676 POPULAR SCIENCE MONTHLY. 



physics. ' In 1805 Gay-Lussac and Humboldt determined the com- 

 position of water by volume; in 1808 Gay-Lussac extended these 

 observations, and found that in all compound gases simple volumet- 

 ric relations existed; and in 1811 the entire subject was general- 

 ized into Avogadro's law. Avogadro showed that equal volumes 

 of gases, compared under equivalent conditions, must contain equal 

 numbers of molecules, and although the force of his discovery 

 was not fully appreciated until much later, it is now recognized as 

 one of the fundamental propositions of both physics and chemistry. 

 For the first time the distinction between atoms and molecules was 

 clearly stated, and from the density of a gas the relative weight 

 of its molecule could be calculated. Avogadro's law rounded out 

 and completed the atomic theory, and to its application much of 

 the advance in organic chemistry is due. Equally striking, but 

 less far-reaching in its consequences, was the discovery announced 

 by Dulong and Petit in 1819, when it was shown that the specific 

 heat of an element was inversely proportional to its atomic weight. 

 Otherwise stated, this law asserts that the atoms of all the elements 

 have the same capacity for heat, and an important check upon 

 determinations of atomic weight was thus provided. 



The next twenty years in the history of chemistry were years 

 of detail rather than of permanent generalizations. The multi- 

 tudinous verification of known laws, the development of experi- 

 mental methods, especially methods of analysis, the discovery of 

 new elements, the preparation of numberless new compounds, occu- 

 pied the attention of most workers. This period, which may be 

 called the Berzelian period, was enormously fruitful in results, 

 although but few of the theories then proposed have survived to 

 the present day. During this period the name and influence of 

 Berzelius overshadowed all others, and his marvelous researches, 

 carried out in a laboratory which was hardly more than a kitchen, 

 were of almost incredible variety. For the crude symbols of Dal- 

 ton, Berzelius substituted a system of chemical formulae which 

 could be used in chemical equations; in 1818 and 1826 he published 

 tables of atomic weights, determined with far greater exactness than 

 ever before; he discovered five new elements and a multitude of 

 compounds, devised methods of research, and proposed theories 

 which, though later to be overthrown, for many years dominated 

 chemical science. His electro-chemical experiments led him to his 

 dualistic theory of compounds, which interpreted each compound 

 as made up of two parts — one positive, the other negative. The 

 electro-positive oxides were basic, the electro-negative groups were 

 acid; chemical affinity was electrical attraction between the two 

 opposites; chomioal union implied a neutralization of one by the 



