318 Penfield — Crystallography of Caesium- Mercury Halides. 



a slightly stronger action on polarized light than those of 

 CsHgCl 3 . When lying on the large q faces, both show an 

 extinction parallel to the edges between p, q and d. 



On a crystal of CsHLgCl 3 the faces in the zone p, q, d, r 

 made prisms, which served for the determination of the follow- 

 in dices of refraction. 



Prism of 36° 54', 021 ^ 111, n, 

 Prism of 63 = 2Sf, 131 ^ 111. n, 



Na flame = 1-791 

 " = 1-792 



Li flame = 1-779 



ci .. _ 1 . 779 



The crystal was of course very small and the refracted rays 

 were not very bright, but the latter were well defined and 

 the double refraction was not strong enough to separate them 

 into two distinct rays. 



The author cannot give any satisfactory explanation of these 

 curious forms. They seem to illustrate a tetartohedral devel- 

 opment of the faces of an orthorhombic crystal, resulting in a 

 figure with triclinic symmetry. The mathematical relations 

 have been very carefully determined and the facts given. It 

 is hoped that a further study will throw some light on the 

 subject. 



The different crops of CsHgBr 3 which were examined 

 showed a variety of habits, represented by figs. 21, 22 and 23. 



21. 



22. 



23. 



The forms and angles are as follows 



c, 001. e, 201, — 2-1 



m, 110, / d, 101, l-i 



Measured. Calculated, 

 c * m. 001 * 110 = *87° 58i' 

 c~d, 001 ^ 101 =*35 52 

 c~e, 001^201 = 52 2S| 52° 30^' 

 c^o, 001 ~ 111 = 45 40 45 49 



o, 111, 1 y, 261.- 6-3 



p, 221, - 2 x, 131, 3-~3 



Measured. Calculated. 

 m ^ m, 110 ~ llO = *90° 38' 

 d ^x, 101 ^ 131 = 60 41 60° 40' 

 e ^p, 201 ^ 221 = 38 46 38 46£ 

 e *y, 201 a 261 = 67 21 67 28 



The crystals are seldom over 5 mm in diameter and sometimes 

 have a hemimorphic development, although this is not always 

 apparent. In the prevailing type, fig. 22,_there is perhaps a 

 tendency for 221 to predominate over 221 but this is not 

 great. The pyramids x and y were observed only with hemi- 

 morphic development, fig. 23. The crystals were tested for 

 pyro-electricity but no satisfactory results were obtained, 



