Jean Baptiste Perrin ForMemRS[1] (30 September 1870 – 17 April 1942) was a French physicist who, in his studies of the Brownian motion of minute particles suspended in liquids (sedimentation equilibrium), verified Albert Einstein's explanation of this phenomenon and thereby confirmed the atomic nature of matter.

In 1901 he proposed a hypothesis that each atom might have a positively charged nucleus, similarly to Hantaro Nagaoka later, but never developed it further.

[4] Following Albert Einstein's publication (1905) of a theoretical explanation of Brownian motion in terms of atoms, Perrin (along with Joseph Ulysses Chaudesaigues who was working in Perrin's lab) did the experimental work to test and verify Einstein's predictions, thereby providing data that would settle the century-long dispute about John Dalton's atomic theory, before the end of the decade.

[5][6][4] Carl Benedicks argued Perrin should receive the Nobel Prize in Physics; Perrin received the award in 1926 for this and other work on the discontinuous structure of matter, which put a definite end to the long struggle regarding the question of the physical reality of molecules.

He also held memberships with the Royal Society of London and with the Academies of Sciences of Belgium, Sweden, Turin, Prague, Romania and China.

[9][10] It remained for Hans Bethe and Carl Friedrich von Weizsäcker to determine the detailed mechanism of stellar nucleosynthesis during the 1930s.

In 1915 he was appointed the deputy chief of the Directorate of Inventions for National Defense which aimed to coordinate French laboratories in the war effort.

In June 1940, when the Germans invaded France, Choucroun and Perrin escaped to Casablanca on the ocean liner Massilia, with part of the French government.