If you've read Lewis Carroll's "Alice's Adventures in Wonderland" or seen any of the movie adaptations, the Mad Hatter is bound to have left an impression. He's eccentric, to say the least, as he presides over a rollicking tea party that Alice attends.
But the idea of being "mad as a hatter" (in the British sense, "mad" meaning "crazy") didn't come from Carroll. And if you, like Alice, have a tendency to fall down rabbit holes, this phrase is a real treat.
"We're All Mad Here"
Carroll's book was published in 1865, but the Oxford English Dictionary puts the earliest known use of "mad as a hatter" in 1829. That's three and a half decades before any March hares or dormice sipped tea, or the Cheshire cat made his famous claim of general madness. The actual origin of the phrase is unknown, but it's believed to be connected to mercury poisoning in hatmakers.
Several years after the Alice first appeared, in 1883, the phrase "hatter's shakes" was used to describe the condition caused by mercury poisoning. The symptoms included muscle tremors, plus mental and behavioral changes. The Hatter behaves strangely in the novel (as do many other characters), but his friends accept his oddities as being the usual.
Today, mercury poisoning is know to the medical and scientific communities as erethism. The modern list of symptoms including irritability and mania, both of which the Hatter has. But there's also sleep disturbance, depression, visual disturbance, hearing loss and those telltale tremors, which the Hatter doesn't seem to have.
You'll be glad to learn that short-term exposure to mercury can cause erethism, but it usually goes away if you can stay away from touching or inhaling mercury. Long-term exposure, such as dental professionals and chemical workers experience, can mean the symptoms persist. In any case, erethism is a rare disease.
"Then You Should Say What You Mean"
At his trial, the Hatter explains to the King that he has no hats of his own because he sells all the hats he has. Which brings us to the last stop in our rabbit hole: What does mercury have to do with hats?
It's part of a process called "carroting." In order to make felt, which is what many hats are made of, you have to get the fur of a beaver or rabbit to stick together in a mat of thick, stiff fabric. To get the fur off the skin cleanly, mercuric nitrate was used. It came to be known as carroting because the solution would turn the edges of the pelts orange as it dried.
Modern haberdashers use hydrogen peroxide to remove the fur from the skin, which is a slower but much safer process.