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.