678 RICE ART. L 



and the separation of minerals from a mixture by the "flotation" 

 process. 



There is of course an "adsorption equation" for a soHd- 

 fluid interface; it is [675] of Gibbs, or its equivalent, [678]. 

 Reference to adsorption at a solid surface has already been 

 made earlier in this commentary, where an account is given of 

 Langmuir's deduction of his adsorption equation from statistical 

 considerations. Here the experimental results are once more 

 so difficult to interpret that the situation is far from satisfactory 

 as regards proving or disproving any theory. The reader is 

 once more referred to Adam, Chapter VIII, for an adequate 

 account with references. 



XIX. Discontinuity of Electric Potential at a Surface. 

 Electrocapillarity 



[Gibbs, I, pp. 331-337] 



62. Volta's Contact Potential between Two Metals and Its Con- 

 nection with Thermoelectric and Photoelectric Phenomena 



The brevity and caution with which Gibbs refers to these 

 matters is natural when one remembers the date of publication 

 of this memoir. In this connection a letter written to W. D. 

 Bancroft, printed at the end of the volume (Gibbs, I, pp. 425- 

 434) , will prove of interest, especially the paragraph at the top 

 of page 429. The situation has been, of course, radically al- 

 tered since those days, experiment having in the meantime 

 clarified obscurities and removed doubts inherent in any treat- 

 ment undertaken at that time. 



Historically, the question of electrode potentials dates back to 

 Volta's early researches on contact potentials between metals. 

 The discredit into which that hypothesis fell during the nine- 

 teenth century was due, of course, to the extreme insistence by 

 the physical chemists and some physicists on the source of the 

 energy transformations in the cell. This led them to look for the 

 source of the E. M. F. of the cell entirely at the metal-electrolyte 

 interfaces, though it must be remembered that Volta's theory 

 was ably defended by many physicists, among whom must be 

 reckoned Lord Kelvin and Helmholtz. An account of the 



