Explosives. The term is in general applied to such substances, either mixtures or free compounds, which can suddenly, owing to some external impulse, give rise to large volumes of gas. The impulse may be given either by a flame, a sudden concussion, electric spark, or other means. In most cases of ordinary explosives the gases are formed owing to the oxidation of some of the constituents, the explosive itself containing in some of its ingredients the oxygen necessary for the oxidation. The energy of the explosion depends, inter alia, upon the rate with which the chemical change can be propagated, and hence, as would be expected, owing to the greater proximity of the molecules, it is generally greater in the case of explosives which are compounds than in those which are mixtures. The oxygen is usually present in the form of a nitrate, chlorate, or in the group NO2, the latter being especially the case with explosive organic compounds. Some of the principal explosives in ordinary usage are the various varieties of gunpowder, gun-cotton, nitro-glycerine, and dynamite, fulminate of mercury, and preparations of picric acid (such as melinite, lyddite, etc.); and explosives obtained from these compounds. During late years also attention has been given to experiments upon flameless explosives, more especially for use in coal mine blasting. A common mode is the enclosure of the explosive in a water cartridge, but some substances have been prepared which themselves explode with the production of no flame. As examples there are Roburite, Bellite, and Securite. For estimating the strength of an explosive, the following points are the chief to be considered: - (1) the quantity of gas produced; (2) the temperature; (3) the rapidity of evolution. In the case of gunpowder the explosive pressure has been both estimated and measured, with but little difference in the results; but in the case of the stronger explosives it has been hitherto practically impossible to measure their strength. The power of the commoner explosives compared for
Nitro-glycerine ... 100
Dynamite ... 74
Gun-cotton ... 60
Blasting powder ... 17.5
One of the tests often employed is the effect in smashing iron plates when exploded upon their surface. Another is to explode the substance in a small cylinder provided with a form of piston, and measure the compression of a lead plug placed under the piston. Various other methods have been adopted, but none are at all satisfactory. It is unnecessary to state that the manufacture of explosives is one which requires the greatest possible care and precaution, while the sale is regulated by stringent legal acts, etc.