061160 3HN0, = C6H3(NO2)30 The formula of the product may be empirically written C6H3N2O7; it is, like gun-cotton and nitro-glycerine, a tri-nitro substitution product. Only the picrates, or salts of picric acid formed with potassium or ammonium, are used in practice, as possessing more force than the uncombined acid. From starch may 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 NO2 are inserted. The number of nitro-substitution products is very great, many of them being more or less violently explosive. The fulminates are among the most violent of all explosive compounds, their chemical sensibility being very small. Sudden 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 explo sives. Fulminate of mercury is produced by adding alcohol under great pre cautions, to a solution of mercury in nitric acid; a gray crystalline precipitate is obtained, very heavy (sp. gr. 4.4), and so sensitive to friction or percussion that it is kept in the wet state. The results of analysis show one atom of mercury, and two each of carbon, nitrogen, and oxygen ; so that the formula may be empirically written H,....C2N202, or perhaps more correctly the chemical factor is called fulminic acid, but has never been produced separately. Opinions differ as to the precise " rational " formuhe of the fulminates, some chemists considering their process of for mation 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 vapor 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 fuses; its practical value has of late years been immensely increased by the discovery of its power, even in very small quantities, to produce the almost instanta neous 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.
It may be generally concluded that the amount of force exerted by an explosive sub stance depends upon (1) the volume of gas or vapor produced by the transformation, com pared with that of the original substance; and (2) the temperature of explosion, which determines the extent to which the gases are expanded, 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 vapor calculated at 0° C. and 7.60 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 increased by the partial or total " dissociation' of the compound gases, at the high tem peratures concerned; for example, that the carbonic acid (CO2) may be decomposed into carbonic oxide (CO) and oxygen, or the aqueous vapor into oxygen and hydrogen. However, Nobel and Abel demonstrate that, in the former instance, the loss of tempera ture, 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 dissocia tion must take place. We may therefore consider that it has no sensible influence upon the explosive force.