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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 a parent every dayness, it took a long time to understand what heat actually was.

It's a story of brewing vats and steam engines, fridges, the monitors, 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.

We'd need to discuss the signs of heat from the 17th century.

I'll join a Hague, Professor of Atmospheric Physics at Imperial College London,

Professor of Philosophy of Science at University College London, and Simon Chaffer, Professor of History of Science at the University of Cambridge and Fellow of Darwin College.

Simon Chaffer, we're going to concentrate on the developing signs 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 recognize that the classical tradition going back to the Greeks gave those philosophers an enormous range of resources to make sense of what heat 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,

weakly linked candles, oil lamps, life itself obviously has something to do with heat.

The classical tradition, suppose 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 analyzing exactly the nature of heat had been going on for a 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 the theory is developed by the Irish 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,

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