182 R. W. G. Wychoff — Determination of the 



Distinctions between most of the space groups of any- 

 one of these divisions are possible in the first order region 

 of the spectrum (see page 183) . By calculating the A and 

 B terms of the intensity expression [1] for the second- 

 order region for various space groups, using the same 

 procedure previously employed, a few more distinctions 

 between space groups can also be made*. A final classi- 

 fication of all of the cubic space groups on the basis of the 

 difTr action effects produced by corresponding crystals 

 can be written as follows. In this table indistinguishable 

 space groups are placed together on one line. 



r c : 



uncertain 



(V; 

 uncertain 



T h 2 



T d 4 ,O h ! 



O h 2 ; 



CV. 



r '• 



T\TY;> 

 T 4 ; \ 



TAO', 



0'; 



0«,0'; 



rp2 rr\ s . \ 



' rr/V r second order region 



ih ; ) 



T d 2 ,0 3 , ) second order region, O h 6 ; \ second or 

 O*, [ uncertain, O h 7 ; [ region 



T 6 O 6 • ) 



'/-VV r second order region 



r/: 



T 3 ,T 5 ,T h 5 ; 



T h 7 ; . 



T d 3 ,0 5 ; ) second order region, O h 9 ; 

 O 8 ; f uncertain, 



T d 6 ; 

 O h 10 . 



Except where definitely stated as lying in the second- 

 order region, the distinguishing characteristics are to be 

 understood as being first-order effects. If it is assumed, 

 as may or may not be the case, that studies of face 

 development as commonly carried out upon crystals 

 invariably indicate the symmetry of the arrangement of 

 their atoms, then it will be seen that means are at hand 

 for distinguishing between all of the various space 

 groups except the two pairs T 3 and T 5 and O 6 and O 7 . 

 Those distinctions which involve the absence of planes 



