Manchester Memoirs, Vol. Lx. (1916), No. 11. 29 



such high pressure. Also the fraction of the overvoltage 

 produced according to Le Blanc's theory has been neg- 

 lected, but even when these things are taken into account, 

 the pressures we must assume are still incredibly high). 



(iii.) When hydrogen is bubbled round a palladium 

 plate in dilute acid, an alloy or solid solution is produced 

 having a single potential quite different from that of the 

 pure metal. If the same plate is treated cathodically, 

 still more hydrogen is absorbed and the potential may 

 rise 0*5 volt if the current density is kept low, Platinum 

 behaves in much the same way. It seems more than 

 probable therefore that this is true to a greater or less 

 extent with all metals and that part at least of the 

 measured overvoltage is due to the high single potential 

 of an alloy formed on the surface by the discharged 

 ions (free atoms) uniting with the electrode material. 

 This alloy is of course super-saturated with gas which 

 is continually endeavouring to form another alloy of still 

 higher gas content. 



When this alloy is prepared with hydrogen at normal 

 pressure, its potential must be the same as that of a 

 hydrogen electrode in the same liquid, for if it were not, 

 then a current could be obtained capable of doing work 

 by hydrogen dissolving at one electrode and being liberated 

 at the other. Since the same quantity of hydrogen is 

 involved in each case, the process could go on in an 

 isolated system, and perpetual motion would be obtained. 



When the alloy, already saturated with hydrogen at 

 normal pressures, is further treated with gas at higher 

 pressures, other alloys of greater hydrogen content will 

 tend to form, and these will have potentials which cannot 

 be calculated from the gas pressures by Nernst's equation, 

 since the concentration of the hydrogen is not propor- 

 tional to the pressure. 



