46o 



NA TURE 



[August 29, 1907 



ready for decomposition, or, if we like, to decompose and 

 to leave the products of decomposition in the fluid instead 

 of getting rid of them at the electrodes. It really means 

 converting a substance into an electrolytic conductor, and 

 for that process the term is wanted. As to the nature of 

 ionisatiori, the facts to be expressed are that the atoms 

 are perfectly frie to travel, that an extremely minute 

 force will set them in motion. They are either free, there- 

 fore, or else potentially free. The merest trace of E.M.F. 

 suffices to practically transfer them in one direction. To 

 account for this, we may suppose a certain proportion of 

 the atoms are either continually or persistently free, which 

 is improbable ; or that they are constantly interchanging 

 with others, in such a way that they are free between 

 their combinations ; or else that they are combined into 

 such a loose and large aggregate that the attraction in 

 all directions is equal — which is really a case of potential 

 freedom barely distinguishable from actual freedom. If 

 the instinct of chemists really prefers this view to the 

 idea of actual dissociation, then great weight should be 

 attached to that instinct, and the aggregation mode of 

 specification should be preferred ; but for practical pur- 

 poses it makes very little difference, and in treating the 

 matter mathematically, actual dissociation is the idea 

 most easily expressible. 



He was' not surprised that Prof. .Armstrong prefers this 

 hypothesis of aggregation, ina-smuch as he has for twenty 

 years preached the doctrine of residual affinity as explan- 

 atory of solution, of molecular combination, and of many 

 other such things, as distinct from simple and undivided 

 units of valency. Now this doctrine is entirely consistent 

 with the electrical theory of chemical attraction. For the 

 single bond with which, say, sodium is attached to 

 chlorine is, on the electrical view, not a unit of which 

 no fractions are possible, as perhaps it used to be re- 

 garded, but a group or bundle of lines of force which 

 can be split up into any number of fractions, a few lines 

 being easily diverted. Thus a few are thrown off to 

 encounter any water molecules which may come into the 

 molecular neighbourhood ; and although the attraction of 

 each water molecule for the sodium or the chlorine must 

 be much less than the attraction of sodium and chlorine 

 for each other, yet in a dilute solution, when the water 

 molecules are numerous, the cooperation of a lot of them 

 may exert a pull sufficient to balance the attraction of 

 the atoms of the salt itself, and either actually or poten- 

 tially to tear the salt molecule asunder. But whether it 

 is really torn asunder or not, or whether it goes about 

 with an aggregate of water molecules hanging on to it. 

 ready to be torn asunder by the slightest addition of 

 applied electric force, is a matter on which as a physicist, 

 at least from the electrolytic point of view, he would 

 express no opinion. It is possible that some of the pheno- 

 mena now known to physical chemists may afford a 

 criterion for distinguishing between these two extremely 

 adjacent hypotheses. But he wished to testify to the fact 

 that the residual adfinily doctrine, when developed in 

 accordance with the electrical theory of chemical action, 

 is a luminous one, and explains, not only the influence of 

 a solvent, but mass .iction generally ; and that we owe a 

 debt to Prof. .Armstrong for keeping the chemical reality 

 of the facts thus able to be explained always before us. 



As to the objection to ihermodvhamical reasoning: he 

 should allow that thermodynamics is a method of arriving 

 at results, but it is not ultimately to be regarded as the 

 most satisfactory method. For it does not pay attention 

 to the process by which the results are obtained : it is 

 rather a method of proceeding blindfold, or in blinkers, 

 and reaching the result by an ingenious scheme of argu- 

 ment without attention to the process involved. It is re- 

 markable that reasoning can be carried on in this way, 

 but it is undeniable that so it is. 



So far from chemistry being cold-shouldered by physics, 

 it would appear rather to be the other way ; and the 

 great interest of the present state of things is that the 

 facts of chemistry, or at any rate the most fundamental 

 of them, seem now able to be tackled by mathematical 

 methods, and to show signs of being more completely 

 understood than ever before. 



This is a process which, however puzzling it may be 

 in its early .stages, will surely be welcomed by chernists 



NO 1974, VOL. 76] 



in the long run ; and it will be a great day when such 

 an empirical generalisation as the periodic law really 

 yields up its secret to mathematical analysis — based, as 

 that probably will be, on the hypothesis of the electric 

 constitution of the atom. 



With all that had been said about Prof. G. F. FitzGerald 

 he heartily concurred, but he seldom or never found him- 

 self in disagreement with that great man ; and inasmuch 

 as Prof. Armstrong seems to agree with him too, at any 

 rate to a great extent, there appears to be less difference 

 between the physicist and the chemist than might super- 

 ficially appear. 



-Meanwhile, we should not avoid the use of the word 

 " ionisation," but should use it carefully, and should 

 determine the nature of the process which it represents. 



Prof. Abegg reminded Prof. Armstrong that combination 

 between solute and solvent is admitted by everyone. But 

 this alone does not explain the duplication of the number 

 of molecules which is indicated by any method of count- 

 ing, such as lowering of freezing point, osmotic pressure, 

 &c. The difliculty felt by some cheinists in realising how 

 simple solution should effect such a thorough change of 

 molecular state is lessened if it is recognised that even in 

 the solid state the molecules are often dissociated. 

 Internal friction is the cause of the small conductivity of 

 solids, not want of dissociation. No theory explains the 

 same number of facts as the dissociation theory. Prof. 

 Werner emphasised the necessity for postulating associ- 

 ation as well as dissociation, bringing forward his own 

 work on complex inorganic combinations. Without 

 association there is no ionisation. Dr. T. M. Lowry said 

 he had been led to advocate a hydrate theory of ionisation. 

 The molecule does not fall to pieces, but is torn apart 

 by the affinity of the solvent for the ionic nuclei of which 

 the salt is composed (Trans. Faraday Society, July, 

 1905). Dr. Burch said chemists and physicists had 

 attacked the subject from opposite ends, like engineers 

 driving a tunnel, and the question was whether they would 

 meet in the middle, and he thought they would. 



Dr. Senter directed attention to Werner's demonstration 

 that in such compounds as Co(H. 01,01, the water was 

 associated with the positive ion, and that it had also 

 been shown that the positive ions of the alkali and 

 alkaline earth salts were also hydrated to a considerable 

 extent in solution (r/. Senter, Scieyxce Progress, January, 

 iqo7). He considered that Armstrong's theory that the 

 variation of electrical conductivity with dilution could be 

 accounted for by hydration alone was opposed to the law 

 of mass action ; the requirements of this law will be 

 satisfied only if the molecule splits up into two parts 

 which convey the current. 



Dr. N. T. M. Wilsmore claimed that the ionisation 

 theory was a growing one, as was indicated by the recog- 

 nition to-day of the influence of the solvent. Against a 

 Grotthus theory he urged that, since two molecules are 

 involved, the conductivity would vary as the square of 

 the concentration. Dr. Haber inquired how the theory of 

 a concentration cell could be worked out if no dissociation 

 occurs. He also discussed the bearing of solubility, e.g. 

 with regard to the decrement of solubility of AgCl by 

 .addition of sodium the quantitative relations are the same 

 as we deduce from conduction experiments. He would 

 express the solution of a salt such as sodium chloride by 

 the equation 



ClNa + (+)Qz^C\Q + Na0 

 where (+) stands for a unit positive and (-) a unit negative 

 chnrjje. 



Throughout the discussion the term hydration was 

 employed. We may point out that we believe Prof. Arm- 

 strong would not take this term as adequately connoting 

 the formation of the complexes that he postulates. 



Two papers were read by Prof. Rutherford, the first 

 on the production and origin of radium, in which he gave 

 evidence that there must be an intermediate substance 

 formed in the derivation of radium from uranium. Sir 

 O. Lodge drew from Prof. Rutherford the statement that 

 he did not know whether or not this substance was radio- 

 active. The second paper, in collaboration with J. E. 

 Petavel, was on the effect of high temperatures on the 

 activity of the products of radium, in which they detailed 



