But, Mrs. B., though experience daily proves the advantage of cultivation, there is still a difficulty which I cannot get over. A certain quantity of elementary principles exist in nature, which it is not in the power of man either to augment or diminish. Of these principles you have taught us that both the animal and vegetable creation are composed. Now the more of them is taken up by the vegetable kingdom, the less, it would seem, will remain for animals; and, therefore, the more populous the earth becomes, the less it will produce. MRS. B. Your reasoning is very plausible; but experience every where contradicts the inference you would draw from it; for we find that the animal and vegetable kingdoms, instead of thriving, as you would suppose, at each other’s expense, always increase and multiply together. For you should recollect that animals can derive the elements of which they are formed only through the medium of vegetables. And you must allow that your conclusion would be valid only if every particle of the several principles that could possibly be spared from other purposes were employed in the animal and vegetable creations. Now we have reason to believe that a much greater proportion of these principles than is required for such purposes remains either in an elementary state, or engaged in a less useful mode of combination in the mineral kingdom. Possessed of such immense resources as the atmosphere and the waters afford us, for oxygen, hydrogen, and carbon, so far from being in danger of working up all our simple materials, we cannot suppose that we shall ever bring agriculture to such a degree of perfection as to require the whole of what these resources could supply. Nature, however, in thus furnishing us with an inexhaustible stock of raw materials, leaves it in some measure to the ingenuity of man to appropriate them to its own purposes. But, like a kind parent, she stimulates him to exertion, by setting the example and pointing out the way. For it is on the operations of nature that all the improvements of art are founded. The art of agriculture consists, therefore, in discovering the readiest method of obtaining the several principles, either from their grand sources, air and water, or from the decomposition of organised bodies; and in appropriating them in the best manner to the purposes of vegetation. EMILY. But, among the sources of nutritive principles, I am surprised that you do not mention the earth itself, as it contains abundance of coals, which are chiefly composed of carbon. MRS. B. Though coals abound in carbon, they cannot, on account of their hardness and impermeable texture, be immediately subservient to the purposes of vegetation. EMILY. No; but by their combustion carbonic acid is produced; and this entering into various combinations on the surface of the earth, may, perhaps, assist in promoting vegetation. MRS. B.