102 CHEMICAL PROPERTIES AND COMPOSITION 



tion between these two elements in which double or triple 

 linkages are involved. 

 For ethylene H 2 C = CH 2 



the above expression leads to 2x2-9 = 5-8. I however, 

 we change it on account of the double carbon linkage, 

 putting 



(C = C) = 10 4(0 H) = -10-4x4-3 = 27-2, 

 all the hydrocarbons with double linkage are brought 

 within the domain of the formula. 



In the same way for triple linkages, we have from the 

 value for acetylene HC = CH 



[C*.Hj=-53-*, 

 (C = C) = -53-*-*(C-H) = -53-3-3X4-3 = -61-8. 



The correctness of the expression so obtained may be 

 tested by a provisional determination of constitution : we 

 will choose benzene as example. 



For the isomer dipropargyl, with the constitution 



HC = C . CH 2 .CH 2 .0 = CH, 



it is found that 



[C 6 .H 6 ]=- 113-4, 



with which calculation gives good agreement 



2 (CEEC) + 3(C-C) + 6(C-H) = -2x61.8-3x5-7 



+ 6x4-3= 114-9. 

 For benzene the heat of formation is 

 [C 6 .H 6 ]=-i 7 .T, 



and since 6(C H) = 6x4-3 = 2 5'^> there remains for the 

 carbon linkages 



-17-1-25.8 = -42-9. 



If the linkage is alternately single and double we should 

 expect 



3 (C=C) + 3(C-C) = 3 (-27-a-5-7) - -98-7, 



