15. Choice of Explosive.—From these experimental mines it may be concluded that for forming craters in ordinary earth dynamite is not quite so efficient as double its weight of good gunpowder. For breaking up hard rock, blowing up strong masonry, and especially in demolitions where tamping is usually defective, this ratio does not hold; but the relative effect of the high explosive increases continually with the lack of tamping and the intensity of the local blow desired, until a point is reached at which the effect of gunpowder is almost imperceptible, while the high explosive does efficient work. This property of the high explosives renders them extremely valuable for use in hasty demolitions, such as blowing up palisades or barriers, destroying guns, etc., etc.
Owing to their varying values in different conditions the choice of explosive to be used in any particular case must evidently depend upon the circumstances attending it.
In underground explosions both gunpowder and high explosives give out noxious gases which penetrate the soil, and which entering a gallery in sufficient quantity would suffocate the miners. Of these gases the carbonic oxide given off by some of the high explosives is probably the most dangerous to human life, and if mixed with the proper proportion of air forms an explosive mixture, resembling in this respect the fire-damp of the coal-mines. Whether in practical mining operations it would ever be retained in the soil in such quantities as to produce this effect remains to be seen.
Some of the high explosives, on the other hand, seem to produce relatively small quantities of noxious gases. The gases produced by gunpowder, while suffocating in their nature, have the advantage of always making their presence known by their odor.
16. For use in overcharged mines designed to break in the enemy’s galleries, the high explosives, from the violent character of their explosion and from the phenomena exhibited in submarine mining, promise to give relatively greater radii of rupture than gunpowder; but sufficient data are not available to state this positively.[12]
17. Beside the considerations above stated, which refer to the effects produced by the explosive when fired, there are others equally important relating to the safety and facility with which the explosive may be transported, handled, and placed in the mines. The latter will frequently have greater weight than the former in determining the explosive to be used in any particular case which may arise in the practical operations of mining. Of the latter considerations some of the most important in deciding whether to use gunpowder or high explosives are the following, viz.:
Gunpowder is easily obtained, and most enlisted men are more or less familiar with its properties.
It explodes when ignited by fire.
It does not ordinarily explode when struck by a bullet.
It is injured by moisture and destroyed by thorough wetting.