Ignorance of the air is costly. The artist Eva Hesse died of inhaling her fiberglass medium. Thousands were sickened after 9/11 by supposedly "safe" air. The African Sahel suffers drought in part because we fill the air with industrial dusts. With the passionate narrative style and wide-ranging erudition that have made William Bryant Logan's work a touchstone for nature lovers and environmentalists, Air is--like the contents of a bag of seaborne dust that Darwin collected aboard the Beagle--a treasure trove of discovery.