EXPERIMENTAL WORK ON OSMOTIC PRESSURE 633 



solvent. Many, however, in their anxiety to avoid the view 

 which practically neglected the presence of the solvent, have 

 gone to the opposite extreme, and have suggested that the sol- 

 vent alone is responsible for the osmotic effects. It is, of course, 

 true that the primary osmotic effect consists in a passage of the 

 solvent into the solution through the semi-permeable membrane; 

 but inasmuch as the occurrence of the osmotic effect is con- 

 ditional on the presence of the solute, and the magnitude of the 

 effect is determined by the amount of solute in the solution, it is 

 obvious that the part played by the solute must be put in the 

 foreground. From the point of view of thermodynamics it is 

 all the same whether the solute molecules are linked to the 

 solvent molecules or not. Larmor, for example (see Nature^ 

 1897, 55, 545), supposes that each molecule of solute forms a 

 loosely connected complex with the surrounding solvent 

 molecules, and concludes that "provided the solution is so 

 dilute that each such complex is, for very much the greater part 

 of the time, out of range of the influence of the other complexes, 

 then the principles of thermodynamics necessitate the osmotic 

 laws." He adds : " It does not matter whether the nucleus of 

 the complex is a single molecule, or a group of molecules, or 

 the entity that is called an ' ion ' ; the pressure phenomena are 

 determined merely by the number of complexes per unit 

 volume." 



A view that finds acceptance in some quarters at the present 

 time regards osmotic pressure as due solely to the difference 

 in the surface tensions of solvent and solution. The advocates 

 of this conception reach the result that solutions with equal 

 surface tensions are in osmotic equilibrium, even though they 

 may not be of equimolecular concentration — a conclusion which 

 it should be possible to bring to the test of experiment. 



Apart from the problem of the nature or cause of osmotic 

 pressure, there is the question as to the function of the semi- 

 permeable membrane. The substances for which copper ferro- 

 cyanide is non-permeable are substances generally of high 

 molecular weight, and it has accordingly been maintained that 

 the membrane acts as a sieve, allowing small molecules to pass 

 through, but preventing the passage of large molecules. It 

 was with the aid of this conception that Pickering in 1897 inter- 

 preted some osmotic experiments in which he had found that 

 a porous vessel was permeable to propyl alcohol or water 



