Atomic Theory, a theory as to the existence and properties of 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 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 proportion of the constituent elements are definite and invariable; (2) The Law of Combination in Multiple Proportions, according to which the several proportions in which one element unites with another, invariably bear towards each other a simple relation; (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. Without expressly adopting the atomic theory, chemists have followed Dalton in the use of the terms atom and atomic weight, yet in using the word atom it should be held in mind that it merely denotes the proportions in which elements unite.

Entry from Everybody's Cyclopedia, 1912.