The mad stone, in other words, is a variation on the bezoar: a real phenomenon that occurs in ruminants whereby a mass of swallowed matter is compacted into a small, hard orb that is passed through the animal’s digestive tract. The word “bezoar” comes from the Persian for “antidote,” and such objects were long believed to have medicinal properties. In fact, modern chemical analyses have shown that certain bezoars, when immersed in a solution that includes arsenic, can indeed extract the poison from the liquid.