January 12, 1917. THE COLLIERY GUARDIAN 71 BLACKDAMP IN MINES.* By G. A. Burrell, I. W. Robertson, and G. G. Oberfell. The Bureau of, Mines, in pursuing investigations look- ing to greater safety in mining, has analysed samples of the air in many different coal mines in the United States, and has studied the analyses. The present report shows how atmospheric air, after entering a coal mine, loses oxygen and gains carbon dioxide, with resulting formation of so-called blackdamp. The effects of the constituents of blackdamp on men, on the burn- ing of oil and acetylene lamps, and on the explosi- bility of methane, are also discussed. The term “ blackdamp ” was, and still is, widely used to designate accumulations of carbon dioxide, but a more exact definition of blackdamp, as Dr. Haldane has pointed out, is an accumulation of carbon dioxide and nitrogen in proportions larger than those found in atmospheric air. Comparison of Atmospheric Air with Mine Air. When atmospheric air enters a coal mine, it changes in composition according to (1) the velocity with which it traverses the workings; (2) the amount of coal with which it comes in contact—-that is, the extent of the mine workings that it traverses; (3) the gaseous nature of the seam; (4) the tendency of the coal to absorb oxygen; and (5) the temperature and wetness of the mine. As regards the details mentioned, the governing con- ditions are as follow:—(1) Other things being equal, more carbon dioxide and methane are present, and there is a greater deficiency of oxygen at places where the air is still, as at working faces and in old workings. (2) The purity of . the air depends on the distance it has travelled, so that in a well-ventilated mine the air is purer at working faces near the shaft than at those that are remote.. In the same mine the air will be fresher and purer when the mine is ventilated by a split system than when the air traverses each working face and entry. (3) Some mines vary greatly as regards the generation of methane. Methane is not only dangerously inflammable, but, if introduced in large proportions at some parts of a mine, it lowers the oxygen content of the atmosphere to such an extent that the atmosphere will not support the combustion of lamps, or even will not support life. (4) Coals differ as regards their power of absorbing oxygen. Absorption of oxygen is never accompanied by a molecular propor- tional increase in the amount of carbon dioxide pro- duced, for the oxygen combines with certain unsaturated hydrocarbon compounds in the coal. (5) The amount of water vapour in air is principally a function of the temperature of the air; hence, if air that is compara- tively dry enters a mine, if the temperature of the mine is higher than the outside temperature, 'and if the mine is comparatively wet, then the proportion of water vapour taken up by the mine air may be greatly in excess of that in the atmospheric air. ' In mines other than coal mines the chief factors that decrease the oxygen content of the mine air are : Velocity of the air current, the extent of the workings traversed by the air, the amount of oxygen taken from the air, and the amount of carbon dioxide or other suffocating gases added to it by the decay of mine timbers, the burn- ing of lights, the breath of men and of animals, and the oxidation of minerals, and ’also by the gases given off from the rocks penetrated. The report proceeds to deal with the composition of the atmosphere; the transfer of oxygen and carbon dioxide in breathing; the effects, on man, of variations in composition and amount of air; the effect of atmo- spheres low in oxygen on canaries and mice ; the rela- tive value of birds and mice in exploring mines; the effect of temperature and moisture on man; the venti- lation of buildings above ground; the action of moisture on coal; the effect of changing proportions of oxygen and carbon dioxide on the explosibility of fire- damp mixtures; and the changes in the composition of air by breathing and combustion, the results of the work of English and other investigators being given at length. Ventilation Conditions in Mines. In coal mines carbon dioxide arises principally from the action of the air on the coal, and 0-2 to 0-3 per cent, is frequently found in the cool, swiftly moving air cf returns, where 50,000 or more cu. ft. of air is passing per minute, and the wet bulb or dry bulb temperature does not exceed 65 degs. Fahr. The relative humidity may be high, almost 100 per cent, in many mines, owing to artificial methods of watering to allay . coal dust, but where the temperatures are not high, the high humidity has no bad effect. Hence a proportion of carbon dioxide that would be easily tolerated in a coal mine might be indicative of extremely poor venti- lation in the room of a house. A similar statement applies to oxygen. The oxygen content is scarcely ever normal in a coal mine, owing to the absorption of oxygen by the coal; in fact, a dimi- nution of 1 per cent, is not uncommon. Such a diminution in an assembly room would usually be accompanied by intolerable conditions of ventilation. However, the proportion of oxygen cannot be dimi- nished too much even in mines. It is the authors’ experience that as little as 1.9 per cent, of oxygen is rarely found in the moving air of coal mines. How- ever, at some of the working faces of poorly ventilated mines, only 17 per cent, of oxygen is found. In mines other than coal mines, carbon dioxide is given off by the burning of lights, by the decay of mine timbers, and, in some mines, by the rocks penetrated. * United States Bureau of Mines, Bulletin No. 105. At poorly ventilated working places in some metal mines the carbon dioxide content may be dangerously high, or the oxygen content dangerously low, or both conditions may prevail. An oxygen content as low as 17 per cent, is itself without harm, for people live at altitudes where the pro- portion of oxygen Corresponds to this percentage at sea level. The air of many health resorts contains even less oxygen by weight than does the air of Denver, but7 the air at those 'resorts is fresh, moving, and invigbr-' ating, and usually rather dry. Mine air that contains so small a percentage of oxygen is in no sense com- parable to the air of these health resorts. If the air in a part of a certain mine contains only 17 per cent, by volume of oxygen, not enough fresh air is finding access there; that is, the air is more or less stagnant, perhaps with . all ■ the attendant, evils discussed previously. The difference between ventilation conditions in buildings above ground and in coal mines is in the degree of vitiation of the air as regards low oxygen and high carbon dioxide; above ground, relatively small oxygen and carbon dioxide changes—a few tenths of 1 per cent.—usually represent bad air; below ground these small percentages are insignificant. In England, the law requires that the oxygen content shall not fall below 19 per cent., or the carbon dioxide content rise above 1-25 per cent, in any part of the mine. Mines in America can easily meet these requirements. The amount, of fresh air per man introduced into coal mines far exceeds that allotted by 'architects 'and engi- neers for ventilating public buildings. As a basis on which to calculate the air necessary for proper venti- lation of public buildings, one may consider the carbon dioxide produced by an adult as 0-6 cu. ft. per hour. Hence, in 6,000 cu. ft. of air, he will produce per hour a pollution amounting to 1 part of carbon dioxide in 10,000 parts of air. If 'the excess of carbon dioxide were to be kept down to this figure, it would be neces- sary to supply 6,000 cu. ft. of fresh air per hour; if the permissible excess was to be 2 parts in 10,000, half this supply would suffice, and so on. The amount of fresh air allowed by ventilating engineers in America varies from 1,800 cu. ft. per hour per person for school build- ings, auditoriums, theatres, factories, etc., to 12,000 cu. ft. per hour per person in surgical and contagious hospitals. In the non-gaseous bituminous mines of Pennsyl- vania, the minimum quantity of air per man is 150 cu. ft. per minute, or 9,000 cu. ft. per hour. In a mine where explosive gas is generated in dangerous quan- tities, the minimum is 12,000 cu. ft. per hour per man. These quantities exceed the quantity stipulated in the ventilation of large public halls. Consequently, in the moving airways of such mines the air is fresh and wholesome. The -temperature is seldom high, and usually bad ventilation exists only at some working faces where the air does not find ready access. • There has been considerable discussion regarding a scheme proposed in England to lower the oxygen con- tent of the air in mines to a point (about 17 per cent.) where the risk from explosion of mixtures of methane and air is reduced. Most mining men are opposed to such procedure on physiological grounds. The authors’ experience has indicated that the oxygen content has to be reduced below this figure to lessen appreciably the liability of gas explosions. Some investigators have argued that disastrous coal dust explosions of recent years can be attributed largely to the better ventilation, of mines at present as com- pared with the ventilation found years ago. To make such a comparison, just, involves the consideration of too many factors to warrant discussion here. There is truth in the statement, at least as regards mixtures of methane and air, to the extent that greater violence is exerted by an explosive mixture if the latter has a high initial velocity of its own. The authors have seen this demonstrated many times in an explosion gallery of their own construction. But an unbiassed considera- tion of evidence accumulated through long experience must lead one to accept the necessity of plenty of fresh air. Whether’for a miner working underground, a man at a health resort, or an assemblage in a large public building, all the fresh air that can reasonably be obtained is needed. - Specific Gravity and Composition of Blackdamp. The specific gravity of blackdamp varies considerably. When methane is present, the combined gases may be lighter 'than air. Great caution should be exercised when any accumulation of blackdamp lighter than air is found, especially .in mines worked with naked lights, as the lesser density is probably due to the presence of methane. A sample of the following composition was collected by the authors from a cavity in a room of a coal mine : Results of Analysis of Sample of Mine Air of much Less Density than Atmospheric Air. GO Per cent. 0-35 kJV-/2 * v.. 15-82 ^2 gh4 19-32 N2 64*51 100-00 ' Owing to the large amount of methane present, the specific gravity (air = 1) of this sample was only 0*91. The analysis of another sample showed an accumula- tion, of methane and a deficiency of oxygen in an enclosed section of an anthracite mine that had been seated for six days because of a fire in an adjoining section. The fire did not affect the particular area from which, the sample wus obtained because of a heavy inter- vening roof fall; consequently the sample represented the gases trapped in a stagnant section unaffected by fire. The results of analysis follow :—...... Results of Analyses of Samples of Gas in an Enclosed Area of an Anthracite Mine. Per cent. C02 ................................ 2*6 O2 .......................... 3-0 CH4 ................................ 53-0 N2 ........................ 41-4 100-0 The specific gravity of this mixture (air. = 1) was only 0-77, owing to the large proportion of methane. Mixtures containing a large amount of carbon dioxide are often found on the floor and in low-lying workings. It happens occasionally that the gas is in a heavy stratum with lighter air above, and these strata are at times so. sharply defined that a lighted candle is extinguished at once by lowering it only 1 in. below a certain level. Equally distinct stratification of gases may be encoun- tered in metal mines. A mixture containing more carbon dioxide, and being consequently heavier than normal air, may lie near the floor, or a mixture con- taining less oxygen and more nitrogen, and being con- sequently lighter than normal air, may accumulate near the roof or in a raise. Either the heavier or the lighter mixture, dangerous to the miner, may be so well strati- fied that a few inches will make the difference between breathable air and suffocating gases. Before starting some work near the floor, a miner may hang his lamp well up in the roof, where by burning it indicates that the air near it is breathable; while the air near the floor may be so bad that it would extin- guish the lamp immediately. However, except in air directly over a fire area or close to a fire, a large amount of carbon dioxide (over 5 per cent.) is unusual in a coal mine. The small proportion of carbon dioxide nor- mally in mine air is indicated in many analyses of samples, though exceptions are quoted in which the carbon dioxide percentages were 14-42, 14-36, 19-60, and 19*33, and the specific gravities 1*051, 1*051, 1*071, and 1*069. The samples were collected from sealed mines in which there were or had been fires. The authors’’ experience as a result of analysing gas samples collected from many mines has indicated that if ’the carbon dioxide results from the action of the oxygen of the air on the coal (including perhaps a small amount released from the pores of the coal), it is not produced in proportions exceeding more than 3 to 5 per cent., even after the air has long been in contact'with, the coal. ' '.■■.•nm. .,< • The following results of analyses show how small the ' carbon dioxide content may be in the air of a mine that has been sealed for- nine, months':— Results of Analyses of Gas Samples from a Mine Area THAT HAD BEEN Sealed for Nine Months. Sample No. 1. Sample Nb. 2. Per cent. Per cent. C02 / 1-50 1-20 04 .: ; 0-30 0-30 ch4 5-29 ... 5-37 N3 92-91 ... 93-13 Specific gravity 100-00 ... 100-00 0-95 ... ' 0-94 The collectors of the. samples wore breathing apparatus. The oxygen had ’almost entirely disap- peared, but only 1*20 and 1*50 per cent, of carbon dioxide were present. ■ Other points'dilated upon in the report are the effect of contaminated air on. lights and mine-fires; and after giving a large series of analyses.of mine air, the authors conclude by summarising the whole question as follows : Summary. The most satisfactory definition of the term ‘ ‘ black- damp ” is an accumulation of carbon dioxide and nitrogen in excess of the percentage found in pure atmospheric'air. The principal factors that affect the changes in mine air.are: (1) The velocity with which the mine air traverses the mine passages; (2) the amount of coal with which it. comes in contact.; (3) the gaseous (methane) nature of the-seam; (4) the nature of the coal as regards its power to react with oxygen; (5) the temperature and . the wetness of the mine. Carbon dioxide must be present in large proportions before it threatens life. A proportion of 3 to 4 per cent, of carbon dioxide in air affects the breathing of most people. Men may, however,, work for a long time in such an atmosphere, although their.efficiency as work- men will be greatly affected, and they will become fatigued quickly. The presence in air of as little as 1 or 2 per cent, of carbon dioxide is not so much a matter of safety and comfort to. those who breathe it 'as it is of their efficiency as* workmen. ; Distress is caused in some people when the oxygen content falls to less than 13 per cent. Rapid breathing is produced much more quickly by an excess of carbon dioxide than by a corresponding deficiency of oxygen. The important point to remember is that rapid breath- ing caused by carbon dioxide starts long before there is any serious danger, whereas rapid breathing caused by 'a deficiency of oxygen is a grave symptom, and points urgently to serious danger. By acclimatisation people live the year round at high altitudes where the air has an oxygen content, by weight, that is the same as that of an atmosphere at sea level containing 12 per cent, oxygen by volume. People unaccustomed to such atmospheres, if suddenly plunged into them, experience severe distress. In an experi- ment conducted by the authors a man lost consciousness temporarily when the oxygen content of an atmosphere he breathed fell to 7 per cent. Mice and canaries are about as resistive to low oxygen atmospheres as men ; hence they cannot be used by exploring parties to give warning of atmospheres that are dangerously low in oxygen.