T36 Mr. 11 atciiett*s Objervatwns en Bituminous $u?>/!ances. 



the flow combuftion of coal proceeds from the joint effects pro- 

 duced partly by the relative proportions of the bituminous, carbona- 

 ceous, and earthy ingredients, and partly by the more or lefs perfect 

 degree of mixture which connects them together, and which degree*- 

 of mixture, I believe, in many cafes, nearly approaches to chemical 

 union, if not actually fo: when, therefore, the degree of mixture 

 is fo perfect as that every particle of bitumen is connected with- 

 much carbon or earthy matter, it is net furprifing that the rapid 

 combuitible property of the former fhould be checked in a confider- 

 able degree ; and, by a parity of reafoning, when the mixture is grofs 

 and imperfect, fo that it confiftsofa ftony or earthy fubftance, 

 which has fimply imbibed bitumen, it is natural to expect that the 

 bitumen (although lefs abundant than in coal) fhould enter readily 

 into combuftion, which is vehement in proportion to the fhortnefs 

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

 flances, and loofe fand-ftones which are fimply impregnated with bitu- 

 men. — To return, however, to the varieties of coal, I muft obferve, 

 that, from the caufes above-mentioned, the different characters and 

 properties of coal appear to me to be produced. That in this 

 manner, perfect pit-coal paffes into fchiftofe or ilaty coal, and this 

 again, by certain gradations, partes into the varieties of combuftible 

 or bituminous fchiftus, which alfo, by the gradual decreafe of the 

 bituminous ingredient, become at length confounded with the 

 varieties 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 Dorfetihire, where a pecu- 

 liar bituminous fchiftus is found, which is ufed as fuel by the inha- 

 bitants, and is improperly called Kimmeridge coal. 



By the feries of gradations which have been noticed in the fore- ' 



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