ERBIUM, a metallic element belonging to the rare-earth group. It was discovered in 1843 by Mosander who originally named the oxide terbia; the names terbia and erbia were inter changed a few years later by men working in this field. In 1878 Marignac separated erbium into erbium and "ytterbium," and in 1879 Soret and Cleve separated this purified erbium into holmium, erbium and thulium. Erbium occurs with its allied elements in the minerals gadolinite, euxenite, xenotime, etc. It has the symbol Er, atomic number 68, atomic weight 167.7. It is best separated by the crystallization of the bromates and by boiling the nitrate solution with sodium nitrite, when a basic erbium nitrate is pre cipitated before that of yttrium. Erbium forms a rose-pink oxide, which dissolves slowly in many acids, giving rose-coloured salts, solutions of which show a strong characteristic absorption spectrum. (See RARE EARTHS.)