﻿330 Prof. J. N. Mukherjee on 



kinetic energy to overcome the potential of the double layer. 

 The number of collisions is thus not proportional to the 

 concentration but rises more slowly. Near about the point 

 where the surface becomes saturated the value of dxjdc will 

 be almost zero (cp. the shape of the adsorption-isotherms 

 of Freundlich, Arrhenius, and Langmuir). On the other 

 hand, the electrical adsorption increases continually with 

 the concentration and the increase of the charge. It is- 

 apparent that soon a balance will be reached between the 

 chemical adsorption of the anion and the electrical adsorption 

 of the cation. The minimum charge will correspond to the 

 stage when dxjdc for the cation is just equal to dxjdc for the- 

 anion. 



Beyond this concentration the charge will decrease rapidly,, 

 and when the surface has been saturated with the anion the 

 subsequent variation in the charge is simply due to electrical 

 adsorption. The reversal of the charge by electrical adsorp- 

 tion has been discussed in the earlier paper. It is necessary 

 to add that as the electrically adsorbed polyvalent cations 

 impart a positive charge to the surface, the adsorption of the 

 cation decreases and the electrical adsorption of the anion 

 becomes possible. As long as there is a positively charged 

 surface the adsorption of the anion will increase more rapidly 

 with the concentration than that of the cation. A second 

 maximum will thus be reached and a decrease in the charge 

 will follow. The electrical adsorption of the anion is small 

 because of the smallness of the positive charge and an 

 initially existing negatively charged surface. A further 

 reversal of the charge is not possible, and, in fact, has never 

 been observed. 



III. The Actio?i of Acids and Alkalies. 



The works of Perrin and of others (J. Chim. Phys. ii. 

 p. 601 (1904) ; iii. p. 50 (1905) ; Haber and Klemensie- 

 wicz, Z. Phys. Chem. Ixvii. p. 385 (1909) ; Cameron and 

 Oettinger, Phil. Mag. [vi.] xviii. p. 586 (1909)) have shown 

 that hydrogen and hydroxyl ions behave exceptionally in 

 that they impart to the surface a charge of the same sign as 

 they carry. This behaviour is in contrast to that of the 

 other univalent ions. 



Perrin attributes their singular activity to the smallness of 

 their radii. In order to explain the presence of these ions, 

 in excess, in the surface layer, it is necessary to assume some 

 sort of a restraining force acting on them at the surface. 



Haber and Klemensiewicz consider that there is an ad- 

 sorbed layer of water in the surface by virtue of which the- 



