Priestley engineered an audience with Franklin and his fellow Honest Whigs because he had concocted an idea for a book on the history of electricity. As a small-town minister and teacher with a hobbyist’s passion for the new discoveries of “natural philosophy,” Priestley knew that no other field had generated so much scientific and practical innovation in such a short amount of time. But no one had written a popular account of these world-changing discoveries. So he set off to London, hoping to meet the “electricians”—as the scientists were popularly known—and to persuade them to let him tell the story of their genius. Franklin, naturally, was immediately supportive of the idea, and promised the young Priestley open access to his library and correspondence. But he and his friends took one additional step that proved crucial: They encouraged Priestley to conduct his own experiments while writing his history.

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Hearing his idols urging him to write about his own experimental research and investigations opened up a whole new field of possibility for the young man. Priestley had arrived in London as a dabbler in natural philosophy, tinkering in the provinces with his electrical machine and his air pump. By the time he left, he was a scientist.

When The History and Present State of Electricity, With Original Experiments was published in 1767, the book instantly landed Priestley in that upper echelon of electricians that had welcomed him so warmly at the London Coffeehouse. It also played an important role in creating the legend of Franklin as maverick scientist: On page 160 of the original printing, Priestley tells the now canonical story of Franklin and the kite, the first time that story had ever been explicitly associated with Franklin in print.

Priestley would continue to pursue his research into electricity over the coming decades, but soon after the publication of the History, his primary focus became chemistry, specifically the study of air. Where his work in electricity had left him as a disciple of Ben Franklin, with chemistry he would quickly become Franklin’s superior as a scientist.

As a child, growing up in rural Yorkshire, Priestley had amused himself with the slightly sadistic pastime of trapping spiders in sealed glass jars and observing how long it would take the poor creatures to perish. The fact that organisms would invariably expire given a finite supply of air was well known to little boys and scientists alike. But the mechanism behind this process was a mystery. Did the creatures somehow exhaust the air they were breathing—in which case, what was left in the jar? Or were they poisoning their environment in some inexorable way? Strangely, the air in the jar didn’t visibly change after the animal’s final convulsions, though it did have one distinct, and puzzling, new attribute: A candle would consistently fail to light in it.

In the late spring of 1771, Priestley decided to try a new twist on his childhood experiment. If animals died swiftly in a sealed jar, how long would it take a plant to suffer the same fate? Could a plant outlast a mouse or a frog? Or would it prove more feeble in the contained environment of the jar? He went out into the garden, and pulled a small mint plant from the ground. He placed the mint in a glass jar that he had inverted over a pneumatic trough. And he waited, patiently, for the plant to expire.

To Priestley’s surprise, the plant stubbornly refused to die. In fact, the determined sprig of mint continued growing all summer long. And there were other mysteries. A candle would readily light in the jar alongside the mint. A mouse placed inside the jar with the plant could survive happily for ten minutes, while a mouse placed in a jar in which another mouse had previously expired would begin to convulse within seconds. Somehow the plant was disabling whatever it was that snuffed out the candle and suffocated the mouse.

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