Jean Perrin

Jean Perrin (1870–1942) was a French physicist whose experiments on Brownian motion and sedimentation provided strong evidence that matter consists of molecules. He received the 1926 Nobel Prize in Physics.

Brownian motion: The irregular motion of microscopic particles suspended in a fluid, caused by collisions with its molecules. Perrin tested Einstein’s account of this motion to estimate Avogadro’s number.
Kinetic theory of gases: A theory that explains gas properties through the motion and collisions of microscopic particles. Its molecular picture supplied predictions Perrin sought to test experimentally.
Albert Einstein: A theoretical physicist whose 1905 analysis gave Brownian motion a quantitative molecular explanation. Perrin tested predictions derived from Einstein’s treatment of suspended particles.
1926 Nobel Prize in Physics: The Nobel Prize awarded to Jean Perrin for work on the discontinuous structure of matter, especially sedimentation equilibrium. The award recognized the experimental case he built for molecules.
Ultramicroscope: An optical instrument that detects particles too small to resolve by viewing light scattered against a dark background. Perrin used ultramicroscopic observations to track suspended particles.
Einstein–Smoluchowski relation: A relation linking a particle’s diffusion rate to temperature, fluid viscosity, and particle size. It let Perrin compare observed particle motion with molecular theory.
Ludwig Boltzmann: An Austrian physicist who developed statistical mechanics and defended the molecular basis of matter. Perrin’s results supplied evidence for the molecular ideas Boltzmann had championed.
Marian Smoluchowski: A Polish physicist who independently developed a statistical theory of Brownian motion. His work provided a second theoretical route to the phenomena Perrin measured.
Atomic theory: The scientific account of matter as composed of atoms with distinct structures and properties. Perrin’s experiments helped turn atomic theory from a contested model into an empirically grounded account.
Colloid: A mixture in which small particles of one substance remain dispersed in another. Colloidal particles made Brownian motion visible and measurable in Perrin’s experiments.

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