the chemist usually has recourse when he desires to examine the properties of cellulin. Pure cellulin is a white, tasteless substance, insoluble in water, alcohol, ether, or oils. It is heavier than water: its fibres are transparent, and exert a depolarizing influence upon a ray of polarized light. A solution of well-washed freshly precipitated hydrated cupric oxide, or cupric carbonate, in dilute ammonia, dissolves the fibre in most of its forms, though in some cases, as in that of the rice paper, this solution does not take place until the vegetable fibre has been boiled with diluted acids. Cellulin is precipitated from the cupric solution, unaltered in composition, on acidulating the solution with an acid. Cold concentrated sulphuric acid dissolves it, and produces a treacly-looking liquid, converting it, after dilution and boiling, first into dextrin, and subsequently into grape sugar. Weak acids exert but little effect upon cellulin, but the action of these and of all other solvents is materially greater upon the recently formed cellules than upon the old ones. Hydrochloric acid, in its concentrated form, dissolves cellulin, and deposits it on immediate dilution with water; but if it be left undiluted for 2 or 3 days, no precipitate occurs on the addition of water. Alkaline liquids when dilute do not act upon cellulin, but when concentrated they gradually destroy its texture. According to Péligot, if cellulin be moistened with water and submitted to distillation with an equal weight of solid caustic potash, wood spirit distils over, and potassic oxalate is formed in the residue. A solution of chlorine acts but very slowly upon cellulin. Cellulin, in its natural state, is not coloured blue by iodine; but after it has been digested for a short time with concentrated sulphuric acid, it becomes of a fine blue when free iodine is added. This reaction is sometimes serviceable in the microscopic examination of vegetable tissues; cellulin being thus easily distinguished from most tissues into the composition of which nitrogen enters. By the prolonged action of sulphuric acid, the property of being coloured blue by iodine disappears, the dextrin and sugar which are formed not being susceptible of the blue coloration. A remarkable modification in the vegetable fibre is effected by the action of chloride of zinc or of sulphuric acid, in consequence of which paper may be converted into a material which, in toughness and appearance, much resembles parchment, and is known under the name of vegetable parchment. It is more transparent than true parchment. In order to prepare it, thin paper, PYROXYLIN, OR GUN COTTON. 135 which must not be sized, is plunged for a few moments, at a temperature of 59° (15° C.) into a mixture of oil of vitriol with half its bulk of water. The paper must be quickly withdrawn and washed, first with water, then with a weak solution of ammonia, and, lastly, with water again. In this process the outer surface of the fibres appears to have become converted into a glutinous substance by which the fibres are cemented together. This substance, according to Hofmann, is intermediate between cellulin and dextrin, with both of which it is isomeric, having neither acquired nor given up the elements of water, nor entered into permanent combination with sulphuric acid. Hofmann found the toughness of this vegetable parchment to be five times as great as that of the paper which furnished it, and to be about three-fourths that of ordinary parchment. It takes ink well. Water, even at 212°, exerts very little action on it for several hours, but if immersed for many months in water it gradually loses its tenacity. It may be substituted for bladder as a septum in dialysis, and in electrolytic operations, with great advantage. Cellulin, when subjected to destructive distillation, yields acetic acid and a variety of hydrocarbons, mixed with carbonic anhydride and carbonic oxide. When oxidized by means of nitric acid of moderate strength cellulin yields oxalic acid in abundance, but no mucic acid. Concentrated nitric acid produces various remarkable substitution products, some of which we proceed to notice. These reactions with nitric acid about to be described show that the formula of cellulin cannot be less than 18H30015 (1104) PYROXYLIN, or Gun Cotton (Є18H21,9NO2015; Hadow). -When cellulin in any form, such as cotton, tow, linen, sawdust, or paper, is dipped into a mixture of equal measures of oil of vitriol and of nitric acid of sp. gr. 1520, no change of form ensues, but a remarkable chemical alteration takes place a certain number of atoms of hydrogen are abstracted, and an equal number of atoms of peroxide of nitrogen (NO) supply their place. The fibre in undergoing this change becomes increased about 82 per cent. in weight (Hadow), and acquires completely new properties. In order to prepare this remarkable bodycalled gun cotton by its discoverer, Schönbein-1 part of finely carded cotton is immersed in 15 parts of a mixture of equal measures of strong nitric acid (sp. gr. 1'5), and sulphuric acid (sp. gr. 1845). The cotton must be completely immersed in the 136 PYROXYLIN, OR GUN COTTON. mixture, otherwise it becomes so hot as to undergo instant decomposition. After a few minutes' immersion it must be plunged into a large volume of cold water, and then washed, so long as the least trace of acid is perceived when the moist mass is placed upon litmus paper; it is then to be carefully dried at a temperature below 212°. When prepared for military purposes, it is found advisable to prolong the immersion in acid for 48 hours, and to continue the washing for several days. See Report on Gun Cotton, Brit. Assoc. Report, for 1863, p. 1. As thus prepared, pyroxylin (so named from up fire, ¿úλov, wood) scarcely differs from unchanged cotton in appearance; it is white and fibrous, rather harsh to the touch, and when examined by the microscope in a beam of polarized light it is found to have lost the property of depolarization which ordinary cotton possesses. It is somewhat hygroscopic, but when dry it is an excellent insulator of electricity, and becomes highly electrical by the mere operation of pulling it out in the fingers. Pyroxylin is insoluble in water, alcohol, and dilute acids, but it is soluble in acetic ether and in acetate of methyl. Ammonia, when mixed with ether, dissolves it, and leaves it by spontaneous evaporation in a pulverulent form. An aqueous solution of potash dissolves it freely, decomposing it at the same time; the solution contains a quantity of sugar, and a mixture of potassic nitrate, nitrite, and oxalate. The solution in potash also contains a peculiar acid (resembling the saccharic), which yields an insoluble precipitate with acetate of lead. The alkaline solution of pyroxylin, when mixed with ammonia-nitrate of silver, and gently heated, reduces the silver in the form of a film upon the surface of the glass, producing a brilliant mirror-like deposit of the metal. Strong sulphuric acid dissolves pyroxylin in the cold without the evolution of gas, and the liquid is not rendered turbid by dilution. If the pyroxylin be pure, the solution is colourless, but it is brown if any unaltered fibres of cotton remain in the mass. Nitric acid in the cold is without action upon it, but if the acid be heated it gradually dissolves the pyroxylin, and on dilution deposits it again. By boiling it with the acid it is decomposed, red fumes escape, and dilution then causes no precipitate. Pyroxylin, according to the observation of Hadow, may be reconverted into ordinary cotton, by digestion at ordinary temperatures in an alcoholic solution of hydro-potassic sulphide (KHS), whilst potassic nitrite is formed; and the change may be effected still more rapidly if the mixture be heated: the reaction may be indicated by the following equation : 2 (18H21-9 NO2,15) + 18 KHS 2€18H30015+ 18 (KNO) +9. Béchamp states that a similar reduction takes place when the pyroxylin is acted upon by ferrous chloride. The most remarkable property of pyroxylin is the facility with which it takes fire, and the rapid and complete manner of its combustion. In the open air it burns with a flash, but without either smoke or report; a temperature just below that of 300° (150° C.) is sufficient to inflame it, being more than 200° below that required to ignite gunpowder. In the open air, pyroxylin burns more rapidly than gunpowder, but if compressed so as to form a fuse, its rapidity of combustion may be reduced below that of powder, and the more it is compressed the slower is the combustion. If it be fired in a confined space it produces violent explosive effects from the suddenness of its combustion, and from the large quantity of gaseous matter which it furnishes. The extreme suddenness of its inflammation, and the short space of time during which its propulsive force is exerted, produce both greater strain upon the gun-barrel and less effect upon the ball, than gunpowder exerts, but the rapidity of explosion may be regulated by proportioning not merely the weight of the cotton but the volume of the cartridge to the size of the chamber of the gun. In certain cases, such as in mining operations in hard and brittle rocks, it may be beneficially employed, as it may be driven into borings above the head of the miner; it produces much less fume and noxious gases than gunpowder; in addition to which it has the great advantage of leaving no train' by leakage from the vessels in which it is stored up. This compound may even be stored in water for indefinite periods, as indeed is commonly practised for keeping purposes, and when dried is found not to have experienced any diminution of its explosive power. During the combustion of pyroxylin in vacuo a large volume of steam is generated, mixed with a variable proportion of carbonic anhydride, carbonic oxide, nitrogen, marsh gas, and nitric oxide; traces of cyanogen are also sometimes formed; but if fired under pressure, no nitric oxide, but a little free hydrogen is obtained, in addition to the gases already enumerated. Considerable difference prevails in the statements of the earlier experimenters respecting its propulsive power as compared with that of gunpowder, but it is now ascertained that its power varies according to the circumstances in which it is exploded; so 138 PYROXYLIN, OR GUN COTTON. that it may be made to vary from three times to nearly six times that of an equal weight of the best musket powder.* The results of Hadow's experiments (Q. J. Chem. Soc. vii. 208), which have been in all their essential points confirmed by subsequent inquiries by a commission of chemists in Austria, and by Abel at Woolwich (Phil. Trans. 1866), serve to explain the conflicting statements relative to the properties and composition of this remarkable substance.† It appears from this inquiry that the composition of gun cotton differs according to the degree of concentration of the acids employed in its formation, and that at least four different and definite compounds, distinguished as A, B, C, and D, may be formed, having the following composition : H21 2015 ; NO, H B = €188 NO €186 NO31 015 23 Ꮎ, 7 NᎾ, When equal measures of the sulphuric and nitric acids are employed, each in the highest degree of concentration, the compound A is formed; it is highly explosive, soluble in acetic ether, and insoluble in any mixture of alcohol and ether. If the acids used be of a strength intermediate between 2 HNO3+2 H2SO+ 3 H2 and HN ̧+H ̧§Ð ̧+2 H2, the compound B is produced : it is less explosive than A, is insoluble in acetic acid, but soluble in a mixture of seven parts of ether and one part of alcohol. 4 * 100 parts of gun cotton yield, according to Karolyi (Phil. Mag., Oct. 1863, p. 266), the following gases under the two circumstances above mentioned:Products of the Combustion of Gun Cotton. + Pelouze still maintains that gun cotton is a combination of nitric acid, and not a substitution product, but even his own analyses do not support the formula he gives, and it appears that the product analysed was really a mixture. |