_Action of Heat._--We have seen that the oxygen required for the
combustion of the carbon in gunpowder is stored up in the saltpetre. So
long as the saltpetre remains below a certain temperature, it will
retain its oxygen; but when that temperature is reached, it will part
with that element. To fire gunpowder, heat is therefore made use of to
liberate the oxygen, which at once seizes upon the carbon with which it
is in presence. The means employed to convey heat to an explosive have
been described in the preceding chapter. It is necessary to apply heat
to one point only of the explosive; it is sufficient if it be applied to
only one grain. That portion of the grain which is thus raised in
temperature begins to “burn,” as it is commonly expressed, that is, this
portion enters at once into a state of combustion, the saltpetre giving
up its oxygen, and the liberated oxygen entering into combination with
the carbon. The setting up of this action is called “ignition.” The hot
gases generated by the combustion set up ignite other grains surrounding
the one first ignited; the gases resulting from the combustion of these
ignite other grains; and, in this way, ignition is conveyed throughout
the mass. Thus the progress of ignition is gradual. But though it takes
place, in every case, gradually, if the gases are confined within the
space occupied by the powder, it may be extremely rapid. It is easy to
see that the gases evolved from a very small number of grains are
sufficient to fill all the interstices, and to surround every individual
grain of which the charge is composed. But besides this ignition from
grain to grain, the same thing goes on from the outside to the inside of
each individual grain, the grain burning gradually from the outside to
the inside in concentric layers. The successive ignitions in this
direction, however, of layer after layer, is usually described as the
progress of combustion. Thus the time of an explosion is made up of that
necessary for the ignition of all the grains, and of that required for
their complete combustion.
The time of ignition is determined in a great measure by the proportion
which the interstices, or empty spaces between the grains, bear to the
whole space occupied by the powder. If the latter be in the form of an
impalpable dust, ignition cannot extend throughout the mass in the
manner we have described; but we shall have merely combustion proceeding
from grain to grain. If, on the contrary, the powder be in large
spherical grains or pellets, the interstices will be large, and the
first gases formed will flash through these, and ignite all the grains
one after another with such rapidity that ignition may be regarded as
simultaneous. Thus the time of ignition is shortened by increasing the
size of the grains and approximating the latter to the spherical form.
But the time of combustion is determined by conditions contrary to
these. As combustion proceeds gradually from the outside to the inside
of a grain, it is obvious that the larger the grain is, the longer will
be the time required to burn it in. Also it is evident that if the grain
be in the form of a thin flake, it will be burned in a much shorter time
than if it be in the spherical form. Thus the conditions of rapid
ignition and rapid combustion are antagonistic. The minimum time of
explosion is obtained when the grains are irregular in shape and only
sufficiently large to allow a fairly free passage to the hot gases.
There are other conditions which influence the time of combustion; among
them is the _density_ of the grain. This is obvious, since the denser
the grain, the greater is the quantity of material to be consumed. But
besides this, combustion proceeds more slowly through a dense grain than
through an open one. The presence of moisture also tends to retard
combustion.
The progress both of ignition and of combustion is accelerated, not
uniform. In proportion as the grains are ignited, the gases evolved
increase in volume, and as the progress of combustion continues to
generate gases, the tension of these increases, until, as we have seen,
the pressure rises as high as 42 tons to the square inch. As the
pressure increases, the hot gases are forced more and more deeply into
the grains, and combustion, consequently, proceeds more and more
rapidly.
_Detonation._--By detonation is meant the simultaneous breaking up of
all the molecules of which the explosive substance is composed. Properly
the term is applicable to the chemical compounds only. But it is applied
to gunpowder to denote the simultaneous ignition of all the grains. The
mode of firing by detonation is obviously very favourable to the rending
effect required of blasting powder, since it reduces to a minimum the
time of explosion. It is brought about, in all cases, by means of an
initial explosion. The detonator, which produces this initial
explosion, consists of an explosive compound, preferably one that is
quick in its action, contained within a case sufficiently strong to
retain the gases until they have acquired a considerable tension. When
the case bursts, this tension forces them instantaneously through the
interstices of the powder, and so produces simultaneous ignition. A
pellet of gun-cotton, or a cartridge of dynamite, the latter especially,
makes a good detonator for gunpowder. Fired in this way, very much
better effects may be obtained from gunpowder than when fired in the
usual manner. Indeed, in many kinds of rock, more work may be done with
it than with gun-cotton or with dynamite.
The action of a detonator upon a chemical compound is different. In this
case, the explosion seems to be due more to the vibration caused by the
blow than by the heat of the gases from the detonator. Probably both of
these causes operate in producing the effect. However this may be, the
fact is certain that under the influence of the explosion of the
detonator, the molecules of a chemical compound, like nitro-glycerine,
are broken up simultaneously, or at least, so nearly simultaneously,
that no tamping is needed to obtain the full effect of the explosion.
Dynamite is always, and gun-cotton is usually, fired by means of a
detonator. A much larger quantity of explosive is needed to detonate
gunpowder than is required for dynamite, or gun-cotton, since, for the
former explosive, a large volume of gases is requisite. Dynamite
detonators usually consist of from six to nine grains of fulminate of
mercury contained in a copper cap, as described in the preceding
chapter. Gun-cotton detonators are similar, but have a charge of from
ten to fifteen grains of the fulminate. An insufficient charge will only
scatter the explosive instead of firing it, if it be unconfined, and
only explode it without detonation, if it be in a confined space.