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EXPLOSIVES. It lies beyond the object of this article to attempt an estimate of the influence, direct or indirect, upon modern civilization of the introduction of explosive agents for the purposes of war. Some eminent authors have gone so far as to consider the invention of gunpowder as next in importance, in its ultimate effects, to those of printing and the application of steam power. However this may be, it is well to remember that explosive substances are now of immense utility in the arts of peace; indeed, it is not too much to say that without their aid many of the great engineering enterprises of the present day would either be impossible, or else have to be carried out at a vast additional expenditure of time and labour.

The germ of all the knowledge of explosive reaction we possess undoubtedly lay in the probably accidental discovery, many ages ago, of the deflagrating properties of the natural substance nitre or saltpetre (KNO3), when in con-act with incandescent charcoal. To trace the consequences of that discovery, eery gradual as they have been, and intimately bound up with the progress of chemical and mechanical science, belongs rather to an article on gunpowder ; but the foot may be briefly referred to in connexion with the second great epoch in the history of explosive substances. By distilling nitre with oil of vitriol, the alchemists obtained a corrosive fluid which they called aroalortis, now known as nitric acid (HNO3), which parts with its oxygen even more readily than saltpetre ; so that if the strongest nitric acid be poured upon finely powdered charcoal, the latter takes fire at the ordinary temperature. Somewhat less than half a century back, it was discovered by some French chemists that upon treating various organic substances, such as starch, the sugars, cotton fabrics, and even paper, with concentrated nitric acid under proper precautions, the chemical constitution of the substances underwent a great change, and they became endowed with violently explosive properties, while remaining for the most part unaltered in external characteristics. To this discovery we owe a distinct class of explosive compounds, the most powerful for practical purposes as yet known ; their general formation and properties will be noticed in due course.

We will now proceed to examine into those principles of constitution and action which are more or less common to all explosive substances.

As the term is often rather loosely employed, " explosion " may for our purpose be defined as the sudden or extremely rapid conversion of a solid or liquid body of small bulk into gas or vapour, occupying very many times the volume of the original substance, and, in addition, highly expanded by the heat generated during the transformation. This sudden or very rapid expansion of volume is attended by an exhibition of force, more or less violent according to the constitution of the original substance and the circumstances of explosion. Any substance capable of undergoing such a change upon the application of heat, or other disturbing cause, is called " explosive."

The explosive substances that are practically the most f important essentially contain carbon, oxygen, and nitrogen, the last always existing in a state of feeble combination with the whole or part of the oxygen, and thus creating that condition of unstable chemical equilibrium which is necessary. When explosion takes place, the nitrogen parts with its oxygen to the carbon, for which it has a great affinity, forming carbonic acid (CO2) and carbonic oxide (CO) gases, the combination being accompanied with great generation of heat, and the nitrogen gas is set free. In most explosives there is also hydrogen accompanying the carbon, and by its combustion producing an extremely high temperature ; it combines with part of the oxygen to form water in the form of greatly expanded vapour. Other subordinate elements are often present ; in gunpowder, for instance, the potassium binds the nitrogen and oxygen loosely together in the state of saltpetre, and there is sulphur, a second combustible, whose oxidation evolves greater heat than that of carbon. When chlorate of potash is present, the chlorine plays the part of the nitrogen, and is set free in the. gaseous state. Two very unstable and practically useless explosive substances, the so-called chloride and iodide of nitrogen, contain neither carbon nor oxygen; but their great violence is equally caused by the feeble affinities of nitrogen for other elements, large volumes of gaseous matter being suddenly disengaged from a very small quantity of -a liquid and solid body respectively.

Explosives may be conveniently divided into two distinct classes, - (l) explosive mixtures, and (2) explosive compounds.

The first class consists of those explosive substances which are merely intimate mechanical mixtures of certain ingredients, and which can be again separated more or less completely by mechanical means, not involving chemical 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 'aerate and saltpetre, 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 hi 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 sub stance in a very condensed atmosphere of oxygen, we can readily imagine the combustion being 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 a very concentrated form, constituting what may be termed a magazine 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 substances, 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 readily, and their action is comparatively gradual; they are not sensitive to friction or percussion, and hence are tolerably safe. Any of the nitrates will form explosive mixtures with combustible substances, but nitrate of potash (KNO,) is the only one practically employed. The nitrate of soda, called "cubical" or Chili saltpetre, has been 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 corresponding charge of gunpowder, equally safe as regards friction or percussion, and less hygroscopic; it consists of two parts ammonium iiicrate, and three parts saltpetre, 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 t are very sensitive to friction and percussion, and explode with great violence ; chlorate of potash (liC103) 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 fuzes for bursting shells on impact; it is sometimes mixed with sulphur, as a combustible, and sometimes with black sulphide of antimony, which gives a longer flame.

In an explosive " compound," the elements are all iu F chemical combination, presenting a definite explosive" " molecule," which contains, so to speak, both the coobustible 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 intimate 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 (NO2), in substitution for an equivalent amount of hydrogen. A new compound, differing outwardly very little, if at all, from the original substance, is thus formed, but in a very unstable state of chemical equilibrium, because of the feeble union of the nitrogen and oxygen in the NO, 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-glycerin are the leading members of this group, being produced in a precisely similar manner, by the substitution of three molecules of NO, for three atoms of hydrogen (II). 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 :-

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