i 76 Methods for Determining Osmotic Pressures. 
no elaborate apparatus is required,— the capillary tubes can easily 
be made by drawing out ordinary glass tubing. 
There are, however, some disadvantages which must be 
considered. Firstly, the amount of time, which must be allowed to 
elapse between the two readings, is long. This is due to the low 
vapour pressure of water, the solvent used. It was generally found 
necessary to leave the tubes overnight to obtain a definite result. 
However, this length of time can be shortened by raising the 
temperature at which the drops are kept in the manner described 
by Barger and Ewins . 1 Secondly, the tubes require to be filled 
with great care, as the drops of the salt solution and of the cell-sap 
must not be allowed to mix on the sides of the tubes more than is 
unavoidable, a slight mixing being of course inevitable. With a 
little practice the tubes can be filled quite easily. 
There is still another disadvantage which Barger’s method has 
in common with the cryoscopic method and all others that deal with 
sap outside the cell. This is, it does not allow for any chemical 
changes that may take place and which might alter the osmotic 
strength of the sap. For this reason, Fitting 2 comments un¬ 
favourably on the use of the cryoscopic method. While working on 
Salicornia I estimated the osmotic pressure of the sap by plasmo- 
lysis and by Barger’s method, and found that the results obtained 
in both cases varied between similar limits. 
I wish here to express warm thanks to Professor F. W. Oliver, 
in whose laboratories part of this work was done, and to Miss 
E. N. Thomas for her kindness in reading and criticising the 
manuscript of this paper. 
1 “ Application of the Microscopic Method of Molecular Weight Determi¬ 
nation to Solvents of High Boiling Point.” Trans. Chetn. Soc., Vol. 87, 1905, 
p. 1757. 
2 Loc. cit., p. 223. 
