258 



PROF. W. H. BRAGG ON X-RAYS AND CRYSTAL STRUCTURE. 



represented, as crystallographers have shown us, by the " face-centred " cube drawn 

 in full line. The cubic habit may therefore be acquired in this way also. By the X-ray 

 method we can easily tell one from the other. For the spacings of the (100), (110) and 



(ill) planes are, in the case of the simple cube, in the ratio 1 : 7= : - . Consequently 



v 2 v 3 



the series of the corresponding glancing angles are as 1 : \/2 : \/3. We have no real 

 example of this lattice, though potassium chloride (sylvine) accidently gives it. The 

 potassium and chlorine atoms are nearly of equal weight, and weight is all that seems 



Fig. 7. 



to affect the scattering of X-ra3 r s, so that the arrangement of the atoms shown in 

 fig. 7 is effectively equivalent to the simple cubic arrangement. 



On the other hand, it may easily be calculated that the spacings of the face-centred 



1 2 

 cube are in the proportion 1 : = : 7= ; and the series of the corresponding angles 



\/2 v3 

 of reflection are as 1 : \/ 2 : \/3/2. A very good example occurs in the case of copper ; 



Fig. 8. Spectra of Pd rays given by certain planes of a copper crystal. The angles recorded 



refer to the a. ray. 



the angles are given in fig. 8, and the relations can be readily verified. The copper 

 atoms are therefore arranged on the lattice in which the elementary cell has three 

 equal sides meeting at a point and including three equal angles of 60 degrees. 



