  We analyze a 137 ks exposure X-ray spectrum of the Seyfert 1 galaxy NGC 5548
obtained with the XMM-Newton Reflection Grating Spectrometer. Due to the long
exposure time, the spectrum is of higher statistical quality than the previous
observations of this AGN. Therefore, we detect for the first time in NGC 5548
inner-shell transitions from O III to O VI ions, and the Unresolved Transition
Array of M-shell iron. The warm absorber found from this X-ray observation
spans three orders of magnitude in ionization parameter. We detect O III, which
is as lowly ionized as the warm absorber detected in the UV band, to Fe XXIV.
For O VI the column density determined from our X-ray data is an order of
magnitude larger than the column density measured in previous UV observations.
We conclude that there is substantially more low ionized material than
previously deduced from UV observations. However, only a few percent of the
warm absorber detected in the X-rays is lowly ionized. A 99.9 % significant
increase in the derived absorbing column density with higher ionization states
is observed. The outflow velocities determined from the X-ray absorption lines
are consistent with those deduced from the UV lines, evidence, together with
the detection of O VI, that the X-ray and UV warm absorber are different
manifestations of the same phenomenon. From a simple mass conservation
argument, we indicate that our data set is consistent with an outflow with
small opening angle formed due to instabilities in the accretion disk. Possible
due to uncertainties in the radiative transport mechanism, an apparent deviant
iron to oxygen abundance is detected. No strong relativistically broadened
emission lines of O VIII, N VII and C VI were detected.
