Chapter XXXVIII: The Air of Mines

The air of mines of whatever kind is extremely liable to vitiation, the nature and extent of which is of considerable importance in relation to the health and safety of those working in the mine. The impurities met with are known to the miners under such names as “black-damp,” “fire-damp,” “after-damp,” “white-damp,” “gob-stink,” etc.; and these terms will as far as possible be adhered to in the present account of the subject.

In all mines a current of air to all parts is secured by means of the arrangement of the shafts and roads. One shaft, known as the “downcast,” serves for the supply of fresh air, which is distributed by means of “intake” roads to the working places. The more or less vitiated air then passes along a corresponding system of “return” roads or passages to the “upcast” shaft, by which it leaves the mine. In coal mines, where a very large ventilation is necessary, the current is usually maintained by means of a centrifugal fan placed at the top of the upcast, or in some cases by a furnace at the bottom of the upcast. In metalliferous mines the warmth of the mine commonly causes sufficient up-current, without any artificial aid. It is evident that by analysing the return air, and measuring its rate of flow, we can obtain the best idea of the nature and amount of the general vitiation of air throughout the whole mine, while analyses made at particular points in the mine afford more information with regard to vitiation due to special local conditions.

_Black-damp._--So far as is known the impurity known to miners as black-damp is met with to a greater or less extent in all, or nearly all, mines, and in many wells. Black-damp is recognised by the fact that when present in small quantities it causes a candle or lamp to burn dimly, without at the same time producing any distinctly noticeable effect on men: that in larger amounts it extinguishes lights; and that in still larger amounts it causes death by suffocation. It is never explosive when present by itself, and is almost always heavier than air, unless, as very frequently occurs in coal mines, it occurs mixed with fire-damp.

Until lately black-damp was believed to issue from coal and other strata, and was commonly identified with carbonic acid. Investigations by Mr W. N. Atkinson and the writer[A] have shown that this belief is erroneous, and that black-damp is nothing else than the residual gas, resulting from the action of air on oxidisable material present in coal and other materials. Pure black-damp, free from air and other gases, consists of a mixture of nitrogen with usually from 5 to 15 per cent. of carbonic acid.

The following table shows the composition of the black-damp obtained from various coal-mines, metalliferous mines, and wells. The samples were usually mixed to a greater or less extent with air, the amount of which was determined from the percentage of oxygen present:--

ANALYSES OF BLACK-DAMP.

+--------------------------------------------------------+---------+-------------+ | |Nitrogen |Carbonic Acid| | LOCALITY. |per cent.| per cent. | +--------------------------------------------------------+---------+-------------+ |Stopping, Podmore Hall Colliery, North Staffordshire | 87.87 | 12.13 | |Another Stopping, same Colliery | 91.37 | 8.63 | |Main Return, same Colliery | 87.30 | 12.70 | |Sladderhill Colliery, Apedale, North Staffordshire | 85.86 | 14.14 | |Old Road, Talk o’ the Hill Colliery, North Staffordshire| 91.90 | 8.10 | |Main West Return, Talk o’ the Hill Colliery | 80.66 | 10.31 | |South Main Return, Great Fenton Col., N. Staffordshire | 89.31 | 12.69 | |Return, Wallsend Colliery, Newcastle, New South Wales | 89.00 | 11.00 | |Old Workings, Burghlee Colliery, Midlothian | 86.90 | 13.10 | |Upcast Shaft, same Pit | 86.91 | 13.09 | |Upcast Shaft, Tylorstown Colliery, South Wales | 85.97 | 14.03 | |Old Road, Conygre (Timsbury) Colliery, Somerset | 84.92 | 15.08 | |Stopping, Nabb Pit, Lilleshall Colliery | 86.48 | 13.52 | |Stopping, Hamstead Colliery, South Staffordshire | 93.25 | 6.75 | |Upcast Shaft, Hamstead Colliery, South Staffordshire | 93.31 | 6.69 | |Old Road, Forge Ironstone Pit, North Staffordshire | 85.30 | 14.70 | |Issuing from Hole, Foxdale Lead Mine, Isle of Man | 88.07 | 11.93 | |Issuing from another Hole, same Mine | 79.80 | 21.20 | |Return Air-way, same Mine | 80.30 | 19.70 | |Well at Redwick, Gloucestershire | 95.80 | 4.20 | |Coming over Top, Well at Balcombe, Sussex | 88.93 | 11.07 | |Well at Charterhouse School, Surrey | 81.86 | 18.14 | |Well at Northleigh, Oxfordshire | 93.19 | 6.81 | +--------------------------------------------------------+---------+-------------+

The black-damp met with in coal-pits is probably formed chiefly and often exclusively from the oxidation of iron pyrites (FeS_{2}) present in the coal. This is oxidised to sulphuric acid and sulphate of iron, and as carbonate of lime (calcite) is also present in the coal, the sulphuric acid usually combines with the lime, liberating carbonic acid. The whole process may be represented by the following equation:--

4FeS_{2} + 15O_{2} + 8CaCO_{3} = 8CO_{2} + 8CaSO_{4} + 2Fe_{2}O_{3}.

Oxidation occurring according to this equation would yield black-damp consisting of 87.7 per cent. of nitrogen and 12.3 per cent. of carbonic acid, which is nearly the composition usually found. On pieces of coal which have been exposed for some time to air and moisture the red oxide of iron resulting from this reaction may frequently be seen. On other pieces where there is no carbonate of lime, crystals of sulphate of iron, and an acid reaction will be found in the positions previously occupied by iron pyrites, while the coal itself is unchanged. The oxygen of air left in contact with coal gradually disappears. Hence old workings or spaces of any kind left unventilated soon become filled with black-damp. A fall in barometric pressure leads to an issue of black-damp from these spaces, though often there is also a constant issue into the return air-ways on account of a little air being sucked through from the intakes. In the case of wells the influence of barometric pressure on the issue of black-damp is very marked. The black-damp is formed in the pores of the surrounding strata, and issues out up the well whenever a fall of pressure occurs. Thus the well may be fairly clear of black-damp at one time, and shortly afterwards full of it. Accidents to well-sinkers commonly occur through ignorance of this fact. The air is perhaps tested in the morning with a candle and found clear. In the afternoon after dinner a further test is neglected, and if the barometer has meanwhile fallen, the well-sinkers may descend into black-damp and be asphyxiated or fatally injured, or drowned by falling from the ladder. In the case of wells and metalliferous mines the black-damp is probably formed partly by the oxidation of carbonate of iron.

The return air of coal mines always contains black-damp, about 2 per cent. being very commonly present in the air of the upcast shaft. As the air-current passing through a coal mine is enormous, the quantity of black-damp formed in the pit is very considerable, usually amounting to over 2000 cubic feet per minute in a large pit. The oxidation thus represented must liberate a large amount of heat in the mine. Thus in the case of one pit investigated by the writer and Mr F. G. Meachem[133] the amount of heat calculated as being liberated in the mine per minute would have sufficed to raise one ton of water 134° F. or to heat the whole of the air-current (of 100,000 cubic feet per minute) to boiling point. Of this heat only about one-ninth was expended in raising the temperature of the air, two-ninths became latent through evaporation of moisture, and the remaining six-ninths escaped by conduction into the surrounding strata. This slow oxidation is one of the main causes of the high temperatures met with in mines; and apart from it very deep workings could be kept comparatively cool by ventilation. It must, however, be borne in mind that air in descending the shaft of a pit is heated by compression about 5½°F. for every 1000 feet of descent. There is thus a limit to the cooling effect of ventilation. Moreover, at great depths oxidation is apt to be favoured by both the higher temperature and the crushing of the coal by the weight of superincumbent strata, since in coal which is crushed a much larger surface is exposed to the oxygen of the air. When a current of air insufficient to carry off the heat passes through crushed coal spontaneous combustion is very apt to occur, and is a source of constant danger in some seams.

Black-damp is ordinarily recognised by its action in extinguishing a candle or lamp. The percentage which is just extinctive to a tallow candle or miners’ safety lamp was carefully determined by Mr Atkinson and the writer. We found that a candle held vertically will not continue to burn if more than 15.8 per cent. of black-damp is present (corresponding to an oxygen percentage of 17.6 per cent.): that 17.7 per cent. of black-damp extinguished an ordinary safety lamp; and that 18.6 per cent. extinguished a candle held horizontally. The extinction is due, practically speaking, to the reduction in the oxygen percentage of the air and not to the presence of carbonic acid, although dilution of air with carbonic acid extinguishes a light somewhat sooner than dilution with nitrogen. A light will still just burn in a mixture of 75 per cent. of carbonic acid and 25 per cent. of oxygen. The presence of black-damp affects the light given by a candle long before the point of extinction is reached. The flame becomes smaller, and the rate of combustion is diminished. Angus Smith[134] found that when a candle was allowed to burn in air increasingly vitiated by its own combustion and by respiration, the light diminished to 22 per cent. of its original value when the oxygen percentage fell to 18.5 (corresponding to 8.9 per cent. of black-damp).

The effects of black-damp on men are due partly to the carbonic acid, and partly to the diminished oxygen percentage accompanying the admixture of black-damp with the air. The influence of excess of carbonic acid and of deficiency of oxygen must therefore first be described separately.[135]

When carbonic acid is added to air no noticeable effect is produced until about 3 per cent. is present, when the breathing begins to be distinctly deeper and slightly more frequent. No other unpleasant effects are produced, even after a long exposure; and animals kept in air containing 3 per cent. of carbonic acid are unaffected in health. As the percentage of carbonic acid increases, the effects on the breathing become more and more marked, until at about 6 or 7 per cent. there is severe panting, the pulse-rate being also more frequent and vigorous, and the face being flushed. The sensation experienced is similar to that accompanying hard, muscular work. A long stay in air of this composition is followed by frontal headache. With 10 per cent. of carbonic acid the respiratory distress is very great, but with a still higher percentage a narcotic effect is produced, and the mind becomes confused. Animals sometimes die from long exposure to air containing about 25 per cent. of carbonic acid, but even 50 per cent. may not prove fatal for some time.

Diminution of the oxygen percentage of air usually causes no noticeable effect until the percentage falls to about 12, when the respirations begin to be just perceptibly deeper. At 10 per cent. the respirations are usually distinctly deeper, and the lips begin to become slightly bluish. At 8 per cent. the lips and face have more or less of a leaden blue colour, and usually the breathing is deeper and more frequent. In some persons, however, this is not the case, and nothing is noticed by the person breathing this vitiated air, although his face presents to a bystander a most alarming appearance. At 5 or 6 per cent. there is clouding of the senses, and loss of power over the limbs, and often complete loss of consciousness, which, to judge from experiments on animals, would probably soon end in death, either from gradual failure of the respiratory centre or from stoppage of the heart. The symptoms described are those observed when the breathing of the vitiated air is not accompanied by muscular exertion. The danger point is, however, reached much sooner when any muscular exertion, such as that of climbing, or even walking, is made. Even at 15 per cent. of oxygen there is often shortness of breath and dizziness on exertion, and when the oxygen percentage falls much further fainting is apt to occur, and this is probably the cause of many accidents in which men fall off ladders in vitiated air, and are fatally injured or drowned. When the oxygen percentage falls below 8 or 10 per cent. death may occur in consequence of muscular exertion. Air vitiated simply by diminution of the oxygen percentage of the air is exceedingly dangerous, for the reason that there are hardly any warning symptoms before life is imperilled; and were it not that a light is usually carried in such air, and that its extinction gives ample warning, since it occurs at about 17 per cent. of oxygen, accidents would be much more frequent.

Sudden exposure to air containing less than 3 or 4 per cent. of oxygen causes in men loss of consciousness within about forty seconds. This is followed by convulsions, and the respirations soon cease. The heart, however, continues to beat for some time longer, and during this period artificial respiration will still restore life.

A reference to the composition of black-damp will show that the effects produced by it are due, in most cases at least, to carbonic acid as well as to want of oxygen. When there is just sufficient black-damp present to extinguish a light no noticeable effect is, as a rule, produced, since there is 17 per cent. of oxygen, and usually not more than about 2 per cent. of carbonic acid. Occasionally, however, as in the case of two of the samples from Foxdale lead mines (see table), there will be as much as 3 or 4 per cent. of carbonic acid present, in which case the breathing will be slightly affected. With an increasing percentage of black-damp the panting due to carbonic acid will usually become more and more noticeable. Thus with 50 per cent. of black-damp there will commonly be about 6 per cent. of carbonic acid, and 10.5 per cent. of oxygen. The panting due to carbonic acid will, therefore, be very considerable. In some cases, however, as in the black-damp from Hamstead Colliery or Redwick Well, there will still be insufficient carbonic acid to produce panting, and the air will be already dangerous from deficiency of oxygen. With still larger percentages of black-damp, the symptoms from want of oxygen will predominate more and more, until at last with about 75 per cent., or 5 per cent. of oxygen, life can no longer be supported. Death is always due to want of oxygen, and not to the poisonous action of carbonic acid. The presence of carbonic acid diminishes the danger, as the panting caused by it not only gives warning of danger, but also increases the oxygen supply to the lungs, and thus wards off for a time the effects of the deficiency of oxygen.

_Carbonic Acid._--There is no recorded case of evolution of pure carbonic acid in a mine in this country, but in one colliery district in France sudden outbursts of pure carbonic acid have occurred. The locality is a volcanic one, and possibly the carbonic acid may have originated from some such cause as the decomposition of limestone by silicates at high temperatures. The well-known case of the Grotto del Cane is one in which pure carbonic acid is evolved. The composition of a sample recently analysed by the writer from near the floor of the Grotto corresponded exactly to that of a mixture by diffusion of pure carbonic acid and air. The effects of pure carbonic acid have already been described, and are, of course, quite different from those of black-damp. According to Clowes, air containing 15 per cent. of carbonic acid is just sufficient to extinguish lights. Air of this composition would produce violent panting and partial loss of consciousness.

_Fire-damp._--The gas known to miners as fire-damp is recognised by its forming with air explosive mixtures. So far as is known the fire-damp met with in English mines is always pure methane (CH_{4}), although in Germany ethane (C_{2}H_{6}) is also reported to occur in connection with certain kinds of coal. The writer has carefully examined many specimens of fire-damp from various English coalfields, but never found that it consisted of anything else but the methane described by previous observers. The presence of fire-damp in air is recognised by miners from the appearance over the ordinary flames of a lamp of a pale non-luminous “cap” of flame, similar in appearance to the non-luminous flame of a Bunsen burner, though much paler, unless the air is nearly explosive. From the size and distinctness of this cap, the percentage of fire-damp can be approximately determined. In testing for fire-damp the flame of the safety lamp should be lowered until only a small blue flame is left. The pale cap can then be better seen above the lamp flame. With very careful observation about 1 per cent. of fire-damp in the air can just be detected. With a hydrogen flame, as in the Clowes lamp, it is possible to detect as little as 0.2 per cent.[136] With increasing percentages of fire-damp, the cap becomes longer and more distinct, and passes right up the chimney when the air is nearly explosive. Air containing anything between about 5 and 13 per cent. of fire-damp is explosive.

Fire-damp is given off from coal, within which it is contained in a highly compressed state. Different seams of coal give off very different proportions of fire-damp. Those seams which give off much are known to miners as “fiery.” The amount of fire-damp contained in the coal seems to depend in large measure on the extent to which the strata above the coal are gas-tight. If fire-damp can escape upwards it drains off from the coal. Some idea of the enormous amount of fire-damp contained in fiery coal can be formed from the amount of fire-damp which escapes from the mine by the upcast shaft. This is chiefly given off from the coal as it is exposed in working, so that from the output of coal and fire-damp from the mine a rough estimate can be made of the proportion of fire-damp in the coal. In one mine, for instance, about 4500 cubic feet of fire-damp per minute were given off, with an output of about 1200 tons of coal per day, or nearly one ton per minute. It would thus appear that 4500 cubic feet of fire-damp per ton raised were given off, or 150 cubic feet of gas per cubic foot of coal.

The fire-damp met with in mines is commonly mixed with a large proportion of black-damp; and in consequence of this the fire-damp, _though still capable of forming an explosive mixture when less air is present_, may not be recognised unless very carefully looked for, as the lamp is extinguished before a prominent cap is visible. Such a mixture may easily be fired by striking a match in the midst of it, or by a blown-out shot in blasting. The mixture is lighter than air, whereas pure black-damp is nearly always heavier than air.

Fire-damp has no direct action on man. It only acts by diluting the oxygen of the air. When a mixture of 79 per cent. of fire-damp and 21 per cent. of oxygen is breathed it cannot be distinguished from air; and animals may be kept for long periods in a mixture of this composition. The action of a given mixture of fire-damp and air thus depends simply upon the oxygen percentage, any effect produced being merely due to want of oxygen, the symptoms of which have already been described. As fire-damp is lighter than air, a man affected by it will, on losing consciousness, usually fall into better air. Were it not for this, fatal accidents by asphyxiation with fire-damp would be much more common. It often enough happens that a man is temporarily overcome by putting his head upwards into a cavity filled with fire-damp. If the fire-damp contains little or no air loss of consciousness occurs suddenly, and without previous warning. Fatal accidents sometimes occur through a man incautiously advancing without a lamp up a road, during attempts to restore ventilation in a district of a mine which has become filled with fire-damp. So long as a lamp burns in air containing fire-damp not the slightest harm results from breathing the air.

_After-damp._--The gas remaining in a mine at the place where an explosion has occurred is known to miners as “after-damp,” and is much dreaded on account of its poisonous properties. A careful examination, recently undertaken by the writer,[137] of the bodies of the men and horses killed in three colliery explosions resulted in showing that in almost every case, whether or not there were burns or other injuries, the actual cause of death was carbonic oxide poisoning. In many cases, however, the burns or other injuries would certainly have proved fatal apart from the carbonic oxide; and the result of a rough estimate was that, on an average of the three explosions, about 23 per cent. of those killed had received burns or other injuries sufficient to cause death. Only 46 per cent. were burnt or injured.

The symptoms of the rescuers who encountered after-damp were clearly those of carbonic oxide poisoning. Their lamps continued to burn in the poisonous air, and in some cases men were found dead with their lamps either still burning, or with the oil burnt out. The fact that a lamp continues to burn, which is an excellent test of the safety of air vitiated by black-damp or fire-damp, is thus no test of the safety of air vitiated by after-damp.

In the case of the great colliery explosions which cause such wholesale destruction of life, the explosion is almost invariably one of coal-dust, started either by blasting at dry and dusty places in the mine, or by small explosions of fire-damp. Slighter explosions may be due simply to ignition of fire-damp. When fire-damp or coal-dust ignites in presence of excess of air, the products of combustion are simply carbonic acid and water, along with a little sulphurous acid in the case of coal-dust. The reaction occurring when fire-damp ignites under these conditions is represented by the following equation:--

CH_{4} + 2O_{2} = CO_{2} + 2H_{2}O.

The after-damp from such an explosion in air would consist of 88.3 per cent. of nitrogen and 11.7 per cent. of carbonic acid; and with coal-dust a slightly higher proportion of carbonic acid would be present. When, however, the proportion of air present is insufficient for complete oxidation a certain amount of carbonic oxide is produced in place of part of the carbonic acid. As much as 4.5 per cent. may be present in the after-damp of an explosion of fire-damp. In a colliery explosion the fire-damp or dust must always be very irregularly distributed in the air, so that in some places there will be an excess of air, and in others an excess of fire-damp or dust. At many places the excess of gas or dust must be so great that no explosion at all can occur. The after-damp is thus a variable mixture of nitrogen, carbonic acid, and carbonic oxide, together with much air, and, in the case of dust explosions, a little sulphurous acid or sulphuretted hydrogen, and various products of the dry distillation of coal. These latter products give after-damp a characteristic unpleasant smell. The fact that sufficient oxygen to support life is left along the track of an explosion is shown by the fact that the bodies do not present the appearances seen in the rapid death which ensues in an atmosphere devoid of oxygen. Probably about 2 to 5 per cent. of carbonic oxide is usually contained in the pure after-damp of a colliery explosion. The gas met with by rescuers some hours, or perhaps days, after the explosion, is, of course, a mixture of after-damp with the black-damp and fire-damp which has accumulated in consequence of stoppage of the ventilation through the air current being short-circuited.

The action on men and animals of after-damp depends practically upon the carbonic oxide contained in it. Pure after-damp, containing no air, would of course cause death just as rapidly apart from the action of the carbonic oxide: but such after-damp unmixed with air is hardly met with.

Carbonic oxide or carbon monoxide (CO) is a very poisonous gas. Judging from experiments on animals, air containing anything more than 0.4 per cent. would, after a sufficient time, always cause death in a man, though anything over 0.2 per cent. would in many cases prove fatal. It has practically no smell or irritating properties by which its presence might be readily recognised, and its action is peculiarly slow and insidious. The first symptoms of carbonic oxide poisoning are usually dizziness, shortness of breath, and palpitation following any unusual exertion. Sometimes there is drowsiness, and sometimes unusual excitement similar to that produced by alcohol. When more of the gas is absorbed there is great failure of muscular power. The least exertion causes dizziness and fainting, and any serious exertion may prove fatal. At the same time the mind becomes more or less confused, although the person affected is not himself aware of any mental failure. The senses are also affected, and frequently there seems to be greater or less insensibility to pain, as miners who are partially stupefied by carbonic oxide are not infrequently seriously burnt by their lamps. The loss of muscular and sensory power gradually becomes more and more complete, until at last consciousness is entirely lost; and finally, with further absorption, death quietly ensues. There is no pain or serious discomfort at any stage. Recovery from slight carbonic oxide poisoning is usually accompanied by severe headache, nausea, and depression. When consciousness has been lost for a considerable time, recovery is very slow and uncertain. Death may occur from pneumonia after two or three days; and a variety of severe mental symptoms may persist for long periods. In cases where much muscular exertion has been attempted during exposure to the poison the heart appears to be often seriously affected.

In order to understand the very peculiar features presented by carbonic oxide poisoning it is necessary to consider the mode of action of this gas. The oxygen absorbed from the air in the lungs is normally taken up in the form of a loose chemical combination with the red colouring matter (hæmoglobin) of the blood corpuscles, and so carried by the circulation to the tissues, where it is used up. The hæmoglobin not only combines with oxygen, but is also capable of forming a much more stable compound with carbonic oxide, and, as was shown by Claude Bernard, hæmoglobin which is saturated with carbonic oxide cannot take up oxygen. Hence, when the hæmoglobin of a living animal is saturated to a certain extent with carbonic oxide, sufficient oxygen cannot be conveyed from the lungs to the tissues, and death occurs from want of oxygen.[138] Carbonic oxide has no other action than that due to its interference with the oxygen supply through the blood. Apart from its property of combining with the hæmoglobin it is a physiologically indifferent gas, like nitrogen; and it has no action on lower animals which do not possess hæmoglobin. The symptoms produced by it are therefore essentially the same as those described above as due to deficiency of oxygen in the air breathed. No noticeable symptoms are produced until the hæmoglobin is about a third saturated with carbonic oxide, and death does not usually occur until about 70 or 80 per cent. saturation has been reached. When, therefore, not much carbonic oxide is present in the air, a considerable time elapses before a sufficient quantity of the gas has been absorbed to produce symptoms. A man possesses on an average a weight of blood equal to ¹⁄₂₁ of his body-weight, or about 3 litres, and the hæmoglobin of this blood requires about 600 c.c. of carbonic oxide to saturate it. He breathes when at rest about 5 litres of air per minute. Hence, supposing that the air contains 0.2 per cent. of carbonic oxide, which is about the minimum quantity which will produce death, he can absorb only 10 c.c. per minute at the most. It must thus take at least twenty minutes, and actually takes a good deal longer, for distinct symptoms to be produced. During this interval, which will, however, be shorter when the man is breathing more rapidly in consequence of muscular exertion, he may advance far into an atmosphere poisonous from the presence of after-damp, and may consequently be unable to return.

It might be supposed that the presence of any proportion, however small, of carbonic oxide in air would ultimately prove fatal from gradual absorption of the gas by the blood. Actually, however, there is a maximum limit to absorption with any given percentage of carbonic oxide in air; for although the affinity of hæmoglobin for carbonic oxide is much stronger than for oxygen, yet if both gases are present, the hæmoglobin is shared between them in proportion, not only to the relative strengths of their affinities for hæmoglobin, but also to the relative percentages present of the two gases. Hence, although the affinity of carbonic oxide for hæmoglobin is nearly four hundred times as great as that of oxygen, yet if the percentage of carbonic oxide is very minute as compared with the percentage of oxygen, only a little of the hæmoglobin will combine with the carbonic oxide, and consequently no symptoms of poisoning will be produced, however long the exposure may be. Thus with less than about .03 per cent. of carbonic oxide in the air, the blood will never absorb enough of the gas to produce distinct symptoms; and with less than .2 per cent. life will hardly be endangered, although very severe symptoms may be produced.

In recovery from carbonic oxide poisoning, the gas is driven out from the blood through the lungs in consequence of the preponderating influence of the oxygen of the air; and in the course of several hours the blood will be again practically free from carbonic oxide. An hour of breathing fresh air will usually suffice to remove any dangerous excess of carbonic oxide, but if, as often enough happens in persons who have been rendered unconscious, the breathing is shallow, a much longer time may be needed unless artificial respiration has been employed. Carbonic oxide is not oxidised within the body, so that the only way in which it can be got rid of is through the lungs. The blood from which the carbonic oxide has been expelled is in no way injured. The expulsion of carbonic oxide during recovery from poisoning can be greatly hastened by the inhalation of pure oxygen, since its influence in driving out carbonic oxide from the blood is about five times as great as that of air, which only contains 20.9 per cent. of oxygen. Inhalation of oxygen has also another and immediate effect, however. In addition to the oxygen taken up in combination with hæmoglobin, the blood takes up in the lungs a little oxygen in simple solution, just as an equal volume of water would do. When pure oxygen is breathed, the quantity of this dissolved oxygen is increased five times, and is then sufficient to afford an important immediate supply of oxygen to the tissues. If an animal be placed in oxygen at two atmospheres’ pressure, carbonic oxide can be administered without harming it, since although its hæmoglobin becomes completely saturated with carbonic oxide, its blood carries enough oxygen in simple solution to support life.

The cause of death in carbonic oxide poisoning can always be determined by examining a drop of blood taken from the body, and comparing it with normal blood from a healthy person or an animal. The normal blood is diluted with water until the tint of the solution appears yellow; the suspected blood is then diluted until its _depth_ of colour appears about the same. If death was due to carbonic oxide poisoning the latter solution will appear pink instead of yellow. By taking advantage of this difference of tint the percentage saturation of the hæmoglobin can readily be determined. Death often occurs, however, some hours or days after removal from the poisonous atmosphere, and in this case the blood will be free of carbonic oxide. Blood saturated with carbonic oxide has a red colour similar to that of arterial blood, hence the lips, cheeks, and other parts of the bodies of men who have died from carbonic oxide poisoning have often a pink colour similar to that seen in life.

In consequence of the restricted oxygen supply to the tissues during prolonged exposure to carbonic oxide, serious changes may be produced in the brain, heart, and other organs, and as a result of these changes recovery does not occur at once when the carbonic oxide disappears from the blood. Unconsciousness may persist, or relapses may occur, and death is not unfrequent several days after exposure to the poison.

The prompt recognition of the presence of carbonic oxide or after-damp in air is of much practical importance. As already remarked, the fact that a lamp continues to burn is no proof of the safety of air in which after-damp may be present, and elaborate chemical tests are hardly applicable in ordinary practice. A small warm-blooded animal, such as a mouse, or perhaps still better a small bird, may, however, be used to indicate the presence of any dangerous proportion of carbonic oxide. The oxidation processes in the small animal are enormously more rapid than in a man: consequently the small animal breathes and absorbs carbonic oxide much more rapidly. It therefore shows symptoms of poisoning in a fraction of the time necessary in the case of a man, although it is only about equally sensitive to a given percentage of the gas. Hence by watching the animal, timely warning may be obtained of the presence of enough carbonic oxide to cause danger to life. It must be remembered, however, that the animal may show no very evident signs when sufficient carbonic oxide is present to cause very distinct and unpleasant symptoms in a man, and that these symptoms may be aggravated to a dangerous extent by muscular exertion, such as that of hurrying back towards fresh air. Another circumstance which tends to suddenly intensify the symptoms of carbonic oxide poisoning is exposure to cold air, and this should be avoided as far as possible with persons who have begun to feel the effects of the gas.

When a man has been rendered unconscious by exposure to carbonic oxide or any other suffocative gas, the first thing to do, after his removal from the contaminated air, is to apply artificial respiration without a moment’s delay, if the breathing has stopped or is feeble.

_Smoke._--Some of the most disastrous accidents in mines have been due to the poisonous action of smoke from underground fires. Fires may occur from an intensification of the spontaneous oxidation of coal, iron pyrites, cotton waste, etc., from the careless use of lights, from engines underground, or in consequence of an explosion of gas setting fire to brattice cloths, igniting blowers of gas, etc. In any case the occurrence of a fire underground is a source of extreme danger, especially when the timbering has become ignited. If the fire occurs on, or spreads to, an intake road the ventilation current carries the smoke over the mine, killing all those who are unable to avoid it. Smoke which has travelled some distance in a mine appears to lose its pungent smell, and deposits the suspended particles which ordinarily render it visible. This greatly increases the danger, as there is then nothing to give warning of its presence. Thus in the case of the Snaefell accident in 1897 a number of men descended into the shaft without their suspicions being in any way aroused until they had gone too far to be able to return.

The poisonous constituent of smoke is carbonic oxide. This was clearly established in the case of the Snaefell accident, where the timbering had caught fire.[139] A sample of the poisonous air collected by Mr Williams, Her Majesty’s Inspector of Mines, was found by the writer to contain 1.1 per cent. of carbonic oxide. Mr Williams fell over unconscious just after obtaining the sample, and was only restored through the prompt application of artificial respiration by his colleague, Mr Jones. Further evidence that carbonic oxide is the cause of death in underground fires was afforded by the medical examination of the bodies by Dr Burkitt in the recent fire at Whitwick Colliery.[140] There can be little doubt that carbonic oxide poisoning is also one of the most frequent causes of death in fires above ground. The writer recently examined the body of a man found dead in a sitting position in a house which was partially burnt in Oxford. Carbonic oxide poisoning was the cause of death. There were only a few superficial burns which had apparently been inflicted after death. The smoke from burning or smouldering wood is particularly dangerous, as the gas distilled from wood may contain about 30 per cent. of carbonic oxide, the large proportion of oxygen in wood favouring the production of carbonic oxide.

As regards the symptoms produced by smoke nothing need be added to what has already been said under the heading of after-damp.

_White-damp, Gob-stink, Fire-stink._--Under one or other of these names is included by miners the poisonous gas given off from coal which has heated from spontaneous oxidation. Some seams of coal, such as the thick coal in South Staffordshire, or the Bulhurst seam in North Staffordshire, are particularly liable to heating, which readily occurs where the coal has become more or less disintegrated. If the coal is not actually red-hot there may be no distinct smell, and the poisonous gas would come under the designation of “white-damp.” The name “gob-stink” is derived from the fact that the heating usually occurs in the waste coal of a goaf or gob (the area from which the workable coal has been removed). The origin of the name “white-damp” is less clear.

Practically speaking, white-damp and gob-stink, or fire-stink, have the same properties as after-damp, and the poisonous constituent is again carbonic oxide, which the writer has found to be present in various samples. Occasionally, however, sulphuretted hydrogen is also present in formidable proportions. The latter gas is extremely poisonous, as little as .07 per cent. being capable of causing death. Air containing 0.2 per cent. kills warm-blooded animals within one and a half minutes. Its presence may be detected not only by its characteristic smell of rotten eggs, but also by the fact that when present in the proportion of more than about .01 per cent., it causes smarting of the eyes and general irritation of the air-passages.

_Gases from Explosives._--The gases from some explosives are extremely poisonous, from the presence in them of carbonic oxide, sulphuretted hydrogen, or nitric peroxide. In coal mines, when the ventilation is everywhere good, it seldom happens that there is trouble from the gases from explosives; but in metalliferous mines and underground quarries cases of poisoning are not uncommon.

Gunpowder gives off on ignition carbonic acid and nitrogen, along with a variable, though much smaller, proportion of carbonic oxide and sulphuretted hydrogen, either or both of which gases may produce symptoms of poisoning.

Of the “high” explosives, nitro-glycerine, blasting gelatine, and roburite yield on detonation only carbonic acid and nitrogen, whereas gun-cotton, tonite, gelignite, and carbonite also give off carbonic oxide, and therefore require more care when the ventilation is defective.

The most serious accidents in mines from gases from explosives have been due to the accidental burning of high explosives, such as dynamite or gun-cotton. When substances of this class burn quietly instead of detonating, nearly the whole of the nitrogen is given off as nitric oxide (NO) instead of as free nitrogen. The nitric oxide at once combines with the oxygen of the air to form nitric peroxide, which is a very dangerous gas. Even when a charge is detonated for blasting purposes it may happen, particularly with badly made or roughly handled explosives, that part burns quietly and forms nitric peroxide. The latter is an irritant gas, but when sufficiently diluted may be breathed for some time without the person exposed to it being aware of his danger. Under such circumstances bronchitis of a very acute character is apt to occur after a few hours, and death often results. In the gold mines in India and the Transvaal, accidents involving the loss of as many as twenty men at a time have sometimes occurred in this way from dynamite catching fire underground, and serious injury to health may easily be caused by the fumes of imperfectly detonated high explosives where the ventilation is bad.

JOHN HALDANE.