Rowlinson, J.S. (2009) James Joule, William Thomson and the Concept of a Perfect Gas.
http://rsnr.royalsocietypublishing.org/content/64/1/43.full, extract: In 1845 Joule had tried to verify this assumption, which Thomson always called “Mayer’s hypothesis”, by expanding a cylinder of compressed gas into an empty cylinder but, as he later admitted, he failed because the heat capacity of his vessels was too large compared with that of the gas. We now know that for air at atmospheric pressure and temperature the difference between the two terms on the right of this equation is only about 3 parts per thousand of either of them.

has been cited by the following article:

• TITLE: Thermodynamics and Irreversibility: From Some Paradoxes to the Efficiency of Effective Engines

  AUTHORS: Olivier Serret

  KEYWORDS: Heat Transfer, External and Internal Pressure Work, Internal Energy, Reversible and Irreversible, Joule’s Law and Joule’s Experiments, Adiabatic and Free Expansion, Clapeyron Diagram, Carnot Cycle, Engine Efficiency

  JOURNAL NAME: Journal of Modern Physics, Vol.5 No.16, October 20, 2014

  ABSTRACT: The traditional thermodynamic theory explains the reversible phenomena quite well, except that reversible phenomena are rare or even impossible in practice. Here the purpose is to propose an explanation valid for reversible and also irreversible phenomena, irreversibility being common or realistic. It previously exposed points tricky to grasp, as the sign of the work exchange, the adiabatic expansion in vacuum (free expansion) or the transfer of heat between two bodies at the same temperature (isothermal transfer). After having slightly modified the concepts of heat transfer (each body produces heat according to its own temperature) and work (distinguishing external pressure from internal pressure), the previous points are more easily explained. At last, an engine efficiency in case of irreversible transfer is proposed. This paper is focused on the form of thermodynamics, on “explanations”; it does not question on “results” (except the irreversible free expansion of 1845...) which remain unchanged.