REFINING METALS. 515 



prive it of the carbonic acid ; then to free it from 

 other impurities, it must be dissolved in spirits of 

 wine, (which dissolves alcalis and no other salt) and 

 the solution evaporated to dryness. It is then pure 

 and powerfully caustic. 



REFINING METALS. 



The term refining signifies the purification of 

 some substance : but we mean to confine it at pre- 

 sent to the separation of gold, silver, and copper 

 from each other, and obtaining each of them in a 

 pure state. 



Cupellation. 



Gold and silver being the only metals capable of 

 withstanding the action of very strong heat, are 

 therefore called perfect metals. All other metals 

 are reduced to the state of oxides when exposed to 

 a violent fire with access of air. Gold and silver 

 may, therefore, be purified from all the baser metals 

 by keeping them fused till the alloy be destroyed : 

 but this process would be very expensive, from the 

 great consumption of fuel, and would be exceed- 

 ingly tedious. A shorter and more advantageous 

 method of performing this operation has been dis- 

 covered. 



A certain quantity of lead is added to the alloy 

 of gold and silver, and the whole is exposed to the 

 action of the fire. 



Lead is one of the metals which is most quickly 

 converted by heat into an oxide, which is easily 

 melted into a semi- vitrified, and powerful vitrifying 

 matter, called litharge. By increasing the proportion 

 of imperfect metals, it prevents them from being so 



p 4 



