122 PROCEEDINGS OP THE AMERICAN ACADEMY. 



up again under the influence of the external electromotive force. It 

 can precipitate only till equiUbrium is reached, and we shall have the 

 condition referred to by Gibbs,* of a substance present in too small 

 quantities to have the properties of " matter in mass." By making 

 the external electromotive force differ infinitely little from the electro- 

 motive force of the cell, it would be possible, theoretically at any rate, 

 to obtain an infinitely thin film of copper. It is to the separation (jf 

 the ion on the electrode in such small quantities as not to have the 

 properties of matter in mass that is due the gradual change of the 

 polarization, instead of having a sudden change from the initial to 

 the final value. 



The main results of this research may be summed up as follows : — 



1. The potential difference between a metal and an electrolyte is 

 not a function of the concentration of the salt solution, nor of the 

 nature of the positive ion, except in certain special cases. 



2. It is a function of the electrode, of the negative ion, and of the 

 solvent. 



3. In acpieous solutions the potential difference is the sum of the 

 terra due to tlie electrode and the term due to the negative ion in the 

 normal cases. 



4. For most metals in most electrolytes the term due to the nega- 

 tive ion has the same numerical value and the same sign. 



5. For mercury it has the same numerical value, but the opposite 

 sign ; for platinum, neither the same numerical value nor the same 

 sign. 



6. For platinum in a haloid solution the change of the electromotive 

 force with the concentration is given by the formula 



Bi-E.i = l RT nat. log ~ 



if the salt dissociates into three ions ; by the formula 

 if it dissociates into two. 



E^- E^^lRT nat. log -^ 



* Thermodynamisclie Studien, p. 393. 



