The inventor of Mayer's reagent and one of the founders of thermodynamics, Julius von Mayer was the first to enunciate what phrase, known as the first law of thermodynamics?
Julius von Mayer Julius Robert Mayer (November 25, 1814 – March 20, 1878) was a German physician, chemist and physicist and one of the founders of thermodynamics. He is best known for enunciating in 1841 one of the original statements of the conservation of energy or what is now known as one of the first versions of the first law of thermodynamics, namely that "energy can be neither created nor destroyed". In 1842, Mayer described the vital chemical process now referred to as oxidation as the primary source of energy for any living creature. His achievements were overlooked and priority for the discovery of the "mechanical equivalent of heat" was attributed to James Joule in the following year. He also proposed that plants convert light into chemical energy.
Émilie du Châtelet Gabrielle Émilie Le Tonnelier de Breteuil, Marquise Du Châtelet (] ; 17 December 1706  – 10 September 1749) was a French natural philosopher, mathematician, physicist, and author during the early 1730s until her untimely death due to childbirth in 1749. Her most recognized achievement is her translation of and commentary on Isaac Newton's book "Principia" containing basic laws of physics. The translation, published posthumously in 1759, is still considered the standard French translation today. Her commentary includes a profound contribution to Newtonian mechanics—the postulate of an additional conservation law for total energy, of which kinetic energy of motion is one element.