186 Walter Stiles 
allow the substance to pass through it is said to be impermeable to 
the substance. 
Although colloids generally cannot diffuse through membranes 
of colloidal substances, it must be realised that this distinction 
between colloids and crystalloids is by no means absolute. By 
varying the membrane and the substance, practically any degree 
of permeability may be found. Particularly interesting in this 
Fig- 3- Curves to illustrate the penetration of methylene blue through collodion 
thimbles of graded permeability. (Constructed from the data of W. Brown.) 
respect are the observations of W. Brown (1915) who describes 
a method by which membranes of collodion may be prepared 
of any degree of permeability within certain wide limits. Brown’s 
method consists in preparing completely air-dried membranes 
of collodion which are highly impermeable. The membranes are 
then immersed in solutions of alcohol in water for a suitable 
time. After washing in water the membranes then exhibit a per- 
