History is repeating itself by renewing the ancient enthusiasm for decoration of china and earthen ware. Bricks made from clay are found to rival granite in durability, and surpass it in resistance to heat, as was proven in the great fires of Boston and Chicago. It will be observed from the symbol of alumina that it is largely composed of the metal aluminum. If this could be readily liberated from the oxygen with which it is combined, the world would be immensely enriched. Every clay bank or clayey soil contains it in great quantities. Next to oxygen and silicon, it is the most abundant element in the earth. Note its valuable properties. It is but two and one-half times heavier than water, as bright and non-oxidizable as silver, malleable, ductile, tenacious, and can be welded and cast. Who will lay the world under obligation by doing with alumina what has been done with iron ores, cheaply liberate the oxygen? [Illustration: TESTING FOR IRON WITH A BORAX BEAD.—THE COMPOUNDS OF IRON WITH BORAX GIVE A BOTTLE GREEN COLOR.] In this brief enumeration of earth materials, we have intentionally omitted the forms of carbon. They constitute no insignificant portion of the earth’s crust, but belong to the class of organic substances. We introduce, however, an illustration showing one of the shapes in which is cut the diamond—that most costly of all forms of matter,—crystallized carbon. THE COMMON METALS. First in importance is iron. The fact already mentioned that its oxide is the most common coloring matter in the mineral world will also indicate its wide dissemination. Trap rock, gneiss, even granite, sands, clays and other rocks all borrow tints from this source. Iron is never found native except in meteors. It exists most abundantly in the form of three ores, the composition of which is as follows: Black or magnetic oxide (Fe₃O₄), red oxide (Fe₂O₃), hydrated sesquioxide (Fe₂O₃3H₂O). From all of these the oxygen is removed in a blast furnace, by the use of some form of carbon. As thus prepared, it is called cast-iron. Two other varieties are employed in the arts, wrought iron and steel. The last differs from the first in having less carbon, and from the second in having more. The general properties of this material are too well known to require description here. A single property of this substance alone has marvelously affected the commerce of the world; that is, the power first discovered in magnetic iron ore, of attracting iron, and pointing northward. The first compass, it is said, consisted of a piece of this metal placed on a cork floating on water.