METALS. 269 



in any other way, for staining porcelain, or for the manufacture of bluo 

 gla?s. 



Smalt and azure, which have a rich blue color, are made by fusing 

 zaffie with glass; or by calcining a mixture of equal parts of roasted 

 cobalt ore, common potash, and ground glass. The zaffre is used for 

 coloring glass, and for painting enamel and pottery ware. The arsenic 

 volatilized in the above process is condensed in chambers ; it consti- 

 tutes the greater part of the arsenic of commerce. The separation of 

 the nickel from ores rich in this metal, is sometimes effected by exposing 

 the moistened ore to the atmosphere. The nickel is unaltered, while 

 the other metals are oxydized. 



The annual yield of zaffre or smalt, in Saxony, amounts to 8000 

 cwt. ; in Bohemia, mainly from Schlackenwald, 4000 cwt. ; in iht 

 Reisengebirge, in Prussia, 600 cwt. ; at Kongsberg, in Norway 

 4000 cwt. 



19. ZINC. 



Zinc occurs in combination with sulphur, oxygen, silica, 

 carbonic acid, and sulphuric acid. It is also found in com- 

 bination with alumina, constituting one variety of the spe- 

 cies spinel. 



The ores of zinc are infusible, or very nearly so ; but 

 they yield on charcoal, with more or less difficulty, white 

 fumes of the oxyd of zinc. Specific gravity below 4*5. 



blende. — Sulphuret of Zinc. I 



Monometric. In dodecahedrons, octahedrons, and other 



allied forms, with a perfect dodecahedral cleavage. Also 



1 massive ; sometimes fibrous. 



Color wax-yellow, brownish- 

 yellow, to black, sometimes green 

 or red ; streak white, to red- 

 dish-brown. Luster resinous or 

 waxy, and brilliant on a cleavage 

 face ; sometimes submetallic. — 



Transparent to subtranslucent. Brittle. H=3*5 4. Gr= 



4*0- — 4*1. Some specimens become electric with friction, and 

 give off a yellow light when rubbed with a feather. 



Composition : zinc 66-72, sulphur 33*28. Contains fre- 

 quently a portion of sulphuret of iron when dark colored ; 



What are smalt and azure 1 How are they used in porcelain paint- 

 ing? What is said of the ores of zinc? What is the crystallization 

 of blende. What are its luster, color, and other physical characters 1 

 Of what does it consist ? 



