LETTER XXVII. 



MINERAL CHARCOAL. ...OPINIONS RESPECTING... .SUBTERRANEAN 

 COMBUSTION OF PIT-COAL. ...COMBUSTION OF PYRITES.*.. 

 MINERAL 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. 



