To estimate the proportion of silver in lead ore, reduce a known quantity of the clear ore to powder, mix with a little dry soda and a few nails, and heat in a round-bottomed iron pot or crucible. The lead which is obtained should then be put in a cup having ashes at the bottom, and strongly heated in an open furnace. A globule of silver will be left, if any is present, and being weighed, the percentage can be found. Rich silver ores may be reduced by mixing them with ten parts of common salt, and exposing the mass for hours in an open furnace, stirring it frequently. When cold reduce to powder and mix with an equal quantity of quicksilver and enough water to make a paste, and agitate the mixture for two days, when the amalgam will fall to the bottom. The amalgam is then squeezed in a leather bag and washed. Silver glance will yield its metal by heating it before a blow-pipe. ASSAY OF COPPER ORE.--When the ore is native copper and rock, as at Lake Superior, it should be pounded and the earthy matter washed away. Then mix with a little potash or soda and bring to a high heat in a crucible. Other copper ores may be tested by dissolving them powdered in dilute aqua regia. The presence of silver will be shown by a white powder on the bottom. Then add considerable ammonia. If there is any copper a blue liquor will be produced. Strain this through tissue paper, and evaporate to dryness. Dissolve the residue in muriatic acid, and by putting in a piece of iron or zinc, the copper will fall down. Or, add to this solution pure potash; dry and weigh the powder thrown down; every 5 parts of it contains 4 parts of copper. Gray copper and red copper ores may be assayed by heating with charcoal, (both powdered,) in a furnace. Malachite and azurite should be smelted with borax; Copper pyrites and silicate of copper with soda or powdered marble. A ton of copper ore which contains ten per cent. of metal, pays $25 at the furnace. The ore of copper when roasted, turns black; and when thrown into nitric acid makes a sky-blue solution. A clean knife-blade put into this solution will be coated with copper. ASSAY OF IRON ORE.--Take a known quantity of the ore in fine powder and mix thoroughly with dry borax (or with one part of fluor spar, one of charcoal and four of salt,) and expose it for an hour in a covered crucible lined with charcoal to a white heat in a wind-furnace for an hour. A button of iron will be found at the bottom, which determines the percentage. ASSAY OF ZINC ORE.--If the weighed ore is roasted with powdered charcoal, white flowers of zinc will be formed on a piece of cold iron held over it. After thorough roasting, the residue should be weighed; the loss is the oxide of zinc, and every 100 parts of this contain 81 of metal. All the ores of zinc will dissolve in either nitric or hot sulphuric acid.