12 atmospheres would cause no harmful effects due to oxygen. The specific gravity of oxygen at 760 mm. (sea level) and 0° C. (32° F.) is 1.1054, and its weight in grams per liter under the same conditions is 1.4291,13 or 0.0892 pound per cubic foot.

Oxygen becomes a liquid at 183° C. below zero; it also boils or vaporizes at this point.14 Liquid oxygen is used in some forms of self-contained oxygen breathing apparatus and also to an increasing 'extent as an explosive for blasting overburden in stripping; liquid oxygen, however, should not be used for underground blasting in coal mines.

CARBON DIOXIDE (CO2)

Carbon dioxide is a product of the decomposition and combustion of organic compounds and also of the respiration of men and animals. It is a colorless and odorless gas which, when breathed in large quantities, may cause a distinctly acid taste. It will not burn or support combustion. Carbon dioxide is usually found along the floor in low places and abandoned workings and is a normal constituent of mine air, being diffused in the ventilating current. The proportion of carbon dioxide in mine air is naturally increased by the process of breathing, by the burning of flame lamps, by fires and explosions, and by blasting. It has been found emanating from the rock strata in underground workings of certain mines, notably the Cripple Creek (Colo.) and Tintic and Park City (Utah) districts as well as in numerous other metal-mining regions.15 Carbon dioxide is an important constituent of black damp, and traces of it are always present in normal air. It has long been used as a measure of the impurity of air, but its importance as such has probably been overemphasized. One-half of 1 per cent (0.5) of carbon dioxide in normal air causes a slight increase in the lung ventilation; that is, & man exposed to this percentage of carbon dioxide will breathe a little deeper and a little faster than when in pure air. If there is 2 per cent of carbon dioxide in the air, the lung ventilation will be increased about 50 per cent; if 3 per cent, about 100 per cent; if 5 per cent, about 300 per cent, and the breathing is laborious; and if 10 per cent, it can not be endured for more than a few minutes.16 Carbon dioxide in air has these effects when the oxygen content

1 Burrell, G. A., and Seibert, F. M. (revised by G. W. Jones), Sampling and Examination of Mine Gases and Natural Gas: Bull. 197, Bureau of Mines, 1926, p. 102. 14 International Critical Tables, vol. 1, 1926, p. 102.

Sayers, R. R., and others, Mine Rescue Standards: Tech. Paper 334, Bureau of Mines, 1923, 44 pp. McElroy, G. E., Rock-Strata Gases in Mines of the East Tintic Mining District, Utah: Repts. of Investigations, Serial 2275, Bureau of Mines, 1921, Denny, E. H., Marshall, K. L., and Fieldner, A. C., Rock-Strata Gases of the Cripple Creek District and Their Effect on Mining: Repts. of Investigations, Serial 2865, Bureau of Mines, 1928, 24 pp.

3 pp.

10 Sayers, R. R., and others, Mine Rescue Standards: Tech. Paper 334, Bureau of Mines, 1923, p. 22.

remains about normal and the subject is at rest. Moving around or working would naturally increase the symptoms, and they would be much more dangerous than when a man is resting. The specific gravity of carbon dioxide at 760 mm. (sea level) and 0° C. (32° F.) is 1.5291, and its weight in grams per liter under the same conditions is 1.9768,17 or 0.1234 pound per cubic foot. Carbon dioxide becomes a liquid at 78.50° C. below zero; it also boils or vaporizes at this point.18 Carbon dioxide is used in various industries, and liquid carbon dioxide is now employed in a special permissible cartridge for blasting coal.19

METHANE (CH.)

Methane, also known as "marsh gas" and "carburetted hydrogen," is the chief constituent of fire damp. It is colorless, odorless, tasteless, inflammable, and nonpoisonous. Methane is found in almost all coal mines and occasionally in metal and other mines. In coal mines it may issue from the cleats or cracks of the coal, from "blowers" or "feeders," or from overlying or underlying strata and generally is held in the coal when irregularities, such as clay veins, "horsebacks," or faults, are penetrated. In metal mines methane is frequently found in tunneling through carbonaceous shales and occasionally is present by infiltration into metal mines at contacts or near carbonaceous rocks. Its common occurrence and explosibility when mixed with air are directly or indirectly responsible for numerous mine disasters. The specific gravity of methane at 760 mm. (sea level) and 0° C. (32° F.) is 0.5545, its weight in grams per liter under the same conditions 0.7168 20 or 0.04475 pound per cubic foot, and its boiling point 195.8° C.21 As stated, methane is odorless; but because it is frequently accompanied by other admixtures of gases, such as petroleum vapors, the admixture has a distinct odor. Moreover, its common occurrence in the old workings of mines, where the air may be musty from decaying timbers and other impurities, has caused many mining men to believe that it has an odor. Methane is usually found in mines near the roof or in high places. However, after becoming thoroughly mixed with air it may be found in practically equal proportions anywhere in a traveling-air current. It may be detected with a flame safety lamp, and the approximate amount present may be gauged by the cap on the lamp flame. It may also be detected by various other approved detecting devices, such as the Burrell methane indicator, Martienssen methane detector,

17 See footnote 15.

18 See footnote 14.

19 Tiffany, J. E., A New Permissible Blasting Device: Repts. of Investigations, Serial 2920, Bureau of Mines, 1929, 8 pp.

20 See footnote 5.

21 See footnote 14.

or Union Carbide Co.'s methane-indicating detector. Methane has no direct physiological effect upon men, but it may accumulate in mine workings to an extent that will dilute the oxygen of the air to an amount which will cause a man to suffer from oxygen deficiency. Several deaths by asphyxiation have been caused by men unknowingly entering high concentrations of methane. Air that contains 5 to 15 per cent of methane will explode if ignited.22 Methane is not the only cause of mine explosions. Dry coal dust suspended in air is explosive, but coal-dust explosions are propagated more rapidly when methane is present in amounts much below the lower explosive limit. Bureau of Mines engineers believe that a mixture of 200 cubic feet of methane, or possibly less, thoroughly diffused with air, will precipitate a general explosion if the mixture is ignited in the presence of coal dust. The amount of methane should therefore be kept as low as possible by proper ventilation. The safest practice is to keep the methane content of every split below 0.5 per cent at all times.23

BUREAU OF MINES CLASSIFICATION FOR COAL MINES

The Bureau of Mines believes that all coal mines are potentially gassy; but for purposes of administration in respect to prevention of explosions and fires decision No. 3 (1926) of the mine safety board of the bureau recommends the following classification:

Class 1 coal mine.-A practically nongassy mine in which inflammable gas in excess of 0.05 per cent can not be found by systematic search.

Class 2 coal mine.-A slightly gassy mine in which (a) inflammable gas has been found," but in amounts less than 2 per cent in still air in any active or unsealed abandoned workings; (b) inflammable gas can be found, but in amounts less than 4 per cent, in some place from which the ventilating current has been shut off for a period of 1 hour; (c) inflammable gas can be found, but in amounts less than 4 per cent, in a split 2 of the ventilating current; or (d) inflammable gas enters a split 2 of ventilating current at a rate" of not more than 25 cubic feet per minute.

26

26

25

Class 3 coal mine.-A gassy mine in which inflammable gas is found in amount greater than specified for a class 2 coal mine.

General notes-(a) The inflammable gas found in coal mines is, with rare exceptions, methane. In coal-mining fields where natural gas is found in lower geological horizons by deep wells that pass through or nearby the coal mines there have been rare instances of a leakage from the well. Natural

Coward, H. F., and Jones, G. W., Limits of Inflammability of Gases and Vapors: Bull. 279, Bureau of Mines, 1928, p. 87.

Rice, G. S., Safety in Coal Mining: Bull. 277, Bureau of Mines, 1928, p. 28.

By using an approved flame safety lamp, with flame drawn low, or by using an approved gas detector, or by sampling and analyzing with an appproved gas analytical apparatus.

By sampling and analysis with an approved gas analytical apparatus, or by using an approved gas detector.

If but one continuous ventilating current is used in a mine, this shall be considered a split" for the purpose of this definition.

Determined by sampling, analysis, and ventilating-current measurements.

gas is chiefly methane; almost always more than 85 per cent is methane, but it usually contains ethane, propane, and traces of butane. Therefore, if the latter gases are found in mine air it is an indication of leakage. The lower limit of explosibility of methane-air mixtures with about 10 per cent ethane and associated hydrocarbon gases is 4.6 per cent. The limit, therefore, varies with the character of mixture.

(b) To determine the proper classification of a coal mine, it is advisable that systematic testing and sampling be done at least three times in a period not less than 72 hours. All tests and samples of the mine air except one must show an inflammable-gas content less than the maximum limit of the class to which the mine is assigned. In other words, a tolerance of one test or analysis may be permitted to provide for a mistake or a very exceptional occurrence.

(c) When a new mine is being opened in a coal field where existing mines are generally gassy, it is common sense to assume that similar conditions will be found in the new mine, and its development and equipment should be based upon the expectation that it will be assigned to class 3.

CARBON MONOXIDE (CO)

Carbon monoxide, frequently but inappropriately called "white damp," is a product of the incomplete combustion of carbonaceous materials. The term "white damp" is misleading, because the gas is colorless and therefore invisible. Generally speaking, it does not support combustion and can not be detected by a flame safety lamp until the concentration becomes so high that persons without respiratory protection would be overcome almost immediately. The specific gravity of carbon monoxide at 760 mm. (sea level) and 0° C. (32° F.) is 0.9672, and its weight in grams per liter under the same conditions is 1.2504,28 or 0.07805 pound per cubic foot. The boiling point of carbon monoxide is 192° C.29 Carbon monoxide is produced during mine fires, by explosions of gas or coal dust, and in blasting or in the burning of explosives. Air that contains 12.5 to 74 per cent of carbon monoxide will explode if ignited.30 Carbon monoxide in excess of 0.01 per cent may eventually produce symptoms of poisoning, and 0.02 per cent will produce slight symptoms in several hours. When 4 parts in 10,000 (0.04 per cent) is present and the exposure is for 2 or 3 hours, headache and discomfort usually occur. With moderate exercise when 0.12 per cent is present it will produce slight palpitation of the heart in 30 minutes, a tendency to stagger in 11⁄2 hours, and confusion of mind, headache, and nausea in 2 hours.31 According to W. P. Yant, a concentration of 0.20 to 0.25 per cent will usually produce unconsciousness in about 30 minutes. Its effect in high concentrations may be so sudden that a man has little or no warning before he collapses.

28 See footnote 5.

See footnote 14.

30 See footnote 22.

Burrell, G. A., Chart of Properties of Mine Gases: Bureau of Mines, 1918.

HOW CARBON MONOXIDE ACTS

Carbon monoxide exerts its extremely dangerous action on the body by displacing oxygen from its combination with hæmoglobin. Hæmoglobin is the coloring matter of the blood which normally absorbs oxygen from the air in the lungs and delivers the oxygen to the different tissues of the body, which require it to do their work. The affinity of carbon monoxide for hæmoglobin is about three hundred times that of oxygen. Because of this, even when only a small amount of the poisonous gas is present in the air breathed into the Jungs, the hæmoglobin will absorb the carbon monoxide in preference to the oxygen that may be present. When carbon monoxide is absorbed by the hæmoglobin it reduces the capacity of the hæmoglobin for carrying oxygen to the tissues. Due to lack of oxygen, these can not do their work properly. The blood acts as a carrier of oxygen from the lungs to the needed parts of the body.

The blood may be considered as a complex liquid containing millions of particles (corpuscles) which, for analogy, may be thought of as little boats that have on board an iron compound called hæmoglobin, which has a strong attraction for oxygen. These little boats come into the lungs in the blood stream, take on a cargo of oxygen, and carry it to parts of the body where it is needed. After depositing their cargoes of oxygen they take on waste products from the body (mainly carbon dioxide) and return to the lungs, where these products are discharged and more oxygen is taken on board. What happens when a very small amount, say, 0.01 per cent, of carbon monoxide is present in the air which is breathed into the lungs? The hæmoglobin on board the little boats likes carbon monoxide approximately three hundred times as well as it likes oxygen; in other words, if 1 carbon monoxide molecule is present to 300 oxygen molecules it has just as much chance of getting on the boats as the oxygen, and in time 50 per cent of the little boats would be loaded with carbon monoxide molecules. These molecules apparently have roundtrip tickets and do not get off unless crowded out by larger proportions of oxygen; thus they prevent the proper amount of oxygen from being carried to the needed parts of the body. Continued exposure to 0.01 per cent of carbon monoxide will cause about 17 per cent of these boats to be filled with carbon monoxide, so that the remaining 83 per cent must do the work that 100 per cent originally was doing. This may cause the blood-transportation system to be overtaxed and result in a "run-down" condition of the body. When the exposed person gets into fresh air the preponderance of oxygen eventually drives out all the carbon monoxide, although the rate is much slower than that at which the monoxide molecules were taken up by the blood. If there is continued exposure to carbon