PREPARATION.—­Deprive the ferrocyanide of potassium (2KCy + FeCy) of its water by heating it over the spirit-lamp in a porcelain dish.  Mix eight parts of this anhydrous salt with three parts of dry carbonate of potash, and fuse the mixture by a low red heat in a Hessian, or still better, in an iron crucible with a cover, until the mass flows quiet and clear, and a sample taken up with an iron spatula appears perfectly white.  Pour the clear mass out into a china or porcelain dish or an iron plate, but with caution that the fine iron particles which have settled to the bottom, do not mix with it.  The white fused mass must be powdered, and kept from the air.  The cyanide of potassium thus prepared, contains some of the cyanate of potassa, but the admixture does not deteriorate it for blowpipe use.  It must be perfectly white, free from iron, charcoal, and sulphide of potassium.  The solution of it in water must give a white precipitate with a solution of lead, and when neutralized with hydrochloric acid, and evaporated to dryness, it must not give an insoluble residue by dissolving it again in water.

6. Nitrate of Potassa, Saltpetre (KO, NO^{5}).—­Saturate boiling water with commercial saltpetre, filter while hot in a beaker glass, which is to be placed in cold water, and stir while the solution is 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.