DESULPnuftATION OF METAJ-S. 20S 



celve I fan account for the numerous and complicated phe- 

 nomena they exhibit. 



Whatever care be taken to roast t^alena in a roasting test, In the small 

 It is impossible to convert the whole of the sulphur into sul- ^^i^Ja^^foJln*^ 

 phurous acid, and avoid the formation of sulphuric. The 

 resrlt always exhibits a mixture of oxide and sulphate of 

 lead. 



In roastings in the large way, on hearths prepared for the In the large 

 purpose, the proportion of sulphate of lead is still more ^^^5' st»ll more, 

 considerable, being in the ratio of the temperature, and the 

 facility with which the air pervades the ore. Numerous ana- 

 lyses made in the School of Mines lead me to believe, that 

 the roasted ore of the mines at Pezey contains from a third 

 to half its weight of sulphate of lead ; whence it follows, 

 that, even supposing the whole of the galena to have been -^ 



decomposed, the roasting has not separated half the sulphur 

 it contained. 



The reverberatory furnace is employed with great success In the reverbc- 

 to roast ores of sulphuretted lead. In some works indeed, '■^^°''>' t^rnace 

 as at Poullaouen, such a complete separation of the sulphur a complete se- 

 is accomplished in this furnace, that, when the roasting is P^Y'\^'«n ^'f the 



' , . . o sulphur may 



judged to be finished, nothing more than the addition of be effected. 



charcoal is requisite, to obtain directly a large quantity of 

 metallic lead. It cannot be doubted however, but a great Yetsulphatei5 

 deal of sulphate of lead is formed, which, as we have seen, **^'""^^"" 

 is a necessary result of the action of air on galena subjected 

 to a high temperature : besides, the chimneys of the fur- 

 naces are filled with it. The decomposition of this sulphate 

 by the charcoal produces a sulphuret, or lead matt, and 

 though sulphurous acid may be evolved, it is very difficult 

 to explain how the addition of charcoal causes the lead to ^ 



flow immediately in considerable quantity. I have imagined 

 that the sulphate of lead was decomposed during the roast'- 

 ing; and that after this operation nothing remained but an 

 oxide very little mixed : and I think I have found the cause 

 ©f this decomposition in the action of the galena still unde- 

 composed on the sulphate formed. The following experi- 

 ments will make known the nature and result of this action. 



Into a retort I put a mixture of one part of powdered sul- Sulphuret 1 

 phuret of lead and three of sulphate; which at first I heated J^ef^'^'^^^^j^t* 



slowly, 



