By the early 1780s Franz Anton Mesmer's Paris clinic, where patients were treated in group sessions around a tub called a baquet that he claimed concentrated an invisible fluid he termed animal magnetism, had become a sensation and a source of medical controversy, prompting Louis XVI to appoint royal commissions to investigate the claims in 1784.
One commission drew four physicians from the Paris Faculty of Medicine together with five members of the Academy of Sciences, including the visiting American statesman and scientist Benjamin Franklin and the chemist Antoine Lavoisier, while a parallel commission from the Royal Society of Medicine examined the question separately. The Academy commissioners designed a series of blinded trials in which subjects were magnetized without being told, and told they were being magnetized when they were not, finding that the convulsive reactions patients displayed tracked their belief that they were being treated rather than any physical procedure performed on them. Their published report, printed in an edition of 24,000 copies and widely circulated, concluded there was no evidence for a physical magnetic fluid and that any real effects of Mesmer's treatment were attributable to imagination, a verdict often credited as one of the earliest applications of a blinded controlled trial to a medical claim and one that effectively ended Mesmer's standing with the French scientific establishment even as his methods continued to circulate under other names, eventually contributing to the later development of hypnotism.
Sources Report of Dr. Benjamin Franklin and Other Commissioners Charged by the King of France with the Examination of Animal MagnetismBenjamin Franklin, Jean-Sylvain Bailly, Antoine Lavoisier, and the Commission of the Faculty of Medicine and the Academy of Sciences with Mesmerism and the End of the Enlightenment in FranceRobert Darnton