628 SCIENCE PROGRESS 



permeable membrane. Since most solutes will pass, more or 

 less rapidly, through a copper ferrocyanide membrane, and 

 since no superior membrane has yet been found, it follows 

 that the scope for direct determination of the osmotic pressure 

 of aqueous solutions is somewhat limited. But when once it 

 has been shown that the values of the osmotic pressure deduced 

 from the vapour pressure agree closely with those found by 

 direct observation, it will be possible to apply the vapour- 

 pressure method with confidence over a wide field, for this 

 method is subject to no such limiting condition as the existence 

 of a semi-permeable membrane. 



Theoretical Value of the Osmotic Pressure 



One of the most notable and interesting investigations 

 associated with the name of van't Hoff is that in which he 

 deals with the analogy between solutions and gases. The 

 main conclusions he reached were that osmotic pressure obeys 

 the laws of gases, and that the osmotic pressure exerted by 

 a dissolved substance in dilute solution is equal to the pressure 

 it would exert if it were in the gaseous state at the same 

 temperature and occupied the same volume as the solution. 

 When van't Hoff published his memoir in 1887, the experi- 

 mental material available for the verification of his conclusions 

 was somewhat scanty, for the only direct measurements of 

 osmotic pressure recorded up to that date were those made 

 by Pfeffer. There is, therefore, some justification for the 

 criticisms passed in certain quarters on the slender experimental 

 basis on which the gas theory of solutions originally rested. 



In an examination of van't Hofif's conclusions in the light of 

 modern work on osmotic pressure, it must be remembered that, 

 according to van't Hoff himself, they are valid in their simple 

 form only for dilute solutions, in which the volume of the 

 dissolved molecules is negligible compared with the total 

 volume of the solution. Where this condition is not fulfilled, 

 deviations from the simple rule may be expected, just as gases 

 subjected to high pressures exhibit considerable deviations 

 from the behaviour required by the simple gas laws. 



The cane sugar solutions examined by Lord Berkeley and 

 Mr. Hartley were certainly concentrated, and therefore it is 

 not surprising that these investigators found very marked 



