24 MORRIS LOEB 



That the vessel is not exploded by the pressure of the 

 molecules, as Dr. Pupin demands, is due to the same force 

 of solution which prevents their evaporating at the free sur- 

 face. How great this force can become we may guess from the 

 enormous condensation taking place in the absorption of 

 gases by liquids; that the force is a kinetic one is shown by 

 its being a function of temperature, a purely kinetic phenom- 

 enon. Bredig 1 has recently endeavored to show how this 

 force can be made to diminish the external effect of the 

 kinetic energy of the molecules of dissolved substance in 

 such a manner that there is still freedom of action within the 

 mass; and upon this line of reasoning we must depend for a 

 final explanation of the phenomena. 



For van 't Hoff it was, however, sufficient that an osmotic 

 pressure does exist which is dependent upon the kinetic 

 energy of the molecules. By simple application of the 

 method of Carnot's cycle, he shows that the osmotic pressure 

 must be proportional to the absolute temperature, and that, 

 for solutions of gases, it corresponds precisely to the tension 

 of the gas in the solution. A natural inference from all this 

 is that Boyle's, Gay-Lussac's, Henry's, and Avogadro's laws 

 find their counterparts in the laws governing osmotic pres- 

 sure. 



Suppose now solutions of two different substances in the 

 same solvent to possess the same tension for the vapor of 

 the latter; it is necessary that they shall also have the same 

 osmotic pressure. For suppose them separated by a wall 

 which is permeable to the solvent alone, but with their free 

 surfaces in communication through the atmosphere. The 

 vapor tension of both being the same, a little of the solvent 

 might distill from one solution to the other without the per- 

 formance of any work; but if at the same time their osmotic 

 1 Z. phydk. Chem. 4, 444 (1889). 



