On December 13, 1778, Franklin noted in his journal that he had received, from an unknown philosopher, a manuscript on elementary fire written in English “with a little tincture of French idiom,” but he felt he could not pass judgment on it until he had seen the experiments.8 The unknown philosopher turned out to be Jean-Paul Marat, and a number of letters in the early spring of 1779 deal with the experiments in question.

Franklin and Marat had a surprising link: they both held degrees from the University of St. Andrews in Scotland. Franklin’s honorary one went back to 1759 and Marat’s, a degree in medicine, to 1775.9 After ten years in England and Ireland (1767–77), in the course of which he had published a book attacking Helvétius and Voltaire,1 the future revolutionary, now in his mid-thirties, was back in Paris, where he occupied the prestigious position of physician to the gardes-du-corps of the comte d’Artois.

A restless, ambitious man, Marat set out to challenge the views of Lavoisier and other establishment scientists on the nature of fire. In order to prove that from fire there emanates a fluid, for which he used the term fluide igné, and that this fluid can be made visible, he devised a series of one hundred twenty experiments to be carried out in a dark room with the help of an instrument he had perfected, the microscope solaire.2

Those experiments were witnessed by a commission appointed by the Académie des sciences, consisting of Franklin’s old friend Jean-Baptiste Le Roy, Sage, Maillebois, and de Montigny.3 They were held first in the rue de Grenelle, and, as of early March, in the rue de Bourgogne at the hotel of the marquis de l’Aubepine.4 Franklin’s presence was much desired both by Marat, who, in a series of letters signed “the author’s representative,”5 begged him to come, and by Le Roy acting as a go-between.

After several missed appointments, possibly because of his gout, Franklin eventually joined in. Sage left a vivid account of the Doctor’s participation: “M. Franklin, qui était témoin de ces expériences, ayant exposé sa tête chauve au foyer du microscope solaire, nous l’aperçûmes ceinte de vapeurs ondulantes, qui se terminaient en pointes torses; elles représentaient l’espèce de flamme que les peintres ont fait l’attribut de Génie.”6

Le Roy, in his report to the Academy, related the same episode in more sober terms: “M. Francklin qui a assisté avec nous à plusieurs de ces expériences, ayant présenté sa tête, sa main à ces rayons, on en vit s’élever sur la toile des émanations on ne peut pas plus apparentes …” His review sounds somewhat embarrassed, as if he were torn between his wish to be fair to the novelty and ingenuity of the experiments and his doubt as to the validity of the theory they were meant to prove.7 Marat, however, interpreted Le Roy’s statement as an endorsement and used it as an introduction to the essay he published soon after under the title, Découvertes sur le Feu, l’Electricité et la Lumière Constatées par une Suite d’Expériences Nouvelles qui Viennent d’Etre Vérifiées par MM. les Commissaires de l’Académie des Sciences. He presented this work to the Academy on June 19.8 But his next foray against the scientific establishment, an attack on Newton’s theories of color—for which once again in August he sought Franklin’s participation —was sharply rejected and intensified his hatred of all academies.