Page:Popular Science Monthly Volume 21.djvu/800

782 full of water, in which revolves a wheel having strips of steel upon its rim, similar strips projecting from the bottom of the tub. As the wheel revolves, the floating pieces are drawn between the steel strips and thus reduced to a fine pulp. This is again washed for a long time by mechanical means, after which it is taken to the press, where nearly all the water is expelled from it, the final pressure applied being about fifteen thousand pounds to the square inch. When taken from the press, it is in the shape of a disk or cylinder, of a close texture, easily broken when dry, and capable of being cut in a manner not unlike pasteboard. When a small quantity is ignited by a flame, it burns quite rapidly, but quietly, if dry; if wet, it is consumed very slowly. If a large quantity is ignited, there may be sufficient confinement of the inner portions by the outer shell to cause an explosion. If dry loose gun-cotton be ignited, it burns with extreme rapidity, like a flash, but without any violence; in fact, a wisp placed on a small pile of gunpowder and fired will not ordinarily cause its explosion. In order to have a complete explosion of the dry material, fulminate of mercury must be used, and this is accordingly employed in the primers made for the explosion of that which is wet. Two pounds of the dry, detonated by twenty-five grains of fulminate, will cause the detonation of five hundred pounds of the wet. In this wet state it must be carefully guarded from a low temperature, as the expansion of the water in freezing will tend to break up the disks.

Many experiments have been made with a view to its adoption in gunnery practice, but it is not at all likely that either it or any of the high explosives will ever displace gunpowder; their action is so violent and sudden that, before the projectile has time to take up its motion in the bore of the gun, the walls yield and the piece is burst. Its use for military purposes must therefore be confined to mines and torpedoes, as in the case of dynamite, or as a bursting-charge for shells, for which purpose experiments show that it is most admirably adapted. It is used in Europe for torpedo purposes, and is carried for that use by war-vessels of the English and other navies.

The explosive gun-cotton is not the only kind made; another sort (in which a less amount of the hydrogen of the cotton is displaced) is used for making collodion, largely employed by photographers. This variety, called collodion gun-cotton, combined with nitro-glycerine, forms a new explosive* agent called gum-dynamite, or explosive gelatine. Singular to relate, the ordinary gun-cotton used for explosive purposes will not enter into this combination, and hence probably the late discovery of the fact that it requires the collodion variety to do so. This is finely shredded, generally by hand, and placed in small quantities at a time in the nitro-glycerine, which is kept at a temperature of 80 Fahr. by means of a water-bath, the whole being constantly stirred with a wooden spatula; the proportion of materials is seven per cent by weight of the gun-cotton to ninety-three per cent of the nitro-glycerine.