The grains afterwards undergo a process of _glazing_, by friction against each other, in barrels containing nearly 200 lbs., making forty revolutions in a minute, and lasting several hours, according to the fancy of the purchaser. This part of the business we entirely disagree with, as injurious to the quick and _certain ignition_. Gunpowder is finally dried by an artificial temperature of 140° Fahrenheit, which is suffered gradually to decline. The last process is sifting it clear of dust, and then packing it in canisters or otherwise.
The utility of the process of granulation results from the impossibility of firing mealed powder sufficiently simultaneously to effect an explosion; and also from the fact that gunpowder, in a mass, does not explode. Fire a solid piece of mill-cake, and it does not flash off like unto granulated powder, but burns gradually, though with an extreme fury, until the whole is consumed. This arises from its density, the compression in the press; it also teaches us one fact, that to be of the greatest service, the time each grain should occupy in burning should be proportioned to the size of the gun for which it is required; since it is clear that the explosion of a heap of gunpowder is but the rapid combustion of all its parts. This action, as is well known, is so rapid, even in a large quantity of powder, that it appears to be a sudden and simultaneous burst of flame; though philosophically and actually it is not so.
Fine grain, when unconfined, explodes quicker than large, or is sooner burnt out, and consequently generates more force in the same period of time; but when it comes to large quantities, its very quickness is detrimental to its force, by condensing the air around the exterior of the mass of fluid which thus constrains its bound. In small quantities, the proportion of condensation is not so apparent, and hence the reason why greater velocities can be obtained with small arms than with cannon.
There exists a diversity of opinion in regard to the strength or projectile force of gunpowder. Dr. Ure remarks--“If we inquire how the maximum gaseous volume is to be produced from the chemical reaction of the elements of nitre on charcoal and sulphur, we shall find it to be by the generation of carbonic oxide and sulphurous acid, with the disengagement of nitrogen. This will lead us to the following proportions of these constituents: