1904.] Method, of Measuring High Osmotic Pressures. 441 



pressure on the solution less than its osmotic pressure, and gradually 

 increasing the former until the latter is exceeded, then gradually 

 reducing the pressure until again below the osmotic pressure, the 

 application to the corresponding capillary readings of a formula 

 involving the guard-ring and the solution-leak rates will enable one to 

 calculate the sum of the two. 



This sum, deduced in this manner from the capillary readings in an 

 experiment with a solution containing 180 grammes in the litre, was 

 5 '36 cm. The separately determined "guard-ring leak" correction was 

 10*17 cm., and the difference, 4*81, represents the calculated "solution 

 leak." The observed "solution leak," determined by analysis, and 

 calculated on the assumptions enumerated above, gave 5'03 cm. 



Some importance is attached to this method of working as it appears 

 to afford a means of determining the osmotic pressures of solutions of 

 substances for which no truly semi-permeable membranes have yet been 

 found. 



The Pressure Apparatus. — The pressure is obtained by means of a 

 vertical steel plunger working in a steel cylinder. The plunger is forced 

 into the cylinder by an iron lever, at one end of which weights are hung,, 

 and the bottom of the cylinder is connected with C (see fig. 2). The 

 cylinder and pressure tube are filled with the solution whose osmotic 

 pressure is being determined, and the plunger is made to work almost 

 pressure tight by a " dermatine " ring, sleeve and nut, similar to that 

 shown at D, E and F, in fig. 2. A horizontal lever is attached to the 

 plunger and is worked to and fro, at intervals, so as to keep the 

 pressure on. 



The pressure applied when different weights were placed at the end 

 of the lever was determined by connecting the press-tube to a Schaffer 

 and Budenberg standard gauge. 



The Formation of the Semi- Permeable Membranes. — Numerous different 

 ways of forming the copper ferrocyanide membrane were tried ; the 

 most successful was by first depositing the film by diffusion in the 

 manner Pfeffer* recommends and then finishing it by the electrolytic 

 methods due to Morse and Horn.f The porcelain tubes were immersed 

 in the copper sulphate solution (50 grammes of OuBO^aq in 1000 c.c.) 

 in a desiccator, and exhausted free of air ; they were then taken out 

 and their inner and outer surfaces dried with filter paper. The ends 

 were then closed by rubber plugs and they were allowed to dry in the 

 air for from J to 1 hour, after which they were quickly plunged into a 

 solution of potassium ferrocyanide (42 grammes in 1000 c.c). When 

 the membrane was seen to be fairly uniform in colour, the tubes were- 

 transf erred to the electrolytic cell. This consisted of a beaker contain- 

 ing the ferrocyanide solution in which a platinum foil electrode 

 i * Loc. cit. 



f H. Morse and D. W. Horn, 'American Chemical Journal,' vol. 26, p. 80. 



