Element Calcium, Ca, Alkaline Earth Metal

The compounds of calcium have an exceedingly wide distribution in the earth's crust, and form one of the most abundant constituents of the latter. It is the carbonate chiefly that occurs in nature; in the silicates of the earth's crust, also, calcium is seldom absent. The element also takes part in the most varied way in the building up of the organisms.

Metallic calcium was comparatively late in being prepared in the pure condition. Davy and Berzelius, at the beginning of the nineteenth century, attempted to obtain it from the electrolytically prepared amalgam by distilling off the mercury, but they did not obtain a sufficiently pure material to allow of its properties being ascertained. At the present day, however, large quantities of calcium are prepared by the electrolysis of fused calcium chloride. Prepared in this way, or by the decomposition of calcium iodide with sodium (whereby, after removal of the excess of sodium, crystallised calcium is obtained, it is found that this element is a white (not, as previously stated, a yellow) metal of density 1.83 which can be remelted in a vacuum at 760°, and can then be cut; it is, however, not so soft as potassium. It is not affected by oxygen, chlorine, bromine, or iodine at the ordinary temperature, but combines with these only on being heated. When heated in the air, it burns, forming a compound both with the oxygen and the nitrogen. It is only slowly attacked by water, but quickly by dilute acids.

Calcium, always in the combined form, is among the more widely distributed and the more abundant of the elements. It is an important constituent of many rocks, both igneous and sedimentary, occurring chiefly as carbonate, sulphate, silicate, phosphate, and fluoride. There are, in fact, whole ranges of mountains composed largely of calcium carbonate. Calcium is also a common mineral ingredient of most natural waters. Finally, animal skeletons owe their rigidity mainly to calcium phosphate, and the shells of molluscs and of birds' eggs to the carbonate.

The metal forms 3.47 per cent, of the older or primitive part of the earth's crust, or 3.25 per cent, of the outer part of the earth, including crust and sea. The amount of calcium in sea-water is from 0.03 per cent, to 0.05 per cent.

Calcium has also been found in meteorites, and by spectral analysis in the sun and fixed stars.

The chief calcium minerals, together with their more important physical constants, are given in the table on the following page.

Compounds of calcium have been employed for building purposes since very early times. A mortar composed of sand and either lime or burnt gypsum was used by the Egyptians, and in the first century a.d. Dioscorides and Pliny described lime-burning and slaking. In 1693 Homberg heated together sal-ammoniac and lime for the production of Homberg's phosphorus, a phosphorescent calcium chloride. In 1722 Hofmann showed that lime was a distinct earth. Black noted its alkalinity in 1755. Lime was regarded as a simple body until the time of Lavoisier, who suspected it to be an oxide. The metal was isolated by Davy in 1808, and independently by Berzelius and Pontin.