A second guest post by Hanna Szabelska.
Hanna Szabelska writes …
As I indicated in my previous post, the fatal destiny (fatalité), about which Voltaire complained in a letter to Jean-Jacques d’Ortous de Mairan , made Madame du Châtelet’s mind more and more prone to the allure of Leibniz’s metaphysics, in particular his concept of vis viva.
For example, the comparison of fire to living force notwithstanding, the first edition of her essay on heat shows the traces of the influence of de Mairan’s Dissertation sur l’estimation et la mesure des forces motrices des corps. One possible reason for this inconsistency being that de Mairan distanced himself from metaphysics and concentrated on pure laws of motion . In the version submitted for the Academy’s prize competition, du Châtelet added a note criticising Leibniz and praising de Mairan as an advocate of the Cartesian measure of force (mv). Afterwards the Marquise desperately fought for permission to remove it before publication. She argued that this insipid compliment (fadeur) had resulted from her ignorance and was not related to the main theme. But she was unsuccessful .
The Leibnizian measure – mv² – was incorporated only in the second version together with a remarkable passage that unravels a complex interplay between the experimental and the speculative approach in du Châtelet. Having discussed the hypothesis that the Sun is a solid body containing fire and emanating it to the Earth, she concludes:
But this emanation of light is subject to far greater difficulties, and seems impossible to be assumed despite the modern observations that apparently speak in its favour: certain observations are enough to destroy a superstition when they seem contrary to it, but they are not enough to establish it and physical and metaphysical difficulties undermining the [hypothesis of] emanation of light seem so insuperable that without them being removed there are no observations that can induce one to assume it. But this is not the place to discuss them. 
The moral of this digression is that observational data are not enough to establish a hypothesis if there are strong metaphysical objections against it. This is the assumption, although not always articulated, that remains at the core of du Châtelet’s rhetorical vein in the heat of debates, e.g. her discussion with de Mairan about one of Jacob Hermann’s experiments and the measure of force. Remarkably, the exchange with de Mairan was published not only together with the Institutions physiques (1742), the second edition of du Châtelet’s manual of physics, but also with the revised version of her essay on fire.
The experiment in question is as follows :
Let the ball A move with the velocity 2 on a horizontal plane and collide with another ball B=3A, being at rest. The ball A will give the velocity 1 to the ball B and move backwards with the velocity 1. Afterwards, let the ball A with the velocity 1 collide with another body at rest C=A. The ball A will also give to the ball C the velocity 1 and as a result of the second collision, it will come to halt. All this can be easily derived from the very well known rules of the motion of elastic bodies. 
To disprove du Châtelet, de Mairan adds scalar magnitudes (m|v|), and then he goes on to directed ones, i.e. applies the measure he accepted. 
His calculation could be interpreted as a correct addition of momenta , but du Châtelet does not consider it either as an alternative of force measure or a different concept. Here comes into play her rhetorical impetus:
To tell the truth, it is remarkable with what ease this small bar you put in front of the formula for the force of the body A rid you of this 8 of force that even your own calculation gave you after collision instead of 4 that you had expected from it; but, tell me, I beg you, you certainly do not think that this sign minus and this subtraction would take away some part of force from the bodies A and B, and that the effects exercised by these bodies on any obstacles would be diminished by it. I also doubt that you would like to either experience it or find yourself in the path of a body that would bounce back affected by this minus sign with 500 or 1000 of force. 
One may think that du Châtelet did not understand the concept of directed magnitudes but was this really the case? After all, she was a very attentive reader of Willem Jacob ‘s Gravesande, who analyses the paradoxical cases of bodies moving in the opposite directions and compares the effectiveness of Leibnizian force measure with the Cartesian one.
This is the description of ‘s Gravesande’s experiment, somewhat simplified by du Châtelet: 
‘s Gravesande devised an experiment that wonderfully confirms this theory. He fastened a ball of clay in Mariotte’s Machine and he made it collide successively with a copper ball, whose mass was three and velocity one, and with another ball of the same metal, whose velocity was three and mass one, and it happened that the impression made by ball one, whose velocity was three, was always much greater than that made by ball three with the velocity of 1, which testifies to the inequality of the forces. But when these two balls with the same velocities as before collided at the same time with the clay ball freely suspended from a thread, the clay ball was not shaken and the two copper balls stayed at rest and equally sunk in the clay and after measurement these equal impressions were found to be much greater than the impression that ball three with the velocity of one had made, having hit only the fastened clay ball and less than that which had been made by ball 1 with the velocity three. For ball 3 consumed its force to make an impression on clay, and its impression having been augmented by the effort of ball one that pressed the clay ball against ball three, reduced the impression of this ball one. Therefore, soft bodies that collide with velocities in inverse proportion to their masses, stay at rest after the collision, because they consume all their forces to mutually impress their parts. For it is not simple rest that holds these parts together, but a real force, and in order to flatten a body and drive into its parts, this force, named coherence or cohesion, must be overcome, and nothing but the force used to drive into these parts is consumed in the collision. 
For both ‘s Gravesande and du Châtelet force is a positive magnitude . Besides, she obviously agrees with ‘s Gravesande that opposite forces do not destroy each other in a direct manner but their interaction is much more complicated: in the collision of two bodies whose forces are opposite there are two actions and two reactions. 
But there is one crucial difference between them: ‘s Gravesande, a Newtonian converted to the Leibnizian force measure by his experiments, was particularly sensitive to difficulties involved in theorizing observational data. For him, the concept of force is vague and leaves room for alternative measurements:
If the word ‘force’ is given a different meaning, if this different meaning is said to be more natural, I do not object: all I wanted to claim is that this what I have called ‘force’ must be measured by the product of mass and velocity squared. In order to claim that it is possible to assume a different measure of force as considered under a different aspect it is necessary to explain all the experiments conducted with respect to force and collision. This is what we do on our part; and I assure you that this has not been done yet by those who have adopted the contrary opinion. 
Not so Madame du Châtelet. The Marquise’s irony towards de Mairan, sardonic despite her capacity to grasp counterarguments, tempts one to suppose that it is one of the aforementioned difficultés métaphysiques that underlies it. Should the Cartesian force be posited as a metaphysical principle of the Universe, the Universe could potentially be left with a metaphysically embarrassing zero value (like in the case of two moving bodies whose momenta are equal but opposite: p and –p). In this respect, velocity squared in the vis viva formula guarantees its superiority.
What follows from this is that the relationship between the speculative and the experimental in du Châtelet’s arguments is far from being straightforward. On the one hand, rigorous conceptualization of experiments like that of Boerhaave can serve to build up metaphysical principles, e.g. weightless fire as one of the springs of the Creator. On the other, there is sometimes hidden metaphysical bias in interpreting experiments as the example of Hermann’s balls proves. This complex mix is certainly incommensurable with mathematized classical mechanics as taught today. The question that imposes itself here is: are we really able to pin down the slippery Proteus of experimentalism with a Leibnizian tinge?
- MLXXXIV – A M. de Mairan, à Bruxelles, le 1er avril 1741, in Oeuvres complètes de Voltaire, ed. Ch. Lahure, vol. 25 [Paris: Librairie de L. Hachette, 1861], p. 522.
- de Mairan, Dissertation sur l’estimation et la mesure des forces motrices des corps, Nouvelle édition, ed. Deidier [Paris, 1741], pp. 7-8.
- Mary Terrall, “Vis viva Revisited,” History of Science 42 (2004): 189-209.
- cf. Letter 148. To Pierre Louis Moreau de Maupertuis, Les lettres de la Marquise du Châtelet, ed. Theodore Besterman [Genève: Institut et Musée Voltaire, 1958], vol. 1, pp. 266-267; the errata allowed by the Academy contains nothing but a stylistic improvement; note a factual mistake in Du Châtelet, Selected Philosophical and Scientific Writings, ed. Judith P. Zinsser [Chicago: University of Chicago Press, 2009], p. 77, note 54 and p. 110, note 10: “In the errata that she was allowed to submit, she changed a reference to Dortous de Mairan’s formula for force to that of Bernoulli. She had been reading Bernoulli and Leibniz on the nature of collisions and had changed her mind.”
- Dissertation, p. 128.
- du Châtelet describes it on page 459 ff. of the Institutions physiques.
- Jakob Hermann, “De mensura virium corporum,” Commentarii Academiae Scientiarum Imperialis Petropolitanae 1 (1726, published 1728): 14.
- de Mairan, “Lettre sur la question des forces vives,” in du Châtelet, Institutions Physiques, p. 487 ff.
- cf. Leibniz’s “Essay de Dynamique sur les loix du mouvement,” unpublished at the time, in Leibnizens Mathematische Schriften, ed. Carl Immanuel Gerhardt, Bd. 6 [Halle: H. W. Schmidt, 1860], p. 215.
- Institutions physiques, p. 529.
- cf. Boudri’s interesting interpretation. However, ‘s Gravesande mentions this experiment in Essai d’une nouvelle théorie du choc des corps and not in Nouvelles expériences, as Boudri claims. Christiaan Boudri, What Was Mechanical about Mechanics: The Concept of Force between Metaphysics and Mechanics from Newton to Lagrange, trans. Sen McGlinn [Dordrecht: Kluwer Academic Publishers, 2002], p. 108.
- Institutions physiques, pp. 466-467. For this passage, I consulted the translation by I. Bour and J. P. Zinsser; Du Châtelet, Selected Philosophical…, pp. 196-197. There are, however, small inaccuracies. E.g. “He took a firm ball of clay and, using Mariotte’s Machine…” See ‘s Gravesande’s description on p. 236: “…une pièce de bois bien affermie par des vis, dans laquelle il y avoit de chaque côté une cavité en demi-sphère, qui servoit à affermir une boule de terre glaise…” ‘s Gravesande, “Essai d’une nouvelle théorie du choc des corps,” in Oeuvres philosophiques et mathématiques, ed. J. N. S. Allamand [Amsterdam: Rey, 1774], Première Partie, pp. 235-236.
- cf. ‘s Gravesande, Essai d’une nouvelle théorie du choc, p. 219, definition II and du Châtelet’s malicious remark that de Mairan would not like to be hit by a body moving with a considerable force either from the left or from the right side.
- cf. the combination of the loss of velocity and indentation in ‘s Gravesande’s experiment discussed above.
- “Nouvelles expériences,” in Oeuvres philosophiques et mathématiques, Première Partie, p. 284.
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.