 Skeptics about atomism finally became convinced at the beginning of the 20th century by careful experimental and theoretical work on Brownian motion, the fluctuation of particles in an emulsion. With the development of kinetic theory it was suspected that this motion was due to invisible particles within the emulsion pushing the visible particles. But it wasn’t until the first decade of the 20th century that Einstein’s theoretical analysis and Perrin’s experimental work gave substance to those suspicions and provided an estimate of Avogadro’s number, which Perrin famously argued was substantially correct because it agreed with determinations made by several other, independent, methods. This was the decisive argument for the existence of microentities which led most of those still skeptical of the atomic hypotheses to change their views (Einstein 1905; Perrin 1913; Nye 1972; Maiocchi 1990).