AS MAD AS A HATTER

TO BE AS MAD AS A HATTER MEANS TO BE COMPLETLY MAD, BUT WHY? AND WHY HATTERS?
LONG TIME AGO, HATTERS MADE HATS WITH POISONOUS SUBSTANCES, GLUES... SO AS YOU CAN IMAGINE THEIR MENTAL FACULTIES BECAME WORST WITH THE TIME.
IN THE NOVEL ALICE'S ADVENTURES IN WONDERLAND, OR IN THE RECENTLY FILM OF TIM BURTON, WE CAN WATCH A VERY CRAZY HATTER, PERFORMED BY JHONNY DEEP. HE IS EXCELENT TO EXPLAIN THE IDIOM. HE IS TOTALLY MAD, TOTALLY CRAZY. AND AS WE HAVE SAID, IN THAT NOVEL APPEARS A MARCH HARE AND A MAD HATTER.
BY THE WAY, DO YOU KNOW THAT SOME PEOPLE SAY THAT THE HATTER LOOKS LIKE MADONNA? I AM NOT REALLY SURE...