316 PRINCIPLES OF CHEMISTKY 



The application of the law must then be explained or it must be 

 laid aside, because the laws of nature admit of no exceptions. We will 



TVT 



therefore take two such cases, and first one in which the quotient - 



is greater than 2, or the density obtained by experiment is less than 13 

 in accordance with the law. 



It must be admitted, as a consequence of the law of Avogadro- 

 Gerhardt, that there is a decomposition in those cases where the volume 

 of the vapour corresponding with the weight of the amount of a 

 substance entering into reaction is greater than the volume of two 

 parts by weight of hydrogen. Suppose the density of the vapour of 

 water to be determined at a temperature above that at which it is 

 .decomposed, then, if not all, at any rate a large proportion of the water 

 will be decomposed into hydrogen and oxygen. The density of such .a 

 mixture of gases, or of detonating gas, will be less than that of aqueous 

 vapour ; it will be equal to 6 (compared with hydrogen), because 

 1 volume of oxygen weighs 16, and 2 volumes of hydrogen 2 ; 

 and, consequently, 3 volumes of detonating gas weigh 18 and 1 

 volume 6, while the density of aqueous vapour = 9. Hence, if the 

 'density of aqueous vapour be determined after its decomposition, the 



quotient would be found to be 3 and not 2. This phenomenon 



might be considered as a deviation from Gerhardt's law, but this would 

 not be correct, because it may be shown by means of diffusion through 

 porous substances, as described in Chapter II., that water is decomposed 

 at such high temperatures. In the case of water itself there can 

 naturally be no doubt, because its vapour density agrees with the law 

 at all temperatures at which it has been determined. 12 But there are 

 many substances which decompose with great ease directly they are 

 volatilised, and therefore only exist as solids or liquids, and 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- 



12 As the density of aqueous vapour remains constant within the limits of experi- 

 mental accuracy, even at 1,000, when dissociation has certainly commenced, it would 

 appear that only a very small amount of water is decomposed at these temperatures. 

 If even 10 p.c. of water were decomposed, the density would bo 8'57 and the quotient 

 M/D = 2 1 1, but at the high temperatures here concerned the error of experiment is not 

 greater than the difference between this quantity and 2 And probably at 1,000 the 

 dissociation is far from being equal to 10 p.c. Hence the variation in the vapc ur density 

 of water does not give us the means of ascertaining the amount of its dissociation. 



