IG'NIS FATTIIS (Let. "vain or foolish fire") is a luminous appearance frequently seen in marshy places, churchyards, and over stagnant pools, which has puzzled phi losophers from the time of Aristotle. It generally appears a little after sunset, as a pale bluish-colored flame, varying in size and shape; sometimes it shines steadily till morn ing, at other times disappears, and reappears within about half-hourly intervals. It floats in air at about 2 ft. from the ground, is sometimes fixed, and sometimes travels with great rapidity. In general, it recedes on being approached, and rice versa, though several successful attempts have been made to light a piece of paper by it. Many efforts have been made to discover its cause.; but so varied are its appearances, and so void of any common principle, that these attempts have totally failed. Of the various theories advanced we need mention only two. The first is that the ignis fatuns is due to p/098 phureted hydrogen gas (PII,), which possesses the power of spontaneous ignition on coming in contact with dry atmospheric air; the gas would be generated by the decom position of animal matter present in a marshy soil. The motion of the ignis fatuus is accounted for by the flame being communicated along the line of a stream of the gas. The second is that it is due to the combustion of light carbureted hydrogen gas arising from the decomposition of vegetable matter; but though this supposition satis factorily accounts for many appearances connected with the ignis fatuus, the gas itself is not spontaneously combustible, and an additional supposition requires to be made to account for its ignition. The probable conclusion is that a number of phenomena
similar to the eye, but arising from different causes, are aggregated under the term ignis fatuus. The ignis fatuus, however, has never been produced artificially Electricity and phosphorescence can produce the luminous appearance, but, as far as our present knowledge enables us to judge, they are unable further to imitate it.
It is not a common phenomenon, many distinguished naturalists never having seen it; but it is not unfrequently seen in the n. of Germany. the swampy and moorland dis tricts in the s. and n.w. of England, and in the lowlands of Scotland. It is seen in the above places from the middle of autumn till the beginning of November. In former times, the ignis fatuus, under the names of Will-o'-the-wisp, jack-a-lantern, Spunkic, etc., was an object of superstition among the inhabitants of the districts where it appears, and was believed to be due to the agency of evil spirits attempting to lure the traveler to his destruction; and, unfortunately, there are too many instances on record of trav elers mistaking the ignis fatuus for a lamp, and being thus decoyed into marshy places, where they perished.