‖ pl. Ignes fatui [ L. ignis fire + fatuus foolish. So called in allusion to its tendency to mislead travelers. ] 1. 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. It is thought by some to be caused by phosphine, PH3, a sponaneously combustible gas. [ 1913 Webster + PJC ]

Will o'the Wisp -- which also rejoices in the names of Ignis Fatuus or Jack o'Lantern -- is not, as some of you may think, a cartoon character. In mediaeval times this chemical phenomenon struck terror into travellers and, very likely, lured some of them to their deaths in a stinking and marshy grave.
I have never seen this Will o'the Wisp; nor am I likely to do so. It is a flickering flame seen over marshes; marshes are not now common in London, nor indeed anywhere else in Britain. In any case the ephemeral nature of the phenomenon and the enormous amount of ambient light “pollution” found in most areas means that most of us will never see it.
What is this Will o'the Wisp? Popular chemical lore has it that it is marsh gas, or methane, which catches fire when it hits the air because of the presence of either phosphine (PH3) or diphosphine (P2H4) in the gas, both of which are spontaneously flammable in air. Methane is certainly formed in marshes, and bubbles up if the mud is disturbed in a pond, say. It is the same reaction that enables organic materials to produce biogas, methane from the decomposition of sewage, which can be profitably used. But is it this that is burning in Will o'the Wisp?
Almost certainly not. At this point I will say that I have thought for some years off and on as to how one might set up an experiment to test the hypotheses, since the sporadic and rare nature of the natural version renders its investigation a highly intractable problem. However: the combustion of methane under the conditions in a marsh would give a yellow flame, and heat.
Will o'the Wisp is not like this, so it is said. Firstly the flame is bluish, not yellow, and it is said to be a cold flame. The colour and the temperature suggests some sort of phosphorescence; since organic material contain phosphorus, the production of phosphine or diphosphine is scarcely impossible, and maybe it does oxidise via a mainly chemiluminescent reaction. The exact nature of the Will o'the Wisp reaction nevertheless remains, to me at any rate, a mystery. Similar phenomena have been reported in graveyards and are known as corpse candles. If anyone knows anything more, I would love to hear of it. A warning that if you look for it on the Web, you will get a great deal of bizarre stuff. You will also get the delightful picture from a Canadian artist which decorates the top of this page (https://web.archive.org/web/20120828064103/http://www.rod.beavon.clara.net/willo.htm), and a couple of poems at least. One is also by a Canadian, Annie Campbell Huestis, the other by the prolific fantasy poet Walter de la Mare.
The preparation of phosphine in the laboratory (by the teacher!) is fun, and perfectly safe in a fume cupboard. White phosphorus is boiled with aqueous sodium hydroxide solution in an apparatus from which all air must have been removed by purging with, say, natural gas. The phosphine will form marvellous smoke rings if allowed to bubble up through water in a pneumatic trough. This is an experiment for the teacher, needless to say. The experiment is described in Partington J.R., “A Textbook of Inorganic Chemistry”, 6th ed, Macmillan 1957, p 572. (So, inter alia, is a great deal of other interesting chemistry.)
Dr. Rod Beavon
17 Dean's Yard London SW1P 3PB
e-mail: rod.beavon@westminster.org.uk [ PJC ]

2. Fig.: A misleading influence; a decoy. [ 1913 Webster ]

Scared and guided by the ignis fatuus of popular superstition. Jer. Taylor. [ 1913 Webster ]