FUSIBLE METAL, or FUSIBLE AL LOY, any alloy, or metallic mixture, which melts at a comparatively low temperature, that is, below the melting point of tin (442° F.). (See ALLOY; AMALGAM; BISMUTH; CADMIUM ) Fusible metals are either binary, ternary or quarternary alloys of lead, tin, bismuth and cadmium. These metals, constituting what is known as *the fusible form simple al loys, which consist of practically pure metals and eutectics. As eutectics have a lower melt ing point than either of their components, we may, by combining these components in the pro portion necessary to form the eutectic, obtain alloys whose melting point is much lower than any of the combined metals. The eutectic of three metals melts at a lower point than that of two metals, and the eutectic of four metals lower than that of three.
The alloy known as Wood's Metal, or Wood's Alloy melts at the lowest temperature of all the fusible alloys — 145° F. Its com position is: tin, 4 parts; lead, 4 parts; bismuth, 8 parts; and •a little* cadmium. Other fusible alloys melting at very low temperatures are constituted as follows: (1) lead, 25 per cent: tin, 12.5 per cent; bismuth, 50 per cent; cad mium, 12.5 per cent —melting at 150° F.; (2) lead, 26.7 per cent; tin, 13.3 per cent; bis muth, 50 per cent; cadmium, 10 per cent melting at 153° F.; (3) lead, 26.7 per cent ; tin, 14.8 per cent; bismuth, 52,2 per cent ; cadmium, 7 per cent —melting at 156° F. (See BISMUTH). Fusible metals are used in the arts for many purposes. Automatic sprinklers, for example, are capped with alloys of this sort, which are chosen so as to have melting points that are higher than any temperature that would nor mally occur in the room that is to be pro tected. If a fire breaks out, however, the ab normal rise of temperature so produced causes them to melt, the water in the sprinkler pipes being thereby released and the fire extinguished Fusible metals have also been used quite gen erally in the manufacture of 'fusible plugs,' for the protection of steam boilers; such plugs being screwed into the boiler at the height which is considered to be the lowest limit to which the water level in the boiler can be allowed to descend with safety. So long as the inner end
of the plug is covered with water, .the plug itself is thereby kept too cool to melt under the influence of the furnace gases; but when the protecting action of the water is removed by the water level descending below the safety limit, the hot furnace gases melt the material of the plug and the steam in the boiler escapes. Fusible plugs are excellent appliances and in fact they are required by law in some of the States. There is no advantage in filling them with an alloy, however, because in any event the alloy must have a melting point higher than that of the steam that the boiler is to generate (365° F., for a gauge pressure of 150 pounds per square inch), and pure tin, with a melting point of 442° F., is entirely satisfactory for the purpose. Indeed, tin is far superior to any alloy for this purpose, because its melting point remains sensibly constant for an indefinite time (so long as oxidation is prevented), while the melting points of alloys that are continuously exposed to heat for considerable periods become quite uncertain and are often found to be far higher than when the alloy is freshly prepared. The eutectic alloy of lead, tin and bismuth ex pands at the moment of solidification. It is therefore of value in making metallic casts of substances which will not endure the heat of .molten single metals.