tion. In order to account for the expulsion of an α and a β particle with the observed velocities, it is necessary to suppose that the particles are in a state of rapid motion in the system from which they escape. Variation of temperature, in most cases, does not seem to affect the stability of the system.
It is well established that the property of radioactivity is inherent in the radio-atoms, since the activity of any radioactive compound depends only on the amount of the element present and is not affected by chemical treatment. As far as observation has gone, both uranium and radium behave as elements in the usually accepted chemical sense. They spontaneously break up but the rate of their disintegration seems to be, in most cases, quite independent of chemical control. In this respect, the radioactive bodies occupy a unique position. It seems reasonable to suppose that while the radioactive substances behave chemically as elements, they are, in reality, compounds of simpler kinds of matter, held together by much stronger forces than those which exist between the components of ordinary molecular compounds. Apart from the property of radioactivity, the radio-elements do not show any chemical properties to distinguish them from the non-radioactive elements except their very high atomic weight. The above considerations evidently suggest that the heavier inactive elements may also prove to be composite.
Origin of the Radio-elements.
We have seen that the radio-elements are continuously breaking up and giving rise to a succession of new substances. In the case of uranium and thorium, the disintegration proceeds at such a slow rate that in all probability a period of about 1,000 million years would be required before half the matter present is transformed. In the case of radium, however, where the process of disintegration proceeds at over one million times the rate in uranium and thorium, it is to be expected that a measurable proportion of the radium will be transformed in a single year. A quantity of radium left to itself must gradually disappear as such in consequence of its gradual transformation into other substances. This conclusion necessarily follows from the known experimental facts. The radium is being transformed continuously into the emanation which in turn is changed into other types of matter. Since there is no evidence that the process is reversible, all the raduim present must, in the course of time, be transformed into emanation. The rate at which radium is being transformed can be approximately calculated either from the number of α particles expelled per second or from the observed volume of the emanation produced per second. Both methods of calculation agree in showing that in a gram of radium about half a milligram is transformed per year. From analogy with other radioactive changes, it is
