Thus the “atom” of the chemist is a highly elastic body--highly resilient. Moreover, thanks to the genius of Professor Frederick Soddy, of Oxford, and of Sir F. W. Aston, we now know that what we considered but three years ago to be a homogeneous element--e.g., lead, chlorine, carbon, iron, etc.--is in many cases--if not in all cases excepting hydrogen and helium--a compound body containing particles of different weights (atomic mass). On the other hand, as previously stated, we are reverting to the Democritean theory that all atoms are composed of but one primitive element, the hydrogen atom. And the hydrogen atom in turn is composed of--_energy_; and energy itself is apparently of atomic structure. All energy is, in the last analysis, electrical in nature, the stuff that lightning, the stars, fire, rocks and metals, the bodies of plants and animals--perhaps the mind of man--are all made of. With these facts in mind, let us return to Democritus, whose errors--the errors of his age--we may now regard with due allowances. “SOUL ATOMS” With Democritus, as with Pythagoras and Heraclitus, fire had a special importance, though it was not, as asserted by some of the ancient writers, regarded by him as an element. The primitive substance, the source of all things, whether fire, earth, air or water, was the world of homogeneous atoms, eternal, unchangeable. Fire, composed of minute, round, smooth atoms, was, on account of its mobility, the living and moving principle of nature, though not the direct _cause_ of motion. But because of the fiery, mobile nature of its atoms, they become a secondary cause of additional motion, from their impact with and recoil from less mobile atoms. Sensation and consciousness are possible because of the extreme mobility of the fiery atoms. They are the “soul atoms,” but sensibility and consciousness arise only as the result of relatively large aggregations of these excessively minute bodies. “EFFLUENCES” In order to account for the mutual action or influence of bodies upon one another, when separated by space,--the void,--the Atomists had recourse to the Empedoclean theory of “effluences,” or emanations, which were supposed to be emitted by all bodies (Aristotle, _Generatio et Corruptio_, i, 8). Penetrating the organs of sense, they excite the appropriate sensation, and, in the case of the brain, they produce images or ideas of things. Hence sensation is the only possible source of knowledge, the origin of all thought being the passage through the sense organs of “effluences” from external objects or organisms, acting mechanically, by contact.