EXPLORING THE ATOM 



peated and modified his experiments. It is shown 

 that different ions move at varying speed, and that 

 the rate of motion bears a certain relation to the 

 atomic weights of the various atoms. 



The ion theory was early championed and various- 

 ly tested by Professor Wilhelm Ostwald, who not 

 only viewed it in connection with his elaborate in- 

 vestigations of acids, but pointed out its relation to 

 the theory of osmosis advanced at about the same 

 time by the Dutch chemist J. H. Van't Hoff. Ac- 

 cording to this theory, the familiar but hitherto in- 

 explicable phenomena of osmosis, in accordance with 

 which water passes through a membrane from a less 

 concentrated to a more concentrated solution, are 

 due to the pressure on the membrane of the mole- 

 cules in the solvent; which pressure, according to 

 the theory, is precisely the same that would be ex- 

 erted by a corresponding number of molecules mak- 

 ing up an equal bulk of a gas. 



The theory of Van't Hoff met with certain contra- 

 dictions so long as the molecules of a salt were 

 regarded as maintaining their stability in solution, 

 but when these molecules were thought of in con- 

 nection with the ion theory as split up into their 

 component atoms, the theory harmonized with the 

 observed facts. Thus the new ion theory of solution 

 and the theory of osmosis tended to support each 

 other. The fact that each ion of the disassociated 

 molecule conveys a charge of electricity is the essen- 

 tial fact lying back of all chemical activity whatever. 

 Thus the ion theory, or as it is also called the dis- 

 association theory, is of fundamental importance in 



103 



