Nor need we be surprised at the doctrine that the atoms, so all-powerful in the formation of things, are themselves invisible. The same is true of the forest-rending blasts, the 'viewless winds' which lash the waves and overwhelm great fleets. There are odours also that float unseen upon the air; there are heat, and cold, and voices. There is the process of evaporation, whereby we know that the water has gone, {219} yet cannot see its vapour departing. There is the gradual invisible detrition of rings upon the finger, of stones hollowed out by dripping water, of the ploughshare in the field, and the flags upon the streets, and the brazen statues of the gods whose fingers men kiss as they pass the gates, and the rocks that the salt sea-brine eats into along the shore. That there is Empty Space or Void he proves by all the varied motions on land and sea which we behold; by the porosity even of hardest things, as we see in dripping caves. There is the food also which disperses itself throughout the body, in trees and cattle. Voices pass through closed doors, frost can pierce even to the bones. Things equal in size vary in weight; a lump of wool has more of void in it than a lump of lead. So much for Lucretius. For abstract theories on physics, except as an adjunct and support to his moral conceptions, Epicurus seems to have had very little inclination. He thus speaks of the visible universe or Cosmos. [373] The Cosmos is a sort of skyey enclosure, which holds within it the stars, the earth, and all visible things. It is cut off from the infinite by a wall of division which may be either rare or dense, in motion or at rest, round or three-cornered or any other form. That there is such a wall of division is quite admissible, for no object of which we have observation is without its limit. Were this wall of division to {220} break, everything contained within it would tumble out. We may conceive that there are an infinite number of such Cosmic systems, with inter-cosmic intervals throughout the infinity of space. He is very disinclined to assume that similar phenomena, _e.g._ eclipses of the sun or moon, always have the same cause. The various accidental implications and interminglings of the atoms may produce the same effect in various ways. In fact Epicurus has the same impatience of theoretical physics as of theoretical philosophy. He is a 'practical man.' [378]