RATE OF DETONATION. The energy developed by the detonation of permissible explosives, like that of other high explosives, depends on the change of the small solid and liquid particles of the explosive into large volumes of highly heated gases and on the rate of detonation or the rapidity with which these gases are formed. The force exerted by these gases is the means of producing useful effects. The rate of detonation is the governing factor in judging the efficiency of an explosive as well as offering the best means for selecting explosives suitable to meet the varying conditions of coal mining. During the conversion of an explosive into solid, liquid, and gaseous products the cooling produced by the walls of the drill hole tends to lower the temperature of the gases so that the maximum theoretical temperature, or pressure, is never reached. The more nearly instantaneous the occurrence of the explosive reaction, all other conditions being equal, the greater the volume of highly heated gases that will be produced, and the more violent the effect. To meet the varying conditions of coal mining in this country the explosives manufacturers have devised explosives with rates of detonation that range from 4,750 to 14,560 feet (1,447 to 4,439 meters) per second. SUGGESTIONS USEFUL IN SELECTING EXPLOSIVES. It is hoped that with few exceptions the classification given will serve as a useful guide for comparing the practical value of permissible explosives. However, in order to select the most suitable, users of these explosives should conduct a series of experiments in the mine. The information given herein should eliminate the necessity of testing many different classes and brands of explosives. It is evident that for certain work in which a shattering effect is desired, as in driving through or "brushing" rock, or in producing coal for coking purposes, the explosive reaction should be rapid. Hence, permissible explosives having a high rate of detonation should be selected. Similarly, for use in soft friable coal to produce lump or steam coal, selection should be made of a permissible explosive that develops its gases at a slow rate, thus insuring the development of a more prolonged pressure. In medium hard coal an explosive having an intermediate rate of detonation would be expected to be most suitable, but is not always so. Coals vary in hardness and coal beds vary in the number and position of the joints, partings, shale bands, etc. These facts have to be considered in mining coal. An explosive having a very low rate of detonation is not always best suited for mining soft friable coal, because its energy may be lost by its gases escaping through cracks and fractures in the coal. Under such conditions an explosive having an intermediate rate produces the most economical results. Another factor to be considered in connection with an explosive having a high rate of detonation is its possible effect on the roof or overlying strata of the coal bed. When large charges of explosives having a very high rate of detonation are used the local effect is marked. Such charges produce small fissures that may later necessitate extra timbering to prevent falls in rooms or entries, and thus make the operation of a mine more costly. It is well known that the pressure developed by the detonation of explosives in a closed space is directly proportional to the charging density; in other words, a 13-inch drill hole loaded with 14-inch cartridges will produce on the walls of the drill hole about one-half as much pressure per square inch as it would if loaded with cartridges of 13-inch diameter. A limited experience indicates that explosives having a rapid rate of detonation will yield a larger proportion of lump coal if used in a hole of larger diameter than the cartridge. Such air spacing to reduce the shattering effect of an explosive is recommended by the Bureau of Mines, provided moist clay stemming is used to confine the charge and is tamped to the mouth of the drill hole. The Bureau of Mines will be glad to learn the results of more extended experience with air spacing. Other less desirable means of reducing the shattering effect of an explosive are the use of an improper detonator, reducing the amount of stemming used in a drill hole, using an explosive that is frozen or partly frozen, using an explosive in cartridges of less diameter than those originally tested, and introducing foreign substances between the cartridges of an explosive; but these methods are all dangerous. They not only eliminate the safety qualities of the explosives but also increase the chance of a resultant dust or gas explosion and should not be adopted. CLARENCE HALL, Expert in Charge Explosives Section. Approved January 20, 1912. JOSEPH A. HOLMES, Director. PUBLICATIONS ON MINE ACCIDENTS AND TESTS OF EXPLOSIVES. The following Bureau of Mines publications may be obtained free by applying to the Director, Bureau of Mines, Washington, D. C. : BULLETIN 10. The use of permissible explosives, by J. J. Rutledge and Clarence Hall. 1912. 34 pp. 5 pls. BULLETIN 15. Investigations of explosives used in coal mines, by Clarence Hall, W. O. Snelling, and S. P. Howell, with a chapter on the natural gas used at Pittsburgh, by G. A. Burrell, and an introduction by Charles E. Munroe. 1911. 197 pp. 7 pls. BULLETIN 17. A primer on explosives for coal miners, by Charles E. Munroe and Clarence Hall. 61 pp. 10 pls. Reprint of United States Geological Survey Bulletin 423. BULLETIN 20. The explosibility of coal dust, by George S. Rice, with chapters by J. C. W. Frazer, Axel Larsen, Frank Haas, and Carl Scholz. 204 pp. 14 pls. Reprint of United States Geological Survey Bulletin 425. BULLETIN 26. Notes on explosive mine gases and dusts, by R. T. Chamberlin. 67 pp. Reprint of United States Geological Survey Bulletin 383. TECHNICAL PAPER 4. The electrical section of the Bureau of Mines, its purpose and equipment, by H. H. Clark. 1911. 12 pp. TECHNICAL PAPER 6. The rate of burning of fuse as influenced by temperature and pressure, by W. O. Snelling and, Willard C. Cope. 1912. 28 pp. TECHNICAL PAPER 7. Investigations of fuse and miners' squibs, by Clarence Hall and S. P. Howell. 1912. 19 pp. TECHNICAL PAPER 11. The use of mice and birds for detecting carbon monoxide after mine fires and explosions, by George A. Burrell. 1912. 15 pp. TECHNICAL PAPER 12. The behavior of nitroglycerin when heated, by Walter O. Snelling and C. G. Storm. 1912. 14 pp. 1 pl. TECHNICAL PAPER 13. Gas analysis as an aid in fighting mine fires, by G. A. Burrell and F. M. Seibert. 1912. 16 pp. MINERS' CIRCULAR 2. Permissible explosives tested prior to January 1, 1911, and precautions to be taken in their use, by Clarence Hall. 1911. 12 pp. MINERS' CIRCULAR 3. Coal-dust explosions, by George S. Rice. 1911. 22 pp. MINERS' CIRCULAR 4. The use and care of mine-rescue breathing apparatus, by J. W. Paul. 1911. 24 pp. MINERS' CIRCULAR 5. Electrical accidents in mines; their causes and prevention, by H. H. Clark, W. D. Roberts, L. C. Ilsley, and H. F. Randolph. 1911. 10 pp. 3 pls. 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