To explore the challenges scientists faced in understanding heat, my illustrator Carly Sanker and I created a map (at bottom) outlining the evolution of heat theories, excerpted from my book.

The material theories of phlogiston and caloric provided dead-end roads that confused many for many, many years.

The mechanical theories eventually won the day in proving that work could transform to heat, and heat to work, by an exact exchange rate called the “mechanical equivalent of heat.” It was this experimentally proven fact that ultimately drove the stake through the heart of the caloric theory. Important to realize, though, is that the the mechanical form of heat required no assumptions about the nature of heat itself since it was based on the use of a thermometer.

The fact that heat is actually a result of the motion of atoms wouldn’t appear until the kinetic theory of gases and the atomic theory of matter were developed, but even then, the existence of atoms was still considered an unproven hypothesis.

It’s hard for us today, given that we learn about atoms in high school, to appreciate the challenges of all of this. If you want to gain this appreciation, read the publications of the scientists in the early- to mid-1800s. The fog of war comes to mind.

And I haven’t even touched on the fact that heat itself doesn’t even exist! : )

Explore in greater details the evolution of these heat theories in my book, Block by Block – The Historical and Theoretical Foundations of Thermodynamics. This evolution eventually led to the theory of energy and its conservation in 1850.