LEWIS AND JACKSON. — POLARIZATION ON MERCURY CATHODE. 411 



ceivable state of affairs. If further evidence were needed, the fact that 

 no change in the current is observed, when the hydrogen bubble appears 

 at the electrode, would show that the gaseous hydrogen plays no essen- 

 tial part in the phenomenon. 



We can also show the unimportance of the diffusion of hydrogen-ion. 

 Assume that the concentration of hydrogen-ion at a given distance from 

 the electrode is kept constant by stirring or by convection, then, if the 

 current depend on the diffusion of the hydrogen-ion, it would be propor- 

 tional to the difference between that constant concentration and the 

 concentration at the electrode. With the decrease of the latter this 

 difference would approach constancy, and therefore our logarithmic 

 formula would no longer hold. The current, instead of increasing more 

 and more rapidly with increase of potential, would tend to assume a 

 constant value. Brunner,* although recognizing this principle, holds to 

 the view that the depletion of hydrogen-ion in the neighborhood of the 

 electrode is responsible for the polarization, and he cites experiments to 

 justify this view; but besides the evidence against this idea that we have 

 presented, other entirely decisive arguments could be adduced ; thus, for 

 example, when hydrogen is deposited on certain metals, like platinum 

 and iron, the polarization is not destroyed by thoroughly washing the 

 electrode, and only disappears after the lapse of hours or days. It is 

 obvious that the diffusion of hydrogen-ion can have nothing to do with 

 this phenomenon. 



One might ask with some reason whether it is possible that the process 

 consists wholly or in part in a process of diffusion, since we have seen 

 that a stationary condition is established almost instantly, when the ex- 

 ternal electromotive force is applied, while diffusion processes ordinarily 

 reach a stationary condition only after considerable time. The answer 

 must be that, if any part of the polarization is due to a process of diffu- 

 sion, this diffusion must occur through a film cf extremely small thickness. 

 We may add that, since the diffusion must be slow in order to account 

 for the polarization, either the film must present a very high resistance 

 to the diffusion, or the diffusing substance must be at an extremely low 

 concentration. 



We have, then, reason to believe that the process responsible for 

 polarization is not a homogeneous reaction, nor a process of diffusion 

 through any considerable distance. 



The theory of over-voltage, with which the authors began these ex- 



* Zeit. pliys. Chetn., 47, 56 (1904). 



