Today everybody knows that energy can not be created or destroyed. We know this principle as the law of conservation of energy  (or the First Law of thermodynamics). Today we use this law in many situations perhaps without always realizing this. However, there was a time that there was great confusion about what energy, work and heat exactly was and their relation. It was the German physician Mayer who first formulated in 1842 a statement that can be considered as the predecessor of today’s energy law. At the occasion of the 100th anniversary of Mayer’s discovery, a small (91 pages) book was published in 1943 by Prof. Jacob Clay, a physics professor at the university of Amsterdam, entitled: “Onstaan en ontwikkeling van het Energie Beginsel”[1] (the book is in Dutch and the title can be translated by “Origin and Development of the Energy Principle”). The book contains some interesting historical elaborations of how the energy law developed. Because the book is not so easy to get anymore, I will summarize the chapter where Clay describes how Mayer got to his energy law.

Julius Robert Mayer, born in 1814 in Heilbronn (Germany, not too far from the French border) who, after several world voyages, became a medical doctor in 1841 also in Heilbronn. He was probably driven to understand energy issues by the question how chemical energy was transformed in living beings to do things like work[2] and generate heat. Mayer got convinced that perpetual engines did not exist (“…. das mechanische Arbeit sich nicht aus Nichts erzeugen lasse.” translated as “mechanical work can not be created from nothing”). Also, while studying processes where changes happened (such as the decrease of velocity of moving bodies and the simultaneous appearance of heat) Mayer was searching for something that would be constant. His inspiration for the existence of a  constant factor came from his colleagues in chemistry who used the law of conservation of mass (formulated in 1789 by Antoine Lavoisier) quite successfully. Mayer observed that “lebende Kraft” (literally “living force” but nowadays what we call  that kinetic energy) could be transformed into heat[3]. In addition he noted that the amount of heat needed to warm a given quantity of gas was larger at constant volume (no work from the PDV term!) than when heated at constant pressure. This led him to formulate the equivalence of work (or energy) and heat and he made a first estimate of this equivalency factor (which today is known at 1 calorie = 4.184 Joule).

The contributions by Mayer were for quite some years neglected by the scientific community. One reason was that he did not very clearly present his thoughts[4]. In addition he had competition  from an English beer brewer: John Prescot Joule who arrived  -somewhat later though- to similar conclusions but who was better connected to institutes such as the French Scientific Academy[5].

However, from 1862 onwards Mayer’s work became more and more acknowledged and he won several prestigious awards underlining the importance of his contributions. In 1847 Hermann von  Helmholtz published his famous article entitled: “Ueber die Erhaltung der Kraft” (“On the conservation of energy”) and this can be seen as an important milestone of the development of the First Law of thermodynamics. I will come back to that article soon.