Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/343

GUNPOWDER 323 (B.) Wlicn Fired in the Bore of a Gun. (1) The products of com bustion, at all events so far as regards the proportions of solid and gaseous matters, are the same as in the case of powder fired in a close vessel. (2) The work on the projectile is effected by the elastic force due to the permanent gases. (3) The reduction of tempera ture due to the expansion of the permanent gases is in a great measure compensated by the heat stored up in the liquid (afterwards solid) residue. (4) An expression is obtained showing the law con necting the tension of the products of combustion with the volume they occupy. (5) Equations are also deduced for the work that gun powder is capable of peforming in expanding in a vessel impervious to heat, and for the temperature during expansion. Thence the ex perimenters give a table showing the total work gunpowder is capable of performing in the bore of a gun, in terms of the density of the products of combustion, or the number of volumes of expan sion. (6) The total theoretic work of gunpowder when in definitely expanded (for example, in a gun of infinite length) is about 486 foot-tons per pound of powder. They further ascertained that (a) the fine-grain powders furnish decidedly smaller portions of gaseous products than large grain or cannon gunpowders ; (b) the variations in the composition of the products of explosion, in a close vessel, furnished by one and the same powder under different conditions as regards pressure, and by two powders of similar composition under the same conditions of pressure, are so considerable that no chemical expression can be given for the metamorphosis of a gunpowder of normal composition ; and (c) the proportions of the several constituents of the solid residue are quite as much effected by slight accidental conditions of explo sion of one and the same powder in different experiments as by de cided differences in the composition, as well as in the size of grain of different powders. It may, however, be remarked here that, while the pressures given in the bore of a gun are very seriously affected by the size of the grains or pieces of the powder, it was clearly demonstrated by these experiments that the force exerted by fired gunpowder is not affected by the apparent accidental variations in the nature of the secondary chemical changes resulting from the explosions ; this fact renders the exact composition of the products of combustion of less practical importance. The following table gives concisely the chief results arrived at: Authority. Volumes of Permanent Gas Generated. Heat Evolved by Combustion. Tension of the Gases. Pressure in Tons. Robins 244 Cent. Atmospheres 1000 Per sq. in. 6-75 Hutton 250 2000 135 4501 1850 2137 14-3 Bunsen and Schiskoff 193 280 3340 2200 4374 6400 29 4-2 1 As before stated, Piobert gives reasons for halving these figures. M. de Saint Robert determined experimentally the amount of work lost by the heat communicated to the gun to be about 250 framme-units per gramme of powder in the. case of a rifled musket. rom the experiments of Noble and Abel this loss becomes reduced to as little as 25 gramme-units per gramme of powder in a 10-inch gun. They have also calculated the energy in foot-tons from th initial velocity obtained, and hence deduced the percentage of the possible theoretic work which is actually realized for every rifled gun in the British service. This percentage, which they term the "factor of effect," is found to be greatest, viz., 93 per cent., in the ease of _the 38-ton gun, and least, 50 5 per cent., in the 7 -pounder mountain gun, weighing 150 R. Deter- To determine the pressures, Noble and Abel employed two lunation methods: (1) directly, by means of " crusher gauges," inserted at jf pres- various points of the bore ; this is an improvement on the Rodman piston-gauge, the pressure being estimated by the amount of com pression given to a small cylinder of pure soft copper ; (2) indirectly, by means of the chronoscope invented by Captain A. Noble, which measures the velocity of the projectile at given points in the bore, whence the pressure can be calculated (see GUNNERY). These pres sures will vary greatly, other conditions being the same, according to the " explosiveness " of the powder ; the great object is to obtain a powder suited to the particular arm with which it is to be used. Many foreign physicists are of opinion that the phenomenon of ,ion im- dissociation comes into play at the moment of maximum temperature, irobable. causing the carbonic acid gas (C0 2 ) to be decomposed into carbomc aae (LO) and oxygen. However, Noble and Abel show that, if such s supposed to occur, the loss of heat absorbed by the decomposition would more than compensate for the increase of volume. PREPARATION AND PURIFICATION OF THE INGREDIENTS. The three ingredients of gunpowder may be purchased in the market in a prepared or refined state, and this is done to a greater or less extent by many powder makers. However, the royal gun powder factory, as well as some of the great private firms, prepare and purify the materials required from the commencement, with the double object of insuring uniformity in the qualities of the sure in bore. 3issocLi- powders made, and of avoiding, so far as practicable, the introduc tion of the least particle of grit or other foreign matters, which might cause serious accidents. The rationale of the refining process is based upon the fact that Refini saltpetre is far more soluble in hot than in cold water, while the gult- chief saline impurities found in grough nitre are almost equally petre. soluble in either. Water at 212 Fahr. holds about seven times as much nitrate of potash in solution as water at 70 Fahr. ; if, there fore, a saturated solution of saltpetre be made at a temperature of 212 Fahr., and the chlorides of sodium and potassium are con tained in the liquor, as the solution cools to 70 Fahr., six-sevenths of the nitre will be deposited in the form of crystals, which can easily be removed, whereas the foreign salts will still remain in solution. The following is a brief account of the improved mode of refining saltpetre, introduced some years back at Waltham Abbey, and now adopted at the chief private factories in England. The refining coppers are capable of holding about 500 gallons each, and are fitted with false iron bottoms, which are perforated with holes to allow sand and other mechanical impurities to fall through. Being each charged with 280 gallons of water, usually the " washings" of saltpetre previously refined, and 40 cwt. of grough nitre, a fire is lighted beneath. In about two hours the greater part of the saltpetre is dissolved, and the solution begins to boil ; the thick scum formed on the surface is carefully taken off, and cold water from time to time thrown in to induce it to rise, the boiling being continued until there is no more scum ; the coppers are then filled up with cold water, and the solution again made to boil briskly for a few minutes, after which the fires are allowed to go down. In about two hours more the solution will have fallen to the proper temperature, 220 Fahr. t (sp. gr. 1 53), for pumping out ; it is then filtered through dowlas bags suspended on a frame, and conducted by troughs to the "coolers" or crystallizing cis terns at about 180 Fahr. These latter are large shallow pans of copper, in which the liquid is kept agitated either by long-handled wooden hoes, or by machinery ; as it cools, fine crystals fall to the bottom, and are from time to time thrown upon inclined draining frames by means of perforated copper shovels. When the tem perature falls below 70 Fahr. , the agitation is ceased, and any large crystals which may afterwards form are left in the mother liquor. After being thus allowed to get rid of some of the liquor, the crystallized saltpetre, having almost the appearance of snow, and technically called "flour," is raked into the "washing-cis tern ; " it is there subjected to three separate washings with pure water, distilled being preferable, which is allowed to drain off through a plug hole at the bottom of the cistern. These " wash ings " are carefully conducted to an underground tank, and kept either for using in the refining coppers, or boiled down in "eva porating pots " holding about 300 gallons, until the liquid is suffi ciently concentrated for the saltpetre to crystallize, when it is run into small copper pans, and set aside to cool ; this course is also pursued with the mother liquor from the large cooling cisterns, and the resulting crystals treated as grough nitre, for these large crystals always enclose liquid containing impurities. After drain ing for a night the refined saltpetre is removed to store bins, and is ready for use without any further pulverization ; the amount of moisture remaining in it, from 3 to 5 per cent., is then carefully ascertained, and allowed for in the mixing house. Neither the solid residue from the pots and coppers nor the mother liquor from crystallizing pans is thrown away, but by repeated boilings and evaporations every ascertainable portion of saltpetre is extracted, and the residue sold ; it chiefly consists of chlorides of sodium and potassium, with some sulphates. The jute bags in which the salt petre is imported are also boiled down. The large percentage of saltpetre contained in damaged powder makes it worth while to extract the nitre by boiling in large cop pers, filtering and crystallizing in small pans as before ; the re siduum is again boiled, and then thrown away. The sweepings from the powder houses, in various* stages of manufacture, are "extracted" in the same manner. Refined saltpetre for making gunpowder is tested as follows : (1) with blue and red litmus paper, for acids or alkalies ; (2) for the presence of chlorides, with solution of nitrate of silver ; (3) for sulphates, with chloride of barium, forming the insoluble sulphate of baryta. By the old method of refining the solution was left to crystallize in coolers, and the drained crystals fused or melted in iron pots at a heat of about 500 Fahr. ; the molten saltpetre was then poured into moulds, broken up when cold, and ground under edge runners. The wood for charcoal is cut in spring, so that the bark may Char strip off easily, and is stacked for about three years to ^ season it, burn but a considerably shorter time will suffice. Being cut in lengths of 3 feet, the wood is packed in iron cylindrical cases termed " slips," which are then inserted in the " cylinders " or retorts, the latter being built into the wall in sets of three with a furnace under neath, arranged so as to allow of the complete regulation of the heat ; the flames surround the retorts as nearly as possible. An opening

is left in the rear end or lid of the slip, corresponding with a similar