Cadmium, and Mercury. 



343 



several members of the U. S. Geological Survey, who will be 

 specially mentioned as occasion demands. 



I. The Sulphides op Zinc : Sphalerite and Wurtzite. 



The sulphide of zinc occurs in nature crystallized in two 

 different forms, the common sphalerite, or blende, which belongs 

 to the regular system and the comparatively rare wurtzite 

 which belongs to the hexagonal system. Both are transparent 

 and straw-colored* when pure and are distinguished from each 

 other and from amorphous zinc sulphide by the presence or 

 absence of double refractionf and by the magnitude of the 

 indices of refraction. The latter for sphalerite is 2'37, which 

 in sodium light is slightly less than that of wurtzite for the ray 

 vibrating in the vertical axis, while the other index of the latter 

 mineral is 2-35 (Merwin). These properties were determined on 

 a very pure natural sphalerite from Sonora, Mexico, and on the 

 wurtzite formed from it by heating to the proper temperature. 

 The analysis of this mineral was as follows : 



Zn 66-98 



Fe 0-15 



S 32-78 



99-91 

 The specific gravity of the two minerals was also determined 

 on the same material by the pycnometer method at 25°. 



Table I. 

 Specific gravities of Sphalerite and Wurtzite. 



Sphalerite. 1 



Wurtzite. 



I. Sonora, Mex. 



II. Same locality. 



Formed by heating sphalerite 



mineral at 25° 

 water at 25° 



mineral at 25° 

 water at 25° 



mineral at 25' 

 water at 4° 



mineral at 25° 

 water at 25° 



mineral at 25° 

 water at 4° 



4-101 

 4-102 

 4-100 

 4-099 



4-oo: 



4-103 

 4-102 

 4-102 







4-098 

 4-100 

 4-098 

 4-098 

 4-100 







av. 4-100 



av. 4-102 



av. 4-090 



av. 4-099 



av. 4-087 



1 For the determination of sample I, only about 6'5 g. were used ; for 

 ihose of sample II, about 13 g. was available, and the latter values are sub- 

 ject to less error in consequence. 



* The blende from Franklin, N. J., and from Nordmark, Sweden, is 

 described as pure white. (Dana, A Textbook of Mineralogy, 1906, p. 61.) 

 Our purest synthetic products have all been straw-color. 



f For an interesting exception to isotropy in amorphous bodies see micro- 

 scopic part of this paper, p. 383. 



