324 SMITH'S INTERMEDIATE CHEMISTRY 



mineral is heated (air excluded), arsenic passes off as vapor and 

 condenses as a crystalline metallic powder : 



FeSAs->FeS + Ast. 



Most natural sulphides (PbS, FeS 2 , SnS 2 , CuFeS 2 , etc.) contain 

 more or less arsenic, which takes the place of a part of the sulphur. 

 When these ores are oxidized in a draft of air (roasted), as one 

 step towards the ultimate extraction of the metal, the metal, 

 sulphur, and arsenic are all converted into oxides. The sulphur 

 dioxide passes off as gas, but the arsenic trioxide As 2 C>3, a solid, 

 settles in the flues. By distilling the deposit with carbon, free 

 arsenic is obtained: 



As 2 O 3 + 3C->2As I 4- 3CO f . 



Properties and Uses. The element has a silvery luster, but 

 tarnishes quickly. When it is heated, its vapor reaches a pres- 

 sure of 760 mm. before the melting-point is attained, so that the 

 metal sublimes without melting. 



The metal burns in air with a bluish- white, flame, giving clouds 

 of white particles of the trioxide As 2 3 (poisonous). 



In the making of small shot, about 0.5 per cent of arsenic is 

 added to the lead. The latter is then run into a vessel, with a 

 perforated bottom, placed at the top of the shot tower. The 

 arsenic^ like any dissolved substance (p. 119), lowers the freezing- 

 point of the solvent (lead), and delays the solidification of the 

 lead until the drops have assumed perfect spherical form. At 

 the foot of the tower the drops fall into water and are cooled. 

 The arsenic also renders the metal harder than pure lead, and 

 less apt to be deformed during the explosion of the cartridge. 



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 Arsenic Trioxidei^Agg)^ This oxide, formed when arsenic 



burns, is a white crystalline- powder (white arsenic). It is acidic 

 and forms salts with bases. Paris^green and Scheele's green are 

 made by dissolving the oxide in boiling water and adding a copper 



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