Discoveries and Inventions of the Nineteenth Century

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. 748The 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 749into 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. 750The 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