Jean-Baptiste Perrin

Jean-Baptiste Perrin (1870-1942), French Physicist, Nobel Laureate. Perrin proved the discontinuous (or atomic) nature of matter, discovered the sedimentation equilibrium (the mathematical basis for the settling of small particles in liquid), and thus confirmed the existence of molecules. For this work, he was awarded the Nobel Prize in physics in 1926. Born in Lille, France, he studied at the Ecole Normale Superieure, where he received his doctorate in 1897 with a dissertation on cathode rays and X rays. He was a lecturer in physical chemistry at the Sorbonne in Paris and a professor there from 1910 to 1940. During World War I, he worked on acoustic detection of submarines. He created the National Center for Scientific Research and, as France's Under-Secretary of State for Scientific research, popularized science for young people. After Germany's invasion of France during World War II, Perrin fled to New York where he gathered support for the French war effort and helped establish the Franco-Belgian School of Higher Studies.

Perrin's key contribution came from his investigations of Brownian motion. In 1827 botanist Robert Brown observed random movements of pollen grains suspended in water and found that particles in a liquid at “equilibrium” continuously moved, turned, rose, and fell. One suggested cause was bombardment by liquid molecules, but the existence of molecules was disputed at that time and for the rest of the century. In 1905, Einstein published a molecular theory of Brownian motion. Perrin provided the difficult experimental proof during 1908 to 1913 with his studies of sedimentation (sinking of suspended particles, caused by gravity). Perrin demonstrated that the displacement of uniform particles resulted in a certain vertical distribution. He used photography to make thousands of microscopic observations, counting particles at various depths in a drop of water. The number of particles, greatest at the bottom, decreased at each higher level. In undisturbed fluid, these concentrations remained constant, despite the force of gravity. Perrin also measured rotation of particles. His findings confirmed the molecular theory and allowed him to calculate molecular size and Avogadro's number (number of molecules in a mole, or the mass, in grams, equal to the molecular weight of the substance, see Atom). His 1913 book, The Atoms, presented evidence for the discontinuous nature of matter and led to wide acceptance of the existence of atoms and molecules.

The Royal Society of London awarded him its 1896 Joule Prize and elected him a foreign member. He held many honorary degrees and belonged to the scientific academies of several countries, including the French Academy of Sciences, of which he was president in 1938. Perrin died at age 71 in New York, and his remains were placed in the Pantheon monument in Paris.