In the philosophy of thermal and statistical physics, Maxwell's demon is a thought experiment created by the Scottish physicist James Clerk Maxwell to "show that the Second Law of Thermodynamics has only a statistical certainty." It demonstrates Maxwell's point by hypothetically describing how to violate the Second Law: A container is divided into two parts by an insulated wall, with a door that can be opened and closed by what came to be called "Maxwell's demon"; the demon opens the door to allow only the "hot" molecules of gas to flow through to a favored side of the chamber, causing that side to gradually heat up while the other side cools down. (Wikipedia, Maxwell's Demon)
See Also
15.21 - Water Dissociation Demonstration
16.03 - Maxwell misses the mark
Entropy
Figure 16.00 - Maxwell and Thomson
Figure 17.18 - Keelys Levitation Demonstration
Heat
Heat Engine
James Clerk Maxwell
Maxwell Equations
Propositions Demonstrating the Relative Properties of Straight and Curved Lines
Second Law of Thermodynamics