An amalgam is an alloy created by mixing mercury with one or several other metals. Typically amalgams are soft when first mixed, and then they harden into a durable solid material.
Silver based amalgams have been used as dental fillings for cavities since the 1830's. The initial malleability of the amalgam allows it to be packed tightly into the cavity, where it hardens without either expanding or contracting, forming a hard, permanent filling.
Amalgams are also used in gold mining. Mercury is washed over sand or crushed rock where any small particles of gold will bond to it. The amalgam thus created is recovered and the gold is extracted. While this technique is effective, it is also a major source of pollution from mining and gold sluicing operations.
A third use is detection of mercury salts in water. Amalgam probes are made by applying a nitric acid solution to copper foil, and then immersing the foil in a water sample. A positive test will show a spot of silvery appearing mercury amalgam forming on the foil.