392 



Allen and Crenshaw — Sulphides of Zinc, 



Mercuric Sulphide. 



Amorphous. — The tendency to aggregate into spherical 

 masses, which is so marked in amorphous cadmium and zinc 

 sulphides, has not appeared in amorphous mercuric sulphide. 



Crystalline. — At least three crystalline modifications of HgS 

 have been prepared. Of these, cinnabar crystallizes best, in 

 sharply-bounded nearly equi-dimensional, red, hexagonal prisms 



Fig. 8. 



26 



















/ 



/ 





















25 











/ 



A 



/ / 

 / / 



/ 











^ 

































y 



















yU/* 700 000 500 



Fig. 8. Dispersion of greenockite and of amorphous cadmium sulphide. 



or tables having very strong positive double refraction and 

 very high refractive indices. The index a> of crystals formed 

 by sublimation was approximately matched under the micro- 

 scope in Li-light with a glassy mixture of Sb 2 S 3 . and As 2 S 3 . 

 and found' to be equal within the limits of error (about ±*02) 

 to © of natural cinnabar, i.e., 2'81; e u , which is 3 - 14, is too 

 high to be determined microscopically. 



The new form, B'-HgS, is prismatic in habit and has not 

 been produced in crystals exceeding ■003 mm in diameter and 

 •03 mm in length. These crystals always taper toward the ends 



