explosives of this class are somewhat similar in composition to the ordinary low-grade dynamites, except that one or more salts containing water of crystallization are added to reduce the flame temperature. They are easily detonated, produce only small quantities of poisonous gases, and most of them can be used successfully in damp working places.

Class 3. Organic Nitrate Explosives.-To Class 3 belong all the explosives in which the characteristic material is an organic nitrate other than nitroglycerin. The permissible explosives listed under class 3 are nitrostarch explosives. They produce small quantities of poisonous gases on detonation.

Class 4. Nitroglycerin Explosives.-To Class 4 belong all the explosives in which the characteristic material is nitroglycerin. These explosives contain free water or an excess of carbon, which is added to reduce the flame temperature. A few explosives of this class contain salts that reduce the strength and shattering effect of the explosives on detonation. The nitroglycerin explosives have the advantages of detonating easily and of not being readily affected by moisture. On detonation some of them produce poisonous and inflammable gases equal in quantity to those produced by black blasting powder, and for this reason they should not be used in mines or working places that are not well ventilated.

CARE OF EXPLOSIVES

Storing Explosives.-Dynamite cartridges should always be laid on the side and not stood on end, for in the latter position the nitroglycerin may ooze out from the dope and collect in the bottom of the cartridge. Dynamite should never be kept for any length of time (as in storage magazines) at a temperature greater than 75° F. It should be stored in a dry place having a reasonably uniform temperature. Magazines should be heated by means of hot-water or exhaust-steam pipes, never by a stove or live-steam pipes. There should, preferably, be two powder houses or magazines at a mine. The main magazine, holding the stock of explosive on hand, should be built sufficiently far from the plant or with some natural obstruction (such as a hill) between it and the plant that its accidental explosion may not injure the miners' village or the surface equipment. This magazine is commonly under the direct supervision of some one of the higher officials and is usually opened only to receive supplies in carload lots direct from the manufacturer and to withdraw the daily amount required by the men. Near the mine mouth is a smaller magazine, commonly called the powder house, to which the day's supply is taken from the main magazine, and where it is handed out to the men individually. All main magazines should be proof against high-power rifle bullets fired at short range. Storing explosives in large quantities in a mine is a bad and dangerous practice and in most states is prohibited by law.

The effect of storing fuse for several days at temperatures much above or below the normal is to greatly retard its rate of burning; commonly to the extent of 50% or more. It follows that fuse should not be stored in too warm a place as over boilers and steam pipes in winter, or in a tin box exposed to the direct rays of the sun in summer, nor should it be left in an unheated tool house when the temperature is much below the freezing point. Fuse is extremely difficult to dry out after wetting, and, as a general rule, fuse that has been stored in damp places or has in any way become wet should not be used in mining.

Thawing Dynamite.-Dynamite freezes at about 45° F., and when solidly frozen it is exploded with difficulty, and if it is exploded the detonation is only partial. It is dangerous to cut, break, or ram a frozen dynamite cartridge, as the frozen nitroglycerin crystals may explode. No attempt should be made to explode dynamite that has been frozen until it has been thoroughly thawed and is soft and plastic; many accidents occur through failure to observe this precaution. If incomplete detonation occurs, unexploded powder is often found in the holes or in the material blown down by the shot.

In cold weather, the cartridges should not be taken to the place where they are to be used until all the holes are ready to be loaded, and all cartridges should be soft and warm when charged into the holes. Dynamite that has been chilled, but not frozen, looses a large part of its efficiency. Many instances are on record in which some of the holes of a blast were loaded with warm, and others with frozen, or partially frozen, dynamite; the dynamite that had been warmed exploded and that which was frozen did not, and miners have subsequently been killed or injured by drilling into these misshots.

When thawing dynamite, it is necessary to use caution to keep the temperature from rising very high, as each degree rise is that much nearer the danger limit where extreme sensitiveness to shock prevails. The thawing of dynamite by placing it in a tight box surrounded by manure is a good method if the manure is fresh so that it is giving off heat. Dynamite should never be thawed before an open fire, on a shovel, in a tin can, or in an oven, for, while dynamite will very frequently burn in the open and when unconfined, it very often explodes. Also, it should never be thawed by immersion in hot water as that has a tendency to leach out the nitroglycerin and make it dangerous. The common practice of thawing dynamite cartridges by passing them through an oven or over a lighted candle is very dangerous.

When dynamite is being used on a large scale during the winter, it is well to provide a special thawing room, in which 1 or 2 days' supply can be kept ready for use. The room need not be of great size, say 12 ft.X16 ft. The powder needed for the day's consumption can be carried in during the afternoon and left over night, the boxes simply being opened or the explosive taken out and put on shelves, the procedure depending on the time available. A thermometer should be consulted to insure that the temperature does not rise above 85° and is preferably kept between 75° and 80° F. If a brick or stone vault is made below the surface and tightly roofed over and banked with earth, dynamite may be kept in it all winter without freezing.

For handling smaller quantities of frozen dynamite, special thawing kettles are used to advantage. Öne device consists of a metal can having tubes that pass through it. The tubes are surrounded by water, and the whole so arranged that a miner's lamp or candle may be placed underneath the can to keep the water warm. When in use, the tubes are filled with sticks of dynamite, the space surrounding them is filled with water, and the cover slipped over so that the cartridges cannot fall out of the tubes. A lamp or candle placed under the can will soon heat the water sufficiently to thaw out the cartridges. These thawers, being portable, are very convenient and, filled with hot water, will keep dynamite in good condition for some time without being artificially heated. It is, however, of great importance that the water receptacle should always contain water, otherwise an explosion may occur. A double thawing kettle commonly used consists of an outer kettle standing on legs and an inner kettle, which is held up by a bead around the edge, in which the cartridges are placed.

Handling Explosives.-While the dangerous practice still prevails to some extent of daily opening the main magazine and there handing out to the men all the explosives they may demand, this method has been very largely superseded by the much safer plan of taking the estimated total daily mine consumption to the small powder house near the opening where only enough for the day's work is given each miner. The amount of explosive is charged to the men either from entries made by the person in charge of the powder house or from written orders or receipts given by the men themselves. At some mines, the men purchase in advance several dollars' worth of so-called powder checks, which are pieces of metal stamped with a number of value (12) c., 25 c., 50 c., etc.) and which may be exchanged at the powder house for explosives and other blasting supplies. After the men have taken their requirements, the explosive remaining is commonly returned to the main magazine, although this may not be done until late in the afternoon for fear it may be needed.

The amount of explosive that a miner may have in his possession or carry into the mine at one time is commonly regulated by law. In rare instances there is no limit placed upon this amount, although a miner will seldom carry in more than a single keg of 25 lb. Where there is a limit, a keg is purchased. but is kept in the magazine, and from it the miner draws his daily allowance of from 5 to 10 lb., which he carries to his working place in a metal canister, preferably of copper. Where permissible powders are used, a day's supply should not exceed 5 lb., as this amount of powder exploded in properly placed holes will, in a seam of average thickness and when undercut as it should be, bring down all the coal a man can load out on one shift.

It is a dangerous practice to carry large metal cans of black powder into the mine upon the shoulder or in mine cars where electric wires charged with current are strung along the roadway. Cars in which powder is being transported should be hauled by mules when the current is shut off the wires. At the working face, the powder can should be stored in a wooden box separate from the caps and fuse. The box should be kept locked when not in use and should be placed some 100 ft. from the face and 25 ft. from the track if this is possible.

Where shot-firers are employed who charge the holes, the mine is commonly divided into a series of districts of a size that two men can charge and fire all the holes therein in a reasonable time. The explosives estimated to be enough for the various districts are placed in separate boxes at the main magazine, conveyed to the nearest point to where they are to be used in a car hauled by a mule, and are there unloaded by the district shot-firer and his helper.

Precautions When Handling Coal-Mining Explosives.-The Bureau of Mines suggests that the following precautions should be observed in handling black powder:

Never open a metal keg of powder with a pick or metal object; use the opening provided by the manufacturer of the keg.

Never make up charges or handle cartridges or powder with an open light on the head; place the light at least 5 ft. away on the return-air side so that sparks from it will not fall into the powder.

Never allow powder or other explosive to remain exposed; keep it in a welllocked box at least 100 ft. from the working face and in an unfrequented place. Never go nearer than 5 ft. to a powder box or powder when wearing an open light or when smoking.

Never use coal slack or coal spalls for stemming; it is dangerous. Use moistened clay, wet wood pulp, or other noninflammable material; even wet coal slack may, under some circumstances, cause an explosion.

Never withdraw a shot that has missed fire; drill a fresh hole at least 2 ft. from it but parallel to the old hole and fire this new hole. After the shot a careful search should be made for the unexploded charge to prevent its being struck by a pick and perhaps causing an explosion.

Never fire the hole the second time; if the first charge proves useless powder and labor are wasted in loading the hole a second time. Moreover, the first shot often cracks the coal so much that the second shot has a chance to blow out of the cracks, and thus a blown-out shot may result.

Never use iron or steel tampers or needles; have at least 6 in. of hard-drawn copper on the tamping end of the bar or, better still, use a hardwood tamping stick. The needle should be made entirely of hard-drawn copper.

Never tamp shots with an iron or steel scraper and do not push a cartridge into the drill hole with the scraper; the scraper rod should be tipped with at least 6 in. of brass or copper on the scraping end.

Never allow the point of the coal auger to become dull or to become of less than the standard gauge, so that a drill hole may be made with it into which the cartridge may always be pushed freely.

Never drill a hole past the loose end, chance, or cutting in solid shooting; if the coal has been undercut, do not drill beyond the undercutting. It is better to stop at least 6 in. short of the solid coal.

Never bore gripping holes; keep the holes parallel to the ribs or as nearly so as possible. Use the side gear on the machine if you can when boring a hole. Never guess at the quantity of powder to be used; always measure it. This course is cheaper and better than guessing. Use cartridges rather than loose powder and make them of cartridge paper. Don't use newspaper for cartridge making.

Never place black blasting powder in the same drill hole with dynamite or a permissible explosive.

Never use short fuse; always have the fuse long enough to stick out at least 2 in. from the mouth of the drill hole. When the short fuse is lit any gas in the hole may be ignited, and this may result in a premature blast.

Never bite a piece of the match off the squib, nor oil it to make it burn faster.

Never use sulphur and gas squibs at the same working face.
Never light two or more shots at the same time.

Never fire shots in adjoining working faces at the same time.

Never return to a shot that has failed to explode until at least 10 min. after lighting it, if squibs were used, or 12 hr. after lighting if fuse was used. When shots are fired electrically be sure that all wires are disconnected from the battery, and wait at least 5 min. before returning to the face.

Never fire a rib or butt shot before a center or busting shot is fired; the opening shots should be fired first, in order to give the succeeding shots a chance to do their work.

Never drill a hole near the remaining portion of a former shot, nor near cracks and fissures made by previous shots, because there is great danger of the powder gases on explosion flying out of the loose coal or the cracks and igniting gas or dust in the mine air.

Never use squibs or any kind of fuse, except electric fuse, in mines that make inflammable gases.

Never fire a shot without making sure that the coal dust near by is well wet down.

Never light a dependent shot at the same time as another shot, and never fire a dependent shot until the first shot has broken properly.

Never fire a split shot; that is, never fire a hole that has been drilled into a mass of coal, cracked and shattered by a previous shot that failed to dislodge the coal.

The following additional precautions are to be observed in handling permissible explosives:

Never take more than 1 da. supply of permissible explosives into the mine at one time.

Never leave permissible explosives in the mine over night.

Never purchase permissible explosives not suited to the coal bed.

Never use weak detonators.

Never fire a charge until it has been completely and carefully tamped.

Never put black blasting powder and permissible explosives together in the same drill hole.

Never break the covering of a cartridge of a permissible explosive until ready to charge.

Never expect permissible explosives to yield entirely satisfactory results when coal is blasted off the solid.

Never expect the first blast with permissible explosives in a newly opened coal bed to be satisfactory. Several trials are often required before satisfactory results are obtained.

Never forget that permissible explosives are different from dynamite and entirely unlike black blasting powder.

Never use fuse to fire permissible explosives when it is possible to use electric firing.

FIRING EXPLOSIVES

MEANS OF FIRING LOW EXPLOSIVES

Ordinary black blasting powder may be ignited by means of squibs, fuse, and electric squibs. High explosives, including permissible powders, are fired with fuse and caps, or, preferably, by means of electricity, and by means of an electric detonator; in the latter case the current is derived either from a battery or from a dynamo. Fuse and caps are commonly employed for firing single holes, and electric firing is used where several holes are fired at once, or in a volley, although portable electric blasting machines (formerly called batteries) are to be preferred even for firing single holes.

Squibs. A squib (sometimes known as a match, reed, rush, spire, etc.) consists of a small paper tube that is filled with quick-burning powder and has a slow match attached at one end. The burning of the slow match gives the miner time to get to a place of safety between the time that he lights the match and the time that the flame reaches the quick powder. When the quick powder is ignited by the burning match, the squib shoots like a rocket through the hole that has been left in the tamping by the withdrawal of the needle into the blasting powder. Two kinds of squibs are in general use, gas squibs and sulphur squibs. In the gas squib, the match end is impregnated with a composition that does not flame when burning but glows throughout its length, and, for this reason, is supposed to be safe in an explosive mixture of methane and air. In the sulphur squib, the match end is dipped in sulphur and burns with a flame and somewhat faster than a gas squib. Since, in the rocket-like action that is necessary to propel the squib into the charge, large volumes of sparks are given off, neither type of squib can be safe in explosive atmospheres.

Fuse. Fuse, sometimes called safety fuse or Bickford's fuse, from its inventor, consists essentially of a central core of fine-grained gunpowder wrapped about by threads of hemp, jute, or cotton. These threads are wound in two sets, the inner being known as the spinning threads and the outer as the counter-threads or, simply, countering. In single-tape fuse, the threads are wound with tape and then coated with tar and covered with fuller's earth or powdered tale to prevent sticking. Double-tape fuse is single-tape fuse wound with a second layer of tape, which is also tarred and powdered. Cotton or hemp fuse, not tape wound, is made, but is only suitable for use in absolutely

dry places and in hot climates. In cold countries, fuse covered with tar is apt to crack and thus become wet and misfire, while in hot countries it becomes sticky and unfit for use. For these reasons special fuses are manufactured for use in either arctic or tropical regions. For use under water, gutta-percha covered fuse has been made.

According to the work in which it is intended to be used, fuse may be divided into four classes. Fuse of the first class is suitable for dry work such as stump blasting and quarrying; it is usually untaped hemp and cotton fuse. Fuse of the second class is intended for damp work, as in coal mining, or in surface work where mud, rain, or dampness is encountered; it is commonly of the single tape variety. Fuse of the third class is suitable for very wet work, such as tunneling, shaft-sinking, etc. Fuse of the fourth class is designed for submarine work; double-tape, triple-tape, gutta-percha, and taped doublecountered fuse belong to these classes. Owing to the large amount of carbonaceous material in the wrappings of fuse, the gases produced by its burning contain a large proportion of carbon monoxide, CO. Where numerous coils of fuse have caught fire and burned, as has happened in magazines and in the rooms of poorly ventilated mines, the gases evolved have been found to be particularly suffocating and poisonous.

The rate of burning of the better grades of American fuse has been determined by the Bureau of Mines to be very nearly 30 sec, per ft. of length, with a variation of some 10% either way.

Electric Squibs.-For use with black blasting powder only, electric squibs are made. They are similar to electric blasting caps in appearance, but the cap is made of paper instead of copper and the charge does not detonate but shoots out a small flame. They are made with iron wires 4, 5, 6, or 8 ft. long, and with copper wires of the same lengths as well as 10 and 12 ft. The wires are insulated and the space between them in the squib is bridged with fine platinum wire, which glows when the electric current is applied and furnishes enough heat to ignite the powder. They are designed to ignite the powder at the center of the charge, something that is obviously impossible with the ordinary paper squib.

MEANS OF FIRING HIGH EXPLOSIVES

Fuse and Caps.-Dynamite and other detonating explosives, including permissible powders, may be fired by means of detonators or caps, but are best exploded by means of electric detonators of the strength prescribed for each one. Caps, blasting caps, detonators, or exploders, as they are variously called, consist of copper capsules, about as thick as an ordinary lead pencil, that are commonly charged with dry mercury fulminate or with a mixture of dry mercury fulminate and potassium chlorate that is compressed in the bottom of the capsule, filling it to about one-third its length. Several grades of these detonators are on the market, and they are differently designated by different manufacturers. A strong detonator is essential to securing the perfect explosion of permissible powders, etc., and for this purpose those of No. 6 strength, containing 15.4 gr. of charge, are recommended. Fulminate of mercury is extremely sensitive to heat, friction, and blows, and for these reasons blasting caps should be handled with as much care as dynamite, or a violent explosion may result.

The following precautions are recommended in handling them:
Never attempt to pick out any of the composition.

Do not drop caps or strike them with anything hard.

Do not step upon caps or place them where they may be stepped upon. When crimping caps on to the fuse, take care not to squeeze the fulminate, and never crimp with the teeth.

Caps should be stored in a dry place and in a separate building from any other explosives.

Caps should not be carried into the mine with other explosives, or placed near other explosives except

in a bore hole.

Electric Detonators. To overcome the dangers incident to the use of fuse and squibs, electric deto

nators (also called electric blasting caps) have been devised. These are simply ordinary detonators that have been fitted with the means of firing them with the electric current. This is done, as shown in the accompanying figure, by inserting within the caps two copper wires d, joined