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Other Nitro Compounds. Picric acid and TNT are nitro derivatives of phenol and toluene respectively. In fact practi- cally all military high explosives are nitro derivatives of aromatic compounds, which latter are produced from the distillation of coal tar. This source of toluene was supplemented by the use of certain natural petroleums which contain benzene and toluene, and the supplies of phenol were augmented by synthetic pro- duction from benzene, another derivative of coal tar. Neverthe- less, these sources of supply were not equal to the demand, and other means of supplementing them had to be found in the war.

Thus tri-nitro-cresol, which is closely allied to picric acid, was much used by the French, as well as di-nitro-naphthalene and di-nitro-phenol, to supplement picric acid. Tri-nitro-anisol, hexa- nitro-diphenylamine, hexa-nitro-diphcnyl sulphide and others were largely used by the Germans. All of these are derived from coal tar and are consequently limited to the available supply of this raw material. Only by finding some material available in larger quantity, with which these nitro compounds could be mixed, was it possible to cope with the demands. The above nitro compounds have the feature in common that they contain insufficient oxygen for their complete combustion: hence the most suitable admixture is a salt rich in oxygen.

Ammonium Nitrate. Of all the available salts, the one which stands out by reason of its accessibility and suitability for the purpose is ammonium nitrate, a substance known as early as the 1 7th century and yet destined to play a most important part in the development of high explosives in the 2oth century.

Mixtures of nitro derivatives of the aromatic compounds with ammonium nitrate, of which Roburite, Ammonal, and Dread- nought powder are amongst the best known, had long been used commercially for blasting purposes, particularly in fiery mines, where the high temperature of explosion of those containing nitro-glycerine is liable to cause explosion of the fire-damp.

Ammonium nitrate explosives are also cheap and safe both to make and to handle, owing to their great stability and insensi- tiveness. They are useful for many purposes where the greater brisance or shattering power given by nitro-glycerine is not re- quired. One of their main disadvantages is the hygroscopicity or moisture-absorbing power of ammonium nitrate, which necessi- tates suitable protection or " waterproofing " from the air in order to prevent the explosive becoming so damp as to fail to respond to the detonating impulse. Although this protection can be readily given in shells and other articles of ammunition, it was probably their characteristic of deliquescence together with the difficulty of detonating such explosives effectively which was responsible for the delay in their adoption for military purposes, except possibly in Austria, where ammonal was to some extent in vogue. Moreover, the peace-time requirements before the war could be amply met in England from lyddite, of which the properties were well known. When, however, other sources of supply of high explosives in gigantic quantities had to be found, ammonium nitrate opened up the best, if not the only, solution, as far as the resources of Great Britain permitted. Yet ammonium nitrate by itself is hardly an explosive at all. By means of a very powerful detonator it is possible to cause a mild explosion and the disruption of the ammonium nitrate molecule, but under ordinary circumstances no " explosive " precautions need be taken in its manufacture or transport a matter of consid- erable advantage in providing the quantities of several thousand tons a week which were required.

Prior to the World War, ammonium nitrate was made by neu- tralizing nitric acid with ammonia

HNO 3 + NH 3 = NH,NO, Nitric acid. Ammonia. Ammonium nitrate.

but the war demands were such that it was necessary to circumvent the necessity of erecting nitric acid plants on such a large scale. This led to the introduction of methods depending on double decomposi- tion of salts.

(l) A modification of the ammonia soda process, as indicated in the equation:

NaNO 3 + NH 4 -HCO 3 = NaHCOs + NH 4 NO 3 Sodium Ammonium Sodium Ammonium

nitrate. bicarbonate. bicarbonate. nitrate.

(2) From calcium nitrate, made either by the arc process or by the action of calcium chloride on sodium nitrate:

2 NaNO, + CaCI 2 = Ca(NO,) s + 2 NaCl.

Sodium Calcium Calcium Sodium

nitrate. chloride. nitrate. chloride.

Ca(NO,) 2 + (NH 4 ) 2 C0 3 = CaCO 3 + 2 NH 4 NO 3

Calcium Ammonium Calcium Ammonium

nitrate. carbonate. carbonate. nitrate.

(3) From sodium nitrate and ammonium sulphate:

2 NaNO, + (NH 4 ) 2 S0 4 = Na 2 SO 4 +2 ,NH 4 NO 3 Sodium Ammonium Sodium Ammonium

nitrate. sulphate. sulphate. nitrate.

In view of its extreme solubility in water, it is difficult to purify the ammonium nitrate completely from the salts which accompany it. The dry salt is very deliquescent and precautions must be taken after drying to avoid the introduction of moisture. A peculiarity of ammonium nitrate is that it undergoes transitions to different crystalline forms at certain temperatures, for instance at about 32 C. and 85 C. the crystalline form changes and also the specific gravity. This point is of considerable importance in shell-filling.

Amatol. The high explosive which was used in the largest quantities by Great Britain during the war was " Amatol," under which name various mixtures of ammonium nitrate with TNT are comprised. These form powerful high explosives capa- ble of detonation with a considerable velocity. " Amatol 40/60 " contains 40% of ammonium nitrate to 60 of T N T and is sufficiently fluid when heated to permit of its being poured in the molten condition. "Amatol 80/20 " contains 80% of ammonium nitrate, which is approximately the proportion necessary for complete combustion of the TNT. This can be compressed into shells, or forced in in a plastic condition above the melting point of the TNT. Either of these methods is much more expedi- tious than the operation of pouring the molten explosive into the shell and allowing it to solidify.

Apart from amatol, mention should also be made of certain other ammonium nitrate explosives which were used during the war. Ammonal had been used in Austria before the war and con- tained ammonium nitrate, TNT, aluminium powder, and charcoal. At a later stage the aluminium was reduced to 3%, as this metal was in great demand for air-craft purposes, and the charcoal was omitted, the resultant mixture being termed alumatol. Sabulite contained ammonium nitrate, TNT, and calcium silicide.

War Requirements. The extent to which the three main high explosives were employed is well illustrated from the table on the following page of quantities manufactured in Great Britain during successive years of the war (see also MUNITIONS OF WAR). For the purpose of comparison, a table of industrial require- ments is added, showing the amount of explosives used in mines and quarries.

So far as Great Britain was concerned, the war was practically fought on the three high explosives, picric acid (lyddite), TNT, and amatol. These necessitated enormous importations of so- dium nitrate from S. America for the manufacture of nitric acid and ammonium nitrate, as well as the importation of sulphur and pyrites for the equally necessary sulphuric acid. Nitric and sulphuric acids are the life-blood of the explosives manufacture, whether it be high explosives or propellant explosives. Without the command of the sea this would have been impossible.

Perchlorate Explosives are analogous to the ammonium nitrate explosives. The perchlorates were discovered by Stadion in 1815. Sodium perchlorate is obtainable by electrolytic methods from common salt, and is readily converted to ammonium perchlorate. Ammonium perchlorate has the advantage of not being hygro- scopic like ammonium nitrate, but on the other hand explosives made from it are generally more sensitive to shock and friction than those containing ammonium nitrate, and are consequently liable to be exploded by the penetration of a rifle bullet or the shock of discharge in a gun. An example of the ammonium perchlorate type of explosives is blastine, which was used in considerable quantities during the war. It contains ammonium perchlorate, sodium nitrate, di-nitro-toluene and paraffin wax.

Industrial Explosives. Industrial explosives are not liable to be exposed to such severe conditions of mechanical shock as military explosives (shock of discharge of a shell from a gun, hostile enemy fire, etc.). This permits of the application of types