ERBIUM (symbol, Er; atomic weight, 165-166), one of the metals of the rare earths. The first of the rare earth minerals was discovered in 1794 by J. Gadolin and was named gadolinite from its discoverer. In 1797 Ekeberg showed that gadolinite contained another rare earth, which was given the name yttria. Yttria is an exceedingly complex mixture, which has been decomposed, yielding as an intermediate product terbia. This latter substance in its turn has been split by J.L. Soret, P.T. Cleve, Lecoq de Boisbaudran and others into erbia, holmia, thulia and dysprosia, but it is still doubtful whether any one of these four splitting products is a single substance. The rare earth metals are found in the minerals gadolinite, samarskite, fergusonite, euxenite and cerite. They are separated from the minerals by converting them into oxalates, which by ignition give the corresponding oxides. The oxides are then converted into double sulphates which are separated from each other by repeated fractional crystallization or by fractional precipitation with ammonia or some other base. Erbium forms rose-coloured salts and a rose-coloured oxide. The oxide dissolves slowly in acids; it is not reduced by hydrogen and is infusible. The salts show a characteristic absorption spectrum. See J.F. Bahr and R. Bunsen (_Ann._, 1866, 137, p. 1); A. v. Welsbach (_Monats._, 1883, 4, p. 641; 1884, 5, p. 508; 1885, 6, p. 477); P.T. Cleve (_Comptes rendus_, 1879, 89, p. 478; 1880, 91, pp. 328, 381; 1882, 95, p. 1225; _Bull. de la soc. chim._, 1874, 21, p. 196; 1883, 39, p. 287); C. Marignac (_Ann. Chim. phys._, 1849 [3] 27, p. 226); B. Brauner (_Monats._, 1882, 3, p. 13); W. Crookes (_Proc. Roy. Soc._, 1886, 40, p. 502); Lecoq de Boisbaudran (_Comptes rendus_, 1886, 102, p. 1005); A. Bettendorf (_Ann._, 1892, 270, p. 376); M. Muthmann (_Ber._, 1898, 31, p. 1718; 1900, 33, p. 42); G. Kruss (_Zeit. f. anorg. Chem._, 1893, 3, p. 108).