The Commissioner, and the Guillotine

Lavoisier sat on the 1784 commission on animal magnetism alongside Benjamin Franklin and the astronomer Jean-Sylvain Bailly, and the experimental logic of that inquiry is recognisably his. The question was posed as one about an alleged material agent, the tests were arranged so that the agent could be present without the subject knowing and absent while the subject believed it present, and the outcome was read from the difference. It is the same move as weighing a sealed vessel: isolate the variable, then measure.

Bailly wrote the principal report and Lavoisier is generally credited with much of the experimental design. His end came from the other half of his life. He had been a member of the Ferme generale, the private consortium that collected taxes for the crown, an association that funded his laboratory and made him wealthy and that the Revolution treated as a capital offence. He was tried and guillotined on the same day in May 1794, at fifty. The remark attributed to Lagrange, that a moment sufficed to strike off that head and a hundred years might not produce its like, is reported at second hand and is probably too neat, but it has stuck because it is not far wrong.