Volume ,in The volume of gas in coal which has been removed from the mine varies greatly. Samples heated to 212° F. gave the following amounts: Condition.—As the volume of gas in coal is in most cases greater than that of the coal and vastly more than that of the interstices or pores, its physical condition is problematical. There must be condensation under such high pressure. When coal is mined and especially when it is crushed, a large proportion of the gas escapes rapidly but much time is required for the complete liberation of the gas as its tension decreases. Chamberlin found on crush ing coal in a vacuum only one-quarter as much gas was given off in the process as was given i off ip vacuum in six months following.
It would seem that powdering the coal would open all of its pores and liberate practically all the gas, but this is not the case. It was found that even under an applied pressure of 75 pounds to the square inch coal samples con tinued to give off gas. No relation has been found between the proportion of gas and the texture of the coal or of the components of the coal as indicated by chemical analyses. Coals highest in "volatile constituents" are not highest in methane.
It has been shown that methane and other gases escape from coal while it lies in the ground, while it is being mined and for a long time after it is mined. The rate of escape varies with different coals and the proportion of methane in the admixture also varies, but in general in the laboratory it is found that the methane is given off somewhat more slowly than the other gases. Samples under different test conditions gave off the gases very unevenly. Crushed samples of bituminous coal from Monongah, W. Va., gave off 1.5 volumes of methane rapidly and then continued to give off a gradually diminishing amount for six months, one-third additional volume coming out in that time. Fine pulverization hastens the emana tion but in most cases did not add to the ulti mate total.
Relation to Weather and Earth Move ments.— It is an old and deeply-seated idea that there is close connection between the weather and the amount of fire damp in mines, and miners generally regard increased gas emana tion as an indication of the approach of stormy weather. The idea has been prev alent also that explosions in mines are more frequent at times of low atmos pheric pressure. Sir F. Abel has shown that this is not the case for in explosions from 1875 to 1:.:5 involving the loss of 2,729 lives only 17.4 per cent of the mortality was at a time when the barometric pressure was below the average and half of the explosions occurred when the pressure was increasing. It is now known, however, that many of the explosions were caused by dust and had no relation to in crease of gas in the mine. Diminution of atmospheric pressure undoubtedly affects the liberation of gas from coal surfaces and es pecially from old workings and crevices where gas has accumulated, but it cannot materially affect the gas under high pressure in the body of the coal. Investigations in English, French,
Prussian and Austrian mines all showed in crease of methane in the returns when atmos pheric pressure diminished, especially if the change was rapid, but in nearly all • the tests there were extensive old workings containing more or less gas. Tests with coal samples showed no relation up to a diminution of pres sure amounting to one and one-half inches of mercury.
It has been claimed that earthquakes, sun spots and other natural disturbances affect the emanation of gas in coal mines but investiga tion has not shown any such relation. G. H. Darwin has shown that variations in atmos pheric pressure cause up-and-dawn movements of the earth's crust which may amount to sev eral inches in vertical amplitude. These might cause slight cracking in some places and so affect gas emanation. From a consideration of a large amount of evidence the author believes that ordinary earth movements do not materi ally affect the emanation of methane in mines unless possibly to precipitate an outburst that is about ready to take place; ordinarily a blast in a nearby chamber would cause much more local movement than an average earthquake.
Poisonous Gases.-- The physiologic effect of gases in mines varies greatly. Any air in which the oxygen content falls below 7 per cent will not support human life and half the normal amount or 10% per cent is dangerously low, so that whenever air is mixed with an excess of inert gases it may prove fatal. Methane and carbon dioxide are probably not toxic and per haps no more deleterious than the harmless nitrogen, but their proportion may become so great in the mine air that they will not support human life. It is claimed by some physiologists that carbon dioxide has slight toxic effect but this is by no means proved. Carbon monoxide, however, which is produced in certain amounts in the ignition of fire damp and coal dust, in explosions and by mine fires is highly poison ous. It is the toxic agent in the afterdamp resulting from explosions that kills many miners and is very dangerous to those endeavoring to rescue the men imprisoned in the mine. Air containing a very small proportion of carbon monoxide will cause death, especially if the pro portion of the oxygen is diminished. The amount that can be endured by a person de pends mainly on his vitality and the length of time he breathes the gas, but no one can sur vive more than a few minutes in air contain ing one-half of 1 per cent. The effect is cumulative and half this proportion would be fatal if breathed for a long time. No gas masks will satisfactorily remove the gas al though it is absorbed by blood and decom posed by certain copper compounds under fa vorable conditions. Removal from the bad air and vigorous long-continued artificial respira tion, especially with oxygen, may save a person who has succumbed, but the chances of re covery are much less than in suffocation by carbon dioxide or other inert gas.