THE ATOMIC VIEW OF NATURE. 



429 



atom or complex was termed a molecule, and it was 

 assumed that molecules, or smallest individual particles 

 of chemical substances, might be made up of one or more 

 atoms of the same or of different substances. Avogadro 

 was able in this way to explain how a certain number of 

 molecules of hydrogen each made up of two atoms 

 combine with an equal number of molecules of chlorine ; 

 these being likewise composed of two atoms of chlorine, 

 in order to form an equal number of molecules of hydro- 

 chloric acid, each of these consisting of two atoms viz., 

 one of chlorine and one of hydrogen. This view, which 30. 

 Ampere likewise adopted, did not recommend itself to same? 

 chemists for many years ; not indeed till, about the year 

 1840, several eminent chemists notably Laurent were 

 independently led to consider chemical compounds as 

 formed by what is termed substitution instead of simple 

 combination. 1 For, according to Avogadro's view, the for- 



ture. They then found that these 

 quantities stood very nearly in in- 

 verse proportions to the atomic or 

 combining numbers. They at the 

 same time pointed out the un- 

 certainty which in consequence 

 of the law of fixed multiple pro- 

 portions existed regarding the 

 smallest figure which was to deter- 

 mine the combining weights ; they 

 chose those numbers which brought 

 out clearly the physical regularity 

 and coincidence which they had dis- 

 covered ; and they expressed their 

 result in the rule that the atoms of 

 all elementary bodies have the same 

 capacity for heat. Whereas Ber- 

 zelius ignored the theoretical dis- 

 cussions of Avogadro and Ampere, 

 he hailed the experimental data 

 of Dulong and Petit as most useful 

 in helping to fix correctly the real 

 equivalent numbers, a task to which, 



as the fundamental requisite of all 

 chemistry, he devoted so much time 

 and labour. It must, however, be 

 noted that the law of Dulong and 

 Petit, now universally accepted as 

 a fundamental fact in the atomic 

 theory, is, as little as the laws of 

 Boyle, Charles, and Gay - Lussac, 

 rigidly correct : it obtains within 

 certain limits. The experiments 

 of Dulong and Petit were extended 

 to compounds by F. Neumann in 

 1831. The connection of the specific 

 heat or thermal capacity of com- 

 pounds and that of their con- 

 stituents was fully investigated by 

 Regnault. A statement of the diffi- 

 culties and anomalies which still 

 exist will be found in L. Meyer's 

 ' Die Atome und ihre Eigenschaften' 

 (p. 73, &c.) _ 



1 A very important influence in 

 contributing to the gradual recog- 



