knocked out of a molecule at once, the residue of the molecule would possess a corresponding number of unit charges, and if this residue were caught by the oil drop, the latter should be seen to jump forward at the instant of capture because of the destruction of the equilibrium between gravity and the electric field; and, furthermore, from the speed which it assumed, as measured by the time which it took to move over a given number of the divisions in the scale of the eye-piece of the observing telescope, the size of the charge of the captured ions could be determined. The experiment was found to be as interesting and as exciting as trout fishing. The star under observation would often stand perfectly still for five, ten, fifteen or even sixty seconds and then suddenly start forward with a speed which was big or little according to the size of the catch and the size of the drop. When we were using large drops, it was found that two or three adjacent molecules were in occasional instances ionized at once, and therefore two or three separate ions were thrown simultaneously upon the drop, but when the drops were very small, we observed in the course of three months about 500 different catches without finding a single one which corresponded with certainty to the advent of an ion carrying more than one elementary electrical charge, and not more than three or four out of the five hundred which were in any way uncertain. This seems to prove conclusively that the act of ionization by all the types of X-rays and gamma and beta rays of radium which we have been able to try consists in the detachment from a neutral molecule of one single electron.
So far we have considered merely the proof afforded by the present experiments of the atomic theory of electricity. I have not attempted to tell "what electricity is," but have been content with demonstrating, that whatever it is it always appears as an exact multiple of a definite electrical unit. If you ask me to tell you what it is, I should answer by asking you first to tell me what matter is, and if you responded that matter is that out of which this world and the planets and the stars of this universe are made; that it is something which exists in the form of about 100 different units, or atoms, of relative weights between 1 and 240, which atoms unite together in different ways to form molecules; that the average diameter of one of these atoms is two hundred-millionths of a centimeter, then I should answer. Very well, if you are content with that sort of a definition of matter, I will define electricity for you in a similar way and say that electricity is something which is still more fundamental than your atoms of matter since it is a constituent of every one of these hundred different types of atoms which you have been describing. It is something too, which like matter is built up out of definite units, but it is unlike matter, in that all of these units are exactly alike so far as we are able to determine, save, however, that a marked difference is found between the positive and negative units. For
