Langevin was convinced that science cannot advance if it is confiscated by a minority. It is a common good, to be shared. He accordingly spent much time in outreach activities which count as 20% of his scientific publications. But how did he disseminate his broad scientific culture? Did he just bring science to the people in a top-down manner as most science popularisers used to do in the 1930s or did he engage in dialogue with specific audiences? He was engaged in the big official cultural enterprises of 1930s such as the Encyclopédie française, thePalais de la découverte, and the Centre international de synthèse. But above all he treasured cross-disciplinary exchanges such as the Semaines internationales de synthèseSemaines internationales de synthèse that gathered natural scientists and humanities scholars each year to discuss a specific topic. We argue that these exchanges helped him consolidate his own position in the on-going controversies in theoretical physics.