THE HYDRATE THEORY OF SOLUTIONS 215 



ammonia — both of which contain an atom which is capable of 

 assuming a higher valency — possess in a remarkable degree the 

 power of combining with salts, and are amongst the best of the 

 ionising solvents. On the other hand, the chemical theory of 

 ionisation recognises to the full the importance of the specific 

 properties of the solute, the mutual affinity between which and 

 the solvent is the determining factor in the production of active 

 electrolytic complexes. 



{d) It is evident that solvents which are capable of com- 

 bining with a large range of solutes are likely also to possess 

 the property of forming loose complexes composed entirely of 

 solvent molecules. This is not, however, a direct cause or 

 necessary condition of ionisation, and whilst the majority of 

 ionising solvents undoubtedly belong to the group of associated 

 liquids, there is no reason for assuming that electrolysis can 

 only occur in solvents of this group. Sulphur dioxide, for 

 instance, which dissolves a number of iodides with the formation 

 of conducting solutions, possesses the property of forming 

 additive compounds, but appears itself to be a non-associated 

 liquid. (Walden, Zeit. phys. Chem. 1902, 39, 513-96; Walden 

 und Centnerszwer, ibid. 1903, 42, 432-68.) 



The exact nature of the chemical equilibrium involved in the 

 process of ionisation has been the subject of much divergence of 

 opinion. Arrhenius's early suggestion was made in a somewhat 

 vague form : the active part of the solute might be a hydrate, 

 but the converse proposition, that a hydrated solute was 

 necessarily an electrolyte, could obviously not be maintained. 

 Some additional factor must come into play in order to produce 

 ionisation. Armstrong, in 1886 {Proc. Roy. Soc. 1886, 40, 268), 

 attributed the increased molecular conductivity of dilute 

 solutions to the gradual resolution of the more or less poly- 

 merised molecules of the salt into simple molecules or monads, 

 which when combined with the solvent constituted a " composite 

 electrolyte," to which alone the conductivity of the solution was 

 due ; to this he has recently added that " the electrolytically 

 effective monads must be thought of as hydrated in some par- 

 ticularly intimate manner, perhaps as hydroxylated " (ibid, 1907, 

 79, 595), and the conception has been introduced that a solu- 

 tion may contain isomeric hydrates of different constitution, 1 



1 Compare Werner, Neuere Anschamingen in der A?iorg. Chemie, Braunschweig, 

 1905, p. 169. 



