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but without any sulphurous smell, is observed; undoubtedly from ' 
the decomposition of water. Exposed to a porcelain heat for 
several hours, in a closed crucible, it entirely, or almost entirely, 
evaporates like diamond. Under a muffle, it is nearly consumed. 
Distilled, it yields a small quantity of water ; but no air, oil, acid, 
nor alkali. Mixed with sulphate of pot-ash, it yields a liver of 
sulphur ; and projected on red-hot melting nitre, it deflagrates and 
alkalizes it ; hence, Mr. Kirwan remarks, there is no doubt but it is 
a combustible substance, possessing both the external and internal 
characters of mere coal, or charcoal. 
The Kilkenny coal is of this species ; its fracture being foliated, 
and its fragments often coated with whitish illinitions. The culm of 
Wales seems to be a variety of this species, but less pure ; differing 
from it, chiefly, in being more brittle, and emitting, when ignited, a 
disagreeable smell. 
This substance yielded to Dr., Hutton, and the French chemists, 
a strong corroboration of the opinion they had formed, respecting 
the changes which bituminous substances, and particularly coal, 
underwent from the influence of subterranean heat*. Knowing 
that besides the common pit-coal, rich in bitumen, there existed 
this substance also, in which no bitumen was discoverable, they con- 
sidered the former kind, as abounding in the oily matter which had 
been contained in the various organized bodies, from which it had 
derived its origin ; and concluded, the latter to have been exposed 
to a considerable degree of subterranean heat, in consequence of 
which, it had suffered a deprivation, by distillation, of its bitumen ; 
it being thereby left a caput mortuum, or perfect coal. Tlie exist- 
ence of the purer bitumens, in separate situations, has been also 
reckoned in evidence of the propriety of this theory. Thus Dr. 
Hutton mentions, as an instance of the separate existence of these 
Theory of the Earth, with Proofs and Illustrations, by Janies Hutton, M. D. 1795.. 
VOL, I. 
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