On the Preparation and Use of Collodion Osmometers. 
BY 
WILLIAM BROWN, M.A., D.Sc. 
{From the Department of Plant Physiology and Pathology , Imperial College of Science and 
Technology, London .) 
I N an earlier paper 1 the writer described methods of preparing and 
grading collodion membranes for purposes of dialysis. It was there 
shown that by soaking the air-dried membranes in alcohol-water mixtures 
of different strengths and subsequently washing in water a series of mem- 
branes with a wide range of permeability could be obtained. At the one 
end of the series (using a grading mixture with a low concentration of 
alcohol) were membranes which were only slowly permeable to water and in 
a less degree to the simpler electrolytes, such as sodium chloride, potassium 
nitrate, &c. ; at the other end (using a grading mixture with a high alcohol 
content) were those which allowed a slow diffusion of starch, aniline blue, 
and other substances which are highly colloidal in aqueous solution. Be- 
tween these extremes any desired degree of permeability could be 
obtained. As was pointed out in the paper already referred to collodion 
membranes of certain grades are directly applicable to osmotic work, as it 
is possible with certainty to prepare them of such a permeability that they 
will allow a ready passage of water and at the same time hold back such 
solutes as cane sugar, copper sulphate, &c. They may thus be used to 
replace the well-known precipitation membranes (such as the copper ferro- 
cyanide membrane of Traube and Pfeffer), which repeated attempts in this 
laboratory have shown to be difficult to prepare satisfactorily. The type of 
osmometer described in this paper has been used now for some years in the 
plant physiology practical classes in this laboratory, and has been found to 
be accurate and simple in manipulation. The following practical notes will 
serve to show how these osmometers are prepared and used. For con- 
venience in description we shall deal in succession with : 
i. Preparation of air-dried membranes or 'thimbles’. 
2. Grading and method of attachment. 
3. Method of using. 
1. The type of membrane most convenient for the present purpose is 
that known as the thimble, which may be formed either inside or outside 
1 Biochem. Journ. , 1915, ix, p. 591. 
f Annals of Botany, Vol. XXXVI. No. CXLIII. July, 1922.] 
