218 SCIENCE PROGRESS 



the liquefaction of the salt rather than to any chemical interaction 

 with the solvent. 



Hydration of the Ions demanded by the Law of Mass- 

 action 



It was unfortunate that the chemical aspects of ionisation, 

 which were clearly recognised when the problem of ionisation 

 was first discussed in 1883, were to a large extent overlooked 

 and set on one side when the theory of electrolytic dissociation 

 was developed four years later. The earlier idea of association 

 as a cause of ionisation was regarded as incompatible with the 

 newer theory of dissociation, and for some years the only 

 possibilities that were recognised were that the solution might 

 contain either hydrated molecules or anhydrous ions. When 

 at last it was recognised that hydrates and ions might exist in 

 the same solution, 1 the assumption was generally made — in 

 defiance of all the principles of mass-action — that hydrates only 

 existed in concentrated solutions, and were decomposed by 

 further dilution. 2 



It is a remarkable illustration of the hypnotic influence of 

 Arrhenius's theory that those who adopted it were able to blind 

 themselves so completely to the commonplace requirements of 

 chemical theory as to assume — apparently without any scruples 

 or qualms of conscience — that the effect of adding water was to 

 dehydrate the solute, and that if a sufficiently large excess of 

 water was added the salt was rendered completely anhydrous ! 

 Even at the present time it is customary to express the theory 

 of electrolytic dissociation by symbols which represent the ions 

 as anhydrous, and the question of water-combination is treated 

 as of secondary importance. 3 Indeed, it is doubtful whether the 



1 " It might well be the case that complex molecular aggregates were capable 

 of existence alongside of dissociated molecules where ions are present. In the 

 case of solutions of sulphuric acid, for example, it is by no means inconceivable 

 that aggregates of several molecules of sulphuric acid (H.,S0 4 ) n , or of compounds 

 of acid and water, such as H..SO,, . 2H s O, etc., might exist along with the ions of 

 dissociated sulphuric acid, 2H and S0 4 , or more probably H and HS0 4 " (Ramsay, 

 B.A. Report, Leeds, 1890, 325). 



3 For an illustration of the prevalence of this opinion see Jones and Getman 

 (Amer. Chem. Jottrn. 1904, 31, 356). The experimental error on which this con- 

 clusion was based was subsequently corrected in a German translation of the paper. 



3 " The question of hydration, though of great interest, is of secondary im- 

 portance for the electrolytic dissociation theory " (Senter, Science Progress, January 

 1907, footnote p. 386). 



