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 LL.D., F.B.A. 
 
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CHEMISTRY 
 
 BY 
 
 RAPHAEL MELDOLA 
 
 D.SC., LL.D., F.R.S. 
 
 PROFESSOR OF CHEMISTRY IN THE FINSBURY TECHNICAL 
 
 COLLEGE } AUTHOR OF "THE CHEMICAL SYNTHESIS 
 
 OF VITAL PRODUCTS," ETC. 
 
 
 NEW YORK 
 HENRY HOLT AND COMPANY 
 
 LONDON 
 WILLIAMS AND NORGATE 
 

CONTENTS 
 
 CHAFTKK AGE 
 
 INTRODUCTORY . 7 
 
 I THE SCOPE OF CHEMISTRY THB NATURE 
 OF CHEMICAL CHANGE CHEMISTRY AN 
 EXPERIMENTAL SCIENCE . . . 16 
 II CHEMICAL COMBINATION AND MECHANICAL 
 MIXTURE AIR A MIXTURE AND NOT A 
 COMPOUND PHYSICAL SEPARATION OF 
 THE COMPONENTS OF AIR . . . 41 
 
 III CHEMICAL CHANGE IN ITS QUANTITATIVE 
 
 ASPECT THE DEFINITENESS OF CHEMICAL 
 CHANGE THE CONSERVATION OF MASS 
 WATER A CHEMICAL COMPOUND . . 64 
 
 IV ELEMENTARY AND COMPOUND MATTER 
 
 THE CHEMICAL ELEMENTS METALS AND 
 NON-METALS . . . . .91 
 V CHEMICAL EQUIVALENCE ELECTRO -CHEMI- 
 CAL EQUIVALENCE MULTIPLE AND 
 RECIPROCAL EQUIVALENCE THE ATOMIC 
 
 THEORY 115 
 
 VI SYMBOLS AND NOTATION ATOMS AND 
 MOLECULES ATOMIC AND MOLECULAR 
 WEIGHTS THE DEFINITENESS OF 
 CHEMICAL COMBINATION BY VOLUME 
 THE HYPOTHESIS OF AVOGADRO . 137 
 VII THE NUMBER OF ATOMS CONTAINED IN 
 A MOLECULE DISSOCIATION AND ASSO- 
 CIATION AUXILIARY METHODS FOR 
 DETERMINING ATOMIC AND MOLECULAR 
 WEIGHTS THE LAW OF DULONG AND 
 PETIT ATOMIC AGGREGATES IN SOLUTION 163 
 
6 CONTENTS 
 
 CHAPTER PAGE 
 
 VIII DETERMINATION OF THE RELATIVE WEIGHTS 
 OF THE ATOMS THE ISOLATION OF 
 DEFINITE SUBSTANCES CHEMISTRY AS 
 AN EXACT SCIENCE THE STANDARD OF 
 ATOMIC WEIGHTS CHEMICAL ARITH- 
 METIC VOLUMETRIC RELATIONSHIPS . 183 
 IX VALENCY CHEMICAL STRUCTURE THE 
 CHEMISTRY OF CARBON STEREOCHEMIS- 
 TRY . . . . . . .204 
 
 X THE PERIODIC CLASSIFICATION OF THE 
 
 ELEMENTS CONCLUSION . . . 228 
 
 BIBLIOGRAPHY 249 
 
 INDEX ..... 253 
 
CHEMISTRY 
 
 INTRODUCTORY 
 
 THE history of civilization reveals the fact 
 that all highly developed nations in the course 
 of their evolution have passed through phases 
 characterised by the culmination of various 
 human activities, physical and intellectual. 
 Not that it is implied by this statement that 
 the manifestation of extreme activity of a 
 particular kind at one period was accompanied 
 by a decline, or was developed at the expense 
 of all other forms of activity. The lesson of 
 history is that, concurrently with the general 
 national activity, certain ages have witnessed 
 special activities or have attained particular 
 maxima of development which have served 
 to stamp the age with some general charac- 
 teristic. Thus, there was an age of Philosophy 
 in ancient Greece, of Militarism in ancient 
 Rome, of Sacred Art in mediaeval Italy, and 
 of Dramatic Poetry in England during the 
 Elizabethan period. The influence of such 
 epochs has extended perceptibly or imper- 
 
ceptibly from a remote past down to the 
 present time ; the recognition of this influence 
 is embodied in the familiar adage that we 
 are the heirs of all the ages. The special 
 activity of the present time, Science, is one 
 that we believe is destined to influence the 
 future more profoundly than any of those 
 activities which reached their culminating 
 points in former ages. 
 
 In stating that we are now living in an 
 age of Science, it is meant that we are getting 
 into closer communion with Nature than has 
 hitherto been possible. From the time when 
 man became an observing and thinking being, 
 he must have been impressed by natural 
 phenomena ; but at no former period, so far 
 as history has preserved records, has there 
 been such intense activity in the questioning 
 of Nature in the systematized observation 
 of facts, and in the endeavour to arrive at a 
 knowledge of causes. It may be said that, 
 among the more advanced nations, mankind 
 is gradually beginning to grasp that great 
 truth which in former ages was realized only 
 by a few specially gifted individuals the truth 
 that the human race, although the intellectual 
 crown and summit of terrestrial life, is not 
 detached from and independent of its 
 surroundings. The anthropocentric notions 
 
INTRODUCTORY 9 
 
 which dominated thought in early times are 
 slowly being replaced by that broader view 
 which makes man a part of Nature an 
 organism adapted to his environment just 
 like any other organism, but having the 
 supreme advantage of practically unlimited 
 adaptability by virtue of his intellectual 
 development. It is now beginning to be 
 perceived that this power of adaptation is 
 synonymous with a knowledge of Nature's 
 methods in other words, that the present 
 well-being and the future progress of the 
 human race is dependent upon the develop- 
 ment of Natural Science. 
 
 The recognition of the principle that man's 
 dominion is inseparably bound up with 
 scientific progress is embodied in Tennyson's 
 lines : " The crowning race ; Of those that 
 eye to eye shall look on knowledge ; Under 
 whose command is Earth and Earth's ; And 
 in their hand is Nature like an open book." 
 
 This recognition has been brought about in 
 modern times by the labours of those who 
 have devoted and are devoting their lives to 
 the study of Nature at first hand. It is the 
 active army of original investigators who, 
 in the first place, have become cognizant of 
 the supreme importance of their work to the 
 present and future welfare of the race. The 
 
10 CHEMISTRY 
 
 realization of the truth that Nature is to the 
 earnest student " an open book " has become 
 the trumpet call of the present age. The 
 worker in the domain of Science is prompted 
 by the knowledge that his results, directly 
 or indirectly, immediately or prospectively, 
 may be utilized for the benefit of humanity. 
 His achievements, although strictly humani- 
 tarian in their ultimate bearing, cannot, 
 however, be weighed and measured by a 
 narrowly practical standard. The level of 
 natural knowledge which has now been reached, 
 and which is annually being raised, is the 
 result of patient and laborious research, often 
 extending over many years, sometimes over 
 a lifetime. But only a small proportion of 
 the work accomplished is of immediate 
 utility ; and that which is obviously useful 
 to man is but the final stage of a long series 
 of antecedent gropings after truth. The 
 popular appreciation of Science, to be of real 
 value to the nation, should, therefore, be 
 independent of the spirit of narrow utilitarian- 
 ism, for no investigator who enters upon a 
 definite line of work can foresee when or how 
 his results may become of practical value, 
 or whether they will ever lead to any practical 
 applications. If the progress of the nation 
 is dependent as we are now beginning to 
 
INTRODUCTORY 11 
 
 realize upon its general appreciation of 
 Science, that appreciation must be of the 
 highest and broadest character it is Science 
 in the abstract, and not purely utilitarian 
 concrete knowledge, which must be raised to 
 the level of one of the most exalted branches 
 of human culture. 
 
 The modern awakening of the spirit of 
 scientific inquiry has resulted in an activity 
 which is in itself responsible in some measure 
 for the slow progress towards the attainment 
 of that high standard of popular scientific 
 culture which we desire to see established. 
 The active workers are a numerous and ever 
 growing body, and the boundaries of knowledge 
 are being extended with such rapidity in every 
 direction that the educated layman who can 
 follow with intelligence the various develop- 
 ments of Literature or of Art finds himself 
 unable to cope with the progress of Science. 
 Nor is this surprising when we find that even 
 the workers themselves, having necessarily 
 to specialize in order to achieve results of 
 value, are unable to keep pace with the 
 progress of discovery in domains outside their 
 own field of research. Moreover, the constant 
 discovery of new facts and principles, and the 
 concurrent revision or extension of scientific 
 doctrine is apt to discourage the would-be 
 
12 CHEMISTRY 
 
 learner who, without any special scientific 
 training, has had his mind deprived of plas- 
 ticity by an inelastic education in subjects for 
 which the materials are gathered entirely from 
 books and not directly from Nature's records. 
 
 Another obstacle in the way of the general 
 diffusion of scientific culture is the technical 
 language which every branch of science has 
 found it necessary to invent in order to give 
 precision to the description of new facts, and 
 for the formulation of new principles. But, 
 while admitting that the technicalities of 
 modern scientific language from the popular 
 point of view interpose difficulties, it must 
 be borne in mind that for the actual workers 
 they are labour-saving contrivances. Although 
 the terminology may appear formidable 
 to the uninstructed, it must not be for- 
 gotten that every term and every symbol 
 corresponds with some natural reality, or 
 with what according to existing knowledge 
 is believed to be a reality. The reality is 
 generally capable of being expressed in 
 simpler terms than would appear from its 
 symbolical expression ; the underlying idea 
 is generally less complex than the language 
 which has been found necessary to define it 
 with scientific precision. 
 
 But, apart from all such difficulties, in view 
 
INTRODUCTORY 13 
 
 of the daily increasing importance of Science 
 as a prime factor of national development, 
 the educated layman can no longer afford to 
 ignore the achievements of that great inter- 
 national army which is waging perpetual 
 warfare against ignorance of Nature's methods. 
 In this quest for knowledge, there is no dis- 
 tinction of race, or creed, or country all 
 workers are co-operating for the general 
 cause ; a truth wrested from Nature becomes 
 the common property of mankind. Such 
 truths cannot be lightly set aside, or crushed 
 out of existence by the older learning ; they 
 are revelations to man as distinct, as eternal, 
 and as far reaching in their consequences as 
 any proclaimed by the seers and prophets of 
 former ages. 
 
 Granting, therefore, that the reader wishes 
 to be put in possession of the existing state 
 of scientific knowledge, it must at the 
 outset be realized that Science never pauses 
 on her onward march ; there is no " existing 
 state " of knowledge in the sense of finality. 
 She has no dogmatic creed to proclaim ; she 
 is aware of her fallibility ; and her strength 
 lies in her knowledge that it is Nature which 
 is infallible, and man but an interpreter with 
 limited power of observation and reasoning. 
 The ground covered by Science is, moreover, 
 
14 CHEMISTRY 
 
 so vast that it must also be recognized that, 
 for the practical purposes of study and re- 
 search, sub-division into departments is abso- 
 lutely necessary. Not that these sub-divisions 
 are representative of any natural reality ; 
 they are expressive rather of the imperfect 
 state of our knowledge. In view of the 
 limitations of human faculty, it is both necess- 
 ary and expedient that the worker should 
 confine himself to some particular department ; 
 but, in accepting this principle as a matter 
 of convenience, the student must not commit 
 himself to the belief that this sub-division 
 indicates a want of unity in Nature. On the 
 contrary, the most advanced thinkers have 
 come to believe in the unity of Nature and 
 to recognize that the ideal towards which 
 research is tending is the unification of know- 
 ledge into one general Science or system of 
 Philosophy. There may be work for countless 
 generations before this ideal is reached, but 
 even now there are indications in every 
 direction that natural knowledge cannot be 
 confined in water-tight compartments ; the 
 barriers, confessedly artificial, are being broken 
 down, and the inter-relations between the 
 various sciences are becoming both more 
 numerous and more intimate with the progress 
 of discovery. The tendency towards coales- 
 
INTRODUCTORY 15 
 
 cence is shown by the creation in modern 
 times of such subjects as thermodynamics, 
 astrophysics, chemical physics and physical 
 chemistry, electrochemistry, thermochemis- 
 try, biochemistry, and biophysics. 
 
CHAPTER I 
 
 THE SCOPE OF CHEMISTRY THE NATURE OF 
 CHEMICAL CHANGE CHEMISTRY AN EX- 
 PERIMENTAL SCIENCE 
 
 The Scope of Chemistry. The Science of 
 Chemistry, as is, no doubt, already known to 
 the reader in a general way, is essentially a 
 materialistic science in so far as it deals with 
 matter. It belongs to a division known as the 
 physical sciences, a designation applied in 
 order to distinguish such subjects from those 
 which, like Zoology, Botany, Physiology, etc., 
 deal with life, and which are, therefore, 
 grouped as the biological sciences. This 
 classification is convenient as representing 
 the existing state of knowledge ; whether 
 such sub-division corresponds with some 
 underlying fundamental reality is a debate- 
 able question concerning which no dogmatic 
 pronouncement can at present be made. 
 But as with all attempts at rigid classification, 
 so here it will be found that no absolute 
 barrier can be erected between the two 
 
 16 
 
SCOPE OF CHEMISTRY 17 
 
 groups. Living matter, whether lowly or 
 highly organized, is as subject to physical 
 and chemical conditions as non-living matter. 
 The Physics and Chemistry of the living 
 organism are no longer regarded as impene- 
 trable mysteries beyond the scope of legitimate 
 scientific investigation. Modern Chemistry 
 does not recognize that rigid definition which 
 in former times restricted its scope to " dead 
 matter " ; in that borderland between the 
 two main groups of sciences, there is now at 
 work a new school of investigators who are 
 attacking the mysteries of vital chemistry 
 in the same spirit as that which has prompted 
 research into every other department of 
 science. It is realized that the living organism 
 has solved chemical problems which we have 
 as yet been unable to approach by our known 
 methods but this is regarded as an incentive, 
 and no longer as a deterrent, to further 
 inquiry. On the other side, the biologist 
 deals with living organisms not only from 
 the point of view of classification, distribution, 
 bionomics, evolution, etc., but he also con- 
 cerns himself with their physical and chemical 
 activities, with the inner mechanism of the 
 life processes. Physiology, in the broad 
 sense, has now become the meeting-ground 
 of the physical and biological sciences. 
 
18 CHEMISTRY 
 
 In placing Chemistry in the front rank as a 
 science concerned with matter, we thereby 
 associate it with other sciences which also 
 deal with matter, and especially with Physics, 
 its nearest and natural ally. In view of what 
 has been said concerning the conventionality 
 of our schemes of classification, it is perhaps 
 unnecessary to insist upon the impossibility 
 of drawing a sharp line of demarcation 
 between Chemistry and Physics. No other 
 branches of science furnish such numerous and 
 such striking examples of interpenetration. 
 Physical methods of studying the properties 
 of matter are used by chemists, and chemical 
 methods are being adopted by physicists. 
 All that can be said definitely is that those 
 general properties of matter, such as gravity, 
 which are independent of the specific nature 
 or composition of the substance, belong more 
 exclusively to the domain of the physicist. 
 But even in this case, the chemist is dependent 
 upon gravitational effect for the determination 
 of weight the most important of known 
 methods for dealing with matter quantita- 
 tively. 
 
 Whenever we pass from the general to the 
 special, and consider in detail the changes 
 in matter brought about by the action of 
 physical forces, it will be seen that the nature 
 
SCOPE OF CHEMISTRY 19 
 
 or amount of change is generally influenced 
 by the inherent properties of the substance, 
 or, in other words, by those specific characters 
 which are as truly chemical as physical. Thus, 
 heating and cooling cause bodies to expand 
 or contract, a change which, in general terms, 
 may be said to be physical. But in this 
 case it cannot be said that the change is 
 independent of the inherent nature of the 
 substance ; for, when the expansion or 
 contraction is measured quantitatively, it is 
 found to be a specific character, and therefore 
 of chemical significance. Again, a ray of 
 light is bent or refracted in passing from one 
 medium to another of different density ; 
 here, also, the general effect is said to be 
 physical. But the actual amount of bending, 
 as measured quantitatively, is found to be 
 determined by the specific character or 
 composition of the medium ; and the relation- 
 ship is so close that in certain classes of 
 cases the measurement of this quantity gives 
 information to the chemist concerning the 
 nature of the substance. One of the most 
 striking illustrations of the association of 
 Physics with Chemistry is furnished by the 
 results obtained by the study of the action 
 of electricity upon matter in the gaseous 
 state, a field of research which has led in 
 
20 CHEMISTRY 
 
 recent times to suggestions of fundamental 
 importance concerning the constitution of 
 matter. It is evident that such questions 
 are of equal significance to Physics and to 
 Chemistry ; but the modern developments 
 in this direction are as yet hardly mature 
 enough to incorporate with the established 
 body of chemical doctrine. As an earlier 
 illustration, however, nothing is more instruc- 
 tive than the history of the development 
 of spectroscopy. A gas or vapour when 
 raised to a sufficiently high temperature to 
 glow or become self luminous emits light 
 which appears to have colour. In purely 
 physical terms, this is explained by the 
 statement that the colour is the result of the 
 impression produced upon the eye by ether 
 waves of particular lengths or oscillation 
 frequencies. But, here again, when we go 
 into details, and analyse the light emitted 
 by such glowing gases, we find that the 
 particular kind of radiation is a specific 
 character of the radiating matter sufficiently 
 specific for diagnostic purposes, so that 
 particular kinds of matter can be identified, 
 whether here, or in the sun, or in the most 
 distant stars and nebulae. The chemist and 
 physicist and astronomer here join hands ; 
 the science based upon this property of 
 
CHEMICAL CHANGE 21 
 
 matter is both astro-chemical and astro- 
 physical. 
 
 It is obvious from such considerations that 
 Chemistry is all-embracing in its scope ; its 
 branches ramify in every direction, and its 
 roots underlie the most diverse departments 
 of science. Since it concerns itself with matter 
 wherever it exists and can be brought within 
 the ken of its methods, it may be regarded as 
 the common meeting-ground of all the natural 
 sciences. From the purely utilitarian point 
 of view, no other science can be said to be 
 more intimately bound up with the immediate 
 welfare of man. The prime needs of civilized 
 nations the raising of crops for food-stuffs, 
 the utilization of fuel for the generation of 
 power or for warmth, the manufacture of 
 useful products from raw materials, the 
 production of the multifarious necessary 
 materials required in every-day life, all depend 
 at one stage or another upon a knowledge of 
 the principles of chemical science and their 
 practical application. 
 
 The Nature of Chemical Change. Chemistry 
 does not restrict itself to the study of matter 
 simply as it exists, either naturally or arti- 
 ficially ; it is, above all, the science which 
 concerns itself with the changes or trans- 
 formations of matter brought about by the 
 
22 CHEMISTRY 
 
 agency of physical forces, or by the action of 
 one form of matter upon another. In so far 
 as the mere consideration of change in matter 
 is concerned, Chemistry and Physics overlap 
 to a very large extent. For practical purposes 
 it is, however, convenient in the present state 
 of knowledge to distinguish between physical 
 and chemical change. If, under the influence 
 of physical agencies such as heat, light, 
 electricity, etc., matter undergoes some change 
 which is not permanent, it is considered to 
 be a physical change. If these or other 
 agencies bring about a permanent change or 
 transformation of one kind of matter into 
 a totally different substance, the action is 
 claimed as coming within the province of 
 Chemistry. Following up the illustration 
 previously given concerning the action of 
 heat, a bar of iron is longer when it is hot 
 than when it is cold ; but the change, i.e., 
 the increase in length, is not permanent, 
 because the bar regains its original length on 
 cooling. This is an example of physical 
 change, and, even if the expansion were 
 permanent, it would not be regarded as a 
 chemical change, because there is no trans- 
 formation of material the bar is the same 
 substance, iron, both hot and cold. If the 
 iron is heated to a very high temperature, it 
 
CHEMICAL CHANGE 23 
 
 undergoes further changes, first becoming 
 soft, and then, as the temperature rises, 
 becoming liquid, so that it can be poured out 
 of the containing vessel. These, again, are 
 physical changes of state, because the material 
 is still iron ; by the process of fusion its 
 form may be permanently changed, but its 
 substance is unaltered. 
 
 Although such properties of matter are 
 described as " physical," they become of 
 chemical significance when we pass from 
 the general to the special when their study 
 is made comparative. Thus the expansion 
 of iron at a given temperature as compared 
 with the expansion of other substances ajt the 
 same temperature, or the temperature at 
 which iron melts, i.e., passes from the solid 
 to the liquid state, may be claimed as physico- 
 chemical properties, because they are specific 
 and not general ; other forms of matter may 
 or may not have the same degree of expansi- 
 bility and the same point of fusion. When, 
 however, intensely heated iron is freely 
 exposed to the air, or is plunged into water, 
 it undergoes a change in appearance ; the 
 surface becomes covered with an incrustation 
 or scale of something which is no longer iron. 
 In this case, the change is permanent so long 
 as the new substance remains under ordinary 
 
24 CHEMISTRY 
 
 conditions ; iron is there, but in a disguised 
 form, and can only be recovered from the new 
 substance by violent treatment, such as 
 heating to an extremely high temperature in 
 contact with coal or carbonaceous matter. 
 The iron in this case has undergone a chemical 
 change ; it has become transformed into a 
 different substance. Even without the appli- 
 cation of heat, iron on exposure to air and 
 moisture, as every one knows, becomes 
 " rusty," i.e., converted into a reddish sub- 
 stance which, like the " scale " just described, 
 contains iron, but is no longer that material ; 
 in rusting, iron loses all its original physical 
 and physico-chemical properties. It is changed 
 in hardness, tenacity, specific gravity, electric 
 conductivity, fusing point, magnetic character, 
 colour, and so forth. It is with transforma- 
 tions of this kind that Chemistry is especially 
 concerned. 
 
 Examples illustrative of chemical change 
 might, of course, be multiplied indefinitely ; 
 the case selected on account of its familiarity, 
 viz., the transformation of iron into other 
 substances, will, however, serve to put the 
 reader into the position of an inquirer wishing 
 to know what happens to iron under the 
 circumstances specified. Now, the first im- 
 pression produced when such cases are 
 
CHEMICAL CHANGE 25 
 
 thought over seriously is one of marvel at 
 the thoroughness of the change. The more 
 extensive one's experience the larger the 
 number and variety of the transformations 
 studied the more wonderful does the 
 phenomenon appear, even to those whose daily 
 occupation has familiarized them with such 
 manifestations of the properties of matter. 
 Then follows naturally the question whether 
 chemical change really is, as it may appear 
 to be at first sight, the actual transmutation 
 of one form of matter into another is iron 
 scale or iron rust simply transmuted iron ? 
 
 This last question may appear a very simple 
 one ; but it took more than a century's work 
 to answer it, and it was not until the answer 
 was given in general terms that Chemistry 
 began to emerge from the empirical and to 
 pass into the scientific stage. It is impossible 
 within the compass of the present work to 
 attempt the historical treatment of the 
 subject : suffice it to say that the answer to 
 the above question is in the negative. Iron 
 rust or iron scale is not simply transmuted 
 iron, but iron in combination with something 
 else. That something else is supplied by the 
 air or water ; and the proof of this is furnished 
 by the fact that the iron gains in weight when 
 it is changed into rust or scale. The nature 
 
26 CHEMISTRY 
 
 of the other component need not concern us 
 just now ; it is the general principle which is 
 of prime importance : that which at first sight 
 might appear to be a case of transmutation 
 turns out to be a case of transformation due 
 to the combination of the iron with another 
 form of matter. Two further questions thus 
 suggest themselves : Is transmutation in 
 the strict sense of the term known or con- 
 ceivable ? Is all chemical change the result 
 of the combination between different kinds 
 of matter ? In answer to the first question, 
 it may be said that the theoretical develop- 
 ment of modern Physics in connection with 
 the constitution of matter has made the 
 notion of transmutation conceivable, but the 
 decision from this point of view must be left 
 to the physicists. Whether cases of trans- 
 mutation are actually known is a chemical 
 question which is now under consideration. 
 
 The answer to the second question is of 
 more immediate importance for the adequate 
 conception of the nature of chemical change. 
 It has already been stated that the change 
 in the case of iron under the conditions 
 described is the result of combination of the 
 addition to the iron of some other substance. 
 This other component of iron rust and iron 
 scale is supplied by the air or water with which 
 
CHEMICAL CHANGE 27 
 
 the iron is brought into contact, as is proved 
 by the fact that iron may be heated or exposed 
 for any length of time in a closed vessel from 
 which air and water are absent without 
 being transformed into scale or rust. It may 
 at once be stated that this other component 
 is a form of matter known to chemists as 
 oxygen, a substance which under ordinary 
 conditions is a colourless gas, but which 
 condenses to a liquid at an extremely low 
 temperature. The transformation wrought 
 by chemical change appears in a more striking 
 light when it is considered that from solid 
 iron and gaseous oxygen there arises a black, 
 brittle scale, or a reddish, earthy rust, both 
 solid, differing from each other, and absolutely 
 unlike their generators. Moreover, underlying 
 the fact that two different forms of matter 
 can when combined give rise to two distinct 
 substances is another great chemical truth, the 
 bearing of which will be considered subse- 
 quently. The would-be inquirer will also want 
 to know in what form the oxygen exists in air 
 and water respectively, since it has been 
 described as a gas, and, although air is a gas, 
 water is a liquid until its temperature is raised 
 sufficiently to convert it into vapour or steam. 
 So that the suspicion arises that the oxygen 
 may not be in the same condition in air as it 
 
28 CHEMISTRY 
 
 is in water ; and another point is thus raised 
 for future consideration. And if the reader 
 is prompted by that spirit of active inquiry 
 which is the prime instigator of all scientific 
 progress, he will demand a more direct proof 
 of the statement that iron rust and iron scale 
 contain oxygen, because the mere gain in 
 weight proves nothing more than that the 
 iron has taken up something else. But the 
 proof of this assertion necessitates a broader 
 grasp of facts, and must be postponed until 
 the general outlook has been widened. In the 
 meantime, the main point whether all chemi- 
 cal change is the result of combination 
 between different kinds of matter must be 
 dealt with. 
 
 In order to prepare the way for further 
 developments, it may at once be stated that 
 the answer to this last question is in the 
 negative. The reverse process, i.e., the un- 
 doing of the combination between different 
 kinds of matter, may also lead to trans- 
 formations which are as distinctly chemical 
 as those resulting from combination. And 
 when we come to inquire more deeply into 
 the process of combination, even in the most 
 familiar and apparently straightforward cases, 
 such as the rusting of iron, it is found that 
 the conditions are in reality very complex 
 
CHEMICAL CHANGE 29 
 
 so complex that the most delicate methods 
 have recently had to be employed by expert 
 experimenters in order to determine the nature 
 of the transformation, and whether that which 
 at first sight appears to be a simple case 
 of direct combination between iron and 
 oxygen may not be a case of indirect com- 
 bination. In fact, the formation of new 
 substances by direct combination as known 
 to chemists is generally an artificial process, 
 i.e., the result of experimental conditions 
 imposed by the experimenter. Indeed, the 
 very forms of matter which have been referred 
 to for the purposes of illustration are, humanly 
 speaking, artificial products, since iron, 
 although found to a limited extent, as such, 
 in nature, is for all practical purposes obtained 
 by chemical processes from its naturally 
 occurring compounds ; and the fact that the 
 substance of iron scale is found in large 
 quantities as a natural mineral, magnetite, 
 suggests interesting lines of inquiry respecting 
 the past conditions of the earth, under which 
 the iron and oxygen were enabled to combine 
 so as to produce in some cases magnetite, 
 and in other cases the substance of iron rust, 
 which is also found as the mineral haematite, 
 or, in combination with water (hydrated), 
 as limonite. Was magnetite formed by the 
 
30 CHEMISTRY 
 
 direct combination of iron with oxygen, by 
 the action of water upon hot iron, or by some 
 other process ? Was haematite formed from 
 iron by some process analogous to that of 
 rusting ? These questions are not raised for 
 the purpose of answering them, because in the 
 present state of knowledge no decisive answer 
 can be given, but in order to illustrate still 
 further the all-embracing scope of chemistry. 
 From the simple observation that iron forms 
 certain compounds with oxygen, there arise 
 questions which bring Chemistry into the 
 domain of Geology. There is, in fact, a science 
 of Geo-chemistry yet awaiting development. 
 Although chemical change by direct com- 
 bination is now a rare phenomenon in nature, 
 it may have been and, no doubt, was the 
 predominant mode of material transformation 
 during that period of the earth's history when 
 our globe was cooling down from an igneous 
 condition. But high temperature chemistry 
 at the present time is, excepting under 
 volcanic conditions, an artificial chemistry, 
 i.e., brought about by human artifice ; and 
 the study of chemical change in all its phases 
 enables us to state that direct combination 
 is but one out of many possible modes of 
 bringing about such transformations of matter. 
 The further consideration of these various 
 
AN EXPERIMENTAL SCIENCE 31 
 
 modes may for the present be deferred ; 
 but it must at once be realized that there are 
 in operation in nature processes of chemical 
 change other than by direct combination, 
 which are everywhere going on, subtly, 
 silently, and generally imperceptibly light, 
 air, and water on the surface of the earth, 
 and heat and pressure below are slowly 
 effecting such transformations of matter ; 
 every living organism is an active centre of 
 chemical change. In other words, the con- 
 sideration of the nature of chemical change 
 must not be allowed to lead to the belief that 
 the process is purely artificial. There is a 
 natural chemistry both of non-living and of 
 living matter of which our knowledge is still 
 very imperfect, and towards the better 
 understanding of which our laboratory studies 
 are gradually leading us. 
 
 Chemistry an Experimental Science. A very 
 erroneous impression would be gained if it 
 were imagined that the data of chemical 
 science can be obtained by direct observation 
 as in the case of Astronomy, or of the biological 
 sciences, in which large bodies of ready-made 
 facts are offered for investigation. Natural 
 chemistry, as already indicated, is both 
 complicated and recondite. The mineral 
 components of our globe represent the products 
 
32 CHEMISTRY 
 
 of chemical changes which may have taken 
 ages for their completion ; the chemical 
 processes which go on in the living organism 
 are still shrouded in mystery. We can 
 produce artificially in our laboratories large 
 numbers of these products of Nature's labora- 
 tory, mineral, animal, and vegetable ; but, 
 great as are the achievements of modern 
 Chemistry in this direction, it must not be 
 concluded that we have thereby disclosed 
 Nature's methods. We cannot compete with 
 Nature in her scale of working either in time, 
 in mass of material, in temperature, or in 
 pressure. We do not, therefore, go to Nature 
 in the first place for ready-made facts in our 
 endeavour to penetrate the inner secrets of 
 those properties of matter upon which depend 
 its capabilities of undergoing chemical change ; 
 but we impose our own conditions in other 
 words, we cross-examine Nature by experi- 
 ment. By experiment, we mean trial 
 the observation of facts obtained under 
 conditions which are under control, and which 
 can, therefore, be varied in known ways. 
 The principle involved in this method is that 
 which is responsible for the development of 
 all those branches of science which are not 
 dependent upon the direct observation of 
 uncontrollable phenomena. By studying 
 
AN EXPERIMENTAL SCIENCE 33 
 
 simple cases under controllable conditions, 
 and observing the effects of changed con- 
 ditions, we endeavour to connect phenomena 
 such as chemical change with their antecedent 
 phenomena, i.e., to connect effect with cause ; 
 and so, from the simpler and known relation- 
 ships established by the experimental method, 
 we pass to the more complex and unknown 
 relationships which exist under natural con- 
 ditions. 
 
 In this way there has been acquired such 
 knowledge of the inherent properties of 
 matter as could never have been acquired by 
 the direct observation of ready-made facts. 
 Chemistry the most typical of the experi- 
 mental sciences has so far penetrated the 
 inner mysteries of matter as to have called 
 into existence an infinitude of new compounds, 
 i.e., of forms of matter absolutely unknown 
 in nature. In claiming the achievements of 
 synthetical chemistry as triumphs of modern 
 science, it must, of course, be understood 
 that no claim is made to our having mastered 
 Nature in the sense of conquering matter. 
 Our chemistry is not an unnatural chemistry ; 
 all that has been achieved has been made 
 possible only by our having learnt those 
 potentialities of matter which, in their ultimate 
 essence, may for ever elude our methods of 
 
34 CHEMISTRY 
 
 investigation. Every artificial substance pro- 
 duced in our laboratories or factories is but 
 the materialization of potentialities already 
 inherent, and which, so far as we know, may 
 have been present in matter from the begin- 
 ning of the existing order of things : " Yet 
 Nature is made better by no mean, but Nature 
 makes that mean : so, o'er that art, which, 
 you say, adds to Nature, is an art that Nature 
 makes." 
 
 It must be clearly understood, therefore, 
 that Chemistry as a science cannot be learnt, 
 in the strict sense of the word, by merely 
 reading about it. The would-be student may 
 be informed of the existing state of knowledge 
 through books of a descriptive or historical 
 character ; but he must realize that all the 
 information given all the general principles, 
 all the laws and generalizations that we are 
 enabled to enunciate, have been based on facts 
 gleaned by laboratory work. That is why the 
 foundations of modern Chemistry are said to 
 be well and truly laid. The apparently simple 
 observation from which we set out the 
 rusting of iron is in reality an experiment, 
 because the transformation is under control. 
 For instance, we can modify the state of 
 aggregation of the iron by reducing it to fine 
 particles by a file, or, if brittle, we can grind 
 
AN EXPERIMENTAL SCIENCE 35 
 
 it to a fine powder (iron swarf) ; and if some of 
 this finely divided iron is exposed for some 
 days in a confined volume of air, such as is 
 contained in a bottle inverted over water, we 
 can find out that the air as a whole is not 
 absorbed, but only a certain proportion of it, 
 about one-fifth of its volume. So that the 
 oxygen which combines with the iron is picked 
 out by the latter by a selective process, and 
 anything that is not oxygen is left uncom- 
 bined, and the rusting ceases as soon as all 
 the oxygen is used up. An experiment of 
 this kind teaches us, therefore, that chemical 
 change is preferential, inasmuch as the iron 
 combines with only one form of matter present 
 in the air, and, under the conditions of the 
 experiment, refuses to combine with any other 
 substance contained in the bottle. From this 
 it further follows that air contains some- 
 thing besides oxygen ; and the question as 
 to the condition in which oxygen is present 
 in the air, which was previously raised (p. 28) 
 is thus answered to the extent that it is 
 permissible to state that it is in a form easily 
 removed by iron at ordinary temperatures. 
 The bearing of this will be seen subsequently. 
 It must be noted, also, that it has been 
 throughout assumed as a condition of rusting 
 that the air must be moist, i.e., that it con- 
 
36 CHEMISTRY 
 
 tains the vapour of water. That statement is 
 based upon experiment, because if we remove 
 all the water vapour from the air by absorbent 
 substances, of which many are known to 
 chemists, then no rusting takes place, although 
 there may be oxygen present. So that 
 oxygen and water vapour are both essential 
 for the production of the chemical change in 
 this case. Moreover, there is also present 
 in air a small quantity of another gaseous 
 substance, which is known as carbon dioxide 
 the gas which is familiar to all as giving 
 rise to the evolution of bubbles, i.e., the effer- 
 vescence when a bottle of mineral water 
 or " sparkling " wine is opened. This gas 
 is present only to the extent of three or four 
 volumes in ten thousand volumes of air, so 
 that refined and delicate methods of measur- 
 ing its quantity have to be adopted. It is 
 believed by some experimenters that the 
 co-operation of this gas with oxygen and water 
 vapour is also essential for the rusting of iron, 
 while others have come to the conclusion that 
 the iron itself can exist in either a sensitive 
 or insensitive state with respect to its power 
 of combining with oxygen, and that carbon 
 dioxide is not essential. All these facts are 
 mentioned in further illustration of the 
 importance of the experimental method, and 
 
AN EXPERIMENTAL SCIENCE 37 
 
 to enforce the lesson already inculcated, that 
 the apparent simplicity of a familiar chemical 
 change disappears when Nature is rigidly cross- 
 examined under controllable conditions with 
 the object of finding out how and why such 
 transformation takes place in any particular 
 case. 
 
 It must not be imagined that the resources 
 of experiment are exhausted when, as in the 
 foregoing illustration, the chemical trans- 
 formation of iron has been traced to its 
 faculty of combining with oxygen. In addi- 
 tion to the facts, a principle has been revealed 
 the principle that iron, regarded as a form 
 of matter, has its likes and dislikes, since it 
 selects the oxygen from the air, and leaves 
 four-fifths of some uncombined gas behind. 
 That gas, it may at once be parenthetically 
 noted for future reference, consists mainly of 
 another form of matter known as nitrogen. 
 As a deduction from the facts already 
 observed, the question next arises whether 
 iron shows preference for other substances. 
 Thus we might proceed to test this question 
 experimentally by mixing iron with other 
 familiar substances, such as sand, or sugar, or 
 charcoal, or chalk, or sulphur. To make the 
 question more searching we should naturally 
 in such a case give the iron every chance, by 
 
38 CHEMISTRY 
 
 using the finely-divided substance in the form 
 of filings or dust, and we should grind this up 
 with the other materials. Under these cir- 
 cumstances, we should find that nothing more 
 than mixtures would be obtained the pro- 
 ducts would not come within the conception 
 of chemical change, because the iron is still 
 present as such, and could be separated from 
 the sand or sugar, etc., by means of a magnet ; 
 or the sugar could be dissolved out by water, 
 leaving the iron behind ; and so with the 
 other mixtures, separation could be effected 
 by appropriate methods. 
 
 But if iron is ground up with that metallic 
 looking solid known as iodine a substance 
 obtained from the ashes of sea-weeds, and 
 familiar in pharmacy we find that, if a suffi- 
 cient quantity of iodine is used, a grey product 
 is obtained from which iron cannot be readily 
 separated ; the iron and the iodine have lost 
 their individuality through chemical change. 
 Thus, under the conditions in which iron 
 refuses to combine with sugar or charcoal 
 or the other substances, it combines with 
 iodine, and the principle of preferential action 
 is thus extended. This is what in old time 
 chemistry would be expressed by saying that 
 the iron had an " affinity " for oxygen and 
 iodine, but not for nitrogen, or sand, or chalk, 
 
AN EXPERIMENTAL SCIENCE 39 
 
 or any of the other substances. And now, 
 prosecuting the inquiry still further, we 
 should find that the application of heat may 
 bring about combination in the case of the 
 sulphur-iron mixture, but in no other of the 
 mixtures referred to, although we should 
 discover incidentally that the sugar was 
 totally transformed on heating, with the 
 production of charcoal, that the sand and 
 charcoal were unchanged, and that the chalk, 
 although apparently unaltered in appearance, 
 had, if strongly heated, become transformed 
 into another substance, lime. When heated, 
 the mixture of iron and sulphur suddenly 
 begins to glow developing heat by the 
 combination of the materials and the pro- 
 duct, when cold, consists of a dark coloured, 
 brittle solid from which, provided sufficient 
 sulphur has been added, iron cannot be readily 
 separated. Here again, chemical change has 
 taken place. 
 
 It will be obvious from these illustrations 
 that Chemistry is an art as well as a science ; 
 the carrying out even of simple experiments 
 necessitates manual dexterity, skill, judgment, 
 and a knowledge of the various contrivances 
 or forms of apparatus available for particular 
 purposes. And so we are brought back to the 
 fundamental proposition that our scientific 
 
40 CHEMISTRY 
 
 knowledge has been built on a foundation 
 furnished by the art of the experimentalist. 
 We can learn from books what are the general 
 scientific conclusions at any particular period ; 
 but no amount of reading will make a learner 
 into a proficient chemist. 
 
CHAPTER II 
 
 CHEMICAL COMBINATION AND MECHANICAL 
 
 MIXTURE AIR A MIXTURE AND NOT A 
 
 COMPOUND PHYSICAL SEPARATION OF 
 THE COMPONENTS OF AIR 
 
 Chemical Combination and Mechanical Mix- 
 ture. The consideration of those profound 
 modifications in matter which result from 
 chemical change will have made it evident 
 that a clear distinction must be made between 
 the products of chemical combination and 
 mechanical mixtures. This point is very 
 generally misunderstood by the uninstructed, 
 and is often a stumbling-block to the student 
 on his first introduction to chemical science. 
 The difficulty arises from the circumstance 
 that the product arising from a combination 
 of two different substances does not, as 
 common sense might lead us to suppose, 
 partake of the characters of both components, 
 but is a totally distinct form of matter. The 
 common sense notion applies to mechanical 
 mixtures, for these do partake of the characters 
 
 41 
 
42 CHEMISTRY 
 
 of their components their properties are 
 intermediate between those of the substances 
 mixed, and can be made to vary indefinitely 
 by varying the proportions of the ingredients. 
 But these ingredients, whatever they may be, 
 are always present as such in the mixture. 
 However finely we may grind up a mixture of, 
 let us say, iron dust and chalk, we can always 
 pick out the iron by a magnet, because it is 
 a property of iron to be attracted by a magnet, 
 while chalk is not thus attracted. Moreover, 
 a mechanical mixture is never homogeneous : 
 such mixtures may be made apparently 
 homogeneous by grinding to a very fine 
 powder, but, if we examine some of this powder 
 under a microscope, the different particles 
 of the components can easily be distinguished. 
 The product of chemical combination, on the 
 other hand, is always homogeneous its 
 particles, however finely divided by mechani- 
 cal means, are all alike in appearance and 
 properties. The mixture of iron dust and 
 sulphur referred to in the last chapter is 
 visibly a mixture when sufficiently magnified ; 
 but if, after heating, the product of com- 
 bination is ground up to a powder, no 
 magnification will reveal either iron or sulphur 
 provided the materials were present in the right 
 proportions. 
 
COMBINATION AND MIXTURE 43 
 
 This last condition introduces a new set of 
 considerations the quantitative relations be- 
 tween the combining materials. Herein we 
 shall find a characteristic of chemical com- 
 bination of such fundamental importance 
 that this part of the subject must be fully 
 dealt with at a later stage, when the general 
 nature of chemical change has been more 
 thoroughly grasped. In the meantime, it 
 will be instructive to give one or two illus- 
 trations of the popular misconception of 
 chemical transformation. We often see it 
 stated, for example, that metals are extracted 
 from their ores, or that dyes, perfumes, ex- 
 plosives, etc., are extracted from coal-tar. 
 To the uninitiated, these statements might 
 convey the impression that the metal, say 
 iron, is present as such in the ore, or that the 
 dyes, etc., are contained in the tar, and that 
 the process of extraction is nothing more 
 than a sort of mechanical separation of the 
 metal or the dyes from the other materials 
 with which they are mixed. Nothing could 
 be further from the truth than this conception 
 of the process. There is no free iron in iron 
 ore ; there are no ready formed dyes or 
 perfumes in tar. The liberation of iron from 
 the oxygen with which it is chemically com- 
 bined in, let us say, haematite is the result of 
 
44 CHEMISTRY 
 
 chemical transformation which the ore is 
 made to undergo by heating it with some 
 other substance, such as the carbonaceous 
 fuel referred to in the last chapter, which 
 enters into chemical combination with the 
 oxygen. That is why iron was previously 
 declared to be a product of artificial chemistry 
 in this case we call it a metallurgical 
 process, because iron belongs to a class of 
 substances known as metals. So, also, are 
 the dye-stuffs, etc., artificial products ; they 
 are obtained from certain substances con- 
 tained in and separated from the tar, but the 
 generating substances the raw materials 
 are not themselves dye-stuffs. The latter 
 arise from their non-tinctorial generators by a 
 series of operations which involve chemical 
 transformation at every stage. This last 
 case is among the most striking illustrations 
 of the marvellous transformations brought 
 about by chemical change. Even when a 
 naturally occurring ore does contain a metallic 
 substance which, like gold, has to be separated 
 from the other materials with which it is 
 mixed, the extraction is very seldom a purely 
 mechanical process : chemical extraction is 
 generally resorted to. 
 
 Although the fundamental distinction be- 
 tween chemical combination and mechanical 
 
COMBINATION AND MIXTURE 45 
 
 mixture has, no doubt, been made clear by 
 the illustrations given, it is not always easy 
 to decide off-hand whether any given substance 
 is a mixture or a true chemical compound. 
 Let it, in the first place, be realized that since 
 Chemistry has for its scope the study of the 
 transformations of matter in all its forms, 
 the particular state of physical aggregation of 
 the matter concerned in any case of chemical 
 change is of minor importance from the 
 chemical point of view. From every day 
 experience the reader has been made familiar 
 with the existence of matter in the three 
 forms known respectively as solid, liquid, 
 and vapour, or gas. The interconvertibility 
 of these forms is generally dealt with from the 
 physical side in works on Physics, although, 
 as has already been pointed out, the specific 
 properties of different forms of matter with 
 respect to the conditions which determine 
 their state of aggregation are also of chemical 
 significance. Illustrations of chemical change 
 resulting from combination between different 
 kinds of matter in different states of aggrega- 
 tion have already been given. Thus, solid iron 
 combines with gaseous oxygen and with solid 
 iodine giving rise to chemical compounds. 
 The selection of the oxygen from the air by 
 iron wucn it rusts has been referred to as a 
 
46 CHEMISTRY 
 
 fact indicating that the oxygen is present in 
 the air in an easily removable form. But 
 this fact does not in itself decide the question 
 whether the oxygen in the air is chemically 
 combined with the nitrogen or the other 
 substances which are known to be present in 
 air. It does not answer the question Is air 
 a chemical compound, or a mechanical mix- 
 ture ? We know of many true chemical 
 compounds which are gaseous at ordinary 
 temperatures, and we know also that iron 
 and other substances can decompose some of 
 these compounds, i.e., can undo the chemical 
 combination by taking out one of the com- 
 ponents and leaving the other. This will be 
 made clearer as we proceed ; but the bare 
 statement of these facts will suffice at present 
 to enforce the lesson that the scientific inter- 
 pretation of experimental evidence requires 
 both caution and judgment. 
 
 Air a Mixture, and not a Compound. It is, 
 of course, well known that air is not a chemical 
 compound, but a mixture of oxygen and 
 nitrogen with small quantities of other gaseous 
 substances, such as carbon dioxide, water 
 vapour, and traces of a few other gases, which 
 will be spoken of in a later chapter. If, 
 therefore, the observation that rusting iron 
 withdraws the oxygen is not decisive, how, 
 
THE COMPONENTS OF AIR 47 
 
 then, it may be asked, has this conclusion 
 been arrived at ? So far as mere homo- 
 geneity is concerned, the air answers to the 
 definition of a chemical compound : it has 
 the same properties from whatever part of 
 the world it comes. The rusting iron experi- 
 ment gives within quite narrow limits the 
 same quantitative result with air from any 
 quarter of the globe, if the volume of oxygen 
 withdrawn is measured. The conclusion that 
 air is a mixture is based upon convergent lines 
 of evidence, the consideration of every one 
 of which will introduce us to new general 
 principles. Two or three of these lines may 
 be advantageously followed up now. 
 
 In the first place, let us ask whether there 
 is any criterion of chemical combination 
 beyond that general change in character of the 
 transformed substance which has so far been 
 considered. To put the case in another way 
 is there any method of ascertaining, apart 
 from the appearance and properties of the re- 
 sulting product, whether two substances when 
 mixed together enter into chemical combina- 
 tion, or remain simply in admixture ? There 
 is one such indication of chemical combination 
 which, when observed, is a very sure sign 
 that something more than mechanical mixture 
 has taken place, and that is the development 
 
48 CHEMISTRY 
 
 of heat. This is a point of fundamental 
 significance, and will require further elabora- 
 tion. It is not always possible to detect the 
 heat of chemical combination, because in 
 many cases the transformation takes place 
 very slowly, and the heat is dissipated too 
 rapidly to enable us to measure it. The com- 
 bination of iron with oxygen is a case in point. 
 There is no doubt that heat is developed when 
 iron rusts, but that is a case of slow chemical 
 combination. Now, although the state of 
 physical aggregation of the matter entering 
 into combination is, as already stated, of 
 minor importance so far as concerns the 
 nature of the final product, yet it is obvious 
 that the state of physical aggregation may 
 influence the rate of combination. In the 
 case of a solid combining with another solid, 
 or with a liquid or a gas, it is evident that the 
 more intimately the combining materials are 
 brought into contact the more rapid will be 
 the chemical combination. That is why, in 
 such experiments with iron as have been 
 described, it was thought desirable to use the 
 metal in a finely divided form. It is simply 
 a case of presenting as large a surface as 
 possible to the other substance. But, how- 
 ever finely we may divide iron by mechanical 
 processes, it is not possible to increase the rate 
 
THE COMPONENTS OF AIR 49 
 
 of its combination with oxygen to a sufficient 
 extent to enable us to detect the rise of 
 temperature on rusting. All that can be said 
 is that iron sheet would rust more rapidly 
 than a solid block, and that iron filings or 
 iron dust would rust more rapidly than either 
 block or sheet iron. It is possible although 
 not easy practically to prove this experi- 
 mentally by measuring the rate of absorp- 
 tion of oxygen from air by iron in different 
 states of division. It is possible, also, by a 
 chemical process to obtain iron in such an ex- 
 tremely fine state of division that it possesses 
 what are known as " pyrophoric " properties, 
 becoming red hot on being shaken into the 
 air from the tube which contains it. In this 
 case, the combination between the micros- 
 copic particles of iron and the atmospheric 
 oxygen takes place so rapidly that the evolu- 
 tion of heat produces a visible effect. 
 
 The development of heat as a criterion of 
 chemical combination is thus likely to be 
 most observed in cases where the combining 
 materials are brought into the most intimate 
 contact. No state of physical aggregation 
 can insure more intimate contact than the 
 gaseous state, for this is the most mobile 
 condition of matter, and gases mix freely 
 with each other in all proportions. Now, the 
 
50 CHEMISTRY 
 
 main constituents of the air, oxygen and 
 nitrogen, are gases ; they can be obtained in 
 various ways by chemical processes, and they 
 can be mixed together in the same proportions 
 as those in which they exist in the air, viz., 
 four volumes of nitrogen and one of oxygen. 
 The resulting product is a gas having all the 
 properties of air ; no change of temperature 
 takes place, and the characters of the mixture 
 are intermediate between those of its compon- 
 ents. That is one reason why air is regarded 
 as a mixture, and not as a compound. 
 
 The homogeneity of product in this case, 
 therefore, is in a sense accidental it is the 
 result of the extreme mobility of the gaseous 
 form of matter. It may be asked, now, if the 
 homogeneity of air which is supposed to 
 be characteristic of a true chemical com- 
 pound is thus a violation of that criterion 
 of chemical union which has been previously 
 set up. Is the homogeneity real, or is it 
 only apparent ? This question may, in the 
 first place, be answered hypothetically. 
 Supposing, by some magnification of the 
 power of vision, it were possible to see the 
 actual particles of which a gas is made up, 
 different gaseous forms of matter might be 
 expected to present different appearances. 
 The particles might be quite dissimilar to an 
 
THE COMPONENTS OF AIR 51 
 
 imaginary being with such an exalted sense 
 of vision they might differ in size or shape or 
 weight, or they might be moving about with 
 different velocities. Thus, our hypothetical 
 being might be supposed to know an oxygen 
 particle from a nitrogen particle ; and if a 
 sample of air were submitted to him for 
 examination, what would he find ? He would 
 see the oxygen particles and the nitrogen 
 particles mixed up, moving about among 
 each other, and bombarding the sides of the 
 containing vessel, colliding and rebounding 
 all in a higgledy-piggledy way ; but the two 
 kinds of particles would throughout their 
 evolutions remain distinct, each after its 
 kind ; there would be no fusion together or 
 combination, and each kind, even if tem- 
 porarily deformed by collisions, would preserve 
 its weight as well as its average rate of 
 movement. 
 
 This ideal picture of the inner state of affairs 
 in gaseous matter is a physical conception, and 
 is in harmony with all those general properties 
 of gases which the student learns from the 
 science of Physics. If the picture be a true 
 representation of the facts, it follows that the 
 homogeneity of air is apparent, and not real ; 
 its particles, could we see them, would not 
 be all alike as they would be if it were a 
 
52 CHEMISTRY 
 
 true compound. The only approach towards 
 homogeneity that could be realized by such 
 a mixture would be the possession by equal 
 volumes taken at random of absolutely the 
 same number of particles of the two gases. 
 It is for the sake of simplifying the argument 
 that attention has been concentrated upon 
 these two components, because the oxygen and 
 nitrogen together make up the main bulk of 
 the air. But what has been said with respect 
 to these two gases is true for the other minor 
 components, such as water vapour and carbon 
 dioxide, and all the other gases which exist 
 in mere traces. Our imaginary being with 
 supernatural power of vision would be likewise 
 capable of distinguishing between and follow- 
 ing the migrations of the fewer particles of 
 water vapour and carbon dioxide in the course 
 of their wanderings among the greater crowd 
 of oxygen and the still greater crowd of 
 nitrogen particles. With these other com- 
 ponents, there is no combination in the 
 chemical sense. 
 
 So much for the hypothetical answer to the 
 question whether air is really homogeneous. 
 Of course, the ideal supernatural vision is 
 unattainable by any human contrivance, so 
 that direct proof of heterogeneity is not to 
 be looked for by any such means. But if 
 
THE COMPONENTS OF AIR 53 
 
 heterogeneity can be proved experimentally, 
 then it will be admitted that there is justifica- 
 tion for the hypothetical answer. There are 
 many such proofs that which is unrealizable 
 visually, viz., the discrimination between the 
 different kinds of particles, can be realized 
 by other means ; and it is of the utmost 
 importance to note that the means about to 
 be indicated are non-chemical. The import- 
 ance of this reservation will become apparent 
 when it is restated that any attempt to 
 separate the components by the action of some 
 other form of matter which exerts a selective 
 action as in the case of rusting iron is 
 always open to the suspicion that there may 
 have been chemical decomposition. As al- 
 ready pointed out, the removal of the oxygen 
 or nitrogen or of any other constituent gas by 
 chemically combining it with some other 
 substance leaves the question of the original 
 condition of the oxygen, etc., in the air an 
 open one. 
 
 A simple observation will serve to show that 
 one of the components, viz., the water 
 vapour, is not chemically combined. It is, no 
 doubt, a familiar fact with those who wear 
 spectacles or eye-glasses that, on coming 
 from the cold outer air suddenly into a warm 
 room, the glasses become dimmed by a deposit 
 
54 CHEMISTRY 
 
 of moisture a deposition of dew upon the 
 glass. The same thing is observed if we take 
 a glass of iced water into a warm room ; the 
 outside of the glass becomes covered with 
 moisture. That means that our supernatural 
 being was right when he observed particles of 
 water vapour moving about among the oxygen 
 and nitrogen particles. The interpretation of 
 the observation is that the water vapour is 
 only retained in the air because the air is 
 warm ; warm air contains more water than 
 cold air, so that, when warm air is cooled by 
 contact with cold glass, it deposits some of its 
 water vapour on the glass in the form of 
 droplets of liquid water or dew. The separa- 
 tion of water as such from the air by the mere 
 lowering of temperature indicates that the 
 water was there, although in an invisible 
 or vaporous state : had the water been 
 chemically combined with any other con- 
 stituent of the air it could not possibly have 
 been liberated by the simple process of cooling. 
 The proof that the nitrogen and oxygen 
 in air are not chemically combined involves 
 the application of methods which are probably 
 unfamiliar to the general reader. Their very 
 unfamiliarity makes them instructive, because 
 their consideration will bring us into contact 
 with other fundamental properties of matter. 
 
THE COMPONENTS OF AIR 55 
 
 Let it be borne in mind that, in this particular 
 case, we are dealing with matter in the gaseous 
 form. The particles of matter in this state 
 possess, as we have already explained, perfect 
 freedom of movement ; and our imaginary 
 being has been supposed to see the different 
 kinds of particles moving about with different 
 velocities. Now, the average speed of these 
 particles is a property which is dependent 
 upon the nature of the gaseous substance ; 
 it is, in fact, a physico-chemical property, 
 and is connected with the relative weights 
 of the particles. We shall have to consider 
 later how these weights are ascertained ; but 
 it is easy even at this stage to form a mental 
 picture of light and heavy particles all mixed 
 up together, the lighter particles moving more 
 rapidly than the heavier particles. If, there- 
 fore, the oxygen and nitrogen particles are 
 independent entities, and not chemically 
 combined, and if the two kinds of particles 
 have different weights, and are within a given 
 volume of air moving with different velocities, 
 then a process of mechanical sorting seems 
 conceivable. 
 
 The foregoing conception can be verified 
 experimentally, because it happens that the 
 nitrogen particles are a little lighter than the 
 oxygen particles in fact, a nitrogen particle 
 
56 CHEMISTRY 
 
 has seven-eighths the weight of a comparable 
 oxygen particle. If we could pass air through 
 a sieve with very small meshes small enough 
 to bear comparison with the actual size of the 
 particles, and not large enough to allow the 
 whole mixture of particles to pass en masse 
 through the interstices then more of the light 
 than of the heavy particles would get through 
 in a given time, because the lighter particles 
 are moving the more rapidly. The air which 
 passed through such a sieve ought, conse- 
 quently, to be richer in nitrogen, and the air 
 which was left behind ought to be richer in 
 oxygen. Now, the fine-meshed sieve which 
 enables this conception to be verified may be 
 any substance with extremely minute pores, 
 such as unglazed porcelain, or plaster of 
 Paris. Of course, the reader is familiar with 
 the physical fact that air cannot be confined 
 in a porous vessel if it is under pressure, or if 
 there is no air outside the vessel to balance 
 the pressure of the air within. If, therefore, 
 air be drawn through a glass tube containing 
 a plug or diaphragm of some fine-pored 
 material, the proportions of the gases will be 
 altered we should draw out of the tube an 
 air containing more nitrogen than the normal 
 proportion, and there would be left behind an 
 air containing more than one-fifth of its volume 
 
THE COMPONENTS OF AIR 57 
 
 of oxygen. Such a purely mechanical separa- 
 tion as this is another proof that the nitrogen 
 and oxygen in air are not chemically combined. 
 
 It will be noted that, throughout this 
 discussion of the question whether air is a 
 mixture or a compound, it has been assumed 
 that the gaseous substances, oxygen, nitrogen, 
 etc., consist of " particles." This conception 
 of the constitution of gases has been referred 
 to as a physical conception, and the further 
 development of the idea in its physical 
 aspects must be pursued in works on Physics. 
 We are here brought face to face with one of 
 the most striking examples of the inter- 
 dependence of two branches of science. So 
 far as the conception has been made use of 
 in this chapter, the main object has been to 
 introduce the reader to the current view that 
 the substances, nitrogen, oxygen, etc., although 
 gaseous under ordinary conditions, are not 
 to be looked upon as continuous, but as 
 discontinuous in structure. The supposed 
 magnification up to the stage of visibility has 
 been imagined to reveal a discrete or granular 
 constitution the gases have been supposed 
 to consist of extremely minute particles. 
 This is the modern view of the constitution of 
 matter in all states of aggregation. 
 
 It is, historically speaking, a very ancient 
 
58 CHEMISTRY 
 
 conception, but it has been put upon a scientific 
 basis in modern times by the joint labours of 
 chemists and physicists. All forms of matter, 
 solid, liquid and gaseous, if we could but see 
 into their inner constitution, would be found 
 to consist of particles, these having perfect 
 freedom of movement in gases, more res- 
 trained powers of movement in liquids, and 
 comparatively little freedom of movement in 
 solids. It will be seen subsequently that 
 these minute components of matter which 
 have hitherto been defined by the intention- 
 ally vague term " particle " must, from the 
 chemical point of view, be defined with much 
 greater precision. The broad physical con- 
 ception of matter as composed of discrete 
 particles has been translated into more 
 concrete terms by modern Chemistry with 
 such marvellous success that we may be said 
 to have made some progress towards the 
 realization of those supernatural powers of 
 vision which have been ascribed to our 
 imaginary being. 
 
 Physical Separation of the Components of 
 Air. The transition from the solid to the 
 liquid, or from the liquid to the gaseous form 
 of matter, or the reverse series of changes, are 
 physical phenomena made most familiar to 
 us in the case of water, which we all know in 
 
THE COMPONENTS OF AIR 59 
 
 the forms of ice, water, and steam, or water 
 vapour. The precise temperatures at which 
 these changes of state take place are physico- 
 chemical characters of the various substances, 
 and are dependent upon the heat imparted to 
 or withdrawn from the substance, the heat 
 so imparted or withdrawn being generally 
 measured on the scale of a thermometer. 
 Thus, the point at which a change of state 
 takes place is said to be the melting-point or 
 point of fusion of the substance if it passes 
 from the solid to the liquid state, or the freez- 
 ing-point or point of solidification if the change 
 is from the liquid to the solid state. The 
 temperature at which a liquid overcomes the 
 atmospheric pressure and passes suddenly 
 into the state of vapour is known as the 
 boiling-point. The measurement of tempera- 
 ture, the determination of melting-points 
 and boiling-points, the various thermometers 
 and their scales, the effects of pressure, etc., 
 are all dealt with in books on Physics and on 
 practical Chemistry. It will suffice for the 
 present to state that in Chemistry the Centi- 
 grade scale is always used, and on this scale 
 the zero-point (0) is the freezing-point of 
 water, and the boiling-point of water under 
 the average atmospheric pressure of 76 centi- 
 metres is marked 100. 
 
60 CHEMISTRY 
 
 Thus the description of the various states 
 of physical aggregation of matter is made 
 more definite when we are enabled to asso- 
 ciate with each substance its specific physico- 
 chemical characters of melting-point or boiling- 
 point. We say, for example, that above 100 
 water exists as vapour, and at as ice. And 
 what is true for water is true for other liquids 
 and for other gases ; for it is now known that 
 those forms of matter which, like nitrogen or 
 oxygen, are gaseous at ordinary temperatures 
 are really the vapours of liquids whose boiling- 
 points are so low that they are never reached 
 under any natural terrestrial conditions. In 
 other words, we should have to cool air down 
 to a temperature of more than 190 below the 
 freezing-point of water in order that its 
 physical state might be changed and the 
 gaseous mixture condensed to a liquid. Now, 
 the liquefaction of air and of other gaseous 
 forms of matter has been effected on the large 
 scale in recent times a feat that must be 
 regarded as among the great achievements 
 of modern science. The principle made use 
 of is essentially physical, and cannot be 
 considered in detail here. All that need be 
 said is that it is a self-cooling process, for, 
 when a highly compressed gas is allowed to 
 escape suddenly from its containing vessel, 
 
THE COMPONENTS OF AIR 61 
 
 it cools itself by expansion, and the cooled air 
 can then be made to cool another lot of 
 escaping gas, and so on by a summing-up of 
 coldness in a continuously cooling cycle until 
 the point of liquefaction is reached. Air is 
 thus obtained as a limpid liquid not unlike 
 water in appearance. 
 
 Starting, then, with air reduced to a liquid 
 in some suitable apparatus by the method 
 described, it is obvious that, when such a 
 liquid is allowed to rise in temperature by 
 exposure to the atmospheric temperature, 
 which in this country averages about 200 
 above the boiling-point of liquid air, the latter 
 will begin to boil, and so resume again the 
 gaseous state. But if the oxygen and nitrogen 
 of the air are not chemically combined, then 
 liquid air must consist, not of a definite 
 compound, but of a mixture of liquids ; and 
 if the various constituents have different 
 boiling-points, then, as the liquid air passes 
 into gas, it might be expected that the 
 constituent which passed most readily into 
 gas, i.e., which had the lowest boiling-point, 
 would boil off more rapidly than the con- 
 stituents which had the higher boiling-point. 
 
 To make this point quite clear, the reverse 
 process may be considered, viz., the cooling 
 down of a mixture of gases to the point of 
 
62 CHEMISTRY 
 
 liquefaction. In this case, the most easily 
 condensible gas would be that which had the 
 highest boiling-point, so that the liquid 
 obtained from such a mixture would be richer 
 in that component. Now, the boiling-points 
 of the main constituents of the air, oxygen 
 and nitrogen, are about 183 and 196 below 
 the freezing-point of water respectively ; in 
 other words, nitrogen boils about 13 lower 
 than oxygen. Consequently, on the assump- 
 tion that air is a mixture and not a compound, 
 the mere physical act of liquefaction might 
 be expected to upset the composition, since 
 the oxygen with the higher boiling-point 
 would condense more readily than the 
 nitrogen with the lower boiling-point. This 
 is actually found to be the case the gaseous 
 air recovered from liquid air contains nearly 
 double the quantity of oxygen contained 
 in normal air. Moreover, if liquid air is 
 allowed to boil by exposure to the ordinary 
 temperature, the nitrogen boils off more 
 rapidly than the oxygen, and there is 
 finally left a liquid residue which, on being 
 allowed to gasify, is found to be still richer 
 in oxygen, so that a continuous process of 
 separation is effected without calling in the 
 agency of any other form of matter capable of 
 removing the oxygen or the nitrogen by 
 
THE COMPONENTS OF AIR 63 
 
 chemical combination. It is a case of 
 separation by purely physical means a 
 separation which would have been impos- 
 sible if the oxygen and nitrogen had 
 been chemically combined. Thus there is 
 furnished another proof that the air is a 
 mixture, and not a chemical compound, 
 
CHAPTER III 
 
 CHEMICAL CHANGE IN ITS QUANTITATIVE 
 ASPECT THE DEFINITENESS OF CHEMICAL 
 CHANGE THE CONSERVATION OF MASS- 
 WATER A CHEMICAL COMPOUND 
 
 Chemical Change in its Quantitative Aspect. 
 Air has been selected as a type of a mechani- 
 cal mixture because of its familiarity as the 
 medium in which we live. All the chemical 
 processes which go on in the world of life, 
 animal and vegetable, and which are mani- 
 festations of " vitality," are dependent upon 
 one or another of the constituents of that 
 gaseous envelope which enwraps our globe. 
 Thus, animals require oxygen and plants 
 carbon dioxide, and both forms of life require 
 nitrogen, which is supplied directly or in- 
 directly from the atmosphere. We speak 
 now of the gaseous components of the air as 
 forms of matter with the same confidence that 
 we refer to a solid, such as a lump of iron, 
 or a liquid such as water, as forms of matter. 
 The only consideration that affects the 
 
 64 
 
CHEMICAL CHANGE 65 
 
 chemist in this case is the purely practical 
 one of difference in methods of manipulation 
 which, when dealing with gaseous matter, 
 at first presented difficulties not met with 
 when dealing with solids or liquids ; and the 
 early history of our science will be found to 
 contain most instructive records of the groping 
 after the truth not only that gases were to be 
 considered material substances, but that 
 there were different kinds of gases or " airs " 
 as distinct from one another as iron from 
 chalk, or as chalk from sulphur. The difficulty 
 arose, of course, from the circumstance that 
 gases such as oxygen, nitrogen, and carbon 
 dioxide, being all colourless, transparent, 
 and invisible substances, do not directly 
 reveal to our senses their individual characters 
 or specific properties. The modern student 
 learns at a very early period to distinguish 
 between such gases, a jar of oxygen, for 
 example, causing a splinter of glowing wood 
 thrust into the gas to burst at once into flame 
 owing to its energetic character as a promoter 
 of combustion, while nitrogen and carbon 
 dioxide extinguish flame. The latter gas 
 also has the property of producing turbidity 
 in a clear solution of lime in water, which 
 property is not possessed by either oxygen 
 or nitrogen. And, as his experience increases, 
 
66 CHEMISTRY 
 
 the student will become acquainted with 
 various other gases to which diagnostic tests 
 can be applied, some, which burn in the air, 
 being said to be combustible, and others 
 possessing characteristic colours and odours, 
 and so forth. 
 
 Admitting, therefore, once and finally, that 
 the physical state of aggregation of matter 
 is of secondary importance to its study from 
 the chemical point of view, we may now pass 
 on to the consideration of some of the other 
 characteristics of chemical change. In many 
 of the cases already referred to, such as the 
 combination of iron with sulphur or with 
 iodine which, it will be remembered, requires 
 the application of heat in the case of sulphur, 
 and which takes place spontaneously in the 
 case of iodine the condition laid down for 
 the production of a new and homogeneous 
 product was that the materials should be 
 brought together in certain proportions (p. 42). 
 A quantitative notion of chemical change is 
 thus introduced ; and with this notion the 
 student of Chemistry must become thoroughly 
 imbued. When chemical change occurs, either 
 as the result of combination, or of decom- 
 position, or by any other of the processes 
 known to chemists, it is found that the actual 
 quantities of matter concerned in the change 
 
CHEMICAL CHANGE 67 
 
 bear a definite relationship towards each 
 other. Herein will be found one of the prime 
 characteristics of chemical change as dis- 
 tinguished from mechanical mixture. It is 
 obvious that, in a mixture, there is no such 
 restriction with respect to quantities. Gases, 
 liquids that are miscible, and solid powders 
 may be mixed together in any proportions, 
 and the product partakes of the character 
 of its components. The chemist is concerned, 
 therefore, not only with the quality, i.e., 
 the specific properties of the substances, but 
 also with the relative quantities of the 
 materials which undergo transformation. 
 
 In physical terms, quantity of matter means 
 mass ; and for practical purposes the most 
 convenient measure of mass is the gravi- 
 tational measure weight. In dealing with 
 gases, and, under certain circumstances, 
 also with liquids, the bulk or volume is 
 a convenient measure ; but in such cases 
 the volume measure is generally for ultimate 
 purposes expressed in terms of weight. The 
 reader who studies the history of Chemistry 
 will learn that our science only began to take 
 rank as an exact science from the time when 
 chemical change was studied quantitatively 
 by means of that delicate weighing machine 
 known as the chemical balance. 
 
68 CHEMISTRY 
 
 The process of weighing, the description of 
 the balance, and of the standards of weight 
 in use, are all dealt with in works on practical 
 Physics and practical Chemistry; and every 
 student is now familiarized with weighing 
 operations at the outset of his practical work. 
 The chief points which the general reader 
 must realize are that the weighings carried 
 out by the chemist are with smaller quantities, 
 and with a degree of precision unknown to 
 those who are accustomed only to the com- 
 paratively rough scales or weighing machines 
 in ordinary use. For scientific purposes, 
 also, the metric system of weights and 
 measures is universally employed. In this 
 system the standard of length is the metre, 
 equal to 39-37 inches, and the unit of weight 
 is the gram, which is the thousandth part 
 of a standard mass of metal (platinum) kept 
 in Paris, and known as the kilogram. The 
 kilogram of pure water at its maximum 
 density of 4 Centigrade occupies a volume of 
 1000 cubic centimetres (one litre), so that 
 the unit weight of one gram is the weight of 
 one cubic centimetre of pure water at 4. 
 Those who are familiar only with our cumbrous 
 English system of weights and measures will 
 acquire more definite notions when it is 
 pointed out that the gram is equal to 15-43 
 
CHEMICAL CHANGE 69 
 
 grains, the kilogram to 2-2 pounds, and the 
 litre to 61-03 cubic inches = 0-22 gallon. 
 In ordinary quantitative chemical work, 
 we seldom deal with quantities exceeding 
 100 grams generally with much less, and a 
 good chemical balance will weigh accurately 
 to l-10,000th of a gram. Such refinement in 
 the process of weighing would have appeared 
 incredible to the early pioneers of quantitative 
 chemistry, whose most accurate operations 
 in the eighteenth century were carried out 
 with balances which would now be regarded 
 as relatively coarse. 
 
 The Definiteness of Chemical Change. By 
 means of the balance the results of chemical 
 transformation can, therefore, be followed 
 quantitatively. In stating that the material or 
 materials which undergo such change bear a 
 definite quantitative relationship towards each 
 other, it is meant that, when a new substance 
 is formed or when substances arise from the 
 combination or from the decomposition of 
 other kinds of matter, the relative weights, 
 both of the original materials which undergo 
 transformation and of the new product or pro- 
 ducts, are always constant for each particular 
 kind of matter. Since combination and de- 
 composition are generally reversible processes, 
 it will be simpler at this stage to consider 
 
70 CHEMISTRY 
 
 combination only. From this point of view 
 the actual weight of one substance which can 
 combine chemically with another substance 
 to produce a different kind of matter is as 
 much a specific chemical property of the 
 substance as any other character. The only 
 qualification that must be borne in mind is 
 that different substances may have the 
 property of combining with each other in 
 more than one proportion by weight ; but 
 this fact does not contravene the principle of 
 definite combination it only enlarges the 
 definition of the principle so as to include 
 more than one possibility. 
 
 An example illustrative of the foregoing 
 statement has already been furnished in the 
 case of iron (p. 23). It will be remembered 
 that this substance forms " scale " when the 
 heated metal is acted upon by water or air, 
 and rust when it is exposed to moist air at 
 ordinary temperatures. Now, iron " scale " 
 and iron rust (when dry) consist of the same 
 two substances, iron and oxygen, so that we 
 have here two dissimilar compounds arising 
 from the combination of the same two 
 materials. There is, of course, no mystery 
 about this ; the balance can be made to prove 
 that the difference is due to the fact that, 
 in " scale," the iron and oxygen are present in 
 
CHEMICAL CHANGE 71 
 
 different proportions from those in which they 
 are present in rust. And, in order to render 
 the story more complete, it may be mentioned 
 that we know a third compound containing 
 only iron and oxygen which is easily obtainable 
 by chemical methods, and which is quite 
 distinct as a form of matter from both scale 
 and rust, and in which the proportions of iron 
 and oxygen are again different. Since com- 
 pounds formed by the combination of oxygen 
 with other substances are generally known 
 in Chemistry as oxides, the case will be made 
 more definite by stating that there exist 
 three oxides of iron three different sub- 
 stances all arising from the combination of 
 the same two materials in different pro- 
 portions. It is quite easy to ascertain by 
 methods which must be considered hereafter 
 the relative proportions of iron and oxygen 
 contained in the three oxides ; and it will give 
 greater precision to the notion of chemical 
 transformation in its quantitative aspect if 
 it is stated that the iron oxide of rust contains 
 70 per cent, of iron, the iron oxide of " scale " 
 72-4 per cent., and the third oxide 77-8 per 
 cent, of iron. 
 
 The main principle the definiteness of 
 chemical change is thus upheld, because 
 whenever iron is made to combine with 
 
72 CHEMISTRY 
 
 oxygen, directly or indirectly, we always 
 get one or another of these three substances. 
 There are no definite intermediate substances ; 
 the defmiteness is chemically rigid. It is a 
 chemical truth expressed in numerical terms 
 that cannot be tampered with. If it were 
 desired to produce a substance containing 
 iron and oxygen in some other proportions 
 than those indicated, it could not be done 
 by chemical means, but only by making a 
 mechanical mixture of some or all of the three 
 oxides to each of which Nature has attached 
 the brand of individuality. 
 
 It will now be understood why the pro- 
 duction of a definite homogeneous substance 
 by chemical change is conditioned by the 
 relative quantities of the materials brought 
 into combination. The relative weights of 
 the materials which combine being fixed in 
 the sense indicated, if too much or too little 
 of one of the components is used the product 
 of the transformation is necessarily a mixture 
 consisting of the new compound, i.e., the 
 chemical product, plus the excess of unchanged 
 material. For example, the compound formed 
 by heating iron with sulphur (p. 39) is a 
 sulphide (note the analogy of the term with 
 oxide) of iron containing iron and sulphur 
 in the ratio 4:3. If more iron is put into 
 
CHEMICAL CHANGE 73 
 
 the original mixture, the product would 
 consist of the sulphide mixed with the excess 
 of unconverted iron. Again, the iron iodide 
 formed by the direct union of iron with 
 iodine (p. 38) is a product resulting from 
 the combination of 7 parts by weight 
 of iron with 31.7 parts of iodine. These 
 materials cannot be made to combine in the 
 chemical sense in any other proportions ; 
 any excess either of iron or of iodine would 
 be left uncombined, and could be separated 
 from the mixture by appropriate methods. 
 
 The very definiteness of chemical change 
 thus prohibits a gradual transition from one 
 form of matter to another. There is neces- 
 sarily an abruptness about the transforma- 
 tions ; there can be no half measures 
 each material must have its full complement 
 of its associated material. If it is not supplied 
 with as much as it can combine with, there 
 is a residue of unappropriated and unchanged 
 material. In speaking of chemical change 
 as abrupt, it is not meant that the transforma- 
 tion takes place suddenly. There are chemical 
 changes of every degree of velocity, from a 
 practically instantaneous explosion to a trans- 
 formation which may take days, weeks, years, 
 or geological periods. Direct combination is 
 generally, but not invariably, a rapid process. 
 
74 CHEMISTRY 
 
 But, whether rapid or slow, the definite 
 character of chemical change is universally 
 maintained. Indeed, the measurement of 
 the velocity or rate of chemical change is 
 only made possible in those cases where it is 
 measurable by the definiteness of the result, 
 because the quantity of chemically trans- 
 formed material (as distinguished from the 
 unchanged material) which is produced at 
 measured intervals of time can be determined 
 either directly or indirectly by means of the 
 balance. 
 
 Some of these points are illustrated by the 
 examples of chemical change already cited. 
 Thus, the rusting of iron is a slow process ; 
 the conversion of iron into " scale " by the 
 action of water or oxygen upon the heated 
 metal is a sudden change. When a mixture 
 of iron and sulphur is heated to the point 
 at which combination takes place, the whole 
 mass suddenly glows, and the conversion into 
 sulphide is so rapid that it may practically 
 be considered to be instantaneous. In some 
 cases, both of slow and of rapid change, it has 
 been observed that the final stage is reached 
 through a series of intermediate steps, but 
 the definiteness of the transformation is not 
 in any way interfered with each stage is 
 as definite as its successor ; it is simply a 
 
CHEMICAL CHANGE 75 
 
 case of a definite, stable, final stage being 
 reached through one or more unstable but 
 equally definite intermediate stages. In some 
 cases it is possible to arrest the process at 
 an intermediate stage, and to isolate the 
 intermediate product. The latter is in all 
 cases the result of combination (or of decom- 
 position) in as definite proportions by weight 
 as is the final product the end result. In 
 other words, the course of the chemical 
 change can in many cases be followed through 
 successive phases, each perfectly definite ; 
 and it has accordingly been suggested with 
 much plausibility that all chemical change 
 takes place in stages, these stages in the 
 majority of cases being passed through too 
 rapidly to enable them to be detected by our 
 present methods. Chemical change has thus 
 been aptly compared to a drama, the scenes 
 of which are shifted with great rapidity, the 
 spectator seeing only the final act. Applying 
 this metaphor to the present discussion, the 
 main point to be borne in mind is that 
 each scene in the drama even if passed 
 through with explosive velocity is a perfectly 
 definite picture from the chemical point of 
 view, although the eye of the chemist may 
 not at present be capable of appreciating it. 
 A great step in the development of the 
 
76 CHEMISTRY 
 
 science of Chemistry will have been made 
 when new methods are discovered or known 
 methods applied to the study of what may 
 be called transition phases. 
 
 The Conservation of Mass. It will have 
 been made evident to the reader that, although 
 chemical change is characterised by those 
 general criteria which have been discussed, 
 there is still an individuality about the 
 process which makes it essential both for 
 scientific and for practical purposes to study 
 particular cases in detail. It is, in fact, by 
 such detailed studies that our generalized 
 statements have been made possible. The 
 vastness of the prospect thus opened out to 
 the gaze of the student of Chemistry will now 
 begin to loom. Before proceeding to further 
 developments, however, there yet remains an 
 aspect of chemical change of fundamental 
 importance which the student will be brought 
 to face at the very beginning of his studies. 
 It will have been realized that Chemistry is 
 concerned with the transformations of matter, 
 and that these transformations are, so far as 
 concerns the relative masses of the materials 
 which undergo transformation, of a perfectly 
 definite character. It is but another step to 
 the conclusion based, of course, upon experi- 
 mental evidence that during such trans- 
 
CONSERVATION OF MASS 77 
 
 formations there is neither gain or loss of 
 material. This is tantamount to the proposi- 
 tion that matter is indestructible by any known 
 chemical process. There is profound modifi- 
 cation or transformation as the result of 
 chemical change, but the whole quantity of 
 matter, i.e., the mass, remains constant ; the 
 sum total of the weights of the materials 
 employed is the same at the end of the 
 transformation as it was before the change 
 occurred. Seven unit weights of iron heated 
 with four unit weights of sulphur give eleven 
 unit weights of iron sulphide with the arith- 
 metical accuracy expressed by 7+ 4= 11, and 
 so on for all other cases of chemical change, 
 either by combination or by decomposition. 
 If, for example, the above process could be 
 reversed (as it can by indirect methods), and 
 the iron sulphide resolved into its components, 
 eleven parts of the sulphide would give seven 
 parts of iron and four of sulphur or, arith- 
 metically, 11 = 7+4. 
 
 The statement that you only get out of a 
 given weight of matter as much as you start 
 with may appear at first sight such a self- 
 evident proposition that no special proof need 
 be adduced. But there is a deeper significance 
 in the statement than might be imagined on 
 superficial consideration. In the first place, 
 
78 CHEMISTRY 
 
 it -involves the great principle that matter can 
 neither be created nor destroyed by chemical 
 agencies. In the next place, it opposes what 
 might be considered the common-sense view 
 that such deep seated and startling trans- 
 formations as are wrought by chemical 
 change must necessarily be accompanied by 
 loss or gain of material. That such a view 
 should have been entertained in former times 
 is not surprising when it is remembered that 
 the truth of the doctrine can only be de- 
 monstrated when all the products of the 
 change are collected and weighed. If, as is 
 often the case, one of the products is gaseous 
 and escapes in an invisible form, it is under- 
 standable that, at a time when the material 
 nature of gases had not been thoroughly 
 recognized, it should have been believed that 
 chemical change might be accompanied by a 
 destruction of matter. Although the scope 
 of this work does not admit of the historical 
 treatment of the subject, it must be recognized 
 as marking an epoch in the history of Chemis- 
 try, that the conversion of the scientific world 
 to the doctrine of the indestructibility of 
 matter was brought about mainly by the 
 researches of the illustrious French chemist 
 Lavoisier (1743-1794), who fell a victim to 
 the Revolution. The student of the history 
 
CONSERVATION OF MASS 79 
 
 of our science will learn that, although the 
 materials for the establishment of this doctrine 
 were in hand, its acceptance had been 
 retarded by the prevalence of certain erroneous 
 theoretical views which were only finally 
 overthrown by the experiments and reasoning 
 of Lavoisier and his disciples. 
 
 A principle so fundamental as that of the 
 indestructibility of matter, which now per- 
 meates every branch of science, must enter 
 into the mental constitution of the would-be 
 chemist. Through every phase of chemical 
 change it is known that this principle holds 
 good, although in ordinary practical work 
 it is never realized unless the most refined 
 methods are employed for ascertaining the 
 weights of all the products. In every-day 
 laboratory or factory experience, there is 
 always more or less loss due to the imperfec- 
 tion of methods ; and the theoretical ideal 
 that the sum total of the weights of the final 
 product or products is equal to the sum total 
 of the weights of the initial materials is 
 seldom realized. But the loss arising from un- 
 avoidable manipulative causes no longer shakes 
 faith in the doctrine, although, for certain 
 philosophical reasons, it has been thought 
 necessary to retest its truth with the utmost 
 refinement of modern scientific resources. 
 
80 CHEMISTRY 
 
 Water a Chemical Compound. In the light 
 of these principles the definiteness of chemi- 
 cal change, and the indestructibility of matter 
 all chemical transformations may be con- 
 sidered and illustrative examples multiplied 
 indefinitely. One other case may be introduced 
 at this stage, both on account of its historical 
 interest and because it will enable fresh 
 materials to be added to the small store of facts 
 which have thus far been found to furnish a 
 sufficient basis for the discussion of the broader 
 generalities. Attention is invited to another 
 very familiar substance, water, which, since it 
 forms iron scale when brought into contact 
 with the heated metal (p. 23), may be con- 
 sidered to have been proved to be a substance 
 containing oxygen. Tlie question whether 
 the oxygen in air and water may not -be 
 present in a different condition in the two 
 substances has already been raised (p. 27). 
 Air has been proved to be a mixture ; water 
 has been proved to contain oxygen, and, 
 since water differs so fundamentally from 
 oxygen in all its properties, chemical and 
 physical, it is evident that it must contain 
 something in addition to oxygen. What is 
 that other constituent, or what are the other 
 constituents if more than one; and is the 
 oxygen simply mixed with the other con- 
 
COMPOSITION OF WATER 81 
 
 stituent or constituents, or is it chemically 
 combined ? The answer to these questions 
 will furnish further instructive illustrations of 
 facts, methods, and principles. 
 
 The fact that heated iron takes oxygen out 
 of water suggests the use of this metal in order 
 to find out what is left after the oxygen 
 has been removed. In forming iron scale by 
 plunging red-hot iron into water, it might be 
 noticed that with the escaping steam there 
 is a gas which, unlike the steam, does not 
 condense on cooling. This gaseous product 
 of the action of water upon hot iron is not 
 easy to detect by such a rough and ready 
 experiment ; but by a refinement of method, 
 and without in any way interfering with the 
 principle by altering the materials, this gas 
 can be readily obtained in any desired 
 quantity. In practice, instead of using water, 
 we use steam, which is the same substance in 
 a different state of physical aggregation ; and 
 the iron, in coarse fragments, such as wire 
 or nails, is enclosed in a tube of porcelain 
 or any suitable material that will stand the 
 heat. The steam is passed through the heated 
 tube, and the emergent gas collected by 
 appropriate methods in any suitable vessel. 
 At the end of such an operation, the iron will 
 be found to have been more or less converted 
 
 F 
 
82 CHEMISTRY 
 
 into scale ; and the new gas, on comparison 
 with oxygen or nitrogen, will be found to 
 possess quite different properties it is a 
 different kind of gaseous matter. This gas is 
 known as hydrogen ; like nitrogen or carbon 
 dioxide (p. 36), it is colourless and transparent, 
 and it extinguishes flame, but, unlike these 
 gases, it is combustible, burning in the air 
 with a barely perceptible, but very hot flame. 
 Hydrogen is the lightest form of matter known 
 on this earth, being, bulk for bulk, 14 J times 
 lighter than air, 16 times lighter than oxygen, 
 and 11,000 times lighter than water. In 
 view of this fact, the gas is difficult to deal 
 with practically, since it tends to escape 
 from all vessels, tubes, joints, etc., having the 
 slightest porosity. 
 
 Water is thus shown to contain hydrogen 
 as well as oxygen ; and, since by no physical 
 process, such as diffusion (p. 56), is it possible 
 to separate hydrogen from water, it is evident 
 that the gas is chemically combined. The 
 question whether the oxygen in air and water 
 may not be present in different states is thus 
 definitely answered. Moreover, a mixture of 
 hydrogen with oxygen is not water, but a 
 gas intermediate in properties between it?' 
 two components. The oxygen in water i/n 
 chemically combined, and its separation by 
 
COMPOSITION OF WATER 83 
 
 hot iron is a chemical process. It will be 
 noticed that the removal of oxygen from water 
 by iron differs materially as a process from 
 that which takes place when iron rusts in air : 
 in the first case the iron has to be red-hot, 
 while in the second case the combination 
 takes place at the ordinary temperature. 
 Chemical combination has to be overcome 
 in the case of water, and not in the case of 
 air, which is simply a mixture. 
 
 From this illustration we can develop 
 further the principle of preferential combina- 
 tion (p. 35), because there are other metals 
 besides iron which liberate hydrogen from 
 water, while many other metals exert no such 
 decomposing action. Thus, a familiar metal, 
 zinc, when heated in steam liberates hydrogen 
 at a lower temperature than iron, and another 
 less familiar metal, magnesium, at a still 
 lower temperature ; while some metals, which 
 are not generally familiar, and which will be 
 afterwards referred to, will decompose even 
 liquid water and liberate hydrogen at the 
 ordinary temperature. So we could con- 
 struct a graduated scale of preferences 
 for oxygen as measured by the temperature 
 at which the hydrogen is liberated from 
 water. On such a scale the inverse prefer- 
 ential order would be iron, zinc, magnesium, 
 
84 CHEMISTRY 
 
 and then those metals which decompose cold 
 water. 
 
 Then again, as already stated, some metals 
 do not liberate hydrogen from water at all, 
 although they are capable of combining with 
 oxygen. Such a metal is the familiar sub- 
 stance copper, which, like iron, when heated 
 unites with oxygen to form an oxide, a black 
 substance totally unlike the red lustrous 
 metal and the gaseous oxygen from which 
 it is formed. The removal of oxygen from 
 the air by passing the latter through a tube 
 containing heated copper is one of the 
 recognized methods of obtaining nitrogen. 
 These facts furnish further proof of water 
 being a chemical compound, because one 
 metal (iron), which has a sufficiently strong 
 liking for oxygen to combine with this 
 substance at the ordinary temperature, has to 
 be made red-hot before it can take the oxygen 
 out of water, while another metal (copper), 
 which has sufficient liking for oxygen to 
 form an oxide when heated in contact with 
 that gas, fails to remove the oxygen from 
 water even when red-hot. From which facts 
 it follows, also, that on the inverted scale of 
 preferences for oxygen given above, copper 
 would precede iron. 
 
 And now there remains to be answered 
 
COMPOSITION OF WATER 85 
 
 the question whether water contains anything 
 besides oxygen a question which, of course, 
 relates only to water as an individual form of 
 matter, and not to water as we find it in 
 nature, in rivers, or in rain, or in the sea, 
 because such water always contains other 
 substances dissolved in it. Natural water is, 
 in fact, an aqueous solution of substances 
 derived from the air and the earth through 
 which it percolates. When a liquid like 
 water takes up other substances, either solid, 
 liquid, or gaseous, and forms a homogeneous 
 mixture, the latter is technically described 
 as a solution ; the water in this case is des- 
 cribed as the solvent and the other (dissolved) 
 substances as the solutes. This point is raised 
 in a preliminary way now, in order that 
 it may be realized that we are at present 
 concerned only with water, and not with any 
 adventitious matter that it may contain. 
 Returning now to the initial question, and 
 referring to the statement that a mixture 
 of oxygen and hydrogen is not water, it only 
 remains to be added that a mixture of these 
 gases explodes when a flame is applied, 
 and the product has been found to be water 
 and nothing but water. This is an old and 
 now a well known fact, but the reader must 
 again be reminded that a discovery of this 
 
86 CHEMISTRY 
 
 importance was made only by skilful experi- 
 ment carried out with all the refinements of 
 method available at the time by Cavendish 
 (1731-1810). In practice, the gases are en- 
 closed in a strong glass vessel (eudiometer) 
 with platinum wires sealed through the side, 
 so that the explosive combination may be 
 brought about by passing an electric spark 
 through the mixture. 
 
 Since oxygen and hydrogen give nothing 
 but water when combined, the question is 
 therefore answered water as such contains 
 no other form of matter. So that, as the 
 result of chemical transformation, a gas, 
 hydrogen, of which the boiling-point is about 
 252 below the freezing-point of water, in 
 combination with another gas, oxygen, of 
 which the boiling-point is about 183 below 
 the freezing-point of water, gives a liquid 
 1,400 times denser than hydrogen and boiling 
 at 100 above the freezing-point of water. 
 The profound nature of chemical change is 
 again exemplified ; but its definiteness has 
 not yet been made apparent in the case under 
 consideration. Supposing, then, that into a 
 suitable vessel, such as a strong glass tube 
 graduated by marked divisions which enable 
 us to measure the volume of gas, and having 
 wires sealed through to enable an electric 
 
COMPOSITION OF WATER 87 
 
 spark to be passed, we introduce hydrogen 
 and oxygen in known volumes. It is not easy 
 speaking strictly it is impossible to form 
 an accurate notion of the method of conduct- 
 ing such an experiment as this by simply 
 reading a description of it. The student of 
 Chemistry, however, learns how to manipulate 
 gaseous matter with the same facility that 
 he deals with solids or liquids, and the methods 
 are given in practical works on the subject. 
 But the principle is quite intelligible, for we 
 have only to consider that we have a mixture 
 of hydrogen and oxygen in which the actual 
 volume of each gas is known. The mixture 
 is exploded by an electric spark, and some of 
 the gas disappears when the vessel has cooled 
 down. In other words, there is shrinkage of 
 volume, because the water which is formed, 
 being liquid at ordinary temperatures, con- 
 denses to a dew on the inside of the vessel. 
 But the whole of the gas does not disappear 
 in an experiment of this kind ; there is 
 always a residue, and, by testing that residue, 
 it is easily shown that it is either oxygen or 
 hydrogen, according to the proportions of the 
 gases in the original mixture. By such obser- 
 vations it is found that, for every unit volume 
 of oxygen, two unit volumes of hydrogen 
 disappear, or conversely, that for every unit 
 
88 CHEMISTRY 
 
 of hydrogen half a unit volume of oxygen 
 disappears. 
 
 Hereby is once more illustrated the definite- 
 ness of chemical change ; under the conditions 
 imposed, the gases refuse to combine in any 
 other proportions there are no intermediate 
 proportions ; it is two to one, or nothing. 
 Any excess of either gas over and above these 
 proportions is left uncombined, and that is 
 why in such experiments there is always 
 some residual gas ; it is only when a mixture 
 of exactly two volumes of hydrogen with one 
 volume of oxygen is exploded that there is 
 no gas left the sole result of the combination 
 is then water, and the vessel in which the 
 explosion takes place is found to be practically 
 vacuous when cold, because the condensed 
 water occupies but a very small volume. The 
 defmiteness in this case manifests itself volu- 
 metrically, but that is really the same thing as 
 defmiteness by weight, because the gases 
 have perfectly definite weights as compared 
 with each other under comparable conditions, 
 the relative weights of equal volumes of 
 oxygen and hydrogen being about 16 : 1. 
 As one volume of oxygen combines with 
 two volumes of hydrogen, the relative weights 
 of the two substances contained in water are 
 accordingly 16 : 2 =8:1. 
 
COMPOSITION OF WATER 89 
 
 Other methods of determining the com- 
 position of water are well known, but these 
 involve new principles, and cannot be dis- 
 cussed now. The chemical change which 
 takes place when hydrogen and oxygen 
 combine is, like all other chemical changes, 
 a redistribution or rearrangement of matter, 
 but the weight of the materials remains 
 constant the principle of the Conservation 
 of Mass is maintained. If the vessel contain- 
 ing the gases is weighed before and after the 
 explosion, there will be found neither loss nor 
 gain a fact which in this instance is made 
 more striking because of the total disappear- 
 ance of the gases as such when the mixture 
 contains the correct proportions. 
 
 The principle of the Conservation of Mass, 
 which lies at the root of the science of Chemis- 
 try, will remind the student of Physics of the 
 analogous principle of the Conservation of 
 Energy. To the chemist, this last doctrine 
 is of equal importance with the first, but the 
 reader must beware of straining the analogy 
 between the two doctrines. The different 
 forms of energy are interconvertible, but it 
 by no means follows that different forms or 
 kinds of matter are interconvertible. Were 
 such interconvertibility possible, we should 
 have realized " transmutation " as distin- 
 
90 CHEMISTRY 
 
 guished from transformation. Transmuta- 
 tion was the moving principle prompting 
 the work of the old alchemists, gold being 
 the form of matter striven for. It is now 
 recognized that alchemy failed in this particu- 
 lar quest, and the idea of transmutation 
 passed out of Chemistry when it became a 
 science. In modern times, and in the light 
 of new discoveries, the idea has been rein- 
 stated in another form, and there is some 
 evidence in its favour how much is a matter 
 of judgment and a question for future investi- 
 gation. But all this belongs to another story 
 which cannot be narrated here, and the 
 reader must be referred for further information 
 to works dealing with Radioactivity, or to 
 the volume on " Matter and Energy " by 
 Mr. F. Soddy, in this series. 
 
CHAPTER IV 
 
 ELEMENTARY AND COMPOUND MATTER THE 
 CHEMICAL ELEMENTS METALS AND NON- 
 METALS 
 
 Elementary and Compound Matter. Matter is 
 comprised under the general term " stuff," 
 an expression which finds its equivalent in 
 the German Stoff. Thus we speak of food- 
 stuffs, dyestuffs, etc., and the German for 
 hydrogen, Wasserstoff, indicates, like the 
 Greek roots of the English name for this 
 substance, that it is the stuff from which water 
 is produced. For the purpose of illustrating 
 the nature of chemical change, appeal was 
 made in the preceding chapters to a number 
 of well-known things, such as air and water, 
 iron, sulphur, iodine, sugar, charcoal, and 
 chalk. By the chemical study of some of 
 these " stuffs," we have been made acquainted 
 with their less familiar components, such as 
 the gases oxygen, hydrogen, and nitrogen, 
 which do not come within the popular notions 
 of matter unless attention is specially directed 
 91 
 
92 CHEMISTRY 
 
 to their material nature. It is obvious that 
 many of the substances referred to are 
 compounds made up of at least two kinds of 
 matter, such, for example, as iron rust and 
 iron scale composed of iron and oxygen, iron 
 sulphide made up of iron and sulphur, or 
 water composed of hydrogen and oxygen. 
 
 It thus falls within the province of Chemistry, 
 which is concerned with the transformations 
 of matter in the sense which has now been 
 made clear, to determine, in the first place, 
 whether a substance which is not a mixture 
 is composite whether other forms of matter 
 can be got out of it by chemical or physical 
 processes. It may be well to take advantage 
 of this opportunity for pointing out that to 
 the chemist the various forms of energy, heat, 
 light, electricity, etc., are as much chemical 
 agencies as matter itself, in so far as these 
 forms of energy are capable of producing 
 chemical change. We investigate the com- 
 position of matter both by physical processes 
 and by purely chemical processes, meaning 
 by the latter the action of one form of matter 
 upon another. This last case is also ulti- 
 mately resolvable into terms of energy ; but 
 for the present the distinction will be found 
 convenient. The study of the composition of 
 matter can be conducted in two ways 
 
ELEMENTS AND COMPOUNDS 93 
 
 a substance may be resolved by physical or 
 chemical means into its components, or the 
 components may be brought into combination, 
 and the substance built up. The first process 
 is, in a general way, described as analysis, and 
 the complementary process as synthesis. Very 
 often the composition of some particular stuff 
 te proved in both ways. Thus, when water 
 is shown by the action of hot iron to 
 contain oxygen and hydrogen, its composition 
 is proved by analysis ; when water is formed 
 by the combination of oxygen and hydrogen, 
 its composition is proved by synthesis. The 
 more complex the substance the greater the 
 number of different forms of matter which 
 enter into its composition the more important 
 and, it may be added, the more difficult does 
 the proof of its composition by both methods 
 become. In the case of extremely complex 
 substances, it is often not possible in the 
 present state of knowledge to complete the 
 synthetical evidence ; but, as we are approach- 
 ing the subject gradually, it will be advisable 
 at this stage to concentrate attention upon 
 the simpler cases. 
 
 For at least a century and a half, chemists 
 have thus been trying by every possible 
 method to pull all available materials to pieces. 
 The component parts which the chemist has 
 
94 CHEMISTRY 
 
 striven for are not simply proximate, but 
 ultimate as ultimate as his methods can 
 carry him. The mineralogist, for instance, by 
 microscopic examination or by other methods, 
 can determine the proximate components of 
 a mineral he may resolve granite into quartz, 
 felspar, and mica ; the biologist may dissect 
 an animal or a plant into various organs and 
 tissues, and, by pushing his studies still further 
 into the domain of cytology, he may study 
 the individual cells of which all organisms are 
 composed, and, by the application of the 
 highest magnifying power and the use of 
 dyes which stain the different component 
 materials of the cell selectively, he may effect 
 a microscopic analysis of even the minutest 
 organized units of animals and plants. Or 
 the mechanic may separate a machine into 
 its component parts to find out how it works, 
 and he may put the parts together again and 
 restore the mechanism to its working condi- 
 tion ; his analysis need go no further for his 
 purpose than the separation of wheels, cranks, 
 levers, and so forth. But the chemist pushes 
 his inquiries deeper than this ; he can tell the 
 mineralogist what the components of granite 
 are themselves composed of, he can supply 
 the biologist with information concerning the 
 different complex forms of matter of which 
 
ELEMENTS AND COMPOUNDS 95 
 
 the organized units are composed, and he can 
 resolve the component parts of a machine 
 into a few metals, upon the specific properties 
 of which the efficiency of the machine depends. 
 
 The general result of the study of matter 
 from this chemical point of view has been 
 the discovery that the process of resolution 
 in every case reaches a limit. From whatever 
 materials we set out, there are finally obtained 
 some forms of matter which cannot be further 
 decomposed from which nothing different 
 can be produced by any known process of 
 resolution ; substances whch are said to be 
 elementary. So that from the chemist's 
 standpoint all matter is either resolvable or 
 unresolved ; the Universe is built up of 
 elementary and compound matter. Whether 
 elementary matter is in its ultimate nature 
 unresolvable is a question for the future to 
 decide ; the point is raised in connection with 
 recent researches in Radioactivity. But for 
 all practical purposes it may be assumed that, 
 whether further decomposition of elementary 
 matter is possible or not, no such resolution 
 takes place in the course of any ordinary case 
 of chemical change. 
 
 The Chemical Elements. The component 
 parts of anything may, therefore, be said to 
 be its elements, the degree of resolution 
 
96 CHEMISTRY 
 
 depending very much upon the particular 
 branch of science. Thus, to the anatomist, 
 the limbs, organs, skeleton, etc., are the 
 elements of an animal ; the bones are the 
 elements of the skeleton ; the histologist 
 separates tissues into elementary cells ; and 
 to the cytologist the various components of 
 the cell constitute its elements. Or, to the 
 engineer, the wheels, cranks, etc., of a machine 
 are its elements. The separation of matter 
 by chemical methods is evidently more 
 fundamental ; and the chemical elements are 
 recognized in every department of science. 
 The evolution of the idea of an element in 
 Chemistry forms an interesting chapter in the 
 history of the science, and the student will 
 find the story most fascinating. It will be 
 readily understood that, with the discovery 
 of new methods of decomposing matter chemi- 
 cally, substances apparently elementary were 
 resolved. In this way, the list of elements 
 was from time to time revised by the substitu- 
 tion of new elements for their previously 
 unresolved compounds. Then, again, with 
 increasing activity in the study of rare 
 minerals and out-of-the-way materials with 
 refinement of methods of separation, and 
 with improvements in physical and chemical 
 methods of detection and discrimination 
 
CHEMICAL ELEMENTS 97 
 
 the number of elements has been increased 
 until at the present time about eighty 
 according to the latest census eighty-two 
 of these unresolved " stuffs " are known. 
 When the reader hears of " new " chemical 
 elements being discovered, he will, of course, 
 understand that what is really meant is the 
 detection of some element that had previously 
 escaped notice by virtue of its rarity, or on 
 account of its being difficult to separate from 
 associated matter, or because of its lacking 
 obtrusively distinct characters. No chemist 
 has ever done more than bring to light those 
 raw materials of the Universe which were 
 already in existence ages before the advent of 
 man upon this earth. 
 
 It is the business of the chemist to know 
 as much as possible about these elements ; 
 to acquaint himself with their mode of occur- 
 rence in nature, with the methods of isolating 
 them, and with their characteristic properties 
 as individual forms of matter in short, with 
 their natural history. The student need not 
 be appalled at the magnitude of the field 
 thus opened out ; he will not be called upon 
 to commit to memory long tables of facts and 
 figures all has been systematized and simpli- 
 fied by scientific generalization. The chemical 
 elements play very different parts in the 
 
98 CHEMISTRY 
 
 economy of nature, and in their utility in the 
 arts and manufactures. Some are abund- 
 antly distributed both in the free and combined 
 state ; others are also abundant, but occur 
 only in combination ; others are rare in the 
 free state, while their compounds are common ; 
 and others, again, are extremely scarce in any 
 state. It is not proposed, even if it were 
 possible, in this little volume to set forth more 
 than a few generalities concerning the eighty- 
 two elements known to science, since any 
 invidious selection would necessarily convey 
 a false impression of relative importance. In 
 applied chemistry, in the economy of life, 
 in their capability of forming multitudinous 
 compounds, every set of elements has its order 
 of importance ; in the light of pure science, 
 there is no absolute scale of importance ; 
 every element has its own story to tell, and 
 one which occurs only in infinitesimal traces 
 the newly isolated radium has opened up 
 some of the most fundamental questions 
 concerning the ultimate constitution of matter 
 that have ever been raised since the individ- 
 uality of the elements became a recognized 
 scientific doctrine. 
 
 Acquaintance with some of the chemical 
 elements has already been made in the pre- 
 ceding chapters ; and it will have been 
 
CHEMICAL ELEMENTS 99 
 
 realized that the conception of an element as 
 an unresolved form of matter is quite inde- 
 pendent of the physical state of aggregation. 
 Thus, oxygen, hydrogen and nitrogen are 
 gaseous elements under ordinary conditions ; 
 sulphur and iodine are solids. Iron, copper, 
 zinc and magnesium are all elements, and 
 have been mentioned as belonging to the 
 category of metals they are metallic elements, 
 and are all solid at ordinary temperatures, 
 but have definite points of liquefaction, i.e., 
 melting-points. The familiar metal mercury, 
 or quicksilver, is an element that is liquid 
 under ordinary conditions, but it solidifies 
 at 39-5C., and is a colourless vapour above 
 357. It would be possible to make a classifi- 
 cation of the elements based on their physical 
 state as familiarly known to us ; but nothing 
 would be gained by such a classification 
 beyond mnemonical assistance. Chemical clas- 
 sification naturally goes deeper, and is directed 
 towards the establishment of chemical rela- 
 tionships the association of elements which 
 are allied in their chemical characters. Solid 
 sulphur and gaseous oxygen, for example, 
 have many chemical properties in common. 
 No classification of the elements based on 
 any set of purely physical characters is of any 
 use from the chemical point of view, although, 
 
100 CHEMISTRY 
 
 as will be seen subsequently, the chemical 
 classification is of the greatest value from the 
 physical point of view. 
 
 The reader is invited to consider another 
 example which will serve to introduce a set 
 of four closely related elements, one of which, 
 iodine, has already been made use of in illus- 
 tration of chemical change (p. 38). Of these 
 four elements, the most abundant is chlorine, 
 which is a component of common salt ; and, 
 as common salt is contained in and was 
 originally obtained from sea water, the group 
 has received the name of Halogens, which 
 simply means generators of salt like sea-salt. 
 The halogens, then, are fluorine, chlorine, 
 bromine and iodine. Fluorine occurs in 
 combination as a constituent of the minerals 
 fluorite (fluor spar) and cryolite, the other 
 constituents of these minerals being certain 
 metallic elements which may stand over for 
 further consideration. Chlorine occurs in 
 combination in common salt, vast deposits of 
 which are found " bedded " in certain geo- 
 logical strata ; this salt is also the main com- 
 ponent of the solid residue left by the drying 
 down (evaporation) of sea water. Bromine 
 also is found in combination as a salt in the 
 same geological deposits with common salt ; 
 and iodine is found in combination in the 
 
CHEMICAL ELEMENTS 101 
 
 earthy residue or ash left when sea-weeds are 
 burnt. There are also found in Chili deposits 
 of a certain salt known as Chili Saltpeter, 
 which is largely used as a fertilizer for crops ; 
 with this salt certain compounds of iodine are 
 found in admixture. Thus, the halogens are 
 found in nature only in a state of combina- 
 tion ; they have been " discovered," i.e., 
 isolated, by chemical methods, and their 
 elementary character has been established by 
 chemical research. The question now arises 
 why are these four elements grouped together ? 
 That the classification is independent of 
 physical considerations is shown by the 
 fact that fluorine and chlorine are gases at 
 ordinary temperatures, bromine a deep red 
 heavy liquid, and iodine a metallic looking 
 solid. The relationship between the members 
 of this family is primarily chemical ; with 
 the same element they form compounds of 
 absolutely similar types and characters. To 
 simplify matters at this stage, it may be 
 considered that the element which is most 
 commonly found in combination with the 
 halogens under natural conditions is the 
 metallic element known as sodium. Common 
 salt is a compound of chlorine and sodium ; 
 and, just in the same way that we speak of 
 compounds of oxygen as oxides (p. 71), we 
 
102 CHEMISTRY 
 
 say that salt is sodium chloride. The other 
 halogens form similar compounds sodium 
 fluoride, bromide and iodide respectively. It 
 may be mentioned incidentally that fluorite 
 is a compound of fluorine with another metallic 
 element, calcium, of which quicklime (p. 39), 
 is an oxide ; while cryolite contains, in addi- 
 tion to the fluoride of sodium, the fluoride of 
 the metallic element aluminium, of which 
 element the mineral substance clay is one of 
 the most familiar compounds. Now, all the 
 sodium compounds of the halogens are alike 
 in their general properties ; they are all 
 colourless salts, crystallizing in the same 
 cubical form, and having the same chemical 
 characters. What is true for sodium is true 
 for calcium and for other elements, metallic 
 or non-metallic, so far as concerns the simi- 
 larity of the four compounds in each set. 
 Thus the four halogens form a series of four 
 compounds with hydrogen, all colourless 
 gases at ordinary temperatures, and all having 
 similar chemical properties ; the fluoride, 
 chloride, bromide and iodine of hydrogen are 
 as comparable among themselves as are the 
 sodium compounds. 
 
 Thus, in the halogens we have a natural 
 group or family of elements which are asso- 
 ciated together because of their chemical 
 
CHEMICAL ELEMENTS 103 
 
 relationship. The profound significance of 
 chemical relationship will be more and more 
 realized as the student becomes more and 
 more familiar with the natural history of the 
 individual elements. All the elements can be 
 grouped into families, the members of which 
 possess among themselves certain characters 
 in common. Such classification is the first 
 step towards the systematic study of the 
 elements as a whole. It is unnecessary now 
 to adduce other illustrations ; but this same 
 group will enable us to lay hold of another 
 principle of great importance the principle 
 of gradation of character. To the chemist, 
 this means gradation of chemical character. 
 We can, for instance, arrange the halogens 
 in a series in the order of their chemical 
 activity. Some notion of what is meant by 
 chemical activity has already been given 
 (p. 83). The order from this point of view is 
 (1) fluorine, (2) chlorine, (3) bromine, (4) iodine. 
 Fluorine is the most energetic, iodine the least ; 
 fluorine combines so energetically with other 
 elements, and forms such stable compounds, 
 that it is one of the most difficult elements to 
 isolate, the difficulty arising from the circum- 
 stance that it attacks all the ordinary materials 
 of which chemical vessels are made ; and its 
 isolation was only made possible by using 
 
104 CHEMISTRY 
 
 an apparatus constructed of the metal plat- 
 inum, which is so inert towards all the chemical 
 elements that it resists the attack of the 
 intensely active element (Moissan, 1886), 
 The relative activities can be illustrated by 
 the statement that fluorine decomposes chlor- 
 ides, bromides and iodides, turning out the 
 halogen from these compounds and forming 
 fluorides ; chlorine similarly displaces bromine 
 and iodine ; and bromine displaces iodine. 
 
 From considerations of this sort we are thus 
 enabled to form the graduated series given 
 above. Having formed this series on chemical 
 grounds, then, physical properties reveal them- 
 selves as also gradational. Take the boiling- 
 points for example : fluorine, 187; chlorine, 
 - 35 ; bromine, 59 ; iodine, 184. Or the 
 melting-points : fluorine, about 223 ; chlor- 
 ine, -- 102; bromine, - 7-3; iodine, 114. 
 Or the colour : fluorine, pale greenish yellow ; 
 chlorine, deeper greenish yellow ; bromine, 
 deep red ; iodine, violet vapour. From such 
 facts as these and other chemical families 
 also show gradation of characters it follows 
 that chemical relationships are expressive of 
 some deep-seated properties inherent in the 
 ultimate constitution of matter. What these 
 ultimate properties are may be brought to 
 light by future research ; in the present state 
 
CHEMICAL ELEMENTS 105 
 
 of knowledge we simply deal with them as 
 they are presented to us. The gradation, 
 as will be seen later, is also associated with 
 another fundamental attribute, mass, as 
 measured by weight ; but this belongs to 
 another chapter in which the classification of 
 the whole body of chemical elements has to be 
 dealt with. 
 
 It will be now seen that there is justifi- 
 cation for the statement made above that 
 the chemical classification of the elements 
 is of importance in Physics, because in a 
 graduated series the physical properties can 
 be inferred within certain broad limits from 
 the position of the element in the series. Thus, 
 knowing the boiling-points of fluorine and 
 bromine, it could be predicted that the boiling- 
 point of chlorine would lie somewhere between, 
 or, knowing the descending order of boiling- 
 points from iodine to chlorine, it could have 
 been foreseen that the boiling-point of fluorine, 
 when determined, would be found to be the 
 lowest of the series. Still more potent as a 
 scientific weapon would be any larger scheme 
 of classification which comprised all the 
 smaller natural groups of elements, and enabled 
 the prevision of properties to be made with a 
 greater degree of accuracy with a precision 
 measured by narrow instead of by broad 
 
106 CHEMISTRY 
 
 limits. That such a scheme exists will be 
 made clear in a later chapter. 
 
 Metals and Non-Metals. In the course of 
 the preceding section, a general grouping of 
 the elements has tacitly, if not explicitly, been 
 adopted. Several elements, such as iron, 
 copper, zinc, magnesium, sodium, calcium, 
 and aluminium, have been spoken of as 
 metals. It is customary to speak of the 
 elements as metallic or non-metallic as 
 metals or non-metals. Here again, we have 
 a classification in which chemical properties 
 are expressive of some correlated physical dis- 
 tinction between the groups. It is not possible 
 to draw a hard and fast line between the two 
 divisions ; but in a general way, and as the 
 result of even the most casual observation, 
 the reader cannot have failed to have asso- 
 ciated certain characters with the term metal. 
 Of course, many familiar metals are not ele- 
 mentary, but mixtures so-called alloys or, 
 to speak more accurately, either simple 
 mixtures or solid solutions of two or more 
 elementary metals. Brass, for instance, is 
 an alloy of copper and zinc ; bronze, gun- 
 metal, and bell-metal are alloys containing 
 the elements copper and tin, etc. ; the 
 " silver " of the coinage is not the element 
 silver, but an alloy of silver and copper ; 
 
METALS AND NON-METALS 107 
 
 the " gold " of our coinage is an alloy con- 
 taining 2 parts of copper in 24 parts of gold. 
 Pewter and soft solder are alloys containing 
 the elements tin and lead ; steel, one of the 
 materials most widely used in constructive 
 engineering work, is the metallic element 
 iron containing a small percentage of carbon. 
 The physical properties of the metals are 
 profoundly modified by this physical associa- 
 tion with other metallic or non-metallic 
 elements, so that many of these alloys are of 
 enormous industrial importance for purposes 
 for which the pure metals would be useless. 
 We are not specially concerned here with this 
 branch of applied chemistry it belongs to a 
 large and important subject which is studied 
 under the designations Metallurgy and Metal- 
 lography, and special works must be consulted 
 for detailed information. 
 
 From the chemical point of view, one of the 
 chief distinctions between the two groups is 
 to be found in the nature of the compounds 
 which they form with oxygen their oxides 
 (p. 71). Of the elements already referred to, 
 hydrogen, nitrogen, carbon, sulphur, and the 
 halogens are non-metals. Now, these elements 
 can all be made to combine with oxygen (which 
 is also a non-metal) sometimes directly, 
 as in the case of hydrogen, sulphur, carbon 
 
108 CHEMISTRY 
 
 and nitrogen, and sometimes indirectly, as 
 in the case of the halogens. All these oxides 
 of the non-metals, however formed, are, with 
 the exception of water, which is a neutral 
 compound, possessed of certain properties 
 which are described as acid. Their solutions 
 in water are sour to the taste, and redden 
 the blue vegetable colouring-matter litmus. 
 These acids, moreover, have the property 
 of combining with the metallic oxides by which 
 the said acids are more or less neutralized, 
 giving rise to the formation of those extremely 
 important compounds known by the general 
 name of salts, about which more remains to 
 be said. The oxides of the metals are known 
 as bases (in contradistinction to acids) ; so 
 that the general distinction between the 
 groups may be summarized by the statement 
 that the non-metals form acid oxides which 
 in aqueous solution are sour and redden 
 litmus, while the metals form basic oxides 
 which neutralize the acids to form salts and 
 which restore the blue colour of litmus. Of 
 course, the distinction is not absolute there 
 are some acid metallic oxides, but there are 
 no basic non-metallic oxides. Neither must 
 it be inferred that the property of conferring 
 acidity exclusively pertains to oxygen, since 
 the compounds of the halogens with hydrogen 
 
METALS AND NON-METALS 109 
 
 (p. 102) are also acids, although they contain no 
 oxygen. But in general terms the distinction 
 is sound ; it corresponds with the facts, and 
 is further borne out by another distinctive 
 character which will introduce a new set of 
 considerations. 
 
 Beginning, as before, with a simple case, 
 common salt may be taken as a type of 
 the class of compounds termed salts. It has 
 already been stated that this substance is a 
 compound of the metal sodium with the non- 
 metal chlorine. Although metallic and non- 
 metallic oxides combine as just stated to form 
 salts, sodium chloride is also a salt in the 
 chemical sense, so that there may be salts 
 with or without oxygen haloid-salts and 
 oxy-salts. The various methods by which 
 salts can be formed do not now enter into 
 consideration ; they can be produced by 
 other means than the combination of acids 
 and bases, but, however formed, they are 
 capable of being decomposed by electricity. 
 A salt, either in solution in water or in a state 
 of fusion by heat if it can be fused is 
 resolved by a current of electricity ; it is 
 said to be an electrolyte, and to undergo 
 electrolysis. The same is true, it may be said 
 parenthetically, of acids and bases ; these 
 also are electrolytes. Now, electrolysis is a 
 
110 CHEMISTRY 
 
 process of chemical decomposition by physical 
 agency (p. 92) ; and the products of electrolysis 
 are the original components of the salt, acid, 
 or base, or the secondary products of the 
 chemical change resulting from the interaction 
 of the primary products and the solvent. 
 Thus, if one of the products of electrolysis is 
 a metal which decomposes water at ordinary 
 temperatures, it is evident that we should not 
 get that metal by electrolysing an aqueous 
 solution of one of its salts, but the products 
 of the interaction of the metal and water, 
 one such product being hydrogen (p. 83). 
 Sodium is one of the metals which decompose 
 water at ordinary temperatures, so that, if 
 a current of electricity is passed through an 
 aqueous solution of common salt, the products 
 are chlorine and hydrogen not chlorine and 
 sodium. If dry fused sodium chloride is 
 electrolysed, the products are sodium and 
 chlorine. 
 
 The general result is that the primary 
 products may always be regarded as a metal 
 and the other component of the salt, whatever 
 that may be. If we are dealing with a solution 
 of an oxy-salt, the products are hydrogen 
 and oxygen. But the oxygen is in this case 
 a secondary product, since the salt is resolved 
 into metal (or hydrogen, if the latter decom- 
 
METALS AND NON-METALS 111 
 
 poses water), and the whole oxygen-containing 
 group, which latter group decomposes water 
 with the liberation of oxygen. The oxygen- 
 containing group is the electro-negative 
 equivalent of the halogen in sodium chloride. 
 The details of electrolytic decomposition and 
 their theoretical explanation are dealt with 
 in works on electro-chemistry. The main 
 fact which this introduction is intended to 
 bring out is that the metal and the halogen 
 or oxygen-containing group always travel in 
 opposite directions the one component of 
 the salt appears at one pole or electrode, and 
 the other component (or the secondary pro- 
 ducts) at the other electrode. And, in accord- 
 ance with the principles of electrical science, 
 the metal, which always appears at the 
 negative electrode (or kathode) is said to be 
 electro-positive, and the halogen or other 
 acid group, which is liberated at the positive 
 electrode (or anode), is said to be electro- 
 negative. So that there is a physical classifi- 
 cation of the elements thus made possible 
 according to the behaviour of their compounds 
 on electrolysis ; and in broad terms this 
 classification agrees with the division into 
 metals and non-metals, the former being as a 
 group electro-positive, and the latter electro- 
 negative. It must be noted that the non- 
 
112 CHEMISTRY 
 
 metal hydrogen is an exception, since it is 
 distinctly electro-positive. But this excep- 
 tion has a special significance, because, in 
 most of its chemical relationships, hydrogen 
 is more closely allied to the metals than is 
 any of the other non-metallic elements ; and 
 it would not be straining matters unduly even 
 now to class hydrogen with the metals if 
 only its chemical characters are taken into 
 consideration. 
 
 Out of the whole list of elements, eighteen 
 may be labelled non-metals. Here, again, there 
 is an indication that chemical character is 
 expressive of something profound and inher- 
 ently constitutional in matter, because, as 
 groups, the metals and the non-metals differ 
 in many well-known characters. Thus, the 
 metals are opaque (excepting when in very 
 thin films) and, when in a massive state, as 
 distinguished from a pulverulent condition, 
 possessed of " lustre " a property well 
 exemplified by the familiar appearance of 
 polished silver, gold, or copper. Among the 
 non-metals, iodine is the most pronounced 
 exception, as its crystals are possessed of 
 distinctly metallic lustre. In contradistinc- 
 tion to the opacity of the metals, ten of the 
 non-metals are transparent gases at ordinary 
 temperatures. The metals as a class are, 
 
METALS AND NON-METALS 113 
 
 in varying degrees, conductors of heat and 
 of electricity, while the non-metals are bad 
 conductors or non-conductors. Among the 
 solid non-metallic elements, such metallic 
 properties as malleability or tenacity are not 
 met with. 
 
 With respect to the distribution of the ele- 
 ments, it is of interest to note that more than 
 three-fourths of the accessible crust of this 
 globe upon which we live is made up of the two 
 non-metals, oxygen and silicon, about one-half 
 being oxygen. Nothing affords more striking 
 evidence of the marvel of chemical change 
 than the contemplation of this geo-chemical 
 fact, that the superficial " solidity " of the 
 earth is due to the predominance of those 
 mineral constituents into the composition of 
 which gaseous oxygen and the non-metal 
 silicon enter to a preponderating extent. 
 The whole crust of the earth with which 
 geology deals is composed to the extent of 
 more than 99 per cent, of only about twenty 
 out of the eighty-two elements. This will give 
 an idea of the rarity of some of the materials 
 which the chemist has had to deal with. 
 Out of the twenty elements which predominate 
 in the earth's crust, four non-metals and seven 
 metals exist in quantities which have been 
 estimated to constitute more than 99 per cent. 
 
114 CHEMISTRY 
 
 of the whole crust in the proportions by weight 
 given below (F. W. Clarke) : 
 
 Oxygen,* 49-98 ; Silicon,* 25-3 ; Alum- 
 inium^ 7.26; Iron,f 5-08; Calcium,f 3-51; 
 Magnesium,")* 2-5 ; Sodium, "j- 2-28 ; Potas- 
 sium^ 2-23; Hydrogen,* 0-94; Titanium, f 
 0-3 ; Carbon,* 0-21. 
 
 The actual mode of combination of the 
 chemical elements as found in nature, i.e., 
 the chemical composition of the materials 
 composing the crust of the globe, is dealt 
 with in works on Mineralogy. 
 
 * Non-metals. f Motala. 
 
CHAPTER V 
 
 CHEMICAL EQUIVALENCE ELECTRO-CHEMICAL 
 EQUIVALENCE MULTIPLE AND RECIPRO- 
 CAL EQUIVALENCE THE ATOMIC THEORY 
 
 Chemical Equivalence. From the principles 
 of the definiteness of chemical change (p. 69) 
 and the Conservation of Mass (p. 76), it is 
 but a short step to the principle of chemical 
 equivalence. As with all broad scientific 
 generalizations, the conception which the 
 reader is now asked to grasp is simple enough 
 as an abstract notion, although some difficulty 
 may at first be experienced in mastering it. 
 It is, perhaps, needless to repeat that by 
 laboratory work the principle has been 
 deduced, and by quantitative manipulation 
 only can its significance be fully realized. 
 It is instructive as a chapter in the history of 
 Chemistry to read how nearly some of the 
 earlier investigators approached without actu- 
 ally reaching the principle of equivalence. 
 The historical treatment, however, cannot be 
 attempted here it must suffice to ascertain 
 
 115 
 
116 CHEMISTRY 
 
 the meaning of the term as used now, without 
 going over the steps by which the present 
 position has been reached. Sufficient material 
 in the way of facts and illustrations has been 
 given to enable the subject to be dealt with 
 in a general way. 
 
 In the first place, it must be noted that the 
 terms " equivalence " or " equivalent " are 
 expressive of facts only, and are independent 
 of any theory or explanation. The facts in 
 this case are those ascertained experimentally 
 and enunciated in the previous chapters the 
 idiosyncrasies of the chemical elements, by 
 virtue of which each element has its own 
 special faculty of entering into partnership 
 with other elements only in certain fixed 
 proportions by weight. Out of these facts, 
 there has, however, been developed one of the 
 most illuminating theories that has been 
 introduced into our science, so that the 
 importance of equivalence has always to be 
 realized by the student at a very early 
 stage. In approaching this subject, it must 
 be pointed out that we are not dealing 
 with the likes and dislikes of the elements 
 why the elements show preferences such, 
 for example, as that of iron for oxygen while 
 this same metal refuses to combine under 
 similar conditions with nitrogen (p. 37). In 
 
EQUIVALENCE 117 
 
 considering the principle of equivalence we 
 have nothing to do with the question why in 
 particular cases chemical union is possible or 
 impossible, but only with the quantitative 
 relationship between what we may call the 
 contracting parties, i.e., the elements and 
 compounds which can and do enter into 
 combination directly, or which can be made 
 to combine by indirect methods. 
 
 From the facts that combination does take 
 place in fixed proportions by weight, and that 
 the product of chemical union is homogeneous 
 (Chap. II.), it follows that any such product, 
 or, in other words, any particular chemical 
 compound, however and whenever produced, 
 always consists of the same elements com- 
 bined in the same proportions by weight. 
 This appears almost a truism now, because 
 it is obvious that, if there were any latitude or 
 variability in the ratio in which two or more 
 elements combined to form a compound, 
 there would be no homogeneity we should 
 have a mixture of two or more compounds. 
 The principle is generally formulated as the 
 law of Constancy of Composition ; and although 
 it may appear obvious now, it was at one time 
 the subject of much controversy, arid this 
 chapter of history is well worthy of considera- 
 tion as an illustration of the method by which 
 
118 CHEMISTRY 
 
 troth is wrong from Nature by rigid experi- 
 ment and logical reasoning. From the point 
 of view of this law, it will be recognized that 
 a chemical compound may be looked upon 
 as being as true and as definite an individual 
 form of matter as a chemical element. And 
 it will be further realized that, as the com- 
 plexity of chemical compounds increases, it 
 may become more and more difficult to prove 
 the individuality in particular cases. But 
 the law is not thereby violated there may be 
 apparent indefiniteness, but this is the result 
 of the imperfection of our practical methods, 
 and the modern chemist still has faith in the 
 principle. 
 
 For the elucidation of the principle of 
 equivalence, a few simple cases may be 
 considered. Oxygen and the halogens all 
 form compounds with hydrogen, directly 
 or indirectly. Oxygen and hydrogen explode 
 when ignited to form water; fluorine and 
 hydrogen explode spontaneously when mixed ; 
 chlorine and hydrogen explode when heated, 
 or on exposure to bright light ; and bromine 
 and iodine can also be combined with hydrogen 
 directly or indirectly. We are not now 
 concerned with methods, bat with products. 
 The proportions by weight of the elements 
 contained in these compounds are known 
 
EQUIVALENCE 119 
 
 with great accuracy, both from analytical and 
 synthetical evidence. In round numbers, 
 and referring all the weights to the standard 
 of one part of hydrogen, water contains 8 
 parts of oxygen, hydrogen fluoride 19 parts of 
 fluorine, hydrogen chloride 35-2 parts of 
 chlorine, hydrogen bromide 79-4 parts of 
 bromine, and hydrogen iodide 126 parts of 
 iodine. All these different weights of the 
 respective elements, therefore, satisfy the 
 same weight of hydrogen they are equivalent, 
 or equal in value, from the point of view of 
 combining with a unit weight of hydrogen. 
 Or take the case of those metals which 
 decompose water (p. 83). If this process is 
 followed quantitatively, it is found that, for 
 every unit weight of hydrogen liberated, there 
 are used up 12-2 parts of magnesium, 21 parts 
 of iron, 23 parts of sodium, and 32-7 parts 
 of zinc. Here, again, there is equivalence 
 these various weights represent quantities of 
 different elementary substances which are of 
 the same chemical value as measured by 
 their capacity for displacing the same weight 
 of hydrogen. Moreover, since this same 
 weight of hydrogen is equivalent to 8 parts of 
 oxygen, these equivalent weights of the 
 metals are also equivalent to 8 parts of 
 oxygen. And, since the compounds formed 
 
120 CHEMISTRY 
 
 by the action of water upon these metals 
 under the conditions specified are oxides, 
 or, as in the case of sodium, may be looked 
 upon as arising from the combination of the 
 oxide with water, it follows that the ratio 
 between the weights of the metal and oxygen 
 can also be expressed numerically on the 
 oxygen scale : Magnesium oxide, 12-2 : 8 ; 
 iron oxide (" scale "), 21 : 8 ; sodium oxide, 
 23 : 8 ; zinc oxide, 32-7 : 8. And so, by 
 ascertaining the proportion of metal to oxygen 
 in other oxides, the equivalents of other 
 metals with reference to oxygen and (by 
 implication) to hydrogen could be found. 
 
 The doctrine of equivalence is, therefore, 
 nothing more than the numerical expression 
 of the definiteness of chemical change (Chap. 
 III.). Perhaps it would be more correct, if 
 we were dealing with the subject in historical 
 sequence, to say that the definiteness of 
 chemical change is the expression of the 
 principle of equivalence, since the definiteness 
 was established by quantitative studies of the 
 kind illustrated above. But the order of 
 statement is immaterial so long as the principle 
 is understood. The " equivalent " is, thus, 
 in abstract terms, a number expressing the 
 parts by weight in which an element combines 
 with or displaces some other element ; and 
 
EQUIVALENCE 121 
 
 since hydrogen has the lowest equivalent, 
 it will be convenient at this stage to refer 
 these weights to the hydrogen standard. 
 It is obvious that the conception as thus 
 formulated is an idealized one, because 
 there are many elements which neither 
 combine with nor displace hydrogen ; so 
 that in such cases the equivalent can only be 
 determined indirectly by reference to some 
 other element of known equivalence. It will 
 be shown, also, that the conception is not 
 restricted to elements : since compounds are 
 formed by the combination of elements in 
 equivalent weights, it follows that if com- 
 pounds combine among themselves such, for 
 example, as acid oxides and basic oxides 
 (p. 108) there must also be equivalence in 
 such cases. 
 
 Electro-chemical Equivalence. An electric 
 current from a chemical point of view, and 
 not regarded simply as a stream of electricity 
 flowing through a metallic conductor, is a 
 decomposing agent (p. 109). In accordance 
 with the doctrine of the Conservation of 
 Energy, a given quantity of electricity has 
 its equivalent in terms of chemical work. In 
 the case of electrolysis, the actual weights of 
 elements liberated in a given time by a 
 measured quantity of electricity measure the 
 
122 CHEMISTRY 
 
 chemical work done, so that the determination 
 of the weight of, let us say, a metal deposited 
 electrolytically in a given time is also a 
 measure of the electrical energy used up. 
 This is of importance in Physics as furnishing 
 a chemical method for the measurement of 
 electric quantity ; and the electro-chemical 
 equivalents of some of the metals such, e.g., 
 as silver and copper, have from this point of 
 view been determined with extreme accuracy. 
 But the absolute weight of an element liberated 
 in a given time by a measured current acquires 
 also a chemical significance when we pass 
 from the absolute to the relative when 
 the relative quantities of different elements 
 liberated in the same time by the same 
 current are considered. It is immaterial 
 in this case how the electricity is made to do 
 its work ; the current may decompose a salt, 
 or an acid, or a base in solution or in the dry 
 fused state (p. 109). Fused common salt, for 
 example, yields 23 parts of sodium and 35-2 
 parts of chlorine ; a solution in water of 
 hydrogen chloride gives for 1 part of hydrogen 
 35-2 parts of chlorine ; water, made to yield 
 hydrogen and oxygen indirectly by an electro- 
 lysable acid in solution (p. 109), gives in round 
 numbers 8 parts of oxygen for 1 part of hydro- 
 gen ; the volume ratio 2 of hydrogen : 1 of 
 
EQUIVALENCE 123 
 
 oxygen is revealed electrolytically, as it is 
 proved by chemical synthesis (p. 88). It is 
 unnecessary to multiply examples at this stage. 
 The point of supreme importance brought out 
 is that these relative weights of electrically 
 liberated elements are identical with the 
 chemical equivalents, a discovery due to 
 Faraday (1833). The establishment of this 
 law has led to some of the most fundamental 
 modern developments both of Physics and of 
 Chemistry ; but its adequate discussion cannot 
 be attempted within the limits of this work. 
 Our consideration must be restricted to its 
 chemical significance in the narrow sense. 
 
 Multiple and Reciprocal Equivalence. The 
 notion of equivalence would be a very 
 simple one if it could be stated in general 
 terms that the numbers representing the 
 equivalents of, let us say, the elements 
 A, B, C, D were always representative of 
 reciprocal equivalence that whenever any 
 pair of these elements combined to form a 
 binary compound (i.e., a compound containing 
 two elements) the latter could always be 
 formulated as AB, AC, AD, BC, BD, CD, etc., 
 in which the juxtaposition of letters may 
 be taken as indicating chemical combination, 
 and the letters as representing equivalent 
 weights. But this statement would be too 
 
124 CHEMISTRY 
 
 sweeping it is not generally true, but only 
 partially true. The principle of reciprocity 
 holds good for the simple cases referred to in 
 the preceding sections ; but, when the whole 
 of the facts are taken into consideration, 
 complications arise owing to the circumstance 
 that many elements can combine with each 
 other in more than one proportion, each 
 compound so formed being a perfectly definite 
 chemical individual. One example of this 
 has already been adduced in the case of iron, 
 which can form three different oxides (p. 71). 
 In all three oxides, the equivalent may be 
 considered to be the weight of iron which 
 combines with one equivalent, i.e., 8 parts of 
 oxygen. On this standard, the equivalent of 
 iron in " scale " is 21 ; in rust (which contains 
 70 per cent, of iron, and therefore 30 per cent, 
 of oxygen), the equivalent of iron is 18 f ; 
 and in the other oxide, which contains 77-8 
 per cent, of iron and 22-2 per cent, of oxygen, 
 iron has the equivalent 28. Iron, therefore, 
 in relation to oxygen has not one equivalent, 
 but three equivalents. 
 
 Any number of cases could be adduced ; 
 and, since the principle of multiple equivalence 
 is of fundamental importance, a few other 
 examples may be given. Twenty-three parts 
 of sodium displace 1 part of hydrogen from 
 
EQUIVALENCE 125 
 
 water, and combine with 8 parts of oxygen 
 (p. 110). The equivalent of sodium with refer- 
 ence to oxygen ( = 8) is in this case 23. But 
 when sodium is heated to about 300 in pure 
 dry air, it forms another oxide in which 11 J 
 parts of sodium are combined with 8 parts 
 of oxygen. This element has, therefore, 
 with reference to oxygen two equivalents, 
 11 1 or 23. The oxide of copper formed when 
 copper is heated in oxygen (p. 84) contains 
 31 1 parts of copper to 8 of oxygen. By 
 chemical methods it is possible to prepare 
 another oxide which contains 63 parts of 
 copper to 8 of oxygen ; so that the equivalent 
 of copper may be 31 1 or 63. Carbon burns in 
 oxygen to form that dioxide which was 
 referred to as a constituent of the atmosphere 
 (p. 36) ; this gaseous compound of carbon 
 contains 3 parts of carbon and 8 parts of 
 oxygen. When carbon dioxide is heated in 
 contact with more carbon, another (gaseous) 
 oxide is formed which contains 6 parts of 
 carbon to 8 of oxygen. Moreover, when 
 carbon is intensely heated by an electric arc 
 in an atmosphere of hydrogen, it combines 
 with that element to form a gas a compound 
 of carbon and hydrogen, and therefore called 
 a hydrocarbon which is known as acetylene. 
 In this gas 1 part of hydrogen is combined 
 
126 CHEMISTRY 
 
 with 12 parts of carbon, so that this last 
 element has the three equivalents, 3, 6, 
 and 12. 
 
 It is clear, therefore, that the conception 
 of equivalence is not so simple as might be 
 imagined on first approaching the subject. 
 The numbers representing the equivalents are 
 not absolute, but relative ; an element A 
 may have towards some other element B a per- 
 fectly fixed equivalent, but towards another 
 element C, it may have the same equivalent 
 that it has with reference to B and likewise 
 another or other equivalents. A may have, 
 also, a certain equivalent with reference to 
 B or C under one set of conditions of com- 
 bination, and other equivalents under other 
 conditions of combination. All this has 
 been established by quantitative analysis and 
 synthesis by determining with the greatest 
 possible accuracy the relative weights of the 
 component elements contained in chemical 
 compounds, or the relative weights of the 
 elements which enter into combination in 
 synthetical operations. That these numerical 
 results are expressive of some underlying 
 physical reality is shown, also, by the co- 
 incidence of the chemical equivalents with the 
 electro-chemical equivalents; because when 
 an element has two equivalents, i.e., forms 
 
EQUIVALENCE 127 
 
 two distinct compounds with another element, 
 it has been proved to possess two electro- 
 chemical equivalents in all those cases where 
 the two electrolysable compounds could be 
 practically dealt with. 
 
 But, although the conception of equivalence 
 is complicated by the multiple equivalence of 
 many of the elements, a glance at the numbers 
 given in the preceding examples will show 
 that there is simplicity in the apparent 
 complexity. Thus, the equivalent of carbon 
 is 3, 6, or 12 ; of copper 31 \ or 63 ; of sodium 
 11 \ or 23, and so forth. These numbers stand 
 in the relationship of simple multiples, so that 
 the notion of equivalence must be widened 
 by making allowance for the ratios of the 
 combining weights being not only A : B, but 
 also some simple multiple of A or B, or of both, 
 such as 2 A : B, A : 2 B, 2 A : 3 B and so forth. 
 This is what is known in Chemistry as the 
 law of Multiple Proportions ; and in this 
 sense only is reciprocity of equivalence or 
 combining weight recognizable. Thus, the 
 weights A and B which combine with a certain 
 weight, C, of a third element need not neces- 
 sarily be the same weights as those which 
 exist in the compound AB, but may be 
 some simple multiple such as 2 A with C, 3 B 
 with C, etc. 
 
128 CHEMISTRY 
 
 One illustration should give a clear notion 
 of this principle of Reciprocal Equivalence. 
 In one of the oxides of iron, the equivalent 
 weights are iron : oxygen = 28 : 8. In iron 
 sulphide (p. 72), this same weight of iron is 
 combined with 16 parts of sulphur. If there 
 were general reciprocity of equivalence, it 
 would be said, therefore, that 8 parts of 
 oxygen should combine with 16 parts of 
 sulphur ; but, in fact, this is not the case. 
 Sulphur burns readily in oxygen to form an 
 oxide the so-called sulphurous acid gas, 
 or sulphur dioxide which is the oxide 
 containing the smallest proportion of oxygen, 
 viz., 8 parts of oxygen to 8 of sulphur. 
 No lower oxide is known, so that 8 parts of 
 oxygen and 16 of sulphur are not in this case 
 equivalent ; in order to express the actual 
 facts, we have to take two equivalents, i.e., 
 8x2 parts of oxygen. Moreover, there is 
 another oxide of sulphur in which 16 parts 
 of sulphur are combined with 24 parts, i.e., 
 8x3 parts of oxygen. The doctrine of 
 equivalence is, therefore, only in harmony 
 with the facts when sufficient elasticity is 
 given to the conception to allow of the 
 inclusion of cases of multiple equivalence and 
 of multiple reciprocal equivalence ; and the 
 main point brought out for assimilation 
 
THE ATOMIC THEORY 129 
 
 by the reader is the simple and integral 
 character of the numbers expressing this 
 multiplicity. 
 
 The Atomic Theory. The doctrine of equi- 
 valence, even in its most elastic form, is still 
 nothing more than a quantitative expression 
 of the facts of chemical composition. Of 
 course, there must be some underlying prin- 
 ciple some explanation of this simplicity 
 of multiplicity. Such explanation was first 
 definitely formulated in 1807-08 by John 
 Dalton, who not only discovered the law of 
 Multiple Proportions, but suggested a theory, 
 the introduction of which marks one of the 
 greatest epochs in the history of Chemistry. 
 The reason why combination takes place in 
 definite proportions by weight, and why, 
 when the same element has more than one 
 equivalent, the principle of integral multiples 
 is maintained is, according to Dalton's 
 explanation, because the combination is 
 between the ultimate particles of which 
 elementary matter is composed. This is the 
 notion of the discontinuity or discreteness of 
 matter referred to in a former chapter (p. 57), 
 and now re-introduced in more exact terms. 
 The " particles " of which matter is composed 
 whatever its state of aggregation are, 
 from Dalton's point of view, ultimate in the 
 
130 CHEMISTRY 
 
 sense of being indivisible. For this reason 
 ho called them atoms. 
 
 The defmiteness of combination by weight 
 is attributable to the weights of the atoms 
 those elements of the elements. The atomic 
 theory as thus introduced into Chemistry was 
 the revival of very ancient views concerning 
 the constitution of matter ; but these 
 older speculations had failed to influence the 
 progress of science until given a definite 
 quantitative meaning by Dalton. From the 
 point of view of this theory, one atom 
 having a certain weight can combine with 
 another atom having also a certain weight, 
 or one atom can combine with two, three, 
 etc., other atoms ; or again, two atoms 
 can combine with three, three with four, 
 and so forth. Any degree of complexity 
 may exist ; but, the atom being (by hypo- 
 thesis) the ultimate unit, the passage from 
 one compound AB to another containing the 
 same elements in different proportions cannot 
 take place but by whole multiples ; fractional 
 parts of A or B are inadmissible. It will be 
 noted that at this stage the theory deals only 
 with one fundamental attribute of matter, 
 mass, as measured by weight. But, instead 
 of dealing with this attribute in the gross as a 
 general physical property of matter irrespec- 
 
THE ATOMIC THEORY 131 
 
 live of individuality, the conception of the 
 indivisible atom brings the sciences of Physics 
 and Chemistry into an indissoluble partner- 
 ship, for the atom of each element is, from 
 this standpoint, a particle of matter which is 
 indivisible, and which has a certain fixed 
 weight, which weight is specific for each 
 kind of atom, and is, therefore, a physico- 
 chemical attribute. 
 
 The atomic theory as thus broadly outlined 
 is, therefore, based entirely upon the observed 
 facts respecting combination by weight. It 
 postulates homogeneity among the atoms in 
 the sense that the atoms of each element are 
 assumed to be all alike all cast in one mould, 
 all possessed of the same weight, size, shape, 
 and structure. That is why our imaginary 
 being (p. 51), was supposed to be able to 
 follow the movements of the different " par- 
 ticles " in a mixture of gases. Of course, 
 the atoms must on this view have absolute 
 weights, definite dimensions, shapes, etc. 
 In its extreme form, the theory has been 
 pushed so far as to claim that the atom is 
 the ultimate component of the material 
 universe imperishable and eternal. The 
 reader must, however, discriminate between 
 ascertained concordance of facts with theory 
 and speculative developments of theory, 
 
132 CHEMISTRY 
 
 however plausible in principle or stimulating 
 in prompting further research. For the 
 general purposes of chemical study, the atom 
 may be regarded as indivisible. From the 
 purely chemical point of view, the absolute 
 weight is immaterial ; it answers all practical 
 purposes to deal with the weights, as did 
 Dalton, as relative. By physical methods, 
 it is possible to ascertain approximately the 
 absolute weights and dimensions of atoms, 
 but this part of the subject is beyond our 
 scope ; suffice it to say that the chemical 
 atom is a particle of matter infinitesimally 
 small of a minuteness of size and weight 
 which, even if expressed numerically, is 
 beyond human conception. 
 
 The discussion of the truth of the atomic 
 theory whether the atom is a physical entity 
 belongs to the domain of Philosophy ; 
 we, as chemists, have only to deal with the 
 doctrine in so far as it is in accordance with all 
 the evidence which our science can produce, 
 and with the verification of deductions drawn 
 from it. It accounts for the definiteness of 
 chemical combination by weight, and, as will 
 appear later, it correlates and explains large 
 bodies of facts which have been accumulated 
 since its introduction by Dalton. It is the 
 " head stone of the corner " in theoretical 
 
THE ATOMIC THEORY 133 
 
 Chemistry ; and yet, as it stands, we all 
 realize that it is incomplete that vast 
 developments may yet be looked for from 
 both the physical and the chemical side. The 
 mysteries of chemical change, the absolute 
 transformation resulting from chemical com- 
 bination, the likes and dislikes of the elements, 
 their activities and inactivities, yet await 
 explanation in terms of the atomic theory. 
 The conception of the atom has simply shifted 
 the responsibility of individual idiosyncrasies 
 from matter in the gross to matter in detail 
 from the bulk to the ultimate particle. 
 
 The possibility of the atom being itself 
 resolvable did not at first enter into considera- 
 tion ; it was assumed to be the final limit of 
 the divisibility of matter. But the early 
 crude notions concerning the nature of the 
 atom have gradually undergone development, 
 chiefly as the result of the study of the action 
 of electricity upon matter in the gaseous state 
 (p. 19) inaugurated in this country by Crookes 
 and J. J. Thomson. Physical modes of deal- 
 ing with matter undreamt of in the time 
 of Dalton have led to the conclusion that 
 the atom is not a structureless particle, but 
 a complex, internally balanced mechanism, 
 capable under certain conditions of being 
 broken down into its smaller component 
 
134 CHEMISTRY 
 
 parts ; and it is accepted by many physicists 
 and chemists as a plausible hypothesis that 
 the atom is built up of particles of electricity 
 (" electrons "). On this view, the conception 
 of the material nature of the atom would have 
 to be recast in terms of energy ; and it is 
 for this reason that the possibility of matter 
 coming into or going out of existence as the 
 result of chemical change was considered 
 worthy of being re-tested (p. 79). So far there 
 is no direct evidence in support of this view 
 the most carefully conducted experiments 
 have only served to confirm the doctrine of 
 the Conservation of Mass. 
 
 The history of the development of the 
 atomic theory will furnish fascinating reading 
 for a future generation experiments and 
 hypotheses are now being pushed forward with 
 a rapidity and with a boldness which entitle 
 the chemist to say that the time is not yet 
 ripe for attempting to complete the story. 
 But it is along these lines the study of the 
 inner mechanism of the atom that Physics 
 and Chemistry are moving towards their 
 point of convergence. No single attribute 
 assigned to the atom during the early period 
 of the theory has remained unchallenged. 
 The idea of absolute uniformity of weight 
 has been called in question ; and it has been 
 
THE ATOMIC THEORY 135 
 
 suggested that the property is an average one 
 that there may be a certain range of varia- 
 tion in weight among the atoms of an element, 
 and that the apparent constancy in weight 
 is the statistical result of the practical 
 necessity of dealing with myriads in our 
 laboratory operations. Of such variability, 
 however, we have at present no experimental 
 evidence. 
 
 Of the shapes and textures of the atoms we 
 know nothing with certainty. The questions 
 whether they are hard and rigid, or elastic, 
 compressible, and deformable have been 
 discussed from time to time from the physical 
 side ; but no conclusion directly affecting the 
 chemical requirements of the theory has as 
 yet been arrived at. The notion of an 
 imperishable atom has also been challenged, 
 first from the purely theoretical side regarding 
 the atom as a mechanism capable of reaching 
 a point of internal instability when a certain 
 degree of complexity has been attained, and 
 then from the observational side owing to 
 the discovery that one of the elements, 
 radium, is capable of giving rise to another 
 element, helium, by a process of spontane- 
 ous disintegration (Rutherford; Ramsay and 
 Soddy, 1903). Here we are, therefore, 
 brought again directly into the domain of 
 
136 CHEMISTRY 
 
 Chemistry ; and the alchemical dream of 
 " transmutation " (p. 26) has, at least in this 
 case, been realized. But the consideration of 
 this subject belongs to a late chapter in the 
 history of Chemistry, and the reader must 
 accept this brief statement by way of pre- 
 liminary introduction to the newly developed 
 science of Radioactivity. (See, for instance, 
 the volume on " Matter and Energy," by 
 Mr. F. Soddy, in this Library.) 
 
CHAPTER VI 
 
 SYMBOLS AND NOTATION ATOMS AND MOLE- 
 CULES ATOMIC AND MOLECULAR WEIGHTS 
 
 THE DEFINITENESS OF CHEMICAL COM- 
 BINATION BY VOLUME THE HYPOTHESIS 
 OF AVOGADRO 
 
 Symbols and Notation. The transition from 
 the crude and primitive notions concerning 
 the chemical atom to the current conception 
 and use of the atomic theory represents a 
 period of more than a century. It was a 
 period teeming with interest from the histori- 
 cal point of view ; but we must perforce pass 
 over the developmental phases and deal 
 with the science of Chemistry as it stands 
 to-day in the light of the theory. And, 
 before any substantial progress can be made, 
 it is necessary that the reader should master 
 at least the rudiments of chemical language 
 they are simple, easy to understand, and 
 replete with meaning. Our alphabet is no 
 more than a set of shorthand symbols for the 
 atoms of the chemical elements. Symbols 
 137 
 
138 CHEMISTRY 
 
 in the form of letters of the alphabet have 
 already been made use of in the last chapter 
 in order to give general expression to the 
 conception of equivalence ; all that is now 
 required of the reader is to attach a specific 
 meaning to these symbols. The chemical 
 symbol as at present used is the initial lettei 
 or letters in the name of the element, and thia 
 symbol stands for one atom of the element. 
 There is no more mystery about this than in 
 writing W. for Walter and Wm. for William ; 
 the only difference is that there may be 
 multitudes of Williams and Walters, and that 
 each individual is distinguished by the addi- 
 tion of one or more names. But we require 
 no binomial system in Chemistry, because 
 each symbol stands for one element only, 
 and for one definite weight of that element 
 the weight of the atom with reference to 
 some common standard. 
 
 The chemical elements have not been 
 named on any coherent system ; some of the 
 names are survivals from a remote past, while 
 those isolated in later times have received 
 names indicating either some characteristic 
 property, the natural source, or the country 
 in which the isolation was effected. All that 
 can be said is that the names of the more 
 recently discovered metals have been given the 
 
SYMBOLS AND NOTATION 139 
 
 terminal syllable um. Each name (Latinized 
 in a majority of cases) is contracted into 
 an initial letter, or, if several elements 
 have the same initials, into two letters. All 
 the elements named in the previous chapters 
 could have been made to tell their story more 
 concisely if symbols had been used. A few 
 examples will serve to illustrate the use of 
 the chemical alphabet. Thus H, O, N, F, 
 I stand respectively for one atom of the 
 non-metals hydrogen, oxygen, nitrogen, fluor- 
 ine, and iodine. We cannot use C for chlorine, 
 because there are several other elements 
 beginning with C, so we write Cl for chlorine, 
 C for carbon, Cu for copper (cuprum), Ca 
 for calcium, and so forth. We use Br for 
 bromine because B stands for boron ; Bi for 
 bismuth, and Ba for barium. S stands for 
 sulphur, Si for silicon, and Se for selenium. 
 Iron is Fe (ferrum), mercury or quicksilver 
 is Hg (hydrargyrum), silver is Ag (argentum), 
 gold is Au (aurum), sodium is Na (natrium), 
 and so forth. The association of the symbol 
 with the name of the element is only a matter 
 of practice, and no great strain of memory 
 is required. A complete list is given for 
 reference at the end of this volume. 
 
 Having mastered the alphabet, however, 
 the next step is to learn how to use it ho\v 
 
140 CHEMISTRY 
 
 to read a chemical formula. Here, again, the 
 principle has already been made use of in 
 order to illustrate equivalence ; the only 
 difficulty that the uninitiated reader is likely 
 to meet is the confusion arising from the mix- 
 ing up of chemical symbols with algebraical 
 expressions, because, in chemical notation, 
 the symbols are used in a different way. Thus, 
 in algebra AB means A x B, but the chemical 
 formula AB stands for a chemical compound 
 in which one atom of A is combined with 
 one atom of B, each atom having its specific 
 weight. So ABC in Chemistry stands for a 
 compound containing one atom of each of the 
 elements A, B, and C, and not, as in algebra, 
 for A x B x C. In algebra the numerical 
 coefficient multiplies any literal symbols that 
 follow; thus2A=2 x A; 2AB=2 x A x B. 
 In Chemistry the number of atoms is 
 multiplied in the same way, so that 2A 
 means two atoms of A ; but in the case of 
 compounds the whole weight of the compound 
 is multiplied by the numerical coefficient. 
 Thus, 2AB in chemical language means twice 
 as much of the compound as is represented 
 by AB, SAB three times as much, and so 
 forth. It is as though we wrote the equation 
 2AB = 2( A -f- B) in which the + sign is made 
 to stand for chemical combination. The 
 
SYMBOLS AND NOTATION 141 
 
 number of individual atoms in compounds 
 is indicated by a small number placed below 
 the symbol, to avoid confusion with the 
 algebraical system of indicating the " power " 
 by a small number placed above the symbol. 
 Thus A 2 B 3 means AxAxBxBxB; 
 A 2 B 3 means a compound containing two 
 atoms of A combined with three atoms of B. 
 A general chemical formula for a compound 
 containing three elements would be A x B y C z ; 
 and the multiplication of the whole weight 
 represented by this formula would be 
 2A x B y C z , or 3A x B y C z , or n A x B y C z . 
 
 Atoms and Molecules. The congeries of 
 atoms which elements form when they com- 
 bine with themselves or with the atoms of 
 other elements are termed molecules. This 
 is a point of fundamental importance in 
 modern chemical language, because it is 
 impossible on this system to have an atom 
 of a compound, although we might conceive 
 the existence of an atom of an element. The 
 justification for this distinction will be given 
 immediately ; but we are now concerned only 
 with notation, and it will be sufficient to point 
 out to the student of the history of Chemistry 
 that immense confusion was caused in the 
 early period of the atomic theory through the 
 failure to recognize that atoms and molecules 
 
142 CHEMISTRY 
 
 were different things. The atom is the 
 smallest weight of an element that enters 
 into the composition of the molecule of a 
 compound, so that while A -f B stands for 
 one atom of A mixed (not combined) with one 
 atom of B, AB stands for one molecule of the 
 resulting compound, 2 AB for two molecules, 
 and so forth. And, since an atom can com 
 bine with one or more similar atoms to form 
 a molecule of the element, A 3 stands for 
 one diatomic molecule of A, A 3 for one 
 triatomic molecule ; and 2A 2 , 3A 2 , 2A 3 , etc., 
 stand in these cases for two, three, and 
 two of the molecules represented by A 2 
 and A 3 respectively. Moreover, the molecule 
 of a compound may in some cases combine 
 with one or more similar molecules to form 
 compounds of double, treble, etc., molecular 
 weight ; and this state of affairs is conveni- 
 ently represented by such formulae as (AB) 2 , 
 (AB) 3 , etc., which therefore have a meaning 
 different from that expressed by 2AB and 
 SAB, which indicate two and three distinct 
 molecules of AB, whereas (AB) 2 , (AB) 3 , mean 
 A 2 B 2 , A 3 B 3 , i.e., one molecule in each case. 
 
 It may be of interest to point out that the 
 introduction of the atomic theory and the 
 clear recognition of the distinction between 
 atoms and molecules has done more than any 
 
ATOMS AND MOLECULES 143 
 
 other conception to bring the kindred sciences 
 of Chemistry and Physics into intimate associa- 
 tion. In the direction of both the higher and 
 the lower limits of divisibility of matter, the 
 chemist and the physicist join hands, since 
 physical properties are associated with mole- 
 cular constitution and chemical properties 
 with the chemical atom ; while the newer 
 researches into the constitution of the atom 
 are laying a yet deeper foundation for both 
 sciences. 
 
 Atomic and Molecular Weights. Since a 
 molecule is composed of atoms, its weight 
 as a concrete particle referred to the same 
 standard as that to which are referred the 
 weights of the atoms must be the sum of the 
 weights of the atoms of which it is composed. 
 But, in view of the principle of multiple 
 equivalence, it is evident that the determina- 
 tion, either by analysis or synthesis, of the 
 relative proportions by weight of the elements 
 contained in a compound does not decide the 
 question of the actual weight either of the 
 atoms or of the molecule. Let us consider 
 one of the previous cases. One part of hydro- 
 gen combines with 8 parts of oxygen to form 
 9 parts of water. The simplest representa- 
 tion of this fact is the symbol HO, which now 
 stands for a molecule of water with a molecular 
 
144 CHEMISTRY 
 
 weight of 9 referred to hydrogen. Since 23 
 parts of sodium displace and are therefore 
 equivalent to 1 part of hydrogen (p. 124), 
 the corresponding oxide would be NaO with 
 the molecular weight 23 + 8 = 31. In sodi- 
 um peroxide, 11 J parts of sodium are com- 
 bined with 8 parts of oxygen (p. 125) what 
 then is the atomic weight of sodium, 11 J or 
 23 ? If 11 J, then the formula for the first 
 oxide must be Na 2 O = (11 J x 2) + 8 = 31, 
 and of the peroxide NaO = 11 J + 8 = 19J to 
 meet the arithmetical requirements. But 
 11 J cannot be taken as the atomic weight of 
 sodium in view of the fact that twice that 
 weight is the smallest quantity which is 
 equivalent to the standard unit of hydrogen. 
 If we take 23 as the atomic weight, then the 
 formula of the peroxide becomes NaO 2 = 23 
 + (8x2), and the molecular weight 39. And 
 so, on the 23-sodium scale, the lowest admis- 
 sible atomic weight for sodium, we have the 
 two oxides NaO and NaO 2 , with the atomic 
 weight of oxygen taken as 8. But this view 
 of the composition of the oxides introduces 
 another equivalent for oxygen, viz., 1C, since 
 the equivalents are respectively 23 : 8 and 
 23 : 16 ; and the question now assumes the 
 form which, if either, is the atomic weight of 
 oxygen ? If 8, then the formula of water is 
 
ATOMS AND MOLECULES 145 
 
 as stated above, HO = 9 ; if 16, the formula 
 must be H 2 O =18. On this same scale, the 
 oxides of sodium obviously become Na 2 O and 
 Na 2 O 2 respectively. 
 
 This last alternative now raises another 
 point. In the peroxide, the ratio Na : O 
 is 23 x 2 : 16 x 2, i.e., 46 : 32 ; and this can, 
 of course, be halved while still maintaining 
 the atomic weight of 16 for oxygen. The 
 formula then becomes NaO. Moreover, with 
 the same atomic weight for oxygen, the 
 formula of water might be H 2 O, H 4 O 2 , or, 
 generally H 2n O n without upsetting the ratio 
 or violating the fundamental canon of an 
 indivisible atom. It is clear, therefore, that 
 the equivalent may or may not represent the 
 weight of the atom ; and it is equally plain 
 that the answer to the questions : which of 
 the equivalents is to be taken as the weight of 
 the atom ? how many atoms are there in the 
 molecule? cannot possibly be given without 
 further independent evidence. 
 
 It will be seen that the last question could 
 be answered so far as concerns the hydrogen if 
 it were possible to ascertain the relative weight 
 of the molecule of water, because we should 
 then know whether it was 9, 18, or 9n times as 
 heavy as an atom of hydrogen. We might 
 thus find out how many atoms of hydrogen 
 
146 CHEMISTRY 
 
 the molecule actually contained, because, the 
 ratio being always 1 : 8, a molecular weight 
 of 9 would indicate that it contained one 
 atom of hydrogen, a molecular weight of 18 
 that it contained two atoms of hydrogen, and 
 so forth. But the question of the number of 
 oxygen atoms in the molecule is still left open, 
 even when the number of hydrogen atoms 
 has been found, because, supposing we had 
 found the molecular weight to be 18 and the 
 number of hydrogen atoms 2, the molecular 
 formula might be H 2 O with one atom of 
 oxygen of atomic weight 16, or H 2 O 2 with 
 two atoms of oxygen of atomic weight 8. 
 Here, again, further information is required 
 before it can be definitely decided whether 
 the molecule of water contains 3 or 4 atoms. 
 In the early days of the atomic theory, no 
 such information was obtainable or, to state 
 the case accurately, such information was 
 available, but it had not been utilized ; the 
 far-reaching consequences of the study of 
 chemical combination between elements and 
 compounds in the gaseous state had not been 
 fully realized. As a result of this lack of 
 evidence, equivalent weights were often taken 
 for atomic weights ; no direct method for the 
 determination of the value of n in the equa- 
 tion n x equivalent = atomic weight had been 
 
ATOMS AND MOLECULES 147 
 
 found. Methods of determining molecular 
 weights had not been applied in Dalton's 
 time the distinction between atoms and 
 molecules had not been clearly appreciated. 
 
 The Definiteness of Chemical Combination 
 by Volume. With the exception of the rela- 
 tions between the weights and volumes of 
 hydrogen and oxygen in water (p. 88), 
 chemical combination by weight has alone 
 been considered up to this stage. Now, the 
 additional evidence required in order to decide 
 definitely the relative weight of the atom of an 
 element, when the relative weight of the mole- 
 cule of a compound containing that element 
 has been ascertained, is supplied, as hinted 
 above, by the study of chemical combination 
 by volume. This presupposes that we are 
 dealing with elements or compounds which 
 are or can be made gaseous it is bulk as 
 distinguished from mass that has now to be 
 dealt with. But, before this part of the 
 subject can be considered, the reader must 
 be reminded of certain elementary physical 
 principles. All gases expand when heated, 
 or when the pressure upon them is reduced ; 
 they contract when cooled, or when the pres 
 sure is increased. This is common knowledge ; 
 but it can be stated in more precise terms. The 
 actual change in volume which takes place on 
 
148 CHEMISTRY 
 
 heating or cooling is 2^3 per degree Centigrade 
 a given volume of gas measured at and the 
 pressure remaining constant, increasing by this 
 fraction of its bulk on being heated, and 
 diminishing to the same extent on being cooled 
 through 1C. So that at -273, i.e., 273 below 
 the freezing-point of water, the gaseous state is 
 presumably non-existent. That point is known 
 as the absolute zero ; it is a mathematical abs- 
 traction, and has never yet been reached, but 
 recent researches on the liquefaction of the gases 
 by Dewar and Onnes have brought us within 
 three or four degrees of it. The measurement 
 of the coefficient of expansion of gases is due 
 to Charles and Gay-Lussac, and the " law " 
 thus discovered is generally associated with 
 the name of Charles. 
 
 The effect of pressure upon gases is expressed 
 quantitatively by the statement that the 
 volume of a gas, the temperature remaining 
 constant, varies inversely as the pressure. 
 This means that, if the pressure is doubled, 
 the volume is halved ; if the pressure is 
 halved, the volume is doubled, and so forth. 
 In mathematical terms : Pressure x Volume 
 = Constant. This is known as the law of 
 Boyle (1626-1691). 
 
 Neither the law of Charles nor the law of 
 Boyle is obeyed absolutely by all gases ; 
 
VOLUMETRIC COMBINATION 149 
 
 there are departures depending primarily upon 
 chemical idiosyncrasies upon the specific 
 nature of the element or compound. There 
 are gases which approach perfection, and 
 there are others which are imperfect gases. 
 The laws in their most generalized form are 
 physical, but in their detailed application 
 they are equally chemical. For the present 
 purpose, however, it is sufficient to regard 
 them as purely physical. It is evident that 
 in dealing with matter so sensitive as are 
 gases to the influences of temperature and 
 pressure, the conditions must be specified ; 
 no comparison can be made between volumes 
 of gases unless the conditions are comparable. 
 Theoretically, any selected temperature and 
 pressure might be adopted ; but practically 
 it is convenient to suppose that the volumes 
 are measured at 0C, and at the average 
 atmospheric pressure of 760 mm. of mercury. 
 Of course, the measurements need not be, 
 and in fact very seldom are, made at this 
 temperature and pressure ; in practice the 
 volume is measured, and the temperature 
 and the pressure (as indicated by the barom- 
 eter column) recorded. Then, by the laws 
 of Charles and Boyle, the volume is reduced 
 by calculation to the volume which the gas 
 would occupy at 0C. and 760 mm. ; the 
 
150 CHEMISTRY 
 
 so-called normal temperature and pressure. 
 Of course, the method is imperfect ; it ignores 
 the idiosyncrasies of the gases ; but, for 
 practical purposes in comparing one gas 
 with another, the errors thus included in the 
 calculation are too small to seriously affect 
 the result. The reader will now understand 
 why in comparing gases it is a sine qua non 
 that the conditions should be comparable. 
 
 Now, when chemical combination takes 
 place between gases, it is found that there is as 
 much definiteness about the process as there 
 is about combination by weight. The ratios 
 between the combining volumes are as fixed 
 as are the combining weights ; and the 
 numbers expressing these ratios are always 
 simple, such as 1:1, 1:2, 1:3, etc. And 
 the same numerical simplicity is observed 
 when the volume of the product is compared 
 with the volumes of the components. Thus, 
 there may be combination between equal 
 volumes giving two volumes of the product : 
 1+1=2. But the rules of arithmetic are 
 not always obeyed ; one volume may com- 
 bine with two, giving rise to two volumes of 
 product, or one volume may combine with 
 three, giving rise to two volumes of product. 
 In these last cases, there is, of course, shrink- 
 age or contraction. But it will be noticed 
 
VOLUMETRIC COMBINATION 151 
 
 that, here also, the numerical relationships 
 are simple. All that need be considered is 
 the ratio between the volume of the mixed 
 gases before and that of the product after 
 combination. The results then come out in 
 this way: (1+1=)2:2; (2-fl=)3:2; 
 (3+ 1 = )4 : 2, etc., so that in the first case 
 there would be no contraction after com- 
 bination ; in the second case there would be 
 contraction to the extent of | of the volume 
 of the mixed gases, and in the third case 
 contraction to the extent of J. 
 
 These are observed facts they were worked 
 out with the greatest skill by Gay-Lussac 
 (1778-1850), and the law of definite volume 
 combination is associated with his name. 
 A few examples will give greater precision 
 to the conception of the principle. The halo- 
 gens combine with hydrogen (p. 102). If the 
 volumes of the elements are measured before, 
 and of the products after combination all 
 under comparable conditions it is found that 
 equal volumes, say of hydrogen and chlorine, 
 combine to give two volumes of hydrogen 
 chloride, 1+1=2; there is no contrac- 
 tion. Hydrogen and oxygen are contained 
 in water in the proportion of two volumes of 
 the former to one of oxygen (p. 88). If the 
 water resulting from the combination of 
 
152 CHEMISTRY 
 
 measured volumes of hydrogen and oxygen is 
 measured in the form of steam at 100 
 (because water is solid at and 760 mm.) 
 and compared with the volume of its com- 
 ponents under similar conditions, it is found 
 that it occupies the same space as the two 
 volumes of hydrogen. So that in this case 
 three volumes shrink to two a contraction 
 of one third. 
 
 In many cases, although compounds con- 
 taining certain elements are well known and 
 perfectly definite, yet it is difficult or even 
 impossible to form such compounds by direct 
 combination. For example, nitrogen forms 
 with hydrogen a compound which is the familiar 
 substance, ammonia, contained in " smelling 
 salts " a pungent smelling, strongly basic 
 compound, gaseous at ordinary temperatures, 
 liquid below 34 and solid at - - 77. It 
 is possible but not easy to bring about direct 
 combination between nitrogen and hydrogen 
 in such a way as to measure the volume 
 relationships. In this case, which is typical 
 of many others, it is much easier to decompose 
 a measured volume of ammonia gas, say by 
 a stream of electric sparks, and to measure 
 the resulting elements. This is the analytical 
 as distinguished from the synthetical method. 
 It is thus found that from one volume of 
 
HYPOTHESIS OF AVOGADRO 153 
 
 ammonia there are produced two volumes of 
 a mixture of nitrogen and hydrogen the 
 volume is doubled ; and it is therefore con- 
 cluded that when nitrogen and hydrogen com- 
 bine there is contraction to the extent of one 
 half. And, since the mixed gases resulting 
 from the decomposition contain three volumes 
 of hydrogen to one of nitrogen, it follows 
 that the ratio of combining volumes is 3:1, 
 and the ratio of the volume of the whole 
 mixture to that of the product 4:2. Here 
 again the validity of the Gay-Lussac law 
 is noticed. Furthermore, the relative weights 
 of hydrogen and nitrogen in ammonia are 
 3 : 14, so that the equivalent of nitrogen in 
 this compound is 4|, that being the weight 
 which combines with one part of hydrogen. 
 
 From the consideration of such cases there 
 arise almost spontaneously the questions : 
 Is there any relationship between definite- 
 ness of combination by weight and by 
 volume ? If such relationship exists, is it 
 explicable in terms of the atomic theory ? 
 Does it enable us to state definitely how 
 many atoms of each element are present in 
 the molecule of a compound when the relative 
 weight of the latter is known ? 
 
 The Hypothesis of Avogadro. The answer 
 to all these questions can now be given. In 
 
154 CHEMISTRY 
 
 the first place, there is an obvious relationship 
 between combination by weight and by 
 volume. If, as stated above, one volume of 
 chlorine combines with an equal volume of 
 hydrogen, and if, as also stated (p. 119), the 
 ratio of combination by weight is 1 : 35-2, 
 it follows that one volume of chlorine weighs 
 35-2 times as much as an equal volume of 
 hydrogen. Again, in the case of water, two 
 volumes of hydrogen combine with one 
 volume of oxygen and the weights are 1 : 8, 
 so that 8 parts by weight of oxygen occupy 
 half the space occupied by one part of hydro- 
 gen. For equal volumes, therefore, we have 
 the relative weights J : 8, which means that 
 oxygen, referred to hydrogen as unity, is 16 
 times heavier, bulk for bulk, under comparable 
 conditions. And, yet again, in ammonia 
 three volumes of hydrogen combine with one 
 volume of nitrogen, and the relative weights 
 are 1 : 4f ; from which it follows (since 4f 
 parts of nitrogen occupy J the space occupied 
 by 1 part of hydrogen) that the relative weights 
 of equal volumes are J : 4f , which means, 
 when expressed in whole numbers, that nitro- 
 gen is, bulk for bulk, 14 times heavier than 
 hydrogen. All this amounts, therefore, to the 
 statement that the densities of the elements 
 chlorine, oxygen and nitrogen are, on the 
 
HYPOTHESIS OF AVOGADRO 155 
 
 hydrogen scale, 35-2, 16 and 14 respectively. 
 The relationship between weight and volume 
 thus conies out in the form that the densities 
 of the gaseous elements are equal to or are 
 simple multiples of the equivalents : chlorine, 
 35-2 x 1 ; oxygen, 8x2; nitrogen, 4| x 3. 
 These facts can, of course, be and have been 
 ascertained experimentally, not only by deter- 
 mining the relative weights of the elements 
 present in the respective compounds by 
 analysis, but also by direct weighing of the 
 gases under comparable conditions-^-i.e., by 
 determining their vapour densities. 
 
 Turning, in the next place, from the ele- 
 ments to the compounds resulting from their 
 union, the densities can also be found from 
 the quantitative composition and the volume 
 occupied by the gas, or by direct weighing. 
 Thus, 36-2 parts of hydrogen chloride resulting 
 from the combination of 1 part of hydrogen 
 with 35-2 parts of chlorine, occupy twice the 
 volume of the hydrogen, as just explained. 
 The density of hydrogen chloride is, therefore, 
 I of 36-2 = 18-1. The 9 parts of water 
 resulting from the combination of 1 part of 
 hydrogen with 8 parts of oxygen occupy the 
 same volume as the hydrogen, so that the 
 density of water vapour is 9. The 5| parts 
 of ammonia resulting from the combination of 
 
156 CHEMISTRY 
 
 1 part of hydrogen with 4| parts of nitrogen 
 occupy f of the volume of the hydrogen (be- 
 cause three volumes of hydrogen and one 
 volume of nitrogen contract to two volumes 
 of ammonia on combination), so that the ratio 
 of the weights of equal volumes of hydrogen 
 and ammonia is f : 5 j = 1 : 8} ; in other 
 words, the density of ammonia on the hydro- 
 gen scale is 8J. 
 
 Looking at these results in a broad way, it 
 will be seen, in the first place, that we have 
 really under consideration two distinct classes 
 of particles or aggregates elementary and 
 compound. The former might be atoms or 
 congeries of similar atoms ; the latter must 
 of necessity be molecules. It will be seen, 
 also, that the densities of the elements bear 
 some definite relationship to the weights of 
 the atoms ; but what that relationship really 
 is could not be stated in numerical terms 
 unless we knew the relative numbers of atoms 
 in equal volumes. If it were assumed, for 
 example, that these densities did actually 
 represent the respective weights of the atoms, 
 then we are confronted with the apparent 
 paradox that the weight of the molecule of a 
 compound is less than the sum of the atomic 
 weights of its components. On the assump- 
 tion that the atoms of chlorine, oxygen, and 
 
HYPOTHESIS OF AVOGADRO 157 
 
 nitrogen weigh 35-2, 16 and 14, the minimum 
 weight of the molecules of hydrogen chloride, 
 water vapour, and ammonia must be 36-2, 
 18 and 17 respectively. But the densities 
 are 18-1, 9 and 8J. The weight of water 
 vapour contained in a volume which contains 
 1 part of hydrogen or 16 parts of oxygen is 9 ! 
 Clearly 9 cannot be the weight of the molecule 
 of water, or 16 cannot be the atomic weight of 
 oxygen. The weight of ammonia which goes 
 into the same space as 1 part of hydrogen or 
 14 parts of nitrogen is 8| ! The molecular 
 weight of ammonia must be greater than 8J, 
 or the atomic weight of nitrogen must be less 
 than 14. The weight of the whole cannot be 
 less than the weight of its parts, unless matter 
 is annihilated during chemical combination. 
 The vapour densities of elements and com- 
 pounds when compared do not decide the 
 molecular weights of the latter any more 
 than do the combining weights or equivalents 
 of the elements contained in the compound 
 unless there is made a certain assumption or 
 hypothesis. 
 
 Such an assumption was made and enun- 
 ciated as a hypothesis in 1811 by the Italian 
 physicist Amadeo Avogadro. In the light of 
 this hypothesis, all the above discrepancies 
 disappear, and the vapour density of a com- 
 
158 CHEMISTRY 
 
 pound can be made to decide the weight of 
 the molecule. According to Avogadro, equal 
 volumes of all gases, elementary and com- 
 pound, contain, under comparable conditions, 
 the same number of molecules. It will be 
 noted that the particles recognized by the 
 hypothesis are molecules not atoms ; there 
 is thus introduced the conception of the 
 molecules of elements, as well as of com- 
 pounds. This view of the constitution of 
 gases harmonizes completely with those physi- 
 cal properties which find quantitative expres- 
 sion in the laws of Charles and of Boyle. But 
 this side of the subject is dealt with in works 
 on Physics under the kinetic theory of gases, 
 and need not be enlarged upon here. Nearly 
 half a century elapsed before the significance 
 of the hypothesis in Chemistry was realized ; 
 it is mainly due to the advocacy of the late 
 Prof. Cannizzaro in 1858 that it has become 
 the foundation of modern chemical theory. 
 Let us consider some of the previous examples 
 in the light of Avogadro's conception. 
 
 One volume of hydrogen, which may be 
 taken as the unit of comparison and assigned 
 unit weight, combines with one volume of 
 chlorine of weight 35-2 to form two volumes 
 of hydrogen chloride of weight 36-2. The 35-2 
 parts of chlorine or the 1 part of hydrogen 
 
HYPOTHESIS OF AVOGADRO 159 
 
 occupy half the volume occupied by the 
 product. Therefore the latter the product 
 volume must, by hypothesis, contain twice 
 as many molecules as there were of chlorine 
 or of hydrogen in the original mixture. But 
 the molecules of chlorine, although only half 
 as numerous as the molecules of hydrogen 
 chloride, have become equally distributed 
 among the molecules of the compound, every 
 one of which contains its full share of chlorine, 
 i.e., 35-2 parts by weight. Each molecule of 
 chlorine has accordingly split into two equal 
 parts it is the molecule which has divided 
 and not the atom, and the atoms are the 
 components of the diatomic molecule, C1 2 , 
 which is composed of two atoms of a weight 
 35-2 each. By precisely similar reasoning 
 applied to the hydrogen molecule, we arrive 
 at the conclusion that this also is a diatomic 
 molecule, H 2 , composed of two atoms of unit 
 weight. Therefore, there is no violation of the 
 atomic theory the atom is still an indivisible 
 particle ; the combination between the 
 elements is an interchange of partners, and 
 not a simple juxtaposition of atoms. 
 
 From the same point of view, consider the 
 case of water. The oxygen molecules must 
 distribute themselves uniformly among double 
 the number of water molecules, because the 
 
160 CHEMISTRY 
 
 water vapour occupies double the space 
 occupied by the oxygen which enters into its 
 composition. Here again, it is the molecule 
 which divides, and the formula of the molecule 
 must be O 2 , irrespective of the question 
 whether the atom weighs 8 or 16. So also 
 with respect to ammonia, a given volume of 
 nitrogen distributes itself uniformly through- 
 out double the volume of ammonia, because 
 one volume of nitrogen combines with three 
 volumes of hydrogen to form two volumes 
 of ammonia. This, in terms of the hypothesis, 
 means that the nitrogen molecule is N 2 , and 
 that it divides into two equal parts when it 
 combines with hydrogen. 
 
 Now sum up the facts and arguments set- 
 ting out from the observation that the 
 densities of the elements under consideration 
 are H = 1, Cl = 35-2, O = 16, N = 14 : 
 
 (a) 1 part by weight of hydrogen occupies 
 half the volume of 36-2 parts of hydrogen 
 chloride ; therefore 2 parts of hydrogen occupy 
 the same volume as 36-2 parts of hydrogen 
 chloride. 
 
 (b) 35-2 parts of chlorine occupy half the 
 volume occupied by 36-2 parts of hydrogen 
 chloride ; therefore 70-4 parts of chlorine 
 occupy the same volume as 36-2 parts of 
 hydrogen chloride. 
 
HYPOTHESIS OF AVOGADRO 161 
 
 (c) 16 parts of oxygen occupy half the 
 volume occupied by 18 parts of water vapour ; 
 therefore 32 parts of oxygen occupy the same 
 volume as 18 parts of water vapour. 
 
 (d) 14 parts of nitrogen occupy half the 
 volume occupied by 17 parts of ammonia ; 
 therefore 28 parts of nitrogen occupy the 
 same volume as 17 parts of ammonia. 
 
 To compare equal volumes of elements and 
 compounds we have, therefore, to double the 
 volumes and, by implication, the weights of 
 the elements. Then we have a series of 
 weights and volumes which are strictly 
 comparable, the volumes all containing the 
 same number of molecules. On this scale the 
 hydrogen standard becomes 2, and the vapour 
 densities of the other elements and of their 
 compounds do represent the relative molecular 
 weights. In other words, the volumes occu- 
 pied by the molecular weights of elements 
 and compounds are equal, and as this holds 
 good for all elements and compounds (with 
 certain exceptions which must be considered 
 later) and irrespective of the number of atoms 
 which form the compound molecule, we arrive 
 at the generalized statement : Vapour density 
 (referred to hydrogen as 1) x 2 = molecular 
 weight, or molecular weight -j- 2 = vapour 
 density (rej erred to hydrogen as 1). And so the 
 
162 CHEMISTRY 
 
 question whether the relative weight of the 
 molecule can be determined by ascertaining 
 the vapour density is answered definitely in the 
 affirmative. The apparent paradox previously 
 raised that the molecule of a compound can 
 weigh less than the sum total of the atoms 
 of its components disappears when, as we 
 now find, the molecular weight of hydrogen 
 chloride becomes 36-2, of water 18, and of 
 ammonia 17. 
 
CHAPTER VII 
 
 THE NUMBER OF ATOMS CONTAINED IN A MOLE- 
 CULE DISSOCIATION AND ASSOCIATION 
 
 AUXILIARY METHODS FOR DETERMINING 
 
 ATOMIC AND MOLECULAR WEIGHTS THE 
 
 LAW OF DULONG AND PETIT ATOMIC 
 
 AGGREGATES IN SOLUTION 
 
 The Number of Atoms Contained in a Molecule. 
 The chemical molecule may be regarded in 
 the light of these conclusions as a weight of 
 matter representing in the case of compounds 
 an irreducible minimum, because any further 
 division must obviously lead to the removal 
 of atoms, i.e., to decomposition, and the com- 
 pound, as such, ceases to exist. And, since 
 the weight of the molecule is the sum total of 
 the weights of its component atoms, we have 
 now to face the next question whether, the 
 relative weight of the molecule being known, 
 it is possible to decide therefrom the numbers 
 of the atoms of the elements of which it is 
 composed. We know, for instance, that a 
 molecule of water has the relative weight 18, 
 163 
 
164 CHEMISTRY 
 
 and that it contains two atoms of hydrogen 
 because it contains the divisible quantity of 
 that element represented by H 2 . The water 
 molecule, therefore, contains 182=16 parts 
 of oxygen ; and the question is how many 
 atoms of oxygen does this represent ? Similar 
 questions arise concerning all the compounds 
 with which we have dealt. 
 
 It has already been pointed out that the 
 formula H 2 O 2 , with an atomic weight of 8 for 
 oxygen, satisfies the arithmetical require- 
 ments ; and it will now be seen that it also 
 satisfies the molecular requirements, because 
 the molecule of oxygen may be supposed to 
 consist of 4 atoms, in which case its formula 
 would be O 4 , and the splitting of the oxygen 
 molecule would be represented by the equa- 
 tion O 4 = 2 O 2 The answer to the question 
 now raised is really in principle a very simple 
 one. It is given by taking a consensus of 
 evidence by a kind of Referendum to all 
 the known gasifiable compounds of the ele- 
 ment in order to find out, by comparing the 
 weights of the element contained in the mole- 
 cules of the various compounds, the smallest 
 weight contained therein. That weight is 
 reasonably taken to be the irreducible mini- 
 mum of the element, i.e., its atomic weight. 
 It corresponds with the definition of the atom 
 
ATOMIC AGGREGATES 165 
 
 previously given (p. 142) the smallest weight 
 of the element which enters into the composi- 
 tion of the molecule, or which can be separated 
 from the molecule by decomposition. In this 
 way we find that the smallest weights of 
 chlorine, oxygen, nitrogen, etc., contained 
 in the molecules of any of their compounds 
 are, respectively, 35-2, 16 and 14 ; and we take 
 these as the weights of their atoms we find 
 the value of n in the equation n x equivalent 
 = atomic weight (p. 146). In this case for 
 Cl, n = 1 ; for O, n = 2 ; and for N, n = 3. 
 And thus we are enabled to write the formulae 
 of the compounds HC1, H 2 O and NH a , and 
 to indicate the actual numbers of atoms 
 composing their molecules. Furthermore, we 
 can represent the formation of these com- 
 pounds from their elements by equations 
 which are both chemically and arithmetically 
 true : 
 
 H 2 + Cl a = 2 HC1 ; 2H 2 + O a = 2 H a O ; 
 N 2 +3H 2 = 
 
 It has already been claimed that the sym- 
 bolical language of Chemistry is full of 
 meaning. The reader will now perceive the 
 wealth of truth embodied in these symbols. 
 The formula for a chemical compound, 
 A x B y C z , stands for one molecule for a weight 
 
166 CHEMISTRY 
 
 of the compound made up of the sum of the 
 weights of x atoms of A, y atoms of B, and z 
 atoms of C. It represents also a weight of the 
 compound which in the gaseous state if it is 
 gasifiable occupies the same volume as the 
 molecule of hydrogen, i.e., H 2 . It will be seen, 
 also, that the symbols for the atoms of those 
 elements which possess diatomic molecules 
 represent half volumes ; the atomic weights of 
 H, Cl, O, N, etc., all occupy the same volume 
 in the gaseous state. The law of definiteness 
 of combination by volume (p. 147) is thus 
 explained the atomic weights must combine 
 in equal volumes, or in some simple multiple 
 of these volumes. 
 
 Dissociation and Association. There are 
 certain elements and compounds whose vapour 
 densities are apparently unconformable ; 
 there are discrepancies between the numbers 
 obtained for their atomic or molecular weights 
 and the numbers deduced from the law of 
 uniformity of atomic and molecular volumes. 
 But this nonconformity is not paradoxical 
 on the contrary, it is instructive in the 
 highest sense. From the story of such cases 
 we get a glimpse into new principles. Let 
 us consider some of the facts. 
 
 The atomic weight of sulphur as deduced 
 from the analysis and vapour densities of its 
 
DISSOCIATION AND ASSOCIATION 167 
 
 gaseous compounds is 32. If it were conform- 
 able, its molecular weight should, therefore, 
 be 64, and its vapour density 32. In fact, 
 this element has two vapour densities, accord- 
 ing to the temperature at which the vapour 
 is weighed. At its boiling-point under 
 atmospheric pressure (448), its vapour density 
 is 128 ; and at 1700 its vapour density is 
 normal, i.e., 32. Translate these facts into 
 terms of the atomic-molecular theory, and 
 consider their meaning. At 448 the volume 
 of sulphur vapour which occupies the space 
 occupied by 1 part by weight of hydrogen 
 weighs 128, and its molecular weight is accord- 
 ingly 256. Similarly, its molecular weight 
 at 1700 is 64. There is really no mystery 
 about this. If, as the evidence shows, the 
 atom weighs 32, then at 448 the molecule 
 must contain 8 atoms, i.e., 32 x 8 = 256 ; 
 the formula is S 8 . At 1700 the molecule 
 must for the same reason be S 2 ; and, as the 
 vapour cools down from 1700 to 448, the 
 simpler aggregate of 2 atoms condenses to 
 the more complex aggregate of 8 atoms. This 
 brings out the principle of Dissociation and 
 Association terms which, in the light of the 
 foregoing example, should be self-explanatory. 
 Again, phosphorus the highly combustible 
 non-metallic element used in the manufacture 
 
168 CHEMISTRY 
 
 of luteifer matches has an atomic weight of 
 about 31, as determined by the usual methods. 
 At 313 the vapour density gives a molecular 
 weight of 31 x 4 = 124, and the molecule is 
 P 4 . As the temperature of the vapour is 
 raised, the P 4 dissociates, and at 1700 it is 
 largely resolved into P 2 . 
 
 In a similar way, and by means of modern 
 appliances which are described in works 
 dealing with practical Chemistry, the molecu- 
 lar weights of many other elements reveal 
 this principle of dissociation and association. 
 Thus, iodine of atomic weight about 127 at 
 448 is normally I 2 ; at 1700 it completely 
 dissociates into I the molecule and the atom 
 at this temperature are the same ; the molecule 
 is monatomic. Sodium, zinc, mercury, etc., 
 have by a similar method been shown to exist 
 at high temperatures as Na, Zn, Hg, i.e., as 
 monatomic molecules. 
 
 So also with regard to compounds. Hydro- 
 gen fluoride (p. 102) is chemically analogous 
 to hydrogen chloride, the vapour density of 
 which accords with the formula HC1. But the 
 vapour density of the fluoride indicates that 
 below 30 its molecule is (HF) 2 , i.e., H 2 F 2 , 
 and that at 88 dissociation has taken place ; 
 as the temperature falls association takes 
 place: 2HF^H 2 F 2 . The reader will inci- 
 
DISSOCIATION AND ASSOCIATION 169 
 
 dentally notice the potency of our symbolical 
 language. It is only necessary to call atten- 
 tion to a certain oxide of nitrogen, to the 
 compound which phosphorus forms with 
 chlorine (phosphorus pentachloride), or to the 
 compound, ammonium chloride, formed by 
 the direct union of one molecule of ammonia 
 with one molecule of hydrogen chloride, as 
 further illustrations of the principle and to 
 write the story of their dissociation and 
 association by means of such schemes as 
 these: N 2 O 4 , ^ 2NO 2 ; PC1 6 ; PC1 3 + C1 2 ; 
 NH 4 C1 ^ NH 3 + HC1. 
 
 The only information not included in the 
 symbolical representation of the process is 
 the temperature at which dissociation takes 
 place in each case, and the influence of pres- 
 sure upon the amount of dissociation. The 
 combination between compound molecules, 
 such as ammonia and hydrogen chloride, 
 furnishes an illustration of the extended 
 principle of equivalence (p. 121), since 17 parts 
 of ammonia and 86-2 parts of hydrogen 
 chloride represent equivalent weights of these 
 two compounds. 
 
 Auxiliary Methods for determining Atomic 
 and Molecular Weights. The reader who has 
 followed the development of the principles 
 of chemical science up to this point will now 
 
170 CHEMISTRY 
 
 realize that the methods employed for the 
 determination of the relative weights of 
 atoms and molecules enable us to state defin- 
 itely that, under such or such conditions, 
 such or such atomic aggregates exist. All 
 the gaseous elements referred to in former 
 chapters form diatomic molecules. Our imag- 
 inary being of the second chapter who was 
 capable of following the gyrations of the 
 " particles " of air would, in fact, see vast 
 crowds of twin pairs of atoms of nitrogen, 
 smaller numbers of twin pairs of atoms of 
 oxygen, and still smaller numbers of triatomic 
 groups constituting the molecules of water 
 (H 2 O) and carbon dioxide (CO 2 ). And he 
 would also see now and again solitary mona- 
 tomic nomads of certain very rare gases 
 (argon, neon, krypton, xenon), which have 
 already been stated to have been detected 
 in late years as constituents of the atmos- 
 phere. But, great as is the insight into the 
 constitution of matter gained by this com- 
 bination of chemical and physical methods of 
 attacking the problem, it must be realized 
 that the results are true only within the 
 limiting conditions of the observations. The 
 state of aggregation revealed is true within 
 the ranges of temperature and pressure avail- 
 able for the determination of atomic and 
 
AUXILIARY METHODS 171 
 
 molecular weights ; but it must not be inferred 
 that no further disaggregation is possible at 
 extreme temperatures (say celestial), or under 
 the influence of other disintegrating forces. 
 Neither can it be asserted that in the other 
 direction in the passage from the gaseous 
 to the liquid or solid state there is no 
 greater complexity than that indicated by the 
 molecular formula as ascertained by the 
 above methods. 
 
 Take the case of the elements. Under 
 infinitesimal pressure, i.e., in a vacuous space 
 containing comparatively few molecules, there 
 is physical evidence that the atoms are cap- 
 able of being broken down by an electric 
 discharge into smaller particles " corpus- 
 cles," or " electrons," or whatever they may 
 be (p. 134). But this ultra-dissociation by 
 ultra-chemical means in no way conflicts 
 with the views concerning the atom derived 
 from the study of its chemical and physical 
 attributes under ordinary conditions. It is 
 supplementary information concerning the 
 inner mechanism of the atom as a discrete 
 particle of matter, of which the direct bearing 
 upon the nature of chemical change has yet 
 to be deciphered. The atom may be knocked 
 to pieces by an electric discharge in a high 
 vacuum, or may be disintegrated in the 
 
172 CHEMISTRY 
 
 atmosphere of the hottest stars (Lockyer) ; 
 but in the course of all the ordinary chemical 
 transformations which matter undergoes on 
 this earth it may still be regarded as the 
 indivisible particle. 
 
 So also in the other direction in the way 
 of increased complexity the molecule, let 
 us say, of vaporous water, H 2 O, may not be 
 the molecule of liquid water, which is cer- 
 tainly (H 2 O) n . The value of n cannot be said 
 to have been definitely established yet. So 
 generally in the case of solids, we have no 
 criterion of the state of molecular aggregation 
 based on vapour density determinations only. 
 The three oxides of iron, for example (p. 124), 
 have the formulae Fe 2 O 3 (rust), Fe 3 O 4 (scale), 
 and FeO respectively ; the sulphide (p. 128) has 
 the formula FeS, and the iodide (p. 73) Fel a . 
 But these are minimum formulae we cannot 
 convert any one of these compounds into 
 vapour ; and the atomic weights of the 
 elements composing these molecules have 
 been determined independently of these 
 particular compounds. The molecules of the 
 solid compounds may be, and probably are, 
 multiples of these formulae. Then, again, in 
 the case of the elements, the molecule of 
 the liquid or solid might be a multiplex aggre- 
 gate of gaseous molecules. Moreover, there 
 
AUXILIARY METHODS 173 
 
 are many elements which, like carbon, for 
 example, cannot be gasified at any manage- 
 able temperature ; and there are elements 
 which are not only non-volatile, but which 
 form no gasifiable compounds. It is evident, 
 therefore, that other evidence is wanted ; 
 the method based upon the hypothesis of 
 Avogadro is inapplicable in such cases as 
 those referred to above auxiliary methods 
 are needed. 
 
 The Law of Dulong and Petit. The reader 
 will learn from Physics that different kinds 
 of matter require different quantities of heat 
 to raise equal weights through the same range 
 of temperature. This is, therefore, a specific 
 property of matter ; and the quantity 
 of heat required to raise some particular 
 substance through some definite range of 
 temperature is said to be the specific heat of 
 that substance. The standards in use are 
 generally 1C. and the specific heat of water 
 as unity ; the quantity of heat required to 
 raise one gram of water through 1C. is known 
 as the calorie. We cannot deal here with 
 the physics of this property of matter why 
 different substances should require different 
 quantities of heat to produce the same visible 
 effect upon a thermometer ; neither is it 
 necessary to describe the methods of deter- 
 
174 CHEMISTRY 
 
 mining specific heat, as these are dealt with in 
 works on Physics. From the chemical point 
 of view, this property of matter can be 
 utilized as an auxiliary method for checking 
 atomic weights. It was discovered by Dulong 
 and Petit (1818) that if, instead of comparing* 
 the specific heats of equal weights of the 
 chemical elements in the solid condition, the 
 specific heats of weights of the elements 
 proportional to their atomic weights are 
 compared, then the fact is revealed that all 
 the elements have the same atomic heat. In 
 other words, the property in question is a 
 property of the atom ; the atoms all require 
 the same quantity of heat to raise them 
 through the same range of temperature. The 
 specific heats of the elements are thus inversely 
 as the atomic weights, so that Atomic Weight 
 x Specific Heat = a Constant. The value of that 
 constant is between 6 and 7, and averages 6-4, 
 so that 6-4 -r Specific Heat = Atomic Weight. 
 
 This law is only approximative, and there 
 are deviations from it some to a considerable 
 extent. But these abnormalities arise from 
 the circumstance that in the case of certain 
 elements the specific heat only approaches the 
 normal value at high temperatures. Passing 
 over these exceptional cases, the remainder 
 of the elements conform closely with the law* 
 
AUXILIARY METHODS 175 
 
 But, although the latter is only approximative, 
 it follows from the relationship between 
 equivalents and atomic weights that there 
 is a wide margin within which the numerical 
 results may be allowed to fluctuate. The 
 equivalents are exact as accurate as can be 
 determined by the most refined and delicate 
 methods of analysis. But, after we have ascer- 
 tained the equivalent, its relationship to the 
 weight of the atom has, as already pointed 
 out, to be settled by independent evidence 
 In cases where the molecular weight cannot 
 be determined directly, or where the element 
 forms no gasifiable compound, the specific 
 heat gives valuable information. The whole 
 point to be decided is the same as that which 
 arises in the case of the gaseous elements, viz., 
 whether the equivalent A represents the 
 weight of the atom, or whether the atomic 
 weight is 2A, 3A, etc. That is why this 
 auxiliary method based on the law of Dulong 
 and Petit admits of considerable numerical 
 latitude. A few illustrations will enable the 
 reader to appreciate its value : 
 
 The vapour density of mercury on the 
 hydrogen scale is 100, so that its molecular 
 weight is 200. If the molecule consisted of 
 two atoms, the atomic weight would be 100. 
 But the specific heat of solid mercury is -032 
 
176 CHEMISTRY 
 
 (water = 1) and 6-4 -f- -032= 200, and so the 
 atomic weight of mercury is taken as 200, and 
 that is one of the reasons why the molecule 
 is considered to be^monatomic (p. 168). Again, 
 which of the equivalents of iron (p. 124) 
 represents the weight of the atom ? The 
 specific heat of iron is -112, and 6-4 -f- '112 = 
 57-1. None of the three equivalents, 21, 
 18 1, or 28, represents the weight of the atom, 
 but the nearest is evidently 28 x 2 ; and it 
 is for this, among other reasons, that the 
 atomic weight of iron is, in round numbers, 
 accepted as 56. Copper forms two oxides, 
 and has, therefore, the two equivalents 31-5 
 and 63 (p. 125). Which of these, if either, 
 represents the weight of the atom ? The 
 specific heat of copper is -994, and 6-4 -f- -994 
 = 64-4. There is no doubt, therefore, in 
 spite of the deviation, that the second of these 
 equivalents represents the weight of the atom. 
 Atomic Aggregates in Solution. The re- 
 sources at the disposal of chemists for deter 
 mining the molecular weights of compounds 
 which cannot be gasified have been added to 
 in recent years by the introduction of other 
 auxiliary methods. These newer methods 
 are based upon physical principles, which 
 cannot be considered in detail here ; but a 
 general notion can be outlined. Let us begin 
 
AUXILIARY METHODS 177 
 
 with some facts. Everybody is familiar 
 with the fact that certain liquids, such as 
 water, dissolve certain solids (p. 85), such as 
 sugar or salt. Other liquids, such as alcohol 
 (spirit of wine), will mix with, i.e., dissolve in 
 water in all proportions. So, also, gases 
 dissolve in liquids like water, and such solu- 
 tions give up the gas again when heated. 
 This property of liquids may be looked upon 
 as a physico-chemical property, because it is 
 specific ; solubility and insolubility, or the 
 degree of solubility under given conditions, 
 are properties dependent upon the nature of 
 the solvent and solute. By " nature " in this 
 case, chemical nature is meant. For the 
 present purpose water may be taken as a 
 typical solvent, and sugar as a typical solute. 
 This familiar substance is composed of the 
 three elements, carbon, hydrogen, and oxygen, 
 in the atomic proportions indicated by the 
 formula C 12 H 2 2O n . This is the result of the 
 analysis of sugar, which contains in 100 parts 
 by weight 42-1 parts of carbon, 6-4 of hydro- 
 gen, and 51-5 of oxygen. The formula, as 
 in all such cases, is simply the percentage 
 composition translated into terms of atomic 
 weights (in round numbers, C = 12 ; H = 1 ; 
 O = 16). But this formula is a minimum 
 formula ; the sum total of the atomic weights 
 
178 CHEMISTRY 
 
 will be found to be 342, which may or may 
 not be the molecular weight. The molecule 
 cannot weigh less than this, because if we 
 halve it we introduce the inadmissible formula, 
 C 6 H n O 5i , containing a half atom of oxygen. 
 Neither can it be decided whether the mole- 
 cular weight is a multiple of 342 by determin- 
 ing the vapour density, because sugar is 
 completely decomposed by heat. This illus- 
 trates a class of cases in which the newer 
 methods supply the required information. 
 
 The point on a thermometer scale at which 
 a liquid when cooled passes into the solid 
 state is said to be the freezing-point ; and the 
 point at which the liquid when heated passes 
 into vapour under any given pressure is the 
 boiling-point at that pressure. Thus, the 
 freezing-point of water is (273 absolute), 
 and, under ordinary atmospheric pressure, 
 the boiling-point is 100 (373 absolute). 
 Now, it is a well known fact that a solution 
 freezes at a lower temperature, and boils at 
 a higher temperature, than the pure solvent. 
 The physical interpretation of this fact cannot 
 be discussed here ; the two properties referred 
 to are really different aspects of the same 
 physical principle, and so the freezing-point 
 only may be considered in order to simplify 
 matters. 
 
AUXILIARY METHODS 179 
 
 It has been established by experiment 
 (Raoult, 1883) that there is a definite relation- 
 ship between the depression of freezing-point 
 of a solution and the molecular weight of 
 the solute. That relationship is broadly ex- 
 pressed by the statement that, for any particu- 
 lar solvent and with equal concentration (i.e., 
 percentage of solute in solvent), weights of 
 different substances corresponding to the 
 molecular weights produce the same amount 
 of depression. It is not possible to enter 
 further into details either of principle or of 
 technique, but it will be seen in a general way 
 that the introduction of this principle adds to 
 the methods available for the determination 
 of molecular weights. If the depression 
 produced by any particular solvent with 
 various solutes of known molecular weight 
 (i.e., determined by other methods) in known 
 degrees of concentration is ascertained experi- 
 mentally and the more dilute the solution 
 the more concordant the results then the 
 molecular weight of a substance of unknown 
 molecular weight can be determined by a 
 simple calculation when the depression of 
 freezing-point produced by a known weight of 
 the substance in a known weight of the solvent 
 is ascertained by observation. 
 
 It must be understood that this method is 
 
180 CHEMISTRY 
 
 applicable only in cases where there is no 
 chemical action between the solvent and solute. 
 It must also be pointed out that the method 
 is inapplicable to acids, bases, or salts in 
 other words, to electrolytes (p. 109) because 
 there is evidence that in the case of these 
 compounds solution, at any rate in water, is 
 accompanied by a resolution of the com- 
 pound into component parts corresponding 
 with those which travel to the respective 
 poles during the process of electrolysis (p. 111). 
 In such solutions there are, therefore, con- 
 tained a larger number of component parts 
 known as ions than is the case with un- 
 resolvable non-electrolytes, such as sugar ; 
 and the depression of freezing-point produced 
 by compounds which undergo this ionic 
 dissociation is, therefore, incomparable with 
 that produced by compounds of the other 
 type. 
 
 From the practical side, the reader must 
 realize, also, that the depression which is 
 measured in these cases is extremely small, 
 and necessitates a refinement in thermometric 
 methods quite beyond the ordinary experience 
 of the casual observer. To give one illustra- 
 tion, it may be mentioned that a solution of 
 one part of sugar in 100 parts of water pro- 
 duces a depression of -058, i.e., the solution 
 
AUXILIARY METHODS 181 
 
 solidifies at 0-058 instead of at 0. The 
 depressing value of water being known for 
 many compounds of known molecular weight, 
 it will be found on calculation that the de- 
 pression produced by sugar corresponds most 
 closely with the molecular weight 342, and 
 that the formula C 12 H 22 O 11 is, therefore, the 
 molecular formula. This means that the 
 molecule of sugar contains 12+22+11 = 45 
 atoms, and that its weight on the hydrogen 
 scale is 342. The state of atomic aggregation 
 in which sugar exists in solution is the same 
 as that which it would possess in the state of 
 vapour, were it possible to convert this com- 
 pound into vapour without decomposing it. 
 
 The contemplation of such conclusions as 
 have now been set forth conclusions based 
 upon experimental observations interpreted 
 by hypothesis will assuredly serve to justify 
 the claim of Chemistry to take rank as a 
 science which is penetrating more and more 
 deeply into the inner mysteries of matter. 
 The atoms and molecules which are dealt 
 with by such methods as have been considered 
 methods which are by no means exhaustive 
 of all our resources are legitimately con- 
 ceived as physical entities ; and every advance 
 in our knowledge of the physical and chemical 
 properties of matter has served to strengthen 
 
182 CHEMISTRY 
 
 the reality of this conception. Some further 
 developments will be discussed as far as 
 possible in the short space remaining at our 
 disposal. It may be fairly asserted that the 
 atoms and molecules which modern chemical 
 philosophy has called into existence " out of 
 the void and formless infinite " have become 
 the common property of thinkers and workers 
 in every department of science. 
 
CHAPTER VIII 
 
 DETERMINATION OF THE RELATIVE WEIGHTS 
 OF THE ATOMS THE ISOLATION OF DE- 
 FINITE SUBSTANCES CHEMISTRY AS AN 
 
 EXACT SCIENCE THE STANDARD OF 
 ATOMIC WEIGHTS CHEMICAL ARITHMETIC 
 VOLUMETRIC RELATIONSHIPS 
 
 Determination of the Relative Weights of 
 Atoms. The fundamental units of matter 
 which have thus been made the basis of the 
 modern interpretation of chemical phenomena 
 exist in the 82 different modifications re- 
 presenting the known chemical elements. 
 The varying characters of the individual 
 elements are qualitative expressions of the 
 idiosyncrasies of the atoms ; with the pro- 
 gress of discovery, we may hope to be able 
 to express these differences more and more 
 precisely in quantitative terms. It has been 
 shown in the previous chapters that the main 
 attribute of the atom which is at present dealt 
 with quantitatively is the relative weight. 
 The question now arises how is this weight 
 
 183 
 
184 CHEMISTRY 
 
 ascertained ? The relative weight of an 
 atom is a " Constant of Nature " in the same 
 sense as, let us say, the length of an ethereal 
 wave corresponding to a particular colour, 
 or the number of vibrations corresponding to 
 a particular musical note. It is evident, 
 therefore, that a character of such fundamental 
 importance as the atomic weight must be 
 expressed numerically with the utmost obtain- 
 able accuracy. 
 
 In the foregoing chapters, certain atomic 
 weights have already been assigned to some 
 of the elements. The reader has been given 
 to understand in a general way that these 
 numbers are found by determining the relative 
 quantities of the elements present in com- 
 pounds, and then ascertaining the molecular 
 weight of the compound by one or another of 
 the available methods. It must be realized 
 that the physical methods, such as vapour 
 density determination, depression of freezing- 
 point, raising of boiling-point, determination 
 of specific heat, etc., are not methods of 
 precision in the strict sense, but methods of 
 control they decide only the total number 
 of atoms composing a molecule, and the 
 particular equivalent or combining weight 
 which represents the weight of the atom. 
 If, therefore, these methods are methods of 
 
ATOMIC WEIGHTS 185 
 
 control, what is it that they do control ? 
 evidently the equivalents determined by 
 analysis or synthesis. 
 
 The determination of the atomic weight of 
 an element resolves itself into a question of 
 quantitative composition as ascertained with 
 all the precision attainable by human skill. It 
 is not a question as to whether the atomic 
 weight of oxygen is 8 or 16, of sulphur 16 or 
 32, of carbon 3 or 12, of nitrogen 4| or 14, of 
 iron 28 or 56, of mercury 100 or 200 these 
 broad issues may be taken as settled by the 
 methods of control. It is now a question of 
 accuracy in the decimal places, for these atoms 
 are the gifts of Chemistry to universal science, 
 and the account of their attributes must be 
 rendered with all the precision demanded by 
 the exact sciences. 
 
 It is beyond the scope of this work to 
 describe in detail the methods of determining 
 atomic weights. The chemical changes dealt 
 with are for the most part simple. For 
 example, direct combination between a metal 
 and a halogen when giving rise to a definite 
 weighable compound gives the ratio : Metal : 
 Halogen. As an actual case, the determination 
 of the equivalent of silver with respect to 
 chlorine may be cited. A known weight of 
 silver converted into chloride by combination 
 
186 CHEMISTRY 
 
 with chlorine gives so much silver chloride. 
 Using symbols, and expressing the change in 
 the form of an equation, we have : 2 Ag + 
 C1 2 = 2 Ag Cl. The gain in weight represents 
 the chlorine combined with the silver ; the 
 ratio Ag : Cl has been found. It will be noted 
 incidentally that no molecular formula has 
 been assigned to the silver, because the state 
 of atomic aggregation of the solid metal is 
 unknown (p. 172) : it may be Ag x , the value of 
 x being unknown. 
 
 Again, most of the elements can be made 
 to combine either directly or indirectly with 
 oxygen. When the oxide is a definite com- 
 pound, the weight of oxide obtained from a 
 known weight of the element gives the ratio : 
 Element : Oxygen. Conversely, an oxide of de- 
 finite composition when resolvable, say by 
 heat, into element and oxygen, gives the same 
 information. Many metallic oxides which do 
 not part with their oxygen on heating alone 
 are reduced to the metallic state when heated 
 with some element which can combine with 
 the oxygen. Carbon and hydrogen are such 
 reducing agents. The oxides of iron, for 
 example, can all be reduced to iron by strongly 
 heating them with charcoal or some other 
 form of carbon. That is why the metal iron 
 was referred to as an artificial product 
 
ATOMIC WEIGHTS 187 
 
 (p. 87), because the metal is obtained from its 
 ores by such treatment. The oxides can also 
 be reduced by heating them in an atmosphere 
 of hydrogen, and this completes the proof that 
 these oxides consist only of metal and oxygen 
 (p. 71). The loss of weight thus undergone by 
 the oxide on reduction by hydrogen might be 
 made to give the ratio : Metal : Oxygen. And, 
 since in this case the oxygen of the oxide forms 
 water with the hydrogen, the weight of water 
 gives also the ratio O : H in water. To take 
 another example : The oxide of copper formed 
 when copper is heated in oxygen (p. 84) is a 
 perfectly definite oxide, which is reduced to 
 copper on heating with carbon or in hydrogen, 
 the change, under the latter condition, being 
 CuO + H 2 = Cu + H 2 O. Here again it will be 
 noted that no molecular formula is assigned 
 to the oxide or to the metal, the reason being 
 as above given our ignorance of the mole- 
 cular weights of solid elements and com- 
 pounds. In this example, a known weight of 
 oxide loses so much on reduction ; the loss 
 represents oxygen. This known weight of 
 oxygen gives so much water, the gain in 
 weight being due to the hydrogen combined 
 with that quantity of oxygen the ratio O : H 
 in water has been determined. 
 
 There is no royal road to the determination 
 
188 CHEMISTRY 
 
 of atomic weights every available method is 
 utilized : it is entirely a question of practica- 
 bility. Since, in the case of some of the 
 elementary gases, the densities referred to 
 hydrogen represent the atomic weights (p. 
 166), a direct determination of density gives 
 the necessary information with a degree of 
 precision limited only by the accuracy of the 
 experimental methods. If a metal can be 
 deposited in a weighable condition by the 
 electrolytic decomposition of its salts, the 
 electro-chemical equivalent (p. 121), gives the 
 required information. It can be asserted as a 
 general principle that for the determination of 
 atomic weights the main requirement is 
 purity the materials used, elements and 
 compounds, must be chemical individual 
 substances in the strictest sense realizable. 
 
 The Isolation of Definite Substances. The 
 whole development of Chemistry is intimately 
 bound up with the practical necessity of 
 isolating the various forms of matter, element- 
 ary and compound, in such a state of purity 
 that a chemical individual substance is 
 obtained. The foundations of our science are 
 based on the study of individual substances ; 
 and the degree of exactness that has been 
 reached is a measure of the success of our 
 laboratory methods. The reader who ap- 
 
ATOMIC WEIGHTS 189 
 
 preaches Chemistry from the purely literary 
 side must thoroughly grasp this fundamental 
 reality he must realize to the full extent the 
 significance of the statement that Chemistry 
 is an art as well as a science (p. 31). In no 
 branch of practical work is the standard of 
 individuality, i.e., of purity, of more vital 
 importance than in the materials used for the 
 determination of atomic weights. It is for 
 this reason that the general question of the 
 isolation of individual substances has been 
 brought forward here. The discussion of 
 practical methods is beyond the limits of this 
 work ; they cannot be mastered by simply 
 reading about them, but only by that com- 
 bination of manual skill, judgment, and 
 resourcefulness which is essential for accuracy 
 in this kind of work. The separation of 
 individual substances in other words, their 
 purification, is effected by processes which 
 are familiar enough as laboratory operations. 
 If a substance has a definite boiling-point 
 (p. 59), it can be distilled, and a fraction 
 having a constant boiling-point isolated ; if 
 it can be vaporized by heat and condensed in 
 the solid form, it can be purified by sublima- 
 tion ; if it separates from its solution in a 
 crystalline form when the solvent is evapor- 
 ated, the associated impurities can in this 
 
190 CHEMISTRY 
 
 way be removed by a sufficient number of 
 crystallizations ; if the substance in solution 
 can by interaction with some other substance 
 be converted into an insoluble compound it 
 can be purified by precipitation. 
 
 But, in spite of all our resources, an abso 
 lutely pure substance is so extremely difficult 
 to obtain that it may almost be regarded 
 as a mathematical abstraction. Matter pure 
 enough to withstand chemical examination 
 may still be shown by more delicate physical 
 tests, such as by means of the spectroscope, to 
 contain traces of foreign substances. The 
 atomic weights in use are necessarily of different 
 degrees of exactness, and finality has not yet 
 been reached the work is still in progress, 
 and accuracy is being pushed further along the 
 line of decimal places. The results obtained 
 by different experimenters are considered by 
 an International Commission, under whose 
 auspices a list of atomic weights is published 
 annually. The list for 1912 is given at the end 
 of this volume. 
 
 Chemistry as an Exact Science. It is not 
 claimed that Chemistry ranks with the exact 
 sciences in the sense of having reached the 
 purely deductive stage ; but in every direction 
 in which quantitative treatment is possible 
 the relative weights of the atoms come sooner 
 
ATOMIC WEIGHTS 191 
 
 or later into consideration. Hence the neces- 
 sity for accuracy in these constants, the 
 responsibility for the determination of which 
 is necessarily thrown ultimately upon the 
 chemical balance. This instrument, com- 
 pared with those used in some of the most 
 delicate physical measurements, may perhaps 
 be considered coarse. But, for all practical 
 purposes, accuracy to the l-10,000th of a 
 gram, i.e., l-10th of a milligram, is sufficient; 
 for the more refined work sensitiveness to 
 1 -200th of a milligram is obtainable. The time 
 may come when, for the investigation of the 
 more recondite attributes of the atom, a 
 higher degree of precision will be necessary ; 
 and it may be well to point out, therefore, that 
 there has recently been added to the resources 
 of the physicist and chemist a micro-balance 
 constructed of quartz capable of weighing 
 the almost inconceivably small quantity of 
 1-10, 000th of a milligram. 
 
 But the difficulty now rests not so much 
 with the instrument as with the matter ; 
 ultra refinement in weighing unless the sub- 
 stance is really " individual " is suggestive of 
 " straining at a gnat and swallowing a camel." 
 Some of the atomic weights are confessedly 
 uncertain ; on account of those difficulties 
 of purification which have just been indicated, 
 
192 CHEMISTRY 
 
 it is recognized that in many cases revision is 
 necessary. In no case, however, is the 
 responsibility of fixing the atomic weight 
 thrown upon the analysis or synthesis of one 
 compound of an element when several com- 
 pounds are available. The degree of precision 
 reached is evidently dependent upon the num- 
 ber of independent sets of observations ; all 
 those atomic weights which have been deter- 
 mined with the greatest accuracy are based 
 upon converging lines of evidence. In modern 
 times the science of Chemistry largely owes its 
 advance towards exactness in this direction 
 to the life-long labours of men like Stas, 
 Morley, Richards and Clarke. 
 
 The Standard of Atomic Weights. Although 
 hydrogen, having the lowest atomic weight, 
 was at first naturally taken as the standard, for 
 practical purposes this element is by no means 
 a convenient one. It is evident that any 
 element of which the equivalent with respect to 
 hydrogen has been determined with precision 
 may be made the standard ; the translation to 
 the hydrogen scale of the atomic weight deter- 
 mined with reference to such other element 
 then becomes a matter of calculation. Hydro- 
 gen forms but few compounds with other 
 elements which admit of satisfactory manipula- 
 tion for quantitative purposes. Moreover, the 
 
ATOMIC WEIGHTS 193 
 
 gas itself is so light (0-0695 when air = 1) that 
 it tends to diffuse out of all vessels, and is 
 difficult to weigh, so that the direct deter- 
 mination of its atomic weight by the observa- 
 tion of its density (p. 166) is liable to error. 
 Oxygen, on the other hand, forms compounds 
 with most of the elements ; and, in fact, many 
 of the equivalents which have been deter- 
 mined are based upon the analysis of oxygen 
 compounds. This gas has also the advantage 
 of being heavier than air (1-105 when air =1), 
 so that its density can be determined with less 
 liability to error than hydrogen. For these 
 and other reasons, oxygen is now made the 
 standard of atomic weights ; and in the 
 international list the numbers adopted are 
 relative to that element. It will be under- 
 stood, therefore, that the weight of the atom 
 of oxygen with reference to hydrogen is a 
 matter of extreme importance. The ratio of 
 oxygen to hydrogen in water obviously furn- 
 ishes the necessary data ; and the concentra- 
 tion of patience and skill which has been 
 brought to bear upon the determination of 
 that ratio by Morley and others will rank in the 
 future history of Chemistry among the greatest 
 achievements in scientific precision. 
 
 The refined experimental methods made 
 use of in fixing this ratio cannot be described 
 
194 CHEMISTRY 
 
 here. The equivalent 8 for oxygen when 
 hydrogen = 1 (p. 119) must be corrected in the 
 light of the most precise evidence to 7-94 ; 
 and so the atomic weight of oxygen on this 
 scale becomes 15-88. Or, if 8 is taken as the 
 equivalent of oxygen, then the atomic weight 
 of hydrogen becomes 1-008, because 1 : 7-94 
 = 1-008 : 8 (neglecting the last decimals). The 
 molecular weight of oxygen is thus 31-76 on 
 the scale H= 1, or 32 on the scale H= 1-008 ; 
 and the molecular weight of hydrogen is then 
 2-016. These results, be it remembered, are 
 based on experimental determinations ; and 
 the atomic weights will, therefore, differ 
 according to the standard adopted, but 
 and here the whole mystery should disappear 
 the relative weights of the atoms towards 
 each other remain unchanged, and that is all 
 that we are concerned with in considering 
 the weight of an atom from the chemical 
 point of view. Consider a case. On the scale 
 H = 1 and O = 15-88, the atomic weight of 
 sulphur is 31-83 ; on the scale H = 1-008 and 
 O = 16, it is 32-07. There is no real discrep- 
 ancy here it is simply a question of stand- 
 ard ; the ratios remain the same : neglecting 
 last decimals, 31-83 : 15-88 = 32-07 : 16, or 
 31-83 : 1 = 32-07 : 1-008 ; and so with refer- 
 ence to the comparison of the weight of the 
 
CHEMICAL ARITHMETIC 195 
 
 sulphur atom with the weight of any other 
 atom on the two scales. The two sets of 
 atomic weights now in existence should thus 
 be easily understood ; the standard O = 16 
 has been adopted for the reasons already 
 stated. The atomic weights assigned to the 
 elements referred to in the previous chapters 
 have, as far as possible, been taken as whole 
 numbers. It will now be understood that 
 these numbers were used only for the sake 
 of simplicity, and in order to illustrate general 
 principles. 
 
 Chemical Arithmetic. The symbolical lan- 
 guage which was introduced in a former 
 chapter (p. 137) is obviously quantitative. 
 The formulae assigned to atoms and molecules 
 represent so much by weight of the respective 
 elements or compounds, the degree of numeri- 
 cal precision attaching to the symbols being 
 the degree of accuracy with which the relative 
 weights of the atoms have been determined. 
 It will be seen, therefore, that when we know 
 the formula of a compound, or when we know 
 the composition of the products arising from 
 chemical reaction between materials of known 
 composition, we must necessarily bring such 
 symbols into the domain of arithmetical 
 treatment. The construction of a formula 
 from the results of analysis is in itself an 
 
196 CHEMISTRY 
 
 arithmetical problem. For example, water 
 has been said to contain (in round numbers) 
 oxygen and hydrogen in the proportion 8 : 1 
 this is the outcome of analysis and synthe- 
 sis ; and, as is the universal custom, the 
 actual numerical results are in the first place 
 stated on the percentage scale ; oxygen 
 88-9, and hydrogen 11-1 per cent., approxi- 
 mately. That gives the ratio 8:1. But for 
 the formula we want not the weight ratios 
 only, but the ratios between the numbers of 
 atoms in the molecule. The number of times 
 the atomic weights of oxygen and hydrogen 
 are contained in these percentage numbers 
 will evidently give the required information. 
 Thus 88-9 -M6 = 5-55, and 11-1 -^ 1 = 11-1, 
 and so the ratio between the numbers of atoms 
 is 5-55 : 11*1 ; then, since the atom is by 
 hypothesis indivisible, we take the nearest 
 whole numbers, and so make the ratio 1 : 2. 
 The formula is thus found to be H 2 O ; and if 
 it should happen to be a multiple of this 
 (which in this case it is in the liquid state ; p. 
 172), it makes no difference to the relative 
 numbers the minimum formula only is 
 found by this calculation, and the molecular 
 formula of the vapour is settled by the 
 determination of the molecular weight as 
 already explained. 
 
CHEMICAL ARITHMETIC 197 
 
 The calculation of a formula from the 
 percentage composition is, therefore, simple 
 enough ; the percentage numbers divided by 
 the atomic weights and the results expressed 
 in the simplest set of whole numbers give the 
 relative numbers of the respective atoms in 
 the molecule ; the actual numbers can only 
 be found when the molecular weight can be 
 determined. Conversely, the formula being 
 known, the percentage composition can be 
 easily calculated. Sugar, for example, is 
 found by analysis to contain carbon, hydro- 
 gen, and oxygen in such proportions as to 
 lead to the formula C 12 H 22 O n (p. 177). The 
 sum total of the weights of the atoms being 
 in round numbers 342, that weight contains 
 144 parts of carbon, 22 parts of hydrogen 
 and 176 parts of oxygen, from which fact 
 the weights of carbon, hydrogen and oxygen 
 contained in 100 parts of sugar can obviously 
 be calculated by a very familiar process. 
 
 It is unnecessary here to set problems or to 
 work out examples ; we are concerned only 
 with those broad principles which serve to 
 illustrate the enormous power which the 
 atomic theory has placed in the hands of the 
 chemist. An obvious extension of the fore- 
 going principles enables us, for instance, to 
 calculate the theoretical yields of products 
 
198 CHEMISTRY 
 
 resulting from chemical change. Such changes 
 chemical reactions are capable of being 
 represented in the form of equations showing 
 the arrangement of the materials before and 
 after the reaction (pp. 169 and 187). Consider, 
 by way of illustration, the burning of iron in 
 oxygen to form scale (p. 23), the decomposition 
 of water by heated iron to form this same 
 oxide (p. 81), or the reduction of copper 
 oxide to copper by means of hydrogen just 
 referred to (p. 187). The equations are : 
 
 3Fe+20 2 = Fe 3 4 ; 
 
 3 Fe+ 4 H 2 O = Fe 3 O 4 + 4 H 2 
 
 CuO + H 2 = H 2 O + Cu. 
 
 It is obvious that, given the atomic weights, 
 all these reactions can be dealt with arith- 
 metically. Using the nearest whole numbers, 
 
 3 x 56 parts of iron yield 3 x 56 + 16 x 4 parts 
 of magnetic oxide ; 3 x 56 parts of iron use up 
 
 4 x (16+ 2) parts of water to form the weight 
 of oxide represented by Fe 3 O 4 , and liberate 8 
 parts of hydrogen ; the weight of copper oxide 
 represented by CuO furnishes the weight of 
 copper represented by the atomic weight, i.e., 
 63-6 when O = 16, or 63-1 when H = 1. From 
 such data it is, of course, easy to find out how 
 much magnetic oxide can be obtained from 
 any given weight of iron, how much hydrogen 
 
CHEMICAL ARITHMETIC 199 
 
 a known weight of iron will yield, or how 
 much copper and how much water can be 
 obtained from a certain quantity of copper 
 oxide. 
 
 It is needless to enlarge upon this topic, 
 but the practical bearing of theory upon 
 industrial processes finds no better illustration 
 in the whole domain of applied science than 
 in this power of prevision which the chemist 
 is thus enabled to wield. From known 
 weights of materials reacting in a known 
 way and giving rise to known products, it is 
 possible to calculate what the yields should be. 
 Not that theoretical yields are often actually 
 realized in practice there are generally 
 secondary changes, unavoidable losses, and 
 so forth ; but the nearer the theoretical yield 
 the more successful the industry the trans- 
 formation of industrial empiricism into scienti- 
 fic procedure is measured quantitatively 
 by the percentage of the theoretical yield. 
 It is only in the most refined laboratory 
 operations that the requirements of theory 
 and the results of experiment coincide. 
 
 Volumetric Relationships. The volumes 
 occupied by known weights of gaseous com- 
 pounds being known, it is evident that 
 here also arithmetical treatment is possible. 
 Facts and arguments have already made it 
 
200 CHEMISTRY 
 
 clear to the reader that in the gaseous state 
 the molecular weights of all non-dissociable 
 elements and compounds occupy the same 
 volumes. This is the obverse side of the 
 hypothesis of Avogadro (p. 157). In order 
 to simplify matters, we can, therefore, take 
 as reference unit some standard element of 
 which the volume of a given weight is known 
 with accuracy. In fact, the molecule of 
 hydrogen in grams, 2 x 1-008 = 2-016 (p. 194), 
 occupies 22-4 litres at the standard tempera- 
 ture and pressure of and 760 mm. The 
 gram-molecule of every non-dissociable element 
 and compound, therefore, under these com- 
 parable conditions fills a space of 22-4 litres. 
 The rest is simple arithmetic. A given weight, 
 say of iron, yields so much by weight of 
 hydrogen : 2-016 grams of hydrogen at 
 and 760 mm. occupy 22-4 litres ; therefore 
 the volume of hydrogen liberated by the 
 given weight of iron will be so many litres 
 under similar conditions. Again, carbon 
 (C = 12) when heated in oxygen burns, i.e., 
 undergoes combustion with the formation of 
 the dioxide, CO 2 (p. 125). The reaction is 
 C -|- O a = CO 2 , whence 12 grams of carbon 
 give 12 +(16 x 2) =44 grams of the dioxide. 
 That weight occupies the same volume as the 
 molecule of hydrogen under comparable con- 
 
CHEMICAL ARITHMETIC 201 
 
 ditions it is the gram-molecule of carbon 
 dioxide ; so that if 12 grams of carbon give 
 22-4 litres of the dioxide under the conditions 
 specified, the volume of gas given by any 
 specified weight of carbon can be calculated. 
 
 From the conception of equimolecular 
 weights, and therefore of equal numbers of 
 molecules of elements and compounds occupy- 
 ing equal volumes under comparable condi- 
 tions, it is but a natural step to the conception 
 of solutions containing equal numbers of 
 molecules in equal volumes. Such solutions 
 can, of course, be prepared. The molecular 
 weight, or some known fraction of that weight, 
 in grams of any compound AB dissolved in 
 some standard volume of a liquid, say water, 
 contains a known weight of AB per cubic 
 centimetre. The standard used is the litre 
 (1000 cubic centimetres), and a solution 
 containing the gram molecule AB per litre is 
 called a normal solution. This solution can 
 be diluted with water if necessary, and made 
 up to any required volume so as to be semi- 
 normal (J AB per litre), deci-normal (T\T AB 
 per litre), and so forth. The point is that 
 such solutions contain known quantities of 
 AB per cubic centimetre, and can be measured 
 out with great accuracy by suitable apparatus. 
 
 Supposing, now, that the compound AB is 
 
202 CHEMISTRY 
 
 capable of reacting with another compound 
 CD in solution, so as to form new substances 
 by interchange of components, then we have 
 a chemical change brought about by what is 
 termed double decomposition. Such change 
 can be expressed by the usual equation : 
 AB + CD = AC -f BD, and in this form can 
 be dealt with arithmetically. It will be seen 
 that we have in this case quantities of two 
 compounds which may be regarded as chemi- 
 cally equivalent (p. 121). If, therefore, the 
 precise point could be detected at which, on 
 adding the solution of AB, the whole quantity 
 of CD present was transformed by interaction 
 with AB into the two new substances, we 
 should be enabled to determine the weight 
 of CD in a solution containing an unknown 
 quantity of that compound. In other words, 
 if a solution of AB of known value is mixed 
 continuously with the solution of CD and the 
 addition arrested at the precise point when the 
 equivalent weight of AB has been added, 
 we can calculate the quantity of CD present 
 in its solution. The weight of AB is known 
 from the measured volume delivered ; the 
 weight of CD contained in a measured volume 
 of its solution is at first unknown, but the 
 weight equivalent to so many grams or frac- 
 tions of grams of AB is settled by the weight 
 
VOLUMETRIC ANALYSIS 203 
 
 of AB added. So many cubic centimetres of 
 the solution of CD required so many cubic 
 centimetres of the solution of AB to hit the 
 point of equivalence the rest is a matter of 
 arithmetic. 
 
 All these conditions can be realized ex- 
 perimentally. There are large numbers of 
 chemical reactions of this type which are so 
 sharp that the exact point when equivalence 
 between the reacting materials has been struck 
 can be indicated by the change of colour of 
 some added substance which is not chemically 
 concerned in the reaction (known as an 
 indicator), or by other means. On this 
 principle which is necessarily outlined here 
 only in a very broad way there is founded 
 a largely used and beautifully delicate method 
 of determining with very great precision the 
 actual weights of substances contained in 
 solutions. It is the method appropriately 
 known as Volumetric Analysis. 
 
CHAPTER IX 
 
 VALENCY CHEMICAL STRUCTURE THE 
 CHEMISTRY OF CARBON STEREO- 
 CHEMISTRY 
 
 Valency. In its simplest aspect, chemical 
 combination is, from the point of view now 
 reached, union between elementary atoms. 
 The equivalent being that weight of an element 
 which combines with or replaces one part by 
 weight, i.e., one atom of hydrogen (p. 119), 
 it follows that the number of times the 
 equivalent is contained in the atomic weight 
 of any particular element will represent the 
 capacity of that particular atom for combining 
 with hydrogen. If the element does not form 
 compounds with hydrogen, then the number 
 expresses the combining capacity of the atom 
 for some other atom which is equivalent to 
 the hydrogen atom. The principle is a 
 familiar one atoms which are equivalent 
 to the same atom (hydrogen) are equivalent 
 to one another. A new property of the 
 chemical atom is thus brought out, viz., its 
 204 
 
VALENCY 205 
 
 value as measured by the number of atoms 
 of hydrogen or equivalent atoms with which it 
 can combine. This property is appropriately 
 described as the valency of the atom. If the 
 atomic weight contains the equivalent once, 
 i.e., if the equivalent and atomic weight 
 are identical, that atom can combine only 
 with one atom of hydrogen or of chlorine, 
 bromine, etc. The formulae of the com- 
 pounds HC1, HBr, etc. (p. 165), express this 
 fact. If the equivalent is contained twice 
 in the atomic weight, then that atom can 
 obviously combine with two atoms of hydro- 
 gen, chlorine, etc. ; if it is contained three 
 times in the atomic weight, the combining 
 capacity or valency of the atom is three, and 
 so forth. 
 
 In the light of this principle, consider some 
 of the compounds which have already been 
 made use of for the purpose of illustrating 
 other principles. The atomic weight of oxy- 
 gen (16) contains two equivalents (8) ; the 
 atom of oxygen can therefore combine with 
 two atoms of hydrogen, as in water, H 2 O. 
 The atomic weight of nitrogen (taken as 14) 
 contains the equivalent (4|) three times, so 
 that the nitrogen atom combines with three 
 atoms of hydrogen, as in ammonia, NH 3 . 
 Again, the atomic weight of carbon (taken as 
 
206 CHEMISTRY 
 
 12) contains the lowest equivalent (3) four 
 times ; the simplest known compound of 
 carbon with hydrogen is methane, or marsh 
 gas, CH 4 , the gas which is formed by the 
 decay of vegetable matter under water, and 
 which rises in bubbles to the surface when the 
 mud at the bottom of a stagnant pond is 
 stirred up. Moreover, since one atom of car- 
 bon can combine with or be saturated by four 
 atoms of hydrogen, and since two atoms of 
 hydrogen combine with one atom of oxygen, 
 two atoms of oxygen are equivalent in com- 
 bining value to four atoms of hydrogen ; and 
 so the oxide of carbon referred to as the product 
 of the burning of carbon in oxygen (p. 125) is 
 the dioxide, CO 2 . 
 
 The broad principle of valency should now 
 become clear without further illustration. 
 It will be seen that we have in this doctrine a 
 means of classifying the atoms irrespective 
 of their nature, i.e., of the kind of elementary 
 matter built up of such atoms. Hydrogen, 
 chlorine, etc., are univalent elements, oxygen 
 bivalent, nitrogen tervalent, carbon quadri- 
 valent, and so forth. It will be seen, also, 
 that the valency is not always a fixed number, 
 but, since an element can have more than one 
 equivalent, in some cases the atom must 
 have more than one valency. Carbon, for 
 
VALENCY 207 
 
 example, is generally quadrivalent, but in 
 some of its compounds it may be bivalent ; 
 iron in the oxide FeO and in the iodide, 
 FeI 2 (p. 38) is bivalent, in the oxide of rust, 
 Fe 2 O 3 , it may be tervalent or quadrivalent. 
 Sulphur is quadrivalent in the dioxide, which 
 is formed when the element burns in air, 
 SO 2 (p. 128) ; the latter can be made to com- 
 bine with another atom of oxygen to form a 
 trioxide, SO 3 , (p. 128), in which the sulphur 
 atom, being combined with 3 bivalent oxygen 
 atoms, is itself sexavalent. Moreover, sulphur 
 forms a gaseous compound with hydrogen, 
 hydrogen sulphide or sulphuretted hydrogen, 
 in which one atom of sulphur is combined 
 with two atoms of hydrogen, SH 2 ; the 
 atom is bivalent. 
 
 The conception of valency may, perhaps, 
 on these grounds, be regarded as somewhat 
 vague ; nevertheless, as will be seen immedi- 
 ately, with all its imperfections it has played 
 an exceedingly important part in the develop- 
 ment of modern Chemistry. It corresponds 
 with some underlying reality ; whether it is 
 an inherent property of the atom as an 
 individual particle, or the result of the inter- 
 action of reciprocal forces exerted between 
 combining atoms, cannot at present be 
 decided. It is an empirical doctrine as it 
 
208 CHEMISTRY 
 
 stands it is descriptive rather than ex- 
 planatory ; yet it describes some faculty 
 potentially or actually present in the atomic 
 mechanism. It serves as a check upon chemi- 
 cal formulation, and yet it fails to explain 
 why any particular formula is possible. In 
 other words, it enables us to assert that, when 
 combination does take place, certain rules 
 more or less elastic are obeyed. But it does 
 not tell us why this, that, or the other atom 
 can combine with certain atoms and not 
 with others ; it takes no account of selective 
 idiosyncrasies beyond representing the facts 
 which are the expressions of these idiosyn- 
 crasies. The fact that valency is associated 
 with certain physical properties of the atom is 
 sufficient indication that there is an under- 
 lying reality. 
 
 Consider, for instance, the fact that the 
 same electric current causes the liberation of 
 equivalent weights of chemical elements (p. 
 121). This may be interpreted as an ind ication 
 that equivalent weights of the elements are 
 associated with equal quantities of electricity 
 that chemically equivalent weights are also 
 electrically equivalent. One step further, and 
 we arrive at the conclusion that, if an 
 element of which the equivalent and atomic 
 weights are identical is associated with on* 
 
VALENCY 209 
 
 unit of electricity, an atom of which the 
 atomic weight is n times the equivalent must 
 carry n units of electricity. In other words, 
 a hint that valency may be connected with, 
 if not actually expressive of, the electric 
 constitution of the atom is hereby suggested 
 (Helmholtz, 1881). 
 
 Again, if the capacity for combination 
 possessed by various atoms is expressed as 
 a geometrical conception, it may be con- 
 sidered that the space required for the group- 
 ing of the attached atoms to the central 
 attracting atom will be larger in proportion 
 with the increase of valency of the latter. 
 The " sphere of influence "of a bivalent 
 atom would be double that of a univalent 
 atom, of a quadrivalent atom four times 
 that of a univalent atom and so forth. Here, 
 also, there is correspondence with fact ; the 
 packing in space of the atoms which build 
 up a molecule, finds external expression in the 
 regular crystalline form which characterises 
 the majority of definite chemical compounds. 
 There is correspondence between the archi- 
 tecture of the solid crystal, regarded as a 
 structure built up of molecules, and the 
 volume occupied by these molecules regarded 
 as assemblages of atoms, each occupying its 
 own " sphere of influence " (Barlow and 
 
210 CHEMISTRY 
 
 Pope, 1906). The conception of the valency 
 of the atom, evolved originally from the study 
 of chemical compounds (Frankland, 1853 ; 
 Kekule, 1860), has thus been extended into 
 the domain of Crystallography, that branch 
 of science which is concerned with the study 
 of the definite geometrical forms assumed 
 by elements and compounds under appropriate 
 conditions. 
 
 Chemical Structure. The actual grouping 
 of the atoms composing a molecule may at 
 first sight appear to be a problem beyond 
 human ken. Nevertheless the doctrine of 
 valency, in spite of its incompleteness, has 
 enabled chemists to make enormous advances 
 in this direction. The study of chemical 
 composition and decomposition of the results 
 of dissecting the molecule piecemeal and, 
 when possible, of reconstructing it from its 
 component atoms renders it possible to 
 form a mental picture of the way in which 
 the atoms are grouped. That mental picture 
 was a blurred and imperfect representation 
 until, at the touch of the conception of 
 valency, definiteness was substituted for 
 vagueness, and the chemist provided with a 
 means of translating his crude imagery into 
 manageable formulae. 
 
 This development is made possible by 
 
CHEMICAL STRUCTURE 211 
 
 the simple device of joining the atoms together 
 not in an arbitrary way, as has been done 
 in the preceding chapters, but in the order 
 which represents the actual state of their 
 combination. Furthermore, the combination 
 between the atoms is, so to say, visualised by 
 giving each atom its valency, and by imagin- 
 ing that the lines of force which bind the 
 atoms are real lines. These " bonds " are, of 
 course, imaginary ; they have no more real 
 existence than the " lines of force " round the 
 poles of a magnet. It is not even necessary in 
 practice to use lines at all ; dots will do 
 equally well, and enable the formulae to be 
 written much more concisely. Begin with 
 an abstract case, and the principle will 
 become clear. 
 
 A molecule is composed, say, of AB 2 C. 
 The mere juxtaposition of symbols here 
 indicates chemical union, but the formula 
 does not show how the atoms are grouped 
 whether A is combined with B or with 
 C, or with both ; whether B is combined 
 with B or with A or C, and so forth. Suppos- 
 ing now that it was known definitely from 
 the mode of decomposition of the molecule 
 that C was combined with A, and also with 
 both atoms of B, then the formula would more 
 nearly express the facts if it were written 
 
212 CHEMISTRY 
 
 ACB 2 . But each of the atoms concerned 
 in building up this molecule has its propel 
 valency, and can therefore be " bonded " with 
 its associates in such a way as to indicate 
 the full structure or mode of grouping thus, 
 
 or, more briefly, A:C:B 2 , in which 
 
 formulae it will be seen that A is bivalent, C 
 quadrivalent and B univalent. This is a type 
 of what is known as a structural or constitu- 
 tional formula. Such a formula is obviously 
 more expressive of the reality of structure 
 than the empirical formula AB 2 C, which tells 
 us nothing more than that the molecule 
 contains one atom of A, two of B, and one 
 of C. All the compounds which have been re- 
 ferred to in former chapters can be formulated 
 on this principle. Thus, H - Cl ; H - O - H, 
 N-H 3 , CnH 4 , C = 0, O = C = O, Fe = I 2 , 
 Fe = S, etc., 'stand as structural formulae 
 for hydrogen chloride, water, ammonia, 
 methane, carbon monoxide, carbon dioxide, 
 ferrous iodide, ferrous sulphide. The valen- 
 cies of the atoms are evident in such formulae ; 
 not only the simple valencies of elements in 
 which the atomic and equivalent weights are 
 identical, but also the variable valencies due 
 to multiple equivalence, as in the case of the 
 above oxides of carbon, in which the? atom 
 
CHEMICAL STRUCTURE 213 
 
 may be bivalent or quadrivalent, or in sul- 
 phuretted hydrogen and the oxides of sulphur 
 (p. 128) : 
 
 H-S-H, = S = 0, OnS, in which 
 
 sulphur is bivalent, quadrivalent, or sex- 
 avalent. 
 
 Structural or constitutional formulae must 
 obviously increase in complexity with increase 
 in the number of atoms composing the mole- 
 cules. They may be written fully spread out 
 with all the bonds represented by lines, and 
 so expressed as " graphic formulae," although 
 in practice the chemist soon becomes accus- 
 tomed to substitute dots for lines, and to 
 pack up the symbols so as to occupy as little 
 space as possible. The significance of the 
 formula is in no way impaired by such con- 
 densation. It will be realized that this method 
 of formulation corresponds with something 
 very real, since it expresses the sum total of 
 ascertained fact it is the pictorial representa- 
 tion of the mode of attachment of the various 
 atoms in a molecule as ascertained by experi- 
 ment. The chemical structure of a mole- 
 cule is not a purely mathematical problem in 
 permutations and combinations ; it is some- 
 thing more than this it is a problem in 
 permutations and combinations controlled 
 
214 CHEMISTRY 
 
 by fixed molecular architectural require- 
 ments necessitated by the valencies of the 
 atoms. 
 
 The conception of chemical structure is 
 obviously of the same order as the conception 
 of valency upon which it is founded it is 
 descriptive rather than explanatory. The 
 experimental evidence upon which a structural 
 formula is based is often difficult to obtain, 
 and still more frequently difficult to interpret. 
 The final aim of the investigator is to be 
 able to represent the atomic structure of 
 molecules by such formulae. The progress 
 which has been made in this direction is 
 synonymous with the progress of modern 
 Chemistry. By every available method of 
 attack is this problem approached by pulling 
 down and building up by chemical methods, 
 or by comparative physical methods starting 
 from certain measurable physical properties 
 correlated with simple compounds of known 
 structure, and extending the results to mole- 
 cules of greater and greater complexity. Such 
 imperfection as exists in our structural 
 formulae is due partly to imperfect informa- 
 tion concerning the actual mode of grouping 
 of the atoms, and partly to that empiricism 
 which at present attaches to the conception 
 of valency. In a molecule composed of a few 
 
CHEMICAL STRUCTURE 215 
 
 atoms, the possibilities of grouping are neces- 
 sarily limited ; when, as in the case of many 
 of the highly complex compounds of carbon 
 which build up the tissues of living organisms, 
 we have molecules composed of hundreds of 
 atoms, we have passed into a new order of 
 things a domain at present beyond the 
 resources of structural formulation. 
 
 So, also, with respect to the graphic repre- 
 sentation of definite compounds of known 
 structure, there are limitations imposed by 
 our ignorance of the true cause of valency. 
 Thus, when a structural formula has been 
 assigned to a particular compound, and all 
 the maximum valencies of the various atoms 
 have been accounted for, the molecule as a 
 whole may still possess the faculty of com- 
 bining with other molecules. In other words, 
 molecules, the valencies of the atoms of which 
 are all apparently in chemical language 
 saturated, may still behave as unsaturated. 
 Such molecular compounds are quite definite, 
 but a new order of combination appears to 
 come into play. Many metallic salts, for 
 instance, combine with definite quantities of 
 water of crystallization, or with definite 
 numbers of molecules of ammonia or other 
 compounds. The oxide of iron of " scale," 
 Fe 3 O 4 , may be a molecular compound of 
 
216 CHEMISTRY 
 
 FeO and Fe 2 O 3 . Thus it has been found 
 necessary to extend the main doctrine by 
 recognizing residual affinities, partial valencies, 
 auxiliary valencies, etc. A new field has, in 
 fact, been opened up in this direction ; and 
 the completion of the theory of valency cannot 
 be looked for until this field has been more 
 extensively cultivated. The pioneers are at 
 work, and considerable advances have been 
 made of late years. 
 
 But the defects of the theory which have 
 been indicated in no way detract from its 
 utility as a means of representing chemical 
 structure as far as its applications can be 
 pushed. The bare recognition of the principle 
 that chemical character and atomic configura- 
 tion are interdependent marks an epoch in 
 chemical thought. Thus, it has long been 
 known that certain elements are capable of 
 existing in different forms the so-called 
 allotropic modifications. Oxygen, for instance, 
 under the influence of the silent electric dis- 
 charge assumes a different and more active 
 form known as ozone. In this case there is 
 no transformation of matter ; ozone is still 
 at basis oxygen. It is known that the change 
 in character is due to a difference in the state 
 of aggregation, the molecule of ozone being 
 triatomic, O 3 , whereas oxygen is diatomic, 
 
CHEMICAL STRUCTURE 217 
 
 O 2 (p. 160). This is proved by the vapour den- 
 sity of ozone, or by the conversion of a known 
 quantity of ozone into oxygen by the action 
 of heat. Oxygen being bivalent or quad- 
 rivalent, the structural formula of oxygen 
 would be O : O, and of ozone (jO'o or O : O : O. 
 Carbon, again, as an element is the same 
 " stuff " whether transparent and crystalline, 
 as in the diamond, or black and opaque as in 
 graphite and charcoal. Phosphorus, also, 
 may be an exceedingly inflammable wax-like 
 solid with a low melting-point, or a compara- 
 tively inert reddish powder : The two forms 
 are interconvertible. 
 
 Such cases are possibly explained by 
 differences in the state of atomic aggregation 
 they may belong to the same category as 
 oxygen and ozone, only the molecular weight 
 of solid carbon or red phosphorus cannot, for 
 the reasons already set forth (p. 172), at present 
 be determined ; and so the actual numbers 
 of atoms contained in the different modifica- 
 tions cannot be definitely fixed. But inde- 
 pendently of the question of molecular weight, 
 the conception of structure asserts itself. A 
 difference of molecular aggregation in a sense 
 implies difference of structure ; but it will 
 also be seen that there might be difference of 
 structure due to different configurations of 
 
218 CHEMISTRY 
 
 atoms within the same molecule. Sulphur, 
 for example, exists in several modifications, 
 four solid and crystalline, one a yellow mobile 
 liquid (119 160), one a dark viscous sub- 
 stance (160 - 448-5), and a vapour above 
 the latter temperature. The vapour exists, as 
 we know (p. 167), in different states of atomic 
 aggregation. The differences between the 
 crystalline forms are probably due to differ- 
 ences of molecular arrangement, i.e., to physi- 
 cal structure ; the difference between the 
 solid and the other forms may be due to 
 differences of atomic aggregation, as with 
 oxygen and ozone ; while the difference 
 between the liquid and viscous forms may be 
 due either to differences of atomic aggregation 
 or to differences of atomic grouping within 
 the molecule. It is evident that the decision 
 between molecular grouping, atomic aggrega- 
 tion, and differences of intra-molecular struc- 
 ture can only be given when the molecular 
 weights of the solid and liquid forms can be 
 determined. 
 
 The Chemistry of Carbon. While the con- 
 ception of structure can at present be borne 
 in mind in its possible application to such 
 cases as the above, the triumph of the theory 
 finds full expression in the domain of what is 
 called Organic Chemistry. This term is a 
 
CHEMISTRY OF CARBON 219 
 
 survival from a period when the compounds 
 dealt with under this division were supposed 
 to be producible only through living agency, 
 i.e., by organisms belonging to the animal or 
 vegetable kingdom. With the progress of 
 science, this meaning of the term " organic " 
 disappears, since large numbers of these 
 compounds identical with the natural pro- 
 ducts are now produced by synthetical 
 methods in our laboratories, and many of them 
 of technical use are manufactured on a colossal 
 scale. Organic Chemistry is, in fact, the 
 chemistry of carbon, since it is this element 
 which, in combination with a few other 
 elements, notably hydrogen, oxygen, and 
 nitrogen, gives rise to the vast multitude of 
 compounds of every degree of complexity 
 which constitute the materials which, with 
 certain mineral or inorganic constituents, 
 build up all animals and plants. The ques- 
 tion whether " vitality " may be a function 
 of the special chemical and physical properties 
 due to the association of carbon with certain 
 other elements is of profound interest, but 
 cannot be discussed here. To the chemist, 
 organic matter is carbonaceous matter ; mod- 
 ern science knows of no " vitalism " apart 
 from carbon compounds. 
 
 The division of Chemistry into inorganic 
 
220 CHEMISTRY 
 
 and organic is now only a matter of con- 
 venience arising from the enormous mass of 
 material supplied by carbon chemistry. Over 
 150,000 definite " organic " compounds are 
 known, a small fraction only of this number 
 being the products of " vitality." The re- 
 mainder are all artificial synthetical com- 
 pounds unknown in nature till called into 
 existence by the exercise of the power of 
 chemical science over the inner constitution of 
 molecules, a power made possible mainly by 
 the theory of structure based upon valency. 
 In recording this triumphant success of syn- 
 thetical chemistry, it must be emphasized 
 that there has as yet been produced in the 
 laboratory no compound possessing in the 
 least degree those characters which pertain 
 to living organic matter. The great and 
 fundamental problem of bridging the gap 
 between living and dead matter still remains 
 unsolved ; it may remain unsolved for all 
 time, or it may not. For aught that we 
 know, Nature may be solving this problem 
 daily under our eyes, but her methods have 
 as yet remained unrevealed. Any revelation 
 in this direction that awaits the science of 
 the future must perforce be through Organic 
 Chemistry. 
 
 The potency of structural formulation as 
 
CHEMISTRY OF CARBON 221 
 
 a means of giving expression to molecular 
 structure is manifest among carbon com- 
 pounds in every direction. The applications 
 are so numerous the field is so vast that 
 only the barest hints can be thrown out in this 
 volume. It will be perceived as a general 
 principle that, with increase in complexity, 
 i.e., with increase in the number of atoms 
 composing the molecule, there must result a 
 continually and rapidly augmenting number 
 of possible formulae. Are these formulae 
 real ? does each different configuration of 
 atoms correspond with some definite chemical 
 compound ? in brief, how far do such graphic 
 representations express the facts of chemical 
 science ? 
 
 The answer to these questions carries with 
 it the vindication of the claim of the theory of 
 structure to run parallel with, if not actually 
 to represent, the underlying physical reality 
 of the atomic configuration of molecules. 
 So close is the correspondence between theory 
 and observation that, given the number of 
 atoms composing a molecule, it is possible 
 to predict the number of compounds that 
 ought to exist, with full confidence that such 
 compounds may actually be obtainable. In 
 hundreds of the simpler cases, the corre- 
 spondence of theory with fact is complete 
 
222 CHEMISTRY 
 
 every compound required by theory has been 
 prepared. There, thus emerges the great 
 principle of isomerism, which simply indicates 
 that totally different compounds may be built 
 up of the same numbers of atoms of the 
 same elements. It is not only the total 
 number of> atoms, but the grouping of the 
 atoms within the molecule that is the deter- 
 mining cause of individuality. A phenomenon 
 which at first appeared at variance with all 
 common sense notions the fact which stag- 
 gered the early investigators in this field, 
 viz., that two or more molecules might have 
 precisely the same ultimate composition, and 
 yet be quite distinct forms of matter, has 
 become a commonplace doctrine in modern 
 science in the light of the theory of chemical 
 structure. Moreover, the development of 
 structural chemistry has of late years led to 
 the recognition of the internal mobility of 
 atoms within molecules of structural con- 
 figurations so delicately balanced that the 
 atoms constituting the molecule may assume 
 one or another of two quite different configura- 
 tions according to the conditions to which 
 it is exposed. To this phenomenon, the 
 general term tautomerism is applied. 
 
 Stereochemistry. Yet another step in the 
 theory of chemical structure, and we are 
 
STEREO-CHEMISTRY 223 
 
 face to face with one of the most brilliant of 
 modern achievements in the direction of 
 bringing Chemistry into the category of the 
 deductive sciences. The atoms composing 
 a molecule must obviously form a group in 
 space the configuration is not that of a 
 congeries of points all lying in one plane, but 
 a system occupying tridimensional space. 
 This conception was first definitely applied 
 to the structural formulation of carbon com- 
 pounds by Le Bel and van't Hoff in 1874. 
 The four " bonds " of the carbon atom, for 
 example, may be represented by lines radiating 
 symmetrically from the carbon atom as a 
 centre. This is expressed geometrically by 
 supposing that the carbon atom is in the 
 centre of a regular tetrahedron, the points of 
 the angles of which represent the terminations 
 of the bonds to which are attached the other 
 atoms or groups of atoms which build up 
 the molecule. If the carbon atom, regarded 
 from this point of view, is combined with four 
 different atoms or groups of atoms, there then 
 arises as a geometrical necessity the existence 
 of two different space groupings of the same 
 molecule which are non-superposable, and 
 which are related to each other in the same 
 way that an object is related to its reflected 
 image in a mirror a right and a left-handed 
 
224 CHEMISTRY 
 
 isomerism quite incapable of being represented 
 by any formula which ignores the space con- 
 figuration of the atoms. This conception in 
 its modern developments may almost be said 
 to complete the theory of isomerism ; large 
 numbers of cases in which the differences 
 between compounds of the same ultimate 
 composition cannot be expressed by plane 
 surface structural formulae are now known to 
 be cases of stereochemical isomerism. This 
 newer development of the atomic theory 
 known as Stereochemistry is gradually per- 
 vading and making its influence felt through- 
 out the whole domain of our science. The 
 fundamental idea of space grouping is not 
 easy to follow at first without the aid of models, 
 but the modern student is being taught to 
 handle these formulae which, by virtue of their 
 rationality, are bound to dominate more and 
 more all our notions concerning the structure 
 of molecules. 
 
 The possibilities of isomerism, regarded 
 from the stereochemical point of view, natur- 
 ally become more complex with the increase 
 in the number of carbon atoms which comply 
 with the conditions of asymmetry just defined. 
 Here, again, is there close parallelism between 
 deduction from the theory and observed facts 
 an everlasting testimony to the fertility 
 
STEREO-CHEMISTRY 225 
 
 of the idea. Thus, the acid of sour milk, lactic 
 acid, contains an asymmetric carbon atom, 
 and exists in two stereochemical forms, as 
 required by theory ; tartaric acid contained 
 in the juice of grape and other fruits contains 
 two asymmetric carbon atoms, and exists in 
 three stereo-isomeric forms as required by 
 theory. The group of sugars typified by grape 
 sugar, or glucose, comprised under the formula 
 C 6 H 12 O 6 , contain four asymmetric carbon 
 atoms, and can exist in sixteen stereo-isomeric 
 forms. Many of these sugars are natural pro- 
 ducts ; and nearly the whole series of sixteen 
 required by theory has been synthesized by 
 Emil Fischer and his colleagues a veritable 
 triumph of modern carbon chemistry. 
 
 One of the chief points of interest arising 
 from space formulation is the correspondence 
 between configuration and a certain physical 
 property, viz., that of optical activity, by 
 virtue of which certain compounds possess 
 the power of causing the rotation of polarized 
 light in either a right-handed (dextro) or left- 
 handed (laevo) direction. It is, in fact, by 
 this character alone that the stereo-isomerism 
 is in most cases revealed, since such isomer- 
 ides, unlike ordinary isomerides, are alike in 
 all other physical and chemical characters. In 
 the light of Stereo-chemistry, optical activity 
 
226 CHEMISTRY 
 
 is shown to be associated with this intra- 
 molecular asymmetry. Physics, Chemistry, 
 and Biology herein find another common 
 meeting ground, since it is to Physics that we 
 look for the explanation of the mechanism of 
 the connection between asymmetry and opti- 
 cal activity ; while the chemical processes 
 which go on in the living organism frequently 
 result in the apparently direct production 
 of optically active carbon compounds an 
 achievement which some chemists believe to 
 be an essential privilege of " vitalism." But 
 laboratory syntheses also result in optically 
 active compounds ; the lactic and tartaric 
 acids, the 6-carbon-atom sugars, and hosts of 
 other compounds have all been synthesized 
 in their stereo-isomeric optically active forms. 
 The main difference is that biochemical 
 synthesis is directive in the sense of leading 
 to the final production of the optically active 
 compound, while laboratory synthesis is at 
 present without such directive power the 
 two possible configurations are produced 
 simultaneously, and the final product is, there- 
 fore, optically inactive by compensation. But 
 such inactive compounds can be afterwards 
 separated or resolved into their stereo-iso- 
 merides by temporary combination with other 
 active compounds of vital origin. With the 
 
CHEMISTRY OF CARBON 227 
 
 solution of the fundamental problem of con- 
 trolling laboratory synthesis so as to suppress 
 the production of the one or the other of the 
 possible intra-molecular configurations, the 
 temporary aid of the optically active vital 
 compound would be dispensed with. There 
 would then disappear another of the barriers 
 which have been erected between living and 
 dead organic matter. 
 
 The asymmetry possible for a quadrivalent 
 atom, such as carbon, is obviously conceivable 
 in the case of other atoms. The hint given 
 by carbon chemistry has been taken with all 
 the seriousness which attaches to what the 
 man of science knows to be a great truth. 
 Other quadrivalent elements, such as tin, 
 silicon, and sulphur; quinquevalent elements 
 such as nitrogen and phosphorus, and, quite 
 recently, the atoms of certain metals such as 
 cobalt, chromium, platinum, and rhodium, 
 have been made to form optically active 
 stereo-isomerides. The reader will realize 
 that this new and vast domain which is being 
 opened up by many workers in many lands 
 invests the atom of modern Chemistry with a 
 reality so vivid that the correspondence 
 between mental imagery and observed fact 
 cannot be said to be surpassed in any of the 
 purely deductive sciences. 
 
CHAPTER X 
 
 THE PERIODIC CLASSIFICATION OF THE 
 ELEMENTS CONCLUSION 
 
 A SCIENCE which, in its purely materialistic 
 aspects, claims for its subject matter the 
 study of some eighty odd elements, and all the 
 compounds capable of being formed by these 
 elements, would be but a heterogeneous jum- 
 ble of facts without guidance from general 
 principles. It has only been possible within 
 the compass of this volume to give the reader 
 a glimpse here and there into some of these 
 principles. The treatment has perforce been 
 narrowly materialistic ; and yet it must not 
 be concluded that the chemist takes only into 
 consideration the matter of which the universe 
 is composed. The energy associated with 
 this matter its distribution during chemical 
 change, the development or absorption of 
 heat as concomitants of chemical transforma- 
 tions, the production of electricity, of light, 
 and, generally, the physical manifestations 
 
 228 
 
PERIODIC CLASSIFICATION 229 
 
 resulting from this redistribution of energy are 
 as much within the province of modern Chem- 
 istry as is the natural history of the elements 
 and their compounds. An introductory frag- 
 ment only has been offered in the hope that a 
 stimulus may be given to the desire for fuller 
 and more specialized study. 
 
 With reference to matter as such, it will be 
 readily seen that any scheme which enables 
 the whole body of elements to be grouped and 
 classified according to their natural relation- 
 ships must mark an advance towards the sys- 
 tematization of our knowledge of the highest 
 order of importance. That natural relation- 
 ships exist among the members of chemical 
 families, which possess certain characters in 
 common, and which also show regular grada- 
 tions of properties when considered in series 
 arranged in the order of their atomic weights, 
 has already been illustrated by reference to 
 the halogens (p. 100). Any other family, non- 
 metals or metals, might have been made to 
 enforce the same lesson. But such classifica- 
 tion is restricted ; a wider and more compre- 
 hensive scheme which embraces all the natural 
 groups or families of elements was first sug- 
 gested by J. A. R. Newlands in 1864, and was 
 elaborated and put upon a scientific basis by 
 Mendeleeff and Lothar Meyer in 1869. Brief 
 
230 CHEMISTRY 
 
 | o consideration to this scheme may 
 
 2 ,| g foe given in this concluding chapter. 
 If the elements are arranged in 
 
 So co rl o the order of their atomic weights, 
 
 ~ j| a remarkable recurrence will be 
 
 a, noticed after a certain number of 
 
 !> 3 i g elements have been passed through. 
 
 ^ S I* Thus, omitting hydrogen which 
 
 ^ stands in a unique position a 
 
 g OT series of eight members must be 
 
 <"! 
 
 ^ arranged in order to bring out the 
 
 fact that with the ninth there 
 g .1 ,_, begins another series, in which the 
 j$ I S5 chemical and physical properties 
 <j of the first series recur with that 
 modification due to gradational 
 transition illustrated in the case 
 e* of the halogens. The two series 
 are given here for comparison. 
 
 Reading these lists in the first 
 place in horizontal sequence, it 
 will be seen that there is con- 
 tinuous increase in atomic weight 
 from right to left. In the next 
 2 si place, it will be noticed that 
 1 If 1 l c recurrence f characters takes 
 ~ 1 & place after the eighth element 
 ^ 'I (fluorine), this recurrence being 
 ^ ^ made evident by reading the 
 
PERIODIC CLASSIFICATION 231 
 
 series vertically each vertical column as 
 here arranged consists of a pair of elements 
 belonging to the same family. Now, if the 
 whole of the chemical elements are arranged 
 on this principle, there results a general 
 scheme which brings out the fact that not only 
 are the properties of the elements connected 
 with their atomic weights, but that there is 
 periodicity in the relationships. The two 
 series given above consist of eight members 
 each ; after these, the periods become longer, 
 but the relationship between the members 
 of the vertical columns is still maintained 
 the grouping in these columns is into natural 
 families with ascending atomic weights and 
 consequent gradation of characters. The 
 law expressing this periodicity is known as the 
 Periodic Law ; and the general scheme is the 
 Periodic Classification. The Table (A; next 
 page) gives the arrangement at a glance, the 
 symbols of the elements being used for the 
 sake of compactness. The atomic weights 
 can be supplied from the international list 
 appended to this chapter. 
 
 The first point to which attention is directed 
 is naturally the periodic arrangement, which 
 is the basis of the whole scheme. The Roman 
 numerals in the first horizontal column are 
 group numbers labelling the groups below 
 
232 
 
 CHEMISTRY 
 
 o 
 
 u 
 
 2 
 
 
 I 
 
 
 PQ 
 
 O 
 
 c8 
 CO 
 
 I 
 
 I 
 
 cc 
 
 Q 
 
 Q 
 
 CO 
 
 
 
 c8 
 
 O 
 
 C/3 
 
 N 
 
 
 PQ 
 
 
 S3 
 
 W 
 
 J-l 
 CO 
 
 PQ 
 
 tf 
 
 bo 
 
 I 
 
 CM 
 
 O 
 
 w 
 
PERIODIC CLASSIFICATION 233 
 
 them in the vertical columns. The first two 
 (short) periods, consisting of eight members 
 each, are given on p. 232. The third (long) 
 period begins with argon, and ends with 
 bromine ; the fourth (long) period begins 
 with krypton, and ends with iodine. It will 
 be noticed that in these cases the elements of 
 the long periods are arranged in two horizontal 
 series, so as to bring the members of the same 
 family into their respective groups (vertical 
 columns). After the fourth group, the sys- 
 tematic uniformity is no longer maintained, 
 because, following lanthanum, there has to be 
 interpolated a whole cohort of elements which 
 are most closely related among themselves. 
 These are the so-called " rare earth " metals ; 
 and for the sake of compactness they are sim- 
 ply inserted en bloc in the order of their atomic 
 weights. The list of these metals is probably 
 still incomplete ; new members may be 
 isolated as the result of further research. 
 The known metals of this group exist as 
 compounds in a complex mixture of minerals 
 known as monazite sand, which is found in 
 various parts of the world, and which is 
 worked up on the large scale for the manufac- 
 ture of the earthy mantles used as incandes- 
 cent gas burners. These mantles are com- 
 posed mainly of the oxides of thorium and 
 
234 CHEMISTRY 
 
 cerium, supported by a suitable framework. 
 After these elements, the horizontal series 
 become fragmentary they are suggestive of 
 incomplete periods. 
 
 Considering the scheme, in the next place, 
 with reference to the natural families or 
 groups in the vertical columns, it will be 
 seen that the relationships are more faithfully 
 expressed by a further division into two sub- 
 groups. This simply indicates that, while 
 the members of the group as a whole are 
 closely related chemically, there is a still 
 closer inner relationship between the members 
 of the sub-group. These points will be made 
 clear by a few examples. Thus, in Group I. 
 will be found the very natural family of metals, 
 beginning with lithium and ending with 
 caesium, known as the alkali metals. Their 
 oxides form strongly basic, alkaline solutions 
 (p. 108) ; and they all form similar types of 
 compounds. The sub-group copper, silver 
 and gold is related to the alkali metals by 
 virtue of certain types of compounds which 
 the metals named are capable of forming ; 
 but the relationship, although confessedly 
 not very close, is more intimate between the 
 three metals themselves they resemble each 
 other more closely in their general characters 
 than they do the metals of the alkalis. In 
 
PERIODIC CLASSIFICATION 235 
 
 Group II., again, will be found the metals of 
 the "alkaline earths " (calcium, barium, etc.), 
 the oxides of which are alkaline earthy sub- 
 stances typified by lime (p. 102). The metals 
 of the sub-group (glucinum, magnesium, etc.) 
 are naturally related to these, but still more 
 closely related to each other. The halogens 
 (p. 100), as will be seen, fall into Group VII., 
 and are (somewhat remotely) related to 
 manganese. 
 
 The general nature of the scheme should be 
 made evident by these examples. It will be 
 seen that, on the whole, chemical dissimilarity 
 characterizes the members of the horizontal 
 series until the period recurs. It would 
 appear as though something added to the mass 
 of the atom caused it to differ qualitatively 
 in its chemical characters from its left-hand 
 neighbour until a certain number of addi- 
 tions had been made, when the difference 
 becomes quantitative instead of purely qualita- 
 tive. And yet this principle is not generally 
 complied with. The triplets of Group VIII., 
 for example, are separated out because, 
 among other reasons, they show close re- 
 semblance of character among themselves. 
 Still more closely related among them- 
 selves are the metals of the " rare earths," 
 the separation of which has necessitated 
 
236 CHEMISTRY 
 
 the most laborious work carried on for 
 very many years. In these cases, there is no 
 abrupt change of character in passing from 
 element to element along the horizontal 
 series. Other discrepancies occur notably 
 with the elements argon (39.88) and potassium 
 (39.1), which are out of place if the strict order 
 of atomic weights is followed. Iodine (126.92) 
 and tellurium (127.5), have likewise to be 
 taken out of order to bring them into the 
 groups to which they naturally belong. It 
 may be that such discrepancies indicate that 
 the atomic weights need further revision. 
 
 Passing over these and certain minor 
 discrepancies as problems yet awaiting solu- 
 tion, the significance of the scheme as a 
 comprehensive whole must be fully realized in 
 order that its systematizing influence upon 
 our science may be adequately appreciated. 
 That it corresponds with some underlying 
 reality is made manifest by the coincidence of 
 periodicity in several distinct sets of charac- 
 ters. The horizontal series, for instance, clas- 
 sified originally simply in the order of the 
 atomic weights, will be found to correspond 
 also with a classification according to valency 
 due allowance being made for the variability of 
 this property (p. 206). Measuring the valency 
 of the atom by the maximum number of 
 
PERIODIC CLASSIFICATION 237 
 
 hydrogen atoms with which it combines if it 
 forms hydrogen compounds, or with halogens 
 if it forms no hydrogen compounds, the 
 valencies of the atoms of the first series 
 may be indicated by attaching small Roman 
 numerals to the symbols : 
 
 He ; Li 1 ; Gl u ; B m ; C iv ; N ui ; O u ; F 1 . 
 
 The periodicity here shown is observed 
 also in a precisely similar way in the next 
 series. If the valency is measured by the 
 number of oxygen atoms in the acid and basic 
 oxides in any series which comprises such a 
 set of oxygen compounds, a regular increase 
 is observed : 
 
 2nd Series : Ne ; Na^O 11 ; Mg u O u ; Al^Oj} ; 
 Si iv O ; PIOj* ; S vi O!j ; Cl O?. 
 
 The scheme thus emphasizes the association 
 of chemical character with valency distinct- 
 ness marking the transition from member 
 to member along the horizontal series and 
 similarity among the natural groups in the 
 vertical columns. In general terms, the Roman 
 numerals heading the eight columns may be 
 regarded as indexes of valency not, of course, 
 in too rigid a sense, but as indicating maximum 
 valency. The zero sign over the first column 
 is to be taken literally as meaning no valency. 
 The elements from helium downwards form 
 
238 CHEMISTRY 
 
 no compounds with other elements ; they are 
 all chemically inert monatomic gases (p. 170) 
 contained in minute quantities in atmospheric 
 air (p. 46). Their separation has necessarily 
 been effected by physical methods, since, 
 chemically speaking, they are dead matter. 
 Helium, it may be mentioned incidentally, 
 is present in the atmosphere of the sun, and 
 has been found in small quantities in some 
 terrestrial minerals and also dissolved in 
 certain mineral waters. The history of this 
 element is intimately connected with the 
 subject of radioactivity. 
 
 The key-note periodicity having been 
 struck, the characters of the elements generally 
 will be found to respond. Notice how each 
 period begins with an inert element, and then 
 passes through a series, one extreme of which 
 is an intensely electro-positive base-forming 
 metal, and the other extreme an intensely 
 electro-negative acid-forming non-metal (halo- 
 gen), the extremes being connected by inter- 
 mediate elements of decreasingly electro- 
 positive and increasingly electro-negative 
 characters. 
 
 Another physical property of the ele- 
 ments, viz., their specific gravity, can also 
 be shown to be conformable with the periodic 
 classification. This conformability is most 
 
PERIODIC CLASSIFICATION 239 
 
 strikingly revealed by taking, instead of the 
 specific gravity, the number obtained by 
 dividing the atomic weight by the specific 
 gravity the so-called " atomic volume," 
 which may be regarded as the volume occu- 
 pied by the atom of the solid or liquid element. 
 The reader must be careful to distinguish 
 between this " atomic volume " of solid and 
 liquid elements and the volumes occupied by 
 the atomic weights of the elements in the 
 gaseous state (p. 166), in which state only does 
 the hypothesis of Avogadro (p. 153) apply. If 
 the numbers representing the atomic weights 
 (arranged in ascending order) are taken as 
 abscissae and the numbers representing the 
 atomic volumes as ordinates, the points of 
 intersection, when joined up in the usual way 
 familiar in co-ordinate geometry, give a curve 
 which reveals the periodicity at a glance. 
 The curve thus constructed has a wave-like 
 form ; and the chemically related elements 
 of the natural families occupy corresponding 
 positions on the curve. Thus, the strongly 
 electro-positive alkali metals of Group I. are 
 at the wave summits, the electro-negative 
 halogens of Group VII. at corresponding 
 positions on the ascending slopes, the alkaline 
 earthy metals of Group II. on the descending 
 slopes, and the high melting-point metals of 
 
240 CHEMISTRY 
 
 lower chemical activity belonging chiefly to 
 Group VIII. in the hollows between the 
 waves. 
 
 Further details arising from the periodic 
 classification, the critical discussion of its 
 imperfections, and the various suggested 
 amendments must be sought for in the stand- 
 ard works. The value of the scheme apart 
 from philosophical considerations is due not 
 only to its systematizing influence, but quite 
 as much to its suggestiveness. It not only 
 consolidates existing knowledge, but it has 
 indicated and still points to gaps in the 
 series which may at some future time be 
 filled. In other words, where the small 
 increment in the numbers representing suc- 
 cessive atomic weights suddenly becomes large, 
 a hint is given that one or more elements 
 yet remain to be discovered, or that some of 
 the elements do not occur on this earth. 
 Moreover, since all the properties of the 
 elements and their compounds are gradational 
 in the ascending series of natural groups or 
 families, and since these families all fall into 
 the general scheme, it is possible to predict 
 within narrow limits the properties of missing 
 elements. Two examples of such gaps are 
 inserted in the form of queries in the tabular 
 scheme in Group VII., where there is an 
 
PERIODIC CLASSIFICATION 241 
 
 indication of an element having an atomic 
 weight intermediate between molybdenum 
 (96) and ruthenium (101-7) and belonging to 
 the sub-group containing manganese ; and of 
 another element belonging to the same 
 sub-group between tungsten (184) and osmium 
 (190-9). Hydrogen, which, as already stated, 
 stands at present alone, might be regarded 
 as the only known representative of a 
 series, the members of which, both of higher 
 and (possibly) of lower atomic weight, are 
 missing. 
 
 A scheme which harmonizes so many 
 distinct sets of properties might legitimately 
 be used deductively. The prediction of the 
 properties of missing elements and their com- 
 pounds illustrates such use ; and the triumph 
 of the scheme dates from the period when 
 certain of the gaps were filled in by newly- 
 discovered elements, the properties of which 
 as foretold by Mendeleeff were found to 
 correspond closely with the prediction. Thus, 
 gallium (Lecoq de Boisbaudran, 1875), scand- 
 ium (Nilson, 1879), and germanium (Winkler, 
 1886) all found appropriate niches awaiting 
 them on their discovery. So also, used 
 deductively, the classification has led both 
 to the revision of atomic weights and to the 
 resortment of elements, i.e., to the transference 
 Q 
 
242 CHEMISTRY 
 
 of elements from positions where there was no 
 gap to be filled to positions where they were 
 wanted. This is tantamount to determining 
 the group to which a doubtful element belongs, 
 and so to the determination of its natural rela- 
 tionships and its valency. The scheme thus 
 becomes available as another method of 
 control for deciding in doubtful cases the 
 relationship between the equivalent and the 
 atomic weight (p. 173). 
 
 In its philosophical aspects, the periodic 
 classification is naturally suggestive of evolu- 
 tion. Clearly, the elements have not been 
 launched haphazard into existence as inde- 
 pendent entities : the contemplation of their 
 various relationships and inter-relationships 
 causes to arise spontaneously the question 
 whether the known forms of elemental matter 
 may not be genetically related whether the 
 regularities arising from the successive addi- 
 tions to the mass of the atom may not be 
 interpreted in terms of the development of the 
 elements from some primordial " stuff." It 
 may be so it would only be in harmony with 
 the whole scheme of Nature that such should 
 be the case. The consideration of the various 
 attempts which have been made to prove 
 elemental genesis would, however, take us 
 too far into the region of speculation. The 
 
PERIODIC CLASSIFICATION 243 
 
 " Periodic Law " as it stands is, in strict 
 philosophical terms, but an empirical summary 
 its interpretation has yet to be found. Is 
 it too great a stretch of prophecy to suggest 
 that the ultimate coalescence of Physics and 
 Chemistry will be brought about through the 
 interpretation of the principle of periodicity ? 
 The actual evidence in support of genetic 
 relationship is at present scanty ; but it is 
 slowly and surely accumulating in the field of 
 radioactivity. Here, again, the scheme, as 
 far as it goes, harmonizes with the latest 
 discoveries. The three elements of highest 
 atomic weight radium, thorium and uranium, 
 with possibly " radium emanation " or niton 
 fall naturally into in the last series. That 
 series may be incomplete there are gaps 
 which may or may not be filled up by future 
 discovery ; but it is in this series that the 
 atoms appear to have reached the limit of 
 internal stability. It is these elements which 
 are undergoing that process of spontaneous 
 decay or disintegration with the liberation of 
 the enormous store of energy which manifests 
 itself in the phenomena of radioactivity. The 
 final material product of this atomic disinteg- 
 ration which has thus far been definitely 
 identified is the inert gas helium. It is in this 
 sense, as already stated (p. 90), that transmu- 
 
244 CHEMISTRY 
 
 tation, as distinguished from transformation, 
 receives recognition in modern Chemistry. 
 
 CONCLUSION. The reader who has followed 
 the development of the subject up to this 
 stage will now realize that he has been 
 brought to the threshold of a great edifice 
 crowded with departmental chambers. The 
 various compartments are not watertight 
 divisions there is free intercommunication 
 by means of cross passages more or less broad 
 and numerous, according to the particular 
 label on the door of the chamber. In this 
 small volume it has not been possible to do 
 more than to point to some of these labels in 
 the hope that the reader may be tempted to 
 explore in greater detail the contents of the 
 various chambers. The inscriptions on a few 
 of these unopened doors are worthy of being 
 noted. 
 
 The redistribution of energy which accom- 
 panies chemical change is dealt with in works 
 on Thermochemistry. When heat is developed 
 as the result of chemical combination, such as 
 when sulphur and iron combine (p. 39), or 
 when carbon burns in oxygen to carbon dioxide 
 (p. 125), and, generally, in all kinds of burning 
 or combustion, the products of such combina- 
 
CONCLUSION 245 
 
 tion are said to be exothermic. Combustion is 
 in fact energetic oxidation. The develop- 
 ment of heat in such cases means that the 
 products of combustion contain less energy 
 than the materials which combine that the 
 system has run down in energy to the extent 
 represented by the heat evolved. On the 
 other hand there are compounds that can only 
 be formed from their elements when energy is 
 supplied from without, because the products 
 contain more energy than their components. 
 These are known as endothermic compounds. 
 And thus we enlarge our conception of chemical 
 change by associating the material transforma- 
 tion with the accompanying redistribution of 
 energy. Moreover, the heat evolved or ab- 
 sorbed is as fixed and definite in quantity for 
 each particular chemical change as is the 
 weight of the matter concerned. The change 
 2H 2 -|- O 2 = 2H 2 O to the chemist means the 
 development of 136,800 calories (p. 173) quite 
 as explicitly as it does that four parts by weight 
 of hydrogen and 32 parts of oxygen give 36 
 parts of water. Thus we come once again 
 into the presence of the chemical atom, now 
 as a definite store of available energy as well 
 as a material particle possessing definite 
 weight. 
 
 As the result of the study of chemical change 
 
246 CHEMISTRY 
 
 from the above point of view a quantitative 
 measure of chemical activity has been pro- 
 vided. The " heat of formation " of com- 
 pounds becomes a measure of the activities of 
 the atoms of the combining elements. It must 
 be noted however that the heat evolved repre- 
 sents but a small fraction of the total internal 
 energy of the atoms. In no ordinary chemical 
 change can it be said that there is tapped 
 more than a definitely limited quantity of this 
 vast store the greater part of the energy 
 locked up in the chemical atom is still unavail- 
 able. The practical solution of the problem 
 of liberating this store of energy if it is ever 
 solved would mark the dawn of a new era 
 for the human race. 
 
 Then again with respect to the conditions 
 which determine chemical change there is a 
 great field which we have left untrodden. 
 Some of the apparently simplest cases of 
 direct combination reveal their inner com- 
 plexity through the fact that water vapour 
 if only a minute trace is essential for the 
 reaction. Thus 2H 2 + O 2 = 2H 2 O ; H 2 + C1 2 
 = 2HC1; and even NH 3 + HC1^NH 4 C1 (p. 
 169) represent chemical changes which do not 
 take place when the gases are absolutely dry. 
 Other chemical changes which at first sight 
 would appear to be quite simple, such, e.g., 
 
CONCLUSION 247 
 
 as SO 2 + O = SO 3 (p. 207), take place only in 
 the presence of heated finely divided platinum 
 or other metals which remain unchanged at 
 the end of the reaction. Metals and other 
 substances which exert this mysterious in- 
 fluence are said to act by contact or cata- 
 lytically. It may be that the presence of a 
 catalyst of some kind is a necessary condition 
 of all chemical change. 
 
 Among other unconsidered conditions of 
 chemical change is the influence of the active 
 masses of the reacting materials upon the 
 velocity of the reaction (p. 73), upon the 
 direction in which the change takes place in a 
 reversible reaction such as 3Fe 4- 4H 2 O ^ Fe 3 
 O 4 + 4H 2 (pp. 187 and 198), and in determining 
 the actual quantities of the products present 
 when the system reaches equilibrium under 
 various conditions, i.e., when the reversible 
 reaction becomes balanced owing to the velo- 
 cities of the change in each direction being 
 equal. The contents of the chambers la'belled 
 Chemical Statics and Dynamics are well 
 worthy of detailed exploration, for here the 
 reader will find that modern Chemistry has 
 been brought within the reach of mathematical 
 treatment. 
 
248 CHEMISTRY 
 
 1912. International Atomic Weights. 
 
 Aluminium .... 
 Antimony .... 
 
 O = 16. 
 Al 27-1 
 Sb 120-2 
 A 39-88 
 As 74-96 
 Ba 137-37 
 Bi 208-0 
 B 11-0 
 Br 79-92 
 Cd 112-40 
 Cs 132-81 
 Ca 40-07 
 C 12-00 
 Ce 140-25 
 Cl 35-46 
 Cr 62-0 
 Co 58-97 
 Cb 93-5 
 Cu 63-57 
 Dy 162-5 
 Er 167-7 
 Eu 152-0 
 F 19-0 
 Gd 157-3 
 Ga 69-9 
 Ge 72-5 
 Gl 9-1 
 Au 197-2 
 He 3-99 
 H 1-008 
 In 114-8 
 I 126-92 
 Ir 193-1 
 Fe 55-84 
 Kr 82-92 
 La 139-0 
 Pb 207-10 
 Li 6-94 
 Lu 174-0 
 Mg 24-32 
 Mn 54-93 
 Hg 200-6 
 Mo 96-0 
 
 Neodymiuin . . 
 Neon 
 
 
 
 Nd 
 Ne 
 Ni 
 Nt 
 ition) 
 N 
 Os 
 
 Pd 
 P 
 Pt 
 K 
 Pr 
 Ra 
 Rh 
 Rb 
 Ru 
 Sa 
 Sc 
 Se 
 Si 
 Ag 
 Na 
 Sr 
 S 
 Ta 
 Te 
 Tb 
 Tl 
 Th 
 Tm 
 Sn 
 Ti 
 W 
 U 
 V 
 Xe 
 Yb 
 i) 
 Yt 
 Zn 
 Zr 
 
 = 16. 
 144-3 
 
 20-2 
 58-68 
 222-4 
 
 14-01 
 190-9 
 16-00 
 106-7 
 31-04 
 195-2 
 39-10 
 140-6 
 225-95 
 102-9 
 85-45 
 101-7 
 150-4 
 44-1 
 79-2 
 28-3 
 107-88 
 23-00 
 87-63 
 32-07 
 181-5 
 127-5 
 159-2 
 204-0 
 232-4 
 168-5 
 119-0 
 48-1 
 184-0 
 238-5 
 51-0 
 130-2 
 172-0 
 
 89-0 
 65-37 
 90-6 
 
 Nickel 
 
 
 Niton 
 
 
 (radium eman, 
 Nitrogen .... 
 Osmium 
 
 Bismuth . . 
 
 Boron 
 
 Bromine 
 
 Oxvsen . 
 
 Cadmium 
 
 Palladium .... 
 Phosphorus . . 
 Platinum .... 
 Potassium .... 
 Praseodymium 
 Radium 
 Rhodium .... 
 Rubidium .... 
 Ruthenium . . 
 Samarium .... 
 Scandium .... 
 Selenium 
 
 Cspsium 
 
 
 Carbon 
 
 Cerium . . 
 
 Chlorine 
 
 Chromium .... 
 Cobalt 
 
 Columbium .... 
 Copper 
 
 Dysprosium 
 Erbium 
 
 Europium .... 
 Fluorine . . . 
 
 Silicon 
 
 Silver 
 
 Gadolinium .... 
 Gallium 
 
 Sodium 
 
 Strontium .... 
 Sulphur 
 
 Germanium .... 
 Glucinuin 
 
 Tantalum .... 
 Tellurium .... 
 Terbium 
 
 Gold .... 
 
 Helium 
 
 Hydrogen .... 
 Indium . 
 
 Thallium 
 Thorium 
 
 Iodine .... 
 
 Thulium . . 
 
 Iridium 
 
 Tin 
 
 
 Titanium 
 Tungsten .... 
 Uranium .... 
 Vanadium .... 
 Xenon 
 
 Krypton 
 
 Lanthanum .... 
 Lead 
 
 Lithium 
 
 Lutecium 
 
 Ytterbium 
 (Neoytterbiun 
 Yttrium 
 
 Magnesium .... 
 Manganese 
 Mercury 
 Molybdenum . . 
 
 Zinc .... 
 
 Zirconium .... 
 
BIBLIOGRAPHY 
 
 HlSTOBIOAL 
 
 History of Chemistry / by Sir T. E. Thorpe. Watts & 
 
 Co. A concise resume. 
 Essays in Historical Chemistry s by the same author. 
 
 Macmillan & Co. A series of biographies of the 
 
 founders of the science. 
 A History of Chemistry from earliest times to the present 
 
 day, being also an Introduction to the Study of 
 
 the Sciences by Ernst v. Meyer, translated by G. 
 
 Mc'Gowan. Macmillan & Co. A larger and fairly 
 
 exhaustive work. 
 
 GENERAL, PHYSICAL, AND INORGANIC CHEMISTRY 
 
 Introduction to Chemistry / by W. Ostwald, translated 
 by Hall & Williams. Wiley & Sons. A very lucid 
 elementary work. 
 
 Outlines of General Chemistry ; by the same author, 
 translated by W. W. Taylor. Macmillan & Co. A 
 general statement of the existing state of knowledge 
 of the science suitable for advanced readers. 
 
 Theoretical Chemistry from the Standpoint of Avogadro's 
 Rule and Thermodynamics : by W. Nernst, trans- 
 lated by H. T. Tizard. Macmillan & Co. An 
 advanced work dealing with the subject more 
 mathematically. 
 
 249 
 
250 BIBLIOGRAPHY 
 
 First Principles of Chemical Theory : by C. H. Mathewson. 
 Wiley & Sons. A concise introduction to theoretical 
 chemistry. 
 
 Inorganic Chemistry : by E. J. Lewis. Cambridge 
 University Press. 
 
 Text- Book of Inorganic Chemistry : by Holleman, trans- 
 lated by Cooper. Wiley & Sons. 
 
 Good introductory works to this branch of the 
 science. 
 
 Text- BooTcs of Physical Chemistry : by various authors, 
 edited by Sir Win. Ramsay. Longmans, Green & 
 Co. A useful series of monographs, each volume 
 dealing with some special branch of the subject. 
 
 STEREOCHEMISTRY 
 
 The Arrangement of Atoms in Space : by J. H. van't 
 Hoff, translated by Arnold Eiloart. Longmans, 
 Green & Co. A work of great historical interest 
 as representing the views of one of the founders 
 of this subject and of his disciples. 
 
 Stereochemistry: by A. W. Stewart. Longmans, Green 
 & Co. One of the Text-Books of Physical Chemistry. 
 
 ORGANIC CHEMISTRY 
 
 The Rise and Progress of Organic Chemistry t by C. 
 Schorlemmer. Macmillan & Co. A concise historical 
 introduction. 
 
 Organic Chemistry : by W. H. Perkin and F. S. Kipping. 
 
 R. & W. Chambers. 
 Text- Book of Organic Chemistry : by Holleman, edited by 
 
 A. J. Walker and O. E. Mott. Wiley & Sons. 
 Good systematic introductory works. 
 
BIBLIOGRAPHY 251 
 
 Modern Organic Chemistry : by C. A. Keane. The Walter 
 Scott Publishing Co. A good elementary exposition 
 of the later developments of this branch. 
 
 Organic Chemistry for Advanced Students : by J. B. 
 Cohen. Edward Arnold. A more elaborate historical 
 treatment of the subject. 
 
 Modern Research in Organic Chemistry : by P. G. Pope. 
 Methuen & Co. A concise account of the newer 
 lines of research. 
 
 APPLIED CHEMISTRY 
 
 Chemistry in Daily Life : by Lassar-Cohn, translated by 
 
 Pattison Muir. Grevel & Co. 
 The Romance of Modern Chemistry / by J. C. Philip. 
 
 Seeley & Co. 
 The Chemistry of Commerce : by R. Kennedy Duncan. 
 
 Harper & Bros. 
 Chemical Research and National Welfare : by Emil 
 
 Fischer. Society for Promoting Christian Knowledge. 
 Four popular works giving an account of the part 
 
 played by Cliemistry in the arts and manufactures and 
 
 in human \v elf are generally, 
 
 NOTE : No attempt has been made to include in this list 
 more than a few selected types of the multitudinous 
 elementary and advanced text- books written for 
 students. The great standard works of reference 
 familiar to all workers in Chemistry would naturally 
 be beyond the scope of the ordinary reader who is 
 only desirous of making himself acquainted with the 
 existing state of the science. 
 
INDEX 
 
 ACETYLENE, 125 
 
 Acid oxides, 108 
 
 Activity, Chemical, 83, 103 
 
 Agencies, Chemical, 92 
 
 Aggregation, Physical state, 45 
 
 Agriculture and chemistry, 21 
 
 Air, Composition of, 46 
 
 Alchemists, 90 
 
 Alkali metals, 234 ' 
 
 Alkaline earths, 235 
 
 Allotropic modification, 216 
 
 Alloys, 106 
 
 Aluminium, 102 
 
 Ammonia, 152 
 
 Analysis, 93 
 
 Arithmetic, Chemical, 195 
 
 Art of chemistry, 39 
 
 Association, 166 
 
 Asymmetric atoms, 223 
 
 Atmospheric moisture, 35 
 
 Atomic heat, 174 
 
 Atoms in molecules, 163 
 
 Atomic theory, 129 
 
 volumes, 166, 239 
 
 weights, 143, 164, 183 
 
 Auxiliary valency, 216 
 Avogadro's hypothesis, 157 
 
 Balance, Chemical, 191 
 Barlow and Pope, 209 
 Basic oxides, 108 
 Bell-metal, 106 
 Biochemical synthesis, 226 
 Boiling-point, 59, 178 
 Bonds, 211 
 Boyle's law, 148 
 Brass, 106 
 Bromine, 100 
 
 Calcium, 102 
 Calorie, 173 
 Cannizzaro, 158 
 Carbon dioxide in air, 36 
 Carbon compounds, 220 
 . , Oxides of, 125 
 
 253 
 
 Catalysts, 247 
 
 Centigrade thermometer, 59 
 
 Cerium, 234 
 
 Chalk, changed to lime, 39 
 
 Charcoal, 217 
 
 Charles' law, 148 
 
 Chemical change, 21 
 
 combination, 41 
 
 equivalents, 120 
 
 Chili saltpetre, 101 
 Chlorine, 100 
 Classification, 100 
 Coal-tar, 43 
 Coinage, Gold, 107 
 
 , Silver, 106 
 
 Combination, Direct, 29 
 
 , Quantitative, 66 
 
 Combustion, 244 
 
 Compounds, Equivalence of, 121 
 Copper oxides, 84, 125 
 Constancy of composition, 117 
 Conservation of energy, 89 
 
 of mass, 76 
 
 Contact action, 247 
 Crookes, 133 
 Cryolite, 100 
 Crystallography, 210 
 
 Dalton, 129 
 
 Decomposition, Quantitative, 66 
 
 Definiteness of chemical change, 71 
 
 Densities of gases, 154 
 
 Depression of freezing-point, 178 
 
 Dew, 54 
 
 Dewar, 148 
 
 Diamond, 217 
 
 Diffusion of gases, 56 
 
 Discrete structure of matter, 58 
 
 Disintegration, Atomic, 243 
 
 Dissociation, 166 
 
 Double decomposition, 202 
 
 Dulong and Petit's Law, 173 
 
 Dyea from tar, 43 
 
 Earth's crust, Composition of, 114 
 
254 
 
 INDEX 
 
 Effervescence, 38 
 Electro-chemical equivalence, 121, 
 
 127 
 
 Electro-negative elements, 111 
 Electro-positive elements, 111 
 Electrolysis, 109 
 Electrons, 134 
 Elemental genesis, 242 
 Elements, Chemical, 95 
 Endothermic compounds, 245 
 Exothermic compounds, 245 
 Explosives from tar, 43 
 Expansion by heat, 22 
 Experimental method, 31 
 Equations, 165, 198 
 Equivalence, 115 
 
 Faraday's law, 123 
 Fluorine, 100 
 Fluorite, 100 
 Formulae, 140, 197 
 Frankland, 210 
 Freezing point, 59, 178 
 
 Gallium, 241 
 Gaseous state, 49 
 Gases, Different, 65 
 Gay Lussac's law, 151 
 Geochemistry, 30 
 Germanium, 241 
 Gold extraction, 44 
 Gradation of properties, 104 
 Grain-molecules, 200 
 Granite, 94 
 Grape sugar, 225 
 Graphic formulae, 213 
 Graphite, 217 
 
 Haematite, 29 
 
 Haloid salts, 109 
 
 Halogens, 100 
 
 Heat of chemical combination, 48 
 
 of formation, 246 
 
 , Expansion by, 19 
 
 Helium, 238, 243 
 
 Helmholtz, 209 
 
 Homogeneity of chemical change, 
 
 Hydrogen, 82, 112 
 
 chloride, 154 
 
 sulphide, 207 
 
 Inert gases, 238 
 Iodine, 100 
 Ionic dissociation, 180 
 Iodine and iron, 38, 172 
 Iron, oxides, 71, 172 
 Iron rust, 24 
 
 Iron scale, 23 
 Isomerism, 222 
 
 Kekule, 210 
 Kinetic theory, 51 
 
 Lactic acid, 225 
 Lavoisier, 78 
 Le Bel, 223 
 
 Lecoq de Boisbaudran, 241 
 Light, refraction of, 19 
 Lime, 39, 102 
 Limonite, 29 
 Liquefaction of gases, 60 
 Lockyer, 172 
 Lothar Meyer, 229 
 
 Magnesium, 83 
 
 Magnetite, 29 
 
 Mass action, 247 
 
 Mass, Conservation of, 76 
 
 Matter, Discrete structure, 58 
 
 Melting-point, 59, 178 
 
 Mendel6eff, 229, 241 
 
 Mercury, 99, 175 
 
 Metallic elements, 99, 106 
 
 Methane, 206 
 
 Metric system, 68 
 
 Mixture, Mechanical, 41 
 
 Moissan, 104 
 
 Molecules, 141, 159 
 
 Molecular compounds, 215 
 
 volumes, 166 
 
 weights, 143, 161 
 
 Monatomic molecules, 168 
 Monazite sand, 233 
 Multiple proportions, 127 
 equivalence, 123 
 
 Newlands, 229 
 Nilson, 241 
 Niton, 243 
 Nitrogen, 37 
 
 dioxide, 169 
 
 Non-metals, 106 
 
 Onnes, 147 
 Optical activity, 225 
 Organic chemistry, 218 
 Ores, Metallic, 43 
 Oxides, 71, 108 
 Oxygen, 27 
 Oxy-salts, 109 
 Ozone, 216 
 
 Partial valency, 216 
 Particles, Visualized, 50 
 Perfumes from tar, 43 
 
INDEX 
 
 255 
 
 Periodic classification, 230 
 Pewter, 107 
 Physical change, 22 
 Phosphorus, 167 
 Preferential action, 35, 37, 83 
 Purity, Chemical, 188 
 Pyrophoric iron, 49 
 
 Quantitative transformation, 67 
 Quicklime, 102 
 
 Radioactivity, 95, 136, 243 
 Radium, 98, 243 
 Ramsay and Soddy, 135 
 Raoult, 179 
 Rare earths, 233 
 Reciprocal equivalence, 123 
 Residual valency, 216 
 Reversible reaction, 247 
 Rusting of iron, 25 
 Rutherford, 135 
 
 Salt, Common, 100 
 Salts, 108 
 Scandium, 241 
 Silicon, 113 
 Sodium, 101 
 
 oxides, 125, 144 
 
 Solder, 107 
 Solution, 85 
 Specific heat, 173 
 Spectroscopy, 20 
 Steel, 107 
 Stereochemistry, 222 
 
 Structure, Chemical, 210 
 
 Sugar, 177 
 
 Sulphur, combination with iron, 39 
 
 oxides, 128 
 
 , vapour density of, 167 
 
 Symbols, 137 
 
 Synthetical chemistry, 33, 93 
 
 Tartaric acid, 225 
 Tautomerism, 222 
 Thermochemistry, 244 
 Thomson, 133 
 Thorium, 233, 243 
 Transmutation^ .26, 90, 243 
 Transition phases, 75 
 
 Uranium, 243 
 
 Valency, 204 
 
 Van't HotF, 223 
 
 Velocity of chemical change, 73 
 
 Vitalism, 219, 226 
 
 Volumelrio analysis, 203 
 
 combination, 147 
 
 Water, Composition of, 80 
 , Formula of, 143 
 
 vapour in air, 53 
 
 , volumetric composition, 88, 
 
 151 
 
 Weights and measures, 68 
 Winkler, 241 
 
 Zero, Absolute, 143 
 Zinc, 83 
 
The Home University 
 
 -jDrarV of Modern Knowledge 
 
 s made up of absolutely new books by leading authorities. 
 
 The editors are Professors Gilbert Murray, H. A. L. Fisher, 
 W. T. Brewster, and J. Arthur Thomson. 
 
 ]loth bound, good paper, clear type, 256 pages per volume, 
 bibliographies, indices, also maps or illustrations where 
 needed. Each complete and sold separately. 
 
 50c 
 
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 AMERICAN HISTORY 
 
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 47. The Colonial Period (1607-1766). 
 
 By CHARLES McLEAN ANDREWS, Professor of American History, Yale. 
 The fascinating history of the two hundred years of "colonial times." 
 
 82. The Wars Between England and America 
 (1763-1815). 
 
 By THEODORE C. SMITH, Professor of American History, Williams 
 College. A history of the period, with especial emphasis on The 
 Revolution and The War of 1812. 
 
 37. From Jefferson to Lincoln (1815-1860). 
 
 By WILLIAM MACDONALD, Professor of History, Brown University. 
 The author makes the history of this period circulate about constitu- 
 tional ideas and slavery sentiment. 
 
 15. The Civil War (1854-1865). 
 
 By FREDERIC L. PAXSON, Professor of American History, University 
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GENERAL HISTORY AND GEOGRAPHY 
 
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 By CHARLES TOWER. Describes the constitution and government of 
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 By HERBERT FISHER, Vice-Chancellor of Sheffield University. Author 
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 33. The History of England. 
 
 By A. F. POLLARD, Professor of English History, University of 
 London. "Professor Pollard is to be ranked among the few leading 
 English historians of the times. One of the most brilliant little vol- 
 umes." Springfield Republican. 
 
 3. The French Revolution. 
 
 By HILAIRE BELLOC. "For the busy man it would be difficult to 
 name another work better suited for the purpose of conveying an 
 intelligent idea of the greatest political event of modern times." 
 San Francisco Chronicle. 
 
4. A Short History of War and Peace. 
 
 By G. H. FERRIS, author of Riissia in Revolution, etc. The Hon. 
 James Bryce writes: "I have read it with much interest and pleas- 
 ure, admiring the skill with which you have managed to compress so 
 many facts and views into so small a volume." 
 
 20. History of Our Time (1885-1911). 
 
 By G. P. GOOCH. A "moving picture" of the world since 1885. 
 
 22. The Papacy and Modern Times. 
 
 By REV. WILLIAM BARRY, D. D., author of The Papal Monarchy, 
 etc. The story of the rise and fall of the Temporal Power. 
 
 8. Polar Exploration. 
 
 By DR. W. S. BRUCE, Leader of the "Scotia" expedition. Empha- 
 sizes the results of the expeditions, not in miles traveled, but in 
 valuable information brought home. "Of enormous interest." 
 Chatauqua Press. 
 
 18. The Opening-up of Africa. 
 
 By Sir H. H. JOHNSTON. The first living authority on the subject 
 tells how and why the "native races" went to the various parts of 
 Africa and summarizes its exploration and colonization. (With 
 maps.) 
 
 19. The Civilization of China. 
 
 By H. A. GILES, Professor of Chinese, Cambridge, author of A His- 
 tory of Chinese Literature, etc. A vivid outline of history, manners 
 and customs, art, literature, and religion. 
 
 36. Peoples and Problems of India. 
 
 By SIR T. W. HOLDERNESS, Secretary of the Revenue, Statistics, and 
 Commerce Department of the British India Office. "The best small 
 treatise dealing with the range of subjects fairly indicated by the 
 title." The Dial. 
 
 7. Modern Geography. 
 
 By DR. MARION NEWBIGIN. Those to whom "geography" suggests 
 "bounded on the north by," etc., will gain a new view of the world 
 from this book. It shows the relation of physical features to living 
 things and to some of the chief institutions of civilization. 
 
 51. Master Mariners. 
 
 By JOHN R. SPEARS, author of The History of Our Navy, etc. A 
 history of sea craft and sea adventure from the earliest times, with 
 an account of sea customs and the great seamen. 
 
 SOCIAL SCIENCE 
 
 77. Co-Partnership and Profit Sharing. 
 
 By ANEURIN WILLIAMS, Chairman, Executive Committee, Interna- 
 tional Co-operative Alliance, etc. Explains the various types of co- 
 partnership or profit-sharing, or both, and gives details of the ar- 
 rangements now in force in many of the great industries both here 
 and abroad. 
 
 75. Shelley, Godwin and Their Circle. 
 
 By H. N. BRAILSFORD, author of "Adventures in Prose," etc. A his- 
 tory of the immediate influence of the French Revolution in England. 
 
79. Unemployment. 
 
 By A. C. PIGOU, M. A., Professor of Political Economy at Cam- 
 bridge. The meaning, measurement, distribution, and effects of un- 
 employment, its relation to wages, trade fluctuations, and disputes, 
 and some proposals of remedy or relief. 
 
 80. Common-Sense in Law. 
 
 By PROF. PAUL VINOGRADOFF, D. C. L., LL. D. Social and Legal 
 Rules Legal Rights and Duties Facts and Acts in Law Legislation 
 Custom Judicial Precedents Equity The Law of Nature. 
 
 49. Elements of Political Economy. 
 
 By S. J. CHAPMAN, Professor of Political Economy and Dean of 
 Faculty of Commerce and Administration, University of Manchester. 
 A clear statement of the theory of the subject for non-expert readers. 
 
 11. The Science of Wealth. 
 
 By J. A. HOBSON, author of Problems of Poverty. A study of the 
 structure and working of the modern business world. 
 
 1. Parliament. Its History, Constitution, and 
 Practice. 
 
 By SIR COURTENAY P. ILBERT, Clerk of the House of Commons. 
 "Can be praised without reserve. Admirably clear." New York Sun. 
 
 16. Liberalism. 
 
 By PROF. L. T. HOBHOUSE, author of Democracy and Reaction. A 
 masterly philosophical and historical review of the subject. 
 
 5. The Stock Exchange. 
 
 By F. W. HIRST, Editor of the London Economist. Reveals to the 
 non-financial mind the facts about investment, speculation, and the 
 other terms which the title suggests. 
 
 10. The Socialist Movement. 
 
 By J. RAMSAY MACDONALD, Chairman of the British Labor Party. 
 "The latest authoritative exposition of Socialism." San Francisco 
 Argonaut. 
 
 28. The Evolution of Industry. 
 
 By D. H. MACGREGOR, Professor of Political Economy, University 
 of Leeds. An outline of the recent changes that have given us the 
 present conditions of the working classes and the principles involved. 
 
 29. Elements of English Law. 
 
 By W. M. GELDART, Vinerian Professor of English Law, Oxford. A 
 simple statement of the basic principles of the English legal system 
 on which that of the United States is based. 
 
 32. The School: An Introduction to the Study of 
 Education. 
 
 By J. J. FINDLAY, Professor of Education, Manchester. Presents 
 the history, the psychological basis, and the theory of the school with 
 a rare power of summary and suggestion. 
 
 6. Irish Nationality. 
 
 By MRS. J. R. GREEN. A brilliant account of the genius and mission 
 of the Irish people. "An entrancing work, and I would advise every 
 one with a drop of Irish blood in his veins or a vein of Irish sym- 
 pathy in his heart to read it." New York Times' Review. 
 
NATURAL SCIENCE 
 68. Disease and Its Causes. 
 
 By W. T. COUNCILMAN, M. D., LL. D., Professor of Pathology, Har- 
 vard University. 
 
 85. Sex. 
 
 By J. ARTHUR THOMSON and PATRICK GEDDES, joint authors of The 
 Evolution of Sex. 
 
 71. Plant Life. 
 
 By J. B. FARMER, D. Sc., F. R. S., Professor of Botany in the Impe- 
 rial College of Science. This very fully illustrated volume contains 
 an account of the salient features of plant form from the point of 
 view of function. 
 
 63. The Origin and Nature of Life. 
 
 By BENJAMIN M. MOORE, Professor of Bio Chemistry, Liverpool. 
 Perhaps the chapters on "The Origin of Life" and "How Life Came 
 to Earth" will attract most attention, as throwing the newest light 
 upon matters of very ancient controversy. 
 
 53. Electricity. 
 
 By GISBERT KAPP, Professor of Electrical Engineering, University of 
 Birmingham. 
 
 54. The Making of the Earth. 
 
 By J. W. GREGORY, Professor of Geology, Glasgow University. 38 
 maps and figures. Describes the origin of the earth, the formation 
 and changes of its surface and structure, its geological history, the 
 first appearance of life, and its influence upon the globe. 
 
 56. Man: A History of the Human Body. 
 
 By A. KEITH, M. D., Hunterian Professor, Royal College of Sur- 
 geons. Shows how the human body developed. 
 
 74. Nerves. 
 
 By DAVID FRASER HARRIS, M. D., Professor of Physiology, Dalhousie 
 University, Halifax. Explains in non-technical language the place 
 and powers of the nervous system, more particularly of those regions 
 of the system whose activities are not associated with the rousing of 
 consciousness. 
 
 21. An Introduction to Science. 
 
 By PROF. J. ARTHUR THOMSON, Science Editor of the Home Univer- 
 sity Library. For those unacquainted with the scientific volumes in 
 the series, this would prove an excellent introduction. 
 
 14. Evolution. 
 
 By PROF. J. ARTHUR THOMSON and PROF. PATRICK GEDDES. Explains 
 to the layman what the title means to the scientific world. 
 
 23. Astronomy. 
 
 By A. R. HINKS, Chief Assistant at the Cambridge Observatory. 
 "Decidedly original in substance, and the most readable and informa- 
 tive little book on modern astronomy we have seen for a long time." 
 
 Nature. 
 
 24. Psychical Research. 
 
 By PROF. W. F. BARRETT, formerly President of the Society for 
 Psychical Research. A strictly scientific examination. 
 
9. The Evolution of Plants. 
 
 By DR. D. H. SCOTT, President of the Linnean Society of London. 
 The story of the development of flowering plants, from the earliest 
 zoological times, unlocked from technical language. 
 
 43. Matter and Energy. 
 
 By F. SODDY, Lecturer in Physical Chemistry and Radioactivity, 
 University of Glasgow. "Brilliant. Can hardly be surpassed. Sure 
 to attract attention." New York Sun. 
 
 41. Psychology, The Study of Behaviour. 
 
 By WILLIAM McDoUGALL, of Oxford. A well digested summary of 
 the essentials of the science put in excellent literary form by a lead- 
 ing authority. 
 
 42. The Principles of Physiology. 
 
 By PROF. J. G. MCKENDRICK. A compact statement by the Emeritus 
 Professor at Glasgow, for uninstructed readers. 
 
 37. Anthropology. 
 
 By R. R. MARETT, Reader in Social Anthropology, Oxford. Seeks to 
 plot out and sum up the general series of changes, bodily and mental, 
 undergone by man in the course of history. "Excellent. So enthusi- 
 astic, so clear and witty, and so well adapted to the general reader." 
 American Library Association Booklist. 
 
 17. Crime and Insanity. 
 
 By DR. C. A. MERCIER, author of Text-Book of Insanity, etc. 
 
 12. The Animal World. 
 
 By PROF. F. W. GAMBLE. 
 
 15. Introduction to Mathematics. 
 
 By A. N. WHITEIIEAD, author of Universal Algebra. 
 
 PHILOSOPHY AND RELIGION 
 69. A History of Freedom of Thought. 
 
 By JOHN B. BURY, M. A., LL. D., Regius Professor of Modern His- 
 tory in Cambridge University. Summarizes the history of the long 
 struggle between authority and reason and of the emergence of the 
 principle that coercion of opinion is a mistake. 
 
 55. Missions : The^r Rise and Development. 
 
 By MRS. MANDELL CREIGHTON, author of History of England. The 
 author seeks to prove that missions have done more to civilize the 
 world than any other human agency. 
 
 52. Ethics. 
 
 By G. E. MOORE, Lecturer in Moral Science, Cambridge. Discusses 
 what is right and what is wrong, and the whys and wherefores. 
 
 65. The Literature of the Old Testament. 
 
 By GEORGE F. MOORE, Professor of the History of Religipn, Harvard 
 University. "A popular work of the highest order. Will be profit- 
 able to anybody who cares enough about Bible study to read a serious 
 book on the subject." American Journal of Theology. 
 
 50. The Making of the New Testament. 
 
 By B. W. BACON, Professor of New Testament Criticism, Yale. An 
 authoritative summary of the results of modern critical research 
 with regard to the origins of the New Testament. 
 
35. The Problems of Philosophy. 
 
 By BERTRAND RUSSELL, Lecturer and Late Fellow, Trinity College, 
 Cambridge. 
 
 44. Buddhism. 
 
 By MRS. RHYS DAVIDS, Lecturer on Indian Philosophy, Manchester. 
 A review of that religion and body of culture which is to a large 
 part of the human race, chiefly situated in Southern Asia, what 
 Christianity is to us of the West. 
 
 46. English Sects: A History of Nonconformity. 
 
 By W. B. SELBIE, Principal of Manchester College, Oxford. 
 
 60. Comparative Religion. 
 
 By PROF. J. ESTLIN CARPENTER. "One of the few authorities on this 
 subject compares all the religions to see what they have to offer on 
 the great themes of religion." Christian Work and Evangelist. 
 
 LITERATURE AND ART 
 73. Euripides and His Age. 
 
 By GILBERT MURRAY, Regius Professor of Greek, Oxford. Brings 
 before the reader an undisputedly great poet and thinker, an amaz- 
 ingly successful playwright, and a figure of high significance in the 
 history of humanity. 
 
 81. Chaucer and His Times. 
 
 By GRACE E. HADOW, Lecturer Lady Margaret Hall, Oxford; Late 
 Reader, Bryn Mawr. 
 
 70. Ancient Art and Ritual. 
 
 By JANE E. HARRISON, LL. D., D. Litt. "One of the 100 most impor- 
 tant books of 1913." New York Times Review. 
 
 61. The Victorian Age in Literature. 
 
 By G. K. CHESTERTON. The most powerfully sustained and brilliant 
 piece of writing Mr. Chesterton has yet published. 
 
 59. Dr. Johnson and His Circle. 
 
 By JOHN BAILEY. Johnson's life, character, works, and friendships 
 are surveyed; and there is a notable vindication of the "Genius of 
 Boswell." 
 
 58. The Newspaper. 
 
 By G. BINNEY DIBBLEE. The first full account, from the inside, of 
 newspaper organization as its exists to-day. 
 
 62. Painters and Painting. 
 
 By SIR FREDERICK WEDMORE. With 16 half-tone illustrations. 
 
 64. The Literature of Germany. 
 
 By J. G. ROBERTSON. 
 
 48. Great Writers of America. 
 
 By W. P. TRENT and JOHN ERSKINE, of Columbia University. Gives 
 the essential facts as to the lives and works of Franklin, Washington 
 Irving, Bryant, Cooper, Hawthorne, Poe, Emerson, and the other 
 Transcendentalists, Oliver Wendell Holmes and the other New Eng- 
 land poets. Motley and the other historians, Webster and Abraham 
 Lincoln, Mrs. Stowe, Walt Whitman, Bret Harte, and Mark Twain. 
 
40. The English Language. 
 
 By L. P. SMITH. A concise history of the origin and development 
 of the English language. "Has certainly managed to include a vast 
 amount of information, and, while his writing is clear and lucid, he 
 is always in touch with life." The Athenaeum. 
 
 45. Medieval English Literature. 
 
 By W. P. KER, Professor of English Literature, University College, 
 London. "One of the soundest scholars. His style is effective, sim- 
 ple, yet never dry." The Athenaeum. 
 
 27. Modern English Literature. 
 
 By G. H. MAIR. From Wyatt and Surrey to Synge and Yeats. "A 
 most suggestive book, one of the best of this great series." Chicago 
 Evening Post. 
 
 2. Shakespeare. 
 
 By JOHN MASEFIELD. "One of the very few indispensable adjuncts 
 to a Shakespearean Library." Boston Transcript. 
 
 31. Landmarks in French Literature. 
 
 By G. L. STRACHEY, Scholar of Trinity College, Cambridge. "For a 
 survey of the oustanding figures of French literature with an acute 
 analysis of the contribution which each made to his time and to the 
 general mass there has been no book as yet published so judicially 
 interesting." The Chautauquan. 
 
 38. Architecture. 
 
 By PROF. W. R. LETHABY. An introduction to the history and 
 theory of the art of building. "Professor Lethaby's scholarship and 
 extraordinary knowledge of the most recent discoveries of archaeo- 
 logical research provide the reader with a new outlook and with new 
 facts." The Athenaeum. 
 
 66. Writing English Prose. 
 
 By WILLIAM T. BREWSTER, Professor of English, Columbia Univer- 
 sity. "Should be put into the hands of every man who is beginning 
 to write and of every teacher of English that has brains enough to 
 understand sense." New York Sun. 
 
 83. William Morris: His Work and Influence. 
 
 By A. GLUTTON BROCK, author of Shellev: The Man and the Poet. 
 William Morris believed that the artist should toil for love of his 
 work rather than the gain of his employer, and so he turned from 
 making works of art to remaking society. 
 
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