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 in 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 profitably be used. But is it this that is burning in Will o’the Wisp?

Maybe, maybe 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.

The combustion of methane in the laboratory with a poor oxygen supply gives 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. On the other hand it is possible that under some conditions a cool blue flame is obtainable from methane, observed first by Humphry Davy when he was developing the miners' safety lamp in late 1815. In the course of some hazardous experiments, which resulted in numerous explosions, Davy discovered that if the methane was burnt in a thin metal tube much of the heat was conducted rapidly away and a cool blue flame results.¹ Perhaps the flame of Will o’the Wisp, being on the surface of the water, has the heat conducted away by the marsh and a blue flame results?

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 New Age spirit nonsense. (Please do not bother to send me this - my Universe does not work this way.) You will also get the delightful picture from Elizabeth Adela Armstrong Forbes (Canadian), which decorates the top of this page, 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. ³

Bibliography:

1. Holmes, R: 'The Age of Wonder', HarperPress 2008, p364.

2. Will o'the Wisp; 27" x 44" triptych, oil on canvas, ca 1900. Elizabeth Adela Armstrong Forbes (Canadian, 1859 - 1912). The National Museum of Women in the Arts, Washington, DC.

3. Partington, JR: ‘A Textbook of Inorganic Chemistry’, 6^th ed, Macmillan 1957, p 572.

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