264 



edges of the coal, each separate lamina will he found to be com- 

 posed of bituminous concretions, exhibiting the conchoidal frac- 

 ture, and shining with a considerable degree of lustre. 



On letting a piece of coal fall on any hard body, thus breaking 

 it in such a manner as will not direct its fracture, but will allow it 

 to take place in such directions as accord with the natural divisions 

 formed by the interposed films, the fragments will in general be 

 found to assume the form of rhomboids or of parallelipipeds ; the 

 very forms which a body, whose parts were thus separated and dis- 

 posed, might be expected to exhibit, on being fractured. 



The separating pellicles or interposed septa, in those specimens 

 which I have examined, appear to be formed of sulphate of lime, 

 containing a small proportion of alumine, and sometimes also of 

 sulphuret of iron. The presence of these substances in coal, is 

 manifested by the analysis which has been made of this substance. 

 Mons. Fourcroy has ascertained that the ashes of coal contain the 

 sulphates of iron, of magnesia, of lime, and of alumine; and, even 

 irj asphaltum, both the sulphuric and muriatic acids were found, 

 by Du Ble, in union with calcareous earth *. 



By such an arrangement of the bituminous particles, and by the 

 frequent interposition of pellicles of incombustible matter^ which 

 have been just described, not only is the combustion of coal mode- 

 rated, but it is likewise so regulated that, in its employment for 

 common domestic purposes, the internal parts of even small por- 

 tions of coal are so protected, from the too rapid access of the 



* How strongly the water, which was diffused through the bituminous mass, was im- 

 pregnated with these several particles, may be inferred from the frequent presence of 

 calcareous spar in the interstices and cavities of coal. Thus, in a specimen of coal now 

 before me, about nine inches in length, and about three inches in width, the substance 

 of the coal is beautifully intersected by numerous white streaks of spathose matter ; and 

 a large interstice, the whole length of the specimen, is covered with very white and per- 

 iect crystals of the dog-tooth spar, forming a beautiful contrast with the including coal. 



