which they are embedded and flying about. In so far as an atom is impenetrable to other atoms, its parts act on the sentinel principle, not on the crowd principle. There are two ways of keeping hostile people out of an open building: one is to fill it with your own supporters, another is to place an armed policeman at every door. The electrons are, extremely energetic and forcible, though in bulk mere specks or centers of force. Every speck is exactly like every other, and each is of the size and weight appropriate to the electron. Different atoms, that is atoms of different kinds of matter, are all believed to be composed in the same sort of way; but if the atoms of a substance are such that each possesses 23 times as many electrons as hydrogen has, we call it sodium. If each atom has 200 times as many as hydrogen, we call it lead or quicksilver. If it has still more than that, it begins to be conspicuously radioactive.
It would seem as if the excessive radiation which follows upon an overcrowded condition were caused by the probability of collision or encounter between the parts of an atom: just as every now and then among the stars in the sky two bodies encounter each other, and a great blaze of radiation, or temporary star, results. Even in atoms of which the parts are sparsely distributed such occurrences are not impossible, though they are less frequent, and accordingly it is to be expected that every kind of matter may be radioactive to a very small extent: a probability which is now justified for most metals, by direct experiment with very sensitive means of detection.
Indeed so far as radiation necessarily accompanies any change of motion of an electron, and in so far as in every atom some electrons are describing orbits and are therefore subject to centripetal acceleration, a certain amount of atomic radiation is inevitable, on the electric theory of matter. In most cases it is imperceptibly small, but it must be there, and accordingly an atom must be slowly undermining its own constitution by the gradual emission of its internal or intrinsic energy in the form of ether-waves.
Thus then it is reasonable to expect that, every now and then, an atom will break up or collapse or divide into parts. This process has been observed by Rutherford of Montreal. The radiation from many of the radioactive substances, on being analyzed by a magnet, is found to be separable into three parts:
(1) the so-called ß rays, which are the shot-off electrons already mentioned;
(2) some, ??? rays, which appear to represent an ethereal pulse,---an analogue as it. were of the sound-wave caused by the explosion or act of firing; and
(3) more important than either, a third kind of projectile called the ??? rays, which are newly formed atoms of foreign matter or new substance. These are pitched away with extraordinary violence as the atom breaks up, they produce by their bombardment of zinc sulphide the bright little flashes seen in Crookes's, spinthariscope, and they likewise generate heat when