or less pure), and a slag containing sulphides of lead and sodium; and
again, if sulphide of lead be fused with an excess of oxide of lead, a
button of lead will be got, and a slag which is simply oxide of lead
(with whatever it may have taken up from the crucible), or if a
sufficient excess has not been used, oxide of lead mixed with some
sulphide. When (as is most frequently the case) the desire is to prevent
the formation of regulus, these reactions may be taken advantage of, but
otherwise the use of a flux having any such tendency must be avoided. A
good slag (from which a regulus may be easily separated) may be obtained
by fusing, say, 20 grams of ore with borax 15 grams, powdered glass 15
grams, fluor spar, 20 grams, and lime 20 grams; by quenching the slag in
water as soon as it has solidified, it is rendered very brittle.

Sulphide of iron formed during an assay will remain diffused through
the slag, instead of fusing into a button of regulus, if the slag
contain sulphide of sodium. The same is true of other sulphides if not
present in too great a quantity, and if the temperature is not too high.

_Speises_ are compounds of a metal or metals with arsenic. They are
chiefly of interest in the metallurgy of nickel, cobalt, and tin. They
are formed by heating the metal or ore in covered crucibles with arsenic
and, if necessary, a reducing agent. The product is fused with more
arsenic under a slag, consisting mainly of borax. They are very fusible,
brittle compounds. On exposure to the air at a red heat the arsenic and
the metal simultaneously oxidize. When iron, cobalt, nickel, and copper
are present in the same speise, they are eliminated in the order
mentioned.

_Slags_ from which metals are to be separated should not be too acid; at
least, in those cases in which the metal is to be reduced from a