242 Chemistry. [ Lect tire 3 2 . 



the operation of reduction becomes therefore ne- 

 cessary. It is often proper to add earths to the 

 ores, as they frequently contain earths not so 

 easily fusible, but which, by mixture with others, 

 become so. The fire being kept up for some 

 time, the ore melts, and as it passes through the 

 fuel, which is generally charcoal of wood, the 

 oxygen which the calx contained is drawn off by 

 the charcoal, forming carbonic acid gas, and the 

 metal falls into the bason constructed for that 

 purpose in the furnace. 



Thus the metal is obtained free from earthy 

 and stony matter, and generally from arsenic and 

 sulphur, but it sometimes contains other metals ; 

 thus copper has always with it more or less of 

 iron, silver, &c. 



Some, as lead ores when rich, are treated by 

 immediate fusion, without previous roasting: 

 for though it would give a greater quantity of 

 the metal, it would be too expensive. There are 

 many ores in which the metal exists in the state 

 of an oxide. Here previous roasting would be 

 of no advantage. The ores of silver and gold 

 require certain additions to them, to attract the 

 sulphur and arsenic, and to melt the other matter 

 which is mixed with them, so as to dispose them 

 to separate. 



To this account of ores must be added some 

 observations on assaying. It seldom happens 

 that the same person takes the ore out of the 

 mine and refines it. But to know the value of 



