Penfield — Crystallography of Cesium-Mercury Halides. 321 



The forms and angles are as follows : 



CsHgoClBr 4 

 &, 010, i-% 

 in, 110, / 

 d, Oil, l-i 



Measured. Calculated. 

 to Am, 110 a 110 = *60° 44' 

 I a b, twin = 60 35 60° 44' 



0sHg 2 Br 5 



6, 010, i-i 

 n, 120, i-2" 



d, Oil, «'-? 



e, 014, l-% 



Measured. Calculated. 

 b*b, twin = *61° 5' 

 &acZ, 010 a 011 = *41 

 Sa», 010a 12C = 40 20 40° 17' 

 b~e, 010 a 014= 73 25 73 57 



CsHg- 5 Cln, 

 CsHg 5 ClBr 1( 



Type 1 : 5. 

 a : b : c 

 Monoclinic 0-7233 : 1 : 0-4675 

 07111 : 1 : 0-4561 



(3 = 85° 51' 40" 

 (3 = 85 29 



31. 



The chloride was made in prismatic crystals, fully 

 10 mm long, and having the habit shown in fig. 31. 

 The forms and angles are as follows : 



TO, no, / 



d, oil, i-i 



e, 101, l-i 



/, 101, 



The dome/* was usually wanting. 



Measured. Calculated. 

 TO a m, 110 a 110 = *71° 37' 

 d acZ, Oil a 011 = *50 

 TOAe, 110 a 101 = *66 8 

 d ~e, 01 1a 101= 4121 41° 20+' 



■l-i 



Measured. Calculated. 



m*d, 110 a 011 = 72° 31' 72° ai'-J- 



TOacZ, 110 a Oil = 78 49 78 48" 



d a/, Oil a 101 =39 30 39 29* 



e a/, 101 a 101 = 65 43 65 42" 



The chlorobromide CsHg B ClBr I0 is much 

 more insoluble than the chloride and was 

 made in crystals, which were not over -§- mm 

 in greatest diameter. The habit is shown 

 in fig. 32 and is very different from that of 

 the chloride. The forms and angles are as follows 



TO, no, i" 



Measured. 

 m, 110 a 110 = *70 o 40' 

 d, Oil a 011 = *48 54 



d, 011, l-i e, 101, l-i 



Measured. Calculated. 

 d Ae, 011 aIOI = *40° 58' 

 toa^, 110a011= 72 40 72° 40' 



The crystals are strongly double refracting and the little 

 tables show in convergent polarized light a bisectrix nearly 

 normal to e. The plane of the optical axes is the clinopina- 

 coid and the optical axial angle is small. The interference 

 phenomena are very interesting when observed through colored 

 glasses. In the hyperbola position the figure is almost uni- 

 axial when viewed through red glass, while with blue the 

 hyperbolae are separated, probably as much as 15°-20°. 



Sheffield Scientific School, New Haven, June, 1891. 



