Permeability 105 
soon be reached where the force of attraction between the excess of 
oppositely charged ions would prevent any further separation. Such 
differences in potential due to inequalities of differently adsorbed 
ions are called adsorption potentials (see Freundlich, 1909). 
The charge at a surface may itself be responsible for adsorption. 
By the principle that free energy will always reduce itself to a mini¬ 
mum, the electrical energy at a surface will tend to diminish when¬ 
ever this is possible. Now if a surface is thereat of a negative charge, 
the deposition of any particle or ion carrying a positive charge will 
reduce the electrical energy of the surface, and such deposition will 
therefore tend to take place. That this is actually the case has been 
shown by Perrin (1904), Bayliss (1906), Lachs and Michaelis (1911) 
and others. For instance Bayliss has shown that a negatively charged 
surface such as that of filter paper will adsorb large quantities of an 
electropositive substance such as night blue, but only a trace of 
substance carrying a negative charge such as congo red. In some 
cases of such electrical adsorption the charge on the surface may 
actually be reversed; it seems likely that in these circumstances 
we are dealing with a complex effect in which both mechanical and 
electrical adsorption are involved. 
Adsorption may obviously have an effect in regard to chemical 
reactions taking place at the surface. Thus if two substances which 
react together are adsorbed on the surface of a third, combination 
will take place on the surface of this third substance, which itself 
may remain unaltered. In a similar way reactions may take place 
if two substances mutually adsorb one another as in the case of 
barium hydroxide added to colloidal silica (van Bemmelen, 1910). 
A white substance is precipitated containing both substances, and 
from the mixture barium silicate slowly forms. 
In such cases the active mass of reacting substances is the number 
of molecules adsorbed to the surface, and this number is proportional 
to the extent of the surface. Hence in such cases the rate of reaction 
is proportional to the surface. This is for example very generally the 
case in enzyme actions (see Bayliss, 1914). 
Finally it must never be forgotten that all the phenomena of 
surfaces occur not only at the obvious surfaces of the cell, but 
throughout the colloidal substance of which the protoplasm is 
composed. Protoplasm is a heterogeneous system of more than one 
phase and throughout the system whenever there is a boundary 
surface between the disperse phase and the dispersion medium the 
surface phenomena described in this chapter must occur. A general 
