MEASUREMENT OF OSMOTIC PRESSURE 567 



the membrane and the solution in the pot was frequently 

 changed. When at the end of about two hours, the resistance 

 of the tube had risen to a steady value, the tube was washed 

 and soaked with distilled water, during about ten days, until 

 every trace of copper sulphate had been washed away. 



After washing, the loose ferrocyanide was rubbed off with 

 pumice-stone and the membrane remade electrolytically at 

 intervals of a few days until the resistance of the tube rose to 

 some 50,000 ohms. The tube was then tested in the osmotic 

 apparatus with a solution of cane sugar containing 660 grammes 

 in a litre and giving an osmotic pressure of about 100 atmo- 

 spheres. After washing, remaking and testing several times, 

 steady values for the osmotic pressure were reached; but 

 out of some 100 tubes of various makes which were tried only 

 two reached the highest state of efficiency, although over 400 elec- 

 trolyses were made. As a great improvement was effected when 

 the membranes were exposed to pressure, a special apparatus 

 was devised in which the electrolytic deposition of the 

 membrane could be carried out under a pressure of 130 

 atmospheres outside the tube and atmospheric pressure inside. 

 It was also found to be a great advantage to deposit the 

 membranes at o° C. and to keep them at o° C. until required for 

 measurements at this temperature. 



The Measurements. — Three operations were involved in the 

 measurement of the " equilibrium pressure," i.e. the hydrostatic 

 pressure which was required exactly to balance that set up 

 by osmosis. 



(a) Guard-ring leak. As the semi-permeable membrane is 

 never quite on the surface of the porcelain tube, it is impossible 

 to get perfect contact between it and the dermatine packing ; 

 there is therefore always a leakage of the compressed solution 

 past these guard-rings. This leakage would not matter but for 

 the fact that the hydrostatic pressure on the solution gradually 

 diminishes as it oozes out until it finally escapes under a 

 pressure that is only atmospheric. Minute portions of the 

 membrane are therefore in contact with uncompressed solution 

 and through these water is steadily drawn from the tube into 

 the solution. This effect was reduced by making the guard- 

 rings overlap the ends of the tube and its magnitude was 

 rendered constant by filling the metallic extension-tubes VV 

 with solution so as to provide an ample reservoir of uncom- 



