The French Marquise Who Produced the Definitive French Translation of Newton's Principia, Wrote It While Pregnant at 42, and Died Six Days After Childbirth

Gabrielle Émilie Le Tonnelier de Breteuil, marquise du Châtelet (1706–1749) was born into the higher French court aristocracy. Her father Louis-Nicolas Le Tonnelier de Breteuil was the protocol officer at Versailles. She was, by his decision, given an academic education — mathematics, Latin, Greek, and music — that was unusual for a French aristocratic daughter and that was selected by her father specifically over her mother’s objections.

She married the marquis Florent-Claude du Chastellet (the family name spelling Châtelet was a later phonetic respelling) in June 1725 at age 18. The marriage was a conventional aristocratic arrangement that produced three children, took the Marquis du Chastellet to a regimental command in Burgundy, and left Émilie free to pursue her intellectual interests in Paris. She took mathematics lessons from the senior French Newtonian Pierre-Louis Moreau de Maupertuis and the analyst Alexis Clairaut. By age 27 she was capable of independent mathematical research.

In May 1733 she began a relationship with Voltaire, then 38 and recently returned from his English exile during which he had become a Newtonian. The Voltaire-du Châtelet relationship — both intellectual and romantic — lasted approximately fifteen years and produced a substantial joint scientific output.

The Cirey years

From 1735 to 1740 Voltaire and du Châtelet lived together at the marquis du Châtelet’s family seat Cirey on the Lorraine-Champagne border. The marquis maintained his regimental command elsewhere. The Cirey establishment was the most active private scientific research centre in early-18th-century France: a chemistry laboratory, an astronomical observatory, an experimental-physics workshop, and a library of approximately 21,000 volumes.

Du Châtelet’s 1740 Institutions de physique was a 450-page French-language textbook of Newtonian physics with extensive Leibnizian metaphysical commentary. It became the standard French Newtonian physics textbook for two decades and was used in the curriculum at the École royale militaire from 1751 onwards.

Her 1737 Dissertation sur la nature et la propagation du feu, submitted independently of Voltaire’s competing entry to the Académie des Sciences’s prize competition on the nature of fire, was the first scientific paper formally entered by a woman in any French Académie competition. (Neither du Châtelet nor Voltaire won; the prize went to Euler.)

In 1745 she began the work that would become her most consequential output: a French translation, with mathematical commentary, of Isaac Newton’s Philosophiæ Naturalis Principia Mathematica (1687).

The Principia translation

The English-language Principia had no complete French translation in 1745. The 1739–1742 edition by the Père Le Seur and the Père Jacquier (the Jesuit Principia) gave the Latin text with extensive Latin scholia but no French version. The French Newtonian community — including the Académie des Sciences and the French royal observatory — was working from the original Latin or from the Le Seur and Jacquier scholia.

Du Châtelet’s translation was more than literal. She produced a full French rendering of the Latin text together with an analytic mathematical commentary that converted Newton’s geometric demonstrations into the algebraic and infinitesimal notation that had become the standard form of mathematical analysis in the four decades since the Principia’s publication. The Newtonian geometric proofs are difficult to read in modern notation; du Châtelet’s commentary was the bridge that made the Principia accessible to mid-18th-century French mathematicians who had been trained in algebraic rather than geometric analytical methods.

The translation work took her four years.

Lunéville, summer 1749

In May 1748 du Châtelet had begun a new relationship with the young French poet Jean-François de Saint-Lambert, an aide-de-camp at the Lorraine court of the dispossessed Polish king Stanisław Leszczyński (Louis XV’s father-in-law). By winter 1748–1749 she was pregnant by Saint-Lambert. She was 42.

The pregnancy in the mid-18th century was, given her age and her three previous pregnancies, a serious medical risk. Du Châtelet had assessed the risk realistically; she had told Voltaire in late 1748 that she did not expect to survive the birth. She accelerated the Principia translation through spring and summer 1749 to ensure it would be complete before the delivery.

She moved to Stanisław’s residence at Lunéville in late June 1749 for the final months of the pregnancy and the delivery. She completed the Principia translation manuscript on the evening of 1 September 1749. She put the manuscript in the keeping of the Bibliothèque du Roi librarian to ensure its post-mortem publication. She gave birth to a daughter on 4 September 1749.

She died of puerperal fever on 10 September 1749, six days after the birth. She was 42.

The daughter died approximately twenty months later.

The book

The completed Principia translation circulated in manuscript among the Académie des Sciences for the next ten years. It was published in two volumes in 1759 as Principes mathématiques de la philosophie naturelle, par feue Madame la Marquise du Chastellet (“Mathematical Principles of Natural Philosophy, by the late Madame the Marquise du Chastellet”). The 1759 publication date was selected because Halley’s Comet had returned that year, almost exactly as Halley had predicted from Newtonian calculations — the most spectacular contemporary observational vindication of Newtonian gravitational theory.

Du Châtelet’s Principia became and remained the standard French Newton for the next two centuries. The first competing modern French translation was published in 1985.

Voltaire’s preface to the 1759 edition contains the following:

Two great men, the late Madame du Chastellet and the late Newton, are no longer; this work is what they left to the world. The world will be very long in finding equivalents to either of them.