state of maturity, when taken as fuel, is pent up in its substance,
ready, when fire is applied, to escape slowly and continuously. In
the case of the coal, after the growth of the plant from which it was
formed, the material underwent changes which enabled it to conserve more
forces, and to exhibit more energy when fire is applied to its mass; and
hence the distinction between wood and coal.

Our analysis thus far has developed the source of the power moving
the steamboat as existing in the gradual action of forces influencing
vegetation, concentrated and 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