Maxwell's Demon: Why Warmth Disperses and Time Passes. Hans Christian von Baeyer. 256 pp. Random House, 1998. $25.

The successful popularizer of science should guide the reader through a jungle of ideas, some of them accessible to the uninitiated, others too complex for brief treatment. The secret of good science writing is in the choosing. By carefully selecting targets for the narrowly focused flashlight beam of clear exposition, a good popular science book presents a well-lighted path of conceptual milestones, while steering the reader past tangles and thickets of unnecessary complication. Maxwell's Demon: Why Warmth Disperses and Time Passes is such a book.

Named for the theoretical homunculus capable of sorting hot atoms from merely warm ones, the book explores the three laws of thermodynamics, in particular the Second Law. Energy inexorably flows downhill, from hot to cold; from a state of orderly concentration toward one of dilution, dissipation and disorderliness: entropy. As a thought experiment, James Clerk Maxwell devised the idea of a microscopic demon who can reverse the flow toward entropy using intelligence about atoms. Operating a sliding door in a wall that separates two containers of lukewarm air, Maxwell's demon opens the door to admit hot, fast atoms but closes it to slow, cool atoms. Thus, the hypothetical demon concentrates useful heat in one container and increases order in the universe, violating the Second Law.

Writing in a style as light and fast-paced as a novel, the author deftly introduces and explains Maxwell's demon in a historical context that breathes life into both the science and the scientists. The reader empathizes with scientists as real people struggling to unravel the mysteries of thermodynamics. Reminiscent of Isaac Asimov, von Baeyer incorporates personal experiences to make a point. For example, he recruits his teenage daughters to flip coins and graph the results to explain the bell-curve distribution of molecule speeds. These vivid, human touches stand the reader in good stead when the time comes to venture deeper into the jungle. By then, almost effortlessly, the reader has gained a clear understanding of Boltzmann's epitaph: "Entropy is the logarithm of probability."

The book suffers from some unevenness of style, occasionally deteriorating from brilliant into merely very good. Also, the discussion of recent theories about the demon, entropy and information theory is tantalizingly brief. Still, the book stands as a masterful distillation of information. Like Maxwell's demon, von Baeyer manages to bring order from chaos by making good choices.—Randall Black, Irvine, California