6; and
also by C. Meinecke, who estimated the amount of silver, chromium and
oxygen in silver chromate, the amount of oxygen in potassium bichromate,
and the amount of oxygen and chromium in ammonium bichromate (_Ann._,
1891, 261, p. 339), the mean value obtained being 51.99.

Chromium forms three series of compounds, namely the chromous salts
corresponding to CrO, chromous oxide, chromic salts, corresponding to
Cr2O3, chromium sesquioxide, and the chromates corresponding to CrO3,
chromium trioxide or chromic anhydride. Chromium sesquioxide is a
basic oxide, although like alumina it acts as an acid-forming oxide
towards strong bases, forming salts called chromites. Various other
oxides of chromium, intermediate in composition between the
sesquioxide and trioxide, have been described, namely chromium
dioxide, Cr2O3.CrO3, and the oxide CrO3.2Cr2O3.

Chromous oxide, CrO, is unknown in the free state, but in the hydrated
condition as CrO.H2O or Cr(OH)2 it may be prepared by precipitating
chromous chloride by a solution of potassium hydroxide in air-free
water. The precipitate so obtained is a brown amorphous solid which
readily oxidizes on exposure, and is decomposed by heat with
liberation of hydrogen and formation of the sesquioxide. The
sesquioxide, Cr2O3, occurs native, and can be artificially obtained in
several different ways, e.g., by igniting the corresponding hydroxide,
or chromium trioxide, or ammonium bichromate, or by passing the
vapours of chromium oxychloride through a red-hot tube, or by ignition
of mercurous chromate. In the amorphous state it is a dull green,
almost infusible powder, but as obtained from chromium oxychloride it
is deposited in the form of dark green hexagonal crystals of specific
gravity 5.2. After ignition it becomes almost insoluble in acids, and
on fusion with silicates it colours them green; consequently it is
used as a pigment for colouring glass and china. By the fusion of
potassium bichromate with boric acid, and extraction of the melt with
water, a residue is left which possesses a fine green colour, and is
used as a pigment under the name of Guignet's green. In composition it
approximates to Cr2O3.H2O, but it always contains more or less boron
trioxide. Several forms of hydrated chromium sesquioxide are known;
thus on precipitation of a chromic salt, free from alkali, by ammonia,
a light blue precipitate is formed, wh