Calcium (syn. Ca, atomic weight 40) is the metal present in chalk, stucco, and other compounds of lime. It may be obtained by passing a powerful current of electricity through fused chloride of calcium, , when the metal separates in minute globules. It is a yellowish-white metal, can be rolled into sheets, and hammered into leaves, and is intermediate between lead and gold in hardness. It has a specific gravity of 1.578; being more than a half denser than water. At ordinary temperatures, it slowly tarnishes by oxidation; and when placed in contact with water, it rapidly decomposes the water, , forming lime, , whilst hydrogen escapes. To be retained bright, calcium must be kept under the surface of naphtha. At a red heat, it melts and burns with a dazzling white light, accompanied by scintillations.
Calcium is of no value in the arts, but many of its compounds are of the highest importance. When oxidised, either by exposure to air, or by combustion, it forms an oxide, Lime (q.v.), having the composition of 40 parts of calcium and 16 parts of oxygen. This, when united with water, yields a hydrate, slaked lime. For the sulphide, see LUMINOUS PAINT. Sulphate of calcium, , is the chief constituent of Gypsum (q.v.). It is present in most drinking-waters, rendering them permanently hard, because the sulphate of calcium is not precipitated by boiling the water. Temporary hardness, due to the presence of carbonate of calcium, , or chalk, is removable by boiling, when the carbonate, which is retained in solution as bicarbonate, is readily decomposed into Carbonic Acid (q.v.), which escapes as gas, and chalk, which falls to the bottom.
The salts of calcium do not seem to possess any medicinal properties characteristic of the metal itself. They are, however, extensively used—e.g. the phosphate, to supply phosphorus for the bones and nerves, the carbonate as an antacid, and so on, with other salts.