Fulminating gold, when exposed by Bertbollet to a very gentle heat in a cop per tube, with the pneumatical apparatus of mercury, was deprived of its fulminat :rig quality, and converted into an oxide, at the same time that ammoniacal gas was disengaged. From this dangerous expe riment it is ascertained, that fulminating gold consists of oxide of gold combined with ammonia. The same eminent philo sopher caused fiulminatinggold to explode in copper vessels. Nitrogen gas was dis engaged, a few drops of water appeared, and the gold was reduced to the metallic form. In this experiment he infers, that the ammonia was decomposed ; that the nitrogen, suddenly assuming the elastic state, caused the explosion, while the oxygen of the oxide united with the hy drogen of the alkali, andformed the water.
This satisfactory theory was still far ther confirmed by the decomposition of fulminating gold, which takes place in consequence of the action of the concen trated sulphuric acid, of melted sulphur, fat oils, and ether ; all of which deprived it of its fulminating quality, by combining with its ammonia.
Sulphurets precipitate gold from its solvent; the alkali uniting with the acid, and the gold falling down combined with the sulphur ; of which, however, it may be deprived by moderate heat.
Most metallic substances precipitate gold from aqua-regia lead, iron, and sil ver, precipitate it of a deep and dull pur ple colour ; copper and iron throw it down in its metallic state ; bismuth, zinc, and mercury, likewise precipitate it. A Plate of tin, immersed iu a solution of gold, affords a purple powder, called the purple powder of Cassius, which is used to paint in enamel. There are various methods of managing this process. That described by Macquer consists in dissolv ing tin by very small portions at a time, i without heat, an aqua-regia composed of two parts of nitric and one of muriatic acid, previously weakened with water equal in weight to both the acids. The first small portion of tin must be suffered to be entirely dissolved before a second is added. This addition must be continued till the acid has acquired a yellow colour, and scarcely acts at all upon the tin last added.
On the other hand, the purest gold must be dissolved in an aqua-regia composed of three parts of nitric and one of muriatie acid. This solution may be made, as ex peditiously as the operator chooses, by the assistance of the heat of a sand bath.
The solution of tin must then be large ly diluted, as, for example, with one hun dred parts of distilled water ; and a small quantity of this may then be assayed, by separating it in two parts, and diluting one of the parts still farther. Upon trial of both, by letting fall a drop of the solu tion of gold into each, it will be seen which affords the most beautiful purple precipitate. The whole of the solution of
tin must accordingly be altered, if neces sary, by adding more water. Pour into this solution, in a large glass or earthen vessel, nearly half as much of the solution of gold as it contains of solution of tin„ stirring the mixture with a glass stick. In a short time the liquor will become of a beautiful red colour, which will gradually disappear on the subsidence of the pre cipitate. By adding a small quantity of the solution of tin, it will be seen whether the whole of the gold is precipitated. The clear liqour must then be decanted, and the precipitate washed. It consists of me tallic gold and oxide of tin, at a maximum in combination, and is the only known substance which has the property of com municating a purple colour to glass. This purple powder is perfectly soluble in am monia. Nitric acid boiled on it brightens it to a tint approaching that of cinnabar.
The difficulties attending the prepara tion of this article appear to depend on the state of the tin. If the solution of this metal be made with heatand rapidity, it becomes too much oxyded to adhere to the acid, or to precipitate the gold ; and the combination of the two metals, which falls down, varies in colour according as this term is approached: these are the chief circumstances ; but there is tic doubt that a complete examination of the process would indicate others worthy of notice.
Ether, naphtha, and the essential oils, take gold from its solvent, and form li quors which have been called potable gold. The gold which is precipitated by evaporation of these fluids, or by the ad dition of sulphate of iron to the solution of gold, is of the utmost purity.
In the dry way, gold resists the action of neutral salts, more especially nitre, which deflagrates with the imperfect me tals. Nitre, however, does not afford an expeditious way of purifying gold, be cause this metal in some measure protects and covers the alloys from its action. It is remarked that borax, used as a flux with gold, renders it paler ; and that this alter ation of colour disappears by the addition of nitre or commou salt. As the acid of borax forms a compound with gold, which falls to the bottom when this acid is added to the metal in solution, it is probable that the paleness produced by borax may arise from the combination of a small portion of its acid with the gold, which might be driven off by a continuance of the heat, and united by stronger affinity with the alkali of the nitre, or of the common salt, in proportion as their acids are dissipated by heat.