These data show strikingly the effects produced by the several physical soil constituents upon some of its physical properties. _Chemical Nature._—While humus artificially produced by the action of caustic alkalies upon sugar or cellulose is free from nitrogen, all naturally occurring humus contains the latter. It is not, however, present in the form of ammonia, as it cannot be set free by treatment in the cold with lime or alkalies. When, however, natural humus is _boiled_ with these substances, ammonia is slowly given off, but the process continues indefinitely and it seems to be impossible to expel all the nitrogen in this manner. This behavior being characteristic of amido-compounds, it is presumable, in view of the slightly acid nature of the humus substances, that natural humus is largely of an amidic constitution. Artificial humic acid, formed by the action of caustic alkalies upon sugar, gums or cellulose, combines with ammonia as with other bases, and at first the ammonia can be readily expelled from this as from other ammonia salts. But after the lapse of some time it seems that the amidic condition is assumed, so that caustic lye acts but very slowly and cannot expel the whole of the nitrogen present. This is very important in connection with the practice of fertilization, as any ammonia taken up by or generated in the soil is thus in the course of time rendered comparatively inert, and unavailable to vegetation until nitrified. _Progressive Changes._—The natural neutral humin and ulmin, as found, _e. g._, in the lower portions of peat beds, are in the course of time by oxidation converted into ulmic and humic acids, capable of combining with bases; by still farther oxidation they form apocrenic and crenic acids, readily soluble in water and in part forming soluble salts with lime, magnesia and other bases. These acids act strongly upon the more readily decomposable silicates of the soil, and in the course of time may dissolve out, and aid in the removal by leaching, of most of the plant-food ingredients as well as the ferric hydrate of a soil. Thus red or rust-colored soils may be rendered almost white by continued “swamping” with stagnant water, and be greatly impoverished; and it is doubtless largely through this agency that the underclays of coal beds and the lower portions of peat beds, as well as peat and coal ashes, are almost wholly destitute of mineral plant food. _The Phases of Humification._—The progressive changes involved in the process of humification of vegetable matter are illustrated in the table below,[42] together with the farther changes by which such matter may ultimately be transformed into the several varieties of coal, and finally into anthracite, which already represents nearly pure carbon, but in nature has sometimes been still farther transformed into graphite (black-lead) and diamond.