Lewis Carroll’s 1865 novel “Alice’s Adventures in Wonderland” famously features an eccentric character called the Hatter, who’s referred to in the story as “mad” and became popularly known as the Mad Hatter. However, the phrase “mad as a hatter,” used to describe someone who’s crazy or prone to unpredictable behavior, didn’t originate with Carroll. Instead, the expression is linked to the hat-making industry and mercury poisoning. In the 18th and 19th centuries, industrial workers used a toxic substance, mercury nitrate, as part of the process of turning the fur of small animals, such as rabbits, into felt for hats. Workplace safety standards often were lax and prolonged exposure to mercury caused employees to develop a variety of physical and mental ailments, including tremors (dubbed “hatter’s shakes”), speech problems, emotional instability and hallucinations.

In Connecticut, mercury-induced tremors were called the Danbury shakes, after the city of Danbury, which was a leading center for hat making during the 19th century and into the early years of the 20th century (by the 1920s, only a handful of headwear manufacturers remained in the place once billed as the “Hat Capital of the World”). In the U.S., the use of mercury in the production of felt finally was banned in the early 1940s.

Researchers have suggested that Boston Corbett, a hat industry worker who killed John Wilkes Booth, President Abraham Lincoln’s assassin, might’ve suffered from poor mental health due to mercury poisoning. Corbett, who’d been employed as a hat maker since he was a young man, became a religious zealot and in 1858 castrated himself with a pair of scissors as a way to curb his libido. He went on to serve in the Union Army during the Civil War, and after Lincoln was shot by Booth on April 14, 1865, at Ford’s Theatre in Washington, D.C., Corbett and his regiment, the 16th New York Cavalry, were sent to track down the gunman, who was on the lam. On April 26, the soldiers surrounded Booth in a Virginia barn; however, Corbett disobeyed orders to capture the fugitive alive and instead shot and killed him. Corbett was cleared of blame by the military and lauded by many in the public as a hero for his role in avenging the president’s death. Eventually, he resumed working in the hat industry in the Northeast before moving to Kansas in 1878, where he lived a solitary existence as a homesteader. In 1887, he landed in a mental asylum after threatening a group of people at the Kansas Statehouse with a gun. The following year, this possible “mad hatter,” who was then in his 50s, escaped the facility and soon disappeared for good.