The ceaseless change and decay in everything around them was doubtless one of the causes which led men to this conception of atoms as indivisible, indestructible substances which could never wear out and could never be changed. But even here rest could not be found; the mind was obliged to regard these atoms as always in motion. The dance of the dust-motes in the sunbeam was to Lucretius the result of the more complex motion whereby the atoms which compose that dust are agitated. In his dream as told by Tennyson-- "A void was made in Nature: all her bonds Cracked: and I saw the flaring atom-streams And torrents of her myriad universe, Ruining along the illimitable inane, Fly on to clash together again, and make Another and another frame of things For ever." The central quest of the physicist, from the days of Democritus to the present time, has been to explain the conception of "atom"--to develop more clearly the observed properties of the things which are seen and which may be handled as dependent on the properties of those things which cannot be seen, but which yet exist. For two thousand years he has been trying to penetrate beneath the ever-changing appearances of Nature, and to find some surer resting-place whence he may survey these shifting pictures as they pass before his mental vision. The older atomists thought to find this resting-place, not in the atoms themselves, but in the wide spaces which they supposed to exist between the worlds:-- "The lucid interspace of world and world Where never creeps a cloud, or moves a wind, Nor ever falls the least white star of snow, Nor ever lowest roll of thunder moans, Nor sound of human sorrow mounts to mar Their sacred everlasting calm." To the modern student of science the idea of absolute rest appears unthinkable; but in the most recent outcome of the atomic theory--in the vortex atoms of Helmholtz and Thomson--he thinks he perceives the very "foundation stones of the material universe." Newton conceived the atom as a "solid, massy, hard, impenetrable, movable particle." To the mind of D. Bernoulli the pressure exerted by a gas on the walls of a vessel enclosing it was due to the constant bombardment of the walls by the atoms of which the gas consisted. Atomic motion was the leading idea in the explanation of heat given by Rumford and Davy, and now universally accepted; and, as we have seen, Dalton was himself accustomed to regard all "elastic fluids" (_i.e._ gases) as consisting of vast numbers of atoms.