OSMOTIC PKESSUEE IN PLANTS 81 



vided only that it is truly semi-permeable. It is therefore 

 permissible to consider the membrane as a bubble of vapour 

 contained by two rigid walls which prevent the hydrostatic 

 pressures of the liquid columns from acting upon it. Since 

 distillation of vapour does not take place, the vapour 

 pressures of solvent and of solution must be equal on either 

 side of ra; but as that of the solution is normally lower 

 than that of the solvent, it is evident that their equality 

 in this case is due to an increase in the vapour pressure of 

 the solution brought about by the application of a hydro- 

 static pressure. This is occasioned by the difference in 

 level of solvent and solution in the tubes. 



Starting from the deduction that for dilute solutions 

 the osmotic pressure was equal to the gaseous pressure 

 which the solute would exert if considered as occupying 

 the actual volume of the solution, Van't Hoff suggested 

 that such a pressure might be similar in nature to that 

 exerted by a gas viz., that it was due to the impinging of 

 the solute molecules upon the membrane. His equation 

 was, however, in no way dependent upon this hypothesis, 

 and he clearly recognized that the phenomenon might be 

 explained by the attraction between solvent and solute. 

 The gas pressure theory has now been largely abandoned, 

 as it fails to account for the behaviour of concentrated 

 solutions satisfactorily. Its simplicity and applicability 

 to dilute solutions both contributed to its continued em- 

 ployment in explaining the phenomena of osmosis. 



The generally accepted view now is that osmotic pressure 

 is a hydrostatic pressure, of such a magnitude as to bring 

 the vapour pressures of solvent and solution into equilib- 

 rium. For the numerous hypotheses as to the manner in 

 which this pressure is produced Findlay's book should be 

 consulted. At present there is some evidence in favour of 

 the view that it is due to the inactivation of the solvent 



