178 R. W. G. Wycfcoff — Crystallo graphic and 



is so difficult to practical crystallography that a further 

 investigation of the symmetry of ammonium chloride 

 seems imperative. 



The observed Symmetry of Ammonium Chloride. — 

 Ammonium chloride is assigned to the enantiomorphic 

 hemihedry of the cubic system both upon the basis of 

 observed face development and because of the symmetry 

 of etch-figures. Pentagonal icositetrahedral forms such 

 as (875) and (943) have been reported on crystals from 

 different sources. The type of etch-figure described as 

 occurring upon such faces as (211) points definitely to the 

 absence of any planes of symmetry. 1 



The X-ray Data upon Ammonium Chloride. — Both 

 spectrometer measurements 2 and powder reflections 3 con- 

 clusively indicate that the ratio n s /m = 1, where n is the 

 order of the reflection and m is the number of chemical 

 molecules within the unit cell. On this basis a "body- 

 centered arrangement" has been assigned to the nitro- 

 gen and chlorine atoms of the crystal. From the results 

 of the theory of space groups it is readily shown that if 

 there is thus one molecule in the unit, the placing of the 

 four hydrogen atoms associated with each nitrogen atom 

 requires that the symmetry of this arrangement be that 

 of the space group Td 1 . According to this structure the 

 co-ordinate positions of the atoms in a unit cube are : 



(1) CI: "-Hi, 



N": 000, 



H: uuu\ uuu\ uuu\ uuu. 



This structure is tetrahedral cubic, a degree of symmetry 

 incompatible with its recorded crystallographic charac- 

 teristics. 



The diffraction data concerning ammonium chloride 

 which have been collected have been used for no other 

 purpose towards determining its crystal structure than to 

 evaluate 4 the ratio of n 3 /m. From these data there is no 



1 A sumraar y of the crystallographic information is given in Groth, Ohem- 

 ische Krystallographie, I, 182 (Leipzig, 1906). 



2 W. H. and W. L. Bragg, X-rays and Crystal Structure, p. 110 (London, 

 1918). 



3 G. Bartlett and I. Langmuir, J. Am. Chem. Soc, 43, 84, 1921. 



4 It is readily shown by approximate calculations of intensity carried out 

 in a manner previously outlined (Ealph W. G. Wyckoff, this Journal, 2, 

 239, 1921) that all of the diffraction data are in accord with this simple 



