matter and are liberated in its excavation; but all who have worked coal
mines know that such accumulations are not sufficient to supply the
enormous and continuous flow which comes from all parts of the mass
penetrated. We have ample proof, moreover, that coal, when exposed to
the air, undergoes a kind of distillation, in which the evolution
of carbonic acid and hydrocarbon gases is a necessary and prominent
feature.

The gas makers know that if their coal is permitted to lie for months or
years after being mined, it suffers serious deterioration, yielding a
less and less quantity of illuminating gas with the lapse of time.
So coking coals are rendered dry, non-caking, and valueless for this
purpose by long exposure.

Carbureted hydrogen, olefiant gas, etc., are constant associates of the
petroleum of springs or wells, and this escape of gas and oil has been
going on in some localities, without apparent diminution, for two or
three thousand years. We can only account for the persistence of this
flow by supposing that it is maintained by the gradual distillation of
the carbonaceous masses with which such evolutions of gas or of liquid
hydro-carbons are always connected. If it were true that carbureted
hydrogen and petroleum are produced only from the primary decomposition
of organic tissue, it would be inevitable that at least the elastic
gases would have escaped long since.

Oil wells which have been nominally exhausted--that is, from which the
accumulations of centuries in rock reservoirs have been pumped--and
therefore have been abandoned, have in all cases been found to be slowly
replenished by a current and constant secretion, apparently the product
of an unceasing distillation.