"We are... concerned with the evolution of tar and gases during the plastic state of coal. In this... consists of two processes in series: diffusion through the molten coal to some internal surface, that of a bubble or a pore; and transport with the bubble or through the pore to the surface of the particle. The role of preexisting pores is not well understood. ...[A] certain fraction of preexisting pores (< 60 Å) collapse during pyrolysis perhaps due to effects. Pores... 60-300 Å were preserved... but one could not distinguish preexisting pores and pores generated by the evolution of bubbles. It appears likely... the major... mass transfer occurs via bubbles while preexisting bubbles play a... minor role."