684 RICE ART. L 



is to discharge the charged surface of the plates. It does so by 

 means of the ions arising naturally from its own dissociation. 

 Indeed Volta had vague notions of the same kind, although 

 naturally he could have no prevision, in his time, of modern 

 ideas of dissociation and energy. 



Of course this changed attitude towards the Volta effect does 

 not carry with it a denial of the existence of a P.D. at a metal- 

 electrolyte interface; it merely asserts that the metal-electrolyle 

 discontinuities in potential do not account for the whole of the 

 E.M.F. of a cell. 



63. Discontinuity of Potential between a Metal and an Electrolyte 



As is well known, the hypothesis of Nernst concerning the 

 origin and magnitude of the potential discontinuity at a metal- 

 electrolyte interface has been accepted until recently by most 

 physical chemists as an adequate formulation. Nernst's proof 

 of his formula is thermodynamical, and he deduces the result 



M 

 Ve = — (log p, - log Pa) , 



where po is the osmotic pressure of the ion which is the common 

 component of electrolyte and electrode, ps its "solution pres- 

 sure" in the metal, v the valency of the ion, and Ve the excess 

 of the potential of the electrode above that of the electrolyte. 

 The "solution pressure" in the metal cannot be intuitively 

 apprehended like the pressure in a gas, or even like an osmotic 

 pressure, which at all events is open to observation by means 

 independent of all considerations of electrode potentials. It is 

 merely brought into the proof to provide a work term in a usual 

 isothermal cycle when electrons occupying volume v in the 

 metal pass into a volume v' in the solution, The proof is well 

 known and can be found in standard texts (e.g., F. H. Newman's 

 Electrolytic Conduction, London, 1930, pp. 184-185). The great 

 objection to the hypothesis is the perfectly monstrous values 

 of solution pressure which must be postulated to make the 

 formula fit the facts. Thus for zinc Ps is almost 10"^^ atmos- 

 pheres, while for palladium it is about 10~^^ atmospheres; in 



