It is most important to distinguish between explosive force and explosive effect, the latter in great measure depending upon the rapidity with which the metamorphosis takes place, while the same amount of force may be exerted suddenly or gradually. We may, therefore, consider that the explosive effect varies directly as the volume of gas produced and the temperature of explosion, and inversely as the time required for the transformation. But the time, and, to a certain extent, the products and temperature, will vary with (a) the physical state of the explosive substance; (b) the external condi tions under which it is fired; (c) the mode of firing or exploding.
The physical or mechanical state of the explosive substance has a most important bearing upon the effect obtained from it. To prove this, it is only necessary to point to the very different results given by made with the same proportions of the three ingredients, but varying in density, and in shape and size Of grains or pieces.
Gun-cotton is even more affected by variations in mechanical condition. In the form of loose wool, it burns so rapidly that gunpowder in contact with it is not inflamed; plaited or twisted tightly, its rate of combustion in air is greatly modified. This is due to the fact that the inflammable carbonic oxide, which is evolved by the decomposition from the want of sufficient stored-up oxygen to oxidize completely all the carbon of the gun-cotton, cannot penetrate between the fibers and accelerate the combustion, but burns with a bright flame away from the surface of the twisted cotton; when the yarn is yet more compressed by any means, the temperature is not kept up to the height necessary for the combustion of the carbonic oxide, so that it escapes unconsumed, abstracting heat, and yet more retarding the rate of burning. For the same reason, pulped and compressed gun-cotton burns comparatively slowly in air, even when dry; in the wet state, it merely smolders away, as the portion in contact with the the suc cessively becomes dried. Yet this same wet compressed gun-cotton can be so used as to constitute one of the most powerful explosives known.
It is well known that gunpowder behaves differently when in the open air and under strong confinement; not only the rate of burning, but even, to a certain extent, the pro ducts of combustion are altered. We have discussed the effect of tightly plaiting or compressing gun-cotton; but when confined in a strong envelope, the whole of the inflamed gas, being unable to escape outwards, is forced into the interstices under immense pressure, and the decomposition is greatly aceeldrated. The amount of con finement or restraint needed by any explosive depends, however, upon the nature of the substance and the mode of exploding it, becoming very much less as the transforma tion is inure rapid, until it may be said to reach the vanishing-point. For example, the very violent explosive chloride of nitrogen is usually surrounded, when exploded, with a thin film of water. Abel states that if this film, not exceeding in. in thickness, be removed, the explosive effect is much lessened. Nitro-glycerine, again, when detonated by a fulminate, is sufficiently confined by the surrounding atmosphere.
By the same means, gun-cotton may be exploded unconfined, if compressed, the mechanical cohesion affording sufficient restraint. In the ease of wet compressed gun cotton, which can be detonated with even fuller effect than dry, the mechanical resist ance is greater. the air-spaces being filled with incompressible fluid.
The manner in which the explosion is brought about has a most important bearing upon the effect produced. This may be done by the direct application of an ignited or heated body, by the use of an electric•current to heat a fine platinum wire, or by means of percussion, concussion, or friction, converting mechanical energy into heat. A small quantity of a subsidiary explosive, such as a composition sensitive to friction or per cussion, is often employed, for tile sake of convenience, to ignite the main charge, the combustion spreading through the mass with more or less rapidity, according to the nature of the substance.
Although subsidiary or initiatory explosives were at first used merely to generate sufficient heat to ignite the charge, and are often still so employed, they have of late years received an application of far wider importance. Mr. Alfred Nobel, a Swedish engineer, while endeavoring to employ nitro-glycerine for practical purposes, found considerable difficulty in exploding it with certainty; he at length, in 1804, by using a large •percussion cap, charged with fulminate of mercury, obtained an explosion of great violence. This result led to the discovery that many explosive substances, when exploded by means of a small quantity of a suitable initiatory explosive, produce an effect far exceeding anything that can be attributed to the ordinary combustion, how ever rapid, of the body in question; in fact, the whole mass of the explosive is con verted into gas with such suddenness that it may, practically, be considered instan taneous. This sudden transformation is termed "detonation." Of the substances 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-glycerine, are detonated or not, according to the mode of explosion, Indeed, Abel has proved that most explo sives, including gunpowder, can be detonated, provided the proper initiatory charge be employed. Roux and Sarrau have divided explosions into two classes or orders " detonations" or explosions of the first order, and "simple explosions" of the second order. They made a series of experiments with the object of determining the compar ative values of various explosive substances, detonated, and exploded in the ordinary manner; the method employed was to ascertain the quantity of each just sufficient to produce rupture in small spherical shells of equal strength. The following tablf; gives the comparative results for the three most important explosive substances: