Today, a historical footnote. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

I doubt you've heard of William Cleghorn. He was born in 1751. His father died young, so he and eight siblings were raised by his uncle, George Cleghorn. Uncle George was a noted physician at the University of Dublin. He'd once served with the British army on the Mediterranean island of Minorca and his book on the island's epidemiology was still around in the nineteenth century.

Young William was bright and, in 1779, he finished a doctoral dissertation at the University of Edinburgh. He died just four years later, and history barely acknowledges his existence. One can read about Cleghorn's work in the published lecture notes of his teacher, the great Scottish chemist Joseph Black. 

Black was a major figure in the process of sorting out the mysteries of heat. He explained the difference between the intensity of heat, or temperature, and the amount of heat. He invented the concept of specific heat to explain the different amounts of thermal energy that different materials absorb when their temperatures increase. He'd measured latent heats -- the thermal energy you have to transfer into a liquid when you boil it. 

James Watt had also tripped onto the idea of latent heat in his steam engine work, and Black had confirmed the concept for him. All around Black it was becoming clear that heat was not some phlogiston -- some component of matter. Rather, it was separate from matter. It flowed in and out of matter. (Black's contemporary, Lavoisier, had suggested the name caloric for this phantom fluid.)

Then brilliant, short-lived, young Cleghorn appeared on the scene to create a systematic description of caloric. It had to be a subtle invisible fluid. It had to be elastic with particles that repelled each other (to explain thermal expansion.) Cool bodies attracted caloric to different extents. That explained heat conduction and specific heats. Caloric took a latent form when you boiled water at 212°F, or a form that raised a material's temperature. Cleghorn said caloric must have weight because metals gained weight when you heated them. (He didn't know about oxidation.) 

Black, however, remained cautious, he knew this was not the whole story. But he pointed out that Cleghorn's rules for caloric explained experiments reported by Benjamin Franklin and others. And, Black added, the theory is "the most probable of any that I know."

We used Cleghorn's caloric for the next sixty years. Not until we understood atoms could we see that heat is just the way we perceive our body's overall response to atomic motion. Even today, we still talk about heat as though it were caloric. We speak of heat flow, and about bodies holding their heat. 

So William Cleghorn served us for a season, and he served us well. Then we left caloric behind, and he became a small footnote. The Roman Horace, could've been describing Cleghorn when he wrote:

Nor has he lived amiss, 
who from birth to death has lived obscurely.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

J. Black, Lectures on the Elements of Chemistry. (J. Robison, ed.) London: Mundell and Son, 1803. Vol. I, General Doctrines of Chemistry, Part I, General Effects of Heat. See especially pp. 33-34. John Robison was another of Black's students. He published the notes four years after Black died in 1799.

W. Cleghorn, Disputatio physica inauguralis, theoriam ignis complectens. (Inaugural physical argument, a comprehensive theory of fire.) given at Edinburgh, 1779.

See also the Dictionary of National Biography entry for George Cleghorn. (There is no entry for his son William.)

The outro music is from the Largo movement of Haydn's Symphony No. 22, The Philosoher, written in 1764, when Cleghorn was thirteen. (Orpheus Chamber Orchestra, Deutsche Gramophone, 1989.

No image of William Cleghorn is available.  Caloric impression above, by John Lienhard, is based upon the Butler Plaza Fountain at The University of Houston