(a priv., and ἀτομον), a particle of matter, so minute as to admit of no division. Atoms are the minima. nature, and are conceived to be the first principles or component parts of all physical magnitude.

**ATOMIC Theory.** See Chemistry.

**ATOMICAL Philosophy,** or the doctrine of atoms, a system which, from the hypothesis that atoms are endowed with gravity and motion, accounted for the origin and formation of all things. This philosophy, first broached by Leucippus, was developed by Democritus, and afterwards cultivated and improved by Epicurus, whence it is denominated the Epicurean Philosophy. See Epicurean Philosophy.

**ATONEMENT.** See Theology.

**ATOOI,** one of the larger Sandwich Islands, in the South Pacific Ocean. Towards the north-east and north-west the country is rugged and broken, but to the southward it is more level. The hills rise from the sea with a gentle acclivity, and at a little distance back are covered with wood. The produce of this island is the same with that of the rest of the cluster; but its inhabitants greatly excel their neighbours in the management of their plantations. It is nearly 30 miles in length, and contains about 10,000 inhabitants. Long. 159° 40' W. Lat. 21° 57' N.

**ATRATO,** a river of New Granada, in South America, which, after a course of 250 miles, falls into the Gulf of Darien. The gold and platinum mines of Choco were on some of its affluents, and its sands are still auriferous.

**ATREBATII,** or Atrebates, a people of Britain, who inhabited part of Berkshire and Wiltshire, next to the Dobuni. This was one of those Belgic colonies which came out of Gaul into Britain. Their capital was Nemetocenna, or Nemetacum, now Aras. They are mentioned by Cæsar among the nations which composed the Belgic confederacy against him; and the quota of troops which they engaged to furnish on the occasion to which he refers was 15,000. Commius of Aras was a king or chieftain among the Atrebati of Gaul in Cæsar's time; and he seems to have possessed some influence over the Atrebati of Britain, for he was sent by Cæsar to persuade them to submission. This circumstance renders it probable that this colony of the Atrebati had not been settled in Britain very long before that time. The Atrebati were amongst the British tribes which submitted to Cæsar; nor do we hear of any remarkable resistance they made against the Romans at their next invasion under Claudius. It is indeed probable that, before the time of this second invasion, they had been subdued by some of the neighbouring states; perhaps by the powerful nation of the Cattivellani; which may also be the reason why they are so little mentioned in history. Calleva Atrebatum, mentioned in the seventh, twelfth, thirteenth, and fourteenth Itineraries of Antoninus, and called by Ptolemy Calcula, seems to have been the British capital of the Atrebati. But our antiquaries differ respecting the situation of this ancient city; some of them placing it at Wallingford, and others at Silchester.

**ATREUS,** in Ancient History, the son of Pelops and Hippodamia, grandson to Tantalus, and the father of Agamemnon and Menelaus, is supposed to have been king of Mycenæ and Argos about 1228 years before the Christian era. He drove his brother Thyestes from court for having criminal intercourse with Erope his wife; but, understanding that two children were the fruit of this connection, he recalled his brother, and served up their flesh to him at a banquet; at which enormity the sun, it is said, withdrew his light.—Hygin. Fab.; Æschyl. Agamemnon; Soph. Ajax.

**ATRI,** or **ATRA,** the ancient Hadria, a town of Naples, in the province of Abruzzo Ulteriore I., situated on a steep mountain 5 miles from the Adriatic, and 18 miles southeast of Teramo. It is the see of a bishop, and has a cathedral, a parish church, several convents and hospitals, and contains about 6000 inhabitants. In its vicinity are some remarkable ancient excavations.

**ATRIENSES,** in Antiquity, a kind of servants or officers in the great families of Rome, who had the care and inspection of the atria and the things lodged therein.

**ATRIUM,** in Roman Architecture. See Architecture.

**ATRUM,** in Ecclesiastical Antiquity, denotes an open place or court before a church, making part of what was called the narthex or ante-temple. The atrium in the ancient churches was a large area or square plat of ground, surrounded with a portico or cloister, situated between the porch or vestibule of the church and the body of the church. Some have erroneously confounded the atrium with the porch or vestibule, from which it was distinct; others with the narthex, of which it was only a part. The atrium was the mansion of those who were not suffered to enter farther into the church; and more particularly, it was the place where the first class of penitents stood to solicit the prayers of the faithful as they went into the church.

**ATROPA,** a genus of plants belonging to the natural order of Solanaceæ, most of which are active poisons. *A. Belladonna* (deadly nightshade), and *A. Mandragora,* are well-known examples.