Penfield — Crystallography of Caesium-Mercury Ralides. 315 



the simple habit shown in fig. 14. The faces were rounded 

 and uneven so that only approximate measurements could be 

 made. 



The forms and angles are : 



a, 100, i-l 



b, 010, i-i 



c, 001, O 

 m, 110, I 



Cs 2 HgI 4 

 Measured. Calculated. 



c A a, 001, 100 = *69° 56' 

 to a m, 110 -s 110 = *77 58 

 a ~ d, 100 a 201 = *41 25 

 c ~e, 001 „ 011= 40 55 41° 1' 



(Z, 201, 2-1 

 e, 011, 1-i 



Cs 2 HgBr 2 I 2 



Measured approximately. 



66° 41' to 66° 47' 



77 16 to 77 57 



The cleavage of both salts is perfect parallel to the base, 

 less so parallel to the clinopinacoid. With Cs 2 HgI 4 the plane 

 of the optical axes is at right angles to the symmetry plane and 

 clinopinacoid cleavage sections show in convergent polarized 

 light an obtuse bisectrix, which is axis of least elasticity. The 

 axis of greatest elasticity makes an angle of about 50° with 

 the vertical axis in the acute angle /3. 



CsHgCls 



CsHgClBr.2 



CsHgBr 3 " 



CsHgBrI, 



CsHgr 3 not measured. 



Type 1:1. 



a : b : c 

 Isometric and Orthorhombic 0-57735: 1 : 0-40884 



" " " approximately like the above. 



" " Monoclinic 1-0124 : 1 : 0-70715, fi — 87° V 



" 0-978 : 1: 0-743 , /3 = 87° 3£' 



The first three compounds are dimorphous and, from solu- 

 tions containing an excess of alkali halicle, they all crystallize 

 in cubes. These sometimes have their edges truncated by 

 small dodecahedron faces, less often beveled by 210, i-2. The 

 crystals show a slight action on polarized light and give an ex- 

 tinction parallel to the diagonals of the cube, but this anomaly 

 is probably due to some internal tension, for when crushed 

 the fragments are isotropic. No cleavage could be detected. 



CsHgClg was repeatedly recrystallized from water and 

 always two types were observed. One of these was confined 

 to those crystals which were attached to the sides of the beaker, 

 while those which grew 

 more in the interior had 

 an entirely different habit. 

 The crystals of the first 

 type averaged about 2 mm 

 in greatest diameter and 

 had the habit shown in figs. 15 and 16. The forms and angles 

 are as follows : 



