ibly evolved into the atmosphere, and their fixed remains mixed
with their kindred earth.

Wood, when exposed to moisture, also undergoes the putrid fermentation
and becomes rotten.

EMILY.

But I have heard that the _dry rot_, which is so liable to destroy the
beams of houses, is prevented by a current of air; and yet you said that
air was essential to the putrid fermentation?

MRS. B.

True; but it must not be in such a proportion to the moisture as to
dissolve the latter, and this is generally the case when the rotting of
wood is prevented or stopped by the free access of air. What is commonly
called dry rot, however, is not I believe a true process of
putrefaction. It is supposed to depend on a peculiar kind of vegetation,
which, by feeding on the wood, gradually destroys it.

Straw and all other kinds of vegetable matter undergo the putrid
fermentation more rapidly when mixed with animal matter. Much heat is
evolved during this process, and a variety of volatile products are
disengaged, as carbonic acid and hydrogen gas, the latter of which is
frequently either sulphurated or phosphorated. --When all these gases
have been evolved, the fixed products, consisting of carbon, salts,
potash, &c. form a kind of vegetable earth, which makes very fine
manure, as it is composed of those elements which form the immediate
materials of plants.

CAROLINE.

Pray are not vegetables sometimes preserved from decomposition by
petrification? I have seen very curious specimens of petrified
vegetables, in which state they perfectly preserve their form and
organisation, though in appearance they are changed to stone.

MRS. B.

That is a kind of metamorphosis, which, now that you are tolerably well
versed in the history of mineral and vegetable substances, I leave to
your judgment to explain. Do you imagine that vegetables can be
converted into stone?

EMILY.

No, certainly; but they might perhaps be changed to a substance in
appearance resembling stone.

MRS. B.

It is not so, however, with the substances that are called petrified
vegetables; for these are really stone, and generally of the hardest
kind, consisting chiefly of silex. The case is this: when a vegetable is
buried under water, or in wet earth, it is slowly and gradually
decomposed. As each successive particle of the vegetable is destroyed,
its place is supplied by a particle of siliceous earth, conveyed thither
by the water. In the course of time the ve