  Gravitational microlensing has proven to be a powerful probe of both the
structure at the heart of quasars and the mass function of compact objects in
foreground lenses. This paper examines the potential of gravitational
microlensing in probing the scale of structure in absorbing material within the
lensing galaxy. We find that, in this high optical depth regime, significant
variations in the equivalent width of absorption features can be induced,
although the details of these are dependent upon the scale of structure of the
absorbing material. The paper concludes with an examination of the absorption
line variability observed in the gravitationally lensed quasar PKS1830-211,
demonstrating how this may indicate the presence of small scale structure in
the cold molecular gas present within the lensing galaxy.
