​TERBIUM [symbol Tb, atomic weight 159.2 (O=16)], a metallic chemical element belonging to the rare earth group; it was originally called erbia by its discoverer Mosander (see Rare Earth). Pure terbium compounds were obtained by G. Urbain (Compt. rend., 1904 seq.) by fractional crystallization of the nickel double nitrates, the ethyl sulphates, and the bismuth double nitrates of the terbium earths. Terbium appears to be trivalent. The oxide is a black or brown powder according as it is prepared from the exalate or sulphate, and when pure it is non-fluorescent, but mixed with gadolinia or alumina it possesses this property. It yields colourless salts; the crystallized sulphate has the formula Tb₂(SO₄)₃·8H₂O.