Chap. 1 8 .] Smelting cf Metals-. i 1 7 



planes or tables, the ftony matter being lighter is 

 wafnecl away, while the metallic remains behind. The 

 ropfling or burning is intended to expel the volatile 

 matters. Ores which contain much fulphur muft be 

 roafted in the operi air, but fuch as contain bur little 

 may be roafted in the furnaces which afterwards ferve 

 to fufe them. Some ores are fufible alone, others re- 

 quire to be -mixed with different fluxes. The methods 

 ct refining metals are extremely various, and depend 

 en particular chemical affinities, which will be men- 

 tioned under the head of each metal. 



The falts, and of them the acids in particular, have 

 great effect on metals. Metals unite with acids into 

 compounds, many of which cryftallize. The corro- 

 fivenefs of the acid is abated by its union with the 

 metal, but not in fo great a degree as by its union with 

 the alkalies or earths. Neither is the point of fatnra- 

 tion fo well marked in the union of an acid with a 

 metal as with an alkali. The fame acid may, in 

 many cafes, be united to the fame metal in different 

 proportions; when the acjd is in excefs the mafs is 

 deliquefcentj, when it is deficient k ieems to produce* 

 little other effect on the metal than to deftroy its tex- 

 ture, and reduce it to a friable and earthlike (late. 



Every metal, however, is not difpofed to unite 

 with every acid, though fome unite with all; others 

 with only one acid. The order alfo-in which the acids 

 attract the metals is different from that in which they 

 attract the alkalis. Metals attract the muriatic acid 

 mod ftrongly, next the vitriolic, and lad the nitrous. 

 Metals which diflblve in the fame acid differ very 

 much in the force with which they adhere to it, fo that 

 they may be employed to precipitate one another. 

 Thus, if we add to the folution of filver in aqua-fortis, 

 {juickfilv-er, it precipitates the filvcr ; copper, the 

 I 3 quickfilveri 



