British Military Requirements, 1914-8.
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1914 from Aug. 4
1915
1916
1917
1918
Total
short
tons
Picric acid (Lyddite), tons (2,000 Ib.) TNT, tons (2,000 Ib.) Ammonium nitrate, tons (2,000 Ib.)
331 143
2,475 7-347 9,184
18,492
5i,i39 62,512
32,053 94,796 147,648
15,160 84,979 204.457
68,511 238,404 423,801
Toted, short tons
474
19,006
132,143
274,497
304,596
730,716
British Industrial Requirements, 1915-8.
1915
1916
1917
1918
Total short tons
Gunpowder, tons (2,000 Ib.) Permitted explosives, tons (2,000 Ib.) Other explosives, tons (2,000 Ib.)
6,827 4,918 3,36l
6,288 5,505 3,606
6,318 5,778 3,771
5,618 5,68o 3,636
25,051 21,881 '4,374
Total, short tons
15,106
15.399
15.867
14.934
61,306
of high explosives containing nitro-glycerine (dynamites, etc.), and potassium chlorate (cheddites), which are of great utility. Moreover, in industrial work great violence is not always re- quired; in many cases it is desired to dislodge the material with as little shattering as possible, and this leads to a range of explo- sives differing widely in their velocities of internal combustion.
For individual classes of work special requirements have to be met; thus for blasting in enclosed spaces it is important to avoid the formation of poisonous gases such as carbon monoxide; for work in coal-mines, where inflammable dust and gases may be present, it is important to avoid explosives which give a powerful flame, and might thus ignite the coal-dust or gases.
As examples of the types of industrial explosives in use we have (i) explosives in which liquid nitro-glycerine is absorbed in wood-pulp, kieselguhr, etc., with the addition of nitrates and other salts; (2) blasting gelatine, gelatine dynamite and gilegnite, in which the nitro-glycerine is gelatinized with nitro-cotton; (3) ammonium nitrate mixtures; (4) gunpowder and various allied mixtures; (5) cheddites containing potassium chlorate mixed with castor oil and nitro compounds; (6) mixtures containing potassium and ammonium perchlorates.
In recent years the methods of liquefying air have undergone great advances, and this has led to the use of liquid oxygen for explosive purposes. A cartridge of carbonaceous material is dipped into liquid oxygen and is then inserted into the bore-hole and detonated. This forms a very direct means of supplying the oxygen necessary for the combustion of the carbon, but, as the liquid oxygen evaporates quickly, the mixture must be ex- ploded within a few minutes, or it loses its explosive properties.
Powerful explosives may also be made by mixing liquefied nitric peroxide with combustible materials. Nitric peroxide is readily liquefied, and as its boiling point lies at 2iC., there is no difficulty in keeping it in the liquid condition. On explosion, the oxygen passes from the nitric peroxide to the combustible substances. Since, however, nitric peroxide* is very poisonous, precautions must be taken in its manufacture and handling.
An interesting development in industrial explosives is their application to agricultural purposes. Useful results have been obtained, especially in America and South Africa, in breaking up hard ground and in removing tree stumps.
Action of High Explosives. The destructive effect of high ex- plosives in munitions and in the blasting of rocks is due to the very sudden pressure of gases developed, and also in the case of shells to the projection of fragments of the envelope itself.
An explosive may be denned as a substance containing potential energy which can be suddenly released through its rapid decom- positions into hot gaseous products. These tend to occupy a far greater volume than the original substance from which they sprang, and in doing so exert great pressure on the containing vessel or any material with which they are in contact.
Explosives may be solids, liquids or gases, but those used for industrial or military purposes are for the most part solid, for the sake of convenience in handling, and almost invariably contain oxygen. The transformation into gas is usually due to a process of internal oxidation or burning, and the heat evolved tends further to
expand the gases. Under ordinary circumstances, combustion can only take place slowly, being limited by the rate at which fresh air or oxygen can be supplied to the burning material. In explosives, on the other hand, oxygen in a loosely combined form is present in intimate contact with the combustible materials. This proximity may be merely mechanical as in the case of gunpowder, or " inter- atomic "as for example inT NT. Gunpowder is a purely mechanical mixture of three ingredients, potassium nitrate (saltpetre), charcoal and sulphur, each of which is in itself non-explosive. The oxygen in the potassium nitrate is striving to attach itself to the charcoal and sulphur, which form the combustible materials, and is enabled to do so as soon as external energy, usually in the form of a flame, is applied sufficient to break down the link or bond between the nitro- gen or oxygen in the molecule of nitrate. Combustion can then only proceed at the points where the particles of charcoal and sulphur come into contact with the oxygen supplied to them. This explains the ad vantage of good incorporation and the greater rapidity of fine grain powders, but at best the proximity of the three ingredients to one another cannot be closer than is obtainable by purely mechanical means. In a high explosive such as T N T, which is a definite chem- ical compound (Cer^NCyjCHs), the oxygen, loosely linked to the nitrogen, is available to unite with the carbon and hydrogen in the same molecule. The elements are therefore in atomic proximity and, consequently, the disruption of the molecule of T N T into hot gases can proceed at a rate which, in comparison with that of gunpowder, is almost instantaneous. The greater the volume of gases produced the greater will be the pressure formed. The rapidity with which the transformation takes place determines the disruptive effect or brisance of the explosion a very high rate of explosion corre- sponding to a sudden blow and a low rate to a prolonged push. In high explosives the explosion wave is propagated with an accelera- tion until it reaches its maximum, or " velocity of detonation."
In order that the velocity may reach its maximum, it is necessary that a sufficiently powerful initiating impulse be given. In general the explosive must be confined in a metal tube in order to detonate with its maximum velocity; and since the communication of the explosion from particle to particle is retarded by many air-gaps in the mass of the explosive, some compression is usually necessary in order to enable the maximum rate of detonation, which is a definite physical constant for each explosive, to be attained.
The effect of physical condition on the mode of explosion is seen
- n modern smokeless propellants. Nitro-cellulose in its ungelatinized
condition is a high explosive, but when gelatinized by solvents so as to form a horny compact mass (with or without nitro-glycerine) it burns relatively slowly from the exterior surface, instead of being resolved en masse into gas.
There is no definite line of demarcation between a " high " and what may for the want of a better term be called a " low " explosive, such as gunpowder. No intensity of initial impulse can cause the latter to explode at a greater velocity than about 300 metres per second. In the recognized high explosives the velocity of detonation may reach to about 8,000 metres per second and is never less than two or three thousand. In a slow explosive, such as gunpowder, good tamping is requisite to obtain the best results, as the gases nave time to find the line of least resistance, but with a high explo- sive the inertia of the explosive itself and of the super-imposed atmosphere offers almost as much resistance to the intensely sudden evolution of gases as does a solid body. In consequence high ex- plosives are sometimes said to " strike downwards." This of course is an erroneous expression, as at the moment of detonation the force of the explosion must be equal in all directions, but the tangible result of a crater blown in the ground is visible to the senses, whilst the considerably larger hole blown in the air is not. It is this un- tamped effect of high explosives which makes them so much more effective for most military purposes than gunpowder.
