R. W. G. WycJcoff — Ammonium Chloride. 177 



Akt. -XIII. — The Cryst alio graphic and Atomic Sym- 

 metries of Ammonium Chloride; by Ralph W. G. 

 Wyckgff. 



Introduction. — It was early observed that the maximum 

 symmetry of the arrangement of the atoms of certain 

 crystals, as determined from a study of their X-ray dif- 

 fraction patterns, was not in accord with the symmetry 

 assigned to these crystals as a result of the purely crystal- 

 lographic observations of the occurrences of faces and of 

 etch-figure formation. Typical of these apparent dis- 

 crepancies between the results of the formal crystal- 

 lography and of the X-ray investigations were potassium 

 chloride (sylvine) and cuprous oxide (cuprite). In both 

 of these instances, however, the symmetry of the arrange- 

 ment of the atoms was greater than that arising from the 

 crystallographic observations, so that it was always pos- 

 sible to avoid a direct conflict between the results of the 

 two methods by assuming that one or more of the kinds of 

 atoms entering into these compounds had about them 

 fields of force of such a shape as to impart to the atomic 

 aggregate the necessary lower degree of symmetry. In 

 view of the absence of any information concerning the 

 shapes of atoms such observations clearly had a certain 

 measure of justification. 



The determination of the probable structure of ammo- 

 nium chloride, however, brings forward another case of 

 discrepancy which cannot be dismissed by similar assump- 

 tions. It is the purpose of this paper to show that not 

 only is the structure which has been assigned to this 

 crystal incompatible with its described symmetry; but 

 that there is no possible structure, possessing the sym- 

 metry required by the recorded crystallographic observa- 

 tions and at the same time permissible from the stand- 

 point of the chemistry of ammonium chloride, which is in 

 even approximate agreement with these X-ray data. As 

 a consequence it seems necessary to conclude either (1) 

 that the very considerable data relating to the symmetry 

 of- ammonium chloride are incorrect, or else (2) that 

 information concerning the occurrences of forms upon 

 crystals and more especially the symmetry of etch-figures 

 does not in all cases furnish an indication of the internal 

 symmetry of the crystal. The second of these conclusions 



