PROPERTIES OF NITRE.

397 this wall for lixiviation, and the washed earth, mixed with a fresh portion of animal matter, is returned systematically to the other side of the heap. The washing of the earth charged with saltpetre is conducted in a systematic manner (589), so as to avoid using a larger quantity of water than is actually needed to dissolve the saltpetre.

Besides the natural and artificial sources of nitre just described, this salt occurs also in solution in the sap of certain plants, among which the sunflower, the tobacco plant, and common borage may be enumerated.

Properties.-Nitre usually crystallizes in long six-sided striated prisms, terminated by dihedral summits; but it is a dimorphous salt, and is occasionally obtained by spontaneous evaporation of small drops of its solution in microscopic rhombohedra, isomorphous with those of nitrate of sodium: it is soluble in 3 times its weight of cold water, and in about a third of its weight of boiling water; it is insoluble in alcohol: its taste is cooling and saline. If paper be dipped in a solution of nitre and dried, it forms what is well known as touch-paper, which, when once kindled, steadily smoulders away till consumed, and hence it is largely employed in firing trains of powder, fireworks, &c. Nitre fuses easily without decomposition at a temperature of 642°, and when cast into moulds, solidifies to a white, fibrous, translucent, radiated mass, known as sal prunelle. When heated to redness, part of its oxygen is expelled, and a deliquescent mass of nitrite of potassium is formed. By a still stronger heat the nitrite is decomposed, nitrogen mixed with oxygen escapes, and a mixture of potash and peroxide of potassium remains.

Owing to the facility with which nitre parts with oxygen, it is a powerful oxidating agent, and is in frequent demand in the laboratory for this purpose: when thrown upon glowing coals it produces a brisk scintillation. If nitre be intimately mixed with any metallic sulphide in fine powder, such as sulphide of antimony, and thrown, in small quantities at a time, into a red-hot crucible, the sulphur burns with a brisk deflagration, or rapid combustion, at the expense of the oxygen of the nitre, and with a portion of its potassium, forms sulphate of potassium, whilst the metal at the same time becomes oxidized to the maximum. In the case of antimony, the oxide produced possesses acid characters, and it also enters into combination with the potassium. Advantage is frequently taken of this oxidizing action of nitre in order to convert small quantities of sulphur in bodies of organic origin into sulphuric acid, for the purpose of estimating the proportion of sulphur which

398

REFINING OF SALTPETRE.

they contain. The quantity of sulphuric acid thus produced, admits of easy and accurate determination in the form of sulphate of barium.

(571)-Refining of Saltpetre.-The impurities of most frequent occurrence in nitre are sulphates and chlorides of potassium and sodium, and nitrates of calcium and magnesium: after it has been fused, unless the heat has been cautiously regulated, a little nitrite of potassium is liable to be formed; in the latter case a fragment of the salt, when moistened with solution of sulphate of copper, becomes of a bright green colour. Nitre may be quickly deprived of chlorine by moistening the powdered salt with pure nitric acid and gently heating it, until a portion of it, when dissolved in water, gives no precipitate with nitrate of silver. Nitre, when pure, is not deliquescent, and its solution gives no precipitate with solutions of chloride of barium, of nitrate of silver, or of carbonate of potassium.

In the refining of nitre, advantage is taken of the circumstance that whilst the solubility of nitrate of potassium increases rapidly as the temperature rises, that of the chloride of sodium is scarcely affected by it. It was formerly the practice to purify the salt by three successive crystallizations; but the same object is now effected by a single operation, in the following manner :— In a deep copper boiler, about 50 gallons, or 500 lb., of water is placed, and twice its weight of crude nitre is added: this salt gradually becomes dissolved, and fresh nitre is added, until, when the water has attained the boiling-point, a quantity of nitre has been added equal to three times the weight of the water employed; when the liquid has been rendered clear by a few minutes' ebullition, it is strained through bag filters, and allowed to run into the crystallizing pan.

The crystallization is effected in a shallow vessel, the bottom of which is formed by two inclined planes which meet in the middle. In this vessel the solution is kept in continual agitation,

*The quantity of chloride of sodium in East Indian nitre is generally small; but in the artificial nitre obtained from the beds' it often rises to a large amount in such a case the liquid is skimmed from time to time, and the chloride of sodium, a large proportion of which remains undissolved, is removed by means of perforated ladles; as soon as nitre equal to about 5 times the weight of the water has been added, the solution is diluted with twice the quantity of water at first employed, after which, if the liquid be strongly coloured, 2 lb. of glue, dissolved in hot water, are added, and thoroughly incorporated by briskly stirring; the coagulum which is formed rises in a scum to the surface, collecting the greater part of the organic impurities derived from the nitre heap; this is carefully removed, and the operation is afterwards continued as above described.

in order to prevent the formation of large crystals: such crystals would mechanically retain the mother-liquor, from which they could not be subsequently freed without recrystallization. The chloride of sodium, being nearly as soluble in cold water as in hot, remains almost entirely in the solution, whilst the saltpetre is deposited in extremely small crystals, technically termed saltpetre flour; these are allowed to drain, and are then removed to tanks provided with false perforated bottoms, where they are deprived of the mother-liquor with which they are saturated. For this purpose, the tanks are completely filled with the crystals, and upon them is poured a solution of saltpetre saturated in the cold; this liquid dissolves the chlorides, but leaves the pure nitrate of potassium undissolved. In the course of a few hours, the liquid is drawn off, and the tanks are then filled up with pure water; this becomes charged with nitre containing traces of chlorides, whilst the undissolved salt is almost chemically pure: the solution of nitre thus obtained serves to wash a fresh portion of the crystals; the refined saltpetre is then dried, and is fit for use.

(572) Gunpowder.—The principal consumption of nitre is in the manufacture of gunpowder, which consists of an intimate mechanical mixture of nitre, sulphur, and charcoal, in proportions approaching to 1 atom of sulphur, 2 of nitre, and 3 of charcoal :

The proportions used vary a little in different countries, as will be seen from the following table :

Composition of Gunpowder in 100 parts.

An excess of sulphur is carefully avoided, on account of its injurious action upon the metal of the gun. The great explosive power of gunpowder is due to the sudden development of a large volume of gaseous bodies, chiefly consisting of nitrogen and

carbonic anhydride, which, at the ordinary temperature of the air, would occupy a space equal to about 300 times the bulk of the powder used; but from the intense heat developed at the moment of the explosion, the dilatation amounts to at least 1500 times the volume of the gunpowder employed. Supposing the mixture to be made in the proportion of 1 atom of sulphur, 2 atoms of nitre, and 3 of carbon, the reaction is often approximatively represented thus:

S2+6 €+4 KNO ̧=6 ¤O2+2 N2+2 K2S.

The actual results, however, cannot readily be represented by any simple formula, and the solid residue, instead of consisting chiefly of sulphide of potassium, contains but a small quantity of this substance, whilst sulphate and carbonate of potassium are found in abundance: they become volatilized by the heat of the explosion, and constitute the white smoke observed when gunpowder is fired.*

Much care is requisite in the selection of the materials for the manufacture of gunpowder. The charcoal must be burned thoroughly, but not at too high a temperature: that from the willow, alder, or dogwood is preferred for the purpose, dogwood charcoal being employed only in making rifle powder. The charcoal always contains a notable proportion of hydrogen and oxygen. In the Government works at Waltham Abbey, sulphur is never used in the state of flowers of sulphur; in this condition it is preferred for fireworks; but distilled sulphur, reduced to a fine meal by grinding, is always used for gunpowder. The flowers of sulphur always contain sulphurous acid, which becomes speedily converted into sulphuric acid and attracts moisture. Nitre of the purest quality is alone employed. These three materials having been separately ground and sifted, are mixed in powder in the proper proportions, and are intimately blended in a revolving drum; they are then made into a stiff paste with water, and ground for some hours under edge-stones in the

* In burning gunpowder in a copper tube with the view of collecting the gases over mercury, Chevreul found a small proportion of nitric oxide, of carbonic oxide, and of carburetted hydrogen, with a little sulphuretted hydrogen, mixed with the nitrogen and carbonic acid. Bunsen, and Linck, obtained results somewhat different, but the temperature, and consequently also the results of the combustion procured by this regulated action, are different from those attending the firing of ordnance. Karolyi, however (Phil. Mag. Oct. 1863), has succeeded in analysing the gases of gunpowder which had been fired in conditions closely resembling those which occur in active artillery practice. For this purpose he enclosed a charge of powder in an iron cylinder of such strength that it just burst when the powder was fired by means of the electric spark. This charged cylinder was suspended in a hollow spherical

GUNPOWDER-PRODUCTS OF ITS COMBUSTION.

401

incorporating mill; the slightly coherent mass thus procured is broken up, and spread, in layers of about an inch in thickness, between plates of gun-metal; it is then subjected to the action of a hydraulic press which exerts a force of 70 tons upon the square foot a hard, sonorous mass, termed press cake, is thus

bomb, from which the air was exhausted before firing. After the explosion had been produced, the gases and the solid residue of the powder were submitted to analysis. The results obtained were the following:

3. Products of Combustion by volume in 100 of Gas.

These gases contained a sufficient amount of carbonic oxide, and of hydrogen and its compounds, to take fire on the application of a lighted match. The formation of sesquicarbonate of ammonium, and of carbonate of potassium in such large proportion is remarkable. The results obtained by the analysis of sporting powder by Bunsen and Schischkoff (Poggend. Annal. cii. 321) do not materially differ from those quoted above; but are slightly modified by the excess of nitre used in the preparation of this kind of powder.

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