terms of energy. In the introductory paragraphs, it is pointed out that the necessary forms of thought, space, time, and mode, lead to the conception of a unitary external world. Science is unable to give any adequate expression of the reality of the external world, but is forced to assume it to be unitary, continuous, and dynamic. All attempts at scientific construction of the world fall under three heads: atomism, plenism, and energism. The author next gives an historical account of the development of atomism from Democritus to the present day. In general, atomism means the conception of substance as composed of discrete and indivisible units, and is the result of an arithmetic type of thought. The modern hypothesis of atomism may be traced to the speculations of Newton and Dalton, which were largely inspired by the discovery of the relations between volume and mass in gases. The further development of the theory was carried out to meet the demands of chemical problems. To chemistry is due the conception of the molecule as a complex of atoms in definite relation, of atomic weights, and of an ultimate unit underlying both the molecule and atom. This last assumption is further borne out by certain phenomena of electricity and radio-activity. Of much speculative significance also are the laws of periodicity and of valence, and the correspondences between valences, atomic weights, and properties of elements. These correspondences, which are all in terms of force and energy, would seem to indicate that the concept of the atom as a bit of matter is inadequate. The insufficiency of the atomic hypothesis becomes more apparent when applied to the problems of physics. The phenomena of the transmission of radiant energy have necessitated the assumption of an imponderable and continuous ether, which is fundamentally incompatible with the conception of the atomic constitution of the universe. Attempts at reconciliation are resulting in a reconstruction of the concept of the atom as a vortex ring or a center of etheric strain. Finally, the recent discovery of such elements as radium, and of new forms of radio-activity, have made the old concept of the atom no longer tenable, and are rapidly forcing scientific speculation in the direction of energism. The author next takes up the hypothesis of plenism. The difficulties inherent in atomism, notably the problem of action at a distance, have led a certain type of mind to take refuge in plenism. The conception of a space-filling substance dynamic in character is found as far back as Anaximander. In modern times plenism has been adopted in the form of a continuous elastic ether to explain radiant transmission. Even if the difficulty of reconciling continuity and homogeneity with elasticity and the heterogeneity demanded by vortex motion were overcome, the author holds that this theory would have no advantage over energism. The final division of the article is concerned with energism itself. Traces of this theory are to be found throughout the history of thought. The habit of thought which makes it difficult to conceive the universe in non-material terms may be overcome, if it is remembered that the concept of matter is itself an abstraction. The great advantage of energism in the eyes of its advocates is pre-
