finely divided state it rapidly combines with oxygen, the
compact metal requiring a strong heating to bring about this
combination. It decomposes steam at a red heat, and slowly dissolves in
dilute hydrochloric and sulphuric acids, but more readily in nitric
acid. Cobalt burns in nitric oxide at 150 deg. C. giving the monoxide.
It may be obtained in the pure state, according to C. Winkler (_Zeit.
fur anorg. Chem._, 1895, 8, p. 1), by electrolysing the pure sulphate in
the presence of ammonium sulphate and ammonia, using platinum
electrodes, any occluded oxygen in the deposited metal being removed by
heating in a current of hydrogen.

Three characteristic oxides of cobalt are known, the monoxide, CoO,
the sesquioxide, Co2O3, and tricobalt tetroxide, Co3O4; besides these
there are probably oxides of composition CoO2, Co8O9, Co6O7 and Co4O5.
Cobalt monoxide, CoO, is prepared by heating the hydroxide or
carbonate in a current of air, or by heating the oxide Co3O4 in a
current of carbon dioxide. It is a brown coloured powder which is
stable in air, but gives a higher oxide when heated. On heating in
hydrogen, ammonia or carbon monoxide, or with carbon or sodium, it is
reduced to the metallic state. It is readily soluble in warm dilute
mineral acids forming cobaltous salts. Cobaltous hydroxide, Co(OH)2,
is formed when a cobaltous salt is precipitated by caustic potash in
the absence of air. A blue basic salt is precipitated first, which, on
boiling, rapidly changes to the rose-coloured hydroxide. It dissolves
in acids forming cobaltous salts, and on exposure to air it rapidly
absorbs oxygen, turning brown in colour. A. de Schulten (_Comptes
Rendus_, 1889, 109, p. 266) has obtained it in a crystalline form; the
crystals have a specific gravity of 3.597, and are easily soluble in
warm ammonium chloride solution. Cobalt sesquioxide, Co2O3, remains as
a dark-brown powder when cobalt nitrate is gently heated. Heated at
190-300 deg. in a current of hydrogen it gives the oxide Co3O4, while at
higher temperatures the monoxide is formed, and ultimately cobalt is
obtained. Cobaltic hydroxide, Co(OH)3, is formed when a cobalt salt is
precipitated by an alkaline hypochlorite, or on passing chlorine
through water containing suspended cobaltous hydroxide or carbonate.
It is a brown-black powder soluble in hydrochloric acid, chlorine
being simultaneously liberated. This hydroxide is solubl