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.

To use the comparison of a writer on energy, we have hitherto spoken only of the laws of working of that machine called the physical universe; let us now endeavour to study the structure of that material of which it is composed.

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.

The foremost place must of course be taken by the old Greek notion of the Atom.