Chromium

Chromium (Cr., atomic weight, 52.1), a metallic element, of which the chief natural compound is chrome iron ore, FeOCr₂0₃. It also occurs in chrome ochre, crocoisite and some other rare minerals. The colour of emerald and the green in serpentine are due to small quantities of this element. It is a very hard metal of a grey or greenish colour, with a specific gravity 6.8. It forms three oxides, CrO, Cr₂0₃. and Cr0₃. Two series of chromium salts exist corresponding to the two first, but the highest oxide has an acid nature and yields the chromates. The chromic oxide, Cr₂O₃, is used in preparation of green glasses, enamels, etc., and as a pigment under the name of emerald green. Combined with water it is also known as Guignet's green. The salts of all yield solutions coloured green or violet, and many show a remarkable change from the one tint to the other by warming. A double salt of chromium and potassium sulphate is known as chrome alum (K₂SO₄ + Cr₂(S0₄)₃ + 240H₂). This forms dark purple octahedra, largely used in calico-printing, dyeing, and tanning.