Before considering farther the subject of animal excrement, it is necessary to examine a class of manures known as _absorbents_. These comprise all matters which have the power of absorbing, or soaking up, as it were, the gases which arise from the evaporation of solid and liquid manures, and retaining them until required by plants. The most important of these is undoubtedly _carbon_ or charcoal. CHARCOAL. _Charcoal_, in an agricultural sense, means all forms of carbon, whether as peat, muck, charcoal dust from the spark-catchers of locomotives, charcoal hearths, river and swamp deposits, leaf mould, decomposed spent tanbark or sawdust, etc. In short, if any vegetable matter is decomposed with the partial exclusion of air (so that there shall not be oxygen enough supplied to unite with all of the carbon), a portion of its carbon remains in the exact condition to serve the purposes of charcoal. [What is the first-named effect of charcoal? The second? Third? Fourth? Explain the first action.] The offices performed in the soil by carbonaceous matter were fully explained in a former section (p. 79, Sect. 2), and we will now examine merely its action with regard to manures. When properly applied to manures, in compost, it has the following effects: 1. It absorbs and retains the fertilizing gases evaporating from decomposing matters. 2. It acts as a _divisor_, thereby reducing the strength (or intensity) of powerful manures--thus rendering them less likely to injure the roots of plants; and also increases their bulk, so as to prevent _fire fanging_ in composts. 3. It in part prevents the leaching out of the soluble parts of the ash. 4. It keeps the compost moist. The first-named office of charcoal, _i. e._, absorbing and retaining gases, is one of the utmost importance. It is this quality that gives to it so high a position in the opinion of all who have used it. As was stated in the section on soils, carbonaceous matter seems to be capable of absorbing every thing which may be of use to vegetation. It is a grand purifier, and while it prevents offensive odors from escaping, it is at the same time storing its pores with food for the nourishment of plants. [Explain its action as a divisor. How does charcoal protect composts against injurious action of rains? How does it keep them moist?]