| Gunpowder | 1·00 |
| Gun-cotton | 2·57 |
| Dynamite | 4·23 |
| Nitro-glycerine | 5·71 |
Hence, for a given height of charge in a bore-hole, gun-cotton exerts about 21⁄2 times the force of gunpowder, and dynamite about 41⁄4 times that force.
Section IV.—Means of Firing the Common Explosive Agents.
Action of Heat.
—We have seen that the oxygen required for the combustion of the carbon in gunpowder is stored up in the saltpetre. So long as the saltpetre remains below a certain temperature, it will retain its oxygen; but when that temperature is reached, it will part with that element. To fire gunpowder, heat is therefore made use of to liberate the oxygen, which at once seizes upon the carbon with which it is in presence. The means employed to convey heat to an explosive have been described in the preceding chapter. It is necessary to apply heat to one point only of the explosive; it is sufficient if it be applied to only one grain. That portion of the grain which is thus raised in temperature begins to “burn,” as it is commonly expressed, that is, this portion enters at once into a state of combustion, the saltpetre giving up its oxygen, and the liberated oxygen entering into combination with the carbon. The setting up of this action is called “ignition.” The hot gases generated by the combustion set up ignite other grains surrounding the one first ignited; the gases resulting from the combustion of these ignite other grains; and, in this way, ignition is conveyed throughout the mass. Thus the progress of ignition is gradual. But though it takes place, in every case, gradually, if the gases are confined within the space occupied by the powder, it may be extremely rapid. It is easy to see that the gases evolved from a very small number of grains are sufficient to fill all the interstices, and to surround every individual grain of which the charge is composed. But besides this ignition from grain to grain, the same thing goes on from the outside to the inside of each individual grain, the grain burning gradually from the outside to the inside in concentric layers. The successive ignitions in this direction, however, of layer after layer, is usually described as the progress of combustion. Thus the time of an explosion is made up of that necessary for the ignition of all the grains, and of that required for their complete combustion.
The time of ignition is determined in a great measure by the proportion which the interstices, or empty spaces between the grains, bear to the whole space occupied by the powder. If the latter be in the form of an impalpable dust, ignition cannot extend throughout the mass in the manner we have described; but we shall have merely combustion proceeding from grain to grain. If, on the contrary, the powder be in large spherical grains or pellets, the interstices will be large, and the first gases formed will flash through these, and ignite all the grains one after another with such rapidity that ignition may be regarded as simultaneous. Thus the time of ignition is shortened by increasing the size of the grains and approximating the latter to the spherical form.
But the time of combustion is determined by conditions contrary to these. As combustion proceeds gradually from the outside to the inside of a grain, it is obvious that the larger the grain is, the longer will be the time required to burn it in. Also it is evident that if the grain be in the form of a thin flake, it will be burned in a much shorter time than if it be in the spherical form. Thus the conditions of rapid ignition and rapid combustion are antagonistic. The minimum time of explosion is obtained when the grains are irregular in shape and only sufficiently large to allow a fairly free passage to the hot gases. There are other conditions which influence the time of combustion; among them is the density of the grain. This is obvious, since the denser the grain, the greater is the quantity of material to be consumed. But besides this, combustion proceeds more slowly through a dense grain than through an open one. The presence of moisture also tends to retard combustion.
The progress both of ignition and of combustion is accelerated, not uniform. In proportion as the grains are ignited, the gases evolved increase in volume, and as the progress of combustion continues to generate gases, the tension of these increases, until, as we have seen, the pressure rises as high as 42 tons to the square inch. As the pressure increases, the hot gases are forced more and more deeply into the grains, and combustion, consequently, proceeds more and more rapidly.
Detonation.
—By detonation is meant the simultaneous breaking up of all the molecules of which the explosive substance is composed. Properly the term is applicable to the chemical compounds only. But it is applied to gunpowder to denote the simultaneous ignition of all the grains. The mode of firing by detonation is obviously very favourable to the rending effect required of blasting powder, since it reduces to a minimum the time of explosion. It is brought about, in all cases, by means of an initial explosion. The detonator, which produces this initial explosion, consists of an explosive compound, preferably one that is quick in its action, contained within a case sufficiently strong to retain the gases until they have acquired a considerable tension. When the case bursts, this tension forces them instantaneously through the interstices of the powder, and so produces simultaneous ignition. A pellet of gun-cotton, or a cartridge of dynamite, the latter especially, makes a good detonator for gunpowder. Fired in this way, very much better effects may be obtained from gunpowder than when fired in the usual manner. Indeed, in many kinds of rock, more work may be done with it than with gun-cotton or with dynamite.