(1870-1942) French physicist who produced the crucial evidence that finally established the atomic nature of matter. Assuming the atomic hypothesis, Perrin demonstrated how the phenomenon of Brownian motion could be used to derive precise values for Avogadro's number. Nobel Prize 1926.
Perrin also contributed to the discovery that cathode rays are electrons. His experiments included imposing a negative electric charge on a fluorescent screen onto which various rays were focused. As the negative charge was increased, the intensity of fluorescence fell.
Perrin was born in Lille and studied in Paris at the Ecole Normale Supérieure. He worked at the Sorbonne from 1897, becoming professor 1910, but in 1940, during World War II, his outspoken antifascism caused him to flee the German occupation. He went to New York.
His book Les Atomes/Atoms 1913 describes his Nobel prizewinning study of Brownian motion.