The easiest book yet on the intriguing and still largely mysterious phenomenon of bioluminescence, first observed by Aristotle to be ""heatless light"" and used today in detecting cancer and anemia, drugs and explosives, air and water pollution and deep sea life. Jacobs describes several luminescent land and sea animals and plants, from species of glowworm or firefly (actually neither worm nor fly but beetle) and the dinoflagellates of Bahia Fosforescente off Puerto Rico to the ""foxfire"" mushroom and the pony fish whose abdomen shines with the light of symbiotic bacteria. Also compactly surveyed are the scientific investigation and application of the creatures' two-part system (for each organism, a different light-giving chemical called luciferin, acted upon by its own luciferase enzyme) and its experimentally important dependence on the molecule ATP. Except for a too definitive and unqualified classification of bacteria among luminescent plants, it's satisfactorily illuminating.