THE 
NEW PHYTOLOGIST 
Vol. XX, No. 5 December 31, 1921 
PERMEABILITY 
By WALTER STILES 
CHAPTER V 
THE PERMEABILITY OF MEMBRANES 
T hat colloids are generally incapable of penetrating into a 
colloidal system was mentioned at the end of the last chapter, 
and the incapacity of colloids to diffuse through colloidal membranes 1 
is the phenomenon which led Graham to distinguish between colloids, 
substances which are incapable of passing through such membranes 
or which do so very slowly, and crystalloids, which pass through 
these membranes readily. By utilising this principle Graham was 
able to separate colloids from crystalloids, and so introduced the 
method of separation and purification of colloids known as dialysis. 
The permeability of a membrane depends on the composition of 
the membrane, but the latter is not equally permeable to all sub¬ 
stances. For example, caouchouc allows pyridine to pass through it 
while water is kept back. When a membrane is in contact with a 
solution the permeability may, and generally does, differ as regards 
the solvent and the solute. Thus parchment paper allows water to 
pass readily, but the solute in the case of an aqueous solution of 
sucrose diffuses through the membrane very slowly. In speaking of 
the permeability of a membrane it is thus necessary to define the 
system with which the membrane is in contact. 
A membrane which allows a substance to pass through it readily 
is said to be permeable to the substance, while one which does not 
1 The term “membrane” may be limited to thin solid structures, while 
the term “film” is used for thin layers of liquid. In biology it is often difficult 
or impossible to distinguish between solid membranes and liquid films, and the 
term “ membrane ” will therefore be used to include films. 
Phyt. xx. v. 
5 
