314 Penfielcl — Crystallography of Caesium- Mercury Halides. 



The angles of Cs„HsrBr„ are 



Measured. 





Measured. 



Calculated. 



m^m, 110, 110=*59° 25' 



m^p, 110 a 221 



= 33° 58£' 



33° 58' 



b*d, 010 a Oil =*H4 12 









On this salt the dome d is always small and frequently want- 

 ing. The pyramid^ was only observed on a few crystals. In 

 convergent polarized light a bisectrix may be seen normal to 

 5, 010. The plane of the optical axes is the macro-pinacoid 

 and their divergence is so large that they cannot be measured 

 in air but in a-monobromnaphtaline the following values were 

 obtained : 



2H = 80° 12' for yellow, Na flame. 

 2H = 85 23 for red, Li flame. 



The dispersion is strong p^>v. The acute bisectrix is axis 

 of least elasticity, the double refraction is therefore positive. 

 The only angles on Cs 2 HgCl 2 Br 2 which were measured are : 



m*m, 110 ^ llO = *59° 6' and 110 a 130 = approx. 30° 53', calculated 30° 59' 



In convergent polarized light a bisectrix may be seen normal 

 to o, 010. The plane of the optical axes is the macropinacoid, 

 and their divergence is large. The axis of greatest elasticity 

 is normal to b. 



Only very fine needles of Cs a HgCl„I 2 were obtained, which 

 were too small for measurement. These appeared under the 

 microscope as striated prisms, with their obtuse edges rounded 

 by oscillatory combinations. In polarized light they show a 

 parallel extinction and in convergent light a biaxial interfer- 

 ence figure, the plane of the optical axes being the vertical 

 pinacoid. The acute bisectrix is axis of least elasticity. 



The crystals of Cs 2 HgT 4 , which were frequently several cen- 



13. 



timeters in diameter, showed a variety of 

 habits represented in figs. 11, 12 and 13. The 

 crystals of the latter habit are usually attached 

 at one end and taper toward the free extrem- 

 ity, owing to a tendency to develop vicinal 

 pyramids in the zone d-h. 



The crystals of Cs„HgBr 2 I 2 , which were 

 examined, were about 2 mm in diameter and had 



