Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/291

Rh GUMRI, or as it is now more frequently called, in honour of the empress Alexandra, a town of Russian Armenia in the province of Erivan, on the old frontier of the Turkish territory which was formed by the j river Arpachai. It is situated at a height of 5268 (accord ing to Abich, 4819) feet above the sea, on an eminence which commands the environs but is somewhat defective in a military point of view because the neighbouring valleys form a kind of screened approaches. The fortifications, erected in 1837 by command of Nicholas I., are the strongest in that part of Asia with the exception of those of Kars, and would be quite impregnable to an Eastern army. Like the ordinary houses of the town, they are built of limestone ; and they consist of regular bastions case- mated and mounted with guns and surrounded with a ditch. There are several handsome Armenian churches in Gumri, one of the finest of which was built in 1872. The town passed to Russia by the peace of Adrianople. In 1840 it was made a district town of the Gruzeno-Imeritian govern ment, and in 1850 it was incorporated with the Erivan government. In 1832 it contained only about 60 houses, but they had increased to 1200 in 1838. At present its population is about 16,000.  GUM-TREE. See.  GUN-COTTON (PyroxUin, Coton poudre, Fidmi coton, Schiessbaumwolle). In 1838 Pelouze observed that when cotton fabrics or paper were immersed in cold concentrated nitric acid for a short time, the free acid being subsequently removed by washing, these materials became, without im portant alteration of structure, converted into substances possessed of explosive properties. These were at the time accepted as closely allied to the substance named xyloidin, described some years previously by Braconnet, which is obtained by adding water to a solution of starch in cold nitric acid. But subsequent observation established the identity of these explosive products with the explosive cotton, or gun-cotton, of which in 1845 Schonbein an nounced the discovery, and which he at once proposed as a substitute for gunpowder. Soon after this announcement Bottger and Otto published the method of producing gun- cotton by immersing carded cotton in cold concentrated nitric acid, and subsequently Knop introduced the more advantageous method of treating the cotton wool with a mixture of nitric and sulphuric acids, the latter being used as a dehydrater of the nitric acid, and as an absorbent of the water eliminated by the nitrification of the cellulose or cotton fibre. The composition of gun-cotton was subse quently made the subject of study by Bottger, Pelouze, Peligot, Von Kirchhoff, Sobrero, Bechamp, Porret, Crum, Gladstone, Hadow, and others, and various formulae were proposed as representing its composition. The divergence of opinion on this point arose partly from difficulties attend ing the preparation of uniform products, and the obtaining of trustworthy analytical results with these, and partly from differences of opinion regarding the nature of the chemical reaction, whereby the cellulose becomes converted into an explosive body. The products obtained in the earlier investigations differed very much as regards their solubility in mixtures of alcohol and ether, and also with respect to the proportion which their weight bore to that of the cotton wool employed in the experimental operations. Crum was the first to entertain the view that gun-cotton might be re garded as cellulose, in which the two or three atoms of hydrogen are replaced by their equivalent of nitric peroxide. This view was afterwards also advanced by Gerhardt, and it received strong support from the researches of Hadow, whose results established the fact that several distinct varieties of pyroxilin could be produced by varying the pro portions of nitric and sulphuric acids used, and who definitely established the composition of three of these, the most explosive of which constituted the chief proportion of the product ordinarily obtained as gun-cotton, and had the composition expressed by the name trinitrocellulose. This highest nitro-product in its pure state is insoluble in mix tures of ether and alcohol, whereas the lower products (one of which is the so-called collodion gun-cotton, used for photo graphic purposes, see, ) differed in regard to their ready solubility in different mixtures of those solvents. Crum s formula for pyroxilin, thus con firmed by Hadow, was afterwards strongly supported by Schrotter, Redtenbacher, and Schneider, in their joint investigation of gun-cotton manufactured in Austria by the improved process of Von Lenk, and though again disputed by Pelouze and Maury, and by Champion and Pellet, the correctness of the formula C 6 H 7 N 3 O n (or C 6 H 7 5 3N 2 ), originally proposed by Crum, was conclusively established in 1866 by the exhaustive analytical and synthetical experi ments of Abel. In the manufacture of gun-cotton, even when most carefully conducted, the most explosive product, trinitrocellulose, is never obtained in a condition even approaching purity ; it always contains an admixture (rang ing from 4 to 10 per cent, in the products of highest quality) of the lower nitrocellulose products, i.e., the soluble varieties of gun-cotton. In addition to these im purities it contains, even when the cotton employed has been submitted to purification by treatment with alkali, small proportions of nitrogenized matters, soluble in alcohol, formed from resinous or fatty substances retained within the cotton fibre. These substances are very much more prone to undergo decomposition (with development of nitrogen acids) by exposure to heat or light than the cellulose derivatives themselves ; and Abel s experiments demonstrated that the uncertain stability of gun-cotton,, which brought this material into bad repute not long after its discovery, from the occurrence of disastrous explosions arising apparently from its spontaneous decomposition, was ascribable primarily to the development of free acid in the gun-cotton by the action of comparatively moderate heat or of light upon these impurities. The occurrence of a violent explosion at the works of Messrs Hall of Faversham, not long after they had com menced the manufacture of Schonbein s gun-cotton woo), followed by a similar casualty in France, led to the abandon ment of endeavours to apply this substance, within a brief period of its discovery, except in Austria, where Von Lenk persevered in attempts to devise means for obtaining it in a purer and therefore more stable condition, as well as for bringing its explosive action sufficiently under control to permit of its advantageous employment as a substitute for gunpowder, not only for destructive but also for projectile purposes. The system of manufacture elaborated by Von Lenk consisted in loosely spinning long staple cotton into yarn of various sizes and different compactness j this yarn was converted into gun-cotton by very careful treatment with a large excess of the strongest nitric and sulphuric acids, the product being immersed for many weeks in run ning water, and then treated with weak alkali ; the gun- cotton yarn and thread were either wound more or less compactly on reels or cores, for employment in firearms, or made up into very compact ropes with hollow cores, or into plaits, of lamp-wick form, for employment in shells or mines. The rapidity of explosion of the gun-cotton, in open air, or under slight confinement, was thus brought to a great extent under control, but if the resistance opposed to the expansion of the highly heated gases upon the first ignition of the confined gun-cotton developed sufficient pressure to cause them at once to penetrate the inner struc ture of gun-cotton fibre which composed a charge, a sudden and violent explosion was thus brought about. Hence no practical advance was made in the reduction of the violence 