692 RICE ART. L 



materials.* Assuming that the distribution is actually on 

 the surfaces in analogy to the distribution in an ideal plane 

 condenser, it appears that Q = csV, where c is a constant, viz., 

 the capacity of the double-layer condenser per unit area. Hence 



and 



da 



Thus 



ais, F, t) = (Tr, - IcV^ 

 or 



a{s,E,t) = am- hc{V, - E)\ 



This leads to two results: (1) that the graph of a and E should 

 be a parabola; (2) that Em = Fq. The first conclusion is 

 certainly only true in a very limited number of cases, while 

 the second, although it has served for some time as the basis for 

 a method of determining absolute electrode potentials, is unques- 

 tionably not exact. It was Helmholtz who suggested the 

 method in question. It consisted in measuring the E.M.F. of 

 a cell with one electrode of mercury and the other of the metal 

 whose P.D. against a given salt was required; the desired P.D. 

 was then calculated on the assumption that the potential at the 

 mercury electrode was that given by the value of Em, obtained 

 as above. Shortly after, he suggested the use of the dropping 

 electrode, a method based on a similar physical picture of the 

 phenomenon. 



67. Recent Developments in the Thermodynamical Treatment of 



Electrocapillarity 



Since those days the developments of the theory have been 

 along two main lines. We can do little more than make verv 



♦ Monatsber. Akad. Wiss., Berlin, 945 (1881). Cf. A. Konig, Ann. 

 Phys. u. Chem., 16, 31 (1882). See also Planck, Ann. Phys., 44, 385 (1891). 



