Air sustains the living. Every creature breathes to live, exchanging and changing the atmosphere. Water and dust spin and rise, make clouds and fall again, fertilizing the dirt. Twenty thousand fungal spores and half a million bacteria travel in a square foot of summer air. The chemical sense of aphids, the ultraviolet sight of swifts, a newborn's awareness of its mother's breast—all take place in the medium of air. 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.