A guest post by Hanna Szabelska.
Hanna Szabelska writes …
Gabrielle Émilie le Tonnelier de Breteuil, la Marquise du Châtelet (1706–1749), ambitious femme savante and Voltaire’s muse had an unusual penchant for physics and mathematics, which pushed her towards conducting and discussing experiments.
By way of an example, to show that heat and light, as opposed to rarefaction – the distinctive property of fire – are nothing but its modes that do not necessarily accompany each other, she made use of the phenomenon of bioluminescence while imitating René-Antoine Ferchault de Réaumur’s experiment:
Dails [pholads] and glowworms are luminous without giving off any heat, and water does not extinguish their light. M. Réaumur even reports that water, far from extinguishing it, revives the light of dails [pholads]. I have verified this on glowworms, I have plunged some in very cold water, and their light was not affected. 
Since she held Newton’s experimental precision in the Opticks in high esteem, to the point that she acquired knowledge to do experiments about different degrees of heating among primitive colours on her own , du Châtelet had reservations about Charles du Fay’s attempt to reduce the seven primitive colours to three.
The following passages are characteristic of her reliance on experiments. Letter 152. To Pierre Louis Moreau de Maupertuis [about the first of December 1738]
I know the Optics by Mr Newton nearly by heart and I must confess that I did not think it possible to call into question his experiments on refrangibility.
A tremendous series of experiments [une furieuse suite d’expériences] is necessary to undermine the truth that Mr Newton seems to have felt with all his senses. However, since I have not seen du Fay’s experiments I suspend my judgement… 
However, as much as she was fascinated by the potential of experimental philosophy, du Châtelet had an acute awareness of her own limitations and those of available apparatus. For example, she ventures the generalization that the tactile sensations of various colours differed analogically from the visual ones but admits her inability to conduct a decisive experiment and confides this task to the judges of her essay on fire .
Moreover, one can detect irony in her remarks about a defective camera obscura designed for optical experiments. In a letter to Algarotti she complains that:
The abbé Nollet has sent me my camera obscura, more obscure than ever; he claims that you have found it very clear in Paris: the sun of Cirey must be, therefore, unfavourable to it. 
Imperfect instruments could distort the results of experiments but so could an experimenter’s understanding of them if, like Locke or Leibniz, one takes the camera obscura as a metaphor of both visual perception and ideas based on it. Such epistemological doubts were also preying on du Châtelet’s mind, giving her natural philosophy a metaphysical depth. Thus, having enumerated some great names of experimental philosophy, she comes to the conclusion that:
It seems that a truth that so many competent natural philosophers have not been able to discover is not to be known by humanity. With regard to first principles, only conjectures and probabilities are within our reach. 
Interestingly, for Voltaire, this amalgam of the experimental and the speculative, imbued with the venustas muliebris of style, as Cicero would put it, was just the Marquise’s way of life, expression of her complex personality, philosophical to the backbone, but not easy to deal with.
The Marquise’s experimental inclination, under the spell of Leibnizian speculative philosophy, gave rise to sophisticated arguments, that often elude the language of modern physics. The devil is, as usual, in the details so let’s analyse some of them.
One of the most instructive stories is du Châtelet’s disagreement with Voltaire on the nature of fire, in particular on the question of its weight. While assisting with his experiments (cf. Peter Anstey’s post), she came to different conclusions and started working on her own essay in secret.
Voltaire evidently tried hard to interpret his results through the lens of a Newtonian experimentalist: to show that fire has weight and is subject to the force of gravity. Therefore, he downplays Herman Boerhaave’s reservations concerning the acquisition of weight by heated bodies  and opts for Peter van Musschenbroek‘s interpretation .
I visited an iron forge to do an experiment [exprès] and whilst I was there I had all the scales replaced. The [new] iron scales were fitted with iron chains instead of ropes. After that I had both the heated and the cooled metal within the range of one pound to two thousand pounds weighed. As I never found the smallest difference in their weights I reasoned as follows: the surface of these enormous masses of heated iron had been enlarged due to their dilation, therefore they must have had less specific gravity. So I can conclude – even from the fact that their weight stays the same irrespective of whether they are hot or cool – that fire had penetrated the masses of iron adding precisely as much weight as dilation made them lose, and consequently, fire has real weight. 
To save his Newtonian face, Voltaire jumps to hypotheses in a rather non-Newtonian manner:
However, although no experiment to date seems to have shown beyond any doubt the gravity and impenetrability of fire, it is apparently impossible not to assume them. 
Despite his efforts, Voltaire’s conclusion remains caught in a limbo between mere hypothesis and a proposition deduced from phenomena and generalized by induction.
Of course, Newton would not have been himself had not his rejection of hypotheses been nuanced  but even so the leap in Voltaire’s reasoning seems a hidden thorn in his Newtonian flesh.
The conceptualization of Boerhaave’s experiment offered by du Châtelet is, on the contrary, more consistent with the data than that of her companion . But on the other hand, it opens the way for establishing fire as one of the grand metaphysical principles of the Universe:
…but claiming that fire has weight is to destroy its nature, in a word, to take away its most essential property, that by which it is one of the mainsprings of the Creator. 
The action of fire, whether it is concealed from us or perceptible, can be compared to force vive [living force] and force morte [dead force]; but just as the force of bodies is perceptively stopped without being destroyed, so fire conserves in this state of apparent inaction the force by which it opposes the cohesion of the particles of bodies. And the perpetual combat of this effort of fire and of the resistance bodies offer to it, produces almost all the phenomena of nature. 
The passages above are to be found in both versions of du Châtelet’s essay on fire: the original (1739, reprinted in 1752 by the Academy) and the revised one from 1744 (published by the Marquise’s own assumption by Prault, fils). However, it is worth noting that her conceptual framework became more consistently Leibnizian with time. It is this development that I will discuss in my next post.
- Trans. Isabelle Bour and Judith P. Zinsser; Du Châtelet, Selected Philosophical and Scientific Writings, ed. J. P. Zinsser, Chicago: University of Chicago Press, 2009, p. 64.
- Dail is an obsolete French term for pholade, pholas dactylus. (Du Châtelet, Dissertation sur la nature et la propagation du feu, Paris: Chez Prault, Fils,1744, p. 4.)
- Dissertation, p. 69.
- du Fay, Observations physiques sur le meslange de quelques couleurs dans la teinture, “Histoire de l’Académie royale des sciences … avec les mémoires de mathématique & de physique,” 1737, p. 267.
- Les lettres de la Marquise du Châtelet, ed. Theodore Besterman [Genève: Institut et Musée Voltaire, 1958], vol. 1, pp. 273–274.
- Dissertation, pp. 70–71.
- Letter 63. To Francesco Algarotti, in Cirey, the 20th [of April 1736], Les lettres de la Marquise du Châtelet, vol. 1, p. 112.
- Trans. I. Bour and J. P. Zinsser; Du Châtelet, Selected Philosophical…, p. 71.; Dissertation, p. 17.
- Hermannus Boerhaave, “De artis theoria,” in: Elementa chemiae, Tomus primus, editio altera [Parisiis: Apud Guillelmum Cavelier, 1733], p. 193 ff.
- Petrus van Musschenbroek, Elementa physicæ conscripta in usus academicos, editio prima Veneta [Venetiis: Apud Joannem Baptistam Recurti, 1745], p. 323 ff.
- cf. Bernard Joly, “Voltaire chimiste: l’influence des théories de Boerhaave sur sa doctrine du feu,” Revue du Nord 77, No 312 (1995): 817–843.
- Voltaire, “Essai sur la nature du feu et sur sa propagation,” in Recueil des pièces qui ont remporté le prix de l’Académie royale des Sciences en 1738, par M. Rouillé de Meslay [Paris: de l’Imprimerie Royale, 1739], p. 176.
- Voltaire, “Essai sur la nature du feu,” Recueil, p. 180.
- cf. e.g. William L. Harper, Isaac Newton’s Scientific Method: Turning Data into Evidence about Gravity and Cosmology (Oxford: Oxford University Press, 2011), p. 44.
- Dissertation, p. 24, 33 ff.
- Trans. I. Bour and J. P. Zinsser; Du Châtelet, Selected Philosophical…, p. 80; Dissertation, p. 40.
- Trans. I. Bour and J. P. Zinsser; Du Châtelet, Selected Philosophical…, pp. 84–85; Dissertation, p. 52.