This metal is found native in the state of oxide, in the state of sulphuret, and in that of salt, combined with carbonic, muriatic, phosphoric, and arsenic acids. Native copper sometimes contains gold, silver, or iron. It may be dissolved in nitric acid: the gold remains in the state of a blackish or rather violet-coloured powder; the silver may be separated by a polished plate of copper (or it may be precipitated from a separate portion of the solution by common salt); the iron may be separated by boiling the solution to dryness, and treating the residuum with water. By this process, the nitrate of iron is decomposed; the oxide of iron remains, while the water dissolves the nitrate of copper; this last salt may be decomposed by boiling it with potash; the precipitate dried in a red heat is black oxide of copper; one hundred parts of it denote eighty of the metallic copper. Sulphuret of copper may be dissolved in dilutic nitric acid; part of the sulphur remains unaltered, and may be estimated by weighing it, and burning it off; part is acidified, and may be precipitated by nitrate of barytes, 100 parts of the dried precipitate indicating 14.5 of sulphur.
By evaporation to dryness, and solution in water, the iron is separated, and the copper may be estimated as in the last paragraph, or muriatic acid may be used instead of nitric acid, but in that case it is more difficult to obtain a complete solution. The usual process employed in our Cornish mining districts, for reducing copper ores to the metallic state, are described under the article Copper, (which see.)