ix SULPHUR AND PHOSPHORUS 173 



Compounds of this type are of frequent occurrence 

 amongst minerals, such as : 



Pyrites (Fig. 8) (Iron or copper and sulphur) 



Cinnabar (Mercury and sulphur) 



Galena (Fig. 38) (Lead and sulphur) 



Blende (Zinc and sulphur) 



The presence of sulphur was probably detected first in 

 pyrites, a mineral which is very rich in sulphur, and yields 



FIG. 38. LARGE CUBE OF GALENA. 

 The triangular faces on the top corners are parts of an octahedron. 



considerable quantities of this element when distilled : in 

 other cases its presence was suspected on account of the 

 similarity of the mineral to artificial compounds of the 

 metals with sulphur. 



The burning of sulphides. When a metallic sulphide 

 is heated in air, the metal usually burns to the calx or oxide, 

 whilst the sulphur escapes in the form of sulphurous 

 anhydride. In this way the presence of sulphur may be 

 detected, even when it cannot be separated in the free state. 



The burning of sulphide to oxide was used by Berzelius 



