312 Penfield — Crystallography of GoBsium-Meroury Halides. 



The crystals have ortho-rhombic optical properties. When 

 lying on their prismatic faces all show in polarized light an ex- 

 tinction parallel to the vertical axis, and in convergent light a 

 trace of the ring system can be seen, indicating that the plane 

 of the optical axes is the base. 



"We have here an excellent illustration of monoclinic hemi- 

 hedrism in the orthorhombic system. Among all of the crys- 

 tals which were examined, there was not one which had aholo- 

 hedral termination. The forms d' and 7/, when present, were 

 always smaller than the corresponding forms d and p, while e 

 was only observed to the right above. Also the right handed 

 vertical edge of the prism showed a tendency toward a skeleton- 

 like growth, which was not observed to the left. In measuring 

 the crystals great pains were taken to detect a monoclinic char- 

 acter by the angles, but none could be found. Of course the 

 three salts may be regarded as monoclinic, with an angle /3, 

 differing so little from 90° that it can not be detected by the 

 goniometer, but against such a supposition are the arguments, 

 that the crystals have orthorhombic optical properties and, 

 while there is a variation in the axial ratios of the series as 

 bromine is substituted for chlorine, there is no change in the 

 angle /3, as would be expected if the salts were monoclinic. 

 In this connection it is interesting to note that while the chlo- 

 ride and bromide are very similar in their axial ratios, the 

 chemically intermediate chloro-bromide is not crystallographi- 

 cally intermediate. 



At the present time there seems to be no other known com- 

 pound which illustrates this hemihedrism. Different substances, 

 which have been referred to this class, as datolite or wolfra- 

 mite, for example, have been shown by accurate measurement, 

 or a study of their optical properties, to be truly monoclinic. 

 Professor P. Groth, in the last edition of his Physikalische 

 Krystallographie, has not mentioned this hemihedrism as a pos- 

 sibility in the orthorhombic system, although in the former 

 edition of his work and in most treatises on crystallography it 

 is recognized. 



