=126.= An account will now be given of some experiments which have thrown much light, not only on the nature of the processes which serve to maintain the radio-activity of the radio-active bodies, but also on the source of the energy continuously emitted by those bodies. In this chapter, for simplicity, the radio-activity of uranium and thorium will alone be considered, for it will be seen later that the changes taking place in these two substances are typical of those which occur in all radio-active substances. We have seen (section 23) that there is some doubt whether the radio-activity of thorium is due to that element itself, or to an unknown radio-active constituent associated with it. This uncertainty, however, will present no serious difficulty when we are discussing the radio-activity of thorium, for the general conclusions are, for the most part, independent of whether thorium is the primary radio-active constituent or not. For simplicity, however, it will be assumed for the present that the radio-activity is due to thorium itself. If future research should definitely show that the radio-activity, ordinarily observed in thorium, is due to a new radio-active element mixed with it, the radio-active processes considered will refer to this new element. =127. Uranium X.= The experiments of Mme Curie show that the radio-activity of uranium and radium is an atomic phenomenon. The activity of any uranium compound depends only on the amount of that element present, and is unaffected by its chemical combination with other substances, and is not appreciably affected by wide variations of temperature. It would thus seem probable, since the activity of uranium is a specific property of the element, that the activity could not be separated from it by chemical agencies.