But now, as we have both those species of coal in the
vicinity of each other, and even the same strata of coal part charred,
while the rest is not, this natural appearance, so far from being a
stumbling block, as it must be to the opposite theory, is most clearly
explained by the partial escape of vapours from the mineral regions, and
thus confirms the theory with regard to the efficacy of compression.
It is owing to the solidity of those natural charred coals, and the want
of oil, that they are so very difficult of kindling; but, when once
kindled in sufficient quantity, they make a fire which is very durable.
There are even some of them which, to common observation, seem to be
altogether incombustible. I have of this kind a specimen from a stratum
at Stair, which shall be afterwards mentioned.
M. Struve, in the Journal de Physique for January 1790, describes a
mineral which he calls _plombagine charbonneuse ou hexaedre_; and gives
for reason, _parce qu'elle ressemble extremement au charbon de pierre
schisteux, ou d'hexaedre_. He says farther, "Il est tres commun, dans
une roche qui forme un passage entre les granits et les breches, qu'on
n'a trouvee jusqu'a present qu'on masses roulees dans le pays de Vaud.
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