Wikimedia
Wikipedia: Democritus
This encyclopedia entry surveys the life and thought of the 5th century BCE Greek philosopher Democritus, who proposed the existence of the atom.
Other
Greek Philosophy: Leucippus and Democritus
This article provides an overview of the atomistic philosophy of the pre-Socratic philosophers Leucippus and Democritus in the 5th and 4th centuries, BCE.
Stanford University
Stanford Encyclopedia of Philosophy: Empedocles
Article about the Greek philosopher Empedocles from the Stanford Encyclopedia of Philosophy. Includes a brief overview of his work and more detailed discussion of his two major works: "On Nature" and "Purifications."