With those metals (tin,
antimony, &c.) which pass quickly into the slag, the contrary is true;
hence with these it is necessary to have enough lead present, so that
the slag formed at the outset shall contain enough oxide of lead to make
it fluid. As silver is so much less easily oxidised than copper, we
should reasonably expect that the proportion of silver carried off in
the oxide of lead would be considerably less than that of the copper
indicated in the above example. Indeed, there are one or two facts
which tend to encourage the hope that the operation may be conducted
without any loss. If a piece of pure silver foil is exposed on a cupel
to air at the usual temperature of cupellation, it undergoes very little
change; it does not even fuse; it loses nothing in weight, and does not
oxidise. In fact, even if oxide of silver were formed under these
conditions, it could not continue to exist, for it is decomposed into
silver and oxygen at a temperature considerably below redness. On the
other hand, oxide of silver is not reduced to metal by heat alone, when
mixed with an excess of oxide of lead; while metallic silver is
converted into oxide when heated with the higher oxides of lead, copper,
and some other metals. That silver, and even gold (which is more
difficult to oxidise than silver), may be carried off in the slag in
this way, is in agreement with general experience.
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