aluminium, and other moderately thin obstacles. That is because they are extremely small, much smaller than the atoms of matter.
If a magnet be brought near a stream of flying electrons they are deflected by the magnetic force, as a rifle-bullet is deflected by a wind; they will then miss the target at which they were aimed, and may strike another. By measuring their deflection when their speed is known it is possible to estimate the mass of each particle; and if any stream consisted of particles of different masses it would be possible thus to sort or fan or winnow them out: the massive ones keeping nearly straight and the lighter ones being blown aside, somewhat as a cork projectile is more easily deflected than a bullet.
Determinations made in this sort of way, supplemented by many other refined and most ingenious measurements conducted in the Cavendish Laboratory, Cambridge, England, have resulted in the following knowledge:—
(1) Each electron has a definite charge of electricity, viz. the same charge as is conveyed by each single atom when a current is passed through a chemically conducting liquid. Every electron has also a definite and uniform mass, which is about 1-800th of that of an atom of hydrogen—hitherto the lightest known form of matter.
(2) From every kind of material the same and no other kind of electron can be obtained, and we have reason for asserting that no other kind exists.
(3) Electric currents are always due to the locomotion of these little electric charges, they permeate and make their way through metals, being handed on from one atom to the next, as a fire-bucket is passed from hand to hand. This is metallic conduction. Liquid conduction is different: the electrons travel with the atoms in liquids, and hence travel slowly, being jostled by the crowd, and being laden with the heavy atom which they convey or propel, as a pony (or a flea—in mass a pony, but in bulk a flea) might drag a heavy wagon through crowded streets; until, at the terminal station, it is unharnessed and allowed to trot into its stable: which is what happens when the boundary between liquid and metallic conductors is reached. Electrons become still more emancipated however in rarefied gases, which act as a cleared race-course or like a free range for flight; and then it is possible to find them flying at prodigious speed, even as high as a hundred thousand miles per second, and sometimes faster still, but never quite so fast as light.
(4) Whenever an electron is suddenly started or stopped, or made to turn a corner, it disturbs the ether through which it had been quietly moving, and excites a ripple in it. These ethereal ripples constitute radiation, and the best-known variety of them we call "light." With this we have been familiar for a long time, because of our happening to possess eyes—instruments for the ready appreciation of ethereal ripples. We used not to know the reason however for the production of light, we know now that it is due to the sudden change of motion, either in speed or direction, of an electron; and probably to no other cause.
(5) An electric charge possesses the extraordinary property of self-induction, by reason of the magnetic field which it generates wherever it moves; and so far back as 1881 J. J. Thomson showed that this was equivalent to the possession of mass or inertia, and calculated its value. The mass or massiveness of an electric charge depends upon its concentration, the more concentrated it is the greater is its effective inertia. The charge in an electron is very small but is extremely concentrated, that is to say it exists only as a very minute nucleus; and in order to explain the manifestation of the observed mass of 1-800th part of a hydrogen atom, by so trifling a quantity of electricity, it is necessary to suppose that it is concentrated into a space one-hundred-thousandth of the diameter of a material atom. This is the size which is at present accepted for an electron. It is quite the smallest thing known. Eight hundred of them would, so to speak, "weigh" as much as a hydrogen atom, and would deal the same blow if stopped, and generally be equivalent to it; but they have remarkably little bulk, for if they were packed tightly together—an amount of packing probably quite impossible even to approach—a thousand million million of them would be required
