EXPLOSIVES 809 We have no certain means of directly estimating the temperature of explosion, but if it be assumed, as is usually done, to be the temperature the total products of combus tion would attain to if the whole amount of heat generated by the explosion were applied to raise them, under con stant volume, from absolute zero, we have the relation H-Tc,, in which H represents the number of units of heat generated, and c the specific heat under constant volume of the united products, gaseous or otherwise. The quan tity of heat can be obtained by experiment, and this livided by the specific heat, will give the temperature. The chief sources of error lie in the assumption that the specific heat remains constant throughout the great range of tem perature in question, and in the additional quantities of heat disengaged by secondary reactions. The value of T thus found probably will therefore be higher than the real temperature. Having regard to the above reasoning, it may be generally concluded that the amount of force exerted by an explosive substance depends upon (1) the volume of yas or vapour produced by the transformation, compared with that of the original substance ; and (2) the temperature of explosion, which determines the extent to which the gases are ex panded, or their tension increased ; or, in other words, the explosive force is directly proportional to the heat of combustion, and the volume of gas and vapour calculated at C. and 7 GO mm. pressure, and inversely proportional to the specific heat of the mixed products. It has been supposed by Berthelot and others that the volume of gas produced may possibly be still further in creased by the partial or total "dissociation" of the compound gases, at the high temperatures concerned ; for example, that the carbonic acid (CO ) may be decomposed into carbonic oxide (CO) and oxygen, or the aqueous vapour into oxygen and hydrogen. However, Noble and Abel demonstrate that, in the former instance, the loss of temperature, consequent upon the absorption of heat by the decomposition, would more than compensate for the increase of volume by dissociation. It must also be remembered that, if the temperature be extremely high, so also is the pressure under which dissociation must take place. "We may therefore consider that it has no sensible influence upon the explosive force. It is most important to distinguish between explosive force and explosive effect, the latter in great measure depending upon the rapidity with which the metamor phosis 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; (6.) the external conditions under which it is fired; (c.) the mode of firing or exploding. The physical or mechanical state of the explosive sub- stance 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 gunpowders 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 fibres and accelerate the com bustion, 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-cottou burns comparatively slowly in air, even when dry ; in the wet state, it merely smoulders away, as the portions in contact with the fire successively become 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 External when fired in the open air and under strong confinement ; condi- not only the rate of burning, but even, to a certain tions * extent, the products 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 out wards, is forced into the interstices under immense pressure, and the decomposition greatly accelerated. The amount of confinement 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 transformation is more rapid, until it may be said to reach the vanishing point. For example, the very violent ex plosive chloride of nitrogen is usually surrounded, when exploded, with a thin film of water ; Abel states that if this film, not exceeding -j-jnnr ^ ncu ^ n thickness, be removed, the explosive effect is much lessened. Nitro- glycerin, again, when detonated by a fulminate, is sufficiently confined by the surrounding atmosphere. By the same means, gun-cotton may be exploded unconfined, if com pressed, the mechanical cohesion affording sufficient restraint. In the case of wet compressed gun-cotton, which can be detonated with even fuller effect than dry, the mechanical resistance is greater, the air-spaces being filled with incompressible fluid. The manner in which the explosion is brought about Mode of has a most important bearing upon the effect produced. f x pl^- This may be done by the direct application of an ignited 1Dg or heated body, by the use of an electric current to heat a fine platinum wire, or by means of percussion, con cussion, or friction, converting mechanical energy into heat. A small quantity of a subsidiary explosive, such as a com position sensitive to friction or percussion, is often em ployed, for the 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 Detona- first used merely to generate sufficient heat to ignite the tlou - 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 endeavour ing to employ nitro-glycerin for practical purposes, found considerable difficulty in exploding it with certainty; he at length, in 18G4, by using a large percussion cap, charged with fulminate of mercury, obtained an explosion of great violence. This result led to the discovery thab 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 attri buted to the ordinary combustion, however rapid, of the body in question ; in fact, the whole mass of the explosive is converted into gas with such suddenness that it may, practically, be considered instantaneous; this sudden trans formation is termed "detonation." Of the substances
VIII. 102