314 PRINCIPLES OF CHEMISTRY 



of vapour, according to the law, a volume equal to that occupied by the 



molecules of other bodies, be indicated by the letters M,, M 2 



or, in general, M, and if the letters D,, D 2 , or, in general, D, 



stand for the density or' weight of a given volume of the gases or 

 vapours of the corresponding substances under certain definite con- 

 ditions of temperature and pressure, then the law requires that 



M,_M 2 M p 



B; is-; 1 = > = 



where is a certain constant. This expression shows directly that the 



volumes corresponding with the weights M,, M 2 M, are equal 



to a certain constant, because the volume is proportional to the weight 

 and inversely proportional to the density. The magnitude of C is 

 naturally conditioned by and dependent on the units taken for the 

 expression of the weights of the molecules and the densities. The 

 weight of a molecule (equal to the sum of the atomic weights of 

 the elements forming it) is usually expressed by taking the weight 

 of an Atom of hydrogen as unity, and hydrogen is now also chosen 

 as the unit for the expression of the densities of gases and vapours ; 

 it is therefore only necessary to find the magnitude of the constant 

 for any one compound, as it will be the same for all others. Let us 

 take, water. Its reacting mass is expressed (conditionally and 

 relatively) by the formula or molecule H 2 0, for which M = 18, if H=l, 

 as we already know from the composition of water. Its vapour 

 density, or D, compared to hydrogen = 9, and consequently for water 

 C = 2, and therefore and in general for the molecules of all substances 



M =2 

 D 



Consequently the weight of a molecule is equal to twice its vapour 

 density expressed in relation to hydrogen, and conversely the density of 

 a gas is equal to half the molecular weight referred to hydrogen. 



The truth of -this may be seen from a very large number of 

 observed vapour densities by comparing them with the results obtained 

 by calculation, As an illustration, we may point out that for ammonia, 

 NH 3 , the weight of the molecule or quantity of tho reacting sub- 

 stance, as well as the composition and weight corresponding with the 

 formula, is expressed by the figures 14 + 3 = 17, Consequently M = 17. 

 Hence, according to the law, D = 8*5. And this result is also obtained 

 by experiment. The density, according to both formula and experiment, 

 of nitrous oxide, N 2 O, is 22, of nitric acid 15, and of nitric peroxide 23. 

 In the case of nitrous anhydride, N 2 3 , as a substance which dissociates 

 into NO -f N0 2 , the density should vary between 38 (so long as th$ 



