2 The reality of atoms
This simulation shows an "atmosphere" of Brownian particles suspended in a liquid and falling under gravity. The concentration of particles decreases exponentially as the height increases, which Jean Perrin measured directly to demonstrate that Brownian particles obey Boltzmann’s equipartition of energy theorem - just like they would if they were very large molecules. Perrin then went on to confirm Einstein’s "square root law" and ultimately proved that atoms were not just convenient theoretical abstractions.