On the Desulphuration of Metals. 79 



more considerable, it is regulated by the temperature and 

 by the facility with which the air penetrates the ore; nu- 

 merous experiments made in VEcole des Mines incline me 

 to think that the roasted schlich of the Pezey ore contains 

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

 even supposing the whole galena to be decomposed, the 

 roasting has not separated the half of the sulphur it con- 

 tains. 



The reverberating furnace may be employed with great 

 success in roasting the sulphurized ores of lead. In some 

 foundries, they produce in this kind of furnace so complete 

 a separation of the sulphur, that it is sufficient, when the 

 roasting is supposed to be finished, to add some charcoal, in 

 order to obtain instantaneously a great quantity of metallic 

 lead. It cannot be doubted, however, that a great quantity 

 of sulphate of lead is formed ; which, as we have already- 

 seen, is a necessary result of the action of the air upon ga- 

 lena exposed to a high temperature; the chimneys of the 

 furnaces are likewise filled with the above substance : the 

 decomposition of this sulphate by charcoal produces a suj- 

 phuret or a matte of lead; and although sulphurous acid 

 may be disengaged, it is very difficult to explain why the 

 addition of charcoal makes the lead flow instantly in a con- 

 siderable quantity. I thought that the sulphate of lead was 

 decomposed during the roasting, and that nothing remain- 

 ed after this operation, but an oxide a little mixed; and I 

 thought I discovered the cause of this decomposition in the 

 action of the galena, as yet undecomposed, upon the sulphate 

 formed. The following experiments will show the nature 

 and result of this action. 



I put into a retort a mixture composed of one, part of 

 pulverized sulphuret of lead, and three of sulphate *, and I 

 heated it at first but slowly. When the retort was red-hot, a 

 considerable disengagement of sulphurous acid gas took place 

 which lasted an hour ? when the retort melted; the residue 

 presented a mixture of oxide and of sulphate of lead. I 

 ascertained that the sulphurous acid which had been collect- 

 ed in the water was not mixed with sulphuric acid. 



* This mixture was made in the humid way. 



This 



