2^2 
LETTER XXVII. 
mineral charcoal OPINIONS RESPECTING SUBTERRANEAN 
COMBUSTION OF PIT-COAL COMBUSTION OF PYRITES MINE- 
RAL CHARCOAL. 
Having already taken some pains to prove, that the formation of 
coal depends on the process of bituminization, it will of course 
appear to you, that I have rather an ungracious task to execute, 
when I now inform you that this letter must be devoted to the 
description and history of a substance, mineral charcoal, which, 
though considered and described as a coal, does not manifest, on the 
nicest analysis, the least trace of bitumen. 
This substance, which possesses most of the external characters 
of pit-coal, but, at the same time, agrees in all its chemical pro- 
perties with charcoal, is thus described by Mr. Kirwan*, who, 
anticipating the discoveries of Guyton de Morveau, conjectured it 
to be the natural carbonic principle, as pure as nature generally 
affords any principle. 
Its colour is black, or greyish black ; its lustre approaching to 
the metallic; and its hardness, although it yields to the nail, ex- 
ceeds that of chalk. It possesses no transparency, and breaks into 
tabular fragments ; its fracture being foliated or slaty, and its cross 
fracture flat conchoidal. It stains the fingers. Specific gravity, 
from 1.4 to 1 . 53 . 
This substance is insoluble in acids. In a strong heat, it reddens, 
but does not flame : in a crucible, however, a slight bluish flame,. 
* Elements of Mineralogy, by Richard Kirwan, Esq. 1796, vol. ii. p. 49. 
