  The 2200 A bump is a major figure of interstellar extinction. Extinction
curves with no bump however exist and are, with no exception, linear from the
near-infrared down to 2500 A at least, often over all the visible-UV spectrum.
The duality linear versus bump-like extinction curves can be used to
re-investigate the relationship between the bump and the continuum of
interstellar extinction, and answer questions as why do we observe two
different kinds of extinction (linear or with a bump) in interstellar clouds?
How are they related? How does the existence of two different extinction laws
fits with the requirement that extinction curves depend exclusively on the
reddening E(B-V) and on a single additional parameter? What is this free
parameter?
  It will be found that (1) interstellar dust models, which suppose the
existence of three different types of particles, each contributing to the
extinction in a specific wavelength range, fail to account for the
observations; (2) the 2200 A bump is very unlikely to be absorption by some yet
unidentified molecule; (3) the true law of interstellar extinction must be
linear from the visible to the far-UV, and is the same for all directions,
including other galaxies.
  In extinction curves with a bump the excess of starlight (or the lack of
extinction) observed at wavelengths less than lambda=4000 A is due to a large
contribution of light scattered by hydrogen on the line of sight. Although
counter-intuitive this contribution is predicted by theory. The free parameter
of interstellar extinction is related to distances between the observer, the
cloud on the line of sight, and the star behind it (the parameter is likely to
be the ratio of the distances from the cloud to the star and to the observer).
The continuum of the extinction curve or the bump contain no information
concerning the chemical composition of interstellar cloud.
