THE MOLECULAR COMPOSITION OF MATTER 83 



similar conditions of pressure and temperature, equal 

 numbers of molecules. As a matter of fact, Gay- 

 Lussac found that gases combined in simple proportions 

 by volume. He was unable, however, to reach the 

 conclusion that the number of molecules hi equal 

 volumes were equal, because of apparently conflicting 

 experimental evidence. For example, if equal volumes 

 of hydrogen and chlorine gas are combined the result- 

 ing volume under the same conditions of pressure and 

 temperature is found to be the sum of the original 

 volumes. If each hydrogen molecule combined with a 

 chlorine molecule there should be only as many mole- 

 cules as there were originally of either chlorine or 

 hydrogen, and hence we should expect the combination 

 to occupy only the volume originally occupied by either. 

 Avogadro, however, assumed that the number of 

 molecules must be proportional to the volume and, 

 since the volume of the compound was twice that 

 of each constituent, that there were twice as many 

 molecules of hydrochloric acid. But, since a mole- 

 cule of hydrochloric acid must contain a particle of hy- 

 drogen and a particle of chlorine, there must have 

 been originally twice as many particles of both hydrogen 

 and chlorine as there appeared to be from the volume 

 they occupied. If each particle of hydrogen really 

 consisted of two smaller particles, and similarly for 

 chlorine, then the experimental facts of the formation 

 of hydrochloric acid 1 would not be contradictory to his 



1 In chemical notation the formation of hydrochloric acid is ex- 

 pressed as H 8 +Clj=2HCl, indicating that each molecule of hydrogen 

 and of chlorine is diatomic and that the combination of a molecule 

 of each kind results in two molecules of hydrochloric acid. 



