ALLOY. A compound formed of two or more metals fused together. Thus bronze is an alloy of copper and tin ; brass an alloy of copper and zinc : they all have lustre, are sonorous, elastic, duc tile, and malleable, like simple metals. Metals do not alloy indifferently with each other, but are governed by peculiar affinities. Silver unites readily with lead, copper, andgold, but will scarcely 'alloy with When hen a metal is united with mercury, it generally receives the title of an amalgam. When metals are united in an alloy the specific gravity of the new compound is not the mean of its constituents, but occasionally is greater —in other instances, less : its melting point also is not the mean of the melting points of the two metals, but it is gene rally somewhat lower in temperature— the fusibility of an alloy is increased. Although the number of metals is very great (43),yet only a feW are extensively found or of much use—perhaps the num ber frequently employed is not more than twelve : where purity of a metal is not required, but some property which a single metal does not possess, an alloy is found to supply the want. Even the property of an alloy itself may be varied by uniting with a portion of a third. metal. Thus in the case of the alloy brass ; when it is required to have brass fit for turning, a small quantity of lead is added. This improves it for that pur pose, but renders it unfit for hammenng. The number of useful metals can be thus multiplied, as it were, by the formation of alloys.
Alloys can only be properly formed by fusion, as by melting the two metals to gether in a crucible. They require to be stirred well while melting, lest the metals separate from each other, the heavier taking the bottom of the vessel. The strength or cohesion of alloys is greater than that of its constituents. The most
refractory metals, which can scarcely be fused in a crucible at the greatest heat of the furnace, melt down with ease when surrounded by the more fusible metals. The surfaces of the superior metal are melted down or washed away, layer by layer, until the whole becomes liquified. Nickel is nearly as difficult to melt as iron ; but it is usefully employed with copper in German silver, to which it gives whiteness and hardness, and renders the alloy less fusible. Platinum cannot be melted at the highest heat of a furnace, but it combines so readily with zinc, tin, and arsenic that it is dangerous to heat one of these substances in a platinum spoon, for an alloy would be formed and the spoon destroyed.
An alloy, remarkable for its easy fusi bility, is made by melting together eight parts of bismuth, five of lead, and three of tin. This melts in boiling water, even in water of the temperature 198° Fahr. It is on this account called fusible metal. The proportions may be varied to make a more or less fusible compound. Safety plugs for valves of steam-boilers are made of this material : a hole made in the boiler is stopped with one of these plugs, so that when from any derange ment of the valve steam above the usual pressure and temperature be formed, it would melt the plug and force its way out through the aperture rather than burst the boiler. When quicksilver is added to this alloy, it becomes more fusible ; and is used by dentists for stuffing de cayed teeth.
Solders are alloys, and generally con tain a portion of the metal they are used to connect. (See SOLDER.)