Democritus

Ancient writers believed that Democritus was the student of Leucippus. They are associated together as the first philosophers to hypothesize that invisible material objects—atoms—make up the universe. Although, like Leucippus, little survives of Democritus’s writings, the theory of atoms was widely discussed by ancient authors; hence much was known about Democritus and his theories even if only fragments of his writings survived. The breadth of his interests is astonishing.

Russell M. Lawson, Science in the Ancient World: An Encyclopedia (Santa Barbara, CA: ABC-CLIO, 2004), 63.