Atomic Laws of Thermochemistry . 27 



calories in the values which they obtain for the heat of 

 combustion of a gramme-molecule of the same substance, it 

 cannot be conceded that Thomsen's values attain an accu- 

 racy warranting the retention of his figures for hundredths 

 of kilocalories, especially as his desire to obtain heats of 

 combustion for substances only in the form of vapours con- 

 strained him to use experimental methods liable to greater 

 experimental error than belongs to other methods. Accord- 

 ingly Thomsen's numbers will be reproduced here only to 

 the nearest tenth of a kilocalorie for the gramme molecule. 

 But it must be remembered that even this degree of refine- 

 ment is only retained because, the experiments being all 

 carried out by the one experimenter, there is a chance that 

 even if absolute values are not correct to this degree the 

 differences for related bodies may often be correct to the 

 nearest tenth of a kilocalorie per gramme-molecule. In 

 calculating heats of formation from amorphous carbon and 

 the other elements as gases at 18° C. and a pressure of 

 one atmo from the heats of combustion, Thomsen uses for the 

 heat of formation of C0 2 the value 96'96 kcal., and for the 

 formation of liquid H 2 from its elements 68'36. To obtain 

 the heat of formation of the gramme-molecule at constant 

 volume, it is necessary to subtract *29(n--2) kcal., where n 

 is the number of atoms other than C in the molecule. It 

 must be remembered that the heat of formation of the mole- 

 cule of an organic compound is, as a rule, only a fraction of 

 its heat of combustion : for instance, the heat of combustion 

 of C 6 H U is about 1000, while the heat of formation is about 

 60 ; and this fact is the main cause of the difficulty in the 

 advance of organic thermochemistry, for a relatively small 

 error in a heat of combustion becomes relatively large in a 

 heat of formation. We shall have occasion to illustrate this 

 difficulty in the sequel by examples of the conflict of experi- 

 mental authority in values of heats of formation. 



In discussing Thomsen's results we will use the same 

 notation as in Part I. to express the same ideas, namely, that 

 the heat evolved on the combination of two atoms R and S 

 can be analysed into three parts : one (R) depending on R 

 only, another (S) depending on S only, and the third /(RS) 

 depending on both. At the very outset of organic thermo- 

 chemistry a most interesting question arises as to this mutual 

 action of atoms, for when three atoms R, S, and T react we 

 might expect to have the equation 



H(RST) = (R) + (S) + (T) +/(RS) +/(RT) +/(8T) ; 



but if, according to the theory of radicals, ST forms a radical 



