When was atomism first considered a scientific theory?

Question

Of course it is difficult, if not impossible, to know whether classical philosophers that talked about atomism thought that "atoms" could be manipulated by men. However, I was startled to know that even at the beginning the 20th century, there were scientists who disputed the existence of atoms, and that it took the explanation of Brownian effect by Einstein to convince them.

My understanding - but I may be wrong - is that the early chemists at the end of the 18th century talked about substances, non atoms: some chemical compounds were considered basic. So, when did atomism started to gain widespread, even if not global, acceptance?

Answer 1

We can see the SEP entry : Atomism from the 17th to the 20th Century by Alan Chalmers for a useful overview.

See in particular the Concluding Remarks :

If we take atomism to involve the claim that the properties of macroscopic matter arise as a result of the combinations and motions of tiny particles, then it is a position confirmed by the time of the Solvay Conference in 1911 in a way that left little room for sensible doubt.

But if we take atomism in a stronger sense, to mean a theory that explains all of the properties of macroscopic matter in terms of underlying particles with specified properties and governed by specified laws, then it must be denied that atomism had reached its objective in 1911.

There were identifiable inadequacies and gaps in the specification of the properties of atoms and the electrons and protons that compose them and there were to an increasing extent problematic experimental results that were eventually to lead to a radical change in the laws that were presumed to govern the behaviour of atomic and sub-atomic particles.

Answer 2

Atomism as a scientific theory has two origins: development of chemistry and development of statistical physics. In chemistry it was experimentally discovered that that there are elements and compunds, and elements probably consist of atoms which combine into moleculas. Atomic weights were measured in the middle of 19-s century, and then came the periodic system. However in chemistry atoms were a kind of theoretical construction. "Nobody has seen them", as many people in 19-th sentury said.

From statistical physics comes the Avogadro number, and statistical physics was developing during the whole 19-th century. However until the very end of the 19-th century there were serious physicists which denied the existence of moleculas and atoms (as physics defines them, not chemistry). Ernst Mach is a notable example. The crucial thing was the discovery of Brownian motion and developing of its mathematical theory by Einstein and Smoluchowski. This theory permitted to measure the Avogadro number experimentally, and the existence of atoms and moleculas became an undeniable experimental fact. This happened around 1905.