Rio, informs us that it was twice colonized by Ionians from Attica in early times. It soon became one of the greatest commercial towns of Greece, and sent out a large number of colonies to various parts of the world. The peninsula of Chalcidice in Macedonia obtained its name from the number of Chalcidian colonies founded there. Cumae and Rhegium in Italy, and Naxos, Zancle, and Tauropentium in Sicily, are among the cities which owed their origin to Chalcis. The government was originally in the hands of an aristocracy, called hippobotes, who were in all probability the proprietors of the rich plain of Lelantum, between Chalcis and Eretria. This plain was claimed by both of these cities; and at an early period a war took place between them, in which Miletus and Samos took part. After the expulsion of the Pisistratids from Athens, the Chalcidians joined the Boeotians in a war against that city; but the Athenians, in 506 B.C., invaded the island with a large force, and after a complete victory, divided the lands of the nobles among 4000 Athenian settlers. These, however, retired from the island on its invasion by the Persians in 490 B.C. After the Persian war, Euboea joined the Athenian confederacy, and continued in that alliance till 445 B.C., when a general insurrection took place in the island against Athens. It was, however, soon reconquered and reduced to subjection by Pericles, and the aristocracy of Chalcis were deprived of their power. In 411 B.C., after the disastrous end of the Athenian expedition to Sicily, the whole of Euboea again threw off the Athenian yoke, and the bridge across the straits was then first built, in order to secure the communication with Boeotia. The island continued independent for some time, and joined the Theban confederacy against Sparta; but when the Athenian power was again in the ascendant, the cities of Euboea became once more subject to her supremacy, and were governed by tyrants. About 350 B.C., Callias and Taurosthenes, joint tyrants of Chalcis, wishing to obtain the sovereignty of the whole island, asked the assistance of Philip of Macedon, who readily acceded to their request; but Plutarch, tyrant of Eretria, having applied to the Athenians for aid against this attempt, they sent an army under Phocion, by whom the Chalcidians were defeated. The Macedonian party, however, still retained their power in Chalcis; and soon the influence of Philip was predominant throughout the island. Owing to its strength and position, Chalcis was a place of much importance in the contest for the dominion of Greece which took place after the death of Alexander, and it was frequently taken in these wars. In later times it fell successively into the hands of Antiochus and Mithridates and was finally taken and destroyed by the Romans, by whom Euboea was included in the province of Achaea. From this time the island continued undisturbed under the power of Rome, and under that of Constantinople after the division of the empire, until the Latin conquest of the East in 1204 A.D. At that time the island came into the hands of the Venetians, who retained possession of it till 1470, when the city of Negropont, as it was then called, was taken by the Turks, and the inhabitants cruelly massacred. In 1688 the Venetians made an unsuccessful attempt to retake the town. Since the Greek revolution, Euboea has formed part of the modern kingdom of Greece; and as the ancient appellations have been by law restored, Euboea and Chalcis are now the names by which the island and town are most commonly known. Pop. of the island (1852), 65,299; of the town, 5000.

Rio, a river of South America, forming the boundary between Patagonia and the Argentine Confederation, takes its rise in the Andes by two head-streams, one from the north and the other from the south, and flows eastward to the Atlantic, into which it falls in S. Lat. 41°, W. Long. 62°, after a course of from 500 to 600 miles. It is full of small islands and sand-banks; and about the middle of its course it separates into two branches, which inclose an island of considerable size. The river is subject to two annual floods, one of which, in December and January, is occasioned by the melting of the snows of the Andes; and the other, in June and July, by the heavy rains in the interior. The river is about 2 miles broad at its mouth; but at the town of Carmen, about 16 miles from the sea, it does not exceed 300 yards in width. Near its entrance is a bar, over which there are several channels, some of which may be crossed by vessels drawing 11 feet of water. The climate of the district is healthy; and though very warm in summer, ice is frequently formed between the months of April and July. The winds are very variable, and frequently violent. The principal town on the river is Carmen, on the north bank, with a population exceeding 1000.

Rio, one of the principal tributaries of the Amazon, rises in New Granada by two head streams, and flows eastwards till it falls into the Amazon, in S. Lat. 3° 16', W. Long. 59'. Its principal affluents are the Cassiquare, by which it is connected with the Orinoco, the Cababure, Padaviri, and Branco, from the N.; and the Tomo, Zie, Haupes, &c., from the S. and W. In the lower part of its course the river flows through a succession of lakes from 15 to 20 miles broad; at its junction with the Amazon it is about 1½ mile in breadth. The river is flooded during the months of August and September, when it rises about 30 feet above its lowest level. The length is about 1000 miles.