ATOLL', the name given by the Malays to a coral reef which forms an annular island, inclosing a lake of water which is connected with the sea by an open strait. Some A. are nearly 100 m. in circumference, and have from 15 to 60 fathoms of water. They make excellent harbors, with safe entrances, always on the windward side. Some of the reefs sustain considerable vegetation, and are inhabited.
ATOM (Gr. atomos, an indivisible particle; from a, not, and temno, I cut). In ancient philosophy, two theories of the nature of matter were recognized, and these have continued to form subjects of argument among speculative men since the year 510 B.C. to the present time. The one theory is that matter is infinitely divisible. Thus, a needle may be divided into two, and each of the parts may in its turn be broken or cut into two, and each of the latter again and again be subdivided, till the parts become so small that it may be impossible to see them by the naked eye; but these parts are regarded as capable of still further division, without limit or stoppage, provided more or delicate means could be ,employed to act upon them. The second theory regarding the constitution of the matter is that in the repeated division and subdivision of a solid, liquid, or gas, a point will be at length reached when it will no longer be possible, by any conceivable means, to break a molecule in two, the molecule being a real unity, not composed of separate parts—in other words, an atom. The latter theory recognizes the finite divisibility of matter, and considers that all matter is more or less compactly built up of myriads of atoms aggregated together, and having spaces or pores between the several atoms or particles. If it were possible to subject such matter to the scrutiny of a sufficiently powerfulanagnifying-glass, or microscope, and thus exhibit. or behold the atoms so separated by spaces, then an appearance would be presented similar to that which the painter chooses to depict on the canvas when he is representing a snow-storm, and where every little flake of snow is separated from its neighbor one by a space in which there are none; or that which would be observed if, during a hail-storm, some great power were to cry, "Halt!" and that instant every minute hail-stone was arrested in the spot it had reached.
This view of the physical nature of matter is that which is known as the atomic or corpuscular theory, and has in modern times received .some support from the facts embodied in the chemical atomic theory originated by Dalton. Granting, however, that the chemist can prove that his simple and compound forms of matter are built up of chemical atoms, the problem still remains to be solved as to the possible iden tity of physical and chemical atoms. What the chemist regards as an A. in his sci ence, may not be an ultimate and indivisible A. in a physical point of view; the chemical A., though incapable of division as a chemical A., may still be composed or built up of many physical atoms, and may be capable of being subdivided into such. Indeed, whilst the atomic theory of Dalton, when first announced, was eagerly seized upon as the best possible evidence for the existence of both chemical and physical atoms, the tendency of recent researches and discussions in chemistry has been to show that the chemical A. is different from the physical, and does not necessitate the existence of the latter. See ATOMIC THEORY. According to the ordinary aceep• tation of the term, it is a molecule of matter having a definite weight, magnitude, and form, possibly alike for the atoms of the same material, but differing in those of different substances. The form of an A. is supposed by some men of science to be the same as that which the fragments of a substance assume when it is split in the direction of the planes of the cleavage of its crystals (see CRYSTALLOGRAPHY), but a more general belief has been that all atoms are spherical, and that the various crys talline forms are produced by the manner in which the atoms are grouped together In regard to the size of atoms, Sir William Thomson has shown, by three entirely different trains of argument from observed facts; that the diameter of an A. cannot be greater than nor less than Tim , 0 0 0 of an inch. Further consider ations regarding atoms will be found under the head Matter (q.v.); also in the article Vortex (q.v.).