32 DIFFUSION AND OSMOTIC PRESSURE 



given quantity of some solute, it cannot be told a priori 

 whether the resulting solution will occupy the same volume 

 as the original solvent, or a greater or less volume. Into 

 this matter it is unnecessary to go farther than to add that 

 osmotic pressure may be demonstrated as readily in solu- 

 tions occupying less volume than the original solvent as in 

 those occupying more. It is obvious that in the former 

 case there must be a greater number of solvent particles 

 per unit volume than in the pure solvent. Hence, if the 

 above explanation can be retained, there should be no 

 osmotic pressure developed in such a solution; indeed, it 

 should appear on the side of the pure solvent. 



But even if it were possible that the entrance of solvent 

 particles into the solution was due to such a difference in 

 concentration of the solvent on opposite sides of the mem- 

 brane, the explanation just quoted would fail. It is incon- 

 ceivable that the osmotic membrane should be more 

 permeable to solvent particles moving in one direction than 

 to those moving in the other, and it thus becomes impos- 

 sible to suppose that solvent particles which can pass the 

 membrane in one direction "will exert great pressure" upon 

 it in the other. Great hydrostatic pressure cannot be main- 

 tained in a sieve, nor can osmotic pressure be maintained 

 upon a membrane by a solvent to which it is permeable. 

 Any difference between the solvent pressures on the two sides 

 of an osmotic membrane must be very rapidly destroyed by 

 diffusion of the solvent through the membrane. 



III. EXPERIMENTAL DEMONSTRATION OF OSMOTIC PRESSURE 



If a parchment-paper bag, like the one used in the illus- 

 tration of osmotic pressure, were fixed in a firm cage, so that 

 it could not expand except on one side, and were then filled 

 with solution and submerged in pure solvent, bulging would 

 occur on the free side. It would make no difference whether 



