320 Mr. J. Frenkel on the Surface Electric 



and, consequently, the temperature (for a given substance), 

 since <r = W is proportional to ?t 2 , that is, to the square of 

 the density 8. In this, however, it does not differ from the 

 -older theories. 



§ 8. The fact that the energy of the surface electric 

 double-layer is sufficient to account for the surface-tension, 

 indicates that the latter does not directly depend upon 

 cohesive forces. The existence of these forces becomes, 

 however, manifest, when instead of the energy and the 

 •corresponding tension parallel to the surface we consider 

 the stress perpendicular to the surface and measured by the 

 attraction between the two halves of the double-layer. The 

 existence of an ordinary condenser implies that the attraction 

 between the oppositely charged surfaces which form it is 

 balanced by some external forces, and the same, mutatis 

 mutandis, may be said about the molecular double-layer. In 

 the outer half the attraction of the charges is balanced by 

 the centrifugal force due to their rotation. The centres of 

 the atoms or molecules contained in the other half are 

 attracted outwards, and the fact that, nevertheless; they 

 remain in equilibrium (statistical) implies the existence of 

 cohesive forces pulling them in the opposite direction. In 

 the absence of heat-motion, i. e. at absolute zero, the resultant 

 of these cohesive forces P, reckoned per unit surface of the 

 double-layer, must be equal to the corresponding electric 

 stress F. To calculate F we have to integrate over one-half 

 of the double-layer the product of the electric density p x 

 and the electric intensity E x , which in the case of a simple 

 body of valency k gives on account of (1) and (4), § 3, 



F=r^E a: ^=|(^2) 2 .^. . . . (1 



<) 



Comparing this with the expree~"o .: ["J) for the energy 

 W, we get 



F 5 W dynes 



2 /• cm.' 



Thus, the whole volume of the body is in a state of stress, 



measured by F. Taking- for W = <j 100 - v - — -' , for rlO -8 cm., 

 J f cm. ' 



we get F = 2-5xl0 10 ( ^ n J, or about 25,000 kilograms per 



sq. cm. This is of the same order of magnitude as the 

 internal pressure of liquids at ordinary temperatures. If, 

 neglecting the influence of the heat-motion at these temper- 



