62 HISTORY OF CHEMISTRY. [LECTURE IV. 



only into molecules integrantes, which in turn are composed of 

 molecules elementaires. He bases his view upon the following 

 considerations : If nitric oxide, which aiises without contrac- 

 tion from equal volumes of nitrogen and oxygen, contains as 

 many molecules as the mixed gases do, then the combination 

 cannot consist in the union with one another of previously 

 separate molecules, since this would necessarily involve a 

 diminution in the number of particles ; but it must be brought 

 about by an exchange. The molecules both of nitrogen and 

 of oxygen must split into two parts, and these then combine by 

 mutual exchange. 



While, therefore, before the combination, the gaseous 

 mixture consists of dissimilar molecules, of which the one half 

 are composed of two atoms of nitrogen, and the other half of 

 two atoms of oxygen, the product of the combination consists 

 of the same number of molecules, but they are all like molecules 

 and each has been formed by the union of one atom of nitrogen 

 with one of oxygen. Consideration of the volume relations in 

 the formation of ammonia, likewise points to a subdivision of 

 the gas particles of elementary substances. All these discus- 

 sions assume the simplest character when the word molecule 

 (molecule integrante) is employed instead of volume, the two 

 conceptions being identical for the gaseous state, according to 

 Avogadro's definition. From Gay-Lussac's numbers, then, it 

 appears that the molecule of ammonia consists of half a mole- 

 cule of nitrogen and one and a half molecules of hydrogen ; 

 that the molecule of water contains half a molecule of oxygen 

 and one molecule of hydrogen, etc. If the simplest hypothesis 

 with respect to the divisibility of the molecule is adopted, so 

 that it is not necessary to introduce fractions of atoms, then the 

 molecules, not only of hydrogen, but also of oxygen and of 

 nitrogen, must consist of two elementary atoms ; and the 

 proportions, by volume, in which the gases combine, then give 

 the number of chemical smallest particles which go to form the 

 molecule. Avogadro finds, for example, that two atoms of 

 hydrogen and one of oxygen are necessary for the formation of 

 water ; that three atoms of the former gas and one of nitrogen 



