Sulphur turns silver black. Make a paste with carbonate of soda, heat on the charcoal, and removing the mass with the point of a knife lay it on a silver coin and moisten. A black sulphide of silver should show quickly on the coin if sulphur is present. Magnesia gives a faint pink color when heated and treated with nitrate of cobalt on coal. Alumina under the same circumstances give a blue color. Roasting is an oxidizing process, the substance being heated in air, so that it may absorb oxygen. The test by reduction with soda on coal in the R.F. is particularly valuable in the case of copper ore, as little as 1 per cent. being detected. CHAPTER IV. ECONOMIC ORES AND MINERALS. Aluminum is derived from two ores, cryolite and bauxite. This metal has made rapid strides into favor during the past half-dozen years. Although known since 1827, it remained a rare substance in the metallic form, though it is the most abundant of any of the metals in its ore. In ordinary clay there is an inexhaustible source of aluminum. But the ores that yield the metal cheaply are few. Until recently, cryolite, found abundantly in Greenland, was the chief source of the metal, but now bauxite is used in its place. Bauxite is a limonite iron ore in which a part of the iron has been replaced by aluminum. It is found in Alabama, Georgia and Arkansas, as well as in Europe. Aluminum is white, and very light in weight. It does not tarnish easily. The chemical composition of these ores is: Aluminum. Cryolite, Al{2}F{6}.6NaF 12.8 per cent. Bauxite, Al{2}O{3}.3H{2}O 73.9 per cent. In 1895 the production of this metal in the United States was 900,000 pounds. In 1899 it rose to 6,500,000 pounds. The only firm producing aluminum is the Pittsburg Manufacturing Company of Buffalo, N.Y., who reduce the metal from bauxite, which they obtain in the southern states. One of the latest uses for this metal is for gold miners' pans. The French seem to keep ahead of the rest of the world in finding new uses for aluminum. Most of the supply of cryolite comes from Greenland, where it occurs in veins running through gneiss rocks. Glass-makers use it and pay good prices for it. Lately makers of aluminum also buy it, as it contains 13 per cent. of that metal. A new aluminum-bearing mineral, discovered in New Mexico and in Ohio, is called native alum. It gives 50.16 per cent. alumina, and may be treated by solution in warm water, filtration, evaporation and roasting. No estimate has yet been made of the amount available.