AMALGAM (Gr. ἅμα, together, and γαμεῖν, to marry; or according to some, μάλαγμα, an emollient, from μαλάσσειν, to soften), an alloy of two or more metals, one of which must be mercury. This metal has a remarkable power of dissolving most of the other metals and forming combinations that may be applied directly to various uses; and, moreover, as the mercury is easily expelled from them by heat, these combinations are used as a means of bringing other metals into a condition of convenient application to many purposes. Thus, gilding is sometimes effected by washing other metals with a solution of gold in mercury. The mercury is driven off by heat, and the gold remains coating the surface. A process is patented in England for covering iron with zinc, which is based on this principle. A considerable degree of cold is produced in forming some amalgams. Thus, in mixing at a temperature of 65° F. 118 parts of tin and 201 of lead, both in filings, 284 of bismuth in fine powder, and 1,616 of mercury, the temperature falls to 18°. Many of the amalgams are definite compounds, from which the mercury in excess may be squeezed out; but sometimes the liquid that thus escapes is found to be itself an amalgam, containing a smaller proportion of the harder metals, seeming to indicate two definite compounds of different proportions. This is observed with the amalgam of mercury with silver, and also with tin. In tinning mirrors, the glass plates are laid upon smooth stone tables covered with the amalgam. The solid portions adhere in a thin film to the glass; and this is a compound of atomic proportions. The liquid squeezed out by the weight placed upon the glass proves also to be an amalgam containing but a small proportion of tin.—Amalgams are prepared by putting the harder metals, reduced to small size, in mercury, and dissolving them with or without heat, as may be required. When the metals are not easily dissolved, they may be rubbed together or triturated in a mortar, or melted, and the mercury heated and poured into the fused metal. This is the process for preparing an amalgam of 4 parts mercury, 2 zinc, and 1 part tin, for the electrical machine. The zinc is first melted, the tin added, and then the hot mercury stirred in. It is to be shaken till cold, then triturated and sifted in a fine sieve. An amalgam of mercury with iron is prepared by rubbing together in a mortar clean iron filings and zinc amalgam, and adding a solution of perchloride of iron. By rubbing and heating this mixture a bright amalgam of iron and mercury is produced. Some amalgams take a crystalline form, thus indicating combination in definite proportions; and there is also a native amalgam of this character of mercury with silver. This is found in dodecahedral crystals, consisting of 1 atom of silver and 2 of mercury = 36 per cent. of the one and 64 of the other. One part of gold heated with 6 parts of mercury crystallizes on cooling in four-sided prisms. Tin amalgam made of 3 parts of mercury and 1 of tin forms cubic crystals. Amalgams freed from their excess of mercury are, when freshly made, dry pasty substances, which soon become hard like stone. This property makes some of them convenient for filling cavities of teeth, but the injury the mercury may effect upon the system renders their use highly objectionable.