COAL: ITS COMPOSITION 117 



B.t.u. This is almost exactly the heat developed when 

 oxygen is destroyed by gaseous carbon. 



In each case three volumes of gas become two volumes, 

 so there is no difference due to a different degree of con- 

 densation. Let there be next taken the heat of combustion 

 of a series of hydrocarbons: C 2 H 2 , C 2 H 4 , CH 4 , C 2 H 6 and 

 C 6 H 6 . These are shown in the second column in B.t.u. per 

 pound of the hydrocarbon. 



B.t.u. B.t.u. 



C 2 H = 21,850 X 26/80 = 7,003 

 C 2 H 4 = 21,927 X 28/96 = 6,395 

 CH 4 = 24,017 X 16/64 = 6,003 

 C 2 H 6 = 22,338 X 30/112 = 5,983 

 C 6 H 6 = 18,094 X 78/240 = 5,880 



for benzine vapor. 

 C 6 H 6 = 17,930 X 78/240 = 5,827 



for benzine liquid. 



In the third column is the ratio of the oxygen consumed, 

 and in the fourth the heat units per pound of oxygen used. 

 This table gives room for thought. It shows, in the first 

 place, a gradually decreasing result in heat set free per 

 pound of oxygen destroyed. Between C 2 H 2 and C 6 H 6 two 

 hydrocarbons, with exactly the same proportions of carbon 

 and hydrogen, using up exactly the same weight of oxygen 

 per pound of each, there is a difference of heat set free of 

 seventeen per cent, (nearly). Burned as vapor and burned 

 as liquid, benzine, C 6 H 6 , gives a different amount of heat 

 again. The figures become confusing when thus treated, 

 and it is necessary to deal with them by the molecule, as 

 they are treated by the chemist. 



How coal is formed cannot be said with absolute cer- 

 tainty, but the probability is that the coal plants accumu- 

 lated like the accumulation of the peat bogs and became 

 buried in sand and gradually sank to a considerable depth 

 in the earth. There under the influence of heat and pres- 

 sure, the vegetable matter changed its nature. Its watery 

 constituents were driven off and the remaining portions 



