August 8, 1889] 



NATURE 



355 



from the action of many forces and affinities. Carbonaceous 

 matters, for example, burn at the expense of the oxygen of the 

 air, yielding a quantity of heat and forming products of com- 

 bustion, in which it was thought that the affinities of the oxygen 

 with the combustible elements were s-atisfied. But it appeared 

 that the heat of combustion was competent to decompose these 

 products, to dissociate the oxygen from the combustible elements ; 

 and therefore, to explain combustion fully, it is necessary to take 

 into account the equilib ium between opposite reactions, between 

 those which evolve and those which absorb heat. 



In the same way, in the case of the solution of common salt 

 in water, it is necessary to take into account, on the one hand, 

 the formation of compound particles generated by the combina- 

 tion of salt with water, and, on the other, the disintegration or 

 scattering of the new particles formed, as well as of those origin- 

 ally contained. At present we find two currents of thought, 

 apparently antagonistic to each other, dominating the study of 

 solutions : according to the one, solution seems a mere act of 

 building up or association ; according to the other, it is only 

 dissociation or disintegration. The truth lies, evidently, between 

 these views ; it lies, as I have endeavoured to prove by my in- 

 vestigaiinns into aqueous solutions, in the dynamic equilibrium 

 of particles tending to combine and also to fall asunder. The 

 large majority of chemical reactions which appeared to act vic- 

 toriously along one line have been proved capable of acting as 

 victoriously even along an exactly opposite line. Elements 

 which utteily dtcline to combine directly may often be formed 

 into comparatively stable compounds by indirect means, as, for 

 example, in the case of chlorine and carbon ; and, consequently, 

 the sympathies and antipathies, which it was thought to transfer 

 from human rela'ions to those of atoms, should be laid aside 

 until the mechanism of chemical relations is explained. Let us 

 remember, however, that chlorine, which does not form with 

 carbon the chloride of carbon, is strongly absorbed, or, as it were, 

 dissolved by carbon, which lead- us to suspect incipient chemical 

 action even in an external and purely surface contact, and in- 

 voluntarily gives rise to conceptions of that uniiy of the forces 

 of Nature which has been so energetically insisted on by Sir 

 William Grove and formulated in his famous paradox. Grove 

 noticed that platinum, when fused in the oxy hydrogen flame, 

 during which operation water is formed, when allowed to drop 

 into water decom]ioses the latter and produces the explosive 

 oxyhydrogen mixiure. The explanation of this paradox, as of 

 many others which arose during the period of chemical renais- 

 sance, has led, in our time, to the promulgation by Henri St. 

 Claire Deville of the conception of diss elation and of equili- 

 brium, and has recalled the teaching of Berthollet, which, not- 

 withstanding its brilliant confirmation by Heinrich Rose and 

 Dr. Glad>ionf, had not, up to that period, been included in 

 received chemical views. 



Chemical equilibrium in general, and dissociation in particular, 

 are now b>.ing so fully worked out in detail, and applied in such 

 various ways, that I do not allude to them to develop, but only 

 use them as examples by which to indicate the correctne-s of a 

 tendency to regard chemical combinations from points of view 

 differing from those expressed by the term hitherto appropriated 

 todefine chemical forces— namely, "affinity." Chemical equilibria 

 dissociation, the speed of chemical reactions, thermochemistry, 

 spectroscopy, and, more than all, the determination of the in- 

 fluence of m.assesand the search for a connection between the 

 properties and Wfights of atoms and molecules; in one word, 

 the vast mass of the most important chemical researches of the 

 present day, c'early indicates the near approach of the time 

 when chemical doctrines will submit fully and completely to the 

 doctrine which was fiist announced in the "Principia" of Newton. 



In order that the application of these principles may bear 

 fruit, it is evidently insufficient to assume that statical equili- 

 brium reigns alone in chemical s\stf ms or chemical molecules : 

 it is necessary to grasp the conditions of possible states of dy- 

 namical equilib ia, and to apply to them kinetic principles. 

 Numerous considerations compel us to renounce the idea of 

 statical equilibrium in molecules, and the recent yet strongly 

 supported appeals to dynamic principles cnnstitu'e, in my 

 opinion, the fnund.ition of the modern teaching relating to 

 atomicity, or the valency of the elements, which usually forms 

 the basis of invest gitions into organic or carbon compounds. 



This leaching has led to brilliant explanations of vecy many 

 chemical relations and to cases of isomerism, or the difference in 

 the properties of substances having the same composition. It 

 has been so fruitful in its many applications and in the foie- 

 shadowing of remote consequences, especially respecting carbon 



compounds, that it is impossible to deny its claims to be ranked 

 as a great achievement of chemical science. Its practical 

 application to the .synthesis of many substances of the most 

 complicated composition entering into the structure of organized 

 bodies, and to the creation of an unlimited number of carbon 

 compounds, among which the colours derived from coal tar 

 stand prominently forward, surpass the synthetical powers of 

 Nature itself. Yet this teaching, as applied to the structure of 

 carbon compounds, is not on the face of it directly applicable to 

 the investigation of other elements, because in examining the 

 first it is possilile to assume that the atoms of carbon have 

 always a definite and equal number of affinities, while in the 

 combinations of other elements this is evidently inadmissible^ 

 Thus, for example, an atom of carbon yields only one compound 

 with four atoms of hydrogen and one with four atoms of chlorine 

 in the molecule, while the atoms of chlorine and hydrogen unite 

 only in the proportions of one to one. Simplicity is here 

 evident, and forms a ])oint of departure from which it is easy to 

 move forward with firm and secure tread. Other elements aie 

 of a different nature. Phosphorus unites with three and with 

 five atoms of chlorine, and consequently the simplicity and 

 sharpness of the application of structural conceptions are lo t. 

 Sulphur unites only with two atoms of hydrogen, but with 

 oxygen it enters into higher orders of combination. The 

 periodic relationship which exists among all the properties of 

 the elements, such, for example, as their ability to enter into 

 various combinations, and their atomic weights, indica'.e that 

 this variation in atomicity is subject to one perfectly exact and 

 general law, and it is only carbon aid its near analogues which 

 constitute cases of permanently pr. served atomicity. It is im- 

 possible to recognize as constant and fundamental properties of 

 atoms, powers which, in substance, have proved to be variable. 

 But by abandoning the idea of permanence, and of the constant 

 saturation of affinities — that is to say, by acknowledging the 

 possibility of free affinities — many retain a co nprehension of the 

 atomicity of the elements "under given conditions"; and on 

 this frail foundation they build up structures composed of 

 chemical molecules, evidently only because the conception of 

 manifold affinities gives, at once, a simple statical method of 

 estimating; the composition of the most complicated molecules. 



I shall enter neither into details, nor into the various con- 

 sequences following from these views, nor into the disputes 

 which have sprung up respecting them (and relating especially 

 to the number of i-omers possible on the a-sumption of free 

 affinities), because the foundation or origin of theories of this 

 nature suffiers from the radical defect of being in opposition to- 

 dynamics. The molecule, as even Laurent expressed himself, 

 is represented as an architectural structure, the style of which is 

 determined by the fundamental arrangement of a few atoms, 

 while the decorative details, which are capable of being varied 

 by the same forces, are formed by the elements entering into the 

 combination. It is on this account that the term "structural " is so 

 appropriate to the contemporary views of the above order, and that 

 the " constructors " seek to justify the tetrahedric, plane, or pris- 

 matic dispo:iiion of the atoms of car! on in benzole. It is evidei.t 

 that the consideration relates to the statical position of atoms and 

 molecules, and not to their kinetic relations. The atoms of the 

 structural type are like the lifeless pieces on a chess-board : they 

 are endowed but with the voices of living beings, and are not 

 tho.se living beings themselves ; acting, indeed, according to 

 laws, yet each possessed of a store of energy, which, in the 

 present stale of our knowledge, must be taken into account. 



In the days of Haiiy, crystals were considered in the same 

 statical and structural light, but modern crystallographers, 

 having become more thoroughly acquainted with their physical 

 properties and their actual f irmation, have abandoned the earlier 

 views and have made their doctrines dependent on dynamics. 



The immediate object of this lecture is to show that, starting 

 with Newton's third law of motion, it is possible to preserve to- 

 chemistry all the advantages arising from structural teaching, 

 without being obliged to build up molecules in solid and motion- 

 less figures, or to ascribe to atoms definite limited valencies, 

 directions of cohesion, or affinities. The wide extent of the sub- 

 ject obliges me to treat only a small portion of it — namely, of 

 substituAons, without specially considering combinations and 

 decompositions— and, even then, limiting myself to the simplest 

 examples, which, however, will throw open prospects embracing, 

 all the natural complexity of chemical relations. For this reason, 

 if it should prove possible to form groups similar, for example, 

 to II4 or CHfj as the remnants of molecules CII4 or C«Hg, we- 

 shall not pause to consider them, because, as far as we know. 



