In his initial paper  and in the latter portion of his text , Planck correctly derived an expression for the blackbody function. But Planck can never state, based on [section] 35-37, that interiors of all cavities contain black radiation.
Consequently, Schuster demonstrated that when the absorption coefficient of the layer was large with respect to the coefficient of scattering, the radiation observed from a large cloud of gas was the blackbody function: "The radiation in this case becomes equal to that of a completely black surface, which agrees with the well-known law that absorption irrespective of scattering tends to make the radiation of all bodies equal to that of a black body when the thickness is increased" [26, p.
Schuster expressed that the radiation emitted by the absorbing layer was the product of the absorption coefficient, k, multiplied by the blackbody function, E, and the thickness of the layer, dx: kEdx [26, p.
Once Schuster assumed that the blackbody function could be directly applied to represent the emission of the gas, a great thickness guaranteed that blackbody radiation was produced, even if the coefficient of absorption was small, merely because the coefficient of scattering was much smaller (see Eq.