CHAPTER VIII 

 APPLICATION OF THE MOLECULAR HYPOTHESIS IN CHEMISTRY 



Avogadro's Hypothesis. We have seen (p. 48) that the 

 physical behavior of matter, and particularly of gases, may be 

 explained by the conception that matter is composed of molecules. 

 The easy compressibility of gases is, under this view, a consequence 

 of the smallness of the molecules and the relative vastness of the 

 empty space between them. The pressure exerted by a gas is 

 regarded as due to the innumerable blows which the molecules 

 deliver when they strike the boundary walls of the space in which 

 the gas is confined. Now, when gases interact chemically, the 

 volumes required for complete interaction are either equal or 

 stand m the ratio of small whole numbers (Gay-Lussac's law, 

 PL. 72). Although that is a chemical, and not merely a physical 

 fact, the molecular hypothesis can explain it also. 



Since matter is composed of molecules, a chemical action 

 between two kinds of matter must consist really in an interaction 

 of the molecules of each kind. Molecules of the two kinds must 

 meet and they may then either combine to form a compound 

 molecule, or they may exchange material in some fashion. Since 

 equal volumes are often the exact quantities required for the 

 action, it appears most likely that in equal volumes of different 

 gases (at the same temperature and pressure) the numbers of 

 molecules present are equal. This addition to the molecular 

 hypothesis was first suggested by an Italian physicist, Avogadro 

 (1811). When two gases interact in equal volumes (like hydrogen 

 and chlorine), one molecule of each is all that is required for a 

 small sample of the change in question. Since two volumes of 

 hydrogen are required to unite with one volume of oxygen (p. 60), 

 clearly the interaction involves two molecules of hydrogen for 



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