A continuance of decomposition
changes this lignite into common or bituminous coal, chiefly by the discharge of
carbureted hydrogen, or the gas by which we illuminate our streets and houses.
According to Bischoff, the inflammable gases which are always escaping from
mineral coal, and are so often the cause of fatal accidents in mines, always
contain carbonic acid, carbureted hydrogen, nitrogen, and olefiant gas. The
disengagement of all these gradually transforms ordinary or bituminous coal into
anthracite, to which the various names of glance-coal, coke, hard-coal, culm,
and many others, have been given.
There is an intimate connection between the extent to which the coal has in
different regions parted with its gaseous contents, and the amount of
disturbance which the strata have undergone. The coincidence of these phenomena
may be attributed partly to the greater facility afforded for the escape of
volatile matter, when the fracturing of the rocks has produced an infinite
number of cracks and crevices. The gases and water which are made to penetrate
these cracks are probably rendered the more effective as metamorphic agents by
increased temperature derived from the interior.
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