The hydrate of baryta is prepared by mixing six parts of finely powdered heavy-spar (BaO, SO_{3}) with one part of charcoal and one and a half parts of wheat flour, and exposing this mixture in a Hessian crucible with a cover to a strong and continuous red heat.  The cooled chocolate-brown mass must be boiled with twenty parts of water, and, while boiling, there must be added the oxide of copper in sufficient quantity, or until the liquid will not impart a black color to a solution of acetate of lead (PbO, [=]A).  The liquid must be filtered while hot, and as it cools the hydrate of baryta appears in crystals.  These crystals must be washed with a little cold water, and then heated at a low temperature in a porcelain dish until the crystal water is expelled.  The hydrate of baryta melts by a low red heat without losing its water of hydration.

3. Bisulphate of Potassa (KO, 2SO^{3}).—­At a red heat the half of the sulphuric acid of this salt becomes free, and thus separates and expels volatile substances, by which we can recognize lithium, boracic acid, nitric acid, fluoric acid, bromine, iodine, chlorine; or it decomposes and reveals some other compounds, as, for instance, the salts of the titanic, tantalic and tungstic acids.  The bisulphate of potash is also used for the purpose of converting a substance into sulphate, or to free it at once from certain constituents.  These sulphates are dissolved in water, by which we are enabled to effect the separation of its various constituents.

PREPARATION.—­Two parts of coarsely powdered sulphate of potash are placed in a porcelain crucible, and one part of pure sulphuric acid is poured over it.  Expose this to heat over the spirit-lamp, until the whole becomes a clear liquid.  The cooled mass must be of a pure white color, and may be got out of the crucible by inverting it.  It must be kept in a fine powder.

4. Oxalate of Potassa (KO, [=]O).—­Dissolve bioxalate of potash in water, and neutralize with carbonate of potash.  Evaporate the solution at a low heat to dryness, stirring constantly towards the close of the operation.  The dry residue is to be kept in the form of a powder.

The oxalate of potash, at a low red heat, eliminates a considerable quantity of carbonic oxide, which, having a strong affinity for oxygen, with which it forms carbonic acid, it is therefore a powerful agent of reduction.  It is in many cases preferable to carbonate of soda.

5. Cyanide of Potassium (Cy, K).—­In the dry method of analysis, this salt is one of the most efficient agents for the reduction of metallic oxides.  It separates not only the metals from their oxygen compounds, but likewise from their sulphur compounds, while it is converted through the action of the oxygen into carbonate of potash, or, in the latter case, combines with the sulphur and forms the sulphureted cyanide of potassium.  This separation is facilitated by its easy fusibility.  But in many cases it melts too freely, and therefore it is better to mix it, for blowpipe analysis, with an equal quantity of soda.  This mixture has great powers of reduction, and it is easily absorbed by the charcoal, while the globules of reduced metal are visible in the greatest purity.