_Force developed by Gunpowder._--In the combustion of gunpowder, the
elements of which it is composed, which elements, as we have seen, are
carbon, hydrogen, nitrogen, oxygen, potassium, and sulphur, combine to
form, as gaseous products, carbonic acid, carbonic oxide, nitrogen,
sulphuretted hydrogen, and marsh gas or carburetted hydrogen, and, as
solid products, sulphate, hyposulphite, sulphide, and carbonate of
potassium. Theoretically, some of these compounds should not be
produced; but experiment has shown that they are. It has also been
ascertained that the greater the pressure, the higher is the proportion
of carbonic acid produced, so that the more work the powder has to do,
the more perfect will be the combustion, and, consequently, the greater
will be the force developed. This fact shows that overcharging is not
only very wasteful of the explosive, but that the atmosphere is more
noxiously fouled thereby. The same remark applies even more strongly to
gun-cotton and the nitro-glycerine compounds.
The careful experiments of Messrs. Noble and Abel have shown that the
explosion of gunpowder produces about 57 per cent. by weight of solid
matters, and 43 per cent. of permanent gases. The solid matters are, at
the moment of explosion, in a fluid state. When in this state, they
occupy 0·6 of the space originally filled by the gunpowder, consequently
the gases occupy only 0·4 of that space. These gases would, at
atmospheric pressure and 32° F. temperature, occupy a space 280 times
that filled by the powder. Hence, as they are compressed into 0·4 of
that space, they would give a pressure of
280
--- × 15 = 10,500 lb.,
0·4
or about 4·68 tons to the square inch. But a great quantity of heat is
liberated in the reaction, and, as it was shown in a former section,
this heat will enormously increase the tension of the gases. The
experiments of Noble and Abel showed that the temperature of the gases
at the instant of explosion is about 4000° F. Thus the temperature of
32° + 461°·2 = 493°·2 absolute, has been raised
4000
------ = 8·11 times,
493°·2
so that the total pressure of the gases will be 4·68 × 8·11 = 42·6 tons
to the square inch. And this pressure was, in the experiments referred
to, indicated by the crusher-gauge. When, therefore, gunpowder is
exploded in a space which it completely fills, the force developed may
be estimated as giving a pressure of about 42 tons to the square inch.
_Relative Force developed by Gunpowder, Gun-cotton, and
Nitro-glycerine._--Unfortunately no complete experiments have hitherto
been made to determine the absolute force developed by gun-cotton and
nitro-glycerine. We are, therefore, unable to estimate the pressure
produced by the explosion of those substances, or to make an accurate
evaluation of their strength relatively to that of gunpowder. It should,
however, be borne in mind that a correct estimate of the pressure
produced to the square inch would not enable us to make a full
comparison of the _effects_ they were capable of causing. For though, by
ascertaining that one explosive gives twice the pressure of another, we
learn that one will produce twice the effect of another; yet it by no
means follows from that fact that the stronger will produce no more than
twice the effect of the weaker. The rending effect of an explosive
depends, in a great measure, on the rapidity with which combustion takes
place. The force suddenly developed by the decomposition of the chemical
compounds acts like a blow, and it is a well-known fact that the same
force, when applied in this way, will produce a greater effect than when
it is applied as a gradually increasing pressure. But some calculations
have been made, and some experiments carried out, which enable us to
form an approximate estimate of the relative strength of these explosive
substances.
Messrs. Roux and Sarrau give the following as the result of their
investigations, derived from a consideration of the weight of the gases
generated and of the heat liberated. The substances are simply exploded,
and the strength of gunpowder is taken as unity.
---------------+---------+-------------+---------
|Relative |Heat in Units|
Substance. | Weight | liberated |Relative
|of Gases.| from 1 lb. |Strength.
---------------+---------+-------------+---------
Gunpowder | 0·414 | 1316 | 1·00
Gun-cotton | 0·850 | 1902 | 3·00
Nitro-glycerine| 0·800 | 3097 | 4·80
---------------+---------+-------------+---------
The relative strength is that due to the volume of the gases and the
heat, no account being taken of the increased effect due to the rapidity
of the explosion.
Alfred Noble has essayed to appreciate the effects of these different
explosives by means of a mortar loaded with a 32-lb. shot and set at an
angle of 10°, the distances traversed by the shot being taken as the
results to be compared. Considered, weight for weight, he estimates as
follows the relative strengths of the substances compared, gunpowder
being again taken as unity:--
Gunpowder 1·00
Gun-cotton 2·84
Dynamite 2·89
Nitro-glycerine 4·00
The relative strength, bulk for bulk, is, however, of greater importance
in rock blasting. This is easily computed from the foregoing table and
the specific gravity of the substances, which is 1·00 for gunpowder and
compressed gun-cotton, 1·60 for nitro-glycerine, and 1·65 for dynamite.
Compared in this way, bulk for bulk, these explosives range as
follows:--
Gunpowder 1·00
Gun-cotton 2·57
Dynamite 4·23
Nitro-glycerine 5·71
Hence, for a given height of charge in a bore-hole, gun-cotton exerts
about 2½ times the force of gunpowder, and dynamite about 4¼ times that
force.