been shown to possess in kind, though not in degree, the fundamental properties of the original atom of which it had formed a part; and it becomes a reasonable hypothesis to surmise that the whole of the atom may be built up of positive and negative electrons interleaved together, and of nothing else; an active or charged ion having one electron in excess or defect, but the neutral atom having an exact number of pairs. The oppositely charged electrons are to be thought of on this hypothesis as flying about inside the atom, as a few thousand specks like full stops might fly about inside this hall; forming a kind of cosmic system under their strong mutual forces, and occupying the otherwise empty region of space which we call the atom,—occupying it in the same sense that a few scattered but armed soldiers can occupy a territory,—occupying it by forceful activity, not by bodily bulk.
6. The hypothetical part of the statement about the size of an electron is the following. Whereas both the mass and the charge of an electron is known, it is not yet quite certain that the mass is wholly due to the charge. It is possible, but to me very unlikely, that the electron, as we know it, contains a material nucleus in addition to its charge, so in that case it need not be so concentrated, because a portion of its mass would be otherwise accounted for.
I say 'accounted for,' but it would be equally true to say unaccounted for. The mass which is explicable electrically is to a considerable extent understood, but the mass which is merely material (whatever that may mean) is not understood at all. We know more about
