of Gibbs's Theory of Surface-Concentration. 505 
it follows that j 
Note. — It should be noted that the expression obtained above occurs. 
in Freundlich's paper already referred to. From the nature of his 
experiments it has been quite impossible to attempt a verification 
experimentally. 
We have now to prepare a solution of certain strength c 
and measure the quantity adsorbed Y as well as the value of 
-j— . On substitution of the values in the equation we may 
obtain a verification or otherwise of the expression *. It 
may be stated at once that the results of experiments to be 
detailed do not show equality on the two sides of the 
equation — the discrepancy being always in the direction 
of r being many times greater than Tjrp-T - * 
III. Experimental Methods. 
Use of a Liquid-liquid Interface. 
As already stated, previous investigations with the ex- 
ception of Milner's have dealt solely with adsorption at a 
solid surface — which does not allow of its surface-extent or 
surface-tension being determined. As it is necessary to 
evaluate these quantities, the possibility presented itself of 
measuring the adsorption of a solute at the interface between 
its solution and another liquid. This latter liquid must of 
course be absolutely chemically inert towards the solute 
itself and its solution. Choice fell on a hydrocarbon oil, 
since a body such as this is characterized by its chemical 
inertness. The next object was to obtain a substance, 
preferably soluble in water, which would possess the property 
of lowering the interfacial tension between water and oil. 
From their behaviour at the air-liquicl interface it seemed 
likely that the soaps, saponine, and bile-salts would act in 
* While this work was in progress, the results of somewhat similar 
determinations were published by Milner (Phil. Mag. Jan. 1907 [6] vol. xiii. 
p. 96). These referred to equilibrium at the air-liquid surface. He has 
deduced an identical expression to that already given in this paper, and 
has applied it to acetic acid in aqueous solution and aqueous sodium 
oleate. He has found that the actual quantity removed from solution 
(r) is nearly ten times the calculated effect ( ™p y- 1. 
Reference might also be made to experiments by Zawidski (Zeit. Phys. 
Chein. xxxv. p. 77 (1900) ) on the formation of the' foam produced by the 
addition of saponine to aqueous solutions of hydrochloric and acetic acids. 
No measurements applicable to the present case were recorded. 
