[History of ether in the 18th century: George-Louis Lesage's system of corpuscules ultramondains]

The article presents an account of Lesage's theory of corpuscules ultramondains and ether. It shows that essential attributes of the antique concept of ether still belong to his speculative physics. In accordance with this concept, Lesage understood the corpuscules ultramondains as subtle celestial matter and carriers of the primordial movements in the universe which were characterised by their proximity to the divine. On the other hand, Lesage was engaged in the contemporary debate concerning the problem of causa gravitatis. His position turns out to be halfway between dynamic corpuscularity and mechanistic physics. With the former he shared the rejection of vortex theories and the adherence to the corpuscular theory of light and the nutshell theory, with the latter the rejection of action at-a-distance and forces inherent to matter. Contrary to the accusation that he plagiarised Fatio de Duillier's theory of gravitation, it is shown that Lesage formulated a gravitational mechanism essentially different to Fatio's. Lesage used the term ether for the theory of chemical affinities. His attempt to reduce them to gravitational force failed. The complexity of chemical phenomena could not be mastered theoretically by reduction to a uniform natural force.