Atomic Laws of Thermochemistry. 29 



may be taken that the heat of combustion of CH 2 is constant, 

 and therefore the heat of formation and attachment of CH 2 

 to a nucleus is to be taken as constant. Let CH 2 be attached 

 to any nucleus X, then the heat of formation of XCH 2 is 



H(X, C, H 2 ) = (X) + (C) + 2(H) +/(X0) + 2/(GH) , 



where (X) is the heat of formation of X, so that for the heat 

 of formation and attachment of CH 2 to X we have 



(C) + 2(H)+/(XC)+2/(CH), 



which we have just seen is independent of X; so that/(XC) 

 is independent of X — that is, it is constant ; and as the CH 2 

 in homologous series is always attached to a C atom in the 

 nucleus X in such a way as to increase the total number of 

 single bindings between carbon atoms by one, we can write 

 the constant /(XC) as /(C*C), the dot denoting a single 

 binding. The heat of combustion of CH 2 in a compound is 

 therefore 



3(0) +/(C0,) +/(H 2 0-/(0 • C) -2/(0H) ; 



and taking the mean value 157*9, we have 



3(0)+/(C0 2 ) +/(H 2 0) -/(C-U)-2/(0H) = 157-9. (1) 



2. 'Equality of the four Affinities of Carbon. — The four 

 bonds of carbon are proved to have the same thermal value 

 by the fact that the successive replacement of the four H 

 atoms of CH 4 to produce CH 3 CH 3 , CH 2 (CH 3 ) 2 , CH(CH 3 ) 3 , 

 and C(0H 3 ) 4 causes a constant increase in the heat of 

 combustion at each replacement. 



3. The Heat of Combustion of Carbon in its Compounds. — 

 By comparing the heats of combustion of such saturated and 

 unsaturated molecules as CH 4 and C 2 H 4 , or C 2 H 5 OH and 

 C 3 H 5 OH, namely 211-9 for CH 4 and 333*3 for C 2 H 4 , it is 

 shown that the heat of combustion of the surplus carbon 

 atom is on the average 121*1, while the heat of combustion 

 of a gramme-atom of solid amorphous carbon is 96*96. The 

 difference between the two cases is that in the first the carbon 

 atom, according to Thomsen's mode of expression, must be 

 separated from a gaseous molecule by breaking a double 

 binding and then combined with oxygen ; while in the 

 second case the atom has first to get its share of latent heat 

 for the vaporization of the molecule and then the heat neces- 

 sary to break it away from the molecule, after which it is 

 combined with oxygen with the evolution of heat, the alge- 

 braic sum of all the thermal effects being 96*96 in this case 

 and 121*1 in the former. Thomsen denotes the heat of com- 

 bustion of the supposed isolated carbon atom by the symbol 



