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. By surgeons it is employed as an escharotic and astringent, and in the arts it is turned to account in dyeing, printing of cotton, 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 Ramsberg near Goslar, in the Hartz; Anglesea in England; and Fahlun in Sweden.

BLUING, the act or art of communicating a blue colour to bodies.

Bluing of Metals is performed by heating them in the fire till they assume a blue colour. It is particularly practised by gilders, who blue their metals before they apply the gold and silver leaf.

Bluing of Iron, a method of beautifying that metal, sometimes practised for mourning buckles, swords, watchsprings, and the like. It is done thus: Take a piece of grindstone or whetstone, and rub hard on the work to take off the black scurf; then heat it in the fire, and as it grows hot the colour will change 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 of itself. Among sculptors we find mention made of 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.