Atom (Gr. atomos, 'that which cannot be cut') is a term properly belonging to the science of that class of physicists who deny the infinite divisibility of matter. It has a secondary, but unjustifiable, use among chemists.
Much of the very old speculation as to the ultimate constitution of matter has been preserved to us in the remarkable poem of Lucretius (q.v.). He follows the teaching of Democritus and Leucippus, basing his argument for the existence of atoms mainly upon the (supposed) result of observation that 'reproduction is slower than decay,' and therefore that there would now, after infinite ages of decay, be no aggregate left unless there were a limit to breakage. He ridicules the opposite theory, propounded by Anaxagoras (that of Homoæmeria), which asserts that, however far a body may be divided, the parts are still similar to the whole, and therefore capable of yet further division. Epicurus (q.v.), and in modern times Gassendi (q.v.), are also notable representatives of the 'Atomic School.'
Modern science has made no advance towards the decision of this question. Even the Vortex-atom (q.v.) requires to be made up of a species of matter, and the postulates of that theory do not as yet require any speculation as to whether it is infinitely divisible or not. But, if the question of the existence of atoms (in the true sense of the word) has made no progress, that of the finite heterogeneity of all kinds of matter has recently advanced very fast.
Chemical and physical facts, of the most varied and independent character, unite in giving us the information that, even in such apparently texture- less bodies as glass and water, after a certain finite amount of division, a stage is reached at which any further division breaks the body into fragments which are not necessarily similar to one another, nor to the original substance. For the chemical arguments on this subject, see ATOMIC THEORY, CHEMISTRY, and MOLECULE; for the physical ones, see GASES (Kinetic Theory of), and MATTER. In these articles the reader will find the latest data as to the probable size of the 'grain' in different kinds of matter. Further than this, no speculation of any value has yet been ventured upon.