16 SCIENCE PROGRESS. 



van't Hoff showed that if a solution is so dilute that on 

 communicating heat to it the heat is exclusively em- 

 ployed in doing external work, then, if the concentra- 

 tion of the solution remains the same, the osmotic pressure 

 should be proportional to the absolute temperature. Such 

 experimental data as were available were not opposed to 

 this conclusion, which is now spoken of as Gay Lussacs 

 law for dilute solutions, as it is similar to the well-known 

 law of Gay Lussac connecting gaseous pressure at con- 

 stant volume with the temperature. Now, Pfeffer's 

 observations on sugar showed that at constant tempera- 

 ture osmotic pressure was approximately proportional to 

 the concentration. Here there was indication that a law 

 analogous to that of Boyle held for dilute solutions, since 

 Boyle's law for gases may be expressed by stating that 

 at constant temperature pressure is proportional to the 

 mass of gas per unit volume, that is, to the concentration. 



The most striking indication, however, of the close- 

 ness of the relation between solutions and gases was 

 obtained by comparing the magnitude of the osmotic 

 pressure of a sugar solution with that of a gas. Pfeffer 

 found that at 6°*8 the osmotic pressure of a one per cent, 

 sugar solution was "664 atmospheres. The pressure of 

 a volume of hydrogen equal to that of the solution, and 

 containing the same number o{ molecules as there are 

 molecules of sugar in the solution, is '665 atmospheres. 

 The values of the two pressures are, therefore, almost 

 identical. This identity of the values for the osmotic 

 and gaseous pressures, the theoretical and practical support 

 to the idea that they obeyed the same laws, led van't 

 Hoff to the conclusion that both are essentially of the 

 same nature, that both have the same origin. The 

 pressure of a gas is due to impacts of its molecules on 

 the walls of the containing vessel, and according to van't 

 Hoff the osmotic pressure of a solution is the result of 

 the impacts of the molecules of the dissolved substance 

 on the semi-permeable membrane, because when equili- 

 brium results between solvent and solution, the impacts 

 of the molecules of the solvent are equal and opposed, 



