Manchester Memoirs, Vol. Ix. (igi6),No. 11. 27 



1. Ions of gas-substance penetrate into the electrode 

 surface, set up very great pressures which under certain 

 conditions are sufficient to break open the surface, and 

 also set up an excess back E. M. F which falls- when the 

 surface breaks. 



2. Most of the ions in solution are either hydrated or 

 at least are largely influenced by the neutral molecules 

 round them. 



3. In some cases certainly, in all cases probably, an 

 alloy or solid solution is formed from the gas and the 

 electrode material, which has a different single potential 

 difference from that of the original electrode material. 



4. Overvoltage and the ionisation of the escaping gas 

 show an inverse relationship. 



We may now picture to ourselves what goes on during 

 electrolysis in the following way. 



(i.) The ions have to pass through the series of changes 



abed 

 Ion hydrate ->free ion -> free atom -> molecule 



and molecules, the same series in the reverse order. 



Of these changes, b — c takes place at the moment of 

 electrical contact of the ion with the electrode. The other 

 two require a measurable time, and hence there may be 

 produced an excess or deficiency of b and c in the elec- 

 trolyte and at the electrodes respectively, which gives rise 

 to overvoltage. This effect will be proportional to the 

 current density and occurs with all ions, gaseous or 

 metallic. It is the sole cause of metal overvoltage, but is 

 never very great, since metal overvoltages are small com- 

 pared with gas overvoltages. 



(ii.) When the current is first applied, the outer surface 

 of the electrode will rapidly become covered with new 



