IGNIS FATUUS, the name applied to the pale flame, also called will-o'-the-wisp and jack-o'-lantern, sometimes seen flick ering over marshy ground and, it is said, over churchyards. No entirely satisfactory explanation has been put forward but it is generally believed that the effect is due to the spontaneous ig nition of gases (especially methane or marsh gas, pro duced by the disintegration of dead plant and, possibly, animal matter.