A Summary of the Scientific Theories of Heat A pre-human species (Homo erectus) used fire for cooking and heat at least 500,000 years ago (maybe as long as 790 000 years ago), even though we have understood the science of heat for less than 200 years. The early Greeks thought fire (heat) was one of four basic elements, and as one of the four basic elements, it was contained in all substances. Scientists realized that there were more than 4 elements. Heat was then determined to be an invisible liquid called phlogiston. However, when some substances were burned, the ashes weighed more than the original substance. Where did this extra weight come from? By the late 1700’s the caloric theory of heat emerged. Caloric was said to be a massless fluid that occupied all substances. It could not be created, nor destroyed, but it could flow from one substance to another (always from warm to cold). A “calorie” was later defined as “the amount of caloric that would increase the temperature of 1g of substance, by 1oC”. Count Rumford’s Hypothesis was introduced when he was making cannons. Boring holes in metal made very hot shavings and drill bits. He realized that heat was the equivalent to energy. German physician Julius Robert Mayer found evidence to support Rumford’s hypothesis while making observations during a blood letting “treatment”. James Prescott Joule (1818-1889) receives credit for discovering the mechanical equivalent of heat. His developments lead us into the physics of heat and energy. The SI unit of energy,