AMALGAM is the term applied to that class of alloys (q.v.) in which one of the com bining 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 A., which is merely diffused throughout the fluid mass." The fluidity of an A. 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 tempera ture. It has no disposition to unite with iron even when hot. A solid A. 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 A., 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, dis covered in Mexico in 1557 by Bartolome de Medina, is very extensively used in Mexico at the present time, and has lately been 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 frag• ments 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 A. 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. This process is the only known method of separating the finer particles of gold from a mass of rock, and is always used by the gold-crushing companies. Indeed, it is now believed that this truly commercial mode of gold-seeking is the only one which, in a few years, will be had recourse to.
Several amalgams may be regarded as definite chemical compounds. Thus, when gold-leaf is placed in mercury, and the A. so produced filtered by being squeezed in a chamois-leather bag, the uncombined mercury oozes through the skin, but a definite A. of 2 of gold and 1 of mercury remains behind in the leather filter. Tin A. is employed in silvering looking-glasses, and is formed by laying a sheet of tin-foil on a table, cover ing it with mercury, and then placing, by a sliding movement, the sheet of glass over it. This A. contains 3 of mercury and one of tin; glass balls are silvered with an A. of 10 mercury, 1 tin, 1 lead, and 2 bismuth. A silver A. highly crystalline—and from the clusters of crystals somewhat resembling a tree, called arbor Diana, or tree of Diana—is prepared from 3 parts of the strongest solution of nitrate of silver, 2 parts of solution of proto-nitrate of mercury added to an A. of 7 mercury and 1 silver. In a day or two, the arborescent appearance presents itself, and the crystals contain 65 per cent mercury and 35 silver. The A. used for frictional electric machines is made from 1 tin, 1 zinc, and 3 mercury, to which sand is afterwards added.