By cupellation with lead, gold may be deprived of any antimony united with it.
Tin gives gold a remarkable hardness and brittleness; a piece of gold, exposed for some time over a bath of red hot tin, becomes brittle. The same thing happens more readily over antimony, from the volatility of this metal. A two thousandth part of antimony, bismuth, or lead, destroys the ductility of gold. The tin may be got rid of by throwing some corrosive sublimate or nitre into a crucible, containing the melted alloy. By the first agent, perchloride of tin is volatilized; by the second, stannate of potash forms, which is carried off in the resulting alkaline scoriæ.
Gold treated by the process of amalgamation, contains commonly nothing but a little silver. The silver is dissolved out by nitric acid, which leaves the gold untouched; but to make this parting with success and economy on the great scale, several precautions must be observed.
If the gold do not contain fully two thirds of its weight of silver, this metal being thoroughly enveloped by the gold, is partially screened from the action of the acid. Whenever, therefore, it is known by a trial on a small scale, that the silver is much below this proportion, we must bring the alloy of gold and silver to that standard by adding the requisite quantity of the latter metal. This process is called quartation.
This alloy is then granulated or laminated; and from twice to thrice its weight of sulphuric or nitric acid is to be boiled upon it; and when it is judged that the solution has been pushed as far as possible by this first acid, it is decanted, and new acid is poured on. Lastly, after having washed the gold, some sulphuric acid is to be boiled over it, which carries off a two or three thousandth part of silver, which nitric acid alone could not dissolve. Thus perfectly pure gold is obtained.
The silver held in solution by the sulphuric or nitric acid is precipitated in the metallic state by copper, or in the state of chloride by sea-salt. See [Parting].
Not only has the ratio between the value of gold and silver varied much in different ages of the world; but the ratio between these metals and the commodities they represent, has undergone variations, owing to the circumstances in which their mines have been successively placed; since they have always poured a greater quantity of the metals into the market than has been absorbed by use. This quantity has greatly increased since the discovery of America, a period of little more than 300 years. The mines of that continent, rich, numerous, and easily worked, by augmenting the mass of gold and silver, necessarily lessened the value of these metals compared with that of the objects of commerce represented by them, so that every thing else being equal, there is now required for purchasing the same quantity of commodities, much more gold or silver than was necessary in the reign of Henry VII., before the discovery of America. This productiveness of the American mines has had an influence on those of the ancient continent; many of whose silver and gold mines have been abandoned, not because the veins or auriferous sands are less rich than they were; but because their product no longer represents the value of human labour, and of the goods to be furnished in return for their exploitation.
In the 3d. vol. of the Mining Journal, p. 331., we have the following statement as to the produce of the precious metals.—“In 40 years, from 1790 to 1830, Mexico produced 6,436,453l. worth of gold, and 139,818,032l. of silver. Chile, 2,768,488l. of gold, and 1,822,924l. of silver. Buenos Ayres, 4,024,895l. of gold, and 27,182,673l. of silver. Russia, 3,703,743l. of gold, and 1,502,981l. of silver. Total, 1880 millions sterling, or 47 millions per annum.”
The following table shews what proportion the product of the mines of America bears to that of the mines of the ancient continent.