Assay of Gold.—The touchstone is em ployed to determine approximately the quality of the gold. It is a hard, black, silicious or flinty slate known also as Lydian stone. Basalt and black Wedgewood ware are also employed. The metal to be examined is rubbed on the stone (any plating or coloring being first scraped off), and the streak compared with that made by needles of known composition differing from each other by. one-half carat. The streak is also treated with nitric acid anda test acid, and the result of their action observed. Three or more sets of needles are employed, the chief being a gold-copper series, a gold silver series and a gold-silver-copper series. Sometimes five sets are employed, in which the proportions of silver and copper are varied. The series to which the article tested belongs is determined by comparison for color, hard ness and toughness, the latter being inferred from the dryness or greasiness of the streak. The streak is first treated with pure nitric acid (applied with a feather), which is afterward rubbed off. With brass and other spurious cop per alloys the streak is completely and instantly dissolved, while poor gold leaves a very faint impression. Nitric acid does not affect any alloy above 15 carats fine. A test acid consist ing of 98 parts nitric acid (specific gravity 1,34) and two of hydrochloric acid (specific gravity 1.173) is used if the streak has been unaffected by the nitric acid. Gold of 18 carats fine and over is not affected by this mixture in the cold. This method of testing is only used when a rough idea of the richness is all that is necessary for valuation purposes.
Accurate. assays of gold alloys are made by wrapping a weighed quantity (either five grains or 0.5 gram) in sheet-lead, with sufficient sil ver to equal three times the weight of pure gold present. Lead to the amount of 34 times the weight of the sample is used for all alloys con taining less than 50 per cent of gold, and less for richer alloys. The sample is dropped on to a cup. (cupel) previously heated to full redness in a muffle furnace. The copper and all base metals in the alloy are oxidized, and the oxides dissolved in the molten litharge formed by the oxidation of the lead added. The fused oxides are absorbed by the. porous cupel, thus keeping the metallic surface clear, and at the end of the operation only silver and gold remain behind. After cooling, the button is rolled into a ribbon, annealed, coiled up and boiled first in nitiric acid of 1.16 specific grav ity, and afterward in nitric acid of 1.26 specific gravity, to dissolve out the silver, and after washing the coherent cornet of gold is heated to dull redness in an annealing cup and weighed: The addition of silver in assaying is known as inquartation. It is necessary owing to the pro tective action exerted by gold on other metals. r Origin and In nature gold usually occurs native, that is, uncornbined, Less commonly it is found in combination with tellurium, as tellurides, of which calaverite (AuTe,) is a well-known example. The na tive gold is often mixed with iron pyrites, the mixture being called auriferous pyrite. Both
the tellurides and the auriferous pyrite usually occur in fissure veins in the rock, into which position they are usually believed to have been introduced by hot waters from deep seated ig neous activity. The Cripple Creek district rep resents the type of the telluride deposits; and the Mother Lode in California the type of native gold and auriferous pyrite. When gold bearing veins of this sort are exposed at the surface the rock weathers faster than the gold. As the lighter .rock particles are washed, away the heavier gold remains behind associated with gravels in the. form of placer deposits. California and Alaska have been the great placer districts of the United States. See PLACERS AND ORE DEPOSITS ; GOLD MINING AND I World Distribution of Gold.— Gold is very I widely distributed, smaller or larger quantities being found in nearly every country. The cients obtained gold from the Spanish Penin- sula, Greece, Asia Minor and India. The Ophir of the Bible has been variously located. bly it was in East Africa. In more modern times Peru, Bolivia, Brazil, Chile, Mexico and other countries of South and Central America furnished immense supplies of gold after the discovery of America until about 1850. By far the greatest discoveries of gold were made during the 19th century. The discovery of the Californian placers in 1848, and of the Atm. tralian placers in 1851, produced a mad rush to the diggings. In 1858 gold was found in New Zealand, and in 1861 the Otago district became a large producer. Since then Imniense ments have taktn place. Besides California; Colorado Montana, Arizona, South Dakota, Idaho daho and others of the Unted States have furnished, and still furnish, large supplies. and Alaska and the Philippines must also be added. British Columbia is an important source, both alluvial and quartz mining being followed. Canada has also entered the lists as a gold-producer, and the phenomenal deposit! at Nome and in the Klondike repot) in the Yukon Basin continue to be a productive field. The rich finds in western Australia, in the goorlie and Coolgardie districts, some years ago, placed that colony in the front rank as a gold-producer, while Victoria, South .Austrilia and New South Wales have long been large gold-producing countries. In Victoria much energy in the development, more especially •tif "deep lead° mining, is being put forth. The Witwatersrand district of the Transvaal has sprung into importance since 1,886,, and Jo hannesburg is now the largest gold-mining centre of the world. Russia is also an import ant producer, the gold being obtained beyond the Ural Mountains. India also produces a considerable amount. Of the prospective gold fields the most likely are British the hinterland of the Gold Coast, certain parts China and East Africa. Gold has been found in several parts of the United ICingdom, prin cipally around Dolgelly, in Merionethshire, in Sutherlandshire, in the Lead Hills, in the Wick low Mountains and other places in Ireland. For world's production of gold table under GOLD MINING AND METALLURGY.