Space Group of a Cubic Crystal. 187 



tions may not give diffraction effects in certain orders ; 

 so that in cases where most of the heavy atoms are in such 

 positions, the characteristic effects upon which a choice of 

 space groups is made may some of them be relatively 

 weak. Especially in view of this possibility of atoms 

 occupying in some instances coordinate positions which 

 may alter the qualitative character of the resulting dif- 

 fraction effects, it is necessary to emphasize the fact that 

 though these criteria are not ambiguous when used 

 properly they cannot be applied blindly. 



Summary. 



Criteria, which are valid for crystals which have any 

 atoms of appreciable scattering power in general posi- 

 tions, are established for determining from studies of 

 Laue photographs the space group to which a cubic 

 crystal should be assigned. This knowledge is of value 

 to the crystal analyst because it is thus possible to tell 

 how the atoms in many chemically complicated crystals 

 are arranged, even though existing methods are not suffi- 

 cient to locate these atoms with accuracy, and because an 

 assignment of a crystal to a particular space group 

 defines completely the positions of all of its elements of 

 symmetry. Many of the space groups give diffraction 

 effects which are different from those given by any other 

 groups and hence a method is provided, in the cases of 

 crystals assignable to any of these unique space groups, 

 of denning completely crystal symmetry without making 

 use of the older methods such as face development and 

 the like. 



Geophysical Laboratory, 

 April, 1922. 



