Larmor, Joseph (1897), “On a Dynamical Theory of the Electric and Luminiferous Medium, Part 3, Relations with material media”, Philosophical Transactions of the Royal Society190: 205–300, Bibcode: 1897RSPTA.190..205L, doi:10.1098/rsta.1897.0020 “…that of the transmission of radiation across a medium permeated by molecules, each consisting of a system of electrons in steady orbital motion, and each capable of free oscillations about the steady state of motion with definite free periods analogous to those of the planetary inequalities of the Solar System;”
Larmor, Joseph (1897), “On a Dynamical Theory of the Electric and Luminiferous Medium, Part 3, Relations with material media”, Philosophical Transactions of the Royal Society190: 205–300, Bibcode: 1897RSPTA.190..205L, doi:10.1098/rsta.1897.0020 “…that of the transmission of radiation across a medium permeated by molecules, each consisting of a system of electrons in steady orbital motion, and each capable of free oscillations about the steady state of motion with definite free periods analogous to those of the planetary inequalities of the Solar System;”
Navarro, Luis. “Gibbs, Einstein and the Foundations of Statistical Mechanics.” Archive for History of Exact Sciences, vol. 53, no. 2, Springer, 1998, pp. 147–80, http://www.jstor.org/stable/41134058.