~Size of grain determined by size of charge.~
The rapidity with which a charge of gunpowder is consumed will depend not only in a certain degree upon the size of the grain, but on the manner in which the charge is put together, for if a charge is closely pressed, the gases meeting resistance in their endeavours to escape between the interstices, will not propagate the ignition so rapidly. With large charges, there exists a positive advantage for the grains to be rather large, so that the most distant parts of the charge should be reached by the gases as quickly as possible; whilst with that of a rifle, the charge being small, the fineness of the grain does not interfere with the quantity of the gas developed. Whence it may rationally be concluded that the dimensions of the grains should increase in proportion to the quantity of the charges into which they are to enter, that is to say, in proportion to the interstices. Ramming down a charge tightly must therefore interfere with the velocity of combustion.
NOTE--The foregoing on the explosive force of gunpowder was taken from Lieut-Colonel E. M. Boxer’s Treatise on Artillery.
The produce obtained by the decomposition of gunpowder are the gaseous and the solid. The gaseous is chiefly nitrogen and carbonic acid. The solid is sulphur and potassium, mixed with a little charcoal, but the solid produce is nearly entirely volatilized at the moment of explosion through the high temperature.
Fouling is occasioned by the deposition inside the barrel of the solid residue proceeding from the combustion of the powder.
~Conditions of fouling depend on state of atmosphere~
One of the principal of these, namely, the sulphide of Potassa, is deliquescent, or attracts water from the atmosphere. Hence, on a clear day, when the air holds little moisture, the fouling does not attain that semi-fluid state it so speedily attains in a damp day, and it is not so easily removed, and tends to accumulate inside the barrel. Fouling may also be increased or diminished, according to the quality of the powder.
Fouling occasions loss of power from the increased friction, and causes inaccuracy in direction and elevation, by filling the grooves, and thus preventing the proper spiral motion being imparted to the projectile.