Deborah Blum tells the true-life tale of "Mary Ann Cotton, a British arsenic killer born in 1832, who was suspected of killing around twenty people—including three husbands, one lover, and most of her own children":
Up until the mid-1800s, scientists had no real test for arsenic in a human body, much less the myriad of other poisons available. Toxic substances were so easy to acquire and so poorly researched that many poison killers just assumed they wouldn’t be caught. Cotton’s execution—resting neatly on scientific evidence—was among many reminders that the world had changed. Scientists were catching up with poisoners and with the catalog of toxic compounds as well.
The Dish 