EXPERIMENTAL WORK ON OSMOTIC PRESSURE 621 



In this way a regular deposit of copper ferrocyanide, very 

 close to the outer surface of the porcelain tubes, was obtained. 

 The tubes were allowed to soak in the ferrocyanide solution, and 

 were then set up for electrolysis, the electrolysis being carried 

 out on the lines described previously. This electrolytic treat- 

 ment of the membrane seems to contribute materially to its 

 efficiency, and it is even found advisable to "remake" a tube 

 electrolytically after it has been used in an actual measurement 

 of osmotic pressure. 



Another factor that appears to contribute to the efficiency 

 and permanence of a membrane is the presence, in small quantity, 

 of the membrane-forming salts. In the complete absence of 

 these, the membrane tends to dissolve in a colloidal form, and 

 so deteriorates. 



The degree of efficiency of a copper ferrocyanide membrane 

 may be adjudged by several criteria. An actual test made with 

 a strong sugar solution will show whether the pressure developed 

 in the cell is of the expected order of magnitude, and a subse- 

 quent examination of the solvent outside the cell will show 

 whether the membrane is strictly semi-permeable, or whether 

 there has been a leak of the sugar. Again, a determination of 

 the electrical resistance of a membrane will, in a general way, 

 indicate its probable efficiency. This is, however, not an 

 infallible criterion, for, although as a rule the resistance of a 

 good membrane deposited in a porous cell is of the order of 

 100,000 ohms, efficient membranes have been produced the 

 resistance of which was considerably lower. 



The Measurement of the Pressure 



As regards the actual measurement of the osmotic pressure, 

 recent work has brought some new developments. The usual 

 procedure is to put the solution in the porous pot which carries 

 the semi-permeable membrane, to connect a closed manometer 

 with the pot, to immerse the latter in the pure solvent, and then 

 to observe the pressures indicated in the manometer. This 

 sounds simple enough, but in reality the greatest difficulty is 

 experienced in securing an efficient junction between the pot 

 and the manometer, for the pressures developed in the cell, even 

 in the case of fairly dilute solutions, are such as to put all joints 

 to a very severe test. Morse and Frazer, who have adopted 

 this usual method in their recent work, refer to the great 



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