WycJxoff — Crystal Structure of Ammonium Chloride. 475 



the internal symmetry of the crystal itself is only one of 

 a number of important factors which bear npon the 

 nature of these surface phenomena. 13 



For the crystal analyst, at least, the formal part of 

 these difficulties must, however, be met by a greater preci- 

 sion in the definition of what is meant by the symmetry 

 of a crystal. Symmetry, from this point of view, can 

 only mean that of the arrangement of the constituent 

 parts (the atoms) of which a crystal is composed. 



This case of ammonium chloride seems to require that 

 both the crystal analyst and the crystallogsapher must 

 make a considerable revision in their estimate of the 

 value of the usual crystallographic information relating 

 to the symmetry of a crystal, and it is therefore worth 

 while to reemphasize the desirability of a repetition of the 

 purely crystallographic study of ammonium chloride. 



SUMMAEY. 



It is shown that the Laue photographic data obtained 

 from crystals of the low temperature form of ammonium 

 chloride are in agreement with the powder data in assign- 

 ing to it a structure containing one chemical molecule 

 within the unit cube. The consequences introduced by 

 the disagreement between the symmetry of this uniquely 

 determined structure and the symmetry as obtained by 

 ordinary crystallographic means are discussed. It is 

 pointed out that it is not permissible to accept etch figure 

 data and face development as definite indications of the 

 symmetry of the arrangement of the atoms within a 

 crystal. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C. 



October, 1922. 



13 It would seem that Wherry's views (op. cit.) are in accord with these, 

 but it is not at all clear from his discussion that such is the case. 



