At the end of the 17th century, Georg Ernst Stahl (1659-1734), a German chemist, physician, and philosopher, proposed the concept of phlogiston. Phlogiston comes from the principle of combustibility, which simply refers to how a substance burns when exposed to fire or a chemical reaction. Combustion is an exothermic redox reaction, meaning that it is a reaction that releases energy from the system (exothermic) and is a type of reaction in which the oxidation states of atoms change (electrons are transferred, ie redox).
Stahl proposed that phlogiston was a chemical body that combined with other substances and, through chemical reactions, could either be fixed or released. The phlogiston theory essentially stated that there was an additional fire-like element that was released during combustion. Stahl was not the only scientist who believed that fire was a separate substance made of particles, inspiring postulations by other revered scientists, such as René Descartes (1596-1650).
Thus, fire was a hot-button topic of the 17th century, and in 1738, the French Academy of Sciences proposed a competition surrounding the question of the nature and propagation of fire. In April, the winners out of 18 submissions were announced: Leonhard Euler (1707-1783)), the Jesuit Louis-Antoine Lozeran du Fech (1691-1755), and Count Jean-Antoine de Créquy (1699-1762). Surprisingly, the French Academy decided to publish another two essays, one from François-Marie Arouet Voltaire (1694-1778) and the other by “a young lady of high rank” Emilie Du Châtelet (1706-1742). Du Châtelet was the first woman to submit a piece to the Academy. Her “Dissertation on the Nature and Propagation of Fire” described and elaborated on experiments conducted in 1737. Interestingly, at this time, women were still not allowed to attend the Academy’s meetings, but Du Châtelet took advantage of the competition’s requirement of anonymous submissions and became the first female scientist to have her work published by the Academy.
Born Gabrielle Émilie le Tonnelier de Breteuil, she married the Marquis Florent-Claude Du Châtelet-Lomont (1695-1765) in 1725. After having two songs, Du Châtelet quickly became bored and decided to pursue her life’s passion, mathematics. To do so, she hired the best current mathematicians, such as Pierre Louis Moreau de Maupertuis (1698-1759) and Alexis Claude Clairaut (1713-1765) as her tutors. In addition, Du Châtelet quickly developed a very close relationship and intellectual partnership with Voltaire. Voltaire was a renowned French writer, who also held interests in mathematical and scientific ideas.
Voltaire decided to write an essay for the French Academy competition, arguing that fire was made out of particles that follow the rule of universal attraction. However, Du Châtelet disagreed with his hypothesis, so she decided to write her own essay.
Du Châtelet focused on studying fire via its numerous manifestations, namely lights, colors, and heat. She deliberately did not focus on combustion because the impact of combustion varies based on the substances used. Thus, combustion was not solely linked to fire. Dilation or rarefaction, however, is a direct consequence of fire. Rarefaction is defined as “diminution in the density of something, especially air or a gas.”
In addition to studying rarefaction, Du Châtelet used a crystal to diffract light, which allowed her to observe the different colors of the visible light spectrum. In doing so, she noticed that each color yielded different levels of warmth, with the warmth being strongest close to the end of the spectrum. To measure the warmth, she held candlewax close to the light. The wax melted beyond the red end of the visible spectrum, which we now as infra-red.
Finally, Du Châtalet examined one more aspect. She came to the conclusion that the distinctive property of fire was “the increase of the volume of the substances and, when continued for a long time, eventually the decomposition of a substance into its principles.” She experimented with these mass changes by measuring the weight after calcination. Calcination is defined as the heating of solids in order to purify them to a certain degree.
Du Châtelet’s dissertation on fire was a turning point in her career. In 1740, she published a textbook for her son, Louis Marie Florent Du Châtelet, called Institutions de physique (“The Foundations of Physics”). She also looked to expand her own knowledge, engaging in discussions with French scholar Jean-Jacques Dortous de Mairan (1678-1771), who was known for his work in geophysics, astronomy, and chronobiology. The duo explored the concept of “vis viva,” which is Latin for “living force.” This term is now recognized as a name for the first known description of what we now call kinetic energy.
In 1749, Du Châtelet died while giving birth to her daughter, who also passed away. Thus, Voltaire was the one to publish her masterpiece, the Principes mathématiques de la philosophie naturelle, Par feue Madame la Marquise Du Chastellet (“Mathemetical Principles of the Natural Philosophy by the late Madame la Marquise Du Châtelet).