Of the same kind also, most probably, are those small luminous appearances which sometimes appear in houses, or near them, called, in Scotland, Elf-candles, and which are supposed to portend the death of some person about the house, in general these lights are harmless, though not always; for some of them have encompassed stacks of hay and corn, and set them on fire; so that they became objects of great terror to the country people. Of these, it was observed, that they would avoid a drawn sword, or sharp-pointed iron instrument; and that they would be driven away by a great noise.

Several philosophers have endeavoured to account for these appearances, but hitherto with no great success; nor indeed does there seem to be sufficient data for solving all their phenomena. Sir Isaac Newton calls it a vapour shining without heat; and supposes that there is the same difference between a vapour of the ignis fatuus and flame, that there is between the shining of rotten wood and burning coals. But though this seems generally to be the case, there are exceptions, as has been instanced in the vapours which set fire to the stacks of corn. Dr. Priestley supposes that the light is of the same nature with that produced by putrescent substances; others, that the electrical fluid is principally concerned; but none have attempted to give any particular solution of the phenomena.

From the frequent appearance of the ignis fatuus in marshes, moist ground, burying-places, and dunghills, putrefaction seems to be concerned in the production of it. This process is attended with the emission of an aqueous steam, together with a quantity of fixed inflammable and alkaline air, blended together in one common vapour. It is likewise attended with some degree of heat, and there are some vapours, that of sulphur particularly, which becomes luminous with a degree of heat much less than that sufficient to set fire to combustibles. The putrid vapour, therefore, may be capable of shining with a still smaller degree of heat than that of sulphur, and consequently may become luminous by that which putrefaction alone affords. This would account for the ignis fatuus, were it only a steady luminous vapour arising from places where putrid matters are contained; but its extreme mobility, and flying from one place to another on the approach of any person, cannot be accounted for on this principle. If one quantity of the putrid vapour becomes luminous by means of heat, all the rest ought to do so likewise; so that though we may allow heat and putrefaction to be concerned, vet of necessity we must have recourse to some other agent, which can be no other than electricity. Without this, it is impossible to conceive how anybody of moveable vapour should not be carried away by the wind; but so far is this from being the case, that the ignes fatui described by M. Beccari, were but little affected by the wind. It is, besides, proved by undoubted experiment, that electricity is always attended with some degree of heat; and this, however small, may be sufficient to give a luminous property to any vapour on which it acts strongly: not to add, that the electric fluid itself is no other than light, and may therefore by its action easily produce a luminous appearance independent of any vapour. We have a strong proof that electricity is concerned, or indeed the principal agent, in producing the ignis fatuus, from an experiment related by Dr. Priestley, of a flame of this kind being artificially produced.

A gentleman, who had been making many electrical experiments for a whole afternoon in a small room, on goingout of it, observed aflame following him at some little distance. This was doubtless a true ignis fatuus, and the circumstances necessary to produce it were then present, viz. an atmosphere impregnated with animal vapour, and likewise strongly electrified, for the quantity of perspiration emitted by a human body is by no means inconsiderable; and it, as well as the electricity, would be collected by reason of the smallness of the room. In this case, however, there seems to have been a considerable difference between the artificial ignis fatuus, and those commonly met with; for this flame followed the gentleman as he went out of the room, but the natural ones commonly fly from those who approach them. This may be accounted for, from a difference between the electricity of the atmosphere in the one room and the other; in which case the flame would naturally be attracted towards that place where the electricity was either different in quality or in quantity; but in the natural way, where all bodies may be supposed equalty electrified for a great way round, a repulsion will as naturally take place. Still, however, this does not seem to be always the case. In those instances where travellers have been attended by an ignis fatuus, we cannot suppose it to have been influenced by any other power than what we call attraction, and which electricity is very capable of producing. Its keeping at some distance, is likewise easily accounted for; as we know that bodies possessed of different quantities of electricity may be made to attract one another for a certain space, and then repel without having ever come into contact. On this principle we may account for the light which surrounded the wo nan at Milan, but fled from the hand of any other person. On the same principle may we, account for those mischievous vapours which set fire to the hay and corn stacks, but were driven away by presenting to them a pointed iron instrument, or by making a noise. Both these are known to have a great effect upon the electric matter; and by means of either, lightning may occasionally be made to fall upon, or to avoid, particular places, according to the circumstances by which the general mass happens to be effected. On the whole, therefore, it seems most probable, that the ignis fatuus is a collection of vapours of the putrescent kind, very much affected by electricity; according to the degree of which, it will either give a weak or strong light, or even set fire to certain substances. This opinion seems to be confirmed from some luminous appearances observed in privies, where the putrid vapours have been collected into balls, and exploded violently on the approach of a candle. This last effect, however, we cannot so well ascribe to the electricity, as to the ascension of the inflammable air which abounds in such places.