190 R. W. G. Wyckoff — Symmetry and Crystal 





Absent Planes. 





70$ 



.453 



047 



053 



.431 



053 



059 



.432 



059 



506 



.336 



065 



067 



.325 



067 



11,0,6 



.329 



0,6,11 



507 



.460 



075 



079 



.330 



079 



0,7,11 



.400 



0,7,11 



850 



.448 



085 



0,8,11 



.318 



0,8,11 



0,8,13 



.351 



0,8,13 



095 



.304 



095 



7,0,10 



.338 



0,10,7 



0,11,5 



.362 



0,11,5 



From this table it will be seen that the only planes of 

 this type which appear in the first order region are of the 

 forms {JtQl}, where h is even and I is odd ; it is also appar- 

 ent that many planes of the forms {Ohl} and of the forms 

 {Okl}, where both k and I are odd, were in suitable posi- 

 tions to reflect but did not do so. Results in complete 

 agreement with these data and from planes of still differ- 

 ent forms are obtained from the interpretation of a 

 photograph taken with the X-rays approximately normal 

 to a cube face. In comparing the data obtained from two 

 different Laue photographs of either a tetartohedral or 

 paramorphic crystal, it must of course be remembered 

 to choose the H and K axes in the same way in both cases ; 

 this is readily accomplished by observing planes of two 

 forms {hkl} and {kill} which show marked differences in 

 reflecting power and are common to the-two photographs 

 to be compared. 



The data recorded in Table I are seen to be in entire 

 accord with the criteria which determine the space group 

 T h 6 . Since these criteria uniquely distinguish this space 

 group from every other group, it is evident that the 

 symmetry of crystals of zinc bromate hexahydrate is that 

 of T h 6 . From this knowledge of the corresponding space 

 group and the fact that four chemical molecules are to be 



