Dr. F. Tinker on Osmotic Pressure. 429 



flow is very similar to that of vapour flow; diffusion proceeds 

 from pure solvent to solution because the pure solvent induces 

 a greater pressure and concentration inside the membrane 

 than the solution does *. 



In the present paper the subject of osmosis is developed 

 quantitatively from the above basis ; and the osmotic pressure 

 connected with the conditions inside the pure solvent, the 

 solution, and the membrane. But inasmuch as for this 

 purpose the Dieterici equation of state is largely employed, 

 it is to be noted at the outset that the results obtained by 

 the application of this equation are only of value in so far as 

 it represents accurately the conditions inside fluids. The 

 equation in its various forms has, however, been applied to 

 fluids with such a measure of success that the formulae 

 developed herein are in all probability at least approximately 

 accurate t- 



Notation. 



Liquid pressure (i. e. pressure inside liquid) tt 



Vapour pressure p 



Pressure inside semipermeable membrane p 



Hydrostatic pressure n 



Osmotic pressure P 



Molecular volume V 



Work done during evaporation of 1 mol. into vapour phase 



proper A 



Work done during evaporation of 1 mol. into semipermeable 



membrane , B 



Expansion on solution (per solute molecule dissolved) .... e 



Coefficient of compressibility /3 



Heat of dilution Q 



Number of molecules of solvent in solution N 



Number of molecules of solute in solution n 



In addition the suffix 1 is used for the solvent (e. g. Vj=mpl. 

 vol. of pure solvent) and the suffix 2 for the solute. 



Partial pressures, volumes, &c. (in solution) are indicated by 

 dashed symbols. 



When it is necessary to consider the volume as a function 

 of the hydrostatic pressure, an additional suffix is used. Thus 

 V 1(a N represents the molecular volume of pure solvent under 

 the atmospheric pressure a. 



The Dieterici equations employed are : — 



for the pressure in the interior of the liquid 



tt(V-6)=RT; 

 for the external vapour pressure 



A - RT - A 



* Tinker, Proc. Roy. Soc. Contemporary number, 

 p t -F° r the fundamental ideas underlying the Dieterici equation and 

 its application to the determination of the vapour pressures of binary 

 mixtures, the reader is referred to a previous paper by the author. 

 Phil. Mag. xxxii. Sept. 1916, p. 295. 



Phil. Mag. S. 6. Vol. 33. No. 197. May 1917. 2 G 



