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Lord Rutherford: In my address to-day, I shall briefly review some of the main lines of advance in our knowledge of atomic nuclei since the last discussion which I had the honour to open. In the interval, there has been substantial progress in many directions, and new and promising methods of attack on this formidable problem have been opened up. I can only refer in passing to the valuable data obtained by Aston and others on the isotopic constitution of the elements and the relative abundance of the isotopes of many of the elements. This has made it possible to determine the chemical atomic weight of many elements with considerable accuracy by the use of the mass spectrograph. A number of new experiments have been made to determine with accuracy the relative quantities of the isotopes of lead, and in particular of lead obtained from pure uranium and thorium minerals of great geological age. Data of this kind are of much interest and importance, not only from the point of view of radioactivity but also with regard to the fixation of an accurate time scale in geology. It seems certain that the end product of the actinium series—actinium lead—has an atomic mass 207 and that actinium is derived from the transformation of an isotope of uranium. From the relative abundance of actinium-lead and uranium-lead derived from old radioactive minerals, it is possible to deduce the average life of this uranium isotope. I pointed out some time ago in ‘Nature,’ that important inferences could be drawn with regard to the production of elements in the sun from a consideration of the average life of the two uranium isotopes.Optical Methods .—One of the most interesting developments in recent years has been the application of optical methods to determine the presence of isotopes and to throw light on the movements of the nucleus. The study of the band-spectra of the molecules of the lighter elements had disclosed the presence of isotopes existing in small quantity compared with the main isotope. It has been shown that oxygen consists of three isotopes of masses 16, 17, 18, carbon 12, 13, beryllium 8, 9, boron 11, 10, while recent observations of Urey, Brickwedde and Murphy are believed to indicate the presence in small quantity in hydrogen of a new isotope of mass 2. Attempts are in progress to concentrate the new isotope by fractional distillation of liquid hydrogen.

Discussion on the structure of atomic nuclei