Results of Crystal Analysis. 413 



in such a way that the C atoms were placed in two opposite 

 corners of the base planes (0 P A B Q R D, fig. 5). It 

 might also be natural to try to arrange the H atoms on the 

 three sides of this prism which meet at the centre of a 

 C atom. In this case, the positions of the H atoms are fixed 

 by means of two parameters. 



It can, however, be shown that the two latter arrange- 

 ments do not give any satisfactory agreement with the 

 observed intensities. 



This can most easily be shown by means of the spectra 

 from the faces (100) and (110). 



Let us first consider the arrangement (2). 

 The amplitudes of the two faces (110) and (100) are given 

 by the expressions : 



(110) A w = 60 + 28 cos na+ (-l)»4+4 cos n(2a— w), 

 (100) A„= 74 + 22 cos 2n(ir-u). 



The intensity distribution of the (110) face might be ex- 

 plained by putting a = 140°. But this value of a would make 

 the fourth-order spectrum of the face (100) much too strong. 

 The amplitudes calculated from the lattice would be 



100 SS 80 117, 



while those calculated from the observed intensities are 



100 85 98 73. 



The amplitudes of the face (001) would be given by the 

 expression 



A n =(43+(-l) ?1 53)cos7i|-. 



In order to get the observed ratio for the spectra of first 



i 

 and second order, we should have to put ~ equal to 



about 40° ; but then the fourth-order spectrum would be 

 about ten times as strong as that of first order, while the 

 ratio actually observed is merely 1 : 8. 



As the amplitudes of (110) and (100) only depend on a, 

 and those of (001) only on the parameter y' , the test made is 

 independent of the relation between the two parameters. 

 Phil Mag. S. 6. Vol. 33. No. 197. May 1917. 2 F 



