Many practical farmers, I am aware, still doubt the facts and theories of chemistry as applied to agriculture; with them I am free to admit that agriculture is not a science as yet, but an experimental art. With this I have nothing to do directly, my object being to show in this chapter in how far civilized man modifies and influences the soil on which he lives. He, the practical farmer, clings to farmyard manure, which he collects in heaps in his farmyard, or by the roadside, exposed to every change of weather, to drenching rains, to summer heat, and winter’s cold; from it run in streams over the roads the liquid parts of the manure, carrying with them the soluble salts; out of what is left when it has become rotten he hopes to restore to the field what it lost during the previous crop, and to a certain extent he succeeds; on the other hand, the chemist argues that the grand object of modern agriculture is to substitute for farmyard manure, that universal food of plants, their elements, obtained from other and cheaper sources retaining its full efficacy; and this can only be done when we shall have learned, what as yet we know but imperfectly, how to give to an artificial mixture of the individual ingredients the mechanical form and chemical qualities essential to their reception, and to their nutritive action on the plant; for without this form they cannot perfectly supply the place of farmyard manure. All our labours must be devoted to the attainment of this important object. However this may be, and however it may be explained by the chemists, it must be admitted that to the accidental discovery of bone manure England owes many turnip crops, and to the introduction of guano from Peru and Ichaboe crops of wheat which no other manure as yet known could have produced. Peruvian guano, the best of all, is the excreta of a sea bird; these excreta, placed in a clear and perfectly dry atmosphere, have been exposed for centuries to a tropical sun; no rain falls on the heaps, trodden down only by the gentle feet of the birds themselves. That out of such a product there should arise so excellent a manure surpasses all previous reasoning derived from mere science.[38] It is obvious, then, that much still remains to be discovered. Were any proof of this required, we might refer to the agriculture of China, where, as has been reported, human excreta alone are used as manure, and with a success unequalled in any other part of the world. In that singular land they have discovered much, or using perhaps the discoveries of preceding races, have turned them to the best account. Their agriculture is said to be perfect. [38] The guano of sea-birds when exposed to rain is of no value. With such a system of manure and such a population one might predicate a condition of earth, air, and water, incompatible with human life. Now the very reverse happens, at least, in so far as regards the Chinese themselves.