Manchester Memoirs, Vol. Ix. (1916), No. 11. 21 



in appearance, the surface being still bright and polished, 

 but in the second case the surface was dull and the 

 craters already referred to were clearly visible. 



It would be difficult to obtain more conclusive proof 

 than this of the fact that the ions of gas penetrate the 

 surface of the electrode and set up great pressures within 

 the electrode. If the current density is great enough, 

 the internal pressures are sufficient to burst open the 

 surface, thus releasing the compressed gas and lowering 

 the overvoltage. If the current density, or rather the 

 applied voltage, is not great enough to produce sufficient 

 pressure to burst the surface, the gas accumulates in the 

 metal and can only escape by re-dissolving in the ionic 

 form. This it tends to do the more vigorously the greater 

 the pressure, and hence the strong overvoltage. A rest 

 for five minutes with no more ions pressing for admittance 

 is sufficient to allow the greater part of this occluded gas 

 to diffuse back into the solution, while a renewal of the 

 applied E.M.F. rapidly sets up the high pressures again. 



Considerations of this kind would lead to the con- 

 clusion that an increase of external pressure might 

 alter over-voltage if only the pressure can be made great 

 enough. 



It is easy to calculate the effect of such pressures. 



Let E = measured single potential of electrode libera- 

 ting H, 2 . 

 E = normal potential of a H% electrode in same 

 liquid. 



Then by Nernst's well known formula 



where K is a constant, / the osmotic pressure of the 

 hydrogen ions in solution, and p H% the pressure of the 



