Thus we have this coal commonly in rectangular pieces, in which it
is extremely difficult to distinguish the direction of the bed, or
stratification of the mass. By an expert eye, however, this may be in
general, or at least sometimes, distinguished, and then, by knowing
the habit of the coal in burning, a person perfectly ignorant of the
philosophy of the matter may exhibit a wonderful sagacity, or even of
power over future events, in applying this body to fire; for, at his
pleasure, and unknown to those who are not in the secret; he may
apparently, in equal circumstances, make this coal either kindle
quietly, or with violent cracking and explosions, throwing its splinters
at a distance.
The explanation lies in this, that, though the rectangular mass of coal
appears extremely uniform in its structure, it is truly a stratified
mass; it is therefore affected, by the sudden approach of fire in a very
different manner, according as the edge of the stratum, which is seen in
four of the sides of this supposed cube, shall be applied to the fire,
or the other two sides, which are in the line of the stratum, or
parallel to the bed of coal.
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