Caloric Theory and Heat

Pierre-Simon Laplace Viewed Heat to be Comprised of Particles

Pierre-Simon Laplace (1749–1827) imagined heat to be a fluid composed of particles, deemed by Antoine Lavoisier (1743–1794) as caloric. Whereas the particles comprising ordinary matter were considered to be attracted to each other, caloric particles were considered to repel each other (by today’s standards it may seem strange to describe heat as a type of particle but light, another imponderable fluid, was also being promoted as a particle, especially by Newton himself).

The fact that the particles of ordinary matter were attracted to each other seemed to be consistent with experimental results: cooling a gas results in the particles moving towards each other to form the more compact liquid structure, and subsequent cooling moves the particles even closer together resulting in the solid structure when something freezes.

By contrast, the addition of heat to a substance meant you were adding particles of caloric to it, and since they repelled each other, increasing the number meant lessening the attraction between the particles of ordinary matter. Therefore, add enough heat to a solid substance (such as ice) and it will melt; add even more and it will boil. Caloric theory seemed to make sense when it came to these phase transitions.

Antoine Lavoisier Considered Caloric to be an Element

In 1789, Lavoisier published An Elementary Treatise on Chemistry where he describes thirty-three elements. The list begins with (what else) caloric and continues with light, oxygen, nitrogen and hydrogen. Lavoisier also discusses his findings from studying a variety of chemical reactions. In particular, he notes that for the chemical reactions he studied, the mass of the starting materials (reactants) and the final materials (products) are always the same. In other words, regardless of the chemical reaction that is being performed, the mass will be conserved throughout the course of the reaction. In fact, he was able to proved this by simply “weighing” the reactants and products, with a very accurate chemical balance that he had constructed himself. In general he concludes that this is a fundamental property of all the elements and therefore, since caloric made his list of elements, it too must be conserved as far as he was concerned.

That heat was conserved and therefore could not be created nor destroyed was central to the concept of caloric theory. In pragmatic terms it meant if one object lost caloric (heat) another (nearby) object acquired that exact same amount. In a similar fashion, Lavoisier also found that caloric (heat) is weightless. So although caloric is imagined to be a “material” substance that is conserved, it has no weight. Needless to say, this caused suspicions in some people’s minds.

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