REPULSION is that power by which bodies or the particles of bodies are made to recede from one another. Both attraction and repulsion exist in all the particles of material substances, and seem to be properties by which those particles act upon one another when not in contact. The cause of these actions will probably be for ever unknown to ua ; and the terms are only applied in conformity to the phenomena exhibited. At all sensible distances, bodies, small and great, except in certain states with respect to electricity or magnetism, attract one another ; and the intensity of the attraction varies inversely as the square of the distance between the bodies. But the phenomena of light, and of elasticity in general, show that at distances which are not appreciable by the eye (perhaps such as are less than ',inch) both attractions and repulsions take place. [Arruacriox.] In his researches concerning the phenomena of light, Newton, having brought at one time a hair, and at another the edge of a knife, near a small beam of light in a darkened room, found that the particles of light were made to deviate from the rectilinear direction, as if attracted by a force which diminished with the distance of the ray from the hair or knife. The shadow of the latter was bordered with three coloured fringes, of which the nearest to it was formed by inflected rays passing at a distance rather greater than inch from the knife-edge ; and the second and third fringes by rays inflected respectively at greater distances. (` Optices,' lib. iii) (DIFFRACTION OF LIMIT.] From these phenomena Newton was led to the opinion (which he proposes as a query) that all material bodies might be assemblages of particles in equilibrio between their mutual attractions and repulsions. lie imagined also that a subtle ether, pervading material bodies, was the immediate agent in producing such attractions or repulsions, together with all the circumstances of cohesion, and also those of chemical, magnetical, and electrical actions. The phenomena of nature seem to
justify the supposition than an ether pervades all bodies; but it must be admitted that the hypothesis of Newton only removes the difficulty concerning the actions of the particles of bodies a step further, since we are equally at a loss to account for the existence of the powers in those particles, and in the ether itself.
The reality of a distance between the particles of bodies, whether solid, fluid, or admits of no question; • for the differences in the densities of them classes of bodies can only be conceived to arise from the different extent of the intervals between the particles. [HEAT.] By the process of cooling, all bodies, with certain exceptions in particular easel, become contracted in volume ; and the mixing of two given volumes of different fluids (as water and sulphuric acid) pro duces a volume leas than the sum of the two separate volumes. These manifestly depend upon the approach of the particles to one another, and are therefore inconsistent with the supposition that they were originally in contact.
It is natural to ask if there be such a thing as mathematical contact in nature, and it may be answered that we have no evidence of such a condition. [[ionssioN.] Besides the continual diminution of volume produced in the cooling of bodies, the Newtonian experiment of pressing a convex lens of glass upon the surface of a glass mirror affords evidence that the leas, at the point of nearest approach, and under a very great pressure, is not in contact with the mirror ; and it LAS been supposed that the distance between them, at that place, is then not less than , inch. (Robison, ' 3lechan. Phil.') It seems to follow that a vast force of repulsion must be in action between the particles of bodies when they are as near together as mechanical power can bring them ; and it can be easily conceived that such repulsive force may be the immediate cause of the sensation of touch.