efficacy in modifying that operation. Coal strata,
which are in the neighbourhood of each other, being of those two
opposite species, the one fusible and inflammable, the other infusible
and combustible, afford the clearest proof of the efficacy of
compression; for, it is evident, that the coal, which was once
bituminous or fusible, cannot be charred without the distillation of
that substance; therefore, prevent the distillation by compression and
the charring operation cannot proceed, whatever should be the intensity
of the heat; and then, fusion alone must be the effect upon the
bituminous body. 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."
He concludes his paper thus: "Ce fossile singulier ne paroit pas
appartenir a la Suisse seule. J'ai dans ce moment devant les yeux une
substance parfaitement semblable, si on excepte la couleur qui tient le
milieu entre le gris de fer et le rouge modere; elle vient du pays de
Gotha de la Friedrischs-grube, proche d'Umneau. On le regarde comme un
eisenrahm uni a du charbon de pierre."

The specimen which I have from Stair upon the water of Ayr, so far as I
can understand, perfectly resembles this _plombagine_ of M. Struve. It
consumes very slowly in the fire, and deflagrates like plumbago with
nitre. Now this comes f