Nitro-Explosives: A Practical Treatise Part 7

~Manufacture.~--The cotton used is cotton-waste.[A] It is thought by some that Egyptian cotton is preferable, and especially long fibre varieties.

The strength of the acids used is, however, of more importance than the quality of the cotton. The percentage composition of the acid mixture which gives the best results is as follows:--Nitric acid, 23 per cent.; sulphuric acid, 66 per cent.; and water, 11 per cent; and has a specific gravity of 1.712 (about). It can be made by mixing sulphuric acid of specific gravity 1.84 with nitric acid of specific gravity 1.368 in the proportions of 66 per cent. and 34 per cent. respectively. (The production of the penta-nitro-cellulose is aimed at if the collodion-cotton is for use as an explosive.) If the acids are much weaker than this, or pota.s.sium nitrate and sulphuric acid is used, the lower nitrates will be formed. The product, while being entirely soluble in ether-alcohol or nitro-glycerine, will have a low nitrogen content, whereas a material with as high a nitrogen as 12 or 12.6 is to be aimed at.

[Footnote A: Raw cotton is often used.]

The cotton should not be allowed to remain in the dipping tanks for more than five minutes, and the acid mixture should be kept at a temperature of 28 C. or thereabouts; and the cotton should be removed after a few minutes, and should not be pressed out, as in the case of gun-cotton, but at once transferred to the pots and allowed to steep for forty-eight hours. (Some prefer twenty-four hours, but there is more chance in this case of the product containing non-nitrated cellulose.) When the nitration is complete, the collodion-cotton is removed from the pots, and treated in exactly the same manner as described under gun-cotton. The produce should be entirely soluble in ether-alcohol and nitro-glycerine, and contain as near 12.7 per cent. of nitrogen as possible. The theoretical nitrogen is for the penta-nitro-cellulose 12.75 per cent. This will, however, seldom if ever be obtained. The following are some of the results I have obtained from different samples:--

Nitrogen.

(1.) (2.) (3.) German make 11.64 11.48 11.49 per cent.

Stowmarket 12.57 12.60 11.22 "

Walsrode 11.61 12.07 11.99 "

Faversham 12.14 11.70 11.60 "

and the following was the a.n.a.lysis of a sample (No. 1) of German-made collodion-cotton, which made very good blasting gelatine:--

_ Soluble cotton (collodion) 99.118 per cent.| Nitrogen = 11.64 per cent.

Gun-cotton 0.642 " _| Non-nitrated cotton 0.240 "

Total ash 0.25 "

It should contain as little non-nitrated or unconverted cotton and as little gun-cotton as possible, as they are both insoluble in nitro- glycerol. The quality and composition of any sample of collodion-cotton can be quickly inferred by determining the percentage of nitrogen by means of the nitrometer and the use of the solubility test.[A] A high nitrogen content coupled with a high solubility is the end to be aimed at; a high nitrogen with a low solubility shows the presence of gun-cotton, and a low nitrogen, together with a low solubility, the presence of unnitrated cotton. Where complete solubility is essential and the percentage of nitrogen less important, Dr Lunge recommends nitration with a mixture of equal parts of sulphuric and nitric acids containing from 19 to 20 per cent. of water.

[Footnote A: See a.n.a.lysis of Explosives.]

Mr T.R. France claims to have invented some improvements in the manufacture of soluble nitro-cellulose. His object has been to produce an article as uniform as possible. His explanation of the imperfect action of the acids is that, however uniform the mixed acids may be in strength and proportions, and however carefully the operations of nitrating, &c., may be conducted, there are variable elements found in different samples of cotton. The cotton fibre has for its protection a glazed surface. It is tubular and cellular in structure, and contains a natural semi-fluid substance composed of oil or gum, which varies in nature according to the nature of the soil upon which the cotton is grown. The tubes of the fibre seem to be open at one end only when the fibre is of normal length. When, therefore, the cotton is subjected to the action of the mixed acids, the line of least resistance seems to be taken by them, viz., the insides of the tubes const.i.tuting the fibre of the cotton, into which they are taken by capillary attraction, and are subject to change as they progress, and to the increased resistance from the oil or gum, &c., in their progress, and therefore to modified action, the result of which is slower and slower action, or chemical change. He also thinks it is possible that the power of capillary attraction is balanced in the tubes by air contained therein, after a little, sufficiently so to prevent the acids from taking full effect. To get over this, Mr France uses his cotton in a fine state, almost dust, in fact, and then nitrates in the usual mixture of acids at 40 to 90 F., the excess of acids being removed by pressure. He says he does not find it necessary to wash this fine cotton dust in an alkaline solution previous to nitration. His mixed acids consist of 8 parts HNO_{3} = 42 B., and 12 parts H_{2}SO_{4} = 66 B., and he stirs in the dipping tank for fifteen minutes, the temperature being 50 F. to 100 F., the temperature preferred being 75 F.

~"Nitrated" Gun-Cotton.~--The nitrates that are or have been mixed with gun-cotton in order to supply oxygen are pota.s.sium nitrate, ammonium nitrate, and barium nitrate (tonite). The total combustion of gun-cotton by pota.s.sium nitrate corresponds to the equation:--

10[C_{24}H_{18}(NO_{3}H)_{11}O_{9}] + 82KNO_{3} = 199CO_{2} + 41K_{2}CO_{3} + 145H_{2}O + 96N_{2},

or 828 grms. of nitrate for 1,143 grms. of gun-cotton, or 42 per cent.

nitrate and 58 per cent. gun-cotton. The explosive made at Faversham by the Cotton Powder Company, and known as tonite No. 1, consists of very nearly half gun-cotton and half barium nitrate. The relations by weight of total combustion would be 51.6 of gun-cotton to 48.4 of barium nitrate.

The average composition of tonite I have found by a.n.a.lysis to be 51 per cent. gun-cotton to 49 per cent. barium nitrate. The heat liberated is practically the same as for an equivalent weight of KNO_{3}; but the barium nitrate mixture weighs 2,223 grms. instead of 1,971 grms., or one-eighth more. The advantage in mixing a nitrate with gun-cotton is that it supplies oxygen, and by converting all the carbon into carbonic acid, prevents the formation of the poisonous gas carbonic oxide (CO). The nitrates of pota.s.sium and barium are also used admixed with nitro- cellulose in several of the sporting smokeless powders.

~The Manufacture of Tonite.~--The explosive tonite was patented by Messrs Trench, Faure, and Mackie, and is manufactured at Faversham and Melling at the works of the Cotton Powder Company, and at San Francis...o...b.. the Tonite Powder Company. It consists of finely divided and macerated gun-cotton incorporated with finely ground nitrate of barium which has been carefully recrystallised. It is made by acting upon carbonate of barium[A] with nitric acid. The wet and perfectly purified, finely pulped gun-cotton is intimately mixed up between edge runners with about the same weight of nitrate, and the mixing and grinding continued until the whole has become an intimately mixed paste. This paste is then compressed into cartridges, formed with a recess at one end for the purpose of inserting the detonator. The whole is then covered with paraffined paper.

[Footnote A: Witherite, BaCO_{3} + 2HNO_{3} = Ba(NO_{3})_{2} + CO_{2} + H_{2}O.]

The tonite No. 2 consisted of gun-cotton, nitrates of potash and soda, charcoal and sulphur. Tonite No. 3[A] is composed as follows:--Gun-cotton, 19 per cent.; di-nitro-benzol, 13 per cent.; and barium nitrate, 68 per cent. or similar proportions. It is a yellowish colour, and being slower in its explosive action, is better adapted for blasting soft rock.

[Footnote A: Tonite No. 1 was patented by Messrs Trench, Faure, and Mackie, and tonite Nos. 2 and 3 by Trench alone.]

Tonite is extensively used in torpedoes and for submarine blasting, also for quarries, &c. Large quant.i.ties were used in the construction of the Manchester Ship Ca.n.a.l. Among its advantages are, that the English railways will take tonite on the same footing as gunpowder; it is a very dense material; if wetted it can easily be dried in the sun; it very readily explodes by the use of a proper detonator; while it burns very slowly and without the least danger; the cartridges being waterproofed, it can be employed in wet bore holes, and it can be tamped with water; and finally, as it contains sufficient oxygen to oxidise the carbon, no carbonic oxide (CO) gas is formed, i.e., its detonation is perfect. It is a very safe explosive to use, being little susceptible to either blows or friction.

Not long ago, a committee, composed of Prof. P. Bedson, Drs Drummond and Hume, Mr T. Bell, one of H.M. Inspectors of Coal Mines, and others, in considering the problem whether the fumes produced by the combustion of tonite were injurious to health, carried out a series of experiments in coal mines for this purpose. The air at the "intake" was a.n.a.lysed, also the air of the "return," and the smoky air in the vicinity of the shot holes. The cartridge was surrounded by the flame-extinguishing mixture, and packed in a brown paper bag. During the first experiment nineteen shots were fired (= 6.29 lbs. tonite). The "return" air showed only a trace of carbonic oxide gas (CO). At the second experiment thirteen shots were fired (= 4.40 lbs. tonite), and a.n.a.lysis of the air of the "return"

showed that CO was present in traces only, whilst the fumes contained only 1.9 to 4.8 parts per 10,000.

~Dangers in connection with the Manufacture of Guncotton, &c.~--Of all the nitro compounds, the least dangerous to manufacture are gun-cotton and collodion-cotton. The fact that the Stowmarket Factory is within five minutes' walk of the town shows how safe the manufacture of this explosive is regarded. With the exception of the nitration and the compression into blocks or discs, the whole process is worked with a large excess of water, and the probability of an explosion is thus reduced to a minimum. Among the precautions that should, however, be taken, are--first, the careful extraction of the resinous and soluble substances from the cotton before nitration, as it was shown many years ago by Sir F.A. Abel that the instability of the gun-cotton first manufactured in England and Austria was chiefly due to these compounds. They are generally removed by boiling the cotton in a soda solution.

The actual nitration of cotton is not a dangerous operation, but the operations of wringing in the hydro-extractors, and washing the nitro- cotton after it leaves the first centrifugal machine, are somewhat so.

Great care should be taken that the wrung-out nitro-cotton at once comes in contact with a large excess of water, i.e., is at once immersed entirely in the water, since at this stage it is especially liable to decomposition, which, once started, is very difficult to stop. The warmer the mixture and the less water it contains, the more liable it is to decomposition; hence it is that on warm and damp days the centrifugal machines are most likely to fire. The commencement of decomposition may be at once detected by the evolution of red fumes. Directly the gun-cotton is immersed in the large quant.i.ty of water in the beater and poacher it is safe.

In order that the final product may be stable and have good keeping qualities, it is necessary that it should be washed completely free from acid. The treatment in the beater and poacher, by causing the material to a.s.sume the state of a fine pulp, in contact with a large quant.i.ty of water, does a good deal to get rid of the free acid, but the boiling process is absolutely necessary. It has been proposed to neutralise the free acid with a dilute solution of ammonia; and Dr C.O. Weber has published some experiments bearing upon this treatment. He found that after treatment with ammonia, pyroxyline a.s.sumed a slightly yellowish tinge, which was a sure sign of alkalinity. It was then removed from the water, and roughly dried between folds of filter paper, and afterwards dried in an oven at 70 C. After three hours, however, an explosion took place, which entirely destroyed the strong copper oven in which the nitro- cotton (about one oz.) had been drying. The explosion was in some respects remarkable. The pyroxyline was the di-nitro-cellulose (or possibly the penta-nitro?), and the temperature was below the igniting point of this material (40 C. would have been a better temperature). Dr Weber determined the ignition point of his di-nitro-cellulose, and found it to be 194 to 198 C., and he is therefore of opinion that the explosion was due to the treatment of the partially washed material with ammonia. A certain quant.i.ty of ammonium nitrate was probably formed, and subsequently dried upon the nitro-cellulose, in a state of very fine subdivision. The faintest trace of acid would then be sufficient to bring about the explosive ignition of the ammonium nitrate.

The drying of gun-cotton or collodion-cotton is also a somewhat dangerous operation. A temperature of 40 C. (104 F.) should not be exceeded, and thermometers should be placed in the nitro-cotton, and the temperature frequently observed. An electric alarm thermometer is also a useful adjunct to the cotton drying house. Great care must also be taken that there are no exposed hot-water pipes or stoves in the drying house, as the fine gun-cotton dust produced by the turning or moving of the material upon the shelves would settle upon such pipes or stoves, and becoming hot, would be very sensitive to the least friction. The floor also should be covered with linoleum or indiarubber. When hot currents of air are made to pa.s.s over the surface of gun-cotton, the gun-cotton becomes electrified.

It is important, therefore, to provide some means to carry it away. Mr W.F. Reid, F.I.C., was the first to use metal frames, carriers, and sieves, upon which is secured the cloth holding the gun-cotton, and to earth them.

The compression of gun-cotton into blocks, discs, &c., is also attended with considerable risk. Mr O. Guttmann, in an interesting paper upon "The Dangers in the Manufacture of Explosives" (_Jour. Soc. Chem. Ind._, No. 3, vol. xi., 1892), says: "The compression of gun-cotton into cartridges requires far more care than that of gunpowder, as this is done in a warm state, and gun-cotton even when cold, is more sensitive than gunpowder.

When coming out of the centrifugal machines, the gun-cotton should always pa.s.s first through a sieve, in order to detect nails or matches which may by chance have got into it. What has been said as to gunpowder presses applies still more to those for gun-cotton, although the latter are always hydraulic presses. Generally the pistons fit the mould perfectly, that is to say, they make aspiration like the piston of a pump. But there is no metal as yet known which for any length of time will stand the constant friction of compression, and after some time the mould will be wider in that part where the greatest compression takes place. The best metal for this purpose has proved to be a special steel made by Krupp, but this also is only relatively better; for pistons I prefer hard cast iron. If the position of the moulds and pistons is not exactly the same in all cases, what the Germans call 'Ecken' (English 'binding') will take place, viz., the mould will stand obliquely to the piston, and a dangerous friction will result." "Of course, it is necessary to protect the man working the hydraulic valves during compression. At Waltham Abbey they have a curtain made of ship's hawsers, which is at the same time elastic and resistant."

Mr Guttmann has found that a part.i.tion wall 12 inches thick, made of 2-inch planks, and filled with ground cinders, gives very effective protection. A door in this part.i.tion enables the workman to get to the press, and a conical tube penetrates the wall, enabling the man to see the whole work from a safe standpoint. The roof, or one side of the building, should be of gla.s.s, so as to give the explosion a direction.

~Trench's Fire-extinguishing Compound~ is manufactured by the Cotton Powder Company at Faversham, and is the invention of Mr George Trench, F.C.S., the manager of the Company. The object of the invention is to surround the cartridges of tonite, when used in coal mines, with a fire- extinguishing compound. If a charge of tonite, dynamite, or gelatine dynamite is put inside a few ounces of this mixture, and then fired, not the least trace of flame can be observed, and experiments appear to show that there is no flame at all. The compound consists of sawdust impregnated with a mixture of alum and chlorides of sodium and ammonia.

Fig. 22 shows the manner of placing the tonite cartridge in the paper bag, and surrounding it with the fire-extinguishing compound, _aa_. The attachment of the fuse and detonator is also shown.

[Ill.u.s.tration: FIG. 22.--TRENCH'S FIRE-EXTINGUISHING CARTRIDGE.]

The following report (taken from the _Faversham News_, 22nd Oct. 1887) of experiments conducted in the presence of several scientific and mining men will show its value:--"A large wrought-iron tank, of 45 cubic feet capacity, had been sunk level with the ground in the middle of the yard; to this tank the gas had been laid on, for a purpose that will be explained later on. The charges were fired by means of electricity, a small dynamo firing machine being placed from 30 to 40 yards away from the 'mine.'" Operations were commenced by the top of the tank being covered over and plastered down in order to make it air-tight; then a sufficient quant.i.ty of coal gas was placed in it to make it highly inflammable and explosive, the quant.i.ty being ascertained by a meter which had been fixed specially for the purpose. Whilst the gas was being injected the cartridge was prepared.

The first experiment was to try whether a small charge of tonite--fired without the patent extinguisher--would ignite the gas. The gas having been turned on, a miner's lamp was placed in the "tank," but this was extinguished before the full quant.i.ty of gas had gone through the meter.

However, the gas being in, the charge of 1-1/4 oz. tonite was placed in the "mine," the detonator was connected by means of long wires to the dynamo machine, and the word was given to "fire." With a tremendous report, and a flash of fire, the covering of the mine flew in all directions, clearly showing that the gas had exploded. The next cartridge (a similar charge) was prepared with the patent compound. First of all a brown paper case of about 2 inches diameter was taken, and one of the tonite cartridges was placed in the centre of it, the intervening s.p.a.ce between the charge and-the case being packed with the "fire-extinguishing compound." The mine having had another supply of gas injected, the protected cartridge was placed inside and fired. The result was astonishing, the explosion not being nearly so loud, whilst there was not the least flash of fire. "Protected" and "unprotected" charges were fired at intervals, gas being turned into the tank on each occasion. Charges of tonite varying from 1 to 6 oz. were also used with the compound. The report was trifling, whilst no flash could be seen.

~Uses of Collodion-Cotton.~--The collodion or soluble gun-cotton is used for a variety of purposes. The chief use is, however, for the manufacture of the various explosive gelatine compounds, of which blasting gelatine is the type. It is also very extensively used in the manufacture of smokeless powders, both military and sporting--in fact, very few of them do not contain it. In some, however, nitro-lignose or nitrated wood is used instead. This, however, is chemically the same thing, viz., nitro- cellulose, the cellulose being derived from the wood fibre. It is more used in this connection than the higher nitrate gun-cotton. Another use to which it has been applied very extensively, of recent years, is in the manufacture of "celluloid." It is used in photography for the preparation of the films on the sensitised plates, and many other purposes. Dissolved in a solution of two parts ether and one of alcohol, it forms the solution known as collodion, used for a variety of purposes, such as a varnish, as a paint for signals; in surgery, for uniting the edges of wounds.

Quite lately, Mr Alfred n.o.bel, the well-known inventor of dynamite, has patented the use of nitro-cellulose, hydro- or oxy-cellulose, as an artificial subst.i.tute for indiarubber. For this purpose it is dissolved in a suitable non-volatile or slightly volatile "solvent," such as nitro- naphthalene, di-nitro-benzene, nitro-toluene, or its h.o.m.ologues; products are obtained varying from a gelatinous consistency to the hardness of ebonite. The proportions will vary from about 20 per cent. of nitro- cellulose in the finished product, forming a soft rubber, to 50 per cent.

nitrating celluloid, and the "solvent" chosen will depend on the use to which the rubber subst.i.tute is to be put, the liquids giving a more elastic substance, whilst mixtures of solids and liquids may be employed when the product is to be used at high temperatures. By means of rollers steam heated, the incorporation may be accomplished without the aid of a volatile liquid, or the nitro-cellulose may be employed wet, the water being removed after "solution."

It is advisable to use the cellulose nitrated only just enough to render it suitable, in order to reduce the inflammability of the finished product. Mr W. Allen, M.P., of Gateshead, proposed to use celluloid for cartridge cases, and thus to lighten ammunition, and prevent jambing, for the case will be resolved into gases along with the powder. Extractors will also be done away with.

~Celluloid~ is an intimate mechanical mixture of pyroxyline (gun-cotton or collodion-cotton) with camphor, first made by Hyatt, of Newark, U.S.A., and obtained by adding the pyroxyline to melted camphor, or by strongly compressing the two substances together, or by dissolving the const.i.tuents in an appropriate solvent, e.g., alcohol or ether, and evaporating to dryness. A combination of the two latter methods, i.e., partial solution, with pressure, is now usually adapted. The pyroxyline employed is generally the tetra- and penta-nitrated cellulose, the hexa-nitrate (gun-cotton) being but seldom used on account of its explosive properties.

Care is taken to prevent the formation of the hexa-nitrate by immersing the cellulose in only moderately strong nitric acid, or in a warm mixture of nitric and sulphuric acids. The paper, either in small pieces or in sheets, is immersed for about twenty-five minutes in a mixture of 2 parts of nitric acid and 5 parts of sulphuric acid, at a temperature of about 30 C., after which the nitrated cellulose is thoroughly washed with water to remove the last traces of free acid, pressed, and whilst still moist, mixed with the camphor.

In the process of Trebouillet and De Besancele, the cellulose, which may be in the form of paper, cotton, or linen, is twice nitrated--first in the acid mixture employed in a previous operation; and secondly, in a fresh mixture of 3 parts sulphuric acid of 1.83 specific gravity, and 2 parts concentrated nitric acid containing nitrous acid. After each nitration the ma.s.s is subjected to pressure, and is then carefully washed with water, to which, at the last, a small quant.i.ty of ammonia or caustic soda is added to remove the final traces of acid. The impregnation of the pyroxyline with the camphor is effected in a variety of ways.

The usual proportion of the const.i.tuents is 2 parts pyroxyline and 1 part camphor. In Trebouillet and De Besancele's process, 100 parts of pyroxyline are intimately mixed with from 40 to 50 parts camphor, and moulded together by strong pressure in a hot press, and afterwards dried by exposure to air, desiccated by calcium chloride or sulphuric acid. The usual method is, however, to dissolve the camphor in the least possible quant.i.ty of alcohol, and sprinkle the solution over the dry pyroxyline, which is then covered with a second layer of pyroxyline, and the whole again treated with the camphor solution, the addition of pyroxyline and camphor solution being repeated alternately until the requisite amount of celluloid mixture is obtained.

The ma.s.s, which sinks together in transparent lumps, is worked for about an hour between cold iron rollers, and then for the same period between rollers which can be gently heated by steam. The layer of celluloid surrounding the rollers is then cut away and again pressed, the resulting cake, which is now about 1 cm. thick, being cut into plates of about 70 cm. long and 30 cm. broad. These are placed one above the other, and strongly pressed together by hydraulic pressure at a temperature of about 70 for twenty-four hours. The thick cakes are once more cut into plates of the desired thickness, and placed in a chamber heated from 30 to 40 for eight to fourteen days, whereby they become thoroughly dry, and are readily made into various articles either by being moulded while warm under pressure, cut, or turned. Occasionally other liquids, e.g., ether and wood spirit, are used in place of alcohol as solvents for the camphor.

Celluloid readily colours, and can be marbled for manufacturing purposes, &c. It is highly inflammable and not explosive even under pressure, and may be worked under the hammer or between rollers without risk. It softens in boiling water, and may be moulded or pressed. Its specific gravity varies slightly with its composition and with the degree of pressure it has received. It is usually 1.35. It appears to be merely a mixture of its components, since by treatment with appropriate solvents the camphor may be readily extracted, and on heating the pyroxyline burns away while the camphor volatilises.

The manufacture of pyroxyline for the purpose of making celluloid has very much increased during recent years, and with this increase of production improved methods of manufacture have been invented. A series of interesting papers upon the manufacture of pyroxyline has been published by Mr Walter D. Field, of New York, in the _Journal of the American Chemical Society_[A] from which the following particulars are taken:--

[Footnote A: Vol. xv., No. 3, 1893; Vol. xvi., No. 7, 1894; Vol. xvi., No.

8, 1894. Figs. 19, 20, 21, 22, and 23 are taken from Mr Field's paper.]

~Selection of the Fibre.~--Cotton fibre, wood fibre, and flax fibre in the form of raw cotton, scoured cotton, paper, and rags are most generally used, and give the best results. As the fibres differ greatly in their structure, they require different methods of nitrating. The cotton fibre is a flattened hollow ribbon or collapsed cylindrical tube, twisted a number of times, and closed at one end to form a point. The central ca.n.a.l is large, and runs nearly to the apex of the fibre. Its side walls are membraneous, and are readily penetrated by the mixed acids, and consequently the highest nitration results. In the flax fibre the walls are comparatively thick, the central ca.n.a.l small; hence it is to be presumed that the nitration must proceed more slowly than in the case of cotton. The New Zealand flax gives the most perfectly soluble nitrates of any of the flaxes. Cotton gives a glutinous collodion, and calico a fluid collodion. One of the largest manufacturers of pyroxyline in the States uses the "Memphis Star" brand of cotton. This is an upland cotton, and its fibres are very soft, moist, and elastic. Its colour is light creamy white, and is retained after nitration. The staple is short, and the twist inferior to other grades, the straight ribbon-like filaments being quite numerous. This cotton is used carded, but not scoured. This brand of cotton contains a large quant.i.ty of half and three-quarter ripe fibre, which is extremely thin and transparent, distributed throughout the bulk of the cotton (Monie., Cotton Fibre, 67). Mr Field says, "This is a significant fact when it is known that from this cotton an extremely soluble pyroxyline can be produced."

Pyroxyline of an inferior grade as regards colour only can be produced from the cotton wastes of the trade. They must be scoured before they are fit for nitrating. Paper made from the pulps of sulphite and sulphate processes is capable of yielding a very soluble pyroxyline. It can be nitrated at high temperatures and still yield good results. Tissue paper made from flax fibre is also used after being cut into squares.

Mowbray (U.S.P., No. 443, 105, 3rd December 1890) says that a pure cotton tissue paper less than 1/500 inch in thickness, thin as it is, takes on a glutinous or colloid surface, and thus requires some thirty minutes to enable the nitration to take place. With a thicker paper only the surface would be nitrated. He therefore uses a fibre that has been saturated with a solution of nitrate of soda, and afterwards dried slowly, claiming that the salt crystallises in the fibre, or enters by the action termed osmose, and opens up the fibre to the action of the acid. This process would only be useful when the cotton is to be nitrated at a low temperature. At a high temperature it would be unnecessary.