30 DIFFUSION AND OSMOTIC PRESSURE 



essential. A brief discussion of the different theories which 

 have been proposed to account for the action of the membrane 

 will be presented in connection with the treatment of the 

 protoplasmic membranes of vegetable cells (see p. 80). 



II. DIFFUSION TENSION OF THE SOLVENT 



The diffusion tension of the solvent has been mentioned 

 several times during the discussion of solutions, but it is 

 thought well to bring together in this place the ideas con- 

 cerning it. 



If the vapor tension of pure water be determined and 

 then that of an aqueous salt or sugar solution, it will be 

 found that the latter is invariably less than the former, and 

 this in proportion to the concentration of the solution. 

 Therefore it must be that particles of the solute hinder the 

 escape of the solvent molecules. The moving particles of 

 solute perhaps bring this about merely by moving into the 

 path of solvent molecules which would otherwise leave the 

 liquid. It is probable that there exists also an attraction 

 between the particles of solute and solvent. 



If two solutions whose concentrations are different be 

 brought into direct contact, as by placing a weak sugar 

 solution over a stronger one, phenomena similar to those 

 just discussed may be detected. Water molecules pass 

 through the common surface in both directions. They are 

 not vaporized, for they remain in the liquid state, but they 

 diffuse as liquid molecules. Under such conditions the dif- 

 fusion of the solvent is always found to be greater from the 

 weaker to the stronger solution than in the opposite direc- 

 tion; it will be remembered that the most rapid diffusion 

 of the solute takes place from stronger to weaker solution. 

 There is, therefore, a difference in the energy of diffusion 

 (diffusion tension) of the solvent in the two solutions. This 

 corresponds to their difference in vapor tension just de- 



