of the Genus Gypsum-hahide. 803 



Sectile. Thin laminae are flexible in a direction nearly 

 perpendicular to the edges between o, n, and P. 



Hardness = 2.0 . . . 2.5, nearer the latter. The perfect 

 faces of cleavage are even below 2.0. since they may be scratch- 

 ed by rock-salt, particularly when applied in a direction cor- 

 responding to the elongation of the crystals. Sp. Gr. = 2.730, 

 in a number of detached crystals. 



2. Dialonious Gypsum-lidloide. 

 Form prismatic. Fundamental form, a scalene four- 

 sided pyramid. P - 133° 35', 123° 59', 75° 35' Fig. 5. 



a : b : c : = l : v 4.02 : j 2.83. 



Simple forms. (Pr.+l) \m) = 137°41, 61° 27', 137° 35' • 



(Pfiy^zziafl 1 46', 59° 32', 121° 87. P + x (c)=100° 0' j 



Pr (a) = 126" 58' ; Pr+x (d) ; Pr— 1 (g^- 146° 53' ; Pr+1 

 (h) = 80° 8' ; Pr+2 (i) = 45° 36' ; Pr+x (f). 



Combinations. 1. Pr — 1. Pr. Pr+1. r + x. Pr oo . 



Pr+x . Fig. 6. 



2. Pr— I. Pr. Pr + 1 (P+l)*- Pr+2. (Pr+1) 3 . P + x. 

 Pr+co. Pr+x. Fig. ?. 



Cleavage, highly perfect, and easily obtained in the dircc- 



tion of Pr+x • Surface, Pr. smooth, Pr. co smooth, or faintly 

 streaked in a longitudinal direction. The measurement of 



the terminal edge of Pr is on that account much more ac- 

 curate than that of P+x , the faces of which are rather irre- 

 gularly streaked parallel to the axes of the crystals. The ho- 

 l-izontal prisms, belonging to the short diagonal, are almost 

 all rough, particularly Pr — 1, which is at the same time 

 slightly rounded. The pyramids are rounded, though 

 smooth. 



Lustre vitreous. Colour white. Streak white. Trans- 

 parent, in small crystals, tranluscent. Double refraction ob- 

 servable through e, and the opposite face of J\ making an 

 angle of 40\ The two indices of refraction arc about 1.62 

 and 1.67. The less refracted image disappears, when the 

 axis of the tourmaline is perpendicular to the edge of the re- 

 fracting prism. 



