grouping of a certain number of negatively charged particles round a centre composed of positive electricity around which they revolve; and it is the number of these particles that determines the nature of the atom --- whether, for instance, it will be an atom of iron or an atom of hydrogen --- and thus we are brought back to Plato's old aphorism that the Universe consists of Number and Motion.

The size of these etheric particles is small beyond anything but abstract mathematical conception. Sir Oliver Lodge is reported to have made the following comparison in a lecture delivered at Birmingham. "The chemical atom", he said, "is as small in comparison to a drop of water as a cricket-ball is compared to the globe of the earth; and yet this atom is as large in comparison to one of its constituents as Birmingham Town Hall is to a pin's head". Again it has been said that in proportion to the size of the particles, the distance at which they revolve round the centre is as great as the distance from the Earth to the Sun. I must leave the realization of such infinite minuteness to the reader's imagination --- it is beyond mine.