John Scott Haldane (1860-1936) was one of the greatest and most colourful of British scientists, acknowledged as the leading physiologist of the era at a time when physiology and much of medical science was coming into its own. The most successful serial self-experimenter in the history of science, Haldane crawled through the carnage of underground explosions, locked himself in sealed chambers, breathed in lethal cocktails of gases, sampled his own blood, burned and healed his own flesh, and experimented on his own children, in an obsessional push to understand the nature of human respiration. What is expired air? How can you make coalmines safer? What does carbon monoxide do to people? These are just some of the vital questions to which Haldane provided the answers, saving thousands of lives in the process. He also designed the first space-suit and invented the gas-mask, among many other innovations and contributions we still benefit from today. Entertaining and enlightening in equal measure, Martin Goodman's lively and revealing biography casts new light on one of the greatest eccentrics of British scientific and intellectual life.