648 



Prof. W. L. Bragg on th 



oxygen atoms being indicated by the small circles in the- 

 figure. 



Fie. 2. 



^ 



^ 



9 z.'nc O 0x y9 en - 



The atoms of zinc are arranged on two hexagonal space- 

 lattices, their centres corresponding very closely with those 

 of a set of equal spheres in hexagonal close-packing. For a 

 close-packed hexagonal arrangement of equal spheres, the- 

 axial ratio may be calculated to be 



a:c = l: 1*632. 



The axial ratio in the case of zinc oxide is 

 a:c = l : 1*608. 



The positions of the zinc atoms are identical with those of 

 the hexagonal close-packed arrangement of spheres, if the 

 latter be supposed to contract in the direction parallel to the 

 hexagonal axis so as to reduce the ratio c\a from 1*632 to 

 1-608. 



The oxygen atoms are light compared with the zinc atoms,, 

 and the spectra cannot be held to determine their positions 

 exactly. In so far as they do this, they are in agreement 

 with the supposition that the oxygen atoms are on two 

 hexagonal space-lattices identical with those on which the 



