IGNIS FATUUS, Ig'nIs (Latin fire, atmospheric fatuus, foolish), an atmosphec light or a luminous appearance sometimes seen in swamplands, in churchyards and over stagnant waters. The light usually appears shortly after sunset; it is common in the north of Germany, in Italy, in the south and southwest of England, and the west of Scotland, and has been noticed in many other countries in undrained marshy districts.
The appearance generally resembles a flame; seen closely, the color appears as bluish, red dish, greenish or yellowish, merging into purple, but never a clear white. Sometimes the flame seems fixed in position, shining steadily close to the ground or a few feet above it; again, it appears in rapid motion, sometimes nsing high in the air, at others separating into smaller flames, which are seen advancing, retiring, re combining, etc.
Some supposed appearances of the ignis fatuus are probably due to luminous insects, or to the phosphorescence of decaying vegetable matter. By setting all such possible cases aside, both fixed and moving ignes fatui have been proved to exist, although the spectrum of the light seems not to have been fully observed.
The common hypothesis that ignis fatuus is the flame of burning marsh-gas, CH., is untenable, for although this gas is produced abundantly in many marshy places, it cannot ignite spon taneously. The more plausible suggestion that phosphoretted hydrogen, PHs, which is sponta neously inflammable, might be produced in churchyards or marshes where there is decaying animal matter does not account for some of the effects observed. The early supposition of a phosphorescent vapor is more reasonable, al though excepting that of free phosphorus, which could not occur in nature, no such vapor is known to exist. The phenomenon was un doubtedly 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 for the ignis fatuus o'-the-Wisp, Jack-a-Lantern, SpunIcie, etc.— abound in folklore, and the superstitions re garding it are connected with many stories of travelers mistaking the marsh-lights for those of cottage windows, and with tales of evil spirits decoying men into dangerous places, often to their doom.