OSMOTIC PRESSURE 



147 



solutions as "osmotic pressures," were directly proportional to the concentration 

 of the solute and to the absolute temperature. 



The process by which water passes through a membrane from a solution on the 

 one side to another solution on the opposite side had been known, since the time 

 of Dutrochet (1827, p. 393), as "endosmosis" or "exosmosis," so that the pressure 

 due to this passage of water was naturally called " osmotic" 



The experiments made by Pfeffer have served as the starting point of sub 

 sequent work on osmotic 

 pressure, especially as the 

 basis of the important 

 theory of solutions put 

 forward by van't Hoff ; I 

 have therefore given his 

 portrait in Fig. 47. 



The similarity of the 

 process in gases and in 

 solutions is obvious, but 

 the relationship was not 

 made clear until van't 

 Hoff (1885, 1) was led by 

 thermodynamic considera- 

 tions (see Cohen's book, 

 1912, p. 225) to the view 

 that the pressure de- 

 veloped by a substance in 

 solution is identical with 

 that which it would exert 

 if converted into gas of 

 the same volume and tem- 

 perature ; in other words, 

 the solute behaves as if it 

 were in the dispersed 

 molecular condition of a 

 gas and the solvent were 

 absent. 



This statement does not 

 necessarily imply that the 

 state of the solute is actually 

 that of a gas, although, as 

 we shall see later, a kinetic 

 theory, similar to that of 

 gases, gives, on the whole, 

 the most satisfactory explana- 

 tion of the phenomena. It 

 should be kept in mind that 

 the facts of osmotic pressure, 

 their connection with vapour 

 pressure and so on, are inde- 

 pendent of any theory of 

 their origin. FIG. 47. PORTRAIT OF PFEFFER. 



On account of the Signature from Charter Book of the Royal Society. 



importance of van't HofPs 



theory, the actual words of the author himself (1885, 2, pp. 42 and 43) may be 



given : 



1. " Loi de Boyle pour les Solutions. La pression osmotique est proportionelle 

 a la concentration, si la temperature reste invariable. 



2. "Loi de Gay-Lussac pour les Solutions. La pression osmotique est pro- 

 portionelle a la temperature absolue, si la concentration reste invariable. 



Ce sont la les analogies qui ont ete demontrees et verifiees en detail dans le 

 travail cite (the preceding paper, 1885, 1); elles ont rapport a la variation de 

 la pression avec les circonstances. Je vais ajouter maintenant une troisieme 



