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The American Enlightenment Treasures from the Stanford University Libraries

A Letter from Benjamin Franklin

Just before leaving America for England, Benjamin Franklin dashed off this eight-page letter to John Lining, a physician who had immigrated from Scotland. Lining lived in Charles Town (now Charleston), South Carolina, a pestilential swamp regularly visited by yellow fever epidemics. A great believer in the influence of climate on human health, Lining had spent a year meticulously measuring his own fluctuating weight and temperature in the hopes of connecting climate to disease. He reported these observations to the Royal Society in London, which brought his experiments to the attention of the international scientific community, including Benjamin Franklin.

Franklin’s letter addresses their mutual fascination with what we would call metabolism: what makes the human body warm or cold? Was it climate or electricity? Frank­lin bet on electricity, speculating that this “Common Fire” might be a fluid that penetrates some objects such as metal coins better than other objects such as wood. Applying the same idea to human beings, Franklin asked how a warm, living body gets “it’s Quantity of this Fluid called Fire.” The answer: by eating. Digested plants gave off “Fire,” which then powered the human furnace.

Both this letter and the one written by George Washington (included in the case titled “In the Midst of War”) are original in the sense that they were penned by Franklin and Washington. Yet both are also copies: slightly different versions of both exist in other archives. These two letters are testaments to the widespread culture of copying that flourished during this great age of letter-writing. Even though both Franklin and Washington personally wrote thousands of letters, they also made copies of many of those letters for a variety of purposes such as record-keeping.

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