[ no ] 



XVI. The Relation of Circular Polarization, as occurring both 

 in the Amorphous and Crystalline States, to the Symmetry 

 and Partitioning of Homogeneous Structures, i. e. of Crystals. 

 By William Barlow*. 



THE general prevalence of the belief that circular polariza- 

 tion is a property connected with the symmetrical 

 arrangement of small parts of the bodies displaying it, dates 

 from Reusch's famous device for obtaining it with a stack of 

 mica plates arranged as a staircase spiral. As we are now 

 acquainted with the nature of the repetition in space which 

 constitutes homogeneity of structure, and know how structures 

 thus formed may be partitioned symmetrically f, the time 

 seems to have arrived for comparing the various distinct 

 classes of cases in which circular polarization occurs with the 

 geometrical possibilities for homogeneous structures, unbroken 

 and broken, i. e. for substances both in the crystalline and the 

 fluid conditions. 



Adopting substantially the classification given by Pope, in 

 a paper which he has recently published J, we may place the 

 substances which possess the power of converting plane-pola- 

 rized light into circularly-polarized light under five heads. 



1. Those which exhibit circular polarization only in the 

 amorphous state (that is, when dissolved, melted, or converted 

 into gas). 



2. Those among the substances showing circular polariza- 

 tion in the crystalline state only which owe the property to 

 complex grouping or intercalation of crystal individuals, 

 such as occurs in many cases of pseudo- symmetry. 



3. Those other substances showing circular polarization in 

 the crystalline state only in which it is an inherent property 

 of the homogeneous structure itself. 



4. Those among the substances which rotate the plane of 

 polarization both when amorphous and when crystalline 

 which owe the property, as displayed in the solid state, to 

 complex intercalation of individual crystals. 



5. Those other substances which possess the property 

 both in the amorphous and in the crystalline state in which 

 the circular polarization, when the latter state prevails, is a 

 specific property of the homogeneous structure, and is not 

 due to intercalation. 



On the threshold of an inquiry as to the relation between 



* Communicated by the Author. 



t ' Mineralogic.il Magazine,' 1896, xi. p. 119. 



J Trans. Chem. Soc. 1896, p. 971, 



