THE CAMBRIDGE COURSE OF ELEMENTARY PHYSICS. PART FIRST. COHESION, ADHESION, CHEMICAL AFFINITY, AND ELECTRICITY. BY W. J. ROLFE AND J. A. GILLET, TEACHERS IN THE HIGH SCHOOL, CAMBRIDGE, MASS. BOSTON: CROSBY AND AINSWORTH. NEW YORK: O. S. FELT. I 868. Entered according to Act of Congress, in the year 1867, by CROSBY AND AINSWORTH, in the Clerk's Office of the District Court of the District of Massachusetts. UNIVERSITY PRESS: WELCH, BIGELOW, & Co., CAMBRIDGE. PREFACE. AN attempt is made in this Course to present the leading principles of Physics which are established at the present time, in a suitable form for teaching in our High Schools and Academies. No attempt has been made to write textbooks "for schools and colleges," since the authors believe that such books are suitable for neither the one nor the other. The authors have not sought to make these books encyclopaedias of facts, as is too often the case with text-books on Physics, especially on Chemistry. They have sought to develop leading principles in the simplest and clearest manner possible; and as all the principles of Physics rest on facts determined by observation, they have adopted the plan of first giving the experiments which establish the facts, and of then drawing out the principle. The experiments given are, in almost every case, those which one of the authors has used in the class-room. The aim has always been to give the simplest experiment (and the one requiring the simplest apparatus) that will establish the required fact. Some two years ago, one of the authors was appointed teacher of Physics in the Cambridge High School. He at first tried text-books, but soon found that there was no book suitable for High School use, which would enable the scholar to gain any satisfactory knowledge of the present iv PREFACE. state of the physical sciences. The books served only to confuse the scholars, and to give them a distaste for the study of even so interesting a subject as Chemistry. He then determined to throw aside text-books, and to rely wholly on oral instruction. The scholars soon became interested, and made much more rapid progress. But there are obvious objections to instruction wholly oral, especially with young scholars. Hence it becomes desirable to embody this instruction in a book, which the authors trust other teachers may find serviceable. These books are not, however, designed to do away with oral instruction. In our school the lessons, whenever it is possible, are illustrated by experiments before the scholar is called upon to recite them; and the success which has attended this method may justify us in recommending it to other teachers. Part II. of the Course will be devoted to Sound, Light, and Heat; Part III. to Gravity and Astronomy. There will probably be a small supplementary volume on elementary Mechanics. Part III. is nearly ready for the press, and will appear next. Each Part will be complete in itself, and, of course, they can be used in any order; but the order given is believed to be the simplest and best. Under AFFINITY the aim has been, first, to develop the great laws of combination, and then to illustrate the action of affinity in the processes of combustion, respiration, and decay, the growth of plants, destructive distillation, and the manufacture of the alkalies. In this Course an atom is regarded, according to the present theory, as a portion of matter chemically indivisible. The new atomic weights, which, as is well known, are often double the old ones, are used; and sufficient prominence is given to the doctrine of atomicily, or the atom-fixing power of the elementary atoms. The authors have fol PREFACE. V lowed Miller, who is regarded as the best English authority on chemistry, in using both the unitary and dualistic symbols of the ternary salts, according to convenience; also, in using barred symbols for those elements whose atomic weights are now regarded as double the old equivalent weights. In ELECTRICITY, greater prominence is given to voltaic and magneto-electricity than to frictional electricity, since, in the present state of the science, and of its applications in the arts, they are far more important. In frictional electricity, especial care has been taken to develop Faraday's beautiful theory of induction, which binds into one harmonious whole the otherwise heterogeneous mass of electrical phenomena. For this reason frictional electricity is presented last. Electricity is uniformly treated as a force, since the hypothesis of electric fluids is clearly out of date. The most important applications of the electric force are pointed out. The principles of telegraphing, electrotyping, plating, and gilding are fully explained. The decimal system of weights and measures will be mainly used in this Course. The Appendix to Part I. contains full directions for performing all experiments that are at all difficult. Free use has been made of all the material at our command. Many of the woodcuts have been copied from the pages of standard French and English works; the rest have been made from our own drawings. In preparing the sections on COHESION and ADHESION we have made constant use of two most excellent treatises on Chemical Physics; one by Prof. J. P. Cooke of Harvard College, and the other by Dr. W. A. Miller, of King's College, London. No teacher can afford to be without these works. The books most consulted in the preparation of the CHEMICAL AFFINITY are Hofmann's "Modern Chem Vi PREFACE. istry" (London, 1865), Part Second of Miller's "Elements of Chemistry" (3d edition, London, I864), and Roscoe's "Elementary Chemistry" (London, I866). Great assistance has also been derived from a variety of other sources. In addition to the works mentioned above, we would call the attention of teachers to Faraday's "Lectures on the Chemical History of a Candle," on "The Physical Forces," (both reprinted by the Harpers, of New York,) and on "The Non-Metallic Elements"; and Professor Cooke's Lectures on " Religion and Chemistry " (Scribner, New York). These are all most excellent books. The material of the ELECTRICITY has been mainly drawn from Faraday's "Researches in Electricity," Dr. Noad's " Manual of Electricity," and a very valuable elementary work on Electricity prepared for Chambers's Educational Course by Dr. Ferguson, and recently published (i866) at Edinburgh. The authors are jointly responsible for everything in the book. The plan of the Course has been mainly developed by Mr. Gillet, who has also selected, arranged, and partially elaborated the material of this Part; while Mr. Rolfe has given the material its final elaboration, and superintended the passage of the book through the press. There has been mutual consultation at every point. There will be the same division of labor in the preparation of the forthcoming Parts. CAMBRIDGE, May I, I867. TABLE OF CONTENTS. PAGE COHESION.......... SUMMARY OF COHESION..... 8 ADHESION.....21 SUMMARY OF ADHESION....35 CHEMICAL AFFINITY....37 ATOMS AND ATOMIC WEIGHTS... 44 SYMBOLS.......46 MURIATIC ACID........ 49 AMMONIA.....55 MARSH GAS...... 59 COMBINING POWER OF THE ELEMENTS.. 61 COMPOUNDS OF OXYGEN AND NITROGEN. 65 ACIDS, BASES, AND NEUTRALS......68 SULPHIDES AND CHLORIDES OF THE METALS 72 HYDRATES......73 SALTS.... 76 TERNARY SALTS FORMED BY SUBSTITUTION. 82 DOUBLE DECOMPOSITION.... 83 BINARY SALTS.........84 ACTION OF HYDRACIDS UPON BASES. 84 BINARY SALTS FORMED BY SUBSTITUTION.. 86 THE LAW OF DOUBLE DECOMPOSITION.. 86 SUMMARY.......89 COMBUSTION AND ITS PRODUCTS..... 95 RESPIRATION. I COMPOSITION OF VEGETABLE AND ANIMAL SUBSTANCES 113 CONDITION OF OXYGEN DURING DECAY AND RESPIRATION 115 THE GROWTH OF PLANTS.......117 SUMMARY...... 121 DESTRUCTIVE DISTILLATION AND ITS PRODUCTS.. 123 COAL-OILS..... 130 COAL-GAS.......132 Viii TABLE OF CONTENTS. ILLUMINATION......... 138 SUMMARY OF DESTRUCTIVE DISTILLATION AND OF ILLUMINATION......... 143 THE ALKALIES AND THEIR MANUFACTURE... 46 THE MANUFACTURE OF SULPHURIC ACID.. I51 THE MANUFACTURE OF BLEACHING POWDER.. 53 PRODUCTS OF THE SODA-ASH MIANUFACTURE, ETC... 55 SOURCES OF SALT..... 55 SOURCES OF SULPHUR....... I57 SOURCES OF AMMONIA.. 158 SUMMARY OF THE MANUFACTURE OF THE ALKALIES 159 CONCLUSION.... 161 ELECTRICITY. MAGNETISM..... 67 SUMMARY..........72 THE QUANTITY AND THE INTENSITY OF THE ELECTRIC FORCE..... 173 SUMMARY....77 ACTION OF THE CURRENT UPON A NEEDLE.. 178 SUMMARY.......88 ELECTRO-MAGNETISM..... 89 SUMMARY. 212 ELECTROLYSIS....... 2I4 SUMMARY..... 221 THE POWER OF THE CURRENT TO DEVELOP HEAT AND LIGHT......222 SUMMARY..... 227 VOLTAIC ELECTRICITY....... 228 SUMMARY........ 234 MAGNETO-ELECTRICITY.... 236 SUMMARY..... 251 THERMO-ELECTRICITY.... 252 SUMMARY..... 254 FRICTIONAL ELECTRICITY.. 255 SUMMARY...... 274 CONCLUSION.....278 APPENDIX..... 281 FRENCH WEIGHTS AND MEASURES. 303 QUESTIONS FOR REVIEW AND EXAMINATION.. 305 INDEX........... 319 I. CO HESION. 1 A COHESION. i. Matter is made zp of Molecules.- When a piece of ice is heated to a temperature of 32~ it melts and becomes water. The parts of the ice hold together firmly, while those of the water into which it is converted move among themselves with the greatest ease. When ice melts, then, it is evidently resolved into minute particles, which retain but a slight hold upon one another. Wax, resin, lead, iron, gold, and many other substances, also melt when they are heated to a certain degree of temperature. Most solids, then, by means of heat, can be resolved into minute particles, which move freely among themselves. If water be heated to a certain degree of temperature, it boils and becomes steam. Its particles are still further separated from one another. What is true of water in this respect is found by experiment to be true of other liquids. The particles into which a solid is resolved when it melts, and a liquid when it boils, are called molecules. Molecule means a little mass, and of these little masses all matter is supposed to be made up. 2. JMolecules are exceedingly small. - It is impossible to pulverize a solid so finely as to convert it into a liquid. A piece of gold may be divided into particles so small that each can barely be made out with the most powerful microscope, yet the gold is solid still. When heated, however, the pulverized gold is converted into a liquid; that 4 COHESION. is, each minute piece is resolved into particles which move freely among themselves. Hence these molecules are much too small to be seen with the best microscope. 3. The Molecules are not in actual contact. - If a brass Fig. I. ball, which at the ordinary temperature will just pass through a ring, be plunged into a freezing mixture and left until it becomes very cold, it will then pass through the ring very easily, not touching it at all. What is true of a brass ball in this respect is found to be true of every solid. If a bulb with a projecting tube be filled with water up to a certain point on the tube, and Fig. 2. the bulb be then plunged into a freez- ing mixture, the water will fall in the tube; and the same is found to be true if any other liquid be put into the bulb. If a similar bulb be filled with air, i and the end of the tube be held under water, and the bulb be cooled by means of a freezing mixture, the water at once rises in the tube; showing that the air occupies less Fig. 3. space when cooled. The same is X~,~ ~ found by experiment to be true of other gases. We find, then, that solids, liquids, and gases contract when cooled; and there seems to be no limit to this con_5__g traction, for they continue to contract, — = _- however much they are cooled. Now, when a body contracts, its molecules are supposed to come nearer together, and since, so far as we know, a body may continue to contract indefinitely, it follows that the molecules are never in actual contact. COHESION. 5 4. The Spaces between the Molecules are immense in comparison with the Size of the.Molecules. - Though the spaces between the molecules are very minute, since they cannot be discerned even with the most powerful microscope, there are good reasons for believing that they are immense when compared with the molecules themselves. The molecules of a body have been compared to the earth, sun, moon, and stars, and the spaces between the molecules to the spaces between these heavenly bodies. This comparison is probably very near the truth. If we imagine a being small enough to live on one of the molecules in the centre of a stone, as we live on the earth, such a being, on looking out into the space about him, would see here and there, at immense distances, other molecules, as we see the scattered stars in the heavens at night. The molecules, though exceedingly minute, are perfectly distinct and definite masses, like the earth, moon, and stars, and they are separated by spaces many thousand times as great as that occupied by each molecule. 5. A.n Attractive and a Repuzsive Moleczdar Force. -If we attempt to pull any solid asunder, we perceive at once that the particles of which it is composed are held together more or less firmly. That which holds them together is called an attraciveforce. If a glass rod be dipped into water, a drop hangs from its end when taken out. This drop is made up of molecules which are evidently held together. In the case of liquids the molecules are held together but feebly, and the attractive.force seems to be slight. If a rubber bag partially filled with air, and closed so as to be air-tight, be placed under the receiver of an air-pump, and the air exhausted from the receiver, the air within the bag will at once expand, as is shown by the filling out of the bag. The same is found to be true when the bag is partially filled with any other gas; showing that gases when left to themselves expand, that is, their molecules separate. 6 COHESION. The force which separates. the molecules is called a repulsive force. Since these forces act between molecules, they are called molecular forces. 6. These tzeo Forces act together. - A brass ball (see Fig. I) which will just pass through a ring at the ordinary temperature, will not pass through the ring after being heated; showing that the ball expands when heated. By similar experiments it is found that all solids expand when heated. While a solid is heating, then, a repulsive force must be acting, which separates the molecules. If, however, while the solid is heating, we attempt to pull it asunder, it resists; showing that the molecules are still held together by an attractive force. We see by the foregoing experiment that heat is the repulsive force which separates the molecules. As the temperature of the solid rises this repulsive force grows stronger and stronger, until it nearly equals the attractive force, when the solid melts, that is, becomes a liquid. If a glass bulb with a projecting tube (see Fig. 2) be partially filled with any liquid and then heated, the liquid rises in the tube; showing that the liquid expands, and that the repulsive force increases. When the repulsive force exceeds the attractive, the liquid boils, that is, it is converted into a gas. From these facts we conclude that the attractive and repulsive forces are always acting together, and that the different conditions of matter depend upon their comparative strength. 7. The Three States of Matter. —When the attractive force is considerably stronger than the repulsive force, matter is in the solid state; when the two forces are nearly balanced, in the liquid state; and when the repulsive force is the stronger, in the gaseous state. 8. Cohesion and Adhesion. - The force which holds the COHESION. 7 molecules of a solid or liquid together is evidently the excess of the attractive over the repulsive force; for if the two forces were just equal, they would just neutralize each other, and the molecules would not be held together in the least. In the case of iron or water, it is evident that molecules of the same kind are held together. When, however, we mark on a blackboard with a piece of-chalk, or write on paper with ink, it is equally evident thlat molecules of different kinds are held together. The force which holds together molecules of the same kind is called cohesion; that which holds together molecules of different kinds, adhesion. 9. These Forces act only through insensible Distances. - Two" hemispheres of lead will not cohere unless perfectly smooth and clean, and pressed firmly together so as to seem to be in actual contact, when they cohere quite strongly. Plates of glass, from simply resting upon each other in the warehouse, have been known to cohere so firmly that they would break elsewhere as readily as where they came in contact. o.'Solids. - Matter, as we have seen, exists in three states, the solid, the liquid, and the gaseous. The distinguishing characteristic of solids is that the attractive considerably exceeds the repulsive force. In solids, therefore, the cohesion is always considerable. The various properties of solids result from modifications of this molecular force. ii. Tenacity. - We find on trial that it is much easier to pull asunder a rod of lead than a rod of steel of the same thickness; showing that the molecules of some solids cohere more strongly than those of others. When a solid is thus pulled asunder, it is said to be ruptured. The power which a solid has of resisting rupture is called tenacity. The relative tenacity of different solids is determined by finding how much force is required to pull asunder a rod of the same thickness of each of the solids. If it takes twice 8 COHESION. as much force to pull asunder a rod of one of the solids as of another, the first is said to have twice the tenacity of the second. The relative tenacity of solids may be determined by means of a machine called a dynamometer. This name is made up of two Greek words, and means force-measurer. Fig. 4Il i _: or I l / t / ~"'-'"~,,~4!'~'"'J~ff-f~LIItt tutltt;Itt=tttSlllltlfmtSltl,-lmmmr I One form of the machine is represented in Figure 4. It consists of a heavy iron frame, at one end of which is a box containing a stout steel spring. A pointer connected with this spring moves over a graduated arc on the top of the box. On the frame are two movable blocks, or slides, one of which is attached to the spring, while the other may be carried backward and forward by means of a screw and crank. The rod whose tenacity is to be tried is stretched between the two slides, and the crank is then cautiously turned so as to pull upon the rod until it breaks. The force which is thus brought to bear upon the rod bends the spring; and the position of the pointer when the rod breaks shows how much force was required to pull the rod asunder. 12. Hardness and Softness. —If we indent a piece of india-rubber with the finger-nail, or strike a piece of lead a smart blow with a hammer, we see that it is possible to dis COHESION. 9 place the molecules of a solid. When it is easy to displace the molecules, as in the case of wax, the solid is called soft; when it is difficult to displace them, as in the case of glass, the solid is called hard. To find which of two solids is the harder, see which will scratch the other. The one which scratches is always harder than the one scratched. Diamond is the hardest solid known. Because of its hardness it is used for cutting glass, which is also a very hard substance. 13. Elasticity, Brittleness, Ductility, and Mfaleability. - When molecules have been displaced, one of three results must follow, -they will return to their original positions as soon as they are left to themselves, or they will take up permanently new positions, or they will fall entirely asunder. If we bend a steel rod moderately, it straightens as soon as it is released; showing that the molecules sometimes tend to return to their former positions after they have been displaced. This tendency of the molecules to return to their original positions is called elasticity. We find on trial that a rod of glass, or even of pipe-clay, will straighten on being released after being slightly bent. Every solid has been found to be elastic. A steel rod may be bent a good deal, and yet straighten when released; but if it be bent beyond a certain point it will no longer straighten, showing that the molecules, after they have been displaced beyond a certain limit, no longer tend to go back to their original positions. The greatest extent to which the molecules of a solid can be displaced, and yet go back to their original positions, is called the limit of elasticity for that solid. While all solids are elastic, they differ very much in the limit of their elasticity. The molecules of steel and india-rubber can be displaced a good deal and yet return to their original positions, while those of glass and pipe-clay can be displaced but slightly. I* 10 COHESION. If a glass rod be bent within a certain limit it will straighten when released, but if it be bent beyond this limit it will not remain permanently bent, but will break; showing that the molecules of a solid cannot always take up permanently new positions. When the molecules cannot take up permanently new positions, the solid is said to be britfle. Hard solids are likely to be brittle also; but hardness and brittleness are, as we have seen, entirely different things. When the molecules of a solid can take up permanently new positions, it is ordinarily described as malleable or dluctile. It is said to be malleable when it can be hammered or rolled out into sheets; ductile when it can be drawn out into wires. Gold is one of the most malleable of the metals. In the manufacture of gold leaf it is hammered out into sheets so thin that it takes from 300,000 to 350,000 of them to make the thickness of a single inch. The gold is first rolled out into sheets by passing it many times between steel rollers in what is called a rolling-machine. The rollers are so arranged that they can be brought neaier and nearer to each other, pressing the gold into a thinner and thinner sheet every time it is passed between them. After it has thus been rolled out to the thickness of writing-paper, it is cut up into pieces about an inch square. These are piled into a stack with alternate pieces of tough paper, and beaten with wooden mallets. They are again cut up into small pieces and arranged in a stack with alternate squares of gold-beater's skin, and again beaten with mallets. This last process is usually repeated three times. Wire is made by drawing a rod of metal through a series of conical holes in a hardened steel plate.. Each hole is a little smaller than the preceding, so that the rod becomes lengthened and diminished in thickness as it is drawn through one after another. A machine for drawing iron COHESION. I I wire is represented in Figure 5. It consists of a reel on which the coarser wire is wound, a drawing-Slate through Fig. 5. which it is pulled, and a drum on which it is wound again. The drum is turned by wheel-work, which is out of sight under the table. The drawing of iron wire is attended with the following curious result. The molecules are separated in the drawing, yet the tenacity of the iron is greatly increased, so that fine iron wire is the most tenacious of substances. A bar one inch square of the best wrought-iron will sustain a weight of thirty tons; a bundle of wires one tenth of an inch in diameter, containing the same quantity of material, will sustain a weight of from thirty-six to forty tons; and if the wires have a diameter of only one twentieth or one thirtieth of an inch, the same quantity will sustain from sixty to ninety tons. Hence cables made of fine iron wire twisted together are much stronger than bars or chains of the same weight. The cables of suspension bridges are made in this way. The following table gives the most useful metals in the order of their tenacity, malleability (both under the hammer and the rolling-mill), and ductility: 12 COHESION. Tenacity. Malleability under Malleability under Ductility. the Hammer. the Rolling-Mill. Iron Lead Gold Platinum Copper Tin Silver Silver Platinum Gold Copper Iron Silver Zinc Tin Copper Zinc Silver Lead Gold Gold Copper Zinc Zinc Lead Platinum Platinum Tin Tin Iron Iron Lead 14. Solids are someuzeat Compressible. Pieces of oak, ash, or elm, plunged into the sea to the depth of 2,000 metres (about 6,560 feet) and drawn up after two or three hours, have been found to be compressed into about half their former bulk. Some metals are permanently diminished in bulk by hammering; and so also by the pressure to which they are subjected in the process of coining. The stone columns of buildings are frequently shortened by the great weight resting upon them. This was found to be the case with the columns supporting the dome of the Pantheon at Paris. I5. The Arrangement of the Molecules. -If alum be added to hot water as long as it will dissolve, and then the water be allowed to cool slowly, a part of the alum will be deposited on the bottom of the dish, not in a confused mass, but in beautiful and symmetrical forms. Such symmetrical forms are called crystals. If saltpetre, nitrate of baryta, or corrosive sublimate be treated in the same way, beautiful crystals will be formed, but in each case the crystals will have a different shape. If sulphur be melted in a crucible, and then allowed to cool slowly till a crust forms on the surface, on carefully breaking the crust and pouring off the remaining liquid the crucible will be found lined with delicate needle-shaped COHESION. 13 crystals. In the same way crystals of bismuth and many of the metals may be obtained. Fig. 6. The cohesive force, then, not only holds the molecules of a solid together, but when it is free to act it often arranges these molecules in regular order, building them up into forms of great beauty. In the cases of the formation of \ crystals which we have already described, the solid is first brought to the liquid state, and then allowed slowly to return to the solid state again. The solid was first reduced to a liquid that the molecules might have freedom of motion. The building of a crystal out of molecules is much like building a house out of bricks. The bricks must be taken one by one and laid in regular order before they are cemented together. So in forming a crystal, the molecules must be arranged one by one in regular order before they are fastened together by the cohesive force. Large crystals of many solids can be obtained by dissolving as much of the solid as is possible in cold water, and then setting it away in a shallow dish where it will be free from dust and disturbance, and allowing the water to evaporate very slowly. The more gradual the formation, the larger are the crystals. The large crystals seen in cabinets of minerals were probably centuries in forming. The water in which the solid was dissolved found its way into a cavity of a rock and there slowly evaporated. The tendency of the cohesive force to form the molecules into crystals is strikingly shown in cannon which have been many times fired, and in shafts of machinery and axles of car-wheels which are continually jarred. Such bodies often become brittle, and on breaking show the smooth faces of the crystals which have been formed. The continued jar I4 COHESION. ring gives the molecules a slight freedom of motion, and crystals are slowly built up. Many solids are crystalline in structure which do not appear to be so. Thus a piece of ice, as we shall prove elsewhere, is a mass of the most perfect crystals, but they are so closely packed together that we cannot readily distinguish them. There is a large class of solids, however, as the fats, which cannot be crystallized. I6. The Moleczies cohere more strongly on some sides than on others. - It is easy to cleave a piece of mica in one direction, but difficult to cleave it in other directions. The same is true of all crystals. It is much easier to cleave them in certain directions than in others. This is also the case with some substances which are not crystalline, as wood, which splits readily in one direction only. These facts prove that the molecules cohere more strongly on some sides than on others. Iron and other solids are not so tenacious when crystalline in structure as when not crystalline. This is because the molecules in crystals are arranged in layers, so that the weakest sides are brought face to face. I7. Annealing and Tenmpering. —If melted glass be dropped into cold water, it forms the well-known Rzupert's drops, which are so brittle that, if we break off the small end or scratch them slightly with a file, they fly in pieces. When glass is allowed to cool in the air at the ordinary temperature, it is also very brittle. In order to make it tough enough for ordinary use, it must be cooled very slowly. This slow cooling of glass or other substances is called annealinig. Glass is annealed by passing it slowly through a long oven, which is kept very hot at one end and cool at the other. It is usually about two days in passing through the oven. Steel, also, when suddenly cooled from a high temperature, is very hard and brittle, but when slowly cooled it is very COHESION. 15 tough and pliable. The process of bringing steel to the various degrees of hardness requisite for its uses in the arts is called temperingz. Steel is usually tempered in the following manner. It is first heated white hot, and then suddenly cooled by plunging it into cold water. It is thus rendered very brittle. It is then reheated and allowed to cool slowly. When it is to be made quite hard, it is reheated but slightly; when quite soft, it is reheated a good deal. The more it is reheated, the softer it becomes on cooling. These different conditions of glass and steel are probably owing to differences in the arrangement of the molecules. I8. Liquids. The distinguishing characteristic of liquids is that the attractive and repulsive forces acting between the molecules are very nearly balanced, the attractive force being slightly in excess. Hence in liquids the cohesion is slight, and the molecules are free to move among themselves. If a piece of lead be carefully measured, then melted and measured again, it will be found to have increased in bulk. The same is true of nearly all solids. Hence, when any substance is in a liquid state, the molecules are farther apart than when it is in a solid state. This explains why, in moulding bullets, the mould is never quite filled by the bullet. There are, however, a few marked exceptions to this rule. It is well known, for instance, that if a bottle be filled with water and tightly corked, and allowed to freeze, the bottle will be burst. This shows that the molecules of ice are farther apart than those of water. 9. Liquids are bzut slightl3y Conpqressibe. - The apparatus represented in Figure 7 consists of a very thick vessel of glass closed at top and bottom. Within the vessel are a piston which can be moved by the thumb-screw at the top, and a glass bulb which is prolonged by a very fine tube bent as represented. Fill the bulb and tube with any liquid, 16 COHESION. as water, and plunge the end of the tube in the mercury Fig. 7. which covers the bottom of the vessel. Then fill the vessel with water, ~_~^ ~ and apply pressure by turning the screw. The mercury will rise in 4i II ~~:the tube, showing that the liquid nTfi iX in the bulb has been compressed. G//~fS1 This compression, however, is but.il i 11 slight, amounting at most to a few'll' II millionths of the bulk of the liquid.! 1~-, 20. Liquids are perfectly Elastic. _,... ~ — However much the screw, in the I,-IT'above experiment, may be turned | 1 11down, or however long it may be left, on loosening it the mercury __i _ will at once fall inside the tube to __ - __ a level with the mercury outside;...~i, == showing that liquids are perfectly elastic. This elasticity is, however,. -v/.,developed only when the liquid is compressed, that is, when the molecules have been brought nearer together. In whatever other way the molecules may be displaced, they show no tendency to return to their former positions. 21. The Arrangement of folecules in Liquids. - If a mixture of water and alcohol be made so as to be just as heavy as sweet-oil bulk for bulk, and a quantity of the oil be carefully introduced into the centre of this mixture by means of a dropping-tube, the oil will neither rise nor sink, but gather into a beautiful sphere. This experiment shows that when the molecules of a liquid are left to themselves, they at once collect into spheres. What hinders the molecules of liquids from always taking this spherical form will be explained further on. Rain-drops, dew-drops, and the manufacture of shot illus COHESION. 17 trate this tendency of the molecules of liquids. In the manufacture of shot, melted lead is poured through a sieve at the top of a very high tower, and the drops in falling take the form of spheres, which become solid before they reach the bottom. 22. Gases.- In gases, as has already been shown, the repulsive molecular force exceeds the attractive. Hence there is no cohesion in this state of matter, and the molecules move among themselves with greater freedom than those of liquids. The molecules of any substance are farther apart in the gaseous state than in either the solid or liquid state. This may be shown by filling Fig. 8. a test-tube nearly full of water, then closing it tightly with a cork through which a fine tube passes nearly to the bottom of the test-tube. On boiling the water so as to convert a portion of it into steam, i. which is a gas, the water is driven forcibly out of the fine tube; showing that the steam occupies more space than the water from which it comes. Fig. 9. 23. Gases are readily Cornpressible. - The figure represents a U-tube closed at one end and open at the other, with a nipper-tap* at the bend. Pour in mercury enough l _ i1 _^ to cover the bend. The closed end is now filled with air. Pour in more ____.~~-~ ~* See Appendix, I. B I18 SUMMARY OF COHESION. mercury, and this column of air rapidly shortens. The same would be true if the closed end of the tube were filled with any other gas; showing that gases are highly compressible. 24. Gases areperfecty Elastic. — Open the nipper-tap that the mercury may run out, and it is entirely driven out of the closed arm of the tube. To prove that it is the elasticity of the air which drives out the mercury from this arm, fill the closed arm and a part of the open arm with mercury, and open the nipper-tap. The mercury will flow out from the open arm, and not from the closed arm. SUMMARY OF COHESION. Matter is made up of definite but insensible masses, called molecules. (i, 2.) These molecules are not in actual contact with one another. (3.) It is probable that the spaces which separate the molecules are immense in comparison with the size of the molecules themselves. (4.) By trying to pull a solid in two we learn that there is an attractive molecular force, which holds the molecules together. By placing a rubber bag partially filled with a gas under the receiver of an air-pump, and exhausting the air, we find that there is also a repulsive molecular force, which pushes the molecules apart. (5.) Since the molecules of a solid may separate on being heated, and yet hold firmly together, we conclude that these two molecular forces act together, and that the reiulsive molecular force is increased by heat. (6.) SUMMARY OF COHESION. 19 We find that there are three states of matter, depending upon the relative strength of these two forces: the solid state, in which the attractive force is considerably the greater; the liquid state, in which the two forces are nearly equal; and the gaseous state, in which the repulsive force is the greater. (7.) The force which holds together molecules of the same kind is called Cohesion; that which holds together molecules of different kinds, Adhesion. (8.) Cohesion is the excess of the attractive over the repulsive molecular force. In solids, it is comparatively strong; in liquids, it is weak; in gases, it does not exist. The properties of solids depend on the action of the cohesive force. (io.) The tenacity of a solid is its power of resisting rupture. (ii.) A solid is called hard when it is difficult to displace its molecules; soft, when it is easy to displace them. (12.) Elasticity is the tendency of the molecules, on being displaced, to return to their original positions. All solids are elastic, but differ greatly in the limit of their elasticity. A solid is said to be brittle when its molecules cannot take up permanently new positions. It is said to be malleable or ductile when they can take permanently new positions: malleable, when it can be hammered or rolled into sheets; ductile, when it can be drawn into wire. (13.) Solids are somewhat compressible. (I4.) The cohesive force often arranges the molecules of a solid into regular forms, called crystals. (15.) Crystals can be split more easily in some directions than in others, showing that the cohesive force is stronger on some sides of the molecule than on others. (I6.) The molecules are farther apart in the liquid than in the solid state; yet liquids are less compressible than solids. (19.) 20 SUMMARY OF COHESION. Liquids are perfectly elastic; but their elasticity is developed only when the molecules are brought nearer together. (20.) The molecules of a liquid, when acted upon only by cohesion, tend to collect into spheres. (21.) In the gaseous state, the molecules are farther apart than in the liquid state. (22.) Gases are readily compressible, and when compressed are perfectly elastic. (23, 24.) II. ADHESION. AD H ESION 25. Adhesion between Solids and Solids. - Adhesion has already been defined as the force which holds together unlike molecules. The sticking of the chalk to the blackboard, of the graphite of the pencil to paper, and of dust to furniture, prove the existence of this force between solids and solids. The use of the various cements also illustrates this force, and also the fact that some solids adhere to a given solid more strongly than others. If we wish to fasten two pieces of wood together, we use glue; if two bricks or stones, we use mortar, or some calcareous cement; if two pieces of glass, sealing-wax, or some resinous substance. Stone adheres to mortar more strongly than wood or glass, and wood adheres to glue more strongly than stone or glass. When solids are held together by cements, cohesion and adhesion are both brought into play. When, for instance, two pieces of wood are held together by means of glue, the adhesive force holds the wood on each side to the glue, and cohesion holds together the molecules of the glue. When furniture breaks, we often see that the wood splits instead of separating from the glue. So also stones are sometimes cemented together so firmly that the stone itself will break sooner than separate from the cement. These facts show that the adhesive force between two solids is frequently stronger than the cohesive force of the solids themselves. 24 ADHESION. 26. Adhesion between Solids and Liquids. - If we dip the hand in water it comes out wet. This, and similar facts equally familiar, prove that there is also an adhesive force between liquids and solids. 2 7. The Adhesion between a Liquid and a Solid is sometimes not strong enough to overcome the Cohesion of the Liquid. - If a glass disc be suspended from one pan of a balance and counterpoised by weights, and then brought in contact with Fig. io. mercury, it will require additional weight to raise the disc from the mercury, and the disc comes off dry. This proves, first, that there is adhesion between glass and mercury, and, secondly, that this adhesion is not strong enough to overcome the cohesion of the mercury. 28. The Adhesion between a Solid and a Liquid is sometimes strong enough to overcome the Cohesion of the Liquid. -If a glass plate be laid upon the surface of water and then removed, it comes off wet, that is to say, covered with a film of water; showing that the adhesion between a solid and liquid is sometimes strong enough to overcome the cohesion of the liquid. Since adhesion takes place only at the surface, it is evident that we may increase the adhesion of a solid for a liquid by increasing the surface of the solid. ADHESION. 25 If we take any solid, as a stone, and break it in two, the stone evidently has all the surface it had before it was broken, and, in addition, the two surfaces exposed by the breaking. Hence the more a solid is broken up, the more surface it exposes. The readiest way, then, to increase the surface of a solid is to pulverize it. If pulverized bone-black be mixed with ordinary vinegar, or with wine, and the liquid be separated again by pouring the mixture upon a piece of unsized paper placed inside a funnel, every trace of color will be removed from the liquid. All vegetable colors can be removed from liquids in the same way. Removing the color from a liquid in this way is called clarifji/g the liquid. Bone-black is obtained by burning bones in closed vessels. It is pulverized that it may present more surface. Other substances are sometimes used for clarifying liquids. Next to bone-black, or " animal charcoal," as it is sometimes called, ordinary charcoal is the best and the most frequently used. The bone-black evidently removes the coloring matter by means of the adhesive force which exists between the two. It is also evident that the coloring matter adheres to the bone-black more strongly than to the liquid, else the two would not be separated. Use is made of this property of charcoal in the refining of sugar. The dark-colored syrup which is obtained from the cane is first filtered through long sacks filled with coarsely pulverized charcoal. In this way all the color is removed. The colorless syrup is then evaporated and forms white sugar. 2 9. [The Adiesionz bctzeez, a Soia'd anzd a Liqztirs is somzelimles strong ecozough z o overcomc /e e Cohesion of t/e Solidr. -If some Epsom salts be put into water, the salts will speedily be reduced to the liquid state. The adhesive force between the water and the salts has evidently overcome the cohesive force of the solid, since it has reduced the solid to the liquid state. 2 26 ADHESION. 30. Summary. - We have, then, three well-marked cases of adhesion between solids and liquids:ist. When the adhesive force is not strong enough to overcome the cohesion of the liquid. In this case the liquid caCmno we! the solid. 2d. When the adhesive force is strong enough to overcome the cohesion of the liquid. In this case the liquid can wet the solid. 3d. When the adhesive force is strong enough to overcome the cohesion of the solid. In this case the liquid can dissolve the solid. The liquid which dissolves the solid is called a solvezt, and the liquid in which the solid has been dissolved is called a solution. 3. lealt romotes Solution. —We find on trial that Epsom salts will dissolve more rapidly and in greater quantity in hot than in cold water. This is as we should expect, since we have already found that heat tends to overcome cohesive force. As a general rule, solids dissolve in greater quantities and more readily in hot than in cold liquids, but there are exceptions. 32..DZffirenl Solids are not ezqall/y soluble in tle sanme Lizuid. -If we put a piece of sealing-wax into water, it does not dissolve at all; while water will dissolve about twice its bulk of Epsom salts. If we compare other solids in the same way, we shall scarcely find any two which dissolve with equal readiness in the same liquid. 33. The same Solid is not equally soluble in different ZLizids. - Sealing-wax, which does not dissolve in water, dissolves quite readily in alcohol. This shows that the same solid does not dissolve with the same readiness in different liquids. 34. Calillarity.- If one end of a fine and clean glass tube be put into water, the water will be seen to rise inside the tube above the surface of the water outside. If one end of a similar tube be put into mercury, the mercury will ADHESION. 27 be seen to fall inside the tube below the surface of the mercury outside. This action of liquids inside tubes is called cacpi/-ri/y. The force which draws some liquids into tubes and pushes others out, has been called capila;'ry force.. This name is a convenient one, and we shall retain it, though, as we shall show elsewhere, capillarity results from the combined action of certain other forces. We have seen already that water will wel glass, while mercury will iiot. We have, then, two well-marked cases of capillarity, corresponding to two cases of adhesion between solids and liquids; for those liquids which will wet a tube are drawn into it, while those which will not wet it are driven out. Mercury will wet zinc, and it is drawn into a tube of zinc, just as water is into a tube of glass. We find by using glass tubes of different sizes, that the finer the tube, the higher the water rises and the lower the mer- Fi cury falls, that is, the more marked is the capillarity. This fact ex- j i plains the name. The word capif- lary comes from a Latin word (capii/zris) which means hair-like. The force was called capilary because its action is most powerful in hair-like tubes. This force, however, acts in tubes Fig.:2! of every size, and in fact a tube is not necessary for its action; as may be seen by putting two plates of glass _^I together as represented in the figure, and then dipping them into water or mercury. The water will rise between the plates, and the mercury fall. 35. Illstlralions of CapilZarity. A lamp-wick is full of 28 ADHESION. tubes and pores, and capillary force draws the oil up through these to the top of the wick, where it is burnt. When one end of a cloth is put into water, capillary force draws the water into the tubes and pores of the cloth, and the whole soon becomes wet. In the same way a lump of sugar, or other porous substance, soon becomes wet throughout, if a corner of it is put into water. Blotting-paper is full of pores into which the capillary force draws the ink. The use of a towel for wiping anything which is wet depends on the same principle. 36. Strength of the Capillary Force. - It is well known that, when a piece of cloth is wet, it is almost, if not quite, impossible to wring or squeeze it dry. This shows that the capillary force which holds the water into the pores of the cloth is very strong. Some solids, as wood, swell on becoming wet. If holes are drilled into a granite rock, and dry wooden plugs driven into them, and water poured over the ends of the plugs, the capillary force draws the water into the pores of the wood, which swells and splits the rock. This is a striking illustration of the strength of the capillary force. 37. Capillary Force never causes a Liquid to fozo throught a Tube. - If a glass tube be so fine that the capillary force will draw water into it to the height of two inches, and the tube be then lowered so that not more than half an inch shall be above the surface of the water, the water will not overflow the tube. If, however, the water be removed as soon as it comes to the top, more will rise in the tube to take its place. When a lamp is burning, the oil is passing up continually through the wick, because it is burned as soon as it reaches the top; but when the lamp is not burning, the oil does not overflow the wick. The wick of an alcohol lamp must be covered with a cap when the lamp is not burning, otherwise the alcohol will evaporate as fast as it comes to the top of the wick, and so all pass out of the lamp. ADHESION. 29 38. Adhesion between Solids and Gases. - If a small glass jar inverted over mercury be filled with ammonia gas,* and a piece of boxwood charcoal, previously heated to redness and cooled by plunging it into the mercury, be introduced into the jar, the mercury rapidly rises into the jar, and, if the piece of coal is large enough, entirely fills it. The ammonia gas, then, has been drawn into the charcoal by an adhesive force; proving the existence of adhesion between the molecules of a solid and those of a gas. When a gas is taken up in this way by any substance, it is said to be absorbed. If we try other solids instead of charcoal, we shall find that no two absorb ammonia gas with equal readiness. If a piece of boxwood charcoal be introduced into a jar of air inverted over mercury, the mercury rises in the jar very slowly; showing that different gases are not absorbed with equal readiness by the same solid. When the ammonia gas is absorbed by the charcoal, as in the above experiment, it evidently occupies less space than before. When a gas is absorbed by a solid, then, the repulsive force of the gas has to be overcome. We have already seen that cold helps to overcome the repulsive force. Hence we should expect that a solid would absorb a gas when cold more readily than when hot. Experiment shows this to be true. Heat, on the other hand, increases the repulsive force. If a solid which has absorbed a gas be heated, the repulsive force of the gas is increased, so that it finally overcomes the adhesion of the solid for the gas, which then leaves the solid. The charcoal is heated before it is introduced into the jar, in order to drive all the air out of its pores. 39. Adhesion between Liquids and Liquids. - If oil be poured upon water, the oil, which is the lighter, soon rises * See Appendix, 2. 30 ADHESION. to the top and remains entirely separate from the water. If, however, alcohol, which is also lighter than water, be poured into water, the two will thoroughly mix. This may be made evident to the eye by using colored water. The fact that the alcohol remains mixed with the water proves that the molecules of the alcohol must adhere to those of the water, and that this adhesion is strong enough to overcome the cohesion of the liquids. Nearly all liquids will mix when poured together, though some will mix much more readily than others. 40. Diffusion of Liquids.- If some colored alcohol be put into a tall glass jar, and then, by Fig. 13. means of a funnel and a long tube reaching to the bottom of the jar, some water ~:~~7 ~ be carefully poured in, the water will reii main a short time at the bottom of the I I jar, and its separation from the alcohol ^ 0I\ will be sharply defined. On standing a few days, the liquid will become of the same color tlroughout; showing that the lllj alcohol and water have mixed. This mixing of liquids on being merely brought i!lJ... into contact is called diffzsion of liquids. Different liquids diffuse into each other i —l at very different rates, while some, as oil... ~it -- 9. a and water, will not diffuse at all. 41. Osmose of Liquids. - If a bladder be fastened air-tight to the end of a long glass tube, and the bladder be filled with alcohol and introduced into a vessel of water, the liquid will gradually rise in the tube; showing that the water has passed into the bladder. At the same time the alcohol passes slowly out and mixes with the water. The mixing of liquids when separated by a thin membrane or porous substance is called osmose of liquids. ADHESION. 31 Liquids do not mix at the same rate when separated by a thin membrane or porous substance as when they mix by simple diffusion. The rate of mixing is modified in a striking manner by the presence of the membrane or porous substance. If, in the above Fig. experiment, we substitute a small collodion balloon for the bladder, the liquid will fall in the tube, showing that now the alcohol passes out more rapidly than water passes in. The cases of osmose already given have been explained as the cornbined effect of capillarity and diffusion. In the _ first case, the water, which ii i ll 1 will wet the bladder much l more readily than alco-'!iliii hoi, is drawn by capilla- ~ ry force into the minute pores of the bladder, and thus is carried to its inner surface, where, coming in contact with the alcohol, it diffuses into it. As the water is thus removed as fast as it comes to the inner surface, a constant flow is maintained through the pores of the bladder. At the same time the alcohol passes out slowly by diffusing through the water which fills the pores of the bladder. In the second case, the alcohol wets the collodion more readily than the water does; hence the more rapid flow is in the opposite direction. Capillarity and diffusion, which explain these cases of osmose so satisfactorily, will by no means explain all cases. No satisfactory explanation has yet been given of the various ways in which the presence of the membrane affects the rate of mixing. 32 ADHESION. 42. Adhesion between Liquids and Gases. — Let a small glass jar inverted over mercury be filled with ammonia gas, and then some water be poured over the surface of the mercury. If now the jar be carefully raised, the moment the mouth of the jar comes in contact with the water, the latter rises and completely fills the jar; showing that the ammonia has been absorbed by the water, and consequently that the molecules of the gas adhere to those of the water. The same gas is not absorbed with equal readiness by all liquids, as is shown by the fact that ammonia gas, which is absorbed so greedily by water, is not absorbed at all by mercury. The same liquid does not absorb all gases with equal readiness, as is shown by inverting a jar of air over water. The air is absorbed scarcely at all. 43. Cold and Pressure promote Absorplion.- When a gas is absorbed by a liquid, as well as when absorbed by a solid, the molecules are brought nearer together and the repulsive force overcome. Both cold and pressure, as we have seen, help to overcome this force; hence they favor the absorption of a gas by a liquid. The effect of pressure on the absorption of a gas by a liquid is illustrated in soda-water. Soda-water owes its agreeable taste mainly to the presence of carbonic acid gas in the water. Water and carbonic acid are brought into contact in the fountain, and subjected to very great pressure. When the water is drawn from the fountain this pressure is removed, and the carbonic acid which had been taken up by the water escapes in thousands of little bubbles, causing the liquid to foam, or effrvesce. Ordinary liquid ammonia, or aqua ammonm ia, is a gas absdrbed by water. If this liquid be heated, the ammonia gas escapes. The heat increases the repulsive force of the gas, and thus enables it to overcome its adhesive force for the liquid. The ordinary way to free a liquid from an absorbed gas is to heat it. ADHESION. 33 Common spring-water owes much of its pleasant taste to the presence of carbonic acid and other gases which it absorbs from the air. When this water is boiled, these gases escape, and it becomes very insipid. The constant agitation of running water helps it to absorb gases, since it is thus made to present more surface to the air. 44. Dz)fusion of Gases. - Two bottles are connected by alongglasstube. The lower Fig. 5. bottle is then filled with carbonic acid and the upper with hydrogen gas, which is very much lighter than carbonic acid.* After a time the hydrogen will be found to have" passed down and mixed with the heavier carbonic acid, and the carbonic acid to have mixed with the hydrogen in the upper bottle. The presence of the carbonic acid in the upper bottle... may be proved by pouring into it lime-water,+ which,, on shaking becomes of a milk-white color. Hydrogen has no effect upon limewater. The mixing of gases when brought into contact is called difusion of gases. Different gases diffuse into each other at very different rates. As a general thing, the more the gases differ in weight, the more rapidly they diffuse into each other. 45. Osmose of Gases. - A long glass tube is fastened airtight, by means of a cork and sealing-wax, into the open * See Appendix, 3. See Appendix, 4. 2* C 34 ADHESION. end of an unglazed porcelain cup such as is used in a Bunsen's or Grove's battery. The cup is then held so that the end of the tube dips beneath'Fig. i the surface of water, and a large bell-jar of hydrogen* is 0/ lj l7 held over the cup. There is,/ an instant rush of bubbles from the end of the tube up through the water, showing that the hydrogen has passed through the pores of the cup and mixed with the air inside. -o' B t Remove now the jar of hydrogen, and the water at once rises in the tube, showing that the hydrogen inside the cup has passed out through the pores to mix with the air outside. f/ ~-~ ^The mixing of gases when separated by a porous sub-.- stance or thin membrane is called osmose of gases. The diffusion and osmose of gases point to the existence of adhesion between molecules of different gases; but the existence of this adhesion has not been fully established.t - See Appendix, 5. t See Appendix, 6 SUMMARY OF ADHESION. 35 SUMMARY OF ADHESION. Adhesion is the force which holds together molecules of different kinds. It acts between molecules of solids and solids, solids and liquids, solids and gases; also between liquids and liquids, and liquids and gases. It is doubtful whether there is any adhesion between the molecules of different gases. The adhesive force between two solids is sometimes greater than the cohesive force of the solids themselves. (25.) There are three cases of adhesion between solids and liquids: - ist. When the adhesion is not strong enough to overcome the cohesion of the liquid, and the liquid cannot wet the solid. 2d. When it is strong enough to overcome the cohesion of the liquid, and the liquid can wet the solid. 3d. When it is strong enough to overcome the cohesion of the solid, and the liquid can dissolve the solid. (26 -30.) Heat generally promotes solution, since it helps to overcome the cohesion of the solid. (31.) The same liquid dissolves some solids more readily than others; while some liquids dissolve the same solid more readily than others do. (32, 33.) Capillary force is a force acting upon liquids within tubes. Liquids which can wet a tube are drawn into it by the action of this force, while liquids which cannot wet it are driven out of it. The finer the tube, the more marked is the capillarity. (34.) The capillary force is a very strong force; but acting 36 SUMMARY OF ADHESION. alone it never causes a liquid to flow through a tube. (36, 37.) When a gas is absorbed by a solid or by a liquid, the adhesive force between the molecules of the solid or liquid and those of the gas must be strong enough to overcome the repulsive force of the gas. (38, 42.) Heat hinders absorption, since it increases the repulsive force between the molecules of the gas. Hence gases absorbed by solids or liquids can be separated from them by means of heat. (38, 43.) The same solid or liquid absorbs some gases more readily than others; while the same gas is absorbed by some solids or liquids more readily than by others. (38, 42.) The adhesive force between the molecules of different liquids causes the liquids to mix. The mixing of liquids on merely coming in contact with each other is called diffusion of liquids. (40.) Liquids also mix when separated by a thin membrane or porous substance. This mixing is called osmose of liquids. (41.) Gases, like liquids, mix by diffusion and by osmose. (44, 45-) III. CHEMICAL AFFINITY. CHEMICAL AFFINITY. 46. Action of Potassium and Sodium on Water. -If a piece of potassium be thrown upon water, it burns with a rose-colored flame, and rapidly disappears. A piece of Fig.. sodium when thrown upon ~ water moves briskly about i with a hissing sound, and soon disappears, but without flame. When, however,, the sodium is kept still by -.. putting. it upon blotting- -. paper which lies upon the surface of the water, it burns with a yellow flame. In both cases the metal disappears, and the water apparrently remains unchanged. But if a strip of red litmuspaper be put into the water on which the sodium or potassium was burned, it turns at once to a blue color, showing that the water has also been changed; for ordinary water has no effect on red litmus-paper. If a piece of sodium be put into a small metallic cup pierced with holes and provided with a handle, and the cup be quickly introduced bottom upward under the mouth of a test-tube previously filled with water and inverted over a vessel of water with its mouth dipping beneath the surface, bubbles of gas rise from the cup and soon fill the test-tube. 40 CHEMICAL AFFINITY. If now the test-tube be raised from the water and a lighted taper applied to its mouth, the gas takes fire with a slight explosion, and burns with a pale bluish flame.'Fig. i8. This gas is called hydrogen. Its extreme lightness, and the peculiar way in which it takes fire and burns, serve to distinguish it from other gases. Its lightness may be shown by the following simple experiment. Fill a test-tube with hydrogen and hold it mouth downward for some time; then apply a lighted taper, and it will still be found full of hydrogen. Fill it again and hold it mouth upward, and after a little time apply a lighted taper, when the hydrogen will be found to have wholly escaped. It is so light that it rises through the air as a cork rises through water. 47. lWhence does the Hydrogen come? - In Figure 19 we have a U-tube open at both ends. In each of its arms is put a small graduated glass bell-jar. At the bend of the tube directly under each jar a platinum wire passes through the glass. This wire is flattened on the inside and terminates in a hook on the outside. The whole apparatus is CHEMICAL AFFINITY. 41 filled with water, and oneofthe platinum wires is connected with the zinc pole, and the other with the platinum pole of a Grove's battery.@ As soon as these con- Fi I. nections are made, bubbles rise from each platinum wire, and the bell-jars slowly fill with gas. When the jars are full, remove the one which has been connected with the zinc pole, and bring a lighted taper to its -- mouth. The gas takes fire with a slight explosion, and is recognized at once as - hydrogen. Remove the other jar, hold it mouth upward, and plunge into it a splinter of wood with a spark of fire on its end. The splinter at once bursts into a flame, and the gas does not burn. This gas is called oxygen. We conclude, then, that the hydrogen in the previous experiment came from the water, and that it was set free by the sodium; and that besides hydrogen there is another gas in water called oxjgen. 48. Are t/ese the only Substances found in WTater?- If a jet of hydrogen be burned in a jar filled with oxygen,t and the heated gases be conducted through a cold glass tube, moisture collects on the sides of the tube, trickles down, and drops from the end, and may be caught in a wine-glass. This liquid resembles water; and if we drop a little of it on a piece of potassium, the latter takes fire and burns with a rose-colored flame, showing that the liquid really is water. This water can have been formed from nothing else than hydrogen and oxygen, hence there can be nothing else in water. 49. Comozpound Subslanzces and Elemenss.- We see, then, that water can be separated, or deconposed, into two gases, hydrogen and oxygen. Substances which, like water, can * See Appendix, 7. t See Appendix, 8. 42 CHEMICAL AFFINITY. be decomposed into other substances, are called comnpound substances. It has hitherto been found impossible to decompose either hydrogen or oxygen. Substances which, like oxygen and hydrogen, cannot by any known process be decomposed into simpler substances, are called eemenzls. Only sixty-five elements are known, and thirteen out of these sixty-five, combining in various ways, make up almost the whole of the substances found in and about the earth. 50. C/zemicaForce. — The force which causes the elements to combine is called ch/emnicalforce or afi;zity. Chemical force always acts between unlike substances, and in this respect it resembles adhesion; but it differs from it in two or three important particulars: ist. When water adheres to a glass plate, neither the properties of the water nor those of the glass are changed. So, too, when sugar is dissolved in water, the sugar still retains its sweet taste and all its peculiar characteristics. It is simply sugar reduced to the liquid state and diluted with water. But when chemical force causes hydrogen and oxygen to combine, they form rwatler, a substance wholly unlike hydrogen, which is inflammable, and oxygen, which aids combustion. The first characteristic of chemical force is that it changes the properties of the substances which it causes to combine. It not only unites different substances, but it unites them so as to form a new substance. It combines them. 2d. If water be again decomposed in the U-tube by means of the battery, it will be noticed that the tube into which the hydrogen rises fills just twice as fast as the other. In whatever way water is decomposed, we always get just twice as much hydrogen as oxygen by measzure. Figure 20 represents a U-tube, one arm of which is closed, the other open, with a nipper-tap at the bend. Two platinum wires pass through the closed arm near the top CHEMICAL AFFINITY. 43 and almost meet within, while their outer ends are formed into loops for the attachment of battery wires. Introduce into the closed end of this Fsg. 20. tube a mixture of hydrogen and oxy- gen, using first just twice as much hydrogen by measure as oxygen. Combine the gases by means of the electric spark.A Water is of course produced, and all the gases are used up, as is shown by the mercury's rising to the top of the tube. If another mixture is used in which = we have more than two measures of hydrogen to one of oxygen, some hydrogen will be left over. If less than two measures of hydrogen to one of oxygen be used, some oxygen will be left. So that in whatever proportions hydrogen and oxygen be mixed, we find that when they form water two measures of hydrogen always combine with one of oxygen. We may dissolve a teaspoonful of sugar in a wine-glass or in a pitcher of water, and there will still be sugar in every part of the water. Adhesion acts between quantities of matter altogether indefinite. The second characteristic of chemical force is that it always acts between definite quantities of matter. Chemical force, then, may be briefly described as the force which acts betoween definite quantities of different kinds of matter, and causes them to combine to form a new7 substance. 5 I. Chemical Force is stronger between some Substances than between others. -When potassium is thrown upon water (46), it unites with the oxygen and sets the hydrogen free; showing that the chemical force between potassium and oxygen is stronger than between hydrogen and oxygen. In * See Appendix, 9. 44 CHEMICAL AFFINITY. general, if to a compound of two elements, A and B, a third element, C, be added, which has a stronger affinity for A than A has for B, A will leave B and combine with C. Any change which is occasioned by chemical force is called a chemical change. ATOMS AND ATOMIC WEIGHTS. 52. We have already found that two measures of hydrogen always combine with one measure of oxygen to form water. If equal measures of hydrogen and oxygen are weighed, the oxygen is found to weigh sixteen times as much as the hydrogen. There are, then, in water, by wezight, two parts of hydrogen to sixteen parts of oxygen. When ice is melted, then, it is resolved into molecules, each of which contains 2 parts by weight of hydrogen and i6 parts of oxygen. When acted upon, at ordinary temperatures, by either sodium or potassium, i part of this hydrogen is removed by 23 parts of the former, or 39 parts of the latter, giving rise to compounds each molecule of which contains i part of hydrogen, 23 parts of sodium, and 16 parts of oxygen, or i of hydrogen, 39 of potassium, and 16 of oxygen. The first of these compounds is called hydrate of sodium; the second, hydrate of potassium. On heating the first with sodium, the remainder of the hydrogen is displaced by 23 additional parts of sodium, and a compound formed each molecule of which contains 46 parts of sodium and I6 of oxygen. On heating the second with potassium, the remainder of the hydrogen is displaced by 39 parts of potassium, and a compound formed containing 78 parts of potassium and i6 of oxygen. When chlorine (an element with which we shall soon become acquainted) is passed through a red-hot tube along with steam, it removes the hydrogen from the water and sets the oxygen free.* It is found that chlorine, like* sodium * See Appendix, Io. CHEMICAL AFFINITY. 45 and potassium, can displace either one half or the whole of the hydrogen, giving rise to two compounds. It is, however, impossible to replace any fraction of the oxygen of water with chlorine. No substance has yet been formed which will displace any other fraction than one half of the hydrogen from water; and no substance is known which will displace any fraction of the oxygen without displacing the whole. We see, then, that by means of affinity a molecule of water can be divided into three parts, 2 of which are hydrogen and i is oxygen; and that the i part of oxygen weighs i6 times as much as each of the 2 parts of hydrogen. No way is known of further dividing this molecule. It has also been, found that no substance can displace less hydrogen from any of its compounds than sodium or potassium, and that it always requires 23 parts of the one and 39 parts of the other to displace one part of hydrogen. We therefore conclude that the smallest parts into which hydrogen, sodium, and potassium can be divided by affinity have the relative weights of I, 23, and 39; that is, the smallest part into which sodium can be divided by affinity weighs 23 times as much as the smallest part into which hydrogen can be divided by the same means; and the smallest part into which potassium can be divided weighs 39 times as much as the smallest part into which hydrogen can be divided. These smallest parts into which substances can be divided by affinity are called atoms, from a Greek word meaning indivisible, and the relative weights of the atoms of the elements are called alomic weig,ths. Thus i, 23, 39, and I6 are the atomic weights of hydrogen, sodium, potassium, and oxygen respectively. 53. kMasses, Moleczues, and Aloms. - We have, then, a threefold divisibility of matter: — ist. Matter can be divided by mechanical means into minute but sensible portions, or masses. 46 CHEMICAL AFFINITY. 2d. Matter can be divided by means of heat into insensible portions, called molecules. 3d. Matter can be divided by means of chemical force into portions which are by us indivisible, called atoms. SYMBOLS. 54. To indicate briefly the composition of a substance, and also such changes as we have already seen brought about by the chemical force, a system of notation by symbols has been devised. 55. Symzbos of Elements. -The symbol for an element is always the first letter of its name (in some cases its Latin name), unless the names of two or more elements begin with the same letter, in which case a second letter is added to distinguish them. Thus the symbol for hydrogen is H; for oxygen, 0; for potassium (Latin lkaliium), K; for sodium (natrium), Na; for phosphorus, P; etc. The symbol of an element, when used alone, indicates one atom of that element.* Thus H stands not only for hydrogen, but also for one atom, or one part by weight, of hydrogen; -- for i atom or i6 parts by weight of oxygen; KI " " 39 " " " potassium; Na " " 23 " " " sodium; P " " 31 " " " phosphorus. Many of the symbols of the elements have a bar across them, for a reason which will be explained hereafter. 56. Symbols of Comnpounzds.-The symbol for a compound is obtained by joining together the symbols of the elements contained in it, writing after the symbol of each * The symbols are, however, sometimes used merely as abbreviations of the names of the elements; H for hydrogen in general, K for potassium, etc. CHEMICAL AFFINITY. 47 element a figure to indicate how many atoms of that element are contained in a molecule of the compound. Thus the symbol for water is H20; the symbol for oxide of potassium is K-&; the symbol for oxide of sodium is Na2-; for hydrate of potassium, HKO; for hydrate of sodium, HNaO. 57. The Meaning and Use of t/e Snzbols. - The symbol H2- indicates that water is a compound of hydrogen and oxygen; that in each molecule of water there are two atoms of hydrogen and one atom of oxygen, and that by weight there are 2 parts of hydrogen to I6 parts of oxygen. The symbol HNaO indicates that hydrate of sodium is composed of hydrogen, sodium, and oxygen; that in a molecule of this compound there is one atom of each of these elements; and that in this substance there is one part by weight of hydrogen to 23 parts of sodium and i6 parts of oxygen. By means of these symbols chemical changes are very concisely indicated. Thus the change which takes place when potassium is thrown upon water can be indicated thus: H,2 +- K = HKO -+ H. The sign - (plus) indicates merely that the substances are together without combination. The change which takes place when all the hydrogen in water is displaced by potassium is indicated by symbols thus: H2O + 2K = K20 + 2H. When the symbol of an element stands by itself, and we wish to express two or more atoms of it, the figure is placed before the symbol instead of after it. Thus we write 2K rather than K2. When hydrogen and oxygen combine to form water (50), the change is indicated by symbols thus: 2H +t- - H0. 48 CHEMICAL AFFINITY. By means of these symbols we can readily determine what fraction of the weight of the whole compound the weight of each element in it forms. For instance, we wish to find what fraction of the weight of water is hydrogen, and what fraction is oxygen. The symbol for water is H2&. H2 indicates 2 parts by weight, and 0 indicates i6 parts. In water, then, there are i8 parts by weight, 2 of which are hydrogen and i6 oxygen: -2g or - of the weight of water is hydrogen, and -1 or - is oxygen. What fraction of the weight of hydrate of sodium is hydrogen, what fraction is sodium, and what fraction oxygen? The symbol for this compound is HNaO. H = i part by weight of hydrogen; Na - 23 parts " " sodium; 0 = - 6 "' " oxygen; 40 = whole number of parts by weight in HNaO. Then, I of HNa- is hydrogen; i 3'' " " sodium; 1 6 " " c oxygen. It will now be easy to solve such problems as the following: How much hydrogen is there in 45 kilogrammes of water? We have found that - of the weight of water is hydrogen. In 45 kilogrammes of water, then, there are 5 kilogrammes of hydrogen. How many kilogrammes of water could be produced by using 64 kilogrammes of oxygen? We have found that,- of water is oxygen. The question then becomes, if f of the weight is 64 kilogrammes, what is, or the whole? Evidently 72 kilogrammes. How many kilogrammes of oxygen would be required to convert 92 kilogrammes of sodium into oxide of sodium? How many kilogrammes of oxygen and of hydrogen, to convert it into hydrate of sodium? CHEMICAL AFFINITY. 49 23 of Na2O is sodium; ir is oxygen. If 3 _ 92 kilogrammes, 1 - 4 kilogrammes, and - - 32 kilogrammes weight of oxygen required to convert the sodium into oxide of sodium. Again, 1 of HNaO- H; 2 =-Na; -. If 92 kilogrammes, then r -= 4 kilogrammes weight of H required, and 6 =- 64 kilogrammes - weight of - required. 58. Problems.- -. Indicate by symbols the chemical change which takes place when sodium is thrown upon water. 2. Indicate by symbols the change which takes place when the hydrogen of water is wholly displaced by sodium. 3. What fraction of the weight of oxide of potassium is oxygen, and what fraction is potassium? 4. What fraction of the weight-of hydrate of potassium is hydrogen, what potassium, and what oxygen? 5. How much water can be formed by using 75 grammes of oxygen? How much hydrogen would be required? 6. How much potassium and how much oxygen in 235 grammes of oxide of potassium? 7. How much hydrogen would be required to make 3 kilogrammes of hydrate of potassium? To make 3 kilogrammes of hydrate of sodium? 8. If 892 grammes of oxygen were used in making oxide of sodium, how much sodium would be used? 9. If the same amount of oxygen were used in making hydrate of potassium, how much potassium would be used? MURIATIC ACID. 59. We will next examine a liquid which somewhat resembles water in appearance, and which, though not so well known, has for a long time been extensively used in the arts. This liquid is commonly called muriatic acid, a 3 50 CHEMICAL AFFINITY. name which, like water, gives no indication of its composition. Its scientific name will be given when we have found out of what the acid is composed. Put some muriatic acid into a flask, and connect the flask by means of a tube with a glass jar filled with mercury and inverted over the mercury trough, as shown in Fig. 21. the figure.* Boil the acid, and a gas will pass over and fill the jar. If now the jar be removed and its mouth be opened under water, the gas will be quickly absorbed, and the water will have all the properties of muriatic acid. We therefore conclude that ordinary muriatic acid is a gas reduced to the liquid state by being absorbed by water. It is thus made liquid merely for convenience in using and transporting it. When this muriatic acid is boiled, steam will be generated and pass over with the gas. We have seen that a solid absorbs some gases much * See Appendix, I. CHEMICAL AFFINITY. 5 more readily than it absorbs others. If we can find a solid which will absorb steam and not absorb muriatic acid gas, we evidently can separate these two gases by causing them to pass together over this solid. Fused chloride of calcium is such a solid. We break it up into coarse lumps, that it may expose the more surface to the gases, and put it into a glass tube or a tall and narrow glass jar. Such a tube or jar is called a drying-tube or drying-jar. If some muriatic acid be again boiled in a flask, and the muriatic gases which escape be conducted, as shown in the Fig. 22. figure, first through a drying-jar, to remove the steam, and then through a bottle partially filled with sodium amalgam (sodium dissolved in mercury), and then into a jar filled with water, and inverted over the water-trough, the gas which collects in this jar will be found to be wholly unlike muriatic acid gas. It is no longer absorbed by water, and has no effect on blue litmus-paper. Muriatic acid gas is not inflammable, but if this gas be tested by bringing a 52 CHEMICAL AFFINITY. lighted taper to the mouth of the jar after raising it from the trough, it takes fire with a slight explosion, and burns with a pale blue flame. This shows the gas to be hydrogen. 60. Conmosifion of AMriatic Acid. - We see then thati muriatic acid is a compound substance, and that one of the elements of which it is composed is hydrogen. We must next find what element or elements the sodium has removed from the acid. The mercury in the amalgam takes no part in the decomposition of the gas. The sodium is dissolved in the mercury merely that it may present more surface to the gas. Taking the same apparatus used for decomposing water by means of the battery,* fill it with dilute muriatic acid, and connect it with the battery as before. The bell-jar connected with the zinc pole is filled quite rapidly with a gas which is found on trial to be hydrogen. The other jar after some time begins to fill with a gas of a yellowish-green color. 6I. Chlorine. - This gas has a peculiarly suffocating odor, and the remarkable property of bleaching vegetable colors. This latter characteristic may be shown by raising the jar filled with the gas, inverting it, and putting into it a piece of moistened litmus-paper, which very soon loses all its color. This gas from its color is called chlorine; a name derived from a Greek word meaning yellowish-green. Its color and its bleaching properties serve to distinguish it from other gases. Chlorine may be more readily obtained from muriatic acid by mixing it with black oxide of manganese and heating the mixture gently in a flask.+ If a jar of chlorine be inverted over water, it is slowly absorbed. This is the reason that, When muriatic acid is * See Appendix, 12. t See Appendix, I3. CHEMICAL AFFINITY. 53 decomposed by means of the battery, the chlorine does not at first collect in the bell-jar. 62. Are Hydrogen and Chlorine the only Elements in M/uriatic Acid? Take Fig. 23 two glass jars of the same size, with T ground mouths, fill I ~ one with hydrogen and the other with chlorine, and place them together mouth to mouth, as represented in Figure 23. - Then withdraw the * glass plates by which they are closed, shake the jars to mix the gases, and open them over a burning lamp. The mixed Fig. 24. gases take fire, and the appearance of a t >\ white cloud shows i that muriatic acid is formed by their corm- L bustion.* Hence this acid can contain only hydrogen and chlorine. 63. flow much Hydrogen and how much Chlorine in fMuriiatic Acid?- Figure 25 represents a glass tube divided by a glass stop-cock into two unequal parts, and closed at its ends by glass stoppers. Fill the smaller division of the tube with dry chlorine and the larger one with dry hydrogen; then open the stop-cock and expose the tube for some * If a dish of ammonia is standing near by, the white fumes will be much more dense. 54 CHEMICAL AFFINITY. hours to diffused daylight, after which bring it into direct sunlight. Now open one end of the tube under water, and the water will rise and fill a Fig. 25. Fi. 2. space double that which was occupied by the chlorine. The gas which remains is found to be hydrogen. Fill now the smaller division of the tube with hydrogen, the larger one with chlorine, and let the two combine under the influence of light as before. The water will rise in the tube to the same height as before, but the gas which remains will be found to be chlorine.* a-_ X_ __..1. In these two experiments it 7/777/i( \\ is evident that equal measures of hydrogen and chlorine have combined; the excess, whether of the one or the other gas, remaining unchanged. By measure then, hydrogen and chlorine combine in equal parts to form muriatic acid. We find on trial that chlorine weighs 35.5 times as much as hydrogen, bulk for bulk. By weight, then, there is one part of hydrogen to 35.5 parts of chlorine in muriatic acid. 64. Atomic Constitution of Muriatic Acid. - It is found impossible to displace any fraction of either the hydrogen or the chlorine from muriatic acid without displacing the whole. We must hence conclude that in a molecule of muriatic acid there is one atom of hydrogen and one of chlorine, and that the atomic weight of chlorine is 35.5. * See Appendix, 14. CHEMICAL AFFINITY. 55 The symbol for chlorine is C1. That of muriatic acid evidently will be HC1. 65. Problems.- io. In 35 grammes of muriatic acid gas how much hydrogen and how much chlorine? I I. How much muriatic acid can be made by using 13 kilogrammes of hydrogen? By using 22 kilogrammes of chlorine? 12. How much hydrogen is required to convert 46 kilogrammes of chlorine into muriatic acid? How much chlorine to convert 46 kilogrammes of hydrogen into muriatic acid? AMMONIA. 66. The ammonia of commerce, like the muriatic acid, is a gas absorbed by water, as may readily be shown by the method described under muriatic acid. (59.) Fig. 26. ~.. If some ammonia be boiled in a flask, steam and ammonia gas will pass off. If these gases are then passed through 56 CHEMICAL AFFINITY. a drying-jar filled with unslaked lime,'the steam will be retained, and the ammonia gas will pass on. If this gas be then conducted into a test-bulb in which there is a piece of heated potassium, and the gas which leaves the tube be collected in a small jar,* it will be found by the usual test to be hydrogen. The apparatus required for this experiment is shown in Figure 26. Ammonia is then a compound substance, one of whose elements is hydrogen. 67. IVhat Elemeent has been remozed by tlze Polassizm? Fill a tolerably large bottle partly full of strong ammonia, Fig. 27. and close it with a cork through which pass two tubes, one reaching to the bottom of the bottle, and the other passing through the cork. Let a stream of chlorine pass through the first tube. As it enters the ammonia, flashes of light * See Appendix, I5. CHEMICAL AFFINITY. 57 are seen, and other indications of energetic action. The gas which escapes from the bottle through the other tube is passed through a bottle filled with water (called a washbottle), and is then collected in jars. This gas is evidently not ammonia, for it was not absorbed by the water. It is not chlorine, for it is colorless. It is not hydrogen or oxygen, for it neither burns when a lighted taper is applied to it, nor will a lighted taper burn in it. It does not affect either blue or red litmus-paper. It cannot be decomposed, and must therefore be regarded as an element. It is called nitrogen, and its symbol is N. 68. Is there any other Element in Ammonia? - Hydrogen and nitrogen cannot be made to combine directly. We cannot therefore employ this method to determine wheth- er ammonia gas contains more than these two ele- ments. But if a gramme of ammonia gas be decomposed into hydrogen and nitrogen, the weight of the hydrogen and nitrogen together will be just one gramme. Hence it is clear that there is nothing but these two elements in ammonia. 69. Hrow much of each of these Elements in Ammonia? Let a long tube (Fig. 28), closed at one end and divided into three equal parts by marks on the side, be filled with chlorine, and a 3* 58 CHEMICAL AFFINITY. dropping-tube (see Fig. 29) filled with ammonia be con" Fig. 29. nected with it by means of a rubber cork so as to Fig. 29. form an air-tight joint. If now the ammonia be allowed to drop slowly into the tube, the chlorine will decompose it, uniting with the hydrogen and setting the nitrogen free. After enough armo-. nia has passed in to fill the tube to the depth of about half an inch, allow dilute sulphuric acid to pass into the tube by means of the dropping-tube as long as it will. It will fill the tube just two thirds full. T'he other third will be filled with a gas which is found on trial to be nitrogen. While this nitrogen has been set free, how much hydrogen has been taken away? The chlorine combines with the hydrogen to form hydrochloric acid. In this acid there are equal measures of hydrogen and chlorine. As the tube was full of chlorine at first, enough hydrogen to fill the tube must have been taken from the ammonia. By measure, then, there are three parts of hydrogen and one of nitrogen in ammonia. Nitrogen weighs 14 times as much as the same bulk of hydrogen. Hence by weight there are in ammonia 3 parts of hydrogen to I4 parts of nitrogen. 70. Atomic Conzstittion of A4mmonzia. - It is also found that, when ammonia is decomposed by chlorine, three definite compounds can be obtained; one containing one third less hydrogen than ammonia does, one containing two thirds less hydrogen, and one containing no hydrogen. Every time a measure of hydrogen is displaced, an equal measure of chlorine replaces it. We therefore conclude that a molecule of ammonia contains three atoms of hydrogen and one atom of nitrogen; for none of the nitrogen can be displaced unless the whole is. The atoms of hydrogen can be successively displaced CHEMICAL AFFINITY. 59 by atoms of chlorine. The symbol for ammonia, then, will be H3N; and the atomic weight of nitrogen is I4. 7I. Problems. 13. What is the symbol for each of the three compounds that may be formed when ammonia is decomposed by chlorine? 14. What fraction of ammonia is hydrogen, and what fraction is nitrogen? I5. How much hydrogen and how much nitrogen in i86 grammes of ammonia? i6. How much ammonia can be made by using 15 grammes of hydrogen? By using 15 grammes of nitrogen? MARSH GAS. 72. When the mud at the bottom of stagnant ponds and marshes is stirred, bubbles of gas are seen to rise. This gas is called mars/i gas. It may readily be prepared by heating strong vinegar in a flask (of glass, or better of copper or iron), together with a mixture of lime and the caustic soda of commerce, and collecting the gas over water.* Fig. 30o 73. Its Composition. If marsh gas be mixed with chlo* See Appendix, 9g. 60 CHEMICAL AFFINITY. rine in a tall jar, and the mixture be ignited, a black powFig. 31. der will be deposited on the sides of the jar. This powder has been proved / /,%^ to be an element, and is called carbon. It is the same substance as char/@ @^ ll |coal. Its symbol is C. If a jet of marsh gas be burned in a jar of oxygen, moisture will collect on the sides of the jar, showing that water has been formed. Now water contains oxygen and hydrogen.. The hydrogen must I - _ g have come from the marsh /d- m ~ gas which was burned. ^/ / / \ This gas, then, must contain at least two elements, hydrogen and carbon. When a weighed quantity of marsh gas is decomposed, the hydrogen and carbon obtained from it weigh just as much as the gas decomposed; hence this gas can contain only hydrogen and carbon. 74. Atomic Constitution of Marsh Gas. - On careful analysis, marsh gas is found to contain 4 parts of hydrogen by weight to I2 parts of carbon. It is also found by experiment that fourths of the hydrogen can be successively displaced from marsh gas by chlorine, giving rise to four compounds; — the ist containing 3 parts of H, 35.5 of C1, O2 ofC, 2d " 2 " " H, 7I " C1, 12' 2, "3d " I " H, I16.5 " C1, 12 " C, " 4th " 0 " H, I42 " C1, I2 " f. CHEMICAL AFFINITY. 6 Hence we conclude that a molecule of marsh gas contains four atoms of hydrogen and one of carbon; and that the atomic weight of carbon is 12. The atoms of hydrogen can be successively displaced by atoms of chlorine, giving rise to the four compounds mentioned above. 75. Problems. - 17. What is the symbol for marsh gas, and for each of the four compounds mentioned? i8. What fraction of the weight of marsh gas is hydrogen, and what fraction is carbon? i9. In 868 grammes of each of the four compounds mentioned, what weight of hydrogen, of chlorine, and of carbon? COMBINING POWER OF THE ELEMENTS. 76. We see, then, that the atoms of different elements have different combining powers. Thus one atom of chlorine has power to fix in combination only one atom of hydrogen, while one atom of oxygen has power to fix two atoms of hydrogen, one atom of nitrogen to fix three atoms of hydrogen, and one atom of carbon to fixfour atoms of hydrogen. We have also seen that the atoms of different elements can displace each other in compounds. Thus one atom of potassium or sodium can displace one atom of hydrogen from water, forming hydrate of potassium or of sodium; that is, H2-I + K = HKO + H. But one atom of barium can displace two atoms of hydrogen from water. Thus, H2O + Ba BaO -1- 2-1H. Whenever either sodium or barium can displace hydrogen from its compounds, one atom of sodium is always able 62 CHEMICAL AFFINITY. to displace one atom of hydrogen, and one atom of barium tzeo atoms of hydrogen. It is also found that one atom of aluminium is able to displace thzree atoms of hydrogen from certain of its compounds, and one atom of tin to displace four atoms of hydrogen. One atom of chlorine or of potassium, then, seems to have the same combining or " atom-fixing' power as one atom of hydrogen; one atom of oxygen or of barium, the same combining power as tzo atoms of hydrogen; one atom of nitrogen or of aluminium, the same as t/ree atoms of hydrogen; and one atom of carbon or of tin, the same as four atoms of hydrogen. 77. Mlonads, Dyads, Triads, and Tetrads. -An element whose atom has the same combining or "atom-fixing" power as one atom of hydrogen is called a monzatoonic element, or, more briefly, a monad; names derived from the Greek word for one. An element whose atom has the same combining or " atom-fixing " power as two atoms of hydrogen is called a diatomlic element, or a dyad,; from the Greek word for two. An element whose atom has the same combining or " atom-fixing " power as tzire atoms of hydrogen is called a riatonmic element, or a triad, from the Greek word for three. An element whose atom has the same combining power as four atoms of hydrogen is called a tetratonzic element, or a tetrad;, from the Greek word forfour. Every element now known is supposed to belong to one of these four groups. The combining or "atom-fixing" power of the hydrogen atom is regarded as unity. Then each atom of a mzonad will have one unit of combining power; one atom of a dyad will have twzo units of combining power; one atom of a triad will have tAree units of combining power; and one atom of a tetrad will havefour units of combining power. CHEMICAL AFFINITY. 63 78. Safurated Coponundzs. - A compound is regarded as sa/lrated when the combining power of all its atoms is satisfied. Thus carbonic acid (-CO-) and marsh gas (H4f) are saturated compounds; since in the first the 4 combining units of carbon are satisfied by the 4 units of the two atoms of oxygen, and in the second the four units of carbon are satisfied by the 4 units of the 4 atoms of hydrogen. But carbonic oxyde (C-O) and olefiant gas (H-C) are non-saturated compounds; since in the first the 2 combining units of the oxygen satisfy only 2 of the 4 combining units of the carbon, and in the second only 2 of the 4 combining units of the carbon are satisfied by the 2 units of hydrogen. The atom-fixing power of the different elements is sometimes indicated thus: C"" indicates that i atom of carbon has a combining power of 4 units, N"' " " I " nitrogen " 3 units, 0" "c i oxygen 2 units H " " I " hydrogen " i unit. 79. Classificalion of the Elements. - The following is a table of the most important elements arranged into groups according to Miller. The names of the elements are given along with their symbols and atomic weights. Those in the first division of each group are the so-called non-meta/lic elements; those in the second division of each group are metZa/s. MONADS. Hydrogen H I Fluorine F 19 Chlorine C1 35.5 Bromine Br 80 Iodine I 127 64 CHEMICAL AFFINITY. Potassium (Kalium) K 39 Sodium (Natrium) Na 23 Silver (Argenfum) Ag I08 DYADS. Oxygen O 16 Sulphur S 32 Barium Ba 137 Cadmium -Cd 12 Calcium -Ca 40 Chromium -Cr 52.5 Cobalt Co 59 Copper (Cuprum) -Cu 63.5 Iron (Ferrum) Fe 56 Lead (Plumbum) Pb 207 Magnesium Mg 24.3 Manganese Mn 55 Mercury (Hydrargyrum) Hfg 20 Nickel Ni 59 Strontium -r 87.5 Uranium U: I20 Zinc Zn 65.5 TRIADS. Nitrogen N 14 Phosphorus P 31 Arsenic As 75 Boron B 10.9 Aluminium (or Aluminum) Al 27.5 Antimony (Stibium) Sb 122 Bismuth Bi 210 Gold (Auruzm) Au I96.6 TETRADS. Carbon -C 12 Silicon Si 28 CHEMICAL AFFINITY. 65 Tin (Stanznm) in 118 Platinum Pt 197 It will be noticed that there is a bar across the symbols of all the elements of the dyad and tetrad groups in the above table, while of those belonging to the nmonad and triad groups the symbol of Aluminium is the only one thus marked. COMPOUNDS OF OXYGEN AND NITROGEN. 80. Nitric Oxide. - If one part by weight of saltpetre, 8 parts of copperas, and 8 parts of dilute sulphuric acid be gently heated in a large flask, a gas will be rapidly disengaged which, when collected in a jar over water, is colorless. If in a porcelain cup floating on water a small piece of phosphorus be ignited, and ajar filled with oxygen be inverted over it, the phosphorus will burn with great brilliancy, and the jar will be filled with a dense white cloud.* This is soon absorbed by the water, which rises in the jar; showing that some of the oxygen has united with the phosphorus to form the white fumes. If now we repeat the experiment, filling the jar with the gas just obtained, the phosphorus burns just as it did in the oxygen, and the same white fumes are formed. There must, then, be oxygen in this gas. But the gas is not pure oxygen, since a lighted taper put into it is speedily extinguished. Let a mixture of equal volumes of this gas and hydrogen be passed through a glass tube containing some platinized asbestos,t and the tube be heated to dull redness by means of a lamp. Hold a piece of red litmus-paper in the gas which escapes from the tube, and its color is at once * See Appendix, I6. t See Appendix, 17. E 66 CHEMICAL AFFINITY. changed to blue. Now ammonia is the only gas that changes red litmus-paper to blue. Hence, when the mixed gases pass over the heated asbestos ammonia is formed. The asbestos remains unchanged, and may be used for years. There must then be nitrogen in the gas which was mixed with the hydrogen and passed through the tube. That there is nothing else than nitrogen and oxygen in this gas can be proved by the method already described in the case of ammonia and marsh gas (68, 73). This composition of the gas is indicated by its name, nitric oxide. 81. Nitrous Acid and Nitric Acid. - If a jar be filled with nitric oxide and another jar of about half the capacity filled with oxygen be inverted over it,* the two gases on mixing will at once become of a bright cherry-red color. They evidently combine and form a new substance. Boil a little aquafortis in a flask, t and let the steam pass through a porcelain tube heated to dull redness. The gas which comes from the tube will be of a bright red color, and will be at once recognized as the compound of nitrogen and oxygen with which we have just become acquainted. Fig. 32. If the gases which issue from the porcelain tube be col* See Appendix, IS. t See Appendix, i9. CHEMICAL AFFINITY. 67 lected in a jar over water, the red gas will be absorbed by the water, and the jar will be filled with a colorless gas, which by the usual tests is found to be oxygen. The steam of aqua-fortis is then decomposed in the heated tube into the red gas and oxygen. Hlence aquafortis must be a compound of nitrogen and oxygen, since the red gas has been found to be a compound of those elements. The scientific name of aqua-fortis is nitric acid, and that of the red compound is nitrous acid. 82. Atomic Constitution of the Compounds of Nitrogen and Oxygen. - The three compounds, nitric oxide, nitrous acid, and nitric acid, which are so unlike in properties, differ in composition only in the proportion of oxygen which each contains. Nitric oxide is converted into nitrous acid by taking up more oxygen, while nitric acid is converted into nitrous acid by losing a part of its oxygen. Hence nitrous acid contains more oxygen than nitric oxide does, and nitric acid more than nitrous acid. In the compounds of hydrogen which we have examined we have found hydrogen combining with each element in only one proportion; here we find oxygen combining with nitrogen in several proportions, and forming compounds no less unlike one another than the compounds of hydrogen with different elements. It is one of the peculiarities of hydrogen that it seldom combines with an element in more than one proportion, or, in other words, that it seldom forms other than saturated compounds, while it is no less a peculiarity of oxygen that it usually combines with the same element in several proportions, or, in other words, that it readily forms non-saturated compounds (78). It combines with nitrogen in five different proportions, forming compounds whose symbols and names are as follows:N2O, Nitrous Oxide, NOf, Nitric Oxide, 68 CHEMICAL AFFINITY. N2-3, Nitrous Acid, N-2, Hyponitric Acid, N205, Nitric Acid. 83. Problems.- 20. What three things does each one of these symbols indicate? 2T. What fraction of the weight of each of these compounds is oxygen? What fraction of each is nitrogen? 22. How much oxygen and how much nitrogen in 756 grammes of each of these compounds? 23. How much of each of these compounds could be made by using 62 kilogrammes of nitrogen, and how much oxygen would be required in each case? 24. How much of each compound could be made, by using 62 kilogrammes of oxygen, and how much nitrogen would be required in each case? ACIDS, BASES, AND NEUTRALS. 84. When phosphorus is burned in oxygen, the jar is filled with a white cloud, which is soon absorbed by the water (80). If now a piece of red litmus-paper be dipped in the solution, its color will not be changed. If, however, a piece of blue litmus-paper, whose color is not affected by a solution of oxide of sodium, be dipped in this solution, its color is at once changed to red. It will be remembered that the solution of the hydrate of sodium formed by replacing the hydrogen of water with sodium (46) has the power to turn red litmus-paper to blue. It will also be remembered that water has no effect on either red or blue litmus-paper. We see, then, that oxygen combines with certain elements, as potassium and sodium, to form a compound whose solution changes red litmus-paper to blue; with certain others, like phosphorus, to form a compound whose CHEMICAL AFFINITY, 69 solution changes blue litmus-paper to red; and with others, like hydrogen, to form a compound which has no effect on either the red or the blue paper. The first class of compounds are called bases, the second class, acids; and the third class, neutrals. 85. The Nlames of Acids.-The acid just formed by the combination of oxygen and phosphorus is called phosphoric acid. The acid compounds of oxygen are always named from the element with which the oxygen combines, as phosphoric acid from phosphorus. We have seen in studying the compounds of nitrogen and oxygen, that oxygen often combines with the same element to form more than one acid. In such cases, the acids are distinguished by terminations or by prefixes. The termination ous indicates less oxygen than ic. Thus nitrous acid contains less oxygen than nitric acid. The prefix hyipo indicates less oxygen, and the prefix hyper more oxygen, than there is in the acid to whose name either is prefixed. Thus hyponitric acid contains less oxygen than nitric acid, and hypochlorous acid less oxygen than chlorous acid; while hyperchloric acid contains more oxygen than chloric acid. 86. Names of Bases and Neutrals. - The basic and neutral compounds of oxygen have the common name of oxides. The bases are named from both elements. Thus oxygen and sodium combine to form a base called oxide of sodium; oxygen and potassium, to form the base oxide of potassium. The compounds of nitrogen and oxygen also show that oxygen sometimes combines with the same element to form more than one oxide. In such cases, the name of the one which contains the more oxygen takes the ending ic, and the name of the other takes the ending ous. Thus N2contains less oxygen in proportion to the nitrogen than NO, and the former is called nitrous oxide, the latter nitric 70 CHEMICAL AFFINITY. oxide. So Fe- is called ferrous oxide, and Fe2O3 ferric oxide; Hg2O mercurous oxide, and Hg-O mercuric oxide. The compounds of nitrogen and oxygen also illustrate the fact, that when oxygen combines with the same element in several proportions, the higher compounds (that is, those containing the most oxygen) are likely to be acids, while the lower are oxides (bases or neutrals). 87. Oxyacids and Hydracids. -In the case of muriatic acid, we have, as we have seen, an acid made up of hydrogen and chlorine, and containing no oxygen. There are several other acids which contain hydrogen as a common element instead of oxygen. The acids whose common element is oxygen are called oxyacids, and those whose common element is hydrogen are called hydracids. 88. Names and Symbols of the Leading Hydracids. - Hydrogen combines with all the non-metallic elements of the monad group, and with sulphur, to form hzydraids. The symbols and names of these acids are as follows:HC1, hydrochloric acid (muriatic acid); HF, hydrofluoric " HBr, hydrobromic " HI, hydriodic " H2S, hydrosulphuric" (sulphide of hydrogen, sulphuretted hydrogen). It will be noticed that the hydracids are named from both their elements. 89. Names and Symbols of the Leading Oxyacids. - Oxygen often combines with a metal to form one or more acids, but the most important oxyacids are compounds of oxygen and a non-metallic element. The following are the most important: - N2-3, nitrous acid; N206, nitric " CHEMICAL AFFINITY. 71 P2&3, phosphorous acid; P285, phosphoric " C1203, chlorous C1205, chloric As2e3, arsenious " As2O5, arsenic B2-3, boracic " S02, sulphurous' Se0, sulphuric " O~-2, carbonic " Si02, silicic " -Cre3, chromic " go. The Bases of the Four Groups. - The ordinary bases are compounds of oxygen and a metal. As oxygen is a diatomic element, the bases of the metals of the first group will regularly contain two atoms of the metal to one of oxygen. Their symbols and names are as follows:Ag2-, oxide of silver; K20, " potassium (potassa); Na2O, " sodium (soda). The bases of the metals of the dyad group will regularly contain one atom of the metal to one of oxygen. Their symbols and names are as follows:Ba0, oxide of barium (baryta); CaO, " calcium (lime); -CdO, " cadmium; CfoO, " cobalt; f-uO, " copper; Mg&, " magnesium (magnesia); MnnO, " manganese; NiO, " nickel; PbO, " lead; r-y, " strontium (strontia); ZnO, " zinc; 72 CHEMICAL AFFINITY. -Hg2, mercurous oxide; Hg-e, mercuric " FeO, ferrous " Fe2-3, ferric " -CrO, chromous " -Cr.e8e, chromic " UO, uranous " U2-&3, uranic c It will be noticed that mercury, besides its regular base, forms another base analogous to those of the metals of the monad group. Also, that iron, chromium, and uranium, besides their regular bases, form others analogous to those of the metals of the triad group. The bases of the triad group regularly contain two atoms of the metal to three of oxygen. They are the following:A121O3, oxide of aluminium (alumina); Sb2,-O, " antimony; Bi203), " bismuth; Au203, " gold. Each of the metals of the tetrad group forms two oxides:Sn-O, stannous oxide; Sn-O2, stannic " PtO, platinous " PtO-2 platinic " SULPHIDES AND CHLORIDES OF THE METALS OF THE FOUR GROUPS. gi. Sulpiides. — Sulphur combines with all the metals of the four groups. As sulphur is a diatomic element, the sulphides have the same composition as the oxides. The metals which have two oxides have usually two corresponding sulphides. CHEMICAL AFFINITY. 73 92. Chlorides. - As chlorine is a monatomic element, the chlorides of the metals of the monad group regularly contain one atom of the metal to one of chlorine. Those of the metals of the dyad group will contain one atom of the metal to two of chlorine; of the triad group, one of the metal to three of chlorine; and of the tetrad group, one of the metal to four of chlorine. The metals which have two oxides have usually two corresponding chlorides. 93. Problems.- 25. Write the symbols and names of the sulphides of the metals of the four groups. 26. Write the symbols and names of the chlorides of the metals of the four groups. HYDRATES. 94. Hydrates of the Acids. -The oxyacids which have the composition represented above (89) are rare substances. These acids, with the exception of carbonic acid, ordinarily exist in combination with one or more molecules of water, and are then called hydrates. When not combined with water they are usually called anhydrous acids, or more briefly anhydrides, —names derived from the Greek, and meaning without water. 95. Symbols of the Hydrates of the Acids. -The symbols of these hydrates are formed by writing the symbol of water before the symbol of the anhydrous acid, with a comma between the two. Thus the symbol of ordinary sulphuric acid (oil of vitriol) is H2&, -S3, and that of ordinary nitric acid (aqua-fortis) is H20, N2O5. When more than one molecule of water combines with a molecule of the anhydride to form the hydrate, the number of the molecules of water is indicated by a figure placed before the symbol I2&. Thus the symbol for ordinary phosphoric acid, 3H2&-, P2-O-, indicates that three molecules of water 4 74 CHEMICAL AFFINITY. and one of phosphoric anhydride combine to form a molecule of the hydrate. 96. Dualistic and Unitary Symbols. - It is evident that the ultimate composition of the hydrate of sulphuric acid can be indicated by the symbol H2SO4, since 1H2-, SOA -H25-4. So the ultimate composition of hydrate of nitric acid can be represented by HN-O, since H2O, N2-5 = H2N2O6 -2HNO3; and that of the hydrate of phosphoric acid by H3PO4, since 3H20, P2O5- H6P2O8 2H3PO4. The symbol H2,-, SO3 represents a molecule of the hydrate of sulphuric acid as made up of a molecule of water and a molecule of sulphuric' anhydride. But we may regard a molecule of this hydrate as merely containing the elements hydrogen, sulphur, and oxygen; and may represent it, therefore, by the symbol H2S-04. The first method of writing the symbol is called the dualistic method; the second, the unitary method. If we have two small piles of bricks we can make them into one, either by bringing them together without disarranging the bricks in either pile, or we can take the bricks one by one from each pile and arrange them in a new pile. So the atoms in a molecule of water and a molecule of sulphuric acid may be combined into a new molecule, either by bringing them together without disarranging the atoms which make them up, or by taking the atoms from each of the two molecules and arranging them into a new one. The dualistic symbol represents a molecule of the hydrate as made up in the first way; the unitary, in the second way. Since the dualistic symbol indicates at a glance that the hydrate is formed by the union of water and the anhydride, it is often convenient to use it, although it may not correctly represent the molecular constitution of the hydrate. The hydracids never form hydrates. 97. Hydrates of' te Bases. -The bases also are usually CHEMICAL AFFINITY. 7 found in combination with water, and are then called hydrates. Thus Na2O, H2O is the hydrate of sodium; Baa, H2O is the hydrate of barium; and A1203, 3H20 is the hydrate of aluminium. 98. Symbols of these Hydrates. The symbol for the hydrate of a base is formed in the same way as that for the hydrate of an acid, except that the symbol of the water is put after that of the base. These symbols also may be written according to the unitary method. Thus HNaO may represent the hydrate of sodium, since Na2-, HO-m 2HNa-; H2Ba-O2 may represent the hydrate of barium, since Ba-O, H2-&= HaO&2; and H3AI-,3 the hydrate of aluminium, since A1203, 3H20 2H3Al&3. 99. Hydrate of Ammonia. -Ammonia gas also combines with water"to form a hydrate analogous to the hydrate of sodium. H3N + H20- = H3N, H20. The resemblance of this hydrate to the hydrate of sodium is shown by writing the symbol thus: H(H4N)-. This symbol will of course correctly represent the composition of the hydrate, since I3N, H2& = H(H4N)-. Comparing this formula with the formulas for hydrate of potassium and sodium, HKO and HNaO, we see that the group of atoms (HUN) takes the place of the atom of potassium in the one hydrate, and of the atom of sodium in the other. oo. Conmpound Radicals. - Ammonium. -There are many groups of atoms which seem in compounds to play the part of a single atom. Such groups of atoms are called compound radicals. Some of them are capable of existing by themselves, or in a free state; others are not. The group (H4N) is called ammonium. It has never yet been obtained in a free state. In combining with other elements it always plays the part of a monad metal. There is also a compound of nitrogen and carbon, which acts like a non-metallic monad element. It is called cy 76 CHEMICAL AFFINITY. anogen and its symbol is -CN, or Cy. It forms a hydracid, H-N, or HCy, called hydrocyanic acid. Cyanogen can exist in a free state. oI. Some Basesform more t/an one Hydrate. In many cases more than one hydrate is formed. Those given above are regarded as the regular or normal hydrates. The normal hydrates of the bases of the monad group have a composition corresponding to that of the hydrate of sodium; those of the dyad group, a composition corresponding to that of the hydrate of barium; and those of the triad group, a composition corresponding to that of the hydrate of aluminium. 102. Problems. 2 7. Write the dualistic and the unitary symbols of the hydrates of the bases of the monad, dyad, and triad group. 28. What fraction of the weight of hydrate of sulphuric acid is water? What fraction is sulphuric anhydride? 29. In the same hydrate, what fraction of the weight is hydrogen? sulphur? oxygen? 30. How much phosphoric anhydride can be obtained from 135 grammes of hydrate of phosphoric acid? How much phosphorus? 31. How much hydrate of sodium may be made from 3 kilogrammes of sodium? How much water will be required? How much oxygen? SALTS. 103. If dilute nitric acid be added to a solution of hydrate of sodium, and the mixture be slowly evaporated, a white crystalline solid will be formed. This substance is found on examination to be wholly unlike either nitric acid or hydrate of sodium. Its solution has no effect on either red or blue litmus-paper. If a platinum wire be dipped into the solution and held in the flame of an alcohol or Bunsen's lamp, the intense yellow color of the flame is at CHEMICAL AFFINITY. 77 once recognized as that which sodium gives to a flame (46). Hence this compound must contain sodium. If a solution of ferrous sulphate (copperas, or green vitriol) be added to dilute nitric acid, the mixture will at once become dark brown. This color will pass off on heating the liquid. If we add strong sulphuric acid to the solution of the crystalline substance obtained above, and then add some ferrous sulphate, the mixture becomes dark brown; but on heating it, this color passes off. This substance then contains nitric acid. On careful analysis the substance is found to contain sodium, oxygen, and nitrogen in the proportions indicated by the formula NaN-3. What changes, then, took place on mixing the nitric acid and the hydrate of sodium? They can be best shown by the following equation: H2&,N2e5 + Na2O, H - =Na2O, N205 + 2H2-; or, using the unitary symbols: HN-3 + HNa-O = NaN-Os + H.20 In these equations it is seen that the hydrogen of the nitric acid has changed places with the sodium of the hydrate of sodium. The first equation shows at a glance that this compound contains the elements of oxide of sodium and nitric anhydride. From its composition it is called nitrale of sodium. A substance which contains the elements of an anhydrous base and acid is called a salt. 104. Symbols of Salts.- The dualistic symbol for a salt is obtained by writing the symbol of the acid after the symbol of the base, with a comma between them. Thus the dualistic symbol of nitrate of sodium is Na2-, N205. The unitary symbol is obtained by writing the symbols of the elements in succession, placing the symbol of the metal 78 CHEMICAL AFFINITY. first, and that of oxygen last; as, for nitrate of sodium, NaN03. The dualistic symbol represents a molecule of the salt as made up by the combination of one or more molecules of a base with one or more molecules of an acid. The unitary symbol represents the salt as made up of three elements, without reference to the arrangement of the atoms in the molecule. The dualistic symbol indicates at once that a salt is made up of the elements of a base and an acid, and the wellknown fact that the salt can be again separated into a base and acid. It also represents the exact composition of the salt. But salts may be decomposed in other ways than into a base and an acid. Thus, when the electric current is sent through a solution of nitrate of sodium, NaNO-, it is separated into sodium, or Na, and NO3. To represent this and similar decompositions of the salt, the unitary symbol is the better. Both symbols will be used in this book according to convenience. 105. Names of Salts.- The name of the salt is formed by changing the ending ic of the name of the acid into ate, or the ending ous into ite, and adding the name of the metal of the base. Thus nitric acid and oxide of sodium form nitrate of sodium, and nitrous acid and oxide of potassium would form nitrite of polassizm. o6. The Hydrates are really Salts. —It will be seen from the above formulas that the hydrates of the acids are salts in which water plays the part of the base, and that the hydrates of the bases are salts in which water plays the part of the acid. 107. The Action of the Hydrates of the Acids on the Hydrates of the Bases. -When the hydrates of the acids act on the hydrates of the bases, the metal of the base always changes place with the hydrogen of the acid, giving rise to a salt and water. As these salts contain three elements, they are called ternary salts. CHEMICAL AFFINITY. 79 The following equations represent the action of the hydrates of nitric and sulphuric acid upon the hydrate of a base in each of the first three groups:(i.) K2&, H20 +- H2e, N25 - 2H2+ K20, N205, or THKO + HNQe3 2 H20 + KNO3; Na2e, H2 - + H2O, S-3 - 2H20- + Na2O, SO3, or 2HNaO + H2iO4 - 2 HO - + Na2SO4. (2.) BaO, H1, + H2&, N2,0 - = - 2O + Ba, N2,-,, or H2BaO2 + 2HN-3 - 2I H1- + BaN2s; BaO, H20 + H20, SO3 2 H20- + Baa, S, or HB2aO2 + H2S-O 4 2 H,2 + BaSe4. The symbol for;ztrate of bcariumz is very often written Ba(N —3)2, or Ba2NO3, instead of BaN2-O6. We have seen that the atom of potassium has the same atom-fixing power as an atom of hydrogen. And here the group of atoms N-O has the same atom-fixing power as an atom of hydrogen, since they combine with one, and only one, atom of potassium. But an atom of barium has the same atomfixing power as two atoms of hydrogen; hence it requires double the group of atoms NO- to satisfy it. This fact is at once indicated by the symbol Ba(NO-3)2 or Ba2N-3. The group of atoms S&4, on the other hand, has the same atom-fixing power as two atoms of hydrogen; hence it satisfies two atoms of sodium, but only one of barium. (3.) Als3, 3H2O- + 3(H2O, N2-s) 60H20 + A12O3, 3 N2s,, or HAIOt-3 + 3HNe — 3 HsO + AI1N39 or AI(N33)3; Al123, 3H,2 + 3(1,2, S03) = 6 H2, + A12,O, 3SO3, or 2H3A113 + 3H2+S3104 6 H2& + Al23-012 or A12(ISTO)3. The symbols and names of the ternary salts obtained in the above equations are as follows: — 80 CHEMICAL AFFINITY. Dualistic Symbol. Unitary Symbol. Name. K20, N2O5 KN-O Nitrate of potassium Ba&, N205 Ba(NO3)2 " barium A12e-3, 3N20O5 AI(NO3)3 6 aluminium Na2O, eS Na2S&4 Sulphate of sodium BaOOe BaaS04 " barium A1203, 3&sO- A1l2(S4)I " aluminium The atom-fixing power of the groups of atoms S&4 and NOe is strikingly shown by the following table:K20 KO2SO4 KC1 KNO3 BaO BaSO4 BaC12 Ba(NO3)2 A12&3 A12(ISO,)3 AlCI3 Al(N&3)3 Neither of the groups of atoms S94 and NO3 can exist in an uncombined state. The above formulas are not intended to convey the idea that SO4 and NOb exist in the various salts as distinct compounds, but merely to express the fact that the group of atoms represented by S-O is equivalent in combination to one atom of oxygen or two of hydrogen, and the group of atoms represented by N03 is equivalent in combination to one atom of hydrogen. io8. Normal, Acid, and Basic Salts. —When the hydrate of sulphuric acid acts upon the oxide of sodium, half of the hydrogen of the acid may be displaced by the sodium. This change is best represented by unitary symbols: Na2 +- H2S-4 = HNaO- + HNaSO4. This compound is regarded as a salt, and is called an aczi salt, or sier-salt; while the' salt Na2SO4, in which all the hydrogen of the acid has been replaced by sodium, is called a normal salt. A normal salt, then, is one in which all the hydrogen of the acid has been replaced by an equivalent of a metal. It will be seen by the above formula for acid sulphate of sodium, that it contains more oxygen in proportion to the sodium than the normal salt Na2SO4. Hence an acid salt CHEMICAL AFFINITY. 81 may be defined as one which contains more oxygen in proportion to the metal than a normal salt. Salts sometimes contain more of the metal in proportion to the oxygen than a normal salt, and in that case are called basic salts, or sub-salts. o09. Monobasic, Dibasic, and Tribasic Acids.- An acid, like the nitric, which can form only one salt with potassa or soda, is called monobasic. An acid, like the sulphuric, which can form two salts with potassa or soda, is called dibasic, or bibasic. An acid which can form three salts with the base is called iribasic. Phosphoric acid is tribasic. The hydrate of the acid and the three salts which the acid forms with oxide of sodium are represented by the following formulas:HPO&4, phosphoric acid; HNaPO4, monobasic phosphate of sodium; HNa2PO4, dibasic " " Na3PO4, tribasic " " or 3H20-, P205, phosphoric acid; Na2O, 2H20, P20,, monobasic phosphate of sodium; 2Na20, H20, P2-&5, dibasic 3Na2-, P205, tribasic IIo. Problems. - 32. Show the action of the hydrates of sulphuric and nitric acid on the hydrates of the bases of the monad, dyad, and triad groups, using both the dualistic and unitary formulas. 33- What is the name of each of the ternary salts formed? 34. In each of these salts, what fraction of the whole weight does each element form? 35. Show by the unitary formulas the action of the hydrates of sulphuric and phosphoric acids on the oxides of potassium, sodium, and ammonium (ioo), giving all the salts that may be formed. W In these problems it is understood that normal salts are to be obtained, unless otherwise stated. 4* F 82 CHEMICAL AFFINITY. TERNARY SALTS FORMED BY SUBSTITUTION. I I. Ternary salts are formed, not only by the action of the hydrates of the acids upon the hydrates of the bases, but also by the action (I.) of the hydrates of bases upon ternary salts, (2.) of the hydrates of acids upon ternary salts, and (3.) of ternary salts upon each other. I. Thus when a solution of hydrate of sodizn is added to a solution of nitrate of lead, the sodium and the lead change places, and hydrate of lead and nitrate of sodium are formed. Na.&, H20 + PbO, N2 5 P- bO, H2O + Na20, N2,5; or, 2HNaO: + Pb(N0), == H2PIbO2 + 2NaN-3. The hydrate of lead is insoluble, and separates as a solid, while the nitrate of sodium remains in solution. When a solid separates in this way from a solution, it is called a precipitate. 2. When hydrate of sulphuric acid is added to a solution of nitrate of lead, the hydrogen and the lead change places, forming szulpate of lead, which is insoluble, and hydrate of nitric acid. H20, S03 + PbO, N205 = Pb, SO -3 + H,, N2,0; or, H2SO4 + Pb(N-3)2- PobS-O4 -+ 2HN-3. 3. When a solution of sulphate of calcizum is added to a solution of nitrate of barium, the calcium and barium change places, forming sulphate of barium and nitrate of calcizm. fCa-, +SO -- B-aO, N.O, BaOl, S3 + -a- O, N25; or, -aS-&4 + Ba(N)2 D Ba&4, + -a(N 3)2. It appears in each of the above equations that the new compounds are formed by substitution. CHEMICAL AFFINITY. 83 DOUBLE DECOMPOSITION. I 2. When sodium is thrown upon water, an atom of sodium, as we have seen (52), replaces an atom of hydrogen in the molecule of water, giving rise to a molecule of hydrate of sodium. H2- + Na = HNae + H. Here, as only one substance is decomposed, the change which takes place may be described as substitution by single decomposition. In the cases given above (III), it will be noticed that both substances are decomposed; hence the change is described as substitution by double decomposition. Experiments have brought to light the following important law: When the solutions of two compounds are mixed, decomposition always takes place when thereby an insoluble or gaseous compound can beformed. In such cases, i atom of a monad is regularly replaced by i atom of a monad; i atom of a dyad by i atom of a dyad or 2 of a monad; i atom of a triad by I of a triad or 3 of a monad, or 2 atoms of a triad by 3 of a dyad; and i atom of a tetrad by i of a tetrad, 4 of a monad, or 2 of a dyad, or 3 atoms of a tetrad by 4 of a triad. II3. Affinity modified by Cohesion. -Few solid compounds will act upon each other when brought together, while many of them will readily act upon each other in solution. The cohesion of the solid acts against the affinity between the elements of the two substances, preventing them from entering into combination. When the solids are dissolved this cohesion is overcome. 84 CHEMICAL AFFINITY. BINARY SALTS. II4. The action of hydrochloric acid upon hydrate of sodium is shown in the following equation: Na2&, H,- + 2HCI = 2NaCl + 2H2&; or, HNaO +- HCI - NaCl + H2O. When a hydracid acts upon the hydrate of a base, a binary compound is formed. This binary compound, since it results from the action of an acid upon a base, is called a binary salt. I 5. Names and Symbols of Binary Salts. - The name of a binary salt is formed by changing the ending of the name of the non-metallic element into ide and adding the name of the metal. Thus the salt obtained above is called chloride of sodium. The salt obtained by the action of hydriodic acid on hydrate of potassium is called iodide of potassium. Hydrocyanic acid (ioo) forms binary salts called cyanides. The symbol of a binary salt is formed by writing the symbol of the non-metallic after the metallic element. The symbols for the two salts just mentioned are NaC1 and KI. ACTION OF HYDRACIDS UPON BASES OF THE MONAD, DYAD, AND TRIAD GROUPS. I i6.. The action of hydrochloric and hydrosulphuric acids upon hydrate of potassium is represented by the following equations: HKO + HCI = KCI + H2O; 2HKO + H2S = K2- + 2H20. The action of these acids upon other bases of the monad group is precisely like their action on hydrate of potassium. CHEMICAL AFFINITY. 85 2. The action of the same acids on hydrate of barium is as follows: H2Ba&2 + 2HC1 = BaC12 + 2H20. H2Bae2 + HS- BaS + 2 H2. Their action upon the other regular bases of the dyad group is the same. 3. Their action upon hydrate of aluminium is as follows: H3AI3 +- 3HC1 = A1CI3 + 3H2. 2H3Af103 + 3H2S = A12S-3 + 6H20. Their action on other triad bases is the same. The action of hydrobromic, hydrocyanic (ioo), hydrofluoric, and hydriodic acids upon bases is like that of hydrochloric acid. It will be noticed in the first of the above equations that only one molecule of the base HK- and one of the at,id HC1 are represented, since only one atom of potassium and one of chlorine are needed to form a molecule of chloride of potassium, KC1. When hydrosulphuric acid, H2S-, is represented as acting on the same base, HKO, two molecules of the base are represented, since two atoms of potassium are required to combine with one atom of sulphur to form a molecule of sulphide of potassium, K2S. Chloride of barium, BaC12, contains two atoms of chlorine; hence two molecules of hydrochloric acid are represented as acting upon one molecule of the base H2Ba-2. But since sulphide of barium, BaS, contains but one atom of sulphur, only one molecule of hydrosulphuric acid, H2S, is represented as acting upon the same base, H2Ba&2. For a like reason, three molecules of hydrochloric acid, HC1, are represented as acting upon one of the base H3Al-3, and three molecules of hydrosulphuric acid, H2S, as acting upon two molecules of H3A1O3. II7. Problems.- 36. Show the action of hydrochloric 86 CHEMICAL AFFINITY. and hydrosulphuric acids upon the bases of the monad, dyad, and triad groups, using the unitary formulas. 37. How much metal in 50 kilogrammes of each binary salt formed? 38. How much iodine in a kilogramme of iodide of potassium? How much chlorine in the same amount of chloride of potassium? BINARY SALTS FORMED BY SUBSTITUTION. 118. Binary salts are also formed by the action (i.) of hydracids upon binary salts, (2.) of binary salts upon each other, (3.) of hydracids upon ternary salts, and (4.) of binary salts upon ternary salts. The following equations will serve to illustrate the different cases: (I.) 2AsCl3 + 3H2S As2S3 + 6HCI; (2.) BiC13 + 3KI = 3KC1-+ BiI3; (3.) Pb(NO3)2 + 2HC1= 2HN03 + PbCI2; (4.) 2AgN-3 + -CaCI2 = -a(N3,)2 + 2AgC1. THE LAW OF DOUBLE DECOMPOSITION. I 9. In the hydrates of acids it is evident that hydrogen plays the part which the metal does in a ternary salt, while in hydracids it plays the part which the metal does in a binary salt. For these and other reasons hydrogen has been regarded by many chemists as a metal. If hydrogen in the two cases just named be regarded as a metal, and if, consequently, the hydrates of the acids and bases be regarded as ternary salts, and the hydracids as binary salts, all the cases of double decomposition given above may be brought under this one general rule: — When two salts on the mixture of their solutions undeigo mutual decomposition, the metals change places. CHEMICAL AFFINITY. 87 As before stated, the salts always undergo this mutual decomposition when an insoluble or volatile compound can thus beformed. I20. Problems. - 39. Show the reaction between nitrate of lead and hydrate of sodium. il- Anychemical change is called a reaction. 40. Show the reaction between nitrate of silver and hydrate of ammonium. 4I. Nitrate of silver and chromate of potassium. 42. Nitrate of lead and chromate of potassium. 43. Mercurous nitrate and chromate of potassium. 44. Nitrate of silver and hydrate of sulphuric acid. 45. Nitrate of lead and hydrate of sulphuric acid. 46. Sulphate of copper and hydrate of potassium. 47. Sulphate of copper and chromate of sodium. 48. Sulphate of manganese and hydrate of sodium. 49. Sulphate of zinc and hydrate of potassium. 50. Nitrate of cobalt and hydrate of sodium. 51. Sulphate of nickel and hydrate of potassium. 52. Sulphate of manganese and carbonate of ammonium. 53. Nitrate of barium and carbonate of ammonium. 54. Sulphate of calcium and carbonate of ammonium. 55. Nitrate of strontium and sulphate of calcium. 56. Nitrate of barium and chromate of sodium. 57. Sulphate of aluminium and hydrate of ammonium. 58. Hydrate of ammonium and sulphate of chromium. 59. Ferrous nitrate and hydrate of ammonium. 60. Hydrochloric acid and nitrate of silver. 6I. Hydrochloric acid and nitrate of lead. 62. Hydrochloric acid and mercurous nitrate. 63. Hydrosulphuric acid and nitrate of silver. 64. Hydrosulphuric acid and nitrate of lead. 65. Hydrosulphuric acid and mercurous nitrate. 66. Nitrate of silver and iodide of potassium. 67. Iodide of potassium and nitrate of lead. 88 CHEMICAL AFFINITY. 68. Iodide of potassium and mercurous nitrate. 69. Stannous chloride (SnC12) and hydrosulphuric acid. 70. Stannic chloride (SnCl4) and hydrosulphuric acid. 7I. Chloride of bismuth and hydrosulphuric acid. 72. Stannous chloride and hydrate of potassium. 73. Chloride of gold and hydrosulphuric acid. 74. Chloride of platinum and iodide of potassium. 75. Chloride of gold and iodide of potassium. 76. Mercuric chloride and sulphide of hydrogen. 77. Chloride of bismuth and sulphuretted hydrogen. 78. Chloride of cadmium and hydrosulphuric acid. 79. Sulphate of copper and hydrosulphuric acid. 80. Chloride of cadmium and hydrate of potassium. 8. Mercuric chloride and iodide of potassium. 82. Sulphate of copper and iodide of potassium. 83. Chloride of cadmium and chromate of potassium. 84. Chloride of aluminium and hydrate of ammonium. 85. Chloride of barium and carbonate of ammonium. 86. Chloride of strontium and carbonate of ammonium. 87. Chloride of calcium and carbonate of ammonium. 88. Chloride of strontium and sulphate of calcium. 89. Chloride of barium and hydrate of sodium. go. Chloride of barium and carbonate of sodium. 9I. Chloride of barium and silicate of potassium. 92. Chloride of calcium and silicate of potassium. 93. Chloride of calcium and carbonate of sodium. 94. Nitrate of silver and carbonate of potassium. 95. Nitrate of silver and silicate of potassium. 96. Chloride of barium and fluoride of potassium. 97. Chloride of calcium and fluoride of sodium. 98. Nitrate of silver and chloride of sodium. 99. Nitrate of silver and bromide of potassium.+ * See Appendix, 28. SUMMARY. 89 SUMMARY. It is possible to decompose water into hydrogen and oxygen; but there is no way known by which hydrogen and oxygen can be decomposed into simpler substances. We therefore divide substances into two classes:ist. Elements, or substances which cannot be decomposed by any known process. Of these only 65 are known. 2d. Compounds, or substances made up of elements. (46-49.) The force which causes elements to combine is called Affnity. Water is wholly unlike either hydrogen or oxygen, or the mere mechanical mixture of the two gases. This illustrates the first characteristic of affinity; that it changes the properties of the substances which it causes to combine. Hydrogen and oxygen always exist in water in the proportion of two parts of hydrogen by volume to one part of oxygen, or two parts of hydrogen by weight to sixteen parts of oxygen. This illustrates the second characteristic of affinity; that it always causes substances to combine in fixed and definite quantities. This law of combination is called the Law of Definite Proportions. (50.) Sodium and potassium in acting upon water set the hydrogen free from the oxygen, and take its place in combination with the oxygen. This illustrates the third characteristic of affinity; that a given element combines with some elements in preference to others.* (5 I.) Potassium or sodium can displace half of the hydrogen from water, or the whole, giving rise to two compounds; the first containing potassium (or sodium), hydrogen, and oxygen; the second, potassium (or sodium) and oxygen. * See Appendix, 20. 90 CHEMICAL AFFINITY. But there is no known substance which will displace any other fraction of hydrogen from a molecule of water, or any fraction of the oxygen without displacing the whole. Hence a molecule of water is made up of three parts which are indivisible by affinity, and which are hence called atoms. A molecule of water is made up of two atoms of hydrogen and one of oxygen. (52.) Matter is, then, divisible in a threefold way:ist. By mechanical means into minute but sensible masses. 2d. By means of heat into insensible portions, called molecules. 3d. By means of affinity into atoms. (53.) If the weight of the hydrogen atom be taken as unity, the weight of the oxygen atom will be i6. Since 23 parts by weight of sodium and 39 of potassium are required to displace one part of hydrogen, the atomic weights of those elements are 23 and 39 respectively. (52.) The composition of substances and the chemical changes which they undergo can be briefly indicated by certain signscalled symbols. (54.) The symbol of an element is the first letter of its name, a second letter being added to distinguish names beginning with the same letter. The symbol of an element always stands for one atom of the element. (55.) The symbol of a compound indicates its composition. It is formed by writing together the symbols of the elements of the compound, with a small figure after each symbol expressing the number of atoms of that element found in a molecule of the compound. The symbol of a compound always stands for one molecule of the compound. (56.) The chemical changes, or reactions, which substances undergo, are indicated by equations made up of these symbols. (57.) On examining the three compounds, muriatic acid, am SUMMARY. 91 monia, and marsh gas, it appears that a molecule of the first is made up of one atom of hydrogen and one of chlorine; that a molecule of the second is made up of three atoms of hydrogen and one of nitrogen; and that a molecule of the third is made up offour atoms of hydrogen and one of carbon. (59 - 74.) The atoms of different elements, then, have power to fix in combination a different number of atoms of hydrogen. Thus, i atom of chlorine can fix i atom of hydrogen; I " oxygen " 2 atoms I " nitrogen " 3 " " I' carbon " 4" " Hence we classify the elements into four groups: Ist. Monatomic elements, or /fonads; those whose atom has the same atom-fixing power as one atom of hydrogen. 2d. Dialomic elements, or Dyads,; those whose atom has the same atom-fixing power as two atoms of hydrogen. 3d. Triatonzic elements, or Triads; those whose atom has the same atom-fixing power as three atoms of hydrogen. 4th. Tetratomic elements, or Tetrads, those whose atom has the same atom-fixing power asfour atoms of hydrogen. (76 - 79.) The compounds of nitrogen and oxygen show that the same elements may combine in more than one proportion, and that in such cases the proportions of the elements in the compounds are always mulfiples of the atomic weight of the elements. This law of combination is known as the Law of Multiple Proportions. (8o- 82.) On examining the compounds of oxygen it is found that they can be arranged in three classes:ist. Acids; which in solution have the power to turn blue litmus-paper to red. 2d. Bases; which in solution turn red litmus-paper to blue. 92 CHEMICAL AFFINITY. 3d. Neutrals; whose solutions have no effect on either red or blue litmus-paper. (84.) Of Acids there are two classes: ist. Oxyacids (or oxygen acids), whose common element is oxygen. 2d. Hydracids (or hydrogen acids), whose common element is hydrogen. (87.) Oxyacids are named from the element with which the oxygen combines; hydracids from both elements. (88, 89.) Hydrogen never forms more than one acid with the same element. Oxygen often forms two or more acids with the same element. When oxygen combines with the same element in several proportions, the higher compounds are likely to be acids, while the lower are either bases or neutrals. (86.) When oxygen forms more than one acid with the same element, they are distinguished by the endings ic and ous, and by the prefixes hypo and hyper, and in some cases by other prefixes. (85.) Bases and neutrals are called oxides; When two oxides are formed with the same element, they are distinguished by the endings ic and ous, and often by means of prefixes. (86.) The oxyacids and the bases are usually combined with water, and are then called hydrates. (94.) The composition of these hydrates can be represented by means of symbols in two ways:Ist. By writing together the symbol of the anhydrous acid or base and the symbol of water, separating the two by a comma. 2d. By writing together the symbols of the elements contained in the hydrate. The first method is called the dualistic method, since it represents the hydrate as formed by the combination of two compounds. SUMMARY. 93 The second is called the unitary method, since it represents the hydrate as a compound containing three elements. (95 -99.) The hydrogen of the hydrate of an acid may be replaced by a metal, giving rise to a compound called a salt. The composition of a salt, like that of a hydrate, may be represented by the dualistic and the unitary methods. The former represents the salt as made up of an anhydrous base and an acid; the latter represents it as a compound of three elements. (103, I04.) The salts which contain three elements are called ternary salts. (107.) Ternary salts may be classified as follows:ist. Normal salts; those in which the hydrogen of the hydrate of the acid is wholly replaced by an equivalent of the metal. 2d. Acid salts; those.which contain less metal in proportion to the oxygen than a normal salt. 3d. Basic salts; those which contain more metal in proportion to the oxygen than a normal salt. (Io8.) Ternary salts are named from the acid and base which combine to form them; by changing the ending ic of the name of the acid into ate, and ous into ite, and adding the name of the metal of the base. (IoS.) Certain groups of elementary atoms act in compounds like single elementary atoms. Such groups of atoms are called compound radicals. Some compound radicals can exist in a free state, others cannot. (Ioo.) When the hydrogen of the hydracids is replaced by a metal, a compound containing two elements is formed, which is called a binary salt. ( 114.) Binary salts-are named from the two elements which combine to form them; by changing the ending of the name of the non-metallic element into ide, and adding the name of the metal. (115.) The hydrates of the acids may be regarded as ternary salts 94 CHEMICAL AFFINITY. in which hydrogen plays the part of a melal; and the kydracids may be regarded as binary salts, in which hydrogen is the metal. (io6, Io7, I19.) When the solutions of the salts are mixed, mutual decomposition takes place whenever an insoluble or gaseous compound can thus be formed. In this decomposition the metals of the salts merely change places; hydrogen being regarded as a metal. Hence the decomposition is said to take place by substitution; and, since both the original salts are decomposed, it is called double decomposition. (III, II2, II9.) CHEMICAL AFFINITY. 95 COMBUSTION AND ITS PRODUCTS. I2I. When a lighted taper is held to a jet of ordinary coal-gas, the latter takes fire. In what does the burning of the coal-gas consist? I22. The Products of the Burning of Coal-Gas are Car-. bonic Acid andi Water. - We will first find what is produced when coal-gas burns. Invert a bottle over a gas-burner a short time; then remove it, pour in a little lime-water, close the mouth of the bottle with the hand, and shake it. The lime-water becomes milky-white. If lime-water is shaken in a bottle filled with air, it remains unchanged. Some new substance then has been formed by the burning of the gas. Fasten a bit of charcoal to a wire, light it, and plunge it into a jar of oxygen. The charcoal glows brightly for a short time, and is then extinguished. If now we pour some lime-water into the jar and shake it, the liquid becomes milky-white; showing that the same substance is produced when carbon burns in oxygen as when gas burns in the air. This substance is of course a compound of carbon and oxygen, and since it will turn moistened blue litmus-paper to red, it is called carbonic acid. (84, 89.) Carbonic acid, then, is one product of the burning of coal-gas. If a jet of coal-gas be burned under a tin funnel connected with a long glass tube (which must be kept cold), moisture will soon collect on the inside of the tube. After a short time, enough will have collected to trickle down 96 CHEMICAL AFFINITY. and drop from the end of the tube. If a bit of potassium be put into this liquid, it will burn with a rose-colored flame; showing that the liquid is water. Fig. 33Water, then, is another product of the burning of coal-gas. Water, then, is another product of the burning of coal-gas. It has been found that no other substance is produced, except in very minute quantities, by the burning of coal-gas. 123. Does Coal-Gas in burning remove anything from the Air? - Arrange a gas-burner so that it can be covered with a bell-jar whose mouth dips beneath the surface of water.* If now we light the gas and cover it with the jar, it soon ceases to burn, and the water rises in the jar. The coalgas, then, in burning, does remove something from the air. 124. The Coal-Gas removes Oxygen from the Air. — InFig. 34. vert a jar of air over a jar of nitric oxide, and the gases on mixing become cherry-red, showi' ing that there is oxygen in the air (80, 8i). Burn a piece of phosphorus in a jar of air over water.+ The jar is filled with dense white fumes, which are soon absorbed by the water, which rises and partially fills the jar. j Introduce now some nitric oxide into the jar while still over the water, and the gas shows scarcely a trace of red color. It is evident, * See Appendix, 21. t See Appendix, 22. CHEMICAL AFFINITY. 97 then, that the free oxygen has been almost wholly removed from the air by the burning phosphorus. Introduce a jet of burning coal-gas into air from which the oxygen has been removed in this way, and it is at once extinguished. Coal-gas in burning, then, removes oxygen from the air. 125. The Burning of Coal-Gas consists in the Combination of thefree Oxygen of the Air with the Carbon and Hydrogen of the Gas. - Carbonic acid, one of the products of the burning of coal-gas, is, as we have learned (I22), a compound of carbon and oxygen; water, the other product, we know to be a compound of hydrogen and oxygen. Of the three elements in these produits, the oxygen, as we have seen, comes from the air; the carbon and hydrogen exist in the coal-gas. The force, then, which causes the coal-gas to burn, is affinity, and the burning consists in the combination of the oxygen of the air with the elements of the gas. 126. All ordinary Combustion consists in the Combination of the Oxygen of the Air wizh the burning Substance. - We have already seen that carbon and phosphorus, as well as coal-gas, in burning combine with oxygen. If any ordinary combustible substance, such as a taper, wax, or wood, be ignited and plunged into a jar of air from which oxygen has been removed, it is instantly extinguished; showing that it cannot burn without a supply of oxygen. And if we burn any of these substances in a jar inverted over water, we see, as in the case of the phosphorus (124), that they remove something from the air. We therefore conclude that all ordinary combustion consists in the com-bination of the oxygen of the air with the burning body. 127. Why a Draft is necessary in Stoves and Furnaces. Since combustion is the combination of oxygen with the burning body, we see why our stoves and furnaces must have a draft. As fast as the oxygen is taken from the air 5 G 98 CHEMICAL AFFINITY. by the burning fuel, this air must be removed, and a fresh supply must take its place. In other words, a stream of air must be kept constantly flowing over or through the fuel. We see, also, how the fire can be regulated by means of the draft. If the doors or dampers through which the air is admitted be partially closed, the supply of air will be diminished, and the burning will therefore be retarded. 128. Combustibles and Szfiporlers of Combustion. - Any substance, as coal-gas, which can be made to burn, is called a combustible; while any substance, as air or oxygen, in which it can burn, is called a supporter of coamsbtion. These terms are convenient, though, strictly speaking, the one substance is no more a combustible or a supporter of combustion than the other. Since the burning of coal-gas consists in the combination of the gas with oxygen, the oxygen in reality burns, as well as the gas; and, on the other hand, the gas is as much a supporter of the combustion as the oxygen. The burning must of course take place where the gases come together. A jet of oxygen would appear to burn in an atmosphere of coal-gas, just as a jet of coal-gas appears to burn in an atmosphere of oxygen. Fig. 35. Fit a cork to one i -1 / end of a lamp chimi ney, and let the tip of a gas-burner pass through it, as repJ.1j ^ resented in the figI~^:'1!'S{L ure. Allow the gas ll' l to escape for some ii jF time, and then light it~ll i l it at the top of the I: iiI chimney, It will burn quietly, and the chimney will evidently be filled with coal-gas. Fill a gasbag with oxygen, and fasten to the bag a bent glass tube CHEMIICAL AFFINITY. 99 drawn out into a fine jet. Force the oxygen through the tube in a gentle stream, and introduce the end of the tube through the flame into the chimney. As it passes the flame, the oxygen takes fire and burns brightly in the coal-gas; the oxygen apparently becoming the combustible body, and the coal-gas the supporter of combustion. In both the flames which we have here, it will be seen that gases are burning where they come together; -the coal-gas and the oxygen of the air, where they meet at the top of the chimney; the oxygen from the bag and the coalgas, where they meet at the end of the tube. I29. What Fraction of the Air is free Oxygen? When phosphorus is burned in a jar of air over water, it removes, as we have seen, the free oxygen from the air, and the water rises in the jar to take its place. After all the oxygen has combined with the phosphorus, and the compound formed has been absorbed by the water, the jar is found to be about one fifth filled with water; showing that about one fifth of the air is oxygen. 130. Oxygezn muzzst be Heated before it zuill combine with ordinary Comnlustibles. ~ The jet of coal-gas shows no disposition to burn until a lighted taper is applied to it. The oxygen of the air is at all times in contact with wood and coal, yet they do not burn unless they are first kindled. When even as inflammable a gas as hydrogen is mixed with oxygen, it does not burn unless ignited with a taper or an electric spark (50). At ordinary temperatures oxygen is one of the most passive of substances; but when heated it becomes very active. Its affinity is dormant until it is heated, when it is aroused to the intensest energy. When the lighted taper is held to the coal-gas, it heats the molecules of oxygen in contact with the gas, and rouses them to activity. These molecules then rush into combination with the gas with sufficient energy to develop the light and heat of the flame. I00 CHEMICAL AFFINITY. The passive condition of oxygen at ordinary temperatures, and the energy with which it rushes into combination when once aroused by heat, are shown by the following experiments. Fill a rubber bag with a mixture of two measures of hydrogen and one of oxygen.* Attach a common clay pipe to the bag by a rubber tube, and blow some soap-bubbles with this mixture. Apply a lighted taper to these bubbles to heat the molecules of oxygen within. A violent explosion follows, showing with what energy the oxygen combines with the hydrogen when its affinity is once roused to activity. Fig. 36. Fill one of the bell-jars of the gas-holder represented in the figure with oxygen, and the other with hydrogen. I ll'_!,! )iiii,iiir)a, Connect the bell-jars with a burner by means of rubber tubes. Tahis burner is so s a pe s g l constructed th at it allows the hydrogen to pass through it watc-spring burns with brilliant scintillations. If a bit of zinjust twice as fast as the oxy-!...~purpose. gen, and the gases can mix' I$ only at the end of the jet, as shown in Figure 37. Force,I".': the gases through the tubes by placing weights on the - ~LL~~I bell-jars, and ignite the mixture as it escapes. Hold a copper or iron wire in this flame, and it burns as readily as a pine shaving held in the flame of a lamp. A steel watch-spring burns with brilliant scintillations. If a bit of zinc is placed on a piece of charcoal hollowed out for the purpose, and this flame is directed upon it, the metal quick* See Appendix, z3. CHEMICAL AFFINITY. IO ly melts and burns. Antimony, bismuth, and many other metals, will burn in the same way, each with a characteristic light. Cast-iron burns with a shower of bright Fig 37. sparks. The flame produced in this way is called the ",' " oxy-hydrogen flame. Its intense heat, which is evident from the experiments just given, shows the energy with which the oxygen combines with the hydrogen. 13I. Combustion is Self-sustaining. - It is not necessary to arouse any large amount of oxygen to activity in order to insure the continuance of the combustion. When, for instance, a lighted match is held to the wick of a candle, it excites but a few molecules of oxygen to activity. These few rush into combination with the elements of the candle, and by so doing develop sufficient heat to awaken the activity of more oxygen, which in turn enters into combination and develops more heat. In this way a supply of active oxygen is maintained until the candle is consumed. 132. The Point of Izniion. - Different substances begin to burn at very different temperatures. This is well illustrated in the kindling of a coal fire. Shavings are put into the grate first, then kindling-wood, then charcoal, and finally hard coal. The shavings are lighted by means of a match. The match is a bit of dry, soft wood, one end of which is covered with sulphur and tipped with phosphorus. It is a well-known fact, that when two bodies are rubbed together heat is developed. On striking the match sufficient heat is developed by the friction to ignite the phosphorus, which takes fire at a temperature of about I5o~ Fahrenheit. The 102 CHEMICAL AFFINITY. phosphorus in burning develops heat enough to ignite the sulphur, which burns at a temperature of about 500~. The burning sulphur develops heat enough to ignite the wood of the match; the match, to ignite the shaving; the shaving, the kindling-wood; the kindling-wood, the charcoal; and the charcoal, the hard coal, which requires the temperature of a full white heat to set it on fire. 133. The Products of Combustlion are not zalways Gaseous. - In the burning of coal-gas and of a candle, the products are wholly gaseous, and in the burning of wood they are mainly gaseous; but when metals, as copper and iron, burn in the oxy-hydrogen flame, the products of their combustion are seen to be solid. 134. The Burning of Melals in the Oxy-hydrogen Flame consists in their Combination wilh Oxygen. - Bend a steel watch-spring into the form of a spiral, fasten a bit of wood to one end of it, light the wood, and plunge the spring into a jar of oxygen.' The steel takes fire, and burns with bright scintillations. The molten product of the combustion falls into the water in the bottom of the jar. If this product be carefully collected and weighed, it will be found to weigh more than the spring did at first. It is evident, then, that the iron in burning combines with oxygen. When an iron wire is burned in the oxy-hydrogen flame, if all the products were collected and weighed, they would be found to weigh more than the wire; and since iron cannot be made to burn in hydrogen or in the air from which oxygen has been removed, we conclude that in burning in this flame it combines with oxygen. If a piece of potassium or sodium be placed in a deflagrating spoon,t heated, and plunged into a jar of oxygen, it will burn brightly for a time. The product of the combustion is a white solid, which by the usual tests (84) we find to be oxide of potssium or sodizum. * See Appendix, 24. t See Appendix, 25. CHEMICAL AFFINITY. 103 135.,a(;gnesium, and some other Aletals will burn in the Air. - A magnesium wire will burn brightly in the air. It may be lighted with a match. Calcium, also, burns readily in the air; as aluminium does if pulverized and strongly heated. Silicon, also, a non-metallic element, but somewhat like a metal in appearance, burns very readily in the air. It will be noticed that all the elements just mentioned are rare in their free state. Many of the rare metals, as potassium, sodium, magnesium, and calcium, are as combustible as carbon; but they differ from carbon in giving rise to solid products while burning (133). 136. Oxygen is not the only Szpporler of Combustion. — If a piece of gold-leaf or Dutch foil be dropped into a jar of chlorine, it disappears with a flash of light. Here the burning consists in a combination with chlorine. Tin and copper foil and pulverized antimony will also burn in chlorine. Many metals will burn in the vapor of sulphur; the burning being then a combination of the metal with sulphur. 137. 7hZT Materials of the Eart/'s Crust are chieJy Chemical Compozunds.- If bits of marble are put into a bottle and hydrochloric acid poured over them, a violent effervescence takes place, and a gas is set free which may be colFig. 38. lected over water. If lime-water be poured into a jar of this gas and shaken, it turns milky-white, showing the gas I04 CHEMICAL AFFINITY. to be carbonic acid (122). Marble, then, must contain carbonic acid, since hydrochloric acid contains only hydrogen and chlorine. If the liquid which remains after the marble has been acted upon by the hydrochloric acid be evaporated to dryness, a white solid is obtained. This solid has been decomposed by the electric current into cjzlorine and a yellow metal called calcium; showing that it is a compound of chlorine and calcium. The chlorine evidently comes from the hydrochloric acid, and the calcium must have come from the marble. Marble, then, contains carbonic acid and calcium, and the chlorine of the hydrochloric acid in acting upon the marble sets tie carbonic acid free. The existence of the metal calcium and of carbonic acid in marble seems to indicate that marble is carbonats of calcium. If so, its composition would be -Ca-O3, and the action of hydrochloric acid upon it would be expressed by the equation, -Caf-30 + 2HCI = -CaCI, + He2m + -CO 2Careful analysis has shown that marble is almost pure carbonate of calcium. We find, then, in marble two very combustible substances, calcium and carbon, combined with oxygen. Limestone has the same composition. Calcium, then, which is a very great rarity in a free state, is a very abundant element in nature. Its great rarity in a free state is due to its very great combustibility. It is almost impossible to separate it from oxygen, and, when once separated, to keep it from combining with oxygen again. It has been found that all the rocks and solid matter of the earth are chemical compounds. The rocks are made up chiefly of such combustible elements as potassium, calcium, magnesium, aluminium, and carbon, combined with oxygen. Many of the rarest and most costly metals, then, are the most abundant in nature. The fact that they are CHEMICAL AFFINITY. 105 so rare in a free state is due to their extreme combustibility. 138. Tze Present Materials of the Earth are Products of Combustion. - We see, then, that water, the most abundant of liquids, is made up of the very combustible elements hydrogen and oxygen; while the solid materials of the earth are composed chiefly of the very combustible elements potassium, magnesium, calcium, aluminium, carbon, and silicon, in combination with oxygen. These materials are the products of combustion. There must have been a time when these elements existed together in a free state. Then, by some means unknown to us, the mass took fire, and the conflagration raged until all the materials were consumed. There was more oxygen than was needed for the combustion, and this is now found in the air in a free state. Oxygen was the most abundant of all the elements, since it alone makes up half the weight of the solid earth, eight ninths the weight of the water, and one fifth the weight of the atmosphere. Most of the materials of the solid earth are, as has been stated, binary or ternary compounds of oxygen. Some of the metals are found combined with chlorine and sulphur. This is as we should expect, since many of the metals can burn in chlorine or in the vapor of sulphur (I36). 139. Free Oxygen is not necessary to the Support of Combustion. -We have seen (46)' that potassium and sodium will burn on water. Here the combustion is supported by the combined oxygen of the water. If a mixture of pulverized charcoal and saltpetre be heated in a small crucible, the charcoal burns with great brilliancy. About half the weight of saltpetre is oxygen. When heated with carbon or other combustible substances, it gives up a part of its oxygen very readily. The oxygen thus set free. rushes into combination with the combustible substance, causing it to burn vigorously. 5* io6 CHEMICAL AFFINITY. Gunpowder is an intimate mixture of pulverized charcoal, sulphur, and saltpetre. A moderate heat causes the carbon and oxygen to combine and the powder to explode. If zinc be melted in an iron ladle and powdered saltpetre be thrown upon it, the zinc will burn with great brilliancy. Here, also, the oxygen is supplied by the saltpetre. 140. Reduction of Metallic Ores by means of Carbon. - The affinity of carbon for oxygen at a high temperature is so great, that this element will burn when strongly heated with the metallic oxides. The carbon in burning combines with the oxygen of the oxide, and leaves the metal in a free state. Advantage is taken of this fact in the reduction of metallic ores. The metals, as has been stated, are found in nature chiefly in combination with oxygen, sulphur, and chlorine. These compounds are called ores. The oxides are commonly reduced to the metallic state by heating them with carbon. The sulphides are first roasted; that is, heated while exposed to the air. The oxygen of the air combines with the sulphur of the ore to form a gas called su4/hurous acid, and the metal also combines with the oxygen of the air to form the oxide of the metal. This oxide is then reduced to the metallic state by heating it with carbon. The roasting is done in what is called a reverberatory Fig. 39. CHEMICAL AFFINITY. Io7 furnace, represented in Figure 39. The ore is put into the hoppers H y;, from which it falls into the chamber C, where it is spread out on the bed c c. The fuel is burned on a hearth at A, separated from the ore by the bridge b. The heated gases rising from the burning fuel are reverberated, or reflected, by the arched roof of the furnace, and driven down upon the ore, and then pass off through the flue f. When the ore is sufficiently roasted, it is allowed to fall through openings, d d, into the chamber E. The ore is stirred from time to time to expose fresh surfaces to the action of the air and the flame. The reduction of the metallic ores is best illustrated by the reduction of iron ore. The bast furnace employed for this purpose is rep- Fig. 40. resented in Figure 40. These furnaces are usu- ally about fifty feet high, and about fifteen feet in i diameter in the widest part of the cavity C D. - The lowest part, F, is called the crzucible, or Am hearth. I I are the /111ll' zfuyeres, or pipes through which air is forced by powerful bellows. KK and AL are arched gal- N leries for thle conven- ience of workmen em- i ployed about the furnace. When working regularly, the furnace is charged from a door at the end of the gallery near the top, first with coal, and then with a mixture of roasted ore and limestone broken into Io8 CHEMICAL AFFINITY. small pieces. As the fuel burns away and the materials gradually sink, fresh supplies of fuel and of ore are added; so that the furnace is kept filled with alternate layers of each. The oxygen of the air from the bellows combines with the carbon of the fuel, forming carbonic oxide, which rises through the porous mass, and, taking the oxygen from the ore, becomes converted into carbonic acid. The iron mixed with the earthy matter of the ore settles down into the hottest part of the furnace, where both are melted. The iron, being the heavier, sinks to the bottom, where it is drawn off at intervals through a tap-hole in the floor H. The lighter earthy matter, or slag, floats on the surface of the iron, like oil on water, and flows off through an opening above the lymp-stone I. The limestone aids in liquefying the earthy matter, and unites with it to form the slag. The separation of metals from the ores, then, is seen to be a process of combustion. If the ore be a sulphide (or an arsenide) of the metal, this is first burned at the expense of the oxygen of the air, and the sulphur (or arsenic) is converted into sulphurous (or arsenious) acid, and the metal into an oxide. This oxide (or the original ore, if it is an oxide) is mixed with carbon and heated, and the carbon burns at the expense of the oxygen of the ore, and the metal is left free. I41. Slow Combuslion. - We have already seen that when iron and other metals are burned in the air or in oxygen, they are converted into oxides. If potassium or sodium be exposed to the air, they soon become coated with a white solid, which is found to be the same as is formed when they are burned in oxygen. This change, or oxidation, has taken place quietly, without development of light, and apparently without development of heat. So, too, when iron is exposed to moist air, it becomes CHEMICAL AFFINITY. 109 covered with a grayish red film, which resembles the powder which collects on the sides of the jar when iron is burned in oxygen. It is, in fact, the same substance. Here, too, the oxidation takes place quietly, and no light is developed, and apparently no heat. This very gradual burning of a substance without development of light, is called slow combustion. When the metals are thus slowly oxidized, the process is called rustizng, and the oxide formed is called rust. All the familiar metals, except silver, gold, and platinum, are tarnished on exposure to the air; that is, they become covered with a film of rust, or oxide. 142. RuZsting is attended with/ Development of Ieat. -This slow oxidation of the metals, as has been stated, takes place apparently without development of heat. But heat is really developed during rusting, though usually so slowly that it is not perceived. If a large pile of iron-filings be moistened and exposed to the action of the air, they rust rapidly, and the temperature rises perceptibly. A remarkable case of heat developed by rusting occurred in England during the manufacture of a submarine electric cable. The copper wire of the cable was covered with gutta-percha, tar, and hemp, and the whole enclosed in a casing of iron wire. The cable as it was finished was coiled in tanks filled with water; these tanks leaked, and the water was therefore drawn off, leaving about 163 nautical miles of cable coiled in a mass 30 feet in diameter (with a space in the centre 6 feet in diameter) and 8 feet high. It rusted so rapidly that the temperature in the centre of the coil rose in four days from 66~ to 79~, though the temperature of the air did not rise above 66~ during the period, and was as low as 590 part of the time. The mass would have become even hotter, had it not been cooled by pouring on water. 143. Decay is Slow Combustion. - Light a splint of IIO CHEMICAL AFFINITY. wood and hold it while burning under a funnel held in an inverted bottle. After a little time pour lime-water into the bottle and shake it. The lime-water becomes milkywhite; showing that carbonic acid has been produced by the burning of the wood. Hold a cold glass vessel over burning wood, and moisture collects on the inside; showing that water is produced by the combustion. Carbonic acid and water, then, are two of the products of ordinary combustion. Nitrogen has been found to be another product. Put some peas in a flask, cover them with water, and connect the flask by a glass Fig. 4'. tube with a second containing i^mh li ~a little water. The end of the tube dips under the water in the second flask. Let the.;^ _..... flasks stand for some time in a moderately warm place, and ^~^~-^= —r_^=- = bubbles of gas will be seen to escape into the second flask. If this gas is tested with lime-water, it is found to be carbonic acid. After the peas have soaked a considerable time in the water, remove them and add hydrochloric acid to remove all vegetable matter. Filter a portion of the liquid into a test-tube, add oxide of sodium, and cover the mouth of the test-tube with moistened red litmus-paper. Heat the liquid, and the litmus-paper becomes blue; showing that the liquid contains ammonia. Carbonic acid and ammonia, then, are two of the products of the decay of the peas. Water is known to be another product. We see, then, that the products of the ordinary combustion and of the decay of vegetable substances are nearly CHEMICAL AFFINITY. III identical; - the former being chiefly carbonic acid, water, and nitrogen; the latter, carbonic acid, water, and ammonia. Hence we conclude that combustion and decay are analogous processes; decay being, in fact, a kind of slow combustion. I44. The Decay of Vegetable Substances develops Heat. The oxidation of dead vegetable substances takes place so slowly at the ordinary temperatures, that the heat developed is not perceptible. When, however, charcoal which has been finely pulverized for making gunpowder is exposed in large heaps, the oxygen of the air combines with it slowly at first; but, as the heat developed accumulates, the oxidation becomes more rapid, until in some cases the mass takes fire and burns. So too, when cotton or tow, which has been used for wiping machinery, and has become saturated with oil, is laid aside in heaps, it begins to oxidize slowly; but the heat developed causes the combustion to go on more and more rapidly, until sometimes the heap bursts into a flame. This rapid combustion, developed gradually from slow combustion, is called spontaneous combustion. RESPIRATION. I45. We have seen that, at a high temperature, vegetable substances combine rapidly with the oxygen of the air, developing intense light and heat, and giving rise to the gaseous products water, carbonic acid, and nitrogen; and that dead vegetable substances, at the ordinary temperatures, combine slowly with the atmospheric oxygen, developing no light and little heat, and giving rise to the gaseous products water, carbonic acid, and ammonia. But it is well known that a large amount of vegetable matter is consumed as food by man and other animals. AWhat becomes of this vegetable matter? 112 CHEMICAL AFFINITY. 146. Animals in Breathing remove Oxygen from the Air. It has been found that if a small animal, as a rat or a mouse, is placed on a float on the surface of water, and a bell-jar of air is placed over it, the water slowly rises into the jar; showing that the animal in. breathing removes something from the air. After a time, the animal will die; showing that the air has become unfit for respiration. It has also been found, that, if a burning candle be put under the bell-jar with the rat, he will not live half so long as when under the jar alone; showing that animals in breathing remove the same substance from the air as bodies in burning, namely, oxygen. I47. Carbonic Acid is a Product, but not the only Product, of Respiration. - If we breathe through a glass tube into lime-water, the liquid becomes milky. Hence, carbonic acid is one of the products of respiration. It has been seen that carbon, when burnt in oxygen, gives rise to carbonic acid. Now it has been found that, when carbon is burned in a jar of oxygen, the volume of the gas is neither increased nor diminished; that is, when oxygen and carbon are converted into carbonic acid, the carbonic acid occupies just the same space as the oxygen which it cGntains occupied when in a free state. But when a rat breathes in confined air, it has been seen that the volume of the air is diminished. It is evident, then, that the oxygen removed from the air in respiration does not all reappear in the carbonic acid sent out in the breath. A part of the oxygen taken from the air appears in wzater, which is a second product of respiration; as may be shown by breathing upon any cold substance. A portion of the products of respiration are removed from the body in a liquid state, by the kidneys. The liquid is found to be highly charged with luea, a compound which contains nitrogen, and which, by the action of the air, gives rise to ammonia and carbonic acid. CHEMICAL AFFINITY. I I3 148. The Products of Respiration are nearly the same as those of the Combustion and the Decay of Vegetable Substlzces. - We see, then, that of the three chief products of the rapid combustion of vegetable substances, of the decay of such substances, and of respiration, two are identical, namely, water and carbonic acid; while the third product, in the first of these processes, is nitrogen; in the second, ammonia, a compound of nitrogen and hydrogen; and in the third, zrea, a compound of carbon, oxygen, hydrogen, and nitrogen..149. Respiration is a Slow Combustion. - Respiration is thus seen to be a process of slow combustion, and the warmth of the body is due to the heat developed by this combustion. COMPOSITION OF VEGETABLE AND ANIMAL SUBSTANCES. I50. From the products of the different kinds of combustion of vegetable substances, it is clear that the three elements, carbon, hydrogen, and nitrogen, are found in these substances. The presence of hydrogen and nitrogen may be shown by heating some vegetable substance, as beans or macaroni, in a test-tube, with hydrate of sodium or hydrate of potassium. If moistened red litmus-paper is held at the mouth of the test-tube, it is at once turned blue. This proves that ammonia has been produced from the vegetable matter. Ammonia, as we know, contains hydrogen and nitrogen; hence the vegetable matter must have contained those elements. The existence of carbon and oxygen in vegetable substances may be shown by taking any one of the three most abundant vegetable substances, - starch, sugar, and woody fibre, - and heating it in a test-tube, closed by a cork, with H I 14 CHEMICAL AFFINITY. a glass tube passing through it, and conducting the gas produced into a bottle of lime-water. The latter becomes milky, showing that carbonic acid has come from the test-tube. As the same product is obtained when the test-tube is first filled with nitrogen instead of air, we must conclude that both the carbon and the oxygen of the carbonic acid came from the vegetable matter. Hence, vegetable substances contain four elements, carbon, hydrogen, oxygen, and nitrogen. They are composed chiefly of the three, carbon, hydrogen, and oxygen. Woody fibre, starch, and sugar contain only these three elements; but we find in every part of plants small quantities of another class of compounds, containing the four elements just mentioned, and called nitrogezozus or nitrogenized compounds, from the fact that they contain nitrogen. They are also sometimes called quaterznary compounds (that is, compounds made up of four elements), to distinguish them from the other, which are ternary compounds. The nitrogenized compounds are found most abundantly in the seeds of plants, such as peas, beans, grain, etc. It appears, then, that in rapid combustion the destruction of vegetable substances is most complete. All the carbon and the hydrogen are separated from the nitrogen, and converted into carbonic acid and water. In the case of decay, the destruction is less complete. All the carbon is separated from the nitrogen and converted into carbonic acid, and all the oxygen also is separated from the nitrogen; but a part of the hydrogen remains combined with the nitrogen as ammonia. In respiration, the destruction is even less complete, since portions of the carbon, hydrogen, and oxygen remain combined with nitrogen in the form of urea. I51. Animal Substances also conainz Carbon, Hydrogen, Nitrogenr, and Oxygen/. - Animal substances are also found to contain the same four elements which exist in vegetables. CHEMICAL AFFINITY. II5 This is what we should expect, since all animals live either upon vegetables, or upon the flesh of other animals which have lived upon vegetables. Animal substances differ from vegetable, in being mostly quaternary compounds. Nitrogen is one of the most abundant elements in animal compounds. CONDITION OF OXYGEN DURING DECAY AND RESPIRATION. 152. We have seen that the oxygen of the air at ordinary temperatures is wholly passive, and that during rapid combustion it is wholly active, and that it is roused to activity by means of heat. During decay and respiration the oxygen of the air is in an intermediate state, or one of partial activity. 53. iMoisture is necessary to the Beginlning of Decay. How then is oxygen roused to this state of partial activity? It is found that, when wood is kept perfectly dry, it may be preserved for any length of time without showing the least disposition to decay. Moisture is necessary to cause the decay to begin. The nitrogenized compounds, which exist in small quantities in all parts of vegetables (I50), are so unstable, that the mere presence of moisture is sufficient to cause them to break up into simpler compounds. The oxygen of the air takes no part in this process, but the molecules of oxygen, in contact with the compounds, appear to be rendered partially active by their breaking up. Being thus made active, they combine with the carbon and hydrogen of the ternary vegetable compounds, and the decay begins. Once begun it is self-sustaining, like ordinary combustion (I3I). The molecules of the oxygen about the decaying substance are rendered partially active by the process of decay. So, too, in respiration, the oxygen seems to be roused to partial activity by the slow combustion taking place within the blood. I 6 CHEMICAL AFFINITY. I54. Ozone. - If one or two sticks of phosphorus, carefully scraped clean, are put into a jar (which must be perfectly free from grease) and partially covered with water, they are slowly oxidized. If the jar be loosely covered and allowed to stand for two or three hours, and the air inside be then examined, it will be found to have undergone a remarkable change.* Dip a piece of unsized paper first into a thin solution of starch, and then into a solution of iodide of potassium. If this paper be exposed to the air for hours, its color will scarcely change at all; but if it be moistened and introduced into the jar in which the phosphorus has been undergoing slow combustion, it instantly becomes of a deep blue color. If a piece of silver-foil be put into a jar of this air, it soon crumbles with rust, though silver is scarcely tarnished by long exposure to ordinary atmospheric oxygen. Hence, we see that the oxygen inside the jar has been rendered partially active by the slow combustion of the phosphorus. Oxygen in this state is called ozone. There are several ways by which oxygen can be converted into ozone, but the most common way is by slow combustion. It would seem that during all processes of decay oxygen is converted into ozone. In what way ozone differs from ordinary oxygen has not yet been well established. 155. Allotropic States. There are several elements which can exist in states wholly unlike in appearance and in many of their properties. No three substances could be more unlike in appearance than diamond, graphite, and charcoal. Yet each of the three is nothing but carbon, as can be proved by burning each in oxygen gas,t when they all give rise to one and the same product, carbonic acid. So, too, when sulphur is heated, it first melts into a lim* See Appendix, 26. t See Appendix, 27. CHEMICAL AFFINITY. 17 pid, light-colored liquid; then becomes dark and thick; and then light and limpid again. If now it be suddenly cooled, by pouring it in a fine stream into cold water, it becomes soft and plastic, like wax, while ordinary sulphur is hard and brittle. If left to itself, this plastic sulphur returns after a time to its ordinary hard and brittle state. These different states of the same element are called allotropic states. The fact that the same element can exist in states so wholly unlike, is. attracting a good deal of attention among scientific men. It seems to point to the conclusion that all the elements may after all be only allotropic states of a single element, or at most of a few elements. THE GROWTH OF PLANTS. 156. The Constituents of the Air are Constant. - Notwithstanding that enormous quantities of oxygen are daily removed from the air in the processes of combustion, decay, and respiration, and that the oxygen is in each of these cases replaced by carbonic acid and watery vapor, the relative quantities of oxygen, carbonic acid, and watery vapor in the atmosphere are nearly constant. The amount of each of these substances found in the air in different places, and at different times, varies but slightly. How is this to be explained? 157. Rain and Dew. - The amount of watery vapor which the air at any given temperature can hold is limited. When the air has taken up all the vapor it can hold, it is said to be saturated. As soon as the air has become saturated, the watery vapor begins to return to the liquid state in the form of rain and dew. It is thus prevented from accumulating to any great extent in the atmosphere. 58. Plants in growing remove Carbonic Acid from the Air, and replace it with Oxygen. - If a leafy plant be placed un 118 CHEMICAL AFFINITY. der a glass vessel and set in the sunshine, and a stream of carbonic acid be made to pass slowly over it, and the gas, as it escapes from the vessel, be collected and examined, it will be found that a part of the carbonic acid has been removed and replaced by oxygen. While, then, the destruction of plants, by the various processes of combustion, decay, and respiration, is removing oxygen from the air and pouring carbonic acid into it, plants in growing are silently removing this carbonic acid from the air and replacing it with an equal bulk of oxygen; and since plants grow as rapidly as they are destroyed, the relative quantities of oxygen and carbonic acid in the air remain unchanged. I59. The Growtz of Plants is a Chemical Process. -We have seen that growing plants are continually removing carbonic acid from the air, and giving out oxygen. We conclude, then, that plants derive their carbon from this carbonic acid. In the leaves of the plant, under the influence of sunlight, the carbonic acid is decomposed, the carbon stored away in the plant, and the oxygen given back to the air. We have already noticed that the most abundant metals are very rare and costly substances in their free state, owing to the difficulty of separating them from the oxygen or chlorine with which they are combined. And it is only by the aid of such rare metals as potassium and sodium, which have a very strong affinity for oxygen, that we can separate carbon from its oxygen in carbonic acid. Hence, if we were obliged to obtain carbon by separating it from its combination with oxygen, this useful element would be as rare and as costly as potassium and sodium. And at best we can obtain carbon from carbonic acid only by removing the oxygen and locking it up in combination with some element which has a stronger affinity for it. The chemist knows no way of separating the carbon, and letting CHEMICAL AFFINITY. 9 the oxygen go free. But this is continually done by the sunbeam in the delicate laboratory of the leaf. Plants thus obtain their carbon from the carbonic acid of the air. It is pretty well established, also, that they obtain their hydrogen, oxygen, and nitrogen from the two compounds water and ammonia, which, as we have seen, are products of their decay. The ammonia is washed out of the air by the rain, and, thus dissolved in water, is taken up by the roots of plants, and conveyed thence to the leaves. It is probable that plants obtain a part of their carbonic acid also through the roots, since this substance is soluble in water. The growth of plants is thus seen to be a chemical process, just the opposite of that of their decay. The three compounds carbonic acid, water, and ammonia are decomposed in the leaves of the plant; a part of the oxygen is given back to the air; and the remainder of the oxygen and the other elements are rearranged so as to form the various vegetable compounds which serve as food for man and other animals, and as fuel for our fires. I60. lNitrogen in the Air. -The oxygen may be removed from the air by means of phosphorus, as we have seen, or by passing the air through a red-hot iron tube filled with copper turnings. At this high temperature the copper combines with the oxygen. If the air from which the oxygen has been removed be passed through lime-water, and then through a tube filled with fused chloride of calcium (59), the carbonic acid and the watery vapor will be taken from it. After doing this we still have about four fifths of the air left, and this large remainder we find to be pure nitrogen. We have already seen (I29) that about one fifth of the air is oxygen. It is evident, then, that there is but a small amount of watery vapor and carbonic acid in the air at any one time, though immense quantities of both are continually passing through it. 120 CHEMICAL AFFINITY. 161. Why Nitrogen can exist in the.Air in a Free Slate. It is at first surprising to find a free element in the air along with oxygen, especially when we remember that the present materials of the earth are the result of a great conflagration, in which oxygen was the chief agent. But we remember that the leading characteristic of nitrogen is its inertness. It will neither burn itself, nor allow a body to burn in it. It is owing to this inertness that nitrogen can exist in the air in a free state. It is with the greatest difficulty that nitrogen can be made to combine directly with any element except boron. Its inertness also explains the fact that nitrogen is found to so small an extent in the compounds of the earth. It exists in the solid crust of the earth only in the nitrates of potassium and sodium, and these nitrates we find only in small quantities. Nitrate of sodium is found chiefly in Peru, and nitrate of potassium in India. These salts are of the greatest importance, since it is from them that nitric acid and the various nitrates are prepared. * The chief use of nitrogen is to moderate combustion. Were not the activity of oxygen tempered by the inactivity of nitrogen, it would be impossible to confine or control our fires. Our stoves would burn as readily as our coal now does. 162. Atmospheric Elements. — We find in the air four elements, hydrogen, oxygen, nitrogen, and carbon; the oxygen and the nitrogen in a free state, and the hydrogen and carbon in combination. These elements are, therefore, called atmospheric elements. It will be noticed that they stand at the head of the four groups of elements (79).:Since these elements mainly make up all animal and vegetable substances (or all organic substances, as they are called), they are also known as organic elements. * See Appendix, 29. SUMMARY. 2 I SUMMARY. Ordinary combustion, decay, and respiration are chemical processes, differing from one another mainly in the rapidity and the completeness with which they take place. The division of substances into combustibles and supporters of combustion is convenient, though merely conventional, since combustion consists in the chemical union of two or more elements, and in reality one of the elements is just as combustible as another. (128.) The present materials of the earth, being mainly compounds of hydrogen and carbon with oxygen, and of the metals with oxygen, chlorine, and sulphur, must be regarded as products of combustion. (138.) The metallic oxides may be reduced by heating them in contact with carbon, which burns at the expense of the oxygen of the oxide, forming carbonic acid, and setting the metal free. (I40.) The metals thus set free return to the state of oxides again by rusting, which is a process of slow combustion. (I4I.) The products of combustion, decay, and respiration are chiefly carbonic acid, water, and ammonia. (I48.) The growth of plants is a chemical process, just the opposite of that of their decay. In growing, plants remove from the air carbonic acid, water, and ammonia. These compounds are broken up; the oxygen i* part given back to the air; and the other elements, with the remaining oxygen, rearranged so as to form the various vegetable compounds. (158, I59.) Animals derive all their food, directly or indirectly, from plants; while plants derive their food, directly or indirectly, from the air. (I5I, I59.) 6 122 CHEMICAL AFFINITY. Animal and vegetable (or organic) compounds contain the four elements carbon, hydrogen, nitrogen, and oxygen. Animal compounds differ from vegetable in being nearly all quaternary, while vegetable compounds are chiefly lernary. (150, 15I). The atmosphere contains oxygen, nitrogen, carbonic acid, and water. The four elements oxygen, nitrogen, carbon, and hydrogen are consequently called atmospheric elements. Since they mainly make up animal and vegetable substances, they are sometimes called organic elements. They head the four groups of elements. (162.) Oxygen exists in three conditions: (i.) wholly passive, (2.) partially active, and (3.) wholly active. Its partially active state is regarded as an allotropic state, and is called ozone. (152, 154.) Several elements are characterized by allotropic states. Those of carbon and sulphur are well marked. (155.) Hydrogen, at a high temperature, combines with oxygen with great energy, as is shown by the violent explosion of a mixture of the two gases, and by the intense heat of the oxy-hydrogen flame. (130.) Nitrogen is remarkable for its inertness, to which it owes its existence in the atmosphere. It is rarely found in combination in nature. In this respect, it is in striking contrast to oxygen, which is found in combination with most of the elements. (i6o, i6.) CHEMICAL AFFINITY. 123 DESTRUCTIVE DISTILLATION AND ITS PRODUCTS. 163. We have already seen how vegetable substances can be destroyed by the various kinds of combustion. When vegetable substances are heated in closed vessels, so as to exclude the oxygen of the air, they are broken up into a number of compounds, which vary with the temperature to which they have been exposed. In combustion these organic compounds are broken up by the affinity of oxygen brought to bear upon them from without; while in the other case they are broken up by the internal action of heat. In the first case, new compounds are formed by the addition of new material; in the second case, by subdivision without any addition of new material. The first process is called combustion; the second, destrzuctive distillation. 164. The Preparation of Charcoal. - One of the simplest cases of destructive distillation is to be seen in the preparation of charcoal. This process may be illustrated by putting pieces of dry wood into a test-tube, closed with a cork, through which a glass tube passes. Heat the testtube, and the wood turns black. As has already been shown, carbonic acid will pass off (I50). Hold a lighted taper to the tube, and the escaping gas takes fire. Hold a cold glass rod in the flame, and it becomes covered with soot, showing the presence of carbon in the flame. Hold a cold glass vessel over the flame, and moisture collects upon it, showing that water is a product of the burning of this gas. Besides the carbonic acid, then, an inflammable gas issued from the tube; and this gas must be a compound of hydrogen and carbon, since carbon is found in the flame, and water is one of the products of the burning gas. 124 CHEMICAL AFFINITY. A part of the carbon, then, combines with oxygen to form carbonic acid; another part with hydrogen, to form the inflammable gas; while the greater part remains behind as a black solid. This black residue is charcoal, a form of carbon. If the heating is continued, and the gaseous products are conducted into a cold receiver, a liquid product is also obtained. One way of preparing charcoal is to place billets of wood in. an iron cylinder, which is closed air-tight and heated to dull redness. The volatile products are driven off and allowed to escape through a flue, and the solid charcoal remains behind. A ruder method is practised in the country, where wood is plenty. A stake is set in level ground, and brushwood heaped around its base. Wood is then stacked round the stake, so as to form a mound some 20 or 30 feet in diameter. This mound is then covered, first with leaves or turf, and then with earth, leaving only a small opening at the bottom, through which the mound is set on fire. When the fire is well under way, the mound is covered more deeply and allowed to burn slowly out. This requires about a month. The burning of a part of the wood furnishes heat for charring the rest. 165. The Prooducts of the DistilZation of Wood. - When hard wood, as beech, is subjected to destructive distillation in a retort, and the volatile products are condensed in a suitable vessel, four principal classes of substances are formed: (i.) gases; (2.) a watery fluid; (3.) a dark resinous fluid; (4.) charcoal. (i.) This product is a mixture of inflammable gases, the most important of which are the two hydrocarbons (or compounds of hydrogen and carbon) marsh-gas, H4-C, and olefian/gas, H2-C. (2.) This product is an acrid liquid, known aspyroligneous acid, or wood vinegar. From this acetic acid is obtained, CHEMICAL AFFINITY. I25 which is used in large quantities in the preparation of the acetates of iron, lead, and soda, which are much employed in dyeing and calico-printing. (3.) This product is wood tar, a thick liquid, insoluble in water, but soluble in alcohol. Its chief use formerly was for tarring and calking ships, but recently it has become an important source of both illuminating and lubricating oils. These oils will be more fully treated hereafter. (4.) This product is the charcoal remaining in the retort. It is used chiefly as fuel and in reducing metallic ores. i66. Ingredients of Wood Tar.- When beech-wood tar is distilled, a light oil passes over first, called euzpion, or wood napht/za. It is now often sold under the name of benzole, and used as a burning-fluid, for removing oil-stains from clothes, and for countless other purposes. It burns with a brilliant white flame, free from smoke; but its extreme inflammability makes it a dangerous liquid for lamps. After this light oil has distilled over, a heavy oil follows. It contains various ingredients, the chief of which are creosote and paraffine. 167. Creosote. -This is an oily, colorless liquid, with a peculiar smoky odor. It has remarkable anliseptic (or preservative) properties. A piece of flesh steeped in a very dilute solution of it dries up into a mummy-like substance, which refuses to decay. Meat, as tongues or hams, may be almost instantly cured by dipping it into a solution containing one part of creosote to Ioo parts of water or brine. It is this substance which imparts to wood-smoke its property of preserving meat. It is a compound of carbon, hydrogen, and oxygen. 168. Paraffine.-This is a pearly-white, tasteless, and odorless solid. The most corrosive acids and alkalies have no effect upon it. Hence its name, from jarum, little, and affinis, from which affinity is derived. It burns with a bright white flame, without smoke. It is 26 CHEMICAL AFFINITY. now much employed as a material for candles, which for purity and lustre are not surpassed by even the best and most costly wax-candles. Unsized paper, after having been soaked in paraffine, may be kept for weeks in concentrated sulphuric acid without undergoing the slightest alteration. Hence it is an excellent coating for the labels of bottles in which acids are kept. I69. A4sphalt. - Asphalt, or pitch, is the residue left after distilling tar. It is used for varnishes, and as a material for making lamp-black. 170. Products of the Decay of Vegetable Substances when Air is excluded. — When vegetable substances are consumed by the slower process of decay, with a partial or complete exclusion of air, the products are somewhat different from those of ordinary decay. The gas obtained by stirring the mud in marshes and at the bottom of stagnant pools (72) is formed in this way, and is made up chiefly of marsh gas, H4-C, and carbonic acid, -C92. I17. The Formation of Mineral Coal.-In tropical swamps where vegetation is rank, vast masses of vegetable matter accumulate, and gradually decay under water. In some cases the land at the bottom of these swamps is slowly sinking, and the bed of peat, as it sinks with it, becomes covered with mud and sand, which numerous streams are washing down upon it. This goes on, year after year and century after century, until the bed is buried hundreds of feet beneath the surface. The vegetable matter thus sunk in the earth, and subjected to enormous pressure, gradually undergoes a process of internal combustion similar to that which takes place under water. In many cases the decomposition is hastened by the agency of the internal heat of the earth. It is probable that the vast beds of coal found in various parts of the earth have been thus formed. All this coal is the remains of an ancient vegetation, and it CHEMICAL AFFINITY. 127 undoubtedly required millions upon millions of years to complete its conversion into coal. 172. Hard and Soft Coals. - The mineral coals may be conveniently divided into hard, or anthracite, and soft, or bituminous coal, and there are several varieties of each. The main differences between the two are these: hard coal is almost pure carbon, while soft coal contains also considerable hydrogen and some oxygen; hard coal still retains the cellular structure of the wood, which is clearly seen under the microscope, while in soft coal this cellular structure is almost entirely wanting; hard coal burns without flame, soft coal with flame. It is found by experiment that, if vegetable matter be enclosed in an apparatus made of wet clay, and subjected for a long time to great pressure and to a high temperature, a variety of coal will be formed closely resembling hard coal. In this case the gases which are formed escape through the porous clay. If strong iron cylinders are used instead of clay, a variety of coal resembling soft coal is formed. In this case the gases have no means of escape. In the first case the coal retains the cellular structure of the wood; while in the second case this cellular structure is entirely destroyed, and the carbon appears to have been dissolved in the liquid and gaseous products formed at the same time. It is probable that, in the slow decomposition of the vegetable matter buried in the earth and subjected to great pressure, both these conditions have existed. In some cases the gaseous products were free to escape as they were formed, and hard coal is the result; in other cases the gases could not escape, and bituminous coal, or soft coal, is the result. This bituminous coal seems once to have been in a liquid state, and afterwards to have hardened. If, when in the liquid state, it was surrounded by porous strata, it was absorbed by these and afterwards hardened. This seems to have been the origin of the bituminous shales and 128 CHEMICAL AFFINITY. slate which occur in immense beds in various parts of the earth. These are loose, clayey rocks, impregnated with bituminous matter. I73. Products of Me Distillation of Bintzzinzous Coal.The products obtained by the destructive distillation of coal are still more numerous than those obtained from wood. Wood, containing much oxygen and comparatively little nitrogen, furnishes compounds which contain much acetic acid and little ammonia, and which, therefore, have an acid reaction. Coal, on the other hand, contains much nitrogen and little oxygen, and gives products rich in ammonia, and having consequently an alkaline reaction. When coal is distilled at high temperatures, an abundance of an inflammable gas is obtained, and also a large amount of liquid products, which are then called lars. When coal is distilled at a low temperature, but little gas is obtained, and the liquid products are then called oils. I74. The Composition of Coal-Tar. - Coal-tar has been found to contain three classes of substances:(i.) Acid oils, soluble in alkalies. (2.) Alkaline oils, soluble in acids. (3.) Neutral oils, not affected by alkalies and some acids. (I.) The most important and abundant ingredient of the acid oils is carbolic acid, f-Ho6. It is analogous to creosote, and is sometimes called coal creosote. It is now largely employed as a permanent dye-stuff for silk and woollen goods. This dye-stuff is prepared by heating carbolic acid moderately with nitric acid. This mixture is called picric acid. On evaporating this liquid, yellow scaly crystals are obtained. Like all the tar colors, its dyeing qualities, when in solution, are most intense. Silk and woollen goods put into the solution, even when cold, assume a rich yellow color, far surpassing that obtained from other dyes. (2.) The alkaline oils constitute but a small fraction of the tar. Their most important ingredients are ammonia and aniline. CHEMICAL AFFINITY. 129 Aniline is an oily substance which, when acted upon by compounds which readily part with oxygen, furnishes a complete series of the most brilliant dyes. The preparation of these rich dyes from aniline is one of the most interesting discoveries of modern times, and has caused almost a revolution in the arts of dyeing and calico-printing. It is still more surprising when we consider that these brilliant colors are obtained from what was until recently a disagreeable waste product of the gas-works. When first prepared, they were worth their weight in gold; now, they can be bought at a comparatively moderate price. Their dyeing qualities are so intense that a little material goes a great way; so that, notwithstanding their high price, they are more economical than any ordinary dye-stuffs. (3.) The neutral oils are the coal-oils proper. They contain a great variety of compounds, both liquid and solid, the latter being held in solution. Of the liquids, benzole, toluole, and cumole are the most important; and of the solids, paraffine and naptthaline. 175. Benzole and Nitro-benzole. - Benzole, which has been already mentioned (i66), is a very important compound, as it is the material from which aniline is usually prepared. The symbol for benzole is f-6H6. When mixed with concentrated nitric acid, it is converted into nitrobenzole, -C6H5NO2. 2e6H6 + N20 - 2eCH5NO2" + H2O. Nitro-benzole is the artificial oil of bitter almonds, and is much used in the art of perfumery. Its most important use, however, is in the preparation of aniline. When heated with acetic acid (-CH402) and iron filings, it loses two atoms of oxygen and takes up two of hydrogen, and is converted into aniline. -C6H5Ne2 - -2 + H2 = -C6HN (aniline). 6* 130 CHEMICAL AFFINITY. I76. Tozlole and Cumole. - Toluole, -7H8s, and cunole, ~91112, are the chief ingredients of the well-known illuminating or lamp oils obtained from coal. I77. NVaphZhaline.-Naphthaline is a beautiful, pearlywhite solid. It is inflammable, but burns with a smoky flame and a disagreeable odor. Brilliant red and blue colors, rivalling those prepared from aniline, have lately been obtained from this solid. When vegetable matter is distilled at a high temperature, benzole and naphthaline are formed in great abundance, with but small quantities of toluole, cumole, and paraffine. When, on the other hand, the distillation is conducted at a low temperature, toluole, cumole, and paraffine are formed in large quantities, with but little benzole and paraffine. COAL- OILS. I78. At the beginning of the present century, the means of lighting our dwellings consisted, in the main, of poor tallow-candles and dim and dirty oil-lamps. On the continent of Europe, whale-oil and other animal oils were costly, and there consequently resort was had to natural tar and bituminous slate, in order to obtain illuminating oils. For more than twenty years past, lamp-oils have been extensively prepared from wood, rosin, and bituminous matter. In Great Britain and in this country the manufacture of coal-oils is of much more recent growth, because the extensive whale-fisheries supplied all the wants of the market. The manufacture of coal-oil was introduced into this country in I853, and was at first confined to those districts where bituminous coal could be mined at a cheap rate. Soon after this manufacturLe was established in this country, and after the value of coal-o is came to be fully recognized, attention was drawn to pefrokeumz, or rock-oil, as a CHEMICAL AFFINITY. I3 ready means of supplying these oils cheaply. On examination this oil was found to be analogous, in its composition and its properties, to that obtained from bituminous materials. I79. The Origin of Petroleum. - We have seen (172) that, in the original decomposition of the vegetable matter of former ages, portions of it were probably reduced to a liquid state, and afterwards hardened into bituminous coal. When in this liquid state, it must have closely resembled petroleum. We have also seen that petroleum closely resembles the coal-oils obtained by the destructive distillation of soft coal and other bituminous substances. All scientific men are agreed that the petroleum found in the earth results from the decomposition of organic matter, and nearly all are agreed that it results mainly from the decomposition of vegetable matter. It is, however, a disputed point, whether it results from the original decomposition of the vegetable substances, or from the action of the internal heat of the earth on the bituminous coal at a subsequent period. It is probable that the petroleum now found in the earth is the product both of the original decomposition and of subsequent distillation. Petroleum is, however, rarely found in contact with bituminous strata of any kind. It is more often found in fissures in sand rocks; rocks in which no oil could ever have been generated, since whatever organic matter they might have contained was too much exposed to atmospheric oxygen to admit of its being bitumenized, or made bituminous. It is not only impossible that the oil could have originated in these sand rocks, or in the sandy shales which underlie them in the Oil Region in Western Pennsylvania, but it is most probable that the oil ascended from still lower rocks in the form of vapor, which condensed in the cavities above. Since, then, petroleum is seldom found where it originated, 132 CHEMICAL AFFINITY. but ordinarily in cavities of rocks higher up, it seems probable that it is mainly the product of distillation. The chemical conditions essential to the generation of oil have evidently existed over a very wide area; but the oil is found only where fissures exist in the rocks. These fissures serve two purposes: one, to give space for the formation and expansion of the hydrocarbon vapor; the other, to furnish receptacles for the condensed oils. These fissures must connect with the sources of the oil. If they have any outlets at the surface of the earth, by which the more volatile portions of the oil may escape as gas, the oil within them becomes thicker and heavier. Hence, as a general rule, the oil found near the surface is heavy, the cavities containing it being likely to have outlets. It may, of course, happen that a deeply-seated fissure has such an outlet. i80. How Petroleum is obtained. The oil is obtained by piercing one of these cavities by a well. It often happens that the upper part of the cavity is filled with pent-up uncondensible gases. In this case, if the well happens to pierce the lower part of the cavity, the expansive force of the confined gases will drive the oil from the well in a continuous stream. Oil is often forced from a new well with such velocity, that it rises in a jet a hundred feet high. It sometimes happens that the lower part of the cavity is filled with brine, upon which the oil floats. If, in this case, the well pierces the lower part of the cavity, brine is the first product. After a time the salt-well may change to an oil-well. COAL-GAS. I8I. The History of Gas-lighting. -The idea of turning hydrocarbon gases to the practical purposes of illumination occurred at about the same time to Murdock, in England, and Lebon, in France. CHEMICAL AFFINITY. I33 As early as I69I, Dr. Clayton discovered that an inflammable gas could be obtained from the destructive distillation of coal; but no one thought seriously of using the gas for illumination till about I790, when Mr. William Murdock, afterwards connected with Messrs. Bolton and Watt's engineering workshops at Soho, turned his attention to this subject. In I792, Mr. Murdock lighted his own house and office with gas which he distilled from coal. In 1802 he made a public exhibition of gas-lighting at the Soho foundry; and Mr. Matthews, an eyewitness, says: " The illumination of the Soho works on this occasion was of the most extraordinary splendor; the whole of the front of that extensive building was ornamented with a great variety of devices, that admirably displayed many of the various forms of which gas-light is susceptible. This luminous spectacle was as novel as astonishing, and Birmingham poured forth its numerous population to gaze at and admire the wonderful display of the combined effects of science and art." The subject now gained the attention of other eminent scientific and practical men. But for several years only private gas-works were erected, for cotton-mills and similar establishments. In i808, Mr. Clegg, an able mechanic, to whose ingenuity are due many of the main features of the present system of gas-lighting, first introduced the method of purifying the gas by passing it through milk of lime. In 1804, Mr. Winsor endeavored to form an incorporated company for the full development of gas-lighting, so that the streets, shops, and private dwellings should enjoy its advantages. "But gas-lighting, like every other great innovation, was looked upon by the public with excessive distrust. In the event of its success, several branches of industry and commerce were doomed to suffer; many interests were supposed to be at stake; some of the chemical 134 CHEMICAL AFFINITY. properties of the gas were unknown; great doubts existed as to its safety, and fears as to its salubrity; indeed, the danger of explosion was magnified to the extent that it was asserted, and belieyed, that a town could be destroyed by the explosion of the main pipes in the street; and interested parties, in order to prevent the establishment of gaslighting, did not scruple to appeal to the naval glory of the nation, and this shortly after Nelson had achieved his great victories.'If,' said the opponents of the new light,'this becomes successful, then our naval supremacy is gone, for at present we obtain our artificial light principally from the whale fisheries; these are the nurseries of our best sailors; so, if we destroy the one, the other must be affected; if the fisheries no longer exist, our navy must degenerate.' At length, after having struggled, during four years, singlehanded, and as it were against the opinion of the world, - having by his letters, pamphlets, and lectures proved the advantages of gas, - Mr. Winsor succeeded, in 1807, in obtaining a capital of -20,000, by means of subscribers, preliminary to the formation of a company." Mr. Winsor and his subscribers could not obtain a charter of incorporation till 1812. Even then the enterprise was looked upon as so visionary, that the act of incorporation was said to be granted in order to make a great experiment of a plan of such extraordinary novelty. In December, 1813, Westminster Bridge was first lighted with gas. From this time gas-lighting made the most rapid progress in England; and now the consumption of gas in London alone amounts to more than seven billions of cubic feet annually. To make this gas, eight hundred thousand tons of coal are required; while the length of the main pipes in the streets of the city is more than two thousand miles. Paris was first lighted with gas in 1820. There, as in England, strong prejudices had to be overcome. 182. AManzfactlre of Coal-Gas. - The most essential CIEMICAL AFFINITY. 135 parts of the apparatus used in the making of coal-gas are represented in Figure 42, k;;ll D Of course, it is onlv soft or bituminous coal that can be used for makin gas. This coal is distilled in long iron Of course, it is only soft or bituminous coal that can be used for nmaking gas. This coal is distilled in long iron 136 CHEMICAL AFFINITY. retorts, seen at the left of the figure. When charged with coal, these retorts are closed air-tight. They are then heated to a very high temperature by the furnaces beneath. The gaseous and volatile compounds formed by the distillation of the coal pass up through pipes (one of which may be seen leading from each retort) into a long horizontal pipe, called the hydraulic main. This is half full of water; and it will be noticed that the pipes leading from the retorts dip beneath the surface of this water. The gas readily passes from the pipes by bubbling up through the water; but, when it has once passed into the main, it cannot pass back again through the water. The gas passes on from the hydraulic main, through the pipe leading towards the right, into a tank, called the tar cistern. By this time the more condensible gases have returned to the liquid state, and collect in the smaller vessel into which the pipe passes, and from which they overflow into the larger tank. From the latter they are drawn off at intervals. These condensed products are coal-tar and a lighter liquid highly charged with ammonia, and called ammoniacal liquor. The uncondensed gases pass on through the series of upright pipes shown in the figure. Here they become still further cooled, and all the remaining condensible gases are reduced to the liquid state. This system of pipes is called the condenser. After leaving the condenser, the gas still contains, besides the compounds fit for illuminating purposes, the noxious compounds carbonic acid and sulphide of hydrogen, or hydrosulphuric acid (88). These are removed in the next part of the apparatus, called the-purzier. It consists of a chamber with several perforated shelves, which are covered with slaked lime. In passing over this lime, the carbonic and hydrosulphuric acids are absorbed, while the CHEMICAL AFFINITY. I37 purified gas passes along into the gasometer, or gas-holder, seen at the extreme right of the figure. This gasometer consists of a large sheet-iron bell-jar, which dips into a cistern of water. The latter is deep enough to allow the bell to be completely submerged, and filled with water. The bell is counterpoised with weights, and rises as the gas passes into it. From the gasometer the gas passes out, through the pipe shown in the figure, into the streets and houses of the city. The gas received into each house is made to pass through a self-acting instrument, called a gas-meter, by which it is measured. One of the most common forms of the instrument (see Figure 43) con-'ig. 43 sists of an outer case, a a, filled with water up to the horizontal dotted line, and, within it, a re- a e f volving drum, divided into four.. compartments, c, d, e,f, by as many bent partitions. The bending of d the partitions forms acentral space,.;/9 g, and thus the gas can pass from ~ one division into the next. The gas enters at the back of the outer case by a pipe which passeinto the central space, where it rises a little above the surface of the water. As one compartment gets filled with gas, it becomes lighter and rises, thereby causing the drum to perform a fourth of a revolution. In the figure, the gas is passing into the division c. As this fills and rises, d comes into the same position; then e; and thenf, which being filled, and rising, completes one revolution. It will be seen that, as each compartment rises above the level of the water, the gas contained in it can pass out through a slit in the rim of the drum into the outer case, and from the top of the case a pipe conveys it to the burners. 138 CHEMICAL AFFINITY. Thus, while one compartment is losing its gas, another one is filling, and so on. The revolution of the drum gives motion to a train of wheels, which in turn move the hands on dial-plates, and thus register the number of cubic feet of gas that have passed through the meter. ILLUMINATION. 183. Nature of Flame. - It has already been noticed that coal-gas burns with flame, while the solid carbon burns without flame. All combustible gases burn with flame. Many solids, as wood, wax, and tallow, appear to burn with flame. We have seen that wood, when heated in a closed tube, gives off an inflammable gas. If either wax or tallow be heated in the same way, an inflammable gas is given off. Thus it has been found to be true, that every solid which appears to burn with flame can be converted into an inflammable gas by means of heat. When these solids begin to burn, the heat developed is sufficient to generate this inflammable gas. It is this gas, and not the solid, which burns with flame. Flame, then, is gas burning. I84. The Lz/ \ set of ivory balls hung side by side, as shown in Figure 56. If i the first ball at the left be raised and let fall again, all the balls remain at rest except the last which flies off. Obviously the force imparted to the first has passed from ball to ball throughout the line. 224. The Electric Battery. -When several cells are joined together, they constitute a battery. There are two ways in which the cells may be joined: (i.) the zinc of the first cell may be joined to the carbon of the second, and the zinc of the second to the carbon of the third, and so on throughout, and the free carbon, or positive pole, of the first cell joined to the free zinc, or negative pole, of the last, by means of a wire; or (2.) the zincs may all be joined together, and the carbons all joined together, and then the zincs and the carbons joined by a wire. These two ways of arranging the battery are shown in Figures 57 and 58. ELECTRICITY. 175 Fig. 57. Fig. 58. 225. Quantity and Intensily. -Let a battery of three cells be arranged according to the first method, and the opposite ends, orpoles, be connected by a short, thick copper wire, which passes over a needle, and let the number of degrees the needle is deflected be noted. Then let the same cells be arranged into a battery according to the second method, and the zinc and carbon be connected by the same wire,'and the needle will be deflected more than in the former case. When the battery, then, is arranged according to the second method, the electricity which it generates has a greater power to deflect the needle than when it is arranged according to the first method. If now the cells be again arranged as at first, and a piece of fine steel wire, five or six inches long, be introduced into the circuit, the needle will be deflected considerably less than when the circuit is completed with the short, thick copper wire; showing that the current is resisted in passing through the fine wire. If now the same piece of fine wire be introduced into the circuit, and the battery be arranged according to the second method, the needle will be deflected considerably less than when the battery is arranged according to the first method; showing that the electricity generated by the first form of battery has a greater power of overcoming resistance in the circuit than that generated by the second form. 176 ELECTRICITY. The power of the current to deflect a needle is called its quantity, and its power to overcome resistance in the circuit is called its intensity, or its tension. The first form of the battery is seen to develop electricity of the greatest tension, and hence it is called a battery of tension, or zintensity battery; while the second form of the battery develops electricity in the greatest quantity, and hence is called a battery of quzantity, or quantity battery. When, therefore, much resistance is to be overcome, the tension battery had better be used; Fig. 59. when but little resistance is to be overcome, the quantity battery is to be preferred. $ When a considerable quantity of electricity is required, and at the same time electricity of considerable tension, the two methods of arranging the battery are combined, as shown in Figure 59. 226. Conductors and Non-Conductors. - If a piece of glass, sealing-wax, or dry wood be introduced into the circuit, the needle will show that no current passes. Substances which will not allow the electric force to pass through them are called non-condZucors; while other substances are called conductors. Metals are generally good conductors. Copper is one of the best conductors; hence copper wires are used in all the ordinary experiments with electric currents. When the circuit is composed entirely of conductors, it is called a closed circuit; when there is a non-conductor in any part of the circuit, it is called an open circuit. SUMMARY. 177 SUMMARY. A Bunsen cell consists of a piece of coke carbon plunged in strong nitric acid contained in a porous cup, which is placed inside of a cylinder of zinc, plunged in dilute sulphuric acid. The carbon and the zinc are called the poles of the cell; the carbon the positive pole, and the zinc the negative pole. That which is employed to connect the poles is called the circzuit. A force called electricity resides in a wire which connects the poles. (222.) Since this force seems to flow through the wire, it is called a current. We always consider the current as starting from the positive pole, and passing to the negative pole. (223.) When two or more cells are connected, the apparatus is called a battery. The carbon of one cell may be connected with the zinc of the next, and so on throughout; or the carbons may all be connected, and also all the zincs. In the first case, the free zinc of the first cell and the free carbon of the last are the poles of the battery; in the second, the united carbons constitute one pole, and the united zincs the other. (224.) When fine steel wire is introduced into the circuit, the current meets with resistance. The power of the current to turn a needle is called its quantity, and its power of overcoming resistance is called its intensity. The electric force does not always possess these two qualities in the same proportions. The battery arranged in the first form gives electricity of 8* L 178 ELECTRICITY. greater inltensity than the battery arranged in the second form; while the latter gives electricity of greater zquanztity than the other. Hence the first form of battery is called an ilztelsity battery; the second, a quantziiy battery. (225.) Substances which allow the current to pass readily through them are called condtuctors; while those which will not allow it to pass are called oon-cond, ctors. The metals are generally good conductors, while dry wood, glass, resin, and gases are non-conductors. Copper is one of the best conductors. When the circuit is made up entirely of conductors, it is said to be closed, when there is a non-conductor anywhere in the circuit, it is said to be open. (226.) ACTION OF THE CURRENT UPON A NEEDLE. 227. Tze Rheolome az te al e 1 eoro2rpe. - In studying the action of the current upon a magnetic needle, it is desirable to have some ready way of stopping or breaking the current, and of changing its direction. An instrument for breaking the current is called a rheotome, a name derived from two Greek words, and signifying cuzrret/-ctr//er. An instrument for changing the direction of the current is called a r-co/rope, that is, a curretn-tZlurner. These two instruments are often combined in one. One of the most convenient forms of this apparatus is shown in Figures 60 and 6i. It has two binding-screws, by which it may be connected with the two poles of the battery. These binding-screws are connected with copper springs, which press against a key. -R represents one of these springs, which are shown better at m and nz in the end view given on a smaller scale in Figure 6I. A copper strip runs from each end of the ELECTRICITY. I79 key, each strip terminating in a binding-screw, to which is attached the remainder of the circuit. The key (shown in section in Figure 6o) is the essential part of the apparatus. It is made chiefly of ebony, which is a nonconductor of electricity. A metallic axis passes a little Fig. 60. A C -44way into each end of the key. Each of these metallic pieces is connected by conductors with the copper strips running from the ends of the key. On opposite sides of the key are two pieces of copper (a and a in Figure 6i), one of which is connected by a brass Fig. 6i. screw A to the metallic axis at one end of the key, and the other by B with the metallic axis at the other end. In one position of rLx the key, the copper springs press against ebony, and the current is thus broken; in another position, the copper springs press against the pieces of copper on each side of the key, and the circuit is thus closed. The current then passes up the copper spring R connected with the positive pole of the battery, along the piece of copper D B on that side of the key, through the brass screw B, to the metallic axis X at one end of the key; thence, through the copper 180 ELECTRICITY. strips and wire M, to the other end of the key 0, through the brass screw A at that end, to the piece of copper A C on the opposite side of the key, down the copper spring, and on to the negative end of the battery. Turn the key half-way round, and the other piece of copper on the key is brought against the spring connected with the positive pole of the battery, and the current must pass out from the key at the opposite end of the axis, and, through the remainder of the circuit, must take a course just the opposite of that in which it first passed. Hence, by turning the key halfway round, the direction of the current can be changed. 228. A iMjagnetic Needle tends to place itself at right angles with a Wire through which a Current is fowing. - If now the current be made to flow over a needle from its north end to its south end, the north pole of the needle will turn to the left hand of an observer who is facing that pole. If it be made to pass over the needle from its south pole to its north, its north pole will turn to the right. If it be made to flow under the needle, the north pole will turn in just the opposite direction; that is, to the right, when it flows from its north to its south pole, and to the left, when it flows from its south to its north pole. If a needle free to move in a vertical plane be held to the right of the wire, its north pole will point downward when the current flows from the north to the south pole of the needle, and upward when it flows in the opposite direction. If such a needle be held to the left of the wire, its north pole will point upward when the current flows from its north to its south pole, and downward when it flows in the opposite direction. 229. Magnetic Rotations. -We see, then, that the needle always tends to place itself at right angles to a wire through which a current is flowing. We see, moreover, that, if a needle which is free to move in both a horizontal and a vertical plane be held above a wire through which a cur ELECTRICITY. 18I rent is flowing from north to south, its north pole turns to the west; and that, if the needle be held on the west side of the wire, its north pole turns downward; and that, if it be held below the wire, its north pole turns to the east; while, if it be held to the east side of the wire, its north pole turns upward. It would seem, then, that this pole would move round the wire, if it were free to do so. The piece of apparatus represented in Figure 62 consists of a glass cup filled with mercury, an iron wire passing up through the bottom of the cup, with a bar magnet fastened to it by means of a string, and a copper wire dipping into the mercury above. As the mercury is a conductor of electricity, and offers no serious resistance to the movement of the magnet, the upper pole of the latter is Fig. 62. free to move around the wire. When the iron and copper wires are connected with the opposite poles of the battery, the upper end of the magnet actually begins to rotate around the wire. If the upper end of the magnet is a north pole, and the copper wire is connected with the positive pole of the battery, the magnet rotates around the wire to the right; if the upper end of the magnet is a south pole, it rotates in Fig. 63. just the opposite direction. - If the magnet be made stationary, and the wire movable, as shown in Figure 63, the wire will rotate around the pole of the magnet. 230. The Rheosrope, or Galvanometer. - Any instrument used for detecting or meas- -- uring a current is called a r/zeoscoype, or a galvanometer. The first name means a current-exzaminer; the second, a measurer of galvanism. Current electricity is often called galvanism, from its discoverer, Galvani. Thus far, in our I82 ELECTRICITY. examination of the existence and the strength of the electric current in the circuit, we have used a simple needle. If this needle moves over a graduated arc, it will be found to move a greater number of degrees when the current passes entirely round it, than when it merely passes over it or under it. It is evident that this should be so. The current above and below the wire must, in this case, pass in opposite directions, and must, therefore, both tend to move the needle in the same direction. The effect is the same as if two currents of equal strength were passing over or under the wire, and both in the same direction. Every time, therefore, that the wire conducting the current is coiled round the needle, the effect of the current is multiplied. A current which is too weak to deflect the needle by simply passing over or under it, may be made to deflect the needle decidedly by coiling the conducting wire a great number of times round the needle. 231. The Astalic Needle. - When a single needle is deflected by the current, not only must the resistance caused by the weight of the needle be overcome, but also the directive action of the earth. It is this directive action of the earth which causes the needle to take its north and south position; and it offers a greater resistance to the deflection of a light needle than its weight does. This directive action can, Fig. 64. however, be completely neutralized by the arrangement shown in Figure 64. Two needles of equal magnetic 5- 1_____.. strength are fastened together, so that _I l __ the north pole of one faces the south n S pole of the other. The earth will evidently pull each end of such a combination of needles both towards the north and towards the south with equal strength. It will therefore have no tendency to point north and south. Such a system of needles is called an astatic ELECTRICITY. I83 needle (from a Greek word meaning nsteadcy), that is, one hazcin no directive power. 232. 7le Astatic Galvanometer. - If now copper wire be coiled a great many times around the lower needle, and an electric current be sent through the coil, it will act on both needles so as to turn both in the same way. For, suppose that the current passes over the lower needle from its north to its south pole, it will turn the north pole of that needle to the left. It will then pass under the upper needle from its south to its north pole, and will turn its south pole to the left of an observer facing that pole. If, then, an observer is facing the north pole of the under needle, the current between the needles will so act upon both as to turn the ends towards him to the left. As the current has only to overcome the resistance caused by the weight and the friction of the needles, they will be very sensitive to its action. An instrument arranged as above described is called an astalic galvvanometer. When the needles are light, and delicately hung, such a rheoscope is exceedingly sensitive. It serves to detect and measure the feeblest current. Fig. 65. Figure 65 represents a galva- _ nometer of this kind. The astatic needle is placed within a coil of l fine copper wire carefully insulated with silk, and is suspended by a cocoon thread to a hook sup- ported by a brass frame. It hangs freely without touching the coil, and the upper needle moves on a graduated circle. The whole is enclosed in a glass case, and rests on a stand supported by three levellingscrews. I84 ELECTRICITY. 233. The Tangent Galvanometer. —This instrument is shown in Figure 66. It consists of a thick strip of copper Fig. 63. bent into the form of a circle from one to two feet in diameter, with a small needle moving on a graduated circle at its centre. It can be used for measuring the strongest currents; and, since the current in passing through the thick copper ring is resisted scarcely at all, it has the advantage of measuring the current without diminishing its strength. VWe have seen that the astatic galvanometer is used for measuring very feeble currents. For measuring currents of ordinary strength, a galvanometer of the same kind, but of less delicate construction, is used. 234. The Needle Telegraph..-Since the electric current passes with comparatively little resistance through thick conductors of almost any length, it is now much used in transmitting signals between distant stations. An instrument for sending such signals is called a telegraph. The word literally means writing at a distance. Four things are essential in every kind of electric telegraph: (i.) a battery for generating the electricity; (2.) wires for conducting the electricity; (3.) an instrument for sending the message; and (4.) an instrument for receiving the message. It has already been found that the opposite poles of the battery must be connected by a continuous conductor in order to obtain a current of electricity. It has been found that, if one end of the battery be connected with a copper plate sunk in the ground deep enough to be in contact ELECTRICITY. 185 with moist earth, and if a wire from the other end of the battery be connected with a copper plate similarly situated at a distant station, the current will pass just as well as when a wire passes from one end of the battery to the distant station, and then back to the other end of the battery. The earth in this case takes the place of one half of the conductor. In practice the circuit is always completed by the earth. Suppose now, that at one station a rheotrope is introduced into the circuit, and at the other a galvanometer. Every time the circuit is closed by the rheotrope, the needle of the galvanometer is deflected; and every time the current is reversed, the motion of the needle is reversed. The operator at the rheotrope can thus readily make the needle at the distant station move any number of times either'to the right or to the left. The galvanometer is placed in a vertical position, and the lower end of the needle is loaded, so that, when the current is broken, the needle quickly takes a vertical position. Out of the movements of the needle an alphabet can be readily arranged. Thus two movements to the left may represent a; three to the left,,b four to the left, c; one to the left and one to the right, d; and so on. When the alphabet has once been agreed upon, the operator by means of the rheotrope can easily send a message, and the operator at the distant station can, by noticing the movements of the needle, as easily read it. Since this telegraph depends on the power of the current to turn a needle, it is called the needle telegraph. It was invented by Cooke and Wheatstone, and put into operation in England in I837. It is not used in this country, but is still quite extensively used in England. 235. Resistazzce to t/e Current. -In the tangent galvanometer we have a ready means of measuring the strength of electric currents, and we find that the dimensions and the material of the substances included in the circuit exercise a I86 ELECTRICITY. marked influence on the strength of the current. It is of the greatest importance, therefore, to ascertain the relative resistance of conductors of various forms and materials. An instrument called the rheostaz (that is, an instrument for making the current steady, or of zunform strenglh) is - Fig. 67. used for this purpose. It was d invented by Wheatstone, and is constructed so as to intro-',_~'~ duce into or withdraw from cn the circuit a considerable amount of highly resisting ~ ~___ current. It is shown in Figure 67. It consists of two cylinders, one of brass, the other of well-dried wood, turning on their axes by a crank. The wooden cylinder has a spiral groove cut into it, in which is placed a fine metallic wire; the brass cylinder is smooth. The end of the wire attached to the wooden cylinder is connected by means of a brass ring, with a binding screw for the attachment of a battery wire. A metallic spring pressing against the brass cylinder is connected with the other binding screw. If now a current be sent through the wire, it will pass through all that portion of it which is wound at the time upon the wooden cylinder, but it will not pass through the portion wound upon the brass cylinder, but through the cylinder instead, since the latter is a better conductor than the fine wire. The wire wound upon the brass, then, is withdrawn from the circuit. When the rheostat is to be used, all the wire is wound upon the wooden cylinder, and put into the circuit of a battery along with a galvanometer. If now the resistances of two wires are to be tested, the galvanometer is read be ELECTRICITY. 187 fore the first is put into the circuit. After it is introduced, the needle falls back in consequence of the increased resistance, and then as much of the rheostat wire is withdrawn from the circuit as will bring the needle back to its former place. The quantity thus withdrawn is shown by a scale, and is obviously equal in resistance to the wire introduced. The first wire is then removed, and the second wire is tested in the same way as the first. If 40 inches were withdrawn in the first case, and 60 inches in the second, the resistance offered to the current by the first wire is to that offered by the second as 40 is to 6o; or, in other words, the former is two thirds of the latter. By means of the rheostat it has been proved that the resistances of wires of the same material and of uniform thickness are in the direct ratio of their lengths, and in the inverse ratio of the sqgiares of their diameters. Thus a wire of a certain length offers twice the resistance of its half, thrice that of its third, and so forth. Again, wires of the same metal, whose diameters are in the ratio of I, 2, 3, etc., offer resistances which are to each other as i, -, 3, etc. Therefore, the longer the wire, the greater the resistance; the thicker the wire, the less the resistance. The same holds true of liquids, but not with the same exactness. The following, according to Becquerel, are the speczfic resistances of some of the more common substances, or the resistance which a wire of each, so to speak, of the same dimensions, offers at the temperature of 54~ F.:- Copper, I; silver,.; gold, 1.4; zinc, 3.7; tin, 6.6; iron, 7.5; lead, i; platinum, II.3; mercury (at 570), 50.7. For liquids the resistances are enormous compared with metals. With copper at 320 F. as I, the following liquids stand thus: saturated solution of sulphate of copper at 48~, I6,885,520; ditto of common salt at 56, 2,903,538; sulphate of zinc, I5,861,267; sulphuric acid, diluted to -T1, at 68~, I,032,020; nitric acid at 55~, 976,000; distilled water at 59~, 6,754,208,ooo. i88 ELECTRICITY. SUMMARY. AN instrument for breaking the current is called a rheotome, an instrument for changing the direction of the current, a rheotrope. (227.) A magnetic needle tends to place itself at right angles to a wire through which a current is passing, The direction in which a needle turns when brought near a wire through which a current is flowing, depends on the direction in which the current is passing, and upon whether the needle is held above, below, to the right, or to the left of the wire. (228.) One end of a magnet free to move will rotate round a wire through which a current is passing. The direction of the rotation depends upon the direction of the current, and upon the nature of the pole which is free to move. If the magnet is fixed, and the wire through which the current flows is free to move, the latter will rotate round the former; the direction of the rotation depending upon the direction of the current, and upon the nature of the pole about which the wire rotates. (229.) An instrument for indicating and measuring the current is called a rheoscope or galvanometer. (230.) When a simple needle is deflected by a current, its tendency to remain at rest, and the magnetic attraction of the earth, must be overcome. The latter resistance is neutralized in the astatic needle, which is therefore more sensitive to the action of the current than a simple needle. (231.) When a current passes entirely round the needle, it deflects the latter as much as two currents of the same strength and direction flowing over the needle. Hence, the effect of the current upon a needle is multiplied by coiling the wire round the needle. (230.) ELECTRICITY. 189 The aslatic galvanometer consists of an astatic needle with a coil of wire round the lower needle of the system. The tangent galvanometer consists of a small magnetic needle in the centre of a circle of thick copper wire. A very delicate astatic galvanometer is suitable for measuring very weak currents; a tangent galvanometer for measuring very strong currents. For currents of intermediate strength an astatic galvanometer less delicately made is suitable. (232, 233.) A telegraph, is an instrument for writing at a distance. Four parts are essential to every telegraph: (I.) a battery for generating, and (2.). a wire for conducting the current; (3.) a sending instrument, and (4.) a receiving instrument. The needle telegcraph depends upon the power of the current to turn a needle. The sending instrument is a rheotrope, and the receiving instrument a galvanometer. The letters are indicated by the movements of the needle. (234.) The relative resistance offered to the current by different conductors is measured by means of the rheostat. The resistances of wires of the same material and thickness are directly as their lengths,'and inversely as the squares of their diameters. (235.) ELECTRO-MAGNETISM. 236. The Current can make Iron magnetic. - If a part of the wire of the circuit be wound into a coil, and a piece of soft iron be placed inside this coil, it becomes strongly magnetic while the current is passing; as may be shown by bringing bits of iron near the ends of the iron inside the coil. Such a magnet is called an electro-magnet. The coil 9go ELECTRICITY. is called a helix. If the coil is a left-hand coil (see Figure 68), the end at which the current enters the coil will be Fig. 68. found by means of the magnetic needle to be the north pole. So that, by reversing the current, the poles of the electro-magnet will be reversed. If the coil is a right-hand Fig. 69.' ~~S N one (see Figure 69), the end at which the current enters is found to be the south pole. When the current is broken, the soft iron instantly loses its magnetism, as is shown by the falling of the bits of iron which before clung to it. If a steel rod is used, instead of a soft iron one, it retains its magnetism after the current is broken. If the wire is wound around the iron in several layers, the strength of the magnet is greatly increased. Steel magnets are now usually made by means of the electric current. A short, thick coil of copper wire and a powerful battery are used for this purpose. A steel bar is inserted into the coil, and moved backwards and forwards, and when the middle part is a second time in the coil, the circuit is opened, and the bar may then be withdrawn, perfectly magnetized. The strongest electro-magnets are of the horseshoe form. They far exceed ordinary magnets in power. Small electro-magnets have been made by Joule in England, which support 3500 times their own weight; and a large one was constructed by Professor Henry, of the Smithsonian Institution, which supports a weight of 2500 pounds. These magnets are much stronger when provided with a ELECTRICITY. 191 keeper, or armat/re, that is, a piece of soft iron which connects the two poles, as shown in Figure 70. Electro-magnets are sometimes mounted in frames Fig. 70. (see Figure 71), which is a convenient form for showing their strength. 237. Th/e Wie tzroughz which at Current is passing is a 11agnet. - If the current be sent through a coil such as is shown in Figure 72, and the end of a rod of soft iron be brought near the + opening in the centre, it is at once drawn into the coil. Coils have been constructed powerful enough to draw up and sustain a weight of 600 pounds. Fig. 71. Fig. 7 The electric current, then, is not only able to develop magnetism in soft iron, but the coil itself, through which the current is passing, is magnetic. If the wire which joins the poles of a battery is brought in contact with fine ironfilings, they adhere to the wire; showing that any wire through which the current is flowing is magnetic. 192 ELECTRICITY. 238. EZeclricity as a Source of Mechanical Power. - A great variety of electro-magnetic machines have been constructed with a view to applying electric force to the work now done by steam. They all depend on the property of an electro-magnet instantly to acquire or to lose its magnetism on the passage or the interruption of the current; or to reverse its poles when the direction of the current is changed. Page's rotating machine, shown in Figure 73, illustrates one method of applying the electric force to do mechanical Fig. 73. work. It consists of a horseshoe i? magnet, in the axis of which is an upright shaft. On this shaft, and at right angles with it, is fixed a piece of soft iron, with its ends facing the poles of the magnet. The soft iron is surrounded with a coil of copper wire, so that it is an electro-magnet. The ends of the wires of the coil are _~-__ i ifastened to two metallic strips, which are so formed that each nearly half surrounds the shaft. These strips are fastened to the opposite sides of the shaft, but separated from it and from each other by a nonconducting substance. The current comes to the coil through two metallic springs, which press against the strips. When the shaft has performed half a revolution, the springs pass from one strip to the other, so that the current is made to enter the coil at the end of the wire opposite to that which it entered at first; and so the poles of the electro-magnet are reversed. The machine is so arranged that, at starting, the poles of the two magnets facing each other are of the same kind. They therefore repel each other, and when the shaft is once started, they send it around a quarter of the way; then un ELECTRICITY. 193 like poles begin to approach each other, and their attraction causes the shaft to complete half of a rotation. The current then changes its direction through the coil, the poles of the electro-magnet are reversed, and like poles again face each other and are repelled. The rotation is kept up by the self-acting rheotrope. Such a shaft may be made to rotate 2000 times a minute, causing 4000 reversals of polarity in that short time. The horseshoe magnet may, of course, be an electro-magnet; and then engines of considerable power may be constructed. One of the best electro-magnetic engines is that devised and constructed by Froment. It is shown in Figure 74. Fig. 74. It consists of six electro-magnets fastened to an iron frame. Only four of these magnets appear in the figure: there are two above, corresponding to the two below. The poles of these magnets face a wheel, upon whose rim are eight pieces of soft iron, placed at equal distances. As the magnets are also placed at equal intervals, only two of the pieces of soft iron can be opposite the poles of the magnets 9 M 194 ELECTRICITY. at once. As the wheel stands in the figure, one of the pieces of iron is just above the magnet seen on the right, and another just below the magnet on the left. If now a current passes through the coils of these magnets, they become active, and each draws the piece of iron nearest it towards itself; and the wheel is thus made to turn till the pieces of iron are brought directly opposite the poles of the magnets, thus carrying the next pieces of iron very near the next magnet. If now the current is stopped in the first pair of coils, and sent through the next, the next pieces of iron will be drawn opposite the poles of these magnets; thus carrying the wheel onward, and bringing the next pieces of iron near the next magnets. The current is then stopped in the magnets through which it was last sent, and is sent through the coils of the next, and so on. In this way the wheel is made to rotate rapidly. The current is sent through the coils in succession, by means of the cog-wheel seen at the end of the axis of the wheel. One pole of the battery is connected with this wheel, and the other with the arc below, which carries the three springs that are seen to press against the cog-wheel. As the wheel turns, the teeth strike the springs one after another, and thus close the circuit. The coils of the electro-magnets are connected in such a way, that every time a tooth touches one of the springs, the current passes through the coils of two magnets directly opposite each other. As soon as the pieces of soft iron are drawn opposite the poles of these magnets, the tooth leaves this spring, and another tooth touches the spring which closes the circuit in which the coils of the next pair of magnets are included, and so on. Electro-magnetic engines have never yet been constructed of above eight or ten horse-power, though there is apparently nothing to limit them to this low power. The great obstacle to the success of these engines is the ELECTRICITY. 19 expense of generating the electricity to run them. It costs some forty or fifty times as much to generate electric force as to generate the same amount of steam force. Yet, for certain kinds of work, where rapid motion and comparatively little force are required, electric engines have been found to answer better than small steam-engines. 239. Foucault's Self-atiing Rheotome. - This instrument, shown in Figure 75, illustrates one of the many applications Fig. 75. I H, of the electric force to doing mechanical work. It consists of a beam, a d, supported by a standard, C G, which acts as a spring. At one end of the beam there is a keeper of soft iron; at the other end, two iron rods, which plunge into cups A, B, partially filled with mercury. Under the iron keeper is an electro-magnet, D. One end of the wire of the helix of this magnet connects with one pole of a Bunsen's cell. The other pole of this cell is connected with the mercury cup, B. The other end of the wire of the helix is connected with the beam by means of the standard; so that the circuit of the Bunsen's cell is closed when the iron rod dips into the mercury, and is open when it is out I96 ELECTRICITY. of the mercury. It is best to cover the mercury with alcohol, which is a non-conductor. When the rheotome is to be worked, the iron rod is so adjusted that its end is just above the surface of the mercury. That end of the beam is then depressed by the hand so as to bring the rod into the mercury. This closes the circuit, and renders the electro-magnet active, and the keeper at the end of the beam is drawn down upon it. This carries the other end of the beam up and the rod out of the mercury, opens the circuit, and renders the electro-magnet inactive. The elasticity of the standard throws this end of the beam back and lowers the rod into the mercury, closing the circuit again. Everything is now as at the first, and the same succession of movements is repeated indefinitely. This instrument is made to open and close a second circuit in the following manner. One pole of the battery of this circuit is connected with the beam, and so with the iron rod, which dips into the second cup of mercury, A, which is connected with the other pole of the battery; so that this circuit is closed when the rod dips into the mercury, and open when it is out of the mercury. But if the point of the rod is so adjusted as to be just above the surface of the mercury, it is drawn out of it every time that the keeper is drawn down to the electro-magnet, and is plunged into it every time that the keeper is thrown back by the spring. Fig. 76. 240. Electric Clocks. - The elec~+\ - tric force has also been used to regulate the movements of clocks. The clocks thus regulated are called copying clocks. They are of the usual construction, except that the pendulum N SK — S N balls are hollow coils of copper wire, B so that they become magnetic when a current is sent through them. In Figure 76, R represents a part of the ELECTRICITY. 197 rod, and B the ball, of such a pendulum. Permanent magnets, 1S and SV, are fastened against the sides of the clock-case opposite the ends of the coil B, with like poles towards the coil. The hollow of the coil, as it swings, can pass a little way up the length of each magnet. If the south poles of the magnets are turned towards the coil, as in the figure, and a current is sent through the wire, one end of the coil becomes a north pole, which is attracted by the magnet near it, and the other end a south pole, which is repelled by the magnet near it. This attraction and repulsion both tend to send the coil in one direction. If, now, at the instant that B is drawn to one side, the direction of the current is changed, the poles of the coil are reversed, and it is carried to the other side. The pendulum thus vibrates every time the current is reversed. This is done by means of a standard or regulating clock. Every time the pendulum of this clock vibrates, the direction of the current is reversed; so that the pendulums of all the copying clocks vibrate exactly at the same rate as the pendulum of the regulating clock. In this way, by means of one accurate clock, any number of copying clocks, of the most ordinary construction, can be made to keep accurate time. Fig. 77. ll I ^ I 0.!.d 0 A Figure 77 shows one of the ways in which the pendulum, A, of the regulating clock can change the direction of the 198 ELECTRICITY. current. The spring e is connected with the negative pole of the battery G, and the spring d with the positive pole of the battery F. The other poles of these batteries are connected with the plates m and n, buried in the earth. B and C are the pendulums of the copying clocks. When the regulating pendulum touches the spring d, the current flows through the wire from A to B and C; when it touches the spring e, the current flows first through the earth from n to o, and then through the wire from C to A. The permanent magnets connected with the pendulums B and C are not represented in the diagram. 24I. Morse's Electro-ma=~ ~ and those of the other, s s, with a galvanometer s' s" G. The instant the cir~P~ P~f- ^'cuit of the battery is completed, and the current sent through p, a current in the opposite direction is induced in the wire s s, as is shown by the galvanometer. This induced current lasts but a moment, for though the current continues to circulate in pp, the needle soon falls back to its original position. If now the battery current in 9p be interrupted, another momentary current in ss is shown by the galvanometer, but in this case in the same direction as the inducing current. The inducing wire and current are called primary; the other wire and current, secondctry. If the primary wire be movable, so that it can suddenly be brought near to and withdrawn from the secondary, while the battery current flows steadily, currents are induced as before, the approach of the wire being marked by an inverse current, and its withdrawal by a direct one. As long, however, as the primary wire remains in any one position, all electricity in the secondary wire disappears; but if in this position the strength of the primary current should be increased or diminished, momentary currents in the secondary wire would again mark the changes in the primary, the increase causing an inverse, and the decrease a direct current. Hence we conclude that a currei w/Z iczh begins, a currenzt wOhich acpproaches, or a cuzrreit whcich increases zin strengtz, induces an inverse momentary current il a neighborizg circuit; and that a cturrent owhich stops, a current /which retires, or a current which decreases in stre-ngtz, induces a direct momentary current in a neighboring circuit. ELECTRICITY. 249 In experiments like the above it is better to wind the primary and secondary wires on bobbins, so as to form coils, as shown in Figure I05. Fig. I05. The primary coil P is made of coarse wire, and the second p s P ary coil S of fine wire, as in the induction coils already described (274, 275). If the primary coil be placed in the S circuit of a cell or battery, and e - if the secondary coil be con- nected with a galvanometer, a momentary inverse current appears in S when P is put inside it, and a momentary direct one when it is taken out; or if, while P remains in S, the strength of the primary current be altered, the galvanometer indicates the induction of currents according to the principles already stated. Current induction is probably only a phase of magnetic induction, since we have seen (237) that any wire through which a current is passing is slightly magnetic, and that such wires are powerfully magnetic when wound in coils. We have seen also (27I) that, when a magnet is brought near to or removed from a conducting wire, it excites an inverse or a direct current in the wire; and that when soft iron placed within a helix is gaining or losing magnetism, momentary currents, inverse or direct, are excited in the wire. 279. Extra Current. -Not only does a galvanic current induce electricity in a neighboring current, but it also acts inductively on itself. When contact is broken in a battery circuit a spark is seen. When the wire is short the spark is feeble, but it grows brighter with the length of the circuit, especially when the wire is wound in a coil. The current is not stronger in the latter case, but weaker, as the galvanometer will show; and the real cause of the brighter spark II* 2 50 ELECTRICITY. with the longer circuit is to be found in the induction of the primary current on the various parts of the circuit, exciting extra currents, as they are called, in the primary wire. Experiments have shown that at the instanit a galvanic current begins and ends, extra currents are induced by the action of the severalparts of its circuit upon each other, thal at the beginning of the current being inverse, and that at the enzd irect. The effect of the extra current on the direct induced current of the secondary coil is to lessen its tension very decidedly. It does this by prolonging the cessation of the magnetism of the core and of the current in the primary coil, and thus impairing the suddenness of this change. 280. The Magnetism of Rotation. - It was long ago observed that, when a magnetic needle was made to oscillate above a copper plate, it came to rest sooner than it did otherwise. The oscillations were made in the same time as when away from the plate, but they were less in extent; the plate seeming to act as a damper to the motions of the needle. Arago reasoned from this that the needle at rest would be influenced by the plate in motion, and experiment confirmed the surmise. He made a copper disc revolve with great rapidity under a needle, the middle of the needle being directly above the centre of the disc. As expected, the needle was deflected in the direction of the motion of the disc; and the deflection increased with the rapidity of the motion, until at length the needle turned round after the disc. This action of the revolving disc was called ti,3 magnetisnm of rotation, and the name has been since retained. It was first explained by Faraday, who proved it to arise from the reaction of currents induced in the disc by the magnet. As the copper plate is a continuous conductor, currents will be started in it by its rotation under the magnet (271). These currents are foulnd to flow in the neighborhood of the needle in such a direction as to deflect it in the direction in which the plate is moving. SUMMARY. 251 SUMMARY. Electricity can be developed by magnetism, either by moving a conductor near a constant magnet, or the magnet near the conductor; or by changing the strength of the magnetism in a magnet which is near a conductor. Electricity developed by magnetism is called magnetoelectricity, and an instrument for developing it is called a magnzeto-electric machine. In all ordinary machines of this kind the electricity is induced by developing and destroying magnetism in soft iron placed inside a helix. This may be effected by using a permanent magnet, or by means of the electric current. (271, 272.) When the magnetism is developed and destroyed by means of the current, the instrument is usually called an induction coi. The most important machine of this kind is the iznductorium, or Ruhmkorff's induction coil. It consists of a bundle of iron wires surrounded by a helix of thick copper wire, through which the primary current is sent in a succession of rapid impulses. This primary coil is surrounded by a much longer coil of very fine wire, in which the secondary current is induced. (275.) The most important magneto-electric machine of the first class is the one recently invented by Wilde. In this machine the current developed by rotating an armature between the poles of a series of permanent magnets is made to develop much more powerful magnetism in an electromagnet, which in turn is made to develop a current by means of a second rotating armature. In this way a magnet indefinitely weak may be made to develop a current of indefinite strength. (273.) Magneto-electricity has much greater intensity than voltaic electricity. Magneto-electricity may also be developed by means of 252 ELECTRICITY. the current by alternately carrying continuous conductors near to and away from a wire through which a current is passing; or by leaving the conductor stationary near the wire, and alternately stopping and starting a current in the wire. This is called current induction. It is probably only magnetic induction, since the wire through which a current is passing is always magnetic. (278.) The'extra current which appears on stopping or starting a current in a wire is due to the inductive action of one part of the circuit upon another. (279.) The movements of a magnetic needle when suspended over a rotating copper disc are explained by the action of the currents induced in the disc by its rotation under the magnet. (280.) THERMO- ELECTRICITY. 281. Electricity developed by Heat. -We have seen the power of the current to develop heat (261), and we shall now see that heat has power to develop an electric current. The electricity so developed is called thermo-electricity (heat electricity). Take a copper wire, cut it in two, and fix each half in one of the binding-screws of a. galvanometer. Heat one of the free ends to redness, and press it against the other, and a current will be generated, passing at the junction from the hot to the cold end, as shown by the needle. Perform the same experiment with two pieces of platinum wire, and the current is stronger. In almost all cases where portions of the same metal at different temperatures are pressed together, a current is produced. Currents also appear when two portions of the same metal or piece of metal have different structures, and the ELECTRICITY. 253 point where both structures meet is heated. If, for instance, one piece of wire be hard-drawn and the other part annealed (I7), a current is produced when the point of separation between the hard and soft part is heated. The same takes place if one part of the wire be hammered or twisted, and the other part not. Fig. io6. When the point of junction of any two metals G is heated, a current is always produced. When a bar of antimony, A, is soldered to a bar of bismuth, B, (see Figure Io6,) and their free ends are connected with a galvanometer, G, a current passes from the bismuth to the antimony B A when the junction is heated. When S is cooled by applying ice, or otherwise, a current in the opposite direction is produced. Such a combi-,k 6. nation of metals is called a t/ermo-electric pair. Metals like antimony and bismuth, which have a crystalline structure, are best suited for a thermo-electric pair. 282. Thermo-electric Battery. -One bismuth-antimony pair has very little power. To obtain a stronger current several pairs are united, as shown in Figure I07. The heat in this case must be applied only to Fig. 07. one row of soldered faces. The strength of the current depends on the difference of temperature of the two sides; and to increase it to the maximum the one series must be kept in ice or in a freezing mixture, whilst the other is exposed to an B l A intense heat. As in the galvanic battery, the electric force is proportionate to the number of pairs. At best, however, it is r small, and the galvanometer used to measure it must be a very delicate one. When a great number of pairs are formed into a battery, they are usually arranged as shown in Figure Io8, which 254 ELECTRICITY. Fig. Io8. shows one of thirty pairs. The odd faces, i, 3, 5, etc., are exposed on one side, and the even faces, 2, 4, 6, etc., on the other. The terminal bars are connected with the binding-screws. The interstices of the bars are filled with gypsum to keep them separate, and the whole is put in a frame of non-conducting material. Such a battery, in connection with a sensitive galvanometer, forms a most delicate thermometer; showing, however, only differences of temperature between the two faces. So long as the opposite faces are exposed to the same temperature, no current is produced; but if the temperature of one side becomes higher than that of the other, a current is at once indicated. If the hand, for instance, be brought near one side, the needle shows a current; or if a piece of ice be held near, a current is also shown, but moving in the opposite direction. SUMMARY. Take pieces of two metals, and connect one end of each by a wire, and bring the other ends together. On heating the point of contact, a current is developed. Electricity thus generated is called therno-electricity, and the metals thus connected constitute a thermo-electric pair. Such a pair can be formed of two pieces of one metal, provided these are in different conditions. Antimony and bismuth form the best thermo-electric pair. (281.) Several such pairs connected form a thermo-electric pile, or battery. Such a pile is a very sensitive thermometer, since a current is developed by the slightest difference of temperature between the two faces. (282.) ELECTRICITY. 255 FRICTIONAL ELECTRICITY. 283. Electricity developed by Friction. -When a cat's back is stroked on a cold, dry day, in a darkened room, sparks are obtained which at once indicate the development of electricity. If a well-dried rod of glass or guttapercha be rubbed with a piece of silk or flannel, similar sparks appear. Hence electricity may be developed by friction. Such electricity is called frictional electricity. It is found by experiment that, when any two dissimilar bodies are rubbed together, electricity is developed; but when the substances are conductors of electricity, the force thus developed passes off silently through the hands and body. In order to detect it, the substances rubbed together must be held by insulating handles, that is, handles which do not conduct electricity. 284. _ _The Electrical Machzine. - In studying frictional electricity it is desirable to have an apparatus suitable for generating it. Such an apparatus is called an electrical machine. One of the best forms of it is shown in Figure o09. It consists of a thick plate of glass, insulated from Fig. o09. _______ _ I-L — I-I-. — > 256 ELECTRICITY. the floor, and turned by a crank. At one end there is a glass standard surmounted by a brass ball. From this standard project two brass strips in the form of a clamp, which hold the rubbers against the glass plate. These rubbers are pieces of wash-leather or woollen cloth, covered with an amalgam of mercury, lead, and tin. At the opposite end, on a glass support, is a long cylinder of brass with rounded ends. This cylinder is the prime or positive conductor. The brass ball connected with the rubber is the negative conductor. It is necessary that the plate and conductors of the machine be well insulated. In dry and frosty weather glass insulates very well. At all other times it becomes covered with a very thin, scarcely visible layer of moisture, which very much impairs its insulating power. In order to insure dryness it is necessary to rub the standards with a warm flannel before using the machine. The deposition of moisture is greatly lessened by coating the glass with shel-lac. 285. Quantity and Intensity of Frictional Electricity. With a medium-sized electrical machine of this kind, sparks are readily obtained two inches long by presenting a conducting substance to the ball of the prime conductor. Machines have been constructed powerful enough to give a spark two feet in length. We thus see that frictional electricity has great tension (225). Its quantity, on the other hand, is next to nothing. This is shown by connecting the positive conductor with one end of the wire of a moderately delicate galvanometer, and the negative conductor with the other end, and working the machine. The needle will be deflected only one or two degrees. The great intensity and the small quantity of frictional electricity place it in striking contrast with voltaic electricity. If a galvanic pair consisting of an iron or copper wire about,g- of an inch in diameter, immersed about an inch ELECTRICITY. 257 in a little water to which has been added one drop of sulphuric acid, be connected with the above galvanometer, it will cause the needle to move six or eight degrees; but the electricity has so little tension that it cannot effect a discharge through the air even at a microscopic distance. Faraday has calculated that a wire of platinum and one of zinc, I- of an inch thick, immersed a of an inch in four ounces of water acidulated with one drop of sulphuric acid, will produce as great a quantity of electricity in three seconds as thirty turns of a so-inch plate machine. The positive conductor of an electrical machine answers to the positive pole of a galvanic battery, and the negative conductor to the negative pole, and the friction on the plates to the chemical action in the cells. With the galvanic battery an enormous quantity of electricity is obtained of slight tension; with the electrical machine, a small quantity of enormous tension. 286. Tze Electroscope. If a pith ball hung by a silk thread from a glass rod be brought near the ball of a prime conductor, it is at first briskly attracted and then as briskly repelled. This power of attracting light bodies is one of the most striking features of frictional electricity, and deserves especial study. It grows out of its high tension. The electricity at the unconnected poles of a powerful galvanic battery can be detected only by the most delicate apparatus, yet it can be shown to exist. The power of the electricity developed by friction to attract and repel light bodies furnishes the most ready means of detecting the presence of this electricity, as the needle furnishes the most ready means of detecting the presence of voltaic electricity. An instrument constructed on this principle for the detection of frictional electricity is called an electroscope. Figure Iio represents a common and convenient form of electroscope. It consists of a brass conducting-rod supporting a graduated semicircle, in the centre of which is Q 258 ELECTRICITY. a movable index made of very light wood, with a pith ball at the end. When it is attached to the prime Fig. II. conductor of the electrical machine, the pith ball is repelled as soon as the plate is turned. Figure III shows a more delicate electroI scope. It consists of a hollow glass ball, the neck of which is covered by a brass cap. Through this cap, but insulated from it, passes a brass rod having a brass ball at its upper end and two narrow strips of gold-leaf suspended from its lower end. If the brass ball is brought near a body charged with electricity, the strips of gold-leaf repel each other, as in Fig. the figure. As the gold-leaves are very easily torn, care must be taken not to communicate to them too strong a charge. In using this electroscope an instrument called a proof plane (see Figure 112) is often convenient. It consists of a small disc of gilt paper insulated by a glass rod. It is used by bringing it in contact with Fig. 112. the electrified body, and then with the brass ball of the electroscope. The gold-leaves will immediately diverge, and, as only a small charge can thus be communicated, there is no danger of injuring them. 287. The Electric Forces on Ate Positive and Negative Conductors act in Opposite Directions. - Insulate both conductors of the machine, and charge them with electricity by turning the plate. Bring a pith ball suspended by a silk thread in contact with the positive conductor, and it is soon repelled. Take it now to the negative conductor, and it is strongly attracted. Discharge now the pith ball by taking it in the hand, and again bring it in contact with the negative conductor, and it is repelled; ELECTRICITY. 259 but on taking it to the positive conductor it is attracted. We see then that a ball which is repelled by the force on one conductor is attracted by the force on the other. In other words, the forces on the two conductors act in opposite directions. These opposite electrical forces are called 5osilive and negative forces. 288. Both Electrical Forces are always developed together. -It is found to be impossible to develop one of these forces without at the same time developing both. The positive force always appears upon one of the substances rubbed together, and the negative force always appears upon the other. The force that acts in the same way as that upon the prime conductor of an ordinary electrical machine is called positive electricity, and the opposite force is called negative electricity. Of course, in order that both the forces should be detected, both of the substances rubbed together must be insulated. The force that appears upon each of the substances depends upon their nature. When any substance, as glass, is rubbed with different substances, the same force does not always appear upon it. 289. Induction. - If an insulated copper ball is connected with the prime conductor of the machine, and a small insulated con- Fig. I 3. ductor is placed near it (see Figure I3), on developing electricity and examining the condition of the insulated conductor, opposite electrical forces will be found to be developed upon its ends. On the end next the ball, negative force will be found; on the end farthest from the ball, positive force. This action of a charged body upon a body near it is called induction. The insulated conductor. is said to be polarized. 260 ELECTRICITY. A charged body polarizes all insulated conductors near it; that is, it develops upon them opposite electrical forces at opposite points. It always develops on the part of the conductor nearest it a force opposite to that with which it is itself charged; and on the part of the conductor farthest from it, a force the same as that with which it is charged. If one copper ball is connected with the positive conductor and another with the negative conductor, and a row of insulated conductors is arranged between these balls, as Fig. 114. X C C, V shown in Figure 114, on developing the electric force each of these insulated conductors will become polarized. The ends of the conductors towards the ball connected with the positive conductor are all negative, and the opposite ends positive. Place three insulated conductors end to end between the two copper balls. First arrange the conductors quite near each other, and develop electricity. Sparks will pass between the ends of the conductors. Completely discharge each conductor by passing the hand over it, and place the middle conductor quite near the conductor on one side, and at some distance from the conductor on the other side. Again develop electricity, and a spark passes between the middle conductor and the conductor nearest to it. Let the condition of the conductors between which the spark has passed be now examined by means of a light pith ball or a proof plane and electroscope. Previous to the passage of the spark there were two forces on each conductor; but after the passage of the spark only one force is found on ELECTRICITY. 261 each conductor. The force on each conductor is the same as that first developed on the ends farthest from each other; and of course those forces are of opposite kinds. The opposite forces, then, upon the ends of the conductors facing each other, have passed from the conductors and become neutralized. Let now the conductors on each side be brought up so as almost to touch the ends of the middle conductor, and the balls so placed as almost to touch the ends of the outside conductor. On turning the machine sparks pass between the conductors, and on examining them scarcely any force is found upon them. If the conductors and balls are brought quite in contact, no force is found upon them when the machine is turned. We see, then, that when insulated conductors are placed end to end near a charged body, opposite forces are developed at the opposite ends of each conductor; and that, when two conductors are so arranged that the spark can pass between them at one end and not at the other, only one force is found on each, and that one the force which was first developed on the ends between which the spark did not pass; also that, when several conductors are so arranged between the balls that a spark can readily pass from each end, no force remains on each. When the two opposite forces exist on a conductor, it is said, as we have already learned, to be polarized; when only one force exists on it, to be charged; and when no force exists on it, to be neutral. When a force which has been developed on an insulated conductor passes off, it is said to be discharged. When an insulated conductor is brought near a charged body, it is first polarized, and the nearer it is brought, the higher the polarization rises. If the conductor is so situated that it can discharge its force at only one end, it becomes charged with the same electric force as the body originally charged; if it discharges from the opposite end, 262 ELECTRICITY. it becomes charged with the force opposite to that on the originally charged body. If the conductor is so situated that it can discharge quite readily at both ends, but more readily at one end than at the other, there will be three steps in the process. The conductor will first become polarized, then charged, and finally neutralized. If the conductor is so situated that it can discharge quite readily, and with equal readiness, at each end, there will be only two steps in the process. It will be first polarized, and then neutralized. 290. The Polarization of the Insulated Conductor depends on the non-condutcting Medium which sej)arates it from the Char