a kind of light, supposed to be of an electrical nature, appearing frequently in mines, marshy places, and near stagnant waters. It was formerly thought to have something ominous in its nature, and to presage death and other 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 of that kind. 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.

**IGNITION** properly signifies setting fire to any substance; but the sense is sometimes limited to that kind of ignibiles burning which is not accompanied with flame, such as that of charcoal, cinders, metals, stones, and other solid substances. For the explanation of the phenomena of ignition, see Chemistry.