Gun-cotton.

Gun-cotton. There are a very large number of explosive nitro-compounds which may be divided into two main classes—viz. (1) Those containing Nitro-glycerine (q.v.), in which is included the great dynamite class, and (2) those not containing nitro-glycerine. Gun-cotton is an explosive nitro-compound of the latter class, and is by far the most important of the class.

So long ago as 1832 it was discovered by Braconot that woody fibre and similar substances could be converted into highly combustible bodies by the action of concentrated nitric acid; six years later Péroux extended this discovery to cotton and other organic substances; he was followed by Dumas, who treated paper in a similar way, and he proposed to make cartridges with paper so treated, the idea being that no residue would be left in the barrel after firing such cartridges. But no practical result followed these discoveries until in 1845 Schönbein, a German chemist, having hit upon the proper mode of treating cotton with nitric and sulphuric acids, announced the discovery of gun-cotton, which he proposed as a substitute for gunpowder. He claimed for it that the advantage it had over gunpowder was that it burned without leaving any residue, and consequently without smoke. He prepared it by immersing carded cotton wool in a mixture of nitric and sulphuric acids, and the equation for its formation may be stated thus:

It will be observed that no mention is made of sulphuric acid in this equation, the presence of which is, however, essential in the production of gun-cotton, for although it takes no active chemical part in the action, it absorbs the water which is formed by the chemical transformation, and thus keeps the nitric acid up to its full strength. Schönbein's discovery gave a great impetus to the question, and experiments continued to be made by many eminent chemists in nearly every country in Europe with the idea of utilising the new explosive for military purposes. It was first manufactured in England on a large scale in the year 1847 by Messrs Hall & Son of Faversham; but, in addition to minor accidents, a terrible explosion took place in their works, which created so much distrust that its manufacture in England was discontinued for several years, as the cause of the explosion, with the then imperfect knowledge possessed of the subject, could not be satisfactorily accounted for. The first country to turn Schönbein's discovery to practical account was Austria. General Von Lenk, an Austrian artillery officer, after extensive trials succeeded in greatly improving the method of manufacture, by which means he was enabled to moderate and ensure a uniform rate of combustion of gun-cotton in air; his discovery was considered of so much importance that in the year 1852 several batteries of Austrian artillery were armed with gun-cotton cartridges. But it soon fell into disrepute, not only on account of its unstable nature, but also because it was found that Von Lenk's improvements were of no practical utility when the gun-cotton was confined in the bore of a gun; the great heat generated caused the inflamed gas to penetrate rapidly through the whole cartridge, so that there was little or no retardation in the rate of combustion, and the rapid combustion caused excessive pressure in the bore, besides giving very unequal results when fired.

Since the failure of the Austrian cartridges gun-cotton has not been used as a propelling agent on a large scale. But its utility as a disruptive agent has been enormously increased by the discoveries of Professor Sir Frederick Abel and the late Mr E. O. Brown. Nothing daunted by the failure of the Austrian experiments, nor by the explosion at Messrs Hall's works, Sir Frederick Abel continued his experiments, and he ultimately discovered a method of manufacture whereby not only a complete purification from free acid is assured, but the material is converted into thoroughly compact homogeneous masses. As a result of his experiments the method of manufacture adopted in England may be briefly described as follows: the best white cotton waste alone is employed; this is first thoroughly cleansed from all grease by boiling with alkalies; it is then picked over by hand and all foreign substances removed, after which the fibre is separated and all knots and lumps opened out by passing the cotton waste through a 'teasing' machine; it is then cut into 2-inch lengths, thoroughly dried, and divided into charges weighing $1\backslash frac\{1\}\{2\}$ lb. each, which are kept in air-tight tin boxes till ready for dipping. The acids used in the manufacture of gun-cotton are nitric acid having a specific gravity of 1.52 and sulphuric acid of 1.84 sp. gr.; these are mixed in the proportion of one part by weight of nitric acid to three of sulphuric acid, and allowed to cool down in iron tanks. The mixed acid is run off into the dipping pans into which a $1\backslash frac\{1\}\{2\}$-lb. charge of cotton is immersed and left in for about five minutes, in which time it will have absorbed about 14 lb. of acid. The charge is now allowed to cool down, after which the waste acid is extracted by means of an 'acid-extractor,' and the charge thoroughly washed to remove all the free acid. It is now pulped and pressed under hydraulic presses to one-third its bulk, and moulded into slabs of various sizes and shapes for storing. The method of manufacture as here described is perfectly safe, as the gun-cotton throughout is in a wet state.

The properties of gun-cotton, as compared with gunpowder, are mainly as follows: (1) It can be ignited at a temperature of about $300^\{\backslash circ\}$, whereas gunpowder requires a temperature of about $600^\{\backslash circ\}$ to ensure ignition; (2) its combustion leaves no solid residue, and is unattended by smoke; (3) the action of gun-cotton is much more rapid than that of gunpowder, and, as has already been pointed out, it is this rapidity of combustion which renders it unsuitable to be used as a propelling agent in cannon; (4) whereas gunpowder is greatly influenced and injuriously affected by moisture, gun-cotton on the contrary is perfectly uninjured, and may be kept for any length of time in water without change. For military purposes this is a most important consideration. Apart from the question of using gun-cotton as a propelling agent, its value for destructive purposes was incontestable, but it was thought to be necessary, in order to develop its full power, that the charge should be strongly confined. Experiments, however, conducted by Mr E. O. Brown clearly demonstrated the fact that compressed gun-cotton could be fully detonated in a totally unconfined state by fulminate of mercury. This discovery was thought to apply to dry gun-cotton only, but Mr Brown continuing his experiments ascertained that wet compressed gun-cotton could be detonated by using a small primer of the dry material. Still further discoveries were made with regard to the detonation of gun-cotton; it was ascertained that detonation, being established at one end of a continuous row of distinct masses of compressed gun-cotton, travels along the whole length of the row, even if a space of half an inch is left between the discs. These discoveries have raised gun-cotton to the highest rank as a military explosive, as the necessity for storing it in a dry state, which is so highly dangerous, is entirely obviated; it is now always stored in a wet state, the gun-cotton containing about 20 per cent. of water, and is packed in air-tight metal cases, so that the necessity for rewetting seldom occurs; in this condition it can be transported with perfect safety.

The discovery with regard to its detonation when in a wet state has led to this material being used as the charge for torpedoes and submarine mines. The first pattern of Whitehead torpedo was 14 feet long and 16 inches in diameter; the speed of the torpedo was 9 knots for 200 yards, and the charge was 118 lb. of compressed wet gun-cotton. Several subsequent patterns of torpedoes have been introduced, the latest being 14 feet long and 14 inches in diameter, and by reducing the charge of gun-cotton to 80 lb. the high speed of 27 knots for 600 yards has been attained. The immense importance of this increased speed can be readily appreciated, as it enables a torpedo to strike the vessel at which it is discharged before she has time to get out of the way. The torpedoes are fired by a striker actuated by a spring which is released on the torpedo striking the side of the ship; the striker is pointed, and penetrates a cap charged with 38 grains of fulminate; this cap is embedded in an 8 oz. disc of dry gun-cotton, enclosed in a hermetically sealed case, and placed as nearly as possible in the centre of the wet gun-cotton charge containing 12 per cent. of water. Gun-cotton is also used as the charge for submarine mines, the charge consisting of from 50 to 500 lb. of wet compressed gun-cotton.

There are various descriptions of marine mines. (1) Ground mines: in these the charge is contained in a case of sheet steel, with cast-iron sinks attached to it to keep it at the bottom of the harbour or river; these mines are fired electrically by observation from the shore when an enemy's ship passes over them. (2) Buoyant mines: these are anchored a few feet below the surface of the water by a steel rope attached to a sunken weight; they are connected with the shore by electric wires; a buoy with a signalling apparatus is attached to the mine, and when a ship strikes a buoy it rings a bell in the signalling-room on shore; if the ship is a friendly one it is allowed to pass, but if it is an enemy's ship the mine is fired by electricity and the ship blown up. (3) Electro-contact mines: these are used only in places where an enemy's ship would pass. When the mine is struck by a passing ship a steel spring or pendulum moves towards the point of impact and thus closes the circuit and fires the mine automatically.

A powder made by the Explosives Company, and generally known under the name of E.C. Powder, is another form in which gun-cotton can be used. There are two descriptions of this powder—viz. sporting and rifle powder; they are both essentially granulated gun-cotton, and consist of small rounded granules, the sporting powder being coloured orange with aurine, and the rifle powder yellow with picric acid.

Schultze Powder may be mentioned here as, although not strictly speaking a gun-cotton powder, it belongs to the class of explosive compounds not containing nitro-glycerine. The process of manufacture consists in macerating soft timber from which all resinous and fatty matter has been extracted by chemical means, the residue being pure finely-divided cellulose; this is saturated with nitric and sulphuric acids, and thoroughly purified by washing. The nitro compound thus formed is finely ground and waterproofed, and then sifted into the various sizes of grain required. Schultze powder has been manufactured since about the year 1860, but E.C. is a powder of more recent date. Both these powders are now largely used for sporting purposes. The great advantages they possess over the ordinary black powder are that an equal velocity is obtained with a very much smaller charge, that they do not foul the gun, and that they are nearly smokeless. But against this must be set the disadvantage that under certain conditions the strain on the breech of the gun is greater. But hitherto the results obtained from these powders when used in military firearms are not sufficiently uniform or regular to justify their adoption for military purposes. There can, however, be but little doubt that a smokeless powder of some sort will, before long, be universally adopted by all the great powers of Europe, not only for rifles, but also for artillery purposes; it is no longer a question as to whether a smokeless powder should be adopted or not, but which of all the smokeless powders experimented on is the best for adoption. Smokeless powders, in order to be suitable for military purposes, must not be too violent in their action, they must be able to stand extremes of heat and cold, they must not be very hygroscopic, and they must keep well in store without deteriorating in quality; and the problem to solve is to find a powder which will fulfil these conditions. Nearly all smokeless powders consist essentially of gun-cotton, or other lower forms of nitro-cotton, acted on by a solvent such as acetic ether or acetone, which reduces the nitro-cellulose to a viscid paste; the paste is then rolled out into sheets, and the solvent allowed to evaporate; the sheets are left as a dense horny substance, and are cut first into strips, and then the strips are cut crosswise into grains of any required size; or the substance can be left in strips or in a fibrous form.

The French, in 1887, were the first to adopt a smokeless powder for the cartridges for their new small-bore rifle, the Lebel. It is known as Vieille's powder, or 'Poudre B;' its exact composition has been kept a secret, but it is believed that picric acid is mixed with the paste as described above. The ballistics attributed to this powder when first introduced were remarkable; a charge of about 70 grains imparted a muzzle velocity of 2000 feet per second, to a bullet weighing 230 grains, fired from a rifle-barrel whose calibre was '.315". But it was found that the powder rapidly deteriorated, and that these results were only attainable with recently manufactured powder. Hence in 1889 the French adopted another powder, the French B.N. Powder, the exact composition of which is kept secret.

The English government, after very exhaustive trials with various kinds of smokeless powders, eventually adopted in 1891 the powder known as cordite for use in Her Majesty's land and naval forces. The name comes from its being made in the form of string or cord, the size of the cord being dependent on the size of the gun for which it is required. It is a nitro-glycerine powder, and consists of 58 per cent. of nitro-glycerine, 37 per cent. of gun-cotton, and 5 per cent. of mineral jelly, acetone being used as a solvent. These ingredients are thoroughly mixed or incorporated in a machine, in which are two revolving blades somewhat in the form of the screw of a steamer. This process, which takes seven hours, converts the mixture into the form of a thick paste, the paste being the same for all sizes of cordite. For rifle cordite, the paste is pressed through a small hole, '.0375" in diameter, in a cylinder, and is wound off on to reels, each reel holding 1 lb. of cordite. The cordite is then dried in a drying-room at a temperature of about 105° Fahrenheit to drive off the acetone, and in this form is ready for use for loading into rifle-cartridge cases. For larger guns, the cordite paste is pressed through plates having holes of various diameters—the larger the gun the larger the diameter of the cord. The sizes at present in use for quick-firing guns are as follows:

For heavier natures, the diameter varies from '.4" to '.5", and length of cord is also greater. The weight of a charge of cordite varies from one-half to two-thirds of that of black powder, and with this charge a higher velocity is obtained with a lower pressure.

The German military powder is also a nitro-glycerine compound. It is very similar to Nobel's ballistite, in which the proportion of nitro-glycerine and gun-cotton are about equal, benzol being used as a solvent; and instead of being pressed into cords, it is rolled under rollers into sheets, and then broken up into grains of various sizes.

The Smokeless Powder Company have also made a powder suitable for military rifles, called riflite; as also a very good sporting smokeless powder known as S.S. These powders are made of nitro-cellulose in the form of nitro-lignine, the various shooting qualities being obtained by the mixture of the higher and lower nitrates of lignine, nitro-benzol being used as a solvent.

The following powders are extensively used for sporting purposes—viz.: Ballistite, already mentioned; Cannonite, which is a nitro-cellulose powder, the gun-cotton being dissolved in ether, and formed into a plastic mass, in which form it is pressed through a cylinder with very small holes in the bottom plate, somewhat in the form of rifle-cordite. These thin strings when dried are then broken into grains by being passed through revolving rollers. Amberite is another nitro-cellulose powder, the gun-cotton being mixed with paraffin and shellac. Walsrode is a German powder, and is a pure gelatinised nitro-cellulose. The gun-cotton being completely dissolved in solvents, the plastic mass being divided into grains by rotation in a barrel.

This powder is known as a condensed powder, the charge necessary to produce the required velocity occupying a very small space. The Normal powder is very similar to Walsrode.

One of the most powerful explosives known is blasting gelatine, made by dissolving 7 per cent. of gun-cotton in 93 per cent. of nitro-glycerine; it forms a gelatinous mass somewhat resembling honey in colour, and varying in consistency from a tough leathery material to a soft substance like stiff jelly. It is stronger than dynamite, as the nitro-cellulose itself is explosive, and, if made with great care, and if absolutely free from all impurities, is a safe and stable explosive. But, unless the ingredients of which it is composed are absolutely pure and free from all foreign matter, it becomes exceedingly dangerous when stored in large quantities, and is liable to spontaneous combustion.

See Wardell, Gunpowder and Gun-cotton (1889); Cundill, Dictionary of Explosives (1895); Eissler, Modern Explosives (1889); Guttmann, Manufacture of Explosives (1895); Sanford, Nitro-Explosives (1896). See also CELLULOID, RIFLES.