HISTORY OF PETROLEUM OR ROCK OIL. 3J7 



SUPPLEMENTARY NOTE. 



• We have stated in the preceding paper that the different mineral 

 combustibles have been derived from the transformations of vegetable 

 matters, or in some cases of animal tissues analogous to these in com- 

 position. The composition of woody fibre or cellulose, in its purest 

 state, may be represented by C2 4H2o02o? or as a compound of the 

 elements of water with carbon ; the incrusting matter of vegetable 

 cells, to which the name of lignine has been given, contains, how- 

 ever, a less proportion of oxygen and more carbon and h3'drogen than 

 cellulose, so that the mean composition of recent woods, as deduced 

 from numerous analyses of various kinds, may be represented by 

 ^24E[]^g.40jg.^. We may conceive of four different modes of trans- 

 formation of woody fibre, all of which probably intervene to a greater 

 or less degree in the production of mineral combustibles ; and in con- 

 sidering these changes we shall for greater simplicity adopt for the 

 composition of woody fibre the first-named formula C24H2o02o' 



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

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

 of one equivalent of the first for two of the last. We may suppose 

 that for H,, wh'ch is oxydized by 0, from the air, the wood loses 

 CO 2 ; so that while the carbon increases in amount, the proportions of 

 oxygen and hydrogen are unchanged. In this way an equivalent 

 of cellulose, by absorbing sixteen equivalents of ox3^gen and losing 

 eight of carbonic acid, (8 CO2,) and sixteen of water, (16 HO,) would 

 leave C^gH^O^. Such is the nature of the decay of wood when ex- 

 posed to the air, and the process, could it be carried out, would leave 

 a residue of carbon only. If, however, the Avood is deeply buried 

 and excluded from the oxygen of the air, two reactions are conceiva- 

 ble. 



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 residual 

 carbon. The abstraction of ten equivalents of carbonic acid from 

 one of woody fibre would leave a hydrocarbon Cy^H^^y. 



III. Instead of combining exclusively with the carbon, a part of 

 the oxygen of the wood may be set free as water, in combination 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 C„yHg. 



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, 

 CjH^. The amount of this gas evolved from decaying plants sub- 

 merged in water, and the immense quantities of it condensed in coal 

 beds and other rocky strata, (forming fire damp,) show the great ex- 

 tent to which this mode of decomposition prevails. 



In nature these various modes of decomposition often go on to- 

 gether, or intervene at different stages in the decomposition of the 

 same mass ; they are, besides, seldom so complete as we have rep- 



