Priestley wrote Franklin in the summer with an account of a new discovery—but the original letter has been lost, and so we don’t know with certainty that he was reporting on his mint experiments. All we know is that Franklin forwarded Priestley’s news along to John Canton, a member of the Club of Honest Whigs, with a note: “I have just received the enclos’d from Dr. Priestly. And as it contains an Account of a new Discovery of his, which is very curious, and, if it holds, will open a new Field of Knowledge.”

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On Aug. 17, 1771, Priestley made a simple, but essential modification to his experiment. He suspended a lit candle in a cylinder over a fresh sprig of mint floating in a pool of water and then waited for the candle to burn through the supply of air in the container. On his return 10 days later, not only was the mint alive, but when he went to light a candle in the glass, he found that “it burned perfectly well in it.”

This was genuine news. Priestley’s first experiment had shown that plants failed to exhaust or poison the atmosphere the way living creatures did. But the flame burning next to the sprig of mint in the modified experiment suggested a far more radical proposition: that plants were restoring something fundamental to the air, or they were creating the air itself.

After performing numerous variations of the experiment Priestley was confident enough in his results to begin sharing the news with the Honest Whigs in the fall of 1771. “You may depend on the account I sent you of my experiments on the restoration of air made noxious by animals breathing it or putrefying it, which I sent to Dr. Franklin,” he wrote to the Rev. Richard Price on Oct. 3. “Air in which candles have burnt out is also restored by the same means.” By the summer of 1772, Priestley had cycled through a series of different plants to confirm that the restorative effect was not somehow specific to mint.

Franklin traveled to Leeds in June 1772 to visit Priestley, and brought along John Pringle, the Scottish physician who would soon be elected president of the Royal Society, England’s national academy of science. Priestley gave them the full tour of his experiments with restoring air, and the visit seems to have energized him all over again about the importance of what he had discovered. On July 1, he wrote to Franklin: “I have fully satisfied myself that air rendered in the highest degree noxious by breathing is restored by sprigs of mint growing in it. You will probably remember the flourishing state in which you saw one of my plants. I put a mouse to the air in which it was growing on the saturday after you went, which was seven days after it was put in, and it continued in it five minutes without shewing any sign of uneasiness, and was taken out quite strong and vigorous.”

While others might have viewed Priestley’s discovery as a clever parlor trick, Franklin had a hunch that it opened a whole new way of thinking about the planet and its capacity for sustaining life. The first indication of that hunch comes in a note he wrote to Priestley after the June 1772 visit: “That the vegetable creation should restore the air which is spoiled by the animal part of it, looks like a rational system, and seems to be of a piece with the rest. Thus fire purifies water all the world over. It purifies it by distillation, when it raises it in vapours, and lets it fall in rain; and farther still by filtration, when, keeping it fluid, it suffers that rain to percolate the earth. We knew before, that putrid animal substances were converted into sweet vegetables, when mixed with the earth, and applied as manure; and now, it seems, that the same putrid substances, mixed with the air, have a similar effect. The strong thriving state of your mint in putrid air seems to shew that the air is mended by taking something from it, and not by adding to it.”

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