FROM the present state of the subject it is clear that there is one, at least, and that very probably there are two, intermediate products between uranium X and radium giving α rays on disintegration. The direct parent of radium has recently been shown by Boltwood (Amer. Journ. Sci., 1908, xxv., 377) to give α rays of characteristic range. Between the atomic weight of uranium and radium there is a difference of twelve units, corresponding to the expulsion of three α particles, of which only two, that from uranium and that from the parent of radium, so far are known. It is true there is a disposition to regard it as probable that the change of uranium into uranium X is accompanied by the expulsion of two α particles, as Boltwood (loc. cit., p. 285) has shown that the contribution of uranium in minerals to the total α radiation of the mineral is about twice that of any of the succeeding products.