1 8 Newbery, RecenI Work on Overvollage. 



owing to the lesser solubility of most oxides in the former, 

 and partly flbecause the presence of the hydroxylion is more 

 favourable to oxide production. 



If the surface oxides are insulators, and also insoluble in 

 the electrolyte, valve action is produced. Thus two electrodes 

 of tungsten and platinum respectively in nitric acid will allow no 

 appreciable cunent to pass if the tungsten is made the anode, 

 although the cell conducts well when the platinum is the anode. 

 No visible change occurs on the tungsten surface, so that the 

 oxide film must fee a very good insulator, and may be only of 

 molecular thickness. 



Stability oj the overvoltage compounds. 



The chemical stability of (t(he hydrides formed on cathodes 

 is of great importance in certain types of electro-chemical 

 reduction. 



A considerable amount of worlc is still to be done in this 

 direction, since very iittle( is known on the subject, although 

 for s<ome reductions it is more important ro use a cathode 

 which gives a stable hydride than one which has a high over- 

 voltage. 



Up to the present, it appears that copper, nickel, and 

 possibly cobalt from hydrides of exceptional stability, and since 

 electrolytic reduction consists in most cases merely of reduction 

 by. these hydrides, it is evid(ent that greaitly dncreased curranit 

 efficiency, etc., may 'be obtained by the use of these electrodes 

 in many cases. 



Thus oleic acid is easily reduced by a copper cathode to a 

 saturated acid under certain conditions, while a lead or zinc 

 cathode has little or no effect. This is evidently due to the fact 

 that 'the slowly reacting oleic acid requires a definite time 

 for reduction to take, place, which is afforded by the slowly 

 decomposing copper hydride, whilst the mor'e active lead or zinc 

 hydrides are decomposed before they can effect any appreciable 

 reduction. 



The reduction of nitrates to ammonia by copper, and to 

 hydroxylamine by amalgamated lead is probably due to similar 

 causes. 



Metal overvoltage and ion hydration. 



We have still to explain why the cathodic metal over- 

 voltages of iron, nickel, and cobalt are so markedly different 

 from those of the other metals given. 



Lapworth (Trans. Chem. Soc, 191 5, loy, 857) has shown 

 that the hydrogen ion is probably strongly hydrated, that is, 

 combined with the solvent in aqueous or alcoholic solution, 

 while other ions, N,a", NH"^, K', Ag", NO3', CI', etc., are either 

 non-hydrated or their affinity for the solvent is of a quite 

 different order from that of hydrion. 



