520 Mr. W. C. M. Lewis : Experimental Examination 
The following are Gibbs's assumptions in the deduction of 
the equilibrium equation : — 
(1) The adsorbing surface is plane. 
(2) The solvent is to show no concentration at the inter- 
face. 
(3) For the particular equation used in the present paper, 
there is supposed to be only one component capable 
of being adsorbed. 
(4) This component and the solvent in which it is dissolved 
are supposed to form a single phase. 
(1) Planeness of the surface. — It may be readily assumed 
that adsorption measurements made at the surfaces of oil- 
drops of sensible magnitude approximate exceedingly closely 
to those at a plane surface. 
(2) Absence of surface-concentration of the solvent. — This 
was simply assumed to be the case, no means of testing its 
validity having as yet suggested itself. 
(3) Adsorption of a single component. — This follows from 
the general experimental conditions, the solute, the sodium 
giycocholate, being the only substance whose adsorption is 
measured. 
(4) Adsorption from a monophase system. — This assumption 
is implied from the actual example of surface-concentration 
given by Gibbs as exemplifying his theory, viz. : — A mixture 
of mercury- and water-vapours meeting at a liquid mercury 
surface, the water being the component which suffers surface- 
concentration. Mixtures of vapours are essentially monophase 
systems ; and the question is, are we dealing with a mono- 
phase system in the case of an aqueous solution of sod. 
giycocholate ? The evidence given by its osmotic behaviour 
in raising the boiling-points of water and alcohol and lowering 
the freezing-point of water, is strongly in favour of its being 
a true electrolyte, and hence of its solution being a mono- 
phase system. 
Temperature Effects accompanying Adsorption. 
All attempts at a direct determination of heat evolution or adsorption at 
the oil surface were negative. It may be shown, however, indirectly that 
a small evolution of heat must occur, from the observed variation of the 
interfacial tension with temperature. Thus, with a solution of concen- 
tration -3 per cent, and oil B, the following results were obtained : — 
Tension in dynes per cm. 
