﻿324 Prof. J. N. Mukherjee on 



In the case of adsorption of gases by solids, Arrhenius 

 (Medd.f. k. Vef. Nobelinstitut, ii. N. 7 (1911); Theories of 

 Solution, 1912, pp. 55-71) has drawn attention to the 

 parallelism between the van der Waals's coefficient "a" for 

 the different gases and the amounts of these gases adsorbed 

 by charcoal, and he believes that this is definite evidence of 

 the compressed state of the surface layer. At the same time 

 he lays stress on the chemical aspect — namely, that in 

 addition to the attractions between the molecules of the gas 

 in the surface layer, one has to consider the chemical 

 attraction of the surface atoms and the molecules of the gas. 



Recently, Williams (Proc. Roy. Soc. xcvi. A. p. 287 (1919) ; 

 xcviii. A. p. 223 (1920); also Trans. Far. Soc. x. p. 155 

 (1914), in which complete references to the literature on 

 negative adsorption are given) has treated adsorption from 

 the points of view of Lagergren and of Arrhenius in a number 

 of interesting communications. 



It may be mentioned here that the disagreement of ob- 

 servations with calculations from Gibbs's equation is at least 

 in part due to the fact that only one source of change in the 

 free energy of the surface layer is taken into account. In 

 the simplest case of the interface, liquid-saturated vapour 

 (one component system), it is open to objection whether " 7 " 

 denotes the total change in free energy of an isothermal and 

 reversible-formation of unit surface. Bakker (Z. Phys. Chem. 

 lxviii. p. 684 (1910)) has pointed out that if the density of 

 the surface layer is different from that of the liquid in bulk 

 a second term is necessary to represent the change in free 

 energy. 



It is possible that in this particular case this second term is 

 negligible in comparison with " 7," the tension per unit length 

 at low temberatures, but at high temperatures " 7 ,7 has a low 

 value and the saturation pressure is very great, so that the 

 second term may be even more important. 



Williams (Proc. Roy. Soc. (Edinburgh), xxxviii. p. 23 

 (1917-18)) has drawn attention to the effect of the variation 

 of the surface of an adsorbent when adsorbing — a factor 

 which is very often neglected. 



Lewis (Z. Phys. Chem. lxxiii. p. 129 (1910) ; also Par- 

 tington, c Text-book of Thermodynamics,' p. 473 (1913)) 

 has discussed the influence of a variation in the electric 

 density on the surface on the form of Gibbs's equation. 



These may be called the physical theories of adsorption. 

 The difficulty in accepting them as general theories of ad- 

 sorption is that they attempt to explain adsorption in terms 

 of a single physical factor j e. g. diminution in surface energy 



