On one side of the debate is caloric theory, created and developed by French scientists from the 1780s through the 1820s, according to which heat is a kind of stuff. This “caloric fluid” flows steadily from warmer to cooler substances, causing them to become hotter and—newly engorged with fluid—to expand. By 1830, caloric theory has achieved a series of spectacular successes: the accurate calculation of the speed of sound through air, the delineation of a mathematical formula capturing precisely the rate with which heat flows through a material such as a metal rod, and an illuminating theory
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