E. T. Wherry — Amphisymmetric Crystals. 239 



would seem to show that the distortion of the structure is slight. 

 The etch figures of cuprite indicate a holo symmetric [Class 32] 

 crystal, and the crystal does not rotate the plane of polarization 

 of light. It is only on account of the existence of crystals which 

 show a holoaxial hemihedrism that cuprite is assigned to the 

 holoaxial class. 



"These observations apply equally well to the case of potas- 

 sium chloride .... assigned by crystallographers to the holoaxial 

 class of the cubic system .... examination by means of the X-ray 

 spectrometer indicates .... perfect cubic symmetry .... "When 

 a potassium chloride crystal is etched with water, it displays 

 .... holoaxial symmetry .... Again however there is no trace of 

 rotatory polarization. ' ' 



It appears to the writer that in order to avoid misunder- 

 standings and reproaches crystallographers should no 

 longer make arbitrary choices in cases of amphisym- 

 metry, but should face the issue squarely and state both 

 symmetries represented in each case. The mode of state- 

 ment suggested in his cited paper is, however, capable 

 of improvement, and it is the purpose of the present con- 

 tribution to further develop the idea. In this connection, 

 the writer wishes to express his thanks to Dr. Elliot Q. 

 Adams, formerly of this Bureau, for many helpful sug- 

 gestions. 



The only reasonable interpretation of amphisymmetry 

 which suggests itself is that some of the methods for 

 bringing out hidden symmetry yield information as to 

 the symmetry of the structure or point-system as it 

 stands, while others indicate the symmetry which the con- 

 stituent atoms or molecules may exhibit when released 

 from the crystal, — in other words, a symmetry latent in 

 the atoms. It is therefore in order to consider the signi- 

 ficance of the several methods, and the group of alkali 

 halides may w T ell be used throughout by way of illus- 

 tration. 



(A) Physical group. — When pyroelectricity is excited 

 in a crystal, the opposite charges develop at the ends of 

 axes wmich are structurally polar, having different kinds 

 of atoms present at their opposite ends. The alkali 

 halides yield no pyroelectricity, indicating the absence of 

 polar axes in their structure and showing definitely that 

 the structures cannot possess the symmetry of the 

 gyroidal class. 



Ajl Jour. Sci.— Fifth Series, Vol. TV, No. 21. — September, 1922. 

 16 



