Topic: William Logan's book Air.

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.