CHEMICAL HISTORY AND APPLIANCES OF GUN-COTTON. 
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The great reduction in the rapidity of combustion of gun-cotton is not the only result 
observed when small quantities of that substance are exposed to heat under diminished 
atmospheric pressure. In the most highly-rarefied atmospheres (from 05 to 1 inch), 
the only indication afforded of the burning of the gun-cotton is the appearance of a 
beautiful green glow, like a phosphorescence, immediately surrounding that part which 
is undergoing decomposition. When the pressure of the atmosphere is slightly in¬ 
creased, a faint yellow lambent flame appears, beyond the green glow, at a short dis¬ 
tance from the point of decomposition ; and, in proportion as the atmosphere is less 
rarefied, this pale yellow flame increases in volume, while the green phosphorescence 
becomes less and less apparent until it seems to be completely obliterated. Lastly, when 
the pressure of the atmosphere is comparatively great (= 25 or 2G, in inches of mer¬ 
cury), the gun-cotton burns with the ordinary bright flame, though less rapidly, of 
course, than it does under normal conditions of atmospheric pressure. There is no 
doubt that this bright flame is due to an almost instantaneous secondary combustion 
(in the oxygen supplied by the air in the apparatus) of the inflammable gases evolved 
by the decomposition. On the other hand, the production of the small pale flame, 
observed when gun-cotton is burned in more highly rarefied air, or in atmospheres 
of gases which cannot supply oxygen for combustion, is most probably due to the gene¬ 
ration of a mixture of gases (by the change which gun-cotton undergoes under these 
conditions), which contains not only combustible bodies such as carbonic oxide, but 
also a proportion of oxidizing gases (protoxide of nitrogen or even oxygen); such a 
mixture, having self-combustible properties, will receive sufficient heat from the burn¬ 
ing gun-cotton to become ignited, except when the atmosphere in which the change- 
takes place is so highly rarefied that the heat is immediately dissipated and the gases, 
evolved become highly attenuated, as already described. 
It will be readily conceived that the mechanical state of the gun-cotton (i. e. the parti ¬ 
cular form in which it is employed), like other variable conditions which have been alluded, 
to, will greatly influence the nature of phenomena observed, when this substance is 
ignited in air, or in various gases, either at ordinary or diminished pressures. This may 
be exemplified by the following experimental illustrationsIt has been stated that when 
a tuft of carded gun-cotton is ignited in carbonic acid, carbonic oxide, nitrogen, coal-gas, 
hydrogen, and other gases, it burns only with a pale yellow flame; this flame, when 
furnished by equal quantities of gun-cotton, is much smaller in an atmosphere of 
hydrogen than it is, for example, in carbonic acid,—a fact which must be ascribed to the 
comparatively very rapid diffusion of the generated gases when hydrogen is used. In 
operating with pieces of gun-cotton yarn, instead of employing loose tufts, the material, 
when ignited by a red-hot wire in atmospheres of carbonic acid, nitrogen in carbonic 
oxide, burns much more slowly than it does in air under the same conditions; and its 
combustion is accompanied only by a very small jet or pointed tongue of pale flame, 
which is thrown out in a line with the burning extremities of the piece of yarn. In 
the same way, if the yarn is enclosed in a tube or other vessel, through which those 
gases are circulating, and from which one extremity of the gun-cotton protrudes, when 
the latter is lighted it will burn in the ordinary manner only until it reaches the open¬ 
ing of the tube, when the form of combustion will at once be changed to that just 
described. If, however, corresponding experiments are made in atmospheres of hy¬ 
drogen or coal-gas, the gun-cotton yarn will burn in the slow manner described, 
but only for a very brief period ; indeed, it ceases to burn at all almost instantaneously, 
just as it does when ignited in a very highly-rarefied atmosphere. This result is not 
clue to the high diffusive powers of the gas in which the gun-cotton is burned, as it may 
be obtained equally in open and in perfectly-closed vessels; it can therefore only be 
ascribed to the high cooling powers, by convection, of the gases employed. Pure 
nitrogen, as stated just now, allows the gun-cotton yarn to burn in the slow manner, 
but if mixed with one-fourth its volume of hydrogen, it arrests the combustion of the 
the material, just like coal-gas or pure hydrogen. 
A rapid current of air will also effect the transformation of the combustion of gun¬ 
cotton from the ordinary to the slow form, if the yarn be enclosed in a moderately wide 
glass tube, with one end protruding from the tube, so that it may be inflamed in the 
ordinary manner; but unless the current be very rapid, an explosive mixture of air 
and the inflammable gases generated from the gun-cotton may be produced in the tube, 
and become ignited, in which case the gun-cotton will flash into flame instantaneously,. 
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