574 RICE 



ART. L 



the average position of the alcohol molecules)."* As the alcohol 

 in the bulk phase is supposed also to extend up to this dividing 

 surface with the bulk concentration, /or the purpose of calculating 

 r for the alcohol, such a gradual creeping outward of the surface 

 will have the effect of causing only a portion of the outer layer of 

 alcohol molecules to appear as "adsorbed alcohol." This illus- 

 trates very forcibly the difficulties that arise when we begin to 

 "tamper" with the dividing surface for the purpose of getting 

 rid of a term in the true adsorption equation for a binary mix- 

 ture, viz., (at constant temperature) 



da = —Tidni — T2dn2- 



If, however, we keep the dividing surface fixed, say at the depth 

 of the unimolecular layer, we can use the equation referred to 

 earlier, 



"--('— S). 



dfi 



(the equation [515] of Gibbs, slightly modified). This in- 

 cidentally shows us how the right-hand side of the equation 

 diminishes with increasing alcohol concentration; for with an 

 accumulation of water molecules in the layer just inside the 

 fixed dividing surface, Fi is positive and increasing and 72V71' 

 is also increasing in the bulk phase. This is then a way of 

 stating the explanation, alternative to that using the moving 

 surface. It has been suggested by Bradley {Phil. Mag., 7, 

 142, (1929)) that an additional relation, which with the above 

 would enable us to determine both Ti and r2 could be obtained 

 from the alteration in the air-liquid electric potential difference 

 which is dependent on the electric moments of solvent and solute 

 molecules in the surface layer; this would of course change with 

 the change in the composition of the surface. The reader is 

 referred to this paper for further information. 



The difficulty of the situation is clear, and it is possible that 

 similar considerations may be brought to bear on all the 

 apparent failures of the Gibbs law. Unfortunately it is not 



* Quoted from Adam's book, p. 131. 



