This is a bit of a weekend-colour-supplement blog post – a bit longer and more leisurely. So pour yourself a nice hot cuppa and enjoy.

In the past couple of weeks I have been trying to convey to my students something of the glory of thermodynamics. It’s not an easy or popular subject, especially given the mathematical content. But I do love it so!

I know it seems dry: heat, work, entropy, state functions, the second law. The zeroth law for God’s sake. But peel away the yellowing, crackling paper and underneath you will find a vibrant subject that is alive with science and history. What it shows is that concepts we now take almost completely for granted, like work and energy, were actually hard won. I have been telling my students about the origins of the concept of heat and therein hangs quite a tale.

No less a figure than Lavoisier listed heat alongside hydrogen, oxygen and nitrogen in the table of elements from his famous Elements of Chemistry textbook, published in 1789. At that time heat was thought of as a substance, a fluid—igneous fluid or caloric—that pre-existed within matter and could be released in chemical reactions.

But Lavoisier was guillotined five years later during the reign of terror that followed the French revolution and soon after the idea of caloric was also knocked on the head. This latter revolution was not the work of a brutal, faceless committee but due to Benjamin Thompson, an infuriatingly delightful character, also known as Count von Rumford.

American by birth, Rumford sided with the British at the outbreak of the revolutionary war and eventually fled to London. There he proceeded to upset the British authorities and soon found himself working for the Prince-elector of Bavaria, in charge of munitions. In this capacity he could observe closely the manufacture of cannons—which were cast in solid brass and bored out to create the barrel—and “was struck with the very considerable degree of heat which a brass gun acquires, in a short time, in being bored”. These observations stimulated an ingenious series of experiments, which he published in 1798 as a paper entitled “An Inquiry concerning the Source of the Heat which is excited by friction”. Thanks to the good offices of JSTOR you may be able to download the paper as a pdf and it’s a cracking read.

In the manner of the time, Rumford’s article is written rather discursively. Here and there he reflects on how he feels about his experiments and achievements. It’s an insightful style that brings plenty of colour to the usual verbiage of scientific reporting—one that we might do well to re-discover.

From the start it is clear that Rumford is not a man wracked by self-doubt. Addressing the learned fellows of the Royal Society, he advances the view that an ordinary working life affords plenty of opportunities for scientific contemplation and goes so far as to claim that, for him, the happy circumstance of his occupation has more often led to “useful doubts, and sensible schemes for investigation and improvement, than all the more intense meditations of philosophers, in the hours expressly set apart for study.” Way to get an audience on your side!

Switching tack, he tries a little modesty. And fails. Rumford temporarily concedes that his experiments may not warrant such a grand introduction but then avers, “I cannot help flattering myself that that they will be thought curious in several respects, and worthy of the honour of being made known to the Royal Society.”

Rumford’s cannon – plug and borer on the left. Water-filled box not shown.

Rumford details his experimental set-up, which uses a cannon cast from solid brass. The Bavarian cannons were cast vertically, muzzle up, as a solid piece about a third longer than the finished product; the additional weight from the extension served to compress the metal that would form the muzzle of the gun, bolstering its strength. But Rumford put this extension to the service of science. He worked it into a hefty plug connected to the main barrel by a short neck and then hollowed it out to accommodate the borer, blunted to increase friction while reducing the rate of erosion of the brass. In this way, the borer and plug could be encased in a watertight wooden box.

Then comes the boring bit. By which I mean, of course, the really exciting bit.

The box is filled with water, the horses harnessed and the cannon rotated on its jig against the borer at a rate of 32 revolutions per minute. Rumford takes the temperature of the water at measured intervals. But before recounting his observations he cannot stop himself from tantalising us:

“The result of this beautiful experiment was very striking, and the pleasure it afforded me amply repaid me for all the trouble I had, in contriving and arranging the complicated machinery used in making it.”

The water temperature climbs inexorably, reaching 107°F after an hour, 178°F after 2 hours and then, “at 2 hours 30 minutes it ACTUALLY BOILED!”. The capitalization is Rumford’s and you can’t help but smile at his infectious enthusiasm. Ever the showman, it becomes apparent that he has gathered an audience to share his moment of triumph:

“It would be difficult to describe the surprise and astonishment expressed in the countenances of the by-standers, on seeing so large a quantity of cold water heated, and actually made to boil, without any fire.

Though there was, in fact, nothing that could justly be considered as surprising in this event, yet I acknowledge fairly that it afforded me a degree of childish pleasure, which, were I ambitious of the reputation of a grave philosopher, I ought most certainly to hide rather than to discover.”

You have to like this guy. He’s kidding no-one about his ambitions but his sheer enthusiasm for the science spills over and I, for one, can easily forgive the fancy flights of self-regard.

Recovering his composure, Rumford turns finally to the main question: “What is heat?—Is there any such thing as an igneous fluid?”. Piece by piece he discounts the caloric theory as inconsistent with the results of his experiments. Emphasizing the evidently inexhaustible heat output from the cannon as long as the boring continues he concludes:

“any thing which any insulated body… can continue to furnish without limitation, cannot possibly be a material substance: and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of any thing, capable of being excited, and communicated, in the manner the heat was excited and communicated in these experiments, except it be MOTION.”

He seems tantalizingly close to our modern understanding, but this is 1798, well before the atomic and molecular theories of matter gained any traction. Indeed Rumford concedes that even he does not really know what he means by motion, though quite characteristically he goes on to speculate that a full understanding of heat may therefore be beyond the reach of human intelligence! Nevertheless, his scientific instincts are intact and he is careful to underscore that the phenomenon is important enough to warrant further investigation.

To drive home its significance, Rumford draws a telling parallel with Newton. He points out that Newton’s law of gravitation has provided no mechanistic insight into why bodies are attracted to one another and goes on to suggest that the problem of heat is at least as great as that of gravity. The reader is left in little doubt about the implicit parallel between the greatness of Newton and that of Rumford!

And finally, though he may be prepared to mention Newton, no-one who had previously worked on heat is named. After 23 pages of densely argued text, Rumford rather disingenuously claims that the omission of mention of these people “has not been owing to any want of respect for my predecessors, but was merely to avoid prolixity, and to be more at liberty to pursue, without interruption, the natural train of my own ideas.”

The cheeky bugger.

For more dirt on Rumford, check out the book.

 

Benjamin Count of Rumford (1798). An Inquiry concerning the Source of the Heat Which is Excited by Friction. Philosophical Transactions of the Royal Society of London, 88, 80-102