Says Rutherford: “Although thorium is nearly always present in old uranium minerals and uranium in thorium minerals, there does not appear to be any radioactive connection between these two elements. Uranium and thorium are to be regarded as two distinct radioactive elements. “With regard to actinium, there is still no definite information of its place in the scheme of transformations. Boltwood has shown that the amount of actinium in uranium minerals is proportional to the amount of uranium. This indicates that actinium, like radium, is in genetic connection with uranium....” The recently discovered product, _protoactinium_,--isolated by Hahn and Soddy,--is the hitherto missing link between uranium Y and actinium. “This substance emits Alpha rays and has an estimated period of 10,000 years. The actinium series is believed to have its origin in a dual transformation of uranium X. The first branch product, representing about 4% of the total, is believed to be uranium Y, a Beta-ray product of period one day. This is directly transformed into protoactinium.” This element has not yet been obtained in a pure state. Many of the radioactive elements are isotropic with known chemical elements--_i.e._, alike in their chemical properties, but dissimilar in radioactive properties. Since they cannot be distinguished--or separated--from the ordinary elements with which they are isotropic, by any chemical methods, they must occupy the same place in the periodic classification of the elements. Radium and mesothorium, for example (as Soddy was first to show) do not have the same atomic weight, but they cannot be distinguished from each other by any chemical methods. Therefore they both have the atomic _number_ 88, though the atomic _weight_ of radium is 226 and of mesothorium 228. (See Shipley, “Origin and Development of the Atomic Theory,” p. 64, Little Blue Book, No. 608.) Radium D and lead, and thorium and ionium, are examples of radioactive isotropes. The nature of the end-product was first suggested by Boltwood, who pointed out the invariable presence of lead in old radium minerals, and in amount to be expected from their uranium content and geologic age. “Thus,” says Prof. T. W. Richards, of Harvard University, “we must adopt a kind of limited transmutation of the elements,” although not of the immediately profitable type [gold] sought by the ancient alchemists.”