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
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
(Haydn outro music)
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
The Engines of Our Ingenuity is
Copyright © 1988-2004 by John H.