Reappraising Einstein s 1905 application of thermodynamics and statistics to radiation

Einstein's 1905 application of thermodynamics and statistics to radiation, which led to the hypothesis of the corpuscular light quantum, is rendered uncertain by a lack of information as to how radiation behaves when undergoing a statistical fluctuation of volume. The paper examines this issue and appraises the particular assumption made by Einstein. We find that Einstein assumed for radiation a type of behaviour normally reserved for a gas of particles, in which case a conclusion about radiation behaving thermodynamically as though it consisted of particles (of energy) is not surprising.