Permeability 187 
meability which increases with increase in the concentration of 
alcohol employed. 
As an example of the degree of variation in permeability which 
may be obtained by this method an experiment made by Brown 
may be cited. Membranes in thimble form of different degrees of 
permeability were prepared as described by treatment with 70, 72*5, 
75, 77*5 and 80 per cent, alcohol respectively. The thimbles were 
filled with a solution of methylene blue, and immersed in vessels 
containing distilled water. The amount of diffusion was estimated at 
daily intervals by colorimetric estimations of methylene blue in the 
external solution. The curves shown in Fig. 3 are constructed from 
the data given by Brown. They show what a wide range in per¬ 
meability is obtained by grading collodion membranes in aqueous 
solutions of alcohol ranging in concentration from 70 to 80 per cent. 
As practicable membranes can be obtained by the use of alcohol 
solutions ranging in concentration from o to 97 per cent, it will 
at once be clear what an extremely wide range of permeability 
can be obtained at will. The following table gives the values found 
by Brown for what he calls the “alcohol index” of a number of 
substances. The alcohol index of a substance is defined as the number 
which represents the strength of alcohol required to produce a 
membrane which just prevents diffusion of the substance. 
Table X 
Alcohol Indices of a Number of Substances. (Data from W. Brown) 
Substance Alcohol Index 
Water o 
Sodium chloride o 
Potassium permanganate 30-40 
Picric acid 35-40 
Copper sulphate 60-70 
Potassium oxalate 60-70 
Sodium sulphate 60-70 
Bismarck Brown 65- 
Methylene Blue 70- 
Neutral Red 72-5-75 
Safranin 75-77-5 
Dextrin 85-87-5 
Starch 90 
Aniline Blue 92 
Litmus (neutral) 93 
Congo Red 96 
Night Blue >96 
In a later paper Brown (1917) extends his method for obtaining 
membranes of graded permeability to other material (gelatine, agar- 
agar) and a number of grading liquids. Other methods of obtaining 
membranes of graded permeability are described by Bechhold (1907), 
Bigelow and Gemberling (1907), Schoep (1911) and Walpole (1915). 
5—2 
