423

CHAPTER X.

VENTILATION.

Importance. It is most important that mines should be properly ventilated. All coals give off, to a more or less greater extent, a quantity of deleterious gases, which have an injurious effect on the human life, even if they are not explosive. In addition the breathing of men and of other animals, and the burning of illuminants, render air impure. It, therefore, becomes necessary that a vigorous current of cool, fresh air should be circulated through the galleries and working-places; indeed, General Rule I. of the British Mines' Regulation Act, 1887, makes such a current compulsory, but even if such were not so, it really pays to have good ventilation, as men are not only capable of doing a greater quantity of work, but they do it with greater comfort and are far more contented.

The quantity of air required in a given mine depends principally on the volume of gases produced by the coal. The quantity of gas given off does not always increase as the workings become more extensive, as it principally depends on the area of freshly exposed coal surface. If bord and pillar working be adopted, a quantity of gas will be given off while the exploring work is being done, but as the pillars are large blocks of coal, up to 50 yards square or more, the comparatively narrow (2 to 5 yards) places driven to form the pillars, cannot liberate the whole of the gas. When the second or broken workings are proceeding wider places are driven and the coal is got more easily, so that more coal is being worked per man per shift, and more gas is liberated. With long wall working, as the workings get more extensive, a greater area of face is opened out, and consequently more gas will be given off. The best plan is, therefore, to make the amount of air proportionate to the output and to increase the current, if exploratory work is proceeding. It is often stated that the amount of air passing should be regulated by the number of men employed. Of course, if the output increases, the number of men generally increases also, but the better plan is to be guided by the output; this is the practice of the Borinage district of Belgium, which is, probably, the most fiery in the world.

Gases met with in Mines.-Atmospheric air in its pure state is a mixture of two gases, nitrogen and oxygen, but, as generally found in Nature, it contains small quantities of another gas, carbonic acid, and of aqueous vapour. The oxygen is the life-supporting element, the nitrogen acting simply as a diluting agent. Dry air is composed of 79 per cent. by volume of nitrogen gas and 21 per cent. of oxygen. The following table shows the specific gravities, molecular weights, &c., of the gases met with in mines:

In breathing, men and other animals take air into their lungs, and part of the oxygen combines with carbon, forming carbonic acid, whilst the nitrogen is unaltered; the same result follows the burning of lights.

Our knowledge of the composition of the gases met with in mines and of their action on flame and on life has been greatly increased by the researches of Prof. F. Clowes and Dr. J. Haldane. Indeed, the latter's experiments possess an increased value due to the fact that they have in some measure been carried out on human beings. Prof. V. B. Lewes has pointed out that rabbits and other rodents resist the action of gaseous poisoning from carbon monoxide better than carnivorous animals, such as dogs, and it is impossible to argue from the action of the gas upon the one what the action will be upon the other, whilst it has not been made clear that the action upon either is a very safe index as to the action of the gas upon man.

Carbonic Acid is known to the miner as black- or choke-damp, or stythe, but black-damp invariably contains both marsh gas and an excess of nitrogen over the quantity contained in pure air. Its presence is common, and is due to the combustion of illuminants, respiration of men and horses, combustion of blasting explosives, decomposition of pit timber, and often exudation from the coal. the chapter on working, an opinion has already been expressed that oxidation of the coal is one of the chief agents tending to spontaneous combustion, and although matters may not proceed so far as to cause absolute ignition, the action probably goes on in all seams, the oxygen in the air combines with the carbon in the coal and sets free the nitrogen and carbonic acid gas. The latter is chemically composed of carbon and oxygen; its symbol is CO2, and it has a specific gravity of 153. As it is considerably heavier than air, it tends to occupy the lowest part of the mine. In its pure state it has no colour, but has a peculiar sharp, but not sour, odour and taste. Its presence is manifested by its influence on the flame of a candle, as it will not support combustion.

When carbonic acid is mixed in increasing proportions with air, a man breathing the mixture begins to pant long before the point at which a light burning in the same atmosphere is extinguished, as so little as 35 per cent. will affect respiration. This is proved by the experiments conducted by Prof. Clowes * to determine the least proportion of carbonic acid gas in air which is sufficient to extinguish flame. The following table gives the results obtained with naked flame:

*Fed. Inst., vii., 420.

The flames in division I. were burnt from wicks, and show a nearly uniform extinctive proportion of carbonic acid gas, while those in division II., which were burnt from jets, vary to a considerable amount. With an ordinary safety lamp the admixture of 16 per cent. of carbonic aoid with air marks the lower limit of rapidly extinctive atmospheres. As experiments have proved that animals can breathe with impunity in an atmosphere containing 25 per cent. of CO2, it appears that respiration may be maintained in a mixture containing 10 per cent. more carbonic acid gas than is sufficient to extinguish the flame of a safety lamp. A knowledge of this fact should not, however, tempt any one to remain in an atmosphere which will not support combustion, because there are seldom means available underground to readily determine whether 16 per cent. or 60 per cent. of carbonic acid is present.

The above results were obtained by adding carbonic acid gas to air, but black-damp as found in mines contains other gases than carbonic acid, and Dr. Haldane states* that 26 to 27 per cent. is requisite to just affect the breathing, while with about 50 to 55 per cent. the panting is very violent and accompanied with marked distress. Imminent risk of loss of power over the limbs or loss of sensation is reached with about 60 per cent. of black-damp. To produce a mixture which will extinguish a light before it affects the respiration of a man, it is necessary to add to air, along with each volume of carbonic acid, more than three times as much nitrogen. The latter condition of affairs exists underground, as part of the oxygen in the air combines with carbon forming carbonic acid gas, thus decreasing the quantity of oxygen and increasing the nitrogen. Indeed, it is the decrease of oxygen which produces the serious symptoms on breathing blackdamp; confusion of mind, loss of power, and the appearance of a leaden blue colour are undoubtedly due to an insufficient percentage of oxygen to saturate the red corpuscles of blood as they pass through the lungs. On breathing fresh air again recovery is almost instan

*Loc. cit., viii., 849.

taneous, and no after effects were experienced from breathing blackdamp, even after the experimenters had been for a considerable time in an atmosphere which instantly extinguished a lamp.

In some European mines, outbursts of carbonic acid are frequent, and are in every respect similar to those of fire-damp. The blowers are generally found in the vicinity of faults. At Toulane Pit, Rochebelle, one blower filled 500,000 cubic feet of workings in less than ten minutes, and disengaged over 400 tons of coal.

*

Carbonic Oxide is variously known as carbon monoxide and, whitedamp. Its chemical symbol is CO and specific gravity 0 97. Luckily its presence is much less frequent in mines than black-damp, as it is far more poisonous than that gas. As little as a per cent. produces giddiness and faintness, while over 1 per cent. may cause death. Indeed, per cent. breathed for any length of time is fatal. Dr. Haldane states that the smallest percentage of carbonic oxide which visibly affects such small animals as a mouse was o'06 per cent., or roughly speaking one part in 2000. Its presence in the blood could be detected long before it produced any outward symptoms, and it gradually accumulated until a point is reached where the blood was so saturated with carbonic oxide that it would no longer convey sufficient oxygen. Carbonic oxide is known by its sweet and delicate odour and deadly results. Candles burn well in this gas, if anything, a little brighter, although their flame is not elongated until 124 per cent. is present. It is produced by imperfect combustion, and especially by spontaneous ignition and by explosions. From Dr. Haldane's experiments it appears that the first distinct symptoms of poisoning by carbon monoxide, an unusual tendency to giddiness, palpitation, and shortness of breath on exertion, occur when the blood is 30 per cent. saturated. At about 50 per cent. saturation the power over the limbs becomes less and less, and any exertion causes the legs to give way, so that it is hardly possible for a man to make his escape. Finally, death occurs when saturation reaches 80 per cent., consciousness is gradually lost with no pain and little or no mental distress, the action being almost that of a gentle anæsthetic. With about 0.06 per cent. of carbon monoxide in the air, the blood of a man becomes about 30 per cent. saturated after 1 hours; o 10 per cent. would cause helplessness after about an hour; with more than o 20 per cent. life would be endangered.†

Sulphuretted Hydrogen is not found in large quantities. Its chemical symbol is SH, and specific gravity 117. It arises from the decomposition of timber in water containing sulphates in solution, or, to a smaller extent, from blasting, especially if inferior cheap gunpowder is employed. It has an injurious effect on life, does not support combustion, and burns with a blue flame. Its presence is immediately detected by a characteristic and offensive smell.

Light Carburetted Hydrogen.-This gas, known as fire-damp, is the one commonly found in mines, and to which explosions are principally due. It consists, in the pure state, of carbon and hydrogen, its symbol being CH, and specific gravity o559. The fire-damp of the miner is, more correctly speaking, a mixture of several gases, the largest proportion being carburetted hydrogen; other hydrocarbons, with hydrogen, carbonic oxide, oxygen, and nitrogen, are generally *Fed. Inst., ix., 381. + Loc. cit., xi., 506.

present, the quantities of such gases varying considerably in almost every colliery, and often in different districts of the same pit, while its average density is nearly o‘70.

The coal itself is the principal reservoir of the gas, where it is held in the pores or cells in a state of more or less high tension. Different gases exuded from the coal have been described in Chapter i., where the phenomenon of "blowers" was also referred to. Blowers are a more or less steady discharge of gases from the coal, continuing for a long time. Where this discharge is a violent and sudden one of large quantity, only continuing a short time, it is known as an "outburst.

Carburetted hydrogen is colourless, and when pure is odourless, while fire-damp is often detected by its smell. This gas, if breathed in a pure state, would soon cause death; it quickly extinguishes flame or lamps if undiluted by air. When 3 to 4 per cent. is present in air, the gas can easily be detected by the elongation of the flame in a safety lamp. If 6 per cent. be present the flame is not only elongated, but a blue halo or cap, appears above it. This halo is often of a brown colour, produced by the presence of carbonic acid gas with the fire-damp. With 7 to 8 per cent. the mixture becomes explosive, and flame is propagated through the contents of the lamp; with 10 to 12 per cent. the propagation is instantaneous and the explosion attains its maximum amount; with 20 per cent. the mixture no longer explodes, but instantly puts out any flame that may be brought into it.

Messrs. Haldane and Atkinson's experiments* have proved that (1) a mixture of black-damp and fire-damp may, when largely diluted with air, be extinctive to lamps and not explosive, but may become explosive when less diluted; (2) a mixture containing air and 6 per cent. of marsh gas is still explosive, in spite of the presence in it of even a third of its volume of black-damp.

The presence of black damp in a mixture of fire-damp and air, hasthus little or no influence on the explosibility of the mixture, provided that sufficient oxygen is present for the complete combustion of the fire-damp.

After-damp.-Under this head is included all the products resulting from the ignition of an explosive mixture. When fire-damp is exploded, the carbon combines with the oxygen in the air, and forms carbonic acid, while hydrogen also combines with oxygen and forms aqueous vapour, leaving a large amount of free nitrogen without the corresponding amount of oxygen. Carbonic oxide is generally present in after-damp, and is always formed during the combustion of every explosive mixture containing carburetted hydrogen in which the proportion of air is less than 9.5 parts. Messrs. Atkinson† give an analysis of the after-damp gases of the Usworth explosion in 1885. The sample was taken from between two stoppings erected on the fifth and seventh days after the explosion, and contained 2.48 per cent. of carbonic oxide, and 4'54 per cent. of carbonic acid gas; it is probable, however, that the presence of a fire beyond may have had some influence on the production of these gases, the proportions being unusually high.

The combustion of 1 cubic foot of fire-damp renders about 40 cubic * Loc. cit., viii., 559. + Explosions in Coal Mines, London, 1886, 113.