SHEET METALS 449 



521. Use of Zinc. Zinc is used with copper to form brass 

 and othei alloys, and as a covering to protect iron from the 

 action of the atmosphere or of sea water. By placing blocks 

 of zinc in metallic contact with the iron of boilers, corrosion 

 is prevented. Zinc plates are used for many purposes; in 

 roofing they are valuable for their lightness, since they weigh 

 but one-sixth as much as lead. 



522. Chemical Characteristics of Zinc. Air attacks zinc 

 very slowly and even the pressure of moisture forms only a 

 basic carbonate which acts as a protective coating. Conse- 

 quently, only the outer layer is affected. When heated, zinc 

 burns in the air forming zinc oxi'de. Dilute hydrochloric 

 and sulphuric acid act on zinc and form hydrogen in addition 

 to the salts. 



523. Properties of Mercury. Mercury, also called quick- 

 silver because it looks like silver and flows quickly, is the 

 only metal that is liquid at an ordinary temperature. Small 

 drops of pure mercury are sometimes found, but it is usually 

 made from an ore, called cinnabar (mercuric sulphide), 

 made up of mercury and sulphur. The metal is easily 

 obtained by roasting the ore and vaporizing the mercury 

 which is condensed and purified. The reaction is: 



HgS + 2 Hg + S0 2 



Mercuric Oxygen Mercury Sulphur 



Sulphide Dioxide 



The sulphur burns to sulphur dioxide and the mercury 

 passes off as a vapor. The vapor is conducted through 

 tubes into a vessel containing water, where it is condensed 

 into liquid mercury. 



