240 E. T. Wherry — Amphisymmetric Crystals. 



When polarized light traverses a crystal, in any direc- 

 tion whatever if it is isotropic, or along an optic axis if 

 anisotropic, rotation of the plane will occur if the direc- 

 tions of attraction between atoms are to any extent gy- 

 roidally arranged, or if, as it is called in stereochemistry, 

 " asymmetric " atoms are present. As the alkali halides 

 show no trace of rotation of the very sensitive plane of 

 polarized light, their structure can certainly not be to the 

 least extent gyroidal, nor their atoms asymmetric. 



Since crystals affect X-rays in accordance with the 

 spacings between layers of atoms or molecules in various 

 crystallographic directions, this method of study permits 

 the recognition of the character of the structure. The 

 alkali halides behave toward X-rays as if they are in 

 every respect cubic-holosymmetric. To be sure, since the 

 theoretical rate of decrease in intensity of reflection with 

 order is not definitely known, slight deviations might not 

 be recognizable, but it is noteworthy that this result 

 agrees with those of the more certain methods previously 

 discussed. 



(B) Chemical group. — When a solvent is applied 

 momentarily to a crystal face, it starts to act at imperfec- 

 tions, and develops there minute pits, bounded by more 

 or less vicinal faces. Such faces may also be produced 

 during the growth of crystals, especially when rapid. 

 Vicinal faces may be explained most simply by combining 

 the views of the three principal writers on the subject. 4 

 Surface tension tends to make crystal surfaces curved, 

 while the building up of atoms or molecules in regular 

 rows in obedience to the attractive forces acting between 

 them tends to fill out all curves or depressions and pro- 

 duce plane faces with sharp intersections. The actual 

 surface features of a given crystal will be the result of 

 equilibrium between these two opposing tendencies, in 

 such a way that the vicinal faces actually produced repre- 

 sent the maximum deviation from normal angular posi- 

 tion which is possible without increasing the growth rate 

 sufficiently to cause the filling up of the resulting gaps 

 in the surface layers of atoms or molecules. 



In the course of momentary solution as applied in the 

 production of etch figures, the symmetry represented in 



4 G. Wulff, Z. Kryst, Min., 34: 449, 1901; H. A. Miers, Phil. Trans. A, 

 202: 459, 1903; P. Niggli, Z. anorg. allgem. Chem., 110: 55-80, 1920. 



