100 
Walter Stiles 
the solution is in contact with a gas. There are however some sub¬ 
stances which increase the surface tension of water against air, 
among which are most inorganic salts, although these lower it at 
the interface between water and oil (Lewis, 1909). 
An anomalous case of surface concentration is met with in the 
case of sugars. It appears that these substances in solution do not 
lower the surface tension of water. Nevertheless they are adsorbed 
at the surface to a slight extent (Adler and Hertzog, 1908; Hertzog, 
1908; Rona and Michaelis, 1909). The explanation of this phenomenon 
offered by Rona and Michaelis is that the surface layer is in a state 
of compression (see p. 94) and the solubility of the dissolved substance 
is greater in this compressed layer than in the bulk of the solution. 
In many cases the presence of the adsorbed material in the surface 
film of a liquid will increase the viscosity of the liquid at the inter¬ 
face. This may account for the formation of the rigid membranes 
observed by Ramsden. Similar membranes have also been obtained 
by Metcalf (1905) and Zangger (1908). 
As we have seen in Chapter II we are concerned in the living cell 
with a decidedly viscous substance, and we can thus understand 
how the cytoplasmic “membrane” when damaged is immediately 
reformed, as has been often observed, e.g. by Chambers (1917) and 
Seifriz (1918). 
Adsorption is in general a reversible process. In the case of such 
viscous fluids as solutions of proteins, soaps and bile acids, on the 
contrary, the process is irreversible. 
Equations connecting the amount of adsorption at equilibrium 
with the concentration have been obtained empirically. Among the 
better known of these are those due to Kuster (1894) and Schmidt 
(1894); the relation is most usually expressed in the form due to 
Freundlich (1909), which is 
— = kC^ 
m 
where x is the mass of substance adsorbed by a mass m of the ad¬ 
sorbent, C is the concentration of the solution after adsorption has 
reached equilibrium, and k and p are constants. The value of p varies 
between 1-25 and 5, but it is very commonly about 2. This equation 
it will be observed may be written in the form 
log (£E| logC+i? - 
If then the logarithm of the amount of material adsorbed is plotted 
against the logarithm of the concentration at equilibrium, straight 
