Crystalline Structure of Antimony and Bismuth. 163 



number of molecules are activated for the reaction of con- 

 densation to a non-polymerized liquid. Since the latent 

 heat of evaporation is given by the relationship L = N/*v, 

 Trouton's constant is on the above hypothesis merely 

 an approximate expression of Wien's displacement law, 

 Amax.T = constant. 



The value of the constant calculated from radiation data 

 agrees closely with experiment. 



The Chemical Laboratory, 



The University, Cambridge. 



IXV. The Crystalline Structure of Antimony and Bismuth. 

 By A. Ogg, M.A., Ph.D., Professor of Physics, The 

 University of Cape Town *. 



IN 1916 observations were made by me in the laboratory 

 of Sir W. H. Bragg of the reflexions from three 

 planes of an antimony crystal with an X-ray spectrometer. 

 The bulb used had a palladium anticathode. The faces used 

 were natural. 



The positions and intensities of the spectra from the 

 (111), (110), and (100) faces were sufficient for the deter- 

 mination of the number of atoms in the unit rhomb as well 

 as the spacings of one of the sets of planes, but the observa- 

 tions were not sufficient for the complete determination of 

 the structure. The spectra from the (111) face suggested 

 a structure similar to that of the diamond distorted along 

 the trigonal axis t, one set of (111) planes dividing the 

 distance between those belonging to the other in a ratio of 

 about 3 : 2, instead of 3 : 1 as in the diamond. If this were 

 the case, the first order spectrum from a (100) face should 

 almost vanish. Experiment, however, showed that the set 

 of spectra was nearly normal. A distorted diamond struc- 

 ture would not fit the observed facts. 



James and Tunstall + have found that two interpenetrating 

 face-centred lattices explain the observed spectra and satisfy 

 the symmetry of the crystal. 



The positions of the spectra observed by them are in 

 agreement with those observed by me in 1916, as are also 

 the ratios of the intensities of the spectra from the (100) and 



* Communicated by Prof. Sir W. IT. Bragg, K.B.E., F.R.S. 

 t Sir W. H. Bragg- and Prof. W. L. Bragg arrived at this conclusion 

 in 1914. 'X-rays and Crystal Structure,' p. 227, 

 \ James and Tunstall, Phil. Mag. Aug. 1920. 



M 2 



