hat we have to deal with
pure metals, which, if not absolutely non-volatile, are recognized by
the incrustation they form upon charcoal. Some compound substances,
when heated upon charcoal, form white incrustations, resembling that
formed by antimony, and which, when heated, may, in like manner, be
driven from place to place. Among these are certain sulphides, as
sulphide of potassium, and sulphide of sodium, which are formed by the
action of the reducing flame upon the sulphates of potassa and soda,
and are, when volatilized, reconverted into those sulphates, and as
such deposited on the charcoal. No incrustation is, however, formed,
until the whole of the alkaline sulphate has been absorbed into the
charcoal, and has parted with its oxygen. As sulphide of potassium is
more volatile than sulphide of sodium, an incrustation is formed from
the former sooner than from the latter of these salts, and is
considerably thicker in the former case. If the potash incrustation be
touched with the reducing flame, it disappears with a violet-colored
flame; and if a soda incrustation be treated in like manner, an
orange-yellow flame is produced.

Sulphide of lithium, formed by heating the sulphate in the reducing
flame, is volatilized in similar manner by a strong blast, although
less readily than the sulphide of sodium. It affords a greyish white
film, which disappears with a crimson flame when submitted to the
reducing flame.

Besides the above, the sulphides of bismuth and lead give, when heated
in either flame, two different incrustations, of which the more
volatile is of a white color, and consists in the one case of sulphate
of lead, and in the other of sulphate of bismuth. If either of these
be heated under the reducing flame, it disappears in the former case
with a bluish flame, in the latter unaccompanied by any visible flame.
The incrustation formed nearest to the assay consists of the oxide of
lead or bismuth, and is easily recognized by its color when hot and
after cooling. There are many other metallic sulphides, which, when
heated by the blowpipe flame, cover the charcoal with a white
incrustation, as sulphide of antimony, sulphide of zinc, and sulphide
of tin. In all these cases, however, the incrustation consists of the
metallic oxide alone, and either volatilizes or remains unchanged,
when submitted to the oxidizing flame.

Of the metallic chlorides there are many which, when heated on
charcoal with the blowpipe