LEWIS AND JACKSON. — POLARIZATION ON MERCURY CATHODE. 413 



salts, the decomposition of a solution of sodium in liquid ammonia.* It 

 is possible that each of these reactions is catalyzed in a specific way; but 

 it is much easier to believe that iu every case the metal simply catalyzes 

 the slow reaction, 2H ±J H 2 , and that this is also the explanation of the 

 electrolytic reaction that we are considering. It is to be noted that metals 

 like mercury and lead, on which the over-voltage is highest, — that is, 

 which are the poorest catalyzers of the electrolytic reaction, — have little 

 influence on any of the reactions mentioned above. 



Although we have laid stress upon the a priori plausibility of this ex- 

 planation, we are nevertheless forced to admit that, while our experiments 

 are in no way opposed to the theory, they fail at the same time to give 

 any evidence in its favor, except in as far as they enable us to exclude 

 certain other explanations and thus limit the field of possibilities. If the 

 main reaction at the basis of polarization is the formation of ordinary 

 hydrogen from monatomic, then our experiments only show that this 

 reaction does not occur in a single homogeneous phase. Since in all 

 probability the monatomic hydrogen is present in extremely small con- 

 centration, this would account for the slowness of any diffusion process in 

 which it enters, even if the diffusion were through a thin film. 



Let us return to our experimental results. We have seen that they 

 may be represented by a number of almost parallel straight lines, whose 

 equations are of the type E=^A log /+ B> Let us now consider the 

 quantity B. 



"While the cell gave consistent and constant results during the time re- 

 quired for a single series of experiments, there was some variation from 

 day to day, so that the results of two different days gave, when plotted, 

 two parallel but not always coincident lines. That is to say, the value 

 of B was not found to be constant, but for some unknown reason varied 

 irregularly with the time.f 



* The interesting fact that this latter reaction is greatly accelerated by platinum 

 was brought to our notice by Dr. C. A. Kraus. 



t It is not impossible that this variation might have been due to changes in the 

 barometric pressure, for, while small changes of pressure have no appreciable 

 effect upon the potential of the reversible hydrogen electrode, they may have a 

 considerable effect upon the velocity of the irreversible process, which is respon- 

 sible for polarization. In fact, this is rendered highly probable by the experi- 

 ments of Tammann and Nernst (Zeit. phys. Chem., 9, 1, (1892)), who showed the 

 great retarding influence of pressure on the reactions between metals and 

 acids. Since in these reactions the only common feature is the formation of 

 gaseous hydrogen from hydrogen-ion, it is probable that this is the reaction that 

 the pressure retards. It is our intention in the near future to study especially the 

 influence of pressure on the polarization of hydrogen. 



