UNIVERSITY OF CALIFORNIA. GIFT OF ' CLASS OF Received Accession No. . Clots No. ELEMENTARY CHEMISTEY. ILontlOtt : C. J. CLAY AND SONS, CAMBEIDGE UNIVEKSITY PKESS WAKEHOUSE, AVE MAEIA LANE. : DEIGHTON, BELL, AND CO. ILetpjtg: F. A. BROCKHAUS. ELEMENTAKY CHEMISTKY BY M. M. PATTISON MUIR, M.A. FELLOW AND PRELECTOR IN CHEMISTKY OF GONVILLE AND CAIUS COLLEGE, CHARLES SLATER, M.A. M.B. FORMERLY SCHOLAR OF ST JOHN'S COLLEGE, CAMBRIDGE. A COMPANION-VOLUME TO PATTISON MuiR AND CARNEGIF/S PRACTICAL CHEMISTRY. CAMBRIDGE: AT THE UNIVERSITY PRESS, 1887 [All Rights reserved.] ' Cambrttige : PRINTED BY C. J. CLAY, M.A. AND SONS, AT THE UNIVERSITY PRESS. PREFACE. THIS book forms one part of a course of elementary chemistry; the other part is contained in a companion- volume on Practical Chemistry. The two books are intended to be used together^ the one being complemen- tary to the other; their object is to teach the elements of chemical science. A real knowledge of chemistry can be gained only by the intimate blending of a properly graduated course of laboratory-work with the lecture- work and reading of the student. An attempt is made in this volume to present the principles of chemistry as rising out of and resting upon chemical facts, and chemical facts as furnishing the data from which principles are deduced. The book is intended to be used in conjunction with lectures on elementary chemistry, wherein more details will be given concerning important and typical bodies than are found in this volume. The book does not profess to be a descriptive catalogue of chemical facts regarding the properties of the individual elements and compounds. VI PREFACE. The authors entertain views rather different from those which generally prevail regarding the relative importance of the various parts of chemistry ; they have endeavoured to make the teaching given in this book sound so far as it goes; they have tried to bind together the facts and principles of the science, and at the same time to avoid speculation. M. M. PATTISON MTJIR. CHARLES SLATER. CAMBRIDGE, October, 1887. TABLE OF CONTENTS. CHAPTER PAGE I. Chemical Change ....... 1 II. Elements and Not-Elements .... 16 III. Mixtures and Compounds 25 IV. Conservation of Matter 37 V. Laws of Chemical Combination .... 40 VI, Symbols and Formulae 56 VII. Chemical Study of Water and Air . . -JO VIII. Chemical Study of Hydrogen and Oxygen . 91 IX. Acids and Salts ....... 102 X. Chemical Nomenclature 108 XL Chemical Classification . .......... 114 XII. Conditions which modify Chemical Change . 166 XIII. Chemical Affinity . . . ... 175 XIV. Chemical Changes and Changes of Energy . 184 XV. The Molecular and Atomic Theory 194 XVI. Applications of the Molecular and Atomic Theory 218 XVII. Isomerism and Structural Formulae . 236 XVIII. The Periodic Law ^nr^. .uu ---- r-- i ?- 26^, XIX. The Elements of Group II 279 Vlll CONTENTS. CHAPTER XX. The Elements of Group VI. . XXI. The Elements of Group V. . XXII. The Elements of Group I. . XXIII. The Elements of Group VII. XXIV. The Elements of Group III. XXV. The Elements of Group IV. . XXVI. The Elements of Group VIII. , Men of the Perio^ifijjaiv XXVII. Recapitulation and Conclusion and Recapitula- PAGE 290 301 312 323 326 334 347 358 CORRECTIONS AND ADDITIONS. PAGE. 222 240 241 PAR. 317 354 354 to table add MONATOMIC ATOMS Na, K. to list of monovalent atoms add Rb, Cs, Ag. ,, divalent ,, add Mn. trivalent add Al, Cr. to list of gaseous molecules on which classification of atoms is based add, RbCl, Rbl, CsCl, Csl, AgCl ; PbCL,, MnCl 2 ; A1C1 3 , CrCl 3 . ELEMENTAKY CHEMISTRY. CHAPTER I CHEMICAL CHANGE. CHEMISTRY is a branch of natural science. The aim of science is to " see things as they are." But to see things as they are it is necessary to study the relations of things, because in nature nothing is wholly cut off from other things, but everything is either a cause or a consequence of many others, and is related in manifold ways even to things which may seem to be wholly unconnected with it. For the purposes of exact study however some boundary lines must be drawn between what we call the different parts of each natural occurrence. Every natural occurrence, in relation to our powers of comprehending it, is infinitely com- plex ; in order 'to explain we must simplify; and to simplify we must overlook some portions of the complete phenomenon. Chemistry deals with certain portions of one class of material phenomena. The mark of this class of phenomena is, change of properties accompanying change of composition. The object of chemistry is to classify the phenomena it studies in order to discover general laws. The object of this book is to place before the student an outline of the methods by which chemistry proceeds ; to teach him some of the general laws of the science; and above all things to shew him that the laws are gained by studying natural occurrences, that the detailed study of these is the foundation on which the science rests, but that, in so far as it is a real branch of science, chemistry is much more than a descriptive catalogue of interesting facts. M. E. C. 1 2 ELEMENTAEY CHEMISTRY. [CHAP. I. A little observation suffices to shew that all things are undergoing change. Physics and chemistry deal with the phenomena " presented in material changes. Certain aspects of these changes we call physical ; certain aspects of them we call chemical. A fire burns on the hearth : when the fire was kindled the grate was filled with lumps of coal ; as the fire sparkles and blazes up the black coal changes to a light-giving, glowing, mass, radiating heat on all sides ; as the flames cease to play about the glowing coals the colour fades, the ashes accumulate, and the burning slackens ; at last the change stops, there remain only ashes and some pieces of unburnt coal. Many of the changes which pass before us as we watch the progress of a coal-fire are chemical changes. It is with such processes as this that we are concerned. The burning and slow extinction of an ordinary fire is however an extremely complex event ; we must turn to com- paratively simple occurrences if we are to learn the character- istic features of chemical change. When a piece of platinum wire is held in the flame of a Bunsen-lamp it becomes hot and gives out light; when the wire is removed from the source of heat it quickly cools, ceases to emit light, and returns to the same condition as before heating. When a piece of magnesium wire or ribbon is brought into the lamp-flame it also becomes hot and gives out light, but at the same time it rapidly burns away ; when removed from the source of heat it continues to burn with emission of dazzling white light ; after a ,little the burn- ing ceases ; the magnesium has now disappeared and in its place there is formed a white, soft, powder, called magnesia, very unlike the hard, lustrous, magnesium which was placed in the lamp-flame. Some change was here produced in the properties both of the platinum and magnesium. In the case of the platinum, the properties of glowing and of communicating heat to colder bodies brought into contact with it were temporarily added to the other properties hardness, lustre, tenacity, high specific gravity, infusibility, &c. which distinguish platinum from other kinds of matter. When those properties which had been temporarily added were withdrawn, the platinum was found to exhibit the same properties which characterised it before it was brought into the lamp-flame. In the second case, the magnesium also temporarily acquired the properties of glowing 14] CHEMICAL CHANGE. 3 and communicating heat to colder bodies brought into contact with it. But the withdrawal of these properties did not leave the magnesium as it was before heating ; accompanying the exhibition of these properties there was a gradual change of the magnesium into a substance so unlike magnesium as to be at once recognised as a different kind of matter. If a thin copper wire, covered with cotton or silk, is wrapped many times round a piece of soft-iron and an electric current is then passed through the wire, the iron will acquire the property of attracting iron -filings ; if the electric current is stopped the iron-filings cease to be attracted ; if the current is again passed through the wire the soft-iron at once acquires the attractive power. Before, during, and after, the passage of the electric current, the soft-iron exhibits all those pro- perties colour, relative density, tenacity, malleability, texture, &c. which mark it off from other kinds of matter; while the current is passing there is added to the iron the property of attracting iron-filings. If the same piece of soft-iron is exposed to damp air for a considerable time, a portion of it, or if sufficient time be given the whole of it, will be changed into iron-rust y which is a reddish powder unlike the iron in texture, colour, lustre, tenacity, malleability, and many other properties. The change of iron to iron-rust resembles the change of magnesium to magnesia, in that in both cases there is produced a new kind of matter. The temporary addition to iron of the property of attracting iron-filings resembles the temporary addition to platinum of the property of glowing and communi- cating heat to colder bodies, in that in both cases the change consists in the addition of a property which does not destroy or mask the original properties, and which can be withdrawn by reverting to the conditions existent before this property was added. Let a rod of copper and an electric bell be introduced into the circuit of a galvanic battery (s. fig. 1) ; the ringing of the bell shews that the electric current is passing through the rod of copper. The moment the current is broken the bell ceases to ring. The characteristic properties of the copper are not modified by the passage through it of the electric current. Let the rod of copper be now removed; let a piece of sheet- platinum be attached to the end of the wire from the battery, and also to the end of the wire from the electric bell, and let these pieces of platinum be placed, side by side but not touching, in a dilute aqueous solution of copper sulphate to 12 ELEMENTAKY CHEMISTRY. [CHAP. Fig. 1. which some sulphuric acid has been added (s. fig. 2). The ringing of the bell shews that the electric current is passing through the solution of copper sulphate; but the passage of the current is accompanied by the deposition on one of the Fig. 2. 47] CHEMICAL CHANGE. 5 platinum plates of a reddish solid which may be proved to be copper, and by the gradual disappearance of the copper sulphate from the solution. When the current is stopped there remains the new kind of matter, copper, which has been formed by the action of the electric current on the kind of matter originally present, copper sulphate; a certain amount of one kind of matter has disappeared and a certain amount of another kind of matter has been formed. The original matter is not reproduced by reverting to the conditions which existed before the change began ; that is to say, by stopping the passage of the electric current. The three kinds of matter, platinum, soft-iron, and copper, have been changed by temporarily adding to each a property which does not belong to it under ordinary conditions; this property existed only so long as the special conditions which caused its existence continued; the withdrawal of these conditions was accompanied by the withdrawal of the special property ; when this property was withdrawn the platinum, soft-iron, or copper, remained exactly as it was before the change had been effected. On the other hand, the three kinds of matter, magnesium, iron, and copper sulphate, have been changed by each permanently losing certain properties which characterise it, and at the same time permanently acquiring new properties which characterise other kinds of matter. Now we recognise different substances by their properties. One kind of matter is recognised, and distinguished from other kinds of matter, by its colour, texture, brittleness, opacity, relative density, hardness, &c. ; also by its behaviour when heated, when submitted to the action of electricity, placed in direct sunlight, mixed with water, brought into contact with other substances at high and low temperatures, &c. Substances which have markedly different properties are said to differ in kind, or to be different kinds of matter. Thus we say that iron is a kind of matter different from glass; that sand is a kind of matter different from wood, &c. The prominent feature of the change undergone by the magnesium when heated, by the iron when allowed to remain in damp air, and by the copper sulphate when the electric current was passed through it, is, that in each case a kind of matter has been produced different from, and in the place of, that which existed before the change began. The prominent feature of the change undergone by the platinum when heated, by the iron when the current was passed round it, and by the 6 ELEMENTARY CHEMISTRY. [CHAP. i. copper when the current was passed through it, is, that no new kind of matter has been produced, but that the kind of matter which existed before the change began existed also while the change lasted and after the change ceased. The first kind of change is called chemical change; the second is called physical change. The differences and resem- blances between these two kinds of change must be more fully illustrated. Iodine is a definite kind of matter, distinguished from other kinds by its lustre, greyish purple colour, opacity, easy solu- bility in alcohol with production of a reddish brown liquid, and by the fact that when a drop or two of this liquid is added to a very little starch paste a substance is formed which colours the liquid deep blue. Lead nitrate is a heavy, white, crystalline, solid ; it dissolves in a little hot water and separates from this solution, as it cools, in white, lustrous, crystals. Two retorts are arranged with the beaks passing into small dry flasks, as shewn in fig. 3 ; a little iodine is placed in one retort, and a little lead nitrate in the other ; each retort is heated by Fig. 3. a Bunsen-lamp. The iodine soon changes to a dark purple, almost opaque, gas ; but this condenses on the cooler parts of the retort and in the small flask, to a solid, which presents the same appearance, and is possessed of the same properties, as the iodine originally used. The lead nitrate is also changed ; a brownish red gas is produced which does not condense to a liquid or solid ; if the heating is continued so long as this gas is produced, a yellowish coloured solid remains in the retort ; this solid is a different kind of matter from the lead nitrate originally used. The change of solid iodine to gaseous iodine, and of gaseous iodine to solid iodine, is a, physical change; the change of lead nitrate into two new kinds of matter a brownish 710] CHEMICAL CHANGE. red gas called nitrogen oxide, and a yellowish solid called lead oxide is a chemical change. The change which water undergoes when it is boiled is a physical change; if the water is placed in a retort arranged as shewn in fig. 3, the water-gas (or steam) produced by heating the water is condensed to liquid water which is found in the small flask. But water may also be chemically changed. An. electric current is passed through water to which some sulphuric acid has been added. The current passes from one plate of platinum to another; these plates are placed each within an inverted tube full of water and standing in a vessel of water (s. fig. 4). Bubbles of gas rise from each platinum plate and collect in the inverted tubes. If the process is continued the water will at last entirely disappear and in Fig. 4. place of it we shall have two colourless gases. The gas in* each tube is examined as regards its behaviour towards a burning splint of wood : one of the gases takes fire, it is hydrogen; the other does not, but the splint of wood burns in the gas very rapidly and brilliantly, this gas is oxygen. These gases are definite kinds of matter ; each is evidently very different from the water from which both have been produced. The change of water into the gases hydrogen and oxygen is a chemical change. A few pieces of loaf-sugar are placed in a little water in a porcelain dish ; the sugar slowly disappears ; some change has 10 8 ELEMENTARY CHEMISTRY. [CHAP. I. occurred. The water is now removed by boiling ; the solid residue in the dish is sugar, it is characterised by all those properties which mark off sugar from other kinds of matter. The change of solid sugar into a solution of sugar, and of sugar-solution into solid sugar, is a physical change. 11 Into a small quantity of water are thrown one or two pieces of the metal sodium ; the metal swims on the surface of the water ; a gas is produced which may be collected and examined; when the sodium has disappeared the water is boiled off; there remains a white solid, which dissolves in water without formation of any gas, and which is evidently very different from either the sodium or the water by the mutual action of which it has been formed. This change - of water and sodium into a solid body, caustic soda, and a gas, hydrogen, is a chemical change. 12 A little hot concentrated sulphuric acid is poured on to some pieces of loaf-sugar; much heat is produced, steam is given off, and a black, charcoal-like, solid remains. Sugar was only physically changed when it was brought into contact with water : but the mutual action of sugar and hot sulphuric acid is a chemical change; both the visible products of this change, steam and carbon, are different kinds of matter from the bodies, sugar and sulphuric acid, by the interaction of which they have been produced. 13 All those changes which we have classed together as chemical have this in common, that one, or more than one, kind of matter has disappeared, and another kind, or other kinds, of matter has been formed. So far as our experiments could tell us, the new matter formed did not exist as a part of the material system before the change began. Those changes which we have classed together as physical have been characterised by the continued existence, during and after each process, of the same kind of matter which was present before the change began. This matter temporarily acquired a new property, or new properties; but the new property did not prevent us from recognising the other pro- perties by which the special kind of matter was marked off from other kinds. In both classes of occurrences the matter experimented with was subjected to new conditions different from those which existed before the experiments began. When these conditions were removed, in one class of phenomena the physical we had a return to the state of things which prevailed at the beginning of the experiments; we had the 1015] CHEMICAL CHANGE. 9 same kind or kinds of matter exhibiting the same properties : in the other class of phenomena the chemical we had not a return to the original state of things ; we had new kinds of matter exhibiting new properties. If the occurrences we have been considering were very 14 closely and accurately examined it would be found that those we have called chemical include changes which belong to the physical class. The emission of light by the burning magnesium, the conduction of heat through the mass of lead nitrate, the heating and volatilisation of water in the interaction of sugar and sulphuric acid ; these are physical rather than chemical changes. The physical, and the chemical, are different aspects of the complete phenomenon. We try to separate them as far as we can that we may study each more accurately, and so find the general laws which hold good for each ; for the more we understand natural events the more are we convinced that one law of nature is never suspended or stopped by another law, however the effects of one may be modified by the effects of another. But we cannot at present attempt minutely to analyse the phenomena we have to study into chemical and physical parts \ we must be content to learn the broad features of the two classes of occurrences. In reasoning on the data obtained in the experiments 15 already described, certain assumptions have been made, and certain expressions have been used somewhat vaguely. It was asserted that when magnesium was burnt in air, the matter called magnesium disappeared and its place was taken by a new kind of matter called magnesia; that when lead nitrate was heated the matter characterised by the properties summed up in the name lead nitrate disappeared and in its place there were formed two other kinds of matter, nitrogen oxide, and lead oxide ; that the passage of the electric current through copper sulphate solution was accompanied by the disappearance of one kind of matter, copper sulphate, and the formation of another kind of matter, copper; and similarly with the other experiments. Now one may well ask : how can it be proved that the magnesium, or the lead nitrate, or the copper sulphate, really disappeared ? how can it be proved that the place of the magnesium was taken by mag- nesia, or of lead nitrate by lead oxide and nitrogen oxide, or of copper sulphate by copper 1 ? And questions such as these must also be asked : what exactly is meant by saying that the 10 ELEMENTARY CHEMISTRY. [CHAP. I. magnesium, or the lead nitrate, disappeared ? : magnesia took the place of magnesium ; copper, the place of the copper sul- phate ; what is meant by saying one substance takes the place of another? An electric current was passed through water, some of the water disappeared and in its place two gases were produced ; but may not these gases have come from the platinum plates, or from the glass of which the vessel was made 1 Or, assuming that the water was indeed changed in this experiment into the two gases called hydrogen and oxygen, we ask ; what definite meaning is to be put upon the statement water can be changed into hydrogen and oxygen ? 16 The history of alchemy, and of the transition from alchemy to chemistry, teaches the necessity of putting, and of answering, such questions as these. The alchemists not only thought that they could, but asserted that they did, change water into earth or into fire, lead into silver, and copper into iron. Their conception of nature led them to regard all things as under- going continual change ; but they were not able so accurately to study these changes as to discern the unchanging sequences in which they occurred, and to grasp the unchangeable parts of the phenomena they observed. The assertion that water could be changed into earth, or into fire, was based upon such experimental evidence as this. A quantity of water was heated in an open glass vessel ; the water slowly disappeared, and a little white earthy solid matter remained in the vessel. The water had disappeared and an earthy solid had been produced in its place. A piece of red hot iron was plunged into water contained in a bell- shaped glass vessel; bubbles of gas rose through the water and collected in the vessel ; this gas took fire when a lighted taper was brought into it. The water had been changed into 'the matter of fire'. To* prove that copper could be changed into iron, the alchemist placed a piece of copper in aqua fortis (nitric acid); the copper slowly disappeared; in the blue-green liquid thus formed he placed a piece of iron; the iron dis- appeared, and copper was produced in its place. The conclusion which the alchemist drew from such experiments as these was that one kind of matter could be changed into other kinds of matter. But if this were so, why should not any kind of matter be changeable into any other] Heat brought about the change of water to earth ; hot iron, the change of water into l the matter of fire' ; aqua fortis, the change of iron into copper. There must surely be some one thing which would 1517] CHEMICAL CHANGE. 11 effect all transmutations. The pursuit of this One Thing became the central quest of alchemy. "There abides in nature" we read in an alchemical treatise "a certain pure 'matter, which, being discovered and brought by art to perfection, converts to itself proportionately all imperfect bodies that it touches." Alchemy was a fascinating dream ; but chemistry is a more satisfying reality. Let us return to experiment. Let a small weighed quantity of magnesium be burnt in air under conditions such that the whole of the magnesia produced in the burning remains in the vessel in which the burning proceeds. The apparatus shewn in fig. 5 is a simple one for the purpose. Some magnesium ribbon is placed on a piece of wire- gauze and covered with an inverted funnel, the stem of which is con- nected by caoutchouc tubing with another funnel ; the upper funnel is covered with filter-paper. The whole apparatus is counterpoised ; the funnels are removed; the mag- nesium is ignited by allowing a Bunsen-flame to play on to it from above; the funnels are then re- placed. When the burning is com- plete the apparatus is allowed to cool and is then counterpoised. "~ It is found that the magnesia pro- Fig. 5. duced weighs more than the mag- nesium before burning. We therefore conclude that the mag- nesia is produced by adding to, or combining with, the mag- nesium, some other kind of matter. As the change from mag- nesium to magnesia proceeded in air, it is probable that the new kind of matter, which, by our hypothesis, has combined with magnesium and so produced magnesia, is derived from the air. To find whether this conclusion is correct or not, it would be necessary, (1) to burn a known weight of magnesium in a known quantity of air; (2) to determine the weight of magnesia produced, and the diminution in the quantity of air which accompanied the production of this weight of magnesia ; (3) to change the magnesia back to magnesium and air, and to determine the weight of each of these obtained. If the difference between the weight of the magnesia and that of 17 12 ELEMENTARY CHEMISTRY. [CHAP. I. the magnesium, by burning which the magnesia was formed, was equal to the weight of air which disappeared during the burning ; and if the magnesium obtained from the magnesia weighed the same as the magnesium originally burnt ; and if the weight of air obtained from the magnesia was the same as the weight of air which disappeared during burning ; then we should be justified in concluding that the chemical change which occurs when magnesium is burnt consists in the addition to, or combination with, magnesium, of a portion of the surrounding air, and that the new kind of matter produced is composed of two kinds of matter, viz. magnesium and air. "We should further have learned that although the magnesium has disappeared it has not been destroyed; and that although magnesia has been produced it has not been produced from previously non-existent matter. We should have given a definite meaning to the terms ' pro- duced 1 and ' disappeared' ', as regards the change of magnesium to magnesia at any rate ; and we should, to some extent at least, understand what is meant by saying the magnesia has taken the place of the magnesium which was burnt. It is not easy to arrange quantitative experiments by which the chemical change of magnesium to magnesia may thus be examined. But if we use mercury in place of magnesium, we can arrange an experiment which will enable us to find an answer to each of the three questions stated in par. 15. 18 The change of mercury to burnt mercury, or as we now call it oxide of mercury, was examined quantitatively by Lavoisier. A sketch of the essential parts of his apparatus is shewn in fig. 6. Lavoisier placed 4 oz. of mercury in a glass balloon, the neck of which, drawn out and bent, passed under mercury and then into the air contained in a bell -jar ; the bell-jar contained 50 cub. inches of air. The mercury was heated nearly to its boiling point by means of a furnace; red specks appeared on the surface of the mercury and the volume of air in the bell-jar slowly decreased. After some days the production of red solid matter on the surface of the mercury seemed to have ceased ; heating was continued for a few days more (12 days in all), and was then stopped. The air in the bell-jar now measured between 42 and 43 cub. inches, the diminution in volume was therefore between 7 and 8 cub. inches ; the red solid was collected, and was found to weigh 45 grains. These 45 grains of the red solid produced by slowly burning mercury in air were placed in a glass tube 17, 18] CHEMICAL CHANGE. 13 closed at one end and drawn out at the other ; the open end passed, under mercury, a little way into a graduated glass vessel filled with mercury (s. fig. 6). When the red solid was heated mercury was formed and deposited on the colder parts of the tube, and a gas collected in the graduated vessel. The mercury thus formed weighed 41 J grains; the gas measured Fig. 6. between 7 and 8 cub. inches. The gas was proved to be, not air, but oxygen : now 7 J cub. inches of oxygen measured at the temperature and pressure of Lavoisier's experiment, weigh 3J grains. Therefore the 45 grains of red solid formed by slowly burning mercury in air consisted of, or were formed by the chemical combination of, 41 J grains of mercury and 3|- grains of oxygen ; and these 3^ grains (or 7^ cub. inches) of oxygen were originally present in the 50 cub. inches of air contained in the bell-jar. When the mercury was burnt 41 J grains of it disappeared, and at the same time 3J grains of one of the constituents of air disappeared, and 45 grains of a new kind of matter were produced ; but these 45 grains of this new matter were composed of the 41 J grains of mercury and the 3tj grains of oxygen which had disappeared. No loss or destruction of matter occurred during the burning. The hot mercury so interacted with the oxygen contained in the air that there was produced a kind of matter altogether different from either of the interacting bodies. 14 ELEMENTARY CHEMISTRY. [CHAP. I. 19 Now if all the experiments already described were repeated so that the weights of the different kinds of matter taking part in each chemical change were determined, and the weights of each and every new kind of matter produced in each of these changes were also determined, and this has actually been done, we should find that the new kinds of matter formed were formed by the union or combination of the different kinds of matter which constituted the material system at the begin- ning of each experiment. 20 The terms disappeared, and was produced, do not then mean were destroyed, and were created ; they rather mean, ceased to exist under the conditions of experiment as a distinct kind of matter, and, was the product of the chemical interaction of two or more kinds of matter which previously existed each as a distinct kind of matter. Similarly the expression used in previous paragraphs, has taken the place of, is now seen to mean, has been formed by the chemical interaction of ; the expression also implies that the weight of the new matter which has taken the place of that formerly present is equal to the weight of that which it has replaced. In the case of the magnesium burnt to magnesia, it would be correct to say that the magnesia has taken the place of the magnesium and a certain weight of oxygen in the air. We now see more clearly than before what is meant by saying that this or that body has been chemically changed into certain other bodies. But a definite and accurate meaning can be given to this and similar expres- sions only when we have learned more about chemical occur- rences. 21 In the preceding paragraphs the important truth has been assumed that the one fundamental property of matter is its mass or quantity. Moreover it is assumed that the student has learned the proportionality of mass and weight*; that any portions of matter whose masses are equal, however different they may be in other properties, are of equal weights. The mass of any portion of matter is the quantity of matter in that portion; the weight is the force with which that portion is attracted towards the earth's centre. But in all practical problems with which we shall have to deal, the terms mass and weight may be taken as synonymous ; because the relative * If the student is not familiar with the connection between mass and weight he ought to consult a treatise on physics. 1921] CHEMICAL CHANGE. 15 masses of substances are determined in chemistry by weighing them against a standard mass of brass, or other material, called 1 gram, or 1 decigram, or 1 milligram, and the weights of substances as thus determined are independent of variations in the force of gravity. We have now some notion of what is implied in saying that in a chemical change some kinds of matter cease to exist as such, and some other kinds of matter are produced by the interactions of those originally present. To some extent we see that in chemical occurrences change of properties is con- nected with change of composition. CHAPTER II. CHEMICAL COMPOSITION OF DIFFERENT KINDS OF MATTER. ELEMENTS AND NOT-ELEMENTS. 22 WE must now more fully examine the conception expressed in the phrase composition of this or that kind of matter. It has been already proved by experiment that when a specified mass of magnesium is burnt in air, a new body is produced composed of the magnesium and oxygen taken from the air, and that the mass of this product of the burning of magnesium is greater than the mass of the magnesium used. The magnesium has been changed by adding to, or combining with, it, another kind of matter, viz. oxygen. If a little very finely divided iron is weighed, and then heated to redness, the iron will glow brightly ; if the source of heat is then withdrawn it will be seen that a reddish brown substance, quite unlike the original iron, has been produced ; if this substance is weighed, when cold, it will be found to weigh more than the iron used. Iron, like magnesium, has been changed into a new substance, and this change has been effected by causing the iron to combine with some kind of matter different from itself. As we know that the burning of magnesium consists in combination with oxygen, and as the conditions under which the iron has been chemically changed are similar to those which prevailed during the burning of magnesium, we conclude that the change which the iron has suffered probably consists in combination with oxygen. This conclusion has been verified by experiments. 23 A weighed quantity of finely divided copper is dissolved in moderately concentrated warm sulphuric acid; when solution is complete, the greenish blue liquid is evaporated to dryness by steam; the blue solid is kept at 100 until it is perfectly dry, when it is collected and weighed. It weighs more than JIVE'RSITYfr 2225] ELEMENTS AND NOT-ELEMENTS. 1*7 the copper did. The blue solid is now dissolved in water, some sulphuric acid is added, and an electric current is passed through the liquid until it is perfectly colourless. Every particle of the red solid which has formed on one of the platinum plates is washed off the plate into a small basin, where it is repeatedly washed with water, then with alcohol, then dried over oil of vitriol, and weighed. It weighs the same as the copper did at the beginning of the experiment; moreover an examination of the properties of this red solid proves that it is copper. In this series of changes, copper has been con- verted into a blue solid called copper sulphate by causing it to react with warm sulphuric acid, and this copper sulphate has been re-converted into copper by electrolysing it. The copper sulphate was produced by combining some other kind of matter with the copper : the experimentally determined fact, that the mass of the copper obtained from the copper sulphate was equal to the mass of copper dissolved in sulphuric acid, proved that the copper sulphate was produced by the union of the copper with some other substance. A little concentrated nitric acid is heated in a porcelain 24 dish ; after a time the whole of the liquid has disappeared ; it has been entirely gasified. A few scraps of thin tin-foil are now weighed into a porcelain dish and a little concen- trated nitric acid is allowed to fall, drop by drop, on to the tin ; the tin is changed to a loose white powdery solid ; this is heated so long as any gas comes off, then allowed to cool, and weighed. The white powder weighs more than the tin did. But we know that nitric acid is entirely volati- lised by heating in an open dish ; hence we conclude that in the reaction between the two kinds of matter, tin and nitric acid, the tin has probably laid hold of some constituent or constituents of the acid, and that the white powder formed is the product of the union of the tin with this substance. This conclusion has been verified by carefully conducted ex- periments : it has been proved that the change of tin to the white powdery solid produced in the last experiment consists in the combination of the tin with one of the con- stituents of nitric acid, namely, oxygen. The experiments now described have this feature in com- 25 moil : in each a chemical change has occurred ; one kind of matter has been changed into another, and the change has consisted in the combination of the original matter with another kind of matter unlike itself. Each change has been M. E. C. 2 18 ELEMENTARY CHEMISTRY. [CHAP. ii. 26 an interaction between at least two definite kinds of matter ; the product of the change has consisted of the whole of one of the interacting substances, and either the whole, or a part, of the other. The mass of the product has therefore been greater than the mass of that one of the interacting substances which was weighed before the change began. Magnesium and iron combined with oxygen contained in the air surrounding them ; copper combined with sulphur and oxygen obtained from the sulphuric acid with which it interacted; tin combined with oxygen obtained from nitric- acid : the oxide of magnesium, or iron, or tin, thus produced weighed more than the magnesium, or iron, or tin, used ; and the sulphate of copper produced weighed more than the copper used. But the whole of the magnesium, or iron, or tin, or copper, formed a part of the new kind of matter into which it was changed. Let us now turn to some chemical changes which differ from those just considered in that in each of them a specified mass of one definite substance is converted into two or more different substances the mass of each of which is less than that of the original substance. A flask with a good fitting cork and exit tube is arranged as shewn in fig. 7 ; the glass cylinder is graduated, filled Fig. 7. with mercury, and inverted in a vessel containing mercury. A weighed quantity of a white solid called potassium chlorate is placed in the flask : this solid is heated until it melts ; the 2528] ELEMENTS AND NOT-ELEMENTS. 19 gas which comes off is collected in the graduated cylinder. The heating is continued as long as any gas is produced. The apparatus and its contents are allowed to cool, precautions being taken to prevent the mercury from rushing back into the flask. By methods which need not be described here, the whole of the gas which is in the flask and exit tube when the experiment is finished is driven into the graduated cylinder. The white solid in the flask is now weighed ; the small quantity of air in the cylinder (which air was in the flask and exit tube at the beginning of the experiment) is removed by suitable methods, and the gas in the cylinder is measured. This gas is now proved to be oxygen. The properties of the white solid left in the flask are compared with those of the potassium chlorate, i.e. with those of the kind of matter used in the ex- periment : the two substances are easily proved to be very different. The white solid produced in the change is called potassium chloride. As the weight of a specified volume of oxygen has been accurately determined, the weight of oxygen produced in the process is easily calculated from the observed volume of the oxygen. In this experiment, one kind of matter potassium chlorate has been changed, by the agency of heat, into two kinds of matter oxygen, and potassium chloride ; the mass of each of these is less than that of the potassium chlorate, but the sum of the masses of the oxygen and the potassium chloride is equal to the mass of the potassium chlorate. An electric current is passed through acidulated water. 27 The experiment is conducted as described in par. 9. (s. fig. 4). But the water used is weighed, and the water remaining at the close of the experiment is weighed; the volumes of hydrogen and oxygen produced are measured ; and special precautions are taken that no water is spilt or lost, and that all the hydrogen and oxygen produced are collected in the tubes. When certain corrections have been made 011 account of the slight solubility in water of the gases hydrogen and oxygen, the result of this experiment is, that a specified mass of water has been changed into hydrogen and oxygen, that the mass of each of these is less than that of the water used, and that the sum of the masses of the two gases is equal to the mass of the water. A small weighed quantity of a black, solid called copper 28 oxide is placed in a bulb of hard glass arranged as shewn in fig. 8. The U tubes contain calcium chloride, a substance 22 20 ELEMENTARY CHEMISTRY. [CHAP, u. 29 which greedily absorbs water ; these tubes, as well as the little dry bulb a, are accurately weighed ; the bulb containing the Fig. 8. copper oxide is also weighed. Precautions are taken that the entire apparatus is quite air-tight. Pure dry hydrogen is passed slowly in as shewn by the arrow ; after a few minutes- the copper oxide is heated ; drops of a liquid resembling water -begin to trickle down into the bulb a; heating in a slow stream of hydrogen is continued as long as any trace of what seems to be water is produced. When the change is com- plete, the apparatus is allowed to cool ; and the various parts are then weighed. The liquid formed can be proved to be water, and the red solid left can be proved to be copper. Assuming that the proofs are conclusive ; and assuming that 1 part by weight of hydrogen combines with 8 parts by weight of oxygen to produce 9 parts by weight of water the results of the last experiment shew that this is so, and the statement has been amply verified experimentally ; assuming these points, the results of the present experiment teach, (1) that the only products of the interaction of copper oxide and hydrogen are water and copper ; (2) that the water is formed by the union of the hydrogen with oxygen previously combined with copper ; (3) that the mass of the oxygen thus taken away from combination with copper is less than the mass of the copper oxide, and the mass of the copper thus removed from combination with oxygen is also less than the mass of the copper oxide; (4) that the sum of the masses of the copper and the oxygen is equal to the mass of the copper oxide. The results of the experiments described in the preceding paragraphs present certain points of similarity. In each, a specified mass of one kind of matter was changed into two 2830] ELEMENTS AND NOT-ELEMENTS. 21 (or more) kinds of matter, each different from, and each weighing less than, the original matter. Potassium chlorate was changed into potassium chloride and oxygen ; water into hydrogen and oxygen ; copper oxide into copper and oxygen : the potassium chloride, or the oxygen, weighed less than the potassium chlorate ; the hydrogen, or the oxygen, weighed less than the water ; the copper, or the oxygen, weighed less than the copper oxide x . But the sum of the masses of the products of each change was equal to the mass of the kind of matter which was changed into these products : the mass of the ' potassium chloride added to that of the oxygen was equal to the mass of the potassium chlorate changed; the mass of the hydrogen added to that of the oxygen was equal to that of the water; the mass of the oxygen added to that of the copper was equal to that of the copper oxide by the decomposition of which the oxygen and copper were produced. We have now examined two classes of chemical changes. One class presented to us interactions between two, or more, definite kinds of matter, resulting in the disappearance of the interacting substances, and the production in their place of one, or more, substances very unlike the original kinds of matter. We paid attention to the mass of one of the inter- acting substances, and to the mass of that product of the change which contained the whole of this substance (neglect- ing other products if other products there were) ; we found that the parts of the change to which we paid attention consisted in the combination of the whole of one of the interacting .substances with either the whole, or a part, of the other substance. In each experiment a certain kind of matter was changed into a different kind of matter, by entering into combination with some substance different from itself. The other class of chemical changes presented to us decom- positions of one definite kind of matter into two, or more, different substances ; the original kind of matter disappeared, a,nd its place was taken by the new substances formed from it. We found that the mass of any one of the new kinds of matter was less than the mass of the matter from which it was derived, but that the sum of the masses of all the new kinds of matter was equal to the mass of the matter which had been changed into these new kinds of matter. One of the changes considered presented features common to 1 The student should carefully follow the reasoning on which this conclusion was based, (s. par. 28.) 30 22 ELEMENTARY CHEMISTRY. [CHAP. II, both classes of changes. Copper oxide interacted with hydrogen ; water and copper were produced : but the water was itself shewn to be produced by the union of the reacting hydrogen with oxygen separated from the copper oxide. The mass of the water added to that of the copper was greater than that of the . copper oxide ; this was because the water formed was com- posed of the oxygen at first combined with the copper and also the hydrogen which was one of the constituents of the whole changing system. The change of the copper oxide into copper "and oxygen was a change belonging to our second class of chemical reactions ; but this was accompanied by a change belonging to the other class of reactions, viz. the production of water by the combination of the oxygen separated from the copper with the hydrogen, the presence of which hydrogen in contact with heated copper oxide was the condition under which the separation of copper oxide into copper and oxygen was accomplished. 31 When various kinds of matter are examined chemically r it is found that they all belong to one or other of two classes, which we may at present call the hydrogen-class and the water-class. Those kinds of matter which are placed in the hydrogen- class are characterised by this; when any one of them is changed into a totally different kind of matter, the mass of this kind of matter is greater than the mass of the substance belonging to the hydrogen-class which was thus changed. These substances have never been changed by separation into unlike parts. They suffer chemical change by combining with another kind, or other kinds, of matter, the test of this combi- nation being that the substance produced differs from, and weighs more than, the original substance. Those kinds of matter which are placed in the water-class are characterised by this ; any one of them may be chemically changed by separating it into unlike parts, the test of this separation being the production from a specified mass of the original substance of at least two different kinds of matter the mass of each of which is less than that of the original sub- stance, while the sum of their masses is equal to that of the original substance. Most, if not all, of the kinds of matter placed in this class may also be chemically changed by com- bining with some other kind, or kinds, of matter different from themselves, and so producing a new kind of matter weighing more than the original substance. 3032] ELEMENTS AND NOT-ELEMENTS. 23 Those kinds of matter which belong to the hydrogen-class 32 are called Elements: those which belong to the water-class we shall at present call Not-Elements. About seventy different kinds of matter belong to the class Elements. By no experiments hitherto tried have chemists succeeded in separating any one of these into unlike parts. When the elements are brought into contact with each other or with not-elements ; or are subjected to the action of heat, light, electricity, or magnetism, either in the presence or absence of other kinds of matter ; or are compressed, or hammered, or drawn into wires, or otherwise mechanically treated; they either remain unchanged into kinds of matter different from themselves, or they combine with other sub- stances and produce new kinds of matter each weighing more than the element from which it has been produced. In the present state of knowledge then we regard an ele- ment as a completely homogeneous kind of matter. We do not assert that an element is completely homogeneous ; that every attempt to separate an element into unlike parts must neces- sarily fail ; but we say that so far as experimental investiga- tion has gone those kinds of matter which are called elements behave as if each were a distinct substance different in kind from all other substances, and were composed of one kind of matter only. The properties of the elements differ much. The following well-known substances are elements : iron, lead, tin, silver, gold, copper. These are all heavy, lustrous, malleable, solids. A few elements are gases at ordinary temperatures and pressures, viz. oxygen, hydrogen, nitrogen, chlorine, fluorine (f) : two are liquids under ordinary conditions, viz. bromine and mercury : the others are solids. Some of the elements are found uncombined with other elements in rocks, e.g. carbon, iron, tin, copper, gold, platinum, sulphur ; oxygen and nitrogen form the chief constituents of the atmosphere ; hydrogen is sometimes found in volcanic gases. Most of the elements however have been separated from those combinations of them with other elements which are found in rocks, soils, waters, or parts of animal or vegetable organisms. The colour of many elements is grey to white ; a few are yellowish-white, or yellow ; one or two are reddish-brown ; three are colourless gases. Some of the elements are very malleable and very ductile ; others are very brittle : some melt at very low temperatures ; others only at the highest attain- 24 ELEMENTARY CHEMISTRY. [CHAP. II. 32 able temperatures, one or two have never been fused. Most of the elements are heavier, some as much as 20 or 22 times heavier, than an equal volume of water ; a few are specifically lighter than water. Some are very good conductors of heat and electricity ; others are practically nonconductors : most elements are opaque; a few are translucent. Some again very readily react chemically with most of the others to pro- duce new kinds of matter; e.g. compounds of oxygen, of chlorine, of bromine, or of sulphur, with most other elements, are known. On the other hand, some elements e.g. boron and nitrogen, combine directly only with a comparatively small number of other elements. To state the name of an element is to state the composition of the element : the name is a short symbol for certain properties which characterise that kind of matter to which the name is given, and mark it off from other kinds of matter. So far as we know at present the element is composed of itself; i.e. any quantity of it is not made up of, or formed by the union of, two or more different kinds of matter, but is completely homogeneous. CHAPTER III MIXTURES AND COMPOUNDS. THE different kinds of matter classed together as Not- 33 Elements are composed each of two or more elements. But we must attempt a subdivision of the class not-element : we are not specially concerned with all the members of this class. If some finely divided iron is intimately mixed with some powdered sulphur, a heavy, greenish-grey, solid is formed. This solid cannot be an element; the method of its prepa- ration precludes this. It is composed of the two distinct kinds of matter, iron and sulphur. It belongs to the class Not- Elements. But is it one of those not- elements whose properties and composition are studied in chemistry ? Make a very intimate mixture of finely divided iron and sulphur, in the ratio of 1 part sulphur to 1J parts iron by weight. Compare the colour and appearance of this mixture with the colour and appearance of each of its constituents, iron and sulphur : the mixture is neither brownish black like iron, nor yellow like sulphur ; it is not lustrous like iron, nor is its texture that of sulphur. The colour and appearance of the mixture are approximately the mean of the colour and appearance of its constituents. Place a little finely divided iron in water ; the iron sinks : place a little powdered sulphur in water ; part of the sulphur floats to the surface. Bring a magnet under a sheet of white paper on which is strewn a little finely divided iron, and blow (with the mouth) along the surface of the paper ; a good deal of the iron remains held by the magnet, although the paper is between the iron and the magnet : examine the action of the magnet on powdered sulphur under similar conditions ; the sulphur is entirely blown away, none is held by the magnet. Examine some of the iron, and some of the sulphur, 26 ELEMENTARY CHEMISTRY. [CHAP. III. used, under the microscope, and note the differences in their appearance. Pour a little carbon disulphide on to a very little powdered sulphur^ and on to a very little finely divided iron, respectively, and very gently warm each ; the sulphur slowly dissolves in the carbon disulphide, the iron remains unchanged. Pour a little hydrochloric acid on to portions of the iron, and sulphur, used : the iron slowly dissolves and a gas is evolved which can be proved to be hydrogen ; in the case of the sulphur no gas is evolved, nor is there any apparent change. Now turn to the mixture of iron and sulphur, and deter- mine whether the iron in it is characterised by those proper- ties which we have found belong to iron as a definite kind of matter, and whether the sulphur in it exhibits the properties which belong to sulphur when it is unmixed with other kinds of matter. Experiment proves that the mixture may be separated . into iron and sulphur, by shaking it with water, or by dissolving out the sulphur by carbon disulphide, or by holding the iron by a magnet and blowing away the sulphur. Experiment also proves that hydrochloric acid reacts with the mixture to dissolve the iron and leave, the sulphur, and that hydrogen is produced in this reaction. Examination of the mixture under the microscope shews the particles of iron, and the particles of sulphur. Now heat another portion of the mixture of iron and sulphur ; it glows throughout : powder the black mass which remains after cooling, and heat it again. Again powder the heated substance, and determine whether iron or sulphur can be detected in it by making use of those properties of iron and sulphur, respectively, which we know characterise these kinds of matter. The appearance and colour of the substance are distinctly different from those either of iron or sulphur ; the substance is not separated into iron and sulphur by any one of the three methods (water, magnet, carbon disulphide), each of which separated the mixture of iron and sulphur into, its constituents ; the substance appears under the microscope to be homogeneous ; interaction with hydrochloric acid results in solution of the substance as a whole, and production of a gas which is not hydrogen, but is sulphuretted hydrogen, a body easily distinguished from hydrogen by many prominent pro- perties. The substance produced by heating a mixture of one part sulphur with If parts iron is thus proved to be a kind of matter quite different from either iron or sulphur; the sub- 3335] MIXTURES AND COMPOUNDS. 27 stance produced by mixing sulphur and iron in the ratio 1 : 1 j was proved to possess properties characteristic both of iron and of sulphur. Mixing iron and sulphur has evidently not produced a chemical change : heating the mixture of iron and sulphur has produced a chemical change. With a mixture of iron and sulphur, we are not especially 34; concerned in chemistry ; with the new kind of matter, called iron sulphide, produced by heating the mixture of 1 part sulphur with If parts iron, we are especially concerned. Both kinds of matter are Not-Elements : the first is a mixture of different kinds of matter, each of which can be recognised in the mix- ture by properties which characterise it when it is unmixed with other kinds of matter ; the second is a compound formed by the combination of different kinds of matter, none of which can be recognised in the compound by properties which belong to it when uiicombined with other kinds of matter. The class Not-Elements is divided into compounds and mixtures. Chemistry deals with the changes of composition and of properties of compounds. We are at present en- deavouring to understand what is meant by the composition of compounds. In order to learn something about the com- position of compounds we must gain as clear a notion of the differences between compounds and mixtures as we can at this stage of our progress. Ammonia is a colourless, volatile, gas, with a very pungent 35 and penetrating smell ; charcoal is a black, porous, light, solid : these two kinds of matter may be recognised by these properties. Let a quantity of ammonia be confined in a tube over mercury ; and let a few pieces of charcoal (previously heated to remove air from their pores) be passed into the tube. The ammonia is rapidly absorbed by the charcoal, and the mercury rises in the tube. The appearance of the charcoal is not changed ; only it smells strongly of ammonia. The ammonia is easily removed from the charcoal, with which it is mixed, by warming the charcoal in a small dry flask, and allowing the ammonia to collect in a tube filled with mercury and placed mouth downwards in a vessel of mercury. Hydrogen chloride is a colourless, volatile, gas, with a very pungent smell, and most irritating action on the skin. Let a certain volume of ammonia be confined over mercury, and let an equal volume of hydrogen chloride be passed into the vessel ; instantly there is produced a white solid, utterly 28 ELEMENTARY CHEMISTRY. [CHAP. III. unlike either the ammonia or the hydrogen chloride by the interaction of which it has been formed. If this solid is collected and examined it is found to be characterised by none of the properties which mark off ammonia and hydrogen chloride, respectively, from other kinds of matter. Ammonia and charcoal when brought together form a mixture ; both constituents are easily recognised in the mixture by the same properties as those by which they are recog- nised when unmixed. Ammonia and hydrogen chloride when brought together form a compound, called ammonium chloride ; neither constituent can be recognised in the compound by the properties by which it is recognised when apart from other kinds of matter. 36 Water boils at 100 when the pressure on the surface of the water is equal to 760 mm. of mercury. At the same pressure alcohol boils at 7 8 '3. Each of these liquids may be recognised, and differentiated from other substances, by observing its boiling point. A mixture of water and alcohol in about equal parts is placed in a flask, fitted with a thermometer, and connected with a condenser and receiver, as shewn in fig. 9. When the liquid has been heated to boiling the thermometer registers a temperature higher than 7 8 -3 and lower than 100; as boiling continues the temperature rises, but a fixed boiling point is not attained. The liquid in the receiver may be proved to be a mixture of water and alcohol, and the portions which distil over soon after boiling begins may be proved to be richer in alcohol than those which distil over after boiling lias continued for some time. If the liquid which distils over (the distillate) is collected in a series of flasks, so that each contains that quantity which has come over for a temperature- interval of (say) 5 or 8, and if each of these quantities is again distilled, and the distillate for every 3 or 4 is collected in separate vessels, it is possible to effect a rough separation of the original mixture of water and alcohol into two liquids, one of which consists for the most part of water and the other for the most part of alcohol. This separation of the mixture has been effected by making use of a property of each consti- tuent, which property is a characteristic physical property of that constituent when unmixed with other kinds of matter. 37 Butylene is a colourless liquid, boiling at (about) 3. Bromine is a dark reddish brown, heavy, strongly smelling, liquid, boiling at (about) 60. Each of these is a definite kind 3538] MIXTURES AND COMPOUNDS. 29 of matter characterised by definite properties, of which the boiling point is one. When butylene and bromine are mixed in the ratio of 1 part butylene to 2 -86 parts bromine, by weight, a colourless liquid unlike either constituent is pro- duced. The weight of the liquid formed is equal to the sum of the weights of the butylene and bromine. If the liquid is distilled (*. fig. 9) the thermometer registers 160 from the time when boiling begins until the last drops of the liquid Fig. 9. have passed over into the receiver ; moreover the distillate has the same properties as the liquid before distillation. Butylene and bromine have formed a compound (called butylene bromide), whose properties are very different from those of either constituent, and from which neither constituent can be withdrawn by taking advantage of one of the physical pro- perties, viz. boiling point, belonging to each constituent when uncombined with other substances. The constituents of a mixture of gases may frequently be separated by making use of the property which gases have of passing through the fine pores of a mass of dry plaster of 38 30 ELEMENTARY CHEMISTRY. [CHAP. in. Paris. The passage of a gas through such a porous substance as plaster of Paris is called diffusion. If two graduated glass tubes are each stopped at one end with a thin dry plate of plaster of Paris, if one is then filled with hydrogen and the other with oxygen, and if both are at once placed in water with the open ends under the water (s. fig. 10), the water will begin to rise in both tubes. As the gases cannot escape at the lower ends of the tubes, they must be pass- ing outwards through the plates of plaster of Paris, and passing outwards more rapidly than air is passing inwards. If the tubes are of the same section and the ^' same length, and if the level of the water in each is observed after a little time, it will be found that the hydrogen has diffused through the porous plate about 4 times quicker than the oxygen. If a similar experiment is made (with proper precautions) with chlorine, a heavy, yellowish-green, very badly smelling, gas -it will be found that the rate of diffusion of hydrogen is about six times that of chlorine. Now let there be prepared a mixture of two volumes hydro- gen with one volume oxygen. This mixture cannot be distin- guished by the eye from its constituents; if a very little of it is placed in a strong glass tube and a flame is brought near a violent explosion occurs. Let the mixture be collected in a gas-holder from which it may be forced at any desired rate by allowing water to enter the gas-holder from a reservoir above. The gas-holder communicates with an arrangement for drying the gases, and this is connected with a long, dry, clay-pipe placed inside a glass tube arranged as shewn in fig. 11. The mixture of oxygen and hydrogen is caused to pass very slowly through the pipe ; gas issues at a and b ; a tube is filled with the gas issuing at a and another with that issuing at b. The gas collected at b does not burn when a lighted wooden splint is brought near it, but the splint itself burns more brightly ; the gas collected at a burns with a slight explosion. The gas issuing at b consists chiefly of oxygen ; that issuing at a consists chiefly of hydrogen. The mixture of hydrogen and oxygen has been partially 38-40] MIXTURES AND COMPOUNDS. 31 separated into its constituents by making use of a physical property of these gases called rate of diffusion, which property EBEATUM. Elem. Chem. p. 30. Experiment describing diffusion of oxygen and hydrogen should be so arranged that the tubes are covered with a bell-jar filled with carbon dioxide. 11 hydrogen chloride is passed tnrougn. a ary ciay-pipe under conditions similar to those already described it can be proved that no separation into hydrogen and chlorine has occurred, but that the gas which issues at a is identical with that which issues at 6, and that both are hydrogen chloride. The compound of hydrogen and chlorine (hydrogen chloride) cannot be separated into its constituents by making use of a certain physical property viz. rate of diffusion which belongs to, and characterises, each of its constituents when these are uncombined with other kinds of matter. The results of experiments such as those considered in pars. 33 to 39 shew that the substances comprised in the class Not-Elements may be divided into two groups; compounds and mixtures. ftriri7BESIT7 40 30 ELEMENTARY CHEMISTRY. [CHAP. in. Paris. The passage of a gas through such a porous substance as plaster of Paris is called diffusion. If two graduated glass tubes are each stopped at one end with a thin dry plate of plaster of Paris, if one is then filled with hydrogen and the other with oxygen, and if both are at once placed in water with the open ends under the water (s. fig. 10), the water will begin to rise in both tubes. As the gases cannot escape at the lower ends of the tubes, they must be pass- in o- outwards through the plates *> i^ ( \ massing guishea uy W^-^_L_ it is placed in a strong glass tuoe an a violent explosion occurs. Let the mixture be^col gas-holder from which it may be forced at any desired rate by allowing water to enter the gas-holder from a reservoir above. The gas-holder communicates with an arrangement for drying the gases, and this is connected with a long, dry, clay-pipe placed inside a glass tube arranged as shewn in fig. 11. The mixture of oxygen and hydrogen is caused to pass very slowly through the pipe ; gas issues at a and b ; a tube is filled with the gas issuing at a and another with that issuing at b. The gas collected at b does not burn when a lighted wooden splint is brought near it, but the splint itself burns more brightly ; the gas collected at a burns with a slight explosion. The gas issuing at b consists chiefly of oxygen ; that issuing at a consists chiefly of hydrogen. The mixture of hydrogen and oxygen has been partially 38-40] MIXTURES AND COMPOUNDS. 31 separated into its constituents by making use of a physical property of these gases called rate of diffusion, which property Fig. 11. is a characteristic mark of each gas when unmixed with other substances. If now a mixture is made of equal volumes of hydrogen 39 and chlorine, and this mixture is exposed to diffused sunlight for some time, a new gas will be formed ; the new gas is colourless ; chlorine is yellow, hydrogen is colourless : the new gas fumes much in the air neither chlorine nor hydrogen fumes in air ; it has an intensely acrid smell, quite different from the smell of chlorine. This gas is called hydrogen chlo- ride. The weight of hydrogen chloride formed is equal to the sum of the weights of the hydrogen and chlorine which have combined to form it. If hydrogen chloride is passed through a dry clay-pipe under conditions similar to those already described it can be proved that no separation into hydrogen and chlorine has occurred, but that the gas which issues at a is identical with that which issues at 6, and that both are hydrogen chloride. The compound of hydrogen and chlorine (hydrogen chloride) cannot be separated into its constituents by making use of a certain physical property viz. rate of diffusion which belongs to, and characterises, each of its constituents when these are uncombined with other kinds of matter. The results of experiments such as those considered in 40 pars. 33 to 39 shew that the substances comprised in the class Not-Elements may be divided into two groups; compound* and mixtures. 0? THB 32 ELEMENTAEY CHEMISTRY. [CHAP. III. Each constituent of a mixture retains in the mixture the properties which characterise it when unmixed with other substances : the properties of the mixture are, broadly, the sum of the properties of the constituents. No constituent of a compound retains in the compound the properties which characterise it when separated from other substances : the properties of a compound are not the sum of the properties of the constituents ; the compound is a definite kind of matter, as distinct from each of its constituents as these are from one another, and yet formed by the combination of these con- stituents. 41 To say of a mixture, that it contains the bodies by mixing which it has been produced, is to use an expression which conveys a correct notion of the relations of the properties of the mixture to those of its constituents. But it is not so correct to say that a compound contains each of those kinds of matter by the interaction of which it has been formed. Thus, a mixture of iron and sulphur contains iron and contains sulphur ; inasmuch as, not only is the mass of the mixture the sum of the masses of the mixed iron and sulphur, but the properties of the mixture are also the properties of iron added to those of sulphur. The mass of a compound of iron and sulphur is certainly the sum of the masses of the iron and sulphur which have combined to form it ; but the properties of the compound are quite distinct from the properties by which iron or sulphur is marked off from other kinds of matter. The formation of a mixture is a physical process. The properties of every mixture probably differ slightly from the sum of the properties of its constituents ; some change occurs in the formation of the mixture ; nevertheless the pro- perties of each kind of matter in the mixture are so slightly modified by the presence of the other kinds of matter that it is always possible, and generally easy, to recognise each of these kinds of matter by some, or all, of the properties which dis- tinctly mark it off from other kinds of matter. The formation of a compound is a chemical process. The properties of each of those kinds of matter which combine are so largely modified by the presence of the other- combining substances that it is impossible to recognise any of them by the properties which belong to it when uncombined. Iron sulphide is as distinct and definite a kind of matter as iron or sulphur ; ammonium chloride is as distinct and definite 4043] MIXTURES AND COMPOUNDS. 33 a kind of matter as ammonia or hydrogen chloride ; butylene bromide is marked off from other kinds of matter by pro- perties as distinct and definite as those which characterise butylene or bromine ; hydrogen chloride, so far as its physical properties indicate, is as homogeneous and as little formed of unlike parts as either hydrogen or chlorine. Iron sulphide, or ammonium chloride, or butylene bromide, 42 or hydrogen chloride, can be separated into unlike parts ; but this separation is accompanied by the disappearance of all the distinctive properties of the compound, and by the production, in each case, of two kinds of matter iron and sulphur, ammonia and hydrogen chloride, butylene and bromine, hydro- gen and chlorine so unlike the compounds from which they have been produced that the only expression to be used regarding the occurrence is that each compound has ceased to exist and has been replaced by two new kinds of matter. Neither iron nor sulphur has yet been separated into unlike parts ; the methods which succeed in separating iron sulphide into iron and sulphur fail to separate iron or sulphur into kinds of matter different from iron or sulphur. Bromine likewise refuses, at present, to reveal its composition, if composition it has in the sense in which it may be said that butylene bromide is composed of butylene and bromine. But ammonia and hydrogen chloride, which are produced by separating ammo- nium chloride into unlike parts, can, each, be further separated into two kinds of matter totally unlike either ammonia or hydrogen chloride. Ammonia is formed by the union of, and can be resolved into, two colourless, odourless, gases nitrogen and hydrogen \ hydrogen chloride is formed by the union of, and can be resolved into, hydrogen, and another, yellowish- green, badly smelling, gas, chlorine. All attempts to separate nitrogen, or hydrogen, or chlorine, into unlike parts, have hitherto failed. A mixture is separated into its constituents by making 43 use of some property or properties of each constituent which belong to that substance when it exists apart from other kinds of matter. Thus the mixture of iron and sulphur was separated by making use of the fact that iron is attracted by a magnet while sulphur is not attracted ; or of the fact that iron sinks in water, while sulphur floats, at least for a time or of the fact that sulphur is soluble, while iron is not soluble, in carbon disulphide. The mixture of ammonia and charcoal was separated by taking advantage of one of the properties of ammonia viz. M. E. C. 3 34 ELEMENTARY CHEMISTRY. [CHAP. III. that it is a very volatile gas. The property possessed by hydrogen of diffusing four times more rapidly than oxygen through a porous plate gave us a method for approximately separating a mixture of hydrogen and oxygen into its con- stituents. But if a compound is to be separated into unlike parts it is necessary either to act upon it by some natural agency, or form of energy, such as heat, light, or electricity or in some cases mechanical energy or, and this is the more usual method, to cause it to interact under suitable conditions with some other kind, or kinds, of matter. Thus the compound water was separated into hydrogen and oxygen by passing an electric current through the water (s. experiment in par. 9). Similarly ammonia may be separated into nitrogen and hydrogen by passing electric sparks through it. Copper oxide was separated into copper and oxygen (s. par. 28) by causing it to interact with hydrogen at a high temperature ; the results of this interaction were copper and water ; but the results of a previous experiment shewed that water is produced by the combination of hydrogen with oxygen. As we proceed in our study we shall learn more of the methods employed for separating compounds into the different kinds of matter by the combination of which they are produced ; meanwhile it is important to observe that the method does not consist in making use of the physical properties belonging to these different kinds of matter. The formation and decom- position of a compound are chemical processes. 44 We have already learned that the chemist puts in one class all those distinct kinds of matter which he has not been able to separate into unlike parts, and calls them Elements. We now learn that certain Not-Elements are distinct kinds of matter, each marked by its own definite and character- istic properties, yet each capable of being separated into parts, totally unlike each other, and unlike the original. These not-elements the chemist puts in one class, and calls them Compounds. One marked characteristic, viz. the con- stancy of composition, of compounds will be dealt with later, (pars. 58 and 59.) All other substances belonging to the group Not-Elements are classed together and called Mixtures. An infinite number of these exists, or may be formed, by mixing elements with elements, or compounds with compounds, or elements with 4347] MIXTURES AND COMPOUNDS. 35 compounds, or mixtures of any of these with other mixtures ; they are all marked off from elements and compounds by the facts that their properties are, broadly, the sums of the pro- perties of their constituents, and their constituents exist in the mixtures each with its own properties scarcely, if at all, modified by the presence of the other constituent parts. Chemistry deals with certain parts of the phenomena 45 presented in the changes of elements into compounds, and of compounds into simpler compounds or into elements. Chemistry concerns itself but little with the formation of mixtures or the resolution of mixtures into their constituents. By the composition of an element is meant the element 4:6 itself; so far as our knowledge goes at present, each kind of matter placed in the class element is entirely homogeneous. By the composition of a compound is meant, primarily, a statement of the elements by the combination of which the compound is formed and into which it can be resolved, and also a statement of the mass of each element which goes to form, or can be obtained from, a specified mass of the compound. By the composition of a compound is frequently meant a statement of certain less complex compounds, and of the masses of these, which interact to produce a specified mass of the compound in question, or which can be obtained from a specified mass of this compound. Thus, experiment has shewn us (par. 35) that the compound ammonium chloride is produced by the interaction of ammonia and hydrogen chloride ; experi- ment (par. 39) has also told us that hydrogen chloride is itself a compound of hydrogen and chlorine. It may also be proved that ammonia is a compound of nitrogen and hydrogen. The composition of ammonium chloride may be expressed by either of the following statements : (1) 100 parts by weight of ammonium chloride are formed by the combination of 31 '7 7 parts by weight of ammonia and 68*23 parts by weight of hydrogen chloride; (2) 100 parts by weight of ammonium chloride are formed by the combination of 26'17 parts by weight of nitrogen, 748 parts by weight of hydrogen, and 6 6 '35 parts by weight of chlorine. We have now gained a clearer conception of chemical 47 change. We now regard such a change as, either the change of a specified mass of a compound into fixed masses of two or more compounds or elements, or the interaction of fixed masses of two or more elements or compounds to produce 32 36 ELEMENTARY CHEMISTRY. [CHAP. III. 47, 48 definite masses of new elements or compounds. We know that in the first case the mass of each element or compound produced is less than that of the original compound before the change began. In both cases, we know that the sum of the masses of the different kinds of matter produced in the change is equal to the sum of the masses of the different kinds of v matter which suffered change. Our present conception of chemical change requires us to have clear notions of the classes of things called elements and compounds, respectively ; and this, in turn, demands that we have grasped, as far as we can at this stage, the essential points of difference between chemical and physical change, and between elements and compounds, on one hand, and mixtures, on the other. 48 "We have also learned something of the meaning of the term chemical properties of this or that kind of matter, as contrasted with the term physical properties of the same kind of matter. Sulphur, for instance, is a yellow, brittle, solid, twice as heavy as water bulk for bulk ; it crystallises in rhombic octahedra, melts at about 115, boils at about 440, and is a bad conductor of heat and electricity : these are some of the physical properties of sulphur, that is, the properties which are recognised as belonging to this kind of matter when it is examined apart from other kinds of matter. But when we examine the relations of sulphur to other kinds of matter, we enter on the study of its chemical properties. We find that one of the chemical properties of sulphur is its power of combining with iron; we find that when one part by weight of sulphur is heated with 1J parts of iron, 2f parts by weight of a compound of iron and sulphur (iron sulphide) are formed; we find that this compound is totally unlike either iron or sulphur, but that the whole of the iron and the whole of the sulphur have been used in its production. Further in- vestigation would shew us that sulphur combines with oxygen to form two distinct compounds, unlike each other, and both unlike either sulphur or oxygen ; we should find that 2 parts by weight of one of these compounds are produced by the com- bination of one part of sulphur with one part of oxygen, and that 2J parts by weight of the other compound are produced by the union of one part of sulphur with 1 J parts of oxygen. CHAPTER IV. CONSERVATION OF MATTEE. CHEMICAL changes are evidently complex occurrences. 49 What we have learned regarding them has been learned by making quantitative experiments and by reasoning 011 the results of these experiments. So long as our experiments are merely qualitative we can attain to no just conceptions of those changes which it is our business, as chemists, to investi- gate. Chemistry began to be a science, that is a department of exact and systematised knowledge of natural events, when quantitative investigation had superseded qualitative experi- n^ents. Before the time of Lavoisier there was much vague 50 speculation about elementary principles. At one time the commonly accepted view was that all things were composed of the four principles, earth, air, fire, and water. A piece of green wood is burnt : smoke ascends, therefore, it was said, wood contains the element air ; the flame which plays round the wood proves the presence of the element fire ; the hissing sound proves that the element water is present in the wood ; and the ashes which remain demonstrate that the element earth is one of the four constituents of the wood. Such reasoning, and such experiments, were possible only so long as chemists did not measure the quantities of the materials taking part in the changes which they observed. About the middle of the eighteenth century, Black firmly established the fact that chalk and burnt lime have a definite and unalterable composition. By quantitative experiments he proved that when chalk is burnt it is changed into lime and carbon dioxide ; and that when burnt lime is exposed to air, it slowly combines with carbon dioxide and chalk is re-formed. 38 ELEMENTARY CHEMISTRY. [CHAP. IV. Lavoisier carried on the work begun by Black. He gave the true interpretation of very many chemical changes, on the superficial qualitative examination of which the structure of alchemy had been raised. 51 That water had been repeatedly changed into earth was granted by all the alchemists. Water was boiled for a long time in a glass vessel ; the water disappeared, and a considerable quantity of a white earth-like solid remained in the vessel. Lavoisier placed some water in a weighed glass vessel; he closed the vessel and weighed it with its contents; he kept the water hot for 101 days, and then poured out the water into another vessel and boiled it until the whole of it had disappeared ; there remained 20J grains of solid earthy matter ; he then dried and weighed the glass vessel in which the water had been heated, it weighed 17 \ grains less than it had weighed before the water was heated in it. Lavoisier concluded that the earthy matter was produced by the action of the water on the glass; that is to say, that the alleged transmutation of water into earth did not occur, but that the earth was a part of the material of the vessel in which the water was heated. The small difference between 20 J and 17 J grains was due, according to Lavoisier, to experimental errors : this conclusion was fully confirmed when more accurate methods of weighing became possible. From quantitative experiments such as these, Lavoisier drew the all-important conclusion, 'that the total quantity of matter which is concerned in any chemical change is the same at the end of the change as at the beginning. 52 Every accurate investigation conducted since the time of Lavoisier has confirmed this generalisation. Under the name of the principle of the conservation of matter , or sometimes conservation of mass, it is now one of the foundations of all modern science. Experimental proofs of this generalisation have been given in preceding paragraphs. However we may change the form of matter, whatever transmutation we may succeed in accomplishing, there is one thing we cannot change, and that is the quantity, or mass, of matter taking part in each of these transmutations. The statement of this principle, or law, sometimes takes such a form as this ; we cannot create or destroy a single particle of matter, we can only change its form. It is important to notice that the test of creation, or destruction, is here, increase, or decrease, of the total mass of matter. 53 In place of the indefinite and indefinable elementary. 5053] CONSERVATION OF MATTER. 39 principles of the alchemists we have the 70, or so, elements of chemistry. Each element is a definite kind of matter charac- terised by its own properties which can be accurately stated frequently in terms having a quantitative signification. By bringing these elements into contact with each other under various conditions, we can accomplish stranger changes than those which alchemists dreamt of ; but we know that the new kinds of matter thus produced are formed by the combinations of the elements ; we have learned that no particle of any of the interacting elements is destroyed, but that the quantity of matter in the products is always exactly equal to the quantity of matter in the interacting elements. CHAPTER V. LAWS OF CHEMICAL COMBINATION. 54 WE have seen that a mixture may be made of two elements or compounds in different proportions, that the properties of the resulting mixture are the sum, or nearly the sum, of the properties of its constituents, and that the greater the pro- portion of one of the constituents the more nearly do the properties of the mixture resemble those 'of that constituent. A chemical compound, on the other hand, is wholly unlike the elements or simpler compounds from which it is formed ; its properties are perfectly definite and fixed, and are different from those of any of its constituents. Does the compound differ also from the mixture in having a fixed composition ? Do the constituents of the compound combine in definite quantities 1 It is evident that we must quantitatively examine the composi- tion of compounds if we desire to discover the laws of their formation. 55 Let us return to the first experiment by which we gained a rough notion of the difference between chemical and physical change. Let us again burn the element magnesium in air; but let the magnesium be weighed before it is burnt, and let the magnesia which is produced be collected and weighed. The result of this experiment is j 1 gram of magnesium when completely burnt in air, or in oxygen, produces 1 '66 grams of magnesium oxide or magnesia : we already know that the substance produced is a compound of magnesium and oxygen. This result may also be stated thus ; 100 parts by weight of magnesium oxide are formed by the combination of 60 parts by weight of magnesium, and 40 parts by weight of oxygen. 5456] LAWS OF CHEMICAL COMBINATION. 41 There are other ways of preparing magnesia, but 100 parts by weight of this compound, however it has been prepared, can always be resolved into 60 parts of magnesium and 40 parts of oxygen. If the compound of iron and oxygen produced by burning iron in oxygen is analysed it is found that its composition per 1 00 parts is ; iron = 72-41 oxygen = 27 -59. By composition per 1 00 parts is meant a statement of the mass of each of the elements which by their combination produce 100 parts by weight of the compound (s. par. 46). An experiment was already described by which the substance potassium chlorate was proved to be a compound of the element oxygen and the less complex compound potassium chloride. 100 parts by weight of potassium chlorate are resolved by heating into 39-13 parts by weight of oxygen and 60'87 parts by weight of potassium chloride ; if 200 parts of the chlorate are used, 78-26 parts of oxygen and 121-74 parts of potassium chloride are obtained. Potassium chloride is itself produced by the combination of the two elements potassium and chlorine in the ratio 52-41 to 47*59; i.e. 100 parts of the compound are composed of 5 2 '41 parts of potassium and 47-59 parts of chlorine. The composition of either potassium chlorate or chloride is definite and unchangeable. By what- ever method either of these compounds is prepared, it is always composed of the same elements combined in the same proportions. The composition per 100 parts of the iron sulphide produced 56 by heating together iron and sulphur is ; iron = 63-63 sulphur -36-37. In other words the ratio of sulphur to iron is 1 : 1*75. Now if a mixture is made of vejry finely divided sulphur and iron in the ratio L: 2,, and this mixture is heated, a black solid will be formed characterised by the properties of iron sulphide ; *--but it can be experimentally proved that the substance thus produced is not iron sulphide only, but is a mixture of iron sulphide and iron ; and further it can be proved that 2-75 parts by weight of iron sulphide have been formed .and that -25 parts of iron remain uncombined with sulphur. Again, if a mixture of 1 -25 parts of sulphur with 42 ELEMENTARY CHEMISTRY. [CHAP. V. 1*75 parts of iron is heated, 2 -75 parts of iron sulphide are formed and -25 parts by weight of sulphur remain uncombined with iron. The compound known as iron sulphide is thus shewn to have a definite and fixed composition : a certain mass of sulphur combines with a fixed mass of iron ; if there is more iron than this fixed mass, the iron over and above the fixed mass generally called the excess of iron does not combine with the sulphur; if there is an excess of sulphur, some of the sulphur does not combine with the iron. 57 Experiments have been described by which water has been shewn to be a compound of the elements hydrogen and oxygen. If water is a compound, the composition of water must be definite and unchangeable. A tube of stout glass is divided into a number of equal parts, preferably into cubic centimetres ; the divisions - are marked on the outside ; the tube is closed at one end; two platinum wires pass through the walls of the tube near the closed end, and are bent so that the ends of the wires nearly, but not quite, touch inside the tube (s. fig. 12). The tube is filled with mercury, with proper precautions, and is in- verted in a trough of mercury. A small quantity of oxygen is passed into the tube, and the volume of the oxygen is determined; let it be 10 c.c. 20 c.c. of hydrogen are now passed into the tube. The tube is then pressed down on a pad of caoutchouc, and firmly clamped (s. fig. 13). An electric spark from an induction-coil is passed from one platinum wire to the other ; combination of the hydrogen and Fl 8* 12t oxygen occurs instantly, and the inside of the tube is slightly dimmed by the minute quantity of water produced. The tube is now raised slightly from the caoutchouc pad ; mercury rushes in and practically fills the tube. It may be proved con- clusively that water, and nothing but water, is formed in this experiment. The result of this experiment shews that 2 volumes of hydrogen combine with 1 volume of oxygen to produce water. Let the experiment be repeated, but with different volumes of hydrogen and oxygen. (1) Let there be 20 c.c. hydrogen and 20 c.c. oxygen : when the mercury is allowed to rush into the tube 10 c.c. of gas will remain ; this gas may be proved to be oxygen. (2) Let there be 30 c.c. hydrogen and 10 c.c. oxygen : 5658] LAWS OF CHEMICAL COMBINATION. 10 c.c. of gas will remain, which may be proved to be hy- drogen. Fig. 13. (3) Let there be 50 c.c. hydrogen and 25 c.c. oxygen : no gas will remain. [It is assumed that every precaution has been taken in measuring the gases, and that all necessary corrections for changes in temperature and pressure have been made.] The result of these experiments is that hydrogen and oxygen combine to form water in the ratio 2 : 1 by volume, and" in this ratio only. Oxygen is 16 times heavier than hydrogen, bulk for bulk ; hence 1 volume of oxygen weighs 8 times as much as 2 volumes of hydrogen, measured at the same temperature and pressure ; hence the results of these experi- ments shew that hydrogen and oxygen combine to form water in the ratio 1 : 8 by weight, and in this ratio only. The composition of several compounds has now been ex- amined quantitatively ; in every case it has been found that a specified mass of the compound has been produced by the combination of fixed and invariable masses of two, or more than two, elements. What is stated regarding the quantita- 58 44 ELEMENTARY CHEMISTRY. [CHAP. V. tive composition of these compounds has been found to hold good for all compounds. Every compound is a definite kind of matter, characterised by certain properties which mark it off from other kinds of matter; every compound is produced by the combination of two or more simpler compounds, or two or more elements ; and these simpler compounds, or these elements, always com- bine in the same proportion to form the specified compound. This result of the examination of the quantitative compo- sition of compounds is of fundamental importance in chemistry. It at once enables us to draw a marked distinction between mixtures and compounds. The composition of a mixture is not unalterable ; that of a compound is fixed and definite. 59 This fact regarding the composition of compounds is usually called The law of constant, or definite, proportions: or the law of fixity of composition. It may be stated in various ways ; thus, The proportions in which bodies unite together chemically are definite and constant. A given chemical compound is always formed by the union of the same elements in the same proportions. The masses of the constituents of every compound stand in an unalterable proportion to each other, and also to the mass of the compound formed. 60 The evidence in support of this statement is really the whole body of chemical facts which are at present known. But special experiments have been conducted with the view of testing the law of fixity of composition. The experiments made by Stas were characterised by the most rigorous and scrupulous accuracy. Stas prepared the com- pound salammoniac, or ammonium chloride, by four distinct methods ; he purified each preparation with the utmost care, and then determined its composition. Ammonium chloride is a com- pound of the three elements nitrogen, hydrogen, and chlorine. When an aqueous solution of this compound is mixed with a solution of silver in nitric acid, the ammonium chloride is decomposed and the whole of the chlorine formerly combined with nitrogen and hydrogen enters into combination with the silver to form silver chloride. Silver chloride is a heavy white solid ; when it is formed as described from ammonium chloride it settles down to the bottom of the vessel in which the experi- 5862] LAWS OF CHEMICAL COMBINATION. 45 ment is conducted, and may be collected, washed, and accu- rately weighed. In each experiment Stas added 100 parts by weight of pure silver, prepared with the greatest care and weighed with the greatest accuracy, to a solution of ammonium chloride prepared by one or other of four distinct methods ; he collected, and most carefully weighed, the silver chloride pro- duced ; thus he determined the mass of ammonium chloride which was wholly decomposed by 100 parts of silver. The fol- lowing numbers are selected from the results obtained by Stas. 100 parts by weight of silver were required to remove, and enter into combination with, all the chlorine from x parts by weight of ammonium chloride : x = 49-600; 49-599; 49-597; 49-598; 49-593; 49-5974; 49-602; 49*597; 49-592. Every experiment is attended with certain unavoidable errors. The results obtained by Stas prove beyond doubt that the quantitative composition of the ammonium chloride examined by him was the same, by whatever method that ammonium chloride had been prepared. We must more fully examine the composition of compounds 61 with the view of learning more of the laws of combination. We found that the composition of magnesia is denned by the statement magnesium = 60 oxygen 40 magnesia =100 The analytical results thus expressed tell that masses of magnesium and oxygen combine to form magnesia in the ratio 60 : 40 - 6 : 4 - 3 : 2 = 120 : 80, &c. But two elements often combine to produce two, or more 62 than two, distinct compounds. For instance carbon and oxygen combine to form two compounds. The composition of these oxides of carbon is represented, in parts per 100, thus ; I. II. carbon = 42'85 27'27 oxygen = 57*15 72-73 carbon oxide =100-00 carbon oxide = 100-00 But these analytical results may be stated in another form. We may ask, how many parts by weight of oxygen are com- bined with one part by weight of carbon in each compound 1 46 ELEMENTARY CHEMISTRY. [CHAP. v. The answers are easily found I. 42-85 : 1 = 57-15 : x. II. 27'27 : 1 - 72-73 : x. x= 1-33. x= 2-66. Here we see that the mass of oxygen which has combined with unit mass of carbon to form compound II. is exactly double that which has combined with unit mass of carbon to form compound I. Carbon and hydrogen combine to form many compounds ; let us select four of these and state their compositions in parts of carbon and hydrogen per 100 parts of each compound. The results are as follows ; Compound I. is called acetylene, II. is called ethylene, III. ethane, and IY. methane. i. ii. in. iv. carbon = 92 -3 857 80 -0 75-0 hydrogen = 77 14-3 20*0 25-Q 100-0 100-0 100-0 100-0 If these results are treated as we treated the analyses of the oxides of carbon, we find that 1 part by weight of carbon is combined with 083 parts by weight of hydrogen in compound I. 1 with 2 x -083 hydrogen II. 1 3x-083 hydrogen III. 1 4x-083 hydrogen IV. Five compounds of the two elements nitrogen and oxygen are known; if the composition of each is determined and is stated as parts by weight of oxygen combined with 1 part by weight of nitrogen, we have this result ; Compound I. -57 parts by weight of oxygen combined with 1 part by weight of nitrogen. ,, II. 2 x -57 oxygen with 1 of nitrogen. III. 3 x -57 IY. 4 x -57 ,, Y. 5 X "57 ,, ,, j, ,, By examining the composition of series of compounds of the same two elements and tabulating the results as we have done for the compounds of carbon and oxygen, carbon and hydrogen, and nitrogen and oxygen, we arrive at the second law of chemical combination, which is generally known as 6266] LAWS OF CHEMICAL COMBINATION. 47 The law of multiple proportions. This law may be stated thus; When one element combines with another in several propor- tions, these proportions bear a simple relation to one another. Or, better, thus; When two elements combine to form more than one compound, the masses of one of the elements which combine with a constant mass of the other element bear a simple relation to each other. In the cases considered we have kept the mass of one 64 of the combining elements constant and have taken this mass as unity, and we have found that the masses of the other element which combine with this constant mass are all whole multiples of one quantity, viz. the smallest mass of the other element which combines with the constant mass of the standard element. The relation between the combining masses of the second element is evidently in these cases a very simple one. But this relation is not always so simple. Thus iron and oxygen combine* to form three distinct oxides of iron; the ratio of the quantities by weight of oxygen which severally combine with one part by weight of iron is 1 : 1-33 : 1-5. Again lead and oxygen combine to form four distinct compounds ; the masses of oxygen which combine with 1 part by weight of lead are, severally, 077, -103, -116, and -154. The ratio of these is 1 : 1-33 : 1-5 : 2. The law of multiple proportions confirms, and also goes 65 beyond, the law of constant proportions. The latter law is the statement of the fundamental fact that the composition of every compound is definite and unchangeable ; the former law generalises the composition of series of compounds of pairs of elements, and states that such compounds are produced by the combination, with a constant mass of one element, of masses of the other element which are simple multiples of the smallest of these masses. We are now ready to advance a step further, and to con- gg sider the composition of compounds of one element with various other elements, and compounds of these other elements with each other. 48 ELEMENTARY CHEMISTRY. [CHAP. v. We shall begin by considering compounds of the element potassium with (1) chlorine, (2) iodine; and then (3) the com- pound of chlorine with iodine. The percentage compositions of these compounds are ; I. II. III. Potassium chloride. Potassium iodide. Iodine chloride. potassium = 52-4 potassium = 23 -6 iodine = 78*1 chlorine - 47-6 iodine = 76*4 chlorine = 21-9 100-0 100-0 100-0 Let us find the masses of chlorine and iodine which seve- rally combine with the same mass of potassium : this may be done by finding, (1) the mass of iodine combined with 52-4 of potassium, or (2) the mass of chlorine combined with 23-6 of potassium ; (1) 23-6 : 52-4-76-4 : x. (2) 52-4 : 23-6 = 47-6 : x'. We shall choose (2) ; ' = 21-4. That is, with 23-6 parts by weight of potassium there combine, (1) 76-4 parts of iodine to form potassium iodide, (2) 21*4 parts of chlorine to form potassium chloride. Now let us turn to the compound of chlorine and iodine. Let us ask ; what is the mass of chlorine which is combined with 76-4 parts of iodine 1 78-1 : 76-4 = 21-9 : x. a; = 21-4. We have now this result ; 2 3 '6 parts by weight of potassium combine with 76*4 parts by weight of iodine. 21*4 ,, ,, chlorine. 7 6 '4 parts by weight of iodine combine with 21-4 parts by weight of chlorine. Or, stated more generally, the masses of chlorine and iodine which severally combine with a constant mass of potassium are also the masses of chlorine and iodine which combine with each other. 67 If another element is used instead of potassium, will a similar result be obtained 1 Chlorine combines with hydrogen to form hydrogen chloride ; iodine also combines with hydrogen 66 69] LAWS OF CHEMICAL COMBINATION. 49 to form hydrogen iodide. If the compositions of these com- pounds are tabulated we have the following results ; Hydrogen iodide. Hydrogen chloride. hydrogen = 0'79 hydrogen = 2-77 iodine = 99-21 chlorine = 97'23 100-00 100-00 Treating these results as before, we find that 79 parts by weight of hydrogen combine with 9 9 '21 parts by weight of iodine. 27-8 chlorine. Then we inquire; how much iodine combines with 27-8 chlorine 1 The answer to this is found from the composition of iodine chloride; it is 99-21. So that we complete the fore- going statement by adding 99-21 parts by weight of iodine combine with 2 7 -8 parts by weight of chlorine. Or, stated more generally, the masses of chlorine and iodine which severally combine with a constant mass of hydrogen are also the masses of chlorine and iodine which combine with each other. Hydrogen combines with oxygen to form water ; hydrogen 68 also combines with sulphur to form hydrogen sulphide ; oxygen combines with sulphur to form oxide of sulphur. Let us examine the compositions of these compounds. We need not state the composition of each in parts per 100 ; let it suffice to state the results thus 1 part by weight of hydrogen combines with 8 parts by weight of oxygen. 16 ,, ,, sulphur. Then we inquire; how many parts by weight of sulphur combine with 8 of oxygen 1 Experiment tells that 8 parts by weight of sulphur combine with 8 parts by weight of oxygen. We have then this result 1 part by weight of hydrogen combines with 8 parts by weight of oxygen. 16 ,, ,, sulphur. 8 parts by weight of oxygen combine with 8 parts by weight of sulphur. Phosphorus combines with hydrogen to form phosphorus 69 M. E. C. 4 50 ELEMENTARY CHEMISTRY. [CHAP. v. hydride ; we know that chlorine combines with hydrogen to form hydrogen chloride ; phosphorus also combines with chlorine to form phosphorus chloride. We know that phosphorus combines with hydrogen, and that oxygen combines with hydrogen ; phosphorus also com- bines with oxygen to form phosphorus oxide. If we determine the compositions of these various com- pounds, and treat the results as before, always in the case of a hydrogen compound determining the mass of the other element combined with 1 part by weight of hydrogen, we have these results : 1 part by weight of hydrogen combines with 10 '3 parts by weight of phosphorus. 35 '5 ,, ,, chlorine. 10*3 parts by weight of phosphorus combine with 35-5 parts by weight of chlorine. 1 part by weight of hydrogen combines with 10 '3 parts by weight of phosphorus. 8 oxygen. 10 '3 parts by weight of phosphorus combine with 8 parts by weight of oxygen. Stating these results generally, we find that the masses of phosphorus and chlorine, or the masses of phosphorus and oxygen, which severally combine with a constant mass of hydrogen, are also the masses of phosphorus and chlorine, or of phosphorus and oxygen, which combine with each other. We also find that the masses of oxygen and sulphur which combine with each other bear a simple relation to the masses of these elements which severally combine with a constant mass of hydrogen. . We have learned that ( 10*3 parts by weight of phosphorus. 1 part by weight of hydro- I 8 ,, oxygen, gen combines with ] 16 ,, sulphur. ( 35-5 chlorine. Also that 10 '3 parts by weight of phosphorus, combine with 8 parts by weight of oxygen. 35'5 ,, chlorine. Phosphorus forms two compounds with sulphur ; when the 6972] LAWS OF CHEMICAL COMBINATION. 51 composition of that mass of each of these which is produced by combining 1O3 parts of phosphorus with sulphur is stated, we have / (1) 8 x 2 (= 16) parts by weight of 10-3 parts by weight of phos- I sulphur. phorus combine with ] (2) 8x3(=26'6) ,, ,, sulphur. Chlorine and oxygen form two compounds, the compositions of which are ; r (1) 8 parts by weight of 35-5 parts by weight of chlorine I oxygen. combine with ] (2) 8x4 (=32) parts by weight { of oxygen. Chlorine and sulphur form a compound the composition of which is ; 35-5 parts by weight of chlorine combine with 8x4 (=32) parts by weight of sulphur. These results may be stated in more general terms thus : The masses of phosphorus, oxygen, sulphur, and chlorine, which severally combine with a constant mass of hydrogen are also the masses of those elements which combine with each other, or they bear a simple relation to these masses. This statement, or a statement equivalent to this, holds good 71 for all the elements. ' The statement is known as The law of reciprocal proportions. This law may be expressed in various forms ; thus When two elements, A and B, severally combine with a third element, C, then the proportions in which masses of A and B severally combine with C are also the proportions in which A and B combine with each other, or they bear a simple relation to these proportions. Or, better, thus The masses of different elements which severally combine with one and the same mass of another element are also- the masses of those different elements which combine with each other, or they bear a simple relation to these masses. The student should particularly observe that the laws of 72 multiple, and reciprocal, proportions, are generalised statements of facts. He should also familiarise himself with the method by which these laws are deduced from the composition of com- pounds. Statements of the percentage composition of a series of compounds do not suggest the laws in question, although 42 52 ELEMENTARY CHEMISTRY. [CHAP. v. they contain the data from which the laws are deduced. It is necessary to compare the compositions o compounds of each of two, or more, elements with one and the same element ; it is also necessary to state these compositions so that the mass of the element with which the others combine is kept constant throughout all the compounds. Any element may be chosen as the standard element ; and any mass of the standard element may be chosen as the fixed mass with which other elements are to be combined. It is found that the relations between the masses of elements which mutually combine are very clearly and simply exhibited by choosing hydrogen as the standard element, and one part by weight (say 1 gram) of hydrogen as the fixed mass. 73 The following table illustrates this way of looking at the composition of several compounds. Column I. exhibits the composition of three compounds of hydrogen, stated, (a) as parts of each element per 100 parts of the compound, (b) so as to shew the weight of the second element combined with 1 part by weight of hydrogen. Columns II., III., and IV., exhibit the composition of compounds of two elements neither of which is hydrogen, stated, (a) as parts per 100, and (6) so as to shew the weights of those elements which severally combine with that weight of oxygen, sulphur, or chlorine, which has been shewn in I. to unite with 1 part by weight of hydrogen. 74 The masses of oxygen, sulphur, and chlorine, which severally combine with 1 part by weight i.e. with unit mass of hydrogen are 8, 16, and 35-5, respectively. The masses of copper, lead, and thallium, which severally combine with 8 parts by weight of oxygen are 31 '7, 103*5, and 204, respectively; and these are also the masses of those elements which severally combine with 16 parts by weight of sulphur, and with 35*5 parts by weight of chlorine, respectively. Let us call those masses of oxygen, sulphur, &c. the combining weights of oxygen, sulphur, &c. We have then : Combining weights, deduced from, composition of compounds with hydrogen. Oxygen = 8; Sulphur =16; Chlorine = 35 -5. These numbers represent parts by weight of each element which combine with one part by weight of hydrogen. Combining weights, deduced from composition of compounds with oxygen. Copper = 31-7; Lead =103-5; Thallium = 204. 7274] LAWS OF CHEMICAL COMBINATION. 53 3 I II I P CJQ O 2 3. 1-1 Pi II II 0> CTQ P ii ii ii i 1 O O5 I ' O 02 ^ Ox H^ GO tO GO GO I ' O 6 it | HO HO ^CfQ - PI 00 5s 9 1 II 00 O 05 CO tO --T CO -^ o CO GO 6x GO i^ *j to co H-* GO O 8 cp O CO H^ OX Ox o Su ^ 1 00 E 11 co co o i ' Ox O -^o^ 6 o O5 CO O5 CO o Ox Pi tO CD CO O5 L 3 54 ELEMENTARY CHEMISTRY. [CHAP. v. These numbers represent parts by weight of each element which combine with 8 parts by weight of oxygen, 16 of sulphur, or 35 '5 of chlorine; in other words these numbers represent the parts by weight of each element which combine with one com- bining weight of oxygen, or sulphur, or chlorine. The conception of combining weight may be extended to all the elements. The combining weight of an element which forms a compound with hydrogen must be regarded by us at present as a number expressing the mass of the element which combines with unit mass of hydrogen. The combining weight of an element which does not form a compound with hydrogen we shall for the present regard as the mass of that element which combines with one combining weight of oxygen, or of sulphur, or of chlorine ; i. e. with that mass of oxygen, sulphur, or chlorine, which combines with unit mass of hydrogen, i. e. with 8 parts by weight of oxygen, 16 of sulphur, or 35 - 5 of chlorine. 75 The laws of multiple, and reciprocal, proportions may now be put into one statement. The elements combine in the ratios of their combining weights, or in ratios which bear a simple relation to these. To illustrate this mode of expressing the laws of multiple and reciprocal proportions, let us tabulate (1) the combining weights of several elements, (2) the compositions of several compounds of these elements stated as so many combining weights of each element. Combining weights of some elements. A. B. Determined from composition Determined from composition of of compounds with ' compounds with dements in hydrogen. column A. Nitrogen = 4-6 Chromium = 17*46 Oxygen =8 Tin 29*5 Sulphur = 16 Copper = 31-7 Chlorine = 35 '5 Antimony = 40 Bromine = 80 Mercury =100 Iodine =127 Composition of some compounds; stated as number of combining weights of each element. Oxides of nitrogen, c. ws. of oxygen : c. ws. of nitrogen =1:1 in compound a, 1 : 3 in compound b, 2 : 3 in compound c r 4 : 3 in compound d. 74, 75] LAWS OF CHEMICAL COMBINATION. 55 Oxides of chromium, c. ws. of oxygen : c. ws. of chromium = 2:3 in compound a, 8 : 9 in 6, 1 : 1 in c, 4 : 3 in d, 2 : 1 in e. Chlorides of antimony, c. ws. of chlorine : c. ws. of antimony = 1 : 1 in compound a, 5 : 3 in b. Bromides of tin. c. ws. of bromine : c. ws. of tin = 1 : 2 in compound a, 1 : 1 in b. Iodides of mercury, c. ws. of iodine : c. ws. of mercury = 1 : 2 in compound a, 1 : 1 in b. Sulphides of copper, c. ws. of sulphur : c. ws. of copper = 1:2 in compound , 1:1 in b. The composition of all compounds may be stated in this way. Let us use a symbol to represent one combining weight of an element. Let N represent one combining weight of nitrogen; N 2 , two c. ws. of nitrogen; N 3 , three c. ws. of nitrogen ; generally N^, x c. ws. of nitrogen : let represent one c. w. of oxygen ; O^, x c. ws. of oxygen : Or, one c. w. of chromium : Sb, one c. w. of antimony : Sn, one c. w. of tin : Hg, one c. w. of mercury : Cu, one c. w. of copper : Cl, one c. w. of chlorine : Br, one c. w. of bromine : I, one c. w. of iodine : and S, one c. w. of sulphur. Then the compositions of the above compounds may be represented thus ; Oxides of nitrogen. ON, ON 3 , O 2 N 3 , O 4 N 3 . Oxides of chromium. O 2 O 3 , 8 Cr 9 , OCr, 4 0r 3 , 2 Cr. Chlorides of antimony. CISb, Cl B Sb 3 . Bromides of tin. BrSn 2 , BrSn. Iodides of mercury. IHg 2 , IHg. Sulphides of copper. SCu 2 , SCu. CHAPTER VI. SYMBOLS AND FORMULAE. 76 IT is customary to express the composition of compounds in a kind of shorthand by a method the principle of which is the same as that we are at present illustrating. A symbol is given to each element \ this symbol is formed either of the first letter, or of the first and some other letter, of the name of the element. When the names of several elements begin with the same letter that element which has been longest known and best studied generally gets a symbol formed of the first letter only; but there is no universally applicable rule. Some of the symbols are derived from the names by which the elements were known to the ancients or in the middle ages. The symbols of two elements, potassium (K), and sodium (Na), are derived from the names kalium and natrium by which these elements are known to German chemists. The symbol W is given to the element tungsten, it is derived from the name (Wolfram) of the mineral from which tungsten was first obtained. It is of the utmost importance to remember that each of these symbols represents a definite mass of the element ; it represents either one, two, three, four, five, or six, combining weights, as we are at present using the term combining weight, of the element. The following table gives the names and symbols of the elements. 76] SYMBOLS AND FORMULAE. Elements. Mass of 57 Mass of element element Name. Symbol. expressed Name. Symbol. expressed by sym- by sym- bol*. bol 1 . Aluminium Al 27 Molybdenum Mo 96 Antimony Sb 120 Nickel Ni 58-6 Arsenic As 75 Niobium Nb 94 Barium Ba 137 Nitrogen K 14 Beryllium Be 9 Osmium Os 193 Bismuth Bi 208 Oxygen o 16 Boron B 11 Palladium Pd 106 Bromine Br 80 Phosphorus P 31 Cadmium Cd 112 Platinum Pt 194 Caesium Cs 133 Potassium K 39 Calcium Ca 40 Rhodium Rh 104 Carbon C 12 Rubidium Rb 8-5-4 Oerium Ce 140 Ruthenium Ru 104-6 Chlorine Cl 35-5 Scandium Sc 44 Chromium Cr 52-2 Selenion Se 79 Cobalt Co 59 Silicon Si 28 Copper Cu 63-2 Silver Ag 108 Didymium Di 144 Sodium Na 23 Erbium Er. 166 Strontium Sr 87 Fluorine F 19 Sulphur S 32 Gallium Ga 69-9 Tantalum Ta 182 Germanium Ge 72-2 Tellurium Te 125 Gold Au 197 Terbium Tr 148 Hydrogen H 1 Thallium Tl 204 Indium In 113-4 Thorium Th 232 Iodine I 127 Tin Sn 118 Iridium Ir 192-6 Titanium Ti 48 Iron Fe 56 Tungsten W 184 Lanthanum La 139 Uranium U 240 Lead Pb 207 Vanadium Y 51-2 Lithium Li 7 Yttrium Y 89 Magnesium Mg 24 Ytterbium Yb 173 Manganese Mn 55 Zinc Zn 65 Mercury Hg 200 Zirconium Zr 90 1 The values in this table are given in round numbers ; they are only approximately correct. 58 ELEMENTARY CHEMISTRY. [CHAP. VI. 77 That collocation of symbols which expresses the composi- tion of a compound is called the formula of that compound. The formulae BaO, B 2 O 3 , O 2 C1 6 , HI, tell, that barium and oxygen combine to form barium oxide in the ratio 137 : 16 by weight, that boron and oxygen combine in the ratio 22 : 48 (= 11 x 2 : 16 x 3), that chromium and chlorine combine in the ratio 104-4 : 213 (= 52'2 x 2 : 35'5 x 6), and that hydrogen and iodine combine in the ratio 1 : 127. Or, the facts concerning composition which these formulae express may be thus stated; 153 parts by weight of barium oxide are formed by the combination of 137 parts by weight of barium with 16 parts by weight of oxygen ; 70 parts by weight of boron oxide are formed by the combination of 22 parts by weight of boron with 48 parts by weight of oxygen ; 3174 parts of chromium chloride are produced by the combination of 1044 parts of chromium with 213 parts of chlorine; 128 parts of hydrogen iodide are formed by the union of 1 part of hydrogen with 127 parts of iodine. 78 The numbers in the third column of the preceding table are sometimes called the combining weights of the elements. We have already given a meaning to the term combining weight (. par. 74). If that meaning is adopted, the mass of an element expressed by its symbol is seldom the same as the value ob- tained for the combining weight of that element; but when it is not the same, it is a simple multiple of the combining weight. We are not yet in a position to go fully into this matter of combining weights. We have already used the expression combining weight to mean, that mass of an element which combines with unit mass of hydrogen, or, in the cases of elements which do not combine with hydrogen, that mass which combines with 8 parts by weight of oxygen, or 16 of sulphur, or 35 '5 of chlorine. But when we come to apply this definition we meet with many difficulties. Thus, nitrogen and phosphorus each form one compound with hydrogen ; nitrogen forms 5 compounds, and phosphorus 2 compounds, with oxygen. From the composition of each of these compounds a value may be deduced for the combining weight of nitrogen, or for that of phosphorus. Similarly iron forms 3 compounds with oxygen, and 2 with chlorine; from the composition of these, values are found for the combining weight of iron. The values are these. 7779] SYMBOLS AND FORMULAE. 50 Combining weights of nitrogen, phosphorus, and iron. Deduced from composition of hydrides. of oxides. of chlorides. Nitrogen 4-6 2-8, 3-5, 4-6, 7, 14 Phosphorus 10-3 6-2, 10'3 6-2, 10-3 Iron 18-6, 21, 28 18'6, 28 This list might be largely extended; in very few cases should we find but one value for the combining weight (as defined) of an element. To adopt several combining weights for each element would introduce endless confusion into our system of re- presenting the composition of compounds. It is absolutely necessary to adopt one value and one value only, not merely for convenience but also for cogent reasons which will be given later. Sometimes the highest value found by the method al- ready stated is adopted, e.g. for nitrogen (N = 14 ; comp. above results with the table in par. 76) ; sometimes a simple multiple of this highest value is adopted, e.g. for iron (Fe = 56 : s. table in par. 76). If we define combining weight as has been already done, then the definition generally leads to several values for the combining weight of each element. If we call the numbers in the table in par. 76 combining weights, then we cannot accurately define the term combining weight. The best compromise, at any rate for us at present, is to 79 say, that the actually used combining weight of an element is a number which expresses either the largest mass .of the element which combines with 1 part by weight, of hydrogen, or 8 parts of oxygen, or 16 of sulphur, or 35 '5 of chlorine, or it expresses a simple multiple of this mass. The following table presents; in column I., the largest mass of each element which is known to combine with either 1 part by weight of hydrogen, or 8 of oxygen, or 1 6 of sulphur, or 35'5 of chlorine; and in column II., the actually used values for what are generally called the combining weights of the elements. I. II. I. II. Aluminium 9 27 Bismuth 104 208 Antimony 40 120 Boron 3 -6 11 Arsenic 25 75 Bromine 80 80 Barium 68-5 137 Cadmium 56 112 Beryllium 4'5 9 Caesium 133 133 60 ELEMENTARY CHEMISTRY. [CHAP. vi. I. II. Calcium 20 40 Oxygen Carbon 12 12 Palladium Cerium 46-6 140 Phosphorus Chlorine 35-5 35-5 Platinum Chromium 26-1 52-2 Potassium Cobalt 29-5 59 Rhodium Copper 63-2 63-2 Rubidium Didymium 48 144 Ruthenium Erbium 58-6 166 Scandium Fluorine 19 19 Selenion Gallium 23-3 69-9 Silicon Germanium 36-1 72-2 Silver Gold 197 197 Sodium Hydrogen 1 1 Strontium Indium 37-8 113-4 Sulphur Iodine 127 127 Tantalum Iridium 96-3. 192-6 Tellurium Iron 28 56 Terbium Lanthanum 46-6 139 Thallium Lead 103-5 207 Thorium Lithium 7 7 Tin Magnesium 12 24 Titanium Manganese 27-5 55 Tungsten Mercury 200 200 Uranium Molybdenum 48 96 Vanadium Nickel 29-3 58-6 Yttrium Niobium 47 94 Ytterbium Nitrogen 14 14 Zinc Osmium 96-5 193 Zirconium I. 16 106 10-3 97 39 52 85-4 52-3 14-6 39-5 7 108 23 43-5 16 45-5 62-5 49-3 204 58 59 24 46 60 51-2 29-6 57-6 32-5 45 II. 16 106 31 194 39 104 85-4 104-6 44 79 28 108 23 87 32 182 125 148 204 232 118 48 184 240 51-2 89 173 65 90 80 As we advance in our study of chemical events we shall learn that there is no purely chemical, and general, method, by using which a decision may be arrived at regarding the best value to be given to the combining weight of an element. Each case must be discussed by itself ; the result is at best a compromise. But we shall also find that the application of certain physical conceptions to chemical phenomena leads to a generally applicable method, based on one definite principle, whereby values may be obtained for what we at present call the combining weights of the elements. 81 The symbol of an element, then, expresses a definite mass of that element. The formula of a compound expresses the masses 79-^82] SYMBOLS AND FORMULAE. 61 of the elements, stated as a certain number of combining weights of each element, which combine to form a specified mass, of the compound. A number placed beneath (or sometimes, above) the symbol of an element in the formula of a compound tells that the symbol is to be multiplied by this number. A number placed at the beginning of the formula of a compound multiplies the whole of the formula, or if a full stop occurs in the formula the number multiplies all as far as that stop ; sometimes the formula is put in brackets and the multiplier is placed outside the bracket. The following formulae will illustrate these points. Fe = 56, O = 16, S = 32. FeO means 56 + 16 = 72 parts by weight of a compound called ferrous oxide ; this formula also tells that one c. w. of iron combines with one c. w. of oxygen to form ferrous oxide. Fe 2 O 3 means (56 x 2) + (16 x 3) = 160 parts by weight of a compound called ferric oxide ; also that 2 c. ws. of iron combine with 3 c. ws. of oxygen to form ferric oxide. FeS0 4 means 56 + 32 + (16 x 4) = 152 parts by weight of a compound called ferrous sulphate ; also that one c. w. of iron, one c. w. of sulphur, and four c. ws. of oxygen, combine to form ferrous sulphate. Fe 2 .3S0 4 or Fe 2 (S0 4 ) 3 means (56 x 2) + 3 (32 + 64) = 400 parts by weight of a compound called ferric sulphate ; also that two c. ws. of iron, three c. ws. of sulphur, and twelve c. ws. of oxygen, combine to form ferric sulphate. 3Fe 2 3SO 4 or 3Fe 2 (SO 4 ) 3 means 3{(56 x 2) + 3(32 + 64)} = 1200 parts by weight of ferric sulphate. Chemical changes are also expressible in formulae, so far 82 at least as the composition of the elements or compounds before and after such changes is concerned. Thus, we have learned that (1) Sulphur and iron combine when heated in the ratio 1 : 1-75, to form iron sulphide; (2) Hydrogen and oxygen combine in the ratio 1 : 8 to form water. These chemical reactions may be shortly expressed thus ; (1) S + Fe = FeS. (2) 2H + = H 2 O. Fe = 56, S = 32, O = 16. The ratio 32 : 56 = 1 : 175; the ratio 2 : 16 = 1 :8. <62 ELEMENTARY CHEMISTRY. [CHAP. VI. The sign + signifies reacts chemically with \ the sign = signifies with production of. The total mass of matter on one side of the sign = is equal to the total mass of matter on the other side. Let us consider one or two rather more complex reactions. (1) Na + H 2 O + Aq = NaOHAq + H. (2) 3Fe + 4H 2 O - Fe 3 4 + 8H. (3) Zn + H 2 SO 4 Aq = ZnSO 4 Aq + 2H. Na = 23, O = 16, Fe = 56, Zn = 65, 8 = 32. (1) When sodium and water interact, 23 parts by weight of sodium and 18 parts by weight of water disappear, and there are produced 40 parts of sodium hydroxide, which remains dissolved in the water that has not been changed, and 1 part by weight of hydrogen. (2) When iron and water interact, 168 parts of iron and 72 of water are changed to 232 parts of iron oxide and 8 parts of hydrogen. (3) When zinc and a solution in water of sulphuric acid interact, 65 parts by weight of zinc and 98 of the acid are changed into 161 parts of zinc sulphate, which remains in solution, and 2 parts of hydrogen. These chemical equations, as they are called, also represent the compositions of the compounds before and after the change, expressed as so many combining weights of each elementary con- stituent of each compound ; when elements take part in the re- actions, the equations also express the number of combining weights of this or that element which interacts with a certain mass of a compound, or with a certain number of combining weights of another element, and the number of combining weights of this or that element which is produced by the interaction. Thus (1) states, more- shortly than can be done in words, the fact that one c. w. of sodium interacts with 18 parts of water to produce 40 parts of sodium hydroxide and 1 c. w. of hydrogen ; and (3) states that one c. w. of zinc interacts with 98 parts by weight of sulphuric acid dissolved in water (which 98 parts are composed of 2 c. ws. of hydrogen, 1 c. w. of sulphur, and 4 c. ws. of oxygen) to produce 161 parts of zinc sulphate (composed of 1 c. w. of zinc, 1 c. w. of sulphur, and 4 c. ws. of oxygen) which remain in solution, and 2 c. ws. of hydrogen. The symbol Aq is used here, and generally in this book, to mean a large (indefinite) quantity of water; when placed 8284] SYMBOLS AND FORMULAE. 63 after the formula of a compound or element it means that that body is in solution in a large quantity of water. Chemical formulae express other facts regarding chemical 83 changes ; these we shall learn as we advance. It is advisable to note here that these formulae and equations do not say any- thing regarding the conditions under which the chemical inter- actions occur. Thus Fe + S = FeS only tells us that a certain mass of iron combines with a certain mass of sulphur to produce the sum of these masses of iron sulphide. So Na + H 2 O + Aq = NaOHAq + H tells that certain masses of sodium and water interact to produce certain masses of sodium hydroxide [which is dissolved in the excess of water (s. ante par. 56)] and hydrogen, and that the sum of the masses of sodium and water is equal to the sum of the masses of sodium hydroxide and hydrogen. The equations in no way in- dicate the facts that iron and sulphur only combine when heated, but that sodium and water interact at ordinary temperatures. The equation Zn + H 2 S0 4 Aq = ZnS0 4 Aq + 2H expresses certain definite quantitative facts (s. ante) ; but it does not indicate or even suggest that the compositions of the products of the interaction of zinc and sulphuric acid vary with variations in the temperature at which the interaction occurs, and that the interaction proceeds according to the representation given by the equation only at the ordinary temperature. Chemical equations evidently give very incomplete repre- sentations of chemical changes. But nevertheless chemical formulae are of the greatest value, inasmuch as they enable us to exhibit, in a simple and intelligible way, the composition of compounds, and those changes of composition, the study of which forms one part of chemical science. "We have learned that the symbol of an element represents 84 a definite mass, and also one combining weight, of that element. The formula of a compound also represents a definite mass of the compound, and tells the composition of that definite mass, both in parts by weight, and also in combining weights, of each of the elements by the combination of which the compound has been formed. The following are the formulae of some well-known compounds j Water H 2 O. Formic acid H 2 CO 2 . Hydrogen peroxide H 2 O 2 . Hydrogen sulphide H 2 S. Oxalic acid HOC). Sulphur chloride S CL. 224 64 ELEMENTARY CHEMISTRY. [CHAP. VI. Hydrogen chloride HC1. Antimony chloride SbCl 3 . Ferric chloride Fe 2 Cl 6 . Benzene H 6 C 6 . Acetylene H 2 C 2 . Methane H 4 C. These formulae suggest a question, the answer to which is of the utmost importance, but a question to which a satis- factory answer cannot yet be given. Why should we choose to represent hydrogen peroxide as composed of 2 combining weights of hydrogen with 2 c. ws. of oxygen 1 ? Water is represented as produced by the union of 1 c. w. of oxygen with 2 c. ws. of hydrogen; why should not the composition of peroxide of hydrogen be represented by the formula H0 1 The ratio H : O is the same as the ratio H 2 : O 2 . Again, why should the formula of ferric chloride be Fe 2 Cl 6 rather than FeCl 3 1 Chloride of antimony, SbCl 3 , is represented as formed by the union of 1 c. w. of antimony with 3 c. ws. of chlorine ; why should we choose a formula for ferric chloride which represents the composition of that mass of this compound which is formed by the union of 2 c. ws. of iron with 6 c. ws. of chlorine 1 Similar questions are suggested by the other formulae. In some cases the formula appears to be the simplest that could be given to the compound, e.g. H 2 O, H 2 S, HC1, SbCl 3 ; in other cases a needless and foolish complication seems to be introduced. Why not HCQ 2 in place of H 2 C 2 O 4 ; HC in place of H 6 C 6 ; SCI in place of S 2 C1 2 ; HO in place of H 2 O 2 ; FeCl 3 in place of Fe 2 Cl 6 1 Or if the more complex formulae are to be used, why should such formulae not be always used 1 Why 'not H O 2 or H 6 O 3 in place of H 2 O ; H 4 S 2 or H 6 S 3 or H I2 S 6 in place of H 2 S ; Sb 2 Cl 6 in place of SbCl g ; &c. ? 85 There must be some reason for these apparent incon- sistencies. There are several reasons ; but we are not yet in a position fully to understand and appreciate these reasons. We may however gain some notion of the kind of reasoning- employed in determining which of several possible formulae best represents the composition and reactions of a compound. The gist of the matter, as we shall hereafter find, is in the conception expressed by the words composition and reactions. So long as we look only at the composition of compounds we cannot find answers to our questions. If we disregard the composition and look only at the reactions of compounds we cannot find answers to our questions. 8486] SYMBOLS AND FORMULAE. 65 The symbol of an element represents a certain mass of 86 that element usually called its combining weight. Elements combine in the ratios of their combining weights, or in ratios bearing a simple relation to these. The formula of a compound represents the composition of a certain mass of that compound; this mass we propose to call the reacting weight of the compound. Compounds interact in the ratios of their reacting weights, or in ratios bearing a simple relation to these. The reacting weight of water is 18 (H 2 = 2 + = 16 ). The combining weight of sodium is 23 (this, as a matter of fact, is the mass of sodium which combines with 8 parts by weight of oxygen). Let us examine the interaction of water and sodium. When sodium is thrown into water a reaction immediately occurs ; the sodium rapidly disappears and hydrogen gas is produced. When the reaction is finished, let the solution be evaporated ; water passes away as steam, and a white solid (caustic soda) remains. The composition of this solid is represented by the formula NaOH (Na = 23, O = 16), that is to say, this compound is produced by the combination of one combining weight of sodium, one c. w. of hydrogen, and one c. w. of oxygen. We know that water is a compound of hydrogen and oxygen, and that sodium is an element. Hence the oxygen and hydrogen which form part of the caustic soda must have come from the water. But besides caustic soda, hydrogen was produced ; this must also have come from the water. Hence when sodium and water interact, a portion of the hydrogen which was combined with oxygen is evolved as hydrogen gas, and another portion enters into combination with the sodium and the oxygen to produce caustic soda. When this experiment is made quantitative, it is found that 23 parts by weight of sodium interact with 18 parts by weight of water, and there are produced 40 parts by weight of caustic soda and 1 part by weight of hydrogen. The 40 parts of caustic soda are composed of 23 parts of sodium, 16 parts of oxygen, and 1 part of hydrogen. The conclusion from these experiments is, that, as regards the interaction of water with sodium, 18 is the reacting weight of water, and that the decomposition of one reacting weight of water results in the production of 2 combining weights of hydrogen and 1 c. w. of oxygen. But if H = 1 and O 16, the formula H 2 O (18) summarises the results of this experiment. M. E. C. 5 66 ELEMENTARY CHEMISTRY. [CHAP. vi. A quantitative study of the reactions of water, carried out in the way thus briefly indicated, leads to the conclusion that the mass of water which interacts with other compounds and with elements can always be represented as 18, or as a whole multiple of 18. The composition of the hydrocarbon benzene is most simply represented as one c. w. of carbon combined with one c. w. of hydrogen ; therefore the smallest value that can be given to the reacting weight of benzene is 13 (CH ; C= 12, H=l). . Is this the best value to adopt for the reacting weight of benzene 1 Benzene and chlorine react to form a series of compounds, each composed of carbon, hydrogen, and chlorine ; the forma- tion of each of these is accompanied by the formation of hydrogen chloride (HC1). The first of these compounds is composed of 35 -5 parts by weight of chlorine, 72 of carbon, and 5 of hydrogen; therefore (as 0=12, and Cl = 35*5) the simplest formula to be given to this compound is C 6 H 5 C1. The composition of the next compound cannot be represented by a simpler formula than C 6 H 4 C1 2 . The other compounds have compositions which cannot be expressed by formulae simpler than C 6 H 3 C1 3 , C 6 H 2 C1 4 , C 6 HC1 5 , and C 6 C1 6 , respect- ively. Now, as C = 12, and H = 1, and as carbon and hydrogen combine to form benzene in the ratio 12 : 1, the simplest formula which we can use to express the composition of the reacting weight of benzene is C 6 H 6 = 78. When we extend our quantitative study of the reactions of benzene we find that the mass of this compound which interacts with other compounds and with elements is either 78 or a whole multiple of 78. These examples give some notion of the methods used for determining the value to be given to the reacting weight of a compound. There is no generally applicable chemical method. Each compound must be considered apart from other com- pounds. The object of the inquiry is to find the relative weight of the smallest mass of the compound which interacts with other compounds, or with elements, in chemical changes. The composition of this mass is then expressed in the formula of the compound. It will be noticed that in this inquiry the combining weights of the elements are assumed to be known. But we know that great difficulties have to be overcome before the 86, 87] SYMBOLS AND FORMULAE. 67 combining weights of the elements can be determined ; indeed it was stated that the only satisfactory principle on which a method for finding these combining weights has been based is physical rather than chemical. We shall see later on that the same physical principle gives us a means for de- termining the reacting weights of compounds. In addition to the three laws of chemical combination now considered the law of fixity of composition, the law of multiple proportions, and the law of reciprocal proportions there is another generalised statement regarding the volumes of gaseous elements or compounds which interact and the volumes of the gaseous products of these interactions. The lawofvolumes^QY the law of Gay Lussac, states that the volume of a gaseous compound produced by the interaction of gaseous elements or compounds bears a simple relation to the volumes of the gases from which it is produced, and the volumes of the interacting gaseous elements or compounds bear a simple relation to each other, All volumes are measured under the same temperature and pressure. Thus : Vols. of reacting gaseous elements or compounds. 1 vol, hydrogen and 1 vol. conditions of chlorine produce H + C1 = HC1. 2 vols. hydrogen and 1 vol. oxygen produce Vols. of gaseous products. 2 vols. hydrogen chloride. 2 vols. water-gas. 2 vols. ammonia. 3 vols. hydrogen and 1 vol. nitrogen produce 3H + N - H 3 N. 2 vols. carbon oxide and 2 vols. 2 vols. carbonyl chlorine produce chloride. CO + C1 2 - COC1 2 . 2 vols. hydrogen iodide and 2 vols. hydrogen 1 vol. chlorine produce chloride and 1 HI + 01 - HOI -f- 1. vol. iodine-gas. 2 vols. ethane and 2 vols. 2 vols. chlor- chlorine produce ethane and 2 C 2 H 6 + C1 2 = C 2 H 5 C1 + HOI. vols. hydrogen chloride. 87 68 ELEMENTARY CHEMISTRY. [CHAP. VI. 2 vols. alcohol-gas and 2 vols. hydrogen iodide produce 2 vols. iodo-ethane and 2 vols. water-gas. C 2 H 6 + HI = C 2 H 5 I + H 2 0. 88 Hydrogen is taken as the standard gas to which the others are referred. Any specified volume, say 1 litre, is adopted as the standard volume, and this is called one volume. If the weight of this one volume of hydrogen is taken as unity, then it is found that the weight of 1 volume of chlorine is 35-5 : that is, 1 vol. of chlorine weighs 35-5^ oxygen 16: 1 oxygen 16 I nitrogen ,,14: 1 nitrogen 14 f.J iodine-gas 127 : 1 ,, iodine-gas 127 J more than 1 vol. of hydrogen. But the combining weights of chlorine, oxygen, nitrogen r and iodine, are 35*5, 16, 14, and 127, respectively. Hence the numbers which represent the combining weights of these elements also represent the specific gravities of these elements in the gaseous state referred to hydrogen as unity. This statement is applicable to many of the gaseous elements. The composition of the reacting weights of hydrogen chloride, water, ammonia, carbonyl chloride, hydrogen iodide, ethane, chlorethane, alcohol, and iodo-ethane, are represented by the formulae HC1, H 2 O, NH 3 , COC1 2 , HI, C 2 H 6 , C 8 H 5 C1, C 2 H 6 O, C 2 H 5 I, respectively. But these formulae also represent the composition of 2 volumes of each compound in the gaseous state ; i.e. they represent the composition of that volume of each gaseous compound which is equal to twice the volume occupied by 1 part by weight of hydrogen. This statement is applicable to all gaseous compounds. The formula of a gaseous compound represents the composition of the reacting weight of that compound, and this is that weight which occupies twice the volume occupied by 1 part by weight of hydrogen. These statements assume that al! volumes are measured under the same conditions of temperature and pressure. 89 Let us now glance back at what we have learned regarding chemical composition. We have learned that chemical changes involve changes of composition and changes of properties ; that these changes occur when elements interact with elements or compounds, or compounds with compounds; that the composition of every 3789] SYMBOLS AND FORMULAE. 69 compound is definite and unchangeable ; that elements combine, or interact, in the ratios of their combining weights, and compounds in the ratios of their reacting weights, or in ratios bearing a simple relation to these; and that the volumes of gaseous elements and compounds which combine, or interact, are simply related to each other and to the gaseous products of the reactions. We have also gained some notion of the meanings of the terms combining weight, and reacting weight, as applied to elements and compounds respectively ; and we have seen how difficult it is to determine the values of these quantities by purely chemical considerations. The study of the composition of compounds has necessi- tated some study of the properties of elements and compounds. The properties we have found it incumbent on us to examine have not been those exhibited by elements or compounds considered apart from each other, but rather those exhibited in the mutual interactions of elements and compounds. To arrive at any just generalisations regarding the connexions between changes of composition and changes of properties the study of which connexions is the business of chemistry we have found it necessary to study the relations between classes or groups of chemical events. The study of isolated occurrences, or the study of isolated elements or compounds, cannot lead to far-reaching conclusions concerning chemical change. We have repeatedly found that chemical changes are accompanied by physical changes : we have tried to keep our attention fixed on the chemical parts of the phenomena ; but we should always remember that nothing in nature is "defined into absolute independent singleness." CHAPTER VII. CHEMICAL STUDY OF WATER AND AIR. 90 THAT we may become better acquainted with the kind of phenomena which chemistry studies, and that we may apply the principles gained in our study of chemical changes, so far as that study has gone, let us examine some of the phenomena presented to the chemist by the two kinds of matter, air and water. 91 Water. To which of the classes, Element or Not-Element, does water belong ? We have already had an answer to this question. In par. 27 we separated a specified mass of water into two kinds of matter different from, and each weighing less than, itself. This separation, or analysis, was effected (1) by passing an electric current through water ; (2) by the interaction between water and sodium. In the first process, the gases into which water was decomposed, hydrogen and oxygen, were collected separately and examined. In the second process, a portion of one of the gases, hydrogen, was obtained, but the other gas, and the rest of the hydrogen, combined with the sodium to form caustic soda. These proofs that water is a not-element were supplemented by the synthesis of water (1) by passing electric sparks through a mixture of 1 part by weight of hydrogen with 8 of oxygen ; and (2) by the interaction of hydrogen ^with hot copper oxide, whereby water and copper were produced. 92 Having proved water to belong to the class Not-Element, the next question to be answered is ; is water a compound or a mixture 1 The experiments whereby water is proved not to be an element afford an answer to this question. Water is so 9093] CHEMICAL STUDY OF WATER AND AIR. 71 completely unlike either of its constituents, hydrogen and oxygen, that we must consider it a compound, not a mixture, of these. Of course it might be urged that a compound may be formed by the union of these two gases, but that water may be a mixture of this compound with some other substance or substances. In describing the experiments whereby the composition of water has been demonstrated it was assumed (the assumption was noted at the time) that the hydrogen and oxygen used for the synthesis of water were perfectly pure, and that every precaution was taken in the experiments. The statement that water is a compound of hydrogen and oxygen, and that these gases combine to form water in the ratio 1 : 8 by weight, implies, that the whole of the hydrogen and the whole of the oxygen disappear, that water is the only substance produced, and that the mass of the water thus pro- duced is exactly equal to the sum of the masses of the hydrogen and oxygen. Details of the experimental methods by which each of these statements is proved to be correct were not given. Nor need these details be given here. But it will be well briefly to recapitulate the stages in Davy's proof of the fact that when pure water is decomposed by an electric current, hydrogen and oxygen are the only kinds of matter produced. Priestley had proved that when a mixture of air and 93 hydrogen was exploded in a closed vessel, water was found in the vessel after the explosion. Knowing that air contained oxygen, Cavendish thought it probable that the water noticed by Priestley was a product of the union of hydrogen with oxygen in the air. Cavendish proved that this was really the case ; he exploded mixtures of hydrogen and oxygen in various proportions ; the loudest explosion was obtained when two volumes of hydrogen were used to one volume of oxygen, and in this case no trace of either gas remained in the vessel after the explosion. Cavendish found that the water produced by exploding air with hydrogen always contained a little acid; the production of this acid he traced to a constituent of the air other than oxygen ; when he used pure oxygen in place of air, the water produced contained no acid. Davy decomposed water which had been purified by distillation, in glass vessels, by passing an electric current through it; in every case a little acid was produced at the positive electrode, and a little alkali at the negative electrode. He re-distilled the water and again electrolysed it, but the 72 ELEMENTARY CHEMISTRY. [CHAP. vn. result was the same. He noticed that the glass vessels in which the water was decomposed were slightly corroded, so he placed the re-distilled water in agate vessels and passed the electric current through it. But the result was as before ; traces of acid and alkali were produced. He used electrodes made of different materials ; the results were the same. He distilled the water again ; there was rather less alkali, but as much acid as before. After another distillation the alkali had further diminished. Davy concluded that the source of the alkali was some substance in the water itself. He placed the water in gold vessels, and electrolysed it : a very little alkali appeared at the negative electrode ; after the current had passed for some minutes the production of alkali nearly ceased ; but the acid was still produced, and the quantity of it slowly increased as the process of electrolysis continued. By evaporating some of the water used to dry ness in a silver dish, Davy obtained a small quantity of a white solid substance, which, after being heated, was distinctly alkaline. A small quantity of the same water was now electrolysed in a gold vessel ; after a few minutes, when the production of alkali had nearly ceased, a little of the white solid obtained by evaporating the water was placed in the water being electrolysed ; the alkali was at once produced in some quantity at the negative electrode. Davy then distilled some of the water he had been using in a silver retort, and electrolysed a portion of the distillate ; no alkali appeared ; he placed a little bit of glass in the water, alkali began to be formed. He had thus traced the production of alkali to the action of the water and the current on the glass or agate vessels used to contain the water: and he had conclusively proved that water is not changed into alkali by the action of the electric current. But it still remained to account for the production of acid a,t the positive electrode. The acid he found to be nitric acid. He knew that this acid is a compound of three elements ; hydrogen, oxygen, and nitrogen. Hydrogen and oxygen Davy could confidently affirm to be constituents of water ; he knew that nitrogen is present in air. On these facts Davy framed an hypothesis. As the decomposition of the water proceeds in every experiment in contact with air, and as hydrogen and oxygen are produced in this decomposition, the conditions for the production of nitric acid are realised ; the hydrogen and 93, 94] CHEMICAL STUDY OF WATER AND AIR. 73 oxygen combine with the nitrogen of the air to produce nitric acid, and this dissolves in the water. If this hypothesis is correct, removal of nitrogen from contact with the decomposing water should be attended with cessation of the production of nitric acid ; re-introduction of nitrogen should be accompanied by re appearance of nitric acid. Davy placed a gold vessel containing pure water on a plate of glass, and covered it with a strong glass jar connected with an air-pump; he exhausted the air from the jar, admitted hydrogen, again exhausted, and again tilled the jar with hydro- gen ; he continued this treatment until he could feel sure that the whole of the air had been withdrawn from the jar. He then filled the jar with hydrogen, and passed the electric current ; not a trace of acid was produced ; hydrogen and oxygen, and these gases only, appeared at the electrodes. He admitted air into the jar; the acid began to form at the positive electrode. But he had already proved that the production of acid was not connected with the presence of any substance in the water, nor with the nature of the vessels containing the water, nor with the material of the electrodes ; hence the production of acid always accompanied the presence of nitrogen. The latter was the cause of the former. " It seems evident then," says Davy, "that water, chemically pure, is decomposed by electricity into gaseous matter alone, into oxygen and hydrogen." This remarkable research is a type of all scientific inquiry. 94 Facts were noticed and verified, conclusions were drawn and tested by experiments ; hypotheses were framed on the basis of the experimentally determined facts, and were used to explain these facts by suggesting fresh lines of inquiry. The result which Davy obtained was not a barren fact ; it at once prompted him to further discoveries. The electric current had slowly decomposed the glass vessels ; probably it would also decompose other substances more or less resembling glass in composition. Water was electrolysed in cups of gypsum ; lime appeared at one electrode and sulphuric acid at the other. Other substances were employed ; he generally obtained an alkaline body at the negative, and an acid at the positive, electrode. This led Davy to regard many compounds as built up of two parts, one positively, the other negatively, electri- fied. This conception prompted him to make more experiments ; these furnished him .with new hypotheses ; these in turn led 74 ELEMENTARY CHEMISTRY. [CHAP. vii. to further inquiry ; and this reaction of experiment on theory and theory on experiment proceeded until he had framed a general con- ception of the composition of salts, and of the relations between salts, acids, and alkalis, which had a most important influence on the develop- ment of chemistry. The facts noticed during the electrolysis of water also led Davy to investigate the action of the current on various substances which were included in the class of elements ; many of these he succeeded in decomposing ; he obtained new elements, and to a large extent chang- ed the whole course of the chemical study of matter. 95 The synthesis of water by passing hydrogen over hot copper oxide has been already mentioned. The synthesis was carried out in the most accurate manner by Dumas. Fig. 1 4 represents the apparatus employed. A is a flask in which hydrogen is pro- duced by the interaction of zinc and dilute sulphuric acid (the cylinder to the left of A contains mercury ; it, and the tube dipping into it, serve as a means for allowing the hydrogen to pass away without taking the ap- paratus to pieces) : the seven large U tubes contain materials by passing through which the hydrogen is purified and dried: the small U tube B con- tains phosphorus pentoxide, a sub- stance which greedily absorbs moisture ; this tube is weighed before and after each experiment, should it increase in weight after an experiment, the re- sults of that experiment are rejected,, as the increase in the weight of B tells Fig. 14. that the hydrogen was not perfectly dry when it passed into C : C is a bulb of hard glass con- 9497] CHEMICAL STUDY OF WATER AND AIR. 75 taining a weighed quantity of perfectly pure and dry copper oxide ; the neck of this bulb is drawn out and passes into the next bulb D : D is a dry bulb of glass destined to contain the water produced in the reaction ; it is weighed before and after each experiment : the U tubes E contain materials to absorb any traces of water which may not be retained in D : the small tube F also contains drying materials ; it is weighed before and after each experiment ; should it shew an increase in weight after an experiment is finished the results of that experiment are rejected, because a doubt arises as to whether the whole of the water produced has been retained in the apparatus and weighed. Let the weight of the copper oxide before an experiment = x ; the weight of the copper remaining in C after the experi- ment = y the weight of water produced (that is the increase in weight of D and E) = z: then x - y gives the weight of oxygen which has combined with hydrogen to produce the weight z of water ; let this weight of oxygen = a : then z a gives the weight of hydrogen which has combined with a of oxygen to produce z of water. Dumas' result was that 1 part by weight of hydrogen combines with 7 '9804 parts by weight of oxygen, to produce 8 '9804 parts by weight of water. The volumetric synthesis of water has already been briefly 96 described : when this synthesis is conducted with all precautions the result is that one volume of hydrogen combines with half a volume of oxygen to produce water ; as careful determinations have shewn that oxygen is 15 '96 times heavier than hydrogen, the result of the volumetric synthesis, altogether confirms that of the gravimetric synthesis, of water. When two volumes of hydrogen are caused to combine with one volume of oxygen in a vessel the temperature of which is above the boiling point of water, that is when the conditions are arranged so that the water produced is main- tained in the state of gas, the result is that two volumes of hydrogen combine with one volume of oxygen to produce two volumes of water-gas. Water then is a compound, not a mixture, since it has been shewn to be of constant composition, and to conform to the laws of chemical combination. The results of experiments on the composition of water 97 are summed up in the formula H 2 O, and in the equation 76 ELEMENTARY CHEMISTRY. [CHAP. VII. Assuming that the combining weight of oxygen is 16 (in round numbers), and that the symbol O represents 1 6 parts by weight of oxygen, then this formula, and this reaction, tell us, that the masses of hydrogen and oxygen which combine to produce water are in the ratio 1 : 8 (in round numbers) ; that the reacting weight of water is 18, that this reacting weight is composed of two combining weights of hydrogen and one combining weight of oxygen, and that this quantity of water can be decomposed and either one or two combining weights of hydrogen removed, but that if oxygen is removed the whole of the oxygen must be removed ; and that two volumes of water-gas are formed by the union of, or can be resolved into, two volumes of hydrogen-gas and one volume of oxygen-gas (comp. pars. 86 and 87). 98 We may here inquire, what would be the composition of other compounds of hydrogen and oxygen if such compounds existed 1 The law of multiple proportions tells us that the composition of such compounds would be expressed by the general formula H^O,,, where H represents a combining weight of hydrogen, O represents a combining weight of oxygen, and x and y are whole numbers. Two compounds of hydrogen and oxygen are known ; one is water, H 2 O ; the other is hydrogen peroxide H 2 O 2 . 99 But neither the formula H 2 O, nor the equation H 2 + = H 2 0, says anything as to the conditions under which water is pro- duced by the union of hydrogen and oxygen ; they do not tell, or even suggest, the properties of water ; nor do they indicate the physical changes which accompany the chemical change from a mixture of hydrogen and oxygen to the compound water. We have already learned something of the conditions under which the reactions expressed by the equations (1) H 2 + = H 3 0; (2) H 2 = H 2 + proceed ; we know a few of the properties of water ; and we are not wholly ignorant of the fact that the production of water is an event which has a physical as well as a chemical aspect. But we ought now to look a little more closely at some of these points. Conditions under which the equations ( 1 ) H 7 + = H 2 and (2) H O = H 2 + O are realised. (1) When two volumes of hydrogen are mixed with one 9799] CHEMICAL STUDY OF WATER AND AIR. 77 volume of oxygen, and an electric spark is passed through the mixture, or a lighted taper is applied. (2) When an electric current is passed through 18 parts by weight (say 18 grams) of water until the whole of the water has disappeared. Or the water is raised to a very high temperature (say 1500 2000) under conditions such that the oxygen and hydrogen are removed from contact with the yet uiidecomposed water as quickly as they are produced. Or the water in the form of steam is passed over hot finely divided iron, or hot magnesium (or certain other metals) ; the hydrogen is collected ; the oxygen combines with the iron or magnesium to form an oxide of either metal ; by decomposing this oxide by suitable means the oxygen may be obtained. Physical changes which accompany the chemical change from the mixture H 2 + O to the compound H 2 O. (1) A large quantity of heat is produced. When 2 grams of hydrogen combine with 16 grams of oxygen to form 18 grams of water, the heat produced is sufficient to raise the temperature of (in round numbers) 68,000 grams of water from to 1 C. ; in other words 68,000 gram-units of heat are produced. This statement tells that the chemical change H 2 + O = H 2 O, i.e. the change of certain masses of two definite kinds of matter into a mass of another kind of matter equal to the sum of the masses of the two kinds of matter, is accompanied by the degradation of a large quantity of energy (. Chap. xiv.). The system H 2 O possesses much less energy, it can do much less work, than the system H 2 + O ; the difference between the energies of the two systems is approximately represented by 68,000 gram-units of heat. (2) A contraction of volume occurs. The mixture of 2 grams of hydrogen with 16 grams of oxygen occupies about 44,000 c.c. at and 760 mm. ; the 18 grams of water produced occupy about 18 c.c. If the temperature is kept a little above 100, the 18 grams of water-gas produced occupy about 30,000 c.c. (3) The mixture of hydrogen and oxygen is gaseous ; the water formed is a liquid below 100 at 760 mm. (4) The change is accompanied by the production of a flash of light. These are some of the more important conditions under which the changes represented by the chemical equations we 78 ELEMENTARY CHEMISTRY. [CHAP. VII. are considering occur, and some of the more important physical changes which accompany the chemical changes. 100 Properties of water. Here we must distinguish the physical from the chemical properties (v. ante ; pars. 2 to 13, and 35 to 41). The physical properties are those which characterise the substance water considered as a definite kind of matter apart from other kinds of matter. The chemical properties are those which are exhibited by water when it interacts with other kinds of matter. If this definition is accepted, then, strictly speak- ing, water can have no chemical properties ; the properties we shall have to examine are properties exhibited by a changing system composed of water plus something else ; the properties called chemical only come into play when water interacts with other bodies, hence they are the properties neither of the water per se, nor of the other body or bodies per se, but of the system of which water forms a part. The simplest chemical occurrence is at least two-sided. One of the chemical properties of hydrogen is that 1 part by weight of this gas combines with 8 parts by weight of oxygen to produce 9 parts by weight of water. But this chemical occurrence may be stated in terms of oxygen, hydrogen, or water. A chemical property of oxygen is that 8 parts by weight of oxygen combine with 1 part of hydrogen to produce 9 parts of water. A chemical property of water is that 9 parts by weight of it are produced by the union of 1 part of hydrogen with 8 parts of oxygen. 101 We can here only enumerate a few of the more important physical properties of water. At temperatures below C. water is a solid, from to 100 it is a liquid, and above 100 it is a gas. The change from solid water to liquid water is accompanied by a slight decrease of volume; the change from liquid water to water-gas is accompanied by an increase of volume ; each change is accompanied by the disappearance of a considerable quantity of heat. The reverse change from water-gas to liquid water is accompanied by a large condensation of volume, and the change from liquid water to ice is accom- panied by a slight increase of volume; during both changes much heat is produced. The temperature at which each change occurs depends upon the pressure of the atmosphere on the surface of the water; the temperature at which the change from liquid to gaseous water occurs freely varies very con- siderably with variations of pressure. To trace the relations of volume, pressure, and temperature, 99103] CHEMICAL STUDY OF WATER AND AIR. 79 for a given mass of water, is a typical physical inquiry. Under ordinary conditions of pressure, solid water melts at 0, and liquid water boils at 100. Water dissolves very many and very different substances. 102 The solution is sometimes unattended with any chemical change ; e.g. common salt or sugar dissolves in water, on evaporating off the water the salt or sugar remains (v. ante, par. 10). In some cases solution in water is accompanied by chemical change ; e.g. sodium dissolves in water, but on evaporating off the water, not sodium, but a different substance, caustic soda, is obtained (v. ante, par. 86). The phenomena presented during solution of bodies in water are extremely complex ; they cannot be classed wholly as physical or wholly as chemical. We are not yet in a position to examine these phenomena with any prospect of approximately understanding them. Let us rather turn to some of the more important phenomena presented by the interactions of water with other kinds of matter, that is, to the chemical properties of water. Water combines with many compounds, and with one or two 103 elements. When the gaseous element chlorine is passed into ice-cold water, after a time the whole becomes semi-solid ; by filtering off the liquid water at a temperature under 0, crystals having the composition Cl . 5H 2 O (Cl = 35 - 5) are obtained. Heated to a little above chlorine hydrate is wholly decomposed into chlorine and water. The compound cobalt chloride is a blue solid ; its composi- tion is expressed by the formula CoCl 2 (Co = 59, 01 = 35 -5). When this salt is dissolved in water, a reddish liquid is produced from which, after partial evaporation, red crystals separate having the composition CoCl 2 . 6H 2 O. When these red crystals are heated they separate into water, which passes off as steam, and blue cobalt chloride, which remains. When a little water is added to the blue solid, the red compound is reproduced. The equations (1) CoCl 2 + 6H 2 O = CoCl 2 . 6H 2 0, occurring at the ordinary temperature ; (2) CoCl 2 . 6H 2 O = CoCl 2 + 6H 2 O, occurring at about 100 ; represent the two chemical changes. When water is poured on to the compound calcium oxide, or lime, the water disappears, the lime swells up, much heat is produced, and the new compound calcium hydroxide, or slaked 80 ELEMENTARY CHEMISTRY. [CHAP. vn. lime, is formed. The equation CaO + H 2 O = CaO . H 2 O repre- sents the change of composition (Ca = 40). When the solid compound calcium hydroxide is strongly heated say to 400 or 500 it is decomposed into calcium oxide and water ; thus CaO . H 2 O = CaO + H 2 O ; the water passes away and the calcium oxide remains as a solid. Copper sulphate CuSO 4 (Cu = 63-2, S - 32) is a white solid ; when it is exposed to moist air it begins to turn blue ; this change is more quickly effected by pouring a little water 011 to the solid. The blue substance thus produced is a com- pound of copper sulphate and water, called hydrated copper sulphate ; its composition is CuS0 4 . 5H 2 O. When this com- pound is heated to about 220 water passes off as steam, and CuS0 4 remains as a white solid. When solid potassium oxide, K 2 O (K = 39), is placed in water it instantly dissolves ; when the solution is boiled to dryness a white solid remains having the composition K O . H,O, called potassium hydroxide, or hydrated potassium oxide, or (more commonly) caustic potash. This solid is not chemically changed by heat ; at a high temperature it melts, and at a very high temperature it is volatilised ; but the melted sub- stance, or that which is volatilised, has the same composition, K 2 O . H 2 O, as the solid. If an aqueous solution of caustic potash is added to an aqueous solution of copper sulphate, a greenish blue solid compound is formed ; when this is collected, washed, and dried at about 40 to 50, it has the composition CuO . H 2 O (Cu = 63-2). When this greenish blue solid is heated to 100 or 150 water passes off as steam and black copper oxide, CuO, remains. If a little water is now added to this copper oxide no chemical change occurs ; the water and the copper oxide remain mixed. 104 If we glance back over the statements made regarding the compounds of water with chlorine, cobalt chloride, &c. we see that the compounds may be divided into three classes, as follows : (i) Compounds formed by 'bringing together water and the other constituent of the compound, and also decomposed by heat into water and the other constituent; C1.5H 2 O; CoCl 2 .6H 2 O; CaO . H 2 O ; CuSO 4 .5H 2 O. (ii) Compound formed by bringing together water and the other constituent , but not decomposed by heat; K 2 . H 2 0. 103105] CHEMICAL STUDY OF WATER AND AIR. 81 (iii) Compound decomposed by heat into water and another compound, but not formed by bringing together water and that other compound ; CuO.H 2 O. We also notice that the compounds under (i) are decom- posed by heat at different temperatures, varying from a little above in the case of 01. 5HO, to 400 or 500 in the case of CaO.H 2 O. Inasmuch as each substance we have been considering is either formed by the combination of water with another sub- stance, or is resolved into water and another substance, we are justified in calling them all compounds of water with other elements or compounds. But in many of its interactions with elements and com- pounds water is decomposed, and new bodies are formed which cannot be regarded as compounds of water. We have learned that when sodium and water interact the products are hydrogen and sodium hydroxide; the equation Na + H 2 O + Aq = NaOHAq + H expresses the composition of the system before and after this interaction. A similar change occurs when potassium is thrown into water ; K + H 2 O + Aq - KOHAq + H. In each of these reactions much heat is produced ; in the case of potassium the reaction proceeds very rapidly, and the temperature of the hydrogen produced is raised so much that this gas takes fire. 105 Fig. 15. M. E. C. 82 ELEMENTARY CHEMISTRY. [CHAP. VII. When water-gas is passed over hot magnesium, or hot finely divided iron, in an apparatus as represented by fig. 15, hydrogen is obtained, and oxide of magnesium or iron is formed and remains in the tube in which the magnesium or iron was heated. Quantitative experiments have proved that for every 18 parts by weight of water decomposed 2 parts by weight of hydrogen are obtained, and an oxide of mag- nesium or iron is formed by the union of the 16 parts by weight of oxygen, formerly combined with the 2 parts of hydrogen, with 24 parts of magnesium or 42 parts of iron. The combining weights of magnesium and iron are 24 and 56, respectively ; the reacting weights of the oxides formed in the process just described are 40 and 232, respectively, and the compositions of these oxides are represented by the formulae MgO and Fe 3 O t . Knowing that the combining weight of oxygen is 16, and the reacting weight of water is 18 (H 2 0), we can summarise the changes of composition which occur in the reactions between steam and heated magnesium or iron in these equations (Mg = 24, Fe = 56, O = 16) ; (1) Mg + H 2 O = MgO + 2H. (2) Fe 3 + 4H 2 O = Fe 3 4 + 8H. 24 +18 -40 +2. 168 + 72 =232 +8. When the gaseous element chlorine is passed into boiling water and the mixture of steam and chlorine thus obtained is passed through a porcelain tube, loosely packed with pieces of porcelain, and heated to bright redness, oxygen and hydrogen chloride are produced. The apparatus represented in fig. 16 may be used. The exit end of the porcelain tube is connected with a vessel containing caustic potash solution (A). The gases coming from the tube bubble through this solution. Hydrogen chloride is absorbed by caustic potash, but oxygen is not. The gas which is not absorbed by the caustic potash is collected and proved to be oxygen. Quantitative experi- ments shew that the compositions of the interacting substances and of the products of the interaction are expressed by the equation (Cl = 35 "5, O = 16) ; H 2 O + 2C1=2HC1 + O. ' ! 18 +71 =73 +16. For every 18 parts by weight of water decomposed, 71 parts of chlorine are used, and 73 parts of hydrogen chloride and 16 parts of oxygen are produced. As all the substances taking part in this reaction are gases under the conditions 105] CHEMICAL STUDY OF WATER AND AIE. 83 of the experiment, and as we know (i) that the symbol of an elementary gas (with a few exceptions) represents the mass of Fig. 16. it which occupies 1 volume (i.e. the volume occupied by unit weight of hydrogen), and (ii) that the formula of a compound 62 84 ELEMENTARY CHEMISTRY. [CHAP. VII. gas represents the mass of it which occupies 2 volumes (v. ante, par. 88), the foregoing equation tells that 2 volumes of water- gas react with 2 volumes of chlorine gas to produce 4 volumes of hydrogen chloride gas and 1 vol. of oxygen gas. H 2 + 2C1=_2HC1 + O. vols. 2 + 2 give 4 + 1. The volumes on each side of the sign = are not the same ; the masses on each side of the sign = are, and in chemical equations always are, the same. 106 Our study of the properties of water has served to illustrate the nature of chemical change ; to emphasise the distinctions between mixtures, elements, and compounds ; to shew the im- portance of the laws of chemical combination ; to familiarise us with the use of chemical formulae and equations; and to illus- trate the meanings of the terms analysis and synthesis. This study has kept before us the notion of each element and com- pound interacting chemically with other elements and com- pounds in certain definite masses which are all simple multiples of one and the same mass. It seems as if a quantity of water, for instance, were composed of a vast number of little particles of water the masses of all of which are the same, and as if chemical interaction occurred between 1, 2, 3, n of these little particles and a definite number of little particles of the element or compound with which the water interacts. The chemical conception of every element or compound having its own reacting weight leads to some such physical conception as this of small definite particles. Finally, the slight examination we have given' to the chemical properties of water has shewn very clearly how closely interwoven chemical changes are with physical changes, and how impossible it is to arrive at any trustworthy conclusions regarding either otherwise than by quantitative experiments and accurate reasoning. 107 Air. When magnesium is burnt in air magnesia is pro- duced ; but magnesia is a compound of magnesium and oxygen, therefore, the chemical change which occurs during the burn- ing of magnesium in air consists in the combination of magne- sium and oxygen. Therefore, in all probability, oxygen is a constituent of air. When mercury is heated in a measured quantity of air, mercury oxide is produced, and some of the air disappears; when the oxide is collected and strongly heated, oxygen and mercury are formed, and the quantity of oxygen is equal to the quantity of air which disappeared. 105 108] CHEMICAL STUDY OF WATER AND AIR, 85 Therefore the heated mercury combined with a part of the air in which it was heated, and this part was oxygen (v. ante, par. 18). From these experimental results we may conclude that if magnesium or mercury is burnt in air, the air which remains after burning will almost certainly differ in properties and composition from the air which was present before burn- ing began. And if this is so we may further conclude that when any element which is known to combine with oxygen is burnt in an enclosed volume of air, the whole or a part of the oxygen in the air will combine with the element, and the air which remains will most probably differ from the original air. Phosphorus is an element which is easily burnt, and which very readily combines with oxygen. Let an apparatus be arranged as shewn in fig. 17. A is a glass jar; the space from the cork to within about 3 or 4 inches of the open end is divided into 5 equal parts. The jar is placed, open end down- wards, in such a quantity of water that the level of the water stands at the point where the graduation of the jar begins. B is an iron cup supported on an iron pillar with a broad foot. Let a piece of dry phosphorus be placed on B ; let the jar be put over the phosphorus and iron stand ; let the end of the brass chain be highly heated, and then let the chain be brought quickly into the jar, as shewn in the figure, so that the heated part of the chain touches the phosphorus. The phosphorus begins to burn, white clouds of phosphorus oxide fill the jar, and the water slowly rises in the jar. When the burning is finished and the clouds have dis- appeared the phosphorus oxide produced dissolves in the water let water be poured into the outer vessel until the level of the water inside and outside A is the same. It is seen that J of the air has disappeared. Withdraw the cork, and plunge a lighted taper into A; the flame is instantly extinguished. Therefore the air in A after the burning of phosphorus is not the same as the air before the phosphorus was burnt. By a little careful manipulation, portions of the air which remain after the phosphorus is burnt may be transferred from A to glass tubes or bottles, and the properties of this air may Fig. 17. 108 86 ELEMENTARY CHEMISTRY. [CHAP. vil. be examined. It is found to be a colourless, odourless, gas, a very little lighter, bulk for bulk, than ordinary air; it does not support combustion, nor is it combustible ; it reacts chemically with but few elements and compounds. Every attempt to separate a specified mass of this gas into unlike parts has failed. But very many compounds are known of each of which this gas is a constituent. The gas is an element ; it is called nitrogen, We know that oxygen is also an element. Hence we have obtained from air two elements nitrogen and oxygen. 109 Is air a compound or a mixture of these gases 1 If it is a compound, the properties of air must differ considerably from the properties of either oxygen or nitrogen ; and these elements must be united in air in a ratio expressed by the formula N x O y where a; is 1, 2, 3, 4...n times the combining weight of nitrogen (14), and y is 1, 2, 3, 4:...n times the combining weight of oxygen (16). If air is a mixture of nitrogen and oxygen, it must be possible to recognise both of these elements in air by making use of the properties which each possesses when unmixed with other kinds of matter. Whichever hypothesis is adopted as a guide in experi- mental inquiry, we must begin by determining the properties of air, the properties of oxygen, and the properties of nitrogen. 110 We already know some of the properties of oxygen and nitrogen. Both are colourless, odourless, gases; nitrogen is 14 times, and oxygen is 16 times, heavier than hydrogen. Combustible bodies burn rapidly and brilliantly in oxygen, but they cease to burn in nitrogen. The ratios of diffusion of both are nearly equal ; but oxygen passes through a thin sheet of india-rubber about 2J times more rapidly than nitrogen. Oxygen is slightly soluble, nitrogen is less soluble, in water ; 1 vol. of water at 16 dissolves '0295 vols. of oxygen, and 0145 vols. of nitrogen. The combining weight of oxygen is 16, and the combining weight of nitrogen is 14. 111 The prominent physical properties of air are known to all. Accurate analyses of air have shewn that 100 parts by weight of dry air freed from carbon dioxide (v. infra, par. 113) are com- posed of 23 parts of oxygen and 77 parts of nitrogen, by weight. The simplest formula which will fairly accurately represent this composition, assuming air to be a compound, is N 51 O 13 ; this compound, if it existed, would be composed of 22-5 parts of oxygen by weight, and 7 7 '5 parts of nitrogen, per 100 parts. Five definite compounds of nitrogen and oxygen are known ; 108111] CHEMICAL STUDY OF WATER AND AIR. 87 their compositions are represented by the formulae N 2 O, NO, N 2 O 3 , NO 2 , N 2 O 5 . It is improbable that a sixth compound of these elements should exist having as complex a composition as N 51 O 13 . But this is the simplest formula which can be given to air if air is a compound of nitrogen and oxygen. Hence the argument based on analogy of composition leads to the conclusion that air is probably a mixture and not a compound. Assuming air to be a mixture of nitrogen and oxygen, we next inquire, what volume of air ought to be dissolved by 1 vol. of water, say at 16 01 ? The solution of a mixture of gases by a liquid between which and the gases there is no chemical interaction follows the same course as if each gas were dissolved separately in the liquid. The solution of a gaseous compound, on the other hand, in a liquid which does not interact chemically with the compound follows a course of its own ; the vol. dissolved is independent of the vols. of the gaseous constituents of the compound dissolved under the same conditions. 1 vol. of water at 16 dissolves '0295 vols. of oxygen, and 0145 vols. of nitrogen ; now 1 vol. of dry air freed from carbon dioxide (v. infra, par. 113) is composed of '2096 vols. of oxygen and -7904 vols. of nitrogen; therefore, if air is a mixture, 1 vol. of water will dissolve (-0295 x -2096) + (-0145 x -7904) = 01765 vols. of air, at 16. Experiment proves that 1 vol. of water dissolves '0177 vols. of air at 16. 1 vol. of water at 16 dissolves '7535 vols. of nitrous oxide (N 2 O) ; but if this gas were a mixture of nitrogen and oxygen in the ratio in which these gases unite to form 1 vol. of nitrous oxide, water would dissolve -02925 vols. of the nitrous oxide. These calculations and experiments shew that air is dis- solved by water exactly as if the air were a mixture of oxygen and nitrogen and not a compound of these elements. In other words : one of the properties of oxygen is to dissolve in water to a certain definite extent, and one of the properties of nitro- gen is to dissolve in water to a certain definite extent \ but both oxygen and nitrogen retain this property when they are present in air ; therefore air is a mixture, and not a compound, of oxygen and nitrogen. We may carry the inquiry further on the same lines. If air is a mixture of oxygen and nitrogen, and if oxygen passes through a thin sheet of india-rubber about 2| times quicke 88 ELEMENTARY CHEMISTRY. [CHAP. VII. than nitrogen it ought to be possible to effect a partial separa- tion of air into its constituent gases by passing it through a sheet of india-rubber. Experiment proves that when all, or almost all, the air is pumped out of an india-rubber bag, and the bag is closed and left in the atmosphere, air passes into the bag through the walls, and that the composition of the air found in the bag is approximately 40 p. ct. of oxygen and 60 p. ct. of nitrogen, by volume. But the composition of ordinary air is approximately 21 p. ct. oxygen and 79 p. ct. nitrogen, by volume. Therefore the air has been partially separated into its constituents by passing it through a sheet of india-rubber; therefore air is a mixture, not a compound, of oxygen and nitrogen. 112 The argument may be extended to chemical events. If air is a mixture, it ought to interact chemically with other substances both as oxygen interacts and also as nitrogen interacts. If air is a compound, its interactions with other substances ought to be different from those of either oxygen or nitrogen. We have learned that nitrogen is a very inert substance ; it does not support combustion, it is not com- bustible, it combines directly with only a few elements, and it does not react chemically with many compounds. On the assumption that air is a mixture, we should, therefore, expect its chemical properties to resemble those of oxygen, but to be less strongly marked because of the presence of the inert nitrogen. For instance, we should expect substances which burn rapidly and brilliantly in oxygen to burn in air but to burn more slowly and less brilliantly. If we can find an element which combines directly with nitrogen when heated in that gas, we should expect that element to form a compound with nitrogen when strongly heated for some time in air. We need not go into details regarding individual experi- ments, but suffice it to say that these expectations are realised ; that the chemical behaviour of air is exactly what the hypo- thesis of its being a mixture asserts ought to be its behaviour. Air then is a mixture, not a compound, of oxygen and nitrogen. 113 But besides these gases, air contains small quantities of the compound gases, carbon dioxide, ammonia, and water- vapour. The composition of air varies within narrow limits. Thus air has not that fixity of composition which as we have seen characterises chemical compounds. 111113] CHEMICAL STUDY OF WATER AND AIR. 89 The experiment described in par. 108 shewed that the composition of air is, roughly, 4 vols. of nitrogen to 1 vol. of oxygen. In order accurately to determine the volume-com- position of air, a quantity of air is passed into a graduated glass tube fitted with two platinum wires passing through the glass near the closed end ; the tube is filled with mercury, and is then inverted in a trough containing mercury. The air to be analysed is freed from carbon dioxide and ammonia, and is then passed into the tube, and i?he volume is measured by reading off the level of the mercury. A quantity of hydrogen equal in volume to nearly of the volume of air is passed into the tube, and the level of the mercury is again read off; the tube is pressed down on a pad of india-rubber and securely clamped ; an electric spark is then sent from one platinum wire to the other ; the effect of this is that the whole of the oxygen in the air combines with a portion of the hydrogen to produce water which condenses. After a little time the tube is slowly raised from the india-rubber pad; mercury rushes in ; the level of the mercury is read off. As we know that 2 vols. of hydrogen combine with 1 vol. of oxygen, we conclude that -J- of the diminution of volume which occurs when the spark is passed represents the volume of oxygen in the volume of air employed. The volume occupied by the small quantity of water produced is so small that it may be neglected. Many precautions are necessary in carrying out such an analysis as this; corrections must be made for temperature and pressure; the volumes of wet air and wet gas after the explosion must be reduced to the corresponding volumes of dry gases, &c. The carbon dioxide in air maybe determined, (1) by slowly passing a large measured volume of air, freed from ammonia and water-vapour, through a series of weighed U tubes filled with caustic potash, and determining the increase in the weight of these tubes ; or (2) by adding, to a measured volume of air, a known quantity of barium oxide dissolved in water, and determining the quantity of this oxide which remains when the carbon dioxide in the air has all been absorbed by a portion of the barium oxide. The chemical reactions on which these methods are based may be represented in equations thus ; (1) xKOH (moist) + C0 2 - K 2 CO 3 + H 2 O + (a: - 2) KOH. (2) ccBaOAq + C0 2 = BaCO 3 + (x - 1) BaOAq, The potassium carbonate (K 2 CO 3 ) and water (H 2 0) formed 90 ELEMENTARY CHEMISTRY. [CHAP. vil. 113115. in (1) remain in the weighed U tubes along with the potash (KOH) which has not been changed by the carbon dioxide (CO 2 ) : the barium carbonate (BaCO 3 ) formed in (2) is a solid, it settles down in the liquid, and the unchanged barium oxide (BaO) remains in solution and is determined by a method which need not be described here. 114 The quantities of oxygen and nitrogen in average country air freed from water-vapour, ammonia, and carbon dioxide, are : # Percentage by volume. by weight. Oxygen = 20*96 23-0 Nitrogen 7 9 '04 77-0 100-00 100-0 The quantity of carbon dioxide averages about -03 volumes per 100 vols. of air. The quantity of ammonia varies very much ; it may perhaps be taken as about 1 part in 10,000,000 parts of air, by weight. The quantity of aqueous vapour also varies with variations in the season, the district, &c. &c. The fact that the quantities of oxygen and nitrogen in country air vary, although within very narrow limits, has been definitely established. The oxygen sometimes amounts to 20-999 vols. per 100 e.g. in air from the seashore or from inland moors ; in towns the oxygen sometimes falls to 20'82 vols. ; in inhabited rooms and crowded halls it may be as little as 20'28 ; in mines it averages about 20-26. A decrease in the volume of oxygen is usually accompanied by an increase in that of carbon dioxide ; in crowded rooms the volume of this gas may be as large as -3 to '5 vols. per 100. Air which contains as much as -1 vol. carbon dioxide per 100 is unpleasant, and harmful to health. The air of towns contains many gases, liquids, and solids, produced by the changes which go on among the living beings, and also by the manufactures conducted in the towns. 115 Our examination of air has afforded an application of the statements made in Chap. in. regarding the differences between mixtures and compounds; it has shewn us how we may determine to which of these classes a given substance belongs ; it has also made us acquainted with some of the prominent characters of air ; and it has a little familiarised us with the methods pursued in chemical inquiries. CHAPTER VIII. CHEMICAL STUDY OF HYDROGEN AND OXYGEN. HAVING now gained a fairly clear notion of the kind of 116 material phenomena which form the subject matter of chemistry, and of the methods by which the chemical aspects of these phenomena are investigated ; and having arrived at certain fundamental generalisations from facts established by quantitative experiments and quantitative reasoning, we are in a position to proceed with the main subject of our inquiry, which is to establish the relations which exist between changes of composition and changes of properties of the definite kinds, of matter we call compounds, and the relations which exist between the properties of the elementary constituents of com- pounds and those of the compounds themselves. This inquiry branches out in two directions ; it requires us to study (1) the properties of compounds, and the properties of elements as exhibited in their compounds; and (2) the composition of compounds. To do this we must classify ; we must group together those compounds which have similar properties, and those which have similar compositions. We shall begin our attempt to learn how elements and 117 compounds are classified, and to become acquainted with the more important results of this classification, by considering the two elements hydrogen and oxygen and some of the compounds of these elements. Occurrence. Oxygen, as we know, forms about ^ of the 118 atmosphere. Hydrogen is sometimes found in small quan- tities in volcanic gases. Numerous compounds of each element occur in nature ; of these water (H 2 O) is the most abundant. Oxides of aluminium, iron, calcium, magnesium, silicon, and many other elements, are found widely distributed and in 92 ELEMENTARY CHEMISTRY. [CHAP. vin. H9 large quantities. Ammonia a compound of hydrogen and nitrogen and compounds of ammonia, exist in the air and in the soil ; and most of the substances which form the parts of plants and softer tissues of animals are compounds of hydrogen, with carbon, oxygen, and nitrogen. Sp. gr. Sp. hts. (constant pressure; Sp. ht. of equal mass of water Physical properties. Hydrogen. 1 -0693 3-405 Oxygen. 16 1-105 for gas- eous oxygen. 979 for liquid oxygen ( water = 1). 218 Vols. dissolved by 1 -0193 '0295 vol. water at 16 Colour, appear- Colourless, tasteless, Colourless, tasteless, odourless, gas. Lique- fied at about - 140 under pressure of 320 atmos. ance, &c. odourless gas. Lique- fied (and 1 solidified) at very low temp, and great pressure ; approximately 1 40 and 600 atmospheres. Bate of diffusion about 4 times that of oxygen. Very bad conductor of sound. 120 Chemical properties. Hydrogen and oxygen readily combine to form water. If a stream of hydrogen is allowed to flow into oxygen, or into air, and a light is brought to the jet the hydrogen takes fire and burns in the oxygen, and water is produced. If a stream of oxygen is allowed to flow into hydrogen from a narrow tube, and a light is brought to the jet the oxygen takes fire and burns in the hydrogen, and water is produced. In each case the chemical change is the same = HO. Fig. 18 shews a simple arrangement for exhibiting these reactions. A and B are stoppered glass jars ; each is fitted 119121] CHEMICAL STUDY OF HYDROGEN AND OXYGEN. 93 with a cork through which passes a tube narrowed at the end which is to go into the jar; A is filled with oxygen, B with hydrogen; each stands in a little water whereby the Fig. 18. gas inside the jar is isolated from the air outside ; the tube passing through the cork which fits jar A is connected with a gasholder containing hydrogen, the other tube is connected with a gasholder containing oxygen. Hydrogen is caused to pass slowly through one tube and oxygen slowly through the other; after a minute or so (when the air is all driven out of these tubes) the hydrogen jet is lighted, the stopper of A is withdrawn and the cork with its tube is quickly inserted ; the hydrogen burns brilliantly ; the stopper of B is withdrawn and a light is brought near the opening of B, the hydrogen in B burns ; the cork is now very quickly pressed into its place, and the jet of oxygen is seen to burn in the atmosphere of hydrogen. A little consideration shews that the chemical reaction 2H + O = H 2 O must occur, for the most part, at or near the surface of that gas which is flowing into the other, which other is, comparatively, at rest. If the inflowing gas is hydrogen, then, as the flame is visible along the surface of the inflowing gas, we say that the hydrogen burns in the oxygen ; that the hydrogen is burnt and the oxygen supports the combustion. If the inflowing gas is oxygen, the flame being as before visible along the surface of the inflowing gas, we say that the oxygen is burnt, and the hydrogen supports the combustion. Oxygen combines directly with many elements; compounds of oxygen with every other element, except bromine and fluorine, have been prepared, either by direct combination, or as the results of several chemical changes. 121 94 ELEMENTARY CHEMISTRY. [CHAP. vili. I. The elements sodium, potassium, lithium, thallium, phosphorus, and some others, combine with oxygen more or less rapidly at ordinary temperatures. II. Antimony, arsenic, carbon, lead, sulphur, and many other elements, combine with oxygen at temperatures above the ordinary. III. Oxides of calcium, bismuth, chromium, copper, &c. &c. are usually prepared by (i) obtaining compounds of these metals with oxygen and hydrogen, and (ii) heating these hydroxides, and so decomposing them into oxides and water. IY. Oxides of lead, manganese, bismuth, and some other metals composed of much oxygen relatively to the mass of lead &c. are obtained by bringing these metals, or oxides of them composed of the metal united with relatively small masses of oxygen, in contact with two or more compounds which interact to produce oxygen. Y. Oxides of nitrogen, sulphur, tellurium, tfec. are obtained by decomposing, by heat or otherwise, compounds of these elements with oxygen and some other element or elements. The following equations present examples of each of the foregoing methods of preparing oxides : I. 2Na + O = Na 2 O ; 2P + 50 = P 2 O 5 . at ordinary temps. II. at higher temps. III. Ca0 2 H 2 = CaO + H 2 O ; Bi 2 O 6 H 6 = Bi 2 O 3 + 3H 2 O. by action of heat. IY. KClOAq + PbO (heated) = PbO 2 + KClAq Sb 2 3 + 2HN0 3 (heated) - Sb 2 O 4 + H 2 O + 2NO 2 . Y. 2HNO 3 + P 2 O 5 (heated) - N 2 O 5 + 2HPO 3 ; H 2 TeO 4 (heated) - H 2 O + TeO 3 . Many elements form more than one compound with oxygen. Thus, five oxides of nitrogen are known, viz. N 2 O, NO, N 2 O 3 , NO N O four oxides of lead have been prepared, viz. PbO, Pb 3 4 , Pb 2 3 , Pb0 2 . 122 Hydrogen combines directly with a few elements; the combination usually occurs at moderately high temperatures : thus, 2H + S (molten) = H 2 S ; H + Br (heated) = HBr ; 2C + 2H (by passing electric sparks) = C 2 H 2 ; C 2 X , SiBr 4 , Si 2 Cl 6 , &c. 156 Compounds with metallic elements. The elements we are considering combine with most metals to form compounds with similar compositions. The compositions of some of these are represented in the following formulae NaCl, KBr, KI ; CdX 2 , CaX 2 , BaX 2 , ZnX 2 ; BiX 3 ; Cr 2 X 6 : Cu 2 X 2 and CuX 2 ; Fe a X 4 and Fe 2 X 6 ; Hg 2 X 2 and HgX^ ; PtX 2 and PtX 4 ; SnX 2 and SnX 4 . These compounds are usually produced by heating the metal in contact with chlorine, bromine, or iodine ; in some cases however it is necessary to use indirect methods of prepa- ration. The binary compounds of the three elements we are con- sidering are usually called haloid compounds (i.e. compounds resembling common salt, NaCl); the elements themselves are often called halogens. Many of the haloid compounds of the non-metals other than hydrogen and oxygen, and some of those of the metals, can be gasified without decomposition. Most of the non-metallic haloid compounds interact with water to produce solutions of HX (where X = 01, Br, or I), and generally either an oxide or an oxygen-containing acid of the non-metal formerly combined with halogen. The following equations represent some of these changes : 154157] CHEMICAL CLASSIFICATION. 119 2AsCl 3 + 3H 2 O + Aq - As 3 Aq + GHClAq. BC1 + 3H O + Aq = H 3 BO 3 Aq + SHClAq. PBr + 3H O + Aq = H 3 PO 3 Aq + SHBrAq. 2S 2 Br + 3H 2 + Aq = H 2 SO 3 Aq + 3S + 4HBrAq. 2Se 2 I 2 + 3H 2 O + Aq = H 2 SeO 3 Aq + 3Se + 4HIAq. In some cases the products of the interaction of a non- metallic haloid compound and water are HX and a compound of the non-metal with oxygen and halogen ; thus SbI 3 + H 2 O + Aq = SbOI + 2HlAq. Most of the haloid compounds of the metallic elements are chemically unchanged when brought into contact with water ; several dissolve in water. In some cases however chemical change occurs ; the usual products are haloid compounds of hydrogen (HX) and an oxychloride, oxybromide, or oxyiodide, of the metal :- thus, BiCl 3 + 2H 2 O -f Aq = BiOCl + 2HClAq; 2SnCl a + H a O + Aq = Sn a OCl a + 2HClAq. Interactions with water. The three elements dissolve in 157 water, chlorine very freely, bromine less freely, and iodine only in small quantities. By cooling aqueous solutions of chlorine or bromine crystals separate having the composition C1.5H a O and Br. 5H O respectively : no hydrate of iodine i.e. compound of iodine with water- has been obtained. Aqueous solutions of the three elements contain small quantities of hydrochloric, hydrobromic, and hydriodic acids, respectively; i.e. the water and chlorine &c. interact as shewn by the equation 2X + H 2 O + Aq = 2HXAq + O. This reaction proceeds more rapidly when X = Cl than when X = Br. When X = I but very little reaction occurs. These reactions are hastened by sunlight. If some easily oxidised substance is dissolved in water and chlorine is passed into the liquid the substance is usually oxidised \ thus a solution of sulphur dioxide reacts with chlorine to produce sulphur tri- oxide, a solution of phosphorous oxide reacts with chlorine to produce phosphoric oxide : or, in equations (1) SO 2 Aq + H 2 O + 2C1 - 2HClAq + SO 3 Aq. (2) P 2 O 3 Aq + 2H 2 O + 4C1 - 4HClAq + P 2 O 5 Aq. The bleaching action of chlorine depends upon its inter- acting with water to produce oxygen. Dry chlorine does not bleach a piece of madder-dyed cloth ; but if water is present 120 ELEMENTARY CHEMISTRY. [CHAP. xr. the cloth is bleached. The colourless bodies produced are the results of the interaction of oxygen with the colouring matter of the cloth ; this oxygen is produced from the water by inter- action with chlorine as already described. An aqueous solu- tion of bromine bleaches more slowly than a solution of chlorine, and a solution of iodine bleaches very slowly indeed : the bleaching action is more or less rapid according as the element decomposes water rapidly or slowly (v. supra). 158 Interactions with solutions of alkalis. Chlorine, bromine, and iodine interact with cold aqueous solutions of caustic potash, soda, &c. to produce potassium or sodium (&c.) chloride, bromide, or iodide, and also potassiun> (&c.) hypochlorite, hypobromite, or (probably) hypoiodite. Thus in equations (X = C1, Br, or I) GKOHAq + 6X = 3KX Aq + SKXOAq + 3H 2 O. The interaction which occurs between one of the halogens and a hot solution of caustic potash, soda, &c. is expressed thus : GKOHAq + 6X = 5KX Aq + KXO 3 Aq * 3H 2 O. The products are potassium (* pounds, with definite melting definite melting points; WCjjUio) SP T 1 and boiling TeBr 2 and T B 4 \ 8 enera M.Y un - 22 I f*M solid, points; but most of them are sepa- TeI 2 and TeI 4 changed by action of heat. Dex>r 4 i SP! rated by heat into TeCl, has been Qt5-L 4 ) selenion and l^gasifi'ed. ^halogen. 177 Compounds with oxygen and the halogens ; oxyhaloid com- pounds. MOX 2 and M0 2 X 2 (X=C1, Br, I). Several of these compounds have been prepared : as examples we shall take SOC1 2 and SO 2 C1 2 . The former is obtained by reacting on sodium sulphite with phosphoric chloride, the latter by a similar reaction between sulphuric acid and phosphoric chloride. The compositions of the systems before and after the changes may be approximately represented in equations as follows ; (1 ) Na 2 SO 3 + 2PC1 5 - S001 2 + 2POC1 3 + 2NaCl ; (2) H 2 SO 4 + 2PC1 5 - SO 2 C1 2 + 2POC1 3 + 2HC1. The compounds SOC1,, and SO 2 C1 2 interact with water to produce hydrochloric acid, and sulphurous or sulphuric acid ; thus - (1) SOC1 + 2H 2 O + Aq - 2HClAq + H 2 SO 3 Aq ; (2) SO Gi; + 2H 2 O + Aq = 2HClAq + H 2 SO 4 Aq. Certain relations between the oxy chloride SOC1 2 and the acid H 2 SO 3 , and between the oxy chloride SO 2 C1 2 and the acid H 2 SO 4 , are established by the foregoing reactions : these relations are perhaps better suggested if the reactions are stated as follows ; (1) HS0-0H+2C1 = (2) H 2 S0 4 - 2 H 2 -f 201 ^ S0 C1 2 ; S0 2 C1 2 - 201 + 2 H 2 - H 2 S0 4 . (SOC1 8 is not directly obtained from H 2 S0 3 , because this acid only exists in aqueous solution.) 176178] CHEMICAL CLASSIFICATION. 135 If the formulae of sulphurous and sulphuric acids are written as SO.O 2 H 2 [or SO(OH)J, and SO 2 .O 2 H 2 [or SO (OH)J, respectively, the reactions which occur between these acids and phosphoric chloride are suggested by the formulae, (s. chap, xvn.) Compounds with oxygen and hydrogen ; Acids. Several of 178 these acids are known ; the most important are those whose compositions are expressed by the general formulae H 2 MO 3 and H 2 MO 4 . The acids H 2 MO 3 where M = S or Se are produced by dissolving the oxides MO 2 in water ; H 2 Te0 3 is formed by oxidising tellurium by nitric acid in presence of water (s. par. 175). The acid H 2 SO 4 is produced by dissolving the oxide SO 3 in water; H 2 SeO 4 is formed by oxidising sele"niori in presence of water by the interaction between water and chlorine (s. par. 1 75) ; H 2 TeO 4 is formed by decomposing the barium salt of this acid by the proper quantity of sulphuric acid, and removing water by evaporation ; thus BaTeO 4 Aq + H 2 SO 4 Aq = BaSO 4 + H 2 TeO 4 Aq. Of the acids H 2 MO 3 and H 2 MO 4 , H 2 SO 3 is known only in aqueous solutions ; when such a solution is evaporated water and sulphur dioxide (SO 2 ) are produced ; the other acids have been isolated. H 2 SeO 3 and H 2 SO 4 are thick oily liquids at ordinary temperatures ; H 2 TeO 3 and H 2 TeO 4 are solids. These acids are all decomposed by heat into water and the correspond- ing oxides ; thus H MO 3 heated gives H 2 O + MO 2 ; HlMO, H 2 + M0 3 . This change occurs at a higher temperature when M = Te than when M = Se, and at a higher temperature when M = Se than when M = S ; in other words, the stability of the acids towards heat increases as the combining weight of M in- creases. There is an oxide corresponding to each of these acids : this oxide, except in the cases of TeO 3 and TeO 3 , interacts with water to produce the acid ; it is also produced, along with water, when the acid is heated. The nomenclature of these oxides and their corresponding acids is exhibited in the following table. Oxide. Corresponding acid. M0 9 . Sulphur, selenion, or tellurium, \ _ . . f ELMO,. Sulphurous, selenious, dioxide; or sulphurous, selenious, V 2 * ._ \ or tellurous, acid, or tellurous, anhydride. J M0 3 . Sulphur, or tellurium, trioxide ; ) H 2 M0 4 . Sulphuric, selenic, or or sulphuric, or telluric, anhydride ) telluric, acid. 136 ELEMENTARY CHEMISTRY. [CHAP. XL 179 Compounds with electro-positive or metallic elements. The three elements combine with many metals to form sulphides, selenides, and tellurides, of similar compositions. The follow- ing are a few examples : CuM, ZnM, FeM, Bi 2 M s , &c. Most of these compounds interact with aqueous solutions of acids to give salts of the metal and a hydride of M ; thus GuM + 2H01 Aq = CuCl 2 Aq + H 2 M. 180 The elements sulphur, selenion, and tellurium, are evidently similar in their properties. They all combine directly with hydrogen to form hydrides MH 2 , two of which are slightly acidic. They combine with oxygen to form oxides MO 2 and MO S , which are more or less markedly acidic. They form several haloid compounds which are not acidic ; the stability of these towards heat increases as the combining weight of M increases. They form oxyhaloid compounds, each of which interacts with water to form an acid of M, and a haloid acid HX (where X = Cl, Br, or I). They do not interact with acids to form salts. 181 Some of the prominent properties of the elements of the sulphur group are compared with the more prominent proper- ties of the halogens, and of the alkali metals, in the following table. The sulphur group of elements is evidently much more allied to the halogens than to the alkali metals. Sulphur group. Sulphur, Selenion, Tel- lurium. All solids.; naore-orless brittle ; fairly high melting points. Electro - negative to many other elements. Form gaseous com- pounds with hydrogen, MH 2 ; soluble in water giving feebly acid, or, in case of TeH^, neutral solutions. Form oxides by direct union with oxygen ; most of these dissolve in water forming fairly stable acids. Unite with many positive, or metallic, elements ; compounds with negative elements, Halogens. Chlorine, Bromine, Iodine. One gaseous ; one liquid ; one solid. Electro - negative to most other elements. Form gaseous com- pounds with H, MH; very soluble in water forming markedly acid solutions. Oxides formed indi- rectly; very soluble in water forming unstable acids. Combine with all posi- tive, or metallic, ele- ments; most com- pounds with negative Alkali metals. Lithium, Kodinm, Po- tassium, liubidium, Cariufa. Solids ; soft, easily melted. light, Electro-positive to all other elements. Do not combine with hydrogen. Oxides formed directly, at ordinary tempera- tures; very soluble in water giving alkaline solutions. Combine with most negative or non- metallic, but not with metallic, elements ; 179-183] CHEMICAL CLASSIFICATION. 137 Sulphur group. Sulphur, Selenion, Tel- lurium. Halogens. Colorine, Bromine, Iodine. as a class, not very stable as regards action of heat. Do not interact with acids to produce salts. Scarcely, if at all, in- teract with water or team. elements are unstable as regards action of heat. Do not interact with acids to produce salts. Interact with steam to produce acids (HX) and evolve oxygen. Alkali metals. Lithium, Sodium, Po- tassium, Rubidium, Ccesium. compounds stable to- wards heat. Readily interact with acids to produce salts. Interact rapidly with cold water to produce alkalis (MOH) and evolve hydrogen. As regards the variations of properties in each of these 182 three groups we have found that as the combining weights increase the elements become more" positive. In the sulphur and halogen group the elements become heavier and their melting points increase j the oxides, oxyacids, and compounds with chlorine, become more solid, and more stable towards heat ; the hydrides become less stable towards heat. In the halogen group the interaction with water takes place more completely and at a lower temperature, the smaller is the combining weight of the element ; in the alkali-metals group the reverse of this holds good. In the sulphur group, tellurium is more metal-like in appearance and general physical properties than sulphur or selenion ; its oxides do not directly interact with water to form acids ; its hydride is easily decom- posed by heat, and its aqueous solution is not acidic ; it does not exhibit allotropy : in a word, tellurium is more metallic than either sulphur or selenion. The terms reduction and oxidation, also reducers or reducing 183 agents and oxidisers or oxidising agents, have been used in describing some of the chemical changes exhibited by the elements and compounds considered in preceding paragraphs. When chlorine is passed over mercuric oxide the chlorine is oxidised and the mercuric oxide is simultaneously reduced, (s. par. 154.) When nitric acid is heated with tellurium the element is oxidised and the acid is simultaneously reduced. (s. par. 175.) When chlorine is passed into concentrated caustic potash solution potassium hypochlorite (KC1O) is formed, when this is heated oxygen is evolved and potassium chloride (KC1) remains ; when chlorine is passed into warm potash solution holding bismuthous oxide (Bi a O 3 ) in suspension bismuthic oxide (Bi s O ft ) and potassium chloride are formed ; the bismuthous oxide is oxidised and the potassium hypo- 138 ELEMENTARY CHEMISTRY. [CHAP. xi. chlorite (formed by the interaction of the chlorine and potash) is simultaneously reduced, (s. par. 158.) When hydrogen is produced, by the interaction of zinc and dilute sulphuric acid, in contact with sodium sulphite (Na 2 SO 3 ) in solution, the hydrogen is oxidised to water, and simultaneously the sodium sulphite is reduced to sodium oxide (Na 2 0) which reacts with the sulphuric acid present to form sodium sulphate (Na 2 SO 4 ), and hydrogen sulphide (H 2 S). In these, and in very many other, cases, the processes of oxidation and reduction occur together as parts of a chemical change. In the experiment with mercuric oxide and chlorine, the chlorine acted as the reducer or reducing agent, and the mercuric oxide as the oxidiser or oxidising agent. In the experiment with tellurium and nitric acid, the acid acted as the oxidiser and the tellurium as the reducer. When bis- muthous oxide was oxidised by potassium hypochlorite, the latter was the oxidiser, the former the reducer : but the pro- duction of the hypochlorite was due to the interaction of chlorine with potassium hydroxide (2KOH Aq + 201 - KClAq + KClOAq), therefore it might be said that the chlorine was the primary oxidising agent. Similarly, if selenion is suspended in water and chlorine is passed into the liquid, selenic and hydrochloric acids are produced ; thus, Se + 601 + 4H 2 O + Aq = H 2 SeO 4 Aq + GHClAq : the selenion is oxidised by the oxygen which before the change began was combined with hydrogen; therefore the water is i_ , the oxidiser, and the selenion is the reducing agent ; but the interaction of chlorine is required to decompose the water, therefore the primary oxidising agent is chlorine. 184 Many elements and compounds may be classified in accord- ance with their actions as oxidisers or reducers ; or in accord- ance with the conditions under which they are oxidised or reduced. Hydrogen, carbon, sodium, carbon monoxide (CO), sulphur dioxide (SO 2 ), nitrous acid (HNO 2 Aq), stannous chloride (SnCl 2 ), aldehyde (C H 4 O), are some of the more commonly used reducing agents. Oxygen, ozone, chlorine, nitric acid, potassium chlorate (KC1O 3 ), potassium permanga- nate (K 2 Mn 2 O 8 ), are among the commonly used oxidising agents. The following equations present examples of the use of these reducers and oxidisers. 183184] CHEMICAL CLASSIFICATION. a I 1 '3 V tfl T3 I '4$ S 33,5 32 SS sg .. I'll !g 139 'g s^i 1 >(?.! ."S S "73 0> -^ O "^ -PH ^Q f3 O IllgJ O o> &D c3 n n o s ^3 O - ^ " O o ft^ o 133 S O> o> f^ 4 ' 3 F^q - D I'llll! s ^ s- Sf ^ ^ _i o> '3 2lli feifjl" C3.0 ii SH 2 ^ a^ -s^ |J| "5'l O rS 02-sJ fl . 2 |1 o 5 duced duct. Re S o S" M DQ O J w ^o o cTc^ d W^ SwM 1 Original 7?ient or c poun fl Jlf 02 coco 140 ELEMENTARY CHEMISTRY. [CHAP. XI. In all these instances of the action of reducing agents, the reduction of one substance is accompanied by the oxidation of another ; in most of the instances of the action of oxidising agents, the oxidation of one substance is accompanied by the reduction of another. 185 In some cases, e.g. reduction of beryllium chloride by sodium, reduction of mercuric chloride by stannous chloride, oxidation of antimonious chloride by chlorine none of the substances taking part in the reactions is a compound of oxygen. It is customary to apply the term reduction to all chemical changes wherein the negative or non-metallic part of a compound is either wholly or partially removed, and the term oxidation to all chemical changes wherein the negative part of a compound is increased, or a negative ele- ment (or elements) is added to a more positive element (or elements). 186 Those elements which are easily oxidised might be placed in one class, and those which are oxidised with difficulty in another class; compounds which are easily and completely reduced might be classed apart from those which are only reduced at high temperatures and by indirect methods. Such a system of classification would be based, primarily at any rate, on the occurrence or non-occurrence under specified con- ditions of a certain chemical change ; it would be based on a certain power of doing, rather than on the compositions, of the bodies classified. To make this classification fairly satisfactory it would be necessary to examine the compositions of the members of each class, and then to connect these compositions with the performance or non-performance of that chemical reaction which had been made the mark of each class. 187 We have had examples of classification founded on re- actions rather than on composition. Oxides were divided into basic and acidic ; a great many compounds were placed in one class, and called acids, because they all interacted with metals and with basic oxides- to produce salts. Can we connect the composition of those oxides which are called basic, and the composition of those which are called acidic, with the facts that the former interact with acids to produce salts, and the latter interact with water to produce acids'? An answer to these questions will carry with it an answer to this ; can we state in general terms the connexion between the composition of acids and the properties connoted by the term acid 1 184188] CHEMICAL CLASSIFICATION. 141 Let us begin the inquiry by learning a little more about 188 the interactions of acids with metals, basic oxides, and alkalis, to produce salts. Let aqueous solutions of the three acids, hydrochloric (HC1), sulphuric (H 2 SO 4 ), and phosphoric (H 3 PO 4 ), be prepared, each containing a known mass of the acid in a specified volume ; let an aqueous solution of the alkali potassium hydroxide (KOH) be prepared containing a known mass of the compound in a specified volume. Let definite quantities of each acid solution be added to definite quantities of the alkali solution, and let all the products of each reaction be collected, examined, and analysed. The results may be represented as follows. Reactions between aqueous solutions of hydro- chloric acid (HC1) and potassium hydroxide (KOH). (1 gram HC1 used in each case.) Grams KOH Grains salt Grams Grams KOH used. formed; and composition of salt. water formed. remaining unchanged. 1-54 2-04 KC1 5 none 1-54x2 2-04 KC1 5 1-54 1-54 x 3 2-04 KC1 5 1-54x2 1-54x4 2-04 KC1 5 1-54x3 Reactions between aqueous solutions of sulphuric acid (H SO 4 ) and potassium hydroxide. (1 gram H 2 SO 4 used in each case.) Grams KOH remaining unchanged. Grams KOH Grams salt Grams used. formed; and composition of salt. water formed. 57 57 x2 57 x 3 1-39 KHSO 1-77 K 2 S0 4 1-77 K;S0 4 18 18 x 2 18 x 2 57 x 4 1-77KJ9D. 18x2 none none 57 57 x 2 Reactions between aqueous solutions of phosphoric acid (H 3 PO 4 ) and potassium hydroxide. (1 gram H 3 PO 4 used in each case.) 142 ELEMENTARY CHEMISTRY. [CHAP. XI. Grams KOH Grams salt Grams Grams KOH used. formed; and water remaining composition formed. unchanged. of salt. 57 1-39 KH 2 P0 4 18 none 57 x 2 1-77 KHP0 4 18 x2 none 57 x3 2-13 KPO 18 x 3 none 57 x4 2-13 K 3 PO 4 18 x 3 57 57 x 5 2-13 K 3 PO 4 18 x 3 57 x2 We see then (1) that hydrochloric acid and potassium hy- droxide interact to produce one salt (KC1), and that the potash over and above that which interacts to produce this salt remains unchanged; (2) that two salts (KHSO 4 and K 2 SO 4 ) are produced by the interaction of sulphuric acid and potash, and that the production of one or other salt depends upon the relative masses of the alkali and acid, but that if a greater mass of potash is added than is required for the production of the salt K SO 4 the excess of potash remains unchanged ; (3) that three salts (KH 2 PO 4 , K 2 HPO 4 , and K 3 P0 4 ) are pro- duced by the interaction of phosphoric acid and potash, and that the production of one or other salt depends upon the relative masses of acid and alkali, but that any excess of potash beyond that which interacts to produce K 3 PO 4 remains unchanged. Similarly, if the metal potassium had been employed in place of its hydroxide we should have obtained one salt (KC1) in the case of hydrochloric acid, two salts (KHSO 4 and K i ,S0 4 ) in the case of sulphuric acid, and three salts (KH 2 P0 4 , K 2 HP0 4 , and K 3 PO 4 ) in the case of phosphoric acid. 189 -^ the interactions between potassium (or sodium) hy- droxide and many acids are examined we find that the acids may be classified as follows : T IT- T -7 (acids which interact with) , I. Monobasic acids ; { -> -. . , > only one salt. ' (potash or soda to produce/ J II. Dibasic acids; two salts. III. Tribasic acids; three salts. IY. Tetrabasic acids; four salts. Y. Pentabasic acids; five salts. YI. Hexabasic acids; " six salts. An n-basic acid may also be defined as an acid from the reacting weight of which n combining weights of hydrogen can be displaced by sodium or potassium, when the acid interacts 188191] CHEMICAL CLASSIFICATION. 143 with sodium or potassium or the hydroxide of either of these metals. We have thus arrived at a fairly clear notion of the meaning 190 of the term acid so far as the reactions of acids with metals, basic oxides, and alkalis, are concerned. But what kind of compounds are acid 1 ? What are the compositions of these compounds? Have acids any common composition corresponding to their common property of forming salts under specified conditions 1 The following formulae represent the compositions of several compounds which are acids IIC1, HBr, HI, HF, H 2 SO 3 , H 2 S0 4 , HN0 2 , HNO 3 , H 2 d0 4 , H 4 C 2 2 , HCNO, H 2 B 2 4 , HC1O 3 , H,IO C , HBr0 3 , HP0 3 , H S PO 4 , H 2 Mn 2 O 8 , HY0 3 , HSbO 3 , H 3 SbO 4 , H 2 MoO 4 , H 2 W0 4 , H 2 Sn0 3 , H 2 S 2 7 , H 2 SeO 4 , H 2 TeO 3 , H 2 Ta 4 O 7 . These acids are all compounds of hydrogen, and most of them are compounds of hydrogen with oxygen and another element. Are all compounds of hydrogen with oxygen and another element acids 1 We know that aqueous solutions of the following com- pounds are alkalis, i.e. exhibit properties strongly opposed to those of acids; LiOH, NaOH, KOH, RbOH, CsOH. The following compounds are also alkaline ; CaO 2 H 2 , BaO 2 H 2 , SrO H 2 , MgO 2 H 2 . Therefore all compounds of hydrogen with oxygen and another element are not acids. What then are the general characters of those elements the com- pounds of which with hydrogen and oxygen are acids'? Most of the acids in the foregoing list are compounds of hydrogen and oxygen with non-metallic elements (S, N, C, B, Cl, I, Br, P, Se, Te) ; hence it would appear probable that the union of a non-metallic, or negative, element with hydrogen and oxygen would produce an acid. Further investigation gives a general confirmation to this conclusion. By far the greater number of the compounds of the non- 191 metallic elements with oxygen and hydrogen are acids. Some of the acids in the foregoing list are compounds of hydrogen and oxygen with elements which are usually classed as metals; H 2 Mn 2 O fl , H 2 MoO 4 , H 2 WO 4 , H n Ta 4 O 7 , H 2 Sn0 3 , HVO 3 , HSbO 3 , H 3 SbO 4 . The elements tungsten, molyb- denum, tantalum, vanadium, and antimony exhibit many of the physical, and some of the chemical, properties of metals ; but they each form at least one acidic oxide ; they do not 144 ELEMENTARY CHEMISTRY. [CHAP. XL form well marked and stable salts by interacting with acids ; they are electro-positive to all the distinctly non-metallic elements (O, S, N, P, Cl, Br, I, F, B, C, Si, S, Se), but they are electro-negative to most of the distinctly metallic elements. The elements manganese and tin are undoubtedly metals, both physically and chemically ; they interact with acids to form salts ; they form basic oxides ; they do not combine with hydrogen; they react with steam at high temperatures to produce oxides and evolve hydrogen ; they are heavy, lustrous, malleable, solids which conduct heat and electricity fairly well. The compounds (1) H 2 Mn 2 O R and (2) H 2 SnO 3 are however acids; the ratios of the numbers of combining weights of hydrogen, metal, and oxygen in a reacting weight of each of these compounds is H : M : O in (])= 1 : 1 : 4, and in (2) = 1 : J : 1 J. The compound in a reacting weight of which there is relatively the greater quantity of oxygen (H a Mn a O 8 ) is decidedly an acid ; the other compound in a reacting weight of which there is relatively less oxygen is an acid, but at the same time it interacts with concentrated sulphuric acid (and with some other acids) to produce a salt and water ;- thus H 2 Sn0 3 + 2H 2 SO 4 (heated) - Sn(SO 4 ) 2 + 3H 2 O. Certain compounds of metals with hydrogen and a rela- tively large quantity of oxygen are then acids. But in the list of acids given a little further back there appeared compounds of hydrogen with certain non-metallic elements other than oxygen; viz. HC1, HBr, HI, HF. We have already learned something of the elements in question : we know that they are typical non-metals ; that they are strongly electro-negative. 192 It appears then that acids are compounds of hydrogen, generally with markedly electro -negative or non- metallic elements of which oxygen is usually one ; but in some cases with oxygen and another element which, physically and chemically, is neither strongly metallic nor strongly negative ; and in a few cases with oxygen and another element which must be placed in the positive or metallic class. In the last mentioned cases there is usually a large quantity of oxygen combined with hydrogen and the metallic element. 193 We now see that acidic oxides are generally the oxides of the electro-negative or non-metallic elements. The oxides corresponding to the acids HYO 3 , HSbO , H 2 Mo0 4 , H 2 W0 4 , H 2 Ta 4 7 , are V 2 O 5 , Sb 2 O 5 , MoO., WO 8 , and Ta O 5 , respectively : these acidic oxides are the oxides of 191195] CHEMICAL CLASSIFICATION. 145 elements which are both metallic and non-metallic ; they are all t composed of a large quantity of oxygen combined with a comparatively small quantity of the other element. The compositions of these oxides, as regards ratio of oxygen to other elements, may be compared with the compositions of the oxides C1 2 O, N 2 O, P 2 O 3 , all of which are acidic, and all of . which are oxides of strongly negative elements. Oxides of the less positive metals formed by the union of much oxygen with comparatively small masses of the metals may then be acidic. The oxides of the very positive elements (Li, Na, K, Rb, Cs, Ca, Sr, Ba, Mg, &c.) are all basic; the oxides of the less positive, but still distinctly positive, elements (Fe, Zn, Ni, Cd, Hg, &c.) are generally basic ; the lower oxides i.e. those with comparatively little oxygen of the more negative of the metals (SnO, PbO, Pb 3 O 3 , MnO, Mn a O 8 , &c.) are usually basic, but the higher oxides of some of these elements (SnO 2 , Pb0 2 , &c.) are feebly acidic. Chromium is a heavy, hard, lustrous, grey, solid. It con- 194 ducts heat and electricity fairly. This element scarcely reacts with steam ; at a high temperature a very little oxide is pro- duced. It interacts with acids to form salts. It forms stable haloid, 'and also several oxy haloid, compounds. It does not combine with hydrogen. It is positive to most of the distinctly non-metallic elements, but negative to most of the distinctly metallic elements. From what we have learned concerning basic and acidic oxides, we might expect to find the lower oxides of chromium basic, and perhaps an oxide with compara- tively much oxygen, acidic. The elements iron and manganese resemble chromium in many of their properties. It is advisable now briefly to con- sider this group of elements. Chromium. Manganese. Iron. 195 Combining weights 52-4 55 56 Spec, gravities (water = 1) about 6-5 about 7 about 7'5 Melting points above 2000 about 1500 about 15QO Appearance &c. All white, lustrous, solids: chromium and manganese very hard ; iron softer ; all malleable and fairly ductile ; all fair conduc- tors of heat and electricity. Preparation. All obtained by reducing the oxides M 2 3 with finely powdered carbon at a very high temperature. Chemical properties. The elements decompose steam form- ing oxides and hydrogen ; iron at about 100, manganese at a M. E. C. 10 146 ELEMENTARY CHEMISTRY. [CHAP. XI. higher temperature, and chromium at a still higher temperature and then only very slowly. None of these elements combines directly or indirectly with hydrogen. They all interact with oxygen to produce several oxides. The compositions of the chief oxides are expressed by the formulae MO, M 3 O 4 , M 2 O 3 ; the oxides MnO 2 , Cr0 2 , and CrO 3 are also known. 196 The oxides MO are basic ; they react with acids to form salts ; thus, MO + H 2 S0 4 Aq = MSO 4 Aq + H 2 O. The oxides M 3 O 4 , where M = Mn or Fe, also react with acids to form salts ; but the compositions of the salts do not correspond to that of the oxides. With dilute sulphuric acid Mn 3 O 4 forms MnS0 4 and Mn0 2 ; thus Mn 3 O 4 + 2H 2 SO 4 Aq - Mn0 2 + 2MnS0 4 Aq + 2H 2 0. With hot concentrated sulphuric acid MnSO 4 is formed and oxygen is evolved ; thus Mn 3 4 + 3H 2 SO 4 = 3MnS0 4 + 3H 2 + 0. The corresponding oxide of iron reacts with sulphuric acid to form a mixture of the two sulphates of iron ; thus, Fe 3 4 + 4H 2 S0 4 Aq = FeSO 4 Aq + Fe 2 3S0 4 Aq + 4H 2 O. The existence of O 3 O 4 is doubtful. The oxides M 2 O 3 are also basic; their interactions with sulphuric acid may be represented thus, (1) Cr 2 O 3 '+ 3H 2 S0 4 Aq = Cr 2 3S0 4 Aq + 3H 2 O. (2) Mn 2 O 3 + H 2 S0 4 Aq = MnO 2 + MnSO 4 Aq + H 2 O : (or with hot concentrated sulphuric acid ; Mn 2 O 3 + 2H 2 S0 4 = 2MnSO 4 +2H 2 O + 0). (3) Fe 2 O 3 +3H 2 SO 4 Aq - Fe 2 3S0 4 Aq + 3H 2 O. But the oxides M 2 O 3 are also feebly acidic ; salts of the form HO.M 2 O 3 are known, where RO= K 2 O, CaO, ZnO, &c. Of the remaining oxides, CrO 2 has not been fully ex- amined ; it seems to react with acids to form the same salts as are obtained from Cr 2 O 3 , and at the same time to evolve oxygen. MnO 2 with concentrated acids gives salts of the form MnX (where X = SO 4 , 2NO 3 , C0 3 , &c.), and at the same time oxygen is produced ; thus, MnO 2 + H 2 SO 4 Aq = MnSO 4 Aq + H 2 O + O (the amount of water must be small). This oxide also reacts with alkalis to form salts the negative 195196] CHEMICAL CLASSIFICATION. * 147 part of which is formed of the MnO 2 and oxygen ; thus when this oxide is fused with solid potash the reaction is 3Mn0 2 + 2KOH = K 2 O . MnO 2 (= K 2 MnO 4 ) + H 2 + Mn 2 O 3 . The salt potassium manganate (K 2 Mn0 4 ) dissolves in water, and from the solution other salts are obtained ; thus, K 2 MnO 4 Aq + BaCl 2 Aq - BaMnO 4 + 2KClAq. These salts, the composition of which is expressed by the formula MMnO 4 where M = Ba, Ca, K 2 , Na 2 , &c., are called manganates. Their composition is similar to that of sul- phates (MS0 4 ). An aqueous solution of a manganate reacts with dilute acids to form a permanganate; thus, 3K MnO Aq + 2HSO 4 Aq - K 2 MnOAq + 2K 2 SO 4 Aq A series of permanganates is known, MMn 2 O 8 , where M = Ba, Ag 2 , K 2 , &c. The oxide CrO 3 dissolves in water to form an acid liquid from which crystals of the acid H 2 Cr0 4 have been obtained. This acid is called chromic acid', it reacts with alkalis and basic oxides to give a series of chromates MCr0 4 (M = K 2 , Na 2 , Ag 2 , Ba, Ca, &c.). The composition of the chromates is similar to that of the manganates (MMnO 4 ) and sulphates (MS0 4 ). An aqueous solution of a chromate reacts with dilute acids to form a dichromate ; thus 2K 2 Cr0 4 Aq + H 2 S0 4 Aq = K 2 O 2 O 7 Aq + K 2 S0 4 Aq + H 2 O. The dichromates, MCr 2 O 7 , are not similar in composition to the permanganates (MMn 2 O 8 ), but they agree in composition with a series of sulphur salts known as di- (or pyro-) sulphates, MS 2 7 . The oxide CrO 3 also interacts with hot concentrated acids to form salts and oxygen ; thus, 2CrO 3 + 3H 2 SO 4 = Cr 2 3SO 4 + 3H 2 O + 30. The manganates and permanganates react with acids to form manganese salts and oxygen; thus, (1) K 2 MnO 4 Aq + 2H 2 SO 4 Aq = K 2 SO 4 Aq + MnSO 4 Aq + 2H 2 O + 2(X (2) K 2 Mn 2 8 Aq+3H 2 S0 4 Aq = K 2 S0 4 Aq + 2MnS0 4 Aq + 3H 2 O + 5O. 102 148" ELEMENTAKY CHEMISTRY. [CHAP. XI. Similarly, the chromates and dichromates react with con- centrated solutions of acids to form chromium salts and oxygen; e.g. (1) 2K 2 Cr0 4 Aq + 5H 2 S0 4 = 2K 2 SO 4 Aq + Cr 2 3S0 4 Aq + 5H 2 4- 3O, (2) O 4 Aq + Cr 2 3S0 4 Aq + 4H 2 + 30. 197 The salts of chromium, manganese, and iron, i.e. com- pounds derived from acids by replacing hydrogen by chromium,. manganese, or iron, form two series the compositions of which are represented by the general formulae MX and r M 3X, respectively, where M = Or, Mn, or Fe, and X = 2NO 3 , 2C10 3 , S0 4 , S0 3 , C0 3 , f P0 4 , |As0 4 , &c. The salts MX are called chromous, manganous, and ferrous, salts; those of the composition M 2 3X are called chromic, manganic, and ferric salts. A few examples of each class of salts are given : -ous salts. -ic salts. Ferrous sulphate FeSO 4 Ferric sulphate Fe 2 3S0 4 Manganous sulphate MnS0 4 Manganic sulphate Mn 2 3S0 4 Chromous sulphate CrS0 4 Chromic sulphate Cr 2 3S0 4 Ferrous nitrate Fe2N0 3 Ferric arsenate Fe 2 2AsO 4 Manganous chlorate Mn2C10 :< Manganic phosphate Mn 2 2P0 4 Chromous oxalate CrC 2 4 Chromic selenite Cr 2 3Se0 3 . Many iron salts of both series are known; the ferrous salts are all fairly readily oxidised to ferric salts. Most of the known manganese salts belong to the manganous class; the manganic salts are all readily reduced to manganous salts. Very few chromous salts have been prepared ; they are all easily oxidised to chromic salts. 198 Chromium and manganese resemble the halogen elements and the elements of the sulphur group in that each forms at least one acidic oxide. The resemblance between chromium and manganese and the sulphur group of elements is further shewn by the compositions of the salts obtained by the interactions of these acidic oxides with basic, or alkali-forming, oxides. Thus (M = Ba, Pb, Ca, K 2 , Na 2 , Ag 2 , shew that a complete account of chemical change cannot be given by regarding only the affinities of the interacting substances. It is necessary to pay attention also to the relative masses of these substances. 246 I* 1 the early years of this century Berthollet formulated the statement "Every substance which tends to enter into chemical combination with others reacts by reason of its affinity and its mass*." Berthollet taught that a chemical change between substances in solution, wherein neither solids nor gases are formed, results in the production of a system in equilibrium ; that each member of the complete system inter- acts with each other in proportion to its affinity and its mass ; and that therefore the equilibrium of the system may be overthrown by changes in the relative masses of one or more of the members of the system. Berthollet further taught that changes in which solid or gaseous substances are formed are not suitable for the study of chemical affinity, because in these changes all the members of the chemical system are not free to interact, inasmuch as some * " Toute substance qui tend a entrer en combinaison, agit en raison de son affinite et de sa quantite." 244248] CHEMICAL AFFINITY. 177 of them are removed from the sphere of interaction almost as quickly as they are formed. The typical normal case of chemical change, according to Berthollet's view, is one wherein every member of the system is free to interact with all the other members throughout the whole of the change ; those changes wherein a final distribution of the interacting sub- stances is quickly established by the formation of solid precipi- tates or the evolution of gases are special limiting cases. Berthollet's law of mass has been developed in recent years 247 chiefly by the researches of Guldberg and Waage and of Ostwald. Guldberg and Waage formulate the law of mass thus chemical action is proportional to the active mass of each substance taking part in the change. By active mass is meant that quantity of a substance measured in equivalent weights which is present in unit volume of the chemical system. The expression equivalent weights will be explained more fully hereafter (s. Chap. xvn.). We know that the amounts of potash and soda which severally neutralise 3 6 '5 parts by weight of hydrochloric acid (HC1 36*5) are those expressed by the formulae KOH (56) and NaOH (40), respectively. We also know that to neutralise a reacting weight of sulphuric acid (H 2 SO 4 = 98) 112 parts by weight of potash (2KOH) or 80 of soda (2NaOH) are required. So far as neutralising by alkali is concerned, the quantities expressed by the formulae 2HC1 (or H 2 C1 2 ) and H 2 S0 4 are equivalent] so far as neutralising by acid is concerned the quantities KOH and NaOH (or 2KOH and 2NaOH) are equivalent. Suppose that a solution of 112 parts by weight of potash (2KOH), 73 parts by weight of hydrochloric acid (2HC1), and 98 parts by weight of sulphuric acid (H 2 SO 4 ), is diluted with water to a specified volume ; then the active masses of potash, hydrochloric acid, and sulphuric acid, respectively, in this solution are one equivalent of each, provided that by one equivalent is meant the quantity expressed by the formulae 2KOH (or K 2 O 2 H 2 ), 2NaOH (or Na 2 O 2 H 2 ), and H 2 SO 4 , re- spectively. The law of mass-action has been experimentally proved in many different reactions ; it probably holds good in all chemical changes. The principle of the coexistence of reactions states that 248 when several reactions occur simultaneously, each proceeds as if it alone took place. No direct experimental investigation of this principle has been made; but it has been largely M. E. C. 12 178 ELEMENTARY CHEMISTRY. [CHAP. XIII. applied in work on chemical affinity, and numerous results have been obtained in keeping with the principle. 249 Assuming the law of mass-action, and the principle of the coexistence of reactions, let us briefly examine a fairly simple chemical change. Let equivalent quantities of the alkali caustic potash, and the acids hydrochloric and sulphuric, be mixed in dilute aqueous solution ; let the substances be present in the ratio K 2 O 2 H 2 : H 2 C1 2 : H 2 SO 4 . The possible products of the interactions are potassium sulphate (K 2 S0 4 ), potassium-hydrogen sulphate (KHSO 4 ), potassium chloride (KC1), and water (H 2 O). But these substances may interact to reproduce the original substances. We have then certain direct changes and certain reverse changes possible. Chemical equilibrium will result when the velocities of the opposite reactions have become equal, that is, when the quantities of the substances formed in the direct change are equal to the quantities of the substances formed in the reverse change, in unit of time. But we say that each change, the direct and the reverse, is proportional to the affinities, and the masses, of the reacting substances. Now we can measure the mass of each substance present at the beginning of the change, and we can also measure the mass of each substance present when equilibrium is established ; hence we can deduce numerical values for the affinities of the reacting substances. Guldberg and Waage, Ostwald, van 't Hoff, and others, have deduced the necessary equations from the fundamental statements already made. But there is another method by which values for the rela- tive affinities of the substances taking part in a chemical change may be deduced from experimental data. The change may be allowed to proceed to a certain extent only, but not until the system has settled down into equilibrium ; the quantity of each substance present in the system may then be measured, and the velocity of the change may thus be determined. Then, assuming that the change which has occurred is proportional to the affinities and the masses of the interacting substances, we may deduce relative values for these affinities from our measurements of the masses. The necessary equations have been deduced by Guldberg and Waage, Ostwald, and others. 250 One of the great difficulties in applying these methods is to find reactions which are sufficiently simple. Yery many chemical changes which appear to be simple are complicated 248251] CHEMICAL AFFINITY. 179 by the occurrence of secondary reactions among the products of the primary change. Another difficulty is to measure the masses of the substances present when equilibrium is established. Suppose, for in- stance, that potash and sulphuric and nitric acids have been mixed in the ratio K 2 2 H 2 : H 2 N 2 O 6 : H 2 SO 4 , in dilute aqueous solution; how are we to determine how much potassium nitrate, and how much potassium sulphate, is actually present in the solution ? The ordinary methods of analysis are useless here, because they are based on the use of reagents other than the substances in the solution ; but the addition of any reagent is forbidden because the equilibrium of the system would thereby be destroyed. We cannot here go into the methods adopted ; many of them consist in measuring some definite physical change and using this as an index of the chemical change which has occurred. In the case of the two acids, nitric and sulphuric, reacting 251 with potash in equivalent quantities it has been shewn, with a very high degree of probability, that about 10 parts of potash combine with the nitric acid to form potassium nitrate, for each 7 parts which combine with the sulphuric acid to form potassium sulphate, when the system is in equilibrium. Hence it is concluded that the ratio of the affinities for potash of nitric and sulphuric acids is approximately 10 : 7. When the acids are nitric and hydrochloric, and the base is potash, the potash divides itself almost equally between the two acids ; that is one half of the potash reacts with the nitric acid to produce potassium nitrate, and one half with the hydrochloric acid to produce potassium chloride. Hence the affinities of hydrochloric and nitric acids for potash are approximately equal. When the acids are hydrochloric and acetic almost the whole of the potash reacts with the hydrochloric acid ; only about '4 parts of potash react to produce potassium acetate for each 100 parts which react to produce potassium chloride. Hence the ratio of the affinities for potash of hydrochloric and acetic acids is approximately 100 : -4. It is very important to observe what is the exact meaning we are now giving to such a statement as this * the relative affinity for potash of nitric acid is to that of hydrochloric acid as 1 : 1,' or as this 'the ratio of the affinities for potash of hydrochloric and acetic acids is 100 : *4.' When these state- ments are amplified they assert, (1) that if equivalent masses of caustic potash (KOH), hydrochloric acid (HC1), and nitric 122 180 ELEMENTAKY CHEMISTRY. [CHAP, xim acid (HN0 3 ), are allowed to interact freely in dilute aqueous solution, one half of the potash combines with each acid to produce potassium chloride and potassium nitrate, respectively ; (2) that if equivalent masses of caustic potash, hydrochloric acid, and acetic acid (C 2 H 4 O 2 ), are allowed to interact freely in dilute aqueous solution, then for every 100 parts of potash which are changed to potassium chloride only -4 parts of potash are changed to potassium acetate. If these statements are correct, it is evident that when equilibrium results the solution contains in the first case, potassium chloride and nitrate and also hydrochloric and nitric acids ; and in the second case, much potassium chloride, a little potassium acetate, a little hydrochloric acid, and much acetic acid. 252 The experiments of Ostwald and Thomsen have shewn that the relative affinities of acids are almost, if not quite, inde- pendent of the nature of the base ; in other words that if equivalent masses of, say, hydrochloric and nitric acids, are mixed in dilute aqueous solution with an equivalent mass of caustic potash (KOH), or soda (NaOH), or ammonia (NH 4 OH), or caustic lime (CaO 2 H 2 ), or caustic baryta (Ba0 2 H 2 ), &c. one half of the base combines with each acid. Ostwald's experiments have also rendered it very probable that the ratio of the affinities for bases of any two acids is independent of the temperature, at least within such a range as to 60. 253 There are many chemical changes brought about by acids other than those which take place between acids and bases. Some of these changes have been examined with the object of determining whether they are quantitatively conditioned by the same values as have been found to condition the reactions between acids and bases. Thus Ostwald made a number of measurements of the effects of different acids on the velocity of the change of acetamide into ammonium acetate. This change may be represented thus CH 3 CONH 2 Aq + H 2 - CH 3 COONH 4 Aq. (acetamide) (ammonium acetate) The change occurs more or less quickly in the presence of acids ; each acid increases the amount of change in unit time to a certain definite extent. The equations deduced from the fundamental statement, that chemical action is proportional to the relative affinities and the active masses of the interacting 251255] CHEMICAL AFFINITY. 181 substances, shew that the ratio of the affinities in such a change as that under consideration is equal to that of the square roots of the velocities of the reactions. Ostwald measured the velocities of the reaction for various acids, and hence deduced the relative affinities of these acids. The numbers obtained agree well with those found by the examina- tion of the reactions between the same acids and potash, soda, and other bases. Among other chemical changes examined were, the change of methylic acetate in presence of water and an acid into methylic alcohol and acetic acid (CH 3 COOCH 3 Aq + H 2 O - CH 3 OHAq + CH 3 COOHAq) ; and the change of cane sugar in presence of water and an acid into glucose (C 12 H M O n Aq + H 2 O = 2C 6 H 12 O 6 Aq). In each oase the velocity of the change was determined for various acids ; the ratios of the square roots of the velocities were taken (as indicated by theory) as the ratios of the relative affinities of the acids. The numbers agree as well as could be expected among themselves, and also with the numbers found by the study of the reactions between acids and bases. The electrical conductivities of solutions of acids are pro- 254 portional to the velocities of the chemical changes produced by these acids. Hence measurements of the electrical conduc- tivities of acids in aqueous solutions of various concentrations give data from which the relative affinities of these acids may be deduced. Many measurements have been made of the electrical conductivities of acids in aqueous solutions, chiefly by Ostwald ; the values of the relative affinities deduced from these results agree very well with those found by more strictly chemical methods. The outcome of the work which has been done in recent 255 years on the subject of the affinities of acids is to establish the conclusion that it is possible to determine for each acid a specific affinity -constant which quantitatively conditions all the reactions brought about by this acid. Of course when it is said that this or that reaction is brought about by an acid, the reaction is regarded as being more simple than it really is. The reaction is brought about by all the substances which form the chemically changing system. But it seems that we may regard the complete change as made up of various parts each of which occurs in accordance with its own laws. The more completely a specified change is dependent on the character of the acids which take 182 ELEMENTARY CHEMISTRY. [CHAP. XIII. part in that change the more suitable is it for deducing values for the relative affinities of these acids. The interactions of acids and bases in dilute aqueous solu- tions are conditioned only by the characters of the acids and the bases. The results of measurements of these changes render it very probable that each acid has a specific affinity- constant which is independent of the nature of the base inter- acting with the acid, and that each base has a specific affinity- constant which is independent of the nature of the acid inter- acting with the base. The results of measurements of many other reactions which occur only in the presence of acids, and which may justly be said to be caused by these acids, render it very probable that the amount of change occurring in a specified time, or the amount of change which has occurred when the system has settled down into equilibrium, is conditioned by the values of the same specific affinity-constants which condition the inter- actions between these acids and bases in dilute aqueous solutions. 256 If these conclusions are granted and they rest on a large body of carefully verified facts it follows that measurements of the specific affinity-constants of the acids are of the utmost importance. Of the various methods hitherto employed for making these measurements the most promising seems to be that based on the proportionality between the electrical con- ductivities and the velocities of the chemical reactions brought about by acids. This method presents no great experimental difficulties, and it is free, or nearly free, from the disturbing influence of secondary reactions. Most, if not all, purely chemical methods are open to the objection that the primary change to be measured is complicated and modified by the occurrence of other changes, and that the influence of these secondary changes can scarcely be eliminated by any experi- mental arrangements. Many measurements of the electrical conductivities of aqueous solutions of acids have been made, and data have thus been accumulated for comparing the relative affinities of many acids. We shall shortly consider some of these data when we have learned more about the composition of acids (s. Chap, xvn.) Acids with large affinity- values are called strong acids; those with small affinity- values are called weak acids. 257 Answers can then be given to the questions propounded at 255259] CHEMICAL AFFINITY. 183 the close of Chap. xn. ; we have learnt that a number can be found for each acid, and each base, which expresses the amount of chemical change which this acid, or base, is capable of producing under denned conditions. It is probable that, as investigation proceeds, specific affinity- constants will be determined for the members of other classes of compounds besides acids and bases. We have now learnt something about chemical composition 258 and chemical classification. We have also found that many, and probably all, chemical reactions brought about by the compounds classed together as acids are quantitatively condi- tioned by the affinity-constants of these acids. We ought now to inquire into the connexions between the compositions of acids and the values of their affinity-constants. But we are not yet ready for this inquiry ; we must learn more regarding chemical composition, (s. Chap, xvn.) No attempt has been made in this chapter to analyse the 259 meaning of the term affinity ; we have not asked why this body chemically interacts with that ; we have not inquired as to the nature of chemical affinity. We have been content to call affinity that property of elements and compounds by virtue of which they interact to produce new combinations. We have found it possible to assign quantitative values to this property in the cases of acids and bases. Before proceeding to consider in some detail the generally accepted theory regarding the mechanism of chemical change, we shall briefly glance at the relations between chemical changes and the changes of energy which invariably accompany them. CHAPTER XIV. RELATIONS BETWEEN CHEMICAL CHANGES AND CHANGES OF ENERGY*. ogQ EVERY chemical change consists of two parts, a change in the form of combination of the matter of the system, and a change in the total quantity, or in the form, or in both the quantity and form, of the energy of the system. Energy is the power of doing work. Work is the "act of producing a change of configuration in a system in opposition to a force which resists that change." . If one system does work on another system, one loses and the other gains energy ; and the energy lost by one is equal to the energy gained by the other. If both systems are included in a larger, the total energy of this system is unchanged. If one part of a system does work on another part, the total energy of the system is unchanged, although one part has gained and another part has lost energy. The principle of the conservation of energy affirms that; "The total energy of any material system is a quantity which can neither be increased nor diminished by any action between the parts of the system, though it may be transformed into any of the forms of which energy is susceptible." (Clerk Maxwell.) 261 ^^ e ener gi es ^ actually existing material systems depend upon the states of these systems at any moment. The state of a system is conditioned by many variables; among the more important are chemical composition, pressure, tempera- ture, and volume. If we wish to connect changes of energy with changes of chemical composition we must start with chemical systems of * The subject of energy is treated very shortly. The student should refer to a book on Physical principles, e.g. to Clerk Maxwell's Matter and Motion. 260264] CHEMICAL CHANGES & CHANGES OF ENERGY. 185 definite and defined composition, in definite and defined states, and we must cause these to change to other definite and defined states; we must then determine the compositions of the resulting systems, and we must measure the changes of energy which have accompanied these changes of composition and of state. Of two equal quantities of energy one may be more 262 available for doing work than the other. Thus, in order to cause thermal energy to do work it is necessary to allow it to pass from a body at a higher to a body at a lower temperature. A certain body may be at a very low temperature and yet contain thermal energy ; but it may be impossible to cause this energy to do work, because of the impossibility of framing an engine consisting of the cold body and another system at a lower temperature than the cold body. A quantity of heat as it exists in a hot body is more available for doing work than the same quantity of heat as it exists in a colder body. When energy passes from a more available, or higher, to a 263 less available, or lower, form it is said to be degraded. All forms of energy can be directly or indirectly transformed into heat. A given quantity of heat-energy cannot be wholly transformed into one of the higher forms of energy. Every transformation of energy involves the degradation of a portion of the energy. But every chemical change is accompanied by a transformation of energy from the form of chemical energy to other forms of which thermal energy is usually one ; every chemical change therefore is accompanied by a degradation of energy. It is not asserted that the whole of the energy which changes form during a chemical change is necessarily degraded. The chemical system represented by the symbols 2H + O 264 contains more energy than the system represented by the symbol H 2 O. In the passage from one of these systems to the other energy is lost by the changing system ; the energy so lost by the system is gained by neighbouring systems, by the vessel in which the change is accomplished, the surrounding air, &c. But although there is no destruction, there is de- gradation, of energy. If 2H represents 2 grams of hydrogen, O represents 16 grams of oxygen, and H 2 O represents 18 grams of liquid water, all measured at normal pressure and at about 15 16, then the change 2H + O H 2 O is accompanied by the production of 68,360 gram-units of heat. If we assume that the whole of the energy which changes form during the chemical change 2H + O = H 2 appears as heat, then 68,360 186 ELEMENTARY CHEMISTRY. [CHAP. xiv. gram-units of heat represents the difference between the energies of the two systems 2H + O and H 2 O. Whether this quantity of heat does or does not measure the total difference of energy between the two systems, it is certain that the change from the one system to the other is always accompanied by the production of the same quantity of heat. And what is true of this chemical change is true of others also. Each definite chemical change from one system of defined composi- tion, under defined conditions, to another system of denned composition, under defined conditions, is accompanied by the production or disappearance of a fixed quantity of heat. The following examples illustrate this point. In each case the original and final systems are under the normal pressure (760 mm.) and the temperature of each is about 16. The symbols represent the combining, or reacting, weights taken in grams. Original New System Gram-units of heat System. formed. which are produced or disappear. [The sign + signifies produced, the sign - , disappears.] Cl + H HC1 22,000 + Br + H HBr 8,440 + I + H HI 6,040 - S + 2H H 2 S 4,740 + 2C + 4H Crr 24 2,710- C + 2O CO 2 96,960 + K + Cl + 3O KC10 3 95,860 + KC1 + 3D KC10 3 9,750- KC1O 3 + Aq KClO 3 Aq 10,040- K + Cl + 3O + Aq KC10 3 Aq 85,820 + H 2 O + 2C1 + Aq HClOAq 2HClAq + O HClAq + O 10,270 + 9,380 + S + 3O SO 103,240 + S + 3O + H 2 O H 2 S0 4 124,560 + SO + H O HSO 21,320 + H 2 S0 4 Aq + H 2 S H> a 4 3 Aq + I I 2 O 9,320- 265 ^- n some f these changes heat disappears from the system, that is to say, energy is raised from the form of heat-energy to some other more available, or higher, form. Yet it has been asserted (par. 263) that every chemical change is accompanied 264265] CHEMICAL CHANGES & CHANGES OF ENERGY. 187 by a degradation of energy. Take, for example, the change H + 1 = HI ; in this change 6040 gram-units of heat disappear. But in order to bring about the formation of HI from H + 1 it is necessary to heat the system H + 1 to 300 400 ; that is to say it is necessary to add energy from without the system. This added energy is employed in bringing the system H -f I into a condition such that chemical action becomes possible; chemical action results and this action is attended with a degradation of energy. Suppose a stone to rest at the bottom of an inclined plane AB (Fig. 19). Let the stone be moved from A to B ; to perform Fig. 19. this work a certain amount of energy must be used. The stone now possesses more energy, by virtue of its position, than it did when it was at A. Let the stone be moved a very little way over the crest of the plane AB, it will now roll down the inclined plane BC until it comes to rest at C. In the passage from B to C energy has been degraded; but the stone at C possesses more energy than it did at A because the level of C is higher than that of A. If the stone is rolled up the short incline CD, by the expenditure of energy, it will be in a position to descend from D to E -, in this descent energy will be degraded. When the stone reaches E it will possess the same energy as when it was at rest at A. The system H + I corresponds to the stone at A ; to bring the system into such a condition that chemical change can occur, energy must be expended. The system ready to undergo chemical change corresponds to the stone at B. Chemical change occurs ; the system passes from B to (7, from H + 1 to 188 ELEMENTAKY CHEMISTRY. [CHAP. xiv. HI, and this passage is attended with degradation of energy. But the system HI possesses more energy than the system H + 1 did before energy was expended upon it in causing it to pass into that condition in which chemical change could occur ; therefore although the chemical change from H + I to HI has been attended with degradation of energy, yet the whole change from 1 gram of gaseous hydrogen and 127 grams of solid iodine, at say 15 or 16, to 128 grams of gaseous hydrogen iodide at the same temperature, has been attended by a raising of energy from a lower to a higher form. The system HI is readily separated into the elements H + 1. To effect this separation the temperature of the gas is raised; this corre- sponds to the rolling of the stone up the small incline CD. The separation of HI into H + 1 is attended with the de- gradation of a considerable quantity of energy, just as the descent of the stone from D to E is attended with the degradation of energy. 266 In this example the energy required to bring the original system into that state in which chemical action could occur was furnished in the form of thermal energy from without the system. But the necessary energy for bringing a chemical system into that state in which chemical change can take place, and energy can be degraded, is frequently supplied by chemical actions occurring within a larger system of which the special system under consideration forms a part. Thus the change from 71 grams of gaseous chlorine and 16 grams of gaseous oxygen to 87 grams of gaseous chlorine monoxide (201 + O = C1 2 O) is accompanied with the disappearance of 17,900 gram- units of heat. Chlorine monoxide is produced by passing chlorine over mercuric oxide ; the equation representing the chemical change is 2HgO + 401 = C1 2 O + Hg 2 OCl 2 . This change consists of various parts ; ( 1 ) formation of Hg01 2 and O, thus HgO + 2C1 = HgCl 2 + O, this is accompanied by the production of 16,250 gram-units of heat; (2) production of the oxychloride Hg 2 OCl , thus Hg01 2 +HgO = Hg 2 OCl 2 , this is accompanied by the production of 8,900 gram-units of heat ; (3) formation of C1 2 O, thus 2C1 + = C1 2 O, this is accompanied by the dis- appearance of 17,900 gram-units of heat. Now 16,250 + 8,900 = 25,150; and 25,150-17,900 = 7,250. The complete change from 2 HgO + 401 to Hg 2 O01 2 + C1 2 O is accompanied by the production of 7,250 gram-units of heat. To bring the system 201 + O into that state in which 265-266] CHEMICAL CHANGES & CHANGES OF ENERGY. 189 chemical union can take place, work must be done upon the system, energy must be expended; this energy is supplied by the other chemical changes HgO + 201 = HgCl 2 + O and HgO + HgCl 2 = Hg 2 OCl 2 , in each of which energy is degraded. Every chemical change is attended with the degradation o energy ; but to bring a given system into that state in which chemical change can take place it may be necessary to expend energy upon the system ; this energy is sometimes supplied by processes altogether outside the system, sometimes by making the required chemical change one part of a cycle of changes in some of which energy is set free under such conditions as to be directly available for bringing the special part of the whole system into that state in which the wished for chemical change can occur. We have had examples of oxidation accomplished by arranging a series of chemical changes so that oxygen shall be produced in contact with the substance to be oxidised. Thus lead monoxide (PbO) was oxidised to lead dioxide (PbO 2 ) by suspending the monoxide in concentrated warm potash solution and passing chlorine into the liquid : potassium hypochlorite (KC1O) was thus produced, but was at once decomposed to potassium chloride (KC1), and oxygen which combined with the lead monoxide to produce lead dioxide. If lead monoxide is suspended in water or potash solution and oxygen is passed into the liquid no lead dioxide is formed. Now to bring the system PbO + O into that state in which the system PbO 2 can be produced, energy must be expended : but the interaction of potash solution with chlorine, to produce potassium chloride, water, and oxygen, is attended with the setting free of a large quantity of energy ; a portion of this energy is employed in bringing the part of the whole changing system represented by the symbols PbO + O into that condition in which chemical action is possible (s. Chap. xi. par. 158.) When zinc and dilute sulphuric acid interact a solution of zinc sulphate, and hydrogen gas, are produced ; if a solution of sodium sulphite (Na 2 SO 3 ) is added to this changing system, hydrogen sulphide (H 2 S) is evolved. But if hydrogen gas is passed into a solution of sodium sulphite, hydrogen sulphide is not produced (s. Chap. xi. par. 175.) To effect the change Na 2 SO 3 Aq + 6H = Na 2 OAq + 2H 2 O + H 2 S energy must be expended : when Na 2 SO 3 Aq is added to dilute sulphuric acid in contact with zinc, both the material and the energy needed for effecting the chemical change are provided; and 190 ELEMENTARY CHEMISTRY. [CHAP. XIV. moreover the energy is provided exactly in the form in which it is required for accomplishing the chemical work to be done. 267 When energy is degraded in a chemical change, a portion, or perhaps in some cases the whole, of the energy is degraded to the form of heat. But the quantity of heat produced seldom if ever affords a direct measure of the chemical energy degraded. Chemical changes are always accompanied by more or less marked physical changes; the production of the energy which appears as heat is generally due partly to the chemical, and partly to the physical, portion of the complete change. Thus when gaseous hydrogen and oxygen combine to form liquid water a large quantity of energy is degraded, and much heat is produced [2H + O = H 2 O = 68,360 gram-units of heat+]; some of this heat is due to the chemical change from the system 2H + O to the system H 2 O, some of it is due to the physical change from gaseous water to liquid water. Some chemical changes, with their accompanying physical changes, occur with the disappearance of heat. Thus selenion dioxide dissolves in water to form a solution of selenious acid, SeO 2 + H 2 O + Aq = H 2 SeO 3 Aq ; this change is accompanied by the disappearance of 920 gram-units of heat. Similarly iodine pentoxide dissolves in water to form a solution of iodic acid, I 2 O 5 + H 2 O + Aq = 2HIO 3 Aq ; this change is accompanied by the disappearance of 1790 gram-units of heat. [The formulae represent reacting weights taken in grams ; SeO 2 e.g. means here 111 grams of selenion dioxide.] In such cases we may suppose that the whole of the energy degraded in the chemical part of the complete change, and some of the energy degraded in portions of the physical change, are degraded to heat, but that this quantity of heat is wholly used in effecting other portions of the physical change. Or we may suppose that only a portion of the chemical energy is degraded to heat, and that this heat is used, along with other forms of energy produced by the degradation of the chemical energy, in effecting the physical portion of the complete change. Either supposition is in keeping with the fact that the total change occurs with the disappearance of heat, and with the genera- lisation that every chemical change is accompanied by degra- dation of energy. 268 It is important to note once -again that the statement that every chemical change is accompanied by degradation of energy does not assert or imply that the whole of the energy 266269] CHEMICAL CHANGES & CHANGES OF ENERGY. 191 which changes form during a chemical change is degraded : some of it is degraded, but some of it may be raised to a more available form. In chapters xn. and xiu. we saw that many chemical 269 changes may be j ustly regarded as proceeding in two directions simultaneously, and that equilibrium results when the ve- locities of the direct and reverse changes become equal. We saw also that such chemical equilibrium may generally be overthrown by changing the temperature, or sometimes the pressure, of the system, or the relative masses of the interacting substances. The considerations concerning the relations of energy-changes and chemical changes shortly developed in the present chapter may be applied to the conception of chemical equilibrium gained in chaps, xn. and xm. Suppose that the masses of ferric chloride and potassium sulphocyanide shewn by the formulae Fe 2 Cl 6 and K 8 C 6 N 6 S e (= 6KCNS) are mixed in dilute aqueous solution; the system is in a condition in which chemical change can occur; chemical change occurs, and a system is produced the composition of which may be re- presented by the equation Fe 2 Cl 6 Aq+6KCNSAq + xFe 2 Cl 6 Aq + x' KCNSAq - Fe 2 (ONS) 6 Aq + 6KC1 Aq + x Fe 2 Cl 6 Aq + x' KCNSAq (comp. Chap. xn. par. 238). Some energy is degraded in this change ; a portion of this energy appears as heat, a portion is probably employed in effecting some of the physical changes (contraction or expansion of volume &c.) which accompany the chemical change; a portion of the energy degraded in one part of the change is also probably employed in bringing the products of the change into a state in which they can interact to reproduce the original substances. After a very short time the system settles down into a state in which there is equilibrium of energy and of chemical distribution of the interacting substances. A little more potassium sulphocyanide is now added; chemical change again occurs ; energy is degraded ; and after a short time equilibrium is established. Potassium sulphocyanide is added little by little until the whole of the ferric chloride originally present has been changed; the addition of more sulphocyanide cannot now cause chemical change; no more energy can be degraded by chemical processes ; the system has reached its state of final equilibrium. Each addition of potassium sulphocyanide disturbed the equilibrium of the energies of the system, and this disturbance was attended by chemical change ; but a disturbance of the equilibrium 192 ELEMENTARY CHEMISTRY. [CHAP. XIV. of the energies of the system would also be produced by raising the temperature of the system ; hence raising the temperature of the system would also alter the distribution of the elements forming the members of the system, i.e. would cause chemical change to occur. Now consider a chemical change, one of the products of which is a solid under the conditions of the experiment. Suppose aqueous solutions of barium chloride and sodium sulphate to be mixed in the ratio BaCl 2 : Na 2 SO 4 . Chemical change occurs ; energy is degraded ; there is change from liquid to solid, and more energy is degraded. As one of the products of the chemical change (barium sulphate) is removed from the sphere of action, by precipitation in the solid form, none of the energy degraded in the direct change can be used to bring the products of this change into a state in which they can chemically react to reproduce the original substances; hence the whole, or at any rate nearly the whole, of the energy degraded to the form of heat passes out of the system. The system is, so to speak, rapidly rolling down hill. Chemical change proceeds until the whole of the energy which can be degraded to heat has been degraded. The system is now in its final state of equilibrium. And this final state has been reached without adding an excess of either of the interacting substances. 270 We have now gained some fairly clear conceptions re- garding chemical change. Elements and compounds interact to produce other elements and compounds. Numbers are given to the elements ex- pressing the masses of them which combine or interact with unit mass of one element chosen as a standard, and which also interact or combine with each other. These numbers we have called the combining weights of the elements. Numbers are also given to compounds which express the smallest masses of them which chemically interact with each other. These numbers we have called the reacting weights of the compounds. But it is necessary in chemistry to have regard not only to composition but also to properties. Ele- ments are classified in accordance with their properties into metallic or positive, and non-metallic or negative, elements. They are also classified in groups in accordance with the properties and compositions of their oxides, hydrides, haloid and oxyhaloid compounds, &c. This classification of elements 269271] CHEMICAL CHANGES & CHANGES OF ENERGY. 193 carries with it a classification of compounds also. Compounds are classified in accordance with their properties into acids, bases, salts, &c. But with the properties connoted by each of these terms there is associated a certain composition. The term acid, for instance, implies certain common properties, and a certain common composition. But chemistry is not content with finding an answer to the question What is produced in this process and how much of it is produced! it seeks to find an answer to this question also How is it produced? Chemistry therefore examines the conditions and general laws of the interactions of elements and compounds. One substance interacts with another to produce new substances; but the new substances also interact, unless they are prevented by the removal of one or more of them from the sphere of action, and tend to re- produce the original substances. Chemical change results in chemical equilibrium. Each substance taking part in a chemical change, wherein all the substances are free to act and react, probably produces a certain definite and measurable effect on the change, which effect is independent of the interactions of the other substances. In certain classes of changes at any rate it is possible to assign to each of the two primarily inter- acting substances a definite number, a knowledge of which enables us to predict the amount of change which will occur under defined conditions. Chemical change is accompanied by change of energy; there is a redistribution of the matter which undergoes change and also of the energies of the parts of the changing system. Although we have thus gained some fairly clear conception 271 of the general character of chemical change, and of the kind of phenomena studied in chemistry, yet we stand greatly in want of a general theory which shall bring the facts together and bind them into a connected whole. There is a theory which to a great extent does this. This theory we must now endeavour to understand. M. E. C. 13 CHAPTER XV. THE MOLECULAR AND ATOMIC THEORY. 272 THE Greek philosophers Leucippus and Democritus (about 440 400 B.C.) were among the first to give definite shape to the conception that "the bodies which we see and handle, which we can set in motion or leave at rest, which we can break in pieces and destroy, are composed of smaller bodies, which we cannot see or handle, which are always in motion, and which can neither be stopped, nor broken in pieces, nor in any way destroyed or deprived of the least of their properties " (Clerk Maxwell). This doctrine was developed by Epicurus (340 270 B.C.). In the poem De Rerum Natura, Lucretius gives what purports to be an account of the teaching of Epicurus on the subject. The conception of atoms is fully elucidated in this poem, and on it is based a theory of the physical universe, and to some extent also a theory of things moral and spiritual. Lucretius says that nothing exists except atoms and empty space, that the atoms are of different forms and different weights, and that the number of atoms of each form is infinite ; that the atoms are in constant motion, and that all change consists in the separation and combination of atoms. According to Lucretius, every atom is indestructible, and its motion is indestructible likewise. Atoms unite to form different kinds of substances; the properties of the substances so formed depend on the mutual relations of the atoms " it matters much with what others and in what positions the atoms of things are held in union, and what motions they mutually impart and receive."* Nearly complete as it was in many respects, the Lucretian theory failed as a scientific conception ; it did not work. It * Lucretius, De Eerum Natura, n. 1007 9 (Munro's translation). 272275] THE MOLECULAR AND ATOMIC THEORY. 195 did not admit of accurate applications to the facts of nature. It was not a science-producing theory, but rather a speculation about the possible causes of natural events. The teachings of the atomists were opposed by the follow- 273 ers of Aristotle, for whom the names of things were as real or more real than the things themselves. As Aristotelianism prevailed during the middle ages, atomism declined. The atomic theory of the Greek philosophers was revived towards the end of the 16th century by Gassendi. Boyle and Newton were upholders of this theory. Newton's demonstration of the action of fhe force of gravitation made a science of atomic physics possible; but the great difficulty was, and still is, to form a clear image of the action of gravitation in terms of the atomic conception of the structure of matter. Not much was done to advance the applications of the 274 atomic theory, after Newton, until in the early years of the present century Dalton made a serious attempt to determine the conditions under which the atoms of elementary bodies unite to form the atoms of compound bodies. Dalton said that it is possible to find the relative weights of the atoms of elements and compounds, and he indicated the method by which this could be done. Dalton found that the mass of hydrogen which combined 275 with carbon to form a certain compound of these elements was twice as great as the mass of hydrogen which combined with the same quantity of carbon to form another compound of these elements. He found also that a specified mass of carbon combined with a certain mass of oxygen to form one oxide of carbon, and with twice that mass of oxygen to form another oxide of carbon. He noticed similar regularities in the masses of oxygen which combined with a fixed mass of nitrogen. Meanwhile Dalton, led thereto by his physical experiments on the absorption of different gases by water, had been thinking a great deal about the ultimate structure of matter. He pictured to himself a quantity of carbonic acid gas as built up of innumerable minute particles, or atoms, each of which was itself composed of one atom of carbon and two atoms of oxygen ; a quantity of nitrous oxide gas as built up of a vast number of atoms, each of which was itself composed of yet smaller portions of matter, viz. of one atom of nitrogen and one atom of oxygen ; and a quantity of hydrogen gas as built up of minute particles, which were single atoms each composed only of hydrogen. Fig. 20, copied from the original in the 132 196 ELEMENTARY CHEMISTRY. [CHAP. xv. 276 277 New System of Chemical Philosophy, gives Dalton's pictorial presentment of his conception of the atoms of these three gases. Dalton's application of the term atom to compounds, such as water, carbonic acid, &c., shews that he did not use the word atom in its strict ety- mological meaning as 'that which cannot be cut ' but rather as signifying the small- est portion of a body which could exhibit the properties of the body. The atom of water, for instance, could be separated into atoms of hydro- gen and oxygen, but this act of separation into parts pro- duced kinds of matter wholly different from the water. The properties of the parts of the atom of water were quite un- like those of the atom itself; whereas the properties of a quantity of water were re- garded by Dalton as the same as those of the atom of water. The reason of the regular- ities in the compositions of the two oxides of carbon, or the two compounds of carbon and hydrogen, examined by Dalton, was to be found, ac- cording to him, in the nature of the atoms of carbon, hydro- Fig- 20 - gen, and oxygen. It is only necessary to assume that the atoms of these elements do not separate into parts in chemical reactions, then the facts find a simple explanation. One atom of hydrogen combines with one atom of carbon to form one atom of a certain hydride of carbon ; if another compound of these elements is formed containing more hydrogen, relatively to the same 275279] THE MOLECULAR AND ATOMIC THEORY. 197 mass of carbon, the next smallest quantity of hydrogen which can combine with the atom of carbon is two atoms. Similarly with the oxides of carbon. One atom of carbon combines with one atom of oxygen to form an atom of an oxide of carbon : this is the simplest possible compound of the two elements. The next compound which can be formed fay combining more oxygen with the same mass of carbon, must be that the atom of which is composed of one atom of carbon united with two atoms of oxygen. Chemical action was thus conceived by Dal ton to be an 278 action between atoms. A mass of any element, or compound, was regarded as constituted of a vast number of very small particles of matter, all alike, all having the same weight, but .all unlike the atoms of any other element, or compound. The atoms of elements were supposed to be capable of combining together to form atoms of compounds. In some cases the atoms of compounds might combine together to form atoms of more complex compounds. An atom of one element might combine with one, two, three, &c. atoms of another element to form one, two, three, or more, distinct compounds; but the atoms of elements could not separate into parts. The atoms of compounds on the other hand separated into parts when compounds interacted to produce new kinds of matter, and these parts, which were elementary atoms, rearranged them- selves to form atoms of the new compounds produced in the interactions, Dalton pictured to himself an atom of a com- pound as a structure, or building, formed of a definite number of elementary atoms arranged in a more or less definite manner. He used symbols to represent elementary atoms, and he grouped these symbols together to represent compound atoms. Thus the symbol Q represented an atom of oxygen] 0, an atom of hydrogen; (J), an atom of nitrogen; @, an atom of sulphur; , an atom of aluminium; ($), an atom of potassium; and so on. In Fig. 21 are given a few of Dalton's symbols for the atoms of compounds. A represents an atom of potash alum; B an atom of aluminium nitrate; C an atom of barium chloride ; and D an atom of barium nitrate. Thus did Dalton's conception of the atom throw light on the laws of fixity of composition, multiple proportions, and reciprocal proportions. " It is one great object of this work " says Dalton in his 279 New System of Chemical Philosophy, " to shew the importance and advantage of ascertaining the relative weights of the 198 ELEMENTARY CHEMISTRY. [CHAP. xv. ultimate particles both of simple and compound bodies, the number of simple elementary particles which constitute one Fig. 21. compound particle, and the number of less compound particles which enter into the formation of one more compound particle." How then did he determine the relative weights of the ultimate particles of simple bodies ? Let us take the case of oxygen. The atom of oxygen, said Dalton, is 8 times heavier than the atom of hydrogen : let us call the weight of the atom of hydrogen, or the atomic weight of hydrogen, one ; then the atomic weight of oxygen is asserted to be 8. Masses of hydrogen and oxygen combine in the ratio 1 : 8, to form water; but an atom of water is formed by the union of atoms of hydrogen and oxygen ; if it is assumed that an atom of water is formed by the union of one atom of hydrogen with one atom of oxygen, then the atomic weight of oxygen must be 8. In saying that the atomic weight of oxygen was 8, Dalton implicitly made this assump- tion. But it might be assumed* that an atom of water is composed of one atom of oxygen united with two atoms of hydrogen; as an atom of hydrogen is the unit in terms of which the weights of the atoms of other elements are stated, it follows from this assumption that the atomic weight of oxygen is 16; because 2 : 16 = 1 : 8. . Or it might be assumed that an 279280] THE MOLECULAR AND ATOMIC THEORY. 199 atom of water is composed of three atoms of hydrogen united with one atom of oxygen ; in this case the atomic weight of oxygen must be 24. Or it might be assumed that an atom of water is composed of two atoms of oxygen united with one atom of hydrogen; in this case the atomic weight of oxygen is 4. Before the atomic weight of oxygen could be determined 280 from the data of the composition of water, it was necessary to determine the number of atoms of oxygen and hydrogen which united to form an atom of water. Dalton's conception of the atom supplied no method whereby this could be done. To get over this difficulty if possible, Dalton framed several empirical rules regarding the compositions of the atoms of binary com- pounds. He classified compound atoms formed by the union of two elements into binary, ternary, quaternary, &c., atoms. Calling the two elements A and B, he said that a binary atom is formed by the union of one atom of A with one atom of B ; a ternary atom, by the union of, either one atom of A with two of B, or two atoms of 'A with one of B ; a quaternary atom, by the union of, either one atom of A with three of B, or three of A with one of B. He then laid down the following rules : * "I. When only one combination of two bodies [elements] can be obtained, it must be presumed to be a binary one, unless some cause appears to the contrary. II. When two combinations are observed, they must be presumed to be a binary arid a ternary. III. When three combinations are obtained, we may expect one to be a binary, and the other two ternary. IV. When four combinations are observed, we should expect one binary, two ternary, and one quaternary, &c. &c." By applying these rules to water, which was the only compound of hydrogen and oxygen known at the time, Dalton concluded that the atom of water was a binary atom ; but his own analyses had convinced him that hydrogen and oxygen combine nearly in the ratio 1 : 7 to produce water [we now know that the ratio is 1 : 8] ; therefore he concluded that the atomic weight of oxygen was approximately 7. Dalton's rules for determining the compositions of compound atoms were not based on any general principle deduced from his fundamental conception of the atom; this conception could not indeed supply such a principle. If only one com- 200 ELEMENTARY CHEMISTRY. [CHAP. XV. pound of two specified elements was known, the simplest assumption to make was certainly that embodied in Dalton's first rule. But the simplest assumption is not always the best. 281 The law of combination by volumes of gaseous elements enunciated by Gay-Lussac in 1809 (s. Chap. vi. par. 87) may be stated as follows. The gaseous elements combine in the ratios of their combining volumes, or in ratios which bear a simple relation to these. By combining volume is here meant the smallest volume of a gaseous element which combines with unit volume of hydro- gen ; and unit volume of hydrogen is defined to be the volume occupied by unit mass of hydrogen. All measurements of volumes are assumed to be made at the same temperature and pressure. Gay-Lussac interpreted his law, in the light of the Daltonian theory, to mean that the ratios of the masses of the combining volumes of gaseous elements are also the ratios of the masses of the atoms of these elements. Thus; 2 volumes of hydrogen combine with 1 volume of oxygen to form water ; but a volume of oxygen weighs 16 times as much as an equal volume of hydrogen ; therefore an atom of oxygen is 1 6 times heavier than an atom of hydrogen ; but the atomic weight of hydrogen is 1, therefore the atomic weight of oxygen is 16. Again, 1 volume of chlorine combines with 1 volume of hydrogen to form hydrogen chloride; but chlorine is 35*5 times heavier than hydrogen, bulk for bulk; therefore, the atomic weight of chlorine is 35'5. 282 If tlri s interpretation of Gay-Lussac's law is admitted, the law supplies a means for determining the atomic weights of gaseous elements. But Gay-Lussac ventured on the further generalisation that equal volumes of gases (measured at the same temperature and pressure) contain equal numbers of atoms. Dalton shewed that this generalisation was inadmissible. Thus, consider the combination of hydrogen and chlorine. One volume of hydrogen combines with one volume of chlorine to form 2 vols. of hydrogen chloride ; therefore, by Gay- Lussac's generalisation, x atoms of hydrogen combine with x atoms of chlorine, to form 2x atoms of hydrogen chloride. To make the statement more definite, let us assume that x = 1 ; then, a single atom of hydrogen by combining with a single atom of chlorine has produced 2 atoms of hydrogen chloride. 280283] THE MOLECULAR AND ATOMIC THEORY. 201 Hence, each atom of hydrogen chloride is composed of half an atom of hydrogen united with half an atom of chlorine. But, by definition, an atom of an element is not separated into parts when it interacts chemically with atoms of other elements or compounds. Hence either Gay-Lussac's general- isation is wrong, or the Daltonian definition of the elementary atom must be modified. In 1811 the Italian naturalist Avogadro modified the 283 Daltonian atomic theory by introducing the conception of two orders of small particles, the molecule and the atom. The molecule of an element or compound, said Avogadro, is the smallest mass of it which exhibits the characteristic pro- perties of that element or compound. The molecule, he said, is formed of smaller parts ; these are atoms. The atoms which form the molecule of an element are all of one kind ; the atoms which form the molecule of a compound are of two, or more, different kinds. Avogadro's conception of the struc- ture of matter applied to the case of water asserted that, if the separation of a quantity of water could be carried far enough, we should at last come to very minute particles each of which would exhibit the properties of water; but if these particles were separated into parts we should no longer have particles of water, but particles some of which would exhibit the properties of hydrogen and some the properties of oxygen. Similarly, if the separation into parts of a quantity of hydrogen could be carried far enough, the hypothesis asserted that we should at last come to very minute particles each of which would exhibit the characteristic properties of hydrogen ; but if these particles were separated into parts we should no longer have particles of what we know as hydrogen, but particles more or less unlike hydrogen, yet each the same as all the others. In other words, the Avogadrean conception of the struc- ture of elements and compounds asserts, (1) that a quantity of a compound, or of an element, consists of a vast multitude of minute particles each of which possesses the characteristic properties of the compound, or of the element ; these particles are called molecules ; (2) that each of these molecules itself consists of a fixed number of yet smaller particles; these smaller particles are called atoms; (3) that the properties of the atoms which form the molecule of a compound are very different from the properties of the molecule itself ; (4) that the pro- perties of the atoms which form the molecule of an element are also different from the properties of the molecule of that Of TH 202 ELEMENTARY CHEMISTRY. [CHAP. xv. element, but that inasmuch as the atoms which form the mole- cule of an element are all of one kind, and are only more minute portions of the same kind of matter as the molecule itself, there is not so marked a difference between the proper- ties of these atoms and the properties of the molecule formed by their union, as there is between the properties of the atoms of the different elements which form a compound and the pro- perties of the molecule of that compound. 284 Avogadro modified the generalisation of Gay-Lussac, and gave it the following form : Equal volumes of gaseous elements and compounds, measured at the same temperature and pressure, contain equal numbers of molecules. 285 Let us apply this generalisation to the combination (1) of hydrogen and chlorine to form hydrogen chloride, (2) of hydro- gen and bromine to form hydrogen bromide. In each case we shall suppose that a certain volume of hydrogen, which we shall call 1 volume, is caused to combine with the other element, and that the volume of the gaseous compound is measured, the temperature and pressure at which all measure- ments are made being the same. The data are these ; 1 volume of hydrogen combines with 1 volume of chlorine to form 2 volumes of hydrogen chloride. I volume of hydrogen combines with 1 volume of bromine to form 2 volumes of hydrogen bromide. Let there be x molecules of hydrogen in the 1 vol. used, then the data translated into the language of the Avogadrean hypothesis read thus ; x molecules of hydrogen combine with x molecules of chlorine and produce 2# molecules of hydrogen chloride. x molecules of hydrogen combine with x molecules of bromine and produce 2x molecules of hydrogen bromide. Now as every molecule of hydrogen chloride is composed of both hydrogen and chlorine, and as every molecule of hy- drogen bromide is composed of both hydrogen and bromine, the necessary conclusion if we grant Avogadro's hypothesis is that one molecule of hydrogen chloride (or bromide) is composed of half a molecule of hydrogen and half a molecule of chlorine (or bromine) ; in other words, that each molecule of hydrogen, and each molecule of chlorine and bromine, has separated into at least two parts, and that these parts of 283287] THE MOLECULAR AND ATOMIC THEORY. 203 molecules have combined to produce the molecules of the compounds formed in the reactions. If we now tabulate the data for the reverse chemical changes, and translate these data into the language of Avoga- dro's hypothesis, we have the following statements : f volumes of hydrogen chloride produce 1 volume of hydrogen and 1 volume of chlorine, molecules of hydrogen chloride produce x molecules of hydrogen and x molecules of chlorine. /2 volumes of hydrogen bromide produce 1 volume of hydrogen and \ 1 volume of bromine. J2 molecules of hydrogen bromide produce x molecules of hydrogen and x molecules of bromine. As we concluded from the former data that a single mole- cule of hydrogen reacting with a single molecule of chlorine (or bromine) produces 2 molecules of hydrogen chloride (or bromide), so now we conclude that 2 molecules of hydrogen chloride (or bromide), when decomposed produce 1 molecule of hydrogen and 1 molecule of chlorine (or bromine). The outcome of Avogadro's conception of the structure of 286 matter is given in the statement already enunciated ; equal volumes of gases contain equal numbers of molecules. The appli- cation of this generalisation to the interactions between hydro- gen and chlorine, and hydrogen and bromine, has led to the conclusion that the molecules of these elementary gases are composed each of at least two parts, and that these parts part company when the gases interact to form hydrogen chloride and bromide, respectively. Since the time of Avogadro the physical conception of the 287 molecule, as a minute portion of matter, has been much ad- vanced. Every attempt to gain a consistent notion of the mechanism of physical changes has led to the recognition of the grained structure of matter. The hypothesis which asserts that a mass of apparently homogeneous matter is really homo- geneous, that however small are the parts into which the body is divided each part exhibits all. the properties of the body, has failed to explain any large class of physical facts. Physi- cists have fully adopted the view that a quantity of any kind of matter consists of a vast number of very minute particles in constant motion. These minute portions of matter they call molecules. The molecules of a gas are supposed to be continually moving about, frequently colliding against each other and rebounding again, but yet remaining intact during 204 ELEMENTARY CHEMISTRY. [CHAP. XV. these collisions. The physical definition of the molecule of a gas is given in the following words of Clerk Maxwell. "A gaseous molecule is that minute portion of a substance which moves about as a whole, so that its parts, if it has any, do not part company during the motion of agitation of the gas." 288 The theory that every portion of a body we can see or handle is composed of a great number of very minute particles, in constant motion, each of which is possessed of the proper- ties which characterise the body in question, does not assert or deny the infinite divisibility of matter. What this theory asserts, to use the words of Clerk Maxwell, is "that after we have divided a body into a certain finite number of constituent parts called molecules, then any further division of these mole- cules will deprive them of the properties which give rise to the phenomena observed in the substance." 289 The relations between the motions and the space occupied by a number of molecules which are mutually independent have been investigated by mathematical analysis. The equa- tions arrived at, after making a justifiable assumption as to the dynamical meaning of temperature, express with considerable accuracy the observed relations between the volume, tempera- ture, and pressure, of gases considerably removed from their liquefaction-points; that is to say the equations agree well with the laws of Boyle and Charles. The properties of a system of molecules moving about freely, and acting on each other only when they come into contact, have been investigated mathematically. One of the deductions arrived at is the generalisation which was stated by Avogadro in 1811; 'Equal volumes of gases contain equal numbers of molecules.' This generalisation is thus raised from a merely empirical statement to the rank of a deduction, made by dynamical reasoning, from a simple hypothesis regarding the structure of matter, which is itself justified by many classes of experimentally established facts. 290 The generalisation of Avogadro is of fundamental import- ance in chemistry. It is essential that the student should understand that this statement rests on physical evidence and dynamical reasoning ; and also that he should understand that the statement presupposes the physical definition of the molecule of a gas (s. par. 287). When this generalisation is applied to many chemical changes taking place between gaseous elements, it leads to the necessary conclusion that the mole- cules of most gaseous elements are composed of parts, and that 287292] THE MOLECULAR AND ATOMIC THEORY. 205 these do part company when the molecules chemically interact. Hence in chemistry we must recognise two orders of small particles ; molecules, and the parts of molecules or atoms. Avogadro's generalisation, or Avogadro's law* as it is 291 usually called, furnishes a means for determining the relative weights of gaseous molecules. For, if the number of molecules in equal volumes of two gases (at the same temperature and pressure) is the same, it follows that the ratio of the densities of the gases is also ike ratio of the masses of the two kinds of molecules. Now a specified volume of oxygen is 16 times heavier than an equal volume of hydrogen ; therefore a molecule of oxygen weighs 16 times as much as a molecule of hydrogen. There- fore if the weight of the molecule of hydrogen is taken as unity, the molecular weight of oxygen must be 16. But ought the molecular weight of hydrogen to be taken 292 as unity 1 ? We have already found that the application of Avogadro's law to the interactions which occur between hy- drogen and chlorine, and hydrogen and bromine, requires us to assert that each molecule of hydrogen separates, in these reactions, into at least two parts. A similar examination of other reactions between hydrogen and various gaseous elements confirms this conclusion. A molecule of hydrogen then is composed of at least two parts, or atoms. But we agree to call the atomic weight of hydrogen one, and to make this the standard in terms of which the relative weights of the atoms of other elements are to be stated. Hence the smallest value which we can give to the molecular weight of hydrogen is two. Of course we may assume that when hydrogen and chlorine react, each molecule of either element separates into 4, 6, 8, 10, &c. parts or atoms; we must assert that each separates into at least two parts. Suppose the assumption is made that each molecule separates into 4 atoms ; then, as there are twice as many molecules of hydrogen chloride formed as the number of molecules of hydrogen or chlorine taking part in the reaction, it follows that each molecule of hydrogen chloride is composed of 2 atoms of hydrogen and 2 atoms of chlorine. But no chemical reactions of hydrogen chloride are in keeping with this conclu- sion. When this compound is decomposed with separation of * The student should observe that the term law is used here in a sense different from that in which the same term is applied to the facts of chemical combination : s. note at end of Chap. xvi. 206 ELEMENTARY CHEMISTRY. [CHAP. XV. hydrogen or chlorine, the whole of the hydrogen or of the chlorine is removed. But the chemical reactions of a gaseous compound are regarded by the molecular theory as the re- actions of the molecules of that compound ; therefore, when a molecule of hydrogen chloride reacts chemically with other molecules, the whole of the hydrogen, or the whole of the chlorine, is removed. The conclusion is that, most probably, a molecule of hydrogen chloride is composed of one atom of hydrogen and one atom of chlorine. 293 -^ or reasons suc h as these, we conclude that the molecular weight of hydrogen is almost certainly two ; that is, that the molecule of hydrogen is composed of two atoms, the weight of each of which we have agreed to call one. Now oxygen is 1 6 times heavier than hydrogen ; but the molecular weight of hydrogen is 2 ; therefore the molecular weight of oxygen is 32. Similarly, chlorine gas is 35-5 times heavier than hydrogen ; therefore the molecular weight of chlorine is 71. Mercury-gas is 100 times heavier than hydro- gen ; therefore the molecular weight of mercury-gas is 200 : and so on, for all the elements which can be obtained as gases. Similarly with gaseous compounds. Water-gas is 9 times heavier than hydrogen; therefore the molecular weight of water-gas is 18. Ammonia is 8*5 times heavier than hydrogen ; therefore the molecular weight of ammonia is 17. Alcohol-gas is 23 times heavier than hydrogen; therefore the molecular weight of alcohol-gas is 46 ; and so on, for all compounds which can be obtained as gases. 294 We can now define the term molecular weight of a gas. The definition may be stated in various forms of words. The molecular weight of a gaseous element or compound is twice the specific gravity of the gas referred to hydrogen. Or ; The molecular weight of a gaseous element or compound is a number which tells the weight of two volumes of the gas, that is, the weight of that volume of the gas which is equal to the volume occupied (under the same conditions of temperature and pressure} by two parts by weight of hydrogen. Or, inasmuch as air is 14435 times heavier than hydrogen, we may say that ; The molecular weight of a gaseous element or compound is the product obtained by multiplying the specific gravity of the gas referred to air, by 28-87. 295 * The application of Avogadro's law to chemical interactions leads to the recognition of the atom as a particle of matter 292297] THE MOLECULAR AND ATOMIC THEORY. 207 weighing less than the molecule ; it also gives a means for determining the maximum weights of the atoms of those elements which form gaseous compounds. The atom of an element is, by definition, the ultimate particle of the element of which cognisance is to be taken in chemistry ; hence, it is evident that the molecule of a com- pound gas formed by the union of (say) two elements, A and B, must be formed by the union of at least one atom of A with at least one atom of B t Or, in general terms, a molecule of a compound gas must be composed of at least one atom of each of the elements which unite to produce ihe com- pound. This is equivalent to saying, the atom of an element is the smallest mass of that element which combines with other atoms to form a gaseous molecule. As we have agreed to call the mass of one atom of hydro- gen unity, and to state the weights of all other atoms in terms of that of the atom of hydrogen, we arrive at a definition of the maximum value to be given to the atomic weight of an element. The smallest mass of an element, in terms of hydrogen as unity, which is found to combine with other elements to form a gaseous molecule represents the maximum value to be given to the atomic weight of the element in question. Or; The number which expresses how many times heavier than the smallest mass of hydrogen which combines with other elements to form gaseous molecules, is the smallest mass of a specified element which combines with other elements to form gaseous molecules, also expresses the maximum value which can be given to the atomic weight of the element in question. The greater the number of gaseous compounds of the 296 specified element which have been examined, the greater is the probability that the maximum value deduced . for the atomic weight of the element represents the true value. Let it be required to determine the atomic weight of 297 oxygen. The definition of atomic weight tells ; (1) that several gasifiable compounds of oxygen must be prepared ; (2) that these compounds must be gasified and the specific gravity of each, and hence the molecular weight of each, determined; (3) that each compound must be analysed, and the results stated as parts of each element per molecule of the compound. Then the smallest mass of oxygen in any one of these molecules is taken as the atomic weight of oxygen. 208 ELEMENTARY CHEMISTRY. [CHAP. xv. Here are some of the data. Data for determining the atomic weight of oxygen. Gaseous compound. Sp. gravity, Composition of one mole- cular weight. Carbon dioxide Sulphur dioxide Sulphur trioxide 1-53 2-25 2-85 Sp. Gr. x 28-87 corrected (s. par. 300) i.e. molecular weight. 43-89 31-92 oxygen + 11-97 carbon. 63-90 31-92 +31-98 sulphur. 79-86 47-88 +31-98 If no other gaseous compounds of oxygen were known, we should select the number 31-92 for the atomic weight of this element. But the following data shew that this conclusion would be incorrect. Gaseous compound. Sp. gravity air=l. Composition of one mole- cular weight. Carbon monoxide Water Nitric oxide Data for determining the atomic weight of oxygen. Sp. Gr. x 28-87 corrected (s. par. 300) i.e. molecular weight. 27-93 15-96 oxygen + 11-97 carbon. 17-96 15-96 +2 hydrogen. 29-97 15-96 + 14-01 nitrogen. 97 63 1-04 298 As a great many gaseous compounds of oxygen are known, and as a molecule of none of them contains less than 15-96 parts by weight of oxygen, this number is taken to be the atomic weight of oxygen. Some of the data from which the value 35 '37 is deduced for the atomic weight of chlorine are presented in the following table. Data for determining the atomic weight of chlorine. Gaseous compound. Sp. Gr. Sp. GV.X28-87 a ir = 1. corrected i. e. Compos ition of one molecular weight, molecular weight. Carbon tetrachloride 5-33 153-45 141-48 chlorine+ 11'97 carbon. Silicon tetrachloride 5-94 169-48 141-48 + 28 silicon. Phosphorus trichloride Antimony trichloride 4-85 7-8 137-07 226-11 10611 106-11 + 30-96 phosphorus. +120 antimony. Zinc chloride 4-7 135-64 70-74 + 64-9 zinc. Tungsten hexachloride 13-4 395-82 212-22 +183-6 tungsten. Sulphuryl chloride 4-67 134-64 70-74 + 31-92 oxygen+31-98 sulphur. Chloroform 41 119-08 106-11 +1 hvdrogen+ 11-97 carbon. Thallium chloride 8-3 239-01 35-37 +203-64 thallium. Methyl chloride 1-74 50-34 35-37 + 11-97 carbon +3 hy- drogen. Nitrosyl chloride 2-3 65-34 35-37 + 15-96 oxygen +14-01 nitrogen. Hydrogen chloride 1-25 36-37 35-37 ., + 1 hydrogen. 297300] THE MOLECULAR AND ATOMIC THEORY. 209 When only a few compounds of a specified element have 299 been gasified and analysed, the value thence deduced for the atomic weight of that element may be, and very possibly is, too large; it cannot be too small. Thus only three compounds of aluminium have been gasified &c. ; a molecule of each is composed of 54-04 parts by weight of aluminium, combined with 212-22 parts by weight of chlorine, 478-5 of bromine, and 759*18 of iodine, respectively. Hence the atomic weight of aluminium is not greater, but may be less, than 54-04. From other data we know that the atomic weight of this metal is ^^ = 27-02. (a. par. 305.) L Determinations of the specific gravities of gases are subject 300 to several sources of error. But the mass of an element which combines with one part by weight of hydrogen, or eight parts by weight of oxygen, or 35*5 parts by weight of chlorine, or 16 parts by weight of sulphur, i.e. the smallest value of the combining weight of the element (v. ante. Chap. v. pars. 73 to 75), can be determined with great accuracy. It is evident that the molecular weight of an element must be either equal to, or a whole multiple of, the combining weight of the element ; and that the molecular weight of a compound must be either equal to, or a whole multiple of, the sum of the combining weights of the constituent elements. Hence the data required for an accurate determination of the molecular weight of an element are (1) an accurate determination of the combining weight of the element, and (2) a fairly accurate determination of the specific gravity of the element in the gaseous state. Similarly, the data required for an accurate determination of the molecular weight of a compound are (1) accurate determinations of the combining weights of the elements which form the compound, and (2) a fairly accurate determination of the specific gravity of the compound in the gaseous state. Thus 35-37 parts by weight of chlorine combine with 1 part by weight of hydrogen ; therefore the molecular weight of chlorine is ^35 -37, where n is a whole number. A deter- mination of the specific gravity of chlorine shews that this gas is approximately 35 J times heavier than hydrogen ; therefore the molecular weight of chlorine is approximately 35 -5 x 2 = 71. But 35-37 x 2 = 70-74 ; therefore the molecular weight of chlorine is 70-74. Again, 10-32 parts by weight of phos- phorus combine with 1 part by weight of hydrogen to produce M. E. C. 14 210 ELEMENTARY CHEMISTRY. [CHAP. xv. 301 302 phosphorus hydride; therefore the molecular weight of this compound is n 11-32 (n = a whole number). Gaseous phos- phorus hydride is found to be about 17 times heavier than hydrogen; therefore the molecular weight of this gas is about 17 x 2 - 34. But 11-32 x 3 = 33-96 ; therefore the molecular weight of gaseous phosphorus hydride is 33-96. The meaning of the heading of col. in. in the tables in pars. 297 and 298, 'p. Gr. (air = 1) x 28-87 corrected, i.e. molecular weight ' will now be apparent. By applying Avogadro's law, values have been obtained for the atomic weights of rather more than half the elements; gaseous compounds of the remaining elements have not yet been obtained, and hence the atomic weights of these elements have not been determined by the method based on the law of Avogadro. The following table gives the results of the application of Avogadro's law to determining the atomic weights of elements. Maximum Atomic Weights of Elements. (Avogadro's law.) Max. Max. Max. Element Atom. Element Atom. Element Atom. Weight Weight Weight Hydrogen 1 Chromium 52-4 Antimony 120 Beryllium 9-1 [Aluminium * 54-04] Tellurium 125 Boron 10-95 Zinc 64-9 Iodine 126-53 Carbon 11-97 Germanium 72-3 [Copper* 126-8] Nitrogen 14-01 Arsenic 74-9 [Gallium* 138] Oxygen 15-96 Selenion 78-8 Tantalum 182 Fluorine 19-1 Bromine 79-75 Tungsten 183-6 Silicon 28 Zirconium 90 Osmium 193 (?) Phosphorus 30-96 Niobium 94 Mercury 199-8 Sulphur 31-98 Molybdenum 95-9 Thallium 203-6 Chlorine 35-37 [Iron* 111-8] Lead 206-4 Potassium 39-04 Cadmium 112 Bismuth 208 Titanium 48 Indium 113-4 Thorium 232 Vanadium 51-2 Tin 117-8 Uranium 240 Avogadro's law equal volumes of gases contain equal numbers of molecules furnishes chemists with a method whereby they may determine the relative weights of the molecules of all gaseous compounds and elements, and the maximum values to be given to the relative weights of the atoms of all elements which form gaseous compounds. But at present only 14 or 15 elements have been gasified, and gaseous * The atomic weights of these four elements are almost certainly 27-02, 55-9, 63-4, and 69, respectively (*. par. 305). 300304] THE MOLECULAR AND ATOMIC THEORY. 211 compounds of only 42 elements have been prepared and analysed. Hence the application of the method based on the law of Avogadro is limited. This method is at present the only general method for determining the relative weights of the gaseous molecules of elements and compounds. But there is another general method whereby values may be found for the atomic weights of elements. This method is contained in the statement ; The products of the specific heats of solid elements, determined in each case at the temperature-interval for which specific heat is nearly constant, into the atomic weights of these elements, ap- proach a constant, the mean value of which is 6*4. This statement is a modification of the so-called law of Dulong and Petit. From their study of the specific heats of 13 solid elements in the year 1819, these naturalists an- nounced that "the atoms of all simple bodies have exactly the same capacity for heat/' Investigation has shewn that this statement was too absolute. The specific heats of some solid elements, e.g. carbon, boron, silicon, beryllium, vary much with variations of temperature, and become approximately constant only at high temperatures. The specific heat of a solid also varies to some extent with variations in the greater or less compactness of the specimen. The product specific heat of solid element x atomic weight is usually called the atomic heat of the element. The specific heats of a few elements have not yet been 303 determined. Values which may be approximately correct, have been indirectly obtained for some of these ; but too great stress must not be laid on these values. The indirect method in question is based on the assumption, to some extent verified by facts, that the 'molecular heat' of a solid compound, i.e. the product of the specific heat into the mass of the compound expressed by its formula, is equal to the sum of the atomic heats of the elements in the compound; therefore if the 1 molecular heat" 1 (as thus defined) of a solid compound is known, and the atomic heats of all the elements in the com- pound except one are known, the atomic heat of the remaining one element can be calculated. The following statements summarise the present state of QQA knowledge with regard to the atomic heats of the 42 elements maximum values for the atomic weights of which have been determined by applying the law of Avogadro (par. 301). I. Solid elements, 28 in number, the specific heats of which 142 212 ELEMENTARY CHEMISTRY. [CHAP. XV. have been directly determined, and the atomic heats of which are approximately equal to 6 4 : P, S, K, Ti, Or, Al, Zn, As, Se, Br, Zr, Mo, Fe, Cd, In, Sn, Sb, Te, I, Cu, W, Os, Hg, Tl, Pb, Bi, Th, U. II. Solid elements, 6 in number, the specific heats of which have been directly determined, and the atomic heats of which are approximately equal to 5*5 : Be, B, C, Si, Ga, Ge. III. One solid element, the atomic heat of which has been indirectly determined and is probably equal to 6*4: Vanadium. IV. Five gaseous elements, the specific heats of which in solid form have only been determined indirectly and are ex- tremely doubtful : H, N, O, F, 01. V. Two solid elements the specific heats of which have not been determined directly or indirectly : Nb, Ta. These data establish a very fair probability in favour of the statement made in par. 302 regarding the constant value of the atomic heat of the solid elements. If this statement is granted, then an approximate value may be found for the atomic weight of an element by dividing 6 '4 by the specific heat of that element in the solid form. 305 The maximum values found for the atomic weights of aluminium, iron, copper, and gallium, by the use of Avogadro's law were 54-04, 111-8, 126-8, and 138, respectively (. Table, par. 301). Now the spec, heats of these elements are -225, 114, -097, and -08, respectively; dividing 6-4 by each of these numbers we get the quotients, 28*5, . 56'1, 65-9, and 80. Therefore we conclude that the maximum values found for the atomic weights of these elements by applying Avogadro's law must be halved, and we adopt the numbers 27*02, 55 -9, 63-4 and 69, as very probably the true atomic weights of aluminium, iron, copper, and gallium, respectively. 306 There is another physical method which has sometimes been found useful in determinations of atomic weights, but which can only be used as a guide to point the way to experi- mental inquiries. This method is founded on the generalisa- tion, that similarity of chemical composition is usually as- sociated with close similarity of crystalline form. In some cases marked similarity of composition is accompanied by identity of crystalline form; e.g. the oxides of arsenic and antimony, As 2 3 and Sb 2 O 3 , crystallise in identical forms ; they are isomorphous. The difficulties in applying this method generally known as the method of isomorphism lie in the vagueness of the ex- 304307] THE MOLECULAR AND ATOMIC THEORY. 213 pressions ' similarity of chemical composition ' and * similarity of crystalline form.' The following example will indicate how the so-called law of isomorphism has been used as an aid in determining the atomic weight of gallium. Gallium sulphate was found to form a double salt with ammonium sulphate ; the crystalline form of this double sul- phate was identical with that of ordinary ammonia-alum. Therefore the double sulphate in question doubtless belonged to the class of alums. Now the composition of the alums is expressed by the general formula M 2 3SO 4 . N 2 SO 4 . 24H 2 O where M=A1, Fe, Or, or Mn, and N=Na, K, Cs, lib, or NH 4 . In the case of common ammonia-alum M 2 = A1 2 = 2 x 27'02 parts by weight of aluminium ; in the double sulphate of gallium and ammonium M 2 was found to represent 138 parts by weight of gallium. Hence, as 2 atoms of aluminium have been replaced by 138 parts by weight of gallium without altering the crystalline form or the general chemical type of the compound, it was concluded that the atomic weight of gallium was i|-^ 69. This number was afterwards verified by the application of the law of Avogadro, and also by the specific heat method*. There are then two generally applicable methods whereby 307 values may be found for the atomic weights of the elements ; the method founded on the law of Avogadro ; and the method based on the specific heats of solid elements. Besides these, there is another method, arising out of the relations between the chemical composition and crystalline forms of similar compounds, which is useful as a guide in determinations of atomic weights. The first method is applicable to determina- tions of the atomic and molecular weights of elements, and the molecular weights of compounds, but it is restricted to bodies which are gasifiable without decomposition. The second and third methods can be strictly applied only to find values for the atomic weights of solid elements, and to some extent of elements which form solid compounds. All the methods are essentially physical ; they are based on physical conceptions, and they are to a great extent developed by physical reason- ing. Thus the image of the molecule which is called up in the mind by the statement " equal volumes of gases contain equal numbers of molecules " is that of a very small, definite, * We do not propose to go more fully into the method of isomorphism here. The study of this subject is more suited to the advanced student of chemistry. 214 ELEMENTARY CHEMISTRY. [CHAP. XV, portion of matter, moving about without separation into parts, colliding with other like particles of matter, and rebounding after collision. The application of this conception to chemical changes obliges us to admit that in many of these changes the molecule is shattered into parts. Thus we are led to the chemical conception of the atom, as a portion of matter smaller than the molecule, and either itself without parts, or else composed of parts which, so far as we know at present, do not part company during any of the changes which the atom undergoes. The study of the properties of atoms leads to the generalisation that the atoms of all solid elements, at certain temperatures, have equal capacities for heat. The molecular and atomic theory regards the molecule of a gas as the smallest portion of it in which the properties of the gas inhere. Chemical change, it looks on as an interaction between molecules; in most cases of chemical change the interacting molecules are separated into parts and these parts are rearranged to form new molecules ; but in some cases it is probable that one kind of molecules combines with other kinds to form more complex molecules. The Daltonian atomic theory applied the term atom to elements and compounds alike ; but the atom of an element was supposed to have no parts, whereas the atom of a com- pound was separable into unlike parts. The molecular and atomic theory applies the term molecule to elements and compounds alike ; but the molecule whether of an element or a compound is regarded as built up of parts which may be either all of one kind, or of different kinds. 308 The atomic weights of most of the elements have been determined by one or other of the physical methods arising out of the molecular and atomic theory. But there are a few elements no compounds of which have yet been gasified, and the specific heats of which have not yet been determined. The values assigned to the atomic weights of these elements have been gained by studying the chemical analogies between these elements and others to which the methods of the mole- cular and atomic theory are directly applicable. The metal rubidium is a case in point. No compound of this metal has been gasified ; hence the molecular weights of rubidium compounds are not known ; and hence the atomic weight of the element has not been determined by the ap- plication of the law of Avogadro. Nor has the specific heat 307309] THE MOLECULAR AND ATOMIC THEORY. 215 of rubidium been determined. The value given to the atomic weight of rubidium is 85 '2 ; how has this number been ob- tained 1 There can be no doubt that rubidium belongs to the class of elements which comprises sodium and potassium (for details of the properties of this group, s. Chap. xi. pars. 160 168). The atomic weights of sodium and potassium are 23 and 39 (in round numbers) respectively ; 23 parts by weight of sodium and 39 parts by weight of potassium severally combine with (a) 8 parts by weight of oxygen, and (b) 35*5 parts by weight of chlorine ; the specific heats of these metals are, for so- dium -293, for potassium -166; now -293 x 23 = 6*7, and 166 x 39 = 6 -5. But if 23 and 39 are the atomic weights of sodium and potassium, respectively, and if 16 is the atomic weight of oxygen, then analyses of the oxide, chloride, &c. &c. of these metals shew that the formulae of these compounds must be M 2 O, MCI, M 2 SO 4 , M 2 CO 3 , &c. &c. where M = one atomic weight of sodium or potassium. Now the compounds of rubidium are very similar in their properties to the com- pounds of potassium and sodium, hence the oxide, chloride, &c. &c. of rubidium ought to be represented by the formulae Rb 2 O, RbCl, Rb 2 SO 4 , RbCO 3 , &c. &c. where Rb = one atomic weight of rubidium. But in order to do this, the number 85 '2 must be assigned to the atomic weight of rubidium. The method based on a study of the analogies between the chemical properties of a specified element and those of other elements is also frequently used to check the results of the determinations of atomic weights gained by applying the two physical methods. But a fuller examination of this chemical method will be better made when we come to consider the periodic law (s. Chaps, xvm. and xxvi.). In the sketch which has been given of the molecular theory 309 of the structure of matter, the conception of the molecule has been applied only to gases. The theory regards liquids and solids also as built up of minute particles. It asserts that the minute particles of a liquid have less freedom of motion than the molecules of a gas, and that they are so frequently in collision with each other that the paths which they describe are far removed from being straight lines. The minute par- ticles of a . solid are supposed to oscillate about positions of equilibrium, and never to travel far from these positions. The particles of both liquids and solids, moreover, are probably aggregations of smaller particles ; and the complexity of the 216 ELEMENTARY CHEMISTRY. [CHAP. XV. particles of a specified liquid or solid is probably not the same for all the particles, nor even for the same particle at different times. 310 It is customary to apply the term molecule to the particles of liquids and solids, as well as to the much more rigidly denned particles of gases. But no generalisations can at present be made, in terms of the molecular theory, regarding the properties of liquids and solids comparable with those which have been made for gases, and which are known as the laws of Boyle, Charles, and Avogadro. We cannot define the term molecule as applied to a liquid or solid body ; we can define the term when applied to a gas. When therefore we speak of the molecular weight of an element or a compound we ought to mean the relative weight of the molecule of the gaseous element or compound. The expression molecular weight is not always used in chemistry in this strict meaning ; it is frequently applied to what in former chapters we have called the reacting weight of a body. To take an example. No compounds of sodium have been gasified ; therefore we do not know the molecular weights of any compounds of this element. But the atomic weight of this metal has been determined by applying the method of specific heat. The simplest formulae which can be given to the compounds of sodium, when we know that Na = 23, are Na 2 O, NaCl, ISaBr, NaNO 3 , Na 2 CO 3 , Na 2 S0 4 , &c. Moreover these formulae enable us to express the chemical reactions of the compounds in a consistent and satisfactory manner. These formulae are therefore adopted. But we must care- fully observe that the formulae do not necessarily express the atomic compositions of molecules of the compounds : indeed we cannot in strict accuracy speak of a molecule of sodium oxide, or sodium chloride, because these compounds are only known as solids and the term molecule corresponds to an accurately defined conception only when it is applied to 311 We formerly used the term reacting weight of a compound ; thus Na 2 O represents the composition of a reacting weight of sodium oxide. We may now, in the light thrown on chemical interactions by the molecular and atomic theory, widen the meaning we give to this term reacting weight. Although Na 2 O does not certainly represent the atomic composition of a molecule of sodium oxide, yet it almost certainly represents the ratio of the number of atoms which constitute the reacting 309311] THE MOLECULAR AND ATOMIC THEORY. 217 weight of this oxide. The reacting weight of a compound may now mean for us a group or collocation of atoms which interacts chemically with other groups of atoms. The formula of a solid or liquid compound does not necessarily express the number of atoms in this chemically reacting group of atoms indeed the number may vary under different circumstances but in all probability it does represent the ratio between the number of atoms in this reacting group. The atomic compo- sition of the reacting weight of sodium oxide may perhaps be better represented by one of the formulae Na 4 O 2 , Na 6 3 , Na 10 5 , than by the simpler formula Na 2 O ; but 4 : 2 = 6 : 3= 10 : 5 = 2 : 1. CHAPTER XVI. APPLICATIONS OF THE MOLECULAR AND ATOMIC THEORY, CHIEFLY TO CLASSES OF FACTS AND PRINCIPLES ALREADY CONSIDERED. 312 THE molecular and atomic theory asserts that^a quantity of any gaseous element or compound is constituted of a very great number of minute particles, all having the same masses and the same properties, and all in constant motion. These particles, or molecules, are constituted of smaller particles which have a certain freedom of motion among themselves ; these smaller particles, or atoms, are of one kind and of equal masses when the molecule formed by their union is the molecule of an element ; but the atoms are of different kinds and different masses when the molecule formed by their union is the molecule of a compound. Chemical change, according to the molecular and atomic theory, is an interaction between molecules, and it results in the formation of new molecules. In very many cases of chemical change the interacting molecules are separated into their constituent atoms, and these atoms rearrange themselves to form new molecules ; but in some cases the interaction of the original molecules probably consists in the direct formation of more complex molecules. Thus the interactions of hydrogen and chlorine to produce hydrogen chloride, and of hydrogen and oxygen to produce water-gas, are represented thus by the theory of atoms and molecules : (1) H 2 +C1 2 = 2HC1; (2) 2H 2 + O 2 = 2H 2 0. The symbols H 2 , C1 2 , O 2 , HG1, H 2 O, each represents the atomic composition of a molecule of an element or compound. But the interactions of water and cobalt chloride, or water and copper sulphate, are probably best represented by equations which assume the 312-315] APPLICATIONS OF MOLECULAR- ATOMIC THEORY. 219 change to consist in the combination of molecules of the inter- acting substances to produce more complex molecules : thus, ( 1 ) CoCl, + 2H O = CoCl 2 . 2H 2 O ; (2) CuSO 4 + 5H 2 O = CuSO 4 . 5H 2 O. The theory regards most physical changes as changes in the 313 rates of motion, without changes in the atomic compositions, of molecules. But changes usually called physical may result in the coalescence of molecules into more or less complex aggrega- tions which are stable under definite conditions of temperature, pressure, &c. The theory of molecules and atoms does not therefore give us a means of sharply distinguishing between physical and chemical change. The typical chemical change results in a redistribution of the atoms of the interacting molecules so as to form new molecules ; the typical physical change results in changes in the rates of motion of molecules without any redistribution of the parts of these molecules. But there are many changes which cannot be placed wholly in one or other of these classes. Every chemical change is accompanied by physical change : the portion of the change we call chemical is only one part of the complete occurrence. Even if the theory gave a sharp and clear definition of each kind of change, it could not give a means whereby we might classify all actually occurring changes into chemical on the one hand, and physical on the other. The laws of chemical combination find a simple explana- 314 tion in terms of the molecular and atomic theory. The atom is the ultimate particle of matter of which we take cognisance in chemistry. The properties of a molecule depend, among other conditions, on the nature and number of the atoms which form it ; this is the law of fixity of composition. If two or more different kinds of atoms combine to form several different molecules, each molecule must be composed of x atoms of one kind 4- x f atoms of another kind + x" atoms of another kind + x'" atoms of another kind &c., and x, x', x", x f " must be whole numbers, because the atom is, by definition, indi- visible ; this is the law of multiple, and the law of reciprocal, proportions. The molecular and atomic theory throws light on the 315 conceptions of combining and reacting weights. The reacting weight of a gas is the molecular weight of that gas-. The combining weight of an element, as the term was defined in Chap. vi. par. 79, is the atomic weight of that element. 220 ELEMENTARY CHEMISTRY. [CHAP. XVI. Thus we found (s. Chap. vi. par. 86) that the reacting weight of water is 18 ; and that one reacting weight of this compound is composed of two combining weights of hydrogen united with 1 c. w. of oxygen. Translated into the language of the molecular and atomic theory this statement reads as follows : the molecular weight of water-gas is 18, and a molecule of water-gas is formed by the union of 2 atoms of hydrogen with 1 atom of oxygen. We know that 36*5 parts by weight of hydrogen chloride are formed by the combination of 1 part by weight of hydrogen with 35-5 parts by weight of chlorine. In order to express this fact in terms of combining and reacting weights, we say that one c. w. of hydrogen combines with one c. w. of chlorine to produce one reacting weight of hydrogen chloride. The molecular and atomic theory expresses the same fact by saying that one molecule of hydrogen interacts (not combines) with one molecule of chlorine to produce 2 molecules of hydrogen chloride. We formerly applied the term reacting weight to com- pounds only. We now apply the term molecule to elements as well as to compounds. But when we are dealing with solid bodies which have not been gasified, we cannot in strict accuracy speak of the interactions of molecules of these bodies. Thus when boron and aluminium are strongly heated together under proper conditions two compounds, A1B 2 and A1B 12 , are produced. As neither boron nor aluminium has been gasified, and as neither of the borides of aluminium has been gasified, we do not know the molecular weights of any of the bodies taking part, or formed, in this reaction : we cannot therefore say how many molecules of each element have taken part in the change nor how many molecules of each compound have been formed. Again, when solutions of barium chloride and sodium sulphate, in aqueous solutions, are mixed in the ratio BaCl 2 : Na 2 SO 4 , the change of composition which occurs may be represented thus : BaCl 2 Aq + Na ? S0 4 Aq = BaSO 4 + 2NaCl Aq. We may read the equation as meaning : one reacting weight of barium chloride interacts with one reacting weight of sodium sulphate to produce one r. w. of barium sulphate and 2 r. ws. of sodium chloride ; but the equation cannot be read as certainly meaning, one molecule of barium chloride interacts with one niol. of sodium sulphate to produce one mol. of barium sulphate and 2 mols. of sodium chloride. As none of the 315, 316] APPLICATIONS OF MOLECULAK-ATOMIG THEORY. 221 bodies taking part in this change have been gasified we do not know the molecular weight of any of them. We might read the equation thus ; atomic aggregates of barium chloride and sodium sulphate interact to produce atomic aggregates of barium sulphate and sodium chloride. The definition of molecule is a physical definition ; it is stated in terms which have an accurate meaning only when used of gaseous elements and compounds. If we choose to use the term in speaking of the phenomena of liquid and solid bodies we must not forget that the term cannot then be accurately defined. The definition of reacting weight is a chemical definition; but the term reacting weight is much vaguer than the term molecule. The reacting weight of a solid or liquid compound is doubtless an aggregation of atoms which interacts, as a whole, with other aggregations of atoms ; but whether the number of atoms in this aggregation is the same in all chemical changes we do not know. We have seen in Chap. xv. that the atomic weights of 316 most of the elements have been determined, either by the method based on the law of Avogadro, or by that founded on the generalisation 'atomic weight into spec, heat = a constant.' But the molecular weights of only a few elements have been determined. About 70 elements are known; 14 of these have been gasified ; therefore the molecular weights of only 14 are known. The specific gravities in the gaseous state, and hence the molecular weights, of some of these 14 elements are constant through a wide range of temperature; the specific gravities, and hence the molecular weights, of others have very different values at different temperature-intervals. The most probable explanation of the changes in the values of the molecular weights of certain elements is that the atomic compositions of the molecules of these elements are different at different temperatures. Thus sulphur-gas from about 450 to about 600, is 96 times heavier than an equal volume of hydrogen at the same temperature ; therefore the molecular weight of sulphur-gas between 450 and about 600 is ap- proximately 96 x 2 = 192. But from about 800 and upwards sulphur-gas is only 32 times heavier than hydrogen ; therefore the molecular weight of sulphur-gas at temperatures above 800 is approximately 32 x 2 = 64. The atomic weight of sulphur is 31-98; this number is determined by applying Avogadro's law to many gaseous compounds of sulphur, and it is verified by determinations of the spec, heat of sulphur. 222 ELEMENTARY CHEMISTRY. [CHAP. XVI. Now 31-98 x 6 = 191-88, and 31-98 x 2 = 63'96 ; therefore we conclude; (1) that sulphur-gas at 450 to 600 has the mo- lecular weight 191-88, and at 800 and upwards the molecular weight 63'96 ; and (2) that the molecule of gaseous sulphur at 450 to 600 is composed of 6 atoms, or is hexatomic, and that the molecule at 800 and upwards is composed of 2 atoms, or is diatomic. 317 The expression atomicity of a molecule is used to denote the number of atoms which form the gaseous molecule of an element or compound^ The data for classifying the molecules of elements in accordance with their atomicities are presented in the following table. Atomicity of elementary gaseous molecules. Monatomic Diatomic Triatomic Tetratomic Hexatomic Zinc Hydrogen Oxygen Phosphorus) Sulphur Cadmium Chlorine (as ozone) Arsenic \ (about 400 to 600) Mercury Bromine Selenion ( a t temps, below Iodine 7 Iodine (>o to about 8000) whiAeat) (at about 1500<> (200 to about and upwards) 1000) (? Bromime Oxygen at 1800<> and Sulphur upwards) (800 and upwards) Selenion (12000 and upwards) Tellurium Nitrogen Phosphorus) Arsenic \ (at white heat) 318 The molecular weights of some gaseous compounds also vary with variations of temperature. Thus nitrogen tetroxide at very low temperatures is about 46 times heavier than hydro- gen, but at higher temperatures it is only 23 times heavier than hydrogen ; therefore this gas has two molecular weights which are approximately equal to 92 and 46 respectively. Determina- tions of the atomic weights of nitrogen and oxygen, and accurate analyses of the compound nitrogen tetroxide, shew that the composition of the molecule of this gas at very low temperatures is represented by the formula N 2 O 4 (N 2 = 28-02, O 4 = 63'84) = 91 '86, and at higher temperatures by the formula NO 2 =45'93. We shall examine the relations between changes of molecular weight and changes of temperature in more detail later (s. pars. 334 to 337). 319 The conception expressed in the term atomic weight is now 316-321] APPLICATIONS OF MOLECULAR-ATOMIC THEORY. 223 seen to be much more definite than that expressed in the term combining weight. The former term brings before the mind the picture of a small definite portion of matter with definite properties; the latter term merely expresses a ratio. The values of atomic weights are determined by two methods, of general applicability, which are deduced from the principles of a theory of the structure of matter which gives a fairly simple explanation of most, if not all, of the observed physical properties of matter. The value given to the combining weight of an element is a purely empirical value ; it must be determined for each element by methods specially applicable to that element ; it is one of several possible values, and it is selected on the ground of general convenience and expediency. The conception expressed in the term molecular weight is 320 also much more definite than that underlying the expression reacting weight. The molecule of a gas is a perfectly definite quantity of matter with defined properties; it is a physical conception, deduced by dynamical reasoning from a physical theory of the structure of matter. This theory presents us with one generally applicable method for determining the relative weights of gaseous molecules. The reacting weight of a body is said to be ' the smallest relative mass of it which takes part in chemical interactions ' ; but this statement involves terms which cannot, at present at any rate, be accurately defined. The value which, under the circumstances, is the best to be given to the reacting weight of a substance must be deduced for each substance by methods which to a very great extent are empirical, and many of which are ap- plicable only to the special case under consideration. It is true that the molecular and atomic theory has not yet enabled us to define the term molecule as applied to a liquid or solid body; but the theory has thrown light on the chemical conception of reacting weight, and in place of regarding the values of reacting weights merely as numbers, we may now look on them as expressing the relative weights of certain aggregations of atoms which interact with each other to produce new aggregations of atoms. The molecular and atomic theory then regards the properties 321 of a gas as the properties of the molecules of that gas; and the properties of the molecules as dependent, among other con- ditions, on the nature and number of the atoms which form these molecules. But if this view is correct, and if it is true that the number of atoms in the molecule of a gaseous element may 224 ELEMENTARY CHEMISTRY. [CHAP. xvi. vary, we should expect sometimes to find differences in the properties of one and the same element. 322 The descriptions given in Chap. xi. pars. 173, 220, 221, of the properties of sulphur and phosphorus shewed that each of these elements differs very considerably in properties under different conditions. But as both elements exhibit differences in the solid state we cannot say whether these differences are, or are not, connected with differences in the atomicities of the molecules of the elements. 323 Some of the prominent properties of oxygen were described in Chap. vm. pars. 118 to 121. If a quantity of pure dry oxygen is confined over sulphuric acid and a series of electric induction-sparks is passed through the oxygen, the volume of the gas diminishes until it has become about -^ less than it was at the beginning of the experiment. The properties of the gas after the diminution of volume has ceased are markedly different from those of oxygen ; nevertheless it has been con- clusively proved that nothing has combined with the oxygen during the change which has occurred. The new gas is called ozone (because of its smell). The whole of a specified quantity of oxygen cannot be changed into ozone, so that pure ozone has not yet been obtained. But experiments have proved ; (1) that the relation between the volume of that portion of a quantity of oxygen which is changed into ozone, and the volume of the ozone formed is expressed by the ratio 3:2; and (2) that the weight of the ozone produced is equal to the weight of the oxygen which has been changed into ozone. If the gas through which electric sparks have been sent until diminution of volume has ceased is heated to 360 or so, expansion occurs, and the original volume of oxygen is re- produced. The outcome of the experiments on the volume- and mass- relations between oxygen and ozone is that the change of oxygen to ozone, or vice versa, is attended with no change of mass, but that 3 volumes of oxygen condense to 2 vols. of ozone, and 2 volumes of ozone are changed by heat to 3 volumes of oxygen. Now as oxygen is 16 times heavier than hydrogen, it follows that ozone is 24 times heavier than hydrogen ; therefore, as the molecular weight of a gas is twice its specific gravity referred to hydrogen, it follows that the molecular weight of ozone is 24x2 = 48. As ozone is only modified oxygen, and as the atomic weight of oxygen is 16, the molecule of ozone must be composed of 3 atoms of oxygen. 321-326] APPLICATIONS OF MOLECULAR- ATOMIC THEORY. 225 Therefore the atomic composition of the molecule of ozone is expressed by the symbol O 3 , that of the molecule of oxygen being expressed by the symbol O a . Oxygen and ozone are both colourless gases; oxygen is odourless, ozone has a very pronounced odour ; ozone is a very energetic oxidiser, e. g. when passed into mercury at ordinary temperatures it produces mercuric oxide, and it interacts with lead sulphide (PbS) to produce lead sulphate (PbSO 4 ); oxygen does not react with an aqueous solution of potassium iodide (KI), ozone interacts with this salt in aqueous solution and produces potassium oxide, iodine, and oxygen ; thus 2KIAq + 3 = K 2 OAq + 1 2 + O 2 . The existence of the two kinds of molecules, O, and O a , 324 one diatomic and the other triatomic, each characterised by its own properties yet each composed of the same kind of atoms, and of atoms all of which are alike, shews that the properties of some molecules at any rate are conditioned (among other circumstances) by the number of atoms which are combined to form these molecules. The other cases of allotropy (s. Chap. xi. par. 173) exhibited 325 by elements are exhibited by those elements in the solid state. The numbers of atoms in the atomic aggregates which compose the reacting weights of the different solid forms of phosphorus, sulphur, arsenic, &c. may perhaps be different ; but we cannot at present decide whether this is so or not. If the properties of a gas are dependent only on the 326 nature and number of the atoms which form the molecule of that gas, then the existence of more than one gaseous compound having a specified composition must be impossible. But, as a matter of fact, several compounds frequently exist, all having the same composition and the same molecular weight. For instance 3 compounds having the composition C 6 H are known ; these bodies have all been gasified ; their specific gravities as gases, and therefore their molecular weights, are identical. But the molecule of each compound is composed of 5 atoms of carbon united with 12 atoms of hydrogen. Therefore we conclude that the properties of some gaseous molecules are conditioned by other circumstances besides the nature and number of their constituent atoms, and that one of these other circumstances probably is the arrangement of the parts of the molecule relatively to each other. The existence of more than one compound with the same M. E. C. 15 226 ELEMENTARY CHEMISTRY. [CHAP. XVI. molecular composition is called isomerism. We shall return to this subject in the next chapter. 327 We now see how necessary it has become to widen our conception of chemical composition. Restricting ourselves to gases, we have found that there may exist more than one form of the same element we have found for instance that there are two oxygens ; we have also learnt that the same quantities of the same elements may be combined so as to produce different compounds ; and that the same numbers of the same atoms may be combined so as to produce several kinds of molecules, each differing in properties from the others. The properties of an element or compound are the pro- perties of its molecules ; the properties of these molecules are conditioned not only by the nature, but also by the number, of atoms which form them ; but the properties of these mole- cules, it appears, are also probably conditioned by the relative arrangement of their parts. 32g This conception of the relation between chemical proper- ties and composition helps us to understand, more fully than we could do before, the relations between the composition and the properties of such classes of compounds as acids, alkalis, and salts. In Chaps, vni. and ix. we learned that the compounds of hydrogen with oxygen and another negative element, or other negative elements, are generally acids, and that the compounds of hydrogen with oxygen and a markedly positive element are alkalis. We also found that the lower oxides of elements which are neither very markedly positive or negative are usually basic, but that the combination of more oxygen with such oxides produces acidic oxides. We may now translate these statements into the language of the molecular and atomic theory, and say that molecules formed by the union of atoms of hydrogen and oxygen with atoms of negative elements interact, under proper conditions, with molecules of metals, basic oxides, or alkalis, and ex- change some or all of their atoms of hydrogen for atoms of metal. We may also say that molecules composed of fairly positive atoms united with a small number of atoms of oxygen are ready to exchange their positive atoms for atoms of hy- drogen when they interact with acids, under suitable condi- tions; but that molecules composed of many oxygen atoms united with a small number of atoms of fairly positive elements do not thus exchange their positive atoms for hydrogen, but 326-330] APPLICATIONS OF MOLECULAR-ATOMIC THEORY. 227 on the other hand are ready to interact with water to produce acids. As few acids, alkalis, basic oxides, or salts, have been gasified, the term molecule is used in the foregoing paragrapli to include, the aggregates of atoms which form the reacting weights of solid bodies. We know that acids may be classified in accordance with 329 their basicity (s. Chap. xi. pars. 188 and 189). Instead of the statement that ' an n-basic acid is an acid from the reacting weight of which n combining weights of hydrogen can be dis- placed by sodium or potassium under suitable conditions'; we may now say that ' the molecule of an n-basic acid contains n atoms of replaceable hydrogen.' It must be carefully noted that the term molecule is here used with a wider meaning than theory strictly justifies ; it must be taken to mean that aggregate of atoms (this may or may not be a true molecule) which forms the reacting weight of an acid, &c. The account of the reactions of acids given in Chaps, x. and xi. shews what is meant by replaceable hydrogen. The compositions of a few acids and of some of the salts derived from them are presented in the following table ; by considering the relations between the compositions of these acids and their salts, the student will be better able to grasp the meaning of the definition of the basicity of an acid which has been given. The formulae represent reacting atomic aggregates, which in some cases are probably true molecules. Acids. Salts formed. HC1 KC1, NaCl, ZnOL, BiCL, SnCl, &c. H 2 S0 4 KHSCK K 2 S0 4 , EeS0 4 , Fe 2 3SO 4 , Sn2S0 4 , &c. H 3 P0 4 KH 2 P0 4 , K 2 HP0 4 , K 3 P0 4 , CaHP0 4 , Ca 3 2PO 4 , FePO 4 , &c. H 3 P0 3 KH 2 P0 3 , K 2 HP0 3 , &c., (K 3 PO 3 cannot be formed). H 4 C 2 2 KH 3 C 2 2 , NaH 3 C 2 2 , Pb(H 3 C 2 O 2 ) 2 , Fe 2 (H 3 C 2 2 ) 6 , &c. In Chap. xii. we learned a little about the relative affinities 330 of acids. A comparison of the relative affinities of a series of acids the compositions of which differ only by the relative quan- tities of oxygen the acids contain brings out the fact, that the affinity-constants of the acids generally increase as the quantity of oxygen increases ; thus the relative affinities of the three acids H 2 SO 3 , H 2 SO 4 , H 2 S 2 O 6 , are in the ratio 66 : 150 : 178. Again the replacement of hydrogen by a strongly negative 152 228 ELEMENTAKY CHEMISTRY. [CHAP. xvi. element such as chlorine is attended with an increase in the affini ty- constants ; thus the relative affinities of the acids H 4 C 2 2J H 3 C1C 2 2 , H 2 C1 2 C 2 2 , HC1 3 C 2 O 2 (acetic, monochlor- acetic, dichloracetic, trichloracetic, acid) are in the ratio 5J : 38 : 76 : 79. Increase of the number of oxygen atoms relatively to that of the other atoms forming the molecule of an acid is then frequently accompanied by an increase in the affinity of the acids. And similarly, an increase in the affinity accompanies the replacement of atoms of hydrogen by atoms of more nega- tive elements, such as chlorine or bromine. These facts concerning the connexion between the compo- sitions and the properties of acids establish the existence of definite relations between the acidic or non-acidic character of compounds of hydrogen with oxygen and a third element, the basicities of acids, and the values of the affinity-constants of acids, on the one hand, and the nature, number, and probably the relative arrangement, of the atoms in the molecules of these compounds, on the other hand. 331 If we suppose that every atom in a molecule, or reacting atomic aggregate, is directly related in some way to a limited number of other atoms, then it appears that an atom of hydro- gen which is directly related to strongly negative atoms is usually easily replaceable by atoms of positive elements ; or, to use a shorter form of words, we may say that this atom of hydrogen performs an acidic function in the molecule, or is acidic. It also appears that an atom of hydrogen which is directly related to strongly positive atoms, e.g. atoms of potas- sium or sodium, is not acidic, that is cannot be replaced by the atoms of positive elements. But we are not yet in a position to discuss this subject of the connexion of properties with molecular structure otherwise than in the most general way (s. Chap. xvii.). 332 In Chap. xn. we made a slight examination of some of the circumstances which condition the course and final results of a chemical change. We then arrived at the conception of chemical equilibrium as the result of direct and reverse pro- cesses of change occurring simultaneously in the changing system (comp. pars. 233 to 236). The general representation which the molecular and atomic theory puts before us of a system of substances free to act and react chemically for instance of a mixture in dilute aqueous solution of equivalent quantities of potash, soda, and nitric acid 330-333] APPLICATIONS OF MOLECULAR-ATOMIC THEORY. 229 is that of a great many small particles moving about freely, colliding and exchanging parts so as to produce new particles, which also collide with each other and with those of the original particles which remain unchanged, and some of which in so doing are decomposed with the re-formation of the original particles. This redistribution of atoms is accom- panied by a redistribution of energy ; energy is degraded in some of the molecular changes, and raised to a higher form in other parts of these changes, but the net result is a degrada- tion of a portion of the energy of the whole system. The strife of molecules proceeds until equilibrium results ; this equilibrium may be overthrown by introducing a fresh number of molecules of one of the original constituents of the system, or by altering the physical conditions under which the equili- brium was attained. In Chap. xn. we briefly examined some cases of chemical 333 change wherein less complex substances, one at least of which was a gas, were produced from a more complex substance, by the action of heat alone. These changes, which were classed together under the name dissociation, were found to be re- versible ; that is to say, the original more complex body was re-formed when the products of the change were allowed to ool in contact with each other. We learned that' there was a certain distribution of the changing substances at any specified temperature and pressure, but that change of tem- perature or pressure was attended with chemical change either in the direct or reverse direction. The explanation which the molecular and atomic theory gives of dissociation can only be indicated here. Dissociation is regarded by this theory as essentially a change of one kind of molecules into two or more kinds of simpler mole- cules, brought about by adding heat-energy to the system. The explanation is based on a deduction from the ' funda- mental assumptions of the theory, to the effect that there must be differences in the states of motion of individual mole- cules in a mass of gaseous molecules of oile kind. The kinetic energy of the molecules is made up of two parts \ the energy of the motion of the molecules as wholes, and the energy of the rotation of the parts of the molecules. Although the sum of these must be constant as long as temperature is un- changed, yet the distribution of the two motions, and hence of the two energies, may differ much, as regards the individual molecules. The energy due to the rotation of the parts of 230 ELEMENTAEY CHEMISTRY. [CHAP. XVI. some of the molecules may be so great, that collision between these molecules may cause them to separate into parts ; the energy due to the motion of other molecules as wholes may be so much greater than the energy due to the rotation of their parts that a considerable quantity of energy must be added to these molecules from a source external to the system before they separate into parts. When the system is heated, those molecules whose kinetic energy is chiefly due to the motion of rotation of their parts will be at once separated into parts; they will be dissociated. Some of the heat-energy added to the system will be used in increasing the motion of rotation of parts of other molecules, until these molecules also are dis- sociated. The process of dissociation will proceed rapidly for a time, but as the number of molecules which are not separated into parts becomes fewer so will the rate of dissociation be- come less, as temperature rises. But it will be possible for some parts of molecules to reunite and reproduce some of the original kinds of molecules, the rotational energy of which will not be greater than that which brings about the separa- tion of molecules into parts. Reunion of parts of molecules will therefore occur to some extent. If temperature is kept constant, the processes of separation of molecules into parts, and of recombination of parts of molecules, will proceed until both the kinetic energy of the system, and the atoms which form the molecules of the system, are so distributed that equilibrium results. 334 In par. 318 we learned that the specific gravity of gaseous nitrogen tetroxide at low temperatures is about half what it is at high temperatures. The most probable explanation of this fact, in terms of the molecular theory, is that gaseous nitrogen tetroxide has two molecular weights, one half as great as the other. The formulae N 2 4 and NO 2 express the atomic compositions of the two molecules. The action of heat on the molecules N 2 O 4 is to convert them into the mole- cules NO 2 ; the change is represented in an equation thus N 2 O 4 = 2NO 2 . But at any temperature between that at which only N 2 O 4 molecules exist and that at which only NO 2 mole- cules exist, the gas must be composed of a mixture of both N 2 O 4 and NO 2 molecules. If we adopt this explanation of the action of heat on nitrogen tetroxide, then it is evident that determinations of the spec, gravity of the gas at varying temperatures give data from which the relative number of molecules of each kind (N 2 O 4 and NO 2 ) at that temperature 333-336] APPLICATIONS OF MOLECULAR- ATOMIC THEORY. 231 can be calculated. The change from N 2 O 4 to NO 2 is a process of dissociation (s. pars. 233 to 236). The only other explanation of the change of spec, gravity 335 of nitrogen tetroxide is that which asserts that this gas does not even approximately obey the ordinary law of the expan- sion of gases by heat. This explanation obliges us to assume that the rate of expansion of gaseous nitrogen tetroxide as temperature rises differs widely from the normal rate of expansion of gases ; and also that the rate of expansion of nitrogen tetroxide itself is very different at different tempera- tures. To make this assumption is to go against the mass of evidence concerning the relations of the volumes of gases to temperature; whereas the assumption which is made in the statement of the molecular explanation of the facts is wholly in keeping with a large mass of evidence, and at the same time brings the apparently abnormal behaviour of nitrogen tetroxide within the number of those occurrences which find a simple explanation in terms of the molecular and atomic theory. Many other instances of so-called abnormal vapour-densities 336 have been observed. For instance, the gas obtained by heating sulphuric acid, H S0 4 , is about 24*5 times heavier than hydrogen. Now the molecular weight of a gaseous compound is twice the spec, gravity of that compound referred to hydrogen as unity ; therefore, reasoning only from deter- minations of the spec, gravity of the gas obtained by heating sulphuric acid, we should conclude that the molecular weight of gaseous sulphuric acid is approximately 49. But the simplest formula which can express the composition of a molecule of sulphuric acid is H 2 SO 4 = 98, if the atomic weights of sulphur and oxygen are 32 and 16 respectively. The density of the vapour of sulphuric acid seems then to be abnormal. As the definition of the molecular weight of a gas is deduced from the law of Avogadro, we begin to doubt this so-called law. But our doubts are laid when experiment proves that the gas obtained by heating sulphuric acid is a mixture of equal volumes of water-gas and sulphur trioxide. The specific gravity of such a mixture is calculated thus : the formulae H 2 O and S0 3 represent the compositions of molecules of gaseous water and gaseous sulphur trioxide, respectively ; but H 2 1 8, and S0 3 = 80 ; therefore the molecular weights of the two gases are 18 and 80, respectively; but the molecular weight of a gas is the weight of 2 volumes of that gas (comp. 232 ELEMENTARY CHEMISTRY. [CHAP. XVI. Chap. xv. par. 294) ; therefore a mixture of equal volumes of H 2 O and SO 3 is =24*5 times heavier than hydrogen. Therefore the observed specific gravity of the gas obtained by heating sulphuric acid is identical with the specific gravity calculated from the experimentally determined composition of this gas. Of\t*f 337 Much discussion at one time took place as to the specific gravity of phosphorus pentachloride in the gaseous state. Analyses of this compound, and determinations of the atomic weights of phosphorus (31) and chlorine (35 -5), shewed clearly that the simplest formula which could be given to the solid compound was PC1 5 . The specific gravity of this compound in the state of gas, referred to hydrogen as unity, must be 31 + (35*5 x 5) **g ' - = 104'25 ; or, referred to air as unity, the spec. gravity must be 7*2. The following table gives the observed spec, gravities (air=l) at different temperatures of the gas obtained by heating phosphorus pentachloride, PC1 5 . In each case the pressure was 760 mm. Temp. Eatio of g PClgtO ^p^le^ Eatio of Q PCL to SP v e f- totll Temp. gravity possible Eatio of Spec. ^ S ravit y possible PC1 3 PC1 3 PC1 3 182 5-08 42 230 4-30 68 289 3-69 96] 190 4-99 45 250 3-99 80 300 3-65 98 200 4-85 49 274 3-84 98 327 3-65 98 288 3-67 97 336 3-65 98 The specific gravity, and therefore probably the molecular weight, of the gaseous compound varies; but at no tempera- ture does the specific gravity approximate to that required by the formula PC1 6 . Here again we have an instance of so-called abnormal vapour-density. Further investigation how- ever shewed that the numbers are not really abnormal, but that they find a ready and simple explanation in terms of the molecular and atomic theory. Experiments proved that phos- phorus pentachloride is gradually dissociated by heat into phosphorus trichloride and chlorine; PC1 5 =PC1 3 +C1 2 . The gas obtained by heating PC1 5 is therefore a mixture of more than one substance. The specific gravity of a mixture of phosphorus 336-339] APPLICATIONS OF MOLECULAR- ATOMIC THEORY. 233 trichloride and chlorine in the ratio POL : C1 must be 3 2 PCI + Cl 5j 2 = 72- 125 if hydrogen is taken as unity, and 3 -6 if air is taken as unity. The specific gravity of the gas obtained by heating PC1 5 to 300 is practically identical with that of a mixture of PC1 3 and C1 2 ; the specific gravity at different temperatures lower than that at which dissociation is complete is that of a mixture of these two gases with some gaseous PC1 5 in varying proportions. The change of PC1 5 into PC1 3 and C1 2 , brought about by the action of heat, is therefore a normal process of dissociation. A portion of the PC1 5 is volatilised without change, but at the same time some of it is dissociated into the simpler molecules PC1 3 and C1 2 \ as temperature rises the quantity of unchanged PC1 5 decreases, and the quantities of PC1 3 and C1 2 increase, until at about 300 the whole of the PC1 5 is changed to PC1 3 + C1 2 . The numbers in the third columns of the table shew the ratio, at each temperature, of PC1 3 to the total PC1 3 possible as the asumption that the whole of the PC1 5 had been dissociated. There are other cases of so-called abnormal vapour-densities which are not quite so easily explained by the theory of atoms and molecules ; but we cannot discuss these here. The practical outcome of these facts as bearing on deter- 338 minations of molecular weights is, that the specific gravity of a gas must be constant through a considerable interval of temperature before we are justified in deducing an approxi- mate value for the molecular weight, from the observed specific gravity, of this gas. If the specific gravity varies considerably with temperature-changes, then the gas is probably a mixture ; but the expression molecular weight of a mixture has no meaning. In Chap. XL par. 175 we had examples of chemical changes 339 brought about by oxygen and hydrogen, respectively, when these elements were themselves products of one part of the complete cycles of change. Thus, when chlorine is passed into a warm solution of potash (KOH), potassium hypochlorite (KC1O) is formed, but this compound is quickly decomposed with production of potassium chloride (KC1) and oxygen ; if lead monoxide (PbO) is suspended in the warm potash solution, the oxygen, or a portion of the oxygen, produced from the decomposition of the hypochlorite combines with the lead monoxide to form dioxide (PbO 2 ). But if oxygen is passed into warm potash solution holding lead monoxide in suspension 234 ELEMENTARY CHEMISTRY. [CHAP. XVI. no lead dioxide is produced ; the oxygen must be produced by a chemical reaction in the system of which the body to be oxidised forms a part. Similarly, if hydrogen is produced, by the interaction of zinc and dilute sulphuric acid, in a solution containing sodium sulphite (ISTa 2 S0 3 ), hydrogen-sulphide (H 2 8) is produced ; but if hydrogen is passed into a solution of sodium sulphite hydrogen sulphide is not produced : the hydrogen must be produced by a chemical reaction in the system of which the sodium sulphite forms a part. In Chap. xiv. par. 266 we briefly considered the changes of energy which accompany such chemical changes as these. The molecular and atomic theory throws some light on these changes. This theory leads to the view that a system com- posed of atoms of a specified element, could such a system exist, would differ from a system composed of molecules of the same element. It also leads to the view that in very many, if not most, chemical changes, the formation of the molecules of the products of the change is preceded by the breaking up into atoms, or sometimes into groups of atoms, of the mole- cules of the interacting substances. And, lastly, the theory almost obliges us to believe that a system composed of atoms of one of those elements the molecules of which are built up of more than a single atom (s. table in par. 317), if it could exist would be extremely unstable, and would almost at once pass into a system composed of molecules of the same element. The application of these conceptions to the class of changes we are considering affords some explanation of the mechanism of these changes. The explanation may be stated as follows. Under ordinary conditions quantities of oxygen or hydrogen consist of molecules of these gases. Oxygen passed into potash holding lead monoxide in suspension does not oxidise the lead oxide, because the affinity between molecules of oxygen and lead monoxide is not sufficient to produce this change, and there is not sufficient energy available in the system for separating the molecules of oxygen into atoms : but when potash and chlorine interact, atom of oxygen are produced ; these atoms combine with the molecules of lead monoxide to form lead dioxide, and in this change more energy is degraded than would be the case if the atoms of oxygen had combined with each other to form molecules of oxygen. A similar explanation would be given of the interaction between sodium sulphite and the atoms of hydrogen produced by the interaction of zinc and sulphuric acid. 339-340] APPLICATIONS OF MOLECULAR- ATOMIC THEORY. 235 Chemical changes which are brought about by elements only when these are themselves products of a part of the complete change are sometimes classed together as nascent actions. The name has been useful as marking a class of reactions which have a common feature. If the view here taken of these reactions is correct, there is nothing in any way abnormal about them ; they belong to the ordinary type of chemical change. It is evident then that the molecular and atomic theory 340 brings into one point of view, arid gives fairly simple explana- tions of, many classes of chemical occurrences. It also indicates directions in which experiments ought to proceed with the object of discovering and explaining new classes of chemical events. The words law, hypothesis, and theory, have been frequently used in this book. The word law has sometimes been employed as synonymous with a general truth ; for instance the laws of chemical combination are general truths, they summarise many facts. The same term, law, is sometimes used as meaning an abstract truth ; for instance, Newton's laws of motion are truths involved in many phenomena although actually seen in none. The statement ' equal volumes of gases contain equal numbers of mole- cules ' has been called a laic. This statement is really a deduction from a theory. The deduction has a definite meaning when the terms in which it is made are defined, but this can be done only by granting the funda- mental assumptions of the theory. The 'law' stands or falls with the theory. The molecular and atomic theory, like all scientific theories, is based on certain assumptions. The fewer, the simpler, and the more binding, the assumptions on which a theory rests the better is the theory. One of the marks of a satisfactory theory is the impossibility of escaping from discrepancies between observed facts and deductions from the theory by the invention of subsidiary hypotheses which do not follow directly from the assumptions on which the theory rests. The molecular and atomic theory, it must be confessed, has been too elastic in this respect. An hypothesis is specially framed to explain a definite occurrence, or a series of occurrences. For instance, when Davy found that nitric acid was formed at the positive electrode during the electrolysis of water, he framed the hypothesis that the air surrounding the decomposing water was the source of the acid : he was able to prove by direct experiment that this hypothesis was correct. An hypothesis is sometimes stated in very general terms, and is used to explain a great many apparently unconnected facts. For instance, very many facts concerning chemical change are generalised in the hypothesis that 'the amount of a chemical change is proportional to the affinities and the active masses of the substances taking part in the change'. A direct and final experimental proof of such an hypothesis as this can scarcely be given. If the terms can be accurately defined, and if after prolonged inquiry no facts are discovered which negative the hypothesis, it is adopted as a trustworthy guide. CHAPTER XVII. ISOMERISM AND STRUCTURAL FORMULAE. 341 IN the last chapter we had an instance of isomerism ; name- ly, the existence of three different compounds all having the molecular composition expressed by the formula C 5 H 12 . The prominent fact of isomerism is, that two or more compounds sometimes exist having identical compositions, and identical specific gravities in the state of gases, and yet ex- hibiting different properties. The statement of this fact in the language of the molecular and atomic theory is, that two or more gaseous molecules may exist composed of the same number of the same atoms, and yet differing from each other in their properties. 342 An instance of isomerism is furnished by the existence of two compounds having the composition C 2 H 6 O. One of these is ethylic alcohol, the other is inethylic ether. Ethylic alcohol inter- acts with potassium or sodium thus, C 2 H 6 O + K = C 2 H 5 KO + H ; methylic ether and potassium (or sodium) do not interact. Phosphorus pentachloride interacts with both isomerides ; the interactions are these : (1) alcohol; C 2 H 6 O + PC1 5 = C 2 H 5 C1 + POC1 3 + HC1. (2) ether; C 2 H 6 O + PC1 6 = CH 3 C1 + CH 3 C1 + POC1 3 . The alcohol is easily oxidised, first to aldehyde C 2 H 4 O, then to acetic acid C 2 H 4 O 2 ; the ether is not easily oxidised. Ethylic alcohol is a colourless, volatile, liquid, boiling at 78 0< 3 ; methylic ether is a colourless gas which may be condensed by cold to a liquid boiling at - 21. Another instance of isomerism is furnished by the existence of four hydrocarbons having the molecular composition C 8 H 10 . These bodies are all liquids, boiling at 134, 136 137, 341345] ISOMERISM AND STRUCTURAL FORMULAE. 237 137 137-5, and 140 141, respectively. That which boils at 134 is easily oxidised to an acid having the composition C 7 H 6 2 . The other three are oxidised to three different acids all having the composition C 8 H 8 O 2 . The conditions under which these three hydrocarbons are oxidised vary somewhat. The four hydrocarbons C 8 H 10 are evidently very similar in 343 their chemical properties ; the two compounds C 2 H 6 O are less closely related to each other. Compounds which have the same composition and the same molecular weight, but which shew differences in their chemical properties so decided as to require them to be placed in different classes, are sometimes said to be metameric. Metamerism is included in the wider term isomerism. The molecular and atomic theory endeavours to explain 344 isomerism by saying that the properties of molecules depend, among other conditions, on the arrangement of their parts. Is this assertion justified by facts 1 Before we can profitably attempt an answer to this question we must understand what is meant by the phrase ' arrange- ment of the parts of a molecule '. In Chap. xni. par. 247 a very brief account was given of the 345 use of the expression 'equivalent weights of two alkalis'. We must now look more fully at the notion of chemical equiva- lency. 88-8 parts by weight of potash (KOH), 63-5 parts by weight of soda (NaOH), and 38-1 parts by weight of lithia (LiOH), severally neutralise 100 parts by weight of nitric acid. These masses, 88*8, 63*5, 38-1, of the three alkalis are therefore equivalent as regards power of neutralising a specified mass of nitric acid, inasmuch as these masses are of equal value in exchange. 100 parts by weight of sulphuric acid are neutralised by 114-3 parts by weight of potash, or by 81-6 parts by weight of soda, or by 49 parts by weight of lithia. These numbers, 114-3, 81-6, and 49, represent masses of the three alkalis which are equivalent as regards power of neutralising a specified mass of sulphuric acid. Now the ratio 88-8 : 63-5 : 38'1 is the same as the ratio 114-3 : 81-6 : 49. If the masses of these three alkalis required to neutralise 100 parts by weight of each of several acids are determined, it is found that these masses always bear the same ratio to one another. Hence it is possible to assign values to these three alkalis representing those masses of them which are equivalent as regards power of neutralising one and the same 238 ELEMENTARY CHEMISTRY. [CHAP. XVII. mass of any specified acid. Similarly, it is possible to assign values to the acids which shall represent those masses of them which severally neutralise one and the same mass of any specified alkali. In determining the equivalent weights of the alkalis it is customary to take one reacting weight (or we may say one molecule) of hydrochloric acid as the unit mass of standard acid. The reacting weight of hydrochloric acid (HC1) is 3 6 '5 : one reacting weight of this acid is neutralised by (in round numbers) 56 parts by weight of potash, 40 of soda, and 24 of lithia, respectively. The mass of sulphuric acid which is neutralised by each of these masses of potash, soda, or lithia is 49 ; the mass of chloric acid is 84*5 ; the mass of ortho- phosphoric acid is 32-6; the mass of metaphosphoric acid is 80 ; the mass of pyrophosphoric acid is 44 '5 ; &c. &c. The numbers 36-5, 49, 84-5, 32-6, 80, 44-5 represent masses of the acids mentioned which are equivalent as regards power of neutralising 56 parts by weight of potash, or 40 parts of soda, or 24 of lithia. 346 The notion of equivalency may be extended to the ele- ments. If we determine the masses of a series of metals which severally combine with 16 parts by weight of oxygen, we shall have determined the equivalent weights of these metals as regards this particular reaction. Or we might cause a number of metals to interact with hydrochloric acid, and determine the mass of each metal which thus produced 1 gram of hydrogen ; these masses would represent equivalent weights of the metals as regards this particular reaction. 347 When therefore we speak of the equivalent weight of an element or compound there is always implied a comparison of the specified substance with some other substance as regards power of performing a definite chemical operation. Equiva- lent weights represent quantities of elements or compounds which can be exchanged in some specified chemical process. 348 The expression equivalent weight of an element is frequently used somewhat loosely. In order to determine equivalent weights, elements are generally compared as regards their combination with oxygen, and 8 parts by weight is usually chosen as the standard mass of oxygen. Hence the equivalent weight of an element generally means the mass of it which combines with 8 parts by weight of oxygen. In the cases of elements which do not combine with oxygen, hydrogen is 345351] ISOMERISM AND STRUCTURAL FORMULAE. 239 generally chosen as the standard element, and the equivalent weight is taken to be that mass of the element which combines with 1 part by weight of hydrogen, or sometimes (especially in the case of metals) that mass of the element which interacts with a dilute acid to produce 1 part by weight of hydrogen. Let us apply these various definitions of equivalent weight to the metal tin. Experiment proves ; (1) that 59 parts by weight of tin combine with 8 parts by weight of oxygen to produce stannous oxide; (2) that 29 '5 parts by weight of the same element combine with 8 parts by weight of oxygen to produce stannic oxide ; (3) that 59 parts by weight of tin interact with hydrochloric acid to produce 1 part by weight of hydrogen, stannous chloride being formed at the same time. We therefore get two values for the equivalent weight of 349 tin. But had we accurately denned the- meaning to be given to the term equivalent weight we should have got over this difficulty. Let tin and lead be compared as regards the formation of oxides having similar properties and similar compositions. Each metal forms a protoxide MO, and a dioxide MO 2 ; the oxides MO are fairly similar chemically, and so are the oxides MO 2 . Experiment shews that 59 parts by weight of tin are equivalent to 103-5 parts by weight of lead as regards power of combining with 8 parts by weight of oxygen to produce oxides belonging to the type MO; and that 29-5 parts by weight of tin are equivalent to 51-75 parts by weight of lead as regards power of combining with 8 parts by weight of oxygen to produce oxides of the type M0 2 . The notion of the equivalency of elements is fairly simple ; 350 but it is often very difficult to apply it accurately. The difficulty consists in finding a standard chemical change where- in a specified mass of one element may be exchanged for a specified mass of another without altering the essential character of the reaction. The conception of equivalency has been extended to the 351 elementary atoms. Let the standard action be ability to combine with one, and only one, atom of hydrogen to produce a gaseous molecule ; let all atoms which do this be classed together as equivalent. Then the atoms of hydrogen, chlorine, bromine, iodine, and pro- bably fluorine*, are equivalent; the evidence is the existence of the gaseous molecules H 2 , HC1, HBr, HI, [and HF]*; and * There is still some doubt whether the molecule of gaseous hydrogen fluoride is HF or H 3 F 2 . 240 ELEMENTAEY CHEMISTRY. [CHAP. XVII. the non-existence of gaseous molecules composed of one atom of chlorine, bromine, &c. and more than a single atom of hydrogen. 352 The atoms of hydrogen, chlorine, bromine, iodine and fluorine are placed together in one class, and are called mono- valent 'atoms. 353 I* * s ey ident that in asserting these atoms to be equivalent, we have relaxed the strict meaning of the term equivalency. An atom of hydrogen, or an atom of chlorine, or an atom of bromine, tfec. combines with only one atom of hydrogen to produce a gaseous molecule ; in this respect the atoms are of equal value in exchange. In defining the meaning of the terms divalent, trivalent, &c. atoms, we must a little further relax the definition of equivalency. We assume that an atom which combines with -2 atoms, and not more than 2 atoms, of chlorine, &c. is equivalent to an atom which combines with not more than 2 atoms of hydrogen, &c. By doing this we arrive at the definition of divalent atoms as atoms which combine with not more than two monovalent atoms to form gaseous molecules. 354. Applying these definitions of monovalent, divalent, &c. atoms to all the elements compounds of which with hydrogen, chlorine, bromine, iodine, or fluorine, have been gasified, we arrive at the following classification of atoms. Standard Monovalent atoms ; H, F, 01, Br, I. I. Monovalent atoms ; i.e. atoms which combine with one standard monovalent atom to form gaseous molecules K, Tl,Hg. II. Divalent atoms ; i.e. atoms which combine with two standard monovalent atoms to form gaseous molecules O, S, Se, Te, Be, Cd, Zn, Hg, Sn, Pb. III. Trivalent atoms; i.e. atoms which combine with three standard monovalent atoms to form gaseous molecules... ...B, N, P, As, Sb, Bi, In. IV. Tetravalent atoms; i.e. atoms which combine with four standard monovalent atoms to form gaseous molecules 0, Si, Ti, Ge, Zr, Y, Sn, Th, U. Y. Pentavalent atoms; i.e. atoms which combine with five standard monovalent atoms to form gaseous molecules P, Nb, Ta, Mo, W. YI. Hexavalent atoms ; i.e. atoms which combine with six standard monovalent atoms to form gaseous molecules W. 4 , 351357] ISOMERISM AND STRUCTURAL FORMULAE. 241 This table includes about half the elements ; the valencies of the atoms of the other elements cannot yet be determined for want of data. The data on which this classification of atoms is based are presented in the following list of gaseous molecules : KI, T1C1, HgCl; OH 25 OC1 2 , SH 2 , SeH 2 , TeH 2 , BeCl 2 , BeBr 2 , CdBr 2 , ZnCl 2 , HgCl 2 , HgBr 2 , HgI 2 , SnCl 2 , PbCl 2 ; BF 3 , BC1 3 , BBr 3 , NH 3 , PH 3 , POL, AsH 3 , AsCL, AsI 3 , SbCL, SbI 33 BiCl 3 , In01 8 ; CH 4 , CC1 4 , SiF 4 , SiCl 4J SiL, GeCl, GeI 4 TiCl 4 , ZrCl 4 , VC1 4 , SnCl 4) SnBr 4 , ThCl 4 , UBr 4 , UC1 4 ; PF 5 NbCl 5 , TaCl 8 , MoCl 55 WC1 5 ; WCl e . Of the 35 elements classified in the table, four viz. P, Sn, 355 W, and Hg, are found each in two classes. The atom of P is trivalent and pentavalent ; the atom of Sn is di- and tetra- valent ; that of W is penta- and hexa-valent ; and that of Hg is mono- and di-valent. As we found that some elements have more than one equivalent weight, so now we find that the atoms of certain elements are sometimes equivalent to one number, and sometimes to another number, of monovalent atoms. In determinations both of equivalent weights and of the equivalency of atoms, the conception of equivalency is rather vaguely used. We may for the present define the maximum valency of an 356 atom to be, the maximum number of atoms of hydrogen, fluorine, chlorine, bromine, or iodine, with which the specified atom com- bines to form a gaseous molecule. This definition indicates the data which must be obtained before the maximum valency of an atom can be determined. To say that a specified atom is divalent has generally been 357 regarded as synonymous with saying that the atom in question is equivalent to 2 atoms of hydrogen, fluorine, chlorine, bromine, or iodine ; and that, therefore, any atom which combines with one divalent atom is thereby .proved to be itself a divalent atom. Thus, the existence of the gaseous molecules OH 2 and OC1 2 proves the atom of oxygen to be divalent : one atom of carbon combines with one atom of oxygen to form the gaseous molecule CO, and with 2 atoms of oxygen to form the gaseous molecule C0 2 ; hence, it is argued, the atom of carbon is di- valent and tetravalent. Of late years many chemists have abandoned such arguments as this. They have recognised the possibility of determining the maximum valencies of the atoms of elements which form gasifiable compounds with hydrogen, fluorine, chlorine, bromine, or iodine, and of such elements M. E. C. 16 ELEMENTARY CHEMISTRY. [CHAP. XVII. only. They have been content, for the present, to admit that a divalent atom is not necessarily strictly equivalent i.e. of equal value in performing a chemical change to 2 mono- valent atoms, a trivalent atom to 3 monovalent atoms, a tetra- valent atom to 4 monovalent or to 2 divalent atoms, &c. s. also par. 372. 358 Let us now see whether the conception we have gained of valency enables us to give a definite meaning to the phrase * arrangement of the parts of a molecule ' used in par. 344. When it is said that the atom of carbon is tetravalent because the gaseous molecules CH 4 , CC1 4 , CHC1 3 , &c. exist, it is asserted that each of these molecules is formed by the union of one atom of carbon with 4 monovalent atoms. This assertion means that in each of these molecules there is direct interaction of some kind between the atom of carbon and each of the 4 monovalent atoms. What the nature of this inter- action is, we do not know. The molecular formulae CH 4 , CC1 4 , &c. do not prove the existence of this direct interaction ; there may be direct interaction between the atom of carbon and only one, two, or three, of the monovalent atoms; but the hypothesis of direct interaction between the atom of carbon and each of the 4 monovalent atoms is the simplest and most workable hypothesis that has been tried. Adopting this hypothesis, let us indicate the existence of some kind of direct interaction between each monovalent atom and the atom of carbon by the symbols H 01 H I I I H C H, 01 C 01, H C H. I I I H 01 01 It is of the greatest importance that the student should clearly understand what is meant to be conveyed by such symbols as these. The symbols F 01 01 01 01 I \/ V F P F, 01 W 01, 01 W 01, Br Cd Br, /\ I A F F 01 01 01 represent the atomic compositions of certain gaseous molecules; and they assert that there is direct interaction between the 357361] ISOMERISM AND STRUCTURAL FORMULAE. 243 atom of phosphorus and each atom of fluorine, between the atom of tungsten and each atom of chlorine, and between the atom of cadmium and each atom of bromine, in the respective molecules. Now, as no gaseous molecule has been obtained composed of one atom of carbon and more than 4 atoms of hydrogen, fluorine, chlorine, bromine, or iodine; as no gaseous molecule has been obtained composed of one atom of phosphorus and more than 5 atoms of hydrogen, fluorine, &c. ; as no gaseous molecule has been obtained composed of one atom of tungsten and more than 6 atoms of chlorine, &c.; and as no gaseous molecule has been obtained composed of one atom of cadmium and more than 2 atoms of chlorine, &c. ; we seem justified in asserting that an atom of carbon can directly interact with not more than 4 monovalent atoms, an atom of phosphorus with not more than 5 monovalent atoms, an atom of tungsten with not more than 6 monovalent atoms, and that an atom of cadmium can directly interact with not more than 2 mono- valent atoms, in gaseous molecules. We thus arrive at the notion of a limit to the number of 359 atoms between which direct interaction can occur in a gaseous molecule. If we now apply this notion not only to gaseous molecules composed of one polyvalent atom united with monovalent atoms, but to all gaseous molecules, we find ourselves in pos- session of a good working hypothesis regarding the arrange- ment of atoms in molecules. The hypothesis may be stated thus; each atom in a gaseous molecule can directly interact with a limited number of other atoms. Let us at once widen our conception of atomic valency, 360 and say that the valency of an atom is a number expressing the maximum number of other atoms between which and the given atom there is direct interaction in any gaseous molecule. But while thus widening the meaning of the term valency, let us agree to determine the valency of any atom by finding the maximum number of monovalent atoms with which it directly interacts, i.e. in this case with which it combines, in a gaseous molecule. We have thus a perfectly definite method for finding the valencies of atoms, and we have also a wide range of applica- tion for these values. It is customary to place small roman numerals above the 361 162 244 ELEMENTARY CHEMISTRY. [CHAP. XVII. symbol of an element to represent the valency of an atom of that element ; thus O n , C IV , Bi m , W VI , mean a divalent atom of oxygen, a tetravalent atom of carbon, a trivalent atom of bismuth, a hexavalent atom of tungsten, respectively. When a numeral is not placed over the symbol of an atom that atom is taken to be monovalent. 362 The table in par. 354 gives the results of the determina- tions of the valencies of atoms ; the applications of these values will now be shewn by one or two examples. Two compounds exist each having the composition expressed by the formula C 2 H 6 O; as both compounds have been gasified, this formula represents the atomic composition of the molecule of either compound. From the data given in the table in par. 354 it is evident that the atom of carbon is tetravalent, and the atom of oxygen is divalent ; the atom of hydrogen is one of the standard monovalent atoms. In other words an atom of carbon can directly interact with not more than 4 other atoms, an atom of oxygen can directly interact with not more than 2 other atoms, and an atom of hydrogen can directly interact with not more than one other atom, in a gaseous molecule. We have agreed to represent direct interaction between two atoms in a gaseous molecule by the use of lines proceeding from the symbols of these atoms. How then can ' we represent the arrangement of one divalent 2 tetra- and 6 monovalent atoms 1 Let us assume (1) that each carbon atom directly interacts with the other carbon atom ; (2) that neither carbon atom directly interacts with the other carbon atom. On assumption (1) the only possible representation of the atomic arrangement of the molecule C 2 IV H 6 O n is H H I I H C C O H; I I H H we shall call this compound I. On assumption (2) the H H I I only possible representation is H C O C H ; we H H shall call this compound II. 361362] ISOMERISM AND STRUCTURAL FORMULAE. 245 Now two compounds, and only two, exist having each the molecular composition C 2 H 6 O. So far the facts are in keeping with the deduction from the hypothesis of valency. But how can we tell which of the two compounds is I. and which is II. 1 If the chemical properties of a molecule depend, partly, on the arrangement of the atoms which constitute the molecule, the chemical properties of compound I. must differ from those of compound II. To determine which compound is represented by formula I. and which by formula II., we must study the chemical properties of each isomeride C 2 H 6 O. The isomerides in question are called ethylic alcohol, and methylic ether. Each interacts with phosphorus pentachloride, but the products of the interactions are very different : ethylic alcohol interacts thus, C 2 H 6 O + PC1 5 = POC1 3 + HC1 + C 2 H 5 C1 ; methylic ether interacts thus, C 2 H 6 O + PC1 5 = POC1 3 + 2CH 8 C1. The first interaction consists in the withdrawal of an atom of oxygen and an atom of hydrogen from the molecule C 2 H 6 O, and the putting in the place of these atoms of an atom of chlorine; the second interaction consists in the withdrawal of an atom of oxygen from the molecule C 2 H 6 O, the putting in the place of this atom of 2 atoms of chlorine, and the simultaneous separation of the group of atoms C 2 H fi Cl 2 into two parts, each of which is composed of one atom of carbon united with 3 atoms of hydrogen and one atom of chlorine. Now symbol I. represents an atom of oxygen as directly interacting with an atom of carbon and also with an atom of hydrogen ; as the atom of chlorine can directly interact with a single other atom only, the with- drawal of the group of atoms OH, and the substitution for it of the atom 01, seems a very probable change. If this change proceeds the resulting molecule will be represented H H I ! by the symbol H C C 01. On the other hand symbol i . l H H II. represents an atom of oxygen as directly interacting with 2 atoms of carbon, and with these atoms only ; if this atom of oxygen were withdrawn and 2 atoms of chlorine put in its 246 ELEMENTARY CHEMISTRY. [CHAP. XVII. H H I I place we should get the molecule H C 01 Cl C H; H H but this molecule cannot exist (by hypothesis) because the chlorine atom is monovalent; hence the substitution of two atoms of chlorine for a single atom of oxygen in the molecule represented by symbol II. must result in the production of two H I molecules each represented by the symbol H C Cl. H We therefore conclude that the molecule of ethylic alcohol H H I I is represented by the symbol H C C O H, and the I I H H molecule of methylic ether by the symbol H ' H I I H C O C H. I ! H H This conclusion is borne out by a further study of the pro- perties of the two isomerides. Thus, sodium rapidly interacts with ethylic alcohol to produce C 2 H 5 NaO -f- H ; but sodium and methylic ether do not interact. The formula given for ethylic alcohol represents one, and only one, of the 6 hydrogen atoms as directly interacting with an atom of oxygen; we should therefore expect that sodium would replace either 5 atoms, or one atom, of hydrogen from the molecule of ethylic alcohol. As sodium replaces only a single atom of hydrogen we conclude that the atom replaced is that which is represented in the formula as directly interacting with an atom of oxygen. But if this is so, we should conclude that none of the hydrogen atoms in the molecule of methylic oxide would be replaceable by sodium. Another reaction which favours this conclusion regarding the interaction of sodium 362364] ISOMERISM AND STRUCTURAL FORMULAE. 247 with ethylic alcohol is that which occurs between sodium and water. This change is formulated thus H 2 O + Na = NaOH + H. Now the only possible way of representing the molecule H 3 n is H - O - H. The interaction between this molecule and an atom of sodium must be represented thus, the atom of hydrogen which is replaced by an atom of sodium must be represented as directly interacting with an atom of oxygen. Therefore, as only one hydrogen atom in the molecule of ethylic alcohol is represented as interacting directly with an oxygen atom, it is fairly probable that this is the atom of hydrogen which is replaced by sodium. Let us take another example of the application of the 363 conception of atomic valency, that is that each atom in a gaseous molecule can directly interact with a limited number of other atoms. A certain hydrocarbon has the molecular composition C 2 H 6 . Can more than one compound exist having this composition 1 In other words, can we represent the arrangement of the atoms C 2 IV H 6 in more than one way 1 As the atom of carbon is tetravalent, and that of hydrogen is monovalent, we must represent the two atoms of carbon in the gaseous molecule C 2 H 6 as directly interacting; the only possible way of doing this is to write the formula thus, H H I I H C C H. I H H Therefore the hypothesis of valency, when applied to the compound C 2 H 6 , asserts that one and only one compound having this molecular composition can exist. As a matter of fact only one compound C 2 H 6 does exist. There is another hydrocarbon C 2 H 4 ; what are the ways 364 in which 2 tetra- and 4 mono-valent atoms can be arranged *? Each carbon atom in the molecule C 2 H 4 must interact with another carbon atom ; we may write the formula as H H H I I I (1) C C, or (2) H C C H. I I H H H 248 ELEMENTARY CHEMISTRY. [CHAP. XVII. Formula (1) represents each carbon atom in the molecule C 2 H 4 as directly interacting with another carbon atom and with 2 hydrogen atoms, i.e. as directly interacting with 3 other atoms. Formula (2) represents one of the carbon atoms as directly interacting with 2 other atoms, and the other carbon atom as directly interacting with 4 other atoms. In par. 360 we defined the valency of an atom to be the number expressing the maximum number of other atoms between which and the given atom there is direct interaction in any gaseous molecule. In accordance with this definition, we may say that formula (1) represents each atom of carbon as trivalent in the molecule C 2 H 4 , and formula (2) represents one atom of carbon as divalent, and one atom of carbon as tetravalent, in the molecule C 2 H 4 . As it is impossible to represent both atoms of carbon as tetravalent, i.e. as directly interacting with 4 other atoms, in the molecule C 2 H 4 , it is evident that although the maximum valency of a carbon atom is 4, yet the actual valency of an atom of this element in a specified molecule may be less than 4. The student should particularly notice that the statement, an atom of carbon may act in a specified molecule as a trivalent or divalent atom, only holds good when we attach to the term valency of an atom the meaning given in par. 360. The hypothesis of valency then points to the possible existence of two isomerides 2 H 4 . But only one compound C 2 H 4 is known to exist. Which of the two formulae given above shall we assign to this compound 1 ? The compound in question is called ethylene. Ethylene readily combines with chlorine to form the dichloride C 2 H 4 C1 2 . If the formula of H H I I ethylene is C C, the formula of the dichloride is almost i i H H H H I I certainly Cl C C Cl ; if the formula of ethylene is l i H H H I H C C H, the formula of the dichloride is almost i H 364] ISOMERISM AND STRUCTURAL FORMULAE. 249 Cl H I I certainly H C C H. Only two formulae are possible I I Cl H H H I for the molecule C 2 H 4 C1 2 ; the formation of Cl C Cl H H Cl H I I is much more likely than the formation of H C C H I I 01 H H H I I from C C : because, as the maximum valency of a I I H H carbon atom is 4, and as each carbon atom in the molecule H H I I C C is represented as trivalent, it is only necessary I I H H for each carbon atom to interact directly with one of the chlorine atoms brought into contact with the molecule C 2 H 4 H H I- in order to produce the molecule Cl C C Cl ; but if i H H 01 H I the molecule H C C H is produced a considerable ! I Cl H rearrangement of the interactions of the atoms of carbon and hydrogen must occur. The only safe rule to adopt in studying 250 ELEMENTARY CHEMISTRY. [CHAP. XVII. the applications of valency to isomerism is, that no rearrange- ment of the interactions of atoms must be assumed to take place unless the facts absolutely require it. Two compounds having the molecular composition C 2 H 4 C1 2 exist : one is formed by the direct addition of chlorine to ethylene, it is called ethylene chloride; the other is produced by the interaction of chlorine with the hydrocarbon ethane C 2 H 6 , [thus, C 2 H 6 +2C1 2 = C 2 H 4 C1 2 + 2HC1], it is called ethy- lidene chloride. To which of these compounds must we 01 H I I assign the formula H C C H? Ethylidene chloride is 01 H also produced by the interaction of phosphorus pentachloride with ethylic aldehyde; thus C 2 H 4 O + PC1 5 = C 2 H 4 C1 2 + POC1 3 . In this reaction one atom of oxygen has been removed from the molecule C 2 H 4 O and 2 atoms of chlorine have been put in its place ; therefore, unless distinct reasons can be shewn to the contrary, it is likely that the 2 atoms of chlorine in the molecule C 2 H 4 C1 2 are related to the rest of the molecule in a way similar to that in which the atom of oxygen in the molecule C 2 H 4 O is related to the rest of the molecule. We shall now assume that the formula of ethylic aldehyde is H O I I H C C. This formula rests on a large number of I i H H reactions; there is very little doubt as to its correctness. Now the replacement of the atom of oxygen in this mole- cule by 2 atoms of chlorine will produce the molecule* H 01 I I H C 01. But the compound produced is ethy- I I H H lidene chloride; hence the formula of ethylidene chloride is * Compare this interaction of PC1 5 and C 2 H 4 with that of PC1 5 and C 2 H 6 (methylic ether) given in par. 362. 364365] ISOMERISM AND STRUCTURAL FORMULAE. 251 H Cl I I H C C Cl ; and hence the formula of the isomeric I ! H H H H I I ethylene chloride is 01 C C Cl, because these are the I I H H only possible formulae for the molecule C 2 H 4 C1 2 . But ethy- lene chloride is produced by the direct addition of chlorine to H H I I ethylene ; hence the formula of ethylene is C C. I H H Two compounds having the molecular composition C 2 H 4 365 may exist according to the hypothesis of valency : only one actually exists; but derivatives of both, e.g. chlorides, are known. It is possible that the compound the molecule of H I which would have the composition H C C H may be H produced; but it is not likely, because the many attempts made to form it have all resulted in the production of the H H I I isomeric compound C C. I H H This is an illustration of the proposition, that it is not always possible to obtain every one of the isomerides of a given composition the existence of which is indicated by the hypothesis of valency ; or, in other words, that the existence of a compound of specified composition is not conditioned solely by the valencies of the atoms which form the molecule of this compound. 252 ELEMENTARY CHEMISTRY. [CHAP. XVII. 366 The hypothesis of valency leads to the conception of the molecule as a structure, the parts of which are related to each other in a definite manner. Formulae such as those given in the preceding paragraphs for ethylic alcohol, methylic ether, ethylene chloride, and ethy- lidene chloride, are called rational or structural formulae ; they are contrasted with empirical formulae (C 2 H 6 O and C 2 H 4 C1 2 ) which express the percentage and atomic composition of molecules. Structural formulae are attempts to summarise the chief reactions of formation and decomposition of compounds in the highly symbolical language of a special hypothesis resulting from the application of the molecular and atomic theory to the chemical phenomena of isomerism. A structural formula may be found for any gasifiable compound the molecule of which is composed of atoms of known valencies ; but the structural formula to be of any value must be the outcome of many experiments on the interactions of the compound to which it is given. The value of the formula consists in its suggestiveness of reactions, and in the extent to which it exhibits the analogies between the com- pound formulated and other compounds. The structural formulae of carbon compounds have been greatly developed. There can be no doubt that the chemistry of these compounds would not have advanced as it has done without the aid of structural formulae ; indeed the remarkable predictions which have been made, and verified, regarding classes of chemical changes among carbon compounds afford satisfactory evidence that the conceptions on which structural formulae are based are accurate and well founded. 367 It is generally possible to shew that the characteristic properties of a group of similar carbon compounds are con- nected with a certain arrangement of some of the atoms in the molecules of these compounds, which arrangement is common to all the members of the group, and can be expressed in a structural formula. Thus, a great many alcohols behave similarly when oxidised ; the molecule of each loses 2 atoms of hydrogen thereby producing an aldehyde, and this aldehyde is then oxidised to an acid the molecule of which is composed of the same number of carbon atoms as the molecule of the alcohol. Such alcohols are called primary alcohols. The following formulae give examples of the oxidation of primary alcohols. 366367] ISOMEEISM AND STRUCTURAL FORMULAE. 253 Primary alcohol. Oxidation-products. (1) Aldehyde (2) Acid. Ethylic C 2 H 6 C 2 H 4 C 2 H 4 O 2 Propylic C 3 H 8 O C 3 H 6 O C 3 H 6 O 2 Butylic C 4 H ]0 O C 4 HO OHO Allylic C 3 H 6 C 3 H 4 C 3 H 4 O 2 . The study of the reactions of these, and of other, primary alcohols has led to structural formulae in all of which the group H I of atoms C O H appears. I H Thus the structural formulae of the 4 alcohols in the above table are these ; CH 3 .CH 2 .OH; CH 3 .CH 2 .CH 2 .OH; CH 3 .CH 2 .CH 2 .OH 2 .OH; and CH 2 . CH . CH 2 . OH ; respectively. [These formulae are shorter than, but have exactly the same meanings as, the more developed formulae ; H H H H H II III H C C H, H C C C H, II III H H H H H H H H H H H I I I I I I H C C C C H, andC C C O H.1 I I i I III H H H H H H H A primary alcohol is sometimes defined as an alcohol the molecule of which is composed of atoms of carbon and hydrogen in union with the atomic group CH 2 . OH ; or it is sometimes said that the molecules of all primary alcohols contain the group of atoms CH 2 . OH. By these statements we under- stand that the study of the chemical changes undergone by those alcohols which are classed together as primary, because of their behaviour on oxidation, has led to structural formulae which represent the molecules of these alcohols as always containing at least one atom of carbon directly interacting with 2 atoms of hydrogen and one of oxygen, which atom of oxygen also directly interacts with an atom of hydrogen. The examination of the aldehydes obtained by oxidising 254 ELEMENTARY CHEMISTRY. [CHAP. XVII. the primary alcohols has shewn that the best structural formulae which can be assigned to these aldehydes always * contain the group ' C ^ _ [ CHO ] ; and the structural formulae given to the acids obtained by oxidising these aldehydes always ' contain the group ' C <^ _ [CO . OH]. For instance, the oxidation of ethylic alcohol to aldehyde and then to acetic acid, is represented thus in structural formulae ; (1) CH 3 .CH 2 .OH + = CH 3 .CHO + H 2 0. alcohol aldehyde (2) CH 3 . CHO + O = CH 3 . CO . OH. acetic acid. 368 Now suppose a new alcohol is discovered; analyses, and determinations of the spec, gravity of the gaseous alcohol, enable an empirical formula to be given to it. The behaviour of the alcohol on oxidation is now examined ; it is found to lose 2 atoms of hydrogen per molecule and to form an aldehyde, one molecule of which then combines with an atom of oxygen and forms an acid. The new alcohol therefore belongs to the class of primary alcohols. The structural formula of the alcohol will therefore, very probably, 'contain the group' CH 2 . OH. The reactions of the alcohol are studied, and if possible a structural formula is found for it which represents the molecule as containing the atomic group CH 2 . OH. This formula tells a great deal about the alcohol; for instance it suggests the formulae, and therefore many of the reactions, of the aldehyde and acid which are produced by oxidising the alcohol. In applying structural formulae in this way it is always to be remembered that a compound may be produced which exhibits most of the class-marks of a certain group but which nevertheless does not belong to that group. Thus an alcohol might be formed which oxidised to an aldehyde, and then to an acid containing the same number of carbon atoms per molecule as the alcohol, but which was not a true primary alcohol, and could not be justly represented by a molecular formula containing the group CH 2 . OH characteristic of the primary alcohols. 369 "We have spoken of the molecules of certain classes of compounds as all containing the same group of atoms. This conception of a group of atoms forming part of a molecule, 367370] ISOMEKISM AND STRUCTURAL FORMULAE. 255 and exerting a definite influence on the properties of the molecule, is of much importance. The hydrocarbon ethane, C 2 H 6 , interacts with chlorine to form chlorethane and hydrogen chloride ; thus C 2 H 6 + Cl g = C 2 H 5 C1 + HC1 : chlorethane and caustic potash interact to produce ethylic alcohol and potassium chloride ; thus C 2 H 5 C1 + KOH = C 2 H 6 + KC1 : when ethylic alcohol is oxidised ethylic aldehyde is formed, and when this is oxidised acetic acid is produced ; thus C 2 H 6 O + O = C 2 H 4 + H O, and C 2 H 4 O + O = C 2 H 4 2 . We already know the structural formulae of most of the carbon compounds taking part in these changes ; let us write the equations expressing the changes in structural formulae ; (1) CH 3 . CH 3 + C1 2 - CH 3 . CH 2 C1 + HCL (2) CH 3 . CH 2 C1 + KOH = CH 3 . CH 2 . OH + KC1. (3) CH 3 . CH 2 . OH + O = CH 3 . CHO + H 2 0. (4) CH 3 . CHO + O - CH 3 . CO . OH. All the molecules of these carbon compounds are repre- sented as containing the atomic group CH 3 . If the formulae are correct, then the group of atoms CH 3 has remained intact during this series of changes. If the sodium salt of acetic acid is prepared, mixed with solid caustic soda, and heated, we get methane (CH 4 ) and sodium carbonate formed; this change is represented in structural formulae thus CH 3 . CO . ONa + NaOH = CH 3 . H + Na 2 CO 3 . Here again the atomic group CH 3 has remained undecom- posed. A group of atoms which forms a part of several molecules, 370 and which remains undecomposed through a series of reactions undergone by these molecules, is called a compound radicle. The following compounds have been obtained, the passage from one to the other has been effected, and the structural formula given to each is the outcome of quantitative ex- periments on the methods of preparation and the interactions of each compound :C 2 H 5 . Cl, C 2 H 5 . Br, C 2 H 6 .CH 2 .OH, C 2 H 5 .CN, C 2 H 5 .NH 2 , C 2 H 5 .C 2 H 3 2 . The group of atoms C 2 H 5 is therefore an example of a compound radicle. The study of the reactions of acetic acid affords a good example of the meaning of the term compound radicle, and of the use of structural formulae. The empirical formula of acetic acid is C 2 H 4 O 2 . The acid is monobasic; from this we conclude that one of the 4 hydrogen atoms is related to the 256 ELEMENTARY CHEMISTRY. [CHAP. XVII. rest of the molecule differently from the other 3 hydrogen atoms ; we therefore adopt the formula C 2 H 3 O 2 . H. Phos- phorus pentachloride interacts with acetic acid; the inter- action consists in the removal of one oxygen and one hydrogen atom from the molecule C 2 H 4 O 2 and the putting of one chlorine atom in their place; this interaction is thus ex- pressed in an equation, C 2 H 3 O 2 +PC1 5 =C 2 H 3 OC1+POC1 3 +HC1. From this we conclude that one of the oxygen atoms in the molecule C 2 H 4 O 2 directly interacts with an atom of hydrogen, and that the relation of this oxygen atom to the rest of the molecule is different from that of the other oxygen atom to the rest of the molecule ; we therefore adopt the formula 2 H 3 O . OH, and we express the interaction with phosphorus pentachloride thus, C 2 H 3 . OH + PC1 5 - C 2 H 3 . 01 + POC1 3 + HC1. If this interpretation of the mechanism of these changes is correct, then the group of atoms C 2 H 3 O is a compound radicle ; this group remains unchanged when acetic acid interacts with phosphorus pentachloride, and it is common to the 2 molecules 2 H 3 O . OH and C 2 H 3 . 01. If acetic acid has the formula C 2 H 3 O . OH the formula of sodium acetate is C 2 H 3 O . ONa : when this salt is mixed with solid caustic soda and the mixture is heated, sodium carbonate (Na 2 CO 3 ) and methane (OH 4 ) are produced. Therefore in this change the atomic group C 2 H g O is decomposed ; one of the carbon atoms and the oxygen atom are removed, and the remaining CH 3 combines with an atom of hydrogen to form CH 4 . The change is most simply represented thus, CH 3 . CO . ONa + NaOH = Na 2 C0 3 + CH 3 H. Because of this reaction we adopt for acetic acid the structural formula OH 3 . CO . OH ; and we say that the molecule of this acid is composed of the compound radicle CH 3 combined with the other compound radicles CO and OH. The H O I I developed structural formula H C O H perhaps H makes this plainer. Had we stopped this investigation after examining the interaction between PC1 5 and C 2 H 4 O 2 , we should have given 370372] ISOMERISM AND STRUCTURAL FORMULAE. 257 to acetic acid the structural formula C 2 H 3 O . OH, and we should have said that the molecule of this acid is composed of the compound radicles G 2 H 3 O and OH. Further investiga- tion however obliges us to modify this conclusion, inasmuch as it shews that the atomic group C 2 H 3 O is itself composed of the simpler groups CH 3 and CO. The formula CH 3 . CO . OH expresses all that is expressed by the formula C 2 H 3 O . OH, and it also suggests the interaction in which methane is produced from sodium acetate. It appears then that a compound may have more than 371 one structural formula ; that formula is the best which tells most about the characteristic reactions of the com- pound. In Chap. xi. pars. 210 and 211 we glanced at the reactions of compounds of ammonia, NH 3 . We found that these re- actions were analogous to those of the alkali potash, KOH ; to bring out these analogies we wrote the formulae of the compounds produced by the interaction of an aqueous solution of ammonia with acids as compounds of the hypothetical compound radicle ammonium, NH 4 . The interpretation of these reactions given by the molecular and atomic theory is that in the molecule of an ammonium compound e.g. NH 4 C1, NH 4 ,N0 3 , (NH 4 ) 2 SO 4 , (NH 4 ) 2 C0 3 , &c. we have always direct interaction between an atom of nitrogen and 4 atoms of hydrogen ; in other words, we have the compound radicle or atomic group, NH 4 . As we speak of the valency of an atom in this or that molecule, meaning thereby the number of other atoms with which the specified atom directly interacts in the molecule, so we speak of the valency of an atomic group or compound radicle in a molecule. The atomic groups CH 3 , C 2 H 5 , C 3 H 7 , &c. are monovalent ; the group CH 2 OH is also monovalent ; the group CO is divalent ; and so on. In par. 357 it was mentioned that an atom which combines 372 with one divalent atom to form a molecule is often regarded as thereby proved to be itself divalent. Thus the atom of oxygen is divalent because of the existence of the gaseous molecules OC1 2 and OH 2 ; one atom of carbon combines with one atom of oxygen to form the gaseous molecule CO ; therefore, it has been urged, the atom of carbon is divalent in the molecule CO. Again, one atom of carbon combines with 2 atoms of oxygen to form the gaseous molecule CO 2 ; therefore, it is said, the atom of carbon is tetravalent in M. E. C. 17 258 ELEMENTARY CHEMISTRY. [CHAP. xvil. the molecule CO 2 . The formulae C = O and O = C = O are generally used to express these statements. These formulae are evidently based on a meaning of atomic valency different from that we have been giving to this expression in preceding paragraphs. It is rather difficult to grasp the exact meaning of the statement, 'the atom of carbon is divalent in the molecule CO and tetra- valent in the molecule CO 2 .' The statement seems to imply that an atom of carbon is capable of directly combining with either 2 or 4 hydrogen, fluorine, chlorine, bromine, or iodine, atoms, or with that number of other atoms which is equivalent to 2 or 4 atoms of chlorine, &c. The statement seems to assert that one atom of oxygen is truly equivalent sometimes to 2 atoms of hydrogen, or 2 atoms of chlorine, &c. and sometimes to 4 atoms of hydrogen, &c. But this assertion is scarcely capable of proof, because it seems impossible to define the exact meaning to be given to the expression equivalent to, as used with regard to atoms. It has even been asserted that the atom of carbon is tetravalent in the molecule CO. If this is so, then one atom of oxygen is equivalent to 4 atoms of chlorine, ifec. when oxygen and carbon combine to form the com- pound CO : but 2 atoms of oxygen are , equivalent to 4 atoms of chlorine, &c. when oxygen and carbon combine to form the compound C0 2 . We see here the extreme difficulty, if not impossibility, of giving an exact and in- variable meaning to the expression equivalent to, as applied to atoms. 373 I* is generally the custom in writing structural formulae to represent each atom whose maximum valency is greater than one with as many lines proceeding from the symbol as correspond to the maximum valency of the atom. The atom of carbon, for instance, is generally represented with 4 lines proceeding from it, the atom of oxygen with 2 lines, and so on. Thus the structural formulae for ethylic aldehyde and acetic acid are generally written thus ; H H HO ! I II H C C, and H C C. I II II HO H H 372373] ISOMERISM AND STRUCTURAL FORMULAE. 259 Similarly the structural formulae of ethylene and the hypo- thetical isomeride ethylidene are generally written thus; H H H II II I C = C, and H C C H. The structural formula of I I I H H H the hydrocarbon acetylene, C 2 H 2 , is put into this form H C = C H ; whereas the formula H C C H would be used when the meaning given to valency is that explained in par. 360. It would be out of place in an elementary book to discuss the possible meanings of these so called 'double bonds' and ' treble bonds' In the opinion of several chemists they have done much to hinder the advance of chemistry, by leading chemists to trust in names, and in far-fetched analogies, instead of in realities, and in well established and accurately defined points of resemblance and difference. On the other hand the employment of 'double and treble Unkings' or ' 'bonds' has some points in its favour. It continually reminds the chemist of the maximum valency of each atom, and by doing this it suggests the possibility of reactions. H H H H I I I Thus, either of the formulae, (1) C C or (2) C = C, II II H H H H represents each carbon atom as directly interacting with only 3 other atoms, but formula (2) tells us that a carbon atom can directly interact with 4 other atoms, and hence suggests the possibility of adding 2 monovalent atoms to the molecule H H I I C H 4 . So the formula for ethylic aldehyde H C C I II H O visibly suggests the possibility of putting 2 monovalent atoms, e.g. C1 2 , in place of the atom of oxygen. 172 260 ELEMENTARY CHEMISTRY. [CHAP. xvn. H H H H II II The formulae C C and H C C of course suggest II II H H HO the same reactions as the formulae with double bonds, if we remember that the maximum valency of the carbon atom is 4, and that of the oxygen atom is 2. These formulae have the great advantage over those with double or treble bonds that they are based on a definite hypothesis regarding atomic valency. 374 The essential part of the hypothesis of valency is the conception of direct action and reaction between each atom in a molecule and a limited number of other atoms. As the whole molecule is held together by the mutual interactions of the atoms, there probably is what we may call indirect action and reaction between all the atoms which constitute the molecule. The hypothesis gives us a definition of the maximum valency of an atom, as the maximum number of monovalent atoms (i.e. atoms of H, F, Cl, Br, or I) with which the given atom directly interacts (i.e. in these cases combines) in any molecule ; and it teaches that the specified atom never directly interacts with a greater number of other atoms, whatever be their valencies, than is expressed by the maximum valency as thus defined. 375 The hypothesis of valency is meaningless apart from the theory of atoms and molecules ; it is based on this theory, and all the results gained by using it are expressed in the language of the theory. The theory of atoms and molecules is strictly applicable, at present, only to gases ; therefore the hypothesis of valency, and all the terms to which it has given birth, are strictly applicable, at present, only to gases. But just as we made use of the molecular and atomic theory as a general guide in studying the chemical changes which occur among liquid and solid substances, so may we make use. of the hypothesis of valency, provided we exercise sufiicient caution, as a general guide in attempts to learn something regarding the structure of those aggregations of atoms which form the reacting weights of solid and liquid substances. But it would be going too far afield to attempt to indicate here even the lines on which the hypothesis of valency may probably be 373377] ISOMERISM AND STRUCTURAL FORMULAE. 261 usefully employed in discussions regarding the structure of the reacting weights of solid compounds. Our conception of chemical composition has been widened 376 by the examination of the phenomena of isomerism. A statement of the composition of a compound should tell the percentage composition of the compound; it should also tell the composition of a reacting weight stated in numbers of combining weights of each element, and the composition of