AMALGAM, a compound of two or more metals, of which one is always mercury ; and this circumstance distinguishes an amalgam from a mere alloy. Nature presents us with only one amalgam, which is of silver, and is termed by mineralogists native amalgam : it is met with either semifluid, massive, or crystal lized. Maproth found it to exist of 64 parts of mercury and 36 of silver, out of 100 parts. Most metals may be amalgamated with mer cury, and the combination appears to depend on chemical affinity. When the cohesion of a metal is slight, as in the cases of potassium and sodium, or when its affinity for mercury is considerable, as in the instances of gold and silver, amalgamation takes place readily by mere contact. When, on the other hand, the cohesion of a metal is strong, or its affi nity for mercury is weak, heat, or intermediate action, or both, are requisite to effect amalga mation. The density of an amalgam exceeds that of the mean of the metals ; this and the tendency exhibited by one or both metals to oxidize, are indications of chemical combina tion. Antimony offers an example of metals which will not amalgamate without heat ; in order to effect combination, it must be melted, and while liquid mixed with hot mercury. It has been stated, that they may be amalga mated by mixing the filings of the metal with powdered alum, and rubbing them together in a mortar with a little water ; after tritura tion the alum may be washed ;mt. By the intervention of tin or zinc, iron may be com bined with mercury, and a 49.11ile amalgam is formed. Platina also unites pith mercury by the intervention of the amalgam of potassium, but not by direct action.
Amalgams are either liquid, 504, or hard; their form being dependent, in some cases, upon the quantity of mercury employed, and, in others, upon the nature of the metal amal gamated: thus, an amalgam consisting of eighty parts of mercury and one part of so dium, is solid, whilst a compound of fifteen parts of mercury and one part of tin is liquid.
The liquid amalgams resemble mercury appearance, except that the greater part of them flow less readily : solid amalgams are brittle. In general, amalgams are white; they are all crystallizable, and they form compounds of definite proportions. The amalgams of the more oxidable metals, as of potassium and sodium, are decomposed by exposure to the and absorbing oxygen, and they decompose water with the evolution of hydrogen gas ; the double amalgain of iron and zinc does not rapidly undergo any change, and is not attracted by the magnet. All amalgams are decomposed hy a rQd heat, the mercury being volatilized, and the' more fixed metals remaining. The process of amalgamation and decomposition is employed to separate gold and silver from their ores ; the mercury obtained by decomposing the amalgams is distilled, and repeatedly used for the same purpose, with comparatively little loss. The amalgams of geld and silver are employed in the processes pf gilding and plating. (For the Mode silvering the in teriors of hollow glss globes, see illinuou.) The amalgam of tin is largely used in what is termed silvering mirrors, and various amal gams of tin and zinc are employed for ex citing electricity in the machine. Some curious effects result from the action of amalgams upon each other : if mercury he added to the liquid amalgam pf potassium and sodium, an instant solidification ensues, and heat enough to inflame the latter Metals is evolved. When, on the other hand, a solid ftynragran of bis muth is put in centsct with one of lead, they become fluid, and the thermometer sinks during their action.
Some of the French dentists use an amal gam of copper to fill the cavities of hollow teeth. It is gray, very hard, and adheres firmly in its place. There are many useful applications for it as a stopper to machines and chemical apparatus wherever other mate ' rials cannot conveniently be used. It is said to consist of 70 parts of mercury to 30 of copper.