RECENT work has directed attention to the great similarity in the modes of transformation of actinium and thorium. Thorium, probably itself inactive, gives rise to radio-thorium (Hahn, Jahrbuch d. Radioact. u. Eleklron., ii., 3330) which emits a rays; radio-thorium forms thorium X, which is followed by the other well known products, the emanation and the active deposit. Actinium behaves in a very similar way. By the same method, which was successful in separating thorium X from thorium, Gndlewski (Phil. Mag., July, 1905) showed that a new product, actinium X, could be separated from actinium. Actinium X produces the emanation, and this in turn the active deposit. The similarity between these two substances is even closer, for I have found that a new product is present in actinium which is intermediate between actinium and actinium X, and, from analogy to thorium, will be called for convenience “radio-actinium.” This product emits rays, is half-transformed in about twenty days, and is the parent of actinium X.