CLAY. The general name, clay, is commonly applied to all tenacious sub-soils of a homogene ous nature. The name signifies viscous, or sticky. Pure clay is a mixture of silex (of which quartz, flint, and most sand and sand-stones are composed) and alumina—this latter being com posed of the metal aluminum and oxygen. Alumina is the characteristic ingredient in com mon clay, and is sometimes called pure clay. One of the purest kinds of clay is derived from the decomposition of granitic rocks, which rocks have been considered the original source of clay. In relation to varieties of clay, we find that silica combined with alumina, and holding a definite proportion of water, is deposited, and constitutes the purest form of clay, or kaolin. When separated from the uncombined silica and undecomposed feldspar that may be present, the plastic portion consists of alumina, 44.5; sil ica, 40.0; and water, 15.5, in 100 parts. The water, from the difficult solubility of the com pound in acids, does not appear to be chemically combined to form a hydrous silicate of alumina. But this simple compound of one atom of alum ina and one of silica, with two of water, is not met with unmixed with other ingredients which modify the properties of pure clay. Iron almost always manifests its presence, when the clay is burned, by the red color of its peroxide. Carb onates of lime and magnesia are detected by their effervescence with acids, and their rendering the substance fusible, an effect also produced by the salts of potash and soda derived from the feldspar. Mica, in a fine state of division, and some other minerals, may also be present. As clay-beds, in the process of accumulation, are often the habitats of shell-fish, and may con stitute a soil covered with a vegetable growth, organic matters are also often found in it. According to its purity it is infusible, and retains. its color unchanged when burned. In a dry state it adheres to the tongue, is very absorbent of water, and possesses a peculiar odor called argillaceous. Clay, being gradually heated, parts with its water and diminishes in bulk without cracking. Heated to redness, it forms a solid mass, which retains its form even if placed, when cool, in Water and allowed to absorb this into its pores. Free from foreign substances, it bears the most intense furnace-heat without melting, and is hence well suited for the manu facture of crucibles and fire-brick. When impure, it may melt at a high heat and form a, glaze. Substances axe mixed with it to produce
this effect, as in the manufacture of porcelain. In the manufacture of brick and other objects, it is tempered with sand in order to counteract its tendency to shrink. The most important varieties of clay are those used for the manufac ture of porcelain, called China clay, or kao lin; potter's clay, for the manufacture of earth enware ; common clay, for the manufacture of brick ; fire clay for fire-brick and crucibles; pipe clay, and a clay used in Europe, for making light, porous, bricks which float upon water. Brick clay is usually obtained from alluvial deposits, aud the color depends ulion the oxide of iron contained. At Milwaukee, Wisconsin, is a brick clay entirely free from iron, and hence the burned brick are of a straw or light cream color. Its brick has given the city the name of Cream City. Clay soils may be classified as fol lows: Pure clay, or pipe clay, consisting of sixty per cent. of silica and forty per cent. of alumina, with or with out some oxide of iron, and from which no sandy rnatter can be mechanically separated by decan tation.—Brick clay is the most tenacious clayey soil, consisting of pure clay with five to ten per cent. of sand added.—Clay loam. This is pure clay, containing be sides fifteen to twenty per cent. of sand. Loamy clay is composed of pure clay, with thirty to sixty per cent. of sand added. This last is, with the addition of or ganic matter, the most fertile and easily worked of all the soils. Bc sides these mentioned all so-called sandy loams or sandy soils (except pure sands, rarely found) contain more or less clay, some of them containing not more than ten per cent. of clay and yet, from the organic matter naturally con tained or supplied, many of these soils are extremely fertile if plenty of moisture be present, for the snore sandy the soil the less moist ure will it absorb and hold, unless a supply be present below, to be supplied by capillary attraction. Pure sands are always infertile. So are pure clays, but from en tirely different causes: Sand be cause it is too open and porous, and clay because it is too close and compact. Hence pure sands are made fertile by adding clay. Not so pure clays by adding sand, since twenty per cent. of clay renders a sandy soil compact, while it would require eighty per cent. of sand to render the clay frialile.