CHAPTER IV.. Of the Inorganic Constitution of Plants—Their

Of the Inorganic Constitution of Plants—Their immediate Source—Their Nature—Quantity of each in certain common Crops.

SECTION I.—SOURCE OF THE EARTHY MATTER OF PLANTS—SUBSTANCES OF WHICH IT CONSISTS.

When plants are burned, they always leave more or less of ash behind. This ash varies in quantity in different plants, in different parts of the same plant, and sometimes in different specimens of the same kind of plant, especially if grown upon different soils; yet it is never wholly absent. It seems as necessary to their existence in a state of perfect health as any of the elements which constitute the organic or combustible part of their substance. They must obtain it therefore along with the food on which they live: it is in fact a part of their natural food, since without it they become unhealthy. We shall speak of it therefore as the inorganic food of plants.

We have seen that all the elements which are necessary to the production of the woody fibre, and of the other organic parts of the plant, may be derived either from the air, from the carbonic acid and watery vapour taken in by the leaves, or from the soil, through the medium of the roots. In the air, however, only rare particles of inorganic or earthy matter are known to float, and these in a solid form, so as to be unable to enter by the leaves; the earthy matter which constitutes the ash, therefore, must be all derived from the soil.

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.

10. Phosphorus is a soft pale yellow substance which readily takes fire in the air, and gives off, while burning, a dense white smoke. The white fumes which form this smoke are a compound of phosphorus with oxygen obtained from the air, and are called phosphoric acid. In the ash of plants the phosphorus is found in the state of phosphoric acid, though it probably does not all exist in the living plant in that state.

Phosphoric acid forms phosphates with potash, soda, lime, and magnesia. When bones are burned, a large quantity of a white earth remains (bone-earth), which is a phosphate of lime, consisting of lime and phosphoric acid. Phosphate of lime is generally present in the ash of plants; phosphate of magnesia is contained most abundantly in the ash of wheat and other varieties of grain.

11. Chlorine.—This is a very suffocating gas, which gives its peculiar smell to chloride of lime, and is used for bleaching and disinfecting. It is readily obtained by pouring muriatic acid (spirit of salt) on the black oxide of manganese of the shops. In combination with the metallic bases of potash, soda, lime, and magnesia, it forms the chlorides of potassium, sodium (common salt), calcium and magnesium, and in one or other of these states it generally enters into the roots of plants, and exists in their ash.

Potash, soda, lime, and magnesia, are compounds of the metals here named with oxygen. It is a very striking fact, that the suffocating gas chlorine, when combined with sodium, a metal which takes fire when placed upon water, should form the agreeable and necessary condiment, common salt.

Such are the inorganic substances usually found mixed or combined together in the ash of plants. It has already been observed, that the quantity of ash left by a given weight of vegetable matter varies with a great many conditions. This fact deserves a more attentive consideration.