Einstein and the Formal Equivalence of Mass and Energy
  This is a brief look at how Einstein explored formal symmetries between quantized matter and quantized radiation between 1903 and 1925. Specifically he employed thermodynamic comparisons between the ideal molecular gas and the photon gas. His achievements are tied in with a more general pattern in physics to explore formal symmetries between quantized mass and quantized energy. 
Paul A. Klevgard
physics.hist-ph