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LI. The Decline of the Hypothesis of Ionic Dissociation. 

 By F. P. Worley, D.Sc. (London), Professor of Chemistry, 

 Auckland, New Zealand*, 



DURING the past quarter of a century the doctrine of 

 ionic dissociation, formulated by Arrhenius in 1883-87, 

 has been so persistently advocated that few chemists now 

 regard it as a mere hypothesis ; the majority fail to see that 

 no single convincing argument necessitating the hypothesis 

 has ever been brought forward and that its acceptance has 

 been due to the plausibility of the arguments by which, 

 apparently, it may be supported rather than to their cogency: 

 nor is it recognized how far from being secure are not only 

 the minor supports on which it rests but also some, if not all, 

 of those upon which most reliance is placed. 



The very simple explanation the hypothesis apparently 

 afforded of so many chemical phenomena left little doubt in 

 the minds of its early advocates that further investigation 

 would dispose of difficulties such as are presented by con- 

 centrated solutions and strong electrolytes, for example ; but 

 although, of late years, a good deal of fresh evidence has 

 been adduced which has appeared to support the doctrine, 

 difficulties such as those referred to have not been removed! ; 

 indeed, it is now tacitly admitted that such phenomena as the 

 colour of salt solutions and the mutual precipitation of salts 

 can no longer be cited in support of the hypothesis. More- 

 over, much has been done to show that the explanation of 

 the catalytic effect exercised by acids — in the case of changes 

 such as those attending the hydrolysis of cane-sugar and of 

 ethereal salts and the reverse change of etherification — by 

 the hypothesis presents very formidable difficulties and that 

 the effect can be accounted for rationally without its aid J; 

 especially difficult of explanation is the part played by neutral 

 salts in such phenomena. 



One of the strongest arguments put forward by Arrhenius 

 in favour of his hypothesis was the fact that it was possible 

 to correlate the chemical activity of electrolytes with the 

 presumed degree of ionic dissociation as deduced from their 

 molecular electrical conductivities: hence the dissociated 

 ions alone of electrolytes were postulated as concerned in 



* Communicated by Dr. II. E. Armstrong, F.R.S. 



t Arrhenius, ' Theories of Solutions,' p. 172. 



J Roy. Soc. Proc. A. lxxxvii. p. 604 (1912). " Studies of the Pro- 

 cesses Operative in Solutions. XXIV. The Nature of the Hydrolytic 

 Process/' By H. E. Armstrong and F. P. Worley. 



