Von Baeyer (Physics/Coll. of William and Mary) offers assorted views of the —demon— and how he’s been slain over time to preserve the sacred second law of thermodynamics. The second law asserts that the universe is moving toward disorder and an increase in entropy. Given enough time, the universe will achieve a state of thermal equilibrium that has been poetically described as heat death. Practically speaking, the second law says that your cup of coffee always cools, that you can’t go backward in time, and that there is no free lunch as far as energy is concerned. That’s where Maxwell’s demon comes in. The eminent James Clerk Maxwell devised a thought experiment in which a tiny (and clever) demon sat at a trap door between two chambers, each filled with gas at different temperatures. Within each chamber, the molecules moved at various speeds in random directions. Suppose the microscopic demon selected the fastest molecules from the cooler chamber and opened the door to admit them to the warmer chamber, never allowing molecules from the hotter chamber to gain access to the cooler? The hotter chamber gets even hotter, with little or no energy expended--a violation of the second law! But, it turns out, this and other variants on the ways a demon could sort molecules didn—t allow for Brownian motion, in which a “large” object (the trap door) bombarded by molecules starts jiggling and destroying the purposeful activities of the demon. To properly appreciate the cogency of that argument, von Baeyer’s reader is treated to a detailed survey of 19th- and 20th-century physics: The conservation of energy, the concept of atoms, probability theory, and much more. To his credit, von Baeyer, a National Magazine Award winner, is ever a graceful and witty writer, at pains to clarify concepts. But this is challenging stuff. Readers are advised to follow Feynman’s principle: Read until you don’t understand, then go back and reread.