The ignition of gunpowder must be distinguished from its combustion. The pow der is ignited when a portion of it begins to develop light and heat; this in granulated gunpowder communicates from grain to grain with the utmost rapidity; but still, it is important to bear in mind, by successive ignitions. Combustion means the final and total decomposition of each grain separately, and the complete liberation of its com ponent gases. in gunpowder these phenomena follow each other so rapidly, that, unless the mass is spread over a considerable space, they appear simultaneous. The heat spread around by each grain during its combustion suffices to ignite all other grains within a sphere of six times its own diameter. This serves to account for the almost instantaneous communication of the flame throughout the whole quantity exposed. The granulation of gunpowder has a great influence on the rapidity of its ignition; the larger the grain, the more rapid is the ignition, but the slower the combustion. On the other hand powder ignites more slowly, and burns with greater speed. When mewled or finely powdered gunpowder is employed, it readily inflames, from the presence of the ordinary ingredients; but there being no interstices, the conflagration spreads but slowly, and the decomposition is proportionately retarded. It thus happens that mewled powder exhibits less explosive power and less rapidity of combustion the closer it is pressed together, a circumstance taken into account in the manufacture of fireworks.
In rifled guns and muskets, where it is desired that the projectile should expand to the shape of the grooves, it is obviously best to have a powder which, by very rapid ignition, creates the utmost sudden expansion, and which, by continued combustion, maintains an increasing degree of heat, which shall further augment the explosive force of the gasses evolved up to the period of time at which the projectile is driven from the muzzle of the weapon. With this object, large-grained powder is clearly the most suitable, although a contrary view has been long accepted; and that to so great an extent, that some among the recent inventors of rifled arms have had to complain that the only gunpowder they could obtain has been far too good for their purposes.
Procexs of Hanqacture.—The three ingredients being taken to separate mills, arc reduced by successive grindings to impalpable powder. The several materials are then taken to the mixing-house, where they are weighed out into their respective proportions. The charcoal is spread in a trough, and the sulphur and niter being sifted upon it. the whole are incorporated, though imperfectly, by the hands. The next process is in the powder-mill, where the mixture is ground between millstones, and incor porated in a wet state. So dangerous is this part of the manufacture, that makers are forbidden to grind more than 42 lbs. in the same house at one time: all the hearings of the machinery are of copper, lest heat should he generated by the friction. Ar.00rding to the quality intended is the time during which this trituration is continued, from one hour to six being that usually employed, and three hours the period in the government factory. The powder, completely pulverized and caked by the moisture and the pres sure, leaves the mill in small lumps, called mill-cake. This mill-cake is then spread between copper-plates in layers about 3 in, thick, and is subjected to an immense pres sure either by a screw-capstan or by a hydraulic engine.
The next operation is graining:, a process to which, as already explained, gunpowder owes its rapidity of ignition, and its consequent explosive power. This is performed by
forcing the mill-cake through minute holes in a circular parchment sieve, the sieve being kept by mechanism in rapid revolution. The .grains thus formed are, however, of very various sizes; and that the gunpowder should be homogeneous, it is necessary that they should be sorted into the several sizes. This is done by the intervention of a series of sieves of different•degrees of fineness.
Since the introduction of the immense modern cannon, it has been found necessary to make powder with very large grain, even larger than hazel nuts, this is called pebble powder, and is now in general lase.
The last processes are drying, glazing, and freeing from dust. The first is effected by heating the powder to a point sufficient to drive off moisture; the second, by the friction of the grains together in a revolving cylinder; the third, by the centrifugal action caused by the powder being twisted round at great speed in a gauze cyinmer, when the mingt (formed in the polishing) flies off through the gauze, and the bright grains remain as finished gunpowder.
This is the modern system—a system involving considerable personal risk at every stage; for the fine dust becomes so diffused through the atmosphere in the mills that the slightest spark would blow the whole into the air in a moment. The early process of manufacture was, however, even more dangerous. What with the stirring, and pommeling, and spirit (which our forefathers used instead of water, in the idea that a strong fluid would impart strength to the mixture), it was probably to the impurity alone of the materials that the operators were indebted for not being, blown to pieces.
The following are the chief properties of gunpowder. Good powder should he per fectly uniform in texture, and should not present any light specks or glittering points. The grains should be sufficiently hard not to be easily crushed by the fingers, or to soil them, or a piece of paper, 'by mere contact. If inflamed on white paper,-it should blacken it but slightly, should on no account set fire to it, and should leave only im very slight residue. The temperature at which it explodes has been carefully studied by Violette, who obtained the f011owing results: The most combustible of these powders was the one containing the largest amount of sulphur, which is the ingredient most ready to enter into ignition. When guupowder is exposed to a heat of 500°, the whole of its moisture is expelled, and the niter and sulphur are reduced to the fluid form}. Ou cooling, such powder is found to be black, and the grain has become indurated, and is no longer able to imbibe moisture. Powder is inflamed by any burning substance, by red-hot metal, by the electric spark, or by the violent concussion even of comparatively soft bodies, if it be sufficiently powerful. For example, powder placed upon lead, or even on wood, may be ignited by the shock of a leaden bullet fired at it. Its specific gravity is about 1.8.
The nature of the residue which is left after the ignition of the powder has been already explained. The fouling caused by this residue is avoided to sonic extent by the lubrication of the barrel with a little fatty [natter.
The manufacture of gunpowder is carried on to a great extent in Great Britain; the exports in 1875 were 15,071,979 lbs., shipped principally to the colonies, and to western Africa and South America.