Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/846

Rh 810 EXPLOSIVES capable of producing such action, fulminate of mercury is the most important. Some explosives appear always to detonate, in whatever manner they may be exploded, such as chloride and iodide of nitrogen ; the explosive effect is therefore much greater than that of a slower explosive substance, although their explosive force may be less. Again, other substances, such as gun-cotton and nitro-glycerin, are detonated or not accord ing to the mode of explosion. Indeed, Abel has proved that most explosives, including gunpowder, can be de tonated, provided the proper initiatory charge be employed. Roux and Sarrau (Comptes Rendus, 1874) have divided Two explosions into two classes or orders, &quot; detonations &quot; or orders explosions of the first order, and &quot;simple explosions&quot; of ^ e *~ the second order. They made a series of experiments with the object of determining the comparative values of various explosive substances, detonated, and exploded in the ordinary manner ; the method employed was to ascer tain the quantity of each just sufficient to produce rupture in small spherical shells of equal strength. The following table gives the comparative results for the three most im portant explosive substances : Explosive Effect. Second Older. First Order. Gunpowder. i-oo 3-00 4-80 4-34 6-46 10-13 (iim-cotton or nitre-cellulose Nitro-glycerin These experiments, although valuable, cannot be con sidered as affording a precise method of comparison ; the results would be affected, inter alia, by the impossibility of ensuring that the shells were all of the same strength, a point of great importance, considering the very small weights of each explosive used ; also the rate of combustion, and therefore the explosive effect, of gunpowder is materially affected by its mechanical condition, so that different powders would give a varying standard of comparison. However, they afford fair evidence that, when detonated, gun-cotton has about six times, and pure nitro-glycerin about ten times the local explosive effect of gunpowder simply ignited in the ordinary manner ; nitro-glycerin is visually employed in the form of &quot;dynamite,&quot; mixed with some inert absorbent substance, so that its power is pro portionately reduced. Theory The rationale of detonation is not yet understood. If the of de- transformation were due merely to the mechanical energy of Jonation. t j ie p ar ticles of gas, liberated from the initiatory charge at a tremendous velocity, being converted into heat by impact against the mass of the explosive substance, then it would follow that the most powerful explosive would be the best detonating agent; this is, however, by no means the case, for a few grains of fulminate of mercury in a metal tube will detonate gun-cotton, whereas nitro-glycerin, although possessed of more explosive force, will not do so unless used in large quantities. The fact of its being possible to detonate wet gun-cotton is also a proof that the action cannot be due to heat alone. It would rather seem to be what Professor Bloxam terms &quot;sympathetic&quot; explosion; the experiments of Abel, as well as those of Champion and Pellet in France, appear to indicate a vibratory action of the detonating agent upon the ultimate particles of the substance to be exploded. An explosive molecule is most unstable, certain very delicately balanced forces preserving the chemical and physical equilibrium of the compound. If these forces be rapidly overthrown in succession, we have explosion; but when, by a blow of a certain kind, they are instantaneously destroyed, the result is detonation. Just as a glass globe may withstand a strong blow, but be shattered by the vibration of a particular note, so it is con sidered by some authorities that, hi the instance cited, the fulminate of mercury communicates a vibration to which the gun-cotton molecule is sensitive, and which overthrows its equilibrium ; it is not sensitive to the vibrations caused by the nitro-glycerin, which only tears and scatters it mechanically. Although the action of detonation has been spoken of as instantaneous, and may practically be so con sidered, yet a certain infinitesimal duration of time is required for the metamorphosis ; different substances possess, doubtless, different rates of detonation, for we can scarcely conceive of a mechanical mixture, such as gunpowder, being so sensitive to the action of the detonating impulse as a definite chemical compound, and the rate even varies slightly, for the same explosive, with its physical state. It has been shown, by means of Captain A. Noble s chronoscope, that compressed gun-cotton, when dry, is detonated at a velocity of from 17,000 to 18,000 feet a second, or about 200 miles a minute ; by using a small primer of dry gun-cotton, the same substance in the wet state may be detonated at the increased rate of from 18,000 to 21,000 feet a second, or about 240 miles a minute. The following results are taken from experiments on detonation and its applications, carried out by F. A. Abel, C.B., F.11.S.: I. Illustrating some of the conditions ichicli promote the detona tion of an explosive siibstancc (a) Quality of the initial detonation; (b) Resistance to mccJianical dispersion offered by the mass of the substance to be detonated. 1. A fuze containing rather more than 1 oz. gunpowder, strongly confined, exploded in contact with a mass of compressed gun-cotton, only inflames it, although the explosion of the fuze is apparently a sharp one. 2. Forty-five grains of fulminate of mercury, exploded unconfined on the surface of a piece of compressed gun-cotton, only inflames or disperses it. 3. A fuze containing 9 grains fulminate of mercury, strongly confined, exploded in contact with compressed gun-cotton, or dyna mite, detonates it with certainty. 4. An equal quantity of fulminate, similarly confined, does not detonate uncompressed gun-cotton in which it is imbedded, hut merely disperses and inflames it. 5. 150 grains compressed gun-cotton, detonated in proximity to dynamite, detonates the latter. 6. 3 oz. of dynamite, and very much larger quantities, detonated in contact with compressed gun-cotton only disperses it I 1. Tra nsmission of Detonation. 7. Detonation being established at one extremity of a continuous row of distinct masses of compressed gun-cotton, or dynamite, travels the whole length thereof. Stretching insulating wires across the row of discs, at intervals of six feet, their rupture by the detonation gives spark-records on the cylinder of Noble s chrono scope, by means of which the rate of transmission can be calculated. 8. A row of gun-cotton discs, of any length, placed 5 inch apart, can all be detonated from one end. 9. Discs of compressed gun-cotton, weighing about 8 oz. each, being placed 6 inches apart, the detonation of the central dis only blows away or breaks up the neighbouring masses. 10. About 2 oz. compressed gun-cotton being inserted into one extremity of a wrought-iron tube 5 feet long, its detonation is trans mitted to a disc of compressed gun-cotton inserted into the other extremity of the tube, III. Applications of Detonation. 11. A wrought iron rail can be destroyed by detonating 8 oz. of compressed gun-cotton placed uuconfined upon the rail. 12. A piece of wet gun-cotton, quite uninflammable, removed from a fire and detonated upon a block of granite, using a small primer of dry gun-cotton, shatters the block. 13. A stockade can be destroyed by means of a flat charge built up of wet gun-cotton slabs, detonation being established by means of a small portion of the charge in a dry state. 14. A submerged charge of wet gun-cotton, open on all sides to the water, and merely confined around the dry initiative charge, or primer, by means of a net, can be detonated. Many attempts have been made, especially by foreign for(J8 chemists and physicists, to arrive at an exact determination ex pi of the comparative force of explosive substances. The sives.