489 
De sulphur at ion of Metals. 
formed in the interior is in a great measure decomposed by tra- 
versing the whole height of the furnace ftiled with coals, and the 
sulphuret is recomposed ; the latter tends by its gravity to gain 
the basin, where it does not arrive until after a series of decomposi- 
tions, which cannot take place, as we have in fact observed, with- 
out there resulting a considerable loss in metal. 
All these facts seem to leave no doubt as to the following pro- 
position : The decomposition of the metallic sulphurets by roast- 
ing is produced by the oxygenation of its compounds, and the sul- 
phur is separated more or less completely in the state of sulphur- 
ous acid. 
III. De sulphur ation of the Metals independent of the Actiori 
of the Air . 
The varied affinities of sulphur for different mineral substances* 
furnish the means of decomposing certain sulphurets ; and seve- 
ral have been employed in metallurgy with success. In order 
that the decomposition of a metallic sulphuret by any mineral 
should form the basis of a metallurgic process, it is not sufficient 
that the affinity of this mineral for sulphur should be greater than 
that of the metal ; it must, besides the conditions required by 
economy, also possess several other requisites absolutely necessa- 
ry for the success of the operation, which considerably limit the 
number of the agents pointed out by chemistry : for instance, if the 
sulphuret resulting from the decomposition is not fusible, or but 
very little so, if it has the property of combining with the metal 
required to be separated, or rather with the still undecomposed 
sulphuret, it is evident that we cannot effect our purpose, namely, 
the isolation of the metallic substance. Hitherto little else has 
been used except lime and iron. 
De sulphur ation of Mercury. — -It is very easy to decompose the 
sulphuret of mercury ; it being sufficient if we present to the sul- 
phur a substance capable of retaining it, and volatilize the mercury 
alone. It is thus that iron and lime are employed together or se- 
parately in the treatment of the ores of cinnabar. 
De sulphur ation of Copper.- — Pyritous copper is melted in some 
foundries with lime, either in the common or the reverberating 
furnace ; but the process is not well enough known as yet to 
epable us to judge of the efficacy of lime as an agent in this case. 
I was once of opinion, with some metallurgists, that the well 
known superior affinity of iron for sulphur ever that of cupper for 
