In this last hypothesis, Franklin had it half right: The plant was taking and adding at the same time, producing oxygen and absorbing carbon dioxide. But his instincts about the fundamental concept were uncanny: The mint's capacity for rejuvenating "putrid" air was part of a system that extended far beyond an isolated laundry sink in Leeds. Franklin saw the whole story almost immediately: Priestley's discovery was a key to understanding the cycle of life on earth.

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Franklin's speculation about the wider consequences of Priestley's experiment was a fitting continuation of their intellectual duet: Franklin created Priestley, the scientist; Priestley popularized the legend of Franklin, daring electrician; and now Franklin was helping Priestley grasp the full significance of his discovery.

Franklin also advanced Priestley's experiment in one other crucial respect. He took the insight about the connected web of life, and immediately recognized that it would have implications for human actions.

Just two years before, the Scottish physician William Buchan had published a best-selling reference guide, Domestic Medicine, Or a Treatise on the Prevention and Cure of Disease by Regimen and Simple Medicines. The book became an instant fixture in British (and soon after, American) households, turning Buchan into the "Dr. Spock" of his day. Buchan had an entire chapter on the dangers of "unwholesome airs," which included this warning: "Surrounding houses too closely with planting, or thick woods, likewise tends to render the air unwholesome. Wood not only obstructs the free current of the air, but sends forth great quantities of moist exhalations, which render it constantly damp. Wood is very agreeable at a proper distance from a house, but should never be planted too near it, especially in a flat country. Many of the gentlemen's seats in England are rendered very unwholesome from the great quantity of wood which surrounds them."

Franklin's letter to Priestley takes dead aim at this pseudoscience. After noting the global implications of the mint experiment, Franklin goes on to suggest how human behavior will have to be altered in light of the discovery, an embryonic version of modern-day Green politics: "I hope this will give some check to the rage of destroying trees that grow near houses, which has accompanied our late improvements in gardening, from an opinion of their being unwholesome. I am certain, from long observation, that there is nothing unhealthy in the air of woods; for we Americans have every where our country habitations in the midst of woods, and no people on earth enjoy better health, or are more prolific."

In the fall of 1772, with Franklin's encouragement, the Royal Society voted to award Priestley the Copley Medal, the most prestigious scientific prize of its day, "on account of the many curious and useful Experiments contained in his observations on different kinds of Air." In awarding the prize, John Pringle delivered an unusually long speech that centered on Franklin's earth-systems interpretation of the mint experiment: "In this the fragrant rose and deadly nightshade co-operate; nor is the herbage, nor the woods that flourish in the most remote and unpeopled regions unprofitable to us, nor we to them; considering how constantly the winds convey to them our vitiated air, for our relief, and for their nourishment."

One question remains. If the collaboration between Priestley and Franklin produced such a significant breakthrough, why is it so rarely celebrated? Priestley was genuinely the first to discover that breathable air was a concoction of plants, and with Franklin's help, he was able to grasp and describe the far-reaching consequences that process would have on our understanding of earth's environment. These were foundational insights that led to world-changing ideas that rippled through science and society. Yet Priestley himself is far better known for a later set of experiments in which he produced pure oxygen by using a magnifying glass to focus the sun's rays on a sample of the compound mercuric oxide. And Franklin's role in the discovery of plant respiration and its broader implications goes entirely unmentioned in any of the major biographies.

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