concerning Volta's Contact Force. 463 



a proportion which can be calculated from a knowledge of the 

 relative rates of migration : though the observations of the true 

 migration ratios may be complicated by molecular electric 

 endosmose through the porous cell. Indeed, so far as I know 

 at present, the migration velocity of S0 4 has not yet been 

 very well directly determined. 



But, although all this is valid on any view as to the nature 

 of solution, the above mode of statement belongs to the old- 

 fashioned view of chemical combination, the notion of 

 metallic solution which was in vogue a few years ago. It 

 may be well to state the matter also in terms of dissociation 

 ideas. 



The fashionable method is to think of the solvent as little 

 more than a menstrum for a stream of metallic ions to saunter 

 through. We may think of the zinc ions travelling into the 

 liquid and some copper ions leaving it, the adjustment of the 

 proportion between the free zinc and the freed S0 4 being a 

 matter of subsequent diffusion. The solid zinc has, as it were, 

 a certain vapour-tension or osmotic pressure P, the zinc of the 

 zinc sulphate has a certain less solution-pressure jo, less because 

 it is there less concentrated than in the solid. Let there be IS 

 molecules per c.c. of solid and only n active molecules per c.c. 

 of liquid, then the steady potential-difference between the 

 solid and the liquid will be given by 



dV = 1_ 



dp ne 



for in the present case — — - = 1, the anions being supposed 

 Vy -rUi 



stationary at that junction, since they cannot diffuse into the 



metal . 



So putting p = pRT = nm RT 7 



we get ; 



e n 



or 



V-V' = ReTlog - =114 log 10 - volt, 



n ■ ~ n 



as the theoretical difference of potential between a metal and 

 a solution of any one of its salts ; e being written for the 

 electrochemical equivalent m/e. Even if it is a foreign salt 

 or an acid the potential-difference should be the same, pro- 

 vided there is equilibrium, i. e. no " local action." 



Suppose now that one metal, with effective concentration 

 N t (active ionsiper c.c), dips into a solution of concentration % ; 



