d. It deliquesces in the air and
melts readily on heating. By the addition of excess of ammonia to its
aqueous solution, in the complete absence of air, a blue precipitate
of a basic nitrate of the composition 6CoO.N2O5.5H2O is obtained.

By boiling a solution of cobalt carbonate in phosphoric acid, the acid
phosphate CoHPO4.3H2O is obtained, which when heated with water to
250 deg. C. is converted into the neutral phosphate Co3(PO4)2.2H2O (H.
Debray, _Ann. de chimie_, 1861, [3] 61, p. 438). Cobalt ammonium
phosphate, CoNH4PO4.12H2O, is formed when a soluble cobalt salt is
digested for some time with excess of a warm solution of ammonium
phosphate. It separates in the form of small rose-red crystals, which
decompose on boiling with water.

Cobaltous cyanide, Co(CN)2.3H2O, is obtained when the carbonate is
dissolved in hydrocyanic acid or when the acetate is precipitated by
potassium cyanide. It is insoluble in dilute acids, but is readily
soluble in excess of potassium cyanide. The double cyanides of cobalt
are analogous to those of iron. Hydrocobaltocyanic acid is not known,
but its potassium salt, K4Co(CN)6, is formed when freshly precipitated
cobalt cyanide is dissolved in an ice-cold solution of potassium
cyanide. The liquid is precipitated by alcohol, and the washed and
dried precipitate is then dissolved in water and allowed to stand,
when the salt separates in dark-coloured crystals. In alkaline
solution it readily takes up oxygen and is converted into potassium
cobalticyanide, K3Co(CN)6, which may also be obtained by evaporating a
solution of cobalt cyanide, in excess of potassium cyanide, in the
presence of air, 8KCN + 2Co(CN)2 + H2O + O = 2K3Co(CN)6 + 2KHO. It
forms monoclinic crystals which are very soluble in water. From its
aqueous solution, concentrated hydrochloric acid precipitates
hydrocobalticyanic acid, H3Co(CN)6, as a colourless solid which is
very deliquescent, and is not attacked by concentrated hydrochloric
and nitric acids. For a description of the various salts of this acid,
see P. Wesselsky, _Berichte_, 1869, 2, p. 588.

_Cobaltammines._ A large number of cobalt compounds are known, of
which the empirical composition represents them as salts of cobalt to
which one or more molecules of ammonia have been added. These salts
have been divided into the following series:--

Diammine Series, [Co(NH3)2]X4M. In these salts X =