getable is entirely destroyed,
but the silex has completely replaced it, having assumed its form and
apparent texture, as if the vegetable itself were changed to stone.

CAROLINE.

That is very curious! and I suppose that petrified animal substances are
of the same nature?

MRS. B.

Precisely. It is equally impossible for either animal or vegetable
substances to be converted into stone. They may be reduced, as we find
they are, by decomposition, to their constituent elements, but cannot be
changed to elements, which do not enter into their composition.

There are, however, circumstances which frequently prevent the regular
and final decomposition of vegetables; as, for instance, when they are
buried either in the sea, or in the earth, where they cannot undergo the
putrid fermentation for want of air. In these cases they are subject to
a peculiar change, by which they are converted into a new class of
compounds, called _bitumens_.

CAROLINE.

These are substances I never heard of before.

MRS. B.

You will find, however, that some of them are very familiar to you.
Bitumens are vegetables so far decomposed as to retain no organic
appearance; but their origin is easily detected by their oily nature,
their combustibility, the products of their analysis, and the
impressions of the forms of leaves, grains, fibres of wood, and even of
animals, which they frequently bear.

They are sometimes of an oily liquid consistence, as the substance
called _naptha_, in which we preserved potassium; it is a fine
transparent colourless fluid, that issues out of clays in some parts of
Persia. But more frequently bitumens are solid, as _asphaltum_,
a smooth, hard, brittle substance, which easily melts, and forms, in its
liquid state, a beautiful dark brown colour for oil painting. _Jet_,
which is of a still harder texture, is a peculiar bitumen, susceptible
of so fine a polish, that it is used for many ornamental purposes.

_Coal_ is also a bituminous substance, to the composition of which both
the mineral and animal kingdoms seem to concur. This most useful mineral
appears to consist chiefly of vegetable matter, mixed with the remains
of marine animals and marine salts, and occasionally containing a
quantity of sulphuret of iron, commonly called pyrites.

EMILY.

It is, I suppose, the earthly, the metallic, and the saline parts of
coals, that compose the cinders or fixed products of their combustion;
whilst the hydrogen a