In the first place then, if the mixture of those two different
substances had been sufficiently perfect, and the precipitation uniform,
the solid body of coal resulting from this mixture, would not only
appear homogeneous, but might break equally or regularly in all
directions; but the fracture of this coal must visibly differ from the
former, so far as the fracture of this heterogeneous coal cannot have
the polished surface of the pure bituminous body; for, the earthy matter
that is interposed among the bituminous particles must affect the
fracture in preventing its surface from being perfectly smooth. This
imperfect plane of the fracture may be improved by polishing; in which
case the body might be sufficiently smooth to have an agreeable polish;
but it cannot have a perfect polish like a homogeneous body, or appear
with that glassy surface which is naturally in the fracture of the pure
bituminous coal.
But this is also a perfect description of that species of coal which is
called in England Kennel coal, and in Scotland Parrot coal. It is so
uniform in its substance that it is capable of being formed on the
turning loom; and it receives a certain degree of polish, resembling
bodies of jet.
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