according to the proportions in which they exist. For purposes of this
kind borax, or glass, or clay with more or less soda may be used, and of
these borax is by far the most generally useful. An objection to too
basic a slag (and a very important one) is the speed with which it
corrodes ordinary crucibles. These crucibles, consisting of quartz and
clay, are rapidly attacked by lime, soda and bases generally.

[Illustration: FIG. 4.]

In considering what is and what is not a good slag, certain chemical
properties are of importance. If a mixture of many substances be fused
and allowed to solidify in a crucible, there will be found some or all
of the following. At the bottom of the crucible (fig. 4) a button of
metal, resting on this a speise; then a regulus, next a slag made up of
silicates and borates and metallic oxides, and lastly, on the top
another layer of slag, mainly made up of fusible chlorides and
sulphates. In assaying operations the object is generally to concentrate
the metal sought for in a button of metal, speise or regulus, and to
leave the earthy and other impurities as far as possible in the slag;
whether there be one or two layers of slag is a matter of
indifference;[3] but the chemical action of the lower layer upon the
speise, or regulus, or metal, is of great importance.

A _regulus_ is a compound of one or more of the metals with sulphur; it
is usually brittle, often crystalline, and of a dull somewhat greasy
lustre. It is essential that the slag, when solid, shall be so much more
brittle than the regulus, that it shall be easy to crumble, and remove
it without breaking the latter; and it must not be basic. The effect of
fusing a regulus with a basic slag is well seen when _sulphide of lead_
is fused with _carbonate of soda_; the result is a button of metal (more