The First American: The Life and Times of Benjamin Franklin

by H.W. Brands

“Universal space, as far as we know of it, seems to be filled with a subtle fluid, whose motion, or vibration, is called light,” Franklin wrote in a letter read to the American Philosophical Society. The vibrations of light—sunlight, for example—heated objects on which the light fell by causing the particles of those objects to vibrate in turn. Franklin used the word “fire” to denote a combination of electromagnetic, kinetic, and chemical energy—a combination about which he was rather vague (and, in fact, confused). He was not sure whether this “fire” was something material or immaterial (although in this he unknowingly anticipated the Einsteinian equivalence of mass and energy). But he hit on a fundamental law of conservation of mass-energy. “Thus, if fire be an original element, or kind of matter, its quantity is fixed and permanent in the world. We cannot destroy any part of it, or make addition to it; we can only separate it from that which confines it, and so set it at liberty, as when we put wood in a situation to be burnt; or transfer it from one solid to another, as when we make lime by burning stone, a part of the fire dislodged from the wood being left in the stone.”