mite is composed of nitroglycerin and some absorbent or "dope." The flame of exploding dynamite is much hotter than the flame of exploding black blasting powder but is not as long. Formerly considerable dynamite was used in coal mines, especially in entries, but a knowledge of the danger resulting from using it in coal mines has led to its use being forbidden in most coal-mining States. As it is quicker acting than black powder, it generally shatters the coal, and thus reduces the amount of lump coal; also, it crushes the coal near the drill hole into fine dust. Often this dust is ignited by the flame and heat of the exploding dynamite and a dust explosion results. Moreover, the explosive has been handled carelessly. If frozen, it was often partly thawed by being placed in the miner's boot or in his bosom or by being held over the flame of a miner's open lamp or by being held against the gauze of a safety lamp. As a consequence attempts were made to explode this dynamite while partly frozen, with the result that the dynamite burned and did no useful work, but gave off noxious gases. In some instances the dynamite exploded feebly and inefficiently, especially if it had been rubbed on the side of the drill hole. Sometimes dynamite as used in coal mines was exploded by a charge of black blasting powder, and thus two different explosives, acting in entirely different ways, were fired in the same hole, whereby a maximum amount of dangerous gases were set free in the air of the mine. When used alone, dynamite is always exploded by means of a "cap," or detonator, and safety fuse or by an electric detonator and battery.

The "cap " lying in a box with its open end upward is often liable to be and has been exploded by a falling spark or by a sudden jar and through its explosion all other detonators and also any dynamite near it are exploded. When fuse is used, it is usually laced through the stick of dynamite, but this is bad practice, because often the dynamite is ignited and burns before the flame reaches the detonator. This burning causes a loss in the explosive force of the dynamite and sets free dangerous gases. Short fuse is also often used with dynamite and as a consequence premature blasts with fatal results frequently Since dynamite is stronger and quicker than powder the miner is often tempted to use it in shooting difficult or improperly placed shots and these attempts often give rise to dust or gas explosions. The use of safety fuse to fire black-powder shots gives rise to evils which are similar to those attending the use of dynamite in blasting coal. While it is true that dynamite has a shorter flame than black powder and is quicker, the reader should not forget that the hot flame from exploding dynamite may cause a gas or dust explosion. There is no work in a coal mine for which dynamite is used that can not be done with a properly chosen permissible explosive.

occur.

For this reason and because of the dangers above mentioned, dynamite should not be allowed in a coal mine.

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PERMISSIBLE OR SAFETY EXPLOSIVES.

The permissible explosives are often called "safety explosives," since they give an extremely short and relatively cool flame, and are therefore not apt to set fire to or explode dust or gas. The term safety" explosive is, however, not a good one, since no explosive can be used safely in the presence of gas or dust, when the charge used exceeds in amount the limits that have been fixed for it by testing. A better term than "safety" for such explosives is shortflame" or "permissible " explosive. No explosive is "safe." If an explosive were perfectly safe it would be useless, because its value. depends on the fact that it will explode when fired under proper conditions.

CLASSES OF PERMISSIBLE EXPLOSIVES.

The four different classes of permissible explosives are described in Miners' Circular 6 of the Bureau of Mines.

These classes are as follows: Class 1, ammonium nitrate explosives; class 2, hydrated explosives; class 3, organic nitrate explosives; class 4, nitroglycerin explosives.

CLASS 1, AMMONIUM NITRATE EXPLOSIVES.

Nitrate of ammonia (ammonium nitrate) is the principal chemical used in this class of explosives; these explosives develop less heat when exploded than some of the others, but they lose sensitiveness and strength by exposure to the air, largely because they absorb moisture from the air. They can not, therefore, be kept ready to use very long; moreover, they generally require the strongest detonators for their explosion.

CLASS 2, HYDRATED EXPLOSIVES.

The hydrated explosives contain substances in which there is a large quantity of water, such as alum, Glauber's salt, and sulphate of magnesia. When the explosion occurs this water produces steam, which lowers the temperature of the flame and thus lessens the chance of its setting fire to dust and gas. However, the lowering of the temperature of the explosion results in a decrease in the force of the explosive, because the strength of an explosive depends on the quantity and temperature of the gases set free at the time of its explosion.

CLASS 3, ORGANIC-NITRATE EXPLOSIVES.

Nitrostarch is the chief base of the organic-nitrate class of explosives. They produce only small quantities of poisonous gases on detonation.

CLASS 4, NITROGLYCERIN EXPLOSIVES.

The nitroglycerin class of permissible explosives is similar to ordinary dynamite, since the chief explosive ingredient is nitroglycerin. These explosives differ from dynamite in that other chemicals are added to the nitroglycerin, so that the explosives give a much shorter and cooler flame than dynamite. The temperature of the gases produced is less than that of the gases from ordinary dynamite. Nitroglycerin explosives freeze at a temperature of 8° to 13° C., (40° to 45° F.), which is above the freezing point of water. This is a great disadvantage for this class of explosives.

COST OF PERMISSIBLE EXPLOSIVES.

The wholesale cost of permissible explosives as sold in 50-pound cases varies from 12 to 14 cents per pound; they are sold to the miner either in 12-pound cartons or by the stick. Ordinary detonators usually cost about half a cent each; electric detonators vary in cost from three-fourths of a cent to 1 cent each, depending on the strength of the detonator and the length of its wires or "legs."

CARE OF PERMISSIBLE EXPLOSIVES.

Permissible explosives differ from black powder in many particulars, and yet like all other explosives they require careful handling. The paper wrapper in which it is inclosed should not be removed from the cartridge of a permissible explosive, and care should be taken not to break the paraffin coating on the outside of the paper cover, for if the paper is removed, or if the paraffin coating is broken, moisture will reach the explosive and cause such a rapid loss of strength and sensitiveness that the explosive will not explode properly.

Only enough explosive for one day's work should be taken into the mine at one time, and none should be left in the mine over night. Explosives should not be carried in the same box or bag with batteries or detonators. More than one man has been killed by putting dry-cell batteries that did not have safety contact buttons in the same sack with electric detonators and explosives, because the wires of a detonator came in contact with the binding posts of the batteries, so that the current set off the detonator and the explosive in the sack.

HANDLING AND STORAGE OF PERMISSIBLE EXPLOSIVES IN MINES.

Because permissible explosives are less sensitive than dynamite some miners handle them carelessly, but, although less sensitive than dynamite, they are still "high" explosives and should be carried, handled, and stored with care. Detonators and explosives should not

be carried together, nor should they be stored in the same place, either outside or inside the mine. Each should be kept separate and by itself until made into a charge.

Accidents have happened through careless handling of explosives while being made into charges. Sparks from miners' lamps have fallen into and exploded detonators, killing those engaged in the work. Naked lights have touched and set fire to explosives. Burning explosives are liable to explode, and for this reason naked lights should be kept a safe distance away from permissible as well as all other explosives.

During the process of manufacture both high and low explosives are handled with the greatest care. Extreme precautions are taken to insure that nothing can occur that may become the slightest factor in causing an explosion. The employees in certain departments of explosives factories are carefully chosen and are highly paid, in order that the greatest care will be taken by them to avoid explosions. On the contrary, when the explosives arrive at the mines they are often treated with the greatest contempt, often metal powder cans are opened with picks, frozen "high" explosives are frequently thawed by being held near the flame of an open lamp, and in some cases high" explosives are actually exploded in the drill hole without any stemming whatever.

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FORCE OF PERMISSIBLE EXPLOSIVES.

Tests to determine the explosive force of the various kinds of permissible explosives are described on pages 103 to 109 of Bulletin 15, Bureau of Mines, entitled "Investigations of Explosives Used in Coal Mines," to which the reader is referred for detailed information concerning this subject.

EXPLOSIONS OF MAGAZINES.

On several occasions mine magazines containing explosives have been destroyed by explosions that resulted from explosives and detonators being stored together, the detonators probably having been set off by roof falls on the magazine.

Most of the explosions of magazines in mines using permissible explosives, so far as the writer's experience extends, have been caused by the shot firer priming cartridges and making charges in the magazine containing the explosives. In the case of one explosion in particular, though reliable evidence could not be obtained because the shot firer and other eyewitnesses were killed, the direct cause of the explosion seemed to have been sparks dropping into the open boxes of detonators, the shot firer having his lamp either on his head or near by. As the detonators or caps are always packed in the box

with the open end upward, and as they are always packed very closely, a spark falling into the box is sure to explode the entire box, and this explosion sets off the other explosives. A fall of some object on the box or the dropping of the box containing the detonators might also start an explosion. Electric detonators should be used in mining on account of their being far safer because the cnds are closed, but they are apt to be set off by a blow from a falling object or by a sudden jar. If they are used, they should be handled very carefully and should be stored separately from the explosives.

THAWING FROZEN EXPLOSIVES.

In cold weather an explosive, especially a permissible explosive of the nitroglycerin class, in being brought from the magazine to the working place may become frozen. When an explosive is frozen, a wooden nail keg full of fresh horse manure is often used to thaw it. The explosive must not be brought into direct contact with the manure, as that may heat the cartridge enough to allow the nitroglycerin to run out of the cartridge, and this may lead to an explosion, but the cartridge of explosive must be put in a small wooden box, which should be placed in the nail keg, the warm manure being heaped around it. The top of the box must be left open in order that the heat from the manure may not become too great and ignite the explosive. This method will probably not be used generally, as some mine managers may think it dangerous, but it is much better than sending large quantities of a frozen explosive into mines and depending on the warm mine air to thaw it.

Often such frozen explosives are used before they are entirely thawed, but there is always a loss of efficiency in doing so, and accidents have occurred from this cause. Although some explosives can be exploded when partly thawed—that is, when the sticks feel soft on the outside though the interior is still frozen hard-the work they do when so fired is not efficient, and there is great danger of accidents during priming and of misfires. If possible, all explosives should be thawed outside the mine. The nail-keg thawer should be used only in extreme cases, as when it is desired to use frozen explosive found in the mine.

A safety lamp should never be used to thaw frozen explosives, as there is great danger of the lamp being left so long in contact with the explosive as to heat it to the burning point.

The proper methods for miners to use in thawing explosives are described in Bulletin 17, entitled, "A Primer on Explosives for Coal Miners," which may be obtained by applying to the Director of the Bureau of Mines.