242 



PHYSIOGRAPHY. 



Gypsum. 



gives rise to the well known lenticular shapes, if the faces 

 P and / disappear. 



Lustre vitreous. P possesses a pearly lustre, more or less 

 distinct, both upon faces of cleavage and faces of crystalli- 

 zation. 



Color, generally white, sometimes inclining and passing 

 into srnalt-blue, flesh-red, ochre-yellow, honey-yellow, and 

 several shades of grey. Impure varieties assume dark- 

 grey, brick-red and brownish-red tinges. Streak white. 

 Transparent . . . translucent. 



Sectile. Thin lamina are flexible in the direction of 

 those edges which arise from the intersection of P with r. 



Hardness =1*5 . . . 2-0. The lowest degrees upon P, 

 its highest degrees in the direction in which the crystals are 

 rounded. Sp. gr. =2-310. 



* Fig. 223. 



Compound Varieties. Twin- 

 crystals. 1. Axis of revolution 

 perpendicular, face of composi- 

 tion parallel to M. Angle of 

 revolution = 180, (as in the an- 

 nexed figure,) which is under- 

 stood, if we suppose fig. 220. to 

 possess, instead of the edge from 

 the meeting of //, a portion of 

 M, and the bisection to take place 

 through P. 



2. Axis of revolution perpendicular to M ; face of com- 

 position parallel to P : angle of revolution =180. 



M 



