In 1850, Rudolf Clausius was referring to U as an "arbitrary function of T and V".
In 1851, William Thomson was using the phrase "mechanical energy of a body in a given state”.
In 1858, William Rankine, in his Manual of Applied Mechanics, defined internal energy as follows:
- “The total internal energy, actual and potential, of a system of bodies, cannot be changed by their mutual actions. This is a proposition made known partly by reasoning and partly by experiment.”
In 1875, Clausius, in his Mechanical Theory of Heat (second edition), compared his U function to similar variations given by other authors on the subject of heat: William Thomson (1851) calls it the “mechanical energy of a body in a given state”, Gustav Kirchhoff calls it “function of activity” (wirkungsfunction), and Gustav Zeuner calls it “interior heat” (1860) of the body as well as “internal work” (1866) of the body. Clausius, in the end, called U it the "energy of the system".
In 1894, James Ewing, in his Steam Engine and other Heat Engines, citing: Rankine, Thomson, and Clausius, respectively in predominance, was employing the term “internal energy” in essentially the Clausius scheme of things.
The following are related quotes:
- “The total energy of the system will then differ from the sum of the internal energies of the two atoms, i.e. twice the energy of the ground state of H, by a certain ‘interaction energy’, E(R), which will depend on the distance R.”
- — Walter Heitler (1956), Elementary Wave Mechanics (pg. 124)
- Rankine, William. (1858). A Manual of Applied Mechanics (pg. 508). Publisher.
- Clausius, Rudolf. (1875). The Mechanical Theory of Heat (translator: Walter Browne) (energy of system, pg. 20, terminology, pgs. 31-32). Macmillan, 1879.
- Ewing, James. (1894). Steam Engine: and other Heat Engines (Rankine, 19+ pgs; Clausius, 6+ pgs; Thomson, 5+ pgs; internal energy, 16+ pgs). Publisher.
- Note: the full details of this, via analysis of the key authors, over the century, however, has not as of yet been completed.
- Internal energy – Hmolpedia 2020.