Fire-walking is no mystery, mystery, just physics

I WAS hugely entertained by the article on fire-walking in Hong Kong (Sunday Morning Post Magazine, February 20) - or the lack of it.

However, contrary to what the writer was evidently told, scientists can explain very well why most fire-walkers do not get their feet burned at these events. The secret has nothing to do with mental miracles, restructuring body chemistry, or violating natural laws. Nor does it have anything to do with the preparatory pep-talk provided by the seminar marketeers.

The first factor is called the Leidenfrost effect - the insulation provided by a thin film of moisture vaporising between you and the hot embers. You observe the same mechanism when you wet your finger to test a hot iron without burning yourself.

Sweaty feet, however, will not protect you for long enough. The crucial factor is the physics of thermal capacity and conduction. Consider how you can put your hand briefly into a hot oven, and not be burned unless you touch the metal sides - even thoughthe air is at the same temperature as the metal. This is because metal contains more thermal energy than air at the same temperature, and also conducts it from place to place much faster.

Initially, the heating-up of your hand (relatively high thermal capacity and conductivity) cools the air around it faster than the air replaces its own thermal energy from the air between it and the heating element. Of course, if you leave your hand in the oven long enough, it will eventually become hot enough to cook.

Similarly, fire-walkers use materials with relatively low thermal capacity and conductivity - hot embers are, contrary to popular conception, lower in these values than are human feet. (Mystical eastern fire-walkers generally use porous rocks, such as pumice cobbles, that also have low values.) Then, they ensure the feet are in contact with the ''fire'' for a very short time, an observed 1.5 to 1.9 seconds on average. You do not stroll, you walk briskly. This is not long enough to allow the skin to heat up sufficiently to be damaged - unless you dawdle, as some seminarians have discovered to their cost.

If you ever do observe someone ''walking on fire'', watch out for this telltale feature: dark footprints in the embers, lasting for a few moments, where the feet have temporarily cooled the coals more than the coals have heated the feet. I cannot say walking across fire will not make you feel good - especially if you see it as a personal triumph. But the most the seminar salesmen can do is to convince you that you have accomplished a miracle. But there is no miracle; there is only simple physics.

REBECCA BRADLEY Sha Tin