THE theory of atomic disintegration, which affords a philosophical explanation of radio-activity, was based on simple chemical observations of the regeneration of radio-active constituents in substances from which they had been chemically separated, and not, as has sometimes been asserted, upon any physical or chemical theories as to the nature of the atoms of matter. Only two of the large number of new problems originally suggested by this theory remain at present unanswered. One had to do with the nature of the ultimate product or products of the disintegration of the atoms of the two primary elements, uranium and thorium. This problem may be likened to the task of trying to find a meteor alter its flight, when its energy is spent and nothing but the matter remains. Much indirect evidence points to lead as the final product of uranium, although no direct proof has been obtained, whereas for the case of thorium there is still no hint of the answer. The other had reference to the origin of radium. This element in the intensity of its activity, and therefore in the rapidity of its disintegration, resembles the short-lived active constituents uranium X and thorium X, whilst in the apparent permanence of its activity it resembles the primary radio-elements. Even the first rough estimates indicated that the period of average life of radium was not greater than a few thousand years. The present estimate, due to Rutherford, is 2500 years. A few thousand years hence the radium in existence to-day will for the most part have disintegrated. Very little of the radium in existence at the time the Pyramids were being built can still exist. Hence arose one of the most interesting and crucial of the problems of atomic disintegration. Does the regeneration of radio-active constituents, observed in the cases where the period is short corn-pared to the span of human life, apply also to radium -to an element, that is, with a definite spectrum, atomic weight and chemical character, filling a vacant place in the periodic system, and forming one of a family of common elements? After the separation of radium from a mineral does the non-radium part of (he mineral grow a fresh crop with lapse of time, the quantity present before separation being the balance or equilibrium quantity when the rate of production is equal to the rate _of supply? A somewhat similar prediction made with reference to the production of another well-defined element, helium, in the radio-active process had only to be tested, as it was first in 1903 by Sir William Ramsay and myself, to be proved correct. The question, however, of the origin of radium is still, in spite of many discoveries, not entirely solved.