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...