Gold is one of the heaviest of the metals, and not being liable to be injured by exposure to the air, it is well fitted to be used as coin. Its ductility and malleability are very remarkable. It may be beaten into leaves so exceedingly thin that one grain in weight will cover fifty-six square inches, such leaves having the thickness of only 1/282000th part of an inch. It is also extremely ductile; a single grain may be drawn into a wire 500 feet long, and an ounce of gold covering a silver wire is capable of being extended upwards of 1,300 miles. It may also be melted and remelted with scarcely any diminution of its quantity. It is soluble in nitromuriatic acid and in a solution of chlorine. Its specific gravity is 19.3, so that it is about nineteen times heavier than water. The fineness of gold is estimated by carats, pure gold being twenty-four carats fine.

Jeweler's gold is usually a mixture of gold and copper in the proportions of three-fourths of pure gold with one-fourth of copper. Gold is seldom used for any purpose in a state of perfect purity on account of its softness, but is combined with some other metal to render it harder. Standard gold, or the alloy used for the gold coinage of Britain, consists of twenty-two parts of gold and two of copper (being thus twenty-two carats fine).

Articles of jewelry in gold are made of every degree of fineness up to eighteen carats, i. e., eighteen parts of gold to six of alloy. The alloy of gold and silver is found already formed in nature, and is that most generally known. It is distinguishable from that of copper by possessing a paler yellow than pure gold, while the copper alloy has a color bordering upon reddish yellow. Palladium, rhodium and tellurium are also met with as alloys of gold.

Gold has been found in smaller or larger quantities in nearly all parts of the world. It is commonly found in reefs or veins among quartz, and in alluvial deposits; it is separated, in the former case, by quarrying, crushing, washing and treatment with mercury. The rock is crushed by machinery and then treated with mercury, which dissolves the gold, forming a liquid amalgam; after which the mercury is volatilized, and the gold left behind; or the crushed ore is fused with metallic lead, which dissolves out the gold, the lead being afterwards separated by the process of cupellation.

By the "cyanide process," in which cyanide of potassium is used as a solvent for the gold, low-grade ores can be profitably worked. In alluvial deposits it is extracted by washing, in dust grains, laminae or nuggets.

In modern times large supplies of gold were obtained after the discovery of America from Peru, Bolivia, and other parts of the New World. Till the discovery of gold in California, a chief source of the supply was the Ural Mountains in Russia. An immense increase in the total production of gold throughout the world was caused by the discovery of gold in California in 1848, and that of the equally rich gold fields of Australia in 1851. The yield from both sources has considerably decreased. Other sections of the United States have of late years proved prolific sources of gold, especially Colorado, which now surpasses California in yield, and Alaska, which equals it. Canada has gold fields in several localities, the richest being those of the Klondike.

Casting Ingots.

Rolling Room The upper view shows the melting room in the United States Mint, Philadelphia. The man at the right is about to pour hot metal into the iron molds. The lower view is in the coining department, where the ingots such as are seen on the truck in foreground, are rolled into long strips of the thickness of the several coins, and then cut into blanks or planchets..

At present the richest gold field in the world is that of South Africa, which yielded in 1910 a value of $175,000,000, somewhat exceeding the combined yield of the United States and Australia. Russia and Mexico followed these in yield. The total production throughout the world amounted to over $450,000,000, of which the United States produced $96,000,000.