THE NATURAL AND ARTIFICIAL DISINTEGRATION OF THE ELEMENTS AN ADDRESS BY Professor Sir ERNEST RUTHERFORD Kt., D. Sc., LL. D., Ph. D., D. Phys., F. R. S. ON THE OCCASION OF THE CENTENARY CELEBRATION OF THE FOUNDING OF THE FRANKLIN INSTITUTE AND THE INAUGURATION EXERCISES OF THE BARTOL RESEARCH FOUNDATION SEPTEMBER 17, 18, 19, 1924 THE FRANKLIN INSTITUTE PHILADELPHIA THE NATURAL AND ARTIFICIAL DISINTEGRATION OF THE ELEMENTS _By_ Professor Sir ERNEST RUTHERFORD, Kt., D. Sc., LL. D., Ph. D., D. Phys., F. R. S. IT is not my intention in this paper to give a detailed account of the natural disintegration of the radio elements or of the methods employed to effect the artificial disintegration of certain light elements. I shall assume that you all have a general knowledge of the results of these investigations, but I shall confine myself to a consideration of the bearing of these results on our knowledge of the structure of the nuclei of atoms. There is now a general agreement that the atoms of all elements have a similar electrical structure, consisting of a central positively charged nucleus surrounded at a distance by the appropriate number of electrons. From a study of the scattering of _α_ particles by the atoms of matter and from the classical researches of Moseley on X-ray spectra, we know that the resultant positive charge on the nucleus of any atom, in terms of the fundamental unit of electronic charge, is given numerically by the atomic or ordinal number of the element, due allowance being made for missing elements. We know that with few exceptions all nuclear charges, from 1 for the lightest atom, hydrogen, to 92 for the heaviest element, uranium, are represented by elements found in the earth. The nuclear charge of an element controls the number and distribution of the external electrons, so that the properties of an atom are defined by a whole number, representing its nuclear charge, and are only to a minor degree influenced by the mass or atomic weight of the atom. This minute but massive nucleus is, in a sense, a world of its own which is little, if at all, influenced by the ordinary physical and chemical forces at our command. In many respects, the problem of nuclear structure is much more difficult than the corresponding problem of the arrangement and motions of the planetary electrons, where we have a wealth of available information, both physical and chemical, to test the adequacy of our theories. The facts known about the nucleus are few in number and the methods of attack to throw light on its structure are limited in scope.