282 



wherry: CRYSTAi. FORM AND OPTICAL PROPERTIES 



in the other. Though often greatest in the layers perpendicular 

 to the one selected as a, — calling the most prominent pyramid 

 (111), — it may be greatest in those along the alternate one, a', 

 which is obtained by making this pyramid (lOl^ the relation 

 between the two being: a' = a\/2; or, if the structure is so simple 

 that the arrangement is about the same in the layers of atoms 

 traversed in both directions, the refractive effect may even be 

 the mean of those along the two axes, and a hypothetical value 

 a + a' 



a" = 



must be used. 



In the course of the preparation of this paper many helpful 

 suggestions have been received from Dr. H. E. Merwin, of the 

 Geophysical Laboratory, Dr. E. Q. Adams, of the Bureau of 

 Chemistry, and others, to. all of whom the writer's warmest 

 thanks are herewith extended. 



REFRACTION RELATIONS OF ORGANIC COMPOUNDS 



Urea 



CO(NH2)2 Tetragonal-scalenohedral; a : c = 1 : 0.833 

 The simplest organic compound known to crystaUize tetra- 

 gonal is urea, or carbamide; it was accordingly selected as the 

 iirst subject for study. The refractive indices of this substance 



TABLE 3 



Refraction Relations of Urea 



Refractivity, based on mean rio = V J^^ = 1.522 andp = 1.33: Md =13.77. 

 Refractivity, calculated from data obtained from liquids: Md = 13.67. 



were determined by the immersion method by using essential 

 oils, in which it is but slightly soluble. The results are shown in 

 table 3, which includes (1) the refractive indices for D, and their 

 probable errors; (2) the ratio of the refractions, R, in the hori- 



C02 - 1 /€2 - 1, 



zontal and vertical directions a and c, that is,— ~ / r' 



