August 24, 1882] 



NATURE 



403 



molecules than they represent the solar system; but unfortu- 

 nately we cannot prevent beginners from regarding them as 

 pictures, and moulding their ideas upon them. They present 

 something easily grasped by the infant mind, and schoolmasters 

 are fond of them ; but only those who have each year to combat 

 a fresh crop of misconceptions and false mechanical notions 

 engendered by them, can be aware how much they hinder, I 

 won't say the advance, but the spread of real chemical science. 

 If it be true that the illustrations of an artist like the late Hablot 

 Browne give to our conceptions of the characters of a story a 

 more definite and permanent, though perhaps a much modified 

 form of what the author of the story intended to portray, it is 

 equally true that the illustrations by which some, even great 

 names among us, have tried to make us fancy that we had a true 

 conception of some natural process have become so fixed in our 

 minds, as to prevent our realising the true meaning of nature. 



What, then, is the progre s which I think has been made in 

 physical chemistry? In the first place, notwithstanding the 

 slowness with which new ideas replace old familiar images, the 

 molecular theories developed by Clausius, Clerk- Maxwell, Boltz- 

 mann, and by Sir \V. 1 homson, have been long enough before the 

 world to have greatly loosened the hold upon our minds of many 

 old notions. The rigid, unbreakable, impenetrable atoms of the 

 Epicurean philosophy made familiar to us by Lucretius always 

 presented difficulties whirh w «re only perhaps exceeded by those 

 of the elastic atmospheres w ith which modern philosophers fancied 

 them to be surrounded ; but now the vortex theory, whether we 

 think it probable or not, at least gives us a standing ground for 

 the assertion that the supposed impenetrability of matter and the 

 curious compound of nucleus and atmosphere which has been 

 invented to account for elasticity are m t necessary assumptions. 

 The kinetic theory of gases has analy.ed for us the different 

 motions of the molecules in a mass of matter, and has facilitated 

 the conception of the part which heat plays in chemical action. 

 Hence we have had of late several attempts to reduce to a form 

 susceptible of mathematical calculation the problems of chemistry. 

 Most of these attempts have proceeded on the well-known 

 mechanical principle that the change of vis viva of a system in 

 passing from an initial to a final configuration is independent of 

 the intermediate stages through which it may have passed so long 

 as the external conditions are unaltered ; and on the principle of 

 the dissipation of energy, that is to say, on the condition that the 

 state of the system, if it be a stable one, must be such that the 

 energy run down in reaching it is a maximum. These principles 

 have been applied successfully to the solution of some particular 

 cases of the equilibrium between a mixture of chemicals by Wil- 

 lard Gibbs, Berthelot, and others. By the first-mentioned 

 principle, all consideration of the intermediate stages by which 

 the final re ult is reached are avoided. Quite recently Lemoinne 

 has attacked the same problem on another principle. His 

 principle is that of an equilibrium of antagonistic reactions in a 

 mixture of materials, a mobile equilibrium such as we are now 

 familiar with, dependent on compensating effects; but he does 

 not seem able to solve the problem in any great number of cases. 

 In fact, the difficulty does not now lie so much in expressing 

 mathematically the conditions of the problem as in the defect of 

 knowledge which depends upon experiment. And it is just 

 in this that I think the outlook most hopeful. In some cases the 

 patient work of weighing ai.d measuring and comparing, which 

 is necessary to make our theoretic speculations of any substantial 

 value, has been already done for us. The publication, three 

 years since, of Berthelot's essay on chemical mechanics has given 

 us in a collected form a large quantity of data of the first im- 

 portance; and now I am glad to say that the long labours of 

 another worker in the same field, Thomsen of Copenhagen, are 

 in course of publication in a handy form. I think these two in- 

 vestigators have done more than any one else of late years 

 towards making it possible to give to chemistry the rank of an 

 exact science. But besides the data which they have supplied to 

 us, there are others which are yet wanting. For instance, almost 

 every equation of chemical equilibrium involves an expression 

 depending on the specific heats of the materials. At present we 

 do not know enongh of the law of specific heats to be able to 

 give in most cases a probable value to those expressions ; but 

 these and other data of the kind do not seem out of our reach, 

 and we may hope that the same ingenuity and patience which 

 hss gained for us so much firm ground in thermal chemistry will 

 extend it to the uncertain spots where we have yet no solid 

 foundation. 



Further, the laws of dissociation so ably investigated by De- 



ville have taught us that the force called chemical affinity, by 

 which we suppose the atoms of unlike matters are held together 

 in a compound molecule, follows precisely the same laws as ihe 

 force of cohesion, by which particles of a similar kind are united 

 in molecules. We have long known that the molecules ot 

 sulphur vapour are broken up into simpler molecules by eleva- 

 tion of temperature, and condense again when the temperature 

 is reduced. Other elementary substances behave in a similar 

 way. We have within the last two or three years learnt that 

 iodine is in part dissociated by a high temperature from mole- 

 cules consisting of two chemical atoms into molecules consisting 

 of only one such atom, and the same is true of chlorine and 

 bromine. That some such change must occur in iodine and 

 other metalloids was inferred as long ago as 1864 by the younger 

 Mitscherlich. He argued that iodine is a compound body from 

 the fact that it shows two spectra — one similar in character to 

 those of metallic oxides, and the other similar to the spectra of 

 metals ; and from the analogy in the behaviour of iodine to a 

 metallic oxide in giving the one spectrum at one temperature, 

 and the other at a higher temperature, " from this it would fol- 

 low that iodine at ordinary temperatures and iodine at the tem- 

 perature of a hydrogen flame must be conceived as two different 

 compounds, because the spectrum of iodine formed at ordinary 

 temperatures" "i.e. the absorption spectrum of iodine vapour" 

 " is different from that produced in a hydrogen flame. Also, 

 "that bromine, though it gives no flame spectrum, gives one 

 spectrum by absorption, and another by the electric spark, and 

 must therefore in its ordinary state be regarded as a compound." 

 Also that "the spectra formed by the flames of selenium, tel- 

 lurium, and phosphorus, and those of sulphur and nitrogen 

 given by feeble elecric discharges, all have the character of the 

 iod ne flame spectrum, and these metalloids would therefore, if 

 the above expressed supposition w ith regard to iodine be con- 

 firmed, also be compound bodies" (PAH, Mag., 1864, p. 1S8). 

 Since the paper from which the foregoing sentence is taken was 

 published, not only the metalloids, but many metals have been 

 found to give complicated spectra atone temperature, and much 

 simpler spectra at higher temperatures. Such are the channelled 

 spectra of sodium and potassium first described by Roscoe and 

 Schuster, the channelled spectra of silver, bismuth, and other 

 metals described by Lockyer and Roberts, and the ultra-violet 

 channelled spectrum of tin recently photographed by Prof. 

 Dewar and myself. But Mitscherlich's hypothesis gives us a 

 rational explanation of such multiple spectra produced by the 

 same subsiance, and it has been accepted in one form or another 

 by all spectroscopists since he wrote. 



Nevertheless, the exi-tence of multiple spectra cannot be 

 taken as a 1 roof of allotropic modification, u less the possi- 

 bility of a chemical combir a'ion is excluded. The channelled 

 spectrum which magnesium gives in hydrogen was mistaken by 

 more than one observer for that of some modification of the 

 simple metal, until it was shown that magne-ium in nitrogen 

 and other gases does not give it, provided hydrogen be excluded, 

 and that its persistence in hydrogen at high temperatures 

 depends, as it should if due to a chemical combinations,on the 

 pressure of the gas. If, however, homogeneous molecules are 

 dissociated by heat, so also are heterogeneous molecules, formed 

 as we say by chemical combination, split up by elevation of 

 temperature, to unite again on cooling or by increase of pressure 

 within certain limits. Nor is there any essential difference in 

 character between a chemical compound and an element beyon I 

 that of facility of decomposition. If we could not so easily 

 resolve them into their constituents, and were to disregard the 

 characteristic differences of the spectra, no one would suppose 

 ammonium to be constituted differently from potassium, or 

 cyanogen from chlorine. Indeed, chemists have long been in 

 the habit of considering the union of two atoms in a molecule of 

 ordinary hydrogen or chlorine as a species of chemical combina- 

 tion, but when we find that the combinations of particles of the 

 same kind are as definite as those of particles of different kinds, 

 and that they are both subject to precisely the same mechanical 

 laws, we are hardly justified in regarding the forces by which 

 they are produced as essentially different. To get rid of a gra- 

 tuitous hypothesis in chemistry must be a great gain. 



But it may be asked why stop here? Why may not the 

 chemical elements be further broken up by still higher tem- 

 peratures? A priori and from analogy, such a supposition is 

 extremely probable. The notion that there is but one elemen- 

 tary kind of matter is at least as old as Thales, and underlies 

 Prout's hypothesis that the atomic weights of our elements are 



