Page:Popular Science Monthly Volume 64.djvu/499

Rh from the rate at which the activity of the emanation decays; and, more important still, since it is found to emit both alpha and beta rays while the emanation emits only alpha rays, it seems necessary to conclude that this film of active matter is a product of the emanation rather than the emanation itself. In fact it appears to bear in all respects the same relation to the emanation which the emanation does to radium. That is, it is the result of the disintegration of the atom of the emanation, just as the emanation is the result of the disintegration of the atom of radium.

In the case of thorium this continuous change from one radioactive substance into another has been followed with certainty through as many as four different stages, thus; first, thorium produces thorium X; second, thorium X produces an active gas or emanation which is very like the radium emanation; third, the thorium emanation gives rise to a radio-active substance which is responsible for the induced radio-activity which is observable whenever the emanation comes in contact with a solid object; fourth, this induced radio-active matter due to the thorium emanation gradually loses its radiating power, and hence must undergo at least one further change into some other substance.

We have endeavored to follow step by step the discoveries which have led up to our present knowledge of the nature of radio-activity. These discoveries have seemed to prove conclusively that the atoms of radio-active substances are slowly undergoing a process of disintegration, this disintegration being indicated, first by the fact that there is a continuous projection from them of particles of matter, the alpha and beta rays; and second, by the fact that we are able to detect the presence of new and unstable types of matter accompanying the phenomena of radio-activity. Just why these atoms are disintegrating and just how these new types of matter are formed must of course be largely a matter of speculation. Nevertheless, discovery has gone far enough to enable us to form a reasonably plausible hypothesis as to the probable mechanism of radio-active change. In presenting this hypothesis the first remarkable fact to be noted is that the three permanently radio-active substances thus far discovered, the only ones which can with certainty be classed as elements, namely uranium, thorium and radium, are the substances whose atoms are the three heaviest