compound, as well as separated from earthy impurities. Where the object
is simply to get a button of metal from a substance in which it is
already in the metallic state, but mixed with dross (made up of metallic
oxides, such as those of zinc or iron), from which it is desired to
separate it, an acid flux like borax is best; or, if the metal is easily
fusible, and there would be danger of loss of metal by oxidation or
volatilising, it may be melted under a layer of resin or fat. Common
salt is sometimes used with a similar object, and is often useful. Under
certain conditions, however, it has a tendency to cause the formation of
volatile chlorides with a consequent loss of metal.

In the great majority of cases, the fusion of the metal is accompanied
by reduction from the state of oxide; in these the slag should be basic.
It is not easy to reduce the whole of a reducible oxide (say oxide of
copper or of iron) from a slag in which it exists as a borate or
silicate; there should be at least enough soda present to liberate it.
When the object is to separate one metal, say copper, without reducing
an unnecessary amount of another (iron) at the same time, a slag with a
good deal of borax is a distinct advantage. The slag then will probably
not be free from copper, so that it will be necessary to powder and mix
the slag with some soda and a reducing agent, and to again fuse the slag
in order to separate this residual metal. In all those cases in which
the slag retains an oxide of a heavy metal, this cleaning of the slag is
advisable, and in the case of rich ores necessary. Slags containing
sulphides are especially apt to retain the more easily reducible metals.

The following are the ordinary and most useful fluxes:--

~Soda.~--The powdered bicarbonate, sold by druggists as "carbonate of
soda," is generally used. It gives off its water and excess of carbonic