Jean Baptiste Perrin

Jean Baptiste Perrin was born in 1870 in Lille. He studied at the École Normale Supérieure in Paris and took his doctorate in 1897, having already shown that cathode rays are streams of negatively charged particles — an early piece of evidence for the electron and for the corpuscular nature of matter that would become his life's theme. He spent most of his career as professor of physical chemistry at the Sorbonne.

Between 1908 and 1911 Perrin carried out the experiments for which he is remembered. Using carefully prepared, uniform emulsions of resin particles, he tested Einstein's 1905 predictions for Brownian motion from several independent angles — the distribution of particles with height under gravity, the mean-square displacement over time, and their rotational diffusion. Each route yielded a consistent value for Avogadro's number, and the agreement with values obtained by entirely different methods left no reasonable room to doubt that matter is made of discrete molecules.

Perrin gathered his results and their wider significance in the influential 1913 book Les Atomes, which persuaded the last serious sceptics — among them the chemists who had long resisted atomism — that atoms are real, countable objects. He was awarded the Nobel Prize in Physics in 1926 for this work on the discontinuous structure of matter.

A committed republican and organiser of French science, Perrin helped found the Palais de la Découverte and the CNRS. When France fell in 1940 he fled the German occupation, emigrating to the United States, and died in New York in 1942; his remains were later returned to the Panthéon in Paris.