from Faraday to J .J. Thomson. 385 



ascribed to a blue copper cation. A striking instance of the 

 same kind is afforded by ferric sulphocyanide ; here the strong 

 solution shows a deep red colour, due to the salt itself ; but on 

 dilution the colour disappears, the ions being colourless. 



If it be granted that ions can have any kind of permanent 

 existence in a salt solution, it may be shown from thermo- 

 dynamical considerations that the degree of dissociation must 

 increase as the dilution increases, and that at infinite dilution 

 there must be complete dissociation. For the available energy 

 of a dilute solution of volume V, containing j gramme-molecules 

 of one substance, >/ 2 gramme-molecules of another, and so on, is 

 (as may be shown by an obvious extension of the reasoning 

 already employed in connexion with concentration-cells)* 



r (T) + RT^n r log (UT! V) + the available energy 



possessed by the solvent before the introduction of the solutes, 

 where r (T) depends on T and on the nature of the r th solute, 

 but not on V, and R denotes the constant which occurs in the 

 equation of state of perfect gases. When the system is in 

 equilibrium, the proportions of the reacting substances will 

 be so adjusted that the available energy has a stationary 

 value for small virtual alterations Swj, &^, ...... of the 



proportions ; and therefore 



- SSn r .<t> r (T) + RT2$n r .log (n r jV) 



Applying this to the case of an electrolyte in which the 

 disappearance of one molecule of salt (indicated by the suffix ,) 

 gives rise to one cation (indicated by the suffix 2 ) and one anion 

 (indicated by the suffix 3 ), we have B^ = - 7^ = - Sn* ; so the 

 equation becomes 



= 0, (T) - 2 (T) - 03 (T) + RT log (n, V/n.n,) - RT, 



or 



= a function of T only. 



* Cf. pp. 382-383. 

 2 C 



