288 Wells £ Foote — One Hundred Years of Chemistry. 



sure over mercury. In 1878 Victor Meyer introduced a 

 simpler method depending upon the displacement of air 

 or other gas by the vapor in a heated tube. As refrac- 

 tory tubes, such as those of porcelain or even iridium, 

 could be used in this method, molecular weights at 

 extremely high temperatures were determined with inter- 

 esting results. For instance, it was found that iodine 

 vapor, which shows the molecule I 2 at lower tempera- 

 tures, gradually becomes monatomic with rise in tem- 

 perature, that sulphur vapor dissociates from S 8 to S 2 

 under similar conditions, and that most of the metals, 

 including silver, have monatomic vapors. 



In 1883 and later it was pointed out by Raoult that the 

 molecular weights of substances could be found from the 

 freezing points of their solutions, but this method was 

 complicated from the fact that salts, strong acids and 

 strong bases behaved quite differently from other sub- 

 stances in this respect, and allowances had to be made for 

 the types of substances used. The complication was 

 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 



