In the early days of civilization, heat was a great mystery. Humans considered it an element unto itself, along with earth, air, and water. This idea persisted until the time of Socrates (c. 470-399 b.c.) and Aristotle, who considered heat to be a fundamental substance. It was not until scientists such as Galileo Galilei and, eventually, Daniel Fahrenheit (1686-1736) constructed the first thermometers that humankind began to truly understand the nature of heat.

During the eighteenth century physicists had determined how heat was transferred from a hot object to a cooler one, but had not discovered its true nature. Most scientists subscribed to caloric theory, proposed by the French scientists Antoine Lavoisier and Pierre-Simon Laplace. They suggested that heat was caused by an invisible fluid, called caloric, that was present in all things; as a substance was heated, more caloric would flow into...