ZINC. 313 



Metallic Zinc is obtained by heating in retorts the oxide or 

 carbonate mixed with charcoal, when decomposition takes place. 

 The liberated metal is vaporized, and distils into suitable receivers, 

 where it solidifies. 



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

 becoming coated with a film of oxide and carbonate ; it has a crys- 

 talline structure and is, under ordinary circumstances, brittle ; when 

 heated to about 130-150C. (260-302 F.) '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 300 C. (572 F.), 

 it loses its malleability and becomes so brittle that it may be pow- 

 dered ; at 410 C. (760 F.) 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 used by itself in the metallic state or fused together with 

 other metals (German silver and brass contain it) ; galvanized iron 

 is iron coated with metallic zinc. 



Zinc combines- with mercury forming a crystalline amalgam of the compo- 

 sition Zn 2 Hg. As a constituent of dental amalgam alloys zinc hastens the 

 setting, aids in controlling shrinkage and to some extent prevents discoloration. 

 While zinc unites with tin in all proportions forming excellent alloys for dental 

 dies, it is not suitable for alloying with lead. 



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

 salts, most of which have a white color, 



As has been pointed out in Chapter 24, zinc bears a close chemical relation- 

 ship to magnesium, and both these metals resemble cadmium in their chemical 

 properties. In fact,, the three elements magnesium, zinc, and cadmium form a 

 natural group similar to that of the alkali metals or the alkaline earth metals. 



Zinc oxide, Zinci oxiduni, ZnO = 80.78 (Flores zinci, Zinc-white), 

 may be obtained by burning the metal, but if made for medicinal 

 purposes, by heating the carbonate, when carbon dioxide and water 

 escape and the oxide is left : 



3[Zn(OH) 2 J.2ZnCO 3 = 5ZnO -f 2CO 2 + 3H 2 O. 



It is an amorphous, white, tasteless powder, insoluble in water, 

 soluble in acids; when strongly heated it turns yellow, but on 

 cooling resumes the white color. 



Zinc hydroxide, Zn(OH) 2 , is obtained by precipitating zinc salts 

 with the hydroxide of sodium or ammonium ; the precipitate, how- 

 ever, is soluble in an excess of either of the alkali hydroxides. 



