Page:The American Cyclopædia (1879) Volume VIII.djvu/80

72 may be collected upon a sheet of paper placed beneath the wire; the paper is stained a purplish brown by the deposit of finely divided gold, and a sheet of silver may be thus gilded. When gold is fused in large quantity and allowed to cool slowly, cubical crystals are sometimes observed to form, and crystals of native gold have been found in the form of the regular octahedron. Gold is not acted upon by alkalies or simple acids, except selenic, nor by the oxygen of the air even when long exposed in a fused state. Neither does sulphur affect it; but it is dissolved by bromine and chlorine, or by any combination of acids or other substances in which free chlorine is present. This element, as it is generated in mixtures, is a powerful solvent of gold; and to it is due this property of the compound called aqua regia, formed of 4 parts of hydrochloric and 1 part of nitric acid. Gold forms alloys with most of the metals. Silver or copper increases its hardness and renders it better adapted for wear when used for coins, jewelry, or plate. Such compounds are also more fusible than pure gold. The solder for gold trinkets is 1 part of copper to 5 of gold, or to 4 of gold and 1 of silver. With mercury gold unites to form an amalgam. Mercurial fumes even, coming in contact with gold, instantly combine with and whiten it. The mercury may be driven off by heat. (See .) Gold is obtained from its solutions in various forms. The precipitate by sulphate of iron is a dull brown powder, which by pressure acquires the metallic lustre and color. The precipitate by oxalic acid is yellower and more metallic in appearance. The metallic gold which is left on evaporating a solution of its compound with chlorine and heating the residue is of a spongy character and dull hue; by annealing it becomes more dense and yellow, and by percussion is readily welded together. (For modes of preparing sponge gold and its uses, see .)—Gold is very widely distributed in nature, and late researches have shown that it is present in appreciable quantities in the waters of the ocean, where it is associated with silver. According to Sonstadt, a ton of sea water yields by a simple chemical process a grain of gold; so that the quantity of the precious metal thus held in solution must be vastly greater than all the gold ever yet extracted from the earth. Gold is very generally diffused throughout the solid rocks, though only here and there accumulated in sufficient quantities to be economically available. The workable deposits of this metal are in stratified rocks of different formations, from the oldest crystallines to the postpliocene sands and gravels, and also in veins traversing rocks of various geological periods. The most common veinstone of gold is quartz, but it is also found in bitter spar and disseminated in metallic sulphides, such as iron pyrites, which very often contains sufficient quantities of the metal to be extracted with profit. In this as well as in the

quartzose gangues the gold is sometimes in large grains or crystalline threads or masses, and sometimes disseminated in particles invisible to the eye. The opinion is entertained by many that in pyrites and in other sulphuretted ores the gold is sometimes chemically combined with the other metals and with sulphur. It has been found that the lead of commerce, from whatever source derived, is seldom or never without a trace of gold. Gold is not, as has been erroneously supposed, confined to rocks of any one geological period. The gold of Colorado is found in veins with metallic sulphurets traversing crystalline rocks of eozoic age, and the same is the case in Ontario; while the gold-bearing strata of the Appalachians are in large part if not wholly of prepalæozoic age, as are those of the Alps and the Ural mountains. In Nova Scotia, on the contrary, the gold-bearing rocks are slates and sandstones, supposed to be of lower Cambrian age; and the auriferous strata of Wales as well as those of Australia are of that period. The gold-bearing veins extensively worked in Transylvania traverse sandstones of eocene or early tertiary age, and the gold-bearing quartz of California is said to be found in strata of the Jurassic formation. It is probable, however, that a part of the auriferous rocks of that country will be found to be eozoic, while on the other hand it appears that the silicious deposits now forming from the thermal waters in Nevada contain not only metallic sulphurets but small portions of gold; so that the processes which in former times gave rise to gold-bearing veins in that region are still in operation.—By the disintegration and crumbling away of the rocks which contain the auriferous veins, the contents of these are swept down to lower levels, and the gold by its density always seeks the lowest places among the moving materials. Thus are produced the auriferous gravel deposits in alluvial formations, the golden sands of the rivers; and so have they been gathering for long ages past and forming deposits, some of which are now seen in situations apparently out of reach of such agencies. In these deposits, when stripped of the clay and sands which cover the lower and richer layers, there are found in the irregular-shaped cavities of the surface of the rock, in pockets and in piles against the projecting strata, the accumulated riches of ancient veins, it may be, of vast extent. By washing away the intermixed earthy and stony matters, the metal is obtained in dust, flattened scales, small lumps, and nuggets of all sizes and shapes, the larger pieces rounded by attrition, or ragged from the irregular forms they held in their original hard quartz matrix. Their size is commonly greater than that of gold found in the veins near by, a fact first explained by the late Oscar Lieber of South Carolina to be due to the solution of gold and its subsequent aggregation. Later observations of Genth and Selwyn go to confirm this view. In these deposits the largest lumps