  Voltaire implicitly acknowledged her significant contribution—especially on more technical material—to his 1738 Eléments de la philosophie de Newton. For many years, it was believed that there was one surviving chapter from an otherwise lost work written by Du Châtelet—her “Essai sur l'optique” (ca. 1736). This chapter is housed among Voltaire's papers in Russia's National Library in St. Petersberg. Judging from that chapter on color formation this earlier essay seems to have been a more developed version of the chapter on optics in Voltaire's book, thus indicating her significant contribution to Voltaire's work. As a fascinating side note on how dynamic Du Châtelet studies are, in recent years fully three complete copies of her Essai have been found, one in Bernoulli's papers in Basel (Nagel 2012) and two that have been acquired recently by the Musée des lettres et manuscrits in Paris. In 1737 Du Châtelet entered a competition to explain the nature of fire, conducting her experiments in secret while Voltaire also conducted experiments for his entry to the competition. Both Du Châtelet's and Voltaire's entries aimed to disprove the theory that fire is a material substance, and both were published along with the three winners (including the essay essay by Leonhard Euler, which took the top prize). Du Châtelet returned to this project a number of times thereafter, making significant revisions to the original text as her ideas on the nature of fire matured and changed.