DIDYMIUM (from the Gr. 818vµos, twin), the name given to the supposed element isolated by C. G. Mosander from cerite (1839-1841). In 1879, however, Lecoq de Boisbaudran showed that Mosander's "didymium," contained samarium; while the residual "didymium" after removal of samarium, was split by Auer v. Welsbach (Monats. f. Chemie, 1885, 6, 477) into two components (known respectively as neodymium and praseodymium) by repeated fractional crystallization of the double nitrate of ammonium and didymium in nitric acid. Neodymium (Nd) forms the chief portion of the old "didymium." Its salts are reddish violet in colour, and give a characteristic absorption spectrum. It forms oxides of composition Nd203 and Nd 2 O 5, the latter being obtained by ignition of the nitrate (B. Brauner). The atomic weight of neodymium is 143.6 (B. Brauner, Proc. Chem. Soc., 1897-1898, p. 70). Praseodymium (Pr) forms oxides of composition Pr 2 O 3, Pr205,xH20 (B. Brauner), and Pr407. The peroxide, Pr 4 O 7, forms a dark brown powder, and is obtained by ignition of the oxalate or nitrate. The sesquioxide, Pr203, is obtained as a greenish white mass by the reduction of the peroxide. The salts of praseodymium are green in colour, and give a characteristic spark spectrum. The atomic weight of praseodymium is 140.5.
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