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THE TEMPLE PRIMERS 
 
 MODERN CHEMISTRY 
 Theoretical 
 
 By 
 WILLIAM RAMSAY, D.Sc. 
 
THE HON. ROBERT BOYLE 
 
mODERH 
 CHEMISTRY 
 
 THEORETICAL 
 
 BY*WILUAII2 
 RAlttSAY'D.S? 
 
 *; 
 
 19O3 fla 29 6-50 BEDFORD-STREET' LONDON 
 

 First Edition, October 1900 
 Second Edition ', . March 1903 
 
 All rights reserved 
 

 
 MODERN CHEMISTRY 
 
 FIRST PART 
 THEORETICAL CHEMISTRY 
 
 CHAPTER I 
 INTRODUCTORY 
 
 Elements Phlogiston Discovery of Oxygen Com' 
 bin ing Proportions Dal ton's Laws Gay-Lus- 
 sac's Law of Volumes Avogadro's Hypothesis 
 A tomic Weigh ts M olecular Weigh ts D it long 
 and Petit 's Law Equivalents Isomorphism. 
 
 ONE of the earliest questions asked by an intelligent child 
 is : " What is this made of? " " What is that made of? " 
 And the answer is generally more or less satisfactory. 
 For example, if the question relates to butter, the reply 
 may be, " From cream." It may be explained, besides, 
 that when cream is beaten up, or churned, the butter sepa- 
 rates, leaving skim-milk behind. But the question has riot 
 been answered. The child may ask, " Was the butter in 
 the milk before it was churned ? or has it been made out 
 of the milk by the churning?" Possibly the person to 
 whom the question is addressed may know that the milk 
 contained the butter in the state of fine globules, and that 
 the process of churning breaks up the globules, and causes 
 them to stick together. The original question has not really 
 been answered ; and indeed it is not an easy one to reply to. 
 Precisely such questions suggested themselves to the people 
 of old, and they led to many speculations. 
 
 VOL. i. A 
 
2 MODERN CHEMISTRY 
 
 Elements. One of these speculations was that things 
 which we see around us were built up out of elements, just 
 as a word is built up out of letters. Indeed, the word 
 element, which is the Latin word for element, is pro- 
 bably derived from the letters /, m, and n, and involves 
 that idea. The ancient Greeks surmised that there were 
 four such elements earth, water, air, and fire. But as it 
 was obvious that some things, for instance gold and silver, 
 did not contain either water or air, the word element was 
 often used to signify, not the constituent of a thing, but 
 rather a property of a thing ; and it might have been said 
 that gold partook of the properties of earth and water, 
 because, like earth, it is not altered by being heated, and 
 yet it can take a fluid form like water if heated hot enough. 
 Hence the old word " element" had a double meaning; 
 it was sometimes used in the sense of " constituent," and 
 sometimes more in the sense of " property." 
 
 If a child is given a mechanical toy, his wish to see how 
 it works generally leads to his taking it to bits. This is 
 unfortunately only too easy ; but it is seldom that he 
 succeeds in putting it together again. Now, if we inquire 
 what a piece of wood or stone is made of, we can, after a 
 fashion, take them to bits ; we may pull the wood into 
 fibres, or we may crush the stone, and pick out the pieces 
 that appear to differ from each other in colour, if they are 
 large enough. But the fibres have much the same appear- 
 ance as the piece of wood, and the fragments of stone, 
 though somewhat different from each other, are still pieces 
 of stone. The question is still to be answered, of what do 
 wood and stone consist ? It is evident that some plan must 
 be tried by which the wood and stone will be unbuilt, as it 
 were, and by which they will yield their constituents. 
 
 It had long been noticed that many things are greatly 
 changed when heated. A piece of wood takes fire and 
 burns ; some kinds of stone melt ; some metals, such as lead 
 and iron, change into earthy-coloured powders. Surely 
 these changes ought to lead to some knowledge of the 
 
PHLOGISTON 3 
 
 nature of wood, stone, and metals. It was long, however, 
 before it was recognised that the presence or absence of 
 air made a difference in the result of heating substances. 
 When attention was drawn to this difference, a new sug- 
 gestion was adopted. It was, that things, besides consist- 
 ing of or sharing the properties of earth, water, air, and 
 fire, also consist of, or at least are like, salt, sulphur, and 
 mercury. Salt dissolves when put into water ; so do many 
 other things. These things must either contain a kind of 
 salt to account for this property ; or they must at least 
 share the property of salt, in so far as they dissolve. Simi- 
 larly, other things, especially metals, must either contain or 
 share the property of mercury, seeing that they shine with 
 the same kind of lustre ; and many things resemble sulphur 
 in so far as they burn and produce a smell in burning. And 
 it was often imagined that when things burn, the sulphur which 
 they contain flies away and disappears, just as ordinary sul- 
 phur, when set on fire, burns away completely, leaving nothing 
 behind. About the middle of the seventeenth century, 
 Johann Joachim Becher, a German alchemist, altered 
 somewhat the conception that substances contain, or are 
 like, salt, sulphur, and mercury ; he imagined all things 
 existing on the surface of the globe to contain three earths, 
 namely the mercurial, the glassy, and the fatty, the last 
 implying the property of being able to burn. And in 
 the early years of the eighteenth century, Becher's pupil, 
 George Ernest Stahl, who was Professor of Medicine in 
 Jena, and later in Halle, two small German towns, made 
 an important addition to the ancient theories, namely, that 
 it was possible to restore the " sulphur," or the * fatty 
 earth," as Becher called it, to things which had been 
 deprived of it by burning, by heating them with other 
 substances rich in that constituent. 
 
 Phlogiston. Stahl devised a new name for this com- 
 bustible constituent of substances, in order better to direct at- 
 tention to his new idea ; he called it "phlogiston," a word 
 which may be translated " burnableness," for it is derived 
 
4 MODERN CHEMISTRY 
 
 from a Greek word signifying flame. Thus lead, which 
 when heated in air changes to an earthy dross, or, as it 
 was then termed, " calx," may be restored to the state of 
 metal by heating it with charcoal powder, or with flour, 
 or with any substance rich in " phlogiston ; " that is, with 
 any substance which is itself capable of burning. He sup- 
 posed that the lead was rich in " phlogiston ; " that when 
 it changed to lead-dross, the " phlogiston " escaped ; but 
 that on heating with charcoal, the latter parted with its 
 phlogiston to the lead-dross, changing it back again into 
 lead. It is evident that this idea accounts for some of the 
 facts observed ; and it gained ground rapidly. But it had 
 been shown by the French physician Jean Rey, by the Eng- 
 lish philosopher Robert Boyle, and others, that metals, when 
 they changed into earthy powders on heating, gained weight; 
 and it is at least curious that the lead, on losing one of its 
 constituents, namely "phlogiston," should gain weight : one 
 would have expected that weight would be lost, not gained. 
 The way out of this difficulty was ingenious. We know, it 
 was said, that weight is due to the attraction of the earth ; 
 now, it is not impossible that the earth may repel phlo- 
 giston, instead of attracting it; and in that case a body would 
 grow heavier, instead of lighter, if it parts with phlogiston. 
 Another objection to the theory was that a candle, for 
 example, which is rich in phlogiston, goes out when made 
 to burn under a glass shade ; that is, when air is excluded. 
 True, said the phlogistonists ; that is because the phlogiston 
 cannot escape. And because this theory gave a plausible 
 explanation of the common phenomenon of combustion, it 
 was widely accepted, and survived until the end of the 
 eighteenth century. 
 
 The idea had been steadily gaining ground that know- 
 ledge was to be acquired only by trial and failure. Francis 
 Bacon, Lord Verulam, at the end of the sixteenth cen- 
 tury wrote: "The true kind of experience is not the 
 mere groping of a man in the dark, who feels at random 
 to find his way, instead of waiting for the dawn or striking 
 
DISCOVERY OF GASES 5 
 
 a light. ... It begins with an ordered not chaotic 
 arrangement of facts, deduces axioms from these, and from 
 the axioms again designs new experiments." Many ex- 
 periments were made on the action of heat on various 
 things, either alone or mixed with others. Boyle, who 
 introduced the word " analysis " to denote the art of dis- 
 covering one substance in presence of another, and who 
 contended for the use of the word "element" in the mean- 
 ing of a constituent of, and not as a property of matter, made 
 many such experiments, and systematically put Bacon's ad- 
 vice into practice. And so knowledge of facts gradually 
 accumulated, and speculations acquired some substantial 
 basis. To Priestley, a nonconformist clergyman of Bir- 
 mingham, we owe the discovery of numerous gases, and 
 Scheele, his contemporary, a Swedish apothecary, also en- 
 riched chemistry in this respect. The discovery of oxygen, 
 in 1774, was made simultaneously by both of these illus- 
 trious men. It had been generally supposed that all gases, 
 or, as they were then termed, "airs," were merely modifi- 
 cations of atmospheric air ; and it was not uncommonly 
 held that air, in consequence of its want of substantiality, 
 was but one remove from nothing. Joseph Black, Pro- 
 fessor of Chemistry in Edinburgh in the middle of the 
 eighteenth century, was the first to prepare carbonic acid 
 gas, or, as he termed it, " fixed air," in a pure state ; and 
 by determining the loss of weight on heating its compound 
 with magnesia, to show that it was due to the escape of the 
 gas ; for he succeeded also in absorbing the gas, and re- 
 constituting the carbonate of magnesia, which then possessed 
 practically the same weight as it originally had. In spite 
 of this discovery, made in 1756, the doctrine was still gene- 
 rally held that burning substances lost their constituent 
 principle, " phlogiston ;" and we owe to the French che- 
 mist Lavoisier the true explanation of the phenomenon 
 of combustion. Lavoisier had been informed by Priestley 
 in the autumn of 1774 of his discovery of what, accord- 
 ing to the views then current, he termed " dephlogisticated 
 
6 MODERN CHEMISTRY 
 
 air ; " he proceeded to repeat an experiment which had 
 previously been made by Boyle, in heating metallic tin to 
 redness in a sealed glass vessel ; there was neither gain nor 
 loss of weight, although the tin had been partly converted 
 into " calx ; " but on admitting air, he observed a gain in 
 weight, nearly equal to that which the tin had gained on 
 being calcined. The conclusion was obvious, that the gain 
 in weight was due to the absorption of a portion of the air 
 by the hot tin ; and he subsequently showed that the gain 
 was to be ascribed to the absorption of Priestley's "de- 
 phlogisticated air," of which Priestley had shown common 
 air to contain about one-fifth. And in 1777 Lavoisier 
 published the statements : 
 
 1 i ) Substances burn only in pure air. 
 
 (2) This air is consumed in the combustion, and 
 the increase in weight of the substance burned is 
 equivalent to the decrease in weight of the air. 
 
 (3) The combustible body is, as a rule, converted 
 into an acid by its combination with the pure air, 
 but the metals, on the other hand, are converted into 
 " calces." 
 
 Oxygen. This last statement explains the name which 
 he gave to Priestley's and Scheele's gas, namely oxygen, a 
 word derived from two Greek words, signifying " acid- 
 producer." The compounds of this substance he termed 
 "oxides;" and it is to him that we owe the system of 
 nomenclature now generally in use. Before the end of the 
 century, the doctrines of Lavoisier had gained almost 
 universal acceptance. 
 
 The word " analysis," as has been stated, was suggested 
 by Boyle, to signify the ascertaining the composition of 
 substances. Attempts were made by him, and by other 
 chemists, especially by Black, to perform quantitative 
 analyses during the seventeenth and the first half of the 
 eighteenth centuries. Priestley and Scheele tried to find the 
 relative proportions of oxygen in air with partial success ; 
 
COMBINING PROPORTIONS y 
 
 but it was not until Lavoisier had convinced most chemists 
 that oxygen was a substance, and not the negation of one, 
 like the absence of phlogiston, that serious attention was 
 directed to accurate determinations of quantity. And 
 towards the end of the eighteenth century fairly trustworthy 
 data began to accumulate. 
 
 Combining Proportions. It became evident, chiefly 
 owing to the work of two German chemists, Wenzel and 
 Richter, that when an acid, such as vitriol or vinegar, is 
 mixed with a base, such as potash, and neutralised, as the 
 expression runs that is, rendered incapable of changing the 
 colour of certain vegetable extracts and deprived of its sharp 
 taste the same weight of base was always required to 
 neutralise the same weight of acid. And other examples of 
 apparently constant proportions between the constituents of 
 substances had also been observed. But the processes of 
 analysis were very imperfect, and the results by no means 
 always concordant; and there was some ground for the 
 statement made by Count Berthollet, a contemporary of 
 Lavoisier, in his Researches on the Laws of Affinity , published 
 in 1803, that the composition of chemical compounds was 
 variable, and not constant ; that, in fact, it depended on 
 circumstances, such as the proportions of the substances 
 present, on the temperature, on whether the substance pro- 
 duced was an insoluble solid, and so on. Berthollet's 
 statement was disputed by his countryman Proust, who, by 
 fairly accurate analyses, carried out during eight years of 
 controversy, proved the truth of the doctrine of constant 
 proportions. But in the course of his work, he found that 
 in certain cases two elements form more than one compound 
 with each other ; for example, tin combines with oxygen in 
 two proportions, each of them fixed and constant ; and iron 
 forms similarly two compounds with sulphur. Perhaps the 
 most exact experiments which had at that time been made 
 were those due to the Hon. Henry Cavendish, who having 
 discovered that water was composed of oxygen in union with 
 another gas, to which the name " hydrogen " was subse- 
 
S MODERN CHEMISTRY 
 
 quently given, determined the proportion of these constituents 
 with very great accuracy. He found that two volumes of 
 hydrogen invariably combine with one volume of oxygen to 
 produce water, neither hydrogen nor oxygen being left over. 
 Owing, however, to the method of expressing the composi- 
 tion of compounds, no relation was evident between the 
 proportions of the constituents. Thus Proust expressed the 
 results of his determination of the composition of the two 
 oxides of tin and of copper in parts per hundred : 
 
 Suboxide of Protoxide of Suboxide of Oxide of 
 copper. copper. tin. tin. 
 
 Metal . . . 86.2 80 87 78.4 
 
 Oxygen . . 13.8 20 13 21.6 
 
 1 00.0 100 IOO 1 00.0 
 
 Had he calculated by simple proportion how much oxygen 
 is combined with the same weight of copper and tin in each 
 case, he would have found that the ratio of the oxygen in 
 the suboxide of copper to that in the protoxide is as 13.8 to 
 21.5 ; and in the two oxides of tin as 13 to 24. The 
 correct figures are : 
 
 Suboxide of Protoxide of Suboxide of Oxide of 
 copper. copper. tin. tin. 
 
 Metal . . . 88.8 79.9 88.2 78.9 
 
 Oxygen . .11.2 20.1 11.8 21.1 
 
 100.0 100.0 100.0 100.0 
 
 The ratio should be as I to 2 in each case ; and the fact 
 that Proust did not remark this is to be ascribed partly to his 
 method of stating his results, and partly to the inaccuracy of 
 his analyses. 
 
 Attention was first drawn to the existence of simple 
 proportionality between the amounts of one element forming 
 more than one compound with another by John Dalton, a 
 Manchester schoolmaster, in 1802 and the next succeeding 
 
r DALTON'S LAWS 9 
 
 years. In the year named, he described experiments " On 
 the proportion of the several gases in the atmosphere ; " and 
 he then stated : " The elements of oxygen may combine 
 with a certain portion of nitrous gas, or with twice that 
 portion, but with no intermediate quantity." And he later 
 illustrated the same fact by considering the composition of 
 two compounds of carbon with hydrogen, marsh gas, and 
 defiant gas, the former of which contains twice as much 
 hydrogen as the latter, proportionally to the same weight 
 of carbon. 
 
 The laws relating to the proportions in which various 
 elements combine are therefore usually called Dalton s 
 Laws ; they are : 
 
 Dalton's Laws. The !a<w of definite proportions : 
 When two or more elements combine with each, other 
 to form a compound, they combine in constant pro- 
 portions by weight. 
 
 The law of multiple proportions : When two elements 
 form more than one compound with each other, they 
 combine in simple multiple proportions. 
 
 Thus, if A and B are definite weights of two elements, 
 the proportions in which they combine will be A with B ; 
 or A with zB ; or A with 36 ; or 2A with B ; or 2A 
 with 3& ; or 3A with 2B, &c. 
 
 But Dalton not merely stated these facts ; he devised a 
 theory with a view to explaining them ; he revived and 
 gave definiteness to the ancient conception that all substances 
 which we see around us consist of atoms. This idea is at 
 least as ancient as 400 B.C., and is to be found in the writ- 
 ings of the Greek philosophers. The theory, in the form 
 which Dalton gave it, is as follows : All compounds con- 
 sist of atoms of elements united with each other. An atom 
 is an indivisible (literally " uncuttable") particle, or, 
 more correctly, a particle which resists division. Each 
 atom has its own definite weight ; but as there is no apparent 
 means of determining this weight (for atoms are inconceiv- 
 ably small), we must be contented in determining their 
 
io MODERN CHEMISTRY % 
 
 weights relatively to each other. This we can do by 
 ascertaining the proportion in which they exist in their 
 compounds. Thus, knowing that water consists of oxygen 
 in combination with hydrogen, if the smallest particle of 
 water consists of one atom of each element, the relative 
 weights of the atoms will be found by discovering the 
 proportions by weight in which these elements are combined 
 with each other. Now, it is found that 8 parts by weight 
 of oxygen and I part of hydrogen by weight combine to 
 form 9 parts by weight of water ; hence an atom of oxygen 
 is eight times as heavy as an atom of hydrogen ; and an 
 atom of water is nine times as heavy. 
 
 We must beware of confusing this theory with the facts 
 on which it is founded ; indeed, Dalton' s contemporaries, 
 while accepting the facts, refused in many cases to accept 
 his theory. Sir Humphry Davy used the word " propor- 
 tion" in place of the word "atom;" and Dr. Wollaston 
 preferred the word "equivalent." And even granting the 
 existence of atoms, the problem of determining their relative 
 weights is not so simple as would at first sight appear. For 
 how is it possible to know which of several compounds is 
 the one containing only one atom of each element ? The 
 two compounds of carbon with hydrogen by means of 
 which Dalton illustrated his law will furnish a good example 
 of this difficulty. While one of them, marsh-gas, consists 
 of one part by weight of hydrogen in combination with 
 three parts of carbon, the other consists of one part of 
 hydrogen, in union with six parts of carbon. Which of 
 these two contains one atom of each element ? If the 
 former, then the atom of carbon is three times as heavy as 
 the atom of hydrogen ; if the latter, it is six times. Dalton 
 was quite aware of this difficulty, but could devise no means 
 of overcoming it, and the numbers which he adopted were 
 only provisional. 
 
 The difficulty was solved chiefly by the experimental 
 work of Joseph Louis Gay-Lussac, Professor of Chemistry 
 in the Ecole Polytechnique in Paris, and by P. L. Dulong 
 
GAY-LUSSAC'S LAW ir 
 
 and T. A. Petit, Director of, and Professor in, the same 
 school. The attention of Gay-Lussac having been directed 
 by the celebrated explorer Humboldt to the fact that water 
 is formed by the union of one volume of oxygen with two 
 volumes of hydrogen gas, he followed it up by the discovery 
 that other gases unite in very simple proportions by volume. 
 Of this we shall see many instances hereafter. 
 
 Gay=Lussac's Law of Volumes. Stated in the 
 form of a law, Gay-Lussac's discovery was : The weights 
 of equal volumes of both simple and compound gases 
 are proportional to their combining weights (or, to 
 use Dalton' s term, their atomic weights), or to rational 
 multiples of the latter. This law appeared as if it ought 
 to have some simple relation to Dalton's laws ; but there 
 is an apparent difficulty in reconciling them, which was 
 surmounted in 1 8 1 1 by Amadeo Avogadro, Professor of 
 Physics in Turin. The difficulty is this : 
 
 Imagine a given volume, say a cubic inch, to be filled 
 with oxygen ; suppose it to contain a very large but un- 
 known number of atoms of oxygen, which we will call . 
 This oxygen, if mixed with twice its volume of hydrogen, 
 or two cubic inches, and made to combine with it (which 
 can be done by heating the mixture with an electric 
 spark), yields nothing but water ; and neither of the gases 
 remains uncombined in excess. Let us suppose that n atoms 
 of oxygen combine with 2n atoms of hydrogen ; and as 
 water also, according to Dalton, consists of atoms, there will 
 be atoms of water formed by their union. But experi- 
 ment shows that the water, in the state of water-gas or 
 steam, has a volume equal to that of the hydrogen from 
 which it was formed ; that is, n atoms of water-gas inhabit 
 a volume equal to that inhabited by ^n atoms of hydrogen. 
 From this it would appear that equal volumes of gases 
 do not contain equal numbers of atoms ; and while some 
 chemists supposed, with Dalton, that water consists of one 
 atom of oxygen in union with one atom of hydrogen, others 
 imagined that two atoms of hydrogen were present for each 
 
12 MODERN CHEMISTRY 
 
 atom of oxygen, basing their conclusions on the fact that 
 two volumes of hydrogen combine with one volume of 
 oxygen, and considering it probable that equal volumes of 
 gases contain equal numbers of atoms. This last proba- 
 bility was maintained by Avogadro, and he defended his 
 doctrine by the following suggestion. 
 
 Avogadro* 's Hypothesis. Substances consist of two 
 kinds of particles, each of which has been termed an atom 
 by Dalton. But they are in reality different. The smallest 
 particle, or, as Avogadro named it, molecule, of water, con- 
 sists of three atoms, two of hydrogen and one of oxygen. 
 But hydrogen gas and oxygen gas also consist of molecules, 
 each of them containing two atoms. The act of union of 
 these elements is to be regarded not as a case of combination 
 of atoms, as Dalton supposed, but rather as an exchange of 
 partners ; the atom of oxygen leaving the other atom with 
 which it had been combined, and uniting with two atoms 
 of hydrogen, each of which had similarly left its partner. 
 Thus n molecules of oxygen exchange partners with 2/2 
 molecules of hydrogen, and form 2n molecules of water-gas ; 
 but whereas the molecules of oxygen and hydrogen each 
 contain two atoms, those of water-gas contain three. And 
 this explanation is consistent with the facts ; for while in 
 molecules of hydrogen and n molecules of oxygen, containing 
 together 6n atoms, react to form 2 molecules of water- 
 gas, the latter also contains 6n atoms, for each molecule of 
 water-gas contains three atoms. 
 
 Using the symbols H and O for one atom of hydrogen 
 and one atom of oxygen respectively, Dalton's idea of the 
 combination was 
 
 H + = HO. 
 
 On the assumption that equal volumes of the gases contain 
 equal numbers of atoms, the equation becomes 
 
 2H + O = H 2 O. 
 Lastly, on Avogadro' s hypothesis, that the action is one 
 
DULONG AND PETIT'S LAW 13 
 
 between molecules of hydrogen and molecules of oxygen, 
 each containing two atoms, the equation is : 
 
 Granting Avogadro's hypothesis, the relative weights 
 of the atoms can be ascertained. For, as oxygen is 
 1 6 times as heavy as hydrogen, and as equal volumes of 
 these gases contain equal numbers of molecules, and more- 
 over as each molecule consists of two atoms, it follows that 
 an atom of oxygen is 16 times as heavy as an atom of 
 hydrogen. 
 
 Atomic Weight. This is usually expressed by the 
 phrase the atomic weight of oxygen is 16; for the 
 atomic weight of hydrogen, being the smallest known, was 
 taken as the unit. 
 
 The relative weight of a molecule can also be calculated; 
 as an instance, let us calculate the molecular weight of 
 water-gas. Experiment shows that a given volume of 
 water-gas is 9 times as heavy as an equal volume of hydro- 
 gen at the same temperature and pressure ; again, equal 
 volumes of gases contain equal numbers of molecules ; 
 therefore a molecule of water-gas is 9 times as heavy as 
 a molecule of hydrogen. But a molecule of hydrogen 
 consists of two atoms ; consequently a molecule of water- 
 gas is 1 8 times as heavy as an atom of hydrogen. 
 
 Molecular Weights. This is usually expressed by 
 saying that the molecular weight of water is 1 8 ; and inas- 
 much as it consists of two atoms of hydrogen in union with 
 one atom of oxygen, the weight of a molecule of water-gas 
 is equal to the sum of the weights of the atoms composing 
 it ; for, (2xi) + i6=i8. 
 
 Dulong and Pet it's Law. Let us now consider 
 the discovery of Dulong and Petit, already alluded to. In 
 1819 they made the announcement that the atoms of 
 simple substances, or elements, have equal capacity for heat. 
 It must be explained that equal weights of different sub- 
 stances require different amounts of heat to raise them 
 
14 MODERN CHEMISTRY 
 
 through the same interval of temperature. Thus, if the 
 amount of heat required to raise the temperature of a gram 
 of water from, let us say, o C. to 100 C. be taken as 
 unity, it is found that only one-ninth of that amount is 
 required to raise an equal weight of iron through the 
 same range of temperature. Or, in other words, while 
 the specific heat of water is I, that of iron is -J-, or in 
 decimals, 0.112. The quantity of heat necessary to be 
 imparted to one gram of water to raise its temperature 
 through i C. is termed a heat-unit, or calory; but 
 sometimes the unit is chosen one hundred times as large, 
 and represents the heat required for a rise of temperature 
 from o to 1 00; and the French make use of a unit 
 one thousand times that of the smallest unit. Dulong 
 and Petit's discovery was, that if weights of the solid 
 elements be taken proportional to their atomic weights, 
 equal amounts of heat must be imparted to them in order 
 to raise them through the same interval of temperature. 
 The following table illustrates this fact, and exhibits some 
 of the results obtained by Dulong and Petit : 
 
 Element. Atomic weight. Specific heat. Atomic heat. 
 
 Bismuth . . . 208 0.0288 6.0 
 
 Lead . . . 207 0.0293 6.0 
 
 Gold . . . 197 0.0298 5.8 
 
 Platinum . . 195 0.0314 6.1 
 
 Silver . . . 108 0.0570 6.1 
 
 Copper ... 63 0.0952 6.0 
 
 Iron .... 56 0.1138 6.4 
 
 Sulphur ... 32 0.1776 5.7 
 
 If the specific heat be taken as the heat required to raise 
 the temperature of one gram of each of these substances 
 through one degree, compared with that required for one 
 gram of water, the atomic heats of bismuth, lead, gold, and 
 the others represent the heats required for 208 grams of 
 bismuth, 207 grams of lead, and so on. It is evident 
 that they are all nearly equal. It should follow that the 
 
EQUIVALENTS 15 
 
 specific heat of solid hydrogen must be also 6, since the 
 atomic weight is taken as i. 
 
 These facts, though clearly indicating the numbers which 
 should be taken for the atomic weights of the elements, 
 were neglected, until renewed attention was called to them 
 in 1858 by Cannizzaro, still Professor of Chemistry at 
 Rome. He pointed out that all that can be gained from 
 the analysis of a compound, for example an oxide, is the 
 "equivalent" of the element. And as an element, such 
 as iron, often forms more than one compound with other 
 elements, Jet us say oxygen or chlorine, it therefore may 
 possess more than one equivalent. But granting the atomic 
 hypothesis, its atom can have only one definite weight. 
 That atomic weight may, however, be inferred from its 
 specific heat, or from the density of its gaseous compounds. 
 Let us consider a concrete instance of each of these 
 methods. 
 
 The analysis of two of the oxides of iron leads to the 
 following results : 
 
 Ferrous oxide. Ferric oxide. 
 
 Iron 77-77 70.00 
 
 Oxygen . . . 22.22 30.00 
 
 99.99 100.00 
 
 Equivalent. Now, i gram of hydrogen combines 
 with 8 grams of oxygen in water ; and 8 is therefore 
 chosen as the equivalent of oxygen ; for the definition of 
 an equivalent is that amount of an element which will 
 combine with or replace one part by weight of 
 hydrogen. In ferrous oxide, since 22.22 grams of 
 oxygen combine with 77.77 grams of iron, 8 grams of 
 
 77*7 7 X 8 
 oxygen will combine with L =28 grams of iron: 
 
 and in ferric oxide, 8 grams of oxygen are in combination 
 with = 1 8. 66 grams of iron. Thus the equivalent 
 
16 MODERN CHEMISTRY 
 
 of iron in ferrous oxide is 28, and in ferric oxide 18.66. 
 The question now arises, What is the atomic weight of 
 iron ? We have seen that the specific heat of iron is 
 0.1138; and we know that the specific heat of solid 
 hydrogen is probably 6. And as the specific heats of 
 elements are inversely as their atomic weights, we have 
 the proportion 
 
 Specific heat Specific heat Atomic weight Atomic weight 
 
 of iron. of hydrogen. of hydrogen. of iron. 
 
 o.i 138 : 6 : : i : 52.7 
 
 The number 52.7 is nearly 2 x 28, and nearly 3 x 18.66 ; 
 these products give 56 ; and it must be remembered that 
 Dulong and Petit's law is not absolute, but merely an 
 approximation ; hence 56 is accepted as the true atomic 
 weight of iron. 
 
 The element sulphur forms a compound with hydrogen 
 which has the following composition : 
 
 Sulphur 94.12 per cent. 
 
 Hydrogen ..... 5.87 
 
 100.00 
 
 The gas is 17.1 times as heavy as hydrogen; and this 
 means that a molecule of hydrogen sulphide is 17.1 times 
 as heavy as a molecule of hydrogen. But a molecule of 
 hydrogen is believed to consist of two atoms ; hence a 
 molecule of hydrogen sulphide is 34.2 times as heavy as an 
 atom of hydrogen. Now, if this gas consists of one atom 
 of hydrogen in combination with one atom of sulphur, then 
 the atomic weight of sulphur will be the same as its equiva- 
 lent, viz., ^=16 ; but if it contain two atoms of 
 5.00 
 
 hydrogen, then the atomic weight of sulphur will be 
 ~Q = 32. The first hypothesis is impossible, for then 
 the molecular weight of hydrogen sulphide would be 
 
ISOMORPHISM 17 
 
 16+ i = 17 ; whereas, it has been found to equal 34.2 ; 
 but if two atoms of hydrogen are present in sulphide of 
 hydrogen, the molecular weight is 32 + 2 = 34 ; and this is 
 nearly the same as the number found, viz., 34.2. It is, 
 however, still possible that hydrogen sulphide consists of two 
 atoms of hydrogen in union with two atoms of sulphur, in 
 which case the atomic weight of sulphur might still be 16 ; 
 but many other gaseous or gasifiable compounds of sulphur 
 are known, and in none of them is the molecular weight 
 such that they could be supposed to contain less than 32 
 parts of sulphur for each part of hydrogen, or its equivalent 
 of another element. It is consequently regarded as impro- 
 bable that the atomic weight of sulphur is less than 32 ; and 
 that 32 is the correct number follows also from the deter- 
 mination of its specific heat. 
 
 Isomorphism. A third method of arriving at the 
 correct atomic weight of an element was suggested in 1819 
 by Eilhard Mitscherlich, then Professor in Berlin. When 
 two substances crystallise in the same crystalline form, 
 they are said to be "isomorphous " with each other. It is 
 often the case that such compounds are similar chemically ; 
 that is, they may contain the same number of atoms, and 
 may also closely resemble each other physically. Thus, 
 there is a large class of compounds, named " alums," which 
 are sulphates of two metals. Ordinary alum is a sulphate 
 of aluminium and potassium ; it crystallises in eight-sided 
 regular figures, termed " octahedra." When the rare metal 
 gallium was discovered, it was found to form an "alum ; " 
 it gave a sulphate of gallium and potassium, crystallising in 
 octahedra, and similar in properties to ordinary alum. Now, 
 Mitscherlich's statement was, that when one element takes 
 the place of another in an isomorphous crystal of the same 
 chemical character, the substitution occurs so that one atom 
 of the one replaces one atom of the other. Hence, if the 
 atomic weight of the one element is known, the weight of 
 the other element which replaces it will be proportional to 
 its atomic weight. In the case above mentioned, it was 
 
 VOL. i. B 
 
i8 MODERN CHEMISTRY 
 
 found that 27.1 parts by weight of aluminium were replaced 
 by 69.9 parts of gallium ; and as it was known from experi- 
 ments such as those previously described that the atomic 
 weight of aluminium is 27.1, it follows that 69.9 is the 
 atomic weight of gallium. But care is necessary in using 
 this indication of the atomic weight ; for it may happen that 
 two compounds may contain the same number of elements in 
 the same proportions, and have a similar crystalline form ; 
 and yet Mitscherlich's law may not be applicable. 
 
CHAPTER II 
 
 Gaseous and Osmotic Pressure Boyle's, Gay- 
 Lussac's, Pfeffer's, and Raoult's Laws. 
 
 IF we grant, in accordance with modern views, that matter 
 consists of minute particles, termed molecules, it must also 
 be allowed that the distance between these ultimate particles 
 must be very different, according to whether the matter is 
 in the solid, or liquid, or in the gaseous state. Thus, a 
 cubic centimeter of water at 100 expands, when it is 
 boiled into steam of the same temperature, to 1700 cubic 
 centimeters; and a cubic centimeter of oxygen, measured 
 at its boiling-point, -182, boils into 266 cubic centimeters 
 of oxygen gas of the same temperature. In changing its 
 state, therefore, from liquid or solid to gas, matter under- 
 goes a great alteration of volume. It is accordingly to be 
 expected that the molecules of a gas, being at so much 
 greater a distance from each other than the molecules of a 
 solid or liquid, should yield more readily to pressure, and 
 should decrease in volume when the pressure is raised, 
 much more than solids or liquids. It is also found, as 
 appeared probable, that the expansion of a gas is much 
 greater than that of a solid or a liquid, by a definite rise 
 of temperature. 
 
 Boyle's Law. The law relating to the compres- 
 sibility of gases was discovered by Boyle. It is, that if 
 temperature be kept constant, the volume of all gases 
 is inversely as the pressure. Thus, if the pressure of the 
 atmosphere, which is equal to 1033 grams on each square 
 
20 MODERN CHEMISTRY 
 
 centimeter of the earth's surface at sea-level, or approxi- 
 mately 15 Ibs. on each square inch, be doubled, the volume 
 of a given weight of air, or of any other gas, will be halved ; 
 on trebling the pressure the volume is reduced to one-third, 
 and so on. As the length of a column of mercury, one 
 square centimeter in cross-section, must be 76 centimeters 
 in order that its weight shall be 1033 grams, 76 centi- 
 meters is taken as the " normal " height of the barometer. 
 And if the height of the mercury in a gauge or "mano- 
 meter" is 152 centimeters, the pressure which produces 
 that rise in the mercurial column will halve the volume 
 of a gas exposed to it. 
 
 Gay=LtUSsac's L,aw. The law connecting the 
 volume of a gas with the temperature was discovered by 
 Gay-Lussac, and independently by Dalton; but it is gene- 
 rally attributed to the former chemist. It is : Provided 
 pressure be kept constant, the volume of a gas, mea- 
 sured at C., increases by ^J^, for each rise of 1. 
 Or i volume of gas at o will become 1.00367 volume at 
 i; 1.0367 volume at 10 ; 1.367 volume at 100, and 
 so on. Generally stated, if / stand for a temperature, I 
 volume of gas will become I +0.00367^ when heated from 
 o to that temperature. 
 
 A third law may be deduced from these two ; it is, 
 that if the volume of a gas be kept constant, the 
 pressure of a gas will increase ^j of its initial 
 value at for each rise of 1. 'This is evident from 
 the following consideration : Suppose that I volume of 
 a gas is heated from o to I ; the volume will increase 
 to 1.00367 volume. To reduce the volume again to its 
 initial value, I, the pressure must be raised by 0.00367 
 of its original amount. If the initial pressure corresponded 
 to that of 76 centimeters of mercury, it would have to 
 be increased to 76 + (76 x 0.00367) centimeters, or to 
 76.279 centimeters in order that the gas should resume 
 its original volume of i. The same consideration will 
 hold if the gas is cooled instead of being heated ; but 
 
DIFFUSION 21 
 
 of course in that case the pressure will be reduced, in- 
 stead of being raised. It follows from this, that if the 
 temperature could be reduced to 273 below o C., the 
 gas v/ould exert no pressure. This temperature, 273, 
 is termed " absolute zero." As a matter of fact, so low 
 a temperature has never been reached ; and, moreover, it is 
 certain that all gases would change to liquids before that 
 temperature was attained. But it serves as the starting- 
 point for what is termed the " absolute scale of tempera- 
 ture." Gay-Lussac's law may therefore be stated thus: 
 The volume of a gas at constant pressure increases 
 as the absolute temperature ; and its corollary, thus : 
 Tlie pressure of a gas at constant volume increases 
 as the absolute temperature. For o C. corresponds 
 with 273 on the absolute scale; and 273 volumes of gas 
 will become 274, if the temperature is raised from 273 
 absolute to 274 absolute. Similarly, the pressure of a gas 
 will increase in the proportion 273 : 274 if the absolute 
 temperature is increased from 273 to 274. 
 
 Pressure Proved by Diffusion. When a solid 
 is dissolved in a liquid, as, for example, sugar in water, the 
 particles of sugar its molecules must obviously be 
 separated from each other to a greater or less extent, ac- 
 cording as much or little water be added. And it has 
 been noticed that if the sugar be placed in the water, and 
 not stirred up, the sugar will dissolve at the bottom of the 
 vessel, and the strong solution of sugar will slowly mix up 
 with the upper layer of water, and in course of time be 
 equally distributed through the water ; just as a heavy gas, 
 such as carbonic acid gas, if placed in an open jar, will 
 gradually escape into the lighter air above it. This pro- 
 cess of mixing of two liquids or gases is termed "diffusion." 
 It is now generally held that the pressure of a gas on the 
 walls of the vessel which contains it is produced by the 
 impacts of its molecules against the walls ; and as the 
 molecules are extremely numerous, and in a state of very 
 rapid motion, they escape from an open vessel ; so that 
 
22 MODERN CHEMISTRY 
 
 even a heavy gas, like carbon dioxide, will escape upwards 
 into a lighter gas ; and similarly, a light gas, like hydro- 
 gen, will escape downwards into a heavy gas, owing to the 
 unceasing motion of its molecules. The fact that the 
 molecules of sugar, which, by the way, becomes itself a 
 liquid when dissolved in water, travel upwards, and diffuse 
 through the lighter water, shows that they too are in 
 motion ; but the slowness of the diffusion, compared with 
 the rate of diffusion of a gas, indicates that their motion 
 is much impeded by the molecules of water, with which 
 they are constantly coming into collision. And just as the 
 motion of the molecules of a gas produces pressure, and 
 causes the gas to escape through an opening, so the motion 
 of the molecules of sugar, which causes them to rise through 
 water against the attraction of the earth, may be taken to 
 imply that they also exert a kind of pressure. But the 
 molecules of wajer, with which the molecules of sugar are 
 mixing, must also be held to exert pressure of the same 
 kind, since they disperse themselves through the molecules 
 of sugar. How is it possible to distinguish the pressure due 
 to the sugar from that due to the water ? A parallel case 
 with gases will help us to reply to this question. 
 
 Dalton's Law of Partial Pressures. Suppose a 
 vessel of one litre capacity to be filled with oxygen gas at 
 o, and under the atmospheric pressure of 76 centimeters of 
 mercury. The oxygen will exert pressure on its walls 
 equal to that of the atmosphere, for the vessel may be 
 placed in communication with the atmosphere, in order to 
 equalise pressure, before it is closed. Now let half a litre 
 of hydrogen be introduced by means of a force-pump. As 
 temperature and volume remain the same, the pressure will 
 be increased to 76 cms. + 38 cms. Introduce another half- 
 litre of hydrogen, and the initial pressure will be doubled; it 
 will now be 152 cms. Let another litre of hydrogen be 
 introduced, and the initial pressure will be trebled. We 
 might introduce a third gas, say nitrogen, into the vessel, and 
 the pressure would be increased proportionately to the quan- 
 
LAW OF PARTIAL PRESSURES 23 
 
 tity introduced. Each constituent of the gaseous mixture, 
 accordingly, exerts pressure on the walls of the containing 
 vessel proportionally to its relative amount. For example, 
 the pressure of the nitrogen of the oxygen and of the argon 
 in air is proportional in each case to the amounts of these 
 constituents, viz., oxygen, about 21 per cent. ; nitrogen, 78 
 per cent. ; and argon, i per cent. This statement is known 
 as Dalton's law of partial pressures. If the pressure of 
 the air is 76 cms., that of the nitrogen is T 7 ^ y x 76 ; of the 
 oxygen, -f^ x 76 ; and of the argon, y^ x 76 cms. In the 
 case of liquids, however, such a method fails. For while 
 in some instances the volume of a solution is nearly equal to 
 that of the solvent, plus that of the dissolved substance, in 
 others the volume is less, and in a few instances greater. 
 
 A device has, however, been discovered, by which it is 
 possible to measure the partial pressure of the dissolved sub- 
 stance ; and again an example will first be given from the 
 behaviour of gases. The rare metal palladium is permeable 
 at high temperatures by hydrogen, but not by other gases. 
 Now, if a vessel made of palladium be filled with a gas that 
 cannot escape through its walls for example, with nitrogen 
 at atmospheric pressure equal to that of 76 cms. of mercury 
 and at a high temperature, say 300 C. ; and if it be 
 then surrounded with hydrogen gas, also at atmospheric 
 pressure, the pressure of the gases in the interior of the 
 vessel will rise to two atmospheres, owing to the entry of 
 hydrogen through the walls, which are permeable to that gas 
 alone. The mercury in the gauge connected with the 
 palladium vessel will rise, until it stands at a height of 76 cms., 
 showing that the original atmospheric pressure has been 
 doubled. As there is no opposition to the passage of the 
 hydrogen inwards or outwards through the walls of the 
 vessel, hydrogen will enter until th.e pressure of the hydrogen 
 in the interior is equal to that on the exterior of the vessel. 
 But the nitrogen cannot escape, hence it exerts its original 
 pressure of 76 cms. of mercury. 
 
 Osmotic Pressure. The partial pressure of the dis- 
 
24 MODERN CHEMISTRY 
 
 solved substance in a solution has been measured by a similar 
 plan, devised by the German botanist Pfeffer. It was 
 necessary for this purpose to discover a " semi-permeable 
 membrane," through the pores of which water could pass 
 freely, but which would be impermeable to the dissolved 
 substance. A slimy precipitate, produced by adding 
 potassium ferrocyanide to copper sulphate, is not permeated 
 by dissolved sugar, though water freely penetrates it. But a 
 diaphragm of this nature is far too tender to withstand any 
 pressure. Pfeffer succeeded in depositing the slimy ferro- 
 cyanide of copper in the interior of the walls of a pot of 
 porous unglazed earthenware, and so constructing a vessel 
 which could be closed with a glass stopper, with the help of 
 cement. The stopper, which was hollow, was placed in 
 connection with a gauge containing mercury ; and after the 
 pot and stopper had been filled with a solution of sugar, the 
 stopper was connected with the gauge, which thus registered 
 the pressure upon, and consequently exerted by, the liquid. 
 The pot was then immersed in a large vessel of water, which 
 could be heated to any desired temperature, not so high as 
 to soften the cement. It was found that the water slowly 
 entered the pot, and consequently raised the mercury in the 
 gauge ; but after a certain quantity had entered, the ingress of 
 water stopped, and the pressure ceased to rise. 
 
 The pressure thus raised has been termed " osmotic 
 pressure." The numbers which follow were obtained by 
 Pfeffer: 
 
 Concentration. Pressure. Ratio. 
 
 1 percent. 53.5 cms. 53.5 
 
 2 101.6 50.8 
 4 >, 2 8 - 2 5 2 - 1 
 6 37-5 5 T -3 
 
 When a gas occupying a certain volume is increased in 
 quantity by pumping in an equal volume of gas, it is clear 
 that the number of molecules in the volume is doubled ; 
 und experiment shows that, in accordance with Boyle's law, 
 
OSMOTIC PRESSURE 25 
 
 the pressure is doubled. The concentration of a solution 
 is expressed by the weight of dissolved substance in 100 
 parts of the solution ; and it is evident from PfefFer's 
 numbers that, on doubling the number of molecules of 
 sugar in a given volume of the solution, the osmotic 
 pressure is also doubled. The osmotic pressure, in fact, 
 increases directly as the concentration, exactly as with 
 gases. 
 
 PfefFer also made experiments at different temperatures. 
 Owing to the softening of the cement with which the semi- 
 permeable pot was closed, he was not able to use high 
 temperatures ; but some of his results are given below : 
 
 Temperature Temperature Pressure 
 
 C. Abs. Pressure. Calculated. 
 
 i4.2 + 273 = 287.2 51.0 cms. 51.0 cms. 
 
 15.5 - 288.5 52.1 51.2 
 
 32.0 = 35- 54-4 >, 54-i 
 
 36.0 = 39- 5 6 -7 >, 54-9 99 
 
 The results are meagre, but, so far as the^ go, in reasonably 
 good accord. Experiments of this kind have seldom been 
 made, owing to the difficulty in preparing satisfactory 
 membranes. The calculation has been made on the assump- 
 tion that the osmotic pressure, like the gaseous pressure, 
 increases directly as the absolute temperature. 
 
 A striking proof of the correctness of the analogy 
 between osmotic and gaseous pressure is derived from the 
 following consideration : A gram of oxygen gas, measured 
 at o C. and 76 cms. pressure, has been found to occupy 
 699.4 cc. ; now, 32 grams of oxygen form a gram- 
 molecule, for the atomic weight of oxygen is 16, and 
 there are two atoms of oxygen in a molecule of the gas, 
 as we have seen on p. 13. The volume of 32 grams is 
 accordingly 699.4x32 = 22,380 cc. The simplest for- 
 mula for cane-sugar is C 12 H 22 O n , and as the atomic 
 weight of carbon is 12, the molecular weight of sugar is 
 at least (12 x 12) + (22 x i) + (i i x 16) = 342. If it 
 
26 MODERN CHEMISTRY 
 
 
 
 were possible for cane-sugar to exist in the state of gas, it 
 might be expected that 342 grams in 22,380 cc. would 
 exert the same pressure as 32 grams of oxygen, viz., 
 76 cms., since 342 grams of sugar are likely to contain 
 as many molecules as 32 grams of oxygen. But sugar 
 chars when heated, and decomposes. However, it is 
 possible to calculate, by means of Boyle's and Gay- 
 Lussac's laws, the pressure which a i per cent, solution 
 of sugar ought to exert at 14.2 C. If there were 
 223.8 grams in 22,380 cc., the solution would be one 
 of I per cent. And the pressure which it should exert 
 
 would be x 76, or 51.66 cms. at o C., or 273 
 
 Abs. And at 14.2 C., or 287.2 Abs., this pressure 
 should be increased in the proportion 273 : 287.2 ; giving 
 a theoretical pressure of 52.5 cms.; the actual pressure 
 measured was 51 cms. a fairly close approximation. It 
 may, therefore, be taken that sugar in solution in water 
 exerts the same osmotic pressure on the walls of a semi- 
 permeable vessel, as the same number of molecules would 
 do, if it were in the state of gas, occupying the same 
 volume, and at the same temperature. 
 
 Experiments with semi-permeable diaphragms arc very 
 difficult ; the diaphragm seldom receives sufficient support 
 from the pipe-clay walls of the pot, and is usually torn 
 when the pressure rises to even a very moderate degree. 
 But it is not necessary to attempt such measurements ; for 
 the Dutch chemist, J. H. van't Hoff, now Professor of 
 Physical Chemistry in Berlin, pointed out in 1887 that 
 very simple relations exist between the osmotic pressure of 
 solutions and the lowering of the freezing-point of the 
 solvent, due to the presence of the dissolved substance, 
 and also the rise of boiling-point of the solvent, produced 
 by the same cause. A proof of this connection will not be 
 attempted here, but the facts may be shortly stated. 
 
 Measurement of Osmotic Pressure by Lower* 
 ing of Freezing-point. All pure substances have a 
 
DEPRESSION OF FREEZING-POINT 27 
 
 perfectly definite melting-point ; thus, ice melts at o C., 
 sulphur at 120, tin at 226, lead at 325, and so on. 
 These temperatures are also the freezing-points of the 
 liquids, provided some of the solid substance is present. 
 If this is not the case, then it is possible to cool the liquid 
 below its freezing-point without its turning solid. Accord- 
 ingly, water freezes at o if there is a trace of ice present ; 
 melted tin solidifies at 226 if there is a trace of solid tin 
 added to the cooled liquid ; and if, for example, water be 
 cooled without the presence of ice, until it has a tempera- 
 ture lower than o, say 0.5 below o, on addition of a 
 spicule of ice a number of little crystals of ice begin to 
 form in the liquid and the temperature rises to o. But 
 if there is some substance dissolved in the liquid, as, for 
 example, sugar in the water or lead in the tin, then the 
 freezing-point is lowered below that of the pure substance. 
 And when the solvent freezes, in general the solid consists 
 of the solid solvent, none of the dissolved substance crystal- 
 lising out with it. It is owing to this fact that travellers in 
 Arctic regions manage to get water to drink; for the ice 
 from salt water is fresh, and when melted yields fresh water. 
 It has been observed that with the same solvent the 
 freezing-point is lowered proportionally to the amount of 
 dissolved substance present, provided the solution is a 
 dilute one. Thus, a solution of cane-sugar in water, 
 containing 3.42 grams of sugar in 100 grams of the 
 solution, froze at 0.185 below zero; and one contain- 
 ing half that quantity, 1.71 grams, froze at 0.092 below 
 zero. Again, the same lowering of the freezing-point is 
 produced by quantities proportional to the molecular 
 weights of the dissolved substances. Malic acid, an acid 
 contained in sour apples, has the molecular weight 134, 
 while it will be remembered that the molecular weight of 
 cane-sugar is 342. Now, a solution of 1.34 grams of 
 malic acid in water, made up with water so that the whole 
 solution weighed 100 grams, froze at 0.187 below zero, a 
 number almost identical with that found for sugar. 
 
28 MODERN CHEMISTRY 
 
 Solvents other than water may also be used ; but in 
 that case the lowering of the freezing-point is different. 
 Acetic acid, which is vinegar free from water, is often 
 employed ; so also is benzene, a compound separated from 
 coal-tar, produced in the manufacture of coal-gas. The 
 freezing-point of acetic acid is 17; that of benzene is 
 4.9. It was found in 1884 by Raoult, Professor of 
 Chemistry in Grenoble in the South of France, that while 
 1.52 grams of camphor (the hundredth part of its mole- 
 cular weight) dissolved in benzene (100 grams of solution) 
 lowered the freezing-point of the benzene by 0.514, the 
 same quantity of camphor, forming a solution in acetic acid 
 of the same strength, lowered the freezing-point of the 
 latter by 0.39. And he also noticed that the lowering of 
 the freezing-point is proportional, at least in some cases, to 
 the molecular weights of the solvents. Thus, the mole- 
 cular weights of acetic acid and benzene are respectively 
 60 and 78; and as 0.39 : 0.514 : : 60 : 79, the pro- 
 portionality is very nearly exact. 
 
 It is possible by this means to determine the molecular 
 weight of any substance which will dissolve in any solvent 
 for which the depression produced in the freezing-point is 
 known. Thus, for example, Beckmann, the deviser of 
 the apparatus with which such determinations are made, 
 found that a solution of naphthalene, a white compound of 
 carbon and hydrogen contained in coal-tar, in benzene, the 
 solution containing 0.452 per cent, of naphthalene, lowered 
 the freezing-point of benzene by 0.140. A I per cent, 
 solution would therefore cause a lowering of 0.309. And 
 as 0.309 : 0.39 :: 100 : 126, this is therefore the molecular 
 weight of naphthalene. The simplest formula for naphtha- 
 lene is C 5 H 4 , for its percentage composition is carbon, 
 93.75, hydrogen, 6.25 ; and to find the relative number 
 of atoms, the percentage of carbon must be divided by the 
 atomic weight of carbon, and that of hydrogen by its atomic 
 
 weight, thus : "3*75 _ y^gj^ an d _ v5 _ 5^5 and these 
 
RISE OF BOILING-POINT 29 
 
 numbers are to each other in the proportion 5 : 4. But 
 a substance with the formula C 5 H 4 must have the molecular 
 weight (5xi2)4-(4Xi)=64; whereas the molecular 
 weight found is 126. Now, 126 is nearly twice 64; 
 hence the formula of naphthalene must be C 10 H g . The 
 method is not exact, but it affords evidence which, taken 
 in conjunction with the analysis of the compound, enables 
 the molecular weight to be determined. 
 
 Measurement of Osmotic Pressure by Rise 
 of Boiling-point. A method for determining the mole- 
 cular weights of substances by the rise of boiling-point of 
 their solutions was also devised by Beckmann, and it is 
 frequently used. The process is analogous to that in 
 which the depression of freezing-point is made use of. 
 Every pure substance has a perfectly definite boiling-point, 
 provided that pressure is constant ; but if any substance is 
 dissolved in a pure liquid, the boiling-point of the latter 
 is raised ; and it is found that the rise of boiling-point is 
 proportional to the number of molecules of the dissolved 
 substance present. As an example, let us calculate the 
 molecular weight of iodine dissolved in ether from the 
 rise in the boiling-point of the ether. The rise caused by 
 the hundredth part of the molecular weight of a substance 
 taken in grams, and dissolved in 100 grams of ether, is 
 0.2105. Now, Beckmann found that 1.513 grams of iodine 
 dissolved in 100 grams of ether raised the boiling-point 
 of the ether by o. 1 26. And to raise the boiling-point by 
 0.2105, 2.53 grams of iodine would have been necessary; 
 2.53 is therefore the hundredth part of the molecular 
 weight of iodine. It is possible to weigh iodine in the 
 state of gas, for it is an easily volatilised element ; and its 
 vapour has been found to be 126 times as heavy as hydro- 
 gen. We have seen that this statement implies that a 
 molecule of iodine gas is 1 26 times as heavy as a molecule 
 of hydrogen gas ; and as a molecule of hydrogen consists 
 of two atoms, a molecule of iodine gas is 252 times as 
 heavy as an atom of hydrogen, or its molecular weight 
 
30 MODERN CHEMISTRY 
 
 is 252. The number obtained from the density of the gas 
 is accordingly almost identical with that obtained from the 
 rise in the boiling-point of ether. 
 
 We have now studied four methods by means of which 
 the molecular weights of elements and compounds have 
 been ascertained ; they are : 
 
 1 I ) By determining the density of the substance in the 
 state of gas with reference to hydrogen, and doubling the 
 number obtained ; for molecular weights are referred to the 
 weight of an atom of hydrogen, while a molecule, it is 
 believed, consists of two atoms. 
 
 (2) By measuring the osmotic pressure exerted by a 
 solution of the substance, and comparing the pressure with 
 that exerted by an equal number of molecules of hydrogen, 
 occupying the same volume, at the same temperature. 
 
 (3) By comparing the depression in freezing-point of a 
 solvent containing the substance in solution, with the de- 
 pression produced by the hundredth part of the molecular 
 weight in grams of a substance of which the molecular 
 weight is known, and by then making use of the known 
 fact that equal numbers of molecules produce equal depres- 
 sion in the freezing-point of a solvent. 
 
 (4) By a similar method applied to the rise in boiling- 
 point of a solvent caused by the presence of a known 
 weight of the substance of which the molecular weight is 
 required. 
 
CHAPTER III 
 
 Dissociation Electrolytic Dissociation or 
 lonlsatlon. 
 
 Dissociation. A certain number of substances are 
 known which apparently do not conform to the laws which 
 have been explained in the last chapter. For example, the 
 compound of ammonia with hydrochloric acid, which has 
 the formula NH 4 C1, should have the density 26.75, f r 
 the atomic weights of the elements it contains are N= 14 ; 
 H= i ; Cl= 35.5 ; and the molecular weight is the sum 
 of 14 + 4+35-5 = 5 3 5- But the found density is only 
 one quarter of this number, viz., 13.375. It was at first 
 imagined that this discrepancy was to be explained by 
 abnormal expansion of the gas ; but with such a supposition, 
 of course, Avogadro's law could not hold. Other sub- 
 stances which show the same " abnormal densities " are 
 pentachloride of phosphorus and sulphuric acid. To ex- 
 plain this abnormality, Henri Saint-Claire Deville pro- 
 pounded the idea that such substances do not go into the 
 state of gas as compounds, but that they split into simpler 
 components, each of which has its usual density, and a 
 mixture of the components will exhibit a mean density. 
 Thus, if ammonium chloride be imagined to decompose into 
 ammonia and hydrogen chloride on changing into gas, then 
 the density of the supposed ammonium chloride gas will be 
 the mean of the densities of its two constituents. Ammonia 
 has the formula NH 3 , and hydrogen chloride, HC1 ; the 
 former has the density 8.5, and the latter, 18.25 > an< ^ tne 
 
 31 
 
32 MODERN CHEMISTRY 
 
 mean of these two numbers is 13.375. Phosphoric chloride, 
 which has the formula PC1 5 , splits in a similar manner into 
 PC1 3 and C1 2 ; and sulphuric acid, H 2 SO 4 , into water, 
 H 2 O, and sulphuric anhydride, SO 3 . To this kind of 
 decomposition, where the bodies which are decomposed by 
 a rise of temperature re-unite on cooling to form the origi- 
 nal substance, Deville gave the name dissociation. It has 
 been found possible, by taking advantage of the fact that 
 light gases, like ammonia, pass out through an opening, 
 or, as it is termed, " diffuse" more rapidly than heavier 
 gases, like hydrogen chloride, to separate these gases, 
 and thus to prove that they exist as such in the vapour 
 of ammonium chloride ; for compounds are not decom- 
 posed into their constituents by diffusion ; hydrogen chlo- 
 ride diffuses as such, and is not split into hydrogen and 
 chlorine. 
 
 Let us look at this dissociation from another standpoint. 
 We know that if 2 grams of hydrogen, or 32 grams of 
 oxygen, or 28 grams of nitrogen, or, in fact, the molecular 
 weight of any gas expressed in grams, be caused to occupy 
 22,380 cubic centimeters at o C., the pressure exerted by 
 the gas will be 76 centimeters of mercury. If the tempe- 
 rature is higher, the pressure will be increased proportionally 
 to the increase in absolute temperature. Thus, suppose the 
 temperature were 300 C., the pressure would be increased 
 in the proportion 273 Abs. : 573 Abs. :: 7 6 cms. : 1 60 cms. 
 Now, if 53.5 grams of ammonium chloride were placed in 
 a vacuous vessel of 22,380 cc. capacity, and the temperature 
 were raised to 300 C., and if no dissociation were to 
 take place, one would expect a pressure equal to that of 
 1 60 cms. of mercury. It has been found, however, that 
 the actual pressure is twice that amount, or 320 cms. In 
 order to account for the doubled pressure, the supposition 
 that dissociation has taken place must again be made ; that 
 is, in order that the pressure must be doubled, twice as many 
 molecules must be present as one would have supposed from 
 the weight taken. The fact of dissociation may accordingly 
 
DISSOLVED SALTS 33 
 
 be inferred either from a diminished density or from an 
 increased pressure. 
 
 "Dissociation" of Salts in Solution. Few 
 
 measurements of the osmotic pressure of salts have been 
 made, owing to the difficulty in producing a membrane which 
 shall allow water to pass, and which shall be impermeable 
 to salts. But very numerous measurements of the depression 
 in freezing-point and the rise in boiling-point of solutions of 
 salts have been made ; and it has been already explained 
 that these quantities are proportional to the osmotic pressure 
 of the dissolved substances. It has been experimentally 
 discovered that in all such cases the fall in freezing-point, 
 or the rise in boiling-point is too great for the supposed 
 molecular weight of the salt. It must be concluded that 
 the osmotic pressure would also be increased, were it possible 
 to measure it. But the fall in freezing-point or the rise in 
 boiling-point does not imply a doubled osmotic pressure, 
 when thre is reason to expect it, unless the solution is very 
 dilute. Now, if the pressure were doubled, we might argue 
 from such cases as ammonium chloride that dissociation into 
 two portions had occurred ; but in moderately concentrated 
 solutions, as the pressure is not doubled, it must be concluded 
 that the dissociation is not complete ; it is only in very 
 dilute solutions that complete dissociation can be imagined to 
 have taken place. Cases are known where substances in the 
 state of gas undergo gradual dissociation, and then the pres- 
 sure does not attain its maximum until the temperature has 
 been sufficiently raised or the pressure sufficiently re- 
 duced. The reason that this is not noticed with ammonium 
 chloride is that the temperature of complete dissociation has 
 been reached before the substance turns to gas. 
 
 Common salt is chloride of sodium ; its formula is 
 NaCl ; and for long the suggestion that it dissociated 
 into an atom of sodium and an atom of chlorine on being 
 dissolved in water was received as too improbable to be 
 worth consideration. There is, of course, another way 
 out of the difficulty ; it is to suppose that a molecule of 
 
 VOL. i. c 
 
34 MODERN CHEMISTRY 
 
 salt has the formula Na.,Cl 2 ; in that case, 58.5 grams of 
 salt (2 x 11.5) + (2 x I 775) dissolved in 1 0,000 grams 
 of water should produce the normal lowering of freezing- 
 point ; or, if it produced a larger lowering, it might be 
 supposed that these complex molecules had split more or 
 less completely into the simpler molecules, NaCl. But 
 though the explanation suggested might account for this 
 instance, it is incapable of accounting for the fact that 
 chloride of barium, which is known to possess the formula 
 BaCl 2 (or a multiple thereof), gives, in sufficiently dilute 
 solution, a depression three times that which one would 
 have expected from its supposed molecular weight, or that 
 ferricyanide of potassium and ferrocyanide of potassium, 
 the formulae of which are respectively K 3 Fe(CN) 6 and 
 K 4 Fe(CN) 6 , should give four and five times the expected 
 depression. But these results are quite consistent with the 
 hypothesis that 
 
 NaCl + Aq decomposes into Na. Aq and Cl. Aq ; 
 BaCl 2 + Aq decomposes into Ba. Aq and Cl. Aq, and Cl. Aq ; 
 K 3 Fe(CN) 6 + Aq decomposes into K.Aq + K. Aq + K. Aq, 
 
 and Fe(CN) 6 .Aq; and 
 K Fe(CN) 6 + Aq decomposes into K.Aq + K.Aq + K ,Aq 
 
 + K.Aq, and Fe(CN) 6 .Aq. 
 
 (The symbol " Aq " stands for an indefinite but large 
 amount of water "aqua.") Here again we are face to 
 face with facts and an attempted explanation. The facts are 
 that certain compounds, which have long been known as 
 " salts," give too great a depression of the freezing-point 
 or too great a rise of boiling-point of the solvent in which 
 they are dissolved, corresponding to too great an osmotic 
 pressure. It has been observed that when the dilution is 
 sufficient the depression in each case reaches a maximum, 
 and that that maximum is two, three, four, or five times 
 what might be expected ; and in each case it is possible to 
 divide the salt into two, three, four, or five imaginary por- 
 tions, which often consist of atoms, though frequently of 
 groups of atoms. 
 
ELECTROLYSIS 35 
 
 Electrolytic Conductivity of Salt Solutions. 
 
 This hypothesis, that a kind of dissociation takes place in 
 salt solution, might have failed to gain acceptance had it 
 not been for a very remarkable coincidence. It appears 
 that all solutions which show this behaviour allow an electric 
 current to pass through them, whereas all solutions of com- 
 pounds such as cane-sugar, do not permit the passage of a 
 current of electricity. The latter class of compounds is 
 called " non-conducting ; " the former class contains com- 
 pounds which are " conductors " of electricity. But metals 
 and certain compounds, chiefly consisting of the sulphides 
 of the metals, are also conductors of electricity, with this 
 difference, however : while the latter are apparently un- 
 altered by the passage of the electric current, solutions of 
 salts undergo profound change. In some cases, oxygen 
 appears in bubbles at the plate connected with the positive 
 pole of the battery/ while hydrogen is evolved from that 
 connected with the negative pole ; in others, when the dis- 
 solved substance is a salt of such metals as copper, silver, or 
 mercury, the metals themselves are deposited on the negative 
 pole, or, as it is usually termed, the " kathode ; " while if 
 chlorine, bromine, or iodine is one of the constituents of 
 the salt, it is evolved at the " anode " or positive pole. 
 
 Faraday's Law. It was discovered in 1833 by 
 Michael Faraday, Professor of Chemistry in the Royal 
 Institution in London, that if an electric current be passed 
 simultaneously through different solutions, the weights of metals 
 deposited or of elements or groups of elements liberated are 
 proportional to their equivalents ( see p. 1 5 ) . If the same cur- 
 rent be passed, for example, through a solution of dilute sul- 
 phuric acid, copper sulphate, and iodide of potassium, each 
 contained in its own vessel, provided with plates of platinum 
 or some other unattackable metal dipping into the solution, 
 for every gram of hydrogen evolved from the kathode in 
 the vessel containing sulphuric acid, 8 grams of oxygen are 
 evolved from the anode; 31.8 grams of copper are de- 
 posited on the kathode dipping into the copper solution. 
 
36 MODERN CHEMISTRY 
 
 while 8 grams of oxygen rise in bubbles from the anode ; 
 and lastly, 127 grams of iodine are liberated from the anode 
 in the vessel containing potassium iodide, I gram of hydrogen 
 rising from the kathode. The evolution of hydrogen 
 instead of the deposition of potassium is due to the fact 
 that the metal potassium is unable to exist in presence of 
 water, but immediately displaces its equivalent of hydrogen. 
 All these numbers are in the proportions of the equivalents 
 of the elements. And without the liberation of these 
 elements no current passes. The elements may, there- 
 fore, in a certain sense, be said to convey the electricity ; 
 and as the same quantity of electricity passes through each 
 solution, liberating equivalents of the elements in each case, 
 it would appear that the same quantity of electricity is con- 
 veyed by quantities of elements proportional to their equiva- 
 lents. The equivalent of an element, it will be remembered, 
 is the weight of the element which can combine with or re- 
 place one part by weight of hydrogen ; it may be identical 
 with, or it may be a fraction of the atomic weight. In 
 the instances given above, the equivalents of iodine and of 
 potassium are identical in numerical value with their atomic 
 weighty ; but those of oxygen and of copper, 8 and 31.8, 
 are half their atomic weights, which are respectively 1 6 and 
 63.6. It would follow, therefore, that an atom of copper 
 or of oxygen is capable of conveying a quantity of electri- 
 city twice as great as that conveyed by an atom of hydrogen 
 or of iodine. 
 
 But how is it known that the atoms " convey " quantities 
 of electricity ? Must they be imagined as like boats, taking 
 in their load of electricity at one pole, and ferrying it over 
 to the other, and there discharging ? It was at one time 
 held that the process rather resembled the method of 
 loading a barge with bricks, where a row of men, who 
 may stand for the atoms, pass bricks, representing the 
 electricity, from one to the other. But it was proved 
 by Hittorf that the charged atoms actually travel or 
 "migrate" from one pole to the other, carrying with 
 
TRANSPORT OF ELECTRICITY 37 
 
 them their electric charges. And the charged atoms, for 
 which the name " ions," or "things which go," was de- 
 vised by Faraday, do not always move at the same rates. 
 The rate of motion depends on the friction which the ions 
 undergo on moving through the water or other solvent in 
 which the salt is dissolved. This friction is different for 
 different ions ; it also depends on the particular solvent em- 
 ployed ; and it is diminished if the temperature is raised. 
 The force which impels the ions is the same as that commonly 
 known as electric attraction and repulsion ; the negatively 
 charged atoms or "kations" being repelled from the negative 
 and attracted by the positive electrode dipping into the solu- 
 tion, while the positively charged atoms or "anions" are 
 repelled by the anode and attracted by the kathode. 
 
 When the anions touch the kathode, they are discharged ; 
 and similarly, when the kations touch the anode, they lose 
 their charge. And for every anion discharged, a kation 
 must simultaneously lose its charge. The result of this 
 is that the number of anions remaining in solution must 
 always be equivalent to the number of kations. It need 
 not always be the same, for it is possible for a kation 
 like copper to carry twice the charge of an anion like 
 chlorine ; but the number of " electrons," or electric 
 charges, must always be the same, although some ions 
 are capable of carrying more than one electron. There 
 can never, therefore, be an excess of, say, copper ions 
 in solution ; for they are always balanced by the requisite 
 number of anions. Thus, if the solution be evaporated, 
 the remaining salt has its usual composition ; though, of 
 course, there is less of it than if none had been decomposed. 
 
 Hittorf s Migration Constants. The fact that ions 
 move at different rates can be demonstrated in two ways, 
 one direct, the other indirect. The indirect method was 
 devised by Hittorf; the direct method, which is much 
 more recent, was first suggested by Lodge. 
 
 It is always advisable to form a mental picture, if 
 possible, of any physical phenomenon, pour prec'uer les 
 
38 MODERN CHEMISTRY 
 
 i flees 9 as the French say ; and a trivial illustration will be 
 now given which may render HittorPs conception clearer. 
 Imagine a ball-room with a door at each end. Suppose 
 the partners to be all separated from each other ; and 
 suppose an order to be given that the men shall march to 
 one door at twice the rate at which the ladies make for 
 the other door ; but that at the same time, for every man 
 who passes through the one door, only one lady shall 
 pass through the other door. At a given signal, say when 
 half the ballroom has escaped, let the condition of the 
 room be examined. It is easy to see that there will be 
 an equal number of men and women in the room, but 
 that there will be a greater number round the door at 
 which the men issue than round that at which the ladies 
 are trying to escape. And the rates of motion will be 
 proportional to the relative numbers in each half of the 
 ballroom, for the greater the rates at which the men move 
 proportionally to the ladies, the greater will be the number 
 in that part of the room at which the men are escaping. 
 
 This is a conception in close analogy with Hittorfs. 
 The men and women are the anions and kations ; and on 
 analysing the solutions round the anode and kathode, he 
 found that the concentration was, as a rule, altered, so 
 that he was forced to conclude that the rate of motion 
 towards the pole at which the concentration was increased 
 was more rapid than that towards the pole at which he 
 found the concentration to be diminished. By comparing 
 the concentrations, too, he calculated the relative rates of 
 motion of the anions and the kations towards the kathode 
 and the anode respectively. 
 
 Lodge's direct method has recently been improved by 
 Orme Mas son, and very accurate results have been ob- 
 tained by him. His plan is to trace the rate of motion 
 of the anions by following them up with a coloured anion, 
 such as the copper ion, which is blue, and can be seen, 
 while the rate of motion of the kation is indicated by 
 following it up with a coloured kation : the one he used 
 
MIGRATION OF IONS 39 
 
 for this purpose is the chromate ion, which is orange-yellow. 
 The apparatus which Masson employed consisted of two 
 flasks connected together by a narrow tube. This tube 
 is filled with a solution of the salt of which the rate 
 of migration of the ions is to be determined, but in order 
 to prevent diffusion of the liquid, or escape owing to 
 currents produced by differences of temperature, the water 
 in which the salt is dissolved contains enough gelatine to 
 make it set into a jelly when cold. It is found that the 
 gelatine does not appreciably interfere with the motion of 
 the ions. The one flask was charged with a solution of 
 copper chloride, and the anode plate was of copper. The 
 other flask was charged with a dilute solution of a mixture 
 of chromate and bichromate of potassium, and the kathode 
 was of platinum. The connecting tube was filled with a 
 warm solution of the salt to be examined, say potassium 
 chloride, in water containing gelatine, and after it had 
 cooled and set it was placed in position. On passing the 
 current, the potassions migrate towards the kathode, and 
 are followed closely by the blue cuprions, which serve to 
 mark the position of the rearmost of the potassions. The 
 chlorions, on the other hand, migrate towards the anode, 
 followed by the orange-yellow chromations, which reveal 
 their position. The rates can be measured by following 
 the advance of the colour in the tubes. If the ions have 
 equal velocity, as is nearly the case with potassions and 
 chlorions, the meeting-place of the blue and the orange is 
 nearly at the middle of the tube ; but if, as in most other 
 cases, the rates are different, the point of junction will be 
 at one side or other of the middle point of the tube. The 
 distances traversed in the same time give a direct measure 
 of the relative velocities of the anion and kation. Having 
 established this ratio, another salt, say sodium chloride, 
 having a different anion but the same kation, can be em- 
 ployed, and so the relative rates of potassion and sodion 
 may be compared. 
 
 The table which follows gives the rates of migration of 
 
40 MODERN CHEMISTRY 
 
 a few ions compared with that of potassion, which is taken 
 as 100. 
 
 K Na Li NH 4 Mg/2 Cl SO 4 /2 
 100 65.6 45.0 ioo 40.5 97.0 87.7 
 
 As the conductivity for a current depends on the velocity 
 both of the anion and the kation, relative numbers for the 
 conductivity may be obtained for any salt by adding the 
 numbers of the individual ions given above. Thus, if it is 
 required to find the conductivity of lithium sulphate, which 
 
 SO 
 
 has the formula LJ 2 SO 4 , we have Li = 45, and 4=87.7, 
 
 together equal to 132.7. 
 
 Measurement of the Extent of lonisatlon. It 
 
 is found in practice, however, that the conductivity of salts 
 agrees with the numbers deduced from the velocity of their 
 ions only when the solution is a very dilute one, and even 
 then not always. This can be ascribed to either or to both 
 of two causes. If the solution is a strong one, the mole- 
 cules of salt may bear an appreciable ratio to the molecules 
 of water, and may interfere by their possibly greater fric- 
 tion with the free transit of the ions. Or, on the other 
 hand, some of the molecules may not be resolved into ions, 
 and there may be fewer " boats " to carry across the elec- 
 trons, the progress of which towards the anode and the 
 kathode must consequently be slower, for the non-ionised 
 molecules take no share in the conveyance of electricity. 
 And as the conveyance of electricity depends on the number 
 of ions and on the rate at which they move, if the latter is 
 known, the relative number of ions may be calculated from 
 measurements of the conductivity of the solution. 
 
 Conductivity of Electrolytes. To measure the 
 conductivity of a solution, a " gram-molecular-weight," 
 i.e. the molecular weight of the salt taken in grams, is 
 dissolved in a litre of water. A small quantity of this 
 solution is placed in a small beaker immersed in a large 
 tank of water, so that the temperature may not vary ; for 
 
ELECTRIC CONDUCTIVITY 41 
 
 the conductivity increases with rise of temperature, owing 
 to the smaller resistance offered by hot water to the passage 
 of the ions, than by cold. Two circular platinum plates, 
 the surfaces of which have been roughened by having 
 platinum deposited on them, are immersed in the solution, 
 so that one lies at the bottom of the beaker, while the other 
 is one centimeter distant from it, higher up in the liquid. 
 The wire connected with the lower plate is protected by a 
 glass tube, in order that the current may pass only between 
 the plates. To measure the conductivity of this solution an 
 arrangement termed a " Wheatstone's bridge " is employed, 
 
 the construction of uhich will be understood from the an- 
 nexed diagram. B is a battery, actuating a small toy coil, C, 
 from the secondary terminals of which T T wires proceed 
 to the measuring-bridge Mb, which consists of a straight 
 piece of nickel-silver wire stretched along a scale. From 
 one end of the bridge, , the current traverses the solution ; 
 towards the other, M, the current passes through a resist- 
 ance-box, R, containing bobbins of wire of known resistance, 
 the number and resistance of which can be varied at will 
 until the resistance is nearly equal to that of the solution. 
 If they are exactly equal, and if the pointer P is exactly in 
 
42 MODERN CHEMISTRY 
 
 the middle of the bridge-wire Mb, then no sound can be 
 heard in the telephone T. The resistance of the solution 
 can then be read from the box. If, as generally happens, 
 the resistance of the solution is not equal to that in the box, 
 it is necessary to move the pointer P until the resistances are 
 equal. Having thus ascertained the resistance of the solu- 
 tion, a portion is diluted with water, so that its strength is 
 exactly half of the former, and the resistance is again deter- 
 mined. Successive dilutions in which the volume of the 
 solution is doubled, and again doubled, are made, and in 
 this manner the resistance due to an equal number of mole- 
 cules in each case is calculated. The conductivity is the 
 reciprocal of the resistance, and it is found that the mole- 
 cular conductivity increases with the dilution up to a certain 
 point. Thus Kohlrausch, the deviser of this method, found 
 the following numbers for sodium chloride : 
 
 Concentration : Molecular Relative number of ions 
 
 58.5 grams in conductivity, per TOO molecules of salt. 
 
 1 litre 69.5 67.5 
 
 2 litres 75.7 73.6 
 10 86.5 84.1 
 
 100 96.2 93.5 
 
 1,000 100.8 98.0 
 
 10,000 102.9 100.0 
 
 50,000 102.8 100.0 
 
 It is evident that 58.5 grams of salt dissolved in 10,000 
 litres of water give a maximum conductivity, for the dilu- 
 tion to 50,000 litres is attended by no further increase. 
 That the altered velocity is not influenced by the frictional 
 resistance of the water is obvious, for the solution of 58.5 
 grams of salt in 10 litres of water does not differ much 
 from pure water in this respect. The increase in conducti- 
 vity must accordingly be attributed to an increase in the 
 number of ions at the expense of the molecules ; and, as 
 a dilution in 50,000 litres of water produces no greater 
 conductivity than in 10,000, it must be concluded that 
 
CONDUCTIVITY OF WATER 43 
 
 complete ionisation has taken place. The figures in the 
 last column are obtained by simple proportion, thus : As 
 102.9 : 69.5 :: 100 : 67.5. 
 
 The extent of ionisation calculated from the conducti- 
 vities of salt solutions agrees well in the main with that 
 calculated from the depressions of freezing-point. 
 
 Conductivity of Pure Water. Two points remain 
 to be mentioned. One has reference to the conductivity of 
 pure water. It is no easy matter to prepare pure water ; 
 even after the water has been distilled, it contains traces 
 of substances which are ionised, such as carbonic acid or 
 ammonia. It is possible, by employing special precautions, 
 to add to the water before distillation substances which form 
 non-volatile compounds with these impurities, and by making 
 use of vessels of gold or platinum, which are not attacked, 
 however slightly, by water, to produce almost pure water. 
 Such water is not wholly devoid of conductivity, but its 
 resistance to the passage of electricity is very great. It 
 must be presumed that the water is for the most part mole- 
 cules of H 9 O, or perhaps even more complex molecules, 
 such as H 4 t) g , H 6 O 3 , &c. But there are, besides, a few 
 
 ions of H and OH ; so that water is capable of reacting in 
 certain cases where ions might be suspected. 
 
 Conductivity of Fused Salts. Another fact which 
 is well known, and largely put to practical use, is that fused 
 salts are, as a rule, good electrolytic conductors of electri- 
 city. Even when the salt is as pure as it can be made, it 
 still conducts in the molten state. Although the conducti- 
 vity of fused, salts has not been investigated with the same 
 completeness as that of solutions, yet it cannot be doubted 
 that the salt must be more or less ionised. The ionisation 
 appears to increase with rise of temperature, for the salt 
 becomes a better conductor. This, however, may be due, 
 in part at least, to the smaller frictional resistance which it 
 offers to the passage of the ions towards the electrodes. 
 But recent experiments have shown that the molecules of 
 
44 MODERN CHEMISTRY 
 
 some salts, at least those in which measurement is possible 
 are more complex than would be supposed from their 
 formulas. Thus, 'nitre, KNO 3 , consists of molecules of 
 four times that complexity, or K 4 N 4 O 12 ; and it is not 
 improbable that among these complex molecules there are 
 
 + 
 
 some ions of K and of NO 3 capable of conveying an elec- 
 tric charge. It may indeed be stated that those liquids 
 which possess complex molecules have the power of ionis- 
 ing salts dissolved in them. Water is one of the most 
 striking examples ; and it is to be presumed that such com- 
 plex molecules are able to surround and prevent ions from 
 at once discharging into one another by protecting them 
 from each other. 
 
 To sum up : Certain substances, in the state of gas, 
 exhibit dissociation that is, they decompose into simpler 
 constituents, which combine again on cooling. This disso- 
 ciation is favoured by a rise of temperature or by a lowered 
 pressure, and reversed by a fall of temperature or a rise of 
 pressure. From a determination of the density of the mix- 
 ture of gases the extent of the dissociation can be calculated. 
 Certain substances, in like manner, and such substances are 
 generally named " salts," when dissolved in water or certain 
 other solvents, undergo electric dissociation or ionisation ; 
 this dissociation is often increased by a rise of temperature, 
 and always by dilution. The constituents of such solutions, 
 the anions and? kations, can be urged in opposite directions 
 by an electric current ; they usually " migrate " at different 
 rates ; and when they discharge, by contact with the elec- 
 trodes, they are sometimes liberated in the free state, as, for 
 example, many metals, and bromine and iodine ; but some- 
 times the discharged ion is incapable of existing in the free 
 state in contact with the solvent, and in this case they 
 react with the water, and form new secondary compounds. 
 The amount of this ionisation can be measured by deter- 
 minations of the depression of freezing-point, or of the 
 conductivity, of the solution. 
 
CHAPTER IV 
 
 Elements -.Methods of Preparation; Classifi- 
 cation ; Valency. Compounds : Structural 
 Formulae; Classification; Nomenclature. 
 
 WE have seen, in Chapter I., how the idea of an " element " 
 as a constituent of compounds gradually became more de- 
 fined. As fresh discoveries were made, it was found that 
 certain substances could not further be decomposed, yielding 
 new constituents. But it is not easy always to determine 
 whether or no a substance is an element. For certain 
 compounds are very stable, that is, are very difficult to de- 
 compose ; and it has happened several times that such com- 
 pounds were mistaken for elements. A remarkable instance 
 is a copper-coloured body, found in the debris left in the 
 hearth of an old iron furnace, which was for long supposed 
 to be the element titanium ; more careful investigation, 
 however, proved it to be a compound of titanium with 
 nitrogen and carbon. 
 
 Methods of Preparing Elements. There are 
 three methods by which elements have been prepared, and 
 all elements have been made by one of these methods. 
 They are : 
 
 (i) Separation of the Element by Means of an 
 Electric Current. We have already seen that the com- 
 pound must be ionised, and this is attained only by dissolv- 
 ing it in water or some other appropriate solvent, or by 
 fusing it. It is the act of solution or of fusion which 
 ionises the compound ; and the effect of the current is to 
 
 45 
 
46 MODERN CHEMISTRY 
 
 direct the ions towards one or other electrode, and dis- 
 charge them ; they then assume the form of the free ele- 
 ment. It is necessary, in order that this method shall 
 succeed, that the discharged ion shall not act on the solvent, 
 nor on the electrode. It is impossible, for instance, to 
 deposit sodium from an aqueous solution of any of its salts, 
 for no sooner is the sodwn discharged than it is attacked by 
 the water ; hydrogen is evolved in equivalent amount to the 
 sodium, and sodium hydroxide is produced, in which the 
 sodium has taken the place of one of the hydrogen atoms in 
 water ; its formula is therefore NaOH. Chlorine, too, 
 cannot be produced by the electrolysis of a chloride, if the 
 anode is of iron, for example, for it at once unites with the 
 iron, and forms a chloride of that metal instead of coming 
 off as an element. 
 
 (2) Separation of an Element from a Compound by 
 Heat. There appears to be little doubt that at a suffi- 
 ciently high temperature all compounds would be decom- 
 posed into their elements : in the sun, which possesses a 
 temperature much higher than can be reached by any means 
 at our disposal, it is probable that all compounds are decom- 
 posed. But certain compounds, like silica or quartz, for 
 example, are so stable that they resist the highest tempera- 
 ture which we can give them, without any change, except 
 fusion and volatilisation. There is, moreover, another 
 reason why this process often fails to isolate an element. 
 The compound may be decomposed on heating, but its con- 
 stituents may re-unite on cooling, unless one of them is 
 more volatile than the other, and removes itself from the 
 sphere of action. For these reasons this process of obtain- 
 ing elements is of somewhat limited application. But it 
 forms the most convenient method of preparing oxygen ; 
 for example, if oxide of mercury be heated, it decomposes 
 into gaseous oxygen, the boiling-point of which lies far 
 below atmospheric temperature, 182; while the mer- 
 cury, which boils at 358, although it volatilises at the 
 temperature requisite to effect the decomposition of the 
 
SEPARATION OF ELEMENTS 47 
 
 oxide, condenses in the flask or tube in which the oxide is 
 heated. Sulphide of gold, too, can be separated into gold and 
 sulphur on being heated ; for while sulphur boils at 446, the 
 boiling-point of gold is probably not much below 2000. 
 
 (3) Separation of an Element from a Compound 
 by Displacement. On heating one element with a com- 
 pound of another element, it not infrequently happens that 
 the element in combination is displaced and liberated, while 
 the other element takes its place in the compound. This is 
 doubtless an ionic phenomenon ; one element that in com- 
 bination being ionised, and hence electrically charged, 
 exchanges its charge with the added element, which in its 
 turn becomes ionised. A solution of iodide of sodium, for 
 
 + 
 
 example, contains wdions and sodions, I and Na. On 
 adding tc it a solution of chlorine in water, in which there 
 are certainly many non-ionised chlorine molecules, C1 2 , mole- 
 
 + 
 
 cular iodine, I I is set free, while ionised chlorine, CJ, 
 goes into solution. The free iodine forms a brown solu- 
 tion, or, if much is present, a black precipitate. Again, 
 when metallic sodium is heated with magnesium chloride to 
 a red heat, globules of metallic magnesium are set free, 
 while the sodium enters into combination with the chlorine. 
 It may be supposed that on fusion the magnesium chloride 
 contains some ions of chlorine and magnesium ; the non- 
 ionised sodium takes the charge of the ionised magnesium, 
 while the latter metal is liberated in an non-ionised state. 
 But it may be objected that only those magnesium ions 
 which exist as such should exchange their charges with the 
 sodium ; that is true ; but when they have done so others 
 become ionised and undergo a similar change ; for if the 
 temperature be kept constant, the ratio between the number 
 of the ionised atoms of magnesium and the non-ionised 
 atoms of magnesium in the chloride must remain constant, 
 so that when the magnesium ions are replaced by sodium 
 ions, other molecules of magnesium chloride become ionised 
 to keep up the balance. 
 
48 MODERN CHEMISTRY 
 
 The element carbon is most frequently used to displace 
 other elements. In its case, little or nothing is known of 
 the electrical actions ; but if analogy may be taken as a 
 guide, its action may be attributed to a similar exchange 
 of electric charge between the displaced element and the 
 carbon. But here the carbon, as soon as it unites with the 
 oxygen which was previously in combination with the dis- 
 placed element, escapes in the form of gas. and the oxide of 
 carbon is certainly not an ionised compound. 
 
 An essential condition for the preparation of elements by 
 the method of displacement is that the element which it is 
 proposed to prepare in the free state shall not itself com- 
 bine with the element which is used to displace it. Thus, 
 chlorine cannot be used to displace either carbon or sulphur 
 from the compound of carbon with sulphur, bisulphide of 
 carbon, since it itself combines with both the carbon and 
 the sulphur, yielding chloride of sulphur together with 
 chloride of carbon. In general, however, this difficulty 
 does not occur. 
 
 The elements which are generally used for the displacement 
 of others from their compounds are : 
 
 1. Free hydrogen at a red heat which displaces 
 
 elements from their oxides or chlorides. 
 
 2. Ions of hydrogen, on the point of being discharged 
 
 electrically, or hydrogen " in the nascent state," i.e. 
 hydrogen being set free from its compounds by the 
 action of a metal ; it also displaces elements from their 
 oxides or chlorides, or, in general, from their salts. 
 
 3. Metallic sodium, which displaces elements from their 
 
 chlorides or fluorides. 
 
 4. Metallic magnesium, which displaces elements from 
 
 their chlorides or oxides. 
 
 5. Metallic aluminium, which displaces elements from 
 
 their oxides. 
 
 6. Metallic iron, which displaces elements from their 
 
 sulphides. 
 7 Fluorine, which displaces oxygen from water ; 
 
CLASSIFICATION 49 
 
 cMorihe in sunlight, which acts slowly in the same 
 way ; chlorine displaces bromine, and bromine,. 
 iodine. 
 
 8. Carbon, which is the most generally employed agent 
 for replacing other elements ; it combines with oxy- 
 gen, forming carbonic oxide or carbonic anhydride 
 gases, and liberating the element with which the 
 oxygen was combined. 
 
 The question of cost or of convenience often decides as 
 to which of these methods is used. In the sequel, only 
 the more generally used plan will be described. It must 
 be remembered, too, that the employment of these processes 
 does not always lead to the isolation of the element ; in 
 many cases a compound is produced, containing less of the 
 element which it was intended to remove ; and it is some- 
 times difficult to decide whether or not an element has really 
 been set free. Experiments on its compounds are often 
 required to decide the question. 
 
 Classification of Elements. For long it had been 
 noted that certain elements displayed a marked similarity 
 with each other. Thus the metals sodium and potassium, 
 discovered by Sir Humphry Davy, are both white, soft, 
 easily oxidisable metals, forming soluble salts with almost 
 all acids ; these salts resemble each other in colour, in 
 crystalline form, and in other properties. The subsequently 
 discovered metals, lithium, rubidium, and caesium, have also 
 a strong resemblance to potassium and sodium. Their 
 atomic weights also increase progressively ; thus we have 
 the series, Li = 7, Na= 23, K = 39.1, Rb = 85, and Cs = 
 133. Similar series had been noticed with calcium, stron- 
 tium, and barium ; magnesium, zinc, and cadmium ; and 
 so on. It was not until 1863 ^ at J onn - Newlands called 
 attention to the fact that if the elements be arranged in the 
 order of their atomic weights a curious fact becomes notice- 
 able. It is that, omitting hydrogen, the first, the eighth, 
 the fifteenth, and, in short, all elements may be so arranged 
 that the " difference between the number of the lowest 
 member of a group and that immediately above it is 7 ; in 
 
50 MODERN CHEMISTRY 
 
 other words, the eighth element starting from a given one is 
 a kind of repetition of the first, like the eighth note of an 
 octave in music." This idea was subsequently discovered 
 independently and elaborated by Lothar Meyer and by D. 
 Mendele'eff, and it has now been adopted, in spite of some 
 difficulties, as the ground- work of classification of chemical 
 substances. 
 
 The table may be given in the following form, although 
 there are many ways of representing the order in which the 
 elements lie : 
 
 The Atomic Weights of the Elements arranged 1 
 according to the Periodic System. 
 
 H 
 
 He 
 
 Li 
 
 Be 
 
 B 
 
 C N O 
 
 i 
 
 4 
 
 7 
 
 9 
 
 1 1 
 
 12 14 it 
 
 F 
 
 Ne 
 
 Na 
 
 Mg 
 
 Al 
 
 Si P S 
 
 19 
 
 20 
 
 23 
 
 24 
 
 27 
 
 28 31 32 
 
 Cl 
 
 A 
 
 K 
 
 Ca 
 
 Sc 
 
 Ti *' cSe 
 
 35 
 
 4 o 
 
 39 
 
 40 
 
 44 
 
 o r ^ 
 
 4 A vi79 
 
 Br 
 
 Kr 
 
 Kb 
 
 Sr 
 
 Y 
 
 Zr ^^ $Te 
 
 80 
 
 82 
 
 85 
 
 $7 
 
 89 
 
 9 10 2 l f' 
 
 I 
 
 X 
 
 Cs 
 
 Ba 
 
 La 
 
 Ce , \ 
 
 127 
 
 128 
 
 '33 
 
 -37 
 
 142 
 
 140 
 
 
 
 
 
 
 Yb 
 
 Os 1 
 
 
 
 
 
 J 73 
 
 IJ ^ 
 
 
 
 
 
 
 Th ; 
 
 
 
 
 
 
 232 
 
 i 
 
 
 
 i 
 
 ii 
 
 iii 
 
 IV V 
 vlll 
 
 to 
 
 
 
 
 
 to to 
 
 vii 
 
 
 
 
 
 
 
 
 
 
 
 1 11 
 
 1 It is a matter of indifference which elerherifls^pTa 
 
 list. The most convenient form to give the diagram is that of two 
 
PERIODIC TABLE 51 
 
 It will be noticed that the number of elements in the first 
 two horizontal rows is not seven, but eight, and that, con- 
 sequently, every ninth element, and not every eighth, pre- 
 sents similarity with its predecessor in the vertical columns. 
 This is owing to the recent discovery of the elements in 
 the second vertical column. It will also be seen that it is 
 possible, by folding the projecting slip to one side or other, 
 to bring new sets of elements in the third and succeeding 
 horizontal rows beneath the elements in the first and second. 
 The first and second rows are termed " short periods," the 
 others, " long periods/' It appears that by so arranging 
 the elements, analogies are brought out more striking than 
 if such long and short periods were not adopted. 
 
 Valency. The Roman numerals below the vertical 
 columns refer to what is termed the " valency." An 
 element capable of combining with or replacing one atom 
 of hydrogen, or, in other words, of which the equivalent 
 and atomic weight are identical (see p. 15), is termed a 
 monad, or is said to be monovalent. Thus, 23 grams of 
 sodium replaces I gram of hydrogen in water or in hydro- 
 gen chloride, to form hydroxide or chloride of sodium ; 
 and as 23 is known to be the atomic weight of sodium 
 from determinations of its specific heat, the atomic weight 
 of sodium is expressed by the same number as its equiva- 
 lent. It is therefore a monad. The element oxygen is a 
 dyad or divalent, because, in water, two grams of hydrogen 
 are combined with 1 6 grams of oxygen ; its equivalent is 
 therefore 8. But oxygen is 16 times as heavy as hydro- 
 gen that is, a molecule of oxygen is 16 times as heavy as 
 a molecule of hydrogen ; and as a molecule of each of 
 these substances is believed to consist of two atoms, an 
 atom of oxygen is 16 times as heavy as an atom of 
 hydrogen. The atomic weight is therefore 1 6 ; but as 
 
 cylinders, on which the elements follow spiral lines, so that oxygen 
 and fluorine, sulphur and chlorine, follow each other round the 
 smaller cylinder, while selenium and bromine, tellurium and iodine, 
 &c. , are conspicuous round the larger cylinder. 
 
52 MODERN CHEMISTRY 
 
 the equivalent is 8, the atomic weight is twice the equiva- 
 lent. Hence the name " dyad." Similarly, there are tri- 
 talent elements, or triads ; Mravalent elements, or tetrads ; 
 pentavalent elements, or pentads ; hexavalent elements, or 
 hex ads ; heptavalent elements, or kept ads ; and possibly one 
 octovalent element, or octad. 
 
 Valency of Elements. As elements may have more 
 than one equivalent (see p. 15), so they may have more 
 than one valency. Certain elements, however, so far as is 
 known, possess only one valency ; examples of this are 
 found in the lithium, the beryllium, and the boron columns. 
 But the majority of elements exhibit more than one valency, 
 according to circumstances. Thus, compounds of nitrogen 
 are known possessing the formulae NO, NH g , NO 2 , and 
 NH 4 C1, in which one atom of nitrogen is combined with 
 one atom of dyad oxygen, and is therefore also a dyad ; 
 with three atoms of monad hydrogen, and is accordingly a 
 triad ; with two atoms of dyad oxygen, whence nitrogen is 
 here a tetrad ; and with four atoms of monad hydrogen 
 and one atom of monad chlorine in all, with five monads 
 and in this case nitrogen must be accounted a pentad, 
 The atomic weight of nitrogen is known from its density 
 to be 14; and its equivalents in these compounds are 
 respectively ^-, A^, ^, and ^-. This peculiarity makes 
 the classification of some of the elements a difficult task. 
 
 But there is an additional difficulty which meets us in 
 attempting to ascribe the valency to an element. It is 
 connected with what is known as the " structure" of 
 compounds. As this subject will be frequently alluded to 
 in succeeding chapters, enough will only be said here to 
 give an idea of the problem which faces us in attempting a 
 rational classification of the elements. 
 
 We are ignorant of the form of the atoms. It is true 
 that various speculations have been made which may 
 possibly lead to a true conception of their appearance and 
 motions, but these are not sufficiently definite and sup- 
 ported by facts to require more than a passing allusion 
 
STRUCTURE 53 
 
 here. For all practical purposes, we are content, in 
 default of a better conception, to regard atoms as spheres, 
 hard and elastic, and compounds as formed by the 
 juxtaposition of these spheres. That this conception 
 is far from reality is more than probable, but it has 
 to suffice. Certain deductions, however, may be drawn 
 regarding the methods of combination of the atoms in the 
 molecule. It is certain that molecules must occupy space 
 of three dimensions ; but just as it is possible to represent 
 solid objects on a plane surface by the help of perspective, 
 so it is allowable to picture molecules as made up of atoms 
 spread over a plane surface, until we find facts which 
 demand space of three dimensions. We shall see later 
 that in certain cases such solid models of molecules are 
 necessary, but, as a rule, they can be dispensed with. 
 And instead of attempting to picture the atoms as circles 
 or projected spheres, the symbols alone will be employed. 
 The fact of combination will be indicated by a dash uniting 
 the atoms ; thus, a monad will have one, and only one, 
 dash, proceeding from it ; a dyad, two ; a triad, three, and 
 so on. 
 
 Structural Formulae. The simplest case which 
 we can consider is that of a compound consisting of two 
 monovalent atoms, such as hydrogen chloride. Here we 
 have the structural formula, H Cl. A compound of a 
 dyad with two monad atoms, such as water, or its analogue, 
 hydrogen sulphide, must have the formula, H O H, 
 or H S H. The compound of a triad with three 
 monad atoms, as, for example, ammonia, would be written 
 H H, H 
 
 H N<f ; and of a tetrad with four monads, 
 
 \H H 
 
 where an atom of carbon is the tetrad, and the compound 
 is named methane, or "marsh-gas." The atom of sulphur 
 is, however, not always divalent ; it is sometimes tetra- 
 valent, as in its compounds with chlorine and with oxygen. 
 In the first case, tetra-chloride of sulphur has the formula, 
 
54 MODERN CHEMISTRY 
 
 CL /Cl 
 
 ySS ; and in the latter, sulphur dioxide is repre- 
 CK \C1 
 
 sented by the formula O = S = O. Sulphur dioxide unites 
 directly with chlorine on exposure of a mixture of the two 
 gases to sunlight, forming a compound named sulphuryl 
 chloride, which has the empirical formula, SO 9 C1 ; in 
 this compound sulphur is regarded as a hexad, hence the 
 
 Cl O 
 
 structural formula must be /S<^ Now, sulphuryl 
 
 CK ^O 
 
 chloride reacts at once with water when they are brought 
 into contact, and sulphuric acid is produced along with 
 hydrogen chloride. - This change can be represented 
 structurally by the equation : 
 
 .H O H CL ,O H Cl HOv /O 
 
 H O H Cl/ ^O H-C1 H O/^O 
 
 The chlorine atoms of the sulphuryl chloride have com- 
 bined with two of the hydrogen atoms of two molecules of 
 water, leaving the residues O H, which are termed 
 " hydroxyl groups ; " these have taken the place of the 
 chlorine atoms, forming sulphuryl hydroxide, or, as it is 
 commonly termed, sulphuric acid. If the foregoing repre- 
 sentation is correct, then an intermediate substance should 
 exist, which may be named " sulphuryl hydroxy-chloride," 
 and which should contain a chlorine atom and a hydroxyl 
 group, each in union with sulphuryl. . Such a body has been 
 prepared by the direct union of sulphur trioxide, where 
 sulphur is in combination with three atoms of oxygen, with 
 hydrogen chloride. But here there must be a transposition 
 of the hydrogen atom, as is evident from the equation 
 
 H /,O H O, ,O 
 
 ci/ V)" 
 
STRUCTURAL PROBLEMS 55 
 
 In a similar manner to the above schemes, the relations of 
 the atoms in compounds may be traced out, but sometimes 
 it is difficult to decide regarding the structure. Here is an 
 instance. The specific heat of the element barium shows 
 that it possesses an atomic weight not far removed from 
 137 ; the analysis of its chloride leads to the fact that 
 137/2 grams of barium are in combination with 3'5-5 grams 
 of chlorine, and 35.5 is known to be the equivalent of 
 chlorine; hence 63.5 is the equivalent of barium, and 
 63.5x2=137 is its atomic weight. Ordinary oxide of 
 barium corresponds with this, for it contains 137 grams of 
 barium in combination with 16 grams of oxygen ; hence we 
 accept barium as a dyad. But if barium oxide be heated to 
 dull redness in a current of oxygen, another atom of oxygen 
 combines with the oxide, and in the compound BaO 9 , 137 
 grams of barium are combined with 32 grams of oxygen. 
 Is barium a tetrad ? 
 
 Among all the numerous compounds of barium, no one is 
 known in which one atom of barium is combined with more 
 than two atoms of a monad ; when barium dioxide is 
 treated with hydrochloric acid, for example, two atoms of 
 oxygen are not replaced by four atoms of chlorine, but the 
 change is 
 
 Ba0 2 + 2 HC1 = BaCl 2 -i- H 2 2 . 
 
 Hydrogen dioxide is produced. Now the formula of 
 hydrogen dioxide has been proved by the freezing-point 
 method to be H 2 O 2 , and not HO ; hence it may be sup- 
 posed that it consists of two hydroxyl groups in union with 
 each other, thus : H O O H ; in this case, barium 
 
 yQ 
 
 dioxide would be Ba<^ | , the two atoms of oxygen being 
 
 themselves united together ; and there are many instances 
 of similar union. But it may also be held that one of 
 the atoms of oxygen is a tetrad, the other remaining a 
 
56 MODERN CHEMISTRY 
 
 H \ 
 
 dyad, thus : J>O = O ; whence barium dioxide would be 
 
 H/ 
 
 Ba=O=O. Both of these views can be supported by 
 arguments, and it is an open question which has a claim to 
 preference. It is certain, however, that barium is not a 
 tetrad. 
 
 In other instances, it must be confessed that the evidence is 
 by no means so clear, and there is then considerable doubt as 
 regards the correct classification of the elements concerned. 
 
 It must not be forgotten that we have as yet no clear 
 conception as to the cause of valency ; at present we accept 
 the facts, and endeavour to use them as a guide to the 
 classification of compounds. 
 
 Were all the elements to be capable of combining with 
 each other, it is easily seen that the number of compounds 
 would be prodigious, and that no mind could possibly hope 
 to grapple with them ; but it happens that only a certain 
 number of elements forms well-defined compounds with the 
 rest, and the grouping of compounds is thus not so diffi- 
 cult a task as might be supposed. The classes are the 
 following : 
 
 Classification of Compounds : 
 
 1. Hydrogen combines with a few elements, forming 
 hydrides. 
 
 2. Fluorine, chlorine, bromine, and iodine combine with 
 most elements, forming fluorides, chlorides^ bromides, and 
 iodides ; this group of elements is called the halogen group, 
 and their compounds are often termed bolides. 
 
 3. Oxygen and sulphur also combine with most elements, 
 and their compounds are named oxides and sulphides. The 
 comparatively rare elements selenium and tellurium form 
 similar compounds, named selenides and tellurides. 
 
 A very numerous and important class of compounds con- 
 sists of those in which oxygen is combined partly with 
 hydrogen, partly with another element. These compounds 
 can be divided into two distinct classes, according to their 
 
ACIDS AND BASES 57 
 
 behaviour in aqueous solution. Members of both classes 
 are ionised, but they yield different ions according to the 
 class to which they belong. An example of the first class 
 is the compound H O Cl, known only in solution in 
 water, for it decomposes when an attempt is made to free it 
 from water. The aqueous solution is only slightly ionised, 
 
 + 
 
 but the ions present are H and O Cl. The hydro- 
 gen may be displaced by metals, forming " salts," which 
 are also ionised, and indeed much more completely than 
 H O Cl. Thus we have K O Cl, Ca=(O Cl).,, 
 
 + 
 and other similar salts, which are ionised in solution to K 
 
 + + 
 
 and O Cl, and to Ca and (O Cl) 2 respectively. Such 
 hydroxides are named acids. 
 
 It appears, however, that elements which form this class 
 of hydroxide are, as a rule, incapable of retaining in com- 
 bination many hydroxyl groups at a time ; hence compounds 
 of this nature are generally mixed oxides and hydroxides. 
 It might, for example, be imagined that triad nitrogen 
 should be capable of retaining in combination three 
 
 ,0-H 
 hydroxyl groups, to form H O N<^ ; but the 
 
 \O H 
 
 compound is unstable, and loses water, giving a mixed 
 hydroxide and oxide, H O N=O. The ions in this 
 
 + 
 
 case are H and O N=O. Another similar instance is 
 that of sulphuric acid ; as it contains hexad sulphur, it 
 might be supposed that the corresponding hydroxide of 
 sulphur would be S(OH) 6 ; but by loss of two molecules 
 
 H-O X o 
 
 of water, sulphuric acid has the formula /$v\ > 
 
 H-0/ ^O 
 + + 
 as already shown. Its ions are H, H, and SO 4 , or some- 
 
 + 
 times H and HSO 4 . The salts of these acids are respec- 
 
53 MODERN CHEMISTRY 
 
 lively M O N=O and (M O) 2 =SO 2 , where M 
 stands for any monad metal. 
 
 Nomenclature of Compounds. The nomencla- 
 ture of this class of bodies is due to a committee of which 
 Lavoisier was a member. After his discovery of the true 
 nature of oxygen, he was led, not unnaturally, to ascribe to 
 it the chief function in the formation of compounds, and the 
 acids and salts were named without introducing any syllable 
 to signify that oxygen was one of the constituents. In 
 general, the best known or the first discovered acid was 
 given a name terminating in " ic," such as " chloric," " sul- 
 phuric/' "nitric/' The salts of these acids were termed 
 "chlorates," "sulphates," and "nitrates." The acid con- 
 taining one atom of oxygen less was named with the final 
 syllable " ous," thus : " chlorous acid," " sulphurous acid," 
 " nitrous acid ; " and the salts were termed " chlorites," 
 "sulphites," and "nitrites." Acids containing still less 
 oxygen were named with the prefix " hypo," thus : 
 " hypochlorous acid " and " hypochlorites ; " and acids and 
 salts containing more oxygen than those which had names 
 terminating in " ic " and "ate" were distinguished by the 
 prefix "per," thus: "perchloric," " persulphuric " acids, 
 forming " perchlorates " and " persulphates." This no- 
 menclature is still retained. It is illustrated in the table 
 which follows : 
 
 Hypochlorous acid, . . . HOC1. 
 
 Chlorous acid, . . . HOC1O. 
 
 Chloric acid, . . . HOC1O 2 . 
 
 Perchloric acid, .... HOCIO^. 
 
 Potassium hypochlorite, . . KOC1. 
 
 chlorite, . . KOC1O. 
 
 chlorate, . . KOC1O 2 . 
 
 perchlorate, . . KOCKX. 
 
 The second class of hydroxides is named " hydroxides." 
 Members of this class, however, yield ions, one of which is 
 
ACIDS AND BASES . 59 
 
 always hydroxyl, OH. As examples, we may select 
 sodium hydroxide, Na O H, and calcium hydroxide, 
 Ca=(O H) 9 . Here solutions of these compounds in 
 
 + + + 
 
 water contain the ions Na and OH, and Ca and (OH) 2 . 
 Such hydroxides are termed bases ; but the name is also 
 indiscriminately applied to oxides when they unite with 
 water to form bases. Thus CuO and Cu(OH) 9 are each 
 termed bases. 
 
 The same elements may sometimes form bases or acids, 
 according to the valency. The element chromium is an 
 instance. Chromous oxide has the formula Cr=O, cor- 
 responding to the chloride Cr=Cl ; the hydroxide is ana- 
 logous with the chloride, and has the formula Cr=(OH) 2 . 
 But there is also an oxide, CrO a , where chromium is a 
 hexad ; the hydroxide is not known, but the acid is like 
 
 H ~~\ ^ 
 sulphuric acid in formula, viz., >Cr/^ . This is an 
 
 H-O/ ^O 
 
 instance of the rule, of very wide application, that the 
 character of a compound is influenced both by the nature of 
 the elements contained in it, as well as by its structure and 
 the valency of these elements. 
 
 Sulphur, and in a less degree selenium and tellurium, 
 resemble oxygen in forming salts of nature similar to these 
 described, as well as acids and bodies analogous to hydrox- 
 ides. The nomenclature follows that of the oxides, except 
 that the syllables "sulpho" or "thio" are interposed for 
 the sulphur compounds. Thus we have a carbonate, 
 K t> CO 3 , and a sulpho- or thiocarbonate, K 2 CS 3 . In the 
 somewhat rare cases where selenium or tellurium play a 
 similar part, the words " selenio- " or " telluric- " are 
 interposed. Compounds analogous to the hydroxides are 
 termed " hydrosulphides," " hydroselenides," or " hydro- 
 tellurides." 
 
 4. Compounds of nitrogen, phosphorus, arsenic, and 
 antimony, with other elements, are termed nitrides, phos- 
 
60 MODERN CHEMISTRY 
 
 phides, arsenides^ and antimonldes. And just as double 
 oxides of hydrogen and other elements exist, so too nitrides 
 of hydrogen and other elements are known. The compound 
 of nitrogen and hydrogen, ammonia, which has the formula 
 NH 3 , unites with acids, forming salts. For example, 
 ammonium chloride, NH 4 C1, is produced by the direct 
 union of ammonia, NH g , with hydrogen chloride, HC1, 
 if a trace of moisture is present. In aqueous solution it 
 
 + 
 
 undergoes partial ionisation, and the ions are NH 4 and Cl. 
 In this it resembles sodium chloride, NaCl, and the name 
 '* 'ammonium" has been devised to exhibit this similarity. 
 When ammonium chloride or similar compounds are formed 
 by the union of ammonia with acids, it is believed that the 
 nitrogen atom changes its valency from triad to pentad, 
 
 thus: H N=H 9 and Cl N^f \ 
 
 ^H 2 
 
 Even in " substituted ammonias," this property of com- 
 bining with acids is distinctive. For example, copper 
 chloride, CuCl 2 , unites directly with ammonia, giving 
 Cu=(NH 2 ) 2 2HCl or Cu=(NH 3 ) 2 Cl 2 . It will be seen 
 that the atom of dyad copper has replaced two atoms 
 of monad hydrogen in two molecules of ammonium 
 chloride. 
 
 It is possible, too, for a group of elements to replace the 
 hydrogen of ammonia, just as it is possible for a group to 
 take the place of the hydrogen of water. Referring back 
 to the formula of sulphuric acid, H 2 SO 4 , it is plain that it 
 may be written SO 2 =(OH) 2 , and regarded as two mole- 
 cules of water, in which two atoms of hydrogen have been 
 replaced by the dyad group, SO 2 . This is identical with 
 the structural formula already given on p. 54. So, too, 
 with nitrogen ; the compound SO 2 =(NH ) 2 is also known, 
 and it may be regarded as derived from two molecules of 
 ammonia by the replacement of two atoms of hydrogen by 
 the dyad group, SO 2 ". 
 
CARBIDES AND SILICIDES 6r 
 
 Compounds of phosphorus, arsenic, and antimony after 
 this pattern are not known. 
 
 5. Compounds of carbon and silicon with other elements 
 are termed carbides and filicides. The carbides are extra- 
 ordinarily numerous owing to the power possessed by carbon 
 of forming compounds in which two or more atoms of car- 
 bon are in combination with each other. We have seen 
 that a molecule of hydrogen consists of two atoms ; a mole- 
 cule of oxygen also consists of two atoms ; but we know of 
 no compounds of oxygen in which more than three atoms 
 of oxygen are in combination with each other. With 
 carbon, however, the case is different. Considering 
 the combination with hydrogen alone, we have not 
 
 H H 
 
 only CH 4 , but H 3 C-CH 3 , H 3 C-C-C-CH 3 , in all 
 
 H H 
 
 of which the element carbon is a tetrad, being either in 
 combination with hydrogen or with another atom of carbon. 
 And these compounds may have their hydrogen replaced 
 by other elements ; for example, an atom of chlorine may 
 take the place of an atom of hydrogen in any one of these 
 compounds, or one or more atoms of hydrogen may be re- 
 placed by groups of hydroxyl, OH, or two atoms of 
 monad hydrogen may be replaced by an atom of dyad 
 oxygen, or three atoms by one of triad nitrogen, and so on. 
 This makes the chemistry of the carbon compounds very 
 complicated, but at the same time it affords the means 
 whereby the numerous constituent compounds of the tissue 
 of plants and animals can be built up, for they consist, for 
 the most part, of carbon compounds, containing at the same 
 time other elements in combination with the carbon atoms. 
 This branch of chemistry is commonly termed " Organic 
 Chemistry," and treated separately. 
 
 6. Many elements of the metallic class, such as iron, 
 lead, copper, sodium, &c., form compounds with each 
 other. These compounds are usually named " alloys ; J> 
 
62 MODERN CHEMISTRY 
 
 but it must be mentioned that the name " alloy " is often 
 applied to solid mixtures of the metals where no actual 
 compound exists. 
 
 Such are the classes into which chemical compounds may 
 be divided. In a short sketch like the present, it is, of 
 course, impossible to do more than consider a few of these 
 compounds, and those will be chosen which are best adapted 
 to illustrate the nature of the various groups. They will be 
 considered in the second volume. 
 
CHAPTER V 
 
 Methods of Determining the Equivalents of the 
 Elements Of Ascertaining their Molecular 
 Weigh ts A 11 o tropy. 
 
 THE meaning of the word " equivalent " has already been 
 explained on p. 15, and we shall now consider how the 
 equivalent of an element may be determined. As already 
 stated, some compound of the element is analysed, prefer- 
 ably one with hydrogen, oxygen, or chlorine, and the 
 weight of the element which is in combination with, or 
 which replaces 8 parts by weight of oxygen, is termed 
 the equivalent of the element. But it is seldom that a 
 direct method of estimating the equivalent can be practised, 
 for it is not always possible to obtain a compound of the 
 element with hydrogen, or to deprive its oxide of oxygen, 
 or its chloride of chlorine. In fact, each element has to 
 be specially studied, and a method devised which will lead 
 to the required information. It is, above all, necessary 
 that the compounds dealt with shall be pure that is, that 
 they shall not contain any other elements than those which 
 it is desired to estimate, and that their composition shall be 
 definite. For instance, if it were desired to find the equiva- 
 lent of barium by estimating the proportion of chlorine in 
 its chloride, it would be essential to obtain barium chloride 
 free from the very similar elements calcium and strontium, 
 and it would also be of the first importance to make sure 
 that in weighing the chloride, the specimen should be free 
 from water adhering to the powdered substance. 
 63 
 
64 MODERN CHEMISTRY 
 
 Methods of Determining the Equivalents of 
 Elements. It is not always necessary to determine both 
 constituents of the compound ; for example, the ratio of 
 silver to chlorine can be found by dissolving a known 
 weight of pure silver in nitric acid, and then adding to the 
 solution some soluble chloride, such as ammonium chloride ; 
 silver chloride is then precipitated thus : 
 
 AgNO 3 .Aq. + NH 4 Cl.Aq. AgCl + NH 4 NO 3 . Aq. 
 
 The silver chloride is collected on a filter, thoroughly 
 washed, and after being dried, weighed. The Belgian 
 chemist, Stas, working in this way, obtained from 108.579 
 grams of silver 144.207 grams of silver chloride. The 
 relation between the atomic weight of oxygen, taken as 
 the standard and placed equal to 16, and the formula- 
 weight of silver chloride was ascertained by heating to 
 redness 138.789 grams of silver chlorate, 2AgCJO 3 = 
 2AgCl + 3O 9 ; the weight of the residual silver chloride 
 was 103.9795 grams, and that of the oxygen evolved 
 taken as difference is 34.8095. The proportion 
 
 Oxygen Silver chloride Q Formula weight 
 
 lost. remaining. of AgCl. 
 
 34.8095 : 103.9795 :: 48 : i43-3 8l 7 
 
 gives the formula weight of silver chloride. The pro- 
 portion of silver it contains is found by the equation 
 
 144.207 : 108.579 :: ! 4 3-3 8l 7 107.9583 
 
 Subtracting from 143.3817 the weight of the silver it 
 contains, 107.9583, the remainder is the atomic weight 
 of chlorine, which, for reasons already given, is identical 
 with its equivalent, namely, 35.4234; and 107.96 is the 
 equivalent of silver. 
 
 Knowing these facts, the atomic weight of, say, barium 
 may be determined by dissolving a known weight of its 
 chloride in water, and adding to the solution a solution 
 
ATOMIC WEIGHTS 65 
 
 of silver nitrate, so as to obtain a precipitate of silver 
 chloride, which can be weighed, and from it the weight 
 of the chlorine in the barium chloride deduced. Sub- 
 tracting this from the weight of the barium chloride taken, 
 the remainder is the equivalent of barium. To determine 
 whether or not this number is identical with its atomic 
 weight, a determination of its specific heat must be made, 
 as described on p. 14. 
 
 In some instances the process is a more direct one. To 
 determine the equivalent of nickel, a weighed quantity of 
 the metal has been heated in oxygen, and the gain in 
 weight noted. Then, as this weight is to the weight of 
 nickel taken, so is the equivalent of oxygen to that of 
 nickel. 
 
 These examples will suffice to give a general idea of the 
 processes used in determining atomic weights, though, as 
 before stated, each element requires special treatment, and 
 the selection of the best method is often a very difficult 
 task. It is usual, moreover, to make determinations by 
 several methods, if that be possible, so as to avoid any 
 permanent source of error. Many observers, too, have 
 made such determinations, and it is not always easy to 
 eliminate a personal element from the results which they give. 
 A committee of the German Chemical Society has recently 
 published a table of atomic weights, reproduced below (with 
 a few alterations and additions), in which the last digit of 
 each number may in all probability be accepted as correct. 
 A second column is added, containing the atomic volumes 
 of the elements, so far as they are known. They represent 
 the volumes in cubic centimeters occupied by the atomic 
 weight of the element taken in grams, thus 197.2 grams 
 of gold occupy 10.2 cubic centimeters. As the elements 
 expand on rise of temperature, these results are not always 
 comparative, but at present they are the best that can be 
 obtained. 
 
66 MODERN CHEMISTRY 
 
 Table of Atomic Weights and Atomic Volumes. 
 
 Atomic Atomic 
 
 Weight. Volume. 
 
 Aluminium . . Al 27.1 10.1 
 
 Antimony. . . Sb 120 17.9 
 
 Argon . . .A 39.9 32.9 
 
 Arsenic . . .As 75 13.3 
 
 Barium . . . Ba 137.1 
 
 Beryllium . . .Be 9.1 4.3 
 
 Bismuth . . . Bi 208.5 21.2 
 
 Boron . . . B n.o 4.1 
 
 Bromine . . . Br 79.96 25.1 
 
 Cadmium . . . Cd 112 13.0 
 
 Caesium . . Cs 133 
 
 Calcium . . . Ca 40 25.3 
 
 Carbon . . . C 12.00 3.4 
 
 Cerium . . Ce 140 20.8 
 
 Chlorine . . . Cl 35-45 
 
 Chromium . . Cr 52.1 7.7 
 
 Cobalt . . .Co 59.0 6.7 
 
 Copper . . . Cu 63.6 j.i 
 
 Erbium . . . Er 166 ? 
 
 Fluorine . . F 19 
 
 Gadolinium . . Gd 156 
 
 Gallium . . . Ga 70 n.8 
 
 Germanium . Ge 72 
 
 Gold . . . Au 197.2 10.2 
 
 Helium . . . He 4 
 
 Hydrogen. . . H 1.007 
 
 Indium . . .In 114 25.7 
 
 Iridium . . . Ir 193.0 8.6 
 
 Iodine . . .1 126.85 25.7 
 
 Iron . , Fe 56.0 6.6 
 
 Krypton . ." . Kr 81.5 37.8 
 
 Lanthanum . . La 138 22.9 
 
ATOMIC WEIGHTS 
 
 67 
 
 
 
 Atomic 
 
 Atomic 
 
 
 
 Weight. 
 
 Volume. 
 
 Lead 
 
 . Pb 
 
 206.9 
 
 18.2 
 
 Lithium . 
 
 . Li 
 
 7-03 
 
 11.9 
 
 Magnesium 
 
 - Mg 
 
 24.36 
 
 J 3-3 
 
 Manganese 
 
 . Mn 
 
 55.0 
 
 7-7 
 
 Mercury . 
 
 - Hg 
 
 200.3 
 
 14.8 
 
 Molybdenum 
 
 . Mo 
 
 96.0 
 
 
 Neodymium 
 
 . Nd 
 
 143-5 
 
 
 Neon 
 
 . Ne 
 
 20 
 
 
 Nickel 
 
 . Ni 
 
 58.7 
 
 6.7 
 
 Niobium . 
 
 . Nb 
 
 94 
 
 14.5 
 
 Nitrogen . 
 
 . N 
 
 14.04 
 
 
 Osmium . 
 
 . Os 
 
 191 
 
 8.9 
 
 Oxygen 
 
 . O 
 
 16.000 
 
 (standard). 
 
 Palladium . 
 
 . Pd 
 
 1 06 
 
 9-3 
 
 Phosphorus 
 
 . P 
 
 31.0 
 
 17.0 
 
 Platinum . 
 
 . Pt 
 
 195.2 
 
 9.1 
 
 Potassium . 
 
 . K 
 
 39- J 4 
 
 45-5 
 
 Praseodymium . 
 
 . Pr 
 
 141 
 
 
 Rhodium . 
 
 . Rh 
 
 103.0 
 
 9-5 
 
 Rubidium . 
 
 . Rb 
 
 85.4 
 
 56.3 
 
 Ruthenium 
 
 . Ru 
 
 101.7 
 
 9.2 
 
 Samarium . 
 
 Sm 
 
 150 
 
 
 Scandium . 
 
 . Sc 
 
 44 
 
 
 Selenium . 
 
 . Se 
 
 79.1 
 
 18.5 
 
 Silicon 
 
 . Si 
 
 28.4 
 
 11.4 
 
 Silver 
 
 Ag 
 
 107.93 
 
 10.3 
 
 Sodium 
 
 . Na 
 
 23.05 
 
 2 3-7 
 
 Strontium . 
 
 . Sr 
 
 87.6 
 
 34-5 
 
 Sulphur 
 
 . S 
 
 32.06 
 
 iS-7 
 
 Tantalum . 
 
 . Ta 
 
 183 
 
 17.0 
 
 Tellurium . 
 
 . Te 
 
 127.6 
 
 20.3 
 
 Thallium . 
 
 . Tl 
 
 204.1 
 
 17.2 
 
 Thorium . 
 
 . Th 
 
 232 
 
 29.8 
 
 Thulium . 
 
 . Tu 
 
 170 ? 
 
 
 Tin . 
 
 . Sn 
 
 119.0 
 
 16.2 
 
68 MODERN CHEMISTRY 
 
 Atomic Atomic 
 
 Weight. Volume. 
 
 Titanium . . Ti 48.1 
 
 Tungsten . . W 184 9.6 
 
 Uranium , . . U 240 13.0 
 
 Vanadium, . .V 51.2 9.3 
 
 Xenon . . . X 128 35.9 
 
 Ytterbium. . . Yb 173 
 
 Yttrium . Y 89 
 
 Zinc . . , Zn 65.4 9.5 
 
 Zirconium . , Zr 90.6 21.9 
 
 Molecular Weights of the Elements. The 
 
 molecular weights of some of the elements have been 
 successfully determined ; in certain cases by their density 
 in the gaseous state, in others by the lowering of the 
 vapour-pressure of mercury, caused by the presence of a 
 known weight of a dissolved metal, and again in others by 
 the depression of the freezing-point of certain metals, caused 
 by the presence of others in known amount. These will 
 be considered in their order. 
 
 (a) Vapour- densities. For reasons already explained 
 on page 13, a molecule of oxygen is believed to contain two 
 atoms, and inasmuch as the equivalents of most elements 
 have been determined with reference to oxygen, by analysis 
 or by synthesis of their oxides or of their chlorides, and 
 as the ratio of the equivalent of chlorine to that of oxygen 
 has been very accurately determined, it has been agreed to 
 refer the atomic weights of the elements to the standard 
 of oxygen instead of to that of hydrogen. But the atomic 
 weight of oxygen is assumed as 16, and the same standard 
 is applied to the densities of gases ; instead of referring them 
 to the standard of H = i, they are referred to O = 16. To 
 find the molecular weights, the number expressing the 
 density must be doubled in order to compare with the 
 molecular weight of oxygen, which is 32. 
 
 Hydrogen. The density referred to this standard is 
 
MOLECULAR WEIGHTS 69 
 
 ioOo6 or 1.007. There is not yet an absolute certainty, 
 but it is clear that the molecular weight of hydrogen must 
 be approximately 2, i.e. the molecule is di-atomic. 
 
 Nitrogen. Lord Rayleigh found the density of nitrogen 
 to be 14.001 ; its molecular weight is therefore 28, and its 
 formula N . 
 
 Oxygen. Taken as 16; formula O . As these gases 
 keep their relative densities up to a temperature of 1700, 
 it is to be presumed that they all remain diatomic, for it is 
 much more likely that no one of them dissociates than that 
 all dissociate to an equal extent on rise of temperature. 
 The case is different with fluorine, chlorine, bromine, 
 and iodine. The density of fluorine at atmospheric tem- 
 perature is 18.3; the theoretical density for F 2 is 19. It 
 follows, therefore, that fluorine must consist of a mixture 
 of monatomic and diatomic molecules. Now, 19 is the 
 molecular weight of F I , for the atom and the molecule are 
 identical, and 38 that of F ; and the gas must contain 
 x molecules of F I +(I x) molecules of F 2 . Hence, 
 19x4- 38(1 - x) = 18.3 x 2 ; and x = 0.073, ** * n ever y 
 1000 molecules of the gas there are 73 molecules of F I 
 and 927 molecules of F . 
 
 Chlorine at 200 was found to have the density 35.45, 
 the same as its atomic weight, but at 1000 the density 
 was 27.06, and at 1560 23.3. At low temperatures, 
 therefore, the formula of chlorine is C1 , but at 1560 the 
 gas consists of 61 per cent, of molecules of Cl r Similar 
 results have been found for bromine, and for iodine, which 
 also has the formula I 2 at low temperatures, the density 
 was found to be 63.7, corresponding to the molecular 
 weight 127.4 at 1500 under low pressure ; for reducing 
 the pressure also increases dissociation. As the atomic 
 weight of iodine is 126.85, tne S as at I S OO consists almost 
 entirely of molecules of I r 
 
 Thallium has been weighed as gas at 1730; the density 
 was 206.2, a sufficient approximation to 204.1 to warrant 
 the conclusion that its molecule is diatomic. 
 
70 MODERN CHEMISTRY 
 
 Bismuth at 1640 gave the density 146.5, showing, as 
 its atomic weight is 208.5, a P art i a l dissociation from Bi 2 
 to Bi 1 . 
 
 Phosphorus and arsenic give densities which indicate 
 the presence in their gases of more complicated molecules. 
 At 313 the density of phosphorus gas is 64, and there 
 is a gradual decrease with rise of temperature, until at 1708 
 the density is 45.6. As the atomic weight of phosphorus 
 is 31.0, the density 62 would correspond to the existence 
 of molecules of P 4 , while at 1708 there must be a con- 
 siderable admixture of molecules of a smaller complexity, 
 probably P 2 . Arsenic gas had the density 154.2 at 644, 
 and 79.5 at 1700 ; the atomic weight of arsenic being 75, 
 the density 150 would correspond to the formula As 4 , and 
 at 1700 the molecules are almost all As 2 , only a small 
 admixture of molecules of As 4 remaining undecomposed. 
 The density of antimony gas, 141.5 at 1640, implies the 
 presence of some molecules of St> 4 among many molecules 
 of Sb 2 , for the atomic weight is 120. 
 
 The elements sulphur, selenium, and tellurium show 
 signs of even greater molecular complexity. Dumas found 
 the density of sulphur gas at 500 to be 94. 8 ; now, the atomic 
 weight of sulphur is 32.08, and 96 is 32 x 3 ; hence, it 
 was for long supposed that a molecule of gaseous sulphur 
 consisted of 6 atoms ; but it has been recently found that 
 at 193, of course under a very small pressure, 2.1 mms. 
 (for the boiling-point of sulphur at normal pressure is 446), 
 the density reached the high number 125.5 ; now, 32 x 4 is 
 128, and it must be concluded that the molecular weight 
 of sulphur in the gaseous state is 256, and its formula at 
 low temperatures S g . At 800 its formula is S 2 , and at 
 1719 the density 31.8 was found, showing no sign of 
 further molecular simplification. Selenium, of which the 
 atomic weight is 79.1, has the density 1 1 1 at 860, imply- 
 ing some molecular complexity, and at 1420 the density 
 is reduced to 82.2, corresponding to the formula Se 2 ; 
 and tellurium, at about 1400, has the gaseous density 130; 
 
MOLECULAR WEIGHTS 71 
 
 it appears, therefore, to consist of molecules of Te , since 
 its atomic weight is 127.6. 
 
 These examples show that the molecules of many ele- 
 ments in the gaseous state are more or less complex. It is 
 probable that sulphur, selenium, and tellurium would exist 
 as octo-atomic molecules could the temperature be suffi- 
 ciently reduced ; even with sulphur at its boiling-point under 
 normal pressure, the temperature is so high that many of 
 these complex molecules are already decomposed. Proba- 
 bility is also in favour of the supposition that elements of 
 the phosphorus group, phosphorus, arsenic, antimony, and 
 possibly bismuth, have molecules consisting of 4 atoms ; 
 these too dissociating with rise of temperature into di-atomic 
 molecules. Oxygen, nitrogen, and hydrogen consist of 
 di-atomic molecules, no sign of dissociation having been 
 remarked even at the highest attainable temperatures; but 
 fluorine, though consisting mostly of di-atomic molecules, 
 contains some mono-atomic ones ; and chlorine, bromine, 
 and iodine, though probably C1 , Br , and I 2 at low ' 
 temperatures, dissociate into molecules identical with their 
 atoms if the temperature is sufficiently raised. The fact of 
 reduction in the molecular complexity of the molecules of 
 elements prepares us for the existence of elements which in 
 the gaseous state are already mono-atomic ; and many such 
 are known. 
 
 Mono-atomic 
 
 
 
 
 Cad- 
 
 
 elements. 
 
 Sodium. 
 
 Potassium. 
 
 Zinc. 
 
 mium. 
 
 Mercury. 
 
 Gas-density 
 
 12.7 
 
 18.8 
 
 34- !5 
 
 57.01 
 
 100.94 
 
 Temperature 
 
 Red heat 
 
 Red heat 
 
 1400 
 
 1040 
 
 446 and 1730* 
 
 Atomic weights 
 
 23-05 
 
 39-14 
 
 6 5-4 
 
 112. 
 
 200.3 
 
 Density x 2 
 
 25-4 
 
 37-6 
 
 68.3 
 
 114.02 
 
 201.88 
 
 The presumption from these numbers is that the 
 elements are all mono-atomic. It must be remembered 
 that their specific heats all point to the atomic weights 
 given. 
 
 There is, however, another argument for the mono- 
 atomicity of gaseous mercury. On the assumption of the 
 
72 MODERN CHEMISTRY 
 
 " kinetic theory of gases," that the pressure of a gas on the 
 walls of the vessel containing it is due to the bombardment of 
 the sides by repeated and enormously numerous impacts of 
 the molecules, it can be calculated that the amount of heat 
 necessary to raise the temperature of the molecular weight 
 expressed in grams of an ideal gas the molecules of which 
 are supposed to be hard smooth elastic spheres, must be 3 
 calories, provided the gas be not allowed to expand. If, 
 however, it be allowed to expand, it will cool itself, and 
 more heat must be added to restore the temperature ; this 
 extra amount of heat is two additional calories. To heat 
 the molecular weight of the gas in grams through i, allow- 
 ing it to expand at constant pressure, requires therefore 5 
 calories. The " molecular heat at constant volume " is 
 thus 3 calories ; the " molecular heat at constant pressure " 
 is 5 calories. The ratio between the two is 3 : 5, or 
 I : 1.66. This has been found to be the case for mercury 
 gas, the mono-atomicity of whose molecule is proved on 
 other grounds; and the inactive gases of the atmosphere, 
 helium, neon, argon, krypton, and xenon, exhibit the same 
 ratio between their atomic heats. It therefore follows that 
 the atoms of these gases are also identical with their mole- 
 cules ; and that their atomic weights are to be deduced from 
 their densities by doubling the numbers representing the 
 latter. Confirmatory of this view, the ratio between the 
 molecular heats of oxygen, hydrogen, nitrogen, and gases 
 which are known to be di-atomic, like NO, CO, &c., is as 
 5 : 7 or i : 1.4. Such gases require more heat to raise 
 their temperature than an equal number of molecules of the 
 mono-atomic gases do ; the reason is, that the heat applied 
 to di- or poly-atomic gases is used, not merely in transport- 
 ing the atoms from place to place and raising pressure by 
 causing them to bombard the walls of the containing vessel, 
 but some heat is required to cause the atoms to move within 
 the molecule, in some rotatory or vibratory manner ; and 
 consistently with this it has been found that gases consisting 
 of a greater number of atoms in the molecule require still 
 
MOLECULAR WEIGHTS 73 
 
 more heat to raise the temperature of weights proportional to 
 their molecular weights ; in other words, their molecular 
 heats at constant volume are higher the greater the number 
 of atoms in the molecule. 
 
 For these reasons the densities of the inactive gases must 
 be multiplied by 2 to obtain their atomic weights. The 
 data are : 
 
 Helium. Neon. Argon. Krypton. Xenon. 
 
 Densities 2 10 20 41 64 
 Atomic and 
 
 molecular weights 4 20 40 82 128 
 
 (b] Lowering of Freezing-Point, or Lowering of 
 Vapour-Pressure of Solvent. The molecular weights 
 of some of the elements have been determined by Raoult's 
 method, either by the lowering of the vapour-pressure 
 of mercury, or by the depression in the freezing-point 
 of some other metal or solvent in which the element has 
 been dissolved. Lithium, sodium, potassium, calcium, 
 barium, magnesium, cadmium, gallium, thallium, manganese, 
 silver, and gold appear to be mono-atomic, while tin, lead, 
 aluminium, antimony, and bismuth show tendency in con- 
 centrated solution to associate to di-atomic molecules. 
 These results were obtained by measuring the lowering of 
 vapour-pressure of mercury produced by known weight of 
 the metals named. By measurement of the depression in 
 the freezing-point of tin, in which metals were dissolved, 
 zinc, copper, silver, cadmium, lead, and mercury appeared 
 to be mono-atomic, while aluminium was found to be di- 
 atomic. These results, however, are not to be regarded 
 with the same confidence as those obtained by means of 
 measurements of the vapour-density, for it is not certain 
 whether the molecular weight of the solvent should be taken 
 as identical with its atomic weight. All that can be certainly 
 affirmed is, that the molecular weights of the elements which 
 have been placed in the same class above correspond to 
 formulae with the same number of atoms in the molecule. 
 Thus, if zinc is mono-atomic, so is cadmium ; if di-atomic, 
 
74 MODERN CHEMISTRY 
 
 cadmium has also a di-atomic molecule ; and similarly with 
 the rest. 
 
 A method has also been devised, depending on the capil- 
 lary rise of liquids in narrow tubes, by means of which it is 
 possible to estimate the molecular complexity of liquids. 
 This method is applicable to only a few elements ; but by 
 its use it has been found that in the liquid state bromine 
 consists chiefly of di-atomic molecules mixed with a few 
 tetra-tomic molecules ; and that phosphorus in the liquid, 
 as in the gaseous condition, forms molecules corresponding 
 to the formula P 4 . 
 
 Allotropy. Closely connected with this question is 
 the phenomenon of allotropy. This word, which signifies 
 " other form," is applied to the existence of elements in 
 more than one condition. Thus phosphorus, which is 
 usually a yellow, waxy substance, with a low melting- 
 point, changes its appearance when heated, and becomes 
 converted into a red amorphous powder, insoluble in the 
 usual solvents for phosphorus, such as carbon disulphide, 
 and melting at a much higher temperature than the yellow 
 variety ; moreover, the red form is much less easily in- 
 flamed than the yellow form. These two forms are said 
 to be allotropic, and the element is said to display allo- 
 tropy. 
 
 The elements which display allotropy are : carbon, 
 silicon, tin, phosphorus, arsenic, antimony, oxygen, sulphur, 
 selenium, iridium, ruthenium, rhodium, silver, gold, and 
 iron. These will be considered in their order. 
 
 Carbon. Diamonds, as was discovered by Lavoisier, 
 yield on combustion nothing but carbon dioxide ; their 
 identity with carbon was thus proved. When pure, they 
 are colourless ; they are the hardest of all known sub- 
 stances, and possess a density of 3.514 at 18. When 
 heated in absence of air in an electric arc, a diamond 
 changes to a coke-like black substance. Diamonds of 
 any appreciable size have not been formed artificially, but 
 minute diamonds have been made by Moissan by dissolving 
 
ALLOTROPY OF CARBON 75 
 
 carbon in molten iron heated to its boiling-point in an 
 electric furnace, and then suddenly cooling the iron by 
 plunging it into molten lead ; the external surface of the 
 iron solidifies, and encloses a molten interior. As iron 
 possesses a greater volume in the solid than in the liquid 
 state, the molten iron, containing carbon in solution, when 
 it solidifies is under great pressure, for it is confined and 
 hindered from expanding by the crust of solid iron ; under 
 this pressure the carbon separates out in the liquid form, 
 and in solidifying crystallises in octahedra with curved facets 
 characteristic of natural diamonds. If, on the other hand, 
 the iron is allowed to cool without any device to compress 
 the interior, the carbon crystallises out in the form of 
 graphite or plumbago, or, as it is sometimes termed, " black- 
 lead." This variety of carbon is also found native; it 
 forms hexagonal plates, is soft, and is slippery to the touch. 
 Lastly, many compounds of carbon when heated to redness 
 decompose, and leave the carbon in an amorphous or non- 
 crystalline form. Varieties of these are gas-carbon, deposited 
 in the necks of gas-retorts ; oil- coke, left as a residue after 
 the distillation of certain oils ; sugar-charcoal, the residue 
 on heating sugar in absence of air ; and wood-charcoal, the 
 product of the distillation of wood. All of these are black, 
 more or less hard substances. When heated to whiteness 
 in an electric arc, they are transformed into graphite. They 
 all contain a trace of hydrogen, from which they can be 
 freed by heating to redness in a current of chlorine. At 
 the temperature of the electric arc, carbon volatilises with- 
 out fusion and condenses as graphite ; it is only when it 
 is heated under pressure, as described, that it can be made 
 to melt. 
 
 Silicon. This element exists in three forms, two of 
 them crystalline, the third amorphous. The amorphous 
 modification when dissolved in molten zinc or aluminium 
 crystallises out in either black lustrous tablets resembling 
 graphite or in iron-grey prisms. It is not known what 
 circumstances determine the formation of the one or the 
 
76 MODERN CHEMISTRY 
 
 other form. Silicon melts at a bright red heat, and can 
 be cast into rods ; they have the graphite-like crystalline 
 form. 
 
 Tin. This metal, when kept at a low temperature 
 about -30 changes to a grey powder. On heating the 
 powdery modification to above 20, it is converted back 
 into ordinary metallic tin, the more quickly the higher the 
 temperature. If the powder be left in contact with ordinary 
 tin at atmospheric temperature, the metal is slowly changed 
 into its allotropic modification, and articles of tin fall to 
 pieces. 
 
 Phosphorus. Three forms are known for phosphorus. 
 The first, or ordinary form, is a waxy solid, melting at 
 44.4. It is soluble in carbon disulphide, and crystallises 
 from it in rhombic prisms. It is luminous in the dark 
 in presence of air, but if the pressure of the air be raised 
 it ceases to shine ; it is also non-luminous in oxygen. It 
 is very easily inflamed, and burns to its oxide, P 9 O 5 . It is 
 poisonous when swallowed. The liquid obtained by melting 
 it is nearly colourless. When this variety is heated to 240 
 in a vessel from which oxygen is excluded, it changes to 
 a red substance, generally termed amorphous phosphorus. 
 This body is insoluble in carbon disulphide and the other 
 solvents which dissolve ordinary phosphorus. It is not lumi- 
 nous in the dark, and is not easily oxidisable. When heated 
 to a temperature higher than 240, it volatilises and condenses 
 to ordinary phosphorus, and if air be present it takes fire. 
 It is soluble in lead, and when the molten lead cools it 
 crystallises out in nearly black crystals. Indeed, its colour 
 depends on the temperature at which it is formed. If 
 produced at 260, it is deep red, and has a glassy fracture ; 
 at 440 it has a granular fracture and is orange; at 550 
 it is grey, and it fuses at 580, and on solidifying it 
 forms red crystals. It is possible, though not probable, 
 that a mixture of several allotropic forms is the cause of all 
 these changes. 
 
 Arsenic. When arsenic is distilled, it passes directly 
 
ALLOTROPY OF OXYGEN 77 
 
 from the gaseous to the solid state on condensing. The por- 
 tion which cools most quickly is a black powder ; that 
 which condenses in the warm part of the tube has a grey 
 metallic lustre. The black variety can be converted into 
 the crystalline metallic variety on heating to 360. When 
 arsenic is heated in an indifferent atmosphere under great 
 pressure, its boiling-point is raised above its melting-point, 
 and it melts ; on solidification, it forms the metallic variety. 
 
 Antimony. The usual form of antimony is a white 
 brittle metal with a faint bluish tinge. If deposited from 
 a strong solution of its chloride by electrolysis, a grey 
 powdery deposit is formed, which has the curious property 
 of exploding when heated or struck ; it then changes into 
 the metallic variety. It has a lower density than the 
 ordinary antimony. 
 
 Oxygen. The allotropic variety of oxygen is named 
 ozone ; it was discovered by Schoenbein, and is obtained by 
 causing a shower of minute electric sparks (the " silent 
 electric discharge ") to pass through oxygen, preferably 
 cooled to a low temperature. One of the best forms of 
 "ozoniser" is a tube about I cm. in diameter, partially 
 evacuated, and traversed by a wire from end to end ; this 
 tube is contained in a wider one, and the space between the 
 two tubes contains a set of metallic annuli, connected 
 together by a wire. Oxygen is passed slowly through the 
 space between the two tubes, while the two wires are 
 connected with the secondary terminals of a coil ; sparks 
 pass through the inner glass and the space between the two 
 tubes. On first passing the current the oxygen expands, but 
 almost at once contraction ensues, and ozone issues at the 
 further end of the tube. It is not possible, in dealing with 
 ozone, to use indiarubber connections of any sort, for the 
 rubber is at once attacked. Ozone is also formed when 
 phosphorus slowly oxidises in moist air ; when the vapour of 
 ether or benzene is stirred with a hot glass rod in presence of 
 air ; when sulphuric acid acts on barium dioxide or potas- 
 sium permanganate, or when sulphuric acid is electrolysed. 
 
78 MODERN CHEMISTRY 
 
 It is also produced in large amount when fluorine comes into 
 contact with water. 
 
 Its name refers to its most striking property its strong 
 disagreeable smell. It condenses when cooled by liquid air 
 to a dark blue liquid, which is very explosive ; and its gas, 
 when seen in a long tube, has also a blue colour ; it shows 
 characteristic spectral bands. The blue liquid boils at 
 i 06, whereas the boiling-point of oxygen is -i 82. Liquid 
 oxygen is also blue, but has quite a pale tint. When heated 
 to 250, ozone is re-converted into ordinary oxygen; but 
 oxygen cannot be transformed into ozone by heat alone. 
 Ozone is a much more active body than oxygen ; it liber- 
 ates iodine from potassium iodide (iKI.Aq + O 3 + H 2 O = 
 2KOH.Aq + 1 2 + O 2 ) ; it oxidises metallic silver and 
 mercury ( Hg + O g = HgO + O ) ; and it changes lead sul- 
 phide into sulphate (PbS + 4 O 3 = PbSO 4 + 4 O 2 ). When 
 passed through a solution of hydrogen dioxide, oxygen 
 is evolved (H 2 O 2 .Aq + O 3 = H 2 O.Aq+ 2O 2 ) ; and it 
 bleaches indigo and other colouring matters. 
 
 Its density is 24, that of oxygen being 16 ; whence its 
 formula is O g . Its rate of diffusion into air bears to that 
 of chlorine, of which the density is 35.47, the ratio of 
 /24 /35-5 another proof of its density. When oxygen 
 is converted into ozone, the portion which is changed con- 
 tracts in the proportion 3:2; and conversely, when ozone 
 is heated and converted into oxygen, that portion of the 
 gas which consists of ozone increases in volume from 2 : 3. 
 All these proofs demonstrate that the formula of ozone 
 is0 8 . 
 
 Ozone is a poison ; it excites coughing, and in large 
 quantity asphyxiates, the blood becoming venous. It is 
 very doubtful whether ozone has been found in the atmos- 
 phere, except, perhaps, after a thunderstorm. 
 
 Sulphur. The allotropy of gaseous sulphur has already 
 been alluded to ; that of liquid and of solid sulphur is no less 
 striking. When sulphur is melted, it forms a mobile, light 
 brown liquid. On raising the temperature, the liquid be- 
 
ALLOTROPY OF SULPHUR 79 
 
 comes viscous, so much so, indeed, that the vessel contain- 
 ing it can be inverted without spilling the liquid ; and at a 
 still higher temperature it again becomes mobile, but has 
 a deep brown colour. On cooling, these changes are re- 
 versed. If viscous sulphur be poured into water, it hardens 
 to a substance resembling indiarubber ; this form, if kept for 
 some hours, falls into minute octahedral crystals. When 
 molten sulphur is allowed to cool slowly, it solidifies at 120, 
 forming long monoclinic needles of a pale brown colour. 
 This variety is also deposited on evaporating a solution of 
 sulphur in ether or in benzene. These needles, on standing 
 for a few hours, become opaque and spontaneously fall into 
 minute rhombic octahedra. Octahedral crystals of a large 
 size may be produced by allowing a solution of sulphur in 
 carbon disulphide to evaporate spontaneously ; this variety 
 melts at 115; its colour is bright yellow; it is in this 
 form that sulphur occurs native. Its density is 2.07, 
 whereas that of monoclinic sulphur is 1.97 at o. Two 
 other varieties of sulphur are known. If sulphur vapour be 
 quickly cooled, it condenses in a dusty form, termed 
 " flowers of sulphur ; " this powder, if treated with carbon 
 disulphide, leaves an insoluble residue, distinct from all other 
 forms, which are all soluble in disulphide. And lastly, if 
 sulphur be produced in presence of water by the decompos- 
 ing action of water on sulphur chloride, or by the action of 
 hydrogen dioxide on hydrogen sulphide, the sulphur does 
 not separate, but remains in a state of " pseudo-solution " in 
 the water ; it can be precipitated on addition of salts such 
 as calcium chloride. It is thus evident that the chemistry 
 of the element sulphur is very complicated. 
 
 Selenium. This element has three allotropic forms ; 
 when precipitated from selenious acid by sulphurous acid 
 H 2 SeO 3 . Aq + 2H 2 SO 3 .Aq = Se + 2H 2 SO 4 . Aq + H,O 
 it forms a red powder, soluble in carbon disulphide, and 
 crystallising therefrom in dark red crystals, a non-conductor 
 of electricity. Either the amorphous red variety or these 
 crystals, if kept at 210 for some time, change into a black 
 
So MODERN CHEMISTRY 
 
 crystalline variety, insoluble in carbon disulphide, and con- 
 ducting electricity on exposure to light. These varieties 
 also differ in density and melting-point. 
 
 Ruthenium, Bhodium, and Iridium are grey-white 
 metals, hard and fusible only at a very high temperature. 
 They are insoluble in hydrochloric, nitric, or sulphuric 
 acid. On alloying them with zinc or lead, and then dis- 
 solving out the alloyed metal with acid, the ruthenium, 
 rhodium, or iridium is left as a black powder, exploding 
 when gently warmed, and going back into the ordinary 
 form of the metal. 
 
 Iron. It has been known for many centuries that the 
 properties of iron are profoundly modified when it contains 
 a small percentage of carbon ; it is then termed steel. 
 Steel has a fine granular fracture, and is not fibrous like 
 pure wrought iron, or coarsely crystalline like cast-iron, 
 which contains a greater proportion of carbon than steel, 
 the latter containing from 0.8 to 1.9 per cent. When steel 
 is heated and then suddenly cooled, an operation termed 
 " tempering," it becomes very hard; this is due to a change 
 which takes place in the molecular structure of iron at 850. 
 At that temperature its specific heat suffers a considerable 
 change ; and if the iron contains a small percentage of 
 carbon, the allotropic state persists after the cooling has 
 taken place, if produced sufficiently rapidly. The various 
 qualities of steel, elastic as in springs, hard as in razors, 
 brittle and extremely hard as in files, are due to admixture 
 of more or less of the allotropic modification with ordinary 
 iron, which is a comparatively soft metal. 
 
 Silver and Gold. Several metals, among them silver, 
 gold, and platinum, when precipitated from aqueous solutions 
 of their salts by some reducing agent, such as sodium 
 formate, form apparent solutions of the metal in water. 
 That of platinum is grey, of silver blue or red, and of gold 
 purple. The colour, however, depends on the state of 
 division of the metal, and it may vary greatly with the 
 same metal. If the " pseudo-solution " of platinum is 
 
PHASES Si 
 
 warmed, the metal is precipitated as a black powder, known 
 as " platinum-black." This substance readily absorbs 
 gaseous oxygen or hydrogen ; when heated, it is converted 
 into a grey powder, obviously finely divided ordinary plati- 
 num, termed "platinum sponge." On evaporating the 
 pseudo-solutions of silver or gold, the metal remains as a 
 coloured residue ; on warming or rubbing, it changes into 
 the ordinary metal. 
 
 These are the known cases of allotropy. In some cases, 
 as when the gases of ozone or sulphur are weighed, a direct 
 clue to the molecular weight, and therefore the cause of 
 isomerism, is revealed ; but in others, where the different 
 modifications are liquid or solid, there is no obvious means 
 of tracing the cause of the allotropy. Yet in some instances 
 a reasonable theory can be formed. 
 
 Phases. We know that a liquid and its gas can exist 
 together at different temperatures, and that at high tem- 
 peratures the gas exerts a greater pressure than at low. 
 To each temperature corresponds a definite pressure. If 
 the temperature be named, it is termed the "boiling-point" 
 at that pressure ; if the pressure for any particular tem- 
 perature be alluded to, it is called the " vapour-pressure." 
 According to the molecular theory, the vapour-pressure is 
 reached when as many molecules leave the liquid surface in 
 unit time as return to it by the condensation of the gas. 
 There is a state of equilibrium ; but on raising the tem- 
 perature the equilibrium is disturbed, and more vapour is 
 given off to restore it. The gaseous state and the liquid 
 state are termed two phases of the same kind of matter, 
 and they can coexist at various temperatures. 
 
 When the pressure is reduced below a certain amount 
 for water to 4.6 mms. the boiling-point is lowered to 
 o. At this temperature water usually freezes under a 
 pressure of one atmosphere. As the pressure is 4.6 mms., 
 the freezing-point of the water will be at 0.007 above 
 zero ; for it is found that the freezing-point of water is 
 lowered by that amount for each rise of one atmosphere 
 
 VOL. I. F 
 
82 MODERN CHEMISTRY 
 
 pressure ; and if the atmosphere pressure be removed, the 
 freezing-point will be raised. At this temperature, there- 
 fore, ice, water, and water-vapour are all in equilibrium 
 with each other and can coexist. The point is called the 
 " triple point." The states of water, ice, and steam can 
 be represented by a diagram. 
 
 B 
 
 PRESSURE 
 
 SOLID 
 
 L V 
 
 VAPOUR 
 
 A 
 
 TEMPERATURE 
 
 Let pressures be measured up the vertical line AB, 
 and temperatures along the horizontal line AC. The point 
 O corresponds to the temperature 0.007 and to the pres- 
 sure 4.6 mms. Along the line OLV (LV standing for 
 liquid-vapour) the liquid and the vapour can coexist ; it is 
 usually termed the " vapour-pressure curve." The line O 
 SL (solid-liquid) shows the alteration in the melting-point 
 of ice as the pressure rises ; its slope is greatly exaggerated 
 in order to make it visible. It shows that as the pressure is 
 raised the melting-point of ice becomes lower and lower. 
 Lastly, the line OSV (solid-vapour) indicates the coexist- 
 ence of the solid and the vapour phases ; the pressure is 
 
PRASES OF WATER 83 
 
 below 4.6 mms., and the temperature below o. The 
 regions shown correspond to those conditions of temperature 
 and pressure where the substance can exist as solid, as liquid, 
 or as gas. The dotted line WO represents the condition of 
 a super-cooled liquid. It is possible to cool water below o ; 
 for example, if it be pure and kept at rest, its vapour- 
 pressure is then greater than that of ice at the same tempera- 
 ture. It has not been found possible to heat ice above its 
 melting-point without its melting, but it is possible to cool 
 steam somewhat below its condensing temperature without 
 condensation occurring, as shown by the dotted line OP. 
 These states of relative instability have been called " meta- 
 stable." Shaking the water or introducing particles of 
 dust into the steam at once induce freezing or condensation ; 
 the water changes to ice or the steam condenses. 
 
 The condition of allotropy ,can be similarly represented. 
 But the problem is more complicated ; in the case of 
 sulphur, for example, there are two solid phases, the rhombic 
 and the monoclinic, besides more than one liquid phase. As 
 the two solid phases, the rhombic melting at 115, and the 
 monoclinic melting at 1 20, are well known, attention will 
 be confined to them. 
 
 If sulphur be allowed to crystallise from fusion, it assumes 
 the monoclinic form of long prisms. These crystals, however, 
 change spontaneously at the ordinary temperature, and in a 
 few hours fall into minute rhombic octahedra. At the 
 temperature 95.6, however, this change no longer takes 
 place ; the two crystalline forms can coexist in presence of 
 each other without one form turning into the other. The 
 rhombic variety gives off vapour which of course exerts 
 pressure at that temperature and at lower temperatures at 
 which the rhombic variety is stable ; and the vapour- 
 pressure curve is indicated by the line PRV (rhombic- 
 vapour). This temperature is termed the "transition 
 temperature " for rhombic and monoclinic sulphur and 
 sulphur-vapour. It may be compared with the melting- 
 point of ice under 4.6 mms. pressure, which, it will be 
 
8 4 
 
 MODERN CHEMISTRY 
 
 remembered, is 0.007, where water, ice, and steam are in 
 equilibrium. But it differs inasmuch as it is possible to 
 heat rhombic sulphur above the transition-point P without 
 immediate change. PO is the vapour-pressure curve for 
 monoclinic sulphur, which melts at 120; and each of these 
 curves must meet in the transition-point P, for at that tempe- 
 rature both modifications can coexist. Below 95.6 the 
 monoclinic form is in the metastable condition, and the line 
 
 Pressure 
 
 PMV, which is a continuation of the line OP, expresses 
 the vapour-pressure of the monoclinic variety below the 
 transition temperature. The rhombic variety above 95.6 
 is in the metastable condition, and its vapour-pressure is 
 shown by the line PRV, the upper portion of which is 
 dotted. If the sulphur be compressed, the transition-point 
 rises, and the line PQ typefies this. At 120 there must be 
 another transition-point, for here rhombic sulphur, liquid 
 sulphur, and sulphur-vapour may exist in presence of each 
 
PHASES OF SULPHUR 85 
 
 other. Now the melting-point of sulphur is raised by 
 pressure instead of being lowered, as in the case of water. 
 This is the more usual ; the lowering of melting-point of the 
 solid water depends on the fact that the density of ice is less 
 than that of water, but that of solid sulphur is greater than 
 that of molten sulphur. Hence the rise of the transition- 
 point along the line PQ. At O three curves meet : OP, 
 representing the vapour-pressure of monoclinic sulphur ; O 
 LV, the vapour-pressure of liquid sulphur; and OQ, the 
 effect of pressure in raising the melting-point of rhombic 
 sulphur. The lines PQ and OQ happen to meet at Q, 
 which is also a transition-point ; it lies at 131 ; and here 
 rhombic, monoclinic, and liquid sulphur can all coexist, 
 though the pressure is too high for vapour to exist along 
 with them. At higher temperatures and pressures mono- 
 clinic sulphur is incapable of existence. As we have seen, 
 the metastable states of sulphur are capable of existence for 
 some time. Rhombic sulphur can be heated to its melt- 
 ing-point, 115, which lies above its transition tempera- 
 ture. At this temperature it and the liquid resulting from 
 its fusion are both in a metastable condition. And the 
 effects of pressure in raising the melting-point of rhombic 
 sulphur is shown by the dotted line RVQ, which is 
 continued in QLR (liquid -rhombic) at temperatures at 
 which the monoclinic variety is no longer capable of exist- 
 ence. 
 
 Although the allotropy of other elements has not been 
 so minutely studied as that of sulphur, it is certain that the 
 various conditions can all be represented in a similar manner. 
 For example, the transition-point of the grey and metallic 
 modifications of tin is 20 ; below that temperature metallic 
 tin is in the metastable condition ; if cooled sufficiently, it 
 may change spontaneously into the grey powder, but the 
 change may not take place, just as water may be kept 
 super-cooled without freezing. But just as the addition 
 of a crystal of ice to super-cooled water causes it to 
 crystallise, so the contact of grey tin below 20 with 
 
86 MODERN CHEMISTRY 
 
 metallic tin induces the change ; the surface of the tin 
 becomes covered with spots like pimples, and, if time be 
 given, all the tin falls to powder. The change is the more 
 rapid, up to a certain point, the lower the temperature. If 
 the grey tin be raised in temperature above 20, it is recon- 
 verted into metallic tin, the more quickly the higher the 
 temperature. 
 
 The yellow waxy condition of ordinary phosphorus, too, 
 appears to be a metastable condition, for if its temperature 
 is raised under pressure, red phosphorus is produced. On 
 the other hand, if red phosphorus be heated under ordinary 
 pressure, it v -Utilises and condenses as yellow phosphorus. 
 Nevertheless, at the very highest temperatures, the vapour- 
 pressure curves would indicate that yellow phosphorus is 
 the stable form. 
 
 T/ e are still in the dark as to the precise reason of such 
 allotropic changes. From cases which can be investigated, 
 owing to the liquid or gaseous states of the allotropic modi- 
 fications, the cause would appear to consist in a greater or 
 less molecular complexity, but this is not proved for solids. 
 It is possible that the cause of allotropy is to be found, in 
 some cases at least, in a different arrangement of the mole- 
 cules in the solid, and this suggestion falls in with the fact 
 that allotropy often consists in different crystalline forms, 
 but it is also conceivable that a different crystalline form 
 may correspond with difference in molecular complexity 
 as well as with different molecular arrangement. Until 
 some method is discovered whereby the molecular weights 
 of solids can be determined, it is not probable that certainty 
 will be attained. 
 
CHAPTER VI 
 
 Isomerism Polymerism Optical and Crystal- 
 log raphic Isomerism Stereo- Isomerism 
 Tautomerism. 
 
 CLOSELY connected with allotropy is what is termed 
 isomerism. Attention was first called to the existence of 
 compounds with identical composition, so far as it could 
 be ascertained by analysis, but possessing different physical 
 properties, such as melting-point, boiling-point, and crys- 
 talline form, and different chemical properties, by Wohler 
 and by Liebig. Faraday, too, drew attention to a similar 
 phenomenon which has received the name of polymerism. 
 All food contains the elements carbon, hydrogen, 
 oxygen, and nitrogen, besides sulphur, phosphorus, and 
 other elements. The main constituents of food are starch, 
 which is the chief component of bread, and which is 
 devoid of nitrogen ; and albumen and allied bodies, of 
 which flesh mainly consists, which is rich in nitro- 
 gen ; dried meat, indeed, contains from I o to 12 
 per cent, of that element. During the passage of food 
 through the system, the carbon and hydrogen are mainly 
 eliminated by the lungs as carbon dioxide, CO 2 , and water, 
 H 2 O ; while the largest portion of the nitrogen passes 
 away through the kidneys, in the form of a compound 
 named urea, of the formula CON 2 H 4 . Now Wohler 
 succeeded in 1827 in forming urea artificially by preparing 
 a compound of ammonia, NH 3 , with an acid termed cyanic 
 acid, HNCO, itself producible from the elements which 
 87 
 
S8 MODERN CHEMISTRY 
 
 it contains. On heating this compound, ammonium cyanate, 
 NH 4 NCO, to the temperature of boiling water, it under- 
 goes an " isomeric change." Before such heating, if the 
 ammonium cyanate be warmed with a solution of caustic 
 potash, the smell of ammonia is at once apparent ; this is 
 the usual test for ammonium ions ; but after the change 
 into urea has taken place, ammonia is not revealed by this 
 test. Moreover, the compound formed, urea, forms salts 
 with acids ; it unites with hydrogen chloride, for example, 
 forming CON 2 H 4 .HC1. The compound from which it 
 is derived, ammonium cyanate, on treatment with hydro- 
 chloric acid, is converted into ammonium chloride, NH 4 C1, 
 and cyanic acid, which itself undergoes further change, 
 unnecessary to allude to here : 
 
 NH 4 NCO + HC1. Aq = NH 4 C1. Aq + HNCO. 
 
 It is evident that here there are two compounds containing 
 the same elements in the same proportion by weight, and 
 yet having very different properties. 
 
 Faraday, in experimenting with oil-gas, produced by 
 heating the vapour evolved from oil at a high temperature, 
 attempted to condense it to a liquid by application of cold 
 and pressure. In this he was successful, and the compound 
 he obtained was identical in composition with the well- 
 known olefiant gas, now termed ethylene, C 2 H 4 , which is 
 the product on heating a mixture of alcohol with concen- 
 trated sulphuric acid. But Faraday's product possessed a 
 density twice as great as that of ethylene. While ethylene 
 has a density approximately 14 times that of hydrogen, imply- 
 ing the molecular weight 28 (and C 2 = 24 + H 4 = 4 are to- 
 gether equal to 28), Faraday's gas, which is now known 
 as butylene, was found to have the density 28, implying a 
 molecular weight of 56, and involving the formula C 4 H g . 
 Hence it appeared that two compounds could exist, of 
 which one might possess a molecular weight twice as great 
 as the other, yet of the same percentage composition ; and 
 to this the name polymerism was applied ; the substance 
 
ISOMERISM AND POLYMERISM 89 
 
 of higher molecular weight is termed the polymer of that 
 of the simpler molecule. 
 
 It is chiefly among compounds of carbon that the pheno- 
 mena of isomerism and polymerism have been observed ; 
 although there are some well-marked cases of the latter 
 among compounds of other elements ; the best known is 
 that of NO 2 and N 9 O 4 ; the former, which exists only at 
 a high temperature, is a dark red gas ; the latter is almost 
 colourless, and is produced by cooling the former. But in 
 this case the two compounds, to both of which the name 
 nitric peroxide is applied, are very easily transformed from 
 one state into the other, and do not differ in their chemical 
 reactions. Instances of isomerism among compounds of 
 elements other than carbon are rare, but are not unknown. 
 
 The explanation of this curious phenomenon was sought 
 for in the arrangement of the atoms in the molecule. The 
 cases given, although they were the first noticed, are not 
 the simplest ; an instance will therefore be chosen from 
 compounds containing only the two elements carbon and 
 hydrogen. 
 
 One of the constituents of coal-gas is named methane, 
 or marsh-gas ; it escapes from the mud at the bottom of 
 stagnant pools when it is stirred with a stick ; its formula 
 is CH 4 . This gas, when mixed with its own volume of 
 chlorine, and exposed to diffuse light for some hours, ex- 
 changes one of the atoms of hydrogen which it contains for 
 an atom of chlorine ; at the same time the displaced hydro- 
 gen unites with another atom of chlorine, forming hydrogen 
 chloride, thus: CH 4 + C1 2 = CH 3 C1 + HC1. If it be 
 assumed that the hydrogen in marsh-gas is in union with 
 the carbon, and that that union can be pictured by a stroke 
 or "bond" between the atoms, the formula of methane can 
 H 
 
 -C H, 
 
 be written H C H, and the above equation 
 
90 MODERN CHEMISTRY 
 
 H H 
 
 | Cl I H 
 
 H C H + | - H C Cl + | , 
 
 I Cl Cl 
 
 H H 
 
 on the similar assumption that a molecule of chlorine 
 consists of two atoms, of which one replaces hydrogen in 
 methane, while the other unites with that displaced hydrogen 
 to form H CL 
 
 Now only one compound of the formula CHgCl is known ; 
 hence it may be argued that the hydrogen atoms and the 
 chlorine atoms are symmetrically grouped round the carbon 
 atom in chloromethane for so the compound is termed. 
 
 The element sodium readily reacts with chlorine ; indeed, 
 if hot sodium be plunged into a jar of chlorine gas, the 
 metal burns brightly, and a white compound of the two 
 is produced, which is none other than common salt, or 
 sodium chloride, Nad. It is possible to withdraw the 
 chlorine from chloromethane by bringing the gas, dissolved 
 in ether (on which neither it nor sodium have any action) 
 in contact with chips of sodium. Another gas is produced, 
 of which the analysis and vapour-density show it to possess 
 the formula C 2 H 6 ; and it is reasonable to suppose that it 
 has been produced by the union of the two groups, CH 3 , 
 left after the removal of the atom of chlorine from CH 3 C1 ; 
 this change can be thus expressed : 
 
 H H 
 
 - 
 
 H C ; Cl + Na j + ; Na + Cl j C H = 
 
 : ................. : : ................ : A 
 
 H H 
 2Na Cl + H C C H. 
 
 u 
 
ISOMERISM AND POLYMERISM 91 
 
 The new gas is called ethane. It, too, exists in only 
 one modification, and it is legitimate to suppose that the 
 atoms of hydrogen are symmetrically arranged with reference 
 to the two atoms of carbon. 
 
 Like methane, it can be attacked by chlorine with simi- 
 lar results ; the equation is : C 2 H 6 + CL = C 2 H 5 C1 + HC1. 
 And here again only one chlorethane, C 9 H 5 C1, is known ;. 
 another argument in favour of symmetry. 
 
 If a mixture of chloromethane and chlorethane, dissolved 
 in ether, be treated with sodium, a third " hydrocarbon J> 
 is formed, possessing the formula C 3 H g ; it is named 
 propane. Its formation may be expressed thus : 
 
 H H H 
 
 + Naj + ! Na + Cl I C C I 
 
 H C ; Cl + Nai + i Na + Cl I C C H 
 
 H H H 
 
 H H H 
 
 I I I 
 ,__C1 + H C C C H. 
 
 2Na 
 
 Again, only one propane is known. But if it is exposed 
 to the action of chlorine, two monochloropropanes are formed, 
 each of which has the empirical formula C 3 tLCl. The 
 chlorine atom in each isomer may be replaced by the 
 hydroxyl group, OH ; the resulting products, C 3 H 7 OH, 
 are termed propyl and isopropyl alcohols. It is possible 
 to reconvert each of these compounds into its respective 
 chloropropane ; and it is also possible to obtain from them, 
 by oxidation, very different products, Propyl alcohol, 
 when oxidised by boiling with a solution of chromic acid, 
 a compound which easily loses oxygen, is converted first 
 into propionic aldehyde, C g H 6 O ; but on continued oxi- 
 dation the aldehyde is changed to propionic acid, C 3 H 6 O 2 . 
 On the other hand, isopropyl alcohol is oxidised by similar 
 
MODERN CHEMISTRY 
 
 treatment to a compound called acetone, possessing the same 
 empirical formula as propionic aldehyde, viz., C 3 H fi O, but 
 differing entirely from the latter in properties ; and if the 
 process of oxidation be continued, the acetone is broken up 
 into acetic acid and carbon dioxide, both simpler com- 
 pounds, containing fewer atoms of carbon than acetone. 
 These changes are easily represented by the following 
 formulas, which are termed graphic, or structural, or consti- 
 tutional, inasmuch as they represent to some extent the struc- 
 ture of each molecule. 
 
 H H H 
 H C C C H 
 
 U A 
 
 Propane. 
 H Cl H 
 
 H C C C H 
 
 H H H 
 Isochloropropane. 
 
 H H H 
 
 LLL 
 
 H 
 
 O 
 
 H H 
 Propionic Aldehyde. 
 
 H OH H 
 H C C C H 
 
 u A 
 
 Isopropyl Alcohol. 
 
 H H H 
 H C C C Cl 
 
 Ui 
 
 Chloropropane. 
 H H H 
 
 H C C C O H 
 
 H H H 
 
 Propyl Alcohol. 
 
 H H O H 
 H C C C=0 
 
 H.i 
 
 Propionic Acid. 
 H O H 
 
 H-LU-H 
 
 Acetone. 
 
ISOMERISM AND POLYMERISM 93 
 
 It is evident that while propane itself is a symmetrical 
 substance, inasmuch as all the atoms of hydrogen are 
 symmetrically arranged as regards the three atoms of carbon, 
 as soon as an atom of hydrogen is replaced by an atom of 
 chlorine, two possibilities are open ; the atom of chlorine 
 may attach itself either to one of the end atoms of carbon, 
 or to the middle one. In each case a different compound 
 is produced ; and this is shown not only by the different 
 boiling-points of the two chloro-compounds, but also by 
 their behaviour with reagents. 
 
 The relation of propane to ethane may be conceived to 
 be due to the replacement of an atom of hydrogen in the 
 latter by the group - CH g , which is termed the methyl 
 group. Ethane may be viewed as methyl -methane, 
 H 3 C CH 3 , and propane may also be regarded as di- 
 methyl-methane, H 3 C CH 2 CH 3 , the central group 
 CH 2 being taken as the methane molecule, of which 
 two atoms of hydrogen have been replaced by two methyl 
 groups. If the structural formula of propane be again 
 inspected, it is evident that two butanes must be possible 
 one, a methyl-propane ; the other, a trimethyl-methane, 
 thus : 
 
 H 
 
 ^ H H Y H C H 
 
 H C C C C H 
 
 H 
 
 H H H H H C C C H 
 
 H 
 
 H H 
 
 Methyl- Propane. Trimethyl- Methane. 
 
 These two butanes may yield similar derivatives. It is 
 evident on inspection that the first will furnish two mono- 
 chlorobutanes, according as an atom of chlorine replaces 
 one of hydrogen of either of the two end atoms of carbon ; 
 
94 MODERN CHEMISTRY 
 
 the other, if the chlorine atom is attached to either of the 
 middle atoms of carbon. And the second butane can also 
 yield two chlorobutanes, according as a chlorine atom re- 
 places hydrogen of one of the three CH 3 groups, or 
 hydrogen of the ^CH group. 
 
 Such forms of isomerism are very common, and instances 
 might be multiplied indefinitely. 
 
 Another form of isomerism arises when an element such 
 as nitrogen, which has more than one valency, is contained 
 in the molecule. The atom of nitrogen can be made to 
 occupy one of two positions as regards an atom of carbon. 
 A common instance of this is the isomerism of the nitrites 
 .and the carbam'mes. A nitrile is a compound in which an 
 atom of nitrogen replaces three atoms of hydrogen, all of 
 which were attached to the same atom of carbon. Thus, 
 from ethane, C 2 H 6 , acetonitrile is derived thus : 
 
 H H H 
 
 H C C H H C C=N 
 
 .. 
 
 Ethane. Acetonitrile. 
 
 H 
 
 I x/0 
 H C C^ 
 
 \0 H 
 H 
 Acetic Acid. 
 
 It is seen to be closely allied to acetic acid, in which 
 three of the atoms of hydrogen of ethane are also replaced ; 
 but this time two by an atom of oxygen, and the third by 
 the hydroxyl- group, OH. This close connection is made 
 obvious by boiling the acetonitrile with dilute alkali ; the 
 nitrogen is evolved as ammonia, while oxygen and hydroxyl 
 take the place of the atom of nitrogen : 
 
ISOMERISM AND POLYMERISM 95 
 
 H H 
 
 I H\ Q ! ,0 /H 
 
 H c C-N + HX =H c cf +N-H 
 
 H O H X)H X H 
 
 H II 
 
 But an isomer of acetonitrile is also known, to which 
 the formula CH 3 N=C is ascribed ; for, on causing it 
 to react with water, the products are methylamine, 
 
 o NH 2 , and formic acid, HC^ thus : 
 
 \OH, 
 
 H H H 
 
 I I /H ,0 
 
 H C NsC+O + H O H = H C N< +H-Cf 
 
 X H ^OH 
 
 H H H 
 
 From this it is inferred that in the latter case the atom 
 of nitrogen is in direct union with the atom of carbon of the 
 CH 3 group ; it remains united even after attack by water 
 in presence of acid, the latter accelerating the action. 
 
 From these instances it is evident that the position of the 
 elements or groups in a molecule, as well as the composition 
 of the molecule, determine its nature ; and this fact is even 
 more strikingly shown by isomerism in the group of com- 
 pounds related to benzene, a liquid hydrocarbon of the 
 formula C fi H 6 . 
 
 It is found that when this compound is attacked by chlorine, 
 so that an atom of hydrogen is replaced by an atom of chlorine, 
 the resulting oily liquid, named chlorobenzene, having the 
 formula C 6 H 5 C1, exists in only one modification. But if two 
 atoms of hydrogen are replaced by two atoms of chlorine, 
 there are three compounds produced, to each of which the 
 empirical formula C 6 H 4 C1 2 may be ascribed. To what is 
 this isomerism due ? 
 
 The generally accepted graphic or structural formula for 
 benzene, first suggested by Kekul, lately professor of 
 chemistry at Bonn, is 
 
96 MODERN CHEMISTRY 
 
 H H 
 C=C 
 
 /I 2\ 
 
 HC6 3 CH, 
 
 \5 4^ 
 C C 
 H H 
 
 in which the six atoms of carbon are arranged as a ring, 
 and each in combination with one atom of hydrogen. Bear- 
 ing in mind that a symmetrical replacement cannot produce 
 isomerism, it is obvious that it is a matter of indifference 
 whether an atom of chlorine replaces one of hydrogen at- 
 tached to any of the six atoms of carbon, numbered I to 6. 
 But when two atoms of hydrogen are replaced by two atoms 
 of chlorine, the case is different. The substituting atoms of 
 chlorine may have three distinct positions relatively to each 
 other ; they may replace hydrogen atoms combined with the 
 carbon atoms I and 2, or with I and 3, or, lastly, with 
 i and 4. Obviously the numbering I and 2 expresses only 
 any two contiguous atoms ; it is identical with 2 and 3, 3 
 and 4, &c. So, too, I and 3 is identical with 2 and 4, 3 
 and 5, &c.; and I and 4 is the same as 2 and 5 or 3 and 6. 
 These chlorine derivatives may be converted into numerous 
 others by replacing the atoms of chlorine by other atoms 
 or groups of atoms ; and so three series of compounds are 
 obtainable, all of which belong to three separate groups. 
 It has been found possible to determine the positions relative 
 to each other of the entering atoms by the following ingenious 
 device. Expressing, for shortness' sake, the structural 
 formula above given by a simple hexagon, and assumim 
 that carbon atoms are situated at the angles of the hexagoi 
 and, where not otherwise indicated, in combination with 
 hydrogen ; but indicating by the symbol Cl that an atom of 
 hydrogen has been replaced by one of chlorine, we have the 
 following three formulae for the three dichlorobenzenes ; 
 
ISOMERISM AND POLYMERISM 97 
 
 Cl Cl Cl_ Cl_ 
 
 / \ci / \ 
 
 \ x cl \ / 
 
 1 
 
 Ortho ; Meta ; Para ; 
 
 Now, if each of these compounds be separately treated with 
 chlorine, trichlorobenzenes are formed ; these are shown in 
 the lower line ; and it will be noticed that while the first 
 dichlorobenzene (that designated by the prefix "ortho-") 
 can yield two, and only two, trichlorobenzenes, the " meta-" 
 dichlorobenzene may yield three trichlorobenzenes, but the 
 " para-" dichlorobenzene only one ; or in numbers, I 2 may 
 yield 123 and 124 trichlorobenzenes ; I 3 may yield 
 I 2 3, i 3 4, and 135 trichlorobenzenes ; while I 4 di- 
 chlorobenzenes can yield only 124 trichlorobenzenes ; the 
 last is obviously identical with I 3 4, or with i 4 5, or 
 with 146 trichlorobenzene. In this way the actual 
 position of the chlorine atoms, relatively to each other, in 
 the three dichlorobenzenes, was established. 
 
 All these formulae, it will be noticed, are written on the 
 assumption that the atoms of the elements lie in a plane. 
 Now this assumption is exceedingly improbable. It is true 
 that a map represents only the length and breadth of a 
 country ; not the height of the mountains, unless contour 
 lines be made use of. And yet a map renders great service, 
 T -for we use it, allowing for this important omission. So 
 these structural formulas may be advantageously employed, 
 r so long as we remember that they do not represent all the 
 structure. This fact must have been in the minds of many 
 chemists for more than ten years before J. A. LeBel 
 and J. H. van't Hoff pointed out independently, in 1874, 
 the necessity of employing formulae of three dimensions in 
 
 VOL. i. G 
 
98 MODERN CHEMISTRY 
 
 order to explain certain cases of isomerism which were 
 inexplicable on the assumption that the elements were dis- 
 tributed on a plane surface, as in the instances already given. 
 This doctrine of the arrangement of the atoms of a 
 molecule in space of three dimensions has been termed 
 Stereo-chemistry. 
 
 Stereo* chemistry. The deposit found in wine-casks, 
 named tartar or argol, is the potassium salt of four acids, to 
 which the generic name tartarlc has been given. But it 
 has long been known that although all these acids are re- 
 presented by the formula C 4 H 6 O 6 , they differed in physical 
 properties. In order to understand the nature of this differ- 
 ence, a short explanation must be given of the nature of 
 polarised light. 
 
 A mineral named tourmaline is known, which is some- 
 times used as a gem. Its colour is usually green, and it 
 occurs in fairly large transparent crystals. If a slice of 
 this mineral be cut, it is to all appearance quite transparent, 
 allowing light to pass with only a slight diminution in its 
 intensity, as would be the case if the light were to pass 
 through a plate of somewhat dull greenish glass. Through 
 two parallel plates of tourmaline, if held in a certain 
 position, light still passes with scarcely diminished intensity ; 
 but if the one plate be slid round on its neighbour, so that 
 it has performed a partial revolution of 90, the two plates 
 in this position are no longer transparent, but cut off light 
 and become practically opaque. The light, after passing 
 through the first plate, is said to be polarised ; and it is 
 possible to extinguish this polarised light by interposing 
 a second plate placed in a certain position. 
 
 It is now certain that the phenomena of light are to be 
 attributed to the propagation of waves in a fluid which 
 penetrates all space, even the interior of solids ; a fluid 
 without weight, possibly composed of particles, which, 
 however, must not be confused with the atoms or mole- 
 cules of ponderable bodies. This fluid is termed ether. 
 Now waves may be caused in ether by other agencies than 
 
STEREO-CHEMISTRY 99 
 
 light. When an ordinary Leyden jar is discharged, the 
 process of discharging it is not instantaneous ; the spark 
 which passes between the knob of the electrical " tongs'* 
 used to discharge it and the knob of the jar is merely one 
 of a number which pass to and fro between the knob 
 of the tongs and the knob of the jar until electrical 
 equilibrium is established. The passage of such sparks 
 causes waves to be propagated through the ether, waves 
 which differ from those of light only in their much greater 
 length. Such waves, whether of light or of electric dis- 
 turbance, are propagated at right angles to the direction of 
 their oscillation ; they resemble waves on the sea in this 
 respect, but differ inasmuch as sea-waves oscillate merely 
 up and down, whereas these waves of light or electric 
 disturbance oscillate in all possible directions at right angles 
 to that in which they move forward. The name applied 
 to such electric waves is " Hertzian waves," after their 
 discoverer, the late Professor Hertz of Bonn. 
 
 It has been found that the leaves of a book held with 
 its edge towards the source of electric waves has the 
 effect of polarising these waves. They are not much 
 diminished in intensity by their passage through the book, 
 and if a second book be held edgewise, with its leaves 
 parallel to thost? of the first book, the waves can still pass 
 on. But if the one book be turned round so that its leaves 
 are at right angles to those of the other, the electric waves 
 are blocked, and are no longer able to pass. The Hertzian 
 waves are so long, and are made in such a manner, that 
 it is possible to ascertain in which plane they are oscillat- 
 ing ; indeed, the experiment with the book proves this. 
 For it is known that if the plane of the waves coincides with 
 that of the pages of the book, the waves are annihilated ; they 
 cause electrical currents in each leaf, and are thus absorbed. 
 But as each leaf is separated from its neighbour by a thin 
 layer of air, which is a practical insulator of electricity, if 
 the leaves of the book are turned at right angles to the 
 position in which they annihilate the electric waves, these 
 
100 
 
 MODERN CHEMISTRY 
 
 waves cannot excite currents in the leaves, because each 
 leaf is insulated from its neighbour, and the currents have 
 no scope. After passage through the book, those waves 
 which were originally oscillating in the plane of these 
 leaves are annihilated, and used up in inciting feeble electric 
 currents in each leaf, while those waves originally at right 
 angles to the plane of the book's leaves pass through. 
 Waves oscillating in intermediate planes, e.g. at an angle 
 of 45 to the plane of the leaves, are partly annihilated, and 
 partly pass, in proportion to the angle which they make. 
 
 The coarsely foliated structure of a book, it cannot be 
 doubted, is analogous to the structure of a plate of tourma- 
 line. It is also almost certainly foliated, but the foliations 
 
 FIG. 4. 
 
 are extremely minute, so that the influence they have in 
 transmitting or obscuring light waves is commensurate with 
 the difference in the oscillation-length of light waves and 
 Hertzian waves. The light which has passed through 
 tourmaline, like the electric waves which pass through the 
 leaves of a book held edgewise, has this peculiarity the 
 waves oscillate in one plane, and resemble in that respect the 
 waves of the sea. They are said to be polarised. 
 
 Now it has been found that certain substances, such as 
 crystals of quartz or of chlorate of potassium, have the 
 curious property of rotating the plane of oscillation of 
 polarised light ; that is, light polarised by passage through 
 a plate of tourmaline (or by other means, for there are 
 
STEREO-CHEMISTRY 101 
 
 more convenient plans for polarising light), and then trans- 
 mitted through a plate of crystallised potassium chlorate, 
 is not wholly obscured when it impir o es on a second plate 
 of tourmaline held at right angles to the first ; it is neces- 
 sary to turn the second tourmaline through more than a 
 right angle in order that total obscuration shall result. The 
 plane of polarisation is rotated by the chlorate crystal. If, 
 however, the crystal is dissolved in water, its solution has 
 no such property ; and it is inferred that the rotation is due, 
 not to any arrangement of the atoms in the molecule of 
 chlorate, but to the arrangement of the molecules in the 
 crystal. For if the rotation were due to the former cause, 
 it would be produced by solution as well as by the solid. 
 It is believed, therefore, that the molecules in a crystal of 
 chlorate are arranged with regard to each other like the 
 stones in a spiral staircase. 
 
 The case is otherwise with crystals of tartaric acid. 
 While one variety of tartaric acid crystal rotates the plane 
 of polarisation to the right, in the direction of the hands 
 of a watch, another variety has the opposite effect, and a 
 third and a fourth variety, which can be distinguished by 
 means to be mentioned hereafter, are without action on 
 polarised light. Unlike potassium chlorate, however, solu- 
 tions of these crystals of tartaric acid have the same effect 
 as the crystals themselves ; those which have a right- 
 handed rotatory power retain that power even when dis- 
 solved ; the left-handed ones remain left-handed, and the 
 neutral ones neutral. 
 
 In 1861, Louis Pasteur, at that time assistant to 
 Professor Balard, of Paris, made a most important dis- 
 covery. It was that crystals of dextro-rotatory or right- 
 handed tartaric acid, which had hitherto been believed to 
 be regular, were all characterised by small facets, developed 
 only on one corner. The neutral tartaric acid, known as 
 racemic acid, had no such facets ; but on crystallising a 
 certain double salt of racemic acid containing ammonium 
 and sodium, Pasteur discovered that two kinds of crystals 
 
102 
 
 MODERN CHEMISTRY 
 
 were deposited ; some with facets on the right upper corner 
 (see (a) Fig. 5), and some with facets on the left corner. 
 The facets in question are shaded in the figure. And most 
 singularly, the crystals, after they had been picked out and 
 separated from each other, when dissolved in water each 
 rotated the plane of polarised light, the crystals with right 
 facets to the right, those with left facets to the left. Up to 
 that time only the dextro-rotatory tartaric acid had been 
 known. From this Pasteur drew the inference that the 
 difference between the two varieties must be due to the dif- 
 ferent arrangement of the atoms in the molecules of the two 
 varieties of tartaric acid in space of thrt e dimensions. 
 
 FIG. 5. 
 
 (a) (b] 
 
 Pasteur also devised two other methods of separating the 
 two varieties of tartaric acid contained in racemic acid : 
 one was by preparing a salt of racemic acid with a base such 
 as quinicine, which itself possesses optical properties. The 
 salt of dextro-tartaric acid with this base is much more 
 soluble than that of the left-handed or lasvo-rotatory tartaric 
 acid ; so that the crystals which separate out on evaporating 
 the solution are practically pure lasvo-tartrate. His other 
 method of effecting the separation, or, in this case, the de- 
 struction of the dextro-tartrate, was by allowing a solution of 
 racemate of ammonium to mould. The organism consumes 
 the dextro-tartrate, and the lasvo-tartrate remains after the 
 mould has been suffered to grow for a sufficient time. It 
 
STEREO-CHEMISTRY 103 
 
 is only the dextro-compound, in fact, which serves as food 
 for the mould. 
 
 In 1874, LeBel and van't HofF independently pro- 
 pounded a theory to explain these and similar cases of 
 isomerism ; it is based on the conception that all molecules 
 occupy space of three dimensions, and that the isomerism is 
 caused by the different arrangement of the atoms in the 
 molecule. This arrangement also gives a clue to the be- 
 haviour of such isomers in rotating the plane of polarised 
 light to the left or to the right, and also indicates why 
 crystals composed of such molecules should develop " hemi- 
 hedral " facets, as those represented on only one side of a 
 crystal are termed. 
 
 It is not necessary to consider any particular compound 
 in giving a sketch of this theory ; it may be stated in general 
 terms. 
 
 Marsh-gas, or methane, it has already been remarked, has 
 the formula CH 4 . We have already seen that one of the 
 four atoms of hydrogen which it contains may be replaced 
 by an atom of chlorine, and the chlorine by the group CH 3 , 
 or methyl. It is possible to replace successively all four 
 atoms of the hydrogen of methane by atoms or groups ; 
 these may be all different. If we indicate such atoms 
 or groups by the letters P, Q, R, S, we may have 
 the compounds CP 3 Q, CP 2 Q 2 , CP 2 QR, and CPQRS, 
 as well as CP 4 . The stereo-chemical hypothesis is based 
 on the conception that the carbon atom is situated at 
 the central point of a pyramid built on a triangular base 
 (which is named a tetrahedron}, and that the elements 
 or groups in combination with the atom of carbon are 
 placed at the four corners or solid angles of the figure, 
 The formulas of the compounds, constructed in this 
 manner, would present the appearance in perspective shown 
 in Fig. 6. 
 
 It is evident, on inspection, that no isomerism is possible 
 with the molecules numbered ( I ), (2), and (3), for in each 
 case a mere turning of the tetrahedron into the appropriate 
 
104 
 
 MODERN CHEMISTRY 
 
 position can place any group in any desired position relative 
 to the others. Thus to take (3) ; if it be supposed 
 that isomerism could result from the relative positions of 
 groups R and Q with regard to the two groups P, it is only 
 necessary so to turn the crystal that the position of the two 
 P groups is reversed, when Q will lie at the remote corner, 
 and R at the near corner. The case is different with 
 the configuration in (4). If Q and R are transposed, 
 as in (5), it is impossible so to place (4) that its 
 groups, P, Q, R, and S, correspond in position with 
 those in (5). In fact, (5) may be termed the "mirror- 
 
 image" of (4), for a reflection of (4) in a mirror is 
 identical with (5). To choose a familiar illustration, it 
 is not possible to convert a right hand into a left hand 
 except by reflecting it in a mirror. In this sense, the right 
 hand is a stereoisomer of the left hand. Now it is precisely 
 such compounds, and only such compounds, which display 
 isomerism of the kind described ; one variety of which 
 causes the plane of polarised light to be rotated to the right, 
 the other to the left. One of the most familiar instances 
 of such isomerism has been observed with the acid of sour 
 milk, lactic acid; in it, the carbon atom at the centre of 
 
STEREO-CHEMISTRY 
 
 105 
 
 the tetrahedron is coupled with four different atoms or 
 groups, and is termed the " asymmetric " carbon atom. P 
 may stand for an atom of hydrogen ; S for the hydroxyl 
 group, - OH ; Q represents the methyl group, - CH 3 ; 
 
 ^ 
 
 and S the car boxy 1 group, - C/ a group common to 
 
 X)H, 
 
 all the acids of carbon, and already shown on p. 94 as part 
 of the formula of acetic acid. The ordinary lactic acid of 
 sour milk is optically inactive, but its isomer extracted 
 from flesh-juice is laevo-rotatory. By one of Pasteur's 
 
 methods, however, the acid from milk can be split into two 
 varieties, one laevo-, the other dextro-rotatory. There are, 
 therefore, two forms of lactic acid, both optically active ; a 
 mixture of the two in equal proportions has no claim to be 
 termed a third body, but of course it is without action on 
 polarised light. 
 
 It is possible for a compound to contain two or more 
 asymmetric carbon atoms. Such is the case with the tar- 
 taric acids. The structure of these acids is shown in Fig. 7. 
 It is supposed to be represented by two tetrahedra, placed 
 apex to apex, one of course being inverted. The carbon 
 atoms in the interior of each tetrahedron are united by one 
 " bond " or valency. The other three valencies of each 
 
io6 MODERN CHEMISTRY 
 
 carbon atom are employed in union with three separate 
 atoms or groups, P, Q, and R, and P', Q', and R'. In the 
 case of the tartaric acids, these are respectively an atom of 
 hydrogen, the hydroxyl or OH group, and the carboxyl 
 
 /p 
 
 or _ C\ group. At present there is no means of as- 
 
 \)H 
 
 certaining which configuration of the molecule will corre- 
 spond to laevo-rotation, or to dextro-rotation ; but it is 
 possible that recent discoveries in connection with electrical 
 waves may enable us to come to some decision on the 
 point. It is necessary, therefore, to make some supposition 
 regarding the position of these groups with reference to 
 their action on polarised light. The convention which has 
 been accepted is to suppose that when corresponding groups 
 can be placed vertically one above the other by rotating 
 either the upper or the lower tetrahedron round the vertical 
 axis which joins their centres, the compound is without 
 action on polarised light. This is represented by (2) of 
 Fig 7. Here P and P, Q and Q', and R and R' lie 
 vertically one above the other. As regards the variety 
 which possesses a right-handed rotation, it is a matter of 
 indifference which configuration be chosen. But it is con- 
 ventionally agreed to take the arrangement shown in ( I ) 
 as a dextro-rotatory one ; because regarding the tetrahedra 
 from the central point of junction, the letters P Q R 
 follow each other in the direction of the hands of a watch, 
 when viewed from below ; and so also do the letters 
 P' Q' R', when viewed from above. The last con- 
 figuration (3)5 on the other hand, if it be agreed to re- 
 gard the order of the attached groups represented by the 
 symbols P Q R, and P' Q' R' from the point of junction 
 of the two tetrahedra, will each exhibit a left-handed 
 screw ; for in the upper tetrahedron, the letters P Q R, 
 viewed from below, follow each other in a direction oppo- 
 site to that of the hands of a watch ; while viewed from 
 above, the letters P' Q' R' also suggest laevo-rotation. 
 Hence we have the inactive molecules of racemic acid 
 
STEREO-CHEMISTRY 107 
 
 (for so this variety of tartaric acid is termed) in (2), 
 Igevo-rotatory tartaric acid in (3), and dextro-rotatory in 
 ( i ). It is, of course, not known in which order the groups 
 are placed to produce dextro- or laevo-rotation, but the 
 idea is easily understood. A fourth variety of tartaric acid 
 may, of course, be prepared by mixing equal weights of 
 the dextro- and lasvo- varieties ; it is inactive, but it is a 
 mixture, and not a definite compound, and it must not 
 be confused with the racemic acid of (2). This fourth 
 variety can be separated into its constituents by Pasteur's 
 device of crystallising the sodium ammonium salt, and sepa- 
 rating by hand those crystals which have a right-handed facet 
 from those with a left-handed facet ; but the true racemic 
 acid cannot be thus resolved at ordinary temperatures ; 
 it must be converted into a salt of some optically isomeric 
 base, and heated ; on solution in water, it is now found 
 to consist of a mixture of dextro- and laevo-tartaric acids, 
 and it may be separated by crystallisation of their sodium- 
 ammonium salts, as before described. 
 
 The tetrahedral form appears to be characteristic of the 
 compounds of all tetrad elements ; for W. J. Pope has recently 
 obtained compounds in which the element tin is combined 
 with four different groups, each containing carbon and 
 hydrogen ; and these display optical isomerism when re- 
 solved by appropriate means into their stereo-chemical 
 isomers. The same has been shown by S. Smiles to be 
 true for compounds of a similar nature containing tetrad 
 sulphur, and this observation has been confirmed by Pope. 
 It will probably be found true for similar compounds of all 
 tetrad elements where they hold in union four different 
 elements or groups. 
 
 The stereo-isomerism of compounds of nitrogen has also 
 been proved to hold by J. A. LeBel. As nitrogen is 
 either a triad or a pentad, however, the tetrahedron cannot 
 be the fundamental figureo It is probably a pyramid erected 
 on a square base. LeBel made a curious discovery in this 
 connection : it is that the groups in combination with the 
 nitrogen must have at least a certain degree of complexity, 
 
io8 MODERN CHEMISTRY 
 
 and a correspondingly high molecular weight, otherwise such 
 isomers are not capable of existence. It is conjectured that 
 the groups combined with the nitrogen, if they are not 
 sufficiently large, change places, so as to form the most 
 stable configuration ; it is only where they are large that 
 such molecular rearrangement does not occur. LeBeFs 
 work has been confirmed by Pope. 
 
 Stereo-isomerism due to double linkage. There 
 is another variety of stereo-isomerism which cannot be 
 detected by the rotation of polarised light. It is assumed 
 that in such a compound as tartaric acid (see Fig. 7), the 
 two tetrahedra, shown connected by their apices, are free 
 to revolve round a vertical axis joining the two asymmetric 
 carbon atoms, and passing through the point of junction 
 of the two tetrahedra. Taking (2) of Fig. 7, if, for 
 example, R and R' happen to lie on a line parallel to that 
 axis, we may have a compound different from one in which 
 R and Q' should lie on that line, as in the figure. If, 
 however, such a configuration were to exist, it would not 
 be permanent, for owing to the revolution of the tetrahedra 
 round the vertical axis passing through their apices, the 
 original configuration would be produced, and R and R' 
 would again lie on the same vertical line. Of course this 
 is on the assumption that the relative positions of P', Q', 
 and R' are not changed, otherwise an isomeride is produced 
 capable of acting on polarised light. 
 
 Now, if the atoms of carbon be connected, not singly, 
 as in the instances in Fig. 7, but doubly, such a power of 
 rotation is hindered. Such a configuration is shown in 
 Fig. 8. The figure may be derived from one of those in 
 Fig. 7 by supposing R and R' to be removed by some 
 appropriate reagent ; the tetrahedra will then be joined 
 along one of the edges instead of only at the apices, and 
 the carbon atoms will be "doubly linked." In (i) of 
 Fig. 8 a double tetrahedron, like that shown in Fig. 7, 
 is reproduced; (2) shows the approach of the two solid 
 angles; in (3), R and R' are removed, giving the new 
 
STEREO-ISOMERISM 
 
 109 
 
 configuration. Such a compound is termed " unsaturated." 
 By addition of such an element as bromine, the com- 
 pound again becomes saturated, and ( i ) is reproduced with 
 bromine atoms in place of R and R'. 
 
 (D 
 
 No. (3) of Fig. 8 is reproduced in Fig. 9 (i), but the 
 letters have been changed, so as to represent actual groups 
 present in two acids, named respectively fumaric and maleic 
 
 -C0 2 H 
 
 C0 2 H 
 
 (l) 
 
 C0 2 H 
 
 (4) 
 
 acids. The formula given in ( i ) is that of maleic acid. 
 This acid, when exposed to hydrogen bromide, HBr, com- 
 bines with it ; but the double linkage between the central 
 carbon atoms is thereby broken, and (2) is produced. The 
 
no MODERN CHEMISTRY 
 
 upper tetrahedron is now free to rotate round the axis 
 joining the two central carbon atoms ; and it is supposed 
 that rotation takes place until the position of greatest 
 stability is reached. In (2) we may observe that two 
 hydrogen atoms occupy the left corners ; a hydrogen atom 
 and a bromine atom occupy the solid angles projecting 
 towards the spectator, and two carboxyl groups are situated 
 on the right. By rotation of the upper tetrahedron through 
 an angle of 120 in the inverse direction of the hands of 
 a watch, H' will be vertically above the lower carboxyl 
 group, H" will be above the bromine atom, and the upper 
 carboxyl group will be above the lower atom of hydrogen, 
 as shown in (3). If, now, hydrogen bromide be removed 
 (and this is possible by treatment with caustic potash), 
 the configuration will be that represented in (4) ; and this, 
 it is believed, is the formula of fumaric acid, the other 
 isomer. Fumaric acid, like maleic acid, can also combine 
 with hydrogen bromide, but on its removal fumaric acid is 
 reproduced. 
 
 An acid containing two carboxyl groups often has the 
 property of losing the elements of water when heated, and 
 yielding an anhydride ; in the case before us, C 2 H 9 ( CO O H ) 2 
 = C 2 H 2 (CO) 2 O + H 2 O. Now maleic acid alone has this 
 property ; and it is inferred that maleic acid must there- 
 fore possess the structure ( I ) , seeing that the carboxyl 
 groups are conveniently situated for losing the elements of 
 water, and their carbon atoms for being linked together by 
 an atom of oxygen. To imagine a configuration which 
 would pertain to an anhydride derived from (4) would be 
 difficult. 
 
 This kind of isomerism is also met with among the com- 
 pounds of nitrogen, which, it will be remembered, acts often 
 as a triad. For example, substances named aldoxtmes are 
 known in which nitrogen is doubly linked to carbon ; and 
 it is also united to a hydroxyl group. Such substances are 
 known in two modifications ; and it appears probable that 
 the two varieties possess some such configurations as : 
 
TAUTOMERISM in 
 
 H C CH 3 H C CH 3 
 
 II ^d || 
 
 N OH HO N 
 
 which resemble those of fumaric and maleic acids. 
 
 Tautomerism. One more kind of isomerism remains 
 to be mentioned ; a body which is said to be tautomeric 
 appears to show a different constitution, according to the 
 reagent with which it is treated. One of the earliest 
 instances observed of a tautomeric compound is aceto-acetate 
 of ethyl. Its formula is : 
 
 CH S C H 2 C C O CH 2 CH q , 
 
 O 
 
 as shown by its reaction with caustic potash, when acetone, 
 CH 3 CO CH 3 , is formed, the scission occurring at the 
 dotted line ; but the tautomeric formula 
 
 CH 3 C = CH C O CH 2 CH 3 
 
 O H O 
 
 may also be ascribed to it, for it can be shown to contain 
 
 a hydroxyl group by the action of diethylamine, giving 
 
 CH 3 C ^ CH C O CH 2 CH 3 . 
 
 i ir 
 
 C,H_N-C 2 H 5 O 
 
 Other reactions point to the same possibility of rear- 
 rangement. Examples of tautomerism are not unknown 
 among compounds of elements other than carbon ; it is 
 probable that two sulphurous acids are capable of existence, 
 
 one possessing the formula /S\ and the other 
 
 \OH, 
 
 ,X 
 
 O = S/ . Silver sulphite appears to be a derivative 
 
 \OH 
 
 of the first, and sodium sulphite of the second of these 
 
ii2 MODERN CHEMISTRY 
 
 forms ; and it is probable that the particular form taken 
 depends on the reagent which is presented to the acid. 
 
 Although the application of geometrical formulae has 
 proved useful in exhibiting certain cases of isomerism such 
 as have been considered, it is not to be supposed that 
 formulas to which grouping in space of three dimensions 
 is not usually applied do not also require three-dimensional 
 space. Their use is not common, merely because the 
 spatial relations are sufficiently evident without involving 
 this conception. Most of us are content with a picture 
 as a sufficient memento of our friends ; but if we wish a 
 fuller presentment, a bust or a statue will give it. 
 
CHAPTER VII 
 Energy 
 
 WE have seen in the last chapter that some conception can 
 be made regarding the form of molecules, supposing them to 
 occupy space of three dimensions. It is further imagined 
 that the atoms in the molecule, unlike those in the diagrams 
 given, are not quiescent, but are in motion relatively to each 
 other, and that the molecules themselves also change their 
 relative places ; both atoms and molecules contain what 
 is termed "energy," in virtue of this motion. When a 
 chemical reaction takes place, energy may be lost or gained 
 lost, when aioms or molecules assume a more stable con- 
 dition ; gained, when the state of a resulting compound is a 
 less stable one than that of the substances from which it is 
 formed. 
 
 We must now consider what is meant by this term 
 ~" energy." Energy can exist under various forms ; for 
 example, when a stone falls to the ground under the influ- 
 ence of the earth's attraction, it loses energy after its fall ; 
 when a billiard-ball is set in motion, for instance, by the 
 tension of a spring, the spring loses and the billiard-ball 
 gains energy. Energy can also be communicated to sub- 
 stances in the form of heat when their temperature is raised ; 
 it may be imparted, to a body in the form of an electrical 
 charge, and in various other ways. 
 
 We have already seen (p. 6) that Lavoisier laid down 
 as a maxim that matter can neither be created nor destroyed. 
 This same doctrine holds as regards energy ; but there is a 
 
 VOL. i. "3 H 
 
ii 4 MODERN CHEMISTRY 
 
 difference in kind between matter and energy, for while one 
 form of matter, e.g. iron, cannot be changed into another 
 kind of matter, such as lead, one kind of energy is con- 
 vertible into all other kinds of energy quantitatively, so that 
 no loss of energy occurs during the -conversion. 
 
 An example will suffice to make this clear : In a coal- 
 mine the steam-engine serves to raise the coals from the pit 
 to the surface. The engine expends energy in overcoming 
 the attraction of the earth for the weight. Whence does 
 the engine obtain its energy ? Obviously from the expan- 
 sion of the steam in the cylinder, for steam (or any other 
 gas) loses energy in expanding. The steam is produced 
 by boiling water in the boiler ; water absorbs energy in 
 changing into steam. And this energy reaches the water 
 in the form of heat from the boiler fire ; the heat is pro- 
 duced by the combustion of coal ; and the coal, which is 
 the product of the decay of wood buried under the surface 
 of the earth, must originally have derived its energy from 
 the sun, the rays of which are essential to the growth of 
 plants. 
 
 We have here a long chain of transformations of energy ; 
 the chemical energy of the coal is transformed into heat, 
 the heat causes the expansion of the water into steam, the 
 steam overcomes the resistance of the piston in the cylinder, 
 the motion of the engine raises the weight. In all this 
 chain there is no loss of energy ; it is only transformed from 
 one kind to another. But it must not be imagined that 
 each kind of energy is quantitatively transformed into the 
 other ; for example, when the steam urges the piston forward 
 in the cylinder, some of the energy is lost by the friction of 
 the piston against the walls of the cylinder, and is converted 
 into heat ; and, indeed, energy tends to be degraded^ that is, 
 to be transformed into heat-energy. 
 
 In almost all chemical reactions which take place, either 
 of their own accord or on rise of temperature, heat is spon- 
 taneously evolved. When that is the case the reaction is 
 termed "exothermic;" but " endothermic " reactions are 
 
ENERGY 115 
 
 also known, in which heat is absorbed. Such reactions, 
 however, do not take place spontaneously at ordinary tem- 
 peratures. All the phenomena of combustion are exothermic 
 reactions. We are familiar with many examples of this, as 
 when coal burns, when hydrogen and oxygen explode, when 
 gunpowder is fired all these are examples of exothermic 
 reactions. The interaction of any two or more elements 
 which spontaneously unite to form a compound is of the 
 same nature as combustion. 
 
 Endothermic reactions those in which heat is absorbed 
 are usually only possible at ordinary temperatures when 
 an exothermic reaction proceeds at the same time. But one 
 point must be noticed here ; it is necessary that both the 
 exothermic and the endothermic reaction should be part of 
 the same chemical process. For example, the formation of 
 chloride of nitrogen by the action of chlorine upon a con- 
 centrated solution of ammonia is an exothermic reaction. 
 Chloride of nitrogen is a fearfully explosive body, detonating 
 with the least shock into its elements, chlorine and nitrogen ; 
 but while it is being formed there is formed at the same 
 time ammonium chloride, a substance which is produced 
 with great evolution of heat. These two reactions are part 
 of the same chemical process, and they are expressed 
 by the equation 4 NH 3 + 3C1 2 = NC1 3 + sNH 4 Cl. It is 
 essential that both ammonium chloride and the chloride of 
 nitrogen should be produced by the same chemical reaction, 
 The combination of nitrogen and chlorine would not take 
 place were any other exothermic reaction unconnected with 
 the formation of nitrogen chloride to be going on in the 
 same vessel. The elements nitrogen and chlorine do not 
 form nitrogen-chloride when mixed, even under the influ- 
 ence of a high temperature, nor would they if another exo- 
 thermic reaction were proceeding simultaneously in contact 
 with the nitrogen and the chlorine. Moreover, in order 
 that an endothermic compound may be formed, it is not suf- 
 ficient that an exothermic reaction take place simultaneously ; 
 the heat evolved during the exothermic reaction must usually 
 
u6 MODERN CHEMISTRY 
 
 exceed that absorbed by the formation of the endothermic 
 compound. 
 
 Endothermic compounds readily decompose, often with 
 explosion ; when they do so heat is evolved ; the compound 
 loses energy. This implies that the elements in the free state 
 or any other products of the decomposition of the endother- 
 mic compound contain less energy than the compound before 
 decomposition. On the other hand, in order to decompose 
 exothermic compounds, heat must be imparted to them. 
 The example given on p. 31 of ammonium chloride is a 
 case in point. It will be remembered that in order to de- 
 compose ammonium chloride into ammonia and hydrogen 
 chloride the temperature must be raised, and heat is absorbed 
 by the chloride ; hence its products ammonia and hydrogen 
 chloride in the uncombined state contain more energy than 
 their compound, ammonium chloride. Other substances 
 similar to ammonium chloride are known which dissociate 
 more gradually than that compound, and the characteristic 
 of all such dissociating bodies is this that the higher the 
 temperature the less stable they are. Even water when 
 raised to a temperature approaching 2000 C. dissociates 
 partially into hydrogen and oxygen. Indeed, the rule for 
 all exothermic compounds is that they become less and less 
 stable the higher the temperature. 
 
 The opposite is the case with endothermic compounds ; 
 the amount of heat absorbed by the union of their con- 
 stituents is less the higher the temperature ; and when the 
 temperature surpasses a certain point peculiar to each sub- 
 stance the endothermic compound changes its character and 
 becomes exothermic. But it is not often possible to produce 
 endothermic compounds by bringing the elements together 
 at a high temperature, because in cooling down they separate 
 again into their constituents. It appears necessary to com- 
 municate energy to them in some form other than heat. 
 The formation of ozone, O 3 , is accomplished by passing 
 the silent electrical discharge through oxygen. It is pro- 
 bable that the disruption of the oxygen molecule O 2 into 
 
ENERGY 117 
 
 atoms is produced by the rise of temperature due to the 
 electric sparks, but the combination of some of these atoms 
 into groups of three (as well as for the most part into 
 groups of two) is probably to be ascribed to the energy 
 which they receive in the shape of electric charges. An- 
 other instance is that of the burning of the nitrogen of the 
 air when a high tension current is passed through it. Nitro- 
 gen and oxygen do not unite even at the highest temperature 
 which can be produced by the combustion of carbon, but 
 when a high tension current is passed through a mixture of 
 the two gases a true flame is produced, and combination to 
 nitric peroxide, NO 2 , takes place. This flame can be 
 blown out, and it can be rekindled by the help of a lighted 
 match. It would thus appear that endothermic compounds 
 can be directly formed when energy is communicated to 
 them electrically. 
 
 When a chemical reaction between elements, resulting in 
 the formation of a compound, takes place, it is not always 
 that compound which is formed involving the greatest ex- 
 penditure of energy, or in other words, the greatest evolu- 
 tion of heat. It is quite possible for a compound to be 
 produced which, when appropriately treated, will change 
 into a still more stable configuration. Let us take an 
 example : When chlorine is passed through a solution of 
 caustic soda, the most stable configuration of the elements is 
 the production of sodium chloride, water, and oxygen. 
 But the reaction proceeds by no means so far ; it ceases 
 when the products are sodium chloride, sodium hypochlorite, 
 and water : 
 
 2NaOH. Aq + C1 2 = NaCl. Aq + NaOCl. Aq + H 2 O. 
 
 This solution, when warmed, undergoes a further change, 
 and again loses energy, yielding sodium chlorate and chlo- 
 ride : sNaOCl. Aq = NaClO 3 .Aq + zNaCl. Aq. But the 
 change does not cease here. For on evaporating to dry- 
 ness, and heating it still further, the chlorate is again decom- 
 posed into oxygen and chloride : 2NaClO 3 = 2NaCl + 3O 2 . 
 
ii8 MODERN CHEMISTRY 
 
 This final change is also exothermic. A mechanical analogy 
 for such a series of transformations may be found. 
 
 Imagine a switchback railway on an incline, those 
 portions of the rail which usually slope upwards being 
 nearly level. Further imagine a carriage started over the 
 first incline so as to roll on to the nearly level rail ; it will 
 stop here ; and it will require a further push to send it over 
 the second incline, when it will rest on the second level 
 platform and require another push to cause it to roll over 
 the third incline on to the third level platform. These level 
 platforms may be taken as analogous to the intermediate 
 compounds before chloride of potassium is formed. As the 
 carriage loses energy during each fall but stops several times 
 before all energy is lost, so it is possible to have a number 
 of stages in loss of energy before the final stable stage is 
 reached. Such cases are by no means unfrequent ; it is not 
 always possible to trace their sequence as readily as in the 
 case given, for it is not always possible to stop at the inter- 
 mediate stage ; but intermediate compounds may be made 
 otherwise, and they obviously belong to a series similar to 
 that given. 
 
 It has been frequently mentioned that application of heat 
 is necessary in order to start a reaction ; this is analogous 
 to the push which must be given to the carriage in order 
 that it roll over the incline ; if left alone, the compound is 
 stable, but the imparting to it of an exceedingly small 
 amount of energy suffices to cause it to lose a considerable 
 amount of energy in passing to the next stage. From the 
 molecular point of view it may be imagined that the applica- 
 tion of heat causes a motion of the atoms within some of 
 the molecules of the compound ; these begin to adjust 
 themselves in some new form of combination, and the heat 
 evolved during this readjustment is imparted to those mole- 
 cules which have not already suffered change, and causes 
 them also to assume a new form of combination attended 
 with loss of energy. 
 
 Besides losing energy by loss of heat during the formation 
 
ENERGY 119 
 
 of a compound, energy may be evolved in other forms. It 
 is well known that in order to change a liquid into gas, heat 
 must be imparted to it, or, if the change take place by evapo- 
 rating the liquid in a partial vacuum, the liquid itself will 
 grow cold. 
 
 Conversely, when a gas is condensed into a liquid it 
 parts with the energy which it previously contained. When 
 a solid is changed into a liquid it absorbs energy ; when a 
 liquid is frozen into a solid it loses energy. Now, in 
 many chemical reactions the products have not the same 
 physical state as the substances from which they are formed ; 
 and in this case energy is lost or gained according to cir- 
 cumstances. For example, when carbon dioxide is set 
 free by the action of an acid upon marble, a gas is pro- 
 duced, and the production of this gas is attended with 
 absorption of energy ; in order to measure the amount of 
 this energy it would suffice to condense that gas to liquid 
 and to freeze the liquid to solid and to measure the amount 
 of energy evolved during these transformations. It would 
 then be possible to ascertain the total quantity of energy 
 lost during the chemical change, independently of the 
 change of state which the products undergo on being formed. 
 But this is not all ; for when a gas is produced it occupies 
 space and displaces a certain amount of air. Imagine the 
 gas to be evolved at the bottom of a vertical tube, which, 
 of course, was originally in communication with the atmos- 
 phere and full of air ; the gas would expel this air from the 
 tube, or, in other words, raise it. Now air possesses weight, 
 and presses on the surface of the earth with a weight of 
 1.033 kilograms on each square centimeter, and the work 
 done by the gas in issuing into the atmosphere would 
 depend, in the instance given, on the sectional area of the 
 tube, and the height up the tube to which the carbonic 
 acid reached. Here energy is expended, or, as is usually 
 said, work is done, in raising the weight ; and in estimating 
 the total energy of the reaction mentioned, this work, accom- 
 plished against gravity, must be subtracted from the total. 
 
120 MODERN CHEMISTRY 
 
 Many measurements of the heat evolved or absorbed during 
 chemical reactions have been made, chiefly by M. Berthelot 
 and by Professors Julius Thomsen and Stohinann. The 
 reaction under investigation is caused to take place in a 
 calorimeter, and the heat evolved or absorbed is measured 
 by the rise or fall of temperature of the water which it 
 contains. In order to measure heat evolved by combustion, 
 the combustion is caused to take place in a vessel enclosed 
 in a calorimeter ; and M. Berthelot has introduced a very 
 convenient piece of apparatus in which combustible sub- 
 stances are caused to burn in a steel bomb charged with 
 oxygen at a high pressure, the bomb being itself immersed 
 in a calorimeter. 
 
 Inasmuch as the heat evolved during chemical decom- 
 position of a compound must be precisely equal to that 
 absorbed during its formation, it is possible indirectly to 
 arrive at the heat of formation of many compounds of 
 which the component elements will not combine directly. 
 Let us take, for example, the case of marsh-gas or methane; 
 this compound, which has the formula CH 4 , is made 
 to burn in a vessel enclosed in a calorimeter. The heat 
 evolved on burning 16 grams of methane with 64 grams of 
 oxygen is 211,900 calories; that evolved on burning 12 
 grams of carbon in 32 grams of oxygen to carbon dioxide 
 is 94,300 calories ; 4 grams of hydrogen when burned in 
 oxygen yield 1 36,800 calories. These results are generally 
 expressed by the following equations: 
 
 CH 4 + 4 O = CO 2 + 2H 2 O + 2 T 3,800 c. 
 C + 26 = CO 2 + 94,300 c. 
 
 2H 2 O + 136,800 c. 
 
 Now methane is an exothermic body ; if it were possible 
 to form it from its elements, carbon and hydrogen, heat 
 would be evolved. It is possible to imagine it decomposed 
 into carbon and hydrogen, when heat would be absorbed. 
 The heat of formation of methane, therefore, is obviously 
 
ENERGY 121 
 
 the difference between that which is evolved when methane 
 is burned in oxygen, and that which is evolved when its 
 constituent elements, carbon and hydrogen, are burned. In 
 this case it is the difference between (94,300+ 136,800) 
 213,800 = 17,300. In this case the carbon is imagined 
 to be solid and in the form of graphite, and the hydrogen 
 and oxygen to be gaseous ; if the carbon were a gas, to begin 
 with, it would naturally give out less heat on its combustion, 
 because heat is necessarily absorbed in the conversion of 
 solid into gaseous carbon. 
 
 It might be thought, without due consideration, that a 
 measurement of the heat of formation of a compound in- 
 volves a measurement of the energy which it contains ; but 
 this is not so, for it is obvious that what is measured is only 
 the difference of the energy contained in the elements from 
 which it is formed and in the compound which they pro- 
 duce. We are as yet ignorant of the total amount of 
 energy contained in any element or compound. 
 
 It is possible by suitable appliances to obtain the energy 
 evolved during chemical combination, not as heat, but in 
 the form of an electric current. When two metals are 
 immersed in a conducting liquid or electrolyte, they at 
 once exhibit a difference of electric potential ; or connect- 
 ing by a wire the two portions of the metals which do not 
 dip into the liquid, that metal which has the higher electric 
 potential combines with one of the ions, and the other ion, 
 as already explained on p. 36, travels through the electro- 
 lyte until it touches the other metallic plate, when it, too, 
 is discharged and escapes in the free state ; its charge enters 
 the metallic plate. The result of this action is that the 
 chemical combination of one of the metals with one of the 
 ions of the liquid is attended by the formation of an electric 
 current, and not necessarily by an evolution or absorption of 
 heat. Now it is possible to measure the difference of poten- 
 tial between the two metals and the amount of electricity 
 which passes through the wire, and thus to determine the 
 
122 MODERN CHEMISTRY 
 
 amount of energy in a form other than heat ; by this means 
 the loss of energy which accompanies combination has been 
 frequently measured. 
 
 The process, however, leads us further, for it is possible 
 to arrive by its help at an estimation of what has been 
 termed "chemical affinity;" it is of the same nature as 
 electric potential. The reason for this statement is as 
 follows : 
 
 It has been mentioned that energy is stored up when 
 a gas is compressed ; the amount of energy stored will 
 obviously depend on the mass of the gas and on the rise 
 of pressure. Energy can also be stored by the raising of 
 a weight above the surface of the earth ; here again the 
 amount of energy depends on the mass or the weight 
 of the body raised and the distance through which it is 
 raised. In the case of heat, the two components of that 
 form of energy are temperature and a quantity analo- 
 gous to specific heat. This case requires a little further 
 consideration. The amount of heat absorbed by a 
 piece of any particular metal, say copper, obviously de- 
 pends on the mass of the copper, on the speciiic heat 
 of the copper, and on the temperature through which 
 it is raised ; if the mass be doubled, the amount of heat 
 which that copper will absorb on being raised through 
 the same interval of temperature will be twice the original 
 amount ; if the mass remain the same and the interval 
 of temperature be doubled, the amount of heat will again 
 be doubled. By choosing another metal of which the 
 specific heat is twice that of copper, the heat absorbed 
 by a weight equal to that of the piece of copper, if the 
 second metal is heated through the same interval of tem- 
 perature, will be doubled. We see, therefore, that heat 
 energy may also be regarded as compounded of two factors 
 for unit mass : 
 
 !i ) The specific heat of the substance. 
 2) The interval of temperature through which it is 
 raised. 
 
ENERGY 123 
 
 Electric energy may also be regarded as compounded of 
 two factors 
 
 1 I ) Electric quantity or charge. 
 
 (2) Electric potential. 
 
 Now, when a current passes through a wire, the quantity 
 of electricity passing depends on the potential, or, as it is 
 sometimes called, electric pressure, and on the diameter, 
 length, and material of the wire. The total energy com- 
 municated in the form of an electric current has, as its 
 factors, the quantity of electricity passing, and the potential 
 with which the electricity is urged along its course. 
 
 It is probable that chemical energy may also be conceived 
 to consist of two factors ; the one is generally called atomic 
 or formula weight, for chemical elements and groups enter 
 into and separate out of combinations in quantities pro- 
 portional to these numbers. At the same time it is pro- 
 bable that when two elements unite together they attract 
 each other, and that this attraction depends for its amount 
 on the nature of the elements which are presented to one 
 another ; the chemical attraction has been termed affinity. 
 Now it has already been explained on p. 37 that when a 
 current is passed through a solution of an electrolyte, 
 it is conveyed by the ions present in solution ; and these 
 ions are composed of elements, or groups of elements, each 
 of which carries one, two, or more electrons. It is here 
 evident that the quantity of an element or group which 
 conveys electricity is identical with the quantity which 
 enters into combination ; it may be termed the equivalent, 
 and while the equivalent is that quantity which conveys a 
 unit quantity of electricity, it is also that which serves as 
 the unit of quantity in chemical compounds. It would 
 appear, therefore, that one of the factors of chemical energy 
 is numerically identical with one of the factors of electrical 
 energy, and it follows from this that the other factor must 
 also be proportional ; that is, a measurement of electric 
 potential is equivalent to a measurement of chemical potential 
 
124 MODERN CHEMISTRY 
 
 or affinity. Up till now, very few experiments have been 
 made with the object of measuring the electric potential 
 of systems of chemical elements ; such measurements are 
 much required, for it would then be possible to arrive at 
 an estimate of the force with which chemical elements and 
 groups of elements are retained in combination. 
 
INDEX 
 
 ACETIC acid, 94 
 
 Acetonitrile, 94 
 
 Acids, 57 
 
 Affinity, chemical, 122 
 
 " Airs," 5 
 
 Alcohols, 91 
 
 Aldoximes, in 
 
 Allotropy, 74 
 
 Alloys, 62 
 
 Analysis, 5 
 
 Anode, 35 
 
 44 Aqua," 34 
 
 Argon, 73 
 
 Arsenic, density of, 70 
 
 Arsenic, allotropic, 77 
 
 Atomic weight, 13, 65, 66, 67, 
 
 68 
 
 Atoms, 9, 52 
 Avogadro's hypothesis, 12 
 
 BASES, 59 
 
 Benzene, 96 
 
 Bismuth, vapour-density of, 69 
 
 Boiling-point, rise of, 29, 73 
 
 Boyle's Law, 19 
 
 Bromine, density of, 69 
 
 Butylene, 88 
 
 CALORY, 14 
 
 44 Calx," 4 
 
 Capacity for heat, 13 
 
 Carbides, 61 
 
 Carbon, allotropic, 74, 75 
 
 ,, stereo-chemistry of, 101 
 Chemical-energy, 123 
 Chlorine, vapour-density of, 69 
 Classification of compounds, 56 
 Combining proportions, 7 
 
 Complexity of molecules, 44, 71 
 
 Concentration, 25 
 
 Conductivity, 35, 40 
 
 , , of water, 43 
 
 , , of fused salts, 43 
 
 Constant proportions, 7 
 
 DALTON'S Laws, 9, 22 
 Dephlogisticated air, 5 
 Diffusion, 21 
 Displacement, 47 
 Dissociation, 31, 32, 33 
 Double linkage, 109 
 Dulong and Petit's Law, 13 
 
 ELECTRIC energy, 123 
 Electrolysis, 35, 40 
 Elements, 2 
 
 ,, preparation of, 45, 46, 
 
 47 
 
 ,, classification of, 49 
 Endothermic reactions, 115 
 Energy, 113 
 Equivalent, 15, 63, 64 
 Ethane, 91 
 Exothermic reactions, 115 
 
 FARADAY'S Law, 35 
 Fluorine, density of, 69 
 Formulae, 52 
 Freezing-point, lowering of, 26, 
 
 Fumade acid, 109 
 
 GASES, density of, 16 
 Gay-Lussac's Law, n, 20, 21 
 Gold, allotropic, 80 
 Graphic formulae, 52, 92, 93 
 
126 
 
 INDEX 
 
 HALIDES, 56 
 Heat, atomic, 14 
 Heat energy, 122 
 
 ,, of combustion, 120 
 
 ,, of formation, 120 
 
 ,, specific, 14 
 Helium, 73 
 Hydrides, 56 
 Hydrocarbons, 61, 92, 93 
 Hydrogen, density of, 69 
 
 ,, discovery of, 7 
 
 Hydroxides, 59 
 
 IODINE, vapour-density of, 69 
 lonisation, 40 
 Iridium, allotropic, 80 
 Iron, allotropic, 80 
 Isomerism, 87 
 Isomorphism, 17 
 
 KATHODE, 35 
 Krypton, 73 
 
 " LAW of octaves," 50 
 
 MALEIC acid, 109 
 
 Marsh-gas, 89 
 
 Methane, 80 
 
 Migration of ions, 36, 37, 38, 39 
 
 Molecular weight, 13, 68 
 
 Monatomicity, 71 
 
 NEON, 73 
 
 Nitric peroxide, 89 
 
 Nitrides, 59 
 
 Nitrogen, stereo-chemistry of, 89 
 
 ,, density of, 69 
 Nomenclature, 58 
 
 OSMOTIC pressure, 23, 24, 26 
 Oxides, 56 
 Oxygen, 6 
 
 ,, allotropic, 77 
 
 , , density of, 69 
 Ozone, 77 
 
 PARAFFINS, 89, 90, 91 
 Partial pressures, 22 ^t*~<? 
 
 Periodic table, 50 
 Phases, 81, 82, 83 
 Phlogiston, 3 
 Phosphides, 60 
 Phosphorus, allotropic, 77 
 
 , , vapour-density of, 70 
 Polarised light, 98 
 Polymerism, 87 
 Propane, 91 
 
 RACEMIC acid, 101 
 Rhodium, allotropic, 80 
 Ruthenium, allotropic, 80 
 
 SELENIDES, 56 
 
 Selenium, allotropic, 79 
 
 Silicides, 61 
 
 Silicon, allotropic, 75 
 
 Silver, allotropic, 80 
 
 Solutions, 33 
 
 Specific heat, 13, 16 
 
 Steel, 80 
 
 Stereo-chemistry, 98 
 
 Structure of compounds, 55, 56 
 
 Structural formulae, 52, 92, 93 
 
 Sulphides, 56 
 
 Sulphur, allotropic, 78, 79 
 
 ,, vapour-density of, 70 
 
 ,, phases of, 84 
 
 , , stereo-chemistry of, 107 
 
 TARTARIC acid, 101 
 Tautomerism, in 
 Tellurides, 56 
 Thallium, density of, 69 
 Tin, allotropic, 76 
 ,, stereo-chemistry of, 107 
 
 UREA. 87 
 
 VALENCY, 51, 52 
 Vapour-densities , 68 
 
 WATER, phases of, 82 
 XENON, 73 
 
INDEX OF NAMES 
 
 AVOGADRO, II 
 
 BACON, 4 
 Becher, 3 
 Beckmann, 28 
 Berthelot, 120 
 Berthollet, 7 
 Black, 5 
 Boyle, 4 
 
 CANNIZZARO, 15 
 
 Cavendish, 7 
 
 DALTON, 8 
 Davy, 49 
 Deville, 31 
 Dulong, 10 
 
 FARADAY, 35, 87 
 GAY-LUSSAC, 10 
 HITTORF, 36 
 
 KEKULE", 95 
 Kohlrausch, 42 
 
 LAVOISIER, 5, c8 
 LeBel, 97 
 Liebig, 87 
 Lodge, 37 
 
 MASSON, 38 
 Mendele'eff, 50 
 Meyer, Lothar, 50 
 Mitscherlich, 17 
 
 NEWLANDS, 49 
 
 PASTEUR, 101 
 Petit, ii 
 Pfeffer, 24 
 Priestley, 5 
 Proust, 7 
 
 RAOULT, 28 
 Rey, 4 
 Richter, 7 
 
 SCHEELE, 5 
 Schonbein, 77 
 Stahl, 3 
 Stas, 64 
 Stohmann, 120 
 
 THOMSEN, 120 
 VAN'T HOFF, 26 
 
 WENZEL, 7 
 Wohler, 87 
 
 FND OF VOL. I. 
 
 Printed by BALLANTYNE, HANSON 
 Edinburgh d^> London 
 
 Co. 
 
UNIVERSITY OF CALIFORNIA LIBRARY 
 
 THIS BOOK IS DUE ON THE LAST DA' 
 STAMPED BELOW 
 
 LTE 
 
 1919 
 
 JAN 26 1922 
 JAN 26 1922 
 
 U>R 8 I 
 
 1925 
 
 17 1925 ' 
 
 NOV 
 
 DEO i* 1931 
 
 'AUG 9 2005