y as chrome-ironstone, Cr2O3.FeO,
and occasionally being found as crocoisite, PbCrO4, chrome-ochre, Cr2O3,
and chrome-garnet, CaO.Cr2O3.3SiO2, while it is also the cause of the
colour in serpentine, chrome-mica and the emerald. It was first
investigated in 1789 by L.N. Vauquelin and Macquart, and in 1797 by
Vauquelin, who found that the lead in crocoisite was in combination with
an acid, which he recognized as the oxide of a new metal.

The metal can be obtained by various processes. Thus Sainte Claire
Deville prepared it as a very hard substance of steel-grey colour,
capable of taking a high polish, by strong ignition of chromic oxide and
sugar charcoal in a lime crucible. F. Woehler reduced the sesquioxide by
zinc, and obtained a shining green powder of specific gravity 6.81,
which tarnished in air and dissolved in hydrochloric acid and warm
dilute sulphuric acid, but was unacted upon by concentrated nitric acid.
H. Moissan (_Comptes rendus_, 1893, 116, p. 349; 1894, 119, p. 185)
reduces the sesquioxide with carbon, in an electric furnace; the product
so obtained (which contains carbon) is then strongly heated with lime,
whereby most of the carbon is removed as calcium carbide, and the
remainder by heating the purified product in a crucible lined with the
double oxide of calcium and chromium. An easier process is that of H.
Goldschmidt (_Annalen_, 1898, 301, p. 19) in which the oxide is reduced
by metallic aluminium; and if care is taken to have excess of the
sesquioxide of chromium present, the metal is obtained quite free from
aluminium. The metal as obtained in this process is lustrous and takes a
polish, does not melt in the oxyhydrogen flame, but liquefies in the
electric arc, and is not affected by air at ordinary temperatures.
Chromium as prepared by the Goldschmidt process is in a passive
condition as regards dilute sulphuric acid and dilute hydrochloric acid
at ordinary temperatures; but by heating the metal with the acid it
passes into the active condition, the same effect being produced by
heating the inactive form with a solution of an alkaline halide. W.
Hittorf thinks that two allotropic forms of chromium exist (_Zeit. fuer
phys. Chem._, 1898, 25, p. 729; 1899, 30, p. 481; 1900, 34, p. 385),
namely active and inactive chromium; while W. Ostwald (ibid., 1900, 35,
pp. 33, 204) has observed that on dissolving chromium in dilute acids,
the rate of solution as measured by the evolution of gas is not
continuou