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.
Copper seems to have been one of the few metals known to barbarous peoples. It is found pure, and in combination. Specimens obtained from the Lake Superior region, in mines worked by the mound builders,[10] have led some to believe that they possessed the art of hardening copper. Malachite is a carbonate of copper, of a beautiful mottled green color, and is made into elegant ornaments. Some magnificent specimens were in the Russian exhibit at the Philadelphia Exposition. It is found in great perfection in the Ural mountains.
Tin is obtained from its binoxide (SnO₂). It was known to the ancients. Some historians claim that the Phœnicians procured it long before the time of Christ, from the mines of Cornwall, England. Until recently our country has seemed to be destitute of this valuable metal. Reports now indicate that Dakota is destined to supply this deficiency. It is a handsome metal, but little affected by oxygen, and capable of being rolled into thin sheets.
Zinc is found in two different ores: red oxide (ZnO) and zinc blende (ZnS), from which it can be separated by smelting, in much the same manner as we obtain iron.
Lead constitutes the fifth of the common metals which are preëminently useful. It is found in the sulphide of lead (PbS), the sulphide being expelled by roasting the ore. It forms numerous compounds, some of which are of great value. For example, lead carbonate (PbCO₃), the white lead which furnishes the most valuable ingredient of all paints.
NOBLE METALS.
These are so called because they retain their brilliancy and are not easily affected by other substances. Three of them are specially important: gold, silver and platinum. Gold is mentioned in the second chapter of Genesis: “and the gold of that land is good.” Although constituting an inconsiderable part of the earth, it is much more widely distributed than many suppose, but often exists in such small quantities that its production is not profitable.