SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Brownian motion as noted by french physicist Jean Perrin (1870-1942). The plot shows the positions of three microscopic pollen grains, suspended in water, as observed at 30 second intervals. The grains are seen to be in ceaseless random motion. This phenomenon was first discovered by the Scottish botanist Robert Brown (1773-1858) in 1827. Fifty years later Albert Einstein (1879-1955) theorised that the pollen was being buffeted by the random movements of water molecules in the fluid. In 1908 Perrin's experiments on Brownian motion gave the first direct demonstration of the existence of molecules. Perrin was awarded the 1926 Nobel Prize in Physics for this work.
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