246 INORGANIC CHEMISTRY 



After that, the study of the weight of the different elements which 

 enter into combination could be pursued with success, especially 

 when Mitscherlich's law of isomorphism (1819) and Dulong and 

 Petit's law of specific heat (1819) became known. In this third period, 

 in which experimental precision was carried to its furthest limits, 

 alongside of the researches of Victor Regnault, Faraday, Marignac, and 

 many others, the most important works published on the subject 

 that we are considering are those of Berzelius, Dumas, and Stas. 



The magnificent effort of Berzelius provided a study, as complete 

 as possible, of most of our simple substances. This line of experiment 

 was taken up with the greatest care by Dumas, who first determined 

 the composition, by weight, of water and of air; and then by means 

 of simple and elegant experiments gave us a certain number of atomic 

 weights, among them that of carbon, the pivot of all organic chem- 

 istry. Stas next took up the study of these questions, and, d pro- 

 pos of William Prout's hypothesis of the unity of matter, showed 

 clearly that the atomic weights are not simple multiples of unity. 

 Stas's experiments will remain in our science as models of precision. 



During this splendid period, which requires about a century, the 

 theories by which we bind together the innumerable details of our 

 science had time to change more than once. 



We have already seen how Lavoisier's ideas replaced the theory 

 of phlogiston. Later, Humphry Davy, after his splendid discoveries, 

 assigned a predominating role to electricity and created the electro- 

 chemical theory, which was adopted and modified by Berzelius. 

 Then came the investigations of vapor density, and, after prolonged 

 discussions, many chemists abandoned the numbers of Berzelius, and 

 followed the so-called notation of equivalents, proposed by Wollas- 

 ton and adopted by Gmelin, Liebig, and Dumas. But soon Gerhardt, 

 considering as equivalents the quantities of hydrochloric acid, water, 

 and ammonia, which correspond to equal volumes, proposed a sys- 

 tem of atomic weights, which won as adherents: in France, Laurent, 

 Wurtz, Friedel; in England, Williamson, Frankland; in Germany, 

 Hofmann, Kekule", Baeyer; in Italy, Cannizaro. The hypothesis of 

 Avogadro and Ampere took on new life, and a sharp distinction 

 between atoms and molecules made possible a reconstitution of the 

 atomic theory on the basis of the great law of Dalton. 



Considerably before this time chemistry was divided into two 

 great chapters: inorganic and organic. 



The study of organic chemistry had begun with the investigations 

 of Lavoisier. During the succeeding century and more, chemists 

 tried first of all to isolate the proximate principles of vegetables and 

 animals. These studies, pursued on all sides with varying success, 

 endowed chemistry with a great number of clearly defined com- 

 pounds, some of which possessed important therapeutic properties. 



