128 PHYSIOGRAPHY. 



Chrysoberyl. 



a bluish opalescence when viewed perpendicular to the nar- 

 row lateral plane of the primary. 



Hardness = 8-5. Sp. gr. =3'754, a transparent variety. 



Compound Varieties. Twin-crystals : face of composi- 

 tion perpendicular, axis of revolution parallel to the edge 

 formed by the meeting of planes i and T, Fig. 132 ; an- 

 gle of revolution = 60. 



- 136. 



In Fig. 135, the crystals project beyond the face of com- 

 position only at one extremity ; it is rare to find them pro- 

 jecting at both extremities, although these do sometimes oc- 

 cur at Greenfield, (N.Y.) Occasionally the sides 1 are so 

 deeply replaced as to convert the made into a triangular 

 prism; and when this is the case, the re-entering angle at a is 

 not visible, the composition being detected only by the striae. 

 In Fig. 136, three prisms cross each other, or the compo- 

 sition is repeated ; and the prisms project at each end be- 

 yond the faces of composition. Rarely crystals are found, 

 in which they project only at one end, in which case the 

 form which results is represented by the upper half of Fig. 

 136; and still more rarely they are found in which they 

 project at neither extremity, when a regular hexagonal ta- 

 ble is produced. 



1. It remains unchanged, if exposed alone or with soda to the heat of 

 the blow-pipe, only the surface in the latter case becomes dull. It is 

 with difficulty fusible with borax and salt of phosphorus. 



