gdom it occurs as both
calcite and aragonite in the tests of the foraminifera, echinoderms,
brachiopoda, and mollusca; also in the skeletons of sponges and corals.
Calcium carbonate is obtained as a white precipitate, almost insoluble in
water (1 part requiring 10,000 of water for solution), by mixing solutions
of a carbonate and a calcium salt. Hot or dilute cold solutions deposit
minute orthorhombic crystals of aragonite, cold saturated or moderately
strong solutions, hexagonal (rhombohedral) crystals of calcite. Aragonite
is the least stable form; crystals have been found altered to calcite.

_Calcium nitride_, Ca_3N_2, is a greyish-yellow powder formed by heating
calcium in air or nitrogen; water decomposes it with evolution of ammonia
(see H. Moissan, _Compt. Rend._, 127, p. 497).

_Calcium nitrate_, Ca(NO_3)_2.4H_2O, is a highly deliquescent salt, [v.04
p.0972] crystallizing in monoclinic prisms, and occurring in various
natural waters, as an efflorescence in limestone caverns, and in the
neighbourhood of decaying nitrogenous organic matter. Hence its synonyms,
"wall-saltpetre" and "lime-saltpetre"; from its disintegrating action on
mortar, it is sometimes referred to as "saltpetre rot." The anhydrous
nitrate, obtained by heating the crystallized salt, is very phosphorescent,
and constitutes "Baldwin's phosphorus." A basic nitrate,
Ca(NO_3)_2.Ca(OH)_2.3H_2O, is obtained by dissolving calcium hydroxide in a
solution of the normal nitrate.

_Calcium phosphide_, Ca_3P_2, is obtained as a reddish substance by passing
phosphorus vapour over strongly heated lime. Water decomposes it with the
evolution of spontaneously inflammable hydrogen phosphide; hence its use as
a marine signal fire ("Holmes lights"), (see L. Gattermann and W.
Haussknecht, _Ber._, 1890, 23, p. 1176, and H. Moissan, _Compt. Rend._,
128, p. 787).

Of the calcium orthophosphates, the normal salt, Ca_3(PO_4)_2, is the most
important. It is the principal inorganic constituent of bones, and hence of
the "bone-ash" of commerce (see PHOSPHORUS); it occurs with fluorides in
the mineral apatite (_q.v._); and the concretions known as coprolites
(_q.v._) largely consist of this salt. It also constitutes the minerals
ornithite, Ca_3(PO_4)_2.2H_2O, osteolite and sombrerite. The mineral
brushite, CaHPO_4.2H_2O, which is isomorphous with the acid arsenate
pharmacolite, CaHAsO_4.2H_2O, is an acid phosphate, and assumes monoclinic
forms. The normal salt may be obt