110 PROCEEDINGS OF THE AMERICAN ACADEMY. 



theoretically different in behavior from NaCl or BaCl2j and we 

 must therefore conclude that this discrepancy is also accidental. In 

 passing from dilutions of 101 to those of 1001, there is a distinct in- 

 crease in potential difference between mercury and chloride solutions. 

 With the other solutions the change is either non-existent or much less 

 marked. On the other hand, cadmium shows this behavior only with 

 HI and KI solutions, zinc with HCl, NaCl, HBr, KBr, HI, and KI 

 solutions. The solutions of HBr, KBr, HI, and KI are not the ones 

 where mercury shows a marked change of value with increasing dilu- 

 tion, so that there is no qualitative regularity in the phenomena. As 

 there is also no quantitative connection to be detected between the 

 change of concentration and the change of potential difference, and as 

 the experimental error is very large in the case of determinations with 

 dilute solutions, I see no reason to assume that there is any change of 

 potential difference, at any rate within wide ranges of concentration.* 

 I am led to this conclusion the more strongly because if we admit with 

 Paschen that the potential difference increases with increasing dilution, 

 we must admit that the electromotive force of the cell Cd|KCl|Hg is 

 a function of the concentration, and I have already shown that this is 

 not the case. 



Paschen has pointed out that these potential differences are functions 

 of the metal forming the electrode and of the anion. This can hardly 

 be accounted for on the Ostwald-Nernst hypothesis. If the potential 

 difference between Ilg and KCl or KBr solutions be due to the 

 amount of mercury as ion which has gone into solution, we must say 

 that the amount varies as we change from KCl to KBr, or, in other 

 words, that the negative ion has an effect. This is quite apart from 

 the difficulty of accounting for the sign of the potential difference. I 

 do not see that the relative solubilities of mercurous chloride and bro- 

 mide can be dragged in to help out matters, because we do not have a 

 saturated solution at all, and the difference in the electromotive forces 

 is more likely to be connected with the difference of solubility as 

 cause than as effect. 



There are no experimental data, so far as I know, on potential dif- 

 ferences at the contact surface of reversible electrodes excejjt some 



* This will not hold true till the concentration of the salt becomes zero ; else 

 we should get always the same potential difference, that of the metal in pure 

 water, wliicli is not tlie case. There will certainly be a minimum concentration 

 beyond whicii the dissolved substance will not have the properties of matter in 

 mass, and the potential difference will then be a function of the concentration. 



