EXPLOSIVES are substances which, by sudden decomposition or chemical ac tion, produce large volumes of heated gas. The decomposition can be brought about by heat, by a blow or by other means. All explosives in actual use are those in which the chemical action is one of oxidation, the oxygen being supplied either by a nitrate or chlorate, nitric acid or an NO3 group. The ease with which an explosion can be brought about depends very largely on the physical condition of the explosive. For instance, gunpowder varies widely in its effects, according to the size of the particles composing the mixture. While the force of all explosives is increased by inclos ing them in a small space, confinement is not necessary in the case of those compounds in which decomposition is very rapid, as, for instance, mercury fulminate.
In an article of this length it would be impossible to deal, even briefly, with all substances of an explosive nature, but as far as possible, representative types of all the well-known groups will be described.
1. Explosive mixtures, as distinct from explosive compounds, consist of two or more substances, at least one of which is a combustible, and at least one of which is a supporter of combustion. The ingredients of these mixtures are frequently not themselves explosives, but may be ordinary combustibles such as carbon, or sulphur, and stable salts such as chlorate or nitrate of potash. In the explosive powder, however, these other wise harmless substances are incorpo rated so thoroughly, and are in such a fine state of division, that combustion can be induced almost instantaneously throughout the mass, with a consequent rapid evolution of hot gases, thus pro ducing the phenomenon known as an ex plosion. The best known member of this class is gunpowder, now almost obsolete at all or only a very small percentage. As substitutes for saltpeter, nitrates of sodium, barium, and ammonia have been employed, the latter extensively in the manufacture of "safety explosives" for use in mines.
Ammonal consists of a mixture of ammonium nitrate, aluminum and char coal. Mixtures containing chlorates and perchlorates in place of nitrates have been manufactured and used to a con siderable extent, but, owing to their un stable character, have never very largely replaced the nitrate mixtures.
2. Explosive compounds are those sub stances which are of such a nature as to for military purposes, but still largely used for sporting purposes, in mining, for fuses and for minor military pur poses. It consists of a mixture of nitrate of potash (saltpeter), sulphur, and charcoal, the proportions varying somewhat, but being approximately fifteen parts of potassium nitrate to two parts of sulphur and three parts of char coal. Great care is taken in selecting the sulphur, while the charcoal is pre pared in special retorts, and the process of manufacture is one requiring much skilled supervision. Many modifications of gunpowder have been made from time to time. In place of charcoal, such materials as coal, coke, peat, sawdust, bran, sugar, starch and many others have been used, while powders have been produced containing either no suinhur be themselves explosive. Instead of a chemical reaction taking place between two separate substances, the reaction oc curs within the molecule of a single sub stance. As a consequence explosive com pounds are more powerful in their action than mixtures, and modern explosives used in warfare all belong to this class. For the most part they are produced by the nitration of organic compounds and the best known and most widely used is trinitrotoluene, commonly called T.N.T. This is manufactured by the nitration of toluene in successive stages, mono nitrotoluene being first produced, then dinitrotoluene, and finally, trinitro toluene. It occurs as yellow crystals, which darken on exposure to light, but is frequently used as a fused mass. When held in a flame, it does not explode, but burns with a smoky flame. It can, however, be detonated by means of ful minate of mercury. It is safe to handle and remarkably stable, and while slight ly less powerful than picric acid it has the advantage that it does not attack metals or form unstable compounds with them.