[About the history chemistry and potassium iodide].

Louis Melsen was born at Louvain, he spent four years in Paris, working in Dumas's laboratory. Four letters from Melsens to Chevreul, since 1951 to 1880, are commented on. Two letters relate to Van Helmont and common interest of the two scientists in history of sciences. The others recall Melsens's proposal that potassium iodide can cure and prevent lead and mercury poisoning, and Chevreul's researches about colours seeing.