other which seem to have no means of connexion between them. Can a body act where it is not? If we answer directly in the negative, the spacial limitations of substance are to a large extent removed, and the complication is increased. The simplest solution is involved in a view that has come down from the early period of Greek physical speculation, and forms one of the most striking items in the stock of first principles of knowledge which had been struck out by the genius of that age. In that mode of thought the ultimate reality is transferred from sensible matter to a uniform medium which is a plenum filling all space: all events occur and are propagated in this plenum, the ultimate elements of matter consisting of permanently existing vortices or other singularities of motion and strain located in the primordial medium, which are capable of motion through it with continuity of existence so that they can never arise or disappear. This view of physical phenomena, which was no doubt suggested by rough observation of the comparative permanence and the mutual actions of actual whirls in water and air, was quite probably, even at that time, not the mere idle philosophizing which has sometimes been supposed. It at any rate involves the fundamental consequence that the structure of matter is discrete or atomic — that contrary to à priori impression matter is not divisible without limit: and it perhaps enables us to form some idea of the line of development of those views on the constitution of matter which, as Democritus and Lucretius described them, were considerably ahead of anything advanced in modern times until the age of Descartes and Newton. The same doctrine was probably the ideal towards which Descartes was striving when he identified space and matter, and elaborated his picture of the Solar System as a compound vortex. In Newton's cautious hands, the relation of material atoms to aether is not dealt with: his establishment of an exact law of gravitation indeed originated the school of action at a distance, which held bluntly that matter can be considered as acting where it is not, and whose influence lasted throughout the sevententh century through Boscovich and the French astronomers and mathematicians, until the time of Faraday. This doctrine of the finality
