The reason of this phenomenon now remains
to be considered.
When the edge of the coal is exposed to the fire, the stratification of
the coal is opened gradually by the heat and expanding vapours, as a
piece of wood, of a similar shape, would be by means of wedges placed in
the end way of the timber. The coal then kindles quietly, and quickly
flames, while the mass of this bituminous schistus is opening like the
leaves of a book, and thus exhibits an appearance in burning extremely
like wood. But let the fire be applied to the middle of the bed, instead
of the edge of the leaves, and we shall see a very different appearance;
for here the expanded aqueous vapours, confined between the _laminae_,
form explosions, in throwing off splinters from the kindling mass; and
this mass of coal takes fire with much noise and disturbance.
The ashes of this coal may be determined as to quality, being in general
a subtile white earth; but, as to quantity, the measure of that earth
produces an indefinite variety in this species of coal; for, from the
kennel or parrot coal, which is valuable for its burning with much
flame, to that black schistus which our masons use in drawing upon
stone, and which, though combustible in some degree, is not thought to
be a coal, there is a perfect gradation, in which coal may be found with
every proportion of this earthy alloy.
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