The principal ores of barium are the sulphate or heavy spar (BaS04), and the carbonate or witherite (BaCO3,). Metallic barium may be prepared from the monoxide baryta, from the chloride, and from other salts, by electrolysis in presence of mercury; the amalgam thus formed is heated in a tube containing petroleum, when the barium remains as a silver-white. powder. Benson prefers to perform electrolysis on a pasty compound of barium chloride and very dilute hydrochloric acid, at a temperature of 212° F. (100° C) in presence of mercury; the crystalline amalgam obtained, when heated, leaves the metallic barium as a porous tarnished mass whose cavities sometimes appear silver-white. Mat-thiesaen obtains better results by passing a galvanic current through barium chloride in a state of fusion, using as a negative pole a fine harpsichord wire, on which the metal deposits in little globules. Barium oxidises very readily, and burns with brilliancy when heated in the air. Its melting-point is higher than that of iron.