108 PROCEEDINGS OF THE AMERICAN ACADEMY 



The variation in passing from a chloride to an iodide solution is 

 about 0.3 volt, far more than can be accounted for by any experi- 

 mental error. This necessitates a reconsideration of the Nernst 

 hypothesis to see where the flaw in the reasoning occurs. The 

 assumption made is, that, if a metal he dipped iuto a solution of one 

 of its salts, ions of that metal will go into solution, and the electrode 

 become charged negatively towards the electrolyte if the " solution 

 pressure " of the metal is greater than the osmotic pressure of the cor- 

 responding ion in the solution. If the osmotic pressure of the ion in 

 the solution is greater than the " solution pressure," ions will be pre- 

 cipitated upon the metal, which will become positive to the solution. 

 This same reasoning is applicable to zinc in a solution of potassium 

 chloride, for instance. The initial concentration of the zinc ions in the 

 solution is zero, and the metal will therefore send off ions until the 

 potential difference corresponding to equilibrium is reached. This 

 will not be the case when we consider mercury in a solution of potas- 

 sium chloride. There are no mercury ions in solution to precipitate on 

 the metal, and it remains an unanswered problem how the mercury is 

 to become charged positively in respect to the solution. Yet this 

 takes place, and the value as determined by the drojjping mercury 

 electrode method is a perfectly well defined one. This value should 

 be independent of the nature of the salt solution if Ostwald's assump- 

 tion about log P is correct. This is not the case. In this connection 

 I wish to say that the question as to the value of the dropping mercury 

 electrode as a means of measuring single potential differences does not 

 affect this discussion at all. It is an experimental fact that the sum of 

 the potential differences MJRX and RXIMj, as determined by this 

 method, is equal to the electromotive force of the cell Mi\RX\M.2, and 

 it is immaterial for the present purposes whether the single determina- 

 tions are wrong by a constant amount, as I am only considering varia- 

 tions in the values. I will now try to show what conclusions may be 

 drawn from the measurements of Paschen * on the potential differences 

 between metals and salt solutions not containing the metal of the elec- 

 trode as ion. He points out himself that the potential difference is 

 not a function of the positive ion of the salt solution. It is not a func- 

 tion of the concentration. Paschen inclines to the opposite view; but 

 I think he is wrong, and that his own results as tabulated in Table X. 

 will bear me out. The first column gives the nature and concentration 

 of the solution ; the second, third, and fourth columns give the poten- 



* Wied. Ann., XLIII. 590, 1891. 



