of Gibbs' s Theory of Surface- Concentration. 513 
Having in the foregoing obtained data for the experimental 
determination of the coefficient -7- in the case of two dif- 
dc 
ferent oils, the next procedure was to determine the value 
of r, i. e. the mass adsorbed per cm. 2 of oil-surface in excess 
of that which would have been there had no adsorption 
taken place at all. To express this quantity V more briefly, 
the term "Adsorption Coefficient,''' following the nomenclature 
of Ostwald, has been employed. 
Measurement of tlie Adsorption Coefficient. 
Although the Adsorption Coefficient is defined as the 
excess mass of solute adsorbed, yet since the normal amount 
of solute per square cm. surface (when no adsorption has 
taken place) is very small in the present instance, this 
normal amount may be neglected in comparison to the total 
amount adsorbed per square cm. This is justifiable, as is 
evident from the following figures from an experiment 
detailed later : — 
The bulk concentration of the glycocholate solution 
= *25 per cent, or *0025 grams per c.c. 
Assuming the thickness of the surface layer to be of 
the order of the range of molecular attraction, namely, 
13*4 x 10~ 6 cms. (according to Parks *), while the quantity 
-adsorbed per cm. 2 in this particular case is 5*4 x 10~ 6 gram, 
the layer evidently possesses a concentration of *403 gram 
per c.c. The surface concentration is therefore about 160 
times the bulk concentration. 
It has been assumed that the thickness of the layer is of 
the dimensions of the range of molecular action. Of course 
the actual case is that there is a gradual " shading off " of 
the excess from the surface into the bulk of the solution, 
but practically all the excess is in the molecular surface- 
layer. This follows from the fact that the magnitude of the 
adsorption depends on the surface-tension, which is a pheno- 
menon whose effects do not extend beyond the range of 
molecular attraction. As Gibbs f says : — " It is only within 
very small distances of such a surface that any mass is 
sensibly affected by its vicinity — a natural consequence of 
the exceedingly small sphere of sensible molecular action/'' 
The experimental determination of the adsorption co- 
efficient of sod. glycocholate at the surface of a hydrocarbon 
* Parks, Phil. Mag. [6] vol. v. p. 517 (1903). 
t Gibbs, ' Scientific Papers,' vol. i. p. 219. 
