acid readily and without fusion. Where the melting down is performed
rapidly, the escaping gas is apt to cause trouble by frothing, and so
causing waste of the material. Ordinary carbonate of soda, when hydrated
(soda crystals), melts easily, and gives off its water with ebullition.
It is unfit for use in assaying, but when dried it can be used instead
of the bicarbonate. One part of the dried carbonate is equivalent to
rather more than one and a half parts of the bicarbonate. From two to
four parts of the flux are amply sufficient to yield a fluid slag with
one part of earthy matter. This statement is also true of the fluxes
which follow.

~Borax~ is a hydrated biborate of soda, containing nearly half its
weight of water. When heated it swells up, loses its water, and fuses
into a glass. The swelling up may become a source of loss in the assay
by pushing some of the contents out of the crucible. To avoid this,
_fused_ or _dried borax_ may be used, in which case a little more than
half the amount of borax indicated will suffice. Borax will flux almost
anything, but it is especially valuable in fluxing lime, &c., and
metallic oxides; as also in those cases in which it is desired to keep
certain of the latter in the slag and out of the button of metal.

~Oxide of Lead~, in the form of red lead or litharge, is a valuable
flux; it easily dissolves those metallic oxides which are either
infusible or difficultly fusible of themselves, such as oxides of iron
or copper. The resulting slag is strongly basic and very corrosive; no
crucible will long withstand the attack of a fused mixture of oxides of
lead and copper. With silicates, also, it forms very fusible double
silicates; but in the absence of silicates and borates it has no action
upon lime or magnesia. Whether the lead be added as litharge or as red
lead, it will exist in the slag as monoxide (litharge); the excess of