MOLECULES AND ATOMS 309 



of experiment (for example, equal to hundredths of the density), or 

 within a possible error owing to the laws of gases having an only 

 approximate significance (as is seen from the deviations, for instance, 

 from the law of Boyle and Mariotte), then the fraction proves but 



slightly different from 2 (between 1-9 and 2-2), and such cases 

 as these may be classed among those which ought to be expected 

 from the nature of the subject. It is a different matter if the quotient 



of 1 - be several times, and in general a multiple, greater or less than 



J. 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 



therefore take two such cases, and first one in which the quotient 4j 



i* i/reatcr than 2, or the density obtained by experiment is less than is 

 in accordance with the Itur. 



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 all events a large portion 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, whilst 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 Chap. 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 



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

 mental accuracy, even at 1000, when dissociation has certainly commenced, it must be 

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

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

 quotient M/D = 2'1, but at the high temperatures here concerned the error of experiment 

 is not greater than the difference of this quantity. And probably at 1000 the dissocia- 



