In ''The Chemical Carousel,'' science writer Dirk Hanson takes the reader on a voyage through the heady world of addiction science, from the lab to the clinic to the junky on the street. Hanson explains the workings of common neurotransmitters and documents the direct effect drugs and alcohol produce on the reward pathways of the brain. He shows how scientists and treatment professionals have finally given us an answer to the perennial question about addiction: Why can't those people just say no?