normal sulphite. It forms in monoclinic prisms, but by adding alcohol to the concentrated aqueous solution, it may be obtained in acicular crystals. The salt is very soluble in water, has a neutral or slightly alkaline reaction, and emits the odor of sulphur dioxide. On exposure to air it gradually changes to sulphate.

Potassium Disulphite, or Pyrosulphite, K2S2O5, is formed when sulphur dioxide is passed into a hot saturated solution of potassium carbonate until effervescence ceases and the liquid assumes a greenish tinge. On cooling, the salt separates in granular crystals. It is difficultly soluble in water, and has an acid, disagreeable taste.

Potassium Sulphate, K2SO4. Potassii Sulphas, U. S. P.— This salt has been known since the fourteenth century. It occurs in many mineral waters, in sea water, and in the ashes of many plants. The chief locality of its occurrence is in the Stassfurt salt-beds. Kainite is a double sulphate of potassium and magnesium with magnesium chloride, K,SO,MgSO,MgCl. 5H2O; schoenite is a double sulphate of potassium and magnesium, K2SO42MgSO4.6H2O.

These two minerals form the chief sources of the salt. It may be obtained as a by-product in the manufacture of nitric acid from potassium nitrate and sulphuric acid, in the manufacture of potassium bichromate, in the decomposition of potassium tartrate by calcium sulphate, in the manufacture of sulphuric acid, and in the manufacture of potassium carbonate from potassium chloride in the Leblanc process.

Preparation. In the preparation of the salt from kainite, the mineral is exposed to the air until the more soluble magnesium chloride deliquesces and runs off. The remaining salt is dissolved in boiling water, which decomposes it, and, on cooling, the less soluble potassium sulphate crystallizes first.

Properties.-Potassium sulphate occurs in "hard, colorless, transparent, six-sided, rhombic prisms terminated by pyramids, or a white powder, odorless, and having a somewhat bitter, saline taste. Permanent in the air. Soluble in about 9.5 parts of water at 15°, and in 4 parts of boiling water; insoluble in alcohol. When heated, the crystals decrepitate. At a bright red. heat they fuse, and at a white heat the salt suffers partial decomposition. The aqueous solution is neutral to litmus paper." The specific gravity of the crystals is 2.648.

Potassium sulphate has some uses in medicine, and it is extensively employed in the preparation of potash alum and potassium carbonate.

Acid Potassium Sulphate (Potassium Bisulphate), KHSO.—This salt is found native in the Grotto del Sofo, near Naples, in the form of long acicular crystals. Its usual source and method of preparation are from the residue of the manufacture of nitric acid from potassium nitrate when an excess of sulphuric acid has been used.

When the salt is separated from an acid solution, it is obtained in the form of tabular rhombic crystals, which are easily soluble in water. From dilute solutions of the acid sulphate the neutral sulphate crystallizes; following this, as the solution is concentrated, there separates a salt having the composition KÓSOKHSO, and then the acid salt crystallizes. From this it appears that in dilute solution the acid salt is dissociated into neutral potassium sulphate and sulphuric acid. This is also observed when the acid salt is mixed with alcohol : a decomposition takes place, the neutral sulphate separates, while the sulphuric acid passes into solution in the alcohol.

Acid potassium sulphate fuses at 200°, and above that temperature is converted into potassium pyrosulphate, K2SOŋ. This salt at a temperature of 600° decomposes into neutral sulphate and sulphur trioxide.

Potassium pyrosulphate crystallizes in long needles, and when brought in contact with water evolves much heat and is converted into the acid sulphate.

Acid Potassium Disulphate, KHSO7, is formed when the pyrosulphate is dissolved in fuming sulphuric acid, and from this liquid it is deposited in transparent prismatic crystals.

Acid potassium sulphate is useful as a flux, because in this way it is possible to bring sulphuric acid in contact with many bodies at a temperature considerably above its boiling point.

OXYGEN SALTS OF POTASSIUM AND NITROGEN.

Potassium Nitrate (Saltpetre, Nitre), KNO,. Potassii Nitras, U. S. P.—Saltpetre was known in the eighth century. It is very widely distributed in nature, mixed more or less with other salts, chiefly calcium nitrate. It is found most abundantly in the warm climates of India, Egypt, Hungary, and South America. Formerly most of the salt came from India. It is made there by collecting the efflorescence from stables and other places where nitrogenous matter is undergoing decomposition with free access of air. This efflorescence is obtained from large heaps of manure, on which urine is poured daily. As the surface dries and shows the presence of the salt, it is removed, and this process repeated until the bed, after two or three years, become exhausted. The crude liquors are treated with potassium carbonate, which precipitates the calcium, and the clear liquid is separated and concentrated to crystallize.

The greatest production of the salt now is from Chili saltpetre,

sodium nitrate, by double decomposition with potassium chloride, as follows:

The hot saturated solutions of these two salts are mixed and concentrated until the solution reaches a specific gravity of 1.5, when the sodium chloride, being no more soluble in hot than in cold water, separates out and is removed. The liquid is then drawn off and allowed to cool, when the potassium nitrate crystallizes in fine powder. This is removed and washed with a small quantity of cold water, which removes the last of the sodium chloride. The salt is then purified by recrystallization.

In order more fully to explain this method of preparing the salt, the following solubilities are given :

15o.

100 parts of water dissolve KNO, 26 parts.
100 parts of water dissolve NaNO,. 86.3
100 parts of water dissolve KCl.
100 parts of water dissolve NaCl

100°.
247 parts.
168.2

46

33.4 36

Properties.-Potassium nitrate occurs in "colorless, transparent, six-sided, rhombic prisms, or a crystalline powder, odorless, and having a cooling, saline, and pungent taste. Permanent in the air. Soluble in 3.8 parts of water at 15°, and in 0.4 part of boiling water; very sparingly soluble in alcohol. When heated to 353°, the salt melts. At a higher temperature it is decomposed, giving off oxygen at first, and then some of its nitrogen, leaving a residue of potassium nitrate, nitrite, and oxide. Thrown on red-hot coals, the salt deflagrates. The aqueous solution is neutral to litmus paper."

Uses. Potassium nitrate is used in medicine and as a preservative; in the latter capacity it enters into the composition of most of the meat preservatives, boric acid, borax, and sodium carbonate being the other compounds usually associated with it. The chief use of potassium nitrate is in the manufacture of gunpowder. The average composition of gunpowder is as follows:

In actual practice the composition varies somewhat from the above, since charcoal is not a uniform substance, consequently the character of the charcoal used in each case must be taken into account. The amount of the constituents is also varied according to the purposes to which the powder is to be applied.

Potassium Nitrite, KNO.—This salt is usually prepared by fusing in an iron vessel 1 part of potassium nitrate, to which are then added, in small portions at a time, 2 parts of powdered metallic lead; the latter is rapidly oxidized, as follows:

On cooling, the mass is treated with water, and from the solution of the potassium nitrite any lead is removed by carbon dioxide. The solution is then concentrated to a small bulk and the undecomposed nitrate allowed to crystallize out. The clear solution of now nearly pure nitrite is next evaporated to dryness, which yields the salt in masses of small, indistinct crystals. It is deliquescent in moist air, but nearly insoluble in alcohol. It usually comes in commerce in sticks of a yellowish color.

OXYGEN SALTS OF POTASSIUM AND PHOSPHORUS. There are three potassium salts of orthophosphoric acid. Tripotassium Orthophosphate, K3PO4, is prepared by heating to redness phosphoric acid and potassium carbonate or hydrate. It is readily soluble in water, and separates in small, colorless, acicular crystals.

Dipotassium Orthophosphate, K2HPO4.-This salt is formed when orthophosphoric acid is neutralized with potassium carbonate. The resulting compound is difficult to crystallize.

Monopotassium Orthophosphate, KH,PO4, is produced when the above dipotassium compound is heated with an excess of orthophosphoric acid. It usually separates in large, colorless, rhombic crystals.

There are two potassium salts of pyrophosphoric acid, according as two or four atoms of hydrogen are replaced by the base.

Potassium Pyrophosphate, KP2O7, is obtained by neutralizing pyrophosphoric acid with potassium hydrate, or, perhaps, more conveniently by heating the dipotassium orthophosphate to redness. It is a deliquescent salt which separates from solution in fibrous crystals, containing three molecules of water.

Dipotassium Pyrophosphate, K,H2P2O7.—This is the acid salt, and is prepared by dissolving the neutral salt in acetic acid and precipitating with alcohol.

Potassium Metaphosphate, KPO, is prepared by heating the monopotassium orthophosphate to redness. As thus obtained it is in translucent masses, nearly insoluble in water, but readily soluble in dilute acids. Although monobasic, metaphosphoric acid possesses the property of polymerization, so that there are at least five compounds of potassium with the acid, having the following formulas: KPO,, metaphosphate; K2P2O, dimetaphosphate; KPO,, trimetaphosphate, etc.

Potassium Hypophosphite, KH,PO,. Potassii Hypophosphis, U. S. P.-This salt is prepared by decomposing calcium hypophosphite with potassium carbonate, as follows:

The details of the process are to add 5.75 parts of potassium carbonate, in 8 parts of water, to 6 parts of calcium hypophosphite, in 64 parts of water; allow to stand until the precipitate settles, filter, and evaporate the filtrate at a temperature not exceeding 100° until granulation is effected. By failing to keep below the temperature specified an explosion is liable to occur. The product is purified by dissolving in alcohol, evaporating to a syrupy consistence, and crystallizing.

Potassium hypophosphite occurs in "white, opaque, hexagonal plates, or crystalline masses, or a granular powder, odorless, and having a pungent, saline taste; very deliquescent. Soluble, at 15°, in 0.6 part of water, and in 7.3 parts of alcohol; in 0.3 part of boiling water, and in 3.6 parts of boiling alcohol; insoluble in ether. When heated in a dry test-tube, the salt at first loses moisture, and then evolves spontaneously inflammable hydrogen phosphide gas, which burns with a bright yellow flame."

This salt should not be rubbed with nitrates, chlorates, permanganates, or other oxidizing agents, as, under these circumstances, it will explode violently.

OXYGEN SALTS OF POTASSIUM AND BORON.

Potassium Metaborate, KBO2, is formed when molecular weights of boric oxide and potassium carbonate are fused together. The product is soluble in water and crystallizes with difficulty.

There are several other borates, the most important of which is the tetraborate, K,B4O7.5H2O. It is prepared by adding potassium hydrate to a solution of boric acid until an alkaline reaction is obtained. This compound resembles the corresponding sodium salt, which will be treated more fully.

OXYGEN SALTS OF POTASSIUM AND SILICON.

Potassium Silicate, K,SiO,, is produced when amorphous silica or silicic acid is dissolved in potassium hydrate. It is also formed when silica is fused with potassium carbonate. It occurs in amorphous vitreous masses, which readily deliquesce when exposed to the air and gradually become converted into a transparent jelly, which then shrinks to a hard

mass.

Potassium Tetrasilicate, K,Si409.-This is the soluble potash glass. It is prepared by fusing together 45 parts of quartz with 30 parts of potas