2o 4 SCIENCE PROGRESS 



dissociation of the salt had been overcome, the new theory was 

 so simple as to lend itself readily to dogmatic treatment ; its 

 advocates scarcely seemed able to realise that so excellent a 

 theory might possess limitations, or that facts urged in its 

 support might be capable of alternative interpretations. The 

 ionisation of a salt in dilute solutions was treated as a spon- 

 taneous process depending mainly on volume considerations, 

 and was no longer regarded as a chemical equilibrium in which 

 the presence of an excess of solvent was the dominant factor. 

 The text-books written in exposition of the new theory of 

 solutions scarcely mentioned the solvent except as a mere 

 diluent, and equations such as 



+ 

 NaCl -±_ Na + CI 



were put forward as expressing all the essential features of the 

 process of ionisation. The later history of the theory, and its 

 development into a maturer and more reasonable (even if less 

 dogmatic) form, has indeed consisted almost entirely in a 

 reversion to the earlier idea that " perhaps the active part is 

 a compound of [solute] and solvent," and that " the three con- 

 stituent parts of the solution form a system in chemical equili- 

 brium such that dilution increases the active and diminishes the 

 inactive portion." 



Physical Theories of the Action of the Solvent 



The earliest attempts to account for the widely different 

 conductivities of the same solute when dissolved in different 

 solvents — or in the same solvent at different temperatures — 

 were based on purely physical considerations, amongst which 

 the viscosity or fluidity of the solution has always played a 

 prominent part (see, for example, Wiedemann, Pogg. Ann. 1856, 

 99, 229). It was one of the conspicuous merits of the theory 

 of Arrhenius that it resolved the conductivity of a solution into 

 two factors, ionisation and mobility, which could be determined 

 experimentally and submitted to separate examination. The 

 idea of separate ionic mobilities for anion and kathion, it should 

 be noted, had been introduced by Kohlrausch some years 

 previously {Gottingen Nachrichten, 1876, 213), but he considered 

 that the whole of the solute was directly concerned in electro- 

 lysis, and therefore arranged his values so that the sum of the 



