ALUMINA. Alumina, or alumine, is one of the component parts of alum, from which it was originally extracted. It is the principal constituent of clays, in which it is found gener ally mixed with other subtances that impart to it a color, it being naturally white. It is a com pound of oxygen and the metal aluminum. The precious stones, corundum, sapphire, emery and other varieties, are nearly pure crystallized alu mina, particularly the sapphire, which is very hard, and is cut by means of diamond powder and is polished on lead wheels with the same powder, or that of emery, a less pure variety of the same mineral. The specific gravity of alumina is 2 (water being unity). It is without smell, and has little or no taste, but is not altogether void of astringency. It has a strong affinity for water, and gives it off with readiney; when heated. The tenacious, sticky nature of clay soils is owing to this property, and is marked in proportion as the soil approaches to pure alumina. As it gives off water under the effects of the sun or the wind, it hardens, cracks, and becomes obdurate and unyielding. Of all soils pure clay is therefore the most difficult to man age or subdue by cultivation. Sir Arthur Young has said that no man ever throve upon a stiff soil. Strong teams are required to work it, and this can be done at the proper moment only, as it is not only difficult and expensive, but to a certain extent injurious at other dines. Clay has merits which should not be underrated, such as the retention of fertilizers more effectually per haps than any other material, and its capability of absorbing ammonia when exposed to the air, as is apparent to the smell when a piece of clay is breathed upon, for the peculiar odor emitted on such an occasion is due to the presence of this alkali. Alumina is rather an essential Mement of soils, or purveyor of food, than a constituent of plants; for the ashes of plants rarely contain it, and even then but sparingly. There is reason to suppose that those analyses which give alumina as a constituent in certain plants are erroneous, and that what was rendered as alumina may have been in reality- phosphate of lime. Thus, it is stated that three-fifths of a grain of alumina are found in thirty-two ounces of the grains of wheat, and about four grains in thirty ounces of the grains of barley and of oats. It is also said to constitute 3.72 parts in 100 of the entire plant of the sunflower, 7 11 of the entire plant of Turkey wheat, and 14 of the entire plant of the fumitory. It is seldom or never found uncombined, but rather in the form of silicate, sulphate, or phosphate of alumina. It is an important part of the crystalline rocks. Aluminous minerals, so manifestly essential to the fertility of soils, are extensively diffused throughout the surface of the globe. They occur in all soils susceptible of cultivation, which may be owing to the fact that they are retentive of fertilizing salts; and, according to a paper lately published by Prof. Voelcker, alumi nous earths possess the power of absorbing and retaining fertilizing principles in such a state of combination as to readily yield them to plants, without their being subject to detraction from that combination by rain or water as it usually falls or passes through soils. In order to form a distinct conception of the quantities of alkalies in aluminous minerals, says Liebeg, it must be remembered that feldspar contains 17t per cent. of potash, albite (soda feldspar) 11.43 per cent. of soda, and mica from 3 to 5 per cent., and that zeolite contains from 13 to 16 per cent.
of alkalies. The analyses of Gmelin, Lowe, Fricke, Meyer and Redtenbacher, have also shown that basalt and clinkstone contain from to 3 per cent. of potash, and from 5 to 7 per cent. of
soda; that claystone contains from 2.75 to 3.31 per cent. of potash, and loam from 1i- to 4 per cent. of potash. The quantities of these sub stances present in any specimen can not give a correct estimate of its presence in all portions of the rock; nor are the alkalies the only fertilizers to be anticipated, where analysis has not hereto fore mentioned or detected their presence, since the presence of phosphoric acid has been shown in a variety of rocks which were considered free frorn that substance and from all organic remains, such as clinkstone, phonolites, horn blende, augite, compact basalt, trap rock, pumite stones, obsidian, mica, granite, chlorite-slate, porphyry, mica-slate and gneiss, and in native borax from the East Indies. Soils eminently aluminous, as has been stated, are absorbents of water and retentive thereof. This renders them stiff, waxy and cold, as well as damp, and exceedingly difficult to cultivate and subdue. It has been said that this property of clay soils may be modified by the application of sand; but this is often expensive, and at all times laborious and attended with difficulty in making the ad mixture. The cohesive nature of clays may be overcome, and they made comparatively porous, dry, warm and fertile, by the operation of paring and burning, the particles being thus partly fused and made to cohere and form a gritty mass containing the elements inherent to the clay with the properties of sand Besides this, burnt clays have an increased power for the absorption of ammonia; but clays thus heated lose a great pro portion of their retentiveness of moisture. The most cohesive clays may, by this process, be con verted into a fine, dry, powdery soil as light as an ash bank; and where rushes, and coarse grasses, and semi-aquatic plants have got hold, a new growth will come in, and the soil will become fertile and easily cultivated. The roots and fibers of weeds and plants, and all noxious seeds, may be eradicated by paring and burning; and in a vast number of instances soils of a clayey nature that can not otherwise be brought into economical clilture, may by this operation be rendered fruitful. The alteration in the inert vegetable fiber existing in soils, when subjected to this operation, will often have a decided effect upon its productiveness, by converting useless and injurious matter into assimilable plant food; or a .product may be thus obtained hav,ing an advantageous chemical action upon the inorganic constituents of the soil. The transformation of inert vegetable fiber by charring, or even roast ing, will add to the absorbent quality of the soil in proportion to the amount of those substances in the earth; for it is known that charcoal has surprising qualities of absorption and conden sation, not of water, but of those gases which have an evident influence upon fertility. The paring and burning process, though applicable and advantageous to deep clays that are barren from their closeness and their wetness, would be followed by disastrous consequences if applied to shallow arenaccous lands. In this case it would drive off or destroy even the scanty vegeta ble matter that such a soil might contain, leaving an arid sandy waste. Sandy soils are frequently too porous, and the heat generated by paring and burning could not give consistence to such a material as silicious sand, since silex, however fusible when in contact with bases such as alu mina, lime, magnesia and the alkalies, is refractory by itself.