Explosive Mechanism, in botany, (1) in flowers, an arrangement for the transference of pollen to an insect-visitor by a sudden movement of floral organs. (2) In fruits, an arrangement for the forcible expulsion of seeds from a fruit brought about in various ways (see Sling-fruits). The spores of Ferns, ascospores, and some conidia are also liberated explosively.
Explosives. An explosive is a substance or mixture of substances which, by the action of a blow or of heat, can be converted very easily and suddenly into a more stable substance or substances, usually gaseous, with the simultaneous liberation of a large amount of heat. Explosives are divided into classes according to the uses to which they are put, but the line of demarcation is not always very clear. The industrial and blasting powders may be either low or high explosives. A 'low' explosive explodes by the application of heat, and burns more or less uniformly and slowly, and projects neighbouring objects to a distance. A 'high' explosive explodes under a blow, and the whole of the substance is instantly transformed, and instantly exerts its maximum pressure, creating a violent disturbance in a limited area without necessarily projecting substances to any great distance. Service explosives, for naval and military purposes, are divided into propellants and high explosives. Sporting powders are specially modified propellant powders. Examples of these classes of explosives, with notes on their composition, are given below.
Low Explosives.—The best examples of the low explosive are gunpowder and similar mixtures. The constituents of the 'gunpowder' explosives are generally not explosive alone, but only when mixed. A 'gunpowder' mixture contains carbon or carbonaceous matter like wood-meal, hydrocarbons, starches, and sugars, &c., which burn owing to the presence of highly oxygenated substances like peroxides, chlorates and perchlorates, nitrates, permanganates, chromates and dichromates, all of which convey the necessary oxygen. In addition, there usually is present some very easily ignited substance like sulphur or sulphides, or phosphorus or phosphides, &c. As compared with other explosives, gunpowder or black-powder has certain advantages. It is cheap, easily ignited, insensitive to shock, and stable at moderately high temperature; it burns regularly, and its residue is non-corrosive. But it is weak in power, and produces much smoke. It is excellent for armour-piercing shell and for rings of time-fuses. Gunpowder made in different countries varies in composition, but for rifle, cannon, and sporting powders it, usually contains 74 to 75 parts of saltpetre, 9 to 14 parts of sulphur, 12 to 16 parts of charcoal. For blasting powders less saltpetre and more charcoal is used. Charcoal is made by the carbonization of wood. In England dogwood, alder, and willow woods are used; in Germany alder and willow are used; in France black alder and also white alder, poplar, aspen, birch, and hazel; in Switzerland hazel wood; in Spain oleander, yew, willow, hemp stems, and vine; in Italy hemp stems. The wood is generally carbonized in iron retorts. The product is allowed to cool out of contact with air, else it may inflame. Wood burnt for ordnance powders gives a yield of 20 to 30 per cent charcoal; that for small-arms gives a yield of 40 per cent. The charcoal contains from 68 to 85 per cent carbon, from 2.8 to 3.7 per cent hydrogen, from 12 to 27 per cent oxygen, and may have up to 5 per cent ash. The saltpetre is found naturally in Chile, India, and in other countries, and is refined by crystallization from water. It is a colourless, crystalline solid. Sulphur, a pale-yellow solid, melting-point 113° C., boiling-point 444·5° C., is found in nature, and is refined to a purity of 99·5 per cent and over. It has a low ignition temperature of 261° C., and makes the powder burn more readily. Under the pressure of the press and the incorporating mill it flows and cements the minute particles of charcoal and saltpetre together. The three
ingredients are ground, mixed, sieved, incorporated or mixed in drums or mills, broken down, and then pressed, corned or granulated, and glazed. Cannon powders receive an addition of graphite to reduce the rate of burning. The powder is then dried in a stove, finished in a reel to get rid of the last traces of dust, and blended. In the United States powder for blasting contains sodium nitrate instead of potassium nitrate. The powder is cheaper and stronger, but is hygroscopic. Sprengsaltpetre is largely used in Stassfurt salt-mines, where a mild explosive is required, and consists of 75 parts by weight of sodium nitrate, 10 parts by weight of sulphur, and 15 parts by weight of brown coal. It is cheap, and does not produce poisonous fumes. Bobbinite is largely used in coal-mines. It is black powder with ammonium and copper sulphates, possibly also starch and paraffin wax. When gunpowder explodes, the product consists of 43 per cent gases, 56 per cent solids, and the rest water. The composition of other industrial and blasting powders is given in the sequel.
Propellant Explosives.—The chief propellants arc nitrocellulose, also called nitrocotton or guncotton, and nitroglycerine.
Nitrocellulose.—The chief sources of cellulose are wood and cotton. When cotton is plentiful, nitrocellulose is made as follows. Cotton-waste is hand-picked to get rid of string, wood, &c.; it is opened out by a teasing-machine, which tears off small portions at a time, and the cotton is then dried to about 0.5 per cent moisture content. The cotton is then nitrated with 'mixed acid'—a mixture of about 16 per cent nitric and 75 per cent sulphuric acid and about 8 per cent water—at 15° to 25° C. After the nitration, the acid is removed and the nitrocotton boiled up in water to stabilize it. Generally nitrocotton contains about 12 to 13 per cent of nitrogen. Wet nitrocotton is quite safe although it can be detonated, but dry nitrocotton is very dangerous. To-day, paper is usually made from wood-pulp, and when the cotton supplies of Germany were stopped during the European War, nitrocellulose had to be made from wood-pulp via a form of paper crêpe prepared by the Germans from the pulp. For propellant purposes the nitrocotton is 'gelatinized', either alone or mixed with nitroglycerine, and is then worked up into different forms, such as wire, rods, grains, or tape, when it becomes controllable at will, so that the firing is not dangerous.
Nitroglycerine.—Mixed acid, containing 41 per cent nitric acid and 57.5 per cent sulphuric acid, is brought to 22° C. by cooling coils of brine, and pure glycerine is injected into the acid at such a rate that no glycerine accumulates unchanged, and that the temperature is kept between 15° and 22° C. When all the glycerine has been added, the liquid is allowed to stand, and the nitroglycerine rises to the surface. It is run off to the wash-house, where it is washed free from acid and settled. The process is a dangerous one, and great care must be taken at every stage of the manufacture. The floors of the plant must be free from grit and dirt, no accumulation of liquid should be allowed anywhere, special clothing and rubber boots must be worn, no metallic implements may be used, and the plant should not be handed over for repairs except under the supervision of a responsible person. Nitroglycerine, when absorbed in a porous earth called 'Kieselguhr', is called dynamite. Kieselguhr, or simply guhr, absorbs twice its weight of nitroglycerine; cork charcoal absorbs nine times its weight. Dynamite cartridges are generally exploded by detonators.
Preparation of Cordite; Nitrocellulose Tape (N.C.T.); Ballistite; &c.—For cordite, the nitrocotton, freed from moisture, is mixed with nitroglycerine, and the paste or the cotton itself, if N.C.T. is to be made, is incorporated into a uniform dough with ether and alcohol. Some mineral jelly is added to render the explosive more stable. The dough is pressed through different sizes of dies according to the product desired. For rifle powder fine cords are used; for artillery, thicker cords or flat ribbons of varying thicknesses are required. The cords or tapes from the dies are cut into suitable lengths, the solvents driven off, and the products blended to obtain uniform ballistic quality. For ballistite the nitrocellulose is beaten up with nitroglycerine in water. The paste is freed from water, dried, and worked into horn-like sheets by means of rollers.
High Explosives; Picric Acid.—At the outbreak of the European War the chief high explosive of the Entente Powers was lyddite (in France, mélenite), also called trinitrophenol or picric acid. It is a bright-yellow solid, melting-point 122° C., sparingly soluble in water, and forms easily exploded metallic salts. It is now displaced by trinitrotoluene. Picric acid is made from phenol or carbolic acid. Phenol is obtained from coal-tar, or made synthetically from benzene. The phenol is sulphonated with strong sulphuric acid, and the phenol-sulphonic acid resulting is nitrated with strong nitric acid at about 100° C. Picric acid separates, and is washed free from mineral acid and dried. It may also be made from benzene without converting it into phenol thus: The benzene is chlorinated and gives chlorbenzene. This is nitrated into dinitrochlorbenzene, and is then treated with caustic soda to give dinitrophenol. This is then further nitrated into trinitrophenol or picric acid. Picric acid has a high melting-point, it must be used pure, is dissolved by water, it attacks metals
forming dangerous compounds, and requires troublesome plant for its manufacture. Hence it has been displaced by more suitable substances, notably by trinitrotoluene.