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Tag Archives: Voltaire

Emilie du Châtelet and experimental philosophy I

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. [1][2]

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 [3], du Châtelet had reservations about Charles du Fay’s attempt to reduce the seven primitive colours to three.[4]

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… [5]


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 [6].

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. [7]

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. [8]

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 [9] and opts for Peter van Musschenbroek‘s interpretation [10][11].

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. [12]

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. [13]

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 [14] 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 [15]. 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. [16]


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. [17]

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.


  1. 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.
  2. 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.)
  3. Dissertation, p. 69.
  4. 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.
  5. Les lettres de la Marquise du Châtelet, ed. Theodore Besterman [Genève: Institut et Musée Voltaire, 1958], vol. 1, pp. 273–274.
  6. Dissertation, pp. 70–71.
  7. Letter 63. To Francesco Algarotti, in Cirey, the 20th [of April 1736], Les lettres de la Marquise du Châtelet, vol. 1, p. 112.
  8. Trans. I. Bour and J. P. Zinsser; Du Châtelet, Selected Philosophical…, p. 71.; Dissertation, p. 17.
  9. Hermannus Boerhaave, “De artis theoria,” in: Elementa chemiae, Tomus primus, editio altera [Parisiis: Apud Guillelmum Cavelier, 1733], p. 193 ff.
  10. Petrus van Musschenbroek, Elementa physicæ conscripta in usus academicos, editio prima Veneta [Venetiis: Apud Joannem Baptistam Recurti, 1745], p. 323 ff.
  11. 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.
  12. 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.
  13. Voltaire, “Essai sur la nature du feu,” Recueil, p. 180.
  14. 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.
  15. Dissertation, p. 24, 33 ff.
  16. Trans. I. Bour and J. P. Zinsser; Du Châtelet, Selected Philosophical…, p. 80; Dissertation, p. 40.
  17. Trans. I. Bour and J. P. Zinsser; Du Châtelet, Selected Philosophical…, pp. 84–85; Dissertation, p. 52.


Voltaire: Experimental Philosopher

Voltaire, détail du visage (château de Ferney)

Voltaire, détail du visage (château de Ferney)

Peter Anstey writes …

The French Philosophe Voltaire played an important role in the transmission of experimental natural philosophy to France in the 1730s. That Voltaire regarded the emergence of experimental philosophy as a pivotal moment in history is seen in his history of The Age of Louis XIV (1751). In the Introduction to this work he speaks of the Italians of the Renaissance being ‘in possession of everything that was beautiful, excepting music, which was then in but a rude state, and experimental philosophy, which was everywhere unknown’.

The decisive moment came in the early seventeenth century in the writings of Francis Bacon. For, in his Letters Concerning the English Nation (Oxford, 1994) that appeared in English and French in 1734, Voltaire credits Bacon with being the first experimental philosopher:

He is the Father of experimental philosophy … no one, before the Lord Bacon, was acquainted with experimental Philosophy, nor with the several physical Experiments which have been made since his Time. (pp. 51–2)

But did Voltaire himself take up experimental philosophy or was he merely a herald and conduit for this movement to the French reading public?

Two works suggest that Voltaire fully embraced the new experimental philosophy that he had encountered in England in the 1720s. The first is his Treatise on Metaphysics that he wrote in 1734, the year in which his Letters appeared but which was published posthumously. This work bears the marks of someone who had imbibed the methodological position of the new experimental philosophy both in its rejection of speculative philosophy and hypotheses and the priority it gives to observation and experiment. For example, he says:

It is clear that one should not make hypotheses. We ought not to say ‘Let us begin by inventing some principles with which we will try to explain everything’, but we ought to say, ‘Make an exact analysis of things and then we will try with great diffidence whether they are related to certain principles’.

He goes on to claim ‘when we can help ourselves with neither the compass of mathematics, nor the torch of experiment and natural philosophy, it is certain that we are not able to do anything’ (ibid., p. 301).

The second work is his ‘Essay on the nature of fire and its propagation’, an essay he submitted for the Académie des sciences prize in 1738. As things turned out Leonhard Euler’s essay won the prize, but Voltaire’s submission and that of Madame du Châtelet were published alongside Euler’s winning essay in the Recueil des pieces qui ont remporté le prix de l’Académie royale des sciences in 1739. This is Voltaire’s only serious foray into experimental natural philosophy.

In the Part One of the essay, the part that addresses the nature of heat, he uses the experiments of others to argue for an Aristotelian theory of heat as an element. In doing so he cites the experimental work of Boyle, Newton and Boerhaave. However, in the second article of Part Two of the essay, on the subject of how fire acts on other bodies, Voltaire relates a whole series of experiments that he had performed himself. This is with a view to establishing certain laws by which fire acts, the second of which purported laws is an inverse square law analogous to Newton’s law of gravitational attraction! (p. 201) At one point he tells us:

the comparative degrees of heat of fluids of minerals and of vegetables can, I believe, be known with the aid of a single thermometer constructed on the principles of Mr de Réaumur.

There is only one precaution to take, and this is that the spirit of wine should not boil in the thermometer. To achieve this I plunged only up to half of the ball of the thermometer in the boiling liquors. (p. 207)

Much more could be said about this fascinating essay, but the key point of interest here is that it is a demonstration of Voltaire’s commitment to and practice of experimental natural philosophy.

He may never have experimented again, yet he continued to refer to experimental philosophy, alluding to his essay on heat in his Metaphysics of Newton (La métaphysique de Neuton, Amsterdam, 1740, p. 49) and, most famously, referring to experimental philosophy in his literary works, including Candide (1759) and Micromégas (1752).


Buffon and the Experimental Philosophy

Peter Anstey writes …

The historiography of the Enlightenment over the last fifty years has focused heavily on the influence of the natural philosophy of Bacon and Newton and the philosophy of Locke on the French philosophes.

Surprisingly, however, the thought of Bacon, Locke and Newton has rarely been seen as part of the broader impact of the experimental philosophy movement: the focus has been on individuals and their thought and experimental philosophy has been regarded as an expression of the ‘empiricism’ of these thinkers. See for example, Jonathan Israel’s monumental Radical Enlightenment and Democratic Enlightenment, neither of which lists ‘experimental philosophy’ in its index and which tend to subordinate English experimental philosophy to empiricism. (The term gets a mere 4 entries in the index of his 983-page Enlightenment Contested.)

Now, one text that has been repeatedly cited as early evidence of the important impact of Newton is Buffon’s Translator’s Preface to his 1735 French translation of Stephen Hales’ Vegetable Staticks of 1727. But when we turn to the text itself it’s pretty clear that Newton is merely an exemplar of a broader phenomenon.

As we have argued many times on this blog, the experimental philosophy that emerged in England in the 1660s was characterized by an emphasis on observation and experiment, an aversion to theoretical systems and especially its decrying of hypotheses and principles. Let us look at Buffon’s Preface and see what he has to say about Hales’ book. He says:

    The novelty of the discoveries and the majority the ideas [of Hales’ book] will no doubt surprise natural philosophers. I know nothing better of its kind, and the genre itself is excellent, for it is only experiment and observation.
    … works founded on experiment, merit more than others. I can even say that in natural philosophy, one ought to search out experiments as much as one ought to be afraid of systems. I admit that there is nothing so good as to establish first a single principle, and then to explain the universe, and I am convinced that if one were so happy to divine it, all the pain that it takes to make experiments would be unnecessary. But sensible people see rather how much this idea is vain and chimerical: the system of nature probably depends upon several principles, principles that are unknown to us and their combination even less so.
    … It is by choice experiments, reasoned and followed, that one forces nature to reveal its secret. All the other methods have never succeeded.
    … Collections of experiments and observations are therefore the only books that can augment our understanding. Being a natural philosopher is not a matter of knowing what follows from this or that hypothesis, in supposing, for example, a subtle matter, vortices, an attractive force, etc. It is to know well that by which it comes and to understand that which is presented to our eyes. The understanding of effects will conduct us insensibly to that of their causes and will not trip us up into the absurdities that seem to characterize all systems.
    … It is this method that my author [Hales] has followed. It is that of the great Newton; that which Bacon, Galileo, Boyle, Stahl recommended and embraced; that which the Académie of Sciences has made it a law to adopt … (pp. iii–vi)

Notice the underlined words here: ‘experiment and observation’, ‘systems’, ‘vain and chimerical’, ‘hypothesis’. This passage bears all the hallmarks of an expression of the central doctrines of the experimental philosophy. This is reinforced by the gallery of greats that is listed: Bacon, Galileo, Boyle, Stahl.

The focus is not on individuals such as Newton at all, nor is it on empiricism. It is on the méthode of the experimental philosophy. This is what Voltaire had referred to just one year earlier in 1734 in his letter ‘On the Lord Bacon’ in his Letters concerning the English Nation, where he claimed that Bacon ‘is the Father of experimental philosophy’ and that ‘no one before the Lord Bacon was acquainted with experimental Philosophy’.

It is the experimental philosophy, and not Bacon or Newton, that Buffon is praising and advocating. The experimental philosophy, as discussed by Buffon, Voltaire, d’Alembert and Diderot, needs to become a central notion in our historiography of the Enlightenment.