BLUE VITRIOL, the sulphate of copper; a salt of a fine blue or bluish-green colour, containing 32.13 per cent. of copper, 31.57 of sulphuric acid, and 36.30 of water. It is employed as an escharotic and astringent, and in the arts it is used in dyeing, cotton-printing, and the like. It owes its existence in nature to the decomposition of other minerals, particularly copper pyrites; and, after having undergone the process of purification, forms regular crystals of a blue colour. It reddens litmus paper, and is soluble in about four parts of cold and two of boiling water. Its chief localities are the Rammelsberg near Goslar, in the Hartz; Anglesea in England; and Fahlun in Sweden.