  We used the multifiber spectrograph FLAMES on the ESO Very Large Telescope
UT2 to derive atmospheric parameters, metallicities and abundances of O and Na
for 79 red giant stars in the Galactic globular cluster NGC 6218 (M 12). We
analyzed stars in the magnitude range from about 1 mag below the bump to the
tip of the Red Giant Branch. The average metallicity we derive is
[Fe/H]=-1.31+/-0.004+/-0.028 dex (random and systematic errors, respectively),
with a very small star-to-star scatter (rms=0.033 dex), from moderately
high-resolution Giraffe spectra. This is the first extensive spectroscopic
abundance analysis in this cluster. Our results indicate that NGC 6218 is very
homogeneous as far as heavy elements are concerned. On the other hand, light
elements involved in the well known proton-capture reactions of H-burning at
high temperature, such as O and Na, show large variations, anticorrelated with
each other, at all luminosities along the red giant branch. The conclusion is
that the Na-O anticorrelation must be established in early times at the cluster
formation. We interpret the variation of Na found near the RGB-bump as the
effect of two distinct populations having different bump luminosities, as
predicted for different He content. To our knowledge, NGC 6218 is the first GC
where such a signature has been spectroscopically detected, when combined with
consistent and homogeneous data obtained for NGC 6752 to gain in statistical
significance.
