AKA: Heat is actually an invisible, unsubstantial, self-repellent fluid called 'caloric' that flows from hot places to cold places.
Theory born: 1783
Parent(s): Antoine Lavoisier.
Antoine Lavoisier, b. 1743, d. 1794
Description: When Lavoisier demolished phlogiston theory in 1783, he proposed that heat was caused by a substance that he called 'caloric' which flowed from hotter to cooler places and that existed in a finite quantity in the universe. Although we now know caloric theory to be incorrect, at the time it explained many experimental observations. For example, a hot cup of tea would cool, because the caloric in it was self-repelling and would spread out in the surrounding environment, warming it slightly. A heated gas expands in volume because it is absorbing caloric from the heat source. Water is a difficult substance to boil because it is so deficient in caloric.
In fact, many of the components of caloric theory sound familiar to modern ears. Both Lavoisier and Benjamin Franklin believed that caloric could flow from place to place, but that the total amount would be conserved: exactly like the law of conservation of energy. It seemed that ideas of temperature and of heat had been conclusively settled, but it would be barely fifteen years later that the first cracks would appear.
Theory death: 1798-1843
Cause of theory death: Incompatible with boring cannons (I mean, cannons that were being bored. Never mind). Also...incompatible with Joule's brewery efficiency studies (1840-1843).
Caloric theory had a long, slow death. Even that may be something of an overstatement. When observations started piling up that there was an issue with caloric theory, it was tweaked and modified until it essentially was absorbed into modern thermodynamics.
One of the first clues that something was wrong was Count Rumsford's investigations into cannon manufacture for the Bavarian military in 1798. He found that repeatedly boring out a cannon did not affect its ability to generate huge amounts of frictional heat: clearly 'caloric' was not a conserved substance in the materials if it could be endlessly generated. He postulated that the action of the boring machine with the cannon material was causing the material of both to vibrate invisibly somehow and that the invisible vibration was somehow transmissible to other materials when they came into contact (which is, essentially, modern kinetic theory!) He published his ideas in his paper "An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction" (love those historic scientific paper names!)
The reaction to his finding was hostile: supporters of caloric theory countered that perhaps the fragments displaced by the grinding had lost all their caloric and that caloric was still flowing from the new, exposed material.
It was James Joule's work in the 1840s that finally finished caloric theory off in its current form. Joule had been tutored by John Dalton (he of atomic theory) as a young man and upon reaching adulthood managed the family's brewery. He began to investigate whether the brewery's steam engines should be replaced with the newly-developed electric motor. Combining his desire for efficiency with his keen intellect and enquiring mind, Joule thoroughly analysed the options open to him.
In his unsuccessful efforts to get the efficiency of a battery-driven motor to equal that of a coal-fired engine, Joule discovered a heating effect in the wire at a rate of I²R (which we now recognise in the equation P=I²R). Champions of caloric theory contended that caloric was leaving the battery and being released from the wire. However, further studies from Joule showed the same heating effect occurring when moving a wire through a magnetic field (inducing a current). With no obvious source of caloric to produce this heat, the theory was in trouble. Joule then proved that the heating effect was directly proportional to the work done in moving the wire, then dispensed with electricity entirely and showed the same proportionality when heating water by stirring a paddle in an insulated bucket. All of the temperature increase then seemed to be a direct result of work done on the water.
Joule's Heat Apparatus
There was no longer room for caloric theory. Weakened by Rumsford and Joule's work, it ran straight into the discovery of infrared radiation by Hershel and the confusion over whether electromagnetic radiation was waves or particles. What was caloric: a wave? A particle? Some fluctuation in the aether? Something even more complicated. Caloric theory died a quiet little death, absorbed into theories that much better explained observed phenomenon.
Attributions
Cannon photograph - PHGCOM, Wiki Commons
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