Methods of Firing spies may be made to explode by heating them to their explosion temperatures. This may be accomplished by a flame, a spark, an incandes cent body, friction, percussion, concussion, pres sure or chemical action, provided the resulting temperature is sufficiently high. Gunpowder was originally ignited in muskets, guns and boreholes by the application of a torch, a glow ing tinder or a heated iron rod. Later mus kets were fired by the sparks from a flint strik ing steel. Joseph Egg, an English gunmaker, 1815, invented percussion caps and to-day all fixed ammunition is fired by percussion primers, while the charges in very large guns are fired by friction primers or by electric primers. In firing blasts a train of powder was laid from the mine to a safe distance by which to convey the inflammation. Later quills, straws or paper or wooden tubes were used to hold the train and permit of the charge being tamped so as to secure the advantage which comes from con fining an explosive. Bickford of Tuckingwall, England, 1831, invented the ((running) or "tape° or ((safety) fuse, in which a core of gunpowder is enclosed in a tube of jute yarn impregnated with a waterproof composition, and this is to-day largely used in firing blasts. Dr. Watson of England, 1745, succeeded in igniting gunpowder by means of electric sparks, and in 1749 a bat tery of 11 guns was fired by means of a fric tional electric machine at Annapolis, Maryland. Dr. Robert Hare of Philadelphia, 1832, invented the method of firing gunpowder charges by ineans of the electric current, using low tension fuses, and he had for some time prior to this employed the voltaic battery, in eudiometrical experiments, for igniting explosive mixtures of gases. Sir Charles Wheatstone, 1856, intro duced the magneto-exploder in blasting, and Moses G. Farmer of Newport, R. I., 1871, in vented the dynarno-electric machine with which to generate the current and applied it to firing electric ignitors or detonators, and to-day great guns, military and naval mines, and torpedoes and many blasts, especially those in which sev eral charges are to be fired simultaneously or where the blast is under water, are fired by this means.
It has long been known that when a notable quantity of a mixture of two volumes of hydro gen with one volume of oiygen is ignited at any point, the inflammation extends almost instan taneously throughout the mass and a most vio lent explosion ensues. This phenomenon is called detonation, and this mixture has long been known under the name of detonating gas. Certain chemical compounds such as nitrogen chloride, nitrogen iodide and mercuric fulminate undergo an almost instantaneous decomposition, giving rise.to violent explosive effects, if heated, struck or rubbed, and they are lmown as de tonating explosives. At first g-uncotton, nitro glycerin and dynamite were fired by ignition, as gunpowder had been, but Nobel, 1866, discov ered that if a quantity of mercuric fulminate was detonated in contact with nitroglycerin or dynamite they then underwent a detonating explosion also. E. O. Brown of the chemical department at Woolwich, England, 1868, discov ered that not only could dry compressed mili tary guncotton be detonated in this way, but that if the dry charge, called a primer, was in contact with wet guncotton, the latter was de tonated also, even though it was saturated with or submerged under and in contact with water. There are then two kinds of explosion, namely, explosion by simple combustion and explosion by detonation. Explosives of the gunpowder class undergo only the first kind of explosion. Explosives of the nitric ester or nitrosubstitu tion classes undergo both kinds of explosions. Explosives of the fulminate class practically undergo only the second kind of explosion. A marked difference between the two kinds of ex plosion is found in the velocity with which the explosive reaction is propagated within the mass of the explosive. Thus in detonating gas, which can undergo both lcinds of explosion, Bunsen found, when using very narrow tubes where only combustion could take place, that the velocity of the reaction was 34 metres per sec ond, while Berthelot, using long and wide tubes in which detonation could take place, found that the velocity of the reaction was 2,810 metres per second. Experiments made by lay
ing trains of the materials and firing them showed that while the velocity of combustion of gunpowder in the open air is but from 1.5 to 3.4 metres per second, the velocity of detonation of dynamite and of guncotton is about 6,000 metres per second. Provided each explosive gave the same voltnne of gases, having the same temperature and that the conditions were in all other respects alilce, it is evident that the explosion in wh:ch the reaction velocity was the greater would be the more violent one. But they are not alike, since guncotton, nitroglycerin and mercuric fulminate, besides being endo thermos compounds, are completely resolved into gases, while gunpowder, besides being a mixture, yields but about 44 per cent of gaseous products. The pressure developed by gunpow der when fired in a space completely filled by it is, from Noble and Abel's experiments, 6,150 atmospheres. The theoretical pressure de veloped by guncotton, notroglycerin and mer curic fulminate when detonated in contact with bodies, are respectively 24,000 atmospheres, 25,000 atmospheres and 28,000 atmospheres. In vestigation has shown that no other explosive lmown will give in contact an instantaneous pressure at all comparable with that of mer curic fulminate.
Owing to these differences in behavior ex plosives have been distinguished as high ex plosives and low explosives. Gunpowder and explosives of the nitrate class, together with smokeless powder are low explosives. They act with comparative slowness and are used where it is desired to lift or push a load without cracking it or in propelling projectiles. Nitro glycerin and explosives of the class of nitric esters or nitrosubstitution bodies are high ex plosives. They are shattering and crushing in their effect. Detonators containing mercuric fulminate are used to fire high explosives, the detonators being themselves fired by means of a Bickford fuse or by an electric current. The high explosives are usually put up for use in paper cylinders, and, when loaded for shipment, these are called cartridges or ((sticks.° Usually several sticks are required for one borehole. In this case the detonator is inserted in one of these sticks, usually the last one inserted in the hole, and this stick is then called the priming cartridge or (primer.° Direction in Which Explosives Explode. —There is a very common but erroneous belief that gunpowder explodes upward and that high explosives explode downward. It arises from the fact that if a quantity of dynamite be laid unconfined upon the surface of the rock on de tonating it the rock will often be fractured, whereas if gunpowder is thus placed and fired, it produces no effect whatever upon the rock. The facts are that each explosive tends to act in all directions about the centre of explosion. As exposed in air they are subjected to the weight of the air above them and are to that extent under confinement. When the high ex plosive is detonated the gases are set free so fast that the air acts as a tamping agent; when the gunpowder explodes the gases are generated so slowly that they have time to move the air. Besides the °pressure in contact° exerted by the high explosives is enormously greater than that of the low explosives. That high ex plosives exert pressure in all directions was demonstrated by Munroe by fastening blocks of guncotton of equal size and weight on opposite sides of a plate of iron, but at some distance apart, by means of light cords or wires, sus pending the plate in any position and detonat ing both charges simultaneously when holes were blown through the plate away from each of the charges. Although high explosives usu ally produce a shattering effect when fired un confined upon a body the effect is markedly increased by increasing the confinement. Thus in the method of breaking boulders and large masses of stone called aplastering,° after the charge is laid upon the stone and fused, one or two shovelfuls of earth are placed over it be fore firing. In ublockholing,° when it is de sired to break the rock into fragments of definite size, a small hole is bored in the rock and the explosive inserted in this cavity whereby the gases obtain a greater leverage.