MOLECULAE AND ATOMS 817 



ammoniac, NH 4 C1, is nearly 14 (compared with hydrogen), whilst 

 its molecular weight is not 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 nitro- 

 gen and chlorine, and therefore 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 (about 27) is here double 



the observed density (about 13-4) ; 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 investigated. 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 HC1=18'2, and 

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

 13*4. 13 The actual decomposition of the vapours of sal-ammoniac was 

 demonstrated by Pebal and Than by the same method as the deconi 

 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 aim 

 and practical use of the discovery of the laws of nature consists in, 

 and is shown by, the fact that they enable the unknown to be foretold, 

 the unobserved to be foreseen. The arrangement of the experiment 

 is based on the following reasoning. 14 According to the law and to 

 experiment, the density of ammonia, NH 3 , is 8|, and of hydrochloric 



13 This explanation of the vapour density of sal-ammoniacj 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 waa 

 begun ; it was, for instance, given in my work on Specific Volumes, 1856, p. 99. The 

 formula, M/D = 2, which was applied later by many other investigators, had already been 

 made use of in that work. 



14 The beginner must remember that an experiment and the mode in which it ia 

 carried out must be determined by the principle or fact which it is intended to illustrate, 

 and not vice versa, as some suppose. The idea which determines the necessity of an, 

 experiment is the chief consideration. 



