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In part one of these series we investigated the effect of Newtonian cooling on acoustic-gravitywaves in an isothermal atmosphere for large Prandtl number. It was shown that the atmospherecan be divided into two regions connected by an absorbing and reflecting layer, created by the exponentialincrease of the kinematic viscosity with height, and if Newtonian cooling coefficient goes to infinity thetemperature perturbation associated with the wave will be eliminated. In addition all linear relationsamong the perturbation quantities will be modified. In this paper we will consider the effect of Newtoniancooling on acoustic-gravity waves for small Prandtl number in an isothermal atmosphere. It is shown thatif the Newtonian cooling coefficient is small compared to the adiabatic cutoff frequency the atmospheremay be divided into three distinct regions. In the lower region the motion is adiabatic and the effect ofthe kinematic viscosity and thermal diffusivity are negligible, while the effect of these diffusivities is morepronounced in the upper region. In the middle region the effect of the thermal diffusivity is large, whilethat of the kinematic viscosity is still negligible. The two lower regions are connected by a semitransparentreflecting layer as a result of the exponential increase of the thermal diffusivity with height. The twoupper regions are joined by an absorbing and reflecting barrier created but the exponential increase of thekinematic viscosity. If the Newtonian cooling coefficient is large compared to the adiabatic cutoff frequency,the wavelengths below and above the lower reflecting layer will be equalized. Consequently the reflectionproduced by the thermal conduction is eliminated completely. This indicates that in the solar photospherethe temperature fluctuations may be smoothed by the transfer of radiation between any two regions withdifferent temperatures. Also the heat transfer by radiation is more dominant than the conduction process

Acoustic-gravity waves in a viscous and thermally conducting isothermal atmosphere (part II: For small Prandtl number)