It is also true that we do not always entertain the most correct view of what we term the new power of locomotive and steamboat; and as it may aid us in some further steps connected with the subject of my remarks, a familiar object, such as a steamboat, may be taken as illustrative of the application of power, and we may thus obtain some simple ideas of what power truly is, in Nature.
My travelled friend considers a steamboat as a ship propelled by wheels, the shaft to which they are attached being moved by the machinery. He follows back to the piston of the engine and finds the motor there,—satisfied that he has discovered in the transference of rectilinear to rotatory motion the reason for the progress of the boat. A more inquisitive friend does not rest here, but assumes that the power of the steam flowing through the machine sets in action its parts; and he rests from farther pursuit of the power, where the larger number of those who give any observation to the application of steam are found,—gratified with the knowledge accumulated, and the readiness with which an explanation of the motion of the boat can be traced to the power of steam as its source.
We must proceed a little farther on our backward course from the point where the power is applied, and in our analysis consider the steam as only the vehicle or carrier of the power; and examining the conditions, we find that water acted on by fire, while contained in a suitable vessel, after some time takes up certain properties which enable it to go forward and move the ponderous machinery of the boat. The water evidently here derives its new character of steam from the fire, and we have now reached the source of the movement of steam, and traced it to the fire. In fact, we have found the source of power, in this most mechanical of all mechanical machines, to be removed from the department of knowledge which treats of machines!
But we need not pause here, although we must now enter a little way into chemical, instead of mechanical science. The fire prepares the water to act as a carrier of power; it must contain power, therefore; and what is it which we call fire? In placing on the grate coal or wood, and providing for the contact of a continuous current of air, we intend to bring about certain chemical actions as consequent on a disposition which we know coal and wood to possess. When we apply fire, the chemical actions commence and the usual effects follow. Now, if we for a moment dismiss the consideration of the means adopted, it becomes apparent to every one, that, as the fire will continue to increase with successive additions of fuel, or as it will continue indefinitely with a regular supply, there must be something else than mere motor action here. We cannot call it chemical action, and dismiss the thought, and neglect further inquiry, unless we would place ourselves with those who regard the movement of the steamboat as being due to the machinery.
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.