See Gun-making, and Rifle.

Gun-Cotton. Cotton is one of the numerous forms of lignine, a compound of carbon, oxygen, and hydrogen; but when it is subjected to the action of nitric acid, nitrogen, which exists in most explosive bodies, enters into its composition. The action of nitric acid on lignine had long attracted the attention of chemists; but the nearest approach to the formation of gun-cotton was made by Pelouze, who, in 1838, writes in the Comptes Rendus of the properties of a substance named Xyloloidine, from ξύλον, wood, discovered by Braconnet in 1833:—“It is very combustible, taking fire at 350° Fahr., burning with great rapidity; and almost without residue. This property has led me to an experiment, which I think susceptible of some application, especially in artillery. By plunging paper in nitric acid of sp. gr. 1·5, leaving it there the requisite time for the acid to permeate the paper, which is usually accomplished in two or three minutes, then withdrawing it, and, lastly, washing it in water, we obtain a kind of parchment impermeable to moisture, and extremely combustible.” In 1846, Schönbein exhibited to the British Association at Southampton specimens of cotton, which appeared to be as explosive as gunpowder; but it was not till April 1847, on the enrolment of the patent, that the method of preparing this cotton was known, although, in the interval, Otto of Brunswick, Morel of Paris, and Böttger of Frankfort, published recipes for making explosive cotton. Schönbein’s method consisted in mixing three parts of sulphuric acid, sp. gr. 1·85, with one part of nitric acid, sp. gr. 1·45 to 1·50; and when the mixture had cooled down to between 50° and 60° Fahr., clean rough cotton in as open a state as possible was immersed in the acid; when well soaked, the excess of acid was drawn or poured off, and the cotton pressed lightly in order to separate the principal portion of the acid. The cotton was then covered over and left for half an hour, when it was pressed and thoroughly washed in running water to get rid of all free acid. After being partially dried by pressure, it was washed in an alkaline solution made by dissolving one ounce of carbonate of potash in a gallon of water. The free acid being thus got rid of, it was put into a press, the excess of alkaline solution was expelled, and the cotton left nearly dry. It was then washed in a solution of pure nitrate of potash, one ounce to the gallon, and being again pressed, was dried at a temperature of from 150° to 170°. It was stated, that three parts of the gun-cotton thus prepared were equal in force to eight parts of Tower-proof gunpowder.

Cotton gains considerably in weight by the above treatment, but it is scarcely changed in colour or in general appearance, if the process has been carefully conducted: it is, however, harsh to the touch, and gives a crepitating sound when pressed by the hand. It differs from common cotton by its electric excitability, the slightest degree of friction causing it to be powerfully attracted and repelled by other bodies; and also by its action on a ray of polarized light, which it does not depolarize like ordinary cotton. It explodes at a temperature of from 350° to 400°, with such rapidity as to interfere with its practical application, for if applied to the purposes of artillery, it may burst the gun before it has time to move the shot, and some of the products of its combustion make it also objectionable for firearms. Among these products water may be mentioned, and, should the cotton not have been well washed, nitrous acid. Another great impediment to the use of gun-cotton is its hygrometric condition, for if exposed to a damp atmosphere, it will in an hour or two absorb a considerable portion of moisture. Many attempts have been made to apply it to mining purposes on account of its enormous force, and the small quantity of smoke which it produces; but the objections to its use are numerous, the most fatal objection being its liability to spontaneous ignition.

Nevertheless, gun-cotton continues to be an object of great interest on account of its application to the beautiful art of photography. When the cotton is prepared in such a way as to burn slowly, it is not liable to spontaneous ignition, and in this state it is perfectly soluble in sulphuric ether, which the more explosive cotton is not. If the ethereal solution, called collodion, be poured on the surface of cold water, a paper is produced, which is prepared for the use of the photographer. This paper is a very active electric, and is perfectly soluble in ether. Collodion has also been made use of in surgery, by applying the ethereal solution to a wound, when a thin delicate artificial skin is formed by it, which perfectly excludes the air.

In the preparation of gun-cotton, nitric acid is the active agent in the formation of xyloidine; the sulphuric acid has no direct action on the lignine, its use being to retain the water abstracted from the cotton, and prevent the solution of the compound which takes place to a greater or less extent in nitric acid alone. The purity and exact strength of the acids are matters of great importance. Mr Hadow found that the best mixture for producing collodion wool is obtained by mixing 89 parts by weight of nitric acid, sp.gr. 1·424, with 104 parts by weight of sulphuric acid, sp.gr. 1·833.

On trying the effect of various re-agents on gun-cotton, Mr Hadow found that it could be perfectly restored to the original cotton, without loss of form, by means of an alcoholic solution of hydro-sulphuret of potassium. On this, and other points connected with the chemistry of gun-cotton, we must refer to Mr Hadow's paper, published in the Transactions of the Chemical Society.