Amalgam is the term applied to that class of Alloys (q.v.) in which one of the combining metals is mercury. On the nature of the union, it has been observed that 'on adding successive small quantities of silver to mercury, a great variety of fluid amalgams are apparently produced; but in reality, the chief, if not the sole compound, is a solid amalgam, which is merely diffused throughout the fluid mass.' The fluidity of an amalgam would thus seem to depend on there being an excess of mercury above what is necessary to form a definite compound. Mercury unites readily with gold and silver at the usual temperature. It has no disposition to unite with iron even when hot. A solid amalgam of tin is used to silver looking-glasses.
Amalgamation is employed on a small scale in some processes of gilding, the silver or other metal being overlaid with a film of gold amalgam, and the mercury being then driven off by heat. But its most extensive use is in separating gold, and especially silver, from certain of their ores. The mercury dissolves the particles of the metal, and leaves the earthy particles; it is then easily separated from the gold or silver. This process, discovered in Mexico in 1557 by Bartolomé de Medina, is still used in Mexico, and was introduced with great success into the Californian and Australian gold-fields. The mode of application is to crush the quartz rock which serves as the matrix in which the small particles of gold are imbedded; place the fragments in a barrel or revolving drum with mercury, and agitate for some time. The mercury attaches all the gold particles to itself; and in the apparatus, when fully agitated, there is found a semi-fluid mass, which is the mercury, appearing half-congealed, and containing all the gold. It is only necessary to place this amalgam in a retort and apply heat, when the mercury sublimes over—and can be re-employed for further amalgamation—and leaves the gold in the body of the retort.
Several amalgams may be regarded as definite chemical compounds. Thus, when gold-leaf is placed in mercury, and the amalgam so produced filtered by being squeezed in a chamois-leather bag, the uncombined mercury oozes through the skin, but a definite amalgam of 2 of gold and 1 of mercury remains behind in the leather filter. Tin amalgam is employed in silvering looking-glasses, and is formed by laying a sheet of tinfoil on a table, covering it with mercury, and then placing, by a sliding movement, the sheet of glass over it. This amalgam contains 3 of mercury and 1 of tin; glass balls are silvered with an amalgam of 10 mercury, 1 tin, 1 lead, and 2 bismuth, best prepared by melting together the last three metals, and then adding the mercury.
A silver amalgam, containing about 26 per cent. of metallic silver—and, from the clusters of crystals somewhat resembling a tree, called Arbor Dianæ, or Tree of Diana—is prepared by placing about half a teaspoonful of mercury in a small phial, and filling the bottle with a solution of nitrate of silver of the strength of 25 grains to the ounce. In the course of a few days the arborescent appearance presents itself. The amalgam used for frictional electric machines is made from 1 tin, 1 zinc, and 3 mercury, to which sand is afterwards added.