The probability of levels in the nuclear structure is most clearly seen on the satellite hypothesis, but in our ignorance of the laws of force near the core we are at the moment unable to apply the quantum dynamics directly to the problem. The outlook for further advances in this direction is hopeful, but is intimately connected with a further development of our knowledge of the laws of force that come into play close to the nucleus in the region occupied by the satellites. ARTIFICIAL DISINTEGRATION OF ELEMENTS We have seen that it is believed that the nuclei of all atoms are composed of protons and electrons and that the number of each of these units in any nucleus can be deduced from its mass and nuclear charge. It is, however, at first sight rather surprising that no evidence of the individual existence of protons in a nucleus is obtained from a study of the transformations of the radioactive elements, where the processes occurring must be supposed to be of a very fundamental character. As far as our observations have gone, electrons and helium nuclei, but no protons, are ejected during the long series of transformations of uranium, thorium and actinium. One of the most obvious methods for determining the structure of a nucleus is to find a method of disintegrating it into its component parts. This is done spontaneously for us by nature to a limited extent in the case of the heavy radioactive elements, but evidence of this character is not available in the case of the ordinary elements. As the swift _α_ particle from the radioactive bodies is, by far, the most energetic projectile known to us, it seemed from the first possible that occasionally the nucleus of a light atom might be disintegrated as the result of a close collision with an _α_ particle. On account of the minute size of the nucleus, it is to be anticipated that the chance of a direct hit would be very small and that consequently the disintegration effects, if any, would be observed only on a very minute scale. During the last few years Dr. Chadwick and I have obtained definite evidence that hydrogen nuclei or protons can be removed by bombardment of _α_ particles from the elements boron, nitrogen, fluorine, sodium, aluminum and phosphorus. In these experiments the presence of H nuclei is detected by the scintillation method, and their maximum velocity of ejection can be estimated from the thickness of matter which can be penetrated by these particles. The number of H nuclei ejected even in the most favorable case is relatively very small compared with the number of bombarding _α_ particles, viz., about one in a million.