FOOTNOTES:

[Q] See [page 185].

[R] A Turkish port, situated on the east coast of the Black Sea, capable of holding several large ships when anchored head and stern, but otherwise only a few.

[S] A town situated on the south bank of the Danube, about eight miles from Brailoff.

[T] One of the principal mouths of the Danube.

[U] A place taken from the Russians in the early part of the war, situated on the east coast of the Black Sea.

CHAPTER VIII.
ON EXPLOSIVES.

EXPLOSION may 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 which in addition is highly expanded by the heat generated during the action.

This sudden or very rapid expansion of volume is attended by an exhibition of force which is more or less violent, according to the constitution of the original body and the circumstances of the explosion.

Any substance capable of undergoing such a change on the application of heat or other disturbing cause is called an "explosive."

Explosive Force.—Explosive force is directly proportional to the heat of combustion and the volume of gas, and inversely to the specific heat of the mixed products.

Explosive effect is directly proportional to the volume of gas produced and the temperature of the explosion, and inversely as the time required for the change to take place.

Explosive Effect and Force compared.—Explosive effect depends upon the rapidity with which the conversion is effected, while the same amount of explosive force may act suddenly or gradually.

As before stated, explosions are more or less violent according to the circumstances under which they take place. These may be considered as follows:—

1.—The physical state of the explosive substance.

2.—The external conditions under which the explosive body is fired.

3.—The mode of firing.

The Physical State of the Explosive Substance.—Numerous instances may be cited to show the influence the physical condition of an explosive body has upon its explosion.

Thus, gunpowder may, by merely varying the size, shape, and density of the grain, be made to ignite rapidly but burn comparatively slowly, or be made to ignite more slowly, but once inflamed to burn very rapidly.

Again, gun-cotton in a loose, uncompressed state, will, if ignited, only flash off; if it is spun into threads or woven into webs, its rate of combustion may be so much reduced that it can be used in gunnery or for a quick fuze; while if powerfully compressed and damp it burns slowly. Wet gun-cotton requires a primer of dry gun-cotton and a fulminate fuze to explode; dry, it may be exploded by a fulminate fuze, &c.

Then nitro-glycerine, when exploded by 15 grains of fulminate of mercury, and at a temperature above 40° F., is very violently detonated; below 40° F. it freezes and cannot be similarly exploded.

To obtain the full effect of all explosives, confinement is absolutely necessary.

The more rapid the explosion the less confinement required, approaching in the case of some explosives to so small an amount that it need not, for practical purposes, be considered.

Thus a charge of nitro-glycerine or gun-cotton, when detonated in the open air, will destroy wrought iron rails, large blocks of stones, balks of timber, &c.

In the case of the former body, the confinement of the atmosphere is sufficient.

In the latter, the mechanical cohesion due to compression is sufficient restraint.

Abel states that if the film of atmosphere surrounding the nitro-glycerine, not exceeding 1/1000 inch in thickness, be removed, the explosive effect is much lessened.

A large charge of gunpowder fired in the ordinary way under water requires a strong case to retain the gases until the action has become general, or, owing to its slow rate of burning, the case would be broken before the whole of the charge had been ignited, and part of the charge drowned.

This is often to be noticed when firing fine-grained powder in heavy guns.

Igniting the charge at several points diminishes the confinement needed.

Mode of Firing.—The application of heat, directly or indirectly, is the principal means of causing an explosion.

The flame from a percussion cap or primer, or a platinum wire heated to incandescence by an electric current, will directly ignite a charge. Friction, concussion, &c., will indirectly ignite a charge due to the conversion of mechanical energy into heat.

It would appear that when one explosive body is used as a means of firing another, the resultant explosion is due to the blow suddenly formed by the gas of the firing charge acting percussively upon the mass to be exploded. If such were the case, then the most powerful explosive would be the best agent for causing an explosion. But it is not so.

For example, nitro-glycerine, which is far more powerful than fulminate of mercury, requires more than 1000 grains to explode gun-cotton, while only 15 grains of the latter is needful for the same work, &c.

A small quantity of an explosive substance which is sensitive to friction or percussion is often used to ignite the original charge.

Detonation.—The instantaneous explosion of the whole mass of a body is defined as "detonation."

The essential difference between an explosion and a detonation is the comparative suddenness of the transformation of the solid or liquid explosive substance into gas and vapour.

Some explosive bodies, such as the fulminates, &c., always detonate, while the detonation of others depends on the mode of firing.

Nitro-glycerine always explodes violently, but when fired with an initiatory charge of fulminate of mercury it is much more powerful than when fired with gunpowder.

Compressed gun-cotton in the air-dry state can be detonated by 2 grains of fulminate of mercury embedded in the material, but when it contains 3 per cent. of water over and above the 2 per cent. which exists normally in the air-dry substance, 15 grains of the fulminate will not always do so.

Theory of Detonation.—The theory of detonation is not yet thoroughly understood. That it is not alone due to the heat caused by the impact of the mechanical energy of the particles of gas, set free from the initiatory charge on the principal mass, is proved by the fact of its being possible to detonate wet gun-cotton.

Professor Bloxam terms detonation to be "sympathetic" explosion.

Experiments carried on in England by Professor Abel, and in France by MM. Champion and Pellet, tend to show that it is due to the vibratory action of the detonating agent.

Thus a glass may withstand a strong blow, though a particular note or vibration will smash it.

All explosive compounds and mixtures, including gunpowder, are susceptible of violent explosion through the agency of a detonation.

Roux and Sarrau.—Roux and Sarrau divide explosions into two orders:—

1st order.—Detonations.

2nd order.—Simple explosions.

Simple explosions are produced by direct inflammation, or by a small charge of gunpowder.

Detonations are obtained from nitro-glycerine, gun-cotton, &c., by exploding with fulminate of mercury.

They state that fulminate of mercury does not detonate gunpowder; but if the exploding charge is a small amount of nitro-glycerine, itself detonated by fulminate of mercury, then an explosion of the first order is obtained.

The relative effects were approximately measured by determining the quantities necessary to rupture small cast iron shells of supposed equal strength.

Results of their Experiments.—The following are some of the results:—

Explosive Effect.
2nd Order. 1st Order.
Gunpowder1·00 4·34
Gun-cotton3·00 6·46
Nitro-glycerine4·8010·13

According to the above table, nitro-glycerine is more than ten times, and gun-cotton more than six times, as powerful as gunpowder fired in the ordinary way (2nd order).

The want of reciprocity between two detonating agents is shown in a remarkable degree by the following experiments, carried out by Professor Abel:—

1.—The detonation of 1/4 ounce of gun-cotton (the smallest quantity that can be thus applied) induced the simultaneous detonation of nitro-glycerine, enclosed in a vessel of sheet tin, and placed at a distance of 1 inch from the gun-cotton.

2.—The detonation of 1/2 ounce of gun-cotton produces the same effect with an intervening space of 3 inches between the substances.

3.—The detonation of 2 ounces of nitro-glycerine in close contact with compressed gun-cotton failed to accomplish the detonation of the latter, which was simply dispersed in a fine state of division, in all the instances but one, in a large number of experiments.

Explosive agents are divided into explosive mixtures and compounds.

In the former the ingredients are mechanically mixed, and can be separated by mechanical means.

In the latter the ingredients are chemically combined, and can only be separated by chemical change.

Torpedo Explosive Agents.—The explosive agents that are practically the most important, as far as their employment as torpedo charges are concerned, are as follows:—

Explosive Mixtures.—A.—Explosive mixtures.

1.—Gunpowder. Nitrate class
2.—Ammonium picrate, or picric powder.

Explosive Compounds.—B.—Explosive compounds.