Franz Anton Mesmer

Mesmer held that a subtle fluid pervades the universe and the body, that disease is an obstruction of its flow, and that a practitioner could restore the flow by passes of the hands and by magnetised objects. His Paris practice in the late 1770s and 1780s was a sensation: patients gathered around the baquet, a tub of magnetised water with iron rods, and underwent the convulsive "crisis" that was taken to signal the cure.

In 1784 Louis XVI appointed a royal commission, its members included Benjamin Franklin, the chemist Lavoisier, and the physician Guillotin, and what it did next matters far beyond Mesmer. Rather than debating the theory, they designed experiments to isolate it: subjects were told they were being magnetised when they were not, and were magnetised without being told. Blindfolded patients convulsed at trees they believed had been treated and were unaffected by trees that actually had been. The commission concluded that the effects were real and the fluid was not, and attributed the results to imagination.

This is one of the earliest blinded, controlled trials in the history of medicine, and it was conducted on an occult claim. Robert Darnton's study is the standard account of the episode and its political setting.

Mesmer's reputation collapsed and his practice did not. His follower the Marquis de Puysegur found that magnetising a patient could produce not convulsion but a lucid sleep in which the subject was responsive and afterwards remembered nothing, artificial somnambulism, which is hypnosis under an older name. Alan Gauld traces the line from there through Braid and Charcot to modern hypnotherapy, and separately into spiritualism, psychical research and the nineteenth-century occult revival, where the magnetic fluid reappears as Levi's Astral Light and, later, as any number of subtle energies.

Mesmer's Paris practice was a theatre of therapy, and its properties explain both the crowds and the commission. Patients too numerous for individual treatment gathered around the baquet, an oaken tub filled with bottles of magnetised water bedded in glass and iron filings; jointed iron rods emerged from its lid, which patients applied to the afflicted part, and a cord looped the company together so the magnetic fluid could circulate. The room was mirrored to reflect the fluid, and Mesmer passed among the patients with his hands and an iron wand, while music played, often on the glass armonica, the instrument of rubbed glass bowls whose unearthly sustained tones he had mastered in Vienna and considered a conductor of magnetism.

Treatment aimed at the crisis: convulsions, weeping, laughter or collapse, understood as the obstruction breaking up, with a padded crisis room standing ready. It was in exactly this setting that his pupil the Marquis de Puysegur found, in 1784, a patient who did not convulse but fell instead into a lucid waking sleep, answered questions with unusual insight, and remembered nothing afterward: artificial somnambulism, the phenomenon that would outlive the fluid entirely and descend, through Braid's renaming, to hypnosis. The apparatus was abandoned; the reliable strangeness it produced in human beings proved to be the durable discovery, which is not the vindication Mesmer wanted.