474 Wyckoff — Crystal Structure of Ammonium Chloride. 



raphy itself but it is of the greatest import to students of 

 crystal structure. By making uncertain a choice of the 

 appropriate class of symmetry in advance of X-ray 

 studies, it considerably lengthens the problem of crystal 

 structure determination. 



The difficulties of crystallographic description in the 

 case of crystals to which different symmetry characteris- 

 tics are assigned by different methods of study have 

 recently been emphasized. 9 It has been suggested that 

 such crystals should be especially segregated into a group 

 by themselves and that a complete description of their 

 symmetry will include both a statement of the symmetry 

 of the crystal as obtained by such "physical" methods 

 as a study of the electrical or optical properties or the 

 X-ray diffraction effects and a statement of the "latent" 

 symmetry which is supposed to be made evident by the 

 "chemical" means of etch figures and growth forms. 

 The creation of this additional classification is not 

 satisfactory if only for the reason that the concept 

 of ' ■ latent ' ' symmetry is not precise. The symmetry of 

 a crystal as deduced by the various "chemical" means 

 is different in many instances not only for these various 

 methods but it may even be different for the same method 

 carried out under changed external conditions. Thus 

 the symmetry deduced from the face development upon 

 scheelite (for face development must be reckoned among 

 the "chemical" means of symmetry determination) is 

 hemihedral though the etch figures that have been 

 obtained are strictly holohedral 10 ; or in the case of 

 dolomite etch figures have been produced which show 

 sometimes holohedral 11 and sometimes hemihedral 12 

 characters. From the point of view of those students 

 of crystal structure who have felt the force of the diffi- 

 culties into which such cases as this one of ammonium 

 chloride have led them, the symmetry characteristics 

 which have here been called "latent" are probably deter- 

 mined not by the symmetry of the crystal itself but rather 

 by certain properties inherent in the constituent atoms. 

 Etch figures, face development and the like are therefore 

 to be considered as essentially surface phenomena, and 



6 E. T. Wherry, this Journal, 4, 237, 1922. 



10 H. Traube, Zeitsch, f. Kryst. 30, 398, 1899. 



11 P. Gaubert, Bull. Soc. franc, de Min., 24, 326, 1901. 



12 P. Koller, Neues Jahrb. Min. Beil. Bel., 42, 488, 1919. 



