NAVAL INSTITUTE, ANNAPOLIS, MD.

FEBRUARY, 1884.

NOTES ON THE LITERATURE OF EXPLOSIVES.*

BY PROF. CHARLES E. MUNROE, U. S. N. A.

No. VI.

In No. V of these Notes we have referred to Berthelot's lectures on Explosives. These have now been reprinted by Van Nostrand as No. 70 of his Science Series. We then proposed to present later his discussion of the theories suggested to account for explosions induced by the influence of contiguous explosions, and hence we extract the following.

In discussing the duration and speed of propagation of explosive reactions Berthelot has regarded the development of the explosive reactions either from the point of view of their duration in a homogeneous system in which all the parts are maintained at the same temperature, or else from the standpoint of their propagation in a system equally homogeneous, to which fire is applied directly by means of a body in ignition, or else by a violent shock. In these later years, however, the study of explosive substances has revealed the existence of another method of propagating the reactions from an explosive centre, this propagation taking place at a distance and by the intermediation of the air or certain solid bodies which do not themselves participate in the chemical change.

We shall now speak of what are called explosions by influence, whose existence was formerly suspected from certain known facts connected with the simultaneous explosion of several buildings separated by considerable space from each other, as in catastrophes

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occurring in powder mills. Attention has been especially directed to this class of phenomena by the study of nitro-glycerine and gun

cotton.

We will begin by giving the most important characteristic facts. A dynamite cartridge made to detonate by means of a fulminate cap causes the adjoining cartridges to detonate, not only by contact and by direct shock, but even from a distance. In this way an indefinite number of cartridges, arranged in a regular course, may be made to detonate.

The distances to which the explosion may be propagated are relatively great. Thus, for instance, with cartridges contained in rigid metallic envelopes and placed on a resisting soil, the detonation produced by 100 grams of Vonges dynamite (75 per cent. of nitroglycerine and 25 per cent. randanite, which is very finely divided silica) communicates itself 0.3 meter, according to the experiments of Captain Coville. D being equal to the distance in meters and C the weight of the charge in kilograms, the experiments of this officer show that D= 3.0C. When the caps were laid on a rail D was found to be equal to 7.0C. On soft or ploughed-up earth the distances, on the contrary, are less. When a cartridge is suspended in air there is no detonation by influence, perhaps because the cartridge not being fixed can recoil freely, which diminishes the violence of the shock. Nevertheless there are experiments which show that the air suffices for the transmission of the detonation by influence, although with greater difficulty and requiring a greater mass of the explosive. With a dynamite less rich in nitro-glycerine (55 per cent. of nitro-glycerine and 45 per cent. of the argillaceous ashes of boghead coal), contained in similar cartridges, and placed along the ground, the experiments of Captain Pamard have given the smallest distances: D=0.90 C. If metallic envelopes having less resistance are used, the distance at which the explosion is propagated is likewise diminished. Dynamite simply spread along the ground ceases to propagate the explosion. The experiments performed in Austria have given similar results. They have shown that the explosion is communicated either in the free air with intervals of 4 cm., or else through pine boards 18 mm. thick. In a lead tube with a diameter

0.15 meter and a meter in length, a cartridge placed at one extremity has caused the detonation of a cartridge at the other end. The explosion is still better transmitted through tubes made with wrought iron. The couplings of the tube diminish its aptitude for transmission.

An explosion which is propagated in this manner will go on weakening itself from cartridge to cartridge and even change its character. Thus according to the experiments made by Captain Müntz at Versailles, in 1872, a first charge of dynamite exploded directly, excavated a funnel-shaped hole in the ground with a radius of 0.30 meter; the second charge, detonated by influence, produced an opening of only 0.22 meter; the effect of the detonation was then reduced. This reduction should manifest itself towards the limit of the distance at which the influence ceases. In the same way four tin screens were located 40 mm. apart, a small cylinder of gun-cotton was placed against each of them, and the entire affair arranged on a board; 15 mm. in front of the first screen a similar cylinder was detonated. All of the cylinders detonated, but a progressive diminution was observed in the indentations produced in the board below each cylinder. According to these facts the propagation by influence depends at the same time on the pressure acquired by the gas and on the nature of the support. It is not even necessary that it should be rigid.

Finally, in operating under water at a depth of 1.30 meters, a charge of 5 kilograms of dynamite, brought on an explosion of a charge of 4 kilograms situated at a distance of 3 meters. The water then transmits the explosive shock, at least to a certain distance, as does a solid body. This transmission is so violent that the fish are killed in ponds within a sphere of a certain radius by the explosion of a dynamite cartridge.*

Similar experiments have been made by Abel with compressed gun-cotton. According to his observations the explosion of the first block determines that of a series of similar blocks. The propagation under water has likewise been studied; the explosion of a torpedo charged with fulminating cotton caused the detonation of adjoining torpedoes placed within a certain radius of activity. The sudden pressure transmitted by the water when measured by means of the compression of lead at different distances, such as 2.50 m., 3.50 m., 4.50 m., 5.50 m., goes on decreasing, as would be expected. Besides, experiment has shown that the relative position of the charge and of the "crusher" is of no consequence, which is in harmony with the principle of equal transmission in all directions of hydraulic pres

sures.

For further data, see Abbot's Submarine Mines, pages 54 and 122.

Explosions of fulminating substances which are rapidly propagated to a great number of caps, belong to this same order of explosions by influence. We have previously cited the explosion in the Rue Beranger.* The experiments which M. Sarrau made on that occasion showed that caps of the description which produced this catastrophe may be successively burned in a fire without giving rise to a general explosion; whereas the explosion of a few of these same caps, each containing 10 milligrams of explosive material, if it is provoked by a rapid pressure, determines by influence the explosion of the adjoining packages, even when they are not contiguous and are situated at a distance of 15 centimeters apart. A general explosion may thus easily be produced by influence. It follows then from these facts, and especially from the experiments made under water, that the explosions by influence are not due to inflammation, properly so called, but to the transmission of a shock arising from the enormous and sudden pressures produced by the nitro-glycerine or the gun-cotton.

Let us enlarge upon this explanation; it is the same fundamentally as that which we have already shown as accounting for the influence of the shock which determines the direct detonation of explosive substances.

In an extremely rapid reaction, the pressures may approach to the limit which corresponds to the matter detonating in its own volume, and the commotion due to the sudden development of almost theoretical pressures can be propagated both through the ground and supports as intermediary, or through the air itself, projected en masse, as has been shown by the explosion of certain powder factories and of gun-cotton magazines, and even by some of the experiments with dynamite and compressed gun-cotton. The intensity of the shock propagated either by a column of air or by a liquid or solid mass varies with the nature of the explosive body and its mode of inflammation; it is of greater violence according as the length of the chemical reaction is shorter and develops more gas, that is to say, a higher initial pressure, and more heat, and consequently work, for the same weight of explosive material.

This transmission of a shock is conveyed better by solids than by liquids, better by liquids than by gases; with gases it becomes better as they are more compressed. Through solids it is better propagated according to their degree of hardness, iron transmitting

*Nav. Inst. Proc. 9, 752.

it better than earth, and hard ground better than ploughed soil. All breaks of continuity in the transmitting material tend to weaken it, especially if a softer substance is interposed. Thus it is that the use of a tube made from a goose-quill, as a receiver, stops the effect of mercury fulminate, while a tube or a capsule of copper transmits this effect in all its intensity. The explosion by influence is the better propagated in a series of cartridges according as the envelope of the first detonating cartridge is the more resisting, which allows the gases to attain a greater pressure before the covering is destroyed. The existence of an empty space, that is to say, filled only with air, between the fulminate and the dynamite, on the other hand diminishes the violence of the shock transmitted, and in consequence that of the explosion; generally the effects of breaking powders are lessened when there is no contact. To form a full conception of the transmission of sudden pressures which produce shock by the supporting medium, it is desirable to recall this general principle, in virtue of which, in a homogeneous mass, pressures are transmitted equally in all directions, and are the same on a small element of surface whatever its position. Detonations produced under water with gun-cotton show that this principle is equally applicable to the sudden pressures which produce the explosive phenomena. But it ceases to be true when one passes from one medium to another.

If the inert chemical matter which transmits the explosive movement is fixed in a given situation on the surface of the ground, or better, on the surface of the rail on which the first cartridge was placed, or better still, held by the pressure of a mass of deep water in the midst of which the first detonation is produced, the propagation of the movement in this matter will hardly be able to take place, except under the form of a wave of a purely physical order, and consequently of an essentially different character from the first wave of a chemical and physical order simultaneously developed in the explosive body itself. This new wave propagates the concussion away from the explosive centre all around it, and with an intensity which decreases inversely as the square of the distance. Even in the neighborhood of the centre, the displacements of the molecules may break the cohesion of the mass and disperse it, or crush it by enlarging the chamber of explosion, if the operation is conducted in a cavity. But at a very short distance (the magnitude of which depends on the elasticity of the surrounding medium) these movements, confused at the beginning, arrange themselves in such order as to produce a wave,