Physics Rediscovered #11: The heat is on!

When you hug someone, don’t you just love the feeling of phlogiston flowing between you?

Jeez, who starts a conversation like that, right? Well, people in the 17th century, apparently.

All of because a certain Georg Ernst Stahl could not figure out why things get warm or cold, or why chemical reaction happen in the first place. Not wanting to look stupid he proposed the existence of a magical substance that handles all. Magical substances are really tight(!) so it was an immediate hit - people loved it.

So how did we learn to avoid these awkward “ice-breakers“ and casually talk to friends about heat transfer, like you normally do?

To answer that, I need to take you on a journey. It will get cold before it gets warmer so grab a cup of your favorite phlogiston and let’s turn the heat up.

Let it burn

Some people would like to see the world burn. These people are batshit crazy.

However, many scientists in the 17th century liked to set fire to a lot of stuff in order to figure out what burning is and how it works.

Let’s role-play for a moment. Forget about everything you know and start cooking your steak on a fire or whatever it is you vegans do. What do you see?

I see that the wood I use as fuel gives of fire. That fire seems sustained as long as I supply the wood. That fire is pretty hot and makes my steak hot. The wood turns into coal and, if I overdo it, my steak will also turn into coal.

Overall, something seems to flow, with the help of fire, from the wood to the steak. Given enough flow this something also changes the nature of its source and target.

So, don’t say no immediately, but what if there’s a substance that lives inside all objects and moves from hotter ones to colder ones. Let’s call it phlogiston, because I’m totally into Greek stuff. Moreover, as it leaves a body, that body becomes its original self or dephlogisticated. For example, ash would be dephlogisticated wood. Ash would also be dephlogisticated human when you think about it and recall our recent tale of heliocentrism.

While, we’re at it, we can also say that it’s responsible for fermentation, metals rusting … and things having color, because why not. Can you touch it, or see it? Almost. Soot would be the closest one can get to purest phlogiston.

Great! We have a theory and our work is done here.

Or is it?

Recall that no scientific experiment is complete without tubes and burners, and lasers, and particle accelerators and giant space stations, and … scales.

A reminder, for those of you born after the 19th century, of what a scale is

Scales turned out to be phlogiston’s nightmare. People were weighing burned remnants and they did not seem to lose any mass (the remnants, not people - no one was this naive about their fitness). That’s weird, because the remnants just lost a bunch of phlogiston. Shouldn’t the remnants be lighter?

Phlogiston proponents said that this is fine and that everything is under control. The solution is that phlogiston is simply weightless.

Ok, but what about that remnant over there, which is now heavier?

Uhmm….

“Soooo, maybe phlogiston has negative mass?”, says Robert Boyle. You know, it decreases the mass of an object when its inside and then allows it to have its original, higher mass once outside.

Ughh, sure.

Phlogiston seemed to acquire more wonderful properties as the scrutiny towards it grew. However, at the same time Boyle found something fascinating. He showed that when you compress air and allow it to decompress, while keeping its temperature constant, its pressure is inversely proportional to its volume, like so:

He speculated that air behaves like it is made of a multitude of tiny springs. Just like springs it stores energy when compressed and releases it when decompressed (original work here). Interesting…

Meanwhile a bunch of fellas started making noise that motion is the true source of heat. Well, who? And motion of what?

Well, Galileo, Descartes, Francis Bacon (a great thinker), John Locke (a fantastic thinker), Robert Hooke (Newton’s stalker), Mikhail Lomonosov (first to freeze mercury)… the list goes on. And motion of microscopic particles that form up all matter.

Hmmm. How will phlogiston psychofans handle this? By applying the ancient technique of: “if we ignore them, then maybe they’ll go away”.

So far, it seemed to be working. “Heat is motion“, didn’t really catch on, especially since it involves believing that matter is made up of invisible particles that go crazy and that’s why your coffee gets hot.

Wait, what? Daniel Bernoulli did what?! He derived, what Boyle just found out, using Newton’s laws of MOTION?! (original bedtime reading here).

Ignore harder, damn it!

Let it breathe

Ok, all this ignoring is exhausting. Let’s give it a moment. We need to take a breather and no one is better at breathing than Joseph Priestley.

He is great at producing and isolating “different kinds of air”. Nitrous air, acid air, vitriolic acid air, alkaline air, diminished air, dephlogisticated air, you name it. Today we can give proper chemical names to those airs, but back then it was a mystery to what those are, exactly.

So what do you do when see a dense gas evaporating from a suspiciously looking, volatile chemical? You breathe it in, obviously.

That was Priestley’s motto. He sniffed those vapors like a cocaine fiend running dry for two weeks. He especially liked the dephlogisticated air. It was:

“…five or six times better than common air for the purpose of respiration…“

…and his experimental mice seemed to live longer on it than on the other airs.

Why dephlogisticated? Because it showed an incredible potential to take in phlogiston. In other words, everything it touched burned like crazy.

Priestley had a good show and went on a European tour to perform his air-generating experiments. While, in Paris he caught the attention of Antoine-Laurent Lavoisier.

Already a giant at the time, a humanist, social reformer and a scientist, Lavoisier repeated Priestley’s experiments, minus the sniffing. Initially, he concluded that dephlogisticated air must just be a purer form of common air, but that was about to change soon.

Lavoisier really didn’t like the whole phlogiston thing. He continued his experiments with dephlogisticated air and took careful measures of the volumes and weights of the chemical components used in the reactions. He showed that dephlogisticated air is not just pure air but as significant part of it. Lavoisier was convinced that it was actually the part that participated in combustion because the weight of that air lost was the amount of weight gained by whatever burned.

Jacques-Louis David saw that Lavoisier’s wife didn’t come to nag her husband but to help with the experiments - it’s possible

Ok, time to drop that stupid name. By now you must know that dephlogisticated air is none other than oxygen, which is what Lavoisier called it.

His critics, however, wanted none of that. Phlogiston is the hill they would die on. It turned out that Lavoisier was more than happy to deliver.

He thought that:

“…if I show that these same phenomena can be explained in an equally natural way in the opposite hypothesis, that is to say without suppose that there is fire matter nor flogiston in the matters called fuels…“

then phlogiston is dead.

It wouldn’t be overnight, though. Whatever evidence Lavoisier delivered, the phlogistonians rejected. However, he was undaunted.

Over many years he perfected chemical experimentation to an art form. At the time, government sponsored grants weren’t a thing so most scientists funded themselves. Lavoisier bankrupted himself doing so, but the truth seemed worth it.

Throughout his experiments he showed that water can be made from combining oxygen and “inflammable air“. That was nothing new as other’s have done that before. However, because of careful measurements he could demonstrate that only those two gasses participate and no magic is required. He named the “inflammable air“ hydrogen - the water generator. On top of that, he used water to show how oxygen participates in acid creation and corrosion.

With a simpler explanation provided by Lavoisier, requiring no magical, anti-gravity substances, people had no choice but to accept that phlogiston was indeed dead.

Let if flow

So when you just dismantled a hypothesis of a mysterious fluid responsible for making all things hot, what’s your next step? Do you give yourself fully into the “heat is motion“ concept from the kinetic-theory-of-heat crowd? No.

You invent a better mysterious fluid.

That’s right. The same Lavoisier who just killed phlogiston gave birth to caloric.

This time it’s better, you see. There’s a lot of heat where there’s a lot of caloric. It’s self-repelling, so it flows from warmer to colder bodies. It is intrinsic to all matter and passes through it easily but not freely. From a scientific perspective it’s not that terrible, really. Here’s what it can do:

It immediately explains the cooling of objects, which are just left alone. Heat expansion and cold contraction is also easy. Since we assume caloric is a real substance, then the volume of objects increases with the amount of caloric and vice-versa.

One thing we didn’t mention is that it’s not just fire or touch that carries heat. It can also be radiated away. Ever stood next to a bonfire? Caloric also handles this one. Lavoisier explained it as only having to do the surface of an object, not its composition. For example, in smooth surfaces the material is packed closely together, making it difficult for caloric to flow - poor radiation. On the other hand, a rough surface is porous so caloric is free to go. He was almost right.

Through many successes, caloric turned into a heavy-duty theory. It would take equally heavy artillery to challenge that. No, I mean it. It would take literal cannons.

Fortunately, there was a science enthusiast, engineer, inventor and military veteran in the area, who had limitless access to cannons. Count Rumford (aka Benjamin Thompson) was the man’s name and he was not a fan of caloric.

He submerged the cannons in water and got to boring. He noticed that as long as there is friction, heat is generated and water boils. There should’ve been a finite amount of caloric in the cannons, but as he restarted the drilling, the water boiled again. Where did the caloric come from?

Well, maybe the cannons had a lot of it in general or something. Ok, so if that was the case, then Rumford’s going to significantly cool down the cannons before each drilling, making sure that they had as little caloric as possible. The water still boiled, over and over.

Count Rumford drilled his cannons mercilessly, but there was nothing sexual about it.

Obviously, the caloric could not be a conserved quantity or it was simply endless, which is dumb. Motion, as Rumford stressed, is the more probable explanation of heat generation.

Let it go

You might think this is the end for caloric, but you would be dead wrong. It’s simply too good, you see. It explains so much and does not require the belief in invisible particles comprising all of matter, which is like sooo crazy. Invisible self-repellent fluids are better.

In fact, another victory for caloric is to be delivered by Nicolas Léonard Sadi Carnot. He was really into steam engines and based on caloric alone imagined a perfect engine, that became the idealized standard to this day. Getting into this story, however, is like opening a can of worms, so we will leave that for the next episode.

So let’s recap. Halfway into the 17th century we’ve started by explaining heat and chemistry with phlogiston, which dominated the scene for about a 100 years. Then, Lavoisier’s identification of hydrogen, oxygen and it’s role in combustion made phlogiston obsolete in favor of caloric, which dominated for about another century. At this point we are almost halfway into the 19th century and we still don’t have a good handle on what heat is.

All this time we were somewhat pretending that the idea that heat explained as the motion doesn’t exist. However, we can ignore it no longer.

We will face that challenge in the upcoming episodes. But beware, brave reader, as we will be entering into the realm of madness, where many have perished.

Brace yourself and see you in the next episode.