THE HYDRATE THEORY OF SOLUTIONS 213 



The assumption made in the preceding paragraph makes no 

 attempt to account for the origin of the energy in ionisation, but 

 rather brings out more strongly than ever the magnitude of the 

 forces resisting it. It is, however, important to notice that the 

 opposing electrical forces are shown to be inversely proportional 

 to the inductive capacity of the medium, so that whilst the 

 electrification of a gram-equivalent of hydrogen chloride in water 

 would demand the expenditure of 9,600 calories, the same 

 process carried out in benzene (assuming, provisionally, equal 

 values for the ionic mobilities) would demand an absorption of 

 400,000 calories ! Whilst, therefore, it is necessary in every case 

 to seek for some force powerful enough to overcome the natural 

 attraction of the ions, the task is rendered practically hopeless 

 in the case of a medium of low inductive capacity by reason of 

 the enormous quantity of energy used up in the process of 

 electrification, apart altogether from the chemical affinity of the 

 atoms. 



Whatever differences of opinion may exist as to details, there 

 can be no doubt that, broadly speaking, it is to the solvent that 

 we must look for the main source of the energy in ionisation. 1 

 It is, moreover, more than probable that this energy is rendered 

 available by a chemical interaction between the solvent and 

 solute. The theories which associate ionisation with the poly- 

 merisation of the solvent or with the presence in the solvent 

 molecules of unsaturated atoms such as divalent oxygen or 

 trivalent nitrogen, do not in themselves afford any real explana- 

 tion of the cause of ionisation ; observations of this kind are of 

 the greatest importance as indicating those properties of the 

 solvent which are most likely to come into operation when it 

 interacts with the solute to produce an electrolyte, but from the 

 point of view of energy it is not enough that the molecules of 

 the solvent should play the part of an onlooker — however 

 sympathetic — they must themselves take part in the struggle and 

 contribute their share in the hard work of ionisation. 



1 " It is acknowledged that different solvents have different powers of ionising 

 a given substance, thereby conclusively proving that the function of the solvent 

 cannot be properly described as merely giving the ions space to resolve themselves. 

 And those who speak so acknowledge that it is only an analogy, or a.fa<;on de ftarler. 

 But it seems a very misleading analogy, which leaves out the really active part that 

 the solvent plays, and attributes to it a purely passive part" (Fitzgerald, B.A. 

 Report, Leeds, 1890, p. 327). 



