The residue should be treated with boiling water, which
dissolves the phosphate and the excess of carbonate of soda, while the
silicate of alumina, with some of the soda, is left. The clear liquor
is now treated with acetic acid, and heated over the spirit-lamp, and
a small portion of crystallized nitrate of silver added; a
lemon-yellow precipitate of phosphate of silver is quickly developed.
Previous to the addition of the nitrate, the liquor should be well
heated; otherwise, a white precipitate of dipyrophosphate of silver
will be produced.

If the examination be of any of the metallic phosphides, the
substances should be powdered in the agate mortar, and fused with
nitrate of potassa on the platinum wire; the fused mass should be
treated with soda in the same manner as any substance containing
phosphoric acid. The metal and the phosphorus are oxidized, while the
phosphate of potassa is fused, and the metallic oxide separates.

(5.) _Sulphur_ (S).--Sulphur is found native in crystals It is
frequently found associated with lime, iron, silica, carbon, etc., and
combined extensively with metals.

The principal acid of sulphur (the sulphuric, SO^{3}) occurs combined
with the earths, the alkalies, and the metallic oxides. Native sulphur
is recognized, when heated upon charcoal, by its odor (sulphurous
acid) and the blue color of its flame. The compounds of sulphur may be
detected by several methods. If the substance is heated in a glass
tube, closed at one end, the yellow sublimate of sulphur will subside
upon the cool portions of the tube; if the substance should also
contain arsenic, the sublimate will present itself as a light brown
incrustation, consisting of the sulphide of arsenic.

If the assay is heated in the open glass tube, sulphurous acid will
thus be generated; but, if the gas is too little to be detected by the
smell, a strip of moistened litmus paper will indicate the presence of
the acid.

The assay will give off sulphurous fumes if heated in the flame of
oxidation.

If the powdered substance is fused with two parts of soda, and one
part of borax, upon charcoal, the sulphide of sodium is formed. This
salt, if moistened and applied to a polished silver surface, will
blacken it. The borax serves no other purpose than to prevent the
absorption of the formed sulphide of sodium by the charcoal. As
selenium will blacken silver in the manner above indicated, the
presence of this substance should be first