Structure of Zinc Br ornate Hexahydrate. 189 



dral and cube faces. These showed clearly an absence 

 of planes of symmetry; hence it is evident that the 

 symmetry of the arrangement of the atoms of this crystal 

 is either tetartohedral or paramorphic hemihedral (pyri- 

 tohedral). Interpretation of these photographs in the 

 usual manner 7 showed that in general planes of all sorts 

 appear in the first order region. The fundamental 

 lattice must consequently be the simple cubic lattice. 

 There are four zinc atoms within the unit, and it is both 

 natural and in accord with previous experience to con- 

 sider them equivalent. If, merely to serve as a starting 

 point for considering the various possible space groups, 

 this assumption of the equivalence of the zinc atoms is 

 made, we find that there are four tetartohedral and para- 

 morphic space groups built upon a simple cubic lattice 

 which have as special cases four equivalent positions 

 within the unit, namely the groups T^T^TVSTV 3 . 



An inspection of the criteria for distinguishing 

 between these space groups (see the preceding article) 

 suggests the investigation of those planes having one of 

 the indices zero. Some data for first order reflections of 

 such planes from a Laue photograph with the X-rays 

 roughlv normal to an octahedral face are given in 

 Table I. 



Table 



I. Laue Photographic Data. 





From a Plate taken 



with the X-rays roughly 



normal to (111) 





Appearing Planes. 







Indices of plane 



Wave Length 



Form of plane 



032 



0.480 A. U. 





032 



340 



.285 





034 



540 



.376 





054 



0o6 



.415 





056 



o80 



.313 





058 



074 



.357 





074 



078 



.264 





078 



12,0,5 



.443 





0,5,12 



0,7,10 



.379 





0,7,10 



0,7,12 



.434 





0,7,12 



8,0,11 



.281 





0,11,8 



7 Kalph W. G. Wyckoff, this Journal 50, 317, 1920. 



