1 6 Nevvber\\ Recent Work on Overvoltage. 



It as evident, therefore, that the liigh values obtained in 

 such cases are true hydrogen over voltages, and are due^ to the 

 formation of metallic hydrides on the electrode surface. 



The occlusion of hydrogen in almost all electro-deposited 

 metals is a well-estabhshed fact. It is only reasonable, therefore, 

 to conclude that the very small cathodic overvoltages usually 

 observed during metal deposition, are due to the presence of 

 small quantities of hydrides, wrhich form a dilute solid solution 

 in the deposited metaJ. Similarly the small anodic overvoltages 

 observed are probably due to the formation of traces of higher 

 oxides on the electrodes. The large values obtained when the 

 electrode becomes passive will be referrea to later. 



The mechanis?n of electrolysis. 



We are now in a position to attempt an explanation of the 

 exact mechanism of electrolysis as far as the electrodes are con- 

 cerned at least. 



When a current is passed between two electrodes in an 

 •aqueous electrolyte, the ions carrying electrical charges are 

 attracted towards the electrodes, and move at first with uniform 

 velocity, owing to frictional resistance. When very close to the 

 electrode, however, they must move very rapidly, and probably 

 strike the electrode with! considerable force. The continuous 

 rain of ions On the electrode will thus produce great pressure 

 on the surface. Under the influence of this pressure the chemic- 

 ally active discharged ion tends to combine with the electrode 

 material, and the resultant compound dissolves in the solid — 

 more easily in the amorphous cementing material between the 

 crystals of the metal than in the crystals themselves. 



On releasing the applied pressure by cutting ofif the current, 

 these compounds will tend to separate from the metal in two 

 ways : — 



(i) By ■spontaneous decomposition within the electrode, 

 liberating gas which forces its way through the surface and pro- 

 duces the craters, photographs of which have been shown. 



(2) By direct ionisation, that is, by taking an electrical 

 charge from the electrode and passing into the solution in 

 exactly the same way as any metal does. 



It is the second action which gives rise t'o o\'ervoltage. 



We may therefore state more definitely than before that— 

 (i) Overvoltage is caused by the high single potential differ- 

 ences of hydrides, higher oxides, etc., formed on or in the 

 electrode surfaces. (2) Variations of overvoltage are caused 

 by changes (a) in the chemical constitution of the above co?n- 

 pounds and (b) in the co7ice?itratinn of the solid solutions 

 formed by these compounds in the electrode surfaces. 



