_Metallurgy of Rarer Metals._--Although less in evidence than iron, this has engaged the attention of the scientist from the earliest years of the century. The full list of metals discovered since 1800 may be found under “Chemistry.” The more important only are here given. Palladium and rhodium were reduced by Wollaston in 1804. Potassium and sodium were first separated in metallic form by Sir Humphrey Davy in 1807, through the agency of the voltaic arc; barium, strontium, calcium and boron by the same scientist in 1808; iodine by Courtois in 1811; selenium by Berzelius in 1817; cadmium by Stromeyer in 1817; silicon by Berzelius in 1823, and bromium by Balard in 1826. Magnesium was first prepared by Bussey in 1829. Aluminum was first separated in 1828 by Wohler, by decomposing the chloride by means of potassium. Oersted, in 1827, preceded him with important preliminary steps, and Deville, in 1854, followed in the first commercial applications. In late years the metallurgy of aluminum has made great advances. The Cowles process heats to incandescence by the electric current a mixture of alumina, carbon and copper, the reduced aluminum alloying with the copper. This process is covered by United States patents to Cowles and Cowles, No. 319,795, June 9. 1885, and Nos. 324,658 and 324,659, August 18, 1885. It has, however, for the most parts been superseded by the process patented by Hall, April 2, 1889, No. 400,766, in which alumina dissolved in fused cryolite is electrically decomposed. In the metallurgy of the precious metals probably the most important step has been the _cyanide process_ of obtaining gold and silver. In 1806 it was known that gold was soluble in a solution of cyanide of potassium. In 1844 L. Elsner published investigations along this line, and demonstrated that the solution took place only in the presence of oxygen. McArthur and Forrest perfected the process for commercial application, and it is now extensively used in the Transvaal and elsewhere. It is covered by their British patent, No. 14,174, of 1887, and United States patents No. 403,202, May 14, 1889, and No. 418,137, December 24, 1889, which describe the application of dilute solutions of cyanide of potassium, not exceeding 8 parts cyanogen to 1,000 parts of water: the use of zinc in a fine state of division to precipitate the gold out of solution, and the preparatory treatment of the partially oxidized ores with an alkali or salts of an alkali. By this solution-process gold, in the finest state of subdivision, which could not be extracted by other processes from the earthy matters, may be recovered and saved in a simple, practical and cheap way. [Illustration: FIG. 263.--EDISON MAGNETIC CONCENTRATING WORKS. THE GIANT CRUSHING ROLLS.] [Illustration: FIG. 264.--EDISON MAGNETIC CONCENTRATOR.]