be extremely concentrated; and, unless it is to be utterly masked by the matter with which it is associated, it must be the latter: that is to say it must exist on bodies of far less than ultra-microscopic size. The mass or inertia of a charge depends upon two factors—the quantity of electricity in it, and its potential,—and by concentrating a given charge on to a sufficiently small sphere, the latter factor can be raised theoretically to any value we please, and thus any required inertia can be obtained; unless a stage is reached at which it becomes physically impossible to concentrate it any more.
2. The next thesis is a very simple and familiar one, and dates virtually from the time of Faraday, though the conception has gradually gained in clearness and solidity: it is that every atom of matter can have associated with it a certain definite quantity of electricity called the ionic charge, that some atoms can have double this quantity, some treble, and so on, but that no atom or any piece of matter can have a fraction of this quantity, which therefore appears to be an ultimate unit, a sort of 'atom,' of electricity. The ratio of the charge to the weight of a material atom is measured with accuracy in electrolysis, in accordance with what are called Faraday's laws; and in so far as the mass of the atom itself is otherwise approximately known, the quantity of electricity which can be associated with it is known with a similar degree of approximate accuracy.
3. Now mathematical data were given by J. J. Thomson in 1881 which enable us to say that if the charge of electricity usually associated with a single monad atom of matter were concentrated into a spherical nucleus
