  We determine the extinction curve from the UV to the near-IR for molecular
clouds and investigate whether current models can adequately explain this
wavelength dependence of the extinction. The aim is also to interpret the
extinction in terms of H2 column density.
  We applied five different methods, including a new method for simultaneously
determining the reddening law and the classification of the background stars.
Our method is based on multicolour observations and a grid of model
atmospheres.
  We confirm that the extinction law can be adequately described by a single
parameter, RV (the selective to absolute extinction), in accordance with
earlier findings. The RV value for B 335 is RV = 4.8. The reddening curve can
be accurately reproduced by model calculations. By assuming that all the
silicon is bound in silicate grains, we can interpret the reddening in terms of
column density, NH = 4.4 (\pm0.5) \times 1021 EI-Ks cm-2, corresponding to NH =
2.3 (\pm0.2) \times 1021 \cdot AV cm-2, close to that of the diffuse ISM,
(1.8-2.2) \times 1021 cm-2 . We show that the density of the B 335 globule
outer shells can be modelled as an evolved Ebert-Bonnor gas sphere with {\rho}
\propto r-2, and estimate the mass of this globule to 2.5 Msun
