An eccentric aristocrat who deserted English society to travel the world believed she had found a cure for the plague, letters show.

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 ingredients are not described, but it is thought to have been a homemade concoction.

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.

Written to an unknown recipient, it claims that she acquired the medicine's recipe and then conducted an experiment on a young boy, who came from a family of four stricken with the disease.

"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.