Poisson, steeped in Coulomb's notions about magnetic action at a distance, essayed to build up a theory of magnetism of rotation, affirming that all bodies acquire a temporary magnetism in the presence of a magnet, but that in copper this temporary magnetism took a longer time to die away. In vain did Arago point out that the theory failed to account for the facts. The so-called "magnetism of rotation" threatened to become a fixed idea.