Henderson, John (1757–88), eccentric scholar, was born 27 March 1757 in Co. Limerick (sources say at Ballygarran or Ballegarance, which may be the townland of Ballycarrane), the only son of Richard Henderson, Wesleyan lay preacher, and his first or second wife, whose name may have been Charlotte. Richard Henderson moved with his family in 1762 to Hanham, near Bristol, England, where he kept a school and later a private asylum, met his third wife (who survived him), and was close to John Wesley's (qv) home; the two men were friends for the rest of their lives. John Henderson was educated at Wesley's school at Kingswood, where his amazing mental ability was quickly noted. At the age of eight he was teaching Latin to schoolfellows, and at twelve he was teaching Greek to men twice his age in Lady Huntingdon's college at Trevecca, Wales. Dr Tucker, dean of Gloucester, was so impressed by his abilities that he paid for him to attend Oxford; he entered Pembroke College (1781) and graduated BA (1786). He continued thereafter to study ancient and modern languages as well as science and medicine.
He was well known, as much for his eccentricities as for his erudition. He refused to conform to accepted dress, wearing his hair like a boy aged six or seven, and refusing to allow it to be ‘strewed with white powder’ (‘An account’, 3) or curled, and he never wore a neckcloth or the then fashionably large shoe buckles. He took to sleeping most of the day; his popularity and desire to shine in society may have caused him to seek the extra stimulation of alcohol and opium. He smoked incessantly and is said to have eaten quantities of opium that would have killed twelve other men; he experimented with the effects of eating mercury and other noxious drugs. Henderson was benevolent and good-hearted; he provided free medical advice for poor people, though he refused to study any medical authorities later than the famous Dutch physician Hermann Boerhaave (1668–1738); moreover, his own habit of sleeping in a wet shirt in wringing wet sheets was perhaps not a practice to be imitated. Literary celebrities of the era, Hannah More and Dr Johnson among them, sought his company and enjoyed his conversation, and his life story, talents, and eccentricities were widely reported in magazines of the day. He disappointed his family by refusing to be ordained; instead he became reclusive and his studies increasingly tended towards the occult. He was interested in alchemy, physiognomy, and astrology, and believed that he had communicated with spirits. Henderson returned to live with his father, but on a visit to his old college died (possibly of opium poisoning), after severe pain and illness, on 2 November 1788 and was buried in St George's church, Hanham. His father was distraught; even after the 86-year-old Wesley walked to his house to comfort him, Richard Henderson had his son's grave opened in case there was any chance of life returning. Wesley (who had made some effort to keep John Henderson from excess) made a characteristically succinct judgement in his journal: ‘with as great and good talents as most men in England [he] had lived two and thirty years and had done just nothing’.