1892.] On the Electrolysis of Silver Nitrate in Vacuo. 357 



In looking at the figures it mnst be remembered that the large 

 bowl would, if placed in air, have given a larger deposit than the 

 small one, so that the difference between oxygen and air is really 

 greater than would appear from the numbers. There seems little 

 doubt, therefore, that it is the removal of oxygen which is the cause 

 of the increased deposits in vacuo. 



We have made a number of experiments on the polarisation of the 

 electrodes in our silver voltameters. It does not follow that because 

 there is as much silver dissolved as deposited, there is necessarily 

 complete absence of what is commonly called polarisation. In the 

 first place, the silver is dissolved from a compact sheet which is in 

 a molecular condition different to the crystalline form in which it is 

 deposited. Secondly, the silver is dissolved into a more concentrated 

 solution than that out of which it is deposited, and, as Warburg has 

 pointed out, it is very difficult to distinguish polarisation effects from 

 electromotive forces due to differences of concentration. Our experi- 

 ments have shown a small but very consistent polarisation of 

 0*007 volt, which was the same in vacuo and in air. 



If, after the polarising current has passed, the anode is taken out, 

 and replaced after the liquid has been thoroughly stirred, the polari- 

 sation is reduced, but still exists to the extent of about one-third the 

 original value. The electromotive force of polarisation does not seem 

 to be different when the platinum basin is partially or completely 

 covered with silver ; but the greater the amount of silver the more 

 slowly does the polarisation die out. We cannot draw any veiy 

 definite conclusions from these observations, but it seemed worth 

 while to put them on record. 



We do not wish to enter into a full discussion of the explanation of 

 our results, but only draw attention to two phenomena investigated 

 by Helmholtz and Warburg respectively. It was' shown by Helm- 

 holtz that the small current which passes through water under the 

 action of electromotive forces insufficient to decompose it is due to 

 the presence of dissolved oxygen. If part of the current in a solution 

 of silver nitrate is conveyed by hydrogen atoms, no hydrogen could 

 separate out as gas, but a recombination with the dissolved oxygen 

 could take place. A small fraction of the current might be conveyed 

 precisely in the way described by Helmholtz. In a subsequent paper,* 

 Helmholtz draws from thermo dynamic principles the conclusion that 

 " in very dilute solutions or in acids containing no salts at all, metals, 

 which we otherwise consider unoxidisable in the acid, may dissolve 

 to a small extent with evolution of hydrogen." 



Warburg,t in an important paper, shows that voltaic cells may be 

 formed by two pieces of the same metal, dipped into the same solu- 

 * ' Collected Works,' vol. 2, p. 978. 

 f ' Wiedemann, Annalen,' vol. 38, p. 321. 



