﻿322 Prof. J. N. Mukherjee on 



"free" is equivalent to the amount of ions "chemically, 

 adsorbed." These " free" ions form the second sheet of the 

 double layer. It is evident that as a result of their thermal 

 motion the mean distance between the two layers will be 

 greater than "#." 



The charge of the surface was treated as due to discrete 

 charged particles widely separated from each other compared 

 with molecular dimensions. It was shown in the previous 

 paper that this view gives a rational explanation of the fact 

 that a reversal of the charge of a surface can be brought out 

 only by polyvalent ions of opposite charge. 



The equilibrium conditions were discussed and the equa- 

 tions deduced were shown to be in agreement with the 

 valency rule, the influence of the mobility of the oppositely 

 charged ion, and with the influence of concentration on the 

 charge o£ the surface. Only the theoretically simplest case 

 has been discussed in the earlier paper. In the present paper 

 the more important facts connected with the adsorption of 

 ions are discussed from this point of view, and it will be 

 seen that this view gives a simple explanation of most of 

 the general conclusions already arrived at on experimental 

 grounds. 



1 heories of Adsorption. 



Before proceeding to discuss the adsorption of ions it 

 will be convenient to deal briefly with the different views 

 advanced to account for adsorption in general. The with- 

 drawal -of a solute from a solution by a solid may be the 

 result of the formation, of definite chemical compounds, of 

 solid solutions, of mixed crystals and surface-condensation. 

 In many cases all these changes are simultaneously present. 

 In this paper the word " adsorption " denotes condensation 

 or combination, at the surface only, without the interpenetra- 

 tion of the adsorbed substance throughout the mass of the 

 adsorbent (Mecklenburg's criterion, Z. Phys. Chem. Ixxxiii. 

 p. 609 (1913) ; cp. also the sense in which the term is used 

 in deriving Gibbs's equation). 



Faraday (Phil. Trans, cxiv. p. 55 (1834)) in his well-known 

 explanation of the catalytic combination of hydrogen and 

 oxygen on platinum surfaces,, remarks "that they are de- 

 pendent upon the natural condition of gaseous elasticity 

 combined with the exertion of that attractive force, possessed 

 by many bodies, especially those which are solid, in an 

 eminent degree, and probably belonging to all, by which they 

 are drawn into association more or less close, without at the 

 same time undergoing chemical combination though often 



