the sulphate of copper; a salt of a fine blue or bluish-green colour, containing 32½ per cent. of copper, 31½7 of sulphuric acid, and 36½30 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 Falun in Sweden.

Blueing of Metals is the process of heating them in the fire till they assume a blue colour. The bluing of iron is done thus: A piece of grindstone or whetstone is rubbed hard on the work to take off the black scurf; the iron is then heated in the fire, and as it grows hot the colour changes by degrees, becoming first of a light, then of a darker gold colour, and lastly blue. Sometimes also indigo and salad-oil are ground together, and the mixture rubbed on the work with a woollen rag while it is heating, after which it is left to cool. Among sculptors is used the term bluing a figure of bronze, by which is meant the heating of it, to prepare it for the application of gold leaf; and it is so called because of the bluish cast the metal acquires in the operation.