s, next to iron, the most widely diffused metal on the face of the earth. It occurs in granite, the oldest rock known to us, and in all the rocks derived from it; it is also found in the vein-stones which traverse other geological formations. From other metals it is readily distinguished by its reddish-yellow colour, and from metallic compounds of a similar tint by its high specific gravity, which varies from 19·2 when it is fused, to 19·4 or 19·5 when it is hammered. Its chemical equivalent on the hydrogen scale is generally taken as 98·5, but some prefer to double this and make its atomic weight 197. Its symbol is \( Au \), from the Latin Aurum.
Unlike the great majority of the metals, it does not rust, i.e., oxidize in the air, neither does it, if pure, tarnish by exposure. In this respect, it contrasts strikingly with silver, which, though indifferent to the rusting action of oxygen, is rapidly blackened by the sulphuretted hydrogen of the atmosphere. Exposed gilding tarnishes, but only because it is alloyed with silver and copper, on which this prejudicial gas can act.
Gold is readily crystallizable, and always assumes one or other of the symmetrical shapes, such as the cube, or regular octahedron, which characterize the simplest crystallographic system. It is softer than silver, and nearly as soft as lead, so that in tenacity it is inferior to copper, silver, iron, and platinum, and a wire \( \frac{1}{3} \)th of an inch in diameter will support without breaking only 191 lb. On the other hand, it is the most ductile and malleable of the metals. One grain can be hammered into leaves sufficient to cover 56 square inches, and the thickness of the gold-leaf will not then exceed \( \frac{1}{28,000} \)th of an inch. When of this tenacity it is transparent, and transmits a faint but beautiful bluish-green light.
Gold melts at a high white heat, and remains unchanged in the hottest furnaces. In the focus of a lens, however, it is vaporized by the sun's rays; and the oxyhydrogen blowpipe or a large voltaic battery can also develop heat sufficient to volatilize it. It contracts in the act of solidifying from a state of fusion, and cannot, in consequence, be made to receive sharp impressions by casting it in moulds. Coins, accordingly, and plate are stamped or embossed, and afterwards chased and carved, if necessary, by cutting tools.
Gold does not dissolve in any of the ordinary acids, such as the nitric, sulphuric, hydrochloric, or acetic, but a rare acid, the selenic, can dissolve it. Its best solvent is a mixture of hydrochloric acid, with some oxidizing agent like nitric acid or oxide of manganese, which causes the hydrochloric acid to part with its chlorine. If the gold be in leaf, chlorine at once unites with it, and the resulting chloride is readily soluble in water. Bromine acts in the same way on the metal; and it may also be dissolved by boiling it with sulphur, potash, and water. The older chemists speculated on the possibility of Moses having dissolved the golden calf of the Israelites in this way.
Gold is also soluble in mercury, and advantage is largely taken of this property of quicksilver to dissolve the precious metal from its ores. The gold-amalgam resulting from the union of the metals is also extensively employed in gilding.
The most important chemical compounds of gold are the following, in referring to which the equivalent of gold is taken as 98·5.
The chlorides are two in number. The sesquichloride, \( AuCl_3 \), is prepared in the mode above mentioned. It forms orange-red crystals, but in aqueous solution appears yellow. It is very easily decomposed by heat, light, organic substances, and all deoxidizing or reducing agents. A solution of this salt in sulphuric ether is sometimes used for gilding steel. The aqueous solution is employed in photography, and from it nearly all the other useful preparations of gold are made. When this salt is heated cautiously to about 392° Fahr., it loses two-thirds of its chlorine, and becomes the sub-chloride \( AuCl_2 \).
The oxides correspond to the chlorides, and are obtained from them. The only important one is the sesquioxide \( Au_2O_3 \), prepared by precipitating the corresponding chloride by magnesia, and washing the precipitate with nitric acid and water. This oxide is of a yellow or brown colour, and by solution in potass, in cyanide of potassium, or sulphite of soda, forms a liquid which is used in gilding. A solution of the oxide in hyposulphite of soda is employed to protect and make more visible daguerreotype portraits on silver. The sulphurets of gold are not important.
Gold is readily identified by chemical tests. When its colour and specific gravity cannot be appealed to as means of identification, its behaviour with the stronger liquid reagents is had recourse to. It resists the solvent action of the most powerful acids or alkalies taken singly, but at once dissolves in aqua regia (a mixture of nitric and hydrochloric acid), or in any similar liquid containing free chlorine. The resulting solution is tested,—1. By adding to it a solution of protosulphate of iron. This causes the gold to separate in the condition of a very fine powder, which remains for hours suspended in the liquid, although it is more than nineteen times heavier than water. The finely divided metal appears brown by reflected, and bluish-green by transmitted light, and if dried and rubbed by any smooth solid, acquires the characteristic colour and lustre of the metal in mass. 2. By neutralizing the solution with carbonate of potass or soda, and boiling with excess of oxalic acid, when the gold separates in highly characteristic splendid flakes. 3. By adding to the solution diluted, a few drops of solution of protoclouride of tin, when a rich purple precipitate fills. Very minute traces of gold may be discovered in this way. The precipitate, which has gone for centuries by the name of Purple of Cassius, appears to be a compound of oxide of gold and oxide of tin (\( Au_2O, Sn_2O_3 \)). It is used to stain glass ruby-red, and to give to porcelain and enamel a rose-pink, crimson, and purple colour. (See Precious Metals.)