to us to be at rest, there is notwithstanding, a continual action, and counter-action, constant efforts, uninterrupted or communicated force, and continued opposition? In short, a _nisus_, by which the constituting portions of these bodies press one upon another, mutually resisting each other, acting and re-acting incessantly? that this reciprocity of action, this simultaneous re-action, keeps them united, causes their particles to form a mass, a body, and a combination, which, viewed in its whole, has the appearance of complete rest, notwithstanding no one of its particles really ceases to be in motion for a single instant? These collective masses appear to be at rest, simply by the equality of the motion--by the responsory impulse of the powers acting in them. Thus it appears that bodies enjoying perfect repose, really receive, whether upon their surface, or in their interior, a continual communicated force, from those bodies by which they are either surrounded or penetrated, dilated or contracted, rarified or condensed: in fact, from those which compose them; whereby their particles are incessantly acting and re-acting, or in continual motion, the effects of which are displayed by extraordinary changes. Thus heat rarifies and dilates metals, which is evidence deducible that a bar of iron, from the change of the atmosphere alone, must be in continual motion; that there is not a single particle in it that can be said to enjoy rest even for a single moment. In those hard bodies, indeed, the particles of which are in actual contact, and which are closely united, how is it possible to conceive, that air, cold, or heat, can act upon one of these particles, even exteriorly, without the motion being communicated to those which are most intimate and minute in their union? Without motion, how should we be able to comprehend the manner in which our sense of smelling is affected, by emanations escaping from the most solid bodies, of which all the particles appear to be at perfect rest? How could we, even by the assistance of a telescope, see the most distant stars, if there was not a progressive motion of light from these stars to the retina of our eye?