Results of Crystal Analysis. 



399 



We must be aware that the symmetry properties o£ the 

 crystal need not be expressed in the geometry of the 

 lattice. The symmetry of the crystal may also depend on 

 the symmetry properties of the centres. We know already 

 several instances of this kind. Thus the holohedral lattices 

 of KBr, KI, and Cu 2 give crystals of the pentagon ikosi- 

 tetrahedric class. 



A stereoscopic reproduction of a model corresponding to 

 the hexakistetrahedrical arrangement is given in PI. IX. (a). 



Analysis of N(CH 3 ) 4 I. 



§ 3. The crystals w r ere got by gradual evaporation of 

 an aqueous solution of N(OH 3 ) 4 I. The crystals obtained 

 were, however, quite small (greatest linear extension about 

 5-6 mm.), and had only the faces (111) and (100) deve- 

 loped. The reflexions for the other faces were got from 

 edges — a circumstance which may to a certain extent 

 reduce the accuracy of the intensity measurements for 

 those faces. 



Table III. and fig. 1 give the observed glancing angles 

 and relative intensities for each face. 



Table III. 



Face. 



0. 





Intensities. 



i 

 1 













»=1. 



2. | 3. 



4. 



(100) 



4° 25' 



100 



24 i 11 



4 



(110) 



3 09 



68 



100 76 



18 



(001) 



3 07 



55 



35 ()(?) 



100 



(101) 



3 41 



100 



24 ! 53 



26 



(111) 



4 23 



100 



7-8 , 2(?) 



1 



5 i 



From the observed glancing angles we find 



= 3*94 xlO- 8 cm. 

 and 



d m : d 001 : d m : d m : d m = 1*40 : T42 : 1-199 : 1*008 : 1, 

 or, I approximately, 



The number of molecules in the elementary cell d'i 00 d oi is 

 found to be 1/2. 



2 E 2 



