a name given to the factitious green vitriol. See Chemistry.

The English copperas is made at Deptford, in the following manner, from pyrites. See Pyrite.

A heap of these stones, two or three foot thick, is laid in a bed well rammed; where being turned once in six months, in five or six years, by the action of the air and rain, they begin to dissolve, and yield a liquor which is received in pits, and thence conveyed into a cistern, in a boiling house. The liquor at length being pumped out of the cistern into a leaden boiler, and a quantity of iron added thereto; in two or three days the boiling is completed; care having been taken all along to supply it with fresh quantities of iron, and to restore the boiling, whenever it seems to abate. When boiled sufficiently, it is drawn off into a cooler, with sticks across, where it is left 14 or 15 days to shoot. The uses of copperas are numerous. It is the chief ingredient in the dying of wool, cloths, and hats, black; in making ink, in tanning and dressing leather, &c. and from hence is prepared oil of vitriol, and a kind of Spanish brown for painters. In medicine it is rarely prescribed under the name of copperas, but it is a true salt of iron, and often prescribed under that name, and used instead of the genuine preparation; our chemists in general giving themselves no further trouble about the making of that salt, than to dissolve and purify the common copperas, and shoot it again into crystals.