Émilie du Châtelet's Essay on Fire: Light, Heat, and the Infrared Question

In 1736 the Académie royale des sciences in Paris opened a contest for its physics prize of 1738 on the nature and propagation of fire, with entries due by 1 September 1737.¹ The prize was paid for by a bequest from Rouillé de Meslay, a former member of the Parlement.² When the Academy printed the results in 1739, it admitted in a short notice that the founder’s wishes sometimes forced it to set subjects that give geometry almost no purchase, that fire was probably one of them, and that little more than competing systèmes could have been expected.²

One of those systems came from Émilie du Châtelet, born in 1706 and living at Cirey in Champagne with Voltaire.³ Her essay did not win, but it was printed, and it is now often credited with anticipating infrared radiation. Both the Academy’s printing of 1739 and her revised edition of 1744 survive, so the claim can be checked against her words.

A prize split three ways

The Academy did not find any entry that fully answered the question. It crowned three, built on three entirely different hypotheses, and distinguished them only by order of arrival and number: No. 4 by Leonhard Euler, then a professor at St. Petersburg; No. 10 by the Jesuit Father Lozeran de Fiesc; and No. 11 by the Comte de Créquy.² The Latin title page of Euler’s essay says the prize was divided into three parts, of which he received one.² Popular accounts that call Euler’s essay the “top paper” are not following the Academy’s own notice.⁴

A second notice introduced two more pieces. Their authors had identified themselves. The commissioners could not approve the idea of fire in either, but judged both among the best for their reading and their well-presented facts. Piece No. 6 was “d’une jeune Dame d’un haut rang,” by a young lady of high rank. Piece No. 7 was “d’un de nos premiers Poètes,” by one of our foremost poets.² Voltaire, writing up her essay for the Mercure in June 1739, said nearly thirty dissertations had been entered.⁵

Written at night, against Voltaire

Her letters to the mathematician Maupertuis tell the story. On 21 June 1738 she told him that she had wanted to test her abilities under cover of anonymity. Only her husband knew. She could do no experiments, she wrote, because she was working without Voltaire’s knowledge and could not have hidden them from him; she began only a month before the deadline, worked at night, and was new to the subject. Voltaire’s own essay, nearly finished before she started, had given her ideas, and in her own she said “je combattais presque toutes ses idées dans mon ouvrage”: she was attacking almost all of his ideas.¹ She confessed only when the Gazette showed that neither of them had won.

A later anecdote attributed to Madame de Graffigny, quoted in the notes to Eugène Asse’s edition of her letters from the 1820 Vie privée de Voltaire, describes eight months of nights with her hands in ice water to stay awake.¹ Her own letter, the earlier witness, says one month.

Voltaire then wrote to the academicians Réaumur and Du Fay, naming her as the author of No. 6. On 7 July 1738 she reported that Réaumur had replied that the public must know one entry was by a young woman and another « du plus grand de nos poètes », by the greatest of our poets, and that the Academy had agreed to print both after the winners.¹ Project Vox dates her anonymous submission to 23 August 1737; the contest closed on 1 September 1737.³¹

Heat and light as two different things

Her essay opens with a problem of definition. Heat and light are the effects of fire that strike our senses most, but she asks whether either is essential to it, and answers with examples.⁶ Moonlight gathered in the focus of a burning glass gives a great deal of light and no heat that we can feel. Glowworms plunged in very cold water keep shining. Iron about to ignite will burn a hand while giving no light.⁶ Light travels in straight lines, heat in every direction; veil a burning mirror and the light at its focus vanishes at once, while an object placed there stays hot.⁶

Her conclusion is that light and heat are “deux modes,” two modes, of the being we call fire, independent of each other, and that neither defines it.⁶ What does define it, she argued, is the one effect it produces everywhere, even in a vacuum: it expands bodies and, if it keeps acting, pulls them apart into their elements.⁶ She did not think moonlight was simply without fire. Perhaps, she wrote, someone would one day invent an instrument fine enough to detect in the Moon’s rays the power of rarefaction that seems inseparable from fire.⁶

The most contested part of the essay was about weight. The chemist Homberg had reported that antimony gained weight when calcined with a burning glass, and others, including Boyle and Lémery, had described similar gains that were read as fire lodging in matter. She argued that the gain could come from foreign matter, and that if all the sunlight falling on our hemisphere on the longest day of summer weighed even three pounds, our eyes would be useless and the Earth’s weight would long since have grown enormously.² A list of corrections she requested, printed at the end of the 1739 volume, changes three pounds to three drams; the 1744 edition keeps three pounds and drops the clause about the Earth.²⁶ Fire, she concluded, does not tend toward any center; its parts tend to fly apart, and that makes it “l’antagoniste perpétuel de la pesanteur,” the perpetual adversary of weight.²

This put her squarely against her companion. Voltaire’s essay reports that he had iron weighed hot and cold at a forge, from one pound up to two thousand, found no difference, and reasoned that the fire inside the hot iron must have added exactly as much weight as expansion took away: therefore “le Feu est réellement pesant,” fire really is heavy.² In his Mercure review of her essay he admitted that her reasons had left him “fort ébranlé,” badly shaken.⁵

The 1739 text then went one step further. Among the conclusions of its first part, fire is a being of an in-between nature, “ni esprit, ni matière, ni espace,” neither spirit, nor matter, nor space. Her list of corrections softens this: fire might well be such a being, and it is not impossible that it is neither spirit, nor matter, nor space.² She defended the idea privately to Maupertuis as singular but not untenable.¹ Voltaire thought it explained the result: he wrote that the essay would probably have won “si on n’y avait pas ajouté une opinion trop hardie,” if a too-bold opinion had not been added, namely that fire is neither spirit nor matter.⁵ That is Voltaire’s guess, not the commissioners’ stated reason, but it matches the Academy’s remark that it could not approve her idea of fire.²

The colored cloth and the experiment she did not do

The passage behind the infrared claim sits in a section on how fire acts on solids. Bodies heat more or less quickly according to their color, she wrote: white ones reflect most of the light they receive, black ones absorb it.⁶ To test this beyond black and white, she had a piece of cloth dyed in the seven colors of the prism, wetted it evenly, and set it before the same fire. The colors dried in the order violet, indigo, blue, green, yellow, orange, red, starting with the fastest.⁶

Then comes a proposal, not a result. “Une expérience bien curieuse (si elle est possible),” she wrote, a very curious experiment if it could be done, would be to gather enough rays of a single color to test whether the primary rays that give us the sensations of different colors also have different burning powers, “si les rouges, par exemple, donneroient une plus grande chaleur que les violets,” whether the reds, for example, would give more heat than the violets. “C’est ce que je suis bien tentée de soupçonner”: that is what I am strongly tempted to suspect.⁶ Her reasoning was an analogy: each kind of ray shakes the optic nerve differently, so why not the skin? It seemed likely that reds heat more than violets and yellows more than blues, “car ils font des impressions plus fortes sur les yeux,” because they make stronger impressions on the eyes.⁶ She closed by admitting “je ne me suis pas trouvée à portée de la faire,” she had not been in a position to carry it out, and handing it to the academicians who would judge her essay.⁶ The passage is essentially the same in 1739 and 1744.²⁶

Did she anticipate infrared?

The strong version appears in popular writing: one science-software blog says the essay made her “among the first scientists to introduce the concept of infrared radiation.”⁴ Scholars are more careful. In their 2024 annotated Portuguese translation, Jaene Guimarães Pereira and Ana Paula Bispo da Silva note that her hypothesis about the colored rays is what William Herschel set out to demonstrate in 1800.⁷ That is a narrower claim, and it is a fair one.

Herschel’s 1800 paper shows what was found. He had noticed, while viewing the Sun through colored darkening glasses, that some combinations gave him heat with little light and others light with little heat.⁸ He put thermometers in the separate colors of a prism’s spectrum and found that the heating power of the colors was “very far from being equally divided,” with red the strongest.⁸ So Du Châtelet’s guess that reds heat more than violets was right.

The infrared step came next, and it was Herschel’s. He reported that the brightest part of the spectrum had little more than half the heat of the full red, and that “the full red falls still short of the maximum of heat; which perhaps lies even a little beyond visible refraction.” Radiant heat, he proposed, would “at least partly, if not chiefly” consist “of invisible light; that is to say, of rays coming from the sun, that have such a momentum as to be unfit for vision.”⁸ Nothing like this appears in Du Châtelet’s essay. She proposed comparing visible colors with each other; she said nothing about heat beyond the red. Her remark elsewhere that nature may hold colors unknown to us concerns suns in other planetary systems, not the edge of the spectrum.⁶

Her reason for expecting red to be hotter also points the wrong way. She tied heating power to how strongly a color strikes the eye, so yellow should outrank blue.⁶ Herschel’s central finding was that the maximum of light and the maximum of heat fall in different places.⁸ His paper does not mention her, and this research found no evidence that he knew her essay.⁸

The verdict: Du Châtelet framed a good, testable question about the prismatic colors more than sixty years before anyone answered it, and her guess about red and violet was right. She did not predict invisible rays, and she did not perform the test. Her deeper commitment, that heat and light are distinct modes of fire, was a reasonable reading of the evidence in 1737. Herschel’s results pointed the other way: he proposed that radiant heat consists at least partly, perhaps chiefly, of “invisible light,” refracted by the prism like visible light.⁸

From fire to Leibniz and Newton

Du Châtelet kept working on the essay after 1739, revising it as her views changed.⁹ In 1740 she published the Institutions de physique, which gave a Leibnizian and Wolffian metaphysical foundation to Newtonian physics and defended Leibniz’s measure of force, force vive. That led to a public exchange with Jean-Jacques Dortous de Mairan, secretary of the Academy, in 1741.⁹³ In 1744 the bookseller Prault reissued the fire essay in the same format as the Institutions, adding the Mairan letter and her reply.⁶

The 1744 text shows the change. The “neither spirit nor matter” conclusion is gone. A new passage says the properties we observe in matter are only phenomena, and that the simple beings composing sensible things might combine without producing weight or impenetrability, so experiment must decide whether fire has them. A footnote states plainly: “On sent aisément qu’on suppose ici les principes de la Philosophie Leibnitiene,” the principles of Leibniz’s philosophy are assumed here.⁶ Elsewhere the revision compares the hidden and active states of fire to force morte and force vive.⁶ The physics stayed largely the same; the metaphysics moved.

In the early 1740s she began a French translation of Newton’s Principia with a commentary. She finished it shortly before her death in 1749, and it was published in 1759.⁹ The fire essay, by contrast, contains no geometrical or algebraic derivations, only a few rough numerical estimates. It argues from reported experiments, many of them by Homberg, Boyle, Musschenbroek, Réaumur and others, and from analogy.² There is a small puzzle here: she told Maupertuis she had been unable to do any experiments, yet the essay reports her own trials with glowworms and dyed cloth. They were not slipped in later. Both are already in the body of the 1739 printing, which the Academy says it printed exactly as received, and in November 1738 she told Maupertuis she asked for no change to the body of the work, since the Academy wanted the public to see it as it had been judged. Her later changes appear only in a separate list of corrections.¹²⁶ Her letters do not explain the discrepancy. Taken together, the three works show a natural philosopher moving from a qualitative prize essay toward the mathematical mechanics of the Principia.

For more, see Judith Zinsser’s biography, the English selections edited by Zinsser and Isabelle Bour, Ruth Hagengruber’s volume on Du Châtelet between Leibniz and Newton, and Brigitte Van Tiggelen’s chapter on the essay.^10111213 The French text is on Project Gutenberg.⁶

Footnotes

1. Asse ed. 1882, letters to Maupertuis of 21 May (no. 88), 21 June (no. 91) and 7 July 1738 (no. 92), with notes; 19 November 1738 (no. 104), pp. 242-243. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

2. Pièces qui ont remporté le prix (1739): Avertissement de l’Académie; Euler, title page; second Avertissement; Du Châtelet, pp. 85-168 (glowworms p. 87; weight of fire pp. 100-111; first-part conclusion pp. 117-118; colors pp. 129-130); Voltaire, Essai, weighing at the forge; the Academy’s notice and the corrections requested by the author of No. 6 (entries for pp. 106 and 118). ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴

3. Project Vox, Du Châtelet timeline, 1737-1742. ↩ ↩² ↩³

4. Forrister, COMSOL Blog, 2018. ↩ ↩²

5. Voltaire, Mémoire sur un ouvrage de physique (Moland ed., vol. 23, pp. 65-69). ↩ ↩² ↩³

6. Du Châtelet 1744, Avis du libraire; Part I, §§ I-IV and Conclusion; Part II, § I and § IV (colors); § on the Sun. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹ ↩²⁰

7. Pereira and Silva 2024, translators’ note to the colored-rays passage. ↩

8. Herschel 1800, pp. 255-256, 261-262, 272-273. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

9. Detlefsen, SEP, “Émilie du Châtelet,” § 1. ↩ ↩² ↩³

10. Zinsser 2006 (bibliographic record). ↩

11. Zinsser (ed.) 2009 (bibliographic record). ↩

12. Hagengruber (ed.) 2012 (bibliographic record). ↩

13. Van Tiggelen 2019 (bibliographic record). ↩