484 De sulphur ation of Metals, 
We fnay explain the concentration of the metal, and the general 
result of the flux, which surprised M. Jars very much. This 
process is perhaps the only one in which, at the same time, the 
sulphur and iron are separated in any quantity. 
The de-sulphuration of pyritous copper by roasting is, in my 
opinion, produced, 1 st, by the sublimation of a small portion of 
sulphur, which may be collected or burnt in the air : 2dly, by the 
extrication of sulphurous acid, so much the more abundant as the 
operation is well conducted:* 3dly, by the vaporization of a little 
sulphuric acid, the greatest part of which, however, remains united 
to the copper. 
Roasting' of Galena. 
IT is extremely difficult completely to desulphurate galena by 
roasting : the affinity of its component parts for oxygen does in-* 
deed effect the disunion quickly enough ; but that of the new com- 
pounds, the sulphuric acid and the oxide of lead , gives birth to a 
new combination, which retains the sulphur, and thus forms an ob- 
stacle to the desulphuration : to this same affinity of the oxide of 
lead for the sulphuric acid, we must attribute the facility with 
which this acid is formed in the roasting of galena* 
I shall examine in detail the various processes to which this 
important decomposition gives rise, as I think they will explain nu- 
merous and complex phenomena*. 
Whatever care is taken in roasting galena, it is impossible to 
convert all the sulphur into sulphurous acid, and to avoid the for- 
mation of sulphuric acid ; the result always gives a mixture of 
oxide and of sulphate of lead. 
In coastings performed hpon a large scale, and in a regulated at- 
mosphere, the proportion of sulphate of lead is much more consi- 
derable, it is regulated by the temperature and by the facility with 
which the air penetrates the ore ; numerous experiments made in 
P Ecole 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. 
* Recent experiments of Messrs. Clements and Desormes show, that the 
combustion of sulphur does not produce sulphuric acid so easily as imagined : 
but we know that its formation is determined by various peculiar circunn 
stances, such as the presence of the alkalies, oxyds, See. 
