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The problem of the velocity of the particles emitted from the nuclei of dis integrating radioactive atoms has always attracted considerable attention. It was early established that in the case of the a-rays all the particles from one substance were emitted with the same velocity, and the latest experiments of Briggs* have emphasised the high degree of homogeneity attained. This result, showing that each disintegration involves exactly the same emission of energy, is easily reconcilable wT ith our general ideas of the radioactive processes, and, as is well known, there is undoubtedly some connection between this characteristic energy and the mean life of the body.The behaviour of the (3-ray bodies is in sharp contrast to this. In place of the oc-particles all emitted with the same energy, we find that the disintegration electrons coming from the nucleus have energies distributed over a wide range. For example, in the case of radium E this continuous energy spectrum formed by the disintegration electrons has an upper limit at 1,050,000 volts, rises to a maximum at 300,000 volts, and continues certainly as low as 40,000 volts, and similar results have been obtained for other [3-ray bodies. If this result is interpreted as showing that different disintegrating nuclei of the same substance emit their disintegration electron with different energies, we must deduce that in this case the energy of disintegration is not a characteristic constant of the body, but can vary between wide limits. Many workers have considered this to be so contrary both to the ideas of the quantum theory and the definiteness shown by radioactive disintegration that they have asserted the inhomogeneity must be a result of some secondary process, such as collision with the extranuclear electrons or emission of general y-radiation, and that although we cannot observe them before they become inhomogeneous, the disintegration electrons are actually emitted from the nucleus with a definite characteristic energy as in the case of the a-particles.Such views are plausible and deserve careful consideration, but they meet with the great difficulty that it has up till now proved impossible to discover any evidence of the secondary effects which are presumed to produce the

Considerable advances have been made in recent years in our knowledge of the γ-ray emission of radioactive bodies. It has been established quite definitely that series of definite frequencies are emitted, forming a characteristic line spectrum of the nucleus, and methods of measuring the frequencies have been devised so that at present the γ-ray spectra of most of the radioactive bodies are well known.

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