THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 

 AUGUST 1888. 



XII. The Function of Osmotic Pressure in the Analogy between 

 Solutions and Gases. By Professor J. van't Hoff*. 



DURING an investigation which required some knowledge 

 of the laws regulating chemical equilibrium in solutions, 

 the conclusion has gradually been evolved that a deep analogy 

 — indeed almost an identity — exists between dilute solutions 

 exerting osmotic pressure on the one hand, and gases under 

 ordinary atmospheric pressure on the other. The following 

 pages contain an attempt to explain this analogy ; and the 

 physical properties of such systems will form the first subject 

 of discussion. 



I. Osmotic Pressure — the nature of the Analogy due to this 

 conception. 



In order clearly to realize the quantity referred to as osmotic 

 pressure, imagine a vessel, A (fig. 1), com- pjg # i § 



pletely full of an aqueous solution of sugar, 

 placed in water, B. If it be conceived that 

 the solid walls of this vessel are permeable 

 to water, but impermeable to the dissolved 

 sugar, then, owing to the attraction of the 

 solution for water, water will enter the 

 1 A up to a certain limit, thereby in- 

 creasing the pressure on the walls of the vessel. Equilibrium 

 then ensue>, owing to the pressure resisting further entry of 

 water. This pressure we have termed osmotic pressure. 



* Communicated to the Physical Society; translated by Prof. W. 

 Ramsay, F.Ii.S. : read June ( J, 1888. 



Phil. Mag. S. 5. Vol. 26. No. 150. Aug. 1888. G 



