a light supposed to proceed from phosphuretted hydrogen, appearing frequently in mines, churchyards, marshy places, and near stagnant waters. It was formerly thought to have something ominous in its nature, and to presage death or misfortunes. There have been instances of people being decoyed by these lights into marshy places, where they perished; and hence the names of ignis fatuus, Will-with-a-wisp, and Jack-with-a-lantern, as if this appearance were an evil spirit which took delight in doing mischief. The general opinion is, that this light is produced by the decomposition of animal or vegetable matters, or by the evolution of gases which spontaneously inflame in the atmosphere. See Physical Geography.