In thermodynamics, ergal, symbol J, is an expression, which can be expressed as a function of Cartesian coordinates, for the work W done by forces acting on a system of particles. The generalized expression for the ergal of a single particle acted on by a force is:

W = F(xyz) + const.

The term was introduced in the mid 1870s by German physicist Rudolf Clausius as the negative value of Irish mathematician William Hamilton’s “force function”, deemed to represent a quantity the subtraction of which gives the work performed, a quantity which had been also given the name “potential energy” by Scottish physicist William Rankine. [1]

Internal work
In reference to internal work, according to Clausius, the “internal forces”, i.e. those forces which the atoms and molecules of a body exert among themselves when undergoing a change, “have an ergal”. [1]

References
1. Clausius, Rudolf. (1879). The Mechanical Theory of Heat, (pgs. 11-20, 28). London: Macmillan & Co.

Further reading
● (a) Clausius, Rudolf. (1875). “On the Theorem of the Mean Ergal, and its Application to the Molecular Motions of Gases”. Philosophical Magazine, Vol. L., 4th Series, Jul-Dec.
(b) Translated from a separate impression, communicated by the Author, having been read at the Meeting of the Niederrheinische Gesellschaft fur Natur-und Heilkunde on November 09, 1874.