With local change in material things, as also with quantitative change, growth and diminution of quantity (mass and volume), everyone is perfectly familiar. From the earliest times, moreover, we find both in science and philosophy the conception of matter as composed of, and divisible into, ultimate particles, themselves supposed to admit of no further real division, and hence called _atoms_ (ἄ-τομος, τέμνω). From the days of Grecian atomism men have attempted to show that all change in the Universe is ultimately reducible to changes of place, order, spatial arrangement and collocation, of those hypothetical atomic factors. It has likewise been commonly assumed that change in mass is solely due to change in the number of those atoms, and change in volume (of the same mass) to the relative density or closeness with which the atoms aggregate together; though some have held—and it is certainly not inconceivable—that exactly the same material entity, an atom let us say, may be capable of _real_ contraction and expansion, and so of _real_ change of volume: as distinct from the _apparent_ contraction and expansion of bodies, a change which is supposed to be due to change of density, _i.e._ to decrease or increase in the dimensions of the pores or interstices between the smaller constituent parts or molecules. However this may be, the attempts to reduce all change in physical nature to mere _mechanical_ change _i.e._ to spatial motions of the masses (_molar_ motions), the molecules (_molecular_ motions), and the atoms or other ultimate components of matter (whether vibratory, undulatory, rotatory or translational motions), have never been satisfactory.