RHODIUM, is a metal discovered by Dr. Wollaston in 1803, in the ore of pla tinum. His contained to the amount of three per cent. in the platinum ore of Antioquia in Colombia, near Bar bacons ; it occurs in the Ural ore, and, alloyed with gold, in Mexico. The pal ladium having been precipitated from the muriatic solution of the platinum ore previously saturated with soda, by the cyanide of mercury, muriatie acid is to be poured into the residuary liquid, and the mixture is to be evaporated to dry ness, to expel the hydrocyanic acid, and convert the metallic salts into chlorides. The dry mass is to be reduced to a very fine powder, and washed with alcohol of specific gravity 0.837. This solvent takes possession of the double chlorides which the sodium forms with the platinum, iridium, copper, and mercury, and does not dissolve the double chloride of rhodium and sodium, but leaves it in the form of a powder, of a fine dark-red color. This salt being washed with alco hol, and then exposed to a very strong heat, affords the rhodium. But a better mode of reducing the metal upon the small scale, consists in heating the double chloride gently in a glass tube, while it stream of hydrogen passes over it, and then to wash away the chloride of sodium with water.
Rhodium resembles platinum in appear ance. Any heat which can be produced in a chemical furnace is incapable of fus ing it ; and the only way of giving it co hesive solidity, is to calcine the sul phuret or arseniuret of rhodium in an open vessel at a white heat, till all the sulphur or arsenic be expelled. A but
ton may thus be obtained, somewhat spongy, having the color and lustre of silver. According to Wollaston, the spe cific gravity of rhodium is 11. It is in soluble by itself in any acid ; but when an alloy of it with certain metals, i as pla tinum, copper, bismuth or load, s treat ed with aqua regia, the rhodium dissolves along with the other metals ; but when alloyed with gold or silver it will not dis solve along with them. It may, how ever, be rendered very soluble by mixing it in the state of a fine powder with chlo ride of potassium or sodium, and heat, big the mixture to a dull-red heat, in a stream of chlorine gas. It thus forms a triple salt, very soluble in water. The solutions of rhodium are of a beautiful rose color, whence its name. In the dry way, it dissolves by heat in bisulphate of potassa ; and disengages sulphurous acid gas in the act of solution. There are two oxides of rhodium. Rhodium combines with almost all the metals ; and, in small quantity, melted with steel, it has been supposed to improve the hardness, close ness, end toughness of this metal. Its chief use at present is for making the inalterable nibs of the so-named rhodium pens.