ONE HUNDRED YEARS OF CHEMISTRY 319 



afterwards explained by the ionization theory of Arr- 

 henius. Better apparatus for this method was soon 

 devised by Beckmann, who introduced also a method 

 depending upon the boiling points of solutions, and these 

 two methods are still the standard ones for determining 

 molecular weights in solution. They are very exten- 

 sively employed by organic chemists. 



It has been found that the majority of substances when 

 dissolved have the same molecular weight as in the 

 gaseous condition, provided that they can be volatilized 

 at comparable temperatures. For instance, sulphur in 

 solution has the formula S 8 , iodine is I 2 and the metals 

 are monatomic. 



Van't Hoff's Law and Arrhenius's Theory of Ions. 

 Modern views on solutions date largely from 1886, when 

 van't Hoff called attention to the relations existing 

 between the osmotic pressure exerted by dissolved sub- 

 stances and gas pressure. 



Pfeffer, a botanist, was the first to measure osmotic 

 pressure (1877). Basing his conclusions chiefly upon 

 Pfeffer's determinations, van't Hoff formulated a new 

 and highly important law, which may be stated as fol- 

 lows: The osmotic pressure exerted by a substance in 

 solution is equal to the gas pressure that the substance 

 would exert if it were a gas at the same temperature and 

 the same volume. Further investigations have fully 

 established the fact that molecules in dilute solution obey 

 the simple laws of gases. 



It was pointed out by van't Hoff that salts, strong 

 acids and strong bases showed marked exceptions to his 

 law in exerting much greater osmotic pressures than 

 those calculated for them. 



The next year in 1887, Arrhenius explained this abnor- 

 mal behavior of salts, strong acids and strong bases by 

 assuming that they dissociate spontaneously into ions 

 when they dissolve, and that these more numerous par- 

 ticles act like molecules in producing osmotic pressure. 

 He showed that these exceptional substances all conduct 

 electricity in solution, while those conforming with van't 

 Hoff's law do not, and according to his theory the ions 

 become positively or negatively charged when they are 

 formed, and these charged ions conduct the current. 



