Illustration: (‡ Illustrations of:) 1. EMERALD. 3. FORTIFICATION AGATE. 5. DIAMOND. 2. GARNET. 4. RUBY. 6. ROCK CRYSTAL. All earths are metals in combination with oxygen: that is to say, they can all be separated into a metal and oxygen. The chief earths used are Alumina, found as clay or slate; Lime, found as chalk or limestone; and Silica, found as sand, flint, or rock-crystal. These, in various proportions, combined with some few other matters, form, by far, the greatest portion of the earth we dwell on. The earths, when pure, are all white substances, not very heavy, and having scarcely any of the properties of the metals from which they are derived. The next frequent of the earths are Baryta and Strontia; but these may be said to be useless when compared with the three former. Alumina, in its various forms of clay, is used for brick and tile-making, and for pottery; hardened into the form of slate, it is much used for roofing and for making cisterns. Lime, in the form of carbonate, constitutes the best building stones, including marble, so valuable for ornamental carving and decoration, and when burnt, to separate the carbonic acid, forms lime itself, which is invaluable as a cement when mixed with sand; lime, in union with another acid (sulphuric) forms plaster of Paris, also a most useful article in the arts, &c. Silica, in the form of sand, is very extensively used for glass making, and, in the form of flint, it is ground and used for pottery: all such stones as quartz, rock-crystal, Scotch pebble, agate, cornelian, &c. are but various forms of Silica, either crystallized or deposited in layers. Most of the precious stones are composed of the earths in a crystalline state and colored by some foreign ingredient, such are the emerald, ruby, garnet, &c. The diamond is not an earth but composed of pure carbon. METALS AND ALLOYS. Metals may be known from all other substances by certain properties: they have that peculiar brilliancy, called, for that reason, the “metallic” lustre; they are rapidly heated, and as rapidly cool, hence they are said to be good conductors of heat; they are all opaque, and most of them very heavy; some indeed, as gold and platinum, are the heaviest substances known, being about twenty times heavier than water; they have, moreover, other valuable properties such as the capability of being melted, of being drawn out into wire, of being beaten into thin plates, &c.