404 Comfound Bttumem. — Jet. — Cannel Coal, 



From thefe I am of opinion, that the moft immediate gradation from afphaltum (which is 

 the laft of the fimple bitumens) into thofe which are compound, takes place ia the fub- 

 ftance called 



JET. 



Jet is a fubftance well known to be of a full black, fometimes however inclining t» 

 brown. It is confiderably harder and lefs brittle than afphaltum. It breaks with a con- 

 choidal fradure, and the internal luftre is glafTy. It has no odour except when heated, and 

 it then refcmbles afphaltum. It melts in a ftrong heat, and, when burned, leaves an earthy 

 refiduum. 



Wallerius confidered jet as afphaltum which had become indurated by time, and Mr. 

 Fourcroy is of the fame opinion*. ' Others again have arranged it with the varieties of 

 coalf. I am inclined however to believe, that it is neither afphaltum nor coal, but an in- 

 termediate fubftance which may be regarded as the firft gradation from the fimple bitumens 

 into thofe which are compound. The matter of afphaltum undoubtedly enters into it in a 

 large proportion, and has confequently damped feveral of its characters upon it ; but the in- 

 creafe of carbon, and of the extraneous or earthy matter which is intimately mixed or 

 rather combined with it, has had fo much influence, that the characters of coal are alfo in 

 fome meafure ap,)arent, and are rendered the more ftriking by the fimilarity of certain local 

 circumftances which attend thefe two fubftances. The characters of coal are however by 

 no means fully eftablifhed in jet ; but from this we pafs immediately to another, in which 

 thefe characters cannot be queftioned. 



This is the fubftance called 



CANNEL COAL, 



which is of a full black, of a fmooth, folid, even texture; it breaks in any direCtion, and the 

 tranfverfe fraCture is conchoidal. It 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 lead- 

 ing 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 circumftantial 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 (hall not therefore defcribe the varieties of coal 

 known by divers names in different countries, and even in different provinces, fuch as thofe 

 called in England caking coal, rock coal, fplent coal, &c. &c. i but (hall 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 com- 

 pound poffeffes more or lefs the characters of perfeCl coal, or, by various (hades, paflfes 

 into certain earthy or ftony fubftances, which, although impregnated with bitumen, do 



* El^mens d'Hift. Nat. ct de Chimie, torn. iii. p. 456. 

 t Widenmann's Handbuch der Mineralogie, p. 628. 



not 



