So what is it and how did this guy demonstrate it, it sounds like something not to do at home.
He got his hand wet and then stuck it in molten lead. The water boils, providing a cushion between his skin and the lead, preventing a burn. This is the Leidenfost effect. He claims to have felt no heat at all. To see if it was really the Leidenfrost effect, he put a dry finger into the molten lead briefly. He says it hurt, and I believe him. DON’T try this at home.
The water doesn’t really “boil”, but behaves in a new way as the liquid phase comes in contact with excessive amounts of heat. You can try a version of the Leidenfrost effect at home that actually led Jearl to discover its “practical” application of hand-in-molten-lead variety. Try heating up a skillet and tossing drops of water on it. At first they sizzle and boil away, but eventually if the temperature is hot enough, the water droplets are seen to slide around the skillet like air hockey pucks. This is because the heat is great enough to instantly vaporize the water on the bottom of the droplet and keeps the top insulated froom the hot surface.
Same principal applies when plunging one’s wet hand into molten lead. The water on the outside (closest to the heat) instantly vaporizes forming an insulating layer of water vapor that protects the hand. Incidentally, there are some drawbacks to the trick. Jearl has burned himself so many times, its debatable whether his discoveries in this area are worth it. Every time you plunge a wet anything into molten lead, the splatter effect occurs and tiny bits of molten lead are likely to hit some other part of your body that’s not protected by the Leidenfrost effect. Leave your hand in the lead long enough and the water eventually vaporizes totally, leading to some nasty results.
Thus, I reiterate, DO NOT try this at home.
Wait, wouldn’t the molten lead coat his hand, anyway?
Because it’s the water vapor that the molten lead is interacting with… no.
The whole point is that water vapor is an excellent insulator, being a gas. Also being a gas will prevent the lead from coating the hand. How does a solid stick to a gas?
Note: to do the molten lead trick, you have to dip your hands in liquid ammonia, you know, like the regular household cleaner. This has the proper evaporative qualities, water alone doesn’t work too well. I recall reading some accounts of the original demonstration, maybe from Leidenfrost himself, of pouring ammonia over your hands and being able to hold molten lead briefly in the palm of your hand. One chem professor said that this act should be required of all chemistry graduates as an act of faith in the laws of physics.
Note: do not try this at home. Perform this only under supervision of trained chemistry experts, like someone who would prove it’s safe by doing it themselves.
<off-topic>
I like the one used by Richard Feynmann better - set up a huge pendulum (like thoese Foucault (sp?) pendulums in museiums), bring the bob right up to your face, and release it gently. When it comes back it cannot hit your face, but will come very, very close to doing so.
</off-topic>
I must admit, the ammonia thing is new to me.
“Evaporative qualities”? I’m not sure I follow. Sounds like the chemistry prof may have been pulling your leg.
Point of the matter is, water works! as demonstrated so brilliantly on “The Tonight Show” with Johnny Carson. In fact it works very well.
Perhaps ammonia prevents the dangerous “backsplatter” I mentioned earlier?
You can see the Leidenfrost effect at work in your kitchen - heat a dry frying pan and then drop a small drop of water onto it. The drop will sizzle and rush around the pan like a mad thing. This is the Leidenfrost effect - the water is boiling upon contact with the pan and forming a small cusion of steam upon which the rest of the drop is floating. The steam insulates the drop from the heat and so the drop can last quite a long time…
It is postulated that the same effect helps “firewalkers”,
I read about a Physics professor who would hold a puddle of liquid nitrogen (-195.8 deg C) on his tongue as a demonstration of his faith in this effect. He said that it worked like a charm until one day when his concentration lapsed and it touched his teeth causing them to look “like crazy paving”.
Don’t try this at home!!
Hey, can we get Cecil to demonstrate this? 
Actually this effect doesn’t have much to do with the sizzling and more to do with the rushing around of said drop. The whole point of the Leidenfrost effect is that the boiling stops because the liquid is well insulated from the heat source. I’ve seen the frying pan splatter occur… there’s a little shout from the waterdroplet, then it skids along quietly for a very long time. Pretty amazing, and in fact I wouldn’t believe it happened if I hadn’t seen it for myself.
As a side note, when Jearl was doing his demonstration of firewalking, he knew that what would keep him safe was the sweat on his feet. The first time he tried it, he was really nervous and felt no effect (presumably because he was sweating so much). The second time he was much more confident and, yep, you guessed it!, he received second degree burns on the bottom of his foot.
The moral he proposed was to have physics students as a “final test” walk across hot coals. You pass the ones who get burned. Sort of one of those circular paradox catch-22 deals.
<anecdotal hijack>
grimpixie, my college prof for first tech physics went him one better. One fine morning, he walked into class carrying a bucket that trailed a thick cloud of fog (this was in southern Louisiana, you see, and it was very humid) and set it on the desk. Needless to say, he had everyone’s attention as he reached into a drawer and pulled out a long-stemmed rose. He dunked the rose into the bucket of liquid nitrogen, which frothed and hissed obligingly. He pulled the rose back out and slammed it down on the desk; naturally, it shattered into many tiny little pieces. Then he dunked a paper airplane and threw it against a wall; it shattered as well. No big surprises yet.
Then he started doing something odd–he took off his wedding ring, and scrubbed vigorously at his left hand with a towel until it was perfectly dry (and warm, one assumes). Then he quickly dunked his hand in the nitrogen, yanked it out, and slammed it on the desk–a number of people screamed and hid their eyes. He was quite uninjured. I still remember the opening words of his lecture:
“Today, we will be discussing heat transfer in relation to the Leidenfrost effect. The key is to always remember which side of the equation supplies the vapor.”
</anecdotal hijack>
Nitpick:
Since this asking for clarification on one of Cecil’s columns, shouldn’t it be under “Comments on Cecil’s Columns”?