It is not surprising that this metal should have remained so long unknown, for it has never yet been found in the metallic state. Its ores are often and in a great degree mixed with foreign ingredients; and when they are melted, it sublimes in a metallic form, and is found adhering above to the cool sides of the furnace; but a particular apparatus is necessary, else the reduced metal partly evaporates, and is partly oxidized, by which means it appears like an earth, and exhibits to the eye no traces of metal. That mixture of zinc and copper called at present brass, tomback, pinchbeck, princes-metal, &c., and which was first discovered by ores, abundant in zinc, yielding when melted not pure copper, but brass, was certainly known to the ancients. Mines that contained ores, from which this gold-coloured metal was produced, were held in the highest estimation; when exhausted, the loss of them was regretted; and it was supposed that the metal would never be again found. In the course of time it was remarked, no one knows by what accident, that an ore, which must have been calamine, when added to copper while melting, gave it a yellow colour. This ore was therefore used, though it was not known what metal it contained, in the same manner as oxide of cobalt was employed in colouring glass before mineralogists were acquainted with that metal itself. Aristotle and Strabo speak of an earth of that kind, the use of which in making brass has been retained through every century. Ambrosius, bishop of Milan, in the fourth century; Primasius, bishop of Adrumetum in Africa, in the sixth; and Isidore, bishop of Seville, in the seventh, mention an addition by which copper acquired a gold colour, and which undoubtedly must have been calamine. When in course of time more calamine was discovered, the ancient method of procuring brass from copper-ore that contained zinc was abandoned; and it was found more convenient first to extract from it pure copper, and then to convert it into brass by the addition of calamine.