117. The next portion of the preliminary inquiry necessary to our concluding argument is that which relates to the intimate nature of matter; and more especially of that very wonderful form of matter which is the vehicle of all the energy we receive from the sun, as it is that of all the information we obtain about the position, motion, nature, mass, condition, and properties of the almost infinitely more distant bodies, which are scattered through cosmical space. In other words, we have hitherto spoken only of the laws of working of the machine called the physical universe; let us now endeavour to study the structure of the materials of which it is composed. 118. Various hypotheses have been proposed as to the ultimate nature of matter. To give even a general account of all the less absurd of these would require a large volume, so we content ourselves with a few of the more reasonable or historically more important. (1.) The foremost place must of course be taken by the old Greek notion of the _Atom_. The outlines of the atomic theory were laid down very precisely by Democritus and Leukippus (_circa_ 400 B.C.), who taught that the whole universe is made up of empty space and eternal atoms, differing only in form (as =Α= and =Ν=), order (as =ΑΝ= and =ΝΑ=), and posture (as =Ζ= and =Ν=). The atoms are endued with a primitive motion in virtue of their weight, and, clashing together, produce vortices from which the world is formed. The gradual progress of this whirl of atoms brings similar elements together, as in the sifting of grain, and so the atoms are sorted into homogeneous groups. The great weakness of this theory lay in the very false ideas then held as to the nature of motion by weight, which was supposed to be necessarily in parallel lines, and with a velocity greater for heavy than for light bodies. The difficulty which arose from this notion led Epicurus to give to the atoms a perfectly arbitrary and capricious side movement, as well as the rectilineal motion due to their weight, and thus, in his school, the theory became really a metaphysical one, reducing the order of the universe to pure chance. It is such a medley of physical speculations, with metaphysical notions, that we find in the greatest exponent of the system, the ‘poet philosopher’ Lucretius. With the help of Munro’s splendid edition of the text of Lucretius, and his very valuable translation and notes, it is now a comparatively easy matter to give a concise summary of the principal points of this most remarkable early physical speculation. In attempting to do so we will endeavour, so far as we can, to bear in mind the awful but too often disregarded warning given by the poet himself:— ‘Omnia enim stolidi magis admirantur amantque, inversis quæ sub verbis latitantia cernunt, veraque constituunt quæ belle tangere possunt auris et lepido quæ sunt fucata sonore.’[44]