326 Mr Haidingec's Notice respecting Train/. 



is also a rough face, replacing the obtuse edge between M and 

 T, the inclination of which, however, I could not ascer- 

 tain. 



Cleavage highly perfect, and easily obtained parallel to M ; 

 faint traces also parallel to n and T. Fracture uneven. Sur- 

 face of n and M smooth, of T generally striated in a horizon- 

 tal direction, or parallel to its edges of combination with T. 



Lustre vitreous. Colour white, occasionally inclining to 

 yellowish grey, when impure. Streak white. Transparent, 

 perfectly so in minute crystals; the larger masses translucent. 

 The index of ordinary refraction, measured through the faces 

 M and T, is about 1.43 ; that of the extraordinary one mea- 

 sured in the same plane about 1.52 ; the two images are fine- 

 ly separated. 



Rather brittle. Hardness = 2.5... 2.75, very near that of 

 alum, though a little superior to it. Sp. gr. = 2.112. Taste 

 pungent, alkaline. 



Compound Varieties. — Crystalline coats, consisting of nu- 

 merous crystals fixed to the support in the place of the edges 

 between n and «, and lengthened between M and T, generally 

 thin, and nearly parallel, so as to produce a very distinct ra- 

 diated fracture. 



2. Hemiprismatic Natron-salt. 

 Hemiprismatic. P = j ^ **' j , 154° 31', 115° 22'. In- 

 clination of the axis = 3° 7 , in the plane of the long diagonal. 

 Fig. 7. Refl. Gon. 



a : b : c : d = 19.10 : 34.72 : 13.67 : 1. 



Simple forms. — (P) - 79° 41' ; ~- (t) = 58° 52' ; 



(Pr + 00 ) 5 (M) = 76° S8f ; Pr + oo (r) ; Pr + oo (I). 



P - 



Combinations. 1' '. — : (Pr + oo )•* . Pr -f 00 . Fig. 8. 



"Pr P 



2 • -g- • -g • (Pr + 00 y . Pr + oo . Pr + x . Fig. 9. 



Cleavage distinct, parallel to t, imperfect parallel to I, traces 

 of M. Fracture conchoidal. Surface smooth and even. 



11 



