Calcium

Calcium (Ca; atomic weight, 39.9), a metallic element, which, although its compounds are very numerous, abundant, and widely distributed, is only obtained by difficult chemical processes. When prepared it is a yellowish metal of specific gravity 1.58, very ductile, decomposing water rapidly, and readily tarnishing by exposure to air. It closely resembles barium and strontium (q.v.) in its properties. Many of its compounds are very important in the manufactures and arts. Its oxide is lime, and is obtained by heating the carbonate, which forms the different varieties of limestone, chalk, and marble. Lime unites with water to form a hydroxide, which is then known as "slaked lime." Bleaching powder (q.v.) is a compound of calcium with oxygen and chlorine. Its fluoride occurs native as Fluor spar (q.v.), and occurs associated with other elements in tourmaline and other minerals. The sulphate forms the mineral anhydrite (q.v.), and united with water constitutes selenite, gypsum, and alabaster (q.v.). From these, by heating, "plaster of Paris" is obtained. Its silicate is a prominent component of glass, and occurs native as wollastonite. The phosphate is the principal mineral constituent of bone, and occurs also as the mineral apatite. The sulphide, from its power of shining in the dark, is known as Canton's phosphorus.