JBCS

INTRODUCTION

Ignis fatuus, Will-o'-the-Wisp, Jack-o'-Lantern, Shui têng, Hitodama, and Boitatá are just a few of the names used to denominate a luminous phenomenon witnessed over centuries by observers in various parts of the world. It appears as a pale bluish flame and was generally seen at night in cemeteries and swamps and near stagnant, still waters and canals. Ignis fatuus is the stuff of legend and lure, striking fear in those who traveled by night, inspiring poets and artists, and sparking debate among famed scientists. Former references to ignis fatuus appear in Chinese literature. There are records of 'water lanterns' (shui têng) in the Kuei Hsin Tsa Chih of Chou Mi (13^th century), Li Shih-Chen (+1596) calls it 'yang flames of marshes', and lights in the water are reported in Shan Thang Sio Khao by Phêng Ta-I (+1595).¹ It also appears in the works of William Shakespeare in Henry IV,² Johann Goethe in Faust,³ Emily Dickinson in Those-Dying Then,⁴ Lewis Carroll in Euclid and his modern rivals,⁵ Charlotte Brontë in Jane Eyre⁶ and others, including the well-known stories of Lord of the Rings and Harry Potter.⁷ In music, it appears in the classics, Franz Liszt in The Transcendental Etude No 5 'Feux Follets,⁸ Franz Schubert in Winterreise,⁹ and Frédéric Chopin in Étude in A Minor Op. 10, No. 2.¹⁰ In Brazilian folklore, ignis fatuus has several denominations, including João Galafoice, Cumadre Fulozinha, Mula sem Cabeça, Tocha, Fogo Corredor, Fogo Fátuo, and Boitatá, this last of which is derived from the Tupi indigenous language of mboî tatá and means 'fire snake'. In one of his letters, the European colonizer Father José de Anchieta speaks of a 'ghost' in the form of a 'sparkling beam' that attacks the indigenous people and kills them.¹¹ This particular entity is a character in Brazilian folklore that defends nature against the attack of its predators.¹²

Ignis fatuus today calls our attention not due to its natural mystique, but precisely because the phenomenon is no longer witnessed. Considering how far we have come in our capacity to observe and record natural phenomena, this disappearance constitutes a mystery.¹³ However, understanding ignis fatuus as consisting of a cool flame of methane can provide a reasonable explanation for this disappearance.

A METHANE COOL FLAME

Meaning 'foolish fire' in Latin, ignis fatuus is not quite brilliant enough to illuminate its surrounding environment and glows 'without heat', as noted by Isaac Newton,¹⁴ who distinguished the phenomenon from the flame of a candle or that of burning wood. Joseph Priestley¹⁵ analyzed accounts of ignis fatuus and found that it occurred during a 'dark and calm night', emitting a 'pale inoffensive light'. In 1783, George Washington and Thomas Paine conducted an experiment in the Millstone River to prove their hypothesis that ignis fatuus was produced by methane emanating from swamps.¹⁶ They held torches close to the mud, where they saw bubbles rise, at which point a flash of light broke out across the water. Similarly, Louis Blesson, the author of very detailed accounts of ignis fatuus, also employed torches to produce flames in the air bubbles rising from the marshes in the Gorbitz Forest, Newmark, and other localities in Germany.¹⁷

Swamp gas is composed of approximately two-thirds of methane,^18,19 a product that emerges from the bacterial anaerobic decomposition of cellulose and proteins in vegetation. While the combustion of methane in air generally produces hot flames, it can also produce cool flames.^20,21 Unlike conventional flames, which generate large amounts of heat, carbon dioxide, and water, cool flames result from a slow combustion process. In 1817, Humphry Davy²² observed cool flames by inserting a hot platinum wire into a mixture of air and diethyl ether vapor. According to Davy, 'When the experiment on the slow combustion of ether is made in the dark, a pale phosphorescent light is perceived above the wire...'. Harry Emeléus²³ recorded the first emission spectrum of low intensity, low temperature flame for different fuels, which included ether, aldehydes, and hexane. Vladimir Kondratiev²⁴ identified the spectral bands as being identical to gas phase formaldehyde fluorescence. A cool flame can also be observed in methane under certain conditions. At atmospheric pressure, the temperature of the methane cool flame is below 475 ^oC, whereas the hot flame occurs from 580 ^oC.²⁵ The cool flame of methane produces excited formaldehyde, or CH₂O*, in addition to H₂O₂ and small quantities of CO, H₂, CO₂ and H₂O.²⁰ In fact, the formaldehyde emission spectrum, with peaks at 350 nm and 400 nm,^23,26 corresponds to the same pale bluish color as that of ignis fatuus.

THE IGNITION PROBLEM

Although the methane cool flame hypothesis is quite acceptable, a major problem is related to the spontaneous ignition of swamp gas. The British geologist Alan Mills^27,28 attributed the auto-ignition of swamp gas to the oxidation of phosphine, which is produced in biodegradation processes. While phosphine is indeed present in low concentrations in swamp gas,^18,19 this theory does not explain natural ignition, since phosphine or diphosphine oxidation itself are not spontaneous in air.²⁹ The Italian chemists Luigi Garlaschelli and Paolo Boschetti, 'on the track of the Will-o'-the-wisp', successfully created a weak, cool flame by mixing phosphine with air and nitrogen in certain proportions.³⁰ Those cool flames were greenish in color. Therefore, they do not resemble the natural phenomenon, which emits bluish light. No other component of the swamp gas could initiate the combustion of methane. In 1776, Alessandro Volta considered the interaction between electricity and flammable air to be the master key to understanding the ignis fatuus phenomenon. Accordingly, such an interaction could not only give an account of the ignis fatuus phenomenon, but it could also explain various 'meteors'.³¹ However, it is important to separate ignis fatuus from other cases, such as luminous meteors. Furthermore, there is no connection of the ignis fatuus with external sources of electricity. For a better understanding of the ignition problem, an analysis of the methane cool flame reaction mechanism was performed. The reactions involved in the cool flame of methane and the formaldehyde formation are shown in Figure 1.

Figure 1. Methane cool flame reaction mechanism. The excited formaldehyde (CH₂O*), which is responsible for the blueish emission of the cool flame, is formed in the last step

The first step is the methane oxidation CH₄ + O₂ ⇆ CH₃ + HO₂. The CH₃ radical turns into CH₃O₂ with O₂ addition, and HCO through the reaction CH₃ + O₂ ⇆ HCO + H₂O. Due to the presence of HO₂, CH₃O₂H is produced by the reaction CH₃O₂ + HO₂ ⇆ CH₃O₂H + O₂. The excited formaldehyde CH₂O*, which is responsible for the blueish emission of the cool flame, is formed with the HCO addition CH₃O₂H + HCO ⇆ CH₂O + CH₃O₂.²⁰ The molecular orbital calculations were performed at the Density Functional Theory (DFT) level using the hybrid functional B3LYP and employing the 6-311G basis set.³² As simple molecules are involved in the reaction mechanism, the results obtained with this level of theory are reliable for the present purpose. The bond lengths of the molecules were taken from the experimental database.³³ The optimization of the intermediate state in each of the steps is performed by approximating the two molecules until they reach the top of the potential barrier. As shown in Fig. 1, the activation energy of the chemical reactions is more than a hundred kcal/mol (117.38 kcal mol^-1 in the first step), corresponding to temperatures above 250 °C. This means that, in nature, an extra source of energy is needed to ignite the cool flame of methane. That is, ignis fatuus is not the spontaneous combustion of swamp gas, as has been argued for years.^13,27

CONCLUSIONS

For more than a century, there have been no reliable records of ignis fatuus sightings like those reported in the past. Marshy grounds are no longer common on the planet, which could explain this disappearance. However, marshes are still being preserved today, and there are cemeteries and swamps where methane is naturally produced. A more adequate explanation can be found by understanding ignis fatuus as a cool flame of methane, a nonspontaneous phenomenon in nature, requiring it to be ignited. Washington and Paine in North America,¹⁶ along with Blesson in Europe,¹⁷ were successful in their efforts to employ torches that could produce these flames in the air bubbles that rise from the swamp. Their experiments were valuable in affording us some understanding of the many sightings of ignis fatuus in the past, when torches and lamps were commonly used to guide travelers at night through the forest. It seems that ignis fatuus is not the days of 'cold' lanterns, swamp landfills and light pollution. Although artificial ignition of gases by humans is a reasonable explanation, the mystery of the extinction of ignis fatuus may not be completely solved. Activation energies can be overcome with catalysts that exist in the air in the form of dust, microorganisms and pollutants. In cemeteries, for example, there are certain circumstances different from those in swamps that may require other chemical alternatives to explain the phenomenon. Therefore, it seems premature to proclaim the extinction of the ignis fatuus. At any time, someone may, either accidentally or as planned, announce the return of ignis fatuus, this time no longer to fool people, but perhaps to continue to inspire artists and intrigue scientists.

ACKNOWLEDGEMENTS

This work used resources of the National Center for High Performance Processing in São Paulo (CENAPAD-SP). Thanks to Cavani Rosas, Atelier Recife, Brazil, for the pen ink painting of the Graphical Abstract.

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