MEASUREMENTS OF OSMOTIC PRESSURE 



191 



in precipitating the semi-permeable membrane in the framework of the 

 vessel, where the two liquids met in the porous wall. 



Figure V-7 shows the form of the apparatus used by Pfeffer to measure 

 osmotic pressures. The earthenware pot was a white unglazed porcelain 

 cell z with the glass pieces v and t inserted as air-tight stoppers. The 



Distilled water bath 



Fig. V-7. A precision method for measuring osmotic pressures according to Pfeffer. 



porcelain cell was about 4.6 cm high and had an internal diameter of 

 about 1.6 cm with walls from 1.25 to 2 mm thick. A manometer m was 

 attached to measure the pressure. The tube g was sealed off after air 

 bubbles had been removed at the joints t. The solution whose osmotic 

 pressure was to be determined was introduced into the apparatus through 

 g. The porous cell was then totally immersed in distilled water, and 



