EXPLOSIVES (Fa., Explosives ; GEE., Knal le).
The industrial applications of explosives are for blasting or to give light and sound for signalling purposes ; explosives for the latter purpose are really fireworks on a large scale, and as such will be treated of in the article on Pyrotechnics. The best known explosives are gunpowder, gun-cotton, nitro-glycerine in various forms, known as dynamite, lithofracteur, giant powder ; gun-cotton and its derivatives, cotton powder and Schultes gunpowder, and to a smaller extent various fulminates and picrates.
An explosive mixture generally has two essential ingredients, one readily combustible, and the other containing oxygen, in considerable quantity, which it will part with easily. Carbon, with which hydrogen is usually associated, is almost always the combustible substance, although occasionally other oxidizable bodies, such as sulphur, are used. The carbon is most frequently in the form of charcoal, but any other organic substance eontaining it largely will afford the same action ; the oxidizing agents are almost invariably nitrates or chlorates, but the difference in the readiness with which these two salts give up their oxygen causes one or other of them to be preferred in the particular explosive of which they form a part.
Gunpowder.—The three ingredients of which gunpowder is composed are saltpetre, charcoal, and sulphur. Its exploding point is about 600° F. The qualities which most influence gunpowder are its density, the size and shape of grain, and the condition of the charcoal. It is essential that gun powder should be uniform in action ; the denser it is the slower it will burn, even in single grains, and dense powder will offer a smaller surface to ignition than an equal weight of a less dense kind. For similar reasons, a large grain will burn slower than a number of small grains making up the same weight, and a grain of regular shape such as a cube, a cylinder, or a sphere, will offer lees surface than an irregular one of the same mass ; laminated or flaky forms indicate a violent powder. There are other points, which are alluded to in the following detail of its manufacture.
The chief woods from which is manufactured charcoal for powder-making are the willow, the alder, and what is popularly known as the " black dogwood," but which is really the alder buck thorn, or berry-bearing alder (Rharnnus frangula).
Their adoption was no doubt decided empirically, for it is not easy even to determine why any particular woods are better adapted than others for charcoal for gunpowder. Though various other
woods are used for coarse blasting powder, the three named are generally selected for the best gunpowders.
Small wood of about ten years' growth, is in all cases preferred for powder-making. Alder and willow of this age will be probably 4-5 in. in diameter ; dogwood, about 1 in.; the two former woods in pieces of 3 ft. long, not less than 1 or more than 4 in. in diameter. The wood must be straight, perfectly sound, and entirely free from bark, and must have been felled during the spring of the current year. All wood is stacked on iron sleepers or on rows of brickwork; the alder and willow are provided with no protection from the weather ; the dogwood is covered with straw thatching.
It is desirable that all moisture should be expelled from the wood before the latter is placed in the retorts. This will be the case •after being stacked for a few months in summer. It may be safely asserted that there is no object in keeping wood for a number of years to season ; on the contrary, that provided a supply of wood can always be depended on, there is nothing to be gained by maintaining a large stock of cut wood.
The wood is converted into charcoal in cast-iron retorts, set in brickwork. Fig. 592 is a trans verse section of a set of cylinders ; Fig. 593 is a longitudinal section of one retort. Each retort has two pipes passing out of the inner end. When set, the lower pipe b is closed with brickwork, being intended for use should the cylinder be turned round and reset, the upper pipe a only being made use of. To the upper pipe is attached a branch leading to a horizontal pipe c, extending behind the whole set of retorts, from one end of which another pipe d descends perpendioularly, joining a leading directly into the furnace. Each cylinder has a false bottom of brickwork, in front of which is bolted on a piece of wrought-iron plate, having a circular hole in it corresponding to the uppermost pipe of the cylinder. The cylinders are closed with tight-fitting iron doors, secured by a powerful screw. The wood is placed in small cylinders of sheet-iron R termed slips, which are placed on small iron travelling carriages, on which they can be run up directly to the mouth of the cylinders, and shot into them direct. The back end of each slip is provided with a handle to facilitate withdrawal.