Francis Blandy falls into a coma and dies in his home outside London, England. Later that night, Blandy's daughter Mary offered one of the family's servants a large sum of money to help her get to France immediately. Mary was forced to flee on her own when he refused, but she was chased down and caught by neighbors who had heard that Blandy had been poisoned.
The servants in the Blandy home had been suspicious of Mary because the unmarried 26-year-old had been having an affair with William Cranstoun, a penniless man with a wife back in Scotland, against her father's wishes. Cranstoun was determined to get a piece of the Blandy fortune.
Blandy had initially approved of the match, even allowing Cranstoun to live in their house. But when Cranstoun wrote his wife and kindly asked if she wouldn't mind disavowing their marriage, Mrs. Cranstoun became outraged and caused quite a local stir. Cranstoun was then abruptly tossed out of the house, yet Mary continued to see Cranstoun behind her father's back.
The couple, frustrated at their inability to touch Mary's sizeable dowry, decided to find another route to the money. Mary began slipping small amounts of arsenic into her father's food, slowly poisoning him over a period of months. As Blandy began to suffer from nausea and acute stomach pain, the servants grew suspicious. One found white powder in the bottom of a pan that Mary had used to feed her father. After Blandy eventually died, the cook saw Mary trying to dispose of the white powder and managed to preserve some of it.
Mary was charged with murder and faced trial at Oxford Assizes in March 1752. Doctors testifying for the prosecution agreed that Francis Blandy had been poisoned with arsenic. But the test they used on the powder was rather unscientific: They heated it and smelled the vapors—which everyone agreed was clearly arsenic. It wasn't until 40 years later that chemists finally developed true toxicology tests for arsenic. But the jury remained convinced, and Mary was sent to the gallows. She told the executioner, "Do not hang me too high, for the sake of decency."
Not long after Mary was executed, Cranstoun, who had escaped to France, died in poverty.