EXPERIMENTAL KKOWLEDGE OF THE rROPEETIES OF MATTER. 137 



and of some volatile compounds of these and of otLer less volatile elements. 

 His object was to deduce from his results, by applying to them the atomic 

 theory as expounded by Dalton and the law of Ampere and Avogadro 

 as to the distx'ibution of the molecules of a gas or gasitied body, the mole- 

 cular weights of both the compounds and the elements. In this Dumas was 

 only partially successful, because with Dalton he made the tacit assumption 

 that in the case of elementary substances there was no distinction between 

 an atom and a molecule. Chemists before Dumas' time had taken no account 

 of the law of Avogadro and Ampere in their endeavours to determine the 

 true formula3 of compounds and the atomic or molecular weights to be 

 assigned to elements, with the exception of Gay-Lussac, who was guided 

 by some adumbration of this law in his investigations into the volume 

 relations of bodies composed of gaseous components. 



In the investigation which Dumas records in this paper he not only 

 recognises this law, but takes it as the foundation of the reasoning he-, 

 applies to his experimental results ; thus inaugurating a method of 

 chemical research which was afterwards renewed by Gerhardt in 1843 and 

 carried by him to a more successful conclusion, for Gerhardt was not only 

 able to show how formulae for compounds and especially for very numerous 

 carbon- compounds were consistent with Avogadro's law, but to include 

 the molecules of volatile elements also under the self-same law. These 

 chemical consequences derived from this law are not anticipated by 

 Dalton's atomic theory, and without some such physical conception of the 

 constitution of matter in the gaseous state we could not have had any 

 reason to suppose that the weights of substances in this state in equal 

 volumes were in any relation to the chemical formulee. But the facts, 

 numerous as they were, which Gerhardt found to show this relation have 

 since the publication of his memoir up to the present time been increased 

 to a vast extent ; so that it is beyond question that Avogadro and Ampere 

 expressed, with reference to the number of molecules in a given volume, 

 in the case of bodies in the state of perfect gas, a law which is approxi- 

 mately true of vapours of bodies at temperatures far removed from the 

 point of liquefaction, and which not only physicists can use with safety 

 in explaining physical properties but chemists to find true chemical' 

 formulae. 



There are, it is true, cases of apparent exception, but on examination 

 it is found that in these cases the body, of which we are trying to find the 

 formula by this law, has wholly or partly ceased to exist in the circum- 

 stances of the experiment, being rejjlaced by two or more other bodies 

 resulting from decomposition of the original. The applicability of 

 Avogadro's law which is thus shown, depends of course on the approximate 

 truth of Boyle's and Gay-Lussac's laws, which as approximations are thus 

 indirectly confirmed ; and V. Meyer and others have confirmed these 

 laws by their results for very high temperatures, not only in the cases of 

 hydrogen, oxygen, and nitrogen, but in the cases of mercury, mercuric 

 chloride, arsenicum, phosphorus, arsenious oxide ; aluminium chloride, 

 bromide, and iodide, indium chloride, antimonious oxide, cupric chloride, 

 and cadmium. Moreover, in many other cases where the results do not 

 seem in accordance with these laws at high temperatures we have signs 

 of a decomposition, while in other such cases the result has been shown 

 to be capable, without any straining of the facts, of simple explanation 

 by supposing a molecule to be split into molecules of half the mass and 

 represented by halving the formulae, e.g., in the case of Sn2Cl4 which ap- 



