Atoll
the name applied by geologists and others to any one of the lagoon islands, or annular coral reefs found in the Pacific and the Indian Oceans, the Red Sea, and some other parts of the tropics. An atoll is a ring of coral rock, oval rather than circular in form. On the top of the coral rock, which rises but slightly above the sea-level, is vegetation of some luxuriance. On the convex circumference of the ring is a beach of white sand, exterior to which is a line of breakers. The ring of land, which is less than half a mile across, encircles a lagoon. In the larger atolls there are generally two or three breaks in the ring, affording ship channels into the lagoon; these mark the spots where fresh water, discharged from the old subsiding land into the sea, prevented the coral animals, which are marine, from locating themselves or building.
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Atom
in mental philosophy, a particle of matter so infinitely small that it cannot again be subdivided. In natural philosophy, one of the exceedingly minute ultimate particles of matter, aggregates of an immense number of which, held in their place by molecular forces, constitute all material bodies. In chemistry, the smallest particle into which an element can be divided. An atom cannot exist in a separate state, but unites with one or more atoms to form a molecule. The atoms of different elements have definite relative weights fixed and invariable for each, the weight of an atom of hydrogen being regarded as unity.
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Atomic Philosophy
in mental and natural philosophy, the doctrine of atoms, broached by Leucippus, developed by Democritus and modified by Epicurus. It represented atoms as possessed of gravity and motion, and attributed to their union the formation of all things.
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Atomic Weights
the proportions by weight in which the various elementary substances unite together. It is necessary that one element be selected as the starting-point of the series and an arbitrary sum affixed to it, so that thereby all the other elements can have their sums awarded to them, according to the proportional amounts in which they combine with each other. The second law mentioned under the atomic theory explains the manner in which this can be done, and how far the numbers are arbitrary. In all systems of atomic weights in modern use, the atomic weight of hydrogen is taken as unity, and the atomic weight of the other elements are then fixed, so as to give on the whole the simplest and most consistent formula for their compounds. There are two systems of atomic weights at present in use: (1) The old system, which, after much discussion, was adopted about 1845; and (2) the new system, which is, in many respects, a revival of the system of Berzelius, and which may be said to have come into general use by scientific chemists about 1860. ficial offering made by Christ in expiation of the sins, according to the Calvinists, of the elect only; according to the Arminians, of the whole human race. The Day of Atonement, or the Great Day of Atonement, was on the of the seventh month. It is still observed as a day of humiliation and supplication by Hebrews all over the world.
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Atonement
in theology, the sacri- the bones of his sons, and described the dreadful revenge which he had taken.
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Atossa
Daughter of Cyrus, and wife successively of her brother Cambyses , of Smerdis the Magian, and of Darius Hystaspis, by whom she became the mother of Xerxes .
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Atrato
a river of Colombia, interesting because it has repeatedly been made to bear a part in schemes for a ship canal across the Isthmus of Panama. Rising on the Western Cordillera at an altitude of 10,560 feet above sealevel, it runs for about 400 miles northward through low, swampy country, and falls by several mouths, interrupted by bars, into the Gulf of Darien. It is navigable by steamers for fully 250 miles, being 750 to 1,000 feet wide, and 8 to 70 feet deep. A route, surveyed by the United States Government in 1871, proposed to connect the Atrato and the Jurador, flowing into the Pacific, by a canal 48 miles long. At the Paris International Congress (1879) for deciding the best route for the interoceanic canal, that route was with various others reiected in favor of Panama. Gold-dust is found in and about the Atrato.
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Atrek
a river of Persia, rising in Khorassan, among the Hazár Masjid Mountains, and thence flowing nearly 350 miles westward to the Caspian Sea, from Shatt downward along the boundary with the Russian Empire.
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Atriplex
a genus of plants belonging to the order chenopodiaceæ (chenopods). Eight species are indigenous, and one or two more partially naturalized, in Great Britain. Of the former may be mentioned the A. laciniata, or frosted sea-orache; the A. babingtoni, or spreading fruited; the A. patulla, or spreading halberd-leaved; the A. angustifolia, or narrow-leaved ora orache; and the A. littoralis, or grass-leaved sea-orache. The leaves may be used as pot herbs.
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Atrium
in ancient times, the hall or principal room in an ancient Roman house. It communicated with the street by the vestibule and the front door. There was in the center of its ceiling a large aperture, called compluvium, im, designed to admit light. Beneath it there was scooped out in the pavement a cistern called impluvium. In a large house, rooms opened into the atrium from all sides, and were lighted from it. In medieval times, till the 12th century, a covered court. After the 12th century, the churchyard.
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Atropa
a genus of plants belonging to the order solanaceæ, or nightshades. It contains the well-known species, A. belladonna, or deadly nightshade. It is three or more feet high, has its ovate leaves paired, large and small together, drooping lurid purple flowers, and blackberries of the size of a small cherry, which if eaten produce delirium, dilation of the pupils of the eyes, and death. The flowers and fruit largely used by the homœopathic school. are both powerful medicinal agents. It is
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