Jean Baptiste Perrin and the Brownian Motion

Jean Baptiste Perrin (1870 - 1942)

Jean Baptiste Perrin (1870 – 1942)

On September 30, 1870, French physicist Jean Baptiste Perrin was born. In his studies of the Brownian motion of minute particles suspended in liquids, Perrin verified Albert Einstein’s explanation of this phenomenon and thereby confirmed the atomic nature of matter.

In 1897, Jean Baptiste Perrin earned his PhD and was appointed lecturer at Sorbonne, Paris. Perrin was announced professor at the university in 1910 and held the position until the German occupation of France during World War II. Already in 1895, Jean Baptiste Perrin was able to show that cathode rays were of negative charge and he also determined Avogadro’s number by several methods. He explained solar energy as due to the thermonuclear reactions of hydrogen.

In 1905, Albert Einstein published his theoretical explanation of Brownian motion in terms of atoms. Perrin then conducted experiments in order to test and possibly verify Einstein’s theory. Brownian motion or pedesis is the random motion of particles suspended in a liquid or a gas resulting from their collision with the fast-moving atoms or molecules in the gas or liquid. This transport phenomenon is named after the botanist Robert Brown. In 1827, while looking through a microscope at particles trapped in cavities inside pollen grains in water, he noted that the particles moved through the water. However, he was not able to determine the mechanisms that caused this motion. Atoms and molecules had long been theorized as the constituents of matter, and Albert Einstein published a paper in 1905 that explained in precise detail how the motion that Brown had observed was a result of the pollen being moved by individual water molecules.

Einstein’s first predictions were at first refuted by several scientists including The Svedberg and Henri. However, Chaudesaigues (1908) and Perrin (1909) were able to confirm Einstein’s theory, which constituted empirical progress for the kinetic theory of heat. In essence, Einstein showed that the motion can be predicted directly from the kinetic model of thermal equilibrium. The importance of the theory lay in the fact that it confirmed the kinetic theory’s account of the second law of thermodynamics as being an essentially statistical law.

At yovisto, you can learn more about Atomic Theory in a lecture by Sylvia Ceyer at MIT.

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