366 Mr. Miller on the Crystals 



the planes c, e,f, h, x, y, seldom occur, and are too imperfect to 

 be measured accurately by the reflective goniometer, the angles 

 PC, &c. are therefore given to the nearest degree only. 



Cleavage, parallel to P ; very perfect. 



Twin crystals are common, having the axis of revolution 

 parallel to the intersection of M and T, and the face of compo- 

 sition parallel to h PP,= 150°.58'. (Fig. 2.) 



The axes of double refraction intersect the plane P, and 

 make with each other an angle of about 8° or 9°- 



The crystals were deposited from a solution of the acid in 

 water. 



II. Borate of Ammonia. 



Primary form, a square prism. 



(Fig. 3.) 



Twin crystals occur having the axis of revolution parallel 

 to M, and perpendicular to the intersection of a and a,. (Fig. 4.) 

 represents an assemblage of twin crystals. 



The crystals were obtained from a solution of the salt in 

 water. 



III. Indigo. 

 Primary form, a right rhombic prism. 



(Fig. 5.) 



The crystals were obtained by sublimation. 



