136.0 9=-Mr. Hatcuet?’s Obfervations on Bituminous Subftances. 
the flow combutftion 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 conneéts 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 conneéted with 
much carbon or earthy matter, it is not furprifing that the rapid 
combuftible property of the former fhould be checked in a confider- 
able degree ; and, by a parity of reafoning, when the mixture is grofs 
and daveneet fo that it confifts vay {tony or earthy fubftance, 
which has fimply imbibed bitumen, it is natural to expeét 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- 
ftances, 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, perfeét pit-coal paffes into {chiftofe or flaty coal, and this 
again, by certain gradations, paffes into the varieties of combuftible 
or bituminous {chiftus, which alfo, by the gradual decreafe of the 
bituminous ingredient, become at length eeestnded with the 
varieties of the common or argillaceous Tchiftiis. 
‘We have a remarkable Seated of this in the gradations of 
bituminous fchiftus into argillaceous fchiftus, which are to be 
obferved at Kimmeridge, on the coaft of Dorfetfhire, 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- 
going 
