When did chemists start to believe in atoms?

Few – if any – chemists would doubt today that atoms and molecules exist. Yet, as I have pointed out previously, the validity of this belief is far from evident. In chemistry too, the existence of atoms and molecules was not always uncontroversial. When John Dalton proposed that chemical elements are species of atoms, his theory wasn’t really conceived as literal, not even by himself. He offered a handy visual representation, with images and wooden balls that allowed chemists to understand how molecules are formed. Nevertheless, these were mostly thought of as teaching tools, rather than depictions of real things. 

This attitude allegedly changed decisively with Jean Perrin, who claimed to have empirically established the reality of atoms and molecules. The Nobel prize in physics awarded to him in 1926 was explicitly given on these grounds. His achievement was based on his analysis of Avogadro’s number. He showed that approximately the same value is calculated by different methods applied to a diverse range of phenomena, including the diffraction of light by atmospheric molecules, radioactivity, and black-body radiation.