112 Mr. W. Barlow on the Relation of Circular Polarization 



Class 1. 



As a parallel to those substances which exhibit circular 

 polarization in the amorphous state only, we have 



Enantiomorphous homogeneous structures which while un~ 

 broken contain effective configurations of two opposite hands 

 whose effects just cancel one another, but which, after being 

 symmetrically partitioned and dislocated, contain but one kind, 

 the other being destroyed in the process of dislocation. 



An example from the cubic system will make this clear. 



A homogeneous structure with gyrohedral symmetry of 

 type 8 * is, as we know, not identical with its own mirror- 

 image ; and if such a structure contains effective con figurations, 

 we may suppose that they consist of particles so placed as to 

 form a single kind of Sohncke's 24-point-set, the centre A 

 of the configurations occupying, therefore, the centres of half 

 the cubes of a close-packed system of cubes filling space, sym- 

 metrically chosen, i. e. so chosen that they are in contact at 

 their edges onlyf. 



But it can easily be shown that particles thus placed may 

 equally well be regarded as forming other 24-point-sets 

 whose centres lie at the centres B of the cubes of the other 

 half of the system of cubes. 



And if the distance of a particle from the point A nearest to 

 it is the same, or practically the same, as its distance from the 

 nearest point B, the only material difference between the forms 

 of the 24-point-sets thus related will be that one will be right- 

 handed the other left-handed. 



If therefore the parts or particles of the effective con- 

 figurations occupy positions thus about midway between the 

 two kinds of singular points J A, B, the rotation produced by 

 their arrangement about centres A may be neutralized by the 

 effect of their arrangement, of the contrary hand, about 

 centres B. 



The homogeneous structure, when in the solid or unbroken 

 state, will then, as a whole, produce practically no rotation. 



Kot so, however, if it is symmetrically partitioned into 

 fragments having one or other of the two kinds of singular 

 points A, B for their centres, and then dislocated. 



For it is evident that this will destroy one of the two 

 kinds of effective configurations, and leave the other to pro- 



* Zeitschr.f. Kryst. 1894, xxiii. p. 18. 



t The centres A form therefore a " cubisches flachericentrirtes 

 Baumgitter," or, as Sohncke calls it, a " regular oktaedrisches Rauin- 

 gitter/' 



I Zeitscht\f. Kryst. xxiii. p. 60, 



