Manchester Memoirs, Vol. Ix. (191 6), No. 11. 25 



requiring time. Also the gases liberated penetrate to a 

 definite distance into the electrode, and the product of 

 this distance and the area of the electrode surface gives 

 the " electrode volume." If this volume is great, the 

 metastable particles take longer to come into contact with 

 each other : hence the gas must accumulate to a greater 

 extent and the overvoltage will rise. 



This view is to some extent in opposition to that of 

 Nernst, who considers that overvoltage is low when solu- 

 bility of the gas is high. 



The solubility of hydrogen is high both in platinum 

 and palladium, specially in the latter where the electrode 

 volume of a thin sheet is practically the volume of the 

 sheet itself, while in metallic zinc it is very low. The 

 overvoltage of platinum and palladium are normally very 

 low but can rise to quite high values, while that of pure 

 zinc is the highest of any single metal. These facts 

 cannot be reconciled with Nernst's or Reichenstein's 

 theories. Pring, by depositing films of metal of vary- 

 ing thickness on cathodes of different metals and 

 measuring the overvoltage of the compound strip, was 

 able to determine the approximate depth to which the 

 hydrogen ions penetrate. His results were as follows: 



n Ft 



4 x icr 6 



mm. 



In Sn 



5'5 X io -3 mm. 



, An 



r6 x icr 4 



)> 



„ Ni 



1 -6 x icr' 2 „ 



, Zn 



4'2 X IO" 4 



>' 



,, Pb 



&6 x io -2 „ 



, Cu 



3-4 X IO -3 



jj 







The order in which these metals lie with regard to 

 overvoltage is Ft Au Ni Cu Sn Pb Zn. Hence the order 

 in which they lie with regard to electrode volume does not 

 agree with the overvoltage order and the discrepancy is 

 still greater if the actual values of the overvoltages be 

 taken into account. 



