Explosives consist of certain elements forced into unwillingcombination; nitrogen, eager to release itself from the few grains of powder in which it is imprisoned, and assume its original state of a free gas; oxygen and carbon, kept apart through the medium of this very nitrogen, but eager to unite to form great quantities of carbon-oxide; and hydrogen, ready to combine with the oxygen as soon as the latter is released from its combination with the nitrogen by the disturbance of the complex explosive molecule. Thus occurs the seeming paradox that one of the most inert substances, nitrogen, is the essential ingredient of explosives, for by coaxing it into distasteful association with other elements, so to speak, may be obtained a little brown powder, or perhapsa few drops of an oily liquid, which, docile enough until a shock severs the already strained relations existing between its elements, in a very small fraction of a second transforms itself into immense quantities of gas, producing by the enormous expansion an almost irresistible dynamic force.
An explosion is in fact, an extremely rapid combustion, which is obtained by bringing the oxygen, necessary for the combustion of thecarbon and hydrogen, into the same molecule with them. This is unlike ordinary burning, where the oxygen must be supplied by the air, and the effect in an explosion, of the practically instantaneous liberation of the products of combustion, is roughly analogous to that which would be obtained were a boiler full of water completely converted into steam in a fraction of a second. On the statement of one authority, the modern explosive, T.N.T., produces ten thousand times its volume of gas.
From what has been said, it is obvious that an explosive consists of some material containing the two combustibles, carbon and hydrogen, to which nitrogen combined with oxygen has been added. This addition, known as "nitration," is effected by nitric acid, mixed with sulfuric acid toabsorb the water formed by the nitration. Tri-nitro-toluol (T.N.T.), an explosive once extensively used as a bursting charge for shells, is obtained by distilling toluol, a fragrant limpid liquid, from coal tar. This toluol is nitrated, so as to introduce three nitro groups into its molecule and convert it into one of the most deadly of modern explosives.
Coal tar is also the basis of another common bursting charge - ammonium picrate, a salt of picric acid. Phenol, the carbolic acid which relieves the suffering behind the lines, as a germicide, is a coal tarproduct, which is converted by nitration into picric acid, a powerful explosive. Widespread devastation is not always the only effect of the explosion of a picric acid shell, for the acid is a powerful yellow dye, and if the detonation is not complete, everything in the vicinity is colored a brilliant canary-bird hue, not excepting the men's skin and clothing.
The explosion of T.N.T. shells, which have largely supplanted those filled with picric acid compounds, is accompanied with great volumes of smoke, which has earned for these shells the nickname "Black Marias." This smoke is due to the fact that the T.N.T. is deficient in oxygen, and to overcome this defect, explosives with four nitro groups have been made. Among these latter, about which little has been heard, are T.N.A. and tetryl, but T.N.T. and ammonium picrate, together with such compounds as melinite and schneiderite of the French, lyddite of the British, and shimose of the Japanese, which are cast picrates, remained the chief explosives used asbursting charges in shells, bombs and grenades of the early 20th Century.
All are dependent upon coaltar and are made by nitration of the various products of distillation of by-products of coking, which also furnishes analin dyes. One of the reasons for the supremacy of this type of explosive is its safety; T.N.T. can be dropped from a tall building, hammered, set on fire, shot full of holes and dumped into a furnace without exploding, but the detonation of a cap containing fulminate of mercury produces an immediate explosion.
It is apparent that such high explosives, which detonate or explode instantaneously, would be entirely unsuitable for the propelling charge which forces a projectile from a gun, for their force is produced so rapidly that it would disrupt a gun chamber, even if the latter were entirely open. By nitrating common cotton, however, rather than toluol or phenol, is obtained a compound that is known as nitro-cellulose, or more commonly, gun cotton, which, when colloided, burns rapidly, rather than explodes, and gradually liberates the gaseous products of combustion with calculated speed and energy. The operation of nitration provides it with oxygen, which is thus available for the combustion of the carbon of which the cotton is partially composed, without recourse to the air. The burning becomes a transfer of the oxygen from the nitrogen, which is released, to the carbon. But, as in the case of the T.N.T., the oxygen supply is insufficient, and the volume of smoke released would be the despair of the camoufleurs if gun cotton were used to shoot guns.
However, by colloiding, that is, dissolving the gun cotton in alcohol and ether, is obtained a plastic mass which contains just sufficient oxygen for its combustion and in which the violence and sensitivity can be definitely controlled. This compound is the basis of smokeless powder employed by all nations as a propelling charge, from the infantry rifle cartridge to the 42-centimetre howitzer.
A multitude of shapes are employed such as sticks, shredds, multiperforated hexagonal grains, etc., which have as their object an increase or decrease in the surface exposed to burning as the charge is consumed, so that the pressure in the barrel of the gun remains effective, although the projectile is constantly moving towards the muzzle. Of course the time of combustion is exceedingly brief, but it is so much longer than that of T.N.T., for instance, that smokeless powder is spoken of as a slow-burning explosive.
The pressure existing in the gun at the time, is a vital factor, for a stick of this powder only burns quietly when lighted in the open air. Where the space is limited, the velocity of burning is so greatly increased that an explosion results.
The manufacture of nitro-cellulose or gun cotton calls for extensive equipment and great care. Short cotton fibres are nitrated by the action of mixed nitric and sulfuric acids, the office of the sulfuric acid being simply to absorb the water liberated when the nitric acid acts on the cotton. The resultant nitro cotton must be repeatedly washed and, for the manufactureof smokeless powder, is dissolved in alcohol and ether to secure a plastic mass, which may be worked into suitable shapes, after which the solvents are evaporated. It is seen that nitrogen is the backbone of all explosives. It is nitration that converts the fluffy cotton into gun cotton, that converts phenol, or carbolic acid, into an explosive, and the limpid, peaceful toluol and oily glycerine into instruments of war.
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