34 DIFFUSION AND OSMOTIC PRESSURE 



liquid solvent, the pressure of solvent particles may be imme- 

 diately effective. The surface film is lifted by the pressure 

 exerted from below, as a piston in such a position might be 

 lifted by wire springs situated within the bulb; and, in 

 rising, be the change in level ever so slight, it increases the 

 volume of the solution in the tube, thus decreasing the dif- 

 fusion tension of the solvent within this solution, and also 

 overcoming to some degree the atmospheric pressure on the 

 free surface of the solution; and water enters through the 

 permeable membrane below. The entrance of the water 

 is due mainly to the diffusion tension of the solvent and in 

 part to hydrostatic pressure. In the latter sense the push- 

 ing up of the surface film acts like the raising of a piston in 

 a pump. With a closed water manometer, or an open one 

 of mercury, a pressure far surpassing that of an atmosphere 

 may be obtained where the membrane used is sufficiently 

 strong. Of course, in such cases hydrostatic pressure as a 

 cause for the ascent of the column is to be ruled entirely 

 out of consideration. 



Other methods of demonstrating osmotic pressure are in 

 use, but the explanation just given may be applied, mutatis 

 mutandis, to any of them. 



