﻿the Adsorption of Ions. 331 



solid .acts as a sort of combined hydrogen and oxygen 

 electrode. They treat the subject from the points of view of 

 thermodynamics and Nernst/s theory of electrolytic solution 

 tension. It has been pointed out by Freundlich (and 

 Elissafoff, Z. Phys. Chan. Ixxix. p. '407 (1912)) that 

 hydrogen and hydroxyl ions are not the only ions which 

 impart a charge to the surface. In many cases, acids haye 

 been observed not to reyerse the charge at all. Many sub- 

 stances haye a negative charge in contact with pure water. 

 These facts show that selectiye adsorption of hydroxyl ions 

 has also to be considered. 



This thermodynamic treatment from the point of yiew of 

 Nernst's theory does not attempt to explain electro-endosmosis. 

 For this purpose it is necessary to conceive of an electrical 

 double layer, of which the layer imparting a charge to the 

 surface is fixed relative to the mobile second layer. 



Freundlich, and Freundlich and Rona (Koll. Zeit. xxviii. 

 5, p. 240 (1921); Kgl. Preuss. Akad. Wiss. Berlin, 1920, 

 p. 397, C. 1920, iii. p. 26) have shown that the potential 

 measurements by Haber's method are not in agreement with 

 those measured by electro-endosmotic experiments. They 

 therefore suggest that there are two distinct drops in 

 potential as one passes from the solid to the liquid (glass to 

 water). The first drop is wholly in the solid and is probably 

 of the nature associaied with the Nernst theory of electrolytic 

 solution-tensions. 



The second drop is in the liquid and composes the Helm- 

 holtzian double layer which it is necessary to assume to 

 explain electro-osmosis and cataphoresis. 



At the same time the characteristic effects of hydrogen 

 and hydroxyl ions on neutral substances like barium sulphate, 

 silver chloride, naphthalene, etc., point strongly to the 

 correctness of Haber's fundamental assumption that the 

 explanation is to be sought in the equilibrium between the 

 hydrogen and hydroxyl ions in the adsorbed layer of water 

 and those in the bulk of the liquid. 



Williams (Proc. Roy. Soc. xcviii. A. <p. 223 (1920)) has 

 recently suggested that the layer of water adsorbed on a 

 charcoal surface is under great internal pressure (about 

 10,000 atmospheres). Applying Planck's equation he shows 

 t}iat the effect of this pressure will be to increase the con- 

 centration of hydrogen and hydroxyl ions in this layer. 

 This increased concentration will setup a diffusion potential. 

 He draws attention to the difficulties in accepting this view 

 of the origin of the potential difference at the surface. In 

 the instances considered by Haber and Perrin, the solid has 



