The first to create the concept of a thermodynamic system was the French physicist Sadi Carnot whose 1824 "Reflections on the Motive Power of Fire" studied what he called the "working substance", e.g., typically a body of water vapor, in steam engines, in regards to the system's ability to do work when heat is applied to it. The working substance could be put in contact with either a heat reservoir (a boiler), a cold reservoir (a stream of cold water), or a piston (to which the working body could do work by pushing on it). In 1850, the German physicist Rudolf Clausius generalized this picture to include the concept of the surroundings, and began referring to the system as a "working body". In his 1850 manuscript "On the Motive Power of Fire", Clausius wrote: