4. 2. 2. 29 64 Cu 63.35 4. 3. 2. 42 96 Mo 96 4. 3. 3. 60 144 Nd 144 4. 4. 3. 78 192 Pt 195 While the agreement is far from perfect for all these structures, there is a general accord with observation. If we take the view that some of these structures can grow by the addition of satellites, there is room for adjustment of masses and to include the intervening elements. This point of view is admittedly very speculative and there may well be other types of structure involved. At the same time, the general evidence suggests that there are some basal structures on which the heavier atoms are progressively built up. The failure of the whole number rule for the mass of isotopes, observed in some cases by Aston, _e.g._, between tin and xenon, certainly supports such a conception. From a study of the artificial disintegration of the elements we have seen that carbon and oxygen represent very stable structures probably composed of helium nuclei. It is possible that oxygen nuclei, for example, may be the structural basis of some of the elements following oxygen, but our information is at present too meagre to be at all certain on this point. I think, however, it will be clear from this lecture what a difficult but fascinating problem is involved in the structure of nuclei. Before we can hope to make much advance, it is essential to know more of the nature of the forces operative close to protons and electrons, and we may hope to acquire much information by a detailed study of the scattering of swift _α_ rays and _β_ rays by nuclei. Fortunately, there is now a number of distinct lines of attack on this problem, and from a combination of the results obtained we may hope to make steady, if not rapid, progress in the solution of this, the greatest problem in Physics.