550 RICE AKT. L 



concentrated solutions, viz., 



(1 — acY da 



r = — ^ > 



Rt dc 



where c is the ratio of solute molecules to solvent molecules and 

 a is a factor obtained from the equation 



= log — 



1 — ac J) 



P being the saturation pressure of an adsorbed gas or vapor and 

 p its equilibrium pressure. In this the departure from the 

 simple approximate Gibbs' formula is attributed to the forma- 

 tion of loose compounds between the molecules of the solute 

 and those of the solvent, which is termed solvation. This has 

 the effect of altering the internal pressure of the solution and 

 with it other properties such as surface tension and compressi- 

 bility which depend upon the internal pressure. On account 

 of the existence of this solvation Freundlich has criticized the 

 approximate form of Gibbs' law even for dilute solutions, since 

 this property certainly interferes with the application of the 

 simple van't Hoff laws to them. Langmuir, however, has 

 replied to this criticism by pointing out that there are deriva- 

 tions of the law, e.g. Milner's, in which the gas laws are applied 

 only to the interior of the solution. This, of course, does not 

 invalidate in any case the complete form of Gibbs' law, although 

 even this is almost certainly limited to true solutions and cannot 

 be applied to colloidal solutions. This point has been empha- 

 sized by Bancroft (J. Franklin Inst., 185, 218, (1918)); we have 

 already drawn attention to the feature of the proof which im- 

 plies thermodynamic reversibility of the adsorption process, 

 and that is certainly in doubt in some instances where the 

 equation has been applied. Undoubtedly in true solutions 

 some equation of the form 



holds, where /(c, t) is some function which is positive; but this 



