Some time since, says the Mining Magazine, M. Gaudin obtained artificial rubies by fusing ammoniacal alum before the oxy-hydrogen blow-pipe, with the addition of a small quantity of chromate of potash. It is now said that he has succeeded in preparing perfectly isolated and colorless crystals of alumina, in the form of the sapphire. For this purpose he introduces into a crucible lined with charcoal equal weights of alum and sulphate of potash, both previously refined and reduced to a powder, and exposes the crucible for a quarter of an hour to the full heat of the forge. When the crucible is broken, the crevices of the lining are found to contain a mass consisting of sulphuret of potassium, through which are disseminated the crystals of alumina. The mass is treated with aqua regia, and the crystals left in the form of a fine sand, which is well washed with water. The crystals vary in size according to the mass* of ma terials employed, and the duration of the heat. Those obtained by M. Gaudin, operating on a small scale, were about one millimetre (0-0394 inches) in length. They are colorless, because in this process any metallic oxyds which may be added for the purpose of imitating the natural colors ofpv'4jphires are reduced by the charcoal. T.%, tremely limpid, and surpass the natural ruD) in hardness. The formation of such cr'rjlu_ depends on the solvent actjp1 ret of potassium.