(Chemical symbol Ca). A silvery, metallic element that occurs in nature in shells, limestone and gypsum. In the field of welding, calcium is commonly associated with carbon to form calcium carbide. Calcium has a strong affinity for oxygen and becomes coated with an oxide film when exposed to air. When heated in nitrogen, it forms calcium nitride. It decomposes readily in water with the evolution of hydrogen and formation of calcium hydroxide. Atomic number, 20; atomic weight, 40.07; melting point, 810°C (1490°F); specific gravity, 1.54.
