Copper-smelting
A prodigious quantity of copper is obtained from Lake Superior. Mr. Petherick, the well-known mining engineer, informed Dr. Percy that at Minnesota, in 1854, not fewer than forty men were engaged during twelve months in cutting up a single mass of native copper, weighing about 500 tons! The native copper at Lake Superior in some places occurs curiously intermingled, but generally not alloyed, with native silver. The following anecdote is recounted of the value of the gold in the residue from some South American copper-ores, and which was communicated to Dr. Percy by Dr. Lyon Playfair. At certain large chemical works where sulphate of copper was prepared by dissolving copper in sulphuric acid, an insoluble residue was produced in the process, which had been put aside from time to time, and had fortunately not been thrown away. A small sum was offered by certain persons for this residue, which had not previously been regarded as of much value. Suspicion was excited, especially by the quarter from which the offer proceeded, and it was declined; whereupon the residue was examined, and was found to contain 700l.-worth of gold!