78 
CHEMICAL HISTORY AND APPLIANCES OF GUN-COTTON. 
hausted, to at any rate one-half the ordinary atmospheric pressure, the proportion of 
air, and therefore oxygen, present when the gun-cotton is ignited, does not suffice to 
effect the combustion of any large proportion of the inflammable gases generated, and 
hence the explosion of the gun-cotton is attended only by a small pale flame. If, 
however, the vessel be filled with oxygen, and then exhausted to an equal or even a 
lower degree, it is filled with flame of dazzling brightness directly the ignition of the 
gun-cotton is effected. 
The one modification, just referred to, of the phenomena which attended the ignition 
of gun-cotton in a rarefied atmosphere is not the only result observed in experiments of 
this kind. Various curious effects may be obtained ; their nature being determined by 
the degree of rarefaction of the atmosphere, the mechanical condition of the gun-cotton, 
its position with reference to the source of heat employed, and other variable elements 
in the experiments. A brief account of some of the principal of these phenomena may 
not be without interest. 
In the experiments with a tuft of gun-cotton in rarefied air, spoken of just now, a 
perceptible interval is observed between the first application of heat (by passage of a 
voltaic current through a platinum wire enclosed in the tuft) and the first appearance of 
ignition of the gun-cotton; moreover, the pale flame observed when the latter does 
burn, is of very perceptibly longer duration that than of the bright flash which attends 
the explosion of gun-cotton in air, under ordinary conditions. If instead of using the 
gun-cotton in the form of a tuft, a short piece of the gun-cotton yarn be employed in the 
experiment, and laid on a support so that it rests upon the wire by which it is to be 
ignited, the pale flame of the burning gun-cotton will travel along towards the two 
extremities of the piece of yarn with a degree of slowness corresponding to the extent 
of rarefaction of the atmosphere. These results are in perfect accordance with the 
observation (first made by Quartermaster Mitchell, afterwards fully examined into by 
Frankland, and recently amplified by Dufour), that the rate of burning of time-fuzees is 
influenced by the altitude at which they are burned, or, in other words, by the degree of 
pressure of the atmosphere, the combustion being proportionately slow with every decre¬ 
ment of pressure of the air. When the platinum-wire is first raised to a red-heat, in 
the centre of the tuft of gun-cotton enclosed in a highly-rarefied atmosphere, the pro¬ 
ducts resulting from the decomposition of that portion of the material which is in close 
contact with the wire, immediately distribute themselves through the rarefied space, 
conveying away, and rendering latent by their great expansion, the heat furnished by the 
platinum-wire and that which results from the chemical change. The increase of pres¬ 
sure within the confined space, by the generation of the gases and vapours, on the one 
hand, and, on the other hand, the effect of the heated gases, which escape, upon the par¬ 
ticles of gun-cotton through which they permeate, result, in the course of time, in the 
ignition of the mass ; but even then the gun-cotton burns only slowly, because, in conse¬ 
quence of the rapidity with which the resulting gases and vapours escape and expand, 
much of the heat essential for the maintenance of the combustion is at once conveyed 
away. The latter result is strikingly exemplified by an experiment in which gun¬ 
cotton-yarn is substituted for the tuft of carded gun-cotton ; indeed, if the atmosphere 
be very highly rarefied (to 06 in inches of mercury) and a sufficient length of the gun¬ 
cotton yarn (4 or 5 inches) be employed in the experiment, the burning of the material, 
induced by the heated wire, will proceed so slowly, that the heat resulting from the 
chemical change will be conveyed away from the burning surface, by the gases gene¬ 
rated, much more rapidly than it is developed, so that the gun-cotton will actually 
become extinguished when only a small portion of it has been burned. 
A very similar result is obtained if gunpowder, either in the form of grains or of one 
large mass, is exposed to the action of an incandescent platinum wire imbedded in it, 
the pressure of the atmosphere, in the apparatus in which the experiment is made, 
being reduced to between 06 and 2, in inches of mercury. The portion of gunpowder 
contiguous to the heated wire will fuse ; vapours of sulphur will be evolved in the first 
instance, and, subsequently, the charcoal will be oxidized by the nitre, bubbles of gas 
escaping from the fused mass. The vapours and gases thus generated convey away 
rapidly the heat provided by the wire and developed by the chemical action; and at the 
same time the change which the gunpowder undergoes diminishes its explosive character, 
so that its partial ignition or explosion will only be effected after the lapse of several 
minutes, and, if it be in the form of grains, the explosion of the particles contiguous to 
the wire will have the effect of scattering the remainder without igniting it. 
