[Continued from page 198.] Although by artificial cultivation the quantity of humus in a soil may be increased almost to any degree, still, in spite of this, there cannot be the slightest doubt that a soil must gradually lose those of its constituents which are removed in the seeds, roots, and leaves of the plants raised upon it. The fertility of a soil cannot remain unimpaired, unless we replace in it all those substances of which it has been thus deprived. Now this can only be done by manure. The manures thus used are divided in two classes :— 1. Animal or natural manures. 2. Chemical or artificial manures. Among the most important of the animal manures are the excrements of animals. The peculiar property of earth in absorbing putrid effluvia, and removing disagreeable smells, appears an indication of nature, to lead us to bury putrid animal substances, of which the excrements and dead carcases of animals are the most numerous and obvious. It would require no length of experience to show that wherever this is done, vegetation is more vigorous. There is, therefore, another motive for burying manure than mertly to get rid of a disagreeable substance. From the most ancient times, of which there are any records, the manuring of a field has been an important part of cultivation. We may now inquire whether the excrements of animals are all of a like nature and power, and whether they in every case administer to the necessities ol a plant by an identical mode of action. These points may easily be determined by ascertaining the composition of the animal excrements, because we shall thus learn what substances a soil really receives by their means. According to the common view, the action of solid animal excrements depends on the decaying organic matters which replace the humus, and on the presence of certain compounds of nitrogen, which are supposed to be assimilated by plants, and employed in the production of gluten and other azotized substances. But this view requires further confirmation with respect to the solid excrements ot animals, for they contain so small a proportion of nitrogen, that they cannot, possibly, by means of it, exercise any influence upon vegetation. We may form a tolerably correct idea of the chemical nature ofthe animal excrements, without further examination by comparing the excrements of a dog with its food. When a dog is ted with flesh and bones, both of which consist in great part of organic substances containing nitrogen, a moist white excrement is produced, which crumbles gradually to a dry powder in the air. This excrement consists of the phosphate of lime of the bones, and contains scarcely 1-100 part of its weight of foreign organic substances. The whole process of nutrition of an animal consists in the progressive extraction ot all the nitrogen from the food, so that the quantity of this element found in the excrements must always be less than that contained in the nutriment. When horse excrement is treated with water, a portion of it, to the amount of three or three and a half per cent., is dissolved, and the water is colored yellow. The solution is found to contain phosphate of magnesia and salts of soda, besides small quantities of organic matters. The portion of the excrement undissolved by the water yields to alcohol a resinous substance, possessing all the characters of gall, which has undergone some change ; while the residue possesses the properties of saw dust, trom which all soluble matter has been extracted by water, and burns without any smell. One hundred parts ot the fresh excrement of a horse, being dried at 212 Fah., leave from 25 to 31 parts of solid substances, and contain accordingly 69 to 75 parts of water. From the dried excrements we obtain variable quantities of salt and earthy matters, according to the nature of the food which has been taken by the animal It results, then, that from 3,600 to 4.000 pounds of fresh horse manure, corresponding to 100 pounds of dry manure, we place on the land from 2,784 to 3,000 pounds of water, and from 730 to 800 pounds ot vegetable matter, and also from 100 to 270 pounds of salt and other inorganic substances. The latter are evidently the substances to which our attention should be directed, for they are the same which formed the component parts of the hay, straw, and oats with which the horse was fed. Their principal constituents are the phosphates of lime and magnesia, carbonae of lime, and silicate of potash; the first three ot these preponderating in grains, the latter in hay. Thus, in 1,000 pounds of horse manure, we present to a field the inorganic substances in 6,000 pounds of hay, or 8,300 pounds of oats. The peculiar action, then, of the solid excrements is limited to their inorganic constituents, which thus restore to a soil that which is removed in the form of roots or grain.— When we treat land with the manure of the cow or sheep, we supply it with silicate ot potash and some salts of phosphoric acid ; and when enriched with the manure of the horse, we supply it with silicate of potash and phosphate of magnesia. In the straw which has served lor a litter, we add a further quantity of silicate of potash and phosphates; which, if the straw be putrefied, are in exactly the same condition in which they were before being assimilated. It is evident, therefore, that the soil of a field will alter but little if we collect and distribute the manure carefully. A certain portion of the phosphate, however, must be lost every year, being removed from the land with grain and cattle ; and this portion will accumulate in the neighborhood of large towns. The loss thus suffered must be compensated for in a well managed farm ; and this is partly done by allowing the fields to lie in grass. It is considered that, for every 100 acres ot corn land, there should be 20 acres of pasture land, which produce annu-nually, on an average, 5,000 pounds of hay. Then, assuming that the ashes ofthe excrements of the animals fed with this hay amount to nearly seven per cent., 341 pounds of the silicate of lime, and phosphates of magnesia and lime, must be yielded by these excrements, and will, in a certain degree, compensate for the loss which the land had sustained. We could keep our fields in a constant state of fertility by replacing every year as much as we remove from them in the form of produce; but an increase of fertility, and consequent increase of crop, can only be obtained when we add more to them than we take away. It will be found that of two fields placed under conditions otherwise similar, the one will be most fruitful upon which the plants are enabled to appropriate more easily, and in greater abundance, those contents of the soil which are essential to their growth and development. It will now be easily understood that, for animal excrements, other substances containing their essential constituents may be substituted. In Flanders, the yearly loss of the necessarj richness in the soil is completely restored by covering the fields with ashes of wood or bones, which may or may not have been lixiviated,and of which the greatest part consists of the phosphates of lime and magnesia. The great importance of manuring with ashes has been long known by agriculturists. Now, bone manure possesses a still greater importance in this respect. The primary sources from which the bones of animals are derived are hay, straw, or other substances used as food. If we admit that bones contain 55 per cent, ot the phosphates of lime and magnesia, and that hay contains as much of them as wheat straw, it will follow that eight pounds of bones contain as much phosphate of lime as 1,000 pounds of hay or wheat straw, and two pounds of it as much as 1,000 pounds of the grain of wheat or oats. These numbers express pretty nearly the quantity of phosphates, which a soil yields annually on the growth of hay and corn. Now, the manure of an acre of land with 40 pounds of bone dust is sufficient to supply three crops of wheat, clover, turnips, &c, with phosphates. But the form in which they are restored to a soil does not appear to be a matter of indifference ; for, the more finely the bones are reduced to powder, and the more intimately they are mixed with the soil, the more easily are they assimilated.