In Our Time - Heat

Episode Date: December 4, 2008

Melvyn Bragg and guests discuss the history of scientific ideas about heat. As anyone who’s ever burnt their hand will testify – heat is a pretty commonplace concept. Cups of coffee cool down, microwaves reheat them, water boils at 100 degrees and freezes on cold winter nights.Behind the everyday experience of hot things lies a complex story of ideas spread across Paris, Manchester and particularly Glasgow. It’s a story of brewing vats and steam engines, of fridges, thermometers and the heat death of the universe. But most importantly, it was the understanding and harnessing of heat that helped make the modern world of industry, engineering and technology.With Simon Schaffer, Professor of History of Science at the University of Cambridge and Fellow of Darwin College; Hasok Chang, Professor of Philosophy of Science at University College London and Joanna Haigh, Professor of Atmospheric Physics at Imperial College London

This BBC podcast is supported by ads outside the UK. Every week, we cover the week's tech news on this week in tech. Hi, this is Leo LePort inviting you to join me and my panelists this week. Jason Heiner, Doc Rock, and Mike Elgin will talk about Anthropics' new AI. They say it's too dangerous to release Sam Altman in response to the firebombing of his house. And Samsung jumps profits eightfold thanks to AI. Hi, you'll find Twitter at twit.tv or wherever you get your podcasts. Thanks for downloading the In Our Time podcast.

For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio four. I hope you enjoy the program. Hello, heat is a commonplace concept. Cups of coffee cool down, microwaves reheat them, water boils at 100 degrees and freezes on cold winter nights. But for a thing of such apparent everydayness, It took a long time to understand what heat actually was.

It's a story of brewing vats and steam engines, fridges, thermometers, thermodynamics, and the heat death of the universe. And importantly, it was the understanding and harnessing of heat that helped make the modern world of industry, engineering and technology. With me to discuss the science of heat from the 17th century are Joanna Haig, Professor of Atmospheric Physics at Imperial College London, Hassok Chang, Professor of Philosophy of Science at University College London,

and Simon Schaffer, Professor of History of Science at the University of Cambridge and Fellow of Darwin College. Simon Schaffer, we're going to concentrate on the developing science of heat from the 17th century, but was there much theory about it before then? Well, in order to understand what begins to be said by natural philosophers from the 1600s onwards about heat, one has to recognise that the classical tradition, going back to the Greeks, gave those philosophers an enormous range of resources to make sense of what heat was.

was, as you said, heat is apparently a very domestic common phenomenon. We're thinking of a world lit by fire in which the notion of heat was very often identified with a vast range of phenomena weekly linked, candles, oil lamps. Life itself obviously has something to do with heat. The classical tradition supposed, perhaps more than anything else, that heat was a quality that perhaps cold was another quality. So either a substance was hot or it was cold. And above all, heat was a quality of the elements that make up nature. So that for many classical thinkers,

one spoke of air as a combination of the qualities of heat and damp, as it were. and one spoke of fire as something warm and dry. So that analysing exactly the nature of heat had been going on for very long time. One had a very rich vocabulary for speaking of it. It was the most common domestic phenomenon in kitchens, in distilling, in alchemy, yet it was a puzzle and a hard puzzle to solve.

Did anyone go any way towards solving it before the 17th century, whether theories developed by the Greeks, Romans, by the Arabs and so on? I think the most dominant philosophical model was the model of the elements, that it seemed to make sense that heat could be more or less present in a body. In other words, to use their technical language, it could be more or less intense. And this raises a problem that we'll be coming back to a lot in our conversation, which is the difficulty of distinguishing between heat and temperature.

That's to say something can be intensely hot and one is invited to measure, or at least estimate how hot it is. We're equipped with a rather good instrument for doing that, which is our hand, our sense of touch. And very often, certainly in the Middle Ages, there were attempts to build scales of heat that look to us now like scales of temperature and are very commonly associated with how stuff feels to the touch. Does it feel warm? Does it feel chilly?

Now we've chosen the 17th century to begin the investigation largely because of what you three have told us was going on. So why did it get going in the 17th century? Francis Bacon seems to come into play here, but other others. But why did he get going then? Why did this... I think Bacon's experience with Heat tells us a great deal about how this topic became pressing and urgent

for philosophers and experimenters. As we've said, heat is, amongst many other things, it is a tool. It's a tool in the kitchen, through fire. It was absolutely indispensable for a wide range of extremely practical quasi-industrial processes in distillation, in the manufacture of alcohol, in mining and metallurgy to extract pure metals from their ores,

which were some of the dominant European industries of the 16th and 17th century. And I think what's characteristic of that period is a renewed alliance, or at least set of exchanges, between elite philosophers and natural philosophers, especially, and literate artisans, and bacon absolutely belongs to that tradition. For bacon, heat matters, perhaps more than anything else, because it might offer a key to life. So a lot of the essays that Bacon himself wrote from the early 1600s onwards on what heat is

have titles which point to heat's importance in maintaining life and explaining how life turns into death. Hazak Zhang, Simon mentioned the distinction between temperature and heat. Let's come to that idea then. The 18th century saw the development of various temperature scales we use today with Meso's Fahrenheit and Celsius. Can you take us through when this distinction was made and how temperature came to be measured?

Yes. Simon mentions that heat used to be considered a quality primarily. And the main thing we see happening as the study of heat becomes scientific in the modern sense is quantification. So the first thermometers that we would recognize as such. were built around 1600. Galileo is one of the people who made the first thermometers. There are a few others. And as soon as they begin to make these thermometers,

the question arises, well, actually, what exactly are we measuring? Because what they were doing in making thermometers was noticing that various phenomena in nature were correlated with these sensations of hot and cold. but they all behaved in very different ways from each other. So by the time you get to the middle of the 18th century, there are about 15 different commonly recognized temperature scales,

and that includes the familiar ones of Fahrenheit and Celsius. But they are on completely different basis. Celsius is an interesting case, for example, because he did use the freezing and boiling points of water, as we know, but his scale was actually upside down. So the boiling point was marked zero and the freezing point 100. And it just makes you wonder what exactly was he trying to measure. You say there are many different forms of measurement.

Was this part of the confusion that came out of the distinctions, yet as yet not quite clear, between temperature and heat? Could you tell us about that? Yes. if you asked the philosophers and scientists of the 18th century, okay, what exactly is the relation between temperature and heat? You got very different answers. So some people said, right, heat is this material fluid.

Temperature is just the density of that fluid. That was the most popular common sense notion that people were using about the middle of the 18th century. other people said no heat is a form of motion and temperature is something to do with the velocity of that motion. One idea in that vein was that heat was a vibration of molecules and temperature had to do with frequency of that vibration. So there are many competing ideas

and the operational distinction between heat and temperature really only comes in the second half of the 18th century when people like the Scottish chemist Joseph Black begin to measure specific heats or sometimes call heat capacities Before we come to clarity, let's stay a bit with confusion It might be interesting to know why there were these differences and what they were based on

You said there were 15 different ways, as I understand it, to measure the temperature. Can you just go into that little more? The different scales are based on, first of all, different fixed points, as they were called. these are the points at the phenomena with which you calibrate the thermometric scale. So we commonly know about freezing and boiling points of water, but there were many, many others. There are some amusing

ones such as the melting point of butter. The temperature of deep caves and cellars was a popular one, especially the temperature of the wine cellars at the Paris Observatory, which appears at least in three well-known thermometric scales. But then there are other ways in which they differ. As I mentioned, some of them go upside down, and some of them are calibrated at two points. Others are calibrated with just one fixed point.

Fahrenheit's famous scale was calibrated at three different points. And you just get almost an indefinite array of permutations. Was there any sense that these were converging, Was there any sense that one idea was becoming dominant and that would become one way of measuring the temperature? By the time you get the work of Celsius, which is around 1740, the boiling and freezing points of water are taking over as standard.

But there's another question which they need to settle, which is what to fill their thermometers with? And Fahrenheit uses mercury. A lot of other people are using alcohol, and they actually have different patterns of expansion. So even if you fix these different thermometers with different liquids to read the same at zero and hundred, they will differ in the middle. You mentioned the Scottish chemist Joseph Black there.

Joanna Haig, about 1750 Black, as it were, came on the scene. Can you describe how and what he did and how revolutionary it was? Well, what he was able to show was that temperature and heat are not actually the same thing. And he did this by observing, one way he did it was by observing a bucket of water containing ice and monitoring its temperature.

And he noticed that the ice was melting, so he assumed from that that heat was entering the bucket. But at the same time, he noticed that the temperature wasn't changing. So while there was a heating of the bucket, there was no change in temperature. Now today we'd understand that in terms

of latent heat of melting. But he was able to show for the first time that heat and temperature weren't the same thing. You mentioned latent heat. He's credited with the discovery of latent heat. Yes. Could you tell us what latent heat is? Well, latent heat is the energy that's required to change the phase of a substance. So by phase, I mean, if it's in a solid or a liquid or a gas, it's in a different phase. And you need energy to turn it from one phase into another.

So we've already mentioned melting of ice, and that's called, you need the latent heat of fusion, which gives the structure of the substance, which is sort of fairly rigid as a solid, the energy to start moving around and become more fluid, and then it turns into a liquid. And then at the next stage, you need energy of evaporation to convert the liquid into a gas.

And at each of these phase changes, it takes place at a particular temperature, so it could be the melting point or the boiling point, as Hassox already mentioned. And you need the latency. per unit mass of the substance to convert from one phase to the other. What is important about that discovery of latent heat? So we've got this bucket of ice and it's turning into water

and yet the temperature of that bucket of ice is the same when it's ice and it's water. But something to do with energy is happening. Yes. And so where is the latent? Can you just go once more into where the latent heat is in that? Right, well we have the absorption of heat in this case, in Black's case, from the atmosphere into the bucket. And what happens then is that it's turned into the thermal energy, the heat in the motions of the molecules involved. So they'll start to move around fast.

This is part of one of the arguments that has, of course, already described as historical arguments. And as the heat is absorbed, these molecules are able to absorb, to move. move around faster, and then the phase change means essentially a separation of the molecules to further distances from that which they were. So, for example, if you're going from a liquid to a gas, there's still interactions between the molecules when it's in the liquid phase, but when you give it the latent heat, the molecules have the energy to escape from each other, and in the gas they're almost independent of each other. in what sense did that take this study of the signs of heat forward

what did it allow to happen next what it did was it showed that you couldn't talk about heat as a property of the substance you couldn't say this bucket contains a certain amount of heat because it's only when the heat is transferring into the bucket that it's changing its temperature or not changing its temperature

that you can see the difference between the energy contained in the bucket, which is measured by the temperature, and the heat input is not the same. In the same Glasgow Laboratory as Joseph Black Samanchev was James Watt. He was the instrument man there. This takes us into an area which I find particularly fascinating, where you have practical men doing things, experimenting on the job, as it were,

and they run ahead of theory as well as literally being in the same laboratory. So it's a wonderful double harness we're in at the start of this Industrial Revolution. which is what it becomes. Can you tell us about that? Can you set that out? Yes, I think two things to think about. One is the use of heat and fire industrially is already common in Britain,

for example, by the 1750s and 1760s, spectacularly so because of the development at the end of the 17th century of what we would now call stationary steam engines, but which contemporaries then called fire. machines. And these were pumps. One of the great economic problems of the age was

to pump out water from deep mines in the West Midlands, in Scotland and in Cornwall above all. And that meant that you needed machines which could run in principle for very long times and these being canny men as cheaply as possible.

And from the 1690s fire machines, steam engines had been used to run these pumps. The question was, were they worth it? That's an economic problem. It's also a problem for natural philosophy, because it's a problem about efficiency. How can one most efficiently use the heat that's driving these engines to get the maximum amount of effect for a given amount of fuel? Now, this was very well known, partly because men like Joseph Black, in his lectures at Glasgow, he and his colleagues would have models of these steam engines, which they would show to their their students to demonstrate the properties of heat, fire and temperature. And the brilliant James Watt, Greenock man, working in charge of the instrument-making shop at Glasgow, had, as one of his tasks, to make sure that Professor Black's demonstrations were not running out of steam. Especially so, because Black's income came from his students.

Students paid to go to Black's lectures. so bad demos mean loss of income. Now there are many economic factors in play here. What's aim was to work with the model steam engine in the Glasgow teaching setup in order to increase its efficiency. He in his notebooks begins to describe my perfect engine. In other words, would it be possible to turn this model engine into a device that was first of all working at least as well as real engines, which it wasn't, and then maybe even better. And I think the punchline of that story is the importance of experimental control.

In the tangled conversations between black and what emerges is the immense importance of precision measurement, the indispensable role of very high-class thermometers, and the role, to put it bluntly, of Glasgow Enterprise in making this model engine work much better than any real world engine up till then it worked. We're talking about that with great enthusiasm and precision

and importance, but at the time the French were supposed to be were claimed to be and were seen to be ahead of the game in the inquiry into the signs of heat, Hussachang, and the dominant French theory was the theory of caloric. Can you

explain that to us? Yes. Caloric was a term coined by the great chemist Antoine Lavoisier, and the caloric theory was really the most dominant form of the material theory of heat, which considered heat to be this real substantive fluid that flows from a hot place to a cold place and so on. And Lavoisier's conception to our modern eyes was quite peculiar because he actually considered caloric a chemical. A. Kemalek's substance. Choloric actually occurs in his great table of chemical elements right at the top with light. And his notion of latent heat, for example, which he understood very well and in the same way

phenomenally as Joseph Black did. But when he tried to explain how this happened, Lovewasi said, okay, if you take a block of ice and heat it, what's happening there is caloric is coming in chemically, combining with that ice, making a chemical compound, which was the liquid water. So this was a very strongly chemical view of heat. And he considered this a key piece of his chemistry. The first chapter of his classic textbook of chemistry of 1789 is all about caloric and how caloric by combining with matter affects these state changes that Joanna mentioned. In terms of the theory of the time, that was the dominating theory.

Can I just come to Joanna for a moment, Simon? There was the Frenchman Sadie Kano, who developed the theory of steam engines. So I'd like these two things to run alongside each other for a while. Simon's talked about the practicalities of what. Can you talk about what Kano did over there in France? Well, what Kano, he was considering essentially this question about how efficient you could make a general... So we talk about heat and work, really. A general heat engine.

And he came to understand that the best and most efficient engine that you could obtain was one in which all the heat came in at one temperature and it was released all at one other temperature. And he didn't understand why that was. He didn't understand the theory, but he observed that that was the case, and indeed that is the case, that's hypothetically the best heat engine that you can design. And that's sort of running in parallel with the technological advances

that Simon's been talking about by James Watt and others in developing the steam engine for practical purposes. And this is a sort of theoretical development. So although the Kano was able to say this is hypothetically the best situation that you could have, he wasn't really able to explain it nor to relate it to these technological advances in the steam engine. I think one of the most interesting things about Carnot's model is that it reminds us of the technological basis of steam engineering at the time

because it's clear that the analogy that's in Carnot's mind is the analogy of the water wheel. What happens in a water wheel is that water flows in at a height and drives a wheel and then flows out at a much lower height. And there's obviously some... relationship between the height through which the water's dropping and the efficiency of the water wheel. And one might think, and I think this is very much in the minds of engineers and physicists

at the time, one might think that if you wanted a perfect engine, what's perfect engine, it would be a great idea if all the height was being used to drive the wheel. And that was obviously a dream, but it was also clearly beyond engineer's capacities. It would be as if you had a river delivering all its water to a water wheel so that

the water impossibly flowed out at zero velocity and you'd extracted all the possible work from the river. And Kano's ideal heat engine would spookily be like that. It was also underwritten

by what we've just said about caloric. The idea that heat is a fluid of some sort, that heat flows, and that you're measuring something like the intensity, understand, height as a temperature, that's a very important notion in physicist's mind at this period. How's up, Jane? Yes, it's very interesting you mentioned, Simon, the water wheel analogy, and I think that is essential in Carnot's early thinking because he is thinking in terms of caloric like everybody else at the time. And I mean, towards the end of his life, he begins to entertain doubts about the caloric theory. And then what we inherit is William Thompson taking Carnot's theory and getting rid of caloric from it under the influence of James Jewel, which I'm sure we'll come to later.

But I think this is very interesting because we have Carnot establishing the fundamentals of the modern theory of thermodynamics, but he's doing it on a completely wrong basis from the modern point of view. And I might mention slightly earlier, there's a man who comes up with the idea that we now, in retrospect, recognize as the more correct one, which is the case of Count Rompford, who argued against Lavoisier, that it was not a material substance, but a form of motion. He was not... He was drilling out gun barrels in Bavaria. Yes, the famous story.

Rumford was an American soldier of fortune who had fought on the wrong side in the American War of Independence. So then he fled and came to Europe, ended up in Munich working for the elector of Bavaria, doing all sorts of amazing things like rounding up the base, of Munich into workhouses and creating the English garden. But one of the things he did was he ran the Bavarian army and he was supervising the boring of cannons at the time.

I mean, that was apparently done by first casting a solid brass cylinder and then drilling a hole all the way through it, the drill driven by two horses. And you can imagine what a frightfully noisy process it would have been. but not only noisy was it, but it created an enormous amount of heat, which he noticed. And he actually put on a huge vat of water on top of a cannon being bored and boiled it. And from that experience, he said, look, I can create an indefinite amount of heat just by this fictional motion. So it doesn't not prove that heat is a form of motion?

Joanna Haig. Yes, I mean, it's interesting to look at these historical, views because none of them is entirely wrong. They each have their own element of correctness and although we know that each of them is not precise, good definition nowadays so for example with caloric of course heat is not a

material quantity that's transferred between bodies but we do understand now that heat is just energy in motion. So the idea of the transfer of caloric now we would see is the transfer of heat which is not a substance but it is something that's transferring energy. So you only identify heat

when it's in transit. So that's the correct aspect of the caloric theory. And then we have the molecules in motion, which again is not a measure of heat, it's a measure of temperature. So that that was sort of correct as well. And nowadays we understand that each of these aspects can be combined together in what's called modern thermodynamics.

So at this stage, can I think of the relationship of heat and work as being the conversion of energy at a microscopic level? Well, of course, there's many different practical ways of doing it. But essentially, yes, what happens in a... in a real system is that you put heat into it, or indeed take heat out of it, but let's have the positive. We put heat into a system,

and it might increase its temperature, but it might do other things to it as well. So if we're talking about a thermodynamic system, for example, in a container, it might increase the pressure or expand the gas, in which case there's been work done on the gas. So the temperature hasn't really, risen, but there has heat been gone into the system. And so nowadays we understand that there's an

equivalence between heat and work. So the heat flows into the system and it changes the state of the system, which can be measured by various different thermodynamic parameters. Simon, there are implications to come back to the double harness in all this for its theoretical, for engineering, for military measures, as I sort of pointed out, with the boring of the cannons and so on. So can you just bring that into play as well. Yes, I mean, in thinking about how European elite scientists deal with the problems of heat, work and temperature

in the period, say, 1780 at the end of Black's career through to Carnot's work in the 1820s, one must not forget that this is a period of dramatic, evident, radical industrial transformation and that it was very widely perceived that this transformation was driven by the steam

engine. And the extraction of efficient work from steam engines and similar kinds of working devices was simultaneously a really important physical problem and a really important economic one. And it's not surprising, therefore, that the sites at which these problems are most actively discussed are the same as the sites of major industrial transformation in this country, Glasgow and Manchester. So that brings us to the Manchester Brewer, James Jewel. Yes, Jule is a fascinating example.

A brilliant young Tory brewer growing up in Manchester from the 1820s and 1830s. The eminent preeminent chemist John Dalton was his tutor. He was a member of the family that was brewing more beer in Manchester than any other family. And then let's think about what the industrialisation of beer, one of the great events in human history, involves. one of the things that it involves is the massive, precise control of the temperature of liquids mixing in vast vats, typically underground, in well-controlled, disciplined storage areas, with available to men like Jule, relatively reliable, extremely well-muscled workforce. those are the preconditions of the work that Jules set out to do. To simplify enormously, Jule in one way, does exactly the opposite of what Kano was thinking,

and in another way, as it were, turns Rumford's cannon-boring experiment into a precision operation. What he does is to build a vat, a rather small one, whose temperature he can control and measure. And then inside the vat he has a paddle wheel. And the paddle wheel is driven by a little machine, which is driven itself by falling weights, equipped with superb temperature control over a vat of water, precision measurement of the size of the weights that are driving the paddle wheel, and a thermometer which many of his contemporaries thought was so miraculous that it must be fraudulent,

he was able, was dual and his assistance, to measure the conversion rate between the amount of mechanical work being done and the temperature change in the water. And he was able to show that there is a conversion rate between mechanical work and heat and that that conversion rate is constant and from that being a Tory and a Christian, he was able to deduce the fact

that what God had created, no human could put asunder. That there was an amount of, as it's later going to be called by his friends in Glasgow, energy in the world which stays the same. And that the reason why the conversion rate between mechanical work and heat is a constant is because their expressions are the same kind of thing. Simon mentions the friends of Jewel in Glasgow, and of course the most important of those friends is William Thompson,

who later becomes Lord Kelvin. And I think the collaboration between Jewel and Thompson is such a wonderful case of the more practically oriented scientists and the more philosophically oriented mathematician coming together to create this thing we now call thermodynamics. William Thompson had gone to Paris to learn from the great experimentalist, Victor Renaud.

And during that year spent in Paris, he picked up Carnot's paper, was very impressed by it. He comes back to, Glasgow to take up the chair of natural philosophy at the unlikely age of 22, a chair which he held for 53 years. And Thompson's trying to develop Carnot's ideas. And then Jules hears a presentation by Thompson about this and says,

this is great, but you must change it because you're taking off Carnot's idea that heat is a conserved entity, that heat just passes through the heat engine and in the course of that movement creates mechanical work. Jules' view from the paddle reel experiment that Simon mentions is no. What the heat engine does is it destroys a bit of the heat to turn it into work. And he eventually prevails upon Thompson to make that change. And that is the burden of modern thermodynamics, as we know it. One important thermodynamics idea is the energy concept.

Is that what he did? Is that what Jules? Yes, I mean, Jule is. known as one of the creators of the principle of energy conservation. So he's focusing on the interconvertibility of heat and mechanical work. And then there are other people like Helmholtz in Germany who comes in and says that actually can be extended to all forms of energy, including the chemical. And that's all happening in the 1840s, in this very, very intriguing industrial as well as scientific context.

Hey, Thompson also used the concept of entropy. Can you explain what that is? Entropy is essentially a measure of disorder of a system. So if you, by analogy, think of a tower of building bricks sitting there nice and solid, it's well-ordered, it's got low entropy. If you knock it over, it's disordered, it has higher entropy. And this is related to the concept of heat, because when heat enters a system, its entropy is increased.

And so this is, entropy is another parameter by which we can describe a thermodynamic system. So we have things like temperature and pressure. We also have entropy. And so by understanding this disorder, which relates to the behaviour of the individual molecules, we can therefore relate the heat to the state, the macroscopic state of the system. Simon Chavon, this phrase, heat, death, that comes out of that. I don't intrigue among others, HG Wells. Indeed.

It's a good phrase. Can you tell us more about it? So William Thompson, Kelvin, is a great place to start. Think what Kelvin and Jule are chatting about. They have this puzzle. On the one hand, they know from the French that a really perfect heat engine would be one in which heat would flow from hot to cold.

And as long as the heat is conserved somehow, you generate enormous amounts of work. But what Jule is saying, so Kelvin understands him, is, no, actually the work is the result of the conversion of heat into work, or vice versa, the conversion of work into heat. So Kelvin makes an absolutely splendid note to himself and says, when thermal energy is used up in a process of heat flow, what becomes of the work that energy could do?

And his answer is, Joanne has explained to us, that the work becomes unavailable. That's an absolutely splendid, Scots, prudent way of describing disorder. For a Glasgowian,

disorder is the loss of available work. So Kelvin's argument, and he's joined in this by a group of Scottish and then British natural philosophers, is that work becomes less and less available. And in general, in the processes of exchange of energy that we see in the universe, if we trace them through, say, the great Scottish physicists, they all end up transforming into heat. and the amount of available work in the universe goes down.

One's therefore left with two great principles, the first two laws of thermodynamics, that, to put it bluntly, the best you can do is to break even, and most of the time you lose. And that creates a world vision, which I think is on the one hand characteristic of a certain kind of Victorian set of values, and on the other hand makes some very specific predictions,

one of which is the heat death of the universe, that in the end, one will be faced with a maximally entropic universe. And as you say, one of the things that HG Wells' time traveller, going forward from 1895, into the immensely distant future, sees, is the slow extinction of the sun

and eventually the heat death of the cosmos. Can you give us a summary, then, we've got Kelvin and, We've had the French connection with Calvin studying in Paris, and he brings that to it. We've had black, and what it's all, it's concentrated now in Glasgow and Manchester. We've got the first and second law of thermodynamics, which became, which is constantly haunted. And those who don't know about science ever since, or taunted them, maybe, is a better way. Where are we towards the end of the 19th century?

Has the choleric theory been displaced? Do people know precisely what they mean by the signs of heat then? Yes, the caloric theory basically dies when energy conservation comes in and heat becomes considered a form of energy, which is a mysterious concept in itself, really, but people buy it and they say heat is no longer a substance. But there's another major development we must mention in the second half of the 19th century, starting with people like the German physicist Rudolf Klausius,

working nearly at the same time as Kelvin and Jules and others, and also Maxwell over in Britain. Well, could you talk first of all about Boltzmann and Klausius, the German side? Yeah, yeah. Clausius' main idea that's an innovation on what we've already discussed, is that heat is actually molecular motion. So if you look at Carnot and Kelvin's thermodynamics, it doesn't talk at all about molecules.

It's just how much heat does this body of gas contain, what is its pressure, what is its volume, and what is its entropy, and so. But Clausius says, no, I want the real story about what's going on at the microscopic level. And he sort of reaches back to the ideas of people like Rumsford and says heat is now not vibrations, just of molecules, but in a gas molecules are randomly bouncing about. And that microscopic motion, which he can't sense as such, is sensed as heat. And that's the road towards statistical mechanics, which now we say thermodynamics.

is just a manifestation of... John Hayk, they're coming at the end of the program now, unfortunately. James Clark Maxwell produced... Gene had things to say about this too. Can you tell us, brief them off of us, right? About his contribution, yeah. Well, Maxwell did a lot of work in thermodynamics. Many people know his work in electromagnetism,

but he also did work in thermodynamics and was able to produce mathematical relationships between the different thermodynamic properties of materials that nowadays that we can use to explain behaviour of materials. But may I just go back to what Hassat was saying about Klausius? I mean, he was absolutely fundamental. His name is not as well known perhaps as some of the other people we've been talking about, but what he was able to explain is quite why the Kano cycle,

the Kano engine, is the hypothetically, theoretically most efficient way of running a heat engine. And that some of the, you need it, the work to transfer the heat around the cycle. So it was absolutely fundamental. Sorry, do you want to say, Simon. So finally, have we got there? Do you three, and

the physicists around and so on, did you understand what heat is now? I think that current physicists' models of heat, temperature, work and energy represent some of the triumphs of modern science, of indeed the relationship between classical and modern physical science. That doesn't mean there aren't exciting paradoxes.

Here's one, which Maxwell, Boltzmann, and Clausius all addressed. What's the relationship between the chanceiness and the determinism of nature? That's one of the things thermodynamics is absolutely about, because on the one hand, it seems to represent the universe as a kind of snakes and ladders game, in which even though fundamental processes might be statistical, there is a direction. There might be catastrophic changes of direction, but there's a direction to the universe and that it will end in fire. Thank you very much. Simon Shavut, Joanna Haig and Hasek Chang.

I hate us to be linked up and joined together, but we seem to be. Next week we're going to talk about the Great Fire of London. Thank you for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com.uk forward slash radio 4.