A hypothetical being imagined as controlling a hole in a partition dividing a gas-filled container into two parts, and allowing only fast-moving molecules to pass in one direction, and slow-moving molecules in the other. This would result in one side of the container becoming warmer and the other cooler, in violation of the second law of thermodynamics.
late 19th century: named after J. C. Maxwell (see Maxwell, James Clerk).
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