Mr, Hatrcuery’s Odfrvations on Bituminous Subftances, 135: 
burns well, and is fo compact that it is often employed, like jet, to 
be formed into trinkets, 
The great refemblance which cannel coal has to jet in many of 
its properties, induces me to regard it as the next gradation of the 
compound bituminous fubftances, and as the leading variety of coal 
from which the others follow according to the degree of their 
bituminous character. 
The limits of this paper will not allow me to enter into a cir-- 
cumftantial account of all the other varieties of pit-coal ; neither 
is it neceffary, after the gradations of afphaltum to jet, and of jet 
to coal, have been noticed. I fhall not therefore defcribe the varie- 
ties of coal known by divers names in different countries, and even 
in different provinces, fuch as thofe called in England caking coal, 
rock coal, {plent coal, &c. &c.; but fhall only obferve, that the 
pit-coals. in general appear to be compofed of bitumen intimately 
mixed, or.rather combined, with various proportions of carbon and 
earthy matter ;.and according to the intimacy of the union, and the 
excefs of one or other of the ingredients, fo the compound poffeffes. 
more or lefsthe characters of perfect coal, or, by various fhades, . 
paffes into certain earthy or ftony fubftances, which, although im-- 
pregnated with bitumen, do not merit the appellation of coal, and: 
thefealfo at length gradually lofe the bituminous charaéter *, . 
It is likewife worthy of notice, that the quantity of earthy. mat-.- 
ter does not appear to be the principal caufe why pit-coals do not: 
burn with the rapidity which is tobe perceived in fome other earthy 
fubftances impregnated with bitumen. For we may conclude, that 
* From Mr. Kirwan’s experiments it appears that carbon is a conftituent principle. 
of coal, and that the prefence of it is a principal caufe-of thofe modifications which pro-~ 
duce the fpecies. Iteven feems chiefly to form the Kilkenny coal.—Kirwan’s Elements 
ef Mineralogy, vol. ii. p. 521. 
the 
