36 Mr. W. Sutherland on the Fundamental 



different types of compound included in his data, for as we 

 know the thermal effect due to the addition of CH 2 , we need 

 only discuss the types from which all the homologous series 

 are derived by additions of CH 2 . 



Heat of Formation of Typical Compounds. 



1. Hydrocarbons. — The general formula for these is C a H 26 , 

 and it is to be remembered that Thomsen adds a X 38*4 to the 

 heat of formation of C a H 2& from amorphous solid carbon, 

 which he denotes by p, the sum being denoted by p + ad, and 

 then he writes : — 



p + ad=2br + tv = b30 + nU'2, 



where %v stands for the sum of the thermal values of all the 

 bindings between the carbon atoms, n being the number of 

 single and double bindings. In our notation we have 



p + ad=H.{G a solid, H 26 )-a(Csolid) = H(C a ,H 26 )-a(C) 



=26/(CH) + S/(CC) (26) 



When the heat of formation of a hydrocarbon is known 

 this equation enables us to determine the total number of 

 single and double bindings in the molecule, and in this way 

 it is competent to throw light on the constitution of the 

 molecule. It happened rather unfortunately that Thomsen 

 chose benzene as a good subject on which to illustrate the help 

 that the thermochemical equation can give in determining 

 the constitution of a molecule, arriving at the conclusion that 

 the six carbon atoms of the benzene molecule must be joined 

 by nine single bonds represented by the sides and diagonals 

 of a hexagon ; because Stohmann was able to point out that 

 his own and Berthelot's values for the heat of combustion of 

 benzene agreed in being less than Thomsen' s by some 12 kcal., 

 so that the heat of formation p, and therefore p + ad, should 

 be greater by 1 2 than they were taken to be by Thomsen ; but 

 12 is nearly the 14 which Thomsen takes as the value of a 

 single and double binding, so that his system of nine bindings 

 is possibly in error by the value of one binding ; but at a 

 later date Thomsen, in discussing some data of Stohmann for 

 the benzene, naphthalene, and anthracene groups, treats the 

 benzene nucleus on the Kekule plan, that is with three 

 double bindings and three single, and of course has to assign 

 a larger value to the double binding in aromatic bodies than 

 in aliphatic : but it seems to me to be undesirable first 

 to force the bonds phraseology upon thermochemistry and 



