radium C. In the transformation of the latter, not only are swift alpha rays emitted but also beta rays of great speed. There is some evidence, however, that the substance called radium C is complex, and that the alpha and beta rays arise from two distinct substances.
The successive substances arising from radium C are radium D, radium E and radium F. The two former, like radium B, emit only beta rays; the latter, known generally as polonium, emits only alpha rays. It is believed that the sequence of changes ends with the transformation of radium F, which is supposed to change into the well-known non-radioactive element lead.
According to the transformation theory radium, like all other radioactive products, must be regarded as a changing element, but one whose rate of transformation is very slow compared with its successive products. Boltwood showed experimentally that radium is half transformed in about 2,000 years, and a quantity of radium would practically have disappeared as such in 100,000 years. In order to account for the continued existence of radium in the earth, it is necessary to suppose that it is steadily produced from some other element. Boltwood showed that the parent substance is a radioactive element called ionium, which is itself derived from the transformation of uranium. A quantity of ionium, entirely freed from radium, will grow radium at a slow but constant rate. The primary element of the ionium-radium series is uranium, which we can calculate should be half transformed in 5,000 million years—a period probably long compared with the age of many of the minerals in which uranium is found.
The complete sequence of changes in the uranium-radium series is shown in the diagram. The nature of the radiation and the half period of transformation are added for each element. In addition to uranium, there are two other radioactive elements, thorium and actinium, which are transformed with the appearance of a number of new substances. The time at my disposal, however, is too short to discuss these changes in detail. Thorium is known to be a primary element whose radioactive life is even longer than uranium, but actinium is believed to be a branch descendant from some point of the uranium series, and is thus to be regarded as a product of that element. In all, thirty-four of these radioactive substances have been discovered, and the position of each in the three main radioactive series has been determined.
Each of these new substances is to be regarded as a distinct chemical element in the ordinary sense, but differs from ordinary stable elements in the spontaneous emission of special radiations which accompanies the disintegration of the atoms. The radioactive substances are thus transition elements which have a limited life and which carry within themselves the seeds of their own destruction. Not only are these transition elements distinguished by their types of radiation but also by