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.

See Migne, _Patrol. Graec._ xxxix.; O. Bardenhewer, _Patrologie_, pp.
290-293 (Freiburg, 1894).

DIDYMUS CHALCENTERUS (c. 63 B.C.-A.D. 10), Greek scholar and grammarian,
flourished in the time of Cicero and Augustus. His surname (Gr. [Greek:
Chalkenteros], brazen-bowelled) came from his indefatigable industry; he
was said to have written so many books (more than 3500) that he was
unable to recollect their names ([Greek: bibliolathas]). He lived and
taught in Alexandria and Rome, where he became the friend of Varro. He
is chiefly important as having introduced Alexandrian learning to the
Romans. He was a follower of the school of Aristarchus, upon whose
recension of Homer he wrote a treatise, fragments of which have been
preserved in the Venetian Scholia. He also wrote commentaries on many
other Greek poets and prose authors. In his work on the lyric poets he
treated of the various classes of poetry and their chief
representatives, and his lists of words and phrases (used in tragedy and
comedy and by orators and historians), of words of doubtful meaning, and
of corrupt expressions, furnished the later grammarians with valuable
material. His activity extended to all kinds of subjects: grammar
(orthography, inflexions), proverbs, wonderful stories, the law-tablets
([Greek: axones]) of Solon, stones, and different kinds of wood. His
polemic agains