Lady Hester Stanhope, the niece of the Prime Minister William Pitt the Younger, claimed to have cured a 12-year-old boy with a mysterious remedy called "serpent stone".

The 1814 letter sent from the Convent of Mar Elias, Mount Lebanon, gives an insight into the mind of the aristocrat, who eventually went mad and died a drug addict in 1839, surrounded by feral cats.

"I fixed upon a boy about twelve years old to make the experiment upon, & ordered the Turkish barbar to make a slight incision upon the buboe [an inflamed lymph node], & then apply the stone, which stuck like a leech.

"At the end of four hours it fell off; upon being put into warm milk it discharged its poison & the milk turned sour.

"[I] set off again the next day to pay him another visit. What a dreadful sight: all dead except the boy in question…"

The letter is expected to fetch up to £3,500 at Sotheby's, London, on May 8.