264 
^ 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 
in 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 
oxygen, that only the external part is actually consumed and re- 
duced to an ash ; whilst the other part is rendered a coke or cinder, 
and is capable of being again employed ; and even with increased 
ow strongly the water which was diffused through the bituminous mass, was impregnated 
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 perfect crystals of the dog-tooth spar, 
forming a beautiful contrast with the including coal. 
