Ig"nis fat"u*us (?); pl. Ignes fatui (#). [L. ignis fire + fatuus foolish. So called in allusion to its tendency to mislead travelers.]


A phosphorescent light that appears, in the night, over marshy ground, supposed to be occasioned by the decomposition of animal or vegetable substances, or by some inflammable gas; -- popularly called also Will-with-the-wisp, or Will-o'-the-wisp, and Jack-with-a-lantern, or Jack-o'-lantern.

<-- thought to be caused by phosphine, PH3, a sponaneously combustible gas. -->


Fig.: A misleading influence; a decoy.

Scared and guided by the ignis fatuus of popular superstition. Jer. Taylor.


© Webster 1913.

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