In the history of thermodynamics, the working substance is the term used by French physicist Sadi Carnot in 1824 to define any generalized body, such as a body of water, a metallic bar, an aeriform fluid, etc., that may be configured to do work when put in alternating contact with hot and cold bodies able to supply or absorb heat. [1] In 1850, building on the work of Carnot, German physicist Rudolf Clausius rephrased this concept using the term "working body" [2] Carnot’s original diagram of this “working substance” is pictured adjacent with the hot body A, the cold body B, and the working substance contained within the cylinder and the adjustable piston.

Overview
In opening pages of his 1824 memoir, Carnot outlines the view that: “the phenomenon of the production of motion by heat has not been considered from a sufficiently general point of view”. He continues, “we have considered it only in machines the nature and mode of action of which have not allowed us to take in the whole extent of application of which it is possible.” More to the point “in such machines the phenomenon is, in a way, incomplete. It becomes difficult to recognize its principles and study its laws.” Also, “in order to consider in the most general way the principle of the production of motion by heat”, Carnot states, “it must be considered independently of any mechanism or any particular agent.” Specifically:

“It is necessary to establish principles applicable not only to steam-engines but to all imaginable heat-engines, whatever the working substance and whatever the method by which it is operated”

In an endnote to this comment, Carnot details that “we distinguish here the steam-engine from the heat engine in general … the latter may make use of any agent whatever, of the vapor of water or of any other, to develop the motive power of heat.” In more detail, regarding this any-type-of-substance proposal, Carnot points out that:

“wherever there exists a difference of temperature, wherever it has been possible for the equilibrium of the caloric to be re-established, it is possible to have also the production of impelling power. Steam is a means of realizing this power, but it is not the only one. All substances in nature can be employed for this purpose, all are susceptible of change of volume, of successive contradictions and dilations, through the alternation of heat and cold.”

Here we see the core of the extrapolation of the science of thermodynamics to systems of working bodies on the earth, being alternatively (diurnally) put in contact with a hot body (the day sun) and a cold body (the cool night sky), thus being modified by “successive contradictions and dilations, through the alternation of heat and cold”. Indeed, such a terminology, i.e. “working medium”, was used by Russian bioelectrochemist Octavian Ksenzhek in 2007. [3]

Carnot continues, “a solid body—a metallic bar for example—alternately heated and cooled increases and diminishes in length, and can move bodies fastened to its ends. A liquid alternatively heated and cooled increases and diminishes in volume, and can overcome obstacles of greater or less size, opposed to its dilatation.”

To give some examples, Carnot states “an aeriform fluid is susceptible of considerable change of volume by variations of temperature. If it is enclosed in an expansible space, such as a cylinder provided with a piston, it will produce movements of great extent. Vapors of all substances capable of passing into gaseous condition, as of alcohol, of mercury, of sulphur, etc., may fulfill the same office as a vapor of water. The latter, alternately heated and cooled, would produce motive power in the shape of permanent gases, that is, without ever returning to a liquid state. Most of these substances have been proposed, many have even been tried, although up to this time perhaps without remarkable success.”

To give a visual picture of the relation between the hot body, cold body, and working substance, he states: “to heat any substance whatever requires a body warmer than the one to be heated; to cool it requires a cooler body. We supply caloric to the first of these bodies that we may transmit to the second by means of the intermediary substance.”

References
1. Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power.” Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.
2. Clausius, Rudolf. (1850). "On the Motive Power of Heat, and on the Laws Which can be Deduced From it for the Theory of Heat." Poggendorff's Annalen der Physik, LXXIX, 368, 500.
3. Ksenzhek, Octavian S. (2007). Money: Virtual Energy - Economy through the Prism of Thermodynamics, (pgs. 162, 170). Universal Publishers.