Impenetrability is commonly understood to imply that quality of a body by which it cannot be pierced; but in Physics the term has a different, or more refined signification. It is therein defined to be that property of a body by which it prevents any other body from occupying the space in which the former is. Two particles of matter cannot exist at the same time in the same place, for as long as one retains its place, it must necessarily exclude the other. Indeed, were the case otherwise, each body might be successively absorbed into the substance of another, till the whole frame of the universe, collapsing to a point, were lost in the vortex of annihilation. The fact of the impenetrability of even water is easily demonstrated. If a solid body be plunged into a vessel filled with water, a portion of the water will overflow exactly equal to the bulk of the solid body immersed, which shows that it is only a change of places of the substances.