574 ZINC. 



form,, owing to the volatility of the metal. It consists of a 

 crucible, covered above, with an iron tube in its bottom, of 

 which the upper open extremity is in the crucible, and the 

 other terminates over a vessel of water below the furnace. The 

 gaseous products and vapour of zinc escape by this tube, and 

 the latter is condensed in the water. Zinc may be purified by 

 a second distillation in a porcelain retort, but the first portions 

 of that metal which come over should be rejected, as they gene- 

 rally contain cadmium and arsenic. 



Zinc is a white metal, with a shade of blue, and possessing a 

 bright metallic lustre. It is usually brittle, and its fracture 

 exhibits a crystalline structure. But zinc, if pure, may be 

 hammered into thin leaves, at the usual temperature ; and com- 

 mercial zinc, which is impure and brittle at a low temperature, 

 acquires the same malleability between 210 and 300 : it may 

 then be laminated; and the metal is now consumed in the form 

 of sheet zinc for a variety of useful purposes. At 400 it again 

 becomes brittle, and may be reduced to powder in a mortar of 

 that temperature. The density of cast zinc is 6.862, but it may 

 be increased by forging to 7-21. Its point of fusion is 773, 

 (Daniell.) At a red heat, zinc rises in vapour, and takes fire in 

 air, burning with a white flame like that of phosphorus ; the 

 white oxide produced is carried up mechanically in the air, 

 although itself a fixed substance. Laminated zinc is a valuable 

 substance, from its little disposition to undergo oxidation. 

 When exposed to air, or placed in water, its surface becomes 

 covered with a grey film of suboxide, which does not increase ; 

 this film is better calculated to resist both the mechanical and 

 chemical effects of other bodies than the metal itself, and pre- 

 serves it. Zinc dissolves with facility in dilute hydrochloric, 

 sulphuric and other hydrated acids, by substitution for hydrogen. 

 In contact with iron, it protects the latter from oxidation in any 

 saline fluid. Zinc forms probably three oxides, the suboxide 

 referred to, the protoxide, and a peroxide, when the hydrated 

 protoxide is acted upon by a solution of peroxide of hydrogen ; 

 but of these, the first and last have not been studied, and the 

 protoxide is, therefore, the only well known oxide of zinc. 



Protoxide of zinc ; ZnO; 503.2 or 40.31. May be obtained 

 by the combustion of the metal in a stoneware crucible, as a 

 white powder, or by precipitation from its salts, by an alkali, 

 as a white hydrate. It is of a yellow colour at a high tempera- 



