E. T. Wherry — Amphisymmetric Crystals. 241 



the arrangement of attraction directions around the 

 atoms, whether it is exhibited in the structure or is sup- 

 pressed by their mode of combination, has a chance to 

 make itself felt. In the alkali halides the etch figures 

 occasionally show gyroidal symmetry in their bounding 

 vicinal faces, indicating this arrangement of the attrac- 

 tion directions around one or both kinds of atoms present. 



When there is present, in the solution of a compound, 

 an impurity which is attracted to one of the constituents 

 in directions other than those perpendicular to the princi- 

 pal faces, this impurity will tend to deposit on the grow- 

 ing crystal in the crystallographic positions corresponding 

 to the attractions and, by preventing the normal constitu- 

 ents from building on there, it may produce crystal forms 

 other than those typical of the pure substance. It should 

 be pointed out in this connection that the relation between 

 the attraction directions and the faces developed is the 

 opposite of that shown in the stereochemist's diagrams 

 of atoms. In the latter, valence lines (attraction direc- 

 tions) emerge at apexes of solids, whereas directions 

 along which impurities are attracted lie perpendicular to 

 the resulting faces. The addition of a large amount of 

 magnesium chloride to a solution of one of the alkali 

 halides, potassium chloride, is recorded to produce gen- 

 eral (hkl) forms of varying indices, probably though not 

 certainly gyroidal in arrangement. 5 The magnesium 

 may well be attracted to chlorine along force lines per- 

 pendicular to (hkl) faces, and prevent some potassium 

 from taking up its normal position, so that these faces 

 appear on the crystals. This, like the preceding method, 

 is thus adapted to bring out the symmetry of the attrac- 

 tion directions of atoms. 



The current theory of the constitution of alkali halide 

 crystals is that each metal atom has given over an electron 

 to a halogen, so that the surfaces of both kinds of atoms 

 are completed octets. The structure is then capable of 

 becoming relatively highly symmetrical, and the evidence 

 in fact indicates cubic holosymmetry. Apparently, how- 

 ever, when atoms are dissolving away or when impurities 

 are depositing, the single surface electron of the free 

 metal, or the seven surface electrons of free halogen, 

 being obviously incapable of highly symmetrical arrange- 



5 L. Wulff, Sitzb. Akad. Wiss. Berlin, 1894, 1, 387. 



