Pure gold has a tenacity of about seven tons per square inch, and elongates about 30 per cent before breaking. A wire one-tenth of an inch thick will support nearly 200 pounds. Its alloys with copper and silver are stronger. Standard gold has a tenacity of 18 tons (Aus ten), and extends 34 per cent before breaking. At very low temperatures this is greatly him creased (Dewar). Gold is the most malleable of metals, and can be reduced to thin leaves by hammering. (See GOLD-BEATING). Such leaves sometimes do not exceed rule.. of an inch in thickness, and transmit green light, though presenting an unbroken metallic surface. This is best seen by mounting on glass. The extreme thinness and high lustre of the metal have led to its use as an illustration of the eittreme divisibility of matter. A particle of gold weighing only mho. of a grain is readily visible to the naked eye. A gram of gold can be made to cover nearly 80 square inches of surface. The malleability of gold is seriously affected by the presence of minute quantities of arsenic, antimony, bismuth, lead, sulphur, selenium and tellurium. Of the last 0.2 per cent, and of bismuth 0.5 is sufficient to render the metal crystalline and brittle. Traces of the above elements unfit the metal for gold-beating and coinage. These are removed by passing chlorine gas through the molten metal, or by treating the molten metal with mercuric chloride (corrosive sublimate). By continued hammer ing the metal is slightly hardened, and must be annealed. Gold surpasses all other metals in respect of ductility. The extreme ductility of the metal is shown by the fact that wires less than &tin of an inch thick were obtained by Wollaston by encasing a wire of gold in silver and drawing down the compound wire. The silver was dissolved off by treatment with nitric acid. A length of 500 feet of such wire weighs only one grain. Gold wire is used for malang gold lace. Gold is also extremely flexible and tough. Gold comes from the refinery in the form of bullion bars, and has to be assayed to determine its value. The bulk of it goes to a mint for coining. In the United States, on reaching the mint, the gold is melted with 10 per cent of copper and recast into ingots of a convenient size for coining. See COINAGE.
Chemical Properties.— Gold alloys readily with most metals. It is rapidly attacked by mer cury, and dissolves in excess of that metal. If the liquid amalgam be squeezed through wash leather a yellow pasty mass remains, which may i be used in lwashlo or (fire' gilding. This proc ess, however, has been displaced by electro-gilding, in which the bath consists of the double cyanide of gold and potassium, and is used hot. The metal is unattacked by any of the simple acids, save selenic, but dissolves in any mixture in which chlorine, bromine or iodine is liberated. The common solvent is aqua regia, a mixture of one part nitric acid and three or four of hydrochloric acid. The chlorine liberated from this mixture converts the gold into the tri chloride, an exceedingly soluble body of high tinctorial power, yielding yellow solutions. In the finely-divided state gold is dissolved by chlorine water, bromine water and iodine solu tion or tincture, the trichloride, tribromide and triodide being produced. It also dissolves in
potassium cyanide solutions (in the presence of air) and in cyanogen bromide. These solvents are employed in the extraction of gold from its ores. Oxides of gold can be prepared with some difficulty. The monoxide is thrown down when caustic potash is boiled with gold chloride solution to which a little acetate of soda has been added. A trioxide (Au,08), a dioxide (Au,O,), and a tetroxide (Au30.) are also known. What is known as fulminating gold, Aus0s(NH,), may be prepared by adding am monia to a solution of chloride or by steeping the hydroxide in ammonia. It is a green or brownish powder, which detonates violently when gently heated or when struck. Two classes of gold salts exist. Of the aurous salts, the principal are sodium auro-sulphite, auro-thiosulphate, the cyanide and potassium auro-cyanide. Of the auric salts, the principal are the trichloride and the chlor-aurates. Gold trichloride crystallizes from solution in dark orange-red crystali, AuCluff30. It is extremely soluble water, and volatilizes at 300° in a stream of chlorine gas, but is decomposed on heating to 200° in air with the formation of the monochloride and chlorine, and, at a highee temperature, of gold. It is soluble in ether, naphtha and essential oils.
The chlor-aurates are combinations of gold chloride with sodium, potassium and other alkaline chlorides. The best known are KCI.AuC1821LO, and NaCi.AuC1..21170, salts commonly sold as gold chloride for photo graphic purposes. Gold is precipitated from solution by most metals. Iron, copper and zinc precipitate it readily, generally in a more or less pulverulent form devoid of metallic ap pearance. Oxalic acid, ferrous sulphate, sul phur dioxide and sodium sulphite, carbon,• grape-sugar and many organic reducing agents, precipitate the gold from gold chloride. In some cases the metal is so finely divided that it imparts a ruby color to the liquid and does not settle for months. Purple of Cassius is the fine purple pigment produced by treating gold chloride solution with a mixture of tin c,h10 rides, that is, stannous chloride containing a little stannic salt. It is used in glass staining, pottery and enamel painting, and for coloring artificial gems, imparting a pink, rose or red color. The addition of tin chloride to the solu tion obtained by treating an ore with aqua regia and boiling off the excess acid is a deli cate test for the presence of gold. Finely divided gold imparts to pottery and glass a color varying from pink to ruby. Gold resists chemical action to a greater extent than plat inum or any other common metal, and in alloys protects base metals from the action of acids to a remarkable extent. Owing to the high specific gravity of gold (19.3) it is possible to roughly determine the richness of the alloy by taking the specific gravity of the article. This is im possible where platinum (specificgravity 21.4) is present in the alloy. The specific gravity of standard gold is 17.157, and of 18-carat gold 16.8.