You’ve probably never tried to carry a mercury thermometer or barometer on a plane.

But even if you really wanted to, you can’t in the U. S., unless you happen to work for a government weather agency.

That might seem like another weird restriction, but there’s an important reason for it: That tiny bit of liquid metal is fascinating, but really dangerous. Mercury is the only metal that’s a liquid at room temperature, which is why it’s sometimes called quicksilver — which comes from the Old English for “living silver”, not the X-Men. And it’s especially useful for thermometers because of how much it expands when you heat it.

It has what’s called a high coefficient of thermal expansion, which means that when you heat it up a little, it expands a lot. So that way, you don’t need to use a magnifying glass on your thermometer to see if you need a sweater. Mercury also forms mixtures, or alloys, with a lot of other metals very easily.

An alloy with mercury in it is called an amalgam, and they’re useful for all sorts of things. For example, if you’ve ever gotten a silver filling at the dentist, that’s a harmless amalgam of metals like mercury, silver, and copper. But some amalgams aren’t so harmless.

Like, when mercury comes into contact with pure aluminum, things get pretty quickly. And you definitely don’t want it in your teeth. We make so many things — like airplanes — out of aluminum because it’s so durable.

When most metals are exposed to oxygen, they rust and degrade over time. But when aluminum reacts with oxygen, it forms aluminum oxide, which is non-reactive and protects the pure metal underneath. Normally, this is great.

Unless there’s mercury involved. If mercury can get to the pure aluminum, like through a scratch on the plane, it immediately starts to react and seep into the metal, forming an amalgam. When that amalgam meets the air, it stills turns into aluminum oxide — except this time, because there’s mercury involved, the reaction doesn’t stop.

And the aluminum oxide starts to grow out of the plane like some kind of cyberpunk plant. And the mercury isn’t consumed during this reaction, so it keeps reacting with more aluminum, and the whole cycle continues until either the mercury evaporates or there’s no aluminum left. Which is why you don’t want your old-timey thermometer anywhere near a plane.

If you have enough of it, the mercury can slowly destroy the integrity of the plane. And even though a tiny amount of mercury probably won’t do fatal damage, mercury spills have damaged and even grounded planes in the past. But conveniently, even if you really did want to measure the temperature on your plane, most thermometers these days don’t have mercury in them, since it’s also toxic if it enters your body.

So even though some air travel laws change over time, you probably won’t be bringing your mercury thermometer on board any time soon. And I’m okay with that. Thanks for asking, and thanks to all our patrons on Patreon who keep the answers coming!

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