LECTURE IV.] HISTORY OF CHEMISTRY. 6l 



Dalton shows, moreover, the poor agreement between the 

 results of Gay-Lussac and those of Henry, and this confirms 

 him in his conclusion that the former had not observed 

 accurately. It cannot be denied that Dalton's contention is 

 well founded. If the atomic theory was chosen as the basis for 

 chemical speculation, the law of gaseous volumes, as Gay- 

 Lussac stated it, could only be brought into harmony with it 

 if the assumption were admissible that equal numbers of the 

 smallest particles are present in the same volumes of all gases. 

 This assumption agreed with the physical properties of gases, 

 but was, as Dalton quite rightly concluded from known facts, 

 impossible. The three rules adopted by Dalton likewise told 

 against the accuracy of this hypothesis. In water, for example, 

 it would have been necessary to assume two atoms of hydrogen 

 for one of oxygen ; and it may be that this was also a reason 

 for Dalton's coming forward so decidedly in opposition to Gay- 

 Lussac. 



It will be clear from these explanations that there was a 

 real difficulty in bringing Gay-Lussac's law into harmony with 

 the atomic theory. Avogadro was the first to show how this 

 difficulty can be got over. 33 The Italian physicist distinguishes 

 molecules integrantes and molecules elementaires, which,, for 

 brevity and simplicity, we shall translate by molecule and atom 

 respectively. The physical properties of the gases (especially 

 the similarity in their behaviour towards changes of pressure 

 and of temperature) lead Avogadro to assume in equal volumes 

 of all gases, the same number of molecules ; and the distances 

 of the latter from one another he considers to be so great in 

 proportion to their masses, that they no longer exercise any 

 attraction upon one another. These molecules are not supposed, 

 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, sub- 

 stances (elements and compounds alike) are not converted, in 

 passing into the gaseous state, into indivisible particles, but 



33 Journ. de Phys. 73, 58 ; A.C.R. 4, 28, 



