occurred, through violent friction of the twist, when miners had attempted to drive home a jammed charge. A con siderable gain was secured by the invention of compressed gun-cotton by Professor Abel in 1868. In this form the explosive occupied less than half the space of the rope charges } and the smooth, hard exterior of the cylindrical charges rendered the operation of loading comparatively easy. The compressed charges, moreover, did not burn with the explosive violence of spun gun-cotton even when con fined in the ordinary packing cases. Among further im provements may be noted the cheapening of the material by use of cotton waste, instead of the long staple cotton of high quality that was used in the Austrian manufacture. In 1868 Mr E. 0. Brown discovered that (like nitroglycerine)[1] compressed gun-cotton was susceptible of violent explosion through the agency of a detonation, as well as in the ordinary way. This was important, especially for submarine operations and works of demolition ; for the strong con finement which was always necessary in the other case could be dispensed with ; indeed, with some waste of power, the substance might be exploded completely unconfined. Gun- cotton is not affected in its explosiveness by cold ; and it can be kept any length of time without deterioration in the damp and perfectly unignitable state. The formula that has been assigned for the most explosive gun-cotton is
CcH 7 N 3 O u .In 1864 Mr Nobel s researches called attention to the application of nitroglycerine (discovered by Sobrero in 1846) as an explosive agent. He first showed that the effect of gunpowder was considerably increased through impregna tion with it ; and later, that the liquid itself, which burns slowly in the air on application of a flame with a common fuse, could be exploded by an initiative detonation, con finement by tamping being then unnecessary. In its pure state it was soon proved to be the most powerful explosive yet known ; its destructive force is about ten times that of gunpowder. Its liquid form, high specific gravity, and insolubility in water, are valuable properties in some cases, as in blasting under water or in wet holes. It freezes at a comparatively high temperature (40 Fahr.) ; but the opinion, formed from several grave accidents, that it was more susceptible of detonation in the frozen than in the liquid state, has been shown to be contrary to fact. When frozen it is more liable to recklessly rough usage. The liquid state of nitroglycerine, on the other hand, constitutes a very serious defect, owing to its tendency to leak from vessels in which it is carried, or from the blast-hole, through fissures in the rock, resulting in unexpected explosions, it may be, through some slight concussion. Mr Nobel s ingenious device for rendering nitroglycerine temporarily inexplosive, by dissolving it, viz., in wood spirit, is only partially successful. Nitroglycerine has been extensively used in various mining districts, especially in California. After some terrible accidents, which occurred in 1866-7, its use in England was placed under severe restrictions.
Impressed with the serious disadvantages of this explosive, Mr Nobel was led to the important observation that its readiness to explode by detonation is not diminished, but rather favoured, by mixture with solid substances, in themselves quite inert ; and the dilution did not materially detract from the great superiority of nitro glycerine over gunpowder. In 1867 he brought before the public the substance appropriately called dynamite, which is one of the safest, most powerful, and most convenient explosives for industrial purposes. It consisted of seventy- five parts of nitroglycerine held absorbed by twenty-five parts of a porous, infusorial, silicious earth, known ill Germany as " Kieselguhr." Other absorbents have been employed (precipitated kaolin, tripoli, precipitated alumina, sugar, &c.), but none of them are equal to kieselguhr in power of retaining oil. Dynamite is furnished to the trade in the form of small cylindrical cartridges, in which the material, consolidated by pressure, is enclosed in a single wrapping of parchment paper. It requires no tamping, and can be exploded by detonation under water. It is slow to catch fire, but burns rather fiercely when fired ; and if the quantities are large, or under confinement, an explosion may finally ensue. The trade in it has developed rapidly ; thus, while only 11 tons of it were sold in 1867, the quantity rose to 3120 tons in 1874.
In the preparation known as lithofracteur, which came into notice during the Franco-German War, nitroglycerine is used in considerably smaller proportions than in dynamite ; and the kieselguhr of the latter is partly replaced by materials forming of themselves a feebly explosive mixture. Its properties are very similar to those of dynamite, but it is less powerful.
The less known ammonia powder, invented by Ohlson and Norbin, is much stronger than lithofracteur, and even surpasses dynamite. Its only drawback is the hygroscopic nature of its chief ingredient, which is nitrate of ammonium; but otherwise it is a very superior blasting agent. Numerous other explosives have been tried in blasting, but those we have named are amongst the most important.
In a recent paper to the Society of Arts, Mr Nobel has discussed the relative power of several blasting agents. He finds that when their ballistic power is compared bulk for bulk, the substances experimented with rank as follows : Nitroglycerine, 100; ammonia powder, 80; dynamite (No. 1), 74; lithofracteur, 53; gun-cotton, 45; Curtis and Harvey s blasting-powder (fired by detonator), 17 5. While these figures show the great superiority of nitroglycerine, there are practical circumstances which bring it and dyna mite nearly on an equality. Thus, to get the full benefit of a blast there should be no air-space round the charge. Now, from the danger (as we have seen) of nitroglycerine leaking through imperceptible fissures in a rock, rigid cartridges are required for it, and these always involve a considerable air chamber, whereas dynamite, being highly plastic, can be easily compressed so as to exclude all empty space. The cartridges of compressed gun-cotton are also liable, of course, to the objection jst noticed.
a saving of labour, of tools, and of time by use of the new explosive agents (such as dynamite or gun-cotton). Their shattering and splitting effect in hard rock is much greater ; but in quarrying, the rock is generally not thrown out by them to the same extent. Where a moderate cleaving or splitting effect is desired, with as little local action as possible, gunpowder is best, as in raising large blocks of slate ; also where great displacing action is required. In | submarine blasting of soft rocks the violent explosives disintegrate the rock into a plastic mass within a limited area, but do not shatter or rend it to any great distance. As regards comparative safety, there is no doubt that modern explosives offer a relative immunity from the danger arising from fire, to which gunpowder is subject. Neither dynamite nor gun-cotton can be fired by a spark, and if accidentally fired by a flame, they allow reasonable chances of escape. On the other hand, accidents have often happened in the thawing of nitroglycerine preparations when frozen, a process that requires great care, and for which suitable warming-pans are provided. But miners are slow to understand that a cartridge which firing does not set off cannot be slowly heated with the same
impunity ; hence they will roast the preparations near a- ↑ It "was found, in the ccmrse of these inquiries, that all explosive compounds, even including gunpowder, are susceptible of explosion through a detonation, though the nature and force of the detonation vary considerably with different explosive substances.
