160 METALS AMU THEIR COMBINATIONS. 



27. ZINC. 



Zn'' = 64.9. 



Occurrence in nature. Zinc is chiefly found either as sulphide 

 (zinc-blende), ZnS, or as carbonate (calamine), ZnCO 3 ; it also 

 occurs in combination with silicic acid as silicate. 



Metallic zinc is obtained by heating the oxide or carbonate 

 mixed with charcoal in retorts, when decomposition takes place. 

 The liberated metal is vaporized, and distils into suitable re- 

 ceivers, where it solidifies. 



Zinc is a bluish-white metal, which slowly tarnishes in the 

 air, becoming coated with a film of oxide; it has a crystalline 

 structure and is, under ordinary circumstances, brittle; when 

 heated to about 130-150 it is malleable, and may be rolled or 

 hammered without fracture; zinc thus treated retains this 

 malleability when cold ; the sheet-zinc of commerce is thus 

 made. When zinc is further heated to about 200 it loses its 

 malleability and becomes so brittle that it may be powdered ; 

 at 410 it fuses, and at a bright red heat it boils, volatilizes, and, 

 if air be not excluded, burns with a splendid greenish-white 

 light, generating the oxide. 



Zinc is frequently used in the metallic state by itself or fused 

 together with other metals (German silver and brass contain it) ; 

 galvanized iron is iron covered with a coating of metallic zinc. 



Zinc is a bivalent metal, forming but one oxide and one series 

 of salts, which have all a white color. 



Zinc oxide, Zinci oxidum, ZnO = 80.9 (Oxide of zinc], may be 

 obtained by burning the metal, but if made for medicinal pur- 

 poses by heating the carbonate, when carbon dioxide escapes 

 and the oxide is left : 



ZnC0 3 = ZnO + CO 2 . 



It is a soft, pale yellowish or nearly white, tasteless powder, in- 

 soluble in water, soluble in acids ; when strongly heated it turns 

 yellow, but reassumes the white color on cooling. 



Zinc hydrate, Zn2HO. is obtained by precipitating zinc salts 

 with the hydrate of sodium or ammonium ; the precipitate, 



