When and Where did the Atomic Theory Come From?

[Rev. Rye]

This is actually a very complicated question:

• Leucippus (his historicity possible, but not proven) and his student Democritus developed the theory of the atom around the fifth century BC in an attempt at reconciling Heraclitus and Parmenides. Aristotle rejected atomic theory and so, with his ascendancy, atomic theory becomes dormant for over a millennium.
  
• Surprisingly, thinkers in India, most notably Kanada (and yes, that was his name), author of the Vaisesika Sutra, stumbled upon a very similar atomic theory to that of Democritus, and possibly even doing so before Democritus. That said, since Atoms weren't mentoned in the Vedas, it did not catch on in India until later.
  
• Near the end of the 18th Century, John Dalton, emboldened by the discovery of the Law of Conservation of Mass, starts work on figuring out the various elements, and even begins to figure out what they're made of and their relative size differences. He first does an oral presentation in 1803, publishes a paper in 1805, and fleshes it all out in 1808's A New System of Chemical Philosophy.
  
• And shit just gets more complicated from there...
  

Thanks. Rev. You've given me something I can work with. What about the work of Heraclitus and Parmenides was Democritus trying to reconcile that led him to think in terms of atoms?

Well, Parmenides believed in monism and denied motion, change, and void. Heraclitus believed change was the only constant in the world. Apparently, the theory of atoms seemed to flow from an understanding of the void for reasons that escape me because the philosphy of the Pre-Socratics can be very strange, with some leaps in logic that may or may not be solely because of gaps in the historical record.