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144 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; 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