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for which had also to be maintained during the war period. The main types of these had been standardized for years, but the experiences of the war have had some effect in influencing the uses of industrial explosives. The conditions which have to be met by commercial explosives are not so stringent as in the case of military explosives. Thus the latter are liable at any time to be subjected to hostile fire, and must therefore be very insensi- tive to shock; this precludes many of the explosives which are in use for commercial mining, etc.

Military Uses. High explosives for military purposes are required for the bursting charges of artillery shells, air and trench - mortar bombs, grenades, naval mines, torpedoes, depth charges, as well as for land mines and demolitions in the field. For the two last-mentioned purposes ordinary commercial blasting ex- plosives may on emergency be used, but a serious danger will be involved to the user if the explosive is of such sensitiveness as to be " set off " by the impact of hostile fire. On this account it is generally undesirable to use explosives containing nitro- glycerine, which form such a large part of the blasting explosives produced for industrial purposes.

The choice of explosives for shells requires special care, as the shock of discharge is so great in modern ordnance that only explosives which are very insensitive to shock can be safely used. For this reason gunpowder was regarded for many years as the only safe explosive for the bursting charges of common shell. The premature explosion to which guncotton gave rise had tended to confirm this view; so that gunpowder, in spite of the comparative mildness of its explosion, remained in universal use until the introduction of picric acid by the French in 1885.

Picric Acid. This was discovered by Woulffe in 1771, but its explosive properties remained for a long time unrecognized. Sprengel had demonstrated its capability of detonation in 1871. In 1885 Turpin, a French chemist, applied it to the filling of shell, for which, by reason of its stability and insensitiveness to shock, as well as its extremely violent action when properly detonated, it proved eminently suitable. Shortly after this, pic- ric acid under various names, either with or without the addition of other substances such as collodion or paraffin wax to reduce its sensitiveness, was universally adopted by the Great Powers as a high explosive for shell-filling. Picric acid can be melted and poured into the shell, where it sets into a compact mass the method adopted in the British service. It was first used in actual warfare by the British army in the S. African War of 1899-1902 under the name of " lyddite." Picric acid is also the main or sole constituent of the French melinite, the Japanese shimose powder, and the Austrian ekrasit. Lyddite can hardly be said to have fulfilled in the S. African War the somewhat exaggerated claims made for it, as the shells, especially of the smaller sizes, were uncertain in their detonation, but this was due to the fear still prevailing of premature explosion in the bore, which prevented the use of a sufficiently powerful detonating impulse in the per- cussion fuze being employed. In the World War of 1914-8, after this disability had been removed, through the employment of a fulminate detonator and a suitable exploder system, shells filled with lyddite were amongst the most certain and violent in their action. When completely detonated, these shells give a dense black smoke due to unconsumed particles of carbon through lack of sufficient oxygen for complete combustion. This smoke is of great assistance to the gunner in enabling him to locate their explosion and so to adjust the range as required.

The manufacture of picric acid has been carried out for many years by the so-called " pot process," and this was retained essential- ly unchanged throughout the war. In this process, phenol (carbolic acid) is first heated with sulphuric acid, whereby phenol-sulphonic acids are formed, as for instance in the following equation : C.HsOH+aH^O, = C, H S (SO,H) 2 OH+2H 2 O Phenol. Sulphuric Phenol disulphonic acid,

acid.

This on cooling forms a buttery mass, which is then transferred to earthenware pots and diluted. Nitric acid is allowed to trickle in slowly through glass syphons, and thus converts the sulphonic acids to tn-mtro-phenol or picric acid, which has the formula CeH 2 (NO 2 ) 3 - OH. The residual acid is drained off and the crystals of picric acid are thoroughly washed and then carefully dried on glass plates in a warm chamber. The last operation is the most dangerous part of

the manufacture and is carried out at a distance from the nitration process. The main recent developments have been directed towards increasing the yield of picric acid and economizing acids, on the one hand by recovering the residual sulphuric acid, and on the other hand by collecting the large volumes of nitrous fumes evolved during the nitration process, which were formerly allowed to go to waste, thereby causing a serious contamination of the atmosphere.

At a later stage of the war a continuous process was patented for the manufacture of picric acid. In this process the phenol sulphonic acids were caused to traverse a long trough constructed of acid- proof bricks, and nitric acid was injected through a series of al- uminium jets at intervals along the trough. This method saves a great deal of handling, and is claimed to give very good yields of picric acid. Under ordinary circumstances the yield of picric acid is about 1 80 Ib. from each loo Ib. of phenol.

An alternative process, which was introduced and used with success during the war, was based on the intermediate formation of di-nitro-phenol. This process started out from benzene and passed through the following stages :

C 6 H 6 C 6 H S C1 C 6 H 3 (NO 2 ) 2 C1.

Benzene. Mono-chloro-benzene. Di-nitro-chloro-benzene. CeH 3 (NO 2 ) 2 OH C 6 H 2 (NO 2 ) 3 OH.

Di-nitro-phenol. Picric acid.

The final nitration was effected with concentrated nitric and sujphuric acids; the picric acid being washed free from acid and dried in stoves as in the phenol process.

Trinitrotoluene (T N T), which is known officially as Trotyl, is a high explosive very similar in its action to picric acid and had been discovered by Wilbrand in 1863. Its manufacture in small quantities in Great Britain had been taken up some 15 years before the outbreak of the World War, mainly for export or as ingredient of certain blasting explosives, and about 1893-4 it was made on a more considerable scale in Germany, where its value as a shell-filling became recognized some ten years later. T N T has the advantage of melting at a lower temperature than picric acid, and of not forming sensitive salts (picrates) with metals; added to which it is even less sensitive to shock and consequently less liable to give rise to premature explosions in the bore of the gun. The lower melting point of T N T (81 C.) enables it to be melted in steam-jacketed pans, whereas picric acid (121-6 C.) needs hot-air chambers or oil baths.

The manufacture of T N T in Great Britain prior to the war was very small, and the best methods had to be worked out from first principles after the outbreak of war. The existing processes were slow and wasteful, and it was necessary to find the best conditions for expediting the process and obtaining the highest possible yield of T N T with the greatest economy of sulphuric and nitric acids.

T N T is made from toluene (CH 3 C 6 H 6 ) by the action of nitric acid (HNO 3 ) as indicated in the following equations:

CH 5 C 6 H 6 -|-HNO3 = H 2 O-f-CH 3 C6H 4 (NO 2 ) (mono-nitro-toluene)

CH 3 C 6 H 4 (NO 2 ) +HNO 3 = H 2 O+CH 3 C 6 H 5 (NO 2 ) 2 (di-nitro-toluene)

CH,C,H 3 (N0 2 ) 2 +HNO S = H 2 O+C H, C H 2 (N O 2 ) 3 (tri-nitro-tolu- ene).

A continuous process was introduced during the war, and proved very successful. In this process, mono-nitro-toluene entered at one end of the plant and strong sulphuric acid at the other, the nitric acid being introduced at intermediate points.

In all of these processes the product is a crude T N T of melting point 74 to 77 C. In general this is good enough for explosive purposes, but for special uses it has to be purified by crystallization or by washing with alcohol.

A more recent purification process consists of a treatment with sodium sulphite, which destroys the chief impurities the isomeric tri-nitro-toluenes. There are six possible tri-nitro-toluenes, which differ according to the relative positions of the nitro groups in the molecule. These are all known, but only three of them are formed by direct nitration. The first stage of the nitration gives mainly ortho- and para-nitro-tpluene with about 3 to 4% of meta-nitro- toluene. On further nitration, ortho- and para-nitro-toluene can give the normal symmetrical tri-nitro-toluene ; the meta compound cannot do so and consequently gives other isomers as shown below :

CH,

Ortho CH>

CH, CH,

o _tr

NO, NO,

Para Symmetrical TNT.

NO.

,

and

N o,

Meta-

NO, NO,

Isomers of T N T.