MEASUREMENT OF OSMOTIC PRESSURE 565 



in placing the membrane and the solution on the outside of a 

 porous tube instead of on the inside of a porous pot. The 

 apparatus is shown in Fig. 6. 



AB is a porcelain tube, 15 cm. long, 2 cm. external diameter 

 and i"2 cm. internal diameter with glazed ends, CC is a gun- 

 metal cage against the ends of which the dermatine rings DD 

 are compressed when the parts E and F of the outer gun- 

 metal vessel are screwed together, thus making a tight joint 

 with the tube. Another dermatine ring X provides a tight 



Fig. 6. — Osmotic apparatus complete. 



joint between E and F and allows the solution in EF to be com- 

 pressed without leakage. The water inside the porcelain tube 

 is enclosed between rubber stoppers KK, carrying the brass 

 tubes LL and compressed between the washers MM and the 

 nuts NN. One of these brass tubes carries a glass funnel and 

 tap, the other an open glass capillary or water-gauge, graduated 

 in millimetres and calibrated ; this capillary serves to show the 

 rate of flow of water through the membrane, either from 

 the water inside to the solution outside or under high 

 mechanical pressure from the solution to the water. Two 



