The myth

If you chuck a lit fag-end into a pool of petrol – as we all know from countless filmic explosions – it goes “boom”.

The “truth”

Researchers at the Maryland laboratory of the US Bureau of Alcohol, Tobacco and Firearms set out to test a common arsonists’ excuse: that the fire they’re accused of setting deliberately actually started accidentally when a burning cigarette came into contact with gasoline. Two thousand attempts to set fire to exposed petrol – which included spraying the stuff onto a burning snout – failed to produce a single flame. They tried different brands of ciggy and of petrol, they used a vacuum device to imitate a smoker drawing on a butt, they tried petrol both spread out and concentrated. Nothing. They’re not sure why, given that petrol ignites at 246˚C (475˚F) and cigarettes burn at 700˚C (1,292˚F), but suggest it’s because the fuel has limited contact with the hottest part of the fag.

Other experimenters have had similar results, trying variations of air temperature and humidity. Forty cigarettes at once were thrown into a pan of petrol: it didn’t catch fire. All researchers reported that a match will set fire to spilt petrol, but a tossed cigarette just won’t.

Sources

Scotsman, 28 Feb 2007;
http://film.guardian.co.uk/print/0,,329728298-3156,00.html;
www.intuitor.com/moviephysics/mpmain.html.

Disclaimer

I haven’t tested this one, I don’t intend to, and I suggest you don’t either. Experimenters stress that there are, obviously, circumstances in which a cigarette can start a petrol fire – but it can’t happen, they say, the way it’s shown in films. The experimental data seem clear – the theory less so. Your burning rebuttals and explosive explanations are awaited on the letters page.