Explosives may be conveniently divided into two distinct classes—(1) explosive mixtures, and (2) explosive compounds. The first class consists of those explosive sub stances which are merely intimate mechanical mixtures of certain ingredients, and which can be again separated more or less completely by mechanical means, not involv ing mechanical action. These ingredients do not, as a rule, possess explosive properties in their separate condition. There are, however, explosives which might almost be classed in both categories; for example, picric powder is composed of ammonium picrate and saltpeter, the former of which contains an explosive molecule, but is mixed with the latter to supply additional oxygen, and thus increase the force. If a substance that will burn freely in air, combining gradually with the oxygen of the atmosphere, be ignited in pure oxygen gas, the combustion will be much more rapid, and the amount of heat generated greater, at the ordinary atmospheric pressure. If it be possible to burn the substance in a very condensed atmosphere of oxygen, we can readily, imagine the combustion as very greatly accelerated, and therefore increased in violence; this is what is ordinarily effected by an explosive " mixture." A combustible body and a supporter of combustion are brought into extremely close contact with one another by means of intimate mechanical mixture; also, the supporter of combustion, or oxidizing agent, is present in very concentrated form, constituting what may be termed a Mag azine of condensed oxygen, solid or liquid. In the case of the explosion of a definite chemical compound, the change may be considered as the resolution of a complex body into simpler forms. This is not, however, always the case when a mechanical mixture is concerned; gunpowder, for example, may be said to contain two elementary sub stances, carbon and sulphur, not in chemical union.
The chief explosive mixtures may be subdivided into " nitrate mixtures" and " chlorate mixtures." In the nitrates, the oxygen is held in combination with sufficient force to need a powerful disturbing cause to separate it, so that mixtures made from nitrates do not explode very rapidly, and their action is comparatively gradual ; they are not sensitive to friction or percussion, and hence are to a great extent safe. Any of the nitrates will form explosive mixtures with combustible substances, but nitrate of potash is the only one practically employed. The nitrate of soda, called " cubical" or Chili saltpeter, has beeu used, but absorbs moisture from the air so readily as to give very inferior results. Gunpowder may be taken as the representative of the nitrate explosive mixtures. Picric powder, above referred to, has been proposed by Abel for use as a bursting charge for shells, as being more powerful than a correspond ing charge of gunpowder, equally safe as regards friction or percussion, and less hygroscopic; it consists of two parts ammonium picrate, and three parts saltpeter, incorporated, pressed, and finished very much as ordinary gunpowder.
The chlorates part with their oxygen far more readily than the nitrates, the strong affinities of chlorine for the metals coming into play, and consequently chlorate mixtures are very sensitive to friction and percussion, and explode with great violence; chlorate of potash, is the only one used. Very many chlorate mixtures have been made, some of which are employed in fireworks. " White gunpowder" is a mixture of two parts chlorate of potash, one of yellow prussiate of potash, and one of sugar; it is exploded very easily by friction or percussion. The most important chlorate mixtures are those used for igniting other explosives, such as the composition for friction tubes for firing cannon, percussion-cap composition, and percussion fuses for bursting shells on impact; it is sometimes mixed with sulphur, as a Combustible, and sometimes with black sulphide of antimony, gives a longer flame.
In an explosive "compound," the elements are all in chemical combination, pre senting a definite explosive "molecule," which contains, so to speak, both the combus tible 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 inti mate mechanical mixture. The chief explosive compounds are formed from some organic substance containing carbon, hydrogen, and oxygen, by introducing into it, through the action of concentrated nitric acid, a certain portion of nitric peroxide in substitution for an equivalent amount of hydrogen. A new compound, differ ing outwardly very little, if at all, from the original substance, is thus formed, but in a very unstable state of eheMical equilibrium, because of the feeble union of the nitrogen and oxygen in the NO2 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-glycerine are the leading members of this group. produced in a precisely similar manner, by the substitution of three molecules of NO2 for three atoms of hydrogen (H). As those explosives will be elsewhere described in detail, we give the formation, as a representative member of the group, of nitro phenol, or picric acid, by treating phenol, or carbolic acid, with a mixture of nitric and sulphuric acids, the latter being required to absorb the water and preserve the full strength of the nitric acid: Carbolic acid. Nitric acid. Picric acid. Water.