310 PRINCIPLES OF CHEMISTRY 



many substances which decompose with great ease directly they are 

 volatilised, and which only exist as solids or liquids, but not in a state 

 of vapour. There are, for example, many salts of this kind, besides all 

 definite solutions having a constant boiling point, all the compounds of 

 ammonia for example, all ammonium salts &c. Their vapour 

 densities, determined by Bineau, Deville, and others, show that they 

 do not agree with Gerhardt's law. Thus, the vapour density of 

 sal-ammoniac, NH 4 G1, is nearly 14 (compared with hydrogen), 

 whilst its molecular weight is riot less than 53-5, whence the 

 vapour density should be nearly 27 according to the law. The 

 molecule of sal-ammoniac cannot be less than NH 4 C1, because it is 

 formed from the molecules NH 3 and HC1, and contains single atoms of 

 nitrogen and chlorine, and therefore it cannot be divided ; it, further, 

 never enters into reactions with the molecules of other substances (for 

 instance, potassium hydroxide or nitric acid) in quantities of less than 

 53'5 parts by weight, &c. The calculated density (27) is here double 



the observed density (about 14), hence =4 and not 2. For this 



reason the vapour density of sal-ammoniac for a long time served as 

 an argument for doubting the truth of the law. But it proved other- 

 wise, after the matter had been fully studied. The low density 

 depends on the decomposition of sal-ammoniac, on volatilising, into 

 ammonia and hydrogen chloride. The observed density is not that of 

 sal-ammoniac, but of a mixture of NH 3 and HC1, which should be 

 nearly 14, because the density of NH 3 =8'5 and of HC^IS'S, and 

 therefore the density of their mixture (in equal volumes) should be 

 13'3. 13 The actual decomposition of the vapours of sal-ammoniac was 

 demonstrated by Pebal and Than by the same method as the decom- 

 position of water, by passing the vapour of sal-ammoniac through a 

 porous substance. The experiment demonstrating the decomposition 

 during volatilisation of sal-ammoniac may be made very easily, and is 

 a very instructive point in the history of the law of Avogadro-Gerhardt, 

 because without its aid it would never have been imagined that sal- 

 ammoniac decomposed in volatilising, as this decomposition bears all 

 the signs of simple sublimation ; consequently, the knowledge of the 

 decomposition itself was forestalled by the law. The whole force 



tion is far from being equal to 10 p.c. Therefore, the variation in the vapour denaifij 

 of water cannot be the means of ascertaining the amount of its dissociation. 



13 This explanation of the vapour density of sal-ammoniac, sulphuric acid, and 

 similar substances which decompose in being distilled was the most natural to resort to 

 as soon as the application of the law of Avogadro-Gerhardt to chemical relations 

 was begun; it was, for instance, given in my work on Specific volumes, 1850, 

 p. 99. The formula, M/D = 2, which later was applied by many other investigators, had 

 already been applied in that work. 



