Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/341

Rh GUNPOWDER 321 sity No physical property affects the explosiveness of gun powder as much as its density. By density we mean the [ness. q uan tity of matter actually present in a certain bulk of the powder. Thus, if different quantities of meal powder, con taining the same proportion of moisture, be compressed into equal bulks, say, for example, into cylinders of equal size, that which contains the most meal will be the densest. Hardness has not necessarily a relation to density, for a substance may be hard, and yet possess little density. Increase of density can only be given by compressing the meal into a smaller bulk, while increase of hardness can be arrived at by pressing the meal in a moister condition. Other things being equal, increasing the density decreases the initial velocity, and, vice versa, a less dense powder gives a higher velocity, but also a greater strain to the metal of the gun. This is due to the less dense powder burning more rapidly than that with a dense close texture. If two grains, or pieces of powder, of equal size and similar shape but very unequal density, be burnt upon a glass plate, the loss dense one will be entirely consumed before the denser on3 has finished burning. Freedom from fouling is a very important property in small-arm powders. From the experience gained in select ing a powder for the Martini-Henry rifle, it was found that, with the same description of charcoal, the slower the action of the charge the less fouling took place ; this modification of action was easily obtained by raising the density, at, however, a correspondiug sacrifice of velocity in the bullet. The reason assigned was that the quicker-burning powder caused a rush of gas past the lubricating wad before the latter had time to act properly. A dense hard powder which will take a high polish or glaze will evidently keep better, and bear transport better, than a more porous and therefore more friable grain, which would easily form dust. It will thus be seen that many considerations enter into this question of density. ize of The size of grain is one of the most important points to ains. k e considered as modifying the explosiveness of powder. Although a charge of powder appears to explode instantane ously, yet both ignition and combustion are comparatively gradual ; the flame is communicated from one grain to another, and each burns in concentric layers until it is con sumed, so that the combustion of the grains is not simul taneous. Meal-powder burns more slowly in air than when the powder is granulated, in consequence of the minute ness of the interstices ; dust, especially in fine-grain powder, retards ignition by filling up the interstices. To go to the other extreme, as showing the advantage gained by granulating the press-cake, we may quote an experiment made some years ago. A small piece weighing 1-06 oz. was placed in a mortar, and a light ball placed upon it ; when it was fired, the ball was not thrown out. An equal quantity broken into 15 pieces, projected the ball 3 3 yards; broken into 50 pieces, the ball ranged 10 77 yards; when the sams weight of ordinary grained powder was used, the range was 56 86 yards. It is most unsafe to attempt to apply to the action of gunpowder, when fired under enormous pressure, and especially when employed in large charges, the conclusions arrived at from its combustion in air. We may, however, assert that the weight of charge, the density, and all other conditions being equal, a charge made up of large grains or pieces of gunpowder will burn more slowly, and exert a lesser initial strain upon the gun than one composed of small grains, owing to the total surface of combustion being diminished by increasing the size of the pieces ; this has been abundantly verified by the results of experiments with heavy rifled ordnance. On the other hand, the larger grains afford larger interstices between them for the passage of the flame, thus facilitating the ignition of large charges. Hence, a gunpowder may possess a low rate of combustion and yet a high rate of ignition, and vice versa. For each gun, or charge of gun powder, there is doubtless a size of grain which would pro duce a maximum velocity with a minimum initial strain, but as each kind of cannon powder has to be employed in more than one piece of ordnance, it is necessary to select that size which will best suit all of them. The same quantity of powder meal made into two grains Shape &amp;lt; of equal density, but different shapes, will take different grains, times to bum ; the larger the surface exposed ths quicker will be the combustion of the grain. A sphere being the smallest form in which a given quantity of matter can be placed, it follows that a certain amount of meal powder compressed into a spherical form will take longer to burn than the same quantity made into the shape of a flat scale, exposing a large surface. The rounded form of grain is the most favourable for the transmission of the flame, the interstices being larger and more regular than in the case of elongated or flat grains fitting into one another. Hence we may conclude that, to secure uniformity, it is better to have the constituent grains as nearly as possible of the same shape, and the nearer this shape approaches a sphere the better. The glazing process is one of considerable importance, Glazing both with reference to the explosiveness, and also the keeping qualities of the gunpowder. As regards the former point, it undoubtedly modifies the violence of the combustion, and this it probably does by slightly retarding the ignition, a powder with a rough porous surface affording a better hold to the flame than one possessing a highly polished exterior. With the large &quot; cubical &quot; powder, used for heavy rifled ordnance, there is little or no appearance of what is com monly understood as glaze, or polish, from this process, but the corners and edges of the cubes are rubbed off, and the shape approximates more nearly to that of a sphere than is the case with the much lighter fine-grain powder; there seems to be also a certain hardening of the surface of the grains or pieces, partly the effects of friction, but probably due in part to the sweating the powder undergoes, a con siderable amount of heat being generated in the glazing barrels. It is evident that, by taking away the sharp angles which would otherwise easily be converted into dust, and also by giving the grains a harder exterior, this process renders gunpowder the better able to bear transport, and to resist the deteriorating effect of a damp atmosphere. The addition of a thin coating of the purest graphite to cannon powders, although originally intended merely to modify the explosiveness, also renders the surface of the grains less absorbent. Military small-arm powders ars never dressed with graphite ; good fine-grain gunpowder will take a high finish without it, but, by its aid, a very inferior article can be polished up to a silvery brightness. Moisture in gunpowder reduces the explosiveness by using Effect &amp;lt; up a portion of the heat generated by the combustion, to get rid of the water; therefore the property of withstanding the absorption of moisture is a very important one for gunpowder to possess. All powder will take up from a normally dry atmosphere a certain amount of moisture, which will depend to some extent upon its density, but to a much greater degree upon the description of charcoal from which it is made. Slack-burnt or red charcoal is greatly more hygroscopic than black charcoal, or that burnt at a high temperature ; and the absorbent properties of gun powder made from the former are but little reduced by raising the density. Large cannon powder contains a greater percentage of water than the fine grain, and the actual amount present in any given sample will be affected by the prevailing state of the atmosphere, especially if kept in wooden barrels. To show the considerable effect upon the initial velocity of projectile, and pressure in bore of gun XL 41