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Materials that ordinarily remain in suspension in water, due to the incessant battering of surrounding molecules, can be made to settle out by means of a centrifugal effect. Instruments called centrifuges whirl suspensions and force the suspended material to the side of the vessel away from the center of rotation. In this way, red blood corpuscles can be separated from blood, and cream from milk. (Since cream is less dense than the watery portion of milk, it collects on the inner side of the vessel, closer to the center of rotation.) Ordinary centrifuges do not produce effects large enough to force colloidal particles smaller than red blood corpuscles or droplets of cream to settle out. But the Swedish chemist The Svedberg (1884-1971) developed an ultracentrifuge in 1923, which spun so quickly it developed effects equivalent to a gravity hundreds of thousands of times normal.

At such high rates of spin, the centrifugal effect can force ordinary protein molecules to settle out, and since mixtures of different types of proteins settle at different rates (the greater the molecular weight, the faster the rate of settling), different proteins can to some extent be separated in this way.

The rate of settling allowed the molecular weights of proteins to be determined with reasonable accuracy, and for this work Svedberg was awarded the Nobel Prize for chemistry in 1926.

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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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