_Gunpowder._--The combustion of gunpowder, as we have seen, is gradual
and comparatively slow. Hence its action is rending and projecting
rather than shattering. This constitutes one of its chief merits for
certain purposes. In many quarrying operations, for instance, the
shattering action of the chemical compounds would be very destructive to
the produce. In freeing blocks of slate, or of building stone, a
comparatively gentle lifting action is required, and such an action is
exerted by gunpowder. Moreover, this action may be modified by using
light tamping, or by using no tamping, a mode of employing gunpowder
often adopted in slate quarries. The effect of the violent explosives
cannot be modified in this way.
Gunpowder is injured by moisture. A high degree of moisture will destroy
its explosive properties altogether, so that it cannot be used in water
without some protective covering. Even a slight degree of moisture, as
little as one per cent. of its weight, materially diminishes its
strength. For this reason, it should be used, in damp ground, only in
cartridges. This is, indeed, the most convenient and the most economical
way of using gunpowder in all circumstances. It is true that there is a
slight loss of force occasioned by the empty space around the cartridge,
in holes that are far from circular in shape. But at least as much will
be lost without the cartridge from the moisture derived from the rock,
even if the hole be not wet. But in all downward holes, the empty spaces
may be more or less completely filled up with dry loose sand.
The products of the explosion of gunpowder are partly gaseous, partly
solid. Of the former, the most important are carbonic acid, carbonic
oxide, and nitrogen. The sulphuretted and the carburetted hydrogen are
formed in only small quantities. The carbonic oxide is a very noxious
gas; but it is not formed in any considerable quantity, except in cases
of overcharging. The solid products are compounds of potassium and
sulphur, and potassium and carbon. These constitute the smoke, the dense
volumes of which characterize the explosion of gunpowder. This smoke
prevents the immediate return of the miner to the working face after
the blast has taken place.
_Gun-cotton._--The combustion of gun-cotton takes place with extreme
rapidity, in consequence of which its action is very violent. Its effect
is rather to shatter the rock than to lift it out in large blocks. This
quality renders it unsuitable to many quarrying operations. In certain
kinds of weak rock, its disruptive effects are inferior to those
produced by gunpowder. But in ordinary mining operations, where strong
tough rock has to be dealt with, its superior strength and quickness of
action, particularly the latter quality, produce much greater disruptive
effect than can be obtained from gunpowder. Moreover, its shattering
action tends to break up into small pieces the rock dislodged, whereby
its removal is greatly facilitated.
Gun-cotton may be detonated when in a wet state by means of a small
quantity of the dry material. This is a very important quality, inasmuch
as it allows the substance to be used in a wet hole without protection,
and conduces greatly to the security of those who handle it. When in the
wet state, it is uninflammable, and cannot be exploded by the heaviest
blows. Only a powerful detonation will bring about an explosion in it
when in the wet state. It is, therefore, for safety, kept and used in
that state. Since it is insensible to blows, it may be rammed tightly
into the bore-hole, so as to fill up all empty spaces. The primer of
dry gun-cotton, however, which is to detonate it, must be kept perfectly
dry, and handled with caution, as it readily detonates from a blow.
Gun-cotton, when ignited in small quantities in an unconfined space,
burns fiercely, but does not explode.
The products of the combustion of gun-cotton are:--carbonic acid,
carbonic oxide, water, and a little carburetted hydrogen or marsh-gas.
On account of the insufficiency of oxygen, already pointed out, a
considerable proportion of carbonic oxide is formed, which vitiates the
atmosphere into which it is discharged. Overcharging, as in the case of
gunpowder, causes an abnormal quantity of the oxide to be formed.
_Dynamite._--As combustion takes place more rapidly in nitro-glycerine
than in gun-cotton, the effects of dynamite are more shattering than
those of the latter substance. Gun-cotton holds, indeed, a mean position
in this respect between dynamite, on the one hand, and gunpowder on the
other. Dynamite is, therefore, even less suitable than gun-cotton for
those uses which are required to give the produce in large blocks. But
in very hard and tough rock, it is considerably more effective than
gun-cotton, and, under some conditions, it will bring out rock which
gun-cotton fails to loosen.
Dynamite is unaffected by water, so that it may be used in wet holes;
indeed, water is commonly used as tamping, with this explosive. In
upward holes, where water cannot, of course, be used, dynamite is
generally fired without tamping, its quick action rendering tamping
unnecessary.
The pasty form of dynamite constitutes a great practical advantage,
inasmuch as it allows the explosive to be rammed tightly into the
bore-hole so as to fill up all empty spaces and crevices. This is
important, for it is obvious that the more compactly the charge is
placed in the hole, the greater will be the effect of the explosion.
Moreover, this plastic character renders it very safe to handle, as
blows can hardly produce sufficient heat in it to cause explosion. If a
small quantity of dynamite be placed upon an anvil and struck with a
hammer, it explodes readily; but a larger quantity so struck does not
explode, because the blow is cushioned by the kieselguhr. If ignited in
small quantities in an unconfined space, it burns quietly without
explosion.
If dynamite be much handled out of the cartridges, it causes violent
headaches; and the same effect is produced by being in a close room in
which there is dynamite in the unfrozen state.
Dynamite possesses one quality which places it at a disadvantage with
respect to other explosives, namely, that of freezing at a comparatively
high temperature. At about 40° F. the nitro-glycerine solidifies, and
the dynamite becomes chalky in appearance. In this state, it is exploded
with difficulty, and, consequently, it has to be thawed before being
used. This may be safely done with hot water; performed in any other way
the operation is dangerous.
The products of the combustion of dynamite are carbonic acid, carbonic
oxide, water, and nitrogen. As, however, there is more than a
sufficiency of oxygen in the compound, but little of the oxide is formed
when the charge is not excessive. If, therefore, dynamite be properly
detonated, and overcharging be avoided, its explosion will not greatly
vitiate the atmosphere. But if it be only partially detonated
hypo-nitric fumes are given off, which have a very deleterious effect
upon the health. It is, thus, of the highest importance that complete
detonation should be effected, not merely to obtain the full effect of
the explosive, but to avoid the formation of this noxious gas. This may
be done by using a detonator of sufficient strength, and placing it well
into the primer.
_Firing Points of the Common Explosive Compounds._--The following table
shows the temperatures at which the commonly used compounds explode:--
-------------------+-----------+-----------------
|When slowly| When suddenly
| Heated. | Heated.
+-----------+-----------------
Gunpowder | .. |from 500° to 540°
Gun-cotton | 360° | 482°
Kieselguhr dynamite| 356° | 446°
Cellulose dynamite | 342° | 446°
-------------------+-----------+-----------------
Cotton powder explodes at the same temperatures as gun-cotton, and
lithofracteur at the same temperature as kieselguhr dynamite.