234 CHEMISTRY 



studies of this kind the chemists of the future must devote a large 

 part of their time. 



During the last twenty years the importance of physical chemistry, 

 or rather the recognition of its importance, has steadily increased, and 

 to-day it seems to dominate the entire field of chemical research. 

 Laboratories are equipped for its purposes alone, journals are de- 

 voted to it, and the activity of investigators has become so great 

 that subdivision has already begun, and men are known as thermo- 

 chemists, electro-chemists, and so on. Electro-chemical societies have 

 been formed and are prosperous; specialism is passing into subspe- 

 cialism; in short, chemistry is swiftly assuming an entirely new form. 



In the evolution of any science successes and disappointments 

 are almost equally influential; the former stimulating, the latter 

 tending to arrest research. The fruitful line is followed, and attracts 

 workers; the barren field is deserted or nearly so. Barrenness, how- 

 ever, may be due not to lack of fertility, but to premature effort; 

 and the truth which is beyond our reach to-day may drop into our 

 hands to-morrow. Thermo-chemistry, for example, has so far failed 

 to repay the labor spent upon it, and has fallen into disfavor; but 

 the future may tell a different story. Its importance is obvious, 

 and its general laws cannot elude discovery forever. The thermal 

 changes which accompany all chemical reactions must sometime be 

 interpreted. 



On the other hand, success has followed the physical study of 

 solutions, and thereby chemical theory has been enriched. First, 

 it was found that substances in solution exerted pressure - - a phe- 

 nomenon attended by depression of the melting-point and increased 

 temperature for boiling. This pressure resembled that observed in 

 gases, and a relation between the two was apparent. It was van 't 

 Hoff's privilege to trace the connection, and to develop a kinetic 

 theory of solutions. Avogadro's law was completely paralleled, 

 and equal volumes of solutions at equal osmotic pressures were shown 

 to contain equal numbers of molecules. For both laws, the liquid 

 and the gaseous, however, there were certain apparent exceptions, 

 which, for gases, were easily explained as the result of dissociation. 

 Arrhenius applied this explanation to the exceptional solutions, 

 taking into consideration also the ionic conceptions developed in the 

 study of electrolysis, and the abnormalities vanished. A salt in dilute 

 solution is electrically dissociated into its ions, which remain in 

 equilibrium although separate. From these generalizations several 

 important consequences followed. First, it became a simple matter 

 to determine the molecular weights of soluble substances --a class 

 of measurements that had previously been possible for gases alone. 

 Secondly, much light was thrown upon the subject of reactions 

 between dissolved salts, especially such as involve precipitation or 



