substance, and the rapidity with which these changes proceed throughout
a mass of the material, are greatly modified by the conditions under
which the action takes place. If a red-hot wire be applied to a small
loose tuft of gun-cotton, it goes off with a bright flash without
leaving any smoke or any other residue. Thus, when the substance is
quite unconfined, no explosion occurs. If the cotton-wool be made into a
thread, and laid along the ground, it will burn at the rate of about 6
in. per second; if it be twisted into a yarn, the combustion will run
along at the rate of 6 ft. per second; but if the yarn be enclosed in an
Indian-rubber tube, the ignition proceeds at the rate of 30 ft. in a
second. If to a limited surface of gun-cotton, such as one end of a
length of gun-cotton yarn, a source of heat is applied—the temperature
of which is high enough to set up a chemical change, but not high enough
to inflame the resulting gases (carbonic oxide, hydrogen, &c.)—the
cotton burns comparatively slowly, rather smouldering than inflaming.
If, however, a _flame_ be applied, the gun-cotton flashes off with great
rapidity, because the heat applied sets fire to the gaseous products of
the chemical action. But if the gun-cotton be confined so that the gases
cannot escape, the combustion becomes rapid however set up. The reason
is that if the gases escape into the air, they carry off so much of the
heat produced by the smouldering gun-cotton, that the temperature does
not rise to the extent required to produce the flaming ignition, in
which the products are completely oxidized. If a mass of gun-cotton be
enclosed in a capacious vessel from which the air has been removed, and
the gun-cotton be ignited by means of a wire made hot by electricity,
the cotton will at first only burn in the slow way without flame; but as
the gases accumulate and exert a pressure which retards the abstraction
of heat accompanying their formation, the temperature will rise and
attain the degree necessary for the complete and rapid chemical changes
involved in the flaming combustion. Thus, the more resistance is offered
to the escape of the gases, the more rapid and perfect is the combustion
and explosive force produced by the ignition. Now, the explosion of
gun-cotton has been found to be too rapid when it is packed into the
powder-chamber of a gun, for its tendency is to burst the gun before the
ball has been fairly started. Hence a material like gunpowder, in which
the combustion is more gradual, is better suited for artillery. The
ignition of gunpowder, though rapid, is not instantaneous, and therefore
we can speak of it as more or less gradual. Indeed, in even the most
violent explosives, some time is doubtless required for the propagation
of the action from particle to particle. This extreme rapidity of
combustion, and consequent rending power, which is so objectionable in a
gun-chamber, makes gun-cotton a most powerful bursting charge for
shells, and, when it is enclosed in strong receptacles, for torpedoes
also.
But by the researches of Nobel, Professor Abel, and others, it has been
discovered—and this is, perhaps, the most remarkable discovery in
connection with explosives—that gun-cotton, nitro-glycerine, and other
explosive bodies, are capable of producing explosions in a manner quite
different from that which attends their ignition by heat. The violence
of this kind of explosion is far greater than that due to ordinary
ignition, for the action takes place with far greater rapidity
throughout the mass, and is, indeed, practically instantaneous. It
appears to be produced by the mere mechanical agitation or vibrations
which are communicated to the particles of the substance. Turning back
to the representation of the molecule of nitro-glycerine on page 744, it
will not be difficult to imagine that this may be an unstable kind of
structure; that the atoms of oxygen are prevented from rushing into
union with those of hydrogen and carbon only by the interposition of the
nitrogen; and that an agitation of the structure might shake all the
atoms loose, and leave them free to re-combine according to their
strongest affinities. Nitro-glycerine is by no means so ready to
_inflame_ as is gun-cotton: it is said that the flame of a match may be
safely extinguished by plunging it into the liquid; and when a
sufficient heat is applied to a quantity of the liquid in the open air,
it will burn quietly and without explosion. Even when nitro-glycerine is
confined, the application of heat cannot always be made to produce its
explosion; or, at least, the circumstances under which it can do so are
not accurately known, and the operation is difficult and uncertain. On
the other hand, nitro-glycerine explodes violently even when freely
exposed to the air if there be exploded in contact with it a _confined_
charge of gunpowder, or a detonating compound such as fulminating
powder. Gun-cotton possesses the same property of exploding by
concussion, which appears indeed to be a general one belonging to all
explosive bodies. According to recent researches, even gunpowder is
capable of a detonative explosion. A mass of gunpowder confined with a
certain proportion of gun-cotton, which is itself set off by fulminate
of mercury, is said to exert an explosive force four times greater than
that developed by the ignition of the gunpowder in the ordinary manner.
It has also been found that _wet_ gun-cotton can be exploded by
concussion, and the force of the explosion is unimpaired even when the
material is saturated with water. This makes it possible to use
gun-cotton with greater safety, as it may be transported and handled in
the wet condition without risk, and it preserves its properties for an
indefinite period without being deteriorated by the water. Some
experiments illustrating the extraordinary force of the detonative
explosions of gun-cotton and nitro-glycerine will give the reader an
idea of their power.
A palisade, constructed by sinking 4 ft. into the ground trunks of trees
18 in. in diameter, was completely destroyed in some experiments at
Stowmarket by the explosion of only 15 lbs. of gun-cotton. Huge logs
were sent bounding across the field to great distances, and some of the
trees were literally reduced to match-wood. The gun-cotton, be it
observed, was simply laid on the ground exposed to the air. The
destructive powers of nitro-glycerine are even greater. A tin canister,
containing only a few ounces of nitro-glycerine, is placed, without
being in any way confined, on the top of a smooth boulder stone of
several tons weight; it is exploded by a fuse containing fulminating
powder, which is fired from a distance by electricity. There is a
report, and the stone is found in a thousand fragments. The last
experiment shows one of the advantages of nitro-glycerine over gunpowder
as a blasting material, beyond its far greater power, which is about ten
times that of gunpowder. A charge of gunpowder inserted in a vertical
hole tends to force out a conical mass, the apex of which is at the
space occupied by the charge. With nitro-glycerine, and also with
gun-cotton, which last has almost six times the force of gunpowder, a
powerful rending action is exerted _below_ as well as above the charge.
Again, in blasting with gunpowder the charge must be confined, and the
hole is filled in above the charge with tightly rammed materials,
forming what is termed the _tamping_. But nitro-glycerine requires no
tamping: a small, thin metallic core containing the charge is simply
placed in the drill-hole, or the liquid itself is poured in, and a
little water placed above it. The effect of the explosion of
nitro-glycerine in “striking down,” when apparently no resistance is
offered, will seem very strange to the reader who is oblivious of
certain fundamental principles of mechanics. The force of the explosion
is due entirely to the sudden production of an enormous volume of gas,
which at the ordinary pressure would occupy several thousand times the
bulk of the material from which it is produced. This gas, by the law of
the equality of action and reaction, presses down upon the stone with
the same force that it exerts to raise the superincumbent atmosphere.
The pressure of the gas at the moment of its liberation is enormous; but
the atmosphere cannot instantaneously yield to this, for time is
required to set the mass of air in motion, and the wave of compression
advances slowly compared with the rapidity of the explosion. Hence the
air acts, practically, like a mass of solid matter, against which the
gases press, and which yields less readily than the rock, so that the
blow which is struck takes visible effect on the latter. Now, with
gunpowder, the evolution of gas is less rapid, the atmosphere has time
to yield, and the reaction has not the same violence. The rapidity of
the evolution of gas from the exploding nitro-glycerine is so great,
that the gases, though apparently unconfined, are not so in reality; for
the atmosphere acts as a real and very efficient tamping.
When nitro-glycerine first came into use for blasting purposes, it was
used in the liquid form under the name of “blasting oil;” but the
dangers attending the handling of the substance in this state are so
great, that it is now usual to mix the liquid with some powdered
substance which is itself without action, and merely serves as a vehicle
for containing the nitro-glycerine. To mixtures of this kind the names
“_dynamite_,” “_dualine_,” “_lithofracteur_” &c., have been given.
It is now hardly necessary to point out that the discovery of these new
explosives has largely extended our power over the rocks, enabling works
to be executed which would have been considered impracticable with less
powerful agents. It is true that the most fearful disasters have been
accidentally produced by the new explosives; but such occasional
devastation is the price exacted for the possession of powerful agents.
And just as in other cases—steam, for example—where great forces are
dealt with, so these new powers must be managed with unceasing care, and
placed in the hands of only skilful and intelligent men.
The products of the combustion of gunpowder are not all gaseous, but
include solid compounds, such as carbonate and sulphate of potassium. It
is these that give rise to the smoke seen when a gun is discharged, and
which, in rapid firing, soon obscures the sight of the objects aimed at.
They are also the causes of the fouling of the bore. Gun-cotton is quite
unexceptionable in these respects, and that prompted the attempts made
soon after its introduction to use it instead of gunpowder in fire-arms.
But the explosion of gun-cotton was found too sudden and violent for
guns and rifles, so that many serious accidents in consequence occurred.
The next thing done was to lessen the rapidity of the explosion by using
gun-cotton mixed with ordinary cotton, or twisted in threads round some
inert substance—in fact, to mitigate the violence of the shock by some
mechanical disposition of the material. The introduction of rapid firing
guns and repeating rifles forced on the problem of a smokeless powder;
and as the plan of replacing nitrate of potassium, in ordinary
gunpowder, by nitrate of ammonium was found to be attended with loss of
the keeping quality of the powder, other materials, such as picric acid,
which forms also the basis of the explosive called _mélinite_, have been
proposed. The composition of _mélinite_ was long a mystery, and that of
the smokeless powder adopted by the French was so carefully concealed
that many experiments had to be made by other nations to discover some
similar preparation, which was found possible by combining certain
substances with gun-cotton so as to modify the violence of its
explosion, and produce a manageable material having the required
properties. The British Government, after many experiments and much
careful testing, decided to adopt _cordite_, made of nitro-glycerine, in
which, by the aid of volatile solvent, di-nitro-cellulose is dissolved,
together with a little mineral oil. The semi-fluid composition, forced
through a round hole or die, comes out like a thread or cord, which the
evaporation of the volatile solvent leaves with very much the appearance
of common brown window-cord. This material has the several advantages of
keeping well, of being _uniform_ in its propulsive powers, of being
capable of imparting as high a velocity as a much larger charge of the
ordinary black gunpowder, while at the same time exercising a less
pressure on the chase of the gun.
It will have become obvious from the preceding paragraphs that,
according to the conditions under which an explosive is to be used,
selection must be made of the most suitable. For example, the substances
employed for propelling projectiles from guns must not have the violent
rending power of certain others, which, by this very property, are most
useful for blasting operations; and, again, although explosives of this
last kind are inadmissible as projectile agents, they are of the kind
best adapted for use in shells where it is the disruptive action that is
required. Also in blasting operations, the explosive has to be adapted
to the nature of the work, and it has been found that a substance which
has worked well in driving a heading for a tunnel through one kind of
rock may prove both slow in progress, and more costly in expenditure,
when some different kind of rock is reached. Besides this, regard must
be had in blasting operations to the nature of the effect required,
which is in some instances a shattering of the rock into fragments, in
others a detachment of it in masses. Thus, in the working of a slate
quarry, the explosive used must not be of a nature to shiver the rock
into useless splinters, but must operate in such a manner that compact
masses may be separated from the mountain side in a condition suitable
for cleaving, by appropriate tools, into numberless broad laminæ, which,
trimmed into rectangular shape, constitute our well-known roofing
slates. The blasting used on a coal seam must be so conducted as to
yield the material as much as possible in big lumps or cobbles rather
than in slack. When granite is blasted for the purpose of obtaining
building stones, the explosive must be one that, by its comparatively
slow action, divides the compact rock into the largest possible blocks.
On the other hand, when granite is blasted merely with the object of
removing it, as when a tunnel has to be driven through a mass of it, the
most disintegrating agent is then the best. The common popular
expressions by which the two classes of explosives just referred to are
distinguished are “high explosives” and “low explosives.” Dynamite may
be taken as a type of the former, and gunpowder a type of the latter. As
will be gathered from what is to follow, no definite separation between
these classes can be fixed, but in a general way it may be said that,
where a destructive, rather than a propelling or pressure effect is
required, the explosive used is one brought into operation by a
concussive or detonating priming, and acting mostly by detonation within
itself, such as dynamite, &c.
Whereas, up to nearly the middle of the nineteenth century, gunpowder
was practically the only explosive in use for either civil or military
purposes, the close of the century can show a list of several hundred
preparations that have been proposed or actually used in its stead. The
names by which these are put forward are expressive sometimes of an
ingredient in their composition, such as “ammonia dynamite,”
“cellulosa,” “mica powder,” “dynamite au carbon,” “dynamite de boghead,”
&c.; and sometimes the inventor’s name, as “So-and-so’s powder or
explosive”; sometimes of the strength of the mixture under various
fanciful names, such as “dynamite,” “heraklin,” “vigorite,” &c., &c.;
sometimes the names relate to the appearance of the compound, as “white
gunpowder,” “blasting gelatine,” &c., &c.; and sometimes to other
circumstances, such as “pudrolithe,” “saxifragine,” “safety powder,”
&c., &c. A very long list might be given of the substances severally
used in these various compositions. It will be sufficient to indicate
the general nature of the several classes into which the new explosives
may be divided. By turning back to p. 746, the reader will be reminded
of the composition of gunpowder, and of the part played therein by the
nitre (nitrate of potassium). Now a considerable number of the recently
patented explosives are simply modified gunpowders, which all contain
some nitrate, replacing wholly, or in part, the nitrate of potassium,
while sulphur is an ingredient of nearly all, and in many, the charcoal
of gunpowder is partly or wholly replaced by other carbonaceous
materials, such as sawdust, coal-dust, tan, starch, paraffin,
lycopodium, graphite, peat, flour, bran, &c. Certain mineral salts enter
into the composition of some, such as sulphate of iron, carbonate of
copper, sulphide of antimony, &c., &c.
In another class of the newer explosives chlorate of potassium takes the
place of the nitrate as the oxygen supplying material, with similar
variations as to the carbonaceous matter as are referred to above.
Yellow prussiate of potash and sugar sometimes replace both the charcoal
and sulphur of gunpowder in this class. Explosives of this chlorate
class are usually dangerous to manufacture, and are often very
sensitive, and also liable to changes by keeping, which render them
still more dangerous.
The next class of preparations brings us to the “high explosives,”
properly so called, and it is among these that most notable preparations
are met with. Of all the explosive nitro-compounds, gun-cotton was the
first practically employed (_vide_ p. 741); but very soon after
nitro-glycerine was discovered by Sobrero when working in Pelouze’s
laboratory. This afterwards became known as “blasting oil,” but it was
many years before nitro-glycerine came into use as an explosive, namely,
when, about 1860, Nobel, a Swedish engineer, had established factories
for its production as an agent for blasting. At first there were
difficulties and dangers attending its use, and it was only when Nobel
had discovered the detonation method of setting free its tremendous
energy that the new era of “high explosives” really commenced. Between
1860 and 1870 such a number of appalling catastrophes occurred in the
handling of the new “blasting oil” that in several European countries
its use was entirely prohibited. And, in England at least, this
prohibition remains, for “in a liquid state this explosive cannot be
sold in, or imported into this country. It is manufactured under the
strict provision that it is forthwith made up into dynamite or some
kindred licensed explosive.” ... “The only source, practically speaking,
of nitro-glycerine on a commercial scale in this country is the factory
of Nobel’s Explosive Co. (Ltd.) at Ardeer, in the county of Ayr.”[17]
Nitro-glycerine being so extremely dangerous to handle in the liquid
form led Nobel to propose its use in an altered condition, by causing it
to be absorbed by some inert porous material, the most suitable being a
siliceous earth found in Germany, and there known as _kieselguhr_, of
which one part will absorb three times its weight of liquid
nitro-glycerine. Here we have the original dynamite, but now other
substances are used for absorbing the liquid, and there are, indeed,
dynamites of two different classes:
1. Dynamites with inert absorbents.
2. Dynamites with absorbents which are themselves combustible, or
explosive.
Footnote 17:
Major Cundill, H.M.’s Inspector of Explosives.
Of the latter class there are endless varieties. One that has latterly
been much used is called “blasting gelatine,” and is practically a
combination of nitro-glycerine and nitro-cotton, this last ingredient
being a less nitrated cellulose than gun-cotton. Blasting gelatine
contains a very large percentage of nitro-glycerine (93–95 per cent.),
and has the appearance of stiff jelly of a pale yellow colour. It may be
of interest to remark that this second class of dynamites admits of
well-defined sub-divisions according to the nature of the absorbent, as:
(_a_) Charcoal, or other simple carbonaceous material.
(_b_) Gunpowder, or other nitrate or chlorate mixtures.
(_c_) Gun-cotton, or other nitro-compounds.
[Illustration:
FIG. 340.—_View on the Tyne._
]
MINERAL COMBUSTIBLES.
Certain mineral combustibles may fairly claim attention in a work
treating of the discoveries of the nineteenth century, not because these
bodies have been known and used only in recent times, but for other
reasons. The true nature of coal—that most important of all
combustibles—its relation to the past history of the earth, and to the
present and future interests of mankind; the work it will do; the extent
of the supply still existing in the bowels of the earth; the innumerable
chemical products which it yields—are subjects on which the knowledge
gained during the present century forms a body of discovery of the most
interesting and important kind. Another substance we have to mention,
though not a modern discovery, has lately been found in far greater
abundance, and is now so largely used for various purposes, that it has
become an important article of commerce.
_COAL._
[Illustration:
FIG. 341.—_Fossil Trees in a Railway Cutting._
]
Most persons know, or at least have been told, that coal is fossil
vegetable matter,—the long-buried remains of ancient forests. But
probably many receive the statement, not perhaps with incredulity, but
with a certain vague notion that it is, after all, merely a daring
surmise. And, indeed, nothing is at first sight more unlike stems, or
leaves, or roots of plants than a lump of coal. Then everybody knows
that coal is found thousands of feet beneath the surface of the earth,
whereas plants can grow only in the light of the sun. One begins to
understand the matter only when the teachings of geology have shown him
that, so far from the crust of the earth being, as he is apt to suppose,
fixed and unchangeable, it is in a state of constant fluctuation.
Changes in the levels of the ground are always going on: in one place it
is rising, in another sinking; here a tract of land is emerging from the
ocean, there a continent is subsiding beneath the water. The extreme
slowness with which these changes proceed causes them to escape all
ordinary observation. The case may be compared to the hour-hand on the
dial, which a casual spectator might pronounce quite stationary, since
the observation of a few seconds fails to detect its movement. As the
whole period comprehended in human annals counts but as a second of
geological time, it cannot be wondered at that it required a vast
accumulation of facts, and much careful and patient deduction from them,
before a conclusion was reached so apparently contradictory of
experience. It is, indeed, startling to learn that “the sure and
firm-set earth” is in a state of flow and change. Even the “everlasting
hills” give evidence that their materials were collected at the bottom
of the sea, and we know that the water which runs down their sides is
slowly but surely carrying them back particle by particle. Of the
magnitude of the changes which the surface of the earth has undergone in
times past, and which are still imperceptibly but constantly proceeding,
the ordinary experience of mankind can of itself give no example. But
such changes have sufficed to entomb a vast quantity of relics of the
innumerable forms of vegetation which flourished and waved their
branches in the sun, ages upon ages before the advent of man.
[Illustration:
FIG. 342.—_Impression of Leaf found in Coal Measures (Pecopteris)._
]