"Leucippus of Miletus (ca. 435 BCE) and Democritus of Abdera (ca. 410 BCE) developed the atomic hypothesis. According to them matter can be subdivided only to a certain point, at which only atoms (that which cannot be cut) remain. The world is made up of atoms moving in the void. Atoms differed from each other only in size and shape, and different substances with their distinct qualities were made up of different shapes, arrangements, and positions of atoms. Atoms were in continuous motion in the infinite void and constantly collided with each other. During these collisions they could rebound or stick together because of hooks and barbs on their surfaces. Thus, underlying the changes in the perceptible world, there was constancy (atoms were neither created nor destroyed); change was caused by the combinations and dissociations of the atoms."

Albert Van Helden, Atomism, The Galileo Project, Rice University.
This timeline entry suggestion courtesy of
Lynn Mackenzie, Editor,
Cutting Teeth Magazine, Scotland.



Further info :

Atomism - Rice University, Galileo Project
http://es.rice.edu/ES/humsoc/Galileo/Things/atomism.html