Cadmium, and Mercury. 395 



gone, temperature and acid concentration have proved the defi- 

 nitive factors. The higher the temperature for a given acid 

 concentration the greater is the percentage of sphalerite (the 

 stable form) crystallized ; and the higher the acid concentration 

 for a given temperature the greater is the percentage of 

 wurtzite (the unstable form) crystallized. 



Quantitative work has shown previously that the same rule 

 holds for the disulphide of iron. Here the stable form is 

 pyrite and the unstable, marcasite. 



4. A preliminary study was made of the precipitation of 

 zinc by hydrogen sulphide from solutions of variable acid 

 concentration. 



5. We have obtained only one sulphide of cadmium, the 

 mineral greenockite, whatever method was used in its prepara- 

 tion. Very pure large crystals were prepared by Lorenz' 

 method, viz., the action of hydrogen sulphide on cadmium 



„,,,.,,, , mineral at 25° 

 vapor. 1 he density of these crystals, — -5—, was 4-820. 



The refractive indices were found to be e Na = 2 - 529, o> Na = 

 2-506. 



The various hues of different preparations of cadmium 

 sulphide do not depend, as has been claimed, upon different 

 allotropie forms ; they depend first on whether the substance 

 is crystalline or amorphous. The color of the amorphous 

 products depends chiefly on the size of the grains, the yellow 

 products consisting of more minute particles, but it is also 

 influenced by the nature of the surface of the individual grains, 

 and their forms. 



6. Mercuric sulphide exists in three different crystalline 



forms, viz. : cinnabar, cr-HgS, which is readily prepared by 



digesting any other form of mercuric sulphide with a solution 



of ammonium sulphide or alkali sulphide ; metacinnabar, 



a'-HgS, which is precipitated from dilute acid solutions of 



mercuric salts by sodium thiosulphate; and a new crystal form, 



yS'-HgS, which is obtained from 'more concentrated neutral 



solutions of mercuric salts in a similar way. 



mi . , „ . mineral at 25° „„»:„ 



Ihe density of cinnabar, - — — , = 8-176. 



J ' water at 4° ' 



The specific gravity of a'-HgS at 25° averaged 7 - 60 as 

 compared to about 7 - 7 for the natural mineral. For reasons 

 stated in the text the latter figure is doubtless too high. 



yS'-HgS has only been obtained in the form of a tine crys- 

 talline powder, having practically the same color as vermilion. 

 It is hexagonal. The specific gravity at 25° averaged 7-20. 

 The indices of refraction for 650 /j,fi were: o> Li r=2-61, e Li 

 = 2-85. 



7. Cinnabar is the stable form of mercuric sulphide at all 

 temperatures up to its sublimation point, which is about 580°. 



