GOLD is a yellow metal, of much greater specific gravity than any other body in nature, except plating. It is soft, very tough, ductile, and malleable ; unal terable and fixed, whether exposed to the atmosphere, or to the strongest heat of furnaces. The most powerful burning mirrors arc said to have volatilized it ; and it has been driven up in fumes, in the metallic state, by flame urged upon it by a stream of oxygen gas. The electric shock converts it into a purple oxide, as may be seen by transmitting that commo tion through gold leaf between two plates of glass ; or by causing the explosive spark of three or more square feet of coated glass to fall upon a gilded surface. A strong heat is required to melt it, which does not happen till after ignition. Its colour, when melted, is of a bluish green ; and the same colour is exhibited by light transmitted through gold-leaf.
The limits of the ductility and mallea bility of gold are not known, and its tena city exceeds that of any other metal. A gold wire of one tenth of an inch dia meter requires 500lb. weight to break it.
The method of extending gold, used by the gold-beaters, consists in hammering a number of thin-rolled plates between skins or animal membranes. By the xe.ight and measure of the best wrought gold leaf, it is found, that one grain Is made to cover 56i square inches ; and from the specific gravity of the metal, to. gether with this admeasurement, it fol lows, that the leaf itself is parts of an inch thick. This, however, is not the limit of the malleability of gold; for the gold-beaters find it necessary to add three grains of copper in the ounce to harden the gold, which otherwise would pass round the irregularities of the new est skins, and not over them ; and in us ing the old skins, which are not so per fect and smooth, they proceed so far as to add twelve grains. The wire which is used by the lace-makers is drawn from an ingot of silver, previously gilded. In this way, from the known diameter of the wire, or breadth when flattened, and its length, together with the quantity of gold used, it is found, by computation, that the covering of gold is only one-twelfth part of the thickness of gold-leaf, though it still is so perfect as to exhibit no cracks when viewed by a microscope.
No acid acts readily upon gold but the nitro-muriatic acid, called aqua-regia, and the oxygenized-muriatic acid. The sul
phuric acid, distilled from manganese, has Some action upon it : as have likewise the pale nitric acid, and the phosporic acid when boiling. Chromic acid added to the muriatic enables it to dissolve gold.
The small degree of concentration of which the oxygenized-muriatic acid is susceptible, and the imperfect action of the latter acids, render aqua-regia the most convenient solvent for this metal.
When gold is immersed in aqua-regia, an effervescence takes place with the escape of gas ; the solution tinges animal matters of a deep purple, and corrodes them. By careful evaporation, fine crys tals of a topaz colour are obtained. The gold is precipitated from its solvent by a great number of substances. Lime and magnesia precipitate it in the form of a yellowish powder. Alkalies exhibit the same appearance ; but an excess of alkali redissolves the precipitate. The preci pitate of gold obtained from aqua-regia by the addition of a fixed alkali appears to be a true oxide, and is soluble in the sulphuric, nitric, and muriatic acids ; from which, however, it separates by standing, or by evaporation of the acids. Gallic acid precipitates gold of a reddish colour, very soluble in the nitric acid, to which it communicates a fine blue colour.
Ammonia precipitates the solution of gold much more readily than fixed alka lies. This precipitate, which is of a brown, yellow, or orange colour, possesses the property of detonating with a very consi derable noise, when gently heated. It is known by the name of fulminating gold. The presence of ammonia is necessary to give the fulminating property to the pre cipitate of gold ; and it will be produced by precipitating it with fixed alkali from an aqua-regia previously made, by adding sal ammoniac to nitric acid ; or by preci pitating the gold from pure aqua-regia, by means of sal ammonia, instead of the ammonia alone. The fulminating gold weighs one-fourth more than the gold made use of. A considerable degree of precaution is necessary in preparing this substance. It ought not to be dried but in the open air, at a distance from a fire, be cattse a very gentle heat may cause it to explode. Several fatal accidents have ;risen from its explosion, in consequence of the friction of ground stoppers in bot tles containing this substance, of which a small portion remained in the neck.