Velocity of the transmission of inflammation of the charge.
When a number of grains of powder are placed together as in the charge of a gun, and a few of them are ignited at one end of the cartridge, a certain quantity of gas is developed of a temperature sufficiently high to ignite those in their immediate vicinity. This has also such elasticity as to enable it to expand itself with considerable velocity. Again, the grains which are so ignited continue the inflammation to others in the same manner. The absolute velocity of expansion of this gas is very considerable; but the grains of gunpowder in the charge offer an obstruction to this motion, the gas having to wind its way through the interstices, and consequently the velocity is considerably diminished, but it is quite clear that it must be very much greater than the velocity of combustion. Estimate of Mr. Piobert.Mr. Piobert estimates the velocity of transmission of inflammation of a charge in a gun at about 38 feet per second, and in all probability even this is much under the mark.
Experiments made on this subject.
Many experiments have been made by observing the velocity of transmission of inflammation of trains of powder under various circumstances, but they do not show us what would be the velocity in a confined charge. The velocity increased with the section of the train, and further when at the end first lighted, there was an obstruction to the escape of gas, as in the case of a gun, a much shorter time was required for complete inflammation.
Time of decomposition depends upon form of grain.
When the charge of powder in a gun is ignited the grains being enveloped by the heated gas, we may consider that each grain is ignited over its whole surface at once. If the grains of powder were of equal or regular form, the time each would be consuming, might be easily calculated, but since in ordinary cases they are irregular in form, although the grains may be of the same weight, the time necessary for their complete decomposition will be very different.
Circumstances affecting combustion.
The quickness of combustion will depend upon the dryness of the powder, the density of the composition, the proportion of the ingredients, the mode of manufacture, and the quality of the ingredients.
Combustion of cubical grains considered.
Were a cubical grain to be ignited upon its whole surface, the decomposition may be supposed to take place gradually from the surface to the centre, and the original cubical form to remain until the whole is consumed, the cube becoming smaller and smaller. If, then, the rate of burning be the same throughout, the quantity of gas generated in the first half portion of the time will evidently be considerably more than in the latter half, as in the latter case there will be a much lesser surface under the influence of flame.