No substance has been as constant an ally to insidious scheming as arsenic, the so-called “king of poisons.” In its chemically pure form, it isn’t much of a threat because our bodies don’t absorb it well; it’s when arsenic combines with other elements that things get dangerous. So, how did this substance become such a popular poison? Neil Bradbury digs into the element's sordid and lethal history. Lesson by Neil Bradbury, directed by Luisa Holanda.