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Did Brown see Brownian motion?

The name of Scottich botanist Robert Brown is inextricably linked with the term "Brownian motion," now used to characterize a wide range of physical processes that exhibit small, random fluctuations. But in 1827, when Brown first observed under a microscope the erratic jiggling of pollen grains and other tiny particles suspended in water, he had no convincing explanation of the motion's cause. The notion that these mysterious, random movements resulted from the combined effects of water molecules bombarding the suspended particles came many decades later.

About six years ago, chemist Daniel H. Deutsch of Pasadena, Calif., became curious enough about Brownian motion to try to see it for himself. He discovered that to observe true Brownian motion in water, he needed a powerful microscope well protected from vibrations. Moreover, the motion became readily apparent only when the suspended particles were about 1 micron in diameter -- much smaller than a pollen grain -- and had a density close to that of water. The care required to observe these movements raised a question in Deutsch's mind: Was the vigorous motion observed by Brown really caused by the action of bombarding water molecules?

Deutsch started digging into historical records of Brown's research to find an answer. "The more I looked, the more obvious it became that there was something wrong," Deutsch says. "There are at least half a dozen reasons why it would not have been possible for Robert Brown to have observed Brownian motion."

The microscopes Brown used were quite different from modern instruments, Deutsch says. They were much less rigid and had a norrower field of view. In addition, samples consisting of small water droplets remained uncovered during observation, allowing the water both to jiggle and to evaporate quickly. Instrument vibrations and currents induced by evaporation would cause particles suspended in a water droplet to move, he contends. These particles would quickly dart in and out of the field of view, making it difficult to track their movements.

"Brown was seeing, in effect, flotsam on the ocean, where you have waves knocking things about," Deutsch says. "It's very easy to be deceived. You need the proper equipment to observe Brownian motion."
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Title Annotation:Robert Brown
Author:Peterson, Ivars
Publication:Science News
Date:May 4, 1991
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