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XLV. Osmotic Pressure from the Standpoint of the Kinetic 

 Theory. By T. Martin Lowey, D.Sc* 



van 't Hoff's Gas-Theoey of Osmotic Peessuee. 



IN any discussion of the origin and mechanism o£ osmotic 

 pressure it is necessary to take into account, as a factor 

 of fundamental importance, the fact — now fully established 

 by experimentf — that the osmotic pressure of a dilute solution 

 can be calculated with a very close approach to accuracy by 

 means of the formula P V = RT, which expresses the relationship 

 between the pressure and volume of a gas. Not only is the 

 osmotic pressure proportional to the concentration of the so- 

 lution and to its absolute temperature, but the constant R has 

 the same value in the two cases, so that the osmotic pressure is 

 actually identical in magnitude with the pressure exerted by 

 a gas of equal molecular concentration. This result could be 

 predicted on general grounds in the case of gaseous osmotic 

 pressures (Arrhenius, Zeit. phys. Chem. 1889, iii. p. 119 ; 

 Ramsay, Phil. Mag. 1894, xxxviii. p. 206) ; but the conditions 

 prevailing in a liquid are so far different tbat the gas-analogy, 

 though suggestive, would be utterly insufficient to justify the 

 application of the gas-laws to osmotic pressure unless these 

 could be verified experimentally or established on an inde- 

 pendent theoretical basis. 



When, however, the discovery was made by van 't HofT 

 (Zeit. phys. Chem. 1887, i. pp. 481-508) that osmotic pressure 

 in liquids is actually identical in magnitude with the pressure 

 of a gas exposed to similar conditions of temperature and 

 pressure, it was generally assumed that the two phenomena 

 must be essentially similar in character, and that the osmotic 

 pressure produced by dissolving sugar in water was due, as in 

 gases, to the bombardment of semi-permeable membrane by 

 the molecules of the solute (van't Hoff, loc. cit. p. 482 ; B. A. 

 Report, 1890, p. 336; Zeit. phys. Chem. 1890, v. pp. 174-176). 

 Van 't HofFs theory had the merit of giving a simple quanti- 

 tative explanation of osmotic pressure, and was not' unrea- 

 sonable when applied to the osmotic pressure of gases dissolved 

 in liquid (as well as in gaseous) solvents. The conception 

 was, however, not easy to apply to the more ordinary cases 

 of osmotic pressure, such as those afforded by aqueous sugar 

 solutions. In most cases the solute molecules have a very 



* Communicated by the Faraday Society (Abstract of a contribution 

 to a general discussion on "Osmotic Pressure " held on Jan. 29, 1907). 



f Pfeffer, Osmotische Untersuchungen, Leipzig, 1877 ; Morse & Fraser, 

 Amer. Chem. Journ. 1905, xxxiv. p. 1 (1906) ; compare also the expe- 

 riments of Griffiths on the freezing-points of dilute aqueous solutions. 



