176 RESPIRATION 



conception of osmotic pressure was mistaken. It is neither the con- 

 centration per liter of the solute molecules, nor that of the solvent 

 molecules, that determines osmosis, but the diffusion pressure 

 of the solvent. Water passes through a semi-permeable mem- 

 brane into a solution, because the diffusion pressure of pure water 

 is greater than that of the diluted water in the solution. The 

 osmotic pressure is not the excess of diffusion pressure of water 

 outside the solution, but the external mechanical pressure required 

 to equalize the two diffusion pressures, although in sufficiently- 

 dilute solutions this mechanical pressure is practically the same 

 as the excess of diffusion pressure of water. 



In a solution, just as in a gas mixture, the molecules are free 

 to move about; and, just as in a gas mixture, the mean free 

 space round each molecule is the same because the mean energy 

 of external movement is the same for each molecule. Hence the 

 free space in which water molecules are free to diffuse is in pro- 

 portion to the total number per liter of molecules present. This 

 space is of course greater per molecule of solvent in a solution 

 than in the pure solvent. Hence the pure solvent diffuses into the 

 solution unless the external pressure on the solution is raised 

 sufficiently to equalize the two diffusion pressures. 



When osmotic pressure, vapor pressures, boiling points, etc., 

 are calculated in terms of this theory instead of van't Hoff 's theory, 

 the experimentally ascertained values agree with the theory, 

 whereas this is not the case, as is now well known, with van't 

 Hoff's theory, except in the case of very dilute solutions. Thus 

 for solutions of cane sugar, and allowing for the fact that at 

 temperatures near o°C. cane sugar is present in solution as a penta- 

 hydrate, the osmotic pressures at o°C. calculated from the con- 

 centrations on the new theory and the pressures actually observed 

 by the Earl of Berkeley and Mr. Hartley at Oxford are as follows : 



