The Collaborative International Dictionary
Fusible \Fu"si*ble\, a. [F. fusible. See Fuse, v. t.] CapabIe of being melted or liquefied.
Fusible metal, any alloy of different metals capable of
being easily fused, especially an alloy of five parts of
bismuth, three of lead, and two of tin, which melts at a
temperature below that of boiling water.
--Ure.
Fusible plug (Steam Boiler), a piece of easily fusible alloy, placed in one of the sheets and intended to melt and blow off the steam in case of low water.
Wikipedia
A fusible plug is a threaded metal cylinder usually of bronze, brass or gunmetal, with a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point that flows away if a pre-determined, high temperature is reached. The initial use of the fusible plug was as a safety precaution against low water levels in steam engine boilers, but later applications extended its use to other closed vessels, such as air conditioning systems and tanks for transporting corrosive or liquefied petroleum gasses.