Permeability 99 
The accumulation of a dissolved substance at the surface forming 
the junction between two phases is called adsorption, and when it 
takes place purely as a result of the mechanical surface tension it is 
called mechanical adsorption. 
It will be observed that accumulation of solute in the surface 
layer can only take place if the dissolved substance lowers the surface 
tension of the solvent. If on the other hand the dissolved substance 
should bring about an increase of the surface tension, the concentra¬ 
tion of the surface layer will be less than that of the rest of the 
solution, and negative adsorption will result. 
Attempts to verify the formula of Gibbs have been made by 
Lewis (1909) and by Donnan and Barker (1911) and an approximate 
agreement with the formula has been obtained in some cases. In 
the following table are shown the values found experimentally for F 
do 
compared with the values obtained by determination of an d 
subsequent calculation from Gibbs’s equation. 
Table III 
Values of F found and calculated. (Lewis) 
(In gm. per sq. cm.) 
Substance 
found 
calculated 
Sodium glycocholate 
5 X IO -6 
7 X IO -8 
Congo red ... 
3-7 x io -6 
I-I X IO -7 
Methyl orange 
5-5 x io~ G 
1-2 X IO -7 
Sodium oleate 
IO -6 
IO -8 
Sodium hydroxide 
1*5 x io -7 
7-5 x io~ 9 ' 
Caffeine 
3-7 X IO -8 
2-4 X IO -8 
Sodium nitrate (kation) ... 
2-5 X IO -8 
4-5 x io~ 9 
Potassium chloride (kation) 
5 X IO~ 8 
1*7 x io -9 
,, ,, (anion) 
IO -9 
i*6 x io -9 
Barium chloride (anion) ... 
... not > io -8 
i*6 x io -9 
Copper chloride (anion) ... 
3-5 x io -8 
2 X IO -9 
,, ,, (kation) ... 
2 X IO -8 
2 x IO -9 
Lewis concludes that caffeine probably obeys 
Gibbs’s Law quanti- 
tatively. In other cases the values found experimentally are always 
greater than those obtained by calculation with the exception of 
the anion of potassium chloride. Lewis thinks that some irreversible 
phenomenon such as gelatinisation at the surface may be the explana¬ 
tion of the discrepancy. As will be shown later in this chapter 
electrical phenomena are often present at a surface which add to the 
complexity of the matter. 
Any substance dissolved in water lowers its surface tension against 
a solid or immiscible liquid, and this is usually the case also when 
