33 6 HISTORY OF CHEMISTRY. [LECTURE XVI. 



(and especially of compressed gases) more satisfactorily than 

 the original equation, but it is also capable of application to 

 liquids. Moreover, since the constants a and b can be deter- 

 mined in a simple manner from the critical data (volume, 

 pressure, and temperature) or from the behaviour of the gases 

 under high pressure, van der Waals's equation furnishes a mode 

 of giving expression to the entire behaviour of all homogeneous 

 liquid and gaseous substances with respect to changes of pres- 

 sure, temperature, and volume ; and, on this account, it may 

 be regarded as of fundamental significance. Its accuracy has 

 been proved by Young Ga in particular. 



The theory of solution is based upon conceptions that have 

 arisen from the well-known experiments of Pfeffer, 7 which latter 

 only became possible after the discovery by Traube 8 of semi- 

 permeable membranes. 



In explaining osmotic pressure as the result of the impacts 

 of the dissolved molecules upon the walls of the vessel, van 't 

 Hoff arrives at a comparison between substances in the dis- 

 solved condition and in the state of gas. The laws of Boyle- 

 Mariotte, and of Henry-Gay-Lussac, as well as the fundamental 

 hypothesis of Avogadro, can now be applied directly to solu- 

 tions ; so that this branch, which has hitherto been one of the 

 most obscure in the whole subject of chemistry, at once becomes 

 fully accessible to investigation. As a consequence, important 

 results, which are capable of being turned to account throughout 

 the whole range of chemistry, are immediately obtained. 



The important relations subsisting between the depression 

 of freezing point, the diminution of vapour pressure, and the 

 elevation of boiling point on the one hand, and the molecular 

 weight of the dissolved substance on the other (which were 

 ascertained experimentally and formulated by Raoult 9 in 



6a Phil. Mag. [5] 33, 153 ; 34, 505. " Osmotische Untersuchungen, 

 Leipzig, 1877. 8 Archiv. f. Anat. u. Phys. 1867, 87. 9 Ann. Chim. [6] 

 2, 66, 99: 8, 289, 317; 2O, 297; Comptes Rendus. 87, 167; Z. physik. 

 Chem. 9, 343, etc. The literature of the predecessors of Raoult is very 

 fully given in Ostwald's Lehrbuch der Allgemeinen Chemie, Second 

 Edition, I, 705 and 741. 



