60 Modern Cannon Powder. | [January, 
material of the cannon would be broken before the inertia 
of the shot could be overcome. 
Now supposing one grain or particle alone to be ignited, 
it will be first inflamed over its whole surface, and the pro- 
gressive combustion will take place from the exterior to the 
interior. Its vate of combustion will therefore depend upon 
both its shape and size, leaving out entirely for the present 
the question of density and hardness. A particle of spherical 
or cubical form will expose less surface to ignition, in pro- 
portion to its volume,than one of an elongated or flat shape, 
and will consequentlyrequirea longer period forthe combustion 
of its entire mass: the larger the particle also, the longer 
will be the time required for its consumption. Looking, then, 
at one grain of powder by itself, we may safely say that the 
larger it is, and the more nearly does its form approach to 
that of a sphere, the longer will its combustion take, and the 
slower will be the evolution of the gas. When, however, 
we come to regard the action of an aggregation of such’ 
particles, as in the charge of a gun, the rate of ignition of the 
whole charge is also affected by the size and shape of the 
grains. The part of the charge first ignited is that near 
the vent, or touch-hole, and the remainder is inflamed by 
contact with the heated gas generated by the combustion of 
this portion, so that the rate of ignition of the whole mass 
will be regulated by the greater or less facility with which 
the gas can penetrate throughout the charge, which is 
itself dependent upon the size and shape of the interstices 
between the grains. If the grains be spherical and regular 
in form, the interstices will be comparatively large and 
uniform, and the gas will penetrate the mass with facility ; 
again, the larger the grains, the larger the interstices between 
them. If, on the other hand, they be flat or flaky and 
irregular in shape, the passage of the gas will be more 
difficult, and the rate of inflammation of the charge reduced. 
We see, therefore, that the considerations which affect the 
more or less rapid combustion of an individual grain of 
gunpowder also affect the rate of ignition of a charge of 
such grains, but in an opposite direction ; so that a form of 
grain which will individually burn rapidly may offer an 
increased resistance to the passage of the heated gas through 
the charge, and thereby retard its ignition, while a grain 
which will burn more slowly may allow of the charge being 
more rapidly ignited. By varying the size and shape of the 
grain alone, a powder may therefore be obtained a charge of 
which shall be ignited rapidly throughout but burn com- 
paratively slowly, or one which shall be ignited more slowly, 
