291. Soils.--Beds of sand, clay, etc., are disintegrated rock. Sand is chiefly SiO2; clay is decomposed feldspar, slatestone, etc. Soils are composed of these with an added portion of carbonaceous matter from decaying vegetation, which imparts a dark color. The reddish brown hue so often observed in soils and rocks results from ferric salts. 292. Minerals, of which nearly 1000 varieties are now known, may be simple substances, as graphite and sulphur, or compounds, as galena and gypsum. Only seven systems of crystallizations are known, but these are so modified as to give hundreds of forms of crystals. See Physics. A given chemical substance usually occurs in one system only, but we saw in the case of S that this was not always true. Crystals of some substances deliquesce, or take water from the air, and thus dissolve themselves. Some compounds cannot exist in the crystalline form without a certain percentage of water. This is called "water of crystallization"; if it passes into the air by evaporation, the crystal crumbles to a powder- and is then said to effloresce. 293. The Earth's Interior.--We are ignorant of the chemistry of the earth's interior. The deepest boring is but little more than a mile, and volcanic ejections probably come from but a very few miles below the surface. The specific gravity of the interior is known to be more than twice that of the surface rock. From this it has been imagined that towards the center heavy metals like Fe and Au predominate; but this is by no means certain, since the greater pressure at the interior would cause the specific gravity of any substance to increase. 294. Percentage of Elements.--Compute the percentage of O in the following rocks, which compose a large proportion of the earth's crust: SiO2, Al2SiO4, CaCO3. Find the percentage of O in pure water. In air. Taking cellulose, C16H30O15, as the basis, find the percentage of O in vegetation. An estimate, based on Bunsen's analysis of rocks, of the chief elements in the earth's crust, is as follows:-- O, 46 per cent Ca, 3 per cent Si, 30 per cent Na, 2 per cent Al, 8 per cent K, 2 per cent Fe, 6 per cent Mg, 1 per cent More than half the elements are known to exist in sea-water, and the rest are thought to be there, though dissolved in such small quantity as to elude detection. What four are found in the atmosphere?CHAPTER LIV. ORGANIC CHEMISTRY.