locked up in the fuel. For the purpose of
illustrating the subject of this essay, we require no farther progress
in this direction. A moment of thought at this point and we shall cease
to consider steam-power as _new_; for, long before man appeared on this
earth, the vegetation was collecting and condensing those ordinary
natural powers which we find in fuel. In our time, too, the rains and
dews, heat, motion, and gaseous food, are being stored up in a wondrous
manner, to serve as elements of power which may be used and applied now
or hereafter.

In this view, too, we may include the winds, the falling of rain, the
ascent and descent of sap, the condensation of gases,--in short,
the natural powers, exerted before,--as the cause of motion in the
steamboat.

Passing from these considerations not unconnected with the subject, let
us inquire what saltpetre is, and how it is formed.

The term Saltpetre is applied to a variety of bodies, distinguished,
however, by their bases, as potash saltpetre, soda saltpetre, lime
saltpetre, etc., which occur naturally. They are all compounds of nitric
acid and bases, or the gases nitrogen and oxygen united to bases, and
are found in all soils which have not been recently washed by rains, and
which are protected from excessive moisture.

The decomposition of animal and of some vegetable bodies in the soil
causes the production of one constituent of saltpetre, while the earth
and the animal remains supply the other. Evaporation of pure water from
the surface of the earth causes the moisture which rises from below to
bring to the surface the salt dissolved in it; and as this salt is not
volatile, the escape of the moisture leaves it at or near the surface.
Hence, under buildings, especially habitations of men and animals, the
salt accumulates, and in times of scarcity it may be collected. In all
cases of its extraction from the earth several kinds of saltpetre are
obtained, and the usual course is to decompose these by the addition of
salts of potash, so as to form from them potash saltpetre, the kind most
generally consumed.

In this decomposition of animal remains and the formation of saltpetre
the air performs an important part, and the changes it effects are
worthy of our attention.

Let us consider the aerial ocean surrounding our earth and resting upon
it, greatly larger in mass and extent than the more familiar aqueous
ocean below it, and more closely and momentarily affecting