308 PRINCIPLES OF CHEMISTRY 



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 the 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 the formula and experiment, 

 of nitrous oxide, N 2 0, is 22, of nitric acid 15, and of nitric peroxide 23. 

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

 into NO + N0 2 , the density should vary between 38 (so long as the 

 N 2 O 3 remains unchanged), and 19 (when NO + NO 2 is obtained). 

 There are no figures of constant density for H.,O 2 , NHO 3 , N 2 O 4 , and 

 many similar compounds which are either wholly or partially decom- 

 posed in passing into vapour. Salts and similar substances either have 

 no vapour density because they do not pass into vapour (for instance, 

 potassium nitrate, KNO 3 ) without decomposition, or if they pass 

 into vapour without decomposing, their vapour density is observed 

 with difficulty only at very high temperatures. The practical de- 

 termination of the vapour density at these high temperatures (for 

 example, for sodium chloride, ferrous chloride, staniious chloride, etc.), 

 requires special methods which have been w 7 orked out by Sainte-Claire, 

 Deville, Crafts, Nilson and Pettersson, Meyer, Scott, and others. 

 Having overcome the difficulties of experiment, it is found that the 

 law of . Avogadro-Gerhardt holds good for such salts as potassium 

 iodide, beryllium chloride, aluminium chloride, ferrous chloride, <tc. 

 that is, the density obtained by experiment proves to be equal to half 

 the molecular weight naturally within the limits of experimental 

 error or of possible deviation from the law. 



Gerhardt deduced his law from a great number of examples of 

 volatile carbon compounds. We shall become acquainted wdth certain of 

 them in the following chapters ; their entire study, from the complexity 

 of the subject, and from long-established custom, forms the subject 

 of a special branch of chemistry, termed 'organic ' chemistry. With all 

 these substances the observed and calculated densities are very similar. 



When the consequences of a law are verified by a great number of 

 observations, it should be considered as confirmed by experiment. But 

 this does not exclude the possibility of apparent deviations. They may 



evidently be of two kinds : the fraction - may be found to be either 



greater or less than 2 that is, the calculated density may be either 

 greater or less than the observed density. When the difference between 

 the results of experiment and calculation fall within the possible errors - 



