Atom

(Gr. a privative, and temno, cut; indivisible). Primarily, the smallest particle of matter which can exist; the ultimate and smallest division of matter, in physics, sometimes the smallest particle to which a substance can theoretically be reduced; in chemistry, the smallest particle of matter that can exist in combination with other atoms building up or constituting molecules. Two opposite doctrines of the constitution of matter were held by the ancient philosophers. One was that matter was infinitely divisible without losing its distinctive and individual properties. This is the doctrine of continuity or homoeomery. Anaxagoras is given as the founder of this view of the constitution of things. According to it any substance, such as wood or water, can by no process of subdivision, however far it might be carried, be made to be anything but a mass of wood or water. Infinite subdivision would not reach its limit of divisibility. Democritus and others held that there were ultimate particles of matter which were indivisible, and these were called atoms. This is the doctrine of atomacity, upheld by Epicurus, and enlarged on by Lucretius in his "De Rerum Natura". The early atomists held that the atoms were not in contact, but that voids existed between them, claiming that otherwise motion would be impossible. Among the moderns, Descartes and Spinoza adhered to continuity. Leibniz upheld atomicity, and Boscovich went to the last extreme of the theory, and defined atoms as centres of force denying them the attribute of impenetrability. +