Fox-fire

On the inside is fixed a Barometer, by which he can tell the depth he is under water; a Compass, by which he knows the course he steers. In the barometer and on the needles of the compass is fixed fox-fire, i.e. wood that gives light in the dark.

["I]t won't burn our fingers, though, if we take it up. It is nothing but fox-fire. Humph! an old rotten stump:" and groping round for a stick, he struck it and knocked it all about, and wherever it lay it looked like burning coals.

[...] Tom said we got to have some light to see how to dig by, and a lantern makes too much, and might get us into trouble; what we must have was a lot of them rotten chunks that's called fox-fire and just makes a soft kind of a glow when you lay them in a dark place.

[Nathaniel] Hawthorne in one of his books records a remarkable personal encounter with this weird fox-fire, and one which cost him dearly. He was on a journey by canal-boat, which had stopped en route for a brief period at midnight. During the interval he had stepped ashore, and was decoyed into a neighboring wood by the bright glow, which proved to be a fallen tree ablaze with phosphorescence. In his surprise and interest he lost all account of time, and thus missed his boat, [...] Almost any damp woods, especially after a rain, is likely to disclose its fox-fire, but it occasionally appears under circumstances where we little expect it.