[1] C. Truesdell (1973) - The efficiency of a homogeneous heat engine, «Journal of Mathematical and Physical Sciences» (Madras) 7, 349-371, and 9 (1975), 193-194; Sul rendimento delle macchine termiche omogenee, «Accad. Naz. dei Lincei, Rend. Cl. Sci. Fis. Mat.», (8) 53 (1972), 550-553 (1973); Improved estimates of the efficiencies of irreversible heat engines, «Annali di Mat. Pura ed Applic.» (4) 108 (1976), 305-323.

[2] R.L. Fosdick and J. Serrin (1975) - Global properties of continuum thermodynamic processes, «Arch. Rational Mech. Anal.», 59, 97-109. | Zbl 0332.73004

[3] C. Truesdell and R.G. Muncaster (1980) - Fundamentals of Maxwell's Kinetic Theory of a Simple Monatomic Gas, treated as a Branch of Rational Mechanics, New York etc., Academic Press. See Section (v) of Chapter I, where we prove the statement in the text above under the assumption that there are no sources of heat but without requiring the field $\tau$ to be constant at each time. If $\tau$ is a function of time alone, it is easy to include sources of heat.

[4] These bodies seem to have been introduced into thermodynamics in a wasted lecture, «Thermodynamics for beginners», pp. 373-389 of «Proc. IUTAM Symposia Vienna 1966»; Wien and New York, Springer, 1968. An improved treatment may be found in Chapter I of my Rational Thermodynamics, A Course of Lectures on Selected Topics, New York etc., McGraw-Hill, 1969, and further results in my paper in the Annali di Matematica, cited in reference 1.

[5] For an account of Clausius' work see § 11D of my Tragicomical History of Thermodynamics, 1822-1854, New York, Springer-Verlag, 1980.