CYTHNUS(Κύθνος: Eth. Κύθνιος: Thermia), an island in the Aegaean sea, one of the Cyclades, lying between Ceos and Seriphos. (Strab. 10. p. 485; Dicaearch. p. 462, ed. Fuhr.; Scylax, p. 22, ed. Hudson; Plin. Nat. 4.12. s. 20; Mela, 2.7; Ptol. 3.15.28.) It was colonised by the Dryopes, whence it was also called Dryopis. (Hdt. 8.46; Steph. B. s. v.) Its name rarely occurs in antiquity. The Cythnians sent a trireme and a penteconter to the battle of Salamis. (Herod. l. c.) After the Peloponnesian war they became the subject allies of Athens, together with the other islanders in the Aegaean; but they never acquired power or wealth. (Comp. Dem. Περὶ Συντάξεως, p. 176.) The only native of the island mentioned by the ancient writers, was Cydias the painter; and its chief celebrity in (antiquity was owing to its excellent cheeses. (Steph. B. s. v.;Eustath. ad Dionys. Per. 525; Athen. 12. p. 516; Plin. Nat. 13.24. s. 27.) Its political constitution, however, had not escaped the attention of Aristotle. (Harpocrat. s. v. Κύθνιοι.) In the war between Philip and the Romans in B.C. 200, Cythnus was occupied by a Macedonian garrison. Attalus and the Rhodians laid siege to the city; but being unable to take it immediately, they quitted the island at the end of a few days, as the capture of the place was hardly worth the trouble. (Liv. 31.15, 45.) After the death of Nero, Cythnus is mentioned as the place where a false Nero made his appearance, and gathered around him many adherents. (Tac. Hist. 2.8, 9.)
Cythnus contained a town of the same name, situated about the middle of the western coast of the island, upon the summit and sides of a hill at least 600 feet in height. Its harbour was formed by a small rock lying in front of the town. The ruins of the ancient town are now called Hebraeokastron. The circuit of the walls may still be traced, though the greater part of them has disappeared. Within this circuit Ross noticed two large rectangular substructions, divided by a passage a few feet in width; they were probably the foundations of two temples or other public buildings. From the above-mentioned passage a flight of steps appears to have been cut out of the rock, leading down to the sea. Near these steps on the descent to the sea are three chambers cut out of the rock, standing alongside of one another; they were probably a sanctuary, as there is nothing to indicate that they were sepulchres.
The modern name of the island, Thermia, is derived from some hot springs on its north-eastern side, which are now much frequented from various parts of Greece, for the cure of diseases. They are not mentioned by ancient writers, but appear to have been used in antiquity, as some ancient remains are found near them. (Tournefort, Voyage, vol. 1. p. 251, transl.; Ross, Reisen auf den Griech. Inseln, vol. 1. p. 105; Fiedler, Reise durch Griechenland, vol. 2. p. 95.) COIN OF CYTHNUS.