392 SCIENCE PROGRESS 



summarise this part of the subject by the remark that, whilst 

 the evidence in favour of association between solvent and 

 solute seems quite conclusive, no method is known of 

 differentiating between the hydration of the undissociated 

 molecules and the ions, and very little is known of the absolute 

 amount of hydration. 



Solvents other than Water 



In recent years a great amount of experimental material has 

 been collected, more particularly by Walden and by Kahlenberg 

 and their co-workers, with the object of elucidating the nature 

 of non-aqueous solutions. The guiding idea in this work, from 

 the point of view of the electrolytic dissociation theory, has 

 been the suggestion of J. J. Thomson 1 and Nernst 2 that, since 

 the attraction between contrary electric charges is inversely 

 as the specific inductive capacity or dielectric constant of the 

 medium, a solvent of high dielectric constant should have great 

 ionising power. The results of numerous experiments show, 

 as a matter of fact, that there is an undoubted parallelism, 

 though not direct proportionality, between the two properties. 

 Thus Centnerszwer 3 found that the conductivity of potassium 

 iodide in liquid hydrocyanic acid, which has a higher dielectric 

 constant than water, is about four times as great as in the latter 

 solvent, whilst Jones 4 has shown by the boiling-point method 

 that the same salt is rather more than half as much ionised 

 in methyl alcohol as in water, the dielectric constants being 

 in the ratio 32*5 to 8ro. There are, however, a good many 

 exceptions to this rule. Dutoit and Aston 5 consider that the 

 ionising power of a solvent is connected with its capacity for 

 polymerisation, and other observers, more particularly Briihl, 6 

 maintain that the property in question depends on the 

 unsaturated character of the solvent, which is almost, but not 

 quite, the same thing as its capacity for polymerisation. Most 

 solvents which are unsaturated have also a high dielectric 

 constant. 



1 Phil. Mag. 1893 [v.], 36, 320. 



2 Zeit. pkysikal. C/iem. 1893, 11, 220. 



3 Ibid. 1 901, 39, 217. 



4 Ibid. 1 899, 31, 129. 



5 Compt. Rend., 1897, 125, 240. 



6 Zeit. physikal. Chem. 1898, 27, 319; 1899, 30, I. 



