INSTRUMENTS AND (INIMICAL TESTS 579 



nm>t he carried to tin- cud without interruption. In cupellation 

 thr lead is oxidi/ed, the oxide is both volatili/ed and absorbed ly 

 the Itone ash. As the percentage of lead decreases, the globule 

 become> more spherical and a thin iridocent film of oxide of lead 

 is constantly moving over its surface; as the last of the lead is 

 driven off, this film disappears, the button brightens, and a dis- 

 tinct change in color is noticed. This change must be carefully 

 watched and oxidation stopped, as silver is volatile also, and con- 

 siderable of it may be lost if the heating is continued after all the 

 lead has been oxidized. The silver button will be white unless 

 it contains considerable gold, when it will be yellowish. It will 

 also contain the metals of the platinum group if the ore contained 

 any. If the ore contained no silver, the lead will disappear en- 

 tirely in the bone ash, but the spot where it disappeared should 

 always be examined with a lens. 



6. Wet test. The powdered mineral is dissolved in nitric 

 acid, the solution cooled and filtered; a drop of HC1 is added, 

 when if silver is present a white precipitate of silver chloride, 

 AgCl, is formed, becoming dark on exposure to light. Lead and 

 mercurous mercury will also yield white chlorides. Lead chloride 

 is soluble in hot water ; if the solution containing the precipitate is 

 boiled, lead chloride will redissolve ; or it is filtered and washed with 

 boiling water, when if the precipitate dissolves, it is lead ; when 

 insoluble, ammonia is poured over it on the filter ; if it blackens it 

 is mercury ; if there is no effect it is silver. 



Illustration. Any ore containing silver may be used for these 

 tests. 



Mercury, Hg. Atomic weight, 200.6. Fusing point, - 38.85 C. 



a. The substance is mixed in powder with 3 parts dry soda, 

 placed in a closed tube, with a little pure dry soda placed on top 

 of the charge. The tube is now heated in the burner flame or 

 O. F. The mercury is reduced and volatilized, forming a gray 

 sublimate on the cold walls of the tube. This sublimate is com- 

 posed of minute globules of mercury. They may be rubbed to- 

 gether, poured out in a watch glass, and examined with a lens. 



Illustration. Use cinnabar, HgS. 



Lead, Pb. Atomic weight, 206 . 9. Fusing point, 327 C. 



a. Reduction test. Lead is reduced to a malleable gray metal 

 with soda and borax in R. F. on coal. The powdered mineral is 

 mixed to a stiff paste with water, 4 parts soda, 1 part borax, and 

 some coal dust ; this charge is fused in R. F. on coal ; the resulting 



