Our farther progress in this analysis will soon open a wide field of knowledge and inquiry; but it is sufficient for our present purpose, if, by a careful study of the composition and chemical disposition of the proximate compounds of the coal and the wood fuel, we arrive at the conclusion that both are the result of forces which, very slight in themselves at any moment, yet when acting through long periods of time become laid up in the form of coal and wood.  All that effort which the tree has exhibited during its growth from the germ of the seed to its 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 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.