88 



HAKRY C. JONES 



which, frankly expressed, is that we do not know and at present 

 have no means of finding out what really causes osmotic pressure. 

 It seems probable from the splendid work of Morse, that when 

 osmotic pressure is explained we will have to take into account 

 the fact that the colloidal semi-permeable membrane used in 

 measuring it, probably takes up water from the outside and 

 becomes hydrated, and that the solution in some unknown way 

 dehydrates this membrane on the inside. But after all this is 

 not very much more than words. 



What we do know, and in my judgment about all that we do 

 know at present in reference to the nature of osmotic pressure, is 

 that it is in some entirely unknown way connected with the sur- 

 face-tension of the solutions between which it manifests itself. 

 Whenever we have a satisfactory theory of osmotic pressure, it 

 seems highly probable that that theory must take into account 

 the phenomena which manifest themselves in capillary tubes, 

 which we call surface-tension, and this probably exerts itself in 

 the fine capillaries in the semipermeable membranes. 



Although we do not today know the cause of osmotic pressure 

 yet we know its result. Its magnitude has now been accurately 

 measured and its significance in connection with solutions pointed 

 out by Van't Hoff and Arrhenius. 



I hope some idea as to the importance of osmotic pressure in 

 the development of general chemistry may be gained from what 

 has already been said. All in all, it is to be regarded as the most 

 important property possessed by matter in the dissolved state. 



