Downloaded from 362 L. H. Gray. investigated, the new quantum mechanics leads to an accurate description of the interaction between a quantum and a free electron. From the standpoint of the older quantum theory all electrons could be regarded as " free " since the binding energy of even the K electrons in lead could be neglected in com parison with the energy of the quantum, which for the y-rays usually investigated was between 1 and 2 million volts. The scattering power per electron (the

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... ring coefficient divided by the number of electrons per unit volume) of all substances should therefore be the same. The early measurements of J. A. Gray referred to above showed that this was roughly true of carbon, iron and lead over a limited angular range. This conclusion was confirmed for light elements, with varying degrees of accuracy, by the subsequent measurements of Florance, Ishino and A. H. Compton. In each case the scattering power of tin and lead, per electron, appeared to be rather smaller than that of the light elements, but no signifi cance was attached to this result since it might reasonably have been due to an error in correcting for the reabsorption of the scattered radiation. Owing no doubt, on the one hand, to the strong theoretical grounds for the assumption that each electron scatters equally and independently, and, on the other hand, to the difficulty of the experimental measurement, no accurate and systematic investigation has ever been undertaken of the relative scatter ing power of different elements. Two systematic investigations have been made of the total absorption coefficient per electron, which includes the energy removed from the primary beam as recoil electrons as well as the energy of the scattered quanta. Kohlrausch* .using hard radium C y-rays found that for the 20 elements investigated between carbon and molybdenum, the coefficient was constant within the limits of experimental error, though the accuracy of measurement was in some cases only about 5 per cent. Ahmadf confirmed this result for certain light elements with an accuracy of 1 or 2 per cent, using a y-ray beam of much smaller average quantum energy. Neither of these investiga tions, however, yields any definite information about the scattering power of heavy elements, since the absorption coefficient then includes a third term of unknown magnitude referring to photoelectric absorption. The experimental results presented by Tarrant in the previous paper show that the absorption coefficient per electron of the hard thorium C" y-rays varies irregularly from element to element in the periodic system, a result