A-tōm'ic or A-tōm'ic-al, a.

a. [Fr. atomique.] Pertaining to, or consisting of, atoms; extremely minute.

Atomical philosophy, or doctrine of atoms, a system which, assuming that atoms are endowed with gravity and motion, accounted for the origin and formation of all things. This philosophy was first broached by Leucippus, was developed by Democritus, and afterward improved by Epicurus, and hence is sometimes denominated the Epicurean philosophy.—Atomic theory, or the doctrine of definite proportions (Chem.), teaches that chemical combinations take place between the supposed ultimate particles or atoms of bodies, in some simple ratio, as of one to one, two to three, or some other, always expressible in whole numbers.—Atomic weight, the weight of the atom of an element as compounded with that of the atom of another element, ascertained from the proportions by weight, in which they combine; or, leaving out of view the hypothetical idea of an atom, it is the number expressing the proportions by weight in which the elements combine, one of the elements, either hydrogen or oxygen, being assumed as the unit for comparison with the others. Oxygen and hydrogen combine to form water in the ratio of 1 of hydrogen to 8 of oxygen; and 1 and 8 are therefore the combining proportions of hydrogen and oxygen; also called, to avoid hypothesis, their combining equivalents. Dana.