70 Mr David Forbes on the Chemical 



From which form approximative measurements were obtained 

 by the common goniometer. 



M : T 



=z 



147° 



+ o : s 



— 



149° 



M : n 



=z 



125° 



a: M 



= 



122° 



- o :T 



— 



153°30' 



— o : a 



= 



143°30' 



The angle M : T is the average of measurement from four 

 different crystals. As the junction of T : T is always cut off 

 by the plane M, and all the crystals hitherto obtained are 

 hemitropes, so that the planes T : T, which form an acute 

 angle, belong each to its own half of the crystal, the direct 

 determination of the angle T : T is too uncertain, but reckoned 

 from M : T = 147°, it will be 114°. The angle a = 58°. 



From these data Mr Hansteen has had the kindness to cal- 

 culate the following values : — 



Axes 



a : b : c = 0.835 



: 1 : 0-766, and 



a: S 



— 



140°, 42' 





M : n = 123°, 27' 



S: +o 



=z 



149°, 14' 





a: T = 114°, 25', 43 



+ o : T 



— 



135°, 11', 17" 







T : o 



— 



151°, 18', 43" 







- o : oP 



= 



143°, 34' 











720° 





The observed forms are — 











a = 







P 







+ o = 



+ 



P 







~T" ° ~ 



_i_ 



P 







r = 



2 P 







s = 



+ 



4? 







n = 





P CO 







M = 



CO 



Pco 



The positive terminal planes have on most of the crystals a 

 strong vitreous lustre, as if polished by friction, and have a 

 great number of small furrows arranged in rows parallel to 

 the edges between T and + o. The vertical prismatic planes 

 are smooth, but possess much less lustre. The negative ter- 

 minal planes are rough, and irregular by reason of an oscil- 

 lating combination between the planes — o and T, by which 



