In the seventeenth century, scientists held a clear association between heat and motion of constituent particles. Heat became recognized as a fluid that flowed from hot objects to cold ones. During Galileo's time, this heat fluid was known as phlogiston and was considered the soul of matter. Phlogiston had mass and was released by or absorbed by an object when burning. In the late eighteenth century Antoine Lavoisier refined the view that heat was a liquid, overthrew the current phlogiston theory, and developed the caloric theory of heat. In 1787 Lavoisier coined the term caloric to represent the heat fluid. Caloric was thought to be massless, colorless and conserved in total in the universe. The individual particles making up the fluid were elastic and repelled each other but were attracted by particles of other substances, the magnitude of the attraction being different for different substances. It was thought that caloric could be "sensible" in that it diffused among the particles of the material it was acting upon thereby surrounding each particle with an atmosphere of caloric.
It could also combine with the particles of the material in a manner similar to chemical combinations and be "latent." By the beginning of the nineteenth century most scientists accepted the caloric theory as the correct theory of heat. The theory offered many plausible explanations regarding heat transfer where other theories had failed. It was a simple theory and had many successful applications. These facts made the theory widely accepted but also made it extremely difficult to overthrow.
Because the caloric theory was so powerful, it took some 50 years to overthrow. A dispute rumbled concerning whether caloric had weight. Benjamin Thompson showed that cooling and heating of a substance had no detectable effect on its weight. He studied the heat produced by friction in boring of cannons in 1798 and showed that just as much heat was produced when a blunt boring tool was used and no metal was cut as when a sharp instrument was employed. It appeared that the heat produced by friction was inexhaustible, and, therefore, not a conserved quantity as required by caloric theory. Thompson continued his attacks on the caloric theory into the early nineteenth century. Eventually, in the 1840s, James Joule explained the source of heating in Thompson's experiments and recognized that heat is another form of energy, resulting from the motion or kinetic energy of atoms and molecules. This led to the downfall of the caloric theory and formation of the principle of conservation of energy, and the kinetic theory of heat.
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