spoon. Different substances melt, or liquefy, at different +temperatures; for example, ice melts at 0 deg. C., and tin at 233 deg. C., +while iron requires the relatively high temperature of 1200 deg. C. Most +substances have a definite melting or freezing point which never +changes so long as the surrounding conditions remain the same. + +But while most substances have a definite melting point, some +substances do not. If a glass rod is held in a Bunsen burner, it will +gradually grow softer and softer, and finally a drop of molten glass +will fall from the end of the rod into the fire. The glass did not +suddenly become a liquid at a definite temperature; instead it +softened gradually, and then melted. While glass is in the soft, +yielding, pliable state, it is molded into dishes, bottles, and other +useful objects, such as lamp shades, globes, etc. (Fig. 20). If glass +melted at a definite temperature, it could not be molded in this way. +Iron acts in a similar manner, and because of this property the +blacksmith can shape his horseshoes, and the workman can make his +engines and other articles of daily service to man. + +[Illustration: FIG. 20.--Molten glass being rolled into a form +suitable for window panes.] + +33. Strange Behavior of Water. One has but to remember that bottles +of water burst when they freeze, and that ice floats on water like +wood, to know that water expands on freezing or on solidifying. A +quantity of water which occupies 100 cubic feet of space will, on +becoming ice, need 109 cubic feet of space. On a cold winter night the +water sometimes freezes in the water pipes, and the pipes burst. Water +is very peculiar in expanding on solidification, because most +substances contract on solidifying; gelatin and jelly, for example, +contract so much that they shrink from the sides of the dish which +contains them. + +If water contracted in freezing, ice would be heavier than water and +would sink in ponds and lakes as fast as it formed, and our streams +and ponds would become masses of solid ice, killing all animal and +plant life. But the ice is lighter than water and floats on top, and +animals in the water beneath are as free to live and swim as they were +in the warm sunny days of summer. The most severe winter cannot freeze +a deep lake solid, and in the coldest weather a hole made in the ice +will show water beneath the surface. Our ice boats cut and break the +ice of the river, and through the water beneath our boats daily ply +their way to and fro, independent of winter and its blighting blasts. + +While most of us are familiar with the bursting of water pipes on a +cold night, few of us realize the influence which freezing water +exerts on the character of the land around us. + +Water sinks into the ground and, on the approach of winter, freezes, +expanding about one tenth of its volume; the expanding ice pushes the +earth aside, the force in some cases being sufficient to dislodge even +huge rocks. In the early days in New England it was said by the +farmers that "rocks grew," because fields cleared of stones in the +fall became rock covered with the approach of spring; the rocks and +stones hidden underground and unseen in the fall were forced to the +surface by the winter's expansion. We have all seen fence posts and +bricks pushed out of place because of the heaving of the soil beneath +them. Often householders must relay their pavements and walks because +of the damage done by freezing water. + +The most conspicuous effect of the expansive power of freezing water +is seen in rocky or mountainous regions (Fig. 21). Water easily finds +entrance into the cracks and crevices of the rocks, where it lodges +until frozen; then it expands and acts like a wedge, widening cracks, +chiseling off edges, and even breaking rocks asunder. In regions where +frequent frosts occur, the destructive action of water works constant +changes in the appearance of the land; small cracks and crevices are +enlarged, massive rocks are pried up out of position, huge slabs are +split off, and particles large and small are forced from the parent +rock. The greater part of the debris and rubbish brought down from the +mountain slopes by the spring rains owes its origin to the fact that +water expands when it freezes. + +[Illustration: FIG. 21.--The destruction caused by freezing water.] + +34. Heat Necessary to Dissolve a Substance. It requires heat to +dissolve any substance, just as it requires heat to change ice to +water. If a handful of common salt is placed in a small cup of water +and stirred with a thermometer, the temperature of the mixture falls +several degrees. This is just what one would expect, because the heat +needed to liquefy the salt must come from somewhere, and naturally it +comes from the water, thereby lowering the temperature of the water. +We know very well that potatoes cease boiling if a pinch of salt is +put in the water; this is because the temperature of the water has +been lowered by the amount of heat necessary to dissolve the salt. + +Let some snow or chopped ice be placed in a vessel and mixed with one +third its weight of coarse salt; if then a small tube of cold water is +placed in this mixture, the water in the test tube will soon freeze +solid. As soon as the snow and salt are mixed they melt. The heat +necessary for this comes in part from the air and in part from the +water in the test tube, and the water in the tube becomes in +consequence cold enough to freeze. But the salt mixture does not +freeze because its freezing point is far below that of pure water. The +use of salt and ice in ice-cream freezers is a practical application +of this principle. The heat necessary for melting the mixture of salt +and ice is taken from the cream which thus becomes cold enough to +freeze. + + + + +CHAPTER IV + +BURNING OR OXIDATION + + +35. Why Things Burn. The heat of our bodies comes from the food we +eat; the heat for cooking and for warming our houses comes from coal. +The production of heat through the burning of coal, or oil, or gas, or +wood, is called combustion. Combustion cannot occur without the +presence of a substance called oxygen, which exists rather abundantly +in the air; that is, one fifth of our atmosphere consists of this +substance which we call oxygen. We throw open our windows to allow +fresh air to enter, and we take walks in order to breathe the pure air +into our lungs. What we need for the energy and warmth of our bodies +is the oxygen in the air. Whether we burn gas or wood or coal, the +heat which is produced comes from the power which these various +substances possess to combine with oxygen. We open the draft of a +stove that it may "draw well": that it may secure oxygen for burning. +We throw a blanket over burning material to smother the fire: to keep +oxygen away from it. Burning, or oxidation, is combining with oxygen, +and the more oxygen you add to a fire, the hotter the fire will burn, +and the faster. The effect of oxygen on combustion may be clearly seen +by thrusting a smoldering splinter into a jar containing oxygen; the +smoldering splinter will instantly flare and blaze, while if it is +removed from the jar, it loses its flame and again burns quietly. +Oxygen for this experiment can be produced in the following way. + +[Illustration: FIG. 22.--Preparing oxygen from potassium chlorate and +manganese dioxide.] + +36. How to Prepare Oxygen. Mix a small quantity of potassium +chlorate with an equal amount of manganese dioxide and place the +mixture in a strong test tube. Close the mouth of the tube with a +one-hole rubber stopper in which is fitted a long, narrow tube, and +clamp the test tube to an iron support, as shown in Figure 22. Fill +the trough with water until the shelf is just covered and allow the +end of the delivery tube to rest just beneath the hole in the shelf. +Fill a medium-sized bottle with water, cover it with a glass plate, +invert the bottle in the trough, and then remove the glass plate. Heat +the test tube very gently, and when gas bubbles out of the tube, slip +the bottle over the opening in the shelf, so that the tube runs into +the bottle. The gas will force out the water and will finally fill the +bottle. When all the water has been forced out, slip the glass plate +under the mouth of the bottle and remove the bottle from the trough. +The gas in the bottle is oxygen. + +Everywhere in a large city or in a small village, smoke is seen, +indicating the presence of fire; hence there must exist a large supply +of oxygen to keep all the fires alive. The supply of oxygen needed +for the fires of the world comes largely from the atmosphere. + +37. Matches. The burning material is ordinarily set on fire by +matches, thin strips of wood tipped with sulphur or phosphorus, or +both. Phosphorus can unite with oxygen at a fairly low temperature, +and if phosphorus is rubbed against a rough surface, the friction +produced will raise the temperature of the phosphorus to a point where +it can combine with oxygen. The burning phosphorus kindles the wood of +the match, and from the burning match the fire is kindled. If you want +to convince yourself that friction produces heat, rub a cent +vigorously against your coat and note that the cent becomes warm. +Matches have been in use less than a hundred years. Primitive man +kindled his camp fire by rubbing pieces of dry wood together until +they took fire, and this method is said to be used among some isolated +distant tribes at the present time. A later and easier way was to +strike flint and steel together and to catch the spark thus produced +on tinder or dry fungus. Within the memory of some persons now living, +the tinder box was a valuable asset to the home, particularly in the +pioneer regions of the West. + +38. Safety Matches. Ordinary phosphorus, while excellent as a +fire-producing material, is dangerously poisonous, and those to whom +the dipping of wooden strips into phosphorus is a daily occupation +suffer with a terrible disease which usually attacks the teeth and +bones of the jaw. The teeth rot and fall out, abscesses form, and +bones and flesh begin to decay; the only way to prevent the spread of +the disease is to remove the affected bone, and in some instances it +has been necessary to remove the entire jaw. Then, too, matches made +of yellow or white phosphorus ignite easily, and, when rubbed against +any rough surface, are apt to take fire. Many destructive fires have +been started by the accidental friction of such matches against rough +surfaces. + +For these reasons the introduction of the so-called safety match was +an important event. When common phosphorus, in the dangerous and +easily ignited form, is heated in a closed vessel to about 250 deg. C., it +gradually changes to a harmless red mass. The red phosphorus is not +only harmless, but it is difficult to ignite, and, in order to be +ignited by friction, must be rubbed on a surface rich in oxygen. The +head of a safety match is coated with a mixture of glue and +oxygen-containing compounds; the surface on which the match is to be +rubbed is coated with a mixture of red phosphorus and glue, to which +finely powdered glass is sometimes added in order to increase the +friction. Unless the head of the match is rubbed on the prepared +phosphorus coating, ignition does not occur, and accidental fires are +avoided. + +Various kinds of safety matches have been manufactured in the last few +years, but they are somewhat more expensive than the ordinary form, +and hence manufacturers are reluctant to substitute them for the +cheaper matches. Some foreign countries, such as Switzerland, prohibit +the sale of the dangerous type, and it is hoped that the United States +will soon follow the lead of these countries in demanding the sale of +safety matches only. + +39. Some Unfamiliar Forms of Burning. While most of us think of +burning as a process in which flames and smoke occur, there are in +reality many modes of burning accompanied by neither flame nor smoke. +Iron, for example, burns when it rusts, because it slowly combines +with the oxygen of the air and is transformed into new substances. +When the air is dry, iron does not unite with oxygen, but when +moisture is present in the air, the iron unites with the oxygen and +turns into iron rust. The burning is slow and unaccompanied by the +fire and smoke so familiar to us, but the process is none the less +burning, or combination with oxygen. Burning which is not accompanied +by any of the appearances of ordinary burning is known as oxidation. + +The tendency of iron to rust lessens its efficiency and value, and +many devices have been introduced to prevent rusting. A coating of +paint or varnish is sometimes applied to iron in order to prevent +contact with air. The galvanizing of iron is another attempt to secure +the same result; in this process iron is dipped into molten zinc, +thereby acquiring a coating of zinc, and forming what is known as +galvanized iron. Zinc does not combine with oxygen under ordinary +circumstances, and hence galvanized iron is immune from rust. + +Decay is a process of oxidation; the tree which rots slowly away is +undergoing oxidation, and the result of the slow burning is the +decomposed matter which we see and the invisible gases which pass into +the atmosphere. The log which blazes on our hearth gives out +sufficient heat to warm us; the log which decays in the forest gives +out an equivalent amount of heat, but the heat is evolved so slowly +that we are not conscious of it. Burning accompanied by a blaze and +intense heat is a rapid process; burning unaccompanied by fire and +appreciable heat is a slow, gradual process, requiring days, weeks, +and even long years for its completion. + +Another form of oxidation occurs daily in the human body. In Section +35 we saw that the human body is an engine whose fuel is food; the +burning of that food in the body furnishes the heat necessary for +bodily warmth and the energy required for thought and action. Oxygen +is essential to burning, and the food fires within the body are kept +alive by the oxygen taken into the body at every breath by the lungs. +We see now one reason for an abundance of fresh air in daily life. + +40. How to Breathe. Air, which is essential to life and health, +should enter the body through the nose and _not through the mouth_. +The peculiar nature and arrangement of the membranes of the nose +enable the nostrils to clean, and warm, and moisten the air which +passes through them to the lungs. Floating around in the atmosphere +are dust particles which ought not to get into the lungs. The nose is +provided with small hairs and a moist inner membrane which serve as +filters in removing solid particles from the air, and in thus +purifying it before its entrance into the lungs. + +In the immediate neighborhood of three Philadelphia high schools, +having an approximate enrollment of over 8000 pupils, is a huge +manufacturing plant which day and night pours forth grimy smoke and +soot into the atmosphere which must supply oxygen to this vast group +of young lives. If the vital importance of nose breathing is impressed +upon these young people, the harmful effect of the foul air may be +greatly lessened, the smoke particles and germs being held back by the +nose filters and never reaching the lungs. If, however, this principle +of hygiene is not brought to their attention, the dangerous habit of +breathing through the open, or at least partially open, mouth will +continue, and objectionable matter will pass through the mouth and +find a lodging place in the lungs. + +There is another very important reason why nose breathing is +preferable to mouth breathing. The temperature of the human body is +approximately 98 deg. F., and the air which enters the lungs should not be +far below this temperature. If air reaches the lungs through the nose, +its journey is relatively long and slow, and there is opportunity for +it to be warmed before it reaches the lungs. If, on the other hand, +air passes to the lungs by way of the mouth, the warming process is +brief and insufficient, and the lungs suffer in consequence. +Naturally, the gravest danger is in winter. + +41. Cause of Mouth Breathing. Some people find it difficult to +breathe through the nostrils on account of growths, called adenoids, +in the nose. If you have a tendency toward mouth breathing, let a +physician examine your nose and throat. + +Adenoids not only obstruct breathing and weaken the whole system +through lack of adequate air, but they also press upon the blood +vessels and nerves of the head and interfere with normal brain +development. Moreover, they interfere in many cases with the hearing, +and in general hinder activity and growth. The removal of adenoids is +simple, and carries with it only temporary pain and no danger. Some +physicians claim that the growths disappear in later years, but even +if that is true, the physical and mental development of earlier years +is lost, and the person is backward in the struggle for life and +achievement. + +[Illustration: FIG. 23.--Intelligent expression is often lacking in +children with adenoid growths.] + +42. How to Build a Fire. Substances differ greatly as to the ease +with which they may be made to burn or, in technical terms, with which +they may be made to unite with oxygen. For this reason, we put light +materials, like shavings, chips, and paper, on the grate, twisting the +latter and arranging it so that air (oxygen in the air) can reach a +large surface; upon this we place small sticks of wood, piling them +across each other so as to allow entrance for the oxygen; and finally +upon this we place our hard wood or coal. + +The coal and the large sticks cannot be kindled with a match, but the +paper and shavings can, and these in burning will heat the large +sticks until they take fire and in turn kindle the coal. + +43. Spontaneous Combustion. We often hear of fires "starting +themselves," and sometimes the statement is true. If a pile of oily +rags is allowed to stand for a time, the oily matter will begin to +combine slowly with oxygen and as a result will give off heat. The +heat thus given off is at first insufficient to kindle a fire; but as +the heat is retained and accumulated, the temperature rises, and +finally the kindling point is reached and the whole mass bursts into +flames. For safety's sake, all oily cloths should be burned or kept in +metal vessels. + +44. The Treatment of Burns. In spite of great caution, burns from +fires, steam, or hot water do sometimes occur, and it is well to know +how to relieve the suffering caused by them and how to treat the +injury in order to insure rapid healing. + +Burns are dangerous because they destroy skin and thus open up an +entrance into the body for disease germs, and in addition because they +lay bare nerve tissue which thereby becomes irritated and causes a +shock to the entire system. + +In mild burns, where the skin is not broken but is merely reddened, an +application of moist baking soda brings immediate relief. If this +substance is not available, flour paste, lard, sweet oil, or vaseline +may be used. + +In more severe burns, where blisters are formed, the blisters should +be punctured with a sharp, sterilized needle and allowed to discharge +their watery contents before the above remedies are applied. + +In burns severe enough to destroy the skin, disinfection of the open +wound with weak carbolic acid or hydrogen peroxide is very necessary. +After this has been done, a soft cloth soaked in a solution of linseed +oil and limewater should be applied and the whole bandaged. In such a +case, it is important not to use cotton batting, since this sticks to +the rough surface and causes pain when removed. + +45. Carbon Dioxide. _A Product of Burning._ When any fuel, such as +coal, gas, oil, or wood, burns, it sends forth gases into the +surrounding atmosphere. These gases, like air, are invisible, and were +unknown to us for a long time. The chief gas formed by a burning +substance is called carbon dioxide (CO_2) because it is composed of +one part of carbon and two parts of oxygen. This gas has the +distinction of being the most widely distributed gaseous compound of +the entire world; it is found in the ocean depths and on the mountain +heights, in brilliantly lighted rooms, and most abundantly in +manufacturing towns where factory chimneys constantly pour forth hot +gases and smoke. + +Wood and coal, and in fact all animal and vegetable matter, contain +carbon, and when these substances burn or decay, the carbon in them +unites with oxygen and forms carbon dioxide. + +The food which we eat is either animal or vegetable, and it is made +ready for bodily use by a slow process of burning within the body; +carbon dioxide accompanies this bodily burning of food just as it +accompanies the fires with which we are more familiar. The carbon +dioxide thus produced within the body escapes into the atmosphere with +the breath. + +We see that the source of carbon dioxide is practically inexhaustible, +coming as it does from every stove, furnace, and candle, and further +with every breath of a living organism. + +46. Danger of Carbon Dioxide. When carbon dioxide occurs in large +quantities, it is dangerous to health, because it interferes with +normal breathing, lessening the escape of waste matter through the +breath and preventing the access to the lungs of the oxygen necessary +for life. Carbon dioxide is not poisonous, but it cuts off the supply +of oxygen, just as water cuts it off from a drowning man. + +Since every man, woman, and child constantly breathes forth carbon +dioxide, the danger in overcrowded rooms is great, and proper +ventilation is of vital importance. + +47. Ventilation. In estimating the quantity of air necessary to keep +a room well aired, we must take into account the number of lights +(electric lights do not count) to be used, and the number of people to +occupy the room. The average house should provide at the _minimum_ 600 +cubic feet of space for each person, and in addition, arrangements for +allowing at least 300 cubic feet of fresh air per person to enter +every hour. + +In houses which have not a ventilating system, the air should be kept +fresh by intelligent action in the opening of doors and windows; and +since relatively few houses are equipped with a satisfactory system, +the following suggestions relative to intelligent ventilation are +offered. + +1. Avoid drafts in ventilation. + +2. Ventilate on the sheltered side of the house. If the wind is +blowing from the north, open south windows. + +48. What Becomes of the Carbon Dioxide. When we reflect that carbon +dioxide is constantly being supplied to the atmosphere and that it is +injurious to health, the question naturally arises as to how the air +remains free enough of the gas to support life. This is largely +because carbon dioxide is an essential food of plants. Through their +leaves plants absorb it from the atmosphere, and by a wonderful +process break it up into its component parts, oxygen and carbon. They +reject the oxygen, which passes back to the air, but they retain the +carbon, which becomes a part of the plant structure. Plants thus serve +to keep the atmosphere free from an excess of carbon dioxide and, in +addition, furnish oxygen to the atmosphere. + +[Illustration: FIG. 24.--Making carbon dioxide from marble and +hydrochloric acid.] + +49. How to Obtain Carbon Dioxide. There are several ways in which +carbon dioxide can be produced commercially, but for laboratory use +the simplest is to mix in a test tube powdered marble, or chalk, and +hydrochloric acid, and to collect the effervescing gas as shown in +Figure 24. The substance which remains in the test tube after the gas +has passed off is a solution of a salt and water. From a mixture of +hydrochloric acid (HCl) and marble are obtained a salt, water, and +carbon dioxide, the desired gas. + +50. A Commercial Use of Carbon Dioxide. If a lighted splinter is +thrust into a test tube containing carbon dioxide, it is promptly +extinguished, because carbon dioxide cannot support combustion; if a +stream of carbon dioxide and water falls upon a fire, it acts like a +blanket, covering the flames and extinguishing them. The value of a +fire extinguisher depends upon the amount of carbon dioxide and water +which it can furnish. A fire extinguisher is a metal case containing a +solution of bicarbonate of soda, and a glass vessel full of strong +sulphuric acid. As long as the extinguisher is in an upright position, +these substances are kept separate, but when the extinguisher is +inverted, the acid escapes from the bottle, and mixes with the soda +solution. The mingling liquids interact and liberate carbon dioxide. +A part of the gas thus liberated dissolves in the water of the soda +solution and escapes from the tube with the outflowing liquid, while a +portion remains undissolved and escapes as a stream of gas. The fire +extinguisher is therefore the source of a liquid containing the +fire-extinguishing substance and further the source of a stream of +carbon dioxide gas. + +[Illustration: FIG. 25.--Inside view of a fire extinguisher.] + +51. Carbon. Although carbon dioxide is very injurious to health, +both of the substances of which it is composed are necessary to life. +We ourselves, our bones and flesh in particular, are partly carbon, +and every animal, no matter how small or insignificant, contains some +carbon; while the plants around us, the trees, the grass, the flowers, +contain a by no means meager quantity of carbon. + +Carbon plays an important and varied role in our life, and, in some +one of its many forms, enters into the composition of most of the +substances which are of service and value to man. The food we eat, the +clothes we wear, the wood and coal we burn, the marble we employ in +building, the indispensable soap, and the ornamental diamond, all +contain carbon in some form. + +52. Charcoal. One of the most valuable forms of carbon is charcoal; +valuable not in the sense that it costs hundreds of dollars, but in +the more vital sense, that its use adds to the cleanliness, comfort, +and health of man. + +The foul, bad-smelling gases which arise from sewers can be prevented +from escaping and passing to streets and buildings by placing charcoal +filters at the sewer exits. Charcoal is porous and absorbs foul gases, +and thus keeps the region surrounding sewers sweet and clean and free +of odor. Good housekeepers drop small bits of charcoal into vases of +flowers to prevent discoloration of the water and the odor of decaying +stems. + +If impure water filters through charcoal, it emerges pure, having left +its impurities in the pores of the charcoal. Practically all household +filters of drinking water are made of charcoal. But such a device may +be a source of disease instead of a prevention of disease, unless the +filter is regularly cleaned or renewed. This is because the pores soon +become clogged with the impurities, and unless they are cleaned, the +water which flows through the filter passes through a bed of +impurities and becomes contaminated rather than purified. Frequent +cleansing or renewal of the filter removes this difficulty. + +Commercially, charcoal is used on a large scale in the refining of +sugars, sirups, and oils. Sugar, whether it comes from the maple tree, +or the sugar cane, or the beet, is dark colored. It is whitened by +passage through filters of finely pulverized charcoal. Cider and +vinegar are likewise cleared by passage through charcoal. + +The value of carbon, in the form of charcoal, as a purifier is very +great, whether we consider it a deodorizer, as in the case of the +sewage, or a decolorizer, as in the case of the refineries, or whether +we consider the service it has rendered man in the elimination of +danger from drinking water. + +53. How Charcoal is Made. Charcoal may be made by heating wood in an +oven to which air does not have free access. The absence of air +prevents ordinary combustion, nevertheless the intense heat affects +the wood and changes it into new substances, one of which is charcoal. + +The wood which smolders on the hearth and in the stove is charcoal in +the making. Formerly wood was piled in heaps, covered with sod or sand +to prevent access of oxygen, and then was set fire to; the smoldering +wood, cut off from an adequate supply of air, was slowly transformed +into charcoal. Scattered over the country one still finds isolated +charcoal kilns, crude earthen receptacles, in which wood thus deprived +of air was allowed to smolder and form charcoal. To-day charcoal is +made commercially by piling wood on steel cars and then pushing the +cars into strong walled chambers. The chambers are closed to prevent +access of air, and heated to a high temperature. The intense heat +transforms the wood into charcoal in a few hours. A student can make +in the laboratory sufficient charcoal for art lessons by heating in an +earthen vessel wood buried in sand. The process will be slow, however, +because the heat furnished by a Bunsen burner is not great, and the +wood is transformed slowly. + +A form of charcoal known as animal charcoal, or bone black, is +obtained from the charred remains of animals rather than plants, and +may be prepared by burning bones and animal refuse as in the case of +the wood. + +Destructive Distillation. When wood is burned without sufficient +air, it is changed into soft brittle charcoal, which is very different +from wood. It weighs only one fourth as much as the original wood. It +is evident that much matter must leave the wood during the process of +charcoal making. We can prove this by putting some dry shavings in a +strong test tube fitted with a delivery tube. When the wood is heated +a gas passes off which we may collect and burn. Other substances also +come off in gaseous form, but they condense in the water. Among these +are wood alcohol, wood tar, and acetic acid. In the older method of +charcoal making all these products were lost. Can you give any uses of +these substances? + +54. Matter and Energy. When wood is burned, a small pile of ashes is +left, and we think of the bulk of the wood as destroyed. It is true we +have less matter that is available for use or that is visible to +sight, but, nevertheless, no matter has been destroyed. The matter of +which the wood is composed has merely changed its character, some of +it is in the condition of ashes, and some in the condition of +invisible gases, such as carbon dioxide, but none of it has been +destroyed. It is a principle of science that matter can neither be +destroyed nor created; it can only be changed, or transformed, and it +is our business to see that we do not heedlessly transform it into +substances which are valueless to us and our descendants; as, for +example, when our magnificent forests are recklessly wasted. The +smoke, gases, and ashes left in the path of a raging forest fire are +no compensation to us for the valuable timber destroyed. The sum total +of matter has not been changed, but the amount of matter which man can +use has been greatly lessened. + +The principle just stated embodies one of the fundamental laws of +science, called the law of the _conservation of matter_. + +A similar law holds for energy as well. We can transform electric +energy into the motion of trolley cars, or we can make use of the +energy of streams to turn the wheels of our mills, but in all these +cases we are transforming, not creating, energy. + +When a ball is fired from a rifle, most of the energy of the gunpowder +is utilized in motion, but some is dissipated in producing a flash and +a report, and in heat. The energy of the gunpowder has been scattered, +but the sum of the various forms of energy is equal to the energy +originally stored away in the powder. The better the gun is, the less +will be the energy dissipated in smoke and heat and noise. + + + + +CHAPTER V + +FOOD + + +55. The Body as a Machine. Wholesome food and fresh air are +necessary for a healthy body. Many housewives, through ignorance, +supply to their hard-working husbands and their growing sons and +daughters food which satisfies the appetite, but which does not give +to the body the elements needed for daily work and growth. Some foods, +such as lettuce, cucumbers, and watermelons, make proper and +satisfactory changes in diet, but are not strength giving. Other +foods, like peas and beans, not only satisfy the appetite, but supply +to the body abundant nourishment. Many immigrants live cheaply and +well with beans and bread as their main diet. + +It is of vital importance that the relative value of different foods +as heat producers be known definitely; and just as the yard measures +length and the pound measures weight the calorie is used to measure +the amount of heat which a food is capable of furnishing to the body. +Our bodies are human machines, and, like all other machines, require +fuel for their maintenance. The fuel supplied to an engine is not all +available for pulling the cars; a large portion of the fuel is lost in +smoke, and another portion is wasted as ashes. So it is with the fuel +that runs the body. The food we eat is not all available for +nourishment, much of it being as useless to us as are smoke and ashes +to an engine. The best foods are those which do the most for us with +the least possible waste. + +56. Fuel Value. By fuel value is meant the capacity foods have for +yielding heat to the body. The fuel value of the foods we eat daily is +so important a factor in life that physicians, dietitians, nurses, +and those having the care of institutional cooking acquaint themselves +with the relative fuel values of practically all of the important food +substances. The life or death of a patient may be determined by the +patient's diet, and the working and earning capacity of a father +depends largely upon his prosaic three meals. An ounce of fat, whether +it is the fat of meat or the fat of olive oil or the fat of any other +food, produces in the body two and a quarter times as much heat as an +ounce of starch. Of the vegetables, beans provide the greatest +nourishment at the least cost, and to a large extent may be +substituted for meat. It is not uncommon to find an outdoor laborer +consuming one pound of beans per day, and taking meat only on "high +days and holidays." + +[Illustration: FIG. 26.--The bomb calorimeter from which the fuel +value of food can be estimated.] + +The fuel value of a food is determined by means of the _bomb +calorimeter_ (Fig. 26). The food substance is put into a chamber _A_ +and ignited, and the heat of the burning substance raises the +temperature of the water in the surrounding vessel. If 1000 grams of +water are in the vessel, and the temperature of the water is raised 2 deg. +C., the number of calories produced by the substance would be 2000, +and the fuel value would be 2000 calories.[A] From this the fuel value +of one quart or one pound of the substance can be determined, and the +food substance will be said to furnish the body with that number of +heat units, providing all of the pound of food were properly digested. + +[Footnote A: As applied to food, the calorie is greater than that used +in the ordinary laboratory work, being the amount of heat necessary to +raise the temperature of 1000 grams of water 1 deg. C., rather than 1 gram +1 deg. C.] + + TABLE SHOWING THE NUMBER OF CALORIES FURNISHED BY + ONE POUND OF VARIOUS FOODS + ---------------------------------------------------- + |FOOD |CALORIES|FOOD |CALORIES| + ---------------------------------------------------- + |Leg of lean mutton | 790|Carrots | 210| + ---------------------------------------------------- + |Rib of beef | 1150|Lettuce | 90| + ---------------------------------------------------- + |Shad | 380|Onion | 225| + ---------------------------------------------------- + |Chicken | 505|Cucumber | 80| + ---------------------------------------------------- + |Apples | 290|Almonds | 3030| + ---------------------------------------------------- + |Bananas | 460|Walnuts | 3306| + ---------------------------------------------------- + |Prunes | 370|Peanuts | 2560| + ---------------------------------------------------- + |Watermelons | 140|Oatmeal | 4673| + ---------------------------------------------------- + |Lima beans | 570|Rolled wheat | 4175| + ---------------------------------------------------- + |Beets | 215|Macaroni | 1665| + ---------------------------------------------------- + +57. Varied Diet. The human body is a much more varied and complex +machine than any ever devised by man; personal peculiarities, as well +as fuel values, influence very largely the diet of an individual. +Strawberries are excluded from some diets because of a rash which is +produced on the skin, pork is excluded from other diets for a like +reason; cauliflower is absolutely indigestible to some and is readily +digested by others. From practically every diet some foods must be +excluded, no matter what the fuel value of the substance may be. + +Then, too, there are more uses for food than the production of heat. +Teeth and bones and nails need a constant supply of mineral matter, +and mineral matter is frequently found in greatest abundance in foods +of low fuel value, such as lettuce, watercress, etc., though +practically all foods yield at least a small mineral constituent. When +fuel values alone are considered, fruits have a low value, but because +of the flavor they impart to other foods, and because of the healthful +influence they exercise in digestion, they cannot be excluded from the +diet. + +Care should be constantly exercised to provide substantial foods of +high fuel value. But the nutritive foods should be wisely supplemented +by such foods as fruits, whose real value is one of indirect rather +then direct service. + +58. Our Bodies. Somewhat as a house is composed of a group of +bricks, or a sand heap of grains of sand, the human body is composed +of small divisions called cells. Ordinarily we cannot see these cells +because of their minuteness, but if we examine a piece of skin, or a +hair of the head, or a tiny sliver of bone under the microscope, we +see that each of these is composed of a group of different cells. A +merchant, watchful about the fineness of the wool which he is +purchasing, counts with his lens the number of threads to the inch; a +physician, when he wishes, can, with the aid of the microscope, +examine the cells in a muscle, or in a piece of fat, or in a nerve +fiber. Not only is the human body composed of cells, but so also are +the bodies of all animals from the tiny gnat which annoys us, and the +fly which buzzes around us, to the mammoth creatures of the tropics. +These cells do the work of the body, the bone cells build up the +skeleton, the nail cells form the finger and toe nails, the lung cells +take care of breathing, the muscle cells control motion, and the brain +cells are responsible for thought. + +59. Why we eat so Much. The cells of the body are constantly, day by +day, minute by minute, breaking down and needing repair, are +constantly requiring replacement by new cells, and, in the case of the +child, are continually increasing in number. The repair of an ordinary +machine, an engine, for example, is made at the expense of money, but +the repair and replacement of our human cell machinery are +accomplished at the expense of food. More than one third of all the +food we eat goes to maintain the body cells, and to keep them in good +order. It is for this reason that we consume a large quantity of food. +If all the food we eat were utilized for energy, the housewife could +cook less, and the housefather could save money on grocer's and +butcher's bills. If you put a ton of coal in an engine, its available +energy is used to run the engine, but if the engine were like the +human body, one third of the ton would be used up by the engine in +keeping walls, shafts, wheels, belts, etc., in order, and only two +thirds would go towards running the engine. When an engine is not +working, fuel is not consumed, but the body requires food for mere +existence, regardless of whether it does active work or not. When we +work, the cells break down more quickly, and the repair is greater +than when we are at rest, and hence there is need of a larger amount +of food; but whether we work or not, food is necessary. + +60. The Different Foods. The body is very exacting in its demands, +requiring certain definite foods for the formation and maintenance of +its cells, and other foods, equally definite, but of different +character, for heat; our diet therefore must contain foods of high +fuel value, and likewise foods of cell-forming power. + +Although the foods which we eat are of widely different character, +such as fruits, vegetables, cereals, oils, meats, eggs, milk, cheese, +etc., they can be put into three great classes: the carbohydrates, the +fats, and the proteids. + +61. The Carbohydrates. Corn, wheat, rye, in fact all cereals and +grains, potatoes, and most vegetables are rich in carbohydrates; as +are also sugar, molasses, honey, and maple sirup. The foods of the +first group are valuable because of the starch they contain; for +example, corn starch, wheat starch, potato starch. The substances of +the second group are valuable because of the sugar they contain; sugar +contains the maximum amount of carbohydrate. In the sirups there is a +considerable quantity of sugar, while in some fruits it is present in +more or less dilute form. Sweet peaches, apples, grapes, contain a +moderate amount of sugar; watermelons, pears, etc., contain less. Most +of our carbohydrates are of plant origin, being found in vegetables, +fruits, cereals, and sirups. + +Carbohydrates, whether of the starch group or the sugar group, are +composed chiefly of three elements: carbon, hydrogen, and oxygen; they +are therefore combustible, and are great energy producers. On the +other hand, they are worthless for cell growth and repair, and if we +limited our diet to carbohydrates, we should be like a man who had +fuel but no engine capable of using it. + +62. The Fats. The best-known fats are butter, lard, olive oil, and +the fats of meats, cheese, and chocolate. When we test fats for fuel +values by means of a calorimeter (Fig. 26), we find that they yield +twice as much heat as the carbohydrates, but that they burn out more +quickly. Dwellers in cold climates must constantly eat large +quantities of fatty foods if they are to keep their bodies warm and +survive the extreme cold. Cod liver oil is an excellent food medicine, +and if taken in winter serves to warm the body and to protect it +against the rigors of cold weather. The average person avoids fatty +foods in summer, knowing from experience that rich foods make him warm +and uncomfortable. The harder we work and the colder the weather, the +more food of that kind do we require; it is said that a lumberman +doing heavy out-of-door work in cold climates needs three times as +much food as a city clerk. Most of our fats, like lard and butter, are +of animal origin; some of them, however, like olive oil, peanut +butter, and coconut oil, are of plant origin. + +[Illustration: FIG. 27.--_a_ is the amount of fat necessary to make +one calorie; _b_ is the amount of sugar or proteid necessary to make +one calorie.] + +63. The Proteids. The proteids are the building foods, furnishing +muscle, bone, skin cells, etc., and supplying blood and other bodily +fluids. The best-known proteids are white of egg, curd of milk, and +lean of fish and meat; peas and beans have an abundant supply of this +substance, and nuts are rich in it. Most of our proteids are of animal +origin, but some protein material is also found in the vegetable +world. This class of foods contains carbon, oxygen, and hydrogen, and +in addition, two substances not found in carbohydrates or +fats--namely, sulphur and nitrogen. Proteids always contain nitrogen, +and hence they are frequently spoken of as nitrogenous foods. Since +the proteids contain all the elements found in the two other classes +of foods, they are able to contribute, if necessary, to the store of +bodily energy; but their main function is upbuilding, and the diet +should be chosen so that the proteids do not have a double task. + +For an average man four ounces of dry proteid matter daily will +suffice to keep the body cells in normal condition. + +It has been estimated that 300,000,000 blood cells alone need daily +repair or renewal. When we consider that the blood is but one part of +the body, and that all organs and fluids have corresponding +requirements, we realize how vast is the work to be done by the food +which we eat. + +64. Mistakes in Buying. The body demands a daily ration of the three +classes of food stuffs, but it is for us to determine from what +meats, vegetables, fruits, cereals, etc., this supply shall be +obtained (Figs. 28 and 29). + +[Illustration: FIG. 28.--Table of food values.] + +[Illustration: FIG. 29.--Diagram showing the difference in the cost of +three foods which give about the same amount of nutrition each.] + +Generally speaking, meats are the most expensive foods we can +purchase, and hence should be bought seldom and in small quantities. +Their place can be taken by beans, peas, potatoes, etc., and at less +than a quarter of the cost. The average American family eats meat +three times a day, while the average family of the more conservative +and older countries rarely eats meat more than once a day. The +following tables indicate the financial loss arising from an unwise +selection of foods:-- + + FOOD CONSUMED--ONE WEEK +|===========================|=======================================| +| FAMILY No. 1 | || FAMILY No. 2 | +|---------------------------|---------------------------------------| +|20 loaves of bread | $1.00 ||15 lb. flour, bread | +|10 to 12 lb. loin steak | || home made (skim milk used) | $.45 +| or meat of similar cost | 2.00 ||Yeast, shortening, and | +|20 to 25 lb. rib roast | || skim milk | .10 +| or similar meat | 4.40 ||10 lb. steak (round, Hamburger| +|4 lb. high-priced cereal | || and some loin) | 1.50 +| breakfast food, 20c | .80 ||10 lb. other meats, boiling | +|Cake and pastry purchased | 3.00 || pieces, rump roast, etc. | 1.00 +|8 lb. butter, 30c | 2.40 || 5 lb. cheese, 16c | .80 +|Tea, coffee, spices, etc. | .75 || 5 lb. oatmeal (bulk) | .15 +|Mushrooms | .75 || 5 lb. beans | .25 +|Celery | 1.00 ||Home-made cake and pastry | 1.00 +|Oranges | 2.00 || 6 lb. butter, 30c | 1.80 +|Potatoes | .25 || 3 lb. home-made shortening | .25 +|Miscellaneous canned goods | 2.00 ||Tea, coffee, and spices | .40 +|Milk | .50 ||Apples | .50 +|Miscellaneous foods | 2.00 ||Prunes | .25 +|3 doz. eggs | .60 ||Potatoes | .25 +| |-------||Milk | 1.00 +| |$23.45 ||Miscellaneous foods | 1.00 +| | || 3 doz. eggs | .60 +| | || -|----- +| | || $|11.30 +| -----------------------|-----------------------------------------|--- +| -----------------------|-----------------------------------------|--- + +"The tables show that one family spends over twice as much in the +purchase of foods as the other family, and yet the one whose food +costs the less actually secures the larger amount of nutritive +material and is better fed than the family where more money is +expended."--From _Human Foods_, Snyder. + +The Source of the Different Foods. All of our food comes from either +the plant world or the animal world. Broadly speaking, plants furnish +the carbohydrates, that is, starch and sugar; animals furnish the fats +and proteids. But although vegetable foods yield carbohydrates mainly, +some of them, like beans and peas, contain large quantities of protein +and can be substituted for meat without disadvantage to the body. +Other plant products, such as nuts, have fat as their most abundant +food constituent. The peanut, for example, contains 43% of fat, 30% of +proteids, and only 17% of carbohydrates; the Brazil nut has 65% of +fat, 17% of proteids, and only 9% of carbohydrates. Nuts make a good +meat substitute, and since they contain a fair amount of carbohydrates +besides the fats and proteins, they supply all of the essential food +constituents and form a well-balanced food. + + + + +CHAPTER VI + +WATER + + +65. Destructive Action of Water. The action of water in stream and +sea, in springs and wells, is evident to all; but the activity of +ground water--that is, rain water which sinks into the soil and +remains there--is little known in general. The real activity of ground +water is due to its great solvent power; every time we put sugar into +tea or soap into water we are using water as a solvent. When rain +falls, it dissolves substances floating in the atmosphere, and when it +sinks into the ground and becomes ground water, it dissolves material +out of the rock which it encounters (Fig. 30). We know that water +contains some mineral matter, because kettles in which water is boiled +acquire in a short time a crust or coating on the inside. This crust +is due to the accumulation in the kettle of mineral matter which was +in solution in the water, but which was left behind when the water +evaporated. (See Section 25.) + +[Illustration: FIG. 30.--Showing how caves and holes are formed by the +solvent action of water.] + +The amount of dissolved mineral matter present in some wells and +springs is surprisingly great; the famous springs of Bath, England, +contain so much mineral matter in solution, that a column 9 feet in +diameter and 140 feet high could be built out of the mineral matter +contained in the water consumed yearly by the townspeople. + +[Illustration: FIG. 31.--The work of water as a solvent.] + +Rocks and minerals are not all equally soluble in water; some are so +little soluble that it is years before any change becomes apparent, +and the substances are said to be insoluble, yet in reality they are +slowly dissolving. Other rocks, like limestone, are so readily soluble +in water that from the small pores and cavities eaten out by the +water, there may develop in long centuries, caves and caverns (Fig. +30). Most rock, like granite, contains several substances, some of +which are readily soluble and others of which are not readily soluble; +in such rocks a peculiar appearance is presented, due to the rapid +disappearance of the soluble substance, and the persistence of the +more resistant substance (Fig. 31). + +We see that the solvent power of water is constantly causing changes, +dissolving some mineral substances, and leaving others practically +untouched; eating out crevices of various shapes and sizes, and by +gradual solution through unnumbered years enlarging these crevices +into wonderful caves, such as the Mammoth Cave of Kentucky. + +66. Constructive Action of Water. Water does not always act as a +destructive agent; what it breaks down in one place it builds up in +another. It does this by means of precipitation. Water dissolves salt, +and also dissolves lead nitrate, but if a salt solution is mixed with +a lead nitrate solution, a solid white substance is formed in the +water (Fig. 32). This formation of a solid substance from the mingling +of two liquids is called precipitation; such a process occurs daily in +the rocks beneath the surface of the earth. (See Laboratory Manual.) + +[Illustration: FIG. 32.--From the mingling of two liquids a solid is +sometimes formed.] + +Suppose water from different sources enters a crack in a rock, +bringing different substances in solution; then the mingling of the +waters may cause precipitation, and the solid thus formed will be +deposited in the crack and fill it up. Hence, while ground water tends +to make rock porous and weak by dissolving out of it large quantities +of mineral matter, it also tends under other conditions to make it +more compact because it deposits in cracks, crevices, and pores the +mineral matter precipitated from solution. + +These two forces are constantly at work; in some places the +destructive action is more prominent, in other places the constructive +action; but always the result is to change the character of the +original substance. When the mineral matter precipitated from the +solutions is deposited in cracks, _veins_ are formed (Fig. 33), which +may consist of the ore of different metals, such as gold, silver, +copper, lead, etc. Man is almost entirely dependent upon these veins +for the supply of metal needed in the various industries, because in +the original condition of the rocks, the metallic substances are so +scattered that they cannot be profitably extracted. + +[Illustration: FIG. 33.--Mineral matter precipitated from solution is +deposited in crevices and forms veins.] + +Naturally, the veins themselves are not composed of one substance +alone, because several different precipitates may be formed. But there +is a decided grouping of valuable metals, and these can then be +readily separated by means of electricity. + +67. Streams. Streams usually carry mud and sand along with them; +this is particularly well seen after a storm when rivers and brooks +are muddy. The puddles which collect at the foot of a hill after a +storm are muddy because of the particles of soil gathered by the water +as it runs down the hill. The particles are not dissolved in the +water, but are held there in suspension, as we call it technically. +The river made muddy after a storm by suspended particles usually +becomes clear and transparent after it has traveled onward for miles, +because, as it travels, the particles drop to the bottom and are +deposited there. Hence, materials suspended in the water are borne +along and deposited at various places (Fig. 34). The amount of +deposition by large rivers is so great that in some places channels +fill up and must be dredged annually, and vessels are sometimes caught +in the deposit and have to be towed away. + +Running water in the form of streams and rivers, by carrying sand +particles, stones, and rocks from high slopes and depositing them at +lower levels, wears away land at one place and builds it up at +another, and never ceases in its work of changing the nature of the +earth's surface (Fig. 35). + +[Illustration: FIG. 34.--Deposit left by running water.] + +[Illustration: FIG. 35.--Water by its action constantly changes the +character of the land.] + +68. Relation of Water to Human Life. Water is one of the most +essential of food materials, and whether we drink much or little +water, we nevertheless get a great deal of it. The larger part of many +of our foods is composed of water; more than half of the weight of the +meat we eat is made up of water; and vegetables are often more than +nine tenths water. (See Laboratory Manual.) Asparagus and tomatoes +have over 90 per cent. of water, and most fruits are more than three +fourths water; even bread, which contains as little water as any of +our common foods, is about one third water (Fig. 36). + +[Illustration: FIG. 36.--Diagram of the composition of a loaf of bread +and of a potato: 1. ash; 2, food; 3, water.] + +Without water, solid food material, although present in the body, +would not be in a condition suitable for bodily use. An abundant +supply of water enables the food to be dissolved or suspended in it, +and in solution the food material is easily distributed to all parts +of the body. + +Further, water assists in the removal of the daily bodily wastes, and +thus rids the system of foul and poisonous substances. + +The human body itself consists largely of water; indeed, about two +thirds of our own weight is water. The constant replenishing of this +large quantity is necessary to life, and a considerable amount of the +necessary supply is furnished by foods, particularly the fruits and +vegetables. + +But while the supply furnished by the daily food is considerable, it +is by no means sufficient, and should be supplemented by good drinking +water. + +69. Water and its Dangers. Our drinking water comes from far and +near, and as it moves from place to place, it carries with it in +solution or suspension anything which it can find, whether it be +animal, vegetable, or mineral matter. The power of water to gather up +matter is so great that the average drinking water contains 20 to 90 +grains of solid matter per gallon; that is, if a gallon of ordinary +drinking water is left to evaporate, a residue of 20 to 90 grains will +be left. (See Laboratory Manual.) As water runs down a hill slope +(Fig. 37), it carries with it the filth gathered from acres of land; +carries with it the refuse of stable, barn, and kitchen; and too often +this impure surface water joins the streams which supply our cities. +Lakes and rivers which furnish drinking water should be carefully +protected from surface draining; that is, from water which has flowed +over the land and has thus accumulated the waste of pasture and +stable and, it may be, of dumping ground. + +[Illustration: FIG. 37.--As water flows over the land, it gathers +filth and disease germs.] + +It is not necessary that water should be absolutely free from all +foreign substances in order to be safe for daily use in drinking; a +limited amount of mineral matter is not injurious and may sometimes be +really beneficial. It is the presence of animal and vegetable matter +that causes real danger, and it is known that typhoid fever is due +largely to such impurities present in the drinking water. + +70. Methods of Purification. Water is improved by any of the +following methods:-- + +(_a_) _Boiling_. The heat of boiling destroys animal and vegetable +germs. Hence water that has been boiled a few minutes is safe to use. +This is the most practical method of purification in the home, and is +very efficient. The boiled water should be kept in clean, corked +bottles; otherwise foreign substances from the atmosphere reenter the +water, and the advantage gained from boiling is lost. + +(_b_) _Distillation_. By this method pure water is obtained, but this +method of purification cannot be used conveniently in the home +(Section 25). + +(_c_) _Filtration_. In filtration, the water is forced through +porcelain or other porous substances which allow the passage of water, +but which hold back the minute foreign particles suspended in the +water. (See Laboratory Manual.) The filters used in ordinary dwellings +are of stone, asbestos, or charcoal. They are often valueless, because +they soon become choked and cannot be properly cleaned. + +The filtration plants owned and operated by large cities are usually +safe; there is careful supervision of the filters, and frequent and +effective cleanings are made. In many cities the filtration system is +so good that private care of the water supply is unnecessary. + +71. The Source of Water. In the beginning, the earth was stored with +water just as it was with metal, rock, etc. Some of the water +gradually took the form of rivers, lakes, streams, and wells, as now, +and it is this original supply of water which furnishes us all that we +have to-day. We quarry to obtain stone and marble for building, and we +fashion the earth's treasures into forms of our own, but we cannot +create these things. We bore into the ground and drill wells in order +to obtain water from hidden sources; we utilize rapidly flowing +streams to drive the wheels of commerce, but the total amount of water +remains practically unchanged. + +The water which flows on the earth is constantly changing its form; +the heat of the sun causes it to evaporate, or to become vapor, and to +mingle with the atmosphere. In time, the vapor cools, condenses, and +falls as snow or rain; the water which is thus returned to the earth +feeds our rivers, lakes, springs, and wells, and these in turn supply +water to man. When water falls upon a field, it soaks into the ground, +or collects in puddles which slowly evaporate, or it runs off and +drains into small streams or into rivers. That which soaks into the +ground is the most valuable because it remains on the earth longest +and is the purest. + +[Illustration: FIG. 38.--How springs are formed. _A_, porous layer; +_B_, non-porous layer; _C_, spring.] + +Water which soaks into the ground moves slowly downward and after a +longer or shorter journey, meets with a non-porous layer of rock +through which it cannot pass, and which effectually hinders its +downward passage. In such regions, there is an accumulation of water, +and a well dug there would have an abundant supply of water. The +non-porous layer is rarely level, and hence the water whose vertical +path is obstructed does not "back up" on the soil, but flows down hill +parallel with the obstructing non-porous layer, and in some distant +region makes an outlet for itself, forming a spring (Fig. 38). The +streams originating in the springs flow through the land and +eventually join larger streams or rivers; from the surface of streams +and rivers evaporation occurs, the water once more becomes vapor and +passes into the atmosphere, where it is condensed and again falls to +the earth. + +Water which has filtered through many feet of earth is far purer and +safer than that which fell directly into the rivers, or which ran off +from the land and joined the surface streams without passing through +the soil. + +72. The Composition of Water. Water was long thought to be a simple +substance, but toward the end of the eighteenth century it was found +to consist of two quite different substances, oxygen (O) and hydrogen +(H.) + +[Illustration: FIG. 39.--The decomposition of water.] + +If we send an electric current through water (acidulated to make it a +good conductor), as shown in Figure 39, we see bubbles of gas rising +from the end of the wire by which the current enters the water, and +other bubbles of gas rising from the end of the wire by which the +current leaves the water. These gases have evidently come from the +water and are the substances of which it is composed, because the +water begins to disappear as the gases are formed. If we place over +each end of the wire an inverted jar filled with water, the gases are +easily collected. The first thing we notice is that there is always +twice as much of one gas as of the other; that is, water is composed +of two substances, one of which is always present in twice as large +quantities as the other. + +73. The Composition of Water. On testing the gases into which water +is broken up by an electric current, we find them to be quite +different. One proves to be oxygen, a substance with which we are +already familiar. The other gas, hydrogen, is new to us and is +interesting as being the lightest known substance, being even "lighter +than a feather." + +An important fact about hydrogen is that in burning it gives as much +heat as five times its weight of coal. Its flame is blue and almost +invisible by daylight, but intensely hot. If fine platinum wire is +placed in an ordinary gas flame, it does not melt, but if placed in a +flame of burning hydrogen, it melts very quickly. + +74. How to prepare Hydrogen. There are many different methods of +preparing hydrogen, but the easiest laboratory method is to pour +sulphuric acid, or hydrochloric acid, on zinc shavings and to collect +in a bottle the gas which is given off. This gas proves to be +colorless, tasteless, and odorless. (See Laboratory Manual.) + + + + +CHAPTER VII + +AIR + + +75. The Instability of the Air. We are usually not conscious of the +air around us, but sometimes we realize that the air is heavy, while +at other times we feel the bracing effect of the atmosphere. We live +in an ocean of air as truly as fish inhabit an ocean of water. If you +have ever been at the seashore you know that the ocean is never still +for a second; sometimes the waves surge back and forth in angry fury, +at other times the waves glide gently in to the shore and the surface +is as smooth as glass; but we know that there is perpetual motion of +the water even when the ocean is in its gentlest moods. Generally our +atmosphere is quiet, and we are utterly unconscious of it; at other +times we are painfully aware of it, because of its furious winds. Then +again we are oppressed by it because of the vast quantity of vapor +which it holds in the form of fog, or mist. The atmosphere around us +is as restless and varying as is the water of the sea. The air at the +top of a high tower is very different from the air at the base of the +tower. Not only does the atmosphere vary greatly at different +altitudes, but it varies at the same place from time to time, at one +period being heavy and raw, at another being fresh and invigorating. + +Winds, temperature, and humidity all have a share in determining +atmospheric conditions, and no one of these plays a small part. + +76. The Character of the Air. The atmosphere which envelops us at +all times extends more than fifty miles above us, its height being far +greater than the greatest depths of the sea. This atmosphere varies +from place to place; at the sea level it is heavy, on the mountain top +less heavy, and far above the earth it is so light that it does not +contain enough oxygen to permit man to live. Figure 40 illustrates by +a pile of pillows how the pressure of the air varies from level to +level. + +[Illustration: FIG. 40.--To illustrate the decrease in pressure with +height.] + +Sea level is a low portion of the earth's surface, hence at sea level +there is a high column of air, and a heavy air pressure. As one passes +from sea level to mountain top a gradual but steady decrease in the +height of the air column occurs, and hence a gradual but definite +lessening of the air pressure. + +[Illustration: FIG. 41.--The water in the tube is at the same level as +that in the glass.] + +77. Air Pressure. If an empty tube (Fig. 41) is placed upright in +water, the water will not rise in the tube, but if the tube is put in +water and the air is then drawn out of the tube by the mouth, the +water will rise in the tube (Fig. 42). This is what happens when we +take lemonade through a straw. When the air is withdrawn from the +straw by the mouth, the pressure within the straw is reduced, and the +liquid is forced up the straw by the air pressure on the surface of +the liquid in the glass. Even the ancient Greeks and Romans knew that +water would rise in a tube when the pressure within the tube was +reduced, and hence they tried to obtain water from wells in this +fashion, but the water could never be raised higher than 34 feet. Let +us see why water could rise 34 feet and no more. If an empty pipe is +placed in a cistern of water, the water in the pipe does not rise +above the level of the water in the cistern. If, however, the pressure +in the tube is removed, the water in the tube will rise to a height of +34 feet approximately. If now the air pressure in the tube is +restored, the water in the tube sinks again to the level of that in +the cistern. The air pressing on the liquid in the cistern tends to +push some liquid up the tube, but the air pressing on the water in the +tube pushes downwards, and tends to keep the liquid from rising, and +these two pressures balance each other. When, however, the pressure +within the tube is reduced, the liquid rises because of the unbalanced +pressure which acts on the water in the cistern. + +[Illustration: FIG. 42.--Water rises in the tube when the air is +withdrawn.] + +[Illustration: FIG. 43.--The air supports a column of mercury 30 +inches high.] + +The column of water which can be raised this way is approximately 34 +feet, sometimes a trifle more, sometimes a trifle less. If water were +twice as heavy, just half as high a column could be supported by the +atmosphere. Mercury is about thirteen times as heavy as water and, +therefore, the column of mercury supported by the atmosphere is about +one thirteenth as high as the column of water supported by the +atmosphere. This can easily be demonstrated. Fill a glass tube about a +yard long with mercury, close the open end with a finger, and quickly +insert the end of the inverted tube in a dish of mercury (Fig. 43). +When the finger is removed, the mercury falls somewhat, leaving an +empty space in the top of the tube. If we measure the column in the +tube, we find its height is about one thirteenth of 34 feet or 30 +inches, exactly what we should expect. Since there is no air pressure +within the tube, the atmospheric pressure on the mercury in the dish +is balanced solely by the mercury within the tube, that is, by a +column of mercury 30 inches high. The shortness of the mercury column +as compared with that of water makes the mercury more convenient for +both experimental and practical purposes. (See Laboratory Manual.) + +78. The Barometer. Since the pressure of the air changes from time +to time, the height of the mercury will change from day to day, and +hour to hour. When the air pressure is heavy, the mercury will tend to +be high; when the air pressure is low, the mercury will show a shorter +column; and by reading the level of the mercury one can learn the +pressure of the atmosphere. If a glass tube and dish of mercury are +attached to a board and the dish of mercury is inclosed in a case for +protection from moisture and dirt, and further if a scale of inches or +centimeters is made on the upper portion of the board, we have a +mercurial barometer (Fig. 44). + +[Illustration: FIG. 44.--A simple barometer.] + +If the barometer is taken to the mountain top, the column of mercury +falls gradually during the ascent, showing that as one ascends, the +pressure decreases in agreement with the statement in Section 76. +Observations similar to these were made by Torricelli as early as the +sixteenth century. Taking a barometric reading consists in measuring +the height of the mercury column. + +79. A Portable Barometer. The mercury barometer is large and +inconvenient to carry from place to place, and a more portable form +has been devised, known as the aneroid barometer (Fig. 45). This form +of barometer is extremely sensitive; indeed, it is so delicate that +it shows the slight difference between the pressure at the table top +and the pressure at the floor level, whereas the mercury barometer +would indicate only a much greater variation in atmospheric pressure. +The aneroid barometers are frequently made no larger than a watch and +can be carried conveniently in the pocket, but they get out of order +easily and must be frequently readjusted. The aneroid barometer is an +air-tight box whose top is made of a thin metallic disk which bends +inward or outward according to the pressure of the atmosphere. If the +atmospheric pressure increases, the thin disk is pushed slightly +inward; if, on the other hand, the atmospheric pressure decreases, the +pressure on the metallic disk decreases and the disk is not pressed so +far inward. The motion of the disk is small, and it would be +impossible to calculate changes in atmospheric pressure from the +motion of the disk, without some mechanical device to make the slight +changes in motion perceptible. + +[Illustration: FIG. 45.--Aneroid barometer.] + +In order to magnify the slight changes in the position of the disk, +the thin face is connected with a system of levers, or wheels, which +multiplies the changes in motion and communicates them to a pointer +which moves around a graduated circular face. In Figure 45 the real +barometer is scarcely visible, being securely inclosed in a metal case +for protection; the principle, however, can be understood by reference +to Figure 46. + +[Illustration: FIG. 46.--Principle of the aneroid barometer.] + +80. The Weight of the Air. We have seen that the pressure of the +atmosphere at any point is due to the weight of the air column which +stretches from that point far up into the sky above. This weight +varies slightly from time to time and from place to place, but it is +equal to about 15 pounds to the square inch as shown by actual +measurement. It comes to us as a surprise sometimes that air actually +has weight; for example, a mass of 12 cubic feet of air at average +pressure weighs 1 pound, and the air in a large assembly hall weighs +more than 1 ton. + +We are practically never conscious of this really enormous pressure of +the atmosphere, which is exerted over every inch of our bodies, +because the pressure is exerted equally over the outside and the +inside of our bodies; the cells and tissues of our bodies containing +gases under atmospheric pressure. If, however, the finger is placed +over the open end of a tube and the air is sucked out of the tube by +the mouth, the flesh of the finger bulges into the tube because the +pressure within the finger is no longer equalized by the usual +atmospheric pressure (Fig. 47). + +[Illustration: FIG. 47.--The flesh bulges out.] + +Aeronauts have never ascended much higher than 7 miles; at that height +the barometer stands at 7 inches instead of at 30 inches, and the +internal pressure in cells and tissues is not balanced by an equal +external pressure. The unequalized internal pressure forces the blood +to the surface of the body and causes rupture of blood vessels and +other physical difficulties. + +81. Use of the Barometer. Changes in air pressure are very closely +connected with changes in the weather. The barometer does not directly +foretell the weather, but a low or falling pressure, accompanied by a +simultaneous fall of the mercury, usually precedes foul weather, while +a rising pressure, accompanied by a simultaneous rise in the mercury, +usually precedes fair weather. The barometer is not an infallible +prophet, but it is of great assistance in predicting the general trend +of the weather. There are certain changes in the barometer which +follow no known laws, and which allow of no safe predictions, but on +the other hand, general future conditions for a few days ahead can be +fairly accurately determined. Figure 48 shows a barograph or +self-registering barometer which automatically registers air pressure. + +[Illustration: FIG. 48.--Barograph.] + +Seaport towns in particular, but all cities, large or small, and +villages too, are on request notified by the United States Weather +Bureau ten hours or more in advance, of probable weather conditions, +and in this way precautions are taken which annually save millions of +dollars and hundreds of lives. + +I recollect a summer spent on a New Hampshire farm, and know that an +old farmer started his farm hands haying by moonlight at two o'clock +in the morning, because the Special Farmer's Weather Forecast of the +preceding evening had predicted rain for the following day. His +reliance on the weather report was not misplaced, since the storm came +with full force at noon. Sailing vessels, yachts, and fishing dories +remain within reach of port if the barometer foretells storms. + +[Illustration: FIG. 49.--Isotherms.] + +82. Isobaric and Isothermal Lines. If a line were drawn through all +points on the surface of the earth having an equal barometric pressure +at the same time, such a line would be called an isobar. For example, +if the height of barometers in different localities is observed at +exactly the same time, and if all the cities and towns which have the +same pressure are connected by a line, the curved lines will be called +isobars. By the aid of these lines the barometric conditions over a +large area can be studied. The Weather Bureau at Washington relies +greatly on these isobars for statements concerning local and distant +weather forecasts, any shift in isobaric lines showing change in +atmospheric pressure. + +If a line is drawn through all points on the surface of the earth +having the same _temperature_ at the same instant, such a line is +called an isotherm (Fig. 49). + +83. Weather Maps. Scattered over the United States are about 125 +Government Weather Stations, at each of which three times a day, at +the same instant, accurate observations of the weather are made. These +observations, which consist of the reading of barometer and +thermometer, the determination of the velocity and direction of the +wind, the determination of the humidity and of the amount of rain or +snow, are telegraphed to the chief weather official at Washington. +From the reports of wind storms, excessive rainfall, hot waves, +clearing weather, etc., and their rate of travel, the chief officials +predict where the storms, etc., will be at a definite future time. In +the United States, the _general_ movement of weather conditions, as +indicated by the barometer, is from west to east, and if a certain +weather condition prevails in the west, it is probable that it will +advance eastward, although with decided modifications. So many +influences modify atmospheric conditions that unfailing predictions +are impossible, but the Weather Bureau predictions prove true in about +eight cases out of ten. + +The reports made out at Washington are telegraphed on request to +cities in this country, and are frequently published in the daily +papers, along with the forecast of the local office. A careful study +of these reports enables one to forecast to some extent the probable +weather conditions of the day. + +The first impression of a weather map (Fig. 50) with its various lines +and signals is apt to be one of confusion, and the temptation comes to +abandon the task of finding an underlying plan of the weather. If one +will bear in mind a few simple rules, the complexity of the weather +map will disappear and a glance at the map will give one information +concerning general weather conditions just as a glance at the +thermometer in the morning will give some indication of the probable +temperature of the day. (See Laboratory Manual.) + +[Illustration: FIG. 50. weather Map] + +On the weather map solid lines represent isobars and dotted lines +represent isotherms. The direction of the wind at any point is +indicated by an arrow which flies with the wind; and the state of the +weather--clear, partly cloudy, cloudy, rain, snow, etc.--is indicated +by symbols. + +84. Components of the Air. The best known constituent of the air is +oxygen, already familiar to us as the feeder of the fire without and +within the body. Almost one fifth of the air which envelops us is made +up of the life-giving oxygen. This supply of oxygen in the air is +constantly being used up by breathing animals and glowing fires, and +unless there were some constant source of additional supply, the +quantity of oxygen in the air would soon become insufficient to +support animal life. The unfailing constant source of atmospheric +oxygen is plant life (Section 48). The leaves of plants absorb carbon +dioxide from the air, and break it up into oxygen and carbon. The +plant makes use of the carbon but it rejects the oxygen, which passes +back into the atmosphere through the pores of the leaves. + +Although oxygen constitutes only one fifth of the atmosphere, it is +one of the most abundant and widely scattered of all substances. +Almost the whole earth, whether it be rich loam, barren clay, or +granite boulder, contains oxygen in some form or other; that is, in +combination with other substances. But nowhere, except in the air +around us, do we find oxygen free and uncombined with other +substances. + +A less familiar but more abundant constituent of the atmosphere is the +nitrogen. Almost four fifths of the air around us is made up of +nitrogen. If the atmosphere were composed of oxygen alone, the merest +flicker of a match would set the whole world ablaze. The fact that the +oxygen of the air is diluted as it were with so large a proportion of +nitrogen, prevents fires from sweeping over the world and destroying +everything in their path. Nitrogen does not support combustion, and a +burning match placed in a corked bottle goes out as soon as it has +used up the oxygen in the bottle. The nitrogen in the bottle, not only +does not assist the burning of the match, but it acts as a damper to +the burning. + +Free nitrogen, like oxygen, is a colorless, odorless gas. It is not +poisonous; but one would die if surrounded by nitrogen alone, just as +one would die if surrounded by water. The vast supply of nitrogen in +the atmosphere would be useless if the smaller amount of oxygen were +not present to keep the body alive. Nitrogen is so important a factor +in daily life that an entire chapter will be devoted to it later. + +Another constituent of the air with which we are familiar is carbon +dioxide. In pure air, carbon dioxide is present in very small +proportion, being continually taken from the air by plants in the +manufacture of their food. + +Various other substances are present in the air in very minute +proportions, but of all the substances in the air, oxygen, nitrogen, +and carbon dioxide are the most important. + + + + +CHAPTER VIII + +GENERAL PROPERTIES OF GASES + + +85. Bicycle Tires. We know very well that we cannot put more than a +certain amount of water in a tube, but we know equally well that the +amount of air which can be pumped into a bicycle or automobile tire +depends largely upon our muscular energy. A gallon of water remains a +gallon of water and requires a perfectly definite amount of space, but +air can be compressed and compressed, and made to occupy less and less +space. While it is true that air is easily compressed, it is also true +that air is elastic and capable of very rapid and easy expansion. If a +puncture occurs in a tire, the compressed air escapes very quickly; +that is, the compressed air within the tube has taken the first +opportunity offered for expansion. + +[Illustration: FIG. 51.--By squeezing the bulb, air is forced out of +the nozzle.] + +The fact that air is elastic has added materially to the comfort of +the world. Transportation by bicycles and automobiles has been greatly +facilitated by the use of air tires. In many hospitals, air mattresses +are used in place of hair, feather, or cotton mattresses, and in this +way the bed is kept fresher and cleaner, and can be moved with less +danger of discomfort to the patient. Every time we squeeze the bulb of +an atomizer, we force compressed or condensed air through the +atomizer, and the condensed air pushes the liquid out of the nozzle +(Fig. 51). Thus we see that in the necessities and conveniences of +life compressed air plays an important part. + +86. The Danger of Compression. Air under ordinary atmospheric +conditions exerts a pressure of 15 pounds to the square inch. If, now, +large quantities of air are compressed into a small space, the +pressure exerted becomes correspondingly greater. If too much air is +blown into a toy balloon, the balloon bursts because it cannot support +the great pressure exerted by the compressed air within. What is true +of air is true of all gases. Dangerous boiler explosions have occurred +because the boiler walls were not strong enough to withstand the +pressure of the steam (which is water in the form of gas). The +pressure within the boilers of engines is frequently several hundred +pounds to the square inch, and such a pressure needs a strong boiler. + +87. How Pressure is Measured in Buildings. In the preceding Section +we saw that undue pressure of a gas may cause explosion. It is +important, therefore, that authorities keep strict watch on gases +confined within pipes and reservoirs, never allowing the pressure to +exceed that which the walls of the reservoir will safely bear. + +[Illustration: FIG. 52.--A pressure gauge.] + +Pressure in a gas pipe may be measured by a simple instrument called +the pressure gauge: The gauge consists of a bent glass tube containing +mercury, and so made that one end can be fitted to a gas jet (Fig. +52). When the gas cock is closed, the mercury stands at the same level +in both arms, but when the cock is opened, the gas whose pressure is +being measured forces the mercury up the opposite arm. If the pressure +of the gas is small, the mercury changes its level but very little. It +is clear that the height of a column of mercury is a measure of the +gas pressure. Now it is known that one cubic inch of mercury weighs +about half a pound. Hence a column of mercury one inch high indicates +a pressure of about one half pound to the square inch; a column two +inches high indicates a pressure of about one pound to the square +inch, and so on. + +This is a very convenient way to measure the pressure of the +illuminating gas in our homes and offices. The gauge is attached to +the gas burner and the pressure is read by means of a scale attached +to the gauge. (See Laboratory Manual.) + +In order to have satisfactory illumination, the pressure must be +strong enough to give a steady, broad flame. If the flame from any gas +jet is flickering and weak, it is usually an indication of +insufficient pressure and the gas company should investigate +conditions and see to it that the consumer receives his proper value. + +87. The Gas Meter. Most householders are deeply interested in the +actual amount of gas which they consume (gas is charged for according +to the number of cubic feet used), and therefore they should be able +to read the gas meter which indicates their consumption of gas. Such +gas meters are furnished by the companies, and can be read easily. + +[Illustration: FIG. 53.--The gas meter indicates the number of cubic +feet of gas consumed.] + +The instrument itself is somewhat complex. It will suffice to say that +within the meter box are thin disks which are moved by the stream of +gas that passes them. This movement of the disks is recorded by +clockwork devices on a dial face. In this way, the number of cubic +feet of gas which pass through the meter is automatically registered. + +89. The Relation between Pressure and Volume. It was long known that +as the pressure of a gas increases, that is, as it becomes compressed, +its volume decreases, but Robert Boyle was the first to determine the +exact relation between the volume and the pressure of a gas. He did +this in a very simple manner. + +Pour mercury into a U-shaped tube until the level of the mercury in +the closed end of the tube is the same as the level in the open end. +The air in the long arm is pressing upon the mercury in that arm, and +is tending to force it up the short arm. The air in the short closed +arm is pressing down upon the mercury in that arm and tending to send +it up the long arm. Since the mercury is at the same level in the two +arms, the pressure in the long arm must be equal to the pressure in +the short arm. But the long arm is open, and the pressure in that arm +is the pressure of the atmosphere. Therefore the pressure in the short +arm must be one atmosphere. Measure the distance _bc_ between the top +of the mercury and the closed end of the tube. + +[Illustration: FIGS. 54, 55.--As the pressure on the gas increases, +its volume decreases.] + +Pour more mercury into the open end of the tube, and as the mercury +rises higher and higher in the long arm, note carefully the decrease +in the volume of the air in the short arm. Pour mercury into the tube +until the difference in level _bd_ is just equal to the barometric +height, approximately 32 inches. The pressure of the air in the closed +end now supports the pressure of one atmosphere, and in addition, a +column of mercury equal to another atmosphere. If now the air column +in the closed end is measured, its volume will be only one half of its +former volume. By doubling the pressure we have reduced the volume one +half. Similarly, if the pressure is increased threefold, the volume +will be reduced to one third of the original volume. + +90. Heat due to Compression. We saw in Section 89 that whenever the +pressure exerted upon a gas is increased, the volume of the gas is +decreased; and that whenever the pressure upon a gas is decreased, the +volume of the gas is increased. If the pressure is changed very +slowly, the change in the temperature of the gas is imperceptible; if, +however, the pressure is removed suddenly, the temperature falls +rapidly, or if the pressure is applied suddenly, the temperature rises +rapidly. When bicycle tires are being inflated, the pump becomes hot +because of the compression of the air. + +The amount of heat resulting from compression is surprisingly large; +for example, if a mass of gas at 0 deg. C. is suddenly compressed to one +half its original volume, its temperature rises 87 deg. C. + +91. Cooling by Expansion. If a gas expands suddenly, its temperature +falls; for example, if a mass of gas at 87 deg. C. is allowed to expand +rapidly to twice its original volume, its temperature falls to 0 deg. C. +If the compressed air of a bicycle tire is allowed to expand and a +sensitive thermometer is held in the path of the escaping air, the +thermometer will show a decided drop in temperature. + +The low temperature obtained by the expansion of air or other gases is +utilized commercially on a large scale. By means of powerful pistons +air is compressed to one third or one fourth its original volume, is +passed through a coil of pipe surrounded with cold water, and is then +allowed to escape into large refrigerating vaults, which thereby have +their temperatures noticeably lowered, and can be used for the +permanent storage of meats, fruits, and other perishable material. In +summer, when the atmospheric temperature is high, the storage and +preservation of foods is of vital importance to factories and cold +storage houses, and but for the low temperature obtainable by the +expansion of compressed gases, much of our food supply would be lost +to use. + +92. Unexpected Transformations. If the pressure on a gas is greatly +increased, a sudden transformation sometimes occurs and the gas +becomes a liquid. Then, if the pressure is reduced, a second +transformation occurs, and the liquid evaporates or returns to its +original form as a gas. + +In Section 23 we saw that a fall of temperature caused water vapor to +condense or liquefy. If temperature alone were considered, most gases +could not be liquefied, because the temperature at which the average +gas liquefies is so low as to be out of the range of possibility; it +has been calculated, for example, that a temperature of 252 deg. C. below +zero would have to be obtained in order to liquefy hydrogen. + +Some gases can be easily transformed into liquids by pressure alone, +some gases can be easily transformed into liquids by cooling alone; on +the other hand, many gases are so difficult to liquefy that both +pressure and low temperature are needed to produce the desired result. +If a gas is cooled and compressed at the same time, liquefaction +occurs much more surely and easily than though either factor alone +were depended upon. The air which surrounds us, and of whose existence +we are scarcely aware, can be reduced to the form of a liquid, but the +pressure exerted upon the portion to be liquefied must be thirty-nine +times as great as the atmospheric pressure, and the temperature must +have been reduced to a very low point. + +93. Artificial Ice. Ammonia gas is liquefied by strong pressure and +low temperature and is then allowed to flow into pipes which run +through tanks containing salt water. The reduction of pressure causes +the liquid to evaporate or turn to a gas, and the fall of temperature +which always accompanies evaporation means a lowering of the +temperature of the salt water to 16 deg. or 18 deg. below zero. But immersed +in the salt water are molds containing pure water, and since the +freezing point of water is 0 deg. C, the water in the molds freezes and +can be drawn from the mold as solid cakes of ice. + +[Illustration: FIG. 56.--Apparatus for making artificial ice.] + +Ammonia gas is driven by the pump _C_ into the coil _D_ (Fig. 56) +under a pressure strong enough to liquefy it, the heat generated by +this compression being carried off by cold water which constantly +circulates through _B_. The liquid ammonia flows through the +regulating valve _V_ into the coil _E_, in which the pressure is kept +low by the pump _C_. The accompanying expansion reduces the +temperature to a very low degree, and the brine which circulates +around the coil _E_ acquires a temperature below the freezing point of +pure water. The cold brine passes from _A_ to a tank in which are +immersed cans filled with water, and within a short time the water in +the cans is frozen into solid cakes of ice. + + + + +CHAPTER IX + +INVISIBLE OBJECTS + + +94. Very Small Objects. We saw in Section 84 that gases have a +tendency to expand, but that they can be compressed by the application +of force. This observation has led scientists to suppose that +substances are composed of very minute particles called molecules, +separated by small spaces called pores; and that when a gas is +condensed, the pores become smaller, and that when a gas expands, the +pores become larger. + +The fact that certain substances are soluble, like sugar in water, +shows that the molecules of sugar find a lodging place in the spaces +or pores between the molecules of water, in much the same way that +pebbles find lodgment in the chinks of the coal in a coal scuttle. An +indefinite quantity of sugar cannot be dissolved in a given quantity +of liquid, because after a certain amount of sugar has been dissolved +all the pores become filled, and there is no available molecular +space. The remainder of the sugar settles at the bottom of the vessel, +and cannot be dissolved by any amount of stirring. + +If a piece of potassium permanganate about the size of a grain of sand +is put into a quart of water, the solid disappears and the water +becomes a deep rich red. The solid evidently has dissolved and has +broken up into minute particles which are too small to be seen, but +which have scattered themselves and lodged in the pores of the water, +thus giving the water its rich color. + +There is no visible proof of the existence of molecules and molecular +spaces, because not only are our eyes unable to see them directly, but +even the most powerful microscope cannot make them visible to us. They +are so small that if one thousand of them were laid side by side, they +would make a speck too small to be seen by the eye and too small to be +visible under the most powerful microscope. + +We cannot see molecules or molecular pores, but the phenomena of +compression and expansion, solubility and other equally convincing +facts, have led us to conclude that all substances are composed of +very minute particles or molecules separated by spaces called pores. + +95. Journeys Made by Molecules. If a gas jet is turned on and not +lighted, an odor of gas soon becomes perceptible, not only throughout +the room, but in adjacent halls and even in distant rooms. An uncorked +bottle of cologne scents an entire room, the odor of a rose or violet +permeates the atmosphere near and far. These simple everyday +occurrences seem to show that the molecules of a gas must be in a +state of continual and rapid motion. In the case of the cologne, some +molecules must have escaped from the liquid by the process of +evaporation and traveled through the air to the nose. We know that the +molecules of a liquid are in motion and are continually passing into +the air because in time the vessel becomes empty. The only way in +which this could happen would be for the molecules of the liquid to +pass from the liquid into the surrounding medium; but this is really +saying that the molecules are in motion. + +From these phenomena and others it is reasonably clear that substances +are composed of molecules, and that molecules are not inert, quiet +particles, but that they are in incessant motion, moving rapidly +hither and thither, sometimes traveling far, sometimes near. Even the +log of wood which lies heavy and motionless on our woodpile is made +up of countless billions of molecules each in rapid incessant motion. +The molecules of solid bodies cannot escape so readily as those of +liquids and gases, and do not travel far. The log lies year after year +in an apparently motionless condition, but if one's eyes were keen +enough, the molecules would be seen moving among themselves, even +though they cannot escape into the surrounding medium and make long +journeys as do the molecules of liquids and gases. + +96. The Companions of Molecules. Common sense tells us that a +molecule of water is not the same as a molecule of vinegar; the +molecules of each are extremely small and in rapid motion, but they +differ essentially, otherwise one substance would be like every other +substance. What is it that makes a molecule of water differ from a +molecule of vinegar, and each differ from all other molecules? Strange +to say, a molecule is not a simple object, but is quite complex, being +composed of one or more smaller particles, called atoms, and the +number and kind of atoms in a molecule determine the type of the +molecule, and the type of the molecule determines the substance. For +example, a glass of water is composed of untold millions of molecules, +and each molecule is a company of three still smaller particles, one +of which is called the oxygen atom and two of which are alike in every +particular and are called hydrogen atoms. + +97. Simple Molecules. Generally molecules are composed of atoms +which are different in kind. For example, the molecule of water has +two different atoms, the oxygen atom and the hydrogen atoms; alcohol +has three different kinds of atoms, oxygen, hydrogen, and carbon. +Sometimes, however, molecules are composed of a group of atoms all of +which are alike. Now there are but seventy or eighty different kinds +of atoms, and hence there can be but seventy or eighty different +substances whose molecules are composed of atoms which are alike. When +the atoms comprising a molecule are all alike, the substance is called +an element, and is said to be a simple substance. Throughout the +length and breadth of this vast world of ours there are only about +eighty known elements. An element is the simplest substance +conceivable, because it has not been separated into anything simpler. +Water is a compound substance. It can be separated into oxygen and +hydrogen. + +Gold, silver, and lead are examples of elements, and water, alcohol, +cider, sand, and marble are complex substances, or compounds, as we +are apt to call them. Everything, no matter what its size or shape or +character, is formed from the various combinations into molecules of a +few simple atoms, of which there exist about eighty known different +kinds. But few of the eighty known elements play an important part in +our everyday life. The elements in which we are most interested are +given in the following table, and the symbols by which they are known +are placed in columns to the right: + + |Oxygen |O |Copper |Cu |Phosphorus |P | + |Hydrogen |H |Iodine |I |Potassium |K | + |Carbon |C |Iron |Fe |Silver |Ag | + |Aluminium Al |Lead |Pb |Sodium |Na | | + |Calcium |Ca |Nickel |Ni |Sulphur |S | + |Chlorine |Cl |Nitrogen |N |Tin |Sn | + +We have seen in an earlier experiment that twice as much hydrogen as +oxygen can be obtained from water. Two atoms of the element hydrogen +unite with one atom of the element oxygen to make one molecule of +water. In symbols we express this H_2O. A group of symbols, such as +this, expressing a molecule of a compound is called a _formula_. NaCl +is the formula for sodium chloride, which is the chemical name of +common salt. + + + + +CHAPTER X + +LIGHT + + +98. What Light Does for Us. Heat keeps us warm, cooks our food, +drives our engines, and in a thousand ways makes life comfortable and +pleasant, but what should we do without light? How many of us could be +happy even though warm and well fed if we were forced to live in the +dark where the sunbeams never flickered, where the shadows never stole +across the floor, and where the soft twilight could not tell us that +the day was done? Heat and light are the two most important physical +factors in life; we cannot say which is the more necessary, because in +the extreme cold or arctic regions man cannot live, and in the dark +places where the light never penetrates man sickens and dies. Both +heat and light are essential to life, and each has its own part to +play in the varied existence of man and plant and animal. + +Light enables us to see the world around us, makes the beautiful +colors of the trees and flowers, enables us to read, is essential to +the taking of photographs, gives us our moving pictures and our magic +lanterns, produces the exquisite tints of stained-glass windows, and +brings us the joy of the rainbow. We do not always realize that light +is beneficial, because sometimes it fades our clothing and our +carpets, and burns our skin and makes it sore. But we shall see that +even these apparently harmful effects of light are in reality of great +value in man's constant battle against disease. + +99. The Candle. Natural heat and light are furnished by the sun, but +the absence of the sun during the evening makes artificial light +necessary, and even during the day artificial light is needed in +buildings whose structure excludes the natural light of the sun. +Artificial light is furnished by electricity, by gas, by oil in lamps, +and in numerous other ways. Until modern times candles were the main +source of light, and indeed to-day the intensity, or power, of any +light is measured in candle power units, just as length is measured in +yards; for example, an average gas jet gives a 10 candle power light, +or is ten times as bright as a candle; an ordinary incandescent +electric light gives a 16 candle power light, or furnishes sixteen +times as much light as a candle. Very strong large oil lamps can at +times yield a light of 60 candle power, while the large arc lamps +which flash out on the street corners are said to furnish 1200 times +as much light as a single candle. Naturally all candles do not give +the same amount of light, nor are all candles alike in size. The +candles which decorate our tea tables are of wax, while those which +serve for general use are of paraffin and tallow. + +[Illustration: FIG. 57.--A photograph at _a_ receives four times as +much light as when held at _b_.] + +100. Fading Illumination. The farther we move from a light, the less +strong, or intense, is the illumination which reaches us; the light of +the street lamp on the corner fades and becomes dim before the middle +of the block is reached, so that we look eagerly for the next lamp. +The light diminishes in brightness much more rapidly than we realize, +as the following simple experiment will show. Let a single candle +(Fig. 57) serve as our light, and at a distance of one foot from the +candle place a photograph. In this position the photograph receives a +definite amount of light from the candle and has a certain brightness. + +If now we place a similar photograph directly behind the first +photograph and at a distance of two feet from the candle, the second +photograph receives no light because the first one cuts off all the +light. If, however, the first photograph is removed, the light which +fell on it passes outward and spreads itself over a larger area, until +at the distance of the second photograph the light spreads itself over +four times as large an area as formerly. At this distance, then, the +illumination on the second photograph is only one fourth as strong as +it was on a similar photograph held at a distance of one foot from the +candle. + +The photograph or object placed at a distance of one foot from a light +is well illuminated; if it is placed at a distance of two feet, the +illumination is only one fourth as strong, and if the object is placed +three feet away, the illumination is only one ninth as strong. This +fact should make us have thought and care in the use of our eyes. We +think we are sixteen times as well off with our incandescent lights as +our ancestors were with simple candles, but we must reflect that our +ancestors kept the candle near them, "at their elbow," so to speak, +while we sit at some distance from the light and unconcernedly read +and sew. + +As an object recedes from a light the illumination which it receives +diminishes rapidly, for the strength of the illumination is inversely +proportional to the square of distance of the object from the light. +Our ancestors with a candle at a distance of one foot from a book were +as well off as we are with an incandescent light four feet away. + +101. Money Value of Light. Light is bought and sold almost as +readily as are the products of farm and dairy; many factories, +churches, and apartments pay a definite sum for electric light of a +standard strength, and naturally full value is desired. An instrument +for measuring the strength of a light is called a photometer, and +there are many different varieties, just as there are varieties of +scales which measure household articles. One light-measuring scale +depends upon the law that the intensity of illumination decreases with +the square of the distance of the object from the light. Suppose we +wish to measure the strength of the electric light bulbs in our homes, +in order to see whether we are getting the specified illumination. In +front of a screen place a black rod (Fig. 58) which is illuminated by +two different lights; namely, a standard candle and an incandescent +bulb whose strength is to be measured. Two shadows of the rod will +fall on the screen, one caused by the candle and the other caused by +the incandescent light. The shadow due to the latter source is not so +dark as that due to the candle. Now let the incandescent light be +moved away from the screen until the two shadows are of equal +darkness. If the incandescent light is four times as far away from the +screen as the candle, and the shadows are equal, we know, by Section +100, that its strength is sixteen candle power. If the incandescent +light is four times as far away from the screen as the candle is, its +power must be sixteen times as great, and we know the company is +furnishing the standard amount of light for a sixteen candle power +electric bulb. If, however, the bulb must be moved nearer to the rod +in order that the two shadows may be similar then the light given by +the bulb is less than sixteen candle power, and less than that due the +consumer. + +[Illustration: FIG. 58.--The two shadows are equally dark.] + +102. How Light Travels. We never expect to see around a corner, and +if we wish to see through pinholes in three separate pieces of +cardboard, we place the cardboards so that the three holes are in a +straight line. When sunlight enters a dark room through a small +opening, the dust particles dancing in the sun show a straight ray. If +a hole is made in a card, and the card is held in front of a light, +the card casts a shadow, in the center of which is a bright spot. The +light, the hole, and the bright spot are all in the same straight +line. These simple observations lead us to think that light travels in +a straight line. + +[Illustration: FIG. 59.--The candle cannot be seen unless the three +pinholes are in a strait line.] + +We can always tell the direction from which light comes, either by the +shadow cast or by the bright spot formed when an opening occurs in the +opaque object casting the shadow. If the shadow of a tree falls +towards the west, we know the sun must be in the cast; if a bright +spot is on the floor, we can easily locate the light whose rays stream +through an opening and form the bright spot. We know that light +travels in a straight line, and following the path of the beam which +comes to our eyes, we are sure to locate the light. + +103. Good and Bad Mirrors. As we walk along the street, we +frequently see ourselves reflected in the shop windows, in polished +metal signboards, in the metal trimmings of wagons and automobiles; +but in mirrors we get the best image of ourselves. We resent the image +given by a piece of tin, because the reflection is distorted and does +not picture us as we really are; a rough surface does not give a fair +representation; if we want a true image of ourselves, we must use a +smooth surface like a mirror as a reflector. If the water in a pond +is absolutely still, we get a clear, true image of the trees, but if +there are ripples on the surface, the reflection is blurred and +distorted. A metal roof reflects so much light that the eyes are +dazzled by it, and a whitewashed fence injures the eyes because of the +glare which comes from the reflected light. Neither of these could be +called mirrors, however, because although they reflect light, they +reflect it so irregularly that not even a suggestion of an image can +be obtained. + +Most of us are sufficiently familiar with mirrors to know that the +image is a duplicate of ourselves with regard to size, shape, color, +and expression, but that it appears to be back of the mirror, while we +are actually in front of the mirror. The image appears not only behind +the mirror, but it is also exactly as far back of the mirror as we are +in front of it; if we approach the mirror, the image also draws +nearer; if we withdraw, it likewise recedes. + +104. The Path of Light. If a mirror or any other polished surface is +held in the path of a sunbeam, some of the light is reflected, and by +rotating the mirror the reflected sunbeam may be made to take any +path. School children amuse themselves by reflecting sunbeams from a +mirror into their companions' faces. If the companion moves his head +in order to avoid the reflected beam, his tormentor moves or inclines +the mirror and flashes the beam back to his victim's face. + +If a mirror is held so that a ray of light strikes it in a +perpendicular direction, the light is reflected backward along the +path by which it came. If, however, the light makes an angle with the +mirror, its direction is changed, and it leaves the mirror along a new +path. By observation we learn that when a beam strikes the mirror and +makes an angle of 30 deg. with the perpendicular, the beam is reflected in +such a way that its new path also makes an angle of 30 deg. with the +perpendicular. If the sunbeam strikes the mirror at an angle of 32 deg. +with the perpendicular, the path of the reflected ray also makes an +angle of 32 deg. with the perpendicular. The ray (_AC_, Fig. 60) which +falls upon the mirror is called the incident ray, and the angle which +the incident ray (_AC_) makes with the perpendicular (_BC_) to the +mirror, at the point where the ray strikes the mirror, is called the +angle of incidence. The angle formed by the reflected ray (_CD_) and +this same perpendicular is called the angle of reflection. Observation +and experiment have taught us that light is always reflected in such a +way that the angle of reflection equals the angle of incidence. Light +is not the only illustration we have of the law of reflection. Every +child who bounces a ball makes use of this law, but he uses it +unconsciously. If an elastic ball is thrown perpendicularly against +the floor, it returns to the sender; if it is thrown against the floor +at an angle (Fig. 61), it rebounds in the opposite direction, but +always in such a way that the angle of reflection equals the angle of +incidence. + +[Illustration: FIG. 60.--The ray _AC_ is reflected as _CD_.] + +[Illustration: FIG. 61.--A bouncing ball illustrates the law of +reflection.] + +105. Why the Image seems to be behind the Mirror. If a candle is +placed in front of a mirror, as in Figure 62, one of the rays of light +which leaves the candle will fall upon the mirror as _AB_ and will be +reflected as _BC_ (in such a way that the angle of reflection equals +the angle of incidence). If an observer stands at _C_, he will think +that the point _A_ of the candle is somewhere along the line _CB_ +extended. Such a supposition would be justified from Section 102. But +the candle sends out light in all directions; one ray therefore will +strike the mirror as _AD_ and will be reflected as _DE_, and an +observer at _E_ will think that the point _A_ of the candle is +somewhere along the line _ED_. In order that both observers may be +correct, that is, in order that the light may seem to be in both these +directions, the image of the point _A_ must seem to be at the +intersection of the two lines. In a similar manner it can be shown +that every point of the image of the candle seems to be behind the +mirror. + +[Illustration: FIG. 62.--The image is a duplicate of the object, but +appears to be behind the mirror.] + +It can be shown by experiment that the distance of the image behind +the mirror is equal to the distance of the object in front of the +mirror. + +106. Why Objects are Visible. If the beam of light falls upon a +sheet of paper, or upon a photograph, instead of upon a smooth +polished surface, no definite reflected ray will be seen, but a glare +will be produced by the scattering of the beam of light. The surface +of the paper or photograph is rough, and as a result, it scatters the +beam in every direction. It is hard for us to realize that a smooth +sheet of paper is by no means so smooth as it looks. It is rough +compared with a polished mirror. The law of reflection always holds, +however, no matter what the reflecting surface is,--the angle of +reflection always equals the angle of incidence. In a smooth body the +reflected beams are all parallel; in a rough body, the reflected beams +are inclined to each other in all sorts of ways, and no two beams +leave the paper in exactly the same direction. + +[Illustration: FIG. 63.--The surface of the paper, although smooth in +appearance, is in reality rough, and scatters the light in every +direction.] + +Hot coals, red-hot stoves, gas flames, and candles shine by their own +light, and are self-luminous. Objects like chairs, tables, carpets, +have no light within themselves and are visible only when they receive +light from a luminous source and reflect that light. We know that +these objects are not self-luminous, because they are not visible at +night unless a lamp or gas is burning. When light from any luminous +object falls upon books, desks, or dishes, it meets rough surfaces, +and hence undergoes diffuse reflection, and is scattered irregularly +in all directions. No matter where the eye is, some reflected rays +enter it, and the various objects are clearly seen. + + + + +CHAPTER XI + +REFRACTION + + +107. Bent Rays of Light. A straw in a glass of lemonade seems to be +broken at the surface of the liquid, the handle of a teaspoon in a cup +of water appears broken, and objects seen through a glass of water may +seem distorted and changed in size. When light passes from air into +water, or from any transparent substance into another of different +density, its direction is changed, and it emerges along an entirely +new path (Fig. 64). We know that light rays pass through glass, +because we can see through the window panes and through our +spectacles; we know that light rays pass through water, because we can +see through a glass of clear water; on the other hand, light rays +cannot pass through wood, leather, metal, etc. + +[Illustration: FIG. 64.--A straw or stick in water seems broken.] + +Whenever light meets a transparent substance obliquely, some of it is +reflected, undergoing a change in its direction; and some of it passes +onward through the medium, but the latter portion passes onward along +a new path. The ray _RO_ (Fig. 65) passes obliquely through the air to +the surface of the water, but, on entering the water, it is bent or +refracted and takes the new path _OS_. The angle _AOR_ is called the +angle of incidence. The angle _POS_ is called the angle of refraction. + +[Illustration: FIG. 65.--When the ray _RO_ enters the water, its path +changes to _OS_.] + +The angle of refraction is the angle formed by the refracted ray and +the perpendicular to the surface at the point where the light strikes +it. + +When light passes from air into water or glass, the refracted ray is +bent toward the perpendicular, so that the angle of refraction is +smaller than the angle of incidence. When a ray of light passes from +water or glass into air, the refracted ray is bent away from the +perpendicular so that the angle of refraction is greater than the +angle of incidence. + +The bending or deviation of light in its passage from one substance to +another is called refraction. + +108. How Refraction Deceives us. Refraction is the source of many +illusions; bent rays of light make objects appear where they really +are not. A fish at _A_ (Fig. 66) seems to be at _B_. The end of the +stick in Figure 64 seems to be nearer the surface of the water than it +really is. + +[Illustration: FIG. 66.--A fish at _A_ seems to be at _B_.] + +The light from the sun, moon, and stars can reach us only by passing +through the atmosphere, but in Section 76, we learned that the +atmosphere varies in density from level to level; hence all the light +which travels through the atmosphere is constantly deviated from its +original path, and before the light reaches the eye it has undergone +many changes in direction. Now we learned in Section 102, that the +direction of the rays of light as they enter the eye determines the +direction in which an object is seen; hence the sun, moon, and stars +seem to be along the lines which enter the eye, although in reality +they are not. + +109. Uses of Refraction. If it were not for refraction, or the +deviation of light in its passage from medium to medium, the wonders +and beauties of the magic lantern and the camera would be unknown to +us; sun, moon, and stars could not be made to yield up their distant +secrets to us in photographs; the comfort and help of spectacles would +be lacking, spectacles which have helped unfold to many the rare +beauties of nature, such as a clear view of clouds and sunset, of +humming bee and flying bird. Books with their wealth of entertainment +and information would be sealed to a large part of mankind, if glasses +did not assist weak eyes. + +By refraction the magnifying glass reveals objects hidden because of +their minuteness, and enlarges for our careful contemplation objects +otherwise barely visible. The watchmaker, unassisted by the magnifying +glass, could not detect the tiny grains of dust or sand which clog the +delicate wheels of our watches. The merchant, with his lens, examines +the separate threads of woolen and silk fabrics to determine the +strength and value of the material. The physician, with his invaluable +microscope, counts the number of infinitesimal corpuscles in the blood +and bases his prescription on that count; he examines the sputum of a +patient to determine whether tuberculosis wastes the system. The +bacteriologist with the same instrument scrutinizes the drinking water +and learns whether the dangerous typhoid germs are present. The +future of medicine will depend somewhat upon the additional secrets +which man is able to force from nature through the use of powerful +lenses, because as lenses have, in the past, been the means of +revealing disease germs, so in the future more powerful lenses may +serve to bring to light germs yet unknown. How refraction accomplishes +these results will be explained in the following Sections. + +110. The Window Pane. We have seen that light is bent when it passes +from one medium to another of different density, and that objects +viewed by refracted light do not appear in their proper positions. + +When a ray of light passes through a piece of plane glass, such as a +window pane (Fig. 67), it is refracted at the point _B_ toward the +perpendicular, and continues its course through the glass in the new +direction _BC_. On emerging from the glass, the light is refracted +away from the perpendicular and takes the direction _CD_, which is +clearly parallel to its original direction. Hence, when we view +objects through the window, we see them slightly displaced in +position, but otherwise unchanged. The deviation or displacement +caused by glass as thin as window panes is too slight to be noticed, +and we are not conscious that objects are out of position. + +[Illustration: FIG. 67.--Objects looked at through a window pane seem +to be in their natural place.] + +111. Chandelier Crystals and Prisms. When a ray of light passes +through plane glass, like a window pane, it is shifted somewhat, but +its direction does not change; that is, the emergent ray is parallel +to the incident ray. But when a beam of light passes through a +triangular glass prism, such as a chandelier crystal, its direction is +greatly changed, and an object viewed through a prism is seen quite +out of its true position. + +Whenever light passes through a prism, it is bent toward the base of +the prism, or toward the thick portion of the prism, and emerges from +the prism in quite a different direction from that in which it entered +(Fig. 68). Hence, when an object is looked at through a prism, it is +seen quite out of place. In Figure 68, the candle seems to be at _S_, +while in reality it is at _A_. + +[Illustration: FIG. 68.--When looked at through the prism, _A_ seems +to be at _S_.] + +112. Lenses. If two prisms are arranged as in Figure 69, and two +parallel rays of light fall upon the prisms, the beam _A_ will be bent +downward toward the thickened portion of the prism, and the beam _B_ +will be bent upward toward the thick portion of the prism, and after +passing through the prism the two rays will intersect at some point +_F_, called a focus. + +[Illustration: FIG. 69.--Rays of light are converged and focused at +_F_.] + +If two prisms are arranged as in Figure 70, the ray _A_ will be +refracted upward toward the thick end, and the ray _B_ will be +refracted downward toward the thick end; the two rays, on emerging, +will therefore be widely separated and will not intersect. + +[Illustration: FIG. 70.--Rays of light are diverged and do not come to +any real focus.] + +Lenses are very similar to prisms; indeed, two prisms placed as in +Figure 69, and rounded off, would make a very good convex lens. A lens +is any transparent material, but usually glass, with one or both sides +curved. The various types of lenses are shown in Figure 71. + +[Illustration: FIG. 71.--The different types of lenses.] + +The first three types focus parallel rays at some common point _F_, as +in Figure 69. Such lenses are called convex or converging lenses. The +last three types, called concave lenses, scatter parallel rays so that +they do not come to a focus, but diverge widely after passage through +the lens. + +113. The Shape and Material of a Lens. The main or principal focus +of a lens, that is, the point at which rays parallel to the base line +_AB_ meet (Fig. 71), depends upon the shape of the lens. For example, +a thick lens, such as _A_ (Fig. 72), focuses the rays very near to the +lens; _B_, which is not so thick, focuses the rays at a greater +distance from the lens; and _C_, which is a very thin lens, focuses +the rays at a considerable distance from the lens. The distance of the +principal focus from the lens is called the focal length of the lens, +and from the diagrams we see that the more convex the lens, the +shorter the focal length. + +[Illustration: FIG. 72.--The more curved the lens, the shorter the +focal length, and the nearer the focus is to the lens.] + +The position of the principal focus depends not only on the shape of +the lens, but also on the refractive power of the material composing +the lens. A lens made of ice would not deviate the rays of light so +much as a lens of similar shape composed of glass. The greater the +refractive power of the lens, the greater the bending, and the nearer +the principal focus to the lens. + +There are many different kinds of glass, and each kind of glass +refracts the light differently. Flint glass contains lead; the lead +makes the glass dense, and gives it great refractive power, enabling +it to bend and separate light in all directions. Cut glass and toilet +articles are made of flint glass because of the brilliant effects +caused by its great refractive power, and imitation gems are commonly +nothing more than polished flint glass. + +114. How Lenses Form Images. Suppose we place an arrow, _A_, in +front of a convex lens (Fig. 73). The ray _AC_, parallel to the +principal axis, will pass through the lens and emerge as _DE_. The ray +is always bent toward the thick portion of the lens, both at its +entrance into the lens and its emergence from the lens. + +[Illustration: FIG. 73.--The image is larger than the object. By means +of a lens, a watchmaker gets an enlarged image of the dust which clogs +the wheels of his watch.] + +In Section 105, we saw that two rays determine the position of any +point of our image; hence in order to locate the image of the top of +the arrow, we need to consider but one more ray from the top of the +object. The most convenient ray to choose would be one passing through +_O_, the optical center of the lens, because such a ray passes through +the lens unchanged in direction, as is clear from Figure 74. The point +where _AC_ and _AO_ meet after refraction will be the position of the +top of the arrow. Similarly it can be shown that the center of the +arrow will be at the point _T_, and we see that the image is larger +than the object. This can be easily proved experimentally. Let a +convex lens be placed near a candle (Fig. 75); move a paper screen +back and forth behind the lens; for some position of the screen a +clear, enlarged image of the candle will be made. + +[Illustration: FIG. 74.--Rays above _O_ are bent downward, those below +_O_ are bent upward, and rays through _O_ emerge from the lens +unchanged in direction.] + +If the candle or arrow is placed in a new position, say at _MA_ (Fig. +76), the image formed is smaller than the object, and is nearer to the +lens than it was before. Move the lens so that its distance from the +candle is increased, and then find the image on a piece of paper. The +size and position of the image depend upon the distance of the object +from the lens (Fig. _77_). By means of a lens one can easily get on a +visiting card a picture of a distant church steeple. + +[Illustration: FIG. 75.--The lens is held in such a position that the +image of the candle is larger than the object.] + +[Illustration: FIG. 76.--The image is smaller than the object.] + +115. The Value of Lenses. If it were not for the fact that a lens +can be held at such a distance from an object as to make the image +larger than the object, it would be impossible for the lens to assist +the watchmaker in locating the small particles of dust which clog the +wheels of the watch. If it were not for the opposite fact--that a lens +can be held at such a distance from the object as to make an image +smaller than the object, it would be impossible to have a photograph +of a tall tree or building unless the photograph were as large as the +tree itself. When a photographer takes a photograph of a person or a +tree, he moves his camera until the image formed by the lens is of the +desired size. By bringing the camera (really the lens of the camera) +near, we obtain a large-sized photograph; by increasing the distance +between the camera and the object, a smaller photograph is obtained. +The mountain top may be so far distant that in the photograph it will +not appear to be greater than a small stone. + +[Illustration: FIG. 77.--The lens is placed in such a position that +the image is about the same size as the object.] + +Many familiar illustrations of lenses, or curved refracting surfaces, +and their work, are known to all of us. Fish globes magnify the fish +that swim within. Bottles can be so shaped that they make the olives, +pickles, and peaches that they contain appear larger than they really +are. The fruit in bottles frequently seems too large to have gone +through the neck of the bottle. The deception is due to refraction, +and the material and shape of the bottle furnish a sufficient +explanation. + +By using combinations of two or more lenses of various kinds, it is +possible to have an image of almost any desired size, and in +practically any desired position. + +116. The Human Eye. In Section 114, we obtained on a movable screen, +by means of a simple lens, an image of a candle. The human eye +possesses a most wonderful lens and screen (Fig. 78); the lens is +called the crystalline lens, and the screen is called the retina. Rays +of light pass from the object through the pupil _P_, go through the +crystalline lens _L_, where they are refracted, and then pass onward +to the retina _R_, where they form a distinct image of the object. + +[Illustration: FIG. 78.--The eye.] + +We learned in Section 114 that a change in the position of the object +necessitated a change in the position of the screen, and that every +time the object was moved the position of the screen had to be altered +before a clear image of the object could be obtained. The retina of +the eye cannot be moved backward and forward, as the screen was, and +the crystalline lens is permanently located directly back of the iris. +How, then, does it happen that we can see clearly both near and +distant objects; that the printed page which is held in the hand is +visible at one second, and that the church spire on the distant +horizon is visible the instant the eyes are raised from the book? How +is it possible to obtain on an immovable screen by means of a simple +lens two distinct images of objects at widely varying distances? + +The answer to these questions is that the crystalline lens changes +shape according to need. The lens is attached to the eye by means of +small muscles, _m_, and it is by the action of these muscles that the +lens is able to become small and thick, or large and thin; that is, to +become more or less curved. When we look at near objects, the muscles +act in such a way that the lens bulges out, and becomes thick in the +middle and of the right curvature to focus the near object upon the +screen. When we look at an object several hundred feet away, the +muscles change their pull on the lens and flatten it until it is of +the proper curvature for the new distance. The adjustment of the +muscles is so quick and unconscious that we normally do not experience +any difficulty in changing our range of view. The ability of the eye +to adjust itself to varying distances is called accommodation. The +power of adjustment in general decreases with age. + +117. Farsightedness and Nearsightedness. A farsighted person is one +who cannot see near objects so distinctly as far objects, and who in +many cases cannot see near objects at all. The eyeball of a farsighted +person is very short, and the retina is too close to the crystalline +lens. Near objects are brought to a focus behind the retina instead of +on it, and hence are not visible. Even though the muscles of +accommodation do their best to bulge and thicken the lens, the rays of +light are not bent sufficiently to focus sharply on the retina. In +consequence objects look blurred. Farsightedness can be remedied by +convex glasses, since they bend the light and bring it to a closer +focus. Convex glasses, by bending the rays and bringing them to a +nearer focus, overbalance a short eyeball with its tendency to focus +objects behind the retina. + +[Illustration: FIG. 79.--The farsighted eye.] + +[Illustration: FIG. 80.--The defect is remedied by convex glasses.] + +A nearsighted person is one who cannot see objects unless they are +close to the eye. The eyeball of a nearsighted person is very wide, +and the retina is too far away from the crystalline lens. Far objects +are brought to a focus in front of the retina instead of on it, and +hence are not visible. Even though the muscles of accommodation do +their best to pull out and flatten the lens, the rays are not +separated sufficiently to focus as far back as the retina. In +consequence objects look blurred. Nearsightedness can be remedied by +wearing concave glasses, since they separate the light and move the +focus farther away. Concave glasses, by separating the rays and making +the focus more distant, overbalance a wide eyeball with its tendency +to focus objects in front of the retina. + +[Illustration: FIG. 81.--The nearsighted eye. The defect is remedied +by concave glasses.] + +118. Headache and Eyes. Ordinarily the muscles of accommodation +adjust themselves easily and quickly; if, however, they do not, +frequent and severe headaches occur as a result of too great muscular +effort toward accommodation. Among young people headaches are +frequently caused by over-exertion of the crystalline muscles. Glasses +relieve the muscles of the extra adjustment, and hence are effective +in eliminating this cause of headache. + +An exact balance is required between glasses, crystalline lens, and +muscular activity, and only those who have studied the subject +carefully are competent to treat so sensitive and necessary a part of +the body as the eye. The least mistake in the curvature of the +glasses, the least flaw in the type of glass (for example, the kind of +glass used), means an improper focus, increased duty for the muscles, +and gradual weakening of the entire eye, followed by headache and +general physical discomfort. + +119. Eye Strain. The extra work which is thrown upon the nervous +system through seeing, reading, writing, and sewing with defective +eyes is recognized by all physicians as an important cause of disease. +The tax made upon the nervous system by the defective eye lessens the +supply of energy available for other bodily use, and the general +health suffers. The health is improved when proper glasses are +prescribed. + +Possibly the greatest danger of eye strain is among school children, +who are not experienced enough to recognize defects in sight. For this +reason, many schools employ a physician who examines the pupils' eyes +at regular intervals. + +The following general precautions are worth observing:-- + +1. Rest the eyes when they hurt, and as far as possible do close work, +such as writing, reading, sewing, wood carving, etc., by daylight. + +2. Never read in a very bright or a very dim light. + +3. If the light is near, have it shaded. + +4. Do not rub the eyes with the fingers. + +5. If eyes are weak, bathe them in lukewarm water in which a pinch of +borax has been dissolved. + + + + +CHAPTER XII + +PHOTOGRAPHY + + +120. The Magic of the Sun. Ribbons and dresses washed and hung in +the sun fade; when washed and hung in the shade, they are not so apt +to lose their color. Clothes are laid away in drawers and hung in +closets not only for protection against dust, but also against the +well-known power of light to weaken color. + +Many housewives lower the window shades that the wall paper may not +lose its brilliancy, that the beautiful hues of velvet, satin, and +plush tapestry may not be marred by loss in brilliancy and sheen. +Bright carpets and rugs are sometimes bought in preference to more +delicately tinted ones, because the purchaser knows that the latter +will fade quickly if used in a sunny room, and will soon acquire a +dull mellow tone. The bright and gay colors and the dull and somber +colors are all affected by the sun, but why one should be affected +more than another we do not know. Thousands of brilliant and dainty +hues catch our eye in the shop and on the street, but not one of them +is absolutely permanent; some may last for years, but there is always +more or less fading in time. + +Sunlight causes many strange, unexplained effects. If the two +substances, chlorine and hydrogen, are mixed in a dark room, nothing +remarkable occurs any more than though water and milk were mixed, but +if a mixture of these substances is exposed to sunlight, a violent +explosion occurs and an entirely new substance is formed, a compound +entirely different in character from either of its components. + +By some power not understood by man, the sun is able to form new +substances. In the dark, chlorine and hydrogen are simply chlorine and +hydrogen; in the sunlight they combine as if by magic into a totally +different substance. By the same unexplained power, the sun frequently +does just the opposite work; instead of combining two substances to +make one new product, the sun may separate or break down some +particular substance into its various elements. For example, if the +sun's rays fall upon silver chloride, a chemical action immediately +begins, and as a result we have two separate substances, chlorine and +silver. The sunlight separates silver chloride into its constituents, +silver and chlorine. + +121. The Magic Wand in Photography. Suppose we coat one side of a +glass plate with silver chloride, just as we might put a coat of +varnish on a chair. We must be very careful to coat the plate in the +dark room,[B] otherwise the sunlight will separate the silver chloride +and spoil our plan. Then lay a horseshoe on the plate for good luck, +and carry the plate out into the light for a second. The light will +separate the silver chloride into chlorine and silver, the latter of +which will remain on the plate as a thin film. All of the plate was +affected by the sun except the portion protected by the horseshoe +which, because it is opaque, would not allow light to pass through and +reach the plate. If now the plate is carried back to the dark room and +the horseshoe is removed, one would expect to see on the plate an +impression of the horseshoe, because the portion protected by the +horseshoe would be covered by silver chloride and the exposed +unprotected portion would be covered by metallic silver. But we are +much disappointed because the plate, when examined ever so carefully, +shows not the slightest change in appearance. The change is there, but +the unaided eye cannot detect the change. Some chemical, the +so-called "developer," must be used to bring out the hidden change and +to reveal the image to our unseeing eyes. There are many different +developers in use, any one of which will effect the necessary +transformation. When the plate has been in the developer for a few +seconds, the silver coating gradually darkens, and slowly but surely +the image printed by the sun's rays appears. But we must not take this +picture into the light, because the silver chloride which was +protected by the horseshoe is still present, and would be strongly +affected by the first glimmer of light, and, as a result, our entire +plate would become similar in character and there would be no contrast +to give an image of the horseshoe on the plate. + +[Footnote B: That is, a room from which ordinary daylight is +excluded.] + +But a photograph on glass, which must be carefully shielded from the +light and admired only in the dark room, would be neither pleasurable +nor practical. If there were some way by which the hitherto unaffected +silver chloride could be totally removed, it would be possible to take +the plate into any light without fear. To accomplish this, the +unchanged silver chloride is got rid of by the process technically +called "fixing"; that is, by washing off the unreduced silver chloride +with a solution such as sodium thiosulphite, commonly known as hypo. +After a bath in the hypo the plate is cleansed in clear running water +and left to dry. Such a process gives a clear and permanent picture on +the plate. + +[Illustration: FIG. 82.--A camera.] + +122. The Camera. A camera (Fig. 82) is a light-tight box containing +a movable convex lens at one end and a screen at the opposite end. +Light from the object to be photographed passes through the lens, +falls upon the screen, and forms an image there. If we substitute for +the ordinary screen a plate or film coated with silver chloride or any +other silver salt, the light which falls upon the sensitive plate and +forms an image there will change the silver chloride and produce a +hidden image. If the plate is then removed from the camera in the +dark, and is treated as described in the preceding Section, the image +becomes visible and permanent. In practice some gelatin is mixed with +the silver salt, and the mixture is then poured over the plate or film +in such a way that a thin, even coating is made. It is the presence of +the gelatin that gives plates a yellowish hue. The sensitive plates +are left to dry in dark rooms, and when the coating has become +absolutely firm and dry, the plates are packed in boxes and sent forth +for sale. + +Glass plates are heavy and inconvenient to carry, so that celluloid +films have almost entirely taken their place, at least for outdoor +work. + +123. Light and Shade. Let us apply the above process to a real +photograph. Suppose we wish to take the photograph of a man sitting in +a chair in his library. If the man wore a gray coat, a black tie, and +a white collar, these details must be faithfully represented in the +photograph. How can the almost innumerable lights and shades be +produced on the plate? + +The white collar would send through the lens the most light to the +sensitive plate; hence the silver chloride on the plate would be most +changed at the place where the lens formed an image of the collar. The +gray coat would not send to the lens so much light as the white +collar, hence the silver chloride would be less affected by the light +from the coat than by that from the collar, and at the place where the +lens produced an image of the coat the silver chloride would not be +changed so much as where the collar image is. The light from the face +would produce a still different effect, since the light from the face +is stronger than the light from the gray coat, but less than that from +a white collar. The face in the image would show less changed silver +chloride than the collar, but more than the coat, because the face is +lighter than the coat, but not so light as the collar. Finally, the +silver chloride would be least affected by the dark tie. The wall +paper in the background would affect the plate according to the +brightness of the light which fell directly upon it and which +reflected to the camera. When such a plate has been developed and +fixed, as described in Section 121, we have the so-called negative +(Fig. 83). The collar is very dark, the black tie and gray coat white, +and the white tidy very dark. + +[Illustration: FIG. 83.--A negative.] + +The lighter the object, such as tidy or collar, the more salt is +changed, or, in other words, the greater the portion of the silver +salt that is affected, and hence the darker the stain on the plate at +that particular spot. The plate shows all gradations of intensity--the +tidy is dark, the black tie is light. The photograph is true as far as +position, form, and expression are concerned, but the actual +intensities are just reversed. How this plate can be transformed into +a photograph true in every detail will be seen in the following +Section. + +124. The Perfect Photograph. Bright objects, such as the sky or a +white waist, change much of the silver chloride, and hence appear +dark on the negative. Dark objects, such as furniture or a black coat, +change little of the chloride, and hence appear light on the negative. +To obtain a true photograph, the negative is placed on a piece of +sensitive photographic paper, or paper coated with a silver salt in +the same manner as the plate and films. The combination is exposed to +the light. The dark portions of the negative will act as obstructions +to the passage of light, and but little light will pass through that +part of the negative to the photographic paper, and consequently but +little of the silver salt on the paper will be changed. On the other +hand, the light portion of the negative will allow free and easy +passage of the light rays, which will fall upon the photographic paper +and will change much more of the silver. Thus it is that dark places +in the negative produce light places in the positive or real +photograph (Fig. 84), and that light places in the negative produce +dark places in the positive; all intermediate grades are likewise +represented with their proper gradations of intensity. + +[Illustration: FIG. 84.--A positive or true photograph.] + +If properly treated, a negative remains good for years, and will serve +for an indefinite number of positives or true photographs. + +125. Light and Disease. The far-reaching effect which light has upon +some inanimate objects, such as photographic films and clothes, leads +us to inquire into the relation which exists between light and living +things. We know from daily observation that plants must have light in +order to thrive and grow. A healthy plant brought into a dark room +soon loses its vigor and freshness, and becomes yellow and drooping. +Plants do not all agree as to the amount of light they require, for +some, like the violet and the arbutus, grow best in moderate light, +while others, like the willows, need the strong, full beams of the +sun. But nearly all common plants, whatever they are, sicken and die +if deprived of sunlight for a long time. This is likewise true in the +animal world. During long transportation, animals are sometimes +necessarily confined in dark cars, with the result that many deaths +occur, even though the car is well aired and ventilated and the food +supply good. Light and fresh air put color into pale cheeks, just as +light and air transform sickly, yellowish plants into hardy green +ones. Plenty of fresh air, light, and pure water are the watchwords +against disease. + +[Illustration: FIG. 85--Stems and leaves of oxalis growing toward the +light.] + +In addition to the plants and animals which we see, there are many +strange unseen ones floating in the atmosphere around us, lying in the +dust of corner and closet, growing in the water we drink, and +thronging decayed vegetable and animal matter. Everyone knows that +mildew and vermin do damage in the home and in the field, but very few +understand that, in addition to these visible enemies of man, there +are swarms of invisible plants and animals some of which do far more +damage, both directly and indirectly, than the seen and familiar +enemies. All such very small plants and animals are known as +_microorganisms_. + +Not all microoerganisms are harmful; some are our friends and are as +helpful to us as are cultivated plants and domesticated animals. Among +the most important of the microoerganisms are bacteria, which include +among their number both friend and foe. In the household, bacteria are +a fruitful source of trouble, but some of them are distinctly friends. +The delicate flavor of butter and the sharp but pleasing taste of +cheese are produced by bacteria. On the other hand, bacteria are the +cause of many of the most dangerous diseases, such as typhoid fever, +tuberculosis, influenza, and la grippe. + +By careful observation and experimentation it has been shown +conclusively that sunlight rapidly kills bacteria, and that it is only +in dampness and darkness that bacteria thrive and multiply. Although +sunlight is essential to the growth of most plants and animals, it +retards and prevents the growth of bacteria. Dirt and dust exposed to +the sunlight lose their living bacteria, while in damp cellars and +dark corners the bacteria thrive, increasing steadily in number. For +this reason our houses should be kept light and airy; blinds should be +raised, even if carpets do fade; it is better that carpets and +furniture should fade than that disease-producing bacteria should find +a permanent abode within our dwellings. Kitchens and pantries in +particular should be thoroughly lighted. Bedclothes, rugs, and +clothing should be exposed to the sunlight as frequently as possible; +there is no better safeguard against bacterial disease than light. In +a sick room sunlight is especially valuable, because it not only kills +bacteria, but keeps the air dry, and new bacteria cannot get a start +in a dry atmosphere. + + + + +CHAPTER XIII + +COLOR + + +126. The Rainbow. One of the most beautiful and well-known phenomena +in nature is the rainbow, and from time immemorial it has been +considered Jehovah's signal to mankind that the storm is over and that +the sunshine will remain. Practically everyone knows that a rainbow +can be seen only when the sun's rays shine upon a mist of tiny drops +of water. It is these tiny drops which by their refraction and their +scattering of light produce the rainbow in the heavens. + +The exquisite tints of the rainbow can be seen if we look at an object +through a prism or chandelier crystal, and a very simple experiment +enables us to produce on the wall of a room the exact colors of the +rainbow in all their beauty. + +[Illustration: FIG. 86.--White light is a mixture of lights of rainbow +colors.] + +127. How to produce Rainbow Colors. _The Spectrum._ If a beam of +sunlight is admitted into a dark room through a narrow opening in the +shade, and is allowed to fall upon a prism, as shown in Figure 86, a +beautiful band of colors will appear on the opposite wall of the room. +The ray of light which entered the room as ordinary sunlight has not +only been refracted and bent from its straight path, but it has been +spread out into a band of colors similar to those of the rainbow. + +Whenever light passes through a prism or lens, it is dispersed or +separated into all the colors which it contains, and a band of colors +produced in this way is called a spectrum. If we examine such a +spectrum we find the following colors in order, each color +imperceptibly fading into the next: violet, indigo, blue, green, +yellow, orange, red. + +128. Sunlight or White Light. White light or sunlight can be +dispersed or separated into the primary colors or rainbow hues, as +shown in the preceding Section. What seems even more wonderful is that +these spectral colors can be recombined so as to make white light. + +If a prism _B_ (Fig. 87) exactly similar to _A_ in every way is placed +behind _A_ in a reversed position, it will undo the dispersion of _A_, +bending upward the seven different beams in such a way that they +emerge together and produce a white spot on the screen. Thus we see, +from two simple experiments, that all the colors of the rainbow may be +obtained from white light, and that these colors may be in turn +recombined to produce white light. + +[Illustration: FIG. 87.--Rainbow colors recombined to form white +light.] + +White light is not a simple light, but is composed of all the colors +which appear in the rainbow. + +129. Color. If a piece of red glass is held in the path of the +colored beam of light formed as in Section 127, all the colors on the +wall will disappear except the red, and instead of a beautiful +spectrum of all colors there will be seen the red color alone. The red +glass does not allow the passage through it of any light except red +light; all other colors are absorbed by the red glass and do not reach +the eye. Only the red ray passes through the red glass, reaches the +eye, and produces a sensation of color. + +If a piece of blue glass is substituted for the red glass, the blue +band remains on the wall, while all the other colors disappear. If +both blue and red pieces of glass are held in the path of the beam, so +that the light must pass through first one and then the other, the +entire spectrum disappears and no color remains. The blue glass +absorbs the various rays with the exception of the blue ones, and the +red glass will not allow these blue rays to pass through it; hence no +light is allowed passage to the eye. + +An emerald looks green because it freely transmits green, but absorbs +the other colors of which ordinary daylight is composed. A diamond +appears white because it allows the passage through it of all the +various rays; this is likewise true of water and window panes. + +Stained-glass windows owe their charm and beauty to the presence in +the glass of various dyes and pigments which absorb in different +amounts some colors from white light and transmit others. These +pigments or dyes are added to the glass while it is in the molten +state, and the beauty of a stained-glass window depends largely upon +the richness and the delicacy of the pigments used. + +130. Reflected Light. _Opaque Objects._ In Section 106 we learned +that most objects are visible to us because of the light diffusely +reflected from them. A white object, such as a sheet of paper, a +whitewashed fence, or a table cloth, absorbs little of the light which +falls upon it, but reflects nearly all, thus producing the sensation +of white. A red carpet absorbs the light rays incident upon it except +the red rays, and these it reflects to the eye. + +Any substance or object which reflects none of the rays which fall +upon it, but absorbs all, appears black; no rays reach the eye, and +there is an absence of any color sensation. Coal and tar and soot are +good illustrations of objects which absorb all the light which falls +upon them. + +131. How and Why Colors Change. _Matching Colors._ Most women prefer +to shop in the morning and early afternoon when the sunlight +illuminates shops and factories, and when gas and electricity do not +throw their spell over colors. Practically all people know that +ribbons and ties, trimmings and dresses, frequently look different at +night from what they do in the daytime. It is not safe to match colors +by artificial light; cloth which looks red by night may be almost +purple by day. Indeed, the color of an object depends upon the color +of the light which falls upon it. Strange sights are seen on the +Fourth of July when variously colored fireworks are blazing. The child +with a white blouse appears first red, then blue, then green, +according as his powders burn red, blue, or green. The face of the +child changes from its normal healthy hue to a brilliant red and then +to ghastly shades. + +Suppose, for example, that a white hat is held at the red end of the +spectrum or in any red light. The characteristics of white objects is +their ability to reflect _all_ the various rays that fall upon them. +Here, however, the only light which falls upon the white hat is red +light, hence the only light which the hat has to reflect is red light +and the hat consequently appears red. Similarly, if a white hat is +placed in a blue light, it will reflect all the light which falls upon +it, namely, blue light, and will appear blue. If a red hat is held in +a red light, it is seen in its proper color. If a red hat is held in a +blue light, it appears black; it cannot reflect any of the blue light +because that is all absorbed and there is no red light to reflect. + +A child wearing a green frock on Independence Day seems at night to be +wearing a black frock, if standing near powders burning with red, +blue, or violet light. + +132. Pure, Simple Colors--Things as they Seem. To the eye white +light appears a simple, single color. It reveals its compound nature +to us only when passed through a prism, when it shows itself to be +compounded of an infinite number of colors which Sir Isaac Newton +grouped in seven divisions: violet, indigo, blue, green, yellow, +orange, and red. + +We naturally ask ourselves whether these colors which compose white +light are themselves in turn compound? To answer that question, let us +very carefully insert a second prism in the path of the rays which +issue from the first prism, carefully barring out the remaining six +kinds of rays. If the red light is compound, it will be broken up into +its constituent parts and will form a typical spectrum of its own, +just as white light did after its passage through a prism. But the red +rays pass through the second prism, are refracted, and bent from this +course, and no new colors appear, no new spectrum is formed. Evidently +a ray of spectrum red is a simple color, not a compound color. + +If a similar experiment is made with the remaining spectrum rays, the +result is always the same: the individual spectrum colors remain +simple, pure colors. _The individual spectrum colors are groups of +simple, pure colors._ + +[Illustration: FIG. 88.--Violet and green give blue. Green, blue, and +red give white.] + +133. Colors not as they Seem--Compound Colors. If one half of a +cardboard disk (Fig. 88) is painted green, and the other half violet, +and the disk is slipped upon a toy top, and spun rapidly, the rotating +disk will appear blue; if red and green are used in the same way +instead of green and violet, the rotating disk will appear yellow. A +combination of red and yellow will give orange. The colors formed in +this way do not appear to the eye different from the spectrum colors, +but they are actually very different. The spectrum colors, as we saw +in the preceding Section, are pure, simple colors, while the colors +formed from the rotating disk are in reality compounded of several +totally different rays, although in appearance the resulting colors +are pure and simple. + +If it were not that colors can be compounded, we should be limited in +hue and shade to the seven spectral colors; the wealth and beauty of +color in nature, art, and commerce would be unknown; the flowers with +their thousands of hues would have a poverty of color undreamed of; +art would lose its magenta, its lilac, its olive, its lavender, and +would have to work its wonders with the spectral colors alone. By +compounding various colors in different proportions, new colors can be +formed to give freshness and variety. If one third of the rotating +disk is painted blue, and the remainder white, the result is lavender; +if fifteen parts of white, four parts of red, and one part of blue are +arranged on the disk, the result is lilac. Olive is obtained from a +combination of two parts green, one part red, and one part black; and +the soft rich shades of brown are all due to different mixtures of +black, red, orange, or yellow. + +134. The Essential Colors. Strange and unexpected facts await us at +every turn in science! If the rotating cardboard disk (Fig. 88) is +painted one third red, one third green, and one third blue, the +resulting color is white. While the mixture of the spectral colors +produces white, it is not necessary to have all of the spectral colors +in order to obtain white; because a mixture of the following colors +alone, red, green, and blue, will give white. Moreover, by the mixture +of these three colors in proper proportions, any color of the +spectrum, such as yellow or indigo or orange, may be obtained. The +three spectral colors, red, green, and blue, are called primary or +essential hues, because all known tints of color may be produced by +the careful blending of blue, green, and red in the proper +proportions; for example, purple is obtained by the blending of red +and blue, and orange by the blending of red and yellow. + +135. Color Blindness. The nerve fibers of the eye which carry the +sensation of color to the brain are particularly sensitive to the +primary colors--red, green, blue. Indeed, all color sensations are +produced by the stimulation of three sets of nerves which are +sensitive to the primary colors. If one sees purple, it is because the +optic nerves sensitive to red and blue (purple equals red plus blue) +have carried their separate messages to the brain, and the blending of +the two distinct messages in the brain has given the sensation of +purple. If a red rose is seen, it is because the optic nerves +sensitive to red have been stimulated and have carried the message to +the brain. + +A snowy field stimulates equally all three sets of optic nerves--the +red, the green, and the blue. Lavender, which is one part blue and +three parts white, would stimulate all three sets of nerves, but with +a maximum of stimulation for the blue. Equal stimulation of the three +sets would give the impression of white. + +A color-blind person has some defect in one or more of the three sets +of nerves which carry the color message to the brain. Suppose the +nerve fibers responsible for carrying the red are totally defective. +If such a person views a yellow flower, he will see it as a green +flower. Yellow contains both red and green, and hence both the red and +green nerve fibers should be stimulated, but the red nerve fibers are +defective and do not respond, the green nerve fibers alone being +stimulated, and the brain therefore interprets green. + +A well-known author gives an amusing incident of a dinner party, at +which the host offered stewed tomato for apple sauce. What color +nerves were defective in the case of the host? + +In some employments color blindness in an employee would be fatal to +many lives. Engineers and pilots govern the direction and speed of +trains and boats largely by the colored signals which flash out in the +night's darkness or move in the day's bright light, and any mistake in +the reading of color signals would imperil the lives of travelers. For +this reason a rigid test in color is given to all persons seeking such +employment, and the ability to match ribbons and yarns of all ordinary +hues is an unvarying requirement for efficiency. + + + + +CHAPTER XIV + +HEAT AND LIGHT AS COMPANIONS + + "The night has a thousand eyes, + And the day but one; + Yet the light of the bright world dies + With the dying sun." + + +136. Most bodies which glow and give out light are hot; the stove +which glows with a warm red is hot and fiery; smoldering wood is black +and lifeless; glowing coals are far hotter than black ones. The +stained-glass window softens and mellows the bright light of the sun, +but it also shuts out some of the warmth of the sun's rays; the shady +side of the street spares our eyes the intense glare of the sun, but +may chill us by the absence of heat. Our illumination, whether it be +oil lamp or gas jet or electric light, carries with it heat; indeed, +so much heat that we refrain from making a light on a warm summer's +night because of the heat which it unavoidably furnishes. + +137. Red a Warm Color. We have seen that heat and light usually go +hand in hand. In summer we lower the shades and close the blinds in +order to keep the house cool, because the exclusion of light means the +exclusion of some heat; in winter we open the blinds and raise the +shades in order that the sun may stream into the room and flood it +with light and warmth. The heat of the sun and the light of the sun +seem boon companions. + +We can show that when light passes through a prism and is refracted, +forming a spectrum, as in Section 127, it is accompanied by heat. If +we hold a sensitive thermometer in the violet end of the spectrum so +that the violet rays fall upon the bulb, the reading of the mercury +will be practically the same as when the thermometer is held in any +dark part of the room; if, however, the thermometer is slowly moved +toward the red end of the spectrum, a change occurs and the mercury +rises slowly but steadily, showing that heat rays are present at the +red end of the spectrum. This agrees with the popular notion, formed +independently of science, which calls the reds the warm colors. Every +one of us associates red with warmth; in the summer red is rarely +worn, it looks hot; but in winter red is one of the most pleasing +colors because of the sense of warmth and cheer it brings. + +_All light rays are accompanied by a small amount of heat, but the red +rays carry the most._ + +What seems perhaps the most unexpected thing, is that the temperature, +as indicated by a sensitive thermometer, continues to rise if the +thermometer is moved just beyond the red light of the spectrum. There +actually seems to be more heat beyond the red than in the red, but if +the thermometer is moved too far away, the temperature again falls. +Later we shall see what this means. + +138. The Energy of the Sun. It is difficult to tell how much of the +energy of the sun is light and how much is heat, but it is easy to +determine the combined effect of heat and light. + +[Illustration: FIG. 89.--The energy of the sun can be measured in heat +units.] + +Suppose we allow the sun's rays to fall perpendicularly upon a metal +cylinder coated with lampblack and filled with a known quantity of +water (Fig. 89); at the expiration of a few hours the temperature of +the water will be considerably higher. Lampblack is a good absorber of +heat, and it is used as a coating in order that all the light rays +which fall upon the cylinder may be absorbed and none lost by +reflection. + +Light and heat rays fall upon the lampblack, pass through the +cylinder, and heat the water. We know that the red light rays have the +largest share toward heating the water, because if the cylinder is +surrounded by blue glass which absorbs the red rays and prevents their +passage into the water, the temperature of the water begins to fall. +That the other light rays have a small share would have been clear +from the preceding Section. + +All the energy of the sunshine which falls upon the cylinder, both as +heat and as light, is absorbed in the form of heat, and the total +amount of this energy can be calculated from the increase in the +temperature of the water. The energy which heated the water would have +passed onward to the surface of the earth if its path had not been +obstructed by the cylinder of water; and we can be sure that the +energy which entered the water and changed its temperature would +ordinarily have heated an equal area of the earth's surface; and from +this, we can calculate the energy falling upon the entire surface of +the earth during any one day. + +Computations based upon this experiment show that the earth receives +daily from the sun the equivalent of 341,000,000,000 horse power--an +amount inconceivable to the human mind. + +Professor Young gives a striking picture of what this energy of the +sun could do. A solid column of ice 93,000,000 miles long and 2-1/4 +miles in diameter could be melted in a single second if the sun could +concentrate its entire power on the ice. + +While the amount of energy received daily from the sun by the earth is +actually enormous, it is small in comparison with the whole amount +given out by the sun to the numerous heavenly bodies which make up the +universe. In fact, of the entire outflow of heat and light, the earth +receives only one part in two thousand million, and this is a very +small portion indeed. + +139. How Light and Heat Travel from the Sun to Us. Astronomers tell +us that the sun--the chief source of heat and light--is 93,000,000 +miles away from us; that is, so far distant that the fastest express +train would require about 176 years to reach the sun. How do heat and +light travel through this vast abyss of space? + +[Illustration: FIG. 90.--Waves formed by a pebble.] + +A quiet pool and a pebble will help to make it clear to us. If we +throw a pebble into a quiet pool (Fig. 90), waves or ripples form and +spread out in all directions, gradually dying out as they become more +and more distant from the pebble. It is a strange fact that while we +see the ripple moving farther and farther away, the particles of water +are themselves not moving outward and away, but are merely bobbing up +and down, or are vibrating. If you wish to be sure of this, throw the +pebble near a spot where a chip lies quiet on the smooth pond. After +the waves form, the chip rides up and down with the water, but does +not move outward; if the water itself were moving outward, it would +carry the chip with it, but the water has no forward motion, and hence +the chip assumes the only motion possessed by the water, that is, an +up-and-down motion. Perhaps a more simple illustration is the +appearance of a wheat field or a lawn on a windy day; the wind sweeps +over the grass, producing in the grass a wave like the water waves of +the ocean, but the blades of grass do not move from their accustomed +place in the ground, held fast as they are by their roots. + +If a pebble is thrown into a quiet pool, it creates ripples or waves +which spread outward in all directions, but which soon die out, +leaving the pool again placid and undisturbed. If now we could quickly +withdraw the pebble from the pool, the water would again be disturbed +and waves would form. If the pebble were attached to a string so that +it could be dropped into the water and withdrawn at regular intervals, +the waves would never have a chance to disappear, because there would +always be a regularly timed definite disturbance of the water. Learned +men tell us that all hot bodies and all luminous bodies are composed +of tiny particles, called molecules, which move unceasingly back and +forth with great speed. In Section 95 we saw that the molecules of all +substances move unceasingly; their speed, however, is not so great, +nor are their motions so regularly timed as are those of the +heat-giving and the light-giving particles. As the particles of the +hot and luminous bodies vibrate with great speed and force they +violently disturb the medium around them, and produce a series of +waves similar to those produced in the water by the pebble. If, +however, a pebble is thrown into the water very gently, the +disturbance is slight, sometimes too slight to throw the water into +waves; in the same way objects whose molecules are in a state of +gentle motion do not produce light. + +The particles of heat-giving and light-giving bodies are in a state of +rapid vibration, and thereby disturb the surrounding medium, which +transmits or conveys the disturbance to the earth or to other objects +by a train of waves. When these waves reach their destination, the +sensation of light or heat is produced. + +We see the water waves, but we can never see with the eye the heat and +light waves which roll in to us from that far-distant source, the sun. +We can be sure of them only through their effect on our bodies, and by +the visible work they do. + +140. How Heat and Light Differ. If heat and light are alike due to +the regular, rapid motion of the particles of a body, and are +similarly conveyed by waves, how is it, then, that heat and light are +apparently so different? + +Light and heat differ as much as the short, choppy waves of the ocean +and the slow, long swell of the ocean, but not more so. The sailor +handles his boat in one way in a choppy sea and in a different way in +a rolling sea, for he knows that these two kinds of waves act +dissimilarly. The long, slow swell of the ocean would correspond with +the longer, slower waves which travel out from the sun, and which on +reaching us are interpreted as heat. The shorter, more frequent waves +of the ocean would typify the short, rapid waves which leave the sun, +and which on reaching us are interpreted as light. + + + + +CHAPTER XV + +ARTIFICIAL LIGHTING + + +141. We seldom consider what life would be without our wonderful +methods of illumination which turn night into day, and prolong the +hours of work and pleasure. Yet it was not until the nineteenth +century that the marvelous change was made from the short-lived candle +to the more enduring oil lamp. Before the coming of the lamp, even in +large cities like Paris, the only artificial light to guide the +belated traveler at night was the candle required to be kept burning +in an occasional window. + +With the invention of the kerosene lamp came more efficient lighting +of home and street, and with the advent of gas and electricity came a +light so effective that the hours of business, manufacture, and +pleasure could be extended far beyond the setting of the sun. + +The production of light by candle, oil, and gas will be considered in +the following paragraphs, while illumination by electricity will be +reserved for a later Chapter. + +142. The Candle. Candles were originally made by dipping a wick into +melting tallow, withdrawing it, allowing the adhered tallow to harden, +and repeating the dipping until a satisfactory thickness was obtained. +The more modern method consists in pouring a fatty preparation into a +mold, at the center of which a wick has been placed. + +The wick, when lighted, burns for a brief interval with a faint, +uncertain light; almost immediately, however, the intensity of the +light increases and the illumination remains good as long as the +candle lasts. The heat of the burning tallow melts more of the tallow +near it, and this liquid fat is quickly sucked up into the burning +wick. The heat of the flame is sufficient to change most of this +liquid into a gas, that is, to vaporize the liquid, and furthermore to +set fire to the gas thus formed. These heated gases burn with a bright +yellow flame. + +143. The Oil Lamp. The simple candle of our ancestors was now +replaced by the oil lamp, which gave a brighter, steadier, and more +permanent illumination. The principle of the lamp is similar to that +of the candle, except that the wick is saturated with kerosene or oil +rather than with fat. The heat from the burning wick is sufficient to +change the oil into a gas and then to set fire to the gas. By placing +a chimney over the burning wick, a constant and uniform draught of air +is maintained around the blazing gases, and hence a steady, +unflickering light is obtained. Gases and carbon particles are set +free by the burning wick. In order that the gases may burn and the +solid particle glow, a plentiful supply of oxygen is necessary. If the +quantity of air is insufficient, the carbon particles remain unburned +and form soot. A lamp "smokes" when the air which reaches the wick is +insufficient to burn the rapidly formed carbon particles; this +explains the danger of turning a lamp wick too high and producing more +carbon particles than can be oxidized by the air admitted through the +lamp chimney. + +One great disadvantage of oil lamps and oil stoves is that they cannot +be carried safely from place to place. It is almost impossible to +carry a lamp without spilling the oil. The flame soon spreads from the +wick to the overflowing oil and in consequence the lamp blazes and an +explosion may result. Candles, on the other hand, are safe from +explosion; the dripping grease is unpleasant but not dangerous. + +The illumination from a shaded oil lamp is soft and agreeable, but the +trimming of the wicks, the refilling of bowls, and the cleaning of +chimneys require time and labor. For this reason, the introduction of +gas met with widespread success. The illumination from an ordinary gas +jet is stronger than that from an ordinary lamp, and the stronger +illumination added to the greater convenience has made gas a very +popular source of light. + +144. Gas Burners and Gas Mantles. For a long time, the only gas +flame used was that in which the luminosity resulted in heating +particles of carbon to incandescence. Recently, however, that has been +widely replaced by use of a Bunsen flame upon an incandescent mantle, +such as the Welsbach. The principle of the incandescent mantle is very +simple. When certain substances, such as thorium and cerium, are +heated, they do not melt or vaporize, but glow with an intense bright +light. Welsbach made use of this fact to secure a burner in which the +illumination depends upon the glowing of an incandescent, solid +mantle, rather than upon the blazing of a burning gas. He made a +cylindrical mantle of thin fabric, and then soaked it in a solution of +thorium and cerium until it became saturated with the chemical. The +mantle thus impregnated with thorium and cerium is placed on the gas +jet, but before the gas is turned on, a lighted match is held to the +mantle in order to burn away the thin fabric. After the fabric has +been burned away, there remains a coarse gauze mantle of the desired +chemicals. If now the gas cock is opened, the escaping gas is ignited, +the heat of the flame will raise the mantle to incandescence and will +produce a brilliant light. A very small amount of burning gas is +sufficient to raise the mantle to incandescence, and hence, by the use +of a mantle, intense light is secured at little cost. The mantle saves +us gas, because the cock is usually "turned on full" whether we use a +plain burner or a mantle burner. But, nevertheless, gas is saved, +because when the mantle is adjusted to the gas jet, the pressure of +the gas is lessened by a mechanical device and hence less gas escapes +and burns. By actual experiment, it has been found that an ordinary +burner consumes about five times as much gas per candle power as the +best incandescent burner, and hence is about five times as expensive. +One objection to the mantles is their tendency to break. But if the +mantles are carefully adjusted on the burner and are not roughly +jarred in use, they last many months; and since the best quality cost +only twenty-five cents, the expense of renewing the mantles is slight. + +145. Gas for Cooking. If a cold object is held in the bright flame +of an ordinary gas jet, it becomes covered with soot, or particles of +unburned carbon. Although the flame is surrounded by air, the central +portion of it does not receive sufficient oxygen to burn up the +numerous carbon particles constantly thrown off by the burning gas, +and hence many carbon particles remain in the flame as glowing, +incandescent masses. That some unburned carbon is present in a flame +is shown by the fact that whenever a cold object is held in the flame, +it becomes "smoked" or covered with soot. If enough air were supplied +to the flame to burn up the carbon as fast as it was set free, there +would be no deposition of soot on objects held over the flame or in +it, because the carbon would be transformed into gaseous matter. + +Unburned carbon would be objectionable in cooking stoves where +utensils are constantly in contact with the flame, and for this reason +cooking stoves are provided with an arrangement by means of which +additional air is supplied to the burning gas in quantities adequate +to insure complete combustion of the rapidly formed carbon particles. +An opening is made in the tube through which gas passes to the burner, +and as the gas moves past this opening, it carries with it a draft of +air. These openings are visible on all gas stoves, and should be kept +clean and free of clogging, in order to insure complete combustion. So +long as the supply of air is sufficient, the flame burns with a dull +blue color, but when the supply falls below that needed for complete +burning of the carbon, the blue color disappears, and a yellow flame +takes its place, and with the yellow flame the deposition of soot is +inevitable. + +146. By-products of Coal Gas. Many important products besides +illuminating gas are obtained from the distillation of soft coal. +Ammonia is made from the liquids which collect in the condensers; +anilin, the source of exquisite dyes, is made from the thick, tarry +distillate, and coke is the residue left in the clay retorts. The coal +tar yields not only anilin, but also carbolic acid and naphthalene, +both of which are commercially valuable, the former as a widely used +disinfectant, and the latter as a popular moth preventive. + +From a ton of good gas-producing coal can be obtained about 10,000 +cubic feet of illuminating gas, and as by-products 6 pounds of +ammonia, 12 gallons of coal tar, and 1300 pounds of coke. + +147. Natural Gas. Animal and vegetable matter buried in the depth of +the earth sometimes undergoes natural distillation, and as a result +gas is formed. The gas produced in this way is called natural gas. It +is a cheap source of illumination, but is found in relatively few +localities and only in limited quantity. + +148. Acetylene. In 1892 it was discovered that lime and coal fused +together in the intense heat of the electric furnace formed a +crystalline, metallic-looking substance called calcium carbide. As a +result of that discovery, this substance was soon made on a large +scale and sold at a moderate price. The cheapness of calcium carbide +has made it possible for the isolated farmhouse to discard oil lamps +and to have a private gas system. When the hard, gray crystals of +calcium carbide are put in water, they give off acetylene, a colorless +gas which burns with a brilliant white flame. If bits of calcium +carbide are dropped into a test tube containing water, bubbles of gas +will be seen to form and escape into the air, and the escaping gas may +be ignited by a burning match held near the mouth of the test tube. +When chemical action between the water and carbide has ceased, and gas +bubbles have stopped forming, slaked lime is all that is left of the +dark gray crystals which were put into the water. + +When calcium carbide is used as a source of illumination, the crystals +are mechanically dropped into a tank containing water, and the gas +generated is automatically collected in a small sliding tank, whence +it passes through pipes to the various rooms. The slaked lime, formed +while the gas was generated, collects at the bottom of the tanks and +is removed from time to time. + +The cost of an acetylene generator is about $50 for a small house, and +the cost of maintenance is not more than that of lamps. The generator +does not require filling oftener than once a week, and the labor is +less than that required for oil lamps. In a house in which there were +twenty burners, the tanks were filled with water and carbide but once +a fortnight. Acetylene is seldom used in large cities, but it is very +widely used in small communities and is particularly convenient in +more or less remote summer residences. + +Electric Lights. The most recent and the most convenient lighting is +that obtained by electricity. A fine, hairlike filament within a glass +bulb is raised to incandescence by the heat of an electric current. +This form of illumination will be considered in connection with +electricity. + + + + +CHAPTER XVI + +MAN'S WAY OF HELPING HIMSELF + + +149. Labor-saving Devices. To primitive man belonged more especially +the arduous tasks of the out-of-door life: the clearing of paths +through the wilderness; the hauling of material; the breaking up of +the hard soil of barren fields into soft loam ready to receive the +seed; the harvesting of the ripe grain, etc. + +[Illustration: FIG. 91.--Prying a stone out of the ground.] + +The more intelligent races among men soon learned to help themselves +in these tasks. For example, our ancestors in the field soon learned +to pry stones out of the ground (Fig. 91) rather than to undertake the +almost impossible task of lifting them out of the earth in which they +were embedded; to swing fallen trees away from a path by means of rope +attached to one end rather than to attempt to remove them +single-handed; to pitch hay rather than to lift it; to clear a field +with a rake rather than with the hands; to carry heavy loads in +wheelbarrows (Fig. 92) rather than on the shoulders; to roll barrels +up a plank (Fig. 93) and to raise weights by ropes. In every case, +whether in the lifting of stones, or the felling of trees, or the +transportation of heavy weights, or the digging of the ground, man +used his brain in the invention of mechanical devices which would +relieve muscular strain and lighten physical labor. + +If all mankind had depended upon physical strength only, the world +to-day would be in the condition prevalent in parts of Africa, Asia, +and South America, where the natives loosen the soil with their hands +or with crude implements (Fig. 94), and transport huge weights on +their shoulders and heads. + +[Illustration: FIG. 92.--The wheelbarrow lightens labor.] + +Any mechanical device (Figs. 95 and 96), whereby man's work can be +more conveniently done, is called a machine; the machine itself never +does any work--it merely enables man to use his own efforts to better +advantage. + +[Illustration: FIG. 93.--Rolling barrels up a plank.] + +150. When do we Work? Whenever, as a result of effort or force, an +object is moved, work is done. If you lift a knapsack from the floor +to the table, you do work because you use force and move the knapsack +through a distance equal to the height of the table. If the knapsack +were twice as heavy, you would exert twice as much force to raise it +to the same height, and hence you would do double the work. If you +raised the knapsack twice the distance,--say to your shoulders +instead of to the level of the table,--you would do twice the work, +because while you would exert the same force you would continue it +through double the distance. + +[Illustration: FIG. 94.--Crude method of farming.] + +Lifting heavy weights through great distances is not the only way in +which work is done. Painting, chopping wood, hammering, plowing, +washing, scrubbing, sewing, are all forms of work. In painting, the +moving brush spreads paint over a surface; in chopping wood, the +descending ax cleaves the wood asunder; in scrubbing, the wet mop +rubbed over the floor carries dirt away; in every conceivable form of +work, force and motion occur. + +A man does work when he walks, a woman does work when she rocks in a +chair--although here the work is less than in walking. On a windy day +the work done in walking is greater than normal. The wind resists our +progress, and we must exert more force in order to cover the same +distance. Walking through a plowed or rough field is much more tiring +than to walk on a smooth road, because, while the distance covered may +be the same, the effort put forth is greater, and hence more work is +done. Always the greater the resistance encountered, the greater the +force required, and hence the greater the work done. + +The work done by a boy who raises a 5-pound knapsack to his shoulder +would be 5x4, or 20, providing his shoulders were 4 feet from the +ground. + +The amount of work done depends upon the force used and the distance +covered (sometimes called displacement), and hence we can say that + + Work = force multiplied by distance, + or _W = f x d_. + +151. Machines. A glance into our machine shops, our factories, and +even our homes shows how widespread is the use of complex machinery. +But all machines, however complicated in appearance, are in reality +but modifications and combinations of one or more of four simple +machines devised long ago by our remote ancestors. These simple +devices are known to-day, as (1) the lever, represented by a crowbar, +a pitchfork; (2) the inclined plane, represented by the plank upon +which barrels are rolled into a wagon; (3) the pulley, represented by +almost any contrivance for the raising of furniture to upper stories; +(4) the wheel and axle, represented by cogwheels and coffee grinders. + +[Illustration: FIG. 95.--Primitive method of grinding corn.] + +Suppose a 600-pound bowlder which is embedded in the ground is needed +for the tower of a building. The problem of the builder is to get the +heavy bowlder out of the ground, to load it on a wagon for +transportation, and finally to raise it to the tower. Obviously, he +cannot do this alone; the greatest amount of force of which he is +capable would not suffice to accomplish any one of these tasks. How +then does he help himself and perform the impossible? Simply, by the +use of some of the machine types mentioned above, illustrations of +which are known in a general way to every schoolboy. The very knife +with which a stick is whittled is a machine. + +[Illustration: FIG. 96.--Separating rice grains by flailing.] + +[Illustration: FIG. 97.--The principle of the lever.] + +152. The Lever. Balance a foot rule, containing a hole at its middle +point _F_, as shown in Figure 97. If now a weight of 1 pound is +suspended from the bar at some point, say 12, the balance is +disturbed, and the bar swings about the point _F_ as a center. The +balance can be regained by suspending an equivalent weight at the +opposite end of the bar, or by applying a 2-pound weight at a point 3 +inches to the left of _F_. In the latter case a force of 1 pound +actually balances a force of 2 pounds, but the 1-pound weight is twice +as far from the point of suspension as is the 2-pound weight. The +small weight makes up in distance what it lacks in magnitude. + +Such an arrangement of a rod or bar is called a lever. In any form of +lever there are only three things to be considered: the point where +the weight rests, the point where the force acts, and the point called +the fulcrum about which the rod rotates. + +The distance from the force to the fulcrum is called the force arm. +The distance from the weight to the fulcrum is called the weight arm; +and it is a law of levers, as well as of all other machines, that the +force multiplied by the length of the force arm must equal the weight +multiplied by the length of the weight arm. + + Force x force arm = weight x weight arm. + +A force of 1 pound at a distance of 6, or with a force arm 6, will +balance a weight of 2 pounds with a weight arm 3; that is, + + 1 x 6 = 2 x 3. + +Similarly a force of 10 pounds may be made to sustain a weight of 100 +pounds, providing the force arm is 10 times longer than the weight +arm; and a force arm of 800 pounds, at a distance of 10 feet from the +fulcrum, may be made to sustain a weight of 8000 pounds, providing the +weight is 1 foot from the fulcrum. + +153. Applications of the Lever. By means of a lever, a 600-pound +bowlder can be easily pried out of the ground. Let the lever, any +strong metal bar, be supported on a stone which serves as fulcrum; +then if a man exerts his force at the end of the rod somewhat as in +Figure 91 (p. 154), the force arm will be the distance from the stone +or fulcrum to the end of the bar, and the weight arm will be the +distance from the fulcrum to the bowlder itself. The man pushes down +with a force of 100 pounds, but with that amount succeeds in prying up +the 600-pound bowlder. If, however, you look carefully, you will see +that the force arm is 6 times as long as the weight arm, so that the +smaller force is compensated for by the greater distance through which +it acts. + +At first sight it seems as though the man's work were done for him by +the machine. But this is not so. The man must lower his end of the +lever 3 feet in order to raise the bowlder 6 inches out of the ground. +He does not at any time exert a large force, but he accomplishes his +purpose by exerting a small force continuously through a +correspondingly greater distance. He finds it easier to exert a force +of 100 pounds continuously until his end has moved 3 feet rather than +to exert a force of 600 pounds on the bowlder and move it 6 inches. + +By the time the stone has been raised the man has done as much work as +though the stone had been raised directly, but his inability to put +forth sufficient muscular force to raise the bowlder directly would +have rendered impossible a result which was easily accomplished when +through the medium of the lever he could extend his small force +through greater distance. + +154. The Wheelbarrow as a Lever. The principle of the lever is +always the same; but the relative position of the important points may +vary. For example, the fulcrum is sometimes at one end, the force at +the opposite end, and the weight to be lifted between them. + +[Illustration: FIG. 98.--A slightly different form of lever.] + +Suspend a stick with a hole at its center as in Figure 98, and hang a +4-pound weight at a distance of 1 foot from the fulcrum, supporting +the load by means of a spring balance 2 feet from the fulcrum. The +pointer on the spring balance shows that the force required to balance +the 4-pound load is but 2 pounds. + +The force is 2 feet from the fulcrum, and the weight (4) is 1 foot +from the fulcrum, so that + + Force x distance = Weight x distance, + or 2 x 2 = 4 x 1. + +Move the 4-pound weight so that it is very near the fulcrum, say but 6 +inches from it; then the spring balance registers a force only one +fourth as great as the weight which it suspends. In other words a +force of 1 at a distance of 24 inches (2 feet) is equivalent to a +force of 4 at a distance of 6 inches. + +[Illustration: FIG. 99.--The wheelbarrow lightened labor.] + +One of the most useful levers of this type is the wheelbarrow (Fig. +99). The fulcrum is at the wheel, the force is at the handles, the +weight is on the wheelbarrow. If the load is halfway from the fulcrum +to the man's hands, the man will have to lift with a force equal to +one half the load. If the load is one fourth as far from the fulcrum +as the man's hands, he will need to lift with a force only one fourth +as great as that of the load. + +[Illustration: FIG. 100.--A modified wheelbarrow.] + +This shows that in loading a wheelbarrow, it is important to arrange +the load as near to the wheel as possible. + +[Illustration: FIG. 101.--The nutcracker is a lever.] + +The nutcracker (Fig. 101) is an illustration of a double lever of the +wheelbarrow kind; the nearer the nut is to the fulcrum, the easier the +cracking. + +[Illustration: FIG. 102.--The hand exerts a small force over a long +distance and draws out a nail.] + +Hammers (Fig. 102), tack-lifters, scissors, forceps, are important +levers, and if you will notice how many different levers (fig. 103) +are used by all classes of men, you will understand how valuable a +machine this simple device is. + +155. The Inclined Plane. A man wishes to load the 600-pound bowlder +on a wagon, and proceeds to do it by means of a plank, as in Figure +93. Such an arrangement is called an inclined plane. + +The advantage of an inclined plane can be seen by the following +experiment. Select a smooth board 4 feet long and prop it so that the +end _A_ (Fig. 104) is 1 foot above the level of the table; the length +of the incline is then 4 times as great as its height. Fasten a metal +roller to a spring balance and observe its weight. Then pull the +roller uniformly upward along the plank and notice what the pull is on +the balance, being careful always to hold the balance parallel to the +incline. + +When the roller is raised along the incline, the balance registers a +pull only one fourth as great as the actual weight of the roller. That +is, when the roller weighs 12, a force of 3 suffices to raise it to +the height _A_ along the incline; but the smaller force must be +applied throughout the entire length of the incline. In many cases, it +is preferable to exert a force of 30 pounds, for example, over the +distance _CA_ than a force of 120 pounds over the shorter distance +_BA_. + +[Illustration: FIG. 103.--Primitive man tried to lighten his task by +balancing his burden.] + +Prop the board so that the end _A_ is 2 feet above the table level; +that is, arrange the inclined plane in such a way that its length is +twice as great as its height. In that case the steady pull on the +balance will be one half the weight of the roller; or a force of 6 +pounds will suffice to raise the 12-pound roller. + +[Illustration: FIG. 104.--Less force is required to raise the roller +along the incline than to raise it to _A_ directly.] + +The steeper the incline, the more force necessary to raise a weight; +whereas if the incline is small, the necessary lifting force is +greatly reduced. On an inclined plane whose length is ten times its +height, the lifting force is reduced to one tenth the weight of the +load. The advantage of an incline depends upon the relative length and +height, or is equal to the ratio of the length to the height. + +156. Application. By the use of an inclined plank a strong man can +load the 600-pound bowlder on a wagon. Suppose the floor of the wagon +is 2 feet above the ground, then if a 6-foot plank is used, 200 pounds +of force will suffice to raise the bowlder; but the man will have to +push with this force against the bowlder while it moves over the +entire length of the plank. + +Since work is equal to force multiplied by distance, the man has done +work represented by 200 x 6, or 1200. This is exactly the amount of +work which would have been necessary to raise the bowlder directly. A +man of even enormous strength could not lift such a weight (600 lb.) +even an inch directly, but a strong man can furnish the smaller force +(200) over a distance of 6 feet; hence, while the machine does not +lessen the total amount of work required of a man, it creates a new +distribution of work and makes possible, and even easy, results which +otherwise would be impossible by human agency. + +157. Railroads and Highways. The problem of the incline is an +important one to engineers who have under their direction the +construction of our highways and the laying of our railroad tracks. It +requires tremendous force to pull a load up grade, and most of us are +familiar with the struggling horse and the puffing locomotive. For +this reason engineers, wherever possible, level down the steep places, +and reduce the strain as far as possible. + +[Illustration: FIG. 105.--A well-graded railroad bed.] + +The slope of the road is called its grade, and the grade itself is +simply the number of feet the hill rises per mile. A road a mile long +(5280 feet) has a grade of 132 if the crest of the hill is 132 feet +above the level at which the road started. + +[Illustration: FIG. 106.--A long, gradual ascent is better than a +shorter, steeper one.] + +In such an incline, the ratio of length to height is 5280 / 132, or +40; and hence in order to pull a train of cars to the summit, the +engine would need to exert a continuous pull equal to one fortieth of +the combined weight of the train. + +If, on the other hand, the ascent had been gradual, so that the grade +was 66 feet per mile, a pull from the engine of one eightieth of the +combined weight would have sufficed to land the train of cars at the +crest of the grade. + +Because of these facts, engineers spend great sums in grading down +railroad beds, making them as nearly level as possible. In mountainous +regions, the topography of the land prevents the elimination of all +steep grades, but nevertheless the attempt is always made to follow +the easiest grades. + +158. The Wedge. If an inclined plane is pushed underneath or within +an object, it serves as a wedge. Usually a wedge consists of two +inclined planes (Fig. 107). + +[Illustration: FIG. 107.--By means of a wedge, the stump is split.] + +A chisel and an ax are illustrations of wedges. Perhaps the most +universal form of a wedge is our common pin. Can you explain how this +is a wedge? + +159. The Screw. Another valuable and indispensable form of the +inclined plane is the screw. This consists of a metal rod around which +passes a ridge, and Figure 108 shows clearly that a screw is simply a +rod around which (in effect) an inclined plane has been wrapped. + +[Illustration: FIG. 108--A screw as a simple machine.] + +The ridge encircling the screw is called the thread, and the distance +between two successive threads is called the pitch. It is easy to see +that the closer the threads and the smaller the pitch, the greater the +advantage of the screw, and hence the less force needed in overcoming +resistance. A corkscrew is a familiar illustration of the screw. + +160. Pulleys. The pulley, another of the machines, is merely a +grooved wheel around which a cord passes. It is sometimes more +convenient to move a load in one direction rather than in another, and +the pulley in its simplest form enables us to do this. In order to +raise a flag to the top of a mast, it is not necessary to climb the +mast, and so pull up the flag; the same result is accomplished much +more easily by attaching the flag to a movable string, somewhat as in +Figure 109, and pulling from below. As the string is pulled down, the +flag rises and ultimately reaches the desired position. + +If we employ a stationary pulley, as in Figure 109, we do not change +the force, because the force required to balance the load is as large +as the load itself. The only advantage is that a force in one +direction may be used to produce motion in another direction. Such a +pulley is known as a fixed pulley. + +[Illustration: FIG. 109.--By means of a pulley, a force in one +direction produces motion in the opposite direction.] + +161. Movable Pulleys. By the use of a movable pulley, we are able to +support a weight by a force equal to only one half the load. In Figure +109, the downward pull of the weight and the downward pull of the hand +are equal; in Figure 110, the spring balance supports only one half +the entire load, the remaining half being borne by the hook to which +the string is attached. The weight is divided equally between the two +parts of the string which passes around the pulley, so that each +strand bears only one half of the burden. + +We have seen in our study of the lever and the inclined plane that an +increase in force is always accompanied by a decrease in distance, and +in the case of the pulley we naturally look for a similar result. If +you raise the balance (Fig. 110) 12 feet, you will find that the +weight rises only 6 feet; if you raise the balance 24 inches, you will +find that the weight rises 12 inches. You must exercise a force of +100 pounds over 12 feet of space in order to raise a weight of 200 +pounds a distance of 6 feet. When we raise 100 pounds through 12 feet +or 200 pounds through 6 feet the total work done is the same; but the +pulley enables those who cannot furnish a force of 200 pounds for the +space of 6 feet to accomplish the task by furnishing 100 pounds for +the space of 12 feet. + +[Illustration: FIG. 110.--A movable pulley lightens labor.] + +162. Combination of Pulleys. A combination of pulleys called block +and tackle is used where very heavy loads are to be moved. In Figure +111 the upper block of pulleys is fixed, the lower block is movable, +and one continuous rope passes around the various pulleys. The load is +supported by 6 strands, and each strand bears one sixth of the load. +If the hand pulls with a force of 1 pound at _P_, it can raise a load +of 6 pounds at _W_, but the hand will have to pull downward 6 feet at +_P_ in order to raise the load at _W_ 1 foot. If 8 pulleys were used, +a force equivalent to one eighth of the load would suffice to move +_W_, but this force would have to be exerted over a distance 8 times +as great as that through which _W_ was raised. + +[Illustration: FIG. 111.--An effective arrangement of pulleys known as +block and tackle.] + +163. Practical Application. In our childhood many of us saw with +wonder the appearance and disappearance of flags flying at the tops +of high masts, but observation soon taught us that the flags were +raised by pulleys. In tenements, where there is no yard for the family +washing, clothes often appear flapping in mid-air. This seems most +marvelous until we learn that the lines are pulled back and forth by +pulleys at the window and at a distant support. By means of pulleys, +awnings are raised and lowered, and the use of pulleys by furniture +movers, etc., is familiar to every wide-awake observer on the streets. + +164. Wheel and Axle. The wheel and axle consists of a large wheel +and a small axle so fastened that they rotate together. + +[Illustration: FIG. 112.--The wheel and axle.] + +When the large wheel makes one revolution, _P_ falls a distance equal +to the circumference of the wheel. While _P_ moves downward, _W_ +likewise moves, but its motion is upward, and the distance it moves is +small, being equal only to the circumference of the small axle. But a +small force at _P_ will sustain a larger force at _W_; if the +circumference of the large wheel is 40 inches, and that of the small +wheel 10 inches, a load of 100 at _W_ can be sustained by a force of +25 at _P_. The increase in force of the wheel and axle depends upon +the relative size of the two parts, that is, upon the circumference of +wheel as compared with circumference of axle, and since the ratio +between circumference and radius is constant, the ratio of the wheel +and axle combination is the ratio of the long radius to the short +radius. + +For example, in a wheel and axle of radii 20 and 4, respectively, a +given weight at _P_ would balance 5 times as great a load at _W_. + +165. Application. _Windlass, Cogwheels._ In the old-fashioned +windlass used in farming districts, the large wheel is replaced by a +handle which, when turned, describes a circle. Such an arrangement is +equivalent to wheel and axle (Fig. 112); the capstan used on shipboard +for raising the anchor has the same principle. The kitchen coffee +grinder and the meat chopper are other familiar illustrations. + +Cogwheels are modifications of the wheel and axle. Teeth cut in _A_ +fit into similar teeth cut in _B_, and hence rotation of _A_ causes +rotation of _B_. Several revolutions of the smaller wheel, however, +are necessary in order to turn the larger wheel through one complete +revolution; if the radius of _A_ is one half that of _B_, two +revolutions of _A_ will correspond to one of _B_; if the radius of _A_ +is one third that of _B_, three revolutions of _A_ will correspond to +one of _B_. + +[Illustration: FIG. 113.--Cogwheels.] + +Experiment demonstrates that a weight _W_ attached to a cogwheel of +radius 3 can be raised by a force _P_, equal to one third of _W_ +applied to a cogwheel of radius 1. There is thus a great increase in +force. But the speed with which _W_ is raised is only one third the +speed with which the small wheel rotates, or increase in power has +been at the decrease of speed. + +This is a very common method for raising heavy weights by small force. + +Cogwheels can be made to give speed at the decrease of force. A heavy +weight _W_ attached to _B_ will in its slow fall cause rapid rotation +of _A_, and hence rapid rise of _P_. It is true that _P_, the load +raised, will be less than _W_, the force exerted, but if speed is our +aim, this machine serves our purpose admirably. + +An extremely important form of wheel and axle is that in which the two +wheels are connected by belts as in Figure 114. Rotation of _W_ +induces rotation of _w_, and a small force at _W_ is able to overcome +a large force at _w_. An advantage of the belt connection is that +power at one place can be transmitted over a considerable distance and +utilized in another place. + +[Illustration: FIG. 114.--By means of a belt, motion can be +transferred from place to place.] + +166. Compound Machines. Out of the few simple machines mentioned in +the preceding Sections has developed the complex machinery of to-day. +By a combination of screw and lever, for example, we obtain the +advantage due to each device, and some compound machines have been +made which combine all the various kinds of simple machines, and in +this way multiply their mechanical advantage many fold. + +A relatively simple complex machine called the crane (Fig. 116) maybe +seen almost any day on the street, or wherever heavy weights are being +lifted. It is clear that a force applied to turn wheel 1 causes a +slower rotation of wheel 3, and a still slower rotation of wheel 4, +but as 4 rotates it winds up a chain and slowly raises _Q_. A very +complex machine is that seen in Figure 117. + +[Illustration: FIG. 115.--A simple derrick for raising weights.] + +[Illustration: FIG. 116.--A traveling crane.] + +167. Measurement of Work. In Section 150, we learned that the amount +of work done depends upon the force exerted, and the distance +covered, or that _W_ = force x distance. A man who raises 5 pounds a +height of 5 feet does far more work than a man who raises 5 ounces a +height of 5 inches, but the product of force by distance is 25 in each +case. There is difficulty because we have not selected an arbitrary +unit of work. The unit of work chosen and in use in practical affairs +is the foot pound, and is defined as the work done when a force of 1 +pound acts through a distance of 1 foot. A man who moves 8 pounds +through 6 feet does 48 foot pounds of work, while a man who moves 8 +ounces (1/2 pound) through 6 inches (1/2 foot) does only one fourth of +a foot pound of work. + +[Illustration: FIG. 117.--A farm engine putting in a crop.] + +168. The Power or the Speed with which Work is Done. A man can load +a wagon more quickly than a growing boy. The work done by the one is +equal to the work done by the other, but the man is more powerful, +because the time required for a given task is very important. An +engine which hoists a 50-pound weight in 1 second is much more +powerful than a man who requires 50 seconds for the same task; hence +in estimating the value of a working agent, whether animal or +mechanical, we must consider not only the work done, but the speed +with which it is done. + +The rate at which a machine is able to accomplish a unit of work is +called _power_, and the unit of power customarily used is the horse +power. Any power which can do 550 foot pounds of work per second is +said to be one horse power (H.P.). This unit was chosen by James Watt, +the inventor of a steam engine, when he was in need of a unit with +which to compare the new source of power, the engine, with his old +source of power, the horse. Although called a horse power it is +greater than the power of an average horse. + +An ordinary man can do one sixth of a horse power. The average +locomotive of a railroad has more than 500 H.P., while the engines of +an ocean liner may have as high as 70,000 H.P. + +169. Waste Work and Efficient Work. In our study of machines we +omitted a factor which in practical cases cannot be ignored, namely, +friction. No surface can be made perfectly smooth, and when a barrel +rolls over an incline, or a rope passes over a pulley, or a cogwheel +turns its neighbor, there is rubbing and slipping and sliding. Motion +is thus hindered, and the effective value of the acting force is +lessened. In order to secure the desired result it is necessary to +apply a force in excess of that calculated. This extra force, which +must be supplied if friction is to be counteracted, is in reality +waste work. + +If the force required by a machine is 150 pounds, while that +calculated as necessary is 100 pounds, the loss due to friction is 50 +pounds, and the machine, instead of being thoroughly efficient, is +only two thirds efficient. + +Machinists make every effort to eliminate from a machine the waste due +to friction, leveling and grinding to the most perfect smoothness and +adjustment every part of the machine. When the machine is in use, +friction may be further reduced by the use of lubricating oil. +Friction can never be totally eliminated, however, and machines of +even the finest construction lose by friction some of their +efficiency, while poorly constructed ones lose by friction as much as +one half of their efficiency. + +170. Man's Strength not Sufficient for Machines. A machine, an inert +mass of metal and wood, cannot of itself do any work, but can only +distribute the energy which is brought to it. Fortunately it is not +necessary that this energy should be contributed by man alone, because +the store of energy possessed by him is very small in comparison with +the energy required to run locomotives, automobiles, sawmills, etc. +Perhaps the greatest value of machines lies in the fact that they +enable man to perform work by the use of energy other than his own. + +[Illustration: FIG. 118.--Man's strength is not sufficient for heavy +work.] + +Figure 118 shows one way in which a horse's energy can be utilized in +lifting heavy loads. Even the fleeting wind has been harnessed by man, +and, as in the windmill, made to work for him (Fig. 119). One sees +dotted over the country windmills large and small, and in Holland, the +country of windmills, the landowner who does not possess a windmill is +poor indeed. + +For generations running water from rivers, streams, and falls has +served man by carrying his logs downstream, by turning the wheels of +his mill, etc.; and in our own day running water is used as an +indirect source of electric lights for street and house, the energy of +the falling water serving to rotate the armature of a dynamo (Section +310). + +[Illustration: FIG. 119.--The windmill pumps water into the troughs +where cattle drink.] + +A more constant source of energy is that available from the burning of +fuel, such as coal and oil. The former is the source of energy in +locomotives, the latter in most automobiles. + +In the following Chapter will be given an account of water, wind, and +fuel as machine feeders. + + + + +CHAPTER XVII + +THE POWER BEHIND THE ENGINE + + +171. Small boys soon learn the power of running water; swimming or +rowing downstream is easy, while swimming or rowing against the +current is difficult, and the swifter the water, the easier the one +and the more difficult the other; the river assists or opposes us as +we go with it or against it. The water of a quiet pool or of a gentle +stream cannot do work, but water which is plunging over a precipice or +dam, or is flowing down steep slopes, may be made to saw wood, grind +our corn, light our streets, run our electric cars, etc. A waterfall, +or a rapid stream, is a great asset to any community, and for this +reason should be carefully guarded. Water power is as great a source +of wealth as a coal bed or a gold mine. + +The most tremendous waterfall in our country is Niagara Falls, which +every minute hurls millions of gallons of water down a 163-foot +precipice. The energy possessed by such an enormous quantity of water +flowing at such a tremendous speed is almost beyond everyday +comprehension, and would suffice to run the engines of many cities far +and near. Numerous attempts to buy from the United States the right to +utilize some of this apparently wasted energy have been made by +various commercial companies. It is fortunate that these negotiations +have been largely fruitless, because much deviation of the water for +commercial uses and the installation of machinery in the vicinity of +the famous falls would greatly detract from the beauty of this +world-known scene, and would rob our country of a natural beauty +unequaled elsewhere. + +[Illustration: FIG. 120.--A mountain stream turns the wheels of the +mill.] + +172. Water Wheels. In Figure 120 the water of a small but rapid +mountain stream is made to rotate a large wheel, which in turn +communicates its motion through belts to a distant sawmill or grinder. +In more level regions huge dams are built which hold back the water +and keep it at a higher level than the wheel; from the dam the water +is conveyed in pipes (flumes) to the paddle wheel which it turns. +Cogwheels or belts connect the paddle wheel with the factory +machinery, so that motion of the paddle wheel insures the running of +the machinery. + +[Illustration: FIG. 121.--The Pelton water wheel.] + +One of the most efficient forms of water wheels is that shown in +Figure 121, and called the Pelton wheel. Water issues in a narrow jet +similar to that of the ordinary garden hose and strikes with great +force against the lower part of the wheel, thereby causing rotation of +the wheel. Belts transfer this motion to the machinery of factory or +mill. + +173. Turbines. The most efficient form of water motor is the +turbine, a strong metal wheel shaped somewhat like a pin wheel, +inclosed in a heavy metal case. + +[Illustration: FIG. 122--A turbine at Niagara Falls.] + +Water is conveyed from a reservoir or dam through a pipe (penstock) to +the turbine case, in which is placed the heavy metal turbine wheel +(Fig. 122). The force of the water causes rotation of the turbine and +of the shaft which is rigidly fastened to it. The water which flows +into the turbine case causes rotation of the wheel, escapes from the +case through openings, and flows into the tail water. + +The power which a turbine can furnish depends upon the quantity of +water and the height of the fall, and also upon the turbine wheel +itself. One of the largest turbines known has a horse power of about +20,000; that is, it is equivalent, approximately, to 20,000 horses. + +174. How much is a Stream Worth? The work which a stream can perform +may be easily calculated. Suppose, for example, that 50,000 pounds of +water fall over a 22-foot dam every second; the power of such a stream +would be 1,100,000 foot pounds per second or 2000 H.P. Naturally, a +part of this power would be lost to use by friction within the +machinery and by leakage, so that the power of a turbine run by a 2000 +H.P. stream would be less than that value. + +Of course, the horse power to be obtained from a stream determines the +size of the paddle wheel or turbine which can be run by it. It would +be possible to construct a turbine so large that the stream would not +suffice to turn the wheel; for this reason, the power of a stream is +carefully determined before machine construction is begun, and the +size of the machinery depends upon the estimates of the water power +furnished by expert engineers. + +A rough estimate of the volume of a stream may be made by the method +described below:-- + +Suppose we allow a stream of water to flow through a rectangular +trough; the speed with which the water flows through the trough can be +determined by noting the time required for a chip to float the length +of the trough; if the trough is 10 feet long and the time required is +5 seconds, the water has a velocity of 2 feet per second. + +[Illustration: FIG. 123.--Estimating the quantity of water which flows +through the trough each second.] + +The quantity of water which flows through the trough each second +depends upon the dimensions of the trough and the velocity of the +water. Suppose the trough is 5 feet wide and 3 feet high, or has a +cross section of 15 square feet. If the velocity of the water were 1 +foot per second, then 15 cubic feet of water would pass any given +point each second, but since the velocity of the water is 2 feet per +second, 30 cubic feet will represent the amount of water which will +flow by a given point in one second. + +175. Quantity of Water Furnished by a River. Drive stakes in the +river at various places and note the time required for a chip to float +from one stake to another. If we know the distance between the stakes +and the time required for the chip to float from one stake to another, +the velocity of the water can be readily determined. + +The width of the stream from bank to bank is easily measured, and the +depth is obtained in the ordinary way by sounding; it is necessary to +take a number of soundings because the bed of the river is by no means +level, and soundings taken at only one level would not give an +accurate estimate. If the soundings show the following depths: 30, 25, +20, 32, 28, the average depth could be taken as 30 + 25 + 20 + 32 + 28 +/ 5, or 27 feet. If, as a result of measuring, the river at a given +point in its course is found to be 27 feet deep and 60 feet wide, the +area of a cross section at that spot would be 1620 square feet, and if +the velocity proved to be 6 feet per second, then the quantity of +water passing in any one second would be 1620 x 6, or 9720 cubic feet. +By experiment it has been found that 1 cu. ft. of water weighs about +62.5 lb. The weight of the water passing each second would therefore +be 62.5 x 9720, or 607,500 lb. If this quantity of water plunges over +a 10-ft. dam, it does 607,500 x 10, or 6,075,000 foot pounds of work +per second, or 11,045 H.P. Such a stream would be very valuable for +the running of machinery. + +176. Windmills. Those of us who have spent our vacation days in the +country know that there is no ready-made water supply there as in the +cities, but that as a rule the farmhouses obtain their drinking water +from springs and wells. In poorer houses, water is laboriously +carried in buckets from the spring or is lifted from the well by the +windlass. In more prosperous houses, pumps are installed; this is an +improvement over the original methods, but the quantity of water +consumed by the average family is so great as to make the task of +pumping an arduous one. + +The average amount of water used per day by one person is 25 gallons. +This includes water for drinking, cooking, dish washing, bathing, +laundry. For a family of five, therefore, the daily consumption would +be 125 gallons; if to this be added the water for a single horse, cow, +and pig, the total amount needed will be approximately 150 gallons per +day. A strong man can pump that amount from an ordinary well in about +one hour, but if the well is deep, more time and strength are +required. + +The invention of the windmill was a great boon to country folks +because it eliminated from their always busy life one task in which +labor and time were consumed. + +177. The Principle of the Windmill. The toy pin wheel is a windmill +in miniature. The wind strikes the sails, and causes rotation; and the +stronger the wind blows, the faster will the wheel rotate. In +windmills, the sails are of wood or steel, instead of paper, but the +principle is identical. + +[Illustration: FIG. 124.--The toy pin wheel is a miniature windmill.] + +As the wheel rotates, its motion is communicated to a mechanical +device which makes use of it to raise and lower a plunger, and hence +as long as the wind turns the windmill, water is raised. The water +thus raised empties into a large tank, built either in the windmill +tower or in the garret of the house, and from the tank the water +flows through pipes to the different parts of the house. On very windy +days the wheel rotates rapidly, and the tank fills quickly; in order +to guard against an overflow from the tank a mechanical device is +installed which stops rotation of the wheel when the tank is nearly +full. The supply tank is usually large enough to hold a supply of +water sufficient for several days, and hence a continuous calm of a +day or two does not materially affect the house flow. When once built, +a windmill practically takes care of itself, except for oiling, and is +an efficient and cheap domestic possession. + +[Illustration: FIG. 125.--The windmill pumps water into the tank.] + +178. Steam as a Working Power. If a delicate vane is held at an +opening from which steam issues, the pressure of the steam will cause +rotation of the vane (Fig. 126), and if the vane is connected with a +machine, work can be obtained from the steam. + +When water is heated in an open vessel, the pressure of its steam is +too low to be of practical value, but if on the contrary water is +heated in an almost closed vessel, its steam pressure is considerable. +If steam at high pressure is directed by nozzles against the blades of +a wheel, rapid rotation of the wheel ensues just as it did in Figure +121, although in this case steam pressure replaces water pressure. +After the steam has spent itself in turning the turbine, it condenses +into water and makes its escape through openings in an inclosing case. +In Figure 127 the protecting case is removed, in order that the form +of the turbine and the positions of the nozzles may be visible. + +[Illustration: FIG. 126.--Steam as a source of power.] + +[Illustration: FIG. 127.--Steam turbine with many blades and 4 +nozzles.] + +A single large turbine wheel may have as many as 800,000 sails or +blades, and steam may pour out upon these from many nozzles. + +The steam turbine is very much more efficient than its forerunner, the +steam engine. The installation of turbines on ocean liners has been +accompanied by great increase in speed, and by an almost corresponding +decrease in the cost of maintenance. + +179. Steam Engines. A very simple illustration of the working of a +steam engine is given in Figure 128. Steam under pressure enters +through the opening _F_, passes through _N_, and presses upon the +piston _M_. As a result _M_ moves downward, and thereby induces +rotation in the large wheel _L_. + +[Illustration: FIG. 128.--The principle of the steam engine.] + +As _M_ falls it drives the air in _D_ out through _O_ and _P_ (the +opening _P_ is not visible in the diagram). + +As soon as this is accomplished, a mechanical device draws up the rod +_E_, which in turn closes the opening _N_, and thus prevents the steam +from passing into the part of _D_ above _M_. + +But when the rod _E_ is in such a position that _N_ is closed, _O_ on +the other hand is open, and steam rushes through it into _D_ and +forces up the piston. This up-and-down motion of the piston causes +continuous rotation of the wheel _L_. If the fire is hot, steam is +formed quickly, and the piston moves rapidly; if the fire is low, +steam is formed slowly, and the piston moves less rapidly. + +The steam engine as seen on our railroad trains is very complex, and +cannot be discussed here; in principle, however, it is identical with +that just described. Figure 129 shows a steam harvester at work on a +modern farm. + +[Illustration: FIG. 129.--Steam harvester at work.] + +In both engine and turbine the real source of power is not the steam +but the fuel, such as coal or oil, which converts the water into +steam. + +180. Gas Engines. Automobiles have been largely responsible for the +gas engine. To carry coal for fuel and water for steam would be +impracticable for most motor cars. Electricity is used in some cars, +but the batteries are heavy, expensive, and short-lived, and are not +always easily replaceable. For this reason gasoline is extensively +used, and in the average automobile the source of power is the force +generated by exploding gases. + +It was discovered some years ago that if the vapor of gasoline or +naphtha was mixed with a definite quantity of air, and a light was +applied to the mixture, an explosion would result. Modern science uses +the force of such exploding gases for the accomplishment of work, such +as running of automobiles and launches. + +In connection with the gasoline supply is a carburetor or sprayer, +from which the cylinder _C_ (Fig. 130) receives a fine mist of +gasoline vapor and air. This mixture is ignited by an automatic, +electric sparking device, and the explosion of the gases drives the +piston _P_ to the right. In the 4-cycle type of gas engines (Fig. +130)--the kind used in automobiles--the four strokes are as follows: +1. The mixture of gasoline and air enters the cylinder as the piston +moves to the right. 2. The valves being closed, the mixture is +compressed as the piston moves to the left. 3. The electric spark +ignites the compressed mixture and drives the piston to the right. 4. +The waste gas is expelled as the piston moves to the left. The exhaust +valve is then closed, the inlet valve opened, and another cycle of +four strokes begins. + +[Illustration: FIG. 130.--The gas engine.] + +The use of gasoline in launches and automobiles is familiar to many. +Not only are launches and automobiles making use of gas power, but the +gasoline engine has made it possible to propel aeroplanes through the +air. + + + + +CHAPTER XVIII + +PUMPS AND THEIR VALUE TO MAN + + +181. "As difficult as for water to run up a hill!" Is there any one +who has not heard this saying? And yet most of us accept as a matter +of course the stream which gushes from our faucet, or give no thought +to the ingenuity which devised a means of forcing water upward through +pipes. Despite the fact that water flows naturally down hill, and not +up, we find it available in our homes and office buildings, in some of +which it ascends to the fiftieth floor; and we see great streams of it +directed upon the tops of burning buildings by firemen in the streets +below. + +In the country, where there are no great central pumping stations, +water for the daily need must be raised from wells, and the supply of +each household is dependent upon the labor and foresight of its +members. The water may be brought to the surface either by laboriously +raising it, bucket by bucket, or by the less arduous method of +pumping. These are the only means possible; even the windmill does not +eliminate the necessity for the pump, but merely replaces the energy +used by man in working it. + +In some parts of our country we have oil beds or wells. But if this +underground oil is to be of service to man, it must be brought to the +surface, and this is accomplished, as in the case of water, by the use +of pumps. + +An old tin can or a sponge may serve to bale out water from a leaking +rowboat, but such a crude device would be absurd if employed on our +huge vessels of war and commerce. Here a rent in the ship's side would +mean inevitable loss were it not possible to rid the ship of the +inflowing water by the action of strong pumps. + +Another and very different use to which pumps are put is seen in the +compression of gases. Air is forced into the tires of bicycles and +automobiles until they become sufficiently inflated to insure comfort +in riding. Some present-day systems of artificial refrigeration +(Section 93) could not exist without the aid of compressed gases. + +Compressed air has played a very important role in mining, being sent +into poorly ventilated mines to improve the condition of the air, and +to supply to the miners the oxygen necessary for respiration. Divers +and men who work under water carry on their backs a tank of compressed +air, and take from it, at will, the amount required. + +There are many forms of pumps, and they serve widely different +purposes, being essential to the operation of many industrial +undertakings. In the following Sections some of these forms will be +studied. + +[Illustration: FIG. 131.--Carrying water home from the spring.] + +182. The Air as Man's Servant. Long before man harnessed water for +turbines, or steam for engines, he made the air serve his purpose, and +by means of it raised water from hidden underground depths to the +surface of the earth; likewise, by means of it, he raised to his +dwelling on the hillside water from the stream in the valley below. +Those who live in cities where running water is always present in the +home cannot realize the hardship of the days when this "ready-made" +supply did not exist, but when man laboriously carried to his +dwelling, from distant spring and stream, the water necessary for the +daily need. + +What are the characteristics of the air which have enabled man to +accomplish these feats? They are well known to us and may be briefly +stated as follows:-- + +(1) Air has weight, and 1 cubic foot of air, at atmospheric pressure, +weighs 1-1/4 ounces. + +(2) The air around us presses with a force of about 15 pounds upon +every square inch of surface that it touches. + +(3) Air is elastic; it can be compressed, as in the balloon or bicycle +tire, but it expands immediately when pressure is reduced. As it +expands and occupies more space, its pressure falls and it exerts less +force against the matter with which it comes in contact. If, for +example, 1 cubic foot of air is allowed to expand and occupy 2 cubic +feet of space, the pressure which it exerts is reduced one half. When +air is compressed, its pressure increases, and it exerts a greater +force against the matter with which it comes in contact. If 2 cubic +feet of air are compressed to 1 cubic foot, the pressure of the +compressed air is doubled. (See Section 89.) + +[Illustration: FIG. 132.--The atmosphere pressing downward on _a_ +pushes water after the rising piston _b_.] + +183. The Common Pump or Lifting Pump. Place a tube containing a +close-fitting piston in a vessel of water, as shown in Figure 132. +Then raise the piston with the hand and notice that the water rises in +the piston tube. The rise of water in the piston tube is similar to +the raising of lemonade through a straw (Section 77). The atmosphere +presses with a force of 15 pounds upon every square inch of water in +the large vessel, and forces some of it into the space left vacant by +the retreating piston. The common pump works in a similar manner. It +consists of a piston or plunger which moves back and forth in an +air-tight cylinder, and contains an outward opening valve through +which water and air can pass. From the bottom of the cylinder a tube +runs down into the well or reservoir, and water from the well has +access to the cylinder through another outward-moving valve. In +practice the tube is known as the suction pipe, and its valve as the +suction valve. + +In order to understand the action of a pump, we will suppose that no +water is in the pump, and we will pump until a stream issues from the +spout. The various stages are represented diagrammatically by Figure +133. In (1) the entire pump is empty of water but full of air at +atmospheric pressure, and both valves are closed. In (2) the plunger +is being raised and is lifting the column of air that rests on it. The +air and water in the inlet pipe, being thus partially relieved of +downward pressure, are pushed up by the atmospheric pressure on the +surface of the water in the well. When the piston moves downward as in +(3), the valve in the pipe closes by its own weight, and the air in +the cylinder escapes through the valve in the plunger. In (4) the +piston is again rising, repeating the process of (2). In (5) the +process of (3) is being repeated, but water instead of air is escaping +through the valve in the plunger. In (6) the process of (2) is being +repeated, but the water has reached the spout and is flowing out. + +[Illustration: FIG. 133. Diagram of the process of pumping.] + +After the pump is in condition (6), motion of the plunger is followed +by a more or less regular discharge of water through the spout, and +the quantity of water which gushes forth depends upon the speed with +which the piston is moved. A strong man giving quick strokes can +produce a large flow; a child, on the other hand, is able to produce +only a thin stream. Whoever pumps must exert sufficient force to lift +the water from the surface of the well to the spout exit. For this +reason the pump has received the name of _lifting pump_. + +[Illustration: FIG. 134.--Force pump.] + +184. The Force Pump. In the common pump, water cannot not be raised +higher than the spout. In many cases it is desirable to force water +considerably above the pump itself, as, for instance, in the fire +hose; under such circumstances a type of pump is employed which has +received the name of _force pump_. This differs but little from the +ordinary lift pump, as a reference to Figure 134 will show. Here both +valves are placed in the cylinder, and the piston is solid, but the +principle is the same as in the lifting pump. + +An upward motion of the plunger allows water to enter the cylinder, +and the downward motion of the plunger drives water through _E_. (Is +this true for the lift pump as well?) Since only the downward motion +of the plunger forces water through _E_, the discharge is intermittent +and is therefore not practical for commercial purposes. In order to +convert this intermittent discharge into a steady stream, an air +chamber is installed near the discharge tube, as in Figure 135. The +water forced into the air chamber by the downward-moving piston +compresses the air and increases its pressure. The pressure of the +confined air reacts against the water and tends to drive it out of the +chamber. Hence, even when the plunger is moving upward, water is +forced through the pipe because of the pressure of the compressed +air. In this way a continuous flow is secured. + +[Illustration: FIG 135.--The air chamber _A_ insures a continuous flow +of water.] + +The height to which the water can be forced in the pipe depends upon +the size and construction of the pump and upon the force with which +the plunger can be moved. The larger the stream desired and the +greater the height to be reached, the stronger the force needed and +the more powerful the construction necessary. + +The force pump gets its name from the fact that the moving piston +drives or forces the water through the discharge tube. + +185. Irrigation and Drainage. History shows that the lifting pump +has been used by man since the fourth century before Christ; for many +present-day enterprises this ancient form of pump is inconvenient and +impracticable, and hence it has been replaced in many cases by more +modern types, such as rotary and centrifugal pumps (Fig. 136). In +these forms, rapidly rotating wheels lift the water and drive it +onward into a discharge pipe, from which it issues with great force. +There is neither piston nor valve in these pumps, and the quantity of +water raised and the force with which it is driven through the pipes +depends solely upon the size of the wheels and the speed with which +they rotate. + +Irrigation, or the artificial watering of land, is of the greatest +importance in those parts of the world where the land is naturally too +dry for farming. In the United States, approximately two fifths of the +land area is so dry as to be worthless for agricultural purposes +unless artificially watered. In the West, several large irrigating +systems have been built by the federal government, and at present +about ten million acres of land have been converted from worthless +farms into fields rich in crops. Many irrigating systems use +centrifugal pumps to force water over long distances and to supply it +in quantities sufficient for vast agricultural needs. In many regions, +the success of a farm or ranch depends upon the irrigation furnished +in dry seasons, or upon man's ability to drive water from a region of +abundance to a remote region of scarcity. + +[Illustration: FIG. 136.--Centrifugal pump with part of the casing] +cut away to show the wheel. + +[Illustration: FIG. 137.--Agriculture made possible by irrigation.] + +The draining of land is also a matter of considerable importance; +swamps and marshes which were at one time considered useless have been +drained and then reclaimed and converted into good farming land. The +surplus water is best removed by centrifugal pumps, since sand and +sticks which would clog the valves of an ordinary pump are passed +along without difficulty by the rotating wheel. + +[Illustration: FIG. 138.--Rice for its growth needs periodical +flooding, and irrigation often supplies the necessary water.] + +186. Camping.--Its Pleasures and its Dangers. The allurement of a +vacation camp in the heart of the woods is so great as to make many +campers ignore the vital importance of securing a safe water supply. A +river bank may be beautiful and teeming with diversions, but if the +river is used as a source of drinking water, the results will almost +always be fatal to some. The water can be boiled, it is true, but few +campers are willing to forage for the additional wood needed for this +apparently unnecessary requirement; then, too, boiled water does not +cool readily in summer, and hence is disagreeable for drinking +purposes. + +The only safe course is to abandon the river as a source of drinking +water, and if a spring cannot be found, to drive a well. In many +regions, especially in the neighborhood of streams, water can be +found ten or fifteen feet below the surface. Water taken from such a +depth has filtered through a bed of soil, and is fairly safe for any +purpose. Of course the deeper the well, the safer will be the water. +With the use of such a pump as will be described, campers can, without +grave danger, throw dish water, etc., on the ground somewhat remote +from the camp; this may not injure their drinking water because the +liquids will slowly seep through the ground, and as they filter +downward will lose their dangerous matter. All the water which reaches +the well pipes will have filtered through the soil bed and therefore +will probably be safe. + +But while the careless disposal of wastes may not spoil the drinking +water (in the well to be described), other laws of health demand a +thoughtful disposal of wastes. The malarial mosquito and the typhoid +fly flourish in unhygienic quarters, and the only way to guard against +their dangers is to allow them neither food nor breeding place. + +The burning of garbage, the discharge of waters into cesspools, or, in +temporary camps, the discharge of wastes to distant points through the +agency of a cheap sewage pipe will insure safety to campers, will +lessen the trials of flies and mosquitoes, and will add but little to +the expense. + +187. A Cheap Well for Campers. A two-inch galvanized iron pipe with +a strong, pointed end containing small perforations is driven into the +ground with a sledge hammer. After it has penetrated for a few feet, +another length is added and the whole is driven down, and this is +repeated until water is reached. A cheap pump is then attached to the +upper end of the drill pipe and serves to raise the water. During the +drilling, some soil particles get into the pipe through the +perforations, and these cloud the water at first; but after the pipe +has once been cleaned by the upward-moving water, the supply remains +clear. The flow from such a well is naturally small; first, because +water is not abundant near the surface of the earth, and second, +because cheap pumps are poorly constructed and cannot raise a large +amount. But the supply will usually be sufficient for the needs of +simple camp life, and many a small farm uses this form of well, not +only for household purposes, but for watering the cattle in winter. + +If the cheapness of such pumps were known, their use would be more +general for temporary purposes. The cost of material need not exceed +$5 for a 10-foot well, and the driving of the pipe could be made as +much a part of the camping as the pitching of the tent itself. If the +camping site is abandoned at the close of the vacation, the pump can +be removed and kept over winter for use the following summer in +another place. In this way the actual cost of the water supply can be +reduced to scarcely more than $3, the removable pump being a permanent +possession. In rocky or mountain regions the driven well is not +practicable, because the driving point is blunted and broken by the +rock and cannot pierce the rocky beds of land. + +[Illustration: FIG. 139--A driven well.] + +[Illustration: FIG. 140.--Diagram showing how supplying a city with +good water lessens sickness and death. The lines _b_ show the relative +number of people who died of typhoid fever before the water was +filtered; the lines _a_ show the numbers who died after the water was +filtered. The figures are the number of typhoid deaths occurring +yearly out of 100,000 inhabitants.] + +188. Our Summer Vacation. It has been asserted by some city health +officials that many cases of typhoid fever in cities can be traced to +the unsanitary conditions existing in summer resorts. The drinking +water of most cities is now under strict supervision, while that of +isolated farms, of small seaside resorts, and of scattered mountain +hotels is left to the care of individual proprietors, and in only too +many instances receives no attention whatever. The sewage disposal is +often inadequate and badly planned, and the water becomes dangerously +contaminated. A strong, healthy person, with plenty of outdoor +exercise and with hygienic habits, may be able to resist the disease +germs present in the poor water supply; more often the summer guests +carry back with them to their winter homes the germs of disease, and +these gain the upper hand under the altered conditions of city and +business life. It is not too much to say that every man and woman +should know the source of his summer table water and the method of +sewage disposal. If the conditions are unsanitary, they cannot be +remedied at once, but another resort can be found and personal danger +can be avoided. Public sentiment and the loss of trade will go far in +furthering an effort toward better sanitation. + +In the driven well, water cannot reach the spout unless it has first +filtered through the soil to the depth of the driven pipe; after such +a journey it is fairly safe, unless very large quantities of sewage +are present; generally speaking, such a depth of soil is able to +filter satisfactorily the drainage of the limited number of people +which a driven well suffices to supply. + +[Illustration: FIG. 141.--A deep well with the piston in the water.] + +Abundant water is rarely reached at less than 75 feet, and it would +usually be impossible to drive a pipe to such a depth. When a large +quantity of water is desired, strong machines drill into the ground +and excavate an opening into which a wide pipe can be lowered. I +recently spent a summer in the Pocono Mountains and saw such a well +completed. The machine drilled to a depth of 250 feet before much +water was reached and to over 300 feet before a flow was obtained +sufficient to satisfy the owner. The water thus obtained was to be the +sole water supply of a hotel accommodating 150 persons; the proprietor +calculated that the requirements of his guests, for bath, toilet, +laundry, kitchen, etc., and the domestics employed to serve them, +together with the livery at their disposal, demanded a flow of 10 +gallons per minute. The ground was full of rock and difficult to +penetrate, and it required 6 weeks of constant work for two skilled +men to drill the opening, lower the suction pipe, and install the +pump, the cost being approximately $700. + +[Illustration: FIG. 142.--Showing how drinking water can be +contaminated from cesspool _(c)_ and wash water _(w)_.] + +The water from such a well is safe and pure except under the +conditions represented in Figure 142. If sewage or slops be poured +upon the ground in the neighborhood of the well, the liquid will seep +through the ground and some may make its way into the pump before it +has been purified by the earth. The impure liquid will thus +contaminate the otherwise pure water and will render it decidedly +harmful. For absolute safety the sewage discharge should be at least +75 feet from the well, and in large hotels, where there is necessarily +a large quantity of sewage, the distance should be much greater. As +the sewage seeps through the ground it loses its impurities, but the +quantity of earth required to purify it depends upon its abundance; a +small depth of soil cannot take care of an indefinite amount of +sewage. Hence, the greater the number of people in a hotel, or the +more abundant the sewage, the greater should be the distance between +well and sewer. + +By far the best way to avoid contamination is to see to it that the +sewage discharges into the ground _below_ the well; that is, to dig +the well in such a location that the sewage drainage will be away from +the well. + +In cities and towns and large summer communities, the sewage of +individual buildings drains into common tanks erected at public +expense; the contents of these are discharged in turn into harbors and +streams, or are otherwise disposed of at great expense, although they +contain valuable substances. It has been estimated that the drainage +or sewage of England alone would be worth $ 80,000,000 a year if used +as fertilizer. + +A few cities, such as Columbus and Cleveland, Ohio, realize the need +of utilizing this source of wealth, and by chemical means deodorize +their sewage and change it into substances useful for agricultural and +industrial purposes. There is still a great deal to be learned on this +subject, and it is possible that chemically treated sewage may be made +a source of income to a community rather than an expense. + +189. Pumps which Compress Air. The pumps considered in the preceding +Sections have their widest application in agricultural districts, +where by means of them water is raised to the surface of the earth or +is pumped into elevated tanks. From a commercial and industrial +standpoint a most important class of pump is that known as the +compression type; in these, air or any other gas is compressed rather +than rarefied. + +Air brakes and self-opening and self-closing doors on cars are +operated by means of compression pumps. The laying of bridge and pier +foundations, in fact all work which must be done under water, is +possible only through the agency of compression pumps. Those who have +visited mines, and have gone into the heart of the underground +labyrinth, know how difficult it is for fresh air to make its way to +the miners. Compression pumps have eliminated this difficulty, and +to-day fresh air is constantly pumped into the mines to supply the +laborers there. Agricultural methods also have been modified by the +compression pump. The spraying of trees (Fig. 143), formerly done +slowly and laboriously, is now a relatively simple matter. + +[Illustration: FIG. 143.--Spraying trees by means of a compression +pump.] + +190. The Bicycle Pump. The bicycle pump is the best known of all +compression pumps. Here, as in other pumps of its type, the valves +open inward rather than outward. When the piston is lowered, +compressed air is driven through the rubber tubing, pushes open an +inward-opening valve in the tire, and thus enters the tire. When the +piston is raised, the lower valve closes, the upper valve is opened +by atmospheric pressure, and air from outside enters the cylinder; the +next stroke of the piston drives a fresh supply of air into the tire, +which thus in time becomes inflated. In most cheap bicycle pumps, the +piston valve is replaced by a soft piece of leather so attached to the +piston that it allows air to slip around it and into the cylinder, but +prevents its escape from the cylinder (Fig. 144). + +[Illustration: FIG. 144.--The bicycle foot pump.] + +191. How a Man works under Water. Place one end of a piece of glass +tube in a vessel of water and notice that the water rises in the tube +(Fig. 145). Blow into the tube and see whether you can force the water +wholly or partially down the tube. If the tube is connected to a small +compression pump, sufficient air can be sent into the tube to cause +the water to sink and to keep the tube permanently clear of water. +This is, in brief, the principle employed for work under water. A +compression pump forces air through a tube into the chamber in which +men are to work (Fig. 146). The air thus furnished from above supplies +the workmen with oxygen, and by its pressure prevents water from +entering the chamber. When the task has been completed, the chamber is +raised and later lowered to a new position. + +[Illustration: FIG. 145.--Water does not enter the tube as long as we +blow into it.] + +Figure 147 shows men at work on a bridge foundation. Workmen, tools, +and supplies are lowered in baskets through a central tube _BC_ +provided with an air chamber _L_, having air-tight gates at _A_ and +_A'_. The gate _A_ is opened and workmen enter the air chamber. The +gate _A_ is then closed and the gate _A'_ is opened slowly to give the +men time to get accustomed to the high pressure in _B_, and then the +men are lowered to the bottom. Excavated earth is removed in a similar +manner. Air is supplied through a tube _DD_. Such an arrangement for +work under water is called a caisson. It is held in position by a mass +of concrete _EE_. + +[Illustration: FIG. 146--The principle of work under water.] + +[Illustration: FIG. 147--Showing how men can work under water.] + +In many cases men work in diving suits rather than in caissons; these +suits are made of rubber except for the head piece, which is of metal +provided with transparent eyepieces. Air is supplied through a +flexible tube by a compression pump. The diver sometimes carries on +his back a tank of compressed air, from which the air escapes through +a tube to the space between the body and the suit. When the air has +become foul, the diver opens a valve in his suit and allows it to pass +into the water, at the same time admitting a fresh supply from the +tank. The valve opens outward from the body, and hence will allow of +the exit of air but not of the entrance of water. When the diver +ceases work and desires to rise to the surface, he signals and is +drawn up by a rope attached to the suit. + +192. Combination of Pumps. In many cases the combined use of both +exhaust and compression pumps is necessary to secure the desired +result; as, for example, in pneumatic dispatch tubes. These are +employed in the transportation of letters and small packages from +building to building or between parts of the same building. A pump +removes air from the part of the tube ahead of the package, and thus +reduces the resistance, while a compression pump forces air into the +tube behind the package and thus drives it forward with great speed. + + + + +CHAPTER XIX + +THE WATER PROBLEM OF A LARGE CITY + + +193. It is by no means unusual for the residents of a large city or +town to receive through the newspapers a notification that the city +water supply is running low and that economy should be exercised in +its use. The problem of supplying a large city with an abundance of +pure water is among the most difficult tasks which city officials have +to perform, and is one little understood and appreciated by the +average citizen. + +Intense interest in personal and domestic affairs is natural, but +every citizen, rich or poor, should have an interest in civic affairs +as well, and there is no better or more important place to begin than +with the water supply. One of the most stirring questions in New York +to-day has to do with the construction of huge aqueducts designed to +convey to the residents of the city, water from the distant Catskill +Mountains. The growth of the population has been so phenomenally rapid +that the combined output of all available near-by sources does not +suffice to meet the increasing consumption. + +Where does your city obtain its water? Does it bring it to its +reservoirs in the most economic way possible, and is there any +legitimate excuse for the scarcity of water which many communities +face in dry seasons? + +194. Two Possibilities. Sometimes a city is fortunate enough to be +situated near hills and mountains through which streams flow, and in +that case the water problem is simple. In such a case all that is +necessary is to run pipes, usually underground, from the elevated +lakes or streams to the individual houses, or to common reservoirs +from which it is distributed to the various buildings. + +[Illustration: FIG. 148.--The elevated mountain lake serves as a +source of water.] + +Figure 148 illustrates in a simple way the manner in which a mountain +lake may serve to supply the inhabitants of a valley. The city of +Denver, for example, is surrounded by mountains abounding in streams +of pure, clear water; pipes convey the water from these heights to the +city, and thus a cheap and adequate flow is obtained. Such a system is +known as the gravity system. The nearer and steeper the elevation, the +greater the force with which the water flows through the valley pipes, +and hence the stronger the discharge from the faucets. + +Relatively few cities and towns are so favorably situated as regards +water; more often the mountains are too distant, or the elevation is +too slight, to be of practical value. Cities situated in plains and +remote from mountains are obliged to utilize the water of such streams +as flow through the land, forcing it to the necessary height by means +of pumps. Streams which flow through populated regions are apt to be +contaminated, and hence water from them requires public filtration. +Cities using such a water supply thus have the double expense of +pumping and filtration. + +195. The Pressure of Water. No practical business man would erect a +turbine or paddle wheel without calculating in advance the value of +his water power. The paddle wheel might be so heavy that the stream +could not turn it, or so frail in comparison with the water force that +the stream would destroy it. In just as careful a manner, the size and +the strength of municipal reservoirs and pumps must be calculated. The +greater the quantity of water to be held in the reservoir, the heavier +are the walls required; the greater the elevation of the houses, the +stronger must be the pumps and the engines which run them. + +In order to understand how these calculations are made, we must study +the physical characteristics of water just as we studied the physical +characteristics of air. + +When we measure water, we find that 1 cubic foot of it weighs about +62.5 pounds; this is equivalent to saying that water 1 foot deep +presses on the bottom of the containing vessel with a force of 62.5 +pounds to the square foot. If the water is 2 feet deep, the load +supported by the vessel is doubled, and the pressure on each square +foot of the bottom of the vessel will be 125 pounds, and if the water +is 10 feet deep, the load borne by each square foot will be 625 +pounds. The deeper the water, the greater will be the weight sustained +by the confining vessel and the greater the pressure exerted by the +water. + +[Illustration: FIG. 149.--Water 1 foot deep exerts a pressure of 62.5 +pounds a square foot.] + +Since the pressure borne by 1 square foot of surface is 62.5 pounds, +the pressure supported by 1 square inch of surface is 1/144 of 62.5 +pounds, or .43 pound, nearly 1/2 pound. Suppose a vessel held water to +the depth of 10 feet, then upon every square inch of the bottom of +that vessel there would be a pressure of 4.34 pounds. If a one-inch +tap were inserted in the bottom of the vessel so that the water flowed +out, it would gush forth with a force of 4.34 pounds. If the water +were 20 feet deep, the force of the outflowing water would be twice as +strong, because the pressure would be doubled. But the flow would not +remain constant, because as the water leaves the outlet, less and less +of it remains in the vessel, and hence the pressure gradually sinks +and the flow drops correspondingly. + +In seasons of prolonged drought, the streams which feed a city +reservoir are apt to contain less than the usual amount of water, +hence the level of the water supply sinks, the pressure at the outlet +falls, and the force of the outflowing water is lessened (Fig. 150). + +[Illustration: FIG. 150.--The pressure at an outlet decreases as the +level of the water supply sinks.] + +196. Why the Water Supply is not uniform in All Parts of the City. +In the preceding Section, we saw that the flow from a faucet depends +upon the height of the reserve water above the tap. Houses on a level +with the main supply pipes (Figs. 148 and 151) have a strong flow +because the water is under the pressure of a column _A_; houses +situated on elevation _B_ have less flow, because the water is under +the pressure of a shorter column _B_; and houses at a considerable +elevation _C_ have a less rapid flow corresponding to the diminished +depth _(C)_. + +Not only does the flow vary with the elevation of the house, but it +varies with the location of the faucet within the house. Unless the +reservoir is very high, or the pumps very powerful, the flow on the +upper floors is noticeably less than that in the cellar, and in the +upper stories of some high building the flow is scarcely more than a +feeble trickle. + +[Illustration: FIG. 151.--Water pressure varies in different parts of +a water system.] + +When the respective flows at _A_, _B_, and _C_ (Fig. 151) are measured, +they are found to be far lower than the pressures which columns of +water of the heights _A_, _B_, and _C_ have been shown by actual +demonstration to exert. This is because water, in flowing from place +to place, expends force in overcoming the friction of the pipes and +the resistance of the air. The greater the distance traversed by the +water in its journey from reservoir to faucet, the greater the waste +force and the less the final flow. + +In practice, large mains lead from the reservoir to the city, smaller +mains convey the water to the various sections of the city, and +service pipes lead to the individual house taps. During this long +journey, considerable force is expended against friction, and hence +the flow at a distance from the reservoir falls to but a fraction of +its original strength. For this reason, buildings situated near the +main supply have a much stronger flow (Fig. 152) than those on the +same level but remote from the supply. Artificial reservoirs are +usually constructed on the near outskirts of a town in order that the +frictional force lost in transmission may be reduced to a minimum. + +[Illustration: FIG. 152.--The more distant the fountain, the weaker +the flow.] + +In the case of a natural reservoir, such as an elevated lake or +stream, the distance cannot be planned or controlled. New York, for +example, will secure an abundance of pure water from the Catskill +Mountains, but it will lose force in transmission. Los Angeles is +undertaking one of the greatest municipal projects of the day. Huge +aqueducts are being built which will convey pure mountain water a +distance of 250 miles, and in quantities sufficient to supply two +million people. According to calculations, the force of the water will +be so great that pumps will not be needed. + +197. Why Water does not always flow from a Faucet. Most of us have +at times been annoyed by the inability to secure water on an upper +story, because of the drawing off of a supply on a lower floor. +During the working hours of the day, immense quantities of water are +drawn off from innumerable faucets, and hence the quantity in the +pipes decreases considerably unless the supply station is able to +drive water through the vast network of pipes as fast as it is drawn +off. Buildings at a distance from the reservoir suffer under such +circumstances, because while the diminished pressure is ordinarily +powerful enough to supply the lower floors, it is frequently too weak +to force a continuous stream to high levels. At night, however, and +out of working hours, few faucets are open, less water is drawn off at +any one time, and the intricate pipes are constantly full of water +under high pressure. At such times, a good flow is obtainable even on +the uppermost floors. + +In order to overcome the disadvantage of a decrease in flow during the +day, standpipes (Fig. 153) are sometimes placed in various sections. +These are practically small steel reservoirs full of water and +connecting with the city pipes. During "rush" hours, water passes from +these into the communicating pipes and increases the available supply, +while during the night, when the faucets are turned off, water +accumulates in the standpipe against the next emergency (Figs. 151 and +154). The service rendered by the standpipe is similar to that of the +air cushion discussed in Section 184. + +[Illustration: FIG. 153.--A standpipe.] + +198. The Cost of Water. In the gravity system, where an elevated +lake or stream serves as a natural reservoir, the cost of the city's +waterworks is practically limited to the laying of pipes. But when the +source of the supply is more or less on a level with the surrounding +land, the cost is great, because the supply for the entire city must +either be pumped into an artificial reservoir, from which it can be +distributed, or else must be driven directly through the mains (Fig. +154). + +[Illustration: FIG. 154.--Water must be got to the houses by means of +pumps.] + +A gallon of water weighs approximately 8.3 pounds, and hence the work +done by a pump in raising a gallon of water to the top of an average +house, an elevation of 50 feet, is 8.3 x 50, or 415 foot pounds. A +small manufacturing town uses at least 1,000,000 gallons daily, and +the work done by a pump in raising that amount to an elevation of 50 +feet would be 8.3 x 1,000,000 x 50, or 415,000,000 foot pounds. + +The total work done during the day by the pump, or the engine driving +the pump, is 415,000,000 foot pounds, and hence the work done during +one hour would be 1/24 of 415,000,000, or 17,291,666 foot pounds; the +work done in one minute would be 1/60 of 17,291,666, or 288,194 foot +pounds, and the work done each second would be 1/60 of 288,194, or +4803 foot pounds. + +A 1-H.P. engine does 550 foot pounds of work each second, and +therefore if the pump is to be operated by an engine, the strength of +the latter would have to be 8.7 H.P. An 8.7-H.P. pumping engine +working at full speed every second of the day and night would be able +to supply the town with the necessary amount of water. When, however, +we consider the actual height to which the water is raised above the +pumping station, and the extra pumping which must be done in order to +balance the frictional loss, it is easy to understand that in actual +practice a much more powerful engine would be needed. The larger the +piston and the faster it works, the greater is the quantity of water +raised at each stroke, and the stronger must be the engine which +operates the pump. + +In many large cities there is no one single pumping station from which +supplies run to all parts of the city, but several pumping stations +are scattered throughout the city, and each of them supplies a +restricted territory. + +199. The Bursting of Dams and Reservoirs. The construction of a safe +reservoir is one of the most important problems of engineers. In +October, 1911, a town in Pennsylvania was virtually wiped out of +existence because of the bursting of a dam whose structure was of +insufficient strength to resist the strain of the vast quantity of +water held by it. A similar breakage was the cause of the fatal +Johnstown flood in 1889, which destroyed no less than seven towns, and +in which approximately 2000 persons are said to have lost their lives. + +Water presses not only on the bottom of a vessel, but upon the sides +as well; a bucket leaks whether the hole is in its side or its bottom, +showing that water presses not only downward but outward. Usually a +leak in a dam or reservoir occurs near the bottom. Weak spots at the +top are rare and easily repaired, but a leak near the bottom is +usually fatal, and in the case of a large reservoir the outflowing +water carries death and destruction to everything in its path. + +If the leak is near the surface, as at _a_ (Fig. 155), the water +issues as a feeble stream, because the pressure against the sides at +that level is due solely to the relatively small height of water +above _a_ (Section 195). If the leak is lower, as at _b_, the issuing +stream is stronger and swifter, because at that level the outward +pressure is much greater than at _a_, the increase being due to the +fact that the height of the water above _b_ is greater than that above +_a_. If the leak is quite low, as at _c_, the issuing stream has a +still greater speed and strength, and gushes forth with a force +determined by the height of the water above _c_. + +[Illustration: FIG. 155.--The flow from an opening depends upon the +height of water above the opening.] + +The dam at Johnstown was nearly 1/2 mile wide, and 40 feet high, and +so great was the force and speed of the escaping stream that within an +hour after the break had occurred, the water had traveled a distance +of 18 miles, and had destroyed property to the value of millions of +dollars. + +If a reservoir has a depth of 100 feet, the pressure exerted upon each +square foot of its floor is 62.5 x 100, or 6250 pounds; the weight +therefore to be sustained by every square foot of the reservoir floor +is somewhat more than 3 tons, and hence strong foundations are +essential. The outward lateral pressure at a depth of 25 feet would be +only one fourth as great as that on the bottom--hence the strain on +the sides at that depth would be relatively slight, and a less +powerful construction would suffice. But at a depth of 50 feet the +pressure on the sides would be one half that of the floor pressure, or +1-1/2 tons. At a depth of 75 feet, the pressure on the sides would be +three quarters that on the bottom, or 2-1/4 tons. As the bottom of the +reservoir is approached, the pressure against the sides increases, and +more powerful construction becomes necessary. + +Small elevated tanks, like those of the windmill, frequently have +heavy iron bands around their lower portion as a protection against +the extra strain. + +Before erecting a dam or reservoir, the maximum pressure to be exerted +upon every square inch of surface should be accurately calculated, and +the structure should then be built in such a way that the varying +pressure of the water can be sustained. It is not sufficient that the +bottom be strong; the sides likewise must support their strain, and +hence must be increased in strength with depth. This strengthening of +the walls is seen clearly in the reservoir shown in Figure 152. The +bursting of dams and reservoirs has occasioned the loss of so many +lives, and the destruction of so much property, that some states are +considering the advisability of federal inspection of all such +structures. + +[Illustration: FIG. 156.--The lock gates must be strong in order to +withstand the great pressure of the water against them.] + +200. The Relation of Forests to the Water Supply. When heavy rains +fall on a bare slope, or when snow melts on a barren hillside, a small +amount of the water sinks into the ground, but by far the greater part +of it runs off quickly and swells brooks and streams, thus causing +floods and freshets. + +When, however, rain falls on a wooded slope, the action is reversed; a +small portion runs off, while the greater portion sinks into the soft +earth. This is due partly to the fact that the roots of trees by their +constant growth keep the soil loose and open, and form channels, as it +were, along which the water can easily run. It is due also to the +presence on the ground of decaying leaves and twigs, or humus. The +decaying vegetable matter which covers the forest floor acts more or +less as a sponge, and quickly absorbs falling rain and melting snow. +The water which thus passes into the humus and the soil beneath does +not remain there, but slowly seeps downward, and finally after weeks +and months emerges at a lower level as a stream. Brooks and springs +formed in this way are constant feeders of rivers and lakes. + +In regions where the land has been deforested, the rivers run low in +season of prolonged drought, because the water which should have +slowly seeped through the soil, and then supplied the rivers for weeks +and months, ran off from the barren slopes in a few days. + +Forests not only lessen the danger of floods, but they conserve our +waterways, preventing a dangerous high-water mark in the season of +heavy rains and melting snows, and then preventing a shrinkage in dry +seasons when the only feeders of the rivers are the underground +sources. In the summer of 1911, prolonged drought in North Carolina +lowered the rivers to such an extent that towns dependent upon them +suffered greatly. The city of Charlotte was reduced for a time to a +practically empty reservoir; washing and bathing were eliminated, +machinery dependent upon water-power and steam stood idle, and every +glass of water drunk was carefully reckoned. Thousands of gallons of +water were brought in tanks from neighboring cities, and were emptied +into the empty reservoir from whence it trickled slowly through the +city mains. The lack of water caused not only personal inconvenience +and business paralysis, but it occasioned real danger of disease +through unflushed sewers and insufficiently drained pipes. + +The conservation of the forest means the conservation of our +waterways, whether these be used for transportation or as sources of +drinking water. + + + + +CHAPTER XX + +MAN'S CONQUEST OF SUBSTANCES + + +201. Chemistry. Man's mechanical inventions have been equaled by his +chemical researches and discoveries, and by the application he has +made of his new knowledge. + +The plain cotton frock of our grandmothers had its death knell sounded +a few years ago, when John Mercer showed that cotton fabrics soaked in +caustic soda assumed under certain conditions a silky sheen, and when +dyed took on beautiful and varied hues. The demonstration of this +simple fact laid the foundation for the manufacture of a vast variety +of attractive dress materials known as mercerized cotton. + +Possibly no industry has been more affected by chemical discovery than +that of dyeing. Those of us who have seen the old masterpieces in +painting, or reproductions of them, know the softness, the mellowness, +the richness of tints employed by the old masters. But if we look for +the brilliancy and variety of color seen in our own day, the search +will be fruitless, because these were unknown until a half century +ago. Up to that time, dyes were few in number and were extracted +solely from plants, principally from the indigo and madder plants. But +about the year 1856 it was discovered that dyes in much greater +variety and in purer form could be obtained from coal tar. This +chemical production of dyes has now largely supplanted the original +method, and the industry has grown so rapidly that a single firm +produced in one year from coal tar a quantity of indigo dye which +under the natural process of plant extraction would have required a +quarter million acres of indigo plant. + +The abundance and cheapness of newspapers, coarse wrapping papers, +etc., is due to the fact that man has learned to substitute wood for +rags in the manufacture of paper. Investigation brought out the fact +that wood contained the substance which made rags valuable for paper +making. Since the supply of rags was far less than the demand, the +problem of the extraction from wood of the paper-forming substance was +a vital one. From repeated trials, it was found that caustic soda when +heated with wood chips destroyed everything in the wood except the +desired substance, cellulose; this could be removed, bleached, dried, +and pressed into paper. The substitution of wood for rags has made +possible the daily issue of newspapers, for the making of which +sufficient material would not otherwise have been available. When we +reflect that a daily paper of wide circulation consumes ten acres of +wood lot per day, we see that all the rags in the world would be +inadequate to meet this demand alone, to say nothing of periodicals, +books, tissue paper, etc. + +Chemistry plays a part in every phase of life; in the arts, the +industries, the household, and in the body itself, where digestion, +excretion, etc., result from the action of the bodily fluids upon +food. The chemical substances of most interest to us are those which +affect us personally rather than industrially; for example, soap, +which cleanses our bodies, our clothing, our household possessions; +washing soda, which lightens laundry work; lye, which clears out the +drain pipe clogged with grease; benzine, which removes stains from +clothing; turpentine, which rids us of paint spots left by careless +workmen; and hydrogen peroxide, which disinfects wounds and sores. + +In order to understand the action of several of these substances we +must study the properties of two groups of chemicals--known +respectively as acids and bases; the first of these may be represented +by vinegar, sulphuric acid, and oxalic acid; and the second, by +ammonia, lye, and limewater. + +202. Acids. All of us know that vinegar and lemon juice have a sour +taste, and it is easy to show that most acids are characterized by a +sour taste. If a clean glass rod is dipped into very dilute acid, such +as acetic, sulphuric, or nitric acid, and then lightly touched to the +tongue, it will taste sour. But the best test of an acid is by sight +rather than by taste, because it has been found that an acid is able +to discolor a plant substance called litmus. If paper is soaked in a +litmus solution until it acquires the characteristic blue hue of the +plant substance, and is then dried thoroughly, it can be used to +detect acids, because if it comes in contact with even the minutest +trace of acid, it loses its blue color and assumes a red tint. Hence, +in order to detect the presence of acid in a substance, one has merely +to put some of the substance on blue litmus paper, and note whether or +not the latter changes color. This test shows that many of our common +foods contain some acid; for example, fruit, buttermilk, sour bread, +and vinegar. + +The damage which can be done by strong acids is well known; if a jar +of sulphuric acid is overturned, and some of it falls on the skin, it +eats its way into the flesh and leaves an ugly sore; if it falls on +carpet or coat, it eats its way into the material and leaves an +unsightly hole. The evil results of an accident with acid can be +lessened if we know just what to do and do it quickly, but for this we +must have a knowledge of bases, the second group of chemicals. + +203. Bases. Substances belonging to this group usually have a bitter +taste and a slimy, soapy feeling. For our present purposes, the most +important characteristic of a base is that it will neutralize an acid +and in some measure hinder the damage effected by the former. If, as +soon as an acid has been spilled on cloth, a base, such as ammonia, is +applied to the affected region, but little harm will be done. In your +laboratory experiments you may be unfortunate enough to spill acid on +your body or clothing; if so, quickly apply ammonia. If you delay, the +acid does its work, and there is no remedy. If soda (a base) touches +black material, it discolors it and leaves an ugly brown spot; but the +application of a little acid, such as vinegar or lemon juice, will +often restore the original color and counteract the bad effects of the +base. Limewater prescribed by physicians in cases of illness is a +well-known base. This liquid neutralizes the too abundant acids +present in a weak system and so quiets and tones the stomach. + +The interaction of acids and bases may be observed in another way. If +blue litmus paper is put into an acid solution, its color changes to +red; if now the red litmus paper is dipped into a base solution, +caustic soda, for example, its original color is partially restored. +What the acid does, the base undoes, either wholly or in part. Bases +always turn red litmus paper blue. + +Bases, like acids, are good or bad according to their use; if they +come in contact with cloth, they eat or discolor it, unless +neutralized by an acid. But this property of bases, harmful in one +way, is put to advantage in the home, where grease is removed from +drainpipe and sink by the application of lye, a strong base. If the +lye is too concentrated, it will not only eat the grease, but will +corrode the metal piping; it is easy, however, to dilute base +solutions to such a degree that they will not affect piping, but will +remove grease. Dilute ammonia is used in almost every home and is an +indispensable domestic servant; diluted sufficiently, it is +invaluable in the washing of delicate fabrics and in the removing of +stains, and in a more concentrated form it is helpful as a smelling +salt in cases of fainting. + +Some concentrated bases are so powerful in their action on grease, +cloth, and metal that they have received the designation _caustic_, +and are ordinarily known as caustic soda, caustic potash (lye), and +caustic lime. These more active bases are generally called alkalies in +distinction from the less active ones. + +204. Neutral Substances. To any acid solution add gradually a small +quantity of a base, and test the mixture from time to time with blue +litmus paper; at first the paper will turn red quickly, but as more +and more of the base is added to the solution, it has less and less +effect on the blue litmus paper, and finally a point is reached when a +fresh strip of blue paper will not be affected. Such a result +indicates infallibly the absence of any acid qualities in the +solution. If now red litmus paper is tested in the same solution, its +color also will remain unchanged; such a result indicates infallibly +the absence of any basic quality. The solution has the characteristic +property of neither acid nor base and is said to be neutral. + +If to the neutral solution an extra portion of base is added, so that +there is an excess of base over acid, the neutralization is +overbalanced and the red paper turns blue. If to the neutral solution +an extra portion of acid is added, so that there is an excess of acid +over base, the neutralization is overbalanced in the opposite +direction, and the solution acquires acid characteristics. + +Most acids and bases will eat and corrode and discolor, while neutral +substances will not; it is for this reason that soap, a slightly +alkaline substance, is the safest cleansing agent for laundry, bath, +and general work. Good soaps, being carefully made, are so nearly +neutral that they will not fade the color out of clothing; the cheap +soaps are less carefully prepared and are apt to have a strong excess +of the base ingredient; such soaps are not safe for delicate work. + +205. Soap. If we gather together scrapings of lard, butter, bits of +tallow from burned-out candles, scraps of waste fat, or any other sort +of grease, and pour a strong solution of lye over the mass, a soft +soapy substance is formed. In colonial times, every family made its +own supply of soap, utilizing, for that purpose, household scraps +often regarded by the housekeeper of to-day as worthless. Grease and +fat were boiled with water and hardwood ashes, which are rich in lye, +and from the mixture came the soft soap used by our ancestors. In +practice, the wood ashes were boiled in water, which was then strained +off, and the resulting filtrate, or lye, was mixed with the fats for +soap making. + +Most fats contain a substance of an acid nature, and are decomposed by +the action of bases such as caustic soda and caustic potash. The acid +component of the grease partially neutralizes the base, and a new +substance is formed, namely, soap. + +With the advance of civilization the labor of soap making passed from +the home to the factory, very much as bread making has done in our own +day. Different varieties of soaps appeared, of which the hard soap was +the most popular, owing to the ease with which it could be +transported. Within the last few years liquid soaps have come into +favor, especially in schools, railroad stations, and other public +places, where a cake of soap would be handled by many persons. By +means of a simple device (Fig. 157), the soap escapes from a +receptacle when needed. The mass of the soap does not come in contact +with the skin, and hence the spread of contagious skin diseases is +lessened. + +[Illustration: FIG. 157.--Liquid soap container.] + +Commercial soaps are made from a great variety of substances, such as +tallow, lard, castor oil, coconut oil, olive oil, etc.; or in cheaper +soaps, from rosin, cottonseed oil, and waste grease. The fats which go +to waste in our garbage could be made a source of income, not only to +the housewife, but to the city. In Columbus, Ohio, garbage is used as +a source of revenue; the grease from the garbage being sold for soap +making, and the tankage (Section 188) for fertilizer. + +206. Why Soap Cleans. The natural oil of the skin catches and +retains dust and dirt, and makes a greasy film over the body. This +cannot be removed by water alone, but if soap is used and a generous +lather is applied to the skin, the dirt is "cut" and passes from the +body into the water. Soap affects a grease film and water very much as +the white of an egg affects oil and water. These two liquids alone do +not mix, the oil remaining separate on the surface of the water; but +if a small quantity of white of egg is added, an emulsion is formed, +the oil separating into minute droplets which spread through the +water. In the same way, soap acts on a grease film, separating it into +minute droplets which leave the skin and spread through the water, +carrying with them the dust and dirt particles. The warmer the water, +the better will be the emulsion, and hence the more effective the +removal of dirt and grease. This explanation holds true for the +removal of grease from any surface, whether of the body, clothing, +furniture, or dishes. + +207. Washing Powders. Sometimes soap refuses to form a lather and +instead cakes and floats as a scum on the top of the water; this is +not the fault of the soap but of the water. As water seeps through +the soil or flows over the land, it absorbs and retains various soil +constituents which modify its character and, in some cases, render it +almost useless for household purposes. Most of us are familiar with +the rain barrel of the country house, and know that the housewife +prefers rain water for laundry and general work. Rain water, coming as +it does from the clouds, is free from the chemicals gathered by ground +water, and is hence practically pure. While foreign substances do not +necessarily injure water for drinking purposes (Section 69), they are +often of such a nature as to prevent soap from forming an emulsion, +and hence from doing its work. Under such circumstances the water is +said to be hard, and soap used with it is wasted. Even if water is +only moderately hard, much soap is lost. The substances which make +water hard are calcium and magnesium salts. When soap is put into +water containing one or both of these, it combines with the salts to +form sticky insoluble scum. It is therefore not free to form an +emulsion and to remove grease. As a cleansing agent it is valueless. +The average city supply contains so little hardness that it is +satisfactory for toilet purposes; but in the laundry, where there is +need for the full effect of the soap, and where the slightest loss +would aggregate a great deal in the course of time, something must be +done to counteract the hardness. The addition of soda, or sodium +carbonate to the water will usually produce the desired effect. +Washing soda combines with calcium and magnesium and prevents them +from uniting with soap. The soap is thus free to form an emulsion, +just as in ordinary water. Washing powders are sometimes used instead +of washing soda. Most washing powders contain, in addition to a +softening agent, some alkali, and hence a double good is obtained from +their use; they not only soften the water and allow the soap to form +an emulsion, but they also, through their alkali content, cut the +grease and themselves act as cleansers. In some cities where the water +is very hard, as in Columbus, Ohio, it is softened and filtered at +public expense, before it leaves the reservoirs. But even under these +circumstances, a moderate use of washing powder is general in laundry +work. + +If washing powder is put on clothes dry, or is thrown into a crowded +tub, it will eat the clothes before it has a chance to dissolve in the +water. The only safe method is to dissolve the powder before the +clothes are put into the tub. The trouble with our public laundries is +that many of them are careless about this very fact, and do not take +time to dissolve the powder before mixing it with the clothes. + +The strongest washing powder is soda, and this cheap form is as good +as any of the more expensive preparations sold under fancy names. +Borax is a milder powder and is desirable for finer work. + +One of the most disagreeable consequences of the use of hard water for +bathing is the unavoidable scum which forms on the sides of bathtub +and washbowl. The removal of the caked grease is difficult, and if +soap alone is used, the cleaning of the tub requires both patience and +hard scrubbing. The labor can be greatly lessened by moistening the +scrubbing cloth with turpentine and applying it to the greasy film, +which immediately dissolves and thus can be easily removed. The +presence of the scum can be largely avoided by adding a small amount +of liquid ammonia to the bath water. But many persons object to this; +hence it is well to have some other easy method of removing the +objectionable matter. + +208. To remove Stains from Cloth. While soap is, generally speaking, +the best cleansing agent, there are occasions when other substances +can be used to better advantage. For example, grease spots on carpet +and non-washable dress goods are best removed by the application of +gasoline or benzine. These substances dissolve the grease, but do not +remove it from the clothing; for that purpose a woolen cloth should be +laid under the stain in readiness to absorb the benzine and the grease +dissolved in it. If the grease is not absorbed while in solution, it +remains in the clothing and after the evaporation of the benzine +reappears in full force. + +Cleaners frequently clean suits by laying a blotter over a grease spot +and applying a hot iron; the grease, when melted by the heat, takes +the easiest way of spreading itself and passes from cloth to blotter. + +209. Salts. A neutral liquid formed as in Section 204, by the action +of hydrochloric acid and the alkali solution of caustic soda, has a +brackish, salty taste, and is, in fact, a solution of salt. This can +be demonstrated by evaporating the neutral liquid to dryness and +examining the residue of solid matter, which proves to be common salt. + +When an acid is mixed with a base, the result is a substance more or +less similar in its properties to common salt; for this reason all +compounds formed by the neutralization of an acid with a base are +called salts. If, instead of hydrochloric acid (HCl), we use an acid +solution of potassium tartrate, and if instead of caustic soda we use +bicarbonate of soda (baking soda), the result is a brackish liquid as +before, but the salt in the liquid is not common salt, but Rochelle +salt. Different combinations of acids and bases produce different +salts. Of all the vast group of salts, the most abundant as well as +the most important is common salt, known technically as sodium +chloride because of its two constituents, sodium and chlorine. + +We are not dependent upon neutralization for the enormous quantities +of salt used in the home and in commerce. It is from the active, +restless seas of the present, and from the dead seas of the +prehistoric past that our vast stores of salt come. The waters of the +Mediterranean and of our own Great Salt Lake are led into shallow +basins, where, after evaporation by the heat of the sun, they leave a +residue of salt. By far the largest quantity of salt, however, comes +from the seas which no longer exist, but which in far remote ages +dried up and left behind them their burden of salt. Deposits of salt +formed in this way are found scattered throughout the world, and in +our own country are found in greatest abundance in New York. The +largest salt deposit known has a depth of one mile and exists in +Germany. + +Salt is indispensable on our table and in our kitchen, but the amount +of salt used in this way is far too small to account for a yearly +consumption of 4,000,000 tons in the United States alone. The +manufacture of soap, glass, bleaching powders, baking powders, washing +soda, and other chemicals depends on salt, and it is for these that +the salt beds are mined. + +210. Baking Soda. Salt is by all odds the most important sodium +compound. Next to it come the so-called carbonates: first, sodium +carbonate, which is already familiar to us as washing soda; and +second, sodium bicarbonate, which is an ingredient of baking powders. +These are both obtained from sodium chloride by relatively simple +means; that is, by treating salt with the base, ammonia, and with +carbon dioxide. + +Washing soda has already been discussed. Since baking powders in some +form are used in almost all homes for the raising of cake and pastry +dough, it is essential that their helpful and harmful qualities be +clearly understood. + +The raising of dough by means of baking soda--bicarbonate of soda--is +a very simple process. When soda is heated, it gives off carbon +dioxide gas; you can easily prove this for yourself by burning a +little soda in a test tube, and testing the escaping gas in a test +tube of limewater. When flour and water alone are kneaded and baked +in loaves, the result is a mass so compact and hard that human teeth +are almost powerless to crush and chew it. The problem is to separate +the mass of dough or, in other words, to cause it to rise and lighten. +This can be done by mixing a little soda in the flour, because the +heat of the oven causes the soda to give off bubbles of gas, and these +in expanding make the heavy mass slightly porous. Bread is never +lightened with soda because the amount of gas thus given off is too +small to convert heavy compact bread dough into a spongy mass; but +biscuit and cake, being by nature less compact and heavy, are +sufficiently lightened by the gas given off from soda. + +But there is one great objection to the use of soda alone as a +leavening agent. After baking soda has lost its carbon dioxide gas, it +is no longer baking soda, but is transformed into its relative, +washing soda, which has a disagreeable taste and is by no means +desirable for the stomach. + +Man's knowledge of chemicals and their effect on each other has +enabled him to overcome this difficulty and, at the same time, to +retain the leavening effect of the baking soda. + +211. Baking Powders. If some cooking soda is put into lemon juice or +vinegar, or any acid, bubbles of gas immediately form and escape from +the liquid. After the effervescence has ceased, a taste of the liquid +will show you that the lemon juice has lost its acid nature, and has +acquired in exchange a salty taste. Baking soda, when treated with an +acid, is transformed into carbon dioxide and a salt. The various +baking powders on the market to-day consist of baking soda and some +acid substance, which acts upon the soda, forces it to give up its +gas, and at the same time unites with the residue to form a harmless +salt. + +Cream of tartar contains sufficient acid to act on baking soda, and is +a convenient and safe ingredient for baking powder. When soda and +cream of tartar are mixed dry, they do not react on each other, +neither do they combine rapidly in _cold_ moist dough, but as soon as +the heat of the oven penetrates the doughy mass, the cream of tartar +combines with the soda and sets free the gas needed to raise the +dough. The gas expands with the heat of the oven, raising the dough +still more. Meanwhile, the dough itself is influenced by the heat and +is stiffened to such an extent that it retains its inflated shape and +spongy nature. + +Many housewives look askance at ready-made baking powders and prefer +to bake with soda and sour milk, soda and buttermilk, or soda and +cream of tartar. Sour milk and buttermilk are quite as good as cream +of tartar, because the lactic acid which they contain combines with +the soda and liberates carbon dioxide, and forms a harmless residue in +the dough. + +The desire of manufacturers to produce cheap baking powders led to the +use of cheap acids and alkalies, regardless of the character of the +resulting salt. Alum and soda were popular for some time; but careful +examination proved that the particular salt produced by this +combination was not readily absorbed by the stomach, and that its +retention there was injurious to health. For this reason, many states +have prohibited the use of alum in baking powders. + +It is not only important to choose the ingredients carefully; it is +also necessary to calculate the respective quantities of each, +otherwise there will be an excess of acid or alkali for the stomach to +take care of. A standard powder contains twice as much cream of tartar +as of bicarbonate of soda, and the thrifty housewife who wishes to +economize, can make for herself, at small cost, as good a baking +powder as any on the market, by mixing tartar and soda in the above +proportions and adding a little corn starch to keep the mixture dry. + +The self-raising flour, so widely advertised by grocers, is flour in +which these ingredients or their equivalent have been mixed by the +manufacturer. + +212. Soda Mints. Bicarbonate of soda is practically the sole +ingredient of the soda mints popularly sold for indigestion. These +correct a tendency to sour stomach because they counteract the surplus +acid in the stomach, and form with it a safe neutral substance. + +Seidlitz powder is a simple remedy consisting of two powders, one +containing bicarbonate of soda, and the other, some acid such as cream +of tartar. When these substances are dissolved in water and mixed, +effervescence occurs, carbon dioxide escapes, and a solution of +Rochelle salt remains. + +212_a_. Source of Soda. An enormous quantity of sodium carbonate, or +soda, as it is usually called, is needed in the manufacture of glass, +soap, bleaching powders, and other commercial products. Formerly, the +supply of soda was very limited because man was dependent upon natural +deposits and upon ashes of sea plants for it. Common salt, sodium +chloride, is abundant, and in 1775 a prize was offered to any one who +would find a way to obtain soda from salt. As a result of this, soda +was soon manufactured from common salt. In the most recent methods of +manufacture, salt, water, ammonia, and carbon dioxide are made to +react. Baking soda is formed from the reaction. The baking soda is +then heated and decomposed into washing soda or the soda of commerce. + + + + +CHAPTER XXI + +FERMENTATION + + +213. While baking powder is universally used for biscuits and cake, +it is seldom, if ever, used for bread, because it does not furnish +sufficient gas to lighten the tough heavy mass of bread dough. Then, +too, most people prefer the taste of yeast-raised bread. There is a +reason for this widespread preference, but to understand it, we must +go somewhat far afield, and must study not only the bread of to-day, +but the bread of antiquity, and the wines as well. + +If grapes are crushed, they yield a liquid which tastes like the +grapes; but if the liquid is allowed to stand in a warm place, it +loses its original character, and begins to ferment, becoming, in the +course of a few weeks, a strongly intoxicating drink. This is true not +only of grape juice but also of the juice of all other sweet fruits; +apple juice ferments to cider, currant juice to currant wine, etc. +This phenomenon of fermentation is known to practically all races of +men, and there is scarcely a savage tribe without some kind of +fermented drink; in the tropics the fermented juice of the palm tree +serves for wine; in the desert regions, the fermented juice of the +century plant; and in still other regions, the root of the ginger +plant is pressed into service. + +The fermentation which occurs in bread making is similar to that which +is responsible for the transformation of plant juices into +intoxicating drinks. The former process is not so old, however, since +the use of alcoholic beverages dates back to the very dawn of history, +and the authentic record of raised or leavened bread is but little +more than 3000 years old. + +214. The Bread of Antiquity. The original method of bread making and +the method employed by savage tribes of to-day is to mix crushed grain +and water until a paste is formed, and then to bake this over a camp +fire. The result is a hard compact substance known as unleavened +bread. A considerable improvement over this tasteless mass is +self-raised bread. If dough is left standing in a warm place a number +of hours, it swells up with gas and becomes porous, and when baked, is +less compact and hard than the savage bread. Exposure to air and +warmth brings about changes in dough as well as in fruit juices, and +alters the character of the dough and the bread made from it. Bread +made in this way would not seem palatable to civilized man of the +present day, accustomed, as he is, to delicious bread made light and +porous by yeast; but to the ancients, the least softening and +lightening was welcome, and self-fermented bread, therefore, +supplanted the original unleavened bread. + +Soon it was discovered that a pinch of this fermented dough acted as a +starter on a fresh batch of dough. Hence, a little of the fermented +dough was carefully saved from a batch, and when the next bread was +made, the fermented dough, or leaven, was worked into the fresh dough +and served to raise the mass more quickly and effectively than mere +exposure to air and warmth could do in the same length of time. This +use of leaven for raising bread has been practiced for ages. + +Grape juice mixed with millet ferments quickly and strongly, and the +Romans learned to use this mixture for bread raising, kneading a very +small amount of it through the dough. + +215. The Cause of Fermentation. Although alcoholic fermentation, and +the fermentation which goes on in raising dough, were known and +utilized for many years, the cause of the phenomenon was a sealed book +until the nineteenth century. About that time it was discovered, +through the use of the microscope, that fermenting liquids contain an +army of minute plant organisms which not only live there, but which +actually grow and multiply within the liquid. For growth and +multiplication, food is necessary, and this the tiny plants get in +abundance from the fruit juices; they feed upon the sugary matter and +as they feed, they ferment it, changing it into carbon dioxide and +alcohol. The carbon dioxide, in the form of small bubbles, passes off +from the fermenting mass, while the alcohol remains in the liquid, +giving the stimulating effect desired by imbibers of alcoholic drinks. +The unknown strange organisms were called yeast, and they were the +starting point of the yeast cakes and yeast brews manufactured to-day +on a large scale, not only for bread making but for the commercial +production of beer, ale, porter, and other intoxicating drinks. + +The grains, rye, corn, rice, wheat, from which meal is made, contain +only a small quantity of sugar, but, on the other hand, they contain a +large quantity of starch which is easily convertible into sugar. Upon +this the tiny yeast plants in the dough feed, and, as in the case of +the wines, ferment the sugar, producing carbon dioxide and alcohol. +The dough is thick and sticky and the gas bubbles expand it into a +spongy mass. The tiny yeast plants multiply and continue to make +alcohol and gas, and in consequence, the dough becomes lighter and +lighter. When it has risen sufficiently, it is kneaded and placed in +an oven; the heat of the oven soon kills the yeast plants and drives +the alcohol out of the bread; at the same time it expands the +imprisoned gas bubbles and causes them to lighten and swell the bread +still more. Meanwhile, the dough has become stiff enough to support +itself. The result of the fermentation is a light, spongy loaf. + +216. Where does Yeast come From? The microscopic plants which we +call yeast are widely distributed in the air, and float around there +until chance brings them in contact with a substance favorable to +their growth, such as fruit juices and moist warm batter. Under the +favorable conditions of abundant moisture, heat, and food, they grow +and multiply rapidly, and cause the phenomenon of fermentation. Wild +yeast settles on the skin of grapes and apples, but since it does not +have access to the fruit juices within, it remains inactive very much +as a seed does before it is planted. But when the fruit is crushed, +the yeast plants get into the juice, and feeding on it, grow and +multiply. The stray yeast plants which get into the sirup are +relatively few, and hence fermentation is slow; it requires several +weeks for currant wine to ferment, and several months for the juice of +grapes to be converted into wine. + +Stray yeast finds a favorable soil for growth in the warmth and +moisture of a batter; but although the number of these stray plants is +very large, it is insufficient to cause rapid fermentation, and if we +depended upon wild yeast for bread raising, the result would not be to +our liking. + +When our remote ancestors saved a pinch of dough as leaven for the +next baking, they were actually cultivating yeast, although they did +not know it. The reserved portion served as a favorable breeding place +to the yeast plants within it; they grew and reproduced amazingly, and +became so numerous, that the small mass of old dough in which they +were gathered served to leaven the entire batch at the next baking. + +As soon as man learned that yeast plants caused fermentation in +liquors and bread, he realized that it would be to his advantage to +cultivate yeast and to add it to bread and to plant juices rather than +to depend upon accidental and slow fermentation from wild yeast. +Shortly after the discovery of yeast in the nineteenth century, man +commenced his attempt to cultivate the tiny organisms. Their +microscopic size added greatly to his trouble, and it was only after +years of careful and tedious investigation that he was able to perfect +the commercial yeast cakes and yeast brews universally used by bakers +and brewers. The well-known compressed yeast cake is simply a mass of +live and vigorous yeast plants, embedded in a soft, soggy material, +and ready to grow and multiply as soon as they are placed under proper +conditions of heat, moisture, and food. Seeds which remain on our +shelves do not germinate, but those which are planted in the soil do; +so it is with the yeast plants. While in the cake they are as lifeless +as the seed; when placed in dough, or fruit juice, or grain water, +they grow and multiply and cause fermentation. + + + + +CHAPTER XXII + +BLEACHING + + +217. The beauty and the commercial value of uncolored fabrics depend +upon the purity and perfection of their whiteness; a man's white +collar and a woman's white waist must be pure white, without the +slightest tinge of color. But all natural fabrics, whether they come +from plants, like cotton and linen, or from animals, like wool and +silk, contain more or less coloring matter, which impairs the +whiteness. This coloring not only detracts from the appearance of +fabrics which are to be worn uncolored, but it seriously interferes +with the action of dyes, and at times plays the dyer strange tricks. + +Natural fibers, moreover, are difficult to spin and weave unless some +softening material such as wax or resin is rubbed lightly over them. +The matter added to facilitate spinning and weaving generally detracts +from the appearance of the uncolored fabric, and also interferes with +successful dyeing. Thus it is easy to see that the natural coloring +matter and the added foreign matter must be entirely removed from +fabrics destined for commercial use. Exceptions to this general fact +are sometimes made, because unbleached material is cheaper and more +durable than the bleached product, and for some purposes is entirely +satisfactory; unbleached cheesecloth and sheeting are frequently +purchased in place of the more expensive bleached material. Formerly, +the only bleaching agent known was the sun's rays, and linen and +cotton were put out to sun for a week; that is, the unbleached +fabrics were spread on the grass and exposed to the bleaching action +of sun and dew. + +[Illustration: FIG. 158.--Preparing chlorine from hydrochloric acid +and manganese dioxide.] + +218. An Artificial Bleaching Agent. While the sun's rays are +effective as a bleaching agent, the process is slow; moreover, it +would be impossible to expose to the sun's rays the vast quantity of +fabrics used in the civilized world of to-day, and the huge and +numerous bolts of material which daily come from our looms and +factories must therefore be whitened by artificial means. The +substance almost universally used as a rapid artificial bleaching +agent is chlorine, best known to us as a constituent of common salt. +Chlorine is never free in nature, but is found in combination with +other substances, as, for example, in combination with sodium in salt, +or with hydrogen in hydrochloric acid. + +The best laboratory method of securing free chlorine is to heat in a +water bath a mixture of hydrochloric acid and manganese dioxide, a +compound containing one part of manganese and two parts of oxygen. The +heat causes the manganese dioxide to give up its oxygen, which +immediately combines with the hydrogen of the hydrochloric acid and +forms water. The manganese itself combines with part of the chlorine +originally in the acid, but not with all. There is thus some free +chlorine left over from the acid, and this passes off as a gas and can +be collected, as in Figure 158. Free chlorine is heavier than air, and +hence when it leaves the exit tube it settles at the bottom of the +jar, displacing the air, and finally filling the bottle. + +Chlorine is a very active substance and combines readily with most +substances, but especially with hydrogen; if chlorine comes in contact +with steam, it abstracts the hydrogen and unites with it to form +hydrochloric acid, but it leaves the oxygen free and uncombined. This +tendency of chlorine to combine with hydrogen makes it valuable as a +bleaching agent. In order to test the efficiency of chlorine as a +bleaching agent, drop a wet piece of colored gingham or calico into +the bottle of chlorine, and notice the rapid disappearance of color +from the sample. If unbleached muslin is used, the moist strip loses +its natural yellowish hue and becomes a clear, pure white. The +explanation of the bleaching power of chlorine is that the chlorine +combines with the hydrogen of the water and sets oxygen free; the +uncombined free oxygen oxidizes the coloring matter in the cloth and +destroys it. + +Chlorine has no effect on dry material, as may be seen if we put dry +gingham into the jar; in this case there is no water to furnish +hydrogen for combination with the chlorine, and no oxygen to be set +free. + +219. Bleaching Powder. Chlorine gas has a very injurious effect on +the human body, and hence cannot be used directly as a bleaching +agent. It attacks the mucous membrane of the nose and lungs, and +produces the effect of a severe cold or catarrh, and when inhaled, +causes death. But certain compounds of chlorine are harmless, and can +be used instead of chlorine for destroying either natural or +artificial dyes. One of these compounds, namely, chloride of lime, is +the almost universal bleaching agent of commerce. It comes in the form +of powder, which can be dissolved in water to form the bleaching +solution in which the colored fabrics are immersed. But fabrics +immersed in a bleaching powder solution do not lose their color as +would naturally be expected. The reason for this is that the chlorine +gas is not free to do its work, but is restricted by its combination +with the other substances. By experiment it has been found that the +addition to the bleaching solution of an acid, such as vinegar or +lemon juice or sulphuric acid, causes the liberation of the chlorine. +The chlorine thus set free reacts with the water and liberates oxygen; +this in turn destroys the coloring matter in the fibers, and +transforms the material into a bleached product. + +The acid used to liberate the chlorine from the bleaching powder, and +the chlorine also, rot materials with which they remain in contact for +any length of time. For this reason, fabrics should be removed from +the bleaching solution as soon as possible, and should then be rinsed +in some solution, such as ammonia, which is capable of neutralizing +the harmful substances; finally the fabric should be thoroughly rinsed +in water in order that all foreign matter may be removed. The reason +home bleaching is so seldom satisfactory is that most amateurs fail to +realize the necessity of immediate neutralization and rinsing, and +allow the fabric to remain too long in the bleaching solution, and +allow it to dry with traces of the bleaching substances present in the +fibers. Material treated in this way is thoroughly bleached, but is at +the same time rotten and worthless. Chloride of lime is frequently +used in laundry work; the clothes are whiter than when cleaned with +soap and simple washing powders, but they soon wear out unless the +precaution has been taken to add an "antichlor" or neutralizer to the +bleaching solution. + +220. Commercial Bleaching. In commercial bleaching the material to +be bleached is first moistened with a very weak solution of sulphuric +acid or hydrochloric acid, and is then immersed in the bleaching +powder solution. As the moist material is drawn through the bleaching +solution, the acid on the fabric acts upon the solution and releases +chlorine. The chlorine liberates oxygen from the water. The oxygen in +turn attacks the coloring matter and destroys it. + +[Illustration: FIG. 159.--The material to be bleached is drawn through +an acid _a_, then through a bleaching solution _b_, and finally +through a neutralizing solution _c_.] + +The bleached material is then immersed in a neutralizing bath and is +finally rinsed thoroughly in water. Strips of cotton or linen many +miles long are drawn by machinery into and out of the various +solutions (Fig. 159), are then passed over pressing rollers, and +emerge snow white, ready to be dyed or to be used as white fabric. + +221. Wool and Silk Bleaching. Animal fibers like silk, wool, and +feathers, and some vegetable fibers like straw, cannot be bleached by +means of chlorine, because it attacks not only the coloring matter but +the fiber itself, and leaves it shrunken and inferior. Cotton and +linen fibers, apart from the small amount of coloring matter present +in them, contain nothing but carbon, oxygen, and hydrogen, while +animal fibers contain in addition to these elements some compounds of +nitrogen. The presence of these nitrogen compounds influences the +action of the chlorine and produces unsatisfactory results. For animal +fibers it is therefore necessary to discard chlorine as a bleaching +agent, and to substitute a substance which will have a less disastrous +action upon the fibers. Such a substance is to be had in sulphurous +acid. When sulphur burns, as in a match, it gives off disagreeable +fumes, and if these are made to bubble into a vessel containing water, +they dissolve and form with the water a substance known as sulphurous +acid. That this solution has bleaching properties is shown by the fact +that a colored cloth dipped into it loses its color, and unbleached +fabrics immersed in it are whitened. The harmless nature of sulphurous +acid makes it very desirable as a bleaching agent, especially in the +home. + +Silk, lace, and wool when bleached with chlorine become hard and +brittle, but when whitened with sulphurous acid, they retain their +natural characteristics. + +This mild form of a bleaching substance has been put to uses which are +now prohibited by the pure food laws. In some canneries common corn is +whitened with sulphurous acid, and is then sold under false +representations. Cherries are sometimes bleached and then colored with +the bright shades which under natural conditions indicate freshness. + +Bleaching with chlorine is permanent, the dyestuff being destroyed by +the chlorine; but bleaching with sulphurous acid is temporary, because +the milder bleach does not actually destroy the dyestuff, but merely +modifies it, and in time the natural yellow color of straw, cotton, +and linen reappears. The yellowing of straw hats during the summer is +familiar to everyone; the straw is merely resuming its natural color +which had been modified by the sulphurous acid solution applied to the +straw when woven. + +222. Why the Color Returns. Some of the compounds formed by the +sulphurous acid bleaching process are gradually decomposed by +sunlight, and in consequence the original color is in time partially +restored. The portion of a hat protected by the band retains its +fresh appearance because the light has not had access to it. Silks and +other fine fabrics bleached in this way fade with age, and assume an +unnatural color. One reason for this is that the dye used to color the +fabric requires a clear white background, and loses its characteristic +hues when its foundation is yellow instead of white. Then, too, +dyestuffs are themselves more or less affected by light, and fade +slowly under a strong illumination. + +Materials which are not exposed directly to an intense and prolonged +illumination retain their whiteness for a long time, and hence dress +materials and hats which have been bleached with sulphurous acid +should be protected from the sun's glare when not in use. + +223. The Removal of Stains. Bleaching powder is very useful in the +removal of stains from white fabrics. Ink spots rubbed with lemon +juice and dipped in bleaching solution fade away and leave on the +cloth no trace of discoloration. Sometimes these stains can be removed +by soaking in milk, and where this is possible, it is the better +method. + +Bleaching solution, however, while valuable in the removal of some +stains, is unable to remove paint stains, because paints owe their +color to mineral matter, and on this chlorine is powerless to act. +Paint stains are best removed by the application of gasoline followed +by soap and water. + + + + +CHAPTER XXIII + +DYEING + + +224. Dyes. One of the most important and lucrative industrial +processes of the world to-day is that of staining and dyeing. Whether +we consider the innumerable shades of leather used in shoes and +harnesses and upholstery; the multitude of colors in the paper which +covers our walls and reflects light ranging from the somber to the +gay, and from the delicate to the gorgeous; the artificial scenery +which adorns the stage and by its imitation of trees and flowers and +sky translates us to the Forest of Arden; or whether we consider the +uncounted varieties of color in dress materials, in carpets, and in +hangings, we are dealing with substances which owe their beauty to +dyes and dyestuffs. + +The coloring of textile fabrics, such as cotton, wool, and silk, far +outranks in amount and importance that of leather, paper, etc., and +hence the former only will be considered here; but the theories and +facts relative to textile dyeing are applicable in a general way to +all other forms as well. + +225. Plants as a Source of Dyes. Among the most beautiful examples +of man's handiwork are the baskets and blankets of the North American +Indians, woven with a skill which cannot be equaled by manufacturers, +and dyed in mellow colors with a few simple dyes extracted from local +plants. The magnificent rugs and tapestries of Persia and Turkey, and +the silks of India and Japan, give evidence that a knowledge of dyes +is widespread and ancient. Until recently, the vegetable world was +the source of practically all coloring matter, the pulverized root of +the madder plant yielding the reds, the leaves and stems of the indigo +plant the blues, the heartwood of the tropical logwood tree the blacks +and grays, and the fruit of certain palm and locust trees yielding the +soft browns. So great was the commercial demand for dyestuffs that +large areas of land were given over to the exclusive cultivation of +the more important dye plants. Vegetable dyes are now, however, rarely +used because about the year 1856 it was discovered that dyes could be +obtained from coal tar, the thick sticky liquid formed as a by-product +in the manufacture of coal gas. These artificial coal-tar, or aniline, +dyes have practically undisputed sway to-day, and the vast areas of +land formerly used for the cultivation of vegetable dyes are now free +for other purposes. + +226. Wool and Cotton Dyeing. If a piece of wool is soaked in a +solution of a coal-tar dye, such as magenta, the fiber of the cloth +draws some of the dye out of the solution and absorbs it, becoming in +consequence beautifully colored. The coloring matter becomes "part and +parcel," as it were, of the wool fiber, because repeated washing of +the fabric fails to remove the newly acquired color; the magenta +coloring matter unites chemically with the fiber of the wool, and +forms with it a compound insoluble in water, and hence fast to +washing. + +But if cotton is used instead of wool, the acquired color is very +faint, and washes off readily. This is because cotton fibers possess +no chemical substance capable of uniting with the coloring matter to +form a compound insoluble in water. + +If magenta is replaced by other artificial dyes,--for example, +scarlets,--the result is similar; in general, wool material absorbs +dye readily, and uniting with it is permanently dyed. Cotton material, +on the other hand, does not combine chemically with coloring matter +and therefore is only faintly tinged with color, and loses this when +washed. When silk and linen are tested, it is found that the former +behaves in a general way as did wool, while the linen has more +similarity to the cotton. That vegetable fibers, such as cotton and +linen, should act differently toward coloring matter from animal +fibers, such as silk and wool, is not surprising when we consider that +the chemical nature of the two groups is very different; vegetable +fibers contain only oxygen, carbon, and hydrogen, while animal fibers +always contain nitrogen in addition, and in many cases sulphur as +well. + +227. The Selection of Dyes. When silk and wool, cotton and linen, +are tested in various dye solutions, it is found that the former have, +in general, a great affinity for coloring matter and acquire a +permanent color, but that cotton and linen, on the other hand, have +little affinity for dyestuffs. The color acquired by vegetable fibers +is, therefore, usually faint. + +There are, of course, many exceptions to the general statement that +animal fibers dye readily and vegetable fibers poorly, because certain +dyes fail utterly with woolen and silk material and yet are fairly +satisfactory when applied to cotton and linen fabrics. Then, too, a +dye which will color silk may not have any effect on wool in spite of +the fact that wool, like silk, is an animal fiber; and certain +dyestuffs to which cotton responds most beautifully are absolutely +without effect on linen. + +The nature of the material to be dyed determines the coloring matter +to be used; in dyeing establishments a careful examination is made of +all textiles received for dyeing, and the particular dyestuffs are +then applied which long experience has shown to be best suited to the +material in question. Where "mixed goods," such as silk and wool, or +cotton and wool, are concerned, the problem is a difficult one, and +the countless varieties of gorgeously colored mixed materials give +evidence of high perfection in the art of dyeing and weaving. + +Housewives who wish to do successful home dyeing should therefore not +purchase dyes indiscriminately, but should select the kind best suited +to the material, because the coloring principle which will remake a +silk waist may utterly ruin a woolen skirt or a linen suit. Powders +designed for special purposes may be purchased from druggists. + +228. Indirect Dyeing. We have seen that it is practically impossible +to color cotton and linen in a simple manner with any degree of +permanency, because of the lack of chemical action between vegetable +fibers and coloring matter. But the varied uses to which dyed articles +are put make fastness of color absolutely necessary. A shirt, for +example, must not be discolored by perspiration, nor a waist faded by +washing, nor a carpet dulled by sweeping with a dampened broom. In +order to insure permanency of dyes, an indirect method was originated +which consisted of adding to the fibers a chemical capable of acting +upon the dye and forming with it a colored compound insoluble in +water, and hence "safe." For example, cotton material dyed directly in +logwood solution has almost no value, but if it is soaked in a +solution of oxalic acid and alum until it becomes saturated with the +chemicals, and is then transferred to a logwood bath, the color +acquired is fast and beautiful. + +This method of indirect dyeing is known as the mordanting process; it +consists of saturating the fabric to be dyed with chemicals which will +unite with the coloring matter to form compounds unaffected by water. +The chemicals are called mordants. + +229. How Variety of Color is Secured. The color which is fixed on +the fabric as a result of chemical action between mordant and dye is +frequently very different from that of the dye itself. Logwood dye +when used alone produces a reddish brown color of no value either for +beauty or permanence; but if the fabric to be dyed is first mordanted +with a solution of alum and oxalic acid and is then immersed in a +logwood bath, it acquires a beautiful blue color. + +Moreover, since the color acquired depends upon the mordant as well as +upon the dye, it is often possible to obtain a wide range of colors by +varying the mordant used, the dye remaining the same. For example, +with alum and oxalic acid as a mordant and logwood as a dye, blue is +obtained; but with a mordant of ferric sulphate and a dye of logwood, +blacks and grays result. Fabrics immersed directly in alizarin acquire +a reddish yellow tint; when, however, they are mordanted with certain +aluminium compounds they acquire a brilliant Turkey red, when +mordanted with chromium compounds, a maroon, and when mordanted with +iron compounds, the various shades of purple, lilac, and violet +result. + +230. Color Designs in Cloth. It is thought that the earliest +attempts at making "fancy materials" consisted in painting designs on +a fabric by means of a brush. In more recent times the design was cut +in relief on hard wood, the relief being then daubed with coloring +matter and applied by hand to successive portions of the cloth. The +most modern method of design-making is that of machine or roller +printing. In this, the relief blocks are replaced by engraved copper +rolls which rotate continuously and in the course of their rotation +automatically receive coloring matter on the engraved portion. The +cloth is to be printed is then drawn uniformly over the rotating roll, +receiving color from the engraved design; in this way, the color +pattern is automatically printed on the cloth with perfect regularity. +In cases where the fabrics do not unite directly with the coloring +matter, the design is supplied with a mordant and the impression made +on the fabric is that of the mordant; when the fabric is later +transferred to a dye bath, the mordanted portions, represented by the +design, unite with the coloring matter and thus form the desired color +patterns. + +Unless the printing is well done, the coloring matter does not +thoroughly penetrate the material, and only a faint blurred design +appears on the back of the cloth; the gaudy designs of cheap calicoes +and ginghams often do not show at all on the under side. Such +carelessly made prints are not fast to washing or light, and soon +fade. But in the better grades of material the printing is well done, +and the color designs are fairly fast, and a little care in the +laundry suffices to eliminate any danger of fading. + +Color designs of the greatest durability are produced by the weaving +together of colored yarns. When yarn is dyed, the coloring matter +penetrates to every part of the fiber, and hence the patterns formed +by the weaving together of well-dyed yarns are very fast to light and +water. + +If the color designs to be woven in the cloth are intricate, complex +machinery is necessary and skillful handwork; hence, patterns formed +by the weaving of colored yarns are expensive and less common than +printed fabrics. + + + + +CHAPTER XXIV + +CHEMICALS AS DISINFECTANTS AND PRESERVATIVES + + +231. The prevention of disease epidemics is one of the most striking +achievements of modern science. Food, clothing, furniture, and other +objects contaminated in any way by disease germs may be disinfected by +chemicals or by heat, and widespread infection from persons suffering +with a contagious disease may be prevented. + +[Illustration: FIG. 160.--Pasteurizing apparatus, an arrangement by +which milk is conveniently heated to destroy disease germs.] + +When disease germs are within the body, the problem is far from +simple, because chemicals which would effectively destroy the germs +would be fatal to life itself. But when germs are outside the body, as +in water or milk, or on clothing, dishes, or furniture, they can be +easily killed. One of the best methods of destroying germs is to +subject them to intense heat. Contaminated water is made safe by +boiling for a few minutes, because the strong heat destroys the +disease-producing germs. Scalded or Pasteurized milk saves the lives +of scores of babies, because the germs of summer complaint which lurk +in poor milk are killed and rendered harmless in the process of +scalding. Dishes used by consumptives, and persons suffering from +contagious diseases, can be made harmless by thorough washing in thick +suds of almost boiling water. + +The bedding and clothing of persons suffering with diphtheria, +tuberculosis, and other germ diseases should always be boiled and hung +to dry in the bright sunlight. Heat and sunshine are two of the best +disinfectants. + +232. Chemicals. Objects, such as furniture, which cannot be boiled, +are disinfected by the use of any one of several chemicals, such as +sulphur, carbolic acid, chloride of lime, corrosive sublimate, etc. + +One of the simplest methods of disinfecting consists in burning +sulphur in a room whose doors, windows, and keyholes have been closed, +so that the burning fumes cannot escape, but remain in the room long +enough to destroy disease germs. This is probably the most common +means of fumigation. + +For general purposes, carbolic acid is one of the very best +disinfectants, but must be used with caution, as it is a deadly poison +except when very dilute. + +Chloride of lime when exposed to the air and moisture slowly gives off +chlorine, and can be used as a disinfectant because the gas thus set +free attacks germs and destroys them. For this reason chloride of lime +is an excellent disinfectant of drainpipes. Certain bowel troubles, +such as diarrhoea, are due to microbes, and if the waste matter of a +person suffering from this or similar diseases is allowed passage +through the drainage system, much damage may be done. But a small +amount of chloride of lime in the closet bowl will insure +disinfection. + +233. Personal Disinfection. The hands may gather germs from any +substances or objects with which they come in contact; hence the hands +should be washed with soap and water, and especially before eating. +Physicians who perform operations wash not only their hands, but their +instruments, sterilizing the latter by placing them in boiling water +for several minutes. + +Cuts and wounds allow easy access to the body; a small cut has been +known to cause death because of the bacteria which found their way +into the open wound and produced disease. In order to destroy any +germs which may have entered into the cut from the instrument, it is +well to wash out the wound with some mild disinfectant, such as very +dilute carbolic acid or hydrogen peroxide, and then to bind the wound +with a clean cloth, to prevent later entrance of germs. + +234. Chemicals as Food Preservatives. The spoiling of meats and +soups, and the souring of milk and preserves, are due to germs which, +like those producing disease, can be destroyed by heat and by +chemicals. + +Milk heated to the boiling point does not sour readily, and successful +canning consists in cooking fruits and vegetables until all the germs +are killed, and then sealing the cans so that germs from outside +cannot find entrance and undo the work of the canner. + +Some dealers and manufacturers have learned that certain chemicals +will act as food preservatives, and hence they have replaced the safe +method of careful canning by the quicker and simpler plan of adding +chemicals to food. Catchup, sauces, and jellies are now frequently +preserved in this way. But the chemicals which destroy bacteria +frequently injure the consumer as well. And so much harm has been done +by food preservatives that the pure food laws require that cans and +bottles contain a labeled statement of the kind and quantity of +chemicals used. + +Even milk is not exempt, but is doctored to prevent souring, the +preservative most generally used by milk dealers being formaldehyde. +The vast quantity of milk consumed by young and old, sick and well, +makes the use of formaldehyde a serious menace to health, because no +constitution can endure the injury done by the constant use of +preservatives. + +The most popular and widely used preservatives of meats are borax and +boric acid. These chemicals not only arrest decay, but partially +restore to old and bad meat the appearance of freshness; in this way +unscrupulous dealers are able to sell to the public in one form or +other meats which may have undergone partial decomposition; sausage +frequently contains partially decomposed meat, restored as it were by +chemicals. + +In jams and catchups there is abundant opportunity for preservatives; +badly or partially decayed fruits are sometimes disinfected and used +as the basis of foods sold by so-called good dealers. Benzoate of +soda, and salicylic acid are the chemicals most widely employed for +this purpose, with coal-tar dyes to simulate the natural color of the +fruit. + +Many of the cheap candies sold by street venders are not fit for +consumption, since they are not only made of bad material, but are +frequently in addition given a light dipping in varnish as a +protection against the decaying influences of the atmosphere. + +The only wise preservatives are those long known and employed by our +ancestors; salt, vinegar, and spices are all food preservatives, but +they are at the same time substances which in small amounts are not +injurious to the body. Smoked herring and salted mackerel are +chemically preserved foods, but they are none the less safe and +digestible. + +235. The Preservation of Wood and Metal. The decaying of wood and +the rusting of metal are due to the action of air and moisture. When +wood and metal are surrounded with a covering which neither air nor +moisture can penetrate, decay and rust are prevented. Paint affords +such a protective covering. The main constituent of paint is a +compound of white lead or other metallic substance; this is mixed with +linseed oil or its equivalent in order that it may be spread over wood +and metal in a thin, even coating. After the mixture has been applied, +it hardens and forms a tough skin fairly impervious to weathering. For +the sake of ornamentation, various colored pigments are added to the +paint and give variety of effect. + +Railroad ties and street paving blocks are ordinarily protected by oil +rather than paint. Wood is soaked in creosote oil until it becomes +thoroughly saturated with the oily substance. The pores of the wood +are thus closed to the entrance of air and moisture, and decay is +avoided. Wood treated in this way is very durable. Creosote is +poisonous to insects and many small animals, and thus acts as a +preservation not only against the elements but against animal life as +well. + + + + +CHAPTER XXV + +DRUGS AND PATENT MEDICINES + + +236. Stimulants and Narcotics. Man has learned not only the action +of substances upon each other, such as bleaching solution upon +coloring matter, washing soda upon grease, acids upon bases, but also +the effect which certain chemicals have upon the human body. + +Drugs and their varying effects upon the human system have been known +to mankind from remote ages; in the early days, familiar leaves, +roots, and twigs were steeped in water to form medicines which served +for the treatment of all ailments. In more recent times, however, +these simple herb teas have been supplanted by complex drugs, and now +medicines are compounded not only from innumerable plant products, but +from animal and mineral matter as well. Quinine, rhubarb, and arnica +are examples of purely vegetable products; iron, mercury, and arsenic +are equally well known as distinctly mineral products, while cod-liver +oil is the most familiar illustration of an animal remedy. Ordinarily +a combination of products best serves the ends of the physician. + +Substances which, like cod-liver oil, serve as food to a worn-out +body, or, like iron, tend to enrich the blood, or, like quinine, aid +in bringing an abnormal system to a healthy condition, are valuable +servants and cannot be entirely dispensed with so long as man is +subject to disease. + +But substances which, like opium, laudanum, and alcohol, are not +required by the body as food, or as a systematic, intelligent aid to +recovery, but are taken solely for the stimulus aroused or for the +insensibility induced, are harmful to man, and cannot be indulged in +by him without ultimate mental, moral, and physical loss. Substances +of the latter class are known as narcotics and stimulants. + +237. The Cost of Health. In the physical as in the financial world, +nothing is to be had without a price. Vigor, endurance, and mental +alertness are bought by hygienic living; that is, by proper food, +fresh air, exercise, cleanliness, and reasonable hours. Some people +wish vigor, endurance, etc., but are unwilling to live the life which +will develop these qualities. Plenty of sleep, exercise, and simple +food all tend to lay the foundations of health. Many, however, are not +willing to take the care necessary for healthful living, because it +would force them to sacrifice some of the hours of pleasure. Sooner or +later, these pleasure-seekers begin to feel tired and worn, and some +of them turn to drugs and narcotics for artificial strength. At first +the drugs seem to restore the lost energy, and without harm; however, +the cost soon proves to be one of the highest Nature ever demands. + +238. The Uncounted Cost. The first and most obvious effect of opium, +for example, is to deaden pain and to arouse pleasure; but while the +drug is producing these soothing sensations, it interferes with bodily +functions. Secretion, digestion, absorption of food, and the removal +of waste matters are hindered. Continued use of the drug leads to +headache, exhaustion, nervous depression, and heart weakness. There is +thus a heavy toll reckoned against the user, and the creditor is +relentless in demanding payment. + +Moreover, the respite allowed by a narcotic is exceedingly brief, and +a depression which is long and deep inevitably follows. In order to +overcome this depression, recourse is usually had to a further dose, +and as time goes on, the intervals of depression become more frequent +and lasting, and the necessity to overcome them increases. Thus +without intention one finds one's self bound to the drug, its fast +victim. The sanatoria of our country are crowded with people who are +trying to free themselves of a drug habit into which they have drifted +unintentionally if not altogether unknowingly. What is true of opium +is equally applicable to other narcotics. + +239. The Right Use of Narcotics. In the hands of the physician, +narcotics are a great blessing. In some cases, by relieving pain, they +give the system the rest necessary for overcoming the cause of the +pain. Only those who know of the suffering endured in former times can +fully appreciate the decrease in pain brought about by the proper use +of narcotics. + +240. Patent Medicines, Cough Sirups. A reputable physician is +solicitous regarding the permanent welfare of his patient and +administers carefully chosen and harmless drugs. Mere medicine +venders, however, ignore the good of mankind, and flood the market +with cheap patent preparations which delude and injure those who +purchase, but bring millions of dollars to those who manufacture. + +Practically all of these patent, or proprietary, preparations contain +a large proportion of narcotics or stimulants, and hence the benefit +which they seem to afford the user is by no means genuine; examination +shows that the relief brought by them is due either to a temporary +deadening of sensibilities by narcotics or to a fleeting stimulation +by alcohol and kindred substances. + +Among the most common ailments of both young and old are coughs and +colds; hence many patent cough mixtures have been manufactured and +placed on the market for the consumption of a credulous public. Such +"quick cures" almost invariably contain one or more narcotic drugs, +and not only do not relieve the cold permanently, but occasion +subsequent disorders. Even lozenges and pastilles are not free from +fraud, but have a goodly proportion of narcotics, containing in some +cases chloroform, morphine, and ether. + +The widespread use of patent cough medicines is due largely to the +fact that many persons avoid consulting a physician about so trivial +an ailment as an ordinary cold, or are reluctant to pay a medical fee +for what seems a slight indisposition and hence attempt to doctor +themselves. + +Catarrh is a very prevalent disease in America, and consequently +numerous catarrh remedies have been devised, most of which contain in +a disguised form the pernicious drug, cocaine. Laws have been enacted +which require on the labels a declaration of the contents of the +preparation, both as to the kind of drug used and the amount, and the +choice of accepting or refusing such mixtures is left to the +individual. But the great mass of people are ignorant of the harmful +nature of drugs in general, and hence do not even read the +self-accusing label, or if they do glance at it, fail to comprehend +the dangerous nature of the drugs specified there. In order to +safeguard the uninformed purchaser and to restrict the manufacture of +harmful patent remedies, some states limit the sale of all +preparations containing narcotics and thus give free rein to neither +consumer nor producer. + +241. Soothing Sirups; Soft Drinks. The development of a race is +limited by the mental and physical growth of its children, and yet +thousands of its children are annually stunted and weakened by drugs, +because most colic cures, teething concoctions, and soothing syrups +are merely agreeably flavored drug mixtures. Those who have used such +preparations freely, know that a child usually becomes fretful and +irritable between doses, and can be quieted only by larger and more +frequent supplies. A habit formed in this way is difficult to +overcome, and many a child when scarcely over its babyhood had a +craving which in later years may lead to systematic drug taking. And +even though the pernicious drug craving is not created, considerable +harm is done to the child, because its body is left weak and +non-resistant to diseases of infancy and childhood. + +Many of our soft drinks contain narcotics. The use of the coca leaf +and the kola nut for such preparations has increased very greatly +within the last few years, and doubtless legislation will soon be +instituted against the indiscriminate sale of soft drinks. + +242. Headache Powders. The stress and strain of modern life has +opened wide the door to a multitude of bodily ills, among which may be +mentioned headache. Work must be done and business attended to, and +the average sufferer does not take time from his vocation to +investigate the cause of the headache, but unthinkingly grasps at any +remedy which will remove the immediate pain, and utterly disregards +later injury. The relief afforded by most headache mixtures is due to +the presence of antipyrin or acetanilid, and it has been shown +conclusively that these drugs weaken heart action, diminish +circulation, reduce the number of red corpuscles in the blood, and +bring on a condition of chronic anemia. Pallid cheeks and blue lips +are visible evidence of the too frequent use of headache powders. + +The labels required by law are often deceptive and convey no adequate +idea of the amount of drug consumed; for example, 240 grains of +acetanilid to an ounce seems a small quantity of drug for a powder, +but when one considers that there are only 480 grains in an ounce, it +will be seen that each powder is one half acetanilid. + +Powders taken in small quantities and at rare intervals are apparently +harmless; but they never remove the cause of the trouble, and hence +the discomfort soon returns with renewed force. Ordinarily, hygienic +living will eliminate the source of the trouble, but if it does not, a +physician should be consulted and medicine should be procured from him +which will restore the deranged system to its normal healthy +condition. + +243. Other Deceptions. Nearly all patent medicines contain some +alcohol, and in many, the quantity of alcohol is far in excess of that +found in the strongest wines. Tonics and bitters advertised as a cure +for spring fever and a worn-out system are scarcely more than cheap +cocktails, as one writer has derisively called them, and the amount of +alcohol in some widely advertised patent remedies is alarmingly large +and almost equal to that of strong whisky. + +[Illustration: FIG. 161.--Diagram showing the amount of alcohol in +some alcoholic drinks and in one much used patent medicine.] + +Some conscientious persons who would not touch beer, wine, whisky, or +any other intoxicating drink consume patent remedies containing large +quantities of alcohol and thus unintentionally expose themselves to +mental and physical danger. In all cases of bodily disorder, the only +safe course is to consult a physician who has devoted himself to the +study of the body and the methods by which a disordered system may be +restored to health. + + + + +CHAPTER XXVI + +NITROGEN AND ITS RELATION TO PLANTS + + +244. Nitrogen. A substance which plays an important part in animal +and plant life is nitrogen. Soil and the fertilizers which enrich it, +the plants which grow on it, and the animals which feed on these, all +contain nitrogen or nitrogenous compounds. The atmosphere, which we +ordinarily think of as a storehouse of oxygen, contains far more +nitrogen than oxygen, since four fifths of its whole weight is made up +of this element. + +Nitrogen is colorless, odorless, and tasteless. Air is composed +chiefly of oxygen and nitrogen; if, therefore, the oxygen in a vessel +filled with air can be made to unite with some other substance or can +be removed, there will be a residue of nitrogen. This can be done by +floating on water a light dish containing phosphorus, then igniting +the phosphorus, and placing an inverted jar over the burning +substance. The phosphorus in burning unites with the oxygen of the air +and hence the gas that remains in the jar is chiefly nitrogen. It has +the characteristics mentioned above and, in addition, does not combine +readily with other substances. + +245. Plant Food. Food is the course of energy in every living thing +and is essential to both animal and plant life. Plants get their food +from the lifeless matter which exists in the air and in the soil; +while animals get their food from plants. It is true that man and many +other animals eat fleshy foods and depend upon them for partial +sustenance, but the ultimate source of all animal food is plant life, +since meat-producing animals live upon plant growth. + +Plants get their food from the air, the soil, and moisture. From the +air, the leaves take carbon dioxide and water and transform them into +starchy food; from the soil, the roots take water rich in mineral +matters dissolved from the soil. From the substances thus gathered, +the plant lives and builds up its structure. + +A food substance necessary to plant life and growth is nitrogen. Since +a vast store of nitrogen exists in the air, it would seem that plants +should never lack for this food, but most plants are unable to make +use of the boundless store of atmospheric nitrogen, because they do +not possess the power of abstracting nitrogen from the air. For this +reason, they have to depend solely upon nitrogenous compounds which +are present in the soil and are soluble in water. The soluble +nitrogenous soil compounds are absorbed by roots and are utilized by +plants for food. + +246. The Poverty of the Soil. Plant roots are constantly taking +nitrogen and its compounds from the soil. If crops which grow from the +soil are removed year after year, the soil becomes poorer in nitrogen, +and finally possesses too little of it to support vigorous and healthy +plant life. The nitrogen of the soil can be restored if we add to it a +fertilizer containing nitrogen compounds which are soluble in water. +Decayed vegetable matter contains large quantities of nitrogen +compounds, and hence if decayed vegetation is placed upon soil or is +plowed into soil, it acts as a fertilizer, returning to the soil what +was taken from it. Since man and all other animals subsist upon +plants, their bodies likewise contain nitrogenous substances, and +hence manure and waste animal matter is valuable as a fertilizer or +soil restorer. + +247. Bacteria as Nitrogen Gatherers. Soil from which crops are removed +year after year usually becomes less fertile, but the soil from which +crops of clover, peas, beans, or alfalfa have been removed is richer in +nitrogen rather than poorer. This is because the roots of these plants +often have on them tiny swellings, or tubercles, in which millions of +certain bacteria live and multiply. These bacteria have the remarkable +power of taking free nitrogen from the air in the soil and of combining +it with other substances to form compounds which plants can use. The +bacteria-made compounds dissolve in the soil water and are absorbed into +the plant by the roots. So much nitrogen-containing material is made by +the root bacteria of plants of the pea family that the soil in which +they grow becomes somewhat richer in nitrogen, and if plants which +cannot make nitrogen are subsequently planted in such a soil, they find +there a store of nitrogen. A crop of peas, beans, or clover is +equivalent to nitrogenous fertilizer and helps to make ready the soil +for other crops. + +[Illustration: FIG. 162.--Roots of soy bean having tubercle-bearing +bacteria.] + +248. Artificial Fertilizers. Plants need other foods besides +nitrogen, and they exhaust the soil not only of nitrogen, but also of +phosphorus and potash, since large quantities of these are necessary +for plant life. There are many other substances absorbed from the soil +by the plant, namely, iron, sodium, calcium, magnesium, but these are +used in smaller quantities and the supply in the soil does not readily +become exhausted. + +Commercial fertilizers generally contain nitrogen, phosphorus, and +potash in amounts varying with the requirements of the soil. Wheat +requires a large amount of phosphorus and quickly exhausts the ground +of that food stuff; a field which has supported a crop of wheat is +particularly poor in phosphorus, and a satisfactory fertilizer for +that land would necessarily contain a large percentage of phosphorus. +The fertilizer to be used in a soil depends upon the character of the +soil and upon the crops previously grown on it. + +[Illustration: FIG. 163.--Water cultures of buckwheat: 1, with all the +food elements; 2, without potash; 3, without nitrates.] + +The quantity of fertilizer needed by the farmers of the world is +enormous, and the problem of securing the necessary substances in +quantities sufficient to satisfy the demand bids fair to be serious. +But modern chemistry is at work on the problem, and already it is +possible to make some nitrogen compounds on a commercial scale. When +nitrogen gas is in contact with heated calcium carbide, a reaction +takes place which results in the formation of calcium nitride, a +compound suitable for enriching the soil. There are other commercial +methods for obtaining nitrogen compounds which are suitable for +absorption by plant roots. + +Phosphorus is obtained from bone ash and from phosphate rock which is +widely distributed over the surface of the earth. Bone ash and +thousands of tons of phosphate rock are treated with sulphuric acid to +form a phosphorus compound which is soluble in soil water and which, +when added to soil, will be usable by the plants growing there. + +The other important ingredient of most fertilizers is potash. Wood +ashes are rich in potash and are a valuable addition to the soil. But +the amount of potash thus obtained is far too limited to supply the +needs of agriculture; and to-day the main sources of potash are the +vast deposits of potassium salts found in Prussia. + +Although Germany now furnishes the American farmer with the bulk of +his potash, she may not do so much longer. In 1911 an indirect potash +tax was levied by Germany on her best customer, the United States, to +whom 15 million dollars' worth of potash had been sold the preceding +year. This led Americans to inquire whether potash could not be +obtained at home. + +Geologists say that long ages ago Germany was submerged, that the +waters slowly evaporated and that the various substances in the sea +water were deposited in thick layers. The deposits thus left by the +evaporation of the sea water gradually became hidden by sediment and +soil, and lost to sight. From such deposits, potash is obtained. +Geologists tell us that our own Western States were once submerged, +and that the waters evaporated and disappeared from our land very much +as they did from Germany. The Great Salt Lake of Utah is a relic of a +great body of water. If it be true that waters once covered our +Western States, there may be buried deposits of potash there, and +to-day the search for the hidden treasure is going on with the energy +and enthusiasm characteristic of America. + +Another probable source of potash is seaweed. The sea is a vast +reservoir of potash, and seaweed, especially the giant kelp, absorbs +large quantities of this potash. A ton of dried kelp (dried by sun and +wind) contains about 500 pounds of pure potash. The kelps are +abundant, covering thousands of square miles in the Pacific Ocean, +from Mexico to the Arctic Ocean. + + + + +CHAPTER XXVII + +SOUND + + +249. The Senses. All the information which we possess of the world +around us comes to us through the use of the senses of sight, hearing, +taste, touch, and smell. Of the five senses, sight and hearing are +generally considered the most valuable. In preceding Chapters we +studied the important facts relative to light and the power of vision; +it remains for us to study Sound as we studied Light, and to learn +what we can of sound and the power to hear. + +250. How Sound is Produced. If one investigates the source of any +sound, he will always find that it is due to motion of some kind. A +sudden noise is traced to the fall of an object, or to an explosion, +or to a collision; in fact, is due to the motion of matter. A piano +gives out sound whenever a player strikes the keys and sets in motion +the various wires within the piano; speech and song are caused by the +motion of chest, vocal cords, and lips. + +[Illustration: FIG. 164.--Sprays of water show that the fork is in +motion.] + +If a large dinner bell is rung, its motion or vibration may be felt on +touching it with the finger. If a tuning fork is made to give forth +sound by striking it against the knee, or hitting it with a rubber +hammer, and is then touched to the surface of water, small sprays of +water will be thrown out, showing that the prongs of the fork are in +rapid motion. (A rubber hammer is made by putting a piece of glass +tubing through a rubber cork.) + +If a light cork ball on the end of a thread is brought in contact with +a sounding fork, the ball does not remain at rest, but vibrates back +and forth, being driven by the moving prongs. + +[Illustration: FIG. 165.--The ball does not remain at rest] + +These simple facts lead us to conclude that all sound is due to the +motion of matter, and that a sounding body of any kind is in rapid +motion. + +251. Sound is carried by Matter. In most cases sound reaches the ear +through the air; but air is not the only medium through which sound is +carried. A loud noise will startle fish, and cause them to dart away, +so we conclude that the sound must have reached them through the +water. An Indian puts his ear to the ground in order to detect distant +footsteps, because sounds too faint to be heard through the air are +comparatively clear when transmitted through the earth. A gentle +tapping at one end of a long table can be distinctly heard at the +opposite end if the ear is pressed against the table; if the ear is +removed from the wood, the sound of tapping is much fainter, showing +that wood transmits sound more readily than air. We see therefore that +sound can be transmitted to the ear by solids, liquids, or gases. + +Matter of any kind can transmit sound to the ear. The following +experiments will show that matter is necessary for transmission. +Attach a small toy bell to a glass rod (Fig. 166) by means of a rubber +tube and pass the rod through one of two openings in a rubber cork. +Insert the cork in a strong flask containing a small quantity of water +and shake the bell, noting the sound produced. Then heat the flask, +allowing the water to boil briskly, and after the boiling has +continued for a few minutes remove the flame and instantly close up +the second opening by inserting a glass stopper. Now shake the flask +and note that the sound is very much fainter than at first. As the +flask was warmed, air was rapidly expelled; so that when the flask was +shaken the second time, less air was present to transmit the sound. If +the glass stopper is removed and the air is allowed to reenter the +flask, the loudness of the sound immediately increases. + +[Illustration: FIG. 166.--Sound is carried by the air.] + +Since the sound of the bell grows fainter as air is removed, we infer +that there would be no sound if all the air were removed from the +flask; that is to say, sound cannot be transmitted through empty space +or a vacuum. If sound is to reach our ears, it must be through the +agency of matter, such as wood, water, or air, etc. + +252. How Sound is transmitted through Air. We saw in Section 250 +that sound can always be traced to the motion or vibration of matter. +It is impossible to conceive of an object being set into sudden and +continued motion without disturbing the air immediately surrounding +it. A sounding body always disturbs and throws into vibration the air +around it, and the air particles which receive motion from a sounding +body transmit their motion to neighboring particles, these in turn to +the next adjacent particles, and so on until the motion has traveled +to very great distances. The manner in which vibratory motion is +transmitted by the atmosphere must be unusual in character, since no +motion of the air is apparent, and since in the stillness of night +when "not a breath of air" is stirring, the shriek of a railroad +whistle miles distant may be heard with perfect clearness. Moreover, +the most delicate notes of a violin can be heard in the remotest +corners of a concert hall, when not the slightest motion of the air +can be seen or felt. + +In our study of the atmosphere we saw that air can be compressed and +rarefied; in other words, we saw that air is very elastic. It can be +shown experimentally that whenever an elastic body in motion comes in +contact with a body at rest, the moving body transfers its motion to +the second body and then comes to rest itself. Let two billiard balls +be suspended in the manner indicated in Figure 167. If one of the +balls is drawn aside and is then allowed to fall against the other, +the second ball is driven outward to practically the height from which +the first ball fell and the first ball comes to rest. + +[Illustration: FIG. 167.--Elastic balls.] + +[Illustration: FIG. 168.--Suspended billiard balls.] + +If a number of balls are arranged in line as in Figure 168 or Figure +169, and the end ball is raised and then allowed to fall, or if _A_ is +pushed against _C_, the last ball _B_ will move outward alone, with a +force nearly equal to that originally possessed by _A_ and to a +distance nearly equal to that through which _A_ moved. But there will +be no _visible_ motion of the intervening balls. The force of the +moving ball _A_ is given to the second ball, and the second ball in +turn gives the motion to the third, and so on throughout the entire +number, until _B_ is reached. But _B_ has no ball to give its motion +to, hence _B_ itself moves outward, and moves with a force nearly +equal to that originally imparted by _A_ and to a distance nearly +equal to that through which _A_ fell. Motion at _A_ is transmitted to +_B_ without any perceptible motion of the balls lying between these +points. Similarly the particles of air set into motion by a sounding +body impart their motion to each other, the motion being transmitted +onward without any perceptible motion of the air itself. When this +motion reaches the ear, it sets the drum of the ear into vibration, +and these vibrations are in turn transmitted to the auditory nerves, +which interpret the motion as sound. + +[Illustration: FIG. 169.--Elastic balls transmit motion.] + +[Illustration: FIG. 170.--When a ball meets more than one ball, it +divides its motion.] + +253. Why Sound dies away with Distance. Since the last ball _B_ is +driven outward with a force nearly equal to that possessed by _A_, it +would seem that the effect on the ear drum should be independent of +distance and that a sound should be heard as distinctly when remote as +when near. But we know from experience that this is not true, because +the more distant the source of sound, the fainter the impression; and +finally, if the distance between the source of sound and the hearer +becomes too great, the sound disappears entirely and nothing is heard. +The explanation of this well-known fact is found in a further study of +the elastic balls (Fig. 170). If _A_ hits two balls instead of one, +the energy possessed by _A_ is given in part to one ball, and in part +to the other, so that neither obtains the full amount. These balls, +having each received less than the original energy, have less to +transmit; each of these balls in turn meets with others, and hence the +motion becomes more and more distributed, and distant balls receive +less and less impetus. The energy finally given becomes too slight to +affect neighboring balls, and the system comes to rest. This is what +occurs in the atmosphere; a moving air particle meets not one but many +adjacent air particles, and each of these receives a portion of the +original energy and transmits a portion. When the original disturbance +becomes scattered over a large number of air particles, the energy +given to any one air particle becomes correspondingly small, and +finally the energy becomes so small that further particles are not +affected; beyond this limit the sound cannot be heard. + +If an air particle transmitted motion only to those air particles +directly in line with it, we should not be able to detect sound unless +the ear were in direct line with the source. The fact that an air +particle divides its motion among all particles which it touches, that +is, among those on the sides as well as those in front, makes it +possible to hear sound in all directions. A good speaker is heard not +only by those directly in front of him, but by those on the side, and +even behind him. + +254. Velocity of Sound. The transmission of motion from particle to +particle does not occur instantaneously, but requires time. If the +distance is short, so that few air particles are involved, the time +required for transmission is very brief, and the sound is heard at +practically the instant it is made. Ordinarily we are not conscious +that it requires time for sound to travel from its source to our ears, +because the distance involved is too short. At other times we +recognize that there is a delay; for example, thunder reaches our ears +after the lightning which caused the thunder has completely +disappeared. If the storm is near, the interval of time between the +lightning and the thunder is brief, because the sound does not have +far to travel; if the storm is distant, the interval is much longer, +corresponding to the greater distance through which the sound travels. +Sound does not move instantaneously, but requires time for its +transmission. The report of a distant cannon is heard after the flash +and smoke are seen; the report of a near cannon is heard the instant +the flash is seen. + +The speed with which sounds travels through the air, or its velocity, +was first measured by noting the interval (54.6 seconds) which elapsed +between the flash of a cannon and the sound of the report. The +distance of the cannon from the observer was measured and found to be +61,045 feet, and by dividing this distance by the number of seconds, +we find that the distance traveled by sound in one second is +approximately 1118 feet. + +High notes and low notes, soft notes and shrill notes, all travel at +the same rate. If bass notes traveled faster or slower than soprano +notes, or if the delicate tones of the violin traveled faster or +slower than the tones of a drum, music would be practically +impossible, because at a distance from the source of sound the various +tones which should be in unison would be out of time--some arriving +late, some early. + +255. Sound Waves. Practically everyone knows that a hammock hung +with long ropes swings or vibrates more slowly than one hung with +short ropes, and that a stone suspended by a long string swings more +slowly than one suspended by a short string. No two rocking chairs +vibrate in the same way unless they are exactly alike in shape, size, +and material. An object when disturbed vibrates in a manner peculiar +to itself, the vibration being slow, as in the case of the long-roped +swing, or quick, as in the case of the short-roped swing. The time +required for a single swing or vibration is called the _period_ of the +body, and everything that can vibrate has a characteristic period. +Size and shape determine to a large degree the period of a body; for +example, a short, thick tuning fork vibrates more rapidly than a tall +slender fork. + +[Illustration: FIG. 171.--The two hammocks swing differently.] + +Some tuning forks when struck vibrate so rapidly that the prongs move +back and forth more than 5000 times per second, while other tuning +forks vibrate so slowly that the vibrations do not exceed 50 per +second. In either case the distance through which the prongs move is +very small and the period is very short, so that the eye can seldom +detect the movement itself. That the prongs are in motion, however, is +seen by the action of a pith ball when brought in contact with the +prongs (see Section 250). + +[Illustration: FIG. 172.--The pitch given out by a fork depends upon +its shape.] + +The disturbance created by a vibrating body is called a wave. + +256. Waves. While the disturbance which travels out from a sounding +body is commonly called a wave, it is by no means like the type of +wave best known to us, namely, the water wave. + +If a closely coiled heavy wire is suspended as in Figure 173 and the +weight is drawn down and then released, the coil will assume the +appearance shown; there is clearly an overcrowding or condensation in +some places, and a spreading out or rarefaction in other places. The +pulse of condensation and rarefaction which travels the length of the +wire is called a wave, although it bears little or no resemblance to +the familiar water wave. Sound waves are similar to the waves formed +in the stretched coil. + +[Illustration: FIG. 173.--Waves in a coiled wire.] + +Sound waves may be said to consist of a series of condensations and +rarefactions, and the distance between two consecutive condensations +and rarefactions may be defined as the wave length. + +257. How One Sounding Body produces Sound in Another Body. In +Section 255 we saw that any object when disturbed vibrates in a manner +peculiar to itself,--its natural period,--a long-roped hammock +vibrating slowly and a short-roped hammock vibrating rapidly. From +observation we learn that it requires but little force to cause a body +to vibrate in its natural period. If a sounding body is near a body +which has the same period as itself, the pulses of air produced by the +sounding body will, although very small, set the second body into +motion and cause it to make a faint sound. When a piano is being +played, we are often startled to find that a window pane or an +ornament responds to some note of the piano. If two tuning forks of +exactly identical periods (that is, of the same frequency) are placed +on a table as in Figure 174, and one is struck so as to give forth a +clear sound, the second fork will likewise vibrate, even though the +two forks may be separated by several feet of air. We can readily see +that the second fork is in motion, although it has not been struck, +because it will set in motion a pith ball suspended beside it; at +first the pith ball does not move, then it moves slightly, and finally +bounces rapidly back and forth. If the periods of the two forks are +not identical, but differ in the slightest degree, the second fork +will not respond to the first fork, no matter how long or how loud the +sound of the first fork. If we suppose that the fork vibrates 256 +times each second, then 256 gentle pulses of air are produced each +second, and these, traveling outward through the air, reach the silent +fork and tend to set it in motion. A single pulse of air could not +move the solid, heavy prongs, but the accumulated action of 256 +vibrations per second soon makes itself felt, and the second fork +begins to vibrate, at first gently, then gradually stronger, and +finally an audible tone is given forth. + +[Illustration: FIG. 174.--When the first fork vibrates, the second +responds.] + +The cumulative power of feeble forces acting frequently at definite +intervals is seen in many ways in everyday life. A small boy can +easily swing a much larger boy, provided he gives the swing a gentle +push in the right direction every time it passes him. But he must be +careful to push at the proper instant, since otherwise his effort does +not count for much; if he pushes forward when the swing is moving +backward, he really hinders the motion; if he waits until the swing +has moved considerably forward, his push counts for little. He must +push at the proper instant; that is, the way in which his hand moves +in giving the push must correspond exactly with the way in which the +swing would naturally vibrate. A very striking experiment can be made +by suspending from the ceiling a heavy weight and striking this weight +gently at regular, properly timed intervals with a small cork hammer. +Soon the pendulum, or weight, will be set swinging. + +[Illustration: FIG. 175.--The hollow wooden box reenforces the sound.] + +258. Borrowed Sound. Picture frames and ornaments sometimes buzz and +give forth faint murmurs when a piano or organ is played. The waves +sent out by a sounding body fall upon all surrounding objects and by +their repeated action tend to throw these bodies into vibration. If +the period of any one of the objects corresponds with the period of +the sounding body, the gentle but frequent impulses affect the object, +which responds by emitting a sound. If, however, the periods do not +correspond, the action of the sound waves is not sufficiently powerful +to throw the object into vibration, and no sound is heard. Bodies +which respond in this way are said to be sympathetic and the response +produced is called _resonance_. Seashells when held to the ear seem to +contain the roar of the sea; this is because the air within the shell +is set into sympathetic vibrations by some external tone. If the +seashell were held to the ear in an absolutely quiet room, no sound +would be heard, because there would be no external forces to set into +vibration the air within the shell. + +Tuning forks do not produce strong tones unless mounted on hollow +wooden boxes (Fig. 175), whose size and shape are so adjusted that +resonance occurs and strengthens the sound. When a human being talks +or sings, the air within the mouth cavity is thrown into sympathetic +vibration and strengthens the otherwise feeble tone of the speaker. + +259. Echo. If one shouts in a forest, the sound is sometimes heard a +second time a second or two later. This is because sound is reflected +when it strikes a large obstructing surface. If the sound waves +resulting from the shout meet a cliff or a mountain, they are +reflected back, and on reaching the ear produce a later sensation of +sound. + +By observation it has been found that the ear cannot distinguish +sounds which are less than one tenth of a second apart; that is, if +two sounds follow each other at an interval less than one tenth of a +second, the ear recognizes not two sounds, but one. This explains why +a speaker can be heard better indoors than in the open air. In the +average building, the walls are so close that the reflected waves have +but a short distance to travel, and hence reach the ear at practically +the same time as those which come directly from the speaker. In the +open, there are no reflecting walls or surfaces, and the original +sound has no reenforcement from reflection. + +If the reflected waves reach the ear too late to blend with the +original sound, that is, come later than one tenth of a second after +the first impression, an echo is heard. What we call the rolling of +thunder is really the reflection and re-reflection of the original +thunder from cloud and cliff. + +Some halls are so large that the reflected sounds cause a confusion of +echoes, but this difficulty can be lessened by hanging draperies, +which break the reflection. + +260. Motion does not always produce Sound. While we know that all +sound can be traced to motion, we know equally well that motion does +not always produce sound. The hammock swinging in the breeze does not +give forth a sound; the flag floating in the air does not give forth a +sound unless blown violently by the wind; a card moved slowly through +the air does not produce sound, but if the card is moved rapidly back +and forth, a sound becomes audible. + +Motion, in order to produce sound, must be rapid; a ball attached to a +string and moved slowly through the air produces no sound, but the +same ball, whirled rapidly, produces a distinct buzz, which becomes +stronger and stronger the faster the ball is whirled. + +261. Noise and Music. When the rapid motions which produce sound are +irregular, we hear noise; when the motions are regular and definite, +we have a musical tone; the rattling of carriage wheels on stones, the +roar of waves, the rustling of leaves are noise, not music. In all +these illustrations we have rapid but irregular motion; no two stones +strike the wheel in exactly the same way, no two waves produce pulses +in the air of exactly the same character, no two leaves rustle in +precisely the same way. The disturbances which reach the ear from +carriage, waves, and leaves are irregular both in time and strength, +and irritate the ear, causing the sensation which we call noise. + +The tuning fork is musical. Here we have rapid, regular motion; +vibrations follow each other at perfectly definite intervals, and the +air disturbance produced by one vibration is exactly like the +disturbance produced by a later vibration. The sound waves which reach +the ear are regular in time and kind and strength, and we call the +sensation music. + +To produce noise a body must vibrate in such a way as to give short, +quick shocks to the air; to produce music a body must not only impart +short, quick shocks to the air, but must impart these shocks with +unerring regularity and strength. A flickering light irritates the +eye; a flickering sound or noise irritates the ear; both are painful +because of the sudden and abrupt changes in effect which they cause, +the former on the eye, the latter on the ear. + +The only thing essential for the production of a musical sound is that +the waves which reach the ear shall be rapid and regular; it is +immaterial how these waves are produced. If a toothed wheel is mounted +and slowly rotated, and a stiff card is held against the teeth of the +wheel, a distinct tap is heard every time the card strikes the wheel. +But if the wheel is rotated rapidly, the ear ceases to hear the +various taps and recognizes a deep continuous musical tone. The +blending of the individual taps, occurring at regular intervals, has +produced a sustained musical tone. A similar result is obtained if a +card is drawn slowly and then rapidly over the teeth of a comb. + +[Illustration: FIG. 176.--A rotating disk.] + +That musical tones are due to a succession of regularly timed impulses +is shown most clearly by means of a rotating disk on which are cut two +sets of holes, the outer set equally spaced, and the inner set +unequally spaced (Fig. 176). + +If, while the disk is rotating rapidly, a tube is held over the +outside row and air is blown through the tube, a sustained musical +tone will be heard. If, however, the tube is held, during the rotation +of the disk, over the inner row of unequally spaced holes, the musical +tone disappears, and a series of noises take its place. In the first +case, the separate puffs of air followed each other regularly and +blended into one tone; in the second case, the separate puffs of air +followed each other at uncertain and irregular intervals and the +result was noise. + +Sound possesses a musical quality only when the waves or pulses follow +each other at absolutely regular intervals. + +262. The Effect of the Rapidity of Motion on the Musical Tone +Produced. If the disk is rotated so slowly that less than about 16 +puffs are produced in one second, only separate puffs are heard, and a +musical tone is lacking; if, on the other hand, the disk is rotated in +such a way that 16 puffs or more are produced in one second, the +separate puffs will blend together to produce a continuous musical +note of very low pitch. If the speed of the disk is increased so that +the puffs become more frequent, the pitch of the resulting note rises; +and at very high speeds the notes produced become so shrill and +piercing as to be disagreeable to the ear. If the speed of the disk is +lessened, the pitch falls correspondingly; and if the speed again +becomes so low that less than 16 puffs are formed per second, the +sustained sound disappears and a series of intermittent noises is +produced. + +263. The Pitch of a Note. By means of an apparatus called the siren, +it is possible to calculate the number of vibrations producing any +given musical note, such, for example, as middle C on the piano. If +air is forced continuously against the disk as it rotates, a series of +puffs will be heard (Fig. 177). + +If the disk turns fast enough, the puffs blend into a musical sound, +whose pitch rises higher and higher as the disk moves faster and +faster, and produces more and more puffs each second. + +The instrument is so constructed that clockwork at the top registers +the number of revolutions made by the disk in one second. The number +of holes in the disk multiplied by the number of revolutions a second +gives the number of puffs of air produced in one second. If we wish to +find the number of vibrations which correspond to middle C on the +piano, we increase the speed of the disk until the note given forth by +the siren agrees with middle C as sounded on the piano, as nearly as +the ear can judge; we then calculate the number of puffs of air which +took place each second at that particular speed of the disk. In this +way we find that middle C is due to about 256 vibrations per second; +that is, a piano string must vibrate 256 times per second in order for +the resultant note to be of pitch middle C. In a similar manner we +determine the following frequencies:-- + + |do |re |mi |fa |sol |la |si |do | + |C |D |E |F |G |A |B |C' | + |256 |288 |320 |341 |384 |427 |480 |512 | + +[Illustration: FIG. 177.--A siren.] + +The pitch of pianos, from the lowest bass note to the very highest +treble, varies from 27 to about 3500 vibrations per second. No human +voice, however, has so great a range of tone; the highest soprano +notes of women correspond to but 1000 vibrations a second, and the +deepest bass of men falls but to 80 vibrations a second. + +While the human voice is limited in its production of sound,--rarely +falling below 80 vibrations a second and rarely exceeding 1000 +vibrations a second,--the ear is by no means limited to that range in +hearing. The chirrup of a sparrow, the shrill sound of a cricket, and +the piercing shrieks of a locomotive are due to far greater +frequencies, the number of vibrations at times equaling 38,000 per +second or more. + +264. The Musical Scale. When we talk, the pitch of the voice changes +constantly and adds variety and beauty to conversation; a speaker +whose tone, or pitch, remains too constant is monotonous and dull, no +matter how brilliant his thoughts may be. + +While the pitch of the voice changes constantly, the changes are +normally gradual and slight, and the different tones merge into each +other imperceptibly. In music, however, there is a well-defined +interval between even consecutive notes; for example, in the musical +scale, middle C (do) with 256 vibrations is followed by D (re) with +288 vibrations, and the interval between these notes is sharp and well +marked, even to an untrained ear. The interval between two notes is +defined as the ratio of the frequencies; hence, the interval between C +and D (do and re) is 288/256, or 9/8. Referring to Section 263, we see +that the interval between C and E is 320/256, or 5/4, and the interval +between C and C' is 512/256, or 2; the interval between any note and +its octave is 2. + +The successive notes in one octave of the musical scale are related as +follows:-- + + |Key of C |C |D |E |F |G |A |B |C' | + |No. of vibrations | | | | | | | | | + |per sec. |256 |288 |320 |341 |384 |427 |480 |512 | + |Interval |9/8 |5/4 |4/3 |3/2 |5/3 |15/8 |2 | | + +The intervals of F and A are not strictly 4/3 and 5/3, but are nearly +so; if F made 341.3 vibrations per second instead of 341; and if A +made 426.6 instead of 427, then the intervals would be exactly 4/3 and +5/3. Since the real difference is so slight, we can assume the simpler +ratios without appreciable error. + +Any eight notes whose frequencies are in the ratio of 9/8, 5/4, etc., +will when played in succession give the familiar musical scale; for +example, the deepest bass voice starts a musical scale whose notes +have the frequencies 80, 90, 100, 107, 120, 133, 150, 160, but the +intervals here are identical with those of a higher scale; the +interval between C and D, 80 and 90, is 9/8, just as it was before +when the frequencies were much greater; that is, 256 and 288. In +singing "Home, Sweet Home," for example, a bass voice may start with a +note vibrating only 132 times a second; while a tenor may start at a +higher pitch, with a note vibrating 198 times per second, and a +soprano would probably take a much higher range still, with an initial +frequency of 528 vibrations per second. But no matter where the voices +start, the intervals are always identical. The air as sung by the bass +voice would be represented by _A_. The air as sung by the tenor voice +would be represented by _B_. The air as sung by the soprano voice +would be represented by _C_. + +[Illustration: FIG. 178.--A song as sung by three voices of different +pitch.] + + + + +CHAPTER XXVIII + +MUSICAL INSTRUMENTS + + +265. Musical instruments maybe divided into three groups according +to the different ways in which their tones are produced:-- + +_First._ The stringed instruments in which sound is produced by the +vibration of stretched strings, as in the piano, violin, guitar, +mandolin. + +_Second._ The wind instruments in which sound is produced by the +vibrations of definite columns of air, as in the organ, flute, cornet, +trombone. + +_Third._ The percussion instruments, in which sound is produced by the +motion of stretched membranes, as in the drum, or by the motion of +metal disks, as in the tambourines and cymbals. + +266. Stringed Instruments. If the lid of a piano is opened, numerous +wires are seen within; some long, some short, some coarse, some fine. +Beneath each wire is a small felt hammer connected with the keys in +such a way that when a key is pressed, a string is struck by a hammer +and is thrown into vibration, thereby producing a tone. + +If we press the lowest key, that is, the key giving forth the lowest +pitch, we see that the longest wire is struck and set into vibration; +if we press the highest key, that is, the key giving the highest +pitch, we see that the shortest wire is struck. In addition, it is +seen that the short wires which produce the high tones are fine, +while the long wires which produce the low tones are coarse. The +shorter and finer the wire, the higher the pitch of the tone produced. +The longer and coarser the wire, the lower the pitch of the tone +produced. + +[Illustration: FIG. 179.--Piano wires seen from the back.] + +The constant striking of the hammers against the strings stretches and +loosens them and alters their pitch; for this reason each string is +fastened to a screw which can be turned so as to tighten the string or +to loosen it if necessary. The tuning of the piano is the adjustment +of the strings so that each shall produce a tone of the right pitch. +When the strings are tightened, the pitch rises; when the strings are +loosened, the pitch falls. + +What has been said of the piano applies as well to the violin, guitar, +and mandolin. In the latter instruments the strings are few in number, +generally four, as against eighty-eight in the piano; the hammer of +the piano is replaced in the violin by the bow, and in the guitar by +the fingers; varying pitches on any one string are obtained by sliding +a finger of the left hand along the wire, and thus altering its +length. + +Frequent tuning is necessary, because the fine adjustments are easily +disturbed. The piano is the best protected of all the stringed +instruments, being inclosed by a heavy framework, even when in use. + +[Illustration: FIG. 180.--Front view of an open piano.] + +267. Strings and their Tones. Fasten a violin string to a wooden +frame or box, as shown in Figure 181, stretching it by means of some +convenient weight; then lay a yardstick along the box in order that +the lengths may be determined accurately. If the stretched string is +plucked with the fingers or bowed with the violin bow, a clear musical +sound of definite pitch will be produced. Now divide the string into +two equal parts by inserting the bridge midway between the two ends; +and pluck either half as before. The note given forth is of a +decidedly higher pitch, and if by means of the siren we compare the +pitches in the two cases, we find that the note sounded by the half +wire is the octave of the note sounded by the entire wire; the +frequency has been doubled by halving the length. If now the bridge is +placed so that the string is divided into two unequal portions such as +1:3 and 2:3, and the shorter portion is plucked, the pitch will be +still higher; the shorter the length plucked, the higher the pitch +produced. This movable bridge corresponds to the finger of the +violinist; the finger slides back and forth along the string, thus +changing the length of the bowed portion and producing variations in +pitch. + +[Illustration: FIG. 181.--The length of a string influences the +pitch.] + +[Illustration: FIG. 182.--Only one half of the string is bowed, but +both halves vibrate.] + +If there were but one string, only one pitch could be sounded at any +one time; the additional strings of the violin allow of the +simultaneous production of several tones. + +268. The Freedom of a String. Some stringed instruments give forth +tones which are clear and sweet, but withal thin and lacking in +richness and fullness. The tones sounded by two different strings may +agree in pitch and loudness and yet produce quite different effects on +the ear, because in one case the tone may be much more pleasing than +in the other. The explanation of this is, that a string may vibrate in +a number of different ways. + +Touch the middle of a wire with the finger or a pencil (Fig. 182), +thus separating it into two portions and draw a violin bow across the +center of either half. Only one half of the entire string is struck, +but the motion of this half is imparted to the other half and throws +it into similar motion, and if a tiny A-shaped piece of paper or rider +is placed upon the unbowed half, it is hurled off. + +[Illustration: FIG. 183.--The string vibrates in three portions.] + +If the wire is touched at a distance of one third its length and a bow +is drawn across the middle of the smaller portion, the string will +vibrate in three parts; we cannot always see these various motions in +different parts of the string, but we know of their existence through +the action of the riders. + +Similarly, touching the wire one fourth of its length from an end +makes it vibrate in four segments; touching it one fifth of its length +makes it vibrate in five segments. + +In the first case, the string vibrated as a whole string and also as +two strings of half the length; hence, three tones must have been +given out, one tone due to the entire string and two tones due to the +segments. But we saw in Section 267 that halving the length of a +string doubles the pitch of the resulting tone, and produces the +octave of the original tone; hence a string vibrating as in Figure 183 +gives forth three tones, one of which is the fundamental tone of the +string, and two of which are the octave of the fundamental tone. +Hence, the vibrating string produces two sensations, that of the +fundamental note and of its octave. + +[Illustration: FIG. 184.--When a string vibrates as a whole, it gives +out the fundamental note.] + +When a string is plucked in the middle without being held, it vibrates +simply as a whole (Fig. 184), and gives forth but one note; this is +called the fundamental. If the string is made to vibrate in two parts, +it gives forth two notes, the fundamental, and a note one octave +higher than the fundamental; this is called the first overtone. When +the string is made to move as in Figure 183, three distinct motions +are called forth, the motion of the entire string, the motion of the +portion plucked, and the motion of the remaining unplucked portion of +the string. Here, naturally, different tones arise, corresponding to +the different modes of vibration. The note produced by the vibration +of one third of the original string is called the second overtone. + +The above experiments show that a string is able to vibrate in a +number of different ways at the same time, and to emit simultaneously +a number of different tones; also that the resulting complex sound +consists of the fundamental and one or more overtones, and that the +number of overtones present depends upon how and where the string is +plucked. + +[Illustration: FIG. 185.--A string can vibrate in a number of +different ways simultaneously, and can produce different notes +simultaneously.] + +269. The Value of Overtones. The presence of overtones determines +the quality of the sound produced. If the string vibrates as a whole +merely, the tone given out is simple, and seems dull and +characterless. If, on the other hand, it vibrates in such a way that +overtones are present, the tone given forth is full and rich and the +sensation is pleasing. A tuning fork cannot vibrate in more than one +way, and hence has no overtones, and its tone, while clear and sweet, +is far less pleasing than the same note produced by a violin or piano. +The untrained ear is not conscious of overtones and recognizes only +the strong dominant fundamental. The overtones blend in with the +fundamental and are so inconspicuously present that we do not realize +their existence; it is only when they are absent that we become aware +of the beauty which they add to the music. A song played on tuning +forks instead of on strings would be lifeless and unsatisfying because +of the absence of overtones. + +It is not necessary to hold finger or pencil at the points 1:3, 1:4, +etc., in order to cause the string to vibrate in various ways; if a +string is merely plucked or bowed at those places, the result will be +the same. It is important to remember that no matter where a string of +definite length is bowed, the note most distinctly heard will be the +fundamental; but the quality of the emitted tone will vary with the +bowing. For example, if a string is bowed in the middle, the effect +will be far less pleasing than though it were bowed near the end. In +the piano, the hammers are arranged so as to strike near one end of +the string, at a distance of about 1:7 to 1:9; and hence a large +number of overtones combine to reenforce and enrich the fundamental +tone. + +270. The Individuality of Instruments. It has been shown that a +piano string when struck by a hammer, or a violin string when bowed, +or a mandolin string when plucked, vibrates not only as a whole, but +also in segments, and as a result gives forth not a simple tone, as we +are accustomed to think, but a very complex tone consisting of the +fundamental and one or more overtones. If the string whose fundamental +note is lower C (128 vibrations per second) is thrown into vibration, +the tone produced may contain, in addition to the prominent +fundamental, any one or more of the following overtones: C', G'', C'', +E'', C''', etc. + +The number of overtones actually present depends upon a variety of +circumstances: in the piano, it depends largely upon the location of +the hammer; in the violin, upon the place and manner of bowing. +Mechanical differences in construction account for prominent and +numerous overtones in some instruments and for feeble and few +overtones in others. The oboe, for example, is so constructed that +only the high overtones are present, and hence the sound gives a +"pungent" effect; the clarinet is so constructed that the +even-numbered overtones are killed, and the presence of only +odd-numbered overtones gives individuality to the instrument. In these +two instruments we have vibrating air columns instead of vibrating +strings, but the laws which govern vibrating strings are applicable to +vibrating columns of air, as we shall see later. It is really the +presence or absence of overtones which enables us to distinguish the +note of the piano from that of the violin, flute, or clarinet. If +overtones could be eliminated, then middle C, or any other note on the +piano, would be indistinguishable from that same note sounded on any +other instrument. The fundamental note in every instrument is the +same, but the overtones vary with the instrument and lend +individuality to each. The presence of high overtones in the oboe and +the presence of odd-numbered overtones in the clarinet enable us to +distinguish without fail the sounds given out by these instruments. + +The richness and individuality of an instrument are due, not only to +the overtones which accompany the fundamental, but also to the +"forced" vibrations of the inclosing case, or of the sounding board. +If a vibrating tuning fork is held in the hand, the sound will be +inaudible except to those quite near; if, however, the base of the +fork is held against the table, the sound is greatly intensified and +becomes plainly audible throughout the room. + +The vibrations of the fork are transmitted to the table top and throw +it into vibrations similar to its own, and these additional vibrations +intensify the original sound. Any fork, no matter what its frequency, +can force the surface of the table into vibration, and hence the sound +of any fork will be intensified by contact with a table or box. + +This is equally true of strings; if stretched between two posts and +bowed, the sound given out by a string is feeble, but if stretched +over a sounding board, as in the piano, or over a wooden shell, as in +the violin, the sound is intensified. Any note of the instrument will +force the sounding body to vibrate, thus reenforcing the volume of +sound, but some tones, or modes of vibration, do this more easily than +others, and while the sounding board or shell always responds, it +responds in varying degree. Here again we have not only enrichment of +sound but also individuality of instruments. + +271. The Kinds of Stringed Instruments. Stringed instruments may be +grouped in the following three classes:-- + +_a_. Instruments in which the strings are set into motion by +hammers--piano. + +_b_. Instruments in which the strings are set into motion by +bowing--violin, viola, violoncello, double bass. + +_c_. Instruments in which the strings are set into motion by +plucking--harp, guitar, mandolin. + +[Illustration: FIG. 186.--1, violin; 2, viola; 3, violoncello; 4, +double bass.] + + _a_. The piano is too well known to need comment. In + passing, it may be mentioned that in the construction of the + modern concert piano approximately 40,000 separate pieces of + material are used. The large number of pieces is due, + partly, to the fact that the single string corresponding to + any one key is usually replaced by no less than three or + four similar strings in order that greater volume of sound + may be obtained. The hammer connected to a key strikes + four or more strings instead of one, and hence produces a + greater volume of tone. + + _b_. The viola is larger than the violin, has heavier and + thicker strings, and is pitched to a lower key; in all other + respects the two are similar. The violoncello, because of + the length and thickness of its strings, is pitched a whole + octave lower than the violin; otherwise it is similar. The + unusual length and thickness of the strings of the double + bass make it produce very low notes, so that it is + ordinarily looked upon as the "bass voice" of the orchestra. + + _c_. The harp has always been considered one of the most + pleasing and perfect of musical instruments. Here the + skilled performer has absolutely free scope for his genius, + because his fingers can pluck the strings at will and hence + regulate the overtones, and his feet can regulate at will + the tension, and hence the pitch of the strings. + + Guitar and mandolin are agreeable instruments for amateurs, + but are never used in orchestral music. + +[Illustration: FIG. 187.--A harp.] + +272. Wind Instruments. In the so-called wind instruments, sound is +produced by vibrating columns of air inclosed in tubes or pipes of +different lengths. The air column is thrown into vibration either +directly, by blowing across a narrow opening at one end of a pipe as +in the case of the whistle, or indirectly, by exciting vibrations in a +thin strip of wood or metal, called a reed, which in turn communicates +its vibrations to the air column within. + +The shorter the air column, the higher the pitch. This agrees with the +law of vibrating strings which gives high pitches for short lengths. + +[Illustration: FIG. 188.--Open organ pipes of different pitch.] + +The pitch of the sound emitted by a column of air vibrating within a +pipe varies according to the following laws: + +1. The shorter the pipe, the higher the pitch. + +2. The pitch of a note emitted by an open pipe is one octave higher +than that of a closed pipe of equal length. + +3. Air columns vibrate in segments just as do strings, and the tone +emitted by a pipe of given length is complex, consisting of the +fundamental and one or more overtones. The greater the number of +overtones present, the richer the tone produced. + +273. How the Various Pitches are Produced. With a pipe of fixed +length, for example, the clarinet (Fig. 189, 1), different pitches are +obtained by pressing keys which open holes in the tube and thus +shorten or lengthen the vibrating air column and produce a rise or +fall in pitch. Changes in pitch are also produced by variation in the +player's breathing. By blowing hard or gently, the number of +vibrations of the reed is increased or decreased and hence the pitch +is altered. + +[Illustration: FIG. 189--1, clarinet; 2, oboe; 3, flute.] + +In the oboe (Fig. 189, 2) the vibrating air column is set into motion +by means of two thin pieces of wood or metal placed in the mouthpiece +of the tube. Variations in pitch are produced as in the clarinet by +means of stops and varied breathing. In the flute, the air is set into +motion by direct blowing from the mouth, as is done, for instance, +when we blow into a bottle or key. + +The sound given out by organ pipes is due to air blown across a sharp +edge at the opening of a narrow tube. The air forced across the sharp +edge is thrown into vibration and communicates its vibration to the +air within the organ pipe. For different pitches, pipes of different +lengths are used: for very low pitches long, closed pipes are used; +for very high pitches short, open pipes are used. The mechanism of the +organ is such that pressing a key allows the air to rush into the +communicating pipe and a sound is produced characteristic of the +length of the pipe. + +[Illustration: FIG. 190.--1, horn; 2, trumpet; 3, trombone.] + +[Illustration: FIG. 191.--1, kettledrum; 2, bass drum; 3, cymbals.] + +[Illustration: FIG. 192.--The seating arrangement of the Philadelphia +orchestra.] + +In the brass wind instruments such as horn, trombone, and trumpet, the +lips of the player vibrate and excite the air within. Varying pitches +are obtained partly by the varying wind pressure of the musician; if +he breathes fast, the pitch rises; if he breathes slowly, the pitch +falls. All of these instruments, however, except the trombone possess +some valves which, on being pressed, vary the length of the tube and +alter the pitch accordingly. In the trombone, valves are replaced by a +section which slides in and out and shortens or lengthens the tube. + +274. The Percussion Instruments. The percussion instruments, +including kettledrums, bass drums, and cymbals, are the least +important of all the musical instruments; and are usually of service +merely in adding to the excitement and general effect of an orchestra. + +In orchestral music the various instruments are grouped somewhat as +shown in Figure 192. + + + + +CHAPTER XXIX + +SPEAKING AND HEARING + + +[Illustration: FIG. 193.--The vibration of the vocal cords produces +the sound of the human voice.] + +275. Speech. The human voice is the most perfect of musical +instruments. Within the throat, two elastic bands are attached to the +windpipe at the place commonly called Adam's apple; these flexible +bands have received the name of vocal cords, since by their vibration +all speech is produced. In ordinary breathing, the cords are loose and +are separated by a wide opening through which air enters and leaves +the lungs. When we wish to speak, muscular effort stretches the cords, +draws them closer together, and reduces the opening between them to a +narrow slit, as in the case of the organ pipe. If air from the lungs +is sent through the narrow slit, the vocal cords or bands are thrown +into rapid vibration and produce sound. The pitch of the sound depends +upon the tension of the stretched membranes, and since this can be +altered by muscular action, the voice can be modulated at will. In +times of excitement, when the muscles of the body in general are in a +state of great tension, the pitch is likely to be uncommonly high. + +Women's voices are higher than men's because the vocal cords are +shorter and finer; even though muscular tension is relaxed and the +cords are made looser, the pitch of a woman's voice does not fall so +low as that of a man's voice since his cords are naturally much +longer and coarser. The difference between a soprano and an alto voice +is merely one of length and tension of the vocal cords. + +Successful singing is possible only when the vocal cords are readily +flexible and when the singer can supply a steady, continuous blast of +air through the slit between the cords. The hoarseness which +frequently accompanies cold in the head is due to the thickening of +the mucous membrane and to the filling up of the slit with mucus, +because when this happens, the vocal cords cannot vibrate properly. + +The sounds produced by the vocal cords are transformed into speech by +the help of the tongue and lips, which modify the shape of the mouth +cavity. Some of the lower animals have a speaking apparatus similar to +our own, but they cannot perfectly transform sound into speech. The +birds use their vocal cords to beautiful advantage in singing, far +surpassing us in many ways, but the power of speech is lacking. + +276. The Ear. The pulses created in the air by a sounding body are +received by the ear and the impulses which they impart to the auditory +nerve pass to the brain and we become conscious of a sound. The ear is +capable of marvelous discrimination and accuracy. "In order to form an +idea of the extent of this power imagine an auditor in a large music +hall where a full band and chorus are performing. Here, there are +sounds mingled together of all varieties of pitch, loudness, and +quality; stringed instruments, wood instruments, brass instruments, +and voices, of many different kinds. And in addition to these there +may be all sorts of accidental and irregular sounds and noises, such +as the trampling and shuffling of feet, the hum of voices, the rustle +of dress, the creaking of doors, and many others. Now it must be +remembered that the only means the ear has of becoming aware of these +simultaneous sounds is by the condensations and rarefactions which +reach it; and yet when the sound wave meets the nerves, the nerves +single out each individual element, and convey to the mind of the +hearer, not only the tones and notes of every instrument in the +orchestra, but the character of every accidental noise; and almost as +distinctly as if each single tone or noise were heard alone."--POLE. + +[Illustration: FIG. 194.--The ear.] + +277. The Structure of the Ear. The external portion of the ear acts +as a funnel for catching sound waves and leading them into the canal, +where they strike upon the ear drum, or tympanic membrane, and throw +it into vibration. Unless the ear drum is very flexible there cannot +be perfect response to the sound waves which fall upon it; for this +reason, the glands of the canal secrete a wax which moistens the +membrane and keeps it flexible. Lying directly back of the tympanic +membrane is a cavity filled with air which enters by the Eustachian +tube; from the throat air enters the Eustachian tube, moves along it, +and passes into the ear cavity. The dull crackling noise noticed in +the ear when one swallows is due to the entrance and exit of air in +the tube. Several small bones stretch across the upper portion of the +cavity and make a bridge, so to speak, from the ear drum to the far +wall of the cavity. It is by means of these three bones that the +vibrations of the ear drum are transmitted to the inner wall of the +cavity. Behind the first cavity is a second cavity so complex and +irregular that it is called the labyrinth of the ear. This labyrinth +is filled with a fluid in which are spread out the delicate sensitive +fibers of the auditory nerves; and it is to these that the vibrations +must be transmitted. + +Suppose a note of 800 vibrations per second is sung. Then 800 pulses +of air will reach the ear each second, and the ear drum, being +flexible, will respond and will vibrate at the same rate. The +vibration of the ear drum will be transmitted by the three bones and +the fluid to the fibers of the auditory nerves. The impulses imparted +to the auditory nerve reach the brain and in some unknown way are +translated into sound. + +278. Care of the Ear. Most catarrhal troubles are accompanied by an +oversupply of mucus which frequently clogs up the Eustachian tube and +produces deafness. For the same reason, colds and sore throat +sometimes induce temporary deafness. + +The wax of the ear is essential for flexibility of the ear drum; if an +extra amount accumulates, it can be got rid of by bathing the ear in +hot water, since the heat will melt the wax. The wax should never be +picked out with pin or sharp object except by a physician, lest injury +be done to the tympanic membrane. + +279. The Phonograph. The invention of the phonograph by Edison in +1878 marked a new era in the popularity and dissemination of music. Up +to that time, household music was limited to those who were rich +enough to possess a real musical instrument, and who in addition had +the understanding and the skill to use the instrument. The invention +of the phonograph has brought music to thousands of homes possessed +of neither wealth nor skill. That the music reproduced by a phonograph +is not always of the highest order does not, in the least, detract +from the interest and wonder of the instrument. It can reproduce what +it is called upon to reproduce, and if human nature demands the +commonplace, the instrument will be made to satisfy the demand. On the +other hand, speeches of famous men, national songs, magnificent opera +selections, and other pleasing and instructive productions can be +reproduced fairly accurately. In this way the phonograph, perhaps more +than any other recent invention, can carry to the "shut-ins" a lively +glimpse of the outside world and its doings. + +[Illustration: FIG. 195.--A vibrating tuning fork traces a curved line +on smoked glass.] + +The phonograph consists of a cylinder or disk of wax upon which the +vibrations of a sensitive diaphragm are recorded by means of a fine +metal point. The action of the pointer in reporting the vibrations of +a diaphragm is easily understood by reference to a tuning fork. Fasten +a stiff bristle to a tuning fork by means of wax, allowing the end of +the point to rest lightly upon a piece of smoked glass. If the glass +is drawn under the bristle a straight line will be scratched on the +glass, but if the tuning fork is struck so that the prongs vibrate +back and forth, then the straight line changes to a wavy line and the +type of wavy line depends upon the fork used. + +In the phonograph, a diaphragm replaces the tuning fork and a cylinder +(or a disk) coated with wax replaces the glass plate. When the speaker +talks or the singer sings, his voice strikes against a delicate +diaphragm and throws it into vibration, and the metal point attached +to it traces on the wax of a moving cylinder a groove of varying shape +and appearance called the "record." Every variation in the speaker's +voice is repeated in the vibrations of the metal disk and hence in the +minute motion of the pointer and in the consequent record on the +cylinder. The record thus made can be placed in any other phonograph +and if the metal pointer of this new phonograph is made to pass over +the tracing, the process is reversed and the speaker's voice is +reproduced. The sound given out in the this way is faint and weak, but +can be strengthened by means of a trumpet attached to the phonograph. + +[Illustration: FIG. 196.--A phonograph. In this machine the cylinder +is replaced by a revolving disk.] + + + + +CHAPTER XXX + +ELECTRICITY + + +280. Many animals possess the five senses, but only man possesses +constructive, creative power, and is able to build on the information +gained through the senses. It is the constructive, creative power +which raises man above the level of the beast and enables him to +devise and fashion wonderful inventions. Among the most important of +his inventions are those which relate to electricity; inventions such +as trolley car, elevator, automobile, electric light, the telephone, +the telegraph. Bell, by his superior constructive ability, made +possible the practical use of the telephone, and Marconi that of +wireless telegraphy. To these inventions might be added many others +which have increased the efficiency and production of the business +world and have decreased the labor and strain of domestic life. + +[Illustration: FIG. 197.--A simple electric cell.] + +281. Electricity as first Obtained by Man. Until modern times the +only electricity known to us was that of the lightning flash, which +man could neither hinder nor make. But in the year 1800, electricity +in the form of a weak current was obtained by Volta of Italy in a very +simple way; and even now our various electric batteries and cells are +but a modification of that used by Volta and called a voltaic cell. A +strip of copper and a strip of zinc are placed in a glass containing +dilute sulphuric acid, a solution composed of oxygen, hydrogen, +sulphur, and water. As soon as the plates are immersed in the acid +solution, minute bubbles of gas rise from the zinc strip and it begins +to waste away slowly. The solution gradually dissolves the zinc and at +the same time gives up some of the hydrogen which it contains; but it +has little or no effect on the copper, since there is no visible +change in the copper strip. + +If, now, the strips are connected by means of metal wires, the zinc +wastes away rapidly, numerous bubbles of hydrogen pass over to the +copper strip and collect on it, and a current of electricity flows +through the connecting wires. Evidently, the source of the current is +the chemical action between the zinc and the liquid. + +Mere inspection of the connecting wire will not enable us to detect +that a current is flowing, but there are various ways in which the +current makes itself evident. If the ends of the wires attached to the +strips are brought in contact with each other and then separated, a +faint spark passes, and if the ends are placed on the tongue, a twinge +is felt. + +282. Experiments which grew out of the Voltaic Cell. Since chemical +action on the zinc is the source of the current, it would seem +reasonable to expect a current if the cell consisted of two zinc +plates instead of one zinc plate and one copper plate. But when the +copper strip is replaced by a zinc strip so that the cell consists of +two similar plates, no current flows between them. In this case, +chemical action is expended in heat rather than in the production of +electricity and the liquid becomes hot. But if carbon and zinc are +used, a current is again produced, the zinc dissolving away as before, +and bubbles collecting on the carbon plate. By experiment it has been +found that many different metals may be employed in the construction +of an electric cell; for example, current may be obtained from a cell +made with a zinc plate and a platinum plate, or from a cell made with +a lead plate and a copper plate. Then, too, some other chemical, such +as bichromate of potassium, or ammonium chloride, may be used instead +of dilute sulphuric acid. + +Almost any two different substances will, under proper conditions, +give a current, but the strength of the current is in some cases so +weak as to be worthless for practical use, such as telephoning, or +ringing a door bell. What is wanted is a strong, steady current, and +our choice of material is limited to the substances which will give +this result. Zinc and lead can be used, but the current resulting is +weak and feeble, and for general use zinc and carbon are the most +satisfactory. + +283. Electrical Terms. The plates or strips used in making an +electric cell are called electrodes; the zinc is called the negative +electrode (-), and the carbon the positive electrode (+); the current +is considered to flow through the wire from the + to the-electrode. As +a rule, each electrode has attached to it a binding post to which +wires can be quickly fastened. + +The power that causes the current is called the electromotive force, +and the value of the electromotive force, generally written E.M.F., of +a cell depends upon the materials used. + +When the cell consists of copper, zinc, and dilute sulphuric acid, the +electromotive force has a definite value which is always the same no +matter what the size or shape of the cell. But the E.M.F. has a +decidedly different value in a cell composed of iron, copper, and +chromic acid. Each combination of material has its own specific +electromotive force. + +284. The Disadvantage of a Simple Cell. When the poles of a simple +voltaic cell are connected by a wire, the current thus produced +slowly diminishes in strength and, after a short time, becomes feeble. +Examination of the cell shows that the copper plate is covered with +hydrogen bubbles. If, however, these bubbles are completely brushed +away by means of a rod or stick, the current strength increases, but +as the bubbles again gather on the + electrode the current strength +diminishes, and when the bubbles form a thick film on the copper +plate, the current is too weak to be of any practical value. The film +of bubbles weakens the current because it practically substitutes a +hydrogen plate for a copper plate, and we saw in Section 282 that a +change in any one of the materials of which a cell is composed changes +the current. + +This weakening of the current can be reduced mechanically by brushing +away the bubbles as soon as they are formed; or chemically, by +surrounding the copper plate with a substance which will combine with +the free hydrogen and prevent it from passing onward to the copper +plate. + +[Illustration: FIG 198. The gravity cell.] + +In practically all cells, the chemical method is used in preference to +the mechanical one. The numerous types of cells in daily use differ +chiefly in the devices employed for preventing the formation of +hydrogen bubbles, or for disposing of them when formed. One of the +best-known cells in which weakening of the current is prevented by +chemical means is the so-called gravity cell. + +285. The Gravity Cell. A large, irregular copper electrode is placed +in the bottom of a jar (Fig. 198), and completely covered with a +saturated solution of copper sulphate. Then a large, irregular zinc +electrode is suspended from the top of the jar, and is completely +covered with dilute sulphuric acid which does not mix with the copper +sulphate, but floats on the top of it like oil on water. The hydrogen +formed by the chemical action of the dilute sulphuric acid on the zinc +moves toward the copper electrode, as in the simple voltaic cell. It +does not reach the electrode, however, because, when it comes in +contact with the copper sulphate, it changes places with the copper +there, setting it free, but itself entering into the solution. The +copper freed from the copper sulphate solution travels to the copper +electrode, and is deposited on it in a clean, bright layer. Instead of +a deposit of hydrogen there is a deposit of copper, and falling off in +current is prevented. + +The gravity cell is cheap, easy to construct, and of constant +strength, and is in almost universal use in telegraphic work. +Practically all small railroad stations and local telegraph offices +use these cells. + +[Illustration: FIG. 199.--A dry cell.] + +286. Dry Cells. The gravity cell, while cheap and effective, is +inconvenient for general use, owing to the fact that it cannot be +easily transported, and the _dry cell_ has largely supplanted all +others, because of the ease with which it can be taken from place to +place. This cell consists of a zinc cup, within which is a carbon rod; +the space between the cup and rod is packed with a moist paste +containing certain chemicals. The moist paste takes the place of the +liquids used in other cells. + +[Illustration: FIG. 200.--A battery of three cells.] + +287. A Battery of Cells. The electromotive force of one cell may not +give a current strong enough to ring a door bell or to operate a +telephone. But by using a number of cells, called a battery, the +current may be increased to almost any desired strength. If three +cells are arranged as in Figure 200, so that the copper of one cell is +connected with the zinc of another cell, the electromotive force of +the battery will be three times as great as the E.M.F. of a single +cell. If four cells are arranged in the same way, the E.M.F. of the +battery is four times as great as the E.M.F. of a single cell; when +five cells are combined, the resulting E.M.F. is five times as great. + + + + +CHAPTER XXXI + +SOME USES OF ELECTRICITY + + +288. Heat. Any one who handles electric wires knows that they are +more or less heated by the currents which flow through them. If three +cells are arranged as in Figure 200 and the connecting wire is coarse, +the heating of the wire is scarcely noticeable; but if a shorter wire +of the same kind is used, the heat produced is slightly greater; and +if the coarse wire is replaced by a short, fine wire, the heating of +the wire becomes very marked. We are accustomed to say that a wire +offers resistance to the flow of a current; that is, whenever a +current meets resistance, heat is produced in much the same way as +when mechanical motion meets an obstacle and spends its energy in +friction. The flow of electricity along a wire can be compared to the +flow of water through pipes: a small pipe offers a greater resistance +to the flow of water than a large pipe; less water can be forced +through a small pipe than through a large pipe, but the friction of +the water against the sides of the small pipe is much greater than in +the large one. + +So it is with the electric current. In fine wires the resistance to +the current is large and the energy of the battery is expended in heat +rather than in current. If the heat thus produced is very great, +serious consequences may arise; for example, the contact of a hot wire +with wall paper or dry beams may cause fire. Insurance companies +demand that the wires used in wiring a building for electric lights be +of a size suitable to the current to be carried, otherwise they will +not take the risk of insurance. The greater the current to be carried, +the coarser is the wire required for safety. + +289. Electric Stoves. It is often desirable to utilize the electric +current for the production of heat. For example, trolley cars are +heated by coils of wire under the seats. The coils offer so much +resistance to the passage of a strong current through them that they +become heated and warm the cars. + +[Illustration: FIG. 201.--An electric iron on a metal stand.] + +Some modern houses are so built that electricity is received into them +from the great plants where it is generated, and by merely turning a +switch or inserting a plug, electricity is constantly available. In +consequence, many practical applications of electricity are possible, +among which are flatiron and toaster. + +[Illustration: FIG. 202.--The fine wires are strongly heated by the +current which flows through them.] + +Within the flatiron (Fig. 201), is a mass of fine wire coiled as shown +in Figure 202; as soon as the iron is connected with the house supply +of electricity, current flows through the fine wire which thus becomes +strongly heated and gives off heat to the iron. The iron, when once +heated, retains an even temperature as long as the current flows, and +the laundress is, in consequence, free from the disadvantages of a +slowly cooling iron, and of frequent substitution of a warm iron for a +cold one. Electric irons are particularly valuable in summer, because +they eliminate the necessity for a strong fire, and spare the +housewife intense heat. In addition, the user is not confined to the +laundry, but is free to seek the coolest part of the house, the only +requisite being an electrical connection. + +[Illustration: FIG. 203.--Bread can be toasted by electricity.] + +The toaster (Fig. 203) is another useful electrical device, since by +means of it toast may be made on a dining table or at a bedside. The +small electrical stove, shown in Figure 204, is similar in principle +to the flatiron, but in it the heating coil is arranged as shown in +Figure 205. To the physician electric stoves are valuable, since his +instruments can be sterilized in water heated by the stove; and that +without fuel or odor of gas. + +A convenient device is seen in the heating pad (Fig. 206), a +substitute for a hot water bag. Embedded in some soft thick substance +are the insulated wires in which heat is to be developed, and over +this is placed a covering of felt. + +[Illustration: FIG. 204.--An electric stove.] + +290. Electric Lights. The incandescent bulbs which illuminate our +buildings consist of a fine, hairlike thread inclosed in a glass bulb +from which the air has been removed. When an electric current is sent +through the delicate filament, it meets a strong resistance. The heat +developed in overcoming the resistance is so great that it makes the +filament a glowing mass. The absence of air prevents the filament from +burning, and it merely glows and radiates the light. + +[Illustration: FIG. 205.--The heating element in the electric stove.] + +291. Blasting. Until recently, dynamiting was attended with serious +danger, owing to the fact that the person who applied the torch to the +fuse could not make a safe retreat before the explosion. Now a fine +wire is inserted in the fuse, and when everything is in readiness, +the ends of the wire are attached to the poles of a distant battery +and the heat developed in the wire ignites the fuse. + +[Illustration: FIG. 206.--An electric pad serves the same purpose as a +hot water bag.] + +292. Welding of Metals. Metals are fused and welded by the use of +the electric current. The metal pieces which are to be welded are +pressed together and a powerful current is passed through their +junction. So great is the heat developed that the metals melt and +fuse, and on cooling show perfect union. + +293. Chemical Effects. _The Plating of Gold, Silver, and Other +Metals._ If strips of lead or rods of carbon are connected to the +terminals of an electric cell, as in Figure 208, and are then dipped +into a solution of copper sulphate, the strip in connection with the +negative terminal of the cell soon becomes thinly plated with a +coating of copper. If a solution of silver nitrate is used in place of +the copper sulphate, the coating formed will be of silver instead of +copper. So long as the current flows and there is any metal present in +the solution, the coating continues to form on the negative electrode, +and becomes thicker and thicker. + +[Illustration: FIG. 207.--An incandescent electric bulb.] + +The process by which metal is taken out of solution, as silver out of +silver nitrate and copper out of copper sulphate, and is in turn +deposited as a coating on another substance, is called electroplating. +An electric current can separate a liquid into some of its various +constituents and to deposit one of the metal constituents on the +negative electrode. + +[Illustration: FIG. 208.--Carbon rods in a solution of copper +sulphate.] + +Since copper is constantly taken out of the solution of copper +sulphate for deposit upon the negative electrode, the amount of copper +remaining in the solution steadily decreases, and finally there is +none of it left for deposit. In order to overcome this, the positive +electrode should be made of the same metal as that which is to be +deposited. The positive metal electrode gradually dissolves and +replaces the metal lost from the solution by deposit and +electroplating can continue as long as any positive electrode remains. + +[Illustration: FIG. 209.--Plating spoons by electricity.] + +Practically all silver, gold, and nickel plating is done in this way; +machine, bicycle, and motor attachments are not solid, but are of +cheaper material electrically plated with nickel. When spoons are to +be plated, they are hung in a bath of silver nitrate side by side with +a thick slab of pure silver, as in Figure 209. The spoons are +connected with the negative terminal of the battery, while the slab of +pure silver is connected with the positive terminal of the same +battery. The length of time that the current flows determines the +thickness of the plating. + +294. How Pure Metal is obtained from Ore. When ore is mined, it +contains in addition to the desired metal many other substances. In +order to separate out the desired metal, the ore is placed in some +suitable acid bath, and is connected with the positive terminal of a +battery, thus taking the place of the silver slab in the last Section. +When current flows, any pure metal which is present is dissolved out +of the ore and is deposited on a convenient negative electrode, while +the impurities remain in the ore or drop as sediment to the bottom of +the vessel. Metals separated from the ore by electricity are called +electrolytic metals and are the purest obtainable. + +295. Printing. The ability of the electric current to decompose a +liquid and to deposit a metal constituent has practically +revolutionized the process of printing. Formerly, type was arranged +and retained in position until the required number of impressions had +been made, the type meanwhile being unavailable for other uses. +Moreover, the printing of a second edition necessitated practically as +great labor as did the first edition, the type being necessarily set +afresh. Now, however, the type is set up and a mold of it is taken in +wax. This mold is coated with graphite to make it a conductor and is +then suspended in a bath of copper sulphate, side by side with a slab +of pure copper. Current is sent through the solution as described in +Section 293, until a thin coating of copper has been deposited on the +mold. The mold is then taken from the bath, and the wax is replaced by +some metal which gives strength and support to the thin copper plate. +From this copper plate, which is an exact reproduction of the original +type, many thousand copies can be printed. The plate can be preserved +and used from time to time for later editions, and the original type +can be put back into the cases and used again. + + + + +CHAPTER XXXII + +MODERN ELECTRICAL INVENTIONS + + +296. An Electric Current acts like a Magnet. In order to understand +the action of the electric bell, we must consider a third effect which +an electric current can cause. Connect some cells as shown in Figure +200 and close the circuit through a stout heavy copper wire, dipping a +portion of the wire into fine iron filings. A thick cluster of filings +will adhere to the wire (Fig. 210), and will continue to cling to it +so long as the current flows. If the current is broken, the filings +fall from the wire, and only so long as the current flows through the +wire does the wire have power to attract iron filings. An electric +current makes a wire equivalent to a magnet, giving it the power to +attract iron filings. + +[Illustration: FIG. 210.--A wire carrying current attracts iron +filings.] + +[Illustration: FIG. 211.--A loosely wound coil of wire.] + +Although such a straight current bearing wire attracts iron filings, +its power of attraction is very small; but its magnetic strength can +be increased by coiling as in Figure 211. Such an arrangement of wire +is known as a helix or solenoid, and is capable of lifting or pulling +larger and more numerous filings and even good-sized pieces of iron, +such as tacks. Filings do not adhere to the sides of the helix, but +they cling in clusters to the ends of the coil. This shows that the +ends of the helix have magnetic power but not the sides. + +If a soft iron nail (Fig. 212) or its equivalent is slipped within the +coil, the lifting and attractive power of the coil is increased, and +comparatively heavy weights can be lifted. + +[Illustration: FIG. 212.--Coil and soft iron rod.] + +A coil of wire traversed by an electric current and containing a core +of soft iron has the power of attracting and moving heavy iron +objects; that is, it acts like a magnet. Such an arrangement is called +an electromagnet. As soon as the current ceases to flow, the +electromagnet loses its magnetic power and becomes merely iron and +wire without magnetic attraction. + +If many cells are used, the strength of the electromagnet is +increased, and if the coil is wound closely, as in Figure 213, instead +of loosely, as in Figure 211, the magnetic strength is still further +increased. The strength of any electromagnet depends upon the number +of coils wound on the iron core and upon the strength of the current +which is sent through the coils. + +[Illustration: FIG. 213.--An electromagnet.] + +[Illustration: FIG. 214.--A horseshoe electromagnet is powerful enough +to support heavy weights.] + +To increase the strength of the electromagnet still further, the +so-called horseshoe shape is used (Fig. 214). In such an arrangement +there is practically the strength of two separate electromagnets. + +297. The Electric Bell. The ringing of the electric bell is due to +the attractive power of an electromagnet. By the pushing of a button +(Fig. 215) connection is made with a battery, and current flows +through the wire wound on the iron spools, and further to the screw +_P_ which presses against the soft iron strip or armature _S_; and +from _S_ the current flows back to the battery. As soon as the +current flows, the coils become magnetic and attract the soft iron +armature, drawing it forward and causing the clapper to strike the +bell. In this position, _S_ no longer touches the screw _P_, and hence +there is no complete path for the electricity, and the current ceases. +But the attractive, magnetic power of the coils stops as soon as the +current ceases; hence there is nothing to hold the armature down, and +it flies back to its former position. In doing this, however, the +armature makes contact at _P_ through the spring, and the current +flows once more; as a result the coils again become magnets, the +armature is again drawn forward, and the clapper again strikes the +bell. But immediately afterwards the armature springs backward and +makes contact at _P_ and the entire operation is repeated. So long as +we press the button this process continues producing what sounds like +a continuous jingle; in reality the clapper strikes the bell every +time a current passes through the electromagnet. + +[Illustration: FIG. 215.--The electric bell.] + +298. The Push Button. The push button is an essential part of every +electric bell, because without it the bell either would not ring at +all, or would ring incessantly until the cell was exhausted. When the +push button is free, as in Figure 216, the cell terminals are not +connected in an unbroken path, and hence the current does not flow. +When, however, the button is pressed, the current has a complete path, +provided there is the proper connection at _S_. That is, the pressure +on the push button permits current to flow to the bell. The flow of +this current then depends solely upon the connection at _S_, which is +alternately made and broken, and in this way produces sound. + +[Illustration: FIG. 216.--Push button.] + +The sign "Bell out of order" is usually due to the fact that the +battery is either temporarily or permanently exhausted. In warm +weather the liquid in the cell may dry up and cause stoppage of the +current. If fresh liquid is poured into the vessel so that the +chemical action of the acid on the zinc is renewed, the current again +flows. Another explanation of an out-of-order bell is that the liquid +may have eaten up all the zinc; if this is the case, the insertion of +a fresh strip of zinc will remove the difficulty and the current will +flow. If dry cells are used, there is no remedy except in the purchase +of new cells. + +299. How Electricity may be lost to Use. In the electric bell, we +saw that an air gap at the push button stopped the flow of +electricity. If we cut the wire connecting the poles of a battery, the +current ceases because an air gap intervenes and electricity does not +readily pass through air. Many substances besides air stop the flow of +electricity. If a strip of glass, rubber, mica, or paraffin is +introduced anywhere in a circuit, the current ceases. If a metal is +inserted in the gap, the current again flows. Substances which, like +an air gap, interfere with the flow of electricity are called +non-conductors, or, more commonly, insulators. Substances which, like +the earth, the human body, and all other moist objects, conduct +electricity are conductors. If the telephone and electric light wires +in our houses were not insulated by a covering of thread, or cloth, or +other non conducting material, the electricity would escape into +surrounding objects instead of flowing through the wire and producing +sound and light. + +In our city streets, the overhead wires are supported on glass knobs +or are closely wrapped, in order to prevent the escape of electricity +through the poles to the ground. In order to have a steady, dependable +current, the wire carrying the current must be insulated. + +Lack of insulation means not only the loss of current for practical +uses, but also serious consequences in the event of the crossing of +current-bearing wires. If two wires properly insulated touch each +other, the currents flow along their respective wires unaltered; if, +however, two uninsulated wires touch, some of the electricity flows +from one to the other. Heat is developed as a result of this +transference, and the heat thus developed is sometimes so great that +fire occurs. For this reason, wires are heavily insulated and extra +protection is provided at points where numerous wires touch or cross. + +Conductors and insulators are necessary to the efficient and economic +flow of a current, the insulator preventing the escape of electricity +and lessening the danger of fire, and the conductor carrying the +current. + +300. The Telegraph. Telegraphy is the process of transmitting +messages from place to place by means of an electric current. The +principle underlying the action of the telegraph is the principle upon +which the electric bell operates; namely, that a piece of soft iron +becomes a magnet while a current flows around it, but loses its +magnetism as soon as the current ceases. + +In the electric bell, the electromagnet, clapper, push button, and +battery are relatively near,--usually all are located in the same +building; while in the telegraph the current may travel miles before +it reaches the electromagnet and produces motion of the armature. + +[Illustration: FIG. 217.--Diagram of the electric telegraph.] + +The fundamental connections of the telegraph are shown in Figure 217. +If the key _K_ is pressed down by an operator in Philadelphia, the +current from the battery (only one cell is shown for simplicity) flows +through the line to New York, passes through the electromagnet _M_, +and thence back to Philadelphia. As long as the key _K_ is pressed +down, the coil _M_ acts as a magnet and attracts and holds fast the +armature _A_; but as soon as _K_ is released, the current is broken, +_M_ loses its magnetism, and the armature is pulled back by the spring +_D_. By a mechanical device, tape is drawn uniformly under the light +marker _P_ attached to the armature. If _K_ is closed for but a short +time, the armature is drawn down for but a short interval, and the +marker registers a dot on the tape. If _K_ is closed for a longer +time, a short dash is made by the marker, and, in general, the length +of time that _K_ is closed determines the length of the marks recorded +on the tape. The telegraphic alphabet consists of dots and dashes and +their various combinations, and hence an interpretation of the dot and +dash symbols recorded on the tape is all that is necessary for the +receiving of a telegraphic message. + +The Morse telegraphic code, consisting of dots, dashes, and spaces, is +given in Figure 218. + +[Illustration: + + |A .- |H .... |O . . |U ..- | + |B -... |I .. |P ..... |V ...- | + |C .. . |J -.-. |Q ..-. |W .-- | + |D -.. |K -.- |R . .. |X .-.. | + |E . |L --- |S ... |Y .. .. | + |F .-. |M - - |T - |Z ... . | + |G --. |N -. | | | + +FIG. 218.--The Morse telegraphic code.] + +The telegraph is now such a universal means of communication between +distant points that one wonders how business was conducted before its +invention in 1832 by S.F.B. Morse. + +[Illustration: FIG. 219.--The sounder.] + +301. Improvements. _The Sounder._ Shortly after the invention of +telegraphy, operators learned that they could read the message by the +click of the marker against a metal rod which took the place of the +tape. In practically all telegraph offices of the present day the +old-fashioned tape is replaced by the sounder, shown in Figure 219. +When current flows, a lever, _L_, is drawn down by the electromagnet +and strikes against a solid metal piece with a click; when the current +is broken, the lever springs upward, strikes another metal piece and +makes a different click. It is clear that the working of the key which +starts and stops the current in this line will be imitated by the +motion and the resulting clicks of the sounder. By means of these +varying clicks of the sounder, the operator interprets the message. + +[Illustration: FIG. 220.--Diagram of a modern telegraph system.] + +_The Relay._ When a telegraph line is very long, the resistance of the +wire is great, and the current which passes through the electromagnet +is correspondingly weak, so feeble indeed that the armature must be +made very thin and light in order to be affected by the makes and +breaks in the current. The clicks of an armature light enough to +respond to the weak current of a long wire are too faint to be +recognized by the ear, and hence in such long circuits some device +must be introduced whereby the effect is increased. This is usually +done by installing at each station a local battery and a very delicate +and sensitive electromagnet called the _relay_. Under these conditions +the current of the main line is not sent through the sounder, but +through the relay which opens and closes a local battery in connection +with the strong sounder. For example, the relay is so arranged that +current from the main line runs through it exactly as it runs through +_M_ in Figure 217. When current is made, the relay attracts an +armature, which thereby closes a circuit in a local battery and thus +causes a click of the sounder. When the current in the main line is +broken, the relay loses its magnetic attraction, its armature springs +back, connection is broken in the local circuit, and the sounder +responds by allowing its armature to spring back with a sharp sound. + +302. The Earth an Important Part of a Telegraphic System. We learned +in Section 299 that electricity could flow through many different +substances, one of which was the earth. In all ordinary telegraph +lines, advantage is taken of this fact to utilize the earth as a +conductor and to dispense with one wire. Originally two wires were +used, as in Figure 217; then it was found that a railroad track could +be substituted for one wire, and later that the earth itself served +equally well for a return wire. The present arrangement is shown in +Figure 220, where there is but one wire, the circuit being completed +by the earth. No fact in electricity seems more marvelous than that +the thousands of messages flashing along the wires overhead are +likewise traveling through the ground beneath. If it were not for this +use of the earth as an unfailing conductor, the network of overhead +wires in our city streets would be even more complex than it now is. + +303. Advances in Telegraphy. The mechanical improvements in +telegraphy have been so rapid that at present a single operator can +easily send or receive forty words a minute. He can telegraph more +quickly than the average person can write; and with a combination of +the latest improvements the speed can be enormously increased. +Recently, 1500 words were flashed from New York to Boston over a +single wire in one second. + +In actual practice messages are not ordinarily sent long distances +over a direct line, but are automatically transferred to new lines at +definite points. For example, a message from New York to Chicago does +not travel along an uninterrupted path, but is automatically +transferred at some point, such as Lancaster, to a second line which +carries it on to Pittsburgh, where it is again transferred to a third +line which takes it farther on to its destination. + + + + +CHAPTER XXXIII + +MAGNETS AND CURRENTS + + +304. In the twelfth century, there was introduced into Europe from +China a simple instrument which changed journeying on the sea from +uncertain wandering to a definite, safe voyage. This instrument was +the compass (Fig. 221), and because of the property of the compass +needle (a magnet) to point unerringly north and south, sailors were +able to determine directions on the sea and to steer for the desired +point. + +[Illustration: FIG. 221.--The compass.] + +Since an electric current is practically equivalent to a magnet +(Section 296), it becomes necessary to know the most important facts +relative to magnets, facts simple in themselves but of far-reaching +value and consequences in electricity. Without a knowledge of the +magnetic characteristics of currents, the construction of the motor +would have been impossible, and trolley cars, electric fans, motor +boats, and other equally well-known electrical contrivances would be +unknown. + +305. The Attractive Power of a Magnet. The magnet best known to us +all is the compass needle, but for convenience we will use a magnetic +needle in the shape of a bar larger and stronger than that employed in +the compass. If we lay such a magnet on a pile of iron filings, it +will be found on lifting the magnet that the filings cling to the ends +in tufts, but leave it almost bare in the center (Fig. 222). The +points of attraction at the two ends are called the poles of the +magnet. + +[Illustration: FIG. 222.--A magnet.] + +If a delicately made magnet is suspended as in Figure 223, and is +allowed to swing freely, it will always assume a definite north and +south position. The pole which points north when the needle is +suspended is called the north pole and is marked _N_, while the pole +which points south when the needle is suspended is called the south +pole and is marked _S_. + +A freely suspended magnet points nearly north and south. + +A magnet has two main points of attraction called respectively the +north and south poles. + +[Illustration: FIG. 223.--The magnetic needle.] + +306. The Extent of Magnetic Attraction. If a thin sheet of paper or +cardboard is laid over a strong, bar-shaped magnet and iron filings +are then gently strewn on the paper, the filings clearly indicate the +position of the magnet beneath, and if the cardboard is gently tapped, +the filings arrange themselves as shown in Figure 224. If the paper is +held some distance above the magnet, the influence on the filings is +less definite, and finally, if the paper is held very far away, the +filings do not respond at all, but lie on the cardboard as dropped. + +The magnetic power of a magnet, while not confined to the magnet +itself, does not extend indefinitely into the surrounding region; the +influence is strong near the magnet, but at a distance becomes so weak +as to be inappreciable. The region around a magnet through which its +magnetic force is felt is called the field of force, or simply the +magnetic field, and the definite lines in which the filings arrange +themselves are called lines of force. + +[Illustration: FIG. 224.--Iron filings scattered over a magnet arrange +themselves in definite lines.] + +The magnetic power of a magnet is not limited to the magnet, but +extends to a considerable distance in all directions. + +307. The Influence of Magnets upon Each Other. If while our +suspended magnetic needle is at rest in its characteristic +north-and-south direction another magnet is brought near, the +suspended magnet is turned; that is, motion is produced (Fig. 225). If +the north pole of the free magnet is brought toward the south pole of +the suspended magnet, the latter moves in such a way that the two +poles _N_ and _S_ are as close together as possible. If the north pole +of the free magnet is brought toward the north pole of the suspended +magnet, the latter moves in such a way that the two poles _N_ and _N_ +are as far apart as possible. In every case that can be tested, it is +found that a north pole repels a north pole, and a south pole repels a +south pole; but that a north and a south pole always attract each +other. + +[Illustration: FIG. 225.--A south pole attracts a north pole.] + +The main facts relative to magnets may be summed up as follows:-- + +_a_. A magnet points nearly north and south if it is allowed to swing +freely. + +_b_. A magnet contains two unlike poles, one of which persistently +points north, and the other of which as persistently points south, if +allowed to swing freely. + +_c_. Poles of the same name repel each other; poles of unlike name +attract each other. + +_d_. A magnet possesses the power of attracting certain substances, +like iron, and this power of attraction is not limited to the magnet +itself but extends into the region around the magnet. + +308. Magnetic Properties of an Electric Current. If a +current-bearing wire is really equivalent in its magnetic powers to a +magnet, it must possess all of the characteristics mentioned in the +preceding Section. We saw in Section 296 that a coiled wire through +which current was flowing would attract iron filings at the two ends +of the helix. That a coil through which current flows possesses the +characteristics _a_, _b_, _c_, and _d_ of a magnet is shown as follows:-- + +_a_, _b_. If a helix marked at one end with a red string is arranged so +that it is free to rotate and a strong current is sent through it, +the helix will immediately turn and face about until it points north +and south. If it is disturbed from this position, it will slowly swing +back until it occupies its characteristic north and south position. +The end to which the string is attached will persistently point either +north or south. If the current is sent through the coil in the +opposite direction, the two poles exchange positions and the helix +turns until the new north pole points north. + +[Illustration: FIG. 226.--A helix through which current flows always +points north and south, if it is free to rotate.] + +_c_. If a coil conducting a current is held near a suspended magnet, +one end of the helix will be found to attract the north pole of the +magnet, while the opposite end will be found to repel the north pole +of the magnet. In fact, the helix will be found to behave in every +way as a magnet, with a north pole at one end and a south pole at the +other. If the current is sent through the helix in the opposite +direction, the north and south poles exchange places. + +[Illustration: FIG. 227.--A wire through which current flows is +surrounded by a field of magnetic force.] + +If the number of turns in the helix is reduced until but a single loop +remains, the result is the same; the single loop acts like a flat +magnet, one side of the loop always facing northward and one +southward, and one face attracting the north pole of the suspended +magnet and one repelling it. + +_d_. If a wire is passed through a card and a strong current is sent +through the wire, iron filings will, when sprinkled upon the card, +arrange themselves in definite directions (Fig. 227). A wire carrying +a current is surrounded by a magnetic field of force. + +A magnetic needle held under a current-bearing wire turns on its pivot +and finally comes to rest at an angle with the current. The fact that +the needle is deflected by the wire shows that the magnetic power of +the wire extends into the surrounding medium. + +The magnetic properties of current electricity were discovered by +Oersted of Denmark less than a hundred years ago; but since that time +practically all important electrical machinery has been based upon one +or more of the magnetic properties of electricity. The motors which +drive our electric fans, our mills, and our trolley cars owe their +existence entirely to the magnetic action of current electricity. + +[Illustration: FIG. 228.--The coil turns in such a way that its north +pole is opposite the south pole of the magnet.] + +309. The Principle of the Motor. If a close coil of wire is +suspended between the poles of a strong horseshoe magnet, it will not +assume any characteristic position but will remain wherever placed. +If, however, a current is sent through the wire, the coil faces about +and assumes a definite position. This is because a coil, carrying a +current, is equivalent to a magnet with a north and south face; and, +in accordance with the magnetic laws, tends to move until its north +face is opposite the south pole of the horseshoe magnet, and its south +face opposite the north pole of the magnet. If, when the coil is at +rest in this position, the current is reversed, so that the north pole +of the coil becomes a south pole and the former south pole becomes a +north pole, the result is that like poles of coil and magnet face each +other. But since like poles repel each other, the coil will move, and +will rotate until its new north pole is opposite to the south pole of +the magnet and its new south pole is opposite the north pole. By +sending a strong current through the coil, the helix is made to rotate +through a half turn; by reversing the current when the coil is at the +half turn, the helix is made to continue its rotation and to swing +through a whole turn. If the current could be repeatedly reversed just +as the helix completed its half turn, the motion could be prolonged; +periodic current reversal would produce continuous rotation. This is +the principle of the motor. + +[Illustration: FIG. 229.--Principle of the motor.] + +It is easy to see that long-continued rotation would be impossible in +the arrangement of Figure 228, since the twisting of the suspending +wire would interfere with free motion. If the motor is to be used for +continuous motion, some device must be employed by means of which the +helix is capable of continued rotation around its support. + +In practice, the rotating coil of a motor is arranged as shown in +Figure 229. Wires from the coil terminate on metal disks and are +securely soldered there. The coil and disks are supported by the +strong and well-insulated rod _R_, which rests upon braces, but which +nevertheless rotates freely with disks and coil. The current flows to +the coil through the thin metal strips called brushes, which rest +lightly upon the disks. + +When the current which enters at _B_ flows through the wire, the coil +rotates, tending to set itself so that its north face is opposite the +south face of the magnet. If, when the helix has just reached this +position, the current is reversed--entering at _B'_ instead of +_B_--the poles of the coil are exchanged; the rotation, therefore, +does not cease, but continues for another half turn. Proper reversals +of the current are accompanied by continuous motion, and since the +disk and shaft rotate with the coil, there is continuous rotation. + +If a wheel is attached to the rotating shaft, weights can be lifted, +and if a belt is attached to the wheel, the motion of the rotating +helix can be transferred to machinery for practical use. + +The rotating coil is usually spoken of as the armature, and the large +magnet as the field magnet. + +310. Mechanical Reversal of the Current. _The Commutator_. It is not +possible by hand to reverse the current with sufficient rapidity and +precision to insure uninterrupted rotation; moreover, the physical +exertion of such frequent reversals is considerable. Hence, some +mechanical device for periodically reversing the current is necessary, +if the motor is to be of commercial value. + +[Illustration: FIG. 230.--The commutator.] + +The mechanical reversal of the current is accomplished by the use of +the commutator, which is a metal ring split into halves, well +insulated from each other and from the shaft. To each half of this +ring is attached one of the ends of the armature wire. The brushes +which carry the current are set on opposite sides of the ring and do +not rotate. As armature, commutator, and shaft rotate, the brushes +connect first with one segment of the commutator and then with the +other. Since the circuit is arranged so that the current always enters +the commutator through the brush _B_, the flow of the current into the +coil is always through the segment in contact with _B_; but the +segment in contact with _B_ changes at every half turn of the coil, +and hence the direction of the current through the coil changes +periodically. As a result the coil rotates continuously, and produces +motion so long as current is supplied from without. + +311. The Practical Motor. A motor constructed in accordance with +Section 309 would be of little value in practical everyday affairs; +its armature rotates too slowly and with too little force. If a motor +is to be of real service, its armature must rotate with sufficient +strength to impart motion to the wheels of trolley cars and mills, to +drive electric fans, and to set into activity many other forms of +machinery. + +The strength of a motor may be increased by replacing the singly +coiled armature by one closely wound on an iron core; in some +armatures there are thousands of turns of wire. The presence of soft +iron within the armature (Section 296) causes greater attraction +between the armature and the outside magnet, and hence greater force +of motion. The magnetic strength of the field magnet influences +greatly the speed of the armature; the stronger the field magnet the +greater the motion, so electricians make every effort to strengthen +their field magnets. The strongest known magnets are electromagnets, +which, as we have seen, are merely coils of wire wound on an iron +core. For this reason, the field magnet is usually an electromagnet. + +When very powerful motors are necessary, the field magnet is so +arranged that it has four or more poles instead of two; the armature +likewise consists of several portions, and even the commutator may be +very complex. But no matter how complex these various parts may seem +to be, the principle is always that stated in Section 309, and the +parts are limited to field magnet, commutator, and armature. + +[Illustration: FIG. 231.--A modern power plant.] + +[Illustration: FIG. 232.--The electric street car.] + +The motor is of value because by means of it motion, or mechanical +energy, is obtained from an electric current. Nearly all electric +street cars (Fig. 232), are set in motion by powerful motors placed +under the cars. As the armature rotates, its motion is communicated by +gears to the wheels, the necessary current reaching the motor through +the overhead wires. Small motors may be used to great advantage in the +home, where they serve to turn the wheels of sewing machines, and to +operate washing machines. Vacuum cleaners are frequently run by +motors. + + + + +CHAPTER XXXIV + +HOW ELECTRICITY MAY BE MEASURED + + +312. Danger of an Oversupply of Current. If a small toy motor is +connected with one cell, it rotates slowly; if connected with two +cells, it rotates more rapidly, and in general, the greater the number +of cells used, the stronger will be the action of the motor. But it is +possible to send too strong a current through our wire, thereby +interfering with all motion and destroying the motor. We have seen in +Section 288 that the amount of current which can safely flow through a +wire depends upon the thickness of the wire. A strong current sent +through a fine wire has its electrical energy transformed largely into +heat; and if the current is very strong, the heat developed may be +sufficient to burn off the insulation and melt the wire itself. This +is true not only of motors, but of all electric machinery in which +there are current-bearing wires. The current should not be greater +than the wires can carry, otherwise too much heat will be developed +and damage will be done to instruments and surroundings. + +The current sent through our electric stoves and irons should be +strong enough to heat the coils, but not strong enough to melt them. +If the current sent through our electric light wires is too great for +the capacity of the wires, the heat developed will injure the wires +and may cause disastrous results. The overloading of wires is +responsible for many disastrous fires. + +The danger of overloading may be eliminated by inserting in the +circuit a fuse or other safety device. A fuse is made by combining a +number of metals in such a way that the resulting substance has a low +melting point and a high electrical resistance. A fuse is inserted in +the circuit, and the instant the current increases beyond its normal +amount the fuse melts, breaks the circuit, and thus protects the +remaining part of the circuit from the danger of an overload. In this +way, a circuit designed to carry a certain current is protected from +the danger of an accidental overload. The noise made by the burning +out of a fuse in a trolley car frequently alarms passengers, but it is +really a sign that the system is in good working order and that there +is no danger of accident from too strong a current. + +313. How Current is Measured. The preceding Section has shown +clearly the danger of too strong a current, and the necessity for +limiting the current to that which the wire can safely carry. There +are times when it is desirable to know accurately the strength of a +current, not only in order to guard against an overload, but also in +order to determine in advance the mechanical and chemical effects +which will be produced by the current. For example, the strength of +the current determines the thickness of the coating of silver which +forms in a given time on a spoon placed in an electrolytic bath; if +the current is weak, a thin plating is made on the spoon; if the +current is strong, a thick plating is made. If, therefore, the exact +value of the current is known, the exact amount of silver which will +be deposited on the spoon in a given time can be definitely +calculated. + +[Illustration: FIG. 233.--The principle of the galvanometer.] + +Current-measuring instruments, or galvanometers, depend for their +action on the magnetic properties of current electricity. The +principle of practically all galvanometers is as follows:-- + +A closely wound coil of fine wire free to rotate is suspended as in +Figure 233 between the poles of a strong magnet. When a current is +sent through the coil, the coil becomes a magnet and turns so that its +faces will be towards the poles of the permanent magnet. But as the +coil turns, the suspending wire becomes twisted and hinders the +turning. For this reason, the coil can turn only until the motion +caused by the current is balanced by the twist of the suspending wire. +But the stronger the current through the coil, the stronger will be +the force tending to rotate the coil, and hence the less effective +will be the hindrance of the twisting string. As a consequence, the +coil swings farther than before; that is, the greater the current, the +farther the swing. Usually a delicate pointer is attached to the +movable coil and rotates freely with it, so that the swing of the +pointer indicates the relative values of the current. If the source of +the current is a gravity cell, the swing is only two thirds as great +as when a dry cell is used, indicating that the dry cell furnishes +about 1-1/2 times as much current as a gravity cell. + +314. Ammeters. A galvanometer does not measure the current, but +merely indicates the relative strength of different currents. But it +is desirable at times to measure a current in units. Instruments for +measuring the strength of currents in units are called ammeters, and +the common form makes use of a galvanometer. + +A current is sent through a movable coil (the field magnet and coil +are inclosed in the case) (Fig. 234), and the magnetic field thus +developed causes the coil to turn, and the pointer attached to it to +move over a scale graduated so that it reads current strengths. This +scale is carefully graduated by the following method. + +If two silver rods (Fig. 208) are weighed and placed in a solution of +silver nitrate, and current from a single cell is passed through the +liquid for a definite time, we find, on weighing the two rods, that +one has gained in weight and the other has lost. If the current is +allowed to flow twice as long, the amount of silver lost and gained by +the electrodes is doubled; and if twice the current is used, the +result is again doubled. + +As a result of numerous experiments, it was found that a definite +current of electricity will deposit a definite amount of silver in a +definite time, and that the amount of silver deposited on an electrode +in one second might be used to measure the current of electricity +which has flowed through the circuit in one second. + +A current is said to be one ampere strong if it will deposit silver on +an electrode at the rate of 0.001118 gram per second. + +[Illustration: FIG. 234.--An ammeter.] + +In marking the scale, an ammeter is placed in the circuit of an +electrolytic cell and the position of the pointer is marked on the +blank card which lies beneath and which is to serve as a scale (Fig. +235). After the current has flowed for about an hour, the amount of +silver which has been deposited is measured. Knowing the time during +which the current has run, and the amount of deposit, the strength of +the current in amperes can be calculated. This number is written +opposite the place at which the pointer stood during the experiment. + +The scale may be completed by marking the positions of the pointer +when other currents of known strength flow through the ammeter. + +[Illustration: FIG. 235.--Marking the scale of an ammeter.] + +All electric plants, whether for heating, lighting, or for machinery, +are provided with ammeters, such instruments being as important to an +electric plant as the steam gauge is to the boiler. + +315. Voltage and Voltmeters. Since electromotive force, or voltage, +is the cause of current, it should be possible to compare different +electromotive forces by comparing the currents which they produce in a +given circuit. But two voltages of equal value do not give equal +currents unless the resistances met by the currents are equal. For +example, the simple voltaic cell and the gravity cell have +approximately equal voltages, but the current produced by the voltaic +cell is stronger than that produced by the gravity cell. This is +because the current meets more resistance within the gravity cell +than within the voltaic cell. Every cell, no matter what its nature, +offers resistance to the flow of electricity through it and is said to +have internal resistance. If we are determining the voltages of +various cells by a comparison of the respective currents produced, the +result will be true only on condition that the resistances in the +various circuits are equal. If a very large external resistance of +fine wire is placed in circuit with a gravity cell, the _total_ +resistance of the circuit (made up of the relatively small resistance +in the cell and the larger resistance in the rest of the circuit) will +differ but little from that of another circuit in which the gravity +cell is replaced by a voltaic cell, or any other type of cell. + +With a high resistance in the outside circuit, the deflections of the +ammeter will be small, but such as they are, they will fairly +accurately represent the electromotive forces which produce them. + +Voltmeters (Fig. 236), or instruments for measuring voltage, are like +ammeters except that a wire of very high resistance is in circuit with +the movable coil. In external appearance they are not distinguishable +from ammeters. + +[Illustration: FIG. 236.--A voltmeter.] + +The unit of electromotive force is called the _volt_. The voltage of a +dry cell is approximately 1.5 volts, and the voltage of a voltaic cell +and of a gravity cell is approximately 1 volt. + +316. Current, Voltage, Resistance. We learned in Section 287 that +the strength of a current increases when the electromotive force +increases, and diminishes when the electromotive force diminishes. +Later, in Section 288, we learned that the strength of the current +decreases as the resistance in circuit increases. + +The strength of a steady current depends upon these two factors only, +the electromotive force which causes it and the resistance which it +has to overcome. + +317. Resistance. Since resistance plays so important a role in +electricity, it becomes necessary to have a unit of resistance. The +practical unit of resistance is called an ohm, and some idea of the +value of an ohm can be obtained if we remember that a 300-foot length +of common iron telegraph wire has a resistance of 1 ohm. An +approximate ohm for rough work in the laboratory may be made by +winding 9 feet 5 inches of number 30 copper wire on a spool or +arranging it in any other convenient form. + +In Section 299 we learned that substances differ very greatly in the +resistance which they offer to electricity, and so it will not +surprise us to learn that while it takes 300 feet of iron telegraph +wire to give 1 ohm of resistance, it takes but 39 feet of number 24 +copper wire, and but 2.2 feet of number 24 German silver wire, to give +the same resistance. + + NOTE. The number of a wire indicates its diameter; number + 30, for example, being always of a definite fixed diameter, + no matter what the material of the wire. + +If we wish to avoid loss of current by heating, we use a wire of low +resistance; while if we wish to transform electricity into heat, as in +the electric stove, we choose wire of high resistance, as German +silver wire. + + + + +CHAPTER XXXV + +HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE + + +318. The Dynamo. We have learned that cells furnish current as a +result of chemical action, and that the substance usually consumed +within the cell is zinc. Just as coal within the furnace furnishes +heat, so zinc within the cell furnishes electricity. But zinc is a +much more expensive fuel than coal or oil or gas, and to run a large +motor by electricity produced in this way would be very much more +expensive than to run the motor by water or steam. For weak and +infrequent currents such as are used in the electric bell, only small +quantities of zinc are needed, and the expense is small. But for the +production of such powerful currents as are needed to drive trolley +cars, elevators, and huge machinery, enormous quantities of zinc would +be necessary and the cost would be prohibitive. It is safe to say that +electricity would never have been used on a large scale if some less +expensive and more convenient source than zinc had not been found. + +319. A New Source of Electricity. It came to most of us as a +surprise that an electric current has magnetic properties and +transforms a coil into a veritable magnet. Perhaps it will not +surprise us now to learn that a magnet in motion has electric +properties and is, in fact, able to produce a current within a wire. +This can be proved as follows:-- + +[Illustration: FIG. 237.--The motion of a magnet within a coil of wire +produces a current of electricity.] + +Attach a closely wound coil to a sensitive galvanometer (Fig. 237); +naturally there is no deflection of the galvanometer needle, because +there is no current in the wire. Now thrust a magnet into the coil. +Immediately there is a deflection of the needle, which indicates that +a current is flowing through the circuit. If the magnet is allowed to +remain at rest within the coil, the needle returns to its zero +position, showing that the current has ceased. Now let the magnet be +withdrawn from the coil; the needle is deflected as before, but the +deflection is in the opposite direction, showing that a current +exists, but that it flows in the opposite direction. We learn, +therefore, that a current may be induced in a coil by moving a magnet +back and forth within the coil, but that a magnet at rest within the +coil has no such influence. + +An electric current transforms a coil into a magnet. A magnet in +motion induces electricity within a coil; that is, causes a current to +flow through the coil. + +A magnet possesses lines of force, and as the magnet moves toward the +coil it carries lines of force with it, and the coil is cut, so to +speak, by these lines of force. As the magnet recedes from the coil, +it carries lines of force away with it, this time reducing the number +of the lines which cut the coil. + +[Illustration: FIG. 238.--As long as the coil rotates between the +poles of the magnet, current flows.] + +320. A Test of the Preceding Statement. We will test the statement +that a magnet has electric properties by another experiment. Between +the poles of a strong magnet suspend a movable coil which is connected +with a sensitive galvanometer (Fig. 237). Starting with the coil in +the position of Figure 228, when many lines of force pass through it, +let the coil be rotated quickly until it reaches the position +indicated in Figure 238, when no lines of force pass through it. +During the motion of the coil, a strong deflection of the galvanometer +is observed; but the deflection ceases as soon as the coil ceases to +rotate. If, now, starting with the position of Figure 238, the coil is +rotated forward to its starting point, a deflection occurs in the +opposite direction, showing that a current is present, but that it +flows in the opposite direction. So long as the coil is in motion, it +is cut by a varying number of lines of force, and current is induced +in the coil. + +_The above arrangement is a dynamo in miniature_. By rotation of a +coil (armature) within a magnetic field, that is, between the poles of +a magnet, current is obtained. + +In the _motor_, current produces motion. In the _dynamo_, motion +produces current. + +321. The Dynamo. As has been said, the arrangement of the preceding +Section is a dynamo in miniature. Every dynamo, no matter how complex +its structure and appearance, consists of a coil of wire which can +rotate continuously between the poles of a strong magnet. The +mechanical devices to insure easy rotation are similar in all respects +to those previously described for the motor. + +[Illustration: FIG. 239.--A modern electrical machine.] + +The current obtained from such a dynamo alternates in direction, +flowing first in one direction and then in the opposite direction. +Such alternating currents are unsatisfactory for many purposes, and to +be of service are in many cases transformed into direct currents; that +is, current which flows steadily in one direction. This is +accomplished by the use of a commutator. In the construction of the +motor, continuous _motion_ in one direction is obtained by the use of +a commutator (Section 310); in the construction of a dynamo, +continuous _current_ in one direction is obtained by the use of a +similar device. + +322. Powerful Dynamos. The power and efficiency of a dynamo are +increased by employing the devices previously mentioned in connection +with the motor. Electromagnets are used in place of simple magnets, +and the armature, instead of being a simple coil, may be made up of +many coils wound on soft iron. The speed with which the armature is +rotated influences the strength of the induced current, and hence the +armature is run at high speed. + +[Illustration: FIG. 240.--Thomas Edison, one of the foremost +electrical inventors of the present day.] + +A small dynamo, such as is used for lighting fifty incandescent lamps, +has a horse power of about 33.5, and large dynamos are frequently as +powerful as 7500 horse power. + +323. The Telephone. When a magnet is at rest within a closed coil of +wire, as in Section 319, current does not flow through the wire. But +if a piece of iron is brought near the magnet, current is induced and +flows through the wire; if the iron is withdrawn, current is again +induced in the wire but flows in the opposite direction. As iron +approaches and recedes from the magnet, current is induced in the wire +surrounding the magnet. This is in brief the principle of the +telephone. When one talks into a receiver, _L_, the voice throws into +vibration a sensitive iron plate standing before an electromagnet. The +back and forth motion of the iron plate induces current in the +electromagnet _c_. The current thus induced makes itself evident at +the opposite end of the line _M_, where by its magnetic attraction, it +throws a second iron plate into vibrations. The vibrations of the +second plate are similar to those produced in the first plate by the +voice. The vibrations of the far plate thus reproduce the sounds +uttered at the opposite end. + +[Illustration: FIG. 241.--Diagram of a simple telephone circuit.] + +324. Cost of Electric Power. The water power of a stream depends +upon the quantity of water and the force with which it flows. The +electric power of a current depends upon the quantity of electricity +and the force under which it flows. The unit of electric power is +called the watt; it is the power furnished by a current of one ampere +with a voltage of one volt. + +One watt represents a very small amount of electric power, and for +practical purposes a unit 1000 times as large is used, namely, the +kilowatt. By experiment it has been found that one kilowatt is +equivalent to about 1-1/3 horse power. Electric current is charged for +by the watt hour. A current of one ampere, having a voltage of one +volt, will furnish in the course of one hour one watt hour of energy. +Energy for electric lighting is sold at the rate of about ten cents +per kilowatt hour. For other purposes it is less expensive. The meters +commonly used measure the amperes, volts, and time automatically, and +register the electric power supplied in watt hours. + + + + + INDEX + + + Absorption, of heat by lampblack, 143-144. + of gases by charcoal, 57. + of light waves, 135-138. + + Accommodation of the eye, 123. + + Acetanilid, 259. + + Acetylene, as illuminant, 152-153. + manufacture of, 152-153. + properties of, 220. + + Acid, boric, 253. + carbolic, 152, 251, 252. + hydrochloric, 55, 80, 227, 238, 241. + lactic, 230. + oxalic, 247, 248. + salicylic, 253. + sulphuric, 55, 80, 240, 241, 307. + sulphurous, 242. + + Acids, action on litmus, 220. + + Adenoids, 51. + + Adulterants, detection of, 16. + + Air, characteristics of, 81-83, 86, 189. + compressibility of, 91. + expansion of, 10-11. + humidity, 38, 39. + pumps, 201-205. + transmits sound, 269. + weight of, 86. + _See_ Atmosphere. + + Alcohol, 234. + in patent medicines, 260. + + Alizarin, 248. + + Alkali, 222. + + Alternating current, 351. + + Alum, 247. + in baking powder, 230. + + Ammeter, 341, 343. + + Ammonia, 152. + a base, 221-222. + in bath, 226. + in manufacture of ice, 98. + neutralizing chlorine, 240. + + Ampere, 342. + + Anemia, 259. + + Angle, of incidence, 110. + of reflection, 110. + of refraction, 114. + + Aniline, 152, 245. + + Animal charcoal, 58. + + Animal transportation, 132. + + Antichlor, 240. + + Antipyrin, 259. + + Armature, 319, 320. + dynamo, 350. + motor, 335. + + Artificial lighting, 148-153. + + Atmosphere, 81. + carbon dioxide in, 54-55. + height of, 81. + nitrogen and oxygen in, 262. + pressure of, 82-86. + water vapor in, 36-38. + weight, 86. + _See_ Air. + + Atmospheric pressure, 82-86. + + Atomizer, 92. + + Atoms, 102. + + Automobiles, gas engines, 185. + + Axis of a lens, 119. + + + Bacteria, 133. + as nitrogen makers, 263. + destroyed by sunlight, etc., 133, 250, 251. + diseases caused by, 133. + in butter and cheese, 133. + + Baking powder, 229-230. + + Baking soda, 227-229. + + Barograph, 87. + + Barometer, aneroid, 84-85. + mercury, 84. + use in weather predictions, 86-87. + + Bases, action on litmus, 221-222. + properties, 220-222. + + Battery, electric, 311. + + Beans, as food, 66. + roots take in nitrogen, 263. + + Bell, electric, 319-321. + + Benzine, 150. + as a cleaning agent, 227. + + Benzoate of soda, 253. + + Bicarbonate of soda, in fire extinguisher, 55, 56. + in Rochelle salt, 227. + in soda mints, 231. + in seidlitz powder, 231. + + Bicycle pumps, 202. + + Blasting, by electricity, 314. + + Bleaching, 237-243. + by chlorine, 238-240. + + Bleaching powder, 239-240. + + Body, human, 63-64. + a conductor of electricity, 292. + + Boiling, 31. + amount of heat absorbed, 31-32. + of milk, 32. + of water, 77. + point, 15. + + Bomb calorimeter, 61. + + Borax, as meat preservative, 253. + as washing powder, 226. + + Boric acid, as meat preservative, 253. + + Boyle's law, 95-96. + + Bread, 232-233. + unleavened, 233. + + Bread making, 232-235. + + Breathing, hygienic habits of, 50. + by mouth, 50-51. + + Burns, treatment of, 52-53. + + Butter, adulteration test, 16. + bacteria in, 133. + + Buttermilk, 230. + + + Caisson, 203-204. + + Calcium carbide, 152-153. + in making nitrogenous fertilizer, 264. + + Calico printing, 249. + + Calorie, 27-28, 61-62. + + Calorimeter, 61. + + Camera, 128-129. + films, 129. + lens, 129. + plates, 129. + + Camping, water supply, 195-197. + + Candle, 148-149. + as standard for light-measure, 104-105. + + Candle-power, 105-107. + + Carbide, calcium, 152-153, 264. + + Carbohydrates, 64-65, 149. + + Carbolic acid, 152. + as disinfectant, 251. + + Carbon, 56, 66. + in voltaic cells, 308. + + Carbon dioxide, 53. + as fire extinguisher, 55-56. + commercial use, 55-56. + in baking soda, 228. + in fermentation, 234. + in health, 54. + in plants, 55. + preparation of, 55. + source of, 53. + test for, 228. + + Catarrh, 259. + + Caustic lime, 222.. + + Caustic potash, 222. + + Caustic soda, 218, 222. + to make a salt, 227. + + Caves and caverns, 71. + + Cell, dry, 310. + gravity, 309-310. + voltaic, 306-308, 310. + + Cells of human body, 63, 64, 66. + + Centigrade thermometer, 15. + + Central heating plant, 19. + + Chalk, in making carbon dioxide, 55. + + Charcoal as a filter, 57. + commercially, 57. + preparation, 57-58. + + Chemical action, and electricity, 307, 315-317. + and light, 126, 127. + + Chemistry, in daily life, 218, 219. + + Chills, 38. + + Chloride of lime, in bleaching, 240. + disinfectant, 251. + + Chlorine, and hydrogen, 239. + effect upon human body, 239. + in bleaching, 238-240. + influence of light upon, 126. + presence in salt, 227. + + Circuit, electric, 321. + local, in telegraph, 325-326. + + City water supply, 206-212. + + Clarinet, 297. + + Cleaning of material, 226, 243. + + Climate, influenced by presence of water, 29, 40. + + Clover, nitrogen producers, 263. + + Coal, 30. + + Coal gas, 150, 151. + by-products, 152. + + Coal oil, 149, 150. + + Coal tar dyes, 152, 218, 245. + + Cogwheels, 170. + + Coil, current-bearing, 320. + magnetic field about, 331-333. + + Coke, 152. + + Cold storage, 97. + + Color, 134-141. + and heat, 142, 143. + influenced by light, 137. + of opaque bodies, 136, 137. + of transparent bodies, 135, 136. + + Color blindness, 140, 141. + designs in cloth, 248, 249. + + Colors, compound, 138, 139. + essential, 139-140. + primary, 135. + simple, 138. + spectrum, 134-135. + variety in dyeing, 247, 248. + + Combustion, heat of, 45. + spontaneous, 52. + + Commutator, 335. + + Compass, 328. + + Compound colors, 138, 139. + + Compound machine, 171. + + Compound substances, 103. + + Compression of air, 91, 92. + cause of heat, 96. + + Compression pumps, 201, 205. + + Concave lens, 118. + + Condensation, 33. + heat set free, 40. + + Conduction of heat, 25. + + Conductivity metals, 321. + + Conductors, electric, 321, 322. + + Conservation, of energy, 58, 59. + of matter, 58, 59. + + Convection, 24, 25. + + Convex lens, 118. + + Cooling, by evaporation, 35-36. + by expansion, 97. + + Copper, in electric cell, 307. + + Core, iron, 319. + + Corn, bleached with sulphurous acid, 242. + + Cotton, mercerized, 218. + bleaching, 241. + dyeing, 245-247. + + Cough sirup, 258. + + Crane, compound machine, 172. + + Cream of tartar, 229. + + Creosote oil, 254. + + Crude petroleum, 149, 150. + + Current, electric, 306, 312. + alternating, 349. + induced, 346-347. + measurement of, 340. + resistance, 312, 343, 345. + strength, 339, 340, 344. + + + Dams, 214-216. + + Decay, 49. + + Decomposition of soil by water, 70-74. + + Degrees Fahrenheit and Centigrade, 15. + + Density, 11. + + Designs in cloth, printed, 248, 249. + woven, 249. + + Developer in photography, 128. + + Dew, 36, 37. + + Dew point, 38. + + Diarrhea, 251. + + Diet, 62, 66. + economy on table, 66-69. + + Discord, reason for, 271. + + Disease, and surface water, 76. + relation of light to, 131-132. + + Disease disinfectants, 250, 251, 252. + + Distillation, 34-35. + in commerce, 35. + of petroleum, 149-150. + of soft coal, 150. + of water, 34, 35, 77. + + Diving suits, 204. + + Door bells, 319-321. + + Drainage, of land, 194, 195. + sewage, 196, 198, 199, 201. + + Drilled well, 199. + + Drinking water, 75-77. + in camping, 195-196. + and rural supplies, 198, 201. + + Driven well, 196-197. + + Drought, 217. + + Drugs, 255, 260. + + Dry cell, 312. + + Dyeing, 244-249. + color designs, 248. + + Dyeing, direct, 245. + home, 247. + indirect, 247. + variety of color, 247. + + Dyes, 218, 244, 245. + + Dynamo, 346. + alternating current, 349. + source of energy, 346-347. + + + Ear, in man, 301-303. + care of, 303. + + Earth, conductor of electricity, 326. + + Echo, 277. + + Economy in buying food, 66-69. + + Effort, muscular, 155, 160. + + Electric, battery, 311. + bell, 319-321. + bread toasters, 314. + conductors and non-conductors, 321-322. + cost of, energy, 352. + current, 306, 312. + flatiron, 313. + heating pad, 314. + lights, 314. + street cars, 337. + + Electricity, heat, 312-315, 339. + as a magnet, 319, 331-333. + practical uses of, 312-317. + + Electrodes, of cell, 308. + + Electrolytic metals, 317. + + Electromagnets, 319. + + Electromotive force, 308. + unit of, 344. + + Electroplating, 315. + + Electrotyping, 317. + + Elements, 102-103. + + Emulsion, 224. + + Energy, conservation of, 58, 59. + transformations of, 58, 59. + + Engine, steam, 183-185. + gas, 185-186. + horse power, 173. + + Erosion, 73-74. + + Essential colors, 139-140. + + Evaporation, 35-39. + cooling effect, 35-36. + effect of temperature on, 35, 36. + effect of air on, 38. + freezing by, 98. + heat absorbed, 36. + of perspiration, 38. + + Expansion, of air, 10, 11. + cooling effect of, 97. + disadvantage and advantage of, 11-13. + of liquids, 9-11. + of solids, 10, 11. + of water, 9, 10, 11, 12. + Eye, 122-125. + headache, 124, 125. + how focused, 122, 123. + nearsighted and farsighted, 123. + strain, 125. + + + Fahrenheit thermometer, 15. + + Fats, 65. + in soap making, 223. + + Fermentation, 232-236. + by yeast, 234-236. + + Ferric compounds, 248. + + Fertilizers, 262-265. + nitrogen, 262. + phosphorus, 263, 264. + potash, 263-265. + + Field magnet, 336. + + Filings, iron, 329. + + Film, photographic, 129. + + Filter, charcoal, 57. + + Filtering water, 77. + + Fire, 9. + and oxygen, 45, 47. + and tinder box, 47. + making of, 51. + primitive production of, 47. + produced by friction, 47. + spontaneous combustion, 52. + sores and burns, 52-53. + extinguisher, 55, 56. + + Fireless cooker, 25, 26. + + Fireplaces, 17, 18. + + Fixing, in photography, 128. + + Flame, hydrogen, 80. + + Flood, Johnstown, 214, 215. + relation to forests, 217. + + Flour, self-raising, 231. + + Flume, 177. + + Flute, 297. + + Focal length, 118. + + Focus, of lens, 118. + + Fog, 37. + + Food, 60-69. + carbohydrates, 64, 65. + economy in buying, 66-69. + fats, 65. + fuel value of, 60-62. + need of, 63, 64. + preservatives, 252. + proteids, 66. + value, 67. + waste, 60. + water in, 75. + + Foot pound, 172. + + Force and motion, 156, 157. + and work, 156, 157. + magnetic lines of, 329-331, 334. + muscular, 155, 160. + + Force pumps, 192, 193. + + Forests and water supply, 216-217. + + Forging of iron, 40, 41. + + Formaldehyde, 253. + + Freezing, effect of salt, 44. + effect on ground and rocks, 42. + expansion of water on, 41. + ice cream freezer, 44. + + Frequency in music, 273, 275. + + Fresh air, 22-24, 49. + amount consumed by gas burner, 22. + and health, 49, 50. + in underground work, 202. + in work under water, 203-205. + + Friction, 173, 174. + losses by, 174, 210. + source of heat and fire, 47. + + Frost, 36, 37. + + Fruit, canned, bleached with sulphurous acid, 242. + colored with coal tar dyes, 253. + + Fuel value of foods, 60-62. + table of fuel values, 67. + + Fulcrum, 159, 160. + + Fumigation, 251. + + Fundamental tone, 290, 291, 292. + + Furnace, hot air, 19. + + Fuse, 340. + + Fusion, heat of, 40. + + + Galvanometer, 341. + + Gas, acetylene, 152, 153. + and unburned carbon, 151. + coal, 151, 152. + effect of heat on volume, 96, 97. + effect of pressure on volume, 95-96. + engine, 185-186. + for cooking, 151, 152. + illuminating, 92, 93, 150, 151. + liquefaction, 97, 98. + meter, 93, 94. + natural, 152. + + Gasolene, 149, 150. + as cleaning agent, 227, 243. + in gas engine, 185, 186. + + Gauge, pressure, 92-94. + + Gelatin, plate and film, 129. + + Glass, kinds of, 119. + molding of, 40. + non-conductor, 321. + + Grape juice, fermented with millet, 233. + + Gravity cell, 309, 310. + + Grease, and lye, 221. + and soap making, 223. + + Gulf Stream, 24. + + + Hard water, and soap, 225. + + Harp, 295. + + Headache, 124, 125. + powders, 259. + + Health, effect of diet, 62, 64. + + Heat, 9. + absorbed in boiling, 31-32. + and disease germs, 250. + and food, 252. + and friction, 47. + and light, 142, 147. + and oxidation, 45, 48, 49. + and wave motion, 145-147. + conduction, 25. + convection, 24, 25. + from burning hydrogen, 80. + from electricity, 312-315, 339. + needed to melt substances, 39. + of fusion, 40. + of vaporization, 32. + produced by compression, 96. + relation of water to weather, 29, 40. + set free by freezing water, 40. + sources of, 29-30. + specific, 28-29. + temperature, 27. + unit of, 27, 28. + + Heating effect of electric current, 312-315. + + Heating of buildings: central heating plant, 19. + fireplaces, 17-18. + + Heating, furnaces, 19. + hot water, 19-22. + + Helix, 318. + + Horse power, 173, 351. + + Hot water heating, 19-22. + + Hues, primary, 135. + + Humidity, 38. + proper percentage for health and comfort, 38, 39. + + Humus, 216, 217. + + Hydrocarbons, 149. + + Hydrochloric acid, composition, 227. + in bleaching, 241. + to make a salt, 227. + to make carbon dioxide, 55. + to make chlorine, 238. + to make hydrogen, 80. + + Hydrogen, 65, 66. + and chlorine, 239. + and water, 79. + chemical conduct, 126-127. + flame, 80. + in voltaic cell, 307. + peroxide, 53, 252. + preparation, 80. + to liquefy, 97. + + + Ice, lighter than water, 42. + manufacture of, 98, 99. + + Ice cream freezers, 44. + + Illuminating gas, manufacture of, 150, 151. + measurement of quantity consumed, 93, 94. + test of pressure, 92, 93. + + Illumination, intensity of, 105, 106. + + Image, in mirror, 108, 111. + + Incandescent lighting, 107, 314. + + Incidence, angle of, 110. + + Inclined plane, 162-166. + screw, 166. + wedge, 166. + + Indigo, 218. + + Induced current, 346-347. + + Ink spots, removal of, 243. + + Insoluble substances, 71. + + Insulators, electric, 324. + + Intensity, of light, 105-107. + of sound, 270-271. + + Interval, in musical scale, 283. + + Iron, forging, 41. + filings, 329. + galvanizing, 49. + oxidation of, 48. + + Irrigation, 193-194. + + Isobaric lines, 88, 91. + + Isothermal lines, 89, 91. + + + Johnstown flood, 214, 215. + + + Kerosene, 149, 150. + + Kilowatt, 351. + + + Lactic acid, 230. + + Leaves, 132, 262. + + Lens, 117-121. + concave, 118. + converging, 118. + crystalline, of eye, 122. + focal length, 118. + material, 119. + refractive power, 119. + + Lever, 158-162. + examples, 160-162. + fulcrum, 159, 160. + + Life, and carbon dioxide, 54. + and nitrogen, 261. + and oxygen, 49, 54. + + Lifting pumps, 189-192. + + Light, absorption, 135-138. + and heat, 142-147. + a wave motion, 145-147. + bent rays, 113, 114. + chemical action, 126-127. + disease, 131-132. + essential to life, 131, 132. + fading illumination, 105, 106. + influence on color, 134. + reflection of, 109-112. + refraction of, 113-125. + travels in a straight line, 108. + white, composed of colors, 134. + + Lighting, artificial, 148-153. + + Lime, chloride of, 240, 251. + + Limewater, 220. + and carbon dioxide, 228. + + Linen, bleaching, 241. + dyeing, 245-247. + + Lines, of force, 329-331, 334. + isobaric, 88, 91. + isothermal, 89, 91. + + Liquefaction of gases, 97, 98. + + Liquid air, 98. + + Liquid soap, 223, 224. + + Litmus, action of acids, 220. + action of bases, 221, 222. + action of neutral substance, 222. + + Logwood dyes, 245, 247, 248. + + Los Angeles aqueduct, 211. + + Lye, 221, 222. + + + Machines, compound, 171. + inclined plane, 162-166. + lever, 158-162. + pulley, 166-169. + wheel and axle, 169-171. + + Madder, for dyes, 245. + + Magnet, 328. + electro-, 319. + field of, 329-331. + lines of force about, 329-331. + poles of, 330-332. + properties of electricity, 318. + + Magnetic, needle, 328. + poles, 329-331. + + Magnifying power, of a lens, 115. + of a microscope, 115. + of a telescope, 115. + + Mammoth Cave of Kentucky, 71. + + Manganese dioxide, 46. + chlorine made from, 238. + oxygen made from, 46. + + Marble, for carbon dioxide, 55. + + Matches, 47. + safety, 47-48. + + Matching colors, 137. + + Matter, conservation of, 58, 59. + + Meat, 66. + preservation of, 253. + + Mechanical devices, 154, 155. + + Melting, 39, 40. + + Melting point, 40. + + Melting substances without a definite melting point, 40. + + Mercerized cotton, 218. + + Mercury, barometer, 84. + thermometer, 14-17. + + Metals, electroplating, 317. + preservation by paint, 253-254. + veins deposited by precipitation, 72, 73. + welding, 315. + + Meter, gas, 93, 94. + + Microoerganisms, 132, 133. + + Microscope, 115. + + Milk, boiling point, 32. + Pasteurized, 250. + + Minerals, in foods, 62, 63. + in water, 70, 71. + + Mirrors, 108-112. + distance of image behind mirror, 111. + distance of object in front of mirror, 111. + image a duplicate of object. 111. + + Molding of glass, 40. + + Molecule, 100-103. + + Mordants, 247, 248, 249. + + Morphine, 257. + + Morse, telegraphic code, 324. + + Motion, in sound, 266, 278, 280. + in work, 156. + + Motor, electric, 336. + principle of, 333. + street car, 337. + + Mouth breathing, 50. + cause of, 51. + + Movable pulley, 167, 168. + + Music, 278. + + Musical instruments, percussion, 299. + stringed, 284-295. + wind, 295, 299. + + Musical scale, 282. + + + Naphtha in gas engines, 185. + + Naphthalene, 152. + + Narcotics, 255. + + Natural gas, 152. + + Needle, magnetic, 328. + + Negative, electrode, 308. + photographic, 130. + + Neutral substance, 222. + and litmus, 222. + + Neutralization, 222. + + Niagara Falls, 176. + + Nitrogen, 66. + and bacteria, 263. + and plant life, 261. + in atmosphere, 261. + in fertilizer, 262-265. + in food, 66. + preparation of, 261. + properties of, 261. + + Noise in music, 280. + + Non-conductors, of electricity, 321-322. + of heat, 25. + + Nutcracker, as a lever, 162. + + + Oboe, 297. + + Octave, 284. + + Odors, 101. + + Ohm, unit of resistance, 345. + + Oil, gasoline, 149, 150. + kerosene, 149, 150. + lubricating, 174. + olive, 16. + + Orchestra grouping, 299. + + Ore, 72. + + Organ pipes, 297. + + Overtones, 290-293. + + Oxalic acid, 247, 248. + + Oxidation, 45-59. + and decay, 49. + heat the result of, 49-52. + in human body, 49, 53. + of iron, 48. + + Oxygen, 66. + and bleaching, 239. + and combustion, 45. + and food, 66. + and plants, 55. + and the human body, 50. + and water, 79, 80. + in the atmosphere, 45. + preparation of, 46. + + + Paint, as wood and metal preservatives, 253, 254. + removal of stains, 243. + + Paper making, 219. + + Paraffin, 150, 321. + + Pasteurized milk, 250. + + Patent medicines, 257-260. + + Peas, sources of nitrogen, 263. + + Pelton wheel, 177. + + Percussion instruments, 299. + + Period of a body, 273. + + Peroxide of hydrogen, 53, 252 + + Petrolatum, 150. + + Petroleum, 149, 150. + + Phonograph, 303-305. + + Phosphorus, in fertilizer, 263, 264. + in making nitrogen, 261. + in matches, 47, 48. + poisoning by, 47. + + Photography, 127-131. + + Photometer, 107. + + Pianos, 284-292. + + Pin wheel, 181. + + Pitch of sound, 280, 281. + cause of, 282. + in wind instruments, 296-299. + + Plane, inclined, 162-166. + + Plants, and atmosphere, 55. + and light, 131-132. + and nitrogen, 261. + + Plate developing, photographic, 128. + + Pneumatic dispatch tube, 205. + + Poles, magnetic, 330-332. + of cell, 308. + + Positive electrode, 308. + + Potash, in fertilizer, 263-265. + + Potassium chlorate and oxygen, 46. + permanganate, 100. + tartrate and Rochelle salt, 227. + + Power, candle, 105-107. + electric, 351. + horse, 173, 351. + sources of, 174, 175, 185. + transmission by belts, 171. + water, 176-180. + + Precipitation, 72, 73. + + Preservatives, food, 252. + wood and metal, 253-254. + + Pressure, atmospheric, 82-86. + calculation of atmospheric, 83, 84. + calculation of gas, 92, 93. + calculation of water, 94. + gauge, 92-94. + of illuminating gas, 93. + relation of pressure of gas to volume, 95, 96. + water pressure, 208-211, 214-216. + within the body, 86. + + Primary colors, 135. + + Print, photographic, 131. + + Printing, color designs in cloth, 248, 249. + electrotype, 317. + + Prisms, 135. + refraction through, 117. + + Proteids, 66. + + Pulleys, 166-169. + applications of, 169. + + Pump, 187-205. + air, 201-205. + force, 192, 193. + lifting, 189-192. + + Pupil of the eye, 122. + + Pure food laws, bleaching, 242. + preservatives, 252. + + Purification of water, 77, 196. + + Push button, 321. + + + Radiator, 19-21. + + Railroads, grading of, 165-166. + + Rain, 36, 37. + + Rainbow, 134. + + Rain water, 225. + + Reflection, angle of, 110. + of light, 109-112. + of sound, 278, 279. + + Refraction, angle of, 114. + by atmosphere, 114. + of light, 113. + uses of, 115-116. + + Relay, telegraph, 325. + + Reservoir, 214. + artificial, 211. + construction of, 214-216. + natural, 211. + + Resistance, electrical, 312. + internal, of cell, 343. + unit of, 345. + + Resonance, 276. + + River, volume and value of, 180. + + Roads, application of inclined plane to, 165-166. + + Rochelle salt, 227, 231. + + Rocks, effect of freezing water on, 42-43. + water as a solvent, 71. + + Rosin, obtained by distillation, 35. + + + Safety matches, 47-48. + + Salicylic acid, 253. + + Salt, 227-228. + + Salts, 227. + general properties, 227. + in ocean, 227. + smelling, 222. + + Saturation of air, 37. + + Scale, musical, 282. + + Screw, and inclined plane, 166. + + Seaweed, 265. + + Seidlitz powder, 231. + + Self-raising flour, 231. + + Sewage, disposition of, 198-199. + of camps, 196. + source of revenue, 201. + + Sewer gas, 57. + + Silk, bleaching, 241. + dyeing, 245-247. + + Silver chloride, 127-131. + + Simple colors, 138. + + Simple substances, 103. + + Siren, 280. + + Smelling salts, 222. + + Snow, 36-37. + + Soap, 222-224. + and hard water, 225. + liquid, 223-224. + preparation, 223. + + Soda, baking, 227, 228-229. + benzoate, 253. + caustic, 218, 222, 223, 227. + washing, 225, 226, 229. + + Soda mints, 231. + + Sodium, bicarbonate, 56, 227, 228, 230-231. + carbonate, 228. + chloride, 228. + + Soil, deposited by streams, 73. + + Solenoid, 318. + + Solution, 70. + + Soothing sirup, 258. + + Sound, and motion, 266, 278. + musical, 278. + nature of, 266. + reflection, 277. + speed of, 271-272. + transmission of, 267-271. + velocity of, 271-272. + waves, 272-274. + + Sounder, telegraph, 324. + + Sounding board, 277. + + Sour milk in cooking, 230. + + Specific heat, 28-29. + + Spectrum, 134-135. + + Speed, of sound, 271, 272. + + Spontaneous combustion, 52. + + Stains, removal of, 226, 243. + + Standpipes, 212. + + Starch, 65. + + Steam, and work, 183-184. + engine, 183-185. + heat of vaporization, 32. + heating by, 33. + turbine, 183-184. + + Steel, forging and annealing, 16. + + Stoves, 18-19. + + Streams, carriers of mud, 73. + volume of, 179-180. + + Street cars, electric, 337. + + Stringed instruments, 284-295. + + Strings, vibrating, 286-290. + + Sugar, 16, 65. + fermented by yeast, 234. + + Sulphur, 66. + as disinfectant, 251. + in making sulphurous acid, 242. + + Sulphuric acid, in bleaching, 240,241. + in fire extinguisher, 55. + in making of hydrogen, 80. + in voltaic cell, 307. + + Sulphurous acid, in bleaching, 242. + preparation, 242. + + Sun, energy derived from, 143-144. + source of heat, 29-30. + + Sunlight, 135. + and bacteria, 133. + and chemical action, 126-127. + + Sympathetic vibrations, 274-277. + + + Tallow, 105, 148. + + Tartar, cream of, 229. + + Telegraph, 322. + long distance, 327. + relay, 325. + sounder, 324. + + Telephone, 350-351. + + Temperature, 13-14. + as measurement of heat present, 27. + in detecting adulterants, 17. + in forging steel, 16. + in making sirups, 16. + measurement of, 14-15. + + Thermometer, 14-17. + Centigrade, 15. + Fahrenheit, 15. + + Tinder box, 47. + + Transmission, of light, 145-147. + of sound, 267-271. + + Tuning fork, 266, 273, 278, 290. + + Turbine, steam, 183. + water, 178. + + Turpentine, and grease, 226. + by distillation, 35. + + + Unleavened bread, 233. + + + Vacuum, sound in, 268. + + Vapor, in atmosphere, 36-38. + + Vaporization, heat of, 32. + + Varnish, on candies, 253. + + Vegetable matter, and coal, 30. + and gas, 30. + and oil, 30. + + Veins, formation in rock, 72-73. + + Velocity, of sound, 271-272. + + Ventilation, 21-24, 54. + need of, 38. + + Vibration, of strings, 286-290. + sympathetic, 274-277. + + Viola, 295. + + Violin, 295. + + Violoncello, 295. + + Vocal cords, 300. + + Voice, 300. + + Volt, 344. + + Voltage, 345. + + Voltaic cell, 306-308, 310. + + Voltmeter, 344. + + Volume, of a stream, 179-180. + relation of pressure of a gas, 95-96. + + + Washing powders, 224-226. + soda, 229. + + Water, action in nature, 70-74. + amount used daily per person, 181. + and hydrogen, 79. + and oxygen, 79, 80. + as solvent, 70-71. + boiling, 77. + boiling point, 15. + composition, 79-80. + condensation, 33. + dams and reservoirs, 214-216. + density, 11. + distilled, 34, 77. + drinking, 75-77, 195-201. + electrolysis, 79-80. + evaporation, 33-34. + expansion, 9-10, 41-42. + filtration, 77. + freezing, 40-41. + hard, 225. + heat of fusion, 40. + impurities, 76-77. + in atmosphere, 36-38. + in food, 75. + in human body, 75. + in vegetables, 75. + influence on climate, 29, 40. + irrigation, 193-194. + minerals in, 70-71. + ocean, 265. + power, 176-180. + precipitates, 72, 73. + pressure, 208-211, 214-216. + purification, 77. + rain, 225. + running, value of, 178-180. + source of, 78. + steam, 32. + waves, 145-147. + weight, 208-209, 215. + wells, 195-201. + wheels, 176-180. + work under, 203-205. + + Water supply, and forests, 216-217. + cost, 212-214. + of city, 206-212, 217. + + Watt, 351. + + Waves, heat, 145-147. + light, 145-147. + sound, 268, 272-274. + water, 145-147. + + Weather, bureau, 87-91. + forecasts, 38-39, 86-88. + relation of water to, 29, 40. + + Weather maps, 89-91. + + Wedge, and inclined plane, 166. + + Weight, of air, 86. + of water, 208-209, 215. + + Welding, by electricity, 315. + + Wells, 195-201. + drilled, 199. + driven, 196-197. + + Wheel and axle, 169-171. + cogwheels, 170. + windlass, 169. + + Wheelbarrow as lever, 160-161. + + White light, nature of, 135. + + Wind instruments, 297-301. + + Windlass, 169. + + Windmill, 174-175, 180-182. + + Winds, 24. + + Wine, 232, 234. + + Wood, as source of charcoal, 58. + ashes in soap making, 223. + in paper making, 219. + preservation, 253-254. + + Wool, bleaching, 241. + dyeing, 245-247. + + Work, 156-186. + and steam, 183-184. + and water, 176-180. + conservation, 174-175. + formula, 157. + machines, 157-175. + unit of, 172-173. + waste, 173. + + Woven designs in cloth, 249. + + + Yeast, 234-236. + wild, 235-236. + + + Zinc, in galvanizing iron, 49. + in making hydrogen, 80. + in voltaic cell, 307-308. + + + + +PLANT LIFE AND PLANT USES + +By JOHN GAYLORD COULTER, Ph. D. + +$1.20 + + +An elementary textbook providing a foundation for the study of +agriculture, domestic science, or college botany. But it is more than +a textbook on botany--it is a book about the fundamentals of plant +life and about the relations between plants and man. It presents as +fully as is desirable for required courses in high schools those large +facts about plants which form the present basis of the science of +botany. Yet the treatment has in view preparation for life in general, +and not preparation for any particular kind of calling. + +The subject is dealt with from the viewpoint of the pupil rather than +from that of the teacher or the scientist. The style is simple, clear, +and conversational, yet the method is distinctly scientific, and the +book has a cultural as well as a practical object. + +The text has a unity of organization. So far as practicable the +familiar always precedes the unfamiliar in the sequence of topics, and +the facts are made to hang together in order that the pupil may see +relationships. Such topics as forestry, plant breeding, weeds, plant +enemies and diseases, plant culture, decorative plants, and economic +bacteria are discussed where most pertinent to the general theme +rather than in separate chapters which destroy the continuity. The +questions and suggestions which follow the chapters are of two kinds; +some are designed merely to serve as an aid in the study of the text, +while others suggest outside study and inquiry. The classified tables +of terms which precede the index are intended to serve the student in +review, and to be a general guide to the relative values of the facts +presented. More than 200 attractive illustrations, many of them +original, are included in the book. + +AMERICAN BOOK COMPANY + + + + +A NEW ASTRONOMY, $1.30 + +By DAVID TODD, M. A., Ph. 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