It's a dark evening, and the barometer indicates an approaching storm. As you walk by the side of the bog, a dim, flickering light catches your eye deep in the gloom over the morass. It flits closer, then away, then back again, and darts to the side, as if beckoning you to approach. It disappears; then it gleams again, to one side. Not a whisper is heard, only a rustle from the wind in the grass.

You have seen Will-o'-th'-Wisp, the "Spunkie" dreaded of superstitious Scottish children, he who wishes to lead them away into the swamp, never to return. Many of their elders shudder, pray and quicken their steps to put the bog behind them, and seek out the warmth of their hearths. The spectre is also called Jack-o'-Lantern in England, and Ignis Fatuus, the "fool's fire," by those with some Latin. A wisp is a handful of tow, broken flax waste, ignited to make a light, carried by the terrible William. Or else it's some changeling Jack, gesturing with a lantern to ensorcell you. In the States, a Jack-o'-Lantern is a hollowed-out pumpkin, with facial features pierced through the shell, and containing a candle. They no longer scare people, even small children, because they are too familiar, and their meaning has been forgotten.

Decaying vegetable matter in the stagnant water and muck of the swamp has used up all the oxygen, so anaerobic bacteria go to work to see if they can make a living extracting oxygen from the sulphates, carbonates, phosphates and carbohydrates in the dark mass, leaving peat, mainly carbon, that can be dug, dried and used as a pungent fuel. They discharge their flatulence in the form of simple gases, hydrogen sulphide, H2S, of the rotten-egg odor, phosphine PH3 and its pyrophoric relative P2H4--all quite poisonous but warning with their pungent odors, like the frightened rattlesnake with its castanets. Also appearing, and probably in the largest amounts, is methane, CH4, "marsh gas" of some fame, odorless and nonpoisonous, but inflammable, as all these gases are. Carbon dioxide, CO2 probably occurs in the mix as well, but it is inert. Bubbles form and coalesce deep in the muck, then insinuate their ways to the surface and fresh oxygen.

When the bubbles reach plain air and burst, the P2H4 spontaneously ignites, forming phosphorus pentoxide and water, but also inflaming the phosphine, hydrogen sulphide and methane, which flares up and then fades as this bubble and that break at the surface. A ghostly flame that flares up appears to approach the observer; one that faints appears to draw farther away. A new bubble bursts to one side, and the flame appears to dart to that place. Will or Jack is only the random ignition of bubbles here and there, and perhaps the burning of a larger reservoir of gas for a time. Not every bubble that bursts succeeds in becoming a flame. None of these gases produce a luminous flame; the phosphine makes a little smoke. The low atmospheric pressure warning of the approach of a storm coaxes the gas out of the bog more readily than usual, so this is the best time to look.

There are no swamps near where I live, unfortunately, so I cannot go out and search for Will in dark, low-barometric nights. If you are lucky enough to live by a swamp, by all means keep alert for an encounter, and report it accurately, capturing some of the gas if possible (you would have to make special arrangements for this, perhaps carrying bottles exhausted with a vacuum pump). Although the ghostly flames have been observed many times, there are never enough reports by skilled observers. I would, of course, be delighted to hear of any observations.


W. R. Corliss, Handbook of Unusual Natural Phenomena (Glen Arm, MD: The Sourcebook Project, 1977). pp. 130-136.

J. Mitchell, Observations on Ignis Fatuus, American Journal of Science 1:16 246-249 (1829). Ignis fatuus in Connecticut.

D. P. Penhallow, A Blazing Beach, Science 22 794-796 (1905). Ignis fatuus at Kittery Point, Me.

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Composed by J. B. Calvert
Created 27 February 2003
Last revised