Jean Perrin (1870–1942)

Jean Perrin tested the molecular interpretation of Brownian motion by counting suspended particles at different heights in an emulsion. Their equilibrium number density decreases with height as $n(z)\propto e^{-m_{\mathrm{eff}}gz/(k_{B}T)}$, where the effective mass includes the correction for buoyancy. A microscope thus connects a visible spatial distribution with the microscopic Boltzmann constant. Comparing such measurements with particle size and temperature supplied independent estimates of the Avogadro constant, turning the molecular hypothesis into a quantitative experimental question.^[ref]

Physics (also physical chemistry).^[ref]