  We investigate here the origin of the gas observed in extended emission line
regions surrounding AGNs. We use the technique of calcium depletion as a test
to prove or disprove the existence of dust in such a gas in order to
discriminate between two main theories: (1) a cooling process from a hotter
X-ray emitting phase surrounding the galaxy, (2) merging or tidal interaction
between two or more components. We have obtained long slit spectroscopy of a
sample of objects representative of different galaxy types although our main
interest focus on radio galaxies. The spectral range was set to always
includethe [CaII]$\lambda\lambda$7291,7324 doublet. The faintness or absence of
such lines is interpreted as due to the depletion of calcium onto the dust
grains and, therefore, is a proof of the existence of dust mixed with the gas
in the EELRs.
