AMADEOAVOGADRO 179 



on combination, which would appear to be otherwise inexplicable. 

 Setting out from this hypothesis, it is apparent that we have the 

 means of determining very easily the relative masses of the molecules 

 of substances obtainable in the gaseous state, and the relative number 

 of these molecules in compounds ; for the ratios of the masses of the 

 molecules are then the same as those of the densities of the different 

 gases at equal temperature and pressure, and the relative number of 

 molecules in a compound is given at once by the ratio of the volumes 

 of the gases that form it. For example, since the numbers 1. 10359 

 and 0.07321 express the densities of the two gases oxygen and hydro- 

 gen compared to that of atmospheric air as unity, and the ratio of 

 the two numbers consequently represents the ratio between the masses 

 of equal volumes of these two gases, it will also represent on our 

 hypothesis the ratio of the masses of their molecules. Thus the mass 

 of the molecule of oxygen will be about 15 times that of the molecule 

 of hydrogen, or, more exactly, as 15.074 to i. In the same way the 

 mass of the molecule of nitrogen will be to that of hydrogen as 

 0.96913 to 0.07321, that is, as 13, or more exactly 13.238, to i. On 

 the other hand, since we know that the ratio of the volumes of hydro- 

 gen and oxygen in the formation of water is 2 to i, it follows that 

 water results from the union of each molecule of oxygen with two 

 molecules of hydrogen. Similarly, according to the proportions by 

 volume established by M. Gay-Lussac for the elements of ammonia, 

 nitrous oxide, nitrous gas, and nitric acid, ammonia will result from 

 the union of one molecule of nitrogen with three of hydrogen, nitrous 

 oxide from one molecule of oxygen with two of nitrogen, nitrous gas 

 from one molecule of nitrogen with one of oxygen, and nitric acid 

 from one of nitrogen with two of oxygen. 



11. 



There is a consideration which appears at first sight to be opposed 

 to the admission of our hypothesis with respect to compound sub- 

 stances. It seems that a molecule composed of two or more elemen- 

 tary molecules should have its mass equal to the sum of the masses 

 of these molecules ; and that in particular, if in a compound one mole- 

 cule of one substance unites with two or more molecules of another 

 substance, the number of compound molecules should remain the same 



