When rain falls, the saline matter is dissolved, and descends again to the subsoil,—in dry weather it reascends. Thus the surface soil of any field will contain a larger proportion of soluble inorganic matter in the middle of a hot season than in one of even ordinary rain; and hence the fine dry weather which, in early summer, hastens the growth of corn, and later in the season favours its ripening, does so, among its other modes of action, by bringing up to the roots from beneath a more ready supply of those saline compounds which the crop requires for its healthful growth. 2. _The earthy or insoluble portion._—The earthy or insoluble portion of soils rarely constitutes less than 95 lbs. in a hundred of their whole weight. It consists chiefly of _silica_ in the form of _sand_, of _alumina_ in the form of _clay_, and of _lime_ in the form of _carbonate of lime_. It is rarely free, however, from one or two per cent. of oxide of iron; and where the soil is of a red colour, this oxide is present in a still larger quantity. A trace of magnesia also may be almost always detected, and a minute quantity of phosphate of lime. The principal ingredients, however, of the earthy part of all soils are sand, clay, and lime; and soils are named or classified according to the quantities of each of these three they may happen to contain. If an ounce of soil be boiled in a pint of water till it is perfectly softened and diffused through it, and, after shaking, the heavy parts be allowed to settle for a few minutes, the sand will subside, while the clay—which is in finer particles, and is less heavy—will still remain floating. If the water and clay be now poured into another vessel, and be allowed to stand till the water has become clear, the sandy part of the soil will be on the bottom of the one vessel, the clayey part on that of the other, and they may be dried and weighed separately. If 100 grains of dry soil leave no more than 10 of clay, it is called a _sandy soil_; if from 10 to 40, a _sandy loam_; if from 40 to 70, a _loamy soil_; if from 70 to 85, a _clay loam_; from 85 to 95, a _strong clay soil_; and when no sand is separated at all by this process, it is a pure _agricultural clay_. The _strong clay soils_ are such as are used for making tiles and bricks; the pure _agricultural clay_ is such as is commonly employed for the manufacture of pipes (pipe-clay). Soils consist of these three substances _mixed_ together. The pure _clay_ is a chemical _compound_ of silica and alumina, in the proportion of about 60 of the former to 40 of the latter. Pure clay soils rarely occur—it being well known to all practical men, that the strong clays (tile clays) which contain from 5 to 15 per cent. of sand, are brought into arable cultivation with the greatest possible difficulty. It will rarely happen, therefore, that arable land will contain more than 30 to 35 of alumina.