Penfield — Crystallography of Ccesium-Mercury Halides. 319 



which is perhaps owing to their small size. 

 The crystals of CsHgBrI 2 were about 2 ram 

 in diameter and had the habit shown in fig. 

 24, which is quite different from that of the 

 bromide. The forms and angles are as follows : 



24. 



6, 010, i-i 



c, 001, 



I, 320, i- 



s, 034, f-t 



Measured. 

 I ~ I, 320 a 320 = *66° 8' 

 5 as, 010 a 034 = *60 54 



Measured. Calculated. 

 ZaS, 320 a 034 = *72° 22' 

 I a s, 320 a 034= 76 16 76° 50' 



The basal planes were curved and uneven so that no satis- 

 factory measurements could be made from them, and the 

 other faces, although bright, did not give very satisfactory 

 reflections. The crystals show in convergent polarized light 

 an optical axis, almost normal to the base, the plane of the 

 optical axes being the clinopinacoid. 



Type 2 : 3. 



b: 



Cs 2 Hg 3 T 8 . Monoclinic, hemiliedral. 0-3438 : 1 : 0-3544, (3 = 7 1° 55£' 

 25. 26. 



'27.. 



The crystals of this salt have a curious development. 

 Some of the most conspicuous forms are triangular plates, fig. 

 25, while fig. 26 is a projection of the same upon the clino- 

 pinacoid. These crystals are terminated above by a basal 

 plane and below by pyramidal faces, which gives a curious 

 hemimorphic development in the direction of the sym- 

 metry plane. A variety of habits was observed, long pris- 

 matic, skeleton forms and simple shapes like fig. 27, but in 

 almost all of these the hemiliedral character was prominent. 

 The crystals frequently measured over 10 mm in greatest diam- 

 eter. The faces were bright and gave excellent reflections. 

 The forms and angles are as follows : 



6, 010, i-i c, 001, to, 110, I p, 111, 1 



The pyramid was observed only with hemihedral develop- 

 ment. 



Measured. 

 C a m, 001 a 110 = *72° 51' 

 TOaTO, 110a110 = *36 12 



Measured. Calculated. 

 TOaJ>, 110 a 111 =*50 c 26' 

 & Ap, 010 a 11- = 74 17 74° 14' 



