1 Jamie: Today we're testing the myth that working with traditional hatmaking chemicals can make you go mad.
2 Adam: I have some felt impregnated with mercury. Let me just inhale the fumes...
2 [sound]: Sniiiiff!! {Adam holds the felt to his nose and inhales deeply}
3 {scene change: Adam is now standing on the Infinite Featureless Plane of Death, next to a Death wearing a fine top hat.}
3 Adam: I'm seeing things, Jamie! I think I've gone mad!
4 Death of Inhaling Hatmaking Chemicals: NO GUV. YOU AIN'T MAD. YOU'RE DEAD.
4 Adam: Aha! Myth busted!

Hatters in the 19th century typically made hats using felt that had been made using mercury nitrate. The inhalation of the residual vapours from the felt could potentially lead to mercury poisoning, one of the symptoms of which is brain damage and dementia.

It is often said that this is the origin of the phrase "mad as a hatter" and also the character of the Mad Hatter from Lewis Carroll's Alice's Adventures in Wonderland. It's not entirely clear that this is unambiguously the case, as there are also competing theories.

Anyway, I can assure you that it is the origin of the character of Death of Inhaling Hatmaking Chemicals. I thought it was time we saw more of this guy.

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2015-11-18 Rerun commentary: The role of the mercury nitrate in the felt-making process was to assist in the removal of the animal hair fibres from the skin, and to modify the fibres so that they matted together more easily to form the felt material. A weak solution of mercury nitrate was brushed on pelts of rabbits, beavers, and whatever other small animals they were using. The pelts were then dried slowly in an oven, and the mercury compound caused the thin fur around the edges of the pelts to turn orange - so this process was called "carroting"*. The fur was then shaved off and pressed to make felt.

So hatmakers would get their hands on the felt after all of this, with residual bits of mercury still in the fibres. The hatmaking process involved steaming the felt to block it into the shapes of hats, and the steam carried elemental mercury vapour into the air - which the hatmakers then breathed.

* March Hare, anyone?