of themselves, apart from work done, than there is for a similar wasting away in the case of a mineral body such as a stone. When an animal does practically no work, as in the instance of our desert-snail, his body actually does not waste, but remains throughout just as big as ever. So we must look a good deal more closely into the problem if we want to understand it, and not rest content with vague generalities about food and fuel. Such half knowledge is really worse than no knowledge at all, because it deludes us into a specious self-deception, and makes us imagine that we comprehend what in fact we have not taken the least trouble to examine for ourselves.
Let us begin, then, by clearly realizing what is the use of fuel to the steam-engine. Obviously, you say, to set up motion. But where does the motion come from? "From the coal," answers the practical man, unhesitatingly. "Well, not exactly," says the physicist, "but from the coal and the air together." All energy or moving power, as we now know, is derived from the union of two bodies which have affinities or attractions for one another. Thus, if I wind up a clock, moved by a weight, I separate the mass of lead in the weight from the earth, for which it has the kind of affinity or attraction known as gravitation. This attraction then draws together the weight and the earth; and, in doing so, the energy I put into it is given out as motion of the clock. Similarly with coal and air: the hydrogen and carbon of the coal have affinities or attractions toward the oxygen of the air, and when I bring them together at a high temperature (of which more hereafter) they rush into one another's embrace to form carbonic acid and water, while their energy is given off as heat or motion of the surrounding bodies. We might have whole minefuls of coal at our disposal; but if we had no oxygen to unite with it, the coal would be of no more use than so much earth or stone. In ordinary life, however, the supply of oxygen is universal and abundant, while the supply of coal is limited; and so, as we have to lay in coals, while we find the oxygen laid in for us, we always quite disregard the latter factor in our fires, and speak as though the fuel were the only important element concerned. Yet one can easily imagine a state of things in which oxygen might be deficient; and in a world so constituted it would have to be regularly laid on in pipes, like gas or water, if the people wished to have any fires.
All energy, then, is derived from the separation of two or more bodies having affinities for one another. So long as the bodies remain separate, the energy is said, in the technical slang of physics, to be potential; as soon as the bodies unite, and the energy is manifested as motion, it is said to be kinetic. But these words are rather mystifying to ordinary readers, and frighten us by their bigness and their abstract sound; so I shall take the liberty of altering them for our present purpose to dormant and active respectively, which are terms quite as well adapted to express the meaning intended, and not half so likely to land us in an intellectual cul-de-sac, or to envelop us in a logical fog.
