CALCIUM is a metallic element, symbol Ca, and was so named by Sir H. Davy because of its occurrence in chalk (Latin calx) ; atomic number 20, atomic weight 40.07 (two isotopes, 4o and 44, the former greatly predominating, according to F. W. Aston). It has a bright silvery appearance when freshly cut, and tarnishes rapidly and becomes yellowish on exposure to air. It does not occur naturally in the free state but is widely dis tributed in combination, being the fifth most abundant constitu ent of the earth's crust. Thus the sulphate constitutes the minerals anhydrite, alabaster, gypsum, and selenite, and the carbonate oc curs as the minerals chalk, marble, calcite, and aragonite, and in combination with magnesium carbonate as dolomite. Fluorspar is the fluoride, which also occurs with the phosphate as apatite. The phosphate is the main constituent of bones, and the carbonate of egg-shells.
The development of electrolytic methods resulted in the com mercial production of the metal. O. Ruff and W. Plato employed a mixture of the chloride and fluoride (fluorspar), which was fused in a porcelain crucible and electrolysed by a carbon anode and an iron cathode ; this method did not admit of commercial applica tion, but W. Rathenau and C. Suter achieved this object by a slight modification. A graphite vessel, which contained the fused mixture, served as the anode, and the cathode was an iron rod which could be gradually raised, and which is known as the "con tact cathode." This just touches the surface of the fused mixture, and as the calcium forms at the point of contact the rod is raised, whereby the calcium is produced as a rod forming a prolongation of the iron rod. It is marketed in this form and has a purity of More recent methods (e.g., that of Brace, J. Inst. Metals, 1921, xxv., p. 16r) tend to revert to the electrolysis of concentrated calcium chloride solutions with a mercury cathode, and subsequent distillation of the mercury.
Properties.—A. W. Hull has shown that the crystal structure of the metal is a face-centred cube, the edge being 5.56 A.U. in length. Calcium reacts slowly with the oxygen and nitrogen of the air to form a yellow surface layer of hydroxide and nitride. It is rapidly acted upon by water, but the resulting layer of hydroxide tends to protect the metal from further attack owing to its sparing solubility. Alcohol is practically without action, and for this reason calcium is used as the best means of removing the last traces of water from alcohol. The chemical properties of calcium, strontium, and barium are very similar, showing a regu lar gradation throughout the series ; these elements together with magnesium constitute the so-called "alkaline earths." Calcium combines directly with most elements, including nitrogen with which it forms the nitride, Its combination with oxygen is so intense that if effected at 3oo°C. the heat evolved suffices to volatilise the lime produced. Its specific gravity is about 1.54, depending upon its previous treatment. It melts at 8io°C., but sublimes at a lower temperature; its specific heat is 0.169 at 20°C. ; it is malleable and can be drawn into wire.
Whereas calcium chloride, bromide, and iodide are very deli quescent soluble substances, the fluoride is insoluble in water; this constitutes a striking contrast with the silver salts for the fluoride is the only soluble one in this case. Calcium fluoride, CaF2, occurs as fluorspar or may be prepared as an insoluble white precipitate by mixing solutions of calcium chloride and sodium fluoride; it requires about 6o,000 parts of water for solution at 18°. The chloride, occurs in many natural waters and as a by-product in many manufacturing processes. According to conditions the crystals may contain 2, 4, or 6 mole cules of water; the anhydrous chloride is obtained by heating the crystals in a current of hydrogen chloride above 200°, and is used as a desiccating agent (but not for ammonia or alcohols, with which it forms compounds). It melts at about 775°. The hexahydrate dissolves in water with absorption of heat, and if mixed with three-quarters of its weight of snow, it produces a temperature of —55°C. At 100 parts of water will dissolve 297 parts of and the solution boils a few degrees higher. Concentrated solutions find a limited use for laying dust. The bromide and iodide closely resemble the chloride.
"Bleaching powder" is said to be a compound of the chloride and hypochlorite, but it has been shown (E. A. O'Connor, 1927) that such a compound does not exist, so that its constitution is still indefinite. It is made by exposing thin layers of slaked lime to chlorine at 30-40°C. (see ALKALI MANUFAC TURE and BLEACHING).
Calcium carbide, CaC2, manufactured by heating lime and car bon in the electric furnace, is of great importance since it gives rise to acetylene (q.v.) on reacting with water. It reacts with the nitrogen of air in the electric furnace to give calcium cyanamide (see CYANAMIDE).
Calcium carbonate, occurs in minerals (above) and in natural waters, in which it is kept in solution by atmospheric carbon dioxide as the bicarbonate, The loss of this carbon dioxide by evaporation or heating of the solution results in the deposition of and hence the "hardness" of the water due to this cause is called "temporary." The chalk thus deposited is called "fur" in boilers or kettles, but if it is produced by the dripping of water in caves it forms stalactites and stalagmites. It also occurs in the skeletons of sponges and corals. The solu bility in water containing the amount of carbon dioxide normally derived from the atmosphere is 1 part in 16,000 parts, but under higher pressures of the gas it is much more soluble, and under a pressure of 15 atmospheres it seems to be entirely converted into the bicarbonate. "Precipitated chalk" is prepared by mixing solu tions of calcium chloride and a soluble carbonate (usually that of sodium) .
Calcium nitride, is formed by heating calcium (prefer ably slightly impure, for the pure metal is almost inert) in nitro gen; it is a greyish-yellow powder, and is readily decomposed by water, with formation of ammonia.
Calcium nitrate, is a highly deliquescent salt; from its occurrence in limestone caverns and its disintegrating action on mortar, it is variously called "lime-saltpetre" and "saltpetre-rot." The anhydrous nitrate is phosphorescent and is known as "Bald win's phosphorus." The hydrate, is obtained in the "flaming arc" process for "fixing" atmospheric nitrogen, and is used as an artificial manure either directly or after conversion into ammonium nitrate.
Calcium phosphide, is obtained as a reddish substance by passing phosphorus vapour over strongly heated lime. It is used as a marine signal fire ("Holmes lights") owing to its prop erty of giving a spontaneously inflammable hydrogen phosphide on reacting with water. It may also be prepared by direct com bination of calcium metal with phosphorus vapour.
Normal calcium phosphate, is the principal inorganic constituent of bones and hence of "bone-ash" (see PHOSPHORUS) ; it occurs as the concretion known as coprolites, and also in vast masses in Florida and in certain Pacific Islands. It may be ob tained artificially as a gelatinous precipitate by mixing solutions of calcium chloride, sodium phosphate, and ammonia. It is in soluble in water, but soluble in solutions of hydrochloric or nitric acids. Basic (or Thomas) slag, a by-product of the steel industry, owes its use as a fertilizer to the calcium phosphate which it con tains. The acid salt, CaH4(PO4)2, obtained by evaporating a solution of the normal salt in hydrochloric or nitric acid, is very soluble, and together with calcium sulphate constitutes the arti ficial manure "superphosphate of lime," which is obtained by treating ground bones or coprolites with sulphuric acid. Other phosphates occur in guano and elsewhere. It is used in medicine; as a source of phosphorus; polishing powder; ceramics; porcelain manufacture ; enameling; manufacturing milk glass.
Calcium sulphide, CaS, is formed by heating the sulphate with charcoal, or by heating lime in a current of sulphuretted hydrogen; after being heated or exposed to the sun, it shows a phosphores cence, apparently in virtue of minute traces of impurities, espe cially compounds of the heavy metals, which it contains; hence it is known as "Canton's phosphorus" (John Canton, 1718-1772). The hydrosulphide, is obtained by saturating a cold suspension of lime with sulphuretted hydrogen; it is used as a depilatory. The di- and penta-sulphides, and are formed when milk of lime is boiled with flowers of sulphur; they form the basis of Balmain's luminous paint. They are also con tained in "liver of sulphur," a complex mixture prepared by heat ing together lime and flowers of sulphur. It is also used in medicine.
Calcium sulphide, is prepared by passing sulphur dioxide into milk of lime; excess of the former leads to a solution of "bisulphite of lime," used for bleaching wood pulp in the manu facture of paper.
Calcium sulphate, CaSO4, occurs as certain minerals and in water, in which it is one of the salts causing "permanent hard ness," i.e., it is not removed on boiling. As a white crystalline pre cipitate it is obtained on mixing solutions of calcium chloride and sodium sulphate; i oo parts of water dissolve 0.24 parts at 15°C. The ordinary dihydrate is converted into a semihydrate, CaSO,,1H2O, on being heated to a suitable temperature (120 180°) ; this hydrate is called Plaster of Paris, having first been made from the gypsum of Montmartre.
There are numerous calcium silicates in various minerals, of which the most important are zeolites and felspars. H. Le Chate lier obtained Ca2SiO4, and artificially.
Calcium salts are not toxic, except possibly in very large doses; deficiency of calcium in the diet leads to "rickets," owing to the lack of material for bone formation. (A. D. M.)