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 (KNO 3 ), when in con-act with incandescent charcoal. To trace the consequences of that discovery, very 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 fact 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 aquafortis, now known as nitric acid (HNO 3 ), 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 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 (CO 2 ) 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 guupowder, 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,—(1) 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 picrate 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 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 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 3 ) 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 picrate, 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 are very sensitive to friction and percussion, and explode with great violence; chlorate of potash (KClO 3 ) 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 in chemical combination, presenting a definite explosive “molecule,” which contains, so to speak, both the combustible 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 (NO 2 ), 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 2 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 2 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 :—

C 6 H 6 O + 3HNO 3 = C β H 3 (NO i ) 3 O + 3H 2 O. Carbolic acid. Nitric acid. Picric acid. water.

The formula of the product may be empirically written C 6 H 3 N 3 O 7 j it is, like gun-cotton and nitro-glycerin, 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 can 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 NO 2 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 stability 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 explosives.

Fulminate of mercury is produced by adding alcohol (C 2 H 6 O), under careful precautions, to a solution of mercury in nitric acid; a grey 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 HgC 2 N 2 O 2 , or perhaps more correctly HgO.C 2 N 2 O; the chemical factor C 2 N 2 O is called fulminic acid, but has never been produced separately. Opinions differ as to the precise “rational” formulae of the fulminates, some chemists considering their process of formation 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 proportions 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 ga3 and vapour 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 fuzes; its practical value has of late years been immensely increased by the discovery of its power, even in very small quantities, to proαuce the almost instantaneous 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.

The difficulties in the way of estimating, with any accuracy, the force of explosive substances are very great, especially as no definite standard of comparison can be laid down. However, by means of theoretical considerations, combined with the results of actual experiment, a tolerably fair approximation may be arrived at.

When an explosive substance is exploded in a closed vessel sufficiently strong to resist rupture, the tension attains its maximum value in an extremely short space of time and gradually decreases from the heat being conducted away by the metal envelope, and dispersed by radiation. It has, however, been demonstrated that, at the moment of maximum tension, the loss of pressure due to the communication of heat to the vessel, if the latter be filled with the explosive, is less than one per cent. The products of combustion, after cooling down, can easily be determined by analysis, and are then either (a) wholly gaseous, as for chloride or iodide of nitrogen; (δ) gaseous and liquid, in the case of gun-cotton and nitro-glycerin; or (c) gaseous and solid, as with gunpowder. It is certain that, at the moment of explosion, the products of the more violent explosive compounds are wholly in a state of gas or vapour, but we should arrive at incorrect results by making the same assumption in the case of a mechanical mixture like gunpowder. The experiments of Noble and Abel on “Fired Gunpowder” (Phil. Trans. Poy. Soc., 1874), which are the most complete ever undertaken, show that the ultimately solid residue is, at the moment of explosion, in a liquid state, and most probably in a very finely divided condition; moreover, that, at that instant, it occupies a space the ratio of which is about ’ 6 that of the original volume, supposing the substance to fill the vessel in which it is exploded.

Provided the laws concerned can be supposed to hold good at such high temperatures, we may assume for the gaseous products of combustion the well-known equation of the elasticity and dilatibility of permanent gases—

pv=Bt (1), where R is a constant, and t reckoned from absolute zero ( - 273° C). For the sake of convenience, we will consider that a unit of weight of the explosive substance occupies a unit of volume, and, if P be maximum tension developed by the explosion, we have

P(l- p )≡RT (2), where T is the temperature of explosion, and ρ the ratio of the volume ©f the non-gaseous products, taken as constant; we have also the relation

Po (1 - p) = R∙^73, ...... (3), when the vessel is cooled down to 0° C; therefore, eliminating R between (2) and (3), we get i ~ 273 ( 4 '∙

But permanent gases under the pressure p 0 in atmospheres, at a volume (1 - p), will occupy a space p 0 (1 - p∖if allowed to expand to the normal pressure of 760 mm.; calling this expanded volume V,

P- VT (5)

273(l-p) tλ

The large amount of aqueous vapour produced by the explosion of some compounds must be added to the value of V, its volume being calculated on the supposition that it can remain uncondensed at the temperature of 0° C.

We have no certain means of directly estimating the temperature of explosion, but if it be assumed, as is usually done, to be the temperature the total products of combustion would attain to if the whole amount of heat generated by the explosion were applied to raise them, under constant volume, from absolute zero, we have the relation

H-Tc,, in which H represents the number of units of heat generated, and c the specific heat under constant volume of the united products, gaseous or otherwise. The quantity of heat can be obtained by experiment, and this livided by the specific heat, will give the temperature. The chief sources of error lie in the assumption that the specific heat remains constant throughout the great range of temperature in question, and in the additional quantities of heat disengaged by secondary reactions. The value of T thus found probably will therefore be higher than the real temperature.

Having regard to the above reasoning, it may be generally concluded that the amount of force exerted by an explosive substance depends upon—(1) the volume of gas or vapour produced by the transformation, compared 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 vapour 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 temperatures concerned; for example, that the carbonic acid (CO 2 ) may be decomposed into carbonic oxide (CO) and oxygen, or the aqueous vapour into oxygen and hydrogen. However, Noble and Abel demonstrate that, in the former instance, the loss of temperature, 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 dissociation must take place. We may therefore consider that it has no sensible influence upon the explosive force.

It is most important to distinguish between explosive force and explosive effect, the latter in great measure depending upon the rapidity with which the metamorphosis takes place, while the same amount of force may be exerted suddenly or gradually. We may, therefore, consider that the explosive effect varies directly as the volume of gas produced and the temperature of explosion, and inversely as the time required for the transformation. But the time, and, to a certain extent, the products and temperature, will vary with—(a.) the physical state of the explosive substance; (δ.) the external conditions under which it is fired; (c.) the mode of firing or exploding.

I The physical or mechanical state of the explosive substance has a most important bearing upon the effect obtained from it. To prove this, it is only necessary to point to the very different results given by gunpowders made with the same proportions of the three ingredients, but varying in density, and in shape and size of grains or pieces. Gun-cotton is even more affected by variations in mechanical condition. In the form of loose wool, it burns so rapidly that gunpowder in contact with it is not inflamed; plaited or twisted tightly, its rate of combustion in air is greatly modified. This is due to the fact that the inflammable carbonic oxide, which is evolved by the decomposition from the want of sufficient stored-up oxygen to oxidize completely all the carbon of the gun-cotton, cannot penetrate between the fibres and accelerate the combustion, but burns with a bright flame away from the surface of the twisted cotton; when the yarn is yet more compressed by any means, the temperature is not kept up to the height necessary for the combustion of the carbonic oxide, so that it escapes unconsumed, abstracting heat, and yet more retarding the rate of burning. For the same reason, pulped and compressed gun-cotton burns comparatively slowly in air, even when dry; in the wet state, it merely smoulders away, as the portions in contact with the fire successively become dried. Yet this same wet compressed gun-cotton can be so used as to constitute one of the most powerful explosives known.

It is well known that gunpowder behaves differently when fired in the open air and under strong confinement; not only the rate of burning, but even, to a certain extent, the products of combustion are altered. We have discussed the effect of tightly plaiting or compressing guncotton; but, when confined in a strong envelope, the whole of the inflamed gas, being unable to escape outwards, is forced into the interstices under immense pressure, and the decomposition greatly accelerated. The amount of confinement or restraint needed by any explosive...