Atomic Laws of Thermochemistry, 3 



Lid so as nearly as lie could. But considerable progress can 

 e made in the thermochemistry of carbon compounds without 

 ^ knowledge of the heat of vaporization of carbon ; for 

 many different types of compound involving the same number 

 of carbon atoms in their molecules can be studied, and in the 

 differences of their heats of formation the unknown quantity 

 disappears. Still, in striving for more comprehensive results, 

 Thomsen made certain assumptions in order to obtain, as a 

 single known quantity, the unknown latent heat of the carbon 

 molecule plus its heat of formation from atoms. These have 

 been a stumbling-block to many chemists on account of their 

 arbitrary nature ; and although Thomsen seems to have 

 abandoned them and the value of the latent heat plus heat of 

 formation of the gramme-molecule of carbon as unsound, a 

 certain distrust of even his sound conclusions still lingers. 

 In the second part of this paper Thomson's analysis of the 

 thermochemical data of carbon compounds will be restated in 

 a brief form, with removal of the few unwarranted and 

 unnecessary parts. 



But in the thermochemical data of inorganic compounds, 

 which will be studied in the first part of the paper, the cause 

 of the little progress that has been made in the discovery of 

 general principles amongst them is the fact that it has not 

 been possible to get them into the ideal state — that is, referred 

 to the gaseous condition of both the agents and the products. 

 Of course for a certain number of inorganic compounds the 

 data can be obtained for the gaseous state, but these have 

 been too few to give a clue to any general thermochemical 

 law ; and the data for the compounds of most of the metals 

 hitherto available are not pure thermochemical data at all, 

 but contain, as it were, unknown amounts of impurity in the 

 shape of unknown latent heats. It is obvious, therefore, that 

 the pure thermochemical laws cannot be discovered until 

 these latent heats are known : that is, for instance, until the 

 heat of combination of gaseous Na with gaseous Gl to produce 

 gaseous NaCl is known. Notwithstanding recent progress in 

 chemical manipulation at high temperatures and the promising 

 possibilities of the electric furnace, it may be some time yet 

 before actual experimental values of the latent heat of vapori- 

 zation of a metal like copper, or of a compound like sodium 

 chloride are available. But in the course of a series of 

 'researches on molecular force and of another research on a 

 kinetic theory of solids, I have been led to results which 

 enable approximate values of the latent heat of vaporization 

 v of the metals and their compounds to be calculated. The 

 ^details of the reasoning by which the methods of calculation 



B 2 



