as a Standard of Electromotive Force. 2'2l) 



way that the transference takes place without change in the 

 entropy or volume of either phase. The energy variation of 

 the system would then be 



(%^~) dm * + \^r) dnu ' = &*i -Pi)drn. 



\OmJr t vm 2 \0"*1 /r/v'm't 



But since neither the entropies nor the volumes change, 

 the system neither does external work nor acquires hear. 

 Consequently we must have 



pi—Pi = 0, 

 and, by a similar argument, 



These and two other equations, derived one from each 

 phase at given and p, suffice to define the conditions of 

 equilibrium completely — the essential variables being 



mjv, m 2 /v and mi/v', m 2 '/v' respectively *. 



In assuming, as above, that the total energy of a known 

 mass of each phase, at given 6 and p, is dependent only on 

 its composition, we neglect the possible influence of surface 

 energy. 



"We may continue to suppose that, to a first degree of 

 approximation, the surface tension and electrostatic potential 

 difference between the x per cent, and y per cent, amalgams 

 are negligible and proceed to examine the conditions under 

 which the two amalgams could coexist in presence of the 

 same electrolyte. 



Considering first the equilibrium of fluid amalgam and 

 electrolyte, reversible exchange of Cd or Hg between elec- 

 trode and electrolyte will be possible since the latter is a 

 solution of sulphates of Hg and Cd. But while the masses 

 dm x and dm 2 of the metals may be considered neutral in the 

 amalgam, they are associated with definite positive charges, 

 which we may write k x dm 1 and k 2 dm 2 , when in solution. 



In order that (as in the equations already given) we may- 

 still regard m l and m 2 as independent variables, we assume 

 that when a quantity dm x of mercury leaves the solution, 

 a quantity dm z of anions (S0 4 ) carrying the charge 



k. 6 dm d = — k 1 dm 1 



also leaves the solution and accumulates at the surface 

 separating electrode and electrolyte. We assume also that 



* Cf. § 14, below. 



