See Dilettanti Society, _Ionian Antiquities_, ii. (1821); C. T. Newton, _Hist. of Discoveries_, &c. (1862) and _Travels in the Levant_, ii. (1865); O. Rayet and A. Thomas, _Milet et le Golfe Latmique_ (1877); E. Pontremoli and B. Haussoullier, _Didymes_ (1904). (D. G. H.)
DIDYMIUM (from the Gr. [Greek: didymos], 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 Nd2O3 and Nd2O5, 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 Pr2O3, Pr2O5, xH2O (B. Brauner), and Pr4O7. The peroxide, Pr4O7, forms a dark brown powder, and is obtained by ignition of the oxalate or nitrate. The sesquioxide, Pr2O3, 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.
DIDYMUS (?309-?394), surnamed "the Blind," ecclesiastical writer of Alexandria, was born about the year 309. Although he became blind at the age of four, before he had learned to read; he succeeded in mastering the whole circle of the sciences then known; and on entering the service of the Church he was placed at the head of the Catechetical school in Alexandria, where he lived and worked till almost the close of the century. Among his pupils were Jerome and Rufinus. He was a loyal follower of Origen, though stoutly opposed to Arian and Macedonian teaching. Such of his writings as survive show a remarkable knowledge of scripture, and have distinct value as theological literature. Among them are the _De Trinitate_, _De Spiritu Sancto_ (Jerome's Latin translation), _Adversus Manichaeos_, and notes and expositions of various books, especially the Psalms and the Catholic Epistles.