﻿Electrochemistry of Radioactive Bodies. 641 



of the mixture deposited on the nickel varied from 90 to 

 50 per cent, of the equilibrium amount. These numbers 

 correspond to potentials o£ from €( Hg ) = to 6(H g )= +0*6 volt. 



7. Explanation of ilie Inflexion Curves. 



The curves of figs, 4 and 5 have inflexion points at 

 approximately 50 per cent, of the equilibrium amount 

 of C. This point corresponds to a potential difference of 

 — 0*63 volt measured against the calomel electrode. 



If an electropositive metal, such as zinc, be dipped into a 

 normal ZnS0 4 solution, the latter becomes charged positively 

 and the zinc negatively owing to the passage of zinc ions 

 into the solution. If an electronegative metal, however, 

 such as silver be dipped in a normal AgN0 3 solution, the 

 bolution becomes negative, and the metal positive, because 

 in this case positive ions are deposited on the metal. Again, 

 if a metal be dipped in a solution of one of its salts and 

 becomes charged neither positively or negatively, there is no 

 potential difference between metal and electrolyte. 



Experiments have been carried out with care to determine 

 this absolute zero of electrolytic potential. They show that 

 the value (e(R- g )= +0*56 V absolute), deduced from measure^ 

 ments with the dropping electrode, is only approximately 

 correct. It is, however, quite definitely known that metals 

 like zinc and cadmium are charged negatively with respect 

 to normal solutions of their salts, and metals like copper and 

 silver positively. 



The question as to how far this point of inflexion is bound 

 up with the absolute zero of potential is left an open one ; 

 the conclusion is drawn, however, from the shape of the 

 inflexion curve that B tends to deposit on a negatively 

 charged electrode, C on a positively charged one. Two 

 other explanations, however, are possible. One is to assume 

 that the curves of figs. 4 and 5 relate to decomposition 

 voltages. As the potential increases, more and more of B 

 separates out with the C, and after the decomposition voltnge 

 of the former is reached, both are deposited by electrolysis 

 in the ratio of their concentrations. On this view, however, 

 difficulties arise when we attempt to explain the part of the 

 curve which lies above the point of inflexion, for it seems 

 entirely anomalous for high potentials to bring about the 

 electrolysis of electropositive metals to a greater extent than 

 electronegative. If the valency of thorium were much 

 greater than that of thorium B, this difficulty would be 

 removed, and the above explanation would become possible. 



Phil. Mag. S. (>. Vol. 23. No. 136. April 1912, 2 I 



