482 De sulphur ation of Metals . 
this principle render this assertion very probable ; it is besides 
proved by the chemical examination of the produce of all the roast- 
ing, as well as by the way in which the operation is conducted. 
In place of seeing in the roasting of the sulphurets the volatiliza- 
tion of the sulphur, produced by a well-managed heat , it will be 
the decomposition of a sulphuret by the simultaneous action of the 
air and of caloric : and the well known necessity of not melting 
the ores does not seem to be recommended in consequence of the 
fear of communicating to it, together with liquidity, a force of co- 
hesion which will oppose the separation of the sulphur ; but rather 
because this state will confine the action of the air to a surface, 
which, not being capable of being renewed, will be soon covered 
by the metallic oxyd. The combination of the oxygen with the 
elements of the sulphurets, gives birth to oxyds and to acids, the 
affinities of which have great influence upon the separation of the 
sulphur, and the results of a roasting : the lattter generally present 
a mixture of oxyd, of sulphat, and of indecomposed sulphuret. 
I shall examine separately and in detail the roasting of several 
kinds of sulphurets, because the nature of the metal produces 
great modifications in their results ; and shall presently show, 
why, and in what form, the sulphur is separated. 
Roasting of Pyritous Cofifier. 
We arrange pieces of pyritous copper upon faggots, in such 
away as to make the combustion continue a long time. The first 
application of the heat separates a part of the sulphur, which is 
distilled in some measure, and may be collected ; but afterwards 
it is this combustible which serves, upon burning, to continue the 
operation : sulphureous acid is liberated, the elasticity of which, 
increased by the elevation of the temperature, hinders its combina- 
tion with the metallic oxyds. The sulphuric acid which is form- 
ed, in spite of the care taken to slacken the combination, is united 
to the oxyds of copper and iron, but the sulphate of iron is partly 
decomposed by the hyper-oxydation of the metal. 
The pyrites of iron submitted to the same operation undergoes 
analogous decompositions , the succession of which is in every re- 
spect the same. 
is volatile, and as these substances are fixed, or at least not so volatile as sul- 
phur, the action of heat alone is sufficient to take the sulphur from most me- 
tals.” He seem, however, to have been aware df the importance of the con- 
tact of the atmospheric air in roasting, since he says, when speaking of the 
sulphurets of mercury and of arsenic, “ It will be possible to desulphurate 
them without intermedium, by a well-managed heat and in the open airP 
