196. Of energy in repose we have the following varieties:--(1.) The energy of fuel. (2.) That of food. (3.) That of a head of water. (4.) That which may be derived from the tides. (5.) The energy of chemical separation implied in native sulphur, native iron, &c. Then, with regard to energy in action, we have mainly the following varieties:-- (1.) The energy of air in motion. (2.) That of water in motion. _Fuel._ 197. Let us begin first with the energy implied in fuel. We can, of course, burn fuel, or cause it to combine with the oxygen of the air; and we are thereby provided with large quantities of heat of high temperature, by means of which we may not only warm ourselves and cook our food, but also drive our heat-engines, using it, in fact, as a source of mechanical power. Fuel is of two varieties--wood and coal. Now, if we consider the origin of these we shall see that they are produced by the sun’s rays. Certain of these rays, as we have already remarked (Art. 180), decompose carbonic acid in the leaves of plants, setting free the oxygen, while the carbon is used for the structure or wood of the plant. Now, the energy of these rays is spent in this process, and, indeed, there is not enough of such energy left to produce a good photographic impression of the leaf of a plant, because it is all spent in making wood. We thus see that the energy implied in wood is derived from the sun’s rays, and the same remark applies to coal. Indeed, the only difference between wood and coal is one of age: wood being recently turned out from Nature’s laboratory, while thousands of years have elapsed since coal formed the leaves of living plants. 198. We are, therefore, perfectly justified in saying that the energy of fuel is derived from the sun’s rays;[7] coal being the store which Nature has laid up as a species of capital for us, while wood is our precarious yearly income. We are thus at present very much in the position of a young heir, who has only recently come into his estate, and who, not content with the income, is rapidly squandering his realized property. This subject has been forcibly brought before us by Professor Jevons, who has remarked that not only are we spending our capital, but we are spending the most available and valuable part of it. For we are now using the surface coal; but a time will come when this will be exhausted, and we shall be compelled to go deep down for our supplies. Now, regarded as a source of energy, such supplies, if far down, will be less effective, for we have to deduct the amount of energy requisite in order to bring them to the surface. The result is that we must contemplate a time, however far distant, when our supplies of coal will be exhausted, and we shall be compelled to resort to other sources of energy. _Food._