In his examination of the ceria earths in 1839, Mosander discovered a new constituent, which he called Lanthana; the new oxide was removed in solution when the ignited mixture was extracted with dilute nitric acid, which leaves cerium dioxide undissolved. On examination, the new oxide was found to be heterogeneous; by fractional precipitation with ammonia, and subsequent recrystallisation of the sulphates, he obtained two oxides, which he called respectively Lanthana (λανθανειν, to be hidden), from the absence of colour and specific reactions, and Didymia, (διδυμοι, twins) from their similarity and the occurrence of the two together. Samaria was isolated by Lecoq de Boisbaudran, in 1879, from a specimen of didymia extracted from the mineral samarskite. Two years previously, Delafontaine had shown that the didymia separated from this mineral was not spectroscopically identical with the oxide obtained from other sources, and in 1878 had isolated an oxide which he called Decipia; this was shown later, however, to be a mixture of which samaria was one component. The samaria obtained by de Boisbaudran was by no means pure, being associated with terbia earths; several investigators claimed to have separated from it new oxides, most of these being proved afterwards to have been more or less impure specimens of Europia. In 1885, Auer von Welsbach[238] employed for the first time the method which has now become of paramount importance for the separation of the cerium group, viz. the fractional crystallisation of the double nitrates. By this method he succeeded in resolving Mosander’s didymia into two new oxides, for which he proposed the names Praseodidymia (πρασινος, leek-green), from the colour of the salts, and Neodidymia respectively; the shorter names praseodymia and neodymia are, however, now generally adopted. [238] _Monats._ 1885, ~6~, 477; _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1885, ~92~, II, 317. [Illustration: ~GROUP A~ MIXED DOUBLE NITRATES.