0fi the CombufitoH tf Coal, (Sfc. 2©5 



not merit the appellation of coal, and thefe. alfo at length gradually lofe the bituminous 

 charader *. - 



It is likewife worthy of notice, that the quantity of earthy matter does not appear to be 

 the principal caufe why pit-coals do not burn with the rapidity which is to be perceived in 

 feme other earthy fub(taiv:es impregnated with bitumen. For we may conclude, that the 

 flow combuftion of coal proceeds from the joint efFefls produced partly by the relative pro- 

 portions of the bituminous, carbonaceous, and earthy ingredients, and partly by the more or 

 lei's perfeft degree of mixture which connedls them together, and which degree of mixture, 

 I believe, in many cafes, nearly approaches 1o chemical union, if not adlually fo : when, 

 therefore, the degree of mixture is fo perfect as that every particle of bitumen is connedted 

 with much carbon or earthy matter, it is not furprifing that the rapid combuftible property 

 of the former fliould be checked in a confiderable degree ; and, by a parity of reafoning, 

 when the mixture is grofs and imperfcft, fo that it confifts of a ftony or earthy fubftance, 

 which has fimply imbibed bitumen, it is natural to expeil that the bitumen (although lefs 

 abundant than in coal) fhould enter readily into combuftion, which is vehement in propor- 

 tion to the fliortnefs of its duration ; and this we find to be the cafe in many earthy fub- 

 ftances, and loofe fand-llones which are fimply impregnated with bitumen. — To return, 

 hovvever, to the varieties of coal, I muft obferve, that, from the caufes above-mentioned, 

 the different charadters and properties of coal appear to nie to be produced. That in this 

 manner, perfe(5l pit-coal pafles into fchiftofe or ilaty coal ; and this again, by certain grada- 

 tions, paffes into the varieties of combuftible or bituminous fchiftus ; which alfo, by the 

 gradual decreafe of the bituminous ingredient, become at length confounded with the 

 Tarieties of the common or argillaceous fchiftus. 



We have a remarkable example of this in the gradations of bituminous fchiftus into 

 argillaceous fchiftus, which are to be obferved at Kimmeridge, on the coaft of Dorfetlhlre, 

 where a peculiar bituminous fchiftus Is found, which is ufed as fuel by the inhabitantSj and 

 is improperly called Kimmeridge coal. 



By the feries of gradations which have been noticed In the foregoing pages, the fimple 

 bituminous fubftances appear to pafs into thofe which are compound ; and thefe alfo, by de- 

 clining (hades, at laft pafs into fubftances appertaining to the clafs of earths afid ftones. 

 ■ In the compound bituminous fubftances the prevalent earthy ingredient Is for the greater 

 part generally, if not always, argillaceous; and although certain calcareous grits (fuch as 

 the Portland ftonef) as well as limeftones and marbles are found impregnated with bitumen, 

 yet I know not of any inftance in which this happens to the degree requifite to form a com- 

 buftible fubftance. 



Thiscurfory view of the fimple bitumens, and of their combinations, would be fufficient 

 as an introduftion to the principal fubjeit of this paper ; but, to complete the feries, 1 ftiall 

 make fome obfervations on the vegetable fubftances which contain bitumen, and ftiall after- 

 wards mention the mixtures of bitumen with metallic oxides. 



• From Mr. Kirwan's experiments it appears that carbon is a conftituenf. principle of coal, and tliat the pre- 

 fence of it is a principal caufe of thofe modifications which produce the fpecics. It even feems chiefly to forra 

 the Kilkenny coal. — Kirwan's Elements of Mineralogy, vol. ii. p. 5ZI. 



t The Portland ftone, when recently broken in the quarries, emits a ftrong bituminous odour, like the b!tu» 

 minnus limeftone or ftink-ftone. It is alfo full of extraneous foflils, or at leaftthe veftigcs of them. 



SECT. IV. 



