In 1794 Gaciolin separated a new earth yttria from a black mineral found at Ytterby in Sweden, and in 1804 Berzelius and Hisinger (Klaproth independently) found another new earth in a mineral from Bastnas which they named ceria. In 1839 Mosander showed that ceria consisted of ceria and a new oxide—lanthaila. A little later the same worker showed that lanthana was composed of lanthana and another oxide didymia. Again, in 1843 Mosander showed that yttria was composed of the oxides, viz., yttria, terbia (rose) and a dark oxide, erbia. Berlin and other chemists in 186o found only two oxides, viz., yttria and the rose-coloured oxide which they now called erbia.
Delafontain and Marignac independently showed that Mosan der's three oxides really existed, and, since the name erbia had been given to terbia, the latter was used to designate erbia. In 1879 de Boisbaudran claimed the discovery of a new element in didymium oxide which he called samarium. Marignac found another element in samarium which he later named gadolinium. In 1885 von Weisbach split didymium into praseodymium and neo dymium, and in 1896 Demarcey separated europium from samarium. In 1926 Hopkins announced the discovery of element 6i—illinium. In the same year Corke, James and Fogg obtained "61" independently and measured the lines of the X-ray spectrum.
These discoveries may be represented graphically as in scheme (I.) In 1878 Marignac divided erbia into erbia and a new oxide ytterbia. In 1878 and 1879 Soret and Cleve separated erbia into three oxides, erbia, holmia and thulia ; de Boisbaudran, in 1886, obtained a new oxide, dysprosia, from holmia. In 1906 Urbain claimed the discovery of lutecia in ytterbia. Thus we have scheme (II.).
The rare earths possess a valence of three, and with the excep tion of cerium, do not form salts higher than the type A few, like samarium, form a lower chloride RC12.
The position of these elements in the periodic system presented a great difficulty, which has been solved by N. Bohr's work. Fur ther, it is clear from Moseley's work on the atomic number (q.v.) that all the elements of this series are now accounted for.
For the extraction and separation of the individual members of the group the reader is referred to the following: J. F. Spencer, The Metals of the Rare Earths (i9ii) ; J. N. Friend, Text-book of Inor ganic Chemistry, vol. iv. (1914, etc.) ; S. I. Levy, The Rare Earths (1915, ; J. W. Mellor, Treatise on Inorganic and Theoretical Chemistry, vol. v. (1922). (C. J.)