A study of the long series of transformations which occur in uranium and thorium provides us with a wealth of information on the modes of disintegration of atoms, but unfortunately our theories of nuclear structure are not sufficiently advanced to interpret these data with any detail. The expulsion of high speed _α_ and _β_ particles from the radioactive nucleus gives us some idea of the powerful forces resident in the nucleus, for it can be estimated that the energy of emission of the _α_ particle is in some cases greater than the energy that would be acquired if the _α_ particle fell freely between two points differing in potential by about 4 million volts. The energies of the _β_ and _γ_ rays are on a similar scale of magnitude. Notwithstanding our detailed knowledge of the successive transformation of the radio-elements, we have not so far been able to obtain any definite idea of their nuclear structure, while the cause of the disintegration is still a complete enigma. In comparing the uranium, thorium, and actinium series of transformations, one cannot fail to be struck by the many points of similarity in their modes of disintegration. Not only are the radiations similar in type and in energy, but, in all cases, the end product is believed to be an isotope of lead. This remarkable similarity in the modes of transformation is especially exemplified in the case of the "C" bodies, each of which is known to break up in at least two distinct ways, giving rise to branch products. For example, thorium C emits two types of _α_ rays, 65 percent of range 8.6 cms. and 35 per cent of range 4.8 cms., and in addition some _β_ rays. In order to explain these results, it has been suggested that a fraction of the atoms of thorium C break up first with the expulsion of an _α_ particle and the resulting product then emits a _β_ particle. The other fraction breaks up in a reverse way, first expelling a _β_ particle, while the subsequent product emits an _α_ particle. Similar dual changes occur in radium C and actinium C, although the relative number of atoms in each branch varies widely for the different elements. This remarkable similarity between the "C" bodies is still further emphasized by the recent discovery of Bates and Rogers that both radium C and thorium C give rise in small numbers to other groups of _α_ particles, some of them moving at very high speeds.