The Intensity of Reflection of X-Rays by Powdered Crystals. II. The Effect of Crystal Size

The problem of extinction in its relation to the intensity of reflection of x-rays is studied by a comparison of measurements upon large single crystals with those upon powdered crystals of various particle sizes. The apparatus and method of making the measurements upon powders have been described in the preceding paper. Secondary extinction was not operative in the powder method measurements. Primary extinction is also absent in measurements upon thoroughly ground crystals of substances which, like the alkali halides, naturally form rather imperfect crystals; but a small amount of primary extinction may be produced by heat treatment of the powder. The relative intensities of reflection from the different planes of powdered NaCl, Ca${\mathrm{F}}_2$ and calcite are compared with similar data from single crystal measurements on these substances. In the case of rock-salt, it is found that the single crystal measurements of Bragg, James and Bosanquet, when corrected for secondary extinction, agree with the powder method results, hence primary extinction was not operative in their experiments. The empirical correction for secondary extinction applied by James and Randall to their single crystal measurements upon Ca${\mathrm{F}}_2$ is not quite large enough to produce agreement with the powder method data. Although the powder method measurements upon calcite, when comparedwith single crystal data, show that the latter have been affected by a great deal of extinction, it is not certain that a small amount of primary extinction was not operative even in the powder method measurements, for calcite forms very perfect crystals.