Permeability 145 
representation of the facts over a wide temperature interval; it may 
be sufficiently accurate over the temperature range with which we 
are generally concerned in living organisms. 
We have so far considered the laws of diffusion in a medium 
which at equilibrium is homogeneous. In the organism, as we have 
seen, we have to deal very largely with heterogeneous systems. As 
the simplest case of a heterogeneous system we may consider two 
immiscible liquids separated by a phase boundary. That such 
systems actually occur in the living cell there can be little doubt. 
If a solute is soluble in both the liquids, its distribution when 
diffusion has proceeded to equilibrium will not be uniform throughout 
the system; on the contrary, the solute generally distributes itself 
unequally between the two solvents. This phenomenon was investi¬ 
gated by Berthelot and Jungfleisch (1869-1872) and later by Nernst 
(1891). It is found that if a substance has the same molecular 
complexity in the two solvents the ratio of the concentration of the 
solute in one solvent to its concentration in the other is a constant 
whatever the concentration. Thus if c x is the concentration in one 
solvent and c 2 the concentration in the other solvent, 
where K is a constant and called the partition coefficient or the 
distribution ratio . If several solutes are present together each one 
distributes itself between the solvents according to its own partition 
coefficient independently of the others; that is, K is independent of 
the presence of other solutes. 
If the solute should undergo polymerisation in one of the solvents, 
the partition law becomes modified to 
where n represents the number of molecules associated together in 
one of the solvents. 
With the law of partition coefficients may be compared the law 
governing the distribution of a substance between a solvent and an 
X 
adsorbent at equilibrium. The quantity—in the adsorption equation 
is of the nature of a concentration, and if for this we write c x , and 
if for the concentration of the solute at equilibrium we write c 2 , the 
adsorption equation becomes 
4—5 
