7 JULY 2007, Page 18

Explosive issue

Sir: You will be tempted to commission another exhibition of panicked paranoia from Melanie Phillips or another hysterical hackette (Gaza is another front in Iran's war on the West', 23 June).

Would it not be a more effective blow against terrorists to ridicule those who thought that the limo-bomb of petrol and gas cylinders could ever work? To believe it displays an understanding of the physical universe scarcely worthy even of a PPE undergraduate.

Petrol is not explosive and propane is not explosive; what is explosive is a mixture of these with air, within narrow composition limits (2-9 per cent by volume for propane, rather less for petrol vapour). Large volumes (the car 'bombs' specified hundreds of cubic metres) of such uniform mixture will not form spontaneously under any normal ventilation conditions.

Just as a flooded engine will not start, so an equilibrium mixture of petrol vapour and air, established in a closed car boot, will not ignite: it is too rich. A venting propane cylinder is still less promising, since it will rapidly sweep all air from the boot. In consequence, it was most probably smoke, as reported, that was seen in the Haymarket, because the igniter circuit had functioned flawlessly — but there was nothing flammable it could ignite. So too, I suspect, at Hyde Park.

PG. Urben Kenilworth, Warks Sir: For the terrorists the events of 29-30 June have been an abject failure: two caught, five suspects arrested, no casualties, insufficient damage. What success there is for them is entirely self-inflicted by us: confusion, fear, new rules (no set-down at airports), resulting traffic jams. The 'oxygen of publicity' should be curbed ab initio, instead of building up the image of the terrorists. Journalists, please think ahead!

Arnold von Bohlen und Halbach Horsham, West Sussex