Caloric theory

This idea would later be formalized as entropy.

In the end, caloric theory collapsed because it couldn't explain friction as an endless heat source and it violated the principle of energy conservation.

Kinetic theory and thermodynamics took its place, explaining not only what caloric once did, but also why engines can never be perfectly efficient, why refrigerators must dump heat somewhere, and why perpetual motion machines can never exist.

And yet, caloric still lingers in our language.