11. OSMOTIC PRESSURE MEASUREMENTS 41 



cules. The fact that the postulated pressure of the sokite molecules 

 is restricted in its manifestation to the region occupied by the solvent 

 {i.e., no such pressures are actually existent upon the walls of a vessel 

 containing the solution) has tended to work against the acceptance of 

 the completeness of the analogy. Others have pictured osmotic 

 pressure as resulting entirely from solvent bombardment, the numbei- 

 of solvent molecules reaching the side of the membrane in contact 

 with the pure solvent beng pictured as greater than that from the 

 solution side because of the cross-sectional area occupied by solute 

 molecules on the solution face of the membrane. However, it is 

 pointed out that the effect of the solute is not proportional to the size 

 of the solute molecule but only (in dilute solutions) to the number 

 present per unit volume of the solution. If it is assumed, however, 

 that the solute in some way causes a decrease in the solvent pressure 

 in the solution and that this decrease is proportional to the number of 

 solute particles present per unit of solvent molecules, we arrive at a 

 basis for the thermodynamic treatment of osmotic pressure together 

 with that of the other colligative properties of the solution in which 

 these effects are related quantitatively to the escaping tendency or 

 fugacit}'' of the solvent in the solution compared to that of the solvent 

 in the pure state. There is still to be answered the question as to the 

 actual kinetic mechanism by which this decrease in solvent fugacity is 

 brought about. In the absence of a satisfactory kinetic picture it is 

 necessary only to bear in mind that, for some reason connected with 

 the presence of solute molecules, the partial free energy, escaping 

 tendency, or activity of a molecule of solvent is lower in the solution 

 than in the pure liquid, and that the transfer of solvent from pure 

 liquid to the solution, whether through a membrane semipermeable 

 to the solute or through the vapor phase (which also may be regarded 

 as a medium permeable to solvent molecules but impermeable to 

 solute), will occur with a loss of free energy and will, therefore, be a 

 spontaneous process. External pressure applied to the solution will 

 cause an increase in the vapor pressure of the solvent in the solution, 

 i.e., increase its escaping tendency, activity, or partial free energy. 

 Where a membrane separates the 2 phases and when this pressure is 

 just great enough to bring the activity of the solvent in the solution 

 to that of the pure solvent, no further net transfer of solvent will 

 occur across the membrane and the ajjplied pressure will be a measure 

 uf the osmotic pressure of the solution. 



