1. That the most powerful substances in this table, woollen rags, for example,—2½ lbs. of which are equal in virtue to 100 lbs. of farm-yard manure,—may yet shew less immediate sensible effect upon the crop than an equal weight of sheep’s dung, or even of urine. Such dry substances are long in dissolving and decomposing, and continue to evolve fertilizing matter, after the softer and more fluid manures have spent their force. Thus, while farm-yard manure or rape dust will immediately hasten the growth of turnips, woollen rags will come into operation at a later period, and prolong their growth into the autumn. 2. That besides their general relative value, as represented in the above table, each of these substances has a further special value not here exhibited, dependent upon the kind and quantity of saline and other inorganic matter which they severally contain. Thus three of dry flesh are equal to five of pigeons’ dung, in so far as the _organic_ part is concerned; but the latter contains also a considerable quantity of bone-earth and of saline matter scarcely present at all in the former. Hence pigeons’ dung will benefit vegetation in circumstances where dry flesh would in some degree fail. So the liquid excretions contain much important saline matter not present in the solid excretions,—not present either in such substances as horn, wool, and hair,—and, therefore, each must be capable of exercising an influence upon vegetation peculiar to itself. Hence the practical farmer sees the reason why no one _simple_ manure can long answer on the same land; and why in all ages and countries the habit of employing _mixed_ manures and artificial composts has been universally diffused. SECTION IV.—NATURAL DISTINCTION OR DIFFERENCE BETWEEN ANIMAL AND VEGETABLE MANURES, AND THE CAUSE OF THIS DIFFERENCE. In what do animal manures differ from vegetable manures,—what is the cause of this difference,—how does the digestion of vegetable matter improve its value as a manure? 1. The characteristic distinction between animal and vegetable manures is this,—that the former contain a much larger proportion of nitrogen than the latter. This will be seen at once, by comparing together the tables given in the two preceding sections, in which the numbers represent the relative agricultural values of certain animal and vegetable substances compared with farm-yard manure. The lowest numbers represent the highest value, and the largest amount of nitrogen, and these low numbers are always opposite to the purest animal substances.