Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/828

810 fire, or on hot cinders, or iu other ways really dangerous. Gun-cotton and dynamite prove much safer than nitro glycerine as regards exploding through concussion. There is nob, however (Mr Nobel thinks), that amount of difference between the sensitiveness of nitroglycerine and dynamite which the latter substance generally receives credit for. The main danger of nitroglycerine arose from the sensitiveness to concussion which it acquired through contact with a hard, metallic, strongly vibrating substance, such as the tin canisters in which it was contained. The main safety of dynamite is derived from the absence of any hard vibrating material in immediate contact with the nitroglycerine it contains. As regards danger from concussion in manufacture and transit, gun-cotton ranks first ; but in the hands of miners, the case is reversed, through the rough usage of gun-cotton charges, where, e.g., they are found too large fora bore-hole, for gun-cotton is well known to explode with a blow. The danger most dreaded in modern explosives is from their supposed liability to chemical decomposition productive of heat, which sometimes leads to ignition and explosion. This decomposition is generally due to the presence of acid (chiefly nitric and hyponitric), which every nitrated com pound has a strong tendency to retain. From the ease with which the acid can be washed out from nitroglycerine, both it and dynamite show much greater chemical stability than gun-cotton. Most cases of spontaneous combustion of the latter have prob.ibly arisen either from imperfect washing, or from drying at too high a temperature (by which hyponitric acid is set free). Complaints have often been made of the poisonous fumes given off by the new explosives. &quot;Vhere this occurs, it is probably due to an injudicious use of the substance, resulting in imperfect explosion. If a dynamite cartridge partly burns instead of exploding, the temperature is much lower, and fumes of hyponitric acid are given off, which could not escape decomposition at the higher temperature of perfect explosion. The general mistake consists in not securing carefully the detonator cap to the fuse, and especially the fuse to the cartridge.

Blasting by Electricity.—It is known that electricity has a thermal effect on wire through which it passes ; and the amount of heat produced in any part of the circuit is pro portional to the resistance in that part. Thus a piece of wire of small section and conductivity may be made incan descent by a current. On this principle platinum is some- tunes employed to fire blasting charges. In making a fuse of this sort, two insulated copper wires are twisted together for a length of about 6 inches, leaving the extre mities free for about half an inch, and separated the same distance. A fine platinum (or iron) wire is stretched across, this interval, metallic contact being established with the copper. The other ends of the fuse are connected with a battery. Platinum fuses are not much to be relied on for simultaneous blasting of several charges by one battery; for some of the fuses may take a little more time to reach the ex ploding temperature than others, and thus, as soon as one explodes, the connection between the others and the battery is broken. The batteries to be used with them are such as generate electricity of great quantity. The Bunsen and Leclanche&quot; batteries, in some of their varieties, are well suited for this. Twelve cells of Highton s battery will melt a piece of platinum wire over an inch long. There is, however, another class of fuses, offering cer tain advantages over those just referred to, in which the spark produced by electricity of tension is the means used to effect the explosion. It might naturally be thought that an electric spark must inevitably cause explo sion in a mass of powder or like substance through which it is made to pass ; but this is not the case. The heating power of the spark is often insufficient for explosion.&quot; Tha duration of an induction spark is about the millionth of a second ; whereas, to ignite powder, it is necessary that &amp;lt;i spark should exist for at least the three hundredth part of a second. By interposing, however, a suitable priming com position in the interval which the spark is to cross, and in contact with the charge, explosion may be thus effected. In preparing such a composition, the properties of the ingredients as regards conductivity, inflammability, and explosiveness have to be nicely adjusted, according to the degree of tension of the electricity employed. The com position selected by Professor Abel for his fuses is an inti mate mixture of subsulphide of copper, subphosphide of copper, and chlorate of potassium. It is a mixture which conducts, but conducts with difficulty, and the fuses made with it are very effective. There are several other varieties, e.g., Ebner s fuse, where the priming consists of a mixture of sulphuret of antimony, chlorate of potash, and graphite. For generating electricity of tension with the Voltaic battery, Leclanche s battery is, again, one of the most suitable. The elements of this battery consist of a rod of carbon placed in a porous cell and tightly packed round with a mixture of peroxide of manganese and coke ; the porous cell is placed in a vessel containing a plate of zinc, which forms the electro-positive element, and a solution of sal-ammoniac is used as the exciting liquid. There are some forms of battery for the same purpose so arranged that the contact of the elements with the liquid takes place only at the time of firing ; such are those of Wollaston, Ruhmkorff, and Trouve. Frictional electricity is the kind generally adopted by military authorities in firing charges, the machines for generating it being easily made, simple, portable, and powerful. Bornhardt s frictional machine has found exten sive use in Austria in ordinary blasting operations. It is contained in a small metallic case, and consists of a disc of ebonite, which can be rotated between two cushions, charging a small Leyden jar placed near it. On pressing a little button from the outside, connection is made between the two coatings of the jar in such a way that the charge is sent through two wires by which the box is connected with the fuse, or fuses, at a distance. Some absorbent of mois ture is kept within the box, and it is necessary to see that the machine be kept as warm and dry as possible. Experiments were made by Messrs Wheatstone and Abel, a number of years ago, with Armstrong s hydroelectric machine, as a source of electricity for exploding charges of powder. They state that in very extensive mining opera tions, where a great many charges have to be fired simul taneously, and provided all the necessary appliances for success are at hand, the machine could be used very effec tively. It is a powerful source of electricity of high tension. There are serious objections, however, to its general use. Electro-magnetic induction currents (such as are deve loped in Ruhmkorff s coil) were first applied, and success fully, by Colonel Verdu, a Spanish officer, in 1853. The induction discharge, unlike that of a Leyden battery, is much enfeebled by successive solutions of continuity, so that not more than four mines in a single circuit could certainly be exploded on this system. But M. Savare made an improvement by interposing the fuses in branches of the principal circuit. The mine nearest the apparatus explodes first ; and, owing to the abrupt separation of the wires, the current can no longer pass through this branch ; thus the electric action is augmented in the other branches, and in a similar manner the explosions necessarily take 