The royal commission on animal magnetism

Anton Mesmer, a doctor of medicine from Vienna, taught that an invisible fluid he called animal magnetism pervaded all living things, and that manipulating it could relieve or cure any disease. Some magnetized patients fell into what practitioners called crises, states of convulsion. In March 1784 the king named four physicians of the Paris faculty to examine the practice. At their request he added five members of the Academy of Sciences, among them Benjamin Franklin and Antoine Lavoisier.

Mesmer refused to let anyone test whether the fluid existed. Charles Deslon, a former associate who had broken with him, agreed to demonstrate his methods and cooperate. The commissioners decided to test the fluid and the imagination separately. At Franklin's house at Passy, blindfolded subjects were led to believe that Deslon was magnetizing them when he was not in the room. They fell into crises anyway.

On 19 June 1784 Deslon magnetized an apricot tree in Franklin's orchard. A boy of about twelve, whom Deslon had chosen as sensitive, was led blindfolded to four other trees in turn and made to embrace each. His symptoms grew as he went, and at the fourth tree, which had not been magnetized either, he fainted and his limbs stiffened.

The report, signed in Paris on 11 August 1784 by all nine commissioners, found no proof that the fluid existed. Imagination without magnetism produced convulsions, it said, and magnetism without imagination produced nothing; the crises came from pressure, imagination and imitation. A separate commission from the Royal Society of Medicine signed its own report against animal magnetism on 16 August.

The report speaks only for the commission as a body. Writing for the James Lind Library in 2016, Ian Donaldson pointed to a plan of experiments in Lavoisier's own hand, printed in his collected works in 1865, and argued that the blind tests were most likely his design. Sixteen years later John Haygarth found that fake wooden tractors eased rheumatism about as well as the metal tractors credited with magnetic healing, and he cited the Paris experiments.