808 EXPLOSIVES bustible and the supporter of combustion, in the closest possible union ; we can therefore understand its action being much more sudden and violent than that of the most intimate mechanical mixture. Nitro- The chief explosive compounds are formed from some substitu- organic substance containing carbon, hydrogen, and oxygen, tion pro- by introducing into it, through the action of concentrated ducts. n it r i c acid, a certain portion of nitric peroxide (NO 2 ), in substitution for an equivalent amount of hydrogen. A new compound, differing outwardly very little, if at all, from the original substance, is thus formed, but in a very un stable state of chemical equilibrium, because of the feeble union of the nitrogen and oxygen in the NO 2 molecule. A slight disturbing cause brings into play the stronger affinity of the carbon and hydrogen for the large store of oxygen contained in the new compound. Gun-cotton and nitro-glycerin are the leading members of this group, being produced in a precisely similar manner, by the substi tution of three molecules of NO 2 for three atoms of hydro gen (H). As those explosives will be elsewhere described in detail, we give the formation, as a representative member Picric of the group, of nitro-phenol, or picric acid, by treating acid. > pj ieno ] j or carbolic acid, with a mixture of nitric and sul phuric acids, the latter being required to absorb the water, and preserve the full strength of the nitric acid : C 6 H 6 + 3HN0 3 Carbolic acid. Nitric acid. Picric acid. 3H 2 0. Water. The formula of the product may be empirically written C 6 H 3 N 3 O 7 ; it is, like gun-cotton and nitro-glycerin, a tri-nitro substitution product. Only the picrates, cr salts of picric acid formed with potassium or ammonium, are used in practice, as possessing more force than the uncom- bined acid. From starch can be obtained, in a strictly analogous manner, an explosive called xyloidine, which is a bi-nitro product, two molecules of nitric peroxide being substituted for two atoms of hydrogen. In the case of nitro-mannite, an explosive made from mannite, one of the sugars, as many as six molecules of the NO 2 are inserted. The number of nitro-substitution products is very great, many of them being more or less violently explosive. Ful- The fulminates are among the most violent of all explosive cunates. compounds, their chemical stability being very small. Sud den in action, their effect is great locally ; thus they are well adapted to the purpose, for which alone they are practically used, of igniting, or upsetting the equilibrium of, other explosives. Fulminate of mercury is produced by adding alcohol (C 2 H 0), under careful precautions, to a solution of mercury in nitric acid ; a grey crystalline precipitate is obtained, eery heavy (sp. gr. 4 4), and so sensitive to friction or per cussion that it is kept in the wet state. The results of analysis show one atom of mercury, and two each of car bon, nitrogen, and oxygen, so that the formula may be empirically written HgC 2 N 2 2 , or perhaps more correctly HgO.C 2 N 2 O ; the chemical factor C 2 ISr 2 O is called fulminic acid, but has never been produced separately. Opinions differ as to the precise "rational" formulas of the ful minates, some chemists considering their process of forma tion to be similar to that of the nitro-substitution products. It will be observed that two atoms of nitrogen take the place of hydrogen, being the ratio of combining propor tions of those elements. The products of combustion are carbonic oxide, nitrogen, and metallic mercury, and the violence of action is due to the sudden evolution of a volume of gas and vapour very large in comparison with that of the substance, its density being so great. This fulminate enters into the composition used for percussion caps and electric fuzes; its practical value has of late years been immensely increased by the discovery of its power, even in very small quantities, to procmce the almost in stantaneous decomposition of several explosive substances. Fulminate of silver is prepared in a similar manner, but, being far more sensitive, is of little practical value ; it is employed, in very minute quantities, in making such toys as detonating crackers. The difficulties in the way of estimating, with any accuracy, the force of explosive substances are very great, especially as no definite standard of comparison can be laid down. However, by means of theoretical considerations, combined with the results of actual experiment, a tolerably fair approximation may be arrived at. When an explosive substance is exploded in a closed Maxi vessel sufficiently strong to resist rupture, the tension mum attains its maximum value in an extremely short space of S1 . on i time and gradually decreases from the heat being conducted vesse l ] away by the metal envelope, and dispersed by radiation. It has, however, been demonstrated that, at the moment of maximum tension, the loss of pressure due to the commu nication of heat to the vessel, if the latter be filled with the explosive, is less than one per cent. The products of com bustion. after cooling down, can easily be determined by analysis, and are then either (a) wholly gaseous, as for chloride or iodide of nitrogen ; (b) gaseous and liquid, in the case of gun-cotton and nitro-glycerin; or (c) gaseous and solid, as with gunpowder. It is certain that, at the moment of explosion, the products of the more violent explosive compounds are wholly in a state of gas or vapour, but we should arrive at incorrect results by making the same assumption in the case of a mechanical mixture like gunpowder. The experiments of Noble and Abel on "Fired Gunpowder " (Phil. Trans. Roy. Soc., 1874), which are the most complete ever undertaken, show that the ultimately solid residue is, at the moment of explosion, in a liquid state, and most probably in a very finely divided condition; moreover, that, at that instant, it occupies a space the ratio of which is about - 6 that of the original volume, sup posing the substance to fill the vessel in which it is exploded. Provided the laws concerned can be supposed to hold good at such high temperatures, we may assume for the gaseous products of combustion the well-known equation of the elasticity and dilatibility of permanent gases pv=*Rt ....... (1), where R is a constant, and t reckoned from absolute zero ( - 273 C). For the sake of convenience, we will consider that a unit of weight of the explosive substance occupies a unit of volume, and, if P be maximum tension developed by the explosion, we have (2), where T is the temperature of explosion, and p the ratio of the volume f the non-gaseous products, taken as constant; we have also the relation (3), when the vessel is cooled down to C ; therefore, elimi nating R between (2) and (3), we get (4). But permanent gases under the pressure /> in atmospheres, at a volume (1 - p), will occupy a space p (1 - p^if allowed to expand to the normal pressure of 760 mm.; calling this expanded volume V, P- VT (5) 273 (1-p) The large amount of aqueous vapour produced by the explosion of some compounds must be added to the value of V, its volume being calculated on the supposition that
it can remain uncondensed at the temperature of C.