Boltzmann's theories were apparently rejected by some of his influential contemporaries (the philosopher Ernst Mach, for one) partly on the grounds that there was no direct evidence for the existence of atoms and molecules [1]. It has been suggested that this may have been a contributing factor in his suicide. If so, then it is sadly ironic that his suicide occurred between Einstein's publication of his famous paper that cast Brownian motion as evidence for the existence of atoms and molecules, and the experimental verification of his statistical model of the process by Perrin in 1908 [2], which quickly put an end to the doubting of the reality of atoms.

[1] http://jfi.uchicago.edu/~leop/AboutPapers/Boltzman.pdf [2] https://en.wikipedia.org/wiki/Brownian_motion