High Quality Content by WIKIPEDIA articles! The discovery of the nature of radiation of heat was gradual. An important early contribution was made by Pierre Prévost in 1791. Prévost considered that what is nowadays called the photon gas or electromagnetic radiation was a fluid that he called "free heat". The present writer's translation of Prevost's 1791 definition is as follows: Absolute equilibrium of free heat is the state of this fluid in a portion of space which receives as much of it as it lets escape. Relative equilibrium of free heat is the state of this fluid in two portions of space which receive from each other equal quantities of heat, and which moreover are in absolute equilibrium, or experience precisely equal changes.