2026年4月22日 / Last updated : 2026年4月22日 桑子 研 Science

The Mystery of “Caloric”: From Middle School Myths to the Truth of Heat

I’m Ken Kuwako, your Science Trainer. Every day is an experiment!

Try rubbing an eraser against your desk. Now, let’s take a look at that through a thermography camera.

As you can see, the temperature rises. But why?

How would you answer such a simple question? Believe it or not, this inquiry is the gateway to a grand mystery that humanity pursued for thousands of years: the true nature of heat. Today, let’s unravel a story that moved the history of science, starting with a rather adorable misunderstanding I had back in middle school.

My 14-Year-Old Self’s “Discovery of the Century”
I remember my second year of junior high like it was yesterday. My science teacher asked the class, “Why does a hot object gradually cool down over time?” As the room fell silent, I raised my hand and confidently declared:

“I think it’s because a substance that causes ‘coldness’ moves into the hot object!”

There was a brief pause in the classroom. My teacher looked surprised, yet also a bit delighted, and said, “That’s a very interesting way of thinking.” At the time, I felt embarrassed, thinking I had given an answer they didn’t expect. Looking back now, though, I want to give my younger self a round of applause.

That’s because my simple question followed the exact same path once taken by history’s greatest scientific minds.

The “Caloric Theory” Even Geniuses Believed
In fact, until the early 19th century, the world’s most brilliant scientists thought just like I did. Back then, people didn’t know that heat is actually the vigorous vibration of atoms and molecules. Instead, they believed in something called Caloric.

The sensations of “hot” and “cold” were explained by an invisible, weightless, mysterious fluid living inside matter. This was known as the Caloric Theory.

According to this theory, a freshly brewed cup of tea cools down because the caloric packed inside it slowly leaks out into the cup and the surrounding air. While my middle school guess about “cold substances” moving was technically backwards in direction, I was incredibly close to historical scientific thought by treating heat as a physical substance.

Truth Found in the Boring of Cannons
However, the Caloric Theory had a major flaw. If heat were a physical substance, it would eventually have to run out. The man who noticed this contradiction was Benjamin Thompson, also known as Count Rumford. While observing the process of boring out cannon barrels, he realized something profound.

“As long as we keep grinding the metal, heat keeps coming out infinitely! If heat were a substance like caloric, the supply should eventually go empty.”

This sparked a massive leap forward for science. We eventually reached the conclusion that heat isn’t a flowing fluid, but rather the kinetic energy of invisible, tiny particles.

Take another look at that first thermography image. It looks like “caloric” is springing forth, doesn’t it?

Actually, this very phenomenon is what ended up disproving the Caloric Theory.

The Seeds of Science Live in Curiosity
Modern textbooks teach us that heat is the transfer of energy. When a cold object and a warm object touch, the fast-moving particles in the warm object collide with the particles in the cold one, sharing their energy. Temperature changes as the overall movement becomes uniform.

By modern standards, my middle school answer was “wrong.” However, the ability to look at a phenomenon and build a model—thinking, “Is something moving here?”—is the most exciting part of science.

The next time you wonder “Why?” during a science lesson, try to come up with your own hypothesis before looking for the answer in a textbook. You might just find that your idea is connected to a “Great Discovery” that the geniuses of the past spent their entire lives trying to prove.

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