It is observed by M. Burat, that, as a general fact, applicable to all the methods of exploitation, it is necessary to be watchful that the pillars be not suffered to remain isolated, and for a long time exposed to the action of the air, before pulling them down. Coal alters in the mines almost as much as at the surface; the pyrites, contained therein, decompose, and the hydroxide of iron, which is the result, gives to the coal a rusty stain which depreciates its value. Finally, the schists disintegrate, effloresce, and the selection of the coal becomes much more difficult. It is necessary, therefore, to proportion the excavation, by preparatory works, to the extraction which may be required during the year.

Coal is, without contradiction, among the useful minerals, the one whose exploitation presents the greatest difficulties. In fact, it requires to be extracted in very large masses; its primitive value, scarcely more considerable than the stones of the mines, is, nevertheless, sufficiently important that we should not abandon the smallest possible amount of it. Left in the old workings, it is lost forever; besides, the interior sources of water and of deleterious gases incroach sometimes upon the immense subterranean surfaces that are exposed. It needs all the resources of science and industry to render possible the working of certain basins, which would have remained in abandonment without the modern means of safety and ventilation and the progress of the steam engine.

Obstructions generally develope themselves in connection with the surfaces placed under investigation. The engineer can then unfold, progressively, his means of action in such a manner as to remain always master of the exploitation. But it is the sudden accidents which defy all human prudence, which endanger the safety of the miners, and which, in a few hours, destroy the fruits of long labour and of powerful capital. The most terrible of these accidents arise from the collection of water and of gas, which in nearly all of these basins, are found accumulated in ancient workings, of which tradition has scarcely preserved the remembrance. When a cutting approaches the vicinity of one of these accumulations, a blow of the pick or a blast of the mine suffices to put the works in communication with the danger; and when it manifests itself, there is neither time to fly nor to resist it.

To avoid these sad rencounters, the miners are preceded in the drifts where danger is to be feared, by horizontal borings, sondages, some of them straight, others divergent. These soundings, to furnish sufficient security, ought to be about thirty feet around; if one of them reach a chasm, all the work of excavation ought to cease as soon as it is practicable to ascertain its nature.*

VENTILATION OF COAL MINES.

On the causes which vitiate the air in mines." The means of maintaining in the mines an atmosphere constantly respirable, and of preserving the workmen from the accidents which result from deleterious gases, constitutes one of the capital parts of the art of working, exploiter.

The causes which most frequently vitiate the air, are these: the respiration of the workmen; the combustion of the lamps; the explosions of powder; the spontaneous decomposition of certain mineral substances, such as the sulphurets which change into sulphates; the coal which heats and burns spontaneously; the corruption of the wood; the striking of the tools against

* Burat Géologie appliquée, p. 416.

rocks which contain ores of arsenic or mercury; in addition to which is the natural disengagement of deleterious gases which penetrate the rocks, or are accumulated in the crevices and natural cavities, and sometimes in old workings.

The gas thus produced or disengaged disposes itself in the drifts or galleries according to the order of density, as follows:

Carbonated hydrogen, fire-damp, or inflammable gas,
Azote or nitrogen gas,

Atmospheric air,

Sulphuretted hydrogen,

Carbonic acid, or choke-damp,

Arsenical and mercurial vapours.

Specific gravity. 0.558

0.976

1.000

1.191

1.524

The general precautions employed to get rid of these gases as soon as they are formed, in creating currents sufficiently active to effect their diffusion with the atmospheric air, and to draw the mixture out of the works before it is prejudicial, constitute the art of ventilation-aérage. But these general means do not always suffice, and it is necessary to add special means to avoid, or at least to restrain the sudden disengagements, until the common methods shall have restored the equilibrium. It is necessary, then,

to be able to recognize the presence of each of these gases, in order to destroy them in time, and even, if possible, to diminish the causes of their production.

When the working of a mine, pit or gallery is commenced, if no particular phenomenon facilitate the renewal of air, the respiration alone of the workmen and the combustion of their lamps, are not slow to modify it sensibly. In fact, a workman respires an average of 800 litres = 210 gallons of air per hour, from which he absorbs, in part, oxygen, and substitutes for this oxygen, in the same space of time, 24 to 25 litres, 6 gallons of carbonic acid; his lamp, operating nearly with the same intensity as his respiration produces as much carbonic acid, and augments besides the proportion of unconnected azote.

=

The carbonic acid, or choke-damp, which is thus the most immediate and most general product of the workings in the mine, is recognized by its weight; it always occupies the lowest parts of the excavations; its intermixture with air manifests itself by the difficulty of combustion in the lamps, whose flame diminishes in brilliancy in proportion as the acid increases, and ends by extinction, when the mixture attains to one-tenth.

Upon the miners, the carbonic acid manifests itself by an oppression which overwhelms them; nevertheless, temperament and habit will greatly vary the proportions of the mixture which some men are able to breathe. Certain miners can yet work when the lights have ceased to burn; there are even some whose acquired habit is such that they pass through, we are assured, galleries where there is more than twenty per cent. of carbonic acid. Nevertheless, we should watch, on pain of the greatest dangers, that the lamps can everywhere burn with facility, and that the proportions never exceed five per cent.; for this gas, which the French miners commonly cal mofette, has the greatest tendency to isolate after generation, and will then cause an instantaneous asphyxia.

A single example will demonstrate this energetic action. The workmen of the Creuzot mine descended one morning, the one following the other, in rotation, into a shaft below, in which carbonic acid had accumulated during the night. Arrived at the level of the "bain," at a few yards from

the bottom of the pit, the first fell, struck with asphyxia, without having time to utter ȧ cry; the second followed immediately; the third saw his comrades prostrated on the ground, almost within reach of his arm; he stooped to seize them, and fell himself; another quickly shared the same fate, in his desire to save the others, and the catastrophe would not have been arrested had not the fifth been an experienced master miner, who obliged those who followed him to reascend.

These accidents are often to be dreaded in coal mines, where spontaneous disengagements are capable of producing in a little time large quantities of carbonic acid. In this case, it is necessary to have within reach ammonia, caustic potash, or lime, of which a solution must rapidly be made, to be thrown into the invaded workings, either by letting it fall from a watering pot, if it be in a shaft, or projected from a pump, if it is in a slope or a gallery. It is also necessary to fight incessantly against the production of the carbonic acid, and to prevent its accumulation by leaving no wood in a state of decomposition, and proscribing all combustion beyond that of the lamps necessary for lighting. Finally, it is essential to prevent the spontaneous heating and firing which is so frequent in coal mines. When a fire is ascertained, it should immediately be circumscribed by impermeable walls, called corrois; walls constructed of rubbish with a mortar of clay.

The gases which result from the subterraneous decomposition of the coal, have, besides carbonic acid, carbonic oxide, azote, sulphurous acid, and the carburets of hydrogen, which have a special odour. Before the coal takes fire, the interior air is already heavy and heated by the gaseous disengagements which are the precursors of ignition. As quickly as these symptoms are remarked, the coals already mined should be raised, and we should isolate from the surrounding air the region or the crevices which enclose the fire; employing at this work the labourers whose organization is known to be the best adapted to support the deleterious influence of these gases.

Azote, or nitrogen gas, is much less to be dreaded than the carbonic acid; because its action upon the animal economy is less energetic; besides, its production can only take place by the absorption of oxygen from the air, and it does not naturally exist in the fissures or cavities of the rocks. It has, then, no spontaneous disengagement; but if we penetrate into the works which have been a long time abandoned, and where there has been combustion, the azote will occupy, in consequence of its lightness, the higher parts of the excavations, while the carbonic acid will occupy the lower parts; the respirable air forming the intermediate zone. Azote is found isolated in certain mines, where there exist pyrites in a state of decomposition; the sulphurets changing into sulphates, absorb the oxygen and isolate the azote; the sulphuret of iron is, in this respect, the most active agent.

Azote manifests itself by the red colour of the flame of the lamps, which ends by extinction; it renders respiration difficult, produces a heaviness of the head, and a hissing or singing in the ears, which seems to indicate a mode of action different from that of carbonic acid.

The ordinary lamp of the miner is extinguished when the air contains no more than 15 per cent. of oxygen: [the atmospheric air is composed of 21 per cent. of oxygen and 79 per cent. of azote,] it is also at this proportion of 85 per cent. of azote that asphyxia or suffocation is caused.

Proto-carbonated hydrogen, or inflammable air, designated by the French and Belgian miners under the name of grisou, is of all the gases the most dangerous; that which occasions the greatest number of accidents, not by asphyxia, which it can nevertheless produce when it is not mixed with at

least twice its volume of air, but for its property of igniting when in contact with lighted flames, and of exploding when it is mixed, in certain proportions, with atmospheric air.

This gas is frequent in nature, and often designated under the denomination of marsh gas, because it disengages from the stagnant waters which retain vegetable matters in decomposition. Some muddy volcanoes called salses, emit it in large quantities; it also penetrates certain rocks, such as the coal series and the saliferous strata, where it is accumulated and condensed in caverns and natural vacancies; so that by soundings or borings its true sources can often be determined. There even exist natural or artificial sources, which can be lighted and which have persistance enough to be brought into useful service.

The grisou is more abundant in the fat and friable coals, than in the dry and meagre coals; it particularly disengages itself in the crushed places, éboulements, in the recent stalls whose surfaces are laid bare, and that so vigorously as often to decrepitate small scales of coal and produce a slight rustling noise. The fissures or fractures of the coal, and even the clefts or the roof or the floor, give sometimes outlets to soufflards or jets of gas.

The action of this gas upon the flame of the lamps is the most certain guide in ascertaining its presence and proportion. The flame dilates, elongates, and takes a bluish tint, which can readily be distinguished by placing the hand between the eye and the flame, so that only the top of it can be seen. As soon as the proportion is equal to a twelfth part of the ambient air, the mixture is explosive, and if a lamp be carried, it will produce a detonation proportionate to the volume of the mixture. When, therefore, a miner perceives at the top of the flame of his lamp the bluish nimbus which decides the presence of the fire-damp, he ought to retire, either holding his light very low or even to extinguish it.

The experiments conducted by Sir H. Davy show that the most violent explosions take place when a volume of proto-carbonated hydrogen gas is mixed with seven or eight volumes of atmospheric air.

The chemical effects of an explosion are, the direct production of the vapours of water and carbonic acid and the separation of azote. The physical effects are, a violent dilatation of gas and of the surrounding air, followed by a reaction through contraction. The workmen who are exposed to this explosive atmosphere are burned, and the fire is even capable of communicating to the wood work or to the coal; the wind produced by the expansion is so great that, even at considerable distances from the site of explosion, the labourers are thrown down, or projected against the sides of the excavations; the walls, the timbering, are shaken and broken; and crushing, or falling down, is produced. These destructive effects can be propagated even at the mouths of the pits, from which are projected fragments of wood and rocks accompanied by a thick tempest of coal in the form of dust.

The evil rests not there; considerable quantities of carbonic acid and azote, produced by the combustion of the gas, become stationary in the works, and cause those who have escaped the immediate action of the explosion to perish by suffocation. The ventilating currents, suddenly arrested by this perturbation, are now much more difficult to re-establish, because the doors which served to regulate them are partly destroyed; the fires are extinguished, and often, even the machines fixed at the mouths of the shafts, to regulate the currents, are damaged and displaced, to such an extent that it becomes impossible to convey any help to the bottom of the works.

Some examples will give a just conception of the intensity of these explosions and of their effects.

In a gallery of a coal mine of Saarbruck, in Rhenish Prussia, the explosive air took fire on the arrival of a miner carrying a common lamp. Seven dykes or walls of bricks built in the lateral works and at twenty feet from the gallery, forming with them sharp angles, in such a manner that they could not be struck by the dilatation of the air in the direction of the explosion, but only by contraction, were nevertheless thrown down. We infer from the description that these walls fell inwards; that is, towards the point of explosion. At nine hundred feet from the explosion timber of eight inches diameter were broken; a door for ventilation was torn up, and violent effects of the same nature were manifested even at near 2000 feet distance.

In a mine of Schaumburg, the fire-damp, which filled a gallery and a shaft of 1000 cubic yards in capacity, took fire in 1839. Stones which weighed more than a ton, serving as the foundation of a hydraulic machine of the weight of twelve tons, were displaced, notwithstanding the strong wooden props which consolidated them against the direction of the explosion and which were themselves broken. In another mine of the same principality, the coal was set on fire, and this coal was coked, by that cause, even to the depth of more than a yard.

The explosion, coup de feu, of the mine of Esperance, which occurred at Liege, in June, 1838, does not seem to have produced such fatal effects as analogous phenomena have elsewhere done. The fire being propagated without explosion, to the right and left of a working, by the effect of a blast in the mine, produced an explosion in a distant working. Sixty-nine miners were killed. In the place of explosion they were burned and broken; in that where the fire had first taken place all the corpses were arranged with their heads directed towards the very point where the combustion originated; these unfortunate people having evidently sought to protect themselves thus against the gas which burned behind them. In the other workings, the miners had only perished by asphyxia.

The relation of these accidents suffices to indicate the general precautions which should be taken. Thus, it is essential to place the lamps only near the lowest parts of the excavations; to avoid all methods of working which ascend without outlets; to work, if possible, by descending, rather than by ascending, and to redouble the usual precautions on entering into excavations after an interruption of the work. A great number of accidents have taken place, for example, on Monday mornings, when the miners descend after having quitted the mine on Sunday.

M. Bischof reports that having visited a gallery which had been abandoned for several days, he found the gases liquated to such an extent that they were inflammable in every part of the area; detonating in the middle portion, while the almost pure atmospheric air filled the lower part.

It is very dangerous to allow these liquations to be produced; it is necessary that the current of air be sufficiently active to produce immediately the diffusion of the gas in the air and its withdrawal out of the mine before the mixture has become explosive. But, notwithstanding the precautions of ventilation-aérage-many mines would be completely unworkable if there had not been found the special means of guarding them from the fire damp

Is it not more probable that these bodies were instantaneously thrown into this position, by the great reaction, the collapsing of the air towards the focus or vacuum caused by the explosion -T.