MOLYBDENUM.—­This metal, in powder, is infusible before the blowpipe.  If heated in the outer flame, it becomes gradually oxidized, and incrusts the charcoal, at a small distance from the assay, with molybdic acid, which, near the assay, forms transparent crystalline scales, and is elsewhere deposited as a fine powder.  The incrustation, while hot, is of a yellow color, but becomes white after cooling.  It may be volatilized by heating with either flame, and leaves the surface of the charcoal, when perfectly cooled, of a dark-red copper color, with a metallic lustre, due to the oxide of molybdenum, which has been formed by the reducing action of the charcoal upon the molybdic acid.  In the reducing flame, metallic molybdenum remains unchanged.

SILVER.—­This metal, when fused alone, and kept in this state for some time, under a strong oxidizing flame, covers the charcoal with a thin film of dark reddish-brown oxide.  If the silver be alloyed with lead, a yellow incrustation of the oxide of that metal is first formed, and afterwards, as the silver becomes more pure, a dark red deposit is formed on the charcoal beyond.  If the silver contains a small quantity of antimony, a white incrustation of antimonious acid is formed, which becomes red on the surface if the blast be continued.  And if lead and antimony are both present in the silver, after the greater part of these metals have been volatilized, a beautiful crimson incrustation is produced upon the charcoal.  This result is sometimes obtained in fusing rich silver ores on charcoal.

SULPHIDES, CHLORIDES, IODIDES, AND BROMIDES.

In blowpipe experiments, it rarely occurs that 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.