There's a good outline of the general picture at ChemTeam's website.

Essentially, the ancient speculative philosophers had a difference of opinion on whether, if you started cutting up a piece of matter, whether you would ever reach a point at which you could no longer make smaller pieces out of your pieces. Demokritus and his followers said yes; Aristotle and most others said no. There was no way to prove the truth one way or another.

The atomists had a number of interesting speculative ideas about how atoms functioned, and many thought that atomic shape had something to do with how one perceived what they were made of (for example, atoms of fire must be sharp and spiky).

Atomism was largely discredited by the time of the birth of Christ, but there continued to be those who espoused the concept right up until Dalton showed that atoms were the simplest way to explain the Law of Definite Proportions. Unlike Demokritus, Dalton didn't just pull it out of his head!

There are two very useful pages on the ChemTeam's site:

• A page on the ancient Greek atomists and
• A page on atomistic ideas from Lucretius to Dalton.

Interestingly enough, our current understanding is somewhere between that of Demokritus (who thought that matter was made of indivisible atoms) and that of Aristotle (who thought that matter was continuous and could always be subdivided no matter how small the piece). Think about it...

Dan Berger