MAGNESIUM AND ZINC. IONIC EQUILIBRIA 449 



In the case of zinc, because it is a volatile metal, the heating of 

 the mixture of oxide and coal is conducted in retorts, from which 

 the metal issues as vapor. The mixture is placed in fire-clay 

 cylinders (4 to 5 ft. long), which are arranged in several tiers in an 

 oblong, gas-heated furnace (Fig. 109). A fire-clay receiver is 



FIG. 109 



luted on with clay. The carbon monoxide burns with a blue flame 

 at the nozzle of each receiver, while the zinc condenses to liquid 

 within it. From time to time the liquid metal is rakdek^ut into 

 a traveling iron pot, from which it is poured into moulds. 



Properties and Uses. The metal is silvery and crystalline. 

 At 120 to 150 it can be rolled into sheets between hot rollers, at 

 200 to 300 it becomes brittle, at 419 it melts, and at 950 it boils. 

 The density of the vapor shows it to be monatomic. Zinc vapor 

 burns with a bluish flame, giving ZnO. In air the metal does not 

 rust, being protected by a non-porous coating of a basic carbonate 

 which adheres closely to the surface. 



Sheet zinc is used for gutters and cornices. Iron is coated 

 (galvanized) with zinc by thorough cleaning with dilute sulphuric 

 a~da"or tke sand blast, and dipping in melted zinc. Iron netting, 

 corrugated iron for sheds and roofing, and iron gutters, tanks, and 

 pipes are coated with zinc, either in this way, or by electroplating 

 (see p. 515). Sherardized iron is made by covering the article 



