40 DIFFUSION AND OSMOTIC PRESSURE 



osmotic pressure. This relation is expressed by the follow- 

 ing equation: 



TT-TT' 0.0819 T X 1000 S X 760 l 



77 ~1F~ 



In this P is the osmotic pressure in millimeters at the abso- 

 lute temperature T, TT and TT ' are the vapor tensions observed 

 at that temperature of the solvent and solutions respectively, 

 s is the specific gravity of the solution, and M is the molecu- 

 lar weight of the pure solvent. In the case of dilute 

 aqueous solutions, s may be put equal to unity (the specific 

 gravity of the pure solvent instead of that of the solution), 

 and M is 18 (the molecular weight of water). Making these 

 substitutions in the above equation, we have: 



TT-TT' 0.0819 Tx 1000 X 760 



7T 18 



or 



The determination of the vapor tensions is best made by 

 means of the method devised by Ostwald and Walker. 2 

 Two Liebig potash bulbs, one filled with the solution to be 

 tested and the other with the pure solvent (the latter 

 weighed) , are joined in series and then attached to a weighed 

 U-tube of pumice moistened with sulphuric acid. A slow 

 current of air is passed, for six to twelve hours, through the 

 series. The air first becomes saturated at the tension of the 

 solution, and then, passing through the second bulb, be- 

 comes again saturated at the vapor tension of the pure 

 solvent. A final weighing of the second bulb and of the 



1 NEKNST-PALMEK, Theoretical Chemistry (London, 1895), p. 126; also J. H. 

 VAN'T HOFF, " Die Rolle des osmotischen Druckes in der Analogie zwischen LOsun- 

 gen und Gasen," Zeitschr.f. physik. Chem., Vol. I (1887), pp. 481-508. 



2 J. WALKER, " Ueber eine Methode der Bestimmung der Dampfspannung bei 

 niederen Temperaturen," Zeitschr. f. physik. Chem., Vol. II (1888), pp. 602-5; also 

 OSTWALD-WALKEB, Manual of Physico-Chemical Measurements (London, 1894), 

 p. 188. 



