The answer to this riddle lies in one central fact: The new science unleashed by Priestley’s mint experiment took almost 200 years to evolve into a coherent discipline. What Priestley had hit upon was not a simple element, like oxygen, or a fundamental law, like Franklin’s discovery of the conservation of electrical charge. What Priestley and Franklin had grasped was something more complicated, more inchoate: a global system, a new awareness that our environment was not merely a given of life on earth, but something actively manufactured by other life forms. But it would take more than a century for that vision to resolve itself into a genuine science: The word “ecosystem” wasn’t even coined until the 1930s, when an Oxford botanist named Arthur Tansley asked a colleague to come up with a name for the complex interactions between organisms and their physical environments. The modern environmental movement prefigured in Franklin’s aside about the dangers of chopping down too many trees wouldn’t blossom for an­other 40 years after that. Legends form more easily around epiphanies that produce results shortly after the eureka moment. The pantheon is not always so kind to breakthroughs that take two centuries to change the world.

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It is more understandable that Franklin’s biographers should ignore his contribution to the birth of ecosystems science. In natural philosophy alone, the list of Franklin’s accomplishments is formidable enough, and the original experiment with the mint plant was all Priestley’s design. Yet Franklin’s contribution was essential, and it serves as a reminder that great intellectual breakthroughs rarely come in the form of isolated epiphanies, the solo genius working alone in his lab. More often than not they emerge out of the interplay between different kinds of minds approaching the same problem from different angles. Priestley was brilliant at concocting novel experiments, but he was never a great theorizer; Franklin took the puzzling results of his friend’s experiment and elevated them to a more rarified sphere: a profound new truth about the way the world works.

Franklin described the value of intellectual exchange wonderfully in a letter written in 1753—the year he received the Copley Medal—defending his decision to publish his scientific discoveries as rapidly as possible, in some cases before he could fully confirm their validity: “Even short Hints, and imperfect Experiments in any new Branch of Science, being communicated, have oftentimes a good Effect, in exciting the attention of the Ingenious to the Subject, and so becoming the Occasion of more exact disquisitions…and more compleat Discoveries.”

The “attention of the Ingenious”—the phrase describes perfectly what Priestley secured for his experiments when he first befriended Franklin in the London Coffeehouse. Together they hit upon a “more compleat Discovery,” one that grows increasingly influential, the more we learn about the complex interdependencies of life on earth. In 1767 Priestley grasped the value of popularizing the story of the kite in the lightning storm as an inspiring tale of great scientific ingenuity and bravado. More than two centuries later, it’s time we did the same for the mint in the glass.

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