Mary Ann Cotton was hanged at Durham gaol on March 24, 1873 for the murder of her seven-year-old stepson Charles Edward Cotton. She was almost certainly responsible for around twenty-one deaths over twenty years. One was charged.
She was born Mary Ann Robson at Low Moorsley in County Durham in October 1832, the daughter of a pitman who fell down a shaft when she was eight. She trained as a nurse, married four times, and moved constantly around the coalfields of Durham and Northumberland.
The people around her died of what doctors recorded as gastric fever. Her first husband William Mowbray and most of their children. Her second husband George Ward. The children of her third husband James Robinson. Her friend Margaret Cotton, and then Margaret's brother Frederick, whom she married bigamously, and Frederick's sons. Her lover Joseph Nattrass. In almost every case there was a Prudential policy or a small legacy behind the death.
Two things made it possible for so long. The symptoms of arsenic poisoning, violent vomiting, stomach pain, rapid dehydration, were indistinguishable from the gastric illnesses that killed working people constantly in those towns. And arsenic was sold over the counter, mixed into soft soap and fly paper.
She was caught by a remark. In 1872, at West Auckland, she asked a parish official about getting Charles Edward into the workhouse. Told she would have to go in with him, she said the boy was in her way and that he would go like all the rest of the Cotton family. He died five days later. The official went to the police, the doctor kept samples, and the Reinsch test found arsenic.
She was tried at Durham in March 1873 for that one death. The prosecution was permitted to lead evidence of the other deaths as system, the pattern itself admitted as proof, which is the legal mechanism the case is cited for. The defence argued that the arsenic had come as dust from green wallpaper. The jury convicted in ninety minutes.
She was hanged by William Calcraft, who bungled the drop and left her three minutes on the rope.
One conviction stands against roughly twenty-one deaths. The gap is not an evidential accident: each death had been certified by a doctor at the time, and it took a chance sentence to a workhouse official before anyone looked.