DIDYMIUM (from the Gr. &Segos, 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 . Chem'ie, 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 Nd205, 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 Pr203, Pr206,xH2O (B . Brauner), and Pr407 . The peroxide, Pr407, 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 .