  This paper is concerned with a diffusion-controlled moving-boundary problem
in drug dissolution, in which the moving front passes from one medium to
another for which the diffusion coefficient is many orders of magnitude
smaller. It has been shown in an earlier paper that a similarity solution
exists while the front is passing through the first layer, but that this breaks
down in the second layer. Asymptotic methods are used to understand what is
happening in the second layer. Although this necessitates numerical
computation, one interesting outcome is that only one calculation is required,
no matter what the diffusion coefficient is for the second layer.
