in natural history (from the Saxon word damp, signifying vapour or exhalation), are certain noxious exhalations issuing from some parts of the earth, and which prove almost instantly fatal to those who breathe them.

These damps are chiefly observed in mines and coal-pits: though vapours of the same kind often issue from old lavas of burning mountains; and, in those countries where volcanoes are common, will frequently enter houses, and kill people suddenly without the least warning of their approach. In mines and coal-pits they are chiefly of two kinds, called by the miners and colliers the choke and fire dumps; and both go under one general name of foul air. The choke-damp is very much of the nature of fixed air; and usually infects those places which have been formerly worked, but long neglected, and are known to the miners by the name of waffles. No place, however, can be reckoned safe from this kind of damps, except where there is a due circulation of air; and the procuring of this is the only proper means of preventing accidents from damps of all kinds. The choke-damp suffocates the miners suddenly, with all the appearances found in those that are suffocated by fixed air. Being heavy, it descends towards the lowest parts of the workings, and thus is dangerous to the miners, who can scarce avoid breathing it. The fire-damp, which seems chiefly to be composed of inflammable air, rises to the roof of the workings, as being specifically lighter than the common atmosphere; and hence, though it will suffocate as well as the other, it seldom proves so dangerous in this way as by its inflammable property, by which it often takes fire at the candles, and explodes with extreme violence.

In the Phil. Trans. no 419, there is an account of some explosions by damps of this kind, on which we have the following observations. 1. Those who are in the place where the vapour is fired, suddenly find themselves surrounded with flames, but hear little or no noise; though those who are in places adjacent, or above ground, hear a very great one. 2. Those who are surrounded by the inflamed vapour feel themselves scorched or burnt, but are not moved out of their places, though such as unhappily stand in the way of it are commonly killed by the violence of the shock, and often thrown with great force out at the mouth of the pit; nor are the heaviest machines found able to resist the impetuosity of the blast. 3. No smell is perceived before the fire, but a very strong one of brimstone is afterwards felt. 4. The vapour lies towards the roof, and is not perceived if the candles are held low; but when these are held higher, the damp descends like a black mist, and catches hold of the flame, lengthening it to two or three handfuls; and this appearance ceases when the candles are held nearer the ground. 5. The flame continues in the vault for several minutes after the crack. 6. Its colour is blue, something inclining to green, and very bright. 7. On the explosion of the vapour, a dark smoke like that proceeding from fired gunpowder is perceived. 8. Damps are generally observed to come about the latter end of May, and to continue during the heat of summer. They return several times during the summer season, but observe no certain rule.

Besides these kinds of damps, which are very common, we find others described in the Philosophical Transactions, concerning the nature of which we can say nothing. Indeed the account seems somewhat suspicious. They are given by Mr Jeffop, from whom we have the foregoing observations concerning the fire-damp, and who had these from the miners in Derbyshire. After describing the common damp, which consists of fixed air, "They call the second sort (says he) the peat-bloom damp, because, as they say, it smells like peat-bloom. They tell me it always comes in the summer-time; and those grooves are not free which are never troubled with any other sort of damps. I never heard that it was mortal; the scent, perhaps, freeing them from the danger of a surprize; but by reason of it many good grooves lie idle at the best and most profitable time of the year, when the subterraneous waters are the lowest. They fancy it proceeds from the multitude of red-trefoil flowers, by them called honey-flower, with which the limestone meadows in the Peake do much abound. The third is the strangest and most perilous of any; if all be true which is said concerning it. Those who pretend to have seen it (for it is visible) describe it thus: In the highest part of the roof of those passages which branch out from the main groove, they often see a round thing hanging, about the bigness of a foot-ball, covered with a skin of the thickness and colour of a cob-web. This, they say, if it is broke by any accident, as the splinter of a stone, or the like, diffuseth itself immediately, and suffocates all the company. Therefore, to prevent casualties as soon as they have espied it, they have a way, by the help of a stick and long rope, of breaking it at a distance; which done, they purify the place well with fire, before they dare enter it again. I dare not avouch Damps vouch the truth of this story in all its circumstances, because the proof of it seems impossible, since they say it kills all that are likely to bear witness to the particulars: neither dare I deny but such a thing may have been seen hanging on the roof, since I have heard many affirm it."—Some damps, seemingly of the same nature with those last mentioned, are noticed by the author of the Chemical Dictionary, under the word Damps. "Amongst the noxious mineral exhalations (says he), we may place those which are found in the mines of Sal-gem in Poland. These frequently appear in form of light flocks, threads, and spiders webs. They are remarkable for their property of suddenly catching fire at the lamps of the miners with a terrible noise and explosion. They instantly kill those whom they touch. Similar vapours are found in some mines of fossil coal."

With regard to the formation of damps we have as yet no certain theory; nor, though the experiments of aeroligists are abundantly able to show the composition and manner of forming these noxious airs artificially, have they yet thrown much light on the method by which nature prepares them on a large scale. There are two general ways in which we may suppose this to be done; one by the stagnation of atmospheric air in old waste places of mines and coal-pits, and its conversion into these mephitic exhalations; the other by their original formation from the phlogistic or other materials found in the earth, without any interference of the atmosphere. In favour of the former opinion it may be urged, that old wastes are never free from damps, especially those of the kind resembling fixed air; nor are they always deficient in the inflammable kind. The same is also true of old wells, or even cellars, and in short in every place where the air stagnates for any considerable time. But, on the other hand, we have many instances of fixed air coming out of the earth, and that in vast quantities, where no considerable stagnation of the atmosphere could be suspected; as for instance, in the grotto del Cani in Italy, where a continual stream of it has issued from time immemorial. The same seems to be the case with the tops of some high mountains, particularly Mont Blanc, the highest in Europe; on the top of which M. Saussure found the atmosphere so much impregnated with fixed air, that lime-water exposed to it very quickly gathered a crust on its surface.

Sir William Hamilton, in his account of the eruptions of Vesuvius, informs us, that the inhabitants in the neighbourhood of that mountain are infested with a kind of pestilential vapours named by them mofete, which issue from the old lavas thrown out by the volcano. These are of the nature of the damps in our mines or coal-pits, and issue forth in such quantity as either to infect the atmosphere for a very considerable way round, or to do mischief by being carried from place to place by the atmospherical currents, which are not strong enough to dissipate them for some time. From some late accounts, the famel (or scouring winds, as they have been represented) in the eastern countries, seem to be no other than streams of fixed air of considerable extent, which exert their usual and fatal effects on those who breathe them. A strong argument in favour of this opinion is, that these winds cannot cross a river, it being the nature of water to absorb fixed air, and thus destroy them.

Hence it is rendered probable that these mephitic vapours are often to be met with in the open atmosphere, and consequently cannot always be the effect of stagnation; nor indeed does it at all appear that mere stagnation can affect the quality of the atmosphere either one way or other. This fluid cannot have its properties altered but by something immersed in it upon which it can act, and by means of which action its component parts may be changed or separated. While this process is going on, there is generally, if not always, an absorption of air, accompanied indeed frequently with an emission of some aerial fluid equal in quantity to that which is absorbed. Mr Scheele, in his Essay on Fire, has shown by a number of experiments the effect of exposing certain substances to the action of air, both on the substances themselves and on the aerial fluid. The result of all these is no other than what we might expect from a very slow combustion, and which perhaps may on inquiry be found to be the only way by which air can be decomposed. If the substance exposed to the air was capable of absorbing that part of the fluid which had undergone a change, there was always an evident diminution, but not otherwise. Thus, on inclosing some caustic fixed alkali in a phial of atmospheric air, a considerable diminution took place; and the alkali, by becoming saturated with fixed air, showed that a decomposition had taken place, and that the dephlogisticated part of the air had separated from the other, attached itself to the fixed alkali, and become fixed air by uniting with a certain proportion of phlogistic matter. Hence we may conceive, that in any place where the air was confined over a vast quantity of caustic alkaline salt, it would soon become unfit for the purposes of animal life, and we might say that a damp would be formed. But this would be a damp of a very different kind from that usually met with in mines; for here the dephlogisticated part of the atmosphere being converted into fixed air, and absorbed by the salt, only the poisonous mephitic, or as it is commonly called phlogisticated, air would remain, so that no fixed air could ever be separated from it.

Let us now suppose, that instead of the alkaline salt a quantity of burning charcoal is confined in a place where there is not a proper circulation of air, and we will soon see that a damp of the very same kind with that called by miners the choke-damp will be formed. But this, according to the late discoveries, takes place by reason of the dissipation of the charcoal by heat, and its union with the dephlogisticated part of the atmosphere, which always constitutes fixed air*. In this case, however, the damp must be but of short continuance, and will soon be destroyed after the charcoal is extinguished; but if, instead of the charcoal, we substitute a large quantity of fermenting liquor, from whence the fixed air is naturally emitted, a damp will be formed much more difficult to be dissipated than the former, because it renews itself in a very short time; and, unless there is a very constant circulation of air, it will be dangerous to enter the place where it is.

From the last example we may form an idea of the manner manner in which these damps, consisting chiefly of fixed air, are formed. We know not indeed thoroughly the nature of fermentation; but we are assured, that it is always accompanied by an internal heat; which, in some cases, is raised to the utmost height, insomuch that large quantities of moist vegetable substances, packed together, will sometimes burst out into flame. It is not, however, at all times necessary for the extrication of fixed air, that the heat should come to this extremity. The example of fermenting liquors shows, that in some cases a very moderate heat is sufficient for the purpose. Now, though the comparison may seem somewhat inadequate between the solid substance of the earth and a fermenting liquid, yet we know that a gentle heat constantly takes place in the bowels of the earth; and that almost all terrestrial substances will emit fixed air on being exposed to heat. It is not at all improbable, therefore, that, on the large scale of nature, the quantity of materials may compensate for the weakness of the heat, and thus occasion a constant emission of fixed air; which, though slow in comparison of what is effected in our experiments by a violent artificial heat, may yet accumulate in the narrow spaces of mines in such a manner as to be very troublesome. In volcanic countries, where the heat of the earth is much greater, the emission of fixed air is in proportion; and thus we may account for that continual stream of it, which issues from the grotto del Cani, and perhaps other places.

The mofetes, which are said to proceed from old lavas, can only be accounted for by supposing the heat, which originally took place in them, to be in some measure renewed; or that they have been again, by some means or other, disposed to take fire as formerly; but this we offer merely as a conjecture; there not being as yet sufficient data to determine anything positively upon the subject.

It may be objected to the hypothesis just now laid down, that, if there is a continual deposition in the earth to produce fixed air, the whole surface of it must pour out such a quantity as would destroy every living creature upon it. This indeed might be granted, were the surface of the earth quite bare, and destitute of vegetation; but we know that fixed air is composed of the dephlogisticated kind and phlogiston; and that these two ingredients, after being once joined, may be separated from each other, and reassume their proper characters. There is no absurdity, therefore, in supposing that the fixed air may be continually decomposed by the vegetables which grow all over the surface of the earth; and the atmosphere not only thus preserved from any taint from it, but supplied with a quantity of pure dephlogisticated air, which it is certain that vegetables do emit. It is also certain, that wherever the atmosphere is suffered to be in contact with the bare surface of the ground for some time, a considerable quantity of fixed air will be produced, unless there is a constant circulation of atmospheric air to carry off the former before it has time to produce any sensible effect. Hence we may account for the damps in wells, cellars, and even in the confined places of old castles and ruinous buildings, where the air is not in contact with the surface of the ground itself, but with mere heaps of rubbish and old walls.

With regard to what is called the fire damp, the case seems to be more plain. In the Phil. Trans. no. 136, we have the following account of one of this kind which seemed evidently to issue from the earth. "This work is upon a coal of five yards in thickness, and hath been begun upon about six or eight and thirty years ago. When it was first found, it was extremely full of water, so that it could not be wrought down to the bottom of the coal; but a witchet, or cage, was driven out of the middle of it, upon a level, for gaining room to work, and drawing down the spring of water that lies in the coal to the eye of the pit. In driving of which witchet, after they had gone a considerable way underground, and were scanted of wind, the fire-damps did begin by little and little to breed, and to appear in crevices and fissures of the coal, where water had lain before the opening of the coal, with a small bluish flame, working and moving continually; but not out of its first feet, unless the workmen held their candles to it; and then being weak, the blaze of the candle would drive it with a sudden fizz away to another crevice, where it would soon after appear blazing and moving as formerly. This was the first knowledge of it in this work, which the workmen made but a sport of; and so partly neglected, till it had gotten some strength; and then upon a morning, the first collier that went down, going forwards in the witchet with his candle in his hand, the damp presently darted out so violently at his candle, that it struck the man clear down, singed all his hair and clothes, and disabled him from working for a while after. Some other small warnings it gave them, insomuch that they resolved to employ a man on purpose that was more resolute than the rest, to go down a while before them every morning, to chase it from place to place, and so to weaken it. His usual manner was to put on the worst rags he had, and to wet them all in water, and when he came within the danger of it, then he fell down groveling upon his belly, and so went forward, holding in one hand a long wand or pole, at the head whereof he tied candles burning, and reached them by degrees towards it; then the damp would fly at them, and if it missed of putting them out, would quench itself with a blast, and leave an ill-scented smoke behind. Thus they dealt with it till they had wrought the coal down to the bottom, and the water following, and not remaining as before in the body of it, among sulphurous and brass metal that is in some veins of the coal, the fire-damp was not seen nor heard of till the latter end of the year 1675, which happened as followeth.

"After long working of this coal, it was found upon the rising grounds that there lay another roach of coal at the depth of 14 yards under it, which proved to be 3½ yards thick, and something more sulphurous. This encouraged us to sink in one of the pits we had formerly used on the five-yards coal.—As we sunk the lower part of it, we had many appearances of the fire-damp in the watery crevices of the rocks we sunk through, flashing and darting from side to side of the pit, and showing rainbow-like colours upon the surface of the water in the bottom; but upon drawing up of the water with buckets, which stirred the air in the pit, it would leave burning, till the colliers at work, with their breath and sweat, and the smoke of their candles, thickened the air in the pit, and then it would appear again; they lighted their candles at it sometimes when they went out; and so in this pit it did no further harm."

In another pit, however, it soon appeared, and at last produced a most terrible explosion. This was occasioned by one of the workmen going imprudently down with a lighted candle, after a cessation of work for some days, and the force exerted by it seemed equal to that of gun-powder.

The formation of inflammable air in mines cannot be attributed to any vitiation of the atmosphere; for there is no natural process with which we are acquainted, by which such a change could be accomplished. In one instance, however, we have an example of a fire-damp being produced, not only without any considerable stagnation of atmospheric air, but where there is the best circulation imaginable. This is in large bellows used in metallurgical works, which are sometimes burst by an explosion of inflammable matter proceeding from the rancid matters with which the leather is greased. Dr Priestley has shown, that inflammable air is composed of pure elementary fire, charcoal or phlogiston, and a little water; and that this composition may take place even in vacuo. All these materials are to be met with in the bowels of the earth. Coal, a bituminous substance, is abundantly able to supply the phlogiston; the natural moisture of the earth affords water, and the heat, however gentle, which constantly exists in the bowels of the earth, may be sufficient to produce a quantity of inflammable air, which gradually accumulating in those places where there is not a constant stream of atmospheric air to carry it off, will soon produce the dreadful effects already mentioned.

A much more important consideration than the formation of dumps, however, is the proper method of avoiding their pernicious effects. The inflammability of one kind affords an easy method of preventing it from accumulating, viz. by setting fire to it. This may be done with safety, unless it has been suffered to go too far before the experiment is made: for the inflammable air, being much lighter than any other kind, will naturally rise to the top; so that a man, lying flat on the ground to avoid the force of the explosion, and holding up a lighted candle fixed upon a pole, may at once free the mine from such a troublesome guest. But where it has been allowed to accumulate in too great quantity, so that this method cannot be used, or in the other kind, which is not inflammable, the method commonly practised is to produce a constant circulation of air as much as possible through all parts of the mine. To procure this, they make a perpendicular opening, which they call a bank or shaft, so that the mine may have two or more openings; and thus by reason of the difference of temperature between the open atmosphere and that in the mine, there is a continual draught of air through them both. This current will always be stronger in proportion to the difference between the external atmosphere and that of the mine; and likewise in proportion to the difference between the depth of the two shafts. But as the temperature of the atmosphere is variable, it happens, at certain seasons of the year, that there is not a sufficient difference between that of the atmosphere and in the mine to produce the necessary circulation. This happens principally in the spring and autumn; at which seasons it is necessary to light fires in the shafts, which are always efficacious for the purpose desired.

Among the other uses to which dephlogisticated air might be applied, Mr Cavallo reckons that of securing people from the dangerous effects of dumps in mines, and other subterranean places. "If a large bladder," says he, "into which a solution of lime in water is introduced, be filled with dephlogisticated air, and a small wooden or glass pipe be adapted to its neck, a man may hold that pipe in his mouth, and may breathe the dephlogisticated air; and thus equipped he may enter into these subterranean places, amidst the various elastic fluids contained in them. A large bladder of dephlogisticated air will serve for above a quarter of an hour, which is a length of time sufficient for various purposes; besides, if longer time is required to be spent in these places, a person may have two or more bladders of dephlogisticated air along with him, and may shift as soon as the air of one is contaminated. Without the necessity of any more complicated apparatus, the bladders full of dephlogisticated air may be kept stopped by putting corks into the glass or wooden pipes that are tied to their necks. This air might also be used for diving-bells."