252 Notes on Petroleum 



these changes we shall for greater simplicity adopt for the com- 

 position of woody fibre the first named formula, C2 4H2 0O2 o. 



I. When wood is exposed to the action of moist air, oxygen is 

 absorbed, and carbonic acid and water are evolved in the proportion 

 of one equivalent of the first for two of the last. We may sup- 

 pose that for H2 which is oxydised by O2 from the air, the wood 

 loses C02,so that while the carbon increases in amount the pro- 

 portions of oxygen and hydrogen are unchanged. In this way an 

 equivalent of cellulose, by absorbing sixteen equivalents of oxygen 

 and losing eight of carbonic acid, (8 CO2) and sixteen of water, 

 (16 HO) would leave C16H4O4. Such is the nature of the de- 

 cay of wood when exposed to the air, and the process, could it be 

 carried out, would leave a residue of carbon only. If however the 

 wood is deeply buried and excluded from the oxygen of the air 

 two reactions are conceivable. 



II. The whole of the oxygen of the wood may be given off" in the 

 form of carbonic acid, while the hydrogen remains with the resid- 

 ual carbon. The abstraction of ten equivalents of carbonic acid 

 from one of woody fibre, would leave a hydrocarbon, Cx 4H2 0. 



III. Instead of combining exclusively with the carbon, a 

 part of the oxygen of the wood may be set free as water, in com- 

 bination of the hydrogen. The abstraction from an equivalent of 

 woody fibre of four equivalents of carbonic acid and twelve of 

 water would leave a hydrocarbon C2 oHs. 



IV. These decompositions are however never so simple as we have 

 supposed in II and III, for a portion of hydrogen is at the same 

 time evolved in combination with carbon, chiefly as marsh gas, 

 C2H4. The amount of this gas evolved from decaying plants 

 submerged in water, and the immense quantities of it condensed 

 in coal beds and other rocky strata, (forming fire damp,) shew 

 the great extent to which this mode of decomposition prevails. 



In nature these various modes of decomposition often go on 

 together, or intervene at diSerent stages in the decomposition of 

 the same mass ; they are besides seldom so complete as we have 

 represented them. The first process results in the formation of 

 vegetable mould, which always retains portions of carbon and 

 hydrogen; while the incomplete operation of the processes II, 

 III and IV gives rise to peat, lignite, brown coal, bituminous coal 

 and pyroschists, in all of which the proportion of the oxygen is 

 much less than the hydrogen, so that their composition may be 



