We shall adopt a classification of the subject somewhat different from that found in most works on manures, but the _facts_ are the same. The action of manures is either _mechanical_ or _chemical_, or a combination of both. For instance: some kinds of manure improve the mechanical character of the soil, such as those which loosen stiff clay soils, or others which render light sandy soils compact--these are called _mechanical_ manures. Some again furnish food for plants--these are called _chemical_ manures. Many mechanical manures produce their effects by means of chemical action. Thus _potash_ combines chemically with sand in the soil. In so doing, it roughens the surfaces of the particles of sand, and renders the soil less liable to be compacted by rains. In this manner, it acts as a _mechanical_ manure. The compound of sand and potash,[U] as well as the potash alone, may enter into the composition of plants, and hence it is a _chemical_ manure. In other words, potash belongs to both classes described above. It is important that this distinction should be well understood by the learner, as the words "mechanical" and "chemical" in connection with manures will be made use of throughout the following pages. [What are absorbents? What kind of manure is charcoal?] There is another class of manures which we shall call _absorbents_. These comprise those substances which have the power of taking up fertilizing matters, and retaining them for the use of plants. For instance, _charcoal_ is an absorbent. As was stated in the section on soils, this substance is a retainer of all fertilizing gases and many minerals. Other matters made use of in agriculture have the same effect. These absorbents will be spoken of more fully in their proper places. TABLE. MECHANICAL MANURES are those which improve the mechanical condition of soils. CHEMICAL " are those which serve as food for plants. ABSORBENTS are those substances which absorb and retain fertilizing matters. [Into what classes may manures be divided? What are organic manures? Inorganic? Atmospheric?] Manures may be divided into three classes, viz.: _organic_, _inorganic_, and _atmospheric_. ORGANIC manures comprise all _animal_ and _vegetable_ matters which are used to fertilize the soil, such as dung, muck, etc. INORGANIC manures are those which are of a purely _mineral_ character, such as lime, ashes, etc. ATMOSPHERIC manures consist of those organic manures which are in the form of gases in the atmosphere, and which are absorbed by rains and carried to the soil. These are of immense importance. The ammonia and carbonic acid in the air are atmospheric manures. FOOTNOTES: [U] Silicate of potash. CHAPTER II. EXCREMENTS OF ANIMALS. [Of what is animal excrement composed? Explain the composition of the food of animals. What does hay contain? To what does Liebig compare the consumption of food by animals, and why?]