POWDER.
The incorporation, or grinding together, of the three ingredients that form gunpowder is by far the most important process in the whole manufacture, for unless the minute particles of the ingredients be thoroughly blended and brought into the closest contact with each other, all subsequent operations— however well performed—will not compensate for the error. The incorpor ating mill consists of two large and heavy "hard chill" cast-iron edge run ners, revolving on a circular cast-iron bed; the peculiar action of these run ners or rollers is well adapted for thoroughly grinding and incorporating the several ingredients; their great weight is for crushing the ingredients; which are also ground together by the twisting action produced by the roll ers traveling round in a small circle. Each roller travels over the bed in a separate track, and is assisted by the plough, which mixes the material, so that it is subjected to crushing, grinding, and mixing by the one operation. The composition attains a body in about one hour after the runners are set in motion, and the action of the ploughs in moving the whole of the material on and across the bed thoroughly mixes it, and subjects every particle 10 the same amount of pressure. Each pair of runners is provided with a telltale dial, which shows the attendant the time that the mill has to run, and enables him to judge the condition of the cake from time to time. Front three to four hours is the period a charge should on the mill, providing the engine or water-wheel is maintained at its proper speed. The cake should be of a blackish-gray color, and, when broken, of a uniform appearance, without any white or yellow specks in it; the presence of these would indicate insufficient incorporation or grinding. Furthermore, it should not be more than half an inch in thickness, in order to be thoroughly incorporated, nor should it be less than a quarter of an inch thick to insure safety, because if the runners are allowed to come in contact with the bed, the friction caused by their twist ing action is so g-reat that an explosion would almost certainly be the result. For fine sporting gunpowder the operation of incorporating is continued in some cases for as long as eight hours, and with heavier rollers, but it is doubtful whether the powder is much, if at all, improved thereby.
When grains of powder are united to form a charge, and fire is communi cated to one of them, the heat and expansive gases evolved insinuate them selves into the interstices of the charge, envelop the grains, and unite them one after another. This propagation of ignition is called inflammation, and its velocity the velocity of inflammation. It is much greater than that of combustion, and it should not be confounded with it. When powder is burned in an open train, fine powder inflames more rapidly than coarse; such, however, is not the case in fire-arms, owing to the diminution of the interstices. If a charge were composed of mealed powder, the flame could no longer find its way through the interstices, and the velocity of inflammation and combustion would become the same. Now, supposing one grain or particle alone be ignited, it will be inflamed over the whole surface, and the progressive combustion will take place from the exterior to the interior. Its rate of combustion will therefore
depend upon both its shape and size, leaving- out entirely for the present, the question of density and hardness. A particle of spherical or cubical form will expose less surface to ignition in proportion to its volume than one of an elon gated or flat shape, and will consequently require a long,er period for the com bustion of its entire mass; the larger the particle, also, the longer will be the time required for its combustion. Looking, then, at one grain of powder by itself, we may say that the larger it is, and the more nearly its form approaches a sphere, the longer will its combustion take, and the slower will be the evolution of the gas. When, however, we come to regard the action of an aggregation of such particles, as in the charge of a gun, the rate of ignition of the whole charge is also affected by the size and shape of the grain. The part of the charge first ignited is that near the vent, and the remainder is inflamed by contact with the heated gas generated by the combustion of this portion, so that the rate of ignition of the whole mass will be regulated by the greater or less facility. with which the gas can penetrate throughout the charge, which is itself dependent upon the shape and size of the interstices between the grains. If the grains be spherical and regular in form, the interstices will be comparatively large and uniform, and the gas will penetrate the mass with facility; again, the larg,er the g,rains, the larger the interstices between them. If, on the other hand, they be flat or flaky and irregular in shape, the passage of the gas will be more difficult, and the rate of inflammation of the charge reduced. We see, therefore, that the considerations which affect the more or less rapid combustion of an individual grain of gunpowder, also affect the rate of ignition of a charge of such grains. but in an opposite direction; so that a form of grain which individually burns rapidly tnay offer an increased resistance to the passage of the heated gas through the charge, and thereby retard its ignition, while a {.7rain which will burn more slowly may allow of the charge being more rapidly ignited. fly varying the size and shape of the grain alone, a powder may therefore be obtained, a charge of which shall be ignited rapidly throughout, but burn comparatively slowly, or one which shall be ignited more slowly, but when once inflamed burn very rapidly.. It is necessary to draw a clear distinction between a rapidly igniting and a quickly burning powder. The heat developed increases with the charge, and as the ve locity of the gases increases with their temperature, it is therefore evident that a large charge is consumed quicker than a small one; it is also true that the loss of heat absorbed by the surface of the bore is much less sensible when the charge is greater than when it is small; that is, the quantity ab sorbed is proportional to the surface of the square of the caliber of the gun and the heat developed increases as the cube of the caliber.