Ignis Fatuus (Lat. ignis, 'fire,' fatuus, 'foolish') is a luminous appearance of uncertain nature which is occasionally seen in marshy places and churchyards. The phenomenon has been frequently described, but it has been observed so rarely in favourable circumstances by scientific men that there is no satisfactory explanation. The light usually appears in autumn evenings shortly after sunset; it is common in the north of Germany, in Italy, in the south and north-west of England, and on the west of Scotland, but it has been noticed in many other countries.
Descriptions of ignes fatui vary so much that several different phenomena have evidently been included under the name. The light usually resembles a flame, and is often mistaken at first for the light of a lantern, but seen more closely the colour appears as bluish, reddish, greenish or yellowish, merging into purple, but never a clear white. Some observers describe the flame as fixed in position, shining steadily either close to the ground or a few feet above it, and illuminating the surrounding reeds and grass. Others have seen it in motion bounding rapidly over the country, and sometimes rising high in the air. The light has been seen to divide repeatedly into several smaller flames, which describe complicated movements, advancing, retiring, and combining. The moving light is said to recede from an observer who approaches it, but to follow him if he retires from it.
Some supposed appearances of the ignis fatuus have been proved to be the lights of distant houses seen through trees; others are almost certainly due to luminous insects, such as the glow-worm, or to the phosphorescence of decaying vegetable matter. St Elmo's Fire (q.v.) has also been confounded with it. But setting all these possible cases aside, both fixed and moving ignes fatui have been proved to exist. The spectrum of the light has never been observed, so far as the writer can ascertain. It is said that paper has been ignited by the flame, and if this be so there must be at least two similar phenomena of different nature. List in north Germany passed his hand through the luminous appearance and felt no warmth; near the same locality at a later date Knorr held the metal tip of a walking-stick in the flame of a fixed ignis fatuus (which he could not himself touch on account of the marsh) for a quarter of an hour, but the metal was not warmed. In the former instance a puff of air extinguished the flame, and a very slight explosion was heard when it reappeared; in the latter a strong waft of air only made it flicker slightly, and a light breath produced no effect. No odour was perceptible.
The common hypothesis that ignis fatuus is the flame of burning marsh-gas, , is untenable, for although this gas is produced abundantly in many marshy places it cannot ignite spontaneously. The more plausible suggestion that phosphoretted hydrogen, , which is spontaneously inflammable, might be produced in churchyards or marshes where there is decaying animal matter, does not account for the effect observed by the German physicists, since no gas can burn without giving out heat, and that particular gas has a very penetrating and characteristic smell. Nor could a burning gas, except on the most extravagant assumptions, bound over the country like a ball of fire for half an hour at a time. The early supposition of a phosphorescent vapour is more reasonable, although excepting that of free phosphorus, which could not occur in nature, no such vapour is known to exist. The phenomenon was undoubtedly more common a century ago than it is now, and its disappearance in many localities may be directly traced to the draining of fens and marshes.
Popular names—e.g. Will-o'-the-Wisp, Jack-a-Lantern, Spunkie, &c.—abound in folklore, and are connected with many stories of travellers mistaking the marsh lights for a cottage window, and being decoyed into dangerous places, often with fatal results. A German legend identifies the will-o'-the-wisp with the soul of an unbaptised infant; an Irish, with a soul broke out of Purgatory. For the folklore of the subject, see Notes and Queries, passim.