ioo CHEMICAL PROPERTIES AND COMPOSITION 



pounds, is not available in the region of organic chemistry 

 on account of the in volatility of carbon, hence we are driven 

 to indirect methods. 



In the first place, isomers have nearly the same heat of 

 combustion, e. g. 



propyl alcohol 498-6, isopropyl alcohol 493-3 5 

 whilst the failure of this equality in the case of acetic acid, 

 acetic acid 225-4, methyl formate 248-1, 



is partly to be referred to the double molecules existing in 

 the vapour, in whose dissociation heat is absorbed, and the 

 heat of combustion accordingly reduced. 



Equal differences in composition correspond approxi- 

 mately to equal differences in heat of combustion, such as 

 those shown in a homologous series (the hydrocarbons are 

 burnt in the form of gas or vapour, the water collected 

 liquid) : 



CH 4 C 2 H 6 C 3 Hg C 4 H 10 C 5 H 12 



211-9 370-4 5^9-2 687-2 847-1 



difference 158-5, 158-8, 158, 159-9- 



Passing now from heat of combustion to heat of forma- 

 tion, by means of the law that the heat of a reaction is 

 the sum of the heats of formation of the products diminished 

 by the corresponding sum for the reacting substances, we 

 get from the heat of combustion of CH 4 : 



CH 4 + 2O 2 = C0 2 + 2H 2 O + 2ii-9 calories, 

 211-9 = (C.0 2 ) + 2(H 2 .0)-(C.H 4 ), 



in which (C.0 2 ) and (H 2 .O) stand for the heat evolved in 

 the formation of 44 grams of carbon dioxide and 18 grams 

 of water from the elements : these are (carbon as diamond) 



(0.0,) = 94-3, (H 2 .0) = 6 7 . 4) 



so that 



(C.HJ = 94-3 + 134-8-21 1-9= 17-2, 



or the formation of 16 grams of methane from diamond and 



