Atoll´, the Polynesian name for coral islands of the ringed type enclosing a lagoon in the centre. They are found chiefly in the Pacific in archipelagos, and occasionally are of large size. Suadiva Atoll is 44 miles by 34; Rimsky 54 by 20. See Coral.
Atomic Theory, a theory as to the existence and properties of atoms (see Atoms); especially, in chemistry, the theory accounting for the fact that in compound bodies the elements combine in certain constant proportions, by assuming that all bodies are composed of ultimate atoms, the weight of which is different in different kinds of matter. It is associated with the name of Dalton, who systematized and extended the imperfect results of his predecessors. On its practical side the atomic theory asserts three Laws of Combining Proportions: (1) The Law of Constant or Definite Proportions, teaching that in every chemical compound the nature and relative weights of the constituent elements are definite and invariable; thus water invariably consists of 8 parts by weight of oxygen to 1 part by weight of hydrogen; (2) The Law of Multiple Proportions, according to which the several proportions in which one element unites with a given weight of another invariably bear towards each other a simple relation; thus 1 part by weight of hydrogen unites with 8 parts by weight of oxygen to form water, and with 16 (i.e. 8×2) parts of oxygen to form peroxide of hydrogen; (3) The Law of Combination
in Reciprocal Proportions, that the proportions in which two elements combine with a third also represent the proportions in which, or in some simple multiple of which, they will themselves combine; thus in olefiant gas hydrogen is present with carbon in the proportion of 1 to 6, and in carbonic oxide, oxygen is present with carbon in the proportion of 8 to 6, 1 to 8 being also the proportions in which hydrogen and oxygen combine with each other. The theory that these proportional numbers are, in fact, nothing else but the relative weights of atoms so far accounts for the phenomena that the existence of these laws might have been predicted by the aid of the atomic hypothesis long before they were actually discovered by analysis. In themselves, however, the laws do not prove the theory of the existence of ultimate particles of matter of a certain relative weight; and although many chemists, even without expressly adopting the atomic theory itself, have followed Dalton in the use of the terms atom and atomic weight, in preference to proportion, combining weight, equivalent, and the like, yet in using the word atom it should be held in mind that it merely denotes the combining weights of the elements. These will remain the same whether the atomic hypothesis which suggested the employment of the term be true or false. Dalton supposed that the atoms are spherical, and invented certain symbols to represent the mode in which he conceived they might combine. The latest atomic hypothesis is one which assigns an electrical structure to the atom. See Chemistry; Electricity; Matter. Cf. H. E. Roscoe and A. Harden, New View of Dalton's Atomic Theory; Sir J. J. Thomson, Atomic Theory.
Atomic Weights. See Chemistry; Molecular Weights.
Atomists. See Atoms.
Atoms, for many years regarded as the ultimate indivisible particles of the chemical elements. The idea originated with some of the ancient philosophers (the atomists), more especially Democritus (450 B.C.), Epicurus, and Lucretius (99-55 B.C.), and was developed into a definite theory by Dalton (1804). According to Dalton the atoms of any one element are alike in all their properties, but differ from the atoms of other elements, and when chemical combination occurs it takes place between the atoms of the combining elements (see Chemistry). Various views have been held with regard to the nature of atoms. Newton regarded them as hard, ponderable particles, perfectly unalterable, and concluded that the difference between substances was due to different kinds of atoms. Lord Kelvin propounded the view that the properties of atoms might be explained by those of vortices or vortex rings in a homogeneous frictionless fluid. As a result of the researches of British and French physicists on radium, the latest view is that matter and electricity are closely connected, that atoms are not indivisible, but complex aggregates containing positive and negative electrons, the differences between the atoms depending mainly on the numbers of these electrons and their velocity. See Radium.
Atonement, in Christian theology, the special work of Christ effected by His life, sufferings, and death. The first explicit exposition of the evangelical doctrine of the atonement is ascribed to Anselm, Archbishop of Canterbury, in 1093.
Atrato (a˙-trä´tō), a river of S. America, in the north-west of Colombia, emptying itself by nine mouths into the Gulf of Darien; it is navigable by steamers of some size for 250 miles, and has long been the subject of schemes for establishing water-communication between the Atlantic and Pacific.
Atrauli, a town of India, United Provinces, Aligarh district, clean, well built, and with a good trade. Pop. 16,560.