E. T. Wherry — Amphisymmetric Crystals. 237 



Art. XXIII.— Amphisymmetric Crystals 1 ; by Edgar T. 



Wherry. 



That the symmetry-class to which a given crystal 

 belongs is not always evident from external features is 

 too well known to require discussion. The methods com- 

 monly applied in discovering hidden symmetry may be 

 divided into two groups, designated in a general way as 

 physical and chemical, respectively. The individual 

 methods falling in each of these groups are listed in 

 Table I. 



Table I. — Methods of Discovering Hidden Symmetry. 



(A) Physical: 



(a). Excitation of pyroeleetricity. 



(Z>)- Observation of rotation of the plane of polarized 



light. 

 (c) Study by X-rays. 



(B) Chemical: 



(a) Development of etch-figures by momentary action of 

 solvents. 



(&) Production of growth forms by causing rapid crystal- 

 lization, by adding impurities to the solution, etc. 



"When the results obtained by applying all of these 

 methods agree in placing the crystal in a single symmetry 

 class, the relation is merely that of mimicry or pseudo- 

 symmetry and requires no further attention. However, 

 as pointed out in a previous paper, in a not inconsiderable 

 number of cases the different methods do not agree 2 ; and 

 for this lack of agreement the term amphisymmetry is 

 here suggested. 



In cases of amphisymmetry, crystallographers have in 

 general selected as characteristic of the substance one of 

 the classes indicated by the special methods of study, and 

 have disregarded the other. The most influential factor 

 in making the choice appears to have been the circum- 

 stance that certain forms are common to crystals of two 

 or more symmetry classes, so that if only such forms are 



1 Presented in abstract at the meeting of the Mineralogical Society of 

 America, Amherst, Mass., Dec. 29, 1921. 



2 E. T. Wherry, J. Wash. Acad. Sci., 8: 480, 1918; questioned by A. E. 

 H. Tutton, ibid., 9: 94, 1919. 



