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1. The present paper may be divided into two parts. In the first, some experiments on the intensity of reflexion of X-rays by rock-salt crystals at low temperatures are described. The results of these experiments, when combined with data obtained previously at high temperatures, are compared with the theoretical formulæ of Debye and Waller for the temperature factor of X-ray reflexion. In the second part of the paper we have attempted to get some idea of the actual amplitude of the heat-motions of the atoms in the rock-salt lattice, by analysing the F curves, or curves showing the variation of the atomic scattering power with angle of scattering, using the method of Fourier analysis introduced by Duane and Havighurst, so as to obtain the distribution of electrons in the crystal unit at different temperatures. In connection with this work a new set of absolute determinations of intensity of reflexion has been made, and, from these, the F factors at different temperatures have been calculated, using the results of the experiments described in the first part of the paper.

An X-ray study of the heat motions of the atoms in a rock-salt crystal