constant until the velocity of light is very nearly attained. That is a critical velocity, which apparently cannot be surpassed. When this critical speed is reached, any electrified body becomes suddenly of infinite mass, and something is bound to happen. What that something is, it is not easy theoretically to say; but the partial or incipient disintegration or dissociation of the atom, and the flying away of a portion with a speed comparable to that of light, is no unlikely result.
Out of the whole multitude of atoms, even of the atoms of a conspicuously radio-active substance, it is probable that only a very few get into this unstable or critical condition at any one time; perhaps not more than one in a million million; nevertheless, just as occasional though rare encounters take place in the heavens, followed by the blaze of a new and temporary star, so, though probably not by the same mechanism, here and there a few out of the billions of atoms in any perceptible speck of radium arrive in due time at the unstable condition, and break down into something else, with energetic radio-activity during the sudden collapsing process; emitting in the process of collapse not only the main projected substance, but likewise also a few electrons and those X-rays which always accompany a sudden electric jerk or recoil. And the X-rays so emitted are of the most penetrating kind known, being able to pass through an inch of solid iron in perceptible quantity.
14. The hypothesis concerning radio-activity which is now in the field, then, is that a very small number, an almost infinitesimal proportion, of the atoms are con-
