Theoretically,[A] therefore, 1 part of cellulose should form 1.8 part of gun-cotton. Practically, however, this is never obtained, and 1.6 lb. from 1 lb. of cellulose is very good working. The mixture of acids used is generally 1 to 3, or 25 per cent. nitric acid to 75 per cent. sulphuric acid.

[Footnote A: (594 x 1)/324= 1.83.]

[Ill.u.s.tration: FIG. 11.--TANK FOR DIPPING COTTON.]

[Ill.u.s.tration: FIG. 12.--THE COOLING PITS.]

The dipping is done in cast-iron tanks (Fig. 11), a series of which is arranged in a row, and cooled by a stream of cold water flowing round them. The tanks hold about 12 gallons, and the cotton is dipped in portions of 1 lb. at a time. It is thrown into the acids, and the workman moves it about for about three minutes with an iron rabble. At the end of that time he lifts it up on to an iron grating, just above the acids, fixed at the back of the tank, where by means of a movable lever he gently squeezes it, until it contains about ten times its weight of acids (the 1 lb. weighs 10 lbs.). It is then transferred to earthenware pots to steep.

[Ill.u.s.tration: FIG. 13.--COTTON STEEPING POT.]

~Steeping.~--The nitrated cotton, when withdrawn from the dipping tanks, and still containing an excess of acids, is put into earthenware pots of the shape shown in Figs. 12 and 13. The lid is put on, and the pots placed in rows in large cooling pits, about a foot deep, through which a stream of water is constantly flowing. These pits form the floor of the steeping house. The cotton remains in these pots for a period of forty-eight hours, and must be kept cool. Between 18 and 19 C. is the highest temperature desirable, but the cooler the pots are kept the better. At the end of forty-eight hours the chemical reaction is complete, and the cotton is or should be wholly converted into nitro-cellulose; that is, there should be no unnitrated cotton.

[Ill.u.s.tration: FIG. 14.--HYDRO-EXTRACTOR.]

~Whirling Out the Acid.~--The next operation is to remove the excess of acid. This is done by placing the contents of two or three or more pots into a centrifugal hydro-extractor (Fig. 14), making 1,000 to 1,500 revolutions per minute. The hydro-extractor consists of a machine with both an inner cylinder and an outer one, both revolving in concert and driving outwardly the liquid to the chamber, from which it runs away by a discharge pipe. The wet cotton is placed around the inner cone. The cotton, when dry, is removed, and at once thrown into a large tank of water, and the waste acids are collected in a tank.[A]

[Footnote A: Care must be taken in hot weather that the gun-cotton does not fire, as it does sometimes, directly the workman goes to remove it after the machine is stopped. It occurs more often in damp weather. Dr Schupphaus, of Brooklyn, U.S.A., proposes to treat the waste acids from the nitration of cellulose by adding to them sulphuric anhydride and nitric acid. The sulphuric anhydride added converts the water liberated from the cellulose into sulphuric acid.]

~Washing.~--The cotton has now to be carefully washed. This is done in a large wooden tank filled with water. If, however, a river or ca.n.a.l runs through the works, a series of wooden tanks, the sides and bottoms of which are pierced with holes, so as to allow of the free circulation of water, should be sunk into a wooden platform that overhangs the surface of the river in such a way that the tanks are immersed in the water, and of course always full. During the time that the cotton is in the water a workman turns it over constantly with a wooden paddle. A stream of water, in the form of a cascade, should be allowed to fall into these tanks. The cotton may then be thrown on to this stream of water, which, falling some height, at once carries the cotton beneath the surface of the water. This proceeding is necessary because the cotton still retains a large excess of strong acids, and when mixed with water gives rise to considerable heat, especially if mixed slowly with water. After the cotton has been well washed, it is again wrung out in a centrifugal machine, and afterwards allowed to steep in water for some time.

[Ill.u.s.tration: FIG. 15_a_.--THE BEATER FOR GUN-COTTON.]

~Boiling.~--The washed cotton is put into large iron boilers with plenty of water, and boiled for some time at 100 C. In some works lead-lined tanks are used, into which a steam pipe is led. The soluble impurities of unstable character, to which Sir F.A. Abel traced the liability of gun- cotton to instability, are thereby removed. These impurities consist of the products formed by the action of nitric acid on the fatty and resinous substances contained in the cotton fibres. The water in the tanks should be every now and again renewed, and after the first few boilings the water should be tested with litmus paper until they are no longer found to be acid.

[Ill.u.s.tration: FIG. 15_b_.--WHEEL OF BEATER.]

~Pulping.~--The idea of pulping is also due to Abel. By its means a very much more uniform material is obtained. The process is carried out in an apparatus known as a "Beater" or "Hollander" (Fig. 15, _a, b_). It consists of a kind of wooden tank some 2 or 3 feet deep of an oblong shape, in which a wheel carrying a series of knives is made to revolve, the floor of the tank being sloped up so as to almost touch the revolving wheels. This part of the floor, known as the "craw," is a solid piece of oak, and a box of knives is fixed into it, against which the knives in the revolving wheel are pressed. The beater is divided into two parts--the working side, in which the cotton is cut and torn between the knife edges in the revolving cylinder and those in the box; and the running side, into which the cotton pa.s.ses after pa.s.sing under the cylinder. The wheel is generally boxed in to prevent the cotton from being thrown out during its revolution. The cotton is thus in constant motion, continually travelling round, and pa.s.sing between the knives in the revolving cylinder and those in the box fixed in the wooden block beneath it. The beater is kept full of water, and the cotton is gradually reduced to a condition of pulp. The wheel revolves at the rate of 100 to 150 times a minute.

[Ill.u.s.tration: FIG. 16_a_.--POACHER FOR WASHING GUN-COTTON.]

[Ill.u.s.tration: FIG. 16_b_.--PLAN OF THE POACHER.]

[Ill.u.s.tration: FIG. 16_c_.--ANOTHER FORM OF POACHER.]

When the gun-cotton is judged to be sufficiently fine, the contents of the beater are run into another very similar piece of machinery, known as the "poacher" (Fig. 16, _a, b, c_), in which the gun-cotton is continuously agitated together with a large quant.i.ty of water, which can be easily run off and replaced as often as required. When the material is first run into the poacher from the beater, the water with which it is then mixed is first run away and clean water added. The paddle wheel is then set in motion, and at intervals fresh water is added. There is a strainer at the bottom of the poacher which enables the water to be drawn off without disturbing the cotton pulp. After the gun-cotton has been in the poacher for some time, a sample should be taken by holding a rather large mesh sieve in the current for a minute or so. The pulp will thus partly pa.s.s through and partly be caught upon the sieve, and an average sample will be thus obtained. The sample is squeezed out by hand, bottled, and taken to the laboratory to be tested by the heat test for purity. It first, however, requires to be dried. This is best done by placing the sample between coa.r.s.e filter paper, and then putting it under a hand-screw press, where it can be subjected to a tolerably severe pressure for about three minutes. It is then rubbed up very finely with the hands, and placed upon a paper tray, about 6 inches by 4-1/2 inches, which is then placed inside a water oven upon a shelf of coa.r.s.e wire gauze, the temperature of the oven being kept as near as possible to 120 F. (49 C.), the gauze shelves in the oven being kept about 3 inches apart. The sample is allowed to remain at rest for fifteen minutes in the oven, the door of which is left wide open. After the lapse of fifteen minutes the tray is removed and exposed to the air of the laboratory (away from acid fumes) for two hours, the sample being at some point within that time rubbed upon the tray with the hand, in order to reduce it to a fine and uniform state of division.

Twenty grains (1.296 grm.) are used for the test. (See Heat Test, page 249.)

If the gun-cotton sample removed from the poacher stands the heat test satisfactorily, the machine is stopped, and the water drained off. The cotton is allowed some little time to drain, and is then dug out by means of wooden spades, and is then ready for pressing. The poachers hold about 2,000 lbs. of material, and as this represents the products of many hundred distinct nitrating operations, a very uniform mixture is obtained.

Two per cent. of carbonate of soda is sometimes added, but it is not really necessary if the cotton has been properly washed.

~Compressing Gun-Cotton.~--The gun-cotton, in the state in which it is removed from the poacher, contains from 28 to 30 per cent. of water. In order to remove this, the cotton has to be compressed by hydraulic power.

The dry compressed gun-cotton is packed in boxes containing 2,500 lbs. of dry material. In order to ascertain how much of the wet cotton must be put into the press, it is necessary to determine the percentage of water. This may be done by drying 2,000 grains upon a paper tray (previously dried at 100 C.) in the water oven at 100 C. for three hours, and re-weighing and calculating the percentage of water. It is then easy to calculate how much of the wet gun-cotton must be placed in the hopper of the press in order to obtain a block of compressed cotton of the required weight. Various forms of presses are used, and gun-cotton is sent out either as solid blocks, compressed discs, or in the form of an almost dry powder, in zinc- lined, air-tight cases. The discs are often soaked in water after compression until they have absorbed 25 per cent. of moisture.

[Ill.u.s.tration: FIG. 17.--OLD METHOD. 100 PIECES.]

[Ill.u.s.tration: FIG. 18.--NEW METHOD. ONE SOLID BLOCK.]

At the New Explosives Company's Stowmarket Works large solid blocks of gun-cotton are pressed up under a new process, whereby blocks of gun- cotton, for use in submarine mines or in torpedo warheads, are produced.

Large charges of compressed gun-cotton have hitherto been built up from a number of suitably shaped charges of small dimensions (Fig. 17), as it has been impossible to compress large charges in a proper manner. The formation of large-sized blocks of gun-cotton was the invention of Mr A.

Hollings. Prior to the introduction of this method, 8 or 9 lbs. had been the limit of weight for a block. This process has been perfected at the Stowmarket factory, where blocks varying from the armour-piercing sh.e.l.l charge of a few ounces up to blocks of compressed gun-cotton mechanically true, weighing 4 to 5 cwts. for torpedoes or submarine mines, are now produced. At the same time the new process ensures a uniform density throughout the block, and permits of any required density, from 1.4 downwards, being attained; it is also possible exactly to regulate the percentage of moisture, and to ensure its uniform distribution. The maximum percentage of moisture depends, of course, upon the density. By the methods of compression gun-cotton blocks. .h.i.therto employed, blocks of a greater thickness than 2 inches, or of a greater weight than 9 lbs., could not be made, but with the new process blocks of any shape, size, thickness, or weight that is likely to be required can be made readily and safely. The advantages which are claimed for the process may be enumerated as follows:--(1.) There is no s.p.a.ce wasted, as in the case with built-up charges, through slightly imperfect contact between the individual blocks, and thus either a heavier charge--i.e., about 15 per cent. more gun- cotton--can be got into the same s.p.a.ce, or less s.p.a.ce will be occupied by a charge of a given weight. (2.) The metallic cases for solid charges may be much lighter than for those built-up, since with the former their function is merely to prevent the loss of moisture from wet gun-cotton, or to prevent the absorption of moisture by dry gun-cotton. They can thus be made lighter, as the solid charge inside will prevent deformation during transport. With built-up charges the case must be strong enough to prevent damage, either to itself or to the charge it contains. For many uses a metal case, however light, may be discarded, and one of a thin waterproof material subst.i.tuted. (3.) The uniform density of charges made by this process is very favourable to the complete and effective detonation of the entire ma.s.s, and to the presence of the uniform amount of moisture in every part of the charge. (4.) Any required density, from the maximum downwards, may be obtained with ease, and any required amount of moisture left in the charge. These points are of great importance in cases where, like torpedo charges, it is essential to have the centre of gravity of the charge in a predetermined position both vertically and longitudinally, and the charge so fixed in its containing case that the centre of gravity cannot shift. The difficulty of ensuring this with a large torpedo charge built up from a number of discs and segments is well known. Even with plain cylindrical or prismatic charges a marked saving in the process of production is effected by this new system. The charges being in one block they are more easily handled for the usual periodical examination, and they do not break or chafe at the edges, as in the case of discs and cubes in built-up charges. A general view of the press is given in Fig. 19. The gun-cotton in a container is placed on a cradle fixed at an angle to the press. The mould is swivelled round, and the charge pushed into it with a rammer, and it is then swivelled back into position. The mould is made up of a number of wedge pieces which close circ.u.mferentially on the enclosed ma.s.s, which is also subjected to end pressure. Holes are provided for the escape of water.

[Ill.u.s.tration: FIG. 19.--A 4-CWT. BLOCK OF GUN-COTTON BEING TAKEN FROM HYDRAULIC PRESS.]

~The Waltham Abbey Process.~--At the Royal Gunpowder Factory, Waltham Abbey, the manufacture of gun-cotton has been carried out for many years.

The process used differs but little from that used at Stowmarket. The cotton used is of a good quality, it is sorted and picked over to remove foreign matters, &c., and is then cut up by a kind of guillotine into 2-inch lengths. It is then dried in the following manner. The cotton is placed upon an endless band, which conducts it to the stove, or drying closet, a chamber heated by means of hot air and steam traps to about 180 F.; it falls upon a second endless band, placed below the first; it travels back again the whole length of the stove, and so on until delivered into a receptacle at the bottom of the farther end, where it is kept dry until required for use. The speed at which the cotton travels is 6 feet per minute, and as the length of the band travelled amounts to 126 feet, the operation of drying takes twenty-one minutes. One and a quarter lb. are weighed out and placed in a tin box; a truck, fitted to receive a number of these boxes, carries it along a tramway to a cool room, where it is allowed to cool.

~Dipping.~--Mixed acids are used in the proportion of 1 to 3, specific gravity nitric acid 1.52, and sulphuric acid 1.84. The dipping tank is made of cast iron, and holds 220 lbs. of mixed acids, and is surrounded on three sides by a water s.p.a.ce in order to keep it cool. The mixed acids are stored in iron tanks behind the dipping tanks, and are allowed to cool before use. During the nitration, the temperature of the mixed acids is kept at 70 F., and the cotton is dipped in quant.i.ties of 1-1/2 lb. at a time. It is put into a tin shoot at the back of the dipping tank, and raked into the acids by means of a rabble. It remains in the acids for five or six minutes, and is then removed to a grating at the back, pressed and removed. After each charge of cotton is removed from the tank, about 14 lbs. of fresh mixed acids are added, to replace amount removed by charge. The charge now weighs, with the acids retained by it, 15 lbs.; it is now placed in the pots, and left to steep for at least twenty-four hours, the temperature being kept as low as possible, to prevent the formation of soluble cotton, and also prevent firing. The proportion of soluble formed is likely to be higher in hot weather than cold. The pots must be covered to prevent the absorption of moisture from the air, or the accidental entrance of water, which would cause decomposition, and consequent fuming off, through the heat generated by the action of the water upon the strong acids.

The excess of acids is now extracted by means of hydro-extractors, as at Stowmarket. They are worked at 1,200 revolutions per minute, and whirled for five minutes (10-1/2 lbs. of waste acids are removed from each charge dipped). The charge is then washed in a very similar manner to that previously described, and again wrung out in a centrifugal extractor (1,200 revolutions per minute). The gun-cotton is now boiled by means of steam in wooden tanks for eight hours; it is then again wrung out in the extractors for three minutes, boiled for eight hours more, and again wrung out; it is then sent to the beater and afterwards to the poacher. The poachers hold 1,500 gals. each, or 18 cwt. of cotton. The cotton remains six hours in the poachers. Before moulding, 500 gals. of water are run into the poacher, and 500 gals. of lime water containing 9 lbs. of whiting and 9 gals. of a caustic soda solution. This mixture is of such a strength that it is calculated to leave in the finished gun-cotton from 1 to 2 per cent. of alkaline matter.

By means of vacuum pressure, the pulp is now drawn off and up into the stuff chest--a large cylindrical iron tank, sufficiently elevated on iron standards to allow room for the small gauge tanks and moulding apparatus below. It holds the contents of one poacher (18 cwt.), and is provided with revolving arms to keep the pulp stirred up, so that it may be uniformly suspended in water.

Recently a new process, invented by J.M. and W.T. Thomson (Eng. Pat. No.

8,278, 1903), has been introduced at the Waltham Abbey Factory. The object of this invention is the removal of the acids of nitration from the nitrated material after the action has been completed, and without the aid of moving machinery, such as presses, rollers, centrifugals, and the like.

The invention consists in the manufacture of nitrated celluloses by removing the acids from the nitrated cellulose directly by displacement without the employment of either pressure or vacuum or mechanical appliances of any kind, and at the same time securing the minimum dilution of the acids. It was found that if water was carefully run on to the surface of the acids in which the nitro-cellulose is immersed, and the acids be slowly drawn off at the bottom of the vessel, the water displaces the acid from the interstices of the nitro-cellulose without any undesirable rise in temperature, and with very little dilution of the acids. By this process almost the whole of the acid is recovered in a condition suitable for concentration, and the amount of water required for preliminary washing is very greatly reduced. The apparatus which is used for the purpose consists of a cylindrical or rectangular vessel constructed with a perforated false bottom and a c.o.c.k at its lowest point for running off the liquid. Means are also provided to enable the displacing water to be run quietly on to the surface of the nitrating acids.[A]

[Footnote A: In a further patent (Eng. Pat. 7,269, 1903, F.L. Natham), J.M. Thomson and W.T. Thomson propose by use of alcohol to replace the water, used in washing nitro-cellulose, and afterward to remove the alcohol by pressing and centrifuging.]

The apparatus is shown in Fig. 2O, side elevation, and in Fig. 21 a plan of the nitrating vessel and its accessories is given. In Fig. 20 is shown in sectional elevation one of the trough devices for enabling liquids to be added to those in the nitrating vessel without substantial disturbance.

[Ill.u.s.tration: FIG. 20.--SECTIONAL ELEVATION OF THOMSON'S APPARATUS, _a_, Tank; _b_, False Bottom; _c_, Bottom; _c'_, Ribs; _d_, Draining Outlet; _e_, Grid; _f_, Troughs, with Ap.r.o.ns _g_; _h_, Pipe, with Branches _h'_, leading to Troughs, _f_; _k'_, Outlet Pipe of the Sulphuric Acid Tank _k_; _l_, Water Supply Pipe; _m_, Pipe to supply of Nitrating Acids; _o_, Perforations of Trough _f_; _p_, c.o.c.k to remove Acid.]

In carrying out this invention a rectangular lead-lined or earthenware tank _a_ is employed, having a false bottom _b_, supported by ribs _c'_, over the real bottom _c_, which slopes down to a draining outlet pipe _d_, provided with a perforated grid or plate _e_, adapted to prevent choking of the outlet. Suitably supported near the top of the vessel _a_ are provided two troughs, _f_ having depending ap.r.o.ns _g_, a pipe _h_ has two branches _h'_, leading to the troughs, _f_. This pipe _h_ is adapted to be connected by a rubber pipe either to the outlet pipe _k'_ of the sulphuric acid tank _k_ or the water supply pipe _l_. The nitrating acids are supplied through the pipe _m_. A charge of mixed nitrating acids is introduced into the vessel _a_ say up to the level _n_, and the dry cellulose thrown into the acids in small quant.i.ties at a time, being pushed under the surface in the usual way.

[Ill.u.s.tration: FIG. 21.--PLAN OF THOMSON'S APPARATUS, _a_, Tank; _b_, False Bottom; _c'_, Ribs; _e_, Grid; _f_, Troughs; _g_, Ap.r.o.ns; _h_ and _h'_, Pipes to Troughs _f_; _k_, Sulphuric Acid Tank; _m_, Pipe to Nitrating Acids Tank; _o_, Perforations of Troughs; _p_, c.o.c.k to remove Acid.]

A thin layer, say half an inch, of a suitable liquid, preferably sulphuric acid, of a gravity not exceeding that of the waste acid to be produced, is run carefully on the top of the acids by means of the troughs _f_, which are perforated as shown at _o_, so that the sulphuric acid runs down the ap.r.o.ns _g_, and floats on the nitrating acids. The whole is then allowed to stand till nitration has been completed. Water is then supplied to the troughs by way of the pipes _l_, _h_, and _h'_, and is allowed to float very gently over the surface of the sulphuric acid, and when a sufficient layer has been formed, the c.o.c.k _p_ at the bottom of the apparatus is opened, and the acid slowly drawn off, water being supplied to maintain the level constant. It is found that the rate of displacement of the acids is a factor which exerts a considerable influence on the properties of the resulting nitro-cellulose, and affords a means of regulating the temperature of displacement. A rate of displacement which has been found suitable is about two inches in depth of the vessel per hour when treating highly nitrated celluloses, but this rate may, in some cases, be considerably increased. The flow of water at the top of the apparatus is regulated so that a constant level is maintained. By this means the water gradually and entirely displaces the acids from the interstices of the nitro-cellulose, the line of separation between the acids and the water being fairly sharply defined throughout. The flow of water is continued until that issuing at the bottom is found to be free from all trace of acid. The purification of the nitro-cellulose is then proceeded with as usual, either in the same vessel or another.

In the process above described, the object of the introduction of a small layer of sulphuric acid is mainly to prevent the fuming which would otherwise take place, and is not essential, as it is found it can be omitted without any deleterious effect. In order to use the mixed acids in the most economical manner, the waste acid from a previous operation may be used for a first nitration of the cellulose; being afterwards displaced with fresh acids which carry the nitration to the required degree before they are in turn displaced by water. The apparatus may be used merely for the removal of the acid, in which case the nitration is carried out in other vessels in the usual way, and the nitro-cellulose removed to the displacement apparatus where it is just covered with waste acid, and the displacement then proceeded with as above described. In some cases the process is carried out in an ordinary nitrating centrifugal, using the latter to effect preliminary drying after acid extraction. This gives a great advantage over the usual method of working ordinary centrifugal nitrating apparatus, because the acid being removed before the centrifugal is run, practically all danger of firing therein disappears, and a greater proportion of the waste acid is recovered.

In some cases the acids and water may be supplied by perforated pipes, lying along the edges of the nitrating vessel, and these edges may, if desired, be themselves made inclined, like the sides of the troughs _f_.

In the case of effecting nitration in centrifugals as above, the displacing sulphuric acid and water may thus be supplied round the edges of the machines, or removal troughs such as _f_ may be used. It will be obvious that any inert liquid of suitable specific gravity may be used instead of sulphuric acid, as a separation layer.

~Moulding.~--By means of the small measuring tank above referred to, the gun-cotton pulp is drawn off from the stuff chest, and run into moulds of the shapes and sizes required. Thence a large proportion of the water is drawn off by means of tubes connected with the vacuum engine, the moulds having bottoms of fine wire gauze, in order to prevent the pulp from pa.s.sing through. Hydraulic pressure of about 34 lbs. on the square inch is then applied, which has the effect of compressing the pulp into a state in which it has sufficient consistency to enable it to be handled with care, and also expels a portion of the remaining water.

~Compressing.~--The moulded gun-cotton is now taken to the press house, which is situated at some distance from the rest of the factory. Here the moulds are subjected to powerful hydraulic pressure, from 5 to 6 tons per square inch, and is compressed to one-third of its previous bulk. The slabs or discs thus formed are kept under pressure for a short time, not exceeding a minute and a half, to give the requisite density. It should, when removed, be compact, and just sink in water, and should perceptibly yield to the pressure of the fingers. There are perforations in the press blocks, to allow of the escape of gases, if formed, by reason of sufficient heat being generated. The men working the press are placed under cover, behind strong rope mantlets having eye tubes which command a view of the press.

~Packing.~--The finished slabs and discs are dipped into a solution of soda and carbolic acid, and packed in special wood metal-lined cases. When it is to be sent abroad, the metal lining, which is made of tinned copper, is soldered down, but both the outer wooden and inner metal cases are fitted with air-tight screw-plugs, so that when necessary water can be added without unfastening the cases.

~Reworked gun-cotton~ does not make such good discs as new pulped gun- cotton, probably because the fibrous tenacity of the gun-cotton has been destroyed by the amount of pressure it has previously undergone, so that when repulped it resembles fine dust, and a long time is required to press it into any prescribed form. It is generally boiled for eight hours to open up the fibre and remove alkali, then broken up by hand with wooden mallets, pulped, and then used with fresh gun-cotton in the proportion of 1 to 5 parts.

~Manufacture at Le Bouchet.~--At Le Bouchet gun-cotton was made thus:--200 grms. of cotton were steeped for an hour in 2 litres of a mixture of 1 volume concentrated nitric and 2 volumes sulphuric acid. The cotton was then removed and pressed, whereby 7/10ths of the waste acids was recovered. After this it was washed for one to one and a half hours in running water, strongly pressed again; allowed to lie for twenty-four hours in wood-ash lye; then well washed in running water; pressed, and finally dried on a wide linen sheet, through which was forced air heated to 60 C. The average yield from 100 parts of cotton was 165 parts of gun- cotton. The strong pressings of the gun-cotton, while still impregnated with acids, caused subsequent washings to be difficult and laborious.

~Granulation of Gun-Cotton.~--Gun-cotton is often required in the granulated form for use either alone or with some form of smokeless powder. This is done under the patent of Sir Frederick Abel in the following manner:--The gun-cotton from the poacher is placed in a centrifugal machine, very similar to the hydro-extractors before mentioned, and used for wringing out the acids. In this machine it loses water until it only contains 33 per cent., and is at the same time reduced to a more or less fibrous state. It is then taken to the granulating room, where it is first pa.s.sed through sieves or perforations, which break up the ma.s.s into little pieces like shot. The material is then transferred to a revolving drum made of wood or stout leather, which is kept constantly revolving for some time. The material is occasionally sprinkled with water. The drum in turning, of course, carries the granules partially round with it, but the action of gravity causes them to descend constantly to the lowest point, and thus to roll over one another continually. The speed of the drum must not be too rapid. None of the granules must be carried round by centrifugal force, but it must be fast enough to carry them some little distance up the side of the drum. After removal from the drum the granules are dried upon shelves in the drying house.

Gun-cotton is also dissolved in acetone or acetic ether until it has taken the form of a jelly. It is then rolled into thin sheets, and when dry cut up into little squares. In the manufacture of smokeless powders from nitro-cellulose, nitro-lignine, &c., the various substances are mixed with the gun-cotton or collodion-cotton before granulating.

~Collodion-Cotton.~--In the manufacture of collodion or soluble cotton the finer qualities of cotton-waste are used and the acids used in the dipping tanks are much weaker. The manufacture of collodion-cotton has become of more importance than gun-cotton, by reason of its use for the manufacture of the various forms of gelatine, such as gelatine dynamite, gelignite, forcite, &c., and also on account of its extensive use in the manufacture of many of the smokeless powders. It is also used for the manufacture of "collodion," which is a solution of collodion-cotton in ether-alcohol; for the preparation of celluloid, and many other purposes. It is less explosive than gun-cotton, and consists of the lower nitrates of cellulose. It is soluble in nitro-glycerine, and in a mixture of 2 parts of ether and 1 of alcohol; also in acetone, acetic ether, and other solvents. MM. Menard and Domonte were the first to prepare a soluble gun- cotton, and its investigation was carried on by Bechamp, who showed that its properties and composition were different to those of gun-cotton.