

Osmotic Pressure and the Kinetic Theory. 553 



slight mobility, and in the absence of the solvent are unable 

 to produce any hydrostatic pressure whatever upon the walls 

 of the containing vessel. It is therefore scarcely reasonable 

 to attribute the whole action of the solution to the relatively 

 inert solute, whilst neglecting the very active part played by 

 the solvent (Fitzgerald, B. A. Report, 1890, p. 327). 



The Mechanism of Osmotic Pressure 



There can at the present time be no doubt that osmotic 

 pressure depends essentially on the phenomena of selective 

 solubility. The palladium membrane used by Ramsay to 

 develop a gaseous osmotic pressure acts in virtue of its ability 

 to absorb or dissolve hydrogen, but not nitrogen ; the amount 

 of hydrogen absorbed depends on the pressure, and equilibrium 

 is attained when the partial pressure of the hydrogen inside 

 the vessel is equal to its total pressure outside. A water- 

 membrane has been used by Nernst to develop an osmotic 

 pressure between ether and an ethereal solution of benzene, 

 the former being soluble and the latter insoluble in water. 

 Copper ferrocyanide, which absorbs or dissolves water and 

 certain salts but not sugars, forms an efficient semi-permeable 

 membrane for aqueous sugar solutions, but not for salt 

 solutions. The presence of the sugar diminishes the solubility 

 of the water in the membrane, and a flow of liquid is set up 

 because the membrane when saturated with regard to the 

 water on one side is supersaturated with regard to the solution 

 on the other side. The amount of water taken up by the 

 membrane can, however, be increased by compressing the 

 liquid, and it is thus possible to counterbalance the decrease 

 of solubility due to the sugar, and by equalizing the solubility 

 on the two sides of the membrane to stop the flow of solvent 

 into the solution. The pressure required to equalize the 

 solubilities and stop the flow is the so-called " osmotic 

 pressure,"" and it may again be urged that there is no a priori 

 reason for supposing that this would be the same as that 

 produced by a gas of equal molecular concentration. 



Pickering's theory (Ber. 1891, xxiv. p. 3639) that tho 

 action of the semi-permeable membrane depends on the 

 relative size of the molecules, has not been confirmed. The 

 similar mechanical theory of Sutherland (Phil. Mag. 1897 

 [5] xliv. pp. 493-498), that the membrane consists of 

 " meshes " through which water but not sugar can pass 

 appears to be equally untenable and need not be discussed 

 here. The theories of Traube and of Armstrong are referred 

 to later. 



