MERCURY. 261 



moderate heat, mercury may be gradually converted into 

 red oxide, but a higher temperature expels the oxygen 

 thus absorbed, and the oxide is again converted into 

 metal. This production of a metal from an oxide, by 

 heat alone, is characteristic of the noble metals. They 

 are loth to obscure their splendor in rust ; if it is forced 

 upon them, they need but little assistance of heat to 

 throw it off and re-assume their original beauty. 



653. AMALGAMS GLASS MIRRORS. 



What are 



amalgams? Mercury combines with many metals form- 

 n?rs sUvered? * n & com P oun ds which are called amalgams. 

 When the mercury is in large proportion 

 they are fluid. Gold, silver, and lead, for example, 

 may be dissolved in mercury. This solvent power of 

 mercury is usefully applied in extracting gold from the 

 rocks which contain it. The beautiful silvering of 

 mirrors consist of an alloy of tin and mercury. Tin 

 foil is applied to the glass, and being afterward drenched 

 with mercury, the excess is removed by pressure. The 

 tin has thus absorbed about one-fourth of its own 

 weight of mercury. 



654. A copper coin may be similarly 

 coppercoin be silvered, by rubbing with metallic mer- 

 cur Y> or keeping it well moistened for some 

 time with a solution of mercury in nitric 

 acid. If the solution is quite acid, it must first be 

 nearly neutralized by ammonia. The coin is to be af- 

 terward polished. The chemical action which takes 

 place in this case is similar to that explained in the 

 case of the lead tree. By drawing a line across a thin 

 brass plate, with a pen dipped in solution of mercury, 



