APPLICATION OF THE MOLECULAR HYPOTHESIS 83 



every one of oxygen. Since two volumes of steam are produced, 

 evidently the two molecules of hydrogen and one of oxygen yield 

 two molecules of water. 



This hypothesis of Avogadro helps us to a clearer notion of how 

 these chemical changes take place. The idea that all gases con- 

 tain equal numbers of molecules in equal volumes also explains 

 why all gases behave alike when subjected to equal pressures, 

 or to equal changes in temperature (Boyle's and Charles' laws, p. 

 45). No facts which conflict with this hypothesis are known, and 

 all the known facts confirm it. Hence, Avogadro's hypothesis has 

 been accepted by chemists, and since 1858 has been the keystone of 

 chemical theory. 



Consequences of Avogadro's Hypothesis Molecular 

 Weights. Equal volumes of the same gas (at the same tem- 

 perature and pressure) have equal weights. But equal volumes 

 of different gases have different weights. The differences are often 

 very great. Thus, bulk for bulk, oxygen is sixteen times as 

 heavy as hydrogen, and mercury vapor one hundred times as 

 heavy. Now, if equal volumes of different gases contain equal 

 numbers of molecules, these differences must be due to the differ- 

 ing weights of the several kinds of molecules. Thus, measuring 

 the weights of equal volumes of different gases will give us the 

 relative weights of their molecules. For example, since 22.4 1. of 

 oxygen weigh 32 g. (p. 74), while the same volume of hydrogen 

 weighs 2.016 g. and of water vapor 18.016 g., and these are the 

 weights of equal numbers of molecules, the individual molecules 

 must differ in weight in the ratio 32 : 2.016 : 18.016. These are the 

 relative weights of the three kinds of molecules. In chemistry the 

 weights of 22.4 1. (at and 760 mm.) of various gases are called 

 the molecular weights of those gases. The unit quantities of 

 various substances are therefore spoken of, technically, as the 

 molecular weights of those substances. The unit volume, 22.4 1., 

 is called the gram-molecular volume (G.M.V.). 



