d absorbed within
the charcoal, and ceases to be visible as soon as it is submitted to
the reducing flame, while the external flame is colored violet;
sulphate of potassa, for instance, is reduced by the glowing charcoal
into the sulphide. This latter is somewhat volatile, but by passing
through the oxidation flame, it is again oxidized into the sulphate.
This, being less volatile, sublimes upon the charcoal, but by exposing
it again to the flame of reduction, it is reduced and carried off to
be again oxidized by its passage through the oxidation flame.

Potassa and its compounds give, with soda, borax or microcosmic salt,
as well when hot as cold, colorless beads, unless the acid associated
with the alkali should itself produce a color. When borax is fused
with some pure boracic acid, and sufficient of the oxide of nickel is
added, so that the beads appear of a brown color after being cooled,
and then the bead thus produced fused with the substance suspected to
contain potassa, in the oxidation flame, the brown color is changed to
blue. The presence of the other alkalies does not prevent this
reaction. As it is not possible to detect potassa compounds with
unerring certainty by the blowpipe flame, the the wet method should
be resorted to for the purpose of confirming it.

The _silicates of potassa_ must be prepared as follows, for analytical
purposes by the wet way. Mix one part of the finely powdered substance
with two parts of soda (free from potassa), and one part of borax.
Fuse the mixture upon charcoal in the oxidation flame to a clear,
transparent bead. This is to be exposed again with the pincers to the
oxidation flame, to burn off the adhering coal particles. Then
pulverize and dissolve in hydrochloric acid to separate the silica;
evaporate to dryness, dissolve the residue in water, with the
admixture of a little alcohol, and test the filtrate with chloride of
platinum for potassa.

(_b._) _Soda_ (NaO).--This is one of the most abundant substances,
although seldom found free, but combined with chlorine or some other
less abundant compound. Soda, its hydrate and salts manifest in
general the same properties as their respective potash compounds; but
the salts of soda mostly contain crystal water, which leaves the salts
if they are exposed to the air, and the salts effervesce.

By exposing soda or its compounds upon a platinum wire to the blue
flame, a reddish-yellow color is communicated to the external flame