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Brownian motion and atomic size.

Brownian motion had remained somewhat of a puzzle since Brown had discovered it (see 1827). In 1902 the Swedish chemist Theodor Svedberg (1884-1971) had suggested that the unequal bombardment of small particles by molecules from all sides impelled them to move randomly, this way and that.

Einstein, in 1905, analyzed the possibility of molecular bombardment thoroughly. He reasoned that any sizable object immersed in water (or any fluid) is bombarded from all sides, and surely more from one side at one moment and more from another side at another moment. However, the countless trillions of molecules involved means that any small differences will be so small as to be indetectable.

If we consider a smaller and smaller object, the total number of molecules striking it at a given moment will be smaller, and little deviations will loom larger. By the time objects approach the microscopic in size, an additional molecule from this side or that should be sufficient to give it a noticeable push in this direction or that.

Einstein worked out an equation to describe Brownian motion, one from which it was possible to work out the size of molecules and of the atoms making them up, provided one found a way to measure certain other variables that occur in the equation.

It was not long before Einstein's equation was put to good use.

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
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