The Mine Gases. — Since the ventilation of a mine consists of diluting with air any dangerous gases that may be given off, in order to render them harmless, an adequate amount of fresh air must be continuously circulated through the workings. The gases that are given off naturally from the strata, or are formed in the ordinary process of working are: (1) Black Damp, sometimes called Choke Damp and Stythe; (2) Firedamp, or, as it is sometimes called, Light Carburetted Hydrogen or simply Gas; and (3) Sulphuretted Hydrogen, commonly called Stink Damp. Another gas, Carbon Monoxide, or White Damp, is occasionally met with in mines as the result of blasting, or outbreaks of fire underground. In addition, air must be circulated in order to ventilate the mine.

As the various gases found in the mine have different properties, they require to be diluted with different proportions of air to render the atmosphere containing them respirable or safe.

The Coal Mines Act of 1911 deals with the dilution of certain of these gases. Thus mine air must not contain more than 1.25 per cent. of carbon dioxide, nor must it contain less than 19 per cent. of oxygen. In mines where naked lights are used the men must be withdrawn if at work, or prevented from entering the mine, if more than 1.25 per cent. of firedamp is found in the general body of the air.

In mines where safety lamps are used working must not be commenced or continued should any more than 2.5 per cent. of firedamp be found in the air current. All miners should pay strict regard to these regulations and report to the nearest official if they have reason to believe that the atmosphere of the mine is not as it should be. Such action on the part of the miner, especially in mines liable to outbursts of firedamp, may be the means of preventing accidents.

Air. — Strictly speaking, air cannot be classed as one of the mine gases, as it is not given off nor formed in the mine. It must, however, be circulated through the workings to dilute the gases given off, and for this reason it is desirable to look into its properties so far as these affect the health or safety of the miner. Air is principally a mixture of two gases, nitrogen and oxygen, and not a chemical compound, therefore its composition is not absolutely invariable. Despite this fact the relative proportions of the two chief gases vary but little, and the composition of air by volume is 20.93 per cent, oxygen, 78.10 per cent. nitrogen, 0.94 per cent, argon, and 0.04 per cent, carbon dioxide. There are in addition extremely small quantities of the rare elements helium, neon, krypton, and xenon, but these latter may be disregarded as they are of no consequence so far as the mining aspect of the question is concerned.

Air also contains a small but variable amount of water vapour, which is of great importance from the mining point of view. The amount of water vapour present in the air passing through the mine may be ascertained by means of instruments called hygrometers, and a hygrometer must be placed underground in the main intake and return airways in a conspicuous place near the shafts. Readings of these instruments must be taken at intervals by an authorised person and recorded in a book. Several types of instruments are available, but the one in general use is that known as Mason's Hygrometer. This is a simple instrument consisting of two thermometers placed side by side on a stand, as shown in Fig. 56.

One thermometer is exposed direct to the atmosphere, while the mercury bulb of the other is surrounded with a piece of clean muslin, which hangs down and dips into a small vessel containing water. The evaporation of water from the muslin surrounding the bulb causes absorption of heat, and a consequent lowering of temperature, so that the wet thermometer gives a lower temperature reading than the dry one, provided that evaporation is taking place. Whether evaporation takes place or not depends upon the condition of the air. If the air is already saturated with water vapour, no evaporation is possible; if partly saturated, evaporation takes place, the rate, and therefore the lowering of temperature, being dependent upon the amount of water vapour present in the air. The readings thus obtained from the two thermometers are made the basis for the calculation of the degree of saturation of the air at the point where the instrument is placed. If, then, one hygrometer be placed where the air enters the mine, and another where it leaves it, the different degrees of saturation will show whether water vapour is being condensed in the mine or water is being absorbed from the mine. The degree of saturation of the atmosphere at the point where the readings are obtained can be calculated with the aid of Hygrometrical Tables.

Barometer. — This is an instrument for measuring the pressure of the atmosphere. It consists of a glass tube about 3 feet long, and closed at one end. This tube is carefully filled with mercury, and when quite full the open end is temporarily closed and the tube inverted, the open end being placed under the level of the surface of a small quantity of mercury contained in a basin. When the temporary obstruction is removed the mercury column in the tube sinks down until it is balanced by the weight of the atmosphere pressing on the surface of the mercury in the basin. A scale fitted with a sliding vernier is fitted alongside the tube, and by its means accurate readings of the height of the mercurial column are obtained. These readings vary from 28 to 31 inches at sea level, depending upon the density of the atmosphere. The readings of the barometer become smaller as one ascends to heights above sea level, and increase if the barometer be taken down a mine below sea level. Roughly, one inch difference of reading indicates a difference of height of 900 feet between the two points at which the respective readings have been taken.

Barometer readings are of considerable importance in mining, but to be of value they must be used intelligently. While the reading of the barometer may tell the miner of the possible existence of gas in the mine, it should never be forgotten that the gas is itself much more sensitive to changes in atmospheric pressure than the mercurial column. For this reason a fall of the barometer will not forewarn the miner of an increased issue of gas from the strata unless the fall has taken place while he has been out of the mine. In reading the barometer for ordinary everyday purposes connected with mining, it is usual to disregard the temperature. While this is admissible it should be noted that the density of mercury changes with changes of temperature, and if completely accurate results are desired, a correction for these changes will have to be made.

Homogeneous Atmosphere. — The average pressure of the atmosphere is 14.7 lbs. per square inch, this pressure, as already indicated, becomes less as we ascend, the air being of gradually diminishing density. It is not possible to determine how far the envelope of air which surrounds the globe extends into space unless we consider it as having the same density throughout; when this is done the height is referred to as the height of the homogeneous atmosphere. Since one cubic foot of dry air at 32 degrees Fahrenheit and 29.92 inches barometric pressure weighs 0.8072 lbs., it is easy to find the height of the atmosphere when we assume it to have a uniform density.

which is the height of the so-called homogeneous atmosphere.

Irrespirable Atmosphere. — Air when circulated through the workings of a mine may be robbed of a small portion of its oxygen, and at the same time contaminated with gases given off from the strata, or formed during the process of working the coal. Where the air is badly affected in this way it may become dangerous to breathe it; such an atmosphere is usually spoken of as being an irrespirable one. Provided an atmosphere of this character does not contain an explosive gas, the dulling down of the lamp or candle flame may be taken as an indication that it is unfit to breathe. There is still a margin of safety if this course be followed, as the extinction of an ordinary lamp or candle flame does not necessarily indicate that such an atmosphere will not maintain life. Should circumstances arise which make it necessary for a person to enter a contaminated atmosphere, care should be taken not to proceed too far, and to retreat to good air as soon as any poisonous symptoms show themselves.

Carbon Dioxide. — Symbol CO₂ has a relative weight of 22, that is, when under a similar pressure and temperature it weighs twenty-two times as much as an equal volume of hydrogen. The gas is composed of one atom of carbon combined with two atoms of oxygen. These on uniting form one molecule of carbon dioxide, which has the same weight as that of its combining components. The gas has no colour, no smell, but a slightly acid taste, although this latter property is of little importance to the miner, as it is impossible to recognise such quantities as are likely to be present in mine air by this feature. The gas does not support combustion, is irrespirable, and does not burn. When present in small quantities in air it causes the flame of a lamp to burn dimly, while about 3 per cent, soon extinguishes an ordinary flame. A person can still breathe such an atmosphere, hence the flame of his lamp furnishes timely warning to the miner should this gas be present in excess. Because of this there are few fatal accidents from carbon dioxide in mines.

Carbon dioxide is widely distributed in nature; the air contains .04 per cent., or one part in every 2500. It is found pent up in the strata, and is frequently given off during the process of mining. It is also formed by the complete combustion of all of the numerous forms of carbon. The steady discharge of carbon dioxide into the air is as steadily taken up by plants, which by aid of the green colouring matter contained in their leaves extract the carbon to build up their tissues and liberate the oxygen. By continued repetition of this cycle the amount present in the atmosphere is kept very nearly constant. Carbon dioxide is formed in the mine by the oxidation of the coal, the decay of timber, and other causes. The air passing through the mine is thus robbed of part of its oxygen, and gradually becomes contaminated with carbon dioxide. This lessening of the proportion of oxygen leaves an excess of free nitrogen, and it is this excess which mixed with the carbon dioxide is now usually described as Choke Damp. Strictly speaking, Choke Damp or Black Damp, as it is often named, is really mine air deficient in oxygen, but containing an excess of nitrogen and more or less carbon dioxide. The chemical and physical properties of such a mixture must necessarily depend on the relative proportion of the gases present, and cannot be stated in the same definite way as the properties of a chemical compound.

The Mines Act of 1911 requires the air in the mine to contain not more than 1¼ per cent. of carbon dioxide and not less than 19 per cent. of oxygen. As this can only be determined in a truly satisfactory manner by an analysis or chemical examination, its discussion is beyond the scope of this work.

Firedamp. — This gas is also called Methane, Marsh Gas, and Light Carburetted Hydrogen. It is composed of 12 parts by weight of carbon combined with 4 parts of hydrogen, its symbol being CH4. Its relative weight is eight times that of an equal volume of hydrogen under similar pressure and temperature. It is a gas with no colour, no taste, and when pure no smell. It is highly inflammable, but does not itself support combustion. When burned in air it gives a pale bluish-yellow flame, and forms water and carbon dioxide as the products of combustion, thus:

When mixed with twice its own volume of oxygen or ten times its volume of air, 2(N₄ + O), it is highly explosive. With less than six or more than fourteen times its volume of air it does not explode but burns slowly, due to an insufficient supply of oxygen in the one case and an excess in the other. When mixed with more than twenty times its own volume of air it ceases to be inflammable, but shows a large cap or halo of burning gas round a flame. With an increase in the proportion of air to gas this cap diminishes until it becomes difficult to discern even with a non-luminous flame when the proportion of air is one hundred times that of the gas. Although firedamp will not support life if breathed in an undiluted state, or even when it is diluted with from two to three times its volume of air, it is not a poisonous gas, and the miner may breathe with impunity an atmosphere that would be extremely dangerous because of its explosive properties.

Carburetted hydrogen is formed when decay of vegetable matter takes place under water. Thus, if the decaying vegetation at the bottom of a stagnant pool of water be stirred up bubbles of gas will be liberated, and may be collected by filling a wide-mouthed jar with water and inverting it under the surface of the water in the pool. The examination of a small quantity of gas so collected will show (a) that the gas has no colour, (b) that it has neither taste nor smell, and (c) that it will burn when a light is brought into contact with it. Further, an analysis shows it to be composed of one part of carbon and four parts of hydrogen. The finding of this gas under the circumstances just described gives rise to the name of Marsh Gas.

As this gas is found in most coal mines we can readily form an idea of the changes that have brought about its formation. Chemical and microscopic examination shows coal to consist of the remains of vegetation, and as these ancient forests, which now form our coal-seams were submerged, one of the first products of decay was firedamp. Much of this gas would escape at the time of its formation, but part of it became entangled in the decaying mass of vegetation which has since changed into coal. Thus imprisoned, the gas remains in the coal, until by the process of mining the coal is broken up and the gas set free. Gas so occluded (hidden) in the coal often exists at high pressures, and instances are on record where the gas has burst through many feet of solid strata. Gas is always given off in largest quantities from freshly broken ground, and may come off from faults which drain the ground for considerable distances ahead of the workings. Gas also occurs in what are termed pockets, feeders, and blowers. Pockets may be formed by the fall of a soft bed forming the roof of the seam, especially if such a bed be overlain by a stronger one that does not fall at the same time. The space formed between the two beds may fill with gas, which on the fall of upper bed will be wholly or partly displaced into the workings.

Open joints or fissures in the strata occur in many mines. Gas frequently drains out from these joints, and where the issue takes place quietly the stream of escaping gas is called a feeder. When the issue takes place under pressure the escaping gas emits a hissing or blowing sound, and an escape of this nature is called a blower.

The issue of gas from the mine is largely influenced by atmospheric pressure, hence the need for paying strict attention to the readings of the barometer (see page 115). These readings are particularly important where during the operation of working out the coal large empty spaces are left in the waste. Such spaces are likely to become filled with gas, which a sudden fall of pressure will allow to expand, the excess volume so produced being forced into the air current passing through the workings. Firedamp, as it is found in the mine, rarely consists of pure carburetted hydrogen. Other gases are usually found associated with it, although usually in minute traces only. The chief associates are other gases of the hydrocarbon series: Nitrogen, Carbon dioxide, and, occasionally, Sulphuretted hydrogen. When this latter gas is present the firedamp is tainted to an extent sufficient to enable one to distinguish it by smell. This, however, is not a reliable indication of the presence of firedamp, as much of the gas found in the mine has no smell whatever.

Firedamp does not occur in all mines, although the details of its formation already given show that it must have been produced in all cases during the formation of the coal. In coal-seams lying near the surface, or where the overlying strata are of an open or porous character, the firedamp has already drained off. In deep seams, or where the overlying strata are of a less permeable character, firedamp is always given off in greater or lesser quantities.

The presence of firedamp in mines may, when sufficient quantities are present, give rise to disastrous explosions. Mine explosions may be due entirely to the ignition of a mixture of gas and air when gas is present in sufficient quantity, or they may be due to the presence of fine coal dust in the mine which has been raised and ignited by a local gas explosion or by blasting. As a rule all those disastrous explosions that have occurred in coal mines, where the whole of the workings have been swept by the explosion, are due to the presence of finely divided coal dust, and not directly to firedamp alone. Fine, dry coal dust, when raised in a cloud and mixed with air, is now known to be violently explosive, provided the necessary conditions exist for its ignition. A small explosion of firedamp in any part of a dusty mine may raise, and at the same time ignite the dust, and so become the means of spreading death and devastation throughout the whole mine. It will thus be seen that great care must be exercised to detect the presence of firedamp wherever it occurs. The detection of firedamp is carried out by observing its action on the flame of a safety lamp. To thoroughly understand how this is done it will be necessary to examine the nature of flame, and look into the construction and use of the safety lamp.