216 SCIENCE PROGRESS 



of which two are active in electrolysis and the third inactive 

 {ibid. 1908, 81. p. 86). 



HC1 + OH„^t HC1 : OH 2 ^ H . C1<(q h $ H-.O/jSj 

 inactive active active 



This view of the nature of ionisation has the advantage 

 of assigning a perfectly definite meaning to the coefficient of 

 ionisation, but does not lend itself so readily as the theory 

 of electrolytic dissociation to an explanation of the double 

 osmotic pressure of a binary electrolyte such as KC1, the 

 independence of the ionic mobilities, or the additive properties 

 of salt solutions. The tendency in recent years has, therefore, 

 been to amalgamate the two rival theories of solution by means 

 of the hypothesis of hydrated ions— a hypothesis that retains 

 all the advantages of the theory of electrolytic dissociation, but 

 has the merit of recognising also the combination of solute and 

 solvent which forms the essential feature of the hydrate theory 

 of solutions. 



The Hydrate Theory of Ionisation 

 The hypothesis of hydrated ions has now received somewhat 

 general acceptance, but its significance as an essential factor 

 in the ionisation of aqueous solutions has up to the present 

 received very scanty recognition ; there has also been some 

 difference of opinion as to the exact form which the hydrate 

 theory of ionisation should take. Werner, who was one of the 

 first to detect the existence of complex ions and to recognise 

 their importance in electrolytic dissociation, limited the process 

 of hydration to the metallic radical, and considered that the 

 hydrated kathion was a compound of fixed composition (usually 

 a hexahydrate) analogous to those which are formed by the 

 addition of ammonia to the metal in the cobaltamine and other 

 similar salts. This view, enunciated in 1893 {Zcit. Anorg. Chem. 

 3, 294), was reiterated with the same limitations in the course 

 of the discussion on "Ionisation" in Section A of the British 

 Association at Leicester in 1907. Armstrong, on the other 

 hand, had laid stress on the association of the solvent with the 

 negative radical. 



The form of the theory advocated by the writer, in conjunc- 

 tion with W. R. Bousfield, in a number of recent papers {Phil. 

 Trans. 1904, 204, 282; Trans. Faraday Soc. 1905, 1, 197; 1907, 

 3, 123), agrees with that put forward by Abegg {Zeit. Anorg. Chem. 



