Abstract
The crystal used in the first part of this investigation, an untwinned specimen from Ceylon in the British Museum collection, was measured goniometrically and described in detail by Dr. G. F. Herbert Smith, who gave as its crystallographic constants : Density (p) 5.82 (grs./c.c.). Monoelinic prismatic, a:b:c = 0.9905 : 1:0.5110, β = 80° 32'. Owing to the comparatively largc size of the crystal and its high absorptive power it was unsuited either to the 'rotating crystal' or Laue photographic methods of X-ray examination, Reflections from the faces were observed on the Bragg ionization spectrometer and spacings measured with an accuracy of about 2 per cent.

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