Prof. J. H. Poynting on Osmotic Pressure. 297 



whence M // = M, or at the level of the solvent surface the 

 mobilities are equal. This equality will be maintained if we 

 descend equal distances in the two liquids below that level. 

 So that if we now connect the two vessels at any level by a 

 horizontal tube with a semipermeable membrane in it, the 

 solvent mobilities on the two sides of the membrane are equal, 

 and therefore the solvent diffuses through at equal rates in 

 the two directions. 



We may then explain in the following general terms the 

 rise which occurs when we place a semipermeable vessel con- 

 taining a solution into a solvent. The solvent molecules are 

 entering the membrane on both sides, but the mobility or 

 number set free per second from the pure solvent is greater 

 than the number set free from the solution. The membrane 

 o-oes on absorbing the solvent from each side till it becomes 

 saturated, i. e. holds so much that it returns as many mole- 

 cules as it receives. It is receiving more from the pure 

 solvent side, and therefore when saturated for that side it is 

 supersaturated for the other. Consequently more molecules 

 are sent into the solution than are received from it, and the 

 solution grows until the growing pressure so much increases 

 the mobility that it is equal on both sides of the membrane. 



If the solution and solvent are in two vessels separated by 

 an indefinitely produced vertical and semipermeable membrane, 

 it is evident that ultimately the two will be in equilibrium at 

 every level, whether in liquid or vapour. 



We may apply the same idea to the change of melting- 

 point in a solution. In the solution the solid mobility is 

 unchanged, but that of the solution is lowered by the fraction 



— , where P is the osmotic pressure ; and to find the new 



melting-point, we must find the temperature d6 below the 

 normal melting-point at which this is equal to the difference 

 between the liquid and solid mobilities. 

 Taking pressures to represent mobilities, 



-57 'Uip 



But , Ltrdd 



«r-«r=__, 



whence we obtain the ordinary result 



*-g. ...... (8) 



Comparing the above result with the lowering due to pressure 



