Mercury fulminate (empirical formula, C4N204Hg2) is prepared by dissolving at a moderate heat, in 12 parts of nitric acid of the specific gravity of 1.35, 1 part of mercury, and adding 11 parts of 90 to 92 per cent. alcohol. Liebig recommends a glass flask, the capacity of which is 18 times the volume of the mixture. In this the mercury is dissolved in cold acid, the nitrous fumes being retained in the flask. The solution is poured into a second vessel, containing one half the alcohol; and the mixture is then returned into the first flask, where it reabsorbs the nitrous fumes. In a few moments bubbles rise from the bottom, where a heavy liquid begins to be segregated. By gentle shaking this is mixed with the supernatant liquid, and a tempestuous ebullition takes place, with evolution of white fumes, and some nitrous acid, the mass becoming black from segregated metal. The remainder of the alcohol is gradually added; the black color disappears, and the fulminate is deposited in sparkling brownish gray crystals. The vapors are chiefly carbonic acid and nitrous ether. Mercury fulminate is scarcely soluble in cold water, but dissolves in 180 parts of boiling water, which gives a means of refining it by recrystallization.
It explodes at 186° C, or under friction or percussion between hard substances. When moistened with 5 per cent. of water, only the portion actually struck explodes. In contact with a tightly packed explosive mixture, its detonation explodes the mixture more rapidly and completely than any other method of firing. Hence its universal employment in the manufacture of percussion caps and detonating fusees. According to the French method, one kilo of mercury gives 1 1/4 kilo of fulminate, sufficient for 40,000 caps. It is ground with 30 per cent. of water under a wooden muller on a marble bed, and 6 parts gunpowder are added for every 10 of fulminate. The mixture is dried, granulated, and sized. A drop of gum is introduced into each cap, and the fulminate powder is dropped upon it. Some caps are varnished, to make them water-proof. English fulminating powder consists of 3 parts mercury fulminate, 5 parts chlorate of potassa, 1 part sulphur, and 1 part powdered glass. Gum is sometimes added in the mixture. Nitre is also recommended.
Samuel Guthrie of Sackett's Harbor, N. Y., whose extensive and perilous experiments are described in the American Journal of Science" for January, 1832, found that 1 part oxide of tin with 3 parts mercury fulminate, ground together with a stiff solution of starch, made a very effective compound. During these experiments Mr. Guthrie discovered chloroform, as did French and German investigators at about the same time.-Silver fulminate is more explosive and dangerous than the mercury salt. It may be made like the latter, using fine silver instead of mercury; or by introducing finely pulverized nitrate of silver into concentrated alcohol, shaking it well, and adding an equal amount of fuming nitric acid; or by treating freshly precipitated oxide of silver with ammonia. It is employed in the manufacture of explosive toys. Gold and platinum fulminates are similar compounds to the foregoing, but they are not employed in the arts.-Fulminating aniline, or chromate of diazobenzole, obtained by the action of nitrous acid upon aniline, and the precipitation of the product by the aid of a hydrochloric acid solution of bichromate of potassa, is, according to Caro and Griess, an efficient substitute for fulminating mercury.- General Theory of Explosives. Explosive substances are said to possess potential energy by virtue of certain unsatisfied affinities between the elements of which they are compounded." In the act of explosion these affinities are satisfied, and the potential energy becomes kinetic, taking first the form of heat, which is partially expended in giving elastic force to the new gaseous compounds generated.
Perhaps this statement does not exactly cover cases like the chloride of nitrogen, which explodes by dissociation, leaving free chlorine and nitrogen. The elastic force at any instant of an explosion and the total energy developed are two different things. The intensity of the force depends upon: 1, the amount of actual heat developed; 2, the volume which a unit of the mass of the products occupies at the instant; 3, the specific heat of these products; or, in other words, upon: 1, the volume of the products; 2, their temperature. The total energy is dependent upon: 1, the ratio between final volume of products and original volume of explosive; 2, the total actual heat of the explosion. The maximum intensity depends chiefly upon the rapidity with which the conversion of the explosive into gas takes place, and this depends on varying conditions, no ex-j plosion being absolutely instantaneous. The primary condition is the rapidity with which the chemical reaction among the constituents takes place. Some, as nitrate and chlorate of potassa, require heat for their decomposition; others are probably dissociated by the vibrations produced by percussion or the ex-ploding spark, as nitro-glycerine and chloride of nitrogen.
Some have so little stability that sound alone is sufficient to precipitate the explosion, as iodide of nitrogen, which may be exploded by sounding a tuning fork of the proper pitch in its vicinity. When heat is required, the rapidity of decomposition will depend also upon the rate of ignition throughout the mass. Thus in a charge of granular gunpowder, the flame from the vent passes between the grains, progressively enveloping their surfaces, and through the pores of each into the mass, its progress being much hastened by the enormous tension produced when the explosion is confined. Hence the rate of ignition (and consequently the intensity of the force at a given instant) may be varied by varying the size of pores and interstices in the mass; a fruitful field of experiment and improvement, particularly in gunpowder. It is evident also that the tension is dependent upon the resistance to the expansion of the gases, and will rapidly increase unless the restraint is withdrawn in proportion to their progressive development. The increase of tension brings with it increased rapidity of ignition and decomposition, and this in turn augments the tension, which is thus a self-multiplying quantity.