RELATIONS TO OTHER SCIENCES 245 



side the heat set free. By weighing the simple substances that were 

 combined and also the compound produced, he definitely established 

 the weight equation of the chemical reaction. By measuring with the 

 calorimeter the amounts of heat set free, he separated ponderable 

 matter from the imponderable agents. All these views were, in addi- 

 tion, logically bound up with each other, and it would have been 

 impossible to study properly the phenomena of combustion if he 

 had not been forming an exact idea of the passage of a body from 

 the solid to the liquid and gaseous states. 



We need not here recall his memorable experiments on the com- 

 position of air and of water, on the increase of the weight of metals 

 during their oxidation, on the phenomena of combustion, respiration, 

 and the production of animal heat, and on fermentation, or finally 

 his creation of the nomenclature. These new ideas overthrew the 

 theory of phlogiston. They brought light into the midst of the labori- 

 ous researches of the alchemists, they prepared the way for organic 

 and physiological chemistry, they gave rigor and exactitude to 

 chemical reactions. In a word, they established chemistry in the 

 position of a science. 



Starting from this epoch, we can divide into three great periods 

 the numerous researches that were pursued in different countries. 

 In the first period, the modern idea of elements takes shape; in the 

 second, the chemical laws are established; and in the third, the 

 atomic weights of the elements are determined. 



The first period includes the studies of a great number of investiga- 

 tors, but among them four names emerge above all others, Scheele, 

 whose chemical genius enriched our science; Priestley, a mind at once 

 original and conservative; Cavendish, whose analyses have never been 

 surpassed; and finally Humphry Davy, w r ho, by the discovery of 

 the metals of the alkalies and alkaline earths, explained the compo- 

 sition of the earths and won definitive acceptance for the conception 

 of elements. 



The second period presents to us the legislators of our science. 

 Wenzel, following up the work of Rouelle, gives precision to the 

 knowledge of salts and of double decompositions. Richter pub- 

 lishes the first tables of neutralization for acids and bases. Proust 

 formulates the law of the constancy of proportions (1806); and 

 Dalton, at the same time, gives a complete exposition of the law of 

 multiple proportions, a first sketch of which he had presented, in 

 1803, to the literary and scientific society of Manchester. As we 

 shall see further on, the importance of Dalton's law was not appre- 

 ciated at its full value till much later. Finally, in 1808, Gay Lussac 

 indicated the laws, so simple, of the combination of gases. By their 

 promulgation, Gay Lussac gave to the concept of combination a 

 truly mathematical exactitude. 



