Penjield — Crystallography of Caesium- Mercury Halides. 311 



Art. XL. — On the Crystallography of the Caesium- Mer- 

 curic Halides ; by S. L. Penfield. 



The salts to be described in this paper were prepared by 

 Prof. H. L. Wells, and their chemical description has been 

 given by him in the September number of this Journal. 



The crystals were all measured on a Fuess reflecting goni- 

 ometer, model II, ard great pains were taken to select the best 

 measurements as fundamental. In a few cases, where the 

 crystals were very small and the reflections of the signal, 

 therefore, rather broad, the mean of a series of measurements 

 was used. The axial ratios are given in tabular form at the 

 beginning of each separate chemical type and the fundamental 

 angles, from which these are derived, are marked by an 

 asterisk in the table of angles accompanying each salt. 



Type 3:1. 



a : 



Cs 3 HgCl 5 . Orthorhombic, monoclinic hemihedrism 0-7976 



Cs 3 HgCl 3 Bro " " " 7882 



Cs 3 HgBr 6 " " " 0-7966 



Cs 3 HgI 5 , Orthorhombic. sphenoidal hemihedrism 0*5362 



or | a :~b :% c = 0-8043 : 1 



0-6605 



0-6527 



0-6656 



0-97975 



0-6532 



Cs,HgBr s I s 



The first three salts have ex- 

 actly the same habit and crystal- 

 lize in slender prisms, attached at 

 one end and terminated at the 

 other by faces which are arranged 

 with monoclinic symmetry, figs. 1, 

 2 and 3. The crystals were sel- 

 dom over l mm in diameter, but the 

 faces were perfect and admitted 

 of accurate measurement. The 

 forms and angles are : 



approximately like Cs 3 HgI 5 



m, 110, I 

 d, 011, \-i 



d\ 011, 'l-i 

 e, 021, 2-1 



p, in, i 

 j»", in, 'i 



Cs 3 HgCl 5 Cs 3 HgCl 3 Br» 



Measured. Calculated. Measured. Calculated. 



TO /. TO, 

 TO *p, 

 TO A_p, 

 TO /. d, 



to /. e, 

 d ~ e, 

 d^d' 

 d *p, 



110 



110 



110 =*77 c 



1 1 1 = *43 



76' 

 *43 



33' 



29 



76° 30' 



110 ~ 111 = 

 110 /.011 = 

 110 /s 021 = 

 011 ,.021 = 

 011/. 011 = 

 011 A lll = 



80 41 

 69 55 

 60 8 

 19 29 

 66 53 

 34 39 



69 

 60 

 19 

 66 

 34 



41*' 



54 



11 



26 



53* 



39 



70 14 70 13 



19 28 



*66 16 



34 37 



19 25 



34 44 



Cs 3 HgBr 6 

 Measured. Calculated 

 *77° 5' 



*43 6-i 

 80 37 

 69 49 

 60 10 



67 16 

 34 47 



69 



60 



67 

 34 



36*' 

 48 



Am. Jour. Sci. — Third Series, Vol. XLIV, No. 262.— October, 1892. 

 21 



