When gun-cotton was first discovered, no other idea was entertained respecting its application than that of superseding gunpowder as an explosive agent. Since that period, however, it has, like galvanism, been applied to quite a number of useful purposes, as the principal ingredient of collodion. Common cotton is one of the forms of lignine, which is a compound of carbon, hydrogen and oxygen (the same as wood), but when subjected to the action of nitric acid, nitrogen enters into its composition, and this element is found in a great number of explosive substances. In 1833, M. Braconet, of Paris, made the discovery that starch, sawdust and cotton wool, when treated with concentrated nitric acid, became very inflammable, taking fire at a temperature of 356 Fah., but were not really explosive. This invention remained merely as a chemical curiosity until 1846, when Professor Schonbein, of Vienna, made. the discovery of rendering cotton explosive by the Kse of sulphuric acid combinctl with the nitric, in treating it. The following is a summary of his process for making gun-cotton, described in the specification of his foreign patent, secured in 1847 : — "Take nitric acid of the specific gravity of 1*45, and sulphuric acid of 1*85 specific gravity, and mix them in the proportion of three parts of sulphuric and one of nitric acid, and allow the mixture to cool to 60 Fah. The rough cotton, which should be thoroughly cleansed from extraneous matter, is immersed (in as open a state as possible) in a glazed earthenware vessel, and, when thoroughly soaked in the acid, is lifted, and the excess of acid squeezed out gently by a glass rod. The cotton is now covered over in a glass vessel, and left thus for an hour. It is then washed well in cold water, to remove the free acid, and is squeezed between rollers in a press, or, if a small quantity, between the hands; after which it is washed in a weak solution of carbonate of potash, (one ounce dissolved in a gallon of water,) to insure its freedom from acid, then dipped into a weak solution of saltpeter, (nitrate of potash,) is then pressed dried, in a warm room, and is fit for use." Three parts (by weight) of gun-cotton thus prepared is equal in strength to eight parts of the best gunpowder. Great hopes were once entertained that it would be generally employed for all kinds of fire-arms, because it is so cleanly, and leaves no dirty residue behind it; but it ignites so rapidly, and is so liable to burst fire-arms, that it is dangerous to use, hence gunpowder still maintains its position for army and hunting purposes, except in Austria, where, as we learn by recent accounts, it is used for artillery. For mining purposes, however, it is certainly superior to gunpowder, and is now extensively used in Europe for blasting rocks. Another application of it has become very extended, namely, in preparing collodion—a discovery made by Dr. May-nard, of Boston, Mass., about seven years ago, and first applied as an adhesive plaster for wounds in surgery. This composition is made by dissolving gun-cotton made with the nitrate of potash (a substitute for nitric acid), and sulphuric acid dissolved in ether and alcohol, and is made as follows:— Take finely powdered nitrate of potash, 40 parts, by weight, concentrated sulphuric acid 60 parts, and carded cotton 2 parts. The nitrate and the acid are mixed together in a porcelain vessel, the cotton added, and stirred in it with a glass rod for about four minutes, then the free acid is pressed out, the cotton washed in cold water, anr1 dried in a loose mass, at a moderate heat. Take rectified sulphuric ether 125 parts, by weight, rectified ?alcohol, 8 parts, and cotton, 8 parts. The latter is added first to the ether, in a well stopped bottle, and the mixture shaken for some minutes, when the alcohol is added by degrees. The gun-cotton prepared as stated is very soluble in the ethereal mixture, forming the well known collodion. When applied to wounds, or the surface of any object, the ether rapidly evaporates, leaving the gun-cotton adhering with wonderful tenacity, and forming a nearly transparent skin, air-tight, and almost impervious to water. It is employed in forming the fine cuticle on paper, and on glass plates for photographic purposes, and has been the means of greatly improving the art of sun painting; it is also used for gilding, in architectural decoration, and a patent was recently issued to an inventor in this city for this beautiful application of it. By coloring with pigments, it has been employed for some time in the manufacture of French artificial flowers. It will readily be perceived how it is now extensively used as a valuable substance in surgery, photography, architectural decoration and for personal ornamentation, and may yet be applied to a hundred other different purposes.