The earthy part of the soil, therefore, serves a double use. It is not merely, as some have supposed, a substratum in which the plant may so fix and root itself, as to be able to maintain its upright position against the force of winds and tempests; but it is a storehouse of food also, from which the roots of the plant may select such earthy substances as are necessary to, or are fitted to promote, its growth. The ash of plants consists of a mixture of several, sometimes of as many as eleven, different earthy substances. These substances are the following:— 1. _Potash._—The common pearl-ash of the shops is a compound of potash with carbonic acid; it is a _carbonate of potash_. By dissolving the pearl-ash in water, and boiling it with quicklime, the carbonic acid is separated, and potash alone, or caustic potash, as it is often called, is obtained. 2. _Soda._—The common soda of the shops is a carbonate of soda, and by boiling it with quicklime, the carbonic acid is separated, as in the case of pearl-ash. 3. _Lime._—This is familiar to every one as the lime-shells, or unslaked lime of the limekilns. The unburned limestone is a _carbonate of lime_; the carbonic acid in this case being separated by the roasting in the kiln. 4. _Magnesia._—This is the calcined magnesia of the shops. The uncalcined is a _carbonate of magnesia_, from which heat drives off the carbonic acid. 5. _Silica._—This is the name given by chemists to the substance of flint, quartz, and of siliceous sands and sandstones. 6. _Alumina_ is the pure earth of alum, obtained by dissolving alum in water, and adding liquid ammonia (hartshorn) to the solution. It forms about two-fifths of the weight of porcelain and pipe-clays, and of some other very stiff kinds of clay. 7. _Oxide of Iron._—The most familiar form of this substance is the rust that forms on metallic iron in damp places. It is a compound of iron with oxygen, hence the name _oxide_. 8. _Oxide of Manganese_ is a brown powder, which consists of oxygen in combination with a metal resembling iron, to which the name of manganese is given. It exists in plants, and in soils only in very small quantity. 9. _Sulphur._—This substance is well known. It generally exists in the ash in the state of _sulphuric acid_ (oil of vitriol), which is a compound of sulphur with oxygen. It does not always exist in living plants, however, in this state. _Sulphuric acid_ forms with potash a _sulphate of potash_,—with soda, _sulphate of soda_ (or Glauber’s salts),—with lime, _sulphate of lime_ (gypsum),—with magnesia, _sulphate of magnesia_ (Epsom salts),—with alumina, _sulphate of alumina_,—and with oxide of iron, _sulphate of iron_ or green vitriol. When the sulphate of potash is combined with sulphate of alumina, it forms common alum.