e in well
cooled acids, forming solutions which contain cobaltic salts, one of
the most stable of which is the acetate. Cobalt dioxide, CoO2, has not
yet been isolated in the pure state; it is probably formed when iodine
and caustic soda are added to a solution of a cobaltous salt. By
suspending cobaltous hydroxide in water and adding hydrogen peroxide,
a strongly acid liquid is obtained (after filtering) which probably
contains _cobaltous acid_, H2CoO3. The barium and magnesium salts of
this acid are formed when baryta and magnesia are fused with cobalt
sesquioxide. Tricobalt tetroxide, Co3O4, is produced when the other
oxides, or the nitrate, are heated in air. By heating a mixture of
cobalt oxalate and sal-ammoniac in air, it is obtained in the form of
minute hard octahedra, which are not magnetic, and are only soluble in
concentrated sulphuric acid.

The cobaltous salts are formed when the metal, cobaltous oxide,
hydroxide or carbonate, are dissolved in acids, or, in the case of the
insoluble salts, by precipitation. The insoluble salts are rose-red or
violet in colour. The soluble salts are, when in the hydrated
condition, also red, but in the anhydrous condition are blue. They are
precipitated from their alkaline solutions as cobalt sulphide by
sulphuretted hydrogen, but this precipitation is prevented by the
presence of citric and tartaric acids; similarly the presence of
ammonium salts hinders their precipitation by caustic alkalis.
Alkaline carbonates give precipitates of basic carbonates, the
formation of which is also retarded by the presence of ammonium salts.
For the action of ammonia on the cobaltous salts in the presence of
air see _Cobaltammines_ (below). On the addition of potassium cyanide
they give a brown precipitate of cobalt cyanide, Co(CN)2, which
dissolves in excess of potassium cyanide to a green solution.

Cobalt chloride, CoCl2, in the anhydrous state, is formed by burning
the metal in chlorine or by heating the sulphide in a current of the
same gas. It is blue in colour and sublimes readily. It dissolves
easily in water, forming the hydrated chloride, CoCl2.6H2O, which may
also be prepared by dissolving the hydroxide or carbonate in
hydrochloric acid. The hydrated salt forms rose-red prisms, readily
soluble in water to a red solution, and in alcohol to a blue solution.
Other hydrated forms of the chloride, of composition