A CENTURY OF CHEMISTRY. 91 



posed, however, to constitute the ultimate particles 

 of matter, but are assumed to be capable of further 

 subdivision under the influence of chemical forces. 

 According to Avogadro, therefore, substances (ele- 

 ments and compounds alike) are not converted, in 

 passing into the gaseous state, into indivisible par- 

 ticles, but only into molecules integrantes, v^hich in 

 turn are composed of molecules elementaires.^^ * 

 The conception of a molecule is that of the smallest 

 portion of a substance which possesses all the prop- 

 erties of that substance; it represents a higher cate- 

 gory than atom ; thus the molecule of water is repre- 

 sented by the symbol HgO, which means, in part, 

 that the smallest particle of water consists of two 

 atoms of hydrogen united with one atom of oxygen. 



Avogadro's generalisation has furnished one of the 

 main grounds for determining the atomic weights 

 of the elements ; and it went far to reconcile Gay- 

 Lussac's discoveries as to gases with Dalton's atomic 

 theory. We have only space to mention that another 

 ground for the determination of atomic weights was 

 furnished by the researches of Dulong and Petit 

 (1818), who showed the close relation between the 

 specific heats of the elements and their atomic 

 weights, and concluded that the atomic heats of all 

 elements (specific heats multiplied by atomic 

 weights) are practically identical ; i.e., that all atoms 

 have the same capacity for heat. 



Avogadro's recognition of the proportion between 

 the specific gravity of a gas and its molecular weight 

 was slowly appreciated,f but it has borne much fruit. 

 *Ladenburg, 1900, pp. 61-62. 



f Dr. J. T. Merz notes in regard to this belated recogni- 

 tion that Avogadro's hypothesis (1811) is not mentioned in 

 Whewell's History, nor in Kopp's (1843-1847), nor in Pog- 

 gendorf's Dictionary (1863). 



