use of electricity, and ventilation. A brief outline of a few typical occurrences follows:

1. In Chicago in 1913 several gas explosions occurred while a tunnel was being driven for water; as a result a number of workers were burned and some killed. The gas, which was ignited in one instance by the breaking of an incandescent light globe, may have been leakage from city gas mains.

2. In Milwaukee in 1914, while a water tunnel was being driven several thousand feet under Lake Michigan, a worker ignited a body of methane in lighting a cigaret; 29 men were burned, although, fortunately, none were killed. Samples of air from the tunnel were analyzed and found to contain appreciable percentages of methane.

3. In 1914, while sinking for a foundation for a bridge over the Mississippi River, nine men in a caisson were killed by an ignition of methane, probably caused by an electric arc.

4. In Cleveland in 1916, while driving a water tunnel under Lake Erie, 9 men were killed in an explosion of gas, probably ignited by electrical equipment; an hour later 10 more were killed while trying to reach the first 9 victims. This tunnel was found to contain appreciable amounts of methane, which was seeping into it from the surrounding sedimentary strata. During the remainder of the work on the project strict precautions were taken against gas accumulations as well as against sources of gas ignition.

5. In 1918, while driving a tunnel near San Francisco, five men were killed by an explosion of methane, probably caused by an electric arc from an opentype storage-battery locomotive. Some investigators, however, held that the ignition was due to sparks between the wheels of the storage-battery locomotive and the track rail while an attempt was being made to move a very heavy load.

6. In Kansas City in 1926 an explosion of methane in a tunnel being driven for water resulted in the death of eight men. Ignition of the gas was undoubtedly due to an electric arc, either from an open electric switch or open storage-battery locomotive.

7. In 1927 a number of ignitions of methane occurred in a tunnel being driven at Berkeley, Calif. In one, 11 men were burned rather severely, although none died; the ignition was probably caused by a smoker's match. although smoking was against the rules, and all of the men denied that they had been smoking.

8. In connection with the driving of shafts and tunnels for the Hetch Hetchy water project near San Francisco several ignitions of methane have occurred in which some men have been killed and others burned; one rather serious fire also resulted from this work."

As methane and other gases occur so frequently in shallow shafts and tunnel work in and around cities, especially where the work is done in sedimentary formations or adjacent to or under rivers or lakes, every State should have definite and stringent regulations governing such work. Generally, these matters are left to city or State engineers, few of whom know even the rudiments of the hazards of gas in subsurface workings, of effective methods of ventilating such workings in the event of an occurrence of gas, or of proper means of detecting explosive gas or preventing its ignition if it should be encountered.

HIGH-NITROGEN OR CARBON DIOXIDE AND LOW-OXYGEN GASES

Gases high in carbon dioxide or nitrogen, or both, and low in oxygen are likely to be found in long unventilated dead ends in any kind of mine. This sort of atmosphere may be expected (though it may not always be present) in long unventilated dead-end workings which are raises or dips, especially vertical dips (winzes), or

Harrington, D., Progress in Metal-Mine Ventilation During 1930: Inf. Circ. 6469, Bureau of Mines, May, 1931, 17 pp.

HIGH-NITROGEN AND LOW-OXYGEN GASES

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are level or approximately level. These low-oxygen atmospheres high in carbon dioxide and nitrogen are formed in mines in numerous ways; although they are more likely to be found in metal than in coal mines, they are by no means absent from the latter.

RICO DISTRICT, COLORADO

Recently an occurrence of irrespirable gases in a metal mine in the Rico district, southwestern Colorado, was investigated by the United States Bureau of Mines. Irrespirable gases have been found in this district virtually since the beginning of mining there and have frequently impeded work and constituted a hazard to miners' lives; the breathing of such gases in small amounts probably has affected efficiency as well as the general health of the miners.

The ore is valuable chiefly for its silver and gold content. Considerable pyrite occurs in and near the vein; limestones, shales, and sandstones characterize the rock formation of the region. Gases that would extinguish a carbide lamp and have distinctive odors have been found in parts of the mine for years, occurring in greater quantity during the summer months. About 1,700 feet from the portal of the fourth level is a vertical winze said to be 65 feet deep. At the time of this investigation the heat increased noticeably as one approached the winze, and there was a distinct odor of hydrogen sulphide at the top. The light of a carbide lamp and a candle were extinguished at the top, and apparently some of the gases in the winze were flowing out and into the tunnel and through connections to the third level. Air samples were taken with vacuum bottles about 1 foot below the collar of the winze and analyzed by the Pittsburgh laboratory of the United States Bureau of Mines. The analysis showed 49.2 per cent of carbon dioxide, 10.8 per cent of oxygen, and 40 per cent of nitrogen, with no methane. Although hydrogen sulphide gas was evidently present, as indicated by the characteristic odor of spoiled eggs, the amount was too small to be disclosed by chemical analysis.

At the breast of the tunnel some air samples were taken; on analysis these showed about 6.5 per cent of carbon dioxide and 19.55 per cent of oxygen. One experienced difficulty in breathing the air, evidently due to the carbon dioxide, as the oxygen in the air was ample to support life. This property of increasing the rate of breathing with resultant physical discomfort is characteristic of carbon dioxide, especially where the temperature is high. The amount of sulphur dioxide present as determined by odor was too small to be determined analytically. A dry-bulb temperature of 99° and a wet-bulb temperature of 79° were recorded.

The excessive heat in this mine, especially in the sections with the irrespirable gases, makes it fairly certain that chemical action, with heat liberation, is proceeding rapidly in the strata. In places numerous vugs and open fissures occur in the formation; it is believed that in this mine, as in some other mines investigated by the bureau, the gases formed and heat produced are a result of the action of atmospheric air and mine water upon the sulphides and carbonates in the fissured rock. In this particular mine there was no record of any sudden liberation of large amounts of gases, but gases were present in considerable amounts in parts of the mine

at all times. As in other districts, the mine management had noted that storm conditions and prevailing winds from certain directions increased the trouble from gases. It was stated that a number of years ago inflammable gas had been ignited with an open light at the breast of the third level.

CRIPPLE CREEK DISTRICT, COLORADO

As detailed in published reports of the United States Bureau of Mines, similar gas occurrences were investigated in the Cripple Creek gold-mining district of Colorado." Here the coroner's records show that at least 35 lives have been lost by suffocation in irrespirable gases during some 25 years of mining. Such gases are mainly nitrogen and carbon dioxide, present in varying amounts. rule the mixture of gases is much heavier than air, but occasionally gases high in nitrogen and consequently lighter than air are found at the top of raises rather than near the floors of drifts, levels, and winzes.

Much of the mining in this district is done in or adjacent to a porous breccia, and the veins mined are often fissured and open. This probably accounts for the fact that gases are liberated suddenly and in considerable volume from the strata upon a slight lowering of atmospheric barometric pressure and disappear again almost as quickly when the barometer rises and weather conditions clear.

In Cripple Creek natural ventilation is relied upon to a large extent; numerous deep shafts with water-level tunnels driven to the surface at points considerably below the collars of the shafts aid in bringing about a copious but variable natural circulation of air, especially during certain seasons. Some of the smaller mines particularly troubled by gases have for years used a pressure system of holding the gases back in the strata; a slight pressure induced by a blower fan or an injector is usually sufficient. To maintain such pressure the tops of shafts are bulkheaded and fitted with doors which may be opened temporarily as ore and rock are raised. Similar systems have been used in drifts in which gases were encountered; one or more bulkheads with doors were erected at points near the mouth of the drift, and a fan or injector was installed to maintain the pressure within the bulkheaded area. Fan-pipe installations are also used to dilute the gases and carry them out of the mines, but small units of this type have not always proved satisfactory.

In one gas occurrence investigated by the Bureau of Mines, in which two men lost their lives, air samples collected by means of a vacuum bottle in the shaft showed less than 0.3 per cent of oxygen,

per cent of carbon dioxide, and 81 per cent of nitrogen, although the air in this same shaft had been entirely free of gas a few hours before. One idle mine of considerable extent was entirely free of gas during good weather, but when a storm arose the mine filled with carbon dioxide and nitrogen so that the light of a carbide lamp was extinguished 20 feet outside the portal. Sulphur dioxide was de

Denny, E. H., Marshall, K. L., and Fieldner, A. C., Rock-Strata Gases in the Cripple Creek District and Their Effect on Mining: Rept. of Investigations 2865, Bureau of Mines, 1928, 24 pp.

Denny, E. H., Marshall, K. L., Fieldner, A. C., Emery, A. H., Yant, W. P., and Selvig, W. A., Rock-Strata Gases of the Cripple Creek District, Colorado, and Their Effect on Mining Bull. 317, Bureau of Mines, 1930, 66 pp.

GASES IN TIMBERED MINES

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tected only once and then by odor only. Tests of large samples of various rocks from the district sealed in glass carboys with atmospheric air and ordinary tap water or mine water showed that gases similar to those found in the mine could be produced in the carboys. The action of water and air upon the rocks for two or three days produced appreciable percentages of carbon dioxide and decreased the oxygen content of the air. After several months all of the oxygen in the air in carboys containing certain rocks was consumed; the residual gases were carbon dioxide and nitrogen. The rocks of the Cripple Creek district probably do not contain more than the average amount of pyrite and other sulphides found in other metal-mining districts, but the fissured and porous nature of the rock affords an opportunity for rapid chemical action when moist air comes in contact with such sulphides.

ROCK-STRATA GASES

UTAH

The Bureau of Mines investigated the occurrence of rock-strata gases in the East Tintic mining district in Utah. The gases found in these mines occurred mainly with low barometer and changing weather conditions and caused much trouble until mechanical means of ventilation were introduced. Rock and air temperatures in these mines were abnormally high. Some of the gases contained more than 60 per cent of carbon dioxide, whereas other gases were almost pure nitrogen and consequently so light that they accumulated in raises.

NEVADA

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In several Nevada mines gases high in nitrogen have been found to issue in quantities from freshly cut fissures. They are found in raises and have caused at least one fatality.

OTHER LOCALITIES

Fatalities have resulted from rock gases in Gilpin County, Colo.; Park City, Utah; Picher, Okla.; and many other metal-mining districts in the United States. Occurrences of irrespirable rock-strata gases in metal mines in England, France, New Zealand, and Australia have also been recorded, some of them of relatively recent date.

GASES IN TIMBERED MINES

In addition to metal-mine gases formed by chemical action on the rock strata and released by changes in pressure, few timbered metal mines are free from gases high in carbon dioxide and nitrogen and low in oxygen in long blind ends or winzes. In sections of metal mines in which there is little or no circulation of air, a condition frequently found in damp, timbered places where no work or travel

McElroy, G. E., Rock-Strata Gases in Mines of the East Tintic Mining District, Utah: Rept. of Investigations 2275, Bureau of Mines, 1921, 3 pp.

Gardner, E. D., Rock-Strata Gases in Mines of a Nevada Mining District: Rept. of Investigations 2427, Bureau of Mines, 1922, 4 pp.

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is being done, some sort of reaction is likely to take place between the damp timber and the oxygen of the air whereby the oxygen is absorbed and its place taken largely by carbon dioxide. When this reaction has continued until the oxygen contact of the air is below 10 per cent the condition of the atmosphere is decidedly dangerous, and when the oxygen content is below 5 per cent a few inhalations result in unconsciousness and probably death. Damp, timbered winzes that have been long abandoned but have not filled with water are decidedly dangerous, as the oxygen-depleted air usually is high in carbon dioxide, which seeks the lowest available place. Even if the gas does not form in the winze itself, oxidation may take place in a level or other working above the winze and the heavy carbon dioxide flow like water into the winze. Anyone who goes into an abandoned, unventilated blind end, whether raise, winze, or level, takes a chance of losing his life unless he tests the air thoroughly or has adequate respiratory protection.

High percentages of carbon dioxide and nitrogen and a low percentage of oxygen are found almost invariably at some time or place in connection with fires in metal mines, including not only quick, hot fires but also long-drawn-out partly sealed or sealed fires. One should always keep in mind the fact that if oxygen is deficient no type of gas mask can be worn safely and that the only available respiratory protection against gases low in oxygen is up-to-date oxygen breathing apparatus or some other device that furnishes oxygen as well as excludes the asphyxial gases.

SULPHUR GASES IN METAL MINES AND TUNNELS

In general, sulphur gases occur much more frequently and in much more dangerous concentrations in metal mines and tunnels than in coal mines. There have been a number of occurrences (some relatively recent) of the decidedly dangerous hydrogen sulphide gas in comparatively shallow shafts for foundations for bridges over rivers. In some instances workers have been killed and in many others they have been made decidedly ill because sufficient precautions were not taken against this gas, which is dangerous to life even in very low percentages in the air breathed. Recently hydrogen sulphide caused loss of lives in a shallow shaft in connection with waterworks tunnels at Detroit.

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In blasting material relatively high in sulphur or material in which the sulphur is loosely bound or readily oxidized or altered many workers have been gassed, some fatally, from fumes in which hydrogen sulphide was the most dangerous constituent, although it may have been accompanied in some instances by other noxious gases, such as sulphur dioxide, carbon monoxide, and various oxides of nitrogen. In blasting material whose sulphur content exceeds 30 per cent of the whole explosions of sulphide ore dust (chiefly sulphides of iron and copper) have occurred; the resultant gases are high in hydrogen sulphide and sulphur dioxide, and possibly other sulphur gases. Sometimes sulphur gases are found in explosives fumes in metal-mine blasting, even when there is no sulphur in the material being shot

Gardner, E. D., Jones, G. W., and Sullivan, J. D., Gases from Blasting in Heavy Sulphides Rept. of Investigations 2739, Bureau of Mines, 1926, 8 pp.