ploding 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 of 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 Espérance, 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

*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.

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 Saturday.

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 should 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-grisou. The coal beds, most dangerous, are those which are the most valuable for their good qualities; science and industry have therefore been called on to seek the means of combatting the effects of the grisou, and we proceed to expose those which have been successively employed.

MEANS TO DESTROY OR CHECK THE FIRE-DAMP OR GRISOU, IN SUBTERRANEAN WORKS.

The first idea which presented itself to the explorers was to disembarrass themselves of the gas by allowing the liquation to establish itself and by setting it on fire, so as to burn it, in the absence of the miners. For this purpose, a workman, clothed in vestments of moistened leather, his visage protected by a mask with spectacles of glass, advanced, crawling on his belly, in the galleries where the fire damp was known to exist, and holding forward a long pole, at the end of which was a lighted torch; he sounded thus the irregularities of the roof, the front of the excavations, and set fire to the grisous. This method, which has been employed, within twenty years, in the basin of the Loire, and even occasionally at the present day, in some of the English fiery collieries, has numerous inconveniences. The workmen, whom they called pénitents, were exposed to dangers to such an extent, that a great number perished. When the gas, instead of being simply inflammable, was detonating, the solidity of the mine was constantly compromised by the explosions; the fire attacked the coal and the timbers; the gases, which resulted from the combustion, became stationary in the works, and menaced the workmen with asphyxia; at length it became necessary, in certain mines, to repeat, even three or four times a day, this perilous operation, and yet it in no respect obviated the rapid disengagements which caused these numerous accidents. This method was equally

in use in the English collieries; only the penitent or fireman, instead of carrying the fire himself, caused it to be moved by means of a slider placed over a line of poles connected together, and directed by a system of pullies and cords. The danger was thus diminished for the fireman, who retired into a niche formed in a neighbouring gallery; but in the meanwhile many were still overtaken, and, besides, all the other inconveniences remained.

The method called the eternal lamps was evidently better. It consisted in placing towards the top of the excavation, and in all the points where the fire-damp collected, lamps constantly lighted, which burned the grisou as fast as it was produced; the danger was diminished in a considerable degree, because there could not be formed such large accumulations of inflammable or detonating gas. This mode of proceeding was, however, renounced in a great number of mines, on account of the production of carbonic acid and of azote; a production the more sensitive, since, to facilitate the liquation of the gases, the air ought not to be very strongly agitated.

But

At length it was devised to profit by the property possessed by platina in sponge to facilitate the combustion of the hydrogen with which it was brought in contact, and pellets, composed of one part of platina, and two parts of clay, were made, and were placed near the points at which the grisou or fire-damp concentrated. all these efforts, based upon the incited combustion of the inflammable gas, proved to be only dangerous and incomplete palliatives, which substituted for a great peril a series of other dangers, less imminent, doubtless, but equally distressing.

From that time all the well disposed continued to search for processes based upon another principle. Two only could conduct to a good result: 1. The withdrawal of the gases out of the mine; 2. A mode of lighting different from that which was in use, and which would suffice for the purposes of the miner without compromising his safety.

The principle of withdrawing-entrainement—of the gases by a rapid ventilation is, without contradiction, that which was the most natural to conceive; because it was already applied to all the other deleterious gases. Dr. Véhrle proposed at first to effect the decanting of the gases by making the excavations (stalls?) communicate by ascending passages with a gallery embracing all the works, and uniting with an ascending shaft. But this project, otherwise impracticable, offered a remedy for only a part of these accidents; the execution alone of the necessary works could not have been made without the greatest danger, if these works had been undertaken in the coal; while, in the rocks of the roof, the expenses would have rendered them impracticable. But a good ventilation alone could not suffice to place the miners in security; it was an excellent auxiliary means, but it always left unsolved this important problem: the prevention of the inflammation of the gases which disengage themselves from the surfaces of the stalls.

The lighting alone could conduct to the solution of this problem,

and numerous attempts had been made, under this head, when Davy discovered the safety-lamp. Before him, they had operated with a small number of lights, placed in the lowest positions, and at a distance from the stalls; the workmen kept these lamps in view, and when the blue nimbus, the indication of hydrogen, began to show itself, they extinguished them or withdrew, covering them with their hats. They made use of, also, in the most infected mines, various phosphorescent matters, and particularly a mixture of flour and lime formed from oyster shells, called Canton phosphorus, although the uncertain and ephemeral light which these materials produced, was but a very feeble resource. At length it was observed that the proto-carbonated hydrogen was somewhat difficult of ignition, and that the red heat was insufficient to accomplish it; thus it was practicable to carry a red coal, or a red hot iron into the fire-damp without inflaming it, the white heat alone having the necessary temperature. They profited by this discovery by lighting the stalls by means of a wheel of steel, which was made to turn against a fragment of flint: a workman was detailed to this service, and the sparks, which were thus produced in a continuous manner, sufficed to light the miners. It happened, occasionally, that these sparks set fire to the grisou; but this discovery, imperfect as it was, was not the less a real benefit.

Such was the state of the question, when Davy commenced the series of experiments which conducted him to the object in view. Many mines had been abandoned notwithstanding the palliatives in use, and a number of those which were maintained in activity, only produced coal at the price of the lives of a great number of men. Davy discovered that the gas, contained in a vase, which only communicated with the exterior by long and straight tubes could not be set on fire; that the flame was difficult of transmission in proportion as the tubes were reduced, and that, consequently, the more their diameters were reduced, the more their lengths might be shortened. He thus arrived at the proof that a plate of thin metal, pierced with holes of about one hundred in an inch, did not communicate fire to the exterior gas, although the interior was charged with lighted gas; the cooling produced by the gas in this small passage sufficed to reduce the temperature of the white heat of the interior down to the red heat of the exterior, and the inflammation could not be communicated. Such was the series of ideas which conducted Davy to surround the flame of the lamps with an envelope of metallic gauze, and thus to construct the safety-lamp.*

LOCAL VENTILATION.

In the sinking of a shaft the work would soon be stopped by the want of air, were it not for the plan of dividing its total section into two unequal parts, by means of a partition of planks, the joints of which are hermetically closed with moss, &c. The smallest compartment is reserved for the ladders; and a current of air is established

* Burat Géologie appliquée, 1846, p. 472.