Arago was an influential scientist and powerful advocate for others. Biot and he conducted research across four countries, to fix the exact length of the metre. Humboldt was the first to contact him, after his imprisonment by pirates and other nautical misadventures. He took over Malus’ research after his death, championed his close friend Fresnel’s theories of light transmission, and (eyesight failing) suggested experiments on the velocity of light to Fizeau and Foucault. He corresponded on wave theory and other matters with Young, and got official backing for Daguerre’s photographic work. Gay-Lussac and Delacroix were close friends, Le Verrier a student.
Silliman was responsible for Henry’s appointment at Princeton, despite Henry’s protestation of a lack of formal education. Morse attended Silliman’s lectures on electricity while a student at Yale, and got to know him outside class. Torrey corresponded with him on botany; Whitney knew him for 25 years and was the subject of a memoir he wrote. He met Watt (something of a hero to him) in London; they talked extensively. He knew Priestley when the Englishman came to live in the U.S., and as a hypochondriac was impressed by claims of curative powers for soda-water, which Priestley had invented.
Price and Priestley moved in the same radical circles in London, while Priestley and Hume both welcomed him as a guest. He assisted at Voltaire’s masonic initiation. Pringle was a frequent travelling-companion in Europe; Walpole called on the two in Paris, where Franklin was a regular at d’Holbach’s salon. Casanova sat next to him, listening to him discussing aeronautical balloons with Condorcet (who also later persuaded him that slavery and racial inequality were corrupt). Lavoisier and Franklin fixed the lightning-conductors Franklin had invented to a Paris church; Boswell admiringly quoted him to Johnson.