IN my letter of March 19 I endeavoured to show that the latent heat absorbed in the production of unit volume of stationary vibration of a particular frequency in a mixed beam of radiation, is not equal to 4q/c (where q is the energy stream per sq. cm. per sec., and c the velocity), as would naturally be supposed, but, in consequence of the Doppler effect at the moving mirror or piston, takes the form T(dp/dT), as required by Carnot's principle, where p is the pressure, or the mechanical work per unit volume, and is equal to 2q/c for a directly reflected beam under equilibrium conditions. The latent heat, T(dp/dT), may be represented as the sum (u + p) of the Intrinsic energy or internal latent heat u and the external work p. As a matter of interpretation, u was identified in my letter with some form of stationary vibration which continued to exist in the medium at the frequency at which it was emitted. Further analysis shows that this is not the case, but that the energy left in the medium conforms exactly to the distribution required by the theory of exchanges. The energy density is 2q/c in a directly reflected beam, and is equal to the pressure, but differs from the energy absorbed on emission or evolved on condensation, namely, the latent heat, T(dp/dT), which is the quantity measured experimentally, as previously explained (Phil. Mag., October, 1913, p. 787).