Penfield — Crystallography of Caesium-Mercury Halides. 313 



The different crops of Cs 3 HgI 5 which were examined showed 

 a great variety in habit, represented by figs. 4-8. The hemi- 

 hedral development is not always strongly marked, and forms 

 like figs. 5 and 6 are the commonest. The crystals sometimes 

 measured over 5 mm in diameter and gave excellent reflections. 



Only one crop of Cs 3 HgBr 3 I 2 was examined. The crystals 

 were in the form of sphenoids, fig. 9, some of them over 10 mm 

 in diameter, but the faces were curved and striated and only 

 approximate measurements could be made. 



The forms which were observed are : 



b, 010, i-l d, 102, \-l s, 021, 1-1 p', 111,-1 o' 112,-$ 



c, 001, r, 011, 14 p, 111.1 o, H2,| e, 121,-2-2 



The angles of Cs 3 HgI 5 are : 



Measured. Calculated. Measured. Calculated. 



CaO, 001 a 112 = 46° 1' 46° 2' 



c*p. 001 a. Ill = *64° 15' oao', 112a112 = 39 55 39 46 



p*.p', 111 ^ 111 = *50 23 Oao', 112 ^ 112 = S7 57 87 56 



Cat, 001 a 011 = 44 23 44° 25' o a d, 112 a 102 = 19 50 19 53 



caS, 001 a 021 = 62 58 62 58 p a e, 111 a 121 = 17 48 18 4 



Both Cs 3 HgL and Cs s ITgBr 3 I 2 cleave distinctly, parallel to 

 the base, but the crystals are very brittle and usually break 

 with a conchoid al fracture. Crystals of the former, which are 

 tabular parallel to the base, show in convergent polarized light 

 a bisectrix normal to c, 001 ; the plane of the optical axes is 

 the macro-pinacoid and their divergence is large. 



Type 2 : 1. 



a : b : c 

 Cs 2 HgCl4 not measured. 

 Cs 2 HgBr 4 Orthorhombic 0*5706: 1 : 1-4715 



Cs 2 HgCl 2 Br 2 " 0-567 :1: 



Cs 2 HgCl 2 I 2 " not measured. 



Cs 2 HgI 4 Monoclinic 1-3155:1:0-9260 /3 = 69° 56' 



Cs 2 HgBr 2 I 2 " Approximately like Cs 2 HgI 4 



The crystals of Cs 2 HgCl 4 were too thin to measure. 



Both Cs 2 HgBr 4 and Cs 2 HgCl 2 Br 2 crystallize in thin rectangu- 

 lar plates ; those of the former were sometimes several centi- 

 meters long, but seldom over -§ mm thick, and had the 10. 

 habit shown in fig. 10. The crystals of the latter salt 

 were very much thinner. The plates were often 

 grouped, with the large pinacoid faces slightly diver- 

 gent, and isolated crystals, suitable for exact measure- 

 ment were only occasionally found. 



The forms which were observed on Cs 2 HgBr 4 are : 



b, 010, i-i c, 001, m, 110, / c?, 011, 1-5 p, 221, 2 



i 



and on Cs,HgCl 2 "Br 2 , h, m and a second prism 130, i-S. The 

 end faces could not be made out. 



