THE HYDRATE THEORY OF SOLUTIONS 203 



" The aqueous solution of any hydrogen-compound (acid or 

 base) is composed of two parts besides the water, viz. an active 

 (electrolytic) part and an inactive (non-electrolytic) part. The 

 three constituent parts of the solution form a system in chemical 

 equilibrium, such that dilution increases the active and dimin- 

 ishes the inactive part." 



" How the inactive and active portions differ is not certain, 

 perhaps only physically ; perhaps the active part is a compound 

 of the hydrogen-compound and solvent." 



" To fix ideas we can introduce the notion of a coefficient of 

 activity defined thus : The coefficient of activity of an electrolyte 

 is a number expressing the ratio of the number of ions which 

 are really in the electrolyte to the number of ions which it 

 would enclose if the electrolyte were totally transformed into 

 simple electrolytic molecules " (Recherches sur la Conductabilite 

 galvanique des Electrolytes (152 pages). Par Svante Arrhenius. 

 Memoire presente a VAcad. des Sciences de Snede le 6 juin 1883. 

 Abstracted by Lodge, B.A. Report, Birmingham, 1886, p. 363). 



In the 1887 paper the " coefficient of affinity" was identified 

 with the "dissociation ratio" which Lodge had introduced 

 {B.A. Report, Aberdeen, 1885, p. 756) in order to express the 

 proportion of ions existing in a free state in the solution as 

 postulated by Clausius and by Williamson. The chief feature 

 of Arrhenius's classical paper, " Uber die Dissociation der in 

 Wasser gelosten Stoffe," is therefore not the introduction of the 

 idea of electrolytic dissociation, nor even the idea of a definite 

 dissociation-coefficient, but the promulgation of two independent 

 methods by which the numerical value of the coefficient could 

 be determined, and concordant results obtained from measure- 

 ments of electrical conductivity and of osmotic pressure. From 

 this time onward the " affinity-coefficient " was generally aban- 

 doned in favour of the " dissociation-ratio " ; but it is important 

 to note that it is still possible to express Arrhenius's earlier 

 conception by making use of the term "coefficient of ionisation " 

 to represent the proportion of the solute that is " ionised " 

 (Fitzgerald, B.A. Report, Leeds, 1890, p. 327), or active in 

 electrolysis, since this expression does not necessarily introduce 

 the further conception that the process of ionisation involves the 

 electrolytic dissociation of the salt. 



An incidental consequence of the introduction of the theory 

 of electrolytic dissociation was to cast into the background the 

 consideration of the part played by the solvent in ionisation and 

 electrolysis. When once the initial reluctance to recognise the 



