cooling. The greater part of the saltpetre will crystallize in very
fine crystals. Place these crystals upon a filter, and wash them with
a little cold water, until a solution of nitrate of silver ceases to
exhibit any reaction upon the filtrate. These crystals must be dried
and powdered.

Saltpetre, when heated with substances easy of oxidation, yields its
oxygen quite readily, and is, therefore, a powerful means of
oxidation. In blowpipe analysis, we use it particularly to convert
sulphides (as those of arsenic, antimony, &c.) into oxides and acids.
We furthermore use saltpetre for the purpose of producing a complete
oxidation of small quantities of metallic oxides, which oxidize with
difficulty in the oxidation flame, so that the color of the bead, in
its highest state of oxidation, shall be visible, as for instance,
manganese dissolved in the microcosmic salt.


7. _Biborate of soda, borax_--(NaO + 2BO^{3}).--Commercial borax is
seldom pure enough for a reagent. A solution of borax must not give a
precipitate with carbonate of potassa; or, after the addition of
dilute nitric acid, it must remain clear upon the addition of nitrate
of silver, or nitrate of baryta. Or a small piece of the dry salt,
fused upon a platinum wire, must give a clear and uncolored glass, as
well in the oxidation flame as in the reduction flame. If these tests
indicate a foreign admixture, the borax must be purified by
re-crystallization. These crystals are washed upon a filter, dried,
and heated, to expel the crystal water, or until the mass ceases to
swell up, and it is reduced to powder.

Boracic acid is incombustible, and has a strong affinity for oxides