Theory of Electrocapillarity. 381 



II. Ascending branch. 



Applying to the ascending branch a reasoning- similar to 

 the above, L ^.assuming T K to be zero, we obtain the equation 



y=f(+-%v ] oz c ) 



whence -p.^ 



. , 1\1 , Ci 



It appears that in reality, especially with active * electro- 

 lytes, the assumption T K = does not hold for the ascending 

 branch and that there is an excess of both anions and cations 



in the surface-layer, when ^~ is positive. We are induced 



to admit this if we consider : — 



(1) The position of the maximum. — In solutions of electro- 

 lytes with an active anion like Br', I', OW, SON', the 

 maximum is displaced to the right, as compared with 

 solutions of electrolytes with an inactive anion, like 

 N0 3 ', SO/', OH'. Thus the maximum surface-tension in 

 h/10 KNO3 corresponds to ^ = 0-57 volt, in njl KN0 2 to 

 ifr = 0'61, and in njl Kl to yjr = 0'S7. Let us consider the 

 portion of the eleetrocapillary curve of njl KI between 0*57 

 and 0*87 volt, supposing that the first value of -^ really corre- 

 sponds to the zero potential difference between mercury and 

 solution. If 0-57 <i/r< 0'87, e>0, and, in consequence, the 

 potential of the mercury must be higher than the potential 

 of the nearest layer o£ the solution ; as the whole potential 

 difference between solution and mercury is positive, the 

 potential in the surface-layer must vary with the distance 

 from the mercury surface in a way shown by fig. 3. The 

 rise of potential can be caused only by free positive charges, 

 and in consequence we must assume an excess of anions 

 immediately at the mercury surface and at some distance 

 from it an excess of cations, a circumstance which has already 

 been pointed out by Gouy f. 



* We call an electrolyte active if it gives an eleetrocapillary curve 

 -with a depressed maximum. The activity of anorganic electrolytes 

 depends on the anion. 



t C. B. exxxi. p. 939 (1900). 



