36 DIFFUSION AND OSMOTIC PRESSURE 



a perfect semi- permeable membrane which will withstand the 

 high pressures developed. A number of determinations for 

 the same solution are necessary in order to eliminate erratic 

 cases. 



The direct method of measurement has just been brought 

 again into prominence by the work of Morse and Horn. 1 

 These authors have succeeded in forming much more perfect 

 membranes in porous clay cups than have ever been pro- 

 duced before. Air is first swept out of the pores of the cup 

 by an "endosmotic" current. The cup is filled with a weak 

 solution of K 2 SO 4 and immersed in a vessel of the same 

 solution until the outer level is near the margin of the cup. 

 Then a current from a dynamo is passed between a cylindri- 

 cal copper electrode surrounding the cup and a platinum 

 electrode within it. As the liquid rises in the cup, it is 

 removed, and in a short time the air is all removed from the 

 porous clay. Then the cup is filled with K 4 Fe(CN) 6 and 

 immersed as before, but now in a solution of CuSO 4 . The 

 current is passed again, and thus the Fe(CN) 6 ions are 

 driven into the clay from one side, while the Cu ions are 

 forced in from the other. The resistance of the cup gradu- 

 ally rises as the membrane is formed, being from fifteen 

 hundred to three thousand ohms at the time when the mem- 

 brane is considered as complete. 2 



b) Indirect methods. Owing to the difficulties encoun- 

 tered in the use of the direct method just described, an 

 indirect method is usually resorted to. These indirect 

 methods depend upon the general principles, that depression 



1 H. N. MORSE AND D. W. HORN, "The Preparation of Osmotic Membranes by 

 Electrolysis," Am. Chem. Jour., Vol. XXVI (1901), pp. 80-86. 



2 The most recent investigations into the nature of precipitation membranes are 

 as follows : G. TAMMANN, " Ueber die Permeabilitat der Niederschlags-Membranen," 

 Zeitschr. f. physik. Chem., Vol. X (1892), pp. 255-64; P. WALDEN, "Ueber Diffu- 

 sionserscheinungen an Niederschlags-Membranen," ibid. (1892), pp. 699-732; J. H. 

 MEERBURG, u Zur Abhandlung Tammanns : Ueber die Permeabilitat der Nieder- 

 schlags-Membranen," ibid., Vol. XI (1893), pp. 446-8. 



