In the 1600s the question of 'what is heat?' was one of the more interesting natural philosophy questions that was addressed. The belief at the time was that all the universe was composed of particles and many interesting notions about the mechanical workings of magnetism and other such natural workings.

The caloric fluid was an idea of the 1700's along with other theories based on fluids. The essence of inflammability (called "phlogiston") was thought to be a fluid that was gained or lost during combustion. Electricity and magnetism were also described in terms of fluids.

While many of the ideas posed about the workings of the universe in this time were unverifiable or outright failed, the caloric fluid theory was strongly reinforced by demonstrations which showed conservation of heat in calorimetric experiments. The Caloric Fluid Theory lasted well into the 1800s when it gave way to the Principle of Conservation of Energy.

The caloric fluid was thought to have the following properties:

  1. It is a substance that can be neither created or destroyed
  2. The fluid is elastic, and the particles repel each other but are attracted by the particles of other substances. The magnitude of the attraction is different for different materials.
  3. Caloric can either be "sensible" - diffusing rapidly in the substance or "latent" - combining with the attractive particles in a manner similar to that of chemical reactions

This poses a very reasonable explanation for a number of phenomena that we are familiar with. The fluid would spread out in a substance and increase the volume that such a substance takes up - thus causing it to expand. This can be seen in gas, liquid, and solids. When an object is rubbed or pounded, the caloric fluid is squeezed out from spaces it originally occupied and is now concentrated at the place of action - thus heating it up. Also, just as water runs downhill, the caloric fluid flows from warm things to cold things. In heating a copper bar, the caloric fluid would enter the bar from the source of the heat and flow through it. The caloric fluid also changed color at different concentrations, from red to blue to white.

The attack on the caloric fluid was centered around if it had mass. Some claimed that as bodies were heated they gained weight. Others claimed that water gained weight as it froze (suggesting that the caloric fluid had a negative mass).

Benjamin Thomson (1753 - 1814) who later became known as Count Rumford worked with many thermal phenomena and had improved upon stoves, chimneys, steam heating systems, lamps and coffee-makers. In the course of his investigations he recognized the importance of convection in the transfer of heat.

When the question of the mass of caloric fluid was arose, Rumford looked into the matter. In the court of Elector, he had access to an extremely find balance and set forth to verify the weighing experiments of his peers. Taking extreme care to avoid drafts and changes in the temperature of the arms of the balance and other such sources of error his results showed that the heating and cooling of a substance had no detectable effect upon its weight. This clearly cast doubts on the material nature of the caloric fluid.

Next, Rumford investigated heat by friction. In 1798 with the boring of a cannon the gun acquires a "very considerable degree of heat" in a short time. The chips that were removed from the cannon by the process got much hotter than the gun itself. The small chips had the same specific heat as the metal the cannon was made from so no claims could be made that these chips had a smaller heat capacity. Furthermore, Rumford demonstrated that just as much heating occurred with a blunt tool was used.

The most significant fact from this is that the generation of heat by friction appeared to be inexhaustible. If the caloric fluid did exist, there could only be so much of it - it cannot be created or destroyed. Unfortunately, Rumford was not motivated enough to make quantitative measurements of the relationship between heat and work. However, he did study the transfer of heat through a vacuum, the behavior of water as it expanded at temperatures below 4C. All of these challenged the core of the caloric fluid theory.

By the age of the steam engine, the molecules in motion theory was the accepted explanation of heat. This showed that the micro motions of molecules (heat) could be transformed into macro motions - enough to move the wheel of an engine. The caloric fluid theory had no explanation for how this could happen.

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