The air is the great source from which all the nourishment of the organic creation, whether vegetable or animal, is derived; its carbonic acid is decomposed by the vegetables, which appropriate the carbon, turning over the oxygen to their companions, the animals; the ammonia of the air furnishes all the nitrogen of seeds and other nutritious parts of vegetables, which are eaten by animals; and water, the chief source of all nutriment, passes first from the air before it enters the soil, bringing both carbonic acid and ammonia to fertilise it. That all this nourishment is derived from the air is evidently shown by the formation and increase of mould in forests which have grown for centuries; this black mould is nearly all decayed vegetable matter, formed by the continuous fall and decay of leaves and trees, but which, instead of diminishing, increases. Now, where does all this come from? certainly not out of the earth, for it does not contain the necessary elements. Expose the surface of the bare earth for several centuries, and first small plants, then larger ones, will grow upon it, until the state of things described above takes place, the earth being just as rich in organic matter now as at the beginning, and much richer on the surface, where a thick stratum of black mould forms from the repeated fall and decay of leaves and wood; all the substance which the forest shall have drawn from the earth (with the exception of certain salts and earthy matters) must therefore have been derived from the air, which contains every ingredient necessary for its formation, while in the earth itself no kind of organic matter is ever found. There was formerly a time when the carbonic acid of the air was in much greater abundance, and favoured the growth of those plants which thrive where there is plenty of water, as in swamps and marshes. They grew and decayed for a vast period of time, till a thick stratum of carbonaceous matter was deposited, which, after being buried (by some convulsion of nature) at a great depth, and pressed by the enormous weight of the superincumbent earth into a hard solid substance, is now being dug up by man, and forms that most valuable of all products of the mine, coal (fig. 10). [Illustration: FIG. 10.]