0 Copyright 1992 American Chemical Society
The ACS Joumal of
Surfaces and Colloids DECEMBER 1992 VOLUME 8, NUMBER 12
Dedication Jean Perrin Jean Perrin was born during the (unhappy) days of the 1870 war. He entered the Ecole Normale in 1891. Marcel Brillouin (the father of Ldon Brillouin) taught him statistical mechanics in the Boltzmann sense-a very controversial attitude for that time. In 1896,he became interested in “cathoderays”. Crookes collected charges from the rays, measured them via electrometers,and obtained the wrong sign! Perrin trapped the rays in a long Faraday cylinder and proved the presence of negative charges. This was the base of his Ph.D. work (1897). He then taught a course on chemical physics, and his interest shifted from charges in a vacuum to ions in water, double layers, etc. He looked at colloidal particles and started his classicexperimentson their statistical features. He discovered (with delight) that their vertical distributionwas a Boltzmann exponential-thus obtainingan estimate of Avogadro’s number. The young Langevin pointed out to him the works of Einstein and Smoluchowski (1905-1906) on Brownian motion. Perrin then became their experimentalcounterpart: proving that the root mean square displacements did indeed increase like the square root of time; checking that the friction coefficient of minute spheres (submicrometer)was still followingStokes’law; castingthe first ideas on fractal curves (prompted by Brownian trajectories, which have a fractal dimension equal to 2). On the whole, he was extremely pleased to show-via quantitative measurements of Avogadro’snumber-that atoms did exist. There had been earlier effortsto measure the size of atoms or molecules-starting with Benjamin Franklin pouring a monolayer of oil on a pond of Clapham Common-an experiment which I always describe with enthusiasm to school children. But Perrin had the first precise assessment of what exactly was the weight of one atom. Soon after, X-ray crystallography developed and provided another route. All this spirit is beautifully described in his book Les atomes (1913). Just after this major message, the first world war came, followed by years of doubt and fluctuations. Perrin was strongly involved in national problems. He was an ardent socialist and became the scientific adviser of the 1936 prime minister Leon Blum. This resulted into two major creations: (a) the “Palaisde la DBcouverte” (now still the leading science Museum in France); (b) the “Centre National de la Recherche Scientifique” (CNRS), giving permanent research positions to a selected group of young scientists. My generation owes a great deal to Perrin’s ideas. I, for instance, first encountered science through beautiful optical experimentsat the Palais. More generally,the revival of French physics after the second world war was (I think) due to three main causes: An intelligent policy of the French Atomic Energy (created by F. Joliot), sending young students abroad for their education (A. Messiah, C. Bloch, A. Abragam, et al.). The creation of the Les Houches Summer School (by CBcile de Witt, 1951). The postwar growth of the CNRS-following Perrin’s ideas. Perrin came to New York in 1940 (his son Francis was then teaching at Columbia). He died in 1942. None of us will forget his talent for simple experiments, his love for chemical physics, his enthusiasm, and his ideals.
P.G. de Gennes
0~43-~463/92/2408-2849$03.00/0 0 1992 American Chemical Society
