  A detailed study of the absorbing molecular clouds towards the radio source
PKS1413+135, at a redshift z=0.247, is reported. Physical conditions are
derived for the molecular gas in the frame work of two models: a homogeneous
multicomponent model with equal filling factors, and a two-phase medium
consisting of dense clumps embedded in a more diffuse component. Our absorption
data are consistent with the presence of a diffuse gas component, dominating
the observed opacity, and a dense component, accounting for most of the mass.
We show that without knowledge of the small scale structure of the absorbing
molecular gas, we can only derive lower limits to the column density. We find
that the opacity ratio between two absorbing components in CO has varied by a
factor 2.3 \pm 0.3. We also discuss what can be derived from molecular
absorption line observations concerning the invariance of physical constants.
