CARBONIFEROUS AGE. 363 



is all that comes from external sources. Under this diminished supply, 

 part of the carbon and hydrogen escape oxydation, and a coaly product 

 is left behind. This covering of water prevents a complete combustion 

 of the material, just like the covering of earth over burning wood, 

 when charcoal is made. The air might also be partly or wholly ex- 

 cluded from vegetable debris, by a covering of clay or earth ; and this 

 is generally what happened, sooner or later, in the Carboniferous period. 

 The changes attending the ultimate decomposition under these 

 circumstances depend on the affinity of (1) the carbon for oxygen, 

 making carbonic acid ; (2) of hydrogen for oxygen, producing water ; 



(3) of carbon for hydrogen, making carbo-hydrogen gas or oil ; and 



(4) on the tendency of the carbon and hydrogen, under certain pro- 

 portions, to form, with a portion of the oxygen, the stable compounds 

 included under the term Coal. The carbonic acid and water escape, 

 and also the carbo-hydrogen gas ; and, consequently, under the most 

 favorable circumstances, the wood loses, in the change, much carbon 

 and hydrogen as well as oxygen. It is probable that, in the making 

 of bituminous coal, at least three-fifths of the material of the wood 

 are lost; and, in the making of anthracite, three-fourths. Besides 

 this reduction to two-fifths and one-fourth by decomposition, there is 

 a reduction in bulk by compression ; which, if only to one-half, would 

 make the whole reduction of bulk to one-fifth and one-eighth. On 

 this estimate, it would take five feet in depth of compact vegetable 

 debris to make one foot of bituminous coal, and eight feet to make 

 one of anthracite. For a bed of pure anthracite thirty feet thick, 

 (like that at Wilkesbarre), the bed of vegetation should have been at 

 least 240 feet thick. 



Anthracite coal is a result, as remarked upon beyond, of the action 

 of heat on bituminous coal, under pressure, attending an upturning of 

 the rocks, the heat driving off nearly all volatile matters it could de- 

 velop, and so leaving a coke (the anthracite) behind. Made in this 

 way. the reduction, in the case of anthracite, would be to about one 

 eighth, as above estimated. The average amount of ash in anthracite 

 ought, consequently, to be nearly half greater than in bituminous coal. 



If the vegetable debris were so buried that no external oxygen were concerned in 

 the change attending the decomposition, and if all the oxygen of the wood went to 

 form carbonic acid with part of the carbon, the result would be a kind of mineral oil; 

 for dry wood has approximately the composition 6 H fl 4 ; removing from twice this, 

 C12H1808, 4C0 2 (which would' take off all the oxygen), there would be left Cm™, 

 the composition of a species of the naphtha group. So also, animal oils, on the simple 

 separation of carbonic acid, may become mineral oils. Warren & Storer obtained, by 

 the destructive distillation of the oil of the white-fish, after its saponification by lime, 

 the various oils of the marsh-gas group, besides others of the ethylene and benzole 

 ^erie-. It is well known, also, that similar oils are obtained by the destructive distilla- 

 tion of wood. 



