614 COAL. 



of that rock, they are often converted into cinder? 

 or coke. 



The strata of coal are generally divided by 

 natural joints, more or less perfect and extensive, 

 by which they are formed into cuboidal and pris- 

 matic masses. The most frequent variety of 

 structure which they present is the laminar ; and, 

 in some rare instances, where in contact with 

 trap, they also assume prismatic forms. 



The mineral composition of coal is scarcely a 

 subject for consideration in this work, of which 

 it is not the object to record the chemical nature 

 of the simple minerals. Yet, as it has been ge- 

 nerally misapprehended, it will be useful to sketch 

 it in the briefest possible manner. 



All the bitumens, from naphtha to asphaltum, 

 consist of compounds, apparently indefinite, of 

 carbon and hydrogen principally ; the small quan- 

 tities of oxygen and azote which they contain, 

 appearing to have little or no effect in modifying 

 their mineral characters. In the most fluid, the 

 hydrogen predominates, diminishing progres- 

 sively according to the order of their relative te- 

 nacity or solidity. Where asphaltum ends this 



