Here's a fragment regarding atomic theory from the site that I put in the thread earlier:
Epicurus and the Epicureans
The ancient atomists (Leucippus and Democritus) had already worked out a systematic description of the natural world comprising many particular material particles, whose mechanical interactions account for everything that happens. In the Hellenistic period, attention turned to the consequences of such a view for the conduct of human life.
Epicurus and his followers pointed out (in the Principle Doctrines, for example) that since the indestructible atoms that constitute the material world move, swerve, and collide entirely by chance, everything that happens in the universe lies outside the reach of direct human control. (Notice how this position projects Hellenistic political impotence onto the natural world.) Human life is, therefore, essentially passive: all we can do is to experience what goes on, without supposing ourselves capable of changing it. Even so, Epicurus held that this sort of life may be a good one, if the experiences are mostly pleasant ones.
I think Democritus should be credited with formulating the first atomic theory:
Democritus (460-370 BCE) Presocratic Greek philosopher. As the originator of classical atomism, Democritus maintained in opposition to the Eleatics that the universe comprises a plurality of distinct entities that really do move. The haphazard collisions of these individually indestructible atoms, he believed, account for the formation and dissolution of all observable things. Long before its appropriation by Epicurus, this doctrine produced an attitude toward human life that earned Democritus a reputation as "the laughing philosopher."
hmm...this didn't cut and paste right...here's the page then (scroll down to the second paragraph called Epicurus and Epicureans)