POISONS OF THE PAST: Molds, Epidemics and History by Mary Kilbourne Matossian (Yale University Press: $10, illustrated). Matossian examines the relationships between ergotism and epidemics, population growth and witch burning in an intriguing, if somewhat technical study. Ergot and certain related fungi grow on rye in wet weather: Ingesting the moldy rye--even after it's been milled and baked--can cause blindness, gangrene, impairment of the immune system and death. The molds also produce several pyschotropic alkaloids, including LSD. Matossian links periods of wet weather that would have favored the growth of the molds with outbreaks of witch burning and other irrational behaviors, including the Great Fear of 1789. While ergotism does not completely explain these episodes, she demonstrates that it was probably an important factor: Increased reliance on cheap, moldy rye for bread flour during wet years, when the harvests of other grains were poor, could transform the staff of life into the scepter of death.